INFORMATION TO USERS

This material was produced from a microfilm copy of the original document. While the most advanced technological means to photograph and reproduce this document have been used, the quality is heavily dependent upon the quality of the original submitted.

The following explanation of techniques is provided to help you understand markings or patterns which may appear on this reproduction.

1. The sign or "target" for pages apparently lacking from the document photographed is "Missing Page(s)". If it was possible to obtain the missing page(s) or section, they are spliced into the film along with adjacent pages. This may have necessitated cutting thru an image and duplicating adjacent pages to insure you complete continuity.

2. When an image on the film is obliterated with a large round black me/k, it is an indication that the photographer suspected that the copy may have moved during exposure and thus cause a blurred image. You will find a good image of the page in the adjacent frame.

3. When a map, drawing or chart, etc., was part of the material being photographed the photographer followed a definite method in "sectioning" the material. It is customary to begin photoing at the upper left hand corner of a large sheet and to continue photoing from left to right in equal sections with a small overlap. If necessary, sectioning is continued again — beginning below the first row and continuing on until complete.

4. The majority of users indicate that the textual content is of greatest value, however, a somewhat higher quality reproduction could be made from "photographs" if essential to the understanding of the dissertation. Silver prints of "photographs" may be ordered at additional charge by writing the Order Department, giving the catalog number, title, author and specific pages you wish reproduced.

5. PLEASE NOTE: Some pages may have indistinct print. Filmed as received.

Xerox University Microfilms 300 North Zoeb Road Ann Arbor, Michigan 48106 INTRODUCTION

I. General Background

Boron Is the only element in the periodic table whose hydrides display a systematic chemistry even remotely similar to the chemistry of the carbon atom, "Whereas organic chemistry follows conventional concepts of structure and bonding (tetrahedral sp3 hybridized carbon —

CnH^^.4, trigonal sp2 — CnHn+2, and linear sp hybridization - which apply to some compounds containing boron (for example,BCI3 — sp2 hybridized boron, trigonal planar molecule), the boron hydrides as a class require special considerations of structure and bonding " .

This is because the molecules have more orbitals than the available electrons can f ill following a conventional 2-center bonding scheme) the term electron deficient is frequently applied to these compounds and the hybridization of the boron atoms is variable. For Instance, the boron atoms in the simplest hydride, B2HQ, have been described as hav- 2 5 * ing sp * hybridization 5 similarly,, in order to satisfy the unusual 5 angles of bonding in B5I& and B5H11, Lipscomb provided an explanation based upon a high degree of !p* character in specific boron atoms#

Molecular formulas of the lower and intermediate boron hydrides indicated that an arbitrary division into Bn^ +4 ®n^n+Q 91

TABLE 13

The Mass Spectrum of BaH7:P( 013)3 m/e I n te n s ity m/e In te n s:

26 16 1*7 1*2

27 1*1 1*8 —

28 100 1*9

29 22 50 —

• 3 0 - 51 12

31 — 52 —

32 18 53 —

33 —

3** ----- 55 11

35 8 56 12

36 ll* • 57 68

37 ll* 58 25

38 — 59 100

39 13 60 15

1*0 1*1 61 100

in 100 62 92

42 1*1 63 —

1*3 61 61* —

1*1* 22 65 —

1*5 92 66 —

1*6 26 67 9 92

TABLE 15 (continued)

m/e Intensity m/e Intensity

68 92

69 9 93 70 — Bk

71 13 95 72 96

73 20 97 10

7k 18 98 10

75' 7k 99 to

76 100 100 75

7 7 yj 1 0 1 2 0

78 — 102

79 — ' 105

80 10^ — **

81 — 105

82 — 106

85 — 107

81* — 108

85 — . 109

86 — HO

87 26 111 l 1*

88 30 112 56

89 86 113 100

90 — 111* 100

91 — 115 32 93

TABLE l 4

X-Ray Powder Pattern Data For B3H7:P(CHa)3

R e la tiv e R elative d.(g) In te n s ity d i l l In te n s ity

6. Itl86 vw 2.5081 s

5 .8 6 7 1 vvs 2.3324 ms

5 .5 0 5 0 vw 2 .2 8 1 3 ms

^.9279 vvs 2.0804 wvw

4.6 225 w 2.0402 wvw

t.3 9 5 9 wvw 1 .9811 vvw 4.1520 vvvw 1 .9^50 mw

3.9515 vw, 1.9103 mw

3.7697 vs 1 .8 7 6 9 mw

3.5900 vs 1 .7648 mw

3. 4530 wvw 1 .6894 w

5.3264 wvw 1 .6 5 0 2 vw

3.1865 wvw 1 .5901 vw

2.9980 w 1.5535 wvw

2.9214 vw 1 .5 2 1 1 vw

2.8577 ms

2.6367 mw

v = very, m = medium, s = strong, w «= weak the material had been exposed to the atmosphere i'or about two hours.

The position of the absorptions had not changed.appreciably, but the

peaks were less distinct and not as sharp as the original sample IR.

It is apparent that handling oi* the material should probably be

confined to inert environments.

6. Elemental analysis

Samples of B3H7:P( 013)3 were subjected to hydrolysis in concen­

tr a te d HC1 and NaOH solutions.Both were incomplete with regard to both

boron and hydrogen analysis, even after heating to over 100°C fo r

several days. Some solid remained in the- vessel throughout the hydroly­

ses; it appeared that about 60$ completion had been achieved.

F. Investigation of the tetraborane( 10)-trimethylamine system

Ihe results of this aspect of the work paralleled somewhat the

observations made with the B4H1o-P{CH3)3 system. A white precipitate

formed when H(CH3)3 was gradually added to a stirring solution of B4H10

in pentane at -95°C. The solvent could be removed at a low enough o temperature that the adduct could be isolated neat below -78 C, Around

0.2 .to 0.25 equivalent B2HS could be isolated as the solid was slowly

warmed from - 78 °C to above 0°C. Stoichiometric studies were not

specifically carried out, but there was no evidence to indicate that

the adduct should not be formulated B4H10: K( CH3) 3.

Contrary to observations made with hexaborane( 12)-trimethylamine systems, it proved convenient, though not extremely fruitful, to study solutions of the adduct in HCF2CI by KMR spectroscopy. The boron- 95

11 nMR spectrum o f B4Hi0:N( 0113)3 a t -80 °C consisted of two singlets, c lo se to 3 :1 In area ratio at 20 ppm and 34 ppm, respectively. No appreciable change in the spectrum was produced by either mild changes in temperature or by decoupling experiments.

The original white solid was taken to 0°C very slow ly before th e solvent was added and the above NMR spectrum run. No B2H6 was observed during the warming. Since continued temperature elevation caused some

B2HQ to be liberated from the solid, and the elimination product could be identified by its infrared spectrum as authentic B3H7: N( 0113)3, the eq u atio n

B4H10: N( CH3) 3 ------* iB 2He + B3B7 : N( CH3) 3 ( 35) does appear to govern the system under the conditions described. I

G. The investigation of low temperature reactions between pentaborane-

( 1 1 ) and selected Lewis bases

1. Pentaborane(11) and trimethylphosphine

In a typical reaction, 0 .5 4 mmo B5H11 was transferred to a reaction vessel equipped with an NMR sidearm. About one m illiliter of pentane was added and stirred at - 95°C. One e q u iv a le n t o f P(CH3) 3 was added gradually to the stirring mixture and reacted over a period of 45 minutes. A white precipitate slowly formed which eventually was thick enough to impede stirring. The volatiles were pumped off as the temper- o ature slowly rose to -7 8 C. After several hours of pumping on the con­ te n ts a t -78 °C, 0 ,5 ml of a mixture of CHC1 3 and HCF2CI ( 1 : 1 ) was added 96

to the solid and mixed at low temperatures. The mixture, of a grayish

white color, was poured into the precooled sidearm and sealed off.

The "boron -1 1 NMR spectrum of the sample exhibited an irregularly

shaped resonance of relative area three at 8.8 ppm, an area one

triplet at ppm and an area one singlet at53*2 ppm. Protcn sp in

decoupling experiments showed that the trip let could be collapsed to

a doublet, Indicating the presence of an H-B-P(CH3)3 group. The

downfield resonance was not appreciably affected by the decoupling.

Since this adduct was tentatively identified as BgHn:P( 0113)3,

several attempts were made to produce the elimination product from the

white solid initially formed, according to the equation

BsHh:P(CH3) 3 ------► iB2H6 + B4Hb:P(CH 3)3 ( 36) 1 When the system was monitored as the temperature was allowed to rise

slowly from ~rjQ0Ct no B2Hq was observed to temperatures as high as

- 20°C. If the temperature were raised to between - 10°C and 0°C, th e

solid turned gray and decomposed visibly with little additional discoloration. A small amount of diborane was evolved in the process.

All attempts to isolate a four-boron species produced after BH3 elimination and before decomposition of the vessel's contents failed.

Boron-1 1 NMR spectra of various temperature stage samples of the solid showed the same resonances as the low temperature product, BsHii:P(CH3)3.

Reactions between N(CH3)3 and BsHn:P( 023)3 were carried out under various conditions to determine whether a reaction such as that shown here would occur: 97

BsH ii: P( CH3) 3 + N( CH3) 3 ------► H^BN( CH3) 3 + B4Hs: P( CH3) 3 ( 37 )

No material positively identifiable as B4H^;P(CH3)3 was isolable.

Two factors indicated that pursuit of this technique may be useful;

1) H3BN(CH3)3 was present in a mixture of the reactants as indicated

by the boron -1 1 NMR spectrum,, and 2 ) the same spectrum exhibited

structure in the upfield resonance similar to the triplets seen in

spectra of B^Hq:L adducts previously referred to. If the reaction

can be driven nearly to completion and the desired product isolated

relatively pure, NMR characterization of B4Ea:P(CH3)3 may be possible.

2. Pentaborane(ll) and ammonia

Equivalent amounts of B5HJ.1 and NH3 were mixed in methylene

chloride at as low a temperature as possible.A trace of solid material

was present in the mixture, but an NMR sample could be conveniently poured over which contained very little solid.

The boron -1 1 NMR spectrum of this sample, seemingly best assigned to an adduct of the formula BsHn;NH3, is shown in Figure 1 6 , along with the decoupled spectrum. In comparison, spectra of a sample of

3-CH3B5H10 and NH3 sim ilarly prepared, suggested a mixture of products present> more than one resonance was observed in the upfield (above

Uo ppm) region of the spectrum. Pertinent information which was obtained from the boron -1 1 NMR spectrum of BsHniNH3 is given in Table 15 . D ecoupled 99

TABLE 15

Boron-1 1 NMR Data for BsHn:ItH3 In CHsClg at -75 °C

R el. Chemical Coupling Resonance Area Assignment S h if t( ppm) Constant(Hz)

t r i p l e t 1 BH2 group - 7 .fc 115 ’

do u b let 2 B-H group - 1 .5 150

d o u b let 1 H-B-NH3 group +12.0 150

d o ublet 1 a p ic a l B-H +5 ^ .3 125

The proton NMR spectrum of BsHn:NH3 suggests that the ammonia molecule is involved in adduct formation and not proton abstraction by the position of the resonance attributable to the three equivalent protons on the ammonia molecule; it is positioned at 5*6 tau.

The presence of resonances distinctly attributable to specific basal borons in the boron -1 1 NMR spectrum of BsHn:NH3 indicates that this adduct differs considerably from the BsHi2:NH3 adduct which exhibits a definite fluxional character that averages out the 'basal1 type of resonances under similar conditions. Further investigation of this aspect of the problem is described in the discussion section.

Regarding research with pentaborane( 11 ), elaboration of the work which has been done with regard to this problem would be in order since the chemistry of BsHu has not been extensively explored. This study was limited by factors of time and availability of the starting materials B5H11 and B4H10J the latter compound is critical in that pentaborane(11) can be easily made from it. DISCUSSION and conclusions

I. Nuclear Ifegnetic Resonance Studies of Hexaborane( 1 2 ) -Ammonia

Reaction Mixtures

When it was established that characterization of the unstable white solid m aterial designated previously as HaE(NH3)a+EsHio" was not conveniently possible, the only means available by which data could be gathered for a Bq Hi 2-NH3 system involved NMR spectroscopy of reaction mixtures. The 1 :1 adduct, BQHi2:NH3, proved to be soluble in methylene chloride at a ll temperatures Investigated, but the 2:1 adduct, E6Hi2*2NH3, was soluble only at temperatures below - 85 °C. Even at such a temperature enough information could be obtained that the two species of adducts could be compared.

The boron -1 1 NMR spectrum o f BQHi2*2NH3 shown in Figure 17 ex h ib its a broad basal resonance centered at 10 ppm and a striking doublet very far upfield arising from an apical B-H group. The chemical shift of

63 ppm (J = 95 Hz) is noteworthy because the highest reported chemical shift for an intermediate boron hydride occurs at 55 ppm for the l i e apical boron of B5H11 . The potential relationship between B6Hx2 and B5H1X was alluded to in the introduction. Shifts approaching 60 ppm 68 94 have been observed for BsH^iL and B4Hs:L species, especially when the ligand, L, Is a fluorine substituted phosphine base.

100 classifications might be expedient. In the first group, B2H0, BsHg,

BSH1 0 and- B1 0 H1 4 were among the first materials isolated and char- 6 acterized by Alfred Stock . These compounds were categorized as 'stable',

a description which is applicable in many respects as long as the 7 8 context of the characterization is well defined and understood

The 'unstable' boranes include B*H10j B5H11 and BeHi2 and are

immediately differentiable from the stable boron hydrides by the fact

that the BnI^+6 structures appear to be relatively open, flexible,

and vulnerable.

In time a separate distinction was developed for borane fragments

or intermediates such as those formulated B3H7, B3H9 and B^Hq J th e ir

classification as stable or unstable Species according to formula must

be tempered by recognition that their existence is either of extremely brief duration or predicated upon stabilization through a second coordinated molecule, usually a phosphine, amine or ether base.

It is only within the last decade that preparations for individual boron hydrides have been developed which allow relatively easy separation of the desired product in good yield. Stock's original method of producing boranes involved acid hydrolysis of magnesium boridej complex mixtures of boranes have been produced by treating 9 10 (CHaJ^BsHe or (02% )3NHB3Hq with polyphosphoric acid or by deconrpos- 11 ing B2H6 in a silent discharge . Also, syntheses were developed which involved pyrolysis or copyrolysis of various lower boron hydrides, but in a ll these preparations the yields of any given boron hydride were very small, usually below ten percent, and the separation of products F ig u re 1 7 : The Boron-1 1 NMR Spectrum of B6H12*2rjH3 in CHaCls, - 95°G 102

In the proton NMR spectrum of B6Hia*2NH3j the resonance attributable to the ammonia protons had a temperature dependent chemical shift. Hie peak was situated between 7*0 and 8.0 tau, and was observed to move toward the upfield portion of the spectrum as the temperature was raised until the material in the NMR tube had precipitated from solution and the ammonia signal was obscured.

Nuclear Magnetic Resonance studies of 1 :1 BsHis/NH3 mixtures showed that rearrangement of the borane framework upon ammonia attack could be observed as a stepwise sequence dependent primarily upon temperature. Variations in concentration and mode of reactant mixing occasionally resulted in changes in the spectrum.

Hie B6Hi2:NH3 species was observable in three distinct stages a3 shown in Figures 18 and 2 0 . At a temperature of about - 85 °C th e doublet-triplet-doublet elements of the BQHi2 NMR spectrum were compressed into an area between - 2h ppm and +10 ppm. H iis seemed to reflect a dynamic system since no constant or exact area ratio could be determined.

Upon raising the temperature to above -75 °C a new and ap p aren tly unrelated spectrum appeared) area ratios are ^:1;1 in the spectrum at the bottom of figure 18, no proton spin decoupling. With decoupling, the area four resonance was resolved into two apparently equal area peaks at zero ppm and six ppm. Hie rough doublets, which arose at20 ppm and 31.5 ppm exhibited only fair splitting) the latter peak was somewhat sharper. Hie decoupled proton NMR spectrum of stage II of

BqHx2: M3 is shown in Figure 19, and the information obtained from Stage I

Figure 1 8 : Boron-rll NMR Spectra oi* BqHi2 :NH3j Stages X and. II

* Figure 1 9 : Tne Decoupled Proton NMR Spectrum o r B3H12:NHs, Stage IX, in CH2CI2 *lOT 1 0 5 the spectrum is provided in Table 16,

TABIE 1 6 o Proton ME Data For BQHi2:MHa, Stage II, in CH2CI2 at -7 0 G

Chemical Relative Shift; (tau) Area Assignment

6.15 3 M 3

7.1 3 Ht’B3,5

k Ht - B2,6 8 . 1. 1 Ht -B4

11.2 1 Ht'Bl

11.1*5 2 n-H I 1 1 .9 1 H

VVA// \\ // B B / B r 1 r 8 V « A b A . \ / f /P ^ n h 3

" - B - H B *

(X) ( I I )

Above are shown the two most feasible structures which can be io 6

Y3 - postulated for this BeHi3 analog. Compelling arguments can be made

for both structures based upon the NMR data, but the likelihood of

some fluxional behavior in the molecule precludes positive elimination

of either structure.

The contention that the ammonia molecule is involved in adduct

formation rather than proton abstraction is supported by the position

of the resonance unequivocally assigned to the ammonia protons in the

PMR spectrum of stage II, Although little information is available

concerning the proton resonances of ammonia bonded covalently to

intermediate borane frameworks, the observed resonances in the PMR

spectra of both BQHxa: NH3 and BsHn:NH3 are too far dovnfield to + be regarded as characteristic of either free NII3 or the NH4 ion.

As far as chemical intuition is concerned, structure (I) might be

favored over structure (II) considering that the attack by one molecule

of a weak base such as ammonia which does not effect cleavage could as well be conducive to maintaining the boron-boron bond in the system*

A mechanism for such an attack and rearrangement is suggested on page 107.

The sim ilarities between structure (I) and the tetraborane(10) structure are reflected in both the boron -1 1 and proton NMR spectra of stage II, BqHx2sKH3. The boron atoms which make up the boron-boron bond of (I) could easily be assigned to the upfield doublets in Figure lQ, One of these borons, probably B^, could be responsible for the 107

resonance at 8,lj- tau in the PMR spectrum described. This is very

close to the corresponding tetraborane(1 0 ) B-H pro to n chem ical

shift of 8,0 tau .

\/ \ l a - BU ^ B D

.B — y ?

;-y NH.

•'Inh3t *

vN . t / w n

/ ^ • b A B B NH. J

Xn addition, the PMR spectrum exhibits two resonances of total area three reasonably attributable to bridging protons, although partial overlap make3 area ratio determination and positive assignment d i f f i c u l t . 108

In support of structure (II), ‘both decoupled spectra (boron-1 1 and proton) show a significant amount of symmetry in the molecule, a problem which cannot be resolved by considering (I) only in static terms, or by invoking the concept of coincidental overlap. Also, the rather poorly resolved doublet at 20 ppm could be attributed to the unique H-B-NH3 group of (II) 3 line broadening caused by quadrupolar 14 ,1 0 3 e ff e c ts 9 from the nitrogen bonded to boron could be partially responsible for the appearance of this resonance. Comparing the resonance with the broad doublet at 12 ppm in th e boron-1 1 NMR spectrum of BsHii:NH3 (Figure 16, p. $8) lends credence to the idea that the

H-B-NH3 group is common to both molecules.

Considering the available evidence, the suggestion of a fluxional type of behavior for the B6Ki2:NH3 species serves to satisfy the ob­ servations made without compromising either structure, This contention is supported by examination of the third stage of the BeHxa: NH3 system.

As the temperature of the solution is raised above - 50°C th e two doublets gradually disappear from the boron- 11 NMR spectrum and two new d o u b lets a re seen a t 12 ppm and 6l .5 ppm ( J= 100 Hz), The presence of a true apical boron is obvious in Figure 2 0 ,

The decoupled spectrum, also shown in Figure 2 0 , verifies the assignments and emphasizes the near-equivalence of the basal-type borons, probably BHa groups, which are responsible for the narrow, area four resonance. Thus, stage III, even more than stage II, requires an 1 0 9

Decoupled,

Not Decoupled

Figure 20; Boron-1 1 NMR Spectra o±* BqHi2:NH3/ Stage XXI, in CHaCla I

110

explanation based, upon fluxional behavior to satisfactorily account

for the data} that i s, averaging of specific environments on a time

scale considerably more rapid than that seen by the MMR instrument. .

The PMR spectrum of the same sample reflects wliat appears to be

a change in geometry without a gross change in bonding. The terminal

hydrogen from the apical boron appears to shift upfield in the stage

III adduct from 1 1 .2 ta u to 1 1 ,8 tau, a value which may be regarded

as slightly more characteristic of an apical environment. A resonance

tentatively assigned to one bridging proton also moved upfield about

one tau unit. The remaining resonances were relatively unchanged.

The cleavage of hexaborane( 12 ) according to the equation o BsHxa + 2NH? c^la* a^Hio" ( 38 )

has been established with reservations, but in light of experimental

evidence the white solid product may be closer to a molecular adduct

of the formulation 2NHa than the ionic representation. It would

be very desirable to perform a metathesis reaction with the system at o -7 8 C to see if the ionic compound were indeed present} unfortunately,

the development of the reaction by Dr. Vincent Brice demonstrated

that it could only be reliably run at temperatures warmer than -4> 5 ° C.

Although a white solid is formed when ammonia reacts with BgHxa at

-Jj-5 C, and some HaB( M 3)3 Br“ is produced when the corresponding

metathesis reaction is run, it cannot be positively stated that the 3 la was very difficult. Refinement of chromatographic and high vacuum 13 column distillation techniques had to occur before these mixtures could be easily separated and pure components isolated. Until that time, characterization of some of the intermediate boron hydrides such as

BqHio j Bq Hx 2 and B5H11 was preliminary at best.

It was fortuitous that while methods of synthesizing boron hydrides were improving, techniques for analyzing the properties and behavior of the materials also developed in a progressive fashion. The fact that most of the compounds were of limited stability, toxic, and potentially very hazardous and/or violent required great elaboration of vacuum

„ -‘ue techniques. Instrumental methods became very important when Nuclear

Ifegnetic Resonance (NMR) was shown to be capable of having great impact 14 15 in the field . Perhaps no other treatment can contribute so much information both toward solving problems of structure, bonding and identification and toward elucidation of mechanisms and other details of reaction processes. The usefulness of NMR is demonstrated in the gathering of complex kinetic data or in something as simple as relating doublets, triplets and quartets in boron -1 1 NMR spectra to B-H, BH2 and

BH3 groups, respectively.

The structures of some of the boron hydrides closely related to this investigation are presented below, showing the three center-two electron bonding scheme used today) assignment of boron -1 1 NMR resonances to specific boron atoms is also illustrated in the representative spectra which are depicted; white solid formed at -78°C is genuinely ionic in nature. The fact

that B5H11 is not formed exclusively upon protonation in liquid HG1

would tend to mitigate against the ionic formulation} however, this

"behavior may be a curiosity of the BsHio" ion, since some B5II11 is

formed.

Although little can be determined with regard to the nature of

the bis-ammonia adduct, considering that it is an isoelectronic analog

of the BgHi4 2 species while the B6Hi2; NH3 adduct is an analog of the

B6Hr3 " io n some com parison’ should be a b le to be made through NMR

s tu d ie s .

A set of boron - 11 NMR spectra were run in order to elaborate upon the relationship between the ammonia adducts. In separate reac­

tions an equivalent of ammonia was added to stirring solutions of o stage II and stage III at -95 c » Th® o b je c t o f th e experim ent was to

examine the NMR spectra of the samples after about 40 minutes of reaction time in order to determine whether the spectra were different from t h a t o f B6H i2 '2HH3 (Figure 17 ) as well as to see how the spectra compared to each other.

To summarize briefly, the spectra were essentially identical to each other, sim ilar to the spectrum of stage III, B6Hi2:NH3, and mildly different with regard to both general appearance and chemical shift v alues from th e spectrum observed when two e q u iv a le n ts o f ammonia a re added to g e th e r td a s o lu tio n o f B6H12 in CE2CI2 to make th e -95°C sample. 112

A d d itio n a l d a ta came from th e boron -1 1 Nl-IR spectrum of a solution

of BeHjL2 in CH3CI2 which was poured into the NMR sidearm and to which

one equivalent of ammonia had been added at -196°G before the tube was x> sealed. The sample was allowed to warm in the spectrometer from -196 C

to - 80 °C, and the changes in the spectrum were monitored. The only

signals observed initially were the resonances characteristic of free

B6H12. When the temperature reached -85 °C these signals were diminish­

ing and a fourth peak was growing into the spectrum at about four ppm.

Eventually the new resonance appeared as intense as any of the other

three present} five points should be made with regard to this system:

1) the new resonance remained featureless and decreased in intensity after reaching a maximum as the temperature was continually raised}

2) no other peaks were observed attributable to a B 6Hi2 :M3 species}

3) a white solid was present in the sample tube after the fourth peak had grown in} 4) the unreacted B^Hxa remained even at temperatures well above that at which the white solid was formed} 5) there was no evidence to indicate the presence of any form of BqHi2:M3.

When equimolar amounts of BsHi2 and M3 were mixed in HCFsCl at

- 120°C a white solid formed within minutes upon stirring. The JJMR sample was poured into the sidearm as usual and the boron-11 MMR o spectrum showed only even at temperatures initially below-1 0 0 C.

The information presented above gives a good indication of the reactive nature of the hexaborane( 1 2 ) molecule. The data suggest that

BqHx2«2MH3 Is a real, conditionally stable adduct which may slowly convert to the ionic diammonlate. An alternative equilibrium between 113

the molecular and ionic species is possible but not extremely likely

due to the excessive amount of bond breaking and forming required to

support such a hypothesis. The structure and properties of BsHia:NH3

and BsHia'2MH3 seem to be fairly unrelated, and the preferred forma­

tion of one adduct instead of the other is dependent upon conditions

such as temperature, concentration and mode of reactant mixing, as

well as stoichiometry*

II, The Fluxional Nature of Selected BjjH^ :L Species

- The topic of fluxional behavior in BnHcl+Q: L adducts deserves

additional treatment due to the possibility, demonstrated earlier, .

that such a concept may be widely applicable to Lewis base adducts

of the intermediate boron hydrides.

For instance, the boroh -1 1 NMR spectrum of BsHg*2P(CH3)3> which

exhibits three basal resonances in a 1;2:1 area ratio, can only be

explained in terms of a fluxional type of behavior which allows an averaging of certain environments in the basal plane of the pyramid in 117 solution . Spin decoupling of the protons from the spectrum resulted

in the resolution of boron-phosphorus coupling in the basal resonmce farthest upfieldj previously cited references have pointed out the « general phenomenon that boron atoms bonded to elements such as phos­ phorus are often shifted upfield in a relative sense.

In th e r e la te d compound BsH9*P(CH3)3N(CH3) 3 where i t i s reaso n ab ly certain that the phosphine group is bonded to the apical boron, three basal resonances are also distinguishable. However, it appears that 114

the area two resonance attributed to the borons adjacent to the

nitrogen-bonded boron is the peak farthest downfield. Msre elaborate

spin decoupling may be required to assign the other two peaks} preliminary examination would assign the sharper (more upfield) peak

to the remaining basal boron which is not bonded to the amine. Such

sharpness would not be expected for a B-N group. The spectrum appears

to rule out a symmetrical bridging amine group in the basal plane.

Comparison of the decoupled boron-11 ITMR spectra of BsHia: P( CH3) 3

and _CH3BqHh:P( 0113)3, Figures 6 and 2 1 , suggests that the substituted boron cannot be assigned in the latter spectrum. Xt appears that flux­

ional behavior in both systems renders the boron-11 NMR spectra similan

In contrast, the involvement of an ammonia molecule in adduct formation is more unusual In boron hydride chemistry and the Bq Hi2:NH3 species have already been alluded to} the B5H11: TJH3 adduct was examined

in the experimental section, but several points need elaboration:

1 ) the anticipated attack by ammonia upon one of the BH2 groups has shifted the resonance of the resulting H-B-UH3 group, a broad doublet, upfield by about 20 ppm} in comparison, the resonance of the N-bonded boron atom of B3H7: M3 is 19 ppm upfield IVora the resonance of the other two borons (3^.5 ppm and 10.5 ppm, respectively - Figure 22)}

2) the remaining basal boron resonances of BsHn:KH3 can be assigned ■ with the aid of decoupled spectra to a single BHs group and two equivalent B-H groups} 3) the PMR spectrum of the adduct was too complex to allow unambiguous assignment} the ammonia proton resonance was sharp at 5.6 tau. 115

D ecoupled

Not Decoupled

Figure 2 1 'i Boron-11 NMR Spectra ol* CH3BqHii:P( 0113)3 in CHCI3/HCF2CI Figure 22 ; Ihe Boron-11 HMR Spectrum o±' BgJIyiNHa in (CHaJaO U 7 One possible sequence of proton tautomerism is shown below:

e tc .

(I) (II)

The lifetime of (il) would have to be of sufficient duration that it

would account for the observed boron-11 I'JMR spectrum. The distinct

nature of the basal resonances in the spectrum suggests that this

mechanism differs significantly from the fluxional mechanism which

governs the solution behavior of MH3.

In addition to the Bellas: NH3 species, the structures postulated

for the BqHi3“ ion are useful in discussing the hexaborane(12)-

trimethylphosphine adduct, BqHj.2: P( CH3) 3, as well. The boron-11 NMR

spectrum depicted in Figure 6 indicated the presence of an

H-B-P(CH3)3 group. While this representation \ \ 1/ fits perfectly with the inserted structure, ® ®

it would be reasonable to expect that the high symmetry of the molecule would favor

sharper downfieU resonances than those which

are actually seen in the spectrum, B

Rearrangement of the framework bonding, q /\\ including rupturing of the boron-boron bond by the

influence of the strong. Lewis base, could be rationally described 118 by the following mechanism:

\/ -B

S'Y0 j j- B* A ^ P I C H o ) 3 3 r

Unfortunately, evidence from the PMR spectrum of BaHia: P( CH3) 3

solutions could not be obtained which‘would conclusively determine

the number or precise area of the bridge protons in the system} the

available evidence points strongly toward fluxional behavior,

III, The Diammoniates of BqHi2 and 3-CHaB6H n

In contrast to the certain amount of speculation unavoidable with

arguments involving the fluxional behavior of hexaborane(IS)-Lewis base

adducts, very definite conclusions may be drawn from the investigation 119 of the ’diammoniates1 of hexaborane( 12)-type compounds: l) the •f* ** diammoniate of if it exists as HaE( 11113)2 BsHlcf> is difficult to characterize and is not conducive to clean chemical treatment* 2 )

NMR techniques were not helpful in the characterization of solutions of the white solid, or specifically, the B5H10” i°n 5 3 ) variations in reaction conditions did not appreciably affect the system.

Two pieces of evidence illustrate that simple stoichiometry cannot he applied to the protonation of the solid: first, the reaction pro­ duced three boranes rather than the lone expected product, B5H11J second, the residue which remained in the reaction vessel a iter proton­ ation made up a light colored gum instead of a dry solid. The equation

3IfeB(HH3)afeHio + 3HC1 = 3BaB(NH3)aCl + E5H11 + BqH10 + B4Hxo + Ha

t therefore cannot be strictly applied* however, all the products listed were consistently observed.

The data do indicate that adjustments can be made upon the penta- borane(ll) framework which render a B5H10” derivative or analog unus­ ually stable relative to free BsHXo”» The chemistry of BsH9:P(CH3 )3 -, which will be discussed in another section, is a good example of this.

The effect of placing a methyl group on the framework was that proton-

««• ation of the BsHqCHs species went cleanly to yield a single volatile product, 3-CH3B5H10. Also, H2B(NH3) ^ BsHgCHa was stable to -13°C.

17. Directional Attack By Amine Bases Upon Substituted HnH^+Q Boranes

An additional conclusion to be drawn from the studies of the sub­ stituted boranes concerns the directional nature of the attack upon 120

the frameworks by amine bases. Since the base is presumed to seek out

the boron atom with the greatest relative positive charge in order to

carry out its nucleophilic attack, there have been emphasized two

factors which determine the attack site: the fact that BHa groups are more positive than B-H groups and the observation that the methyl xoe group, on an intermediate borane framework , will release electron

density to adjoining atoms. Thus, the most remote BHa group should

experience the attack.

Using these guidelines, the following scheme reflects the cleavage of 3-CH3BsHii by two equivalents of ammonia to form HaB( 1^3)2 and

B5H9CH3 ions, then protonation in liquid HC1 to give the neutral com­ pound, 3-CH3BsHio> exclusively:

H2E(riH3)2

/I

/I A A

Extending the degradation one step further, cleavage of 3-CH3BSH10 \l H ■ f 1

B ^ B L - H

H / \ Br H /V

i H— B— H / \ B? / H- B — H 121

"by two equivalents of ammonia was shown to result in the formation

of IIsB(I'iH3)aB4%CH3 J this diammoniate gave a good yield of

I-CH3B4H9 upon protonation in liquid HCls

CH

'H2B (m 3) a

In all these systems there was no evidence to suggest that base

attacked a B-H group or the BHa group closer to the substituted boron.

Symmetrical cleavage of 3-CH3BaHn by trimethylamine produced

2-0113%% as the principal product in a slightly more complex reaction.

V. General Observations: Physical and Chemical Properties of the

CH3BnHn+5 Compounds

The single most important discovery made with regard to the nature of these new boron hydride derivatives would have to be the marked in­ crease in stability of the compounds as a result of the substitution.

The successive, high-yieU degradations employed in the preparations of the lower boranes are a reflection of the tractability of the materials. 122

All tliree can be stored at -rjQ °C for periods of weeks, at least.

Looking at 3-CH3BsH n , no simple mechanism can be proposed which accounts for the fact that its synthesis can be effected using either l-CH3BsHa or 2-CH3B5H8 as starting material. Obviously a very flex­ ible intermediate is involved at some point between CH3B6H10” ion formation and the protonation in liquid HOI giving the neutral borane.

An insight into localized environments within the 3-CH3B13H11 and

3-CH3B5H10 frameworks can be grasped by examining the assignments suggested for their boron-11 NMR spectra. The spectrum of ^-CHsBqHh shows that the two BH2 groups are very nearly equivalent} the 'down­ field' triplet is arbitrarily assigned to the BH2 group adjacent to the methyl-substituted boron, This is done keeping in mind the mild, trend which characterizes alkyl-substituted boranes; the substituted boron atom is always shifted downfield in the boron-11 NMR spectrum 115 while the boron atom opposite is usually shifted upfield . Accord­ ingly, the 'upfield1 triplet is assigned to boron number one.

Since the environment about the two BHg groups seems very similar, it must be considered possible that adduct formation under extremely mild conditions could take place as a result of P(CH3)3 attack upon either BH2 group. The non-equivalence of the adducts spectroscopically might then be averaged out by a fluxional mechanism.

In the case of the 3-CH3B5H10 compound, the downfield triplet is similarly assigned to the BHa group closer to the methyl substituent.

Looking briefly at the CH3BQHn;P(CH3)3 adduct, when the white solid was warmed slowly from -78°C in the absence of solvent, diborane 123 was given off} however, at slightly higher temperature (about -25°C) the material in the reaction vessel takes on a grayish cast as the contents slowly decompose. It is apparent that the conditions which bring about BH3 elimination also effect decomposition of the result­ ing solid. Kb KMR spectra of an elimination product could be obtained.

Similar results were observed for the P(CH3) system.

As far as physical observations were concerned, C H a B ^ H g ^ 0113)3 also turns gray before complete decomposition when slowly warmed to approximately zero degrees. It was determined that prolonged pumping cn the contents of the vessel at that temperature failed to result in the elimination of the BE3 unit. The boron-11 NMR spectrum of this gray solid was identical to that of a fresh sample of CH3B4B9:F(CH3)3 as shown in Figure 23. Wide and narrow line spin decoupling of the protons did not assist in clarifying the assignments due to what appeared to be extensive overlap of several borons. The spectrum appears to be far too complex to be rationalized by simply replacing a B-H group with a B-CH3 group in any scheme suggested for B4Hio:P(CH3)3. The spectrum of the unsubstituted adduct is complicated enough — the addition of the methyl group makes it considerably worse.

Keeping all the contributing factors in mind, generalizations can be made regarding systems involving P(CII3)3 and methyl-substituted boranes: l) under similar conditions these reactions required a greater amount of time to go to completion. The same effect could be obtained by running the reaction at slightly higher tenperatures than that used with the parent boranej 2 ) loss of BH3 was hindered because of the 12^4-

D ecoupled

Not Decoupled

Figure 2 }; Boron-11 NMR Spectra oi* CH3B4H9:P(CH3)3 in CHC13/HCF2C1, -80°C 125 presence of the methyl group with two factors most heavily contribut­ ing to this behavior. First, introduction of electron density to the framework by the methyl group resulting in stronger framework bonding75 this would make elimination of the BH3 unit more difficult. Second, interference due to the-possibility of the methyl group residing on the boron atom eventually eliminated (assuming mechanism consistency).

In spite of the complications encountered with trimethylphosphine studies of substituted boranes, these hydrides retain great potential in the realm of synthetic chemistry5 synthesis of other substituted

®n*Wa famlly members is possible.

Regarding the characterizability of the derivatives ^-CHeEs^n and

3-CH3B5H10, hydrolyses of these materials carried out in concentrated

HC1 and HaOH solutions proved to be only 60$ effective. If glacial o acetic acid was employed in the hydrolysis, heated to +115 C for sever­ al days, the percent hydrolysis was increased to 87) however, no agent was found which would totally hydrolyse the materials.

The mass spectra of the liquids exhibited facile loss of Ife in the instrument and a large peak at m/e = 28 due to the formation of BaHe from BH3 fragments. It was impossible to see the parent peaks for these compounds, just as the spectra of BaHxa and B g H n cut off 46, 47,55 before the parent peaks are reached .

VI. Trimethylphosphine Adducts: Unique Solid Phase and Solution Behavior

System one: BeHi2: P( CH3) 3 ------*-^BaHe + BsHs:P( 0113)3

As indicated in the introduction, phenomena such as tautomerism or 126 rearrangement of frameworks have generally been described as being assisted by either solvent or some second agent, usually a mild base.

Gas phase eliminations which are often initiated in high energy systems 118,119 and feature BH3 elimination make up another category , but there have been no well founded reports of solid phase behavior remotely resembling the phenomenon observed with these BnHn+QiL adducts.

Thus, with the larger, more complex Bq -Bs system the delicacy of such a solid phase process is not unexpected. As the size of the system is reduced to the B4-B3 range, the stabilities of the adducts increases to the point where the initial adduct B^HioJFCCH3)3 is briefly stable at room temperature, the sublimable elimination product B3II7: P( CH3) 3 is the most stable of all the adducts made in this study, and B3H7: Mite3 is the only amine adduct which can be handled at room temperature.

Specific reference to BH3 elimination from the BnI^l+Q family was made by Dunbar as a result of ion cyclotron resonance studies . There was no evidence to indicate cleavage of any kind other than symmetrical cleavage of these boranes.

The best insight into the complexities of the Be-Bs system can be gained by examining the proposed mechanism for the elimination and rearrangement process} the mechanism assumes a solid phase structure for BqHi2:P( 01*3)3 such as the one indicated. It might be noticed that this structure is very similar to the structure of hexaborane(12) labeled (II) on page 11 of the introduction. 127

' In spite of some awkwardness inherent in this mechanism, it is the only means discovered so far by which a framework rearrangement can lead to phosphine substitution on the apical boron of BsHg:P( 0113)3.

The scheme also partially explains why it has been so difficult to iso­ late the elimination product, since the mechanism is rather elaborate.

It appears that small amounts of impurity in the product present from outside sources or from a secondary elimination/rearrangement can lead to a chain reaction decomposition of B5H9:P(CH3)3. This agrees with visual observations that the decomposition proceeds gradually from a given area in the vessel, usually around the upper edges, eventually consuming the solid which is adhered to the Teflon-coated stirring bar last. This behavior is somewhat analogous to the facile decomposition of BQHia which occurs if impurities are present.

The fact that the adduct BsH9:P(CH3) 3, when successfully formed, bears only limited stability is reasonable in view of concepts of structure and bonding of boron hydrides. 128

In addition to considerations of purity, temperature must be a factor since only slow (overnight) warming of the system allowed the eventual isolation of BsIl9:P( CH3) 3. When the cold bath 0 o was abruptly removed at any point between -78 C and 0 C during this o procedure, decomposition occurred within minutes. Warming above -50 G in a solvent other than CHCI3 or HCF2C1 resulted in the decomposition of both BQHia:P(£113)3 and B5Hg:P(CH3)3> the white solid dramatically and rapidly would be converted to a gray oil suspended in the solvent.

(11) 73 ■ Two structures are postulated (above) for the B5H10" species, of which BsIIg:P( 0113)3 Is an isoelectronic analog. This adduct, which cannot be made directly from BsHg and P( 0113)3, is extremely important in that it is one of only two analogs or derivatives of the penta- borane(ll) anion whose preparation and characterization have been elucidated.

Proton MMR was decisive in assigning (I) as the correct structure.

Not only were there two bridging resonances seen in a relative area ratio of 2:1, but at a more moderate temperature the three bridging hydrogens were equivalent on the UMR time scale.

Corresponding boron-11 MMR spectra support these data. The narrow basal resonances'even at low temperatures indicated near-equivalence 129

of the basal boron atoms; the fact that the peak was split into a

perfectly symmetrical doublet at higher temperatures indicates that

some means of making the basal borons equivalent should perhaps be

dealt with.

Along this line, there are several reasons why the theory of

apical boron involvement in proton tautomerism has merit. In the

'simple' B s H u molecule, the standard explanation as to why the

apical resonance is a doublet and not a triplet as expected for a BH2

group invokes the concept of partial bridging character of the second

apical hydrogen as part of a three center-two electron bond involving 120 borons number one, two and five. Onak and Leach discussed

stabilization of the pentaborane( ll) framework in terms of dispen­

sation of 'partially relinquished1 electron density due to the

protonic nature of the anomalous hydrogen. As one might expect, the 54 PMR spectrum of B5H11 is very complex .

Another reason is the availability of surplus electron density which can potentially be donated to the neighboring framework area by the phosphorus atom; this would assist the tautomerism by reducing

the electron deficiency of the molecule. The fact that the only sub­

stituent of the apical boron is the phosphine group suggests that there might be likely a molecular configuration which would feature one of the apical boron orbitals being involved in bonding to a bridging proton. Genuine equivalence of the basal borons would require either two or (probably) four bridging protons at some stage.

5Tom the data collected during this investigation, it has become 130 apparent that adding electron density by means of methyl substitution on the framework or by Incorporation of a basic group such as F( 0113)3

into the framework could be regarded as the single most important factor which contributes toward stabilizing these 'adjusted* B5H10" analogs relative to the free B5H10 ion.

System two: CH3) 3 »iB2HQ + BsH7:P( 0113)3

Examination of features common to both adducts will simplify the discussion treatment of this system. An important physical property common to both of these unusually stable adducts is solubility in the

Freon solvent* HCFaCl. It was rather unfortunate that the adducts of the higher molecular formulas could not be observed in solution at very low temperatures} the fact that low temperature MMR spectra of

B4HXo:P(CH3)3 and B3H7: P( Cl-fc) 3 could be obtained was critical in relating the two species and establishing their behavior under a variety of conditions.

From the boron-11 KMR spectra the most important discovery re­ vealed that the (CH3)sP-BH2- group was common to both species at very low temperatures. It became apparent that similar structural principles were likely in eiYect under selected conditions.

Examining the simple B3H7:P( 0113)3 adduct first* a completely unambiguous picture of the solution behavior of the compound can be drawn: l) at moderate temperatures (ca. -8o°C) the seven framework protons are equivalent on the NMR time scale* indicating that a complete proton tautomerism is in eiYect: 5

In contrast, the structures of BgHg and BqHio are compact, closed pyramidal frameworks:

A 'h -B— BgHio

To help grasp the concept of molecular activity (as opposed to reactivity) of various boron hydrides, presented below are descriptions of isotope studies and exchange phenomena involving interactions of boron hydrides in the absence of solvent, except where noted.

Mixtures of BgHg and Da demonstrated that all six diborane protons exchanged with deuterium at the same rapid rate. The rate of exchange * increased with surface catalysis and decreased upon the introduction of gases such as N2 or COa which would compete for surface adsorption la sites . There was no change in the observed rate when helium was included in the mixture. By comparison, the B2H0/B2Dq exchange was described by the same authors as a simple homogeneous gas phase process

(exchange of B2H3 with B(CH3)3 in a similar fashion had been recognized 17. IB by Schlesinger and co-workers in the thirties). 19 Koski and co-workers demonstrated that interaction between BaD0 and BsHg resulted ■ in terminal substitution of deuterium atoms in the 30 product, BgH^Ds. Shapiro, and Keilen showed that no boron exchange 151

etc.

2) at very low temperatures the quartet in the horon-11 NMR spectrum must he assigned to the unique (CH3) sP-BHa- group. Spin decoupling of the protons from the spectrum resulted in the collapse of the quartet to a-doublet, proving the presence of the B-P bond) 3 ) under the same conditions as 2), the upfield portion of the FMR spectrum exhibited two features, an area two peak due to the presence of terminally bonded hydrogens which are part of the previously described group, and an area one resonance in the bridging proton region. Prom these data a static structure for the molecule in solution at -130°C can be arrived at;

1 /

| \

It was unfortunate that the resonance from the nine methyl protons obscured the four terminal hydrogens which reside on the two equivalent boron atoms. The positioning of the upfield resonances does balance, by weighted averaging, the four downfield positions in the masked position*

The reported PMR spectrum of (CellsCHaJsHCHsBaHr did exhibit thermal de­ coupling at a temperature of -80°C., but no information concerning a 132 l a i static low temperature molecular configuration was reported. .

The PMR spectrum of* B3H7:P(CH3)3 displayed, no thermal decoupling.

It was desirable to look at a similar system such as NHa3aH7 or

( CH3) 3NB3IE7- where possibly all the resonances from a static structure could be seen. The trimethylamine adduct proved to be soluble in HCF2CI but the PMR spectrum at -136°C showed no evidence of* behavior which corresponded to what was seen with the low temperature spectra of

B3K7iP( CII3) 3. The ammonia triborane(7 ) adduct was totally insoluble in

HCF2CI5 solutions of the adduct in dimethyl ether were prepared for NMR analysis , but these spectra did not show proton non-equivalence at low temperatures. The boron-11 NMR spectrum of B3H7:NH3 simply showed broadening of the distinct 2:1 singlets as the temperature was lowered; a static H2B-NH3 group (triplet in the NMR spectrum) was not observed.

For comparison,, the boron-11 NMR spectrum of (CH3) 2NPF2B3E7 exhibited a distinct downfield triplet at 12 ppm (area two, J = 11^ Hz) and a complex upfield resonance at 50 ppm (area one); the spectrum not only demonstrated the fixed nature of the compound (2015 styx notation) ea but also suggested stereoisomerism .. The spectrum of F2FHB3H7 was less distinct, but very similar to that described above; the upfield resonance arose at 60 ppm.

Several structure determinations and theoretical studies of tri- borane(7) derivatives (B3H7:L and B 3 % -) have resulted in the accumu­ lation of a large amount of data describing the ways in which substitu­ ent atoms, which are found in quite a complex variety in the literature, 133 are arranged about the three boron fragment. All structures have been shown to fall within constraints placed upon the two structures described by the styx notations U O t and 2013:

\ J r I llCA I / • \ 2013 Y ' A A

70 122 B3H7C0 * ...... 110U 124 B3H7UH3 ...... intermediate 12s B3H7K( Gha) ...... intermediate ZL23 BsEfe” ...... 2013

Solid phase structure determinations performed on the two amine adducts showed the protons bonded to the B-I: group are positioned closer to the boron atom than would be expected for strictly bridging protons. On the 126 other hand, Stevenson used non-enrplrieal molecular orbital calcula­ tions to show that the llOt configuration was the most stable for the

B3H0 ion.

In the study of (CS%CH2)2MCH3B3H7, the authors estimated that the rate of scrambling of protons in uhe complex amine adduct was faster than the rate observed for [( CqH5) 3?J 2CUB3II3.

Even though bond distances provide a good indication of the nature of the pseudo-bridging protons, supportive data from WMR spectra are very desirable. A supplement to the characterization of B3H7:P( 0113)3 would be to perform a structure determination on the adduct, since it

can be neatly sublimed from a +50°C environment and is very stable.

Turning to the B^HxorPCCHsJe adduct, the initial attack by the

phosphlne upon the B4H10 framework should take place on a Blfe group.

An examination of the structures postulated for B4Hi o :P(C 113)3, based

h—B

(i)

H h —B

(11:

upon the isoelectronically analogous B 4 H n “ ion, reveals that there are two possible candidates which exhibit very different features.

Boron-11 and proton NMR spectra support the theory that both structures are present in solution, either as partners in a tautomeric exchange or as focus points in a fluxional mechanism. The evidence:

1) a quartet, verified by decoupling experiments, appeared in the 155 boron-11 NMR spectrum at 37 PP® due to the presence of a BH3 group in solution. This resonance increased in intensity as the temperature was lowered, indicating possibly a longer lifetime for the species un­ der the extreme conditions. The extent of the intensity Increase wad not precisely determinable because of overlap Involving what appeared to be a triplet at 57*5 ppm. The proton resonance of the three BH3 protons was apparently not observed in the PMR spectrum. The likeli­ hood that this resonance was masked by the methyl proton resonance grows when it is recognized that such a resonance for the terminal “ 127 protons in the singly bridged B2H7 ion appears at about 8,5 tau

This is very close to the chemical shift of the nine methyl protons of B4H10!P(CH3)3 — 8 tauj 2 ) The upfield resonance in the boron-11

NMR spectrum was a very poorly resolved doublet at -80°C, Proton spin t decoupling sharpened the resonance immensely, while without decoupling, lowering the temperature of the sample resulted in the conversion of the rough doublet into a quartet at -130°C. This demonstrated the presence of the (CH3)3p-BH2- group under those conditions. Upon elevat­ ing the temperature, the resonance from the phosphorus-bonded boron • did not sharpen, but the two upfield peaks in the spectrum did decrease

in intensity in a reversible fashion relative to the downfield singlet S»l o 3 ) In all PMR spectra examined to temperatures as low as -136 C, only one resonance, apparently of relative area one, was observed in the region characteristic of bridging hydrogen resonances (9*5 tau to 15 tau). The conclusion to be drawn from this piece of information is that there is still proton tautoraerism in effect even at such low temper­

atures 5 it is impossible for only one bridging hydrogen to exist in a

B4H11 analog — there are too many hydrogen atoms5 4 ) Accompanying

the increase in intensity of the peaks around 37 ppm in the boron-11

NMR spectrum of B4H10: P( CH3) 3 was a decrease in relative intensity of

the unresolved downfield resonance which approached a relative area of

one at -136°Cj . 5 ) The only other resonances distinct in the PMR

spectrum of B4H10: P( CH3)3 were sufficiently far downfield that they required assignment as terminal B-H or BBa protons. The closest area ratio which could be determined for the two peaks was 1;2, and their partially overlapping nature might indicate that they arose from very

similar environments. Comparisons with the PMR spectrum of B3H7:P( 0113)3 would suggest that these protons could well be two different

(CHaJsP-BHa- groups in slightly altered geometric configurations. A less likely explanation would be that the resonances arise from one

(CH3)3P-B-H group and one (CHaJsP-BHa- group. It also seems unlikely, but not Impossible, that one of the resonances (that of area one) could be assigned to the BH3 groupj it could not be determined whether the resonance had an intensity which was a function of temperature.

With data analysis complete, there seems to be only one solution to the problem which conforms to the observed spectral phenomena: the adduct B4H10:P(CH3)3 exists in solution in at least.two forms which 137 are related by a tautomeric or fluxional mechanism.

Most likely, the ratio oi' one configuration over the other is relatively unchanged over the temperature range studied, but each one is affected by lowering of the temperature. The single bridging proton,

if that is what it is, is visible on the NMR time scale at -130°C, but some of the remaining protons continue to be engaged in some sort of exchange. The boron bonded to the phosphine group is rather 'frozen' into its environment or environments.

An additional point is that what the NMR instrument sees is the average situation in the sample-at a given moment. The particular solution can be described as considerably active, even at very low temperature. In contrast, the inclusion of chloroform in the solvent system produced some drastic changes in the boron-11 NMR spectrum of i B4H3.0: F( CH3) 3. As the temperature was lowered to near the freezing point the downfield resonance decreased in intensity while the upfield resonances merged into a single peak at about 33 ppm. Some structure is noticeable on the new peak, and at a limiting temperature the entire spectrum consists of one resonance at the 33 ppm position; the down- field peak completely disappeared.

For the solid phase description of B4H10: CH3)3 and the mechan­ ism of BH3 elimination, structure (11) must be emphasized. Proton tautomerism without solvent participation or assistance must be con­ sidered extremely unlikely, and the B3Ht>:P( 0113)3 adduct can be readily produced by loss of the dangling BH3 group. 138

System three; BgHn: P( CH3) 3 - ")<•>-» 2B2H6 + B4lk:P(CH3)3

The inability of the B5H3.1: P( CH3) 3 adduct to non-destructively eliminate a BH3 group can at least be partially explained.by looking at a mechanism which would seem to exclusively govern the process;

p p / l /\ 'I H r

t *

Apparently the exposed nature of the fragment which remains upon BH3 elimination precludes the formation of a stable B4Ha;P( 0113)3 adduct by this means. Whether such a B^Hq analog, successfully formed, would bear reasonable thermal stability would be another matter entirely.

Xt seems unlikely that the initial adduct, BsHn:P( 0113)3, would be so intrinsically stable that it would prefer riot to eliminate the

BH3 unit, although the BsHn;MH3 adduct shows remarkable stability in solution.

It is curious to note that Bslin does not have a boron-boron bond 139 as do B4H10 and BaHj.a, but the importance oi' this factor is question­ able. Holding the framework during rearrangement may be involved.

Preliminary attempts to effect removal of the BH3 group by means of a cleaving agent, 11(0113)3, had limited success. While the cleavage product, H3BN(CH3) 3, was observed in the NMR spectra of solutions of reaction mixtures, the reactions have not been successful in allowing a simple B^Hq: P(CH3)3 adduct to be isolated. Apparent triplet struc­ ture in an upfield (ca. 37 PPm) resonance similar to a feature of the 94 / s spectrum published for B^HgjPIa suggests that some B4lig:P{CH3j3 may be present. Resonances attributable to the initial adduct, BsHu: PM33, were very apparent in the spectrum, however. Thus, a mixture o±* materials is present and additional work with the system might be fruitful. I While it is likely that BgHii:N(CH3)3 could, be formed by an obvious reaction, work done in 1959 indicated that preliminary attempts to isolate the B

VII. The Role of the Phosphlne Group in the Adducts Studied

Several generalities concerning the unique function of the trimethylphosphlne group in both the initial adducts and the elimin­ ation products can be enumerated:

1) The previously described tendency of P(CH 3) 3 to form very stable

adducts with intermediate boron hydrides, especially compared to 't adducts involving amine bases, continued to dominate the chemistry

investigated'in connection with this problem. One dramatic

manifestation of this stabilizing effect was the solid phase lUO

elimination of a BH3 group from an initially formed 1:1 adduct.

Even though the process was self-destructive in some instances,

the mere fact that thermally stable analogs of the B5H10" and

B3Hq " ions could be isolated is a tribute to the versatility of

the P( CH3) 3 group.

2 ) Unlike the findings with the more stable 2P( CH3) 3

compounds, the B Hn+e»P(0113)3 adducts could be made in and Isolated

from both methylene chloride and alkane solvents, as long as the

reaction temperatures were kept below -85°C,

3) In examining the more localized effects caused by placement of

the phosphine group upon borane frameworks, the following chemical

phenomena have varying degrees of *impact depending upon the nature

of the adduct and whether the solid phase or solution chemistry is

being discussed: first, the obvious donation of electron density

from the phosphorus atom to the electron deficient framework)

second, the use of 'p* and 'd' orbitals of the phosphorus atom to

balance the electron density distribution) third, the flexibility

rendered the boron atom to which the phosphine is bonded.

The last item listed in the third section above merits further discussion. An examination of the phosphine adducts prepared during this investigation shows that the group can assume several roles in the framework, depending upon the size of the system and whether the behavior i3 solid, phase or solution in nature.

Earlier, BsH9:P( 0113)3 was discussed in terms of the potential that o took place in the gas phase at 250 C between BsHa and B5H0.

At 200°C, hydrogen-deuterium exchange was observed but the same effect was not seen at room temperature. Similarly, no intermolecular boron 10 21 exchange was observed to take place between BsHq and BqHjlo • This latter work also showed that exchange took place between BeHio and DgO involving bridge positions only, and that low temperature exchange between BeHio and BaHa involved only the five basal terminal positions. 22 Johnson showed that gas phase room temperature exchange between BeHio and DC1 involved bridging positions only, possibly by means of an + 1 + intermediate, formulated BaHioD , analogous to the postulated B e H n 23 species . Uiis observation is in contrast to the discoveries that 24 25 BsHa and B10H14 undergo only terminal exchange with DC1 and that a catalyst is required. Other data include unpublished reports indicating I that abstraction of a proton from decaborane(1^) took place at a bridge site according to the reaction

B10H10D4 + NaH------*-HD + NaBioHioDs (l) and that BioHi^/DeO exchange in dioxane affected four protons out of 2a the fourteen, apparently the bridging species .

A very significant piece of work revealed that 10BaHs underwent complete boron interchange with l'TaBx0Hi3 in diethyl ether solution at 2T lO zero degrees over a period of one hour . Hie authors postulated a BH3 addition to the B10H13 ion followed by framework rearrangement and eventual loss of a 11BH3 group. Numerous repetitions of this process 141 the apical boron had for bridge proton bonding. A bonding scheme which would probably apply to both solid phase and solution considerations can be formulated as S- £+ and probably can be applied to all the adducts described herein.

Estimating the state of hybridization of the boron atom in such a situation would involve a measure of speculation.

With either B4H10: P( CH3)3 or B3H7:P( CH3)3, evidence from very low temperature NMR spectra of HCF2CI solutions demonstrated that a unique four coordinate boron atom exists in a static configuration —

H ( CH3) 3P-B— H

The fourth orbital is involved with a closed, three center-two electron bond to the remaining segment of the framework.

Again, emphasis must be placed upon the fact that much of the information dealing with the characterization of these adducts comes from spectroscopic analysis of solutions of the compounds. The bonding in the solid materials can perhaps be expected to conform more strictly to the postulated structures which have been presented* in the same regard, it is likely that some modifications of the concept of the phosphine*s role in the framework may have to be applied when the sinple solid phase system is being considered.

VIII. Special Considerations for the BeHi2: N(CH3)3-BsHg:N(CH3)3 System 1^2

Supplemental to any analysis of the properties of Lewis base adducts of boron hydrides should be a discussion of the behavior of the system B6Hi2: K( CH3) a ► iBaHe + BgHg: N( C1I3) a ( 29)

Hie phenomena which were discussed in the experimental section are consistent with these assumptions and corollaries: 1) the similarity in the P(CH3)3 and N(CH3)3 systems strongly suggests that the same mechanism may be in effect in both cases5 it would govern the elim­ ination of the BH3 unit and the rearrangement of the remaining frame­ work elements. In addition, there was established the likelihood that the electron density introduced to the framework by the Lewis base strengthened the neighboring bonds such that one of the two distant

BH2 groups would be involved in the elimination} 2) the primary t consequence of the invoked mechanism is that the Lewis base must reside on the apical boron once the BH3 unit has been eliminated to give the BsHgjL species} 3)the structure of the previously described bis-adduct, BsHg«2N(CH3)3, exhibits no substitution on the apical 11T boron } in contrast, the structure of the mixed adduct, BsHg*PMs3lMfe3, was postulated to be similar to that of BgHg* 2P( CH3) 3J namely, featuring phosphine substitution on the apex and amine substitution on the base. A comparison of spectral data supports this deduction.

All the information presented above is consistent with the observations that were made during attempts to prepare and character­ ize B5H9: N( CH3) a# Pii’st of these was that the compound lacks thermal llj-3

stability because of the inability of the trimethylamine group to

stabilize the BsHxo" analog the way the phosphine group canj this is most likely because of the greater basicity of the amine group, as well as the deficiency of pi-type backbonding through 'p* and

(especially) 'd* orbitals. The fact that solutions of amine adducts of BqHi2 show the presence of cleavage products even at very low temperatures reinforces the basicity argument.

Second, the presence of the amine group on the apical boron, as a consequence of the invoked mechanism, precludes the possibility of further expansion of the framework by addition of a second molecule of base to form a thermally stable bis-adduct. It was pointed out that neither BsHq *211(0113) 3 nor BsHg* N( CH3) aP( CH3) 3 could be formed by the addition of the appropriate Lewis base to BsHg: N(CHs) 3, even though some sort of addition or reaction which yielded new, unstable material was indicated in each case.

Thus, it is apparent that the phosphine group can reside on either an apical or basal boron, but the amine can only be present somewhere on the base of the pyramidal framework in order for a thermally stable adduct to result, and that aspects of mechanism and structure pertinent to the problem support these contentions.

IX. Conclusions as to the General Nature of the Adducts Characterized

in This Investigation, and of the Starting l&terial, Hexaborane(12)

. Investigations of bis-adducts made to date has established the fact, subsequently verified by this research, that trimethylphosphine adducts of boron hydrides exhibit greater stability than the corres­ ponding trimethylamine adducts. It was pointed out that the previous

studies were carried out on the closed pyramidal species B5H9 and BqHio* two compounds with p h y sica l and chem ical p rop erties q u ite d iffe r e n t frcm the BnHn+s liydrides. Accordingly, it Is not unexpected that the proper­ ties of the adducts of the latter boranes were different from the prop­ erties of the bis-adducts. It was found that adducts formed between

Lewis bases and hydrides obey a 1:1 stoichiometry, producing -2 B H , analogs in contrast to the BnHn+£1 analogs produced upon the n n+7 j-ire expansion of the BsHg and BQII10 frameworks. Among the reasons one might cite, to explain this behavior would be the factor of crowding in the moleculeJ that is, the ability of the framework not only to satisfy the steric requirements of the resulting adducts, but also to accomo­ date the orbitals and electrons provided by the Lewis base species.

The ’saturated' nature»of the BqHj.3 analog, BQHi2: NH3, was demonstrated in the following way: an equivalent . of HBr was added to a vessel containing a solution of stage III of the adduct in CH2CI2 a t

-196°C. Upon warming to about -90°C, over 0,5 equivalent of H2 was isolated from tne vessel. While this behavior cannot be assigned a simple quantitative explanation, it is likely that the introduction of ■f a proton, H , created a hypothetical BqHi^ analog. This species, presumed to be unstable according to the equation B6Hi4 — *■ BsHis + H2 would be expected to lose hydrogen in order to assume a more stable molecular configuration. In addition, if the ammonia molecule were involved in proton abstraction, hydrogen evolution upon protonation would not be expected. Other observations with hexaborane(12), which was the principal

material under investigation in the course of this problem, reflected

the complexity of the mole exile. In common terms, the tendency of the

molecule to be structurally loose and receptive to attack by base under

very mild conditions (-100°C and below) helped to account for a number

of unpredictable patterns of behavior: l) the rearrangement of the

framework in low temperature solutions upon certain Lewis base attack

to give a structure pyramidal in nature and similar to the tetraborane

structure; 2 ) the formation of unique BsHxa-ammonia adducts in lieu of

proton abstraction; 3) the ability of the solid adduct B6Hia:f(CH3)3

to eliminate BH3 and undergo framework rearrangement without solvent

assistance; k) the eventual formation of a B5H10 analog, BsHqjPC 0113)3^

by the solid phase process while elaborate solution chemistry failed

to provide even decent characterization of the BsHxo” C*n ’^iie ^orm

HaB( IIH3)aBgHio or (n-C^HgJ^irEsHxo) 5 5) the critical condition depen­ dence of the hexaborane( 12)-base reactions, especially P(CH3)3 systems where cleavage and adduct formation could be observed in competition.

In a very real sense, the hexaborane(12) molecule is one of a kind.

The availability of alkyl-substituted derivatives of the compound will

inevitably shed a great deal more light on how some of the reactions

3hown here take place. It is also obvious that substitution consider­ ably alters both the physical and chemical properties of the molecule, and that comparisons of BqHi2 with alkyl derivatives must be made carefully. 146

X, Projections of Research in the Field

An area of research related to this investigation would explore the possibility that adjusting the substituents on the phosphorus and amine bases could allow for the synthesis of additional thermally stable adducts of substituted and/or unsubstituted boron hydrides.

In another realm, the investigative and preparative techniques could be extended to include other members of the family, such as

BsHxs or BaHx-t. It is possible that this study could lead to the dis­ covery of the first seven-boron derivative of significant stability by means of a reaction such as remove EqHi* + F( CH3) 3 ^ BsHx^P( CH3) 3 iBsHe + ByHxx: F( CH3) 3 ( 4 l) warm

Elaboration of the characterization of 3-GIl3BQHxx and 3-CH3B5H10 could lead to a buildup of the chemistry of the alkyl- and halo- apically substituted derivatives of these materials similar to the , * 115, isa chemistry established for similar pentaborane(9) derivatives •

A more specific possibility would be the synthesis of an apically substituted fluorine-pentaborane(11) compound such as l-F,3-CHaBsH9. .

Burg cited the inflexibility of the pentaborane(9) framework as the primary reason why l-FBsHg had not been synthesized, A looser frame­ work might allow an alignment of fluorine and boron orbitals suitable for secondary pi-type bonding between them.

An investigation into the step-wise framework buildup chemistry of the substituted derivatives would have to begin with studies of the deprotonation reactions of 3-CH2B0H11 and 3-CHsBsHio. 147

One specific curiosity might be pursued, which would serve to partially bridge the gap between B j ^ and BnHn+Q Lewis base species.

If the ammonia adduct of hexaborane(12), BeHia: NHa, could be prepared at very low temperature in a solvent such as dimethyl ether, it might be possible to effect deprotoiiation of this species according to the equation B6Hia; HH3 + KH ►He + KBqHuHHo" (42)

This would result in the preparation of an isoelectronic analog of

BeHia"2* The characterization of this species, mostly by NMR, would be

-2 interesting compared to data obtained from other BsHia analogs made by adding two equivalents of base to BeHio, creating BsHi0*2L adducts.

A suggestion based upon the work of Dunbar would be the synthesis of a MBeHia species by reaction of a metal such as potassium or sodium with B6Hi2 to create the BsHxa ion by electron transfer.

Finally, the preparation of hexaborane(12) derivatives with multiple alkyl substitution by methods previously described in equation form should be a serious objective. Preliminary experiments carried out toward that end employed l,2-( CHa^BsKj’- as the precursor for a desired dimethylhexaborane(l2 ) derivative. The boron-11 NMR spectrum of the volatile product obtained upon fractionation into a - 6$ ° C trap showed resonances over the portion of the spectrum characteristic of a

BeHia species (-35 ppm to + 25 ppm)5 however, there were no features to the resonances apparently because the reaction produced a mixture of

(CH3)2BsHio compounds. Delicate separation techniques such as vapor phase chromatography may be needed to isolate the products made. REFERENCES

1) W. H. Eberhardt, B. Crawford, Jr., and W, N. Lipscomb, J. Chem.

Phys., 22, 989( 195U)

2) W. IJ. Lipscomb and R. E. Dickerson, J. Chem. Phys., 2J, 212,(1957)

3) R. Hofftnan and W. N. Lipscomb, J. Chem. Phys.,^6, 2179 7 (1 9 6 2)

k) E. Switkes, I. R. Epstein, J, A. Tossell, R. M. Stevas and W. N.

Lipscomb, J. Amer. Chem. Soc., 38377 (19 7 0 )

5) E. Switkes, M. D. Newton and W. N. Lipscomb, J. Amer. Chem. Soc,,

22, 38 ^7 ,( 1970 )

6 ) A. E. Stock , Hydrides of Boron and Silicon, Cornell Univ. ,1933 I 7) R. V/. Parry and L. J. Edwards, J. Amer. Chem. Soc. .81, 355^,(1959)

8 ) R. C. Dunbar, J. Amer. Chem. Soc,, 93, Ul67,(l97l)

9) H. A. Beall and W. N. Lipscomb, Inorg. Chem. 1783,( 1 96k)

1 0 ) D. F. Gaines and R. Schaeffer, Proc. Chem. Soc., Sept., 19&3, P 267

11) W. V. Kotlenski and R. Schaeffer, J. Amer. Chem, Soc..80. ^517,(195$

12) D. M. Lutz and C. A. Ritter, Canad. J. Chem., ll, 13^,(1963)

13) J. Dobson and R. Schaeffer* Inorg. Chem., _9, 2183,(1970)

1^) J. A. Pople, W. G. Schneider and H. J. Bernstein, High Resolution

Nuclear tfegnetic Resonancej McGraw-Hill Book Co., New York,N.Y. 1959

15) G. R. Eaton and W. N. Lipscomb, NMR Studies of Boron Hydrides and

Related Compounds, W. A, Benjamin, Inc., New York, N. Y. 19^9

16) P. C. ffeybury and W, S. Koski, J. Chem Phys., 2 1 . 7^2,(1953)

Ik8 1k$

17) H. Schlesinger and A. Walker, J. Amer. Chem.Soc., <57, 621,(1935)

18) H. Schlesinger, N. W. Flodin and A. B. Burg, J. Amer. Chem. Soc.,

‘ 61, 1078 ,( 1 9 5 9)

19) w. S. Kbski, J. J. Kaufman and P. C. Lauterbur, J. Amer. Chem. Soc.,

79, 2382,(1957)

20) I. Shapiro and B. Keilen, J, Amer. Chem. Soc., 77> 2663, (l955 )

2 1 ) J. C. Carter and H. L. Mock, J . Amer. Chem. S o c .,91 , 5891,(1969)

2 2 ) H. D. Johnson II, Ph.D. Ihesis, 1969

2 3 ) W. W. Lipscomb, J. Chem. Phys., 2 8 , 170 ,( 1958 )

2k) T. Onak and R. E. W illiam s, Inorg. Chem., J ., 106, 0-962)

25) M. P. Hawthorne, A. R. P ito c h e lli-a n d R. 0. Strahm, J, Amer. Chem.

Soc., 82, 1825,(1960)

26) M. Hawthorne and J, M iller, J. Amer. Chem. Soc., 80, 75^* 0-959

27) R. Schaeffer and F. Tebbe, J, Amer. Chem. Soc., J3^,2020 , (1963)

28) D. F. Gaines and T. Iorns, J, Amer. Chem. S o c ,, 8g, 3575, 0-967)

29) T. Onak, F. Gerhart and R. E. Williams, J. Amer. Chem. Soc.,85.

175^ 0-965)

30) R. G. tfeguire, I . J. Solomon and M, J. Klein, Inorg. Chem. . 2,

1155,(1965)

31) R. E. Williams and F. Gerhart, J. Organometal. Chem., 10.168 ,(1967 )

32) D. A. Franz and R. N, Grimes, J . Amer. Chem. S o c ,, J?3,387 # tl97l)

35) T. Onak and D. Iferynick, J. Chem. Soc,(A), 1797# 0-969)

3k) R. G. tbguire, I. J. Solomon, M. J. Klein and K. Hattori, Inorg.

Chem., _2, 1136,(1963) 35) A. Burg and H. S ch lesin ger, J. Amer. Chem. Soc. ,j>5, U009,( 1933)

36) R. N. Grimes, C. Bramlett and R. Vance, Inorg.Chem., 1066, (1968)

37) A. Norman and R. Schaeffer, Inorg. Chem. ,k, 1225,(1965)

38) A. C. Bond and M. L. Pinsky, J. Amer. Chem. S o c . 7585,(1970)

39) R. Schaeffer and J. D, Odom, Inorg. Chem., J?, 2157, (1970)

40) R. SchaeiTer and A. L. C o llin s, Inoeg, Chem., J9, 2153,(1970)

Ifl) R. Parry, R. Rudolph and D. Shriver, Inorg. Chem. ,_3, 1^79,(196*0 lj-2) R. Schaeffer and A. D. Norman, J. Amer, Chem. Soc.,_88, Il*f3,(l966)

43) A. Stock and W. Siecke, Ber., _57, 566, (192U) bb) W. N. Lipscomb, J. Chem. Phys., 65, 106^,(1961) k5) W. N. Lipscomb and R. Dickerson, J. Chem. Phys.,27, 212,(1957) k6) S. G. Gibbins and I. Shapiro, J. Amer. Chem. Soc.. 82, 2968,( i960) bj) A. B. B ayless, G. A. P re ssle y , J r ., M. E. Gordon and F. E. S tafford ,

J. Amer. Chem. Soc., 88, 929, (1966)

1*8) T. P. Fehlner and W. S. Koski, J. Amer. Chem. Soc., J36, 1012,(196*!-) k$) J. Dobson, R. tforuca and R. Schaeffer, Inorg. Chem., 9, 2l6l,(l970)

50) D. M. Ritter, C. Phillips and C. A. Lutz, Inorg. Chem., J , 1191,0-96^

51) S. R. Gunn and J. H. Kindsvater, J. Phys, Chem,, 70. 111^,(1966)

52) W. R. Deever and D. M. Ritter, Inorg. Chem., _7, 1036,(1968)

53) R* Schaeffer and D. F. Gaines, Inorg. Chem.,_3, **38, (196*0

3b) R. S ch aeffer, J . Leach, T. Onak, J. Spielman, R. R ietz and L. G.

Sneddon, Inorg. Chem,_9, 2170^ 1970)

55) R* Schaeffer, S. J. Steck, G. A. Pressley, Jr., F, E. Stafford and

J. Dobson, Inorg. Chem., 9, 21*52,(1970)

56) R. Schaeffer, private communication regarding electron diffraction

stu d ies o f BqHi2 would result in an isotopic distribution of boron atoms in an

equilibrium situation.

-/ 28 In studying the BsHe /DC1 system, Gaines and lorns maintained that

M--i)BsHa exhibited no intermolecular exchange and that any type of

terminal proton-bridge deuterium exchange had to be assisted by a Lewis

base species. They found that weak bases such as tetrahydrofuran( THF) 29 promoted intramolecular exchange5 Onak, Gerhart and Williams had described the rapid equilibration of deuterium in all positions of

BsHaD with the assistance of a strong base such as 2, 6-dimethylpyridine

(2,6-DMP) four years earlier. Rearrangement of large boron hydride frameworks by Lewis base assistance w ill be discussed later.

The fact that B4H3.0 and BsHxx were intrinsically unstable/reactive

In the vapor phase made it difficu lt to undertake meaningful experiments similar to some of those described above for the more stable boron hydrides. Gas phase reactions involving B4H3.0 and BgHxx with acetylenes and/or ethylenes suggested the existence of the B4H8 intermediate 30-32 species as part of the reaction mechanism in each case :

BgHxi------B*Hs + BH3 (2 )

B A o ------B 4H8 + Ha (3 )

The concept of describing the pentaborane(11) molecule in terms of a labile BH3 group draws support from several studies of exchange phenomena in volvin g BsHxx* Evidence supported the follow in g gas phase equilibria which depict steady state concentrations of reactive fragments: 151

57) W. N. Lipscomb, Inorg. Chem., _3, 1683,( 1964)

58) J. Dobson and R. Schaeffer, Inorg. Chem., 7, 402, (1968)

59) S. Kettle and V. Qtomllnson, J. Chem. Soc.(A), 15, 2007,(1969)

60) H. Beall, C. Bushweller, W. Dewkett, M. Grace, J. Amer. Chem. Soc.,

22, 3484,(1970)

61) C. Bushweller, H. Beall, M. Grace, W. Dewkett and H. Bilofsky,

J. Amer. Chem. Soc., 95, 2l45,(l97l)

62) W. N. Lipscomb, J. Inorg. Nucl. Chem., 11, 1,( 1959)

65) R. E. Williams, J. Inorg. Nucl. Chem., 20, 198, (L96l)

64) W. Phillips, H. Miller and E. L. Muetterties, J. Amer. Chem. S o c .,

81, 4496,(1959)

65) M. A. Ring, E. Witucki and R. Greenough, Inorg, Chem. ,6, 395,(1967)

66) J. B. DeRoos and J. P. O liver, Inorg. Chem., 4 , 17^1,(1965)

67) R. W, Parry et al., J. Amer. Chem. Soc., 8l, 3530-3560,(1959)

68) W. Deever, E. Lory and D. Ritter, Inorg, Chem.. 8. 1263/ 1969)

69) B. M. Graybill and J. K. R uff, J. Amer. Chem. S o c ., 84, 1062,( 1962)

70) R. T. Paine and R. W. Parry, Inorg. Chem., 11, 268, (1972)

71) R. Schaeffer, P. Tebbe and C. Phillips, Inorg. Chem.,J5, 14-75,(1964)

7 2 ) E. R. Lory and D. M. Ritter, Inorg, Chem., 10. 959,(1971)

73) W. N. Lipscomb, Boron Hydrides, W. A. Benjamin, Inc., New York, N. Y,

1963, P. 175

74) H. D. Johnson II, R. Geanangel and S. G. Shore, Inorg. Chem., 9,

908,( 1970) .

75) D. P. Gaines and T. V. Iorns, J. Amer. Chem. Soc., 89, 3375,(1967) 152

76) R. Geanangel and S. G. Shore, J. Amer. Chem. Soc., J39, 6771,(1967)

77) V. T. Brice and S. G, Shore, Inorg. Chem., 12, 509,(1973)

78) V. T. Brice, H. D. Johnson, II, and S. G. Shore, J. Chem. Soc. Chem.

Comm., 1128,(1972)

79) V. T. B rice, H. D. Johnson II and S. G. Shore, Inorg. Chem.,

approved for publication

80) T. Onak, J. Amer. Chem. Soc.,83, 2584, (1961)

81) T. Onak and F. Gerhart, Inorg. Chem.,JL, 7^2 , ( 1962)

82) L. J. Edwards, W. V. Hough and A. F. Stang, J. Amer. Chem. S o c .,

8 5 , 831 ,( 1 9 6 3) 83) R. W. Parry and P. C. Mbews, Jr., Inorg. Chem., 5, 1553,(1966)

84) R, Schaeffer, J. N. Shoolery and R. Jones, J. Amer. Chem. Soc,,

80, 2670, (195E)

85) D. F. Gaines, J. Amer. Chem. Soc., _91, 6503, (1969)

86) W. S. Koski, Avd. Chem. Ser., No. 12. ACS Washington, D.C., 1961, 78

87) J. L. Boone and A. B. Burg, J. Amer. Chem. S o c ., 8jl, 1766, (l959)

88) J. L. Boone and A. B. Burg, J. Amer. Chem. S o c ., .80, 1519, (1958)

89) T. Onak, A. Fratiello and R. Schuster, J. Amer. Chem. Soc., ^0, 1194,

(1 968)

90) G. Kodama and R. W. P a r r y ,’J. Amer. Chem. S o c ., 82 , 6250,(1960)

91) G. Kodama and R. W. Parry, J. Amer. Chem. S o c .,_ 93, 3372, 0-97^)

92) A. Burg and J. R. Spielman, J. Amer. Chem, Soc., 8l, 3479, 0-959)

93) R. W. Parry e t al., J. Amer. Chem. Soc., 84. 1767,(1962)

94) A. B. Burg e t al., Inorg. Chem., . 2 , 1139,(1963)

95) H. D. Johnson, II andS. G, Shore, J. Amer. Chem. Soc. ,93,3798,(L971)

96) H. D. Johnson II V. Brice and S. G. Shore, Inorg.Chem. ,12, 689,@-973) 155

97) H. D. Johnson, II, S. G. Shore, N. L, Mock and J. C. Carter,

J. Amer. Chem. Soc., j?l, 2131, (1969)

98) H. D. Johnson and S. G. Shore, J. Amer. Chem. Soc., 93, 7586,(1970)

99) H. D. Johnson, II, private communication

100) A. C. Bond and M. L. Pinsky, Inorg. Chem., 12, 603,(1973)

101) H. D. Johnson, II, S. G. Shore, N. L. Mock, J. C. Carter and F.

Swicker, J. Amer. Chem. Soc., 92, 72l6, (197^

102) V. T. Brice, D. L. Denton, S. G. Shore and H. D. Johnson, II,

Inorg. Chem., 11, 1135^(1972)

103) S. G. Shore, C. W. Hickam and D. Cowles, Jr., J. Amer. Chem. Soc.,

_87, 2773,(1963)

104) C. Hall and S. G. Shore, J. Amer, Chem. Soc., _8 8 , 53^6, (1 9 6 6 )

105) D. E. Young and S. G. Shore, J. Amer. Chem. Soc., 91, 3^97,(1969)

106) F. Yungfleisch, Ph.D. Dissertation, Ohio State University, 1973

107) G. Brubaker, Ph.D. Dissertation, the Ohio State University, 1971

108) G. Kodama and R. W. Parry, J. Amer. Chem. Soc., j?2, 3^82, (1970)

109) G. L. Brubaker, M. L. Denniston, S. G. Shore, J. C. Carter and

F. Swicker, J. Amer. Chem. Soc., 92, 7216, CL9T0)

110) R. T. Sanderson, Vacuum Manipulation of Volatile Compounds, John

Wiley and Sons, Inc., New York, N. Y., 19^8

111) D. F. Gaines, R. Schaeffer and F. Tebbe, Inorg. Chem. ,2, 3 2 6 ,(1 9 6 3 )

112) A. C. Bond and M. L. Pinsky, J. Amer. Chem. Soc, J?2, 32, (1970)

113) F. Mann and A Wells, J. Chem. Soc., 1938, 702 5 M. A. Beg and H. C.

Clark, Canad. J, Chem., 38, 119,(1960) 15J+

Ilk WADD Technical Report 60-262, April, i9 6 0 , p. 23

115 T. Onak, G. Dunks, I. Searcy, and. J. Spielman, Inorg, Chem., 6 , . 11*65,( 1967 )

1 1 6 R. E. Williams and P. Gerhart, Inorg. Chem. ,_4, 1239,(1965)

1 1 7 M. L. Denniston and S. G. Shore, unpublished data

1 1 8 T. P. Fehlner and W. S. Koski, J. Amer. Chem. Soc., J36, 2733#( 196*0

119 D. F. Munro, J. E. Ahnell and W. S. Koski, J. Phys. Chem., J£2,

2 6 8 2 ,(1 9 6 8 )

1 2 0 T. Onak and J. Leach, J. Amer. Chem. Soc., _92, 3513,(1970)

1 2 1 W. J. Dewkett, H. Beall and C. H. Bushweller, Inorg, Hucl. Chem.

Lett., 7, 633/1971)

122 J. Glore, J. Rathke and'R. SchaeiTer, in press

123 C. R. Peters and C, E. Nordman, J. Amer. Chem. Soc. ,82, 5758, @-960)

12k C. E. ITordman, Adv. Chem. Ser., 32, 20lf, (1 9 6 1 )

1 2 5 C. E. Nordman and C. Reimann, J. Amer. Chem. Soc., _8 l, 3538,(1959)

126 P. E. Stevenson, J. Amer. Chem. Soc., 95, 5^,(1973)

1 2 7 R. Hertz and S. G. Shore, Inorg. Chem., in press

128 T„ Onak# P. M. Tucker and J. B. Leach, Inorg. Chem. ,_9, 1**30, (197^

1 2 9 A. B. Burg, J. Amer. Chem. Soc., 90# l!*07,(1968) 8

B5H11 + RB2Hs ”•= = * . BsHioR + B2Hq R=CH3, C2H5 (4)

B4K3.0 + RB2H5 « » B4H9R + B2Ha R=CH3, C2Hs (5 )

B4H10 + ~§( H2BCH3)3 «=_.., »H2 + B5H3.0CH3 ( 6 )

12 Ritter and. Lutz proposed that the methyl group of EsHxoCHs was situated on the frontal pyramid-base boron atom, and that based upon proton NMR spectra, geometric isomers of the compound were present. 33 Onak and Marynick cited the conformations (exo- and endo-) of

BsHi0 CH3 in discussing ring current models for NMR chemical shift correlations in pyramidal boron hydride compounds. Solomon and co- 30 34 workers stated that in the B5 Hxi/C2 HsB2 H5 system all borons and hydrogens were involved in the scramble in a reversible fashion.

Pentaborane(ll) has also been discussed in terms of gain and loss I 35 of hydrogen, depending'upon conditions. Burg and Schlesinger indicated that B5H11 readily absorbed hydrogen to form B4H10 and B2HqJ Grimes and 36 co-workers described the tendency of B s H u to hydroborate (form alkyl- boranes) and to hydrogenate an alkyne to an alkene or alkane, reflecting facile loss of hydrogen:

% H u + Calls J ? > 60°C ^ 2-C2HbBsIfe + H2 (7 )

B5 H1 1 + C2 H4 T=; ~ ^ . C > C2 H5 BsH1 0 + H4 C2 B4 Hq + (8)

C2 HsB2 Hs + other materials 9

Some isotope studies involving the ®nHn+Q hydrides have been 37 carried out. Norman and SchaeiTer made fi -DB4Hg from the following

reaction B5H11 + 2D2O ► B ^ D + B(OD)3 + 2HD .(9)

As shown by successive infrared spectra, the deuterium atom gradually

began migrating from the bridge position until a random distribution

was indicated by the infrared spectrum. This work also is significant

in that the authors suggested that deuterium transfer was effected + 30 within a H q B(D z O ) ^ B4H9 species. Bond and Pinsky also observed

migration of the deuterium atom in p, -DB4Hg made by reacting LiB^Hg

with DC1 . 39 SchaeiTer and Odom reacted BqHi2 with and prepared

unspecifically labeled BqHi o J deuterium atoms were present in both ' <40 bridging and terminal positions. In another work , reaction of BqHi2,

one of the primary materials studied in the course of this problem, o with liquid BaD6 at -30 C resulted in deuteration of the two BH2 groups

of the molecule exclusively. If the temperature were raised to 25°C,

total deuteration of the hexaborane(l2 ) framework was seen. There was o no boron exchange at -30 C. ■41 Two final Investigations merit discussion. Parry and co-workers

cleaved B4H10 with NaBD4, produced B2H3D3 and HaBaHrjrD, and eventually proved that the reaction proceeded first by symmetrical cleavage to give the B3H7 species. In later work, Schaeffer and Norman reacted

B4HaC0 with B2B0 and identified the product as B^HeDa by mass spectro­ metry j bridging and terminal positions were substituted in the product. 10

Under different conditions, reacting B2H6 with ^B^HqCO produced a labeled pentaborane(11) molecule enriched with 1XB in the basal positions.

However, within ten minutes at room temperature a statistical distribution of the lIB isotope over the entire framework was reported, indicating that some mode of basal/apical interchange was in operation.

II. Hexaborane(lS)

Hexaborane(12 ) has been one of the rarest boron hydrides discovered to date, primarily because of the difficulties which have been encountered in synthesizing the compound by so-called traditional methods. As a result, very little information is available in the chemical literature regarding the physical or chemical properties of the molecule. 43 The discovery of BsHj.2 was reported in 1924 by Stock and Siecke 6 and at later dates both withdrawn and resuggested . Lipscomb, established as a pioneer in the field- of boron hydride chemistry from a theoretical and predictive standpoint, suggested three valence structures for the 44,45 elusive molecule (I-III, page 11) , one of which was similar to the structure postulated for the hypothetical B5H14 molecule (IV). Unfortu­ nately, Lipscomb's attempts to complete a low temperature single crystal structure determination have been thwarted by the inability of B©Hia to freeze in any state other than a glass.

One of the first indications that the molecule did exist was . 40 furnished by Gibblns and Shapiro in i960 as a result of mass spectra taken of a mixture of boron hydrides. The mixture was known to contain ri

74-11,005

LONG, John Richard, 1945- TOE REACTIONS OF HEXABORANE(12), FENTABORANE(11) AND TETRABORANE(10) WITH SELECTED LEWIS BASES.

The Ohio State University, Ph.D., 1973 Chemistry, inorganic

University Microfilms, A XEROX Company, Ann Arbor, Michigan & ■J

THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED. Structure I Structure II

\ / \ / / Y \

■ k < / / . IV ■ \ f / B s H H

Structure III- ' 1 ^ Structure IV see page 4 / y - , I \ H M

/\ primarily BeHxo> “but inconsistencies in vapor pressure measurements suggested that other "boron hydrides were present.

Until 1971* the "best preparation for BeHj.2 involved the treatment 1 0 of ( CH3)4HB3Hq with polyphosphoric acid 5 however, the yield of B6Hi2> about five percent, did not provide sufficient quantities of the material 12

suitable for an extensive investigation. An analysis and vapor density

molecular weight determination were described. Other preparations,

which produced numerous other boranes as w e ll as BqHx2 j included the ■47 integral furnace pyrolysis of B4II10 t the shock tube pyrolysis of 4a BsHq , the elevated temperature reaction between B5H3 and B2H6 in a 49 52 flow-quench system and the reaction between BF3 and (CHa)20B3H7 . 50 In the mid-sixties, Ritter and co-workers elaborated upon the properties of hexaborane(l2), suggesting that since other boron hydrides

induce its decomposition, the material should be considered very reac­ tive rather than intrinsically unstable. It was found that after stand­ ing in the liquid phase for three hours at 25°C, B6Hi2 could be recov­ ered almost quantitatively, if very pure originally. In comparison, only

37 ft recovery was achieved when a sample of B6Hx2 was maintained in a solution of B2H6 for k'f hours. BgHxs and BxoHi4 were produced by that decomposition. Hexaborane(12) vapor was completely destroyed when kept o n at 100 C for five minutes'. At 25 C for one hour, the vapor showed the presence of B5H11 plus traces of B4H3.0 and B^Hq. Decomposition of the o liquid at 25 C resulted in the formation of products in the order of abundance B2He> Egllxx^ B4.Hxo* ffeintaining BaHx2 for three hours in an equivalent of B2H6 at ten atmospheres resulted in BsHxx and B4HX0 formation. Other decompositions were shown to yield Eslls and BxoHi4 also. 51 Gunn and Kindsvater determined the heat of formation of BqHx2 to be +26,5 kcal/mole, and the heat of reaction to be -8 kcal/mole for

BqHx2 ------» BsHg + ^B2Hq ( 10) (e) (s) (e) 13

by exploding mixtures of BqHi s and stibine (SbHa).

Preliminary reactions of B^Hia showed that the molecule was

cleaved by diethyl ether symmetrically to produce pentaborane(9 )j and 53 was.degraded by water at zero degrees to B^Hio > in addition,

derivatives of pentaborane(9) and diborane(6) were produced from

reactions of BqHi s with excess amines and nitriles, while initial 40 attempts to brominate the molecule were unsuccessful • 53 The first boron-11 NMR spectrum of BeHie verified (III) as the

structure of the molecule (page 4). This preliminary NMR data was 54 supplemented by later boron-11 and proton NMR (FMR) studies . In

addition to determining chemical shift and coupling constant values

(table one), the PMR spectrum of BqHi2 showed that the BH2 protons gave «

TABLE 1

______Hexaborane( 12) Boron-11 and Proton NMR Data______

1XB Assignment .Chemical Shift Coupling Constant

B^6 doublet -22.6 ppm 156 Hz

Bl,4 triplet -7*9 ppm 133 Hz

132,5 Sublet +23.6 ppm 158 Hz

XH ®3,6 terminal 4.9 tau 160 Hz

ax/eq 5*8 tau 120 Hz Bj_ h 9 eq/ax 6.1 tau 138 Hz

®2,5 terminal 7*9 tau 158 Hz

|d -hydrogens- 10.2 tau Ik

rise to two sets of resonances in the spectrum, indicating the existence o f a x ia l and eq u atorial environments. Sim ilar phenomena were observed 54 with high resolution NMR spectra of B4H10 and BgHn .

A recen t m olecular beam mass sp e c tr a l a n a ly sis o f BgHia 55 established its ionization potential as 9.75 eV and described the

•|< J . Bs Hjj and B ,*!^ envelopes as one-third and one-fourth the. intensity of the parent envelope, respectively.

Supplemental to a discussion of hexaborane(12) are considerations of the hypothetical BqHi * molecule. A postulated structure was depicted 56 on page 11, and its similarity to the verified configuration of BqHi2 lends credence to the argument that the elusive nature of the hydrogen- rich hexaborane(lU) is due to some mode of spontaneous hydrogen loss to produce the more sta b le B6Hip m olecule. 57 Lipscomb predicted severe crowding in the BqHi ^ molecule as part of his discussion of inter- and intramolecular steric requirements of various boron hydrides. This behavior would be analogous to the well characterized behavior of the BaHi* molecule* this species loses o 58 hydrogen spontaneously at -50 C to form BaHi2 . 59 Kettle and Tomlinson calculated stabilization energies for various boron hydrides, including BeHi2 and BsHi^, in terms of coulomb and resonance integrals. 15

III. Behavior and Reactivity of Boron Hydrides in Solution.

A careful distinction was made in section one of the introduction

regarding the emphasis which was being placed upon the term activities

of the boron hydrides in various situationsj these considerations are

rather apart from the discussion of the reactivity of the intermediate

boron hydrides which commences here. 4,5 Lipscomb and co-workers summarized an extensive set of principles

which are generally applicable as a guideline for reactions of the boron hydrides. The more pertinent points are 1) apical borons have a greater relative negative charge than basal borons, thus the apex is

subject to electrophilic attack under appropriate conditions 2 ) BHa borons have a greater relative positive charge than B-H borons, thereby being subject to nucleophilip attack 3) terminal hydrogens bear a relative negative charge while bridge hydrogens bear a relative positive charge and are open to abstraction by a suitable agent 4) the unique proton shared by the three frontal (two basal, one apical) boron atoms of the B5H11 framework is intermediate in nature between a bridging and a terminal proton 5) bridge protons between B-H and BH2 groups are closer (more strongly bonded) to the B-H group.

Applying these principles to pyramidal boron hydrides is a relative­ ly straightforward process. For example, electrophilic substitution would be expected to take place at the apical boron, while attack by an amine base or an ether would be expected to occur at a BHe group. Tetraborane 16

olTers a slightly different case, however> free radical substitution would be favored at borons number one and three (B-H groups) but direct bromination of the molecule has shown to result in the preparation of 2-BrB^Ife • Thus, the guidelines are available from the theoretical basis, but care must be exercised in applying them to specific cases.

Turning to some solution chemistry, recent investigations by 60,61 Bushweller and co-workers not only gave a complete picture of intramolecular exchange of protons in the BsHs” ion, but also showed that ,quadrupole induced thermal decoupling could be observed in the

PMR spectra of solutions of the ion. Samples of TIB3HB, (CHsJ^UBaHa and

[(CgHs)npj 2 CUB3HQ exhibited moderate temperature (-1 0 ° to -5 0 °)

* coalescence of the spectrum followed by distinct sharpening at temperatures between -100° and -150°, Special mention was made of the cation dependence of the spectra and of fluxional behavior in the anion.

Examining the literature reports of the triborane(7) derivatives provides further evidence not only of intramolecular exchange or 62,63 tautomerism of protons , but also of varying degrees of intermolec- ular activity.

The boron-H NMR spectrum of (C2 Hs)2 0 BaH7 indicated that all 64 borons were identical on the MMR time scale , and the spectrum of

THP*B3 H7 exhibited one poorly resolved octet from the seven tautom- erizing protons coupling with the boron atoms whether the adduct was 65 dissolved in TUP or benzene . In contrast, the spectrum of (CHaJsNB;^ showed a superposition of two octets separated by less than two parts per million (ppm)j this indicated non-equivalence of the boron atoms 17

since the amine was too strongly coordinated, to the borane to

exchange intermole cularly. It should be noted that trimethylamine ex- 6S change has been observed in spectra of solutions of (CH3 ) ^3aN( CH3 ) 3 . • *1 Displacement of coordinating ligands from the triborane(7) species

is well documented. Early studies demonstrated the displacement of 68 amine from (CH3 )3NB3 H7 by (C6 Hs)3P and of ether from (C2 H5 )2 OB3H7 by 69 70 FF3 . Parry and Paine facilitated the process by introducing a Lewis

acid species to complex with the displaced base (usually ether) and

allow the synthesis of unique stable triborane(7 ) adducts such as

F3PB3 H7 . The authors also offered an explanation for the observation

that (CH3 ) 3 HB3 H7 could be isolated from the reaction

B4H10 + 2E( CH3) 3 -----» HaBNC CH3) 3 + ( CH3) 3KB3H7 ( H) 1 but a similar system employing PF3 as the base resulted in the formation

of (FsP)2 B2 H4 . Central to the hypothesis was the suggestion that the

amine stabilized the B3S7 fragment by inductive electron donation 72 thereby making the two BH2 groups less electrophilic .

The articles published in 1971 and 1972 also presented data from

1 1 B, 3 1 P, 1 SF, and NMR spectra of numerous triborane(7) derivatives.

Each boron-11 M R spectrum displayed an area two resonance downfield - between 9 ppm and 22 ppm (relative to BF3 «etherate) and an area one

upfield resonance between ^0 ppm and 6 0 ppm} reference was made to the

'i’ixed1 nature of the structures of the derivatives which were examined. 18

Two additional areas deserve mention in connection with the concept ol‘ inter- and intramolecular activity in the intermediate boron hydrides.

First is the area of low temperature quenching of proton tautomerism as monitored by NMR spectroscopyj second is the area of molecular rearrangement or conversion through Lewis base assistance.

With regard to the first, discussions of the boron-11 NMR spectra 63,73 J M _74-76 of BqHxo , BqH9 and B5 H3 have centered around the concept of bridge hydrogen tautomerism, possibly with assistance from an ether solvent, which renders the basal borons equivalent in the boron-11 NMR spectrum over a wide range of temperatures. Recently, very low temper­ ature NMR investigations have demonstrated a quenching of such phenomena _77 73 79 78 79 79 in the case of 2 -CH3BSH7 , BaH^o f > 2 -CH3 BqH8 and 2 -BrBaHa and the determination of some static structures was possible. , so In the second area, Onak described the conversion of apically substituted alkyl-pentaborane(9 ) derivatives to basal alkyl-penta- borane(9) species by means of 2,6-DMP assistance. A rather harsh thermal 61 rearrangement was subsequently submitted along with suggestions of possible mechanisms, including one cleavage mechanism. Later elaborations made upon the phenomenon of base assisted rearrangements included 82 observations of isomerizations assisted by as strong a base as N( 0113)3 • 83 Parry and tfoews found that 2 ,6-DMP rearranged 1,2-(CH3)2BsHt' to

SjMCHsJaEfeHT but not to 2,J+-(CH3)2BsH7.

In addition, spontaneous rearrangement of methyl groups in

2 ,2 -(CHsJsB^Hq takes place very rapidly and is dependent only upon 85 8 6 temperattire . Kbski has also described the pecularities of wandering substituent atoms on boron hydride frameworks in some detail. 19

Since electron deficient molecules such as boron hydrides are potentially very vulnerable to attack by basic (nucleophilic) agents, any solution chemistry of the boranes must take into account the nature of the solvent medium. This consideration is reflected in the observation that much of the chemical literature which deals with solution chemistry of BeHio and Bslfo describes reactions carried out in ether solutions.

In contrast, the BnHn + 6 family of boranes has exhibited instability in or reactivity with ethers, usually dependent upon temperature conditions. ar For example, Boone and Burg reported that B5H n reacted with bases as weak as ethers to give hexaborane( 10). The same authors also described the quantitative conversion of BsHu to B4Hxo according to the equation BsHix + 3HaO -----». B(0H ) 3 + B4 H1 0 (12) as well as the conversion of B5 H1 1 to B5H9 (close to 50$) and BqHio

(3.6$) through the action of (MeaKjaBH88. It was also interesting to note that while small amounts of B4 H 1 0 were formed in this last conversion, as much as 2 5 $ BsHg could be formed from B4 H1 0 By treating it with

(Me2 N)2 BH. These processes were significant in that they took place between the temperatures of -78°C and 0°C.

The behavior of diborane in various ether solutions was 8 9 investigated by Onak and co-workers . He found solvent exchange to be frequently slow enough to permit observation of proton resonances of both bulk solvent and molecules in the first coordination sphere.

Results of specific experiments again demonstrated dependence upon temperature. Under appropriate conditions THF cleaved diborane(6 ) 20 symmetrically to give the TUF*BH3 species, solutions of B2 HQ in diethyl ether suggested that no change occurred, and dimethyl ether

could be observed to coordinate with the IfeHe molecule, the six equivalent protons of which gave rise to a septet in the PMR spectrum of the solution.

Symmetrical cleavage reactions between tetraborane and ethers which produced etherates of the triborane(7 ) species have been alluded 67 to and the factors which determine how a given boron hydride will undergo cleavage upon various base attacks will be discussed in a later section. In a related study, the dependence of tetraborane cleavage reactions upon conditions of temperature was illustrated in a study 71 published by Schaeffer, Tebbe and Phillips . Siey found that reactions run at -53°C exhibited unsymmetrical cleavage by an ether, and the ■

B3H7 species was formed by hydride ion transfer from a BaH©- ion to the H2 B counter ion when the temperature was raised.

90 As was the case with tetraborane , low temperature solutions of pentaborane(1 1 ) in diethyl ether demonstrated sufficient stability that cleavage of the borane framework by a second agent, such as ammonia, could be effected91.

A factor which is perhaps most closely associated with boron hydride-ammonia systems is the amount of time in which a given system is allowed to react. Throughout the early literature ammonia adducts of various boron hydrides exhibited stoichiometries which would be regarded as unusual or unjustified by modern understandings. Kodama and THE REACTIONS OF HEXABORANE( 12), EENTABORANE( 11)

AND TETRABORANE(10) WITH SEIECTED IEWIS BASES

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the graduate School of the Ohio State University

by

John Richard Long , B. S.

The Ohio State University 1973

Reading Committee:

Dr. Russell M. .Pitzer Approved by

Dr. Devon W. tfeek

Dr. Sheldon G. Shore

• Adviser Department of Chemistry 21

9 0 Parry placed, early emphasis on the effects of aging in the

B4 H1 0 /2 NH3 system, and the synthesis of other 'diararaoniates* described

reaction times in terms of weeks. A corollary to this feature is the

general observation that recovery of all excess ammonia above the

established stoichiometry from a system is usually not possible.

It is apparent that great care is required in investigating any

ammonia-boron hydride reaction.

Item the investigations into cleavage reactions of the boron

hydrides, enough information is available that factors can be discussed

which determine whether cleavage proceeds in a symmetrical or

unsymmetrical fashion. This discussion will deal with representative

examples for the sake of brevity; extensive treatment of diborane(6 )-

amine cleavage reactions has been carried out by Shore and co-workers 1 0 a-ice and several references are very valuable.

The following reactions demonstrate the two types of cleavage:

B2He + 2 HH3 — » h2b(NH3) 2 BH4" (unsym) (13)

B2He + 2N( CH3) 3 — — * 2 ( CH3)3MBH3 (syra) W

B4H10 + 2NH3 ■ - » HaB(MH3 )a B2B5 (unsyra) (15)

B4HX0 + 2N( CH3) 3 » HaBNt CH3) 3 + ( CH3) (sym) (1 1 )

Apart from specific reaction conditions, the factors which seem to have the greatest bearing upon the nature of the cleavage are : 1 ) a standard inductive effectj 2 ) the electron donating' abilities of the cleaving base; 3) steric requirements. 23ius, it is obvious that 22

ammonia always cleaves in an unsymmetrical fashion while trimethylamine

always cleaves in a symmetrical fashion.

The unsymmetrical cleavage of tetraborane (reaction 15) was 90 investigated by Kodatna and Parry in i9 6 0 and gave one of the first'

indications of the importance of choice of solvent and allowance of

extended reaction time. 83 Parry and tfoews later studied the cleavage of tetramethyl-

diborane(6 ), (CHsJ^BsHsj with ammonia and identified the cation from + » the cleavage product by forming the unique salt (CH3 )2 B( NH3 ) 2 Cl via

a protonation reaction.

Similar unsymmetrical cleavage was observed with the pentaborane(11) 91 + - ammonia system 5 the reaction produced HsB( M 3 ) 2 B4 H9 and reinforced

the contention that the base attacks and abstracts a BH2 group from 1 the borane.

In contrast to these systems, two examples of unsymmetrical

cleavage of boron hydrides which do not feature any BHa groups have been 108 documented. Kodama and Parry aged a mixture of ammonia and penta-

borane( 9 )j 2 : 1 in diethyl ether, for over a week at -7 8 °C before isolat- + 107* ing a white solid which proved to be HaB(KH3)a B4H7 . Brubaker

showed that BsHio also reacted with two equivalents of ammonia to give an unstable cleavage product.

A second category of boron hydride-Lewls base reactions merits

some discussion$ it involves the production of usually solid adducts of varying stabilities by incorporation of one or two molecules of a

Lewis base into an expanded boron hydride framework. Adducts made by a 23

direct addition process include B5Hg*2N(CH3)3 and BsHio* 2P( CH3) 3109,

and require specific conditions of solvent and temperature to allow

their formation) other adducts such as B4Hq PP3N(CH3)2 and B4HSPF3

have to be made by indirect procedures since symmetrical cleavage of

B s H u by these and other bases have not been successful in 42,93,94 synthesizing the respective derivatives

One significant conclusion drawn from studies of Lewis base

reactions with intermediate boron hydrides is that an order of stability X09 of adducts of any given boron hydride was readily determinable :

P(C6Hs )3 adducts > P{ CH3) 3 adducts »N ( C H 3)3 adducts

A careful examination of information presented earlier demonstrated that procedures used to synthesize B5H11 and Bq Hj.2 predominantly ■ involved vigorous or high temperature processes which by their very nature limited the yields of boron hydrides. Thus, it was of monumental 95 significance when Johnson and Shore succeeded in developing a high- yield synthetic procedure which may have extremely general applicability as far as step-wise synthesis of boron hydrides and their derivatives' is concerned.

The following reactions illustrate how B s H u and BqHih can be prepared by this method from B4H10 and B5H9, respectively:

~78°C B4H10 + KH ^ y^ Hg + K B 4 % (16)

KB4H9 + |BHHQ ^ ! | j ^ K B s H 12 ( 17) 2 k o KB5H12 C..»Hg + KC1 + BsHxl ( 18 ) nOl

BsHg + KH(.c-^Q °g.„ Hg -f KBgHa (1 9 )

KBs Ha + feHa ^g^*KB6Hn ( 30)

o KB0H11 C > BqH12 + KCl (2 1 )

As long as the ether solvent can be removed from the intermediate salts produced, and separated from the desired product, such low temperature syntheses are Infinitely preferable in terms of convenience, speed and percent yield of product to previously published syntheses.

In addition to the synthesis of BnHn+e hydrides, BH3 addition to a borane anion in solution accounted for a vastly improved method of preparing hexaborane(10) according to the following equations: t

fl0 2 -BrBsHs + KH Ha + KBsH7Br (22 )

KBsHrBr + iB2H6 KBaHxoBr (23 )

KBQHioBr - L > BqHxo + KBr (2l|) neat

Perhaps the key to these syntheses was the development of deprotonation reactions of varitius intermediate boron hydrides. In 71 75 1967 Geanangel and Shore as well as Gaines and Iorns provided most of the characterization of the octahydropentaborate( -1) ion which resulted from the reaction of pentaborane( 9) with strongly basic anions such as the hydride and methide ions. Shortly thereafter analogous 25

reactions with BqHio were described, by Shore and co-workers74*57 along with an assessment of the importance of choice of solvent,

temperatures and deprotonating agents.

' Even though deprotonation of B4H10 could be carried out in a 33 . 98 straightforward manner • , there have been no published reports of quantitative deprotonation of the other members of the series, B5H11 and BqHi2J preliminary indications are that these compounds undergo 00 deprotonation to the extent of 75$ and 50$ respectively •

A natural consequence of the deprotonation reactions was the determination of relative orders of acidity for the boron hydrides. Prom the information procured from several studies which featured NMR techniques74*77*100, the following sequences of Bronsted acidities can be affirmed: B10H14 > B4H10 5 BsHio y B5H9

1 -CIBsHs > B5H9 > I-CH3B5H8 = 2 -CH^BgHa

These orders, which obviously represent solution phenomena, can be rationalized in terms of polyhedral framework size, the abilities of the resulting frameworks to be stabilized by tautomeric mechanisms, and simple inductive effects.

A unique set of deprotonation reactions have been characterized which proceed in a conditionally reversible fashion through proton 98,i o x abstraction by a molecule of the weak base, ammonia . Hi is is consistent with the suggestion by Parry and Edwards that bridge hydrogens of the lower boron hydrides interact with ammonia. Not only 26 were B4H10, EsHg and BqHio observed to experience this behavior* but

Johnson and Shore also obtained evidence for similar behavior when a 1:1 mixture of ammonia and pentaborane( 11) underwent a metathesis X02 . N reaction with (n-C^HgJ^NI to produce NH4I, among other materials.

This reaction would be represented by the equation

B s H u + MH3 + (n-C4H9)4NI ---- *■ (n-C^HgJ^KBsHxo + M U I (2 5 )

The boron-11 NMR studies which allowed the ammonia reactions to be monitored very effectively also brought up the point that was reflected in other studies of boron hydride reactions, namely, the importance of conditions such as temperature and solvent properties in these reactions.

IV. Structures and Predictions I Before elucidating a predictive chemistry for the hexaborane(12) molecule, it would be appropriate to explain how geometries and valence structures of various boron hydrides and their ionic derivatives can be ascertained. This information is vital in relating MMR spectra to valence structures, especially since more than one structure can be postulated for most boron hydrides and their derivatives (see page 11 for structures of BeHia). 73 Lipscomb derived a system for determining permissible (though not necessarily probable) structures, each of which was described by a four digit s*t*y»x notation. Hie first digit designates the number of bridge hydrogens In the structure, the second shows the number of three center two electron B-B-B bonds (open or closed), the third, the number of boron-boron two center bonds, and the fourth digit represents the

number of BHs units in the structure, tfore strictly, the last number

indicates the number of hydrogen atoms terminally attached, above and

beyond the allowed minimum of one (which would be a B-H group). For

example, if a structure contained all B-H groups except for one BH3

group, the last number in the s»t«y«x notation would be two. If there

were one EHa group and one BH3 group in the molecule, the x notation

would be three.

To convert Lewis base adducts to simple horon hydride derivatives

or analogs for purposes of valence structure determination, one only

need consider each molecule of base to be equivalent to a hydride ion, —2 H . Thus, BsH9*2rr(CH3)3 is equivalent to B5H11 , B6Hi0*becomes

-2 - BqHi2 and B a H y L is analogous to B s Hq . The flexibility of the system

lies in the fact that there is usually more than one molecular model

to which data from techniques such as NMR analysis can be fitted . The

true structure must accomodate not only the spatial requirements of the

new adduct, but also orbitals and electrons which the base introduces

into the framework.

Based upon the information available in the literature and under­

standing the reactive/unstable nature of the hexaborane(12) molecule,

certain predictions about the reactions of selected Lewis bases with

BsH12 can be offered: 1) attack by ammonia would be predicted to give unsymmetrical cleavage, and the product from the reaction would have

the formulation HsB(NH3)2+B5Hio" 5 the individual ions should be 28 identifiable upon protonation in liquid HC1 , giving H2E(WH3)2C1 and

B5H115 2) trimethylaraine should cleave in a symmetrical fashion to produce H3BN(CH3)3 and BsHgJ j5) trimethylphosphine could cleave in a symmetrical fashion, but adduct formation of unknown stoichiometry and stability would be more likely under mild conditions. Either

B6Hi3" or BqHj.4-2 analogs would be produced, and a similar tri- methylamine adduct might be isolable. This similarity would most likely be in terms of observable properties, but structural similarity would not be required, and might be the exception rather than the rule (see p. 1^2 ).

V. Statement of the Problem 35 The reactions developed by Johnson and Shore established the first high yield synthesis of BQHi2 as well as the best preparation of B5H11 to date. While characterization of the latter material is partial at best, this is the first opportunity made available for the extensive investigation of the chemistry of hexaborane(12 ). The course of the research was systematic: 1) verily and develop the . predictive chemistry of hexaborane(12)5 2) investigate any phenomena which contradict or supplement the predictive chemistry)

3 ) extend the principles established for the hexaborane(12)-Lewis base systems to the other members of the family, pentaborane(ll) and tetraborane(10). EXPERIMENTAL

I. Apparatus

A. Vacuum system

The fact that the boron hydrides employed in this investigation

were sufficiently volatile that they could be manipulated under vacuum

allowed for the use of a vacuum system similar in design to that 1 1 0 described by Sanderson . Since most of the materials were toxic,

sensitive to moisture, and required at least an inert atmosphere

environment to maintain their integrity, as many procedures as possible,

including preparations of starting materials, reactions, separations,

purifications, solvent drying, washings and extractions, quantitative measurement of moles of gases, and storage, were carried out on the vacuum line.

The vacuum system consisted of a mechanical forepump capable of maintaining a vacuum close to ten microns, a mercury diffusion pump -6 which reduced the vacuum to at least 10 Torr, a main manifold and a transfer manifold, two reaction trains (one in combination with a * calibrated distillation train), a storage manifold and a calibrated

Toeppler system used for transfer and measurement of noncondensible gases such as hydrogen or methane. The two pumps were protected by liquid nitrogen cold traps. The distillation train and the Toeppler system featured glass stopcocks exclusively} the valves of the reaction

29 30

trains anl the storage system employed, Teflon k mm stopcocks and

0 -ring joints in order to provide a grease free system. The degree of

vacuum in the line could. be measured with a McLeod gauge, and mercury

manometers were used for direct pressure measurements.

Each reaction train included several stations consisting of a

stopcock, a 14/35 inner joint and (usually) a mercury blowout which

prevented a pressure buildup of greater than one atmosphere. Each

reaction train had two 18/9 socket joints to which a demountable U-

trap could be connected in order to effect isolation, destruction and

removal of waste volatiles. In the case of the combined manifold, this

U-trap could be used as part of a five trap fractionation system. By

employing the transfer manifold which .served to interconnect all parts

of the system below the main manifold, volatile material removal could be carried out at either or both of the U-trap locations, and ffom whatever direction desired. One reaction train had a station specifi­

cally designed for attachment of the extracting apparatus.

As indicated, the volumes of the permanent traps of the distilla­

tion train were known and applied in the measurement of gases in the traps5 the other principal purpose of the train was the low temperature fractionation of volatile materials.

The Toepler system, which had access to three expansion bulbs of various volumes in order to accomodate a wide range of gas volumes, consisted of an automatically controlled mercury piston which contin­ uously displaced noncondensibles into the calibrated volume selected. VITA tfey 16 , 1 9 ^ 5 ...... Born, Toledo, Ohio

1 9 6 7 ...... B. S., the University of Toledo

1967-1969 ...... Teaching Assistant, Department of Chemistry, The Ohio State University, Columbus, Ohio

1969-1971 ...... Full-time Assistant, Department of Chemistry, The Ohio State University, Columbus, Ohio

1971-1973 ...... Research Associate, Department of Chemistry, The Ohio State University, Columbus, Ohio

PUBLICATIONS

Heat of Sublimation and Dipole Moment .of Cyelotriborazane, D. R. Leavers J. R. Long, S. Shore and W. Taylor, J. Chem. Soc.(A), 1580, 1969

Boron Heterocycles Part VII. Halogeno-, Mothyl-, Phosphino- and Amino- Derivatives of 1 ,3,2 -Dioxaborolane and 1 ,3 ,2-Dlthiaborolane Ring Sys­ tems, S. G, Shore, J.' Crist, B. Lockroan, J. R. Long and A. D. Coon, J. Chem. Soc., Dalton Transactions, 1123, 1972

Reactions of Hexaborane(12) With Selected Lewis Bases, J. R, Long and S. G. Shore, Abstracts, 164th National Meeting of the American Chem­ ical Society, New York, II. Y., September, 1972, No. INOR 1^3

FIELDS OF STUDY tfejor Field; Inorganic Chemistry

Studies in Non-metal Chemistry - Professors Sheldon G. Shore and Eugene P. Sehram

Studies in Transition Matal Coordination Chemistry - Professors Daryle H, Busch, Devon W. Ifeek and Andrew A. Wojcicki

i i 31 B. Inert atmosphere enclosure

Three inert atmosphere glove boxes were available for manipulating sensitive compounds. With each, the port was evacuated and refilled with gas from the box before the contents of the port were admitted into the box, A stainless steel box made by the Kewanee•Scientific

Company was continuously purged with nitrogen dried by passage through columns containing Linde ^ A molecular sieves, calcium hydride and phosphorus pentoxide. A small glove box with a static helium atmosphere which was constructed within the chemistry department employed stirring sodium/potassium alloy as a desiccant and scavenger. A large

Vacuum/Atmospheres glove box provided an atmosphere constantly purged by a recirculating purification system conducted through copper tubing.

The water and oxygen levels in the Ife atmosphere were maintained at one ppm by the use of Linde 13ft molecular sieves and Dow*s Q-l, respectively.

C. Reaction vessels

Reactions were carried out in Fyrex tubes and bulbs of various sizes equipped with standard taper joints and fitted to stopcock adap­ ters which could be attached to the vacuum line. Frequently one or two glass sidearms were connected to the reaction vessels in order to allow the transfer of solutions into. HMR tubes for spectroscopic analysis.

These adaptations were constructed so that the entire apparatus below the stopcock could be immersed in a cold bath. In addition, an extra long sidearm was frequently attached to the vessel in combination with a standard sidearm so that samples of a given system could be examined at different stages of the reaction. Transferring a portion of neat 32

solid to the long tube and sealing it oil* with a torch permitted the

solvent to be added to it separately while the original material in

the reaction vessel could be treated in any desired fashion.

All glassware was cleaned by soaking several hours in a solution oi* potassium hydroxide and denatured alcohol, rinsing with distilled o water and acetone, and dried in an oven at 130 C before use. Occasion­ ally a 3$ HF solution was used briefly to supplement the cleaning.

D. Infrared spectra

-Spectra in the Infrared region between 2.5 and 40 microns were

obtained on a Perkin-Elmer model 457 spectrometer. Gas samples were examined by means of a ten centimeter glass cell equipped with polished

KBr windows which had been attached to the specially cut glass by means of Kel-F wax. Hie cell was dttached to the vacuum line by means of a

Teflon valve standard taper joint.

Solid samples were ground in an inert atmosphere and mixed with

Nujol or hexachlorobutadlene (HCBD) and run as mulls between KBr windows.

E. Jfess Spectra

Mass spectra were obtained on an AEI MS 902 high resolution mass spectrometer which was equipped with a direct insertion probe. Relative intensities which were below eight were not included in tables herein.

F. X-ray powder diffraction patterns

X-ray powder patterns were obtained using a North American

Phillips X-ray generator and a 11,46 cm diameter camera of the Debye- 33

Scherrer type. A copper target (K = 1 .5^lS A) and a nickel filter were

used on the generator, which was operated at 12 Ma current and 32 Kv.

Solid samples were ground until the powder could be transferred

to a 0.5 ran Lindeman glass capillary in the dry box. The open end of

the tube was sealed with silicone grease and the tuba was removed from

the box for sealing with a small flame. The sample was mounted in the

camera and the pattern was recorded on Eastman Kodak X-ray film which

was developed in the laboratory darkroom.

G. Nuclear magnetic resonance (NMR)

Boron-11 NMR spectra were obtained at 52.08 MHz in the HR mode using a Varian model HA-100 high resolution spectrometer. Sideband

frequencies were supplied either internally or by an external variable

frequency oscillator used ty drive a Varian model 3521 integrator unit.

Boron-11 NMR spectra of one sample was supplied courtesy of Professor

J. C. Carter, using high field instrumentation at the University of

Pittsburgh,

The proton NMR (PMR) spectra were obtained on the Varian HA-100 spectrometer in the HA mode operating at 100 MHz. Methylene chloride, chloroform, dimethyl ether or Freon-22 (HCFaCl) were included in each sample to serve as a lock signal.

Heteronuclear boron-11 and proton spin decoupling was achieved by irradiation of the sample with an RF signal generated by a General Radio

Company 1161+-A coherent decade frequency synthesizer and a Hewett Packaid

3772 A noise generator. The decoupler, which was assembled by Mr. John

Kelley, also employed a . Hewett Packard h6l A amplifier and an Electron­ ic Navigation Industries 320L RF power amplifier. All spectra were obtained from neat liquid samples or from material dissolved in an inert solvent. Sample tubes were 5 mm pre­

cision ground glass tubes which were sealed with a torch under vacuum after sanqple introduction. Boron-11 chemical shifts were measured rela

tive to external standards containing F3B:0(CsHs)2 or by BCI3 tube

interchange; the chemical shift of BCI3 was determined to be -46.8 ppm

Shifts were determined after averaging data obtained from sweeping both upfield and downfield in order to eliminate drift effects. They are expressed as parts per million (ppm) relative to F3B:0(C2Hs)2»

'The PMR chemical shifts are expressed as tau units where the standard, Tt

All figures which depict NMR spectra exhibit increasing field as the spectrum is scanned from left to right; in other words, the upfield portion of the spectrum is to the right and the downfield portion is to the left side of the page.

II. Starting fthterials

A H solvents were dried over lithium aluminum hydride and vacuum distilled into glass storage bulbs equipped with Teflon stopcocks.

Pentane, diethyl ether, methylene chloride, THF and heptane were used most frequently and stored at room temperature. Dimethyl ether was stored at -78°C, and chloroform was stored in a vessel shielded from light to retard decomposition. The Freon solvent, HCF2CI (F-22 ), was purchased from the tfatheson Company and used as received. Volatile reagents were purii’ied "by means of the techniques listed

below and were distilled on the vacuum line until their vapor pressures

agreed with accepted literature values. Reagent grade chemicals were

used in all reactions.

1) BqHB5H11J B4H10 and B2HQ

Hexaborane(1 2 ) and pentaborane(ll) were prepared from B5H9 and

respectively, employing the stepwise framework expansion 95 reactions described elsewhere • Ttetraborane(10) was made by treatment 111 . u s of KaBsHg with a roughly stoichiometric amount of HC1 . Several

milliliters of B4H10 were also isolated from a nearly empty tank of

diborane(6) which had been purchased from the Callery Chemical Co,

« The storage of pentaborane(11 ) and tetraborane(10) was maintained

at liquid nitrogen temperature, -136°Cj hexaborane(l2) was stored in o a calibrated bulb at -78 C and used within a month of its preparation. 2) BsHa

Pentaborane( 9 ) was purchased from the Callery Chemical Company and

vacuum distilled from a -78°C trap before use.

3) p (c h 3)3

Trimethylphosphine was prepared by slowly dropping PCI3 into a

freshly prepared slurry of CHsffel in ether, the product eventually being

isolated and stored as the silver iodide complex until its use was 113 anticipated .

4 ) H(CH3)3, kh3 •

Trimetbylamine was purchased from the ffetheson Company and dried

over lithium aluminum hydride before use. Anhydrous ammonia was 36 purchased, from the Matheson Company and. stored, over sodium or NaK at -78°C.

5 ) KH

Potassium hydride was obtained pure in a mineral oil dispersant from [fetal Hydrides, Inc. Hie oil was removed by washing repeatedly with pentane in an extractor. The dry powder was stored in the dry box (glove box).

6) (n-C^HgJ^NBr and (n-C

-Tetra-n-butylammonium halide salts were obtained from the Eastman

Chemical Company and washed in an extractor with ether and dried under vacuum before use.

7 ) HC1 , HBr

Hydrogen chloride and tlydrogen bromide were purchased from the

[fetheson Company and used as received.

III. Analytical Procedures

A. Hydrolyzable hydrogen analysis

A sample of the material to be analyzed was transferred to a preweighed reaction vessel either by vacuum condensation or by mechanical means in the dry box. Hie mass of the material was determined and the tube was evacuated. Depending upon the nature of the material, a 50 $ solution of either HC1 or NaOH } or a five milliliter portion of glacial acetic acid was introduced into o the vessel while the contents were kept at -196 C, The hydrolysis o tube was then sealed with a torch and heated to at least 100 C for a period of several days. Upon completion of the hydrolysis, the vessel was attached to the vacuum line by means of an adapter which could be used to fracture the break tip which had been incorporated as part of the vessel*s structure. This action would allow the noncondensible gas within the vessel to be collected and measured in the Toepler system. Prom this procedure the percentage of hydrolyzable hydrogen could be calculated.

B. Boron analysis

The hydrolysis solution which had been frozen in the vessel while the noncondensibles were removed was warmed and quantitatively transferred to a volumetric flask and diluted to a known concentration.

The percentage boron was determined by transferring a ten milliliter aliquot to a beaker where the pH was adjusted to 6.8 by adding dilute

NaOH, saturating the solution with mannitol, and back titrating to the pH of 6.8 using 0.0500 N NaOH. The pH values were read from a

Photovolt pH meter. The percentage boron could then be calculated from the amount of standard base which was required to achieve neutralization of the solution. The average value of several precise trials was taken as the acceptable value for the number of milliliters of base required. IV. Reactions

A. Determination of the density o±* hexaborane( 12)

In order to facilitate manipulation and measurement of the start­ ing material,, it was convenient to determine the density of BqHi2 at

0°C. Volume measurement-was made in a calibrated tube reading in hundredths of m illiliter units and was equipped with an 18/9 £ ball joint> this tube could be attached directly to the vacuum line or coupled with a stopcock adapter fo r storage. Whenever p o s s ib le , tra n s­ fer of borane starting materials was carried out between stations on the vacuum line which did not have mercury blowouts in order to elim i­ nate condensation of mercury or gases trapped therein with the borane.

The measuring tube was coupled with a stopcock adapter which could be attached to the vacuum line by a Fischer-Porter 9 nun O-ring joint, establishing a grease free connection. The apparatus was evacuated, weighed and returned to the vacuum line where 0.592 ml (0°C) BqHx2 was transferred into it. The mass of the BeHia was determined to be 0.396 gram. PVom th ese numbers the d en sity o f BgHia was c a lc u la ted , and the following conversion factors employed throughout the investigation:

66.9 mg/ml BqHi 2

O.O87 mmo/ml BaHxa (mmo = m illim ole)

0.115 ml/mmo BqHi2

It appears that BqHx2 is slightly, less dense than BsHio, which has been ca lib ra ted to O.IO85 ml/mmo107. 29

B. Investigation of the Hexaborane( 12) -Ammonia System

1, Determination of stoichiometry

Since tensimetric titrations undertaken in methylene chloride at o -78 C did not satisfactorily establish the stoichiometry.for the

BeHi2-NH3 system, a rough value was determined by measurement of excess

ammonia recovered from the system after product formation was complete.

A O.76 mmo sample of BeHxa was condensed into a reaction vessel

from the calibrated tube and about two m illiliters of methylene chloride

were mixed in at -78°G. Exactly 2.606 mmo NH3 were measured out In the

fractionation train and were Introduced to the stirring mixture at -78 C

in increments over a period of two days5 no obvious break was noted in

the titration curve and precipitate formation did not occur until after

the mole ratio of 1.0 (KHajBgHia) was reached. When the entire amount

of ammonia had been added, a thick white precipitate was present. Re­

moval and separation of the volatiles by fractional condensation from o the reaction vessel, whicn was kept at or near -7° C at a ll times,

resulted in the eventual isolation of O.856 mmo NH3. From these data,

1.75 mmo NH3 had reacted with O.76 mmo BqHi2, a mole ratio of 2.3:1.

The product of this reaction, a white solid insoluble in CH2CI2 and

tentatively identified as H2B(NH3)2 B5H10 > decomposed upon warming to

about -Uo°C without yellow in g. The only d isc o lo r a tio n noted was a

frequent graying which was independent of the presence of mercury.

Warming the solid in the presence of solvent to about -45°C resulted in a distinct color change to an off-white, accompanied by coagulation and partial dispersal of the precipitate. If the reactants were mixed at -50°C, the precipitate which rapidly formed was almost white in color and more flocculent than the ’original’ solid warmed from -78° to -50°C.

2 . Characterization of the ’diammoniate1 of hexaborane(12)

Characterization of the material focused upon three treatments:

1) identification of the. anion by low temperature protonation in liquid

HClj 2 ) identification of the cation by isolation of an HaB(M3)2X salt j_3 ) low uemperauure NMR spectroscopy of solutions of the B5H10" species, utilizing various solvent systems and cation exchange techniques.

Results of an initial attempt to protonate the anion are described in the following equation which lists the volatile materials isolated:

* H2B( NH3) 2+ BsHio"' + HC1 ~i:L2 -C» Bg H n + BqHio + B ^ o + H2 (26) (1)

The three boranes produced were present in a mole ratio very close to

1 :1:1 according to the boron-11 NMR spectrum of the volatile samplej the amount of hydrogen given off was slightly less than one equivalent.

Systematic variation of reaction time and temperature were carried out in order to observe changes in the relative amounts of volatiles produced. While the ratio of boranes remained fairly constant, the amount of Ife evolved increased as the reaction time increased. Table 2 illustrates the conditions which were employed and the results observed.

As described by equation 26 , the identity of the cation should be verified by establishing the presence of N ^ N H s J a C l in the residue tab ie o p c o n te n ts

Page

V i m ...... ii

TABIE OF CONTENTS ...... i i i

ILLUSTRATIONS ...... , ...... v

TABLES ...... v i i

INTRODUCTION ...... / ...... 1

I. General Background ...... 1

II. Hexaborane( 12) ...... 10 « III. Behavior and Reactivity of Boron Hydrides in Solution ...... 15

IV. Predictions and Structures ...... 26

V. Statement of the Problem ...... 28

EXPERIMENTAL ...... 29

I. Apparatus...... 29

II. Starting M aterials ......

III. Analytical Procedures...... * . . . . . 56

IV. Reactions ...... 58

A. Determination of the Density of Hexaborane(12) ...... 58

B. Investigation of the Hexaborane( 12)-Ammonia System ...... 59

C. Investigation of the Hexaborane( 12) -Trimethylamine System • 56

D. Investigation of the Hexaborane(12)-Trimethylphosphine

S y ste m...... 58 i i i 1+1

TABIE 2

The Effect of Variations in Time and Temperature upon the Unsymmetrical

Cleavage Reaction Between BqHi 2 and 2 NH3 •

R eaction R eactio n R eaction Number o f Boron-11 NMR and sc a le Temperature Time equivalents spectrum and (mmo BgHia) (degrees C) ( h o u rs) H2 evolved other comments

A 1.18 -78 2 0 0.62 B-iHioj B5 H1 1 and

B 0.5U -78 1+0 0.75 BaHio were close to c 0.87 -78 210 0 . 8 9 1 :1:1 mole ratio in

D £ d + -78 650 O.96 reactions A - D

E 1 .10 -78 550 1.0 B5 H9» B SH1 1

F 0.88 -50 2 1 .5 somewhat less B5 H9

G 0.80 -50 2 I .06 more Bq Hio V H 1.26 -78 1.7 0 .81+ ty p ic a l -50 0 .3

Systems E and F involved protonation of solids derived from metathesis reactions with tetra-n-butylammonium bromide; thus, their formulation might be consistent with (n-C^HsJ-iMBsHioj theoretically. •

The appearance of a considerable amount of pentaborane(9) among the volatile materials produced upon protonation of this salt was not unusual in light of observations that protonation of (n-C 4 H9 ) 4 NB6 H9 xoa in liquid EC1 produced mostly B5 H9 k2

deposited in the reaction vessel after protonation. However, the solid

material which remained in the vessel after extended room temperature

pumping was most intractable. Extensive washing of the 'gum' with

CH2 CI2 provided a small amount of dry solid material on the fl*it of

the extracting apparatus from which a powder pattern could be obtained.

I t showed that HaBfNHaJsCl was present1 1 4 along with unidentified

material which contributed extraneous lines to the powder pattern,

Ear greater success was experienced with metathesis reactions

which resulted in an exchange of cations in the H2 B(UH3 ) 2 B5 H io-

( n-C^HaJ^HBr system in methylene chloride. When this technique was

exploited in an extractor, a large quantity of HaB(NH3 ) 2 B r was is o la te d 114 on the frit and identified by means of-its powder pattern . Thus,

the presence of the H2 B(NH3 ) 2 group in the system was confirmed.

It was hoped that a better understanding of the situation could be

obtained by gathering data by means of boron-11 NMR spectroscopyj however, this first entailed getting the solid into solution at a temperature at which the spectrum was well defined without evidence of decomposition. It has been shown that ether solutions of B^H^g hydrides bear limited stability, so a search was made for an inert solvent out­ side the ether family. The solid was found to be extremely insoluble in a ll solvents -cried, including methylene chloride, toluene, chloroform, methylcyclohexane and pentanej attempts to improve the solubility by means of cation exchange techniques resulted in very slight improvement of the spectra taken — gross insolubility was still apparent. A very poorly resolved spectrum of 'Ha^WHaJaBsHxo* was obtained in dimethyl

ether which consisted of a broad hump centered at approximately ten ppm

and about 3 5 ppm wide and an apical resonance at roughly5 7 ppm, also

unresolved. Since these resonances are similar to those of the spectra

which were observed with molecular adducts between ammonia and hexabor-

a n e ( l 2 ) which will be discussed later, it is possible that the cited

spectrum does not represent the ionic compound HaE( NH3 )a+EsHio in any

definitive fashion. Spectra taken of the metathesis product showed

greater detail but were extremely noisy and appeared to undergo

decomposition while the spectrum’was being run.

5. Preparation and characterization of 3-methylhexaborane(12)

Based upon the equations describing the preparation of the

unsubstituted hexaborane( 1 2 )' (e q u a tio n s 1 6 , 1J and 1 8 ), it was reasoned

that by starting with a methyl-substituted pentaborane(9 ) d e r iv a tiv e ,

reasonable yields of a sim ilarly substituted hexaborane( 1 2 ) d e riv a tiv e

should be produced by a like sequence of reactions. Subsequently, it

was e s ta b lis h e d th a t both I-CH 3B5 H0 and 2 -CH3 BsHe could be carried

through an identical reaction series which produced 3 -CH3B0 H1 1 in yields

up to 8 0 $, It was observed that regardless of the reaction scale, all

preparations of 3 -CH3B0 H1 1 resulted in better yields with less hydrogen

evolution (from side reactions) than corresponding BqHi2 syntheses. 25ie

white powdery solid which remained in the reaction vessel after protonation and removal of the volatiles was identified to be KC1 by its X-ray powder pattern. The colorless liquid 3-CH3BqHxx was purified by fractional distillation through a -h5°C trap into a -63°C trap until a constant vapor pressure of 6 Torr was attained.

1. Vapor pressure and heat of vaporization data for 3-CHaBeHn

Vapor pressures were recorded for the following temperatures:

0°C - 6 Torr, 10°C - 11 Torr, 20°C - 21 Torr. By plotting the logarithm of the vapor pressure versus the reciprocal of the temperature in degrees Kelvin, the heat of vaporization could, be determined from the slope of the straight line: aH = -2,3(R)(slope).

The heat of vaporization of J-CHsBqHxi was found to be 9* 9 kcal/m ole

2. The boron-11 NMR spectrum of 3-CH3BqHxx

A p u r if ie d sample o f 3-CH3Bq Hx i was passed into a U-trap at the bottom of which was attached a 5mm precision NMR tube. The trapped liquid, maintained at a temperature below -75°C, was allowed to settle to the bottom of the tube which was then frozen and sealed.

The spectrum, which was unchanged between 0°C and -85°C except for a loss of resolution of two overlapping triplets at very low temper­ ature, is reproduced with assignments and structure in figure 1. The chemical shifts and coupling constants are listed in Table 3. The values tabulated were the same when measured for a sample of 3 -CH3BqHxi d isso lv e d in HCFaCl.

3. The infrared spectrum of 3-CH3BqHx i

A sample o f 3-CH3B6Hx i was transferred to a gas cell and the spectrum was run. The sample was allowed to sit at room temperature for three hours whereupon a second spectrum was obtained. The two *5

\l

r / ^ / B"Tj >c h ”/C

B6 BU b i

Figure lj Structure, Assignments and Boron-11 NMR Spectrum oi* 3-CHaBeHn k e

TABLE 3

Boron-11 NMR Data For 3-CH3B8Hii and 3 -CH3 B5 H1.0

S-CHsBqHix

Assignment Chemical Shil't(ppm) Coupling Constant(Hz)

B-CH3 -3 3 .8 u n s u b s titu te d B-H -2 0 .9 130

adjacent BHa - S U 115

opposite BHa - 6.k 110

B-H +18.5 150

B-H +23.3 150

3-CH3B5H310

B-CH3 -21

adjacent BHa - 8.6 120

opposite BHa - 1.8 120

b a s a l B-H + 8.0 1&5

a p ic a l B-H +51.0 135 spectra were identical, indicating that no decomposition had taken place. The spectrum is pictured in Figure 2, and the absorptions are presented in Table 4.

4. The vapor density molecular weight of 3 -CH3 B6 H n .

From the formula M = gRT/PV, using a 0.0605 gram sample which exerted a pressure of 22 Torr in a volume of 527 ml at 296°K, the molecular weight of 3 -CH3 BQHn was calculated to be 87.2 grams p e r mole, experimentally.

This agrees well with the theoretical molecular weight of 90.86 grams per mole.

5. The reaction of ^-CH sBq Hh with trimethylamine

In a reaction vessel equipped with an MMR sidearm, 0. 6 5 mmo o f o 3 -CH3 BqHh was mixed with 0.5 ml CH2 CI2 and cooled to -95 C. One equivalent of N( CH3 ) a was added gradually with stirring at that temperature. After a few minutes the sample was poured over and sealed.

Present in the boron-11 MMR spectrum of the sample after warming above -80°C were resonances readily assignable to HsBN( 0 1 1 3 )3 ( a Quartet JL X5 a t 8 ppm) and 2 -CH3BsH© •

6 . The reaction of 3 -CH3 B6H n with ammonia

* ■ A 1.4 mmo sample of 3 -CH3BQHn was measured into a standard reaction vessel. After about four m illiliters of methylene chloride had been added and stirred at -78°C two equivalents of ammonia were slowly added. Some faint cloudiness was present as the reactants mixed} however, this disappeared after a few minutes and a copious precipitate was not X X (cm”1) 2500 2000 1800 f500 1200 J00Q 600

Figure 2 : 2he Infrared. Spectrum of 3-t,fethylhexaborane( 12 ), 3-CH3B6H n ■t- 00 TABIE 4

Infrared. Absorptions of j-CHsBeHn -1 Wavenumber(cm ) intensity Wavenumber (cm 1) Intensj

^600-2800 mwjbr 1325 s

3040 w 1170 s

2980 mw 1130 mw

2950 mw 1095 mw

2850 w 1050 s ,sh

2600 vs 1030 s

2575 vs 965 ms

2505 vs 945 ms,sh

2530 w 905 mw

2005 mjSh 810 w

1900 ms,br 748 mw

1870 w,sh 705 mw

1620 w,sh 600 w

1513 vs,br 545 m

1A30 s,br 465 mw

1385 ms 50

visible until after an additional hour had elapsed. It was shown that

stirring was considerably impeded if the solution weren't kept fairly

dilute (about 0,4 molar).

This mixture was stirred at -78°C for approximately two days

at which point the solvent was removed from -78°C until a dry solid was

left in the vessel. Warming the contents to -45°C for several hours

accelerated the removal of last traces of the solvent. Two milliliters

of HC1 were condensed into the vessel at -196°C and completely dis­

solved the white solid upon warming to -110°C. Little hydrogen evolution

was observed as the solution was stirred for less than a minute,

Eractionation of the volatiles from the reaction vessel resulted

in the isolation of a colorless liquid in the -95°C trap which proved

to be 3-CHsBgHioj exclusively. This material passed through -78°C very slowly.

The white powdery solid which remained in the reaction vessel after total removal of volatiles was taken into the dry box without washing, and a sample of the material was transferred into a 0.5 mm X-ray capillary tube in order that the powder pattern of this residue might be obtained. It was identified as HaB( NH3)sCl11*4.

Characterization of 5-CH3B5H10

1 . The boron-11 NMR spectrum of jS-CHsBgHio

A sample of jS-CHaBsHio was fractionally condensed until a constant zero degree vapor pressure of 16 Torr was attained. About 0.15 ml of this portion was transferred into an NMR tube in the manner previously Page ft E. Investigation of the Tetraborane( 10)-Trimethylphosphine

System ...... * 78

P. Investigation of the Tetraborane(10)-Trimethylamine

System...... 9^

G. Low Temperature Lewis Base-Pentaborane(ll) Reactions 95

DISCUSSION AND CONCLUSIONS ...... 100

I. NMR Studies of Hexaborane(12)-Ammonia Reaction Mixtures 100

II. The Fluxional Nature of Selected Adducts...... 113

III. The Nature of the Diammoniates of Hexaborane(l2 ) and

3-Methylhexaborane( 12) ■*,...... 118

IV. Directional Nature of the Attack by Amine Bases on

Substituted Compounds...... 119

V. General Observations of the Derivatives...... 121

VI. Solid Phase and Solution Chemistry of the Phosphine Adducts. 125

VII. The Role of the Phosphine Groups in the Adducts Studied ... 139

VIII. Considerations of the BeHiajNtCHaJo-BsHgjNC 0113)3 System .. 1*41

IX. General Conclusions ...... 1^3

X. Projections of Research in the Field ...... 1^6

REFERENCES...... M

i v 51

described, for 3-CH3B6Hn. The boron-11 MMR spectrum, with assignments and structure, is shown in Figure 3 j and the chemical shift and approximate coupling constant values were listed in Table 3.

2 . The infrared spectrum of 3-CHsBsHxo

A pure sample of 3-CH3B5H10 was allowed to expand into a gas

infrared cell to a pressure of about 20 Torr. The infrared spectrum

of the sample, which was unchanged over a period of two hours at room

temperature, is depicted in Figure 1*, and the absorption wavenumbers and relative intensities are listed in Table 5 *

3 . The vapor density molecular weight of 3-CH3B5H10

Using the gas law formula and a 0.035 gram sample which exerted a

2h Torr vapor pressure, at 295°K In a volume of 326 ml, the molecular weight of 3-CH3B5H10 was calculated to be 79-0 grams per mole, experi­ mentally.

This is in excellent agreement with the theoretical value of

79-05 grams per mole.

If. The heat of vaporization of 3-CH3B5H10

Vapor pressures o±’ 3-CH3BSH10 were determined for 0°C - l6 Torr,

9°C - 26 Torr, and 20.5°C - 1*3.5 Torr. In the manner described previous-

* ly the heat of vaporization was calculated to be J .6 kcal/mole.

5 . The reaction of3 -CH3BsHj.o with ammonia

Approximately three millimoles of 3-CHsBsHio were stirred in _o 10 ml CHaCl2 at -78 C as exactly two equivalents of ammonia were 52

v ^ C H , \ / ^ \ / 3

At V A i A

Figure 3: Structure, Assignments and Boron-11 NMR Spectrum of 3-CH3B5HJ0

s ______I______i______I______I------I------:------1------2500 2000 1600 1400 1100 8 0 0 (cm*3)

F igu re 4 : The Infrared Spectrum of 3-Jfethylpentaborane(1 1 ), 3-CHa^Hio

\ji 5h

TABIE 5

The infrared absorptions of 3-CH3B5H10

Wavenumberf cm ) Intensity Wavenumber( cm" ) Intensity

2980 raw 1235 w

2580 vs 1175 m

2h95 vs 1325 m

2070 w 1020-1070 br,raw

19^0 vw 960 m

1600 mw 900 w

1480 sh,m 765 s

1390 vs, hr 750 s

1328 ms 1 705 m

1280 m 580 w

1262 m ^95 w slowly added. Visual observations paralleled those made with the

3-CH3B6Hh-2NH3 cleavage reaction. After two days of stirring at -78°C

solvent removal was carried out as before. About three milliliters of

HC1 were added at -196°C and thawed to -110°C to allow a brief reaction

The volatile materials were fractionated. o A fairly volatile liquid which slowly passed through a -95 C trap

was isolated in a -110°C trap) after purification by fractional

condensation the material was transferred to an NMR tube and sealed.

The bpron-11 NMR spectrum, which did not change upon sitting at

0°C for twenty minutes, is consistent with the formulation I-CH3R4H9.

The molecular structure, which is pictured

at the right, accounts for the three resonances

which are seen in the spectrum: an area two

triplet, assigned to the two equivalent BHa groups (Bg at k .6 ppm ,(J = 131 Hz)) an area

one singlet at 29.0 ppm arising from B-j_) and | an area one doublet at ^0.7 Ppm (J = 158 Hz) which accounts for the number three boron, '

No extensive characterization of l-CHaB^Hs was undertaken. The

total gathered data were sufficient to satisfactorily describe the directional nature of the Lewis base attack upon the substituted

®n^n+e 1'ramevrorks> and to answer some of the questions which arose from

the investigation.of the 'diamntoniates' of BqIIx2, J-CHaBeHn and

3-CH3B5H10. 56

C. Investigation of the hexaborane(12 )-trimethylamine system

1 . Determination of stoichiometry

The predicted stoichiometry of the hexaborane(12)-trimethylamine system was partially verified by means of a tensimetric titration carried out in pentane at -1*5°C. In the initial stages of the titration some white precipitate was visible. The break in the plotted curve occurred at a mole ratio of 5:1 N(CH3)3 "to BqHi2 > this is consistent with a two step reaction sequence:

BqHi2 + N( CH3) ---- - HsBN( CH3) 3 + B5H9 ( 27 )

B5 H9 + 2N{ CH3) 3 ----- *BsH 9 *2 N{CH3 ) 3 (28)

After the pentane was removed at -15°C following the completion of the titration, about 0.5 ml CH2C12 was introduced to the vessel and mixed with the residue at -78°C. An NMR sample was poured over, frozen and sealed. The boron-11 NMR spectrum indicated the presence of

HsBN(CH3)3 5 the presence of BsH9»2N(CH3)3 was confirmed by examination of the infrared and horon-11 NMR spectra of the non-volatile solid which remained in the reaction vessel.

2 ) Determination of temperature and solvent dependence

When one equivalent of N(CH3)3 was gradually added to a stirring solution of BeHi2 in pentane at -97°C, a white precipitate formed over a period of one hour. It was shown that additional N(CH3)3 did not react with this adduct, formulated BeHi2:N(CH3)3, since the excess amine could be recovered as the ammonium halide salt. In pentane, the solid was stable to approximately -70°Cj in the absence of solvent, the adduct 57

could be slowly warmed to about -30°C before any changes in the system

could be detected.

Ety monitoring the volatile materials pumped from the vessel

* containing the solid BaHi2:N( CH3) 3 moderate amounts ( 0.2 to 0.3

equivalent) of diborane(6) were isolated as the temperature rose to

around - 30°C. Any appreciable increase in the temperature above that

level resulted in the visible decomposition of the solid with little

discoloration. One of the decomposition products was H3BW(01(3)3.

3) Attempted characterization of BsHg:W( 01(3)3

The presence of free diborarie being liberated from the white solid,

possibly according to the equation

BeH12: N( CH3) 3 ------► ?B2H6 + BgHg: W( CH3) 3 (29)

suggested undertaking the chemical characterization of the elimination

product, BsHg: W( 01(3)3, by expanding the framework to accomodate a second molecule of Lewis base. If the base were N( 01(3)3, the previously 117 characterized bis-adduct, BsHg* 2K( 01(3)3, would be produced in this way.

Accordingly, a slowly warming sample of BsHi2:N(CH3)3 was monitored until liberation of diborane appeared to be finished. The white solid which remained in the vessel was frozen to -196°C and about two milli- o liters of pentane were added. The contents were warmed to -97 C and

stirred for about ten minutes in order to thoroughly mix the system be­ fore one equivalent of N(CH3)3 was added. The mixture was stirred as the

cold bath temperature rose to about -Uo°C> at this point, an off-white colored solid was present in the vessel. Removal of the solvent at that temperature was carried out and the contents slowly allowed to warm.

The solid was observed to undergo decomposition when a temperature of approximately -5°C was reached.

In a parallel reaction, a second equivalent of base in the form of P( 0113)3 was added to 'BsHgrNf 0113)3' in an attempt to produce a 2:1 _2 mixed adduct which was an isoelectronic analog of the B s H u ion.

The results of this experiment were essentially identical with the results obtained for the previous trial with N( 0113)3,

Neither BsHia:N( 0113)3 nor BsHg:N( 0113)3 were characterizable by means of NMR spectroscopy. For example, when the former was placed in solution, the NMR spectrum exhibited resonances attributable to BgHg among other resonances which were present of lesser intensities.

While BeHi3;N(0113)3 could be readily prepared in alkane solvents, it was found that the adduct could not be made and isolated from methylene chloride.

D. Investigation of the hexaborane(1 2 )-trimethylphosphine system

1 . Determination of stoichiometry

Tensimetric titrations involving P( 0113)3 and B3H12 undertaken in o , 0 pentane at temperatures between 0 C and -45 C were not definitive as far as pinpointing the stoichiometry of the system was concerned. Qhe titrations at higher temperature regions resulted in the decomposition of the solid material formed as evidenced by the rapid discoloration of the reaction mixture. At temperatures below -22°G, mixture of the reactants resulted in the formation of a white solid which did not 59 discolor with time. A mild, temporary break at a 1:1 stoichiometry was observed but was not reproducible. In addition, no sharp 3:1 break was reliably found.

If a gross excess (ca. 5:1 P(CH3)3 to B g H x s ) oi> trimethylphosphine was stirred into a solution of BqHj.2 in heptane at - h ^ ° C over a period of one hour, fractionation and isolation of excess P(CH3)3 from the volatiles reduced the stoichiometry to 3* ^-3 to 1, phosphine to borane.

When the volatile materials produced by a 3:1 P( CH3) 3-BqHx2 reaction were removed from the vessel, the infrared spectrum of the unwashed solid residue exhibited absorptions assigned to the known adduct, BgHg* 2p(CH3)3. In ah attempt to establish whether the following reaction sequence accurately described the hexaborane(12)-trimethyl­ phosphine system under fairly mild conditions, an NMR sample of a 1:1

BqHi2 + P(CH3) 3 ------*-H 3BP(CH3)3 + B5H9 ( 30)

BsH9 + 2P( CH3) 3 ---- » BsH9 • 2P( CH3) 3 ( 31) mixture of the reactants in methylene chloride was prepared and run.

The boron-11 NMR spectrum which was obtained is shown in Figure 5 } i'b" does show the presence of BgHg (resonances labeled X) and H3BP(CH3)3

(resonance labeled Y), but it also showed the presence, in considerable quantity, of a third material (resonances labeled Z?). Thus, it appears that equations 30 and 31, which are analogous to equations 2J and 28 , are inadequate. Even though little solid material was observed in the reaction vessel or the NMR tube for this last system, the evidence suggests the likelihood of a competition between cleavage and adduct formation under the specific conditions investigated, at least. Figure 5 : The Boron-11 NMR Spectrum of a 1:1 m ix tu re, P(CH3 ) 3 : BeHi2 ,

in CH2 C1 2 a t -60°C ILLUSTRATIONS

Figure Page

1. The Eoron-11 NMR Spectrum, Structure and Assignments

of 3-CHsBqHii...... k5

2. The Infrared Spectrum of 3“CH3BeH n ...... 48

3. The Boron-11 NMR Spectrum, Structure and Assignments

o f 3 -GH3BSH1 0 ...... 52

4. The Infrared Spectrum of 3 -CH3B5 H1 0 ...... 53

5. The Boron-11 NMR Spectrum of a Mixture of Hexaborane( 12)

and Trimethylphosphine 1:1 in CH2C12, -60°C ...... 6 0

6 . Boron-11 NMR Spectra o,f BqHi2 :P(CH3 ) 3 in CHCI3/HCF2 CI .... 6 3

7. The Boron-11 NMR Spectrum of EsHg:P( 0113)3 in CHC13, -35°0. 6 6

8 . Infrared Spectra of BsHg: P(CH3 ) 3 and EgHg* 2P( CH 3 ) ...... 6 7

9. The Decoupled Boron-11 NMR Spectrum of

B5 Hg’P(CH3 ) 3 N(CH3 ) 3 in CHC13 a t -5 0 ° C ...... 74

10. The In fra re d Spectrum o f EsHg*P( 0113) 3^1( 0 1 1 3 ) 3 ...... j6

11. Boron-11 NMR Spectra of B ^ o : P( CH3) 3 in HCF2 C1, -80°C ... 8 l

1 2 . Boron-11 NMR Spectra of B4 HiO:P ( 0 H3 ) 3 in HCF2 C1, -130°C .. 82

13. Proton NMR Spectra of B^o*. P( CH3) 3 in H0FaC l ...... 84

14. Boron-11 NMR Spectra of B3 H7 :P(CH3 ) 3 in HCF2 C 1 ...... 8 7

v 61

o When the reaction temperature was lowered, to -95 C, it was found

that equivalent molar quantities of BgHx2 and P( 0113)3 reacted slowly

to form a white solid which was extremely insoluble in pentane or

methylene chloride. The reaction could be carried out successfully in

either solvent, and at temperatures below -78°C no additional P( 0113)3

could be made to react with the system.

In a sample trial, 0.82 mmo BqHi2 was mixed into about 2 ml heptane

and h,15 mmo P(CH3)3 were introduced with stirring at -78°C over a period

of two hours. Fractionation of the volatiles from the reaction vessel^

which contained a copious white solid, resulted in the isolation of

3.26 mmo unreacted P( 0113)3. This experiment established the stoichiometry

of the reaction which took place under.these conditions to be 1.08:1

P( 0113)3 to B6Hi2> or 1 :1 . In this fashion the resulting adduct was assigned the formula BqHx2: P( CH3) 3.

Characterization of BeHi2:P( 0113)3

It was observed that BeHx2tP( 0113)3 was not stable in the absence o 0 of solvent above -30 C. If the solid were warmed slowly ±r*om -78 C, less

than 0.U equivalent B2HS could be isolated from the reaction vessel, while a white solid remained.

In methylene chloride or pentane the solid decomposed, as shown by o boron-11 NMR spectra, if the temperature were raised to about -50 C. It was significant that B6Hx2:P(CH3)3 could be prepared in and isolated from either methylene chloride or pentane. For purposes of taking NMR spectra, the adduct was completely soluble in CHCI3, and stable at low temperatures in mixtures of chloroform and HCFaCl. The solid was 62

relatively insoluble in the Freon solvent alone.

The only characterization which could be carried out on

BqH13:P(CH3)3 was low temperature NMR studies. This work suggested a

fluxional nature for the adduct in solution. The boron-11 NMR spectrum

is presented in Figure 6 and the decoupled spectrum is shown for

comparison.

Basal type borons account for the downfield resonances at 5*7 a&d-

17*5 ppm* while the resonances at 39*2 and 1*7.1 ppm are attributable

to B-H and H-B-P(0113)3 groups, respectively. A structure is suggested

for this species in a later discussion.

The proton NMR spectrum was run for several samples of the adductj

however, each time it proved impossible to assign the resonances based

upon any feasible static structure postulated for a BQHi3" ion.

Characterization of the elimination product, BsHg:P( 0113)3

1 . General considerations

As described, the conversion of BsHiatPCCHs^ to BsHg:P(CH3)3

takes place in the absence of solvent by the slow elimination of a

BH3 unit from the solid phase molecular framework according to the simple equation

BsHi2:P(CH3)3 ----- ► BsHaiPCCHaJa + £b2Hq (32)

It should be noted that 0.3 equivalent of dlborane(6) was never isolated from the system) 0.36 equivalent was the most that was ever observed.

Unfortunately, the elimination was frequently observed to take place in a destructive fashion, resulting in the total consumption of 63

Not

Decoupled.

Decoupled.

Figure 6: Boron-11 NMR Spectra of BqH-|p:P( CHa)n in CHCI3/HCF2CI, -70 C 6k the solid material in the reaction vessel. Systematic variations in the way in which the reaction was carried out did not provide substantial evidence as to why the isolation of BgHgjPC 0113)3 was not consistently possible. It was shown that slow warming (usually overnight) was a positive factor in getting She synthesis to succeed.

The white solid adduct is stable for at least an hour in vacuo, but in a dry box atmosphere the compound gradually turned yellow. If the solid were exposed to the air, there was no change in appearance, but the infrared spectrum of the material showed marked change, with loss of bridge hydrogen absorptions especially noticeable.

The adduct was isolable from the precursor, B6Hi2:P(CH3)3 , whether the initial adduct had been made in methylene chloride or alkane sol­ vent. The pentaborane( 9 ) derivative was very soluble in chloroform and stable in chloroform solution to 20°C, if purej it was sparingly soluble in HCFgCl.

Hie melting point of a sample of the adduct was determined to be 1 o 74 C by slow warming in a sealed capillary.

2 . The boron-11 NMR spectrum of BbHg:P(0113)3

The boron-11 NMR spectrum of BsH9:F(CH3)3 in chloroform at -35°C exhibited a fairly narrow, symmetrical resonance of relative area four at 3*8 ppm and a doublet of relative area one at 51*5 ppm (J=129 Hz).

At ambient temperature with high field Instrumentation the downfield peak was split into a symmetrical doublet (J=117 Hz). Spin decoupling experiments also showed that the upfield doublet arose from phosphorus coupling with the apical boron. The low temperature spectrum, not 65 decoupled, is shown In Figure J,

3. The proton NMR spectrum of BsHg:P( 0113)3

The PMR spectrum of BgHgzPC 0113)3 showed four resonances apart from the resonance attributed to the methyl protons in the decoupled spectrum at -65°C: an area two peak at 5*92 tau, an area four peak at 6.81 tau, an area two peak at 10.1 tau, and an area one peak at 11.2 tau. At -4o°C the two upfield peaks, assigned to bridging protons, were shown to merge into a single resonance at 10.5 tau, approximately the weighted average of the lower temperature bridge proton chemical shifts.

h . The infrared spectrum of BsHg:P( 0113)3

The infrared spectrum of BsHg:P( 0113)3 was very similar to that recorded for the bis-adduct, BsHg*2P(CH3)3j significant differences arose only in the region of terminal B-H stretching at 2500 cm"1 to

2600 cm-1 and in the region of bridge hydrogen absorption. Figure 8 shows a comparison of these key portions of the spectra of the two adducts. All the infrared absorptions of BsHg:P( 0113)3 are listed in

Table 7 *

5 . The mass spectrum of BsHg:P( 0*3)3

The mass spectrum of BsHg:P( 013)3 exhibited a sharp cutoff at the expected m/e value of I k O for the parent peak of ^BsHg: 31P( 0113)3.

Again, similarities to the mass spectrum of the bis-adduct were evident.

The summation of relative intensities and m/e values, obtained for the mass spectrum of BgHg:P( 0113)3 is presented in Table 7 *

6. Boron analysis

Carefully weighed samples of BgHg:P( 0113)3 were subjected to acid 66

Figure 7i The Boron-11 NMR Spectrum of BsHs: P( CII3 ) 3 in CHCI3 a t -55 C 6 7

B H •P(CH ) 5 9 3 3

2500 2000 1800 cm

Figure 8 : Infrared. Absorptions of BsHg:P( 0113)3 ancl BsH9*2p( 0113)3 68

TABLE 6

The Infrared Absorptions of BsHgjPfCHaJa

Wavenumberf cm"1) I n te n s ity Wavenumber( cm ) I n te n s ity

2980 w 1295 s

2920 w 1160 s

25^0 vs,br 1080 w

2500 vs,br 10^5 ra

2425 vs 995 m

I89O mwjbr 960 s,br

1830 inw,br 920 in,sh I 1^28 m,sh 860 m

llfl7 ms 755 m

13^0 w 715 w

1310 mw 680 w 69

TABLE 7

The ^ s s Spectrum oi* BsHgrPCCHsja m/e Intensity m/e Intensity

32 58 52 —

33 -- 53 10

34 12 54 18

33 10 55 25

36 16 56 25

37 14 57 100

38 — 58 52

39 — 59 100

40 44 60 71

4l 100 61 100

42 8 62 84

43 10 63 71

44 27 64 57

45 100 65 ** M

46 38 66 --

47 62 67 10

48 30 68 —

49 11 69 —

50 11 70 —

51 *24 71 ** 7 0

HABLE 7 (continued) m/e Intensity m/e Intensity

72 — 93 --

73 26 9h —

71* 26 95 —

75 100 96 —

76 100 97 —

77 h o 98 —

78 — 99 20

79 — 100 38

80 — 101 —

81 — 102 —

82 — 103 —

83 — 10lf

Q k — 105 —

85 «• 106 —

86 10 107 8

87 36 108 12

88 1j8 109 I k

89 89 110 13

90 — 111 h3

91 112 6k

92 — 113 3h Figure Page

15. Proton HMR Spectra of B3H7 :P(CH3 ) 3 in HCF2 CI ...... 8 9

1 6 . Boron-11 RMR Spectra of BgHujNHa in ClfeCla...... $Q

17. The Boron-11 MMR Spectrum of BsHi2 *2 KH3 in CHaCl2, -95°C. 101

18. Boron-11 NMR Spectra of Stages I and II, BeHi2 :NH3 in

Ifethylene Chloride ...... 105

19. The Decoupled PMR Spectrum of BQHia:NH3, Stage I I .....104

20. Boron-11 NMR Spectra* of B6 Hi2 : NH3, Stage III in CH2 CI2 • • 109

21. Boron-11 T O Spectra of CH^B6H 1;l:P( CHs) 3 in CHCI3/HCF2 CI. 115

22. The Boron-11 Jit© Spectrum of BsH7 :NH3 in (CH3) a O .... 116 m 25. Boron-11 T O Spectra of CH3B4 H9 : P( CH3) 3 in CHCI3/HCF2 CI.. 124

v i 71

■EABIE 7 (continued)

m/e Intensity m/e Intensity

llU 33 128 l b

115 ~ 129

116 — 130

117 — 131 H

118 — 132 22

119 — 133 k 3

120 — 13^ 50

121 15 135 h o

122 36 136 52

123 69 1 137 8o

12k 98 138 100

125 100 139 76

126 100 1U0 22

127 23 1^1 72 hydrolysis and the percent boron determined by previously described techniques. The calculated percent boron in BsHg:P(CH3)3 was 38.8 *

The experimentally determined percent boron values were 38.^ and 38.3 *

7 . Chemical characterization of BsH9:P(CH3)3

The chemical characterization of the adduct involved taking advantage of the potential for expansion of the borane framework upon introduction of a second molecule of Lewis base. This would produce a -2 well characterized BsHu analog from B5H9:P(CH3)3, a B5H10 analog,

.In a reaction designed"to examine this possibility, a sample of

BsH9:P(CH3)3 was frozen to -196°C in a standard reaction vessel and about five milliliters of pentane were added. The contents were warmed to -97°C and stirred as a second equivalent of P(CH3)3 was added and maintained as the methylene chloride slush bath slowly warmed overnight o to about -1*0 C. It was determined, by boron-11 NMR and infrared spectra, that the reaction produced EsHg*2P(CH3) 3.

In addition to this behavior, it was demonstrated that the first known example of a thermally stable mixed 2;1 adduct of pentaborane( 9) could be prepared by mixing an equivalent of N( CH3)3 into the stirring

BgHg:P(CH3 ) 3-pentane mixture in a similar fashion. The mixed adduct, which was characterized in a preliminary manner only, can be formulated

B5H9.P(CH3)3N(CH3)3. It has a melting point of 86°C, can be manipulated in the dry box, and appears to be close to BsH9*2N(CH3)3 in stability. o The boron-11 -NMR of the mixed adduct in chloroform at -55 C consisted of three overlapping downfield (basal) resonances of total 73

area four which could be distinguished with proper narrow line spin

decoupling at 32.1 MHz, and an apical resonance of relative area one.

The decoupled boron-11 NMR spectrum is shown in Figure and

approximate chemical shifts, assignments, and a proposed fluxional

type of structure are presented in Qhble 8 .

The most striking characteristic of the Infrared spectrum of

£5119 * P( CH 3.) qN( CH3 ) 3 is the complex absorption pattern displayed for

the C-H stretching frequencies of the various methyl groups. The

spectrum is pictured in Figure 10, and the absorptions and intensities

listed in Table 9»

Several samples of the mixed adduct were subjected to mass

spectral analysis, and each time the spectrum displayed no peaks of

relative intensity over 10 fbr m/e values greater than 1^0. The fact

that the intensity of the m/e value equal to 3 9 j corresponding to the

( CH3)3N*" among other ions, was registered as 100+ would suggest that

the adduct readily lost the trimethylamine group under the conditions

experienced in the instrument.

Investigation of the 3-CH3BeHn-P( CH3)3 system

A brief inspection of this* system indicated that although the reaction proceeded similarly to the observed reaction involving the unsubstituted hexaborane( 1 2 ), it was not possible to isolate a thermally

stable solid product under any circumstances. One reinforcement of a previously suggested explanation of an observed phenomenon came to light: there were marked sim ilarities between the boron-11 NMR spectra of the adducts BsHi2 : P( CH3 ) 3 and CHaBsHu: P( CH3 ) 3 . The suggestion of Figure 9: The Decoupled. Boron-11 NMR Spectrum oi* BsHg• P( CH3) 3N( CHa) 3 in CHCI3

-3 Table 8

Boron-11 WMR Data and a Suggested Structure for BsHg*P( 0113)3^1(0113)3

Assignment Relative Area Chemical Shift

2 7.7 ppm

N-bonded Bg 1 22.0 ppm

1 29.6 ppm

B^ 1 56.8 ppm

\l .. \/

B v i» > B H ' V „

Y'\ ______1______I______I------1------1------1------3 0 0 0 2500 1900 1500 1200 900 (cm"1)

Figure 1 0 ; 2Jie Composite Infrared Spectrum of BsHg*P( CHa)3N( CH3) 3

-a ON 77

TABLE 9

The Infrared Absorptions ol‘ BsH9«r( CHs)3N( 0113)3

Wavenumberfcm X) Intensity Wavenumberf cm-1) Intensity

30if0 - — ■ raw ll*10 m

3000 ms 1310 mw

2960 s 1290 m

_2920 s • 1255 mw

2895 sh,w 1160 ms

281*0 w 111*5 w

2815 w 1120 vw

2^90 s 1090 w

2^35 . vs 975 ms

21*00 sh 950 ms

2315 vs 880 m

2220 sh 850 w

1890 vs,br 750 w

ll*85 m 725 vw

lU60 ms 690 w

11*20 m 650 m fluxional behavior occurring in solutions of both adducts is treated

in the discussion section.

E. Investigation of the tetraborane( 10)-trimethylphosphine system

Determination of stoichiometry

Studies of the tetraborane(10)-trimethylphosphine system were limited to very low temperature reactions, therefore tensimetric titrations were not employed.

Attempts to recover unreacted B4 H1 0 from low temperature 1:1 reactions between B 4H1 0 and P( 0113)3 in pentane were unsuccessful if sufficient reaction time were allowed (about one hour at -95°C for a one millimole scale reaction).

It was possible to isolate up to 0.38 equivalent B2 H3 which was eliminated from the in itial or precursor adduct upon slow warming in the absence of solvent, according to the equation

B*H10: P( CH3) 3 ------* ^B2Hq + B3 H7 : p( CH3)3 ( 33)

It should, be pointed out that this is the highest percent yield of diborane ( 0 . 3 8 / 0 .5 0 o r 76 $ of theory) observed for any system governed by the equation V W L ------sBaH. + B ^ H ^ L (31*)

In a second reaction, two equivalents of P(CH 3 ) 3 were slowly stirred into a solution of B^Hio in pentane at -95°C. A white precip­ itate slowly formed over a period of one hour. Hie volatiles were pumped 79

o out of the vessel, through a -78 C trap, and into a trap maintained at

-196°C as the temperature of the reaction vessel was slowly raised to

room temperature overnight. It was observed that there was no material,

liquid or solid, in the -78°C trap at that point.

The contents of the -196°C trap were allowed to warm briefly, and

were fractionated again through the -78°C trap. This time, crystals

o f H3BP(CH3) 3 were isolated in the -78°C trap, indicating that the

excess phosphine had reacted with the liberated diborane when the

contents of the -196°C trap were warmed. Thus, it was apparent that under the conditions employed, the second equivalent of F(CH3) 3 d id

not serve either to effect bis-adduct formation or to act as an agent

to remove a BH3 unit from the framework of the in itial adduct,

henceforth referred to as B^xoiPC CH3)3. i

Characterization of B4Hio:P(CH3)3

1 . General observations

As stated above, the adduct formed at low temperatures in alkane

solvents and formulated as B4H2.0*. P( CH3) 3 was observed to undergo spontaneous loss or a BH3 unit upon controlled warming to temperatures o between +5 C and ambient tenperature. In fact the material, on occasion but without reproducible consistency, could be manipulated in the drybox with little or no loss of diborane for periods of time less than one hour. In working with the system it was therefore desirable to monitor the diborane produced until the elimination was complete. 80

2. The b o ro n -U NMR spectrum o f B^H^o: **( CH 3 ) 3

The adduct had to be isolated from the alkane solvent before a

suitable solvent, such as chloroform and/or HCF 2 CI could be intro­

duced and the NMR sample prepared. Samples containing chloroform were

shown to give convergence of the boron-11 NMR spectrum of the adduct

at temperatures approaching the freezing point of the solution. Samples

made up in the ITeon solvent alone (f.p. of about -l60°C) showed no

convergence; instead, significant changes in m ultiplicity of

characteristic resonances were seen. The spectra presented in Figures

1 1 and 1 2 show the changes which were observed to be attributable to

temperature variation and to proton spin decoupling.

Coupling constant and chemical shift data are detailed in Table

1 0 which combines boron - 1 1 NMR data for B^IIioJPCCHaJs and BaH7 :P( 0113) 3 .

3. The p ro to n NMR spectrum of B4 H1 0 : P( CH3 ) 3

The proton NMR spectrum of B ^ioi P( CH3 ) 3 was also temperature dependent as the decoupled spectra in Figure 13 show. The same sample provided both the boron-11 and proton NMR spectra cited. The chemical shifts determinable from the latter spectra are given in Table 11 as well as data from the proton NMR spectra of B3 H7 tP( 0113) 3 .

4 . Supplemental spectral analysis

Rapid glove bag transfer techniques were employed in order to try to further characterize the B4 Hio:P(CH3 ) 3 adduct. In a typical instance, the solid product of the low temperature reaction was slowly warmed from the original temperature of to zero degrees overnight with TABLES

Table Page

1 . Hexaborane(12 ) Boron-11 and Proton HI© Data ...... 13

2 . The Effect of Variations in Time and Temperature upon the

Unsymmetrical Cleavage of Hexaborane(l2 ) by Ammonia,... ij-1

3. Boron-11 M R Data for_ 3-CH3BsH n and 3-CH3B5H10...... ^6

Infrared Absorptions of 3-CH3BbHh ...... k $

3 . Infrared Absorptions of 3-CH3B5HX0...... 5k

6. Infrared Absorptions of BsHg:P(CH3) ...... 68

7 . The Ifess Spectrum of BsHg: P( 0113)3...... 69

8 . Eoron-11 HI© Data and a Suggested Structure for

EsHg* P( CH3) 3N( CH 3) ...... 75

9 . Infrared Absorptions of B5Hg*P(CH3)3H(CH3)3 ...... 77

10. Boron-U M R Data for B^HiorPtCHa) 3 and B 3H7 :P(CH3) 3 ..... 83

1 1 . Proton HI© Data for B4Hio:P(CH3).3 and B2H7:P(CH3)3 ..... 85

1 2 . Infrared Absorptions of B 3H7 :F(CH3)3 ...... 90

1 3 . The Ifess Spectrum of B3H7:F(CH3)3...... 91

1^. Tlie X-ray Powder Pattern of B3H7:P(CH3)3 ...... 93

15 . Boron-11 HI© Data for BsHn:HH 3 in C H a C l a ...... 99

16. Proton HI© Lata for Bellia:HH3J Stage II* in C H a C l a 105

vii Hot Decoupled.

Decoupled. 82

Not Decoupled

Decoupled

Figure 12 ; Boron-U NMR Spectra of F(CH3) 3 in IICFSC1, -135 C 83

TABLE 10

Data From Boron-11 NMR Spectra of B4Hio:P( H3)3> A, and P(CH3)3, B

Compound and Relative Chemical Coupling Description Assignment Area Shift( ppm) Constant( Hz) and conditions

A, B-P group U5.6 120 doublet obscure at -80°C5 well resolved with spin decoupling

Aj (CHaJsPBHa- same as above 110 resolved to quartet group below -125°C

A, B K z group temp. 37.3 overlaps with a quar­ dependent tet at 37 PProj area increases as temp, decreases

A, BH3 group temp. 37.0 110 quartet all temps.5 dependent area increases as temp, decreases

B, BHa groups 16.0 unresolved no splitting at any temperatures

B, B-P group 1*.8 no splitting, -90°C

same resonance 110 quartet at -130°C

same resonance 120 resonance collapses to doublet upon • hydrogen spin de­ coupling, all temps. 8U

—80

W

-1 3 6

Figure 13: Decoupled PMR Spectra o f J3

TABLE 11

Data From PMR Spectra of B4 Hio:P(CHs)3 ,A, and BsH7 :P( CH3) 3, B

Compound and R e la tiv e Chemical Assignment Area Shi±'t( tau) Description and Conditions

A* a ll protons '10' 8.5 singlet, sharp - all protons on framework equivalent at - 7 0 degrees

A, terminal BHa •61 unobtainable several protons masked by the methyl proton resonance at temperatures below -115°C

A* m ethyl H*s 9 * 8.1

A* terminally 1 8.85 approximate area ratios5 in th e bonded H 's 2 9.0 region expected for protons bonded as part of a (CHsJiP-B-H^ « group, x= 1 o r 2

A, bridge proton 1 (?) 10.85 present only at temperatures below -125°C

B, a ll protons .7, 9.1 -80°C a ll framework protons on framework equivalent, tautomerizing

B, BHa groups 8.5 -120°C terminally bonded protons, masked by methyl proton peak

B, CH3 protons 9 8.5

B, ( CH3 ) 3P-BH2 - 2 9.55 terminal protons on boron which group is bonded to phosphorus, -150°C

B, bridge proton 1 11 .1*5 -130°C, static single bridging proton* no tautomerism constant evacuation into a -196°C trap. A trace o±' BaHe was isolated

from this system. The contents of the vessel were removed in a

nitrogen-filled glove hag and samples i’or mass spectral analysis and

infrared analysis were transferred to appropriate devices within

seconds. !

The mass spectrum showed an envelope typical of boron-containing

compounds which exhibited fairly weak intensities around a cutoff m/e

value of 128 (calculated parent m/e value for 11B4Hio: 31P(OH3)3 was

129.1)> the rest of the spectrum was identical to that recorded for

the elimination product, BsE7:F( 0113)3.

All mull infrared spectra which were obtained in the manner

described showed no appreciable difference i’rom the spectrum of

B3H7: F( CH3) 3. ,

Characterization of B3H7:P( 0113)3

1 . The boron-11 NMR spectrum of B^H7 :P( 0113)3

It was found that the very stable elimination product was soluble

in Freon-22 so that low temperature NMR spectra could be readily obtained. Q3ie temperature dependent boron-11 NMR spectrum, with and without decoupling, is shown in Figure ll±, and the chemical shift and coupling constant data are described in the combined table previously referred to.

2. The proton' NMR spectrum of B3H7:P( 0113)3

The temperature dependent proton NMR spectrum of BsH7:P( 0113)3 is 87

Not Decoupled

Decoupled

Figure 14 : Boron-11 IIMR Spectra of B3H7:P( 0113)3 in HCF2CI, -lj50 C 88

shown in Figure I5) pertinent data calculated from the spectra are

listed in Table 11 .

3. The infrared spectrum of B3H7:P( 0113)3

The solid was transferred to a standard XR cell and made into

mull suspensions with Nujol and HCBD, The values of the absorptions

and intensities which were obtained from these spectra are combined

and presented in Table 12.

4 . The mass' spectrum of B3H7:P( 0113)3

The mass spectrum of B3H7:P(CH3)3 was determined by the standard

capillary insertion technique) the exact mass determination was

performed on the CsBsHi^P* peak - calculated m/e : llU.111279^>

observed m/e : llU.11130782.

Facile hydrogen loss prevented the examination of the parent peak.

The recorded m/e values and relative intensities are presented

in Table 13.

5 . The X-ray powder pattern of B3H ?-:P(CH3)3

Xt was found that a sample of the elimination product could, be

sublimed from an approximate +30°C environment to the cooler, ambient

temperature portion of the subliming vessel, in vacuo. Small, well defined crystals were obtained, and some of these were ground in the drybox and transferred to a 0.5 mm capillary tube. The d values and relative intensities of the lines are listed in Table lU. The lines were somewhat grainy because the solid has the consistency of a fine granulated sugar rather than a powdery, well packed solid.

An infrared spectrum of the sublimed B3H7:P( CH3)3 was taken after 89

Figure 1 5 : Decoupled PMR Spectra oi’ B3Hr:F(CHa) 3 HCFaCl 90

TABLE 12

Inl*rared Absorptions 01* BsHyiPf 0113)3

Wavenumberf cm"1) Intensity Wavenuiriber( cm ) Intensity

2980 w 1150 ms

2910 w 1065 m

2500 vs 1040 mw to and 2450 br 9k 0 s,br

2250 m,sh 890 m

2200 m 845 m I 1450 s 750 m

1510 ms 720 mw

1290 s 680 mw

1170 sh 645 nw