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Xerox University Microfilms 300 North Zoob Road Ann Arbor, Michigan 48106 75-19,451 HOLLANDER, Or1n, 1946- REACTIONS OF AMMONIA WITH DECAB0RANE(14) AND S THE FORMATION OF A STABLE ION-DIPOLE COMPLEX I OF DECABORANE; PREPARATION AND ISOLATION OF TETRACARBONYLIRON HEPTABORANE(ll), ELECTROPHILIC STABILIZATION OF AN UNSTABLE BORON HYDRIDE. The Ohio State University, Ph.D., 1975 Chemistry, Inorganic i ! Xerox University Microfilms Annr Arbor, Michigan 48106

THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED. REACTIONS OF AMMONIA WITH DECABORANE (14) AND THE FORMATION OF A STABLE ION-DIPOLE COMPLEX OF DECABORANE; PREPARATION AND ISOLATION OF TETRACARBONYLIRON HEPTABORANE(11) , ELECTROPHILIC STABILIZATION OF AN UNSTABLE BORON HYDRIDE

DISSERTATION

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

By

Orin Hollander, B.A.

The Ohio State University 1975

Reading Committee: » Approved by

Sheldon G. Shore Daryle H. Busch Gary G. Christoph 7 " Advisor Department of Chemistry ACKNOWLEDGMENTS

I would like to express my appreciation for the finan­ cial support of the National Science Foundation, the Conti­ nental Oil Company, and the Department of Chemistry.

I am especially grateful to Dr. Sheldon 6 . Shore fot his guidance and suggestions which made these studies pos*- sible.

i I deeply appreciate the moral and financial supportj i of my parents, David and Marion. Their continued trust and understanding have been a source of great strength.

j I am deeply grateful to my fiance, Terri, for her patience and help in typing the draft of this manuscript.!

I especially appreciate the many helpful discussion^ i and suggestions of Dr. Vincent T. Brice and my roommate I Dr. Ihor Jaworiwsky, a Uniquely Knowledgeable Experimental­ ist.

i ji

■ i i

ii VITA

April 15, 1946 ...... Born - Chicago, Illinois

1968 ...... B.A., Northwestern University, Evanston, Illinois

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

1971-1972 ...... Continental Oil Fellow, The Ohio State University, Columbus, Ohio

1970-1975 ...... Research Assistant, Department of Chemistry, The Ohio State University, Columbus, Ohio

Publications

Metalloboranes Derived from the Lower Boron Hydrides, S.G. Shore, D.L. Denton, V.T. Brice, R.K. Hertz, and Orin Hol­ lander, International Boron Symposium, Leeds, England (1974).

Preparation and Properties of a Stable Ion-Dipole Complex of Decaborane (14) , B^H,.!” ; Reactions of Decaborane with Ammonia, 0. Hollander ana S.G. Shore, Abstr. Papers, 166th Meeting Amer. Chem. Soc., Chicago, Illinois (1973).

Electrophilic Stabilization of an Unstable Boron Hydride: Preparation and Isolation of B 7H.,Fe(CO)4 , 0. Hollander and S.G. Shore, Abstr. Papers, 167th Meeting Amer. Chem. Soc., Los Angeles, Calif. (1974).

Preparation, Isolation, and Structure of (C.Hg),jN+ y-Fe(CO)^- B-Hi j ** ?Preparation and Isolation of B^H^-jFe(CO) Electro- pnillc Stabilization of Heptaborane Speca.es, 0. Hollander, W.R. Clayton, and S.G. Shore, Chem. Comm., 604 (1974).

Fields of Study

Major Field: Inorganic Chemistry

iii Studies in Non-metal Chemistry: Professors Sheldon G. Shore and Eugene P. Schram

Studies in Transition Metal Chemistry: Professors Daryle H. Busch, Devon W. Meek, and Andrew A. Wojcicki TABLE OP CONTENTS Page

ACKNOWLEDGMENTS ...... ii

VITA ...... i U

TABLES ...... vii

ILLUSTRATIONS ...... viii

INTRODUCTION ...... 1

Decaborane(14) ...... 1

Hexaborane(10) ...... 12

Metalloboranes ...... 21

Heptaborane ...... 37

EXPERIMENTAL ...... 43

I. Apparatus and Procedure...... 42

II. Starting Materials ...... 51

III. Analytical Procedures ...... 33

IV. Reactions of NH3 with B1QH14 ...... 56

V. Reactions of BioH14 w*th ^C4H9^4NI and (C6H5)3PCH3I ...... 63

VI. Studies of p-Fe (CO) 4-B6H1(J ...... 72

CONCLUSIONS AND DISCUSSION ...... 93

I. Reactions of Ammonia with Decaborane .. 93

II. Reactions of BinH,,. with (C.Hq).NI and (c6h5)3pch3i 100 Page

III. Studies of p-Fe(CO)4-B6H 10 ...... Ill

REFERENCES ...... 146

vi TABLES Table Page

1. Electron Charge Densities for B^qH^4 ...... 6

2. X-ray Powder Diffraction Data for B10H1 2 (NH3>2 ...... 3. X-ray Powder Diffraction Data for 1

4. X-ray Powder Diffraction Data for [ CC4H9) 4N 1 tp-Fe (CO) 4“B6H9 1...... 79

5. X-ray Powder Diffraction Data for [

6. Mass-Spectral Data for Fe(CO) 87

7. Mass-Spectral Data for M-Fe(CO)4-2-CH3B6H9 ...... 90

8. Boron-11 NMR Data for B1QH,4 and ...... 7..7...... 96 10 13 9. Boron-11 NMR Data for B1QH14l” Complexes ... 106

10. Boron-11 NMR Data for Iron-Hexaborane(lO) Complexes ...... 125

11. Infrared Data for Iron-Hexaborane(10) Complexes 132

vii ILLUSTRATIONS

Figure “ Page

1 Molecular Structure o£ B^oH14 ...... ^ 2 Boron-11 NMR Spectrum of B^qH^4 ...... 4

3 Structure of B2.0H142~ ...... 4 Boron-11 NMR Spectrum of B 10H12 2 ...... 10 5 Structure of ......

6 Molecular Structure of BgH10 ...... 14

7 Static Structures for 2-CH^Bf-Hg and 2-BrBgHg ...... 7 . : . ? ...... 24

8 Structure of Beryllium Borohydride ...... 24

9 Structure of B3H g ~ ...... 30

10 Structure of (CH3) 2AlB3Hg ...... 3®

11 Structure of Mn(CO)3B3Hg ...... 32

12 Structure of [ (CgHg) ^^PtB-jH^ ...... 32

13 Structure of Mn(CO)3BgH^2 ...... 33 14 Topological Structure of B^H^1 ...... 38

15 Tensimetrie Titration Apparatus ...... 47

16 Infrared Spectrum of B^qH^ (NH3) 2 59

17 Boron-11 NMR'Spectrum of B10H12(NH3>2 ...... 61 18. Infrared Spectrum of [(CcHc)«PCHi] W l O ^ O 0*1!...... 67

viii Page 19. Boron-11 NMR Spectrum of t 3PCH3][b10h13] ...... 71

20. Tensimetric plot of the addition- of B2H6 to K [y-Fe(CO)4-B6H9'] in ether .... 75

21. Infrared Spectrum of I(C.HQ).N) [y-Fe(CO)4-B6H9] ...... 78 22. Tensimetric plot of the addition of B-H, to (C4H9)4N y-Fe(GO)4-B6H9 in CH2C12 .... 80 23. Infrared Spectrum of [(C.Hq).N] [U-Fe(CO)4-B7H12] ...... 81

24. Infrared Spectrum of B7H^Fe(CO)4...... 85

25. Infrared Spectrum of p-Fe(CO)4-2-CH3B6H9 ...... 59

26. Boron-11 NMR Spectrum of [NH4][B1qH13] ...... 94

27. Boron-11 NMR Spectrum of KB3qH13 ...... 95

28. Boron-11 NMR Spectrum of 1,2,3,4-B10H1qD4 102

29. Boron-ll NMR Spectrum of [(C4H9> 4N][B10H10D4I] ...... 103

30. Boron-11 NMR Spectrum of [(C6H5)3PCH3][B10H10D4I] ...... 104

31. 80.2 Mhz. Boron-11 NMR Spectrum Of t(C4H9)4N][B10H10D4I] ...... 105

32. Raman Spectrum of B 1QH14 and I 3PCH3 J [BiqH,^] in the Terminal B-H ancl Bridging B-H-B region ...... 108

33. Proposed Structure of B10H14I- Complex ...... 109

34. Boron-11 NMR Spectrum of yi—Fe(CO)4~BgH3Q 112

ix Page

35. Temperature-dependent boron-11 nmr spectrum of K [p-Fe CCO) b6 h9 " 1 ...... 113 36. Possible static structures for p-Fe(CO)4-B6H9" ...... 115

37. (a) Boron-11 nmr spectrum of the reaction of BfiH10 with p-Fe (CO).-BgHq“ (B) Boron-11 nmr spectrum of the reaction of BgH9" with p-Fe(CO)4-B6H1q ...... 118

38. Boron-11 nmr spectrum of p-Fe(CO)4-B?H12 ...... 120

39. 100MHz proton magnetic resonance spectrum of p-Fe (CO)4-B7H12~in the bridge-proton region ...... 121

40. Infrared spectra of p-Fe(CO).-BgHq“ and p-Fe (CO) 4-B7H. 2 in the terminal B-H and CO stretching region ...... 123

41. Molecular structure of p-Fe(CO) 4“ByH12‘ 124 42. Possible 3-center borane-metal bonding modes ...... 126 43. Molecular structure of p-Fe(CO)4-B-H. 2“ as viewed along the axial carbonyls .... 127

44. Boron-11 nmr spectrum of B7H^Fe(CO)4 • • • 130 45. Infrared spectra of p-Fe(CO)4-BgH10 . and B7H,jFe(CO)4 in the terminal BH and CO stretching region ...... 131

46. Mass spectra of p-Fe(CO)4-BgH, * and B?Hi^Fe(CO)4 in the parent mass region. 133

47. Topological structure of B^H^Fe (CO) 4.. • • 135

48. Boron-11 nmr spectrum of p-Fe (CO) 4-2-CH3B6H9...... 139

x Page

100 MHz proton magnetic resonance spectrum of u-Fe(CO) 4~2-CH3BgHg in the methyl proton region ...... * Possible isomers of v»-Pe(CO)4- 2-CH3BgH9 ...... 142 INTRODUCTION

—Decaborane *■■ ■ ■■ ■ ■ ■ 4 Decaborane(14) is an extremely versatile compound and undergoes a wide variety of reactions. The derivative chemis­ try of B10H14 is the most extensive of all the binary boron hydrides. The commercial availability of decaborane since

1950 has made this compound the most exhaustively studied of all the boron hydrides.

Stock initially prepared decaborane(14) via pyrolysis of B2Hg or B^H^q, and it is the chief product of the decom­ position of BcjHg at room temperature.^* Other means of produc­ ing B^qH^4 are via ultraviolet irradiation of BjHg or B^H^q.

The best yields of decaborane are obtained by heating diborane to 120° for 47 hours. The low volatility of decaborane makes it easy to separate from the other volatile boron hydrides, and yet it is volatile enough to be easily separated from the non-volatile products. Mercury must be excluded from the system since it is difficult to remove from the hydride in the co-condensate.

Decaborane(14) is a colorless air-stable crystalline solid at room temperature and has a fetid odor. It melts at

99.7°, boils at 213° with decomposition, and has a vapor pres­ sure of 0.3 torr at 55°.^

1 2

Insoluble in water, decaborane dissolves in alcohol, ethers, benzene, carbon disulfide, hydrocarbons and halog- enated hydrocarbons. Decaborane may be purified by subli­ mation or recrystallization from methylcyclohexane.

Stock'1' reported that decaborane reacts with halo­ gens via substitution of hydrogens. He prepared compounds of the formula BiQH^2Et2 and B10HllBr3* Decaborane(14) readily dissolved in liquid ammonia at -75° but did not react. However, at 120° a mixture of and ammonia reacted to give B^oH13 N H 2* In basic solutions decaborane dissolves with the evolution of hydrogen. Permanganate is reduced by decabor­ ane but concentrated nitric acid fails to react. 2 3 4 The crystal structure of ' 1 shows a mole­ cule of C2v symmetry. Ib may be considered as a fragment of an icosahedron with two adjacent borons removed. The resulting structure resembles a double-pointed basket.

All boron-boron distances are in the normal range of O 1.7-1.8 A except for the B5-10 and B7-8 bond distance O which at 2.01 A is the longest known boron-boron bond.

The bridge hydrogens are unsymmetrical, lying closer to the borons 5,7,8 and 10. Figure 1 shows the structure and framework bonding in decaborane(14).

The boron-11 nmr of decaborane (14) has been ob­ tained at 12.3^ Mhz., 16.26 Mhz., 32.1 Mhz.^, and 64.16 p a Mhz. Proton spectra at 220 Mhz. of brominated and

deuterated derivatives allow unequivocal assignments for

the spectrum of biqH14* Q At 64 Mhz. the boron-11 nmr spectrum (Figure 2)

consists of a non-symmetric triplet of area 2 centered at

-10.5 ppm., a symmetrical doublet at -017 ppm. of area 2f

and a symmetrical doublet of area 1 at +35.8 ppm. (all' chemical shift values relative to BF-j.OfCjHg^ unless

otherwise noted). The doublet at highest field was as­

signed to the 2,4 boron since it collapsed to a singlet in

the spectrum of 2,4-l2B^0H^2 .^° The resonance at -0.7 ppm. was assigned to borons 5,7,8, and 10, and the triplet at

lowest field is due to the partial overlap of borons 1,3

and 6,9.^ The lowest field component of this triplet is

due to boron 1,3 since in 1,2,3,4-B^qH10D^ a singled ap-.

pears at highest and lowest fields.** No evidence is seen

for coupling of the bridging protons to borons 5,7,8 and

10 even though the bridges are displaced towards these 2 3 4 borons. ' 1 The 220 Mhz proton nmr of 2-BrB^QH^3 and 7 1,2,3,4-B10H1QD4 allow assignments of the bridging protons

at 11.It terminal protons 2,4 at 9.4t ; 5,6,8,10 at 7.0t ;

1,3 at 6.6x; and the 6,9 protons at 6.2t (measured in

tetrahydrofuran).

The boron-11 chemical shifts do not follow the charge

densities on the various boron atoms. Molecular Orbital 4

Figure 1: Molecular Structure of B1QH14

Figure 2: Boron-11 NMR Spectrum of B1QH14 calculations give a charge distribution for biqh14 summar­ ized in Table 1. The calculated dipole moment from the charges in column 4 is 3.6 debyes11 (the experimental value is 3.52 debyes), and the" reactions of decaborane are con­ sistent with this charge distribution.

Decaborane acts as a monoprotic acid and reacts 12 with aqueous hydroxide to produce yellow solutions. Upon 13 14 acidification b ^qh14 was recovered from these solutions. ' Titration in acetonitrile gave a curve typical of a mono- 13 protic acid with a pKa of 3.5. The yellow solutions absorb in the visible region at 267 and 335 my (e “ 2.5 x 3 -3 2 15 10 and 1.7x10 cm /mole respectively).

The acidic proton in decaborane was shown to be the 16 17 18 bridge proton, as infrared ' and nmr studies indicated • • t that bridging protons were exchanged rapidly in D20-dioxane solutions made slightly basic. The reaction of DC1 with

NaB^gHj^ involved bridge deuteration as shown by infrared studies.^ However, exchange with DC1 in the presence of 18 AlCl^ resulted in terminal deuteration only. This is in sharp contrast to the equilibration in D20-dioxane in which deuterium enters the bridge position first and then slowly migrates to the 5,7/8 and 10 positions.1®

The b i o h 13_ lon ls Produced reaction of NaH 16 19 with b ioh14 in ether, ' but was highly solvated, and was not isolated as a solid. The first isolated salt of

B10h 13- was °btainec* from the reaction with triphenyl- 20 methylene phosphorane according to equation 1. Table 1: Electron Charge Densities for B^qH ^

VB LCAO Ia LCAO IIb LCAO IIIC LCAO-MOd

Bl,3 0 +0.05e +0.046e -0 .102e -0.03e

B2,4 -0.67e -0.25e -0.254e -0.329e -0.46e

B5,7,8,10 -0.03e +0.069e +0.Olle +0 ,10e +0.33e

B6/ 9 0 +0.27e +0.486e +0.410e +0.29e

aneglects bridge hydrogen charges

^assigns a value of +0 .20e to the bridge hydrogens

ceguates boron Coulomb integrals with valence state ionization energies

^based on 111 valence bond structures

91 The reaction of decaborane with a Grignard Reagent produces the BiQH^MgX species which can itself act as a 21 22 Grignard. ' Treatment of B^gH^Mgl with alkyl and aryl 21 22 halides and dialkyl sulfates produces the corresponding alkylated decaborane, BioH13Rr w^ere tlie a^kyl group was 21 shown by nmr studies to be in the 6,9 position, except 22 for diethylsulfate which results in the formation of

5-ethyldecaborane. Benzyl bromide reacts with N a B ^ H ^ to 23 give mixtures of 6-and 1-benzyldecaborane.

Electrophilic substitution in decaborane(14) takes place mainly at the 2,4 and 1,3 positions in that order of reactivity. Reaction of decaborane with I2 or Br2 Pro“ 0 A duced the 2,4-dihalodecaborane. Halogenations carried out in the presence of Friedel-Crafts catalysts result in 25 substitution at the 1,2,3 and 4 positions. Friedel-

Crafts alkylations of decaborane follow the general reaction 26 shown.in equation 2. When R=CH.j a mixture of 1? 2; 2,4;

1,2,4; and 1,2,3,4-methylated decaboranes was obtained and

the relative yields suggested greater reactivity at the

2,4 position.

A1C1- RBr + — > RB1AH,- + HBr (2) Nucleophilic attack on b ^qH14 occurs Primarily at the 6,9 position. Reaction with RLi (R=CH3 or C2H 5)2^

•gives e-RB^H^. B*CH3 some 6,9-dimethyldecaborane * as well as 5,6-dimethyldecaborane is formed. Reaction

with Lewis bases generally lead to formation of b i o H12*L 2

2— complexes with are isoelectronic with the ion. 2— The b i o H14 *on was ^*rst postulated by Lips- 28 comb. It has the structure shown in Figure 3. The

ion has been produced by the reaction of decaborane with 29 30 sodium in liquid ammonia, ' and via reduction of

B10H14 with bh4~*31 The structure of the b^qb142-

B10H14 + 2Na ---- » ^a2B10H14 (3>

B10H14 + BH4~ + 4H2° -----* B10H142” + H30+ + B(0H)3 +

H2 (4)

ion has not been determined, but the single-crystal X- 21 33 ray structure of the isolectronic (NCCH3)2 '

was obtained and agreed with the structure proposed for

B10H14 where one B"H bond at the 6,9 position is replaced 33 by a B-N bond. The.acetonitrile moiety is linear indicat­

ing bonding by way of the lone pair on the nitrogen, thus

making the ligand formally equivalent to H". A number of

other analogous b i o h 12i*2 comPounda have been prepared where L =* phosphines, dialkyl sulfides, amides, tertiary 37 amines, and others. Methods of preparation include reflux- ing decaborane with the appropriate ligand in benzene or displacement of one ligand with another of greater nucleo- philicity.

The boron-11 nmr spectrum of B^gH^2 ^C2H5NC^ 2 at 32.1 Mhz (Fig. 4) consists of a doublet at 1.3 ppm. of area 2 assigned to B2,4; a broad doublet at 18.3 ppm. of area 4 due to B5,7,8,10 and an unsymmetrical triplet of area 4 arising from the overlap of doublets from B6,9 (39.9 ppm.) and Bl, 3 (44.7 ppm.). Where the less basic diethylsulfide 39 ligand is used the 86,9 resonance moves downfield to 38 26 ppm. as expected based on inductive effects.

2- A class of compounds similar to B^gH^ and B^qL ^ I ^ are the B^gH^I*” ions. The reaction of excess diethylamine with decaborane yielded a salt formulated as B^qH^ * 2 40 ^C2h5^2nh* Further investigation of this compound showed it to be a salt of the B^qH^3(C2H5)2NH~ ion.4*r^2 Addi­ tion of one equivalent of HC1 in dioxane precipitated diethylamxnonium chloride. The remaining material was shown to be B^gH^ (C2H5)2NH and probably is structurally analo­ gous to the B^qH^jj” ion, but it was not isolated.

The 12.1 Mhz. boron-11 nmr of B^gH^fCjHg)2NH~ was similar in its gross features to that of the B^qH^2L 2 A *7 compounds, indicating a similar structure. The B^gli^g"

2 - .ion was prepared via treatment of B^gH^ ions with one 43 equivalent of acid. The boron-11 nmr spectrum consisted 0 =bh

2- Pigure 3: Structure of B^QH ^

Figure 4: Boron-11 NMR Spectrum of B^ 11 of three signlets at +9.5 ppm., + 16.8 ppm., and +25.1 ppm.

No other features or assignments were reported. The in­ frared spectrum possessed B-H stretching bands at 2330,

2380, and 2530 cm.”^ which suggests the presence of BH2 units.

A series of substituted biqh13~ ions have been pro­ duced of general formula biqH12X** where x = CN"*» CNO- , CNS”, and (CNJgC"".1*^ These products were obtained by re- fluxing decaborane with the appropriate salt in ethereal solvents with the evolution of hydrogen.

R,0 B 10H14 + NaX --- — > NaB10H12X + H2 (5)

No specific structure determinations were made, however, solvolysis of B^q H^NCS"” methyl alcohol*^ most likely produced the 6-BgH^NCS” ion, which would indicate a parent compound formulated as 6-B^H^NCS-.

Although amines react with decaborane(14) to give

B10H12L2 cotnP°un^s (vide supra), the reaction of decaborane with triethylamine produced in addition to the expected bis-

ligand compound an isomeric material which had a saltlike 37 37 character. Chemical studies and molecular weight 46 2— determination showed this salt to be that of the B^oH10

ion. The boron-11 nmr spectrum of this salt consists of two doublets of area ratio 1:4 at -2.0 and + 27.0 ppm. 47 respectively. This spectrum is consistent with a D. 12 structure (Eigure 5). This structure may be described as two square pyramids joined at the base with one pyramid rotated 45° with respect to the other. Thus there are two sets of equivalent borons: an apical set consisting of two borons each split by a single terminal proton and an equatorial set of eight borons each split by a single ter­ minal proton. Confirmation of this structure came with 48 49 the X-ray structure determination of Cu2B^qH^q. '

The crystal structure of Cu2B^qH1q^® gave evidence

2- for a covalent interaction of Cu(I) with the B1QH10 ion; the first reported example of a three-center inter­ action of a metal ion with a boron hydride. This compound will be more fully discussed in the section on metallo- boranes.

Hexaborane(10)

Hexaborane was first prepared by Stock^ from the acid hydrolysis of magnesium boride in extremely low yields.

Due to the difficulty in preparing hexaborane its chemistry remained largely unknown. Improved yields were achieved 50 51 from this reaction and one involving ,BgH ^ . Other 42 53 syntheses involved silent discharge ' and base-catalyzed 54 conversions from other boranes. However, it was not until an improved synthesis from B,.Hg55, which led to rela­ tively high yields (ca. 20%), that a study of the chemistry of hexaborane(10) was feasible. The preparation of BgH10 13 in 70% yields from 1-BrBgHg reported by Johnson, Brice and 56 Shore made studies of hexaborane(10) most convenient.

Hexaborane(lO) is a clear colorless liquid (m.p.

-65.1°) with a vapor pressure of 7.2 torr at 0°.* It has an odor of burning rubber and does not flame upon contact with air in contrast to the behavior of B^Hg. At room temperature hexaborane decomposes slowly in the liquid phase to give hydrogen, diborane and decaborane; but seemed quite stable in the gas phase at 300°. Aqueous alkaline solutions of hexaborane are stable.1

The structure of BgH^g was determined by Lipscomb, 57 58 59 et al. * 1 and was shown to be a pentagonal pyramid with each boron possessing a single terminal proton. There are four bridging hydrogens and one boron-boron bond in the base. All boron-boron distances are in the normal range 0 of 1.7-1.8 A except for the unique basal boron-boron bond O with a separation of 1.6 A. The boron-p-hydrogen distances O are ca. 0.3 A longer than the sum of the covalent radii of boron and hydrogen. Figure 6 shows the structure of BgH^g.

The boron-11 nmr spectrum of hexaborane has been observed at 12.8,60 32.1,61 and 70.6®2 Mhz. The spectrum consists of two doublets in the area ratio of 5:1 at -13.7 and +52.2 ppm. respectively, each of which collapses to a singlet upon irradiation at the frequency. The 220 Mhz. 62 proton magnetic resonance spectrum consists of a 1:1:1:1 quartet of area 5 and a similar quartet of area 1 (which 14

0 =bh

2_ Figure 5: Structure of

0 =bh

Figure 6: Molecular Structure of 15

overlaps a large singlet) at 5.8, 11.2 and 11.It respec­ tively. When irradiated at the boron-11 frequency the downfield quartet collapses to a singlet, the upfield quartet vanishes and the singlet at 11.It sharpens and intensifies. These spectra may be understood in terms of the nmr equivalence of all five basal borons as well as their terminal protons. Coupling of the bridging protons to the basal borons or the basal terminal protons is not observed. 57 58 59 Since hexaborane(10) possesses C symmetry ' ' 0 in the solid state, and the spectral data are consistent £ 1 £ 0 with C q ' symmetry, the apparent equivalence of the v basal borons must arise either from the coincidental over­ lap6^ of all basal boron resonances (as well as those of the basal terminal protons in the proton spectrum), or a fluxional behavior47'64 which places the boron-boron bond in the base adjacent to each boron at a speed which is rapid compared to the nmr timescale.

Variable temperature nmr experiments have verified the second hypothesis. Below -70° in the boron spectrum the signal due to the five basal borons collapses to a singlet and broadens. As the temperature is progressively lowered a new peak appears upfield giving an area ratio for the downfield peaks of 4:1.®^ At -80°®® the proton spectrum (basal borons decoupled) shows a single resonance 16 arising from the four bridge protons, however at -147° the basal terminal resonance resolves into three signals in the area ratio of 2:2:1 while the bridge proton signal splits into two resonances of equal area. The averaged chemical shifts of the peaks agrees with the chemical shifts of the single peaks observed at higher temperature.

Thus the low temperature spectrum is completely consistent with the solid state structure.

Chemical and spectral evidence suggest that only the bridging protons are involved in the tautomerism. The fact that the bridge and terminal proton signals are al­ ways observed in the area ratio of 4:5 plus the lack of spin-coupling of the bridge protons to the basal borons mitigates against terminal proton participation in the tautomerism. Exchange of B2Dg with hexaborane yields

B6H5D5 the deuterium is found in terminal posi- 67 68 tions exclusively. ' Gas phase equilibration of BgH10 with DCl yields a hexaborane deuterated in the bridge positions solely.6^'®® 4 7 The mechanism for this tautomeric process probably

involves breaking of the boron-bridge proton bond adjac­ ent to the basal boron-boron bond as the initial step. 67 The proton, which lies below the basal plane (see Fig­ ure 6), may then swing around to form a bridge at the pre­ vious site of the boron-boron bond. The position vacated by the bridge proton becomes the site of the new boron- 17

•boron bond. Since only bridge protons are favorably

located to undergo this exchange the terminal protons are

excluded from the process. 4 Studies of substituent effects on the tautomerism66'^0

suggest that an inductive effect determines the position of

the boron-boron bond in the static structure. The proton

magnetic resonance spectrum of 2-CH2BgHg at +25° exhibits

one bridge proton resonance indicating the magnetic equiv­

alence of all bridging protons on the nmr timescale. At

-50° the bridge resonance splits into two resonances indi­

cating a structure where the basal boron-boron bond is

restricted to the 3-4, 4-5, and 5-6 positions. At -125°

there are four distinct bridge resonances indicating a

static structure in which the boron-boron bond is frozen

in the 3-4(5-6) position. In 2-BrBgHg similar features

are observed. However, at -120° the bridge protons give

rise to three signals in the area ratio of 1:2:1. This

would be in accord with a structure undergoing partial

tautomerism of the boron-boron bond between the 2-3 and

2-6 positions. Thus the presence of a normally electron-

withdrawing substituent (Br) directs the boron-boron bond

to an adjacent position while the presence of a methyl

group directs the boron-boron bond to non-adjacent posi­

tion. If methyl is electron-donating with respect to

hexaborane it is reasonable to expect the electron-rich

boron-boron bond to reside at the most distant position. The fact that this bond resides only one boron removed

from the methylated boron may be due to an entropy ef­

fect. Figure 7 shows the static structures of 2-CHgBgHg

and 2-BrBgHg. 47 Molecular orbital calculations give charge distri­

butions for BgH^Q as follows: B^ (-0.72e), Bg (0.36e),

°3 6 (°*35e)' and b4 5 (-0.17e). Furthermore, SCF-MO 71 calculations for higher boron hydrides indicate that

the bridge protons have positive character. SCF calcula- 72 tions using Slater orbitals yielded a charge distribu­

tion of -0.02e on Bl; 0.07e on B2; 0.06e on B3,6; 0.04e

on B4,5; -0.07e on HI; -0.04e on H2, 2,6; -0.09e on H4,5;

0.03e on p-H23,26; and 0.023e on y-H34,56. A topological 73 approach based on reaction intermediates predicts SE1

and S^l reactions at Bl; S^l and S£2 at B2,4,5; Sgl and

S„jl at B3,6, and S^.1 at B4,2.

Hexaborane is amphoteric, exhibiting both Lewis and

Bronsted acidity as well as Lewis basicity. Hexaborane is

deprotonated by metal hydrides, MH {M = Li, Na, K)61,74

to yield the nonahydrohexaborate(-1) ion, BgHg-. Proton

competition studies established the order of acidity of 74 the BnHn+4 boroi) hydrides as B10H14 > B6h10 > B5H9 (equations 6 and 7). Ammonia in a 1:1 mole ratio has 19

B6H10 + B5H8 ------> B6H9 + B5H9 (7)

65 67 been shown to deprotonate hexaborane as well. ' This

is in contrast to the behavior of B5Hg which is deproton- 76 ated by ammonia only in liquid ammonia.

Hexaborane reacts with Lewis bases to form addition

compounds. These complexes take the form BgH^QL2 where

L *■ trimethylamine, trimethylphosphine and triphenylphos- 75 phine. A 1:1 complex of and triphenylphosphine 77 has also been reported.

The Lewis basicity of hexaborane(10) appears to be

associated with the basal boron-boron bond. This bond

iB most likely bent and extends outward and down from the basal plane.5<7r58,59,78 The exchange Qf hydrogen with deuteriodiborane at low temperature results in deuteration 67 at basal terminal sites only. Since the only other

examples of such exchange occurs in boron hydrides contain**

ing BH2 units the boron-boron bond is believed to play a 67 part in the exchange with BgH^g. Furthermore, hexaborane

rapidly exchanges bridge hydrogens with DC1 in the gas 68 phase at room temperature. This is in sharp contrast 79 to the exchange of DC1 with pentaborane(9) and decabor-

ane(14)*5 which exchange terminal hydrogens only and

requires a catalyst. The mechanism of the exchange with eg + DC1 y is presumed to occur via the intermediate BgH^D

/ 20

which may then lose a bridge proton to give y-BgHgD ac­ cording to equations 8 and 9.

DCl + B6H1q ---- >«B6H1QD+ Cl“ (8)

B6H10D+ Cl~ -----> ^”B 6H9D + HC1

The stability of the intermediate B g H ^ + was predicted by 80 + - Lipscomb, and the stable species ( B g H ^ ] [BC14 ] was 81 isolated and studied by Shore and co-workers. ■ The stability of this ion is due to the large proton affinity of hexaborane which was shown by chemical ionization mass 82 spectrometry to be the highest of the known boron hy­ drides. The basicity of this bond was emphatically demon- 83 84 strated by Davison and co-workers, ' in the synthesis of

M-Fe(CO)4-B6H1084 and (BgH1())2ptCl2.83'85. Extended dis­ cussion of these compounds will be presented in the section on metalloboranes.

Derivative chemistry of B6H10 is rather limited to date, but in addition to the adducts, 2-methylhexaborane,^8 70 86 87 2-bromohexaborane, 2 ,3-dimethylhexaborane and 1- 88 trimethyl M(IV) hexaborane (the first apically substi­ tuted hexaborane(10)) have been characterized.

The nonahydrohexaborate(-l) ion which results from the removal of a proton at a bridging site is structurally i the same as the parent compound, BgH^g. ' The boron-11 21

nmr spectrum of KBgHg in THF does not change appreciably

in the temperature range + 30° to -80°.®^ There are two

symmetrical doublets in the area ratio 1:5 at 48.3 and

-9.5 ppm. respectively. The BgHg” ion is considerably more

stable in solution than the octahydropentaborate(-1) ion,

and the isolated solid, KBgHg shows only partial decom- 61 position after one week at room temperature under a dry

nitrogen atmosphere. The tetrabutylaminium salt of BgHg

is prepared via metathesis from KBgHg and is thermally more stable than the alkali metal salts of BgHg- . The

decomposition of BgHg- in solution yielded primarily

B11H14~ and BH4~*89

Metalloboranes

Since the synthesis about ten years ago of the first

boron hydride with a boron-to-metal bond the field of metal-

loboranes has been an area of active interest. A wide

variety of compounds has since been synthesized and char­

acterized which exhibit interesting structures and proper­

ties. . The bonding characteristics of metalloboranes vary

from purely ionic to covalent and include two-center, three-

center and hydrogen-bridged species. The nature of the

metalloborane depends upon the nature of the boron hydride,

the metal atom and the ligands. Metal derivatives of boron

hydrides have been made with Group I, II, III, and IVa; I

and lib; and recently group VIII metals. Boron's similarity 22

to carbon in its size and electronegativity make the field of metalloborane chemistry potentially as rich and as varied as that of organometallics. Finally, the tendency of boron to form polyhedral structures renders many metal- loboranes formally similar to metal cluster compounds.

There are four main bonding types observed in metalloboranes. These are: (a) ionic compounds such as

MBH4, MB3Hg, MBgHg and MBgHg (M = Li, Na, K); (b) covalent compounds bonded via hydrogen bridges; (c) covalent com­ pounds bonded via direct boron-metal bonds either of the two-center or three-center variety; and (b) the pi-bonded or "sandwich" compounds.

The nature of the bonding in metalloboranes has been elucidated via chemical means and instrumental tech­ niques as well as by X-ray or electron diffraction methods.

The simplest of the metalloboranes are the tetra- 90 hydroborates recently reviewed by James and Wallbridge.

The stability of the metal tetrahydroborates is governed by the electronegativity of the metal. Only those metals less electronegative than boron can form tetrahydroborates, and attempts to form tetrahydroborates with metals of higher electronegativity resulted in the formation of the corresponding metal hydride due to the more successful competition for the hydride ion. However, if such metals possess ligands which tend ot reduce the electronegativity of the metal the tetrahydroborate may be stable. Thus, 91 copper(1) tetrahydroborate is stable only below -12°, whereas bis-triphenylphosphinecopper(I) borohydride is 92 stable at ambient temperature. Another feature of sta­ bility is the ionicity of the compound. That is, those hydroborates which are less ionic than diborane are ex- 93 pected to be unstable.

The main structural feature exhibited by the metal hydroborates is the presence of metal-hydrogen bridges.

Some electron-diffraction and X-ray studies have verified this type of interaction, but the main evidence is spectro scopic. Aluminum borohydride has a planar arrangement of borons connected to the metal via double hydrogen brid- 94 95 ges. ' Bis-triphenylphosphinecopper(I) borohydride has a double hydrogen bridge although the large P-Cu-P angle 96 of 123° indicates some direct Cu-B interaction. Triple hydrogen bridges were first seen in zirconium borohydride.

There are four borons in a tetrahedral arrangement around the metal with twelve hydrogens as nearest neighbors in 97 icosahedral positions. The structure of beryllium boro- 98 hydride was a matter of some controversy. Early work suggested a linear B-Be-B structure, but other electron 99 diffraction studies indicate a structure involving a BeH moiety bridge bonded to each of the borons of a diborane fragment as shown in Figure 8. Figure 7: Static Structures for 2-CH3BgHg and 2-

* Bo 0*B

Figure 8: Structure of Beryllium Borohydride. 25

Infrared spectral evidence has proven to be a useful tool in elucidating structures of tetrahydroborates. Sod­ ium borohydride, which is purely ionic and thus has no « bridge bonds, possesses infrared bands at 2270 and 1080 cm-1 assigned to B-H stretching and BH2 deformation modes respectively.100 Diborane, on the other hand, exhibits these bands at ca. 2500 and 1175 cm 1 plus additional bands at 1860 and 1600 wavenumbers assigned to out-of-plane bridge expansion and in-plane bridge stretching respective­ ly* Aluminum hydroborate shows the terminal modes at 2500 and 1100 wavenumbers, but the bridge mode bands are now located at 2050 and 1400 cm”1 .103,104 These shifts toward the terminal band positions have been interpreted as arising from the increased ionicity of A 1 (BH4)3 vis-a- vis so that the bridge modes should move progressively more towards the terminal band positions in increasingly ionic compounds, eventually becoming degenerate with them.

Zirconium borohydride,which was shown to possess a triply 97 hydrogen-bridged structure should lack BH2 deformation modes, and indeed, the band near 1100 cm-1 is absent in the spectrum of this compound.1®5 90 The Group I hydroborates are all ionic and are air- stable. Acid hydrolysis is slow, and gives boric acid and hydrogen according to equation 10.

H+ BH4 + 3H20 — =---> H3B03 + 4H2 (10) 26

Tetrahydroborates of the Group II metals Be and Mg

are distinctly covalent, being volatile enough to be sub­

limed, and BeB2Hg inflames in air as well. Calcium, stron­

tium and barium borohydrides are ionic and resemble the

corresponding alkali metal compounds. The Group III tetra­

hydroborates are all covalent, and aluminum borohydride is

the most volatile Al(III) compound known. Indium hydro­

borate has been isolated as the THF adduct only1^6; loss

of the solvent at -10° resulted in decomposition to indium,

diborane and hydrogen. Tin and lead hydroborates are the 107 only Group IV hydroborates to be prepared. The Sn(II) 107 hydroborate is unstable above -65°, and the lead corn- 108 pound, (CHg)gPbBH^, has been isolated as the ammoniate.

Transition metal hydroborates have been made with

Ti, Zr, Hf, V, Nb, Cr, Mn, Fe, Co, Ni, Cu, Ag, Au, Zn

and Cd. Titanium borohydride is a volatile green material

that decomposes rapidly at 15°. The molecule *l5“ (C5H5)2” TiBH^*® has a tetrahedral structure and has two hydrogen

bridges. Zirconium and hafnium hydroboratesare vola­

tile, colorless solids which inflame upon contact with 109 air. Their physical properties are almost identical.

The structure of ZrfBH^)^ was previously discussed (vide

supra). Vanadium borohydride has been reported^with

few details. The only niobium hydroborate, h5-(C5H5)2-

NbClBHj, has been isolated as a red-violet solid soluble 112 in methylene chloride and benzene. Chromium hydro- 113 borate was prepared by Parry and co-workers as

[Cr(NH3)6] (BH4)*3-5NH3. This material is stable to 60° under vacuum in contrast with the Co (III) analogue which decomposes at 25° if the ammonia pressure is less than

50 torr. The slow hydrolysis of the Cr complex indicates an ionic interaction of the metal and BH^. The reaction of MnCl2 with LiBH^ yields a complex material formulated as Li2 [Mn(BH4)C12] nO(C2H 5)2.114 Manganese borohydride has been reported as has Mn(CO)gBH^ , 3,3,5 formed from the reaction of Mn(CO)gBr and Al(BH4 )g. Copper borohydride was previously discussed, and a silver analogue, bis- triphenylphosphinesilver(I) borohydride3'1,6 has also been 117 118 prepared. Zinc and cadmium borohydrides, (M(BH4)2), have been characterized. The zinc compound reacts vigor- 119 ously with water which is a good indicator of covalent meta1 -hydrogen bonds.

Ionic and covalent complexes of the octahydrotribor- ate(-l) ion have been prepared. Sodium triborohydride 120 was first reported by Hough, Edwards and McElroy, from the reaction of sodium amalgam and diborane (equation II).

121 - The structure of the BgHg ion is given in Figure 9, and shows two sets of borons and three different kinds of hydrogen (actually five sets if axial and equatorial 122 distinctions are taken into account). The boron-11 nmr, 28

2Na + 2B2H6 ------> NaB^Hg + NaBH^ (11) however, shows seven arms of a regular nonet, suggesting the equivalence of all eight protons. A copper compound 1OA (03p)2CuB3h8' ' shows evidence of being covalently 124 bonded. Conductivity measurements show no evidence 124 of dissociation in chloroform; infrared bands in the bridge hydrogen region near 2100 cm-* and the proton mag- 125 netic resonance spectrum shows distinct evidence of non-equivalent protons at -97°. The single-crystal X-ray t 26 structure confirms the hydrogen bridging and shows a smaller P-Cu-P angle than in (jJgP^CuBH^ (120° vs. 123°), 126 which was interpreted as further evidence of a covalent bond between Cu and the borane. The greater electroneg­ ativity for BH4~ as opposed to B^Hg” should cause a great­ er contraction of the Cu-B bond overlaps allowing more room for the expansion of the phosphine moiety. A silver 123 127 analogue to the copper compound was made, * but had a lower binding tendency, and gold failed to form a stable 127 complex. Aluminum and gallium form volatile complexes of formula (CH^) 2MB3H8 have H-H-B stretching bands 128 in the region 2100-2200 wavenumbers. The boron-11 nmr spectrum at -23° consists of a triplet of area 1 and a quartet of area 2. The triplet arises from the coupling of the two terminal protons to the unique boron, and the 29 quartet is due to the coupling of one terminal and one bridge hydrogen to the two bridged borons. Coupling of the remaining bridging protons to the borons is not ob­ served. Figure 10 gives the structure for (CH3)2AlB3Hg.

Reaction of Cr, Mo and W hexacarbonyls with tetra- alkylammonium salts of B 3H0” resulted in air-stable salts • 1 oq 1 "Jn formulated as M(CO)3B3Hg . ' Infrared and boron-11 131 nmr spectra and X-ray structure determinations estab­ lish a double hydrogen bridge bond to the metal which lies in a pseudo-octahedral environment.

Reaction of (CO),-MBr (M = Mn, Re) with B^Hg” produced volatile liquids formulated as (CO)^MB^Hg which were shown 132 to be isostructural with the Cr compound previously 132 discussed. However, irradiation of these complexes produced a unique molecule in which one additional CO was lost by the metal and a third hydrogen bridge was formed to occupy the open coordination site. The structure, shown in Figure 11, represents the first example of

B3H8“ acting as a tridentate ligand. The protons not engaged in bridging to the metal aton are equivalent on the nmr timescale indicatubg a partial intramolecular exchange.

An even more unique bonding arrangement was observed 133a in the reaction of CsB3Hg with (03P)2PtCl2. The product, formulated as (03P)2PtB3H7, was resistant to acid . 30

Figure 9: Structure of BgHg”.

0 = c o = H

Figure 10: Structure of {CHg^AlBgHg 31 hydrolysis and no infrared band was observed in the M-H-B region. Nuclear magnetic resonance evidence suggested, 1 11K and the X-ray structure confirmed a pi-allyl type of bonding. Figure 12 shows the structure of (03P) 2PtB3H.j.

Another example of a tridentate boron hydride ligand is the M(CO) 3^9^13** (M=Mn, Re) or the isoelectronic

M{CO)3B9H12l “ THF, ether). The structure resem­ bles that of where the metal occupies the 6-posi­ tion and forms hydrogen bridges with borons 5 and 7, and forms a direct metal-boron bond to boron 2. An analogue — 135 of , Mn(CO)3BgH^2 involves three hydrogen bridges from the borane to the metal as shown in Figure 13. Other borane ions which act as bidentate ligands via hydrogen - - - - 127 bridges are BgH^^ , BgH^2S , ®io^i3 und B^qH^^ •

Of increasing interest in recent years are the com­ pounds involving direct metal-boron bonds. These bonds may be of two types; terminal two-center two-electron bonds and three-center two-electron bonds. The simplest of these complexes are the metal carbonyl anions to which BH3 has been added: H3BM(CO)3 and H3BM(CO)4P(CgH^)3 136 (M«Mn,Re). Gaines and lorns prepared the complexes 137 2-M(CO)3B3Hg (M=Mn,Re) from the reaction of the metal carbonyl anion and the chloro- or bromopentaborane.

Regardless of whether the.l- or 2-halopentaborane was used the product was always the 2-substituted isomer. No o =CO Mn o B o H

Figure 11: Structure o£ MntCOJ^B^Hg,

0 = p ® -p» O = B o = H

Figure 12: Structure of [(CgH5)gP]2PtB2Hy. Figure 13: Structure of explanation was given, but since the reactions were car­ ried out in THF or ether the exclusive 2-substituttion is probably related to the base-catalyzed rearrangement of 130 1-substituted pentaboranes to the 2-isomer. Other complexes of this type are 2-R.jMBgHg (R= H, CH3 and CjH,.; 139 M» Si, Ge) which are made from the corresponding y-R^M-BgHg via Lewis base-catalyzed rearrangements. Con­ trary to these reaction routes is the reaction between 1- or 2- (CHg) (M=Si, Ge) and H2BC1 which results in 98 the exclusive formation of 1- (CH3)3MBgHg .

Oxidative addition reactions of BgHg and BrBgHg (1- or 2- isomer) with irCl(CO)(PMe3)2 results in the forma­ tion Of 2-[Ir(CO)HCl(PMe3)2]B5Hg or 2-[IrBr(C0)C1(PMe3)2)

BgHg respectively.1*® The carbonyl stretching frequency and the magnitude of the trans-effect suggest that the

M-B interaction is similar to that of a a-alkyl.

Three-center boron-metal bonds were first postulated 48 49 141 for Cu2BioH10' ' however Lipscomb, et al. have recently reinvestigated the data and proposed a hydrogen bridging interaction. An infrared stretching band from

2100 to 2300 cnT1 suggests a Cu-H-B bond, and a Cu-B dis- O tance of 2* 2 A could indicate a bridge bond. This last point must be advanced with caution since the lower bond order of a B-Cu-B three-center bond could also account for the bond lengthening. A neutron-diffraction study 35 which locates the hydrogens might settle the question. 142 143 Brice and Shore ' prepared y-(03P) 2Cu”B 5H8 which the metal is inserted into the basal boron-boron _ 144 1 4 c bond in the BgHg ion. Denton and Shore ' have prepared CHgMBgHg (M= Zn, Cd, Mg), Mg (BgHg )2 and BgHgMgX by the action of (CHg)2M or CH^MgX on BgH1Q. The nmr evidence indicates that the metals are inserted into the basal boron-boron bond of BgHg” . At ambient temperature the borons are equivalent on the nmr timescale whereas at low temperature the tautomerism is slowed enough to see the non-equivalence of the basal terminal and bridging protons in contrast to LiBgHg which showed equivalence 69 of these sets of protons down to -130°. A blue, para­ magnetic complex, hg-(CgHg)2TiBgHg was also prepared and found to be bridge-substituted.^^

The basicity of the basal boron-boron bond in hexa- 65 borane(10) has been employed to displace weaker bases to form metal complexes of the type P-I*nM-BgH10 and y-L M-(BCH,ft)0. Davison and co-workers^have syn- n o iu z thesized y-Pe(CO)4-BgH^ from the reaction of Fe2(CO)g and BgH^g. This yellow, sublimable solid has three basal boron resonances in the area ratio of 1:2:2 demonstrat­ ing the Cg symmetry of the molecule. Compounds of the

formula M(B6H^q)2C12 (M- Pt, Rh, Ir) are prepared by the reaction of two equivalents of hexaborane with Zeise's salt, Pt(C2H4)Cl3+, displacing the trans-chlorine and the QC ethylene. The Pt complex is square-planar with the chlorines trans to each other. The boron-11 nmr spectrum is identical to that of BgH^g indicating a possible weak interaction of the metal and the borane in solution.

It is clear that metal complexes of boron hydrides is a rich field in which many unusual structures are found. The valence-bond descriptions of many of these com­ plexes are still unclear as are the structures themselves.

All of the bonding modes of organometallic compounds are observed and the three-center metal ligand interaction is a unique feature not observed in the organometallic fam­ ily. The synthetic techniques are flexible, involving:

(a) borane plus metal hydride; (b) borane plus metal alkyl or aryl; (c) borane plus metal complex; (d) borane anion plus metal complex; (e) metathesis; (f) borane anion plus metal halide; and (g) borane plus metal carbonyl.

Many of these metalloboranes may be viewed as metal clus­ ter analogues in view of the incorporation of the metal into the boron framework. The possibility of catalytic activity of many of these compounds, especially in light of the rapid hydrogen tautomerism in some complexes is an area yet to be extensively investigated and one which is full o£ promise. 37

Heptaborane

The nido-boron hydrides have the general formula

BnHn+^, and have been synthesized and characterized where n-2-6,1'35'146 8,147'148 9,149 10124 and 18.148-150 Of these, B3H7, B^Hg and BgH^ are not isolated as the binary hydride, but rather, require a Lewis base ligand to stabilize them.^5'14**'*4^ Heptaborane has not been iso­ lated. 47 i i Lipscomb, et al., ' have developed a topo­ logical approach to predicting the structures of boron hydrides. From geometric and valence bond considerations a designation, STYX, may be assigned to possible boron hydride structures where S= the number of hydrogen brid­ ges, T= the number of three-center boron bonds, Y= the number or boron-boron two-center bonds, and X= the number of B-H bonds in excess of one per boron. Based on the calculations from the theory, B.jH ^ may have structures

4320, 3411, and 2502. No satisfactory structures could be drawn for the first two designations due to violations of one or more topological rules such as the restriction of having a boron atom form two- and three-center bonds to the same boron atom. A satisfactory structure based on the 2502 topology is shown in Figure 14.

Heptaboranes of type B ^ H ^ or B ^ H ^ have been re­ ported occasionally, but the characterization is dubious 38

Figure 14: Topological Structure of 39 and none are stable enough to exist much above -196°. 156 Quayle observed a boron hydride envelope which could be attributed to B ^ H ^ in the mass spectrum of pen- taboranes. The gas was admitted to the ionization cham­ ber by vapor diffusion but no other details were given. 157 Schaeffer and co-workers reported a mass spectrum of tetraborane which showed peaks having a cutoff at m/e= 89.

They attributed this to a B^H^ species which probably arose from a B?H13 parent compound. However, the envelope shown was not characteristic for boron hydrides of type 158 BnHn+4 or BnHn+6* Gibbins and Shapiro report the repeated observation of heptaborane peaks at m/e=83 and

87 when they subject diborane to an electric discharge.

« In a mass spectral study of the pyrolysis of di- 159 borane with pentaborane(9), Norman, et al. showed that

B,jHg was an intermediate in the formation of B^QH^4.

Since the data supported a stepwise sequence of addition of diborane followed by loss of H2, a B?H13 species must exist as an intermediate.

B 5h 9 + b 2H 6 ---£--- > B 7H13 <12> 2

In the shock-tube pyrolysis of diborane Fehlner and

Koski^**® trapped the volatile products at 77°K. The products were then separated on a column with a temperature gradient from 77 to 300°K and subjected to mass-spectral 40 analysis. The materials which came off the column at -43 and -36°C were identified as and respectively.

In a reinvestigation of the work by Gibbins and Shapiro, 161 Williams and co-workers obtained a sample of the mater­ ial identified as a heptaborane and purified this sample by gas chromatography. The material was positively iden­ tified as ethylpentaborane and triethoxyborane, which gives a strong rearrangement peak at m/ea 85. In the molecular beam mass spectrum of the pyrolysis of B4H^Q a 162 variety of heptaboranes was observed, and in the copy- 163 rolysis of pentaborane{9) and diborane a material iden­ tified as a B6 of B7 hydride was isolated. The mass spectrum showed envelopes possibly due to a B7 hydride; the boron-11 nmr spectrum consisted of doublets at -14.2,

+13.9 and +51.2 ppm. The area ratio of the tv/o downfield doublets to the upfield doublet was 5:1.

The only definitive chemical evidence for a B7 hy- 164 dride was achieved by Johnson and Shore in the addi­ tion of diborane to BgHg". A tensimetric titration showed a sharp break at CB2Hg]/[BgH9”] » 0.5. Attempts to iso­ late from solution were unsuccessful, however, and treatment with HC1 gave hexaborane(lO) in 80% yields and

B2H6 .

2B7Hi2 + 2HC1 > 2B6H10 + B2H6 + 2C1 (13) 41

The existence of heptaborane has not been adequate­ ly established. Mass spectrometric evidence is open to other interpretations, and in at least one case the pre­ sumed heptaborane was positively identified as ethylpen- taborane and other species. Furthermore, no heptaborane of formula B ^ H ^ or B7Hi3 has been characterized.

Statement of the Problem

Ammonia is known to deprotonate BgH^Q, B^Hg, and 65 75 76 B4H10 at *,ow ten*Peratures • Since BiOH14 m j acidic than BgH^g it was of interest to see if B^QH ^ would be deprotonated by NHg. A metathesis reaction car­ ried out to isolate B^qH ^ " salts indicated a reaction of l“ with the decaborane cage in the presence of large cations. The nature of this reaction was to be studied and the.resultant species was to be isolated and charac­ terized.

The preparation of the hexaborane(10) adduct, u- 8 3 84 Fe(CO)4"BgH^Q by Davison and co-workers ' provided a metal complex of B^H^q in which the system of bridging protons was undisturbed. It was of interest to determine if this species could be deprotonated and to study the nature of the resulting ion.

Since BHg has been added to other boron hydride anions,^®'164 the addition of BH3 to the ion resulting 42 from the deprotonation of p-Fe(CO) 4-B6H10 was to be stud­ ied , and an attempt was to be made to convert this species to a neutral hydride. Single-crystal X-ray diffraction studies were to be carried out in order to elucidate the nature of the metal-boron 3-center bond. Mass spectra, infrared spectra, and boron-11 nmr spectra were to be used to characterize and to study the resulting species.

Studies by Brice, Johnson, and Shore66'70 on

2-CH^BgH^ indicate a partial tautomerism of the boron- boron bond in the base. To further study and confirm this mechanism y-FefCO^^-CH^BgHg was to be prepared and studied. Since the Fe(CO)^ moiety is bonded to the basal 84 boron-boron bond, it could be used as a probe to deter­ mine the placement of this bond relative to the methyl group. EXPERIMENTAL

4 I . Apparatus and Procedure

A. Vacuum System

Due to the sensitivity of many boron hydrides to air and moisture, all reactions were carried out on a vacuum line. Volatile materials were manipulated by trap-to-trap distillation and non-volatile materials were handled in a glove box where they were placed in a reaction vessel fit­ ted with an appropriate stopcock adaptor and removed to the vacuum line for evacuation.

The vacuum line consisted of a pumping station, a main manifold, three reaction manifolds, a distillation train and a toepler pump. -5 Pressures of less than 5x10 torr were achieved by a rotary forepump and a two-stage mercury diffusion pump.

Both pumps were protected from reactive chemicals by placing

traps maintained at -196° before each pump.

All manifolds and the toepler system were connected to

the main manifold by standard high-vacuum stopcocks with ground glass tapered plugs. Reaction manifolds consisted of a series of reaction stations equipped with standard taper 14/35 inner joints attached to the manifold via high

43 44 vacuum stopcocks with 4mm ground glass tapered plugs on two of the manifolds. A third reaction manifold employed

4am Fischer-Forter teflon stopcocks in order to provide a grease-free system. All reaction stations were equipped with mercury safety blowouts.

The distillation train consisted of four calibrated traps serially connected by high-vacuum stopcocks with ground glass tapered plugs, and a mercury manometer. The arrangement of traps and stopcocks provided for a means of pumping through only two or three or all four traps as needed. Two reaction stations previously described, em­ ploying ground glass stopcocks were used to introduce materials into the distillation manifold. In addition, the distillation manifold was connected to each of the reaction manifolds via ground glass high vacuum stopcocks.

The toepler system consisted of an Eck and Krebs 11. toepler pump, three calibrated traps serially linked, and a mercury manometer. Gas could be introduced from any reaction or distillation manifold and condensible gases were removed at a -196° trap. A recirculating system em­ ploying a 3-way ground glass high-vacuum stopcock was used to insure complete removal of condensible gases from the toepler system.

All calibrated volumes were determined using CO2 measured in a volumetric bulb which was calibrated with distilled water. 45

All high-vacuum ground glass stopcocks were lubri­ cated with a 50-50 mixture of Apiezon N and T greases.

B. Glove Boxes

Three glove boxes were used in the course of these studies.

A glove box manufactured by Kewanee Scientific Com­ pany of stainless steel construction was continuously flushed with nitrogen dried by passage through columns con­ taining molecular sieve, calcium hydride and phosphorous pentoxide. The airlock was evacuated and refilled from the box atmosphere twice before entering.

A Vacuum/Atmospheres glove box used a recirculating system made of copper tubing. Ultrapure nitrogen was con­ tinuously recirculated in a closed loop through a cannis- ter containing molecular sieve and Dow's Q-l catalyst.

The port was evacuated and refilled from the box atmos­ phere twice before entering.

A third box was used for mounting crystals of lim­ ited air stability, but of low thermal stability. A

Kewanee Scientific Company glove box was continuously flushed with nitrogen dried by passage through columns containing molecular sieve, calcium hydride and silica gel.

A reservoir of phosphorous pentoxide was maintained in the box. Samples were placed in a dish and packed with crushed dry ice, and placed in the port. The port was 46

.allowed to flush with the C02 for ten minutes prior to

entering the box. To maintain low temperatures while

mounting the crystals, a cake of dry ice was placed in

the box atop a wood insulator. A petri dish was placed

atop a plate of half-inch aluminum, which was placed atop

the cake of dry ice. In this way, crystals could be held

at -78° while mounting in capillaries.

C. Reaction Vessels

Reaction vessels consisted of test tubes or round-

bottom containers equipped with standard taper glass

joints. These were attached by means of appropriate stop­

cock adapter to the vacuum line. Nmr tubes were sealed to

sidearms when necessary. Samples were prepared by tip­

ping a solution into the tube and sealing off with a torch.

Reaction mixtures were stirred by placing a teflon-coated magnet in the vessel and employing a motor-driven stirrer with a magnetic head.

Tensimetric titrations were carried out in the

vessel shown in Figure 15. The substrate was placed in a

reaction vessel attached at A. Volatile samples were

condensed into reservoir B at -196°. Stopcock C was

closed and stopcock D was opened allowing mixture of the

reactants. Pressure was monitored by measuring mercury

levels in the manometer with a cathetometer. 47

B

FRONT SIDE

Ct=r T

MANOMETER

Figure 15: Tensimetric Titration Apparatus 48

D. Nuclear Magnetic Resonance Spectra

Boron-11 nmr spectra were obtained on a Varian HA-

100 high resolution spectrometer operating at 32.08 Mhz.

4 in the HR mode. External oscillators operating at £re- « quencies from 2500 to 3000 hz were employed to generate sidebands. A Bruker F-T 90 nmr spectrometer operating at

28.9 Mhz. in the F-T mode was also used. The spectra were run unlocked for 100-500 pulses and tube interchange with BCl^ was used to obtain chemical shifts. An 80.2

Mhz nmr spectrometer at the Carnegie-Mellow Institute in

Pittsburgh was operated by Dr. J. C. Carter; no details of the operation were given.

Samples were sealed in high-precision 0.5 cm o.d. glass tubes. External references BCl^ or BF-j-t^Hg^O were placed in sealed 1.0 mm glass capillaries and were employed as needed. Chemical shifts were measured by the sideband technique (HA-100) and reported relative to

BF^*(C2Hg)2O. Area ratios of peaks were measured using a polar planimeter.

Proton spectra were all recorded on the Varian HA-

100 spectrometer in the HA mode at 100 Mhz. Chloroform,

Methylene Chloride, Benzene or deuterodimethyl ether were used as solvents. Chloroform or methylene chloride were used as lock references. All chemical shifts were re­ ported in t units. 49

Heteronuclear proton and boron decoupling was achieved in broad-band or narrow-line modes by means of a

General Radio Company 1164-A frequency synthesizer and a

Hewlett-Packard 3722-A noise generator. A Hewlett-Packard

461-A amplifier and an Electronic Navigation Industries

320 RF power amplifier were used to amplify the signal.

E. Infrared Spectra

All infrared spectra were recorded on a Perkin-Elmer

457 spectrometer. Polystyrene film was used for the cal­ ibration. Solid samples were ground into mulls using

Nujol or hexachlorobutadiene and placed between polished

KBr plates; KBr pellets were used for some samples. Air- sensitive samples were mulled in the glove box and placed between KBr plates which were then inserted into a 20 mm penton coupling. Air was excluded by means of 38 mm

0-rings around the KBr plates. Volatile samples were expanded into a 10 cm glass cell with polished KBr plates sealed at the ends using Kel-P wax or glyptal paint cured at 110° for two hours.

F. Visible Spectra

Spectra in the range 250 to 400 mu were recorded on

a Cary-14 Recording Spectrophotometer in 1 cm quartz cells. 50

G. X-Ray Powder Diffraction Patterns

X-ray patterns were obtained on a Debye-Scherer camera of 11.46 cm diameter. The X-ray generator was a

North American Phillips X-ray generator with a copper target and a nickel filter. Exposure times varied from

6 to 28 hours at 32 kv and 12mA.

Samples were finely ground in an agate mortar and placed in 0.3 or 0.5 mm quartz capillaries. Air sensitive samples were prepared in the glove box and the capillaries sealed with silicone grease for removal from the box.

The sample was then flame-sealed to a convenient length.

H. Single-Crystal X-Ray Studies

Single Crystals were mounted in 0.5 or 0.7 mm quartz capillaries in the glove box and sealed as described in section G. Data were collected by Dr. W. R. Clayton on a

Picker FACS III Four-circle Diffractometer using Mo radia­ tion.

temperature-sensitive crystal were mounted as quick­ ly as possible and the data were collected at -50° using a Nonius low temperature maintenance device.

I. Molecular Weight Determination

Molecular weights were determined cryoscopically 165 using a standard cryoscopic cell previously described.

Samples were distilled into a vessel and weighed.

The solvent was then distilled in and 15 ml aliquots were 51 syringed into the cell in the glove box and removed to the cryoscope,

J. Mass Spectra

Mass spectra were obtained on an AEZ MS-9 double- focusing mass spectrometer operated by Mr. C. R. Weisen- berger. Samples were admitted via vapor diffusion or via direct insertion into the beam. Source temperatures were lowered to 55° for some unstable materials. Heptacosa- flourotributylamine was used to calibrate the spectra.

II. Starting Materials

A * B 2H 6 Diborane(6) was purchased from the Gallery Chemical

Company and was used without further purification.

B. B5H 9

Pentaborane(9) was purchased from the Callery Chem­ ical Company and was used without further purification.

C * B 6H 10

Hexaborane(lO) was prepared via reaction of B2Hfi with K(l-BrB5H7) in (CH2)20 according to the reaction of

Brice, Johnson and Shore.The hexaborane was purified via fractionation until the vapor pressure at 0° was

7.5 mm. and was stored in a glass vessel with a teflon stopcock at -196°. 52

D. 2-CH3BgH9

2-Methylhexaborane(10) was prepared via reaction of

BjHg with K(l-Br-2-CH3B,jHg) in (CH3) 20 according to the 56 reaction reported by Brice, Johnson, and Shore. 2-Methyl- hexaborane(lO) was fractionated until the vapor pressure at

0° was 6 mm. and was stored at -78° in a glass vessel with a teflon stopcock.

E. 1,2,3,4-B10H10D4

Tetradeuterodecaborane was prepared by bubbling DC1 through a solution of 5.0 g biqH14 60 ml of CS2 to which 5.0 g of A1C13 had been added. The DC1 pressure was maintained at 60 mm above ambient pressure using a mercury bubbler. Reaction was carried out at 27° for 4 hours. The solution was filtered and CS2 removed by a rotary evaporator. The remaining solid was sublimed at

45°.

F. HCl

Hydrogen Chloride was obtained from Matheson, Cole­ man and Bell and used directly from the cylinder.

<3. PCI

Deuterium chloride was prepared via reaction of

S0C12 and P20 99.76% and stored in a stainless steel cylinder. 53

H. NH3

Ammonia was obtained from Matheson, Coleman and

Bell and was dried over Na/K amalgam and stored in a glass vessel at -78°.

I. KH

Potassium hydride was obtained from KOC/RIC as a

50% suspension in mineral oil. The oil was removed by repeated washing with pentane, and dry KH was stored in the glove box. Potassium hydride activity was determined via methanolysis and measurement of the evolved H 2. The

KH was 96-97% active.

J-

Tetrabutylammonium Iodide, tetrabutylammonium bro­ mide, and potassium chloride were obtained from Matheson,

Coleman and Bell and were used without further purifica­ tion.

K. (C6H5)3PCH3I

Triphenylmethylphosphonium iodide was generously supplied by R. K. Hertz.

L. Fe2 (CO)9

Diiron enneacarbonyl was made according to the method of Braye and Hubei;1®® thirty ml of Fe{C0)3 was 54 irradiated in 220 ml glacial acidic acid for 6 hours under nitrogen. The product was filtered and washed with water, ethanol and pentane; dried, and stored under Ng in a glass vessel wrappecl in aluminum foil to exclude light.

M. p-Fe(CO)4-B6H10

Tetracarbonyliron(O)-hexaborane(10) was prepared ac­ cording to the method of Davison, Traficante and Wre- 83 84 ford. ' An excess of hexaborane was distilled onto

Fe2 (C0)g in a sublimator. Pentane was distilled in and the mixture was stirred at room temperature for 12 hours or until all FegtCOjg was dissolved. Solvent, excess hexaborane and Fe(CO)5 were distilled away and the product was sublimed at room temperature. The yellow solid was stored under vacuum at -78°.

N. Solvents

Diethyl ether, tetrahydrofuran, methylene chloride, chloroform and pentane were dried over LiAlH^ and stored in glass bulbs with teflon spotcocks. Dimethyl ether was dried over KH and stored at -78° in a glass bulb with a teflon stopcock. Benzene for molecular weights was dried and stored over molecular sieve in a glass vessel with a teflon stopcock. 55

III.. Analytical Procedures

A. Hydrolyzable Hydrogen

The material to be analyzed was weighed into a glass vessel fitted with a seal-off constriction and a break-tip. The vessel was evacuated, and degassed meth­ anol was distilled in at -196*. One ml. of liquid HC1 was then distilled in and the vessel was sealed with a torch.

The solution was heated over a steam bath for 24 hours and then reattached to the vacuum line using a tip breaker. After the vessel was cooled to -196* and the break-tip was broken, the non-condensible gas was meas­ ured in the toepler system. Equation 13 gives the general formula for the acid methanolysis of a boron hydride.

H + + 3nCH-0H — ---> nB(OCH,K + m+3n/2 H- (14) n m 3 3 3 i

Thus would yield 22 equivalents of hydrogen gas.

B. Iodine A sample of the material to be analyzed was weighed into a test tube along with a small quantity of clean sodium metal. The test tube was heated over a Bunsen bur­ ner until the sodium metal melted. Distilled water was then added slowly until the excess sodium had completely reacted. The resulting clear solution was diluted with 5 distilled water to a convenient volume and filtered. A

solution of 1M AgNO^ was added dropwise until precipita­

tion of Agl was complete. The precipitate was filtered and dried at 100° and weighed.

IV. Reactions of NH3 with B1QH14

A. Deprotonation of B10H14

Decaborane was allowed to react with equimolar quan­ tities of ammonia in ethereal solvents at low temperatures resulting in the formation of B^qh 13* A 0.105 g quantity of B^qH14 (0.85 mmoles) was weighed into a reaction tube fitted with an nmr side arm and a teflon spinbar. The tube was fitted with a stopcock adaptor and evacuated on the line. Tetrahydrofuran (2.0 ml) and 0.86 mole of NH3 were distilled in at -196°. The O vessel was warmed to -78 and stirred for several hours.

Within minutes the solution became bright yellow, indica­ tive of B^q H13-. An nmr sample was tipped into the side- arm and sealed off with a torch.

Solvent was removed by pumping at -20°. The bright yellow gum obtained would not melt, but in the temperature range 60-80° the yellow color disappeared and colorless needles formed which then melted at 99-100°.

The boron-11 nmr was obtained from -40° to + 15°.

At low temperature there was a broad resonance centered 57 near 0 ppm. and a well-defined doublet at -35.4 ppm. At

+15° (Fig. 26) all signals sharpened. The upfield doublet moved to +36.1 ppm. and the downfield resonance consisted of an asymmetric triplet at -3.7 ppm. (J=128 hz) and a symmetrical doublet at +5.3 ppm. (J-135 hz).

An authentic sample of KB^qH^ was prepared by react­ ing 0.21 g B1qH14 (1.72 mmoles) with 0.07 g KH (1.75 mmoles) in 5.0 ml tetrahydrofuran at -78°. Hydrogen evo­ lution was monitored and reached 98% of the calculated value within 20 minutes. A clear yellow liquid remained and was sealed off in an nmr tube.

The boron-11 nmr of Kbiqh13 (Fig. 27) was qualita­ tively similar to that found for the reaction of ammonia with decaborane, and chemical shifts differed by less than

0.5 ppm.

A similar reaction of NH3 and biqb14 was carried out in diethylether and proceeded in a similar manner, but ammonia failed to react with decaborane in methylene chlor­ ide at -78°.

B. Reaction of b^qb^4 with Excess NH^

In a typical reaction, 0.09 g biqh14 (°*72 mmole) was weighed into a reaction tube which was then evacuated followed by addition of 8 ml tetrahydrofuran and 1.44 mmoles NH^. The mixture was warmed to -78° for one hour and stirred. A bright yellow solution was obtained. The 58

reaction vessel was then wanned to room temperature and stirred for 25 hours. At the end of this period, the solution was colorless and a white precipitate had formed. 4 Non-condensible gas had evolved and was measured to be

0.65 mmoles. The solvent was then pumped through a trap maintained at -78° to a trap maintained at -196°. No material was found in the -196° trap and the liquid at the -78° trap was determined to be free of ammonia by infrared analysis. The white precipitate was insoluble in THF,‘ ether, acetonitrile, dioxane and water. In an acetone-water mixture the material dissolved with evolu­ tion of gas.

Ammonia was then allowed to react with decaborane in a 3:1 mole ratio. A 0.14 g sample of (1.22 mmoles) was stirred with 3,7 mmoles NH3 in 5 ml THF at room temperature for 24 hours. The initial yellow color disappeared completely in 10-12 hours and was replaced by a white precipitate. The quantity of H2 evolved was 1.2 mmoles, and ammonia was separated from the solvent by pumping through a trap at -78° to one at -196°. In this way 0.53 mmoles of ammonia were recovered and identified by infrared.

The infrared spectrum of the white precipitate,

(nh3)2b 10H12 9*ven Figure 16. Have Number (CM-1) 1000 800 600 3000 2500 2000 1800 1600 1400 1200 ■ - «~ i T TT "I -I

Figure 16: Infrared Spectrum of B^qHj2*2NH3 60

The boron-11 nmr of (NH3)2BioH12 (p*9ure was determined in dimethylsulfoxide. The spectrum consisted of a broad singlet of area 1 at +8.6 ppm. an irregular singlet of area 6 centered at +21.0 ppm. and a sharp doublet of area 2 at +42.0 ppm. Upon irradiation at the

frequency the high-field doublet collapsed to a sing­ let; then low-field resonance sharpened and the large irregular singlet had a shoulder.

C. Metathesis of NH4B1QH13 with {C4H9>4NI

In a reaction tube fitted with an nmr tube and a solid addition sidearm 0.15 g BiOH14 nunoles) was added. In the sidearm, 0.454 g (C4Hg)4NI was added. The apparatus was assembled and evacuated. A 0.5 ml quan­ tity of THF was distilled in along with 1.21 mmoles NH^.

The reaction mixture was stirred at -78° for 2 hours and the ether mixture was cooled to -196°. Approximately

2.0 ml of CH2CI2 was then distilled in and the (C4Hg)4NI was tipped in. The mixture was then rewarmed to -78° and stirred for 2 hours with the appearance of a white precipitate.

The precipitate was allowed to settle and an nmr sample of the supernatant liquid was sealed off. The boron-11 nmr spectrum was identical to that previously given for the reaction of biqH14 a^d NH3* At +15° a downfield assymetrical triplet of area 4 was centered 61

Figure 17: 32.1 Mhz. Boron-11 nmr Spectrum of 3iqh12* (a) Undecoupled (b) Decoupled 62

Table 2: X-Ray Powder Diffraction Data for B^oH12* 2

0 d-Spacinq (A) Intensity

6.66 VW

6.02 W

5.75 VS

5.16 H

4.98 W

4.65 VW

4.47 s 3.87 M

3.54 H

3.38 M

3.01 W 2.83 w 2.66 VW

2.44 VW

2.17 VW 63 near -3.8 ppm., a sharp doublet of area 4 at +5.0 ppm.

and a doublet of area 2 at +36.4 ppm. were also seen.

The reaction vessel was warmed to room temperature 4 and the contents filtered in the air. The white precip­

itate was washed several times with CHjClj and dried

and weighed. The precipitate was identified by X-ray

powder pattern to be NH^I (Calculated 0.175 g; found

0.168 g, or 96.4%). The yellow filtrate was dried in

vacuo but solvent removal could not be completed. A

paler yellow, wet, solid remained even when warmed to

50° under vacuum.

V. Reaction of B^0H14 with tC4Hg)4NI and (CgHgJ.jPCH.jI

A. Reaction of biqh14 with (C4Hg)4NI

During the metathesis of n h 4b i o h13 tetrabutyl- ammonium iodide, it was noticed that if the two solids were mixed, a yellow color began to appear in the solid

state. Addition of solvent resulted in the formation of

a bright yellow solution.

Decaborane and tetrabutylammonium iodide was placed

in equimolar amounts in a reaction tube and the vessel

was evacuated. Tetrahydrofuran was distilled into make 0 a 0.5M solution and the mixture was stirred at -78 .

The solution became bright yellow in a matter of minutes.

A similar reaction was carried out in ether and

another in methylene chloride. In all cases, similar 64

results were seen. Boron-11 nmr samples were prepared.

The spectra, run at 32.1 Mhz. in THF showed an irregular

triplet o£ area 4 at -11.7 ppm., a symmetrical doublet

of area 4 at +2.45 ppm. and a doublet of area 2 at +

33.4 ppm.

B. Reaction of 1,2,3,4-B^q H j^q D^ with (C^Hg)^NI and (C6H5)PCH3I, Preparation of ((CgH5)3PCH3)(B1QH14I)

To investigate the nature of the reaction between

tetrabutylammonium iodide or tetriphenylmethylphosphon-

ium iodide reactions were carried out using decaborane

deutered in the 1,2,3, and 4 positions.

The reactions were carried out by stirring equi-

molar quantities of the reactants in enough tetrahydro-

furan to make 1M solutions. All reactions were carried out at room temperature and were stirred for 2-3 hours

before sealing off nmr samples. The boron-11 nmr of the

two compounds were identical, with only minor differ­

ences in chemical shifts. The spectra are shown in

Figures 29 and 30 and compared with the spectrum of 1,2,

3,4-BiqHiod4 (Figure 28). A high resolution spectrum of

[ (C4Hg) 4n] [B^qHjlqD4I1 at 80.2 Mhz. is shown in Figure 31.

.Additional reactions of decaborane with KI on

(C^H^)^ NBr showed no sign of reaction and the boron-11

nmr spectra were identical to that of decaborane. 65

In order to isolate the solid product of the reac­ tion Of B1(JH14 and (CgHg) 3PCH3I, 0.06 g B10H1QD4 (0.5 nmoles) and 0.20 g (CgHg)gPCHgl (0.5 mmoles) were placed in a vacuum extractor and 0.5 ml CH2Cl2 was distilled in. The mixture was warmed to room temperature and stirred for several hours. After cooling the solution to -196°, 0.5 ml of n-pentane was distilled in. The solution was then warmed to -78° and stirred vigorously.

A yellow precipitate was formed and this solution was filtered at -78° under nitrogen. A clean receiver was attached and the precipitate was dried by pumping at room temperature to form a bright yellow free-flowing powder. The precipitate, formulated as ((CgHg)gPCHg)

<®10H10D4I>' was washed with portions of n-pentane at room temperature, dried and any excess decaborane was removed by sublimation at 40°. Only trace amounts of decaborane appeared on the cold finger.

This material when redissolved in tetrahydrofuran, gave a boron-11 nmr spectrum identical to that obtained previously from the reaction of (CgHg)gPCHgl and

B10h I0D4* The elemental analysis of Schwartzkopf Microanaly- tical Laboratories fit a formulation of [(CgHg)gPCHg]

lB10H10D415* Calculated: B, 20*42; C, 43*0; H, 6*79; I, 23*95; P, 5*84. Found: B, 20*64; C, 42*44; H, 5*91;

I, 22*86; P, 5.85. 66

The X-ray powder pattern of [(CgHg)3PCH3][B^qH^q

D^X] presented in Table 3 shows no lines attributable

to (C6H5)3PCH3I or B10H1qD4 .

The infrared spectrum (Figure 18) has the follow­

ing bands attributable to B^qH^q D^I- : 2590 (S), 1930 (M) ,

845 (M) , 822 (W, Sh) , 650 (W), and 555 (VW) cm**1. Bands

assigned to (CgHg)3PCH3+ are: 3050 (W), 3005 (W), 2992

(Sh), 2900 (W), 2940 (H), 2889 (M), 1589 (M), 1482 (M),

1439 (S), 1397 (M), 1337 (W), 1319 (W), 1162 (M), 1111

(S), 1098 (Sh), 994 (M), 921 (M), 904 (S), 784 (M), 750

(Sh), 741 (S), 718 (S), 688 (S), 509 (SH), 503 (S), 478

(M), 431 (W), and 373 (W) cm -1.

The visible spectra showed two absorptions at 265

and 335 my; the peak at 265 my being approximately twice

as intense as the one at 335 my. Absorption coefficients were not measured. Visible spectra obtained in CHClg,

CH2Cl2f tetrahydrofuran, ethyl ether and N,N-dimethyl-

formamide showed no changes in the position of the ab­

sorption.

Single crystals of [ (CgHg) 3PCH3J [B^qH-^qD^I] were

grown by placing ca. O.lg of the powder in a vial. After

addition of 1 ml. n-pentane, methylene chloride was

added dropwise with agitation until a pale yellow liquid was formed. This saturated solution was carefully pip-

petted into another vial, sealed and placed in a freezer

at -20° for several days, after which pale yellow Have Number (CM ) 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 800 . 400

Figure 18: Infrared Spectrum of [ (CgH5) 3PCH3] fB10H1(JD4I] crystals appeared.

The crystal density was determined by floatation

in CCl^ and n-CgH14 and was found to be 1.3315 g/cm . 0 3 The cell volume was 670A giving a molecular weight .

(Z-l) of 536 (calculated for [ (CgHg) 3PCH3] [B10H1(JD4I],

530). The X-ray structure showed the crystal to be in the triclinic system and the structure was determined with an R-factor of only 0.31 due to broad, poorly- shaped diffraction peaks. The B5-10 distance was found O to be 3.6A and the iodine atom was symmetrically placed above and between B6 and B9 at a B6-I distance of ap- O proximately 4.8A. Hydrogens were not located because of the poor resolution.

Excess potassium hydride reacted with [ (CgH,-) .jPCHg]

Ib10H1qD4I1 tetrahydrofuran to give guantiative yields of hydrogen. The resulting solutions were tubid. Fil­

tration under nitrogen yielded a clear yellow solution

and the precipitate was identified as KI by X-ray powder diffraction analysis.

The boron-11 nmr of the resulting solution is shown

in Figure 19. The most striking feature of this spec­

trum is the splitting of the resonances for Bl,3 and

B2,4. Mass spectral analysis of the evolved non-conden-

sibles showed mainly H2 and only trace amounts of HD or . Table 3: X-Ray Powder Diffraction Data for [ (CgHg)3PCH31 [®io h i o d 41^ o o d-Spacing (A) . Intensity d-Spacing (A) Intensity

11.79 W 3.95 W S

10.32 W 3.74 W

8.71 VS 3.62 W

8.10 S 3.47 S

7.26 M 3.18 H

6.37 M 3.06 M 5.87 VS 2.93 W 5.50 w 2.82 H 5.02 w 2.71 VW 4.77 s 2.62 H

4.42 w 2.52 VW 4.09 s 2.44 w

01 M> 70

C. Reaction of I (C4Hg)^N][B^qH14IJ with Excess NH^

In a typical reaction 0.221g biqH14 (1*75 mmoles) and 0.646g (C^Hg)4N I (1.75 mmoles) were weighed into a reaction flask which was attached to an extractor and evacuated. Approximately 3 ml. of THF and 5.0 mmoles of NH^ were distilled in and the mixture was warmed to room temperature with stirring. The reaction was al­ lowed to proceed for 12 days and the evolved non-conden- sibles were periodically measured by toeplerization.

Initially the reaction mixture was a clear yellow solution, but after five days the yellow color had com­ pletely disappeared and was replaced by a white precipi­ tate. After five days 0.82 mmoles of hydrogen has been evolved, and after ten days a total of 1.3 mmoles were collected.

After 12 days the solution was filtered and 14 mmoles HBr was added to the filtrate. The resulting

NH^Br was isolated, dried and weighed. Analysis was by

X-ray powder diffraction (Found: 1.27 mmoles or 91% based on the hydrogen evolved).

The precipitate from the reaction NH3 with biqh14i" was filtered and washed with ether, hexane, and water.

The material was insoluble in most solvents but dissolved in acetone-water or in methanol with decomposition

(evolution of hydrogen). Iodine analysis of the white 71

Figure 19: 32.1 Mhz. Boron-11 nmr Spectrum of [ (CgH5)3PCH3]

tB10H13] 72

powder yielded 84.6 mole % I based on a formula

I(c4h9)4n ][b10h12i•2nh3].

*

VI. Studies of p-Fe(CO)4-BgH10

A. Deprotonation of p-Fe(CO)4_BgH^o

In order to generate the ion p-Fe(CO)4-BgHg~,

0.24 g p-Fe(CO)4-BgH10 (1 mmole) and O.OSg KH (1.25 mmoles) were weighed into a reaction vessel in the glove box. The vessel was attached to an extractor and the apparatus was evacuated on the vacuum line. Dimethyl ether (1.0 ml) was distilled in and the vessel was warmed

to -78°. A vigorous reaction commenced which subsided in about 5 minutes leaving a slightly turbid yellow solu­ tion. After stirring for one hour the hydrogen was toep- lerized and 0.95 mmoles were collected (95% yield).

Filtration of the solution at -78° gave a clear yellow-brown solution, and an nmr sample was obtained.

Boron-11 nmr spectra were obtained in the tempera­ ture range -50° to +30° (Figure 35).

Teracarbonyliron(0)-hexaborane(10) is deprotonated by KH in diethyl ether only above -20°, and complete reaction requires approximately one hour when run on a 1 mmole scale. 73

B. Relative Acidity of BgH^Q and y-Fe(CO)4-BgH^0

Tetracarbonyliron(o)-hexaborane(10) was deproton- ated in methyl ether and an equimolar quantity of BgH^Q was added. The boron-11 nmr of the reaction mixture was studied.

A 0.24g sample of y-Fe(CO)4-BgH1Q (1 mmole) and. exactly 0.04g KH were weighed into reaction vessel and were stirred at -78° in (CH^^O (1.0 ml) for one hour.

After removal and measurement of the hydrogen 1.0 mmole of was distilled in and the solution was stirred for an additional hour.

Boron-11 nmr spectra at +20° {^H decoupled) ex­ hibited a large singlet at -13.9 ppm., small singlets at

-9.4, -2.5, and +52.0 ppm (Fig. 37).

In a second reaction 0.24g y-Fe(CO)4-BgH^Q was placed in an addition sidearm equipped with a teflon stopcock and was attached to a reaction vessel contain­ ing 0.04g KH. After evacuation 1.0 mmole BgH1Q and 1.0 ml (CH^)2° were distilled in. Warming to -78° resulted in rapid deprotonation of BgH^. When the solution was clear the hydrogen was removed and measured (Found: 0.98 mmoles), and the solvent was removed by pumping at -78° until only a white powder remained. The vessel was then warmed to room temperature with pumping to insure complete removal of excess BgH10- The solvent was then replaced 74 and the material in the addition tube was washed into the reaction mixture. The resulting yellow solution was stirred for one hour at -78°.

The boron-11 nmr (^H decoupled) of this solution consisted of a large singled at -14.3 ppm., and smaller singlets at -10.4, -3.9, and +52.2 ppm (Pig. 37).

C. Addition of B 2Hg to K+ [p-Fe(CO)4-BgH9“ ]

A measured quantity of the iron compound was quan­ titatively deprotonated in (C2Hg)20. The resulting solu­ tion was filtered in an extractor at low temperature.

After cooling to -196° the reaction vessel was removed under N2 and attached to a titration apparatus (Figure

15). Diborane was added to increments and the reaction warn stirred at -78°. Pressure was monitored at 10 min­ ute intervals and when no pressure change occurred after two successive readings the pressure was recorded and another increment of B2Hg was added. The process was repeated until a large excess of diborane had been added.

A plot of the pressure vs. [B2Hg]/[u7Fe(CO)4-

BgHg~] is shown in Figure 20.

D. Preparation of K+ ly-Fe(CO)4-b7H12 ]

In a reaction vessel attached to an extractor was placed 0.37g u-Fe(CO) -B 6H10 (1.52 mmoles), and 0.15g KH

(3.6 mmoles). The extractor was evacuated and 1.4 ml 50

40 "o* X £30

20

Cl to

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Solvent ((^Hg^O Mole Rotio [BjHgJ/K'L-FetCO^-BgHgl

Figure 20: Tensimetric Plot of Addition of B2Hg to K+ Ip-Fe(CO) cn 76

(CD3^2° were distilled in; reaction commenced after warm­ ing to -78°. When reaction ceaBed the hydrogen was re­ moved and measured (Foun$: 1.52 mmoles).

The solution was filtered at -78°, and 0.75 mmoles of BjHg were distilled in. The solution was then stirred for another three hours.

The boron-11 nmr spectrum of K+ Iy-Fe(C0) ^ByH^"*! decoupled) is shown in Figure 38. Proton nmr spectra in the bridge region are presented in Figure 39.

E. Preparation of [ (C4Hg)4n][y-Fe(CO)4BgHg] and [(C4Hg)

4N] [u.-Fe(CO)4-B7H12]

A metathesis reaction of tetrabutylammonium iodide and the potassium salt of the iron tetracarbonylnonahydro- hexaborate(-l) ion was carried out. The resulting mater­ ial was isolated and converted to the corresponding By salt by addition of diborane.

In a typical reaction one mmole quantities of y-Fe (CO) 4“BgH^Q and KH were reacted in 1 ml. (CH^^O.

After removal and measurement of the hydrogen 6.0 ml.

CH2Cl2 was added and slightly less than one mmole

(C4Hg)4NI was added from a sidearm. The solution was stirred at -22° for 1/2 hour to produce a turbid brown solution. This solution was filtered at -78° and the precipitate was identified as KI by X-ray powder dif­ fraction. The yield was essentially quantitative based 77

on the tetrabutylammonium iodide.

To the clear brown filtrate was added ca. 8 ml.

^C2H5^2°* uP°n standing at -78° for 6 hours red-brown crystals formed. Transfer to another extractor under N 2 followed by filtration at -78° yielded 0.215g (or 69%)

[(C4h9)4N][y-p e (c°)4“BgH9] (MW= 312.7). In another reaction employing 1.8 mmoles of the starting iron borane complex the yield was 73%.

The infrared spectrum of [ (C^Hg) 4N] [ji-Fe (CO) 4-

BgHg] is shown in Figure 21.

Boron-11 nmr spectra in dimethyl ether were iden­ tical to those for the potassium salt.

Diborane was added tensimetrically to the tetra- butylammonium salt in CH2C12 in a manner previously des­ cribed (section C). A plot of the pressure vs the mole ratio is shown in Figure 22.

Following the titration an amount of ether equal to twice the volume of the CH2C12 was added and tan crystals of f(C4H9)4N] [jj-Fe(CO)4-B?H12J appeared upon standing at -78° for several hours. The precipitate was filtered and washed with ether at low temperature and dried. When 0.279g of the starting Bg salt (0.88 mmoles) was used, 0.190g of the By salt were obtained (MW=326.5) for a yield of 66%.

The infrared spectrum of [(C4Hg)4N][y-Fe(CO)4~

B 7 H1 2 ] is shown in Figure 23. Wav* Nuaber (caT1) 3500_____ 300D 3 £ £ 2 ___ 22°fi___ ISpS__ i6yo 1^00 lyo injm npn Ktjn

Figure 21: Infrared spectrum of [(C^Hg)^N][y-Fe(CO) 79

Table 4: X-Ray Powder Diffraction Data for [(C^HgJ^N] [p-Fe(CO)4-B6H9]

o d-Spacing (A) Relative Intensity 12.63 H 9.82 VS 8.54 w 7.72 s 6.93 w 5.81 s 5.22 s 4.91 M 4.60 s 4.37 H 4.21 W 3.60 s 3.50 VW . 2.84 VW 6 0

50

4 0

3 0

CO CO 220

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Solvent CH2CI2 Mole Ratio [ b2H ^/ £N(C4H^(tt-Fe(C0)4-B6Hj

Figure 22: Tensimetric Plot of Addition of B2Hg To I(C4Hg)4N][y-Fe(CO)4B6Hg ] oo o H a w Number (CH~*) 4000 3500 3000 2500 2000 1600 1600 1400 1200 1000 800 600 400

Figure 23: Infrared Spectrum of t(C4Hg)4H] [(i-Pe(C0)4 - B?H12J 82

Table 5: X-Ray Powder Diffraction Data for [(C.HO.Nl [M-Fe(CO)4-B7H12] *

o A o d-Spacing {A) Intensity d-Spacing (A) Intensity

11.95 W 3.92 W

10.53 w 3.60 W 8.67 vs 3.45 S 8.00 s 3.18 H 7.28 w 3.02 M

6.39 M 2.93 W

5.84 VS 2.84 M

5.48 M 2.70 VW

5.07 W 2.63 M

4.77 S 2.20 VW

4.44 M 2.16 w 4.26 S 1.98 M 83

Boron-11 spectra (*H decoupled) in CH2C12 were identical to that reported for the corresponding potas- # sium salt.

Single crystals of I(C4Hg)4N][u-Fe(CO)4-B7H12I were grown by diffusing ether onto a 0 .1M solution of the salt in CH2C12 at -78°. After 48 to 96 hours clear, pale- yellow crystals were formed. The mother liquor was de­ canted and the crystals were mounted in capillaries at low temperature.

The crystal and molecular structure were determined by Dr. W.R. Clayton. Data were collected at -50°, how­ ever thermal decomposition at this temperature occurred over a period of two weeks.

Crystals were of the space group P2 yQ i aaio.891(5), b=ll.656(6), c= 23.803(9) A, 0= 90.34(1)°, Z=4. A conven­ tional R-factor of 0.11 was obtained and there was partial disorder in the cation.

Upon standing at room temperature for a few hours the crystals become red-orange and give an infrared spec­ trum identical to that for the corresponding Bg salt.

F. Preparation of Fq (CO) 4B 7H ^

Addition of HC1 at low temperature to K+ [p-Fe(CO)4~ b6H9~^ resulted in evolution of hydrogen and produced a species formulated as Fe(CO)4B7H^^. 84

In a typical reaction 0.175g y-Fe (CO) 4**BgH^0

(0.72 mmoles) was deprotonated by an equimolar .amount of

KH in 1 ml. of (CH3)20. The solution was filtered at

-78° after reaction was completed. To the filtrate 0.36 mmoles B2Hg was distilled in and the solution was stir­ red at -78° for several hours.

Solvent was removed by pumping at -78° until no further solvent was observed to come off. The vessel was briefly warmed to room temperature and the last traces, of solvent were removed. The solvent was anal­ yzed by infrared spectroscopy and methanolysis for the presence of diborane, but none was detected.

Approximately 1 ml. of HC1 was condensed onto the solid K+ [y-Fe (CO) an^ vessel was warmed to

-110°. Rapid evolution of H2 commenced and was completed in ca. 5 minutes (found: 0.58 mmoles, 80.5%). Yields of hydrogen varied from 67 to 88% in similar reactions.

Volatiles were removed by pumping at -110° to remove excess HC1, and at -78° for other volatiles. Analysis for B2Hg yielded negative results. KC1 was isolated and identified by X-ray powder diffraction.

The red-brown solid that remained melted at 20-25°.

• The infrared spectrum of Fe (CO) 487**^ shown in

Figure 24. Wave Number (CM~*)

3500______3000 2500______2<]00 1800 1600 IjOQ If 00 y m n ann fipn

Figure 24: Infrared Spectrum of Fe(CO)4B7H11

00 in 8 6

The mass spectrum was obtained by direct inser­ tion of the sample in the beam with the source tempera- ture lowered to 55° and is displayed in Table 6. The cutoff occurred at m/e«255 (calculated for ^®Fe^*2C4^ 0 4

*= 255) . Envelopes characteristic for nido boron hydrides appeared at 28, 56, and 84 mass units be­ low the parent envelope. The largest peak in each en­ velope appears at 3 mass units below the envelope cut­ off.

Boron 11-nmr spectra of FetCOJ^B^H^ (*H decoupled) is shown in Figure 44. The area ratio of the upfield singlet to the downfield multiplet was 1:6.2.

G. Preparation of y-Fe(CO)4-2-CH3BgHg

In a typical reaction 0.847g Fe2 (CO)g (2*33 mmoles) was stirred at room temperature with 4 mmoles 2-CH^BgH^ in 5 ml. CH2C12 in a sublimator. After 2 hours a clear brown solution was formed. Solvent, excess 2-CH^BgHg and Fe(CO)g were distilled away at -30° and at 0°. Red- brown crystals of y-Fe (CO)4-2-CH3BgHg were sublimed and stored at -78° under vacuum (yield: 0.48g; 79.9%). The melting point of y-Fe(CO)4-2-CH3BgHg was 30-32°.

The infrared spectrum is presented in Figure 25.

Decomposition was noticeable after 1/2 hour at room tem­ perature when exposed to the air and was essentially complete after 6 hours. 87

.Table 6 : Mass Spectral Data for Fe (CO) :

Inten­ Inten­ Inten- m/e sity m/e sity m/e sity

257 0.2 202 0.2 158 23.1 256 0.2 201 0.2 157 9.3 255 0.3, 200 2.2, 156 2.6 254 0 .4 199 3.8 155 0.8 253 0.2 198 2.9 154 0.3 252 0.05 197 1.5 153 0.3 251 0.03 196 1.8 152 0.2 250 0.05 195 0.3 151 0.6 194 0.1 246 0.1 193 0.4 149 0.1 245 0.4 192 0.1 148 0.4 244 191 0.1 147 0.4 3 ' 3 a 243 4 .3 190 0.2 146 0.3 242 3.0 145 1.7 241 1.2 189 0.5 144 5.9 240 0.4 188 6.0 143 9.8 239 0.1 187 8.9 142 8.9 238 0.4 186 6.0 141 6.2 185 2.5 140 7.3 232 0.1 184 0.9 139 5.8 231 1.1 183 0.3 138 6.9 230 0.03 183 0.2 137 7.0 229 0.2 136 6.3 228 1 -9« 177 0.1 135 4.8 227 • 2.9 176 0.05 134 3.5 226 2.5 175 0.1 133 2.8 225 1.0 174 0.1 132 48.6 224 0.4 173 0.4 131 71.4' 223 0.1 172 6.2 130 51.4 171 10.0a 129 28.6 218 0.1 170 8.5 128 28.9 217 0.4 169 5.5 127 23.4 216 4 .9_ 168 4.1 126 28.6 215 7.3 167 0.5 125 24.5 214 ’ 5.0 166 0.4 124 19.6 213 2.2 • 165 0.2 123 16.9 212 0.4 122 17.5 211 0.2 163 0.3 121 12.3 210 0.05 162 0.6 120 7.0 161 1 -1» 119 3.0 204 0.2 160 23.6a 118 12.4 203 0.2 159 11.7 117 3.4 88

Table 6 : (Continued)

Inten­ Inten­ Inten­ m/e sity m/e sity m/e sity

116 3.8 88 1.2 60 1.7 115 8.4 87 0.5 59 8.8 114 12.3 86 0.3 58 15.8 113 22.2 85 0.3 57 17.2 112 25.1 84 4.5 56 17.1 111 16.9 83 34.3a 55 100.0a 110 7.5 82 2.8 54 11.1 109 2.9 81 1.9 53 6.3 108 1.0 80 1.9 52 3.0 107 0.6 79 1.9 51 1.3 106 0.4 78 1.2 50 3.3 105 0.4 77 0.6 49 2.4 104 0.7 76 0.4 48 0.8 103 13.4a 75 15.2 47 2.9 102 15.8a 74 24.3 46 4.4 101 10.8 73 25.7 45 4.8 100 7.0 72 31.4 44 3.0 99 2.3 71 40.0 43 2.0 98 1.0 70 51. 4a 42 2.2 97 0.4 69 38.6 41 1.2 96 0.3 68 28.6 40 1.5 95 0.3 67 24.5 39 0.2 94 0.7 66 23.9 38 0.8 93 1.0 65 22.2 37 0.3 92 2 *9a 64 13.4 36 0.7 91 2 .4 63 4.7 35 1.1 90 2.9 62 1.0 34 1.8 89 2.5 61 1.2 33 1.3

Most intense peak in the envelope Nava Nuaber (CM-^)

4000 3500 3000 2500 2000 1800 1600 1200 1000 800 600 4001400

Figure 25: Infrared Spectrum of y-Fe(CO)

oo VO 90

Table 7: Mass Spectral Data for y-Fe(CO) ^-2-CH^BgK^

Inten­ Inten­ Inten­ m/e sity m/e sity m/e sity

260 0.3 214 0.3 171 16.0 259 0.4 213 0.1 170 5.1 258 3 *9a 212 0.1 169 1.5 257 5.3 211 0.1 168 3.9 256 3.6 210 0.1 167 0.3 255 1.9 209 0.1 166 .0.4 254 0.5 208 0.1 165 0.5 253 0.2 207 0.2 164 0.3 252 0.1 206 0.3 163 0.7 205 0.4 162 0.5 250 0.1 204 0.4 161 0.6 249 0.1 203 1.0 160 1.0 248 0 •1 202 12.9 159 1.2 247 0.3 201 18.2a 158 1.0 246 0.2 200 11.9 157 1.0 245 0.3 199 5.3 156 3.4' 244 0.2 198 2.2 155 0.5 243 0.2 197 0.7 154 0.4 242 0.1 196 1.9 153 0.3 241 0.1 195 0.1 152 0.2 • 194 0.1 151 0.6 236 0.1 193 0.2 150 0.6 235 0.2 192 0.3 149 0.8 234 0.3 191 0.4 148 0.7 233 0.4 190 0.3 147 3.4 232 0.3 189 0.4 146 62.2 231 0.7 188 0.6 145 77.0 230 11.2 187 0.7 144 66.2 229 15.0a 186 0.6 143 37.8 228 9.7 185 0.4 142 23.7 227 4.4 184 0.3 141 18.7 226 1.0 183 0.2 140 15.8 225 0.4 182 0.1 139 12.6 224 0.1 181 0.1 138 8.7 180 0.2 137 6.8 222 0.1 179 0.4 136 5.8 221 0.1 178 0.4 135 4.8 220 0.2 177 0.6 134 2.9 219 0.3 176 0.5 133 1.4 218 0.4 175 2.3 132 4.1 217 0.7 174 32.4 131 4.8 216 0.3 173 47.3a 130 6.3 215 0.3 172 32.4 129 7.8 91

Table 7: (Continued)

Inten­ Inten­ Inten­ m/e sity m/e sity m/e sity

128 14.8 96 1.5 64 1.7 127 18.4a 95 2.9 63 1.5 126 15.5 94 1.0 62 6.3 125 11.6 93 1.2 61 7.3 124 8.5 92 3.2 60 13.6 123 6.1 91 3.4 59 22.6 122 3.9 90 45.9 58 21.8 121 1.7 89 72.9 57 20.3a 120 1.0 88 70.3 56 24.2a 119 1.2 87 60.8 55 12.1 118 9.2 86 81.8_ 54 4.4 117 9.7 85 100.0a 53 2.4 116 10.7 84 89.2 52 1.0 115 12.4 83 78.4 51 1.0 114 13.6a 82 54.0 50 1.5 113 11.6 81 29.7 49 5.1 112 10.9 80 24.3 48 8.7 111 3.6 79 23.0 47 8.7 110 1.7 78 21.6 46 6.3_ 109 2.2 77 13.3 45 12.6a 108 1.0 76 5.3 44 4.1 107 1.0 75 6.8 .43 6.3 106 1.0 74 23.8 42 3.4 105 1.5 73 21.6 41 6.8 104 11.4 72 28.4 40 1.0 103 13. la 71 33.8 39 2.2 102 8.7 70 36.5a 38 0.5 101 4.6 69 27.0 37 9.2 100 2.2 68 25.7 36 6.1 99 1.0 67 19.9 35 2.4 98 1.2 66 10.7 97 2.4 65 4.6

amost intense peak in the envelope 92

The mass spectrum is shown in Table 7. An exact mass determination of the cutoff peak yielded a value of m/e* 257.0628 {calculated for 56Pe12C5160410B11B51H12 =

257.0675).

The boron-11 nmr spectrum (^H decoupled) is shown in Figure 49. The spectrum in the region 8.5 to 10 t is presented in Figure 50. CONCLUSIONS AND DISCUSSION

I. Reaction of Ammonia with Decaborane

A. Deprotonation of B10H14 167 Ammonia is capable of deprotonating b4hiq# 168 75 BgHg and • Since biqhi4 has been shown to be

b4h10 + nh3 ---- >[nh41[b4h9] (15)

b5h9 + nh3 ---- >[nh4]Ib5h 8] (16)

75 a stronger Brjrfnsted acid than these species, it was reasonable to assume that decaborane might also be depro- tonated by ammonia.

In order to study this possibility, decaborane and ammonia were allowed to react in equimolar amounts at low temperatures. Within minutes of allowing the frozen reactants to warm to >78°, a bright yellow color _ 12 which is characteristic of the biqH^3 ion appeared.

Boron-11 nmr spectra of these yellow solutions confirm the presence of B^qH^”. Comparison with the spectrum of Kbiqh13 (Figures 26 and 27) show a close similarity in the features of the spectra. The close agreement of the chemical shifts (Table 8) also strongly

93 Figure 26: 32.1 Mhz. Boron-11 nmr Spectrum of INH4]IB1QH13]

\0 Figure 27-. 32.1 Mhz. Boron-U nmr Table 8 : Boron-11 Chemical Shifts for biqH 1 4 and B^qH^ a

• Bl,3 B6,9 B2,4 B5,7,8,10

+35.8 B10H14 -11.2 -0.7

-3.7b +35.8 +5.0 raiOH13

[nh4][Biohi3 [ -3.7b +36.1 +5.34

I(c4h9)4n][b10h13] —3.8b +36.4 +5.0

I(c6h5)pch3][b10h13] -3.5b +36.0 +5.1

aall spectra measured in THF

^center peak of an unsummetrical triplet

vo 91 97 supports the presence of similar species. The upfield shifts of all resonances when compared to those of support the presence of the Bi0Hl3” species.

The spectra of biqh13~ ar© relatively uninforma­ tive below 0°, consisting of very broad resonance at low field and a sharp doublet at higher field. However, above 0° the downfield resonance begins to resolve into a series of signals until at +15° the maximum resolution if achieved.

Isolation of [NH^][bioh13^ was not possible* At temperatures below 25° gummy, yellow material remained, and no amount of pumping could remove the remaining sol­ vent. However, if the solid was redissolved, the boron-

11 nmr spectrum was identical to that of the original solution.

If pumping was carried out at elevated tempera­ tures (40° to 60°), the yellow color would disappear with the last traces of solvent and only would remain. Recovery of ammonia from the mixture was es­ sentially quantitative.

The deprotonation of decaborane by ammonia is reversible. This deprotonation can be carried out

HH3 + B10H14 tNH41 [B10H13] (18) 25° 98 in sufficiently basic solvents such as THF, diethyl ether and dimethyl ether, but was not possible in methy­ lene chloride or hexane.

B. Metathesis Reaction of [NH^] (B^gH^l

In order to unequivocally establish the deprotona­ tion of decaborane by ammonia the following metathesis reaction was carried out:

THF/CH Cl [NH4] [b10h13] +(c4h9)4n i ------— NH4I + —78® (19)

[

Reaction was carried out by generating [NH41 IB^qH^] in situ and then adding the tetrabutylammonium salt. The initially clear yellow solution became quite cloudy and the precipitate was quantitatively recovered and iden­ tified as NH4I. The yield of the salt was 96% establish­ ing the quantitative deprotonation of decaborane by ammonia.

The yellow solution of [(C4Hg)4N][B^qH131 gave boron-11 nmr spectra essentially identical to that for

[NH4][b10h13]. Again solvent removal was not complete even at temperatures of 80°. Heating above this temper­ ature resulted in decomposition of the salt and forma­ tion of black tars. C. Reaction of b1 qh 1 4 with Excess NH^

If solutions of [NH^] IB^qH ^ ] were allowed to stand at room temperature for several hours white pre­ cipitates would form accompanied by the evolution of hydrogen.

Accordingly, reactions were carried out in which excess NH^ was stirred with ether solutions of B^qH^4 at room temperatures. Approximately one equivalent of

H2 was evolved in about a day from this reaction. If the ratio of NH^ to Bj.0H14 was 2:1, no ammonia was re­ covered from the system. However, if the 2:1 ratio was exceeded, then ammonia was recovered, but stoichiomet­ ric yields were not achieved.

The product is formulated, based on the stoich­ iometry as and appears to be similar to the 169 material reported by Hough.

That this material is not (NH^) 2 (B^qH-^q) is con­ firmed by the boron-11 nmr (Figure 17) which is qualita­ tively similar to those reported for other B10H12*L2 38 systems.

It seems, then, that B-^q H ^ reacts with ammonia via two pathways. A low temperature reaction results in a reversible deprotonation (equation 18), so long as ammonia is present in 1:1 mole ratio or greater. At ambient temperatures and above, however, the principal 1 0 0

.reaction is adduct formation, and the reaction is irre­ versible .

2NH3 + B10H14 — > B10H12-2NH3 + H2 (20)

This behavior is consistent with that observed for some lower boron hydrides. Tetraborane, as was seen, is 167 deprotonated by ammonia at -78°, but also undergoes 170 cleavage in ether solutions at -45°. Decaborane does not possess a BH2 unit and thus does not undergo cleav­ age, but rather adduct formation.

II. Reaction of B1QH14 with (C4H9)4NI and (CgH5 )3PCH3I

During the metathesis of [NH4] IB^qH^] with (C4Hg)4

HI, it was noticed that if decaborane and the salt were initially mixed, a yellow solution would form prior to addition of ammonia. Since there was no likelihood of dep­ rotonation of decaborane by a guarternary ammonium salt, some other reaction was taking place.

Yellow solutions of the two reactants in 1:1 mole ratio absorbed in the visible region at 267 and 335 mu.

The position of the absorption did not change if differ­ ent solvents were used thus casting doubt on the forma­ tion of a charge-transfer complex. 1 0 1

It was found additionally that (CgH,.) 3PCH3I and

^C6H5*4PI would undergo a similar reaction with but KI or (C^Hg)^NBr showed no evidence of reaction.

Boron-11 nmr spectra of these solutions were gen­ erally not very informative. The spectra showed gross similarities to those of Bj.0h13~ but the chemical shifts were different. The resonance assigned to B2,4 moved downfield from that in B^QH^4 to 33.4 ppm whereas the

B5,7,8,10 signal moved upfield to +2.45 ppm. The center of the triplet moved down field to -11.7 ppm (all values are for THF solutions).

Decaborane deuterated in the 1*2,3 and 4 position was allowed to react with the appropriate salts and the boron-11 nmr spectra obtained were considerably more informative. The signal for B6,9 clearly lies upfield of the singlet due to Bl,3 in decaborane (Figure 28).

However, in the complexes (Figures 29 and 30) this sig­ nal is now downfield of that for Bl,3. The 80.2 Mhz spectrum (Figure 31) clearly shows a doublet arising from borons 6 and 9.

The solid product of the reaction between (CgH5) 3

PCK3I and B^qH^qD^ was isolated and the chemical anal­ ysis fit a formulation of [(CfiH5).JPCH3I]•Ib1qH1qD4]. Molecular weight determination by crystal density and unit cell volume supported this formulation as well.

Figure 29: 32.1 Mhz. Boron-11 nmr Spectrum of [(C^H^)^N]lBioHioD4I]

o u 104

Figure 30: 32.1 Mhz. Boron-11 nmr Spectrum of [ (CgHg)3PCH3] 105

31, 80.2 Hhz. Boron-11 nmr Spectrum of t (C 4H 9 ) 4N ] IB10H10D4 1] Figure Table 9: Boron-11 Chemical Shifts of B^qH ^ I Complexes

Blr 3 B6,9 85,7,8,10 B2,4 I B10H14a -11.3 -11.1 0 • +35.8

B10H13“ -3.7b . +5.0 +35.8 I(c4h9)4n][b10h14x] -11.7b +2.45 +33.4

[ (c6h5)3pch3][b10h14i] -11.7b +2.51 +32.9

I(c4h9 )4n][b10h10d4i] -9.0 -14.5 +2.42 +33.4 a

aFrom Ref 8

bcenter peak of an unsymmetrical triplet 107

The Raman spectra of solid and the com­ plex with (CgHg)are shown in Figure 32. The region of the bridging hydrogen stretches ca. 1800 cm“^ shows marked differences for the two materials. Two strong bands are seen for the bridge stretch in whereas these bands are completely absent in the spec­ trum of the complex.

Chemical evidence supports the interaction of l“ with the decaborane cage. No decaborane could be re­ covered from the isolated solid [ (CgH5 )3PCH3I]•[B1QH14] by physical means. Reaction with ammonia to produce the bis-ammonio derivative of decaborane proceeds at a drastically reduced rate (R1/2 8 5 days) and the iso­ lated product was shown to contain iodine. Furthermore, the isolated product could not be dissolved in any sol­ vent system without decomposition and production of non- condensible gases. Reaction with KH resulted in quanti­ tative deprotonation to form the I(CgH^)3PCH31 [B10H ^3 1 species and the removal of l“ from the cage.

A single crystal study was attempted, but the quality of the crystals was very poor and the refine­ ment of the structure was not adequate to resolve details with a high degree of accuracy. However, some features were noted. The iodide seems to be associated more with the cage than with the cation, lying at approx- o o imate distances of 4.8 A and 6.9 A respectively. 1800 1900 2000 2500 2600 2000

(b)

Figure 32: Raman Spectrum of b i q h14 ^ an<* ^ C6H5^3PCH3^

tSlO1*^ 1! Figure 33 1 1 0

Furthermore, the iodide ion was located above the cage and symmetrically between borons 6 and 9.

The spectral and phemical evidence support an in­ teraction of l“ with the decaborane cage in the 6 and 9 position. The attenuation of B-H-B stretches in the complex suggest some sort of hydrogen bonding with the I~.

This is further supported by the reaction with NH^. Since the formation of the bis-ammonio adduct of decaborane requires a migration of the bridging protons from the

B5-B6 (and equivalent) positions to the Bg-B^g (and B7-

B 8) positions, the presence of a hydrogen-bridged iodide ion would be expected to greatly increase the activation energy for this process. The removal of I~ in the de­ protonation is consistent with electrostatic repulsions that would occur if I~ were interacting with the bridge protons, the site of a negative charge in the B.qH.,,”

The fact that the B6 , 9 resonance is shifted to a greater extent in the boron-11 spectrum than the B5,7,

8,10 resonance is somewhat puzzling in view of the fact that the bridging protons lie closer to the latter bor- 2 3 4 ons. ' However, since the 6 and 9 positions are most 27 susceptible to nucleophilic attack, any electron charge transferred from i” to the cage might be expected to reside at the 6 and 9 position. Ill

- **44 In the reaction of B]_oHi4 w^-th SCN or CN some similar features were seen but evolution of an equiv­

alent of hydrogen established the species as biqhi2X~* No hydrogen was evolved in the reactions with the iodide

salts and thus a formulation as b^qH^4i” is supported.

III. Studies of p-Pe (CO)4-B6H1q

A. Deprotonation of y-Fe(CO)4“BgH^Q

83 84 The boron-11 nmr spectrum of y-Fe(CO)4-BgH^Q# '

(I),“ (Figure 34) shows the C_ s structure of this molecule. Since all bridging sites are now occupied, there is no possibility of a tautomerism of the bridge protons. In

BjjHg-, however, the removal of a bridge proton results

in a tautomerism making all basal borons magnetically equivalent.^

Compound I. was smoothly deprotonated in dimethyl ether or THF by KH at -78°. The reactions were com­ pleted in a matter of minutes when carried out on a 1-2 millimole scale (1M solutions were used). However, in diethyl ether, the deprotonation would occur only above

-20° and required periods of about one hour to go to

completion. Deprotonations were quantitative in all cases, evolving 98-100% of the theoretical yield.

Boron-11 spectra (^H decoupled) of the ionic

species K+ [y-Fe(CO)4-BgH9“ l, (II), (Figure 35) showed Figure 34: 32.1 Mhz. Boron-11 nmr Spectrum (^H Decoupled) of y-Fe(CO)^-BgH^g 1X3

Figure 35: 32.1 Ilhz. Boron-11 nmr Spectrum {^H Decoupled)

Of K+ [M-Fe(CO)4-B6H9_] 11 4 a marked temperature dependence. At temperatures below

—50° there was a sharp singlet at highest field due to

Bl, and broad unsymmetrical peak near -5 ppm. However, at -40° the downfield peak resolved itself into three peaks of area ratio 1 :2:2 (from lowest to highest field).

Further warming resulted in a loss of resolution of the downfield set of peaks and at 0 ° a symmetrical singlet was evident. At +30° the singlet was quite sharp and the area ratio of this peak to the upfield singlet was

5:1. The chemical shift of the downfield singlet came at the weighted average of the three peaks seen at -40°.

Solutions of 3^ were thermally stable; no changes were apparent in the boron-11 spectrum at +30° over a period of one hour after which decomposition began to be­ come evident. The solution changed in color from a clear yellow to a turbid brick-red in 1-2 hours at +30°.

Evidently, a tautomerism ensues with the removal of a bridge proton from I . This tautomerism is rapid on the nmr timescale at ambient temperatures and a static structure is obtained at -40®. This is in marked con­ trast to BgH^Q in which the tautomerism is only partially quenched at -80® and is completely quenched at -147®.65,66

The static structure of II would seem to indicate the location of the bridge bond in a partial tautomerism across from the iron carbonyl moiety (Figure 36a) result­

ing in Cs symmetry, but electrostatic considerations would 0JelCOU -0 *<“ >* 0M C ° U (b) (a)

Figure 36: Possible static structures for y-Fe(CO)4-BgHg 116

require the basal bond, which is electron-rich to reside adjacent to,the electron-withdrawing Fe(CO)4 group (Fig­ ure 36b).

This latter structure would preclude a partial tautomerism of the boron-boron bond into the other ad­ jacent position without involving total tautomerism.

Although the evidence is not unequivocal, struc­ ture a (Figure 36) is more likely based on the nmr evi­ dence .

The presence of a basal boron-boron bond in .II raises the possibility of introducing a second species into a bridging position. However, reactions involving

II and Fe2 (CO)9 or of BgH9” with two equivalents of

Fe2 (CO)g, resulted in rapid decomposition and formation of brick-red solutions which gave boron-11 nmr spectra exhibiting broad, shapeless humps

(C2He)20 Decomposition K+[p—Fe(CO)4~B6H^q] + Fe2 (CO)9 ------» Products (21)

(C2Hc)Decomposition B6H9" + 2Fe2 (CO)9 ------) Products (22)

If the thermally more stable species [(C4H9)^N][y-Fe(CO)4-

BgHg], (III), was used the same results were observed.

CH Cl [ (23) Decomposition Products 117

B. Relative Acidity of BgH^Q and p-Fe(CO)4-BgH1Q

The electron-withdrawing Fe(CO)4 moiety in the framework of BgH1Q made it likely that I would be more acidic than BgH^Q*

Proton competition reactions were run to deter­ mine the relative acidities. A solution of II and BgH^Q showed the presence of BgH^g and only a small amount of

BgHg“ . If I_ was reacted with BgHg- , again B6H10 was the major species present and BgHg~ was present in small amounts only. In each reaction 11 was the only iron- hexaborane species seen.

y-Fe(CO)4-B6H9” + BgH1Q > p-Fe (CO) 4-BfiH9" + BgH1()

(24)

U-Fe (CO) 4”BgH^Q + BgHg > Ji-Fe (CO) 4-BgHg + BgH^Q (25)

The presence of trace amounts of BgH9~ in each case may be due to production of this species from the presence of some excess KH in the case of reaction (24) or excess BgH9~ in reaction (25). Another possibility was that the difference in acidity between I and B6H10 is small enough that an equilibrium is established in which the positions of equilibrium is far to the left.

M-Fe

< * >

(b)

Figure 37: 32.1 Mhz. Boron-11 nmr Spectrum (^H Decoupled) of [U-Fe(CO)4-B6H 9” ]-[B6H10] System (a) from reaction 24 (b) from reaction 25 119

In no case was evidence seen for the existence of I.

However, boron-11 nmr is much less sensitive to I and

II than to BgH^Q or BgHg“ so that the presence or ab­ sence of 1^ in the reaction mixture is not established.

If reactions were run insuring that no excess

BgHg~ was produced, then the presence of excess I ob­ scured the region of the basal signal for BgH9~.

C. Addition of B2Hg to u-Fe(CO)4-B6H9”

Previous attempts to add diborane to BgH9~ re­ sulted in solutions that were not characterized, and no species corresponding to B^H^2 were isolated.

However, when B2Hg was added to 11^ or III tensi- raetrically a sharp break occurred near a mole ratio of 0.5

b 2H6 to ~ (or II3C)• The boron-11 nmr spectra of the resulting species

(Fig. 38) showed the presence of a new species which showed no sign of undergoing tautomerism at +30°. The

1H decoupled spectrum of K+ [p-Fe(CO) 4-B?H12"J ,(IV) , ex­ hibited seven signlets in the approximate area ratio of

1:1 for all peaks. The chemical shifts of the peaks were at -15.1, + 2.5, + 7.6, + 16.9, + 23.1, + 40.4 and + 48.5

5 ppm. The *^B decoupled *H spectrum in the bridge reg­ ion showed three fairly symmetrical peaks at 9.9, 10,3 and 11.4 t (Fig. 39). Figure 38: 32.1 Mhz. Boron-11 nmr Spectrum (*H Decoupled) of

K+ [y-Fe(CO)4-B7H12“]

N> O Figure 39: 100 Mhz. Proton Magnetic Resonance Spectrum ( ^ B Decoupled) of + * the Bridge Hydrogen Region of K [p-Fe (CO) 1

M t o The isolated salt [(C4H9)4N]tp-Fe(CO)4-B?H12],

(V), was a light tan color as compared to the reddish- yellow color of III. Infrared spectra of III and V

(Fig. 40) reveal significant differences in the B-H ter­ minal stretch region and the CO stretch region. The carbonyl stretching frequencies are often very sensitive to electronic changes in molecules. The shift to higher frequencies in V supports expansion of the borane frame­ work with a corresponding decrease in electron density on the iron atom.

The X-ray structure of IV was determined and showed several important features (Fig. 41). The struc­ ture of IV is derived from a nido-type icoshedral seg­ ment and not an arachno-type segment. The added boron is present as a BH^ group and there are only 3 bridge protons. The molecule may be viewed as a Lewis-acid adduct of III.

The nature of the metal-boron bond has been the subject of some discussion.^®»49,141 In a(j

Figure 40: Infrared Spectra of [(C4Hg)4N][p-Fe(CO)4-BgHg] and [(C4Hg)4N][p-Fe (CO)4-B7H12l, BH and CO Stretching Region 123 Figure 41: Molecular Structure of ti-Fe(CO) 4-B7H12 Table 10: Boron-11 Chemical Shifts of Iron-Hexaborane(lO) Complexes

B1 B2 B3,6 (B4f5) B4,5 (B3,6)

U-Fe(CO)4-B6H1Q +54.1 -11.0 -5.0 0

K+ [M-Fe(CO)4-B6H9“ ] +53.0 -15.la -2.9a +2.4a

[(c4h9 )4n] -3.2b [p-Fe (CO) 4-B6H9] +52.9 -15.la -3.2a +2.3a

ameasured at -40°

bat +30° 125 Figure 42: Possible 3-Center Borane-Metal Atom Bonding Modes 127

Figure 43: Molecular Structure of i*Fe(CO)View Along Axial Carbonyls. 1 128

The structure of V (Fig. 43) shows that the local

symmetry around the iron atom is pseudo-D- in which h there are two axial carbonyls separated by an angle of

114.9° in addition to the boron ligand. This would seem

to confirm the closed three-center bond type of Figure

42 since the other bond type would require a pseudo-

octahedral geometry around iron. A previous X-ray struc- 85 ture of (B^H^q)2PtCl2 did not unequivocally establish

the nature of the metal-borane bond since the resulting

square-planar complex could be derived from either bond­

ing mode.

D. Preparation of Fe(CO)4B7H ^

Addition of HC1 to IV resulted in nearly quantita­

tive yields of H 2 with little or not production of

If methyl ether was present the yields of hydrogen were

— _ n n ° K [p-Fe(CO)4-B7H12 ] + HC1 - ■-■■■■ > H2 + Fe(CO>4B7H11 +

KC1 (27)

« significantly lower (ca. 35-50%) and the product con­

tained large amounts of I. If the HC1 was.added to ether solutions of iv only >20% of the theoretical amount of H2 was produced whereas neat addition of HC1 to V in solu­

tion resulted in no observable reaction and V could be

recovered from these reactions. 129

The boron-11 spectrum C^H decoupled) of Fe (CO)

B7H11' ^ s^own **n F^9ure 44. Chemical shifts for the peaks were -14.4 (shoulder), -8.3, -3.2, and +54.5 ppm. with the area ratio of the downfield multiplet to the upfield singlet being 6:1 .

Again, infrared spectral comparisons with :i (Fig.

45} show significant differences in the B-H stretching regions and the CO stretching region. Shifts of CO bands to higher wavenumbers in VI^ are indicative of polyhedral expansion of the borane with a corresponding decrease of the electron density on iron. Table 11 summarizes the infrared data for 1^ II, IV, and VI.

The mass spectral data support a structure similar 83 to that for 1. Davison, et al., reported a series of four envelopes for I_ corresponding to the parent compound with*the loss of four CO groups. The same behavior is seen for VI except that the corresponding envelopes are

12 mass units above those for 3^, arising from the pres­ ence of an extra BH unit (Fig. 46)*. The presence of envelopes for I is probably due to decomposition to this i • more stable species in the source chamber, which was at

55°.

The thermal stability of VI is limited, showing signs of pronounced decomposition after 1/2 hour at room temperature, but the material was stable at -78° Figure 44: 32.1 Mhz. Boron-11 nmr Spectrum (^H Decoupled) of Fe(CO)4B7H 1]L

u o ii-FefCO^-B^

1— I— I I I I— I— I— I— I— I— ]— I I I I' T 1

f«(co)4b7h u

3000 2500 2000 IBOO

Figure 45: Infrared Spectra of p-Fe (CO) 4”B6H10 an<* Fe(C0)^B7H ^ ; BH and CO Stretching Region 131 Table 11: Infrared Streching Frequencies for the Iron-Borane Complexes

fp -F e(C O )4”BgHg” ] 2562, 2510, 2435 2085, 2041, 1989 •

[p-Fe(CO)4-B7H12“] 2571, 2522, 2400 2090, 2053, 2021 p-Fe(C O )4-B 6H10 2593, 2562, 2619 2072,2019, 1983, 1942

Fe(CO)4B7Hu 2600, 2568, 2511, 2440 2101, 2075, 2044, 2020-1960 132 - 1 0 0 %

56 -50%

J I I i r ---- 1---- r i l

m/e 170 190 230 250

Figure 46: Mass Spectra of li-Fe (CO) ^-BgH^g and Fe(CO)

Parent Mass Region 133 X34

under nitrogen for days.

Crystals were grown from 1M ether solutions at

-78°, but the crystals, which are whisker-like and red- 4 brown, become opaque when removed from the solvent.

A proposed structure for VI is shown in Figure

47. This topological formulation is based on the equa­ tions worked out by Lipscomb, et al., ^7,153,154,155

assuming four bridge protons and no BH2 units. This

structure was considered by Lipscomb to be unsatisfac- 47 tory for and instead he proposed the 2502 structure

previously discussed.

The 4320 structure of Figure 47 presents certain

difficulties, chief among which is the negative excess

connectivity of -1 for B7. This boron would normally

be expected to be a BH2 unit, but a structure based on

this topology cannot be drawn which is in agreement with

Lipscomb's equations of balance. Furthermore, there is

no evidence of a BH2 unit in the boron-11 spectrum, but

since the undecoupled spectrum is very uninformative no

flat dismissal of a BH2 unit at Bg is possible.

. The 2502 (Fig. 14) structure can be flatly ruled

out however, since this structure provides no coordina­

tion site for the Fe(CO)^ moiety, which is clearly

indicated as present in the infrared and mass spectrum

of VI. In addition the rough similarity of the boron-11

spectrum of VI to that for indicated a similar 4320

Figure 47: Topological Structure of Fe(CO) 136

framework, and thua the 4320 structure remains the best

choice. The relative placement of the Pe(CO)4 moiety and

B7 is based on the X-ray structure for V and the assump-

tion that no significant skeletal rearrangement occurs

in the conversion of IV to VI at low temperatures.

The 4320 structure is a static one, lacking the

high degree of symmetry of most of the nido-boron hy­

drides. There is only one valence-bond structure possi­

ble for this topological representation, all of which

contributes to the low stability of such a species and

the failure to isolate the binary hydride species, ByH^.

The stability conferred by the Fe(C0)4 moiety may be based on its electron-withdrawing properties although

®7**11 *s an electron-deficient species and other unstable boron hydrides (e.g. B^Hy,^5 B4H Q,1^6 and BgHi3)149 are

stabilized by the presence of electron-donating groups.

Thus, this appears to be the first example of electro-

philic stabilization of an unstable boron hydride.

Most likely the Pe(C0)4 moiety serves to "freeze"

the basal boron-boron bond into position thus stopping

any possible tautomerism which might, ironically, de­

stabilize the B7 framework. A tautomeric structure that would place the boron-boron bond adjacent to Bg or Bg

would impose severe bond strain on these borons as well

as leaving other borons coordinatively unsaturated.

Since the only reasonable valence-bond tautomer of a 137

4320 structure requires the basal boron-boron bond to re­ side between borons 3 and 4 the F e ( C O ) 4 group at this bridging position stabilizes the molecule.

E. . Preparation and NMR Studies of u-Fe(CO) 4-2-CH3BgHg

The method of preparation of p-Fe(CO)4-2-CH3BgHg,

(VII), is completely analogous to that given by Davison and co-workers for p-Fe(CO) ^-B^H.^.84 However, reaction is completed in about two hours on a three millimole scale for VII as evidenced by the complete disappearance of Fe2 (CO)g. By comparison, the formation of I requires more than 12 hours when run on this scale.

CH-C1, Fe2 (CO)g + 2-CH3B6Hg — =— 4 y-Fe(CO) 4-2-CH3B6H9 +

Fe(CO) 5 (28)

The product was established as p-Fe(CO)4-2-

CH3BgHg by infrared spectrum which exhibited C-H stretch­ ing bands in the region near 3400 cm"*1 as well as B-H stretching bands near 2500 cnT'*’ and carbonyl stretching bands from 1900 to 2000 era”*. Mass spectral data estab­ lished the molecular weight as 257 and revealed the presence of four carbonyl groups. Nearly quantitative isolation of Fe(CO)5 from the reaction mixture estab­ lished the stoichiometry. 138

The boron-11 nmr spectrum (FI9. 48) revealed the presence of two singlets at -23.8 and -19.6 ppm which were assigned to CH^-B groups. The area ratio of these two peaks was 1:2 respectively.

There are two peaks assignable to methyl protons at 9.07 and 9.13 t in the proton magnetic resonance spec­ trum of VII (Fig. 49). These peaks were in the approx­ imate area ratio of 1:2 respectively. An exact measure­ ment was not possible due to the near coincidence of the two peaks.

The assignment of the resonances at -23.8 and

-19.6 ppm in the boron-11 spectrum was based on the as­ signments given by Brice, Johnson, and Shore for

2-CH3BgHg6®'70 in which the CH^-B resonance occurred at

-29.0 ppm. However, the basal borons in B g H ^ resonate at -14 ppm whereas the weighted average of the basal boron resonances in 1 is -5 ppm or about 9 ppm up- field from that of BgH^Q. Therefore the assignment of

CHg-B resonances for VII at -24 and -20 ppm is reason­ able. The methyl proton resonance in 2-CH^BgHg occurs at 9.17 t . 70

These results are best explained by postulating two isomers for VII in which the relative positions of the CH3 and Fe(C0)4 moieties differ. There are three possible isomers that can occur, but the spectral evi­ dence supports the existence of only two of these isomers, Figure 48: 32.1 Mhz. Bo'ron-11 nmr Spectrum (^H Decoupled) of

y-Fe(CO)4-2-CH3B6H9 139 140

Figure 49 r 100 Mhz. Proton Magnetic Resonance Spectrum of Vi-Fe(CO) in the Methyl-Hydrogen Region 141 those shown In Figure 50 (a) and (b). The basis for these assignments is as follows.

There exists the possibility of all three isomers existing with the coincidental overlap of the resonances fox: two of them. However, this would require the unlikely occurrence of coincidental overlap in both the boron-11 and the proton magnetic resonance spectra. That there is,no other CH^-B resonance in the boron-11 spectrum hidden under the large peak is established by the fact that the sum of the integrated areas for the peaks at -24 and -20 ppm is equal to the area for the upfield peak due to the apical boron.

Structure c in Figure 50 is not considered likely since this would require placement of the boron-boron bond adjacent to the methyl group. Jaworiwsky and 171 — Shore studied the 1-CH3B4H8 system and showed that the bridge protons are adjacent to the methyl group in 66 70 the static structure. Brice, Johnson, and Shore, ' in studies of 2-CH3BgH8 found that at low temperatures a partial tautomerism occurred which placed the methyl group at least two borons away from the boron-boron bond.

At still lower temperatures a static structure was ob­ tained in which the boron-boron bond was located in the

3-4 position.

Since the Fe(CO)4 group is coordinated to the basal boron-boron bond only those structures would be 'je(CO),

6H3 6%

(c) (a)

Figure 50: Possible Isomers of u - F e ( C O ) ^^-CH^BgHg 142 obtained for VII in which the boron-boron bond existed at that relative position from the methyl group for a significant amount of tiipe on the reaction timescale.

At the temperature at which reaction (28) is car­ ried out (25-30°) the tautomerism of the bridging pro­ tons is complete and rapid on the nmr timescale. How­ ever, it must be considered likely that the residence time of the boron-boron bond in the 3-4, 4-5, and 5-6 positions is much greater than the residence time in the remaining positions due to the directive effect of the methyl group, and that relatively slow "shutter speed" of the nmr experiment does not reflect such a difference in residence times.

The rate-determining step in reaction 29^ is the dissociation of Fe2 (CO)g, but once the Fe(CO)^ species is generated it reacts almost immediately with the borane

Fe^ (CO) 3 — > Fe(CO) 4 + Fe(CO)s (29) almost immediately with the borane. Since the predom­ inating species of 2-CH3BgHg at any time is the one in which the boron-boron bond is in the 3-4, 4-5, or 5-6 position, these are the main isomers of VII to be formed.

It is possible that trace amounts of the 2-3 or the 2-6 isomer are formed although steric hinderance may pre­ clude the formation of even trace amounts. 144

The residence times of the boron-boron bond in

the 3-4, the 4-5, and the 5-6 positions are nearly equal

as evidenced by the intermediate temperature spectrum of 70 2-CH2BgHg, so that the formation of these isomers of

VII should be present in nearly equal amounts. However,

the 3-4 and the 5-6 isomers are identical, giving rise

to only one signal in the proton and the boron-11 spec­

tra. The 2:1 ratio of the formation of the isomers of

VII is the result of this statistical effect.

Fe2(C0)9 + 2-CH3BeHs

Mass spectral studies were not useful in unequiv­

ocally establishing the absence of the 2-3 isomer of

VII since there were no peaks in the mass region of frag- *4* ments such as CH3BFe(CO)4 . Likewise, it was not possi­

ble to unequivocally establish the identity of the other

isomers by a similar process.

Physical separation of the isomers was not achieved.

The extremely close, if not identical, vapor pressures

of the two isomers precluded fractional distillation. Some separation of the isomers was achieved on silica gel using pentane as the eluent but complete separation was not achieved. If a more polar solvent such as ether or THF was used no separation was seen. REFERENCES

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