This dissertation has been microfilmed exactly as received 66-15,151

W E L C H , Cletus Norman, 1937- SYNTHESIS, CHARACTERIZATION, AND REACTIONS OF SELECTED HETERONUCLEAR DIBORON RING SYSTEMS,

The Ohio State University, Ph.D., 1966 Chemistry, inorganic

University Microfilms, Inc., Ann Arbor, Michigan SYNTHESIS, CHARACTERIZATION, AND REACTIONS OF

SELECTED HETERONUCLEAR DIBORON RING SYSTEMS

DISSERTATION

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

By

Cletus Norman Welch, B.S., M.Sc,

The Ohio State University 1966

Approved by

Adviser Department of Chemistry ACKNOWLEDGMENTS

This investigation was financially supported by a grant from the National Science Foundation.

I would like to take this opportunity to express my appreciation to Dr. Sheldon G„ Shore for his continued interest and guidance throughout the duration of this investigation and in the preparation of this dissertation.

A special note of thanks goes to my colleagues Mr.

Russell A. Geanangel, Mr. David E. Young, and Dr. Roger

K. Bunting for their generous imparting of beneficial assistance, equipment, and technical information.

My wife, Delores, deserves a sincere expression of appreciation for her unselfish desires, understanding, and moral support to the completion of this investigation,

11 VITA

February 2, 1937 Born— Convoy , Ohio

1 9 6 1 ...... B.S., Bowling Green State University, Bowling Green, Ohio

1961-1954. . . . Teaching Assistant, Department of Chemistry, The Ohio State University, Columbus, Ohio

1964 ...... M. Sc., The Ohio State University, Columbus, Ohio

1964-1966. . . . Research Fellow, Department of Chemistry, The Ohio State University, Columbus, Ohio

PUBLICATIONS

"New Organic Boron Heterocycles Contain Oxygen or Sulfur Atoms in the Ring," C & EN News, p. 42, April 4, 1966.

FIELD OF STUDY

Major Field; Inorganic Chemistry

Studies in Non-Metal Chemistry. Professor Sheldon G. Shore

Studies in Transition-Metal Coordination Chemistry, Professor Daryle H. Busch, Devon W, Meek, and Andrew A. Wojcicki

111 CONTENTS

Page

ACKN0WLEDGI4ENTS...... ii

VITA ...... iii

TABLES ...... vii

ILLUSTRATIONS...... viii

INTRODUCTION ...... 1

I, Background of Diboron Chemistry ...... 1

II. General Preparation of Diboron Compounds. . . 2

III. Tetrahalodiborons ...... 3

IV. Tetra-(amino)-diborons...... 16

V. Tetraalkoxydiborons ...... 25

VI. Tetraalkyl- or Tetraaryldiborons...... 30

VII. Tetrathiodiborons ...... 32

VIII. Tetra-(hydrido)-diboron ...... 32

IX. Mixed Diboron Systems ...... 33

STATEMENT OF PROBLEM ...... 44

DISCUSSION AND CONCLUSIONS ...... 47

I. Syntheses ...... 47

XV CONTENTS - Cont'd.

Page

II. R e a c t i o n s ...... 67

III. Nuclear Magnetic Resonance...... 80

EXPERIMENTAL ...... 92

I. Apparatus ...... 92

II. Starting Materials...... 104

III. Analytical Procedures ...... 109

IV. Synthesis and Properties of Heteronuclear Diboron Ring Compounds Considered in This Investigation ...... 113

A. B 2 (02C2H^)2...... 113

B. B 2 CI 2 (O2 C 2 H 4 ) ...... 121

C. 12 3

D. B2(02CgHg)2« .....«..•«••••Cl 127

E. Bg (S2 C 2 H 4 ) 2 12 8

F. B 2 CI 2 (S2 C 2 H 4 ) ...... 134

G. B 2 [N(CH3 )2 ]2 (S2 C 2 H 4 ) ...... 136

H. B 2 [(NCH3)2C2H4]2 ...... 137

I. B 2 [(NH)2 C g H 4 ] 2 ...... 143

J. B2(S2C2H4)2*2 NH(CH3)2 ...... 147 CONTENTS - Cont'd.

Page

V. Chemistry of Diboron Compounds Synthesized in This Investigation...... 152

A. Reactions of B 2 CI 2 (O2 C 2 H 4 ) ...... 153

B. Reactions of B 2 CI 2 (S2 C 2 H 4 )...... 158

C. Reactions of B 2 [N (CII3) 2 ] 2 .... 162

D. Reactions of B 2 (0 2 C 2 H 4 )2 ...... 170

E. Reactions of B 2 (S2 C 2 H 4 ) 2 ...... 174

F. Reactions of B 2 [(NH)2 C 5 H 4 ] 2 ...... 177

SUMMARY ...... 179

APPENDIX...... 183

BIBLIOGRAPHY...... 186

VI TABLES

Table Page

1. X-Ray Powder Diffraction Pattern Data of

B 2 (0 2 C 2 H 4 ) 2 and B 2 C l 2 (0 2 C 2 H 4 ) ...... 119

2. X-Ray Powder Diffraction Pattern Data of

B2(02CeH4)2 and B 2 (0 2 C 3 H g ) 2 ...... 125

3. X-Ray Powder Diffraction Pattern Data of

B 2 (S2 C 2 H 4 ) 2 and B 2 CI 2 (S2 C 2 H 4 ) ...... 132

4. X-Ray Powder Diffraction Pattern Data of

B 2 [N(CH3 ) 2 ] 2 (S2 C 2 H 4 ) and B 2 [ (NCH 3 )2 C 2 H 4 ] 2 . 138

5. X-Ray Powder Diffraction Pattern Data of

B 2 [(NH)2 C e H 4 ] 2 and B 2 (S2 C 2 H 4 ) 2 * 2 NH(CH 3 ) 2 . 146

6 . Boron-11 N.M.R. Chemical Shift Data and Physical Properties of Synthesized Diboron Compounds ...... ,..,149

7. Proton N.M.R. Data of Synthesized Diboron C o m p o u n d s ...... 151

8 . X-Ray Powder Diffraction Pattern Data of B 2 C l 4 * 2 N H ( C H 3 ) 2 and (CH3 ) 2 H 2 NCI...... 165

9. Boron-11 N.M.R. Chemical Shift Data of other Synthesized Boron Compounds ...... 178

V I 1 ILLUSTRATIONS

Figure Page

1. Discharge Cell for the Preparation of Diboron Tetrachloride...... 9 4

2. Schematic Diagram for Automatic Arc Dis­ charge System...... 96

3. Electrical Circuit for Automatic Operation of Discharge System...... 97

4. Molecular Weight Cryoscopy Apparatus . . . 99

5. Reaction Vessels ...... 101

6 . Vacuum Filtering Apparatus ...... 116

7. Infrared Spectrum of B 2 (0 2 C 2 H 4 ) 2 ...... 120

8 . Infrared Spectrum of B 2 CI 2 (O2 C 2 H 4 ) .... 120

9. Infrared Spectrum of B 2 (0 2 CgH 4 ) 2 ...... 126

10. Infrared Spectrum of 6 2 (0 2 0 3 1 1 5 ) 2 ...... 126

11. Apparatus for Reactions with Hydrogen C h l o r i d e ...... 130

12. Infrared Spectrum of B 2 (B2 O 2 H 4 ) 2 ...... 133

13. Infrared Spectrum of B 2 CI 2 (S2 C 2 H 4 ) .... 133

14. Infrared Spectrum of B 2 [N(CH3 )2 )2 (S2 C 2 H 4 ) . 139

15. Infrared Spectrum of B 2 C 1 ^ * 2 NH(CH 2 )2 * • • 139

16. Infrared Spectrum of B 2 [ (NCH3 )2 0 2 6 4 )2 • • « 142

17. Infrared Spectrum of B 2 [ (NH)2O 6 H 4 ] 2 . . . 142 viii INTRODUCTION

I. Background of Diboron Chemistry

This introduction is concerned with the chemistry of (diboron compounds, those compounds with a single boron-boron bond excluding the boron hydrides. A detailed description of more than sixty diboron compounds are reported in the literature. The present treatment of diboron chemistry is not exhaustive since recent reviews appear elsewhere (1,2). Comprehensive reviews which include only boron subhalide chemistry have also been compiled (2,3).

Diboron chemistry had its origin with Stock's original preparation of diboron tetrachloride in 1925 (4).

For the next several years, very little research was initiated with diboron tetrachloride due largely to its difficulty in synthesizing. In 1948, Professor Schlesinger and associates undertook a systematic study of the prep­ aration and reactions of diboron tetrachloride (5). Prior to 1960, diboron chemistry was limited to diboron tetrachloride and its derivatives. At this time, two independent groups developed the techniques for the preparation of tetrakis-(dialkylamino)-diboron compounds in macro quantities using large bench-scale equipment

(6,7,1), With this development, an increase in interest and diversification in diboron chemistry resulted.

II. General Preparation of Diboron Compounds

The preparation of diboron compounds can be grouped under three general headings; (a) the electric discharge of a simple borane such as boron trihalides to yield diboron tetrahalides, (b) the coupling of bis-substituted- haloboranes with an active metal such as the reaction of bis-(dimethylamino)-chloroborane with sodium, and

(c) the reaction of a boron-boron bond compound with the appropriate reagent to yield the desired product. Exam­

ples of this type are diboron tetrachloride or tetrakis-

(dimethylamino)-diboron reacting with amines and alcohols.

Of these, the latter two methods are less specific and

most applicable to the preparation and study of diboron

compounds. Further considerations of each method will be given later. III. Tetrahalodiborons

Diboron tetrafluoride,-tetrachloride,-tetrabromide

and -tetraiodide have been known for some time. Of these, diboron tetrachloride was first prepared and its chemistry

has been more completely elucidated.

A. Diboron tetrachloride

1. Properties and structure

Diboron tetrachloride is a colorless liquid at

room temperature, which ignites in dry air with the

emission of light and undergoes partial decomposition

at 0°C or above (8,9,10). The decomposition products

are mainly BCI 3 , B^Cl^, and B]_2 Cli]_ (11,12). Interest

in B 2 CI 4 is created by its extremely high reactivity

toward Lewis bases and rr-electron systems. The possibil­

ity exists for participation in both addition and substi­

tution reactions. The instability and high reactivity

of B 2 CI 4 can be attributed to the direct bonding of two

boron atoms which have vacant p-orbitals. Diboron

tetrachloride has a melting point of -92.6°C and a

boiling point of 65.5°C with a heat of vaporization of

80 29 calories per mole (13), The thermochemical bond

energy is 106.4 kilocalories per mole for the B-Cl bond and 79.0 kilocalories per mole for the B-B bond (14).

The available evidence indicate that B 2CI 4 has a planar structure in the solid state and a non-planar structure in the liquid and vapor states (15,16,17,18). The

B-Cl bond distance is 1.73 A “ and the B-B bond distance is 1.75 A° (18,19).

2. Preparation

Diboron tetrachloride was first prepared by Stock in 1925 by the passage of BCI 3 through a zinc-arc dis­ charge (4). Much improved yields were obtained by

Schlesinger's group using an a.c. mercury-arc discharge under continuous and automatic recycling of the BCI 3

(8,9). Others have more recently reported improved yields by using direct current, higher amperage and volt­ age, and the insertion of copper wool as a reducing agent in the discharge cell (20,21,22). Microwave excitation of BClg in an electrodeless discharge is reported to yield B 2 CI 4 (23,24). An emission spectrograph examination of the glow discharge indicates the presence of BCl radi­ cals (25,26). The isolation of higher boron subhalides (BCl)n/ in the mercury-arc discharge preparation of B2 CI 4 suggests the following mechanism;

BCI 3 + 2 Ilg --- 5- Hg 2 C l 2 + BCl

n BCl s- (BCl)n

BCl + BCI 3 > B2 CI 4

More recently, a chemical route to the synthesis of BgCl^ has been developed, as shown by the following reaction scheme (17,28,29).

2 [(CHgigNjgBX + 2 Na --- ? - 8 3 [N (CH3 ) 3 ] 4 + 2 NaX

4 HCl B2[N(CH3)2]4 + >B2(0H)4 + 4 (CH3)2H2NC1 4 H20 220°C 8 2 (0 8 ) 4 ----- =?► 2 (BO)x + 2 H2 O

£ (BO) + 4 B C I 3 ----- 9» 3 B2 CI 4 + 2 B 2 O 3 X X

The yields, however, are very low and the procedure necessary for the isolation of B^Cl^ is very time- consuming, The more direct and obvious route however,

does not yield B 2 CI 4 (30).

B 2 (N(CH3 )2 ] 4 + 4 B C I 3 + 4 (CH3 )2 NBCl 2 It appears that the best method still available for the preparation of B^Cl^ is with the mercury-arc discharge cell.

3. Reactions

The high reactivity of diboron tetrachloride has enabled it to participate in replacement reactions, addition reactions, and reactions where the boron-boron bond is cleaved.

a. Adducts of diboron tetrachloride.— Diboron tetrachloride serves as a good Lewis acid and several attempts have been made to form adducts with Lewis bases.

Diboron tetrachloride reacts with excess acetone to yield a white 1 : 2 adduct which slowly loses acetone above -2 3°C (31) . With dimethyl- and diethyl ether, both a monoetherate and a solid dietherate are formed at

-80°C, which decompose at room temperature with the evo­ lution of alkyl chlorides (8,9,32,33,34).

n BgCl^ + 2n RgO 4n RCl + 2 (BO)^

where R — CH^, C 2 H^

Ethylene oxide, unlike diethyl ether, does not form an adduct but yields a rearranged product which reacts slowly and incompletely with R^CHg)^ at 25°C (33,35). B 2 CI 4 + 4 (CH2)20 ----> B2(0C2H4C1)4

Analogous results are obtained with tetrahydrofuran (34).

With nitrogen donors, stable molecular adducts of 2 0 * 3 2 0 1 4 are readily formed where D = (CH2 ) 3 N,

C 5 H3 N, C H 3 CN, or HON (34, 36). The trimethylamine adduct

appears to be a tetramer [3 2 0 1 4 * 2 N(OH 3 )3 ] 4 (9). When difunctional amines such as hydrazine or ethylenediamine are reacted with B 2 OI 4 , 1 : 1 adducts are not formed but replacement reactions take place, forming a type of boron nitride species which contains boron-boron bonds

(36).

B2OI4 + 5 N2H4 --^ 4 N2H5OI + I (BN)n

It has also been reported that the reaction between

B 2 OI 4 and ethylenediamine led to polymeric products (34).

With N,N,N',N',-tetramethylethylenediamine and B 2 OI 4 in a one molar ratio, a non-volatile polymeric adduct is formed (36). Dinitrogen tetrafluoride reacts violently with B 2 CI 4 , resulting in boron-boron bond cleavage instead of yielding the anticipated adduct (37,38).

3 N 2 F 4 + 6 B 2 CI 4 -- > 8 B C I 3 + 4 B F 3 + 3 N 2

Adducts of B 2 CI 4 with molecules containing phosphorus donor atoms have been isolated with PH3 , PCI 3 , and P2 (CH3 )^ but not with P2 CI 4 , With phosphine, both a monophosphine

and a diphosphine were observed at -78.5“C to 0°C (32,39).

The PCI 3 1:2 adduct is stable at room temperature and is

the first example of a complex outside of the transition metal series in which PCI 3 acts as an electron donor.

The 6 3 0 1 ^ .P2 (CH3 )^ adduct forms at -78°C and is undis­

sociated at 150°C (38). No reaction was observed when

BgCl^ was added to white phosphorus (10).

Diboron tetrachloride reacts with hydrogen sulfide yielding BgCl^'HgS and B^Cl^' 2 H2 S at -79°C (10,39).

Above this temperature in the presence of excess H^S,

quantitative cleavage of the boron-boron bond occurs with accompanying evolution of hydrogen. The complex

product mixture included BCI 3 , B2 S 3 , Cl^BSH, and B 3 S 3 CI 3 .

With dimethyl sulfide, B^Cl^ forms both 1:1 and 1:2

adducts which are analogous to the dialkyl ethers (32),

However, the dimethyl sulfide adducts were much more

stable than the related oxygen derivatives. The di-

adduct, 8 3 0 1 ^*2 CH^SH, was formed when methyl mercaptan

was added to B 3 CI 4 at low temperature (34). Warming to

room temperature with excess mercaptan yielded B 2 CI 3 -

(SCH2 ) 3 . Continued reaction with excess mercaptan resulted in cleavage of the boron-boron bond and the for­ mation of hydrogen.

b. Reaction of diboron tetrachloride with unsat­ urated hydrocarbons.— Diboron tetrachloride, a system with adjacent boron atoms can constitute in effect a

"vacant ir-orbital." Olefins and unsaturated hydrocar­ bons, being systems of high electron density can donate

their n-electrons into the "vacant ir-orbital" of B2 CI 4 .

The first compound prepared from this type of interaction was 1 ,2 -bisdichloroborylethane, CI 2 BC 2 H 4 BGI 2 , through the reaction of B 2 CI 4 with ethylene (40). This com­ pound appeared to show significant thermal stability; it decomposed only partially after several days in vacuum at 200°C (41). With trimethylamine, 1,2-bis-

dichloroborylethane formed a stable 1 : 2 adduct which further added two moles of hydrogen chloride without decomposition (2 0 ).

Since the initial study of B 2 CI 4 with C 2 H 4 , several unsaturated compounds have been treated with

B 2 CI 4 . Diboron tetrachloride reacts in a 1:1 molar ratio with propene, butene- 2 , cyclopropane, acetylene. 10

allyl halides, and 4-chlorobutene over a temperature range

of -45°C to -78°C (33,40,42,43,44). More recently, a

small amount of a material which contains B2 Cl^ and C 2 H 2

in a 2:1 ratio has been isolated (45). It is of general consensus that after one mole of E^Cl^ is bonded to the

acetylene group, the chlorine atoms on the boron deact­ ivate the TT-electrons by electron withdrawing effects which are sufficiently large to prevent further reaction.

From this argument, one might also predict that halo-

ethylenes would not form stable reaction products. This has been found to be the case with vinyl chloride, vinyl

fluoride, 1 , 1 difluoroethylene, tetrafluoroethylene and

trichloroethylene (42,4 4), However, some type of inter­

action is suggested, as concluded from proton n.m.r.

results and the observed thermal stability of BgCl^ in the

presence of haloethylenes (42). Butadiene reacts with

BgCl^ in both a 1 : 1 and a 1 : 2 molar ratio to form the

expected stable products (42,43). Diboron tetrafluoride

undergoes similar reactions with olefins but a higher

temperature is required for reaction to occur (42),

Feeney looked further at the addition of B^Cl^

to ir-electron systems (44). The rapidity of the 11

reaction of many olefins with B2 CI 4 at low temperature indicates that initial fission of the B-B bond and then addition is unlikely. Instead, probably the first stage is donation from the olefin into the two vacant p-type orbitals of the boron atoms. This would give a n-complex

(I) which might either dissociate again to give reactants or suffer fission of the B-B bond to give structure (II), as is found with ethylene.

CI 9 B BCl? CloB——BCl? C1?B BCl? , _ Î , —^ I I , 1^2^ * CR2 R2C CR2 R2C--- CR2

(I) (II)

The stability of the complex (I) with respect to dis­ sociation into the reactants may be decreased (a) if there is repulsion or steric interference between the two sub­ stituents R and R ' and the Cl atoms or (b) if the donat­ ing ability of the olefin is reduced by the mesomeric or inductive effect of R or R' either because of decreased electron density at the bond or because the latter is made asymmetrical when R ^ R'. The available evidence suggests that substituent effects are not critical when

R = alkyl, but are important when R = halogen. The latest evidence shows that when a proton n.m.r. spectrum 12

is taken of a B2 CI 4 -CI 3 C2 H mixture, two-thirds of the mixture consists of a substituted ethane while the re­ mainder consists of an intermediate which contains a double bond. This, then, probably explains why no stable adducts are formed with haloethylenes but stabil­

izes against the decomposition of B2 CI 4 ,

Diboron tetrachloride also reacts with aromatic hydrocarbons. With naphthalene, the usual type of addition across double bonds takes place very slowly in a 2 : 1 molar ratio, yielding the following product

(46,47) ;

CUB H ^ \ /

H ecu

Reacting the product with additional B 2 CI 4 over a one- year period resulted in no further uptake of B2 CI 4 .

Benzene reacts with B^Cl^ over a period of many days at room temperature, yielding phenyldichloroborane

(45,47). Here a typical aromatic substitution reaction occurs and differs from the olefin additions or the naphthalene reaction discussed above. 13

c. Miscellaneous reactions of diboron tetrachlor­ ide.— Studies were undertaken to consider the possibility of B 2 CI 4 reacting with simple »-bonded molecules such as

N2 , O2 f CO, and NO to form 1:1 adducts (10,4 8 ). Nitro­ gen and CO were found not to react at all. Oxygen forms

boric oxide and BCI 3 ; the formation of an intermediate adduct is postulated (48). I C l 2 B-OiO-BCl 2 2 B 2 CI 4 + O2 ^ 2 [CI2 B-O-BCI 2 ] --- 1

2 BCI 3 + 2(B0C1)

Nitric oxide forms a 1:1 adduct which decomposes at

-40°C to give B 2 (NO)3 *BCl 3 . In vacuum, this compound releases boron trichloride to form B^(N 0 ) 3 (48). With cyanogen, B 2 CI 4 reacts to yield a brown solid with an empirical composition of B 2 CI 4 «1. 5 (CN)2 , which does not appear to contain a boron-boron bond (49).

Diboron tetrachloride does not react with hydro­ gen chloride, hydrogen iodide, sulfur, methane, or nickel tetracarbonyl (32,50). Diboron tetrachloride reacts with Br 2 , C I 2 but not with I2 to yield the approp­ riate boron trihalide (32). Cleavage of the B-B bond results also with hydrogen, borohydrides, aqueous hydroxide, and other inorganic oxidizing agents. 14

Hydrogen reacts with to yield BCI 3 and 6 3 % (8,9).

Wartik believes that an intermediate of HBCI 2 is formed which decomposes slowly to these products (4 7);

B 2 CI 4 + H 2 ^ 2 (HBCl2 )

6 (HBCl2 ) --- > 4 BC I 3 + B 2 Hg

Diboron tetrachloride reacts rapidly with lithium and aluminum borohydrides, yielding diborane, tetraborane, pentaborane-9, traces of decaborane plus the metal chlorides (8,9). No reaction was observed with lithium hydride, lithium aluminum hydride, or calcium aluminum hydride (8,9,51). Aqueous base hydrolysis of diboron tetrachloride or any other diboron compound results in a quantitative evolution of hydrogen (8 ).

B 2 CI 4 + 6 NaOH -- 5- 2 NaB 0 2 + 4 NaCl + 2 H 2 O + H 2

Aqueous hydrolysis of B 2 CI 4 yields B 2 (OH) 4 . Diboron tetrachloride is oxidized by silver ion, permanganate ion, nitric acid as well as chlorine and bromine as discussed earlier (10,39).

Reactions with B 2 CI 4 where the chlorine atoms are replaced by donor atoms such as nitrogen, oxygen, or sulfur will be discussed below under appropriate head­ ings . 15

B. Diboron tetraf luoride,,-tetrabromide , -tetraiodide

Diboron tetrafluoride is a gas which is extremely explosive in the presence of oxygen and moisture (52,53).

It can be prepared by essentially three methods: (a) the

passage of BF 3 through an electrical discharge similar to the preparation of B 2 CI 4 (54), (b) by using a halide

exchange mechanism where SbFg is reacted with B 2 CI 4 to

yield B 2 F 4 plus SbClg (54) ,

3 B 2 CI 4 + 4 SbFg --- > 3 B 2 F 4 + 4 SbCl] and (c) by a relatively new chemical method where sulfur tetrafluoride reacts with boron monoxide (55).

2 (BO)n + 2 n S F 4 --- 5- n B 2 F 4 + 2 n SOF 2

Diboron tetrafluoride is stable at room temperature, quite unlike B 2 CI 4 . The chemistry of B 2 F 4 is very similar to B 2 CI 4 , The adducts formed by B 2 F 4 are, how­ ever, much more stable than those formed by B 2 CI 4 .

Very little research has been conducted on diboron tetrabromide and diboron tetraiodide. The least studied,

^ 2 ® ^ 4 ' can be prepared in very small yields by the pas­ sage of BBrg through an electric discharge, producing a colorless thermally unstable liquid (22,56), Diboron 16 tetrabromide can be prepared in high yields by an exchange reaction between B 2 CI 4 and BBr^ (8,9), Diboron tetra­ iodide can be prepared by subjecting boron triiodide to an electrodeless radio-frequency discharge at room temp­ erature (57). It is a pale yellow crystalline solid which shows slow decomposition at room temperature.

The only reaction reported is that with aqueous base to yield the anticipated products (5 7).

IV, Tetra-(amino)-diborons

A, Preparation

Tetra-(amino)-diboron compounds were initially pre­ pared from the interaction of the appropriate amine with

B 2 CI 4 ,

The first tetra-(alkylamino)-diboron compounds were prepared by the interaction of methylamine and dimethyl- amine with diboron tetrachloride (34,40).

B 2 CI 4 + 8 NHR(CH3)— 5-B 2 [NR(CH3 ) l4 + 4 (H3 ORH 2 NCI

where R = H,CH 3

However, a much improved synthetic route was made avail­ able through direct syntheses from amino-haloboranes which are readily prepared. This method, the reaction of 17 bis-(dialkylamino)-haloboranes with active metals, repre­ sents a major synthetic improvement, and large quantities of diboron compounds can now be prepared conveniently in bench-scale equipment (6,7,58).

2(R2N)2BX + 2 M -----> B 2 (NR2 ) 4 + 2 MX

R — CHg,

X = Cl, Br

M = Na, K, Na/K, Mg

The bromoboranes appear to react more rapidly than the

chloroboranes. The bis-(dialkylamino)-haloboranes can be obtained easily from the interaction of tris-(dialkyl­ amino) -borane with the appropriate boron trihalide (5).

2 B( N R 2 ) 3 + B X 3 --- > 3 BX(NR 2 ) 2

In addition to the simple bis-(dialkylamino)-haloboranes

described above, several related cyclic derivatives have

also been converted to diboron compounds (59).

R R R ( r ^ 2 (CH2)n BCl + 2 M -- > (CH2)n .B-B (CH2 ) n + 2 MCI V N N --- ^ R R R

M = Na/K

n = 2; R = CHg, CgH^, i-C^H^

n =■ 3; R = CH 3 18

Although not true tetra-(amino)-diborons, the boron- boron bonded diborazines can be considered here. The diborazine shown below is synthesized by adding mono-

chloroborazine to a highly dispersed mixture of molten

sodium in xylene (1 ).

R' R R' R R R' ^B-N^ yB-N\ 2 RN B-Cl + 2 Na 5-RN^ B-B NR

^B— '^'B—N^ —B R' R R' R R R'

+ 2 NaCl

R = CH-

R * — n —C

The analogous reaction with B-chloropentamethyIborazine

gave a product which could not be purified (1 ).

Attempts to prepare B 2 (NH2 ) 4 by the interaction of

B 2 CI 4 with ammonia resulted in a polymeric product (9).

n B 2 CI 4 + 6 n N H 3 --^ 2 (BNH)^ + 4n N H 4 CI

The reaction of B 2 CI 4 with ethylenediamine did not yield

a monomeric species but a polymer (34);

/ill I \ kB-B-N-CH 2 -CH 2 -N 4— yT

B. Reactions

Most other tetra-(amino)-diboron compounds are

prepared conveniently by the reaction of, B 2 [N(CH3 )2 ] 4 / 19 tetrakis-(dimethylamino)-diboron with the appropriate amine (6,60). This reaction is directly related to the general transaminations of simple tris-(amino)-boranes

(61) . The limiting factors in tliese transaminations appear to depend upon the size of the entering and the substituent amino groups. Some primary amines which undergo transamination very smoothly with B2[N(CH3)2]4 are methylamine, n-hexylamine, aniline and N,N-dimethyl- hydrazine (1).

B2 [N(CH3)2]4 + 4 RNHg B2 (NHR) 4 + 4 (CH3)2NH

R = CH3, CH3(CH2)4CH2, CgHg, (CH3)2 N

Only recently triethylsilylamine has been shown to undergo reaction very slowly v/ith B2[N(0143)3] 4 to give tetrakis-(triethylsilylamino)-diboron in low yields (62).

B2[N(CH3)2]4 + 4(C2H5)3SiNH2 — > B 2 [ N H S i ( C 2H 5 ) 3)4

+ 4 N H ( C H 3)2

The transamination of B2[N (0113)2] 4 with several secondary amines yielding stable diboron compounds is shown below

(6,60).

B2[N(CH3)2]4 + 4 RR'NH — B2 (NRR') 4 + 4 (OH3) 2NH

R = R* = 0H2 (0H 2)2CH 3, OgHg

R = OH3; R' = OgHg 20

In some cases, it is necessary to add an amine hydrochlor­ ide to serve as a catalyst to obtain complete evolution of dimethylamine.

The transamination of tetrakis-(dimethylamino)-di­ boron with diamines has led to monomeric products. Brown has shown that with a given diamine, identical products are obtained from the coupling reaction of the approp­ riate diamine-haloborane as that from the transamination reactions (59), No evidence was obtained for a fused- ring type structure for the transaminated products.

R R R

^ NH (■--- N N --- B2[N(CH3)2]4 + 2(CH2>n — > (CH^)n (CH^)^

NH------^--- N N --- ' R R R

+ 4 (CH3 )2 NH

n = 2; R = C H 3 , C 2 H 5 , i- C 3 H?

n = 3; R=H, C H 3

When the diamine is ethylenediamine, an indefinite possibly polymeric product results. The transamination of tetrakis-(dimethylamino)-diboron with o-phenylenedi-

amine in a 1 : 2 molar ratio has led to what appears to be a monomer (1). The structure is not known, but by 21

analogy to the aliphatic diamines, the polycyclic struc­ ture would be preferred. Similar reactions with m- and p-phenylenediamine led to unidentified polymeric products,

Acid hydrolysis of tetra-(amino)-diboron compounds, in general, yields tetrahydroxydiboron without B-B bond cleavage.

The majority of the chemistry of the tetra-(amino)- diborons has been carried out with B2 [N(CH3)2]4.

Tetrakis-(dimethylamino)-diboron upon reacting with boron trichloride or with boron trichloride-trimethyl-

amine yields CI 2 BN(0113)2 the partially halogenated

derivative B 2 CI 2 [N(CH3 )2 ) 2 f by the mechanism shown

(1,30,63) .

Clo-B-Cl Cl-B-Clo I I '

(CH3 ) 2 N^ N ( C H 3 )o 01^ Cl / \ / B —B S> B —B^ + 2 (CH3)o NBC1 o

It has also been reported that with BCI 3 at 25°C, a diboron tetrachloride adduct of dichloro-(dimethylamino)-

borane, [B2 CI 4 •(CH3 )2 NBCI 2 ] can be isolated (64).

Tetrakis-(dimethylamino)-diboron is reported to have been converted to B 2 CI 4 with B C I 3 , but no details of this 22

reaction were given (65). Other attempts to reproduce this work have failed (30).

Hydrogen chloride reacts with B 2 [N(CHg)2 ] 4 in a series of reaction ratios to yield stable products in anhydrous diethyl ether at low temperatures (6 3,66).

Bg [N(CHg)2]4 + 2 HCl— ?-B2C1 [N(CH3)2 ]3 + (CH3)2H2NC1 (1)

B2[N(CH3)2]4 + 4 HCl — >B2Cl2 [N (CH3 ) 2 )2 + 2(CH3)2H2NC1 (2)

Bg [N(CH3 )2 ] 4 + 6 HCl-^B2Cl4*2 NH(CH 3 ) 2 + 2(CH3)2H2NC1 (3)

The dimethylamine adduct of diboron tetrachloride, product of reaction (3) can also be prepared by reacting the product from reaction (2 ) with hydrogen chloride in

a ratio of 1 :2 .

B 2 CI 2 [N(CH3)2]2 + 2 HCl— »B2Cl4*2 NH(CH3)2

The dibromo derivative, B 2 Br 2 [^(0113) 2 12 / has also been prepared by reacting boron tribromide or dibromo-(methyl)-

borane with B 2 [N(CH3 )2 ] 4 (1 ).

Malhotra has recently reinvestigated the dimethyl­ amine adduct of B 2 CI 4 and concluded from infrared spectra that the compound was indeed covalent in character in­ stead of salt-like as previously postulated (6 7). With other strong and weak acids, such as hydrogen bromide or hydrogen cyanide, adducts of a similar type are formed

(67) . 23

X X

6 HBr \ / Eg [N(CH3 ) 2 ]4 + 6 H C l — > (CH3 )2 HN — >B— NH (0^)2 4 HCN / \ X X

X = Br, Cl, CN

If B2 [N(CH^ ) 2 3^ is treated with a mixture of HCN and HCl,

a mixed adduct of B2 C l 2 (CN)2 * 2 NH(CH 2 ) 2 can be isolated.

A rather novel compound can be prepared when B 2 [N(CH2 ) 2 3 4 reacts with HCl and H 2 S in a molar ratio of 1:4:2,

respectively (6 7) .

(CH3 )2 N^ ^N(CH3)2

4 HCl / \ B2[N(CH3)2]4 + --- > S S + 4 (CH3)2H2NC1

2 H 2 S \ / B — B

(CH3 )2 N'^ ^N(CH3)2

Acid hydrolysis of B 2 [N(CH3 )2 ] 4 yields tetrahydroxy­ diboron which can be dehydrated to give boron monoxide

(29). The chemistry of boron monoxide has been described in Section III. The reaction of B 2 [N(CH3 )2 ] 4 with ammonia at 50°C under pressure did not yield B 2 (NH2 )4 , but a polymeric species resulted:

N(CH3)2 N(CH3)2 H I I I B ------B N , ^

Pyrolysis of this polymeric material led to continued evolution of amines and eventual conversion to a boron nitride-like structure (1 ). 24

A new type of chemistry for diboron compounds has been shown by the insertion reaction of phenyl isocyanate with

BatNfCHaig], (6 8 ).

Ç 6 H 5

B2[N(CH3) 2 34 + 4 CgHgNCO ^ B 2 [N - C-N(CH3)2]4

0

Tetrakis-(dimethylamino)-diboron does not react with molecular hydrogen at high temperatures and pressures,

but B 2 CI 2 [N(CH3 )2 ]2 gives a quantitative yield of dimeric [^(CHa) 2 NB (H)C]^ (1) . Silver ion, permanganate ion, bromine, alkaline peroxide, and nitric acid all oxidize

B 2 [N(CH3 )2 l 4 (1). Tetrakis-(dimethylamino)-diboron does not react with trialkyIborates to yield the respective tetra-(alkoxy)-diboron compounds (1). The reaction of

alcohols and phenols with B 2 [N(CH3 ) 2 ] 4 will be discussed in Section V.

C. Stability

The tetra-(amino)-diborons are the most stable of all the diboron compounds. Tetrakis-(dimethylamino)-di­ boron is stable up to its boiling point of 206°C (6 ),

At 300“C, decomposition occurs with the formation of a 25

considerable amount of [ (CH3 )2 N] 2 BH, some (CH3 )2 NBH 2 , and a residue which contains boron-carbon bonds. Di­ boron derivatives of primary amines appear to be less stable than those from secondary amines. Pyrolysis of primary amine diboron derivatives in general results in partial elimination of the amine itself. The large thermal stability of tetra-(amino)-diboron compounds suggests a significant amount of back-coordination of the non-bonding electron pair on nitrogen into the empty p-orbitals of boron. This assumption is supported by infrared studies on a series of these compounds (6 9).

V. Tetraalkoxydiborons

A. Preparation

Tetraalkoxydiboron compounds were first prepared by Wiberg who coupled chlorodialkoxyboranes with sodium amalgam (70).

2 (R0 )2 BC 1 + 2 N a / H g ---> B 2 (0 R ) 4 + 2 NaCl + 2 Hg

R = C H 3 , C 2 H 5

This was the first example of a diboron compound being

synthesized by techniques other than discharge methods.

The use of potassium as the active metal has enabled 26 the preparation of diboron derivatives of higher molecular weight alcohols such as n-propyl and isoamyl (7). In all cases, the corresponding borate is produced in significant amounts. Tetraalkoxydiboron compounds have been isolated from the reaction of diboron tetrachloride with the appropriate alcohol (8,9,34).

BgCl^ + 4 ROH — ^BgCOR)^ + 4 HCl

where R = CH_, CgH^, ClCH^CH^

The methoxy derivative could not be isolated in a pure form.

The preparation of tetraalkoxydiborons from

BgtNCCHgjg]^ has been studied more extensively and is probably the best general method currently available.

Alcoholysis of with the appropriate alcohol has yielded tetramethoxy-, tetraethoxy-, and tetra-n-

butoxydiboron (6 ). Alcoholysis of the boron-boron bond is usually not too serious.

B^ [N(CH^)^]^ + 4 ROH— î-B^COR)^ + 4 (CH^jgNH

R = CH], CgHg, n-C^Hg

Others found that it was difficult to remove all of the amine groups in the reactions. However, if an equimolar amount of anhydrous hydrogen chloride to alcohol is 27 added, tetraalkoxydiborons can be prepared rapidly and in good yields with quantitative elimination of dimethylamine

(71,72).

B2[N(CH3)2]4 + J >B2(0R)4 + 4 (CH3)2H2NC1

R = C H 3 , C 2 H 5 , 1 -C 3 H 7 , CgHg, N'

Some cleavage of the boron-boron bond results. Similarly,

tetraphenoxydiboron and tetra-(quinolin- 8 -oxy)-diboron,

although not strictly tetraalkoxydiborons, can be pre­

pared as indicated (1 ).

Tetraalkoxydiborons have also been prepared by

transestérification reactions (73).

B 2 (0 ^ 3 ) 4 + 4 ROH — ^ B 2 (OR) 4 + 4 C H 3 OH

R = C 2 H 3 , i—C 3 H ‘y

The reaction apparently can be used as a general method

for the preparation of tetraalkoxydiborons derived from

higher boiling alcohols provided it is not limited by

steric factors. Reactions with t-butyl alcohol failed

to yield tetra-t-butoxydiboron. No evidence was obtained

for boron-boron bond cleavage in transestérification

reactions (73). 28

Several attempts have been made to prepare cyclic alkoxy diborons. IVhen 2-halo-l,3,2~dioxaborolane was

reacted with lithium, zinc or zinc plus zinc-copper alloy, and later highly dispersed sodium, no evidence was found for a compound with a boron-boron bond (1,51).

0 \ B— + 2 MX

0'^ 0 —J

2-Chloro-l,3,2-benzodioxaborole is reported to react with highly dispersed sodium with a catalytic amount of tri-

ethylamine in refluxing xylene to yield the required diboron compound (1,74).

1 B— Cl + 2 Na — > I B —B^

+ 2 NaCl

However, the product appears to be somewhat impure.

2-Chloro-l,3,2-dioxaborinane reacts rapidly with sodium

to yield the required diboron derivative but again in

an impure form (1,74). A reaction between B 2 CI 4 and

ethylene glycol has been reported but no products were

identified (51). 29

B. Reactions

Probably the most fruitful reaction of tetraalkoxy­ diborons is the transestérification reaction described earlier without boron-boron bond cleavage (73), Tetra­ alkoxydiborons can be converted to tetrahydroxydiboron upon reaction with water except with tetramethoxydiboron where boron-boron cleavage results (1,29).

32 (OR) 4 + 4 H 2 O -- > 3 2 (OH) 4 + 4 ROH

Tetraalkoxydiborons react with oxygen, yielding the respective orthoborate (71). No reaction has been ob­ served between olefins or benzene with tetraalkoxydiborons

as well as with B2 [N(CH2 )2 ] 4 (1). Tetraethoxydiboron does not react with hydrogen chloride at 25°C but does react with sulfur tetrafluoride as does tetrahydroxy­ diboron to yield B2 F4 (1,55,75). Tetramethoxydiboron does not react with dichlorophenylborane to yield B2 CI4 .

Reaction of tetramethoxydiboron with BCl^ gave a white solid adduct, B2 (OCH3 )4 • 2 BCI3 , which is stable at

0°C but decomposed at 25°C with the evolution of methyl chloride (1). With boron trifluoride, a highly associated alkoxy-(halo)-diboron of unknown structure was obtained

(1) . 30

C. Stability

Tetraalkoxydiborons are less stable than tetra-

(amino)-diborons. Tetramethoxy- and tetraethoxydiboron are stable to about 130°C and 90°C, respectively (71).

Above these temperatures, decomposition takes place by a complex mechanism, yielding the appropriate ortho­ borate, B( 0 R) 3 , a polymer (BOR)^/ and the respective dialkyl ether. Higher carbon chains, especially branched chains, are difficult to distill without decomposition.

VI, Tetraalkyl- or Tetraaryldiborons

A, Preparation and stability

To date, no stable tetraalkyl- or tetraaryldiborons have been isolated. Numerous attempts have been reported to prepare tetramethyIdiboron. The reaction of B 2 CI4 with dimethylzinc or dime thyIcadmium yielded only trimethyl- borane and a black residue which could not be identified

(9,51). The interaction of dimethylhaloboranes with a variety of metallic reducing agents produced similar results (64,76,77). It appears that tetramethyIdiboron forms but disproportionates readily as shown.

n B 2 (CH3 ) 4 — :>n B( C H 3 ) 3 + (BCH3 )n 31

No identifiable product was isolated from the reaction of diboron tetrachloride with phenyl lithium (51), In the reaction of chlorodiphenylborane with sodium, some evi­ dence was obtained for the formation of sodium salts of tetraphenyldiboron (1,78). Dibutylchloroborane reacts with sodium-potassium alloy in a two-step reaction to yield the metal chlorides and a material with an empirical formula of [B(n-C^Hg)2 ] which decomposed to B^n-C^Hg)^ and a polymer [n-C^HgB]^ (79). It appears perhaps that the unstable diboron compound, B 2 (n-C^Hg)^ , was first

formed and then decomposed into the borane and polymer as indicated. The trimethylamine complex of tetramethyl- diboron from the reaction of bromodimethylborane with metallic sodium or silver in trimethylamine has apparently been isolated (51).

2 Na N(CH 3 ) 3

2 (CH3 ) 2 BBr + or ------> B 2 (CH3 ) 4 - 2 N( C H 3 ) 3 2 Ag 2 Na Br + or 2 Ag Br

Attempts to remove the trimethylamine resulted in decom­ position.

An unsaturated tetraalkyIdiboron has been postulated by thermal disproportionation of a trialkenylborane (80). 32

2 B(CH^CH=CH^)^-^B^(CH^CH=CH^) ^ + (CH^=CHCH^) ^

The thermal stability of 116°-117°C is unexpected for this type of compound and it is conceivable that the actual structure has bridging propyl groups,

VII. Tetrathiodiborons

A. Preparation and stability

The only tetrathiodiboron compound reported to date is tetra-(methylthio)-diboron which was prepared by the interaction of diboron tetrachloride with sodium methylmercaptan (33).

BgCl^ + 4 NaSCHg — > 8 2 (SCH^) ^ + 4 NaCl

The stability and chemical behavior of this sulfur- substituted diboron compound has not been reported.

VIII, Tetra-(hydrido)-diboron

A, Preparation and stability

To date, all attempts to prepare boron hydride,

B 2 H^, have been unsuccessful and the available evidence suggests that this compound will not be stable under normal conditions. Reaction of B^Cl^ with hydrogen or lithium borohydride failed to yield as indicated 33

earlier (9). Two examples of adducts of B 2H 4 have been reported. When triphenylphosphine was added to trimethyl- amine triborane, the diphosphine adduct was apparently

formed (81) .

3 PfCgHg)] + (CH^)

+ <°6“ 5> 3'-“ 3

A stable adduct which has been assigned the formula

is reported to be a product from the reaction of tetraborane with pyridine at 0°C (82).

IX. Mixed Diboron Systems

With the advent of coupling techniques in preparing diboron compounds, a more generalized method for pre­ paring mixed diboron compounds became available. This is a very important class of compounds and several ex­ amples will be given. Mixed diboron compounds are charac­ terized by two different substituents groups on the di­ boron framework which may have the general formula of

A. Preparation

There are essentially four methods of preparing mixed symmetrical alkyl-(amino)- and aryl-(amino)-diborons: 34

(a) dehalogenation of (CHg) 2NRBX with an active metal,

(b) interaction of B 2 [N(CH3 ) 2 l 4 with BR3, (c) alkylation

of B 2 X 2 [N(CH3 ) 2 ] 2 with organolithium compounds, and (d)

transamination of dialky1-bis-(dialkylamino)-diborons with ortho-diamines and amines.

Alkyl- and aryl-(amino)-diboron derivatives have been prepared in good yields by the reaction of the

appropriate haloboranes with active metals (83,84,85,86).

R R

2 (CH3)2NB-X + 2 M — > (CH3 ) ^N - B — B-N (CH^) ^ + 2 MX

X = Cl, Br

M = Na, Na/K

R = C Ü 3 , C 2 H 5 , n - C 3Hy, n-C^Hg, CgH^

Interaction of B 2 [N(CH3 )2 ] 4 with tributylborane has

resulted in the mixed 1,2-dibutyl-l,2-bis-(dimethylamino)-

diboron (64).

B 2 [N(CH 3 )2 ] 4 + 2 B(n-C 4 H g ) 3 — > B 2 (n-C4Hg)2 [N(CH3 ) 2 ]2

+ 2 (n-C4Hg)2BN(CH3)2

l,2-Dichloro-l,2-bis-(dimethylamino)-diboron which is

prepared from B 2 [N(CH3 )2 ] 4 and hydrogen chloride in a

molar ratio of 1:4 as described in Section IV, can be 35

alkylated with n-butyllithium to yield B2 (n-C^Hg)2 (N(CH2 )2 ] 2

(66) .

Transamination of B2 (^2 % ) 2 [N (CHg) 2 ]2 with di-n- propyl- and di-n-butylamines led to impure products. The

related reaction of 1 , 2 di-n-propyl- 1 ,2 -bis-(dimethylamino)- diboron with n-butylamine led to the expected di-n-buty 1 - amino derivative (8 6 ). The reaction of B 2 (n-C^Hy) 2 [N(CH^) 2 ] 2 with ammonia yielded unidentified products, while no reac­ tion occurred with unsym-dimethy1-hydrazine at 60°-80°C

(8 6 ). Transamination of some dialky1-bis-(dialkylamino)- diborons with o-phenylenediamine has yielded the naphtha­ lene analog structure (A) (1,64,86).

R = CH3, C2H5, n-CaHg, CgHg

The diphenyl derivative led to extensive cleavage of the boron-boron bond, resulting in the formation of

CgHg-B-NH-CgH^-NH and hydrogen evolution. These are some of the first examples of a boron-boron bond existing in a six-membered heteronuclear ring system. 36

Analogs of anthracene and phenanthrene have been prepared by the transamination of the appropriate

1,2-dialkyl-l,2-bis-(dimethylamino)-diborons v;ith 2,3- and 1,2-diaminonaphthalenes yielding structure (B) and

(G), respectively (8 6 ),

(B) (C)

R = n-C^H^, n-C^Hg R' = n-C^Hg

The only reported example of a phosphinodiboron compound has been prepared, as shown by the following series of equations (87). This compound is unique in that it is monomeric with covalent boron-phosphorus b o n d s ,

(C^Hg)^NBCl2 + 2 L iP(C^H^)g " » (^2 ^^)g^B[P(C^H^)^

+ 2 LiCl

(C^Hg)^NB[P(C 2 H^)^]^ + HCl— > (C^Hg)g^BClP(C^Hg)^ 37

P(C2H5)2 (C2 Hs) 2 P P(C 2 H s )2 I \ / 2 (CoHc)2 N - B - Cl + 2 M > B — B +2 MCI / \ (C2H5)2N N(C2H5)2

M = Na/K in petroleum ether

Mixed alkoxy-(amino)-diborons have been prepared by active metal coupling techniques (64) .

NR' RAN NRA I 2 ^ I ^ 2 ROB-Cl + 2 M --- > ROB— BOR + 2 MCI

R — CH^f n—C^Hg

M = Na/K

These compounds cannot be prepared readily by partial

displacement of amines from B 2 [N(CH2 )2 ] 4 hy alcohols.

However, the analogous reaction with higher boiling mercaptans does yield a mixed diboron species (64).

B2[N(CHg)2]4 + 2 n-C^Hg S H B 2 [ (CH3 ) 2 ] 2 (S-n-C4Hg)2

+ 2 (CH3 )2 NH

Displacement of more than two amine groups has proved to be very difficult.

Symmetrical B 2 CI 2 [N(CH3 )3 ]2 has been isolated from the reaction of B 2 [N(CH3 )2 l^ with BCI 3 , (CH3 ) 3 N'BCl 3 ,or

HCl, as shown earlier (1,2,63,66). A similar reaction of B 2 [N(CH3 )2 ]^ with B F 3 yielded smaller amounts of the 38

dihalide [(CH^)2 NBF 2 ], but no fluorinated diboron com­

pounds were isolated (1 ). The bromoanalog, B 2 Br 2 [N(CH^) 2 ]2 / has also been isolated from the interaction of BBr^ or

B r 2 BCH 2 with B 2 [N (CII^) 2 ] 4 (1). Other compounds in which three of the four substituents are identical, B^RgR'

have been isolated. These include B 2 H[N(CH 3 )2 ]3 ,

B2CI [N(CH3)2]3, and B2R[N(CH3)2]3 (1,63).

B. Reactions

Some of the reactions of mixed diboron systems have already been considered above as methods of preparing other mixed diborons. This includes transamination with amines and diamines and reactions with organo-lithium reagents.

The reaction of 1,2 dialkyl-1,2-bis-(dimethylamino)- diboron with alcohols usually results in boron-boron bond cleavage with hydrogen and dimethylamine evolution or in disproportionation (1,83,86).

B2R2 [N(CH3)2l2 + 4 R'OH — > 2 BR(0R')2 + 2 (CH3 )2 NH + H2

R = C 2 H 5 ; R> = C H 3 , C 2 H 5

R — R* — n— 39

B2 (n“C 4Hg ) 2 [N (CH3 ) 2 ] 2 ^ n-C^HgOPI — n-C^HgB (O-n-C^Hg ) 2

+ 1 /n (B-n-C^Hg)^ + 2 (CH3 )2 NH

With a stoichiometric amount of hydrogen chloride added to aid in removal of the dimethylamine groups, dispro­

portionation still takes place. The reactions of

B 2 (n-C4 H g )2 [N(CH3 )2 ]2 with catechol and o-aminophenol

led to boron-boron bond cleavage and alkylboron polymers

(86) .

-XH B2(n-C4Hg)2 [N(CH3)2]2 + B-n —C4 Hg -OH

+ 1 /n (B-n-C 4 Hg)n + 2 NH(CH 3 ) 2

X = 0,N

The products probably result from the disproportionation

of an unstable intermediate which was also isolated in

the reaction with catechol.

2 NH(CH^)^

•n - C^Hg

Boron trifluoride reacts with B 2 (C2 H 5 ) 2 [N(CH3 )2 l2

in 2 : 1 molar ratio to yield one molar equivalent of

C 2 H 5 BF 2 and approximately two molar equivalents of 40

[F2 BN(CîÎ2 )2 ] (83). A possible reaction scheme can be shown as follows;

C 2 H 5 C 2 H5 CgHg C 2 H 5 \ / \ / B— B + 2 BF- — ^ B — B / \ / \ (CH3)2N N(CH3)2 (CH3 >2 H H(CB3)2

BF3 BF3

(D)

C 2 H 5 C 2 % \ / (D) --- > B—B + 2 [(CH3 )2 NBF 2 ] / \ F F

C 2 H 5 \ / B — B ^ > C 2 HsBF 2 + 1 /n (C2 H5 B)n / \ F F

The last equation is the usual type of disproportionation reaction observed with various diboron compounds.

Anhydrous hydrogen chloride reacts readily with

1,2 dialkyl-1,2-bis-(dimethylamino)-diborons. With the dimethyl- and diethyl-, stable 2 : 1 adducts were formed

(1). However, with the di-n-butyl-, decomposition resulted above -30°C yielding largely n-butyl-(chloro)- dimethylaminoborane (1,86). A similar product is 41

observed when the di-n-butyl- derivative interacts with bromine or iodine (85),

Hydrolysis of mixed diborons, B2R2 [N(CH3)2]2 » to the respective hydroxy-derivatives, B2R2(0H)2, have failed due to cleavage of the boron-boron bonds (83).

In essentially all other reported diboron compounds, alkoxy, amino, and halo substituent groups are hydrolyzed much more readily than the boron-boron bond.

The mixed dialkyl diamino diborons in general do not react with oxygen at room temperature but react at higher temperatures to give unidentified products (83).

1,2-Dichloro-l,2-bis-(dimethylamino)-diboron reacts with lithium hydride and lithium borohydride but the products

were not identified (1 ).

C. Stability

The dialky1-diamino-diborons in general are more thermally stable than the tetraalkoxydiborons but less stable than the tetra-(amino)-diborons. An overall order of thermal stabilities for tetra-(substituted)- diborons can now be given.

B2 (NR2)4 > B2 (NR2)2R2 ^ 82 (OR) 4 > B2X4 > B2R4 > B2H4 42

1,2-Di-n-buty1-1,2-bis-(dimethylamino)-diboron is reported

to be more stable than either B 2 (C5 H5 ) 2 [N (CH3 >2 ]2 or

B2 (C2 H5 )2 [N(CH3 >2 ]2 (85). The decomposition of

B 2 (n-C^Hg )2 [N (CH3 ) 2 ]2 at. 220°-230°C led predominantly to (CH3 )2 N(n-C^Hg)BH and butene-1 (1,85). Decomposition

of B 2 (C2 H 5 )2 [N(CH3 )2 l2 at 200°C or 300“C gave small amounts of hydrogen and ethane, traces of dimethylamine, and a significant amount of (C2 H 5 )2 BN(CH 3 )2 (83).

X. Spectra of Diborons

Several diboron compounds have been subjected to

detailed infrared and Raman studies. In B 2 CI 4 , the

B-Ll- B^ ^ a n d B-l^ B^^ stretching modes, which are degen­ erate in the infrared spectrum, have been observed as polarized lines in the Raman spectrum at 1131 and 1162 cm ” l, respectively (17,18) . The B-B stretching frequency for 3 2 (001-1 3 ) 4 was assigned as a polarized line in the

Raman spectrum at 642 cm“^ and that for 3 2 (^(0 3 3 ) 3 ] 4 was 586 cm“ ^ (69).

Very few B^^ nuclear magnetic resonance spectra of diborons have been obtained. The chemical shifts reported are wi-th respect to the diethyl etherate of boron trifluoride as the zero reference point. 43

S(p.p.m.) Reference

B 2 [N(CH3 ) 2 ] 4 — 35.1 57

B2Cl4*2 NH(CH 3 ) 2 " 7.7 67

B2H4*2 P(CgH 5 ) 3 34.9 81

B4[N(CH3)2]4S2 “ 6.9 67

B 2 (0 0 ^ 3 ) 4 -30±3 88 STATEMENT OF PROBLEM

From 1925-1960, diboron chemistry was restricted to ? diboron tetrachloride and the derivatives it formed.

Difficulty in the preparation of diboron tetrachloride probably limited the number of investigators in this area. However, with the advent of improved techniques for synthesizing boron-boron bond species such as tetrakis-(dimethylamino)-diboron on a large scale, a renewed interest in diboron chemistry resulted.

Several examples of heteronuclear diboron ring species have recently been reported. These compounds were prepared either by the direct transamination of tetrakis-(dimethylamino)-diboron or a dialky 1 -bis-

(dimethylamino)-diboron with an appropriate diamine or by the coupling of a 2 - halo- 1 ,3,2 -diazaborolidine with an active metal (1,59,64,86). All of the pure, stable, heteronuclear diboron ring species reported to date have contained nitrogen as the donor atom.

44 45

Apparent attempts to prepare heteronuclear diboron

ring systems from diboron tetrachloride are limited.

There are only two examples reported of diboron tetra­

chloride reacting with bidentate organic species which contain at least one replaceable hydrogen on each donor

atom, Schlesinger reported that diboron tetrachloride

reacts with ethylene glycol; however, the product was not identified (51) . Fox showed that ethylenediamine

reacts with diboron tetrachloride to form a polymeric

material (34) . It is evident from the limited amount

of information available that additional investigations

concerning the reaction of diboron tetrachloride with

bidentate organic species is in order.

Initially, the problem was to reconsider the reac­

tion of diboron tetrachloride with ethylene glycol in

the presence of a non-coordinating solvent at low

temperatures. If the product appeared simple in struc­

ture, further reactions with selected bidentate species would then be attempted. The problem was approached

primarily from a synthetic point of view in that the

main objectives were the preparation, isolation, and

characterization of the reaction products. If 46

monomeric heteronuclear diboron ring compounds could be

isolated, they would be of interest with respect to the

following considerations: (a) possible conclusions

concerning the degree of multiple-bonding between boron

and the donor atom, (b) extent of delocalization of the non-bonding electron pair on the donor atoms in the heteronuclear ring, (c) the ability to act as Lewis

acids, (d) establishment of chemical shift data

and subsequent correlation with electron density around

the boron atom, (e) subsequent reactions of synthesized

diboron compounds in an attempt to better understand diboron chemistry, and (f) establishment of structures by chemical or physical means. DISCUSSION AND CONCLUSION

Syntheses

Several boron heterocycles have been synthesized and characterized over the past few years. It appeared appropriate to extend these types of systems to encom­ pass heteronuclear diboron ring systems which are characterized by a boron-boron bond intact. Diboron tetrachloride, B2 CI 4 , and tetrakis-(dimethylamino)-di­

boron, B2 [N(CH3 )2 ]4 , contain a boron-boron bond and appeared most appropriate to serve as reagents for the types of reactions under consideration. Monomeric heteronuclear diboron ring systems were isolated when these reagents reacted with simple bidentate organic species which contain at least one replaceable hydrogen on each donor atom. The organic species considered in this investigation which yielded simple monomeric prod­ ucts were ethylene glycol, ethanedithiol, catechol,

1,3-propanediol, sym-dimethylethylenediamine, and o-phenylenediamine. The physical properties of these

47 48 products have been in general characterized by melting point, sublimation point, chemical analyses, molecular weight, proton and boron - 1 1 nuclear magnetic resonance spectroscopy, x-ray powder diffraction pattern data, and infrared spectroscopy. The n.m.r. chemical shifts, melting points, and sublimation points of these products are summarized in Tables 6 and 7.

Diboron tetrachloride reacts with ethylene glycol and ethanedithiol in a stoichiometric 1 ; 1 molar ratio to yield six-membered ring systems, I and II, which are characterized by a boron-boron bond in the ring.

Cl Cl \ / B 2 CI 4 + C 2 H 4 (XH) 2 ---> + 2 HCl

X X w

I; X = 0

II; X = S

These compounds are white solids and react rapidly with moisture. Elemental analyses are in good agreement with the formulas for I and II. Molecular weight determina­ tions of I in chloroform are in good agreement with a 49 monomeric species. Compound II was not sufficiently sol­ uble in an appropriate solvent for molecular weight deter­ minations. The boron-11 n.m.r. spectra of these com­ pounds indicate equivalent boron atoms. This confirms the six-membered ring structure as indicated and mini­ mizes the possibility of the existence of the five- membered ring isomer which has the following structure;

/Cl B — B, Cl

The B^^ chemical shifts were recorded at 30,8 and 6 7.8 p.p.m. downfield from F 2 B' 0 (C2 H g )2 for I and II, respectively. No experimental data are currently avail­ able concerning the stability of heteronuclear diboron ring systems. Results from studies of a somewhat related heteronuclear boron ring system show that the five- membered ring of 1 ,3,2 -dioxaborolane, O(CH 2 )2 Q-B-H, is slightly more stable, thermodynamically and kinetically, toward disproportionation than the six-membered ring of

1,3,2-dioxaborinane, O(CH^) 3 O-B-H (89).

The reaction of diboron tetrachloride with ethylene

glycol or ethanedithiol in a molar ratio of 1 : 2 yields 50

monomeric products. III and IV, which can assume one of two possible structures, the bicyclic (A) or fused ring (B) .

B2CI4 + 2 C2 H4 (XH)2 B — B or X ^ ^X

(A)

+ 4 HCl

XT ^X

(B)

III; X = 0

IV; X = S

Elemental analyses and molecular weight determinations in benzene are consistent with the formulas shown.

Boron-11 n.m.r. spectra show that each compound has equivalent boron atoms, with chemical shifts of - 31.5 and - 58.3 p.p.m. with respect to F3B»0(C2H5)2 for III and IV, respectively. These compounds are white crystalline solids of which the oxygen derivative is considerably more stable, thermally and hydrolytically, than the sulfur derivative. Schlesinger has previously reported that diboron tetrachloride reacts with ethylene glycol, but no products were identified (5 1 ). 51

Attempts have been made to prepare the bicyclic isomers, III-A and IV-A, by coupling techniques. If discrete products could be isolated, their properties would be compared to the products obtained from the diboron tetrachloride reactions, thereby enabling structural assignment. Several types of coupling reactions have been carried out in this laboratory as well as by other research groups. In this investiga­ tion, the attempted coupling of 2-halo-l,3,2-dioxaboro- lane or -dithiaborolane using an active metal has failed to yield the required bicyclic compound (III-A or IV-A). The halogen was either chlorine or bromine with the active metal being dispersed metallic sodium, sodium amalgam, metallic lithium, or magnesium. Dif­ ferent solvent systems and reaction conditions were used for a given metal. No evidence of the formation of a boron-boron bond was found in any of the reactions,

■X X—I

] B—B + 2 MY ^ X ' X X'

where X 0,8

Y = Cl, Br

M Na, Na/Hg, Li, Mg 52

When temperatures above 40°-50°C were used, extensive decomposition of the halo-heterocycles generally re­ sulted. Others have reported attempts to couple

2-halo-l,3,2-dioxaborolane with metallic sodium, zinc, or zinc-copper alloy but were also unsuccessful (1 ,51).

In view of the existence of compounds I and II, the fused ring structure would be favored for compounds III and IV. Additional reactions of compounds I and II will be considered later and may provide evidence for the establishment of the fused-ring or bicyclic type struc­ ture.

Crystalline, white solid products, V and VI, are obtained when diboron tetrachloride reacts with catechol or 1 ,3-propanediol in a 1:2 molar ratio.

B— '0'^ O' (A)

BgCl^ + 2 (V) + 4 HCl

(B) 53

/ 0\ B2CI4 + 2 CsHgfOHig -- > B O 0

(A)

or + 4 HCl

(B)

(VI)

Elemental analyses and molecular weight determinations in benzene are consistent with the formulas shown.

Boron-11 n.m.r, spectra show equivalent boron nuclei for each compound. The B^^ chemical shifts were deter­ mined at 30.7 and 28.6 p.p.m. downfield from F3B»0(C2H5)2 for V and VI, respectively. Again, two possible struc­ tures can be postulated for each compound. The analo­ gous reaction of 1,3-propanedithiol with diboron tetra­ chloride in a 2:1 ratio yielded a white insoluble solid which was apparently polymeric as indicated by analyses.

It is of interest to point out that diborane forms a monomeric species with 1 ,3-propanediol, H-B-0 (CH2 jgO, but does not with 1 ,3-propanedithiol (89).

Compound V-A is reported to have been prepared by 54 the coupling of 2-chloro-l,3 ,2-benzodioxaborole with molten dispersed sodium (1 ,74).

0\ 1) B -C l + 2 Na B — B ' ç / '^0'^

+ 2 NaCl

The product, however, was impure. A catalytic amount of triethylamine was added to the reaction mixture.'

This results in quaternization of the boron atom, which should reduce the boron-chlorine bond strength, and thereby enhance coupling. If triethylamine is not present, coupling does not take place. We have repeated this experiment using nearly the same procedure and were also unable to isolate a pure product. Low boron analy­ ses resulted after recrystallization and sublimation of the product. A B^^ n.m.r. spectrum of the product gave a chemical shift of - 14.2 p.p.m. with respect to

F3B»0(C2H^)2. The position of the chemical shift is at considerably higher fields than other compounds which have identical or nearly an identical atomic environment

around each boron. The chemical shifts for B2(02C2H4)2

and B2(02CgH^)2 are - 31.5 and - 30.7 p.p.m., respectively.

The coupled product in a nitrogen atmosphere becomes 55 moist and difficult to handle. It was concluded from the above results that a considerable amount of triethylamine is coordinated to the boron atoms but in a non-stoichio­ metric ratio. Other workers have attempted to prepare compound VI-A by the coupling of 2- c h l o r o - l ,3 ,2-dioxa- borinane with sodium; however, a pure compound could not be isolated (1 ,74).

The reaction of diboron tetrachloride with either catechol or 1 ,3-propanediol in a 1:1 molar ratio did not yield monomeric species. Analyses, molecular weight, and n.m.r. data suggest that polymeric materials of

small chain length were formed. The possibility of partial disproportionation followed by polymerization

can not be excluded. A possible structure for a small chain polymer without B-B bond cleavage is shown below.

/Cl /O' (R) B - B (R) ^ 0 0 *'---(R)-- ^ I (R)]-- ) X

where X = 0 ,1,2

(R) = CgH^, C^Hg

The diboron products III, IV, V, and VI described

above can also be synthesized from 56 tetrakis-(dimethylamino)-diboron in the presence of hydro­ gen chloride with the appropriate organic species in a

1 :4:2 molar ratio, respectively,

4 HCl -Xh X /— XH

2 ( C H 2 ) n I / 1------^----X X \—■XH ' or

III; n = 2 , X = 0 ^--- X v ^ ^ X-

IV; n = 2 , X = S (CH2 )n ? (C%2 )n I \ I V; (CH2 )n = CgHa, X = 0 ^---- X ^ ^ X ---

VI; n = 3 , X = 0 + 4 (CIi3)2H2NCl

It is postulated that the reactions take place in a two-step fashion yielding an intermediate of 1,2-di- chloro-1,2-bis-(dimethylamino)-diboron which forms at low temperatures (approximately -96°C) and then reacts with the organic donor species at a somewhat higher temperature to yield the required products. The inter­ mediate was actually isolated; subsequent reaction with ethylene glycol or ethanedithiol in a 1:2 ratio yielded products III and IV, respectively. 57

[N + 4 HCl — [N(CH2)2 ]2 + 2 (CHglgHgNCl

B^Cl^ [N(CH^)2]2 + 2 C^H^ (XH)^-- >B^ (X^CgH^)^ + 2 (CH^)^-

Hg NCI

X = 0 ,S

Tetrakis-(dimethylamino)-diboron was found to react with ethanedithiol and hydrogen chloride in a 1:1:2

molar ratio, respectively, to yield compound VII.

2 HCl (CHgigN N(CH3)2 r-S H \ / B2[N(CH3)2l4 +| > B—B + 2 (CH3)2H2NC1 1-SH / \ s s

VII

Elemental analyses and molecular weight data are consis­

tent with the molecular formula shown. The B^^ n.m.r.

spectrum indicated equivalent boron nuclei which led

to the assignment of the six-membered ring structure as

shown, instead of the isomeric five-membered ring, non­

equivalent boron atom species:

(CH3)2N\ S- B — B (€^3)2%/ S-

The B^^ chemical shift for VII was recorded at 43.7 p.p.m.

downfield from F3B«0(C2H3)3. The analogous reaction with 58

ethylene glycol yielded 3 2(0^2^4)2/ dime thy lainmonium chloride, and a polymeric material, with no evidence for B2 [N (CH3) 2 ]2 • From analyses, a postulated structure for the polymeric material is shown below,

N(CH3)2 N(CH3)2 CH3 \ B ------B ------N ----r-- Compounds II, IV, and VII are the first examples of heteronuclear diboron ring systems v;ith sulfur in the ring. Compounds I, II, and VII are examples of a new type of system where the boron-boron bond exists in the ring itself. These compounds contain reactive

functional groups on each boron atom which should enable

them to undergo further interesting chemical reactions.

All other reported stable systems with a boron-boron bond in the ring contain nitrogen as the donor atom

(1 ,6 4 ,86). Only one other compound is reported where the boron-boron bond exists in the ring of an oxygen

containing system. The compound

n-C/iIi

B , I *2 NHfCHglg B q/ ^ n-C^Hg 59 is an adduct, which was an unstable intermediate product and disproportionated to a large extent (86), The only other reported sulfur-containing diboron compounds are

BgtSCHg)^, BgClgfSCHglg, Bud Bg (S-n-C^Hg)^ [NfCHgigjg

(1 ,33,34).

In addition to ethylene glycol, only one other reaction between diboron tetrachloride and a bidentate organic species, which has at least one replaceable hydrogen on each donor atom, has been reported. Fox indicates that the reaction between B2CI4 and ethylene- diamine yields a polymeric product with the following postulated structure (34).

-B—B —N—C2H^—N- ^ ^

The reaction of B^Cl^ with other selected diamines was examined in this work.

Diboron tetrachloride interacts with sym-dimethyl- ethylenediamine in a 1:4 molar ratio to yield a low melting crystalline white solid. This product, when compared to that synthesized by the transamination of tetrakis-(dimethylamino)-diboron with sym-dimethy1- ethylenediamine, was physically identical. A B^^ n.m.r. 60

spectrum of the product indicated equivalent boron atoms with a chemical shift of - 33.7 p.p.m, with respect to

F 3B*0 (C2H 5)2 . CH3 CH3 — B2CI4 + 4 C 2H 4 (NHCH3)2 -- > B — B^ — CÎI3 CH3

+ 2 C2H4 [K(CH3)H2C 1]2

VIII

CH3 CH3 /N-] B2[N(CH3) 2] 4 + 2 C2H4(NHCH3)2 — > B — B^ — CH3 CH3

+ 4 NHfCHg) 2

VIII

Brown has shown that with a given diamine, an ident­

ical product is obtained from the coupling reaction of

the appropriate diamine-haloborane as that from the

transamination reaction of B2 [N (CH3) 3] 4 (59). The

products were shown to be identical by comparison of

infrared spectra and physical properties. Those

diamines considered were sym-dimethyl-, sym-diethyl-,

sym-di-isopropylethylenediamine, and sym-dimethyltri­

me thy lenedi amine . 61

R H R -N. ,---- N, f~\ f“ \ / I 2 (CHn)„ B-Cl + 2 M B—B (CH_)^ + 2 MCI

iliy■N------iliy''-----N \„_j N- R R R

M = Na-K

n = 2 ; R = CH3, C2H5, i-CgH?

n = 3 ; R = CH3

R R R H N \------/---- N. N- 1 r ~ \ / ^ B2 [N(CH3)2]4 + 2 (0% ^ ) % --- B — B (CHgln

H N ^ ^ N N ----- ^ R R R

+ 4 NH(CH3)2

n = 2 ; R = CHgp C^Hgp 1—C^Hy

n = 3 ; R = H, CH3

The product from the reaction of tetrakis-(dimethyl­ amino) -diboron or diboron tetrachloride with sym-dimethyl- ethylenediamine must therefore have the bicyclic struc­ ture as required by the coupled product.

Brotherton has found that the transamination of tetrakis-(dimethylamino)-diboron with o-phenylenediam­ ine yields a high melting crystalline solid, which is very insoluble in organic solvents, and thus makes 62 accurate molecular weight determinations difficult (1).

By analogy to the aliphatic diamines, a polycyclic

structure would be preferred for the product. The

unusual stability of the product, both thermal and hydrolytic, suggests additional stabilization energy which perhaps a fused-ring type structure could offer.

li H

B 2 tN<=H3>2 h + 2 T J .

H H

+ 4 (CHglgNH

IX

We have repeated this work and observed similar results.

Elemental analyses indicate an empirical formula of

^12^12^4^2* n.m.r. spectrum shows equivalent

boron nuclei with the chemical shift being solvent

dependent. This suggests solvent interaction, and the

validity of the reported chemical shifts is questionable

in the solvents shown. Several examples are reported

where o-phenylenediamine exists in six-membered diboron

ring systems, which necessitates the naphthalene type

structure (1 ,5 4 ,86). 63

R — CHg, CgHg

The dimethyl and the di-n-butyl derivatives are stable, whereas the ethyl and phenyl derivative tend to dis­ proportionate .

Diboron tetrachloride reacts with o-phenylenediamine very slowly in a 1:6 molar ratio. A material was isolated from the reaction mixture which gave an x-ray powder diffraction pattern in which the "d" values for the major lines were identical to those of the product obtained from the tetrakis-(dimethylamino)-diboron reaction. Soma weaker lines indicated that the product was not pure. Isolation of a pure product was not attained.

'NHg BgClj + 6 B2 [(NH) 2CgH^]2 + 4 CgH^ (NII2) NH3CI

+ other unidentified solids

A reaction in which the molar ratio was controlled at

1:4 gave no evidence for the required product.

Other fused ring type species have been reported. 64

The analogs of anthracene and phenanthrene have been synthesized by the transamination of l,2-dialkyl-l,2- bis(dimethylamino)-diboron with 2,3-diaminonaphthalene and 1,2-diaminonaphthalene where alkyl is n-C^H^ or n-C^I-Ig (85) . Attempts to prepare related boron-oxygen heterocyclic compounds containing boron-boron bonds by reactions with catechol and o-aminophenol generally led to extensive cleavage of the boron-boron bond (86).

,XH B2(n-C ^H g)2 [N(CHg) 2)2 •OH

I I B-n-C^Hg •O'

+ 1/n (B-n-C^Hg)^

+ 2 NH(CH2)2

X = 0 , NH

An unexpected spontaneous reaction occurred between

B2[N (0112)2] 4 and ethanedithiol in a 1:2 molar ratio in the absence of a solvent at 25°C. A white solid material was isolated from the reaction mixture, which analyzed for the dimethylamine adduct of 32(52^2^^)2'

B2 [N(CH3)2]4 + 2 C 2H 4 (SH)2--5> B2 (S2C2H4) 2* 2 NH(CH2)2

+ 2 NH(CH3)2 65

Equivalent boron atoms were observed in the b H n.m.r.

SiJBCtrum, with a chemical shift of -11.8 p.p.m. with re­ spect to F2B*0{C2H5)2• The high field value for the chemical shift confirms that an adduct is formed instead of an isomeric compound with trigonal borons, such as

B2(SC2H^SH)2 [N(CH^)2 ]2• identical product was isolated from the reaction of ^2(S2C2H^)2 with dimethyl- amine. A preliminary investigation has also shown that

1,3-propanedithiol reacts with B2[N(CH^)2]4, however only very slowly, to yield a white solid which slowly turned a light brown color. Noth has reported the isolation of di-n-butylmercapto-bis-(dimethylamino)- diboron from the interaction of n-butyl mercaptan with

B2[N(CH2)2]4 in refluxing benzene (64).

B2 [N(CH3 ) 2]4 + 2 C4H9SH > B2 [N (CH3 ) g ] 2 (n-SC4Hg ) 2

+ 2 NH (0143)2

Attempts were made to synthesize monomeric species by the reaction of diboron tetrachloride with organic ligands which contain non-equivalent donor atoms such as; o-aminophenol, CgH4(OH)(NH2); 2-mercaptoethanol,

C2H4(SH)(OH); and styrene glycol, C2H3(CgH^)(OH)2, in a 1:2 molar ratio. It was anticipated that if monomeric 66 species were formed, n.m.r. would perhaps enable structural assignment of these heteronuclear diboron ring systems. From B^- n.m.r. spectra, x-ray powder diffraction pattern data, and boron analyses, the prod­ ucts appear to be polymeric.

The reaction of E^Cl^ with 2-mercaptoethanol in a

1:4 ratio resulted in the formation of a non-volatile liquid which gave a B^^ chemical shift of -29.9 with respect to F3B*0(C2H5)2. A possible formula for the compound is B2(OC2H4SH)4. The position of the chemical shift is consistent with compounds which have two oxy­ gen atoms around each boron atom such as B2 (0202114)2,

^2(03^6^4) 2 ? ^2 ^^*“2^ 5 ^ 4 *

A more detailed interpretation of boron-11 and proton n.m.r. data obtained from the above synthesized diboron compounds will be discussed later.

A quantitative study of the hydrolytic and thermal stability of the above synthesized diboron compounds was not initiated. However, with these compounds, it is believed that the following general order of stabil­ ity exists: B2 [(NH)2C6H412> B2 [ (NCH3)2C2H4]2 >

B2 (02C 6» 4)2 > B 2 (02C 2H 4)2 = B 2 (02C 3H g ) 2> B 2 [N(CH3)2 ]2 - (S2C2Hj)> B2(S2C2H^)2> B2CI2(02C2H^) > B2CI2(S2C2H4). 67

This order of stability is in agreement with the extent of multiple-bonding betv/een boron and the donor atom where B-N>B- 0 >B-S. This implies that nitrogen donates its non-bonding pair of electrons back into the empty p-orbital of boron to a greater extent than does oxygen, which in turn, is greater than sulfur.

II. Reactions

The first part of this section is concerned with attempts to prepare mixed heteronuclear diboron ring systems. It was anticipated that if such species did exist, this would be good indirect evidence for the fused-ring type structure of the oxygen and sulfur systems. There have been no previously reported mixed heteronuclear diboron ring systems. The second part is concerned with attempts to prepare stable adducts and to carry out displacement type reactions on those compounds described in the above Syntheses Section.

The types of reactions considered are examples of reactions which simple boranes and other diboron com­ pounds have been shown to undergo successfully. The results of these reactions will help to characterize 68 the chemical properties of the new diboron compounds prepared in this investigation.

The reaction of B2CI2(02C2H^) with a slight excess of ethanedithiol yielded B2(02C2H^)2 plus what appeared to be some polymeric dithio-diboron compound.

Cl^ /Cl I— SH + B2 (C2C2H4) 2 + dithio-diboron 0 0 •SH polymer + HCl \ _ v

When catechol and B2C12(O2C2H4) were allowed to react in a 1:1 ratio, the reaction product contained an equi- molar mixture of B2(02-2h4)2 ^2 2•

C l \ /Cl / B — B ^ + I --- ^ 1/2 B2 (‘^2‘^2^ 4^ 2 0 O \ ____/ t 1/2 B2 (02C g H 4)2 + 2 HCl

The reaction of catechol with B2CI2{S2C2H4) in a 1:1 molar ratio yielded B2(02CgH4)2 with unreacted

B2CI2(S2C2H4) also appearing in the reaction product,

-OH ^ B 2 (02C g H 4)2 ^ 'OH

\ / + B—B^ + HCl s s v _ / similarly, ethylene glycol and B2CI2(S2C2H4) in a 1:1 molar ratio yielded B2(Û2C2H4)2 along with unreacted 69

B2CI2 (S2C2H4) . No evidence of the existence of a mixed heteronuclear diboron ring system was observed in any of the above reactions. When B2Cl2(02C2H4) serves as a reactant, one can postulate tiiat initially the mixed diboron ring system is formed which then rearranges through intermolecular base-association to form a more stable or symmetrical system.

B 2 <°2= 6“ 4>2

This rearrangement can best be envisaged if the products of the rearrangement assume the fused-ring structure.

With B2CI2 (S2C2H^) as a reactant, the mixed fused ring is probably formed first followed rapidly by base dis­ placement of the sulfur groups with oxygen groups. This scheme can be rationalized in terms of the very limited solubility of B2CI2(S2C2H4) in the reaction solvent of

CH2CI2. 70

An attempt was made to prepare a mixed diboron ring

system by the transamination of B2 [N(CH2>2’2 with o-phenylenediamine (1 :1) in benzene. However, the

species reacted slowly at room temperature to yield

B2 [ (NH)2CgH^]2 in addition to unreacted B2 [NCCH^)2^2”

(S2C2H^). Ethanedithiol and dimethylamine were also

liberated in the reaction mixture.

(CH3)2N\ / N(CH 3)2 B—B + 1 1/2 B2 [(NH)2C g H 4]2 \

N(CH 3)2 w (CH3)2N\ / 1/2 ;b —B\ s' w

+ NH(CH3)2 + 1/2 C2H4(SH)2

The mechanism of the reaction is probably complex. A

possibility is the formation of a dimethylamine adduct

intermediate which then rearranges to form the products

indicated.

HN^ ,NH (CH3)2HN ------— ■NH(CH3)2 products s''' S

The slow step is probably the formation of the adduct

which may or may not be stoichiometric with respect to

dimethylamine. Once the intermediate is formed, re­

arrangement rapidly takes place to give products. 71

It appeared feasible to extend this type of reac­ tion in an attempt to prepare B2 ( (NH)2C2H/j ]2 , which could not be synthesized by the reaction of ethylene- diamine with diboron tetrachloride or tetrakis-(dimethyl- amino)-diboron (34,59). The 1:1 molar reaction of

B2 [N(CH3)2 ly(^2^2^4^ with ethylenediamine yielded a product which was polymeric and did not appear to con­ tain any sulfur groups.

Substitutional reactions were carried out on

B2CI2(02C2H^) and B2CI2(S2C2H^) with ethyl alcohol and dimethylamine. Ethyl alcohol reacts with B2CI2 (0^02^1^) in a 2:1 ratio to yield B2(02C2H^)2 y triethyIborate, and what appears to be tetraethoxydiboron with the latter two in a ratio of approximately 3 :1 , respectively,

Since the identification of the latter two compounds was by B^^ chemical shift data, the presence of small amounts of B2 (002% ) 2 (*^2^2^4 ) could not be ruled out entirely. When the identical reaction was carried out with B2CI2(S2C2H4), the reaction product consisted of triethyIborate, a small amount of unreacted

B2CI2(S2C2H4), plus an amorphous solid, with no evi­ dence for the diethoxv-diboron derivative. Here 72 again, rearrangement takes place with extensive boron- boron bond cleavage. Boron-boron bond cleavage has been observed in several other cases when diborons re­ act with alcohols (1 ,6 ,9 ,71).

Excess ethylene glycol when treated with

B2CI2(O2C2H4) yielded B2(02C2H^)2; however,

B2CI2{S2C2H/J did not form B2(S2C2H^)2 when allowed to react with ethanedithiol in CH2CI2. Perhaps if the reaction was allowed to take place in a solvent which would initially dissolve B2CI2 (S2C2H/, ) , the desired product could be isolated. It is noteworthy that

B2(S2C2H4)2 is not observed as a product from any of the above rearrangement reactions in which ethanedithiol or a dithiol derivative serves as a reactant.

Dimethylamine reacts v/ith B2CI2 (S2C2H4) in a 4:1 molar ratio to yield a product which is identical to that obtained from the reaction of tetrakis-(dimethy1- amino)-diboron, ethanedithiol, and hydrogen chloride in a 1:1:2 molar ratio, respectively, as described earlier under Syntheses. 73

+ 2 (CHg) H NCI

VII

This is further evidence that 82012(3202^1^) is indeed monomeric (since a molecular weight determination was not feasible because of low solubility in appropriate solvents) and has the structure previously indicated.

The analogous reaction of dimethylamine with

2^012(02028^) yielded a small amount of ^2 ^*^2*“2^4^ 2 '

(08^)282801, in addition to an amorphous white solid.

A possible structure for the amorphous material is shown below.

N (083)2 N (083)2 \ I I B ------B ------O-O284-O 4—^

As shown previously, the bis-amino compound,

B2 [N(O83)2]2(O2O284), could not be prepared from the reaction of B 2 [8(083)2)4 with ethylene glycol and 801 in a 1 :1:2 ratio, respectively. With trimethylamine,

B2CI2 (S2O284 ) and 82012(020284) reacted incompletely to yield non-stoichiometric adducts. however, the mole ratio of amine to acid is greater for the sulfur compound than for the oxygen derivative. 74

At elevated temperatures, some type of interaction occurs between acetylene or diphenylacetylene with

B2CI2(X2C2H4) (where X = 0 ,S), but discrete stable compounds could not be isolated. At the temperatures used, the possibility of some decom.position or rearrange­ ment of B2CI2(X2C2H4) can not be excluded. Generally, no evidence was obtained for boron-boron bond cleavage as is observed when B2CI4 reacts with acetylenes or olefins.

The stoichiometry of the reaction between

B2 [N(CH3)2]2(S2C2H4) and hydrogen chloride was deter­ mined to be 1 :4. The product obtained from this reac­ tion was identical to that synthesized from the reaction of B2[N(CH3)2]4 and hydrogen chloride in a molar ratio of 1 :5 , This product has previously been prepared and assigned the dimethylamine adduct of diboron tetra­ chloride structure (6 3 ,6 6 ,67).

B2 [N(CH3 )2]2 (S2C2H4) + 4 HCl ---> B 2Cl4»2 NH(CH3)2

-f C2H4 (SH)2

B2 [N(CH3 )2]4 + 6 HCl >B 2C l 4'2 NH(CH3)2

+ 2 (CH3)2H2NC1

Initially, this compound was assigned the salt-like 75 structure, B2CI2 [N(CH3)2]2*2 HCl (63). Results from more recent investigations indicate a covalent struc­

ture (57). We have shown that when NH(CH3)2

is reacted with ethylene glycol in a 1:2 ratio

B2(02C2H4)2/ dime thy lamm.onium chloride, and hydrogen

chloride are reaction products. This further supports

arguments for covalent character.

B 2C l 4*2 NH(CH 3)2 + 2 C 2H 4 (OH) 2 ---5. B2 (O2C2H4) 2

+ 2 (CH3)2H 2NC 1 + 2 HCl

The reaction of B2 [N(CH3)3]2(S2C2H4) with hydro­

gen chloride in a 1:2 ratio did not yield either of

the following adducts, B2CI2(S2C2H4)•2 NH(CH3)2 or

B2 [N(CH3)2]2(S2C2H4)“2 HCl, but gave (CH3)2H2NC1 plus

an amorphous white solid. A possible structure for

the amorphous solid is shown below.

Cl N(CH3)2

B — B-S-C^H.-S ‘ /n

Compounds of the form B2R2 [N(CH3)2]2f where R is

methyl or ethyl, have been reported to give stable

di-(hydrogen chloride) adducts, but butyl does not

(1 ,86).

Attempts to carry out an amine exchange reaction 76 between B2 [N(CHg) 2] 2 ammonia at - 78“C re­ sulted in only partial amine replacement. At higher temperatures, replacement of the dithio-group begins resulting in an undefined product. Pyrolysis of the product from the - 78°C reaction resulted in a polymeric material which contained only amine groups on the diboron framework. A possible structure for the poly­ mer can be represented as shown below.

NHo NHo CHo, I I I \ B B N L r Treatment of tetrakis-(dimethylamino)-diboron with ammonia at 50°C has been reported to result in the formation of a polymeric solid which approached the formula shown below (1 ,64).

N(CH3)2 NH2 H I ! I B ------B N

The reaction of B2 [N (CH3) 2] 2 (S2C2H4) with BCI3 in a 1:2 ratio yielded Cl2BN(CH3)2, a small amount of

B2CI2(S2C2H4), and additional unidentified solids which appear to be polymeric. The stoichiometry of reaction between BF3 and 83[N(CH3)2)2(82^2^4) was determined to 77 be 1 .5 3 :1 .0 . A CH^Clg insoluble solid from this reac­ tion gave a boron analysis which approached the theo­ retical value in B2F [N (CH^) 2^ (^202^1^) . An amorphous solid and [F2BN(CH^)2^2 were also products from this reaction. These types of reactions probably proceed through the formation of a four-centered intermediate which has been previously proposed by other workers on related types of compounds (83).

X--BXg I Î I N(CH.)2 \ 4 / B — B, / \ B2 (02C2H_^) 2 reacts slowly and incompletely with trimethylamine, dimethylamine, and methylamine which forms non-stoichiometric adducts. However, as the steric requirements of the base decrease, the per cent uptake of the amine increases. In comparison,

B2(S2C2H^)2 also reacts incompletely with trimethyl­ amine but to a greater extent than does ^2(O 2 C 2 )2.

With dimethylamine, B2(S2C2H^)2 forms a stable 1:2 adduct. This product is identical to that obtained from the reaction of tetrakis-(dimethylamino)-diboron and ethanedithiol in the absence of solvent at room 78 temperature as described previously. Methylamine reacts rapidly with B2(S2C2H^)2/ but partial boron-sulfur bond cleavage appears to result. As of now, the 1:2 adduct has not been isolated in a pure form. It can be con­ cluded from these observations that the formation of a stable adduct depends upon the steric requirements of the amine and the Lewis acid character of the boron in a given diboron compound. Here, as^_^ generally the case for boranes, the sulfur containing derivatives are stronger Lewis acids than their respective oxygen containing analogs.

B2CI2 (S2C2H4) B2CI2 (O2C2H4) B2 (3202114)2 B2 (02C 2H 4)2

decreasing acceptor character ^

increasing hydrolytic and thermal stability ^

Few attempts to prepare amine adducts with diboron compounds have been reported. Diboron tetrachloride forms a stable 1:2 adduct with trimethylamine, which appears to be a tetramer (9 ). The product from B2CI4 and ethylene oxide, B2(OC2H4CI)4, reacts slowly and incompletely with trimethylamine (34). The trimethyl­ amine adduct of tetramethyIdiboron has been isolated 79

(51), Dimethylamine adducts of the type 82X4*2 NHfCHgXz, where X = Br, Cl, or CN, have been reported (67 ). A

2:1 dimethylamine adduct of 32(020584) (n-C4Hg)2 has been synthesized but tends to disproportionate (86),

No amine adducts of a sulfur containing diboron com­

pound have been prepared.

Experiments were proposed to prepare diboron

tetrachloride chemically. Boron trichloride, at high

pressures, was passed slowly over B2(S2C2H4)2 and

82(0202114)2 suspended on an extra coarse frit heated

to 50°-60°0 . Products from the 82(8202114)2 reaction

included OI-B-SO2H4S, 82012(820284), an amorphous

dithio-compound, and unreacted 82(820284) 2» The

82(020284)2 reaction yielded 01 - 8- 0 0 284O, small amounts

of 82012(020 284) , unreacted 82(020284)2, and mainly

amorphous materials. Separation of 82012(020284) from

the amorphous material was not attained. Considerably

more 8^012(820284) was formed than 8^012(020284); how­

ever, yields were very small in both cases. No evidence

of the formation of diboron tetrachloride was obtained.

Attempts to form sodium salts by the reduction of

82(020284)2 and 82 [8(083)2)4 with sodium amalgam failed. 80

Although the available evidence is not conclusive, it appears as if a sodium salt of tetraphenyIdiboron is formed from the interaction of chlorodiphenylborane with sodium (1) .

Hydrogen chloride reacts with B2 [ (NCH^) 2^ 2^^4 ^2 and B2 [ (NH)2CgH4]2 in ratios of 4 .46:1 and 4 .0 2 :1 , respectively. The products from both reactions are a m o r p h o u s .

Ill. Nuclear Magnetic Resonance

A number of studies have shown that the chemical shift of proton resonance can be correlated with a considerable degree of success with the apparent elec­ tron density in the region of a given hydrogen nucleus.

Theoretical justifications of this correlation have been presented (9 0 ) . However, at present there is no theoretical sound reason for believing that a trust­ worthy relationship exists between electron density and chemical shift data in boron resonance. Neverthe­ less, experimental studies have shown that a reasonably good qualitative correlation exists between chemical shift and electron density within the same general type of boron compounds. The chemical shifts of compounds 81

investigated in this work are listed in Tables 6 ^ 7 , and

9 .

Investigations in this laboratory considering boron heterocyclic species have shown that 2-chloro-

1 ,3 ,2-dithiaborolane, CI-B-SC2H4S, is a stronger Lewis

a c i d than 2-chloro*-l,3 ,2-dioxaborolane, Cl-B-OC^H^O, with respect to a given reference base. One can

rationalize this in terms of "pi" type bonding between boron and the donor atom, where B-0 has more "pi" bonding than B-S. If B^^ resonance can be used as a

criterion for electron density, then the chemical

shift of CI-B-SC2H4S should appear at a lower field

than that of CI-B-OC2H4O. Their respective chemical

shifts are - 62,7 and - 31,4 p,p,m, with respect to

F3B*0(C2H5) 2. All other B ^ chemical shift data in

the remainder of this discussion will be referenced to

F^B'O(C2Hg)2 in p,p,m, Hawthorne reports comparable

results when the chemical shifts of sulfur substituted

boranes are compared to borate esters (91). In the

present investigation, similar observations were made

concerning oxygen and sulfur substituted diboron ring

compounds. The chemical shift for 32(32021^4)2 was 82

considerably downfield from 82(020211^)2 with values of

- 68.3 and - 31.5 , respectively. It was also shown that

82(320211^)2 reacted faster and more completely with trimethylamine, dimethylamine, and methylamine than did

82 (0202^1^) 2. F r o m t hese observations, it can be con­ cluded that 82(320211^)2 is a stronger Lewis acid than

^2 (02^2^4) 2 the extent of "pi" bonding is consider­ ably greater for the boron-oxygen derivative. The coordination of a dimethylamine group to each boron in 3 2 (22^ 2^^4^2 resulted in a large upfield shift for both boron and proton resonance, boron at -11*8 with the methylene hydrogens at 7 .06T,

Similar results were observed when the chemical shifts of 82^12(22C2H4) and 82012(020284) were compared,

- 67,8 versus - 30.8 , respectively. It also appeared that trimethylamine reacted more completely with

82012(320284) than with 82012(020284). 8ere, as the case above, the sulfur containing derivative is the better Lewis acid. Replacement of the chlorine atoms with dimethylamino groups in 82^12(S2C284), to yield

82[8(082)2)2(82O284), resulted in an upfield shift to

- 43.7 . This upfield shift presumably results from an 83 increase in electron density around boron which is con­ sistent with theory in that nitrogen is a better "pi" bonder than chlorine. The chemical shift for the

heterocycle 2-dimethylamino-l,3 , 2-dithiaborolane,

(CH3)2N-B-SC2H4S, has been reported at - 45.3 (9 2 ).

The reported chemical shift at - 5.9 for the six-membered ring species of B4[N(CH3)2]4S2 (67 ), which has the same atomic environment around each boron atom as the com­ pound B2 [N(CH3)2l2(S2C2H4), appears to be inconsistent with results from the present study. The trimeric species of [BN(CH3)2S]3 (93) has a chemical shift re­ ported at - 38.1 , which is consistent with the hetero­ cycl e s (CH3) 2^— B — SC2H4S and B2 [N (CH3) 2)2 (^2^2^^4^ • 1^ is noteworthy that the insertion of a mono-substituted boron substituent into the borolane structure forming diboron derivatives results in relatively small changes of the chemical shifts. As a first approximation, this implies that the electron density around boron is essentially unchanged, a result which is unexpected; however, caution should be observed in drawing such a conclusion. A comparison of the chemical shifts is shown below. 84

C h e m i c a l C l Cl C h e m i c a l S h i f t \ / Shift B — B )\ B-Cl - 62.7 / \ - 67 . 8 S S w

I— S \ B — B - 43.7 B-N(CH3)2 - 45,3 / \ S S w

Cl C l \ / r - 0\ B — B B-Cl - 31.4 / \ - 30.8 0 0

If the boron chemical shift is principally related to electron density, it is difficult to understand the position of the chemical shift for B2CI2(O2C2H4), - 30 .8 , compared to B2(Û2C2H4)2, - 31.5, considering the fact that B2CI4 has a chemical shift of - 62 .6 . The chemical shift of B2CI4, however, may appear at a lower field than expected because of its staggered or non-planar structure in the liquid state. The B^^ chemical shifts for BCI3, - 45.6 ; BBrg, - 38,5 ; and BI3, + 8.0 , parallel 85

the electronegativities of the halogens in that they go to higher fields with decreasing electronegativity

(88,94). This is consistent with the theory that as

the electronegativity of the halogen increases, the electron density around boron decreases. The unusual high field value for Big can be rationalized if the high polarizability of iodine is taken into account. How­

ever, the boron chemical shift for BFg, - 9 . 4 , is incon­

sistent with the chemical shifts obtained from the other boron trihalides if only electronegativity is taken into consideration (9 4). A possible explanation of this apparent anomaly is to assume a significant amount of

"back coordination" of the non-bonding electrons on

fluorine into the empty p-orbital of boron. The n.m.r.

results and interpretations are consistent with argu­

ments for back coordination based on the shortening of

the B-X bond distance in BXg and the apparent Lewis

acidity of BXg, where BFg is a weaker acceptor than

BClg or BBrg.

In this investigation, the boron chemical shift

for B2F4 was recorded at -22.8 compared to -62.6 for

B2CI4, These shifts show the same general trend as 86

the values reported for the trihalides, where BF3 is

- 9.4 and BCI3 is - 45.6 . The chemical shift for 2- b r o m o -

1 ,3 ,2-dithiaborolane, Br-B-SC2H4S, is - 59.8 c o m p a r e d to - 62.7 for 2- c h l o r o - l , 3 ,2-dithiaborolane, CI-B-SC2H4S.

The shifts show the same trend as the values for BBrg and BCI3, which are - 38.5 a nd - 4 5 .6 , respectively.

It appears that the same arguments given above to account

for the chemical shifts of the boron trihalides are

satisfactory for the diboron tetrahalides and the

2 - h a l o - l , 3 ,2-dithiaborolanes.

The "tau" values for the methylene hydrogens in

the heterocyclic borolanes are in general consistent

for the diboron compounds.

CI-B-SC2H4S 6.57a

B2CI2(S2C2H4) —--

®2^^2^2^4^2 6.6 8

CI-B-OC2H4O 5.82a I______1 B 2 (02C 2H 4)2 5.85

B2CI2(O2C2H4) 4.74

(CH3)2N-B-SC2H4S 7 .00*

B2 [N(CH3)2]2(S2C2H4) 7.12

aValues taken from reference 9 2 . 87

However, the methylene hydrogens for B2CI2(02^2 ^4 ) appear at a somewhat lower field than expected. Perhaps this can best be explained in that chlorine is a poorer

"pi" bonder than oxygen; therefore, there is a greater drain of electron density from the glycol group to the diboron framework than there is in systems such as

82(03^2^4)2 CI-B-OC2H4O, Since the "tau" values for the methylene hydrogens in Cl-B-SC2H^S and Cl-B-OC^H^O are nearly equal to those for B2 (8202114) 2 82(0202114)2, respectively, this could suggest the latter two compounds assume the bicyclic-type structure.

The stability of the following series of diboron compounds decreases in the order shown; B2(NR2)4 >

82(OR)4 > B2X4 (7 ,83). The reactivity, which exhibits the Lewis acid character of these diboron compounds, has the reverse order. The order of stability and reactivity has been rationalized in terms of the extent of multiple-bonding between boron and the donor atom.

Initially, one would predict that B^^ resonance for boron-nitrogen bonded compounds would occur at a higher field than for boron-oxygen compounds. Consider first some simple boranes such as B[N(0285)213 and B (00285)3, 88

The chemical shifts are - 31.0 (94) and - 18.1 , respectively,

Now consider some tetra-substituted-diboron compounds such as B2[N(CH2)2]4 and B2(OC2H^)^. The chemical shifts for these compounds are - 36.1 a n d - 30.7. C o m p a r e those values with some heteronuclear ring diborons.

The chemical shifts determined for B 2 [(NCH3)2C2H4]2 and 82(02^2^4)2 - 33.7 and - 31.5 , respectively. In all three cases of comparison, the chemical shifts of the boron-oxygen compounds appear at higher fields than the boron-nitrogen compounds.

It is of interest to compare the chemical shifts of the compounds shown in the series below.

C h e m i c a l S h i f t

B F 3 - 9.4a

B(OC2Hs)3 - 18.1

B(N(C 2H 5)2 l3 - 3l.O^

B2F4 -22.8

B 2 (0C 2H s )4 - 30.7

82(020384)2 - 31.5

B 2 [N(CH3)2 l4 - 36.1

^Values taken from reference 88.

^Values taken from reference 94 , 89

B2 [(NCH3)2C 2H 4]2 - 33.7

BCI3 - 4 5 .6^

B(S-n-C4Hg)3 -66.QC

■i-C4H9-B-SC2H4S - 7 0 .0°

B2CI4 - 62.6

B2(S2C2H4)2 — 68.3

Diboron compounds, in general show the same trend in the placement of the chemical shift as the analogous tris-

substituted-boranes. For Period Two substituents bonded

to boron, the trends for electronegativity and "pi" bonding ability are shown below.

N O F

increase in electronegativity ^

------decrease in "pi" bonding 3».

Considering these trends, one might predict that the b H

resonance for the boron-nitrogen bonded species would

appear at the highest field, with boron-fluorine bonded

species at the lowest field. However, the reverse order

is observed. Similar results are observed for sulfur

and chlorine in Period Three. If electron density is

^Values taken from reference 9 1 . 90

to be related to chemical shift, then it is apparent that other factors contribute significantly to the placement of b H n.m.r. resonance for these species.

In conclusion, the sulfur containing diboron com­ pounds are better Lewis acids than the oxygen containing ones. Boron -11 chemical shift data are consistent with electron density by placing the boron-oxygen compounds at considerably higher fields. From the placement of the chemical shift, it appears that the extent of multiple-bonding between boron and sulfur is very small.

The correlation of electron density with chemical shift data for boron-oxygen and boron-nitrogen diboron com­ pounds are inconsistent with respect to their Lewis acid character and stability, but show the same trends as the analogous tris-substituted-boranes. The chemical shifts of the heteronuclear diboron ring compounds in general show the same trend as their borolane counterparts.

Proton n.m.r. spectra for ethylene glycol, 1 ,3-pro- panediol and ethanedithiol were obtained with an external reference of tetramethyl-silane. Experimental data for the molar suspectibilities were not available; therefore. 91

"tau" values could not be recorded. Suitable solvents in which the above ligands were miscible could not be found without solvent-ligand interaction. EXPERIMENTAL

I. Apparatus

A, Vacuum system

Due to the high sensitivity of diboron tetrachlor­ ide and its reaction products to oxygen and moisture, all reactants and products were handled in a standard vacuum system similar to that described by Sanderson

(95). The vacuum apparatus consisted of a pumping section, a main manifold, two reaction trains, and a distillation train. All pressures were measured with mercury manometers. Volumes were calibrated with sulfur hexafluoride by standard procedures. The system was evacuated using a high capacity Duo-Seal forepump.

A cold trap cooled with liquid nitrogen preceded the forepump.

The distillation train consisted of three calibra­ ted traps with a total volume of 35 8 milliliters and a

92 93 mercury manometer. The traps could be used individually or as a unit. The train was used for low temperature fractionation of volatile mixtures and especially for the measurement of condensible gases.

The reaction trains contained several reaction stations consisting of a standard taper 14/35 mal e joint separated from the reaction manifold by a teflon vacuum stopcock. Teflon was used because of the high reactivity of diboron tetrachloride with vacuum stopcock grease. On each station there was a mercury

"blowout" between the joint and the stopcock. On

these reaction trains , materials were introduced into the vacuum system and reactions were carried out.

The reaction trains were connected to each other and in turn each connected separately to the distillation train and the main manifold.

B. Discharge cell and automatic modified toepler pump system

Diboron tetrachloride, B2CI4, was prepared by passing BCI3 at low pressures through a mercury-arc discharge cell which is shown in Figure 1 . B o r o n 94

*Z ^ Tungsten rod \ / K

Copper wool

— 12 cm. - arc path

Mercury pool

Figure I. Discharge cell for preparation of Diboron Tetrachloride. 95

trichloride has a pressure of approximately 3.5 m i l l i ­ meters at - 78.5“C, The arc-path length of the discharge cell should be between 5-15 centimeters. The discharge cell is water cooled and operated by a luminous tube

(neon sign) transformer of 15,000 volts and 30 milli- amperes. Boron trichloride can be continuously recycled through the discharge cell by using an automatic modified

Toepler pump system. Figure 2 gives a schematic diagram of the entire system. Figure 3 shows a wiring diagram for the automatic operation of the system.

The mercury in the modified Toepler pump serves as both a piston and an electrical conductor. Viewing

Figures 2 and 3 , the system operates as follows; when the mercury makes contact with electrodes (1) and (3), the latching relay activates the vacuum pump solenoid causing the mercury to recede to the lower bulb. Boron trichloride then expands into the upper bulb of the pump. When the mercury makes contact with electrodes

(1) and (2), the latching relay activates the air leak solenoid and the luminous tube transformer. This allows the mercury to force the BCI3 from the upper bulb through the operating discharge cell into the - 78,5 ®C tra p To manometer

(3) To T2

To air leak (2) vacuum line

W a t e r BCl cooling both I ' To vocuum Me r c ury V p ump

Figure 2. Automatic arc discharge system.

C\ 97

(2)

O -e- o o 8 l i o o 10 i - 8 |

o (3)

T| — 6 volt filament transformer Tg — 15,000 volt luminous tube transformer (o),(7) — Coils on 4PDT latching relay (4 )— Air leak solenoid (5)— Vacuum pump solenoid (!)— Common electrode on mercury pump (2),(3)— Regulator electrode on mercury pump

Figure 3. Electrical circuit for automatic operation of discharge. 98

completing the cycle. Diboron tetrachloride has only a slight vapor pressure at this temperature.

The discharge cell can also be operated by simply passing BCI3 repeatedly through the discharge cell from a - 78.5°C trap to a - 196°C trap. Yields obtained by both methods are small and are of the order of one milliliter of pure B2CI4 per two weeks of operation.

Increased yields can be obtained by placing two or more discharge cells in parallel. This method of the prep­ aration of B2CI4 is given in more detail elsewhere (8 ,9).

C. Cryoscopic molecular weight determinations

Molecular weight determinations were performed using a standard cryoscopic cell. Figure 4 , The differ­ ential Beckman thermometer was first calibrated with pure dry benzene. The cell was flushed with dry nitrogen before each run. The sample was stirred with a magnetic hopper type, spiral stirrer. The cooling bath was ice-water (0 “C). In a normal determination, the solid sample was placed in a dry 50 milliliter

Erlenmeyer flask, then stoppered with a rubber stopple.

Benzene was transferred to the Erlenmeyer flask with a 99 Solenoid with periodic circuit breaker

Iron bar in stirrer Beckman Gloss differential stirrer t h e r m 0 me te r

24x inner joint Ï joinls glued to thermometer

T 24/ dewor ^ 4 o joint in cell cop i - % o

Close fitting vacuum jacket

Dewar for cooling both

2 0 cm

Figure 4. Cryoscopy apparatus, 100

hypodermic syringe through the stopple without exposure

to the atmosphere. The flask was weighed before and

after the sample was added and then weighed again after

the addition of benzene. The contents of the flask were agitated until the solid was completely dissolved

and the resulting solution homogeneous, A sample, with

the aid of a long needle and syringe, was transferred

to the cryoscopic cell and the cooling curve was deter­

mined, taking temperature measurements every thirty

s e c o n d s .

D, Reaction and transfer techniques

Reactions were carried out in reaction vessels of

the type shown in Figure 5 , These vessels, in addition

permit the transfer of non-volatile materials between

the vacuum system and the dry-box without exposure to

atmospheric conditions. Non-volatile liquid reactants

were transferred in the dry-box directly to a tared

reaction vessel and weighed on an analytical balance.

Volatile reactants were measured in the calibrated trap

section of the vacuum system at room temperature.

Solid reactants, after transferred in the dry-box 101

1-mm teflon vacuum - stopcock 2 4 0- Ring Boll 4 0 joint

Stirrer

Stir bar

A B

Figure 5. Reaction vessels. 102

were weighed in a capped weighing peg before being intro­

duced into a reaction vessel. The dry-box was contin­

uously flushed with dry nitrogen. Reaction products were transferred from the reaction vessels to dry

labeled K-vials.

E. Infrared spectra

Spectroquality KBr was ground and then dried under

v a c u u m at 250“C. The solid products whose spectra were

under investigation were ground with the appropriate

amount of dry KBr in the dry-box. The pellet was then

pressed very rapidly in the open air. Spectra were

obtained on a Perkin-Elmer Model 337 Grating Infrared

Spectrometer, This instrument produces spectra in the

range of 2,5 to 25 microns. The spectra were calibrated

by reference peaks at 6.245 and 11.035 microns of poly­

styrene unless otherwise stated,

F, X-ray measurements

X-ray powder diffraction patterns of the crystal­

line solids were obtained employing a Debye-Scherrer

type camera with a 11,46 centimeter effective diameter

used with a North American Phillips X-ray generator. 103

Samples were loaded in the dry-box into 0.5 millimeter capillary tubes, sealed with stopcock grease, removed from the dry-box and sealed off with a very small flame. The patterns were obtained under the following conditions; C o p p e r targ e t (K 1.5418 A “ ), n i c k e l filter,

32 kilovolts, 12 milliamperes, and a 10-24 hour exposure time depending on the compound.

G. O s m o m e t e r

A Mechrolab Vapor Pressure Osmometer, Model 301A was used for obtaining osmometric data. Chloroform served as the solvent.

H, Nuclear magnetic resonance spectra

B o r o n -11 n.m.r, spectra were obtained with a Varian

HR -60 high resolution spectrometer operating at 19.25 megacycles. Proton n.m.r. spectra were obtained using a V a r i a n A -60 spectrometer operating at 60 megacycles.

Exact concentrations of solutions were not recorded.

Generally, a given quantity of solvent was saturated with the product whose spectrum was under investigation, then a sample of the solution was syringed directly into the n.m.r, tube under inert atmospheric conditions. 104

In most cases, a 15 millimeter tube was used for the boron samples, A capillary filled with boron trifluoride

etherate or trimethylborate was placed in each boron

sample tube as an external reference. The solvent

served as an internal reference for proton spectra.

II. Starting Materials

A. Boron trihalides , BCl3 and BF3

Boron trichloride and boron trifluoride were ob­

tained from the Matheson Company in commercial cylinders

and used without further purification.

B. Diboron tetrachloride, B2CI4

Diboron tetrachloride was prepared by the mercury-

arc discharge method as described previously. After

the discharge had operated for approximately 100-200

hours, the discharge pot was fractionated through a

series of traps maintained at - 6 3 .5°, - 78.5 °, - 96 .7°,

and - 196°C, The B2CI4 was condensed in the -7 8.5°

and - 96. 7°C traps. This was then distilled into a

storage bulb on the vacuum system which was maintained

at - 78.5°C until used. 10 5

C, Tetrakis-(dimethylamino)-diboron, B2 [N(CH3)2]4

Tetrakis-(dimethylamino)“diboron was obtained from the U.S. Borax Company as complimentary samples. A purified dry compound was obtained by warming over

finely divided sodium for several hours and then dis­ tilled under vacuum at low temperatures.

D, Diols

Ethylene glycol, C2H4(OH)2, was obtained from

Matheson Coleman and Bell. 1 , 3-Propanediol , C3Hg(OH)2f was purchased from the Shell Chemical Company. These

glycols were dried by heating vigorously over a mixture

of BaO and CaH2. Because diols react slowly with BaO

and CaH2? only a slight excess of the drying agents were used than the anticipated moisture content. The

purified dry compound was isolated by fractional

distillation under vacuum.

E, Dithiols

Ethanedithiol , C2H4(SH)2, and 1 ,3-propanedithiol,

C3Hg(SH)2, were secured from the Aldrich Chemical

Company. Dithiols were purified and dried as described

for diols. Finely divided sodium was also used as a

drying agent. 106

F. 2~Mercaptoethanol, C2H^(SH)(OH)

2-Mercaptoethanol was obtained from Eastman Chemi­ cal, dried, and purified as indicated for the diols.

Stable non-volatile materials, after being dried and purified, were stored in stoppered bottles in the dry-box where transfer to the reaction vessel took place.

G. C a t e c h o l , C g H 4(OH)2

Catechol was purchased from Eastman Organic and used without further purification. Before use, the sample was pumped on directly for several hours.

H. Styrene glycol, CzHsfCgHs)(0H)2

Styrene glycol was obtained from Arapahoe Chemical

Company and purified by recrystallization from benzene.

The sample was pumped on directly for several hours before using.

I. 0-aminophenol. C6H4(0H)(NH2)

0 -aminophenol was purchased from K & K Laboratories and purified by sublimation. 10 7

J. Sym-dimethylethylenediainine , C 2H 4 (NHCH3)2

Sym-dimethylethylenediamine was obtained from the

Aldrich Chemical Company, It was warmed over divided

sodium for several hours and then distilled under

vacuum at low temperatures,

K, 0 -phenylenediamine , Cg H 4 (NH2)2

0 -phenylenediamine was obtained from The Aldrich

Chemical Company and used without further purification.

Samples were pumped on directly before being used,

L, Aluminum trichloride, AI2CI6

Aluminum trichloride was obtained from Fisher and

was sublimed at 105*- 110°C before using,

M, Antimony trifluoride, SbF]

Antimony trifluoride was purchased from Alfa

Inorganics and purified by sublimation,

N, Diphenylacetylene, C 2 (CgH5)2

Diphenylacetylene was obtained from The Aldrich

Chemical Company and used without further purification. 108

0. Ethyl alcohol, C 2 H 5 OH

Ethyl alcohol was dried by refluxing over CaClg and magnesium ribbon. The purified, dried material was collected at the proper boiling point range.

P. Trimethylamine, dimethylamine, methylamine, and ammonia; NfCH])], NH(CH3)2, NH 2 CH 3 , N H 3

These amines were obtained from The Matheson

Company in commercial cylinders. They were dried over divided sodium in the vacuum system and stored at -78°C until used, Trimethylamine was also dried over CaH 2 .

Q, Hydrogen chloride, HCl

Hydrogen chloride was obtained in a commercial

cylinder and used directly.

R. A c e t y l e n e , C 2 H 2

Industrial acetylene was purified by low tempera­

ture fractionation through a series of traps maintained

at -111°, -140°, and -196°C. The acetylene was iso­

lated in the -196°C trap. 109

S. Solvents

1. Benzene, dichloromethane, chloroform, and pentane: CgHg, CHgClg, CHCI3, C5H12

These solvents were refluxed over CaCl 2 and C a H 2 and then distilled into one liter storage bulbs where they remained until used.

2. Diethyl ether and diglyme; (C2 H 5 )2 0 , (CH3OC 2 H 5 )2O

Diethyl ether and diglyme were warmed with stirring o ver L i A l H 4 . The dry solvent was recovered by vacuum

distillation.

3. Acetonitrile, CH 3CN

Acetonitrile was purified and dried by refluxing

first over (P20s)2 (5 grams per liter of CH 3C N ) , and

then refluxing the distillate over CaH 2 . The pure,

dry C H 3CN was collected at the proper boiling point

range.

III. Analytical Procedures

A. Boron analyses

Boron analyses were performed with the aid of a

series 2320, 8 milliliter, semi-micro Parr Bomb. The

bomb was thoroughly dried in a 110“C oven and introduced 110

into the dry-box. The following quantities of materials constitute the bomb charge; 0 , 2 gram of KCIO 4 (acceler­

ator) , 40-80 milligrams of confectionary powdered sugar or benzoic acid (combustion aid), and 4.0 grams of Na 2Û 2

(oxidizing agent). The materials were thoroughly mixed in the bomb. A weighed sample of 20-40 milligrams was then introduced into the bomb and again thoroughly mixed.

The bomb was then heated for one minute over a pointed flame of a gas-oxygen torch. The cooled open bomb was placed in a 250 milliliter beaker containing approximately 70 milliliters of warm water. After one hour, the bomb was removed from the solution with washing. The solution was made very acid with concen­

trated HCl or HNO 3 and allowed to evaporate to a volume of 50-60 milliliters. With the aid of a Beckman pH meter, the pH of the solution was adjusted to 6 .8 ,

saturated with mannitol, and back titrated to a pH of

6 , 8 with 0.0500 N. NaOH. The percentage of boron

could then be calculated directly.

The quality of results obtained depend largely on

the thoroughness of mixing the bomb charge, the small Ill particle size of the charge, and the reproducibility of the pH meter.

B, Chlorine analyses

Chlorine analyses were obtained using potentio- metric methods. Complete solution of an accurately weighed sample was obtained by digestion in basic aqueous media or by the aid of the Parr Bomb using a similar procedure as outlined under boron analyses.

The bomb charge consists of 0,2 grams of potassium nitrate, 40-80 milligrams of benzoic acid, 4 grams of

Na2Û2» and a sample size of 0.0 4-0.10 grams. Upon complete solution of the sample, the pH of the solution was adjusted to 5.5-5.0 with HNO 3 which was then ready for titration.

In the potentiometric precipitation titration of chloride ion with silver nitrate, a pure metallic silver rod serves as the indicator electrode with a saturated calomel electrode as the reference electrode.

The sample solution is isolated from the reference electrode to prevent chloride ion contamination by a bridge which contains a 4 per cent agar saturated potassium nitrate solution. A pH meter can serve as 112

a potentiometer. The equivalence point can be obtained by plotting volume of silver nitrate versus EMF. For the sample sizes cited, a standardized 0.1000-0.0500 N. silver nitrate solution serves as an appropriate con­ centration. With the collected data, the percentage of chlorine in the sample can be determined.

C. Nitrogen analyses

Nitrogen determinations were made using the Kjeldahl method. A sample of 20-30 milligram size was introduced

in a straight pyrex tube which measured 2 8 mm. diameter by 24 cm. in length. A few milligrams (approximately

15-25 each) of Selenium powder and sodium sulfate were

added to the sample along with 5-6 milliliters of con­

centrated sulfuric acid. The digestion tube was placed

in a sand bath and heated to 220*-230°C until complete

solution resulted. The solution was then transferred

to an appropriate volumetric flask. A 20 milliliter

aliquot was then taken and introduced into a micro-

Kjeldahl distillation apparatus. Excess concentrated

sodium hydroxide was added to the sample and the liber­

ated ammonia was steam distilled into 25 milliliters

of 0.0100 N. hydrochloric acid. The acid sample was 113

back titrated with 0.0500 N. sodium hydroxide to the equivalence point. From the collected data, the per­ centage of nitrogen in the sample could be calculated.

D. Carbon and hydrogen

Carbon and hydrogen analyses were carried out by

Galbraith Laboratories, Inc., Knoxville, Tennessee.

IV. Synthesis and Properties of Heteronuclear Diboron Ring Com­ pounds Considered in This Investigation

A. B2(02C2H4)2*

B 2 (0 2 C 2 H 4 ) 2 has been prepared in ample quantities by two different synthetic routes. (a) Diboron tetra­

chloride reacts with ethylene glycol in a ratio of 1:2

to yield the desired product. In the usual preparation,

dry ethylene glycol (8 millimoles) was transferred in

the dry-box to a tared reaction vessel (as shown in

Figure 5-A), weighed, cooled to -196°C, and evacuated

on the vacuum system. Approximately 25-35 milliliters

of dichloromethane was distilled into the reaction

*See Appendix for nomenclature. 114

vessel along with 4 millimoles of diboron tetrachloride

(measured in the gaseous state). The reaction mixture

was allowed to warm to -78°C where it was stirred for

3 hours and then warmed to 25°C over approximately a

2-3 hour period. The mixture was stirred at 25*C for

an additional 12 hours. Considerable pressure developed

in the reaction vessel due to the liberation of 16

millimoles of hydrogen chloride. The resulting solution was clear, without any insoluble materials. Dichloro­

methane was then slowly distilled away under vacuum,

A white solid product resulted which was removed from

the reaction vessel in the dry-box. If excess ethylene

glycol was used, separation was achieved by fractional

distination-sublimation. If very pure diboron tetra­

chloride was used, nearly 100 per cent yields could be

obtained. The overall reaction is represented as

follows:

B 2CI 4 + 2 C2H4(0H)2 --->B2(02C2H4)2 + 4 HCl

(b) Tetrakis-(dimethylamino)-diboron reacts with

ethylene glycol in the presence of HCl to yield the

desired product. In a typical reaction, 8 millimoles 115

of tetrakis-(dimethylamino)-diboron was weighed out in the reaction vessel. At least 16 millimoles of ethyl­ ene glycol was then syringed directly into the reaction vessel. After cooling to -196°C and evacuating, approx­ imately 25-30 milliliters of dichloromethane and 32 millimoles of hydrogen chloride were distilled into the reaction vessel. The reaction mixture was allowed to warm to -78°C; it was stirred at this temperature for 3-4 hours and then warmed slowly to room temperature over a 2 hour period. The reaction mixture was stirred at this temperature for an additional 12 hours with dimethylammonium chloride appearing as a precipitate in the dichloromethane. The dichloromethane was dis­ tilled away under vacuum, leaving a copious white solid.

The white solid was finely divided, agitated in approx­ imately 75-80 milliliters of benzene, and filtered with the vacuum filtering apparatus as shown in Figure 6 .

The product was soluble in benzene with the insoluble

salt collecting on the frit. The benzene was then

distilled away from the filtrate yielding the desired white solid product in yields of 60-70 per cent. The 116

100 ml.

Glass frit medium porosity To vacuum line

.24, 4 0

eceiver Cham ber

Figure 6. Vacuum filtering apparatus. 117

product can also be isolated from the reaction mixture by sublimation at 45°-50°C. The overall reaction can be represented as follows:

2 C2H4(0H)2 B2[N(CH3)2]4 + > B2(02C2H4)2 4 HCl + 4 (CH3)2 H 2NCI

It appears that the reaction proceeds through the formation of the intermediate, B 2CI 2 [N(CH3 )2 ]2 at very low temperatures (-96° to -78°C), which then reacts with ethylene glycol at slightly higher temperatures to yield products. The above cited intermediate was synthesized as reported elsewhere (63,66). The reaction of the intermediate with ethylene glycol in a 1:2 ratio yielded

B 2 (0302114)2 . The reaction sequence can be represented as:

B 2 [N(CH3 )2 ] 4 + 4 HCl — ^ B 2 CI 2 [N(CH3 ) 3 ] 2

+ 2 (CH3 )2 H 2 NC 1

B2CI2 [N(CH3)2 32 + 2 C2H4(0H)2 --»B2(02C2H4)2

+ 2 (CH3 )2 H 2 NC 1

The products from reaction (a) and (b) give iden­ tical x-ray powder diffraction patterns and are alike upon the comparison of physical properties. It appears 118

that a slight excess of ethylene glycol or hydrogen chloride has no effect on the product formed.

^ white crystalline solid which melts at 165°-167°C, sublimes in vacuo at 40°-45°C, and is soluble in dichloromethane, benzene, ether, and other weak organic coordinating solvents. The product slowly reacts with moisture. Molecular weight deter­ minations in benzene indicate a monomeric species.

The b H n.m.r. spectrum shows equivalent boron atoms with a chemical shift of -31.5 p.p.m. with respect to

FgB'OfCgHgjg in both dichloromethane and benzene. The

n.m.r. spectrum positions the -CHg- hydrogens at

5.85 T in CH 2 CI 2 . The product gives a simple and dis­ tinct x-ray powder diffraction pattern with an exposure time of 12 hours. The relative line intensities and

"d" values are given in Table 1. Refluxing the product vigorously in CgHg gave no change in the x-ray powder diffraction pattern. The infrared spectrum is shown in Figure 7.

Anal. Calcd, for C 4H g B 2 0 ^; C, 33.89; H, 5.69; B, 15.27;

0, 45.15; mol, wt., 141.74, Found: C, 33.74; H, 5.82;

B, 15.31; mol, wt. , 146.2 (0.085 m) in benzene. 119

TABLE I

X-RAY POWDER DIFFRACTION PATTERN DATA

^2 (O2C 2 H 4 )2 B 2 CI 2 (O2C2H4)

Intensity "d" (A*) I n tensity "d" (A°)

S 6.30 S 7.86 w w 5.08 S 4.92 vs 4.28 M 4.03 W 4.32 W 3.75 VS 3.92 VS 3.60 vs 3.48 M 2.974 w 3.20 s 2.814 w 3.12 w 2.858 w 2.755 vw 2.710 M 2.671 w 2.629 w 2.581 M 2.449 M 2.344 W W 2.076 M 2.262 M 2.136 w w 2.029 W W 2.096 w w 1.631 vw 1.965 vw 1.935 w w 1.594 VIV 1. 836 w w 1.535 vw 1.796 M 1.742 w 1.647 VIV 1.572 vw 1.546 w w 1.481 w w 1.423 w w 1.405 w w 1.285 ww 1.268 w w 1.248 w w 1.216

I n t e n s i t y Î S = strong; M = medium; W = weak; V = verj 3 0 0 0 2000 1500 cm 1300 1000 800 600 500 400 00

.20 11.035

.30

.40 I -50 < .60 .70 6.245

CO 2.5 3.0 4.0 5.0 6.0 7.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 Wavelength {microns) Figure 7. Infrared spectrum of .

3000 2000 1500 c m ' 1300 1000 800 600 500 400 0.0

.20

.30

S .40 S .50 < .60 .70

00 2.5 3.0 4.0 5.0 6.0 7.0 80 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 H Wavelength (microns) [O Figure 8 . Infrared spectrum of B2 CI2 ( 0 2 CgH^) . o 121

B. 2 ,3-Dichloro-l,4,2,3-dioxadiborinane, B 2 CI 2 (O2 C 2 H 4 )

2 ,3-Dichloro-l,4,2,3-dioxadiborinane was prepared by the reaction of diboron tetrachloride with ethylene glycol in a 1 : 1 stoichiometry using dichloromethane as the solvent. In a given preparation (a 4-6 millimole reaction), a known quantity of ethylene glycol was transferred to the reaction vessel under inert atmospher­ ic conditions. After cooling to -196“C and evacuating,

35-40 milliliters of dichloromethane and a 1:1 stoichio­ metric amount of B 2 CI 4 were condensed into the reaction vessel. The reaction mixture was allowed to warm to

-78“C with stirring. The mixture was stirred at -78°C for 3-4 hours then warmed slowly to 25°C over a 4-5 hour period and allowed to stir at this temperature for 1 2 hours. The white solid product precipitated out of solution which was isolated by filtration in approxi­ mately 85-90 per cent yields. A small quantity of

^ 2 (0 2 ^ 2 ^ 4 ) 2 plus an unidentified white residue were observed in the filtrate after distilling away the solvent.

B 2 CI 4 + C2H4(0H)2 — > B2Cl2(02C2H4) + 2 HCl 122

2 ,3-Dichloro-l,4,2,3-dioxadiborinane is a white crystalline solid which is only slightly soluble in most organic solvents and commences to melt with apparent decomposition at 175°-180°C. Molecular weight determinations in chloroform using osmometric techniques indicate a monomeric species. The n.m.r. spectrum shows equivalent boron atoms with a chemical shift of

-30.8 p.p.m. with respect to F 3 B* 0 (0 2 1 1 5 ) 2 in chloroform.

The proton n.m.r. spectrum records the -CH 2 - hydrogens at 4.74T. Two other very small singlets also appear in the spectrum at 4.52 and 5.85 7". The higher field value is consistent with the methylene hydrogens in

B2(02C2H4)2* The product gave a simple, distinct x-ray powder diffraction pattern. The "d" values and rela­ tive line intensities are given in Table 1. The infrared spectrum is shown in Figure 8 .

Anal. Calcd. for C 2 H 4 B 2 CI 2 O 2 : C, 15.74; H, 2.64;

B, 14.18; Cl, 46.47; 0, 20.97; mol. wt. , 152.61.

Found: C, 15.98; H, 2.79; B, 13.85; Cl, 45.30; mol. wt. ,

149.5 (0.027 m) in chloroform. 123

C. ^2( 0 2 ^ 6 ^ 4 )2*

Diboron tetrachloride or tetrakis-(dimethylamino)-

diboron reacts with catechol in a stoichiometric molar

ratio of 1:2 to yield the desired product. The condi­

tions under which the reaction is run and techniques

for isolation of the product are very similar to those

of 2 2 (0 2 0 2 ^1 4 ) 2 « If excess catechol is used, the product

is very difficult to separate from the unreacted cate­

chol. The reaction product from B 2 [N(CH2 ) 2 ] 4 appears

to contain an impurity as shown by x-ray powder dif­

fraction pattern data and both and n.m.r, spectra.

The B^^ n.m.r. spectrum shows two singlets with chemical

shifts of -30.8 and -8.1 p.p.m. with respect to

F^B'O(C 2 H 5 )2 » The low field value is consistent with

B 2 (Ü2 0 g H 4 )2 . The high field value is probably due to

dimethylamine coordination. The proton n.m.r. spectrum

verifies this by showing the disubstituted benzene ring

hydrogen resonance occurring at 2.72 T and a small

singlet at 3.17 T. From these observations, it can be

concluded that the reaction product is a physical

*See Appendix for nomenclature. 124

mixture of two solids with the major product being

^ 2 (02*^6^4^ 2 "

B 2 CI 4 + 2 CgHjfOHlg --- >- B2(02CgH4)2 + 4 HCl

2 C6H4(0H)2 B2(N(CH3)2]4 + >■ B2(Û2CgH4)2 4 HCl + 4 (CH3)2H2NC1

B2 (0 2 CgH 4 ) 2 is a white crystalline solid of low density, soluble in dichloromethane and most aromatic solvents, melts at 195°-198°C, and sublimes vacuo at 120°-130°C. Cryoscopic molecular weight determina­ tions in benzene indicate a monomeric species. The b 1 1 n.m.r. spectrum shows equivalent boron atoms with a chemical shift of -30.7 p.p.m. with respect to

F 3 B' 0 (C2 H g ) 2 in dichloromethane. The H^ n.m.r. spectrum in C H 2 CI 2 locates the disubstituted benzene ring hydro­ gens at 2 . 6 9 T . A very sharp and intense x-ray powder diffraction pattern is obtained with a 1 0 - 1 2 hour ex ­ posure, The "d" values and relative line intensities are given in Table 2, The infrared spectrum is shown in Figure 9.

Anal. Calcd. for C 1 2 H8 B2 O 4 ; C, 60.60, H, 3.39; B, 9.10;

0, 26.91; mol. wt., 237.82. Found; C, 60.35; H, 3.61; 125

TABLE 2

X-RAY POWDER DIFFRACTION PATTERN DATA

B 2 (O2 C 6 H 4 )2 B2(02C3Hg)2

Intensity "d" (A°) Intensity "d" (A°)

S 8.27 VS 5.61 M 6.44 W W 4.98 VS 5.37 M 4.77 w 4.56 VS 4.36 w w 4.31 VS 3.94 M 4.10 s 3.45 M 3.77 w 3.38 VS 3.59 M 3.26 s 3.35 VW 2.912 vs 3.11 vw 2.801 M 2.683 M 2.629 M 2.588 VW 2.485 VW 2.449 M 2.344 VW 2.330 M 2.238 w 2.230 WW 2.153 w 2.166 ww 2 . 1 0 8 w w 2.053 w 2.062 w w 1.961 w w 2.006 w w 1.922 ww 1.969 ww 1 . 860 w 1.910 w 1.797 ww 1.814 w w 1.743 vw 1.786 w w 1.716 w w 1.722 w 1.671 ww 1.658 w w 1.590 w w 1.628 w w 1.520 w w 1.567

Intensity; S =: Strong; M == medium; W = weak; V = very, 300 0 2000 1500 cm 1300 1000 800 600 5 00 400 00

.10

. 2 0

° .30

** .50 .60 .70

1.0 -

00 ______I______I_ 2.5 3.0 4 0 50 6 0 7.0 8.0 100 12.0 14.0 16.0 18.0 20.0 22.0 24.0 Wavelength (microns) Figure 9 . Infrared spectrum of8 2 1 0 2 ^6 ^4)2 '

3 00 0 2000 1500 cm 1300 1000 800 600 500 400 0.0

.10

S .20

.30

001--- 1— 2 .5 3.0 4.0 5.0 5.0 7.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 N> Wovelength (microns) G\ Figure 10. Infrared spectrum of B2(02C^Hg)2 - 127

B, 9.09; mol. wt., 241.0 (0.043 m) in benzene. Base hydrolysis gave 0,96 moles of hydrogen per mole of B-B compound.

D. B2(02C3H6)2*

8 2 (0 2 ^ 3 ^ 6 ) 2 been prepared by the reaction of diboron tetrachloride or tetrakis-(dimethylamino)-diboron with 1,3-propanediol in a molar ratio of 1:2 or with a slight excess of 1,3-propanediol using dichloromethane as a solvent. The same techniques and procedure as outlined for B 2 (Û2 C 2 H ^ ) 2 were used here.

B 2 CI 4 + 2 C3Hg(0H)2 -- >B2(02C3Hg)2 + 4 HCl

2 C3Hg(OH)2 B2[N(CH3)2l4 + > ^2^02^3^6)2 4 HCl + 4 (CH3)2H2NC1

^2 (0 2 0 3 H g ) 2 is a white crystalline solid which melts at 158“-162“C, sublimes ^ vacuo without decomposition at 45“-50®C, and is soluble in benzene, dichloromethane, and other non-coordinating organic solvents. The B^^ n.m.r. spectrum shows equivalent boron atoms with a chemical shift of -28.4 p.p.m. in benzene and -28.6 p.p.m. in C H 2 C]g with respect to F 3 B * 0 (C2 H 3 )2 . Molecular weight

*See Appendix for nomenclature. 128

determinations in benzene indicate a monomeric species.

The n.m.r. spectrum gives a quintet for the center

methylene group being split by the two end methylene

groups and a triplet for the end methylenes being split by the center methylene group. The y values are 8.12

and 6,12, respectively, with 11.1 and 5.7 c.p.s. coup­

ling constants. A very sharp and distinct x-ray powder

diffraction pattern was obtained with a 10-12 hour

exposure. The "d" values and relative line intensities

are given in Table 2. The infrared spectrum is shown

in Figure 10.

Anal. Calcd. for CgHQ^2 B 2 0 4 î C, 42.44; H, 7.13;

B, 12.75; O, 37.69; mol. wt., 169.80. Found: C , 42.13;

H, 7.09; B, 12.60; mol. wt., 165.1 (0.098 m) in benzene.

E.

The method of preparing ^2(^^^ 2 ^ 4 ) 2 with B^Cl^ is

virtually the same as for B2(02C2H^)2« It has been

found that when the 1:2 reaction is run with B 2 CI 4 and

ethanedithiol, a slight excess of ethanedithiol is

necessary to obtain the desired product. A

*See Appendix for nomenclature. 129

stoichiometric reaction of 1 ; 2 [B2 CI 4 /C 2 H 4 (SH)2 ] in

C 5 H 1 2 yielded a polymeric material. Dichloromethane was found to be the best suitable solvent,

B 2 CI 4 + 2 > B2(S2C2H4)2 + 4 HCl

Tetrakis-(dimethylamino)"diboron and ethanedithiol can not be added directly together at room temperature without side reactions occurring. The apparatus used

for this reaction is shown in Figure 11, A weighed quantity of tetrakis-(dimethylamino)-diboron (8 . 0 mmoles) was added to vessel (A) which was cooled to

-196°C, evacuated, and 25-30 milliliters of dichloro-

methane distilled in. This was then placed on the

"transfer apparatus" (B).

Bulb (C) was then attached which contained 16.2

mmoles of ethanedithiol. The ethanedithiol was then

distilled over into the reaction vessel (A) at low

temperatures. The required amount of HCl (32.1 mmoles) was distilled into bulb (C) and then allowed to expand

over into reaction vessel (A) which was stirred at

-96“C. The separation and isolation of the product

is identical to that of ^ 2 ^^2 *"2 ^^4 ^ 2 give yields of To 130 vacuum line

4 o

Stirrer

Figure II. Set-up for reactions with Hydrogen Chloride. 131

85-90 per cent. A slight excess of ethanedithiol or hydrogen chloride has no apparent effect on the product formed.

2 C H.(SH)

^ ^ ^ ^ 2 ^^2^ 2 4 ^ 2

+ 4 (CHglgHgNCl

® 2 ^^2 *"2 ^ 4 ^ 2 ^ crystalline white solid which is very soluble in benzene and dichloromethane but inter­ acts with most coordinating solvents. It melts with apparent decomposition (by color change) at 170°-175°C and appears to sublime slowly in vacuo at 85°-95°C; however, the sublimate was not checked for purity.

The compound appears to decompose readily when exposed to atmospheric conditions. Molecular weight determina­ tions in benzene indicate a monomeric species.

The n.m.r. spectrum indicates equivalent boron atoms with a chemical shift of - 6 8.3 p.p.m. with respect to FgB'OfCgHgjg in dichlorome thane. The n.m.r. spectrum shows the -CHg- hydrogens at 6 . 6 8 7 in C H 2 CI 2 .

The x-ray powder diffraction pattern exhibits distinct lines with a 12-16 hour exposure. The "d" values and relative line intensities are given in Table 3. The infrared spectrum is shown by Figure 12. 132

TABLE 3

X-RAY POWDER DIFFRACTION PATTERN DATA

B 2CI 2 (S2C2H4)

Intensity "d" (A°) Intensity "d" (A°)

VS 6.61 M 10.28 VS 5.01 S 8.27 w 4.73 VS 7.02 vs 4.08 s 6.26 vs 3.83 w 5.28 M 3.49 w 5.01 . M 3.36 s 3.83 VW 2.998 M 3.37 M 2.730 M 3.13 W 2.644 M 2.629 W 2.571 M 2.556 M 2.508 VW 2.301 WW 2.439 VW 2.232 WIV 2.270 VW 2.151 w w 2.222 VW 2.023 M 2.163 VW 1.951 WW 1.953 WW 1 . 888 w w 1.857 w w 1.799 w w 1.710 w w 1.671 w w 1.616 w w 1.579 w w 1.494 w w 1.453 vw 1.416

Intensity: S = strong; M = medium; W = weak; V = very. 5000 2000 1500 c m '' 1300 1000 800 600 400500 00

.10

.20

2 .30

« .40 < .50 .60 .70

2.5 3.0 5.040 6.0 7.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 Wavelength (microns) Figure 12.Infror'ed spectrum of 82 (S2 C2 H^)2 -

3000 2 0 0 0 1500 cm “ i 1300 1000 800 600 500 4 0 0 0.0

.20

^ .30

40 .50 .60 .70

1.0

2.5 3.0 4.0 5.0 6.0 7.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22 0 24.0 Wovelength (microns) W Figure 13. Infrared spectrum of2 Cl B2 {S2C2H4 ). w 134

Anal. Calcd. for C 4 H 3 B 2 S 4 : C, 23.32; H, 3.91; 10.50;

S, 62.27; mol. wt., 206.02. Found; C, 21,41; H, 3.77;

B, 10.05; mol. wt., 209.0 (0.047 m) in benzene.

F. 2 ,3-Dichloro-l,4,2,3-dithiadiborinane, B2Cl2(S2C2H4)

Diboron tetrachloride reacts with ethanedithiol

in a 1 : 1 stoichiometric molar ratio to yield 2 ,3-dichloro-

1,4,2,3-dithiadiborinane. Conditions and techniques for

synthesis of the product are very similar to those described for the preparation of B 2 CI 2 (O2 C 2 H 4 ) . The

resulting reaction mixture contained a dichloromethane

insoluble white solid. Upon filtration, the white

solid product was collected on the frit in approximately

70-75 per cent yields. After distilling away the di­

chloromethane from the filtrate, a white residue remained which gave an amorphous x-ray powder diffraction pattern

and a B ^ n.m.r. spectrum which showed two non-equivalent

boron atoms. The two peaks in the B^^ n.m.r. spectrum

gave chemical shifts of -68,3 and -63.3 p.p.m. with

respect to F 3 B»0 (C2 H 5 ) 2 and an approximate ratio of

areas under the curves of 3:1, respectively. 135

B 2 CI 4 + C2H4(SH)2— ^B 2 Cl 2 (S2 C 2 H 4 ) + 2 HCl

2,3-Dichloro-l,4,2,3-dithiadiborinane is a white,

very finely divided, crystalline solid which is vir­

tually insoluble in all non-coordinating organic sol­ vents, Molecular weight determinations could not be

obtained on this species due to insufficient solubility

in an appropriate solvent. It does not sublime and

melts with apparent decomposition starting at 175°-180°C,

The compound is very susceptible to moisture. The B^l n.m.r. spectrum shows equivalent boron atoms with a

chemical shift of -5 7.8 p.p.m. with respect to

F 3 B*0 (C2 H 5 ) 2 in diglyme. The n.m.r. spectrum in

diglyme could not be obtained, either because of insuf­

ficient concentration or the sample signal was masked

by the solvent signal. The x-ray powder diffraction

pattern is not as discrete as would be desired. The

"d" values and relative line intensities are given in

Table 3. The infrared spectrum is shown in Figure 13.

Anal. Calcd. for C 2 H 4 B 2 CI 2 S 2 S C, 13.00; H, 2.18;

B, 11.71; Cl, 38.39; S, 34.72; mol. wt., 184.75.

Found: C, 13.18; H, 2.40; B, 11,99; Cl, 37.90. 136

G, 2 ,3-Bis(dimethylamino)- 1,4,2,3-dithiadiborinane, B2 [N(CH3)2]2(S2C2H4)

Tetrakis-(dimethylamino)-diboron reacts with ethane­ dithiol in the presence of hydrogen chloride in a stoi­ chiometric molar ratio of 1 :1 :2 , respectively, to yield

2,3-bis(dimethylamino)-1,4,2,3-dithiadiborinane. The conditions, apparatus, and techniques for running the reaction are similar to those described for the prepara­ tion of B2(S2C2H4)2 from B2 [N(CH3) 2] 4. When the prod­ uct is extracted from the reaction mixture with ben­ zene, it is very difficult to obtain crystals from the solvent, but crystallization from a benzene-pentane solvent system afforded crystals in good yield.

C2H4(SH)2 B2[N(CH3)2]4 + > B2 [N(CH3) 2]2(S2C2H4) 2 HCl + 2 (CH3 )2 H 2 NC 1

2,3-Bis(dimethylamino)-1,4,2,3-dithiadiborinane is a white crystalline solid which is soluble in most organic solvents. It sublimes vacuo with slight decomposition at 70“-80°C, melts at 63“-66°C, and reacts readily with moisture. Molecular weight deter­ minations in benzene indicate a monomeric species. It 137 appears, however, that association occurs to some extent at higher concentrations. The n.m.r. spectrum shows equivalent boron atoms with a chemical shift of -43.7 p.p.m. with respect to F 3B» 0 (C2 H 5 ) 2 in benzene and dichloromethane. The Hi n.m.r, spectrum in CH 2 CI 2 shows two singlets with the -CH^ hydrogens at 7.23 T and the -CH 2 - hydrogens at 7.12 7”• A discrete, but what appears to be a complex x-ray powder diffraction pattern is obtained with a 20-22 hour exposure. The

"d" values and relative line intensities are given in Table 4. The infrared spectrum is shown in Figure

14.

Anal. Calcd, for CgHigN 2 B 2 S 2 : C , 35.68; H, 7.99;

N, 13.87; B, 10.71; S, 31.75; mol. w t . , 201.98.

Found; C, 35.34; H, 7.98; N, 13.74; B, 10.50; mol. wt. ,

219 (0.040 m) and 236 (0.064 m) in benzene.

H. 1,1',3,3'-Tetramethyl-2,2*- bi - 1 ,3 ,2 -diazaborolidine, B 2 [(NCH3 ) 2C2H412

1,1',3,3'-Tetramethyl-2,2'-bi-1,3,2-diazaborolidine can be prepared from the reaction of sym-dimethylethyl- enediamine with either diboron tetrachloride or 138 TABLE 4

X-RAY POWDER DIFFRACTION PATTERN DATA

32 [N(CH3)2]2(S2C2H4) 32 [(NCH3)2C2H4]2

Intensity "d" (A°) Intensity "d" (A°)

M 7.69 vs 8.46 VS 6.68 s 7.25 M 6.05 vs 6.44 VS 5.56 WW 6.07 M 5.01 WW 5.10 S 4.18 s 4.86 M 4.00 s 4.37 S 3. 83 s 4.22 VS 3.58 s 4.02 vs 3.36 M 3.83 M 3.17 M 3.75 WW 3.03 VS 3.61 WfJ 2.955 s 3.51 ww 2.726 M 3.38 ww 2.667 VW 3.21 ww 2.603 WW 3.10 vw 2.468 WW 3.05 vw 2.404 WW 2.964 vw 2.292 ww 2.871 ww 2.227 ww 2.801 vw 2.161 ww 2,710 vw 2.113 vw 2.641 ww 2.067 w 2.46 8 vw 1.965 vw 2.386 Vl'J 1.920 ww 2.287 w 1.823 vw 2.049 ww 1.783 ww 1.979 w w 1.664 w w 1.947 ww 1.615 vw 1.796 Wl'7 1.576 w w 1.549 w w 1.522 w w 1.289

Intensity; S = strong; M = medium; W = weak; V = very. 3000 2000 1500 cm"' 1300 1000 800 6 0 0 500 4 0 0 00

.10

.20

« .50 < .60 .70

1.0

00 2.5 3.0 4.0 5.0 60 7 0 8 0 100 12.0 14.0 16.0 18.0 20.0 22.0 24.0 Wavelength (microns) Figure 14.1nfrared spectrum of Bg ( SgCgH^.

3000 2000 1500 cm 1300 1000 800 600 500 400 00

.10

.20

.40 .50 .60 .70 9.724 1.0 001 I_ 2.5 3.0 4.0 50 6.0 7.0 8.0 10.0 12 0 14.0160 18.0 20 0 22.0 24.0 Wavelength (microns) Figure 15.Infrared spectrum of BgC 2 NH(CH^)g. t-" W VO 140

tetrakis-(dimethylamino)-diboron. (a) Diboron tetra­

chloride (5.38 mmoles) was condensed into a system

containing sym-dimethylethylenediamine (2 1 . 6 mmoles)

and 25 ml. of pentane or dichloromethane at -196°C.

Reaction was allowed to occur over a temperature range

of -78° to 25°C for 17 hours which resulted in the

formation of a white precipitate. After distilling

away the solvent, the resulting white reaction product was extracted with benzene. Upon distillation of the

benzene from the filtrate, white crystals of the prod­

uct slowly formed on the walls of the reaction vessel.

A dry crystalline compound was obtained by pumping on

the product directly for several hours,

B 2 CI 4 + 4 C 2 H 4 (NHCH3 ) 2 -- ^ B 2 [(NCH3 )2 C 2 H 4 ] 2

+ 2 C 2 H 4 [N(CH3)H2C1]2

(b) Tetrakis-(dimethylamino)-diboron (23,2 mmoles)

and sym-dimethylethylenediamine (47.1 mmoles) were

added to a small round bottom flask attached to a

reflux condenser equipped with a calcium chloride

drying tube. The reaction mixture was heated at

115°-120°C with stirring for 6 hours. At the end of

this period, the evolution of dimethylamino had ceased. 141

A pure compound was afforded by low temperature crystal­

lization from a benzene-pentane solvent system.

B2[N(CH3)2]4 + 2 C2H4(NHCH3)2 B2 [ (NCH3) 2C2H4 ] 3

+ 4 NH(CH3)2

The products from (a) and (b) are identical in all

physical respects.

1,1',3,3'-Tetramethyl-2,2'-bi-1,3,2-diazaboroli­

dine is a white crystalline solid which is soluble in

most organic solvents. It melts at 45°-47*C and does

not sublime. It decomposes only slowly when exposed

to atmospheric conditions. The n.m.r. spectrum

shows equivalent boron atoms with a chemical shift of

-33.3 p.p.m. in CgHg and -33.7 p.p.m. in CH 2 CI 2 with

respect to F 3 B« 0 (C2 H 5 )3 . Two single peaks were ob­

served in the n.m.r. spectrum with the -CH 2 - hydro­

gens at 6.83 7* and the -CH 3 hydrogens at 7.34 T ,

A sharp and intense x-ray powder diffraction pattern

is obtained with a 8-10 hour exposure. The "d" values

and relative line intensities are given in Table 4.

The infrared spectrum is shown in Figure 16.

Anal. Calcd. for CgH 2 gN 4 B 2 : C, 49.55; H, 10.40; 3 0 0 0 2000 1500 cm 1300 1000 800 6 00 500 400 00

.10

.20

.40 2 50 < .60 .70

1.0 œ 2.5 3.0 4.0 50 6 0 70 8.0 100 12.0 14.0 16.0 18.0 20.0 22.0 24.0 Wavelength (microns) Figure 16.Infraredspectrum of Bg |^(

______3000 2000 1500 cm~i 1300 1000 8 00 600 500 400 0.0

.10 o .20

.40 5 0 - .6 0 - .7 0 -

1.0

CO 4.0 5.0 6.0 7.0 8.0 10.0 120 140 16.0 18.0 20.0 22.0 24.0 Wove length (microns) Figure 17,Infrared spectrum of B2 j{ NH)^ . 143

N, 28.89; B, 11.16; mol. wt., 193.92. Found: C, 49.31;

H, 10.22; B, 11.02.

I. B2[(NH)2CgH4]2*

(a) Tetrakis-(dimethylamino)“diboron (16 millimoles), o-phenylenediamine (32 millimoles), and 75 milliliters of benzene were introduced into a 250 milliliter 3-neck flask equipped with a water-cooled condenser and paddle stirrer under gas flush. The reaction mixture was refluxed vigorously until the evolution of dimethyl- amine had apparently ceased which was about 24 hours.

During the reaction, the white product slowly precipi­ tated out of solution. Upon termination of the reac­ tion, the resulting reaction mixture was filtered which isolated white solid product on the frit. No further recrystallization was necessary. Any unreacted o-phenylenediamine is soluble in benzene.

B2[N(CH3)2]4 + 2 CgH4(NH2)2 — > B 2 [ (NH)2CgH4 ] 2

+ 4 NH(CH3)2

(b) Diboron tetrachloride (2-4 millimoles) was allowed to react with o-phenylenediamine in a molar

*See Appendix for nomenclature, 144

ratio of 1:6 in dichloromethane or benzene. Reaction

occurred very slowly over a temperature range of -78°

to 25°C. The reaction iru.xture was stirred for 16 hours

at 25°C with the formation of a copious white precipi­

tate. The solvent was distilled away leaving a nearly white reaction product. The reaction product was

heated in a horizontal sublimer at 140°-160°C range,

which sublimed out the salt. The residue was finely

divided and reheated to 200°-210°C to obtain complete

removal of the salt. The residue which remained gave

an x-ray powder diffraction pattern of which the major

intense lines corresponded to those obtained from the

tetrakis-(dimethylamino)-diboron reaction. However,

some very weak lines indicated that the residue was

not pure. Isolation of a pure product using diboron

tetrachloride proved to be very difficult. A material

isolated from a reaction with a 1:4 molar ratio, gave

no evidence for the required product.

B2CI4 + 6 CgH4(NH2)2 > B2f(^H)2C6H4]2

+ 4 C g H 4 (NH2 )NH 3 Cl

The product with an empirical formula of 145

B 2 [ (NH)2 C 6 H 4 ]2 / is a low density white solid which ap­

pears aromatic in character. It is extremely stable both thermally and hydrolytically. It does not melt

up to 280“C and sublimes very slowly ^ vacuo at

230°-235*C. It is virtually insoluble in all organic

solvents; therefore, molecular weight determinations were not obtainable. The B Ü n.m.r. spectra in ace-

tonitrile and diglyme indicate equivalent boron atoms but the chemical shifts were inconsistent being -27.9

and -23,1 p.p.m., respectively, with F3B.0(C2Hg)2 as

the zero reference in the solvents indicated. The

sample was insufficiently concentrated to give an

n.m.r. signal in CH 3 CN. The product gave a distinct

x-ray powder diffraction pattern with an exposure time

of 14 hours. The relative intensities and "d" values

for the lines are shown in Table 5. The infrared

spectrum is shown in Figure 17.

Anal. Calcd, for C2^2'^12^4®2* ^ f 61.63; H, 5.17;

N, 23.95; B, 9.25; mol. wt. , 233.86. Found: C, 61.39;

H, 4.96; B, 9.31. 146

TABLE 5

X-RAY POWDER DIFFRACTION PATTERN DATA

B2[(NH)2CgH4]2 B2(S2C2H4) 2 * 2 NH(CH3)2

Intensity "d" (A°) Intensity "d" (A°)

VS 12.03 M 8.93 M 6.61 S 7.53 M 6.05 VS 6.84 VS 4.44 vs 5.74 M 3.93 w 5.37 W 3.78 M 4.98 S 3.63 s 4.28 M 3.32 w 3.98 M 3.17 M 3.65 S 3.07 VW 3.39 S 3.02 W 3.18 w 2.826 w 3.02 w 2.637 M 2.931 w w 2.318 W 2.849 w w 2.214 VW 2.780 vvw 2.125 vw 2.698 w w 2.062 vw 2.629 vvw 1.989 vw 2.417 w w 1.882 w w 2.365 w 1.821 w 2.312 vw 1.726 VW7 1.796 W-J 1.743 w w 1.694 w w 1.637

Intensity; S = strong; M = medium; W = weak; very. 147

J. B2(S2C2H4)2’2 NHfCHg)]

The dime thy lamine adduct of 8 2 (5 2 ^ 2 ^ 4 ) 2 has been prepared by essentially two methods. (a) Tetrakis-

(dimethylamino)-diboron (6,2 mmoles) and ethanedithiol

(12.8 mmoles) were mixed together in a closed system without solvent. The mixture was agitated and allowed

to equilibrate at room temperature. Slowly, white

crystals began to form at the interface of the liquid mixture and the glass walls of the reaction vessel.

Once the reaction starts, it proceeds slowly with the evolution of a small amount of heat. The reaction mixture was allowed to stand overnight which yielded a pinkish solid plus excess ethanedithiol. All the volatile species were distilled away leaving a pinkish solid. Recrystallization of this solid from a benzene- pentane solvent system afforded a white crystalline product. The filtrate from the recrystallization gave a pink gummy solid.

(b) Dimethylamino was added to 3 2 (8 2 0 2 8 4 ) 2 dis­ solved in benzene in a molar ratio of at least 4:1.

The resulting reaction mixture was allowed to stir for 148 several hours at 25°C, then at 35°-40°C for 2-3 hours to yield a very light pink colored solution. Upon concentrating the reaction solution and the subsequent addition of pentane, a white solid product was isolated by filtration which was identical to the product in part (a).

The 2:1 dime thy lamine adduct of 8 2 (8 2 0 2 1 1 4 ) 2 is a crystalline white solid which melts at 108°-114°C, soluble in most organic solvents, and appears to be quite stable. The n.m,r, spectrum shows equivalent boron atoms which are 1 1 , 8 p,p,m. downfield from

F^B'O (0 2 % ) 2 in C H 2 CI 2 , The proton n.m,r, spectrum in C H 2 CI 2 exhibits two singlets with the methyl hydro­ gens at 7,35 T and the methylene hydrogens at 7,06 T ,

The product gave a distinct x-ray powder diffraction pattern with an 18 hour exposure. The "d" values and relative line intensities are given in Table 5.

Anal, Calcd. for 0 2 ^ 2 2 ^ 2 8 2 8 4 : 0, 32,44; H, 7,49;

N, 9,46; B, 7,31; S, 43,31; mol, wt,, 296,22, Found;

C, 32,26; H, 7,33; B, 7.13; mol, wt,, 285 (0,012 m)

in benzene. TABLE 6

BORON-11 N.M.R. CHEMICAL SHIFT DATA AND PHYSICAL PROPERTIES FOR PREPARED DIBORON COMPOUNDS

B Ü n.m.r. Chemical Shift M. Pt. Subl. Pt. p • p • in* Compound F3B.0(C2H5)2 Solvent (°C) (°C/10-2 ram Hg)

B 2 (O2 C 2 H 4 ) 2 -31.5 CH 2 CI 2 , CgHg 165-167 40-45

B 2 CI 2 (O2 C 2 H 4 ) -30.8 C H C I 3 175-180 (decorap.)

^ 2 (02*^6^45 2 -30.7 CH 2 CI 2 195-198 120-130

B 2 (02C3Hg)2 -28.4,-28.6 CgHg,CH 2 Cl 2 158-162 45-50

® 2 (S2C2H4)2 -68.3 CH 2 CI 2 170-175 (decorap.)

B 2 CI 2 (S2 C 2 H 4 ) -67.8 diglyme 175-180 (decorap.)

B2 [N(CH3)2]2(S2C2H4) -43. 7 CH 2 CI 2 ,CgHg 6 3— 66 70-80 (si. decorap.) B2[(NCH3)2C2H4]2 -33.3,-33.7 CgHg,CH 2 Cl 2 45-47

t-" .b. VD TABLE 6 - Cont'd.

n.m.r, Chemical Shift M. Pt. Subl. Pt. >.m.__ Compound F3B.0(C2H5)2 Solvent (°C) (°C/10“2 mm Hg)

B2 [ (NH)2C6H432 -27.9 ,-23.1 CH 3 CN,diglyme > 280 230-235 (slowly)

B 2 (S2 C 2 H 4 )2*2 NH(CH3)2 -11.8 CH 2 CI 2 108-114

B 2 C 1 4 - 2 NH(CH3)2 - 7.9 CH 2 CI 2 194-195 110-120 (decomp.)

Ul o 151

TABLE 7

PROTON N.M.R. DATA OF SYNTHESIZED DIBORON COMPOUNDS

-CH2- -CH3 -CgH4- Sol­ Compound II -j-" II II •J' II II II vent

B2(02C2H4)2 5.85 CH2CI 2

B2Cl2(02C2H4) 4.74 CHCI 3

B2 (O2C 6H 4) 2 2.69 CH2CI 2

B2 (O2C 3H 6)2 8.12 (Center) CH 2CI2 6.12 (End)

B2 {S2C2H 4)2 6,68 CH2CI2

B 2 [N(CH3)2]2(S2C2H4) 7.12 7.23 CH2CI2

B2 [(NCH3)2^2^412 6.83 7.34 CH 2CI 2

B2(S2C2H4)2*2 NH(CH3)2 7.06 7.35 CH2CI2 152

V, Chemistry of Diboron Compounds Syn­ thesized in This Investigation

In this section, some reactions of those diboron compounds cited in Section IV will be described. The reactions considered are essentially of two types; those which enabled the formation of adducts and those which led to substitutional derivatives. In general, reactions were allowed to occur in the types of reac­ tion vessels shown in Figure 5. In those reactions where the pressure approached or exceeded one atmos­ phere, reaction vessel (B) was used exclusively. Where filtration was necessary, the vacuum filtering appara­

tus shown in Figure 6 was employed. All reactions were carried out under vacuum. In those cases where a

temperature range of -78° to 25°C was used, generally

the reaction mixture was stirred at -78°C for 3-4 hours, then it was warmed slowly to room temperature

over a 2-3 hour period where stirring was continued

for the duration of the experiment. 153

A. Reactions of B 2 C l 2 (0 2 C 2 H 4 )

1, Ethanedithiol

Several attempts were undertaken to prepare mixed heteronuclear diboron ring systems. Ethanedithiol

(1.4 mmoles) was distilled into a reaction vessel con­ taining 1.1 mmoles of B 2 CI 2 (O2 C 2 H 4 ) and 15 ml. of

CH 2 CI 2 at low temperatures. The reaction was allowed to proceed over a temperature range of -78° to 25°C for

30 hours. Nearly complete solution of the reaction mixture resulted, A white product remained after dis­ tilling away the solvent. The insoluble residue from a C 5 H 2 2 extraction of the reaction mixture gave a dis­ tinct x-ray powder diffraction pattern and a B^^ chemi­ cal shift of 30.5 p.p.m. downfield from FgB'O(C 2 Hg)2 .

The observed data is consistent for (0 2 C 2 H ^ )2 . An x-ray powder diffraction pattern of the white solid from the CgHi 2 soluble fraction of the extract indicated a slight amount of B 2 (0 2 C 2 H4 ) 2 in addition to an amor­ phous solid. The product from another 1:1 reaction, which was not allowed to warm above -30°C with solvent 154

present, contained '^2 2 No evidence was obtained for the presence of B 2 (O2 C 2 H 4 )(S 2 C 2 H 4 ) or

B2(S2C2H4) 2 ’

B2CI2 (O2C2H4) + 02^14(311)2 ---5^ 2

+ HCl + amorphous solids

2. Catechol

Catechol (1.48 mmoles) and 1.47 mmoles of

B 2 CI 2 (O2 C 2 H 4 ) were added to a reaction vessel in addi­ tion to 15 ml. of C H 2 CI 2 which was distilled in at low temperatures. The reaction was allowed to take place over a temperature range of -78° to 25°C for 24 hours which resulted in complete solution of the reaction mixture. After distilling away the solvent, a white product resulted. An x-ray powder diffraction pattern of the product indicated a mixture of B 2 (0 2 0 2 1 1 4 ) 2 and

B 2 (0 2 0 ^ 1 1 4 )2 . Sublimation of the reaction product at

45°-55°C yielded a white solid sublimate, identified as 3 2 (0 2 0 2 1 1 4 )2 » and a white solid residue shown to be

^2 (° 2 *^6 ^ 4 ) 2 '

The products were identified by comparison of x-ray powder diffraction pattern data and melting 155 points with the known authentic compounds.

B2Cl2(02C2H4) + 05^14(011)2 ---> 1 / 2 B 2 (02C 2H 4)2

+ 1/2 B2(02C5H4)2 + 2 HCl

3. Ethanol

Ethanol (5.74 mmoles) was distilled into a reaction vessel containing 2.87 mmoles of B 2 CI 2 (O2 C 2 H 4 ) and

20 ml. of CH 2 CI 2 at low temperatures. The reaction was allowed to take place over a temperature range of -78° to -22°C for 16 hours with complete solution of the reaction mixture. After distilling away the solvent, a white solid and a non-volatile liquid remained in the reaction vessel. The liquid was separated from the solid by fractionation from a 40°C trap to a

-196°C trap. The solid was identified as B 2 (0 2 C 2 H 4 ) 2 from its x-ray powder diffraction pattern. The B^^ n.m.r. spectrum of the non-volatile liquid indicated two singlets which yielded chemical shifts of -29.8

(broad, small) and -18.7 (sharp, large) p.p.m. with respect to F 3 B* 0 (C2 H g ) 2 with areas under the curves in approximately 1:4 ratio, respectively. The liquid 156

mixture contained boron-boron bonds. The chemical shift of the large peak was consistent with triethylborate and the smaller one was probably 3 2 (0 0 2 1 1 5 ) 4 , but 3 2 (0 2 0 2 1 1 4 )-

(OC2 H 5 ) 2 could not be ruled out without further evidence.

82012(020234) + 2 O2H5OH -----■> 82(0202114)2

+ 8 2 (0 0 2 3 5 ) 4 + 8 (0 0 2 8 5 ) 3

+ 301

4. Ethylene glycol

Excess ethylene glycol (3.5 mmoles) was syringed into a reaction vessel containing 0.75 mmoles of

8 2 0 1 2 (0 2 0 2 3 4 ) cooled to -78°0. After distilling in

15 ml. of 0 3 2 OI 2 , the reaction mixture was allowed to stir over a temperature range of -78° to 25°0 for 8 hours. Oomplete solution of the reaction mixture occurred. A white solid material and a non-volatile liquid remained after distilling away the solvent.

The solid was separated from the liquid by fractional distillation-sublimation at 45°-50°C. The white solid sublimed at this temperature and gave a melting point of 16 4°-16 7°C, From sublimation temperature, melting point, and solubility data, the white solid was 157

identified as 2 ’ ^2^^2 ^^2^^2^4^ is not soluble in C H 2 CI 2 . The non-volatile liquid was assumed to be unreacted ethylene glycol.

B2CI2(O2C2H4) + C2H4(OH)2 (xs) -- ^ B2 (02C2H 4)2

+ 2 HCl

5. Dimethylamino

Dimethylamino (11.52 mmoles) was condensed into a reaction vessel containing 2.88 mmoles of B 2 C l 2 (0 2 C 2 H 4 ) and 20 ml. of C ‘J. ^ l 2 at low temperatures. The reaction mixture was allowed to stir over a temperature range of -78° to 25°C for 24 hours with incomplete solution resulting. A white reaction product was obtained after distilling away the solvent. A benzene extract of the reaction mixture yielded a very small amount of a white solid which was shown to be B 2 (0 2 C 2 H 4 ) 2 by x-ray powder diffraction pattern data. An x-ray powder diffraction pattern of the benzene insoluble material indicated

(CH3)2H2NC1 as the only crystalline material. After subliming out the (CH3)2H2NC1 at 110°-115°C, the residue gave an amorphous powder pattern. No evidence was found for the presence of B 2 [N(CH3 )3 ) 2 • 158

6 . Trimethylamine

A large excess of trimethylamine (approximately

30 mmoles) was condensed into a mixture of benzene and 3 mmoles of B 2 CI 2 (O2 C 2 H 4 ). The reaction mixture was stirred at 25°C for 35 hours. After distilling away all volatile species, a white solid residue re­ mained which gave a distinct x-ray powder diffraction pattern and analyzed for 10.35% B. In another reaction,

B 2 CI 2 (O2 C 2 H 4 ) was allowed to react with excess tri­ methylamine (approximately 2 ml.) in the absence of solvent over a temperature range of 25°-45°C for 20 hours. After the removal of all volatile species, a dry white solid was obtained which slowly became moist when the reaction vessel was opened in the dry-box.

Analyses of this product gave 8 . 6 3% B and 26.75% Cl.

For B 2 CI 2 (O2 C 2 H 4 ) • 2 N^CH])], the elemental compositions

are 7.99% B and 26.12% Cl.

B. Reactions of B 2 CI 2 (S2 C 2 H 4 )

1. Catechol

Catechol (1.42 mmoles) and 1.42 mmoles of

B 2 C l 2 (S2 C 2 H^) were introduced into a reaction vessel. 159

After distilling 20 ml. of CH 2 CI 2 into the vessel at

-196“C, the reaction was allowed to proceed over a

temperature range of -78°C to 25°C for 32 hours. An

insoluble white solid persisted in the reaction mix­

ture. The insoluble white solid was recovered by

filtration and was shown to be B 2 CI 2 (S2 C 2 H 4 ) by x-ray

powder diffraction pattern data. Another white solid

was isolated from the filtrate which was identified

as B 2 (0 2 Cg H 4 ) 2 by x-ray powder .diffraction pattern

data. No evidence was obtained for the presence of

B2(S2C2H4)(O 2 C 6 H 4 ) .

® 2 * " ^ 2 ^^2^2^4^ CgH4 (0 H) 2 ------3*-B2 (02CgH4)2

+ B 2 CI 2 (S2 C 2 H 4 ) +' HCl

2. Ethylene glycol

Ethylene glycol (2.0 mmoles) was syringed directly

into a reaction vessel containing 2,7 mmoles of

B 2 CI 2 (S2 C 2 H 4 ) at -78®C. After distilling in 15 ml. of

CH 2 CI 2 , the reaction mixture was allowed to stir over

a temperature range of -78“ to 25“C for 17 hours.

Incomplete solution of the reaction mixture resulted.

Filtration of the reaction mixture isolated the CH 2 CI 2 160

insoluble white solid which by x-ray powder diffraction pattern data indicated the presence of B 2 CI 2 (S2 C 2 H 4 ) .

A white solid material was recovered from the filtrate which was shown to be 3 2 (0 2 0 2 1 1 4 ) 2 by x-ray powder diffraction pattern and melting point data. No evidence for a species which formulates for B 2 (S2 C 2 H 4 )(O2 C 2 H 4 ) was observed.

B2Cl2(S2C2H4) + C2H4 (OH) 2 — > 8 2 (O2C2H4) g

+ B 2 C l 2 (S2 C 2 H 4 ) + HCl

3. Ethanol

Ethanol (3.50 mmoles) was distilled into a reaction vessel containing 1.72 mmoles of B 2 CI 2 (S2 C 2 H 4 ) and 25 ml. of C H 2 CI 2 at low temperatures. The reaction was allowed to proceed over a temperature range of -78° to 25°C for

35 hours resulting in nearly complete solution. A small amount of CH 2 CI 2 insoluble material was recovered by filtration [probably unreacted B 2 CI 2 (S2 C 2 H 4 )]. A white solid and a non-volatile liquid were isolated from the filtrate by fractionation. The white solid contained sulfur and was amorphous as shown by the absence of a distinct x-ray powder diffraction pattern. The 161

non-volatile liquid, when dissolved in CH 2 CI 2 » gave a singlet in the n.m.r. spectrum with a chemical shift of 18.2 p.p.m. downfield from F 2 B’0 (C2 H^ ) 2 » The va]ue for the chemical shift is in agreement with triethylborate. It appears that extensive cleavage of the boron-boron bond occurred.

4. Dimethylamine

Dimethylamine (4.91 mmoles) was condensed into a reaction vessel containing 1.23 mmoles of B 2 CI 2 (S2 C 2 H 4 ) and 20 ml. of CH 2 CI 2 . Reaction was allowed to occur over a temperature range of -78° to 25°C for 2 8 hours.

After the removal of all volatile species, a benzene extract of the reaction product yielded a white crystal­ line solid which analyzed for 10.2 7% B and gave a discrete x-ray powder diffraction pattern. This com­ pound is identical to that isolated from the reaction of B 2 [N(CH2 )2 ] 4 with C 2 H ^ ( S H ) 2 and HCl in a 1:1:2 molar ratio, respectively. Boron analysis was consistent with the calculated value. The reaction, however, did not appear to be quantitative. 162

C2 H 4 (SH) 2

B2[N(CH3)2]4 + -----> B 2 [N(CH3)2l2 (S2 C 2 H 4 ) 2 HCl + 2 (CH3 )2 H 2 NC 1

B 2 CI 2 (S2 C2 II4 ) + 4 (CH3 )2 NH -- 5-B 2 [N(CH3 )2 ] 2 (S2 C 2 H 4 )

+ 2 (CH3 )2 H2 NC 1

5. Trimethylamine

Approximately 0.8 mmoles of B 2 CI 2 (S2 C 2 H 4 ) was

allowed to stir in 2-3 ml. of trimethylamine, first at

25°C for 12 hours then at 40°-45°C for 4 hours. A white, dry, solid product was obtained after the

removal of all volatile species. The product was

soluble in CH 2 CI 2 , gave a distinct x-ray powder dif­

fraction pattern, and analyzed for 8.13% B and 24.35% Cl.

The calculated elemental values in B 2 CI 2 (S2 C 2 H 4 ) •2 N( C H 3 ) 3

are 7.15% B and 23.40% Cl.

C, Reactions of B 2 [N(CH3 )2 ]2 (2 2 ^ 2 ^ 4 )

1. Hydrogen chloride

Anhydrous hydrogen chloride (32.0 5 mmoles) was

added to 3.13 mmoles of B 2 [N(CH3 )2 ]2(S2 C 2 H 4 ) and allowed

to react at -96“C. After reacting for 5 hours, 19.75

mmoles of hydrogen chloride was recovered from the

reaction mixture. The net number of mmoles of HCl 163

consumed in the reaction was determined to be 12.30.

The stoichiometry of the reaction was then established at 3.94:1.0 [HCl/Bg[N(CH3 )2 I z ( % Co h4 )] . An identical product can be isolated with excess HCl in CH2 CI 2 . The reaction product consisted of a white solid plus a non-volatile liquid which contained sulfur. Separation was attained by fractionation. The following reaction is suggested which yields the dimethylamine adduct of

B2 CI 4 .

B2 [N(CH3 )2 ] 2 (S2 C 2 H 4 ) + 4 HCl ----5^

C l ^ ^C l (CH3 )2 HN — >B— B <— NH(CH3)2 Cl Cl + C2H4(SH)2

The product analyzed for 8.483 B and was shown to be identical by comparison of x-ray powder diffraction pattern data, melting point, and B^^ n.m.r. chemical shift data to that obtained from the direct reaction between B 2 [N(CH3 )2 l4 and.HCl in a 1:6 molar ratio.

This reaction has been previously described (63,66),

B 2 [N(CH3 )2 l4 + 6 HCl — > B2Cl4*2 NH(CH 3 ) 2

+ 2 (CH3 ) 2 H 2 NC 1 164

The dimethylamine adduct of diboron tetrachloride is a crystalline white solid which is soluble in most organic solvents, melts at 194°-196°C, and sublimes in vacuo at 110°-120°C with decomposition. The n.m.r. spectrum shows equivalent boron atoms with a chemical shift of -7.9 p.p.m. with respect to

F^B'O(C 2 H g ) 2 in C H 2 CI 2 . A well defined x-ray powder diffraction pattern is obtained with a 2 0 - 2 2 hour exposure. The "d" values and relative line intensities are given in Table 8 . The infrared spectrum is shown in Figure 15. The calculated per cent boron in

B 2 C l 4 * 2 N H ( C H 3 ) 2 is 8.54.

The dimethylamine adduct of diboron tetrachloride

(1.25 mmoles) and at least 4.0 mmoles of ethylene glycol were introduced into a reaction vessel, Dichloromethane

(15 ml.) was distilled into the reaction vessel at low temperatures and the reaction was allowed to occur over a temperature range of -78° to 25°C for 14 hours. Upon distilling away the solvent, a white reaction product resulted. Hydrogen chloride was evident in the distil­ late. Sublimation of the reaction product at 50°-55°C 165

TABLE 8

X-RAY POWDER DIFFRACTION PATTERN DATA

320x4-2NHfCHg) 2 (CH3 ) 2 H 2 NCI

Intensity "d" (A°) Intensity "d" (A°)

WW 11.79 VVW 7.25 WW 9 .94 w w 6.56 WliJ 8 . 84 vs 4.98 VS 6.58 vw 4.11 WW 5.50 S 4.93 vs 3.65 WW 3.97 w 3.33 M 3.63 w 3.11 vs 2.940 W 3.43 s 2.784 S 3.26 M 3.06 s 2.574 S 2.931 w w 2.485 vvw 2.411 M 2.814 M 2.287 M 2. 714 M 2.171 W W 2.525 w 2.436 VVW 2.108 w 2.045 w 2.365 w 1.987 w 2.222 M 1.890 w 2.178 V(f 1.854 w w 2.110 vw 1.821 w w 2.0 76 vw 1.787 vw 2.017 w 1.729 w w 1.726 w 1.628 w w 1.684 w 1.506 vvw 1.627 vw 1.394 vvw 1.607 w w 1.345 w w 1.254 w w 1.218

Intensity; S = strong; M = medium; W = weak; V = very, 166

yielded a white solid sublimate which was identified as

^ 2 (0 2 ^ 2 ^ 4 )2 x-ray powder diffraction pattern data.

The residue was shown to contain dimethylammonium

chloride by x-ray powder diffraction pattern data.

The over-all reaction can be shown below.

B2Cl4'2 NHfCHgig + 2 ---^ ^ 2 ^°2^2^4^2

+ 2 (CHgigHgNCl + 2 HC1

Hydrogen chloride (4.93 mmoles) was distilled into

a reaction vessel containing 2.46 mmoles of B2 [N(CH3 )2 ]£ ”

(S2 C2 H 4 ) and 25 ml. of CgHg at low temperatures.

Reaction was allowed to occur as the temperature in­

creased slowly to 25°C and was stirred at this tempera­

ture for 16 hours. A considerable amount of white

solid appeared in the reaction media. The white solid

was recovered by filtration. The filtrate afforded a

small amount of an unidentified non-volatile liquid

which contained sulfur. The white solid when sublimed

at 110°-115°C yielded another white solid sublimate

which was identified as (CH3 )2 H 2 NC 1 by x-ray powder

diffraction pattern data. The residue from the sublima­

tion was amorphous, insoluble in most organic solvents,

and contained B-B bonds and sulfur. Another 167 reaction in CH^Cl^ yielded similar results. No evidence was obtained for the presence of 8 2 [N(CH^) 2 ^2 ^ "

2 HCl or B 2 CI 2 (S2 C 2 H 4 ) *2 N H (0 8 3 )2 .

2. Boron trichloride

Boron trichloride (6.62 mmoles) was condensed into a reaction vessel containing 3.2 7 mmoles of

B 2 [N(CH3 )2 ]2 (S2 C2 H 4 ) and 2 0 ml. of CH 2 CI 2 at low tempera­ tures. The reaction was stirred over a temperature range of -78° to 25°C for 32 hours. An insoluble, white solid was recovered from the reaction mixture by filtration. This white solid gave a B^^ n.m.r. chemical shift of -67.5 p.p.m. with respect to F3 B «0(€ 3 ) 2 in diglyme. An x-ray powder diffraction pattern of the white solid appears to contain some B2 CI2 (S2 C 2 H4 ) plus other unidentifiable diffused lines. From the above filtrate, a non-volatile liquid was obtained.

Upon dissolving in CH2 C 1 2 , a solid slowly crystallized out of solution on standing over a several hour period.

The B^l n.m.r. spectrum of this mixture showed two singlets with chemical shifts of -30.5 and -4.7 p.p.m. with respect to F 3 B* 0 (C2 H 5 )2 » Possible compounds which 168

could give such chemical shifts are CI 2 B-N(01^2 ) 2

[Cl2 B-N(CH^)2^2^ respectively. The chemical shift for the monomer has been reported at -30.5 (8 8 ).

B 2 [N(CH3)2]2S2C2H4 + 2 BCI 3 ---5-B 2 CI 2 (S2 C 2 H 4 )

+ [Cl2B-N(CH3)2]

+ other solids

3. Ammonia

B 2 [N(CH3 )2 )2 (S2 C 2 H 4 ) (0.25 grams) was stirred in a large excess of liquid ammonia (approximately 1 0 ml.) at -78°C for 54 hours without complete solution.

Removal of the ammonia left a white crystalline solid which gave a discrete x-ray powder diffraction pattern and contained 12.72% B. The calculated per cent boron in B 2 (NH2 ) 2 (8 2 ^ 2 ^ 4 ) is 14.85 and in B 2 [N(CH3 )2 ]2 (S2 C 2 H 4 ) it is 10.72. The product was too insoluble in CH 2 CI 2

to produce a detectable B Ü n.m.r. signal. When the reaction was allowed to take place at -40° to -45°C, the

resulting white solid product gave a diffuse x-ray powder

diffraction pattern and analyzed for 15.25% B. Sublim­

ation of the product from the -78°C reaction at 110°-

115°C gave a white gummy solid sublimate which reduced 169

Ag ion and gave Ag 2 S when treated with AgNO^. The solid residue from the sublimation gave a boron analysis of

25.25%, indicating loss of dithio-groups. An x-ray powder diffraction pattern of the residue gave broad diffuse lines. The determined boron percentage approaches the boron content in B 2 (NH2 )4 ; however, the product is probably polymeric.

4. Phenylenediamine

An attempt was made to transaminate B 2 [N (CH3 )2 ] 2 ~

(S2 C 2 H 4 ) with phenylenediamine. Phenylenediamine

(8.0 mmoles), 8.0 mmoles of B 2 [N(CHg)2 )2 (S2 C 2 H 4 ) , and

50 ml. of CgHg were added to a 250 ml, flask. Ini­ tially, complete solution resulted. Slowly a white precipitate began to form while stirring at 25°C.

It was anticipated that a temperature of at least refluxing benzene would be required for transamination to occur. A copious amount of precipitate had formed in the reaction mixture after stirring at 25°C for 8 hours. The precipitate was isolated by filtration.

The benzene insoluble solid was shown to be B 2 [ (NH)2 C 5 H 4 ] 2 by x-ray powder diffraction pattern data. A white solid 170

which was isolated from the filtrate gave a n.m.r, chemical shift of -43.7 p.p.m. with respect to

F 3 B* 0 (C2 H 5 ) 2 and a distinct x-ray powder diffraction pattern. The n.m.r. and x-ray data confirmed the presence of B 2 [N(CHg)2l2(2 2 ^ 2 ^ 4 ).

B2 [N(CH3)2]2(S2C2H4) + CgH4(NH2)2 ------

+ 1/2 B2 [N(CH3)2l2(S2C2H4)

+ 1/2 B2 [(NH)2C6H4]2

+ NH(CH3)2 + 1/2 C2H4(SH)2 J

D. Reactions of B2(02C2H4)2

1. Boron trichloride

Approximately 0.7 grams of B 2 (0 2 C 2 H 4 ) 2 was suspended on an extra coarse frit heated to 50°-60°C. Boron tri­ chloride (5 ml.) was passed slowly through the heated zone with approximately 400-500 millimeters pressure maintained at all times. This pressure was maintained by distilling BCI 3 from a reservoir at +5°C to a receiver bulb at -10° to -15°C. The total transfer time was about 35-40 minutes. Fractionation of the condensible species through a series of traps maintained 171

at -78.5°, -96.7°, and -196°C yielded BCI 3 , no BgCl^, and a non-volatile liquid which gave a B^^ n.m.r. chemical shift of -31.4 p.p.m. with respect to

FgB'O(C 2 H 3 )2 . The chemical shift data is consistent with CI-B-OC 2 H 4 O. The residue remaining on the frit had a light tan cast. After extracting with CgHg and CH 2 CI 2 , the residue gave an x-ray powder diffrac­ tion pattern of which the most intense lines indicate the presence of B 2 CI 2 (O2 C 2 H 4 ). An amorphous material was present also. The per cent boron in the residue was found to be 16.22. The calculated boron percentage in B 2 CI 2 (O2 C 2 H 4 ) is 14.18.

Another reaction was run with B 2 (0 2 C 2 H 4 ) 2 in benzene heated to 60°C with a large excess of BCI 3 in a closed system. After six hours, the reaction mixture had turned reddish pink. Upon distilling away all volatile species, the pink residue was extracted with

CH 2 CI 2 . The C H 2 CI 2 insoluble solid gave an x-ray powder diffraction pattern which exhibited faint lines of B 2 CI 2 (O2 C 2 H 4 ), but the majority of the solid was an amorphous material which contained Cl and B-B bonds. 172

Isolation of pure B2 CI 2 (O2 C 2 H4 ) was not attainable in either of the above reactions.

2. Trimethylamine

Excess trimethylamine (at least 12 mmoles) was condensed into a system containing 2-3 mmoles of

^*^2*"2^4^ 2 iO ml. of diethyl ether. The reaction was stirred at 25°C for 46 hours. Removal of all the volatile species left a white crystalline solid which contained 12.51% B. The calculated per cent boron in

& 2 (0 2 ^ 2 ^ 4 )2 ' 2 NfCHg)] is 8.33, while in B 2 (0 2 C 2 H 4 ) 2 it is 15.31. Based upon the boron analysis, about

0.80 moles of NfCHg)^ was consumed per mole of

^ 2 ^^2 *“2 ^ 4 ^ 2 * ^ reaction is carried out with excess

NtCHg)^ (2-3 ml.) in the absence of a solvent, a dry white product is formed which becomes moist when the reaction vessel is opened in the dry-box.

In all subsequent reactions of ^ 2 (0 2 0 2 5 ^ ) 2 or

® 2 ^^2 ^ 2 ^^^4 ^ 2 with an amine, the quantity of diboron compound was usually 1-3 mmoles with the amine in a

5-10 fold excess. In the absence of a solvent, usually the diboron compound was allowed to react in 2-3 ml. of the liquid amine. 173

3. Dixnethylamine

Excess dimethylamine and 8 2 (0 2 ^ 2 ^ 4 ) 2 were allowed to react in benzene at 25°C for 72 hours. Upon removal of all the volatile species, a white residue formed which analyzed for 12.48% B. The theoretical per cent boron in 82(^2^2^4^2'^ NH(CH 2 ) 2 is 9.35. The calculated ratio was 0.9 5 moles of NH (01^2 ) 2 used per mole of

8 2 (0 2 ^ 2 ^ 4 )2 * if the reaction is allowed to take place in the absence of solvent, a gummy solid results which could not be recrystallized.

4. Methylamine

Excess methylamine and B2 (Û2 C 2 H ^ ) 2 were reacted in benzene at 25°C for 38 hours. The white flaky solid which remained, after distilling away all volatile species, gave a boron analysis of 1 2 .1 0 %.

When the reaction was carried out in the absence of solvent, a crystalline solid was obtained which analy­ zed for 11.64% B, The calculated per cent boron in

8 2 (02*^2 ^4 ^ 2 NH2 CH 3 is 10.63.

In any of the above amine reactions, no apparent 174

cleavage of the boron-boron bond or boron-oxygen bond occurred; however, quantitative evidence is not avail­ able.

E. Reactions of 2

1, Boron trichloride

Boron trichloride was passed slowly over B2 (3 2 0 2 1 1 4 ) 2 suspended on an extra coarse frit. The procedure was identical to that described above for B2 (0 2 0 2 1 ^4 )2 ,

Section V-D. Fractionation of the volatile species yielded a non-volatile liquid, BCI 3 , no B2 CI 4 .

The non-volatile liquid gave a B^l n.m.r. spectrum which showed three singlets with chemical shifts of

-29.7, -62.7, and -46.6 p.p.m. with respect to

F 3 B•0 (021^3 ) 2 in C H 2 CI 2 . The chemical shifts of the latter two are consistent with Cl-B-SC^H/S and BCI 3 , I ^ ^ I respectively. The BCI 3 was present in only small amounts. The white solid which remained on the frit was very gummy. The solid was extracted with CH 2 CI 2 and a B^^ n.m.r. spectrum of the extract gave chemical shifts of -4.4, -62.7, and -68.2 p.p.m. with respect to F 3 B' 0 (C2 Hg) 2 . The chemical shift data of the latter 175

two are consistent with CI-B-SC 2 H 4 S and 2 2 (32021114)2 , respectively. The high field value in either of the above two spectra could not be accounted for. The small amount of white solid insoluble in CH 2 CI 2 was isolated by filtration. An x-ray powder diffraction pattern of this solid was identical to that produced by B2 CI 2 (S2 C 2 H 4 ).

2 . Trimie thy lamine

Excess trimethylamine and B 2 (S2 C 2 H 4 ) 2 were allowed to react in benzene at 25°C for 20 hours. After distil­ ling away all the volatile species, a dry light pink product was obtained only after pumping on directly for several hours. The dry product contained 8.87% B.

The calculated per cent boron in 8 2 (8 2 0 2 2 4 )2 "2 N (0 2 3 ) 3 is 6 .6 8 . For 8 2 (8 2 0 2 2 4 )2 , the theoretical value is

10.50 per cent. Based upon the boron analysis, 0.85 moles of N (0 8 3 ) 3 were consumed per mole of 8 2 (8 2 0 2 2 4 )2 .

3. Dimethylamine

Dime thy lamine was added to one case with excess dimethylamine and in another with a stoichio­ metric 2:1 molar ratio. The reactions were carried out 176 in benzene at 25°C for 18 hours then warmed to 35°-40°C for 2-3 hours. Both reactions yielded light pink colored products which gave boron analyses of 6.82 per cent and

6.65 per cent, respectively. Recrystallization of the light pink products from a benzene-pentane solvent system yielded white solids by filtration which gave identical x-ray powder diffraction patterns as that obtained from the reaction of B 2 [N(CH3 )2 ] 4 and ethanedithiol in a less than 1:2 ratio. This product analyzed for the 2:1 dimethylamine adduct of B2 (S2 C 2 H 4 ) 2 as shown in Section

IV-J.

4. Methylamine

Excess methylamine was allowed to react with

®2^^2^2^M^2 benzene at 25°C for 36 hours to yield a light pink dry product. The dry crystalline product gave a boron analysis of 7.35 per cent and a well defined x-ray powder diffraction pattern. The calculated per cent boron in B 2 (S2 C 2 H^) 2 * 2 NH 2 CH 2 is 8,07. It appears

from the boron analysis that some cleavage of the boron- sulfur bond resulted with covalent boron-nitrogen bond

formation. A compound with the following formula could 177

be the existing species; B 2 (SC2 H^SH) 2 (NHCH^)2 *NH2 CH 2 .

If the reaction was rerun stoichiometrically, it is believed that the 1 : 2 adduct could probably be isolated similar to the dimethylamine adduct discussed above.

In all of the amine reactions with B 2 (S2 C 2 H ^ ) 2 , no hydrogen was observed to have been evolved.

F. Reactions of

1. Hydrogen chloride

Hydrogen chloride (8.13 mmoles) was allowed to react with 0.90 mmoles of B 2 [(NH)2 C g H 4 ]2 at - 9 6 °C for

22 hours. The unreacted HCl was recovered from the reaction mixture and was measured to be 4.05 mmoles.

A determined 4.02 moles of HCl was consumed in the reac­ tion per mole of B 2 [(NH)2 C 6 H 4 ]2 . The uptake of the last millimole of HCl was very slow. The white reaction product gave an amorphous x-ray powder diffraction pattern. 178

TABLE 9

BORON-11 N.M.R. CHEMICAL SHIFT DATA OF OTHER SYNTHESIZED BORON COMPOUNDS

Chemical Shift p. p. m. Compound F3B'0(C2H5)2 Solvent

B2(OC 2 H 5 ) 4 -30.7 CH 2 CI 2

32[N(CH3)2]4 -36.1 C 6 % 6

B 2 CI 4 -6 2,6 Neat

B 2 F 4 -22. 8 Neat (-40°C)

Br-B-SC 2 H 4 S -59. 8 Neat

CI-B-SC 2 H 4 S -62.7 CH 2 CI 2 1 t — i r-s S-! B—SC 2 H 4 S—B -64.2 CgHg L-S \ <

CI-B-OC 2 H 4 O -31.4 CH 2 CI 2

CI-B-OC 6 H 4 O -29.1 CH 2 CI 2

B(OC 2 H s ) 3 -18.1 CH 2 CI 2 SUMMARY

Several heteronuclear diboron ring systems have Loen synthesized by the interaction of diboron tetrachloride or tetrakis-(dimethylamino)-diboron with diols, dithiols, or diamines. In general, the species were shown to be monomeric by molecular weight determinations and elemental analyses. The bidentate organic species considered in this investigation were ethylene glycol, ethanedithiol, catechol, 1,3-propanediol, sym-dimethylethylenediamine, and o-phenylenediamine.

The products from the reaction of ethylene glycol or ethanedithiol with diboron tetrachloride in a 1 : 1 molar ratio have equivalent boron atoms as shown by

n.m.r. spectra, suggesting the six-membered ring structure:

C l ^ ^ C 1 ^B —B\ X = 0,8 X X \ ____ /

Products from the interaction of ethylene glycol,

179 180

ethanedithiol, catechol, or 1 ,3-propanediol with diboron tetrachloride or tetrakis-(dimethylaraino)-diboron in a

2 : 1 molar ratio can assume one of two possible structures, the bicyclic or fused ring. The completely substituted diboron products obtained from the reaction of diboron tetrachloride or tetrakis-(dimethylamino)-diboron with sym-dimethylethylenediamine or o-phenylenediamine are believed to assume the bicyclic structure. A species which has the six-membered ring structure as suggested by n.m.r. has been synthesized by the interaction of

tetrakis-(dimethylamino)-diboron with ethanedithiol and hydrogen chloride in a 1 :1 : 2 ratio.

(CH3 )2 Nv, ^N(CH3)2 /B— S S

Proton and boron-11 n.m.r. chemical shift data have

been compiled on these heteronuclear diboron ring systems.

Lewis acid-base studies indicate that the boron-sulfur

species are considerably better Lewis acids than the

boron-oxygen derivatives. Boron-11 n.m.r. chemical

shift data is consistent with these results in that they 181 suggest the extent of "pi" type bonding between boron and sulfur is less than that between boron and oxygen.

Comparison of chemical shift data of the boron-oxygen and boron-nitrogen systems shows the same trends as the analogous borolanes and borane compounds.

Attempts to prepare mixed heteronuclear fused diboron ring species failed. Several other reactions of these heteronuclear diboron ring compounds have been investigated.

A scheme for the preparation and reactions of these heteronuclear diboron ring systems is shown on the following page. 4NH(CH3)2

m 00 ro to GO 1— 1 ro 4HCI z z « o I S 2C,H.(XH)ro 2(HX)^H^02 O o lO r o^ ( f ^ w -è % rvT' o P ro n I ro .■» , I ro n P ro

O /k TS I / Co . ro^

ro œ

CO N ro. to APPENDIX

I. Nomenclature

The chemical names given below for the heteronuclear diboron species considered in this investigation have been adopted by Chemical Abstracts.

A. 3 2 (0 2 0 2 1 1 4 ) 2

B — B 2,2'-Bi-1,3,2-dioxaborolane ■ o'' '^0 —'

2,3,7, 8-Tetrahydro- [1,4,2,3]: I dioxadiborino[2,3-b]3 [1,4,2,3]dioxadiborin

C. B2(02CgH4)2

■ 0\ B — B 2,2'-Bi-1,3,2-benzodioxa borole

183 184

[1,4,2,3]Benzodioxadiborino C [2 ,3-b] [1,4,2 ,3]benzodioxaC diborin

D. B2(02C3Hg)2

2,2'-Bi-1,3,2-dioxaborinane

2 ,3,4,8 ,9 ,1 0 -Iiexahydro- B^ [1,5,2,3,4]dioxadiborino [2,3,4-b][1,5,2,3,4] = dioxadiborin

E. B2(S2C2H4)2

/S B—B 2,2'-Bi-1,3,2-dithiaborolane I— g/ ]

B 2,3,7, 8 -Tetrahydro- [1,4,2,3]0 dithiadiborino[2 ,3-b^C [1,4,2,3]dithiadiborin S'^ '^S 185

I. Bg[(NH)2CgH4]2

1,1',3,3'-Tetrahydro- 2,2'-bi-2H-l,3,2-benzo: diazaborole

5,7,12,14-Tetrahydro- [1,4 ,2,3]benzodiaza 0 diborino[2,3-b][1,4,2,3] benzodiazadiborine BIBLIOGRAPHY

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