ENGINEERING RESEARCH INSTITUTE THE UNIVERSITY OF MICHIGAN ANN ARBOR Quarterly Report No. 3 THE CHEMISTRY OF BORON HYDRIDES AND RELATED HYDRIDES July-September, 1956 R. W. Parry G. Kodama S. Go Shore R. Co Taylor J R Weaver CO E. Nordman E. Alton C. Cluff Project 2469 U. So AIR FORCE WRIGHT AIR DEVELOPMENT CENTER WRIGHT-PATTERSON AIR FORCE BASE, OHIO CONTRACT NO. AF 33 (616) 3343 November 1956

The University of Michigan ~ Engineering Research Institute OBJECTIVE A continuation of fundamental studies on the chemistry of the hydrides of boron and related compounds. ABSTRACT Monomeric ammonia-borane, H3NBH3, has been prepared directly from B2H6 and NH3 by their interaction in dimethyl ether solution. H3NBH3 is monomeric in liquid ammonia and has a dipole moment in dioxane solution of 3,6 ~.1 Debye units. The action of a liquid-ammonia solution of potassium on trimeric methylamino-borane gives evidence for a compound K2 (HCH3NBH2). The slow spontaneous decomposition of [H2B(NH3)2](BH4) in diethyl ether solution containing ammonia has been noted. A microcrystalline form of (H3NBH3)n, believed to be [HB(NH3)3](BH4)2, has been isolated. The Raman spectrum of F3PBH3 is discussed and Raman spectra of H3NBH3 have been obtained but not interpreted. A completely new type of ammonia —boron-hydride coordination compound has been isolated. It has the formula H3NB3H7. Its structure has been determined by the methods of x-ray crystallography. Its structure and proper= ties are reported. ii

The University of Michigan * Engineering Research Institute I. THE REACTIONS AND STRUCTURE OF THE "DIAMMONIATE OF DIBORANE" ANID RELATED COMPOUNDS A. THE PREPARATION OF H3NBH3 DIRECTLY FROM DIBORANE The preparation of H3NBH3 from B2H6 would involve symmetrical cleavage of the diborane bridge, followed by addition of NH3 to the acidic borane group. Arguments previously summarized indicated that a solid-gas phase reaction between NH3 and B2H6 respectively favored nonsymmetrical cleavage to give the "diammoniates of diborane." Earlier attempts by Schlesinger and Burg1 to prepare H3INBH3 by the reaction between solid H3BO(CH3)2 and gaseous NH3 yielded the diammoniate in agreement with the foregoing arguments. On the other hand, evidence has been accumulated which indicates that symmetrical cleavage of the diborane bridge is effected in ether; hence a solution of H3BO(CH3)2 in dimethyl ether can be obtained at low temperature by adding B2H6 to an excess of very cold ether. When NH3 was dissolved in this cold (-78~C) ether solutionH3NBH3 was obtained in yields ranging up to 70% based on the equation B2H6 + 2NH3......, 2NH3B'H3 as solvent This is now the easiest method for large-scale preparation of H3NBH3. B. THE MOLECULAR WEIGHT OF H3NBH3 IN LIQUID AMMONIA Previous studies have shown that H3NBH3 is monomeric in dioxane, in diethyl ether, and in the solid state,3 but data showing its molecular weight in liquid ammonia under conditions comparable to those used for studying the classical "diammoniate of diborane" were not available. Such data have been obtained using previously described4 vapor-pressure depression methods. Data 1. H. J. Schlesinger and A. B. Burg, J. Am. Chem. Soc., 60, 290 (1938). 2. S. G. Shore and R. W. Parry, ibid., 77, 6084 (1955). 3. E. L. Lippent and W. N. Lipscomb, ibid., 78, 503 (1956); E. W. Hughes, ibid., po 502. 4. Ro W. Parry, G. Kodama, and D. R. Schultz, WADC Tech. Rept. 56-318. Final report for Contract AF 33(616)-8, EoO.R.-464 Br-l, Eng. Res. Inst.,Univ. of Mich., June, 1956, p. 82. 1

The University of Michigan ~ Engineering Research Institute for H3NBH3, the classical "diammoniate of diborane," [H2B(NH3) 2](BH4), and socalled "mistreated diammoniate" or "diammoniate (II)," [HB(NH3)21(BH4)2, are shown in Fig. 1. Values of the molecular weights are consistent with the formulas assigned in every case. C. THE DIPOLE MOMENT OF H3NBH3 The dipole moment of H3NBJIH3 has been measured in solution using very pure dioxane as a solvent. A high-precision heterodyne beat apparatus, kindly donated to The University of Michigan by the Chrysler Corporation of Detroit, was employed for evaluating the dielectric constants of the solutions and solvent. A value of 3.6 ~.l Debye units has been obtained for the dipole moment of H3NBH3 under the conditions used. Uncertainties are largely dependent on methods of extrapolation and calculation rather than on uncertainties in actual experimental values. The remaining members in this series, H3NBH2Me, H3NBH(Me)2, and H3NB(Me)3, will be measured in an attempt to gain information as to the effect of methyl groups on electron distribution in the amine-boranes. A more complete report will be given upon completion of the work. D. EVIDENCE FOR THE CLEAVAGE OF (HCH33NBH2)3 BY METALLIC POTASSIUM IN LIQUID AMMONIA-THE ELECTRON PAIR AS A COORDINATING LIGAND IN BOROHYDRIDE-TYPE STRUCTURES In the previous quarterly report from this laboratory the ammonolysis of the residues obtained from the action of excess sodium on the diammoniate of diborane was interpreted in terms of a preliminary reaction: 2Na + H2NBH2 - Na2(H2NBH2) o The product would have the structure -aH2N /H 2Na+ B H in which the electron pair serves as a coordination ligando An extension of the foregoing postulates indicated that the compound (HCH3NBH2)3 described by 5 "'Chemistry of Boron Hydrides and Related Hydrides," Quarterly Report No. 2, Eng. Res. Inst., Univ. of Mich., Project 2469-2-P, WADC Contract No. AF 33(616)-3343, Aug., 1956.

93: Theory 90 80 | -_ O - Original values of Schultz in this lab. | 80 — ~ -l a - Values of Shore in this lab. r 0 (3~~ 70 ~~"Diammoniate (1C or I 70 HB(NH3 )3'"H4 I 2 J 62 2Theory J 60,,_ Values of Stock B Pohland for"clossical diammoniote" W Values of this Cr 5investigotion for + < 50 "classical diammoniate (I)" or 0 m a. <c: [H2B(NH3)2] (l4H2) (NH3 40 ". 30** II 20140 ~ ~ ~ ~ ~ 0: 2c0 I, I I, I, I i,, I I i Y 1.0 2.0 - MOLALITY BASED ON UNITS OF (H3NBH3): Fig. 1l Molecular weight of ammonia-diborane addition compounds in liquid ammonia.

The University of Michigan ~ Engineering Research Institute Bissot and Parry6 might react with metallic potassium in liquid ammonia without hydrogen evolution. Such predictions have been confirmed. A liquid-ammonia solution of (HCH3TNBH2)3 will decolorize a liquid NH3 solution of potassium without hydrogen evolution. The stoichiometry has not been established with certainty, but the following equation is indicated: 2K + (HCH3NBH2)3 ---- K2(HCH3NBH2) Experiments designed to isolate and purify K2(HCH3NBH2) are in progress. E. THE SODIUM SALT OF AMMONIA-BORAI\NE-THE H2N GROUP AS A COORDINATING LIGAND IN BOROHYDRIDE-TYPE STRUCTURES The reaction of H3NBH3 with Na in liquid ammonia should produce NaH2NBH3 or Na B Na+ H2N /H l H H The solid product obtained from this process is extremely water sensitive and it has not been possible to characterize it by x-ray methods without preliminary decomposition. Techniques are being refined to continue the work. F. THE DECOMPOSITION OF THE "DIAMMONIATE OF DIBORA1ME," [H2B(NH3)21(BH4), IN AN ETHER SLURRY In an earlier report2 the preparation of H3NBH3 from [H2B(NH3)2 (BH4) was described in terms of the following equation Trace NH3 NH4C1 + [H2B(NH3)21(BH4) -- H2 + H31NB3 + [H2B(NH3)21 Cl e (1) Diethyl ether The validity of this overall equation was questioned recently when it was found that the solid reaction residues,examined by x-ray diffraction, showed only small amounts of the salt [H2B(NH3)2] C1. The explanation for the discrepancy between theory and experiment is found in a very recent observation. It is now known that the "diammoniate" undergoes a competing decomposition in an ether slurry containing ammonia but NO ammonium chloride: 6. T. C. Bissot and R. W. Parry, J. Am. Chem. Soc., 77, 3481 (1955). 4

The University of Michigan ~ Engineering Research Institute Trace NH3 1 [H2B(NH3)2 (BH4) ra 2 H H3NBH3 + - (2NBH2)n. (2) Diethyl ether Data are summarized in Fig. 2. Reactions 1 and 2 proceeding simultaneously would be consistent with all observed facts. 1.2 [ H28 (NH3)2] (BH4) +NH4CI I.0 0.8' 0.6.1 0.4 0.2 Small amount of NH3 added 0 20 40 60 80 100 HOURS Fig. 2. The decomposition of the "diammoniate of diborane" in an ether slurry. Go EVIDENCE FOR THE CRYSTALLINITY OF THE "DIAMMONIATE OF DIBORANE (II)" "Mistreated diammoniate," "diammoniate of diborane (II),"' or tentatively [HB(NH3)31 (BH4)2, has been obtained in a microcrystalline form which gives a definite powder pattern. The sample is the one used previously for the determination of the molecular weight of diammoniate (II) in liquid ammonia. The application of x-ray methods to this compound looks somewhat more promising. II. RAMAN SPECTRAL STUDIES A. TEE RAMAN SPECTRUM OF PF3BH3 Samples of PF3BH3 and PF3BD3 were prepared and purified as previously

The University of Michigan ~ Engineering Research Institute described.7 Samples of each compound were condensed into capillary Raman tubes which were then sealed off and maintained at liquid-nitrogen temperatures until used. Details of the experimental equipment have been given previously.8 The spectra were obtained for the liquid at -80~C, qualitative polarization measurements being made on the hydrogen compound only, using the two-exposure method and polaroid cylinders. The data reported in Table I represent the averages from several spectra, the estimated probable error being approximately 1 cml, except where indicated. An earlier paper from this laboratory7 proposed an ethane-type structure which would have C3V symmetry. Such a structure was based on known similarities between PF3 and CO as coordinating ligands. The vibrational frequencies of such a structure are twelve in number and can be thought of in terms of the four vibrational frequencies of each of the two halves, considered as free molecules with C3V symmetry, plus four vibrations arising as a consequence of the bond between the apex atoms of the two pyramids. Since free BH3 is not known, reference can be made to the Raman frequencies of BH3CO which have been determined recently.9 In the B-H stretching region, two strong bands appear in the spectrum of PF3BH3 at 2385 and 2455 cm1, which appear to be a1 and e type modes, respectively. Corresponding frequencies are found at 2380 and 2434 cm-1 in the spectrum of liquid BH3CO. Boron-hydrogen deformation bands are observed at 1077 and 1117 cm1l for the al and e modes, which again do not differ greatly from the corresponding bands at 1073 and 1101 cm'l in the carbon-monoxide complex. The frequencies associated with the PF3 group likewise show a close similarity in pattern to those found in the Raman spectrum of liquid PF3, which was also obtained in the present work. In this case, the a1 and e P-F stretching frequencies at 832 and 874 cm-l shift to 994 and 958 cml, respectively, in the complex while the al and e bending frequencies shift from 484 and 351 to 441 and 370 cml', respectively. The remaining modes may be described as a P-B stretch, BH3 and PF3 rocking motions, and the inactive torsional mode. The first was easily identified as the strong polarized band at 607 cm.1 while the low depolarized band at 197 cm' is certainly the PF3 rock. The BH3 rock was assigned to the rather weak band at 697 cm-1. No information as to the torsional frequency was obtained. Two fundamentals of PF3BD3 were not observed directly. The position of the first, the symmetrical B-D bond, was estimated at 842 cm-1 from its 7. Ro Wo Parry and T. C. Bissot, ibid., 78, 1524 (1956). 8. Go L. Vidale and R. C. Taylor, ibid, 294 (1956). 9. R. C. Taylor, "The Chemistry of Boron Hydrides and Related Hydrides," Quarterly Reports No. 1 and 2, Eng. Res. Inst., Univ. of Mich., Project 2469, Contract No. AF 33(616)-3343, May and Aug., 1956. 6

The University of Michigan ~ Engineering Research Institute TABLE Io OBSERVED RAMAN FREQUENCIES AND ASSIGNMENTS FOR LIQUID PF3BH3 AND PF3BD3 AT -80 C Frequency in cm-1 Intensity and PF3BH3 PF3BD3 Polarization 197 169 m, dp e V12 PF3 rock 370 362 vw e V1l F-P-F deformation 441 421 m, p al V5 F-P-F deformation 607 572 s, p al V4 P-B stretch 697~2 - vw e vio BH3 rock 799 vw diborane? 8865 - vw, p? A1 2v5 2 x 441 = 882 920 - w, p A1+A2+E vs-v12 1117 - 197 = 920 944 944~5 m, p? al V3 P-F stretch 957+3 958~2 m, dp e vs P-F stretch 1040+3 - w, p A1 v4+v5 441 + 607 = 1048 1077 - w, p al v2 H-B-H deformation 1117 807 s, dp e v8 H-B-H deformation - 1756 w A1+A2+E v8+v9 807 + 958 = 1765 1797 vvw E v2+v9? 842 + 958 = 1800 - 1980 vvw diborane-d,? 2112 - vvw diborane? 2140+4 1672~2 vw A1 2v2 2 x 1077 = 2154 2247+2 1602 vw A1 2v8 2 x 1117 = 2234 2328+4 - vvw 2v3+V5? calco 2329 2385 1717 vs, p al v1 B-H stretch 2455 1845 vs, dp e V7 B-H stretch - 2431 w B-H stretch (H impurity) 2530 - vvw diborane? 2655+5 - vvw A1+A2+E v7+vl2 197 + 2455 = 2652 7~~~~

The University of Michigan ~ Engineering Research Institute first overtone, and the position of the second, the BD3 rock, was estimated at 603 cm-1 from the comparison with BD3CO. The calculated product rule ratios using these estimated values are 1.98 and 2.53 for the al and e classes, which may be compared with the theoretical values of 1.97 and 2.55. The spectra of both molecules thus are interpreted satisfactorily in terms of the C3V structure. A normal coordinate treatment is in progress and its results together with a more detailed discussion of the assignments will be published on its completion. B. THE RAMAN SPECTRUM OF H3NBH3 Earlier attempts to obtain a Raman spectrum for H3SNBH3 were consistently complicated by strong background scattering resulting from the presence of a colloidal solid which formed in the ether solution. Earlier chemical studies have shown that the formation of such a solid is accelerated by trace quantities of water in the system. By taking great pains to exclude water from the system, a clear solution of ammonia-borane in dimethyl ether was obtained. Relatively good spectra for H3NBH3 have been obtained. Interpretation will be carried out later. III. THE COMPLEXES OF B4H10 A. BACKGROUND In earlier reports9 symmetrical and nonsymmetrical cleavages of the double-bridge bond in B4Hlo were postulated. oH I H H /H/.B- H /H H H, BU-H H B'/ B B H HB H H H HH —B H' H / I HI H Symmetrical Nonsymmetrical Products = H3B and B3H7 Products = H2B+ and B3Is Products expected in each case are indicated above. Evidence for nonsymmetrical cleavage was contained in an earlier report9 to the effect that a diasmmoniate of B4H10 can be obtained 8

The University of Michigan ~ Engineering Research Institute I B H IKLH3 I\H \ or [H2B(NH3)2] / B H B-H H H which reacts with sodium in liquid ammonia to give the expected sodium salt NaB3H8, H2, and H2NBH2. B. SYNTHESIS OF A NEW AMMONIA-BORON-HYDRIDE COMPLEX, H3NB3H7 Observations made in the synthesis of H3NBH3 suggested that a reaction between the ether soluble NaB3H8 and an ether slurry of IH4Cl1 should proceed as follows: diethyl NaB3H8 + NH4C1 -i —--- H2 + NaCl + H3NB3H7 e the r The new type of ammonia —boron-hydride coordination compound, 113NB3H7, would be the ammonia-addition compound of the symmetrical-cleavage product, B3H7. Using the foregoing reaction, the compound H3NB3H7, tentatively named ammonia-triborane, has been obtained. Yields have been poor and rather erratic. Best results were obtained by the following procedure. The reaction was stopped after the evolution of about one mole of hydrogen; the solution was filtered and ether was distilled from the filtrate. From the solid filtrate residues H3NB3H7 was sublimed under high vacuum in yields running up to 30% of the theoretical. The white crystalline compound has been characterized by means of the following data. Analysis. Calculated for H3NB3H7: H(hydridic) = 14.15%, N = 24.8%, B = 57.4%. Found: H(hydridic) = 14.10%, N = 24.4%, B = 57.2%. Molecular weight by vapor-pressure depression in diethyl ether was 55; theoretical for H3NB3H7 is 56.5. The compound dissolves in organic solvents such as benzene, ether acetone, and alcohol without decomposition. It will pick up ether vapor avidly to give an ether solution of the compound. It is only slightly soluble in petroleum ether. The compound also dissolves in water and in liquid ammonia. 9r

The University of Michigan * Engineering Research Institute It is surprisingly resistant to hydrolysis, requiring periods of up to one week at 120~C in 6NHCl for complete reaction. It melts at 735 to 75~C with loss of hydrogen. The solid was attacked by an equimolar quantity of trimethylamine at room temperature to give a stoichiometric quantity of trimethylamine-borane and an unidentified solid residue which slowly gave off hydrogen. H3NB3H7 in liquid ammonia reacts slowly with sodium to give H2o Only NaBH4 was detected in the solid residues. The reaction is being explored farther in the hope of isolating Na(lNH2B3H7) and following the cleavages which occur. C. THE STRUCTURE OF H3NB3H7 AS DETERMINES BY X-RAY DIFFRACTION X-ray powder and single-crystal studies of H3NB3H7 have been conducted in this laboratory by Dr. C. E. Nordman. The solid undergoes two phase transitions between room temperature and -74~C. The room-temperatureoform has a structure as follows: Unit cell = tetragonal a = 6.11 A, c = 6.57 A. There are two formula units per cell. The space group is C4g - 14 mm A structure predicted from the chemical data has been confirmed. The boron atoms form an equilateral triangle with a B-B distance of 1.75 Ao The N atom lies on the normal to the plane of the borons through the center of the triangle. The distance of the N atom from the plane of the boron triangle is 2.53 A. The hydrogen positions have not been established with certainty from the data currently available. A fourfold axis of symmetry runs through the nitrogen atom and the center of the boron triangle. The molecules are either freely rotating or disordered about the fourfold axis. The parameters are being refined by the method of ]_east squares, The structure of the molecule as determined by currently available data is shown in Fig. 3. The hydrogens are placed arbitrarily since current data do not permit their precise location, D. THE ELECTRONIC STRUCTURE OF H3N1B3H7 The compound H3NB3H7 poses problems in so-called "electron deficiency;' as do most of the other boron hydrides, It can be formulated in relatively simple terms, using hybridized atomic orbitals to build uIp appropriate molecular orbitals. Hybridization of sp2 orbitals gives an equilateral triangle with a third p orbital perpendicular to the plane of the triangle~ Combination of the three unhybridized p orbitals of boron permits formation of molecular orbitals pointing up and down from the plane. The bottom hydrogen and the coors dinated ammonia are bound through these orbitalso See Fig. 4,. 10

HH! H N H -4 //~~~~~~~~~v 2.53 -1 (H) B tD B ~B~~~~~~~ 1'~~~~ ~~~~~~.75 sic ~~~~~~~~~~~~~~~'< 2.53 ~~~~~~~~~~~~~~~~~~~ (H) Positions of Hydrogens not N on rbitazet hybrdztino yet established from X-Raypln of boron boro ritl data A ranl 7, /.75 1~~~~~~~~~~~(7 ~~~~~~~~~~u, (H)V Nonhybridiz4Teds t /~~~~~~~tr~~~~~~~Panarlsp ( )5 Positions of Hydrogens not Tp orbitt c I oftio oH yet established from X-Roy plane of boron boron orbitals 71:3 data triangle,. _:3 (H) ~ Fig. 3, tl-ua~t~e cf H~H3H-7.F'ig. o 4 The el, u._onzc str-ucture of,,3PB33H7.