3502
J . Phys. Chem. 1991, 95, 3502-3506
Infrared Matrix Isolation Characterization of Aminoborane and Related Compounds John D. Carpenter and Bruce S. Auk* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: June 12, 1990; In Final Form: October 9, 1990) The reaction products arising from the high-temperature pyrolysis of mixtures of B& and NH3 in argon have been isolated and characterized in low-temperaturematrices. All 1 1 infrared active fundamentals of aminoborane, H2B=NH2, were identified and I5N and 1°B counterparts located for most of the fundamentals. The two lowest frequency fundamentals were observed for the first time. Based on isotopic shifts and a normal-coordinate calculation, reassignment of several of the previously observed gas-phase fundamentals is suggested. Spectra were also obtained of [H2B(NH3)2]+[BH4]and p-aminodiborane, H2NB2HS,and band assignments proposed.
Introduction Diborane, B2H6, is the simplest stable member of the boron hydride family and has long been of interest to chemists as a consequence of its unusual structure and bonding.'-3 B2H6 is electron deficient, and it reacts readily with a range of Lewis bases in solution. The reaction of B2H6 with NH, has been studied extensively and the products found to be sensitive to the solvent. In a wide range of solvents the product is [H2B(NH3)2]+[BH4](also known as the diammoniate of diborane), while in other solvents the 1 :1 adduct H3B.NH3 is Previous investigators have suggested* that heating the diammoniate of diborane would lead to formation of borazine, B3N3H6,possibly via the aminoborane intermediate, H2B=NH2. The gas-phase reaction of B2H6 and NH3 is used extensively in chemical vapor deposition at elevated temperatures (700-1 200 "C) for the production of boron nitride thin films.+" These thin films have applications in the semiconductor industry, as well as protective coatings. However, relatively little is known about the mechanism of this important reaction. Aminoborane was first detected in the mid-1960s in a mass spectrometric study12 of the thermal dissociation of (BH2NHJ2; subsequent studies found it to be present" in the vapor phase above heated H3B.NH3, and as an rf discharge productI4 of borazine. The low-resolution infrared spectrum of solid H2B=NH2 was obtainedI4 in 1970; in 1985, Gerry and co-workersI5 heated H3B.NH3 to 400 "C and reported 9 of the 11 infrared active fundamentals of H2B=NH2 in the gas phase, while paying particular attention to the B=N stretching mode. Sugie and co-workers16 heated a mixture of diborane and ammonia to 500 O C and were able to obtain the microwave spectrum of this compound. Aminoborane has also been the subject of several theoretical investigations concerning its structure, bonding, and vibrational spectrum."-'* While H2B=NH2 has been studied in the gas phase, two infrared-active fundamentals have not yet been identified nor have other possible intermediates in the gas-phase reaction of B2H6 with NH,. The matrix isolation technique was for the characterization of reactive intermediates and should be able to provide information complementary to that previously obtained in the gas phase. Consequently, a study was undertaken to isolate and characterize species formed in the high-temperature pyrolysis of mixtures of B2H6 and NH,. Experimental Section Matrix isolation experiments in the present study were all carried out on conventional equipment which has been described?2 NH3 (Matheson) and IsNH3 (99% 15N, Icon Services) were introduced into the vacuum line from lecture bottles, and purified by freeze-thaw cycles at 77 K. B2H6 was introduced into the vacuum system as a 1% mixture in argon (Matheson) and used without further purification after further dilution with argon. Samples of Ar/B2H6 and Ar/NH, were prepared in separate *Author to whom correspondence should be addressed.
0022-3654191/2095-3502$02.50/0
manifolds to prevent reaction and formation of the diammoniate of diborane. Preparation of N2/B2H6 matrix samples was somewhat more difficult, in that the B2H6 had to be separated from the premixed argon by fractional distillation at 77 K and then warmed and diluted with an appropriate amount of N1. Samples were deposited either in the twin jet mode, where the two gas samples were sprayed simultaneously on the cold window from two separate nozzles, or in the merged jet mode. In this latter approach, the two deposition lines were joined with an Ultratorr tee 145 cm from the cold cell. The two gas samples then mixed during the flight time from the tee to the cold window, allowing for additional mixing time compared to the twin jet mode but without the static equilibration which occurs in the single jet, premixed mode. Each sample was deposited at a rate of 2 mmol/h for 20-24 h before final scans were obtained on a Nicolet IR 42 Fourier transform infrared spectrometer at 1 cm-' resolution. In the merged jet experiments, the 90 cm of the deposition line closest to the cold cell was wrapped with heating tape and could be heated to as high as 360 O C . Gas samples that were pyrolyzed in this manner were then deposited onto the cold window in the usual manner. No rise was noted in the temperature of the cold window when the deposition line was heated. In most experiments, the heated portion of the deposition line was primarily 1/4 in. 0.d. copper, with a short segment of stainless steel near the entrance to the cold cell. In a few experiments, only the stainless steel portion of the line was heated. The infrared spectrum of a static, rmm temperature mixture of B2H6, NH,, and Ar was also obtained by introducing 0.01 mmol B2H6, 0.02 mmol NH3, and 1.0 mmol Ar into a 10 cm path length (1) Stock, A.; Massenez Chem. Ber. 1912, 45, 3539. (2) Muetterties, E. Boron Hydride Chemistry; Academic Press: New York, 1975. (3) Lane, C. F. Chem. Rev. 1976, 76, 773. (4) Perry, R. W.; Edwards, L. J. J . Am. Chem. SOC.1963, 81, 3554. (5) Johnson, H. D.; Shore, S. G. New Results in Born Chemistry; Springer-Verlag: New York, 1970; Vol. 15. (6) Mayer, E. Inorg. Chem. 1972, 1 1 , 866. (7) Purcell, K. F.; Devore, D. D. Inorg. Chem. 1987, 26, 43. (8) Wiberg, E. Naturwissenschaften 1948, 35, 212. (9) Hirayama, M.; Shohno, K. J . Electrochem. SOC.1975, 122, 1671. (10) Hyder. S. B.; Yep, T. 0. J . Electrochem. SOC.1976, 123, 1721. (1 1) Adams, A. C.; Capio, C. D. J . Electrochem. SOC.1980, 127, 399. (12) Bcddeker, B. W.; Shore, S. G.; Bunting, R. K. J . Am. Chem. Soc. 196688,4396. (13) Kuznesof, P. M.; Shriver, D. F.; Stafford, F. E. J . Am. Chem. SOC. 1968, 90, 2557. (14) Kwon, C. I.; McGee, H. A. Inorg. Chem. 1970, 9, 2458. (15) Gerry, M. C. L.; Lewis-Bevan, W.; Merer, A. J.; Westwood, N . P. C. J . Mol. Spectrosc. 1985, 110, 153. (16) Sugie, M.; Takeo, H.; Matsumura, C. Chem. Phys. Lett. 1979, 64, 573. (17) Gropen, 0.; Seip, H. M. Chem. Phys. Letr. 1974, 25, 206. (18) Dill, J. D.; Schleyer, P. V. R.; Pople, J. A. J. Am. Chem. SOC.1975, 97, 3407. (1 9) Craddock, S.; Hinchliffe, A. Matrix Isolation; Cambridge University Press: New York, 1975. (20) Hallam, H . Vibrational Spectroscopy of Trapped Species; Wiley: New York, 1973. (21) Andrews, L. Annu. Rev. Phys. Chem. 1971, 22, 109. (22) Auk, B. S. J . Am. Chem. SOC.1978, 100, 2426.
0 1991 American Chemical Society
Characterization of Aminoborane TABLE I: Fundemhl h a d Podtiom a d Assignments for Matrix-Isolated Aminobonne, H+=NH2 peak position, cm-' Ar matrix N 2 matrix gas I4N I5N 14N I5N phase assignment 3519 3509 3523 3503 3534 u9, N-H antisym str 3437 3432 3439 3433 3451 u l , N-H sym str ul0, B-H antisym str ('OB) 2585 2585 2585 2585 2568 2568 2568 2568 2564 uI0, B-H antisym str 2520 2511 2533 2524 Y,, B-H sym str ('OB) 2499 2494 2506 2499 2495 u2, B-H sym str 1620 1615 1617 1609 1625 u,, NH, sym bend u4. B=N str ('OB) 1365 1361 1377 1371 1334 1329 1346 1340 1337 u4, B P N str u5, BH, sym bend 1131 1125 1132 1125 1120 I 1 1 5 1122 1116 1131 us, BH2 sym bendb u I I , NH2 rockb 1022 1016 1028 1023 1014 1014 1013 1013 u,, BH, out-of-plane wag 1002 1002 1001 1001 1005 Y,, BH, out-of-plane wagb us, NH2 out-of-plane wagb 705 702 723 719 uI2, BH, rock ('OB) 613 uI2, BH, rockb 608 604 647c 642 'sym = symmetric: antisym = antisymmetric; str = stretch. bReassigned relative to previous gas-phase study; see text. CDominantsite of site-split doublet (636, 647 cm-I).
gas cell. A fine white powder settled out quickly on all of the interior surfaces of the gas cell. An infrared spectrum was then recorded at 1 cm-' resolution. p-Aminodiborane was synthesized in one experiment by allowing diborane and ammonia to react in a glass bulb on the vacuum line, forming a powder of the diammoniate of diborane on the walls of the vessel. The excess gaseous reactants were pumped out, and an additional quantity of the argon/diborane mixture was added. The bulb was gently heated with a heat gun, and after several minutes matrix deposition was initiated directly from the glass bulb.
Results Prior to any deposition experiments, blank experiments were conducted with B2H6 and NH3, each alone in argon. Spectra of these samples were in good agreement with literature spectra, and with spectra recorded previously in this laboratory.23*24Additional blank experiments were conducted while heating the deposition line; no distinct differences were observed under these conditions. Samples of Ar/B2H6 = 1000/1 and Ar/NH3 = 1000/1 were codeposited in an initial twin jet experiment and no new absorptions were observed in the resultant spectrum. Sample concentrations were then varied systematically in a series of experiments, all employing twin jet deposition. In none of these experiments were any new infrared absorptions detected. Two merged jet experiments were then conducted with the deposition line at room temperature, one with the tee 40 cm from the cold cell, and one with the tee 145 cm from the cell. Again, no new absorptions were seen. An extensive series of merged jet experiments was then carried out in which the deposition line was heated, initially to 80 OC, and then stepwise to 360 OC. The 145-cm mixing region was employed. At 80 OC, no new absorptions were observed. When a sample of Ar/B& = 250 was merged with a sample of Ar/NH3 = 250/1 and the deposition line heated to 180 OC, a band of medium intensity was observed at 608 cm-I, along with weak bands at 705,1002,1022,1120,1216,1334,1603,2500,2568, and 3519 cm-I. Additional experiments were conducted at this temperature, varying the B2H6/NH3ratio from 2/1 to 1/2. The same set o f product absorptions was observed, with comparable overall intensity. When the temperature of the deposition line was increased to 250 O C , the same bands were observed, with substantially greater intensity. In addition, a number of weak new absorptions were seen and are listed in Tables I and 11. The intensity of the absorptions of parent NH, and B2H6all decreased, except for the (23) Auk, B. S.Chem. f h y s . Lou. 1989, 157, 547. (24) Auk, B. S. Inorg. Chem. 1981, 20, 2817.
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3503 TABLE II: Combination, Overtone, Sitesplit, and Unassigned Bands in the Swctrum of Amiaoborane Isolated in Solid Argon band position,=-' I4N I5N intensity assignment 3509 3500 0.12 OD site splitting 3489 3480 0.08 site splitting 3342 3332 0.10 2667 2657 0.10 2v4 2548 0.30 Y4 + V I ' 2463 2453 0.50 site splitting 2453 2445 0.18 y4 + y5 1603 1600 0.50 site splitting" 1458 0.07 site splitting 1449 0.07 y6 + 1353 1347 0.18 u6 + u12 1225 1216 0.07 242 ('OB) 1216 1209 0.25 2YI2 1 I45 1137 0.07 site splitting 888 888 0.04 636 632 0.25 site splitting 617 617 0.50 site splitting #Tentative assignment; see text.
1.
I ' & '
"
'A'
"
K4"m
I
' 'Ob0
400.0
Figure 1. Infrared spectrum, from 400 to 4000 cm-I, of the products arising from the 360 "C pyrolysis of a mixture of Ar/B2H2/NH3 = 1OOO/1/1 followed by condensation at 14 K.
-.-
1
MA-
d0""dr"''
Figure 2. Infrared spectrum, from 550 to 750 cm-l, of the matrix isolated products arising from the pyrolysis of a sample of Ar/B2H6/I5NH, = 1000/1/1 (trace a) compared to the spectrum of the products of the pyrolysis of a sample of Ar/B2H6/14NH3 (trace b).
intensity of the parent NH3 absorption at 3437 cm-I. Increasing the temperature to 360 "C led to a further increase in product band intensities and decrease in parent band intensities, as shown in Figures 1 and 2. Samples of N2/B2H6were deposited with samples of N2/NH3 in several experiments at elevated temperatures, and comparable product bands were observed. These product bands are also listed in Table I; shifts from argon to nitrogen were generally small, although the argon matrix band near 608 cm-' showed a large shift, to 647 cm-l, with substantial broadening. In general, spectral regions near absorptions of parent NH3 were cleaner and more readily interpreted due to the inhibition the rotation of parent NH3 in N2matrices. Additional experiments were conducted at 250 and 360 O C employing ISNH3;a small amount o f I4NH3was observed due to
3504 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
'
.
I 0.00
h
,
,
,
,
u -
ab'
'
I
'C&
400.0
Figure 3. Infrared spectrum of the white powder formed after gas-phase reaction of Ar/B2H6and NH, in a IO-cm gas cell.
exchange with adsorbed NH3 in the vacuum manifold. When the deposition line was heated to 250 O C and samples of Ar/B2H6 = 250 and Ar/I5NH3 = 250 were deposited in the merged jet mode, medium-to-strong new absorptions were observed at 604, 608,702, 1002, 11 15, 1209, 1329, 1600,2495,2568,3433, and 3509 cm-I. Additional, weak bands were seen and are listed in Tables I and 11. Products due to the reaction of B2H, with residual I4NH3were also weakly observed. When the deposition line was heated to 360 OC, the product bands were seen with increased intensity, and the parent absorptions were greatly diminished. For comparison, similar experiments were conducted in N2 matrices and comparable absorptions noted, as listed in Table I. In a few experiments, dependence of product yield on the deposition line material was examined. When only the relatively short section of stainless steel near the cold cell was heated, the same product bands were observed without any additional product bands. The yield was reduced somewhat in this experiment compared to heating both the short stainless steel section and the longer copper section, yet the yield was still significant. As noted above, the argon/diborane gaseous mixture was combined with NH3 gas in one experiment in a 10-cm gas cell, and the infrared spectrum of the resulting mixture was obtained. Shortly after mixing, a fine white powder settled out on all of the interior surfaces of the gas cell. No diborane was seen in the resultant spectrum, and only a minor amount of NH3(g) was seen. In addition, new broad, moderately intense product absorptions were observed at 1100,1185,1400,1610,2255,2330,2450,3170, and 3250 cm-I, as shown in Figure 3. The synthesis of p-aminodiborane was attempted through the reaction of diborane with the diammoniate of diborane at slightly elevated temperatures in a glass bulb. The spectrum of this resulting mixture showed strong absorptions due to unreacted diborane; in addition, a number of less intense absorptions were observed and are listed in Table 111.
Discussion Product Identification. When diborane and ammonia were allowed to flow through a length of heated deposition line and the sample was then rapidly condensed on the matrix cold window, a number of infrared absorptions were observed which could not be attributed to either parent species. In addition, the intensities of the parent absorptions were significantly reduced, indicating that reaction had occurred. Earlier studied6 have indicated that one of the initial products of the high-temperature reaction of B2H6 with NH3 is aminoborane, H2B=NH2. While no matrix spectrum is available for this compound, the low-resolution gas-phase spectrum has been observed15 above 750 cm-', as well as the high-resolution spectrum of the B=N stretching mode near 1350 cm-I. No spectral data have been previously reported for the I5N isotopomer. Comparison of the gas-phase spectrum of aminoborane with the spectrum obtained here showed numerous strong similarities, as listed in Table I. As a consequence of this close agreement, the product absorptions listed in Table I are assigned to fundamentals of argon matrix isolated H2B=NH2. The previous gas-phase study was unable to identify directly the two lowest energy modes due to spectral limitations but estimated that these
Carpenter and Ault TABLE III: Band Paeitions ( c o i l ) and Tentative Assignments of the Infrared Spectrum of H2N&H5 in Solid Argon H2NB2H5
(CHd2NBJV
3472 3430 340 1 3362 2568 2494 2482 1912 1663 1105 1075 1069 1030 995
2552 2476 2476 1885 1635 1075 1069 1063 945
~
assignment N-H antisym str N-H antisym str (?) N-H sym str N-H sym str (?) B-H antisym str (terminal) B-H sym str (terminal) site splitting B-H sym str (bridging) B-H antisym str (bridging) NH, antisym bend BH2 antisym bend B-N str BH2out-of-plane wag BH, out-of-plane wag
"Taken from ref 35. bands would occur near 600 and near 700 cm-I, through Fermi resonance interactions with higher levels. The spectra obtained here contained two bands, a t 608 and 705 cm-' (shown in more detail in Figure 2), which maintained a constant intensity ratio to bands which clearly could be assigned to H2B==NH2, indicating that they must be assigned to the same absorber. Thus, these two bands can be readily assigned to the two previously missing fundamentals of aminoborane. In addition, experiments with I5NH3led to a comparable set of product absorptions shifted 0-10 cm-' to lower energy and are readily assignable to the I5N counterpart of each of the aminoborane absorptions. These are also listed in Table I. Finally, boron occurs in nature as a 20%/80% mixture of 'OB/lIB, so that the absorptions described above are due to the IlB isotopomer of aminoborane. For a number of the stronger absorptions, an additional weaker band was observed to the blue of the IIB absorption, and can be assigned to the I0B isotopomer of H2B==NH2. These are also listed in Table I. Band Assignments. Comparison of the matrix band positions for H2B=NH2 with the gas-phase results indicates that there is good agreement between the two, for the nine fundamentals observed in the gas phase. The observed matrix shifts, approximately 1% or less of the observed frequency, are not uncommon. However, the magnitude of the I5N and 1°B shifts for several vibrations suggest reassignment of several of the modes. Two vibrations of BI symmetry are anticipated for aminoborane, corresponding to the out-of-plane deformation of the BH2 and NH2 subunits. Gerry et al.15assigned a band with a distinctive C-type contour at 1005 cm-l to the NH2 out-of-plane deformation and suggested that the analogous BH2 mode should come near 700 cm-l in the gas phase, with the same general contour. In the present matrix experiments, the 1002-cm-I band showed a strong 1°B shift (to 1014 cm-I) and no measurable ISNshift. On the other hand, a band was observed at 705 cm-' which was assigned above to the previously unobserved fundamental expected in the gas phase near 700 cm-l. This band showed a several wavenumber ISN shift, and no 1°B shift. The observations suggest that the 1002-cm-I band is better assigned to the BH2 out-of-plane motion and the 705-cm-I band to the NH2 out-of-plane motion. Both should have the characteristic central spike of a C-type contour so that assignment based strictly on band contour may have some intrinsic ambiguity. [Gerry et al. did actually assign the 1005-cm-I band to the BH2 out-of-plane motion in the text but assigned it to the NH2 out-of-plane motion in their summary table.] Finally, Dewar and M C K have ~ ~carried ~ ~ out MNDO calculations for aminoborane and calculated the BH2 out-of-plane motion to come 300 cm-' higher than the NH2 out-of-plane motion, in agreement with the present assignment and opposite of the assignment of Gerry. The intense product band at 608 cm-I in an argon matrix showed a I0B shift to 613 cm-I and a IsN shift to 604 cm-I. This band was remarkably sensitive to the matrix material and appeared ( 2 5 ) Dewar, M. J.
S.;McKee, M. L. J . Mol. Srrucr. 1980, 68, 105.
Characterization of Aminoborane
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3505
as a doublet with the more intense component at 647 cm-I in solid N,. While no distinct 1°B counterpart was observed in N,, the band was broadened significantly on the high-energy side, and the weaker loB counterpart is very likely obscured. Gas-phase spectroscopic studies and MNDO studies both assign this mode as ut,, the antisymmetric BH2bending or in-plane rocking mode. In view of these results, and the isotopic shifts observed here, such an assignment is appropriate, although the I5Nshift indicates some mode mixing. In the gas-phase spectrum, Gerry et al. observed at least three overlapping bands between 900 and 1300 cm-l, the 1005-cm-I band discussed above as well as bands centered near 1131 and 1225 cm-l. They note that the structures of these bands are not clear-cut, and the 1131-cm-l band 'is very confused because of overlapping and additional structure belonging to the loB species". They assign the 1131-cm-l band to the NH2 antisymmetric bending mode and the 1225-cm-I band to the BH, symmetric bending mode. The isotopic data obtained here suggest reassignment. The 1 I3 1-cm-I gas-phase band, observed here at 1120 cm-', showed a strong loB shift and a small but nonzero I5Nshift. As discussed below, this mode is strongly mixed with two other vibrations of A, symmetry and consequently involves motion of both the boron and nitrogen atoms. Consequently, assignment of the 1 131-cm-' band as the BH2 symmetric bend appears appropriate. The analogous BH2 deformation mode of H2B= N(CHJ2 has been observed2' at 1156 cm-', supporting this assignment. It is interesting to note that Gerry et al. assigned the 1 131-cm-I band to the NH2 antisymmetric deformation despite the "overlapping and additional structure belonging to the 1°B species". Two bands of comparable intensity were observed in the matrix spectrum, at 1603 and 1620 cm-I, which might be assigned to u3, the symmetric NH2 bending mode (observed in the gas phase at 1625 cm-'). Both showed ISNshifts, as anticipated. The 1603and 1620-cm-1 bands are most likely site splittings of v3, although a 17-cm-I site splitting is larger than usually observed. Based on proximity of the 1625-cm-' gas-phase band, assignment of the 1620-cm-I band as u3 is made, with the 1603 cm-I as a site-split peak. Only the NH2 rocking mode remains to be assigned. Based on comparison26 to (CH,),B=NH,, this mode is anticipated slightly above 1000 cm-l. The medium-intensity band observed in argon at 1022 cm-I and in N2 a t 1028 cm-' is the most likely candidate. This band showed both a strong I5N shift, indicative of a vibration involving motion of the nitrogen atom, and no 1°B shift. The lack of observation of this band in the gas-phase spectrum is likely due to the close proximity of the intense vS mode, and the spectral congestion in this region. Consequently, the 1022-cm-I band in the argon matrix experiments is assigned as the NH2 rocking mode of aminoborane. Additional weaker bands were also observed in the higher temperature, higher yield experiments and are listed in Table 11. Of particular note is the 1225-cm-I band (observed here in an argon matrix at 1216 cm-l) which showed a 7-cm-I I5N shift and a 9-cm-I 1°B shift. The best candidate for a N2 matrix counterpart is a weak band observed at 1285 cm-l, which represents a very large matrix shift. However, this band is almost exactly double the energy of the BH2 rocking mode in each matrix (608 cm-l in argon and 647 cm-I in N2). Gerry et al. observed a band at 1225 cm-I in the gas phase which they concluded must be of A, symmetry and assigned to the BH2 symmetric deformation. The I5N shift of this band precludes this assignment (the spectra presented here indicate assignment of the BH2 symmetric deformation to the band a t 1120 cm-' as noted above). Moreover, the very large matrix shift and the large 1°B and ISN shifts are best rationalized as an overtone rather than a fundamental. In fact the isotopic shifts of the 1216-cm-I band were almost exactly double the shifts of the 608-cm-I band. Consequently, the (26) Becher, H. J. Specrrochim. Acra 1963, 19, 575. (27) Price, W. C.; Fraser, R. D. B.; Robinson, T. S.;Loguet-Higgins, H. S.Discuss. Faraday SOC.1950, 9, 13 1 .
band
band position,
tentative
position,
cm-I
assignment
cm-l
antisym str sym str antisym str sym str BHL antisym str BH4- antisym str
1610 1400 1190 1100 1065
3250 3170 2550 2450 2370 2330
N-H N-H 9-H 9-H
1020
tentative assignment N H l antisym bend N H l sym bend B H i sym bend BH4- antisym bend BH4- antisym bend BH4- antisym bend
1216-cm-I argon matrix band and its 1285-cm-I N2 matrix counterpart are best assigned as 2uI2,also of AI symmetry. Several additional weak bands can reasonably be assigned to combinations and overtones; Gerry et alei5made note of several of these combinations as well. Two involve the A2 infrared-inactive mode of aminoborane, from which this mode can be located near 744 cm-I in an argon matrix. This compares well to the estimated 763-cm-I gas-phase position and the 738-cm-I solid-phase position. Additional bands may be due to site splittings but cannot be so assigned with certainty. The ratio of intensities of almost all of these bands relative to bands due to aminoborane remain constant, suggesting that they are best assigned to aminoborane and not a second reaction product. Normal-Coordinate Calculations. The I5N shift of the 1334-cm-' band which has been assigned to the B=N stretching mode was much smaller, 5 cm-I, than anticipated for a B-N diatomic, 20 cm-I. At the same time, the experimental 1°B shift was quite large, 31 cm-I. To clarify these observations, and all of the band assignments made herein, normal-coordinate calculations were carried out for H2B=NH2. The experimental geometry obtained from microwave datal6 was employed, with a general valence force field. Initial force constants were taken from literature calculations on similar compounds28 and refined to the experimental frequencies measured here. I4N, ISN,I 1 B, and loB vibrational frequencies were employed in the calculation. The overall fit from this calculation was quite good, given the harmonic nature of the calculation, with an average error of 1.76% for the I4N and ISNspecies. A key feature of the calculation is that, not surprisingly, the three vibrations of AI symmetry which lie between 1100 and 1600 cm-I mix significantly. These are nominally the BH2symmetric bend, the B=N stretch and the NH, symmetric bend. As a consequence of this mixing or coupling of vibrational modes, these labels are very approximate at best. For example, the potential energy distribution for the 1334-cm-l band shows roughly 35% B=N stretching character, 40% BH2 bending character, and 25% NH2 bending character. This, then, accounts for the unexpectedly low ISNshift compared to an isolated B-N oscillator. Finally, the calculations suggest a 4-cm-l I5Nshift for the NH2out-of-plane deformation at 705 cm-', in good agreement with the observed 3-cm-I shift, thus supporting this reassignment. [H2E(NH3),]'[BH4]and H2NE2H5.During the study of the infrared spectrum of H2B=NH2, the spectra of two additional B-N-H species were obtained. While the diammoniate of diborane, [H2B(NH3),]+[BH4]-,is often mentioned in the literature,4-6 there has been no discussion of the infrared spectrum of this compound (and only a brief remark on the Raman spect r ~ m , ~ )The . white powder formed upon mixing B2H6 and NH3 in a 10-cm gas cell is very likely this compound based on the known chemistry of this pair of reactants, and the spectrum is consistent with this identification. The absorptions obtained here are listed in Table IV, with tentative assignments. Two distinct N-H stretching modes were observed, slightly to the red of free NH3, as expected for coordinated NH3. Two B-H stretching modesM of the BH4- anion were observed between 2300 and 2400 cm-l, (28) (a) Goubeau, J. Adu. Chem. Ser. 1963, No. 42. (b) Reuter. D. C.; Thorne, L. R.; Gwinn, W. D. J . Phys. Chem. 1982,86,4737. (c) Smith, J.; Seshadri, K. S.;White, D. J . Mol. Specrrosc. 1973, 45, 327. (29) Taylor, R. C.; Schultz, D. R.; Emery, A. R. J . Am. Chem. Soe. 1958, 80, 27. (30) Emery, A. R.; Taylor, R. C. J . Chem. Phys. 1958. 28, 1029.
3506 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 in agreement with previous studies of this anion. Due to the low symmetry of the cation, reduction in symmetry of the anion from T d is anticipated, with a splitting of the degeneracy of the F2 stretching mode, and an activation of the AI stretching mode. Two B-H stretches were observed at higher energies, at 2450 and 2550 cm-I, which are appropriate for B-H stretches of the cation. Bands at 1400 and 1610 cm-' are readily assigned to NH3 symmetric and antisymmetric bending modes by comparison to many known31 coordination complexes of NH,. The bands between 1000 and 1200 cm-' lie in the bending region of the BH4- anion, as well as perhaps the H-B-H bending region of the cation, and are assigned to such modes. The reaction of B2H6 with [H2B(NH3)2]+[BH4]-at slightly elevated temperatures is known to lead to NH2B2HS,p-aminodib~rane.,~While the infrared spectrum of this compound was determined as part of a Masters thesis,33it has not been published in the literature, except for the low-frequency ring puckering mode.34 This compound was synthesized and deposited into an argon matrix, to determine whether this was a side product of the reaction producing H2B=NH2. A number of absorptions were obtained and are listed in Table 111. Also listed are absorptions of the known3s p-dimethylaminodiborane,for which many vibrations should be quite similar. As is apparent, there is good agreement in the B-H stretching and bending regions, as well as for B-N-B stretch. Bands appropriate for N-H stretching modes of p-aminodiborane were observed above 3350 cm-I; N-H stretching modes, of course, will not occur for the dimethyl derivative. While these assignments are based largely on analogy to the dimethyl derivative, they are likely sound. At the same time, it is clear that H2NB2HSis not a detectable side product in the high-temperature reaction of B2H6 with NH, under the present conditions. Reaction Mechanism. The gas-phase reaction of B2H6 with NH, to form H2B=NH2 requires elevated temperatures and is not likely a one-step reaction. Since, however, only a single reaction product was identified, it is difficult to pin down the reaction pathway. Several researchers3638 have postulated that B2H6 undergoes symmetric scission above 100 OC to form two BH, units (others have disputed this initial ~ t e p ~ ~If, ~BH3 ) . is formed in this fashion, reaction with the strongly basic NH3 should be rapid, forming the adduct H3B.NH3. This species is known to be unstableI3-lsand will eliminate H2 upon heating, yielding the observed aminoborane product. While this pathway would account for the product formation, it does not explain the apparent absence of BH, in the matrix, particularly in the high-temperature blank experiments involving B2H,. Also, since there is considerable (31) Nakamoto, K. Infrared and Roman Spectra oflnorganic and Coordinarion Compounds. 4th ed.;Wiley-Interscience: New York, 1986. (32) Schlesinger, H. 1.; Ritter, D. M.; Burgs, A. D. J . Am. Chem. SOC. 1938, 60, 2297. (33) Gaylord, A. S.M.S. Thesis, Connecticut Wesleyan University, 1972. (34) Gaylord, A. S.; Pringle, W. C. Jr. J . Chem. fhys. 1973, 59, 4674. (35) Mann, D. E. J . Chem. Phys. 1954.22, 70. (36) Long, L. H. Prog. Inorg. Chem. 1972, 15, 1 . (37) Askins, B. S.;Riley, C. Inorg. Chem. 1977, 16, 481. (38) Mappes, G. W.; Fridman, S. A.; Fehlner, T. P. J . fhys. Chem. 1970, 74, 3307. (39) Long, L. H. J . Inorg. Chem. Nucl. Chem. 1970, 32, 1097. (40) Fehlner, T. P. J . Am. Chem. SOC.1%5,87,4200.
Carpenter and Ault debate over the first, symmetric scission step, this pathway seems less likely. Alternatively, direct reaction of NH3 with B2H6 at elevated temperatures could occur, yielding the adduct and BH,. Again, the adduct will lose H2 readily, yielding aminoborane. This mechanism also suggests that free BH, will be formed and should be isolable. However, this would only occur in the reaction mixtures, not in the blank experiments, and with an excess of NH3 (compared to BH,) all of the BH3 produced might be rapidly scavenged. Earlier researchers" formed and studied BH, in argon matrices through the pyrolysis of samples of Ar/BH3C0. They obtained only a quite low yield of BH3 and attributed this to rapid decomposition of BH3 on the wall of the heated deposition line. A similar fate would be expected in the present experiments. The merged jet reaction of B2H6with methylamines has been studied and both the adduct H3B.NH,(CH3), (x + y = 3) and the H2 elimination products were observed (except for experiments involving (CH3),N where only the adduct was seen). If BH3 were formed in the initial step and survived passage to the matrix, then this would be anticipated as a reaction product common to all of these systems. Careful examination of the spectra showed no product bands common to all of the ammonia and methylamine experiments. Consequently, all of the evidence suggests that BH3 is formed but rapidly consumed by wall reaction or by reaction with excess amine prior to matrix deposition. At the same time, experiments during which different sections of deposition line, constructed of different materials, were heated suggested that the reaction is relatively independent of deposition line material (copper or stainless steel). This indicates that the, reaction is most likely a homogeneous gas-phase reaction, although the BH, destruction step may be heterogeneous. A third and less likely possibility is a three-body reaction involving one B2Hs molecule and two NH, molecules. This reaction would lead simultaneously to two molecules of the adduct, which would then eliminate H2 to yield the observed product. However, this pathway should show a strong sensitivity to the ratio of B2H6 to NH, in the reaction mixture; to the contrary, no strong dependence of yield on concentration was observed. Overall, then, it is difficult to deduce the reaction mechanism from the one product, although the pathway involving direct reaction of one NH, with B2H6appears most likely. The variation of product yield, as indicated by band intensities, with temperature of the deposition line suggests that a simple plot of In absorbance versus 1/T might yield an activation energy for the reaction. Such a plot is roughly linear with a slope of -2803 and a correlation coefficient of 0.85. This suggests a low activation energy, 5.6 kcal/mol, for the rate-limiting step in the reaction process. Given the lack of observation of the H3B.NH3 adduct in these experiments, the slow step is likely the initial step involving attack of an NH, molecule on B2H6, although this cannot be deduced with certainty. Acknowledgment. We gratefully acknowledge support of this research by the National Science Foundation under grant C H E 81-21969. (41) Kaldor, A,; Porter, R. F. J . Am. Chem. SOC.1971, 93, 2140. (42) Carpenter, J. D.; Ault, B. S. To be published.
