2623
J. Phys. Chem. 1991,95, 2623-2628 proximately 1:3100 (therefore only 1:3 for AI/SiH, in the sample). Reducing the quantity of silane in the matrix narrowed the absorption bands considerably as is seen in Figure 10 in an attempt to reveal weak satellite bands near the two infrared-active fundamentals in the presence of aluminum (spectrum C). Satellite bands closely proximate to the fundamental vibrational frequencies could be attributable to perturbations of the tetrahedral geometry of silane due to the presence of complexed aluminum. As far as it can be detected, however, spectra B and C are identical with two exceptions: there appear to be two very small bands exclusive to spectrum C at approximately 2100 and 850 cm-I, both of which are very close on the low-energy side to each of the two fundamental vibrational frequencies. These weak bands are indicated in the figure by an asterisk. Upon obtaining spectrum C, much work was carried out enlarging these spectral regions (along with preparing alternate samples having varying dispersion ratios of metal/substrate) to convince ourselves that the weak bands were in fact inherent to the system and not anomalous and that they were reproducible. However, upon irradiation of the sample that gave rise to spectrum C (and the similar samples mentioned above) to photolytically generate the insertion product silylaluminum hydride, SiH,AIH (the subject of part 2 of this study), these bands
-
grew in intensity at equal rates, indicating that they were characteristic of the insertion product and not the ground-state complex. Hence, with the dilution conditions present in the sample of spectrum C, approximately 1:IOOO SiH,/Ar and 1:3 Ai/SiH4, the aluminum-silane complex has not manifested itself with "grating" IR detection. Summary
The experimental and theoretical results of this study demonstrate that A1 atoms in pure silane or silane/argon mixtures at 12 K yield a ground-state complex possibly containing a threecenter, two-electron AI($-SiH,) bonding interaction.
Acknowledgment. The generous financial support of the Natural Sciences and Engineering Resehrch Council of Canada is greatly appreciated. The award of an Ontario Graduate Scholarship (M.A.L.) is deeply appreciated. Valuable computational assistance from Dr. Douglas McIntosh and Dr. Mike Peterson with various aspects of the EPR simulations and ab initio calculations are gratefully acknowledged. Registry No. AI, 7429-90-5; SiH4, 7803-62-5; Ar, 7440-37-1; SiH3AIH, 116233-51-3.
-
Ai(2P)(SiH4]Complex and Photoreversible Oxidative Addition/Reductive Elimination Reaction Ai(2P)(SiH4] SIHSAiH. 2. Ai(*P)(SiH,) SIHSAiH Reaction Michael A. Lefcourt and Geoffrey A. Ozin* Lash Miller Chemical Laboratories, University of Toronto, 80 St. George St., Toronto, Ontario, Canada M5S 1Al (Received: April 26, 1989; In Final Form: August 9, 1990)
Brief 400-nm photolysis of 1: 10 silane/argon matrices containing monatomically isolated aluminum results in the formation of the insertion product silylaluminum hydride (SiH,AIH), most likely via an oxidative addition mechanism. The techniques of UV-vis, EPR, and infrared spectroscopy enabled the identification and characterization of this molecule. The photolytic generation of this species paralleled the formation of methylaluminum hydride (CH3AIH) studied previously in our laboratory. EPR spectral simulations and ab initio self-consistent-fieldmolecular orbital (SCF MO) calculations were employed to help in the characterization of silylaluminum hydride. The EPR spectral parameters extracted from the raw data via the computer simulations, along with subsequently calculated atomic orbital spin densities, determined that the molecule was a bent, orthorhombic species. The SCF MO calculated bond angle at the AI atom was shown to be 1 18.80°. Secondary photolysis of the aluminum/silane matrix sample resulted in the conversion of the insertion product back to that of the ground-state complex most likely by a reductive elimination mechanism.
Introduction
To confidently report the existence of a matrix-isolated aluminum-silane AI(SiH4) ground-state complex, justification must be made to the extent that the spectral features presented in part 1 of this study do not correspond to the more easily envisaged entity, that of the insertion product silylaluminum hydride (SiH3AlH). This latter species is more abundantly precedented in the literature by several analogous matrix systems involving methane and a variety of metals such as Mn,4SS Fe,es Cu,2*4*798 and Mg2*9*'0 to list a few. Each of these metals in atomic (1) Parnis, J. M.; Ozin, G. A. J . Am. Chem. SOC.1986, 108, 1699. (2) Parnis, J. M. Ph.D. Thesis, University of Toronto, 1987. (3) Parnis, J. M.; Ozin. G. A. J. Phys. Chem. 1989, 93, 1204. (4) Billups, W. E.; Konarski, M. M.; Hauge, R. H.;Margrave, J. L. J. Am. Chem. Soc. 1980, 102,7393. ( 5 ) Ozin, G. A.; McCaffrey, J. G.; McIntosh, D. F. Pure Appl. Chem. 1984, 56, 1 1 1 . ( 6 ) Ozin, G. A.; McCaffrcy, J. G. J . Am. Chem. SOC.1982, 104, 7351. (7) &in, G. A.; McIntosh, D. F.; Mitchell, S. A. J. Am. Chem. Soc. 1981, 103. 1514.
form has been shown to oxidatively add to single C-H bonds of methane, resulting in products of the general form CH3MH (where M represents the metal atom). In a few instances, notably with copper, the product methylmetal hydride is subject to secondary photolysis and shows a wavelength-dependent fragmentation to form such radicals as CH3, MH, CH3M, and H.2,7*8With respect to the use of silane as a substrate, two prior gas-phase studies have demonstrated the ability of silane to be insertively attacked by the monatomic species 'PI Mg" and the zPSi ion.I2 Similar to the matrix reaction of Cu with methane, both of these gas-phase systems exhibited immediate fragmentation upon Si-H bond insertion; however, in these cases the exothermicity was at the origin of the fragmentation step. ~
~
(8) Parnis, J. M.; Mitchell, S.A.; Garcia-Prieto, J.; Ozin, G. A. J. Am. Chem. SOC.1985, 107, 8169. (9) McCaffrey, J. G.; Parnis, J. M.; Ozin, G. A.; Breckenridge, W. H. J. Phys. Chem. 1985, 89, 4945. (IO) McCaffrey, J. G. Ph.D. Thesis, University of Toronto, 1987. ( 1 1) Breckenridge, W. H.; Umemoto, H. J. Chem. Phys. 1984,81,3852. (12) Boo, B. H.; Armentrout, P. B. J . Am. Chem. Soc. 1987, 109. 3549.
0022-3654/91/2095-2623$02.50/00 1991 American Chemical Society
2624 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
Lefcourt and Ozin
u n n nu= 400 nm (18 mln.)
*cIu nu- 100 nm (11 mln.)
4
I
200
INCLINE
3(#
460
1
500
nm
U n n nu=
1oQ nm (11 mh.)
Figure 2. UV-vis spectra of aluminum atoms (12 K) upon initial deposition in a 1:lO CH3SiH3/Armatrix (A) and after W n m photolysis for 15 min (B).
-
AND hv= 520 nm (15 h m , 18 min.)
BASELINE
do
do
Jo
SW nm
Figure 1. UV-vis spectra of aluminum atoms (12 K): upon initial deposition in a 1:lO SiH,/Ar matrix (A) after 400-nm photolysis for 11 min (B) and after subsequent 520-nm photolysis for 15 hand 18 min (C).
Baseline spectra were obtained from short depositions of the matrix gas mixture (roughly 0.2 mmol) which yielded flat, absorption-free profiles. Both photolyses employed a monochromator having a 20 nm fwhm exit
U
slit.
U
1
One criterion exists that should instantly indicate that the spectral data of part 1 of this study most likely do not correspond to SiH3AIH formation in a matrix environment or to any resulting fragmentation byproducts. The fact that the aforementioned metal/methane systems exhibited C-H bond insertion only upon photolytic excitation suggests that the ground-state spectra of part 1 of this study represent a chemical entity that is fundamentally different from those anticipated directly or indirectly from Si-H oxidative attack. By use of the well-documented Al/CH4 matrix system'-3 studied earlier in our laboratory as the prototypical photochemical reaction model, this task was accomplished. The previously published reports of ground-state aluminum atom insertion with respect to the C-H bonds of methane'3*14have been deemed, with a reasonable degree of certainty,'-3 to involve erroneous band assignments most likely attributable to A1 atom clusters. Experimental Details
For descriptions pertaining to the basic matrix isolation apparatus, UV-vis, EPR, and IR spectrometers, source metal and matrix gases, and the theoretical computer packages used, the reader is referred to part 1 of this study. Photolytic studies were carried out on matrix-prepared systems with the aid of a 450-W xenon arc lamp (Osram XBO in an Oriel lamp housing). The lamp was situated to the side of the spectrometer such that the emitted light could impinge upon the matrix sample when the cryochamber was elevated vertically out of the spectrometer cavity by several inches. In the case of the EPR experiment photolysis was enabled when the sample was still inserted within the microwave cavity. In all cases the lamp was equipped with a IO-cm water-cooled cell for the purpose of filtering out infrared light to which was attached a 20-cm focal length lens at the front end. A grating monochromator (Oriel 7240) with variable slits (fwhm 2-20 nm) allowed the choice of any photolysis wavelength in the region of interest (generally 800-200 nm). Thermal annealing of prepared matrix samples was accomplished with a small strip heater located just above the cryostat (13) Klabunde, K. J.; Tanaka, Y.J . Am. Ch" Soc. 1983, 105, 3544. (14) Jeong, G.;Klabundt, K. J. J . Am. Chcm. Soc. 1986, 108, 7103.
460 em1 Figure 3. Infrared spectrum of aluminum atoms depited in a 1:lO 2400
1430
SiH,/Ar matrix (12 K). Bands denoted by asterisks appeared upon 400-nm photolysis of the sample for 2 h. The small band indicated by at roughly 1590 cm-l (observed both before and after photolysis) is most likely due to AIH fragments. tip. Temperatures as high as 40 deg above the typical cryogenic operating range (10-12 K) at the sample were accessible via the heater, and thus conditions for the observation of diffusion and agglomeration of matrix-entrapped species were obtainable.
Results A. W-Visible Absorption Studies. Figure 1 shows the UV-vis spectral results when the AI/( 1:lO SiH,/Ar) sample of Figures 2 and 3 of the preceding paper has been exposed to 400-nm irradiation (20 nm fwhm) with a 450-W Xe lamp. Spectra B and C of Figure 1 show respectively the spectra obtained after sample irradiation at 400 nm followed subsequently by 520-nm exposure (also at 20 nm fwhm). It is readily apparent upon the "primary" photolysis (as the initial irradiation process will hereafter be designated) that the major absorption features of spectrum A (band maxima at 400,353,275, and 244 nm) are almost entirely bleached out (with the obvious exception of the 275-nm band) and replaced by a spectrum (B) containing two new absorptions: an easily discernible one at roughly 292 nm and a weak but very broad band with a maximum at approximately 520 nm. A sharp band at 272 nm and a low-intensity absorption in the 240-nm region correspond to the deposition features a t 275 and 244 nm, respectively, therefore revealing incomplete loss of these two bands. The total photolysis time required to generate the new absorption profile was 11 min. The "secondary" photolysis performed to obtain spectrum C was targeted in the low-energy band of spectrum B. Lengthy irradiation (>1S h) at this wavelength (520 nm) resulted in the loss of the two new bands in spectrum B and a partial regeneration of the four deposition absorptions of spectrum A at -50% of their original intensities. In spectrum C these bands (at 400, 350,270, and 242 nm) appear very close to the original band positions in spectrum A listed above (within 5 nm), indicating that the Al/SiH4 ground-state complex is re-
.+
A1(*P)(SiH4)
SiH3AlH Reaction
formed in the same site geometry as that inhabited prior to the primary photolysis. This situation of deposition band depletion upon an initial, brief photolysis to form a new absorption profile, followed by the partial regeneration of the original bands during a second, lengthy photolysis is paralleled in the Al/CH4 matrix system.’-’ Photolysis of a freshly deposited Al/CH4 sample (no rare gas present) at 305 or 368 nm resulted in a “rapid depletion of all features associated with A1 atoms while giving rise to a broad absorption between 450 and 600 nm”.’ It was subsequently determined, via EPR and 1R characterization, that the most abundant species present after the primary photolysis was the insertion product methylaluminum hydride (CH’AIH). Secondary photolysis in the low-energy absorption region (using broad-band irradiation with a cutoff filter at 450 nm) caused the original deposition bands to reappear at approximately 80% of their original intensities after 2.75 h.’ This significantly shortened time frame for the regeneration of the initial spectrum as compared to the situation in the AI/SiH4 system could be a reflection of differences in the quantum yield for the two systems due to relaxation from two distinct photoexcited states. Other possibilities include the existence of a late transition state favoring C-H bond formation (subsequent to AI-C cleavage in CH3AlH) as compared to Si-H bond formation to give back the A1/SiH4 complex or differences in photophysical and photochemical rates. It should be stressed that these comments are purely speculative, however, since none of these phenomena can be identified presently with the current amount of experimental data obtained from the system. Without looking at the EPR and infrared spectral results, it is impossible to assign spectrum B of Figure 1 to the insertion product silylaluminum hydride (SiH3AIH), but we can rightfully assume that a photochemical reaction has taken place. The photochemical sequence displayed in Figure 1 is by no means restricted to the AI/SiH4 or Al/CH4 matrix systems. The photolytic behavior observed here is directly analogous to the situation found for the AI/SiD4 system (deposition spectrum (1:lO SiD4/Ar) in Figure 3 of part 1 of this study) in which four deposition absorption bands (corresponding exactly to the four bands in Al/SiH4) at 395, 349, 272, and 245 nm are replaced by bands at approximately 290 and >500 nm upon brief photolysis (5 min) a t 395 nm. The assumption that the breaking of Si-D and Si-H bonds does occur in the Al/SiD4 and AI/SiH4 systems, respectively, upon initial irradiation is supported by some additional UV-vis data collected from an interesting system in its own right, that of the aluminum-methylsilane deposition matrix. The substrate species methylsilane (CH3SiH3) contains both silyl and methyl functionalities, allowing competition studies to be carried out concerning Si-H vs C-H band activation. Figure 2 shows the UV-vis spectra of this species in the substrate role (1:lO in argon) with respect to monatomic aluminum both before (A) and after (B) the primary photolysis (targeted at 400 nm). The obviously striking observation here, not considering the photochemistry as yet, is the strong resemblance of the deposition spectrum (four band maxima at 403, 365, 268, and 245 nm) with those of the aluminum-silane systems of the preceding paper and the conspicuous absence of the intense atomic aluminum bands observed in the AI/CH4 deposition spectrum of Figure 1 in part 1 of this study. Spectrum A of Figure 2 clearly indicates that an AI/ CH3SiH3ground-state complex involving silyl group interaction is the preferred arrangement instead of the isolated A1 atom situation found in the AI/CH4 spectrum. With respect to Si-H photochemical insertion, it appears obvious from spectrum B that the methylsilane matrix exhibits the same spectral characteristics upon exposure to 400-nm irradiation as the A1/SiH4 system. Two new bands, one moderately intense and the second weak and broad, appear at 305 and >500 nm, respectively (the latter band is not shown), while the two high-energy deposition bands a t 268 and 245 nm in spectrum A appear, after photolysis, a t the slightly shifted positions of 271 and 240 nm. B. Infrared Spectroscopic Studies. Infrared spectra, in conjunction with the EPR results (subject to section C), successfully
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2625
U 24100
U 1-
40
em1
Figure 4. Infrared spectrum of aluminum atoms deposited in a 1 : l O SiD,/Ar matrix (12 K). Bands denoted by asterisks appeared upon 395-nm photolysis of the sample for 4 h. The small band indicated by at roughly 1160 cm-’ (observed after photolysis only due to the noise level in the deposition spectrum) is most likely due to AID fragments.
provided the information by which it was determined that the dominating species obtained upon the primary photolysis was the insertion product SiH3AIH. Unlike attempts to observe the Al/SiH4 ground-state interaction with IR spectroscopy, the determination of the photolytic synthesis of silylaluminum hydride was rather straightforward via IR techniques, based on comparisons with the analogous spectrum reported for methylaluminum h~dride.2~ Figure 3 shows the interesting regions found in the infrared spectrum obtained from an Al/( 1:lO SiH4/Ar) sample both before and after execution of the primary photolysis while Figure 4 shows the same for an AI/( 1:lO SiD4/Ar) sample. The choice of 400-nm irradiation for the silane sample and 395-nm irradiation for the SiD4 matrix was due to the slightly differing positions of the absorption maxima revealed in the UV-vis spectra of Figure 3 from part 1 of this study. Probably the most important observation arising from any of the techniques employed in this study with respect to the determination of silylaluminum hydride, and undoubtedly the most important infrared result, is seen in Figures 3 and 4. Both the SiH4 and SiD4 systems exhibited the growth of three new bands upon the respective primary photolyses: at 21 16, 1784, and 842 cm-l for aluminum in silane and at 1522, 1300, and 628 cm-’ for the Al/SiD4 sample. Each system exhibited equivalent growth rates for its respective three bands during the primary photolysis as well as equivalent rates for their disappearance during the secondary photolysis (not shown in the figures). The wavelength position for the second irradiation process in each system was 520 nm. The fact that the appearance and disappearance of the three bands in each of the two systems constituted synchronized events strongly implies that only one species is being generated and then eliminated in each of the two consecutive irradiation processes. The specific IR data obtained upon the primary photolysis that provided one of the strongest pieces of evidence supporting the existence of SiH3AIH were the wavenumber positions of the two intermediately located bands in the spectra of Figures 3 and 4: those located at 1784 and 1300 cm-l, respectively. These wavenumber positions lie in the same spectral regions as two very conspicuous bands in the spectra of CH3AlH and CD3AlD located at 1746 and 1272 cm-I, respecti~ely,~*~ which have been identified as the A1-H and A1-D stretching frequencies for these two molecules. The AI-H/AI-D wavenumber ratio obtained from these bands is 1.373, a value which is very close to the ratio of 21/2calculated for a theoretical diatomic grouping containing hydrogen and a second atom of infinite mass. The wavenumber ratio obtained from the 1784- and 1300-cm-I bands of the SiH4 and SiD4 samples, 1.372, is essentially identical with the value obtained from the CH3AlH/CD3AlD spectra and therefore constitutes extremely strong support for the assignment of these two bands to those of AI-H and A1-D vibrations. Further support is garnered from unbound AI-H and AI-D species formed in argon matrices which have been shown to yield
2626 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
respective diatomic stretching absorptions at 1593 and 1158 cm-I,I5 giving an isotopic shift ratio (ISR) of 1.376. As well, the actual differences between the AI-H band positions observed in the unbound AI-H case, for CH3AIH, and for the suspected AI-H stretch of the silane-containing matrix show that the local environment assumes a large role in determining the frequency of the vibration in question. The fact that the unbound AI-H fragment absorption differs in frequency by at least 150 cm-' with respect to both values associated with the bound AI-H moieties while these latter quantities exhibit a much smaller difference of less than 40 cm-I between them indicates that the proposed AI-H moiety in the silane sample occupies an environment very similar to that found in the CH3AIH molecule. This discussion is also applicable to the differences observed between the analogous AI-D stretching frequencies across the three systems. The other two infrared bands in each of the SiH4 and SiD4 matrices that manifest themselves upon the initial irradiation process appear as small low-energy satellites of the intense u3 and u4 substrate-only fundamentals. As mentioned in the preceding paper, the u3 band is the T2 asymmetric silane stretch centered at 2180 cm-' (- 1590 cm-l for SiD4) while the u4 band is the T, silane deformation mode situated at approximately 900 cm-' (-670 cm-' for SiD4). The positioning of the satellites between 40 and 70 cm-I to the low-energy side of each of the much more intense silane bands indicates that these smaller bands most likely arise from small quantities of the matrix-deposited silane molecules that have reacted with the AI atoms. The ISR values obtained for the paired (SiH4/SiD4)u3 and u4 satellite bands in the presence of aluminum are 1.390 and 1.341, therefore implying that the bands do arise from Si-H and Si-D vibrations as is expected due to the proximity of the satellites to the substrate-only Si-H/D absorptions. The data presented to this point unequivocally show that AI-H and AI-D bonds are formed upon 400- and 395-nm irradiation of AI/SiH4 and AI/SiD4 matrices, respectively, but it has not been shown whether an insertion of fragmentation process occurs. The presence of a very weak band in the 1590-cm-' region of Figure 3 both before and after photolysis (indicated with the square marker) provides some insight into this question. As was mentioned previously, unbound AI-H molecules isolated in argon matrices were found to absorb in the IR region at 1593 cm-'.I5 The small band in question in the silane sample is therefore most likely due to small quantities of aluminum monohydride, indicating that some fragmentation resulting in SiH3 radicals and AIH molecules does occur. However, due to the observation that this band does not grow upon photolysis implies that the fragmentation process takes place only on deposition. Such a postulation is plausible when one remembers that SiH, radicals were observed in the EPR deposition spectra of Figure 5 from part 1 of this study. Therefore, the primary photochemical reaction appears to arise from a concerted mechanism or a t least not from one that leads to AI-H formation. The analogous diatomic stretching mode for unbound AI-D molecules formed upon deposition of the SiD,containing matrix is positioned at roughly 1160 cm-' (1 158 cm-l in the previously cited work involving argon matricest5)and can be seen upon photolysis in Figure 4. The band is not observed in the deposition spectrum due to the noise level present there. Before leaving the discussion of these infrared results, the photolytically generated bands of Figure 3 should be considered in the context of the discussion found in section E of part 1 of this study. The spectral positions of the Si-H modes in Figure 3 (21 16 and 842 cm-I) correspond very well with the two very weak bands found at approximately 2100 and 850 cm-l in spectrum C of Figure 10 from part 1 of this study (Al/(l:1000 SiH4/Ar) deposition spectrum). This comparison strongly indicates that there is some ground-state formation of the product abundantly produced upon the primary photolysis. An alternate explanation for these weak ground-state absorptions in line with the observation of AI-H fragments upon deposition is one of (1s) Wright, 2215 .
Lefcourt and Ozin 9 I
1
I
"
- - -COMwITIA SIMULATION RMS = 0.007
%I
I I
I
1
,.
1
I
91
I
I'
*
I
1
"4
Afl'RRkv=368nm
2 b 3doo 4dOO QAUSS Figure 5. EPR spectra of aluminum atoms deposited in a 1:lO SiH,/Ar
matrix following 400-nm photolysis for 90 min (A) and in a pure CH, matrix following 368-nm photolysis for 120 min (B, from ref 2, p 285). The silane-containingsample is superimposed with the best-fit computer-optimized band profile. The stick spectra pictorially indicate the orthorhombic g and hyperfine splitting parameters obtained from the simulated spectrum in (A) and the approximately axial parameters previously obtained*in the simulation of spectrum B. The asterisks refer to bands arising from unreacted AI atoms. Spectra A and B were obtained from samples cooled to 12 K. attributing their presence to SiH3 radicals. Infrared silyl radical absorptions arising from photolyzed 1:lOOO SiHJAr matrices at 4 KI6 were found as high as 1999 cm-' and as low as 925 cm-I. Due to the EPR spectra in part 1 of this study, the existence of this radical is unquestionable; however, the IR band shifts from these literature results (75-100 cm-I) are much too large to allow a convincing infrared assignment to be made. A second problem with this latter suggestion as well as with the more believable proposal regarding ground-state formation of the insertion product is that a band attributable to an AI-H stretch (free species or molecularly bound) is not apparent in spectrum C of Figure 10 (part 1). A third alternative consisting of a ground-state process resulting in the formation of SiH3Al and H radicals is negated by an absence of EPR signals arising from isolated H atoms. C. Electron Paramagnetic Resonance Studies and Spectral Simulation. The combination of EPR data and associated spectral simulations obtained via the software package described in part 1 of this study along with the infrared results of the previous section provided an unequivocal determination of the insertion product silylaluminum hydride. Figure 5 shows the EPR spectrum of AI atoms in a 1:lO SiH4/Ar matrix after 400-nm irradiation in spectrum A, overlayed with a computer-simulated plot (rms = 0.067 G). Spectrum B displays the EPR profile of CH3A1H,'-, obtained after 368-nm irradiation of aluminum atoms in pure methane. The separated and scaled down (-0.5 times) narrowband profiles in the center of each spectrum correspond to the respective deposition spectra of Figure 4 (Al/CH4) and Figure 5 (Al/(l:lO SiH4/Ar)) from the preceding paper. The apparent discrepancy between the presence of unreacted matrix components here and the complete absence of deposition UV-vis bands in spectrum B of Figure 1 after photolysis is due to the extremely low spin concentrations detected in the EPR experiment. From the spectral simulations of the EPR band patterns spanning a field range of over 1500 G in Figure 5, it has been determined that both of the spectra represent bent, orthorhombic
R. B.; Bates, J. K.; Gruen, D. M.Inorg. Chem. 1978, 17, (16) Milligan, D. E.;
Jacox, M. E. J . Chem. Phys. 1978, 52, 2594.
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2627
A1(2P)(SiH4J* SiH3AIH Reaction
--
n
/?
I\
v 3-
-COMCUTKR 8IMUUTION
*I
I
4000 QAUSS
Figure 6. Expanded EPR spectral region showing 'parallel-like" aluminum atom resonances from Figure SA split into doublets caused by single hydrogen atom superhyperfinecoupling. The corresponding portion of the best-fit computer-optimized spectrum is indicated by the dashed line. The doublets are indicated by asterisks. TABLE I: EPR Spectral Parameters for Each of the Metbyhluninum Hydride (CHAIH) and Silylaluminum Hydride (SiHAlH) Moleculesa
DarameteP
CHIAIH' 2.002
SHIAlH
2.002 2.000 880
2.003 2.002
723 712 157
753 717
146 154
2.006 890
62 62 62
OThe values were extracted from computer-optimized simulations involving the software packages mentioned in the text. Hyperfine Values (A) in MHz. buncertainty in g (*O.OOI), A (*I MHz). eAs reported in ref 2.
species exhibiting single A1 atom hyperfine and single H atom superhyperfine couplings. Initial approximations to the superhyperfine H atom coupling in the silane system used as input to the computer programs were obtained from the band splittings observed on two of the high-field components of spectrum A. These doublets appear on "parallel-like" A1 atom resonances and are shown in expanded form in Figure 6 along with the corresponding portions of the final optimized spectrum. In order to rule out the existence of multiple trapping sites as the cause of these doublets, thermal annealing of the matrix sample was carried out to 20 K. Spectra obtained upon the warming of the sample to this temperature followed by subsequent cooling back to 12 K did not yield any noticeable changes in the bands comprising the doublets, thus giving no reason to consider the bands in question to arise from anything but superhyperfine interactions. The numerical results for the simulation of spectrum A of Figure 5 in the current study along with those for CH3AlH carried out p r e v i ~ u s l yare ~ ~found in Table I. Although the parameters for both species were determined to be orthorhombic, the data show that an approximately axial description is valid. The average of the very similar A2 and A, hyperfine magnitudes in each spectrum can therefore be equated with an A, value while the AI splittings can be designated as A,,. The stick spectra observed in Figure 5, however, indicate the computer-obtained orthorhombic g values and splittings for spectrum A while the axial stick spectra used in spectrum B for CH3AIH reflect the fact that the gland g2values along with the A2 and A3 splitting magnitudes are even closer to representing axial quantities than those extracted from the simulation of spectrum A. From the spectra of Figure 5 it can be seen that the twoderivative profiles are very similar except that the "parallel-like" and "perpendicular-like" bands of spectrum B are separated with respect to each other whereas the equivalent resonances of spectrum A are not. The photolytically generated bands in the silane-containing matrix are significantly broadened with respect to the corresponding features in the CH3AlH spec-
=-(e)
BOND ANGLE FORMULA:
cow
Figure 7. Approximate geometries of methylaluminum hydride (CHIAIH) and silylaluminum hydride (SiH3AIH)as deduced from the computer simulationsof the respective EPR spectra. The bond angle at the aluminum atom in each molecule was calculated by using a, formula referenced by Weltnerl*shown in the figure. The s value is the fractional s-orbital character at the aluminum atom to which was assigned the 3s orbital values of Table 11: 20% in each case. TABLE 11: Qualitative EPR Atomic Orbital Spin Densities (in Percentages) Calculated from the Spectral Parameters in Table I via the Morton and Preston Axial Approximation (Ref 17)
orbital spin density
CHJAIH 65.0 20.0 11.0 -96
SiH,AIH 62.0 20.0 4.4
-86
trum, and even the characteristically broad aluminum resonances of the ground state Al/SiH4 complex are dwarfed in bandwidth magnitude by the photolytically generated derivative lines. As mentioned, Table I lists the orthorhombic EPR spectral parameters extracted via the computer simulations of spectra A and B of Figure 5 while Table I1 shows the qualitative atomic orbital spin densities calculated by using the Morton and Preston axial approximation." Comparisons of the spin densities reveal that the A1 atom 3p and 3s orbitals contain virtually the same amount of net spin in each of the two systems while the hydrogen 1s orbitals differ noticeably. The total net spin accounted for in each species, roughly 96% for methylaluminum hydride and a p proximately 86% for the silane-containing sample, seems to indicate that there is a transfer of 10% of the spin away from the AI-H region upon replacing the methyl group of CH3AlH with a silyl group (assuming that SiH3AlH is the species so formed). The most likely region for this spin to be transferred would be the silicon 3d atomic orbitals since these constitute the distinguishing electronic feature of silane compared to methane. It was mentioned in the discussion concerning Figure 5 that both spectra A and B correspond to bent geometries. This conclusion was reached as a result of the significant Morton and Preston spin populations residing in the AI atom 3s orbitals: 20% for both species. Linear frameworks would have the unpaired electron in the degenerate pair of aluminum p orbitals perpendicular to the C3 symmetry axis, resulting in a minimal 3s spin density, obviously not the case shown in Table 11. Also, the degeneracy of the p orbitals would probably result in a highly broadened and unobservable EPR spectrum since the spin-lattice relaxation time ( T , ) is greatly reduced in this situation. Figure 7 shows the methylaluminum hydride molecule along with the proposed SiH3AlH species in their approximate geometries as deduced from the EPR spectra. Using a formula referenced by Weltner'* and shown in the figure, we calculated the bond angle (17) Morton, J.
R.;Preston, K.F. J . Magn. Reson. 1978, 30, 577.
2628 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
Lefcourt and Ozin 12K
AI
+ SIHq/Ar-
hrl . a 1 1 0
AI@P){SlH4)=
H3SIAIH
m W U R hrl yo 1111 GEOYOan
Cs ORTHORHOMBIC
-282.060860 H # M a l #("&4)
0
r
w
111140 111.14.
1ir.m
AI
+ CH4-AI
1 2 K hui =
Joarm
+CH4eH3CAIH hrl
0
SSU
Cs ORTHORHOMBIC
Figure 9. Pictorial summary of the ground-state and photochemical phenomena for each of the Al/SiH4 and AI/CH4 matrix systems.
i, -533.099418 H 0W-Q -0
1121c 111" . ( H M 11Figure 8. Final ab initio SCF M O 6-31G** optimized geometries for methylaluminum hydride (CH,AlH, from ref 2, p 296) and silyl-
)w+w
aluminum hydride (SiH,AIH). All the bond lengths and angles are shown for the two molecules along with their converged energies (in hartrees). Both structures represent *A' species. at the aluminum atom for each molecule. The value s in the formula is the fractional s character at the aluminum atom to which we assigned the quantities of Table 11: 20% in each case. Recalling the possible fragmentation pairs discussed in section B,SiH, and AIH, and SiH3AI and H, it is clearly apparent from the experimental data and associated analyses presented to this point that these species are not generated in any sizable amounts. The absence of growth of the AI-H molecule band upon the 400-nm photolysis of an Al/(l:lO SiH,/Ar) sample in the IR effectively rule out fragmentation of the first kind indicated above while the distinct absence of isolated H atom resonances in the EPR spectra (characteristic hyperfine splitting of -500 G about g = gc) eliminates the second choice. Along with these conclusions, the distinctive infrared and EPR spectral data have persuaded us to declare that the photochemistry results in a concerted reaction and that the insertion product silylaluminum hydride is the dominant species formed upon the primary photolysis of AI/SiH4 matrix samples. Likewise, SiD3AlDis the main product upon the corresponding irradiation of AI/SiD4 matrices. D. Ab Initio Self-Consistent-Field Molecular Orbital Calculations. For a better understanding of the bent geometries shown in Figure 7 a b initio SCF molecular orbital calculations were carried out on silylaluminum hydride and the results compared with the analogous calculations previously done in our group on methylaluminum hydride. The optimizations were carried out in stages with the highest level basis set available to us on our computer system, 6-31G**,I9 used for the final iterations. This basis set includes polarization functions on all atoms including hydrogen. Figure 8 shows the final optimized 6-31G** geometries for both methyl- and silylaluminum hydride with all the optimized bond lengths and angles specified along with the respective con~~~~
~
(18) Wcltner, W., Jr. Magnetic Atoms and Molecules; Van Nostrand Reinhold Co: New York, 1983; p 142. (19) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
verged energies. Both structures were found in our calculations to represent true energy minima, unlike the geometries we investigated in the case of the ground-state aluminum-silane complex. In Figure 8 it is indicated that both CH3AIH and SiH3AIH molecules are in the 2A' electronic state, and it is clearly evident that both species are bent at the aluminum atom: CH3AIH having an angle of 118.35O and SiH3AIH showing an almost identical angle of 118.80°. This close agreement is in accord with the estimates obtained from the bond angle formula mentioned in the discussion of Figure 7; however, the similarity in values was not anticipated since the experimental data, particularly the results arising from the Al/SiH4 ground-state complex, suggested that the silicon 3d orbitals play a large role in the interaction of silane with aluminum. The question therefore arises as to whether there is some degree of spin polarization in the u system of the insertion product which would account for the observed difference in hydrogen atom spin density between SiH3A1H and CH,AIH portrayed in Table 11. Such a situation seems possible since the system of SiH3AIH does not match that of symmetry of the i~ the hydrogen 1s orbital.
Summary In summary, the experimental and theoretical results that have been presented in this and part 1 of this study largely support the scheme shown a t the top of Figure 9 for the aluminum/silane system. Aluminum atoms cocondensed in pure silane or in a silane/argon mixture at 12 K yield a ground-state complex possibly containing a three-center, two-electron AI(q2-SiH4) interaction. This species, upon brief irradiation at 400 nm, is converted to the bent orthorhombic molecule silylaluminum hydride, presumably via the oxidative addition of a silicon-hydrogen bond to an aluminum atom. Regeneration of the ground-state complex via a reductive elimination reaction is then possible with 520-nm irradiation over a much longer time period than that of the primary photolysis. This sequence of events is paralleled in the aluminum/methane system shown in the lower half of the figure except that a ground-state complex between aluminum and methane was not observed during the prior investigations carried out on this system in our laboratory. Acknowledgment. The generous financial support of the Natural Sciences and Engineering Research Council of Canada is greatly appreciated. The award of an Ontario Graduate Scholarship (M.A.L.) is deeply appreciated. Valuable computational assistance from Dr. Douglas McIntosh and Dr.Mike Peterson with various aspects of the EPR simulations and ab initio calculations are gratefully acknowledged. Registry No. AI, 7429-90-5; SiH4, 7803-62-5; SiD4, 13537-07-0; SiH,AIH, 116233-51-3.
