Organometallics 1995, 14, 2589-2592
2589
Transmetalation Reactions of Sterically Encumbered Gallium and Indium Halides with Tetrahydrometalates. Synthesis and Structure of a Base-Free Monomeric Aluminum Hydride Alan H. Cowley," Harold S. Isom, and Andreas Decken Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712 Received December 27, 1994@ Summary: The reaction of (Ar*)2GaCl or (Ar"hlnC1 (Ar" = 2,4,6-t-Bu&H.d with LiAlH4 resulted in transmetalation and formation of (Ar*)&lH (61, the first example of a structurally authenticated monomeric base-free aluminum hydride. An X-ray crystallographic study revealed that the monomeric nature of the new hydride is due to the shielding of the AI-H moiety by o-t-Bu groups of the aryl ligands. Crystal data for 6: space group P21/c, a = 10.024(2) A, b = 29.745(4) A, c = 11.459(1)A, /3 = 94.87(1)",V = 3404(2) A3, Z = 4. The X-ray crystal structure of (Ar*)zInCl (5) has also been determined. Crystal data for 5: space group P2 IC,a = 10.328(2) A, b = 22.924 2) A, c = 16.091(1) B= 107.34(1)",V = 3636.6(8) Z = 4.
13,
k,
In order to maximize the chance of obtaining monomeric hydride derivatives, we opted to employ the bulky aryl group 2,4,6-t-Bu3C6Hzas the other group 13 substituent. Herein we report (i) the synthesis and first structural characterization of a base-free monomeric organoaluminum hydride and (ii)facile transmetalation reactions.
Results and Discussion The deployment of sterically demanding ligands can have dramatic effects, not only on the stabilities of maingroup species but also on their patterns of reactivity. In the context of group 13 hydrides, it was discovered recently6 that Ar*GaHz (1;Ar* = 2,4,6-t-Bu3CsHz),the
Introduction In part, our interest in the organometallic chemistry of the heavier group 13 hydrides stems from the fact that AH, and GaH, entities have been detected on surfaces during film growth from organoaluminuml and organogallium2precursors. In order to learn more about the fundamental chemistry of these surface-bound hydrides, it would be desirable to have in hand neutral monomeric species of the types RMH2 and RzMH (M = Al, Ga). A second motivation for preparing these classes of compounds is related to the possibility that they might serve as sources of the corresponding univalent organometallics via reductive elimination of hydrogen or alkane. Thirdly (and somewhat optimistically), we were interested in the possibility of preparing neutral organoindium hydrides. For both the organoaluminum and organogallium hydrides there is a pronounced tendency toward oligomerization on account of the coordinative unsaturation at the metal center. Until recently, therefore, all structurally authenticated examples were dimeric or, in some cases, 0ligomeric.3>~Information concerning the indium hydrides is particularly ~ p a r s e . With ~ exception of an unstable ether adduct of composition [(InH3),*nEt20]all other indium hydrides are anionic. @Abstractpublished in Advance ACS Abstracts, April 1, 1995. (1)Bent, B. E.; Nuzzo, R. G.;Dubois, L. H. J . Am. Chem. Soc. 1989, 111, 1634. (2) Zanella, P.; Rossetto, G.; Brianses, N.; Ossola, F.; Porchia, M.; Williams, J. 0. Chem. Mater. 1991,3,275. Butz, K.W.; Elms, F. M.; Raston, C. L.; Lamb, R. N.; Pigram, P. J. Inorg. Chem. 1993,32,3985. (3) Chemistry ofAluminum, Gallium and Indium; Downs, A. J.,Ed.; Blackie-Chapman Hall: London, 1993. Downs, A. J.; Pulham, C. R. Chem. Soc. Rev. 1994,23,175. (4) Gmelin Handbook of Inorganic and Organometallic Chemistry: Organogallium Compounds; Springer-Verlag: Berlin, 1987;Part 1. (5)Gmelin Handbook of Inorganic and Organometallic Chemistry: Organoindium Compounds; Springer-Verlag: Berlin, 1991; Part 1.
/t-Bu
H
't-Bu
1
,1-B u
2
I
1-B u
,t-Bu
first monomeric organogallium hydride, could be isolated from the reaction of Ar*GaC12 with LiGaH1. Interestingly, the product of the reaction of the corre(6) Cowley, A. H.; Gabbal', F. P.; Isom, H. S.; Carrano, C. J.; Bond, M. R. Angew. Chem., Int. Ed. Engl. 1994,33,1253.
0276-733319512314-2589$09.00/0 0 1995 American Chemical Society
Notes
2590 Organometallics, Vol. 14, No. 5, 1995
Scheme 1
t-Bu
6
1
5
sponding monochloride (Ar*)zGaCl with L i G a was the "aryl-rotated" product 2 rather than the anticipated monohydride (Ar*)zGaH (3h6 Compound 3 has been prepared subsequently by Power et aL7via the reaction of (Ar*)zGaCl with t-BuLi. As indicated in Scheme 1, treatment of (Ar*)zGaCl (4)* with LiAlH4 in Et20 solution afforded an 80% yield of (Ar*)AH (6) via a transmetalation reaction. The IR spectrum of 6 features a single, sharp absorption a t 1869 cm-l, which is close to the terminal AI-H stretching frequency reportedgfor the matrix-isolated monomer (TMP)AH (TMP = 2,2,6,6-tetramethylpiperidinyl). Further support for the monomeric nature of 6 stems from the CI mass spectrum, which exhibits a sharp cutoff a t mlz 517 (M+ - H), and from the detection of a proton resonance a t 6 5.72, which corresponds to an Al-H moiety. It was, however, necessary t o appeal to X-ray crystallography in order to establish the degree of oligomerization definitively. The crystalline state of 6 consists of individual monomers, and there are no conspicuously short intermolecular contacts. The structure is illustrated in Figure 1 along with the atomnumbering scheme, and selected bond distances and angles appear in Table 1. The significance of 6 is that this compound represents the first example of a structure of a base-free aluminum hydride monomer. Heretofore all structurally authenticated aluminum hydrides featured bridging hydride entities. Examples include [Ar*(H)Al@-H)1210and species with two sterically demanding substituents, uiz. [(TMP)~A~(,U-H)I~~ and [(tBu)2Al@-H)13.l1 A n indication of the cause of the monomeric nature of 6 stems from the observation that the o-t-Bu groups partially shield the terminal Al-H (7) Wehmschulte, R. J.; Ellison, J. J.; Ruhlandt-Senge, K.; Power, P. P.Inorg. Chem. 1994,33,6300. (8)Meller, A.; Pusch, S.; Pohl, E.; Haming, L.; Herbst-Irmer, R. Chem. Ber. 1993,126,2255. (9) Klein, C.; Noth, H.; Tacke, M.; Thomann, M. Angew. Chem.,Int. Ed. Engl. 1993,32,886. (10) Wehmschulte, R. J.; Power, P. P. Inorg. Chem. 1994,33,5611. (11)Uhl, W. 2.Anorg. Allg. Chem. 1989,570, 37.
C1341 ..
c1321
C1141
c191
01
Cl291
Figure 1. View of (Ar*)&lH (6) showing the atom-labeling scheme. Thermal ellipsoids are scaled to the 30% probability level. All hydrogens are omitted for clarity except the alane hydrogen. Table 1. Selected Bond Distances (A) and Bond Angles (deg) for (Ar*)zInCI ( 5 ) and (Ar*)AIH ( 6 ) (h* = 8,4,&t-B&Caz) In-C1 In -C(1) In-C(19) Al-H(l) Al-C(l) Al-C(21)
Compound 5 2.523(2) C1-In-C( 1) 2.161(11) C1-In-C(19) 2.148(10) C(l)-In-C(19) Compound 6 1.53(4) H(l)-Al-C(l) 1.976(6) H(l)-Al-C(Bl) 2.007(6) C(l)-Al-C(21)
113.2(3) 99.6(3 144.2(5) 114.3(14) 113.1(14) 131.7(3)
bond, thus thwarting the formation of AI-H-AI bridges. Within experimental error the geometry a t aluminum is trigonal planar (sum of bond angles 359.1(3)"). However, the C-AI-C angle is unusually wide (131.7(3)"),thus furnishing evidence for the existence of strain in this molecule. Power et aL7 have reported a similarly wide angle (131.9(1)9 for the analogous monomeric gallane 3. Steric strain is also evident in the conformation of 6. Thus, there is an angle of 35.7" between the Al-C(21) and (321). 4324) vectors, while in the other ring, which is approximately at right angles, the corresponding tilt angle is close to zero (1.8"). The conformation of 37is virtually identical with that of 6;
Organometallics, Vol. 14, No. 5, 1995 2591
Notes
Table 2. Crystal Data, Details of Intensity Measurement, and Structure Refinement for (Ar*)zInCl (6) and (Ar*)AlH (6) (Ar*= 2,4,6-t-Bu&aHz)
c1311
c114I
5
formula fw cryst dimens mm cryst syst space group a, A b, A C1161
Figure 2. View of (Ar*)zInC1(5)showing the atom-labeling scheme. Thermal ellipsoids are scaled to the 30% probability level. All hydrogens are omitted for clarity.
in this case the tilt angle of one of t h e rings is 33.6'. A further consequence of steric crowding is the presence of relatively short contacts between aluminum and some o-tert-butyl hydrogens e.g. Al***H(9 B) = 2.33 A and Ab..H (9 C) = 2.24 The Al-H bond distance of 1.53(4) A is comparable to those in hydroaluminates such as [HAl(NMe2)31- (1.52(2) Allz but shorter t h a n those in compounds with Al-H-Al bridge bonding such as [Me&p-H)]2 (1.68(2) A).11 The Al-C bond distances, which average 1.991(6)A, are similar to that in t h e recently reported dimer [Ar*(H)A&-H)h (1.966(3) and to those for the Ga-Ar* bonds13 in 1 (1.942(7) A)6 and 2 (1.98307) Our reasons for investigating t h e reactivity of 5 toward tetrahydrometalates were twofold. The major motivation was to explore t h e possibility that the extensive steric blockade represented by two Ar* groups might permit isolation of t h e first example of a neutral indium h ~ d r i d e . ~A, ~second objective was to explore whether an "aryl-rotation" process would occur as observed in t h e formation of 2. It was found that the reaction of 5 with LiAlH4 proceeded in a fashion analogous to that for 4 (Scheme 1) and resulted in transmetalation and formation of the monomeric aluminum hydride 6. Transmetalation was also observed in t h e reaction of 5 with LiGaH4 (Scheme 1);however, in this case it was accompanied by cleavage of an aryl group and production of 1. The fact that no aryl cleavage was observed in the formation of 6 is consistent with t h e order of bond enthalpies Al-C > Ga-C =- InC. Finally, we note that the aryl cleavage has been postulated6 as a component of t h e mechanism of "aryl rotation". Finally, we report t h e X-ray crystal structure of (Ar*)2InCl (5). We have prepared this compound earlier.6 However, at that time we were not able to isolate suitable crystals of this compound. In the interim, Oliver et uZ.l4 have reported t h e X-ray crystal structure of t h e corresponding bromide (Ar*)zInBr (7).The solid state of 5 consists of isolated molecules, and there are no abnormally short intermolecular contacts. The molecular structure is illustrated in Figure 2, and a listing of bond distances and angles appears in Table 1. With the exception of the indium-halogen bond lengths, the metrical parameters for 5 and 7 are almost identical.
A.
~~~
~~
(12) Linti, G.; Noth, H.; Rahm, P. 2.Nuturforsch., B 1988,43,1101. (13) Incomplete shielding by Ga(3d) electrons renders the covalent radii of Al and Ga virtuallv identical. (14)Rahbarnoohi, H.;Heeg, M. J.; Oliver, J. P. Organometallics 1994,13,2123.
c, A
a,deg 8. den Y 9 deg
v, A3
d(calc), g cm-3
z
radiation total no. of d n s no. of obsd rflns no. of ref params wR2/R1 GOF on F
C&&1In 640.5 0.17 x 0.21 x 0.42 monoclinic P21lc 10.328(2) 22.924(2) 16.091(1) 90 107.34(1) 90 3636.6(8) 1.171 4 Mo Ka 7054 6327 364 0.206710.0882 1.096
6 C36HSd
518.8 0.34 x 0.44 x 0.44 monoclinic P21lc 10.024(2) 29.745(1) 11.459(1) 90 94.87(1) 90 3404.3(9) 1.012 4 Mo Ka 6344 5944 339 0.1410/0.1065 1.174
Moreover, the conformations of 6 and 7 a r e similar in t h e sense that one of the aryl rings exhibits a pronounced tilt (angle between t h e In-C(l) and C ( l > *C(4)vectors 34.0') and the two aryl rings are close to orthogonal. Experimental Section General Considerations. All reactions were performed under oxygen-free argon or under vacuum using standard Schlenk line or drybox techniques. All solvents were dried over sodium and distilled from sodium benzophenone under argon before use. The starting materials ( A I - * ) ~ G ~(Ar*)2C~,~ InC1,6and LiGaH415were prepared according to the literature methods; LiAlH4was procured commerically and used without further purification. Physical Measurements. IR spectra were obtained as KBr pellets on a Bio-Rad FTS-40spectrometer. Mass spectra (CI) were run on a Bell and Howell 21-491 instrument, and NMR spectra were measured on a GE QE-300 spectrometer (lH, 300.17 MHz; 13C, 75.48 MHz). NMR spectra are referenced to C6Ds which was dried over Na/K alloy and distilled prior to use. All chemical shifts are reported relative to TMS (0.00 ppm). Melting points (uncorrected) were obtained in sealed capillaries under argon (1 atm), and elemental analyses were performed by Atlantic Microlab, Norcross, GA. Reaction of (Ar*)zGaCl (4) with La&. A 100 mL capacity Schlenk flask was charged with 320 mg of 4 (0.54 mmol) and 21 mg of LMH4 (0.54 mmol). The solids were dissolved in 25 mL of Et20 at -78 "C, and the stirred reaction mixture was warmed immediately t o ambient temperature. After 1 h the volatiles were removed under reduced pressure and the resulting solid was extracted with toluene (3 x 20 mL). After filtration, the volume of the filtrate was reduced by -50%. Storage of the concentrated solution at -20 "C overnight afforded 210 mg (80%yield) of colorless crystalline 6 (mp 144-146 "C). Spectroscopic data for 6 are as follows: IR (KBr, cm-l) 1869 ( V N - H , terminal); 'H NMR (300 MHz, C&, 25 "c; 6 (ppm)) 1.34 (s, 9H, p-Me), 1.45 (s, 18H, o-Me), 5.72 (8, lH, Al-H), 7.46 (s, 2H, ring); l3C(lH}NMFt (75.5 MHz, C6D6, 25 "c; 6 (ppm)) 31.47 @-Me),34.72 (para quaternary), 33.03 @Me), 38.66 (ortho quaternary), 121.65 (CH, ring), 150.54 (C, ring), 159.65 (C, ring), ipso carbon not observed; CIMS (CHI): m l z 517 (M+ - H), 273 (M - AI-*), 246 (AI-* + (15)Shirk, A. E.;Shriver, D. F. Inorg. Synth. 1977,17, 45.
2592 Organometallics, Vol. 14, No. 5, 1995
Notes
Table 3. Atomic Coordinates ( x 104) and Equivalent Isotropic Displacement Parameters (A2 x 10s) for (Ar*)2InC1( 5 ) and (Ar*)2AlH (6) (Ar*= 2,4,6-t-BusCsHd atom
xla
ylb
3350(1) 5381(2) 2301(11) 874(11) 272(13) 980(14) 2345(14) 3011(12) - 140(13) 545(16) -904(15) -1249(15) 294(18) -266(41) 1275(33) -976(40) 436(150) -1217(109) 1256(116) 4449(13) 5116(14) 5376(13)
2583(1) 2529(1) 1760(5) 1760(5) 1300(6) 839(5) 809(5) 1250(5) 2171(6) 2658(7) 1796(6) 2435(6) 379(7) 695(12) -105(15) 124(18) -247(46) 453(49) 448(55) 1078(5) 1521(6) 951(6)
3589(2) 1937(6) 961(6) -233(6) -504(6) 486(6) 1677(6) 1144(7) 24(6) 2483(6) 1174(6) -1807(6) -2462(6) -1511(6) -2815(6) 2705(7) 3322(7) 2068(6) 3860(6)
6271(1) 6610(2) 6446(2) 6677(2) 7082(2) 7257(2) 7034(2) 6021(2) 5683(2) 5786(2) 6162(2) 7342(2) 7444(2) 7784(2) 7084(2) 7302(2) 7662(2) 7424(2) 7024(2)
zlc
U(ed
5352(1) 4789(2) 5277(7) 4911(8) 4354(9) 4174(9) 4645(8) 5206(8) 5163(10) 5774(12) 5671(10) 4368(11) 3476(11) 2605(16) 3435(26) 3731(24) 3834(55) 2990(94) 2742(65) 5771(8) 6478(8) 5220(9) 1467(2) 992(5) 122(5) -144(5) 359(5) 1146(5) 1476(5) -612(6) -535(5) -299(5) -1900(5) 62(6) 1183(6) -515(5) -742(5) 2290(6) 1559(6) 3296(5) 2828(5)
atom
xla
yfb
zlc
4296(14) 3124(12) 3621(12) 3466(13) 2893(12) 2417(12) 2499(12) 4357(16) 3327(26) 5721(29) 4589(29) 5828(65) 3796(78) 5009(84) 2763(14) 3160(21) 1339(19) 3590(21) 1904(13) 2687(16) 434(14) 1858(14)
518(5) 3513(4) 3830(5) 4438(5) 4748(5) 4432(5) 3822(5) 3550(6) 3664(11) 3890(12) 2909(9) 3273(29) 3025(35) 3939(28) 5415(5) 5682(6) 5601(6) 5669(6) 3562(5) 3789(6) 3755(5) 2898(5)
6288(9) 5389(8) 6174(8) 6156(8) 5420(8) 4653(8) 4616(7) 7077(9) 7676(13) 7521(19) 7114(13) 6920(33) 7299(48) 7762(40) 5398(9) 6270(10) 4980(13) 4853(12) 3695(8) 3094(9) 3334(8) 3635(8)
4098(6) 5499(6) 6056(6) 5321(6) 3960(6) 3328(6) 6469(6) 7362(6) 5776(6) 7398(6) 6002(7) 6240(10) 7283(9) 5237(9) 1784(6) 1385(6) llOO(6) 1167(6)
5880(2) 5800(2) 5821(2) 5874(2) 5874(2) 5862(2) 5678(2) 6079(2) 5502(2) 5292(2) 5904(3) 5460(3) 6118(4) 6133(4) 5816(2) 5472(2) 6258(2) 5638(2)
2861(5) 3100(5) 4271(6) 5225(5) 4978(5) 3855(5) 2161(6) 1926(5) 1016(5) 2644(5) 6438(6) 6928(7) 6513(7) 7251(7) 3775(6) 4685(5) 4028(5) 2595(5)
Ues)
isotropic thermal parameters for 5 and 6 are listed in Table 3. The crystals were mounted in thin-walled glass capillaries and sealed under argon. Both data sets were collected at 25 Reaction of (Ar*)ZInCl (5) with L U . A 100 mL "C on an Enraf-Nonius CAD-4 diffractometer. The unit cell capacity Schlenk flask was charged with 350 mg of 5 (0.55 parameters were obtained by centering 25 reflections having mmol) and 19 mg of LiAlH4 (0.54 mmol). The solids were 28 values between 16 and 24". For both structures, the data dissolved in 25 mL of Et20 at -78 "C, and the stirred reaction were corrected for Lorentz and polarization effects. The mixture was warmed immediately to ambient temperature. After 1 h the volatiles were removed under reduced pressure structures were solved by least-squares refinements. All and the resulting solid was extracted with toluene (3 x 20 d). calculations were performed using SHEI.XP and the Siemens After filtration, the volume of the filtrate was reduced by SHELXTL P L U S 7 software package. -50%. Storage of the concentrated solution a t -20 "C overnight afforded 180 mg (69%yield) of colorless crystalline 6, which was identified on the basis of NMR and mass Acknowledgment. We thank the National Science spectroscopy (see above). Foundation and the Robert A. Welch Foundation for Reaction of (Ar*)dnCl (5) with L i G a . A 100 mL generous financial support. capacity Schlenk flask was charged with 350 mg of 5 (0.55 mmol) and a solution of LiGaH4 (10 mmol) in 25 mL of Et20 at -78 "C. The stirred reaction mixture was warmed imSupplementary Material Available: For 5 and 6,tables mediately to ambient temperature. After 1h the volatiles were of crystallographic data, anisotropic thermal parameters, bond removed under reduced pressure and the resulting solid was lengths and angles, and hydrogen atom parameters (12 pages). extracted with toluene (3 x 20 mL). .After filtration, the Ordering information is given on any current masthead page. volume of the filtrate was reduced by -50%. Storage of the concentrated solution at -20 "C overnight afforded 140 mg OM9409884 (88%yield) of colorless crystalline 1,which was identified on the basis of published6 NMR and mass spectroscopic data. (16) Sheldrick, G. M. SHELXL-93; University of Gbttingen, GottinX-ray Crystallography. Details of the crystal data and gen, Germany, 1993. summary of intensity data collection parameters for 5 and 6 (17) Sheldrick, G. M. SHELXTL-Plus(PC)L4.21; Siemens Analytical X-ray Instruments, Inc., 1990. are presented in Table 2. Atomic coordinates and equivalent H). Anal. Calcd for C36H& C, 82.23; H, 11.16.
(5): C, 83.40; H, 11.40. Found:
