PHENOXIDES AND THIOPHENOXIDES OF ALUMINUM AND GALLIUM - EVIDENCE OF A H-1-F-19 COUPLING IN (R2MOC6F5)2 IN SOLUTION - CRYSTAL AND MOLECULAR-STRUCTURES OF (ME2ALOC6F5)2 AND (ME2GASC6F5)2

The reaction of trialkylaluminum and -gallium derivatives with pentafluorophenol and pentafluoro-thiophenol yields the corresponding phenoxides and thiophenoxides, (R2MEC6F5)2 (M = Al, E = O, R = Me (1a), Et (1b), and i-Bu (1c); M = Ga, E = O, R = Me (2a); M = Ga, E = S, R = Me (2b)) quantitatively. These compounds have been characterized by H-1, C-13, and F-19 NMR spectroscopy, and the structures of 1a and 2b were determined by single-crystal X-ray diffraction techniques. 1a was assigned to the orthorhombic cell system, space group Cmcm (No. 63), with cell constants a = 11.058 (2) angstrom; b = 16.908 (2) angstrom, c = 10.764 (2) angstrom, and Z = 4 (dimers). The refinement converged with R = 6.99% (R(w) = 7.87%) based on 541 (I greater-than-or-equal-to 2.5-sigma(I)) observed reflections. The molecule exists as an oxygen-bridged dimer with a central planar four-membered Al2O2 ring. The average Al-O bridge bond distance is 1.895 (5) angstrom. 2b was assigned to the triclinic cell system, space group P1BAR (No. 2) with cell constants a = 9.790 (3) angstrom, b = 10.122 (1) angstrom, c = 11.284 (1) angstrom, alpha = 76.08 (1)degrees, beta = 89.67 (2)degrees, gamma = 86.99 (2)degrees, and Z = 2 (dimers). The refinement converged with R = 4.65% (R(w) = 4.89%) based on 1920 (I greater-than-or-equal-to 3.0-sigma(I)) observed reflections. The molecule exists as a sulfur-bridged dimer with a puckered four-membered Ga2S2 ring. The average Ga-S bridge bond distance is 2.450 (3) angstrom. 1a does not react with diethyl ether or tetrahydrofuran but is cleaved by 4-picoline to give a 1:1 addition complex. H-1 and F-19 NMR spectra of (R2MOC6F5)2 in toluene-d8 or benzene-d6 solutions show H-1-F-19 coupling with a 6J(FH) = 2.0 Hz. In contrast, long-range H-F coupling is not observed in the sulfur derivatives of either gallium or aluminum. Variable-temperature H-1 NMR studies established that the Al2S2 and Ga2S2 ring systems undergo rapid inversion/exchange, which leads to equivalence of the alkyl groups attached to the metal atoms.