KETYL COMPLEXES OF (SILOX)3TI (SILOX = TBU3SIO-)

Treatment of TiCl4(THF)2 or ZrX4 (X = Cl, I) with 3.0 equiv of Na(silox) in THF afforded (silox)3TiCl (1, 85%) or (silox)3ZrX (X = Cl, 2, 68%; I, 3, 65%). According to electrochemical experiments, the reduction of 1-3 was feasible; orange, crystalline (silox)3Ti (4) was produced in 76% yield upon exposure of 1 to Na/Hg in DME, but a Zr(III) analogue could not be isolated. XPS data are used to rationalize the stability of low-valent siloxide complexes. UV-vis and EPR spectra of 4 were consistent with a D3h geometry and 2A1' (d(z)2)1 ground state (g(iso) = 1.9554, g parallel-to = 1.9997, g perpendicular-to = 1.9323; a(iso) = 155 MHz (approximately 56.7 G)). Addition of L to 4 provided various thermally unstable adducts, (silox)3TiL (4-L; L = DME, CNMe, CN(t)Bu, NC(t)Bu, PMe3, NH3), whereas ketones and aldehydes reacted to give ketyls or compounds indicative of ketyl reactivity. Acetone and 4 produced a 1:1 mixture of (silox)3Ti(OCMe2H) (5a) and (silox)3Ti(OMeC = CH2) (6a), while acetaldehyde and 4 afforded a 1:1 mixture of (silox)3TiOEt (5b) and (silox)3TiOCH = CH2 (6b). Sterically hindered substrates (L = (t)Bu2CO, (t)BuCHO, 3,3,5,5-tetramethylcyclohexanone, PhMeCO, 4,4'-dimethylbenzophenone) generated transient ketyls whose EPR spectra are indicative of carbon-centered radicals (4-OC(t)Bu2, g = 1.9985; 4-OCH(t)Bu, g = 2.0001; 4-OC6H6Me4, g = 1.9920; 4-OCMePh, g = 2.002; 4-OC(p-tolyl)2, g = 2.0005). Benzophenone and 4 provided an equilibrium mixture of the ketyl (silox)3Ti(OCPh2.) (4-OCPh2, g = 2.0004) and [(silox)3Ti-(OCPh2)]2 (7), which arises from a C(alpha)-C(p-Ph) coupling reminiscent of trityl radical (K(eq)(25-degrees-C) = [4-OCPh2]2/[7] = 7.5 x 10(-7), DELTA-H(diss) = 18 (1) kcal/mol, DELTA-S = 33 (3) eu). Addition of Ph3SnH to 7 yielded (silox)3TiOCPh2H (8) and Ph3SnSnPh3. Ketyl formation was also reversible, since addition of 4,4'-dimethylbenzophenone to 7 produced some of the mixed diarylketone dimer (silox)3Ti-O(tolyl)2 activated C(H)C(CH)2C(CH = CH) = C(Ph)OTi(silox)3 (9). EPR studies of 4-OCRR' were clearly supportive of the ketyl formulation; tentative evidence for a chair --> chair interconversion in 4-OC6H6Me4 and equilibration of the Ph rings in 4-OCPh2 is also presented. The EPR and UV-vis spectra of 4 are rationalized in terms of a relatively strong ligand field. The g values and hyperfine couplings of the ketyls are used to assess the binding of the R'RC = O substrates, and sterics play the major role.