Investigation of Ketone C=O Bond Activation Processes by Heterobimetallic Zr/Co and Ti/Co Tris(phosphinoamide) Complexes

The reactivity of the reduced Zr/Co and Ti/Co complexes (THF)Zr(MesNP(i)Pr(2))(3)CoN2 (1, Mes = 2,4,6-trimethylphenyl) and (THF)Ti(XylNP(i)Pr(2))(3)CoN2 (7, Xyl = 3,5-dimethylphenyl) toward diaryl ketones is explored in an effort to gain mechanistic insight into C=O bond cleavage processes. Complex 1 reacts with 4,4'-dimethoxybenzophenone to generate ((p-OMeC6H4)(2)CO)Zr(MesNP(i)Pr(2))(3)CoN2 (2), which exists as a mixture of valence tautomers in solution that interconvert via electron transfer from Co-I to the Zr-bound ketone in 2(S) to form a Zr-bound ketyl radical in 2(T). The geometry of 2 in the solid state is most consistent with the singlet ketone adduct tautomer 2(S). Upon removal of the Co-bound N-2 under vacuum, complex 2 cleanly coverts to the u-oxo carbene product (eta(2)-MesNP(i)Pr(2))Zr(MesNP(i)Pr(2))(2)(u-O)Co=C(C(6)H(4)p-OMe)(2) (5) at room temperature in solution. A diamagnetic intermediate, tentatively assigned as ketone-bridged species (eta(2)-MesNP(i)Pr(2))Zr(MesNP(i)Pr(2))(2)Co(u(2),eta(1)eta(2)-OC(p-OMeC6H4)(2)) (6), is observed spectroscopically during the transformation of 1 to 5. Similar reactions between the Ti/Co analogue 7 and diaryl ketones reveal no evidence for electron-transfer to form triplet ketyl radical species. Complex 7 reacts with 4,4'-dimethoxybenzophenone to afford diamagnetic ((p-OMeC6H4)(2)CO)Ti(XylNP(i)Pr(2))(3)CoN2 (8). In contrast, addition of benzophenone to 7 under N-2 generates a mixture of (eta(2)-XylNP(i)Pr(2))Ti(XylNP(i)Pr(2))(2)Co(eta(2)-OCPh2) (9) and (Ph2CO)Ti(XylNP(i)Pr(2))(3)CoN2 (10) in solution, and C-3-symmetric 10 is found to be favored in the solid state. Complex 9 can be generated exclusively and isolated in the absence of N-2. Ti/Co complexes 8-10 are thermally stable and do not undergo C=O bond cleavage even at elevated temperature, in stark contrast to their Zr/Co congeners.