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

Abstract

The reactivity of the reduced Zr/Co and Ti/Co complexes (THF)Zr(MesNPiPr2)3CoN2 (1, Mes = 2,4,6-trimethylphenyl) and (THF)Ti(XylNPiPr2)3CoN2 (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)2CO)Zr(MesNPiPr2)3CoN2 (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 2S to form a Zr-bound ketyl radical in 2T. The geometry of 2 in the solid state is most consistent with the singlet ketone adduct tautomer 2S. Upon removal of the Co-bound N2 under vacuum, complex 2 cleanly coverts to the μ-oxo carbene product (·2-MesNPiPr2)Zr(MesNPiPr2)2(μ-O)Co - C(C6H4p-OMe)2 (5) at room temperature in solution. A diamagnetic intermediate, tentatively assigned as ketone-bridged species (·2-MesNPiPr2)Zr(MesNPiPr2)2Co(μ2,·1·2-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)2CO)Ti(XylNPiPr2)3CoN2 (8). In contrast, addition of benzophenone to 7 under N2 generates a mixture of (·2-XylNPiPr2)Ti(XylNPiPr2)2Co(·2-OCPh2) (9) and (Ph2CO)Ti(XylNPiPr2)3CoN2 (10) in solution, and C3-symmetric 10 is found to be favored in the solid state. Complex 9 can be generated exclusively and isolated in the absence of N2. 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.