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>i</i>Pr₂)₃CoN₂ (<b>1</b>, Mes = 2,4,6-trimethylphenyl) and (THF)­Ti­(XylNP^<i>i</i>Pr₂)₃CoN₂ (<b>7</b>, Xyl = 3,5-dimethylphenyl) toward diaryl ketones is explored in an effort to gain mechanistic insight into CO bond cleavage processes. Complex <b>1</b> reacts with 4,4′-dimethoxybenzophenone to generate ((<i>p</i>-OMeC₆H₄)₂CO)­Zr­(MesNP^<i>i</i>Pr₂)₃CoN₂ (<b>2</b>), which exists as a mixture of valence tautomers in solution that interconvert via electron transfer from Co^–I to the Zr-bound ketone in <b>2</b>^<b>S</b> to form a Zr-bound ketyl radical in <b>2</b>^<b>T</b>. The geometry of <b>2</b> in the solid state is most consistent with the singlet ketone adduct tautomer <b>2</b>^<b>S</b>. Upon removal of the Co-bound N₂ under vacuum, complex <b>2</b> cleanly coverts to the μ-oxo carbene product (η²-MesNP^<i>i</i>Pr₂)­Zr­(MesNP^<i>i</i>Pr₂)₂(μ-O)­CoC­(C₆H₄<i>p</i>-OMe)₂ (<b>5</b>) at room temperature in solution. A diamagnetic intermediate, tentatively assigned as ketone-bridged species (η²-MesNP^<i>i</i>Pr₂)­Zr­(MesNP^<i>i</i>Pr₂)₂Co­(μ₂,η¹η²-OC­(<i>p</i>-OMeC₆H₄)₂) (<b>6</b>), is observed spectroscopically during the transformation of <b>1</b> to <b>5</b>. Similar reactions between the Ti/Co analogue <b>7</b> and diaryl ketones reveal no evidence for electron-transfer to form triplet ketyl radical species. Complex <b>7</b> reacts with 4,4′-dimethoxybenzophenone to afford diamagnetic ((<i>p</i>-OMeC₆H₄)₂CO)­Ti­(XylNP^<i>i</i>Pr₂)₃CoN₂ (<b>8</b>). In contrast, addition of benzophenone to <b>7</b> under N₂ generates a mixture of (η²-XylNP^<i>i</i>Pr₂)­Ti­(XylNP^<i>i</i>Pr₂)₂Co­(η²-OCPh₂) (<b>9</b>) and (Ph₂CO)­Ti­(XylNP^<i>i</i>Pr₂)₃CoN₂ (<b>10</b>) in solution, and <i>C</i>₃-symmetric <b>10</b> is found to be favored in the solid state. Complex <b>9</b> can be generated exclusively and isolated in the absence of N₂. Ti/Co complexes <b>8</b>–<b>10</b> are thermally stable and do not undergo CO bond cleavage even at elevated temperature, in stark contrast to their Zr/Co congeners