M<sup>IV</sup>/Co<sup>–I</sup> (M = Zr, Hf)\nBis(phosphinoamide) Complexes with η<sup>6</sup>- and η<sup>4</sup>‑Arenes

Despite their relevance to catalysis, low-/subvalent\ncobalt complexes\nare difficult to synthesize and isolate. Consequently, very few “cobaltate”\ncomplexes are known and there is a lack of architectural diversity\nin this field. Lewis acidic d<sup>0</sup> group IV metals have been\ndemonstrated to stabilize Co<sup>–I</sup> centers via metal–metal\nbonds. Herein, we report the synthesis of bis(phosphinoamide) heterobimetallic\nM<sup>IV</sup>/Co<sup>–I</sup> arene complexes (M = Zr, Hf).\nThe driving force to maintain the aromaticity of the arenes dictates\nthe hapticity of the Co-bound arene ligands and influences the binding\narrangement of the phosphinoamide ligands. Benzene and toluene were\nobserved to bind η<sup>6</sup> to the Co<sup>–I</sup> center, forcing dissociation of one of the phosphinoamide ligands,\nwhereas η<sup>4</sup>-coordination of anthracene allows both\nphosphinoamide ligands to remain bound to the Co center. The identity\nof both the arene and the group IV metal ion starkly influence the\nlability of the arene. For instance, the Co-bound benzene ligand in\nthe Zr<sup>IV</sup>/Co<sup>–I</sup> benzene complex rapidly\nexchanges with C<sub>6</sub>D<sub>6</sub> in solution, whereas toluene/C<sub>6</sub>D<sub>6</sub> exchange is much slower and appreciable C<sub>6</sub>H<sub>6</sub>/C<sub>6</sub>D<sub>6</sub> exchange is not observed\nfor the Hf analogue. The Zr<sup>IV</sup>/Co<sup>–I</sup> benzene\ncomplex loses benzene upon repeated exposure to vacuum to form an\narene-free tetrametallic dimer.