Coordinatively Unsaturated Cobalt Complexes with Acyclic Aminooxycarbene Ligation: Synthesis and Characterization

Acyclic aminooxycarbene (AAOC) ligands, analogues of N-heterocyclic carbene (NHC), are believed to possess superior σ-donating and π-accepting abilities over those of NHCs. Their coordination chemistry, however, remains largely unexplored. In this study, we demonstrate that AAOC ligands can effectively stabilize coordinatively unsaturated cobalt complexes. The equimolar reactions of CoCl₂ with the AAOC ligands, N,N-diisopropyl-2,6-dimethylphenoxylmethylidene (LiPrXyl) and N,N-diisopropyl-2,6-diisopropylphenoxylmethylidene (LiPrDipp), in THF afford the tetrahedral cobalt(II) complexes [(AAOC)(THF)CoCl₂] (AAOC = LiPrXyl, 1; LiPrDipp, 2) in good yields, whereas the reaction of CoCl₂ with N,N-diisopropyl-1-adamantanoxylmethylidene (LiPrAd) yields the dimeric complex devoid of THF coordination [(LiPrAd)Co(Cl)(μ-Cl)₂Co(Cl)(LiPrAd)] (3). The solvent affects the reaction outcome as the interaction of CoCl₂ with two equiv of LiPrDipp in toluene produces the square planar cobalt(II) complex [trans-(LiPrDipp)₂CoCl₂] (4). Reduction of 4 with Mg in THF yields the three-coordinate cobalt(I) complex [(LiPrDipp)₂CoCl] (5), which further reacts with NaBPh₄ to form the two-coordinate cobalt(I) complex [(LiPrDipp)₂Co][BPh₄] (6). Moreover, the three-coordinate cobalt(0) complexes with AAOC ligation [(AAOC)Co(dvtms)] (dvtms = divinyltetramethyldisiloxane, AAOC = LiPrXyl, 7; LiPrDipp, 8; LiPrAd, 9) proved accessible from the reduction reactions of 1-3 with Mg in the presence of dvtms. Complexes 1-9 have been fully characterized by ¹H NMR spectroscopy, magnetic susceptibility measurement, single-crystal X-ray diffraction, and elemental analysis.