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.