The molecular and electronic structure of an unusual cobalt NNO pincer ligand complex

Abstract

The reaction of two equivalents of [Co(PMe₃)₄] (1) with one equivalent of a neutral NNO pincer ligand (L₁) led to the formation of purple-coloured single crystals. The crystal structure determination reveals the molecular structure as a cobalt dimer [Co₂(L₁)(PMe₃)₅], which is solved in the triclinic P space group. Although this species appears to have a formal zero oxidation state on cobalt ions, careful analysis of the structural parameters of the L₁ reveals that the NNO ligand is reduced by three electrons; this observation has rarely been reported in the literature. Therefore, herein, more accurate description of the molecular formula [(PMe₃)₂Co^II(η⁴-L₁³⁻)Co^I(PMe₃)₃] (2) was proposed. In 2, we observed an unusual η³-π-allyl-type binding mode of the pyridine ring carbon atoms of the L₁³⁻ ligand with the cobalt ion. X-ray photoelectron spectroscopy not only reveals the presence of the mixed valent cobalt ion within 2 but also unambiguously discloses the spin state of these metal ions (Co(I) diamagnetic (low-spin) and Co(II) paramagnetic (high-spin)). The proposed electronic structure is consistent with the magnetic moment measured at room temperature. The electronic structure of 2 was further supported by the Q-band EPR measurements performed on polycrystalline sample of 2 at 5.0 K, and the presence of two independent S = ½ states was revealed. This has been qualitatively rationalized based on the super-exchange coupling pathway observed in 2. The NMR studies performed for 2 (C₆D₆ solvent) evidently showed that the solid-state structure of 2 was maintained in solution.