Bis(imino)aryl NCN pincer cobalt complexes: synthesis and disproportionation

Abstract

Treatment of ^iPr[NCN]Br (2,6-(2,6-ⁱPr₂C₆H₃CN)₂C₆H₃Br) with ⁿBuLi in THF and the subsequent addition of 1 equiv. of CoCl₂, CoCl₂(Ph₃P)₂, and CoBr₂ gave pincer Co(II) complexes {^iPr[NCN]Co(μ-Cl)}₂ (1d), ^iPr[NCN]CoClPh₃P (1d-Ph₃P), and ^iPr[NCN]CoBr₂·Li(THF)₄ (1d-LiBr) respectively in moderate yields, whereas the slow addition of in situ prepared ^iPr[NCN]Li to CoCl₂ in THF afforded an unexpected mixed-valence cobalt(I/II) complex κ²_C,N,η^6-iPr[NCN]Co-κ_N-CoCl₃·Li(THF)₄ (2d). Complex 2d was probably formed via a disproportionation reaction of the ^iPr[NCN]Co(II) species with excess CoCl₂ during the reaction. Nevertheless, addition of CoCl₂ to in situ formed 1d-THF at room temperature did not lead to 2d but gave a trinuclear Co(II) complex {^iPr[NCN]Co(μ-Cl)(μ-Br/Cl)}₂Co (1d-CoCl₂) in moderate yield. Similar reactions using ligands containing small ortho groups in the imine moieties ^R[NCN]Br (2,6-(2,6-Me₂C₆H₃CN)₂C₆H₃Br, ^Me[NCN]Br; 2,6-(2,6-Et₂C₆H₃CN)₂C₆H₃Br, ^Et[NCN]Br; 2,6-(2,4,6-Me₃C₆H₂CN)₂C₆H₃Br, ^Mes[NCN]Br) and CoBr₂, regardless of the reactant addition sequence, afforded mixed-valence cobalt(I/II) complexes {κ²_C,N,η^6-R[NCN]Co-κ_N-CoBr(μ-Br)}₂ (^Me[NCN] (2a), ^Et[NCN] (2b), and ^Mes[NCN] (2c)), suggesting that the bulkiness of the ortho-groups in the imine moieties of the ligands plays an important role in the disproportionation reaction. In the presence of PMe₃, Co(II) complexes κ²_C,N-^R[NCN]CoBr(PMe₃)₂ (3a–d) and a bisligated cobalt(II) complex κ³_N,C,N-κ²_C,N-^iPr[NCN]₂CoPMe₃ (4d) can be prepared respectively in high yields. The molecular structures of 1d-LiBr, 1d-CoCl₂, 2b, 2d, 3a, and 4d were confirmed by X-ray crystallographic analysis and the detailed mechanisms of the disproportionation reaction were proposed.