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Bis(imino)aryl NCN pincer cobalt complexes: synthesis and disproportionation
Li-Cheng Huang1, Jing-Shun Zhang1, Teng Jia1
1College of Chemistry, Jilin University, Changchun 130012, China. gw@jlu.edu.cn.
Abstract:
Treatment of iPr[NCN]Br (2,6-(2,6-iPr2C6H3C[double bond, length as m-dash]N)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 κ2C,N,η6-iPr[NCN]Co-κN-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-Me2C6H3C[double bond, length as m-dash]N)2C6H3Br, Me[NCN]Br; 2,6-(2,6-Et2C6H3C[double bond, length as m-dash]N)2C6H3Br, Et[NCN]Br; 2,6-(2,4,6-Me3C6H2C[double bond, length as m-dash]N)2C6H3Br, Mes[NCN]Br) and CoBr2, regardless of the reactant addition sequence, afforded mixed-valence cobalt(i/ii) complexes {κ2C,N,η6-R[NCN]Co-κN-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 κ2C,N-R[NCN]CoBr(PMe3)2 (3a-d) and a bisligated cobalt(ii) complex κ3N,C,N-κ2C,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.
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