Mixed-Sandwich Cp*Cr Complexes Containing Poly(methimazolyl)borates (Cp* = C₅Me₅): Syntheses and Structural and Electrochemical Studies

Reaction of the scorpionate salts K­[HB­(mt)3], Na­[H2B­(mt)2], and Li­[HB­(mt)2(pz)] with [Cp*CrBr2]2 (1) gave the 15-electron Cr­(III) complexes [Cp*Cr­{κ3-S,S′,S″-HB­(mt)3}]Br (2), [Cp*Cr­{κ2-S,S′-H2B­(mt)2}­Br] (3), and [Cp*Cr­{κ2-S,S′-HB­(mt)2(pz)}­Br] (4) in moderate to high yields. These are the first examples of mixed-sandwich chromium­(III) complexes containing poly­(methimazolylborate) ligands. The tridentate coordination of the monoanionic ligands [H2B­(mt)2] in 3 and [HB­(mt)2(pz)] in 4 can be effected by using a silver salt to remove the Br coligand, thus yielding the complexes [Cp*Cr­{κ3-H,S,S′-H2B­(mt)2}]­PF6 (6) and [Cp*Cr­{κ3-N,S,S′-HB­(mt)2(pz)}]­PF6 (7). It was also found that, in the presence of acetonitrile, the reaction of 3 with AgPF6 afforded [Cp*Cr­{κ2-S,S′-H2B­(mt)2}­(NCMe)]­PF6 (5). The coordination geometries of all the complexes have been determined by X-ray diffraction analyses. Cyclic voltammetric studies of complexes 2, 4, and 7 showed that the oxidation and reduction processes are chemically reversible and that the reduced and oxidized states of complexes 3, 5, and 6 are very short-lived.将蝎型盐K[HB(mt)3]、Na[H2B(mt)2]与Li[HB(mt)2(pz)]与二聚体[Cp*CrBr2]2（记为1）反应，以中等至较高产率得到15电子的三价铬配合物[Cp*Cr{κ3-S,S',S''-HB(mt)3}]Br（2）、[Cp*Cr{κ2-S,S'-H2B(mt)2}Br]（3）与[Cp*Cr{κ2-S,S'-HB(mt)2(pz)}Br]（4）。此类配合物为首例含聚(甲基咪唑硫基硼酸)配体的混合夹心型三价铬配合物。配合物3中的单阴离子配体[H2B(mt)2]与配合物4中的[HB(mt)2(pz)]可通过银盐脱除Br共配体实现三齿配位，进而得到配合物[Cp*Cr{κ3-H,S,S'-H2B(mt)2}]PF6（6）与[Cp*Cr{κ3-N,S,S'-HB(mt)2(pz)}]PF6（7）。研究同时发现，在乙腈存在条件下，3与AgPF6的反应可生成[Cp*Cr{κ2-S,S'-H2B(mt)2}(NCMe)]PF6（5）。所有配合物的配位几何构型均通过X射线衍射分析得以确定。对配合物2、4与7的循环伏安研究表明，其氧化还原过程具有化学可逆性；而配合物3、5与6的还原态与氧化态寿命极短。