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.