Dicationic μ-diborolyl arene triple-decker complexes [CpCo(μ-1,3-C<inf>3</inf>B<inf>2</inf>Me<inf>5</inf>)M(arene)]²⁺ (M = Rh, Ir; Cp = cyclopentadienyl): Synthesis, structures, electrochemistry and bonding

The reaction of the bromide complexes [CpCo(μ-1,3-C3B2Me5)MBr2]2 [M = Rh (1), Ir (2); Cp = cyclopentadienyl] with AgBF4 in acetonitrile affords the tris(acetonitrile) μ-diborolyl triple-decker complexes [CpCo(μ-1,3-C3B2Me5)M(MeCN)3]2+ [Rh (3), Ir (4)]. The labile nitromethane solvates [CpCo(μ-1,3-C3B2Me5)M(MeNO2)3]2+, generated in a similar way, react with benzene and its methylated derivatives to give the arene triple-decker complexes [CpCo(μ-1,3-C3B2Me5)M(arene)]2+ [M = Rh (5), Ir (6); arene = C6H6 (a), 1,2,4,5-Me4C6H2 (b), C6Me6 (c)]. The structures of 5b(BF4)2, 5c(BF4)2, 6b(BF4)2 and 6c(BF4)2 were determined by X-ray diffraction. The electron-transfer ability of the arene complexes was ascertained by electrochemical techniques. In general, they are able to undergo two separate one-electron reductions reversibly. DFT calculations revealed structural changes caused by redox processes and satisfactorily predicted the redox potentials. The second reduction is accompanied by a η6-→-η4 hapticity change of the arene ligand. Energy decomposition analysis revealed that the Rh-benzene bond in cation 5a is weaker than in cyclopentadienyl analogues [(C5R5)Rh(C6H6)]2+; however, 5a proved to be the least reactive in benzene replacement with acetonitrile and mesitylene. The labile nitromethane solvates [CpCo(μ-1,3-C3B2Me5)M(MeNO2)3]2+ {generated from the bromide complexes [CpCo(μ-1,3-C3B2Me5)MBr2]2 (M = Rh, Ir; Cp = cyclopentadienyl) and AgBF4 in MeNO2} react with arenes to give triple-decker complexes [CpCo(μ-1,3-C3B2Me5)M(arene)]2+. X-ray diffraction, electrochemical techniques and DFT calculations were used for the characterization of the complexes obtaine