Synthesis, Structures, and Redox Properties of Mixed-Sandwich Complexes of Cyclopentadienyl and Hydrotris(pyrazolyl)borate Ligands with First-Row Transition Metals

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Abstract

A series of [MCpRTp]n+ (CpR = Cp*(pentamethylcyclopentadienyl) or Cp (cyclopentadienyl), Tp = hydrotris(pyrazolyl)borate) complexes have been synthesized by reaction of a suitable MCpR precursor with KTp (CpR = Cp*; n = 0, M = Cr, Fe, Co, Ni; n = 1, M = Cr, Co, Ni; CpR = Cp; n = 0, M = V, Co, Ni; n = 1, M = V, Co). Oxidation with [FeCp2]+ salts or reduction with CoCp2 where appropriate provided easy access to the corresponding M(III) or M(II) species. All of the complexes studied showed reversible M(III)/M(II) redox couples. Similarly [MCpRTpm]n+ complexes have also been isolated (Tpm = hydrotris(pyrazolyl)methane; CpR = Cp*, n = 1, M = Fe; CpR = Cp, n = 1 or 2, M = Co). Analytical, NMR, IR, and mass spectroscopic data are consistent with the formulation of these species as mixed-sandwich complexes. Oxidation of VCpTp in MeCN solution yields [VCpTp(MeCN)]+, whereas similar reaction in CH2Cl2 solution yields [VCpTp]+. [VCpTp]+ reacts with σ-donor ligands L, where L = CNtBu and PMe3, to form [VCpTpL]+ species, but is unreactive when L = CO, indicating little π-base character to the V center. Crystal structure determinations were performed for various complexes: CoCp*Tp is unique in displaying κ2-Tp and η5-Cp* coordination, whereas [VCpTp]+, [CoCp*Tp]+, NiCp*Tp, and [CoCpTpm]2+ all display mixed-sandwich structures with κ3-Tp binding. The factors determining the relative conformations of the CpR and Tp ligands are discussed. The structures of [VCpTp(MeCN)]+ and [VCpTp(PMe3)]+ were also determined and show that considerable distortion of the sandwich moiety has occurred to accommodate coordination of the extra ligand. For the MCpRTp complexes a dependence of νB-H on Tp hapticity, ancillary ligand, and oxidation state is observed from IR spectroscopic data in the solid state. IR data obtained in solution suggest that CoCp*Tp is in conformational equilibrium, with a κ3-Tp structure also present. Some of the factors affecting 1H and 13C NMR shifts are discussed and compared with relevant homoleptic analogues. Electrochemical data reveal, in general, that MCp2 and MCp*2 species are more electron rich, although comparisons are best confined to a per metal basis.

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