Synthesis and characterization of mixed-ligand cyclopentadienyl-pyrazolylborate complexes of ruthenium

Article abstract of DOI:10.1021/om00133a021

Neutral compounds of Ru(II) that contain a cyclopentadienyl and a pyrazolylborate ligand were synthesized from [CpRu(CH₃CN)₃]PF₆ and [Cp*Ru(CH₃CN)₃]PF₆ (Cp^- = η⁵-cyclopentadienyl; Cp^*- = η⁵-pentamethylcyclopentadienyl). [CpRu(HBpz₃)], [CpRuHB(3,5-Me₂pz)₃], [CpRuBpz₄], and [Cp*Ru(HBpz₃)] were characterized spectroscopically. An X-ray crystallographic study of [CpRu(HBpz₃)] was completed. CpRu(HBpz₃) crystallizes in the P2₁/m space group with Z = 2, V = 749.8 A?³, a = 9.0854 (4) A?, b = 10.074 (4) A?, c = 8.195 (2) A?, and β = 91.50 (3)°. Full-matrix least-squares refinement (112 variables, 2290 reflections) converged to give R and R_w values of 0.043 and 0.052, respectively. The structure consists of discrete (η⁵-Cp)Ru(HBpz₃) units with Ru-C bonds of 2.153 (3) A? and Ru-N bonds of 2.128 (3) A?. Electrochemical measurements for the series of compounds were carried out for comparison with ferrocene and ruthenocene. The compounds exhibit a quasi-reversible, one-electron oxidation that falls in the range 0.145-0.463 V vs. Ag/AgCl. Oxidation of CpRuHB(3,5-Me₂pz)₃ with AgPF₆ allowed the isolation of [CpRuHB(3,5-Me₂Pz)₃]PF₆. The thermal reactivity of these compounds allowed the synthesis of the new complexes [CpRuHB(3,5-Me₂pz)₃(CO)], [CpRuBpz₄(CO)], and [Cp₂Ru₂(CO)₂Bpz₄]PF₆ that exhibit bidentate coordination of pyrazolylborate ligands to ruthenium. While the CpRu pyrazolylborate complexes are formally similar to ruthenocene, the replacement of cyclopentadienyl with a pyrazolylborate ligand significantly alters the chemistry of these new compounds.