Coordinating properties of mixed pyrrolyl/dicarbollide cobalt metallocene-type complexes

Abstract

Mixed-sandwich Co(III) complexes incorporating an η⁵ pyrrolyl unit [η⁵-NC₄H₄]⁻ are very stable when the second η⁵ moiety is a dicarbollide [η⁵-C₂B₉H₁₁]²⁻. The existence of a lone pair on N in the neutral closo-[3-Co(η⁵-NC₄H₄)-1,2-C₂B₉H₁₁] (1) has allowed for the first time a study of the N-σ bonding capacity of a pyrrolyl ligand η⁵ bonded to a metal in an oxidation state higher than two. This capacity has been proven by reacting closo-[3-Co(η⁵-NC₄H₄)-1,2-C₂B₉H₁₁] with Lewis acids (BH₃, BF₃ and Ag⁺) and a Co(III) macrocycle. In all cases the reaction went to completion proving the strength of the σ bond generated. The stability of the [3-Co(η⁵-NC₄H₄)-1,2-C₂B₉H₁₁] complexes vs. oxidation state Co(III)/Co(II) was studied by cyclic voltammetry. It was found that [3-Co(η⁵-NC₄H₄)-1,2-C₂B₉H₁₁]⁻ is not as stable as [3-Co(η⁵-NC₄H₄)-1,2-C₂B₉H₁₁]; however, when coordinated to cobalt(III), both [3-Co(η⁵-NC₄H₄)-1,2-C₂B₉H₁₁]⁻ and [3-Co(η⁵-NC₄H₄)-1,2-C₂B₉H₁₁] are of comparable stability. The stability of the reduced form Co(II) in the mixed pyrrolyl/dicarbollide is enhanced when an electron withdrawing group is bonded to the cluster carbon atom in the dicarbollide.