The surprising lability of bis(2,2′:6′,2′′-terpyridine)chromium(iii) complexes

Abstract

The complex [Cr(tpy)(O₃SCF₃)₃] (tpy = 2,2′:6′,2′′-terpyridine) is readily made from [Cr(tpy)Cl₃] and is a convenient precursor to [Cr(tpy)₂][PF₆]₃ and to [Cr(tpy)(4′-(4-tolyl)tpy)][PF₆]₃ and [Cr(tpy)(5,5′′-Me₂tpy)][PF₆]₃ (4′-(4-tolyl)tpy = 4′-(4-tolyl)-2,2′:6′,2′′-terpyridine; 5,5′′-Me₂tpy = 5,5′′-dimethyl-2,2′:6′,2′′-terpyridine); these are the first examples of heteroleptic bis(tpy) chromium(III) complexes. The single crystal structures of 2{[Cr(tpy)₂][PF₆]₃}·5MeCN, [Cr(tpy)(4′-(4-tolyl)tpy)][PF₆]₃·3MeCN and [Cr(tpy)(5,5′′-Me₂tpy)][PF₆]₃·3MeCN have been determined. Each cation contains the expected octahedral {Cr(tpy)₂}³⁺ unit; in all three structures, the need to accommodate three anions per cation and the solvent molecules prevents the formation of a grid-like array of cations that is typical of many lattices containing {M(tpy)₂}²⁺ motifs. Three reversible electrochemical processes are observed for [Cr(tpy)(4′-(4-tolyl)tpy)][PF₆]₃ and [Cr(tpy)(5,5′′-Me₂tpy)][PF₆]₃, consistent with those documented for [Cr(tpy)₂]³⁺. At pH 6.36, aqueous solutions of [Cr(tpy)₂][PF₆]₃ are stable for at least two months. However, contrary to the expectations of the d³ Cr³⁺ ion being a kinetically inert metal centre, the tpy ligands in [Cr(tpy)₂]³⁺ are labile in the presence of base; absorption and ¹H NMR spectroscopies have been used to monitor the effects of adding NaOH to aqueous and CD₃OD solutions, respectively, of the homo- and heteroleptic complexes. Ligand dissociation is also observed when [Bu₄N]F is added to CD₃OD solutions of the complexes, but in aqueous solution, [Cr(tpy)₂][PF₆]₃ is stable in the presence of fluoride ion.