The surprising lability of bis(2,2':6',2''-terpyridine)chromium(III) complexes

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 d(3) Cr(3+) ion being a kinetically inert metal centre, the tpy ligands in [Cr(tpy)2](3+) are labile in the presence of base; absorption and (1)H 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.