Environmental control of solution speciation in cobalt(ii) 2,2′:6′,2′′-terpyridine complexes: anion and solvent dependence

Abstract

In methanol or chloroform/methanol solutions, reactions of Cltpy or MeOtpy (Rtpy = 4′-R-2,2′:6′,2′′-terpyridine) with CoX₂·xH₂O (X⁻ = Cl⁻, [OAc]⁻, [NO₃]⁻ or [BF₄]⁻) result in the formation of equilibrium mixtures of [Co(Rtpy)₂]²⁺ and [Co(Rtpy)X₂]. A study of the solution speciation has been carried out using ¹H NMR spectroscopy, aided by the dispersion of signals in the paramagnetically shifted spectra; on going from a low- to high-spin cobalt(II) complex, proton H⁶ of the tpy ligand undergoes a significant shift to higher frequency. For R = Cl and X⁻ = [OAc]⁻, increasing the amount of CD₃OD in the CD₃OD/CDCl₃ solvent mixture affects both the relative proportions of [Co(Cltpy)₂]²⁺ and [Co(Cltpy)(OAc)₂] and the chemical shifts of the ¹H NMR resonances arising from [Co(Cltpy)(OAc)₂]. When the solvent is essentially CDCl₃, the favoured species is [Co(Cltpy)(OAc)₂]. For the 4′-methoxy-2,2′:6′,2′′-terpyridine, the speciation of mono- and bis(terpyridine)cobalt(II) complexes depends upon the anion, solvent and ligand : Co²⁺ ion ratio. The ¹H NMR spectrum of [Co(MeOtpy)₂]²⁺ is virtually independent of anion and solvent. In contrast, the signals arising from [Co(MeOtpy)X₂] depend on the anion and solvent. In the case of X⁻ = [BF₄]⁻, we propose that the mono(tpy) complex formed in solution is [Co(MeOtpy)L_n]²⁺ (L = H₂O or solvent, n = 1–3). The formation of mono(tpy) species has been confirmed by the solid state structures of [Co(Cltpy)(OAc-O)(OAc-O,O′)], [Co(MeOtpy)(OAc-O)(OAc-O,O′)], [Co(MeOtpy)(NO₃-O)₂(OH₂)] and [Co(MeOtpy)Cl₂]. The single crystal structure of the cobalt(III) complex [Co(Cltpy)Cl₃]·CHCl₃ is also reported.