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

In methanol or chloroform/methanol solutions, reactions of Cltpy or MeOtpy (Rtpy = 4`-R-2,2`:6`,2 ``-terpyridine) with CoX2 center dot xH(2)O (X- = Cl-, [OAc](-), [NO3](-) or [BF4](-)) result in the formation of equilibrium mixtures of [Co(Rtpy)(2)](2+) and [Co(Rtpy)X-2]. A study of the solution speciation has been carried out using H-1 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 H-6 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 H-1 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 H-1 NMR spectrum of [Co(MeOtpy)(2)](2+) is virtually independent of anion and solvent. In contrast, the signals arising from [Co(MeOtpy)X-2] 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)Cl-2]. The single crystal structure of the cobalt(III) complex [Co(Cltpy)Cl-3]center dot CHCl3 is also reported.