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 CoX(2)·xH(2)O (X(-) = Cl(-), OAc](-), NO(3)](-) or BF(4)](-)) 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 (1)H 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 CD(3)OD in the CD(3)OD/CDCl(3) solvent mixture affects both the relative proportions of Co(Cltpy)(2)](2+) and Co(Cltpy)(OAc)(2)] and the chemical shifts of the (1)H NMR resonances arising from Co(Cltpy)(OAc)(2)]. When the solvent is essentially CDCl(3), 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:Co(2+) ion ratio. The (1)H 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(-) = BF(4)](-), we propose that the mono(tpy) complex formed in solution is Co(MeOtpy)L(n)](2+) (L = H(2)O 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)(NO(3)-O)(2)(OH(2))] and Co(MeOtpy)Cl(2)]. The single crystal structure of the cobalt(III) complex Co(Cltpy)Cl(3)]·CHCl(3) is also reported.