Examination of cobalt, nickel, copper and zinc(II) complex geometry and binding affinity in aqueous media using simple pyridylsulfonamide ligands

The sixteen neutral ML₂ complexes of Co, Ni, Cu and Zn(II) with the p-toluenesulfonamide and trifluoromethylsulfonamide derivatives of 2-aminomethylpyridine (L¹, L²) and its 6-Me homologue (L³, L⁴) have been characterised by low temperature X-ray crystallography (100-120 K). Complexes of Co and Zn invariantly adopted a distorted tetrahedral geometry and whilst Cu(II) complexes of L², L³ and L⁴ also took up a distorted tetrahedral geometry, that with L¹ was square planar. A database survey of the distortion from limiting tetrahedral/square planar geometry has been carried out, aided by a simple geometric analysis. The trifluoromethylsulfonamide ligands (L² and L³) were less basic, e.g. log K₁ 7.51(3) for L² vs. 12.23(6) for L¹ (80% MeOH/H₂O) and afforded a weaker ligand field, exemplified by the position of the visible d-d transition in Cu(II) complexes and the ease of reduction of the Cu(II) centre: E_1/2 values (MeCN vs. Ag/AgCl) are -430, -137, +55 and -240 mV for Cu(L¹)₂, Cu(L²)₂, Cu(L³)₂ and Cu(L⁴)₂. Ligand protonation and stepwise formation constants have been measured for L¹-L³ and derived species distribution diagrams reveal that for complexes with L² and L³, the predominant species present at pH 7.4 when zinc was in the nanomolar range was ZnL₂.