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 (L-1, L-2) and its 6-Me homologue (L-3, L-4) 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 L-2, L-3 and L-4 also took up a distorted tetrahedral geometry, that with L-1 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 (L-2 and L-3) were less basic, e.g. log K-1 7.51(3) for L-2 vs. 12.23(6) for L-1 (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(L-1)(2), Cu(L-2)(2), Cu(L-3)(2) and Cu(L-4)(2). Ligand protonation and stepwise formation constants have been measured for L-1-L-3 and derived species distribution diagrams reveal that for complexes with L-2 and L-3, the predominant species present at pH 7.4 when zinc was in the nanomolar range was ZnL2.