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 L2vs. 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: E1//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.