Examination of cobalt, nickel, copper and zinc(ii) complex geometry and binding affinity in aqueous media using simple pyridylsulfonamide ligandsElectronic supplementary information (ESI) available: experimental details for [M(L2)2], [M(L3)2] and [M(L4)2] (M = Zn, Cu, Ni, Co); species distribution plots. See http://www.rsc.org/suppdata/nj/b2/b206279h/

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 (100â 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 dâ d transition in Cu(ii) complexes and the ease of reduction of the Cu(ii) centre: E12 values (MeCN vs. Ag/AgCl) are â 430, â 137, +55 and â 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 L¹â 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.