Anion recognition by nickel(II) and cobalt(III) complexes with quadridentate ligands based on isothiosemicarbazide

New nickel(II) and cobalt(III) complexes of quadridentate ligands based on S-substituted isothiosemicarbazides [Ni(H2L1)]PF6 (5), [Ni(H2L2)]PF6 (6), [Ni(H2L3)]PF6 (7) and [Co(L-4)Py-2]PF6 (8), where H3L1 = pentane-2,3-dione bis(S-methylisothiosemicarbazone), H3L2 = pentane-2,4-dione bis(S-dodecylisothiosemicarbazone), H3L3 = pentane-2,4-dione bis(S-ethylphenylisothiosemicarbazone) and H2L4 = 3-oxo-pentane-2,4-dione bis(S-methylisothiosemicarbazone), have been prepared from the corresponding iodide salts 1-4 and used for anion binding studies. The crystal structure of 8 has been determined by X-ray crystallography. For hosts 5-7 reasonable binding of chloride anion in DMSO has been found. The stability constants elucidated by H-1 NMR titration are 60-100 M-1. The cobalt(III) host 8 forms a stronger complex with chloride (K-a = 220 M-1) compared to nickel(II) species. In addition much stronger binding of CH3COO- (K-a = 4420 M-1) has been observed. (C) 1998 Elsevier Science Ltd. All rights reserved.