Titration and spectral studies of complexes between first-row transition metals and tris(2-pyridyl)carbinol and bis(2-pyridyl)phenylcarbinol

Abstract

Titrations of tris(2-pyridyl)carbinol (py3COH) and di(2-pyridyl)phenylcarbinol (py2phCOH) in the presence of first-row transition metals form 2 : 1 complexes and one —COH group per complex is made acidic. For py3COH, coordination is via a mixed-mode method, i.e. one ligand is bound in a symmetric N,N′,N″ mode and the second, in which the —COH group is made acidic, in a non-symmetric N,N′,O mode. The cobalt(II) and nickel(II) complexes titrate as a weak acid (pKa 6.6) and the H+ is bound to a non-coordinated pyridyl nitrogen. The iron(II) and manganese(II) titrate as strong acids, even through a pyridyl nitrogen is not coordinated. The copper(II) and zinc(II) have both ligands coordinated in a symmetrical fashion and do not consume OH− until considerably higher pH values. Di(2-pyridyl)phenylcarbinol binds, as it must if it is to be tridentate, exclusively in an N,N′,O fashion and all the complexes titrate as strong acids. The complexes have been characterized by spectroscopic techniques.