Zn and Ni complexes of pyridine-2,6-dicarboxylates: Crystal field stabilization matters!

Abstract

Six reaction products of Zn II and Ni II with pyridine-2,6-dicarboxylic acid (H2Lig 1), 4-chloropyridine-2,6-dicarboxylic acid (H2Lig 2) and 4-hydroxypyridine-2,6-dicarboxylic acid (H2Lig 3) are used to pinpoint the structural consequences of crystal field stabilization by an incomplete d shell. The pseudo-octahedral Zn II coordination sphere in bis(6-carboxypicolinato)zinc(II) trihydrate, [Zn(C 7 H4NO4)2 ]·3H2O or [Zn(HLig 1)2 ]·3H2O, (1), is significantly less regular than that about Ni II in the isostructural compound bis(6-carboxypicolinato)nickel(II) trihydrate, [Ni(C 7 H4NO4)2 ]·3H2O or [Ni(HLig 1)2 ]·3H2O, (2). The Zn II complexes poly[(4-chloropyridine-2,6-dicarboxylato)zinc(II)], [Zn(C 7 H2ClNO4)] n or [Zn(Lig 2)] n, (3), and poly[[(4-hydroxypyridine-2,6-dicarboxylato)zinc(II)] monohydrate], {[Zn(C 7 H3NO 5)]·H2O} n or {[Zn(Lig 3)]·H2O} n, (4), represent two-dimensional coordination polymers with chelating and bridging pyridine-2,6-dicarboxylate ligands in which the coordination polyhedra about the central cations cannot be associated with any regular shape; their coordination environments range between trigonal-bipyramidal and square-pyramidal geometries. In contrast, the corresponding adducts of the diprotonated ligands to Ni II, namely triaqua(4-chloropyridine-2,6-dicarboxylato)nickel(II), [Ni(C 7 H2ClNO4)(H2O)3] or [NiLig2(OH 2) 3)], (5), and triaqua(4-hydroxypyridine-2,6-dicarboxylato)nickel(II) 1.7-hydrate, [Ni(C 7 H3NO 5)(H2O)3]·1.7H2O or [NiLig3(OH 2) 3)]·1.7H2O, (6), feature rather regular octahedral coordination spheres about the transition-metal cations, thus precluding the formation of analogous extended structures.