Transition metal dipicolinates as designer T-shaped building blocks

Abstract

The synthesis of co-ordination polymers derived from T-shaped transition metal dipicolinate fragments [M(dipic)] (M = Co, Ni; dipic2− = dipicolinate, pyridine-2,6-dicarboxylate) has been investigated. The use of mononuclear precursors such as [Co(dipic)(OH2)(OSMe2)2] 1 for the synthesis of 4,4′-bipyridine-bridged complexes showed limited success, direct reaction of individual components being found to be more effective. Thus, reaction of transition metal(II) cations with the dipicolinate anion in the presence of 4,4′-bipyridine (4,4′-bipy) under a variety of reaction conditions affords co-ordination complexes with [M(dipic)]:4,4′-bipy molar ratios of 1 : 1, 2 : 1 and 2 : 3. In the 2 : 1 complexes, {[{Co(dipic)(OH2)2}2{μ-4,4′-bipy}]·nH2O}∞3 (n = 4), 4 (n = 3) the co-ordination geometry at the metal centre is completed by one N-donor bridge and two water molecules. The resulting binuclear building blocks are assembled into 3D matrices by complex hydrogen-bonding networks. In the 1 : 1 polymers, {[{Co(μ-dipic)}(μ-4,4′-bipy)]·n(solvent)}∞ [n(solvent) = 3H2O 5, H2O·MeOH 2, or 2H2O·0.5Me2SO 6] the metal centre co-ordination geometry is completed by two N-donor bridges and an oxygen atom of a bridging dipicolinate anion. The extended structure thus comprises a series of [Co(dipic)]∞ chains bridged by 4,4′-bipyridine molecules to give a 2D structure with (4,4) topology. Π–π stacking interactions between dipicolinate anions of adjacent [Co(dipic)]∞ chains lead to 3D networks with spacious channels occupied by diverse solvent molecules. Replacement of 4,4′-bipyridine by the longer bridge, trans-4,4′-azobis(pyridine) (azpy) in {[{Ni(μ-dipic)}(μ-azpy)]·H2O·azpy}∞8 provides sufficient space in the channels for the inclusion of a non-co-ordinated azpy molecule at the expense of solvent molecules. In the 2 : 3 polymer, {[Ni(dipic)(μ-4,4′-bipy)1.5]·H2O·Me2SO}∞7, the metal centre co-ordination geometry is completed by three N-donor bridges thus generating the classical T-shaped connecting unit, which leads to a 2D structure of (6,3) topology. Π–π stacking interactions analogous to those found in the 1 : 1 complexes result in a distortion of the classical brickwall architecture, each brick adopting a bow-tie-like topology.