Curly–curly, loop–loop: homoleptic metal(ii) complexes of pyridinecarbaldehyde 4′-(2,2′:6′,2″-terpyridyl)hydrazones and their coordination polymers

Abstract

The ligands 4-, 3- and 2-pyridinecarbaldehyde 4′-(2,2′:6′,2″-terpyridyl)hydrazone (1, 2 and 3) have been prepared and characterized. In the homoleptic iron(II) and ruthenium(II) complexes, the nitrogen donor atoms of the pendant pyridine rings subtend an obtuse angle (≈155°) at the metal centre, thus providing an expanded ligand, which can be used for the assembly one-dimensional coordination polymers with an undulating backbone. Protonation of the pendant pyridine rings in [FeL₂]²⁺ (L = 1, 2, 3) or [RuL₂]²⁺ (L = 1, 2) leads to a colour change from purple to blue, or red to pink, respectively, as the MLCT band shifts to a lower energy. The structural data for [Fe(H2)(2)][PF₆]₃·3.5MeCN·2.5H₂O (where [H2]⁺ is the monoprotonated ligand) show that the [Fe(H2)(2)]³⁺ cations form one-dimensional chains supported by N_pyH⋯N_py hydrogen bonds and that the undulating chains pack to give a porous material, with channels that accommodate the anions and solvent molecules. A single-crystal X-ray diffraction determination of {[Fe(NCS)₂(Ru(1)₂)₂][Fe₂(NCS)₆(OEt)₂(EtOH)₂][NCS]₂·4EtOH·H₂O}_n reveals that pairs of [Ru(1)₂]²⁺ cations connect {Fe(NCS)₂} units to give a one-dimensional polymer comprising of linked metallamacrocycles.