Enhanced ferromagnetic interaction in metallacyclic complexes incorporating m-phenylenediamidato bridges.

Abstract

The double stranded Cu^II₂-metallacyclic complex of formula [Cu₂(mbpb)₂]·2H₂O (1) and the triple stranded Ni^II₂-metallacyclic complexes of formula [Ni₂(Hmbpb)₃]PF₆·21H₂O (2), [Co(H₂O)₆][Ni₂(mbpb)₃]THF·10H₂O (3) and {Ag₂(H₂O)[Ni₂(mbpb)₃]}·11H₂O (4) (where H₂mbpb is the bisbidentate dinucleating bridging ligand 1,3-bis(pyridine-2-carboxamide) benzene) have been synthesised and characterised by single-crystal X-ray diffraction. Within the dinuclear molecules, metal ions are bridged by either fully or semideprotonated bisbidentate ligands, which are coordinated through the pyridine and amidato nitrogen donor atoms. In complex 4 the triple stranded dinuclear Ni₂ units are connected to the Ag⁺ cations through O-amidato bridges and Ag–π_(benzene) interactions to afford a 1D bimetallic chain. Cu₂ (1) and Ni₂ (2–4) complexes exhibit ferromagnetic coupling between the metal ions through the bridging ligand with J_Cu–Cu = 21.1 cm⁻¹ and J_Ni–Ni in the range of 2.9–3.6 cm⁻¹, respectively. Amongst copper(II) dinuclear complexes bearing m-phenylenediamidato bridges, complex 1 exhibits the stronger ferromagnetic exchange coupling reported so far. DFT calculations firstly confirm that the spin polarisation mechanism is responsible for the ferromagnetic coupling, and secondly allows us to predict stronger ferromagnetic couplings in Cu^II₂ complexes with larger tetrahedral distortions of the CuN₄ coordination environment.