Magnetic Mn and Co Complexes with a Large Polycyclic Aromatic Substituted Nitronylnitroxide

Abstract

2-(1'-Pyrenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (PyrNN) was reacted with M(hfac)(2) (M = Mn(II) and Co(II), hfac = hexafluoracetylacetonate) to give two isostructural ML(2) stoichiometry M(hfac)(2)(PyrNN)(2) complexes and a ML stoichiometry one-dimensional (1-D) polymer chain complex [Mn(hfac)(2)(PyrNN)]. The ML(2) complexes have similar crystal structures with monoclinic unit cells, in which one NO unit from each PyrNN ligand is bonded to the transition metal on cis vertices of a distorted octahedron. The major magnetic interactions are intracomplex metal-to-radical exchange (J), and intermolecular exchange across a close contact between the uncoordinated NO units (J'). For M = Mn(II) an approximate chain model fit gives g = 2.0, J = (-)125 cm(-1) and J' = (-)49 cm(-1); for M = Co(II), g = 2.4, J = (-)180 cm(-1), and J' = (-)70 cm(-1). Hybrid density functional theory (DFT) computations modeling the intermolecular exchange by using only the radical units across the close contact are in good accord with the estimated values of J'. The chain type complex structure shows solvent incorporation for overall structure [Mn(hfac)(2)(PyrNN)](n)·0.5(CHCl(3))·0.5(C(7)H(16)). Both NO groups of the PyrNN ligand are complexed to form helical chains, with very strong metal to radical antiferromagnetic exchange that gives overall ferrimagnetic behavior.