Abrupt Spin Transition in a Heteroleptic Fe(II) Complex with Pendant Naphthalene Functionality.

ABSTRACT

A heteroleptic spin-crossover (SCO) complex, [Fe(tpma)(xnap-bim)](ClO4)2 (1; tpma = tris(2-pyridylmethyl)amine, xnap-bim = 8,15-dihydrodiimidazo[1,2-a2',1'-c]naphtho[2,3-f][1,4]diazocine), has been obtained by reacting a Fe(II) precursor salt with tetradentate tpma and bidentate xnap-bim ligands. Depending on crystallization conditions, two different solvates have been obtained, 1·2.25py·0.5H2O and 1·py. The former readily loses the interstitial solvent to produce either a powder sample of 1 upon filtration or crystals of 1 if the solvent loss is slowed by placing the crystals of 1·2.25py·0.5H2O in diethyl ether. In contrast, 1·py exhibits higher stability toward solvent loss. The crystal packing of both solvates and of the solvent-free structure features double columns of [Fe(tpma)(xnap-bim)]2+ cations formed by efficient π-π interactions between the pyridyl groups of tpma ligands, as well as by stacks supported by π-π interactions between interdigitated naphthalene fragments of xnap-bim ligands. While both solvates show a gradual SCO between the high-spin (HS) and low-spin (LS) states of the Fe(II) ion, solvent-free complex 1 exhibits an abrupt spin transition centered at 127 K, with a narrow 2 K thermal hysteresis. Complex 1 also shows a light-induced excited spin state trapping effect, manifested as LS → HS conversion upon irradiation with white light at 5 K. The metastable HS state relaxes to the ground LS state when heated above 65 K.