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

A heteroleptic spin-crossover (SCO) complex, [Fe(tpma)(xnap-bim)](ClO 4 ) 2 ( 1 ; tpma = tris(2-pyridylmethyl)amine, xnap-bim = 8,15-dihydrodiimidazo[1,2- a :2',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·0.5H 2 O 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·0.5H 2 O 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.