Observation of the First Spin Crossover in an Iron(II) Complex with an S6 Coordination Environment: Tris[bis(N,N-diethylamino)carbeniumdithiocarboxylato]iron(II) Hexafluorophosphate

For the first time, the spin-crossover (SCO) phenomenon has been observed in an Fe-II-S-6 system in a tris(chelate)-type iron(II) complex with a zwitterionic sulfur donor bidentate, bis(N,N-diethylamino)carbeniumdithiocarboxylate (EtL), [Fe-II(EtL)(3)](PF6)(2) (1), as synthesized by the reaction of a precursor complex [Fe-II(CH3CN)(6)](PF6)(2) with EtL. In the solid state, the high-spin (HS) d(6) state at ambient temperature and the low-spin (LS) d(6) state at temperatures lower than approximately 240 K were evidenced by magnetic measurements with SQUID and Mossbauer spectra in the temperature range 4-290 K. X-ray analyses of the crystals at various temperatures disclosed that the distorted trigonal prismatic coordination environments essentially do not change; however, contraction of Fe-S distances by approximately 10 % (0.22 angstrom), ordering of alkyl groups in EtL and PF6- counteranions, and formation of significant intermolecular S center dot center dot center dot S interactions between adjacent molecules (average distances of 3.59 angstrom) take place during the transition from the HS to the LS state. A large decrease in the volume of the formula unit (78.1 angstrom(3)) might be responsible for the large activation barrier, thereby resulting in a slow phase transition upon cooling.