Unexpected isotope effect on the spin transition of the coordination polymer Fe(C5H5N)2[Ni(CN)4]Dedicated to Patrick Cassoux on the occasion of his retirement.

Abstract

The two-dimensional coordination polymer spin-crossover compound Fe(pyridine) 2 [Ni(CN) 4 ] (1) and its isotope substituted analogues Fe(pyridine-D 5 ) 2 [Ni(CN) 4 ] (2) and Fe(pyridine-15 N) 2 [Ni(CN) 4 ] (3) have been synthesised and the isotope effect on the thermal spin-crossover behaviour has been studied using magnetic susceptibility, 57 Fe Mo ¨ssbauer, Raman and calorimetric techniques. All these methods confirmed that upon isotope substitution-contrary to previous observations-the spin transition temperature shifted downwards from 202 to 194 K. The theoretical analysis of spectroscopic data has revealed that this shift was a result of a subtle balance between different vibrational and electronic factors.

Spin-Crossover Behavior of Hofmann-Type-Like Complex Fe(4,4’-bipyridine)Ni(CN)4·nH2O Depending on Guest Species

“…The sample including C 2 D 5 OD showed small shift to higher temperature region as often observed in deuterated SCO complexes. However, in the sample containing (CD 3 ) 2 CO, a remarkable increase in the hysteresis width was observed [42]. Such increase in hysteresis width coming from deuteration is rare [42].…”

“…In contrast to 1, 2 showed a two-step spin transition with thermal hysteresis, the critical temperatures of which on cooling (T C Ó) were 213 and 187 K, and on warming (T C Ò) were 223 and 197 K. These T C values are close to those observed in the Hofmann-pyridine-type complex Fe(py) 2 Ni(CN) 4 . [25,42] In 3, a spin transition was observed as a one-step transition with hysteresis, the T C Ó and T C Ò of which were 125 and 145 K, respectively. The T C values of 3 are considerably lower than those of Fe(py) 2 Ni(CN) 4 .…”

“…However, in the sample containing (CD 3 ) 2 CO, a remarkable increase in the hysteresis width was observed [42]. Such increase in hysteresis width coming from deuteration is rare [42]. Table 1 lists the transition temperatures including those obtained from the deuterated samples.…”

“…The doublet of the major absorption at room temperature, whose δ and ∆E Q were 1.070(1) mm s´1 and 1.190(2) mm s´1, respectively, converted into a singlet peak with δ of 0.490(2) mm s´1 and ∆E Q of 0.217(6) mm s´1 at 77 K. These observations on the major components agree with the spin transition observed in the magnetic susceptibility measurement. Comparing the Mössbauer parameters with those obtained in Fe(py) 2 Ni(CN) 4 [25,42] and its related complexes [44][45][46][47][48][49][50][51], this major signal is considered to come from the octahedral Fe II N 6 species in the cyanide network. On the other hand, the minor signals, which always appeared as a doublet assignable to Fe II in the HS state in 1, 2, and 3, showed no large change over all measurements.…”

New Hofmann-like spin crossover compound with 3,5-lutidine

“…Special attention is currently paid to coordination polymer iron(II) SCO compounds acting in cooperative behaviour since they can manifest sensory and memory functions. The Hofmann pyridine coordination polymer Fe(py) 2 Ni(CN) 4 1 shows iron(II) SCO behaviour, as revealed by 57 Fe Mössbauer spectroscopy and the SQUID technique [2]. Pressure tuning Raman spectroscopy of 1 was reported [3] and an isotope effect of 1 was found [4].…”