Temperature-scan-rate dependent SCO hysteresis found in a tetramethylphosphonium tripodal ferrate(II) salt

Abstract

A new compound (Me4P)[Fe(py3COH)(NCS)3]•MeOH was prepared and structurally characterized, where py3COH stands for tris(2-pyridyl)methanol. After annealed at 400 K, desolvated (Me4P)[Fe(py3COH)(NCS)3] displayed an abrupt spin-crossover (SCO) at T1/2↑ = 255 K and T1/2↓ = 234 K. Namely, 21 K-wide hysteresis was recorded at a temperature scan rate of 5 K min−1. The χmT vs T profile was temperature-scan rate dependent; the wider hysteresis with the slower scan. The secondary SCO with T1/2↑ = 275 K emerged, and a new concomitant hysteresis loop overlapped the primary one. The total hysteresis width became 39 K at a slow scan with 0.1 K min−1. A mechanism of interplaying spin crossover and structural phase transition is described. Combining a similar result on the corresponding Me4N+ analog, we supposed that the tripod py3COH ligand would have an advantage for hysteretic SCO and possibly for double SCO behavior.