Tunable spin equilibria in four-coordinate iron trisphosphine phosphiniminato complexes

Abstract

A series of four-coordinate iron tris(phosphine)borate phosphiniminato complexes (PhBP3RFeNPR'3) with varying alkyl or aryl substituents (R = Ph, CH2Cy, m-terphenyl, iPr; R' = Ph, Cy, Et) have been synthesized and crystallog. characterized. The spin states of these complexes vary depending on the ligand substituents, and most undergo spin-crossover (S = 0 to S = 2) at temps. near or above room temp. in soln. and in the solid state. The change in spin state also involves a significant change in the degree of π-bonding between Fe and the N of the phosphiniminato ligand as well as dramatic changes in Fe-P bond lengths. Spin-crossover is accompanied by striking changes in the UV-visible absorption spectra, which allowed for quant. modeling of the thermodn. parameters (ΔH and ΔS) of the spin equil. The complexes and their spin equil. have also been studied by paramagnetic NMR, IR spectroscopy, Moessbauer spectroscopy, and soln.- and solid-state magnetometry. These studies lead to qual. correlations between the steric and electronic properties of the ligand substituents and the enthalpy and entropy changes assocd. with the spin equil. in soln. This series of complexes emphasizes the importance of considering sep. entropic and enthalpic effects when attempting to understand and predict trends in spin-crossover temps. (TC).