Pressure-Induced Spin-State Crossovers in Six-Coordinate FeIILnL‘m(NCS)2Complexes with L = L‘ and L ≠ L‘: A XANES Investigation

Abstract

The pressure-induced iron(II) high-spin (HS) → low-spin (LS) conversion has been investigated, by near-edge X-ray absorption (XANES) spectroscopy at room temperature, in a number of parent six-coordinate complexes of the type FeLnL‘m(NCS)2, viz. Fe(phen)2(NCS)2 form I (1) and form II (2), Fe(py)2bpym(NCS)2 (3), Fe(py)2phen(NCS)2 (4) and Fe(py)4(NCS)2 (5), where phen = 1,10-phenanthroline, py = pyridine, and bpym = 2,2‘-bipyrimidine. The spectra of the two spin isomers are interpreted. For compounds 1−4, known to exhibit thermally-induced spin transitions, nLS vs P plots (nLS = LS fraction, P = pressure) are centered around Pc = 0.80 (1), 0.65 (2), 1.00 (3) and 1.55 (4) GPa. For 5, which retains the HS form at any temperature under atmospheric pressure, the Pc value is much higher than the previous ones, lying in the range 5−7 GPa. After pressure was released, the spectra were found to be quite similar to those obtained before pressure was applied. The Pc values of 1−4 were analyzed in terms of spin-transition temperatures (Tc), entropy (ΔS) and crystal volume (ΔVSC) variations associated with the spin change, crystal structures, and volumic compressibility coefficients (kv), insofar as these data were available. The alteration of the spin-conversion development, when subjecting a compound (here 2) several times to increasing then to decreasing pressures, was ascribed to a progressive decrease in the number of crystal defects (vacancies in particular).