title: Abrupt versus Gradual Spin-Crossover in Fe(II)(phen)2(NCS)2 and Fe(III)(dedtc)3 Compared by X-ray Absorption and Emission Spectroscopy and Quantum-Chemical Calculations.
abstract: Molecular spin-crossover (SCO) compounds are attractive for information storage and photovoltaic technologies. We compared two prototypic SCO compounds with Fe(II)N6 (1, [Fe(phen)2(NCS)2], with phen = 1,10-phenanthroline) or Fe(III)S6 (2, [Fe(dedtc)3], with dedtc = N,N'-diethyldithiocarbamate) centers, which show abrupt (1) or gradual (2) thermally induced SCO, using K-edge X-ray absorption and Kβ emission spectroscopy (XAS/XES) in a 8-315 K temperature range, single-crystal X-ray diffraction (XRD), and density functional theory (DFT). Core-to-valence and valence-to-core electronic transitions in the XAS/XES spectra and bond lengths change from XRD provided benchmark data, verifying the adequacy of the TPSSh/TZVP DFT approach for the description of low-spin (LS) and high-spin (HS) species. Determination of the spin densities, charge distributions, bonding descriptors, and valence-level configurations, as well as similar experimental and calculated enthalpy changes (ΔH), suggested that the varying metal-ligand bonding properties and deviating electronic structures converge to similar enthalpic contributions to the free-energy change (ΔG) and thus presumably are not decisive for the differing SCO behavior of 1 and 2. Rather, SCO seems to be governed by vibrational contributions to the entropy changes (ΔS) in both complexes. Intra- and intermolecular interactions in crystals of 1 and 2 were identified by atoms-in-molecules analysis. Thermal excitation of individual dedtc ligand vibrations accompanies the gradual SCO in 2. In contrast, extensive inter- and intramolecular phen/NCS vibrational mode coupling may be an important factor in the cooperative SCO behavior of 1.
authors.affiliation: Institut für Experimentalphysik, Freie Universität Berlin , 14195 Berlin, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Mebs
authors.affiliation: Institut für Chemie, Humboldt-Universität zu Berlin , 12489 Berlin, Germany.
authors.forename: Beatrice
authors.initials: B
authors.lastname: Braun
authors.affiliation: Institut für Experimentalphysik, Freie Universität Berlin , 14195 Berlin, Germany.
authors.forename: Ramona
authors.initials: R
authors.lastname: Kositzki
authors.affiliation: Institut für Chemie, Humboldt-Universität zu Berlin , 12489 Berlin, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Limberg
authors.affiliation: Institut für Experimentalphysik, Freie Universität Berlin , 14195 Berlin, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Haumann