Tuning of Charge Transfer Assisted Phase Transition and Slow Magnetic Relaxation Functionalities in {Fe9–x Co x [W(CN)8]6} (x = 0–9) Molecular Solid Solution

• Published in: Journal of the American Chemical Society 2016-02, Vol.138 (5), p.1635-1646
• Main Authors: Chorazy, Szymon, Stanek, Jan J, Nogaś, Wojciech, Majcher, Anna M, Rams, Michał, Kozieł, Marcin, Juszyńska-Gałązka, Ewa, Nakabayashi, Koji, Ohkoshi, Shin-ichi, Sieklucka, Barbara, Podgajny, Robert
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.5b11924

Precisely controlled stoichiometric mixtures of Co2+ and Fe2+ metal ions were combined with the [WV(CN)8]3– metalloligand in a methanolic solution to produce a series of trimetallic cyanido-bridged {Fe9–x Co x [W­(CN)8]6(MeOH)24}·12MeOH (x = 0, 1, ..., 8, 9; compounds 0, 1, ..., 8, 9) clusters. All the compounds, 0–9, are isostructural, and consist of pentadecanuclear clusters of a six-capped body-centered cube topology, capped by methanol molecules which are coordinated to 3d metal centers. Thus, they can be considered as a unique type of a cluster-based molecular solid solution in which different Co/Fe metal ratios can be introduced while preserving the coordination skeleton and the overall molecular architecture. Depending on the Co/Fe ratio, 0–9 exhibit an unprecedented tuning of magnetic functionalities which relate to charge transfer assisted phase transition effects and slow magnetic relaxation effects. The iron rich 0–5 phases exhibit thermally induced reversible structural phase transitions in the 180–220 K range with the critical temperatures being linearly dependent on the value of x. The phase transition in 0 is accompanied by HSFeII WV ↔ HSFeIII WIV charge transfer (CT) and the additional minor contribution of a Fe-based spin crossover (SCO) effect. The Co-containing 1–5 phases reveal two simultaneous electron transfer processes which explore HSFeII WV ↔ HSFeIII WIV CT and the more complex HSCoII WV ↔ LSCoIII WIV charge transfer induced spin transition (CTIST). Detailed structural, spectroscopic, and magnetic studies help explain the specific role of both types of CN–-bridged moieties: the Fe-NC-W linkages activate the molecular network toward a phase transition, while the subsequent Co–W CTIST enhances structural changes and enlarges thermal hysteresis of the magnetic susceptibility. On the second side of the 0–9 series, the vanishing phase transition in the cobalt rich 6–9 phases results in the high-spin ground state, and in the occurrence of a slow magnetic relaxation process at low temperatures. The energy barrier of the magnetic relaxation gradually increases with the increasing value of x, reaching up to ΔE/k B = 22.3(3) K for compound 9.