Tuning of the phase transition between site selective SCO and intermetallic ET in trimetallic magnetic cyanido-bridged clusters

Abstract

A series of crystalline phases composed of trimetallic 3d-5d-5d′ {Fe₉[Re(CN)₈]_6−x[W(CN)₈]_x(MeOH)₂₄}·yMeOH (x = 1 (1), 2 (2), 3 (3), 4 (4) and 5 (5); y = 10–15) clusters were obtained by altering the octacyanidometalate composition. The temperature dependent studies involving SC XRD, SQUID magnetic measurements, IR spectroscopy and ⁵⁷Fe Mössbauer spectroscopy revealed reversible phase transition with the retention of single crystal character in each congener. The transition was assisted by reversible spin-crossover (SCO) ^HSFe^II ↔ ^LSFe^II transition at the central Fe1(II) site for Fe₉Re₅W₁ (1), Fe₉Re₄W₂ (2), Fe₉Re₃W₃ (3) and Fe₉Re₂W₄ (4). In contrast, the tungsten-rich congener Fe₉Re₁W₅ (5) exhibited nontrivial behavior with the SCO transition being stopped halfway through the cooling process, to be completed with single electron transfer (ET) from the external Fe2(II) center towards one of the neighboring W(V) sites. The critical temperature T_c of SCO has been systematically increased from 193 K (1) to 247 K (4). All experimental data indicate the domination of the Fe(II)–W(V) valence states in all crystals 1–5, however, with increasing quantity of [W(CN)₈]³⁻ (and decreasing quantity of [Re(CN)₈]³⁻), the valence equilibrium Fe(II)–W(V) ↔ Fe(III)–W(IV) was systematically shifted to the right, starting from congener 3. The overall electronic configuration at low temperatures and variable amounts and location of spin carriers along the whole series suggest the remarkable competition between magnetic super-exchange Fe(II)–CN–W(V) interactions and intermolecular interactions. The observed behavior is in line with the information collected previously for the bimetallic congeners Fe₉Re₆ and Fe₉W₆, to shed light on the role of the mixed tri-metallic composition in changing the properties observed for the relevant bimetallic cyanido-bridged skeletons.