A Charge-Transfer-Induced Spin Transition in a Discrete Complex: The Role of Extrinsic Factors in Stabilizing Three Electronic Isomeric Forms of a Cyanide-Bridged Co/Fe Cluster

A series of bimetallic, trigonal bipyramidal clusters of type {[Co(N−N)2]3[Fe(CN)6]2} are reported.
The reaction of {Co(tmphen)2}2+ with [Fe(CN)6]3- in MeCN affords {[Co(tmphen)2]3[Fe(CN)6]2} (1). The
cluster can exist in three different solid-state phases:  a red crystalline phase, a blue solid phase obtained
by exposure of the red crystals to moisture, and a red solid phase obtained by desolvation of the blue solid
phase in vacuo. The properties of cluster 1 are extremely sensitive to both temperature and solvent content
in each of these phases. Variable-temperature X-ray crystallography; 57Fe Mössbauer, vibrational, and
optical spectroscopies; and magnetochemical studies were used to study the three phases of 1 and related
compounds, Na{[Co(tmphen)2]3[Fe(CN)6]2}(ClO4)2 (2), {[Co(bpy)2]3[Fe(CN)6]2}[Fe(CN)6]1/3 (3), and {[Ni(tmphen)2]3[Fe(CN)6]2} (4). The combined structural and spectroscopic investigation of 1−4 leads to the
unambiguous conclusion that 1 can exist in different electronic isomeric forms, {CoIII2CoIIFeII2} (1A), {CoIIICoII2FeIIIFeII} (1B), and {CoII3FeIII2} (1C), and that it can undergo a charge-transfer-induced spin transition
(CTIST). This is the first time that such a phenomenon has been observed for a Co/Fe molecule.