A series of cyanide bridged Fe-Co molecular squares, [Co(2)Fe(2)(CN)(6)(tp*)(2)(dtbbpy)(4)](PF(6))(2)·2MeOH (1), [Co(2)Fe(2)(CN)(6)(tp*)(2)(bpy)(4)](PF(6))(2)·2MeOH (2), and [Co(2)Fe(2)(CN)(6)(tp)(2)(dtbbpy)(4)](PF(6))(2)·4H(2)O (3) (tp = hydrotris(pyrazol-1-yl)borate, tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, bpy =2,2'-bipyridine, dtbbpy =4,4'-di-tert-butyl-2,2'-bipyridine), were prepared by the reactions of [Fe(CN)(3)(L)](-) (L = tp or tp*) with Co(2+) and bidentate ligands (bpy or dtbbpy) in MeOH. In the molecular squares, Fe and Co ions are alternately bridged by cyanide ions, forming macrocyclic tetranuclear cores. Variable temperature X-ray structural analyses and magnetic susceptibility measurements confirmed that 1 exhibits two-step charge-transfer induced spin transitions (CTIST) centered at T(1/2) = 275 and 310 K in the solid state. The Fe and Co ions in 1 are the low-spin (LS) Fe(III) and high-spin (HS) Co(II) ions, described here in the high-temperature (HT) phase ([Fe(III)(LS2)Co(II)(HS2)]) at 330 K, while a low-temperature (LT) phase ([Fe(II)(LS2)Co(III)(LS2)]) with LS Fe(II) and Co(III) ions was dominant below 260 K. X-ray structural analysis revealed that in the intermediate (IM) phase at 298 K 1 exhibits positional ordering of [Fe(III)(LS2)Co(II)(HS2)] and [Fe(II)(LS2)Co(III)(LS2)] species with the 2:2 ratio. In photomagnetic experiments on 1, light-induced CTIST from the LT to the HT phase was observed by excitation of Fe(II) → Co(III) intervalence charge transfer (IVCT) band at 5 K and the trapped HT phase thermally relaxed to the LT phase in a two-step fashion. On the other hand, 2 and 3 are in the HT and LT phases, respectively, throughout the entire temperature range measured, and no CTIST was observed. UV-vis-NIR absorption spectral measurements and cyclic voltammetry in solution revealed that the different electronic states in 1-3 are ascribable to the destabilization of iron and cobalt ion d-orbitals by the introduction of methyl and tert-butyl groups to the ligands tp and bpy, respectively. Temperature dependence of UV-vis-NIR spectra confirmed that 1 exhibited a one-step CTIST in butyronitrile, of which T(1/2) varied from 227 to 280 K upon the addition of trifluoroacetic acid.

A cyanide-bridged molecular square, [Co 2 Fe 2 (CN) 6 (tp*) 2 -(dtbbpy) 4 ](PF 6 ) 2 ¢2MeOH (1) (tp* = hydrotris(3,5-dimethylpyrazol-1-yl) borate and dtbbpy = 4,4¤-di-tert-butyl-2,2¤-bipyridine) was prepared by the reaction of (Bu 4 N) [Fe(CN) 3 (tp*)] with Co(CF 3 SO 3 ) 2 ¢6H 2 O and dtbbpy in methanol. Variable temperature X-ray structural analyses, magnetic and spectroscopic data revealed that 1 exhibited novel thermal two-stepped spin transitions induced by intramolecular electron transfers.Prussian blue analogs (PBAs) are 3-D bulk materials with face-centered cubic structures, where metal ions with various electronic and oxidation states are bridged by cyanide ions. Cyanide ions mediate electronic and magnetic interactions between metal ions, some of which allow PBA to exhibit fruitful physical properties, such as high T c magnets, spincrossover, linkage isomerism, and ferroelectric properties. 3 On the other hand, cyanide-bridged multinuclear complexes have core structures similar to a constituent unit of PBA, and such complexes have potential to show specific molecular functions, such as single-molecular magnetism, multistepped spin-crossover, and multistepped redox behavior. 4Dunbar and co-workers have reported the first observation of gradual CTIST behavior in a discrete molecule, that is, a cyanide-bridged pentanuclear FeCo cluster with a trigonalbipyramidal core. 5 Although there are few reports on other complexes exhibiting one-step CTISTs, 6 to the best of our knowledge, there have been no reports of the two-stepped CTIST behavior in both bulk and discrete molecular systems. We report here a novel cyanide-bridged molecular square of [Co 2 Fe 2 (CN) 6 (tp*) 2 (dtbbpy) 4 ](PF 6 ) 2 ¢2MeOH (1) (tp* = hydrotris(3,5-dimethylpyrazol-1-yl) borate, dtbbpy = 4,4¤-di-tert-butyl-2,2¤-bipyridine), exhibiting a two-stepped CTIST.1 was prepared by the reaction of (Bu 4 N)[Fe III (CN) 3 (tp*)] with Co II (CF 3 SO 3 ) 2 ¢6H 2 O and dtbbpy in methanol. 7 X-ray crystal structure analysis for 1 was performed at 100 K and an ORTEP diagram of the complex cation is depicted in Figure 1. 8 1 has a square-shaped macrocyclic core, in which Fe and Co ions are alternately bridged by cyanide ions. 1 crystallized in a monoclinic space group C2/c and the asymmetric unit is composed of half of the complex cation, [(tp*)Fe(CN) 3 Co-(dtbbpy) 2 ]. The Fe ions are coordinated by tridentate tp* with facial configuration and the remaining coordination sites are occupied by three cyanide carbon atoms.The two bidentate dtbbpy ligands coordinate to the Co ion, and bridging cyanide ions in cis-positions are linked to the neighboring [(tp*)Fe] units. The coordination bond lengths about the Co ion at 100 K are in the range of 1.892(7) 1.944(6) ¡, characteristic of LS Co(III) ions. The average coordination bond length around the Fe centers is 1.959(7) ¡, which corresponds to a LS Fe(II) or LS Fe(III) ion. Although the assignments of oxidation states of LS Fe(II) and Fe(III) ions are difficult only from the structural data, Mössbauer measu...