Cyanide-Bridged Fe−Fe and Fe−Co Molecular Squares: Structures and Electrochemistry of [Fe(μ-CN)4(bpy)8](PF6)4⋅4 H2O, [FeCo(μ-CN)4(bpy)8](PF6)4⋅3 CHCl3⋅2 CH3CN, and [FeCo(μ-CN)4(bpy)8](PF6)6⋅2 CHCl3⋅4 CH3NO2

Abstract

Cyanide-bridged iron-iron and iron--cobalt molecular squares of [Fe(II/4)(mu-CN)4(bpy)8[(PF6)4 x 4H2O (1), [Fe(II/2)Co(II/2)(mu-CN)4(bpy)8](PF6)4 x 3CHCl3 x 2CH3CN (2), and [Fe(II/2)Co(III/2)(mu-CN)4(bpy)8](PF6)6 x 2CHCl3 x 4CH3NO2 (3) (bpy =2,2'-bipyridine) were prepared. X-ray structure analyses for 1-3 were performed and their electrochemistry was studied. In 1-3, four metal ions are bridged by cyanide groups to form tetranuclear macrocycles ("molecular squares"). Each metal ion in the square is six-coordinate: four of the coordination sites are occupied by the nitrogen atoms of two of bpy ligands and the remaining cis coordination sites are occupied by cyanide-carbon or cyanide-nitrogen atoms. In 1, Fe-C (cyanide) (1.899(4)-1.927(4)A) and Fe-N(cyanide) (1.929(4)-1.950(4)A) distances are typical of low-spin Fe2+ ions. In 2, Fe-C(cyanide) and Co(2+)-N(cyanide) bond lengths are in the range 1.919(5)-1.963(5)A and 1.850(5)-2.017(5) A, respectively: in contrast, shorter bond lengths are observed for the metal to cyanide-carbon and cyanide-nitrogen (1.878(7)- 1.893(7) A) in 3. As a result, the molecular squares in 1. 2, and 3 have sides of 4.947(1)4.986(1) A, 5.001(1)-5.053(1) A, and 4.910(1)-4.918(1) A, respectively. Magnetic susceptibility measurements revealed that the Fe2+ and Co3- ions in 1 and 3 are diamagnetic, while the high-spin Co2+ ions in 2 are weakly coupled through the low-spin Fe2 ions. Cyclic voltammograms of the squares are presented, and the electrochemically generated mixed-valent species [Fe(II/2)Fe(III/2)(mu-CN)4(bpy)8]6+ was discussed in terms of the intervalence transfer band.

Kineticomechanistic Study of the Redox pH Cycling Processes Occurring on a Robust Water-Soluble Cyanido-Bridged Mixed-Valence {CoIII/FeII}2 Square

“…The distances between adjacent Fe and Co centers through the cyanide linkers are very similar [4.936(2) and 4.956(2) Å], and the angles Fe1–Co1–Fe1 and Co1–Fe1–Co1 are 88.47(1) and 91.53(1)°, respectively, thus displaying a regular square macrocyclic core, in good agreement with other reported examples of squares containing {Co III –NC–Fe II } edges. ,, The coordination bond lengths about the Co ion are in the range of 1.909(3)–1.989(3) Å, characteristic of low-spin (LS) Co III ions . The Fe–C distances are within 1.880(4) and 1.921(4) Å, confirming the LS character of the Fe ions present in the compound and its divalent oxidation state .…”

Temperature dependence of spherical electron transfer in a nanosized [Fe14] complex

“…The best fit of the experimental data yielded U eff = 943(47) cm −1 with a pre-exponential factor of τ 0 = 5.0(16) × 10 −11 s. Hence, the temperature dependence of the rates of intramolecular electron transfer in the Mössbauer spectroscopic analysis clearly demonstrated that six B-site Fe ions form a class II mixed-valence system according to the Robin–Day classification 29 . Electronic spectroscopy in the domain of intervalence transitions exhibits a wide peak centered at 11,100 cm −1 (900 nm) with a pronounced absorption tail due to the superposition of adjacent IVCT (Fe II -CN-Fe III ) and remote IVCT (Fe II -NC-Fe II -CN-Fe III ), as indicated in the reported cyanide-bridged systems with N-terminal mixed-valence state 24,30,31 . (Supplementary Fig.…”

Stimuli-responsive magnetic materials: impact of spin and electronic modulation

“…2) began with the Oshio et al report in 2000, where a series of cyanide bridged [Fe 2 Co 2 ] and [Fe 2 Fe 2 ] tetrametallic molecular squares were reported without having any bistability. 85 In 2004, Dunbar et al reported a breakthrough in which the first-ever cyanide-bridged discrete Fe/Co pentanuclear [Fe 2 Co 3 ] system where reversible thermo-induced [(Co II HS ) 3 (Fe III LS ) 2 ] ↔ [(Co II HS ) 2 Co III LS Fe II LS Fe III LS ] MMET was observed. 76,77 Later, in 2011, Clérac, Mathonière, and Dunbar et al established modest photomagnetic activity of this [Fe 2 Co 3 ] system upon exposure to moisture, leading to a partial conversion from [(Co II HS ) 2 Co III LS Fe II LS Fe III LS ] ↔ [(Co II HS ) 3 (Fe III LS ) 2 ].…”