The First Structurally Characterized Discrete Dinuclear μ-Cyano Hexacyanoferrate Complex

Abstract

The complex anions hexacyanoferrate(II) and hexacyanoferrate(III) are among the oldest known coordination compounds. The reaction of Fe 3+ aq and [Fe(CN) 6 ] 4-to form the intensely colored Prussian Blue (or Fe 2+ aq with [Fe(CN) 6 ] 3-to give the same products) was employed almost 300 years ago to generate pigments for use in inks and paints. In Prussian Blue, the main structure comprises arrays of high-spin Fe 3+ centers bridged by [Fe(CN) 6 ] 4-units, 2 where a metal-to-metal charge transfer (MMCT, intervalence) transition from [Fe(CN) 6 ] 4-to Fe 3+ gives rise to its intense blue color. If the mixed valence Prussian Blue is oxidized to its Fe 3+ /[Fe(CN) 6 ] 3-analogue (Prussian Brown) or reduced to the Fe 2+ /[Fe(CN) 6 ] 4-relative (Prussian White), the intense blue color is lost. This feature has found application in electrochromic materials, 3 where a system may be switched, reversibly, between two contrasting colored states by a simple electrochemical reaction. The strong coupling between metal centers bridged by cyano ligands has also been exploited in the development of molecular magnetic materials based on ferricyanide and other transition metal analogues. 4 Despite the multitude of cyano complexes in the literature, 5 and the intense degree of interest in the field of hexacyanoferrate complexes in particular, control over the assembly of oligonuclear arrays based on [Fe(CN) 6 ] 4-/3-anions still presents a major challenge. There are six potentially bridging sites at each [Fe(CN) 6 ] 4-unit, and polymeric arrays comprising linear (1D), square (2D), or cubic (3D) structures will generally result if either ferro-or ferricyanide is mixed with a labile metal ion or complex. Kinetic 6 and spectroscopic 7 studies of discrete dinuclear, cyanobridged complexes of hexacyanoferrates have been reported, but remarkably none has been structurally characterized. In several cases, mixed valence complexes containing ferro-or ferricyanide are thermodynamically unstable toward comproportionation 8 or exhibit photolability. 9 This has perhaps hampered attempts to produce X-ray-quality single crystals of these compounds.In order to make discrete dinuclear µ-cyano complexes of ferroor ferricyanide, one must employ a metal complex bearing a single reactive coordination site. We have achieved this goal through reaction 10 of the trans-chloropentaaminecobalt(III) complex of a known 11 macrocycle L 1 with [Fe(CN) 6 ] 4-. The proposed mechanism for the formation of [L 1 Co(µ-NC)Fe(CN) 5 ] -is given in steps (i)-(iv); and is based on the fairly well understood reactions between pentaaminecobalt(III) complexes and ferrocyanide. 12 Typically, in the reactions of acyclic pentaamine analogs, the Co II complex formed in step (iii) dissociates and a precipitate of Co II 3 [Fe III (CN) 6 ] forms unless a scavenging ligand such as EDTA is present. However, the Co II intermediate in the present system is stabilized by strong binding with the macrocycle L 1 . 13The solid state infrared spectrum of Na[L 1 Co(µ-NC)Fe(CN) 5 ] exhibits thre...

Discrete Cyanide‐Bridged Mixed‐Valence Co/Fe Complexes: Outer‐Sphere Redox Behaviour

“…[18] The same sort of changes have been observed for the reduction potential of hexacyanoferrates measured at 1.0  HCl, [19] or when the {L n Co III } moiety is attached to one of the cyanide ligands. [7,8] 2514 the shifting to higher energies, as well as the loss in symmetry, due to protonation ( Figure S1, Supporting Information, see also the footnote on the first page of this article). [20,21] In order for the integrity of the complex to be held, which prevents the formation of Co 3 [Fe III (CN) 6 ] 2 , protonation of the amine groups of the macrocycle must not occur.…”

“…[7] Despite its high standard potential (2010 mV), the electron transfer mechanism of the peroxodisulfate ion prevents the very fast reaction rate that would be expected, [22] producing a k ex of ca. 10 Ϫ18  Ϫ1 s Ϫ1 by the direct application of the Marcus relationship.…”

Tuning the metal-to-metal charge transfer energy of cyano-bridged dinuclear complexes

“…[1][2][3][4] As an example, the complex anion trans-[L 14 CoNCFe(CN) 5 ] − (Chart 1) and its analogues fall into the Robin and Day 5 Class II group of mixed valence compounds, where weak to moderate electronic coupling between the metal centres is present and electronic transitions from the individual Co III and Fe II chromophores are apparent. and acceptor (Co III/II ) redox potentials i.e.…”

Outer‐Sphere Redox Reactions Leading to the Formation of Discrete CoIII/FeII Pyrazine‐Bridged Mixed‐Valence Compounds

“…The process involves the formation of an intermediate Red 1 -Oxd 2 compound, after the electron-transfer reaction indicated in Scheme 1, which undergoes an inner-sphere redox process to produce the final thermodynamically stable covalently bonded Oxd 1 -Red 2 complex. [9,22] A simple substitution process is apparent from a stoichiometric perspective, but the actual mechanism is more sophisticated and dependent on a number of variables.…”