Synthesis and characterization of metal cyanide and heterometallic cyanide-bridged complexes: Applications in biology and chemistry

Metal cyanide complexes have been extensively investigative for many years. The most well known compound, Prussian blue, was discovered over three centuries ago. There are numerous applications of metal cyanide complexes ranging from enzyme systems such as hydrogenases to material science such as polymer catalysts. Recently, we have been able to study the role of the Fe(CO)(CN)₂ unit in [NiFe]hydrogenase via a small organometallic molecule, [K][CpFe(CO)(CN)₂] (Cp = η 5-C₅H₅). Also, Rauchfuss and coworkers have successfully synthesized “molecular boxes” using CpCo(CN) ₃− or Cp*Rh(CN)₃− (Cp* = η5-C₅Me₅). In addition, double metal cyanide complexes can serve as the catalysts for the homopolymerization of epoxide or the copolymerization of CO₂ and epoxides. The applications involving both biology and chemistry draw our attention to the synthesis of iron cyanide complexes, such as CpFe(CO)₂(CN), [K][CpFe(CO)(CN) ₂], and [K]₂[CpFe(CN)₃], as well as their Cp* derivatives, and heterometallic complexes of iron(II) and copper(I) bridged by cyanide ligands. These complexes have been characterized by infrared, 31P NMR, and X-ray crystallography wherever possible.; The nearly identical infrared spectra, similar isotopic label frequency shifts, and X-ray structural matched Fe(CO)(CN)₂ units of the oxidized forms of [NiFe]hydrogenase and [K][CpFe(CO)(CN)₂] illustrate that CpFe(CO)(CN)₂− is a effective model complex for [NiFe]hydrogenase. In addition, diamond-shaped complexes comprised of a Fe₂Cu₂ array, which contains two iron(II) and copper(I) atoms and four bridged cyanide ligands, and a variety of phosphines on copper centers were synthesized using [K][CpFe(CO)(CN)₂]. A channel is created by the overlapping diamonds within the solid, and solvent molecules can occupy the channel when the diamond-shaped core is not blocked by phosphine ligands. Substitution of carbonyl by phosphine on iron centers accompanying with the cleavage of CN-Cu bond to form a bimetallic species occurred as excess phosphine per copper was added. Similar bimetallic monocyanide bridged complexes were synthesized by the reaction of CpFe(CO)₂(CN) and [Cu(CH ₃CN)₄][BF₄] in dichloromethane followed by adding two equiv of PCy₃. Equilibrium between CN bound Cu(I) and uncomplexed Cu(I) species was observed when the reaction was performed in acetonitrile. Unlike the hemecyanide-copper complexes synthesized by R. H. Holm, linkage isomerization was not discovered in the solid-state structure of the bimetallic complex, [CpFe(CO)₂(μ-CN)Cu(PCy₃)₂][BF ₄].