Diamond-shaped heterometallic complexes of iron(II) and copper(I) bridged by cyanide groups containing monodentate or bidentate phosphanes bound to copper(I), including an alternative structure based on the nature of the bidentate phosphane ligand

The diamond-shaped heterometallic cyanide-bridged complex of iron(II) and copper(I), [CpFe(CO)(μ-CN)₂Cu(CH₃CN)₂]₂ (1), has been synthesized from the reaction of K[CpFe(CO)(CN)₂] and [Cu(CH₃CN)₄][BF₄] in acetonitrile. Upon the addition of 2-4 equivalents of phosphane ligands to complex 1, the coordination geometry of the copper centers can be varied from trigonal planar as in [CpFe(CO)(μ-CN)₂Cu(PCy₃)]₂ (2), to tetrahedral as found in [CpFe(CO)(μ-CN)₂Cu(PCy₃)₂]₂ (3). Analogous derivatives containing other phosphane ligands, namely PMe₃, P(p-to-lyl)₃, PMe₂Ph, and PPh₂Me have been synthesized as well. Prolonged exposure of complex 3 in CH₂Cl₂ to excess PCy₃ has provided the bimetallic complex [CpFe(PCy₃)(CN)(μ-CN)Cu(PCy₃)₂] (4), where the metal aggregate is disrupted and PCy₃ has displaced CO at the iron center. Bidentate phosphane analogs of complex 3 have been prepared by adding two equivalents of dcpe [bis(dicyclohexylphosphanyl)-ethane] or dcpp [bis(dicyclohexylphosphanyl)propane] to complex 1. The solid-state structures of several of these diamond-shaped derivatives, in addition to complex 4, have been determined by X-ray crystallography. The overlapping diamond-shaped [Fe₂(CN)₄Cu₂] cores, which form channels within the solids, are blocked by bulky, copper-bound phosphane ligands such as PCy₃. However, in the case of the small phosphane PMe₃, or the chelating phosphanes dcpe and dcpp, these channels are not blocked by the phosphane ligands; solvent molecules occupy the channels created by the overlapping metal cores instead. Alternatively, the complex [CpFe(CO)(CN)(μ-CN)Cu(dcpm)]₂ (13), derived from bis(dicyclohexylphosphanyl)methane(dcpm), possesses two copper(I) centers, each in a distorted trigonal coordination geometry, bridged by two dcpm ligands to form an eight-membered metallacycle with a short Cu^I⋯Cu^I separation of 2.844 Å. The third ligand completing each copper's coordination sphere is a nitrogen-bound bridging cyanide group from a CpFe(CO)(CN)₂^- anion.