Synthesis and Structural Characterization of Double Metal Cyanides of Iron and Zinc: Catalyst Precursors for the Copolymerization of Carbon Dioxide and Epoxides

Abstract

Several synthetic approaches for the preparation of double metal cyanide (DMC) derivatives of iron(II) and zinc(II) are described. These include (1) metathesis reactions of ZnCl₂ or ZnI₂ with KCpFe(CN)₂CO in aqueous solution, (2) reactions of KCpFe(CN)₂CO and its phosphine-substituted analogues with Zn(CH₃CN)₄(BF₄)₂ and subsequent displacement of acetonitrile at the zinc centers by the addition of a neutral (phosphine) or anionic (phenoxide) ligand, and (3) reactions of the protonated HCpFe(CN)₂(phosphine) complexes with Zn(N(SiMe₃)₂)₂, followed by the addition of phenols. All structures are based on a diamond-shaped planar arrangement of the Fe₂(CN)₄Zn₂ core with various appended ligands at the metal sites. Although attempts to replace the iodide ligands in [CpFe(μ-CN)₂PPh₃ZnI(THF)]₂ with acetate using silver acetate failed, two novel cationic mixed-metal cyanide salts based on the [CpFe(PPh₃)(μ-CN)₂Zn(NC₅H₅)]₂²⁺ framework were isolated from pyridine solution and their structures were defined by X-ray crystallography. The anionic ligand bound to zinc in these derivatives, which serve as an anionic polymerization initiator, was shown to be central to the catalytic copolymerization reaction of CO₂/epoxide to provide polycarbonates and cyclic carbonates. The structurally stabilized phosphine-strapped complexes [CpFe(μ-CN)₂Zn(X)THF]₂(μ-dppp), where X = I or phenolate, were shown to be thermally stable under the conditions (80 °C) of the copolymerization reaction by in situ infrared spectroscopy. Both of these derivatives were proposed to serve as mimics for the heterogeneous DMC catalysts in the patent literature, with the derivative where the initiator is a phenolate being more active for the production of polycarbonates.