Interconversion between ladder-type octanuclear and linear tetranuclear copper(I) complexes supported by tetraphosphine ligands

Linearly ordered tetraphosphine, meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm), readily reacted with 4 equiv. of CuX in dichloromethane to afford octanuclear copper(I) complexes, [Cu₈(μ-X)₈(μ-dpmppm)₂] (X = Cl (1a), Br (1b), I (1c)), which involve a ladder-type {Cu₈(μ-X)₂(μ₃-X)₆} core supported by two dpmppm ligands in anti arrangement. Complex 1a was further transformed by treatment with Cu(OTf)₂ into [Cu₈(μ-X)₆(OTf)₂(μ-dpmppm)₂] (2) in which two TfO anions attached to both end of the Cu₈ ladder. When complexes 1 were dissolved in dimethyl sulfoxide (dmso) in the presence of 2 equiv. of dpmppm, the octacopper(I) ladder was unzipped to result in the linear tetranuclear copper(I) complex, [Cu₄X(μ-X)₃(μ-dpmppm)₂(dmso)] (X = Cl (3a), Br (3b), I (3c)). Similar treatment of dpmppm with 2 equiv. of CuX in the presence of coordinative solvents, N,N′-dimethylformamide (dmf) and pyridine (py), yielded the symmetrical tetracopper(I) complexes, [Cu₄(μ-Cl)₃(μ-dpmppm)₂(dmf)₂][CuCl₂] (4) and [Cu₄(μ-I)₃(μ-dpmppm)₂(py)₂]I (5). A series of tetracopper(I) complexes with terminal isocyanide ligands, [Cu₄(μ-X)₃(μ-dpmppm)₂(RNC)₂]PF₆ (X = Cl, R = Xyl (6a); X = Br, R = Mes (7b); X = I, R = Xyl (8a), Mes (8b), ^tBu (8c)), were obtained from reactions of dpmppm with 2 equiv. of CuX in dmf or acetonitrile followed by addition of an excess of RNC and NH₄PF₆. Complexes 3–8 possess a bending Cu₄(μ-X)₃ chain supported by two dpmppm ligands in syn arrangement, where the terminal sites are reactive and accommodate a variety of terminal ligands. When complex 8c was treated with AgPF₆ in a CH₃CN–CH₂Cl₂ mixed solvent, the soft Lewis acidic Ag(I) ions took the copper(I)-binding dpmppm off and the resultant two {Cu₄(μ-I)₃(μ-dpmppm)(^tBuNC)₂}⁺ fragments were zipped up to form the ladder-type octacopper(I) complex, [Cu₈(μ-I)₆(μ-dpmppm)₂(^tBuNC)₂](PF₆)₂ (9). The apparent interconversion between the Cu₈ ladder and the Cu₄ chain might proceed through a labile intermediate with a {Cu₄(μ-X)₃(μ-dpmppm)}⁺ unit, which could be useful synthon of further extended copper(I) halide clusters.