Ni–Cu tetracarbide carbonyls with vacant Ni(CO) fragments as borderline compounds between molecular and quasi-molecular clusters

Abstract

The reaction of [Ni₉C(CO)₁₇]²⁻ with [Cu(CH₃CN)₄][BF₄] (1.1–1.5 equiv.) afforded the first Ni–Cu carbide carbonyl cluster, i.e., [H₂Ni₃₀C₄(CO)₃₄{Cu(CH₃CN)}₂]⁴⁻ ([H₂1]⁴⁻). This has been crystallised in a pure form with miscellaneous [NR₄]⁺ (R = Me, Et) cations, as well as co-crystallised with [H₂Ni₂₉C₄(CO)₃₃{Cu(CH₃CN)}₂]⁴⁻ ([H₂2]⁴⁻) which differs from [H₂1]⁴⁻ by a missing Ni(CO) fragment. By increasing the [Cu(CH₃CN)₄]⁺/[Ni₉C(CO)₁₇]²⁻ ratio to 1.7–1.8, the closely related [H₂Ni₃₀C₄(CO)₃₅{Cu(CH₃CN)}₂]²⁻ ([H₂3]²⁻), [H₂Ni₂₉C₄(CO)₃₄{Cu(CH₃CN)}₂]²⁻ ([H₂4]²⁻), and [H₂Ni₂₉C₄(CO)₃₂(CH₃CN)₂{Cu(CH₃CN)}₂]²⁻ ([H₂5]²⁻) dianions have been obtained. Replacement of Cu-bonded CH₃CN with p-NCC₆H₄CN afforded, after protonation of the tetra-anion, mixtures of [H₃Ni₃₀C₄(CO)₃₄{Cu(NCC₆H₄CN)}₂]³⁻ ([H₃6]³⁻) and [H₃Ni₂₉C₄(CO)₃₃{Cu(NCC₆H₄CN)}₂]³⁻ ([H₃7]³⁻). The species 1–7 display a common Ni₂₈C₄Cu₂ core and differ for the charge, the presence of additional Ni atoms, the number and nature of the ligands, even though they are obtained under similar experimental conditions and often in mixtures. Their nature in solution has been investigated via FT-IR, chemical and electrochemical methods. Electrochemical studies, besides confirming the poly-hydride nature of these clusters, show that they undergo different redox processes with features of chemical reversibility, and this might be taken as proof of the incipient metallisation of their metal cores.