Structural rearrangements induced by acid–base reactions in metal carbonyl clusters: the case of [H3−nCo15Pd9C3(CO)38]n− (n = 0–3)

Abstract

The new bimetallic [HCo₁₅Pd₉C₃(CO)₃₈]²⁻ tri-carbide carbonyl cluster has been obtained from the reaction of [H₂Co₂₀Pd₁₆C₄(CO)₄₈]⁴⁻ with an excess of acid in CH₂Cl₂ solution. The mono-hydride di-anion can be reversibly protonated and deprotonated by means of acid–base reactions leading to closely related [H_3−nCo₁₅Pd₉C₃(CO)₃₈]ⁿ⁻ (n = 0–3) clusters. The crystal structures of the three anionic and the neutral clusters have been determined as their H₃Co₁₅Pd₉C₃(CO)₃₈·2thf, [NEt₄][H₂Co₁₅Pd₉C₃(CO)₃₈]·0.5C₆H₁₄, [NMe₃(CH₂Ph)]₂[HCo₁₅Pd₉C₃(CO)₃₈]·C₆H₁₄ and [NEt₄]₃[Co₁₅Pd₉C₃(CO)₃₈]·thf salts. They are composed of a Pd₉(μ₃-CO)₂ core stabilised by three Co₅C(CO)₁₂ organometallic fragments. The poly-hydride nature of these clusters has been indirectly inferred via chemical, electrochemical and magnetic measurements. Besides, cyclic voltammetry shows that the [H_3−nCo₁₅Pd₉C₃(CO)₃₈]ⁿ⁻ (n = 1–3) anions are multivalent, since they undergo two or three reversible oxidations. SQUID measurements of [HCo₁₅Pd₉C₃(CO)₃₈]²⁻ indicate that this even electron cluster is paramagnetic with two unpaired electrons, giving further support to its hydride nature. Finally, structural studies show that the Pd₉ core of [H_3−nCo₁₅Pd₉C₃(CO)₃₈]ⁿ⁻ (n = 0,1) is a tri-capped octahedron, which becomes a tri-capped trigonal prism in the more charged [H_3−nCo₁₅Pd₉C₃(CO)₃₈]ⁿ⁻ (n = 2,3) anions. Such a significant structural rearrangement of the metal core of a large carbonyl cluster upon protonation–deprotonation reactions is unprecedented in cluster chemistry, and suggests that interstitial hydrides may have relevant stereochemical effects even in large carbonyl clusters.