Functionalised [Ge9Ni] Clusters as Homogeneous Single-Site Catalysts for Olefin Isomerisation Reactions

Single-site catalysis is an increasingly vital strategy to optimise heterogeneous catalytic reactions. In an ideal case, the nature and the population of catalytically active sites are all identical, which is impossible to realise on a solid support. The idea of single site catalysts is transferred from heterogeneous to homogeneous catalysis by the incorporation of a transition metal with oxidation state 0 in the surface of a small, soluble, molecular, homoatomic germanium atom cluster that also comprises main group atoms with oxidation state close to zero. An optimised synthetic protocol for four cluster compounds Hyp(3)Et[Ge9Ni]PR3 (R=Ph, (p)tolyl, Pr-i, Me) is presented, in which the transition metal is embedded in Ge atoms of a polyhedral cluster. The products were characterised by NMR spectroscopy, LIFDI/MS and elemental analysis and also structurally characterised for R=Pr-i and Me by single crystal X-ray structure determination comprising a closo-[Ge9Ni] core. The catalytic potential is investigated for various olefin isomerisation reactions. The formation of an active metal site is realized by ligand dissociation, which is observed for PPh3. DFT calculations of Hyp(3)Et[Ge9Ni]PR3 (R=Ph, Pr-i, Me), Hyp(3)Et[Ge9Ni](tolyl) and Hyp(3)Et[Ge9Ni](C6H12) show that ligand exchange with substrate molecules occurs during the catalytic reaction. Since the same principle, namely a bare metal atom site at the cluster surface, applies as in single-site heterogeneous catalysis and since in contrast to classical homogeneous catalysis the metal atom is embedded in a cluster surface, the abbreviation SSHoC for Single Site Homogeneous Catalyst is proposed for this catalyst type. Since each metal atom is catalytically active, SSHoC enable a considerable increase in efficiency of catalysts and thus allow for sustainable use of expensive and less abundant transition metals.

Automated summary

Single-site catalysis is an important strategy for optimizing heterogeneous catalytic reactions. A synthetic protocol for synthesizing four cluster compounds with transition metals embedded in a polyhedral cluster has been presented. The catalytic potential of the compounds has been investigated for various olefin isomerisation reactions. The results show that these single-site homogeneous catalysts can significantly increase the efficiency of catalysts and allow for sustainable use of expensive and less abundant transition metals.