The chemistry and the geometric and electronic structures of small naked metal clusters prepared using a rotating cryostat and studied by electron paramagnetic resonance

Abstract

A rotating cryostat has been used to generate several new naked metal clusters whose structures and reactions have been studied by EPR spectroscopy. Most alkali-metal trimers adopt an obtuse triangular geometry with a ²B₂ ground state in C_2v symmetry. A new acute Na₃ cluster was formed by interaction with water, Na₃(H₂O)_x where x 1. The first EPR spectrum of a mixed alkali-metal trimer, Na₂Li, has been recorded. The fluxionality of both Na₃ and Li₃ in an adamantane matrix is lost and a static form results with the Li located at the apical position. The first ENDOR (electron nuclear double resonance) spectrum of a naked metal cluster, Li₃, has been recorded. It gives more accurate paramagnetic parameters and shows conclusively that the cluster is a fluxional, pseudo-rotating Jahn–Teller molecule. The ENDOR spectra of Li₃(H₂O)_x complexes were also observed. A new coinage metal trimer, Au₂Ag, has been characterised by EPR spectroscopy and has the silver in the apical position in a static obtuse structure. Reactions of the alkali-metal and coinage-metal trimers with simple molecules such as water and ethene resulted in a change in geometry from an obtuse or fluxional molecule to an acute static entity. The flexible nature of clusters is discussed in the context of modelling the chemical events occurring at the active metallic sites of heterogeneous catalysts.