Electronic spectroscopy of homo- and heterometallic binuclear coinage metal phosphine complexes in isolation

Abstract

Binuclear coinage metal phosphine complexes are examined under ion trap isolation in order to elucidate their noncovalent binding, structural properties and intrinsic electronic spectra. Our survey shows an intriguing order of electronic transitions obtained by in situ synthesis and mass-spectrometrically supported UV photodissociation spectroscopy on a series of six isolated homo- and heterobinuclear complexes of type [MM′(dcpm)₂]²⁺ (M, M′ = Cu^I, Ag^I, Au^I; dcpm = bis(dicyclohexyl-phosphino)methane). This approach provides the unique opportunity to study all possible coinage metal interactions within a fixed ligand framework. A successive blue-shift (33 700–38 500 cm⁻¹; 297–260 nm) of the lowest-energy bright electronic transition energy in gas phase was observed in the order of Cu₂ < CuAu < CuAg < Au₂ < AgAu < Ag₂. This order was reproduced by quantum chemical calculations using a scalar-relativistic GW-Bethe–Salpeter-equation (GW-BSE) approach. Theory ascribes the electronic bands of all complexes to metal-centered ¹MC(dσ*–pσ) transitions revealing a strengthening of metal–metal′ (M–M′) binding upon excitation, in agreement to mass spetrometric results. A test of the correlation of transition energies with M–M′ distance by quantum chemical calculations of single point energies as a function of intermetallic distance indicates qualitative agreement with experimental results. However, the experimentally observed high sensitivity of spectroscopic shifts towards metal composition cannot be accounted for solely by M–M′ distance variation. The differences in electronic transitions are qualitatively rationalized by the varying (n + 1)s (n = 3, 4, 5) orbital contributions (increase from Cu₂via CuAu/CuAg to Au₂/AgAu/Ag₂) within the nd(n + 1)s/p-hybridization for the ground electronic state of the different complexes, whereas the excited state (of (n + 1)p orbital character) shows significantly less variation in energy. In particular, the observed spectroscopic and mass spectrometric sequence for the Ag/Au complexes is traced back to the interplay of Pauli repulsion and variation in metal–ligand bond strength within the orbital hybridization model.