Vibrational Spectroscopy Reveals Varying Structural Motifs in Cu+(CH4)n and Ag+(CH4)n (n = 1-6)

Abstract

Vibrational spectra are measured for Cu+(CH<inf>4</inf>)(Ar)<inf>2</inf>, Cu+(CH<inf>4</inf>)<inf>2</inf>(Ar), Cu+(CH<inf>4</inf>)<inf>n</inf> (n = 3-6), and Ag+(CH<inf>4</inf>)<inf>n</inf> (n = 1-6) in the C-H stretching region (2500-3100 cm-1) using photofragment spectroscopy. Spectra are obtained by monitoring loss of Ar or CH<inf>4</inf>. Interaction with the metal ion produces substantial red shifts in the C-H stretches of proximate hydrogens. The magnitude of the shift reflects the metal-methane distance and the coordination to the metal ion of the methane hydrogens (?2 or ?3). The structures of the complexes are determined by comparing the measured spectra with spectra calculated for candidate geometries using the B3LYP and CAM-B3LYP density functionals with 6-311++G(3df,3pd) and aug-cc-pVTZ-PP basis sets. Because of the d10 electronic configuration of the metal ions, the complexes are expected to adopt symmetric structures, which is confirmed by the experiments. All of the complexes have ?2 hydrogen coordination in the first shell, in accord with theoretical predictions; second-shell ligands sometimes show ?3 hydrogen coordination. The vibrational spectrum of Cu+(CH<inf>4</inf>)(Ar)<inf>2</inf> shows extensive structure due to Fermi resonance between the lowest-frequency C-H stretch and overtones of the H-C-H bends. The Cu+(CH<inf>4</inf>) cluster has a smaller red shift in the lowest-frequency C-H stretch than M+(CH<inf>4</inf>), M+ = Co+ (d8) and Ni+ (d9). Although all three ions have similar binding energies, the metal-ligand electrostatic interaction is largest for Cu+, while the contribution from covalent interactions is largest for Co+. The larger ionic radius of Ag+ leads to a larger metal-ligand distance and weaker interaction, resulting in substantially smaller red shifts than in the Cu+ complexes. The Cu+(CH<inf>4</inf>)<inf>2</inf> and Ag+(CH<inf>4</inf>)<inf>2</inf> clusters have symmetrical structures, with the methanes on opposite sides of the metal, while Cu+(CH<inf>4</inf>)<inf>3</inf> and Ag+(CH<inf>4</inf>)<inf>3</inf> adopt symmetrical, trigonal planar structures with all M-C distances equal. For Cu+(CH<inf>4</inf>)<inf>4</inf>, the tetrahedral structure dominates the observed spectrum, although a trigonal pyramidal structure may contribute; however, only the tetrahedral structure is observed for Ag+(CH<inf>4</inf>)<inf>4</inf>. The structures of Cu+(CH<inf>4</inf>)<inf>n</inf> and Ag+(CH<inf>4</inf>)<inf>n</inf> differ for clusters with n > 4. For copper complexes, these are primarily formed by adding outer-shell methane ligand(s) to the tetrahedral n = 4 core. The observed spectra of the larger Ag+ clusters are dominated by symmetrical structures in which all of the Ag-C distances are similar: Ag+(CH<inf>4</inf>)<inf>5</inf> has a trigonal bipyramidal geometry and Ag+(CH<inf>4</inf>)<inf>6</inf> is octahedral. © 2019 Elsevier B.V., All rights reserved.