Infrared Spectroscopy of Zn(Acetylene)⁺ Complexes: Ligand Activation and Nascent Polymerization

Zinc-acetylene ion-molecule complexes were produced by laser vaporization in a supersonic expansion. These complexes were mass selected and studied with infrared laser photodissociation spectroscopy complemented by computational chemistry. The combined approach of infrared spectroscopy and theory provides information on the structures and bonding of these complexes, as well as evidence for intracluster reactions. Fragmentation patterns demonstrate that the coordination number of strongly bonded ligands is three. Infrared spectra compared to those predicted by theory allow identification of different isomers at each cluster size. The coordination in these complexes varies between η² and η¹ metal-acetylene connections. Structures based on η² bonding form a symmetric D₃ configuration for the n = 3 complex. This unreactive core ion forms larger clusters with only weakly bonded acetylene in solvation structures. Structures based on three η¹-bonded acetylenes form a near-C₃ core ion which is the doorway configuration for subsequent reactions. Electron transfer to the next (fourth) acetylene produces a metal-carbon bond and a trans-bent metal-vinyl structure with a terminal radical site. This radical site attaches a fifth acetylene to produce a vinyl-dimer structure. Evidence for continued reactions in the larger clusters is obscured by solvating acetylenes with more intense IR bands. The asymmetric coordination of zinc cations and the critical configuration with three-fold coordination that leads to reactivity are new features of intracluster metal-molecular reactions.