Theoretical Understanding of Reactions of Rhenium and Ruthenium Tris(thiolate) Complexes with Unsaturated Hydrocarbons: Noninnocent Nature of the Ligand, Mechanism, and Origin of Differential Reactivity

Abstract

In retrospect to the complexity induced by the noninnocent ligands in identifying the transition metal's oxidation state and correlating the ligand's noninnocence with reactivity, the reactions of alkene/alkyne addition to rhenium/ruthenium tris(thiolate) complexes are particularly good cases for shedding light on the chemistry of the dppbt ligand, including its noninnocent nature, ligand-centered mechanism, and origin of differential reactivity. Density functional theory (DFT) combined with the high-level ab initio calculations performed herein demonstrates that, upon alkene/alkyne addition, the orbital symmetry properly regulates the reaction to form ligand-centered cis-interligand dithioethers as the most favorable pathway. The neutral and cationic Re and Ru dithioethers are revealed via DFT calculations to be in a low-spin ground state; on the contrary, high-level ab initio methods confirm that the dicationic Re-dithioethers exhibit obvious multireference character with antiferromagnetic coupling between Re-dyz and S1-py. The metal-stabilized thiyl radicals play a pivotal role in delivering the reactivity of [RuL3]+ and [ReL3]+/2+ toward alkene/alkyne rather than [ReL3], where [RuL3]+ and [ReL3]+/2+ present significant radical characters on ligand S2, yet neutral [ReL3] has little such feature, from which differential reactivity arises. Faster styrene addition to Ru tris(thiolate) in contrast to Re tris(thiolate) has been properly interpreted using DFT calculations with major products assigned. The deeper understanding gained in this work would illuminate further experimental exploration in adding alkene/alkyne to other metal-stabilized thiyl radicals.