An Exploration of the Inorganic, Organometallic, and Catalytic Potential of [Re6(μ3-Se)8]2+ Core-Containing Cluster Derivatives: Synthesis, Characterization, Computational Analysis, and Applications

Abstract

Investigations of small molecules activated through ligation to the Lewis acidic [Re6(μ3-Se)8]2+ cluster core are detailed herein for the purpose of the development of unrealized organometallic and catalytic applications. An introduction is provided including the background needed to comprehend access to the solution chemistry, the general properties, and the current state of development of the [Re6(μ3-Se)8]2+ cluster core. Activation of nitriles is examined, specifically focusing on the site-differentiated cluster acetonitrile solvates of the general form [Re6(μ3-Se)8(PEt3)6-n(MeCN)n](SbF6)2 (n = 1,2) and the hexaacetonitrile solvate [Re6(μ3-Se)8(MeCN)6](SbF6)2. Coordination of acetonitrile to the cluster core yields susceptibility of the nitrile carbon to nucleophilic attack; thus, the formation of the alcohol adducts of these molecules, the stereochemical features, and the reactivity of the resulting imino ester complexes is established. Bifurcated hydrogen bonding between two selenium atoms of the core and the hydrogen atom of an approaching alcohol is proposed to explain the stereoselectivity. Furthermore, the synthesis, characterization, computational analysis, and reactivity of the first organometallic compounds of the [Re6(μ3-Se)8]2+ cluster core, that of the cluster carbonyls [Re6(μ3-Se)8(PEt3)6-n(CO)n](SbF6)2 (n = 1,2), are presented. Spectroscopic, electrochemical, structural, and computational studies verify that backbonding between the [Re6(μ3-Se)8]2+ cluster core and CO exists, that electronic distinction is inherent between cis and trans sites (despite overall delocalization), and that the Se atoms actively contribute to the Re bonding capabilities. Substantiation of the high air, water, thermal, and UV light stability of the carbonyl complexes led to derivatives of the carbonyl compounds, that of the oxidized cluster complex [Re6(μ3-Se)8(PEt3)5Cl](SbF6)2 and the known fused-cluster dimer [Re12Se16(PEt3)10](SbF6)4, demonstrating the reactivity of the cluster carbonyls. Electrophilicty of coordinated CO is exemplified through the synthesis of an acyl complex {Re6(μ3-Se)8(PEt3)5[C(O)(CH3)]}I, an unexpectedly stable product in the pursuit a cluster carbene. Initial investigations of this complex suggest thorough transference of the acyl negative charge to the cluster core prohibiting successive electrophilic addition. The putative acetone solvate and its reactivity to alcohols is briefly examined with aspects that merit further research, including full characterization of the catalytic cycle to liberate free imino esters, exploitation of the proposed hydrogen bonding to generate a carbene compound, and hydrogen storage/activation.