The Sulfur–FluorineGaucheEffect in Coinage-Metal Complexes: Augmenting Conformational Equilibria by Complexation

Abstract

Controlling the rotation about unhindered C(sp3)–C(sp3) bonds by simple structural changes has obvious benefits in molecular design. While the avoidance of nonbonding interactions remains one of the cornerstones of acyclic conformational control, stabilizing stereoelectronic effects have the added benefit that conformer populations can be fine-tuned by augmenting or diminishing the central interaction. Strategies may include adjusting the oxidation state of a substituent or reversible formation of a complex to modulate MO levels. In the case of the sulfur–fluorine gauche effect, the propensity of the S–C–C–F motif to adopt a synclinal arrangement (ΦFCCS = 60°), the conformer population distribution of the three dominant rotamers partitioned by 120° can be biased by oxidation of the S atom. Motivated by the importance of sulfur-based ligands in main structural chemistry, the sulfur–fluorine gauche effect was translated to an organometallic paradigm as a potential tool to achieve structural preorganization. This would allow the influence of coinage-metal complexation on conformer population to be initially assessed. The synthesis and characterization of a model gold(I) and silver(I) metal complex featuring a ligand system containing a freely rotatable SCCF motif is disclosed. In both complexes, the title stereoelectronic effect manifests itself in the expected conformation, with the synclinal-endo conformer being preferred. This was corroborated by X-ray crystallography and DFT analysis, and the molar fraction of rotamers was extrapolated from a detailed solution-phase NMR spectroscopic analysis. Complexation was found to reinforce the sulfur–fluorine gauche effect.