Lewis acid enhancement by juxtaposition with an onium ion: the case of a mercury stibonium complex

Abstract

While diarylmercury derivatives (Ar₂Hg) are usually not Lewis acidic, we have recently observed that bis(μ-1,8-naphthalenediyl)mercury(II)(bisphenylstibonium(V)) ([2]⁺), a compound that possesses a Ar₂Hg moiety flanked on one of its sides by a stibonium unit, readily binds bromide or iodide ligands at the mercury center. To further investigate this behavior and understand its origin, we now report a series of results dealing with the coordination chemistry of [2]⁺. In particular, we show that this cation interacts with neutral donor ligands such as THF and DMAP to afford [2-THF]⁺ and [2-DMAP₃]⁺, respectively, which have been isolated as [PF₆]⁻ salts. ¹H and ¹⁹⁹Hg NMR titration experiments carried out in DMSO-d₆ indicate that the mercury center of [2]⁺ engages heavy halide anions to afford the corresponding complexes 2-Cl, 2-Br and 2-I whose stability constants are equal to 1890 (±10) M⁻¹, 500 (±10) M⁻¹, and 145 (±5) M⁻¹, respectively. In the case of chloride, binding of a second halide ligand at antimony is observed leading to [2-Cl₂]⁻ which has been characterized as a [nBu₄N]⁺ salt. Results obtained from titrating [2]⁺ against F⁻ also indicate the formation of a complex, albeit with antimony as the primary anion binding site. Although the short Hg–Sb distances observed in these complexes (3.04–3.09 Å) remains essentially invariant, NBO calculations show a distinct strengthening of a 6s(Hg)→σ*(Sb–C) donor–acceptor interaction upon coordination of a halide to the mercury center. These NBO results also reveal weak 5d(Hg)→σ*(Sb–C) dative interactions which, as suggested by Hg L₃ and Sb K-edge XANES measurements, are too weak to induce a measurable oxidation of the mercury center. In turn, we conclude that the enhanced Lewis acidity of the diarylmercury unit of [2]⁺ results from the presence of the stibonium moiety which provides a Coulombic pull for the coordination of Lewis bases while also drawing electron density away from the mercury atom via relatively weak orbital interactions.