Lewis acid enhancement by juxtaposition with an onium ion: the case of a mercury stibonium complex
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While diarylmercury derivatives (Ar2Hg) are usually not Lewis acidic, we have recently observed that bis(μ-1,8-naphthalenediyl)mercury(II) (bisphenylstibonium(v)) (+), a compound that possesses a Ar 2Hg 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 +. 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 + 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 + 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 + 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. © 2012 The Royal Society of Chemistry.
author list (cited authors)
Lin, T., Nelson, R. C., Wu, T., Miller, J. T., & Gabbaï, F. P.