σ‑Donor/Acceptor-Confused Ligands: The Case of a Chlorostibine

In
search for new examples of σ-acceptor ligands, we have investigated
the tridentate ligands (o-(iPr2P)­C6H4)2SbPh) (LPh) and (o-(iPr2P)­C6H4)2SbCl) (LCl) which react
with (tht)­AuCl (tht = tetrahydrothiophene) to afford LPhAuCl (1) and LClAuCl (2), respectively.
As suggested by the structure of these complexes, which confirm complexation
of the SbP2 ligands to the gold chloride fragment, and
in agreement with the results of the density functional theory (DFT)
and natural bond orbital (NBO) calculations, the gold and antimony
atom of 1 and 2 are involved in a Au→Sb
donor–acceptor interaction. The magnitude of this interaction
is higher in complex 2 which possesses a chlorinated
and thus more Lewis acidic antimony center. We have also compared
the strength of the Au→Sb interaction present in 2 with the Au→Bi interaction observed in the newly prepared
bismuth analogue [(o-(iPr2P)­C6H4)2BiCl]­AuCl (3). This comparison reveals that 2 possesses a stronger
Au→Pn bond (Pn = pnictogen), an observation reconciled by invoking
the greater Lewis acidity of antimony­(III) halides. Finally, complexes 1 and 2 undergo a clean antimony-centered oxidation
when treated with ortho-chloranyl. These oxidation
reactions afford complexes [(o-(iPr2P)­C6H4)2(o-C6Cl4O2)­SbPh]­AuCl (5) and [(o-(iPr2P)­C6H4)2(o-C6Cl4O2)­SbCl]­AuCl (6). Structural
and computational studies of 5 show that the Au→Sb
bond becomes shorter and more covalent upon oxidation of the antimony
atom. Although the structure of 6 has not been experimentally
determined, spectroscopic and computational results show a similar
effect in this complex. Complex 5 and 6 constitute
rare examples of metalated six coordinate antimony compounds.