Halogen Bonding Involving Gold Nucleophiles in Different Oxidation States.

A single-crystal X-ray diffraction (XRD) study of diaryliodonium tetrachloroaurates (or, in the recent terminology, tetrachloridoaurates), [( p -XC 6 H 4 ) 2 I][AuCl 4 ] (X = Cl, 1 ; Br, 2 ), was performed for 1 (the structure is denoted as 1a to show similarity with the isomorphic structure 2a ) and two polymorphs─ 2a (obtained from MeOH) and 2b (from 1,2-C 2 H 4 Cl 2 ). Examination of the XRD data for these three structures revealed 2-center C-X···Au III (X = Cl and Br) and 3-center bifurcated C-Br···( Cl - Au ) halogen bonding (abbreviated as XB) between the p -Cl or p -Br atoms of the diaryliodonium cations and the gold(III) atom of [AuCl 4 ] - . The noncovalent nature of Au III -involving interactions, the nucleophilicity of the gold(III) atoms, and the electrophilic role of p -X atoms of the diaryliodonium cations in the XBs were studied by a set of complementary computational methods. Combined experimental and theoretical studies allowed the recognition of the d-nucleophilicity of the [d 8 Au III ] atom which, regardless of its rather substantial formal 3+ charge, can function as a d-nucleophilic partner of XB. This conclusion was also supported by theoretical calculations performed for the structures' refcodes BINXOM and ICSD 62511; the obtained data verified the nucleophilicity of Au III toward a K + ions or a σ-(Cl)-hole, respectively. All our results, together with consideration of relevant literature, indicate that gold atoms in the three oxidation states (0, I, and even III) exhibit nucleophilicity in XBs.