Synthesis, Characterization, and Reactivity of Cyclometalated Gold(III) Dihalide Complexes in Aqua Regia

Abstract

A range of N,C‐chelated, cyclometalated gold(III) complexes Au(ppyR)X2 have been prepared and characterized by spectroscopic, crystallographic, and computational means. Here, ppyH is 2‐phenylpyridine dicarboxylic acid (series 1), ppyEt is diethyl 2‐phenylpyridine dicarboxylate (series 2), and X is trifluoroacetate OAcF (a), Cl (b), Br (c), or I (d) anion. The dihalo complexes 1b‐d and 2b‐d are obtained when Au(ppyR)(OAcF)2 (1a and 2a) are treated with HNO3/HX mixtures (aqua regiaX). Good to high yields are obtained with short reaction times (< 30 min) and simple work‐up. Notably, the strongly acidic medium does not cause protolytic cleavage of the Au–C or Au–N bonds in the chelate, nor is ester hydrolysis of complexes 2b‐d seen. Ethylene inserts into an Au–O bond of 1a and 1b, and the resulting trifluoroacetoxyethyl–Au complexes can be further elaborated in aqua regia without cleavage of the two Au–C bonds in the molecule. Facile, mutual halide exchange reactions between complexes with different halides (1b and 1d, 1c and 1d) were observed and led to formation of mixed‐halide complexes Au(ppyH)(X)(Y). These exchange reactions occurred with complete stereoselectivity. The stereoisomer produced was the one expected based on the relative trans influence of the halides (I > Br > Cl), i.e. the highest trans influence halide was located trans to N which is the lowest trans influence end of the chelate. These thermodynamic preferences were also investigated by DFT computations.