Synthesis and characterization of gold(iii) complexes possessing 2,9-dialkylphenanthroline ligands: To bind or not to bind?

In an effort to discover potential alternatives to the anti-cancer drug cisplatin, the synthesis of gold(iii) polypyridyl coordination complexes was pursued. Specifically, this report describes the synthesis and characterization of a series of 2,9-dialkyl-1,10-phenanthroline (^R phen) gold(iii) coordination complexes (R = n-butyl, sec-butyl, and tert-butyl). Due to the steric hindrance imparted by the alkyl substituents, these ligands do not react with HAuCl₄ to form square-planar gold(iii) dichloride complex ions, as is the case with 1,10-phenanthroline, but instead form salts comprised of [AuCl₄]⁻ anions and protonated 2,9-dialkylphenanthroline cations (compounds 1 and 2). In an effort to facilitate direct binding between the substituted phenanthroline and the gold(iii) metal center, reactions were carried out between the ligand and NaAuCl₄ in the presence of a Ag(i) salt. The precipitation of one equivalent of AgCl afforded the formation of neutral, distorted square-pyramidal gold(iii) trichloride complexes (compounds 3 and 4). Primary or secondary substitutions at the alpha carbon of the alkyl substituent allow direct metal–ligand coordination, whereas a tertiary substituent inhibits chelation and results only in the formation of a salt comprised of a protonated phenanthroline cation and a [AuCl₂]⁻ anion (compound 5). Compounds 1–4 have been characterized by¹ H NMR, UV/vis, IR spectroscopy, and X-ray crystallography.