Synthesis and structure of gold(III) complexes of asymmetric dithiolene ligands

• Published in: Polyhedron 2006-01, Vol.25 (2), p.591-598
• Main Authors: Tunney, Josephine M., Blake, Alexander J., Davies, E. Stephen, McMaster, Jonathan, Wilson, Claire, Garner, C. David
• Format: Article
• Language: English
• Subjects: Dithiolene; Gold; Redox properties; Structure
• ISSN: 0277-5387
• DOI: 10.1016/j.poly.2005.09.002

Four square planar [Au(dithiolene) 2] − complexes have been synthesized and characterized; they comprise Au(III) bound to two asymmetrically substituted ene-1,2-dithiolates and both cis and trans isomers are present in solution. Each complex undergoes an oxidation and a reduction, the potentials of which correlate with the electron withdrawing nature of the dithiolene. Four new [Au(dithiolene) 2] − complexes, each involving two asymmetrically substituted dithiolene ligands (SC(H)C(R)S; R = phenyl, pyridin-2-yl, pyridin-3-yl, or pyridin-4-yl), have been synthesized by reacting K[AuCl 4] with the corresponding dithiolene, generated by base hydrolysis of the thione-protected 1,3-dithiolate. Each complex has been isolated as its [PPh 4] + salt and the structures of these [PPh 4][Au(dithiolene) 2] compounds have been determined by X-ray crystallography; each anion possesses a square planar {Au S 4} core with a cis-arrangement of the R groups for R = phenyl and pyridin-2-yl complexes, but a trans-arrangement for R = pyridin-3-yl and pyridin-4-yl. In each compound, the lengths of the C–C (1.34 ± 0.01 Å) and C–S (1.75 0.03 Å) bonds of the metallocycle are consistent with the ene-1,2-dithiolate form of the dithiolene, i.e., these are [Au(III)bis(ene-1,2-dithiolate)] − complexes. Each compound is diamagnetic and 1H NMR studies indicate that, in solution in d 6-acetone, both the cis and trans isomers of the [Au(SC(H)C(R)S) 2] − anion are present and do not interconvert at room temperature. Each [Au(dithiolene) 2] − complex undergoes an oxidation ( 80 < E p a < 230 mV ) and a reduction ( - 1880 < E p c ( red ) < - 2020 mV ; versus [Fc] +/[Fc]); the particular potentials observed correlate with the electron withdrawing ability of the dithiolene substituent (R), as expressed by the Hammett σ p-parameter. Comparison of the information obtained in these studies with that determined for related systems leads to the conclusion that the oxidation is a ligand-based process and that reduction involves addition of an electron to an orbital that possesses both metal and ligand character.