Carbocations as Secondary Coordination Sphere Z-Type Ligands – Anion-Induced Au(I)/Au(III) Oxidation and Impact on Reactivity

Abstract

We describe our efforts towards the design of phosphine-based ligands featuring an electrophilic carbocationic moiety positioned to enter the secondary coordination sphere of the phosphine-bound metal atom. Using simple synthetic protocols, we have been able to obtain and fully characterize the complexes [(<i>o</i>-Ph<sub>2</sub>P(C<sub>6</sub>H<sub>4</sub>)Acr)AuCl]<sup>+</sup> ([<b>3</b>]<sup>+</sup>, Acr = 9-<i>N</i>-methylacridinium) and [(<i>o</i>-Ph<sub>2</sub>P(C<sub>6</sub>H<sub>4</sub>)Xan)AuCl]<sup>+</sup> ([<b>4</b>]<sup>+</sup>, Xan = 9-xanthylium). While only weak interactions occur between the gold atom and the methylium centers of these complexes, the more Lewis acidic [<b>4</b>]<sup>+</sup> readily reacts with chloride to afford a trivalent phosphine gold dichloride derivative (<b>7</b>) in which the metal atom is covalently bound to the former methylium center. This anion-induced Au(I)/Au(III) oxidation is accompanied by a conversion of the Lewis acidic methylium center in [<b>4</b>]<sup>+</sup> into an X-type ligand in <b>7</b>. This transformation leads us to conclude that the methylium center acts as a latent Z-type ligand poised to increase the Lewis acidity of the gold center, a notion supported by the carbophilic reactivity of these complexes.