The auration of 2-hydroxy-pyridine (2-pyridone): preparative and structural studies and a comparison with reactions of related aliphatic O,N-donors

In a study of the isolobal analogy between the proton H+ and the ligand-backed gold(I) cations [(R3P)Au]+, the reaction of the mixture of 2-pyridone/2-hydroxy-pyridine tautomers with [(Ph3P)Au]BF4 has been investigated. It affords the 1:1 complex with the gold atom N-bonded to the 2-hydroxy-pyridine tautomer: {(Ph3P)Au[NC5H4-(OH-2)]}+BF4 −, which is related to known salts with the 2-hydroxy-pyridinium cation such as [HNC5H4(OH-2)]+Cl−. The structure was derived from analytical and spectroscopic data and from a comparison with the salt [(Ph3P)Au(pyr)]BF4, prepared and investigated structurally as a reference compound. An analogue was also prepared with 2-dimethylamino-ethanol as a substrate, which also affords the N-bonded complex [(Ph3P)Au([Me2NCH2CH2OH)]+BF4 −, the structure of which has been determined. The OH group is not attached to the gold atom but engaged in hydrogen bonding with the counterion. By contrast, in the complex [(Ph3P)Au(Me2NCH2CH2NMe2)]+BF4 − synthesized similarly with tmeda and crystallized as the dichloromethane solvate, one nitrogen atom is bonded firmly to the metal atom, but the second nitrogen atom is also weakly engaged in coordinative bonding. The compound is fluxional in solution, where a site exchange is observed which is rapid on the NMR time scale. The reaction of two equivalents of [(Ph3P)Au]BF4 with an alkali 2-pyridinolate, prepared from the above tautomeric mixture and sodium metal or a potassium alkoxide, yields the diaurated product {N,O-[(Ph3P)Au]2(NC5H4–O-2)}BF4. In the crystal structure determination of a sesqui-solvate with dichloromethane it has been shown that one gold atom is attached solely to the nitrogen atom and the other solely to the oxygen atom. The dinuclear cations are associated into cyclic dimers through head-to-tail aurophilic bonding. From the geometrical characteristics of the core unit of the cations the ligand can be assigned a 2-pyridinolate form featuring pyridine and phenolate donor sites.