Ion pairing in cationic NHC and NAC gold(I) η2-alkyne complexes

Gold(I) cationic complexes of general formula [LAu+ X-] [L = NAC (Nitrogen-Acyclic-Carbene) or NHCs (N-Heterocyclic Carbenes), X- = weakly coordinating anion] are successfully employed as catalysts in a large variety of organic reactions involving the activation of unsaturated carbon-carbon bonds. A key role in such reactions is played by the counterion. We have studied the ion pair structure in solution of [NHC-Au(η2-3-hexyne)]BF4 and [NAC-Au(η2-3-hexyne)]BF4 by NOE NMR spectroscopy and relativistic DFT calculation. In our previous work, two main orientations were observed for unsaturated NHC ligands: one with the anion close to the carbene backbone (A, most populated) and another with the anion close to the 3-hexyne (B). Here we focus on the effect of different carbenes on the ion pair structure, comparing the aromatic NHC (1BF4) with 5 different ligands: a non aromatic NHC (2BF4), a polycyclic ligand with an extended aromatic system (3BF4) and three different NAC (4BF4, 5BF4 and 6BF4). For 2BF4, the A:B ratio (observed with NOE NMR spectroscopy) remains almost the same as for 1BF4, whilst the ion pair structure of the 3BF4 becomes mainly non-specific, with a slight preference for the orientation B. Both cases can be explained analyzing the DFT Coulomb potential map, that shows an attractive region on the backbone of 2BF4 and a flat weak potential around the whole 3BF4. An interesting result was obtained for 4BF4 that shows a strong attractive region close to the two N-H that makes the IP 100% specific for the orientation A (figure beside). 5BF4 shows a less specific ion pair due to the presence of only one N-H on the back. For the 6BF4 the -OH group on the arm of the carbene introduce a third ion pair configuration, strongly stabilized by the hydrogen bond between the hydroxyl and the anion.