Ligand Effects on Bonding and Ion Pairing in Cationic Gold(I) Catalysts Bearing Unsaturated Hydrocarbons

Abstract

We critically review recent experimental and theoretical investigations into some key aspects of the chemistry of gold(I) complexes of the type [L-Au-S]X-+(-) (L = NHC carbenes and phosphanes, S = alkenes and alkynes, and X- = weakly coordinating counterion). These systems are important intermediates formed during gold-catalyzed nucleophilic additions to an unsaturated substrate, and their specific activity is largely governed by two fundamental factors: the nature of the gold-substrate bond and the role of the ion-pair structure in solution. Both are crucially influenced by the nature and properties of the auxiliary ligand L, and on this interplay we focus our discussion. The relative anion-cation orientation, investigated by NOE NMR spectroscopy and DFT calculations, shows that the exact position of the counterion is determined by the natures of the ancillary ligand and substrate: the counterion is located near the substrate in the phosphane complexes, while for the NHC complexes the preferred position of the counterion is near the ligand. This tunable interionic structure opens the way to greater control over the properties and activity of these catalysts. The bond between Au-I and the unsaturated substrate is investigated using an original and powerful theoretical method of analysis. Our approach permits a rigorous definition and assessment of the charge-displacement (CD) components at the heart of the Dewar-Chatt-Duncanson model: substrate-to-metal (sigma donation) and metal-to-substrate (back-donation) and how these change with different ligands. The results consistently reveal that back-donation is a large and crucially important component of the Au-I-substrate bond in all systems: back-donation penetrates the external side of coordinated alkynes, where nucleophile attack is directed, thus partially mitigating the electron depletion caused by sigma donation

Catching the role of anisotropic electronic distribution and charge transfer in halogen bonded complexes of noble gases

“…To study the presence of a CT component in these systems, we analyzed the electron density changes due to the interaction between CCl 4 (CF 4 ) and the Ngs by means of the charge displacement function (CDF), 100 which we have successfully used to study intermolecular weak interactions 48,50,[52][53][54][55]101,102 and chemical bonding in several diverse contexts. [103][104][105][106] …”

“…To investigate the occurrence of CT in the Ng-CCl 4 and Ng-CF 4 , including its radial dependence and stereo specificity, we exploited the CDF, recently employed with success to study the chemical bond in several contexts, [103][104][105][106] including weak intermolecular hydrogen bonds. 50,54,55 CDF offers a broad perspective for an assessment of the CT occurrence, free of any charge decomposition scheme.…”

Uncovering Subtle Ligand Effects of Phosphines Using Gold(I) Catalysis

“…1 Quantitative relationships with phosphines have primarily focused on Tolman’s cone angle or electronic parameter, with both being useful measures in correlating direct interactions; 2 however, the indirect interactions involved in determining reaction outcomes are often poorly understood. This is particularly striking in gold(I) catalysis, 3 where fewer steric contacts between substrate and ligand are observed as a function of the linear nature of the gold(I) complexes (Scheme 1).…”

Relationship between the anion/cation relative orientation and the catalytic activity of nitrogen acyclic carbene–gold catalysts

“…12 A remarkable example highlighting the importance of ion-pairing has been reported by Toste and coworkers. 14 We recently demonstrated, by combining 19 F, 1 H-HOESY NMR 15 and relativistic Density Functional Theory (DFT) calculations (Coulomb map potential and/or full ion pair optimization), how the anion/cation relative orientation can be finely tuned by the proper choice of the ligand 16 and how the ion pair structure can be related with the electronic properties of the ligand (Scheme 1). It was be surmised 13 that an even higher enantiomeric excess than reachable with a chiral auxiliary ligand might be achieved when chiral counterion resides near the catalytic site.…”