Disentanglement of orthogonal hydrogen and halogen bonds via natural orbital for chemical valence: A charge displacement analysis

As known, the electron density of covalently bound halogen atoms is anisotropically distributed, making them potentially able to establish many weak interactions, acting at the same time as halogen bond donors and hydrogen bond acceptors. Indeed, there are many examples in which the halogen and hydrogen bond coexist in the same structure and, if a correct bond analysis is required, their separation is mandatory. Here, the advantages and limitations of coupling the charge displacement analysis with natural orbital for chemical valence method (NOCV-CD) to separately analyze orthogonal weak interactions are shown, for both symmetric and asymmetric adducts. The methodology gives optimal results with intermolecular adducts but, in the presence of an organometallic complex, also intramolecular interactions can be correctly analyzed. Beyond the methodological aspects, it is shown that correctly separate and quantify the interactions can give interesting chemical insights about the systems

Monitoring of the pre-equilibrium step in the alkyne hydration reaction catalyzed by au(Iii) complexes: A computational study based on experimental evidences

The coordination ability of the [(ppy)Au(IPr)]2+ fragment [ppy = 2-phenylpyridine, IPr = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene] towards different anionic and neutral X ligands (X = Cl 12, BF4 12, OTf 12, H2 O, 2-butyne, 3-hexyne) commonly involved in the crucial pre-equilibrium step of the alkyne hydration reaction is computationally investigated to shed light on unexpected experimental observations on its catalytic activity. Experiment reveals that BF4 12 and OTf 12 have very similar coordination ability towards [(ppy)Au(IPr)]2+ and slightly less than water, whereas the alkyne complex could not be observed in solution at least at the NMR sensitivity. Due to the steric hindrance/dispersion interaction balance between X and IPr, the [(ppy)Au(IPr)]2+ fragment is computationally found to be much less selective than a model [(ppy)Au(NHC)]2+ (NHC = 1,3-dimethylimidazol-2-ylidene) fragment towards the different ligands, in particular OTf 12 and BF4 12, in agreement with experiment. Effect of the ancillary ligand substitution demonstrates that the coordination ability of Au(III) is quantitatively strongly affected by the nature of the ligands (even more than the net charge of the complex) and that all the investigated gold fragments coordinate to alkynes more strongly than H2 O. Remarkably, a stabilization of the water-coordinating species with respect to the alkyne-coordinating one can only be achieved within a microsolvation model, which reconciles theory with experiment. All the results reported here suggest that both the Au(III) fragment coordination ability and its proper computational modelling in the experimental conditions are fundamental issues for the design of efficient catalysts

Charge Displacement Analysis-A Tool to Theoretically Characterize the Charge Transfer Contribution of Halogen Bonds

Theoretical bonding analysis is of prime importance for the deep understanding of the various chemical interactions, covalent or not. Among the various methods that have been developed in the last decades, the analysis of the Charge Displacement function (CD) demonstrated to be useful to reveal the charge transfer effects in many contexts, from weak hydrogen bonds, to the characterization of σ hole interactions, as halogen, chalcogen and pnictogen bonding or even in the decomposition of the metal-ligand bond. Quite often, the CD analysis has also been coupled with experimental techniques, in order to give a complete description of the system under study. In this review, we focus on the use of CD analysis on halogen bonded systems, describing the most relevant literature examples about gas phase and condensed phase systems. Chemical insights will be drawn about the nature of halogen bond, its cooperativity and its influence on metal-ligand bond components

The chemical bond between Au(I) and the noble gases. Comparative study of NgAuF and NgAu(+) (Ng = Ar, Kr, Xe) by density functional and coupled cluster methods

The nature of the chemical bond between gold and the noble gases in the simplest prototype of Au(I) complexes (NgAuF and NgA

π Activation of Alkynes in Homogeneous and Heterogeneous Gold Catalysis

The activation of alkynes toward nucleophilic attack upon coordination to gold-based catalysts (neutral and positively charged gold clusters and gold complexes commonly used in homogeneous catalysis) is investigated to elucidate the role of the σ donation and π back-donation components of the Au–C bond (where we consider ethyne as prototype substrate). Charge displacement (CD) analysis is used to obtain a well-defined measure of σ donation and π back-donation and to find out how the corresponding charge flows affect the electron density at the electrophilic carbon undergoing the nucleophilic attack. This information is used to rationalize the activity of a series of catalysts in the nucleophilic attack step of a model hydroamination reaction. For the first time, the components of the Dewar–Chatt–Duncanson model, donation and back-donation, are put in quantitative correlation with the kinetic parameters of a chemical reaction

Ion Pairing in Cationic Olefin-Gold(I) Complexes

The relative anion-cation orientation in [(PPh(3))Au(4-Me-styrene)]BF(4) (1BF(4)) and [(NHC)Au(4-Me-styrene)]BF(4) [2BF(4); NHC = 1,3-bis(di-iso-propylphenyl)-imidazol-2-ylidene] has been investigated by combining (19)F, (1)H-HOESY NMR spectroscopy and Density Functional Theory (DFT) calculations incorporating solvent and relativistic effects. It has been found that BF(4)(-) locates on the side of 4-Me-styrene, close to the olefin region that is opposite to the 4-Me-Ph moiety in 1BF(4). In 2BF(4), the counterion approaches the cation from the side of the NHC ligand and is mainly located close to the imidazole ring. In both cases, the counterion resides far away from the gold site, the latter carrying only a small fraction of the positive charge. This indicates that the preferential position of the counterion is tunable through the choice of the ancillary ligand, and this opens the way to greater control over the properties and activity of these catalysts

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

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