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

Back-Donation in High-Valent d0 Metal Complexes: Does It Exist? the Case of NbV

In the last years, some N-heterocyclic carbene (NHC) complexes of high-valent d0 transition-metal halides have been structurally characterized, showing a significant short distance between the carbene carbon and the cis-halide ligands (Clax). Some authors attributed this arrangement to a halide â\u86\u92 Ccarbene unusual "back-donation", whereas, according to others, the M-carbene bond is purely Ï\u83. More, in general, the ability of d0 metal centers to provide back-donation to suitable ligands is still debated, and detailed bond analyses for this class of systems are missing in the literature. In this contribution, we analyze in detail the NbV-L bond within neutral, cationic, and anionic derivatives of NbCl5, with L = NHC, CO, CNH, and CN-. In [NbVCl6-x(NHC)x]x-1 complexes, with NHC being either a model carbene (1,3-dimethylimidazol-2-ylidene, IMe) or a realistic one [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IPr], we demonstrate that the metal center is really capable of back-donation to the carbene ligand by a charge flux that involves the chloride in the trans position and, directly, the metal. In this case, a direct interaction between Clax and Ccarbene can be excluded, while if different Ï\u80-acceptor ligands, such as CO or CNH, are used (instead of NHC), the direct Clax â\u86\u92 L interligand interaction becomes predominant

Tuning the Gold(I)-Carbon σ Bond in Gold-Alkynyl Complexes through Structural Modifications of the NHC Ancillary Ligand: Effect on Spectroscopic Observables and Reactivity

AbstractUnderstanding the features of the gold(I)‐carbon σ bond and its modulation induced by an ancillary ligand has become fundamental for the purposes of ligand design, due to the increasing interest towards gold(I)‐alkynyl complexes and their wide range of applications. We carry out a systematic computational analysis of 16 gold(I)‐acetylide complexes bearing different N‐Heterocyclic Carbenes (NHCs) as ancillary ligands [NHC−Au(I)−CCH]. The results show that the strength and features of the Au−C bond can be efficiently tuned by performing specific structural modifications on the NHC, enabling a more efficient π communication between the alkynyl and the ancillary ligand. We also demonstrate that the effect of the bond modulation can be revealed via NMR spectroscopy, as highlighted by the tight correlation between the computed nuclear shielding constants and the bonding parameters. Finally, we show that, for the dual‐gold‐catalyzed Bergman cyclization as case study, suitable structural modifications on the NHC ligand, which modulate the π‐acidity of the metal fragment σ‐coordinated to an enediyne substrate, could affect the reaction barrier and the thermodynamic stability of the product. All the reported results can be well rationalized in the framework of distortion/interaction analysis, which has been recently extended to the dual (σ,π‐type) Au catalytic systems by Alabugin et al (J. Am. Chem. Soc. 2017, 137, 3406‐3416)

Un profilo di Federico Barocci nelle lettere di Francesco Maria II della Rovere

Nelle intenzioni originarie, questo lavoro voleva esplorare gli archivi di Urbino alla ricerca di documenti inediti relativi alla commissione, esecuzione e collocazione in sito dell’ultimo capolavoro di Federico Barocci “L’ultima cena”, visibile ancor oggi presso la Cappella del S. Sacramento del Duomo di Urbino. Ma fin dall’inizio del lavoro mi accorsi che buona parte della documentazione, abbondantissima sul Barocci, era già stata reperita e pubblicata, prima nell’opera pionieristica del Gronau, poi, in tempi più recenti, dall’Emiliani. Ma dalla lettura di questi documenti nacque un’idea secondaria e tuttavia di non minore interesse: ricavare dall’epistolario di Francesco Maria II della Rovere, assai spesso coinvolto in richieste e sollecitazioni volte a ottenere opere del pittore allora più considerato in Europa, richiesto di intercedere per ottenere impegni o sollecitare la conclusione di opere del pittore urbinate, quindi obbligato a dare risposte ed esprimere valutazioni o consigli sul miglior modo di procedere seco, data la particolare personalità dell’artista, un profilo psicologico dell’artista stesso: come nitida traspare da queste pagine.[... segue nel testo ...]Presentato dal Dipartimento di Scienze di Base e Fondamenti

The application software RIMMF: RDA thinking in action

RIMMF software is grew out of the need to visualize and realize records according to the RDA guidelines. The article describes the software structure and features in the creation of a r­ball, that is a small database populated by recordings of bibliographic and authority resources enriched by relationships between and among entities involved. At first it's introduced the need that led to RIMMF outcome, then starts the software functional analysis. With a description of the main steps of the r-ball building, emphasizing the issues raised. The results highlights some critical aspects, but above all the wide scope of possible developments that open the Cultural Heritage Institutions horizon to the web prospective. Conclusions display the RDF–linked­data development of the RIMMF incoming future.RIMMF software is grew out of the need to visualize and realize records according to the RDA guidelines. The article describes the software structure and features in the creation of a r­ball, that is a small database populated by recordings of bibliographic and authority resources enriched by relationships between and among entities involved. At first, it is introduced the need that led to RIMMF outcome, then starts the software functional analysis. With a description of the main steps of the r-ball building, emphasizing the issues raised. The results highlights some critical aspects, but above all the wide scope of possible developments that open the Cultural Heritage Institutions horizon to the web prospective. Conclusions display the RDF–linked­data development of the RIMMF incoming future

Monomeric gold hydrides for carbon dioxide reduction: Ligand effect on the reactivity

We analyzed the ligand electronic effect in the reaction between a [LAu(I)H]0/− hydride species and CO2, leading to a coordinated formate [LAu(HCOO)]0/−. We explored 20 different ligands, such as carbenes, phosphines and others, carefully selected to cover a wide range of electron-donor and -acceptor properties. We included in the study the only ligand, an NHC-coordinated diphosphene, that, thus far, experimentally demonstrated facile and reversible reaction between the monomeric gold(I) hydride and carbon dioxide. We elucidated the previously unknown reaction mechanism, which resulted to be concerted and common to all the ligands: the gold–hydrogen bond attacks the carbon atom of CO2 with one oxygen atom coordinating to the gold center. A correlation between the ligand σ donor ability, which affects the electron density at the reactive site, and the kinetic activation barriers of the reaction has been found. This systematic study offers useful guidelines for the rational design of new ligands for this reaction, while suggesting a few promising and experimentally accessible potential candidates for the stoichiometric or catalytic CO2 activation

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

Spin-resolved charge displacement analysis as an intuitive tool for the evaluation of cPCET and HAT scenarios

We introduce here the spin-resolved version of the charge displacement function, which is applied to two competing pathways of proton-coupled electron transfer in oxidation catalysis (hydrogen-atom transfer, concerted proton-coupled electron transfer). The difference in charge displacement between the two mechanisms is directly observable and can be translated to electron flow using this new analysis tool

Selectively measuring π back-donation in gold(I) complexes by NMR spectroscopy

Even though the Dewar-Chatt-Duncanson model has been successfully used by chemists since the 1950s, no experimental methodology is yet known to unambiguously estimate the constituents (donation and back-donation) of a metal-ligand interaction. It is demonstrated here that one of these components, the metal-to-ligand π back-donation, can be effectively probed by NMR measurements aimed at determining the rotational barrier of a C-N bond (ΔHr (≠) ) of a nitrogen acyclic carbene ligand. A large series of gold(I) complexes have been synthesized and analyzed, and it was found that the above experimental observables show an accurate correlation with back-donation, as defined theoretically by the appropriate charge displacement originated upon bond formation. The proposed method is potentially of wide applicability for analyzing the ligand effect in metal catalysts and guiding their design