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

A PGSE NMR approach to the characterization of single and multi-site halogen-bonded adducts in solution

We demonstrate here that the Pulsed field Gradient Spin Echo (PGSE) NMR diffusion technique can be effectively used as a complementary tool for the characterization of mono- and multi-site intermolecular halogen bonding (XB) in solution. The main advantage of this technique is that it provides the possibility of unambiguously determining the stoichiometry of the supramolecular adduct, information that is particularly important when multi-site molecular systems are studied. As an example, PGSE NMR measurements in chloroform indicate that hexamethylenetetramine (HMTA), a potentially four-site XB acceptor, actually exploits only two sites for the interaction with the XB donor N-bromosuccinimide (NBS), leaving the other two nitrogen sites unoccupied. Charge displacement calculations suggest that this is due also to the anti-cooperativity of the XB interaction between HMTA and NBS

Charge-displacement analysis as a tool to study chalcogen bonded adducts and predict their association constants in solution

The secondary interaction between a polarized chalcogen atom and different Lewis bases, either anionic or neutral, has been studied by charge displacement analysis. Using charge displacement analysis, the charge rearrangement in the adduct upon the formation of the interaction has been quantified and described in great detail. By comparing the theoretical results with the experimental association con- stants, two linear correlations can be found for anionic and neutral bases. Such correlations can be used to reliably predict the association constants of adducts for which experimental data are not available yet

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

Deep Eutectic Solvents formed by chiral components as chiral reaction media and studies of their structural properties

We present the realization, the use as reaction media/chiral organocatalysts/acid catalysts and the structural properties of novel chiral Deep Eutectic Solvents. These liquids are formed by mixtures of chiral HBD and HBA molecules that are common, relatively cheap and commercially available (the two enantiomers of camphorsulfonic acid as HBD) or easily one-step synthesized molecules from commercially available compounds ((S)- and (R)-N,N,N-trimethyl-(1-phenylethyl)ammonium methanesulfonate as HBA). These liquids proved to be highly-structured as showed by different yields and enantiomeric excesses observed in a probe reaction, suggesting these liquids to form diastereoisomerically different liquids by changing one of the two enantiomers. Their structural features were analyzed via 1H Pulsed Field gradient Spin Echo (PGSE) NMR, NMR titration, 1H NMR analyses of formation and differences in the chemical shifts of the peaks of the liquids. Density Functional Theory (DFT) optimization helped to define the structures of these liquids. The methanesulfonate counterion of HBA molecule showed to be relevant in order to obtain these highly-structured liquids as it interacts specifically with the HBD. These chiral Deep Eutectic Solvents revealed to be promising novel high-structured media for enantioselective reactions

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

Experimental and theoretical DFT investigations in the [2, 3]-wittig-type rearrangement of propargyl/allyl-oxy-pyrazolones

Here we report the synthesis of interesting 3-alkyl-4-hydroxy-1-aryl-4-(propa-1,2-dienyl)1H-pyrazol-5(4H)-ones and 9-alkyl-7-aryl-1-oxa-7,8-diazaspiro[4.4]nona-3,8-dien-6-ones, starting from 1,2-diaza-1,3-dienes (DDs) and propargyl alcohol. The reaction proceeds through a sequence Michael-type nucleophilic attack/cyclization/[2,3]-Wittig rearrangement. In the same way, the reaction between the aforementioned DDs and allyl alcohol furnished 4-allyl-4-hydroxy-3-alkyl-1-aryl-1H-pyrazol-5(4H)-ones. A DFT study was also carried out, in order to have decisive clarifications about the mechanism

Guidelines for a correct evaluation of deep eutectic solvents thermal stability

Deep eutectic solvents (DESs) are a class of versatile and green emerging materials. Despite the huge amounts of applications proposed in the last years, studies on their thermal stability are often missing. In this short review, we propose a guide for a correct evaluation of DES thermal stability, conducted mainly by dynamical thermogravimetry (TGA). We collected all the data reported in the literature on choline chloride (ChCl)-based DESs, as proof of concept to show the potentialities of the technique, highlighting all the parameters that need to be considered for a correct analysis, with particular attention to the possible sources of misleading interpretations (e.g. the adsorbed water, or the formation of undesired products during DES preparation). In many cases, the additional use of isothermal TGA, or TGA coupled with online techniques such as Fourier Infra-Red Spectroscopy or Mass Spectrometry, may help for the data interpretation. Besides, we summarize in a graph the degradation temperatures of many DESs and their precursors, intended as an operative guide to choosing the correct DES for different applications. The findings reported to date, highlight the potentialities of thermal analysis on DESs, as a powerful tool to obtain essential information on their applicability, and to implement the knowledge of their nanostructure from a molecular point of view

Cooperativity between hydrogen- and halogen bonds: the case of selenourea

A combined experimental/computational study of cooperativity between halogen-(XB) and hydrogen bonding (HB) is presented. Selenourea (SeU) has been chosen due to its ability to act at the same time as an XB acceptor toward I(CF2)5CF3 (I1) through the two lone pairs on the selenium atom, and as a HB donor to the benzoate anion through its two amino moieties. All equilibrium constants have been estimated using either diffusion NMR and NMR titration techniques. Experimental results demonstrate that the –NH2⋯anion interaction strongly enhances the Se⋯I one of about one order of magnitude (in terms of formation constant of the adduct), whereas DFT results rationalize such results revealing that the presence of a HB between the benzoate and SeU strongly polarizes the latter, enhancing the negative partial charge on selenium and, consequently, its Lewis basicity and its XB acceptor properties

Theoretical Investigation on Selenium-Based Frustrated Lewis Pairs

The Frustrated Lewis Pairs (FLP) concept recently demonstrated to be a powerful way to activate small molecules and promote a plethora of organic reactions without the help of transition metal-based catalysts. This strategy is based on simple combinations of Lewis acids and bases that are sterically or electronically hindered from forming classical Lewis acid-base adducts. Generally, the Lewis acid is a fluorinated compound of boron or aluminum, and the Lewis base is based in most cases on phosphorous, nitrogen or, seldom, oxygen. To the best of our knowledge, no attempt has been made to isolate, characterize and test in catalysis selenium-based FLPs. In the present contribution, the interaction between sterically encumbered organoselenium compounds, as selenides and selones, and standard Lewis acids, as B(C 6 F 5 ) 3 and less-fluorinated boranes, will be studied by Energy Decomposition Analysis and compared to the interaction of known P-based FLPs. The results indicate that selenium-based FLPs are theoretically possible. For the most promising candidates, the products of the activation of small molecules (H 2 or CO 2 ) will be theoretically studied, to check if the reaction would be thermodynamically favored