Different Reaction Modes Operating in ansa-Half-Sandwich Magnesium Catalysts.

Magnesium-based catalysts are becoming popular for hydroelementation reactions specially using p-block reagents. Based on the seminal report from Schäfer's group (ChemCatChem 2022, 14, e202201007), our study demonstrates that the reaction mechanisms exhibit a far greater degree of complexity than originally presumed. Magnesium has a variety of coordination modes (and access to different hybridizations) which allows this electron-deficient centre to modulate its catalytic power depending on the σ-donor properties of the reagent. DFT calculations demonstrate several reaction channels closely operating in these versatile catalysts. In addition, variations in limiting energy barriers resulting from catalyst modifications were examined as a function of the Hammett constant, thereby predicting enhanced efficiency in reaction conversions.

Redox Activity of IrIII Complexes with Multidentate Ligands Based on Dipyrido-Annulated N-Heterocyclic Carbenes: Access to High Valent and High Spin State with Carbon Donors.

Synthetic strategies to access high-valent iridium complexes usually require use of π donating ligands bearing electronegative atoms (e. g. amide or oxide) or σ donating electropositive atoms (e. g. boryl or hydride). Besides the η5 -(methyl)cyclopentadienyl derivatives, high-valent η1 carbon-ligated iridium complexes are challenging to synthesize. To meet this challenge, this work reports the oxidation behavior of an all-carbon-ligated anionic bis(CCC-pincer) IrIII complex. Being both σ and π donating, the diaryl dipyrido-annulated N-heterocyclic carbene (dpa-NHC) IrIII complex allowed a stepwise 4e- oxidation sequence. The first 2e- oxidation led to an oxidative coupling of two adjacent aryl groups, resulting in formation of a cationic chiral IrIII complex bearing a CCCC-tetradentate ligand. A further 2e- oxidation allowed isolation of a high-valent tricationic complex with a triplet ground state. These results close a synthetic gap for carbon-ligated iridium complexes and demonstrate the electronic tuning potential of organic π ligands for unusual electronic properties.

Stable Two-Legged Parent Piano-Stool and Mixed Diborabenzene-E4 (E=P, As) Sandwich Complexes of Group 8.

A cyclic alkyl(amino)carbene-stabilized 1,4-diborabenzene (DBB) ligand enables the isolation of 18-electron two-legged parent piano-stool Fe0 and Ru0 complexes, [(η6 -DBB)M(CO)2 ], the ruthenium complex being the first of its kind to be structurally characterized. [(η6 -DBB)Fe(CO)2 ] reacts with E4 (E=P, As) to yield mixed DBB-cyclo-E4 sandwich complexes with planar E4 2- ligands. Computational analyses confirm the strong electron-donating capacity of the DBB ligand and show that the E4 ligand is bound by four equivalent Fe-P σ bonds.

A boron, nitrogen-containing heterocyclic carbene (BNC) as a redox active ligand: synthesis and characterization of a lithium BNC-aurate complex.

Stabilization of low oxidation gold anions as aurate or auride by organic ligands has long been a synthetic challenge, owing to the proneness of low-valent gold centres to cluster. Despite being the most electronegative metal, isolable gold(I) aurate complexes have only been obtained from a few σ-withdrawing organo- and organo-main group ligands. Stabilization of highly-reduced gold complexes by π-modulating redox active ligands has only been achieved by cyclic (amino)(alkyl)carbene (CAAC), which is limited to 1e--reduction to form neutral gold(0) complexes. This work reports a simple modular synthesis of a boron, nitrogen-containing heterocyclic carbene (ClBNC) at a gold(I) center through metal-assisted coupling between azadiboriridine and isocyanides. The anionic electrophilic ClBNC ligand in the gold(I) complex [(ClBNC)AuPMe3] (3a and 3b) allows a 2e--reduction to form the first η1-carbene aurate complex [(BNC)AuPMe3]Li(DME) (5a, DME = dimethoxyethane). Single crystal crystallographic analysis and computational studies of these complexes revealed a highly π-withdrawing character of the neutral 4π B,N-heterocyclic carbene (BNC) moiety and a 6π weakly aromatic character with π-donating properties to the gold(I) fragment in its reduced form, showcasing the first cyclic carbene ligand that allows electronic tunability between π-withdrawing (Fischer-type)- and π-donating (Schrock-type) properties.

Proton to hydride umpolung at a phosphonium center via electron relay: a new strategy for main-group based water reduction.

Generation of dihydrogen from water splitting, also known as water reduction, is a key process to access a sustainable hydrogen economy for energy production and usage. The key step is the selective reduction of a protic hydrogen to an accessible and reactive hydride, which has proven difficult at a p-block element. Although frustrated Lewis pair (FLP) chemistry is well known for water activation by heterolytic H-OH bond cleavage, to the best of our knowledge, there has been only one case showing water reduction by metal-free FLP systems to date, in which silylene (SiII) was used as the Lewis base. This work reports the molecular design and synthesis of an ortho-phenylene linked bisborane-functionalized phosphine, which reacts with water stoichiometrically to generate H2 and phosphine oxide quantitatively under ambient conditions. Computational investigations revealed an unprecedented multi-centered electron relay mechanism offered by the molecular framework, shuttling a pair of electrons from hydroxide (OH-) in water to the separated proton through a borane-phosphonium-borane path. This simple molecular design and its water reduction mechanism opens new avenues for this main-group chemistry in their growing roles in chemical transformations.

Accurate Estimation of pKb Values for Amino Groups from Surface Electrostatic Potential (VS,min) Calculations: The Isoelectric Points of Amino Acids as a Case Study.

Theoretical calculation of equilibrium dissociation constants is a very computationally demanding and time-consuming process since it requires an extremely accurate computation of the solvation free energy changes for each of the species involved. By correlating the minimum surface electrostatic potential (VS,min) on the nitrogen atom of several aliphatic amino groups-calculated at the density functional theory (DFT) ωB97X-D/cc-pVDZ level of theory-we obtained regression models for each kind of substitution pattern from which we interpolate their corresponding pKb values with remarkable accuracy: primary R2 = 0.9519; secondary R2 = 0.9112; and tertiary R2 = 0.8172 (N = 20 for each family). These models were validated with tests sets (N = 5) with mean absolute error (MAE) values of 0.1213 (primary), 0.4407 (secondary), and 0.3057 (tertiary). Combining this ansatz with another previously reported by our group to estimate pKa values [Caballero-García, G.; et al. Molecules 2019, 24(1), 79] we are able to reproduce the isoelectric points of 13 amino acids with no titrable side chains with MAE = 0.4636 pI units.