Terminal and bridging end-on coordination of N 2 to transition metal complexes offer possibilities for distinct pathways in ammonia synthesis and N 2 functionalization. Here we elucidate the fundamental factors controlling the two binding modes and determining which is favored for a given metal−ligand system, using both quantitative density functional theory (DFT) and qualitative molecular orbital (MO) analyses. The Gibbs free energy for converting two terminal MN 2 complexes into a bridging MNNM complex and a free N 2 molecule (2ΔG eq °) is examined through systematic variations of the metal and ligands; values of ΔG eq °range between +9.1 and −24.0 kcal/mol per M−N 2 bond. We propose a model that accounts for these broad variations by assigning a fixed π-bond order (BO π ) to the triatomic terminal MNN moiety that is equal to that of the free N 2 molecule, and a variable BO π to the bridging complexes based on the character (bonding or antibonding) and occupancy of the π-MOs in the tetratomic MNNM core. When the conversion from terminal to bridging coordination and free N 2 is associated with an increase in the number of π-bonds (ΔBO eq π > 0), the bridging mode is greatly favored; this condition is satisfied when each metal provides 1, 2, or 3 electrons to the π-MOs of the MNNM core. When each metal in the bridging complex provides 4 electrons to the MNNM π-MOs, ΔBO eq π = 0; the equilibrium in this case is approximately ergoneutral and the direction can be shifted by dispersion interactions.

The cleavage and formation of carbon−carbon bonds have emerged as powerful tools for structural modifications in organic synthesis. Although transition−metal−catalyzed decarbonylation of unstrained diaryl ketones provides a viable protocol to construct biaryl structures, the use of expensive catalyst and high temperature (>140 oC) have greatly limited their universal applicability. Moreover, the direct activation of two inert C − C bonds in diaryl ketones without the assistance of metal catalyst has been a great challenge due to the inherent stability of C − C bonds (nonpolar, thermo-dynamically stable, and kinetically inert). Here we report an efficient light-driven transition-metal-free strategy for decarbonylation of unstrained diaryl ketones to construct biaryl compounds through dual inert C − C bonds cleavage. This reaction featured mild reaction conditions, easy-to-handle reactants and reagents, and excellent functional groups tolerance. The mechanistic investigation and DFT calculation suggest that this strategy proceeds through the formation of dioxy radical intermediate via a single-electron-transfer (SET) process between photo-excited diaryl ketone and DBU mediated by DMSO, followed by removal of CO2 to construct biaryl compounds.

Carboxylic acids are readily available, structurally diverse and shelf-stable; therefore, converting them to the isoelectronic boronic acids, which play pivotal roles in different settings, would be highly enabling. In contrast to the well-recognised decarboxylative borylation, the chemical space of carboxylic-to-boronic acid transformation via deoxygenation remains underexplored due to the thermodynamic and kinetic inertness of carboxylic C-O bonds. Herein, we report a deoxygenative borylation reaction of free carboxylic acids or their sodium salts to synthesise alkylboronates under metal-free conditions. Promoted by a uniquely Lewis acidic and strongly reducing diboron reagent, bis(catecholato)diboron (B2cat2), a library of aromatic carboxylic acids are converted to the benzylboronates. By leveraging the same borylative manifold, a facile triboration process with aliphatic carboxylic acids is also realised, diversifying the pool of available 1,1,2-alkyl(trisboronates) that were otherwise difficult to access. Detailed mechanistic studies reveal a stepwise C-O cleavage profile, which could inspire and encourage future endeavours on more appealing reductive functionalisation of oxygenated feedstocks.

PONOP-Ir-Me (1) and POCOP-Ir-CO (2), R = Bu orPr, are known to undergo acid-catalyzed oxidative addition of H that yields octahedral products with two hydrides in a trans-configuration. We use density functional theory to study the free energies (Δ G) and equilibrium isotope effects (EIE) for H/D addition to 1,2, and related complexes for R = Bu,Pr, and Me. For a given R, reaction of 1 is ∼5 kcal/mol more exergonic than2. For a given subclass of complexes, Δ G is more exergonic for the smaller R. The computed values of Δ G vary between +5.1 and -17.4 kcal/mol. EIE varies between 0.78 and 1.22. Counterintuitively, it is the less-exergonic reactions that afford products with shorter Ir-H bonds, greater symmetric and asymmetric trans-Ir-(H) stretching vibrational frequencies, and more inverse EIE. This disparity is amplified in PONOP-Os-CO, where Δ G is -35.2 kcal/mol, yet the Os-H bonds are long, and the Os-H vibrational frequencies are low as compared with the Ir-H bonds, and EIE is high (1.20). Attempts are made to account for the inverted bond strength-bond length correlation based on the hydricity of the products and the total negative charge on the trans-Ir(H) unit as computed using the Quantum Theory of Atoms in Molecules.