Cyclometalated complexes are an important class of (pre)catalysts in many reactions including hydride transfer. The ring size of such complexes could therefore be a relevant aspect to consider while modulating their catalytic activity. However, any correlation between the cyclometalating ring size and the catalytic activity should be drawn by careful assessment of the pertinent geometrical parameters, and overall electronic effects thereof. In this study, we investigated the vital role of key stereoelectronic functions of two classes of iridacyclic complexes-five-membered and six-membered cycles-in manupulating the catalytic efficiency in a model hydride-transfer reaction. Our investigation revealed that there exists an interesting multidimensional synergy among all the relevant stereoelectronic factors-yaw angle, bite angle, and the electronic properties of both the ligand and the metal center-that governs the hydride donor ability (hydricity) of the complexes during catalysis. Thus the six-membered chelate complexes with small yaw and large bite angles, strong donor ligand, and electron-rich metal were found to be better catalysts than their five-membered analogues. A frontier molecular orbital analysis supported the significant role of the above stereoelectronic synergistic effect associated with the chelate ring to control the hydride donor ability of the complexes.

During the past decade earth-abundant metals have become increasingly important in homogeneous catalysis. One of the reactions in which earth-abundant metals have found important applications is the hydroboration of unsaturated CÀ C and CÀ X bonds (X = O or N). Within these set of transformations, the hydroboration of challenging substrates such as nitriles, carbonates and esters still remain difficult and often relies on elaborate ligand designs and highly reactive catalysts (e. g., metal alkyls/hydrides). Here we report an effective methodology for the hydroboration of challenging C�N and C=O bonds that is simple and applicable to a wide set of substrates. The methodology is based on using a manganese(II) triflate salt that, in combination with commercially available potassium tert-butoxide and pinacolborane, catalyzes the hydroboration of nitriles, carbonates, and esters at room temperature and with near quantitative yields in less than three hours. Additional studies demonstrated that other earth-abundant metal triflate salts can facilitate this reaction as well, which is further discussed in this report.

This work discloses that a simple change in the anion of a copper(II) reagent along with the reaction solvent can dramatically alter the course of a Cp*Rh -catalyzed C-H activation-annulation reaction leading to completely switchable chemoselective products. The nature of the anion in terms of its coordinating ability and basicity, and also the polarity of the solvent have been found to be the crucial factors in the observed divergence.

Imidazolium motif-containing molecules are known to be of significant interest in diverse research areas, but there is a lack of functionalization protocols of these molecules. In a programme to overcome this challenge, recently we developed a unique dual-role of latent imidazolium C-H bond which not only generated a metal-C NHC bond upon activation but also directed further aryl/heteroaryl C-H activation to furnish a library of potentially valuable products. Mechanistic investigation of any newly-discovered catalytic reaction is at the heart of its future development for potential application in diverse fields. Motivated by this philosophy, the presented work delineates the key mechanistic insights of this annulation reaction which unravel the crucial competition of two C-H bonds (imidazolium and aryl C-H) and two M-C bonds (M-C NHC and M-C aryl ) in establishing the rate-limiting step and the alkyne-insertion regioselectivity in the reaction. Through careful isolation and X-ray structural characterization of the key seven-membered inserted intermediate along with DFT rationale, the exclusive regioselectivity of the alkyne-insertion to the M-C NHC bond was established. Kinetics studies were used to evaluate the rate-determining step of the reaction which was found to be the initial nondirected imidazolium C-H activation step. These mechanistic insights seem to be useful in understanding similar C-H activation processes in general which are topical in the area of catalysis.

In parallel to the directing-group-assisted sp 2 C−H bond activation−functionalization of aromatic backbones, a similar exercise with nonaromatic sp 2 C−H bonds is also in high demand in synthetic chemistry despite several challenges pertinent to the latter process. In the presented protocol, Nheterocyclic carbene (NHC) motifs, appended to nonaromatic sp 2 C−H bond-containing organic molecules, have been used for developing a rhodium(III)-catalyzed annulation reaction with internal alkynes to synthesize a class of imidazo[1,2-a]pyridinium architectures. Mechanistic studies highlight the directing role of the NHC ligand during the C−H activation process and intermediacy of the C−H-activated Rh-NHC metallacycle in the catalysis.

A new rhodium­(III)-catalyzed protocol for C–H activation and functionalization of poorly reactive pyridine backbones with the aid of in-built N-heterocyclic carbene ligand has been presented. Mechanistic investigations including the isolation and single crystal X-ray structural characterization of the pre- and postfunctionalized rhodium­(III)-intermediates suggested a plausible pyridine-coordination effect in the present NHC-directed cyclometalative annulation of the C(3)–H bond of the pyridine rings with internal alkynes. The strategy enables us to overcome the challenges of functionalizing robust metal–CNHC backbone concomitant with pyridine C–H activation under an oxidizing environment.

Acceptorless alcohol dehydrogenation is a powerful reaction in sustainable synthesis. When combined in a tandem reaction with dehydrogenative coupling or hydrogen borrowing, acceptorless dehydrogenation can be used for the environmentally benign construction of C−C, C−N, and C−O bonds. While many of these reactions rely on using precious metals, in the past decade the use of earth-abundant metals has become more prevalent. If a green and renewable feedstock could be introduced as well, the sustainability of the acceptorless dehydrogenation could further be increased. Methanol would be such a substrate and could serve as a renewable C1 source when used for the methylation of a wide variety of substrates. Herein, we report the efficient manganesecatalyzed α-methylation of ketones and indoles. The manganese catalyst is based on a PC NHC P pincer platform containing a rare central carbene donor. The reaction supports a diverse set of functional groups, occurs at moderate temperatures (110 °C), and provides the corresponding methylated ketones and indoles in excellent yields. In contrast to previously reported mechanisms, the herein reported mechanism does not depend on metal−ligand cooperativity but rather proceeds via (i) a metal-based mechanism featuring a manganese hydride or (ii) via a ligand-centered mechanism where a manganese-carbonyl acts as catalytic center, depending on the used additive.

Metal–ligand cooperation is an important aspect in earth-abundant metal catalysis. Utilizing ligands as electron reservoirs to supplement the redox chemistry of the metal has resulted in many new exciting discoveries. Here, we demonstrate that iron bipyridine-diimine (BDI) complexes exhibit an extensive electron-transfer series that spans a total of five oxidation states, ranging from the trication [Fe(BDI)] 3+ to the monoanion [Fe(BDI] −1 . Structural characterization by X-ray crystallography revealed the multifaceted redox noninnocence of the BDI ligand, while spectroscopic (e.g., 57 Fe Mössbauer and EPR spectroscopy) and computational studies were employed to elucidate the electronic structure of the isolated complexes, which are further discussed in this report.