Rameswar Bhattacharjee

Complete Transmetalation in a Metal-Organic Framework by Metal Ion Metathesis in a Single Crystal for Selective Sensing of Phosphate Ions in Aqueous Media.

A complete transmetalation has been achieved on a barium metal-organic framework (MOF), leading to the isolation of a new Tb-MOF in a single-crystal (SC) to single-crystal (SC) fashion. It leads to the transformation of an anionic framework with cations in the pore to one that is neutral. The mechanistic studies proposed a core-shell metal exchange through dissociation of metal-ligand bonds. This Tb-MOF exhibits enhanced photoluminescence and acts as a selective sensor for phosphate anion in aqueous medium. Thus, this work not only provides a method to functionalize a MOF that can have potential application in sensing but also elucidates the formation mechanism of the resulting MOF.

Metal‐Free Reduction of CO2 to Methoxyborane under Ambient Conditions through Borondiformate Formation

An abnormal N-heterocyclic carbene (aNHC) based homogeneous catalyst has been used for the reduction of carbon dioxide to methoxyborane in the presence of a range of hydroboranes under ambient conditions and resulted in the highest turnover number of 6000. A catalytically active reaction intermediate, [aNHC-H⋅9BBN(OCOH)2 ] was structurally characterized and authenticated by NMR spectroscopy. A detailed mechanistic cycle of this catalytic process via borondiformate formation has been proposed from tandem experimental and computational experiments.

Strain Control: Reversible H2 Activation and H2/D2 Exchange in Pt Complexes

Experiments have indicated that bulky ligands are required for efficient H2 activation by Pt-Sn complexes. Herein, we unravel the mechanisms for a Pt-Sn complex, Pt(Sn(t)Bu3)2(CN(t)Bu)2 (1a), catalyzed reversible H2 activation. Among a number of Pt-Sn catalysts used to model H2 activation and H2/D2 exchange reactions, only 1a with large strain was found to be suitable because the addition of H2 to 1a requires lowest distortion energy, minimal structural changes, and smallest entropy of activation. The activity of this Pt-Sn complex was compared vis-à-vis its Pt-Ge and Pt-Si analogues, and we predicted that strained Pt-Ge complex can efficiently activate H2 reversibly. Direct dynamics calculations for the rate of reductive elimination of H2, HD, and D2 from Pt(Sn(t)Bu3)(CN(t)Bu)2H3 (4a) and Pt(Sn(t)Bu3)(CN(t)Bu)2HD2 (4a([2D])) shows that H/D atom tunneling contributes significantly, which leads to an enhanced kinetic isotope effect. Strain control is suggested as a design concept in H2 activation.

An Azoaromatic Ligand as Four Electron Four Proton Reservoir: Catalytic Dehydrogenation of Alcohols by Its Zinc(II) Complex

Electroprotic storage materials, though invaluable in energy-related research, are scanty among non-natural compounds. Herein, we report a zinc(II) complex of the ligand 2,6-bis(phenylazo)pyridine (L), which acts as a multiple electron and proton reservoir during catalytic dehydrogenation of alcohols to aldehydes/ketones. The redox-inactive metal ion Zn(II) serves as an oxophilic Lewis acid, while the ligand behaves as efficient storage of electron and proton. Synthesis, X-ray structure, and spectral characterizations of the catalyst, ZnLCl2 (1a) along with the two hydrogenated complexes of 1a, ZnH2LCl2 (1b), and ZnH4LCl2 (1c) are reported. It has been argued that the reversible azo-hydrazo redox couple of 1a controls aerobic dehydrogenation of alcohols. Hydrogenated complexes are hyper-reactive and quantitatively reduce O2 and para-benzoquinone to H2O2 and para-hydroquinone, respectively. Plausible mechanistic pathways for alcohol oxidation are discussed based on controlled experiments, isotope labeling, and spectral analysis of intermediates.

Understanding Thermal and Photochemical Aryl-Aryl Cross-Coupling by the AuI/AuIII Redox Couple

Systematic mechanistic investigations of the gold(I)/gold(III) redox-controlled aryl-aryl cross-coupling reaction have been performed by using both a thermal and photochemical approach. Electron-deficient and electron-rich arenes were considered as the coupling partners of the reaction. Based on transition-state modeling and distortion/interaction analyses, it is shown that AuI prefers to react with electron-deficient arenes whereas AuIII likes to activate electron-rich arenes. This orthogonal reactivity of gold makes it an efficient catalyst for the aryl-aryl cross-coupling reaction. The crucial role of the carboxylate ligand in the reaction has been elucidated through analysis of the transition states. It is shown that due to the presence of two coordination sites, a carboxylate ligand can stabilize the transition state more efficiently than other monodentate ligands such as chloride (Cl- ). Moreover, carbon-boron transmetalation is shown to be favorable over direct C-H metalation, hence reactions initialized by C-B transmetalation are expected to be much faster and selective. Additionally, a dual photoredox/gold catalyst was employed to access the AuI /AuIII catalytic cycle for the cross-coupling reaction. [Ru(bpy)3 ]2+ was used as the photoredox catalyst for the reaction, which, on excitation, transfers an electron to one of the coupling partners, namely a diazonium salt (ArN2+ ), and initializes the cycle.