A Cp*Ir(III) complex (1) of a newly designed ligand L 1 featuring a proton-responsive pyridyl(benzamide) appended on N-heterocyclic carbene (NHC) has been synthesized. The molecular structure of 1 reveals a dearomatized form of the ligand. The protonation of 1 with HBF 4 in tetrahydrofuran gives the corresponding aromatized complex [Cp*Ir(L 1 H)Cl]BF 4 (2). Both compounds are characterized spectroscopically and by X-ray crystallography. The protonation of 1 with acid is examined by 1 H NMR and UV-vis spectra. The proton-responsive character of 1 is exploited for catalyzing α-alkylation of ketones and β-alkylation of secondary alcohols using primary alcohols as alkylating agents through hydrogen-borrowing methodology. Compound 1 is an effec-tive catalyst for these reactions and exhibits a superior activity in comparison to a structurally similar iridium complex [Cp*Ir(L 2 )Cl]PF 6 (3) lacking a proton-responsive pendant amide moiety. The catalytic alkylation is characterized by a wide substrate scope, low catalyst and base loadings, and a short reaction time. The catalytic efficacy of 1 is also demonstrated for the syntheses of quinoline and lactone derivatives via acceptorless dehydrogenation, and selective alkylation of two steroids, pregnenolone and testosterone. Detailed mechanistic investigations and DFT calculations substantiate the role of the proton-responsive ligand in the hydrogen-borrowing process.À OH/=O, À CH 2 /=CH and À NH/ = N motifs on pyridine, [25][26][27] bipyridine, [28][29][30][31][32] bipyrimidine [33] and azole-pyridine/ pyrimidine [34][35][36][37] are particularly effective. Some of the representative examples that have been employed for catalyzing (de) hydrogenation and alkylation reactions are given in Scheme 1. Yamaguchi and co-workers reported a Cp*Ir(III) compound with

H 2 storage in carbon dioxide (CO 2 ) or bicarbonate (HCO 3 − ) in the form of formic acid (HCO 2 H) or formate (HCO 2 − ) and the reverse H 2 liberation allows, in principle, to develop a rechargeable hydrogen carrier system along with a CO 2 -recycling mechanism. The key to such an alluring approach toward the realization of a carbonneutral H 2 -based fuel option is the development of efficient bidirectional catalysts for CO 2 (or HCO 3 − ) hydrogenation and HCO 2 H (or HCO 2 − ) dehydrogenation. With an aim toward (i) structurally robust catalysts under variable reaction conditions, (ii) metal− ligand bifunctionality-triggered heterolytic H 2 splitting and H + /H − transfer during hydrogenation/dehydrogenation reactions, (iii) electron-rich catalytic metal center for facilitating hydride delivery, and (iv) water solubility of the catalysts via second coordination sphere interactions, herein, we applied a series of "cyclic amide−NHC" hybrid bidentate ligand-bound Cp*Ir(III) complexes (Ir-1−Ir-4) in bidirectional hydrogenation−dehydrogenation of CO 2 (HCO 3 − )/HCO 2 H (HCO 2 − ) couple in water as a "green" solvent without the use of organic additives/solvents. Notably, with the catalyst Ir-1, hydrogenation of CO 2 achieving a turnover number (TON) of 16 680 at 60 °C in 6 h and dehydrogenation of formic acid with a turnover frequency (TOF 5min ) of 70 674 h −1 at 80 °C can be efficiently carried out. Key control and mechanistic studies emphasized the following aspects of the current system: (i) pH of the solution played a crucial role in controlling the rate of hydrogenation/dehydrogenation reactions, (ii) H 2 was cleaved readily by the catalyst to form the iridium hydride intermediate, which could react with CO 2 to furnish the formate product, (iii) pH (acid/base)-switchable on-demand formic acid dehydrogenation was devised, and (iv) the liberated H 2 and CO 2 gas from the Ir-1-catalyzed formic acid dehydrogenation reaction were reutilized in secondary reactions in a tandem fashion, signifying the suitability of the system to demonstrate the utility of formic acid as a typical H 2 /CO 2 storage liquid, as it is advocated for.

Polyaromatic N-heterocycles are some of the most common building blocks in natural products and active pharmaceutical ingredients. Significant efforts have been devoted to developing catalytic protocols, including those which use an acceptorless dehydrogenation strategy at elevated temperatures, to produce polyaromatic N-heterocycles like quinolines and naphthyridines. However, photoinitiated catalysis driven by visible light offers a milder and often more selective protocol as an alternative to thermal reactions. Here, we present the catalytic syntheses of quinolines and naphthyridines from ortho-aminobenzyl alcohols and ketones using the photocatalyst [Mn(L 1 H)(CO) 3 Br] (L 1 H = 7-hydroxy-2-methyl-1,8-naphthyridine-N-oxide), bearing a phenolic unit on a 1,8-naphthyridine-N-oxide scaffold, under ambient and aerobic conditions with visible light illumination. We describe a broad, functional group-tolerant substrate scope of >30 examples under modest reaction conditions. A variety of 2-aminobenzyl alcohols containing electron-donating and electron-deficient groups and (2-aminopyridin-3-yl)methanol are converted to the corresponding quinolines and naphthyridines using ambient air as an oxidant in the presence of KOH. We synthesized a wide range of derivatives, including some of the bioactive antimalarial drug chloroquine and the steroids progesterone and pregnenolone to highlight the value-added applications of this catalytic protocol for pharmaceutical ingredient and natural product syntheses. We performed substrate viability, ultraviolet−visible, electron paramagnetic resonance, and X-ray photoelectron spectroscopy studies, as well as density functional theory calculations to gain mechanistic insights to the reaction pathway. The catalytic cycle involves condensation of the amino group in the ortho-aminobenzyl alcohol with the ketone initially, which is followed by aerobic oxidation of the benzyl alcohol to the corresponding benzaldehyde catalyzed by the photoinitiator [Mn(L 1 H)(CO) 3 Br] in the presence of visible light, and finally, a KOH-promoted condensation and cyclization to afford quinolines as the final products.

Three platinum(II)-N-heterocyclic carbene(NHC) compounds [Pt(L1)Cl](PF6) (1), [Pt(L2)(COD)](PF6)2 (2) and [Pt(L2)Cl2] (3) were synthesized bearing pyridyl-functionalized butenyl-tethered (L1H) and n-butyl tethered (L2H) NHC ligands, and their antibacterial activity against clinically relevant...