Dehydrogenation of formic acid over various Ru‐arene complexes containing N‐donor chelating ligands was investigated in H2O and isolated and characterized several important catalytic intermediate species to elucidate the reaction pathway for formic acid dehydrogenation. Among the studied complexes, Ru‐arene complexes, namely [(η6‐C6H6)Ru(κ2‐NpyNH2‐AmQ)Cl]+ (C‐2), [(η6‐C10H14)Ru(κ2‐NpyNH2‐AmQ)Cl]+ (C‐3) and [(η6‐C6H6)Ru(κ2‐NpyNHMe‐MAmQ)Cl]+ (C‐4) [AmQ = 8‐aminoquinoline and MAmQ = 8‐(N‐methylamino)quinoline] were proved to be the efficient catalysts for formic acid dehydrogenation at 90 °C, even in the absence of base. With an initial TOF of 940 h–1, complex C‐4 displayed the highest catalytic activity for formic acid dehydrogenation in H2O and it can be recycled up to 5 times with a TON of 2248. Effect of temperature, pH, formic acid and catalyst concentration on the reaction kinetics were also investigated in detail. Extensive mechanistic investigations using mass spectrometry and NMR evidenced the formation of a coordinatively unsaturated species [(η6‐C6H6)Ru(κ2‐NpyNH‐AmQ)]+ (C‐2A)/[(η6‐C6H6)Ru(κ2‐NpyNMe‐MAmQ)]+ (C‐4A) as the active component during the catalytic dehydrogenation of formic acid. We further characterized the dimer‐form of C‐2A, possibly the catalyst resting state, by single‐crystal X‐ray crystallography.

A cobalt-nitrosyl complex, [(BPI)Co(NO)(OAc)], 1 {BPI = 1,3-bis(2'-pyridylimino)isoindol} was prepared and characterized. Structural characterization revealed that the cobalt center has a distorted square pyramidal geometry with the NO group coordinated from the apical position in a bent fashion. The addition of dioxygen (O) to the dichloromethane solution of complex 1 resulted in the formation of nitro complex, [(BPI)Co(NO)(OAc)], 2. It was characterized structurally. Kinetic studies suggested the involvement of an associative mechanism. FT-IR spectroscopic studies suggested the formation of the intermediate 1a [(BPI)Co(NO)(O)(OAc)] in the reaction. The intermediate 1a decomposed to complex 2 via a presumed peroxynitrite intermediate which was implicated by its characteristic phenol ring nitration reaction.

A Co(II) complex, [Co(L)]Cl, 1 of the ligand L (L = bis(2-ethyl-4-methylimidazol-5-yl)methane) upon reaction with HO in methanol solution at -40 °C resulted in the formation of the corresponding Co(III)-peroxo complex [Co(L)(O)] (2). The addition of NO gas to the freshly generated solution of the complex 2 led to the formation of the Co(II)-nitrato complex 3 through the putative formation of a Co(II)-peroxynitrite intermediate, 2a. The intermediate 2a was found to mediate the nitration of the externally added phenol resembling the nitration of tyrosine in biological systems.

A series of half sandwich arene–ruthenium complexes [(η 6-arene)­RuCl­(κ 2-L)]+ ([Ru]-1–[Ru]-10) containing bis-imidazole methane-based ligands {4,4′-(phenylmethylene)­bis­(2-ethyl-5-methyl-1H-imidazole)} (L1), {4,4′-((4-methoxyphenyl)­methylene)­bis­(2-ethyl-5-methyl-1H-imidazole)} (L2), {4,4′-((2-methoxyphenyl)­methylene)­bis­(2-ethyl-5-methyl-1H-imidazole)} (L3), {4,4′-((4-chlorophenyl)­methylene)­bis­(2-ethyl-5-methyl-1H-imidazole)} (L4), and {4,4′-((2-chlorophenyl)­methylene)­bis­(2-ethyl-5-methyl-1H-imidazole)} (L5) are synthesized. The synthesized and purified complexes ([Ru]-1–[Ru]-10) are further employed for hydrogen production from formic acid in aqueous medium. Among the investigated complexes, [(η 6-p-cymene)­RuCl­(κ 2-L2)]+ [Ru]-2, having Ru­(II) coordinated 4-methoxy phenyl substituted bis-imidazole methane ligand (L2), outperformed over others, displaying a higher catalytic turnover of 8830 and high efficiency (TOF = 1545 h–1) with appreciably high long-term stability for formic acid dehydrogenation in water.

The activation of nitric oxide (NO) by transition metal complexes has attracted a wide range of research activity. To study the role of ligand denticity on the NO reactivity of Co(ii) complexes, three complexes (, and ) were prepared with ligands , and [ = N(1),N(2)-bis(2,4,6-trimethylbenzyl)ethane-1,2-diamine; = N(1)-(2,4,6-trimethylbenzyl)-N(2)-(2-((2,4,6-trimethylbenzyl)amino)ethyl)ethane-1,2-diamine] and = N(1)-(2,4,6-trimethylbenzyl)-N(2),N(2)-bis(2-((2,4,6-trimethylbenzyl)amino)ethyl)ethane-1,2-diamine], respectively. The complexes differ from each other in terms of denticity and flexibility of the ligand frameworks. In degassed methanol solution, they were exposed to NO gas and their reactivity was studied using various spectroscopic techniques. In the case of complex with a bidentate ligand, reductive nitrosylation of the metal ion with concomitant dinitrosation of the ligand framework was observed. Complex with a tridentate ligand did not undergo reductive nitrosylation; rather, the formation of [Co(III)(NO(-))] was observed. The nitrosyl complexes were isolated and structurally characterized. On the other hand, complex with a tetradentate tripodal ligand did not react with NO. This can be attributed to the geometry of the complex as well as due to the accessibility of the corresponding redox potential.

The cobalt porphyrin complex [(ClTPP)Co], 1, {ClTPP = 5,10,15,20-tetrakis(4'-chlorophenyl)porphyrinate dianion} in dichloromethane solution was subjected to react with nitric oxide (NO) gas and resulted in the formation of the corresponding nitrosyl complex [(ClTPP)Co(NO)], 2, having {CoNO} description. It was characterized by spectroscopic studies and single-crystal X-ray structure determination. It did not react with dioxygen. However, in CHCl/CHCN solution, it reacted with HO to result in the Co-nitrito complex [(ClTPP)Co(NO)], 3, with the simultaneous release of O. It induced ring nitration to the added phenol in an appreciable yield. The reaction presumably proceeds through the formation of corresponding Co-peroxynitrite intermediate.

Four new complexes containing the bis­(pentamethylcyclopentadienyl)­thorium­(IV) moiety, Cp*2Th­(L1)­(Me) (Th2), Cp*2Th­(L2)­(Me) (Th3), Cp*2Th­(L1)Cl (Th5), and Cp*2Th­(L2)Cl (Th6), were synthesized in quantitative yields via the protonolysis reaction of the metallocene precursor complexes Cp*2Th­(Me)2 (Th1) and Cp*2Th­(Me)Cl (Th4) and the respective six- and seven-membered N-heterocyclic neutral imine ligands L1H and L2H. The molecular structures of all the complexes were established by single-crystal X-ray structure analyses. The synthesized complexes along with the precursor complexes were employed as catalysts for the cyanosilylation reaction of ketones with trimethylsilyl cyanide (Me3SiCN). The removal of the iminato ligand is necessary to trigger the reaction, allowing the formation of the active catalyst.

Ni(ii) complex of ligand ( = bis(2-ethyl-4-methylimidazol-5-yl)methane) in methanol solution reacts with an equivalent amount of NO resulting in a corresponding Ni(i) complex. Adding further NO equivalent affords a Ni(i)-nitrosyl intermediate with the {NiNO}(10) configuration. This nitrosyl intermediate upon subsequent reaction with additional NO results in the release of N2O and formation of a Ni(ii)-nitrito complex. Crystallographic characterization of the nitrito complex revealed a symmetric η(2)-O,O-nitrito bonding to the metal ion. This study demonstrates the reductive nitrosylation of a Ni(ii) center followed by N2O release in the presence of excess NO.