Electrocatalytic Reduction of CO2 and H2O with Zn(II) Complexes Through Metal-Ligand Cooperation.

Air and water-stable zinc (II) complexes of neutral pincer bis(diphenylphosphino)-2,6-di(amino)pyridine ("PN3P") ligands are reported. These compounds, [Zn(κ2-2,6-{Ph2PNR}2(NC5H3))Br2] (R=Me, 1; R=H, 2), were shown to be capable of electrocatalytic reduction of CO2 at -2.3 V vs. Fc+/0 to selectively yield CO in mixed water/acetonitrile solutions. These complexes also electrocatalytically generate H2 from water in acetonitrile solutions, at the same potential, with Faradaic efficiencies of up to 90 %. DFT computations support a proposed mechanism involving the first reduction of 1 or 2 occurring at the PN3P ligand. Furthermore, computational analysis suggested a mechanism involving metal-ligand cooperation of a Lewis acidic Zn(II) and a basic ligand.

Electrocatalytic H2 Generation from Water Relying on Cooperative Ligand Electron Transfer in "PN3 P" Pincer-Supported NiII Complexes.

Water is the most sustainable source for H2 production, and the efficient electrocatalytic production of H2 from mixed water/acetonitrile solutions by using two new air-stable nickel(II) pincer complexes, [Ni(κ3 -2,6-{Ph2 PNR}2 (NC5 H3 )Br2 ] (R=H I, Me II) is reported. Hydrogen generation from H2 O/CH3 CN solutions is initiated at -2 V against Fc+/0 , and bulk electrocatalysis studies showed that the catalyst functions with an excellent Faradaic efficiency and a turnover frequency of 160 s-1 . A DFT computational investigation of the reduction behavior of I and II revealed a correlation of H2 formation with charge donation from electrons originating in a reduced ligand-localized orbital. As a result, these catalysts are proposed to proceed by a novel mechanism involving electron/proton transfer between a Ni0I species bonded to an anionic PN3 P ligand ("L- /Ni0I ") and a NiI -hydride ("Ni-H"). Furthermore, these catalysts are able to reduce phenol and acetic acid, more active proton sources, at lower potentials that correlate with the substrate pKa .

Organochalcogen ligands in catalysis of oxidation of alcohols and transfer hydrogenation.

Organochalcogen compounds have been used as the building blocks for the development of a variety of catalysts that have been studied comprehensively during the last two decades for several chemical transformations. Transfer hydrogenation (reduction of carbonyl compounds to alcohols) and oxidation of alcohols (conversion of alcohols to their respective ketones and aldehydes) are also among such chemical transformations. Some compilations are available in the literature on the development of catalysts, based on organochalcogen ligands, and their applications in Heck reaction, Suzuki reaction, and other related aspects. Some review articles have also been published on different aspects of oxidation of alcohols and transfer hydrogenation. However, no such article is available in the literature on the syntheses and use of organochalcogen ligated catalysts for these two reactions. In this perspective, a survey of developments pertaining to the synthetic aspects of such organochalcogen (S/Se/Te) based catalysts for the two reactions has been made. In addition to covering the syntheses of chalcogen ligands, their metal complexes and nanoparticles (NPs), emphasis has also been placed on the efficient conversion of different substrates during catalytic reactions, diversity in catalytic potential and mechanistic aspects of catalysis. It also includes the analysis of comparison (in terms of efficiency) between this unique class of catalysts and efficient catalysts without a chalcogen donor. The future scope of this area has also been highlighted.

Visible-Light Photocatalytic Reduction of CO2 to Formic Acid with a Ru Catalyst Supported by N,N'-Bis(diphenylphosphino)-2,6-diaminopyridine Ligands.

Visible-light photocatalytic CO2 reduction is carried out by using a RuII complex supported by N,N'-bis(diphenylphosphino)-2,6-diaminopyridine ("PNP") ligands, an unprecedented molecular architecture for this reaction that breaks the longstanding domination of α-diimine ligands. These competent catalysts transform CO2 into formic acid with high selectivity and turnover number. A proposed mechanism, with combined electron transfer and catalytic cycles, models the experimental rate of formic acid production.

Ultra-small palladium nano-particles synthesized using bulky S/Se and N donor ligands as a stabilizer: application as catalysts for Suzuki-Miyaura coupling.

Two chalcogenated ligands L1 and L2 containing anthracene core and amine functionality have been synthesized. Both the ligands have been characterized using 1H and 13C{1H} NMR techniques. The structure of L1 has also been corroborated by single crystal X-ray diffraction. Application of L1 and L2 as stabilizers for palladium nano-particles (NPs) has been explored and six different types of NPs 1-6 have been prepared by varying the quantity of stabilizer. The nano-particles have been characterized by PXRD, EDX, and HRTEM techniques. The size of NPs has been found to be in the range of ∼1-2 nm, 2-3 nm, 4-6 nm, 1-2 nm, 1-2 nm and 3-5 nm for 1-6 respectively. The catalytic activities of 1-6 have been explored for Suzuki-Miyaura coupling of phenyl boronic acid with various aryl halides. These NPs showed good catalytic activity for various aryl chlorides/bromides at low catalyst loading (5 mg). Among 1-6, the highest activity has been observed for NPs 1, probably due to their relatively small size and high uniformity in the dispersion. The recyclability of the NPs upto 5 catalytic cycles is a distinct advantage.

Electro- and Photocatalytic Generation of H2 Using a Distinctive CoII "PN3 P" Pincer Supported Complex with Water or Saturated Saline as a Hydrogen Source.

Efficient electrocatalytic production of H2 from mixed water/acetonitrile solutions was achieved using three new CoII complexes supported by the neutral pincer ligand bis(diphenylphosphino)-2,6-di(methylamino)pyridine ("PN3 P"). At -1.9 V vs. Fc/Fc+ , these catalysts showed 96 % Faradaic efficiency with added water or saturated aqueous saline at rates of up to 316 L(mol cat)-1 (cm2 )-1 h-1 using a glassy carbon working electrode. The complex [Co(κ3 -2,6-{Ph2 PNMe}2 (NC5 H3 )Br2 ] (1) was also able to photocatalytically reduce water to hydrogen in the presence of a Ru(bpy)32+ photosensitizer and a reductant.

Electrocatalytic generation of H2 from neutral water in acetonitrile using manganese polypyridyl complexes with ligand assistance.

A Mn(i) tris(2-pyridylmethyl)amine complex fac-[Mn(κ3-tpa) (CO)3]+OTf- carries out electrocatalytic hydrogen evolution from neutral water in acetonitrile. Bulk electrocatalytic studies showed that the catalyst functions with a moderate Faradaic efficiency and turn over frequency. DFT computations support the role of the tpa ligand as a shuttle to transfer of protons to the metal center.

New Experimental Insight into the Nature of Metal-Metal Bonds in Digallium Compounds: J Coupling between Quadrupolar Nuclei.

Multiple bonding between atoms is of ongoing fundamental and applied interest. Here, we report a multinuclear ((1) H, (13) C, and (71) Ga) solid-state magnetic resonance spectroscopic study of digallium compounds which have been proposed, albeit somewhat controversially, to contain single, double, and triple Ga-Ga bonds. Of particular relevance to the nature of these bonds, we have carried out two-dimensional (71) Ga J/D-resolved NMR experiments which provide a direct measurement of J((71) Ga,(71) Ga) spin-spin coupling constants across the gallium-gallium bonds. When placed in the context of clear-cut experimental data for analogous singly, doubly, and triply bonded carbon spin pairs or boron spin pairs, the (71) Ga NMR data clearly support the notion of a different bonding paradigm in the gallium systems. Our findings are consistent with an increasing role across the purported gallane-gallene-gallyne series for classical and/or slipped π-type bonding orbitals.

Electrocatalytic reduction of CO2 using Mn complexes with unconventional coordination environments.

New complexes, Mn{κ(3)-[2,6-{Ph2PNMe}2(NC5H3)]}(CO)3(+)Br(-) (1(+)Br(-)) and MnBr{κ(2)-(Ph2P)NMe(NC5H4)}(CO)3 (2), are reported and present new ligand environments for CO2 electrocatalytic reduction to CO. Compound 1(+) presents a unique metal geometry for CO production (96%) in the absence of added water while 2 required addition of water and generated both CO and H2 products.

Cu-Based Nanocomposites as Multifunctional Catalysts.

Herein, we report the synthesis of Cu/Cu2 O nanocomposites by a one-step hydrothermal process at 180 °C, for which the resulting morphology is dependent on the hydrothermal reaction time (24, 72, and 120 h). With a longer reaction time of 120 h, a rod-shape morphology is obtained, whereas at 72 and 24 h assemblies of nanoparticles are obtained. The rod-shaped (120 h) particles of the Cu/Cu2 O nanocomposites show a much higher efficiency (6.3 times) than the agglomerates and 2.5 times more than the assemblies of nanoparticles for the hydrogen-evolution reaction. During the oxygen-evolution reaction, the nanorods produce a current that is 5.2 and 3.7 times higher than that produced by the agglomerated and assembled nanoparticles, respectively. The electrocatalysts are shown to be highly stable for over 50 cycles. As catalysts for organic synthesis, a 100 % yield is achieved in the Sonogashira cross-coupling reaction with the nanorods, which is higher than with the other nanocomposite particles. This result demonstrates the significant enhancement of yield obtained with the nanorods for cross-coupling reactions.

Coinage metal complexes supported by a "PN(3)P" scaffold.

A series of monovalent group 11 complexes, [2,6-{Ph2PNMe}2(NC5H3)]CuBr 1, [2,6-{Ph2PNMe}2(NC5H3)]CuOTf 2, [2,6-{Ph2PNMe}2(NC5H3)]AgOTf 3, and [2,6-{Ph2PNMe}2(NC5H3)](AuCl)24, supported by a neutral PN(3)P ligand have been synthesized and characterized by multinuclear NMR and single crystal X-ray diffraction studies. The variation of the coordination properties were analyzed and electronic structure calculations have been carried out to provide insight on the bonding details in these complexes. The Cu(I) complexes displayed an unusual coordination geometry with a tridentate pincer ligand and an overall four coordinate trigonal pyramidal geometry. In contrast the Ag(I) analogue displayed a bidentate κ(2)-P,P' ligation leaving the pyridyl-N atom uncoordinated and yielding a pyramidalized trigonal planar geometry around Ag. The bimetallic Au(I) complex completed the series and displayed a monodentate P-bonded ligand and a linear coordination geometry.

Efficient catalysis of Suzuki-Miyaura C-C coupling reactions with palladium(II) complexes of partially hydrolyzed bisimine ligands: a process important in environment context.

Potentially hexadentante [O(-),N,E:E,N,O(-)] chalcogenated bisimine ligands L1-L3 have been synthesized by reaction of 1,1'-(4,6-dihydroxy-1,3-phenylene)bisethanone with H2N(CH2)2SPh, H2N(CH2)2SePh and H2N(CH2)2TeC6H4-4-OMe respectively. The L1-L3 react with Na2PdCl4 resulting in their partial hydrolysis, which appears to be metal-promoted. Of the two [(CH3)CN(CH2)2EAr] fragments of L1-L3, one is converted to (CH3)CO and H2N(CH2)2EAr eliminated. The hydrolysis products 1-[C(CH3)N(CH2)2SPh]-3-[C(CH3)O]-4,6-[OH]2C6H2 (L1'), 1-[C(CH3)N(CH2)2SePh]-3-[C(CH3)O]-4,6-[OH]2C6H2 (L2') and 1-[C(CH3)N(CH2)2TeC6H4-4-OMe]-3-[C(CH3)O]-4,6-[OH]2C6H2 (L3') have formed complexes [PdCl(L'-H)] (1, 3 and 5). The other product of hydrolysis H2N(CH2)2EAr (L″) reacted with Na2PdCl4 yielding the complexes [PdL"Cl2] (2, 4 and 6). All the complexes (1-6) were found thermally and air stable. Complexes 1, 3 and 5 have been investigated as catalysts for Suzuki-Miyaura CC coupling reactions. The catalytic activities of 1 and 3 which are palladium complexes of S- and Se-containing Schiff base derivatives respectively, were found good for the Suzuki-Miyaura cross-coupling of aryl bromides with phenylboronic acid under mild reaction conditions. The Pd(II) complex (3) of selenated ligand was found active to catalyze the coupling of 2-chlorobenzaldehyde and 3-chlorotoluene. The activity of Te analog was found to be the lowest one as it failed in catalyzing the coupling of electronically deactivated aryl bromides.

Organosulphur and related ligands in Suzuki-Miyaura C-C coupling.

Suzuki-Miyaura C-C cross coupling (SMC), an important synthetic strategy for many organic molecules, has several advantages such as mild reaction conditions, high tolerance toward various functional groups and ease in isolation of the product. Palladium(II) ligated with phosphines (particularly bulky and electron-rich) and N-heterocyclic carbenes (NHCs) has been found to be efficient in the catalysis of SMC. The drawback with many of these catalysts is their air/moisture sensitivity. Since 2000, palladium(II) complexes of organosulphur and related ligands have emerged as viable alternatives to palladium-phosphine/carbene complexes as they have sufficient thermal stability, air and moisture insensitivity. Moreover synthesis of complexes of such ligands is easy. In this perspective Suzuki-Miyaura C-C coupling reactions catalyzed with palladium(II)-complexes of organosulphur ligands have been reviewed. Catalysis of SMC with palladium(II) complexes of organoselenium and tellurium ligands, studied much less in comparison to those of organosulphur ligands, is also included.

Didocosyl selenide stabilized recyclable Pd(0) nanoparticles and coordinated palladium(II) as efficient catalysts for Suzuki-Miyaura coupling.

First selenium ligand stabilized Pd(0) nanoparticles (∼3-5 nm) catalyze Suzuki-Miyaura C-C coupling in short time and are recyclable (up to 94% yield after 5 reuses). The air stable compound [PdCl(2)(L1)(2)] shows high catalytic efficiency for this coupling as its 3 × 10(-5) mol% is sufficient for activated ArBr.

Palladium(II)-selenated Schiff base complex catalyzed Suzuki-Miyaura coupling: dependence of efficiency on alkyl chain length of ligand.

The new selenated Schiff bases L1-L4 which differ in the chain lengths (longest in L4) of non-coordinating substituents and their square planar complexes [Pd(L-H)Cl] (1-4) [L = L1-L4, behaving as (Se, N, O(-)) ligand] have been synthesized and characterized by multinuclei NMR. The molecular structure of 1 has been elucidated by X-ray diffraction on its single crystal [Pd-Se = 2.3965(9) Å]. All the complexes 1-4 (0.5 mol%) have been found suitable to catalyze Suzuki-Miyaura coupling reactions under mild conditions. The activity of 4 which has ligand L4 has been found highest. The formation of palladium(0) nano-particles (NPs) stablilized by organoselenium species appears to be the catalytic pathway. The length of the pendent alkyl chain present in the complex molecule unprecedentedly controls the dispersion and composition of these particles and consequently the catalytic activity.

Palladium(II)-selenoether complexes as new single source precursors: first synthesis of Pd4Se and Pd7Se4 nanoparticles.

Pd(4)Se and Pd(7)Se(4) nanoparticles (size 38-104 nm), protected by TOP, have been obtained for the first time using Pd(II) ligated with selenated primary and secondary amines (see 1 and 2) as single source precursors respectively. TEM, SEM, powder XRD and photoluminescence have been used to characterize them. 1 and 2 are also the first Pd(II)-selenoether complexes used for the synthesis of nanoparticles containing palladium and selenium.

Palladacycle containing nitrogen and selenium: highly active pre-catalyst for the Suzuki-Miyaura coupling reaction and unprecedented conversion into nano-sized Pd17Se15.

Newly synthesized, air and moisture insensitive palladacycle [PdCl(L-H)] (L = (C(6)H(5))(2-HOC(6)H(4))CHNH(CH(2))(3)SePh; a (N, Se, C(-)) ligand) shows high catalytic activity for the Suzuki-Miyaura coupling reaction of phenylboronic acid with aryl/heteroaryl chlorides/bromides (TON for aryl chlorides up to 9200) and is converted to approximately 8 nm size particles of Pd(17)Se(15) (probably the real catalyst).