Controlling catalyst microenvironments using proton shuttles and hydrogen bond donors in the secondary coordination sphere is a promising approach for developing catalysts that can affect multiproton and multielectron transfer processes. In this context, three palladium calixpyrrole complexes with pendent amine (1), amide (2), and carbamate (3) groups were examined as electrocatalysts for the hydrogen evolution reaction (HER). Building on prior studies showing that the palladium complexes generated catalytically active heterogeneous HER catalysts in the presence of p-toluenesulfonic acid monohydrate, 1-3 were evaluated using the significantly milder proton source anilinium tetrafluoroborate. The active catalytic species for all three systems was found to be solution-based, and kinetic analysis uncovered a first-order dependence on acid, as well as large H/D kinetic isotope effect values, which were consistent with proton-coupled electron transfer being rate-limiting. The calixpyrrole complexes displayed exceptional activity, achieving kobs and turnover frequency values of 4.65 × 106 s-1, 4.19 × 106 s-1, and 3.09 × 106 s-1 for 1, 2, and 3, respectively. These catalytic activities and rate constants approached the diffusion rate limit and ranked among the fastest HER catalysts to date.

In search of new green and renewable energy sources, the use of hydrogen fuel cells continues to be a promising avenue. However, to unlock the full potential of these cells, the production of hydrogen remains to be the limiting factor, rooted in the need for new efficient catalyst development. To this end, we believe that placing proton shuttles (pendent group) in secondary coordination sphere close to catalytically active metal centre, mimicking that of natural enzymes and in some synthetic catalysts. In this, it will provide protons and electrons, which can decrease overpotential (energy to bring electron) and increase catalytical activity.To be discussed are three palladium complexes bearing calixpyrrole ligands with functionalities in the secondary coordination sphere. The pendent groups varying about the aniline, with no substitution (R= NH 2 ; 1A), an acetate (R=NHC(O)CH 3; 1B) and a Boc group (R=NHC(O)OC(CH) 3 ; 1C). Each of these were electrochemically characterized and electrocatalytic activities for hydrogen production were explored. They showed very fast reaction kinetics in acetonitrile, with high turnover frequencies up to 4,630,000 s -1 for 1A at an overpotential of 0.86 V, up to 4, 689, 000 s -1 for 1B at an overpotential of 0.92 V, and up to 3,690,000 s -1 for 1C at an overpotential of 0.94 V with anilinium tetrafluoroborate (pKa=10.62). To check the importance of pendent groups in hydrogen production, Ni complexes bearing calixpyrrole ligands with the same pendent groups (R=NHC(O)CH 3 ; NHC(O)OC(CH) 3 ) and without pendent group (R=H) were synthesized and electrochemically analyzed. The inclusion of Ni metal, rather than Pd, makes these catalysts far more economic. In this, the Ni compound decreased overpotential for hydrogen production from 0.98V (R=H) to 0.81V (R= NHC(O)OC(CH) 3 ) with anilinium tetrafluoroborate and showed high turnover frequency from 32891s -1 (R=H) to 48931s -1 (R=NHC(O)OC(CH) 3 ) and is giving idea of faster reaction kinetics. The results based on the pendent groups give significant insights into controlling secondary coordination sphere and showed how pendent groups can increase the catalytical activity for hydrogen evolution. Figure 1

A ruthenium phenylidene complex bearing a monodentate phosphinimine ligand (Ru1) was investigated as a ring-opening metathesis polymerization (ROMP) catalyst. Building on prior studies showing ultrafast initiation and a bimolecular mechanism of decomposition, Ru1 was evaluated using norbornene (NBE), cyclooctene (COE), and cyclooctadiene (COD) as model substrates. Kinetic analysis revealed first-order dependence on monomer concentration, consistent with catalyst activation proceeding through a rate-limiting interchange ligand substitution reaction. Ru1 displayed exceptional activity, achieving turnover frequencies (TOFs) up to 4.00 × 104 s-1 and turnover numbers (TONs) up to 232,000 for NBE, TOFs up to 9.88 × 103 s-1 and TONs up to 240,000 for COE, and TOFs up to 1.80 × 102 s-1 with TONs up to 2760 for COD. Experimentally determined rate constants reached 7.9 × 108 M-1·s-1 (NBE), 9.6 × 107 M-1·s-1 (COE), and 1.1 × 105 M-1·s-1 (COD); values that approach the diffusion limit and rival the fastest enzymatic systems. To the best of our knowledge, these are the highest catalytic rates reported for ROMP to date, establishing Ru1 as a benchmark system for ultrafast metathesis polymerization.

Designing ligand architectures that can mimic enzyme active sites is a promising approach for developing efficient small molecule activation catalysts for sustainable energy applications. Some key design features include chemically distinct binding pockets for multiple metal centers and a three-dimensional structure that controls the positioning of catalytic sites. With these principles in mind, mono- and bimetallic unsymmetric cofacial palladium complexes, 2 and 3, respectively, bearing ligands with calixpyrrole and salen coordination sites, or "salixpyrrole" ligands, are reported. These species were accessed in a straightforward Schiff-base reaction with appreciable yields. In addition, both 2 and 3 were found to be active hydrogen evolution electrocatalysts using para-toluenesulfonic acid monohydrate as the proton source. The two salixpyrrole species displayed different mechanisms of action, with 2 showing a second-order dependence on acid concentration, whereas 3 exhibited a first-order dependence. Moreover, the bimetallic catalyst was significantly more efficient, with higher turnover frequencies, 4640 s-1 vs 1680 s-1 for 2, and lower overpotentials, 0.39 V vs 0.69 V for 2. The results reported herein provide proof-of-concept that bimetallic catalysts with chemically distinct binding sites demonstrate enhanced catalytic properties in comparison to monometallic or symmetric analogues.

Hydrogen gas and its production from renewable power sources will be an important part of decreasing global reliance on fossil fuels and developing a sustainable energy economy. Efficient electrocatalysis, however, relies on the delivery of both protons and electrons; ideally in a concerted fashion to avoid high energy intermediates, prevent charge build‐up, and circumvent large kinetic barriers. While this can be achieved using ligand design in homogenous molecular transition metal catalysts (specifically incorporating proton shuttles in the secondary coordination sphere), heterogeneous systems are more desirable from an industrial perspective due to their ease of use and enhanced durability. Supporting transition metal catalysts on electrode surfaces is therefore a promising approach for developing next generation electrocatalysts that retain molecular level control in interfacial environments. This review will first cover key design principles from natural systems, such as hydrogenase enzymes, and then survey some representative examples of synthetic homogeneous hydrogen evolution electrocatalysts that incorporate these important features. We will then discuss transition metal species that have been supported on electrode materials, with a focus on recent advances in the field, and the major challenges that remain.

Incorporating design elements from homogeneous catalysts to construct well defined active sites on electrode surfaces is a promising approach for developing next generation electrocatalysts for energy conversion reactions. Furthermore, if functionalities that control the electrode microenvironment could be integrated into these active sites it would be particularly appealing. In this context, a square planar nickel calixpyrrole complex, Ni(DPMDA) (DPMDA=2,2'-((diphenylmethylene)bis(1H-pyrrole-5,2-diyl))bis(methaneylylidene))bis-(azaneylylidene))dianiline) with pendant amine groups is reported that forms a heterogeneous hydrogen evolution catalyst using anilinium tetrafluoroborate as the proton source. The supported Ni(DPMDA) catalyst was surprisingly stable and displayed fast reaction kinetics with turnover frequencies (TOF) up to 25,900 s(-1) or 366,000 s(-1) cm(-2). Kinetic isotope effect (KIE) studies revealed a KIE of 5.7, and this data, combined with Tafel slope analysis, suggested that a proton-coupled electron transfer (PCET) process involving the pendant amine groups was rate-limiting. While evidence of an outer-sphere reduction of the Ni(DPMDA) catalyst was observed, it is hypothesized that the control over the secondary coordination sphere provided by the pendant amines facilitated such high TOFs and enabled the PCET mechanism. The results reported herein provide insight into heterogeneous catalyst design and approaches for controlling the secondary coordination sphere on electrode surfaces.

A series of three calixpyrrole ligands (1a-c) with pendant nitrogen-based hydrogen bond donors, R, were syn-thesized and coordinated to palladium to produce metal complexes (2a-c), where R = NH2 (a); NHC(O)CH3 (b); or NHC(O)OC(CH3)3 (c). The calixpyrrole compounds were generated using a Schiff-base reaction starting with 5,5 & PRIME;-diformyl-2,2 & PRIME;-diphenyldipyrromethane and an aniline precursor. The deprotonated calixpyrrole species were subsequently bound to palladium to generate distorted square planar complexes. The pendant groups were not coordinated to the metal with Pd-N distances of 3.36 to 4.79 & ANGS;. The electrochemical properties of the palladium complexes were also explored, and 2a-c displayed two irreversible oxidations above 0.0 V vs ferrocene/ferro-cenium (Fc/Fc+), as well as two irreversible reductions below -0.70 V vs Fc/Fc+. Interestingly, the free ligands showed similar electrochemical features, suggesting redox non-innocence. Preliminary reactivity studies indi-cated the palladium complexes did not activate small molecules, but they did catalyze H2 evolution in the presence of acid. The onset of catalysis for 2a-c was approximately 0.4-0.5 V more positive than a glassy carbon electrode, and the active species could undergo 500 scans without significant changes in activity. The catalysts were found to be heterogeneous in nature and adsorbed onto the working electrode.

A ruthenium-based olefin metathesis (OM) catalyst bearing a monodentate triphenylphosphinimine ligand, Ru1, was synthesized, characterized, and its activity for the homocoupling of terminal alkenes was investigated. Utilizing 1-hexene as a model substrate, the empirical rate law for Ru1 was found to be first-order in alkene and complex (indicating that both species were involved in the rate-limiting step), with a rate constant of 0.697 +/- 0.050 M-1 s(-1). Moreover, the experimentally determined activation parameters.S. and.H. (-48.7 +/- 5.1 eu and 3.19 +/- 0.15 kcal/ mol, respectively) were consistent with an associative or associative interchange ligand substitution reaction. When considering the.G. (298 K) value of 17.7 kcal/mol, Ru1 ranked among the fastest initiating ruthenium-based OM catalysts reported in the literature. Density functional theory (DFT) calculations were also performed to explore potential catalytic mechanisms. Two pathways were considered: a traditional mechanism where the phosphinimine ligand de-coordinated and an alternative mechanism where the phosphine donor de-coordinated. Although the energy differences between the two pathways were typically fairly small (1.4-3.5 kcal/mol), the alternative pathway with phosphine de-coordination was energetically more favorable. It is anticipated, however, that both cycles are working in tandem during the catalytic reaction. In addition to kinetic studies, the stability of Ru1 was explored using 1-hexene as a model substrate. The phosphinimine catalyst was found to be mildly oxygen-sensitive and moisture-tolerant. Furthermore, Ru1 was determined to be prone to bimolecular decomposition, through the crystallographic characterization of a key degradation product. There was also strong evidence for NH exchange between the tricyclohexylphosphine and triphenylphosphinimine moieties. Lastly, the substrate scope of Ru1 in regard to a-olefins was explored. Catalytic efficiency dropped with more electron-deficient alkenes, as well as with increasing steric bulk on the substrate, which was consistent with the proposed catalytic mechanism.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

A series of eight scorpionate ligands (Ar)(2)CHR (1-4) were synthesized and coordinated to aluminum to produce Al(CH2CH3)((Ar)(2)CHR) (5-8), where R = CH2OCH3 (1 or 5), CH2N(CH3)(2) (2 or 6), imidazole (3 or 7), or N-methylimidazole (4 or 8), and Ar = 2,4-dimethylphenol (a) or 2-tert-butyl-4-methylphenol (b). The bisphenol compounds were generated using a Friedel-Crafts allcylation reaction starting with commercially available reagents. The scorpionate species were subsequently combined with triethylaluminum in order to probe the coordination geometries of 1-4. The resulting complexes 5-8 displayed distorted tetrahedral structures with the bisphenolate ligands acting as tridentate donors. The only exception was Sa, which formed phenolate bridged dimers in the solid state. Additionally, complexes 5-8 were found to be active catalysts for the ring-opening polymerization of epsilon-caprolactone. The aluminum alkyl species affected this transformation both with and without the use of an alcohol co-catalyst, but higher activities and cleaner activations were seen when 1 equiv of iPrOH was employed. The less sterically hindered complexes with more weakly bound hemilabile "tails" showed the highest activities, with nearly 100% conversion after 1 h at 50 degrees C, and a catalyst loading of 1 mol %.

We synthesized Mo(NC6F5)(CHCMe2Ph)(TPPO)(PPhMe2)Cl (TPPO = 2,3,5,6‐tetraphenylphenoxide), Mo(NC6F5)(CHCMe2Ph)(TTBTO)(PPhMe2)Cl (TTBTO = 2,6‐di(3′,5′‐di‐tert‐butylphenyl)phenoxide), and Mo(NC6F5)(CHCMe2Ph)(TPPO)(PPhMe2)(CF3Pyr) (CF3Pyr = 3,4‐bistrifluoromethylpyrrolide), in order to evaluate them as catalysts for the homocoupling of 3‐methyl‐1‐butene. They were compared with Mo(NC6F5)(CHCMe2Ph)(HMTO)(PPhMe2)Cl (HMTO = 2,6‐dimesitylphenoxide), Mo(NC6F5)(CHCMe2Ph)(HIPTO)(PPhMe2)Cl (HIPTO = 2,6‐di(2′,4′,6′‐triisopropylphenyl)phenoxide), and several other Mo and Ru catalysts. In the best cases turnover numbers (TONs) of 400 – 700 were observed for the homocoupling of 3‐methyl‐1‐butene in a closed vessel (ethylene not removed).

Molybdenum imido alkylidene and tungsten oxo alkylidene complexes that contain a tridentate "pincer" [ONO]2– ligand have been prepared and treated with ethylene to give unsubstituted metallacyclobutane complexes that have a 16e count. Both Mo and W metallacyclobutane complexes exchange C2D4 into the metallacyclobutane ring at 22 °C at a rate that is first order in metal and zero order in C2D4. These metallacycles lose ethylene at least 104–105 times slower than reported 14e unsubstituted Mo and W metallacyclobutane complexes that have been explored in the literature that have a TBP geometry with the metallacyclobutane ring bound in the equatorial positions. Our studies suggest that breaking up the metallacyclobutane ring in these 16e d0 Mo or W complexes is slow because a 14e TBP metallacyclobutane complex cannot be accessed readily.

Our group previously reported the development of iron carbonyl catalysts bearing chiral tridentate P N P' ligands for the asymmetric hydrogenation of prochiral ketones in THF. An NMR study into the activation process identified the amine hydride alkoxide complexes Fe(P-NH-Pl(C0)(H)(012.1) with R' = Me, tBu, or tAmyl and P-NH-P' = PPh(2)CH(2)CH(2)NHCH(2)CH(2)PiPr(2) or (S,S)-PPh2CHPhCHMeNHCH2CH2PCy2. These still required treatment with excess KOtBu and H2(g) to be catalytically active in THF. Both experimental methods and density functional theory (DFT) calculations were used to show that this treatment leads to the formation of a hydride amide complex Fe(P N P')(CO)(H), which reacts with dihydrogen to form cis and trans dihydride complexes Fe(P-NH-P')(CO)(H)(2), identified by NMR spectroscopy. In the presence of KOtBu, NaOtBu, or KOtBu/2,2,2-cryptand and H-2(g), these species are active for the catalytic hydrogenation of acetophenone, whereas in the absence of H-2(g), inactive Fe(O) complexes are formed. Ketone hydrogenation is proposed to occur in an outer-sphere stepwise process, and this enantio-determining step has been modeled by DFT. The calculations suggest that the energy barriers for hydride attack on the ketone or dihydrogen splitting either to the nitrogen of the amide complex in the inner coordination sphere or to the oxygen of an alkoxide group in the outer sphere are similar and that either hydride transfer or dihydrogen splitting could determine the turnover frequency depending on the nature of the ketone.

In the interest of preparing molybdenum and tungsten alkylidene complexes for olefin metathesis that are longer-lived at high temperatures (similar to 150 degrees C or above), we synthesized complexes that contain a phenoxide ligand with a 2-pyridyl in one ortho position and a mesityl (Mes) or 2,4,6-i-Pr3C6H2 (Trip) in the other ortho position ([MesON](-) or [TripON](-), respectively). The alkylidene (neophylidene) complexes that were prepared include W(O)(CHCMe2Ph)(Me(2)Pyr)(RON) (R = Mes or Trip), Mo(NC6F5)(CHCMe2Ph)(RON)Cl, Mo(N-2,6- Me2C6H3)(CHCMe2Ph)(RON)Cl, Mo(N-t-Bu)(CHCMe2Ph)(RON)Cl, and M(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(TripON)(OTf) (M = Mo or W). The reaction between Mo(NAr)(CHCMe2Ph)(TripON)(OTf) and ethylene yielded an ethylene complex, Mo(NAr)(C2H4)(TripON)(OTf)(ether). All neophylidene complexes were essentially unreactive toward terminal olefins at 22 degrees C and showed modest homocoupling activity (at 80 or 100 degrees C) and alkane metathesis activity (at 150 and 200 degrees C). W(O)(CHCMe2Ph)(Me(2)Pyr)(MesON) also stereoselectively polymerized several substituted norbornadienes at 100 degrees C.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Asymmetric transfer hydrogenation is an important transformation for the production of fine chemicals. Traditionally, platinum group metals are used to catalyze this reaction, but recent pressure for greener practices has driven the development of base-metal catalysts. Due to the growing interest in this area of research, the underlying concepts for this type of chemistry are suitable for an undergraduate laboratory. A two-part experiment was adapted from original research and tested for use in an upper-division undergraduate inorganic chemistry laboratory. The first part of the experiment explores the template synthesis of a trans-bis-acetonitrile iron complex with a tetradentate ligand. The product from the first laboratory period is used as the starting material in the second laboratory period that focuses on the synthesis of a green catalyst mimic. Students learn about multi-step synthesis, specifically the synthesis of a catalyst for green chemistry, and key inorganic chemistry concepts to help reinforce topics introduced during lectures.

A simple method for synthesizing diphosphine monosulfide species was developed utilizing lithium sulfide and chlorophosphine starting materials. This afforded 1,1,2,2-tetraphenyldiphosphine monosulfide (1), as well as 1,1,2,2-tetracyclohexyldiphosphine monosulfide (2), which could be used as convenient ligand precursors. Upon addition of 1 or 2 to the ruthenium compound Ru(C5Me5)(cod)Cl, the diphosphine monosulfides rearranged to give bidentate bis(ditertiaryphosphino)thioether ligands in Ru(C5Me5)(PPh2SPPh2)Cl (3) and Ru(C5Me5)(PCy2SPCy2)Cl (4).