Crystallization of Arthrobacter sp. strain 1C N-(1-D-carboxyethyl)-L-norvaline dehydrogenase and its complex with NAD+

The novel NAD+-linked opine dehydrogenase from a soil isolate Arthrobacter sp. strain 1C belongs to an enzyme superfamily whose members exhibit quite diverse substrate specificites. Crystals of this opine dehydrogenase, obtained in the presence or absence of co-factor and substrates, have been shown to diffract to beyond 1.8 ? resolution. X-ray precession photographs have established that the crystals belong to space group P21212, with cell parameters a = 104.9, b = 80.0, c = 45.5 ? and a single subunit in the asymmetric unit. The elucidation of the three-dimensional structure of this enzyme will provide a structural framework for this novel class of dehydrogenases to enable a comparison to be made with other enzyme families and also as the basis for mutagenesis experiments directed towards the production of natural and synthetic opine-type compounds containing two chiral centres.     

Organometallic ni pincer complexes for the electrocatalytic production of hydrogen

Nonplatinum metals are needed to perform cost-effective water reduction electrocatalysis to enable technological implementation of a proposed hydrogen economy. We describe electrocatalytic proton reduction and H(2) production by two organometallic nickel complexes with tridentate pincer ligands. The kinetics of H(2) production from voltammetry is consistent with an overall third order rate law: the reaction is second order in acid and first order in catalyst. Hydrogen production with 90-95% Faradaic yields was confirmed by gas analysis, and UV-vis spectroscopy suggests that the ligand remains bound to the catalyst over the course of the reaction. A computational study provides mechanistic insights into the proposed catalytic cycle. Furthermore, two proposed intermediates in the proton reduction cycle were isolated in a representative system and show a catalytic response akin to the parent compound.     

An Unusual Example of Hypervalent Silicon: A Five‐Coordinate Silyl Group Bridging Two Palladium or Nickel Centers through a Nonsymmetrical Four‐Center Two‐Electron Bond

Pd and Ni dimers supported by PSiP ligands in which two hypervalent five‐coordinate Si atoms bridge the two metal centers are reported. Crystallographic characterization revealed a rare square‐pyramidal geometry at Si and an unusual asymmetric M₂Si₂ core (M=Pd or Ni). DFT calculations showed that the unusual structure of the core is also found in a model in which the phosphine and Si centers are not part of a pincer group, thus indicating that the observed geometry is not imposed by the PSiP ligand. NBO analysis showed that an asymmetric four‐center two‐electron (4c‐2e) bond stabilizes the hypervalent Si atoms in the M₂Si₂ core.     

Synthesis of PCP-supported nickel complexes and their reactivity with carbon dioxide

The Ni amide and hydroxide complexes [(PCP)Ni(NH(2))] (2; PCP=bis-2,6-di-tert-butylphosphinomethylbenzene) and [(PCP)Ni(OH)] (3) were prepared by treatment of [(PCP)NiCl] (1) with NaNH(2) or NaOH, respectively. The conditions for the formation of 3 from 1 and NaOH were harsh (2 weeks in THF at reflux) and a more facile synthetic route involved protonation of 2 with H(2)O, to generate 3 and ammonia. Similarly the basic amide in 2 was protonated with a variety of other weak acids to form the complexes [(PCP)Ni(2-Me-imidazole)] (4), [(PCP)Ni(dimethylmalonate)] (5), [(PCP)Ni(oxazole)] (6), and [(PCP)Ni(CCPh)] (7), respectively. The hydroxide compound 3, could also be used as a Ni precursor and treatment of 3 with TMSCN (TMS=trimethylsilyl) or TMSN(3) generated [(PCP)Ni(CN)] (8) or [(PCP)Ni(N(3))] (9), respectively. Compounds 3-7, and 9 were characterized by X-ray crystallography. Although 3, 4, 6, 7, and 9 are all four-coordinate complexes with a square-planar geometry around Ni, 5 is a pseudo-five-coordinate complex, with the dimethylmalonate ligand coordinated in an X-type fashion through one oxygen atom, and weakly as an L-type ligand through another oxygen atom. Complexes 2-9 were all reacted with carbon dioxide. Compounds 2-4 underwent facile reaction at low temperature to form the κ(1)-O carboxylate products [(PCP)Ni{OC(O)NH(2)}] (10), [(PCP)Ni{OC(O)OH}] (11), and [(PCP)Ni{OC(O)-2-Me-imidazole}] (12), respectively. Compounds 10 and 11 were characterized by X-ray crystallography. No reaction was observed between 5-9 and carbon dioxide, even at elevated temperatures. DFT calculations were performed to model the thermodynamics for the insertion of carbon dioxide into 2-9 to form a κ(1)-O carboxylate product and understand the pathways for carbon dioxide insertion into 2, 3, 6, and 7. The computed free energies indicate that carbon dioxide insertion into 2 and 3 is thermodynamically favorable, insertion into 8 and 9 is significantly uphill, insertion into 5 and 7 is slightly uphill, and insertion into 4 and 6 is close to thermoneutral. The pathway for insertion into 2 and 3 has a low barrier and involves nucleophilic attack of the nitrogen or oxygen lone pair on electrophilic carbon dioxide. A related stepwise pathway is calculated for 7, but in this case the carbon of the alkyne is significantly less nucleophilic and as a result, the barrier for carbon dioxide insertion is high. In contrast, carbon dioxide insertion into 6 involves a single concerted step that has a high barrier.     

Secondary coordination sphere interactions facilitate the insertion step in an iridium(III) CO2 reduction catalyst

There is considerable interest in both catalysts for CO(2) conversion and understanding how CO(2) reacts with transition metal complexes. Here we develop a simple model for predicting the thermodynamic favorability of CO(2) insertion into Ir(III) hydrides. In general this reaction is unfavorable; however, we demonstrate that with a hydrogen bond donor in the secondary coordination sphere it is possible to isolate a formate product from this reaction. Furthermore, our CO(2) inserted product is one of the most active water-soluble catalysts reported to date for CO(2) hydrogenation.     

Exploring the reactions of CO2 with PCP supported nickel complexes

The reactions of PCP supported Ni hydride, methyl and allyl species with CO(2) to generate Ni carboxylates are described. Computational studies suggest that all three reactions follow different pathways.     

Synthesis of PCP‐Supported Nickel Complexes and their Reactivity with Carbon Dioxide

The Ni amide and hydroxide complexes [(PCP)Ni(NH₂)] ( 2 ; PCP=bis‐2,6‐di‐tert‐butylphosphinomethylbenzene) and [(PCP)Ni(OH)] ( 3 ) were prepared by treatment of [(PCP)NiCl] ( 1 ) with NaNH₂ or NaOH, respectively. The conditions for the formation of 3 from 1 and NaOH were harsh (2 weeks in THF at reflux) and a more facile synthetic route involved protonation of 2 with H₂O, to generate 3 and ammonia. Similarly the basic amide in 2 was protonated with a variety of other weak acids to form the complexes [(PCP)Ni(2‐Me‐imidazole)] ( 4 ), [(PCP)Ni(dimethylmalonate)] ( 5 ), [(PCP)Ni(oxazole)] ( 6 ), and [(PCP)Ni(CCPh)] ( 7 ), respectively. The hydroxide compound 3 , could also be used as a Ni precursor and treatment of 3 with TMSCN (TMS=trimethylsilyl) or TMSN₃ generated [(PCP)Ni(CN)] ( 8 ) or [(PCP)Ni(N₃)] ( 9 ), respectively. Compounds 3–7 , and 9 were characterized by X‐ray crystallography. Although 3 , 4 , 6 , 7 , and 9 are all four‐coordinate complexes with a square‐planar geometry around Ni, 5 is a pseudo‐five‐coordinate complex, with the dimethylmalonate ligand coordinated in an X‐type fashion through one oxygen atom, and weakly as an L‐type ligand through another oxygen atom. Complexes 2–9 were all reacted with carbon dioxide. Compounds 2 – 4 underwent facile reaction at low temperature to form the κ¹‐O carboxylate products [(PCP)Ni{OC(O)NH₂}] ( 10 ), [(PCP)Ni{OC(O)OH}] ( 11 ), and [(PCP)Ni{OC(O)‐2‐Me‐imidazole}] ( 12 ), respectively. Compounds 10 and 11 were characterized by X‐ray crystallography. No reaction was observed between 5 – 9 and carbon dioxide, even at elevated temperatures. DFT calculations were performed to model the thermodynamics for the insertion of carbon dioxide into 2 – 9 to form a κ¹‐O carboxylate product and understand the pathways for carbon dioxide insertion into 2 , 3 , 6 , and 7 . The computed free energies indicate that carbon dioxide insertion into 2 and 3 is thermodynamically favorable, insertion into 8 and 9 is significantly uphill, insertion into 5 and 7 is slightly uphill, and insertion into 4 and 6 is close to thermoneutral. The pathway for insertion into 2 and 3 has a low barrier and involves nucleophilic attack of the nitrogen or oxygen lone pair on electrophilic carbon dioxide. A related stepwise pathway is calculated for 7 , but in this case the carbon of the alkyne is significantly less nucleophilic and as a result, the barrier for carbon dioxide insertion is high. In contrast, carbon dioxide insertion into 6 involves a single concerted step that has a high barrier.