When a Twist Makes a Difference: Exploring PCET and ESIPT on a Nonplanar Hydrogen-Bonded Donor-Acceptor System.

Bioinspired benzimidazole-phenol constructs with an intramolecular hydrogen bond connecting the phenol and the benzimidazole have been synthesized to study both proton-coupled electron transfer (PCET) and excited-state intramolecular proton transfer (ESIPT) processes. Strategic incorporation of a methyl group disrupts the coplanarity between the aromatic units, causing a pronounced twist, weakening the intramolecular hydrogen bond, decreasing the phenol redox potential, reducing the chemical reversibility, and quenching the fluorescence emission. Infrared spectroelectrochemistry and transient absorption spectroscopy confirm the formation of the oxidized product upon PCET and probe excited-state relaxation mechanisms, respectively.

Comparing the Electronic Structure of Iron, Cobalt, and Nickel Compounds That Feature a Phosphine-Substituted Bis(imino)pyridine Chelate.

It was recently discovered that (PDI)Mn (PDI = pyridine diimine) exists as a superposition of low-spin Mn(II) that is supported by a PDI dianion and intermediate-spin Mn(II) that is antiferromagnetically coupled to a triplet PDI dianion, a finding that encouraged the synthesis and electronic structure evaluation of late first row metal variants that feature the same chelate. The addition of PDI to FeBr resulted in bromide dissociation and the formation of [(PDI)FeBr][Br]. Reduction of this precursor using excess sodium amalgam afforded (PDI)Fe, which possesses an Fe(II) center that is supported by a dianionic PDI ligand.

Multi PCET in symmetrically substituted benzimidazoles.

Proton-coupled electron transfer (PCET) reactions depend on the hydrogen-bond connectivity between sites of proton donors and acceptors. The 2-(2'-hydroxyphenyl) benzimidazole (BIP) based systems, which mimic the natural Tyr-His190 pair of Photosystem II, have been useful for understanding the associated PCET process triggered by one-electron oxidation of the phenol. Substitution of the benzimidazole by an appropriate terminal proton acceptor (TPA) group allows for two-proton translocations.

Reaction of a Molybdenum Bis(dinitrogen) Complex with Carbon Dioxide: A Combined Experimental and Computational Investigation.

Refluxing Mo(CO) in the presence of the phosphine-functionalized α-diimine ligand DI allowed for substitution and formation of the dicarbonyl complex, (DI)Mo(CO). Oxidation with I followed by heating resulted in further CO dissociation and isolation of the corresponding diiodide complex, (DI)MoI. Reduction of this complex under a N atmosphere afforded the corresponding bis(dinitrogen) complex, (DI)Mo(N).

The electronic structure of a β-diketiminate manganese hydride dimer.

The electronic structure of a dimeric manganese hydride catalyst supported by β-diketiminate ligands, [(2,6-iPr2PhBDI)Mn(μ-H)]2, was investigated with density functional theory. A triple bond between the manganese centres was anticipated from simple electron-counting rules; however, calculations revealed Mn-Mn Mayer bond orders of 0.21 and 0.

Proton-coupled electron transfer across benzimidazole bridges in bioinspired proton wires.

Designing molecular platforms for controlling proton and electron movement in artificial photosynthetic systems is crucial to efficient catalysis and solar energy conversion. The transfer of both protons and electrons during a reaction is known as proton-coupled electron transfer (PCET) and is used by nature in myriad ways to provide low overpotential pathways for redox reactions and redox leveling, as well as to generate bioenergetic proton currents. Herein, we describe theoretical and electrochemical studies of a series of bioinspired benzimidazole-phenol (BIP) derivatives and a series of dibenzimidazole-phenol (BIP) analogs with each series bearing the same set of terminal proton-accepting (TPA) groups.

Scope and mechanism of nitrile dihydroboration mediated by a β-diketiminate manganese hydride catalyst.

The manganese hydride dimer, [(BDI)Mn(μ-H)], was found to mediate nitrile dihydroboration, rendering it the first manganese catalyst for this transformation. Stoichiometric experiments revealed that benzonitrile insertion affords [(BDI)Mn(μ-NCHCH)] en route to N,N-diborylamine formation. Density functional theory calculations reveal the precise mechanism and demonstrate that catalysis is promoted by monomeric species.

Solution and Solid-State Characterization of PbSe Precursors.

The addition of lead to diphenyl diselenide in ethylenediamine (en) or pyridine (py) allowed for the observation of the solvento complexes, (en)Pb(SePh) or (py)Pb(SePh), respectively. Performing this reaction in dimethyl sulfoxide and subsequent crystallization was found to afford Pb(SePh). Inductively coupled plasma optical emission spectroscopy revealed a 1:2 lead to selenium ratio for all three complexes.

Efficient Cobalt Catalyst for Ambient-Temperature Nitrile Dihydroboration, the Elucidation of a Chelate-Assisted Borylation Mechanism, and a New Synthetic Route to Amides.

,-Diborylamines have emerged as promising reagents in organic synthesis; however, their efficient preparation and full synthetic utility have yet to be realized. To address both shortcomings, an effective catalyst for nitrile dihydroboration was sought. Heating CoCl in the presence of PDI afforded the six-coordinate Co(II) salt, [(PDI)CoCl][Cl].

A β-diketiminate manganese catalyst for alkene hydrosilylation: substrate scope, silicone preparation, and mechanistic insight.

The dimeric β-diketiminate manganese hydride compound, [(BDI)Mn(μ-H)], was prepared by treating [(BDI)Mn(μ-Cl)] with NaEtBH. This compound was characterized by single crystal X-ray diffraction and found to feature high-spin Mn centres that exhibit strong magnetic coupling by EPR spectroscopy. Once characterized, [(BDI)Mn(μ-H)] was found to mediate the hydrosilylation of a broad scope of alkenes at elevated temperature.

Controlling Proton-Coupled Electron Transfer in Bioinspired Artificial Photosynthetic Relays.

Bioinspired constructs consisting of benzimidazole-phenol moieties bearing N-phenylimines as proton-accepting substituents have been designed to mimic the H-bond network associated with the Tyr-His190 redox relay in photosystem II. These compounds provide a platform to theoretically and experimentally explore and expand proton-coupled electron transfer (PCET) processes. The models feature H-bonds between the phenol and the nitrogen at the 3-position of the benzimidazole and between the 1 H-benzimidazole proton and the imine nitrogen.

GeTiO: a new high-pressure material with rutile-type crystal structure.

Single crystals of a GeO-TiO solid solution with the corresponding composition GeTiO (germanium titanium tetra-oxide) were obtained by devitrification of germania-titania glass at high pressure and temperature. The new compound crystallizes in the rutile structure type (space group 4/), where Ge and Ti share the same position (site symmetry ), with occupancy values of 0.57 (3) and 0.

Carbonyl and ester C-O bond hydrosilylation using κ-diimine nickel catalysts.

The synthesis of alkylphosphine-substituted α-diimine (DI) ligands and their subsequent addition to Ni(COD)2 allowed for the preparation of (iPr2PPrDI)Ni and (tBu2PPrDI)Ni. The solid state structures of both compounds were found to feature a distorted tetrahedral geometry that is largely consistent with the reported structure of the diphenylphosphine-substituted variant, (Ph2PPrDI)Ni. To explore and optimize the synthetic utility of this catalyst class, all three compounds were screened for benzaldehyde hydrosilylation activity at 1.

Isolation of Mn(I) Compounds Featuring a Reduced Bis(imino)pyridine Chelate and Their Relevance to Electrocatalytic Hydrogen Production.

We report the preparation and electronic structure determination of chelate-reduced Mn(I) compounds that are relevant to electrocatalytic proton reduction mediated by [(PDI)Mn(CO)][Br]. Reducing [(PDI)Mn(CO)][Br] with excess Na-Hg afforded a neutral paramagnetic complex, (PDI)Mn(CO). This compound was found to feature a low spin Mn(I) center and a PDI radical anion as determined by magnetic susceptibility measurement (1.

Hydroboration of alkynes and nitriles using an α-diimine cobalt hydride catalyst.

Addition of NaEtBH to (DI)CoCl affords the corresponding monohydride, (DI)CoH. X-ray diffraction and DFT calculations indicate that this compound possesses a radical monoanion α-DI chelate and a Co(ii) centre. Notably, (DI)CoH catalyzes the hydroboration of alkynes and dihydroboration of nitriles under mild conditions.

Mechanistic Investigation of Bis(imino)pyridine Manganese Catalyzed Carbonyl and Carboxylate Hydrosilylation.

We recently reported a bis(imino)pyridine (or pyridine diimine, PDI) manganese precatalyst, (PDI)Mn (1), that is active for the hydrosilylation of ketones and dihydrosilylation of esters. In this contribution, we reveal an expanded scope for 1-mediated hydrosilylation and propose two different mechanisms through which catalysis is achieved. Aldehyde hydrosilylation turnover frequencies (TOFs) of up to 4900 min have been realized, the highest reported for first row metal-catalyzed carbonyl hydrosilylation.

Hydrogen production from water using a bis(imino)pyridine molybdenum electrocatalyst.

In 5.0 M H2O/acetonitrile, [((Ph2PPr)PDI)MoO][PF6]2 produces H2 with 96% Faradaic efficiency at -2.5 V vs.

Isolation of a bis(imino)pyridine molybdenum(i) iodide complex through controlled reduction and interconversion of its reaction products.

Analysis of previously reported [((Ph2PPr)PDI)MoI][I] by cyclic voltammetry revealed a reversible wave at -1.20 V vs. Fc(+/0), corresponding to the Mo(ii)/Mo(i) redox couple.

Two-step C-H, C-P bond activation at an α-diimine iron dinitrogen complex.

Reduction of 6-coordinate ((Ph2PPr)DI)FeBr₂ under N2 results in formation of the terminal dinitrogen complex, ((Ph2PPr)DI)FeN2. Heating this product to 75 °C allows for C-H and C-P activation of the chelate to generate the cisoid and transoid isomers of [(μ-PrPPh-κ(5)-P,N,N,Cγ,P-(Ph2PPr)DI(PrPPh))Fe]2. Mechanistic possibilities for this transformation are discussed.

A Pentacoordinate Mn(II) Precatalyst That Exhibits Notable Aldehyde and Ketone Hydrosilylation Turnover Frequencies.

Heating (THF)2MnCl2 in the presence of the pyridine-substituted bis(imino)pyridine ligand, (PyEt)PDI, allowed preparation of the respective dihalide complex, ((PyEt)PDI)MnCl2. Reduction of this precursor using excess Na/Hg resulted in deprotonation of the chelate methyl groups to yield the bis(enamide)tris(pyridine)-supported product, (κ(5)-N,N,N,N,N-(PyEt)PDEA)Mn. This complex was characterized by single-crystal X-ray diffraction and found to possess an intermediate-spin (S = (3)/2) Mn(II) center by the Evans method and electron paramagnetic resonance spectroscopy.

Conversion of Carbon Dioxide to Methanol Using a C-H Activated Bis(imino)pyridine Molybdenum Hydroboration Catalyst.

Using a multistep synthetic pathway, a bis(imino)pyridine (or pyridine diimine, PDI) molybdenum catalyst for the selective conversion of carbon dioxide into methanol has been developed. Starting from ((Ph2PPr)PDI)Mo(CO), I2 addition afforded [((Ph2PPr)PDI)MoI(CO)][I], which features a seven-coordinate Mo(II) center. Heating this complex to 100 °C under vacuum resulted in CO loss and the formation of [((Ph2PPr)PDI)MoI][I].

Carbon Dioxide Promoted H(+) Reduction Using a Bis(imino)pyridine Manganese Electrocatalyst.

Heating a 1:1 mixture of (CO)5MnBr and the phosphine-substituted pyridine diimine ligand, (Ph2PPr)PDI, in THF at 65 °C for 24 h afforded the diamagnetic complex [((Ph2PPr)PDI)Mn(CO)][Br] (1). Higher temperatures and longer reaction times resulted in bromide displacement of the remaining carbonyl ligand and the formation of paramagnetic ((Ph2PPr)PDI)MnBr (2). The molecular structure of 1 was determined by single crystal X-ray diffraction, and density functional theory (DFT) calculations indicate that this complex is best described as low-spin Mn(I) bound to a neutral (Ph2PPr)PDI chelating ligand.

Substantial production of drosophilin A methyl ether (tetrachloro-1,4-dimethoxybenzene) by the lignicolous basidiomycete Phellinus badius in the heartwood of mesquite (Prosopis juliflora) trees.

Toxic organohalogen pollutants produced as by-products of industrial processes, such as chloroform and polychlorinated dibenzo-p-dioxins, also have significant natural sources. A substantial terrestrial source of halogenated organics originates from fungal decay of wood and leaf litter. Here we show that the lignicolous basidiomycete Phellinus badius deposits up to 30,000 mg of the halogenated metabolite drosophilin A methyl ether (DAME, tetrachloro-1,4-dimethoxybenzene) per kilogram of decayed heartwood in the mesquite Prosopis juliflora.

Antineoplastic agents. 599. Total synthesis of dolastatin 16.

The first 23-step total synthesis of the cyclodepsipeptide dolastatin 16 (1) has been achieved. Synthesis of the dolaphenvaline and dolamethylleuine amino acid units using simplified methods improved the overall efficiency. The formation of the 25-membered macrocycle employing lactonization with 2-methyl-6-nitrobenzoic anhydride completed a key step in the synthesis.

A nickel phosphine complex as a fast and efficient hydrogen production catalyst.

Here we report the electrocatalytic reduction of protons to hydrogen by a novel S2P2 coordinated nickel complex, [Ni(bdt)(dppf)] (bdt = 1,2-benzenedithiolate, dppf = 1,1'-bis(diphenylphosphino)ferrocene). The catalysis is fast and efficient with a turnover frequency of 1240 s(-1) and an overpotential of only 265 mV for half activity at low acid concentrations. Furthermore, catalysis is possible using a weak acid, and the complex is stable for at least 4 h in acidic solution.