Joseph A. Wright

Gold peroxide complexes and the conversion of hydroperoxides into gold hydrides by successive oxygen-transfer reactions

Gold catalysts are widely studied in chemical and electrochemical oxidation processes. Computational modelling has suggested the participation of Au-OO-Au, Au-OOH or Au-OH surface species, attached to gold in various oxidation states. However, no structural information was available as isolable gold peroxo and hydroperoxo compounds were unknown. Here we report the syntheses, structures and reactions of a series of gold(III) peroxides, hydroperoxides and alkylperoxides. The Au-O bond energy in peroxides is weaker than in oxides and hydroxides; however, the Au-OH bond is also weaker than Au-H. Consequently Au-OH compounds are capable of oxygen-transfer generating gold hydrides, a key reaction in a water splitting cycle and an example that gold can react in a way that other metals cannot. For the first time it has become possible to establish a direct connection from peroxides to hydrides: Au-OO-Au→Au-OOH→Au-OH→Au-H, via successive oxygen-transfer events.

Sequential removal of the benzyl-type protecting groups PMB and NAP by oxidative cleavage using CAN and DDQ

Abstract The selective cleavage of the PMB (4-methoxybenzyl) group in the presence of the NAP (2-naphthylmethyl) group was achieved using CAN with a range of mono-saccharides. The NAP group can then be removed selectively in the presence of a benzyl group using DDQ. This provides a strategy for sequential deprotection of hydroxyl groups.

The Third Hydrogenase: A Ferracyclic Carbamoyl with Close Structural Analogy to the Active Site of Hmd

The synthesis of a close structural analogue of the active site of [Fe]-hydrogenase is described (see structure; C gray, H dark blue, Fe green, N light blue, O red, S yellow). Nature most probably constructs the five membered ferracyclic ring to poise the 2-hydroxy pyridine substituent in a position to assist the heterolytic cleavage of dihydrogen, and the accessibility of the analogue should now provide opportunities for probing this.

“Pincer” Pyridine–Dicarbene–Iridium Complexes: Facile CH Activation and Unexpected η2-Imidazol-2-ylidene Coordination†

Anionic 2,6-diphosphinomethylphenyl (P–C–P), 2,6-diphosphinitophenyl (PO–C–OP), and neutral 2,6-diphosphinomethylpyridine (P–N–P) “pincer” complexes of iridium have been studied as catalysts in important organometallic transformations, including alkane dehydrogenation (in the presence or absence of an H2 acceptor), dehydrogenation of primary amines to nitriles, and dehydrogenation of borane– amine complexes. Furthermore, interesting stoichiometric, intermolecular C-H activations of substituted aromatic compounds (anisole, acetophenone, and halobenzenes) have been realized. This remarkable reactivity is, in part, due to the thermal stability and rigidity that the “pincer” imparts on the Ir center, although reports of ligand “non-innocence” have appeared. Intramolecular metalations of ligand C H bonds are known in [Ir(P–C–P)] complexes. Owing to the relevance of these catalytic transformations to the atomand energy-efficient use of organic molecules, the further development of new Ir “pincer” complexes with improved activity and selectivity is a goal of much current research. To this end, we considered the replacement of the P donors of the “pincer” arms by N-heterocyclic carbenes (NHCs, see below), which, based on the accepted analogy between trialkylphosphine and N-heterocyclic carbene (NHC) ligands, would result in novel highly reactive complexes.

Paramagnetic Bridging Hydrides of Relevance to Catalytic Hydrogen Evolution at Metallosulfur Centers

Paramagnetic hydrides are likely intermediates in hydrogen-evolving enzymic and molecular systems. Herein we report the first spectroscopic characterization of well-defined paramagnetic bridging hydrides. Time-resolved FTIR spectroelectrochemical experiments on a subsecond time scale revealed that single-electron transfer to the μ-hydride di-iron dithiolate complex 1 generates a 37-electron valence-delocalized species with no gross structural reorganization of the coordination sphere. DFT calculations support and (1)H and (2)H EPR measurements confirmed the formation an S = ½ paramagnetic complex (g = 2.0066) in which the unpaired spin density is essentially symmetrically distributed over the two iron atoms with strong hyperfine coupling to the bridging hydride (A(iso) = -75.8 MHz).

Determination of the photolysis products of [FeFe]hydrogenase enzyme model systems using ultrafast multidimensional infrared spectroscopy.

Ultrafast transient 2D-IR (T-2D-IR) spectroscopy has been used to study the photolysis products of the [FeFe]hydrogenase enzyme model compound (μ-propanedithiolate)Fe(2)(CO)(6) in heptane solution following irradiation at ultraviolet wavelengths. Observation of coupling patterns between the vibrational modes of the photoproduct species formed alongside examination of the appearance time scales of these signals has uniquely enabled assignment of the photoproduct spectrum to a single pentacarbonyl species. Comparison of the vibrational relaxation rate of the photoproduct with that of the parent is consistent with the formation of a solvent adduct at the vacant coordination site, while anisotropy data in conjunction with density functional theory simulations indicates substitution in an axial rather than equatorial position. No firm evidence of additional short-lived intermediates is seen, indicating that the subsequent chemistry of these species is likely to be strongly defined by the nature of the first solvation shell.

Ferracyclic carbamoyl complexes related to the active site of [Fe]-hydrogenase.

The active site of the [Fe]-hydrogenase features an iron(II) centre bearing cis carbonyl groups and a chelating pyridine-acyl ligand. Reproducing these unusual features in synthetic models is an intriguing challenge, which will allow both better understanding of the enzymatic system and more fundamental insight into the coordination modes of iron. By using the carbamoyl group as a surrogate for acyl, we have been able to synthesize a range of ferracyclic complexes. Initial reaction of Fe(CO)4Br2 with 2-aminopyridine yields a complex bearing a labile solvent molecule, which can be replaced by stronger donors bearing phosphorus atoms to produce a number of derivatives. Introduction of a hydroxy group using this method is unsuccessful both with a free OH group and when this is silyl-protected. In contrast, the analogous reactions starting from 2,6-diaminopyridine does allow synthesis of complexes bearing a pendant basic group.

Femtosecond to microsecond photochemistry of a [FeFe]hydrogenase enzyme model compound

The photochemistry and dynamics of a model compound of the active site of the [FeFe]hydrogenase enzyme system have been studied on a wide range of time scales using a unique combination of femtosecond time-resolved infrared spectroscopy, nanosecond time-resolved infrared spectroscopy, and steady-state UV-FTIR methods. Using three different solvents, heptane, acetonitrile, and cyanoheptane, we have observed the rapid formation of solvent adduct species from the first solvation shell of the solute following photolysis of a carbonyl ligand and global fitting techniques have been employed to provide new insights into the ultrafast dynamics of this process. In addition, the use of solvent mixtures has enabled the observation of competitive ligand substitution processes at the newly created coordination site on time scales of a few nanoseconds, shedding new light on the chemical behavior of these enzyme models.

Solution-phase photochemistry of a [FeFe]hydrogenase model compound: Evidence of photoinduced isomerisation

The solution-phase photochemistry of the [FeFe] hydrogenase subsite model (μ-S(CH(2))(3)S)Fe(2)(CO)(4)(PMe(3))(2) has been studied using ultrafast time-resolved infrared spectroscopy supported by density functional theory calculations. In three different solvents, n-heptane, methanol, and acetonitrile, relaxation of the tricarbonyl intermediate formed by UV photolysis of a carbonyl ligand leads to geminate recombination with a bias towards a thermodynamically less stable isomeric form, suggesting that facile interconversion of the ligand groups at the Fe center is possible in the unsaturated species. In a polar or hydrogen bonding solvent, this process competes with solvent substitution leading to the formation of stable solvent adduct species. The data provide further insight into the effect of incorporating non-carbonyl ligands on the dynamics and photochemistry of hydrogenase-derived biomimetic compounds.

The role of CN and CO ligands in the vibrational relaxation dynamics of model compounds of the [FeFe]-hydrogenase enzyme

The vibrational dynamics of (μ-propanedithiolate)Fe(2)(CO)(4)(CN)(2)(2-), a model compound of the active site of the [FeFe]-hydrogenase enzyme, have been examined via ultrafast 2D-IR spectroscopy. The results indicate that the vibrational coupling between the stretching modes of the CO and CN ligands is small and restricted to certain modes but the slow growth of off-diagonal peaks is assigned to population transfer processes occurring between these modes on timescales of 30-40 ps. Analysis of the dynamics in concert with anharmonic density functional theory simulations shows that the presence of CN ligands alters the vibrational relaxation dynamics of the CO modes in comparison to all-carbonyl model systems and suggests that the presence of these ligands in the enzyme may be an important feature in terms of directing the vibrational relaxation mechanism.