Ferracyclic carbonyl complexes as anti-inflammatory agents.

Reaction of Fe(CO)4Br2 with 2-aminopyridine and 2-aminonapthalene yields ferracyclic iron(ii) complexes bearing two CO ligands. Irradiation with visible light releases these two CO molecules. Substitution of a halide in the parent complexes by thioglucose provides significantly enhanced water solublity and raises the quantum yield for CO release by around five times. The complexes show anti-inflammatory activity in a TNF assay in the dark.

Organoimido-Polyoxometalate Nonlinear Optical Chromophores: A Structural, Spectroscopic, and Computational Study.

Ten organoimido polyoxometalate (POM)-based chromophores have been synthesized and studied by hyper-Rayleigh scattering (HRS), Stark and Resonance Raman spectroscopies, and density functional theory (DFT) calculations. HRS ß0 values for chromophores with resonance electron donors are significant (up to 139 × 10-30 esu, ∼5 times greater than that of the DAS+ cation), but systems with no donor, or the -NO2 acceptor show no activity, in some cases, despite large DFT-predicted ß-values. In active systems with short (phenyl) π-bridges, ß0 values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic ß-values, ß0/N3/2 (where N is the number of bridge π-electrons) thus appear to break empirical performance limits (ß0/N3/2 vs λmax) for planar organic systems. However, ß0 values obtained for extended systems with a diphenylacetylene bridge are comparable to or lower than that of their nitro analogue, N,N-dimethyl-4-[(4-nitrophenyl)ethynyl]-aniline (DMNPEA). Resonance Raman spectroscopy confirms the involvement of the POM in the electronic transitions, whether donor groups are present or not, but Stark spectroscopy indicates that, in their absence, the transitions have little dipolar character (hence, NLO inactive), consistent with DFT-calculated frontier orbitals, which extend over both POM and organic group. Stark and DFT also suggest that ß is enhanced in the short compounds because the extension of charge transfer (CT) onto the POM increases changes in the excited-state dipole moment. With extended π-systems, this effect does not increase CT distances, relative to a -NO2 acceptor, so ß0 values do not exceed that of DMNPEA. Overall, our results show that (i) the organoimido-POM unit is an efficient acceptor for second-order NLO, but an ineffective donor; (ii) the nature of electronic transitions in arylimido-POMs is strongly influenced by the substituents of the aryl group; and (iii) organoimido-POMs outperform organic acceptors with short π-bridges, but lose their advantage with extended π-conjugation.

Muonium Chemistry at Diiron Subsite Analogues of [FeFe]-Hydrogenase.

The chemistry of metal hydrides is implicated in a range of catalytic processes at metal centers. Gaining insight into the formation of such sites by protonation and/or electronation is therefore of significant value in fully exploiting the potential of such systems. Here, we show that the muonium radical (Mu. ), used as a low isotopic mass analogue of hydrogen, can be exploited to probe the early stages of hydride formation at metal centers. Mu. undergoes the same chemical reactions as H. and can be directly observed due to its short lifetime (in the microseconds) and unique breakdown signature. By implanting Mu. into three models of the [FeFe]-hydrogenase active site we have been able to detect key muoniated intermediates of direct relevance to the hydride chemistry of these systems.

Encapsulating Subsite Analogues of the [FeFe]-Hydrogenases in Micelles Enables Direct Water Interactions.

Encapsulation of subsite analogues of the [FeFe]-hydrogenase enzymes in supramolecular structures has been shown to dramatically increase their catalytic ability, but the molecular basis for this enhancement remains unclear. We report the results of experiments employing infrared absorption, ultrafast infrared pump-probe, and 2D-IR spectroscopy to investigate the molecular environment of Fe2(pdt)(CO)6 (pdt: propanedithiolate) [1] encapsulated in the dispersed alkane phase of a heptane-dodecyltrimethylammonium bromide-water microemulsion. It is demonstrated that 1 is partitioned between two molecular environments, one that closely resembles bulk heptane solution and a second that features direct hydrogen-bonding interactions with water molecules that penetrate the surfactant shell. Our results demonstrate that the extent of water access to the normally water-insoluble subsite analogue 1 can be tuned with micelle size, while IR spectroscopy provides a straightforward tool that can be used to measure and fine-tune the chemical environment of catalyst species in self-assembled structures.

Correction: EPR detection and characterisation of a paramagnetic Mo(iii) dihydride intermediate involved in electrocatalytic hydrogen evolution.

Correction for 'EPR detection and characterisation of a paramagnetic Mo(iii) dihydride intermediate involved in electrocatalytic hydrogen evolution' by Christopher Prior, et al., Dalton Trans., 2016, 45, 2399-2403.

PhotoCORMs: CO release moves into the visible.

The potential of carbon monoxide to act as a therapeutic agent is now well-established. Controlled delivery of CO is best achieved using 'CORMs': molecules which release known amounts of carbon monoxide in response to a stimulus. Metal carbonyl complexes will release CO if irradiated with ultraviolet light, but it is only in the past five years that development of true 'photoCORMs' has been explored. Recent exciting developments in this area now show that design of photoCORMs operating well into the visible region is achievable. In this Perspective, we examine the growth of photoCORMs from their origins in the photophysics of metal carbonyls to the latest visible-light agents.

EPR detection and characterisation of a paramagnetic Mo(iii) dihydride intermediate involved in electrocatalytic hydrogen evolution.

EPR spectroscopy and theoretical data show that the slow heterogeneous electron-transfer kinetics associated with the reduction of an 18-electron Mo(IV) acetato dihydride are a consequence of an η(2)-η(1) rearrangement of the carboxylate ligand which gives a unique paramagnetic 17-electron Mo(III) dihydride.

Detection of Transient Intermediates Generated from Subsite Analogues of [FeFe] Hydrogenases.

This article reviews the application of transient techniques in the elucidation of electron, proton, and photon chemistry related to the catalytic subsite of [FeFe] hydrogenase from the perspective of research in this area carried out at the UEA and Strathclyde laboratories. The detection of mixed-valence states, bridging CO intermediates, paramagnetic hydrides, and coordinatively unsaturated species has both informed understanding of biological catalysis and stimulated the search for stable analogues of key structural motifs likely involved in turnover states.

Gold(III)-CO and gold(III)-CO2 complexes and their role in the water-gas shift reaction.

The water-gas shift (WGS) reaction is an important process for the generation of hydrogen. Heterogeneous gold catalysts exhibit good WGS activity, but the nature of the active site, the oxidation state, and competing reaction mechanisms are very much matters of debate. Homogeneous gold WGS systems that could shed light on the mechanism are conspicuous by their absence: gold(I)-CO is inactive and gold(III)-CO complexes were unknown. We report the synthesis of the first example of an isolable CO complex of Au(III). Its reactivity demonstrates fundamental differences between the CO adducts of the neighboring d (8) ions Pt(II) and Au(III): whereas Pt(II)-CO is stable to moisture, Au(III)-CO compounds are extremely susceptible to nucleophilic attack and show WGS reactivity at low temperature. The key to understanding these dramatic differences is the donation/back-donation ratio of the M-CO bond: gold-CO shows substantially less back-bonding than Pt-CO, irrespective of closely similar ν(CO) frequencies. Key WGS intermediates include the gold-CO2 complex [(C^N^C)Au]2(µ-CO2), which reductively eliminates CO2. The species identified here are in accord with Au(III) as active species and a carboxylate WGS mechanism.

An element through the looking glass: exploring the Au-C, Au-H and Au-O energy landscape.

Gold, the archetypal "noble metal", used to be considered of little interest in catalysis. It is now clear that this was a misconception, and a multitude of gold-catalysed transformations has been reported. However, one consequence of the long-held view of gold as inert metal is that its organometallic chemistry contains many "unknowns", and catalytic cycles devised to explain gold's reactivity draw largely on analogies with other transition metals. How realistic are such mechanistic assumptions? In the last few years a number of key compound classes have been discovered that can provide some answers. This Perspective attempts to summarise these developments, with particular emphasis on recently discovered gold(iii) complexes with bonds to hydrogen, oxygen, alkenes and CO ligands.

Zinc calixarene complexes for the ring opening polymerization of cyclic esters.

Reaction of Zn(C6F5)2·toluene (two equivalents) with 1,3-dipropoxy-p-tert-butyl-calix[4]arene (L(1)H2) led to the isolation of the complex [{Zn(C6F5)}2L(1)] (1), whilst similar use of Zn(Me)2 resulted in the known complex [{Zn(Me)}2L(1)] (2). Treatment of L(1)H2 with in situ prepared Zn{N(SiMe3)2}2 in refluxing toluene led to the isolation of the compound [(Na)ZnN(SiMe3)2L(1)] (3). The stepwise reaction of L(1)H2 and sodium hydride, followed by ZnCl2 and finally NaN(SiMe3)2 yielded the compound [Zn{N(SiMe3)2}2L(1)] (4). The reaction between three equivalents of Zn(C6F5)2·toluene and oxacalix[3]arene (L(2)H3) at room temperature formed the compound {[Zn(C6F5)]3L(2)} (5); heating of 5 in acetonitrile caused the ring opening of the parent oxacalix[3]arene and rearrangement to afford the complex [(L(2))Zn6(C6F5)(R)(RH)OH]·5MeCN R = C6F5CH2-(p-(t)BuPhenolate-CH2OCH2-)2-p-(t)BuPhenolate-CH2O(-))(3-) (6). The molecular structures of the new complexes 1, 3 and 6, together with that of the known complex 2, whose solid state structure has not previously been reported, have been determined. Compounds 1, 3-5 have been screened for the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) and rac-lactide. Compounds featuring a Zn-C6F5 fragment were found to be poor ROP pre-catalysts as they did not react with benzyl alcohol to form an alkoxide. By contrast, compound 4, which contains a zinc silylamide linkage, was the most active of the zinc-based calix[4]arene compounds screened and was capable of ROP at ambient temperature with 65% conversion over 4 h.

Tri- and tetra-dentate imine vanadyl complexes: synthesis, structure and ethylene polymerization/ring opening polymerization capability.

Reaction of the ligand 2,4-tert-butyl-6-[(2-methylquinolin-8-ylimino)methyl]phenol (L(1)H) with [VOCl3] in the presence of triethylamine afforded the complex [VOCl2L(1)] (1), whereas use of [VO(OnPr)3] led to the isolation of [VO2L(1)] (2) or [VO2L(1)]·2/3MeCN (2·2/3MeCN). Reaction of 2-((2-(1H-benzo[d]imidazol-2-yl)quinolin-8-ylimino)methyl)-4,6-R(1),R(2)-phenols (R(1) = R(2) = (t)Bu; L(2)H), (R(1) = R(2) = Me; L(3)H) or (R(1) = Me, R(2) = Ad; L(4)H) with [VO(OnPr)3] afforded complexes of the type [L(2-4)VO] (where L(2) = 3, L(3) = 4, L(4) = 5). The molecular structures of 1 to 3 are reported; the metal centre adopts a distorted octahedral, trigonal bipyramidal or square-based pyramidal geometry respectively. In Schlenk line tests, all complexes have been screened as pre-catalysts for the polymerization of ethylene using diethylaluminium chloride (DEAC) as co-catalyst in the presence of ethyltrichloroacetate (ETA), and for the ring opening polymerization (ROP) of ε-caprolactone in the presence of benzyl alcohol. All pre-catalyst/DEAC/ETA systems are highly active ethylene polymerization catalysts affording linear polyethylene with activities in the range 3000-10,700 g (mol h bar)(-1); the use of methylaluminoxane (MAO) or modified MAO as co-catalyst led to poor or no activity. In a parallel pressure reactor, 3-5 have been screened as pre-catalysts for ethylene polymerization in the presence of either DEAC or DMAC (dimethylaluminium chloride) and ETA at various temperatures and for the co-polymerization of ethylene with propylene. The use of DMAC proved more promising with 3 achieving an activity of 63,000 g (mol h bar)(-1) at 50 °C and affording UHMWPE (M(w) ~ 2,000,000). In the case of the co-polymerization, the incorporation of propylene was 6.9-8.8 mol%, with 3 exhibiting the highest incorporation when using either DEAC or DMAC. In the case of the ring opening polymerization (ROP) of ε-caprolactone, systems employing complexes 1-5 were virtually inactive at temperatures <110 °C; on increasing the CL : V ratio at 110 °C, conversions of the order of 80% were achievable.

[FeFe] hydrogenase: protonation of {2Fe3S} systems and formation of super-reduced hydride states.

The synthesis and crystallographic characterization of a complex possessing a well-defined {2Fe3S(µ-H)} core gives access to a paramagnetic bridging hydride with retention of the core geometry. Chemistry of this 35-electron species within the confines of a thin-layer FTIR spectro-electrochemistry cell provides evidence for a unprecedented super-reduced Fe(I)(µ-H)Fe(I) intermediate.

Electronic control of the protonation rates of Fe-Fe bonds.

Protonation at metal-metal bonds is of fundamental interest in the context of the function of the active sites of hydrogenases and nitrogenases. In diiron dithiolate complexes bearing carbonyl and electron-donating ligands, the metal-metal bond is the highest occupied molecular orbital (HOMO) with a "bent" geometry. Here we show that the experimentally measured rates of protonation (kH) of this bond and the energy of the HOMO as measured by the oxidation potential of the complexes (E1/2(ox)) correlate in a linear free energy relationship: ln kH = ((F(c - ßE1/2(ox)))/(RT)), where c is a constant and ß is the dimensionless Brønsted coefficient. The value of ß of 0.68 is indicative of a strong dependence upon energy of the HOMO: measured rates of protonation vary over 6 orders of magnitude for a change in E1/2(ox) of ca. 0.55 V (ca. 11 orders of magnitude/V). This relationship allows prediction of protonation rates of systems that are either too fast to measure experimentally or that possess additional protonation sites. It is further suggested that the nature of the bridgehead in the dithiolate ligand can exert a stereoelectronic influence: bulky substituents destabilize the HOMO, thereby increasing the rate of protonation.

Vanadyl complexes bearing bi-dentate phenoxyimine ligands: synthesis, structural studies and ethylene polymerization capability.

Reaction of [VO(OnPr)3] with the Schiff bases 3,5-(tBu)2-2-OH-C6H2CH(N(x-OR-C6H4)) (R = Me; x = 2, L(1)H; x = 3, L(2)H; x = 4, L(3)H; R = Et (L(4)H), CF3 (L(5)H), Ph (L(6)H)) or 4-methyl-3-(R)-2-(OH)-C6H4C[double bond, length as m-dash]N(2'-(2''-(OR1)C6H4)C6H4) (R = adamantyl, R(1) = Ph (L(7)H) or R = C(Me)2Ph, R(1) = Ph (L(8)H)) afforded the bis(chelate) vanadium(iv) complexes [VO(L(n))2] (n = 1 (1·2MeCN); n = 2 (2); n = 3 (3·2MeCN); n = 4 (4); n = 5 (5); n = 6 (6); n = 6, (7·1.5MeCN); n = 7, (8); n = 8, (9)); in the case of L(6)H, the oxo-bridged vanadium(v) complexes [VO(µ-O)(L(6))]2 (10) was also isolated. By contrast, interaction of 4-methyl-3-(R)-2-(OH)-C6H4C[double bond, length as m-dash]N(2'-(2''-(OR1)C6H4)C6H4) (R = adamantyl, R(1) = Me (L(9)H); R = tBu, R(1) = Me (L(10)H); R = C(Me)2Ph, R(1) = Me (L(11)H)) with [VO(OnPr)3] led to the isolation of the dinuclear complexes [VO(µ-OH)(µ-OnPr)(L(n))]2 (n = 9, (·4MeCN); 10, (12); 11, (13)), respectively. The molecular structures of 1 to 13 are reported. All complexes have been screened as pre-catalysts for the polymerization of ethylene in the presence of the co-catalyst diethylaluminium chloride (DEAC) with or without ethyltrichloroacetate (ETA) present at 1 or 10 bar of ethylene. Under high pressure, all pre-catalysts exhibited high activity and afforded high molecular weight (Mw ≈ 200 000 to 675 000 g mol(-1)), linear polyethylene with activities (in the presence of ETA) in the range 4960-16 400 g mmol(-1) h(-1); at one bar, the products were generally of lower molecular weight. The use of methylaluminoxane (MAO) or modified MAO (MMAO) as co-catalyst led to trace or poor (≤110 g mmol(-1) h(-1)) activity, respectively.

Electrochemistry of Au(II) and Au(III) pincer complexes: determination of the Au(II)-Au(II) bond energy.

The bond energy of the unsupported Au-Au bond in the Au(ii) dimer [(C(∧)N(∧)C)Au]2 and the difference between Au(III)-OH and Au(III)-H bond enthalpies have been determined experimentally by electrochemical methods, with Au-OH and Au-H complexes showing unexpected differences in their reduction pathways, supported by DFT modelling.

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.

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.

Towards alternatives to anodic water oxidation: basket-handle thiolate Fe(III) porphyrins for electrocatalytic hydrocarbon oxidation.

Selective electrocatalytic oxidation of hydrocarbons to alcohols, epoxides or other (higher value) oxygenates should in principal present a useful complementary anodic half-cell reaction to cathodic generation of fuels from water or CO(2) viz. an alternative to oxygen evolution. A series of new basket-handle thiolate Fe(III) porphyrins have been synthesised and shown to mediate anodic oxidation of hydrocarbons, specifically adamantane hydroxylation and cyclooctene epoxidation. We compare yields obtained by electrochemical and chemical oxidation of the thiolate porphyrins and benchmark their behaviour against that of Fe(III) tetraphenyl porphyrin chloride and its tetrapentafluorophenyl analogue.

Encapsulating [FeFe]-hydrogenase model compounds in peptide hydrogels dramatically modifies stability and photochemistry.

A [FeFe]-hydrogenase model compound (µ-S(CH(2))(3)S)Fe(2)(CO)(4)(PMe(3))(2) [1] has been encapsulated in a low molecular weight (LMW) hydrogelator (Fmoc-Leu-Leu). Linear infrared absorption spectroscopy, gel melting and ultrafast time-resolved infrared spectroscopy experiments reveal significant contrasts in chemical environment and photochemistry between the encapsulated molecules and solution phase systems. Specifically, the gel provides a more rigid hydrogen bonding environment, which restricts isomerisation following photolysis while imparting significant increases in stability relative to a similarly aqueous solution. Since understanding and ultimately controlling the mechanistic role of ligands near Fe centres is likely to be crucial in exploiting artificial hydrogenases, these gels may offer a new option for future materials design involving catalysts.