Resonant X-ray photo-oxidation of light-harvesting iron (II/III) N-heterocyclic carbene complexes
Ontology highlight
ABSTRACT:
SUBMITTER: Temperton R
PROVIDER: S-EPMC8590020 | biostudies-literature |
REPOSITORIES: biostudies-literature
Similar Datasets
Project description:The chemistry of N-heterocyclic carbenes with Earth-abundant manganese has largely focused on low-valent systems for reductive catalysis. Here, we have decorated imidazole- and triazole-derived carbenes with phenol substituents to access higher-valent Mn(III) complexes [Mn(O,C,O)(acac)], where acac = acetylacetonato, and O,C,O = bis(phenolate)imidazolylidene (1) or bis(phenolate)triazolylidene (2). Both complexes catalyze the oxidation of alcohols in the presence of tBuOOH as terminal oxidant. Complex 2 is slightly more active than 1 (TOF up to 540 h-1vs. 500 h-1), yet significantly more robust towards deactivation. Secondary and primary alcohols are oxidized, the latter with high selectivity and essentially no overoxidation of the aldehyde product to carboxylic acids unless the reaction time is substantially extended. Mechanistic investigations using Hammett parameters, IR spectroscopy, isotope labelling experiments, and specific substrates and oxidants as probes support the formation of a manganese(V) oxo system as the active species and subsequent turnover-limiting hydrogen atom abstraction.
| S-EPMC10262172 | biostudies-literature
Project description:The ability of resonant X-ray emission spectroscopy (XES) to recover physical oxidation state information, which may often be ambiguous in conventional X-ray spectroscopy, is demonstrated. By combining Kβ XES with resonant excitation in the XAS pre-edge region, resonant Kβ XES (or 1s3p RXES) data are obtained, which probe the 3dn+1 final-state configuration. Comparison of the non-resonant and resonant XES for a series of high-spin ferrous and ferric complexes shows that oxidation state assignments that were previously unclear are now easily made. The present study spans iron tetrachlorides, iron sulfur clusters, and the MoFe protein of nitrogenase. While 1s3p RXES studies have previously been reported, to our knowledge, 1s3p RXES has not been previously utilized to resolve questions of metal valency in highly covalent systems. As such, the approach presented herein provides chemists with means to more rigorously and quantitatively address challenging electronic-structure questions.
| S-EPMC8252016 | biostudies-literature
Project description:The synthesis of new moisture-sensitive imine-functionalized N-heterocyclic carbene (NHC) precursor salts [1-(2-[(hydroxyl-benzylidene)-amino]-ethyl)-3-R-3H-imidazole-1-ium bromide; R = methyl (1a), ethyl (1b), and benzyl (1c)] is reported. Subsequent deprotonation of 1a-c and coordination of the in situ generated NHC ligands to CoBr2 led to the isolation of air-stable six-coordinate Co(III) complexes 2a-c, respectively. All the salts and complexes were fully characterized. Single-crystal X-ray analysis of 2a and 2c showed octahedral Co centers hexacoordinated to two NHC carbons, two imine nitrogen atoms, and two phenolate oxygens in the form [C^N^O(Co3+)C^N^O]. The complexes were used in the catalytic transfer hydrogenation (CTH) of a range of ketones in 2-propanol as the solvent and hydrogen donor. Based on a low catalyst concentration of 0.4 mol %, significant conversions in the range of 70-99% were recorded at high turnover frequencies up to 1635 h-1. A mechanism to account for the steps involved in the CTH of cyclohexanone by complex 2a is proposed and supported by data from cyclic voltammetry, low-resolution mass spectrometry, UV, and IR spectroscopic techniques.
| S-EPMC7033672 | biostudies-literature
Project description:The hyperpolarization (HP) method signal amplification by reversible exchange (SABRE) uses para-hydrogen to sensitize substrate detection by NMR. The catalyst systems [Ir(H)2(IMes)(MeCN)2(R)]BF4 and [Ir(H)2(IMes)(py)2(R)]BF4 [py = pyridine; R = PCy3 or PPh3; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene], which contain both an electron-donating N-heterocyclic carbene and a phosphine, are used here to catalyze SABRE. They react with acetonitrile and pyridine to produce [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4 and [Ir(H)2(NCMe)(py)(IMes)(PCy3)]BF4, complexes that undergo ligand exchange on a time scale commensurate with observation of the SABRE effect, which is illustrated here by the observation of both pyridine and acetonitrile HP. In this study, the required symmetry breaking that underpins SABRE is provided for by the use of chemical inequivalence rather than the previously reported magnetic inequivalence. As a consequence, we show that the ligand sphere of the polarization transfer catalyst itself becomes hyperpolarized and hence that the high-sensitivity detection of a number of reaction intermediates is possible. These species include [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4, [Ir(H)2(MeOH)(py)(IMes)(PPh3)]BF4, and [Ir(H)2(NCMe)(py)2(PPh3)]BF4. Studies are also described that employ the deuterium-labeled substrates CD3CN and C5D5N, and the labeled ligands P(C6D5)3 and IMes-d22, to demonstrate that dramatically improved levels of HP can be achieved as a consequence of reducing proton dilution and hence polarization wastage. By a combination of these studies with experiments in which the magnetic field experienced by the sample at the point of polarization transfer is varied, confirmation of the resonance assignments is achieved. Furthermore, when [Ir(H)2(pyridine-h5)(pyridine-d5)(IMes)(PPh3)]BF4 is examined, its hydride ligand signals are shown to become visible through para-hydrogen-induced polarization rather than SABRE.
| S-EPMC3850244 | biostudies-literature
Project description:N-Heterocyclic carbene (NHC) gold(I) complexes offer great prospects in medicinal chemistry as antiproliferative, anticancer, and antibacterial agents. However, further development requires a thorough understanding of their reaction behavior in aqueous media. Herein, we report the conversion of the bromido[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(I) ((NHC)AuIBr, 1) complex in acetonitrile/water mixtures to the bis[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(I) ([(NHC)2AuI]+, 7), which is subsequently oxidized to the dibromidobis[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(III) ([(NHC)2AuIIIBr2]+, 9). By combining experimental data from HPLC, NMR, and (LC-)/HR-MS with computational results from DFT calculations, we outline a detailed ligand scrambling reaction mechanism. The key step is the formation of the stacked ((NHC)AuIBr)2 dimer (2) that rearranges to the T-shaped intermediate Br(NHC)2AuI-AuIBr (3). The dissociation of Br- from 3 and recombination lead to (NHC)2AuI-AuIBr2 (5) followed by the separation into [(NHC)2AuI]+ (7) and [AuIBr2]- (8). [AuIBr2]- is not stable in an aqueous environment and degrades in an internal redox reaction to Au0 and Br2. The latter in turn oxidizes 7 to the gold(III) species 9. The reported ligand rearrangement of the (NHC)AuIBr complex differs from that found for related silver(I) analogous. A detailed understanding of this scrambling mechanism is of utmost importance for the interpretation of their biological activity and will help to further optimize them for biomedical and other applications.
| S-EPMC7581288 | biostudies-literature
Project description:The electronic structures of pyridine N-heterocyclic dicarbene (iPrCNC) iron complexes have been studied by a combination of spectroscopic and computational methods. The goal of these studies was to determine if this chelate engages in radical chemistry in reduced base metal compounds. The iron dinitrogen example (iPrCNC)Fe(N2)2 and the related pyridine derivative (iPrCNC)Fe(DMAP)(N2) were studied by NMR, Mössbauer, and X-ray absorption spectroscopy and are best described as redox non-innocent compounds with the iPrCNC chelate functioning as a classical ? acceptor and the iron being viewed as a hybrid between low-spin Fe(0) and Fe(II) oxidation states. This electronic description has been supported by spectroscopic data and DFT calculations. Addition of N,N-diallyl-tert-butylamine to (iPrCNC)Fe(N2)2 yielded the corresponding iron diene complex. Elucidation of the electronic structure again revealed the CNC chelate acting as a ? acceptor with no evidence for ligand-centered radicals. This ground state is in contrast with the case for the analogous bis(imino)pyridine iron complexes and may account for the lack of catalytic [2? + 2?] cycloaddition reactivity.
| S-EPMC4195514 | biostudies-literature
Project description:Data from K? resonant inelastic X-ray scattering (RIXS) have been used to extract electronic structure information, i.e., the covalency of metal-ligand bonds, for four iron complexes using an experimentally based theoretical model. K? RIXS involves resonant 1s?3d excitation and detection of the 2p?1s (K?) emission. This two-photon process reaches similar final states as single-photon L-edge (2p?3d) X-ray absorption spectroscopy (XAS), but involves only hard X-rays and can therefore be used to get high-resolution L-edge-like spectra for metal proteins, solution catalysts and their intermediates. To analyze the information content of K? RIXS spectra, data have been collected for four characteristic ?-donor and ?-back-donation complexes: ferrous tacn [Fe(II)(tacn)2]Br2, ferrocyanide [Fe(II)(CN)6]K4, ferric tacn [Fe(III)(tacn)2]Br3 and ferricyanide [Fe(III)(CN)6]K3. From these spectra metal-ligand covalencies can be extracted using a charge-transfer multiplet model, without previous information from the L-edge XAS experiment. A direct comparison of L-edge XAS and K? RIXS spectra show that the latter reaches additional final states, e.g., when exciting into the e(g) (?*) orbitals, and the splitting between final states of different symmetry provides an extra dimension that makes K? RIXS a more sensitive probe of ?-bonding. Another key difference between L-edge XAS and K? RIXS is the ?-back-bonding features in ferro- and ferricyanide that are significantly more intense in L-edge XAS compared to K? RIXS. This shows that two methods are complementary in assigning electronic structure. The K? RIXS approach can thus be used as a stand-alone method, in combination with L-edge XAS for strongly covalent systems that are difficult to probe by UV/vis spectroscopy, or as an extension to conventional absorption spectroscopy for a wide range of transition metal enzymes and catalysts.
| S-EPMC3920600 | biostudies-literature
Project description:Since their first discovery, N-heterocyclic carbenes have had a significant impact on organometallic chemistry. Due to their nature as strong σ-donor and π-acceptor ligands, they are exceptionally well suited to stabilize Au(I) and Au(III) complexes in biological environments. Over the last decade, the development of rationally designed NHCAu(I/III) complexes to specifically target DNA has led to a new "gold rush" in bioinorganic chemistry. This review aims to summarize the latest advances of NHCAu(I/III) complexes that are able to interact with DNA. Furthermore, the latest advancements on acyclic diamino carbene gold complexes with anticancer activity are presented as these typically overlooked NHC alternatives offer great additional design possibilities in the toolbox of carbene-stabilized gold complexes for targeted therapy.
| S-EPMC9219884 | biostudies-literature
Project description:A series of neutral acridine-based gold(III)-NHC complexes containing the pentafluorophenyl (-C6F5) group were synthesized. All of the complexes were fully characterized by analytical techniques. The square planar geometry around the gold center was confirmed by X-ray diffraction analysis for complexes 1 (Trichloro [1-methyl-3-(9-acridine)imidazol-2-ylidene]gold(III)) and 2 (Chloro-bis(pentafluorophenyl)[1-methyl-3-(9-acridine)imidazol-2-ylidene]gold(III)). In both cases, the acridine rings play a key role in the crystal packing of the solid structures by mean of π-π stacking interactions, with centroid-centroid and interplanar distances being similar to those found in other previously reported acridine-based Au(I)-NHC complexes. A different reactivity when using a bulkier N-heterocyclic carbene ligand such as 1,3-bis-(2,6-diisopropylphenyl)-2-imidazolidinylidene (SIPr) was observed. While the use of the acridine-based NHC ligand led to the expected organometallic gold(III) species, the steric hindrance of the bulky SIPr ligand led to the formation of the corresponding imidazolinium cation stabilized by the tetrakis(pentafluorophenyl)aurate(III) [Au(C6F5)4]- anion. Computational experiments were carried out in order to figure out the ground state electronic structure and the binding formation energy of the complexes and, therefore, to explain the observed reactivity.
| S-EPMC9740751 | biostudies-literature
Project description:A series of methylpalladium(II) complexes with pyrimidine-NHC ligands carrying different aryl- and alkyl substituents R ([((pym)^(NHC-R))Pd(II)(CH3)X] with X = Cl, CF3COO, CH3) has been prepared by transmetalation reactions from the corresponding silver complexes and chloro(methyl)(cyclooctadiene)palladium(II). The dimethyl(1-(2-pyrimidyl)-3-(2,6-diisopropylphenyl)imidazolin-2-ylidene)palladium(II) complex was synthesized via the free carbene route. All complexes were fully characterized by standard methods and in three cases also by a solid state structure.
| S-EPMC4979653 | biostudies-literature