Redox-active metal hydrides are of central importance in the development of novel hydrogen generation catalysts. Direct insight into open-shell hydrides is, however, difficult to obtain. One approach to gain this information is to use muonium (Mu• = μ+ e-) as a surrogate for the hydrogen radical. The chemistry of Mu• is analogous to H•; however, the species provides a highly sensitive probe through detection of the positrons arising from the muon decay (with a lifetime of ∼2.2 μs) and can therefore provide unique information about hyperfine couplings and thus molecular structure. Using this approach, we demonstrate here that the high-symmetry {2Fe2S} systems Fe2(edt)(CO)4L2 (edt = ethane-1,2-dithiolato; L = CO, PMe3, CN-) form bridging radicals directly on the time scale of the muon experiment. We also extend our computational approach to detail all of the possible addition sites in solid state samples.

The active sites of metalloenzymes continue to inspire synthetic chemists to create structural models of the intricate structures seen in biology. As well as the fundamental intellectual challenge, this is driven by the potential societal impact that many of these systems offer. Formate dehydrogenases (FDHs) hold such potential: formate is one of the key candidates as a hydrogen carrier for future energy transport. Efforts at mimicking the active site of FDH require the synthesis of (functionalised) molybdenum bis(dithiolene) complexes, principally featuring the MoO unit. In this review, we give an overview of the synthetic routes used to date in these efforts, along with key infrared and electrochemical data for the full collection of synthetic complexes reported to date.

A series of Fe-2(mu-SR)(2)(CO)(6) complexes {R=Me (1(Me)), Et (1(Et)), Pr (1(Pr)), iPr (1(iPr)), tBu (1(tBu)), PhCH2 (1(Bn)) and Ph (1(Ph))} have been synthesised. Complexes 1(Me), 1(tBu), 1(Bn) and 1(Ph) were produced by addition of S2R2 to Fe-3(CO)(12), with all but 1(tBu) giving excellent yields. Two isomers of 1(Me) and 1(Ph) were isolated: the anti- and 'open' syn-products. Complexes 1(Et), 1(Pr) and 1(iPr) were synthesised by treatment of RSH with Fe-3(CO)(12); two isomers of each complex were isolated. Addition of one equivalent of PR'(3) (R'= Me, Cy, Ph) yields the corresponding mono(phosphine) adducts, whilst use of two equivalents of the phosphine (under mild condition, reflux, or irradiation using a deep blue LED depending on SR group) affords the corresponding bis(phosphine) adducts in good to excellent yield. Treatment of 1(Ph) or 1(Me) with two equivalents of PMe3 gives the corresponding bis-substituted phosphines when carried out in the absence of light but leads to oxidative cleavage to Fe(mu-SPh)(2)(PMe3)(2)(CO)(2) and Fe(mu-SMe)(2)(PMe3)(2)(CO)(2), respectively, under blue light irradiation. Treatment of 1(Pr) with two equivalents of PCy3 under blue light irradiation leads to reductive breakdown of the Fe Fe bond to yield Fe(CO)(3)(PCy3)(2), but in the dark at room temperature the desired product Fe-2(mu-SPr)(2)(PCy3)(2)(CO)(4) may be isolated. Single crystal X-ray structures were obtained for most family members of 'butterfly' {Fe2S2} cores. Cyclic voltammetry shows PMe3-containing complexes undergo irreversible oxidation, whereas both PCy3 and PPh3 complexes show one (quasi)reversible oxidation, IR of in situ protonation showed COv blue shifting around 80-100 cm(-1), while P-31{H-1} NMR spectroscopy showed shifting to low field.

The number of methods to study transient paramagnetic hydrides at organometallic centres is extremely limited. The reactivity of {2Fe2S} centres with protons to produce both diamagnetic and paramagnetic systems is of central interest in developing novel catalysts for hydrogen production, inspired by the [FeFe]-hydrogenase enzymes. Here, we show how a combination of spectroscopic and electrochemical techniques is allowing access to detail of the reactivity of key species on these pathways. Electron paramagnetic resonance and infra-red spectroelectrochemical approaches have been used to observe the reduction of pre-generated diamagnetic hydrides. In contrast, avoided level crossing muon spin resonance (ALC-µSR) has been used to form the open-shell species directly and to examine the formation of short-lived intermediates in the reaction process. The combination of these techniques suggests the involvement of terminal hydrides or CO-protonation states on the pathway to the isolable bridging hydride products.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Muons are particles with a spin of ½ that can be implanted into a wide range of condensed matter materials to act as a local probe of the surrounding atomic environment. Measurement of the muon’s precession and relaxation provides an insight into how it interacts with its local environment. From this, unique information is obtained about the static and dynamic properties of the material of interest. This has enabled muon spin spectroscopy, more commonly known as muon spin rotation/relaxation/resonance (μSR), to develop into a powerful tool to investigate material properties such as fundamental magnetism, superconductivity and functional materials. Alongside this, μSR may be used to study, for example, energy storage materials, ionic diffusion in potential batteries, the dynamics of soft matter, free radical chemistry, reaction kinetics, semiconductors, advanced manufacturing and cultural artefacts. This Primer is intended as an introductory article and introduces the μSR technique, the typical results obtained and some recent advances across various fields. Data reproducibility and limitations are also discussed, before highlighting promising future developments.

Reaction of Ru(CO)4Br2 with a series of heterocyclic amines leads to a family of ruthenium carbamoyl complexes. These are analogous to the related ferracyclic systems reported by us as potential photoCORM systems (Chem. Commun., 2020, 56, 4300?4303). In contrast to the iron compounds, the ruthenium carbamoyls release CO only when irradiated in the UV, and show no activity as anti-inflammatory agents.

Phenylacetylene-capped silicon nanoparticles (Phenyl-SiNPs) have attracted interest as a novel thermoelectric material. Here, we report a combined muon spectroscopic (μSR) and computational study of this material in solution to investigate the microscopic electronic structure of this system. For comparison, the model molecular compound tetrakis(2-phenylethynyl)silane has also been investigated. μSR measurements have shown that the muon isotropic hyperfine coupling constant, Aμ, which depends on spin density at the muon, is greatly reduced for the Phenyl-SiNPs system when compared to the model compound. Results have also demonstrated that the temperature dependence of Aμ for the Phenyl-SiNPs is of opposite sign and proportionally larger when compared to the model compound. Ab initio DFT methods have allowed us to determine the muon addition site in the model compound, while a wider computational study using both DFTB+ and CASTEP offers a qualitative explanation for the reduced coupling seen in the Phenyl-SiNPs system and also the contrasting temperature dependence of Aμ for the two materials. Calculations suggest an increase in the density of electronic states at the energy level of the highest occupied molecular state for the Phenyl-SiNPs, even in the presence of an organic cap, suggesting a mechanism for enhanced electron transport in this system when compared to the tetrakis model compound.

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.

The cationic pincer‐type complexes [IrI(CNMeC)L]X {CNMeC = [2,6‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]‐3,5‐dimethylpyridine, L = CO, X = PF64; L = CH3CN, X = PF65; L = pyridine, X = BArF4, ArF= 3,5‐bis‐trifluoromethyl‐phenyl6}, that were obtained from [IrI(CNMeC)Cl] (1) by displacement of the chloride ligand were structurally characterized. Complexes4and5adopt square planar, in‐plane distorted geometries, and in6the metal environment shows substantial pyramidalization. Theoretical calculations of the cations in4and6reproduce the experimental structures and rationalize their features.1undergoes oxidative transformations with CH2Cl2tocis‐[IrIII(CNMeC)(CH2Cl)Cl2] (7) and with PhICl2tomer‐[IrIII(CNMeC)Cl3] (8). The ruthenium derivativestrans‐[RuII(CNC)Cl2L] {CNC = [2,6‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]‐pyridine, L = pyridine,10} and [RuII(CNC)(η2,η2‐nbd)](X)2. 2L (nbd = 2,5‐norbornadiene, L = CH3CN, X = BF411), were prepared by the reaction ofcis‐trans‐[RuCl2(nbd)(py)2] andtrans‐cis‐[RuCl2(nbd)(pip)2] (pip = piperidine) with the ligand CNC, respectively; both adopt distorted octahedral structures. The back‐bonding in11is comparable to that in its precursor complex, indicating minimal contribution of the NHC donors to this effect. Substitution of both chlorides in the knowncis‐[RuII(CNC)Cl2L] (L = PPh3) by azido ligands gavecis‐[RuII(CNC)(N3)2L] (L = PPh3,12), which by photolytic cleavage of the coordinated N3failed to produce well‐defined complexes.

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 β0 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 β-values. In active systems with short (phenyl) π-bridges, β0 values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic β-values, β0/N3/2 (where N is the number of bridge π-electrons) thus appear to break empirical performance limits (β0/N3/2 vs λmax) for planar organic systems. However, β0 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 β 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 β0 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.

AbstractThe 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.

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.

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 η2–η1 rearrangement of the carboxylate ligand which gives a unique paramagnetic 17-electron Mo(iii) dihydride.

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.