Two-dimensional infrared spectroscopy as a tool to reveal the vibrational and molecular structure of [FeFe] hydrogenases.

[FeFe] hydrogenases are Nature's most efficient catalysts for the cleavage and evolution of molecular hydrogen. Despite decades of research, key aspects of the catalytic cycle and the underlying geometrical and electronic properties of the active-site cofactor, called the H-cluster, are not fully understood. Spectroscopic techniques have played a central role in establishing the current state of knowledge on [FeFe] hydrogenases, and further advances in the field depend critically on novel techniques that yield so-far inaccessible insights into structural and mechanistic aspects.

Ultrafast vibrational spectroscopic analysis of the ubiquitous precatalyst [Mn2(CO)10] in different solvents.

Infrared (IR) absorption and time resolved IR (IRpump-IRprobe, 2D-IR) spectroscopies have been combined to study the vibrational dynamics and solvent interactions of the carbonyl ligand stretching vibrational modes of the photocatalyst dimanganese decacarbonyl, [Mn2(CO)10], in solvents with varying physical properties (heptane, cyclohexane, THF, MeCN, DMSO, iPrOH, and MeOH). The presence of a solvent-mediated symmetry breaking mechanism leading to a gain in oscillator strength of formally symmetry-forbidden modes was observed in all solvents, although the effect was more marked in polar solvents. Ultrafast vibrational energy dissipation was found to occur via two solvent dependent relaxation pathways, rapid intramolecular vibrational energy redistribution (IVR ∼ 0.

Isolobal Cationic Iridium Dihydride and Dizinc Complexes: A Dual Role for the ZnR Ligand Enhances H Activation.

The reaction of [Ir(IPr)H][BAr] (; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; BAr = B{CH(3,5-CF)}) with ZnMe proceeds with CH elimination to give [Ir(IPr)(IPr')(ZnMe)H][BAr] (, where (IPr') is a cyclometalated IPr ligand). reacts with H to form tetrahydride [Ir(IPr)(ZnMe)H][BAr], , that loses H under forcing conditions to form [Ir(IPr)(ZnMe)H][BAr], . Crystallization of also results in the formation of its noncyclometalated isomer, [Ir(IPr)(ZnMe)][BAr], , in the solid state.

Ligand-to-metal charge transfer facilitates photocatalytic oxygen atom transfer (OAT) with -dioxo molybdenum(vi)-Schiff base complexes.

Systems incorporating the -Mo(O) motif catalyse a range of important thermal homogeneous and heterogeneous oxygen atom transfer (OAT) reactions spanning biological oxidations to platform chemical synthesis. Analogous light-driven processes could offer a more sustainable approach. The -Mo(O) complexes reported here photocatalyse OAT under visible light irradiation, and operate a non-emissive excited state with substantial ligand-to-metal charge-transfer (LMCT) character, in which a Mo[double bond, length as m-dash]O π*-orbital is populated transfer of electron density from a chromophoric salicylidene-aminophenol (SAP) ligand.

Understanding the [NiFe] Hydrogenase Active Site Environment through Ultrafast Infrared and 2D-IR Spectroscopy of the Subsite Analogue K[CpFe(CO)(CN)] in Polar and Protic Solvents.

The [CpFe(CO)(CN)] unit is an excellent structural model for the Fe(CO)(CN) moiety of the active site found in [NiFe] hydrogenases. Ultrafast infrared (IR) pump-probe and 2D-IR spectroscopy have been used to study K[CpFe(CO)(CN)] () in a range of protic and polar solvents and as a dry film. Measurements of anharmonicity, intermode vibrational coupling strength, vibrational relaxation time, and solvation dynamics of the CO and CN stretching modes of in HO, DO, methanol, dimethyl sulfoxide, and acetonitrile reveal that H-bonding to the CN ligands plays an important role in defining the spectroscopic characteristics and relaxation dynamics of the Fe(CO)(CN) unit.

Metasurface-enhanced mid-infrared spectroscopy in the liquid phase.

Vibrational spectroscopy is an important tool in chemical and biological analysis. A key issue when applying vibrational spectroscopy to dilute liquid samples is the inherently low sensitivity caused by short interaction lengths and small extinction coefficients, combined with low target molecule concentrations. Here, we introduce a novel type of surface-enhanced infrared absorption spectroscopy based on the resonance of a dielectric metasurface.

Reversible Glutamate Coordination to High-Valent Nickel Protects the Active Site of a [NiFe] Hydrogenase from Oxygen.

NAD-reducing [NiFe] hydrogenases are valuable biocatalysts for H-based energy conversion and the regeneration of nucleotide cofactors. While most hydrogenases are sensitive toward O and elevated temperatures, the soluble NAD-reducing [NiFe] hydrogenase from (SH) is O-tolerant and thermostable. Thus, it represents a promising candidate for biotechnological applications.

Reversible photo-isomerization of cis-[Pd(L-κS,O)] (HL = N,N-diethyl-N'-1-naphthoylthiourea) to trans-[Pd(L-κS,O)] and the unprecedented formation of trans-[Pd(L-κS,N)] in solution.

Upon ex situ UV-visible light irradiation, complex cis-bis(N,N-diethyl-N'-naphthoylthioureato)-palladium(ii), cis-[Pd(L-κS,O)2], undergoes isomerization in acetonitrile-d3 and chloroform-d to yield trans-[Pd(L-κS,O)2] which then rearranges thermally to novel trans-[Pd(L-κS,N)2] prior to reverting thermally to the cis isomer in the absence of light. The thermal isomerization rate is highly solvent dependent and harnessed to enable each of these three geometric isomers to be isolated and characterized by 1H NMR spectroscopy, X-ray crystallography, melting point and thermal analysis. The formation of the trans-[Pd(L-κS,N)2] isomer as part of this isomerization has only been observed with the sterically demanding cis-bis(N,N-diethyl-N'-(naphthoylthioureato)palladium(ii) precursor based on our knowledge to date.

Towards measuring reactivity on micro-to-millisecond timescales with laser pump, NMR probe spectroscopy.

We present a quantitative analysis of the timescales of reactivity that are accessible to a laser pump, NMR probe spectroscopy method using para-hydrogen induced polarisation (PHIP) and identify three kinetic regimes: fast, intermediate and slow. These regimes are defined by the relative rate of reaction, k, compared to δω, the frequency of the NMR signal oscillations associated with the coherent evolution of the hyperpolarised 1H NMR signals created after para-hydrogen (p-H2) addition during the pump-probe delay. The kinetic regimes are quantitatively defined by a NMR dephasing parameter, ε = δω/k.

Unlocking a Diazirine Long-Lived Nuclear Singlet State via Photochemistry: NMR Detection and Lifetime of an Unstabilized Diazo-Compound.

Diazirines are important for photoaffinity labeling, and their photoisomerization is relatively well-known. This work shows how hyperpolarized NMR spectroscopy can be used to characterize an unstable diazo-compound formed via photoisomerization of a N-labeled silyl-ether-substituted diazirine. This diazirine is prepared in a nuclear spin singlet state via catalytic transfer of spin order from para-hydrogen.

Photochemical pump and NMR probe to monitor the formation and kinetics of hyperpolarized metal dihydrides.

On reaction of IrI(CO)(PPh) with para-hydrogen (-H), Ir(H)I(CO)(PPh) is formed which exhibits strongly enhanced H NMR signals for its hydride resonances. Complex also shows similar enhancement of its NMR spectra when it is irradiated under -H. We report the use of this photochemical reactivity to measure the kinetics of H addition by laser-synchronized reactions in conjunction with NMR.

Coherent evolution of parahydrogen induced polarisation using laser pump, NMR probe spectroscopy: Theoretical framework and experimental observation.

We recently reported a pump-probe method that uses a single laser pulse to introduce parahydrogen (p-H) into a metal dihydride complex and then follows the time-evolution of the p-H-derived nuclear spin states by NMR. We present here a theoretical framework to describe the oscillatory behaviour of the resultant hyperpolarised NMR signals using a product operator formalism. We consider the cases where the p-H-derived protons form part of an AX, AXY, AXYZ or AA'XX' spin system in the product molecule.

Photochemistry of Transition Metal Hydrides.

Photochemical reactivity associated with metal-hydrogen bonds is widespread among metal hydride complexes and has played a critical part in opening up C-H bond activation. It has been exploited to design different types of photocatalytic reactions and to obtain NMR spectra of dilute solutions with a single pulse of an NMR spectrometer. Because photolysis can be performed on fast time scales and at low temperature, metal-hydride photochemistry has enabled determination of the molecular structure and rates of reaction of highly reactive intermediates.

Activation of B-H, Si-H, and C-F bonds with Tp'Rh(PMe3) complexes: kinetics, mechanism, and selectivity.

The photochemical reactions of Tp'Rh(PMe3)H2 (1) and thermal reactions of Tp'Rh(PMe3)(CH3)H (1a, Tp' = tris(3,5-dimethylpyrazolyl)borate) with substrates containing B-H, Si-H, C-F, and C-H bonds are reported. Complexes 1 and 1a are known activators of C-H bonds, including those of alkanes. Kinetic studies of reactions with HBpin and PhSiH3 show that photodissociation of H2 from 1 occurs prior to substrate attack, whereas thermal reaction of 1a proceeds by bimolecular reaction with the substrate.

Photochemical pump and NMR probe: chemically created NMR coherence on a microsecond time scale.

We report pump-probe experiments employing laser-synchronized reactions of para-hydrogen (para-H2) with transition metal dihydride complexes in conjunction with nuclear magnetic resonance (NMR) detection. The pump-probe experiment consists of a single nanosecond laser pump pulse followed, after a precisely defined delay, by a single radio frequency (rf) probe pulse. Laser irradiation eliminates H2 from either Ru(PPh3)3(CO)(H)2 1 or cis-Ru(dppe)2(H)2 2 in C6D6 solution.

Photochemical Reactions of Fluorinated Pyridines at Half-Sandwich Rhodium Complexes: Competing Pathways of Reaction.

Irradiation of CpRh(PMe)(CH) (; Cp = η-CH) in the presence of pentafluoropyridine in hexane solution at low temperature yields an isolable η-Ccoordinated pentafluoropyridine complex, CpRh(PMe)(η-,CNF) (). The molecular structure of was determined by single-crystal X-ray diffraction, showing coordination by C3-C4, unlike previous structures of pentafluoropyridine complexes that show N-coordination. Corresponding experiments with 2,3,5,6-tetrafluoropyridine yield the C-H oxidative addition product CpRh(PMe)(CNF)H ().

Oxidative addition of ether O-methyl bonds at a Pt(0) centre.

Pt(PCyp3)2 (Cyp = cyclopentyl) undergoes C-O oxidative addition with 2,3,5,6-tetrafluoro-4-methoxypyridine, pentafluoroanisole, 2,3,5,6-tetrafluoroanisole and 2,3,6-trifluoroanisole yielding platinum methyl derivatives. The reactions occur in preference to C-H or C-F activation.

Selective photochemistry at stereogenic metal and ligand centers of cis-[Ru(diphosphine)2(H)2]: preparative, NMR, solid state, and laser flash studies.

Three ruthenium complexes Λ-[cis-Ru((R,R)-Me-BPE)(2)(H)(2)] Λ-R,R-Ru1H(2), Δ-[cis-Ru((S,S)-Me-DuPHOS)(2)(H)(2)] Δ-S,S-Ru2H(2), and Λ-[cis-Ru((R,R)-Me-DuPHOS)(2)(H)(2)] Λ-R,R-Ru2H(2) (1 = (Me-BPE)(2), 2 = (Me-DuPHOS)(2)) were characterized by multinuclear NMR and CD spectroscopy in solution and by X-ray crystallography. The chiral ligands allow the full control of stereochemistry and enable mechanistic studies not otherwise available. Oxidative addition of E-H bonds (E = H, B, Si, C) was studied by steady state and laser flash photolysis in the presence of substrates.

Photosensitized oxidation of alkyl phenyl sulfoxides. C-S bond cleavage in alkyl phenyl sulfoxide radical cations.

The 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ(+)ClO4(-))-photosensitized oxidation of phenyl alkyl sulfoxides (PhSOCR1R2R3, 1, R1 = R2 = H, R3 = Ph; 2, R1 = H, R2 = Me, R3 = Ph; 3, R1 = R2 = Ph, R3 = H; 4, R1 = R2 = Me, R3 = Ph; 5, R1 = R2 = R3 = Me) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Steady-state photolysis showed the formation of products deriving from the heterolytic C-S bond cleavage in the sulfoxide radical cations (alcohols, R1R2R3COH, and acetamides, R1R2R3CNHCOCH3) accompanied by sulfur-containing products (phenyl benzenethiosulfinate, diphenyl disulfide, and phenyl benzenethiosulfonate). By laser irradiation, the formation of 3-CN-NMQ(*) (lambda(max) = 390 nm) and sulfoxide radical cations 1(*+) , 2(*+), and 5(*+) (lambda(max) = 550 nm) was observed within the laser pulse.