AbstractAnalogues of the [2Fe-2S] subcluster of hydrogenase enzymes in which the central group of the three-atom chain linker between the sulfur atoms is replaced by GeR2 and SnR2 groups are studied. The six-membered FeSCECS rings in these complexes (E=Ge or Sn) adopt an unusual conformation with nearly co-planar SCECS atoms perpendicular to the Fe-Fe core. Computational modelling traces this result to the steric interaction of the Me groups with the axial carbonyls of the Fe2(CO)6 cluster and low torsional strain for GeMe2 and SnMe2 moieties owing to the long C−Ge and C−Sn bonds. Gas-phase photoelectron spectroscopy of these complexes shows a shift of ionization potentials to lower energies with substantial sulfur orbital character and, as supported by the computations, an increase in sulfur character in the predominantly metal–metal bonding HOMO. Cyclic voltammetry reveals that the complexes follow an ECE-type reduction mechanism (E=electron transfer and C=chemical process) in the absence of acid and catalysis of proton reduction in the presence of acid. Two cyclic tetranuclear complexes featuring the sulfur atoms of two Fe2S2(CO)6 cores bridged by CH2SnR2CH2, R=Me, Ph, linkers were also obtained and characterized.

•Gas-phase ionization energies correlate directly with solution oxidation potentials.•Remote substituents cause substantial shifts in the metal–metal δ bond ionizations.•Shifts are not caused by direct inductive charge effects or overlap effects.•Shifts are caused by the change in potential field at the metals.•The shifts are moderated in solution by solvent stabilization of the positive ion.The gas-phase ionization energies of a series of Mo2(DPhF)4 paddlewheel complexes (DPhF is the N,N′-diphenylformamidinate anion with p-CH3, p-Cl, m-Cl, p-CF3, or m-CF3 phenyl substituents) have been measured by ultraviolet photoelectron spectroscopy (UPS) and compared with the solution oxidation potentials measured by cyclic voltammetry (CV) reported by Ren and coworkers. A linear relationship was found between the gas-phase ionization energies and the solution oxidation potentials. Density functional theory (DFT) computations clarify the individual electronic and thermodynamic factors that contribute to the correlation. The metal–metal delta bond electron energy is the largest factor in determining the solution oxidation potential. The substituents shift the metal–metal orbital energies by changing the through-space field potential at the metals rather than by an inductive change in charge at the metals or orbital overlap effects. The cation solvation energies determine the extent that the potential shifts are attenuated in solution. The results show that substituent field effects and solvation have major roles in determining the dimetal redox chemistry even when the dimetal unit is protected from direct interaction with the substituent and the solvent.Graphical abstractDimetal paddlewheel complexes display an extraordinarily wide range of electron energies and electrochemical potentials for a single class of molecules. The dimetal tetraformamidinate complexes are ideal for revealing the key electronic factors that determine the electrochemical potentials through a combination of photoelectron spectroscopy, cyclic voltammetry, and density function theory examination.

A series of conformationally constrained 2,6-bisferrocenylphenyl thioethers were synthesized via Suzuki–Miyaura cross coupling reactions. Structural information was obtained using X-ray crystallography and dynamic 1H NMR spectroscopic studies, showing highly constrained m-terphenyl systems. Interaction of the ferrocene moieties through space mediated by the sulfur were studied by ultra-violet photoelectron spectroscopy (UPS), cyclic voltammetry, differential pulse voltammetry, UV–Vis–NIR spectroscopy and DFT computations. Electrochemical results show two, fully reversible 1e− redox processes for the ferrocenes where the separation of peaks is affected by both solvent and supporting electrolyte, suggesting significant electrostatic interaction which is further confirmed in the gas phase by UPS studies.Graphical abstractGeometrically constrained 2,6-diferrocenylphenylthioethers were synthesized and characterized. Through space interaction between the ferrocene moieties mediated by sulfur were evaluated by electrochemistry, photoelectron spectroscopy and UV–Vis–NIR spectroscopy of the mixed valence species. Enhanced interaction mediated by sulfur was ascribed to electrostatic interaction enhanced by the polarizability of sulfur.

The gas-phase He I and He II photoelectron spectra of the propynylruthenium molecule CpRu(CO)2CCMe (Cp = η5-C5H5) and the ethynediyldiruthenium molecule [CpRu(CO)2]2(μ-CC) are compared with the spectrum of CpRu(CO)2Cl to experimentally determine electronic structure interactions of the alkynyl ligands with the metal. The spectra indicate that the interaction between the filled metal-dπ and filled alkynyl-π orbitals dominates the metal-alkynyl π electronic structure, mirroring previously characterized CpFe(CO)2 alkynyls. All valence ionizations of the Ru molecules are stabilized with respect to similar Fe compounds, contrary to the common expectation of lower ionization energies with atomic substitution down a column of the periodic table. Ab initio electronic structure calculations suggest that this stabilization traces to the greater inherent electronic relaxation energy associated with removal of Fe 3d electrons compared to removal of Ru 4d electrons. Destabilization of the first two ionization bands of the diruthenium molecule are a result of filled–filled interactions between alkynyl π-bonds with the symmetric combination of metal–metal-dπ orbitals, showing electronic communication between the metals through the alkynyl bridge. From the photoelectron spectrum, this communication was calculated to have a minimum electron-transfer integral of 0.56 eV. The stabilization of the antisymmetric combination of the metal–metal-dπ orbitals gives a direct and unique experimental measure of the interaction with the alkynyl π∗ orbitals. The stabilization caused by the alkynyl π∗ orbitals was found to be approximately one-third of the destabilization caused by the filled–filled interaction with the alkynyl π-bonds and about one-fourth to one-third the stabilization provided by back-bonding to a carbonyl ligand.UV photoelectron spectroscopy was used to probe the electronic structure of the alkynyl ligand of (η5-C5H5)Ru(CO)2CCMe and [(η5-C5H5)Ru(CO)2]2(μ-CC). The filled/filled interaction between the ligand π and metal d orbitals, the backbonding capacity of the alkynyl ligand, and the electronic communication between the metal centers is quantified from the ionization energies.

Reductive cleavage of disulfide bonds is an important step in many biological and chemical processes. Whether cleavage occurs stepwise or concertedly with electron transfer is of interest. Also of interest is whether the disulfide bond is reduced directly by intermolecular electron transfer from an external reducing agent or mediated intramolecularly by internal electron transfer from another redox-active moiety elsewhere within the molecule. The electrochemical reductions of 4,4′-bipyridyl-3,3′-disulfide (1) and the di-N-methylated derivative (22+) have been studied in acetonitrile. Simulations of the cyclic voltammograms in combination with DFT (density functional theory) computations provide a consistent model of the reductive processes. Compound 1 undergoes reduction directly at the disulfide moiety with a substantially more negative potential for the first electron than for the second electron, resulting in an overall two-electron reduction and rapid cleavage of the S–S bond to form the dithiolate. In contrast, compound 22+ is reduced at less negative potential than 1 and at the dimethyl bipyridinium moiety rather than at the disulfide moiety. Most interesting, the second reduction of the bipyridinium moiety results in a fast and reversible intramolecular two-electron transfer to reduce the disulfide moiety and form the dithiolate. Thus, the redox-active bipyridinium moiety provides a low energy pathway for reductive cleavage of the S–S bond that avoids the highly negative potential for the first direct electron reduction. Following the intramolecular two-electron transfer and cleavage of the S–S bond the bipyridinium undergoes two additional reversible reductions at more negative potentials.

The molecule (η5-Me2Pdl)Mn(CO)3 (η5-Me2Pdl = 2,4-dimethyl-η5-pentadienyl) has been prepared by a new method and used as a starting material to prepare the molecules (η5-Me2Pdl)Mn(CO)n(PMe3)3–n (n = 2, 1) by phosphine substitution for carbonyls. The first carbonyl substitution is achieved thermally in refluxing cyclohexane, and the second carbonyl substitution requires photolysis. At room temperature in benzene the associative intermediate (η3-Me2Pdl)Mn(CO)3(PMe3) that precedes the initial loss of carbonyl is observed. Single-crystal structures are reported for all complexes, including the associative intermediate of the first substitution in which the pentadienyl ligand has slipped to the η3 bonding mode. These molecules offer an opportunity to examine fundamental principles of the interactions between metals and pentadienyl ligands in comparison to the well-developed chemistry of metal cyclopentadienyl (Cp) complexes as a function of electron richness at the metal center. Photoelectron spectra of these molecules show that the Me2Pdl ligand has π ionizations at energy lower than that for the analogous Cp ligand and donates more strongly to the metal than the Cp ligand, making the metal more electron rich. Phosphine substitutions for carbonyls further increase the electron richness at the metal center. Density functional calculations provide further insight into the electronic structures and bonding of the molecules, revealing the energetics and role of the pentadienyl slip from η5 to η3 bonding in the early stages of the associative substitution mechanism. Computational comparison with dissociative ligand substitution mechanisms reveals the roles of dispersion interaction energies and the entropic free energies in the ligand substitution reactions. An alternative scheme for evaluating the computational translational and rotational entropy of a dissociative mechanism in solution is offered.

The redox characteristics of (μ-SMe)2Fe2(CO)6 from the 1+ to 2– charge states are reported. This [2Fe-2S] compound is related to the active sites of [FeFe]-hydrogenases but notably without a linker between the sulfur atoms. The 1+ charge state was studied both by ionization in the gas phase by photoelectron spectroscopy and by oxidation in the solution phase by cyclic voltammetry. The adiabatic ionization is to a cation whose structure features a semibridging carbonyl, similar to the structure of the active site of [FeFe]-hydrogenases in the same oxidation state. The reduction of the compound by cyclic voltammetry gives an electrochemically irreversible cathodic peak, which often suggests disproportionation or other irreversible chemical processes in this class of molecules. However, the return scan through electrochemically irreversible oxidation peaks that occur at potentials around 1 V more positive than the reduction led to the recovery of the initial neutral compound. The dependence of the CVs on scan rate, IR spectroelectrochemistry of reduction and oxidation cycles, chronocoulometry, and DFT computations indicate a mechanism in which stabilization of the dianion plays a key role. Initial one-electron reduction of the compound is accompanied in the same cathodic peak with a second slower electron reduction to the dianion. Geometric reorganization and solvation stabilize the [2Fe-2S]2– dianion such that the potential for addition of the second electron is slightly less negative than that of the first (potential inversion). The return oxidation peaks at more positive potentials follow from rapid pairing of the dianion with another neutral molecule in solution (termed homoassociation) to form a stabilized [4Fe-4S]2– dianion. Two one-electron oxidations of this [4Fe-4S]2– dianion result in regeneration of the initial neutral compound. The implications of this homoassociation for the [FeFe]-hydrogenase enzyme, in which the H-cluster active site features a [2Fe-2S] site associated with a [4Fe-4S] cubane cluster via a thiolate bridge, are discussed.

Two very soluble compounds having W2(bicyclic guanidinate)4 paddlewheel structures show record low ionization energies (onsets at 3.4 to 3.5 eV) and very negative oxidation potentials in THF (−1.84 to −1.90 V vs Ag/AgCl). DFT computations show the correlation from the gas-phase ionization energies to the solution redox potentials and chemical behavior. These compounds are thermally stable and easy to synthesize in high yields and good purity. They are very reactive and potentially useful stoichiometric reducing agents in nonpolar, nonprotonated solvents.

The intramolecular electronic structures and intermolecular electronic interactions of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene), 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]-pentacene (TP-5 pentacene), and 2,2,10,10-tetraethyl-6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (EtTP-5 pentacene) have been investigated by the combination of gas-phase and solid-phase photoelectron spectroscopy measurements. Further insight has been provided by electrochemical measurements in solution, and the principles that emerge are supported by electronic structure calculations. The measurements show that the energies of electron transfer such as the reorganization energies, ionization energies, charge-injection barriers, polarization energies, and HOMO–LUMO energy gaps are strongly dependent on the particular functionalization of the pentacene core. The ionization energy trends as a function of the substitution observed for molecules in the gas phase are not reproduced in measurements of the molecules in the condensed phase due to polarization effects in the solid. The electronic behavior of these materials is impacted less by the direct substituent electronic effects on the individual molecules than by the indirect consequences of substituent effects on the intermolecular interactions. The ionization energies as a function of film thickness give information on the relative electrical conductivity of the films, and all three molecules show different material behavior. The stronger intermolecular interactions in TP-5 pentacene films lead to better charge transfer properties versus those in TIPS pentacene films, and EtTP-5 pentacene films have very weak intermolecular interactions and the poorest charge transfer properties of these molecules.

The electronic interactions between metals and dithiolenes are important in the biological processes of many metalloenzymes as well as in diverse chemical and material applications. Of special note is the ability of the dithiolene ligand to support metal centers in multiple coordination environments and oxidation states. To better understand the nature of metal–dithiolene electronic interactions, new capabilities in gas-phase core photoelectron spectroscopy for molecules with high sublimation temperatures have been developed and applied to a series of molecules of the type Cp2M(bdt) (Cp = η5-cyclopentadienyl, M = Ti, V, Mo, and bdt = benzenedithiolato). Comparison of the gas-phase core and valence ionization energy shifts provides a unique quantitative energy measure of valence orbital overlap interactions between the metal and the sulfur orbitals that is separated from the effects of charge redistribution. The results explain the large amount of sulfur character in the redox-active orbitals and the ‘leveling’ of oxidation state energies in metal–dithiolene systems. The experimentally determined orbital interaction energies reveal a previously unidentified overlap interaction of the predominantly sulfur HOMO of the bdt ligand with filled π orbitals of the Cp ligands, suggesting that direct dithiolene interactions with other ligands bound to the metal could be significant for other metal–dithiolene systems in chemistry and biology.

Several different pKa values for the complex (η5-C5H5)Fe(CO)2H (FpH) in acetonitrile are present in the literature, ranging over 7 orders of magnitude. As a result, the energy of its Fe–H bond is also in dispute, making it difficult to predict the reactivity of this complex. The pKa, bond dissociation enthalpy, bond dissociation free energy, and hydricity of FpH have been measured. The pKa values of analogous group 8 hydrides have been determined, and the energies of the M–H bonds and the hydricity of FpH have been established.

The [FeFe] hydrogenase enzyme active site inspired complexes [Fe2(μ-C6H4S2)(CO)5PTA] (1PTA) and [Fe2(μ-C6H4S2)(CO)4PTA2] (1PTA2) (PTA = 1,3,5-triaza-7-phosphaadamantane) were synthesized and characterized. The ability of 1PTA and 1PTA2 to catalytically produce molecular hydrogen in solution from the weak acid acetic acid was examined electrochemically and compared to previous studies on the all carbonyl containing analogue [Fe2(μ-C6H4S2)(CO)6] (1). Computational methods and cyclic voltammograms indicated that the substitution of CO ligands by PTA in 1 resulted in markedly different reduction chemistry. Both 1PTA and 1PTA2 catalytically produce molecular hydrogen from acetic acid, however, the mechanism by which 1 and 1PTA and 1PTA2 catalyze hydrogen differ in the initial reductive processes.

A new synthetic method for annulating hydroquinones to Fe2S2(CO)6 moieties is reported. Piperidine catalyzed a multistep reaction between Fe2(μ-SH)2(CO)6 and quinones to afford bridged adducts in 26−76% yields. The hydroquinone adducts undergo reversible two-electron reductions. In the presence of acetic acid, hydrogen is produced catalytically with these adducts at potentials more negative than that of the initial reversible reduction. Spectroscopic studies suggest the presence of intramolecular hydrogen bonding between the phenolic OH groups and the adjacent sulfur atoms. Computations, which are in good agreement with the electrochemical studies and spectroscopic data, indicate that the hydrogen bonding is most important in the reduced forms of the catalysts. This hydrogen bonding lowers the reduction potential for catalysis but also lowers the basicity and thereby the reactivity of the catalysts.

The effects of intermolecular interactions on the electronic properties of bis-triisopropylsilylethynyl-substituted (TIPS) anthracene, tetracene, and pentacene are obtained from comparison of the ionization energies measured by solid-phase ultraviolet photoelectron spectroscopy (UPS) with the ionization energies measured by gas-phase UPS, and with the oxidation potentials measured electrochemically in solution. Additional insight is provided by electronic structure calculations at the density functional theory level. The results show that the solution-phase oxidation potentials correlate linearly with the gas-phase first ionization energies of TIPS oligoacenes, and both energies decrease with the increase in acene core size as expected for the increasing delocalization of the HOMO. However, the solid-phase ionization energies are independent of the acene core size, and thus do not follow the trend indicated by the molecular electronic structures and verified by the gas-phase and solution measurements. The solid-phase electronic properties such as charge injection barriers, ionization energies, and HOMO−LUMO energy gaps are greatly affected by the polarization effects of the surrounding molecules in the solid state, which dominate over the changes in molecular electronic properties caused by the change in acene core size.

The one- to two-electron reduction of μ-(1,2-ethanedithiolato)diironhexacarbonyl that has been observed under electrochemical conditions is dependent on scan rate and temperature, suggesting activation of a structural rearrangement. This structural rearrangement is attributed to fluxionality of the [2Fe2S] core in the initially formed anion. Computations support this assessment. Upon an initial one-electron reduction, the inherent fluxionality of the [2Fe2S] complex anion allows for a second one-electron reduction at a less negative potential to form a dianionic species. The structure of this dianion is characterized by a rotated iron center, a bridging carbonyl ligand, and, most significantly, a dissociated Fe−S bond. This fluxionality of the [2Fe2S] core upon reduction has direct implications for the chemistry of [FeFe]-hydrogenase mimics and for iron−sulfur cluster chemistry in general.

The electronic structures of the molecules (tBuO)3M≡N (M = Cr, Mo, W) have been investigated with gas phase photoelectron spectroscopy and density functional calculations. It is found that the alkoxide orbitals mix strongly with the M≡N triple bond orbitals and contribute substantially to the valence electronic structure. The first ionization of (tBuO)3Cr≡N is from an orbital of a2(C3v) symmetry that is oxygen based and contains no metal or nitrogen character by symmetry. In contrast, the first ionizations of the molybdenum and tungsten analogues are from orbitals of a1 and e symmetry that derive from the highest occupied M≡N σ and π orbitals mixed with the appropriate symmetry combinations of the oxygen p orbitals. In this a1 orbital, the oxygen p orbitals mix with the highest occupied M≡N orbital of σ symmetry. This mixing reduces the metal character, consequently reducing the metal−nitrogen overlap interaction in this orbital. From computational modeling, the polarity of the M≡N bond increases down the group such that W≡N has the highest charge separation. In addition to investigation of the effects of the metals, the electronic influences of substitution at the alkoxide ligands have been examined for the molecules (RO)3Mo≡N (R = C(CH3)2H, C(CH3)3, and C(CH3)2CF3). The introduction of CF3 groups stabilizes the molecular orbital energies and increases the measured ionization energies, but does not alter the overall electronic structure. The bonding characteristics of the (tBuO)3M≡N series are compared with those of organic nitriles.

Molecules of the general form Tp*MoO(OR)2 [where Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and (OR)2 = (OMe)2, (OEt)2, and (OnPr)2 for alkoxide ligands and (OR)2 = O(CH2)3O, O(CH2)4O, and O[CH(CH3)CH2CH(CH3)]O for diolato ligands] were studied using gas-phase photoelectron spectroscopy, cyclic voltammetry, and density functional theory (DFT) calculations to examine the effect of increasing ligand size and structure on the oxomolybdenum core. Oxidation potentials and first ionization energies are shown to be sensitive to the character of the diolato and alkoxide ligands. A linear correlation between the solution-phase oxidation potentials and the gas-phase ionization energies resulted in an unexpected slope of greater than unity. DFT calculations indicated that this unique example of a system in which oxidation potentials are more sensitive to substitution than vertical ionization energies is due to the large differences in the cation reorganization energies, which range from 0.2 eV or less for the molecules with diolato ligands to around 0.5 eV for the molecules with alkoxide ligands.

In order to elucidate the influence of the bridging chalcogen atoms in hydrogenase model complexes, diiron dithiolato, diselenolato, and ditellurolato complexes have been prepared and characterized. Treatment of Fe3(CO)12 with 3,3-bis(thiocyanatomethyl)oxetane (1) or a mixture of 2-oxa-6,7-dithiaspiro[3.4]octane (2a) and 2-oxa-6,7,8-trithiaspiro[3.5]nonane (2b) in toluene at reflux afforded the model compound Fe2(μ-S2C5H8O)(CO)6 (3). The analogous diselenolato and ditellurolato complexes, Fe2(μ-Se2C5H8O)(CO)6 (4) and Fe2(μ-Te2C5H8O)(CO)6 (5), were obtained from the reaction of Fe3(CO)12 with 2-oxa-6,7-diselenaspiro[3.4]octane (6) and 2-oxa-6,7-ditelluraspiro[3.4]octane (7), respectively. Compounds 3−5 were characterized by spectroscopic techniques (NMR, IR, photoelectron spectroscopy), mass spectrometry, single-crystal X-ray analysis, and computational modeling. The electrochemical properties for the new compounds have been studied to assess their ability to catalyze electrochemical reduction of protons to give dihydrogen, and the catalytic rate is found to decrease on going from the sulfur to selenium to tellurium compounds. In the series 3−5 the reorganization energy on going to the corresponding cation decreased from 3 to 4 to 5. Spectroscopic and computational analysis suggests that the increasing size of the chalcogen atoms from S to Se to Te increases the Fe−Fe distance and decreases the ability of the complex to form the structure with a rotated Fe(CO)3 group that has a bridging carbonyl ligand and a vacant coordination site for protonation. This effect is mirrored on reduction of 3−5 in that the rotated structure with a bridging carbonyl, which creates a vacant coordination site for protonation, is disfavored on going from the S to Se to Te complexes.

The metal–sulfur bonding present in the transition metal–thiolate complexes CpFe(CO)2SCH3, CpFe(CO)2StBu, CpRe(NO)(PiPr3)SCH3, and CpRe(NO)(PPh3)SCH3 (Cp = η5-C5H5) is investigated via gas-phase valence photoelectron spectroscopy. For all four complexes a strong dπ–pπ interaction exists between a filled predominantly metal d orbital of the [CpML2]+ fragment and the purely sulfur 3pπ lone pair of the thiolate. This interaction results in the highest occupied molecular orbital having substantial M–S π∗ antibonding character. In the case of CpFe(CO)2SCH3, the first (lowest energy) ionization is from the Fe–S π∗ orbital, the next two ionizations are from predominantly metal d orbitals, and the fourth ionization is from the Fe–S π orbital. The pure sulfur pπ lone pair of the thiolate fragment is less stable than the filled metal d orbitals of the [CpFe(CO)2]+ fragment, resulting in a Fe–S π∗ combination that is higher in sulfur character than the Fe–S π combination. Interestingly, substitution of a tert-butyl group for the methyl group on the thiolate causes little shift in the first ionization, in contrast to the shift observed for related thiols. This is a consequence of the delocalization and electronic buffering provided by the Fe–S dπ–pπ interaction. For CpRe(NO)(PiPr3)SCH3 and CpRe(NO)(PPh3)SCH3, the strong acceptor ability of the nitrosyl ligand rotates the metal orbitals for optimum backbonding to the nitrosyl, and the thiolate rotates along with these orbitals to a different preferred orientation from that of the Fe complexes. The initial ionization is again the M–S π∗ combination with mostly sulfur character, but now has considerable mixing among several of the valence orbitals. Because of the high sulfur character in the HOMO, ligand substitution on the metal also has a small effect on the ionization energy in comparison to the shifts observed for similar substitutions in other molecules. These experiments show that, contrary to the traditional interpretation of oxidation of metal complexes, removal of an electron from these metal–thiolate complexes is not well represented by an increase in the formal oxidation state of the metal, nor by simple oxidation of the sulfur, but instead is a variable mix of metal and sulfur content in the highest occupied orbital.A strong dπ–pπ interaction between a filled predominantly metal d orbital and the purely sulfur 3pπ lone pair of the thiolate in the title complexes is linked to geometry and electronic structure changes at the metal, and very effectively buffers the ionization energies to substitutions of ligands on the metal and alkyls on the thiols.

The photoelectron spectrum and a density functional computational analysis of the first p-block paddlewheel complex, Bi2(tfa)4, where tfa = (O2CCF3)−, are reported. The photoelectron spectrum of Bi2(tfa)4 contains an ionization band between the region of metal-based ionizations and the region of overlapping ligand ionizations that is not seen in the photoelectron spectra of d-block paddlewheel complexes. This additional ionization arises from an a1g symmetry combination of the tfa ligand orbitals that is directed for σ bonding with the metals, and the unusual energy of this ionization follows from the different interaction of this orbital with the valence s and p orbitals of Bi compared to the valence d orbitals of transition metals. There is significant mixing between the Bi–Bi σ bond and this a1g M–L σ orbital. This observation led to a re-examination of the ionization differences between Mo2(tfa)4 and W2(tfa)4, where the metal–metal σ and π ionizations are overlapping for the Mo2 molecule but a separate and sharp σ ionization is observed for the W2 molecule. The coalescing of the σ and π bond ionizations of Mo2(tfa)4 is due to greater ligand orbital character in the Mo–Mo σ bond (∼7%) versus the W–W σ bond (∼1%).

Understanding the electronic properties and electron transfer characteristics of functionalized oligoacenes is of great importance for the fabrication of organic electronic devices. Charge transfer parameters of bis-triisopropylsilylethynyl-substituted anthracene, tetracene, and pentacene have been investigated based on the analysis of their ionization energies and radical cation reorganization energies. High-resolution gas-phase photoelectron spectroscopy measurements and first-principles quantum-mechanical calculations at the density functional theory level have been performed. The results indicate that the ionization energies in the gas phase and the inner-sphere reorganization energies are sensitive to the number of fused rings and the substitution by the triisopropylsilylethynyl (TIPS) group. This TIPS functional group shifts the first ionization band of these molecules to lower energy in the gas phase due to mixing between the frontier occupied orbitals of the TIPS group with the highest occupied acene orbital. This mixing adds geometry modifications of the TIPS substituents to those of the acene core that occur with ionization, resulting in a near doubling of the reorganization energies with electron transfer for these molecules.

(η5-C5H5)Fe(CO)2H (FpH) is stable to weak acids such as acetic acid. However, reduction of FpH in acetonitrile in the presence of weak acids generates H2 catalytically. Evidence for the catalytic generation of H2 from just water also is observed. Since reduction of Fp2 generates Fp−, which can be protonated with weak acids, Fp2 serves as a convenient procatalyst for the electrocatalytic production of H2. Electrochemical simulations provide values for the key parameters of a catalytic mechanism for production of H2 in this system. Protonation of Fp− is found to be the rate-determining step preceding H2 production. The wealth of structural, spectroscopic, and thermodynamic information available on the key Fp2, Fp−, and FpH species provide a variety of checkpoints for computational modeling of the catalytic mechanism. The computations give good agreement with the crystal structure of Fp2, the IR spectra of Fp2, Fp−, and FpH, and the photoelectron spectra of Fp2 and FpH. The computations also account well for the reduction potentials and equilibrium constants in the electrochemical simulations. The FpH− anion is found to be susceptible to a direct and rapid attack by a proton to produce H2 and the Fp• radical, which is then reduced and protonated to continue the electrocatalytic cycle. This direct energetically downhill step of metal hydride protonation to produce molecular hydrogen may be common for sufficiently electron rich metal hydrides and/or sufficiently strong acids among many of the hydrogenase mimics reported thus far.

Synthetic analogs, μ-(RS)2Fe2(CO)6, of the active site of [Fe–Fe] hydrogenases do not have the semi-bridged CO and “rotated” structure found in the enzyme. However, recent studies have shown that cations of dithiolatodiiron complexes adopt this rotated structure. This paper reports the use of photoelectron spectroscopy in combination with density functional theory calculations to show that two previously reported complexes: μ-(1,2-benzenedithiolato)Fe2(CO)6 and μ-(1,3-propanedithiolato)Fe2(CO)6 and two new complexes: μ-(2,3-pyridinodithiolato)Fe2(CO)6 and μ-(norbornane-2-exo,3-exo-dithiolato)Fe2(CO)6 favor the “rotated” structure in their corresponding cations. Furthermore, these methods provide a measure of the reorganization energy between the “rotated” and “unrotated” structures in the gas phase. The results provide insight on the entatic state of the dithiolatodiiron site in the enzyme, in which the protein controls the structure of the active site. This structure influences the redox energy and reorganization energy enabling fast electron transfer.

The [FeFe] hydrogenase enzyme active site inspired complexes [Fe2(μ-C6H4S2)(CO)5PTA] (1PTA) and [Fe2(μ-C6H4S2)(CO)4PTA2] (1PTA2) (PTA = 1,3,5-triaza-7-phosphaadamantane) were synthesized and characterized. The ability of 1PTA and 1PTA2 to catalytically produce molecular hydrogen in solution from the weak acid acetic acid was examined electrochemically and compared to previous studies on the all carbonyl containing analogue [Fe2(μ-C6H4S2)(CO)6] (1). Computational methods and cyclic voltammograms indicated that the substitution of CO ligands by PTA in 1 resulted in markedly different reduction chemistry. Both 1PTA and 1PTA2 catalytically produce molecular hydrogen from acetic acid, however, the mechanism by which 1 and 1PTA and 1PTA2 catalyze hydrogen differ in the initial reductive processes.