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Small-molecule catalysts inspired by the active sites of [FeFe]-hydrogenase enzymes have long struggled to achieve fast rates of hydrogen evolution, long-term stability, water solubility, and oxygen compatibility. We profoundly improved on these deficiencies by grafting polymers from a metalloinitiator containing a [2Fe-2S] moiety to form water-soluble poly(2-dimethylamino)ethyl methacrylate metallopolymers (

Optical technologies in the midwave and long wave infrared spectrum (MWIR, LWIR) are important systems for high resolution thermal imaging in near, or complete darkness. While IR thermal imaging has been extensively utilized in the defense sector, application of this technology is being driven toward emerging consumer markets and transportation. In this viewpoint, we review the field of IR thermal imaging and discuss the emerging use of synthetic organic and hybrid polymers as novel IR transmissive materials for this application. In particular, we review the critical role of elemental sulfur as a novel feedstock to prepare high refractive index polymers via inverse vulcanization and discuss the fundamental chemical insights required to impart improved IR transparency into these polymeric materials.

The tetrafluorinated derivative of 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), is of interest for charge transfer complex formation and as a p-dopant in organic electronic materials. Fourier transform infrared (FTIR) spectroscopy is commonly employed to understand the redox properties of F4TCNQ in the matrix of interest; specifically, the ν(C≡N) region of the F4TCNQ spectrum is exquisitely sensitive to the nature of the charge transfer between F4TCNQ and its matrix. However, little work has been done to understand how these vibrational modes change in the presence of possible acid/base chemistry. Here, FTIR spectroelectrochemistry is coupled with density functional theory spectral simulation for study of the electrochemically generated F4TCNQ radical anion and dianion species and their protonation products with acids. Vibrational modes of HF4TCNQ-, formed by proton-coupled electron transfer, are identified, and we demonstrate that this species is readily formed by strong acids, such as trifluoroacetic acid, and to a lesser extent, by weak acids, such as water. The implications of this chemistry for use of F4TCNQ as a p-dopant in organic electronic materials is discussed.

Optical technologies in the long-wave infrared (LWIR) spectrum (7-14 μm) offer important advantages for high-resolution thermal imaging in near or complete darkness. The use of polymeric transmissive materials for IR imaging offers numerous cost and processing advantages but suffers from inferior optical properties in the LWIR spectrum. A major challenge in the design of LWIR-transparent organic materials is that nearly all organic molecules absorb in this spectral window which lies within the so-called IR-fingerprint region. We report on a new molecular-design approach to prepare high refractive index polymers with enhanced LWIR transparency. Computational methods were used to accelerate the design of novel molecules and polymers. Using this approach, we have prepared chalcogenide hybrid inorganic/organic polymers (CHIPs) with enhanced LWIR transparency and thermomechanical properties via inverse vulcanization of elemental sulfur with new organic co-monomers.

[2Fe–2S] organometallic complexes are effective electrocatalysts for the reduction of weak acids in acetonitrile to generate molecular hydrogen (H2) (hydrogen evolution reaction, HER). Inco...

Reviewed herein is the development of novel polymer-supported [2Fe-2S] catalyst systems for electrocatalytic and photocatalytic hydrogen evolution reactions. [FeFe] hydrogenases are the best known naturally occurring metalloenzymes for hydrogen generation, and small-molecule, [2Fe-2S]-containing mimetics of the active site (H-cluster) of these metalloenzymes have been synthesized for years. These small [2Fe-2S] complexes have not yet reached the same capacity as that of enzymes for hydrogen production. Recently, modern polymer chemistry has been utilized to construct an outer coordination sphere around the [2Fe-2S] clusters to provide site isolation, water solubility, and improved catalytic activity. In this review, the various macromolecular motifs and the catalytic properties of these polymer-supported [2Fe-2S] materials are surveyed. The most recent catalysts that incorporate a single [2Fe-2S] complex, termed single-site [2Fe-2S] metallopolymers, exhibit superior activity for H2 production.

Dimetal paddlewheel complexes with bicyclic guanidine ligands are extremely strong thermodynamic electron donors. As a probe of the chemical potential and sites for chemical reactivity of these complexes, the interaction of Mo2(TEhpp)4 with weak acids was investigated (TEhpp is the anion of the bicyclic guanidine 3,3,9,9-tetraethyl-1,5,7-triazabicyclo[4.4.0.]dec-4-ene). Mo2(TEhpp)4 is readily protonated by acetic acid and trifluoroacetic acid as expected, but surprisingly by a mechanism that is more complicated than a simple acid-base proton exchange. Electrochemistry measurements of the shifts in potentials with successive additions of acid reveals that the conjugate base of the acid plays a critical role throughout the reaction. Computations indicate that initially the acid hydrogen bonds to a TEhpp nitrogen atom bound to the metal, and then a facile rearrangement of the conjugate base toward the axial site of the Mo–Mo bond concomitantly results in protonation of the nitrogen atom. Interestingly, the dimetal complex is bifunctional in this process, acting as a nucleophile at the nitrogen atoms of the TEhpp ligands, and as an electrophile at the Mo–Mo axial bond sites. The protonation requires a novel synergism of these disparate bonding interactions, in which protonation is not favored without enhancement by the coordinated base, and coordination of the base is enhanced by the protonation.

Small-molecule catalysts inspired by the active sites of [FeFe]-hydrogenase enzymes have long struggled to achieve fast rates of hydrogen evolution, long-term stability, water solubility, and oxyge...

A multi-technique spectroscopic and theoretical study of the Cp(2)M(benzenedithiolato) (M=Ti,V,Mo; Cp = η(5)-C(5)H(5)) series provides deep insight into dithiolene electronic structure contributions to electron transfer reactivity and reduction potential modulation in pyranopterin molybdenum enzymes. This work explains the magnitude of the dithiolene folding distortion, and the concomitant changes in metal-ligand covalency, that are sensitive to electronic structure changes as a function of d-electron occupancy in the redox orbital. It is shown that the large fold angle differences correlate with covalency, and the fold angle distortion is due to a pseudo-Jahn-Teller (PJT) effect. The PJT effect in these and related transition metal dithiolene systems arise from the small energy differences between metal and sulfur valence molecular orbitals, which uniquely poise these systems for dramatic geometric and electronic structure changes as the oxidation state changes. Herein, we have used a combination of resonance Raman, magnetic circular dichroism, electron paramagnetic resonance, and UV photoelectron spectroscopies to explore the electronic states involved in the vibronic coupling mechanism. Comparison between the UV photoelectron spectroscopy (UPS) of the d(2) M=Mo complex and the resonance Raman spectra of the d(1) M=V complex reveals the power of this combined spectroscopic approach. Here, we observe that the UPS spectrum of Cp(2)Mo(bdt) contains an intriguing vibronic progession that is dominated by a “missing-mode” that is comprised of PJT active distortions. We discuss the relationship of the PJT distortions to facile electron transfer in molybdenum enzymes.

Electrocatalytic [FeFe]-hydrogenase mimics for the hydrogen evolution reaction (HER) generally suffer from low activity, high overpotential, aggregation, oxygen sensitivity, and low solubility in water. By using atom-transfer radical polymerization (ATRP), a new class of [FeFe]-metallopolymers with precise molar mass, defined composition, and low polydispersity, has been prepared. The synthetic methodology introduced here allows facile variation of polymer composition to optimize the [FeFe] solubility, activity, and long-term chemical and aerobic stability. Water soluble functional metallopolymers facilitate electrocatalytic hydrogen production in neutral water with loadings as low as 2 ppm and operate at rates an order of magnitude faster than hydrogenases (2.5×105 s-1 ), and with low overpotential requirement. Furthermore, unlike the hydrogenases, these systems are insensitive to oxygen during catalysis, with turnover numbers on the order of 40 000 under both anaerobic and aerobic conditions.

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.

The gas-phase ionization energies of a series of Mo 2 (DPhF) 4 paddlewheel complexes (DPhF is the N , N ′-diphenylformamidinate anion with p -CH 3 , p -Cl, m -Cl, p -CF 3 , or m -CF 3 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.

The gas-phase He I and He II photoelectron spectra of the propynylruthenium molecule CpRu(CO)2C CMe (Cp = η5-C5H5) and the ethynediyldiruthenium molecule [CpRu(CO)2]2(μ-C C) 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.

International audience; Analogues 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.

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 sca...

Small molecule biomimetics inspired by the active site of the [FeFe]-hydrogenase enzymes have shown promising electrocatalytic activity for hydrogen (H2 ) generation. However, most of the active-site mimics based on [2Fe-2S] clusters are not water-soluble which limits the use of these electrocatalysts to organic media. Polymer-supported [2Fe-2S] systems, in particular, single-site metallopolymer catalysts, have shown drastic improvements for electrocatalytic H2 generation in aqueous milieu. [2Fe-2S] complexes functionalized within well-defined macromolecular supports via covalent bonding have demonstrated water solubility, enhanced site-isolation, and improved chemical stability during catalysis. In this report, the synthesis of a new propanedithiolate (pdt)-[2Fe-2S] complex bearing a single α-bromoester moiety for use in atom transfer radical polymerization (ATRP) is demonstrated as a novel metalloinitiator to prepare water-soluble poly(2-dimethylaminoethyl methacrylate) grafted (PDMAEMA-g-[2Fe-2S]) metallopolymers. Using this approach, metallopolymers with controllable molecular weights (Mn = 5-40 kg mol-1 ) and low dispersity (Đ, Mw /Mn = 1.09-1.36) are prepared, which allows for the first time observation of the effect of the metallopolymers' chain length on the electrocatalytic activity. The ability to control the composition and molecular weight of these metallopolymers enables macromolecular engineering via ATRP of these materials to determine optimal structural features of metallopolymer catalysts for H2 production.

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 =...

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.

Noninnocent ligands that are electronically coupled to active catalytic sites can influence the redox behavior of the catalysts. A series of (μ-dithiolato)Fe2(CO)6 complexes, in which the sulfur atoms of the μ-dithiolato ligand are bridged by 5-substituted (Me, OMe, Cl, t-Bu)-1,4-benzoquinones, 1,4-naphthoquinone, or 1,4-anthraquinone, have been synthesized and characterized. In addition, the bis-phosphine complex derived from the 1,4-naphthoquinone-bridged complex has also been prepared and characterized. Cyclic voltammetry of these complexes shows two reversible one-electron reductions at potentials 0.2 to 0.5 V less negative than their corresponding parent quinones. In the presence of acetic acid two-electron reductions of the complexes result in conversion of the quinones to hydroquinone moieties. EPR spectroscopic and computational studies of the one-electron-reduced complexes show electron delocalization from the semiquinones to the 2Fe2S moieties, thereby revealing the “noninnocent” behavior of the...

Two new complexes in which 2-phenylazopyridine (pap) chelates iron in hydrogenase mimics, 1,2-(μ-benzenedithiolato)-2′-phenylazopyridinediirontetracarbonyl and 1,3-(μ-propanedithiolato)-2′-phenylazo- pyridinediirontetracarbonyl have been synthesized and fully characterized, including X-ray crystal structure determinations. The electronic structures of the two complexes are compared with the analogous 1,2-(μ-benzenedithiolato)diironhexacarbonyl and 1,3-(μ-propanedithiolato)diironhexacarbonyl complexes. Based on comparison of the crystal structures, the overall bonding in the 2Fe2S core of the molecules is little perturbed by replacing two carbonyl ligands with the pap ligand. Also, the coordinated pap ligand retains a similar structure and N˭N bond distance to that of the uncoordinated ligand. However, the charge asymmetry in the 2Fe2S core that results from chelating the pap ligand on one of the iron atoms induces substantial localization of the individual orbital characters in the 2Fe2S core. Most intere...

Analogues 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 GeR

In order to study the influence of sulfur and selenium atoms incorporated into the structure of complexes that model the active site of [FeFe]-hydrogenases, a series of diiron dithiolato and diselenolato complexes of the form (μ-ECH2XCH2E-μ)Fe2(CO)6 have been prepared and characterized, where the diiron bridging atoms E are S or Se, and the linker bridgehead X is CH2, S, or Se. The electron energies have been compared by gas-phase photoelectron spectroscopy, and the oxidation, and reduction behaviors, as well as the ability to reduce protons from acetic acid to form H2, have been compared by cyclic voltammetry. Density functional theory computations agree well with the structures and electron energies of these molecules, and shed additional light on the oxidation and reduction properties. The computations indicate that the HOMO of each molecule where the bridgehead X is S or Se contains substantial chalcogen ‘lone pair’ orbital character. The presence of the bridgehead chalcogen lone pairs favors the Fe(CO)3 ‘rotated’ structures for both the cations and dianions of these complexes, but in different ways. In the cations one Fe(CO)3 rotates to put one carbonyl ligand in a semibridging position, and the bridgehead chalcogen lone pair electrons donate to the vacant coordination site created on the iron to stabilize the positive charge. In the dianions one Fe(CO)3 rotates to put one carbonyl ligand in a fully bridging position, and one bridging chalcogen atom breaks its bond with an iron atom, pulling the bridgehead chalcogen lone pair away from the iron to minimize the electron–electron repulsions.

(Bis-selenolato) and (bis-tellurolato)diiron complexes [2Fe2E(Si)] were prepared and compared with the known (bis-thiolato)diiron complex A to assess their ability to produce hydrogen from protons. Treatment of [Fe3(CO)12] with 4,4-dimethyl-1,2,4-diselenasilolane (1) in boiling toluene afforded hexacarbonyl{μ-{[1,1′-(dimethylsilylene)bis[methaneselenolato-κSe : κSe]](2 −)}}diiron(FeFe) (2). The analog bis-tellurolato complex hexacarbonyl{μ-{[1,1′-(dimethylsilylene)bis[methanetellurolato-κTe : κTe]](2 −)}}diiron(FeFe) (3) was obtained by treatment of [Fe3(CO)12] with dimethylbis(tellurocyanatomethyl)dimethylsilane, which was prepared in situ. All compounds were characterized by NMR, IR spectroscopy, mass spectrometry, elemental analysis and single-crystal X-ray analysis. The electrocatalytic properties of the [2Fe2X(Si)] (X=S, Se, Te) model complexes A, 1, and 2 towards hydrogen formation were evaluated.

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 Cp(2)M(bdt) (Cp = η(5)-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 role of intramolecular OH…S hydrogen bonding in the electrochemical reduction of protons from acetic acid using biomimetically inspired catalysts has been studied. The catalysts, hydroquinone moieties annulated to an Fe2S2(CO)6 core, were synthesized by piperidine-mediated conjugate addition of the dithiol Fe2(SH)2(CO)6 to quinones in 26–76% yields. These complexes catalyze electrochemical H2 production from acetic acid. Evidence for weak intramolecular OH…S hydrogen bonding in the neutral complexes is presented. Such hydrogen bonding becomes stronger as the charge increases on the sulfur in the electrochemically produced dianions due to “charge assistance,” and this has chemical consequences.

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...

[FeFe]-hydrogenase-active-site models containing larger chalcogens such as Se or Te have exhibited greater electron richness at the metal centers and smaller gas-phase ionization energies and reorganization energies relative to molecules containing S atoms. Diiron complexes related to the much-studied molecule [Fe 2 (μ-SC 3 H 6 S)(CO) 6 ] (1) have been prepared with one S atom replaced either by one Se atom to give [Fe 2 (μ-SC 3 H 6 Se)(CO) 6 ] (2) or by one Te atom to give [Fe 2 (μ-SC 3 H 6 Te)(CO) 6 ] (3). The molecules have been characterized by use of mass spectrometry and 13 C{ 1 H} NMR, 77 Se{ 1 H} NMR, IR, and photoelectron spectroscopic techniques along with structure determination with single-crystal X-ray diffraction, electrochemical measurements, and DFT calculations. He I photoelectron spectra and DFT computations of 2 and 3 show a lowering of ionization energies relative to those of the all-sulfur complex 1, indicating increased electron richness at the metal centers that favors electrocatalytic reduction of protons from weak acids to produce H 2 . However, chalcogen substitution from S to Se or Te also causes an increase in the Fe―Fe bond length, which disfavors the formation of a carbonyl-bridged "rotated" structure, as also shown by the photoelectron spectra and computations. This "rotated" structure is believed to be important in the mechanism of H 2 production. As a consequence of the competing influences of increased electron richness at the metals with less favorable "rotated" structures, the catalytic efficiency of the Se and Te molecules 2 and 3 is found to be comparable to that of molecule 1.

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 2...

The molecular and electronic structures of AsP(3) and P(4) have been investigated. Gas-phase electron diffraction studies of AsP(3) have provided r(g) bond lengths of 2.3041(12) and 2.1949(28) A for the As-P interatomic distances and the P-P interatomic distances, respectively. The gas-phase electron diffraction structure of P(4) has been redetermined and provides an updated value of 2.1994(3) A for the P-P interatomic distances, reconciling conflicting literature values. Gas-phase photoelectron spectroscopy provides experimental values for the energies of ionizations from the valence molecular orbitals of AsP(3) and P(4) and shows that electronically AsP(3) and P(4) are quite similar. Solid-state (75)As and (31)P NMR spectroscopy demonstrate the plastic nature of AsP(3) and P(4) as solids, and an extreme upfield (75)As chemical shift has been confirmed for the As atom in AsP(3). Finally, quantum chemical gauge-including magnetically induced current calculations show that AsP(3) and P(4) can accurately be described as strongly aromatic. Together these data provide a cohesive description of the molecular and electronic properties of these two tetraatomic molecules.

The [FeFe] hydrogenase enzyme active site inspired complexes [Fe(2)(mu-C(6)H(4)S(2))(CO)(5)PTA] (1PTA) and [Fe(2)(mu-C(6)H(4)S(2))(CO)(4)PTA(2)] (1PTA(2)) (PTA = 1,3,5-triaza-7-phosphaadamantane) were synthesized and characterized. The ability of 1PTA and 1PTA(2) 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 [Fe(2)(mu-C(6)H(4)S(2))(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 1PTA(2) catalytically produce molecular hydrogen from acetic acid, however, the mechanism by which and 1PTA and 1PTA(2) catalyze hydrogen differ in the initial reductive processes.

We present an overview of the phosphonothioate hydrolytic chemistry promoted by molybdenum organometallics as well as new results on solvent effects. The metallocene bis(� 5 -cyclopentadienyl) molybdenum(IV) dichloride (Cp2MoCl2 Cp = � 5 -C5H5) and the methylated analog, (CH3Cp)2MoCl2 hydrolyzes the compoundO, S-diethyl phenylphosphonothioate (DEPP) whose core functional group mimics the neurotoxin VX. This is one of the few examples of phosphonothioate degradation yielding exclusively the desired P-S bond scission under mild aqueous conditions (pH 7, 25 � C). The solvent composition affects the monomer-dimer equilibrium of aquated molybdocenes which in turn dictates the rate of DEPP hydrolysis. Kinetic and equilibrium results are presented to show that the monomer is the active species. Moreover, photoelectron spectroscopy data indicate that while the methylated (CH3Cp)2MoCl2 has a higher monomer concentration compared to Cp2MoCl2, the former has a diminished electrophilic molybdenum center that is key in phosphonothioate hydrolysis.

Energy measures of the intra- and intermolecular electronic effects of triisopropylsilylethynyl substitution on pentacene have been obtained from the combination of closely related gas phase and solid phase ultraviolet photoelectron spectroscopy (UPS) measurements along with solution electrochemical measurements. The results show that the shift to lower ionization energy that is expected with this substitution and observed in the gas phase measurements becomes negligible in solution and is even reversed in the solid phase. The principles that emerge from this analysis are supported by electronic structure calculations at the density functional theory level. The relation between the gas phase and solid phase UPS measurements illustrated here provides a general approach to investigating the electronic effects acting on molecules in the condensed phase, which in this case are greater than the direct substituent electronic effects within the molecule. Electronic properties such as lower ionization energies built into the single-molecule building blocks of materials and devices may be reversed in the solid state.

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...

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 (O(n)Pr)(2) for alkoxide ligands and (OR)(2) = O(CH(2))(3)O, O(CH(2))(4)O, and O[CH(CH(3))CH(2)CH(CH(3))]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.

This article reviews published literature on the electrochemical reduction and oxidation of complexes containing the Fe 2 S 2 core characteristic of the active site of [FeFe]-hydrogenases. Correlations between reduction and oxidation potentials and molecular structure are developed and presented. In cases where the complexes have been studied with regard to their ability to catalyze the reduction of acids to give dihydrogen, the overpotentials for such catalyzed reduction are presented and an attempt is made to estimate, at least qualitatively, the efficiency of such catalysis.

The displacement of terminal CO ligands in Fe 2 (μ-Se 2 C 3 H, 5 CH 3 )(CO) 6 (1) by triphenylphosphane, trimethyl phosphite, and bis(diphenylphosphanyl)ethane (dppe) ligands is investigated. Treatment of 1 with 1 equiv. of triphenylphosphane afforded Fe 2 (μ-Se 2 C 3 H 5 CH 3 )(CO) 5 (PPh 3 ) (2). The mono- and disubstituted phosphite complexes Fe 2 (μSe 2 C 3 H 5 CH 3 )(CO) S P(OMe) 3 (3)andFe 2 (g-Se 2 C 3 H 5 CH 3 )(CO) 4 -[P(OMe) 3 ] 2 (4) were obtained from the reaction of 1 with excess P(OMe) 3 at reflux in toluene. In contrast, the reaction of 1 with 1 equiv. of dppe in the presence of Me 3 NO·2H 2 0 gave a mixture of Fe 2 (μ-Se z C 3 H 5 CH 3 )(CO) 4 (κ 2 -dppe) (5) and [Fe 2 (μ-Se 2 C 3 H 5 CH 3 )(CO) 5 ] 2 (μ-dppe) (6). The newly synthesized complexes 2-6 were fully characterized by IR, 1 H NMR, 13 C NMR , 77 Se{ 1 H} NMR, and 31 P{ 1 H} NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction analysis. Complex 2 has proved to be a catalyst for the electrochemical reduction of the weak acid, acetic acid, to give molecular hydrogen.

Diiron diselenolato complexes have been prepared as models of the active site of [FeFe]-hydrogenases. Treatment of Fe3(CO)12 with 1 equiv of 1,3-diselenocyanatopropane (1) in THF at reflux afforded...

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...

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 ac...

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.

(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 Cp(2)VCl(2) molecule is a prototype for bent metallocene complexes with a single electron in the metal d shell, but experimental measure of the binding energy of the d electron by photoelectron spectroscopy eluded early attempts due to apparent decomposition in the spectrometer to Cp(2)VCl. With improved instrumentation, the amount of decomposition is reduced and subtraction of ionization intensity due to Cp(2)VCl from the Cp(2)VCl(2)/Cp(2)VCl mixed spectrum yields the Cp(2)VCl(2) spectrum exclusively. The measured ionization energies provide well-defined benchmarks for electronic structure calculations. Density functional calculations support the spectral interpretations and agree well with the ionization energy of the d(1) electron and the energies of the higher positive ion states of Cp(2)VCl(2). The calculations also account well for the trends to the other Group V bent metallocene dichlorides Cp(2)NbCl(2) and Cp(2)TaCl(2). The first ionization energy of Cp(2)VCl(2) is considerably greater than the first ionization energies of the second- and third-row transition metal analogues.

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.

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%).

The molecule Cp(3-Me3Si-6,6-dmch)ZrI2 (Cp = η5-cyclopentadienyl; 3-Me3Si-6,6-dmch = η5-3-trimethylsilyl-6,6-dimethylcyclohexadienyl) has been synthesized, and the molecular and electronic structures have been investigated. Photoelectron spectroscopy shows that substitution of a trimethylsilyl group in place of a hydrogen atom on the 6,6-dmch ligand destabilizes all of the ionizations of Cp(3-Me3Si-6,6-dmch)ZrI2 by ca. 0.1−0.2 eV compared to those of Cp(6,6-dmch)ZrI2. Density functional calculations accurately reproduce the experimental structure of the molecule and agree with the observed shift of all ionizations to lower energies. Interestingly, the ionizations are calculated to shift to lower energies without the expected increase in electron density at the metal center. This apparent contradiction is understood from analysis of one-center and two-center charge effects in the molecule. As a consequence of these charge effects, the silyl substitution makes the 3-Me3Si-6,6-dmch ligand a slightly better do...