Vacant Coordination Site Research Articles

Comparison of S and Se dichalcogenolato [FeFe]-hydrogenase models with central S and Se atoms in the bridgehead chain

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.

Synthesis and Studies of a Water-Soluble and Air-Stable CuI/CuII Open-Shell Funnel Complex

The derivatization of the large rim of a TMPA-capped calix[6]arene (TMPA = tris(2-pyridylmethyl)amine) with three trimethylammonium groups enables the water-solubilization of two air-stable Cu(I)/Cu(II) complexes. These two complexes present a vacant coordination site shielded from the aqueous environment by the calixarene core. The spectroscopic and electrochemical data recorded in pure water indicate that the host-guest properties of the funnel complex are retained in both oxidation states of the copper cation.

Quadruply bonded dimolybdenum complexes with highly unusual geometries and vacant coordination sites

New quadruply bonded dimolybdenum complexes of the terphenyl ligand Ar(Xyl(2)) (Ar(Xyl(2)) = C(6)H(3)-2,6-(C(6)H(3)-2,6-Me(2))(2)) have been prepared and structurally characterized. The steric hindrance exerted by the Ar(Xyl(2)) groups causes the Mo atoms to feature unsaturated four-coordinate structures and a formal fourteen-electron count.

Application of Electron-Withdrawing Coordinatively Unsaturated η6-Arene β-Diketiminato–Ruthenium Complexes in Lewis Acid Catalyzed Diels–Alder Reactions

Utilizing the aza-Wittig reaction involving the ylid 3,5-(CF3)2C6H3NPPh3 and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione, a highly fluorinated and electron-withdrawing β-diketiminate was obtained. Using strong bases, nBuLi, Ag2O, or TlOEt, the corresponding β-diketiminato-Li, -Ag, or -Tl chelated complexes were prepared. Subsequent in situ transmetalation with (Ru(η6-C6H6)Cl2)2 or (Ru(η6-p-cymene)Cl2)2 afforded the half-sandwich chloro-substituted Ru(II) β-diketimino complexes in high yield. The synthesis of the Lewis acidic catalysts featuring a vacant coordination site at the metal center was accomplished using [Na]BArF (BArF = tetrakis[3,5-bis(trifluoromethyl)phenyl]boron). These complexes are active for the Lewis acid catalyzed Diels–Alder reaction between α,β-unsaturated aldehydes, that is, methacrolein, acrolein, and dienes, that is, cyclopentadiene and 2,3-dimethyl-1,3-butadiene, with conversions in the range of 66–98% under mild conditions. Whereas the herein described catalysts generally promote exo ...

Lessons from Isolable Nickel(I) Precursor Complexes for Small Molecule Activation

Small-molecule activation by transition metals is essential to numerous organic transformations, both biological and industrial. Creating useful metal-mediated activation systems often depends on stabilizing the metal with uncommon low oxidation states and low coordination numbers. This provides a redox-active metal center with vacant coordination sites well suited for interacting with small molecules. Monovalent nickel species, with their d(9) electronic configuration, are moderately strong one-electron reducing agents that are synthetically attractive if they can be isolated. They represent suitable reagents for closing the knowledge gap in nickel-mediated activation of small molecules. Recently, the first strikingly stable dinuclear β-diketiminate nickel(I) precursor complexes were synthesized, proving to be suitable promoters for small-molecule binding and activation. They have led to many unprecedented nickel complexes bearing activated small molecules in different reduction stages. In this Account, we describe selected achievements in the activation of nitrous oxide (N(2)O), O(2), the heavier chalcogens (S, Se, and Te), and white phosphorus (P(4)) through this β-diketiminatonickel(I) precursor species. We emphasize the reductive activation of O(2), owing to its promise in oxidation processes. The one-electron-reduced O(2) activation product, that is, the corresponding β-diketiminato-supported Ni-O(2) complex, is a genuine superoxonickel(II) complex, representing an important intermediate in the early stages of O(2) activation. It selectively acts as an oxygen-atom transfer agent, hydrogen-atom scavenger, or both towards exogenous organic substrates to yield oxidation products. The one-electron reduction of the superoxonickel(II) moiety was examined by using elemental potassium, β-diketiminatozinc(II) chloride, and β-diketiminatoiron(I) complexes, affording the first heterobimetallic complexes featuring a [NiO(2)M] subunit (M is K, Zn, or Fe). Through density functional theory (DFT) calculations, the geometric and electronic structures of these complexes were established and their distinctive reactivity, including the unprecedented monooxygenase-like activity of a bis(μ-oxo)nickel-iron complex, was studied. The studies have further led to other heterobimetallic complexes containing a [NiO(2)M] core, which are useful for understanding the influence of the heterometal on structure-reactivity relationships. The activation of N(2)O led directly to the hydrogen-atom abstraction product bis(μ-hydroxo)nickel(II) species and prevented isolation of any intermediate. In contrast, the activation of elemental S, Se, and Te with the same nickel(I) reagent furnished activation products with superchalcogenido E(2)(-) (E is S, Se, or Te) and dichalcogenido E(2)(2-) ligand in different activation stages. The isolable supersulfidonickel(II) subunit may serve as a versatile building block for the synthesis of heterobimetallic disulfidonickel(II) complexes with a [NiS(2)M] core. In the case of white phosphorus, the P(4) molecule has been coordinated to the nickel(I) center of dinuclear β-diketiminatonickel(I) precursor complexes; however, the whole P(4) subunit is a weaker electron acceptor than the dichalcogen ligands E(2), thus remaining unreduced. This P(4) binding mode is rare and could open new doors for subsequent functionalization of P(4). Our advances in understanding how these small molecules are bound to a nickel(I) center and are activated for further transformation offer promise for designing new catalysts. These nickel-containing complexes offer exceptional potential for nickel-mediated transformations of organic molecules and as model compounds for mimicking active sites of nickel-containing metalloenzymes.

Preparation of zirconium phenylphosphonate films with different crystallite orientations on polystyrene substrates by in situ hydrothermal crystallization

Zirconium phenylphosphonate (ZrPP) films with different crystallite orientations have been fabricated on unmodified and surface-sulfonated polystyrene (PS) substrates by an in situ hydrothermal crystallization method. On the unmodified PS substrate, a monolayer film with the hexagonal ab planes of the ZrPP crystallites parallel to the PS can be obtained by restricting the crystallization time. After a longer crystallization time, the ZrPP films adopt a bilayer arrangement, with the ab planes of the ZrPP crystallites in the lower layer being parallel to the substrate, and the ab planes of the ZrPP crystallites in the upper layer being perpendicular to the substrate. On the surface-sulfonated PS, however, the ZrPP crystallites form a monolayer film with their ab planes perpendicular to the substrate irrespective of the crystallization time. We suggest that the different interactions between the ZrPP crystallites and the two substrates, namely the π–π interactions between the phenyl groups in the ZrPP surface and those in the unmodified PS substrate, and the coordinate interactions between the lone pairs of electrons on oxygen atoms in the sulfonate groups on the sulfonated PS and vacant coordination sites on Zr atoms on the edges of ZrPP crystallites, are responsible for the different morphologies of the monolayer films on the unmodified and sulfonated substrates. Two distinct growth mechanisms are proposed which can satisfactorily account for the different crystallite orientations in the two ZrPP films.

Unraveling the Electronic Properties of the Photoinduced States of the H‐Cluster in the [FeFe] Hydrogenase from D. desulfuricans

Abstract[FeFe] hydrogenases belong to a class of enzymes that catalyze the reversible heterolytic splitting of molecular hydrogen. The structure of their active site is rather unusual consisting of a CN– and CO‐coordinated [2Fe] subcluster connected to a ferredoxin‐like [4Fe4S] subcluster through one of the Cys‐S ligands. The iron, distal to the “cubane” has an open coordination site that is believed to be the substrate binding site. In the active forms of the H‐cluster, this site can be inhibited by carbon monoxide, which results in an EPR‐active state, called Hox‐CO. At temperatures below 100 K, illumination in the visible range causes conversion of this state into two other EPR‐active states. One of these is the Hox state, i.e. the catalytically active form, characterized by a vacant external coordination site. The other state (HLI) is probably characterized by dissociation of the bridging CO ligand. The photodissociation of the Hox‐CO state provides a convenient way to study changes in the electronic structure caused by dissociation of various CO ligands. In this article, we show results of an advanced EPR study of the light‐induced species of the active site, in particular, HLI. On the basis of the resolved hyperfine signals of the 1H, 14N, 13C, and 57Fe nuclei, we obtained a detailed picture of the electronic structure of the H‐cluster. It turned out that dissociation of the bridging CO ligand causes a major rearrangement of the structure of the H‐cluster and large changes in the distribution of the unpaired spin. Moreover, it appears that the extent of spin delocalization in the 2Fe subcluster in the case of HLI is intermediate between that of Hox and Hox‐CO.

Lanthanide Clusters with Chalcogen Encapsulated Ln: NIR Emission from Nanoscale NdSex

Ln(SePh)(3) (Ln = Ce, Pr, Nd) reacts with elemental Se in the presence of Na ions to give (py)(16)Ln(17)NaSe(18)(SePh)(16), a spherical cluster with a 1 nm diameter. All three rare-earth metals form isostructural products. The molecular structure contains a central Ln ion surrounded by eight five-coordinate Se(2-) that are then surrounded by a group of 16 Ln that define the cluster surface, with additional μ(3) and μ(5) Se(2-), μ(3) and μ(4) SePh(-), and pyridine donors saturating the vacant coordination sites of the surface Ln, and a Na ion coordinating to selenolates, a selenido, and pyridine ligands. NIR emission studies of the Nd compound reveal that this material has a 35% quantum efficiency, with four transitions from the excited state (4)F(3/2) ion to (4)I(9/2), (4)I(11/2), (4)I(13/2), and (4)I(15/2) states clearly evident. The presence of Na(+) is key to the formation of these larger clusters, where reactions using identical concentrations of Nd(SePh)(3) and Se with either Li or K led only to the isolation of (py)(8)Nd(8)Se(6)(SePh)(12).

High Gas Sorption and Metal‐Ion Exchange of Microporous Metal–Organic Frameworks with Incorporated Imide Groups

Metal-organic frameworks (MOFs), {[Cu(2)(bdcppi)(dmf)(2)]·10DMF·2H(2)O}(n) (SNU-50) and {[Zn(2)(bdcppi)(dmf)(3)]·6DMF·4H(2)O}(n) (SNU-51), have been prepared by the solvothermal reactions of N,N'-bis(3,5-dicarboxyphenyl)pyromellitic diimide (H(4)BDCPPI) with Cu(NO(3))(2) and Zn(NO(3))(2), respectively. Framework SNU-50 has an NbO-type net structure, whereas SNU-51 has a PtS-type net structure. Desolvated solid [Cu(2)(bdcppi)](n) (SNU-50'), which was prepared by guest exchange of SNU-50 with acetone followed by evacuation at 170 °C, adsorbs high amounts of N(2), H(2), O(2), CO(2), and CH(4) gases due to the presence of a vacant coordination site at every metal ion, and to the presence of imide groups in the ligand. The Langmuir surface area is 2450 m(2) g(-1). It adsorbs H(2) gas up to 2.10 wt% at 1 atm and 77 K, with zero coverage isosteric heat of 7.1 kJ mol(-1), up to a total of 7.85 wt% at 77 K and 60 bar. Its CO(2) and CH(4) adsorption capacities at 298 K are 77 wt% at 55 bar and 17 wt% at 60 bar, respectively. Of particular note is the O(2) adsorption capacity of SNU-50' (118 wt% at 77 K and 0.2 atm), which is the highest reported so far for any MOF. By metal-ion exchange of SNU-51 with Cu(II), {[Cu(2)(bdcppi)(dmf)(3)]·7DMF·5H(2)O}(n) (SNU-51-Cu(DMF)) with a PtS-type net was prepared, which could not be synthesized by a direct solvothermal reaction.

A New Family of High-Performance Ti Catalysts for Olefin Polymerization

The syntheses, structures and olefin polymerization behavior of Ti complexes containing a pair of chelating aminotropone [ON] ligands are reported. DFT studies revealed that bis(aminotropone) Ti complexes, when activated, provide a metal alkyl in the cis position to a vacant coordination site for monomer binding, suggesting great potential for the polymerization of olefinic monomers. Unlike Ti-FI catalysts, bis(aminotropone) Ti complexes do not require the presence of steric bulk in close proximity to the anionic donor. Bis(aminotropone) Ti complexes combined with MAO display high ethylene polymerization activities (max. 27 200 kg polymer per mol catalyst and hour) comparable to those obtained with early group-four metallocene catalysts under identical conditions.

IF‐ReS2 with Covalently Linked Porphyrin Antennae

The preparation of inorganic and organic hybrid materials, of metals or semiconductor systems which are functionalized with functional molecules to fabricate devices — nanotechnology — is currently an area of intense activity in both fundamental science and applied science on an international scale. Principally, nanotechnology aims at manipulating atoms, molecules, and nanosize particles in a precise and controlled manner in order to build materials with a fundamentally new organization and novel properties. The embryonic stage of nanotechnology is atomic assembly, whereas the mature form of nanotechnology will be molecular assembly to make nano-building blocks for the design of nanocomposites or self-organizing nanodevices. In order to achieve the outstanding properties of nanomaterials, surface modification is the key to success, as any application in materials and devices is hindered by difficulties in processing and manipulation. For instance, the charge transport process across the interface between a semiconductor metal oxide and an organic dye or a biomolecule forms the basis for improving the performance of many optoelectronic devices, photovoltaic cells, and light emitting diodes (LEDS). Chalcogenide nanotubes and fullerenes are particularly useful components for inorganic hybrid structures because they find many potential applications as solid lubricants, catalysts for the hydrodesulfurization, and they are rigid and structurally well-defined. Many of the useful properties of inorganic layered metal chalcogenides are related to their crystal structures, which are characterized by weak van der Waals forces between the individual MQ2 (Q = S, Se) slabs containing metal atoms sandwiched between two inert chalcogen layers. Furthermore, they may have a broad range of chemical and physical properties that can be chosen according to the desired task. However, in order to utilize chalcogenide nanotubes and fullerenes in hybrid materials, their surfaces must be tailored with reactive groups, i. e., their surface chemistry has to be mastered. But the inertness of the chalcogen surface layer and the associated shielding of the metal atoms from nucleophilic attacks by organic ligands are the main obstacles for the functionalization of chalcogenide nanoparticles and nanotubes. Since direct anchoring of organic ligands to the surface of chalcogenides nanomaterials is difficult, we used a nitrilotriacetic acid that can be used to coordinate to Ni ions which, in turn, can use their vacant coordination sites for binding to the surface sulfur atoms of metal chalcogenides. A seemingly simple question to be addressed is the binding mode of the metal/scorpionate ligand block to the sidewalls of the metal chalcogenides nanoparticles. The most obvious model would assume a binding of the metal to defects on the chalcogenide surface. The second model assumes that the metal (e.g., Ni) is bonded to the ideally closed packed outer sulfur layer of the chalcogenide tube, and the metal is situated in a threefold hollow (or on a twofold coordinating or single site) of the closepacked sulfur layer. Herein, a step further, we report the surface functionalization of ReS2 fullerenes with polymeric ligands, the subsequent binding of protoporphyrin ligands, and the complexation of zinc. Our polymeric ligand incorporates (i) a nitrilotriacetic acid (NTA) anchor that can be used to coordinate to Ni ions which, in turn, can use their vacant coordination sites for binding to the surface S atoms of IF–ReS2; (ii) amine groups which can be used to immobilize protoporphyrins; and (iii) a piperazyl 4-chloro-7-nitrobenzofurazan (pipNBD) fluorescent dye as a recognition molecule to monitor microscopically the functionalization of the polymeric ligand to IF–ReS2. The amino groups were utilized for conjugating the protoporphyrin, which in turn complexates the Zn cation. The resulting functionalized IF– ReS2 particles can be dispersed in water and organic solvents. They were characterized by high-resolution trans-

Determination of the Photolysis Products of [FeFe]Hydrogenase Enzyme Model Systems using Ultrafast Multidimensional Infrared Spectroscopy

Ultrafast transient 2D-IR (T-2D-IR) spectroscopy has been used to study the photolysis products of the [FeFe]hydrogenase enzyme model compound (μ-propanedithiolate)Fe(2)(CO)(6) in heptane solution following irradiation at ultraviolet wavelengths. Observation of coupling patterns between the vibrational modes of the photoproduct species formed alongside examination of the appearance time scales of these signals has uniquely enabled assignment of the photoproduct spectrum to a single pentacarbonyl species. Comparison of the vibrational relaxation rate of the photoproduct with that of the parent is consistent with the formation of a solvent adduct at the vacant coordination site, while anisotropy data in conjunction with density functional theory simulations indicates substitution in an axial rather than equatorial position. No firm evidence of additional short-lived intermediates is seen, indicating that the subsequent chemistry of these species is likely to be strongly defined by the nature of the first solvation shell.

Metallacyclic Pyridylidene Structures from Reactions of Terminal Pyridylidenes with Alkenes and Acetylene

Iridium pyridyl species are key intermediates in the title reactions. Thermal elimination of benzene from complexes 1 (R=Me, Ph) creates a vacant coordination site accessible to unsaturated hydrocarbons. Subsequent intramolecular nucleophilic attack by the pyridyl nitrogen atom to the alkene or vinylidene leads to iridacyclic pyridylidene structures (see scheme).

Probing ‘Spin-Forbidden’ Oxygen-Atom Transfer: Gas-Phase Reactions of Chromium−Porphyrin Complexes

Oxygen-atom transfer reactions of metalloporphyrin species play an important role in biochemical and synthetic oxidation reactions. An emerging theme in this chemistry is that spin-state changes can play important roles, and a 'two-state' reactivity model has been extensively applied especially in iron porphyrin systems. Herein we explore the gas-phase oxygen-atom transfer chemistry of meso-tetrakis(pentafluorophenyl)porphyrin (TPFPP) chromium complexes, as well as some other tetradentate macrocyclic ligands. Electrospray ionization in concert with Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry has been used to characterize and observe reactivity of the ionic species [(TPFPP)Cr(III)](+) (1) and [(TPFPP)Cr(V)O](+) (2). These are attractive systems to examine the effects of spin-state change on oxygen-atom transfer because the d(1) Cr(V) species are doublets, while the Cr(III) complexes have quartet ground states with high-lying doublet excited states. In the gas phase, [(TPFPP)Cr(III)](+) forms adducts with a variety of neutral donors, but O-atom transfer is only observed for NO(2). Pyridine N-oxide adducts of 1 do yield 2 upon collision-induced dissociation (CID), but the ethylene oxide, DMSO, and TEMPO analogues do not. [(TPFPP)Cr(V)O](+) is shown by its reactivity and by CID experiments to be a terminal metal-oxo with a single, vacant coordination site. It also displays limited reaction chemistry, being deoxygenated only by the very potent reductant P(OMe)(3). In general, [(TPFPP)Cr(V)O](+) species are much less reactive than the Fe and Mn analogues. Thermochemical analysis of the reactions points toward the involvement of spin issues in the lower observed reactivity of the chromium complexes.

Capsule Formation, Carboxylate Exchange, and DFT Exploration of Cadmium Cluster Metallocavitands: Highly Dynamic Supramolecules

A family of molecular heptacadmium carboxylate clusters templated inside [3 + 3] Schiff base macrocycles has been isolated and studied by variable temperature solution and solid-state NMR spectroscopy, single-crystal X-ray diffraction (SCXRD), and density functional theory (DFT) calculations. These metallocavitand cluster complexes adopt bowl-shaped structures, induced by metal coordination, giving rise to interesting host-guest and supramolecular phenomena. Specifically, dimerization of these metallocavitands yields capsules with vacant coordination and hydrogen-bonding sites accessible to encapsulated guests. Strong host-guest interactions explain the exceptionally high packing coefficient (0.80) observed for encapsulated N,N-dimethylformamide (DMF). The guest-accessible hydrogen-bonding sites arise from an unusual mu(3)-OH ligand bridging three cadmium ions. Thermodynamic and kinetic studies show that dimerization is an entropy-driven process with a highly associative mechanism. In DMF the exchange rate of peripheral cluster supporting carboxylate ligands is intrinsically linked to the rate of dimerization and these two seemingly different events have a common rate-determining step. Investigation of guest dynamics with solid-state (2)H NMR spectroscopy revealed 3-fold rotation of an encapsulated DMF molecule. These studies provide a solid understanding of the host-guest and dynamic properties of a new family of metallocavitands and may help in designing new supramolecular catalysts and materials.

Neutral and Cationic Hydridoruthenium Tetrakiscarbene Complexes

AbstractStarting from the novel chlorido precursor trans‐[RuCl2(IMe)4] (1, IMe = 1,3,4,5‐tetramethylimidazol‐2‐ylidene), hydridoruthenium complexes trans‐[RuH2(IMe)4] (2) and [RuH(IMe)4][BEt4] (3BEt4) have been synthesized. Complex 2 was isolated from the reaction of 1 with LiAlH4, while ionic compound 3 was obtained when LiBHEt3 was used as the hydride source. Complexes 1–3 have been characterized by X‐ray crystallography, multinuclear NMR, IR, UV/Vis spectroscopy and mass spectrometry. Neutral dihydride 2 displays a tetragonal bipyramidal geometry with four carbene groups coordinated to ruthenium in equatorial positions and two apical hydrogen atoms. DFT calculations suggest that the trans structure observed for 2 is preferred over a cis arrangement for steric reasons. The ruthenium atom of 3 has a tetragonal pyramidal coordination environment with a vacant coordination site sterically protected by the Me substituents of the ligands. Thus, compound 3 should be an attractive target for future coordination studies.

X-ray Crystallographic and NMR Spectroscopic Study of (η2-Alkene)(μ-alkyne)pentacarbonyldicobalt Complexes: Arrested Pauson−Khand Reaction Intermediates

The hexacarbonyldicobalt complexes of a range of 5-alkynyl-5H-dibenzo[a,d]cycloheptenes, 11a−e, readily undergo loss of a carbonyl ligand with concomitant formation of pentacarbonyldicobalt clusters, 12a−e, in which the vacant coordination site on cobalt is now occupied by the C(10)−C(11) double bond of the central seven-membered ring. These (η2-alkene)(μ-alkyne)pentacarbonyldicobalt complexes provide structural models of the first step of the proposed mechanism of the Pauson−Khand process for the formation of cyclopentenones via the coupling of an alkene, an alkyne, and a source of carbon monoxide. X-ray crystallographic data reveal that the distance between the alkene carbons and the nearest alkyne carbon is approximately 2.85 Å, slightly shorter than the theoretically predicted value of ∼2.95 Å. VT NMR data for the interconversion of 11b and 12b yielded activation energies for the forward and reverse processes, 29 and 15 kcal mol−1, respectively, and the enthalpy change for the endothermic process (14 kcal mol−1); these match very well the earlier predictions from DFT calculations. Although the ethynyl-hexacarbonyldicobalt complex, 11f, does not suffer facile elimination of CO, it does participate in an intermolecular PKR with norbornadiene. Likewise, attempts to extend the reach of the alkynyl moiety through incorporation of an additional methylene group yield only an intermolecular PKR product. It is suggested that the pentacarbonyl complexes, 12a−e, do not continue along the PKR pathway because of the unfavorable relative orientation of the cobalt-coordinated alkyne and alkene.

Development of Molecular Electrocatalysts for CO2 Reduction and H2 Production/Oxidation

The conversion of solar energy to fuels in both natural and artificial photosynthesis requires components for both light-harvesting and catalysis. The light-harvesting component generates the electrochemical potentials required to drive fuel-generating reactions that would otherwise be thermodynamically uphill. This Account focuses on work from our laboratories on developing molecular electrocatalysts for CO(2) reduction and for hydrogen production. A true analog of natural photosynthesis will require the ability to capture CO(2) from the atmosphere and reduce it to a useful fuel. Work in our laboratories has focused on both aspects of this problem. Organic compounds such as quinones and inorganic metal complexes can serve as redox-active CO(2) carriers for concentrating CO(2). We have developed catalysts for CO(2) reduction to form CO based on a [Pd(triphosphine)(solvent)](2+) platform. Catalytic activity requires the presence of a weakly coordinating solvent molecule that can dissociate during the catalytic cycle and provide a vacant coordination site for binding water and assisting C-O bond cleavage. Structures of [NiFe] CO dehydrogenase enzymes and the results of studies on complexes containing two [Pd(triphosphine)(solvent)](2+) units suggest that participation of a second metal in CO(2) binding may also be required for achieving very active catalysts. We also describe molecular electrocatalysts for H(2) production and oxidation based on [Ni(diphosphine)(2)](2+) complexes. Similar to palladium CO(2) reduction catalysts, these species require the optimization of both first and second coordination spheres. In this case, we use structural features of the first coordination sphere to optimize the hydride acceptor ability of nickel needed to achieve heterolytic cleavage of H(2). We use the second coordination sphere to incorporate pendant bases that assist in a number of important functions including H(2) binding, H(2) cleavage, and the transfer of protons between nickel and solution. These pendant bases, or proton relays, are likely to be important in the design of catalysts for a wide range of fuel production and fuel utilization reactions involving multiple electron and proton transfer steps. The generation of fuels from abundant substrates such as CO(2) and water remains a daunting research challenge, requiring significant advances in new inexpensive materials for light harvesting and the development of fast, stable, and efficient electrocatalysts. Although we describe progress in the development of redox-active carriers capable of concentrating CO(2) and molecular electrocatalysts for CO(2) reduction, hydrogen production, and hydrogen oxidation, much more remains to be done.

The role of amide ligands in the stabilization of Pd(II) tricoordinated complexes: is the Pd–NR2 bond order single or higher?

The existence of tricoordinated Pd(II) complexes has been a matter of controversy for a long time. The recent X-ray characterization of a family of Pd complexes [PdArXL] allowed to certify the existence of true tricoordinated Pd(II) species. The unique role played by the amido ligand (X = NR2), among a family of X ligands, was noticed in a previous computational work. Here, the influence of the R substituents at the amide and the nature of the Pd–Namido bond are theoretically analyzed. The relative stability of d8 tricoordinated [PdLAr(NR2)] complexes versus d8 tetracoordinated derivatives as a function of the R substituents is studied by analyzing the two most common ways to fill the vacant coordination site in a tricoordinated complex: solvent coordination (with tetrahydrofuran as solvent), or dimerization giving [(μ-NR2)2Pd2L2Ar2]) complexes. The nature of the Pd–N bonding interaction is analyzed using several theoretical schemes as molecular orbitals, QTAIM, ELF and NBO. Each of these schemes suggests that the order of the Pd–N bond in this family of complexes is higher than one. An asymmetric π interaction between the nitrogen lone pair and the LUMO over the tricoordinated Pd center is proposed as an important source of additional stabilization of tricoordinated species provided by amido ligands.

High Capacity Hydrogen Adsorption in Cu(II) Tetracarboxylate Framework Materials: The Role of Pore Size, Ligand Functionalization, and Exposed Metal Sites

A series of isostructural metal-organic framework polymers of composition [Cu2(L)(H2O)2] (L= tetracarboxylate ligands), denoted NOTT-nnn, has been synthesized and characterized. Single crystal X-ray structures confirm the complexes to contain binuclear Cu(II) paddlewheel nodes each bridged by four carboxylate centers to give a NbO-type network of 64.82 topology. These complexes are activated by solvent exchange with acetone coupled to heating cycles under vacuum to afford the desolvated porous materials NOTT-100 to NOTT-109. These incorporate a vacant coordination site at each Cu(II) center and have large pore volumes that contribute to the observed high H2 adsorption. Indeed, NOTT-103 at 77 K and 60 bar shows a very high total H2 adsorption of 77.8 mg g(-)- equivalent to 7.78 wt% [wt% = (weight of adsorbed H2)/(weight of host material)] or 7.22 wt% [wt% = 100(weight of adsorbed H2)/(weight of host material + weight of adsorbed H2)]. Neutron powder diffraction studies on NOTT-101 reveal three adsorption sites for this material: at the exposed Cu(II) coordination site, at the pocket formed by three {Cu2} paddle wheels, and at the cusp of three phenyl rings. Systematic virial analysis of the H2 isotherms suggests that the H2 binding energies at these sites are very similar and the differences are smaller than 1.0 kJ mol-1, although the adsorption enthalpies for H2 at the exposed Cu(II) site are significantly affected by pore metrics. Introducing methyl groups or using kinked ligands to create smaller pores can enhance the isosteric heat of adsorption and improve H2 adsorption. However, although increasing the overlap of potential energy fields of pore walls increases the heat of H2 adsorption at low pressure, it may be detrimental to the overall adsorption capacity by reducing the pore volume.