Inspired by Nature-Functional Analogues of Molybdenum and Tungsten-Dependent Oxidoreductases

Throughout the previous ten years many scientists took inspiration from natural molybdenum and tungsten-dependent oxidoreductases to build functional active site analogues. These studies not only led to an ever more detailed mechanistic understanding of the biological template, but also paved the way to atypical selectivity and activity, such as catalytic hydrogen evolution. This review is aimed at representing the last decade’s progress in the research of and with molybdenum and tungsten functional model compounds. The portrayed systems, organized according to their ability to facilitate typical and artificial enzyme reactions, comprise complexes with non-innocent dithiolene ligands, resembling molybdopterin, as well as entirely non-natural nitrogen, oxygen, and/or sulfur bearing chelating donor ligands. All model compounds receive individual attention, highlighting the specific novelty that each provides for our understanding of the enzymatic mechanisms, such as oxygen atom transfer and proton-coupled electron transfer, or that each presents for exploiting new and useful catalytic capability. Overall, a shift in the application of these model compounds towards uncommon reactions is noted, the latter are comprehensively discussed.

Synthesis, Characterization, and Catalytic Exploration of Mononuclear Mo(VI) Dioxido Complexes of (Z)-1-R-2-(4',4'-Dimethyl-2'-oxazolin-2'-yl)-eth-1-en-1-ates

The coordination chemistry of the title ligands with Mo metal centers was investigated. Thus, the synthesis and characterization (NMR, X-ray diffraction) of four mononuclear formally Mo(6+) complexes of (Z)-1-R-2-(4′,4′-dimethyl-2′-oxazolin-2′-yl)-eth-1-en-1-ates (L: R = –Ph, –Ph-p-NO₂, –Ph-p-OMe and –t-Bu), derived from the part enols (LH), is described. The resulting air-stable MoO₂L₂ complexes (1–4) exist, as shown by single-crystal X-ray diffraction experiments, in the cis-dioxido-trans(N)-κ²-N,O-L conformation in the solid state for all four examples. This situation was further probed using semi-empirical PM6(tm) calculations. Complexes 1–4 represent the first Mo complexes of this ligand class and, indeed, of Group 6 metals in general. Structural and spectroscopic comparisons were made between these and related Mo(6+) compounds. Complex 1 (R = –Ph) was studied for its ability to selectively catalyze the production of poly-norbornene from the monomer in the presence of MAO. This, unfortunately, only resulted in the synthesis of insoluble, presumably highly cross-linked, polymeric and/or oligomeric materials. However, complexes 1–4 were demonstrated to be highly effective for catalyzing benzoin to benzil conversion using DMSO as the O-transfer agent. This catalysis work is likewise put into perspective with respect to analogous Mo(6+) complexes.

Accessing Molecular Dimeric Ir Water Oxidation Catalysts from Coordination Precursors

One ongoing challenge in the field of iridium-based water oxidation catalysts is to develop a molecular precatalyst affording well-defined homogeneous active species for catalysis. Our previous work by using organometallic precatalysts Cp*Ir(pyalk)OH and Ir(pyalk)(CO)₂ (pyalk = (2-pyridyl)-2-propanolate) suggested a μ-oxo-bridged Ir dimer as the probable resting state, although the structure of the active species remained elusive. During the activation, the ligands Cp* and CO were found to oxidatively degrade into acetic acid or other products, which coordinate to Ir centers and affect the catalytic reaction. Two related dimers bearing two pyalk ligands on each iridium were crystallized for structural analysis. However, preliminary results indicated that these crystallographically characterized dimers are not active catalysts. In this work, we accessed a mixture of dinuclear iridium species from a coordination precursor, Na[Ir(pyalk)Cl₄], and assayed their catalytic activity for oxygen evolution by using NaIO₄ as the oxidant. This catalyst showed comparable oxygen-evolution activity to the ones previously reported from organometallic precursors without demanding oxidative activation to remove sacrificial ligands. Future research along this direction is expected to provide insights and design principles toward a well-defined active species.

Electrochemical properties and C-H bond oxidation activity of [Ru(tpy)(pyalk)Cl]+ and [Ru(tpy)(pyalk)(OH)]. Dalton Trans 2018;47:9701-9708. [PMID: 29978176 DOI: 10.1039/c8dt02260g]

[Ru(tpy)(pyalk)Cl]Cl (pyalk = 2-(2'-pyridyl)-2-propanol) was synthesized and characterized crystallographically and electrochemically. Upon dissolution in water and acetonitrile, [Ru(tpy)(pyalk)Cl]Cl was found to form [Ru(tpy)(pyalk)Cl]+ and [Ru(tpy)(pyalk)(OH)]+, respectively. The Ru(ii/iii) couple of [Ru(tpy)(pyalk)Cl]+ was found to be relatively low compared to that of other Ru complexes in acetonitrile, but the Ru(iii/iv) couple was not significantly different than other Ru complexes bearing anionic ligands. Pourbaix diagrams were generated for [Ru(tpy)(phpy)(OH2)]+ (phpy = 2-phenylpyridine) and [Ru(tpy)(pyalk)(OH)]+ in water, and it was found that [Ru(tpy)(pyalk)(OH)]+ has a lower Ru(ii/iii) potential than [Ru(tpy)(phpy)(OH2)]+ under neutral to alkaline pH. [Ru(tpy)(pyalk)(OH)]+ was found to catalyze C-H bond hydroxylation of secondary alkanes and epoxidation of alkenes using cerium(iv) ammonium nitrate as the primary oxidant.

Oxygen atom transfer between DMSO and benzoin catalyzed by cis-dioxidomolybdenum(vi) complexes of tetradentate Mannich bases

Dioxidomolybdenum(vi) complexes of tetradentate ONNO donor Mannich base ligands for the catalytic oxygen atom transfer between benzoin and DMSO are reported.

A Pyridine Alkoxide Chelate Ligand That Promotes Both Unusually High Oxidation States and Water-Oxidation Catalysis

Water-oxidation catalysis is a critical bottleneck in the direct generation of solar fuels by artificial photosynthesis. Catalytic oxidation of difficult substrates such as water requires harsh conditions, so the ligand must be designed both to stabilize high oxidation states of the metal center and to strenuously resist ligand degradation. Typical ligand choices either lack sufficient electron donor power or fail to stand up to the oxidizing conditions. Our research on Ir-based water-oxidation catalysts (WOCs) has led us to identify a ligand, 2-(2'-pyridyl)-2-propanoate or "pyalk", that fulfills these requirements. Work with a family of Cp*Ir(chelate)Cl complexes had indicated that the pyalk-containing precursor gave the most robust WOC, which was still molecular in nature but lost the Cp* fragment by oxidative degradation. In trying to characterize the resulting active "blue solution" WOC, we were able to identify a diiridium(IV)-mono-μ-oxo core but were stymied by the extensive geometrical isomerism and coordinative variability. By moving to a family of monomeric complexes [Ir^III/IV(pyalk)₃] and [Ir^III/IV(pyalk)₂Cl₂], we were able to better understand the original WOC and identify the special properties of the ligand. In this Account, we cover some results using the pyalk ligand and indicate the main features that make it particularly suitable as a ligand for oxidation catalysis. The alkoxide group of pyalk allows for proton-coupled electron transfer (PCET) and its strong σ- and π-donor power strongly favors attainment of exceptionally high oxidation states. The aromatic pyridine ring with its methyl-protected benzylic position provides strong binding and degradation resistance during catalytic turnover. Furthermore, the ligand has two additional benefits: broad solubility in aqueous and nonaqueous solvents and an anisotropic ligand field that enhances the geometry-dependent redox properties of its complexes. After discussion of the general properties, we highlight the specific complexes studied in more detail. In the iridium work, the isolated mononuclear complexes showed easily accessible Ir(III/IV) redox couples, in some cases with the Ir(IV) state being indefinitely stable in water. We were able to rationalize the unusual geometry-dependent redox properties of the various isomers on the basis of ligand-field effects. Even more striking was the isolation and full characterization of a stable Rh(IV) state, for which prior examples were very reactive and poorly characterized. Importantly, we were able to convert monomeric Ir complexes to [Cl(pyalk)₂Ir^IV-O-Ir^IVCl(pyalk)₂] derivatives that help model the "blue solution" properties and provide groundwork for rational synthesis of active, well-defined WOCs. More recent work has moved toward the study of first-row transition metal complexes. Manganese-based studies have highlighted the importance of the chelate effect for labile metals, leading to the synthesis of pincer-type pyalk derivatives. Beyond water oxidation, we believe the pyalk ligand and its derivatives will also prove useful in other oxidative transformations.

Synthesis of pyridine-alkoxide ligands for formation of polynuclear complexes

A series of novel polydentate N,O-donor ligands strongly favour formation of polynuclear complexes.

New Ir Bis-Carbonyl Precursor for Water Oxidation Catalysis

This paper introduces Ir(I)(CO)2(pyalc) (pyalc = (2-pyridyl)-2-propanoate) as an atom-efficient precursor for Ir-based homogeneous oxidation catalysis. This compound was chosen to simplify analysis of the water oxidation catalyst species formed by the previously reported Cp*Ir(III)(pyalc)OH water oxidation precatalyst. Here, we present a comparative study on the chemical and catalytic properties of these two precursors. Previous studies show that oxidative activation of Cp*Ir-based precursors with NaIO4 results in formation of a blue Ir(IV) species. This activation is concomitant with the loss of the placeholder Cp* ligand which oxidatively degrades to form acetic acid, iodate, and other obligatory byproducts. The activation process requires substantial amounts of primary oxidant, and the degradation products complicate analysis of the resulting Ir(IV) species. The species formed from oxidation of the Ir(CO)2(pyalc) precursor, on the other hand, lacks these degradation products (the CO ligands are easily lost upon oxidation) which allows for more detailed examination of the resulting Ir(pyalc) active species both catalytically and spectroscopically, although complete structural analysis is still elusive. Once Ir(CO)2(pyalc) is activated, the system requires acetic acid or acetate to prevent the formation of nanoparticles. Investigation of the activated bis-carbonyl complex also suggests several Ir(pyalc) isomers may exist in solution. By (1)H NMR, activated Ir(CO)2(pyalc) has fewer isomers than activated Cp*Ir complexes, allowing for advanced characterization. Future research in this direction is expected to contribute to a better structural understanding of the active species. A diol crystallization agent was needed for the structure determination of 3.

Synthesis of a highly reactive form of WO2Cl2, its conversion into nanocrystalline mono-hydrated WO3 and coordination compounds with tetramethylurea

A new form of WO₂Cl₂ was prepared from WCl₆. Conversion of WO₂Cl₂ into nanocrystalline WO₃·H₂O occurred upon air exposure at room temperature. The first coordination complexes of WO₂Cl₂ with an alkylurea are reported.

Intramolecular chalcogen-tin interactions in [(o-MeEC6H4)CH2]2SnPh(2-n)Cl(n) (E = S, O, CH2; n = 0, 1, 2) and intermolecular chlorine-tin interactions in the meta- and para-methoxy isomers

Organotin(IV) compounds of the type [(o-MeEC(6)H(4))CH(2)](2)SnPh(2-n)Cl(n) were synthesized, E = O, n = 0 (1), n = 1 (2), and n = 2 (3); E = S, n = 0 (4), n = 1 (5), and n = 2 (6); and E = CH(2), n = 0 (7), n = 1 (8), and n = 2 (9). The dichloro compounds 3 and 6 have been investigated by single-crystal X-ray diffraction and exhibit bicapped tetrahedral geometry at the tin atom as a consequence of significant intramolecular Sn...O (3) and Sn...S (6) secondary bonding, in monomolecular units. Compound 3, when crystallized from a hexane/THF solvent mixture, shows two different conformers, 3' and 3'', in the crystal structure; 3' has two equivalent Sn...O interactions, while 3'' has two nonequivalent Sn...O interactions. Upon the recrystallization of 3 from hexane, only a single structural form is observed, 3'. The Sn...E distances in 3', 3'', and 6 are 71.3, 73.5 and 72.9, and 76.3% of the SigmavdW radii, respectively. The meta- and para-substituted isomers of 3 (10, 11) exhibit a distortion at the tin atom due to self-association via intermolecular Sn...Cl interactions, resulting in polymeric structures. (119)Sn NMR spectroscopy suggests that the intramolecular Sn...E interactions persist in solution for the dichloride compounds 3 and 6.

Mn4, Mn6, and Mn11 clusters from the use of bulky diphenyl(pyridine-2-yl)methanol

The synthesis, crystal structure, and magnetochemical characterization are reported of three new Mn clusters [Mn(4)O(2)(O(2)CBu(t))(5)(dphmp)(3)] (1), [Mn(6)O(4)(OMe)(2)(O(2)CPh)(4)(dphmp)(4)] (2), and [Mn(11)O(7)(OMe)(7)(O(2)CPh)(7)(dphmp)(4)(MeOH)(2)] (3). They were obtained from the use of diphenyl(pyridine-2-yl)methanol (dphmpH), a bulkier version of the 2-(hydroxymethyl)pyridine (hmpH) reagent commonly employed previously in Mn chemistry. The reaction of dphmpH with MnCl(2) x 4 H(2)O and NaO(2)CBu(t) in MeCN/MeOH (30 mL, 5:1 v/v) led to the isolation of tetranuclear complex 1, whereas the analogous reaction with NaO(2)CPh gave hexanuclear complex 2. When the 5:1 solvent ratio in the latter reaction was changed to 1:29, the isolated product was now undecanuclear complex 3. Complexes 1-3 all possess rare or unprecedented Mn(x) topologies: Complex 1 possesses a [Mn(4)(mu(3)-O)(2)](8+) (4 Mn(III)) butterfly core, one edge of which is additionally bridged by an alkoxide arm of a dphmp(-) chelate; complex 2 possesses a [Mn(6)(mu(4)-O)(2)(mu(3)-O)(2)(mu(3)-OMe)(2)](8+) (6 Mn(III)) core with a face-sharing double cubane topology; and complex 3 (Mn(II), 10 Mn(III)) possesses a [Mn(4)(mu(4)-O)(3)(mu(3)-OMe)](5+) cubane unit, attached on one side to a Mn(II) atom by a mu(4)-O atom and alkoxide groups, and on the other side to a [Mn(5)(mu(4)-O)(mu(3)-O)(3)(mu(3)-OMe)(mu-OR)(3)](3+) unit consisting of three face-sharing defective cubanes linked to an additional Mn(III) atom by a mu(3)-O atom. Solid-state dc and ac magnetic susceptibility measurements on 1-3 establish that they possess S = 0, 3, and 5/2 ground states, respectively. ac susceptibility studies on 2 and 3 reveal weak non-zero frequency-dependent out-of-phase (chi(M)'') signals at temperatures below 3 K, possibly indicative of single-molecule magnets with very small barriers. The combined results demonstrate a ligating difference between bulky dphmp(-) and hmp(-), and the resulting usefulness of the former to provide access to a variety of Mn(x) molecular species not known with the latter.

Dioxomolybdenum(VI) and dioxotungsten(VI) complexes supported by an amido ligand

Stepwise addition of one equivalent of n-butyllithium and trimethylsilyl chloride to 2-tert-butylmercaptoaniline affords the new ligand 1-(Me3SiNH)-2-(t-BuS)C6H4 (LH), that reacts with one equivalent of butyllithium to its lithium salt LLi. Dioxodichloromolybdenum [MoO2Cl2] and dioxodichlorotungsten dimethoxyethane [WO2Cl2(dme)] react in tetrahydrofuran solution at low temperature with two equivalents LLi to monomeric dioxomolybdenum(VI) [MoO2L2] (1) and dioxotungsten(VI) complex [WO2L2] (2) employing two bidentate amido thioether ligands. The crystallographic determination of the molecular structures of 1 and 2 show evidence for M...S contacts. The reaction of [MoO2Cl2] with LLi in tetrahydrofuran solution at room temperature leads next to 1 to two compounds where silyl group migration from nitrogen to oxygen atoms occurs forming [Mo(=NL')2(OSiMe)2] (3) and [Mo(=NL')2(OSiMe3)L] (4, L' = N-2-t-BuSC6H4) as determined by NMR spectroscopy. Compound 4 was isolated in low yield and its molecular structure determined by X-ray crystallography. Higher yields of a bisimido complex can be obtained by the direct reaction of one equivalent of LLi with [Mo(NAr)2Cl2(dme)] (Ar = 2,6-Me2C6H4) forming [Mo(NAr)2LCl] (5).

Synthesis and catalytic application of octahedral lewis base adducts of dichloro and dialkyl dioxotungsten(VI)

Complexes of the composition W(O)(2)(Cl)(2)L(2) and W(O)(2)(R)(2)L(2) (R = Me, Et; L(2) = bidentate Lewis base ligand) have been prepared and are fully characterized (including an exemplary X-ray crystal structure of W(O)(2)(Cl)(2)(4,4'-di-tert-butyl-2,2'-bipyridine)). This latter compound crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 8.3198(1) A, b = 13.3224(2) A, c = 18.0415(2) A, and Z = 4. The title complexes are applied as catalysts in olefin epoxidation catalysis with tert-butyl hydroperoxide (TBHP) as the oxidizing agent. The W(VI) complexes display only moderate turnover frequencies but can be reused several times without loss of catalytic activity. The highest activity can be achieved at reaction temperatures of ca. 90 degrees C. Chloro derivatives are somewhat more active than alkyl complexes, and sterically less crowded complexes show also higher activities than their congeners with bulky ligands L(2). Kinetic examinations show that the catalyst formation is the rate determining step and it is observed that tert-butyl alcohol, the byproduct of the epoxidation reaction, acts as a competitor for TBHP, thus lowering the reaction velocity during the course of the reaction but not irreversibly destroying the catalyst.