Catalytic Applications of Vanadium: A Mechanistic Perspective.

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C-C and C-O bond cleavage, carbon-carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

Nuclearity effects in supported, single-site Fe(ii) hydrogenation pre-catalysts.

Dimeric and monomeric supported single-site Fe(ii) pre-catalysts on SiO2 have been prepared via organometallic grafting and characterized with advanced spectroscopic techniques. Manipulation of the surface hydroxyl concentration on the support influences monomer/dimer formation. While both pre-catalysts are highly active in liquid-phase hydrogenation, the dimeric pre-catalyst is ∼3× faster than the monomer. Preliminary XAS experiments on the H2-activated samples suggest the active species are isolated Fe(ii) sites.

Publisher Correction: A molecular cross-linking approach for hybrid metal oxides.

In the version of this Article originally published, Liban M. A. Saleh was incorrectly listed as Liban A. M. Saleh due to a technical error. This has now been amended in all online versions of the Article.

A molecular cross-linking approach for hybrid metal oxides.

There is significant interest in the development of methods to create hybrid materials that transform capabilities, in particular for Earth-abundant metal oxides, such as TiO2, to give improved or new properties relevant to a broad spectrum of applications. Here we introduce an approach we refer to as 'molecular cross-linking', whereby a hybrid molecular boron oxide material is formed from polyhedral boron-cluster precursors of the type [B12(OH)12]2-. This new approach is enabled by the inherent robustness of the boron-cluster molecular building block, which is compatible with the harsh thermal and oxidizing conditions that are necessary for the synthesis of many metal oxides. In this work, using a battery of experimental techniques and materials simulation, we show how this material can be interfaced successfully with TiO2 and other metal oxides to give boron-rich hybrid materials with intriguing photophysical and electrochemical properties.

Synthesis, Structure, and Reactivity of the Sterically Crowded Th3+ Complex (C5Me5)3Th Including Formation of the Thorium Carbonyl, [(C5Me5)3Th(CO)][BPh4].

The Th3+ complex, (C5Me5)3Th, has been isolated despite the fact that tris(pentamethylcyclopentadienyl) complexes are highly reactive due to steric crowding and few crystallographically characterizable Th3+ complexes are known due to their highly reducing nature. Reaction of (C5Me5)2ThMe2 with [Et3NH][BPh4] produces the cationic thorium complex [(C5Me5)2ThMe][BPh4] that can be treated with KC5Me5 to generate (C5Me5)3ThMe, 1. The methyl group on (C5Me5)3ThMe can be removed with [Et3NH][BPh4] to form [(C5Me5)3Th][BPh4], 2, the first cationic tris(pentamethylcyclopentadienyl) metal complex, which can be reduced with KC8 to yield (C5Me5)3Th, 3. Complexes 1-3 have metrical parameters consistent with the extreme steric crowding that previously has given unusual (C5Me5)- reactivity to (C5Me5)3M complexes in reactions that form less crowded (C5Me5)2M-containing products. However, neither sterically induced reduction nor (η1-C5Me5)- reactivity is observed for these complexes. (C5Me5)3Th, which has a characteristic EPR spectrum consistent with a d1 ground state, has the capacity for two-electron reduction via Th3+ and sterically induced reduction. However, it reacts with MeI to make two sterically more crowded complexes, (C5Me5)3ThI, 4, and (C5Me5)3ThMe, 1, rather than (C5Me5)2Th(Me)I. Complex 3 also forms more crowded complexes in reactions with I2, PhCl, and Al2Me6, which generate (C5Me5)3ThI, (C5Me5)3ThCl, and (C5Me5)3ThMe, 1, respectively. The reaction of (C5Me5)3Th, 3, with H2 forms the known (C5Me5)3ThH as the sole thorium-containing product. Surprisingly, (C5Me5)3ThH is also observed when (C5Me5)3Th is combined with 1,3,5,7-cyclooctatetraene. [(C5Me5)3Th][BPh4] reacts with tetrahydrofuran (THF) to make [(C5Me5)3Th(THF)][BPh4], 2-THF, which is the first (C5Me5)3M of any kind that does not have a trigonal planar arrangement of the (C5Me5)- rings. It is also the first (C5Me5)3M complex that does not ring-open THF. [(C5Me5)3Th][BPh4], 2, reacts with CO to generate a product characterized as [(C5Me5)3Th(CO)][BPh4], 5, the first example of a molecular thorium carbonyl isolable at room temperature. These results have been analyzed using density functional theory calculations.

Expanding Thorium Hydride Chemistry Through Th²âº, Including the Synthesis of a Mixed-Valent Th4⁺/Th³âº Hydride Complex.

The reactivity of the recently discovered Th(2+) complex [K(18-crown-6)(THF)2][Cp″3Th], 1 [Cp'' = C5H3(SiMe3)2-1,3], with hydrogen reagents has been investigated and found to provide syntheses of new classes of thorium hydride compounds. Complex 1 reacts with [Et3NH][BPh4] to form the terminal Th(4+) hydride complex Cp″3ThH, 2, a reaction that formally involves a net two-electron reduction. Complex 1 also reacts in the solid state and in solution with H2 to form a mixed-valent bimetallic product, [K(18-crown-6)(Et2O)][Cp″2ThH2]2, 3, which was analyzed by X-ray crystallography, electron paramagnetic resonance and optical spectroscopy, and density functional theory. The existence of 3, which formally contains Th(3+) and Th(4+), suggested that KC8 could reduce [(C5Me5)2ThH2]2. In the presence of 18-crown-6, this reaction forms an analogous mixed-valent product formulated as [K(18-crown-6)(THF)][(C5Me5)2ThH2]2, 4. A similar complex with (C5Me4H)(1-) ligands was not obtained, but reaction of (C5Me4H)3Th with H2 in the presence of KC8 and 2.2.2-cryptand at -45 °C produced two monometallic hydride products, namely, (C5Me4H)3ThH, 5, and [K(2.2.2-cryptand)]{(C5Me4H)2[η(1):η(5)-C5Me3H(CH2)]ThH]}, 6. Complex 6 contains a metalated tetramethylcyclopentadienyl dianion, [C5Me3H(CH2)](2-), that binds in a tuck-in mode.

Synthesis, Structure, and Reactivity of the Ethyl Yttrium Metallocene, (C5Me5)2Y(CH2CH3), Including Activation of Methane.

(C5Me5)2Y(µ-Ph)2BPh2, 1, reacted with ethyllithium at -15 °C to make (C5Me5)2Y(CH2CH3), 2, which is thermally unstable at room temperature and formed the C-H bond activation product, (C5Me5)2Y(µ-H)(µ-η(1):η(5)-CH2C5Me4)Y(C5Me5), 3, containing a metalated (C5Me5)(1-) ligand. Spectroscopic evidence for 2 was obtained at low temperature, and trapping experiments with (i)PrNCN(i)Pr and CO2 gave the Y-CH2CH3 insertion products, (C5Me5)2Y[(i)PrNC(Et)N(i)Pr-κ(2)N,N'], 4, and [(C5Me5)2Y(µ-O2CEt)]2, 5. Although 2 is highly reactive, low temperature isolation methods allowed the isolation of single crystals which revealed an 82.6(2)° Y-CH2-CH3 bond angle consistent with an agostic structure in the solid state. Complex 2 reacted with benzene and toluene to make (C5Me5)2YPh, 7, and (C5Me5)2YCH2Ph, 8, respectively. The reaction of 2 with [(C5Me5)2YCl]2 formed (C5Me5)2Y(µ-Cl)(µ-η(1):η(5)-CH2C5Me4)Y(C5Me5) in which a (C5Me5)(1-) ligand was metalated. C-H bond activation also occurred with methane which reacted with 2 to make [(C5Me5)2YMe]2, 9.

Modification of the degree of branching of a beta-(1,3)-glucan affects aggregation behavior and activity in an oxidative burst assay.

Scleroglucan is a ß-(1,3)-glucan which is highly branched at the 6-position with a single glucose residue. Acid hydrolysis of a high molecular weight scleroglucan gave a medium molecular weight, freely soluble material. Linkage analysis by the partially methylated alditol acetate method showed that the solubilized material had 30% branching. When the material was subjected to partial Smith degradations, the percent branching was reduced accordingly to 12% or 17%. After the percent branching was reduced, the average molecular weight of the samples increased considerably, indicating the assembly of higher ordered aggregate structures. An aggregate number distribution analysis was applied to confirm the higher aggregated structures. These aggregated structures gave the material significantly enhanced activity in an in vitro oxidative burst assay compared to the highly branched material.

Synthesis, structure, and reactivity of crystalline molecular complexes of the {[C5H3(SiMe3)2]3Th}1- anion containing thorium in the formal +2 oxidation state.

Reduction of the Th3+ complex Cp''3Th, 1 [Cp'' = C5H3(SiMe3)2], with potassium graphite in THF in the presence of 2.2.2-cryptand generates [K(2.2.2-cryptand)][Cp''3Th], 2, a complex containing thorium in the formal +2 oxidation state. Reaction of 1 with KC8 in the presence of 18-crown-6 generates the analogous Th2+ compound, [K(18-crown-6)(THF)2][Cp''3Th], 3. Complexes 2 and 3 form dark green solutions in THF with ε = 23 000 M-1 cm-1, but crystallize as dichroic dark blue/red crystals. X-ray crystallography revealed that the anions in 2 and 3 have trigonal planar coordination geometries, with 2.521 and 2.525 ŠTh-(Cp'' ring centroid) distances, respectively, equivalent to the 2.520 Šdistance measured in 1. Density functional theory analysis of (Cp''3Th)1- is consistent with a 6d2 ground state, the first example of this transition metal electron configuration. Complex 3 reacts as a two-electron reductant with cyclooctatetraene to make Cp''2Th(C8H8), 4, and [K(18-crown-6)]Cp''.

Reactivity of organothorium complexes with TEMPO.

Reactions of the 2,2,6,6-tetramethylpiperidin-1-oxyl radical (TEMPO) with thorium metallocenes have been examined to investigate both the radical reaction patterns for organothorium complexes and the coordination chemistry of TEMPO with thorium. (η(5)-C5Me5)2ThMe2 reacts with 2 equiv of TEMPO to generate 1-methoxy-2,2,6,6-tetramethylpiperidine (Me-TEMPO) and (η(5)-C5Me5)2ThMe(η(1)-TEMPO), which contains a TEMPO(-) anion coordinated to thorium through oxygen only. (η(5)-C5Me5)2Th(η(1)-C3H5)(η(3)-C3H5), synthesized from (η(5)-C5Me5)2ThBr2 and (C3H5)MgBr, reacts with 2 equiv of TEMPO to form 1-(2-propen-1-yloxy)-2,2,6,6-tetramethylpiperidine (allyl-TEMPO) and (η(5)-C5Me5)2Th(η(1)-C3H5)(η(1)-TEMPO). Although bis(TEMPO) metallocenes were not obtained in these reactions, the methyl group in (η(5)-C5Me5)2ThMe(η(1)-TEMPO) is reactive with 1 equiv of CuBr to form (η(5)-C5Me5)2ThBr(η(1)-TEMPO). The bis(TEMPO) metallocene (η(5)-C5Me5)2Th(η(1)-TEMPO)2 is accessible in the reaction of [(η(5)-C5Me5)2ThH2]2 with 4 equiv of TEMPO. In contrast, (η(5)-C5Me5)2ThBr2 reacts with 2 equiv of TEMPO by loss of C5Me5 to form (C5Me5)2 and (η(2)-TEMPO)2ThBr2, in which the TEMPO(-) anions bind through oxygen and nitrogen. The bromide ions in (η(2)-TEMPO)2ThBr2 can be replaced by an additional 2 equiv of TEMPO in the presence of 2 equiv of KC8 to form the per(TEMPO) complex Th(η(1)-TEMPO)2(η(2)-TEMPO)2. ThBr4(THF)4 reacts with TEMPO to form ThBr4(THF)2(η(1)-TEMPO), which contains an oxygen-bound TEMPO radical. The Th(3+) complex (η(5)-C5Me4H)3Th is oxidized in the presence of TEMPO, without ligand loss, to afford the Th(4+) species (η(5)-C5Me4H)3Th(η(1)-TEMPO). The reactions show that TEMPO can react with organothorium complexes in several ways including coordination, anion substitution, and cyclopentadienyl replacement.

Enzymatic method to measure ß-1,3-ß-1,6-glucan content in extracts and formulated products (GEM assay).

An enzymatic method to measure ß-glucan content (GEM assay) is applicable in a variety of matrices. The method is composed of swelling the sample with KOH and initial digestion with a lyticase, which is followed by treatment with a mixture of exo-1,3-ß-d-glucanase and ß-glucosidase that converts the ß-glucan to glucose. The glucose generated by the enzymatic hydrolysis is measured by another enzymatic method. The method is shown to be accurate and precise. The method is selective and applicable to both highly branched and unbranched ß-1,3-glucans.