Multinuclear Clusters of Manganese and Lithium with Silsesquioxane-Derived Ligands: Synthesis and Ligand Rearrangement by Dioxygen- and Base-Mediated Si-O Bond Cleavage

The synthesis of manganese cluster complexes templated by polyhedral oligomeric silsesquioxane-derived ligands is described. Mn^II₃(Ph₇Si₇O₁₂)₂Pyr₄ (1) and Mn^II₄(Ph₄Si₄O₈)₂(Bpy)₂(Py)₂ (3) are prepared by replacement of the amide ligands of Mn(NR₂)₂ (R = SiMe₃) via ligand protolysis by the acidic proton of the respective silsesquioxane-derived silanols. Complex 1 is shown to undergo ligand rearrangement by reaction with O₂, which results in oxidation of the cluster to a mixed Mn^II/III cluster, concomitant with cleavage of the Si-O bonds of the ligand, releasing a [Ph₂Si-O]⁺ unit, opening a new ligating siloxide group, and resulting in the formation of Mn₃(Ph₆Si₆O₁₁)₂Pyr₄ (2). The ligand framework of 1 can also be perturbed by a base. The addition of LiOH/BuLi delivers a soluble equivalent of Li₂O to 1, resulting in cleavage of the Si-O bonds and linkage of the resulting exposed silicon atoms by the new oxide, giving a linked ligand variant that templates a Li₂Mn₃ cluster, Mn₃Li₂(Ph₇Si₇O₁₂OPh₇Si₇O₁₂)DMF₅Pyr (4). These systems are characterized by single-crystal X-ray diffraction, absorption spectroscopy, Fourier transform infrared, cyclic voltammetry, and CHN combustion analysis. Mechanistic implications for the Si-O bond cleavage events are discussed.

Reversible PCET and Ambient Catalytic Oxidative Alcohol Dehydrogenation by {V=O} Perfluoropinacolate Complexes

A new air-stable catalyst for the oxidative dehydrogenation of benzylic alcohols under ambient conditions has been developed. The synthesis and characterization of this compound and the related monomeric and dimeric V(IV)- and V(V)-pin^F (pin^F = perfluoropinacolate) complexes are reported herein. Monomeric V(IV) complex (Me₄N)₂[V(O)(pin^F)₂] (1) and dimeric (μ-O)₂-bridged V(V) complex (Me₄N)₂[V₂(O)₂(μ-O)₂(pin^F)₂] (3a) are prepared in water under ambient conditions. Monomeric V(V) complex (Me₄N)[V(O)(pin^F)₂] (2) may be generated via chemical oxidation of 1 under an inert atmosphere, but dimerizes to 3a upon exposure to air. Complexes 1 and 2 display a perfectly reversible V^IV/V couple at 20 mV (vs Ag/AgNO₃), whereas a quasi-reversible V^IV/V couple at -865 mV is found for 3a. Stoichiometric reactions of 3a with both fluorenol and TEMPOH result in the formation of (Me₄N)₂[V₂(O)₂(μ-OH)₂(pin^F)₂] (4a), which contains two V(IV) centers that display antiferromagnetic coupling. In order to structurally characterize the dinuclear anion of 4a, {K(18C6)}⁺ countercations were employed, which formed stabilizing K···O interactions between the counterion and each terminal oxo moiety and H-bonding between the oxygen atoms of the crown ether and μ-OH bridges of the dimer, resulting in {K(18C6)}₂[V₂(O)₂(μ-OH)₂(pin^F)₂] (4b). The formal storage of H₂ in 4a is reversible and proton-coupled electron transfer (PCET) from crystals of 4a regenerates 3a upon exposure to air over the course of several days. Furthermore, the reaction of 3a (2%) under ambient conditions with excess fluorenol, cinnamyl alcohol, or benzyl alcohol resulted in the selective formation of fluorenone (82% conversion), cinnamaldehyde (40%), or benzaldehyde (7%), respectively, reproducing oxidative alcohol dehydrogenation (OAD) chemistry known for VO_x surfaces and demonstrating, in air, the thermodynamically challenging selective oxidation of alcohols to aldehydes/ketones.

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.

A study of DNA/BSA interaction and catalytic potential of oxidovanadium(v) complexes with ONO donor ligands

The study of DNA/BSA interaction and the catalytic potential of five mono- and dinuclear oxidoethoxido vanadium(v) complexes.

Mechanism of alcohol oxidation by dipicolinate vanadium(V): unexpected role of pyridine

Dipicolinate vanadium(V) alkoxide complexes (dipic)V(V)(O)(OR) (OR = isopropoxide (1), n-butanoxide (2), cyclobutanoxide (3), and α-tert-butylbenzylalkoxide (4)) react with pyridine to afford vanadium(IV) and 0.5 equiv of an aldehyde or ketone product. The role of pyridine in the reaction has been investigated. Both NMR and X-ray crystallography experiments indicate that pyridine coordinates to 1, which is in equilibrium with (dipic)V(V)(O)(O(i)Pr)(pyr) (1-Pyr). Kinetic studies of the alcohol oxidation suggest a pathway where the rate-limiting step is bimolecular and involves attack of pyridine on the C-H bond of the isopropoxide ligand of 1 or 1-Pyr. The oxidations of mechanistic probes cyclobutanol and α-tert-butylbenzylalcohol support a two-electron pathway proceeding through a vanadium(III) intermediate. The alcohol oxidation reaction is promoted by more basic pyridines and facilitated by electron-withdrawing substituents on the dipicolinate ligand. The involvement of base in the elementary alcohol oxidation step observed for the dipicolinate system is an unprecedented mechanism for vanadium-mediated alcohol oxidation and suggests new ways to tune reactivity and selectivity of vanadium catalysts.