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  1. Anionic dopants, such as O-atom vacancies, alter the thermochemical and kinetic parameters of proton coupled electron transfer (PCET) at metal oxide surfaces; understanding their impact(s) is essential for informed material design for efficient energy conversion processes. To circumvent challenges associated with studying extended solids, we employ polyoxovanadate–alkoxide clusters as atomically precise models of reducible metal oxide surfaces. In this work, we examine net hydrogen atom (H-atom) uptake to an oxygen deficient vanadium oxide assembly, [V 6 O 6 (MeCN)(OCH 3 ) 12 ] 0 . Addition of two H-atom equivalents to [V 6 O 6 (MeCN)(OCH 3 ) 12 ] 0 results in formation of [V 6 O 5 (MeCN)(OH 2 )(OCH 3 ) 12 ] 0 . Assessment of the bond dissociation free energy of the O–H bonds of the resultant aquo moiety reveals that the presence of an O-atom defect weakens the O–H bond strength. Despite a decreased thermodynamic driving force for the reduction of [V 6 O 6 (MeCN)(OCH 3 ) 12 ] 0 , kinetic investigations show the rate of H-atom uptake at the cluster surface is ∼100× faster than its oxidized congener, [V 6 O 7 (OCH 3 ) 12 ] 0 . Electron densitymore »derived from the O-atom vacancy is shown to play an important role in influencing H-atom uptake at the cluster surface, lowering activation barriers for H-atom transfer.« less
    Free, publicly-accessible full text available May 2, 2024
  2. Free, publicly-accessible full text available November 8, 2023
  3. Hydrogen-atom (H-atom) transfer at the surface of heterogeneous metal oxides has received significant attention owing to its relevance in energy conversion and storage processes. Here, we present the synthesis and characterization of an organofunctionalized polyoxovanadate cluster, (calix)V6O5(OH2)(OMe) 8 (calix = 4- tert -butylcalix[4]arene). Through a series of equilibrium studies, we establish the BDFE(O–H) avg of the aquo ligand as 62.4 ± 0.2 kcal mol −1 , indicating substantial bond weaking of water upon coordination to the cluster surface. Subsequent kinetic isotope effect studies and Eyring analysis indicate the mechanism by which the hydrogenation of organic substrates occurs proceeds through a concerted proton–electron transfer from the aquo ligand. Atomistic resolution of surface reactivity presents a novel route of hydrogenation reactivity from metal oxide surfaces through H-atom transfer from surface-bound water molecules.
    Free, publicly-accessible full text available November 9, 2023
  4. Bipyridyl ligands are commonplace in catalysis. Structurally similar to this ligand class with unique properties is the novel di-(2-pyridyl)methanesulfonate (dpms) ligand, which is prepared and reacted with [Cp*IrCl2]2 to afford Cp*Ir(dpms)Cl (1) in high yield. Its single-crystal X-ray structure indicates an exo–(kappa2) conformation of the ligand, with the sulfonate group directed away from the iridium center. Halogen exchange by treatment of 1 with NaI gives the iodide derivative, Cp*Ir(dpms)I (2). Abstraction of the halogen from 1 using AgPF6 generates [Cp*Ir(dpms)]PF6 (3), which was not found to activate the C-H bonds of benzene.
    Free, publicly-accessible full text available September 20, 2023
  5. The selective uptake of lithium ions is of great interest for chemists and engineers because of the numerous uses of this element for energy storage and other applications. However, increasing demand requires improved strategies for the extraction of this element from mixtures containing high concentrations of alkaline impurities. Here, we study solution phase interactions of lithium, sodium, and potassium cations with polyoxovanadate-alkoxide clusters, [V 6 O 7 (OR) 12 ] (R = CH 3 , C 3 H 7 , C 5 H 11 ), using square wave voltammetry and cyclic voltammetry. In all cases, the most reducing event of the cluster shifts anodically as the ionic radius of the cation decreases, indicating increased stability of the reduced cluster and further suggesting that these assemblies might be useful for the selective uptake of Li + . Exploring the consequence of ligand length, we found that the short-chain cluster, [V 6 O 7 (OCH 3 ) 12 ], irreversibly binds Li + in the presence of excess potassium (K + ) and exhibits an electrochemical response in titration experiments similar to that observed upon the addition of Li + to the POV–alkoxide in the presence of non-coordinating tetrabutylammonium ions. However, inmore »the presence of excess sodium (Na + ), the cluster showed only a modest preference for lithium, with exchange between sodium and lithium ions governed by equilibrium. Extending these studies to [V 6 O 7 (OC 5 H 11 ) 12 ], we found that the presence of the pentyl ligands allows the assembly to irreversibly bind Li + in the presence of Na + or K + . The change in mechanism caused by surface functionalization of the clusters increases the differential binding affinity for more compact cations, translating to improved selectivity for Li + uptake in these molecular assemblies.« less
  6. We report accelerated rates of oxygen-atom transfer from a polyoxovanadate–alkoxide cluster following functionalization with a 4- tert butylcalix[4]arene ligand. Incorporation of this electron withdrawing ligand modifies the electronics of the metal oxide core, favoring a mechanism in which the rate of oxygen-atom transfer is limited by outer-sphere electron transfer.
  7. We report the synthesis and characterisation of a series of siloxide-functionalised polyoxovanadate–alkoxide (POV–alkoxide) clusters, [V 6 O 6 (OSiMe 3 )(OMe) 12 ] n ( n = 1−, 2−), that serve as molecular models for proton and hydrogen-atom uptake in vanadium dioxide, respectively. Installation of a siloxide moiety on the surface of the Lindqvist core was accomplished via addition of trimethylsilyl trifluoromethylsulfonate to the fully-oxygenated cluster [V 6 O 7 (OMe) 12 ] 2− . Characterisation of [V 6 O 6 (OSiMe 3 )(OMe) 12 ] 1− by X-ray photoelectron spectroscopy reveals that the incorporation of the siloxide group does not result in charge separation within the hexavanadate assembly, an observation that contrasts directly with the behavior of clusters bearing substitutional dopants. The reduced assembly, [V 6 O 6 (OSiMe 3 )(OMe) 12 ] 2− , provides an isoelectronic model for H-doped VO 2 , with a vanadium( iii ) ion embedded within the cluster core. Notably, structural analysis of [V 6 O 6 (OSiMe 3 )(OMe) 12 ] 2− reveals bond perturbations at the siloxide-functionalised vanadium centre that resemble those invoked upon H-atom uptake in VO 2 through ab initio calculations. Our results offer atomically precise insight into themore »local structural and electronic consequences of the installation of hydrogen-atom-like dopants in VO 2 , and challenge current perspectives of the operative mechanism of electron–proton co-doping in these materials.« less
  8. We report a rare example of the direct alkylation of the surface of a plenary polyoxometalate cluster by leveraging the increased nucleophilicity of vanadium oxide assemblies. Addition of methyl trifluoromethylsulfonate (MeOTf) to the parent polyoxovanadate cluster, [V 6 O 13 (TRIOL R ) 2 ] 2− (TRIOL = tris(hydroxymethyl)methane; R = Me, NO 2 ) results in functionalisation of one or two bridging oxide ligands of the cluster core to generate [V 6 O 12 (OMe)(TRIOL R ) 2 ] 1− and [V 6 O 11 (OMe) 2 (TRIOL R ) 2 ] 2− , respectively. Comparison of the electronic absorption spectra of the functionalised and unfunctionalised derivatives indicates the decreased overall charge of the complex results in a decrease in the energy required for ligand to metal charge transfer events to occur, while simultaneously mitigating the inductive effects imposed by the capping TRIOL ligand. Electrochemical analysis of the family of organofunctionalised polyoxovanadate clusters reveals the relationship of ligand environment and the redox properties of the cluster core: increased organofunctionalisation of the surface of the vanadium oxide assembly translates to anodic shifts in the reduction events of the Lindqvist ion. Overall, this work provides insight into the electronic effects inducedmore »upon atomically precise modifications to the surface structure of nanoscopic, redox-active metal oxide assemblies.« less