Development of sterically hindered siloxide-functionalized polyoxotungstates for the complexation of 5d-metals.

In this study, we extend the family of organosilyl-functionalized trivacant Keggin polyoxotungstates, [PW9O34(RSiOH)3]3- (R = nPr, iPr, tBu), through the introduction of bulky aryl and aliphatic silanol substituents, namely phenyl, cyclohexyl and biphenyl. This work was performed in order to study the impact of these large functional groups on the accessibility of the well-defined tridentate coordination site. Coordination of hafnium to these type II hybrid polyoxotungstates was conducted in order to study the ability of the bulkier ligand pockets to support larger cations in comparison to those previously reported (e.g. Ti4+, V3+, V5+, Ge4+). Increased steric hindrance around the coordination site from the biphenyl groups resulted in much longer reaction times for the complexation reaction compared to the other functional groups used, but the impact of our design toward stabilizing reactive species proved limited, as all complexes easily undergo hydrolysis of the Hf-OtBu bond in the presence of water. Electrochemical investigations of the ligands and hafnium complexes reveal that the redox events centered on the polyoxotungstate core can be tuned by varying the substituents on the silyl fragment, and exhibit a cathodic shift after coordination of the redox inactive tetravalent cation.

Organic Functionalization of Polyoxovanadate-Alkoxide Clusters: Improving the Solubility of Multimetallic Charge Carriers for Nonaqueous Redox Flow Batteries.

The success of nonaqueous redox flow battery technology requires synthetic advances in charge carrier design to increase compatibility with organic solvents. Herein, previous discoveries related to the development of multimetallic charge carriers are built upon with the high-yielding syntheses of ether- functionalized polyoxovanadate-alkoxide clusters, [V6 O7 (OR)9 (OCH2 )3 CR'] (R=CH3 , C2 H5 ; R'=CH3 , CH2 OCH3 , CH2 OC2 H4 OCH3 ). Like their homoleptic congeners [V6 O7 (OR)12 ] (R=CH3 , C2 H5 ), these clusters exhibit four redox events, spanning nearly a two-volt window, and demonstrate rapid electron-transfer kinetics. The ethoxide derivatives can reversibly cycle two electrons at each electrode in symmetric charging schematics, demonstrating long-term solution stability. Furthermore, ether functionalization yields a twelvefold increase in solubility, a factor which directly dictates the energy density of a redox flow battery.

Versatile Organic Chemistry on Vanadium-Based Multi-Electron Reservoirs.

We report the synthesis, post-functionalization, and redox behavior of two organically functionalized aggregates, [V6 O7 (OMe)9 {(OCH2 )3 C-CH2 N3 }] and [V6 O7 (OMe)9 {(OCH2 )3 C-NH2 }]. All twelve µ2 -oxo groups on the edges of the Lindqvist-type {V6 O19 } core were replaced by alkoxo ligands. The absence of a negative charge and the closed organic shell make these neutral mixed-valence compounds very stable towards hydrolysis and well soluble in almost all common organic solvents. These are important advantages over classical POMs. By post-functionalization through copper(I)-catalyzed Huisgen cycloaddition or imine formation, various organic moieties could be introduced. Even a well-soluble trimer composed of three hexanuclear vanadium units connected through an aromatic triimino core was synthesized and studied. The diverse redox behavior, the versatile reactivity, the good stability, and the excellent solubility make our vanadium compounds highly interesting for applications as building blocks in macromolecular chemistry as well as redox labels in biochemistry.

Functional polyoxometalates from solvothermal reactions of VOSO4 with tripodal alkoxides - a study on the reactivity of different "tris" derivatives.

We report a study on the structure directing effects of functional groups and counterions. The aim was to find a facile and high yielding synthetic procedure to obtain polyoxometalate (POM) building blocks for post-functionalisation. Therefore, solvothermal reactions of VOSO4 with various tris(hydroxymethyl)methane derivatives in alkaline methanolic solutions were investigated. In doing so, new POM fragments were isolated and characterised. The binding modes of the functionalised tripodal alkoxides turned out to be surprisingly different.

[Fe19 ] "Super-Lindqvist" Aggregate and Large 3D Interpenetrating Coordination Polymer from Solvothermal Reactions of [Fe2 (OtBu)6 ] with Ethanol.

The syntheses, crystal structures, and physical properties of [HFe19 O14 (OEt)30 ] and {Fe11 (OEt)24 }∞ are reported. [HFe19 O14 (OEt)30 ] has an octahedral shape. Its core with a central Fe metal ion surrounded by six µ6 -oxo ligands is arranged in the rock salt structure. {Fe11 (OEt)24 }∞ is a mixed-valence coordination polymer in which Fe(III) metal ions form three 3D interpenetrating (10,3)-b nets. The arrangement of the Fe(III) ions can also be compared to that of Si ions in α-ThSi2 . Thus, the described structures are at the interface between molecular and solid-state chemistry.

Online monitoring the isomerization of an azobenzene-based dendritic bolaamphiphile using ion mobility-mass spectrometry.

Ion mobility-mass spectrometry was used to obtain detailed information about the kinetics of the light-induced cis/trans isomerization process of a new supramolecular azobenzene-based bolaamphiphile. Further experiments revealed that the investigated light-induced structural transition dramatically influences the aggregation behaviour of the molecule.

Photoresponsive switches at surfaces based on supramolecular functionalization with azobenzene-oligoglycerol conjugates.

The synthesis, supramolecular complexation, and switching of new bifunctional azobenzene-oligoglycerol conjugates in different environments is reported. Through the formation of host-guest complexes with surface immobilized ß-cyclodextrin receptors, the bifunctional switches were coupled to gold surfaces. The isomerization of the amphiphilic azobenzene derivatives was examined in solution, on gold nanoparticles, and on planar gold surfaces. The wettability of functionalized gold surfaces can be reversibly switched under light-illumination with two different wavelengths. Besides the photoisomerization processes and concomitant effects on functionality, the thermal cis to trans isomerization of the conjugates and their complexes was monitored. Thermal half-lives of the cis isomers were calculated for different environments. Surprisingly, the half-lives on gold nanoparticles were significantly smaller compared to planar gold surfaces.