Use of Solvatochromism to Assay Preferential Solvation of a Prototypic Catalytic Site

Abstract

The composition of the reaction medium near photoactive catalytic sites can be inferred from the solvatochromism of the absorption and emission spectra of the wetted sites, which depend on the polarizability of the fluid. In brief, solvatochromism measures the interaction of the dipole moments of the ground and excited states with the electric field imposed by the solvent shell: a field, which does not relax on the time scale of the absorption or emission events. To establish the utility of the technique for inorganic catalysts that operate in complex reaction media, such as encountered in the upgrading of biogenic fuels, we have measured the solvatochromism of a common, structural feature of metal oxide catalysts, mono-oxide or dioxide of a transition metal prepared by incorporating the OM or O₂M moiety into the framework of a polyhedral oligomeric silsesquioxane (POSS). In toluene, cyclohexene, chloroform and tetrahydrofuran, POSS-ligated oxometalates exhibit strong ligand-to-metal charge-transfer bands in their UV–visible absorption and emission spectra. From the solvatochromism of the chromophores dissolved in toluene-chloroform mixtures we inferred an unexpectedly strong, preferential solvation of the chromophore even when all three components (oxometalate and the two solvents) were highly miscible.

Graphical Abstract

Appendix

Here we present (1) the syntheses of the compounds we studied, (2) details of the spectroscopy of the OVPOSS samples, (3) an exemplary set of spectra showing the effect of solvents on the absorption and emission features, (4) estimations of the polarizabilities of the solvent mixtures, and (5) energies of the optical transitions calculated for the chromophores.

1.1 Synthesis Protocols

1.1.1 OVPOSS Complex

Two separate flasks were charged with starting materials under inert atmosphere, in the glove box, as follows: 0.172 g (1 mmol, 1 eq) of vanadyl (V) trichloride was charged in one flask, and 0.93 g of trisilanolphenyl POSS (1 mmol, 1 eq), was charged into a second flask. Anhydrous toluene was added to both flasks, inside the glove box. The POSS solution (20 mL) was added into the vanadyl solution (20 mL) with stirring. The resulting mixture was refluxed, and after 30 min of reflux, triethylamine (0.33 g) in 5 mL of toluene was added dropwise. The reaction mixture was allowed to reflux overnight (18 h). Next day, the precipitate formed was removed by filtration inside a glove box and the solution was stored in sealed vial inside a glove box. The solvent was not evaporated to prevent dimerization. In a second trial, vanadium oxytripropoxide was used as the vanadium source due to its superior stability as compared to vanadyl trichloride. A 3 mL solution of vanadium oxytripropoxide (0.25 mL, 1.10 mmol) in benzene, was added with vigorous stirring to a suspension of trisilanol phenyl POSS in benzene (1.00 g, 1.07 mmol). Complete dissolution of POSS was obtained in 5–10 min, however the reaction mixture was allowed to stir at room temperature overnight. The amorphous solid formed was isolated by filtration, while the benzene mother liquor was analyzed as is. The solution was characterized by ²⁹Si NMR (three broad signals) and ⁵¹V NMR (−676 ppm). The presence of the signal at −676 ppm indicates the presence of symmetric C_3v vanadyl species.

1.1.2 O₂MoPOSS Complex

Two separate flasks were charged with starting materials under inert atmosphere, in the glove box, as follows: 0.2 g (1 mmol) of molybdenum (VI) dichloride dioxide was charged in one flask, and 0.89 g of disilanolisobutyl POSS (1 mmol), was charged into a second flask. Anhydrous toluene was added to both flasks via cannula under argon, outside the glove box. The POSS solution (20 mL) was cannulated into the molybdenum solution (50 mL) with stirring. The resulting mixture was quickly brought to reflux, and after 30 min of reflux, triethylamine (0.33 g) in 5 mL of toluene was added dropwise. The resulting reaction mixture was refluxed for 18 h (overnight). Next day, the precipitate formed was removed by filtration in ambient atmosphere; the solid washed with dichloromethane and the filtrate was concentrated to an oil, which on standing crystallized. The yield was 0.5 g (~50 %). EIMS (M⁺) m/z: 1017 (parent).

1.1.3 O₂WPOSS Complex

Two separate flasks were charged with starting materials under inert atmosphere, in the glove box, as follows: 0.16 g (0.56 mmol) of tungsten (VI) dichloride dioxide was charged in one flask, and 0.5 g of disilanolisobutyl POSS (0.56 mmol), was charged into a second flask. Anhydrous toluene was added to both flasks via cannula under argon, outside the glove box. The POSS solution (20 mL) was cannulated into the tungsten solution (50 mL) with stirring. The resulting mixture was quickly brought to reflux, and after 30 min of reflux, triethylamine (0.23 g) in 5 mL of toluene was added dropwise. The resulting reaction mixture was refluxed for 18 h (overnight). Next day, the precipitate formed was removed by filtration in ambient atmosphere, the solid washed with dichloromethane and the filtrate was concentrated to an oil, which on standing crystallized. The reaction is sluggish and thus yields are generally low and variable (average 10 %). The POSS can be removed to some extent by recrystallization from hexanes (POSS remains in mother liquor, while the complex precipitates out), and the mixture can be enriched in the O₂WPOSS desired product. EIMS (M⁺) m/z: 1158 (Na salt of parent), 1124 (monohydrate of parent), 1044 (parent with loss of isobutyl group).

1.1.4 Cr-POSS Complex

A procedure analogous to that used to prepare the W-complex, was ineffective in the case of Cr. Instead, the Cr-POSS was prepared via a procedure of Feher and Blanski [49]. A round bottom flask was charged with disilanolisobutyl POSS (0.713 g, 0.8 mmol, 1 eq), CrO₃ (0.2 g) and MgSO₄ (0.5 g, dehydrating agent), under inert atmosphere in the glove box. To the solids, 30 mL CCl₄ were added, outside the glove box, via syringe, under argon, and the resulting slurry was stirred at room temperature overnight. The reaction mixture was sonicated the second day, three times for 30 min at 1-h intervals to ensure dispersion of CrO₃ in the mixture. During handling, the flask was wrapped in aluminum foil to protect the contents from light. The solvent was then removed under reduced pressure, the solid was suspended in acetone and removed by filtration. The filtrate was concentrated to yield an amorphous orange-red glassy solid (38 mg, 5 % yield). EIMS (M⁺) m/z: 991 (hydrate of parent), 971 (parent compound).

1.2 Spectroscopy of OVPOSS Samples

OVPOSS is not stable as a monovanadium complex [49]. The more stable dimer readily forms in solution and that propensity increases with increasing concentration. This is especially true for our investigation where the solutions are prepared by dissolving solid ‘OVPOSS’ as opposed to studying in situ reaction solutions. A series of absorption measurements was used to determine the absorption maximum (λ_abs) of the monomer and the dimer. Both complexes coexisted at all conditions, but the assumption underlying the experiment was that predominantly the dimer exists in solution as the concentration is increased, while the percentage of monomer increases at lower concentrations. The absorption peak, λ_abs, of the monomer was subsequently used as the excitation wavelength for emission measurements of differently concentrated OVPOSS containing solutions. The emission peak (λ_em) of the monomer and the dimer are distinctly different (separated by ~1 eV) with the monomer emitting light at lower energy. Throughout this study we exclusively report λ_abs and λ_em of the monomer. Except for the complication of having two photoactive species in solution, the measurements were identical to the procedure described in the main text.

1.3 Solvatochromism of O₂WPOSS

The solvent composition appeared to mostly affect the position of the emission maximum, exemplified below with the O₂WPOSS complex in chloroform/toluene mixtures. See Fig. 10.

Fig. 10

UV–Visible spectra of O₂WPOSS in mixtures of toluene and chloroform. Left emission spectra excited at (280 nm = 4.43 eV), Right absorption spectra

1.4 Orientation Polarizabilities of the Mixtures

We have used the following definition of, Δf:

$$\Delta f = \frac{\varepsilon - 1}{2\varepsilon + 1} - \frac{{n^{2} - 1}}{{2n^{2} + 1}}.$$

We measured the indices of refraction of the mixtures, n, but approximated the relative permittivities, ε, using the mixing rules described by Wang and Anderko [52]. See Table 3.

Table 3 Measured refractive index and calculated permittivity and orientation polarizibility

1.5 Calculated Optical Transition Energies

The table below presents the energies in eV for the HOMO–LUMO and LUMO–HOMO transitions for the clusters containing the indicated chromophores. Vertical transitions evaluate the energy of promoting (or relaxing) a single electron transfer between energy levels evaluated in the geometry of the starting state. Diagonal transitions evaluate the transition energy between geometry-optimized starting and final states. See Table 4.

Table 4 Calculated optical transition energies, all in eV