Molecular Models of Atomically Dispersed Uranium at MoS2 Surfaces Reveal Cooperative Mechanism of Water Reduction

Single atoms of uranium supported on molybdenum sulfide surfaces (U@MoS2) have been recently demonstrated to facilitate the hydrogen evolution reaction (HER) through electrocatalysis. Theoretical calculations have predicted uranium hydroxide moieties bound to edge-sulfur atoms of MoS2 as a proposed transition state involved in the HER process. However, the isolation of relevant intermediates involved in this process remains a challenge, rendering mechanistic hypotheses unverified. The present work describes the isolation and characterization of a uranium-hydroxide intermediate on molybdenum sulfide surfaces using [(Cp*3Mo3S4)UCp*], a molecular model of a reduced uranium center supported at MoS2. Mechanistic investigations highlight the metalloligand cooperativity with uranium involved in the water activation pathway. The corresponding uranium-oxo analogue, [(Cp*3Mo3S4)Cp*U(=O)], was also accessed from the hydroxide cluster via hydrogen atom transfer and from [(Cp*3Mo3S4)UCp*] through an alternative direct oxygen atom transfer. These results provide an atomistic perspective on the reactivity of low-valent uranium at molybdenum sulfide surfaces toward water, modeling key intermediates associated with the HER of U@MoS2 catalysts.


SUPPORTING INFORMATION TABLE OF CONTENTS
[The v(OH) centered at 3668 cm -1 corresponds to the mono-hydroxide compound 2, v(OH) centered at 3604 and 3645 cm -1 is attributed to the OH groups of bis-hydroxide compound 3, and v(OH) centered at 3704 and 3730 cm -1 is attributed to the OH groups of the proposed cluster-free uranium hydroxide compound      2) and (Cp*3Mo3S4)UCp*(=O) (4) collected via ATR.Although the mono-hydroxide species 2 is stable enough to perform all the required characterizations, the uranium oxo species 4 is highly unstable.We observed rapid decomposition in 1 H NMR while performing the synthesis via either pathway, even at low temperatures (hydrogen atom transfer using Gomberg's dimer or oxygen atom transfer using styrene oxide), which precludes further characterization of the oxo compound 4. In this figure, we have attempted to characterize the uranium oxo compound 4 using FT-IR spectroscopy and observed bands at 760 cm −1 in both products obtained from the HAT and OAT reactions, along with a weak band at 915 cm −1 in the product obtained from the HAT reaction.These bands were absent in compound 2. Comparing these with uranium oxo compounds reported in the literature suggests a close resemblance to UO2 (775 cm −1 ) and UO2 + (952 cm −1 ).This observation is consistent with decomposition observed by 1 H NMR spectroscopy.We argue that the limitations of our model in stabilizing uranium oxo or uranyl derivatives, possibly stemming from weaker U-Ssurf bonds compared to other reported analogous compounds featuring stronger U-X (X = N, O) bonds.

FigureFigure S3 .
Figure S2. 1 H NMR spectrum (400 MHz) of the reaction mixture containing C6D6 solution of (Cp*3Mo3S4)Cp*U and benzene-d6 solution having slightly more than 1 equivalent of water in THF-d8.Signal located at δ = 4.47 ppm is assigned to H2 (~80% yield), while signals at δ = -0.45 and -7.89 are attributed to the resonances of methyl protons of Mo-Cp* and U-Cp* of the starting cluster, (Cp*3Mo3S4)Cp*U.

Figure S9 .Figure S10 .
Figure S9.The relevant zoomed-in portion of the spectra provided in Figure S8.The * signal on the leftpart of the spectra represents the Mo-Cp* methyl protons of 2, while * signal represents the Mo-Cp* methyl protons of 3. The signal on the right part of the spectra represents H2.

Figure S11 .
Figure S11.The selected region of v(OH) (3400-3800 cm -1 ) of the stacked spectra shown in FigureS10[The v(OH) centered at 3668 cm -1 corresponds to the mono-hydroxide compound 2, v(OH) centered at 3604 and 3645 cm -1 is attributed to the OH groups of bis-hydroxide compound 3, and v(OH) centered at 3704 and 3730 cm -1 is attributed to the OH groups of the proposed cluster-free uranium hydroxide compound (Scheme S1)].

Figure
Figure S12. 1 H NMR spectrum (400 MHz) of the reaction mixture containing benzene-d6 solution of (Cp*3Mo3S4)Cp*U and benzene-d6 solution having excess of water in THF-d8.The signal at δ = 8.61 ppm is attributed to the resonance of methyl protons of Mo-Cp* of the free neutral cluster [(Cp*3Mo3S4)], while δ = 4.46 ppm is attributed to H2; solvent impurities located δ = 3.50 and 1.39 ppm correspond to THF.

Figure
Figure S13. 1 H NMR spectrum (400 MHz) of the reaction mixture containing a benzene-d6 solution of (Cp*3Mo3S4)Cp*U(OH) and a benzene-d6 solution with 0.5 equivalent of Gomberg's dimer.The signal at δ = 8.59 ppm is attributed to the resonance of methyl protons of Mo-Cp* of the free neutral cluster (Cp*3Mo3S4), while δ = 7.06 ppm is attributed to the by-product of Gomberg's dimer after abstracting the hydrogen atom (Ph3CH).

(
Figure S14. 1 H NMR spectrum (400 MHz) (-10 o C) (with slow relaxation time) of the reaction mixture containing a toluene-d8 solution of (Cp*3Mo3S4)Cp*U and a toluene-d8 solution with 1 equivalent of styrene oxide.The signal at δ = 8.54 ppm is attributed to the resonance of methyl protons of Mo-Cp* of free neutral cluster, (Cp*3Mo3S4).

Figure S15 .
Figure S15.Stacked 1 H NMR spectra (400 MHz) of A) the reaction mixture containing a Toluene-d8 solution of (Cp*3Mo3S4)Cp*U and a toluene-d8 solution with 1 equivalent of styrene oxide; B) the reaction mixture containing a C6D6 solution of (Cp*3Mo3S4)UCp*(OH) and a benzene-d6 solution with 0.5 equivalent of Gomberg's dimer.

(Figure S16 .
Figure S16.Stacked infrared spectra of (Cp*3Mo3S4)UCp*(OH) (2) and (Cp*3Mo3S4)UCp*(=O) (4) collected via ATR.Although the mono-hydroxide species 2 is stable enough to perform all the required characterizations, the uranium oxo species 4 is highly unstable.We observed rapid decomposition in 1 H NMR while performing the synthesis via either pathway, even at low temperatures (hydrogen atom transfer using Gomberg's dimer or oxygen atom transfer using styrene oxide), which precludes further characterization of the oxo compound 4. In this figure, we have attempted to characterize the uranium oxo compound 4 using FT-IR spectroscopy and observed bands at 760 cm −1 in both products obtained from the HAT and OAT reactions, along with a weak band at 915 cm −1 in the product obtained from the HAT reaction.These bands were absent in compound 2. Comparing these with uranium oxo compounds reported in the literature suggests a close resemblance to UO2 (775 cm −1 ) and UO2 + (952 cm −1 ).This observation is consistent with decomposition observed by 1 H NMR spectroscopy.We argue that the limitations of our model in stabilizing uranium oxo or uranyl derivatives, possibly stemming from weaker U-Ssurf bonds compared to other reported analogous compounds featuring stronger U-X (X = N, O) bonds.