Small-Molecule Models of Hydrogen-Evolving MX2 (M = Mo, W; X = S, Se) Bulk Solids: Composition–Activity Relationships

TL;DR

Researchers studied small-molecule models of hydrogen-evolving materials and found that certain combinations of metals and ligands improve hydrogen production efficiency.

Contribution

The study identifies composition–activity relationships in triangular metal chalcogenide clusters as molecular analogs of MX2 catalysts.

Findings

Clusters with all-sulfide cores and Mo produce higher H2-turnovers than all-selenide cores.

Dichalcogenocarbamate ligands enhance catalytic activity compared to dialkyldithiophosphate ligands.

Electron-donating ligands like iBu2NCS2– improve H2-evolution performance.

Abstract

Triangular metal chalcogenide clusters of the form [M3Q7L3]An (M = Mo or W; Q = S or Se; L
= iBu2NCS2–, (CF3CH2)2NCS2–, iBu2NCSe2–, or iBu2PS2–; An = Cl– or I–) have been investigated as molecular analogues of layered metal
dichalcogenide (MX2) H2-evolution catalysts.
These clusters have been evaluated for their relative H2-evolving ability under a common photolysis protocol implementing
[Ru(bpy)3]2+ as chromophore and Et3N as sacrificial electron donor. With M constant as Mo and with constant
supporting ligand, clusters with an all-sulfide core enable greater
H2-TON than clusters with an all-selenide core. A more
active catalyst is produced by [Mo3S7(S2CNiBu2)3]+I– than its W3 analogue
with the same core sulfide composition and supporting dithiocarbamate
ligands. Dichalcogenocarbamate ligands provide more active catalysts
than dialkyldithiophosphate ligated clusters, and within the dichalcogenocarbamate
set, greater H2-turnovers correlate with more-electron-donating
ligands (i.e., iBu2NCS2– > (CF3CH2)2NCS2– > iBu2NCSe2–). Cluster
cations
with Cl– as counteranion are very similar in activity
H2-evolving levels to identical clusters with I–, ruling out any significant interfering effect by I– upon the electron transfer relay between Et3N and catalyst.
In the aggregate, observations are consistent with a mechanism for
H2 evolution that involves reductive extrusion of H2 from a metal hydride intermediate.

Triangular metal chalcogenide clusters [M3Q7L3]An (M = Mo or W; Q = S or Se; L = iBu2NCS2−,
(CF3CH2)2NCS2−, iBu2NCSe2−, or iBu2PS2−; An = Cl− or I−) are H2-evolution catalysts under photolysis and functional
analogues of layered metal dichalcogenide catalysts. Highest activity
in the variable composition set is observed for [Mo3S7(S2CNiBu2)3]I, ∼300 TON H2 over 2 h. Lower activity
for [W3S7(S2CNiBu2)3]I may arise from a stronger M−H
bond in the intermediate preceding H2 extrusion.

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