Expanding the Role of Dimeric Species: On-Cycle Involvement, Improved Stability, and Control of Stereo-Specificity. A Case Study of Atom-Economic Catalytic Hydrothiolation

A common assumption that dimeric metal complexes in many
catalytic
systems represent a resting state and are not directly involved in
catalytic processes was revised in a combined experimental and theoretical
study. On-cycle participation of dimeric metal complexes, rather than
typically assumed off-cycle involvement, was revealed, and advantageous
performance in terms of improved selectivity was observed. The conceptual
rationalization for the participation of dimeric species in the catalytic
cycle was developed. The Pd-catalyzed hydrothiolation process (where
strong Pd–S binding is well established and a persistent opinion
for the inactive/poisoning role of dimeric species is presumed) was
evaluated as a challenging system to test the concept. Activation
of an (NHC)Pd(Cl)(acac) precatalyst (NHCN-heterocyclic carbene
and acacacetylacetonate) under the reaction conditions produced
monomeric (NHC)Pd(SPh)2 or dimeric (NHC)2Pd2(SPh)4 species depending on the steric bulkiness
of the NHC ligand. Dimeric complexes possessed higher selectivity
and tolerated disulfide impurities in contrast to monomeric complexes.
Quantum chemical modeling suggested that dimeric catalysis proceeds
through the opening of only one (μ-SPh)–Pd bridging bond
with retention of the dimeric structure. The second bridging bond
is maintained, which prevents the monomerization of the complex. Catalytically
active species were detected in a hydrothiolation reaction by high-resolution
mass spectrometry and NMR spectroscopy. Proving the opportunity for
productive homogeneous catalysis via strongly coordinated dimeric
metal species opens new opportunities for catalyst design in the increased
nuclearity dimension.