Computational Insights into the Mechanisms of H₂ Activation and H₂/D₂ Isotope Exchange by Dimolybdenum Tetrasulfide Complexes

Abstract

The mechanisms for H 2 activation by [Cp*Mo] 2 (μ-S) 2 (μ-S 2) (1-a, Cp* = pentamethylcyclopentadienyl) and its reaction product [Cp*Mo] 2 (μ-S) 2 (μ-SH) 2 (2) have been investigated by DFT methods. The reaction of 1-a involves the homolytic addition of H 2 to its μ-S ligands, followed by the cleavage of the S-S bond of the μ-S 2 ligand in a subsequent step. Complex 2 can adopt five conformations that only differ in the stereochemistry of the μ-SH and μ-S ligands; although an isomer with adjacent μ-S ligands (2-a) is formed initially, it then isomerises

Computational Insights on the Mechanism of H2 Activation at Ir2S2(PPh3)4: A Combination of Multiple Reaction Pathways Involving Facile H Migration Processes

“…All in all, the data in this section agree with the NMR experiments, 11 showing that the free energy barriers for the H exchange processes at 4 are all within the range of 19−23 kcal mol −1 . This bears similarities with the results for [FeFe]hydrogenase models 12 and other sulfido clusters, 8,13 all supportive of highly mobile H atoms at metal−sulfur clusters.…”

“…The process requires nonetheless the cleavage of the Ir′−S bond (3.40 Å in TS 4a/4e ) to allow for the rotation of the (μ-SH) ligand, 15 being worth noting that similar transition states have been computed for the analogous process on the binuclear compound [Cp*Mo] 2 (μ-S) 2 (μ-SH) 2 (Cp*= cyclopentadienyl). 8 DFT Studies on the Mechanism of the Reaction between 1 and H 2 To Form 2. Experimentally, the reaction between 1 and H 2 takes place in toluene at room temperature without formation of NMR-detectable intermediates.…”

“…Given that TS A/Bc is higher in energy than TS A/2 , which directly leads to the formation of the observed product 2, it seems unlikely that Bc is formed under the experimental conditions. Finally, an additional possible interaction between 1 and H 2 to form the bis-hydrosulfido compound C is labeled as pathway d. Although the concerted formation of two μ-SH ligands has been evidenced for some dinuclear molybdenum−sulfide complexes such as [Cp*Mo] 2 (μ-S) 2 (μ-S 2 ) (Cp* = pentamethylcyclopentadienyl), 8 in the present case this seems unlike due to both the computed endergonic nature of the process (ΔG r = 21.8 kcal mol −1 ) and its relatively large activation barrier (TS 1/C , G = 41.1 kcal mol −1 , see Figure 3). The activation strain model (ASM) analysis 19 can be used at this point to explain the different behavior of these two systems.…”

“…In fact, recent computational studies have shown that the reaction starts with the homolytic activation of H 2 at two adjacent sulfur centers followed by the isomerization of the resulting species into the observed product. 8 In contrast, the dicationic rhodium complex [Rh 2 (μ-S) 2 (triphos) 2 ] 2+ is known to reversibly activate 2 equivalents of H 2 via consecutive additions across Rh−S bonds. 9 Although monometallic, the third example included in Scheme 1c illustrates another possible H 2 activation pathway available for metal sulfide complexes.…”

“…The understanding of these H 2 activation processes is indeed not straightforward as the possible pathways for each step are complicated by the hydride mobility often observed in hydrogenases 12 and other sulfido clusters, 8,13 which means that the observed species are not necessarily the direct products of H 2 activation but can also originate from H migrations at previously formed intermediates. In line with this, NMR data indicates that intermetallic hydride exchange in 2 and exchange between IrH and SH centers in 4 proceed with relatively low activation barriers.…”