Activation of Molecular Hydrogen over a Binuclear Complex with Rh2S2 Core: DFT Calculations and NMR Mechanistic Studies

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Abstract
The dicationic complex [(triphos)Rh(μ-S)2Rh(triphos)]2+, 1 (modeled as 1c) [triphos = CH3C(CH2PPh2)3], is known to activate two dihydrogen molecules and produce the bis(μ-hydrosulfido) product [(triphos)(H)Rh(μ-SH)2Rh(H)(triphos)]2+, 2 (modeled as 2b), from which 1 is reversibly obtained. The possible steps of the process have been investigated with DFT calculations. It has been found that each d6 metal ion in 1c, with local square pyramidal geometry, is able to anchor one H2 molecule in the side-on coordination. The step is followed by heterolytic splitting of the H−H bond over one adjacent and polarized Rh−S linkage. The process may be completed before the second H2 molecule is added. Alternatively, both H2 molecules are trapped by the Rh2S2 core before being split in two distinct steps. Since the ambiguity could not be solved by calculations, 31P and 1H NMR experiments, including para-hydrogen techniques, have been performed to identify the actual pathway. In no case is there experimental evidence for any Rh−(η2-H2) adduct, probably due to its very short lifetime. Conversely, 1H NMR analysis of the hydride region indicates only one reaction intermediate which corresponds to the monohydride-μ-hydrosulfide complex [(triphos)Rh(H)(μ-SH)(μ-S)Rh(triphos)]2+ (3) (model 5a). This excludes the second hypothesized pathway. From an energetic viewpoint the computational results support the feasibility of the whole process. In fact, the highest energy for H2 activation is 8.6 kcal mol-1, while a larger but still surmountable barrier of 34.6 kcal mol-1 is in line with the reversibility of the process.

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