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

Abstract

The dicationic complex [(triphos)Rh(mu-S)(2)Rh(triphos)](2+), 1 (modeled as 1c) [triphos = CH(3)C(CH(2)PPh(2))(3)], is known to activate two dihydrogen molecules and produce the bis(mu-hydrosulfido) product [(triphos)(H)Rh(mu-SH)(2)Rh(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 d(6) metal ion in 1c, with local square pyramidal geometry, is able to anchor one H(2) 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 H(2) molecule is added. Alternatively, both H(2) molecules are trapped by the Rh(2)S(2) core before being split in two distinct steps. Since the ambiguity could not be solved by calculations, (31)P and (1)H NMR experiments, including para-hydrogen techniques, have been performed to identify the actual pathway. In no case is there experimental evidence for any Rh-(eta(2)-H(2)) adduct, probably due to its very short lifetime. Conversely, (1)H NMR analysis of the hydride region indicates only one reaction intermediate which corresponds to the monohydride-mu-hydrosulfide complex [(triphos)Rh(H)(mu-SH)(mu-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 H(2) 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.

A Counterintuitive Structural Effect of Metal–Metal Bond Protonation and Its Electronic Underpinnings

“…[5] However, there are only a few structural reports on pairs of bimetallic precursors and hydrogenated derivatives, a selection of which is given in reference [6]. Elongation of the M À M disAbstract: Protonation across the metal-metal bond in the complexes [(CO) 2 lengthens with two consecutive protonations, but there are no crystal structure determinations to highlight the effects on the IrÀIr bond. DFT calculations and the analogous cobalt system confirm that the transformation of a two-electron, two-center (2e-2c) bond into a 2e-3c bond is accompanied by the predicted elongation.…”

“…In contrast, Puddephatt et al [7] first pointed out the unexpected shortening of an RhÀCo bond on protonation (from 2.6858(2) to 2.6480 (8) ). The structures of parent dimer 2 Co(CO)] (dppm = Ph 2 PCH 2 PPh 2 ) and its protonated derivative are shown in Figure 1 (data obtained from the Cambridge Structural Database [8] ). Theoretical analysis of this heterodinuclear system, which approximately combines a T-shaped RhL 3 ) and a trigonal-pyramidal CoL 4 fragment, is complicated by the semibridging CO ligand and lack of symmetry.…”

“…In particular, the QTAIM approach seems to have evident difficulties in clarifying the nature of MÀM bonding, especially in the presence of CO ligands, which subtract a significant amount of electron density away from the intermetallic region. [14] The structures of the complexes [(CO) 2 [15] Ru (2) [16] ) and [(CO) 3 …”

“…In this context, we mention our recent DFT and NMR mechanistic study of the double reversible activation of H 2 over the Rh 2 S 2 core. [2] At the biological level, the hydrogenases are comparably functional. In particular, the active site of Fe-only hydrogenases features a cooperative pair of iron centers, which in the early stage involve a bridging hydride intermediate.…”

MRI of Heterogeneous Hydrogenation Reactions Using Parahydrogen Polarization

“…The most common use of p−H 2 polarization is in the study of homogeneous catalysis [4,14,26,27,29,58,64,66,85,96,103,104,120,161]. The large signal enhancement allows one to observe intermediate species and their rearrangements and to monitor reaction kinetics.…”