Supported rhodium catalysts are known as one of the most active catalysts for hydrogenation and dehydrogenation reactions.¹ However, the mode of interaction between Rh sites and H₂ is still not fully understood.² To gain further insight, we developed a series of bimetallic complexes bearing novel Rh(I) dimer moieties such as 1 and 3. In comparison to heterogeneous catalysts, such model compounds allow for characterisation of the products by solution NMR techniques including parahydrogen induced polarization (PHIP) or ¹⁰³Rh NMR. With dppm as ligand, complex 1 adds hydrogen reversibly in solution. Detailed DFT calculations allowed us to confirm that hydrogen activation precedes formation of 2 consistent with in situ NMR experiments (Figure 1, b). Remarkably, 1 activates H₂ in a very similar manner as was calculated for small rhodium clusters.³ It is therefore a viable model compound for heterogeneous rhodium catalysts. Other complexes such as 3 transfer hydrogen to unsaturated C,C bonds in the ligand in a stepwise manner forming 4 and 5 (Figure 1, c). These reactions provide insight into conceivable pathways of hydrogen transfer to a carbon support which may be relevant for Rh/C hydrogenation catalysts.

Figure 1. a) Crystal structure of 1, b) hydrogen activation on 1 leads to dihydride species 2, c) a stepwise H₂ addition to the rhodium-bound triple bonds in 3. L is CD₃CN.

[1] Velina K. Markova, Georgi N. Vayssilov and Notker Rösch, J. Phys. Chem. C, 2015, 119, 1121-1129.
[2] Pedro Serna and Bruce C. Gates, Acc. Chem. Res., 2014, 47, 2612-2620.
[3] Abhijit Dutta and Paritosh Mondal, J. Phys. Chem. C, 2018, 122, 16925-16939.