Stoichiometric and Catalytic Solid-Gas Reactivity of Rhodium Bisphosphine Complexes

The complexes [Rh((Bu2PCH2CH2PBu2)-Bu-i-Bu-i)L-2][BAr4F] [L-2 = C4H6, (C2H4)(2), (CO)(2), (NH3)(2); Ar-F = 3,5-C6H3(CF3)(2)] have been synthesized by solid-gas reactivity via ligand exchange reactions with, in some cases, crystallinity retained through single-crystal to single-crystal transformations. The solid-state structures of these complexes have been determined, but in only one case (L-2 = (NH3)(2)) is the cation ordered sufficiently to enable its structural metrics to be determined by single crystal X-ray diffraction. The onward solid-state reactivity of some of these complexes has been probed. The bis-ammonia complex [Rh((Bu2PCH2CH2PBu2)-Bu-i-Bu-i)(NH3)(2)][BAr4F] undergoes H/D exchange at bound NH3 when exposed to D2. The bis-ethene complex [Rh((Bu2PCH2CH2PBu2)-Bu-i-Bu-i)(C2H4)(2)][BAr4F] undergoes a slow dehydrogenative coupling reaction to produce a material containing a 1:1 mixture of the butadiene complex and a postulated mono-ethene complex. The mechanisms of these processes have been probed by DFT calculations on the isolated Rh cations. All the solid materials were tested as heterogeneous catalysts for the hydrogenation of ethene. Complexes with weakly bound ligands (e.g., L-2 = (C2H4)(2)) are more active catalysts than those with stronger bound ligands (e.g., L = (CO)(2)). Surface-passivated crystals, formed through partial reaction with CO, allow for active sites to be probed, either on the surface or the interior of the single crystal.