Stoichiometric and Catalytic Solid–Gas Reactivity of Rhodium Bis-phosphine Complexes

The
complexes [Rh­(iBu2PCH2CH2PiBu2)­L2]­[BArF4] [L2 = C4H6, (C2H4)2, (CO)2, (NH3)2; ArF = 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 (L2 = (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(iBu2PCH2CH2PiBu2)­(NH3)2]­[BArF4] undergoes H/D exchange at bound NH3 when exposed
to D2. The bis-ethene complex [Rh­(iBu2PCH2CH2PiBu2)­(C2H4)2]­[BArF4] 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., L2 = (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.