Recycling and Delivery of Homogeneous Fluorous Rhodium Catalysts Using Poly(tetrafluoroethylene): “Catalyst-on-a-Tape”

Abstract

The red−orange fluorous rhodium(I) complexes ClRh(P((CH 2 ) m R fn ) 3 ) 3 (m/n = 2/6 (1a), 2/8, 3/6, 3/ 10; R fn = (CF 2 ) n−1 CF 3 ) are essentially insoluble in organic solvents at 20 °C but have measurable solubilities in dibutyl ether at 55−65 °C. Under these conditions, they are effective catalyst precursors for the hydrosilylation of cyclohexanone by PhMe 2 SiH. Upon cooling, the catalyst rest states precipitate, giving colorless solutions of C 6 H 11 OSiPhMe 2 . When this sequence is conducted in the presence of poly(tetrafluoroethylene) (PTFE; Teflon) tape, the catalysts precipitate onto the tape but desorb when used in subsequent cycles. The catalyst precursors can also be precoated onto the tape, allowing quantities to be delivered by length instead of mass. Rate measurements (1a) show an induction period in the first cycle, excellent retention of activity in the second and third cycles, and significant activity loss in the fourth. Rhodium leaching is 0.57% and 5.3% for the first two cycles (atomic absorption spectroscopy inductively coupled plasma analysis); (CF 2 ) 5 CF 3 leaching is 11.4% over the first three cycles ( 19 F NMR). Reactions with added mercury show that metallic rhodium is not responsible for catalysis. Identical protocols are applied to 2-octanone, acetophenone, and benzophenone, albeit with some activity loss in the third cycle. Other forms of PTFE can be similarly employed (e.g., Gore-Tex membrane). However, fluorous/organic liquid/ liquid biphase conditions can give better retention of catalyst activity. Nonetheless, the diverse morphologies of PTFE that are commercially available suggest avenues for further optimization.

Density Functional Theoretical Study on the Electronic Structure of Rh2O7+ with Low Oxidation States

“…Rh is regarded as an important active center, which is widely applied in various syntheses and their catalysis. − In numerous syntheses, it has been used as a catalyst for cyclopropanation, , hydroformylation, , and C–H bond activation; , what is more, it has also been used for manufacturing acetic acid and β2-amino acid . In particular, Shan et al reported a mononuclear rhodium species, which was anchored on a zeolite or titanium dioxide support suspended in an aqueous solution; they catalyzed the direct conversion of methane to methanol and acetic acid using oxygen and carbon monoxide under mild conditions.…”