Synthesis and Characterization of N‐Heterocyclic Carbene Substituted Phosphine and Phosphite Rhodium Complexes and their Catalytic Properties in Hydrogenation Reactions

Abstract

Mixed N-heterocyclic carbene-substituted phosphine and phosphite complexes of rhodium were prepared, starting from [RhA C H T U N G T R E N N U N G (COE) 2 Cl] 2 (COE = cyclooctene) by addition of free N-heterocyclic carbenes (NHC) and PR 3 . All new complexes were characterized by spectroscopy. In addition, the structures of trans-chloro(1,3-dicyclohexylimidazol-2-ylidene)-bis(triphenylphosphite)rhodium(I) (5), chloro-trans-bis(1,3-dicyclohexylimidazol-2-ylidene) (triphenylphosphine)rhodium(I) (6), and chloro(h8) were determined by single crystal X-ray analyses. The hydrogenation of cyclohexene using molecular hydrogen has been optimized for some N-heterocyclic carbene-substituted phosphine and phosphite rhodium complexes by variation of the reaction conditions.

N‐Heterocyclic Carbene Rhodium Complexes and Their Reactions with H2 and with CO

“…30-40 % after 5 h). Our findings generally parallel closely more detailed studies reported by Herrmann et al, [9] who tested RhCl(COD)(NHC) and [Rh(COD)(NHC) 2 ] + in toluene for catalytic hydrogenation of cyclohexene and 1-octene; in these systems, addition of PPh 3 was needed to generate effective catalysts. Nolan's group has reported low activity for RhCl(IMes)(PPh 3 ) 2 for hydrogenation of cyclohexene, [20] but this study was carried out in CH 2 Cl 2 not generally a favoured solvent because hydrogen transfer to the solvent can occur with liberation of HCl.…”

“…[18] In our case, the Rh-H hydrides were not located even though residual electron density around the Rh was evident. The two IPr ligands are mutually trans [177.1 (2) [2,4,5,9,18,28] ). Crudden and co-workers have synthesized complex 5 from the nitrogen complex trans-RhCl(N 2 )(IPr) 2 , and have also obtained a crystal structure for 5 which co-crystallized with the Rh II complex RhCl 2 (IPr) 2 ; their determined crystallographic geometrical parameters for a non-solvated crystal of 5 are close to our values, and they were able also to locate the hydrides (Rh-H = 1.45 Å).…”

“…[17] There is literature on reactions of H 2 and CO, and related catalysis, within NHC-Rh I systems, [5,9,13,[18][19][20][21] but not specifically with the COE/IPr/IMes precursors, and our studies have led to identification of new species, as well as some findings on catalytic hydrogenation and isomerization of COE and 1-octene. Our findings on the O 2 and H 2 reactions were first presented at a Canadian Chemistry conference.…”

“…Hydrogenation of olefinic substrates using Rh-NHC catalysts, either with [9,33] or without [9,30] ancilliary phosphane ligands, has been reported, but we were unaware of any attempts at such catalysis using characterized Rh-hydrido-NHC complexes as catalyst precursors. Thus, we tested via a few preliminary experiments the Rh(H) 2 Cl(NHC) 2 complexes 5 (NHC = IPr) and 5Ј (NHC = IMes) for catalytic hydrogenation of cyclooctene and 1-octene at ambient conditions, and compared data with those we obtained also using the well-known RhCl(PPh 3 ) 3 system.…”

“…For 6, three 13 C{ 1 H} doublets are seen at δ = 186.0 (J Rh-C = 54 Hz), 184.2 (J Rh-C = 72 Hz) and 182.1 ppm (J Rh-C = 45 Hz) for the one carbene-carbon and the two inequivalent carbonyls; signal assignment is not obvious, because J Rh-C values for the carbene carbon atom are typically in the 35-65 Hz range (e.g.ref. [2,[4][5][6][7]9,13,15] [5,30] For 7, the 13 C{ 1 H} spectrum shows only one doublet at δ = 186.0 ppm (J Rh-C = 54 Hz) for the two carbonyl ligands, and one for the carbene-carbon at δ = 179.2 ppm (J Rh-C = 45 Hz), although two ν CO IR bands at 2070 and 1990 cm -1 are consistent with inequivalent carbonyls, at least in the solid-state structure. This phenomenon has been found in other carbenecarbonyl-Rh complexes.…”