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

Abstract

The NHC‐RhI complexes [RhCl(COE)(NHC)]2 1 and 2 [COE = cyclooctene, NHC in 1 = N,N‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) and, in 2, N,N‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes)] react with H2 in hexane to give the dimeric, mono‐carbene dihydrido species [Rh(H)2Cl(NHC)]2 (NHC = IPr (3), IMes (4)). In the presence offurther NHC, the bis‐carbene dihydrido species Rh(H)2Cl(NHC)2 are formed; a crystal structure of the IPr complex 5 is analogous to that of the known IMes analogue. The dihydride‐mixed‐carbene species Rh(H)2Cl(IPr)(IMes) (5a) was also observed but not isolated. A benzene solution of 5 under D2 slowly generates the corresponding dideuteride. Reactions of the mono‐carbenes (1/3, or 2/4) with CO in hexane afford the respective dicarbonyl complexes RhCl(CO)2(NHC) [NHC = IPr (6), IMes (7)], while CO reactions with the bis‐carbene dihydrides give, respectively, the mono‐carbonyl complex RhCl(CO)(IPr)2 (8) and the known IMes analogue. All the complexes are characterized by elemental analysis, 1H and 13C{1H} NMR and IR spectroscopies and, in the case of 5, by X‐ray crystallography. The catalytic activities of 5 and the previously reported Rh(H)2Cl(IMes)2 for hydrogenation of COE and 1‐octene (and isomerization of the latter) are shown to be poor.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Structure and Reactivity of Dinitrogen Rhodium Complexes Containing N‐Heterocyclic Carbene Ligands

“…However, the dihydride appears to be oxidatively sensitive and upon exposure to oxygen reacts to form dioxygen complex 1a, necessitating anaerobic purification of 3. Personal communication with B. R. James [39] indicates that a similar reaction was also observed in his laboratories. X-ray quality crystals of both 3 and 4 were obtained by diffusion of hexane into concentrated THF solutions of the corresponding compounds, and are shown along with relevant bond lengths and bond angles in Figures 2 and 3, respectively (crystallographic parameters of 2-4 are presented in Table 1).…”

Efficient Rhodium‐Catalyzed Multicomponent Reaction for the Synthesis of Novel Propargylamines

“…Treatment of complex 2 with isovaleraldehyde at room temperature led to the formation of the bis‐carbonyl complex [RhCl(CO) 2 IPr] ( 3 ), probably through decarbonylation of the aldehyde 13. This complex has been previously prepared by placing [RhCl(COD)IPr] under an atmosphere of carbon monoxide 10. Remarkably, when two equivalents of both isovaleraldehyde and 2,4,6‐trimethylaniline were added to 2 in toluene, a new complex with a coordinated amine is formed instead of 3 .…”

“…Hexane was either dried by standard procedures and distilled under argon before use or obtained oxygen‐ and water‐free from a Solvent Purification System (Innovative Technologies). The starting complexes [{Rh(μ‐Cl)(IPr)(coe)} 2 ] and [{Rh(μ‐Cl)(H) 2 (IPr)} 2 ] were prepared following the procedures described in the literature 10. All other chemicals were used as purchased from Sigma–Aldrich, Merck, and Acros.…”

1,2,4‐Triazol‐3‐ylidenes with an N‐2,4‐Dinitrophenyl Substituent as Strongly π‐Accepting N‐Heterocyclic Carbenes

“…(4 a-c) in good yields through in situ generation of NHCs 3 a-c, respectively. To estimate the sdonating properties of NHCs 3 a-c, Rh complexes 4 ac were converted to [RhClA C H T U N G T R E N N U N G (NHC)(CO) 2 ] (5 a-c) in quantitative yield by bubbling CO gas through their solutions in CH 2 Cl 2 or [D 6 [7] and [RhCl-A C H T U N G T R E N N U N G (IPr)(CO) 2 ] (5 e) [8] were also prepared for comparison of the electronic properties of the NHCs. In addition, [AuCl-A C H T U N G T R E N N U N G (NHC)] complexes 6 a and 6 c were synthesised by the reactions of triazolium salts 3 a·HOTf or 3 c·HOTf with AuCl-A C H T U N G T R E N N U N G (SMe 2 ) and K 2 CO 3 as base (Scheme 2).…”

“…Complexñ(CO) [a]ñ av (CO) [b] TEP [c] dA C H T U N G T R E N N U N G ( 13 CN 2 ) [d] 5 2086. 6, 2007.5 [3] 2047.1 [3] 2058 -5 e A C H T U N G T R E N N U N G (L=IPr) 2078.9, 1996.0 [3] 2037.5 [3] (3); N1-C1-N3 102.0(2), C1-Rh1-Cl1 88.22 (8), N1-C1-Rh1-Cl1 À98.1(2), C1-N1-C3-C4 À141.7(3), C3-C4-N4-O1 46.4(3), C5-C6-N5-O3 À6.2(4).…”