New Rhodium(I) Carbene Complexes from Carbene Transfer Reactions

Abstract

The Rh(I) complexes RhCl(diene)(NHC) have been synthesized from Ag(I) carbene precursors, where NHC is one of the N-heterocyclic carbenes 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), 1,3bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), 3-methyl-1-phenacylimidazol-2-ylidene (MAI), and 3-methyl-1-picolylimidazol-2-ylidene (MPI), and diene is either NBD or COD. The complexes are characterized by elemental analysis, 1 H NMR and IR spectroscopies, mass spectrometry, and, in the case of RhCl(NBD)(IMes) and RhCl(COD)(MAI), X-ray crystallography. The new Ag(I) carbene complexes [Ag(IPr) 2 ]PF 6 and [Ag(IMes) 2 ]PF 6 are also reported.

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

“…For 3, as well as the high-field doublet resonance for the equivalent hydrides, the 1 H NMR spectrum (in C 6 D 6 ) shows a δ = 6.43 ppm singlet for the NCH proton, a septet signal at δ = 2.92 ppm for the IPr-methine protons, and doublets at δ = 1.34 and 1.02 ppm for two types of Me-groups; the signal integrations and J values support the formulation, as well the 13 C{ 1 H} data, which include a doublet for the carbene carbon at δ = 182.7 ppm (J RhC = 58 Hz), the normal range for carbene-Rh complexes. [13,24] The hydride resonances are also in the range reported for the hydride ligands of Rh(H)Cl(IMesЈ)(IMes), where IMesЈ is the cyclometallated carbene formed from a methyl group via intramolecular C-H activation, [15,18] and of Rh(H) 2 Cl-(IMes) 2 (see below).…”

“…[14] We later reported that (carbene-phosphanyl)-Rh I species undergo "standard" oxidative addition of O 2 to give Rh III -peroxide species, [15] while Crudden and co-workers have recently suggested that such species are best considered as Rh I with coordinated singlet oxygen. [16] Our studies [13][14][15] have thus far mainly utilized the simple, unfunctionalized NHCs, N,N-bis (2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and N,N-bis (2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) (see Scheme 1), in syntheses of the RhCl(diene)(NHC) and [RhCl(COE)(NHC)] 2 complexes, and reactivities of these complexes with O 2 (diene = NBD or COD, and COE = cyclooctene). Because of our interests in catalytic hydrogenation/hydroformylation, we then studied reactions of the [RhCl(COE)(NHC)] 2 complexes with H 2 and with CO, with an ultimate goal of comparing catalytic activity of such sys-afford the respective dicarbonyl complexes RhCl(CO) 2 (NHC) [NHC = IPr (6), IMes (7)], while CO reactions with the biscarbene dihydrides give, respectively, the mono-carbonyl complex RhCl(CO)(IPr) 2 (8) and the known IMes analogue.…”

“…[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.…”

“…Chem., and other recent publications [1][2][3][4][5][6][7][8][9][10][11][12] and the references contained therein. Our initial studies in metal-carbene chemistry [13] developed from attempts to replace oxidizable tertiary phosphane ligands in a Ru system that showed catalytic activity for conversion of N 2 O to N 2 and O 2 . [14] We later reported that (carbene-phosphanyl)-Rh I species undergo "standard" oxidative addition of O 2 to give Rh III -peroxide species, [15] while Crudden and co-workers have recently suggested that such species are best considered as Rh I with coordinated singlet oxygen.…”

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

“…An important additional route for the generation of NHC-substituted rhodium(I) complexes is the carbene transfer reaction starting from NHC-substituted silver(I) complexes, which was first mentioned by Lin. [29,30] …”