Rhodium(III) Peroxo Complexes Containing Carbene and Phosphine Ligands

Abstract

The Rh(I) carbene precursors [RhCl(COE)(NHC)] 2 , where the N-heterocyclic carbene is 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene (IPr) or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), were used to synthesize the RhCl(NHC)(P-N) complexes 4 (NHC ) IPr) and 5 (NHC ) IMes), where P-N is P,N-chelated o-(diphenylphosphino)-N,N-dimethylaniline, and the corresponding cis-RhCl(NHC)-(PPh 3 ) 2 complexes 6 and 7. The synthesis of 4 surprisingly requires the reaction to be carried out under a hydrogen atmosphere and occurs via the intermediate dihydride RhCl(H) 2 (IPr)(P-N) (3). Complexes 4-7 in benzene readily undergo irreversible oxidative addition of O 2 to form the corresponding Rh(III) peroxide complexes 9-12. For comparative purposes, RhCl(PPh 3 )(P-N) (8) was synthesized from RhCl-(PPh 3 ) 3 , and this also added O 2 to form a peroxo complex (13). All of the complexes were generally characterized by elemental analysis and 1 H, 31 P{ 1 H}, and 13 C{ 1 H} NMR and IR spectroscopies and, in the cases of 9, 10, and 13, by X-ray crystallography.

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).…”

“…The expected carbene-carbon resonance of 4 is surprisingly absent in the 13 C{ 1 H} NMR, although the same absence has been noted for 2, the precursor complex. [15] This is not understood but the signal is presumably broadened into the base-line by some exchange process, and reflects in some way the relatively labile and weak metal-carbene bond in this system (formation of 4 from 2 is not reversed on subjecting a solution to vacuum). Of note, rapid exchange of N-heterocyclic ligands in various Ag systems, including between dinuclear Ag 2 and tetranuclear Ag 4 species, is well known.…”

“…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. [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).…”

“…[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.…”

gem‐Selective Cross‐Dimerization and Cross‐Trimerization of Alkynes with Silylacetylenes Promoted by a Rhodium–Pyridine–N‐Heterocyclic Carbene Catalyst

“…Organic solvents were dried by using standard methods and distilled under Ar before use or obtained oxygen-and water-free with a Solvent Purification System (Innovative Technologies). The starting complexes [Rh(m-Cl)(IPr)(h 2 -coe)] 2 (1 a) [16] and 2…”