N-heterocyclic Carbene Complexes Of Rhodium Research Articles

Rhodium Indenyl NHC and Fluorenyl-Tethered NHC Half-Sandwich Complexes: Synthesis, Structures and Applications in the Catalytic C-H Borylation of Arenes and Alkanes.

Indenyl (Ind) rhodium N‐heterocyclic carbene (NHC) complexes [Rh(η 5‐Ind)(NHC)(L)] were synthesised for 1,3‐bis(2,6‐diisopropylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene (SIPr) with L=C2H4 (1), CO (2 a) and cyclooctene (COE; 3), for 1,3‐bis(2,4,6‐trimethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene (SIMes) with L=CO (2 b) and COE (4), and 1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes) with L=CO (2 c) and COE (5). Reaction of SIPr with [Rh(Cp*)(C2H4)2] did not give the desired SIPr complex, thus demonstrating the “indenyl effect” in the synthesis of 1. Oxidative addition of HSi(OEt)3 to 3 proceeded under mild conditions to give the Rh silyl hydride complex [Rh(Ind){Si(OEt)3}(H)(SIPr)] (6) with loss of COE. Tethered‐fluorenyl NHC rhodium complexes [Rh{(η 5‐C13H8)C2H4N(C)C2H x NR}(L)] (x=4, R=Dipp, L=C2H4: 11; L=COE: 12; L=CO: 13; R=Mes, L=COE: 14; L=CO: 15; x=2, R=Me, L=COE: 16; L=CO: 17) were synthesised in low yields (5–31 %) in comparison to good yields for the monodentate complexes (49–79 %). Compounds 3 and 1, which contain labile alkene ligands, were successful catalysts for the catalytic borylation of benzene with B2pin2 (Bpin=pinacolboronate, 97 and 93 % PhBpin respectively with 5 mol % catalyst, 24 h, 80 °C), with SIPr giving a more active catalyst than SIMes or IMes. Fluorenyl‐tethered NHC complexes were much less active as borylation catalysts, and the carbonyl complexes were inactive. The borylation of toluene, biphenyl, anisole and diphenyl ether proceeded to give meta substitutions as the major product, with smaller amounts of para substitution and almost no ortho product. The borylation of octane and decane with B2pin2 at 120 and 140 °C, respectively, was monitored by 11B NMR spectroscopy, which showed high conversions into octyl and decylBpin over 4–7 days, thus demonstrating catalysed sp3 C−H borylation with new piano stool rhodium indenyl complexes. Irradiation of the monodentate complexes with 400 or 420 nm light confirmed the ready dissociation of C2H4 and COE ligands, whereas CO complexes were inert. Evidence for C−H bond activation in the alkyl groups of the NHC ligands was obtained.

Ring-expanded iridium and rhodium N-heterocyclic carbene complexes: a comparative DFT study of heterocycle ring size and metal center diversity

A new series of ring-expanded six- and seven-membered N-heterocyclic carbene precursors (re-NHCs) and their transition metal complexes were synthesized. The basic properties of the synthesized materials were investigated by density functional theory (DFT). The six- and seven-membered re-NHCs were synthesized in good yield via reaction of the corresponding alkyldibromides or alkyldiiodides with N,N′-bis-(2-phenylbenzene)formamidine in the presence of K2CO3 under aerobic conditions. Complexes, represented by the formula [ML1,2(COD)Cl] (where M = Ir or Rh and L is a ring-expanded N-heterocyclic carbene ligand), were synthesized in the presence of the corresponding free carbene and iridium or rhodium metal precursors in tetrahydrofuran. All new re-NHC complexes were characterized by different analytical techniques, including NMR spectroscopy, X-ray diffraction, UV spectroscopy and elemental analysis. According to molecular electrostatic potential calculations, the electrophilic properties of the complexes were aligned, from highest to lowest, as Ir-6-DiPh, Rh-6-DiPh and Ir-7-DiPh. The HOMO, LUMO and energy gaps of the complexes were calculated by DFT. On the basis of the DFT analysis, it can be predicted that Rh-6-DiPh is the most stable complex and Ir-7-DiPh is more reactive than Ir-6-DiPh.

Synthesis of New Chiral Benzimidazolylidene–Rh Complexes and Their Application in Asymmetric Addition Reactions of Organoboronic Acids to Aldehydes

A series of novel chiral N-heterocyclic carbene rhodium complexes (NHC–Rh) based on benzimidazole have been prepared, and all of the NHC–Rh complexes were fully characterized by NMR and mass spectrometry. These complexes could be used as catalysts for the asymmetric 1,2-addition of organoboronic acids to aldehydes, affording chiral diarylmethanols with high yields and moderate enantioselectivities.

Enhancement of N-heterocyclic carbenes on rhodium catalyzed olefination of triazoles.

A few rhodium complexes of N-heterocyclic carbenes were prepared through carbene transfer reactions and their structures were characterized by X-ray diffraction analysis. The rhodium complexes of NHCs are found to be efficient catalysts for vinylation of various triazoles via C-H activation. A number of double vinylated triazoles can be obtained in good yields.

Highly selective directed arylation reactions via back-to-back dehydrogenative C–H borylation/arylation reactions

Dimeric rhodium N-heterocyclic carbene complexes are demonstrated to be effective catalyst precursors for directed C-H borylation reactions at room temperature. The reactions are highly selective for mono-borylation and can be combined with a one-pot Suzuki-Miyaura coupling to give C-H arylation products with exclusive selectivity for mono-arylation without the requirement for steric blocking groups.

Radioiodinated and astatinated NHC rhodium complexes: Synthesis

IntroductionThe clinical development of radioimmunotherapy with astatine-211 is limited by the lack of a stable radiolabeling method for antibody fragments. An astatinated N-heterocyclic carbene (NHC) Rhodium complex was assessed for the improvement of radiolabeling methodologies with astatine. MethodsWet harvested astatine-211 in diisopropyl ether was used. Astatine was first reduced with cysteine then was reacted with a chlorinated Rh-NHC precursor to allow the formation of the astatinated analogue. Reaction conditions have been optimized. Astatine and iodine reactivity were also compared. Serum stability of the astatinated complex has been evaluated. ResultsQuantitative formation of astatide was observed when cysteine amounts higher than 46.2 nmol/μl of astatine solution were added. Nucleophilic substitution kinetics showed that high radiolabeling yields were obtained within 15 min at 60°C (88%) or within 5 min at 100°C (95%). Chromatographic characteristics of this new astatinated compound have been correlated with the cold iodinated analog ones. The radioiodinated complex was also synthesized from the same precursor (5 min. at 100°C, up to 85%) using [125I]NaI as a radiotracer. In vitro stability of the astatinated complex was controlled after 15 h incubation in human serum at 4°C and 37°C. No degradation was observed, indicating the good chemical and enzymatic stability. ConclusionThe astatinated complex was obtained in good yield and exhibited good chemical and enzymatic stability. These preliminary results demonstrate the interest of this new radiolabeling methodology, and further functionalizations should open new possibilities in astatine chemistry. Advances in knowledge and implications for patient careAlthough there are many steps and pitfalls before clinical use for a new prosthetic group from the family of NHC complexes, this work may open a new path for astatine-211 targeting.

Hydroxo–Rhodium–N-Heterocyclic Carbene Complexes as Efficient Catalyst Precursors for Alkyne Hydrothiolation

The new Rh–hydroxo dinuclear complexes stabilized by an N-heterocyclic carbene (NHC) ligand of type [Rh(μ-OH)(NHC)(η2-olefin)]2 (coe, IPr (3), IMes (4); ethylene, IPr (5)) are efficient catalyst precursors for alkyne hydrothiolation under mild conditions, presenting high selectivity toward α-vinyl sulfides for a varied set of substrates, which is enhanced by pyridine addition. The structure of complex 3 has been determined by X-ray diffraction analysis. Several intermediates relevant for the catalytic process have been identified, including RhI-thiolato species Rh(SCH2Ph)(IPr)(η2-coe)(py) (6) and Rh(SCH2Ph)(IPr)(η2-HC≡CCH2Ph)(py) (7), and the RhIII-hydride-dithiolato derivative RhH(SCH2Ph)2(IPr)(py) (8) as the catalytically active species. Computational DFT studies reveal an operational mechanism consisting of sequential thiol deprotonation by the hydroxo ligand, subsequent S–H oxidative addition, alkyne insertion, and reductive elimination. The insertion step is rate-limiting with a 1,2 thiometalation of the alkyne as the more favorable pathway in accordance with the observed Markovnikov-type selectivity.

Dioxygen adducts of rhodium N-heterocyclic carbene complexes

Rhodium complexes functionalized by N-heterocyclic carbene ligands react with dioxygen to form adducts. Depending on the specifics of the ancillary ligands, oxygen binds to Rh either as a peroxide to form a fully oxidized Rh(III) complex, or as singlet dioxygen in a Rh(I) square planar complex. We have shown through analysis of a series of compounds, some previously published and some novel, that the presence of additional ligands that would support the formation of an octahedral geometry, as typically found with Rh(III) complexes, is critical for formation of the peroxide. In addition, we have demonstrated through DFT studies, that the potential energy surface with regard to the O-O bond length is relatively shallow, which provides a rationale for the distribution of bond lengths observed for apparently similar complexes analyzed by crystallography.

N-alkyl functionalised expanded ring N-heterocyclic carbene complexes of rhodium(i) and iridium(i): structural investigations and preliminary catalytic evaluation

A series of new N-alkyl functionalised 6- and 7-membered expanded ring N-heterocyclic carbene (NHC) pro-ligands 3-6 and their corresponding complexes of rhodium(I) and iridium(I), [M(NHC)(COD)Cl] 7-14 and [M(NHC)(CO)2Cl] 15-22 are described. The complexes have been characterised by (1)H and (13)C{(1)H} NMR, mass spectrometry, IR and X-ray diffraction. It is noted from X-ray diffraction studies that the N-alkyl substituents are found to orientate themselves away from the metal centre due to unfavourable steric interactions resulting in low percent buried volume (%V(bur)) values in the solid state. The heterocycle ring size is also found to dictate the spatial orientation of the N-alkyl substituents in the neopentyl functionalised derivatives 10 and 14. The 7-membered derivative 14 allows for a conformational 'twist' of the heterocycle ring with the N-alkyl substituents adopting a mutually trans configuration with respect to each other, while the more rigid 6-membered system 10 does not allow for this conformational 'twist' and consequently the N-alkyl substituents adopt a mutually cis configuration. The σ-donor function of this new class of expanded ring NHC ligand has also been probed by measured IR stretching frequencies of the [M(NHC)(CO)2Cl] complexes 15-22. A preliminary catalytic survey of the hydrogenation of functionalised alkenes with molecular hydrogen under mild conditions has also been undertaken with complex , affording an insight into the application of large ring NHC ancillary ligands bearing N-alkyl substituents in hydrogenation transformations.

Ring-Expanded N-Heterocyclic Carbene Complexes of Rhodium with Bifluoride, Fluoride, and Fluoroaryl Ligands

Thermolysis of Rh(PPh3)4H in the presence of the six-membered N-heterocyclic carbene 1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidine (6-iPr) gave the monocarbene complex Rh(6-iPr)(PPh3)2H as a 1:2 mixture of the cis- and trans-phosphine isomers 1a and 1b. This same isomeric mixture was formed as the ultimate product from treating Rh(PPh3)3(CO)H with 6-iPr at room temperature, although pathways involving both CO and PPh3 loss were observed at initial times. Treatment of 1a/1b with Et3N·3HF generated the bifluoride complex cis-Rh(6-iPr)(PPh3)2(FHF) (2a), which upon stirring with anhydrous Me4NF was converted to the rhodium fluoride complex cis-Rh(6-iPr)(PPh3)2F (3a). Thermolysis of 1a/1b with C6F6 resulted in C–F bond activation to afford a mixture of 3a and the pentafluorophenyl complex trans-Rh(6-iPr)(PPh3)2(C6F5) (5b). Complexes 1b, 2a, 3a, and 5b were structurally characterized.

Double CH Bond Activation of C(sp3)H2 Groups for the Preparation of Complexes with Back‐to‐Back Bisimidazolinylidenes

Two-headed carbenes: N-heterocyclic carbene complexes of rhodium and iridium were obtained by the double CH activation of CH2 groups in N heterocycles (see structure: C gray, N blue, Cl green, Ir pink). The reaction constitutes a valuable approach to new carbene ligands with unprecedented architectures.

Synthesis and characterization of chiral mono N-heterocyclic carbene-substituted rhodium complexes and their catalytic properties in hydrosilylation reactions

Different chiral mono-substituted N-heterocyclic carbene complexes of rhodium were prepared, starting from [Rh(COD)Cl] 2 (COD = cyclooctadiene) by addition of free N-heterocyclic carbenes (NHC), or an in-situ deprotonation of the corresponding iminium salt. All new complexes were characterized by spectroscopy methods. In addition, the structures of chloro( η 4-1,5-cyclooctadiene)(1,3-di-[(1 R,2 R,3 R,5 S)-2,6,6-trimethylbicyclo[3.1.1]hept-3-yl] imidazolin-2-ylidene)rhodium(I) ( 5a), chloro( η 4-1,5-cyclooctadiene)(1,3-di-[(1 R,2 S,5 R)-2-isopropyl-5-menthylcyclohex-1-yl]imidazol-2-ylidene)rhodium(I) ( 5b) and chloro( η 4-1,5-cyclooctadiene)(1,3-di-[(2 R,4 S,5 S)-2-methyl-4-phenyl-1,3-dioxacyclohex-5-yl]imidazolin-2-ylidene)rhodium(I) ( 5i) were analyzed by DFT-calculations. The enantioselective hydrosilylation of acetophenone, ethylpyruvate and n-propylpyruvate with diphenylsilane and hydrolysis was carried out with chiral C 2-symmetrical mono-substituted N-heterocyclic carbene rhodium complexes giving for the first time an enantioselective excess of up to 74% ee in the case of the n-propylpyruvate.

A simple procedure for the polymer-supported N-heterocyclic carbene–rhodium complex via click chemistry: a recyclable catalyst for the addition of arylboronic acids to aldehydes

A novel polymer-supported carbene-rhodium complex was prepared using a simple procedure via click chemistry. This polymer-supported N-heterocyclic carbene-rhodium complex was characterized and used as a catalyst for the addition of arylboronic acids to aldehydes in good to excellent yields.

Synthesis of rhodium N-heterocyclic carbene complexes and their catalytic activity in the hydrosilylation of alkenes in ionic liquid medium

Rhodium complexes bearing N-heterocyclic carbene (NHC) ligands were prepared from bis( η 4-1,5-cyclooctadiene) dichlorodirhodium and 1-alkyl-3-methylimidazolium-2-carboxylate, and the catalytic properties of rhodium complexes prepared in the hydrosilylation of alkenes in ionic liquid media were investigated. It was found that both the catalytic activity and selectivity of the rhodium complexes bearing NHC ligands were influenced by the attached substituents of the imidazolium cation. Additionally, rhodium complexes bearing NHC ligands in ionic liquid BMimPF 6 could be reused without noticeable loss of catalytic activity and selectivity.

A Rhodium IBiox[(−)-menthyl] Complex as a Highly Selective Catalyst for the Asymmetric Hydroarylation of Azabicyles: An Alternative Route to Epibatidine

The synthesis and characterization of a new chiral rhodium N-heterocyclic carbene complex, Rh(IBiox[(-)-menthyl](CO)(2)Cl, is reported. In addition, the very high enantioselectivity exhibited by this complex, as a catalyst for the asymmetric hydroarylation of azabicycles, is demonstrated and applied to the synthesis of epibatidine.

Rhodium N-Heterocyclic Carbene Complexes as Effective Catalysts for [2+2+2]-Cycloaddition Reactions

Rhodium complexes of N-heterocyclic carbenes (NHC) [RhCl(cod)(NHC)], [cod = 1,5-cyclooctadiene, NHC = 1,3-diisopropylimidazolin-2-ylidene, and NHC = 1,3-dimesitylimidazolin-2-ylidene] are tested for intra- and partially intramolecular [2+2+2]-cycloaddition reactions demonstrating their effectiveness in this kind of process. This is the first use of a rhodium-NHC complex in a [2+2+2]-cycloaddition reaction of three alkynes. ESI-MS has been used to detect oxidative addition intermediates using rhodium NHC as the catalytic system.

The synthesis and X-ray structure of a chiral rhodium–NHC complex: Implications for the use of NHCs in asymmetric hydroformylation catalysis

An optically active N-heterocyclic carbene (NHC) has been metallated with [RhCl(COD)] 2 to give a rhodium(I) monocarbene substituted complex. This complex has been fully characterized by 1H and 13C NMR spectroscopy and by single-crystal X-ray analysis. Preliminary tests of hydroformylation of styrene catalyzed with this compound have shown that branched to linear aldehyde ratio was rather low 0.67 (40%) and that the enantiomeric excess measured on the corresponding alcohols was only 10%. In the presence of an excess of triphenylphosphine, the ratio of branched to linear aldehyde increased markedly (91%) whereas the enantioselectivity of the reaction remained almost unchanged (12.5%).

Supported N-heterocyclic carbene rhodium complexes as highly selective hydroformylation catalysts

New supported rhodium(I) catalyst precursors were obtained in reaction of [Rh(OMe)(cod)] 2 with polymeric monoliths containing methyl-imidazolium moieties and applied in 1-hexene hydroformylation at 10 atm of CO/H 2. Under solventless conditions with a small amount of P(OPh) 3 as a modifying ligand, [P(OPh) 3]/[Rh] = 2, ca. 80% of aldehydes with a high n/iso ratio of ca. 6 was obtained in eight successive runs. When P(OCH 2CF 3) 3 was used instead of P(OPh) 3, n/iso ratios up to 49.8 were obtained, however, the systems were active only in two subsequent reactions. With PPh 3 as a modifying ligand, the yield of aldehydes decreased to ca. 20% already in the third run and n/iso was ca. 3.

Quinoline-Tethered N-Heterocyclic Carbene Complexes of Rhodium and Iridium: Synthesis, Catalysis, and Electrochemical Properties

A series of rhodium and iridium complexes of quinoline-tethered hemilabile N-heterocyclic carbenes (NHC∧N) have been synthesized via either deprotonation of imidazolium salts or silver transmetalation. Deprotonation of imidazolium ions by tBuOK in the presence of [Rh(COD)Cl]2 (COD = 1,5-cyclooctadiene) afforded both chelating [Rh(COD)(NHC∧N)]+ and monodentate [Rh(COD)(NHC)2]+ complexes, while only the chelating carbene complexes were obtained for the iridium analogues. Silver transmetalation of this type of carbene to [M(COD)Cl]2 (M = Rh, Ir) consistently afforded M(NHC)(COD)Cl, maintaining the metal chloride and pendant quinoline entity. Carbene transmetalation to (Cp∗IrCl2)2 gave an equilibrium mixture of neutral Cp*Ir(NHC)Cl2 and ionic [Cp*Ir(NHC∧N)Cl]Cl. All these rhodium and iridium cyclooctadiene complexes can undergo one-electron oxidation in cyclic voltammetry. The variable-scan-rate cyclic voltammetry experiments indicate that these compounds undergo slow heterogeneous electron transfer and that the oxidized forms are relatively short-lived. A neutral Rh(COD)(NHC)Cl complex proved to be active in catalyzing the [3 + 2] cycloaddition reactions of diphenylcyclopropenone and internal alkynes. Crystal structures of metal complexes in each category have been reported.

N-Heterocyclic carbene complexes of rhodium: structure, stability and reactivity

The organometallic chemistry of N-heterocyclic carbene complexes of the transition metals has received significant attention over the past ten years, especially with respect to complexes of Pd and Ru. The present Perspective highlights the chemistry of NHC complexes of another important transition metal, Rh. The application of Rh-NHC complexes in the industrially significant reactions of hydrogenation and hydroformylation is described. In order to accurately assess the activity of these complexes, their stability must be carefully determined. Various synthetic routes to Rh-NHC complexes are also described.