Chiral iridium(I) bis(NHC) complexes as catalysts for asymmetric transfer hydrogenation

The common use of NHC complexes in transition‐metal mediated C–C coupling and metathesis reactions in recent decades has established N‐heterocyclic carbenes as a new class of ligand for catalysis. The field of asymmetric catalysis with complexes bearing NHC‐containing chiral ligands is dominated by mixed carbene/oxazoline or carbene/phosphane chelating ligands. In contrast, applications of complexes with chiral, chelating bis(NHC) ligands are rare. In the present work new chiral iridium(I) bis(NHC) complexes and their application in the asymmetric transfer hydrogenation of ketones are described. A series of chiral bis(azolium) salts have been prepared following a synthetic pathway, starting from L ‐valinol and the modular buildup allows the structural variation of the ligand precursors. The iridium complexes were formed via a one‐pot transmetallation procedure. The prepared complexes were applied as catalysts in the asymmetric transfer hydrogenation of various prochiral ketones, affording the corresponding chiral alcohols in high yields and moderate to good enantioselectivities of up to 68%. The enantioselectivities of the catalysts were strongly affected by the various, terminal N‐substituents of the chelating bis(NHC) ligands. The results presented in this work indicate the potential of bis‐carbenes as stereodirecting ligands for asymmetric catalysis and are offering a base for further developments. Copyright © 2010 John Wiley & Sons, Ltd.     

Rhodium complexes with dioximes as catalysts of hydroformylation and hydrogenation of 1-hexene

Rhodium(II) complexes with dioximes [Rh(Hdmg)₂(PPh₃)]₂ [I] (Hdmg=monoanion of dimethylglyoxime) and [Rh(Hdmg)(ClZndmg)(PPh₃)]₂ [II] catalyse hydroformylation and hydrogenation reactions of 1-hexene at 1 MPa CO/H₂ and 0.5 MPa H₂ at 353 K, respectively. Hydroformylation with complex [I] produces 94% of aldehydes (n/iso=2.2) and 6% 2-hexene whereas the second catalyst [II] gives ca. 40% of aldehydes (n/iso=2.1) and 60% of 2-hexene. Corresponding Rh(III) complexes are inactive in hydroformylation except of RhH(Hdmg)₂(PPh₃) [III], which shows activity similar to [I]. Complexes [Rh(Hdmg)₂(PPh₃)]₂ [I], [Rh(Hdmg)(ClZndmg)(PPh₃)]₂ [II], RhH(Hdmg)₂(PPh₃) [III] and [Rh(Hdmg)₂(PPh₃)₂]ClO₄ [V] catalyse 1-hexene hydrogenation with an average TON ca. 18 cycles/mol [Rh]×min. Complex [II] has also been found to catalyse hydrogenation of cyclohexene, 1,3-cyclohexadiene and styrene.     

Synthesis and characterization of half-sandwich iridium(III) and rhodium(III) complexes bearing organochalcogen ligands

Reactions of [Cp*M(μ-Cl)Cl]₂ (M = Ir, Rh; Cp* = η⁵-pentamethylcyclopentadienyl) with bi- or tri-dentate organochalcogen ligands Mbit (L1), Mbpit (L2), Mbbit (L3) and [Tm^Me]⁻ (L4) (Mbit = 1,1′-methylenebis(3-methyl-imidazole-2-thione); Mbpit = 1,1′-methylene bis (3-iso-propyl-imidazole-2-thione), Mbbit = 1,1′-methylene bis (3-tert-butyl-imidazole-2-thione)) and [Tm^Me]⁻ (Tm^Me = tris (2-mercapto-1-methylimidazolyl) borate) result in the formation of the 18-electron half-sandwich complexes [Cp*M(Mbit)Cl]Cl (M = Ir, 1a; M = Rh, 1b), [Cp*M(Mbpit)Cl]Cl (M = Ir, 2a; M = Rh, 2b), [Cp*M(Mbbit)Cl]Cl (M = Ir, 3a; M = Rh, 3b) and [Cp*M(Tm^Me)]Cl (M = Ir, 4a; M = Rh, 4b), respectively. All complexes have been characterized by elemental analysis, NMR and IR spectra. The molecular structures of 1a, 2b and 4a have been determined by X-ray crystallography.     

Kinetics and mechanisms of homogeneous catalytic reactions: Part 7. Hydroformylation of 1-hexene catalyzed by cationic complexes of rhodium and iridium containing PPh3

Cationic rhodium and iridium complexes of the type [M(COD)(PPh₃)₂]PF₆ (M = Rh, 1a; Ir, 1b) are efficient precatalysts for the hydroformylation of 1-hexene to its corresponding aldehydes (heptanal and 2-methylhexanal), under mild pressures (2–5 bar) and temperatures (60 °C for Rh and 100 °C for Ir) in toluene solution; the linear to branched ratio (l/b) of the aldehydes in the hydroformylation reaction varies slightly (between 3.0 and 3.7 for Rh and close to 2 for Ir). Kinetic and mechanistic studies have been carried out using these cationic complexes as catalyst precursors. For both complexes, the reaction proceeds according to the rate law r_i = K₁K₂K₃k₄[M][olef][H₂][CO]/([CO]² + K₁[H₂][CO] + K₁K₂K₃[olef][H₂]). Both complexes react rapidly with CO to produce the corresponding tricarbonyl species [M(CO)₃(PPh₃)₂]PF₆, M = Rh, 2a; Ir, 2b, and with syn-gas to yield [MH₂(CO)₂(PPh₃)₂]PF₆, M = Rh, 3a; Ir, 3b, which originate by CO dissociation the species [MH₂(CO)(PPh₃)₂]PF₆ entering the corresponding catalytic cycle. All the experimental data are consistent with a general mechanism in which the transfer of the hydride to a coordinated olefin promoted by an entering CO molecule is the rate-determining step of the catalytic cycle.     

Rhodium phosphine complexes immobilized on silica as active catalysts for 1-hexene hydroformylation and arene hydrogenation

In immobilizing the rhodium complexes [Rh(acac)(CO)(P)] (1) and [Rh(acac)(P)₂] (2) (P = Ph₂PCH₂CH₂Si(OMe)₃) onto SiO₂, acetylacetone is found to be released through protonation of the acac ligand by the acidic silica-OH groups. The resulting complexes [Rh(O-{SiO₂}(HO-{SiO₂})(CO)(P-{SiO₂})] (1a) and [Rh(O-{SiO₂})(HO-{SiO₂})(P-{SiO₂})₂] (2a) were successfully tested with respect to their catalytic action on 1-hexene hydroformylation as well as benzene and toluene hydrogenation. The reaction outcome, viz. the formation of aldehydes versus isomerization, depends strongly on the presence and concentration of a phosphine co-catalyst. Thus, while 1a gave only a 17% yield of aldehyde in the absence of phosphines, the yield is increased to 54% in the presence of phosphinated silica P-{SiO₂} or even 94% if PPh₃ is added to the solution. Without extra added phosphine, both 1a and 2a effect mainly the isomerization of 1-hexene to 2-hexene. Pre-catalyst 1a catalyzes also the hydrogenation of benzene at 10.5 atm H₂ and 90 °C to give cyclohexane with a TOF of 608 h⁻¹.     

Trichlorostannyl complexes of iridium with both P-donor and N-donor ligands: Preparation and activity as hydrogenation catalysts

Bis(trichlorostannyl) complex IrH(SnCl₃)₂(PPh₃)₂ (1) was prepared by allowing the chloro-derivative IrHCl₂(PPh₃)₃ to react with SnCl₂·2H₂O in ethanol. Instead, treatment of phosphite complexes IrHCl₂P₃ [P = P(OEt)₃ and PPh(OEt)₂] with SnCl₂·2H₂O gave stannyl derivatives IrCl₂(SnCl₃)P₃ (2). Pyrazole-trichlorostannyl complexes IrHCl(SnCl₃)(HRpz)P₂ (3, 4) (R = H, 3-Me; P = PPh₃, PⁱPr₃) were prepared by allowing chloro-derivatives IrHCl₂(HRpz)P₂ to react with SnCl₂·2H₂O. 1,2-Bipyridine-trichlorostannyl complexes IrHCl(SnCl₃)(bpy)P (5) (P = PPh₃, PⁱPr₃) were also prepared. Complexes 1-5 were characterised spectroscopically (IR, ¹H, ³¹P, ¹¹⁹Sn NMR) and a geometry in solution was also established. The trichlorostannyl iridium complexes were evaluated as catalyst precursors for the hydrogenation of 2-cyclohexen-1-one and cinnamaldehyde. The influence of the stannyl group, as well as the steric hindrance of both N-donor and P-donor ligands in the catalytic activity of the complexes is discussed.     

Chiral bisphospholane ligands (Me-ketalphos): synthesis of their Rh(I) complexes and applications in asymmetric hydrogenation

Rhodium complexes of functionalized bisphospholane ligands (S,S,S,S-Me-ketalphos) 1 and (R,S,S,R-Me-ketalphos) 2 have been used as catalyst precursors for the asymmetric hydrogenation of several different types of functionalized olefins and have achieved high enantioselectivities.     

Half-sandwich rhodium (and iridium) complexes as enantioselective catalysts for the 1,3-dipolar cycloaddition of 3,4-dihydroisoquinoline N-oxide to methacrylonitrile

Cationic half-sandwich complexes containing the [(eta(5)-C(5)Me(5))M(Diphos*)] moiety (M=Rh, Ir; Diphos*=chiral diphosphine ligand) catalyze the cycloaddition of the nitrone 3,4-dihydroisoquinoline N-oxide (A) to methacrylonitrile (B) with excellent regio and endo selectivity and low-to-moderate enantioselectivity. The most active and selective catalyst, (S(Rh),R(C))-[(eta(5)-C(5)Me(5))Rh{(R)-Prophos)} (NC(Me)C==CH(2))](SbF(6))(2), has been isolated and fully characterized including the determination of the molecular structure by X-ray diffraction. The R-at-metal epimers of the complexes [(eta(5)-C(5)Me(5))M{(R)-Prophos)}(NC(Me)C==CH(2))](SbF(6))(2) (M=Rh, Ir) isomerize to the corresponding S-at-metal diastereomers. The stoichiometric cycloaddition of A with B is catalyzed by diastereopure (S(M),R(C))-[(eta(5)-C(5)Me(5))M{(R)-Prophos)}(NC(Me)C==CH(2))](SbF(6))(2) with perfect regio and endo selectivity and very good (up to 95 %) ee. The catalyst can be recycled up to nine times without significant loss of either activity or selectivity.     

Synthesis and reactivity of substituted cyclopentadienyl rhodium(I) and (III) complexes

New cyclopentadienyl derivatives of rhodium COD complexes [Cp*=C₅H₄COOCH₂CHCH₂ (1); C₅H₄CH₂CH₂CHCH₂ (2); C₅H(i-C₃H₇)₄ (3)] and carbonyl complex [Cp*=C₅H(i-C₃H₇)₄ (4)] were synthesized from [RhCl(COD)]₂ and [RhCl(CO)₂]₂. 1, 2 and 3 oxidized by iodine gave iodine bridged dimers 5, 6 and 7, respectively. Triphenyl phosphine, carbon monoxide and carbon disulfide molecules broke down the iodine bridged structure easily and produced monomer products Cp*RhI₂L [Cp*=C₅H₄COOCH₂CHCH₂, L=CS₂ (8); L=PPh₃ (9). Cp*=C₅H(i-C₃H₇)₄, L=CO (10)]. All of these new compounds were characterized by elemental analysis, ¹H NMR, IR, UV-Vis and mass spectroscopy. The crystal structure of 1 was solved in the triclinic space group with one molecule in the unit cell, the dimensions of which are a=7.082(9) Å, b=8.392(3) Å, c=13.889(5) Å, α=101.19(3)°, β=99.06(6)°, γ=105.11(5)°, and V=763(1) ų. The crystal structure of 3 was solved in the orthorhombic space group Pn21a with four molecules in the unit cell, the dimensions of which are a=9.748(3) Å, b=16.054(5) Å, and V=2319(1) ų. Least squares refinement leads to values for the conventional R₁ of 0.0251 for 1 and 0.0558 for 3, respectively. Compared to that in 1, a shorter metal-ligand bond length in 3 was observed and this is attributed to the rich electron density on Rh(I) metal center piled up by the C₅H(i-C₃H₇)₄ ligand.     

Study of incidence of DiPAMP ligand modification on the rhodium(I)-catalyzed asymmetric hydrogenation of α-acetamidostyrene

A series of P-stereogenic enantiopure 1,2-bis[(aryl)(phenyl)phosphino]ethane ligands was prepared through an extensive systematic incorporation of various substituents onto the P-o-anisyl rings of Knowles’ DiPAMP (DiPAMP = 1,2-bis[(o-anisyl)(phenyl)phosphino]ethane). The study of incidence of such modification on the Rh(I)-catalyzed hydrogenation of α-acetamidostyrene is reported revealing that substitution on position 3 is detrimental, while it is beneficial on position 5. Namely, a 2.5-fold increased catalyst activity coupled with a higher enantioselectivity (90% ee) was attained with the P-(2-MeO-3-naphthyl)-substituted ligand under mild conditions (1 bar H₂, rt in MeOH).     

Catalytic hydrogenation of CO and CN bonds via heterolysis of H2 mediated by metal-sulfur bonds of rhodium and iridium thiolate complexes

Coordinatively unsaturated rhodium and iridium complexes having a bulky thiolate, [Cp∗M(PMe₃)(SDmp)](BAr^F₄) (1a: M = Rh; 1b: M = Ir; Dmp = 2,6-(mesityl)₂C₆H₃, Ar^F = 3,5-(CF₃)₂C₆H₃), catalyzed the hydrogenation of benzaldehyde, N-benzylideneaniline, and cyclohexanone, under 1 atm of H₂ at low temperatures. In these catalytic reactions, the M-H/S-H complexes [Cp∗M(PMe₃)(H)(HSDmp)](BAr^F₄) (2a: M = Rh; 2b: M = Ir) generated via H₂ heterolysis by 1a or 1b were suggested to transfer both M-H hydride and S-H proton to substrates. The catalytic reactions were terminated by the dissociation of H-SDmp from the metal centers of 2a and 2b that occurs at ambient temperature under H₂ atmosphere.     

铱(I)联萘胺Schiff碱(BPMBNDI)配合物催化苯乙酮的不对称氢转移反应

用旋光活性2, 2'-(1, 1'-联萘)二胺和2-吡啶基甲醛缩合得到的Schiff碱BPMBNDI[N, N'-二(2-吡啶基亚甲基)-(1, 1'-联萘)-2, 2'-二亚胺]为配体与[Ir(COD)Cl]2(COD=1, 5-环辛二烯)反应, 生成了10个光学活性铱配合物。研究它们在异丙醇对苯乙酮不对称氢转移反应中的光学诱导活性时, 发现10个催化剂均具有较高的立体选择性,其中[Ir(COD)(BPMBNDI)I]催化的光学产率高达84%。     

Cationic rhodium(I)–diolefin complexes containing an optically active C2-symmetric bis(sulfoximine) ligand: Synthesis and catalytic activity

A series of cationic rhodium(I) complexes [Rh(diene)(N^N)][BF₄] (diene = 1,5-cyclooctadiene (cod), norbornadiene (nbd), tetrafluorobenzobarralene (tfb)), containing the optically pure bis(sulfoximine) ligand 1,2-bis(S-methyl-S-phenylsulfonimidoyl)benzene, have been synthesized and fully characterized. The structure of the R,R enantiomer of the ligand, and that of its cyclooctadiene–Rh(I) complex, were confirmed by means of single-crystal X-ray diffraction techniques. Studies on the catalytic activity of these complexes in acetophenone hydrosilylation and dimethyl itaconate hydrogenation are also reported.     

Asymmetric transfer hydrogenation of ketones catalyzed by rhodium and iridium complexes with chiral bidentate Schiff's bases

Rhodium and iridium complexes of Schiff's bases derived from (1R,2R)- and (1S,2S)-diaminocyclohexane catalyze asymmetric transfer hydrogenation of alkyl aryl ketones in PrⁱOH at room temperature to give chiral secondary alcohols (up to 65% ee).     

Synthesis and DNA-binding properties of apoptosis-inducing cytotoxic half-sandwich rhodium(III) complexes with methyl-substituted polypyridyl ligands

Half-sandwich organorhodium(III) complexes of the type [(η⁵-C₅Me₅)RhCl(pp)] (CF₃SO₃) containing polypyridyl ligands (pp) represent a promising class of cytostatic agents. Replacement of the polypyridyl ligands of complexes 1 (pp = phen) and 6 (pp = dppz) by methyl-substituted derivatives in 2-5 (pp = 4-Mephen, 5-Mephen, 4,7-Me₂phen, 5,6-Me₂phen) and 7 (pp = Me₂dppz) leads to a significant improvement in their antiproliferative activity towards human MCF-7 and HT-29 cancer cells. For instance, the IC₅₀ value towards HT-29 cells decreases from 4.3 ± 0.2 μM for 6 to 0.98 ± 0.49 μM for complex 7. In contrast, no activity (IC₅₀ > 100 μM) was observed for the HOOC and n-BuNHCO substituted dppz complexes 8 and 9. UV/vis, CD and NMR spectra for mixtures of complexes 7-9 with CT DNA were in accordance with intercalation of the substituted dppz ligands between the base pairs of the double helix and direct evidence for this binding mode was also provided by a 2D NOESY study for complex 7 with the hexanucleotide d(5′-CGTCGG-3′). Each of the methyl-substituted phen complexes 2-5 is significantly more active towards immortalized HEK-293 cells (IC₅₀ values 0.40 ± 0.02 to 0.94 ± 0.02 μM) than towards the cancer cells. Flow cytometric measurements of DNA fragmentation in BJAB cells following an incubation period of 72 h with 1, 5 and 6 indicate that the complexes induce specific apoptotic cell death in the non-adherent lymphoma cells.     

Voltammetric Sensing of Mercury and Copper Cations at Poly(EDTA‐like) Film Modified Electrode

Complexing polymer‐coated electrodes have been synthesized by oxidative electropolymerization of ethylenediamine tetra‐N‐(3‐pyrrole‐1‐yl)propylacetamide (monomer L ). The presence of four polymerizable pyrrole fragments on the same EDTA skeleton was thought to confer enhanced rigidity and controlled dimensionality to the resulting complexing materials, which were used for the electrochemical detection of Hg(II), Cu(II), Pb(II) and Cd(II) ions by means of the chemical preconcentration‐anodic stripping technique. The polyamide electrode material showed particularly a significant selectivity towards mercury ions, even in the presence of a large excess of other metal cations. Moreover, the use of imprinted polymer‐coated electrodes prepared by electropolymerization of L in the presence of metal cations turned out to significantly improve the detection limits, down to 5×10^?10 mol L^?1 for Hg(II) and Cu(II) species.     

Pentamethylcyclopentadienyl rhodium(III) trifluorovinyl phosphine complexes and attempted intramolecular dehydrofluorinative coupling of pentamethylcyclopentadienyl and trifluorovinyl phosphine ligands

The trifluorovinyl phosphine complexes [Cp*RhCl₂{PR_3−x(CFCF₂)_x}] (1x = 1, a R = Ph, b Prⁱ, c Et; 2x = 2, R = Ph) have been prepared by treatment of [Cp*RhCl(μ-Cl)]₂ with the relevant phosphine. The salt [Cp*RhCl(CNBu^t){PPh₂(CFCF₂)}]BF₄, 3, was prepared by addition of Bu^tNC to 1a in the presence of NaBF₄. The salt [Cp*RhCl{κP,κS-(CF₂CF)PPh(C₆H₄SMe-2)}]BF₄ was prepared as a mixture of cis (5a) and trans (5b) isomers by treatment of [Cp*RhCl(μ-Cl)]₂ with the phosphine-thioether (CF₂CF)PPh(C₆H₄SMe-2), 4, in the presence of NaBF₄. The structures of 1a-c and 5a have been determined by single-crystal X-ray diffraction. Intramolecular dehydrofluorinative carbon-carbon coupling between pentamethylcyclopentadienyl and trifluorovinylphosphine ligands of 1a, 3 and 5 has been attempted. No reaction was observed on treatment of the neutral complex [Cp*RhCl₂{PPh₂(CFCF₂)}], 1a, with proton sponge, however, 5a underwent dehydrofluorinative coupling to yield [{η⁵,κP,κS-(C₅Me₄CH₂CFCF)PPh(C₆H₄SMe-2)}RhCl]BF₄, 6. Other reactions, in particular addition of HF across the vinyl bonds of 5, occurred leading to a mixture of products. The cation of 3 underwent similar reactions.     

Zwitterionic iridium complexes with P,N-ligands as catalysts for the asymmetric hydrogenation of alkenes

Several zwitterionic iridium complexes based on chiral P,N-ligands with imidazoline or oxazoline donors and anionic tetraarylborate or aryltrifluoroborate substituents have been synthesized. The corresponding cationic analogues have also been prepared, to evaluate the effect of the covalent linkage between the anion and the cationic metal complex in catalytic reactions. The respective pairs of structurally analogous precatalysts have been compared for their efficacies in the asymmetric hydrogenation of unfunctionalized olefins. In most cases, the anionic derivatization has virtually no influence on the asymmetric induction of the iridium complex. This is in accordance with X-ray structural studies, which have shown that the chiral environment of the cationic metal center is not affected by the anionic substituent. Depending on the nature of the counterion employed, the zwitterionic catalysts proved to be significantly more reactive than their cationic counterparts in nonpolar solvents.     

Synthesis and luminescence of poly(phenylacetylene)s with pendant iridium complexes and carbazole groups

Poly(phenylacetylene)s containing pendant phosphorescent iridium complexes have been synthesized and their electrochemical, photo‐ and electroluminescent properties studied. The polymers have been synthesized by rhodium‐catalyzed copolymerization of 9‐(4‐ethynylphenyl)carbazole (CzPA) and phenylacetylenes (C∧N)₂Ir(κ²‐O,O′‐MeC(O)CHC(O)C₆H₄C?CH‐4) (C∧N = κ²‐N,C¹‐2‐(pyridin‐2‐yl)phenyl (Ir^ppyPA) or κ²‐N,C¹‐2‐(isoquinolin‐1‐yl)phenyl (Ir^piqPA)). In addition, organic poly(phenylacetylene)s with pendant carbazole groups have been synthesized by rhodium‐catalyzed copolymerization of CzPA and 1‐ethynyl‐4‐pentylbenzene. Complex (C∧N)₂Ir(κ²‐O,O′‐MeC(O)CHC(O)Ph) (Ir^piqPh; C∧N = 2‐(isoquinolin‐1‐yl)phenyl‐κ²‐N,C¹) was prepared and characterized. While the copolymers of the Ir^ppy series were weakly phosphorescent, those of the Ir^piq series displayed at room temperature intense emissions from the carbazole (fluorescence) and iridium (phosphorescence) emitters, being the latter dominant when the spectra were recorded using polymer films. Triple layer OLED devices employing copolymers of the Ir^piq series or the model complex Ir^piqPh yielded electroluminescence with an emission spectra originating from the iridium complex and maximum external quantum efficiencies of 0.46% and 2.99%, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3744–3757, 2010