Rhodium Systems Research Articles

Interaction of a Bulky N-Heterocyclic Carbene Ligand with Rh(I) and Ir(I). Double C−H Activation and Isolation of Bare 14-Electron Rh(III) and Ir(III) Complexes

Reactivity and structural studies of unusual rhodium and iridium systems bearing two N-heterocyclic carbene (NHC) ligands are presented. These systems are capable of intramolecular C-H bond activation and lead to coordinatively unsaturated 16-electron complexes. The resulting complexes can be further unsaturated by simple halide abstraction, leading to 14-electron species bearing an all-carbon environment. Saturation of the vacant sites in the 16- and 14-electron complexes with carbon monoxide permits a structural comparison. DFT calculations show that these electrophilic metal centers are stabilized by pi-donation of the NHC ligands.

Luminescent cyclometallated rhodium(iii) bis(pyridylbenzaldehyde) complexes with long-lived excited states

A series of luminescent cyclometallated rhodium(III) diimine complexes containing two aldehyde functional groups [Rh(pba)2(N–N)]Cl (Hpba = 4-(2-pyridyl)benzaldehyde; N–N = 2,2′-bipyridine, bpy (2); 4,4′-dimethyl-2,2′-bipyridine, 4,4′-Me2bpy (3); 1,10-phenanthroline, phen (4); 3,4,7,8-tetramethyl-1,10-phenanthroline, 3,4,7,8-Me4phen (5); 4,7-diphenyl-1,10-phenanthroline, 4,7-Ph2phen (6)) have been synthesised, and their photophysical and electrochemical properties investigated. The X-ray crystal structures of complexes 3, 4 and the precursor complex, [Rh2(pba)4Cl2] (1), have also been determined. Upon photoexcitation, complexes 2–6 display long-lived emission in solutions at 298 K and in low-temperature glass. Remarkably, the luminescence lifetimes of the complexes in solutions at 298 K are extraordinarily long (ca. 4.2–8.7 µs). To the best of our knowledge, there is no precedent for such long emission lifetimes observed in other related cyclometallated rhodium(III) systems. The solution emission spectra show structured bands with emission maxima at ca. 506 nm. The emission is tentatively assigned to an excited state of triplet intra-ligand 3IL (π → π*)(pba−) character, probably mixed with some triplet metal-to-ligand charge-transfer 3MLCT (dπ(Rh) → π*(pba−)) character. On the basis of the facile reaction between the aldehyde group and the primary amine group, to form a secondary amine after reductive amination, complexes 2–6 have been used to label the protein bovine serum albumin. The photophysical properties of the bioconjugates have also been investigated.

Synthesis of large chelate rings with diphosphites built on a cyclodextrin scaffold. Unexpected formation of 1,2-phenylene-capped α-cyclodextrins

Two primary face difunctionalised α-cyclodextrins (α-CDs) bearing AC and AD-positioned triarylphosphite ligands have been synthesised and their ability to form large chelate rings has been evaluated. 6 A,6 D-Bis- O-{2-〚(diphenoxyphosphino)oxy〛phenyl}-2 A,2 B,2 C,2 D,2 E,2 F,3 A,3 B,3 C,3 D,3 E,3 F,6 B,6 C,6 E,6 F-hexadeca- O-methyl-α-cyclodextrin ( L1) was prepared in three steps in 59% overall yield: ( i) treatment of 6 A,6 D-di- O-(methylsulfonyl)-2 A,2 B,2 C,2 D,2 E,2 F,3 A,3 B,3 C,3 D,3 E,3 F,6 B,6 C,6 E,6 F-hexadeca- O-methyl-α-cyclodextrin ( 1a) with sodium 2-(benzyloxy)phenolate, affording a mixture of the corresponding bisaryloxy substitution product 2a; ( ii) cleavage of the benzyl groups of 2a with formation of bisphenol 6a; ( iii) reaction of 2a with chlorodiphenylphosphite to afford L1. Diphosphite L2 was obtained in a similar manner using the AC dimesylate 1b (yield: 54%). In step ( i) of each synthesis, the unexpected formation of a catecholato-capped CD was observed as a result of benzyl loss under the used arylation conditions. The AD-bridged product was characterised by a single crystal X-ray diffraction study. In the solid state, the CD torus adopts the usual circular shape while four glucose rings, although not distorted, are considerably tilted towards the cavity axis. Both ligands, L1 and L2, when reacted with the cationic complexes AgBF 4 or 〚Rh(norbornadiene)(THF) 2〛X (X = BF 4, PF 6), produced selectively chelate complexes having up to 29-membered metallomacrocycles in high yields. Rhodium systems based on either L1 or L2 catalyse effectively the hyfroformylation of 1-octene (TOF up to 1200 mol mol –1 of Rh h –1). Hydrogenation of dimethyl itaconate with a Rh(I)/ L2 complex produced dimethyl ( R)-(+)-methylsuccinate with an EE as high as 83.6%.

Unraveling the origin of regioselectivity in rhodium diphosphine catalyzed hydroformylation. A DFT QM/MM study.

The origin of regioselectivity in rhodium diphosphine catalyzed hydroformilation was investigated by means of hybrid QM/MM calculations using the IMOMM method. The roles of the diphosphine bite angle and of the nonbonding interactions were analyzed in detail by considering rhodium systems containing xantphos-type ligands, for which a correlation between the natural bite angle and regioselectivity has been recently reported. From the pentacoordinated equatorial--equatorial HRh(CO)(alkene)(diphosphine) key intermediate, eight possible reaction paths were defined and characterized through their respective transition states (TS). We succeeded in reproducing the experimentally observed trends for the studied diphosphines. By performing additional calculations on model systems, in which the steric effects induced by the phenyl substituents of xantphos ligands were canceled, we were able to separate, identify, and evaluate the different contributions to regioselectivity. These additional calculations showed that regioselectivity is governed by the nonbonding interactions between the diphenylphosphino substituents and the substrate, whereas the effects directly associated to the bite angle, what we call orbital effects, seem to have a smaller influence.

In situ study of diphosphine rhodium systems in asymmetric hydroformylation of styrene

HPNMR and in situ HPIR (HP = High Pressure) spectroscopic techniques were used to study the species present in the hydroformylation of styrene by rhodium disphosphine systems. Rhodium precursors with BDPP [(2S,4S)-bis(diphenylphosphino)pentane] and CHIRAPHOS [(2R,3R)-bis(diphenylphosphino)butane] as the chiral ligands (P–P) were used. The species observed were compared in order to find a relation between the structures and the enantiomeric excesses obtained for both chiral diphosphines. The effect of the P–P ∶ [Rh] molar ratio and the addition of a monophosphine on the enantiomeric excess were studied. It has been found that oxidation of the coordinated chiral diphosphine leads to formation of rhodium species more active but less regio- and enantio-selective.

Biphasic hydroformylation of olefins using a novel water soluble rhodium polyethylene glycolate catalyst

The highly reactive water-soluble hydroformylation catalyst, rhodium polyethylene glycolate (Rh(PEG) x ), prepared by the reaction of polyethylene glycol and rhodium trichloride hydrate, was used as a catalyst for hydroformylation reactions of olefins, such as dodec-1-ene, 2.4.4-trimethylpent-1-ene and styrene, in biphasic systems. The reactions were done in a broad temperature range and at a pressure range from 7 to 12 MPa. Turnover frequency for the catalytic reaction of the low reactive 2.4.4-trimethylpent-1-ene is 450 (mol aldehyde/mol Rh×h), which is 3 times higher than in comparable homogeneous rhodium systems. The selectivity for aldehydes is excellent (>98%). The dependence of the conversion vs. time was monitored for different partial pressures, pH values, temperatures and catalyst concentrations. Activation parameters have been calculated for the hydroformylation of the olefins in water and polyethylene glycol with rhodium polyethylene glycolate as a precatalyst. The activation energy in water is calculated for the hydroformylation of various olefins (dodec-1-ene E a 95 kJ/mol, 2.4.4-trimethylpent-1-ene E a 30 kJ/mol and styrene E a 34.1 kJ/mol). The hydroformylation reaction with rhodium polyethylene glycolate as a precatalyst is first-order for dodec-1-ene and 2.4.4-trimethylpent-1-ene and zero-order for styrene.

Why Do ManyC2-Symmetric Bisphosphine Ligands Fail in Asymmetric Hydroformylation? Theory in Front of Experiment

Supported by a pure QM and MM treatment, stereoselectivities of rhodium systems containing the C2-symmetric ligands CHIRAPHOS (2,3-bis(diphenylphosphino)butane; 2), BINAP (2,2‘-bis(diphenylphosphino)-1,1‘-binaphthyl; 3), DIOP (2,2-dimethyl-4,5-bis((diphenylphosphino)methyl)-1,3-dioxolane; 4), and NAPHOS (2,2‘-bis((diphenylphosphino)methyl)-1,1‘-binaphthyl; 5) have been calculated with a combined QM/MM method. On the basis of the RSAI (requirement of synchronous asymmetric inductions), which states that all ligand coordination modes favor transition states with the same asymmetric induction, we demonstrate that the performance of C2-symmetric bidentate phosphine ligands is governed by two interdependencies, namely induction influence of the chelate ring and backbone flexibility. In a further step, the NAPHOS derivatives 6 (2,2‘-bis((2-dinaphthylphosphino)methyl)-1,1‘-binaphthyl) and 7 (2,2‘-bis((1-dinaphthylphosphino)methyl)-1,1‘-binaphthyl) which to our knowledge have not been tested experimentally up to ...

Rhodium-diphosphine catalysts for the hydroformylation of styrene: the influence of the excess of ligand and the chelate ring size on the reaction selectivity

This study discusses the hydroformylation of styrene using rhodium systems containing four structurally related diphosphines: 1,2-bis(diphenylphosphine)ethane 1 (dppe), 1,3-bis(diphenylphosphine)propane 2 (dppp), ( R, R)-2,3-bis(diphenylphosphine)butane 3 (chiraphos) and ( S, S)-2,4-bis(diphenylphosphine)pentane 4 (bdpp). A systematic analysis of the effect of the pressure, temperature and the ligand to metal molar ratio for these catalytic systems shows that the five- and six-membered ring chelating diphosphines behave different. The regio- and enantioselectivity observed provide evidence of the catalytic species involved in the process. By analyzing the selectivity of the catalytic systems formed by mixing PEtPh 2 and the chiral diphosphine ligands, we propose a model describing the equilibria among the catalytic species.

New rhodium systems for biphasic hydrogenation and hydroformylation of 1-hexene

The catalytic activity of rhodium complexes formed in reactions of catalyst precursor, [Rh(acac)(CO) 2] with water soluble phosphines: Ph 2PCH 2CH 2CONHC(CH 3) 2CH 2SO 3Li ( PNS), Ph 2PCH 2CH(COOLi)(CH 2COOLi) ( PC), Ph 2PCH 2CH(CH 3)(COOH) ( PH) or Ph 2PCH 2CH(CH 3)(COONa) ( PNa) in hydrogenation and hydroformylation of 1-hexene in mono- and biphasic systems have been studied. The yield of aldehydes obtained in hydroformylation of 1-hexene in the system [Rh(acac)(CO) 2]+PNS strongly depends on the kind of solvent: 24% in toluene, 53–86% ( n/ iso 2.9–4.6) in the toluene–water–ethanol mixture and 77–94% ( n/ iso 2.5–3.8) in water–ethanol solution. The mixture of water–ethanol as a solvent was also found to be the best for hydrogenation of 1-hexene (96% of hexane) with [Rh(acac)(CO) 2]+PNS system. Application of PH phosphine (in hydrogen form) produces ca. 2% of aldehydes in both solvents, toluene only and toluene–water mixture. However, conversion of PH phosphine into its sodium salt, PNa, increased the catalytic activity of rhodium catalyst up to 85% yield of aldehydes in toluene, 92% in toluene–water and 94% in toluene–water–ethanol mixture. Spectroscopic studies of the reaction mixture in situ (IR, 1 H -, 31 P -NMR) allowed to identify following rhodium complexes existing in hydroformylation reaction conditions in the system [Rh(acac)(CO) 2]+PH/PNa: [Rh(acac)(CO)(PH)], [Rh(OH)(CO)(PH) 2], [HRh(CO)(PNa) 3], [Rh 4(CO) 12− x (PH) x ]. [HRh(CO)(PNa) 3] was found to be stable only when sodium hydroxide was introduced to the system.

Comparison of the C−H Activation of Methane by M(C5H5)(CO) for M = Cobalt, Rhodium, and Iridium

The C−H activation reactions of methane by MCp(CO) (Cp = C5H5) for the metals cobalt, rhodium, and iridium have been studied using a variety of methods including a recently developed scaling scheme, different perturbation theory methods, and also hybrid density functional theory methods. The main chemical problems investigated include the recent finding that CoCp(CO) is entirely inert toward alkanes in contrast to the corresponding rhodium and iridium systems. Comparisons are made for CoCp(CO) between the reaction with methane and the reaction with CO, which is found experimentally to proceed fast. Also studied are the isotope effects on the reaction in relation to other recent experiments. At the highest level of treatment, good agreement is found with all present observations for these systems, but it should be pointed out that precise experimental information is lacking for many of the systems studied. Severe deviations between the results obtained at different computational levels are pointed out.

Oxidative-addition reactions on planar chloranilate rhodium systems. Crystal structure of [Rh2(µ-C6Cl2O4)Me2I2(CO)2(PPh3)2

The binuclear chloranilate complexes [Rh2(µ-C6Cl2O4)L4]{L = CO 1 or CNR (R = toluene-p-sulfonylmethyl)2, L4=(CO)2(CNR)23, (CO)2(PPh3)24 or (CO)2[P(OMe)3]25} have been prepared and oxidative addition of the electrophiles MeI and I2 investigated. Addition of XI (X = Me or I) to compounds 4 and 5, in a Rh : XI = 1 : 1 molar ratio, led to the symmetrical complexes [Rh2(µ-C6Cl2O4)X2I2(CO2)(PR3)2](X = Me, R = Ph 6; X = I, R = Ph 7; X = I, R = OMe 8). Interaction of 4 and 5 with 1 mol of XI (X = Me or I) gave equimolecular mixtures of RhI2 and RhIII2 systems. The crystal structure of 6 has been determined by X-ray diffraction methods.

The migratory insertion of carbon monoxide in pentamethylcyclopentadienyliridium (III) complexes. Structural effects on reactivity and mechanism for rhodium and iridium systems

The reactions of complex (C5Me5)Ir(Cl) (CO) (Me) (1a) with cyclohexylisocyanide and phosphines (L=CyNC, PHPh2, PMePh2, PMe2Ph) give the products of alkyl migratory insertion (C5Me5Ir(Cl) (COMe) (L), in toluence or tetrahydrofuran at 323 K or higher temperature. The phenyl analogue (C5Me5)Ir(Cl)(CO)(Ph) or the iodide complexes (C5Me5)Ir(I) (CO) (R) (R=Me, Ph_are not reactive under the same conditions. The reaction of (C5Me5)Ir(Cl)(CO)(Me) with PMePh2 and PMe2Ph in acetonitrile yields the chloride substitution product [(C5Me5)Ir(CO)(L)(Me)]+Cl−. Kinetic measurements for the reactions of (C5Me5)Ir(Cl)(CO)(Me) in toluene are first order in the iridium complex and exhibit a saturation dependence on the incoming donors L. Analysis of the data suggests a two-step process involving (i) rapid formation of a molecular complex [(C5Me5)Ir(Cl)(CO)(Me), (L)], in which the structure of 1a is unperturbed within the limits of spectroscopic analysis, and (ii) rate determining methyl migration. The reaction parameters are K for the pre-equilibrium step (K = 1.5 (CyNC), 7.3 (PHPh2), 7.1 (PMePh2) dm3 mol−1 at 323 K) and k2 for the slow carboncarbon bond formation (k2 (105) = 6.9 (CyNC), 1.2 (PHPh2), 1.0 (PMePh2) s−1 at 323 K). The activation parameters for the methyl migration step in the reaction with PMePh2 obtained between 308 and 338 K, are ΔH≠ = 106±16 kJ mol−1 and ΔS≠ = − 14±5 J K−1 mol−1. The reaction of 1a with PMePh2 proceeds at similar rates in tetrahydrofuran (K = 3.7 dm3 mol−1, k2 (105) = 1.2 s−1, 323 K). The crystal structure of (C5Me5)Ir(Cl)(COMe) (PMe2Ph) has been determined by X-ray diffraction. C20H29ClOPIr: Mr = 544.1, monoclinic, P21/n, a = 8.084 (2), b = 9.030(2), c = 28.715 (3) A, β = 91.41 (3)°, Z = 4, Dc = 1.71 g cm−3, V = 2095.5 A3, room temperatyre, Mo Kα, γ = 0.71069, μ = 65.55 cm−1, F(000) = 1044, R = 0.037 for 2453 independent observed reflections. The complex shows a deformed tetrahedral coordination assuming the η5-C5Me5 molecular fragment as a single coordination site. The iridium-chlorine bond is staggered with respect to two adjacent C(ring)-methyl bonds, while the IrP and the IrCOMe bonds are eclipsed with respect to C(ring)-methyl bonds.

C 1 to acetyls: catalysis and process

Carbonylation of methanol and methyl acetate to acetic acid and acetic anhydride are the only current examples of C 1 processes displacing petrochemical routes for the manufacture of commodity chemicals. Modem carbonylation processes are based upon an iodide promoted, homogeneous rhodium catalyst operating under mild reaction conditions (150°C–200°C, 25–45 bar). This highly selective catalytic cycle is described, and contrasted with other catalytic metals. The embodiment of Rh/I catalysis in commercial BP Chemicals methanol and methyl acetate carbonylation processes is also described, illustrating the process simplicity arising from high selectivity. The now extensive literature on attempts to support the rhodium catalyst in iodide promoted systems is reviewed. In the liquid phase, catalytic behaviour similar to that for the homogeneous system is claimed, although proper comparative data and conclusive evidence for catalyst immobilisation is generally lacking. A number of supported rhodium systems are reported to be active under vapour phase conditions, and activated carbon is also found to promote carbonylation activity of certain base metals, notably nickel. Issues affecting commercial viability include catalyst lifetime, gaseous by-product formation and efficiency of utilisation of precious metal components. Literature on carbonylation in the absence of iodide promoter, and on direct routes to acetyls from synthesis gas or methanol only is also reviewed. All require step changes in catalytic performance in order to approach commercial viability.

Electronic structure and properties of the laves phase compounds YRh 2 and LaRh 2

The band structures of the Laves phase systems YRh 2 and LaRh 2 determined using the LMTO method are reported here. The paramagnetic band structures and the DOS curves are compared with their counterparts YCo 2 and LuCo 2 which exhibit anomalous variation of susceptibility with temperature, while the rhodium compounds are Pauli paramagnetic. The band structure results are used to analyse the reasons for the differences in physical properties between the cobalt and rhodium systems. Further, as the rhodium compounds are found to be superconducting, we also report their theoretically calculated values of the transition temperatures.

Transition metal complexes bound to macroporous resins carrying nitrile groups. Effect of matrix structure on the activity of supported catalysts

Two groups of resins bearing nitrile (CN) groups based on macroporous copolymers of acrylonitrile and divinylbenzene (AN/DVB) and terpolymers of styrene, acrylonitrile and divinylbenzene (S/AN/DVB) have been used as polymer matrices for the immobilization of Rh(I), Pt(II) and Pd(II) complexes. A group of four S/DVB resins functionalized with the CN ligands have been prepared and used for comparison. The resins differ in their chemical and physical structure, local concentration of the CN groups and their availability. Characterization of the heterogeneous complexes by IR spectroscopy confirmed the coordination of the metal ions to the polymer-CN ligands. The catalytic behaviour of the immobilized complex catalysts was tested in the hydrosilylation of 1-hexene. The activity of the polymer-bound catalysts strongly depends on the structure of the support used. The largest effect of the chemical structure of the polymer was found for the catalysts immobilized on the AN/DVB resins, while the polymer morphology played the major role in the high activity of the catalysts attached to the S/N/DVB resins. Lower activity was found for the systems bound to the functionalized S/DVB resins. Both polymer-supported Pt and Rh systems appeared to be highly effective for the hydrosilylation of the CC double bonds, but the platinum catalysts proved to be considerably more active. The rhodium catalysts were found efficient in the hydrosilylation of ketones. The immobilized Pd(II) complex was reduced to the metallic Pd by hydrosilanes. The supported Pt catalyst remained active when recycled 5 times, while the activity of the rhodium systems gradually decreased. The results offer the possibility of choosing the most suitable polymer matrix for the immobilization of metal complex catalysts for use in hydrosilylation and other catalytic reactions.

On the catalytic C-H activation of some silicon dioxide overlayers on thin metal film systems ? An observation on the isokinetic effect

Kinetic data on the exchange reaction betwee hexane and deuterium have recently been reported by Maier and coworkers [1]. The reaction was catalysed by a series of platinum or rhodium systems, some of which had the property that the upper surface was shielded by a silica overlayer. The data are reinterpreted to indicate an isokinetic temperature of 580 K. This observation indicates that all the catalysts behave in a similar manner, which means that the exchange reaction does not require a free metal surface.

ChemInform Abstract: Activation of 1‐Alkynes at Tripodal (Polyphosphine)rhodium Systems. Regioselective Synthesis of Enol Esters from 1‐Alkynes and Carboxylic Acids Catalyzed by Rhodium(I) Monohydrides.

AbstractRegioselective formation of 2‐(benzoyloxy)propene (III) results from the addition of benzoic acid (I) to propyne (II) in the presence of the trigonal‐bipyramidal Rh(I) monohydrides LRhH (L: E(CH2CH2PPh2)3, E: P, N).

Activation of 1-alkynes at tripodal (polyphosphine)rhodium systems. Regioselective synthesis of enol esters from 1-alkynes and carboxylic acids catalyzed by rhodium(I) monohydrides

Regioselective formation of 2-(benzoyloxy)propene results from the addition of benzoic acid to propyne in the presence of the trigonal-bipyramidal Rh(I) monohydrides [(PPh 3 )RhH] (1) and [(NP 3 )RhH] (2, PP 3 =P(CH 2 CH 2 PPh 2 ) 3 , NP 3 =N(CH 2 CH 2 PPh 2 ) 3 ). The reactions are catalytic under relatively mild conditions (catalyst to substrate ratio 1:100, toluene, 100 o C). A detailed experimental study on the reactions of 1 and 2 with carboxylic acids, 1-alkynes, or carboxylic acid/1-alkyne mixtures has allowed us to draw a catalysis cycle involving the 16-electron fragments [(L)Rh] + as catalysts (L=PP 3 , NP 3 ). The catalytic behavior of the precursors 1 and 2 has been compared and contrasted with those of the isostructural and isoelectronic derivatives [(L)Rh(C≡CPh), [(L)RhCl], and [(PP 3 )RhMe]. The novel vinylphosphonium complex [(Ph 2 PCH 2 CH 2 ) 2 P(CH 2 CH 2 PPh 2 )Rh{C=C(H)Ph}(O 2 CPh) has been synthesized and fully characterized by IR and 1 H and 34 P{ 1 H} NMR techniques

Phospholes as ligands for rhodium systems in homogeneously catalysed hydroformylation reactions: Part 2. Optimisation of the reaction parameters and comparison of the different catalytic systems

The influence of various parameters on the rate and products distribu- tion of 1-hexene hydroformylation were studied in the case of the rhodium- 1,2,5-triphenylphosphole system and optimal values were determined. These optimal conditions were then used for an evaluation of various phospholes. Two of them, namely 1,2,5-triphenylphosphole and 1- o-tolyl-2,5-diphenyl- phosphole, have been found superior to triphenylphosphine, the standard phosphane for hydroformylation. Furthermore, the activity and selectivity of the rhodium-1,2,5-triphenylphosphole system are not dependent on the ligand concentration.

Phospholes as ligands for rhodium systems in homogeneously-catalysed hydroformylation reactions: Part 1. Stereoelectronic properties of the ligands and hydroformylation of 1-hexene

The stereoelectronic properties of various phospholes were determined and discussed in terms of Tolman's electronic ν and steric θ parameters. The selected phospholes, associated with rhodium, were used in the catalytic hydroformylation of 1-hexene and compared to triphenylphosphine under typical hydroformylation conditions (80 °C, 20 bar syngas). One phosphole, namely 1,2,5-triphenylphosphole (TPP), has been found to be by far superior to triphenylphosphine (about 10 times more active and 10% more selective). Most importantly, the rhodium-TPP system does not seem to be sensitive to the P/Rh ratio.