Phosphines with 2-imidazolium ligands enhance the catalytic activity and selectivity of rhodium complexes for hydrosilylation reactions

Phosphines with 2-imidazolium ligands can specifically vary their physical and chemical properties by altering the attached substituents. Rhodium complexes (1b-7b) exhibited excellent catalytic activity and selectivity for hydrosilylation of olefins. The selectivity of the β-adduct clearly increased when the length of the alkyl chain bound to the imidazolium cation increased. Rhodium complex 1b in BMimPF₆ can be reused without noticeable loss of catalytic activity and selectivity.     

Synthesis, structure, and catalytic properties of polymer-immobilized rhodium clusters

Immobilized Rh₆ clusters (cyclohexene hydrogenation catalysts) were prepared by the polymer-analogous transformations or copolymerization of cluster-containing monomers and characterized. Intermediates formed in the course of a catalytic reaction were studied using IR spectroscopy, XPS, and atomic force microscopy. It was found that the relative intensity of a low-energy line in the Rh3d _5/2 spectrum of the initial polymer-immobilized cluster in the XPS spectrum of Rh₆ increased in the course of hydrogenation. The catalytic activity of the immobilized complex changed symbatically with both the number of Rh atoms bound to the H(CO) group and the number of Rh atoms, the charge on which was greater than that in the parent cluster. Some experimental evidence was obtained in favor of the hypothesis of cluster fragmentation in the course of hydrogenation with the formation of highly active, most likely, nanosized particles, which are true catalysts, in low concentrations. The surface of macrocomplex particles after hydrogenation became more homogeneous and hydrophilic; this fact is also indicative of an increase in the concentration of polar functional groups in surface layers. This was likely due to Rh-Rh bond cleavage in the polymer-immobilized cluster.     

Synthesis and structural properties of a series of pentacoordinate rhodium nitrosyl complexes: Crystal and molecular structures of cis-dibromonitrosylbis(methoxydiphenylphosphine)rhodium and trans-dibromonitrosylbis(iso-propoxydiphenylphosphine)rhodium

The pentacoordinate rhodium nitrosyl complexes [RhBr₂(NO)L₂ [L = P(OPh)₂Ph, P(OMe)Ph₂ or P(OPrⁱ)Ph₂] have been synthesized and the structures of [RhBr₂(NO){P(OMe)Ph₂}₂] and [RhBr₂(NO){P(OPrⁱ)Ph₂}₂] have been determined X-ray crystallographically. Both of these latter compounds are tetragonal pyramidal with the nitrosyl group apical. The methoxydiphenylphosphine ligands in [RhBr₂(NO){P(OMe)Ph₂}₂] are cis-disposed whereas the larger cis-propoxydiphenylphosphine ligands in [RhBr₂(NO){P(OPrⁱ)Ph₂}₂] are mutually trans. The nitrosyl group in trans-[RhBr₂(NO){P(OPrⁱ)Ph₂}₂] eclipses an Rh-P axis but in cis-[RhBr₂(NO){P(OMe)Ph₂}₂] it is staggered with respect to the P-Rh-P linkage. The isomeric behaviour of nitrosyl complexes of type [RhX₂(NO)L₂] (X = halogen, L = phosphorus donor ligand) is rationalized in terms of the size of the ligand L.     

Palladium and rhodium ureaphosphine complexes: exploring structural and catalytic consequences of anion binding

The addition of a chloride ion to Pd and Rh complexes supported by the ureaphosphine ligand L results in the formation of chelating diphosphine complexes that retain some catalytic activity.     

The synthesis of new chiral rhodium complexes and their crystal structures

Two novel chiral rhodium complexes were successfully synthesized from the reaction of chiral bidentate nitrogen ligands with RhCl₃·3H₂O in ethanol under reflux. Their unusual crystal structures were unambiguously obtained by X‐ray analysis. Copyright © 2001 John Wiley & Sons, Ltd.     

Immobilization of rhodium complexes at thiolate monolayers on gold surfaces: catalytic and structural studies

Chiral rhodium-diphosphine complexes have been incorporated into self-assembled thiolate monolayers (SAMs) on gold colloids. Catalysts of this type are of interest because they combine properties of homogeneous and heterogeneous systems. In addition, it should be possible to influence the catalytic properties of the metal center by the neighboring thiolate molecules. Colloids with a diameter of ca. 3 nm, coated with a mixed monolayer of n-octanethiolates and thiolates with chiral rhodium-PYRPHOS end groups, were studied as hydrogenation catalysts. With methyl alpha-acetamido-cinnamate as substrate, virtually the same enantioselectivities (up to 93% ee) and full conversion were obtained as with the corresponding homogeneous [Rh(COD)(PYRPHOS)]BAr(F) catalyst. The colloids were easily recovered by filtration and reused as catalysts three times without loss of enantioselectivity. STM studies of analogous SAMs on Au(111) gave a detailed picture of the structure and dynamics of mixed monolayers of this type. The STM images showed that the catalyst-bearing thiolates are distributed statistically on the surface and that the ordered structure of the n-octanethiolate SAM can be retained during incorporation of the catalyst-bearing thiols using the place-exchange methodology.     

1,4-dicyanobutene-bridged binuclear rhodium(I) complexes and their catalytic activities

Reactions of Rh(ClO₄)(CO)(PPh₃)₂ with dicyano olefins, cis-NCCHCH-CH₂CH₂CN (c-DC1B), rans-NCCHCHCH₂CH₂CN (t-DC1B), trans-NCCH₂CHCHCH₂CN (t-DC2B), and NCCH₂CH₂CH₂CN (DCB) produce the binuclear dicationic rhodium(I) complexes, [(CO)(PPh₃)₂RhNCACNRh-(PPh₃)₂(CO)](ClO₄)₂ (NCACN = c-DC1B 1), t-DC1B (2), t-DC2B (3), DCB (4). Complexes 1 and 2 are catalytically active for the hydrognation of c-DC1B and t-DC1B, respectively, to give DCB, while complex 3 catalyze the isomerization of t-DC2B to give c-DC1B and t-DC1B, and the hydrogenation of t-DC2B to DCB at 100°C.     

Homogeneous catalytic system for photoinduced hydrogen production utilizing iridium and rhodium complexes

An efficient homogeneous catalytic system for the visible-light-induced production of hydrogen from water utilizing cyclometalated iridium(III) and tris-2,2'-bipyridyl rhodium(III) complexes is described. Synthetic modification of the photosensitizer Ir(C--N) 2(N--N) (+) and water reduction catalyst Rh(N--N) 3 (3+) creates a family of catalysts with diverse photophysical and electrochemical properties. Parallel screening of the various catalyst combinations and photoreaction conditions allows the rapid development of an optimized photocatalytic system that achieves over 5000 turnovers with quantum yields ( (1)/ 2 H 2 per photon absorbed) greater than 34%. Photophysical and electrochemical characterization of the optimized system reveals that the reductive quenching pathway provides the necessary driving force for the formation of [Rh(N--N) 2] (0), the active catalytic species for the reduction of water to produce hydrogen. Tests for system poisoning with mercury or CS 2 provide strong evidence that the system is a true homogeneous system for photocatalytic hydrogen production.     

N-fused pentaphyrins and their rhodium complexes: oxidation-induced rhodium rearrangement

meso-Aryl-substituted pentaphyrins were isolated in the modified Rothemund-Lindsey porphyrin synthesis as a 22-pi-electron N-fused pentaphyrin ([22]NFP5) and a 24-pi-electron N-fused pentaphyrin ([24]NFP5), which were reversibly interconvertible by means of two-electron reduction with NaBH4 or two-electron oxidation with dichlorodicyanobenzoquinone (DDQ). Judging from 1H NMR data, [22]NFP5 is aromatic and possesses a diatropic ring current, while [24]NFP5 exhibits partial anti-aromatic character. Metalation of [22]NFP5 1 with a rhodium(I) salt led to isolation of rhodium complexes 9 and 10, whose structures were unambiguously characterized by X-ray diffraction analyses and were assigned as conjugated 24-pi and 22-pi electronic systems, respectively. In the rhodium(I) metalation of 1, the complex 9 was a major product at 20 degrees C, but the complex 10 became preferential at 55 degrees C. Upon treatment with DDQ, compound 9 was converted to 10 with an unprecedented rearrangement of the rhodium atom.     

Carbene complexes of rhodium and iridium from tripodal N-heterocyclic carbene ligands: synthesis and catalytic properties

Two tripodal trisimidazolium ligand precursors have been tested in the synthesis of new N-heterocyclic carbene rhodium and iridium complexes. [Tris(3-methylbenzimidazolium-1-yl)]methane sulfate gave products with coordination of the decomposed precursor. [1,1,1-Tris(3-butylimidazolium-1-yl)methyl]ethane trichloride (TIMEH(3)(Bu)) coordinated to the metal in a chelate and bridged-chelate form, depending on the reaction conditions. The crystal structures of two of the products are described. The compounds resulting from the coordination with TIME(Bu) were tested in the catalytic hydrosilylation of terminal alkynes.     

Syntheses and structures of heterometallic clusters by the reaction of o-carboranyl cobaltadichalcogenolato complexes

Metalladichalcogenolate cluster complexes [Cp'Co{E(2)C(2)(B(10)H(10))}]{Co2(CO)5} [Cp' = eta5-C5H5, E = S(3a), E = Se(3b); Cp' = eta5-C5(CH3)5, E = S(4a), E = Se(4b)], {CpCo[E(2)C(2)(B(10)H(10))]}(2)Mo(CO)2] [E = S(5a), Se(5b)], Cp*Co(micro2-CO)Mo(CO)(py)2[E(2)C(2)(B(10)H(10))] [E = S(6a), Se(6b)], Cp*Co[E(2)C(2)(B(10)H(10))]Mo(CO)2[E(2)C(2)(B(10)H(10))] [E = S(7a), Se(7b)], (Cp'Co[E(2)C(2)(B(10)H(10))]W(CO)2 [E(2)C(2)(B(10)H(10))] [Cp' = eta5-C5H5, E = S(8a), E = Se(8b); Cp' = eta5-C5(CH3)5, E = S(9a), E = Se(9b)], {CpCo[E(2)C(2)(B(10)H(10))]}(2)Ni [E = S(10a), Se(10b)] and 3,4-(PhCN(4)S)-3,1,2-[PhCN(4)SCo(Cp)S(2)]-3,1,2-CoC(2)B(9)H(8) 12 were synthesized by the reaction of [Cp'CoE(2)C(2)(B(10)H(10))] [Cp' = eta5-C5H5, E = S(1a), E = Se(1b); Cp' = eta5-C5(CH3)5, E = S(2a), E = Se(2b)] with Co2(CO)8, M(CO)3(py)3 (M = Mo, W), Ni(COD)2, [Rh(COD)Cl]2, and LiSCN4Ph respectively. Their spectrum analyses and crystal structures were investigated. In this series of multinuclear complexes, 3a,b and 4a,b contain a closed Co3 triangular geometry, while in complexes 5a-7b three different structures were obtained, the tungsten-cobalt mixed-metal complexes have only the binuclear structure, and the nickel-cobalt complexes were obtained in the trinuclear form. A novel structure was found in metallacarborane complex 12, with a B-S bond formed at the B(7) site. The molecular structures of 4a, 5a, 6a, 7b, 9a, 9b, 10a and 12 have been determined by X-ray crystallography.     

Synthesis and X-ray structures of rhodium complexes with new chiral biaryl-based NHC-ligands

A new series of chiral NHC–rhodium complexes has been prepared from the reactions between [Rh(COD)Cl]₂, NaOAc, KI and dibenzimidazolium salt 4a or monobenzimidazolium salts 4b–d, which are derived from chiral 2,2′-diamino-6,6′-dimethyl-1,1′-biphenyl, 2,2′-diamino-1,1′-binaphthyl or 6,6′-dimethyl-2-amino-2′-hydroxy-1,1′-biphenyl. The steric and electronic effects of the ligand play an important role in the complex formation. For example, treatment of chiral monobenzimidazolium salt 4b (with a NMe₂ group) with 0.5 equiv of [Rh(COD)Cl]₂ in the presence of NaOAc and KI in CH₃CN at reflux gives a chiral Rh(I) complex 5b, while chiral monobenzimidazolium salt 4d (with a MeO group) affords a racemic Rh(I) complex 5d. Under similar reaction conditions, treatment of dibenzimidazolium salt 4a with 0.5 equiv of [Rh(COD)Cl]₂ in the presence of NaOAc and KI gives a racemic Rh(III) complex 5a, while the dibenzimidazolium salt [C₂₀H₁₂(C₇H₅N₂Me)₂]I₂ derived from chiral 2,2′-diamino-1,1′-binaphthyl affords a chiral Rh(III) complex [C₂₀H₁₂(C₇H₄N₂Me)₂]RhI₂(OAc). All compounds have been characterized by various spectroscopic techniques, and elemental analyses. The solid-state structures of the rhodium complexes have been further confirmed by X-ray diffraction analyses.     

IR-photodissociation of size selected molecular clusters and their structures

Infrared photodissociation spectra of (CH₃NH₂) _n clusters were measured fromn=2 ton=6 near the monomer absorption of the C-N stretching mode at 1044 cm^?1 using a cw-CO₂ laser. The clusters were size-selected by scattering from a helium beam. The spectrum of cold dimers shows a red (1038 cm^?1) and a blue (1048 cm^?1) shifted peak which is attributed to the non-equivalent position of the C-N in the open dimer structure. The larger clusters exhibit only one peak between 1045.4 cm^?1 and 1046.0 cm^?1 caused by the equivalent position of the C-N in the cyclic structures of the larger clusters. Structure calculations confirm these results. Secondly, the mixed complexes C₂H₄-CH₃COCH₃ and C₂H₄-(CH₃COCH₃)₂ were investigated. The dimer spectrum, measured around the monomer frequency of the out-of-plane bending mode of C₂H₄ at 949 cm^?1, shows two peaks at 946.2 cm^?1 and 961.3 cm^?1. This splitting is attributed to two different isomers that are found in configuration calculations. A similar behaviour is found for the trimer.     

Heterometallic heterochalcogenmethylarsenide clusters: Synthesis,molecular structures,and thermolysis

Reactions of the arsinechalcogenide complexes [Fe₃(μ₃-X)(μ₃-AsCH₃)(CO)₉] (X = Se (Ia) or Te (Ib)) with (PPh₃)₂Pt(PhC≡CPh) (transmetalation reaction) and Cp₂Cr₂(SCME₃)₂S (Cp = π-C₅H₅) (photochemical reaction) gave the heterometallic (heterochalcogen)(methylarsine) clusters [(PPh₃)₂Pt(μ₃-X)(μ₃-AsCH₃)Fe₂(CO)₆] (II and III, respectively), as well as Fe₃(μ₃-X)(μ₃-AsCH₃)(CO)₈(C₅H₅)₂Cr₂(μ₃-S)(μ₂-S ^t Bu)₂ (IV and V, respectively). The structures of complexes II, IV, and V were determined by X-ray diffraction analysis. Thermolysis of all the complexes yielded no metal carbides or oxides.     

Chemistry of polycarbon species: from clusters to molecular devices

Polycarbon species with pi-conjugated systems interact with attached metal fragments to exhibit a variety of intriguing structural features and chemical properties. In this Perspective, following an introductory account of polycarbon cluster compounds and the unique chemical transformations on them which cannot easily be realized by mononuclear species, the properties of polynuclear complexes connected by pi-conjugated carbon-rich bridging ligands will be discussed with emphasis on those applicable to molecular electronics, e.g. wire-like behavior (molecular wires), higher dimensional systems (branch circuits), and switching functions.