Twisted paddlewheel rhodium complexes: Contribution of central and axial chirality to ECD,VCD, and NMR spectra

Dirhodium complexes bearing N-substituted chiral amino acid ligands are investigated. These complexes have an unusual twisted paddlewheel structure, showing inherent chirality. We would like to demonstrate that parallel application of chiroptical spectroscopic methods (ECD and VCD) and NMR spectroscopy combined with quantum chemical calculations constitutes a powerful tool to determine the configuration of the complexes unequivocally. Two chiroptical methods are needed to determine the absolute configuration: ECD for the coordinated nitrogen atom and VCD for the rhodium core. A quick to use NMR method is also presented: Upon the coordination of small molecules in the axial position, the relative configuration of both the rhodium core and the nitrogen atom can be determined simultaneously by studying spatial proximities provided by 1D NOE spectra.     

Immobilization of rhodium complexes in chiral organic-inorganic hybrid materials

Two new alkoxysilylated derivatives of (-)-(1R,2R)-1, 2-diaminocyclohexane: M = N-[(triethoxysilyl)propyl]-(-)-(1R,2R)-1, 2-diaminocyclohexane and B = N, N'-bis[(triethoxysilyl)propyl]-(-)-(1R,2R)-1,2-diaminocyclohexane have been synthesized. Their complexation with [Rh(cod)Cl]2 in the presence of TEOS = Si(OEt)4, followed by sol-gel hydrolysis-condensation, afforded new catalytic chiral hybrid materials. Evidence for the presence of the organic moieties complexed by rhodium in these solids was obtained by UV-visible spectroscopy, FT-IR studies, solid state 13C and 29Si CP-MAS NMR analysis, energy-dispersive X-ray (EDX) techniques, and elemental analysis. The nitrogen sorption studies and BET analyses ranged these solid gels from nonporous to highly porous materials. The catalytic activities and selectivities of the solid materials have been studied in the asymmetric hydrogen-transfer reduction of prochiral ketones and compared to that of the homogeneous rhodium complexes of the ligands M and B. The hybrid materials appeared interesting supports for enantioselective heterogeneous catalysis leading to chiral alcohols with ee up to 58% in the reduction of acetophenone and up to 98% in the case of the more hindered related ketones. The catalytic properties as a function of the nature of chiral hybrid solid are discussed.     

Recovery of rhodium(III) from solutions and industrial wastewaters by a sulfate-reducing bacteria consortium

A quantitative analysis of the rate of removal of rhodium(III) by a resting sulfate-reducing bacteria (SRB) consortium under different initial rhodium and biomass concentrations, pH, temperature, and electron donor was studied. Rhodium speciation was found to be the main factor controlling the rate of its removal from solution. SRB cells were found to have a higher affinity for anionic rhodium species, as compared to both cationic and neutral species, which become abundant when speciation equilibrium was reached. Consequently, a pH-dependent rate of rhodium removal from solution was observed. The maximum SRB uptake capacity for rhodium was found to be 66 mg of rhodium per gram of resting SRB biomass. Electron microscopy studies revealed a time-dependent localization and distribution of rhodium precipitates, initially intracellularly and then extracellularly, suggesting the involvement of an enzymatic reductive precipitation process. When a purified hydrogenase enzyme was incubated with rhodium chloride solution under hydrogen, 88% of the rhodium was removed within 1 h, whereas with a soluble extract from SRB 77% was removed within 10 min. Due to the low pH of the industrial effluent (1.31), the enzymatic reduction of rhodium by the purified hydrogenase was greatly limited, and it was apparent that an industrial effluent pretreatment was necessary before the application of an enzymatic treatment. In the present study, however, it was established that SRB are good candidates for the enzymatic recovery of rhodium from both aqueous solution and industrial effluent.     

Antitumour action of planar, organometallic rhodium(I) complexes

Four cis-, square planar rhodium(I) organometallic complexes have been tested for antineoplastic properties. Their activity varies depending on the tumour system employed. They cause little or no effect on the growth of solid S180, and only one of them is effective on LI210 leukemia. Their activity on Ehrlich ascites is more pronounced, with low T/C ratios, two derivatives in particular causing some regressions. This greater activity on ascites carcinoma has been correlated with the oxidability and the lability of the leaving groups of these complexes. The most active compound on Ehrlich ascites, when tested for effects on the incorporation of labelled precursors in macromolecules shows a selective inhibition of leucine incorporation into proteins at therapeutically active doses. It is pointed out that these rhodium derivatives, in contrast to platinum complexes, are characterized by π carbon-metal bonds instead of nitrogen donor ligands for the non-leaving groups.     

Synthesis and characterisation of ferrocene-containing β-diketonato complexes of rhodium(I) and rhodium(III)

The synthesis of new β-diketonato rhodium(I) complexes of the type [Rh(FcCOCHCOR)(CO)₂] and [Rh(FcCOCHCOR)(CO)(PPh₃)] with Fc=ferrocenyl and R=Fc, C₆H₅, CH₃ and CF₃ are described. ¹H, ¹³C and ³¹P NMR data showed that for each of the non-symmetric β-diketonato mono-carbonyl rhodium(I) complexes, two isomers exist in solution. The equilibrium constant, K_c, which relates these two isomers in an equilibrium reaction, are concentration independent but temperature and solvent dependent. Δ_rG, Δ_rH and Δ_rS values for this equilibrium have been determined and a linear relationship between solvent polarity on the Dimroth scale and K_c exists. The relationship between RhP bond lengths, d(RhP), and ³¹P NMR peak positions as well as coupling constants ¹J(³¹P¹⁰³Rh) has been quantified to allow calculation of approximate d(RhP) values. Variations in d(RhP) for [Rh(RCOCHCOR′)(CO)(PPh₃)] complexes have also been related to the group electronegativities (Gordy scale) of the terminal β-diketonato R groups trans to PPh₃. A measure of the electron density on the rhodium centre of [Rh(RCOCHCOR′)(CO)(PPh₃)] may be expressed in terms of the IR carbonyl stretching wave number, ν(CO), the sum of the group electronegativities of the R and R′ groups, (χ_R+χ_R′), or the observed pK_a^′ values of the free β-diketones RCOCH₂COR^′. An empirical relationship between ν(CO) and either pK_a^′ or (χ_R+χ_R′) has also been quantified.     

Tripodal oxygen and tripodal nitrogen ligands in hydroformylation reactions: formation of novel rhodium(III) carbonyl bis(acyl) complexes

The rhodium(I) complexes TpmsRh(CO)₂ (1) and TpmsRh(cod) (2) of the tripodal nitrogen ligand tris(pyrazolyl)methanesulfonate, Tpms⁻=[(pz)₃CSO₃]⁻, catalyze the hydroformylation of 1-hexene. Addition of phosphine has a negative effect on the activity. The hydroformylation activity reaches a maximum at about 60 °C. At temperatures above 80 °C hydrogenation becomes an important secondary reaction. When the catalysis is performed at 60 °C in acetone with 1 or 2 as catalyst precursor all of the rhodium is recovered in the form of the rhodium(III) bis(acyl) complex TpmsRh(CO)(COC₆H₁₃)₂ (9). A similar behaviour is observed with rhodium(I) complexes bearing the tripodal oxygen ligand L_OMe⁻=[(cyclopentadienyl)tris(dimethylphosphito-P) cobalt O,O^′,O″]⁻. In this case all of the rhodium is transformed into L_OMeRh(CO)(COC₆H₁₃)₂ (10). These hitherto unknown bis(acyl) rhodium(III) complexes show the same catalytic activity as the rhodium(I) starting compounds.     

Inhibiting prolyl isomerase activity by hybrid organic–inorganic molecules containing rhodium(II) fragments

A small molecule containing a rhodium(II) tetracarboxylate fragment is shown to be a potent inhibitor of the prolyl isomerase FKBP12. The use of small molecules conjugates of rhodium(II) is presented as a general strategy for developing new protein inhibitors based on distinct structural and sequence features of the enzyme active site.     

Five-coordinated rhodium(I) carbonyl compounds

The title compounds were synthesised by the replacement of chlorine in Rh(CO)(PPh₃)₂Cl with monobasic bidentate chelating ligands such as salicylaldehyde, acetylacetone, benzoylacetone, dibenzoylmethane, 8-hydroxyquinoline, benzoylphenyl hydroxylamine, 2-hydroxyacetophenone and 2-hydroxybenzophenone. IR spectral evidence points out that these compounds have a trigonal bipyramidal geometry around rhodium in the solid state. However, in benzene solutions, except for the 8-hydroxyquinoline and 2-hydroxybenzophenone derivatives they all take a square planar structure, as seen from their electronic spectra.     

Recognition of G-U mismatches by tris(4,7-diphenyl-1,10-phenanthroline)rhodium(III).

The coordination complex tris(4,7-diphenyl-1,10-phenanthroline)rhodium(III) [Rh(DIP)3(3+)], which promotes RNA cleavage upon photoactivation, has been shown to target specifically guanine-uracil (G-U) mismatches in double-helical regions of folded RNAs. Photoactivated cleavage by Rh(DIP)3(3+) has been examined on a series of RNAs that contain G-U mismatches, yeast tRNA(Phe) and yeast tRNA(Asp), as well as on 5S rRNAs from Xenopus oocytes and Escherichia coli. In addition, a "microhelix" was synthesized, which consists of seven base pairs of the acceptor stem of yeast tRNA(Phe) connected by a six-nucleotide loop and contains a mismatch involving residues G4 and U69. A U4.G69 variant of this sequence was also constructed, and cleavage by Rh(DIP)3(3+) was examined. In each of these cases, specific cleavage is observed at the residue which lies to the 3'-side of the wobble-paired U; some cleavage by the rhodium complex is also evident in several structured RNA loops. The remarkable site selectivity for G-U mismatches within double-helical regions is attributed to shape-selective binding by the rhodium complex. This binding furthermore depends upon the orientation of the G-U mismatch, which produces different stacking interactions between the G-U base pair with the Watson-Crick base pair following it on the 5'-side of U compared to the Watson-Crick pair preceding it on the 3'-side of U. Rh(DIP)3(3+) therefore serves as a unique probe of G-U mismatches and may be useful both as a model and in probing RNA-protein interactions as well as in identifying G-U mismatches within double-helical regions of folded RNAs.     

Synthesis of rhodium(III), iridium(III) and osmium(III) complexes with tris(2-aminoethanethiolato)cobalt(III)

Polynuclear sulfur bridged complexes where the neutral complex tris(2-aminoethanethiolato)cobalt(III) acts as a tridentate ligand to rhodium(III), iridium(III) and osmium(III) have been prepared. These complexes have been characterized by electronic spectroscopy, vibrational spectroscopy and nuclear magnetic resonance spectroscopy. Along with the previously prepared complexes of iron(III), ruthenium(III) and cobalt(III), these complexes form two series of complexes with the group 8 and group 9 elements from all three transition series.     

Asymmetric polymerizations of chiral 4-benzyl-2-ethynyloxazoline with rhodium catalyst and chiroptical properties of the polymers

Optically active 4-alkyl-2-ethynyloxazoline derivatives (BnEOx) were polymerized with rhodium catalysts. The polymerization in toluene produced polymer with the highest absolute values of specific rotation ([α](D) = -77.3°). The yields, molecular weights, and specific rotations of poly(BnEOx)s were influenced by polymerization conditions. The copolymerization with phenylacetylene (PA) was effective to increase the molecular weight of the copolymer. It is interesting to note that the copolymers exhibited positive specific rotations ([α](D) = +4.7° to +62.5°) despite the fact that [α](D) s of BnEOx and the homopolymer are negative sign. The chiroptical properties were investigated by the chiral/achiral copolymerization of BnEOx with PA. The copolymerizations of BnEOx with PA gave copolymers containing higher order structure such as one-handed helical conformation. Furthermore, induced Cotton effects were observed in the π-π* transition region of conjugated main chain depending a complex of these polymers with zinc triflate salt in tetrahydrofuran solution, indicating the formation of chiral supramolecular aggregates.     

Amphiphilic proline and prolylproline derivatives in the rhodium(I)-catalyzed asymmetric hydrogenation in water: Chiral induction as indication of the location of reactants within the micelle

Proline and prolylproline dipeptide derived surfactants promote the asymmetric hydrogenation of (Z)-methyl α-acetamidocinnamate in water in the presence of the catalytic system [Rh(cod)₂]BF₄ + BPPM. Activity and enantioselectivity are enhanced significantly and the results in water are similar to those obtained with organic solvents. The possibility of a chiral induction was investigated in the presence of the optically active amino acid and peptide amphiphiles and an achiral rhodium catalyst [Rh(bdpb)(cod)]BF₄. The analysis of the low optical induction gave some indications of the site where the reaction takes place within the micelle. Selected critical micelle concentrations (cmc) of the new prepared surfactants were determined by surface tension measurements. Chirality 10:754–759, 1998. © 1998 Wiley-Liss, Inc.     

Supported organometallic complexes. Part 27: novel sol-gel processed rhodium(I) complexes: synthesis, characterization, and catalytic reactions in interphases

A novel T-silyl functionalized cationic (COD)(dppp)rhodium(I) complex was sol-gel processed with various amounts of the co-condensing agents MeSi(OMe)₂(CH₂)₆(OMe)₂SiMe and MeSi(OMe)₂(CH₂)₃(C₆H₄)(CH₂)₃(OMe)₂SiMe to give novel stationary phases for ‘Chemistry in Interphases’. The polysiloxane matrices and the integrity of the rhodium(I) complex centers were investigated by means of multinuclear solid-state NMR (¹³C, ²⁹Si, ³¹P) and EXAFS spectroscopies. Dynamic NMR measurements show an increasing mobility of the matrix and the reactive centers with a higher amount of the co-condensing component. The accessibility of the anchored rhodium(I) centers was scrutinized by the metal catalyzed hydrogenation of 1-hexene. All applied xerogels show remarkable activities and selectivities. An enhancement of the activities is achieved when polar solvents are used. SEM micrographs reveal the morphology of the hybrid materials and energy dispersive X-ray spectroscopy (EDX) suggests that the distribution of the elements is in satisfying agreement with the applied composition.     

Antifungal activity of rhodium, iridium, and ruthenium tripodal phosphine complexes

Twenty-eight rhodium, iridium or ruthenium complexes were evaluated for their in vitro antifungal activities against Candida albicans and Candida tropicalis. Fourteen compounds showed an antifungal activity against C. albicans and C. tropicalis with a range of the minimum inhibitor concentrations (MICs) between 16 and 250 micrograms/mL.     

The structure of 2-carboxyquinolinatobis(triphenylphosphite)rhodium(I)

2-Carboxyquinolinatobis(triphenylphosphite)rhodium (I) was prepared by means of the following reaction: [Rh(Qin)(CO)₂] + 2P(OPh)₃→ [Rh(Qin)(P(OPh)₃)₂] + 2CO It crystallizes in the triclinic space groupP] witha = 12.406,b = 18.702,c = 9.547 Å, α = 76.36, β = 111.35, γ = 97.88^o and Z = 2. The structure was determined from 4520 observed reflections. the final R value was 0.051. The RhP bond distances may indicate (although the difference is only about 3σ) that the nitrogen atom the chelate ring has the largest trans influence. The chelate ring is significantly folded along the N---O axis.     

Mechanism of asymmetric hydrogenation by rhodium complexes with unsymmetrical P-chirogenic bisphosphine ligands

Using a series of the rhodium complexes with (1S,2S)-1-(R(1))methylphosphino-2-(R(2))(R(3))phosphinoethane (R(1), R(2) and R(3) = 1-adamantyl, t-butyl, cyclohexyl, cyclopentyl, methyl; abbreviated as unsymmetrical BisP*), very high enantioselectivities were observed when the di- or tri- substituted and tetra-substituted dehydro-alpha-amino acid derivatives were used as the substrates. The main factor to give high enantioselectivity is the repulsive interaction between the functional groups of the substrate and the bulky substituents of the unsymmetrical BisP*. Since the unsymmetrical BisP* has two independent chiral phosphorous atoms in the vicinity of the active site, the higher enantioselectivity than those by the C2 symmetric BisP* complexes can be obtained. Moreover, the fine-tuning to obtain extremely high enantioselectivity may be possible by changing the combination of the substituents on the two phosphorous atoms of the unsymmetrical BisP*.     

Chirality responsive helical poly(phenylacetylene) bearing L-proline pendants

A novel, optically active, cis-transoidal poly(phenylacetylene) bearing an L-proline residue as the pendant group (poly-1) was prepared by the polymerization of the corresponding monomer using a rhodium catalyst in water, and its chiroptical property was investigated using circular dichroism spectroscopy. Poly-1 showed intense Cotton effects in the UV-visible region of the polymer backbone in water, resulting from the prevailing one-handed helical conformation induced by the covalent-bonded chiral L-proline pendants and exhibited a unique helix-sense inversion in response to external, achiral, and chiral stimuli, such as the solvent and interactions with chiral small molecules. We found that poly-1 could enantioselectively trap 1,1'-2-binaphthol within its hydrophobic helical cavity inside the polymer in aqueous media and underwent an inversion of its helical sense in the presence of one of the enantiomers. The effect of the optical purity of 1,1'-2-binaphthol on the chiroptical properties of poly-1 was also investigated.     

Novel rhodium porphyrin derivatives

Hydrated rhodium trichloride reacts with porphyrins in dimethylformamide to give bis-dimethylamino (DMA) derivatives of general formula [RhP(DMA)₂- X]. Modifications of the spectral properties caused by anion exchange are discussed.     

Structure of a photoactive rhodium complex intercalated into DNA

Intercalating complexes of rhodium(III) are strong photo-oxidants that promote DNA strand cleavage or electron transfer through the double helix. The 1.2 A resolution crystal structure of a sequence-specific rhodium intercalator bound to a DNA helix provides a rationale for the sequence specificity of rhodium intercalators. It also explains how intercalation in the center of an oligonucleotide modifies DNA conformation. The rhodium complex intercalates via the major groove where specific contacts are formed with the edges of the bases at the target site. The phi ligand is deeply inserted into the DNA base pair stack. The primary conformational change of the DNA is a doubling of the rise per residue, with no change in sugar pucker from B-form DNA. Based upon the five crystallographically independent views of an intercalated DNA helix observed in this structure, the intercalator may be considered as an additional base pair with specific functional groups positioned in the major groove.     

A circular dichroism and fluorescence quenching study of the interactions between rhodium(II) complexes and human serum albumin.

Various divalent rhodium complexes Rh2(L)4 (L = acetate, propionate, butyrate, trifluoroacetate and trifluoroacetamidate) have been found to bind to non-defatted human serum albumin (HSA) at molar ratios about 8:1. The circular dichroism measurements showed that the more liposoluble carboxylates, butyrate and trifluoroacetate, caused the major alterations of the secondary structure of HSA. Stern-Volmer constants for the fluorescence quenching of the buried Trp214 residue by these complexes were also higher for the lipophilic metal compounds. In the case of the rhodium carboxylates it was observed that their denaturating and quenching properties could be explained in terms of their liposolubilities: the higher their lipophilic characters, the higher their abilities to penetrate inside the protein framework leading to structural alterations, and the closer they could get to the Trp residue causing fluorescence quenching. The liposoluble amidate complex, Rh2 (tfc)4, presented an intermediate quenching and did not cause structural alterations in the protein, presumably not penetrating inside the peptidic backbone. This study shows that it is possible to design new antitumor metal complexes which bind, to a large extent, to a transport protein causing little structural damage.