Rhodium-catalyzed addition of aryldifluoromethylsilanes to N-sulfonylaldimines

The addition of aryldifluoromethylsilanes to N-sulfonylaldimines was found to be catalyzed by a rhodium complex, [Rh(cod)(MeCN)₂]BF₄, in the presence of potassium fluoride to give the corresponding arylated N-sulfonylamines in good yield. The reaction mechanism would involve the generation of a fluoride-coordinated arylsilicate and the transmetalation between the arylsilicate and the rhodium complex to give the arylrhodium species as a key intermediate.     

Theoretical study on the reaction mechanism of NO and CO catalyzed by Rh atom

The gas-phase reaction mechanism of NO and CO catalyzed by Rh atom has been systematically investigated on the ground and first excited states at CCSD(T)//B3LYP/6-311+G(2d), SDD level. This reaction is mainly divided into two reaction stages, NO deoxygenation to generate N₂O and then the deoxygenation of N₂O with CO to form N₂ and CO₂. The crucial reaction step deals with the NO deoxygenation to generate N₂O catalyzed by Rh atom, in which the self-deoxygenation of NO reaction pathway is kinetically more preferable than that in the presence of CO. The minimal energy reaction pathway includes the rate-determining step about N–N bond formation. Once the NO deoxygenation with CO catalyzed by rhodium atom takes place, the reaction results in the intermediate RhN. Then, the reaction of RhN with CO is kinetically more favorable than that with NO, while both of them are thermodynamically preferable. These results can qualitatively explain the experimental finding of N₂O, NCO, and CN species in the NO + CO reaction. For the N₂O deoxygenation with CO catalyzed by rhodium atom, the reaction goes facilely forward, which involves the rate-determining step concerning CO₂ formation. CO plays a dominating role in the RhO reduction to regenerate Rh atom. The complexes, OCRhNO, RhON₂, RhNNO, ORhN₂, RhCO₂, RhNCO, and ORhCN, are thermodynamically preferred. Rh atom possesses stronger capability for the N₂O deoxygenation than Rh⁺ cation.     

[RhIII(Cp*)]‐Catalyzed ortho‐Selective Direct C(sp2)H Bond Amidation/Amination of Benzoic Acids by N‐Chlorocarbamates and N‐Chloromorpholines. A Versatile Synthesis of Functionalized Anthranilic Acids

A Rh^III‐catalyzed direct ortho‐C?H amidation/amination of benzoic acids with N‐chlorocarbamates/N‐chloromorpholines was achieved, giving anthranilic acids in up to 85 % yields with excellent ortho‐selectivity and functional‐group tolerance. Successful benzoic acid aminations were achieved with carbamates bearing various amide groups including NHCO₂Me, NHCbz, and NHTroc (Cbz=carbobenzyloxy; Troc=trichloroethylchloroformate), as well as secondary amines, such as morpholines, piperizines, and piperidines, furnishing highly functionalized anthranilic acids. A stoichiometric reaction of a cyclometallated rhodium(III) complex of benzo[h]quinoline with a silver salt of N‐chlorocarbamate afforded an amido–rhodium(III) complex, which was isolated and structurally characterized by X‐ray crystallography. This finding confirmed that the C?N bond formation results from the cross‐coupling of N‐chlorocarbamate with the aryl–rhodium(III) complex. Yet, the mechanistic details regarding the C?N bond formation remain unclear; pathways involving 1,2‐aryl migration and rhodium(V)– nitrene are plausible.     

Rhodium(I) Pincer Complexes of Nitrous Oxide

The synthesis of two well‐defined rhodium(I) complexes of nitrous oxide (N₂O) is reported. These normally elusive adducts are stable in the solid state and persist in solution at ambient temperature, enabling comprehensive structural interrogation by ¹⁵N NMR and IR spectroscopy, and single‐crystal X‐ray diffraction. These methods evidence coordination of N₂O through the terminal nitrogen atom in a linear fashion and are supplemented by a computational energy decomposition analysis, which provides further insights into the nature of the Rh–N₂O interaction.     

Structural evolution of a recoverable rhodium hydrogenation catalyst

A recoverable, water soluble, hydrogenation catalyst was synthesized by reacting poly-N-isopropylacrylamide containing a terminal amino group (H₂N-CH₂CH₂-S-pNIPAAm) with [Rh(CO)₂Cl]₂ in organic solvents to form the square planar rhodium complex (Rh(CO)₂Cl(H₂N-CH₂CH₂-S-pNIPAAm)). The catalyst-ligand structure was characterized using in situ multinuclear NMR, XAFS and IR spectroscopic methods. Model complexes containing glycine (H₂NCH₂COOH), cysteamine (H₂NCH₂CH₂SH) and methionine methyl ester (H₂NCH(CH₂CH₂SCH₃)COOCH₃) ligands were studied to aid in the interpretation of the coordination sphere of the rhodium catalyst. The spectroscopic data revealed a switch in ligation from the amine bound (Rh-NH₂-CH₂CH₂-S-pNIPAAm) to the thioether bound (Rh-S(-CH₂CH₂NH₂)(-pNIPAAm)) rhodium when the complex was dissolved in water. The evolution of the structure of the rhodium complex dissolved in water was followed by XAFS. The structure changed from the expected monomeric complex to form a rhodium cluster of up to four rhodium atoms containing one SRR′ ligand and one CO ligand per rhodium center. No metallic rhodium was observed during this transformation. The rhodium-rhodium interactions were disrupted when an alkene (3-butenol) was added to the aqueous solution. The kinetics of the hydrogenation reaction were measured using a novel high-pressure flow-through NMR system and the catalyst was found to have a TOF of 3000/Rh/h at 25 °C for the hydrogenation of 3-butenol in water.     

Hydrogenation of olefins using water and zinc metal catalyzed by a rhodium complex

The hydrogenation of olefins using H₂O or D₂O as a hydrogen source and zinc metal as a reducing agent has been found to be catalyzed by a rhodium complex. α,β-Unsaturated ketones also underwent hydrogenation, affording the corresponding saturated ketones selectively.     

Catalytic asymmetric hydrogenation of indoles using a rhodium complex with a chiral bisphosphine ligand PhTRAP

Highly enantioselective hydrogenation of N-protected indoles was successfully developed by use of the rhodium catalyst generated in situ from [Rh(nbd)₂]SbF₆ and the chiral bisphosphine PhTRAP, which can form a trans-chelate complex with a transition metal atom. The PhTRAP–rhodium catalyst required a base (e.g., Cs₂CO₃) for the achievement of high enantioselectivity. Various 2-substituted N-acetylindoles were converted into the corresponding chiral indolines with up to 95% ee. The hydrogenations of 3-substituted N-tosylindoles yielded indolines possessing a stereogenic center at the 3-position with high enantiomeric excesses (up to 98% ee).     

Transformation of rhodium carbonyl complexes in hydroformylation of 1-hexene studied fromin situ IR spectroscopic data

Rhodium carbonyl complexes that formed from RhCl₃·4H₂O and RhCl₃·4H₂O modified by poly-N,N-dimethyl-N,N-diallylammonium chloride in a methanol—chloroform medium in the hydroformylation of 1-hexene were studied byin situ IR spectroscopy. Along with the rhodium hydrocarbonyl complexes, anionic complexes of the [Rh(CO)₂Cl₂]⁻ type, whose concentrations and rates of formation in an acidic medium are much higher than those in a basic medium, were shown to be the active centers of hydroformylation. The function of the polycation is the stabilization of the catalytically active mononuclear rhodium complexes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 708–710, April, 1999.     

NH3 Synthesis in the N2/H2 Reaction System using Cooperative Molecular Tungsten/Rhodium Catalysis in Ionic Hydrogenation: A DFT Study

The ionic hydrogenation of N₂ with H₂ to give NH₃ is investigated by means of density functional theory (DFT) computations using a cooperatively acting catalyst system. In this system, N₂ binds to a neutral tungsten pincer complex of the type [(PNP)W(N₂)₃] (PNP=pincer ligand) and is reduced to NH₃. The protons and hydride centers necessary for the reduction are delivered by heterolytic cleavage of H₂ between the N₂–tungsten complex and the cationic rhodium complex [Cp*Rh{2‐(2‐pyridyl)phenyl}(CH₃CN)]⁺. Successive transfer of protons and hydrides to the bound N₂, as well as all N_xH_y units that occur during the reaction, enable the computation of closed catalytic cycles in the gas and in the solvent phase. By optimizing the pincer ligands of the tungsten complex, energy spans as low as 39.3 kcal mol^?1 could be obtained, which is unprecedented in molecular catalysis for the N₂/H₂ reaction system.     

Probing the steric limits of rhodium catalyzed hydrophosphinylation. P-H addition vs. dimerization/oligomerization/polymerization

The reactivity of secondary phosphine oxides containing bulky organic fragments in hydrophosphinylation reactions has been investigated using several rhodium based catalysts. Upon heating in a focused microwave reactor, HP(O)(2-C₆H₄Me)₂ adds to prototypical terminal alkynes affording a complex mixture containing 1,2 and 1,1-addition products. Addition of a second ortho-substituent (HP(O)Mes₂) completely suppresses the hydrophosphinylation reaction for alkyl and aryl substituted alkynes. Variations in the temperature, catalyst loading, solvent, and microwave power were unable to induce an addition reaction in the case of HP(O)Mes₂. While this secondary phosphine oxide did not participate in the hydrophosphinylation reaction, it promoted the polymerization of phenylacetylene. HP(O)R₂ substrates are not commonly thought of as innocent ligands for rhodium complexes in reactions involving alkynes due to facile hydrophosphinylation. While this is certainly true for diphenylphosphine oxide, the chemistry presented herein suggests that HP(O)Mes₂ and related bulky secondary phosphine oxides have great potential as valuable ligands for rhodium catalyzed transformations involving alkynes due to their lack of reactivity towards the addition reaction.     

Simple Amides and Amines for the Synergistic Recovery of Rhodium from Hydrochloric Acid by Solvent Extraction

The separation and isolation of many of the platinum group metals (PGMs) is currently achieved commercially using solvent extraction processes. The extraction of rhodium is problematic however, as a variety of complexes of the form [RhCl_n(H₂O)_6-n]⁽ⁿ⁻³⁾⁻ are found in hydrochloric acid, making it difficult to design a reagent that can extract all the rhodium. In this work, the synergistic combination of a primary amine (2-ethylhexylamine, L^A) with a primary amide (3,5,5-trimethylhexanamide, L¹) is shown to extract over 85 % of rhodium from 4 M hydrochloric acid. Two rhodium complexes are shown to reside in the organic phase, the ion-pair [HL^A]₃[RhCl₆] and the amide complex [HL^A]₂[RhCl₅(L¹)]; in the latter complex, the amide is tautomerized to its enol form and coordinated to the rhodium centre through the nitrogen atom. This insight highlights the need for ligands that target specific metal complexes in the aqueous phase and provides an efficient synergistic solution for the solvent extraction of rhodium.     

Remarks on the process of homogeneous carbonylation of rhodium compounds by N,N-dimethylformamide

Reductive carbonylation of rhodium(III) chloride complexes, commercial RhCl₃ · nH₂O neutralized with BaCO₃, (Me₂NH₂)₂[RhCl₅(DMF)], (PPh₄)[RhCl₄(H₂O)₂], RhCl₃(DMF)₃, RhCl₃(CH₃CN)₃, RhCl₃(CH₃CN)₂(DMF), [Rh(CO)₂Cl₃]₂, and rhodium(I) complex, Rh(PPh₃)₃Cl, by N,N-dimethylformamide (DMF) is studied. The data obtained support the conception of direct carbonyl group transfer from DMF molecule to the Rh metal center. The mechanistic scheme of carbonylation process is refined and discussed with regard of new experimental results.     

Rhodium(III), palladium(II), and platinum(II) complexes with 2-(2-hydroxybenzoyl)-N-methylhydrazinecarbothioamide: Syntheses,structures, and spectral characteristics

The syntheses and spectral (IR, UV-VIS, XPS, and ¹H and ¹³C NMR) characteristics of the rhodium(III), palladium(II), and platinum(II) complexes with 2-(2-hydroxybenzoyl)-N-methylhydrazinecarbothioamide (HBMHCTA) are described. The coordination of HBMHCTA to the central metal ion and its intraligand rearrangement in the complex formation of rhodium(III) ions are studied. The structure of the mixed-ligand complex [Pd(H₂L)PPh₃] is determined by X-ray diffraction analysis.     

Reaction of formaldehyde with the Wilkinson complex in the presence of carboxylic acid amides

N,N-Dimethylacetamide and N, N-dimethyformamide react with RhCl(PPh₃)₃ with displacement of PPh₃ and the formation of a complex with the amide. Formamide and N-propylacetamide do not form similar complexes under similar conditions. In contrast to the reaction of RhCl(PPh₃)₃, which leads to the formation of RhCl(CO)(PPh₃)₂ due to decarbonylation of CH₂O, stabilization of the ₂-CH₂O form of the CH₂O coordinated with rhodium is likely in the reaction of formaldehyde with a rhodium complex containing an N-bonded amide. Under the conditions of hydroformylation of CH₂O in a solution of the Wilkinson complex in an unsubstituted amide the dominating pathway of the transformation of formaldehyde is its reaction with the solvent or the ammonia formed via decarbonylation of the unsubstituted amide.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2670–2673, December, 1989.     

Di‐μ‐methanolato‐bis[(η4‐tetrafluorobenzobarrelene)rhodium(I)]

The versatile synthetic precursor methanolate‐bridged title rhodium complex, [Rh₂(CH₃O)₂(C₁₂H₆F₄)₂] or [Rh(μ‐OCH₃)(tfbb)]₂ [tfbb = tetrafluorobenzobarrelene or 3,4,5,6‐tetrafluorotricyclo[6.2.2.0^2,7]dodeca‐2(7),3,5,9,11‐pentaene], has been structurally characterized. The asymmetric unit contains half a molecule that can be expanded via a twofold axis. The title compound has been shown to be a dinuclear rhodium complex where each metal centre is coordinated by two O atoms from two bridging methanolate groups and by the olefinic bonds of a tfbb ligand. Comparison of the bite angles of tfbb, norbornadiene (nbd) and cyclooctadiene (cod) olefins in their η⁴‐coordination to rhodium reveals similarities between the tfbb and nbd ligands, which are much more rigid than cod. The short distance found between the distorted square‐planar metal centres [2.8351 (4) Å] has been related to the syn conformation of the folded core `RhORhO' ring.     

Bridge functionalized bis-N-heterocyclic carbene rhodium(I) complexes and their application in catalytic hydrosilylation

A series of chelating bridge functionalized bis-N-heterocyclic carbenes (NHC) complexes of rhodium (I) were prepared by reacting the corresponding imidazolium salts with [Rh(COD)Cl]₂ in an in-situ reaction. For the N-methyl substituted complex with a PF₆-anion an X-ray crystal structure was exemplary obtained. All complexes were spectroscopically characterized and tested for the hydrosilylation of acetophenone.     

The heterogeneous formation of N2O in the presence of acidic solutions: Experiments and modeling

We investigated the heterogeneous processes that contribute towards the formation of N₂O in an environment that comes as closely as possible to exhaust conditions containing NO and SO₂ among other constituents. The simultaneous presence of NO, SO₂, O₂, and condensed phase water in the liquid state has been confirmed to be necessary for the production of significant levels of N₂O. The maximum rate of N₂O formation occurred at the beginning of the reaction and scales with the surface area of the condensed phase and is independent of its volume. The replacement of NO by either NO₂ or HONO significantly increases the rate constant for N₂O formation. The measured reaction orders in the rate law change depending upon the choice of the nitrogen reactant used and were fractional in some cases. The rate constants of N₂O formation for the three different nitrogen reactants reveal the following series of increasing reactivity: NO < NO₂ < HONO, indicating the probable sequential involvement of those species in the elementary reactions. Furthermore, we observed a complex dependence of the rate constant on the acidity of the liquid phase where both the initial rate as well as the yield of N₂O are largest at pH=0 of a H₂SO₄/H₂O solution. The results suggest that HONO is the major reacting N(III) species over a wide range of acidities studied. The N₂O formation in synthetic flue gas may be simulated using a relatively simple mechanism based on the model of Lyon and Cole. The first step of the complex overall reaction corresponds to NO oxidation by O₂ to NO₂ mainly in the gas phase, with the presence of both H₂O and active surfaces significantly accelerating NO₂ production. Subsequently, NO₂ reacts with excess NO to obtain HONO which reacts with S(IV) to result in N₂O and H₂SO₄ through a complex reaction sequence probably involving nitroxyl (HON) and its dimer, hyponitrous acid. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet: 29 : 869–891, 1997.     

Physicochemical study of rhodium sulfite

Diamagnetic rhodium sulfite Na₁₅(NH₄)₃[Rh₄(μ-SO₃)₆(SO₃)₇(H₂O)₅]·10H₂O was synthesized. An X-ray photoelectron spectroscopy study and chemical analysis revealed that rhodium in this compound was in the oxidation state 2+. The thermal and chemical properties of the compound were investigated. Based on XRPA, IR, and EPR spectroscopy and analytical data about the chemical properties, a hypothesis about the structure of the complex has been formulated.     

Rhodium-catalyzed addition of arylstannanes to carbon-heteroatom double bond

The addition of arylstannanes to the carbon-heteroatom double bond in the presence of a catalytic amount of a cationic rhodium complex ([Rh(cod)(MeCN)₂]BF₄) was examined. The reactions of aldehydes, α-dicarbonyl compounds, and N-substituted aldimines with the arylstannanes gave corresponding alcohols, α-hydroxy carbonyl compounds, and amines, respectively. An arylrhodium complex generated by the transmetalation with the arylstannane was probably the active catalytic species.     

Preparation and characterization of a new SNO ligand derived from ethanebis(thioamide) and 2-hydroxybenzaldehyde, and its Cu(II), Co(II), and Rh(III) metal complexes

A new Schiff base N-[(E)-(2-hydroxyphenyl)methylidene]-N’-[(Z)-(2-hydroxyphenyl)methylidene]ethanebis(thioamide) (L_C) containing sulfur, nitrogen, and oxygen atoms has been synthesized by condensation of ethanebis(thioamide) with 2-hydroxybenzaldehyde. Metal complexes were synthesized by reaction of the new ligand with copper(II) and cobalt(II) as nitrate salts and with rhodium(III) as chloride salt, using hot absolute ethanol as solvent. All the new compounds were characterized by use of different physicochemical techniques including UV–visible spectroscopy, magnetic susceptibility, IR spectroscopy, molar conductance, and determination of metal content. It is proposed the paramagnetic copper and cobalt complexes adopt octahedral geometry whereas the diamagnetic rhodium complex has octahedral geometry.