Rhodium Carbonyl Research Articles

An automated Peak Group Analysis for vibrational spectra analysis

Peak Group Analysis (PGA) is a multivariate curve resolution technique that attempts to extract single pure component spectra from time series of spectral mixture data. It requires that the mixture spectra consist of relatively sharp peaks, as is typical in IR and Raman spectroscopy. PGA aims to construct from individual peaks the associated pure component spectra in the form of nonnegative linear combinations of the right singular vectors of the spectral data matrix.This work presents an automated PGA (autoPGA) that starts with upstream peak detection applied to time series of spectra, combining different window-based peak detection techniques with balanced peak acceptance criteria and peak grouping to deal with repeated detections. The next step is a single-spectrum-oriented PGA analysis. This is followed by a downstream correlation analysis to identify pure component spectra that occur multiple times. AutoPGA provides a complete pure component factorization of the matrix of measured data. The algorithm is applied to FT-IR data sets on various rhodium carbonyl complexes and from an equilibrium of iridium complexes.

Acetylene ligands stabilize atomically dispersed supported rhodium complexes under harsh conditions

Facile sintering of atomically dispersed supported noble metal catalysts at catalytically relevant temperatures, particularly under reducing conditions, poses a challenge for their practical applications. Some ligands, such as carbonyls, aid in improving the stability at the expense of severely suppressing the catalytic activity. Here, we demonstrate that substitution of the carbonyl ligands with reactive acetylene ligands can maintain the atomic dispersion of the supported mononuclear rhodium complex under harsh reducing conditions (>573 K), as confirmed by in-situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. In contrast, the supported rhodium carbonyl complex aggregates into nanoclusters under identical conditions. Furthermore, our results indicate that the acetylene ligands provide this anti-sintering ability while retaining the hydrogenation activity.

Hydroformylation and Hydrogenation of Sinapyl Alcohol, Catalyzed by Cobalt and Rhodium Carbonyl Complexes

Hydroformylation and Hydrogenation of Sinapyl Alcohol, Catalyzed by Cobalt and Rhodium Carbonyl Complexes

In situ FTIR spectroscopic investigations on rhodium carbonyl complexes in the absence of phosphorus ligands under hydroformylation conditions

The composition of mixtures of rhodium carbonyl complexes in the absence of P-ligands under varying conditions relevant for hydroformylation have been studied by in situ FTIR spectroscopy.

A Δ-learning strategy for interpretation of spectroscopic observables.

Accurate computations of experimental observables are essential for interpreting the high information content held within x-ray spectra. However, for complicated systems this can be difficult, a challenge compounded when dynamics becomes important owing to the large number of calculations required to capture the time-evolving observable. While machine learning architectures have been shown to represent a promising approach for rapidly predicting spectral lineshapes, achieving simultaneously accurate and sufficiently comprehensive training data is challenging. Herein, we introduce Δ-learning for x-ray spectroscopy. Instead of directly learning the structure-spectrum relationship, the Δ-model learns the structure dependent difference between a higher and lower level of theory. Consequently, once developed these models can be used to translate spectral shapes obtained from lower levels of theory to mimic those corresponding to higher levels of theory. Ultimately, this achieves accurate simulations with a much reduced computational burden as only the lower level of theory is computed, while the model can instantaneously transform this to a spectrum equivalent to a higher level of theory. Our present model, demonstrated herein, learns the difference between TDDFT(BLYP) and TDDFT(B3LYP) spectra. Its effectiveness is illustrated using simulations of Rh L3-edge spectra tracking the C-H activation of octane by a cyclopentadienyl rhodium carbonyl complex.

A New Process for Efficient Recovery of Rhodium from Spent Carbonyl Rhodium Catalyst by Microreactor.

Triphenylphosphine acetylacetone carbonyl rhodium (ROPAC) is an important catalyst in the petrochemical industry, and its deactivated waste catalyst holds significant value for recovery. This study focuses on the existing forms of rhodium (Rh) in waste catalysts and the current status of traditional processes. A green, efficient, and continuous recovery technique was developed using a sealed stainless steel microchannel reactor. The influence of reaction temperature, reaction time, and phase ratio on the Rh recovery rate was investigated, and the process parameters were optimized using response surface methodology (RSM). The results indicate that the magnitude of the impact on the Rh recovery rate follows the order: reaction temperature > reaction time > phase ratio. The optimized process parameters were determined as follows: a reaction time of 29 min, a reaction temperature of 110 °C, and a phase ratio of 1:1, with a corresponding maximum recovery rate of Rh of 66.06%. Furthermore, secondary treatment was performed on the organic phase after primary recovery using the same process conditions, resulting in an overall Rh recovery rate of 95.6%, indicating satisfactory recovery efficiency. Moreover, the application of FTIR and ICP-OES analysis provided definitive evidence that the oxidative dissociation of the rhodium-phosphine chemical bond by H2O2 within ROPAC leads to the conversion of Rh+ into Rh3+. Subsequently, Rh forms chloroaquorhodium (III) complexes that enter the aqueous phase, enabling effective recovery of Rh.

Tracking C-H activation with orbital resolution.

Transition metal reactivity toward carbon-hydrogen (C-H) bonds hinges on the interplay of electron donation and withdrawal at the metal center. Manipulating this reactivity in a controlled way is difficult because the hypothesized metal-alkane charge-transfer interactions are challenging to access experimentally. Using time-resolved x-ray spectroscopy, we track the charge-transfer interactions during C-H activation of octane by a cyclopentadienyl rhodium carbonyl complex. Changes in oxidation state as well as valence-orbital energies and character emerge in the data on a femtosecond to nanosecond timescale. The x-ray spectroscopic signatures reflect how alkane-to-metal donation determines metal-alkane complex stability and how metal-to-alkane back-donation facilitates C-H bond cleavage by oxidative addition. The ability to dissect charge-transfer interactions on an orbital level provides opportunities for manipulating C-H reactivity at transition metals.

(N-Benzoyl-N-phenyl-hydroxy-laminato)carbon-yl(tri-phenyl-arsine)rhodium(I).

The mol-ecule of the title compound, [Rh(C13H10NO2){As(C6H5)3}(CO)] or [Rh(BPHA)(AsPh3)(CO)] (BPHA is the N-benzoyl-N-phenyl-hydroxy-laminate anion), comprises a bidentate N-benzoyl-N-phenyl-hydroxy-laminate anion coordinating through the O atoms to the soft Lewis acid, rhodium(I), and two monodentate ligands, viz. tri-phenyl-arsine and carbonyl. The resulting CO2As coordination environment around the central RhI atom is distorted square planar.=.

Probing the donor strength of yldiide ligands: synthesis, structure and reactivity of rhodium complexes with a PCylideN pincer ligand.

Control of the metal ligand interaction by changes in the ligand protonation state is vital to many catalytic transformations based on metal-ligand cooperativity. Herein, we report on the coordination chemistry of a new PCy(H)N pincer ligand with a central ylide as donor site, which through deprotonation to the corresponding yldiide changes from a neutral L3-type ligand to an anionic L2X-type PCYN ligand. The isolation of a series of rhodium complexes showed that the strong donor ability of the neutral ylide PCY(H)N is further increased upon deprotonation, as evidenced by one of the lowest reported CO stretching frequencies in complex [(PCYN)Rh(CO)] (2) compared to other known rhodium carbonyl complexes. DFT calculations revealed that the high donor ability mostly results from the antibonding interaction of the pπ orbital at the ylide with the d xz orbital at rhodium, which enhances the backdonation into the π* orbital of the CO ligand. This unique interaction results in a rather long metal-carbon bond, but still a strong activation of the CO ligand in order to minimize repulsion between the filled orbitals at the rhodium and the ylide ligand. Accordingly, CO by phosphine replacement leads to a strong deviation from the square-planar geometry in the analogous phosphine complexes [(PCYN)Rh(PR3)] and an unusual reactivity with small alkyl halides, which upon oxidative addition add to the CO ligand, before inserting into the P-C bond in the pincer ligand. These results demonstrate the unique donor strength of yldiide ligands and their potential in the activation of strong bonds.

Dipyrromethane-Based PGeP Pincer Germyl Rhodium Complexes.

A family of germyl rhodium complexes derived from the PGeP germylene 2,2’‐bis(di‐isopropylphosphanylmethyl)‐5,5’‐dimethyldipyrromethane‐1,1’‐diylgermanium(II), Ge(pyrmP i Pr2)2CMe2 (1), has been prepared. Germylene 1 reacted readily with [RhCl(PPh3)3] and [RhCl(cod)(PPh3)] (cod=1,5‐cyclooctadiene) to give, in both cases, the PGeP‐pincer chloridogermyl rhodium(I) derivative [Rh{κ3 P,Ge,P‐GeCl(pyrmP i Pr2)2CMe2}(PPh3)] (2). Similarly, the reaction of 1 with [RhCl(cod)(MeCN)] afforded [Rh{κ3 P,Ge,P‐GeCl(pyrmP i Pr2)2CMe2}(MeCN)] (3). The methoxidogermyl and methylgermyl rhodium(I) complexes [Rh{κ3 P,Ge,P‐GeR(pyrmP i Pr2)2CMe2}(PPh3)] (R=OMe, 4; Me, 5) were prepared by treating complex 2 with LiOMe and LiMe, respectively. Complex 5 readily reacted with CO to give the carbonyl rhodium(I) derivative [Rh{κ3 P,Ge,P‐GeR(pyrmP i Pr2)2CMe2}(CO)] (6), with HCl, HSnPh3 and Ph2S2 rendering the pentacoordinate methylgermyl rhodium(III) complexes [RhHX{κ3 P,Ge,P‐GeMe(pyrmP i Pr2)2CMe2}] (X=Cl, 7; SnPh3, 8) and [Rh(SPh)2{κ3 P,Ge,P‐GeMe(pyrmP i Pr2)2CMe2}] (9), respectively, and with H2 to give the hexacoordinate derivative [RhH2{κ3 P,Ge,P‐GeMe(pyrmP i Pr2)2CMe2}(PPh3)] (10). Complexes 3 and 5 are catalyst precursors for the hydroboration of styrene, 4‐vinyltoluene and 4‐vinylfluorobenzene with catecholborane under mild conditions.

New mechanistic insights into direct ethylene glycol synthesis from syngas over modified Rh carbonyl catalysts

Direct ethylene glycol (EG) synthesis from syngas as one of typical atom-economic reaction is a big challenge in catalysis, attracting wide attention in scientific research. Here, the mechanistic studies on direct syngas-to-EG conversion over the rhodium carbonyl catalyst Rh4(CO)12 modified by triethylamine (Et3N), 1,3-dimethyl-2-imidalidinone (DMI), and triisopropylphosphine (PiPr3) ligands were performed using combined density functional theory (DFT) calculation and the microkinetic modeling. The results demonstrate the significant improvement of selectivity from syngas to EG on going from Rh4(CO)12 to Rh4(CO)10(Et3N)(DMI) and Rh4(CO)10(DMI)(PiPr3) systems on account of the formation of CH2OH species being more favorable than CH3O. More importantly, the effect of different ligands makes a clear change in the carbon chain growth mechanism from the CO insertion into CH2OH in the Rh4(CO)10(DMI)(PiPr3) to the CHO coupling in the Rh4(CO)10(Et3N)(DMI). According to Bader charge and canonical molecular orbital analyses, it is suggested that the facilitated CHO coupling in the Rh4(CO)10(Et3N)(DMI) is associated with stronger electrostatic attraction and π-π conjugation between two CHO intermediates, and the π and σ bonding interactions between CHO and DMI. Such a fundamental mechanistic understanding reveals the nature of tuning the catalytic performance using different ligands, and also opens a new OHCCHO-mediated pathway for direct EG synthesis from syngas over Rh carbonyl catalysts.

Rhodium(I) carbonyl complexes containing amino acid ester ligands: synthesis, reactivity and DFT studies

Bridge-splitting reaction of the dimer [Rh(CO)2Cl]2 with two molar equivalents of amino acid ester ligands yield the rhodium(I) dicarbonyl complex [Rh(CO)2ClL] (1a-g) {L = glycine methylester (a), alanine methylester (b), alanine ethylester (c), alanine t-butylester (d), phenylalanine methylester (e), phenylalanine ethylester (f), valine methylester (g)}. Complexes 1 undergo oxidative addition (OA) with CH3I to give the five-coordinate Rh(III) complexes of the type [Rh(CO)(COCH3)IClL] (2a–g). The complexes have been characterized by elemental analysis, IR and 1H NMR spectroscopy. Theoretical calculations reveal that the synthesized complexes 1 are very prone to undergo oxidative addition with CH3I with a low activation barrier.

Spectroscopic evidence of the simultaneous participation of rhodium carbonyls and surface formate species during the CO2 methanation catalyzed by ZrO2-supported Rh

ZrO2-supported rhodium nanoparticles prepared by impregnation of RhCl3 are active for CO2 methanation at temperatures above 180 °C. Infrared (IR) spectra recorded during catalysis allowed identification of Rh carbonyls and formate species bonded to the support. To verify their individual involvement in the catalysis, their transformations were investigated by IR spectra measured as a sample of ZrO2-supported Rh was treated with CO and isotopically labelled formic acid. The data indicate two coexisting reaction routes: a dissociative route in which CO2 reacts on the Rh sample to give Rh carbonyls, and an associative route in which the CO2 molecule is activated on the support, in the form of bicarbonate species that are hydrogenated to give formate species prior to methane formation. Our data show that labelled formates are transformed into Rh–13CO, thus connecting the dissociative and associative catalytic routes. The results indicate the existence of a dual mechanism for CO2 methanation.

Structure and reactivity of rhodium(I) carbonyl complexes as model nano-wired assemblies and catalysts

Structure and reactivity of rhodium(I) carbonyl complexes as model nano-wired assemblies and catalysts

Preparation of carbonyl rhodium polyether guanidinium ionic liquids and application in asymmetric hydroformylation based on homogeneous catalysis-biphasic separation system

Four carbonyl rhodium polyether guanidinium ionic liquids (Rh(CO)4-PolyGILs) were firstly synthesized by utilizing the ion exchange strategy. This protocol was achieved by introducing polyether guanidinium ionic liquids (PolyGILs) into K[Rh(CO)4]. Asymmetric hydroformylation (AHF) of styrene was employed as probe reaction to investigate the catalytic performance of the above obtained four functional ionic liquids. The AHF of styrene provided 79.00% conversion, 73.30% yield and 54% enantiomeric excess for 2-phenylpropionaldehyde ([Me(EO)16TMG][Rh(CO)4] served as catalyst, (R)-BINAP served as chiral ligand at 60°C and 2.0 MPa for 4 h). In addition, based on the unique solubility properties of the above obtained four ionic liquids, a highly effective homogeneous catalysis-biphasic separation (HCBS) system was established for the AHF. HCBS system with [Me(EO)16TMG][Rh[(R)-BINAP](CO)2] could be reused four times without significantly decrease in the activity. The structure of [Me(EO)16TMG][Rh[(R)-BINAP](CO)2] was further studied by using the density functional theory (DFT) method, which revealed that the final energy of the molecule was tending to -9460.83 eV and the presence of Ph substituent provided a chiral pocket. The presence of closed rigid chiral pocket could lead to better enantioselectivities.

Synthesis and Characterization of Rhodium Complex Bearing Anionic CNC‐Type Pincer Ligand with Pyrrolide and Imidazo[1,5‐a]pyridin‐3‐ylidene Moieties

AbstractA novel monoanionic CNC‐type pincer ligand featuring a pyrrolide as a central donor and imidazo[1,5‐a]pyridin‐3‐ylidenes as arm donors is designed and synthesized. The reaction of the lithium complex prepared from the pincer ligand with [RhCl(CO)2]2 gave the corresponding rhodium(I) carbonyl complex. X‐ray analysis and IR spectroscopy of the rhodium complex indicate that the newly designed and synthesized pincer ligand has an electron‐withdrawing character with a planar backbone.

Rhodium carbonyl complexes featuring carbodiphosphorane-based pincer ligands

The coordination of a pincer ligand with a protonated carbodiphosphorane (CDP) as central donor group, [(dppm)2CH]+ (1), in the rhodium(I) complex [({dppm}2CH)RhCl(CO)]+ (2) is reported (dppm = 1,1-bis(diphenylphosphino)methane). In contrast to related iridium(I) precursors, no C-H-oxidative addition is observed with [RhCl(CO)2]2 in this reaction. The different reactivity of rhodium(I) and iridium(I) precursors is investigated by means of density functional theory (DFT). Finally, the CDP-based pincer-type complex [({dppm}2C)Rh(CO)]+ (3) was obtained by deprotonation of complex 2.

Chemistry of transition-metal complexes containing functionalized phosphines: synthesis and structural analysis of rhodium(I) complexes containing allyl and cyanoalkylphosphines.

A series of related acetylacetonate-carbonyl-rhodium compounds substituted by functionalized phosphines has been prepared in good to excellent yields by the reaction of [Rh(acac)(CO)2] (acac is acetylacetonate) with the corresponding allyl-, cyanomethyl- or cyanoethyl-substituted phosphines. All compounds were fully characterized by 31P, 1H, 13C NMR and IR spectroscopy. The X-ray structures of (acetylacetonato-κ2O,O')(tert-butylphosphanedicarbonitrile-κP)carbonylrhodium(I), [Rh(C5H7O2)(CO)(C8H13N2)] or [Rh(acac)(CO)(tBuP(CH2CN)2}] (2b), (acetylacetonato-κ2O,O')carbonyl[3-(diphenylphosphanyl)propanenitrile-κP]rhodium(I), [Rh(C5H7O2)(C15H14N)(CO)] or [Rh(acac)(CO){Ph2P(CH2CH2CN)}] (2h), and (acetylacetonato-κ2O,O')carbonyl[3-(di-tert-butylphosphanyl)propanenitrile-κP]rhodium(I), [Rh(C5H7O2)(C11H22N)(CO)] or [Rh(acac)(CO){tBu2P(CH2CH2CN)}] (2i), showed a square-planar geometry around the Rh atom with a significant trans influence over the acetylacetonate moiety, evidenced by long Rh-O bond lengths as expected for poor π-acceptor phosphines. The Rh-P distances displayed an inverse linear dependence with the coupling constants JP-Rh and the IR ν(C[triple-bond]O) bands, which accounts for the Rh-P electronic bonding feature (poor π-acceptors) of these complexes. A combined study from density functional theory (DFT) calculations and an evaluation of the intramolecular H...Rh contacts from X-ray diffraction data allowed a comparison of the conformational preferences of these complexes in the solid state versus the isolated compounds in the gas phase. For 2b, 2h and 2i, an energy-framework study evidenced that the crystal structures are mainly governed by dispersive energy. In fact, strong pairwise molecular dispersive interactions are responsible for the columnar arrangement observed in these complexes. A Hirshfeld surface analysis employing three-dimensional molecular surface contours and two-dimensional fingerprint plots indicated that the structures are stabilized by H...H, C...H, H...O, H...N and H...Rh intermolecular interactions.

Subtle variation of stereo-electronic effects in rhodium(I) carbonyl Schiff base complexes and their iodomethane oxidative addition kinetics

Rhodium(I) carbonyl complexes of the form [RhI(N,O-ScBa)(CO)(PR3)] (R = Ph or Cy), with N,O-ScBaH a mono anionic bidentate Schiff base ligand, 2-(cyclopentyliminomethyl)-5-methylphenol (5-Me-Sal-CyPH), and bearing different phosphine ligands on Rh(I), are reported. N,O-ScBaH was also varied to be 2-(cyclohexyliminomethyl)phenol (Sal-CyH) and 2-(isopropyliminomethyl)-5-methylphenol (5-Me-Sal-IPropH) to investigate the effects of different substituent groups on the periphery of the Schiff bases on metal center reactivity. The structural characterization of two sample complexes is described and an extensive spectroscopic kinetic-mechanistic study utilizing UV/vis, infrared and 31P NMR spectroscopy of the iodomethane oxidative addition is presented. Iodomethane oxidative addition led exclusively to RhIII-alkyl complexes as final products, while an associative activation is inferred from the large negative ΔS ≠ value. The relative reactivity of the [RhI(N,O-ScBa)(CO)(PR3)] complexes when stepwise varying PR3 (PPh3, PPh2Cy, PPhCy2 and PCy3) as tertiary phosphine ligands follow a surprising similar reactivity relationship as observed for a range of isostructural [RhI(X,O-Bid)(CO)(PR3)] complexes, with X = O, N or S in corresponding oxinate, acetylacetonate and thioureate mono anionic X,O-Bid ligands. This suggests systematic and responsive dynamic behavior of the PR3 ligands, independent of the bidentate chelate at the Rh(I) metal center. A variation of more than two orders-of-magnitude in reactivity was observed.

Prospects of Heterogeneous Hydroformylation with Supported Single Atom Catalysts.

The potential of oxide-supported rhodium single atom catalysts (SACs) for heterogeneous hydroformylation was investigated both theoretically and experimentally. Using high-level domain-based local-pair natural orbital coupled cluster singles doubles with perturbative triples contribution (DLPNO-CCSD(T)) calculations, both stability and catalytic activity were investigated for Rh single atoms on different oxide surfaces. Atomically dispersed, supported Rh catalysts were synthesized on MgO and CeO2. While the CeO2-supported rhodium catalyst is found to be highly active, this is not the case for MgO, most likely due to increased confinement, as determined by extended X-ray absorption fine structure spectroscopy (EXAFS), that diminishes the reactivity of Rh complexes on MgO. This agrees well with our computational investigation, where we find that rhodium carbonyl hydride complexes on flat oxide surfaces such as CeO2(111) have catalytic activities comparable to those of molecular complexes. For a step edge on a MgO(301) surface, however, calculations show a significantly reduced catalytic activity. At the same time, calculations predict that stronger adsorption at the higher coordinated adsorption site leads to a more stable catalyst. Keeping the balance between stability and activity appears to be the main challenge for oxide supported Rh hydroformylation catalysts. In addition to the chemical bonding between rhodium complex and support, the confinement experienced by the active site plays an important role for the catalytic activity.