Cationic Rhodium Complexes Research Articles

Non‐Covalent Immobilization of Chiral Rhodium Catalysts on Carbon Nanotubes for Asymmetric Hydrogenation

AbstractSupported chiral catalysts were prepared by ionic anchoring of two cationic rhodium complexes (A and B) onto functionalized carbon nanotubes (CNTs). The immobilization of the Rh complexes was done by applying two ionic anchoring strategies: i) electrostatic interaction with the negatively charged surface of carboxylate‐functionalized CNTs (CNT‐COO−); ii) electrostatic interaction through a heteropoly acid (HPA) spacer previously grafted at the CNTs surface (CNT‐HPA). The characterization of the obtained CNT‐COO−@complex A, CNT‐COO−@complex B, CNT‐HPA@complex A, and CNT‐HPA@complex B materials by ICP, TEM and XPS showed the successful anchoring of the Rh complexes onto the supports. The HPA spacer was found to increase the catalyst's stability and to limit the effect of the CNT surface on the catalytic performances of the immobilized Rh complexes. CNT‐HPA@complexes performed the hydrogenation of dimethyl itaconate into methyl succinate with high conversions (83–100 %) and good enantioselectivities (58–63 %). The recyclability tests showed the catalysts were less active but still enantioselective after recycling.

Deprotonation-induced changes in π-electron systems of iridium(III) and rhodium(III) complexes with azophenine

N,N′,N′′,N′′′-Tetraphenyl-2,5-diamino-1,4-benzoquinonediimines (RAp·H2) are used as unique bridging ligands containing localized π-electron systems at each of the two NCCCNH moieties, which are susceptible to changes on coordination of the N atoms to Lewis acid(s). In this contribution, we disclose deprotonation-induced changes in the π-electron systems, spectroscopic properties, and electronic state of cationic cyclometalated iridium(III) and rhodium(III) complexes with 4-EtAp·H2, i.e., [M(ppy)2(4-EtAp·H2)]Cl (ppy: 2-phenylpyridinate; M: Ir(III) (1·H2+) and Rh(III) (2·H2+)). The cationic complexes possessed localized π-electron systems, but the NCCCNH moieties were isomerized from those in free 4-EtAp·H2. Although 4-EtAp·H2 showed no responsiveness to tetrabutylammonium hydroxide (TBAOH) as a base, the reaction of 1·H2+ and 2·H2+ with TBAOH afforded monodeprotonated neutral complexes [M(ppy)2(4-EtAp·H)] (M: Ir(III) (1·H) and Rh(III) (2·H)). UV–vis absorption spectral titrations of 1·H2+ and 2·H2+ with TBAOH revealed the occurrence of monodeprotonation even in the presence of 4 equivalents of TBAOH. The cationic complexes were regenerated from 1·H and 2·H upon addition of p-toluenesulfonic acid. X-ray crystallography of 2·H showed that the complex has localized and delocalized π-electron systems at the NCCCNH and NCCCN moieties, respectively. The UV–vis absorption spectra of 1·H2+ and 2·H2+ in CH2Cl2 present intense bands around 500 nm, and those of 1·H and 2·H exhibit intense bands around 450 nm and broad bands around 700 nm. Density functional theory (DFT), time-dependent DFT, and natural transition orbital calculations suggest that the intense bands of all complexes and the broad bands of 1·H and 2·H stem from π–π* transitions in the localized and delocalized π-electron systems, respectively, with MLCT transition.

Diphosphene with a phosphineborane tether and its rhodium complex.

A diphosphene ligand possessing a PP bond and a phosphineborane moiety in its molecule was synthesized. The reaction of the new bidentate diphosphene-phosphineborane ligand with [Rh(cod)2]BF4 provided a cationic diphosphene-rhodium complex with cis-coordination through the η1-PP and η2-BH3 moieties. The complex was applied to the coupling reaction of benzimidazole with cyclohexylallene. The complex also underwent a ligand exchange reaction with N-donor reagents such as N-methylpyrrolidine and N,N,N,N-tetramethylethylenediamine. In particular, the addition reaction of pyridine with the rhodium complex provided an equilibrium mixture of the rhodium complex, pyridine, the diphosphene-phosphineborane ligand, and [Rh(pyridine)2(cod)]BF4.

Cationic Rhodium Diphosphane Complexes as Efficient Catalysts for the Semi‐Hydrogenation of Dehydroisophytol

AbstractThe selective reduction of terminal alkynes to alkenes was investigated using common cationic diphosphane rhodium complexes of the type [Rh(PP)(diolefin)]X (PP=diphosphane, X=anion). The effectiveness of the catalyst was demonstrated in the semi‐hydrogenation of dehydroisophytol (DIP), an industrial produced intermediate of vitamin E. The present study highlights the high activity and good selectivity of this simple catalytic system. However, deactivation increases at higher DIP concentrations. Several strategies to circumvent the deactivation are presented.

Synthesis and Reversible H2 Activation by Coordinatively Unsaturated Rhodium NHC Complexes

AbstractWe report the synthesis of coordinatively unsaturated cationic rhodium complexes bearing the sterically encumbered electron‐rich NHC ligand IPr*OMe. The COD (1,5‐cyclooctadiene) complex [Rh(IPr*OMe)(COD)]BF4 adopts a tilted, pseudo‐square planar coordination geometry, where bonding to the ipso‐carbon of the NHC aryl substituent was observed in the solid state. Hydrogenation of this complex afforded a metastable dihydride complex [Rh(IPr*OMe)(H)2]BF4 with an unusual internal coordination to an arene of the ligand. In the absence of a hydrogen atmosphere, spontaneous reductive elimination of H2 afforded a rhodium complex [Rh(IPr*OMe)]BF4 with a single chelating ligand that stabilizes the highly unsaturated metal by two‐fold π‐face donation as suggested by NMR spectroscopy and computational studies. This unusual complex might serve as a versatile precatalyst for a variety of transformations.

Structures, Thermal Properties, and Reactivities of Cationic Rh-cod Complexes in Solid State (cod = 1,5-Cyclooctadiene).

Cationic rhodium complexes with 1,5-cyclooctadiene (cod) ligands are important organometallic compounds that are useful as precatalysts; however, their solid-state structures and thermal properties have not been adequately investigated. In this study, we synthesized [Rh(cod)L]X (L = cod, C6H6, PhMe; X = SbF6, (FSO2)2N (= FSA), CF3BF3, CB11H12) and investigated their phase behaviors, crystal structures, and reactivities. The phase transitions of these salts result in disordered solid-state structures. Moreover, the structural disorder increases with a decrease in the cation symmetry in the SbF6 salts; [Rh(cod)(PhMe)]SbF6 exhibits a rotator phase, and the cations in other salts exhibit a dynamic rotational disorder. In contrast, a lower crystal symmetry with less cation disorder is observed for FSA salts. The thermal stabilities and reactivities of these salts were further investigated. FSA salts with arene ligands produce anion-coordinated complexes upon melting, and SbF6 salts with arene ligands produce [Rh(cod)L'2]SbF6 (L' = MeCN and SMe2) via an in situ single-crystal-to-single-crystal ligand-exchange reaction.

Substitution lability of the perfluorinated Cp* ligand in Rh(I) complexes.

Several cationic rhodium(I) complexes [Rh(COD)L2][C5(CF3)5] have been synthesized through substitution of the weakly bound [C5(CF3)5]- ligand from [Rh(COD)(C5(CF3)5)], further emphasizing its unique reactivity. Besides acetonitrile, pyridine derivatives with varying degrees of fluorination have been employed as ligands in order to investigate the influence of fluorination upon the binding affinity towards the resulting [Rh(COD)]+ fragment and the limit as to which the [C5(CF3)5]- ligand can be displaced. Furthermore, the newly synthesized compounds represent rare examples of rhodium complexes containing fluorinated pyridines as ligands.

β-(Z)-Selective alkyne hydrosilylation by a N,O-functionalized NHC-based rhodium(I) catalyst.

Neutral and cationic cyclooctadiene rhodium(I) complexes with a lutidine-derived polydentate ligand having NHC and methoxy side-donor functions, [RhBr(cod)(κC-tBuImCH2PyCH2OMe)] and [Rh(cod)(κ2C,N-tBuImCH2PyCH2OMe)]PF6, have been prepared. Carbonylation of the cationic compound yields the dicarbonyl complex [Rh(CO)2(κ2C,N-tBuImCH2PyCH2OMe)]PF6 whereas carbonylation of the neutral compound affords a mixture of di- and monocarbonyl neutral complexes [RhBr(CO)2(κC-tBuImCH2PyCH2OMe)] and [RhBr(CO)(κ2C,N-tBuImCH2PyCH2OMe)]. These complexes efficiently catalyze the hydrosilylation of 1-hexyne with HSiMe2Ph with a marked selectivity towards the β-(Z)-vinylsilane product. Catalyst [RhBr(CO)(κ2C,N-tBuImCH2PyCH2OMe)] showed a superior catalytic performance, in terms of both activity and selectivity, and has been applied to the hydrosilylation of a range of 1-alkynes and phenylacetylene derivatives with diverse hydrosilanes, including HSiMe2Ph, HSiMePh2, HSiPh3 and HSiEt3, showing excellent β-(Z) selectivity for the hydrosilylation of linear aliphatic 1-alkynes. Hydrosilylation of internal alkynes, such as diphenylacetylene and 1-phenyl-1-propyne, selectively affords the syn-addition vinylsilane products. The β-(Z) selectivity of these catalysts contrasts with that of related rhodium(I) catalysts based on 2-picolyl-functionalised NHC ligands, which were reported to be β-(E) selective. An energy barrier ΔG‡ of 19.8 ± 2.0 kcal mol-1 (298 K) has been determined from kinetic studies on the hydrosilylation of 1-hexyne with HSiMe2Ph. DFT studies suggest that the methoxy-methyl group is unlikely to be involved in the activation of hydrosilane, and then hydrosilane activation is likely to proceed via a classical Si-H oxidative addition.

Catalytic direct hydrocarboxylation of styrenes with CO2 and H2.

A three-component hydrocarboxylation of an olefin with CO2 and H2 could be regarded as a dream reaction, since it would provide a straightforward approach for the synthesis of aliphatic carboxylic acids in perfect atom economy. However, this transformation has not been realized in a direct manner under mild conditions, because boosting the carboxylation with thermodynamically stable CO2 while suppressing the rapid hydrogenation of olefin remains a challenging task. Here, we report a rhodium-catalysed reductive hydrocarboxylation of styrene derivatives with CO2 and H2 under mild conditions, in which H2 served as the terminal reductant. In this approach, the carboxylation process was largely accelerated by visible light irradiation, which was proved both experimentally and by computational studies. Hydrocarboxylation of various kinds of styrene derivatives was achieved in good yields without additional base under ambient pressure of CO2/H2 at room temperature. Mechanistic investigations revealed that use of a cationic rhodium complex was critical to achieve high hydrocarboxylation selectivity.

Inside Back Cover: Synthesis of Cyclophenacene‐ and Chiral‐Type Cyclophenylene‐Naphthylene Belts (Angew. Chem. Int. Ed. 15/2022)

A naphthalene-embedded conjugated nanobelt is an excellent design for achieving the synthesis of cyclophenacene-type and chiral-type carbon nanobelts. A cationic rhodium complex catalyst is a powerful tool for the (asymmetric) synthesis of these analogues, as reported by Ken Tanaka and co-workers in their Research Article (e202200800). In the picture, the three blue belts in the foreground and the rhodium complex in the back represent the cyclophenylene-naphthylene belts and the key catalyst, respectively.

Stereoselective Access to Conjugated and Cross-Conjugated Dienoates by Rh- and Ru-Catalyzed Isomerizations of Vinylcyclopropanes.

Two complementary catalytic protocols for the isomerization of stereoisomeric mixtures of vinylcyclopropanes are described. A commercially available cationic rhodium complex provides access to conjugated dienoates in high yield with excellent stereocontrol. The combination of a bisphosphine ligand and a ruthenium precatalyst affords cross-conjugated dienoates via a formal 1,3-ring opening. The products are obtained with moderate to high stereoselectivity. The ability of each type of dienoate to engage in [4 + 2] cycloaddition reactions has been demonstrated.

Diastereoselective Three-Component 3,4-Amino Oxygenation of 1,3-Dienes Catalyzed by a Cationic Heptamethylindenyl Rhodium(III) Complex.

The direct oxyamination of olefins is a compelling tool to rapidly access β-amino alcohols-a privileged motif ubiquitous in natural products, pharmaceuticals and agrochemicals. Although a variety of expedient methods are established for simple alkenes, selective amino oxygenation of 1,3-dienes is less explored. Within this context, methods for the oxyamination of 1,3-dienes that are selective for the internal position remain unprecedented. We herein report a modular three-component approach to perform an internal and highly diastereoselective amino oxygenation of 1,3-dienes catalyzed by a cationic heptamethylindenyl (Ind*) Rh(III) complex.

Variation on the π‐Acceptor Ligand within a RhI−N‐Heterocyclic Carbene Framework: Divergent Catalytic Outcomes for Phenylacetylene‐Methanol Transformations

AbstractA series of neutral and cationic rhodium complexes bearing IPr {IPr=1,3‐bis‐(2,6‐diisopropylphenyl)imidazolin‐2‐carbene} and π‐acceptor ligands are reported. Cationic species [Rh(η4‐cod)(IPr)(NCCH3)]+ and [Rh(CO)(IPr)(L)2]+ (L=pyridine, CH3CN) were obtained by chlorido abstraction in suitable complexes, whereas the cod‐CO derivative [Rh(η4‐cod)(IPr)(CO)]+ was formed by the carbonylation of [Rh(η4‐cod)(IPr)(NCCH3)]+. Alternatively, neutral derivatives of type RhCl(IPr)(L)2 {L=tBuNC or P(OMe)3} can be accessed from [Rh(μ‐Cl)(η2‐coe)(IPr)]2. In addition, the mononuclear species Rh(CN)(η4‐cod)(IPr) was prepared by cyanide‐chlorido anion exchange, which after carbonylation afforded the unusual trinuclear compound [Rh{1κC,2κN‐(CN)}(CO)(IPr)]3. Divergent catalytic outcomes in the phenylacetylene‐methanol transformations have been observed. Thus, enol ethers, arisen from hydroalkoxylation of the alkyne, were obtained with neutral Rh−CO catalyst precursors whereas dienol ethers were formed with cationic catalysts. Variable amounts of alkyne dimerization, cyclotrimerization or polymerization products were obtained in the absence of a strong π‐acceptor ligand on the catalyst.

(1,2,5,6-η)-Cyclo-octa-1,5-diene](1-ethyl-3-isopropyl-1,3-imidazol-2-yl-idene)(tri-phenyl-phosphane)rhodium(I) tetra-fluorido-borate.

A new N-heterocyclic cationic rhodium(I) complex with a tetra-fluorido-borate counter-anion, [Rh(C8H14N2)(C8H12)(C18H15P)]BF4, has been prepared and structurally characterized. The cationic complex exhibits a distorted square-planar environment around the rhodium(I) ion. Two connections are made from rhodium(I) to the carbon atom of an N-heterocylic carbene ligand and to the phospho-rus atom of a tri-phenyl-phosphane ligand. The remaining two coordination sites are made via a bidentate inter-action from the two olefinic bonds of cyclo-octa-diene to the rhodium(I) ion. The compound includes an out-sphere tetra-fluorido-borate counter-anion. Within the crystal of the compound exist several weak inter-molecular C-H⋯F inter-actions.

Cationic Rhodium(I)‐Catalyzed Carbonylative [2+2+1] Cycloaddition of Diynes

AbstractWe describe Rh(I)‐catalyzed carbonylative [2+2+1] cycloaddition reactions of diynes with CO to give cyclopentadienone derivatives (CPDs). Previous reports on the use of neutral rhodium(I) complexes, such as [RhCl(cod)]2 and RhCl(PPh3)3, indicate that no CPD products are formed or that they are produced in low yields, even at elevated temperatures. The reaction described herein, which uses a cationic rhodium(I) complex, [Rh(cod)2]BF4, as a catalyst, proceeds smoothly at room temperature to give cyclopentadienone derivatives. The key to this is the use the cationic rhodium(I) complex as a catalyst, which produces an additional coordination site on the rhodium for the substrate (a diyne) to approach, than neutral rhodium complexes.

An Unconventional trans - exo -Selective Cyclization of Alkyne-Tethered Cyclohexadienones Initiated by Rhodium(III)-Catalyzed C–H Activation via Insertion Relay

Different from the established trans-endo-selective cyclization of alkyne-tethered electrophiles that involve an E/Z isomerization process, herein, the authors present a novel strategy to allow tra...

Bidentate Iminophosphorane-NHC Ligand Derived from the Imidazo[1,5-a]pyridin-3-ylidene Scaffold

The synthesis of the bifunctional iminophosphorane-NHC (1) based on the imidazo[1,5-a]pyridin-3-ylidene (IPy) platform is reported. Its imidazo[1,5-a]pyridinium salt precursor [1·H](X) was readily obtained by an efficient three-component coupling reaction between 5-bromoimidazo[1,5-a]pyridinium bromide, sodium azide, and triphenylphosphine according to a SNAr/Staudinger reaction sequence. The stable free carbene 1 was generated by deprotonation of [1·H](X) with potassium bis(trimethylsilyl)amide, and its coordination ability toward various transition-metals was evaluated, either upon direct metalation of the free carbene or by transmetalation from a silver(I) NHC complex. While the ligand 1 is singly bounded through the carbene carbon atom in the latter complex, it behaves as a chelating bidentate ligand in all other complexes that were prepared, including the cationic and neutral palladium(II) complexes [Pd(allyl)(κ2C,N -1)](OTf) ([5](OTf)) and [PdCl2(κ2C,N -1)] (7), and the cationic rhodium(I) complexes...

13C NMR Signal Enhancement Using Parahydrogen-Induced Polarization Mediated by a Cobalt Hydrogenation Catalyst.

The use of a cobalt-based catalyst for the generation of hyperpolarized 13C NMR resonances by parahydrogenation of ethyl acrylate is presented herein. Comparisons of the carboxylate 13C NMR signal enhancement factor of ethyl propionate between using (MesCCC)Co-py and a commonly utilized cationic diphosphine rhodium complex demonstrates that the cobalt system is a viable PHIP catalyst alternative. Furthermore, the operative hydrogenation mechanism of the cobalt system was examined by using 1H, 13C, and parahydrogen-induced polarization NMR spectroscopies to elucidate reaction intermediates affiliated with the observed 1H and 13C NMR signal enhancements in ethyl propionate.

Rhodium-Catalyzed Cascade Synthesis of Benzofuranylmethylidene-Benzoxasiloles: Elucidating Reaction Mechanism and Efficient Solid-State Fluorescence.

A new synthetic route to highly fluorescent benzofuranylmethylidenebenzoxasiloles through cationic rhodium(I)/binap complex-catalyzed cascade cycloisomerization of bis(2-ethynylphenol)silanes has been developed involving 1,2-silicon and 1,3-carbon (alkyne) migrations followed by oxycyclization. The present synthesis requires only three steps, starting from commercially available dichlorodiisopropylsilane, which is markedly shorter than our previous synthesis (eight steps starting from commercially available chlorodiisopropylsilane). Theoretical calculations elucidated the mechanism of the above cascade cycloisomerization. This reaction is initiated by the formation of a rhodium vinylidene not through direct 1,2-silicon migration but rather through an unprecedented stepwise 1,5-silicon migration followed by C-Si bond-forming cyclization from a dearomatized allenylrhodium complex. Subsequent 1,3-carbon (alkyne) migration leading to a η3 -allenyl/propargyl-rhodium complex followed by oxycyclization through π-bond (alkyne) activation with the cationic rhodium(I) complex affords the benzofuranylmethylidenebenzoxasilole product. The structure-fluorescence property relationships of the thus obtained benzofuranylmethylidenebenzoxasiloles were investigated, which revealed that good fluorescence quantum yields were generated in the solution state (φF =69-87 %) by introduction of electron-donating alkyl and phenyl groups on two phenoxy groups. In the powder state, 4-methyl- and 4-methoxy-phenoxy derivatives exhibited efficient blue fluorescence (φF =52 % and 46 %, respectively). Especially, the 4-methylphenoxy derivative was thermally stable, and exhibited strong narrow-band fluorescence in the film state (blue, φF =95 %) and redshifted strong narrow-band fluorescence (green, φF =90 %) in the crystalline state as a result of the formation of an offset π-stacked dimer; the latter was confirmed by X-ray crystallographic analysis and by theoretical calculations.

Potassium Binding Adjacent to Cationic Transition-Metal Fragments: Unusual Heterobimetallic Adducts of a Calix[4]arene-Based Thione Ligand.

The synthesis of cationic rhodium and iridium complexes of a bis(imidazole-2-thione)-functionalized calix[4]arene ligand and their surprising capacity for potassium binding are described. In both cases, uptake of the alkali metal into the calix[4]arene cavity occurs despite adverse electrostatic interactions associated with close proximity to the transition-metal fragment [Rh+···K+ = 3.715(1) Å; Ir+···K+ = 3.690(1) Å]. The formation and constituent bonding of these unusual heterobimetallic adducts have been interrogated through extensive solution and solid-state characterization, examination of the host-guest chemistry of the ligand and its upper-rim unfunctionalized calix[4]arene analogue, and use of density functional theory based energy decomposition analysis.