Rh Center Research Articles

First stabilization of 14-electron rhodium(I) complexes by hemichelation.

Hemichelation is emerging as a new mode of coordination where non-covalent interactions crucially contribute to the cohesion of electron-unsaturated organometallic complexes. This study discloses an unprecedented demonstration of this concept to a Group 9 metal, that is, Rh(I). The syntheses of new 14-electron Rh(I) complexes were achieved by choosing the anti-[(η(6):η(6)-fluorenyl){Cr(CO)3}2] anion as the ambiphilic hemichelating ligand, which was treated with [{Rh(nbd)Cl}2] (nbd=norbornadiene) and [{Rh(CO)2Cl}2]. The new T-shaped Rh(I) hemichelates were characterized by analytical and structural methods. Investigations using the methods of the DFT and electron-density topology analysis (NCI region analysis, QTAIM theory) confirmed the closed-shell, non-covalent and attractive characters of the interaction between the Rh(I) center and the proximal Cr(CO)3 moiety. This study shows that, by appropriate tuning of the electronic properties of the ambiphilic ligand, truly coordination-unsaturated Rh(I) complexes can be synthesized in a manageable form.

Reaction of Dinuclear Rhodium 4,5-Diazafluorenyl-9-Carboxylate Complexes with H2 and CO2

The reactivity of the dinuclear Rh(I) 4,5-diazafluorenyl-9-carboxylate complex 4 with H2 was explored in an attempt to reduce the trapped CO2-derived moiety. After overnight reaction with H2 the only observed species at room temperature by NMR spectroscopy was the decarboxylated 4,5-diazafluorenyl (L–) Rh(III) complex 1. A series of stepwise stoichiometric reactions with H2, NMR experiments at low temperatures, trapping experiments with added PPh3 or CO2, and 13C-labeling experiments were conducted in an attempt to identify the fates of the CO2-derived portion of 4 and the second Rh center.

Rhodium Catechol Containing Porous Organic Polymers: Defined Catalysis for Single-Site and Supported Nanoparticulate Materials

A single-site, rhodium(I) catecholate containing porous organic polymer was prepared and utilized as an active catalyst for the hydrogenation of olefins in both liquid-phase and gas-phase reactors. Liquid-phase, batch hydrogenation reactions at 50 psi and ambient temperatures result in the formation of rhodium metal nanoparticles supported within the polymer framework. Surprisingly, the Rh(I) complex is catalytically active and stable for propene hydrogenation at ambient temperatures under gas-phase conditions, where reduction of the Rh(I) centers to Rh(0) nanoparticles requires at least 200–250 °C under a flow of hydrogen gas. After high-temperature treatment, the Rh(0) nanoparticles are active arene hydrogenation catalysts that convert toluene to methylcyclohexadiene at a rate of 9.3 × 10–3 mol g–1 h–1 of rhodium metal at room temperature. Conversely, single-site Rh(I) is an active and stable catalyst for the hydrogenation of propylene (but not toluene) under gas-phase conditions at room temperature.

Asymmetric hydroformylation catalyzed by RhH(CO)₂[(R,S)-Yanphos]: mechanism and origin of enantioselectivity.

Asymmetric hydroformylation (AHF) catalyzed by transition metal (TM) complexes bearing chiral phosphorus ligands is one of the most powerful synthetic ways that could provide chiral aldehydes directly from alkenes and syngas in one step. Experiments have proved the efficiency of Rh catalyst with hybrid phosphorus ligands owning two different phosphorus moieties in AHF. Herein the origin of enantioselectivity of AHF catalyzed by RhH(CO)2[(R,S)-Yanphos] was studied at M06/BSI level using the density functional theory (DFT) method to unveil a fundamental understanding on factors contributing to the efficiency in AHF. The alkene insertion step is supposed to be the chirality-determining step in the whole catalytic cycle of the Rh-Yanphos system. Four possible pathways of styrene (Sub1) insertion step (pathways R1, S1, R2, and S2) were discussed; the calculated results indicate that pathways R1 and S2 are proposed to be two dominant alkene insertion pathways and that styrene tends to adopt apical coordination mode (A mode) to Rh center in pathways R1 and S2 compared to equatorial coordination mode (E mode) in pathways R2 and S1. The enantioselectivity of AHFs of ten alkene substrates (CH2═CH-R, R═Ph, C(═O)OCH3, Ph-(p)-Me, Ph-(p)-OMe, Ph-(p)-(i)Bu, Ph-(p)-F, Ph-(p)-Cl, Ph-(o)-F, OC(═O)-Ph and O-Ph, corresponding alkenes are abbreviated as Sub1 to Sub10, respectively) were also investigated. The predicted chiralities agree well with experimental results. The present work suggests that the relative stabilities of coordination modes (A/E mode) of alkene to 2 (RhH(CO)[(R,S)-Yanphos]) might be of importance in the enantioselectivity of AHF catalyzed by Rh-Yanphos.

Chemically regulating Rh(i)-Bodipy photoredox switches

The ability of Rh(I) centers to undergo photoinduced electron transfer from discrete metal orbitals to Bodipy fluorophores is mediated through reversible coordination chemistry.

New Supramolecular Structural Motif Coupling a Ruthenium(II) Polyazine Light Absorber to a Rhodium(I) Center

Two new complexes, [(bpy)2Ru(dpp)Rh(I)(COD)](PF6)3 and [(Me2bpy)2Ru(dpp)Rh(I)(COD)](PF6)2(BF4) (bpy = 2,2'-bipyridine, Me2bpy = 4,4'-dimethyl-2,2'-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine, and COD = 1,5-cyclooctadiene), representing a new Ru(II),Rh(I) structural motif, have been prepared and characterized by mass spectrometry, (1)H NMR spectroscopy, electrochemistry, electronic absorption spectroscopy, and emission spectroscopy. These two complexes represent a new type of supramolecular complex with a [(TL)2Ru(dpp)](2+) (TL = terminal ligand) light absorber (LA) coupled to a Rh(I) center and are models for Ru(II),Rh(I) intermediates in the photochemical reduction of water using dpp-bridged Ru(II),Rh(III) photocatalysts. Electrochemical study reveals overlapping reversible Ru(II/III) and irreversible Rh(I/II/III) oxidations and a quasi-reversible dpp(0/-) reduction, demonstrating that the lowest unoccupied molecular orbital (LUMO) is dpp(π*) based. The COD ligand is sterically bulky, displaying steric repulsions between hydrogen atoms on the alkene of COD and dpp about the square planar Rh(I) center. An interesting reactivity occurs in coordinating solvents such as CH3CN, where Rh(I) substitution leads to an equilibrium between the Ru(II),Rh(I) bimetallic and [(TL)2Ru(dpp)](2+) and [Rh(I)(COD)(solvent)2](+) monometallic species. The electronic absorption spectra of both complexes feature transitions at ca. 500 nm attributed to a Ru(dπ) → dpp(π*) metal-to-ligand charge transfer (MLCT) transition that is slightly red-shifted from the Ru synthon upon Rh(I) complexation. The methylation of TL on the Ru impacts the electrochemical and optical properties in a minor but predictable manner. The photophysical studies, by comparison with the model complex [{Ru(bpy)2}2(dpp)](PF6)4 and related Rh(III) complex [(bpy)2Ru(dpp)Rh(III)Cl2(phen)](PF6)3, reveal the expected absence of a Ru(dπ) → Rh(dσ*) (3)MMCT state (metal-to-metal charge transfer) in the title complexes, which is present in Rh(III) systems. The absence of this (3)MMCT state in Ru(II),Rh(I) complexes results in a longer lifetime and higher emission quantum yield for the Ru(dπ) → dpp(π*) (3)MLCT state than [(bpy)2Ru(dpp)Rh(III)Cl2(phen)](PF6)3. Both complexes display photocatalytic hydrogen production activity in the presence of water and a sacrificial electron donor, with the [(bpy)2Ru(dpp)Rh(I)(COD)](PF6)3 possessing a higher catalytic activity than the methyl analogue. Both display low activities, hypothesized to occur due to steric crowding about the Rh(I) site.

Synthesis and Structural Characterization of Metal Compounds Incorporating the Ortho-Quinone Manganese Tricarbonyl Complex as an Organometalloligand

Three π-quinonoid compounds incorporating the organometalloligand ortho-quinone manganese tricarbonyl ([(C6H4O2)Mn(CO)3]−, o-QMTC) have been synthesized and characterized by IR and X-ray crystallography: [Pd(o-QMTC)(PPh3)2]BF4·CH2Cl2 (1), (o-QMTC)Rh(1,5-cyclooctadiene) (2), and Cu(o-QMTC)2 (3). The formally anionic o-QMTC ligand features Mn-arene bonding that is best described as η4-bound. Nevertheless, o-QMTC interplanar angular data and νco data suggest significant contribution from η5 and η6 forms. In each of the compounds, the o-QMTC organometalloligand(s) contribute to a square planar geometry about the Pd(II), Rh(I), or Cu(II) metal center.

A theoretical study on the alkene insertion step in Rh-Yanphos catalyzed hydroformylation

This paper studied the mechanism of the alkene insertion elementary step in the asymmetric hydroformylation (AHF) catalyzed by RhH(CO)2[(R,S)-Yanphos] using four alkene substrates (CH2CHPh, CH2CHPh(p)Me, CH2CHC(O)OCH3 and CH2CHOC(O)Ph, abbreviated as A1–A4). Interestingly, the equatorial vertical coordination mode (A mode) with respect to the Rh center was found for A1 and A2 but not for A3 and A4, although the equatorial in-plane coordination mode (E mode) was found for A1–A4. The relative energy of the E mode of the η2-intermediates is lower than that of the A mode. In the alkene insertion step, Path 1 is more favorable than Path 2 for this system. As for A1 and A2, there could be a transformation between 2eq and 2ax.

Photoswitchable Metal-Mediated Catalysis: Remotely Tuned Alkene and Alkyne Hydroborations

A photochromic dithienylethene-annulated N-heterocyclic carbene (NHC)–Rh(I) complex was synthesized and found to undergo reversible electrocyclic ring closure upon alternate exposure to UV (λirr 313 nm) and visible (λirr >500 nm) radiation. Under ambient light, the Rh catalyst efficiently promoted the hydroboration of alkenes and alkynes with pinacolborane. However, upon UV irradiation to effect a photocyclization within the NHC ligand, the catalytic activity was reduced by up to an order of magnitude. The disparity in the rates was used to photoswitch the rates of a series of hydroboration reactions, thus demonstrating the first examples of photomodulating a transition-metal catalyst by tuning its electronic properties. The rate attenuation observed under UV irradiation was attributed to inhibition of the rate-determining reductive elimination step arising from a decrease in electron-donating ability of the photocyclized NHC ligated to the Rh center.

Stable Ionic Rh(I,II,III) Complexes Ligated by an Imidazolium-Substituted Phosphine with π-Acceptor Character: Synthesis, Characterization, and Application to Hydroformylation

The stable ionic Rh(I,II,III) complexes [RhI(acac)(CO)(L)]PF6 (2), [RhII2(OAc)4(L)2]2PF6 (3), and [RhIIICl4(L)2]PF6 (4) were synthesized through the complexation of RhI(acac)(CO)2, RhII2(OAc)4(H2O)2, and RhIIICl3·3H2O with the phosphine-functionalized ionic liquid (FIL) 1 ([L]PF6, L = 1-butyl-2-diphenylphosphino-3-methylimidazolium), respectively. The cation of L in 1 is an imidazolium-substituted phosphine with a positive charge vicinal to the P(III) atom, which acts as an electron-deficient donor with π-acceptor character to afford the stable complexes 2–4 due to the presence of retrodonating π-binding between Rh–P linkage. Due to the weakened reducing ability of L, the redox reaction between L and RhCl3·3H2O during the complexation is avoided, leading to the formation of 4, in which the Rh center is in the +3 valence state. Single-crystal X-ray analyses show that 2–4 are all composed of a Rh-centered cation and a PF6– counteranion. The cation of 2 possesses structural similarity to RhI(acac)(CO)(PPh3),...

N‐Heterocyclic Carbene Dirhodium(II) Complexes as Catalysts for Allylic and Benzylic Oxidations

AbstractThe experimental conditions (solvent, base, temperature and oxidant) for allylic and benzylic oxidation reactions catalyzed by dirhodium(II)/N‐heterocyclic carbene (NHC) complexes were optimized for the first time in this work. The oxidations of cyclohexene and fluorene were used as model reactions. Two optimized experimental conditions for both types of oxidations were found, which resulted in their ketone [aerobic conditions, 40 °C, 1 equiv. tBuOOH (TBHP)] or tert‐butyl peroxide derivatives (anaerobic conditions, 25 °C, 2 equiv. TBHP). The dirhodium(II) complexes undergo a single‐electron reversible oxidation by cyclic voltammetry in CH2Cl2, which is assigned to the Rh24+/Rh25+ redox couple at an oxidation potential that is lowered upon sequential axial coordination of further ligands to the Rh–Rh centre. The oxidation potential is discussed in terms of the electron‐donor character of the NHC ligand, which was shown to act as an effective electron‐releaser to the Rh24+ centre, and the electrochemical behaviour is compared to the observed catalytic activity.

C–Cl activation of the weakly coordinating anion [B(3,5-Cl2C6H3)4]− at a Rh(i) centre in solution and the solid-state

Addition of H2 to [Rh((i)Bu2PCH2CH2P(i)Bu2)(NBD)][BAr(Cl)4] (NBD = norbornadiene, Ar(Cl) = 3,5-Cl2C6H3) in the solid-state results in the rapid formation of zwitterionic [Rh((i)Bu2PCH2CH2P(i)Bu2){(η(6)-C6H3Cl2)BAr(Cl)3}] by a gas/solid reaction. This undergoes slow C-Cl bond cleavage in the solid-state to ultimately afford the dimeric Rh(III) complex [RhCl((i)Bu2PCH2CH2P(i)Bu2){C6H3Cl(BAr(Cl)3)}]2. This reactivity is mirrored in solution (CH2Cl2). Kinetic data for the C-Cl activation in both the solid-state and solution are reported.

P-oxidation of gem-dicationic phosphines

In spite of the general oxophilic character and λ5 valence state accessibility of phosphorus centers, the oxidation of phosphanes bearing proximal positive charges remains challenging, in particular for dicationic representatives. Previously described diimidazolo- and diimidazolio-phosphane substrates were first resumed in the P-phenylated series, where the reported absence of coordinating ability of the N,N-dimethylated dication towards a neutral Rh(I) center is here confirmed towards a promoted oxygen atom. By contrast, the P-tert-butylated homologue is shown to undergo P-oxidation to give a stable gem-dicationic phosphine oxide (in 91% yield with m-CPBA). The latter could also be obtained by N,N-dimethylation of the neutral diimidazolo-phosphine oxide precursor. The strict covalent vs. dative character of the N2C–P bond in dicationic systems (N2C+)2P(X)R (R = Ph, tBu; X = lone pair, :O) has been investigated by DFT calculations on a model series where (N2C+) = 1,3-dimethyl-1H-imidazolio-2-yl. The relative magnitude of Gibbs free energies of the homolytic and heterolytic dissociation modes shows that diamidinio-phosphines and corresponding P-oxides are not N2C → P donor–acceptor adducts (as mono(amidinio)phosphines are), but rather true gem-dicationic phosphines and phosphine oxides in the strict sense, respectively.

Revealing the P–B coupling event in the rhodium catalysed dehydrocoupling of phosphine–boranes H3B·PR2H (R = tBu, Ph)

We demonstrate that [Rh(η6-FC6H5)(Ph2PCH2CH2CH2PPh2)][BArF4] is a competent catalyst for the dehydrocoupling of H3B·PR2H to give H3B·PR2BH2·PR2H (R = Ph, tBu). The isolation of intermediates (R = tBu, Ph) and kinetic/isotopic labelling experiments (for R = Ph) point to a mechanism, for R = Ph, in which the rate-limiting process for dehydrocoupling involves B–H activation, while the turnover limiting process for catalysis involves the substitution of the oligomeric product at the metal, which forms a strong chelate complex with a Rh(III) centre, by H3B·PPh2H. When R = tBu no chelate product complex forms, and it is speculated that the dehydrocoupling process now becomes turnover limiting. These observations provide, for the first time we believe, data on the P–B bond forming events that are central to the dehydrocoupling and dehydropolymerisation of phosphine–boranes.

1,4-Metal migration in a Cp*Rh(iii) complex

The reaction of [Cp*RhCl(Ph)(PR3)] (R = Ph or Me) with internal alkynes in the presence of NaBAr(F)4 affords the o-vinylaryl complex with a Rh-(vinyl CH) agostic interaction by way of the alkyne insertion followed by 1,4-rhodium migration, providing the first example of the 1,4-migration of a Rh(III) center.

Neutral one-dimensional metal chains consisting of alternating anionic and cationic rhodium complexes

The metallophilic interactions were investigated within chains of oppositely charged rhodium carbonyl complexes. The cationic [Rh(CO)(2)(L)](+) (L = 2,2'-bipyridine and 1,10-phenanthroline) and anionic [RhCl(2)(CO)(2)](-) units were self-assembled into one dimensional rhodium chains supported by electrostatic interactions. The array of Rh centers in {[Rh(CO)(2)(2,2'-bpy)][RhCl(2)(CO)(2)]}(n) was found to be nearly linear with a Rh···Rh···Rh angle of 170.927(11)° and Rh···Rh distances of 3.3174(5) Å and 3.4116(5) Å. The crystal structure of {[Rh(CO)(2)(1,10-phen)][RhCl(2)(CO)(2)]} consisted of two sets of crystallographically independent chains with slightly different Rh···Rh···Rh angles (170.275(9)° and 159.573(9)°). The higher linearity allowed closer packing of the rhodium complexes. The Rh···Rh distances were 3.2734(3) Å and 3.3155(3) Å for the more linear and 3.3498(3) Å and 3.3211(3) Å for the less linear system. The existence of metallophilic interactions was confirmed computationally by TD-DFT and QTAIM analysis. The computational results also indicated that the intermolecular charge transfer from the cation to the anion had a significant contribution to the absorption properties of the chain compounds.

A dimer of bis(N-heterocyclic carbene)rhodium(I) centres spanned by a dibenzo-18-crown-6 bridge from synchrotron radiation

The compound (μ-3,3′,3′′,3′′′-{[2,5,8,15,18,21-hexa­oxatricyclo­[20.4.0.09,14]hexa­cosa-1(22),9,11,13,23,25-hexa­ene-11,12,24,25-tetra­yl]tetra­kis­(methyl­ene)}tetra­kis­(1-methyl-1H-imidazol-2-yl))bis­[(η4-cyclo­octa-1,4-diene)rhodium(I)] bis­(hexa­fluoridophosphate) acetonitrile sesquisolvate dihydrate, [Rh2(C8H12)2(C40H42N8O6)](PF6)2·1.5CH3CN·2H2O, crystallized from acetonitrile under an atmosphere of diethyl ether. In the crystal structure, the complex cation exhibits two square-planar RhI centres, each bound by a cyclo­octa­diene (COD) ligand and by two adjacent imidazolyl­idene N-heterocyclic carbene (NHC) donors from the same phen­oxy ring of the {[dibenzo-18-crown-6-11,12,24,25-tetra­yl]tetra­kis­(methyl­ene)}tetra­kis­(1-methyl-1H-imidazol-2-yl) (L) ligand. The dibenzo-crown ether bridge of L spans the Rh centres and forms hydrogen bonds with water mol­ecules. One water mol­ecule with half occupancy bridges adjacent macrocycles in the lattice. Another water with full occupancy forms weak hydrogen bonds to the crown ether O atoms and is, in turn, part hydrogen bonded by a lattice water with half occupancy. The latter water is within hydrogen-bonding distance of a fourth water also with partial occupancy. The result of these inter­actions is the formation of a layer in the ab plane. Two PF6 − ions, one of which is twofold disordered, and one ordered and one twofold disordered (with 0.5 occupancy) lattice acetonitrile mol­ecules complete the crystal structure.

ChemInform Abstract: Kinetico‐Mechanistic Studies of Cyclometalating C—H Bond Activation Reactions on Pd(II) and Rh(II) Centres: The Importance of Non‐Innocent Acidic Solvents in the Process

AbstractReview: 146 refs.

Synthesis of a Rhodium Complex Featuring the Rh–H–B Linkage via a Hydride Migration from Rhodium to Borane: Study on the Electronic Deviation Induced by the Presence of the Boron Moiety

The rhodium complex [{o-(Ph2P)C6H4}2(Ph)BH]Rh(CO)(PPh3) (3-Rh) ,featuring a B–H–Rh interaction, was synthesized via phosphine ligand exchange between tris(triphenylphosphine)carbonylrhodium(I) hydride, RhH(CO)(PPh3)3 (2-Rh), and the diphosphine–borane {o-(Ph2P)C6H4}2BPh (5). In contrast, the reaction of 5 with the iridium analogue of 2-Rh, IrH(CO)(PPh3)3 (2-Ir), resulted in the formation of the iridium complex [{o-(Ph2P)C6H4}3B]Ir(H)(CO) (4-Ir), with a direct Ir→B interaction. These compounds 3-Rh and 4-Ir were fully characterized using 1H, 11B, and 31P NMR spectroscopy, X-ray diffraction analysis, and elemental analysis. Density functional theory calculations of 3-Rh revealed that the boron accepts a significant amount of electron density from the Rh center through a three-center–two-electron Rh–H–B interaction and that the presence of the boron moiety considerably weakens the Rh–CO bond. The electronic variation induced by the boron moiety can promote the facile CO elimination of 3-Rh. The “boron enhancement effect” observed in the catalytic transfer hydrogenation of ethyl phenyl ketone is also reported.

Isolation and X-ray structures of four Rh(PCP) complexes including a Rh(I) dioxygen complex with a short O–O bond

The reaction of RhCl3·H2O with tBu2P(CH2)5PtBu2 afforded several complexes including [RhIII(H)Cl{tBu2P(CH2)2CH(CH2)2PtBu2}] (1), [RhIIIHCl2{tBu2P(CH2)5PtBu2}]2 (2), [RhICl{tBu2P(CH2)2CHCHCH2PtBu2}] (3) and [RhICl{tBu2PCH2C(O)CHCHCH2PtBu2}] (4). X-ray crystal structures of 3 and 4 showed that the CC bond on the C5 unit of tBu2P(CH2)5PtBu2 is bound to Rh(I) in a η2 configuration. In 4, the Rh atom has a trigonal pyramidal coordination geometry. The X-ray crystal structure of 2 consists of two rhodium(III) centers bridged by two tBu2P(CH2)5PtBu2 ligands with two phosphorus atoms, one from each ligand, trans to one another. The crystal structure of the rhodium oxygen adduct with 1,3-bis(di-t-butylphosphinomethyl)benzene [RhO2{tBu2PCH2(C6H3)CH2PtBu2}] (5) was also investigated. In this species the O2 is η2 coordinated to the Rh(I) center with asymmetric Rh–O bond lengths (2.087(7) and 1.998(8)Å). The O–O bond distance is short (1.337(11)Å) with νO–O of 990.5cm−1. DFT calculations on complex 5 yielded two η2–O2 structures that differed in energy by only 0.76kcal/mol. The lower energy one (5a) had near C2 symmetry, and had nearly equal Rh–O bond lengths, while the higher energy structure (5b) had near Cs symmetry and generally good agreement with the experimental structure. The calculated UV–Vis and IR spectra of complex 5 are in excellent agreement with experiment.