Dinuclear Rhodium Research Articles

Selective dehydrogenation of ammonia borane to borazine and derivatives by rhodium olefin complexes.

This report presents a selective synthetic approach towards borazine from ammonia borane using a dinuclear rhodium olefin homogeneous catalyst. The synthesis and spectroscopic characterization of a dirhodium ammonia borane complex as an intermediate provides insight into a possible mode of activation.

Pathway selection in the self-assembly of Rh4L4 coordination squares under kinetic control

Pathway selection principles in reversible reaction networks such as molecular self-assembly have not been established yet, because achieving kinetic control in reversible reaction networks is more complicated than in irreversible ones. In this study, we discovered that coordination squares consisting of cis-protected dinuclear rhodium(II) corner complexes and linear ditopic ligands are assembled under kinetic control, perfectly preventing the corresponding triangles, by modulating their energy landscapes with a weak monotopic carboxylate ligand (2,6-dichlorobenzoate: dcb–) as the leaving ligand. Experimental and numerical approaches revealed the self-assembly pathway where the cyclization step to form the triangular complex is blocked by dcb–. It was also found that one of the molecular squares assembled into a dimeric structure owing to the solvophobic effect, which was characterized by nuclear magnetic resonance spectroscopy and single-crystal X-ray analysis.

Interplay between Catalyst Complexes and Dormant States: In Situ Spectroscopic Investigations on a Catalyst System for Alkene Hydroformylation

Results from a detailed in situ IR and NMR spectroscopic study on a monophosphite-modified rhodium catalyst system are presented. Equilibria that occur between the catalytically active hydrido rhodium(I) complexes and formally inactive dinuclear rhodium(0) complexes as well as mononuclear ortho-metalated rhodium(I) complexes have been investigated. The latter can be considered as “dormant states”, which are formed at conditions with lower hydrogen pressures or [H2]/[Rh] ratios. It was found that dimer formation is dominant at lower temperatures and higher rhodium concentrations, whereas ligand metalation is effective at higher temperatures. At the appropriate partial pressure of hydrogen, the regeneration of hydrido rhodium(I) complexes takes place on a short time scale from both states. Especially, the temporary stabilization of the catalyst via ligand metalation during a catalyst recycling procedure represents a feasible approach.

Stabilization of Dinuclear Rhodium and Iridium Clusters on Layered Titanate and Niobate Supports.

Atomically dispersed organometallic clusters can provide well-defined nuclearity of active sites for both fundamental studies as well as new regimes of activity and selectivity in chemical transformations. More recently, dinuclear clusters adsorbed onto solid surfaces have shown novel catalytic properties resulting from the synergistic effect of two metal centers to anchor different reactant species. Difficulty in synthesizing, stabilizing, and characterizing isolated atoms and clusters without agglomeration challenges allocating catalytic performance to atomic structure. Here, we explore the stability of dinuclear rhodium and iridium clusters adsorbed onto layered titanate and niobate supports using molecular precursors. Both systems maintain their nuclearity when characterized using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Statistical analysis of HAADF-STEM images revealed that rhodium and iridium dimers had mean cluster-to-cluster distances very similar to what is expected from a random distribution of atoms over a large area, indicating that they are dispersed without aggregation. The stability of dinuclear rhodium clusters supported on titanate nanosheets was also investigated by X-ray absorption fine structure (EXAFS), DRIFTS, and first-principles calculations. Both X-ray absorption spectroscopy and HAADF-STEM simulations, guided by density functional theory (DFT)-optimized structure models, suggested that rhodium dimers adsorb onto the nanosheets in an end-on binding mode that is stable up to 100 °C under reducing conditions. This study highlights that crystalline nanosheets derived from layered metal oxides can be used as model supports to selectively stabilize dinuclear clusters, which could have implications for heterogeneous catalysis.

Rational Design of Mono- and Bi-Nuclear Cyclometalated Ir(III) Complexes Containing Di-Pyridylamine Motifs: Synthesis, Structure, and Luminescent Properties.

Heteroleptic cyclometalated iridium (III) complexes (1–3) containing di-pyridylamine motifs were prepared in a stepwise fashion. The presence of the di-pyridylamine ligands tunes their electronic and optical properties, generating blue phosphorescent emitters at room temperature. Herein we describe the synthesis of the mononuclear iridium complexes [Ir(ppy)2(DPA)][OTf] (1), (ppy = phenylpyridine; DPA = Dipyridylamine) and [Ir(ppy)2(DPA-PhI)][OTf] (2), (DPA-PhI = Dipyridylamino-phenyliodide). Moreover, the dinuclear iridium complex [Ir(ppy)2(L)Ir(ppy)2][OTf]2 (3) containing a rigid angular ligand “L = 3,5-bis[4-(2,2′-dipyridylamino)phenylacetylenyl]toluene” and displaying two di-pyridylamino groups was also prepared. For comparison purposes, the related dinuclear rhodium complex [Rh (ppy)2(L)Rh(ppy)2][OTf]2 (4) was also synthesized. The x-ray molecular structure of complex 2 was reported and confirmed the formation of the target molecule. The rhodium complex 4 was found to be emissive only at low temperature; in contrast, all iridium complexes 1–3 were found to be phosphorescent in solution at 77 K and room temperature, displaying blue emissions in the range of 478–481 nm.

Quadruple C-H Bond Activations of Methane by Dinuclear Rhodium Carbide Cation [Rh2C3

The structure of the [Rh2C3]+ ion and its reaction with CH4 in the gas phase have been studied by infrared photodissociation spectroscopy and mass spectrometry in conjunction with quantum chemical calculations. The [Rh2C3]+ ion is characterized to have an unsymmetrical linear [Rh–C–C–C–Rh]+ structure existing in two nearly isoenergetic spin states. The [Rh2C3]+ ion reacts with CH4 at room temperature to form [Rh2C]+ + C3H4 and [Rh2C2H2]+ + C2H2 as the major products. In addition to the [Rh2C]+ ion, the [Rh213C]+ ion is formed at about one-half of the [Rh2C]+ intensity when the isotopic-labeled 13CH4 sample is used. The production of [Rh213C]+ indicates that the linear C3 moiety of [Rh2C3]+ can be replaced by the bare carbon atom of methane with all four C–H bonds being activated. The calculations suggest that the overall reactions are thermodynamically exothermic, and that the two Rh centers are the reactive sites for C–H bond activation and hydrogen atom transfer reactions.

Frontispiece: Dirhodium Complexes as Panchromatic Sensitizers, Electrocatalysts, and Photocatalysts

Dinuclear rhodium complexes are attractive candidates as homogeneous panchromatic photosensitizers and photocatalysts. Modification of the coordination sphere of the Rh2(II,II) compounds results in photophysical and redox properties that are highly desirable for electro- and photocatalysis. Optimization of the Rh2(II,II) catalysts is crucial to realize their future application in devices or systems designed for the production of fuels from sunlight. For more details see the Minireview by C. Turro and S. Lin on page 5379.

Three Reversible Redox States of Thiolate-Bridged Dirhodium Complexes without Metal–Metal Bonds

An unusual dinuclear rhodium complex with the anionic 2-mercapto-6-methylpyridinate (mmp) bridging ligand is reported which is capable of undergoing significant variations in its structural and coordination environments as a result of two reversible redox events at accessible potentials (E1/2 = 0.014, 0.52 V vs. Ag/AgCl). The large degree of separation between these redox states (ΔE = 0.51 V, KC = 4.17x108) allows for the chemical isolation of three distinct complexes 1, 2, and 3, in which the oxidation states of each Rh center are described as Rh2I,I, Rh2I,II, and Rh2II,II, respectively, and whose structures were elucidated by single crystal X-ray diffraction studies. Complex 2 is an unprecedented type of mixed valence dirhodium species whose EPR spectrum revealed a delocalization of the unpaired electron through the thiolate bridging ligand. Intervalence charge transfer (IVCT) occurs between the Rh centers as evidenced by a broad absorption in the near-infrared region (λmax¬=1187 nm). The structure of 3 is quite rare in that it lacks the typical RhII-RhII σ bond but significant orbital overlap between the Rh 4dz2 and S 3pz orbitals results in strong antiferromagnetic coupling (computed J = -1516.9 cm-1). Complex 3 also absorbs low energy light (λmax¬ = 779 nm) and spectroscopic and magnetic measurements are supported by DFT methods which further elucidate the nature of the ground state energies, frontier orbital characters, excitated state transitions, and the presence of weak Rh-Rh NBO interactions.

Monohydride-Dichloro Rhodium(III) Complexes with Chiral Diphosphine Ligands as Catalysts for Asymmetric Hydrogenation of Olefinic Substrates.

We report full details of the synthesis and characterization of monohydride-dichloro rhodium(III) complexes bearing chiral diphosphine ligands, such as (S)-BINAP, (S)-DM-SEGPHOS, and (S)-DTBM-SEGPHOS, producing cationic triply chloride bridged dinuclear rhodium(III) complexes (1 a: (S)-BINAP; 1 b: (S)-DM-SEGPHOS) and a neutral mononuclear monohydride-dichloro rhodium(III) complex (1 c: (S)-DTBM-SEGPHOS) in high yield and high purity. Their solid state structure and solution behavior were determined by crystallographic studies as well as full spectral data, including DOSY NMR spectroscopy. Among these three complexes, 1 c has a rigid pocket surrounded by two chloride atoms bound to the rhodium atom together with one tBu group of (S)-DTBM-SEGPHOS for fitting to simple olefins without any coordinating functional groups. Complex 1 c exhibited superior catalytic activity and enantioselectivity for asymmetric hydrogenation of exo-olefins and olefinic substrates. The catalytic activity of 1 c was compared with that of well-demonstrated dihydride species derived in situ from rhodium(I) precursors such as [Rh(cod)Cl]2 and [Rh(cod)2 ]+ [BF4 ]- upon mixing with (S)-DTBM-SEGPHOS under dihydrogen.

Backbone-Directed Self-Assembly of Interlocked Molecular Cyclic Metalla[3]Catenanes.

The efficient backbone-directed self-assembly of cyclic metalla[3]catenanes by the combination of tetrachloroperylenediimide (TCPDI)-based dinuclear rhodium(III) clips and 4,4'-diazopyridine or 4,4'-dipyridylethylene ligands is realized in a single-step strategy. The topology and coordination geometry of the cyclic metalla[3]catenanes are characterized by NMR spectroscopy, ESI-TOF-MS spectrometry, UV/Vis-NIR spectroscopy, and X-ray diffraction studies. The most remarkable feature of the formed cyclic metalla[3]catenane is that it contains π-aggregates (ca. 2.6 nm) incorporating six TCPDIs. Further studies revealed that cyclic metalla[3]catenanes can be converted reversibly to their corresponding sodium adducts and precursor building blocks, respectively. This strategy opens the possibility of generating unique supramolecular structures from discrete functional π-aggregates with precise arrangements.

Backbone‐Directed Self‐Assembly of Interlocked Molecular Cyclic Metalla[3]Catenanes

AbstractThe efficient backbone‐directed self‐assembly of cyclic metalla[3]catenanes by the combination of tetrachloroperylenediimide (TCPDI)‐based dinuclear rhodium(III) clips and 4,4′‐diazopyridine or 4,4′‐dipyridylethylene ligands is realized in a single‐step strategy. The topology and coordination geometry of the cyclic metalla[3]catenanes are characterized by NMR spectroscopy, ESI‐TOF‐MS spectrometry, UV/Vis‐NIR spectroscopy, and X‐ray diffraction studies. The most remarkable feature of the formed cyclic metalla[3]catenane is that it contains π‐aggregates (ca. 2.6 nm) incorporating six TCPDIs. Further studies revealed that cyclic metalla[3]catenanes can be converted reversibly to their corresponding sodium adducts and precursor building blocks, respectively. This strategy opens the possibility of generating unique supramolecular structures from discrete functional π‐aggregates with precise arrangements.

Stirring or milling? First synthesis of Rh(I)-(di-N-heterocyclic carbene) complexes both in solution and in a ball mill

An environment-friendly, convenient, fast and solvent-free mechanochemical approach have been accomplished for the synthesis of several diimidazolium salts and the bridging dinuclear rhodium(I)–N-heterocyclic carbene complexes of the type [{RhCl(cod)}2(μ-di-NHC)] derived from them. The compounds were synthesized also by the classical solvent method and the results of the two approaches were compared. A systematic study of both the mechanochemical and the solvent syntheses has also been carried out to determine the effects of various factors influencing the reactions. This is the first report on the mechanochemical synthesis of poly-NHC metal complexes as well as NHC–Rh complexes in ball mill.

First dinuclear rhodium(II) complexes with triazolopyrimidines and the prospect of their potential biological use

Five novel rhodium(II) complexes of general formula [Rh2(μ-OOCCH3)4L2], where L is a triazolopyrimidine derivative, in particular dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (dmtp) for (1), 5,7-diethyl-1,2,4-triazolo[1,5-a]pyrimidine (detp) for (2), 7-isobutyl-5-methyl-1,2,4-triazolo[1,5-a]pyrimidine (ibmtp) for (3), 7-hydroxy-5-methyl-1,2,4-triazolo[1,5-a]pyrimidine (HmtpO) for (4) and 5,7-ditertbutyl-1,2,4-triazolo[1,5-a]pyrimidine (dbtp) for (5) are reported. These first representatives of paddle-wheel dirhodium complexes with triazolopyrimidines have been characterized by IR and NMR spectroscopy as well as by single-crystal X-ray diffraction studies. Three of the new complexes (1), (2) and (5) were thoroughly screened in vitro for their cytotoxicity against human breast cancer cell line MCF-7 and L929 murine fibroblast cells. Favorably, they show significantly less effective inhibition on the cell growth of L929 than cisplatin under identical conditions. Complexes (1) and (5) display moderate cytotoxic activity (IC50 = 16.3–21.5 μM) against MCF-7 cells which is induced via reactive oxygen species-independent pathways. Extensive studies of rhodium complexes (1), (2) and (5) against microorganisms have shown that the tested compounds exhibit antibacterial activity against Gram-positive bacteria (Staphylococcus aureus and Bacillus subtilis) while (5) significantly inhibited the growth of Malassezia furfur. The highest antibacterial, and antifungal activity, was observed for (5).

The Reaction Pathway Leading to Dinuclear Rhodium and Iridium Complexes from Alkyne-Containing Bisphosphine Ligands

Abstract A mechanistic study of a series of dinuclear complexes produced by the complexation of alkyne-containing bisphosphine ligands with Ir and Rh has been performed by experimental analysis as well as density functional theory (DFT) calculations. The key process is oxidative cyclization involving two metals and two alkyne-containing ligands 1. The related alkyne-bridged dimer 7, considered as an intermediate, was determined using X-ray measurements. The DFT calculation supported that the core metals alter the reorganization pathways from alkyne-bridged dimer, leading to the unique structures 4 and 5.

Investigations into the mechanism of the in situ formation of neutral dinuclear rhodium complexes

The often applied in situ formation of neutral dinuclear rhodium precatalysts of the type [Rh(Diphosphine)(μ2-Cl)]2 with the ligands DPEPhos and DIOP has been mechanistically investigated by NMR spectroscopy. The structural characterization of reaction intermediates [Rh2(μ2-Diphosphine)(COD)2(Cl)2] and [Rh2(Diphosphine)(COD)(μ2-Cl)2] was accomplished using X-ray crystallography. So a new and unexpected intermediate was found, which casts a new light on the mechanism of this ligand exchange. In addition, equilibria could be found which can at least influence the formation of neutral dinuclear rhodium precatalysts.

Asymmetric Hydrogenation of Aryl Perfluoroalkyl Ketones Catalyzed by Rhodium(III) Monohydride Complexes Bearing Josiphos Ligands.

The asymmetric hydrogenation of 2,2,2-trifluoroacetophenones and aryl perfluoroalkyl ketones was developed using a unique, well-defined chloride-bridged dinuclear rhodium(III) complex bearing Josiphos-type diphosphine ligands. These complexes were prepared from [RhCl(cod)]2 , Josiphos ligands, and hydrochloric acid. As catalyst precursors, they allow for the efficient and enantioselective synthesis (up to 99 % ee) of chiral secondary alcohols with perfluoroalkyl groups. This system does not require an activating base for the hydrogenation of 2,2,2-trifluoroacetophenones. Additionally, the enantioselective C=O hydrogenations of 2-phenyl-3-(haloacetyl)-indoles, a class of privileged structures in medicinal chemistry, is reported for the first time.

Sequential Double Dearomatization of the Pyrazolate‐Based “Two‐in‐One” Pincer Ligand in a Dinuclear Rhodium(I) Complex

A binucleating pyrazolate‐based ligand providing two tridentate PNN compartments is shown to form a dinuclear rhodium(I) complex [L{Rh(CO)}2](PF6) (1) that can be deprotonated at both side arm methylene groups between pyridine and phosphino donors to give K[L**{Rh(CO)}2] (3). Sequential twofold deprotonation has been monitored by NMR and UV/Vis spectroscopy and proceeds via the neutral intermediate L*{Rh(CO)}2 (2). X‐ray crystallographic characterization of 1 and the K[2,2,2]+ salt 3′ ([2,2,2] is a cryptand) evidences pyridine dearomatization upon deprotonation, and all spectroscopic and structural signatures are in good agreement with those of related mononuclear rhodium complexes based on a common PNN pincer ligand, which corroborates that pyrazolate‐bridged L– is best described as a scaffold with two pincer‐type subunits.

An Entry to Double Phosphanyl‐Bridged Dinuclear Rhodium(II) Complexes

The reaction of [RhCp(coe)2] (1, coe = cis‐cyclooctene) with one equivalent of the secondary phosphines PR2H in refluxing toluene afforded in situ the new complexes [RhCp(coe)(PR2H)] (R = Ph, 2a; R = o‐tolyl, 2b; R = tBu, 2c) in good yields. Further thermolysis of 2a and 2b, respectively, in refluxing xylene resulted in the formation of the dinuclear doubly phosphanyl‐bridged rhodium(II) species [{RhCp(µ‐PR2)}2] (3a and 3b). This thermolytic approach does not work in the case of 2c since the tBu substituents increase the electron density on the rhodium center and impede, therefore, the loss of the olefinic ligand presumably caused by a stronger back donation effect. On the way from 2a to 3a the intermediate rhodium(III) complex [{RhCp(µ‐PPh2)(H)2}2] (4a) was isolated. All new compounds were fully characterized by spectroscopic methods as well as by X‐ray crystallography in the case of 3a, 3b and 4a confirming their molecular structures in the crystal. The compound [{RhCp(µ‐PPh2)}2] did not react with dihydrogen at ambient conditions and not even in refluxing toluene affording the complex 4a by oxidative addition as originally assumed.

Mechanistic Investigation on the Polymerization of Phenylacetylene by 2-Diphenylphosphinopyridine Rhodium(I) Catalysts: Understanding the Role of the Cocatalyst and Alkynyl Intermediates

The mono- and dinuclear rhodium(I) complexes featuring 2-(diphenylphosphino)pyridine ligands, [Rh(cod)(Ph2PPy)]+ and [Rh(nbd)(μ-Ph2PPy)]22+ (cod = 1,5-cyclooctadiene, nbd = 2,5-norbornadiene), have been prepared in order to be evaluated as phenylacetylene (PA) polymerization catalysts. In contrast with compound [Rh(nbd){Ph2P(CH2)2Py}]+, featuring a 2-(2-(diphenylphosphino)ethyl)pyridine ligand, that showed a moderate catalytic activity, both [Rh(diene)(Ph2PPy)]nn+ (n = 1, cod; n = 2, nbd) complexes showed no catalytic activity due to the formation of unusual dinuclear species [Rh2(diene)2(μ-Ph2PPy)(μ-C≡C-R)]+, supported by a Ph2PPy bridging ligand and an alkynyl ligand coordinated in a μ-η1:η2 fashion, which are inactive in PA polymerization. However, compounds [Rh(diene)(Ph2PPy)]nn+ efficiently polymerize PA in the presence of a cocatalyst as iPrNH2 affording highly stereoregular poly(phenylacetylene) (PPA) of Mw = 3.42 × 105 (cod) and 2.02 × 105 (nbd) with polydispersities of 1.39 and initiation efficiencies of 4–7%. NMR studies on the polymerization reaction have allowed identification of the alkynyl species [Rh(C≡CPh)(cod)(Ph2PPy)] as the likely initiating species involved in the generation of the rhodium-vinyl species responsible for the propagation step. The iPrNH2 cocatalyst is possibly involved in the efficient proton transfer from the coordinated PA to iPrNH2 that allows for a significant concentration of the key initiating species [Rh(C≡CPh)(cod)(Ph2PPy)]. The distinct behavior of compounds [Rh(diene)(Ph2PPy)]nn+ as PA polymerization catalysts is a consequence of the binucleating ability of the Ph2PPy ligand in combination with the low basicity of the pyridine fragment which allows for the stabilization of the inactive alkynyl-bridge dinuclear species.

Rhodium–N-Heterocyclic Carbene Catalyzed Hydroalkenylation Reactions with 2-Vinylpyridine and 2-Vinylpyrazine: Preparation of Nitrogen-Bridgehead Heterocycles

Dinuclear rhodium–NHC complexes of formula [Rh(μ-Cl)(NHC)(η2-coe)]2 react with 2-vinylpyridine to yield the chelate compounds RhCl(NHC)(κ-N,η2-CH2═CHC5H4N) {NHC = IPr, 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-carbene; IMes, 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-carbene}. The strained metallacycle can be opened by substitution of the pyridine ring by the small electron-rich PEt3 to give RhCl(IPr){η2-CH2═CH(C5H4N)}(PEt3), whereas π ligands such as olefins or alkynes undergo C–C coupling to yield 2-(butenyl)pyridine or 2-(butadienyl)pyridine RhCl(NHC){κ-N,η2-CH(R)═CH(C5H4N)} complexes by formal hydroalkenylation of the unsaturated bond by vinylpyridine. Reaction of the dinuclear precursors with 2-vinylpyrazine in the presence of pyridine affords the η2 derivative RhCl(IPr){η2-CH2═CH(C4H3N2)}(py). Compound RhCl(IMes)(κ-N,η2-CH2═CHC5H4N) is an efficient catalyst for the hydroalkenylation of a range of alkenes and alkynes with 2-vinylpyridine and 2-vinylpyrazine. The butadienyl-heterocycle derivatives re...