Half-sandwich Rhodium Complexes Research Articles

Half-Sandwich Rhodium Complexes with Releasable N-Donor Monodentate Ligands: Solution Chemical Properties and the Possibility for Acidosis Activation.

Cancer chemotherapeutics usually have serious side effects. Targeting the special properties of cancer and activation of the anticancer drug in the tumor microenvironment in situ may decrease the intensity of the side effects and improve the efficacy of therapy. In this study, half-sandwich Rh complexes are introduced, which may be activated at the acidic, extracellular pH of the tumor tissue. The synthesis and aqueous stability of mixed-ligand complexes with a general formula of [Rh(η5-Cp*)(N,N/O)(N)]2+/+ are reported, where (N,N/O) indicates bidentate 8-quinolate, ethylenediamine and 1,10-phenanthroline and (N) represents the releasable monodentate ligand with a nitrogen donor atom. UV-visible spectrophotometry, 1H NMR, and pH-potentiometry were used to determine the protonation constants of the monodentate ligands, the proton dissociation constants of the coordinated water molecules in the aqua complexes, and the formation constants of the mixed-ligand complexes. The obtained data were compared to those of the analogous Ru(η6-p-cymene) complexes. The developed mixed-ligand complexes were tested in drug-sensitive and resistant colon cancer cell lines (Colo205 and Colo320, respectively) and in four bacterial strains (Gram-positive and Gram-negative, drug-sensitive, and resistant) at different pH values (5-8). The mixed-ligand complexes with 1-methylimidazole displayed sufficient stability at pH 7.4, and their activation was found in cancer cells with decreasing pH; moreover, the mixed-ligand complexes demonstrated antimicrobial activity in Gram-positive and Gram-negative bacteria, including the resistant MRSA strain. This study proved the viability of incorporating releasable monodentate ligands into mixed-ligand half-sandwich complexes, which is supported by the biological assays.

Half-sandwich rhodium complexes with phenylene-based SCS ligands: Synthesis, characterization and catalytic activities for transfer hydrogenation of ketones

A series of half-sandwich rhodium complexes with tridentate phenylene-based bis(thione) (SCS) ligand have been synthesized and characterized. Both half-sandwich rhodium complexes and phenylene-based bis(thione) compounds were fully characterized by 1H and 13C NMR spectra, mass spectrometry and single-crystal X-ray diffraction method. The catalytic activities of half-sandwich rhodium complexes toward the transfer hydrogenation of ketones to their corresponding alcohols were explored using 2-propanol as hydrogen source and solvent. And the half-sandwich rhodium complexes exhibited high catalytic activity for transfer hydrogenation of ketones with a broad functional group tolerance.

Structural and chemical properties of half-sandwich rhodium complexes supported by the bis(2-pyridyl)methane ligand.

[Cp*Rh] complexes (Cp* = pentamethylcyclopentadienyl) supported by bidentate chelating ligands are a useful class of compounds for studies of redox chemistry and catalysis. Here, we show that the bis(2-pyridyl)methane ligand, also known as dipyridylmethane or dpma, can support [Cp*Rh] complexes in the formally +iii and +ii rhodium oxidation states. Specifically, two new rhodium complexes ([Cp*Rh(dpma)(L)]n+, L = Cl-, CH3CN) have been isolated and structurally characterized, and the properties of the complexes have been compared with those of [Cp*Rh] complexes bearing the related dimethyldipyridylmethane (Me2dpma) ligand. Complex [Cp*Rh(dpma)(NCCH3)]2+ displays a quasireversible rhodium(iii/ii) reduction by cyclic voltammetry; related electron paramagnetic resonance (EPR) spectroscopic studies confirm access to the unusual rhodium(ii) oxidation state. Further reduction to the formally rhodium(i) oxidation state, however, is followed by deprotonation of dpma, as observed in electrochemical studies and chemical reduction experiments. This reactivity can be understood to occur as a consequence of the presence of doubly benzylic protons in the dpma ligand, since use of the analogous Me2dpma enables reduction to rhodium(i) without involvement of ligand deprotonation. These findings highlight the important role of the ligand backbone substitution pattern in influencing the stability of highly-reduced complexes, a key class of metal species for study of electron and proton management in catalysis.

Synthesis and reactivity of the cyclohexadienyl rhodium complexes

In a search for an active catalyst for CH-activation reactions we have synthesized the half-sandwich rhodium complexes with a cyclohexadienyl ligand. In particular, the reaction of [(cyclooctene)2RhCl]2 with 1,3,5,5-tetramethylcyclohexadiene followed by addition of bromine gave the cyclohexadienyl polybromide complex [(η5-C6H3Me4)RhBr3]n (1) in 73% yield. Treatment of 1 with ethylene converted it into the Rh(III) bromide complex [(η5-C6H3Me4)RhBr2]2 (2), which is analogous to the classical catalyst [Cp*RhCl2]2. The reaction of 2 with 2 equiv. of 2-electron ligands L produced the expected adducts (η5-C6H3Me4)RhBr2L (L = tBuNC, pyridine, P(OMe)3, PPh3) in 80–90% yields. On the other hand, the reactions of 1 or 2 with excess of tBuNC led to complete replacement of the cyclohexadienyl ligand. The complex 2 was found to be inactive as catalyst (at 10 mol-% loading) for reactions of diphenylacetylene with benzoic acid, acetanilide, O-Boc-phenylhydroxamic acid, or benzophenone oxime. The absence of catalytic activity was attributed to the displacement of the cyclohexadienyl ligand.

Synthesis of rhodacarboranes containing σ- and π-carboranyl ligands in one molecule

16-Electron half-sandwich rhodium complexes containing carborane ligands of two different σ- and π- types 3,3-μ-(1′,2′-C2B10H10-1′,2′-S2-)-4-L-3,1,2-RhC2B9H10 (L = SMe2, NMe3, NMe2CH2Ph) were synthesized for the first time. The solid state structures of 3-(COD)-4-PhCH2Me2N-3,1,2-RhC2B9H10 and 3,3-μ-(1′,2′-C2B10H10-1′,2′-S2-)-4-Me3N-3,1,2-RhC2B9H10 were determined by single crystal X-ray diffraction.

Half-sandwich complexes of rhodium containing cysteine-derived ligands.

The modified cysteine ligand, S-benzyl-α-methyl-l-cysteine (HL2), was prepared from l-cysteine hydrochloride methyl ester. The reaction of commercial S-benzyl-l-cysteine (HL1) or HL2 with the dimer, [{(η(5)-C5Me5)RhCl}2(μ-Cl)2], gives rise to the cationic complexes, [(η(5)-C5Me5)RhCl(HL)]Cl (HL = HL1 (1), HL2 (2)), in which the cysteine ligand exhibits a κ(2)N,S coordination mode. In a basic medium, HL1 or HL2 reacts with [{(η(5)-C5Me5)RhCl}2(μ-Cl)2] to afford mixtures of two epimers at the metal centre of the neutral complexes, [(η(5)-C5Me5)RhCl(κ(2)N,O-L)] (HL = HL1 (3), HL2 (4)), in which amino carboxylate adopts a κ(2)N,O mode of coordination along with variable amounts of the cationic compounds, [(η(5)-C5Me5)Rh(κ(3)N,O,S-L)]Cl (HL = HL1 (6Cl), HL2 (7Cl)), which contain κ(3)N,O,S coordinated cysteine-derived ligands. However, in a basic medium, the N-Boc substituted cysteine S-benzyl-N-Boc-l-cysteine (HL3) only yields the κ(2)O,S coordinated derivative, [(η(5)-C5Me5)RhCl(κ(2)O,S-L3)] (5), as a mixture of two diastereomers depending on the configuration of the metal centre. The bidentate chelate complexes 3-5 react with AgSbF6 to give the hexafluoroantimonates [(η(5)-C5Me5)Rh(κ(3)N,O,S-L)][SbF6] (HL = HL1 (6Sb), HL2 (7Sb), HL3 (8Sb)) with tridentate coordination. Compound 8Sb reacts with NaHCO3 to give the neutral complex [(η(5)-C5Me5)Rh(κ(3)N,O,S-L3-H)] (9), which can also be prepared by reacting the dimer [{(η(5)-C5Me5)RhCl}2(μ-Cl)2] with HL3 in the presence of two equivalents of NaHCO3. The new compounds contain up to four stereogenic centres, namely, Rh, S, N, and C. The absolute configuration of the complexes has been established by spectroscopic and diffractometric investigations, including the crystal structure determination of [(η(5)-C5Me5)RhCl(κ(2)O,S-L3)] (5), [(η(5)-C5Me5)Rh(κ(3)N,O,S-L1)][SbF6] (6Sb), [(η(5)-C5Me5)Rh(κ(3)N,O,S-L2)][SbF6] (7Sb) and [(η(5)-C5Me5)Rh(κ(3)N,O,S-L3-H)] (9). Variable temperature (1)H NMR studies reveal the existence of epimerization processes and theoretical calculations were used to discriminate their nature.

Exploring Oxidation of Half-Sandwich Rhodium Complexes: Oxygen Atom Insertion into the Rhodium–Carbon Bond of κ2-Coordinated 2-Phenylpyridine

The reactions of oxygen atom transfer reagents with Rh(Cp*) complexes, each with a bidentate ligand and an accessible coordination site, are described (Cp* = η5-pentamethylcyclopentadienyl). When [Rh(Cp*)(phpy)(NCArF)][B(ArF)4] (1, phpy = 2-phenylene-κC1′-pyridine-κN, NCArF = 3,5-bis(trifluoromethyl)benzonitrile, B(ArF)4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) was treated with the soluble oxygen atom transfer reagent 2-tert-butylsulfonyliodosylbenzene (sPhIO), oxygen atom insertion into the rhodium–carbon bond of coordinated phpy was observed. This resulted in the formation of a κ2 2-(2-pyridyl)phenoxide ligand. Following insertion to form a new bidentate ligand, a second equivalent of sPhIO, acting as a neutral, two-electron donor ligand, coordinated to the rhodium center through the iodosyl oxygen. Over time, the sPhIO ligand dissociates and dimerization occurs to generate a phenoxide-bridged dinuclear species. The 2-(2-pyridyl)phenoxide ligand could be protonated and cleaved from the mononucl...

Photochemical Reactions of Fluorinated Pyridines at Half-Sandwich Rhodium Complexes: Competing Pathways of Reaction.

Irradiation of CpRh(PMe3)(C2H4) (1; Cp = η5-C5H5) in the presence of pentafluoropyridine in hexane solution at low temperature yields an isolable η2-C,C-coordinated pentafluoropyridine complex, CpRh(PMe3)(η2-C,C-C5NF4) (2). The molecular structure of 2 was determined by single-crystal X-ray diffraction, showing coordination by C3–C4, unlike previous structures of pentafluoropyridine complexes that show N-coordination. Corresponding experiments with 2,3,5,6-tetrafluoropyridine yield the C–H oxidative addition product CpRh(PMe3)(C5NF4)H (3). In contrast, UV irradiation of 1 in hexane, in the presence of 4-substituted tetrafluoropyridines C5NF4X, where X = NMe2, OMe, results in elimination of C2H4 and HF to form the metallacycles CpRh(PMe3)(κ2-C,C-CH2N(CH3)C5NF3) (4) and CpRh(PMe3)(κ2-C,C-CH2OC5NF3) (5), respectively. The X-ray structure of 4 shows a planar RhCCNC-five-membered ring. Complexes 2–5 may also be formed by thermal reaction of CpRh(PMe3)(Ph)H with the respective pyridines at 50 °C.

Stereochemical Nonrigidity of a Chiral Rhodium Boryl Hydride Complex: A σ-Borane Complex as Transition State for Isomerization

Experimental and computational studies are reported on half-sandwich rhodium complexes that undergo B-H bond activation with pinacolborane (HBpin = HB(OCMe2CMe2O)). The photochemical reaction of [Rh(eta5-C5H5)(R,R-phospholane)(C2H4)] 3 (phospholane = PhP(CHMeCH2CH2CHMe)) with HBpin generates the boryl hydride in two distinguishable isomers [(SRh)-Rh(eta5-C5H5)(Bpin)(H)(R,R-phospholane)] 5a and [(RRh)-Rh(eta5-C5H5)(Bpin)(H)(R,R-phospholane)] 5b that undergo intramolecular exchange. The presence of a chiral phosphine allowed the determination of the interconversion rates (epimerization) by 1D 1H EXSY spectroscopy in C6D6 solution yielding DeltaH = 83.4 +/- 1.8 kJ mol-1 for conversion of 5a to 5b and 79.1 +/- 1.4 kJ mol-1 for 5b to 5a. Computational analysis yielded gas-phase energy barriers of 96.4 kJ mol-1 determined at the density functional theory (DFT, B3PW91) level for a model with PMe3 and B(OCH2CH2O) ligands; higher level calculations (MPW2PLYP) on an optimized QM/MM(ONIOM) geometry for the full system place the transition state 76.8 kJ mol-1 above the average energy of the two isomers. The calculations indicate that the exchange proceeds via a transition state with a sigma-B-H-bonded borane. The B-H bond lies in a mirror plane containing rhodium and phosphorus. No intermediate with an eta2-B-H ligand is detected either by experiment or calculation. Complex 3 has also been converted to the [Rh(eta5-C5H5)Br2(R,R-phospholane)] (characterized crystallographically) and [Rh(eta5-C5H5)(H)2(R,R-phospholane)]. The latter exhibits two inequivalent hydride resonances that undergo exchange with DeltaH = 101 +/- 2 kJ mol-1. DFT calculations indicate that the boryl hydride complex has a lower exchange barrier than the dihydride complex because of steric hindrance between the phospholane and Bpin ligands in the boryl hydride.

Half-sandwich rhodium complexes containing both N-heterocyclic carbene and ortho-carborane-1,2-dithiolate ligands

A new route was used to synthesize half-sandwich rhodium complexes containing both N-heterocyclic carbenes (NHC) and carborane ligands. The rhodium carbene complexes Cp ∗Rh(L)[S 2C 2(B 10H 10)] (Cp ∗ = pentamethylcyclopentadienyl, L = 1,3-dimethylimidazolin-2-ylidene; 4) can be obtained from the reaction of Cp ∗Rh(L)Cl 2 ( 2) with Li 2S 2C 2(B 10H 10) or from the reaction of Cp ∗Rh[S 2C 2(B 10H 10)] ( 3) with silver–NHC complex prepared by direct reaction of an imidazolium precursor and Ag 2O. Complexes 2 and 4 were characterized by IR, NMR spectroscopy, element analysis and X-ray structure analyses.

Half-sandwich cyclopentadienyl rhodium complexes bearing pendant sulfur or oxygen ligands and their catalytic behaviors in ethylene polymerization

Two half-sandwich rhodium complexes with sulfur or oxygen functionalized cyclopentadienyl ligands [η 5-C 5H 4(CH 2) 2SCH 2CH 3]RhI 2 3, {[η 5-C 5H 4(CH 2) 2OCH 3]RhI 2} 2 4 have been synthesized and characterized by IR, 1H-NMR spectra and Elemental analyses. The molecular structures of complexes 3 and 4 have been determined by X-ray crystallographic analysis. Complexes 3, 4 with a pendent arm on cyclopentadienyl ligand have been tested as catalysts for ethylene and norbornene polymerization in the presence of MAO. Complexes 3 and 4 kept high activities of ca. 10 6 g PE mol −1 Rh h −1 with morderate molecular weight ( M w ≈ 10 5 g mol −1) of polyethylene in the ethylene polymerization. Catalytic activities, molecular weights of polyethylene have been investigated under the various reaction conditions.

Half‐Sandwich Complexes with Aminocarboxylate Ligands and Their Use as Enantioselective Hydrogen Transfer Catalysts

AbstractFor Abstract see ChemInform Abstract in Full Text.

Half-Sandwich Complexes with Aminocarboxylate Ligands and Their Use as Enantioselective Hydrogen Transfer Catalysts

Chiral-at-metal half-sandwich complexes of rhodium, iridium, ruthenium, or osmium of the general formula [(ηn-ring)M(Aa)Cl] [(ηn-ring)M = (η5-C5Me5)Rh, (η5-C5Me5)Ir, (η6-p-MeC6H4iPr)Ru, (η6-p-MeC6H4iPr)Os; Aa = L-α-aminocarboxylate] can readily be prepared from the corresponding dimers [{(ηn-ring)MCl}2(μ-Cl)2]. The compounds have been prepared as diastereomeric mixtures of the two epimers at the metal center. In general, alkynyl aminocarboxylate derivatives [(ηn-ring)M(Aa)(C≡CR)] are obtained by treating the aforementioned chlorides with the corresponding alkynes in basic media. However, the reaction of the (alaninato)rhodium chloride [(η5-C5Me5)Rh(Ala)Cl] with the alkynes HC≡CR (R = Ph, p-tolyl) produced the alkynylcyclobutadiene complexes [(η5-C5Me5)Rh(η4-C4HR2C≡CR)] (R = Ph, p-tolyl). Treatment of the chlorides [(ηn-ring)M(Aa)Cl] with AgBF4 afforded the cationic trimers [{(ηn-ring)M(Aa)}3](BF4)3. Trimerization occurs with chiral self-recognition: only the (R,R,R) or (S,S,S) configuration at the metal trimers can be detected. The trimers [{(ηn-ring)M(Aa)}3](BF4)3 reacted with tertiary phosphanes, leading to the cationic mononuclear complexes [(ηn-ring)M(Aa)(PR3)](BF4). The assignment of the configuration at the metal center was accomplished by X-ray diffraction, circular dichroism and NMR spectroscopy. Most of the aminocarboxylate derivatives epimerized at the metal center. On the basis of kinetic and spectroscopic data, a general mechanism is proposed for the epimerization process. Neutral [(ηn-ring)M(Aa)Cl] and cationic [{(ηn-ring)M(Aa)}3](BF4)3 complexes are active catalysts for the hydrogen transfer reaction from 2-propanol to acetophenone. Conversions of up to 97% and enantioselectivities up to 75% were achieved. A proposal about the origins of the enantioselectivity is given. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Half-sandwich rhodium polychalcogenide complexes and a carbon insertion into Te-Te bond

Treatment of CptRh(PMe3)Cl2(l) (Cpt = 5η-tBu2C5H3) with [Et4N]2Se6 in DMF solution leads to the formation of cyclo-tetraselenido half-sandwich rhodium complexes CptRh(PMe3)(Se4) (2). The elimination of 2 with excess of nBu3P results in cyclo-diselenido rhodium complex CptRh(PMe3)(Se2) (3). 1 reacts with [nBu4N]2Te5 in DMF solution to yield cyclo-ditellurido rhodium complex Cp_tRh (PMe3)(Te2) (4) in which carbon atom from CH2C12 can be inserted into Te-Te bond to form CptRh(PMe3)(TeCH2Te) (5). The new complexes 2–5 have been characterized by their IR, EI-MS, and 1H, 13C, 31P, 103Rh NMR spectra as well as elemental analysis.

Synthesis of metallocenes of zirconium, hafnium, manganese, iron, tin, lead and half-sandwich complexes of rhodium and iridium containing the ligands (η-C 5R 4CR′ 2PMe 2), where R and R′ may be H or Me

The dimethylphosphino substituted cyclopentadienyl precursor compounds [M(C 5Me 4CH 2PMe 2)], where M=Li + ( 1), Na + ( 2), or K + ( 3), and [Li(C 5H 4CR′ 2PMe 2)], where R′ 2=Me 2 ( 4), or (CH 2) 5 ( 5), [HC 5Me 4CH 2PMe 2H]X, where X −=Cl − ( 6) or PF 6 − ( 7) and [HC 5Me 4CH 2PMe 2] ( 8), are described. They have been used to prepare new metallocene compounds, of which representative examples are [Fe(η-C 5R 4CR′ 2PMe 2) 2], where R=Me, R′=H ( 9); R=H and R′ 2=Me 2 ( 10), or (CH 2) 5 ( 11), [Fe(η-C 5H 4CMe 2PMe 3) 2]I 2 ( 12), [Fe{η-C 5Me 4CH 2P(O)Me 2} 2] ( 13), [Zr(η-C 5R 4CR′ 2PMe 2) 2Cl 2], where R=H, R′=Me ( 14), or R=Me, R′=H ( 15), [Hf(η-C 5H 4CMe 2PMe 2) 2]Cl 2] ( 16), [Zr(η-C 5H 4CMe 2PMe 2) 2Me 2] ( 17), {[Zr(η-C 5Me 4CH 2PMe 2) 2]Cl}{(C 6F 5) 3BClB(C 6F 5) 3} ( 18), [Zr{(η-C 5Me 4CH 2PMe 2) 2Cl 2}PtI 2] ( 19), [Mn(η-C 5Me 4CH 2PMe 2) 2] ( 20), [Mn{(η-C 5Me 4CH 2PMe 2B(C 6F 5) 3} 2] ( 21), [Pb(η-C 5H 4CMe 2PMe 2) 2] ( 23), [Sn(η-C 5H 4CMe 2PMe 2) 2] ( 24), [Pb{η-C 5H 4CMe 2PMe 2B(C 6F 5) 3} 2] ( 25), [Pb(η-C 5H 4CMe 2PMe 2) 2PtI 2] ( 26), [Rh(η-C 5Me 4CH 2PMe 2)(C 2H 4)] 29, [M(η,κ P-C 5Me 4CH 2PMe 2)I 2], where M=Rh ( 30), or Ir, ( 31).

Intrazeolite half-sandwich complexes of rhodium and iridium: spectroscopy and reactivity patterns

The incorporation of the half-sandwich complexes CpM(CO){sub 2}, Cp*M(CO){sub 2}, and CpM(C{sub 2}M{sub 4}){sub 2} (M = Rh, Ir; Cp = C{sub 5}H{sub 5}; Cp* = (CH{sub 3}){sub 5}C{sub 5}), as well as CpRh(C{sub 2}H{sub 4})CO and (acac)Rh(CO){sub 2}, into zeolites has been investigated. Initial studies have focused on the immobilization and characterization of the species, primarily in alkali-metal Y zeolites, through in situ optical reflectance and vibrational spectroscopy. Size exclusion studies in Na{sub 12}-A (4.2 {angstrom}) and K{sub 8}Na{sub 4}-A (3.4 {angstrom}) zeolites demonstrate internal rather than external confinement of the organometallic. Two distinct anchoring sites within the supercages are observed for the carbonyl systems. Site 1 interaction is attributed to a cation-ring coordination, while site 2 is assigned to a single cation interaction with the oxygen atoms of both carbonyl ligands. The population distribution between sites 1 and 2 varies with the nature of the organometallic guest, the exchanged cation, and the Si:Al ratio of the zeolite. The {sup 13}CO exchange reactions of encapsulated CpM(CO){sub 2} and Cp*M(CO){sub 2} show faster rates compared to solution-phase work in those alkali-metal Y zeolites that exhibit population of both sites 1 and 2. Preliminary studies indicate {sup 13}CO pressure dependent exchangemore » kinetics for CpIr(CO){sub 2} in K-Y zeolite. This has been attributed to an intrazeolite associative exchange mechanism which is enhanced by site 1 occupation. Similarly, fast room temperature CO substitution of ethylene from intrazeolite CpM(C{sub 2}H{sub 4}){sub 2} yields the dicarbonyl in the same sites and distributions as encountered for a freshly CpM(CO){sub 2} impregnated sample. The thermal properties of the entrapped organometallics have also been probed and the anchored half-sandwich products, CpM and Cp*M, tentatively identified.« less