Synthesis,characterization, and reactivity of ruthenium(II) complexes containing η6-arene-η1-pyrazole ligands

New ruthenium(II) complexes containing η⁶-arene-η¹-pyrazole ligands were synthesized and characterized by elemental analysis and spectroscopic methods. In addition, the molecular structure of dichloro-3,5-dimethyl-1-(pentamethylbenzyl)-pyrazole–ruthenium(II), [Ru]L_3b, was determined by X-ray diffraction studies. These complexes were applied in the transfer hydrogenation of acetophenone by isopropanol in the presence of potassium hydroxide. The activities of the catalysts were monitored by NMR.     

Aromatic hydrocarbons as ligands. Recent advances in the synthesis,the reactivity and the applications of bis(η6-arene) complexes

Synthetic procedures for bis(η⁶-arene) metal derivatives and aspects of their reactivity are reviewed. Attention is focused on early transition metals (Groups 4–6) but, when necessary, reference will be made to arene derivatives of Groups 7–10, lanthanides and actinides.After a short historical presentation of bis(η⁶-arene) derivatives, aimed at illustrating the relevance of this class of compounds to the origin and the evolution of organometallic chemistry, the synthetic procedures to bis(η⁶-arenes) will be discussed in the light of the most recent results. As far as the reactivity of bis(η⁶-arene) compounds is concerned, particular attention is given to the electron transfer reactions occurring with or without arene displacement; data are reported for the use of low-valent bis(η⁶-arene) compounds as a useful entry into the inorganic and coordination chemistry of the corresponding metal in non aqueous systems.The use of bis(η⁶-arene) derivatives of transition metals in low oxidation states (0, +1) as precursors to catalytic systems for the oligomerisation and polymerization of unsaturated monomers and as starting compounds for the preparation of molecule-based magnets, ordered crystals and new materials and supports is described.     

Electrochemistry and UV/visible spectroscopy of phosphino-substituted bis(η-indenyl)iron(II) complexes

Six phosphino-functionalized diindenyl ferrocenes have been characterized by UV/vis spectroscopy and cyclic voltammetry in dichloromethane. The complexes contain the following ligands: 1-diphenylphosphino- (1), 1-diphenylphosphino-2-methyl- (2), 1-diphenylphosphino-3-methyl- (3), 1-diphenylphosphino-3-trimethylsilyl- (4), 1-diphenylphosphino-2,3-dimethyl- (5), and 1-diphenylphosphino-4,7-dimethyl-indenide (6). The cyclic voltammetry shows an approximately additive relationship between oxidation potential and the type of substituent and its ring position, but with increasing substitution leading to lower than otherwise expected oxidation potentials. The UV/vis spectra show two absorptions with the low energy band moving to lower energy with increasing substitution on the C₅ ring.     

Synthesis and characterization of the first transition-metal fullerene complexes containing bis(η-benzene)chromium moieties

While photochemical reaction of C₆₀ with an equimolar amount of Mo(CO)₄(η⁶-Ph₂PC₆H₅)₂Cr (1) in toluene at room temperature produced bimetallic Mo/Cr fullerene complex fac/mer-(η²-C₆₀)Mo(CO)₃[(η⁶-Ph₂PC₆H₅)₂Cr] (2) in 87% yield, the thermal reaction of an equimolar mixture of C₆₀, M(dba)₂ (M = Pd, Pt; dba = dibenzylideneacetone) and (η⁶-Ph₂PC₆H₅)₂Cr (3) in toluene at room temperature afforded bimetallic M/Cr fullerene complexes (η²-C₆₀)M[(η⁶-Ph₂PC₆H₅)₂Cr] (4, M = Pd; 5, M = Pt) in 88% and 92% yields, respectively. Products 2, 4 and 5 are the first transition-metal fullerene complexes containing bis(η⁶-benzene)chromium moieties. While 2, 4 and 5 were characterized by elemental analysis and spectroscopy, the crystal molecular structures of 4 along with the starting materials 1 and 3 have been determined by X-ray diffraction techniques.     

Synthesis and properties of monometallic, homo- and heterobimetallic complexes based on {(η-arene)RuCl} and {(η-arene)OsCl} fragments with tetrathioether and tetraselenoether ligands

The reaction of [Ru(η⁶-p-cymene)Cl₂]₂ with 2.0 mol equivalents of C(CH₂SMe)₄, C(CH₂SeMe)₄, 1,2,4,5-C₆H₂(CH₂SMe)₄ or 1,2,4,5-C₆H₂(CH₂SeMe)₄ (L₄) and [NH₄][PF₆] in ethanol solution forms the [RuCl(η⁶-p-cymene){κ²-L₄}][PF₆] complexes. Similar Os(II) complexes are obtained starting with [Os(η⁶-p-cymene)Cl₂]₂. Treatment of [RuCl(η⁶-p-cymene){κ²-L₄}][PF₆] with a further 0.5 mol equivalents of [Ru(η⁶-p-cymene)Cl₂]₂ or reaction of [Ru(η⁶-p-cymene)Cl₂]₂ directly with 1.0 mol equivalent of L₄ forms the homobimetalllic [{RuCl(η⁶-p-cymene)}₂{κ²κ′²-L₄}][PF₆]₂. Reaction of [OsCl(η⁶-p-cymene)-{κ²-C(CH₂SeMe)₄}][PF₆] with [Ru(η⁶-p-cymene)Cl₂]₂ or [PtCl₂(MeCN)₂] affords the heterobimetallic [{OsCl(η⁶-p-cymene)}{RuCl(η⁶-p-cymene)}{κ²κ′²-C(CH₂SeMe)₄}][PF₆]₂ and [{OsCl(η⁶-p-cymene)}{PtCl₂}{κ²κ′²-C(CH₂SeMe)₄}][PF₆] respectively. The complexes have been characterised by multinuclear NMR and IR spectroscopy and X-ray crystallography.     

Electronic and resonance Raman spectra of iron(II)—tetraazamacrocyclic complexes containing N-heterocyclic ligands

The adducts of the meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,4,8,11-tetraene iron(II) complex with N-heterocyclic ligands exhibit three characteristic absorption bands in the ranges 690-640, 505-470 and 435-340 nm. Using excitation wavelengths coinciding with the low energy band, a selective enhancement of the Raman vibrational modes of the macrocyclic ligand is observed, supporting the assignment of a metal-to-diimine charge-transfer transition. The high energy band is strongly dependent on the nature and pK_a, of the axial ligands. Excitation at this band leads to the enhancement of the N-heterocyclic vibrational modes in the Raman spectra, indicating a metal-to-(N-heterocycle) charge-transfer transition. The intermediate band exhibits weak resonance Raman activity. Its intensity depends on the proximity of the high energy band and is consistent with the occurrence of an intensity stealing mechanism.     

Novel dithiocarbamate–Hg(II) complexes containing mixed ligands: Synthesis,spectroscopic characterization,and H2 storage capacity

A tetrahedral Hg(II) diethyl dithiocarbamate (Et₂Dt) complex containing triphenylphosphine (PPh₃) of the composition [HgCl(κ²-Et₂Dt)₂(PPh₃)] ( 1 ) is prepared. Furthermore, complex ( 1 ) is used as a synthone to prepare a novel series of complexes of the following composition [Hg(Et₂Dt)L(PPh₃)] {L = saccharinate ( 2 ), thiosaccharinate ( 3 ), benzisothiazolinate ( 4 ), benzothiozole-2-thiolate ( 5 ), and benzooxazole-2-thiolate ( 6 ) anions}. The resulted complexes ( 1 )–( 6 ) are characterized by elemental analysis, molar conductivity, powder X-ray diffraction, fourier transform Infrared, and NMR (¹H and ³¹P) spectroscopic techniques. The Et₂Dt ligand is coordinated as bidentate chelate through the sulfur atoms, whereas the L ligands are bonded as monodentate ligands to afford a tetrahedral geometry around the Hg(II) ion. Nitrogen adsorption–desorption isotherm for two of as-prepared complexes ( 2 ) and ( 3 ) are measured first to get their Brunauer–Emmett–Teller surface area. Moreover, the mentioned two complexes are evaluated for their ability to store hydrogen gas at 77 K. However, the results of the hydrogen storage tests proved that the selected complexes are all capable of storing hydrogen, but in varying degrees, where complex ( 2 ) exhibited a storage capacity of 4.22 wt% under 88 bar.     

Planar chiral (η5-cyclohexadienyl)- and (η6-arene)-tricarbonylmanganese complexes: synthetic routes and application

Planar chiral arenetricarbonylchromium complexes have been intensively investigated and they have been applied as valuable building blocks for asymmetric synthesis and as ligands for asymmetric catalysis. In contrast, in the field of the isoelectronic cationic [(η(6)-arene)Mn(CO)(3)](+) complexes, until these last 10 years, very few studies were published involving nonracemic planar chiral cationic complexes and their potential applications, certainly because of the difficult access to enantiopure starting material. In 2009, however, the discovery of the first resolution of such compounds opened a new area for their application in the field of organic as well as of organometallic enantioselective syntheses. We felt it important to write a review on this subject to give an up-to-date summary of the methodologies used to prepare enantiomerically pure planar chiral neutral [(η(5)-cyclohexadienyl)Mn(CO)(3)] and cationic [(η(6)-arene)Mn(CO)(3)](+) complexes as well as their potential in enantioselective synthesis.     

Structural investigations of copper(II) complexes containing unsymmetrical β-diketonate and monothio-β-diketonate ligands

The crystal structures of the complexes Cu(txhd)₂ and Cu(S-tmhd)₂ (where txhd is the anion of 2,2,6-trimethylheptane-3,5-dione and S-tmhd is the anion of 5-mercapto-2,2,6,6-tetramethyl-4-hepten-3-one) were determined. In the solid state, both complexes are square planar. In each case, only one geometrical isomer (trans or cis) was observed in the crystals; arguments are presented that both isomers are present in bulk samples of Cu(txhd)₂, while from electronic considerations, the monothio-β-diketonate ligands probably have cis geometry in Cu(S-tmhd)₂. Calculations of molecular volumes for structurally similar Cu[t-BuC(O)CHC(O)R]₂ complexes showed that there is a slight decrease in packing efficiency as the steric bulk of R increases. More importantly, strong ring-stacking interactions, such as those found for Cu(acac)₂ are not observed, or are greatly attenuated, in complexes with bulkier peripheral substituents. [Cu(txhd)(μ₃-OEt)]₄, an impurity that co-sublimed with Cu(txhd)₂, was isolated in low yield. The tetrameric structure, which is isomorphous with known [Cu(tmhd)(μ₃-OEt)]₄ (where tmhd is the anion of 2,2,6,6-tetramethylheptane-3,5-dione), has a cubane-like core.     

Synthesis of (η-arene)(η-cyclopentadienyl) iron (II) complexes with heteroatom and carbonyl substituents. Part I: Oxygen and carbonyl substituents

A series of reactions have been used to introduce oxygen substituents into (η-arene)(η-cyclopentadienyl) iron (II) complexes. Photochemical ligand exchange led to the formation of the first recorded trioxygenated complex as well as mono- and di-oxygenated species. Using microwave techniques, reaction times for S_NAr displacement reactions of halobenzene complexes by phenols were reduced from several hours to a few minutes. Phenols protected by either t-butylation or trimethylsilylation were found to give modest yields of the corresponding phenol complexes, using conventional thermal ligand exchange reactions. Without such protection, yields were extremely low. The above method led to the synthesis of the first example of a dihydroxybenzene complex. Some miscellaneous syntheses are also reported.The Nef reaction has been adapted to convert (η⁶-α-nitroalkylarene)(η⁵-Cp) iron (II) salts to corresponding aldehyde and ketone complexes. The α-nitroalkyl arene complexes were synthesised in good yields from (η⁶-halobenzene)(η⁵-Cp) iron (II) complexes using NaOtBu in DMSO. H/D exchange reactions with ²[H]₆-DMSO in the presence of K₂CO₃ showed partial D incorporation in the methyl group for the unreacted α-nitroethylbenzene complex and complete exchange for the carbanion generated by deprotonation. Conversion of the α-nitroalkylarene complexes to the corresponding aldehyde and ketone complexes was accomplished in moderate yields using three methods:

(A)

H₂O₂ and NaOtBu in DMSO followed by reaction with CF₃CO₂H.

(B)

SnCl₂/aq. HCl.

(C)

K₂CO₃ in DMF using microwave-mediated reactions.

In addition, two one-pot syntheses are reported using methods B and C.     

Reactivity studies of (η-arene)ruthenium dimeric complexes towards pyrazoles: isolation of amidines, bis pyrazoles and chloro bridged pyrazole complexes

The complex [(η⁶-p-cymene)Ru(μ-Cl)Cl]₂1 reacts with pyrazole ligands (3a-g) in acetonitrile to afford the amidine derivatives of the type [(η⁶-p-cymene)Ru(L)(3,5-HRR′pz)](BF4)₂ (4a-f), where L = {HNC(Me)3,5-RR′pz}; R, R′ = H (4a); H, CH₃ (4b); C₆H₅ (4c); CH₃, C₆H₅ (4d) OCH₃ (4e); and OC₂H₅ (4f), respectively. The ligand L is generated in situ through the condensation of 3,5-HRR′pz with acetonitrile under the influence of [(η⁶-p-cymene)RuCl₂]₂. The complex [(η⁶-C₆Me₆)Ru(μ-Cl)Cl]₂2 reacts with pyrazole ligands in acetonitrile to yield bis-pyrazole derivatives such as [(η⁶-C₆Me₆)Ru (3,5-HRR′pz)₂Cl](BF₄) (5a-b), where R, R′ = H (5a); H, CH₃ (5b), as well as dimeric complexes of pyrazole substituted chloro bridged derivatives [{(η⁶-C₆Me₆)Ru(μ-Cl) (3,5-HRR′pz)}₂](BF₄)₂ (5c-g), where R, R′ = CH₃ (5c); C₆H₅ (5d); CH₃, C₆H₅ (5e); OCH₃ (5f); and OC₂H₅ (5g), respectively. These complexes were characterized by FT-IR and FT-NMR spectroscopy as well as analytical data. The molecular structures¹ of representative complexes [(η⁶-C₆Me₆)Ru{3(5)-Hmpz}₂Cl]⁺5b, [(η⁶-C₆Me₆)Ru(μ-Cl)(3,5-Hdmpz)]₂²⁺5c and [(η⁶-C₆Me₆)Ru(μ-Cl){3(5)Me,5(3)Ph-Hpz}]₂²⁺5e were established by single crystal X-ray diffraction studies.     

Bis(triethanolamine)cadmium(II) and -­mercury(II) saccharinates: seven-coordinate complexes containing both tri- and tetradentate triethanolamine ligands

The structures of the title triethanol­amine (tea) complexes of Cd^II and Hg^II saccharinates, bis­(triethanol­amine)-κ³O,N,O′;κ⁴O,N,O′,O′′-cadmium(II) 1,2-benziso­thia­zol-3(2H)-onate 1,1-dioxide, [Cd(C₆H₁₅NO₃)₂](C₇H₄NO₃S)₂, (I), and bis­(tri­ethanol­amine)-κ³O,N,O′;κ⁴O,N,O′,O′′-mercury(II) 1,2-benz­iso­thia­zol-3(2H)-onate 1,1-dioxide, [Hg(C₆H₁₅NO₃)₂](C₇H₄NO₃S)₂, (II), or [M(tea)₂](sac)₂, where M is Cd^II or Hg^II and sac is the saccharinate anion, reveal seven-coordinate metal ions in both complexes. Both complex cations, [M(tea)₂]²⁺, adopt a monocapped trigonal prism geometry in which the two tea ligands exhibit different coordination modes to achieve seven-coordination. One tea ligand acts as a tetradentate ligand using all its donor atoms, while the other behaves as a tridentate O,N,O′-donor ligand, with one of its ethanol groups remaining uncoordinated. The H atoms of the free and coordinated hydroxyl groups of the tea ligands are involved in hydrogen bonding with the amine N atom, and with the carbonyl and sulfonyl O atoms of neighbouring sac ions, forming an infinite three-dimensional network. A weak π–π interaction between the phenyl rings of the sac ions also occurs.     

Cytotoxic dimeric half-sandwich Ru(II), Os(II) and Ir(III) complexes containing the 4,4′-biphenyl-based bridging ligands

A series of dinuclear half-sandwich Ru(II), Os(II) and Ir(III) complexes [Ru₂(μ-Lⁿ)(η⁶-pcym)₂Cl₂](PF₆)₂ ( 1 , 4 ), [Os₂(μ-Lⁿ)(η⁶-pcym)₂Cl₂](PF₆)₂ ( 2 , 5 ) and [Ir₂(μ-Lⁿ)(η⁵-Cp*)₂Cl₂](PF₆)₂ ( 3 , 6 ), based on 4,4′-biphenyl-based bridging Schiff base ligands N,N′-(biphenyl-4,4′-diyldimethylidyne)bis-2-(pyridin-2-yl)methanamine (L¹; for 1 – 3 ) and N,N′-(biphenyl-4,4′-diyldimethylidyne)bis-2-(pyridin-2-yl)ethanamine (L²; for 4 – 6 ) is reported; pcym = 1-methyl-4-(propan-2-yl)benzene, Cp* = pentamethylcyclopentadienyl. The complexes were characterized by relevant analytical techniques (i.e. elemental analysis, FT-IR, NMR, ESI-MS), and their in vitro cytotoxicity was assessed at six cancerous and two non-cancerous (healthy) human cell lines. Overall, complexes 4 – 6 , containing the L² bridging ligand, revealed higher cytotoxicity as compared with 1 – 3 and, thus, they were studied in greater detail. The best-performing complex 6 exceeded at least twice the in vitro cytotoxicity of cisplatin and showed high selectivity towards the cancer cells over the normal ones, including the primary culture of human hepatocytes. In contrast to cisplatin, complexes 4 – 6 did not induce the cell cycle modification of the treated A2780 human ovarian carcinoma cells (studied by flow cytometry and Western blot analysis). High levels of superoxide anion were induced by complexes 4 – 6 at the A2780 cells. The levels of activated forms of Caspase-3 and Caspase-8 at the A2780 cells treated by Ru(II) complex 4 were comparable with cisplatin, while complexes 5 and 6 had only a minor effect on activation of these caspases.     

η1-Alkynylplatinum(II) complexes with cycloocta-1,5-diene and tri(1-cyclohepta-2,4,6-trienyl)phosphane ligands

η¹-Alkynylplatinum(II) complexes of the type (cod)Pt(CCR)₂ (1, cod=η⁴-cycloocta-1,5-diene; R=Me (a), ^tBu (b), Ph (c), Fc (d), SiMe₃ (e)) were prepared in good yields from the reaction of (cod)PtCl₂ with either HCCR and NaOEt (R=^tBu, Ph, Fc) or di(1-alkynyl)dimethyltin, Me₂Sn(CCR)₂ (R=Me, SiMe₃). The analogous reaction of [P]PtCl₂ ([P]=tri(1-cyclohepta-2,4,6-trienyl)phosphane, {P(C₇H₇)₂(η²-C₇H₇)}) with Me₂Sn(CCR)₂ (R=Me, ^tBu, Ph, Fc, SiMe₃), afforded selectively the complexes [P]PtCl(CCR) 2a–e in high yield, in which the 1-alkynyl group is in cis position with respect to the phosphorus atom, and one of the C₇H₇ rings is η²-coordinated to platinum through the central CC bond. Complexes 3a–e of the type [P]Pt(CCR)₂ could not be prepared by the reaction of 2 with an excess of the 1-alkynyltin reagents. However, the reaction of 1 with the phosphane P(C₇H₇)₃ gave compounds 3a–e in quantitative yield by substitution of the cod ligand. The molecular structures of 2b and 3d were determined by X-ray structure analysis, and complexes 1–3 were characterised in solution by multinuclear magnetic resonance spectroscopy (¹H-, ¹³C-, ²⁹Si-, ³¹P-, ¹⁹⁵Pt-NMR). The structures of 2 and 3 in solution were found to be fluxional with respect to coordination of the C₇H₇ rings to platinum.     

Synthesis,electrochemical properties and electro-oxidative polymerization of copper(II) and nickel(II) complexes with N′-benzoylthiourea ligands containing pyrrole groups

The synthesis of four N-benzoylthioureas containing pyrrole groups are described. The electrochemical behaviour of their copper(II) and nickel(II) complexes has been investigated in aprotic solvents by coulometry and by cyclic voltammetry which indicates that the electrochemical oxidation of copper complexes leads to the formation of Cu^II-benzylureate complexes. The oxidative polymerization of nickel complexes on platinum and a glassy carbon electrode, has been carried out in MeCN.The redox properties of the polymeric films formed have been examined by cyclic voltammetry. The films are catalytically active in the electroreduction of oxygen.     

Diastereoisomeric (η6-arene)ruthenium(II) chiral Schiff base complexes: crystal structure of a triphenylphosphine adduct

Reaction of [(η⁶-p-cymene)RuCl(L*)] with AgClO₄ in Me₂CO gives a perchlorate complex which on subsequent treatment with PPh₃, γ-picoline or Cl⁻ yields adducts showing that there can be retention as well as inversion of configuration at the metal centre. The (R)_Ru,(S_C absolute configurations of the chiral centres in the triphenylphosphine adduct have been established by an X-ray diffraction study [HL*, (S-α-methylbenzylsalicylaldimine]. The CD spectral study reveals that there is an inversion of configuration during formation of the PPh₃ adduct.     

Intramolecular dehydrofluorinative coupling of η-arene and fluoroarylphosphine ligands in ruthenium complexes

NMR studies of reactions between a series of arene ruthenium(II) fluoroarylphosphine complexes and Proton Sponge have revealed the necessary conditions for intramolecular dehydrofluorinative ligand coupling. The complex must be cationic, and the phosphine need have only one fluoroaryl substituent. The reaction is rapid and clean for [(η⁶-toluene)RuCl(dfppe)]BF₄, [(η⁶-mesitylene)RuCl{(C₆F₅)₂PC₆H₄SMe}]BF₄ and the diastereomer of [(η⁶-toluene)RuCl{Ph₂PC₂H₄PPh(C₅F₄N-4)}]BF₄ in which the tetrafluoropyridyl substituent is close to the η⁶-arene. [(η⁶-p-cymene)RuCl(dfppe)]BF₄ reacts in the presence of Proton Sponge to give a mixture of unidentified compounds. The neutral complex [(η⁶-toluene)RuCl₂{Ph₂P(C₆F₅)}] and the diastereomer of [(η⁶-toluene)RuCl{Ph₂PC₂H₄PPh(C₅F₄N-4)}]BF₄ in which the tetrafluoropyridyl substituent is distant to the η⁶-arene do not undergo reaction.