Η6-arene Ligand Research Articles

Synthesis of Cationic Sandwich-type Ruthenium Complexes with η6-Arene Ligands Bearing Long Alkyl Chains and Their Behavior in Water

Abstract Novel sandwich-type ruthenium complexes [Ru(η6-arene)(p-cymene)](BF4)2, in which the η6-arene ligands have long alkyl chains, were prepared. These complexes were dispersed in water to form corresponding micelles. Alkoxy moieties on the η6-arene ligands were reacted with OH− to afford the corresponding phenolic species. Thus, [Ru(p-undecyloxyanisole)(p-cymene)]2+ was converted to [Ru(η6-p-methoxyphenolate)(p-cymene)]+ through the unexpected demethylation by treatment with Na2CO3 with the micelle remaining. The micelle collapsed when similar ruthenium complex bearing dodecyl 2-(4-methoxyphenoxy)acetate as an arene ligand was treated with Na2CO3 aq.

Arene-ruthenium(II) complexes with tetracyclic oxime derivatives: synthesis, structure and antiproliferative activity against human breast cancer cells

Six new arene-ruthenium(II) complexes containing two different η6-arene ligands – benzene and hexamethylbenzene, with indenoquinoxalinone oxime analogues (11H-indeno[1,2-b]quinoxalin-11-one oxime, 6H-indeno[1,2-b]pyrido[3,2-e]pyrazin-6-one oxime and 6–(hydroxyimino)indolo[2,1-b]quinazolin-12(6H)-one oxime (tryptanthrin-6-oxime)) as N,N’- chelating ligands are reported. The complexes were characterized by elemental analysis, IR, UV–VIS, 1H and 13C NMR spectroscopy and by single-crystal X-ray structure analysis. Complexes adopt a half-sandwich «piano-stool» geometry with oxime ligands in anionic form, the ruthenium(II) center coordinates one arene, one N,N-bidentate oximate and one chloride ligand. The computational analysis of non-covalent intermolecular interactions revealed weak attraction between the oxime oxygen atoms and the nearest hydrogen atoms of the oxime and the arene ligands. The cytotoxic activity of the complexes was evaluated against human breast cancer cell line MCF-7 and cisplatin-resistant human breast cancer cell line MCF-7CR as well as non-cancerous human breast epithelial cell line MCF10A. The cytotoxicity tests show micromolar IC50 values and tryptanthrin-6-oxime hexamethylbenzene-ruthenium(II) complex was found to exhibit the best antiproliferative activity among the studied compounds against the MCF-7 and MCF-7CR cell lines (IC50 9.0 ± 4.4 and 8.9 ± 1.5 µM, respectively).

The rate of substitution from η6-arene ruthenium(II) complexes

The rate and mechanism of substitution in the Ru(II) complexes (C1–C6) by thiourea nucleophiles was studied at pH 2 and rate constants measured as a function of nucleophile concentrations and temperature using spectrometric methods. There is increased electron density at the Ru metal atom of C2 as a result of inductive donation by substituents on the arene ligand, making it less positive and therefore less reactive than C1. For the complexes C3–C6 bearing the 2,2′-bipyridyl ligand, the aqua ligands are located trans to the arene ligands, and hence, their reactivity increases in accordance to the number and type of alkyl substituents on the η6-arene ligands which donate inductively into the π-molecular orbitals, causing increased trans labialisation of the coordinated aquo co-ligand. Compared to the reactivity of triaquo complex (C1), the auxiliary bipyridyl ligand of (C3) complex lowers the rate of substitution for the later complex by a factor of about 100, possibly due to its steric hindrance at the Ru(II) metal centre. The significantly negative activation entropies and positive activation enthalpies suggest an associative mode of substitution. The reactivity of the nucleophiles follow the order DMTU > TU > TMTU.

Synthesis, characterisation and cytotoxicity studies of ruthenium arene complexes bearing trichlorogermyl ligands

The synthesis of five half sandwich ruthenium(II) trichlorogermyl complexes of the type [(η6-Arene)Ru(PR3)Cl(GeCl3)] (PR3 = Phosphane and phosphite ligands; Arene = p-cymene or C6H5-OC2H4OH) is reported: [(η6-p-cymene)Ru(P(OMe)3)Cl(GeCl3)] (1), [(η6-p-cymene)Ru(P(OPh)3)Cl(GeCl3)] (2), [(η6-p-cymene)Ru(PPh3)Cl(GeCl3)] (3), [(η6-p-cymene)Ru(pta)Cl(GeCl3)] (pta = 1,3,5-triaza-7-phosphaadamantane) (4) and [(η6-C6H5-OC2H4OH)Ru(pta)Cl(GeCl3)] (5). The nature of the η6-arene and phosphane ligand was varied and the complexes have been prepared by facile insertion of GeCl2 (from GeCl2.(dioxane)) into the Ru-Cl bonds of the respective easily accessible precursor complexes [(η6-Arene)Ru(PR3)Cl2]. The complexes were fully spectroscopically characterized by 1H, 13C{1H} and 31P{1H} NMR spectroscopy, UV-Vis, ATR-IR, HRMS (ESI) and their thermal behavior elucidated by TGA. Their cytotoxicity to human ovarian carcinoma (A2780) and non-tumorigenic human embryonic kidney HEK293 cell lines is also reported, and represents the first cytotoxic investigations of Ru(II) germyl complexes to date. The first DFT studies (B3LYP; basis set 6-31+G(d,p) for H, C, O, P, Cl, N, and Ge atoms and DGDZVP for Ru atom) on trichlorogermyl ruthenium complexes were carried out on complex 2 and 5 in order to gain insights into the bonding situation between Ru and Ge and are reported.

Synthesis of Ru(II) complexes containing N-heterocyclic carbenes functionalized with secondary donor groups: Catalytic activity toward enantioselective transfer hydrogenation

A series of new N-heterocyclic carbene (NHC)–Ru(II) complexes functionalized with additional hemilabile donor groups, such as a hydroxy-amide group, were successfully synthesized and structurally characterized. Preliminary studies indicated that the complexes acted as precatalysts with moderate activity in the asymmetric transfer hydrogenation at room temperature without temperature control. The investigation of the effect of varying the η6-arene ligands showed that these ligands appeared to significantly alter the activity of the complexes as precatalysts. Furthermore, it was revealed that the hydroxy functional group on the NHC ligand side-arm not only induced the stereoselectivity of the reaction but also enhanced the reaction rate.

A new binding geometry for an ortho-xylylene-linked bis(NHC)cyclophane: a ruthenium(II) complex with a chelating (η1-NHC)2:η6-arene ligand

Using two different reaction procedures, a Ru(II) complex has been isolated that contains an ortho-xylylene-linked bis(NHC)cyclophane (NHC = N-heterocyclic carbene) that binds to the Ru centre through two carbene carbons and one of the arene rings in an eta(6)-mode.

Configurational Flexibility of Epimeric β-Aminothioether-chelated Ruthenium(II) η6-Arene Complex Salts

Five chiral β -aminothioethers were obtained via different routes orientated on literature protocols. Three of these β -aminothioethers were reacted with two di-μ-chloro-bis{chloro[η6-arene]- ruthenium(II)} derivatives, resulting in the title complex salts. The complex cations exhibit three stereocenters, viz. ruthenium and sulfur atoms and the chiral benzylic carbon atom of the chelate ligand backbone. Both, ruthenium and sulfur stereocenters epimerize into a mixture of four NMR distinguishable diastereomers in equilibrium, but the designed chiral benzylic carbon atom is stable under all conditions applied so far. The relative diastereomer concentrations in solution depend mainly on the spatial requirements of the η6-arene ligand rather than on the thioether moiety. Diastereomer ratios and the absolute configurations in solution were studied by NMR and CD spectroscopy. The spectroscopic results fit to the absolute X-ray crystal structure parameters determined for the diastereomers present in the crystalline state.

Dichloro(η6-p-cymene)(triphenylphosphine)ruthenium(II)

The title compound, [Ru(C10H14)Cl2(C18H15P)], crystallizes with two mol­ecules in the asymmetric unit. It adopts the classic pseudo-tetra­hedral piano-stool structure. A comparison of the Ru—P, Ru—Cl, Ru—C(av) bond lengths and the sum of the P—Ru—Cl1, P—Ru—Cl2 and Cl1—Ru—Cl2 angles (for both independent mol­ecules) with those of previously determined compounds [(η6-arene)Ru(PPh3)Cl2] reveals that the nature of the η6-arene ligand has a marginal effect on these structural parameters.

A Stable Neutral Stannaaromatic Compound: Synthesis, Structure and Complexation of a Kinetically Stabilized 2-Stannanaphthalene

A kinetically stabilized 2-stannanaphthalene, the first example of a stable, neutral aromatic compound containing a tin atom, has been synthesized and fully characterized. The results of spectroscopic and crystallographic structural analyses of the compound and theoretical calculations using model compounds strongly suggest that it has a delocalized 10pi-electron ring system as does naphthalene. In addition, it was found to behave as an eta6-arene ligand in the ligand exchange reaction with [Cr(CH3CN)3(CO)3] giving the first stable eta6-2-stannanaphthalene chromium complex.

Ruthenium Catalysts for the Kharasch Reaction

Recent advances in the development of ruthenium catalysts for the atom transfer radical addition of polyhalogenated compounds to olefins (Kharasch reaction) are described. Three classes of homogeneous catalysts are discussed: halfsandwich complexes with η6-arene, η5-cyclopentadienyl or η5-carborane ligands, ruthenium complexes with alkylidene- or vinylidene ligands and polynuclear complexes. Furthermore, first attempts to use immobilized ruthenium complexes as heterogeneous catalysts for the Kharasch reaction are summarized. Keywords: metathesis, N-heterocyclic carbenes (NHC), halfsandwich complexes, polyhalogenated compounds, olefins, anti-Markovnikov adduct

Chemistry of Ruthenium(II) Alkyl Binap Complexes: Novel Bonding, Cyclometalation, and P−C Bond Splitting

Reactions of the bis-isopropyl and bis-cyclohexyl alkyl Binap ligands, 8 and 9, respectively, with [RuCl2(η6-p-cymene)]2 afford new dinuclear chloro-bridged Ru compounds which contain the Binap ligands as six- rather than four-electron donors. A backbone double bond proximate to one of the P-donors complexes the metal atom. NMR details of the olefin bonding plus isomerization reactions involving loss of the olefin complexation are reported. Reactions of 8 or 9 with [Ru(OAc)2(η6-p-cymene)] result in slow P−C bond cleavage and cyclometalation, instead of affording the anticipated [Ru(OAc)2(Binap)] complex. The new cyclometalated complexes, 15 and 16, contain the complexed R2P−O(CO)CH3 ligand and arise (presumably) via acetate attack at phosphorus with the electrons in the P−C bond moving to the ruthenium atom. The solid-state structure of one of these, the cyclohexyl analogue, 16, is reported and represents a rare structural example of a molecule with three different chelate ligands. The complexed R2P−O(CO)CH3 ligand is readily hydrolyzed in wet triflic acid to afford the R2P(OH) donor and an η6-arene ligand (via Ru−C protonation).

2,4,6-Tri-tert-butyl-1,3,5-triphosphabenzene as an η6-Ligand in Transition Metal Complexes

The η6-1,3,5-triphosphabenzene transition metal complexes 12a,b, 14, and 16 have been synthesized for the first time by the displacement of η6-arene ligands (toluene, benzene and naphthalene) with 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene (9). The new complexes have been fully characterized by a combination of elemental analysis, mass spectrometry, and 1H-, 13C- and 31 P-NMR spectroscopy.

2,4,6-Tri-tert-butyl-1,3,5-triphosphabenzene as an η6-Ligand in Transition Metal Complexes

The η6-1,3,5-triphosphabenzene transition metal complexes 12a,b, 14, and 16 have been synthesized for the first time by the displacement of η6-arene ligands (toluene, benzene and naphthalene) with 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene (9). The new complexes have been fully characterized by a combination of elemental analysis, mass spectrometry, and 1H-, 13C- and 31 P-NMR spectroscopy.

A monomeric ruthenium thiolato complex with a chelated η6-arene ligand. The preparation and structure of [Ru{2-(η6-Ph)-6-PhC6H3S}(2,6-Ph2C6H3S)(PPh3)

Reaction of [RuCl2(PPh3)3] with the 2,6-diphenylthiophenolate anion (DPT) gives the complex [Ru(η6- DPT)(DPT)(PPh3)], with one thiol phenyl substituent η6-bonded to the ruthenium. The X-ray crystal structure reveals a distorted piano stool type of geometry. Crystal data: space group P21/c, a=14.204(4), b=9.503(5), c=31.680(9) Å, β=97.40(3)°, Z=4. The η6-arene ligand is labile and can readily be replaced by CO and 1,2- bis(diphenylphosphino)ethane.

Co-ordination chemistry of 2,6-diphenylthiophenol. The syntheses and X-ray crystal structures of some molybdenum and rhodium complexes with σ- or η6-phenyl–metal interactions

Reaction of [MoCl2(CO)4] with the 2,6-diphenylthiophenolate anion (dpt) gives the complex [Mo(η6-dpt)(dpt)(CO)], with one thiol phenyl substituent η6-bonded to the molybdenum. Crystal data: space group P21/c, with a= 13.939(3), b= 10.548(3), c= 20.863(5)Å, β= 106.88(1)° and Z= 4. The η6arene ligand is labile and is reversibly replaced by CO. Replacement of this ligand also occurs with 2,2′-bipyridyl(bipy),1,10-phenanthroline (phen), and 1,2-bis(diphenylphosphino)ethane (dppe) to give the dicarbonyl complexes [Mo(dpt)2(CO)2L](L = bipy or phen) or the monocarbonyl complex [Mo(dpt)2(CO)(dppe)]. The X-ray crystal structure of the bipy derivative and the mechanism of its formation is discussed. Crystal data: space group P21/c with a= 10.567(3), b= 22.843(5), c= 18.341(3)Å, β= 105.2(2)° and Z= 4, Reaction of RhCl3 with dpt gave the dinuclear species [Rh2{µ-SC6H3(C6H4)-2-Ph-6}2(dpt)2(NCMe)2]. An X-ray crystal structure reveals that each bridging thiolate ligand is also σ-bonded to rhodium [Rh–C 2.016(8)Å]via one of the phenyl carbons of the thiolate substituents. Crystal data: space group P21/n, a= 11.991(2), b= 20.426(4), c= 15.910(4)Å, β= 90.42(1) and Z= 2.