Structural and electronic trends in ortho-metalated dirhodium(II) complexes

Properties of chiral dirhodium catalysts with ortho-metalated aryl phosphine ligands have been studied by a computational quantum chemical density functional theory method. The main aim in the current work was to systematically modify the ligand core of the Rh₂(O₂C R)₂(PC)₂ catalysts (PC is ortho-metalated aryl phosphine) in order to find structural and electronic trends involved with the modifications. The strongest impact on the properties of the active rhodium site was found when electron-withdrawing groups were introduced in the ligand core. The computational approach offers a possibility for a stepwise study of the properties of the catalysts and therefore a tool for further design of the most effective structures.     

Axial coordination of NHC ligands on dirhodium(II) complexes: generation of a new family of catalysts

An efficient new methodology for the arylation of aldehydes is disclosed which uses dirhodium(II) catalysts and N-heterocyclic carbene (NHC) ligands. Complexes of Rh 2(OAc) 4 with one and two NHCs attached on the axial positions were successfully isolated, fully characterized, and used as catalysts in the reaction. The saturated monocomplex ((NHC 5)Rh 2(OAc) 4) 31 was shown to be the most active catalyst and was particularly efficient in the arylation of alkyl aldehydes. DFT calculations support participation of complexes with one axial NHC in the reaction as the catalysts active species and indicate that hydrogen bonds involving dirhodium unit, reactants, and solvent (alcohol) play an important role on the reaction mechanism.     

Substituent and isomer effects on structural, spectroscopic, and electrochemical properties of dirhodium(III,II) complexes containing four identical unsymmetrical bridging ligands

Substituent and isomer effects on the structural, spectroscopic, (UV-visible and ESR) and electrochemical properties of dirhodium(III,II) complexes containing four identical unsymmetrical bridging ligands are reported for seven related compounds of the type Rh(2)(L)(4)Cl where L = 2-(2-fluoroanilino)pyridinate (2-Fap), 2-(2,6-difluoroanilino)pyridinate (2,6-F(2)ap), 2-(2,4,6-trifluoroanilino)pyridinate (2,4,6-F(3)ap), or 2-(2,3,4,5,6-pentafluoroanilino)pyridinate (F(5)ap) anion. Rh(2)(2-Fap)(4)Cl exists only in a (4,0) isomeric conformation while Rh(2)(2,6-F(2)ap)(4)Cl, Rh(2)(2,4,6-F(3)ap)(4)Cl, and Rh(2)(F(5)ap)(4)Cl exist as both (4,0) and (3,1) isomers. It had earlier been demonstrated that Rh(2)(L)(4)Cl complexes can adopt different geometric conformations of the bridging ligands, but the current study provides the first example where two geometric isomers of Rh(2)(5+) complexes are obtained for one compound using the same synthetic procedure. The synthesis, structural, spectroscopic, and/or electrochemical properties of (3,1) Rh(2)(2,6-F(2)ap)(4)CN and (4,0) Rh(2)(2,4,6-F(3)ap)(4)(C triple bond C)(2)Si(CH(3))(3) are also reported and the data on these compounds is discussed in light of their parent complexes, (3,1) Rh(2)(2,6-F(2)ap)(4)Cl and (4,0) Rh(2)(2,4,6-F(3)ap)(4)Cl.     

High enantiocontrol in the intramolecular cyclopropanation of diazo ketones catalyzed by dirhodium(II) complexes with ortho-metalated aryl phosphine ligands

[reaction: see text]. Chiral dirhodium(II) complexes, Rh2(O2CCF3)2(PC)2, [PCH = (p-CH3C6H4)3P, (m-CH3C6H4)3P], provide an excellent yield and a high enantiocontrol in the cyclopropanation of alpha-diazo ketones with gamma and delta double bonds. The ee values are significantly dependent on the solvent used; the best results are obtained using pentane.     

Synthesis,structure and in vitro anticancer activity of ruthenium(II) and platinum(II) complexes with chiral aminophosphine ligands

Transition Metal Chemistry - (R)-[Ru(η6-p-MeC6H4iPr)Cl2{Ph2PNHCH(CH3)(C6H4-4-F)}] (1) and cis-(R,R)-[PtCl2{Ph2PNHCH(CH3)(C6H4-4-F)}2] (2) have been obtained by the reaction of the chiral...     

Syntheses, structures, and properties of tetrakis(mu-acetato)dirhodium(II) complexes with axial pyridine nitrogen donor ligands with or without assistance of hydrogen bonds

Eight adducts of Rh2(O2CCH3)4 with axial pyridine derivatives that contain hydrogen-bonding amino and/or steric methyl substituents in the 2- and 6-positions have been prepared and examined by electronic absorption and 1H NMR spectroscopy in solution and by elemental, IR, thermogravimetric, and X-ray diffraction analyses in the solid state. The results indicated that strong hydrogen bonding interactions between Rh2(O2CCH3)4 and axially coordinated pyridine derivatives with a 2- or 6-amino group occur in both solution and the solid state and contribute to the higher thermal stability of the molecular assembly of dirhodium complexes. It was demonstrated that such a combination of coordinate and hydrogen bonds is useful as a building tool in designing and constructing new organic-inorganic hybridized compounds and supramolecular architectures.     

DNA binding and photocleavage in vitro by new dirhodium(II) dppz complexes: correlation to cytotoxicity and photocytotoxicity

Two new dirhodium(II) complexes possessing the intercalating dppz ligand (dppz = dipyrido[3,2-a:2',3'-c]phenazine), cis-[Rh(2)(mu-O(2)CCH(3))(2)(dppz)(eta(1)-O(2)CCH(3))(CH(3)OH)](+) (1) and cis-[Rh(2)(mu-O(2)CCH(3))(2)(dppz)(2)](2+) (2), were synthesized and characterized as potential agents for photochemotherapy. Various techniques show that 1 binds to DNA through intercalation, although some aggregation of the complex on the DNA surface is also present. In contrast, 2 does not intercalate between the DNA bases; however, strong hypochromic behavior is observed in the presence of DNA, which can be attributed to intermolecular pi-stacking of 2 enhanced by the polyanion. The apparent DNA binding constants determined using optical titrations are compared to those from dialysis experiments. Both complexes photocleave pUC18 plasmid in vitro under irradiation with visible light (lambda(irr) >or= 395 nm, 15 min), resulting in the nicked, circular form. Greater photocleavage is observed for 1 relative to 2, which may be due to the ability of 1 to intercalate between the DNA bases. The cytotoxicity toward human skin cells (Hs-27) measured as the concentration at which 50% cell death is recorded, LC(50), was found to be 135 +/- 8 microM for 2 in the dark (30 min), which is significantly lower than those of 1 (LC(50) = 27 +/- 2 microM) and Rh(2)(O(2)CCH(3))(4) (LC(50) = 15 +/- 2 microM). Irradiation of cell cultures containing 1 and Rh(2)(O(2)CCH(3))(4) with visible light (400-700 nm, 30 min) has little effect on their cytotoxicity, with LC(50) values of 21 +/- 3 and 13 +/- 2 microM, respectively. Interestingly, a 3.4-fold increase in the toxicity of 2 is observed when the cell cultures are irradiated (400-700 nm, 30 min), resulting in LC(50) = 39 +/- 1 microM. The greater toxicity of 1 compared to 2 in the dark may be related to the ability of the former compound to intercalate between the DNA bases. The lower cytotoxicity of 2, together with its significantly greater photocytotoxicity, makes this complex a potential agent for photodynamic therapy (PDT). These results suggest that intercalation or strong DNA binding may not be a desirable property of a potential PDT agent.     

DNA-binding,antioxidant activity and in vitro cytotoxicity induced by ruthenium(II) complexes containing polypyridyl ligands

Two new ruthenium(II) polypyridyl complexes, [Ru(dmp)₂(maip)](ClO₄)₂ 1 (maip = 2-(3-aminophenyl)imizado[4,5-f][1,10]phenanthroline and [Ru(dmp)₂(paip)](ClO₄)₂ 2 (paip = 2-(4-aminophenyl)imidazo[4,5-f][1,10]phenanthroline, dmp = 2, 9-dimethyl-1,10-phenanthroline) have been synthesized and characterized. The DNA-binding behaviors of complexes 1 and 2 were studied by viscosity measurements, thermal denaturation, and absorption titration. The results show that the two complexes intercalate between the base pairs of DNA. The DNA-binding constants K _b for complexes 1 and 2 were determined to be 3.23 ± 0.16 × 10⁴ M⁻¹ (s = 0.97) and 4.34 ± 0.65 × 10⁴ M⁻¹ (s = 1.13). The cytotoxicity of these complexes has been evaluated by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. The IC₅₀ values are 35.70, 41.04, 55.25 and 37.50 for complex 1 and 37.02, 103.08, 130.07 and 37.80 for complex 2 against BEL-7402, C-6, HepG-2 and MCF-7 cell lines, respectively. The antioxidant activity against hydroxyl radical (OH•) was also investigated.     

Organoplatinum(II) complexes with phosphite ligands

The phosphite complexes cis-[PtMe₂L(SMe₂)] in which L = P(OⁱPr)₃, 1a, or L = P(OPh)₃, 1b, were synthesized by the reaction of cis,cis-[Me₂Pt(μ-SMe₂)₂PtMe₂] with 2 equiv. of L. If 4 equiv. of L was used the bis-phosphite complexes cis-[PtMe₂L₂] in which L = P(OⁱPr)₃, 2a, or L = P(OPh)₃, 2b, were obtained. The reaction of cis-[Pt(p-MeC₆H₄)₂(SMe₂)₂] with 2 equiv. of L gave the aryl bis-phosphite complexes cis-[Pt(p-MeC₆H₄)₂L₂] in which L = P(OⁱPr)₃, 2a′, or L = P(OPh)₃, 2b′. Use of 1 equiv. of L in the latter reaction gave the bis-phosphite complex along with the starting complex in a 1:1 ratio.The complexes failed to react with MeI. The reaction of cis,cis-[Me₂Pt(μ-SMe₂)₂PtMe₂] with 2 equiv. of the phosphine PPh₃ gave cis-[PtMe₂(PPh₃)₂] and cis-[PtMe₂(PPh₃)(SMe₂)] along with unreacted starting material. Reaction of cis-[PtMe₂L(SMe₂)], 1a and 1b with the bidentate phosphine ligand bis(diphenylphosphino)methane, dppm = Ph₂PCH₂PPh₂, gave [PtMe₂(dppm)], 8, along with cis-[PtMe₂L₂], 2. The reaction of cis-[PtMe₂L(SMe₂)] with 1/2 equiv. of the bidentate N-donor ligand NN = 4,4′-bipyridine yielded the binuclear complexes [PtMe₂L(μ-NN)PtMe₂L] in which L = P(OⁱPr)₃, 3a, or L = P(OPh)₃, 3b.The complexes were fully characterized using multinuclear NMR (¹H, ¹³C, ³¹P, and ¹⁹⁵Pt) spectroscopy.     

Traceable platinum(II) complexes with alkylene diamine-derived ligands: synthesis,characterization and in vitro studies

Diiodido- (6a/6b) and dichloridoplatinum(II) complexes (7a/7b) with fluorescent ligands 2-[(2-aminoethyl)amino]ethyl-2-(methylamino)benzoate (5a) and 2-amino-1-(aminoethyl)ethyl-2-(methylamino)benzoate (5b) were prepared and characterized by elemental analysis, ESI-MS analysis, fluorescence spectrometry, as well as ¹H, ¹³C, and ¹⁹⁵Pt NMR spectroscopy. All compounds have been tested against A2780 ovarian cancer, A549 lung carcinoma, and HT-29 colon cancer cell lines using sulforhodamine-B assay. The activity increased from ligand precursors, diiodido- to dichloridoplatinum(II) complexes, except against HT-29 cell line where diiodido and dichlorido expressed similar activity. These compounds enter the tumor cells and emit a bright fluorescence at ca. 470 nm, mainly targeting nuclei.     

Synthesis,crystal structure,and in vitro antitumor activities of copper(II) complexes containing tetradentate pyridine-based ligands

Several new Cu(II) complexes of Schiff bases obtained by condensation of 2-[N-(α-picolyl)-amino]-benzophenone with different chiral amino acids were synthesized and characterized by physico-chemical and spectroscopic methods. The crystal structure of one of the complexes was determined using single crystal X-ray diffraction. The ligands were coordinated to the metal atom in a tetradentate manner with ONNN donor sets using the carboxyl oxygen, azomethine nitrogen, CON⁻, and pyridine nitrogen. The cytotoxicities of the complexes were evaluated against human cancer cells. The substituents on the aromatic rings strongly influenced the cytotoxicities of the complexes. The complex with bromine substituents on the pyridine rings showed the highest cytotoxicity. The antitumor activities against tumor cell lines were assayed in vitro, and the complexes were found to be highly effective, with six of the nine complexes having inhibition ratios better than that of 5-Fluorouracil. This behavior is indicative of a high ability to circumvent the cellular drug resistance mechanisms.     

Water exchange on seven-coordinate Mn(II) complexes with macrocyclic pentadentate ligands: insight in the mechanism of Mn(II) SOD mimetics

Seven-coordinate manganese(II) complexes [Mn(L)(H2O)2]2+, where L represents an equatorial pentadentate macrocyclic ligand with five nitrogen donor atoms, were studied with regard to their acid-base properties, water-exchange rate constants, and corresponding activation parameters (DeltaH, DeltaS, and DeltaV). Three of the studied complexes without imine bonds in the macrocyclic ligand are proven superoxide dismutase (SOD) mimetics. Their water-exchange parameters were compared with those of the imino groups containing complex [Mn(L1)(Cl)2] (dichloro-2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]-octadeca-1(18),2,12,14,16-pentaenemanganese(II)), which does not show SOD activity. In addition the X-ray crystal structure of a new complex, dichloro-2,6-bis[1-(2-(N-methylamino)ethylimino)ethyl]pyridine-manganese(II) [Mn(L2)(Cl)2], which is the acyclic analog of [Mn(L1)(Cl)2], is reported. Stability constants of the complexes and the pKa values of the ligands were measured by potentiometric titration. The titrations of [Mn(L1)(H2O)2]2+ and [Mn(L2)(H2O)2]2+ led to complicated species distribution curves because of their ligands containing imine bonds. Water exchange was measured by temperature- and pressure-dependent 17O NMR techniques. In addition to the measurements on [Mn(EDTA)(H2O)]2- and its derivatives, this is the only study of water exchange on seven-coordinate manganese complexes. The water exchange rate constants vary between 1.6 x 107 s-1 and 5.8 x 107 s-1 at 25 degrees C and are mainly controlled by the pi-acceptor abilities of the ligands. The exchange rate constant of the diaqua-1,4,7,10,13-pentaazacyclopentadecanemanganese(II) [Mn([15]aneN5)(H2O)2]2+ complex seems to be even higher but could not be exactly determined. On the basis of the obtained activation parameters, the exchange mechanism of the studied seven-coordinate manganese(II) complexes follows a dissociative pathway (Id mechanism). DFT calculations (UB3LYP/LANL2DZp) were performed to obtain the energy required for the dissociation of the coordinated water molecule, that is, the energy difference between the starting seven-coordinate complex and a six-coordinate intermediate. The results have been discussed in terms of the catalytic mechanism of the proven SOD mimetics.     

Conversion of imine ligands in allyl-nickel(II) complexes

Interaction of Ni(allyl)₂ and bidentate nitrogen-containing ligands (phenanthroline-1,10; bis(2,6-diisopropylphenyl)diazabutadiene) has been studied. It has been shown that coordination of diimine ligands proceeds with transfer of an allylnickel group to the diimine frame and formation of a covalent Ni-N bond giving rise to imine(amide)Ni(II) complexes. In the case of phenanthroline dearomatization of one heteroaromatic ring takes place. The low-spin imine(amide)allyl complexes (allyl)Ni(C₁₅H₁₅N₂) (1) and (allyl)Ni(C₂₉H₄₂N₂) (3) have been isolated as crystals and characterized by solution spectroscopy. Combining two molar equivalents of phenanthroline-1,10 with Ni(allyl)₂ results in the transfer of both allyl groups and formation of the high-spin imine(amide)Ni(II) complex Ni(C₁₅H₁₅N₂)₂ (2).     

Azetidines as ligands in the Pd(II) complexes series

The preparation of palladium (II) complexes having sterically congested azetidines as ligands is described. Diastereomerically pure α-alkylamino and α-alkylimino azetidines react with Na₂PdCl₄ to afford the corresponding bidendate Pd(II) complexes, whereas 2-cyano azetidines can be used to access bidendate Pd(II) complexes containing an amino-imidate moiety. Preliminary study of the catalytic activity of these new complexes in the Suzuki cross-coupling reaction is presented.     

Phenyl-cyclohexyl enaminoketone ligands and their Cu(II) complexes

The liquid crystalline properties of 1-(alkylamino)-3-[(4'-hexyl-trans-cyclohexyl-4'-phenyl]-prop-1-en-3-one-s, from methyl to octadecyl, and their copper (II) complexes have been examined by optical, DSC, X-ray and EPR methods. The compounds are enantiotropic nematogens except those having the shortest and the longest terminal chains. Short chains promote the S_A phase in both ligands and complexes, whereas long chains promote S_C and crystal H phases for the ligands or S_A and S_C phases for the complexes. A partly bilayer smectic A_d phase is observed from ligands terminated with short non-polar substituents. Direct isotropisation from the crystal H phase for some of the compounds, as well as other phase transitions have been studied. The molecular shape of the complexes and the organization of their mesomorphic phases are discussed, based on the refractive indices and X-ray data.