Mauro Ghedini

Transition Metals Complexed to Ordered Mesophases: Palladium-Azo Complexes

Abstract The synthesis and characterization of palladium (II) complexes with liquid crystal azobenzene ligands is reported. The new organometallic compounds, having a [sgrave] metal-carbon bond and a metal-metal chloro bridge, display ordered mesophases. The temperatures of the K-N phase transition, in the region of 200°C, are some 130° higher than those for the related free ligands.

Dinuclear cyclopalladated azobenzene complexes : a comparative study on model compounds for organometallic liquid-crystalline materials

The series of dinuclear 4,4′-bis(hexyloxy)azobenzene, [H(Azo-6)], cyclopalladated complexes of general formula [Azo-6)Pd(µ-X)]2, (X = Cl, Br, I, N3, SCN, OAc) and [Azo-6)2Pd2(µ-Ox)] (Ox = oxalate) have been synthesized and investigated for mesomorphism and spectroscopic properties. Single-crystal X-ray analysis of the dinuclear bromo- and iodo-bridged complexes has been performed. The structural data, compared with those of the known homologous chloro compound, show that all the [Azo-6)Pd(µ-X)]2)] (X = Cl, Br, I) molecules crystallize in the monoclinic space group P21/c and are isomorphous. They are arranged in slipped pairs with intermolecular non-bonding Pd–Pd contacts ranging from 3.668(1) A(X = Cl) to 3.758(3) A(X = I). The different nature of the bridging group allows variation of the distance between the palladium atoms and the bond environment experienced by the metal centers. Thus, this comparative study reveals that the effectiveness of the bridging group in promoting thermotropic mesophases is greater for chloride, bromide, azide or oxalate than for iodide, thiocyanate or acetate. The greatest range of liquid-crystal behavior was displayed by [Azo−6)2Pd2(µ−Ox)]. Remarkably, this compound is the first example of a metallomesogen containing the bridging oxalate group. The bimetallic complexes exhibit different absorption spectra (i.e. colors) depending, in general terms, on the nature of the bridge connecting the two cyclometalated [H(Azo-6)] moieties, which can be varied so as to tune the optical properties. Blocking the azo group in the trans position results in several cases in weakly luminescent complexes, with luminescence efficiencies ϕ ≈10−4 and luminescence lifetimes of the order of nanoseconds. Using the data obtained from the 4,4′-bis(hexyloxy)azoxybenzene [H(Azoxy-6)] derivative, [Azoxy-6)Pd(µ -Cl)]2, from the mononuclear acetylacetonate (acac) complexes [(Azo-6)Pd(acac)] and [(Azoxy-6)Pd(acac)], and from the uncomplexed [H(Azo-6)] and [H(Azoxy-6)] ligands, the nature of the excited states relevant to the photophysical behavior are discussed. Copyright

Synthesis and characterization of a homologous series of mononuclear palladium complexes containing different cyclometalated ligands

Abstract New mononuclear palladium(II) complexes formed by with 2-hydroxy-4-( n -hexyloxybenzylidene)-4′- n -hexyaniline ( HL ) and 2-phenylpyridine ( I ), benzo[h]quinoline ( II ), azobenzene ( III ), 2-benzoylpyridine ( IV ) or phenyl-2-pyridylketone-2,4-dinitrophenylhydrazone ( V ) have been synthesized and characterized by analytical and spectroscopic methods and the single-crystal structures of [(IVa)Pd(L)] and [(Va)Pd(L)] have been established. The spectroscopic and diffractrometric data account for molecular structures wherein the palladium(II) center is part of two chelate rings involving HL and one of the I – V ligands which form a five-member N,C{[(Ia)PdL], [(IIa)Pd(L)] and [(IIIa)Pd(L)]}, a six-member N,C{[(IVa)Pd(L)]}, or a six-member N , N ′{[(Va)Pd(L)]} metallacycle. The electronic spectra of [(IIIa)Pd(L)] and [(Va)Pd(L)] show an absorption maxima, at 495 ( e ∼3×10 3 M −1 cm −1 ) and 531 nm ( e ∼2×10 4 M −1 cm −1 ), respectively, which results are significantly affected by the polarity of the solvent. These chromophores should therefore be of interest for the preparation of nonlinear optical materials.

Synthesis and spectroscopic characterization of organometallic chromophores for photoluminescent materials: cyclopalladated complexes

Abstract A homologous series of isostructural and isoelectronic palladium acetylacetonato complexes containing different cyclometalated ligands HL (HL=2-phenylpyridine H(PhPy), benzo[h]quinoline H(BhQ), 9-diethylamino-benzo[a]phenoxazin-5-one H(NR), azobenzene H(Azo), 5-[(4′-methyl)-phenylazo]-8-methoxyquinoline H(QAzo), 2-benzoylpyridine H(BKPy), phenyl-2-pyridylketone-2,4-dinitrophenylhydrazone, H(BHPy)) has been prepared. The photophysical characterization of the newly synthesized products shows that many of them are luminescent in solution at room temperature. The best performances are exhibited by the H(NR) derivative (Φem=0.23, λem=660, in dichloromethane solution), which improves the emission features displayed by the unmetalated ligand (Φem=0.13, λem=602, in dichloromethane solution). H(NR) is a well known laser dye commonly named nile red and to the best of our knowledge [(NR)Pd(acac)] is the first example reported up to now of its incorporation in a transition metal complex. Interestingly, the results of the present investigation prove that “[(NR)Pd]” can be an useful building block for the preparation of luminescent organometallic complexes.

Transition Metal Complexes with Ordered Mesophases: Cyclopalladated Compounds of (p-Ethoxyphenylazo)-p-Phenyl Heptanoate

Abstract The synthesis, characterization and thermal behaviour of dinuclear and mononuclear cyclopalladated derivatives of (p-ethoxyphenylazo)-p-phenyl heptanoate are reported. The dinuclear halo-bridged complexes show nematic phases at temperatures increasing in the order CI < Br < I. The breaking of the halogen bridge by pyridine or quinoline affords a new series of mononuclear liquid crystals in which a layered smectic phase is present. The connection between the nature of the neutral nitrogenous ligand and the thermal properties of the mononuclear complexes is also discussed.

Luminescence of azobenzene derivatives induced by cyclopalladation

Abstract In a series of organometallic azobenzene Pd(II) complexes, which are of interest as mesogenic materials, cyclopalladation blocks the azo group in the trans configuration. Whereas the uncomplexed azobenzene reference compound is not luminescent, the complexes are weakly luminescent (Φ ∼ 10−4, τ = 1–3 ns) owing to the emissive properties of nπ∗ excited states localized on the trans-blocked azobenzene ligand. These findings suggest that cyclopalladated azo-containing compounds may afford luminescent liquid crystalline materials.

Cyclopalladation of 5-(1-hexyl)-2{[4′-(1-undecyloxy)phenyl]}-pyrimidine. Synthesis and characterization of mononuclear complexes

Abstract The synthesis and characterization of a number of mononuclear cyclopalladated 5-(1-hexyl)-2{[4′-(1-undecyloxy)phenyl]}pyrimidine, HL , derivatives are reported. All these new complexes were obtained by bridge splitting reactions from the dimeric [Pd(L)Cl] 2 species.

Cyclopalladated complexes. Synthesis and crystal structure of di-μ-chloro-bis{[2,6-dimethyl-N-(benzylidene) phenylaminato-C2′, N]palladium(II)}

Abstract The X-ray crystal structure of the cyclometallated palladium(II) complex [{Pd(L2)(μ-Cl)} 2 ], L2=2,6-dimethyl- N - (benzylidene)phenylaminate, is described. Crystals were orthorhombic, space group Pna 2 1 with a = 17.307(3) A, b = 17.732(4) A, c = 9.075(2) A, Z = 4, U = 2785.0(10) A 3 . The palladated dimer, with asymmetrically bridging chlorine atoms, exhibits a Pd 2 Cl 2 unit folded at 36.7(1)° and a non-bonding Pd ⋯ Pd separation of 3.425(1) A.

Improving the bioactivity of Zn(II)-curcumin based complexes

New Zn(II)-curcumin based heteroleptic complexes (1-5) have been synthesized and fully characterized, with the aim to improve the bioactivity of the precursor derivative [(bpy-9)Zn(curc)Cl] (A), a potentially intercalating antitumor agent recently reported. Some structural changes have been made starting from the reference complex A, in order to introduce new functionalities, such as electrostatic and/or covalent interactions. In particular, keeping the same N,N chelating ligand, namely bpy-9, two completely different Zn(II) species have been obtained: a tetracoordinated Zn(II) cation with tetrafluoroborate as counterion (1) and a dimeric neutral complex in which the sulfate anion acts as a bridging group through two Zn(II) centres (2). Moreover, by changing the N,N chelating unit, [(L(n))Zn(curc)Cl] complexes (3-5), in which the Zn(II) ion shows the same pentacoordination seen in the precursor complex A, have been obtained. The antitumour activity of all new Zn(II) complexes was tested in vitro against the human neuroblastoma cell line SH-SY5Y in a biohybrid membrane system and the results indicate that all species exhibit strong cytotoxic activity. In particular the ionic tetrafluoroborate Zn(II) complex, 1, and the neutral phenanthroline based Zn(II) derivative, 4, show the strongest growth inhibition, being even more effective than the model complex A. Both complexes have a dose-dependent anti-proliferative effect on cells as demonstrated by the decrease of viability and the increase of Annexin V and PI-positive cells with the increase of their concentration. Cells treated with complexes 1 and 4 undergo apoptosis that involves the activation of JNK, caspase 3 and MMP changes. Finally, complex 1 is more effective in the induction of caspase-3 activation demonstrating its ability to trigger the execution-phase of cell apoptosis.

Transition metals complexed to ordered mesophases: VIII. Cyclopalladated p-azoxyanisole complexes. Crystal structure of [N-(phenyl)-2-salicylideneaminato]-[4′-(methoxyphenyl-NNO-azoxy-N2-4-methoxyphenyl-2-ato]palladium(II)☆

The synthesis and characterization of the cyclopalladated p-azoxyyanisole chloro-bridged dimer [Pd(Azoxy)Cl]2, 1, is reported and its reactions with some salicylideneaniline derivatives (HL1–5) are described. 1H NMR spectral observations indicate that except for [Pd(Azoxy)(L4)] the mononuclear [Pd(Azoxy)(L)] products are ca 5:1 mixtures of N-trans and N-cis isomers. In the HL4 case (HL4 = salicylidene(2,4-dimethyl)aniline) the only product which was detected is the N-trans isomer. The crystal structure of trans-[Pd(Azoxy)(L1)] (HL1 = salicylideneaniline) (3-trans) is monoclinic.