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

Synthesis and crystal structure of [Pd{C6H4(CH2NHCH2Ph)-2-κ2C,N}(μ-I)]2

The binuclear title complex, di-μ-iodido-bis(-{2-[(benzylamino-κN)meth-yl]phenyl-κC1}palladium(II)), [Pd2I2(C14H14N)2], was prepared by reaction of [Pd{C6H4(CH2NHCH2Ph)-2}(μ-OAc)]2 with NaI. It crystallizes with one discrete mol-ecule in the asymmetric unit. The mol-ecule presents an iodide-bridged dimeric structure with a cisoid arrangement with respect to the C,N-cyclo-palladated ligands. Both PdII atoms have a slightly distorted square-planar coordination environment. Weak inter-molecular contacts of the type C-H⋯Pd seem to have a significant influence on the arrangement of the mol-ecules along the b axis in the crystal.

Photoisomerization and optical properties of a subphthalocyanine–azobenzene–subphthalocyanine triad

A novel subPc–AB–subPc triad exhibits on–off switching of the fluorescence emission intensity upon reversible trans ↔ cis photoisomerization of the azobenzene moiety. NMR spectroscopy provides additional evidence for the conformational change.

Insight into luminescent bisazoaromatic CNN pincer palladacycle: synthesis, structure, electrochemistry and some catalytic applications in C–C coupling

The 2-(phenylazo)azobenzene furnished novel palladacycles in excellent yield, which showed luminescence at rt and catalytic activity. The optoelectronic and electrochemical responses were substantiated with DFT and TDDFT.

2,2′-Disilylazobenzenes featuring double intramolecular nitrogen⋯silicon coordination: a photoisomerizable fluorophore

Fluorescent (E)-4,4′-dimethyl-2,2′-bis(trimethylsilyl)azobenzene was synthesized.

Tetrahedral Zinc Complexes with Liquid Crystalline and Luminescent Properties: Interplay Between Nonconventional Molecular Shapes and Supramolecular Mesomorphic Order

Novel metallomesogens with luminescent properties and liquid crystalline behavior at room temperature have been achieved by the preparation of zinc complexes with polycatenar pyrazole and bis(pyrazolyl)methane ligands. Their molecular structures do not have a conventional shape in that they are far from the typical rod-like and flat disc-like geometries of common liquid crystals. They consist of a nonplanar nucleus due to the methylene spacer and/or the coordination to the tetrahedral center, as confirmed by single crystal analysis of the cores. The different numbers and positions of side chains in the pyrazole ligand enabled us to access lamellar and columnar mesophases and, of particular interest, to obtain columnar arrangements at room temperature. Supramolecular models for the organization of the molecules in the mesophases are proposed on the basis of the small-angle XRD diffractograms. The zinc complexes display luminescence in the near UV-blue region with large Stokes shifts. An interplay between non-conventional molecular shapes (due to the tetrahedral core) and the supramolecular mesomorphic order (due to the ligand design) led to materials that interestingly embody two rather opposite properties, a columnar self-organizational ability and luminescence with weak intermolecular interactions.

Synthesis of the most intensely fluorescent azobenzene by utilizing the B–N interaction

A boron-substituted azobenzene, (E)-[2-(4-methoxyphenylazo)phenyl]bis(pentafluorophenyl)borane, presented the most intense fluorescence among the azobenzene derivatives.

Mesomorphism of ionic allylpalladium(II) complexes containing pz(R2)py as ligands and [BF4]-, [PF6]- or [CF3SO3]- as counteranions

The synthesis and thermal behaviour is reported of some ionic liquid-crystalline complexes formed when a mesogenic, two-chained, pyrazolylpyridine ligand is bound to a eta3-allylpalladium(II) fragment. The phase behaviour of the complexes is dominated by the formation of the SmA phase and, while the stability of the crystal phase appears to be controlled by intermolecular solid-state interactions, the stability of the liquid crystal phases is controlled by the length of the ligand alkoxy chains. Some of the complexes also show a complex polymorphism leading to the observation of double-melting behaviour.

A Novel Heteroditopic Terpyridine-Pincer Ligand as Building Block for Mono- and Heterometallic Pd(II) and Ru(II) Complexes

A palladium-catalyzed Stille coupling reaction was employed as a versatile method for the synthesis of a novel terpyridine-pincer (3, TPBr) bridging ligand, 4'-{4-BrC6H2(CH2NMe2)2-3,5}-2,2':6',2' '-terpyridine. Mononuclear species [PdX(TP)] (X = Br, Cl), [Ru(TPBr)(tpy)](PF6)2, and [Ru(TPBr)2](PF6)2, synthesized by selective metalation of the NCNBr-pincer moiety or complexation of the terpyridine of the bifunctional ligand TPBr, were used as building blocks for the preparation of heterodi- and trimetallic complexes [Ru(TPPdCl)(tpy)](PF6)2 (7) and [Ru(TPPdCl)2](PF6)2 (8). The molecular structures in the solid state of [PdBr(TP)] (4a) and [Ru(TPBr)2](PF6)2 (6) have been determined by single-crystal X-ray analysis. Electrochemical behavior and photophysical properties of the mono- and heterometallic complexes are described. All the above di- and trimetallic Ru complexes exhibit absorption bands attributable to (1)MLCT (Ru --> tpy) transitions. For the heteroleptic complexes, the transitions involving the unsubstituted tpy ligand are at a lower energy than the tpy moiety of the TPBr ligand. The absorption bands observed in the electronic spectra for TPBr and [PdCl(TP)] have been assigned with the aid of TD-DFT calculations. All complexes display weak emission both at room temperature and in a butyronitrile glass at 77 K. The considerable red shift of the emission maxima relative to the signal of the reference compound [Ru(tpy)2]2+ indicates stabilization of the luminescent 3MLCT state. For the mono- and heterometallic complexes, electrochemical and spectroscopic studies (electronic absorption and emission spectra and luminescence lifetimes recorded at room temperature and 77 K in nitrile solvents), together with the information gained from IR spectroelectrochemical studies of the dimetallic complex [Ru(TPPdSCN)(tpy)](PF6)2, are indicative of charge redistribution through the bridging ligand TPBr. The results are in line with a weak coupling between the {Ru(tpy)2} chromophoric unit and the (non)metalated NCN-pincer moiety.

Syntheses, X-ray structures, photochemistry, redox properties, and DFT calculations of interconvertible fac- and mer-[Mn(SPS)(CO)3] isomers containing a flexible SPS-based pincer ligand

The lithium salt of the anionic SPS pincer ligand composed of a central hypervalent lambda4-phosphinine ring bearing two ortho-positioned diphenylphosphine sulfide side arms reacts with [Mn(CO)5Br] to give fac-[Mn(SPS)(CO)3]. This isomer can be converted photochemically to mer-[Mn(SPS)(CO)3], with a very high quantum yield (0.80+/-0.05). The thermal backreaction is slow (taking ca. 8 h at room temperature), in contrast to rapid electrode-catalyzed mer-to-fac isomerization triggered by electrochemical reduction of mer-[Mn(SPS)(CO)3]. Both geometric isomers of [Mn(SPS)(CO)3] have been characterized by X-ray crystallography. Both isomers show luminescence from a low-lying 3IL (SPS-based) excited state. The light emission of fac-[Mn(SPS)(CO)3] is largely quenched by the efficient photoisomerization occurring probably from a low-lying Mn-CO dissociative excited state. Density functional theory (DFT) and time-dependent DFT calculations describe the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of fac- and mer-[Mn(CO)3(SPS)] as ligand-centered orbitals, largely localized on the phosphinine ring of the SPS pincer ligand. In line with the ligand nature of its frontier orbitals, fac-[Mn(SPS)(CO)3] is electrochemically reversibly oxidized and reduced to the corresponding radical cation and anion, respectively. The spectroscopic (electron paramagnetic resonance, IR, and UV-vis) characterization of the radical species provides other evidence for the localization of the redox steps on the SPS ligand. The smaller HOMO-LUMO energy difference in the case of mer-[Mn(CO)3(SPS)], reflected in the electronic absorption and emission spectra, corresponds with its lower oxidation potential compared to that of the fac isomer. The thermodynamic instability of mer-[Mn(CO)3(SPS)], confirmed by the DFT calculations, increases upon one-electron reduction and oxidation of the complex.

Simple Route to the Doubly ortho-Palladated Azobenzenes: Building Blocks for Organometallic Polymers and Metallomesogens

A new class of doubly cyclopalladated complexes, {PdCl(dmf)}2(mu-azb) (1) and {PdCl(dmf)}2(mu-aazb) (2), has been prepared in dimethylformamide (dmf) by reaction of azobenzene (azb) and 4-aminoazobenzene (aazb), respectively, with an excess of PdCl2(CH3CN)2 complex. Recrystallization of 1 and 2 in dimethyl sulfoxide (dmso) yields complexes {PdCl(dmso)}2(mu-azb) (3) and {PdCl(dmso)}2(mu-aazb) (4), respectively. The crystal structures of 1 and 4 have been determined by X-ray diffraction. All complexes are characterized by 1H and 13C NMR spectra and elemental analysis. In both crystal structures, solvent molecules are bound to palladium through oxygen atoms and oriented trans to carbon. In view of greater preference of palladium to nitrogen and sulfur atoms, the experimental structures were rationalized by quantum-chemical calculations and confirmed as the most stable isomers.

Gold Liquid Crystals Displaying Luminescence in the Mesophase and Short F⋅⋅⋅F Interactions in the Solid State

Rodlike gold(I) complexes, [Au(C6F4OCmH2m+1)(C(triple bond)NC6H4C6H4OCnH2n+1)] (m=2, n=4, 10; m=6, n=10; m=10, n=6, 10), display interesting features. They are liquid crystals and show photoluminescence in the mesophase, as well as in the solid state and in solution. The single-crystal, X-ray diffraction structure of [Au(C6F4OC2H5)(C(triple bond)NC6H4C6H4OC4H9)] confirms its rodlike structure, with a linear coordination around the gold atom, and reveals the absence of any Au...Au interactions (such interactions are often present in luminescent gold complexes). Well-defined, intermolecular Fortho...Fmeta interactions, with remarkably short intermolecular FF distances (2.66 A), are observed; these interactions seem to be responsible for the crystal packing, which consists of an antiparallel arrangement of molecules. Experiments under different conditions support the explanation that the photoluminescence has an intramolecular origin.

Synthesis and Mesogenic Properties of Binuclear Copper(II) Complexes Derived from Salicylaldimine Schiff Bases

The synthesis and mesomorphic (liquid crystal) properties of new binuclear dihalocopper(II) complexes derived from N- and ring-substituted salicylaldimine Schiff bases are reported, together with the mesomorphic properties of their monomeric precursor complexes. With just N-substituents both the dichlorodicopper(II) binuclear complexes and their mononuclear analogues are waxy solids with melting points that increase with their N-chain length. However, with both N- and ring-substituents in the 4-positions, the mononuclear and binuclear complexes are each liquid crystalline or mesogenic, except in case of the mononuclear complexes where the N-substituent is straight chain alkyl. The other mononuclear complexes exhibit a variety of liquid crystal phases: smectic A, C, and E (SA, SC, and SE, respectively). The liquid crystal phase SA is observed in the binuclears with shorter chain N-substituents p-R-O-C6H4- and shorter chain ring-substituents. The chain lengths were increased until the phase behavior expanded to a further form SC in the case of an N-substituent p-C14H29O-C6H4- and a -OC12H25 ring substituent. This points the way toward achieving multiphase behavior with these binuclear systems. The Cu-Br analogues of the binuclear complexes behave similarly but with significant qualitative differences, specifically lower mesophase stability and higher melting temperatures. The structures of the nonmesogenic binuclears ([Cu(N-dodecylSal)X]2, X=Cl, Br) were determined with the aid of X-ray crystallography. These are prototypes for the structures of the binuclear complexes and especially for the shape of the central Cu2O2 X2 core in the binuclears: distorted planar coordination about the copper with distortion toward tetrahedral measured by a characteristic twist angle tau (0 degrees planar; 90 degrees tetrahedral). The binuclear complexes also show magnetic coupling which can be used to estimate the geometry. For [Cu(N-dodecylSal)X]2 tau>36 degrees, which corresponds to weaker coupling than observed in the mesogenic binuclears where a stronger magnetic coupling indicates a geometry closer to planar (tau=25 degrees). The mesophases were characterized by differential scanning calorimetry (DSC) analysis and optical polarized microscopy.