Unsymmetrical diimine chelation to M(II) (M=Zn, Cd, Pd): atropisomerism, pi-pi stacking and photoluminescence

Zn Coordination and the Identity of the Halide Ancillary Ligand Dramatically Influence the Excited-State Dynamics and Bimolecular Reactions of 2,3-Di(pyridin-2-yl)benzo[g]quinoxaline

The optical properties of coordination complexes with ligands containing nitrogen heterocycles have been extensively studied for decades. One subclass of these materials, metal complexes utilizing substituted pyrazines and quinoxalines as ligands, has been employed in a variety of photochemical applications ranging from photodynamic therapy to organic light-emitting diodes. A vast majority of this work focuses on characterization of the metal-to-ligand charge-transfer states in these metal complexes; however, literature reports rarely investigate the photophysics of the parent pyrazine or quinoxaline ligand or perform control experiments utilizing metal complexes that lack low-lying charge-transfer (CT) states in order to determine how metal-atom coordination influences the photophysical properties of the ligand. With this in mind, we examined the steady-state and time-resolved photophysics of 2,3-di(pyridin-2-yl)benzo[g]quinoxaline (dpb) and explored how the coordination of ZnX₂ (X = Cl^-, Br^-, I^-) affects the photophysical properties of dpb. In dpb, we find that the dominant mode of deactivation from the singlet excited state is intersystem crossing (ISC). Coordination of ZnX₂ perturbs the relative energies of the ππ* and nπ* excited states of dpb, leading to drastically different rates of ISC as well as radiative and nonradiative decay in the [Zn(dpb)X₂] complexes compared to dpb. These differences in the rates change the dominant singlet-excited-state decay pathway from ISC in dpb to a mixture of ISC and fluorescence in [Zn(dpb)Cl₂] and [Zn(dpb)Br₂] and to nonradiative decay in [Zn(dpb)I₂]. Coordination of ZnX₂ and the choice of the halide ligand also have profound effects on the rate constants for excited-state bimolecular reactions, including triplet-triplet annihilation and oxygen quenching. These results demonstrate that metal coordination, even in complexes lacking low-lying CT states, and the choice of the ancillary ligand can dramatically alter the photophysical properties of chromophores containing nitrogen heterocycles.

Evaluation of the Anticancer and DNA-Binding Characteristics of Dichloro(diimine)zinc(II) Complexes

Several metal diimine complexes have been reported to possess anticancer properties. To evaluate the anticancer properties of tetrahedral zinc(II) diimine complexes, six complexes were synthesized with the general formula M(N^N)Cl2 {where M = Zn, Pt and N^N = 2,2’-biquinoline (1), 2,2’-dipyridylketone (2) and 4-((pyridine-2-ylmethylene)amino)phenol (3)}. In general, the intrinsic DNA-binding constants for the different compounds exhibited values within close proximity; the changes in the viscosity of the CT-DNA upon binding to the compounds suggest intercalation-binding mode. Molecular docking study predicted that complexes containing the highly planar ligand 2,2’-biquinoline are capable to establish π–π interactions with nucleobases of the DNA; the other four complexes engaged in donor–acceptor interactions with DNA nucleobases. The six complexes and two reference drugs (cisplatin and sunitinib) were tested against two cancer cell lines (COLO 205 and RCC-PR) and one normal cell line (LLC-MK2), highlighting the better performance of the zinc(II) complexes compared to their platinum(II) analogues. Moreover, zinc(II) complexes have higher selectivity index values than the reference drugs, with promising anticancer properties.

Pyridyldiimine macrocyclic ligands: Influences of template ion, linker length and imine substitution on ligand synthesis, structure and redox properties

A series of 2,6-diiminopyridine-derived macrocyclic ligands have been synthesized via [2+2] condensation around alkaline earth metal triflate salts. The inclusion of a tert-butyl group at the 4-position of the pyridine ring of the macrocyclic synthons results in macrocyclic complexes that are soluble in common organic solvents, thereby enabling a systematic comparison of the physical properties of the complexes by NMR spectroscopy, mass spectrometry, solution-phase UV-Vis spectroscopy, cyclic voltammetry and single-crystal X-ray crystallography. Solid-state structures determined crystallographically demonstrate increased twisting in the ligand, concurrent with either a decrease in ion size or an increase in macrocycle ring size (18, 20, or 22 membered rings). The degree of folding and twisting within the macrocycle can be quantified using parameters derived from the N_pyr-M-N_pyr bond angle and the relative orientation of the pyridinediimine (PDI) and pyridinedialdimine (PDAI) fragments to each other within the solid state structures. Cyclic voltammetry and UV-Vis spectroscopy were used to compare the relative energies of the imine π* orbital of the redox active PDI and PDAI components in the macrocycle when coordinated to redox inactive metals. Both methods indicate the change from a methyl to hydrogen substitution on the imine carbon lowers the energy of the ligand π* system.

Charge-assisted hydrogen bond and nitrile⋯nitrile interaction directed supramolecular associations in Cu(ii) and Mn(ii) coordination complexes: anticancer, hematotoxicity and theoretical studies

Charge-assisted H-bonds and nitrile⋯nitrile interactions directed assemblies in Cu(ii) and Mn(ii) complexes have been analyzed by MEP surface and NCI plot index. Anticancer activities and hematotoxictiy have been investigated.

Non-symmetrical, potentially redox non-innocent imino NHC pyridine ‘pincers’ via a zinc ion template-assisted synthesis

A Zn^II-promoted modular synthesis allows access to new non-symmetrical, redox-active imino NHC pyridine pincer ligands. Radical anionic and dianionic redox states of the ligand are involved in its Fe^II complexes obtained from FeBr₂/KC₈.

Unsymmetrical N-Aryl-1-(pyridin-2-yl)methanimine Ligands in Organonickel(II) Complexes: More Than a Blend of 2,2'-Bipyridine and N,N-Diaryl-α-diimines?

The new organonickel complexes [(R-PyMA)Ni(Mes)X] [R-PyMA = N-aryl-1-(pyridin-2-yl)methanimine; aryl = phenyl, 2,6-Me₂-, 3,5-Me₂-, 2,4,6-Me₃-, 2,6-ⁱPr₂-, 3,5-(OMe)₂-, 2-NO₂-4-Me-, 4-NO₂-, 2-CF₃-, and 2-CF₃-6-F-phenyl; Mes = 2,4,6-trimethylphenyl; X = F, Cl, Br, or I] were obtained as approximate 1/1 cis and trans isomeric mixtures or pure cis isomers depending on the PyMA ligand and X. The [(R-PyMA)Ni(Mes)X] complexes with X = Br or Cl were directly synthesized from the precursors trans-[(PPh₃)₂Ni(Mes)X], while [(PyMA)Ni(Mes)X] derivatives with X = F or I were obtained from [(PyMA)Ni(Mes)Br] through X exchange reactions. Although density functional theory (DFT) calculations show a preference for the sterically favored cis isomers, both isomers could be observed in many cases; in three cases, even single crystals for X-ray diffraction could be obtained for the trans isomers. Possible intermediates for the isomerization were investigated by DFT calculations. All complexes were studied by multiple spectroscopic means, electrochemistry, and spectroelectrochemistry (for the reduction processes). The long-wavelength metal-to-ligand charge-transfer (MLCT) absorptions vary markedly with the R substituent of the ligand and the cathodic electrochemical potentials to a far smaller degree. Both are almost invariable upon variation of X. All of this is in line with Ni-based and π*-based lowest unoccupied molecular orbitals (LUMOs). In line with the unsymmetric character of the N_Py^N_methanimine ligand, electrochemistry and MLCT transitions seem to not correspond to the same type of π* LUMO, making these PyMA ligands more interesting than the symmetric heteroaromatic polypyridine ligands such as 2,2'-bipyridine (bpy; N_Py^N_Py) and N,N-diaryl-substituted aliphatic α-diimines (N_methanimine^N_methanimine) such as the diaza-1,3-butadienes (DAB). First attempts to use these complexes in Negishi-type cross-coupling reactions were successful.

Sugar-boronate ester scaffold tethered pyridyl-imine palladium(II) complexes: synthesis and their in vitro anticancer evaluation

A series of five palladium(ii) pyridyl-imine Schiff base complexes 5a-e containing boronate esters with protected sugar diols derived from d-xylose, l-sorbose and d-mannitol were designed and synthesized starting from pyridyl-imines generated in situ from 3-aminophenyl boronate ester of sugars 3a-e and 2-pyridinecarboxaldehyde, followed by the addition of Pd(cod)Cl2 in dichloromethane solvent. All the complexes are remarkably stable orange/yellow crystalline solids and were obtained in good yields. The complexes were fully characterized by FT-IR, multinuclear NMR ((1)H, (13)C and (11)B), UV-visible spectroscopy, and elemental analysis. The solid state structures of 3a and 5a were established by single crystal X-ray diffraction analysis. The complexes have been tested for their in vitro anticancer activities against human colon cancer (HT-29) and breast cancer (MDA-MB-231) cell lines. All the complexes have shown moderate to good cytotoxicity in both the cancer cell lines with IC50 values ranging from 4.27 to 34.76 μM. Strikingly, 5a displayed selective anticancer activity against both HT-29 and MDA-MB-231 cells with low IC50 values 6.71 and 8.58 μM respectively. Results also demonstrate that some of these complexes are highly potent against HT-29 cells as compared to the other cancer cell lines. In particular, 1,2:5,6-di-O-isopropylidene-d-mannitol complex 5d showed a two-fold higher toxicity against HT-29 cells in comparison with that of cisplatin. In addition, these complexes are less toxic to model non-tumorigenic human embryonic kidney cells (HEK-293T). Furthermore, the interaction of the complexes with calf thymus DNA (CT-DNA) was investigated using spectroscopy and viscosity measurements. It was found that they intercalate with DNA.

Mono- and di-nuclear photoluminescent complexes of zinc(II), cadmium(II) and mercury(II) of a chiral diimine ligand

Reaction of α-pyridoin and N-phenyl-o-phenylenediamine affords 2-(2-(phenylamino)phenylimino)-1,2-di(pyridin-2-yl)ethanol (L) which undergoes cyclization to a chiral diimine, 2-methoxy-1-phenyl-2,3-di(pyridin-2-yl)-1,2-dihydroquinoxaline, L(OMe) (conjugated 14πe system) in the presence of zinc(II), cadmium(II) and mercury(II) ions affording [Zn(L(OMe))Cl2] (1), [Cd2(L(OMe))2Cl4] (2) and [Hg2(L(OMe))2Cl4] (3) complexes. Ligand L and complexes 1-3 are substantiated by elemental analyses, mass, IR, (1)H NMR and UV-vis spectra including the single-crystal X-ray structures of 1 and 3. The possibility of the atropisomerism of L is restricted in cyclic L(OMe). L and complexes 1-3 are fluorescent in fluid solutions at 298 K (CH2Cl2: 1, λ(ex) = 470 nm, λ(em) = 627 nm, Φ = 0.014, τ(avg) = 2.5 ns; 2, λ(ex) = 430 nm, λ(em) = 599 nm, Φ = 0.08, τ(avg) = 7.6 ns; 3, λ(ex) = 415 nm, λ(em) = 600 nm, Φ = 0.021, τ(avg) = 2.8 ns). Time-resolved emission spectra (TRES) established that the two-component lifetimes of 1-3 are due to the existence of two conformers. Density functional theory (DFT) and time dependent (TD) DFT calculations authenticated that 1-3 complexes are fluorescent due to intra-ligand charge transfer (ILCT) to the π(diimines)* orbital.

Dichlorido{2-[(2,6-dimethyl-phen-yl)imino-meth-yl]pyridine-κN,N'}zinc

In the asymmetric unit of the title compound, [ZnCl(2)(C(14)H(14)N(2))], the central Zn(II) ion is four-coordinated in a distorted tetra-hedral environment by two N atoms of the ligand 2-[(2,6-dimethyl-phen-yl)imino-meth-yl]pyridine and two chloride anions. In the crystal, adjacent mol-ecules are connected through C-H⋯Cl hydrogen bonds between a C-H group of the ligand and a Cl(-) anion, leading to a chain-like structure along the b direction.