Kajal Krishna Rajak

Binuclear Copper(II) Complexes with N4O3 Coordinating Heptadentate Ligand: Synthesis, Structure, Magnetic Properties, Density-Functional Theory Study, and Catecholase Activity

The N4O3 coordinating heptadentate ligand afforded binuclear complex [Cu 2(H 2L)(mu-OH)](ClO4)2 (1) and [Cu2(L)(H2O)2]PF6 (2). In complex 1, two copper ions are held together by mu-phenoxo and mu-hydroxo bridges, whereas in complex 2, the copper centers are connected only by a mu-phenoxo bridge. In 1, both the Cu(II) centers have square pyramidal geometry (tau=0.01-0.205), whereas in the case of 2, one Cu(II) center has square pyramidal (tau=0.2517) and other one has square based pyramidal distorted trigonal bipyramidal (tau=0.54) geometry. Complexes 1 and 2 show an strong intramolecular and very weak antiferromagnetic interaction, respectively. Density-functional theory calculations were performed to establish the magneto structural correlation between the two paramagnetic copper(II) centers. Both of the complexes display a couple of one-electron reductive responses near -0.80 and -1.10 V. The complexes show significant catalytic activity at pH 8.5 on the oxidation of 3,5-di- tert-butylcatechol (3,5-DTBC) to 3,5-di- tert-butylquinone (3,5-DTBQ), and the activity measured in terms of kcat=29-37 h(-1).

Enantiopure Tetranuclear Iron(III) Complexes Using Chiral Reduced Schiff Base Ligands: Synthesis, Structure, Spectroscopy, Magnetic Properties, and DFT Studies

Four new tetranuclear iron(III) complexes of formula [{Fe(L)(2)}(3)Fe], 1-4, have been prepared by reacting [Fe(ClO(4))(3)].6H(2)O with H(2)L in methanol. Here, L(2-) is the deprotonated form of N-(2-hyrdoxybenzyl)-l-valinol (H(2)L(1)), N-(2-hyrdoxybenzyl)-l-leucinol (H(2)L(2)), N-(5-chloro-2-hyrdoxybenzyl)-l-leucinol (H(2)L(3)), and N-(2-hyrdoxybenzyl)-l-phenylalaninol (H(2)L(4)). The complexes are prepared in an enantiomeric pure form. The complexes have been characterized with the help of IR, UV-vis, circular dichroism (CD), (1)H, and elemental analyses. The complex [{Fe(L(2))(2)}(3)Fe].CH(3)OH.2H(2)O, 2.CH(3)OH.2H(2)O, crystallizes in enantiomeric pure form containing a propeller-like Fe(4)O(6) core. (1)H and CD spectral studies of the four species are consistent with the structural similarities of the complexes in solution. Variable-temperature magnetic susceptibility of one case shows an intramolecular antiferromagnetic coupling between the Fe(III) ions. Magnetic measurements are in accord with the S = 5 ground state and suggest single molecular magnet behavior. The magnetic exchange coupling constant between the iron centers within the molecule is interpreted using broken-symmetry density functional theory calculation.

Synthesis, structure and properties of mononuclear oxovanadium(V) alkoxides incorporating chelated ethane-1,2-diol and propane-1,3-diol

Abstract Dark coloured complexes of type VO(L)(Heg) and VO(L)(Hpd) have been synthesized in excellent yields by reacting bis(acetylacetonato)oxovanadium(IV) with H2L in the presence of excess ethane-1,2-diol (H2eg) and propane-1,3-diol (H2pd), respectively in acetone. Here L2− are the deprotonated forms of N-(1-hydroxyethyl)naphthaldimine (H2L1) and N-(1-hydroxy-2-methylpropyl)naphthaldimine (H2L2). Heg− and Hpd− form five- and six-membered V(O,O) chelate rings, respectively. The crystal structure of VO(L2)(Heg) is reported. The ONO-coordinating tridentate [L2]2− spans meridionally and consists of two planar segments mutually rotated along a C–N bond. The Heg− ligand forms a non-planar chelate ring in which the dimethylene bridge is in gauche conformation. The five V–O lengths are unequal, the shortest being V–O(oxo) and the longest is V–O(alcoholic) which is subjected to the trans influence of the oxo oxygen atom. In the crystal lattice, the VO(L2)(Heg) molecule forms an infinite chain helical pattern via intermolecular O⋯O hydrogen bonding. Methylene 1H resonances are systematically shifted to lower field compared to those of free H2eg and H2pd, the shift of the CH2OV resonances being particularly large. The complex multiplet 1H structure in CDCl3 is a good indication of the rigidity of the chelate ring in solution. 51V chemical shifts are found to be diagnostic of the alcohol–alkoxide chelate ring size, the shift of VO(L)(Heg) being ∼25 ppm downfield from that of VO(L)(Hpd). Low V(V)/V(IV) reduction potentials (ca. −0.30 V versus SCE) are indicative of the considerable VO3+ stabilization due to two alkoxide coordination.

A novel fluorene based “turn on” fluorescent sensor for the determination of zinc and cadmium: experimental and theoretical studies along with live cell imaging

A new fluorene–pyridine linked imine conjugate, HAFPA, has been synthesized and characterized. HAFPA has been successfully found to recognize Zn2+ and Cd2+ selectively among a wide range of biologically relevant metal ions. The chemosensing behaviour of the ligand has been demonstrated through 1H NMR, ESI-MS, emission, absorption, colour change, luminescence spectra. The probe HAFPA displays a prominent fluorescence enhancement along with a large red shift (∼60 nm and 65 nm), which may be because of the CHEF (chelation-enhanced fluorescence) processes after its interaction with Zn2+ and Cd2+ respectively. The binding of the ligand with Zn2+ and Cd2+ was investigated through ESI-MS, Jobs plot and reversibility studies. The fluorescence property of the ligand after complexation with Zn2+ and Cd2+ can be restored in the presence of EDTA and sulphide ions. This reversibility phenomenon is utilized in the construction of a logic gate. The DFT, TDDFT and triplet excited state calculations were performed in order to demonstrate the structure of the ligand and its complexes and their electronic properties. Importantly, our chemosensor could be used to detect intracellular Zn2+ and Cd2+ with no cytotoxicity in A549 cells, and a colorimetric test strip of the ligand for the detection of both the ions could be useful for all practical purposes.

Synthesis, structure, and spectroscopic properties of chiral oxorhenium(V) complexes incorporating polydentate ligands derived from L-amino acids: a density functional theory/time-dependent density functional theory investigation.

The oxorhenium(V) complexes [Re (V)O(L A)Cl 2] bearing the (N-2-pyridylmethyl) of l-valine (HL A (1)), l-leucine (HL A (2)), and l-phenylalanine (HL A (3)) and [Re (V)O(L B)Cl] containing the {(N-2pyridylmethyl)-(N-(5-nitro-2-hydroxybenzyl)} of l-valine (H 2L B (1)), l-leucine (H 2L B (2)), and l-phenylalanine (H 2L B (3)) are presented in this article. The complexes are isolated in enantiomeric pure form examined from X-ray structure determination. The complexes are characterized by spectroscopic and electrochemical methods. The molecular structures observed in the solid state are grossly preserved in solution ( (1)H, (13)C, and circular dichroism spectra). Gas-phase geometry optimization and the electronic structures of [Re (V)O(L A (1))Cl 2], [Re (V)O(L A (2))Cl 2], and [Re (V)O(L B (2))Cl] have been investigated with the framework of density functional theory. The absorption and circular dichroism spectra of the complexes were also calculated applying time-dependent density functional theory (TDDFT) using the conductor-like polarizable continuum solvent model to understand the origin of the electronic excitations. The chemical shift ( (1)H and (13)C) as well as (1)H- (1)H spin-spin coupling constant were also computed by the gauge-independent atomic orbital method, and the computed values are consistent with the experimental data.

Photophysical property vs. medium: mononuclear, dinuclear and trinuclear Zn(II) complexes

Three complexes [Zn2(L)2(μ1,2-OAc)(μ1,1-OAc)]·C9H8N2 1·C9H8N2, [Zn3(L)4]·(ClO4)2 2 and [Zn(L)2]·CH3OH 3 were synthesized using a common Schiff base 2,4-dimethyl-6-((quinolin-8-ylimino)methyl)phenol (HL) and characterized by various methods such as elemental analysis, 1H NMR, FT-IR, UV-Vis and mass spectroscopy. The single crystal XRD analysis revealed that complex 1 is dinuclear, complex 2 trinuclear and complex 3 mononuclear in nature. The photophysical properties of the ligand and complexes were also investigated by different techniques such as UV-Vis spectroscopy, fluorescence spectroscopy, fluorescence lifetime measurement, DFT, and TDDFT calculations in solvents of different polarity. The complexes exhibited higher quantum yields than the ligand in solution. The trinuclear complex had higher emission intensity than the mono and dinuclear complexes in solution as well as in the solid state. In addition, complex 1 exhibited a unique dissymmetric coordination mode of the oxygen atom in the acetate bridging moiety.

Mononuclear rhenium(I) complexes incorporating 2-(arylazo)phenyl benzyl thioethers: synthesis, structure, spectral, DFT and TDDFT studies

The stoichiometric reaction of [Re(CO)5Cl] with 2-(arylazo)phenyl benzyl thioethers [ArNNC6H4SCH2Ph: Ar = Ph, 4-chlorophenyl, 4-nitrophenyl] (L1–L3) in a ratio of 1:1 in dry toluene under refluxing condition afforded the Re(I) complexes of general formula fac-[Re(L)(CO)3Cl] in excellent yields. The ligands used in this present work are 2-(phenylazo)phenyl benzyl thioether (L1), 2-(4-chlorophenylazo)phenyl benzyl thioether (L2) and 2-(4-nitrophenylazo)phenyl benzyl thioether (L3). The elemental analysis and ESI mass spectroscopic measurements ensure the formation of desired complexes. The crystal structure of the complexes [Re(L1)(CO)3Cl], 1 and [Re(L2)(CO)3Cl], 2 were determined by X-ray diffractometry study. The molecular structures observed in the solid state were retained in the solution (1H and 13C NMR spectra). The ground and excited-state geometries, NMR, absorption, and luminescent properties of three Re(I) complexes were examined by DFT and TDDFT methods. The introduction of an electron withdrawing group at the para position of the phenyl ring attached to the NN bond varies the frontier molecular orbital energies, compositions and optical properties of these molecules significantly. The lowest lying triplet excited is associated with an admixture of the 3MLCT and 3ILCT excited state having a cis conformation of the azoaryl moiety. The emission like transition having mixed 3MLCT and 3ILCT nature was characterized by natural transition orbital (NTO) and spin density difference map analysis. The presence of the electron withdrawing NO2 group leads to the low quantum yield through enhanced non-radiative deactivation. The cis orientation at the lowest lying triplet excited state (T1) may imply some kind of photoinduced isomerization in the ligand frame and as a result there may be more than one emitting species in the solution and hence bi-exponential decay nature was observed for all the complexes.

Synthesis, characterization and DFT study of oxorhenium(V) complexes incorporating quinoline based tridentate ligands

Two tridentate quinoline based Schiff base ligands HL1 and HL2 were prepared by condensation of salicylaldehyde and 2-hydroxy naphthaldehyde with 8-aminoquinoline, respectively in excellent yield. These ligands react with [ReOCl3(OPPh3)(SMe2)] in a ratio of 1 : 1 in acetone to form mononuclear trans-dichloro oxo complexes of general formula [ReVO(L)Cl2]. Here L− is the deprotonated form of 2-((quinolin-8-ylimino) methyl)phenol (HL1) or 1-[(quinolin-8-ylimino)methyl]naphthalene-2-ol (HL2). The elemental analysis and ESI mass spectroscopic measurements ensure the formation of the desired complexes. The molecular structure of mer-[ReVO(L1)Cl2] was confirmed by single-crystal X-ray diffraction. The complexes were also characterized by different spectroscopic techniques and electrochemical methods. The ground state geometry, NMR and absorption of Re(V) complexes were examined by DFT and TDDFT methods. The natural transition orbital (NTO) and spin density difference map analysis reveal the nature of the excitations. It was also found that the complexes act as a catalyst for oxidation of cis-cyclooctene to the corresponding epoxide by tert-butyl hydroperoxide (TBHP). The NMR and ESI mass spectroscopic analysis ensure the formation of the desired product which was obtained from catalysis.

Selective fluorometric detection of F− and Zn(II) ions by a N, O coordinating sensor and naked eye detection of Cu(II) ions in mixed-aqueous solution

Through click chemistry, salicylaldehyde and fluorene groups have been explored to recognize anions through O–H⋯A– hydrogen-bonding complexation followed by an ESIPT mechanism and cation sensing via CHEF and CHEQ mechanisms for Zn2+ and Cu2+ metal ions respectively. Herein, we demonstrate evidence of the interactions between the O–H bond and F− which were studied by fluorescence spectroscopic, UV-Vis spectroscopic, lifetime spectroscopic, 1H NMR spectroscopic titrations and theoretical treatment. This sensor could simultaneously detect two biologically important metal ions (Cu2+ and Zn2+) through colorimetric methods in mixed aqueous solution. The 2:1 binding stoichiometry ratio of the ligand and metal in the complexes was established by UV-Vis, fluorescence, 1H NMR and ESI-MS spectroscopy, and their corresponding association constants, Kassoc observed at 8.13 × 104 and 5.12 × 106 M−1 corresponds to Zn2+ and Cu2+ metal ions in aqueous buffer–CH3OH (2:1, v/v) at pH 7.2. In addition, the electronic structures and photo physical properties of the ligand and complexes were calculated by DFT and time-dependent DFT (TDDFT) methods.

Heteroleptic iridium(III) complexes bearing a coumarin moiety: an experimental and theoretical investigation

Orange coloured complexes with the general formula [Ir(2-pypy)2(L)] have been synthesized in excellent yields by reacting [Ir(2-pypy)2(Cl)]2 with HL in a ratio of 1 : 1 in a 1 : 1 mixture of ethanol and dichloromethane in an argon atmosphere. Herein, L− is the deprotonated aldimine form of 7-hydroxy-4-methyl-2-oxo-2H-chromene-8-carbaldehyde and 1-napthylamine (HL1), 2-aminoanthracene (HL2) and 2-aminofluorene (HL3), respectively. The elemental analysis and ESI mass spectroscopic measurements ensured the formation of the desired complexes. The molecular structure of [Ir(2-pypy)2(L1)] was confirmed by single-crystal X-ray diffraction. The complexes were also characterized by different spectroscopic techniques. The ground and excited-state geometries, NMR, absorption, and phosphorescence properties of the three Ir(III) complexes were examined by DFT and TDDFT methods. The natural transition orbital (NTO) and spin density difference map analysis reveals the nature of excitations. The lowest lying triplet excited state is associated with the 3IL and 3ML excited state. The emission-like transition is consistent with the strong 3ILCT and 3MLCT character.