LMCT Band Research Articles

Ab Initio Calculation of UV-vis Absorption of Parent Mg, Fe, Co, Ni, Cu, and Zn Metalloporphyrins.

Relativistic restricted active space (RAS) second-order multireference perturbation theory (MRPT2) methods, incorporating spin-orbit (SO) coupling perturbatively via state interaction (SO-MRPT2/RASSCF), were used to reproduce the absorption spectra of parent metalloporphyrins containing the Mg2+, Zn2+, Co2+, Ni2+, Cu2+, or FeCl2+ ions in the 12,500-40,000 cm-1 region. Particular attention was paid to the interaction between the porphyrin ring and the metal 3d electrons in states of different multiplicities (we used metal 3d and double d-shell or 3d' orbitals). For this class of compounds, the N-electron valence state perturbation theory (NEVPT2) method is superior to the complete active space perturbation theory (CASPT2) and successfully reproduces the energies of all four characteristic transitions (Q, B, N, and L) of closed-shell metalloporphyrins. Inclusion of SO coupling was found to have very little effect on excitation energies and oscillator strengths. For FeCl2+ porphyrin, we treated ligand-to-metal charge-transfer (LMCT; π,d), metal ligand field (d,d), and metal-to-ligand charge-transfer (MLCT; d,π*) transitions within the same framework. The broad and intense spectral features associated with its B (Soret) band are attributed to multiconfigurational LMCT (d,π*) bands involving strong metal-ligand orbital mixing.

Electronic Structure and Reactivity of Mononuclear Nonheme Iron-Peroxo Complexes as a Biomimetic Model of Rieske Oxygenases: Ring Size Effects of Macrocyclic Ligands.

We report the macrocyclic ring size-electronic structure-electrophilic reactivity correlation of mononuclear nonheme iron(III)-peroxo complexes bearing N-tetramethylated cyclam analogues (n-TMC), [FeIII(O2)(12-TMC)]+ (1), [FeIII(O2)(13-TMC)]+ (2), and [FeIII(O2)(14-TMC)]+ (3), as a model study of Rieske oxygenases. The Fe(III)-peroxo complexes show the same δ and pseudo-σ bonds between iron and the peroxo ligand. However, the strength of these interactions varies depending on the ring size of the n-TMC ligands; the overall Fe-O bond strength and the strength of the Fe-O2 δ bond increase gradually as the ring size of the n-TMC ligands becomes smaller, such as from 14-TMC to 13-TMC to 12-TMC. MCD spectroscopy plays a key role in assigning the characteristic low-energy δ → δ* LMCT band, which provides direct insight into the strength of the Fe-O2 δ bond and which, in turn, is correlated with the superoxo character of the iron-peroxo group. In oxidation reactions, reactivities of 1-3 toward hydrocarbon C-H bond activation are compared, revealing the reactivity order of 1 > 2 > 3; the [FeIII(O2)(n-TMC)]+ complex with a smaller n-TMC ring size, 12-TMC, is much more reactive than that with a larger n-TMC ring size, 14-TMC. DFT analysis shows that the Fe(III)-peroxo complex is not reactive toward C-H bonds, but it is the end-on Fe(II)-superoxo valence tautomer that is responsible for the observed reactivity. The hydrogen atom abstraction (HAA) reactivity of these intermediates is correlated with the overall donicity of the n-TMC ligand, which modulates the energy of the singly occupied π* superoxo frontier orbital that serves as the electron acceptor in the HAA reaction. The implications of these results for the mechanism of Rieske oxygenases are further discussed.

Metallophilic interactions and magnetism in heterobimetallic {Pt,M} lantern complexes

A small series of polymeric heterobimetallic {Pt,M} lantern complexes of the form [PtM(SAc)4DABCO]∞ (SAc = thioacetate, DABCO = 1,4-diazabicyclo[2,2,2]octane; M = Co (1), Ni (2), Zn (3)) has been synthesized, along with the discrete molecular analog of each polymer formed by replacing DABCO with quinuclidine (4–6). All compounds except 3 have been structurally characterized by single-crystal X-ray diffraction. Compound 1 shows a metallophilic Pt…Pt interaction of 3.1204(4) Å, while in 2 the same interaction is even shorter at 3.0944(10) Å. The electronic spectra of all compounds have been measured, which show d-d transitions of the 3d metal, Pt LMCT bands, and metal–metal charge transfer bands. Measurements of the temperature dependence of the magnetic susceptibility of 1 and 2 reveal antiferromagnetic exchange coupling between the 3d metal centers with J = −10 cm−1 for Co-containing 1 and J = −32 cm−1 between Ni centers for 2, both of which access the Pt…Pt metallophilic interactions.

Copper(II)-flavonolate complexes of 2N ligands as functional models for quercetin 2,4-dioxygenase enzymes: The role of axially coordinated water and ligand substitution on dioxygenase activity

Three new copper(II)-flavonolate complexes of the type [Cu(L)(fla)(ClO4)] 1–3, where L is the 2N ligand N-(pyridin-2-ylmethyl)aniline (L1), 2,6-dimethyl-N-(pyridin-2-ylmethyl)aniline (L2) and 2,6-diisopropyl-N-(pyridin-2-ylmethyl)aniline (L3), and H(fla) is 3-hydroxyflavonone, have been isolated as functional models for Cu(II)-containing quercetin 2,4-dioxygenase (2,4-QueD). Single crystal X-ray structure of 3 contains a CuN2O2Oʹ chromophore with a slightly distorted square pyramidal geometry (TBDSP, τ = 0.03). The pyridyl N1 and the tertiary amine N2 nitrogens of the 2N ligand occupy the two corners of the basal plane. The O1 (enolate) and the carbonyl O2 atoms of deprotonated 3-hydroxyflavanone (fla−) occupy the remaining corners while the O3 of perchlorate anion (ClO4−) occupies the axial position at a longer distance. The molecular structures of the complexes 1–3, their aquated species [Cu(L)(fla)(H2O)]+1a–3a and the square planar species [Cu(L)(fla)]+1c–3c have been computed by using DFT method. While 1–3 contain axially coordinated perchlorate 1a–3a contain axially coordinated water molecule with a slightly distorted square pyramidal geometry (CuN2O2Oʹ) with varying trigonality (τ, 0.03–0.05). The significantly high g|| (2.305) and low A|| (150 × 10−4 cm−1) values observed for 1–3 are consistent with π–back bonding of Cu(II) with the conjugated carbonyl group of coordinated fla− and are typical of type-II copper enzymes. The electrochemical reduction of CuII to CuI and oxidation of fla− occur upon dissociation of axially coordinated water molecule in 1a–3a formed in DMF solution to give 1c–3c. Upon exposure to dioxygen, all the Cu(II)-flavonolate adducts undergo oxygenative degradation in DMF solution, as seen from the disappearance of the LMCT band at 430 nm at 80 °C to produce CO and the corresponding acids. The rate of dioxygenation, kO2, of the complexes decrease in the order, 1 (3.31 ± 0.53) > 3 (2.59 ± 0.36) > 2 (1.52 ± 0.13 × 10−1 M−1 s−1), due to electron-donating substituents on the primary ligand.

Mononuclear nickel(ii)-flavonolate complexes of tetradentate tripodal 4N ligands as structural and functional models for quercetin 2,4-dioxygenase: structures, spectra, redox and dioxygenase activity.

Three new nickel(ii)-flavonolate complexes of the type [Ni(L)(fla)](ClO4) 1-3, where L is the tripodal 4N ligand tris(pyrid-2-ylmethyl)amine (tpa, L1) or (pyrid-2-ylmethyl)bis(6-methylpyrid-2-ylmethyl)amine (6-Me2-tpa, L2) or tris(N-Et-benzimidazol-2-ylmethyl)amine (Et-ntb, L3), have been isolated as functional models for Ni(ii)-containing quercetin 2,4-dioxygenase. Single crystal X-ray structures of 1 and 3 reveal that Ni(ii) is involved in π-back bonding with flavonolate (fla-), as evident from enhancement in C[double bond, length as m-dash]O bond length upon coordination [H(fla), 1.232(3); 1, 1.245(7); 3, 1.262(8) Å]. More asymmetric chelation of fla- in 3 than in 1 [Δd = (Ni-Ocarbonyl - Ni-Oenolate): 1, 0.126; 3, 0.182 Å] corresponds to lower π-delocalization in 3 with electron-releasing N-Et substituent. The optimized structures of 1-3 and their geometrical isomers have been computed by DFT methods. The HOMO and LUMO, both localized on Ni(ii)-bound fla-, are highly conjugated bonding π- and antibonding π*-orbitals respectively. They are located higher in energy than the Ni(ii)-based MOs (HOMO-1, dx2-y2; HOMO-2/6, dz2), revealing that the Ni(ii)-bound fla- rather than Ni(ii) would undergo oxidation upon exposure to dioxygen. The results of computational studies, in combination with spectral and electrochemical studies, support the involvement of redox-inactive Ni(ii) in π-back bonding with fla-, tuning the π-delocalization in fla- and hence its activation. Upon exposure to dioxygen, all the flavonolate adducts in DMF solution decompose to produce CO and depside, which then is hydrolyzed to give the corresponding acids at 70 °C. The highest rate of dioxygenase reactivity of 3 (kO2: 3 (29.10 ± 0.16) > 1 (16.67 ± 0.70) > 2 (1.81 ± 0.04 × 10-1 M-1 s-1)), determined by monitoring the disappearance of the LMCT band in the range 440-450 nm, is ascribed to the electron-releasing N-Et substituent on bzim ring, which decreases the π-delocalization in fla- and enhances its activation.

Effect of TBP on the Coordination Process of Eu(III) with T2EHDGA: A Luminescence Spectroscopy Investigation

AbstractIn this report, we explore the precise responsibility of a phase modifier along with the prime ligand in the coordination process of Ln(III) ions using luminescence spectroscopic technique by taking Eu(III) as the probeing metal ion. A well accepted diglyoclamide (DGA) ligand: N,N,N’,N’‐ tetra(ethylhexyl)diglycolamide(T2EHDGA) was used as the prime extractant along with tri‐n‐butylphosphate (TBP) as the phase modifier. Meticulous investigations on Eu(III) complexation with T2EHDGA‐TBP binary mixture was pursued by employing luminescence spectroscopy with the systematic variation of the concentration of TBP in the ligating phase and initial concentration of Eu(III) in the feed phase. The dominancy of the LMCT band over that of the metal centered band signifies the structural difference in the ligands used in the study. The presence of TBP in the organic phase enhances the intensity of 5D0→7F2 and 5D0→7F0 transition. The excitation and emission patterns of the Eu(III)‐solvate species formed in the organic phase both in unirraditaed and irradiated conditions were found to be entirely different, which effectively corroborates the efficacy of the extracting phase. The asymmetry ratio (AR) and the life time data at each experimental conditions were ascertained to realize the coordination behavior of the Eu(III) complex in the organic phase.

Lanthanide Luminescence and Thermochromic Emission from Soft-Atom Donor Dichalcogenoimidodiphosphinate Ligands

The luminescence properties of two divalent europium complexes of the type Eu[N(SPPh2)2]2(THF)2 (1) and Eu[N(SePPh2)2]2(THF)2 (2) were investigated. The first complex, Eu[N(SPPh2)2]2(THF)2 (1), was found to be isomorphous with the reported structure of complex 2 and exhibited room temperature luminescence with thermochromic emission upon cooling. We found the complex Eu[N(SePPh2)2]2(THF)2 (2) was also thermochromic but the emission intensity was sensitive to temperature. Both room temperature and low temperature (100 K) single crystal X-ray structural investigation of 1 and 2 indicate geometric distortions of the metal coordination, which may be important for understanding the thermochromic behavior of these complexes. The trivalent europium complex Eu[N(SPPh2)2]3 (3) with the same ligand as 1 was also structurally characterized as a function of temperature and exhibited temperature-dependent luminescence intensity, with no observable emission at room temperature but intense luminescence at 77 K. Variable temperature Raman spectroscopy was used to determine the onset temperature of luminescence of Eu[N(SPPh2)2]3 (3), where the 615 nm (5D0 → 7F2 transition) peak was quenched above 130 K. The UV-visible diffuse reflectance of 3 provides evidence of an LMCT band, supporting a mechanism of thermally activated LMCT quenching of Eu(III) emitting states. A series of ten isomorphous, trivalent lanthanide complexes of type Ln[N(SPPh2)2]3 (Ln = Eu (3) Pr (4), Nd (5), Sm (6), Gd (7), Tb (8)) and Ln[N(SePPh2)2]3 (Ln = Pr (9), Nd (10, structure was previously reported), Sm (11), and Gd (12) for Q = Se) were also synthesized and structurally characterized. These complexes for Ln = Pr, Nd, Sm, and Tb exhibited room temperature luminescence. This study provides examples of temperature-dependent luminescence of both Eu2+ and Eu3+, and the use of soft-atom donor ligands to sensitize lanthanide luminescence in a range of trivalent lanthanides, spanning near IR and visible emitters.

Copper complexes of strongly electron rich and deficient salen ligands

Three copper(II) salen compounds (Cu(SalR)) bearing strongly electron donating (Cu(SalNMe2)), electronically neutral (Cu(SaltBu)) and strongly electron withdrawing (Cu(SalNMe3)) para-substituents were prepared. Their geometric structures were evaluated by single crystal X-ray diffraction and supported by theoretical DFT calculations. The electronic spectra of Cu(SalR) revealed LMCT bands at 23 000 cm−1–27 000 cm−1 with varying para-substituents, while the d-d bands remain around 17 600 cm−1. The electrochemical behaviour of Cu(SalR), evaluated by cyclic voltammetry, reveal the expected positive shift in redox waves for more electron deficient compounds (Cu(SalNMe2) (CH2Cl2): −0.08, 0.03, 0.55, 0.64 V; Cu(SaltBu) (CH2Cl2): 0.59, 0.74 V; Cu(SalNMe3) (DMF): 0.78 V versus ferrocene/ferrocenium). The additional reversible oxidative couples observed for Cu(SalNMe2) are likely due to the strongly electron donating NMe2 moieties that either enable additional oxidation events to occur, and/or shifting any additional oxidation events within the experimental electrochemical window. One-electron oxidation of Cu(SalNMe2) and Cu(SaltBu) were monitored using low temperature UV–vis spectroscopy: characteristic spectra for metal oxidized [CuIII(SaltBu)]+ and ligand oxidized [CuII(SalNMe2)•]+ species were observed.

Photocatalytic oxidation of Mn(II) on the surface of Bi2.15WO6via the ligand-to-metal charge transfer (LMCT) pathway.

Biotic and abiotic oxidation of Mn(II) in aqueous environments is an important process for the cycling of many elements. However, the mechanism involved in photocatalytic oxidation of Mn(II) has not been clearly elucidated yet. In this study, the photocatalytic oxidation of Mn(II) on the surface of self-doped Bi2+xWO6 (Bi2.15WO6) under visible light was conducted. Kinetics results show that visible light apparently accelerates the oxidation of Mn(II) to Mn(III, IV) oxides on Bi2.15WO6. The average oxidation states (AOS) of manganese reach 2.18 after 80 min of reaction under visible light at pH 8.50. Characterizations indicate the formation of Bi(III)-O-Mn(II) surface complexes between Mn(II) and surface Bi(III) on Bi2.15WO6, which then decreases the bandgap of [Bi2.15WO6 + Mn(II)]light (2.53 eV) compared with those of [Bi2.15WO6 + Mn(II)]dark (2.72 eV) and pure Bi2.15WO6 (2.86 eV), suggesting the contribution of the ligand-to-metal charge transfer (LMCT) pathway to the photocatalytic oxidation of Mn(II). Moreover, the addition of inorganic oxidants with strong oxidizing capacities (such as Cr2O72-, NO3- or NO2-) significantly increases the oxidation rate of Mn(II), further verifying the contribution of the LMCT pathway to Mn(II) oxidation. We therefore suggest that the LMCT pathway is one of the important oxidation routes for Mn(II) oxidation on Bi2.15WO6 under visible light.

Copper(II) hydrazone complexes derived from (Z)-N′-{(2-hydroxynapthalen-1-yl}methylene)acetohydrazide: Synthesis, spectral characterization, electrochemical behaviour, density functional study, in vitro catalytic activity and molecular docking

In order to investigate the structure–activity relationship of complexes, three new copper(II) complexes with (Z)-N′-{(2-hydroxynapthalen-1-yl}methylene)acetohydrazide (HL), [(HL)Cu(µ-ClO4)Cu(L)(H2O)2] 1, [(HL)Cu(µ-SO4)Cu(HL)(H2O)2] 2 and [(HL)Cu(µ-bpm)Cu(HL)](ClO4)2·3H2O 3(where bmp = 2,2-Bipyrimidine) were synthesized.These synthesized complexes were characterized by Elemental analysis, FTIR, UV–vis and single-crystal X-ray diffraction techniques.The absorption bands obtained in the UV region (291–339 nm) are characteristic of the HL ligand. In these complexes, the LMCT bands were observed in the range of 403–410 nm and the d-d band at 670–709 nm. The X-ray diffraction analysis shows the development of supramolecular structures through varied non-covalent interactions viz., CH⋯π and π···π unit interactions. The careful Hirshfeld surface and 2D fingerprint plots yielded a comparative image of the mode of non-covalent interactions. The DFT calculations were carried out and also the results are in smart agreement with experimental results. X-band EPR spectral measurements are dispensed to demonstrate the magneticbehaviour of all complexes. The IC50values for SOD activity values for all complexes show that they are good models for antioxidant compounds. Molecular docking has been performed to predict the antibacterial properties of HL and binuclear complexes.

Mononuclear copper(II) complexes with (Z)-N′-{(2-hydroxynapthalen-1-yl}methylene)acetohydrazide: X-ray single-crystal structures, Hirshfeld analysis, X-band epr spectra, DFT calculations and SOD mimetic activity

The 1:1 reaction of (Z)-Ń-{(2-hydroxynapthalen-1-yl}methylene)acetohydrazide (HL) and copper(II) salt in methanol yields a series of new mononuclear complexes, [Cu(HL)(OH2)]ClO41, [Cu(HL)(OH2]ClO42, [Cu(HL)(OH2)]NO33 and [Cu(L)(Cl)] 4. The room temperature magnetic moments of the complexes 1–4 are 1.79, 1.78, 1.80 and 1.76B.M. respectively. The absorption bands emerging in the UV region (337–341 nm) are characteristic of the HL ligand. In these complexes, the LMCT bands were observed in the range of 503–506 nm and the d-d band at 617–623 nm. The molecular structures of all four complexes have been solved using single-crystal X-ray analysis. The X-ray analysis shows the formation of supramolecular structures through various non-covalent interactions viz., CH∙∙∙π and π···π intermolecular interactions. The detailed Hirshfeld surface and fingerprint plots yielded a comparative picture of the mode of non-covalent interactions. The DFT calculations were performed and the results are in good agreement with experimental results. X-band epr spectral measurements have been carried out to authenticate the paramagnetic behavior of all complexes. The stability of the copper(II) centre was examined using cyclic and differential pulse voltammetry. The IC50 and SOD activity values for all complexes reveal that they are good models among the best values reported for low molecular weight mononuclear copper(II) complexes.

Delocalization tunable by ligand substitution in [L2Al] n- complexes highlights a mechanism for strong electronic coupling.

Ligand-based mixed valent (MV) complexes of Al(iii) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I2P) have been prepared. The MV states containing EW groups are both assigned as Class II/III, and those with ED functional groups are Class III and Class II/III in the (I2P−)(I2P2−)Al and [(I2P2−)(I2P3−)Al]2− charge states, respectively. No abrupt changes in delocalization are observed with ED and EW groups and from this we infer that ligand and metal valence p-orbitals are well-matched in energy and the absence of LMCT and MLCT bands supports the delocalized electronic structures. The MV ligand charge states (I2P−)(I2P2−)Al and [(I2P2−)(I2P3−)Al]2− show intervalence charge transfer (IVCT) transitions in the regions 6850–7740 and 7410–9780 cm−1, respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I2P2−)(I2P3−)Al]2− complexes containing –PhNMe2 or –PhF5 substituents. Minor localization of charge in [(I2P2−)(I2P3−)Al]2− was observed when –PhOMe substituents are included.

Photochemistry of hexachloroosmate(IV) in ethanol.

The photochemistry of the OsIVCl62- complex in ethanol was studied by means of stationary photolysis, nanosecond laser flash photolysis, ultrafast pump-probe spectroscopy and quantum chemistry. The direction of the photochemical process was found to be wavelength-dependent. Irradiation in the region of the d-d and LMCT bands results in the photosolvation (with the wavelength-dependent quantum yield) and photoreduction of Os(iv) to Os(iii), correspondingly. The characteristic time of photosolvation is ca. 40 ps. Photoreduction occurs in the micro- and millisecond time domains via several Os(iii) intermediates. The nature of intermediates and the possible mechanisms of photoreduction are discussed. We believe that the lability of the photochemically produced Os(iv) and Os(iii) intermediates determines the synthetic potential of OsIVCl62- photochemistry.

Synthesis, characterization, reactivity, and catalytic studies of heterobimetallic vanadium(V) complexes containing hydrazone ligands

Six heterobimetallic alkali metal dioxidovanadium(V) coordination polymer complexes {[M6{VO(μ-O)}2(μ-OH)4(μ4-slox/nph)].n DMF}∞ where M = Na, K, and Cs; n = 1 for (1), 0 for (2)-(6) of two dihydrazone ligands, disalicylaldehydeoxaloyldihydrazone (H4slox) and bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone (H4nph) are reported. All the complexes have been characterized by various physicochemical techniques such as elemental analyses, molar conductance, IR, NMR, UV–vis, and cyclic voltammetry. The IR, 1HNMR, and 13CNMR spectral data suggest that the dihydrazones are coordinated through phenolate/naphtholate oxygen, enolate oxygen, and azine nitrogen atoms to the metal centres. The structure of complex {[Na6{VO(μ-O)}2(μ-OH)4(μ4-slox)].DMF}∞ (1) is also determined by single-crystal X-ray data, which revealed that the H4slox coordinated via all possible dative sites to metal centres as tetrabasic octadentate ligand. The vanadium metal centres adopted distorted square-pyramidal coordination geometries, and the sodium atoms are also in five coordination atmospheres. The electronic spectra of the complexes showed LMCT bands in addition to intra-ligand π → π* and n → π* transitions. As evident from the cyclic voltammetry, the complexes showed two metal-centred electron transfer reactions {[(VVVV(slox)2−/VVVIV(slox)3−] and [(VVVIV(slox)3−/VVVIV(slox)4−]}, in addition to the ligand centred electron transfer reactions. Further, bovine serum albumin (BSA interaction studies of the complexes {[Na6{VO(μ-O)}2(μ-OH)4(μ4-slox)].DMF}∞ (1) and [Na6{VO(μ-O)}2(μ-OH)4(μ4-nph)]∞ (4) revealed strong binding affinity. Moreover, the catalytic studies of the complexes (1) and (4) were found to be effective for the oxidation of alcohols into their corresponding aldehydes and ketones and bromination of some organic substrates in the presence of H2O2 as an oxidizing agent.

Valence tautomerism in catecholato cobalt Bis(phenolate) diamine complexes as models for Enzyme–substrate adducts of catechol dioxygenases

Mononuclear cobalt(III) complexes of 3,5-di-tert-butylcatecholato cobalt complex of the substituted bis(phenol) diamine ligands of H2LNEX (CoLNEX(DTBC) which X: BM, Bu and OB stands for 2-(tert-butyl)-4-methyl, 2,4-di-tert-butyl and 2-(tert-butyl)-4-methoxy phenols, respectively, and DTBC is an abbreviation for 2,4-di-tert-butylcatechol, as models for enzyme-substrate adducts of catechol dioxygenases, have been synthesized and characterized by different techniques. X-ray structure analysis has revealed that cobalt centers were surrounded by two phenolates, two amine nitrogens and two oxygen atoms of catechol ligand. For all three compounds, the magnetic susceptibility increases with decreasing temperature as expected for non-interacting, magnetic moments which confirm the monomeric structure of cobalt complexes and valence tautomerism between Co(II) and Co(III) oxidation state. Crystal structures confirm the nature of Co(II)/Co(III) character consistent with the magnetic experiments. In electronic absorption spectra of two complexes, the position of LMCT bands show the phenolate(π)-to- Co(II) or Co(III) (dπ*) charge transfer. The cyclic voltammetry studies, show the ligand-centered oxidation of phenolate group to phenoxyl radical.

Synthesis, DNA binding and in vitro cytotoxicity studies of a mononuclear copper(II) complex containing N2S(thiolate)Cu core and 1,10-phenanthroline as a coligand

We report the synthesis, characterization, spectroscopic properties, redox behavior and biological activities of the Cu(II) complex [Cu(pabt)(o-phen)](ClO4) (1), (Hpabt = N-(2-mercaptophenyl)-2′-pyridylmethylenimine, o-phen = 1,10-phenanthroline). The intense purple solution of 1 exhibits electronic spectrum with a strong LMCT band in the visible region mainly associated with S → Cu(II). It displays a four-line EPR multiplet due to the interaction of the unpaired electron with the central 63/65Cu nucleus (I = 3/2) with the Aiso value of 80 ± 1.5 G at RT suggesting its monomeric nature in solution. It shows irreversible electrochemical behavior suggesting the instability of the reduced Cu(I) species. It shows catechol oxidase activity and strong intercalative DNA binding as revealed from absorption, emission spectral and viscometric studies. It exhibits strong cytotoxicity against human lung cancer A549 and epidermoid carcinoma A431 cell lines as revealed from the MTT assay. The respective IC50 values are: 5.26 μM for A549 and 5.41 μM for A431. The compound is found to be less toxic for the L132 human lung embryonic normal cell line and least toxic for the human keratinocyte HaCaT normal cells. AO/PI dual staining assay with A549 and L132 cells suggests the compound induced cell death via apoptosis in A549 cells.

A Series of 4- and 5-Coordinate Ni(II) Complexes: Synthesis, Characterization, Spectroscopic, and DFT Studies.

A series of four- and five-coordinate Ni(II) complexes Cz tBu(Pyr iPr)2NiX (1-3 and 1·THF-3·THF), where X = Cl, Br, and I, were synthesized and fully characterized by NMR and UV-vis spectroscopy, X-ray crystallography, cyclic voltammetry, and density functional theory calculations. The solid-state structures of 1-3 reveal rare examples of seesaw Ni(II) complexes. In solution, 1-3 bind reversibly to a THF molecule to form five-coordinate adducts. The electronic transitions in the visible region (630-680 nm), attributed to LMCT bands, for 1 → 3 exhibit a bathochromic shift. The thermochromic tendency of the five-coordinate complexes implies the loss of THF coordination at elevated temperatures. Finally, the electronic properties of all Ni(II) complexes were studied by time-dependent density functional theory calculations to characterize the nature of the excited states.

Mössbauer Spectroscopic Characterization of Iron(III)-Polysaccharide Coordination Complexes: Photochemistry, Biological, and Photoresponsive Materials Implications.

While polycarboxylates and hydroxyl-acid complexes have long been known to be photoactive, simple carboxylate complexes which lack a significant LMCT band are not typically strongly photoactive. Hence, it was somewhat surprising that a series of reports demonstrated that materials synthesized from iron(III) and polysaccharides such as alginate (poly[guluronan-co-mannuronan]) or pectate (poly[galacturonan]) formed photoresponsive materials that convert from hydrogels to sols under the influence of visible light. These materials have numerous potential applications in areas such as photopatternable materials, materials for controlled drug delivery, and tissue engineering. Despite the near-identity of the functional units in the polysaccharide ligands, the reactivity of iron(III) hydrogels can depend on the configuration of some chiral centers in the sugar units and in the case of alginate the guluronate to mannuronate block composition, as well as pH. Here, using temperature- and field-dependent transmission Mössbauer spectroscopy, we show that the dominant iron compound detected for both the alginate and pectate gels displays features typical of a polymeric (Fe3+O6) system. The Mössbauer spectra of such systems are strongly dependent on temperature, field, size, and crystallinity, indicative of superparamagnetic relaxation of magnetically ordered nanoparticles. Pectate and alginate hydrogels differ in the size distribution of the iron oxyhydroxy nanoparticles, suggesting that in general smaller nanoparticles are more reactive. Potential biological implications of these results are also discussed.

Charge Transfer Properties of Triarylamine Integrated Dimolybdenum Dyads.

Three quadruply bonded dimolybdenum complexes equipped with a triarylamine pendant, [(DAniF)3Mo2(μ-O2CC6H4N(C6H4CH3)2] (DAniF = N,N'-di(p-anisyl)formamidinate; [OO-ph-N]), [(DAniF)3Mo2(μ-OSCC6H4N(C6H4CH3)2] ([OS-ph-N]), and [(DAniF)3Mo2(μ-S2CC6H4N(C6H4CH3)2] ([SS-ph-N]), have been synthesized and characterized by single crystal X-ray diffraction. In electrochemical measurements, the redox couple for the organic amine group becomes irreversible, reflecting the substantially strong electronic interaction between the dimetal center and organic redox site. The potential difference for the two successive redox events, ca. ΔE1/2(E1/2(2)(N/N•+) - E1/2(1)(Mo2IV/V)), falls in the range of 0.5-0.8 V as estimated from the differential pulse voltammograms. For the monocation radicals [OO-ph-N]+, [OS-ph-N]+, and [SS-ph-N]+, obtained by chemical oxidation of the neutral precursor, a broad ligand (amine) to metal (Mo2) charge transfer (LMCT) absorption band is observed in the near-IR region. Interestingly, analogous to the intervalence charge transfer (IVCT) bands for mixed-valence complexes, the LMCT absorption bands, which are solvent dependent, decrease in energy and bandwidth as the electronic coupling between the two redox sites increases in an order of increasing S content in the chelating group. The electronic coupling matrix elements (Hab) are determined by optical analyses from the generalized Mulliken-Hush (GMH) theory, falling in the range of 400-800 cm-1 in CH2Cl2. These results indicate that in these radical cations the charge is localized. Time-dependent DFT calculations show that the frontier molecular orbitals for these asymmetrical donor-acceptor systems have unbalanced distribution of electron density, and the LMCT bands arise from an electronic transition from the pendant ligand-based to metal-based molecular orbitals, corresponding to donor (N)-acceptor (Mo2) charge transfer.

A study of asymmetrical mixed-valent Mo2-Mo2 complexes in the class III regime.

Three novel asymmetrical dimolybdenum dimers, [Mo2(DAniF)3]2(μ-OOCCOS) (DAniF = N,N'-di(p-anisyl)formamidinate) ([OO-OS]), [Mo2(DAniF)3]2(μ-S2CCO2) ([SS-OO]), and [Mo2(DAniF)3]2(μ-SSCCOS) ([SS-OS]), have been synthesized and characterized by either single-crystal X-ray crystallography or 1H NMR spectroscopy. The structural asymmetry for these compounds gives rise to a redox asymmetry, which enlarges the potential separation (ΔE1/2) between the two [Mo2] units. The mixed-valance (MV) species [OO-OS]+, [SS-OO]+ and [SS-OS]+, prepared by one-electron chemical oxidation of the neutral precursors, exhibit an intense and symmetrical intervalence charge transfer (IVCT) absorption band in the near-IR region, along with the high energy metal (δ) to ligand (π*) (ML) and ligand (π) to metal (δ) charge transfer (LMCT) absorptions. The LMCT band, which is absent in the neutral precursors, is reflective of the cationic [Mo2]+ unit in the MV species; therefore, it is evidenced that in the MV complexes optical electron transfer from the electron donor to acceptor occurs, while the thermal process is energetically unfavorable. The C(1)-C(2) bonds (1.44-1.48 Å) that connect the two [Mo2] units are significantly shorter than a C-C single bond, showing that the two Mo2 centers are strongly coupled. For the series, TD-DFT calculations show that the molecular orbitals have an unsymmetrical charge density distribution over the two dimolybdenum sites. For each of the complex systems, the calculated orbital energy gaps, SOMO(δ - δ)-LUMO(bridging ligand π*), HOMO-8(bridging ligand π)-SOMO(δ - δ) and SOMO(δ - δ)-HOMO-1(δ + δ), are in good agreement with the observed MLCT, LMCT and IVCT absorption band energies, respectively. The consistency in energy between the IVCT band and the SOMO(δ - δ)-HOMO-1(δ + δ) gap permits assignment of the MV complexes to Class III in the Robin-Day scheme.