Advances on the interaction of polypyridyl Cr(III) complexes with transporting proteins and its potential relevance in photodynamic therapy

The present study reports a detailed investigation into the interaction of [Cr(phen)₂(dppz)]³⁺ and [Cr(phen)₃]³⁺ with transferrin, the key protein for the transport of Fe³⁺ in blood plasma; its cycle holds promise as an attractive system for strategies of drug targeting to tumor tissues. This can allow us to understand further the role of both complexes as sensitizers in photodynamic therapy (PDT). Chromium(III) complexes, [Cr(phen)₂(dppz)]³⁺ and [Cr(phen)₃]³⁺, (phen = 1,10-phenanthroline and dppz = dipyridophenazine), where dppz is a planar bidentate ligand with an extended π system, have been found to bind strongly with apotransferrin (apoTf) with an intrinsic binding constant, K_b, of (1.8 ± 0.3) × 10⁵ M^− 1 and (1.1 ± 0.1) × 10⁵ M^− 1 at 299 K, for apoTf-[Cr(phen)₂(dppz)]³⁺ and apoTf-[Cr(phen)₃]³⁺, respectively. The interactions of apoTf with the different Cr(III) complexes were assessed employing UV-visible absorption, fluorescence and circular dichroism spectroscopy. The relative fluorescence intensity of the protein decreased when the increasing concentration of Cr(III) complex was added, suggesting that perturbation around the Trp and Tyr residues took place. The analysis of the thermodynamic parameters ΔG, ΔH, ΔS indicated that the presence of the Cr(III) complex stabilizes the protein with a strong entropic contribution. The binding distances and transfer efficiencies for apoTf-[Cr(phen)₂(dppz)]³⁺ and apoTf-[Cr(phen)₃]³⁺ binding reactions were calculated according to Föster theory of non-radiation energy transfer. All these experimental results suggest that [Cr(phen)₂(dppz)]³⁺ and [Cr(phen)₃]³⁺ bind strongly to apoTf indicating that this protein could act as a carrier of these complexes for further applications in PDT.     

New insights in the DNA-[Cr(phen)2(dppz)] binding and photocleavage properties by the complex with an intercalating ligand

Due to the key role of DNA in cell life and pathological processes, the design of specific chemical nucleases, DNA probes and alkylating agents is an important research area for the development of new therapeutic agents and tools in Biochemistry. Hence, the interaction of small molecules with DNA has attracted in particular a great deal of attention.The aim of this study was to investigate the ability of [Cr(phen)₂(dppz)]³⁺ to associate with DNA and to characterize it as photocleavage reagent for Photodynamic Therapy (PDT).Chromium(III) complex [Cr(phen)₂(dppz)]³⁺, (dppz = dipyridophenazine, phen = 1,10-phenanthroline), where dppz is a planar bidentate ligand with an extended π system, has been found to bind strongly to double strand oligonucleotides (ds-oligo) and plasmid DNA with intrinsic DNA binding constants, K_b, of (3.9 ± 0.3) × 10⁵ M⁻¹ and (1.1 ± 0.1) × 10⁵ M⁻¹, respectively. The binding properties to DNA were investigated by UV-visible (UV-Vis) absorption spectroscopy and electrophoretic studies. UV-Vis absorption data provide clearly that the chromium(III) complex interacts with DNA intercalatively. Competitive binding experiments show that the enhancement in the emission intensity of ethidium bromide (EthBr) in the presence of DNA was quenched by [Cr(phen)₂(dppz)]³⁺, indicating that the Cr(III) complex displaces EthBr from its binding site in plasmid DNA. Moreover, [Cr(phen)₂(dppz)]³⁺, non-covalently bound to DNA, promotes the photocleavage of plasmid DNA under 457 nm irradiation. We also found that the irradiated Cr(III)-plasmid DNA association is able to impair the transforming capacity of bacteria. These results provide evidence confirming the responsible and essential role of the excited state of [Cr(phen)₂(dppz)]³⁺ for damaging the DNA structure. The combination of DNA, [Cr(phen)₂(dppz)]³⁺ and light, is necessary to induce damage. In addition, assays of the photosensitization of transformed bacterial suspensions suggest that Escherichia coli may be photoinactivated by irradiation in the presence of [Cr(phen)₂(dppz)]³⁺. In sum, our results allow us to postulate the [Cr(phen)₂(dppz)]³⁺ complex as a very attractive candidate for DNA photocleavage with potential applications in Photodynamic Therapy (PDT).     

Synthesis and characterization of tris(heteroleptic) diimine complexes of chromium(III)

A preparative procedure of potentially wide applicability is described for the synthesis of previously unreported tris(heteroleptic) [Cr(diimine)₃]³⁺ complexes. The synthetic scheme involves the sequential addition of three different diimine ligands, and employs CrCl₃ · 6H₂O as the initial Cr(III) reagent. The synthesis and characterization of the complexes [Cr(TMP)(phen)(diimine′)]³⁺ are reported (where TMP = 3,4,7,8-tetramethyl-1,10-phenanthroline, phen = 1,10-phenanthroline; and diimine′ is either bpy = 2,2′-bipyridine, Me₂bpy = 4,4′-dimethyl-2,2′-bipyridine, 5-Clphen = 5-chloro-1,10-phenanthroline, or DPPZ = dipyridophenazine). Chiral capillary electrophoresis and electrospray mass spectrometry were essential aids in determining the presence or absence of diimine ligand scrambling. Utilizing emission and electrochemical data obtained on these compounds, the oxidizing power of the lowest lying excited state (²E_g(O_h)) was calculated, and was found to vary in a systematic fashion with diimine ligand type.     

New advances in the study on the interaction of [Cr(phen)2(dppz)]3+ complex with biological models; association to transporting proteins

The present study reports a detailed investigation with the interaction of [Cr(phen)₂(dppz)]³⁺ with serum albumins, the key protein for the transport of drugs in the blood plasma, which allows us to understand further the role of [Cr(phen)₂(dppz)]³⁺ as sensitizer in Photodynamic Therapy (PDT).Chromium(III) complex [Cr(phen)₂(dppz)]³⁺, (dppz = dipyridophenazine and phen = 1,10-phenanthroline), where dppz is a planar bidentate ligand with an extended π system, has been found to bind strongly with bovine and human serum albumins (BSA and HSA) with an intrinsic binding constants, K_b, of (1.7 ± 0.3) × 10⁵ M^− 1 and (2.2 ± 0.3) × 10⁵ M^− 1 at 295 K, respectively. The interactions of serum albumins with [Cr(phen)₂(dppz)]³⁺ were assessed employing fluorescence spectroscopy, circular dichroism and UV-vis absorption spectroscopy. The serum albumins-[Cr(phen)₂(dppz)]³⁺ interactions caused conformational changes with the loss of helical stability of the protein and local perturbation in the domain IIA binding pocket. The relative fluorescence intensity of the albumin (BSA or HSA) bound to the Cr(III) complex decreased, suggesting that perturbation around the Trp 214 residue took place. The analysis of the thermodynamic parameters ΔG, ΔH, ΔS indicated that the hydrophobic interactions played a major role in both BSA-Cr(III) and HSA-Cr(III) association processes. The binding distances and transfer efficiencies for BSA-Cr(III) and HSA-Cr(III) binding reactions were calculated according to the Föster theory of non-radiation energy transfer. All these experimental results suggests that [Cr(phen)₂(dppz)]³⁺ binds to serum albumins, by which these proteins could act as carriers of this complex for further applications in PDT.     

DNA photocleavage activity of cobalt(III) polypyridyl complexes containing dpq ligand

Four cobalt(III) polypyridyl complexes, [Co(phen)_3−n(dpq)_n]³⁺ (phen = 1,10-phenanthroline, dpq = dipyrido[3,2-f:2′,3′-h]-quinoxaline) (n = 0, 1, 2, and 3) were synthesized and the influences of the dpq ligand on the photophysical properties, electrochemical properties, DNA binding affinities, as well as photonuclease activities of the complexes, were examined in detail. The presence of dpq ligand increases the DNA binding affinities of the corresponding complexes remarkably with respect to [Co(phen)₃]³⁺. With the sequential substitution of phen ligand by dpq ligand, the ¹O₂ quantum yields of the corresponding complexes are enhanced greatly. As a result, the photonuclease activities follow the order of [Co(dpq)₃]³⁺ > [Co(phen)(dpq)₂]³⁺ > [Co(phen)₂(dpq)]³⁺ ? [Co(phen)₃]³⁺. It was found all the examined complexes can generate OH upon UV irradiation, and OH is also involved in DNA photocleavage as reactive oxygen species.     

Syntheses, characterization, and DFT investigation of new mononuclear acetonitrile- and chloro-ruthenium(II) terpyridine complexes

A series of mononuclear acetonitrile complexes of the type [Ru(CH₃CN)(L)(terpy)]²⁺ {L = phen (1), dpbpy (3), and bpm (5)}, and their reference complexes [RuCl(L)(terpy)]⁺ {L = phen (2), dpbpy (4), and dpphen (6)} were prepared and characterized by electrospray ionization mass spectrometry, UV-vis spectroscopy, and cyclic voltammograms (CV). Abbreviations of the ligands (Ls) are phen = 1,10-phenanthroline, dpbpy = 4,4′-diphenyl-2,2′-bipyridine, bpm = 2,2′-bipyrimidine, dpphen = 4,7-diphenyl-1,10-phenanthroline, bpy = 2,2′-bipyridine, and terpy = 2,2′:6′,2″-terpyridine. The X-ray structures of the two complexes 2 and 3 were newly obtained. The metal-to-ligand charge transfer (MLCT) bands in the visible region for 1, 3, and 5 in acetonitrile were blue shifted relative to those of the reference complexes [RuCl(L)(terpy)]⁺. CV for all the [Ru(CH₃CN)(L)(terpy)]²⁺ complexes showed the first oxidation wave at around 0.95 V, being more positive than those of [RuCl(L)(terpy)]⁺. The time-dependent-density-functional-theory approach (TDDFT) was used to interpret the absorption spectra of 1 and 2. Good agreement between computed and experimental absorption spectra was obtained. The DFT approach also revealed the orbital interactions between Ru(phen)(terpy) and CH₃CN or Cl⁻. It is demonstrated that the HOMO-LUMO energy gap of the acetonitrile ligand is larger than that of the Cl⁻ one.     

The relationship between the structures of periphery ligands and the DNA binding mode of [Ru(II)(1,10-phenanthroline)(L1L2)dipyrido[3,2-a:2',3'-c]phenazine](n+) (L1=Cl or pyridine and L2=pyridine, n=1,2)

The binding modes of the [Ru(II)(1,10-phenanthroline)(L₁L₂) dipyrido[3,2-a:2′,3′-c]phenazine]²⁺ {[Ru(phen)(py) Cl dppz]⁺ (L₁ = Cl, L₂ = pyridine) and ([Ru(phen)(py)₂dppz]²⁺ (L₁ = L₂ = pyridine)} to native DNA is compared to that of the [Ru(II)(1,10-phenanthroline)₂dipyrido[3,2-a:2′,3′-c]phenazine]²⁺ complex ([Ru(phen)₂dppz]²⁺) by various spectroscopic and hydrodynamic methods including electric absorption, linear dichroism (LD), fluorescence spectroscopy, and viscometric titration. All measured properties, including red-shift and hypochromism in the dppz absorption band, nearly perpendicular molecular plane of the dppz ligand with respect to the local DNA helix axis, prohibition of the ethidium binding, the light switch effect and binding stoichiometry, increase in the viscosity upon binding to DNA, increase in the melting temperature are in agreement with classical intercalation of dppz ligand of the [Ru(phen)₂dppz]²⁺ complex, in which both phenanthroline ligand anchored to the DNA phosphate groups by electrostatic interaction. [Ru(phen)(py)₂ dppz]²⁺ and [Ru(phen)(py) Cl dppz]⁺ complexes had one of the phenanthroline ligand replaced by either two pyridine ligands or one pyridine plus a chlorine ion. They exhibited similar protection from water molecules, interaction with DNA bases, and occupying site that is common with ethidium. The dppz ligand of these two Ru(II) complex were greatly tilted relative to the DNA helix axis, suggesting that the dppz ligand resides inside the DNA and is not perpendicular relative to the DNA helix axis. These observation suggest that anchoring the [Ru(phen)₂dppz]²⁺complex by both phenanthroline is essential for the dppz ligand to be classically intercalated between DNA base-pairs.     

Syntheses, structural and spectroscopic investigation (IR, NMR and luminescence) of new terbium and europium acylpyrazolonates

The synthesis and characterization of new lanthanide complexes of formulae [M(Q)₃(H₂O)(EtOH)], NBu₄[M(Q)₄] and [M(Q)₃(L)] (M = Eu or Tb; HQ = 1-phenyl-3-methyl-4-R-pyrazol-5-one: R = cyclopentylcarbonyl, HQ = HQ^CP; R = cyclopentylpropionyl, HQ = HQ^EtCP; L = 1,10-phenanthroline (phen) or 4,7-diphenyl-1,10-phenanthroline (bathophen)) are reported. The crystal structure of the tetrakis (β-diketonate) complex [NBu₄][Eu(Q^ETCP)₄] containing an eight-coordinated Eu atom in a distorted square antiprismatic environment has been determined. Luminescence studies have been performed on selected derivatives: the data suggested a strong influence of the nature of the acyl moiety in Q ligands and of Ph groups in bathophen (with respect to phen) on the luminescence properties.     

Probing inter-ligand excited state interaction in homo and heteroleptic ruthenium(II) polypyridyl complexes using selective deuteriation

The effect of deuteriation on the photophysical properties of two series of regioselectively deuteriated Ru(II) complexes ([Ru(bipy)_x(ph₂phen)_3−x]²⁺, where x = 0-3 and ph₂phen is 4,7-diphenyl-1,10-phenanthroline and [Ru(bipy)₂(dcbipy²⁻)], where H₂dcbipy is 4,4′-dicarboxy-2,2′-bipyridyl) is reported. Although overall, deuteriation results in an increase in emission lifetime for all complexes, the effect of substitution of hydrogen for deuterium shows strong regioselectivity both in terms of the ligand and the position on individual ligands that are exchanged.     

Praseodymium (III) complexes with 1,10-phenanthroline and cyanamide derivatives as N-donor ligands

The molecular structure of praseodymium (III) complex with 1,10-phenanthroline (phen), [Pr(phen)₂Cl₃·OH₂] (1) was determined by single-crystal X-ray diffraction. Crystal data: crystal system, triclinic, space group P and Z = 2, a = 7.1110(7) ?, b = 10.1716(10) ?, c = 17.2367(18) ?, α = 80.922(5)°, β = 78.759(5)°, γ = 70.151(5)°, R₁ = 0.036; wR₂ = 0.076 for all data. Treatment of aqueous solution of [Pr(phen)₂Cl₃·OH₂] (1) with thallium phenylcyanamide salts yield [Pr(phen)₂(L)₃] (L = pcyd (2), 2-Clpcyd (3), 2,3,5-Cl₃pcyd (4), 2,3,4,5-Cl₄pcyd (5)). Four new praseodymium (III) complexes have been characterized by IR, UV-Vis and ¹H NMR spectroscopy as well as elemental analysis. The ¹H NMR spectra of these complexes show broadening of ligand protons attributed to coordination of paramagnetic center.     

Structural and equilibrium studies on mixed ligand complexes of Co(II), Ni(II), Cu(II) and Zn(II) with N-(2-benzimidazolyl)methyliminodiacetic acid and typical N, N donor ligands

Mixed ligand complexes: [Co(L)(bipy)] · 3H₂O (1), [Ni(L)(phen)] · H₂O (2), [Cu(L)(phen)] · 3H₂O (3) and [Zn(L)(bipy)] · 3H₂O (4), where L²⁻ = two -COOH deprotonated dianion of N-(2-benzimidazolyl)methyliminodiacetic acid (H₂bzimida, hereafter, H₂L), bipy = 2,2′ bipyridine and phen = 1,10-phenanthroline have been isolated and characterized by elemental analysis, spectral and magnetic measurements and thermal studies. Single crystal X-ray diffraction studies show octahedral geometry for 1, 2 and 4 and square pyramidal geometry for 3. Equilibrium studies in aqueous solution (ionic strength I = 10⁻¹ mol dm⁻³ (NaNO₃), at 25 ± 1 °C) using different molar proportions of M(II):H₂L:B, where M = Co, Ni, Cu and Zn and B = phen, bipy and en (ethylene diamine), however, provides evidence of formation of mononuclear and binuclear binary and mixed ligand complexes: M(L), M(H₋₁L)⁻, M(B)²⁺, M(L)(B), M(H₋₁L)(B)⁻, M₂(H₋₁L)(OH), (B)M(H₋₁L)M(B)⁺, where H₋₁L³⁻ represents two -COOH and the benzimidazole N₁-H deprotonated quadridentate (O⁻, N, O⁻, N), or, quinquedentate (O⁻, N, O⁻, N, N⁻) function of the coordinated ligand H₂L. Binuclear mixed ligand complex formation equilibria: M(L)(B) + M(B)²⁺ ? (B)M(H₋₁L)M(B)⁺ + H⁺ is favoured with higher π-acidity of the B ligands. For Co(II), Ni(II) and Cu(II), these equilibria are accompanied by blue shift of the electronic absorption maxima of M(II) ions, as a negatively charged bridging benzimidazolate moiety provides stronger ligand field than a neutral one. Solution stability of the mixed ligand complexes are in the expected order: Co(II) < Ni(II) < Cu(II) > Zn(II). The Δ log K_M values are less negetive than their statistical values, indicating favoured formation of the mixed ligand complexes over the binary ones.     

A new synthetic route towards heterotrimetallic complexes. Synthesis, crystal structure and magnetic properties of a [CuMnCr] trinuclear complex

The heterotrimetallic complex, [{LCuMn(H₂O)}{Cr(phen)(C₂O₄)₂}](ClO₄) · H₂O (1), has been obtained by assembling heterobinuclear cations, [LCuMn]²⁺, with [Cr(phen)(C₂O₄)₂]⁻ ions (H₂L is the compartmental Schiff-base resulting from the stepwise condensation of 2,6-diformyl-p-cresol with ethylenediamine and diethylenetriamine). The copper(II) and manganese(II) ions are hosted into the compartments of the macrocyclic ligand. [Cr(phen)(C₂O₄)₂]⁻ acts as a ligand, being coordinated through one oxalato oxygen atom to the apical position of the square pyramidal copper(II) ion. The cryomagnetic investigation of 1 reveals an antiferromagnetic interaction between Cu^II and Mn^II within the compartmental ligand (J = −39 cm⁻¹). The interaction between Cu^II and Cr^III across the oxalato bridge is negligible. The crystal structure of [LCuPb](ClO₄)₂ · H₂O, a useful precursor in obtaining 3d-3d′ complexes, is also reported.     

A theoretical study of Ru(II) polypyridyl DNA intercalators: Structure and electronic absorption spectroscopy of [Ru(phen)2(dppz)] and [Ru(tap)2(dppz)] complexes intercalated in guanine-cytosine base pairs

The structural and spectroscopic properties of [Ru(phen)₂(dppz)]²⁺ and [Ru(tap)₂(dppz)]²⁺ (phen = 1,10-phenanthroline; tap = 1,4,5,8-tetraazaphenanthrene; dppz = dipyridophenazine ) have been investigated by means of density functional theory (DFT), time-dependent DFT (TD-DFT) within the polarized continuum model (IEF-PCM) and quantum mechanics/molecular mechanics (QM/MM) calculations. The model of the Δ and Λ enantiomers of Ru(II) intercalated in DNA in the minor and major grooves is limited to the metal complexes intercalated in two guanine-cytosine base pairs. The main experimental spectral features of these complexes reported in DNA or synthetic polynucleotides are better reproduced by the theoretical absorption spectra of the Δ enantiomers regardless of intercalation mode (major or minor groove). This is especially true for [Ru(phen)₂(dppz)]²⁺. The visible absorption of [Ru(tap)₂(dppz)]²⁺ is governed by the MLCT_tap transitions regardless of the environment (water, acetonitrile or bases pair), the visible absorption of [Ru(phen)₂(dppz)]²⁺ is characterized by transitions to metal-to-ligand-charge-transfer MLCT_dppz in water and acetonitrile and to MLCT_phen when intercalated in DNA. The response of the IL_dppz state to the environment is very sensitive. In vacuum, water and acetonitrile these transitions are characterized by significant oscillator strengths and their positions depend significantly on the medium with blue shifts of about 80 nm when going from vacuum to solvent. When the complex is intercalated in the guanine-cytosine base pairs the ¹IL_dppz transition contributes mainly to the band at 370 nm observed in the spectrum of [Ru(phen)₂(dppz)]²⁺ and to the band at 362 nm observed in the spectrum of [Ru(tap)₂(dppz)]²⁺.     

Structural, spectroscopic and redox studies of a new ruthenium(III) complex with an imidazole-rich tripodal ligand

The present paper describes a new tripodal ligand containing imidazole and pyridine arms and its first cis-[Ru^III(L)(Cl)₂]ClO₄ complex (1). The crystal structure of 1 shows Ru^III in a distorted octahedral geometry, in which two chloride ions, cis-positioned to each other, are coordinated besides the four nitrogen atoms from the tetradentate ligand L. The cyclic voltammogram of 1 exhibits three redox processes at −67, +73 and +200 mV versus SCE, which are attributed to the Ru^III/Ru^II couple in the cis-[Ru^III(L)(Cl)₂]⁺, cis-[Ru^II(L)(H₂O)(Cl)]⁺ and cis-[Ru^II(L)(H₂O)₂]²⁺, respectively. After chemical reduction (Zn(Hg) or Eu^II) only the cis-[Ru^II(L)(H₂O)₂]²⁺ species is observed in the cyclic voltammetry. Complex 1 absorbs at 470 nm (ε=1.4×10³ mol⁻¹ L cm⁻¹), 335 nm (ε=7.9×10³ mol⁻¹ L cm⁻¹), 301 nm (ε=6.7×10³ mol⁻¹ L cm⁻¹) and 264 nm (ε=9.9×10³ mol⁻¹ L cm⁻¹), in water solution (CF₃COOH, 0.01 mol L⁻¹, μ=0.1 mol L⁻¹ with CF₃COONa). Spectroelectrochemical experiments show a decrease of the bands at 335 and 301 nm, which are attributed to LMCT transitions from the chloride to the Ru^III center and the appearance of a broad band at 402 nm ascribed to MLCT transition from the Ru^II center to the pyridine ligand. The lability of the water ligands in the cis-[Ru^II(L)(H₂O)₂]²⁺ species has been investigated using the auxiliary ligand pyrazine. Reactions in the presence of stoichiometric and excess of pyrazine yield the same species, cis-[Ru^II(L)(H₂O)(pz)]²⁺, which exhibits a reversible redox process at 493 mV versus SCE and absorbs at 438 nm (ε=5.1×10³ mol⁻¹ L cm⁻¹) and 394 nm (ε=4.2×10³ mol⁻¹ L cm⁻¹). Experiments performed with a large excess of pyrazine gave a specific rate constant k₁=(2.8±0.5)×10⁻² M⁻¹ s⁻¹, at 25 °C, in CF₃COOH, 0.01 mol L⁻¹, μ=0.1 mol L⁻¹ (with CF₃COONa).     

Structure and solution behaviour of cyclooctadiene complexes of platinum(II)

The substitution behaviour of [PtCl(R)(COD)] (R⁻ = Me and Fc) complexes, by the stepwise addition of phosphine ligands, L (L = PPh₃, PEt₃ and P(NMe₂)₃), were investigated in situ by ¹H and ³¹P NMR spectroscopy. Addition of less than two equivalents of the phosphine ligand results in the formation of dimeric molecules with the general formula trans-[Pt(R)(μ-Cl)(L)]₂ for the sterically demanding systems where R⁻ = Me/L = P(NMe₂)₃ and R⁻ = Fc/L = PEt₃, PPh₃ and P(NMe₂)₃ while larger quantities resulted in cis- and trans mixtures of mononuclear complexes being formed. In the case of the relatively small steric demanding, strongly coordinating, PEt₃ ligand the trans-[PtCl(R)(PEt₃)₂] mononuclear complexes were exclusively observed in both cases. The crystal structures of the two substrates, [PtCl(R)(COD)] (R⁻ = Me or Fc), as well as the cis-[PtCl(Fc)(PPh₃)₂] substitution product are reported.     

Synthesis, X-ray crystal structure, DFT- and TDDFT-based spectroscopic property investigations on a mixed-ligand chromium(III) complex, bis[2-(2-hydroxyphenyl)benzimidazolato]-(1,10-phenanthroline)chromium(III) isophthalate

A mixed-ligand Cr(III) complex with 2-(2-hydroxyphenyl)benzimidazole, 1,10-phenanthroline and isophthalic acid, [Cr(pbm)₂(phen)]X_0.5 (1X_0.5) (Hpbm = 2-(2-hydroxyphenyl)benzimidazole; phen = 1,10-phenanthroline; H₂X = isophthalic acid) has been prepared by heating in aqueous solution and characterized, and the geometric structure and spectroscopic properties, investigated experimentally and theoretically by using the density functional theory level (DFT) and the time-dependent density functional theory level (TDDFT). The theoretical-experimental agreement is satisfactory. Further theoretical analyses of electronic structure and molecular orbitals have demonstrated that the low-lying absorption bands in UV-Vis spectrum are mainly π → π∗ ligand-to-ligand charge transfer transition (LLCT) and or π → (d_z²-d_x²-y²-d_yz) ligand-to-metal charge transfer transition (LMCT) in nature.     

Synthesis and characterization of Os(II)(dpop′) (dpop′ = dipyrido(2,3-a;3′,2′-j)phenazine) complexes with 2,2′-bipyridine(bpy); 2,2′-bipyrimidine(bpm) and 2,3-bis(2-pyridyl)pyrazine(dpp)

New Os(II) complexes including [Os(dpop′)₂](PF₆)₂ (dpop′= dipyrido(2,3-a;3′,2′-j)phenazine) and a series of mixed ligand [Os(dpop′)(N-N)Cl]PF₆ (N-N = 2,2′-bipyridine(bpy); 2,2′-bipyrimidine(bpm) and 2,3-bis(2-pyridyl)pyrazine(dpp)) were synthesized. The Os dπ → dpop′ π^∗ MLCT transitions for [Os(dpop′)₂]²⁺ are observed at lower energy than for Os dπ → tpy π^∗ (tpy = 2,2′:6′,2″-terpyridine) and Os dπ → tppz π^∗ (tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) (The ligand abbreviations tpd, tpp and tpypz have also appeared in the literature for 2,3,5,6- tetrakis(2-pyridyl)pyrazine in addition to tppz.) MLCT transitions in the comparative [Os(tpy)₂]²⁺ and [Os(tppz)₂]²⁺ complexes. The Os dπ → dpop′ π^∗ MLCT transitions are observed at lower energy in mixed bidentate ligand N-N systems compared with [Os(dpop′)₂]²⁺. Cyclic voltammetry shows more positive osmium oxidation, and less negative ligand reduction potentials for [Os(dpop′)₂]²⁺ as compared to [Os(tpy)₂]²⁺ and [Os(tppz)₂]²⁺ complexes. The osmium oxidation potentials in mixed ligand [Os(dpop′)(N-N)Cl]⁺ complexes are at less positive potential than for the [Os(dpop′)₂]²⁺ ion. NMR results show different chemical shifts for ring protons either trans or cis to dpop′ in mixed ligand systems, and also show two geometrical isomers for the [Os(dpop′)(dpp)Cl]⁺ complex. The [Os(dpop′)(dpp)Cl]⁺ geometric isomer with the pyrazine ring of dpp trans to dpop′ is found more predominate by 1.0/0.7 over the isomer with the pyrazine ring of dpp cis to dpop′ and that inter-conversion of geometric isomers does not occur in room temperature solution on the NMR timescale.     

A Study of the Interaction of Cr(III) Complexes and Their Selective Binding with B-DNA: A Molecular Modeling Approach

Abstract

Molecular modeling and energy minimisation calculations have been used to investigate the interaction of chromium(III) complexes in different ligand environments with various sequences of B-DNA. The complexes are [Cr(salen)(H₂O)₂]⁺; salen denotes 1, 2 bis-salicylideneaminoethane, [Cr(salprn)(H₂O)₂]⁺; salprn denotes 1, 3 bis- salicylideneamino-propane, [Cr(phen)₃]³⁺; phen denotes 1, 10 phenanthroline and [Cr(en)₃]³⁺; en denotes eth- ylenediamine. All the chromium(III) complexes are interacted with the minor groove and major groove of d(AT)₁₂, d(CGCGAATTCGCG)₂ and d(GC)₁₂ sequences of DNA. The binding energy and hydrogen bond parameters of DNA-Cr complex adduct in both the groove have been determined using molecular mechanics approach. The binding energy and formation of hydrogen bonds between chromium(III) complex and DNA has shown that all complexes of chromium(III) prefer minor groove interaction as the favourable binding mode.     

Searching for switchable molecular conductors: Salts of [M(dcbdt)2] (M = Ni, Au) anions with [Fe(sal2-trien)] and [Fe(phen)3]

Salts of [Fe^III(sal₂-trien)]⁺and [Fe^II(phen)₃]²⁺ cations and M[(dcbdt)₂]⁻ anions with M = Ni and Au (dcbdt = dicyanobenzenedithiolate) with formula [Fe(sal₂-trien)] [M(dcbdt)₂] and [Fe(phen)₃] [M(dcbdt)₂]₂ were obtained and characterized by single X-ray diffraction and magnetic measurements. None of these salts shows a clear spin crossover behaviour and their magnetic properties are due essentially to the cations in a high spin S = 5/2 and low spin states for the Fe^III and Fe^II salts respectively. The magnetic Ni sublattices in both compounds appear to have a negligible direct contribution to the magnetization but enhance the AF interactions in the cation sublattice.