Low valence states of iron complexes of aliphatic and mixed aliphatic-aromatic diimine ligands

Electrochemical reductions of iron(II) diimine complexes [FeL₃²⁺, where L = CH₃NC(R)C(R′) NCH₃, aliphatic diimine series with R,R′ = H,H; H,CH₃ and CH₃,CH₃ and L = C₅H₄NC(R)N(R′), mixed diimine series with R,R′ = H,CH₃ and CH₃] were investigated through polarography and cyclic voltammetry in acetonitrile, with tetraethylammonium perchlorate supporting electrolyte (0.2 M) as a function of temperature. In the 0 to ?2.4 V vs. Ag/AgCl potential range two to four polarographic waves were observed for the aliphatic series. The first two waves can be described as one-electron reversible reduction processes. They indicate that low valence states iron(I) and iron(0) are stabilized in acetonitrile. In the mixed ligand series three one-electron reversible reduction waves were observed, indicating that in addition to the low valence states stabilized in the aliphatic diimine series the formal reduction state Fe(–I) is also stabilized. The stabilization of the low oxidation states is due to the electron acceptor properties of the diimine ligands, inherent to the presence of the chromophoric iron diimine group. The half-wave potential data and the stabilization of the low valence states point to the importance of analyzing both σ and π effects. The molecular electronegativity values for the series of iron diimine complexes investigated evidences a synergistic interaction between the metal-ligand σ and π bonds. Diffusion coefficients, temperature effects on the heterogeneous electron transfer step, and electrocapillary curves were obtained for these complexes. No evidence for adsorption of the complexes on mercury electrodes was found for the one-electron reversible steps. When comparing polarographic data with those obtained on platinum disk working electrodes employed in the cyclic voltammetric experiments, we observed that for the symmetric aliphatic diimine ligands the observed cathodic currents are larger than expected on the basis of the previously calculated diffusion coefficients. In addition, the reduction waves are shifted 0.14 V to more negative potentials. The symmetric aliphatic diimine complexes exhibit adsorption of the electroactive species on the surface of the platinum electrodes in this potential range.     

Synthesis of a series of ruthenium and rhodium complexes with tridentate pyridine-based ligands containing hydrogen bonding groups

A series of ruthenium and rhodium complexes with a urea-disubstituted pyridine ligand are reported. The X-ray crystal structures of three of these species, RuCl₂(L¹)(PPh₃) (1), [Ru(MeCN)₂(L¹)(PPh₃)][BF₄]₂ (3) and Rh(CH₂Cl)Cl₂(L¹) (9) (where L¹ = N,N′-(2,2′-(1E,1′E)-(1,1′-(pyridine-2,6-diyl)bis(ethan-1-yl-1-ylidene))bis(azan-1-yl-1-ylidene)bis(ethane-2,1-diyl))diacetamide) have shown that the disubstituted pyridine acts as a tridentate ligand and its urea substituents engage in hydrogen bonding interactions with species coordinated to the metal centres. The reactivity of the ruthenium complexes towards coordination of other anions such as NCS⁻ has been investigated, as well as the oxidative-addition of alkyl chlorides to rhodium(I) centres (to yield species such as 9).     

Antiproliferative activity and QSAR study of copper(II) mixed chelate [Cu(N-N)(acetylacetonato)]NO3 and [Cu(N-N)(glycinato)]NO3 complexes, (Casiopeínas)

Mixed chelate copper(II) complexes patented and mark title registered as Casiopeínas^® are antineoplastic agents with general formulas [Cu(N-N)(α-l-amino acidato)]NO₃ and [Cu(N-N)(O-O)]NO₃, where the N-N donor is an aromatic substituted diimine (1,10-phenanthroline (phen) or 2,2′-bipyridine (bpy)) and the O-O donor is acetylacetonate (acac) or salicylaldehydate (salal). In the present work, the series of complexes [Cu(N-N)(acac)]NO₃ and [Cu(N-N)(gly)]NO₃ with several substituents on the diimine ligand were selected to perform a quantitative structure-activity relationship (QSAR) study. Two main analysis were performed: (1) the study of the influence of the substituents on diimine ligand on physicochemical properties such as half-wave potential (E_1/2) and their relationship with medial lethal dose (LD50) or medial inhibitory concentration (IC50) on several tumor cell lines and (2) the study of the influence of the secondary ligand when acac is changed for glycinate (gly). Results showed that the presence of the central fused aromatic ring in the phen containing complexes is necessary to preserve the antiproliferative activity. The QSAR equations showed a strong relationship between the IC50 and E_1/2; the most active complexes are the weaker oxidants. The change of secondary ligand from acac to gly has less influence on biological activity than the changes on the diimine ligand.     

Tripodal oxygen and tripodal nitrogen ligands in hydroformylation reactions: formation of novel rhodium(III) carbonyl bis(acyl) complexes

The rhodium(I) complexes TpmsRh(CO)₂ (1) and TpmsRh(cod) (2) of the tripodal nitrogen ligand tris(pyrazolyl)methanesulfonate, Tpms⁻=[(pz)₃CSO₃]⁻, catalyze the hydroformylation of 1-hexene. Addition of phosphine has a negative effect on the activity. The hydroformylation activity reaches a maximum at about 60 °C. At temperatures above 80 °C hydrogenation becomes an important secondary reaction. When the catalysis is performed at 60 °C in acetone with 1 or 2 as catalyst precursor all of the rhodium is recovered in the form of the rhodium(III) bis(acyl) complex TpmsRh(CO)(COC₆H₁₃)₂ (9). A similar behaviour is observed with rhodium(I) complexes bearing the tripodal oxygen ligand L_OMe⁻=[(cyclopentadienyl)tris(dimethylphosphito-P) cobalt O,O^′,O″]⁻. In this case all of the rhodium is transformed into L_OMeRh(CO)(COC₆H₁₃)₂ (10). These hitherto unknown bis(acyl) rhodium(III) complexes show the same catalytic activity as the rhodium(I) starting compounds.     

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.     

Synthesis of some ruthenium complexes with mixed diimine ligands

The use of Ru(DMSO)₄Cl₂ and Ru(PY)₄Cl₂ (DMSO=dimethyl sulphoxide; Py=pyridine) in synthesizing mixed ligand complexes is reported. The bidentate ligands used are of the diimine type, namely, 2,2′-bipyrimidine (bpm); 3,6-di-(2-pyridyl)-1,2,4, 5-tetrazine (dpt); 2,3-bis(2-pyridyl)-5,6-dihydropyrazine (dhp); 2,3-bis(2-pyridyl)-pyrazine (dpp); 2,3-bis-(2-pyridyl)-quinoxaline (dpq); 2,3,5,6-tetrakis(2-pyridyl)-pyrazine (tpp). Characterization of the complexes has been accomplished using elemental analysis, conductivity, IR and UV-Vis spectroscopy.     

Axial versus equatorial coordination of thioether sulfur: Mixed ligand copper(II) complexes of 2-pyridyl-N-(2′-methylthiophenyl)-methyleneimine with bidentate diimine ligands

The synthesis, structure and spectral and redox properties of the copper(II) complexes [Cu(pmtpm)Cl₂] (1) and [Cu(pmtpm)₂](ClO₄)₂ (6), where pmtpm is the linear tridentate ligand 2-pyridyl-N-(2′-methylthiophenyl)methyleneimine containing a thioether and two pyridine donors, are described. Also, the mixed ligand complexes [Cu(pmtpm)(diimine)](ClO₄)₂ (2-5), where the diimine is 2,2′-bipyridine (bpy) (2), 1,10-phenanthroline (phen) (3), 2,9-dimethyl-1,10-phenanthroline (2,9-dmp) (4) or dipyrido-[3,2-d:2′,3′-f]-quinoxaline (dpq) (5), have been isolated and studied. The X-ray crystal structures of the complexes 1, [Cu(pmtpm)(2,9-dmp)](ClO₄)₂4 and 6 have been successfully determined. The complex 1 possesses a trigonal bipyramidal distorted square based pyramidal (TBDSBP) coordination geometry in which three corners of the square plane are occupied by two nitrogens and thioether s of pmtpm ligand and the remaining equatorial and the axial positions by two chloride ions. The complex 4 contains a CuN₄S chromophore also with a TBDSBP coordination geometry in which two nitrogens and the thioether sulfur of pmtpm ligand occupy three corners of the square plane. One of the two nitrogens of 2,9-dmp ligand is equatorially coordinated and the other axially to copper. On the other hand, the complex 6 is found to possess a square based pyramidal distorted trigonal bipyramidal (SPDTBP) coordination geometry. The CuN₂S trigonal plane in it is comprised of the pyridine and imine nitrogens and the thioether sulfur of the pmtpm ligand. The pyridine nitrogens of the ligand occupy the axial positions and the second thioether sulfur remains uncoordinated. On long standing in acetonitrile solution the mixed ligand complexes 2 and 3 undergo ligand disproportionation to provide the corresponding bis-complexes of bpy and phen, respectively, and 6. The electronic and EPR spectral parameters and the positive redox potential of complex 4 are consistent with the equatorial location of the thioether sulfur in the square-based coordination geometry around copper(II). On the other hand, the higher g_∥ and lower A_∥ values and lower E_1/2 values for the complexes 2, 3 and 5 are consistent with the axial coordination of the thioether sulfur. Also, the Cu(II)/Cu(I) redox potentials increase with increase in number of aromatic rings of the diimine ligand. The steric and electronic effects of the bidentate diimine ligands in orienting the thioether coordination to axial or equatorial position are discussed.     

Synthesis of a novel pyridine-diamino bridged diphosphine ligand and its macrocyclic metal complexes

A novel long chain diphosphine ligand with a pyridine-diamino bridge, 2,6-bis(N-benzyl-N-diphenylphosphinomethylamino)pyridine (PNP1), was prepared conveniently using the Mannich reaction of HPPh₂ with paraformaldehyde and 2,6-bis(N-benzylamino)pyridine in high yield. Reactions of the ligand with metal complexes, M(COD)Cl₂ (M = Pd, Pt), M(CH₃CN)₄ClO₄ (M = Cu, Ag) and M(CO)₆ (M = Mo, W) afforded the corresponding 10-numbered monometallic macrocyclic complexes with an uncoordinated pyridyl bridge. The monometallic chelate PdCl₂(PNP1) continued to react with Ag⁺ or Cu⁺ giving the μ-Cl bridged bicyclic metallic complex (μ-Cl)₂[PdCl(PNP1)]₂. The diphenylphosphine group coordinated with metal ion in cis-form in all the 10-numbered macrocyclic metal complexes. Ligand PNP1 and another known analogous 2,6-bis(N-diphenylphosphinoamino)pyridine (PNP2) reacted with Au(SMe₂)Cl giving the corresponding bimetallic Au₂Cl₂(PNP1) and Au₂Cl₂(PNP2), respectively. The latter bimetallic complexes continued to react with Ag⁺ and diphosphine ligand to give the corresponding bimetallic macrocyclic complexes Au₂(ligand)₂(ClO₄)₂. All the complexes were characterized and the structures of some complexes were confirmed by X-ray single crystallography determination.     

DNA binding and cleavage activity of [Ru(NH3)4(diimine)]Cl2 complexes

The interaction of a series of mixed ligand complexes of the type [Ru(NH₃)₄(diimine)]Cl₂, where diimine=2,2^′-bipyridine (bipy), 1,10-phenanthroline (phen), 5,6-dimethyl-1,10-phenanthroline (5,6-dmp), 4,7-dimethyl-1,10-phenanthroline (4,7-dmp), 2,9-dimethyl-1,10-phenanthroline (2,9-dmp), 3,4,7,8-tetra-methyl-1,10-phenanthroline (Me₄phen), with calf thymus DNA has been studied using absorption, emission and circular dichroic spectral measurements and viscometry and electrochemical techniques. On interaction with DNA the complexes show hypochromism and red-shift in their MLCT band suggesting that the complexes bind to DNA. The magnitude of the binding constant (K_b) obtained from absorption spectral titration varies depending upon the nature of the diimine ligand: Me₄phen > 5,6-dmp > 4,7-dmp > phen suggesting the use of diimine ‘face’ of the octahedral complexes in binding to DNA. The interaction of phen complex possibly involves phen ring partially inserted into the DNA base pairs. In contrast, the methyl-substituted phen complexes would involve hydrophobic interaction of the phen ring in the grooves of DNA, which is supported by hydrogen bonding interactions of the ammonia ligands with the intrastrand nucleobases. Also the shape and size of the phen ligand as modified by the methyl substituents determine the DNA binding site sizes (0.12-0.45 base pairs). The relative emission intensities (I/I₀) of the DNA-bound complexes parallel the variation in K_b values. Almost all the metal complexes exhibit induced CD bands on binding to B DNA, with the 4,7-dmp and Me₄phen complexes inducing certain structural modifications on the biopolymer. DNA melting curves obtained in the presence of metal complexes reveal a monophasic melting of the DNA strands, the Me₄phen complex exhibiting a slightly enhanced tendency to stabilize the double-stranded DNA. There were slight to appreciable changes in the relative viscosities of DNA, which are consistent with enhanced hydrophobic interaction of the methyl-substituted phen rings. Upon interaction with CT DNA, the Me₄phen, 4,7-dmp and 5,6-dmp complexes, in contrast to bipy, phen and 2,9-dmp complexes, show a decrease in anodic peak current in their cyclic voltammograms suggesting that they exhibit enhanced DNA binding. DNA cleavage experiments show that all the complexes induce cleavage of pBR322 plasmid DNA, the Me₄phen and 5,6-dmp complexes being remarkably more efficient than other complexes.     

Co-ordination compounds derived from dimethylhydrazones of cyclohexane-1,2-dione, 2-acetyl- and 2-benzoylpyridines and 4-bezoylpyridine

The 1,1-dimethylhydrazones of cyclohexane-1,2-dione (CDDMH), 2-acetylpyridine (APDMH) and 2-benzoylpyridine (2BPDMH) from tetrahedral complexes MX₂L (M = Co(II), Zn(II); X = Cl, Br) in which the ligand is chelating through the methylene nitrogen atoms (CDDMH) or one methylene and one pyridine nitrogen atom (APDMH, 2BPMDH). Octahedral complexes CoX₂L₂ (X = Cl, NCS; L = APDMH, 2BPDMH) have also been isolated but no tris-ligand complexes. The ligand 4-benzoylpyridine-dimethylhydrazone (4BPDMH) does not chelate but forms tetrahedral complexes MX₂(4BPDMH)₂ in which the unidentate ligand co-ordinates through the pyridine nitrogen atom.     

Optically active transition metal complexes. 87. Synthesis of porphyrin complexes containing chiral Fe atoms. Demonstration of Fe chirality by H NMR spectroscopy

After transformation of the vinyl groups in hemin into H, CH₂CH₃ and COCH₃, the propionic acid side chains were converted into esters and amides using (-)-menthol, (-)-2-methylbutanol and (-)-1-phenylethylamine. By introducing the CS ligand into the apical position, square pyramidal complexes were obtained, differing only in the Fe configuration, which could not be separated. However, the two diastereomers with different Fe configuration, having optically active 1-phenylethylisonitrile and pyridine (or 4-methylpyridine) as ligands in the axial positions, at −20°C exhibit different chemical shifts demonstrating configurational stability at the Fe atom on the ¹H NMR time scale. At room temperature epimerization at the Fe atom occurs by ligand exchange reactions.     

Optically active transition metal complexes. 87. Synthesis of porphyrin complexes containing chiral Fe atoms. Demonstration of Fe chirality by 1H NMR spectroscopy

After transformation of the vinyl groups in hemin into H, CH₂CH₃ and COCH₃, the propionic acid side chains were converted into esters and amides using (-)-menthol, (-)-2-methylbutanol and (-)-1-phenylethylamine. By introducing the CS ligand into the apical position, square pyramidal complexes were obtained, differing only in the Fe configuration, which could not be separated. However, the two diastereomers with different Fe configuration, having optically active 1-phenylethylisonitrile and pyridine (or 4-methylpyridine) as ligands in the axial positions, at ?20°C exhibit different chemical shifts demonstrating configurational stability at the Fe atom on the ¹H NMR time scale. At room temperature epimerization at the Fe atom occurs by ligand exchange reactions.     

Exploring the properties of mixed valence cyanide bridged dinuclear complexes: Solvent stabilization of electronic isomers

We report here the synthesis and properties of a family of mixed-valence cyanide-bridged dinuclear complex ions trans-[(L′L₄Ru^II(μ-NC)Fe^III(CN)₅]⁻ (with L = pyridine or 4-dimethylaminopyridine (dmap) and L′ = pyridine, 4-methoxypyridine (meopy) or 4-dimethylaminopyridine)) whose properties could be adjusted smoothly by changing the acceptor properties of the solvent and the σ donor properties of the L′ pyridine ligand. In solution these complexes exhibit an intense solvent-dependent MM′CT (Ru^II → Fe^III) absorption in the near infrared region. Analysis of this band in different complexes and solvents suggests an enhanced interaction as the energies of the metal centers come closer. From this trend the anion trans-[(dmap)₅Ru(μ-NC)Fe(CN)₅]⁻ (dmap = 4-dimethylaminopyridine) in water is expected to belong to the class II-III, but its spectral properties indicates a ground state with Ru(III)-Fe(II) character. The stabilization of this electronic isomer is probably related to the better donor properties of the hexacyanoferrate(II) moiety and its stronger interaction with water.     

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.     

Electron transfer studies on Cu(II) complexes bearing phenoxy-pincer ligands

Oxido-pincer ligands with phenolate-groups [2,6-bis(2-methoxyphenyl)pyridine (LOMe₂), 2,6-bis(2-hydroxyphenyl)-pyridine (LOH₂), 2,6-bis-(2,4-dimethoxyphenyl)-pyridine (LOMe₄)] coordinate to Cu^II forming binuclear complexes which can be easily and reliably converted into mononuclear species. Their physical properties were analysed using EPR, optical spectroscopy and (spectro-)electrochemical methods. The results were compared to those of related Ni^II complexes and discussed in view of Cu-containing metalloenzymes. Due to the ligands flexibility the Cu^II/Cu^I redox couple exhibits high reversibility, while the ligand-centred oxidation leads to highly reactive phenoxy radicals. Reduction of the LOH₂ complex leads to sequential deprotonation. The ligand LOMe₄ and the derived complexes show blue luminescence, which can be rationalised from its molecular structure (analysed by XRD).     

Molecular structures of electron-transfer active complexes [Re(XQ)(CO)3(NN)] (XQ=N-Me-4,4-bipyridinium or N-Ph-4,4-bipyridinium; NN=bpy, 4,4-Me2-2,2-bpy or N,N-bis-isopropyl-1,4-diazabutadiene) in the solid state and solution: an X-ray and NOESY NMR study

Crystal and molecular structures of a series of complexes [Re(XQ⁺)(CO)₃(NN)]²⁺ (XQ=N-methyl-4,4^′-bipyridinium (MQ⁺) and N-phenyl-4,4^′-bipyridinium (PQ⁺) and NN=bpy, 4,4^′-dimethyl-bpy (dmb) or N,N^′-bis-isopropyl-1,4-diazabutadiene (ⁱPr-DAB)) in the solid state have been determined by X-ray diffraction. Aromatic rings within the XQ⁺ ligand were found to be highly staggered. The dihedral angles between the pyridine and pyridinium rings were found in the range 39°-45° for MQ⁺ and 28°-46° for PQ⁺. The exceptionally low dihedral angle of 8° in [Re(MQ⁺)(CO)₃(dmb)]²⁺ is due to crystal-packing effects. The pyridinium and phenyl rings of the PQ⁺ ligand are even more staggered, with dihedral angles in the range 40°-55°. The pyridine ring of the XQ⁺ ligand is oriented relative to the equatorial ligands in such a way that it bisects the angles between the equatorial Re-N and Re-C bonds in all complexes, except for [Re(PQ⁺)(CO)₃(ⁱPr-DAB)](PF₆)₂, where it bisects the DAB ligand. Structures of the complexes [Re(XQ⁺)(CO)₃(NN)]²⁺ (NN=bpy, dmb) were also studied in solution using NOESY NMR. It was found that the orientation of the XQ⁺ ligand relative to the equatorial ligands is the same as in the solid state. The XQ⁺ ligands become even more staggered on going to the solution where pyridine-pyridinium dihedral angles range from 42° to 45°. A value of ∼69° was found for the pyridinium-phenyl dihedral angle in PQ⁺ complexes. The structural data obtained are related to electron-transfer activity of XQ⁺ complexes. It follows that any ground- or excited-state electron transfer reactions or optical charge transfer excitation have to be coupled with a major reorganization of the XQ⁺ ligand, namely twisting of its aromatic rings and shortening of the interring C-C bond. This conclusion has important implications for estimates of Marcus inner reorganization energy and electron transfer dynamics.     

Copper phosphates and phosphinates with pyridine/pyrazole alcohol co-ligands: Synthesis and structure

Copper phosphates, [Cu(dtbp)₂(pzet)₂]·H₂O (1) and [Cu(dtbp)₂(pyme)₂] (2), as well as copper phosphinate, [Cu(dppi)₂(pyet)₂] (3) have been synthesized by the reaction of copper acetate with di-tert-butyl phosphate (dtbp) or diphenyl phosphinate (dppi) in the presence of pyridine base having hydroxyl group, namely, 3,5-dimethylpyrazole-2-ethanol (pzet) or 2-(hydroxymethyl)pyridine (pyme) or 2-(2-hydroxyethyl)pyridine (pyet). Single crystal X-ray diffraction studies reveal that copper ion in all the three complexes is bonded to two phosphoryl ions (P(O)O⁻) and two pyridine co-ligands. The crystal structure of 1 reveals that the hydroxyl group of the CH₂CH₂OH moiety of pzet ligand exhibits a positional disorder between the non-bonding position and the bonding position with respect to the central copper ion along the Jahn-Teller axis. Hence, the structure of 1 can be considered to exhibit both ‘square-planar’ and ‘octahedral’ coordination geometries simultaneously for the copper ion in the same complex. A similar situation for the -OH groups has not been observed in the complexes 2 and 3 and hence the coordination geometry around the copper ion is axially elongated octahedron.     

1H nmr studies of complexes of ferric leghemoglobin with substituted pyridines and nicotinic acids

The ¹H nuclear magnetic resonance (nmr) spectra of complexes of soybean ferric leghemoglobin with 3-substituted pyridines and 5-substituted nicotinic acids have been recorded in order to determine the influence of axial ligands on heme electronic structure. The hyperfine shifted resonances of the heme group were assigned by analogy to previous assignments for the pyridine and nicotinic acid complexes of leghemoglobin. The spectra are characteristic of predominantly low-spin ferric heme complexes. For the pyridine complexes, the rate of ligand exchange was found to increase with decreasing ligand pK_A. For many of the complexes, optical and nmr spectra reveal the presence of an equilibrium mixture of high- and low-spin states of the iron atom. The percentage of high-spin component increases with decreasing ligand pK_A Smaller hyperfine shifts are noted for leghemoglobin complexes with ligands capable of weak ligand → metal π bonding. The pattern of hyperfine shifted resonances is similar for all complexes studied and indicates that the overall heme electronic structure is dominated by the bonding to the proximal histidine.     

Labeling of proteins using [105Rh]Rh-4-(p-aminobenzyl)-diethylenetriamine

A bifunctional ligand 4-(p-aminobenzyl)-diethylenetriamine has been synthesized. ¹⁰⁵Rh complexes with this ligand were prepared with an overall yield of 79% at pH 9.0 in bicarbonate buffer. The preformed complex was converted to the isothiocyanate derivative using thiophosgene. Conjugation yields of 75% with IgG and 85% with HSA could be obtained for a 4 h conjugation reaction. Affinity chromatography of human-IgG coupled to the rhodium complex in an anti-human IgG agarose gel indicated no denaturation of the labeled protein. The procedure reported here can be adapted for the preparation of ¹⁰⁵Rh-labeled antibodies for therapeutic applications.     

Synthesis,density functional theory calculations and luminescence of lanthanide complexes with 2,6‐bis[(3‐methoxybenzylidene)hydrazinocarbonyl] pyridine Schiff base ligand

A pyridine‐diacylhydrazone Schiff base ligand, L = 2,6‐bis[(3‐methoxy benzylidene)hydrazinocarbonyl]pyridine was prepared and characterized by single crystal X‐ray diffraction. Lanthanide complexes, Ln–L, {[LnL(NO₃)₂]NO₃.xH₂O (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Er)} were prepared and characterized by elemental analysis, molar conductance, thermal analysis (TGA/DTGA), mass spectrometry (MS), Fourier transform infra‐red (FT‐IR) and nuclear magnetic resonance (NMR) spectroscopy. Ln–L complexes are isostructural with four binding sites provided by two nitro groups along with four coordination sites for L. Density functional theory (DFT) calculations on L and its cationic [LnL(NO₃)₂]⁺ complexes were carried out at the B3LYP/6–31G(d) level of theory. The FT‐IR vibrational wavenumbers were computed and compared with the experimentally values. The luminescence investigations of L and Ln–L indicated that Tb–L and Eu–L complexes showed the characteristic luminescence of Tb(III) and Eu(III) ions. Ln–L complexes show higher antioxidant activity than the parent L ligand.