Donor Atom Set Research Articles

Diaqua-2κ2O-bis(μ-1-oxido-2-naphthoato)-1:2κ3O1,O2:O2′;2:3κ3O2:O1,O2′-bis(1-oxido-2-naphthoato)-1κ1O2,O2;3κ2O1,O2-hexapyridine-1κ2N,2κ2N,3κ2N-trimanganese(II/III) pyridine disolvate dihydrate

The title complex, [Mn3(C11H6O3)4(C5H5N)6(H2O)2]·2H2O·2C5H5N, is a trinuclear mixed oxidation state complex of symmetry. The three Mn atoms are six-coordinated in the shape of distorted octa­hedra, each coordinated with an O4N2 set of donor atoms, where the ligands exhibit mono- and bidentate modes. However, the coordination of the MnII ion located on the inversion centre involves water mol­ecules at two coordination sites, whereas that of the two symmetry-related MnIII ions involves an O4N2 set of donor atoms orginating from the organic ligands. Intramolecular C—H⋯π interactions between neighbouring pyridine ligands stabilize this arrangement. A two-dimensional network parallel to (001) is formed by inter­molecular O—H⋯O hydrogen bonds.

Influence of the ligand donor atoms on the in vitro stability of rhenium(I) and technetium (I)-99m complexes with pyrazole-containing chelators: Experimental and DFT studies

The new pyrazole-containing ligand 3,5-Me 2pz(CH 2) 2S(CH 2) 2COOH ( L 1 H) was synthesized and used to prepare the complexes fac-[M(κ 3-L 1)(CO) 3] (M = Re (1), 99mTc( 1a)), which were obtained in high yield albeit with a low specific activity in the case of 99mTc. The X-ray diffraction analysis of 1 confirmed that L 1 coordinates to the metal as monoanionic and through a (N,S,O) donor atom set. Challenge experiments of 1a against cysteine and histidine showed that this complex suffers considerable transchelation in vitro. This contrasts with the behavior exhibited by the related complex fac-[ 99mTc(κ 3- L 2 )(CO) 3] ( 2a) ( L 2 = 3,5-Me 2pz-(CH 2) 2NH-CH 2-COO), anchored by a (N 2O)-tridentate ligand. Biodistribution studies of 1a and 2a in mice indicated that both compounds have a relatively similar biological profile. Nevertheless, the fastest blood clearance and minor hepatic retention found for 2a has shown that this complex is more adequate to be further explored in radiopharmaceutical sciences. DFT calculations (ADF program) were performed for these neutral complexes and related cationic M(I) (M = Re, Tc) tricarbonyl complexes anchored by pyrazole-containing ligands, in order to have a better understanding of the influence of the donor atom set (N,N,O vs. N,O,S; N,N,N vs. N,N,S vs. N,S,S) on their in vitro stability. The differences of the calculated binding energies are not significant, suggesting that the in vitro behavior of these Re(I)/Tc(I) tricarbonyl complexes is not determined by thermodynamic factors.

Synthesis, characterization and electro-spectroelectrochemical studies of four macrocyclic Schiff-base Co(II) complexes having N2O2 set of donor atoms

Four macrocyclic Schiff-base cobalt complexes, [CoL1][NO3]2 · 3H2O, [CoL2][NO3]2 · 4H2O, [CoL3][NO3]2 · 4H2O and [CoL4][NO3]2 · 2H2O, were synthesized by reaction of salicylaldehyde derivatives with 1,4-bis(3-aminopropoxy)butane or (±)-trans-1,2-diaminocyclohexane and Co(NO3)2 · 6H2O by template effect in methanol. The metals to ligand ratio of the complexes were found to be 1:1. The Co(II) complexes are proposed to be tetrahedral geometry. The macrocyclic Co(II) complexes are 1:2 electrolytes as shown by their molar conductivities (ΛM) in DMF (dimethyl formamide) at 10−3 M. The structure of Co(II) complexes is proposed from elemental analysis, Ft-IR, UV–visible spectra, magnetic susceptibility, molar conductivity measurements and mass spectra. Electrochemical and thin-layer spectroelectrochemical studies of the complexes were comparatively studied in the same experimental conditions. The electrochemical results revealed that all complexes displayed irreversible one reduction processes and their cathodic peak potential values (E pc) were observed in around of −1.14 to 0.95 V. It was also seen that [CoL1][NO3]2 · 3H2O and [CoL2][NO3]2 · 4H2O exhibited one cathodic wave without corresponding anodic wave but, [CoL3][NO3]2 · 4H2O and [CoL4][NO3]2 · 2H2O showed one cathodic wave with corresponding anodic wave, probably due to the presence of different ligand nature even if the complexes have the same N2O2 donor set. In view of spectroelectrochemical studies [CoL3][NO3]2 · 4H2O showed distinctive spectral changes in which the intensity of the band (λ = at 316 nm, assigned to n → π* transitions) decreased and a new broad band in a low intensity about 391 nm appeared as a result of the reduction process based on the cobalt center in the complex.

New Compounds with Potential Liquid Crystal Properties. Copper(II) and Nickel(II) Complexes of N,N′-bis(4-decyloxysalicyliden-N-n-propyl)-Piperazine. Synthesis and Characterization

The new Schiff base N,N′-bis(4-decyloxysalicyliden-N-n-propyl)-piperazine (ZOPP) was designed and synthesized. Structures were confirmed by elemental analysis and IR, 1H-NMR spectroscopy. ZOPP can act as ligand through the two set of donor atoms (one set: two nitrogen atoms and one oxygen) situated on each side of the piperazine bridge. The nature of the complexes depends strongly on the metal to ligand molar ratio. Thus, starting from metal to ligand of 1 to 1 ratio, where M=Cu(II), Ni(II), binuclear complexes with a polymeric structure have been obtained, whereas at metal to ligand molar ratio of 2 to 1, discrete binuclear complexes were formed.

Rhenium and technetium complexes bearing quinazoline derivatives: progress towards a 99mTc biomarker for EGFR-TK imaging

The quinazoline derivatives (3-chloro-4-fluorophenyl)quinazoline-4,6-diamine (2) and (3-bromophenyl)quinazoline-4,6-diamine (3) were labelled with (99m)Tc using the "4 + 1" mixed-ligand system [Tc(NS3)(CN-R)] and the tricarbonyl moiety fac-[Tc(CO)3]+. In the "4 + 1" approach the technetium(iii) is stabilized by a monodentate isocyanide bearing a quinazoline fragment (L1,L2 ) and by the tetradentate tripodal ligand tris(2-mercaptoethyl)-amine (NS3). In the "4 + 1" approach, 99mTc-labelling was performed in a two-step procedure, the complexes [Tc(NS3)(L1)] (7a) and [Tc(NS3)(L2)] (8a) being obtained in about 50-70% yield. In the tricarbonyl approach, the fac-[Tc(CO)3]+ unit is anchored by two different monoanionic chelators bearing the quinazoline derivatives (3-chloro-4-fluorophenyl)quinazoline-4,6-diamine (2) and (3-bromophenyl)quinazoline-4,6-diamine (3). Both chelators have a N2O donor atom set, but one contains a pyrazolyl ring (L5H) and the other contains a pyridine unit (L6H). In both cases the conjugation of the quinazoline to the chelator was done through the secondary amine of the potentially tridentate and monoanionic chelators, the corresponding 99mTc-complexes (10a, 11a) being obtained in quantitative yield. The identities of the 99mTc-labelled quinazolines (7a, 8a, 10a, 11a) were confirmed by comparison with the HPLC profiles of the analogous Re compounds (7, 8, 10, 11). All these Re complexes were characterized by NMR and IR spectroscopy, elemental analysis and in some cases by MS and X-ray diffraction analysis. In vitro studies indicate that the quinazoline fragments, after conjugation to the cyano group (L1, L2) or to the pyrazolyl containing chelator (L5H), as well as the corresponding Re complexes (7, 8, 10) inhibit significantly the EGFR autophosphorylation and also inhibit A431 cell growth. These two effects were also found for the pyridine-containing chelator (L6H) and corresponding Re complex (11), although to a lesser extent.

Structural features of the Cu2+–vancomycin complex

The structure of vancomycin coordinated to Cu2+ ions is presented and structural aspects upon metal coordination are discussed. The asymmetric part of unit cell comprises two independent molecules of vancomycin–Cu2+ complex, one of them is partially disordered. The binding site involves one imino nitrogen atom, two amide nitrogen atoms delivered by peptide bonds, and carboxyl oxygen from the peptide moiety. The identical set of donor atoms is not reflected in identical coordination geometry around individual metal ions. The studied complex presents two distinct types of conformation. Additionally, leucinyl side chain in one conformer is disordered leading to another type of conformation. The complex molecules form heterodimer with antiparallel hydrogen bonding.

First examples of neutral rhenium(V) complexes with a novel semi-rigid ligand containing a P,N,N,S donor atom set: Synthesis, characterisation and crystal structure

A new PN 2S ligand, N-[2-(diphenylphosphino)phenyl]-2-[(S-trityl)acetylamino]ethanamide [Ph-P(Ph 2)N 2S(Trt)], was synthesised and reacted with Re V precursors. The reaction of both tritylated and detritylated ligands with ReOCl 3(PPh 3) 2 gave the same expected neutral complex [ReO{Ph-P(Ph 2)N 2S}] ( 4) in good yield. An unexpected neutral and diamagnetic species, [ReN{Ph-P(Ph 2)N 2S(Trt)}] ( 5), has been isolated during the complexation of the tritylated ligand with ReNCl 2(PPh 3) 2. The complexes, characterized by classical spectroscopic methods and X-ray analysis for 4, are the first examples of neutral semi-rigid-PN 2S rhenium(V) complexes.

Tripodal Bis(imidazole) Thioether Copper(I) Complexes: Mimics of the CuM Site of Copper Hydroxylase Enzymes

Tripodal bis(imidazole) thioether ligands, (N-methyl-4,5-diphenyl-2-imidazolyl)2C(OR)C(CH3)2SR' (BIT(OR,SR'); R = H, CH3; R' = CH3, C(CH3)3, C(C6H5)3), have been prepared, offering the same N2S donor atom set as the CuM binding site of the hydroxylase enzymes, dopamine beta hydroxylase and peptidylglycine hydroxylating monooxygenase. Isolable copper(I) complexes of the type [(BIT(OR,SMe))Cu(CO)]PF6 (3a and 3b) are produced in reactions of the respective tripodal ligands 1a (R = H) and 1b (R = Me) with [Cu(CH3CN)4]PF6 in CH2Cl2 under CO (1 atm); the pyramidal structure of 3a has been determined crystallographically. The infrared (IR) nu(CO)'s of 3a and 3b (L = CO) are comparable to those of the Cu(M)-carbonylated enzymes, indicating similar electronic character at the copper centers. The reaction of [(BIT(OH,SMe))Cu(CH3CN)]PF6 (2a) with dioxygen produces [(BIT(O,SOMe))2Cu2(DMF)2](PF6)2 (4), whose X-ray structure revealed the presence of bridging BIT-alkoxo ligands and terminal -SOMe groups. In contrast, oxygenation of 2b (R = Me) affords crystallographically defined [(BIT(OMe,SMe))2Cu2(mu-OH)2](OTf)2 (5), in which the copper centers are oxygenated without accompanying sulfur oxidation. Complex 5 in DMF is transformed into five-coordinate, mononuclear [CuII(BIT(OMe,SMe))(DMF)2](PF6)2 (6). The sterically hindered BIT(OR,SR') ligands 9 and 10 (R' = t-Bu; R = H, Me) and 11 and 12 (R' = CPh3; R = H, Me) were also prepared and examined for copper coordination/oxygenation. Oxygenation of copper(I) complex 13b derived from the BIT(OMe,SBu-t) ligand is slow, relative to 2b, producing a mixture of (BIT(OMe,SBu-t))2Cu2(mu-OH)2-type complexes 14b and 15b in which the -SBu-t group is uncoordinated; one of these complexes (15b) has been ortho-oxygenated on a neighboring aryl group according to the X-ray analysis and characterization of the free ligand. Oxygenation of the copper(I) complex derived from BIT(OMe,SCPh3) ligand 12 produces a novel dinuclear disulfide complex, [(BIT(OMe,S)2Cu2(mu-OH)2](PF6)2 (17), which is structurally characterized. Reactivity studies under anaerobic conditions in the presence of t-BuNC indicate that 17 is the result of copper(I)-induced detritylation followed by oxygenation of a highly reactive copper(I)-thiolate complex.

Tetrahydroquinoline and tetrahydroisoquinoline mixed ligand rhenium complexes with the SNS/S donor atom set

AbstractNew oxorhenium complexes with 3‐methylazapentane‐1,5‐dithiolate (SNMeS) and thiol functionalized monodentate tetrahydroquinolyl and tetrahydroisoquinolyl derivatives have been synthesized by simultaneous reaction of [PPh3]2[Re(O)Cl3] with tridentate HSNMeSH and the corresponding N‐heterocycle containing thiol. The characterization of complexes involved elemental analysis, IR, 1H and 13C NMR spectroscopy and X‐ray crystallographic analysis. The nature of the heterocycle in monodentate ligand, even situated at the distance of two methylene group length, has been found to have a significant influence on the molecular conformation. Metal complexes were found to be active in psychotropic in vivo and cytotoxicity in vitro screening. Copyright © 2007 John Wiley & Sons, Ltd.

A new bisphosphonate-containing 99mTc(I) tricarbonyl complex potentially useful as bone-seeking agent: synthesis and biological evaluation

Aiming to develop new bone-seeking radiotracers based on the organometallic core fac-[(99m)Tc(CO)(3)](+) with improved radiochemical and biological properties, we have prepared new conjugates with phosphonate pendant groups. The conjugates comprise a chelating unit for metal coordination, which corresponds to a pyrazolyl-containing backbone (pz) with a N,N,N donor-atom set, and a pendant diethyl phosphonate (pz-MPOEt), phosphonic acid (pz-MPOH) or a bisphosphonic acid (pz-BPOH) group for bone targeting. Reactions of the conjugates with the precursor [(99m)Tc(H(2)O)(3)(CO)(3)](+) yielded (mote than 95%) the single and well-defined radioactive species [(99m)Tc(CO)(3)(kappa(3)-pz-MPOEt)](+) (1a), [(99m)Tc(CO)(3)(kappa(3)-pz-MPOH](+) (2a) and [(99m)Tc(CO)(3)(kappa(3)-pz-BPOH)](+) (3a), which were characterized by reversed-phase high-performance liquid chromatography . The corresponding Re surrogates (1-3), characterized by the usual analytical techniques, including X-ray diffraction analysis in the case of 1, allowed for macroscopic identification of the radioactive conjugates. These radioactive complexes revealed high stability both in vitro (phosphate-buffered saline solution and human plasma) and in vivo, without any measurable decomposition. Biodistribution studies of the complexes in mice indicated a fast rate of blood clearance and high rate of total radioactivity excretion, occurring primarily through the renal-urinary pathway in the case of complex 3a. Despite presenting moderate bone uptake (3.04 +/- 0.47% injected dose per gram of organ, 4 h after injection), the high stability presented by 3a and its adequate in vivo pharmacokinetics encourages the search for new ligands with the same chelating unit and different bisphosphonic acid pendant arms.

6,7,9,10,17,18,20,21,28,29,31,32,39,40,42,43-Hexadecahydro-16,44:22,38-di(ethanooxyethano)-5,8,11,19,27,30,33,41-octaoxa-16,22,38,44-tetraazatetrabenzo[h,q,z,i 1]cyclohexatriacontine: a benzoannelated tricyclic cryptand

The tricyclic title compound, C48H64N4O10, consists of two unsymmetric 18-membered diazatetraoxamacrocycles (NONOOO donor atom set) connected through the N atoms by two lateral ethanoxyethano bridges. The bridging subunits, together with one bridge from each monocycle, define a 24-membered ring that forms a central cavity. The 18-membered rings are parallel to each other, are displaced out of overlap with each other, and are oriented in an opposite sense with respect to the plane defined by the donor atoms of the central cavity.

Very Small and Soft Scorpionates: Water Stable Technetium Tricarbonyl Complexes Combining a Bis-agostic (k3-H, H, S) Binding Motif with Pendant and Integrated Bioactive Molecules

The novel trihydro(mercaptoazolyl)borates Na[H(3)B(tim(Me))] (L(1)) (tim(Me) = 2-mercapto-1-methylimidazolyl), Na[H(3)B(tim(Bupip))] (L(2)) (tim(Bupip) = 1-[4-((2-methoxyphenyl)-1-piperazinyl)butyl]-2-mercaptoimidazolyl), and Na[H(3)B(bzt)] (L(3)) (bzt = 2-mercaptobenzothiazolyl) were synthesized by reaction of NaBH(4) with the corresponding azole. Ligands L(1)-L(3) represent a new class of light and soft scorpionates that stabilizes the [M(CO)(3)](+) core (M = (99)Tc, Re) by formation of the complexes fac-[M{kappa(3)-H(mu-H)(2)B(tim(Me))}(CO)(3)] (M = (99)Tc (1), Re (2)), fac-[Re{kappa(3)-H(mu-H)(2)B(tim(Bupip))}(CO)(3)] (3), and fac-[Re{kappa(3)-H(mu-H)(2)B(bzt)}(CO)(3)] (4), respectively. The soft scorpionates are coordinated to the metal in unique (kappa(3)-H, H', S) fashion, as confirmed by X-ray crystallography of 1, 2, and 4. These complexes with bis-agostic hydride coordination are formed in aqueous solution with the two hydrides replacing two coordinating aquo ligands. The agostic hydrogen atoms were located directly, confirming an unprecedented donor atom set combining one sulfur and two hydrogen atoms. Preliminary studies have shown the possibility of preparing some of these complexes at the no carrier added level ((99m)Tc), under conditions as required in radiopharmaceutical preparation. Due to their lipophilicity, small-size, and easy functionalization with adequate biomolecules, the trihydro(mercaptoazolyl)borate technetium tricarbonyl complexes are suitable for the design of CNS receptor ligand radiopharmaceuticals as exemplified with 3, comprising a pendant serotonergic 5-HT(1A) ligand. The integrated design of radiopharmaceuticals involving a bis-agostic scorpionate ligand is demonstrated by the synthesis of 4, with an integrated benzothiazolyl fragment for the recognition of beta-amyloid plaques.

Structural and conformational analysis of neutral dinuclear diorganotin(IV) complexes derived from hexadentate Schiff base ligands

The synthesis of three hexadentate Schiff base ligands has been carried out, which contain two sets of ONO donor atoms. These were reacted with diorganotin(IV) dichloride derivatives (R = Me, nBu, Ph) to prepare seven dinuclear diorganotin(IV) complexes in moderate yields. Aside from IR and NMR ( 1H, 13C, 119Sn) spectroscopic studies, mass spectrometry and elemental analysis, four tin complexes were characterized by X-ray diffraction analysis. The spectroscopic analyses showed that in solution the tin atoms have five-coordinate environments with a distorted trigonal bipyramidal geometry. Each tin atom is coordinated to the nitrogen atom and forms covalent bonds with two oxygen atoms and two carbon atoms. Due to the presence of a methylene group as bridge between the two ONO chelates, the overall molecular structures can have cis or trans conformation, having either mirror or C 2-symmetry. While in solution a fast equilibrium can be supposed, in the solid state different intermediate conformations have been detected. Furthermore, for the dialkyltin derivatives Sn⋯O intermolecular interactions were found allowing for a dimeric or crinkled polymeric organization, whereas for the diphenyltin derivatives no such interactions were observed.

X‐ray Crystal Structure Characterization of Two Mononuclear Low Coordinate Lanthanoid Aryloxides, [La(Odpp)3(dig)]·0.5dig and [Nd(Odpp)3(py)2]·2py, (HOdpp = 2,6‐diphenylphenol, dig = bis(2‐methoxyethyl) ether, py = pyridine)

AbstractStructures of the new complexes [La(Odpp)3(dig)]·0.5dig (Odpp = 2,6‐diphenylphenolate; dig = bis(2‐methoxyethyl) ether – diglyme) and [Nd(Odpp)3(py)2]·2py (py = pyridine), obtained by crystallization of the corresponding [Ln(Odpp)3(thf)2] complex from diglyme or pyridine contain six‐coordinate and five‐coordinate lanthanoid atoms respectively. In the former, two meridional sets of donor atoms, (O3(dig) and O3(Odpp)) are near normal to each other and in a highly distorted octahedral stereochemistry, whereas the latter is trigonal bipyramidal with an oxygen and a nitrogen atom apical and a pronounced lengthening of the axial Nd‐N bond.

Transition metal complexes with thiosemicarbazide-based ligands. Part LIV. Nickel(II) complexes with pyridoxal semi- (PLSC) and thiosemicarbazone (PLTSC). Crystal and molecular structure of [Ni(PLSC)(H 2O) 3](NO 3) 2 and [Ni(PLTSC-H)py]NO 3

This paper describes the synthesis of the first Ni(II) complexes with pyridoxal semicarbazone (PLSC), viz. Ni(PLSC)Cl 2 · 3.5H 2O ( 1), [Ni(PLSC)(H 2O) 3](NO 3) 2 ( 2), Ni(PLSC)(NCS) 2 · 4H 2O ( 3), [Ni(PLSC-2H)NH 3] · 1.5H 2O ( 4), as well as two new complexes with pyridoxal thiosemicarbazone (PLTSC), [Ni(PLTSC-H)py]NO 3 ( 5) and [Ni(PLTSC-H)NCS] ( 6). Complexes 1– 3 are paramagnetic and have most probably an octahedral structure, for complex 2 this was proved by X-ray diffraction analysis. In contrast, complexes 4– 6 are diamagnetic and have a square-planar structure, and in the case of complex 5 this was also confirmed by X-ray structural analysis. In all cases the Schiff bases are coordinated as tridentate ligands with an ONX (X = O, PLSC; X = S, PLTSC) set of donor atoms. With the complexes involving the neutral form of PLSC and the monoanionic form of PLTSC, the PL moiety is in the form of a zwitterion. In addition to the above-mentioned techniques, all the complexes were characterized by measuring their molar conductivities, UV–Vis and partial IR spectra.

Inclusive coordination of F−, Cl−and Br−anions into macrobicyclic polyammonium receptors

The interaction of F−, Cl− and Br− with protonated forms of the cage-like macrobicyclic ligands L1 and L2, containing, respectively, N7 and N6O sets of donor atoms, was studied by means of potentiometric titrations in aqueous solution, achieving the determination of the stability constants of the complex species formed. While L1 forms halogenide complexes [L1HnX](n−1)+ in various protonation states (n = 2–5 for F− and Cl−, n = 1–5 for Br−), only [L2H4X]3+ (X = F−, Cl−, Br−) complexes are formed by L2. For a given anion, the complex stability increases with the ligand charge, in agreement with a fundamental electrostatic character of the anion–receptor interaction. On the contrary, contrasting stability trends are observed when the stability constants of anion complexes with the ligands in a given protonation state are considered. Only in the case of [L1H5X]4+ complexes the stability follows the trend F− > Cl− > Br−, while for less protonated L1 species the trend is Cl− > Br− > F−. In the case of [L2H4X]3+ complexes the stability trend is F− > Br− > Cl−. Despite the fact that [L2H4]4+ forms a largely more stable F− complex than [L1H4]4+, L1 shows clear selectivity in halogenide binding with respect to L2 over all the pH range. L2 displays a marked selectivity in the binding of F− over Cl− and Br− while in the case of L1 inversion of binding selectivity is found upon pH variations, the F− complexes being favoured in very acidic media. Molecular modelling calculations performed in the gas phase and/or by adopting minimal solvation shells, furnished useful information for the interpretation of these stability trends. Anion size and anion desolvation upon coordination are the main factors determining the stability of halogenide complexes with L1 and L2.

Coordination induced 2-(2′-hydroxyphenyl) imidazolidine ring hydrolysis of dinucleating amine–imine–phenol ligands: X-ray structures of hardness-matched mononuclear cobalt(III) complexes as end products having isomeric N 4O 2 coordination spheres

Two dinucleating ligands H 3 L I x (subscript I for imidazolidine; x = 1, 2) having pendant imidazolidine rings have been reacted with cobalt salts in aqueous methanol at room temperature to obtain the mononuclear complexes [Co IIIL 1]Cl · 2H 2O ( 1 · 2H 2O) and [Co IIIL 2]ClO 4 · H 2O ( 2 · H 2O) of hydrolyzed ligands H 2L x . Expected dicobalt complexes were not obtained due to the coordination induced hydrolytic cleavage of the central substituted imidazolidine rings. The complexes 1 · 2H 2O and 2 · H 2O were structurally characterized by X-ray crystallography. Both mer- and fac-ON 2 binding modes of the two halves of the hexadentate ligands have been identified in these complexes. The mononuclear complexes have CoN 4O 2 coordination spheres with a N 4O 2 donor atom set of the two hexadentate ligands, forming isomeric distorted octahedral coordination geometries around the metal atoms. As a result, the imine nitrogen pairs coordinated trans to each other in [CoL 1] +, whereas the phenolic oxygens are trans in [CoL 2] +. In complex 1 · 2H 2O the average Co–N(imine), Co–N(amine), and Co–O(phenolate) distances are 1.910, 1.959, 1.894 Å, respectively, and in 2 · H 2O the corresponding average distances are 1.944, 1.991 and1.887 Å, respectively. Both the cobalt(III) complexes are diamagnetic. The UV–Vis spectra of the end products, after the imidazolidine ring hydrolysis, is different from the spectrum of the initial reaction mixture containing the μ-bis(tetradentate) ligands, H 3 L I x ( x = 1 , 2 ) and metal salts.

Corroborative Models of the Cobalt(II) Inhibited Fe/Mn Superoxide Dismutases

Attempting to model superoxide dismutase (SOD) enzymes, we designed two new N3O-donor ligands to provide the same set of donor atoms observed in the active site of these enzymes: K(i)Pr2TCMA (potassium 1,4-diisopropyl-1,4,7-triazacyclononane-N-acetate) and KBPZG (potassium N,N-bis(3,5-dimethylpyrazolylmethyl) glycinate). Five new Co(II) complexes (1-5) were obtained and characterized by X-ray crystallography, mass spectrometry, electrochemistry, magnetochemistry, UV-vis, and electron paramagnetic resonance (EPR) spectroscopies. The crystal structures of 1 and 3-5 revealed five-coordinate complexes, whereas complex 2 is six-coordinate. The EPR data of complexes 3 and 4 agree with those of the Co(II)-substituted SOD, which strongly support the proposition that the active site of the enzyme structurally resembles these models. The redox behavior of complexes 1-5 clearly demonstrates the stabilization of the Co(II) state in the ligand field provided by these ligands. The irreversibility displayed by all of the complexes is probably related to an electron-transfer process followed by a rearrangement of the geometry around the metal center for complexes 1 and 3-5 that probably changes from a trigonal bipyramidal (high spin, d7) to octahedral (low spin, d6) as Co(II) is oxidized to Co(III), which is also expected to be accompanied by a spin-state conversion. As the redox potentials to convert the Co(II) to Co(III) are high, it can be inferred that the redox potential of the Co(II)-substituted SOD may be outside the range required to convert the superoxide radical (O2*-) to hydrogen peroxide, and this is sufficient to explain the inactivity of the enzyme. Finally, the complexes reported here are the first corroborative structural models of the Co(II)-substituted SOD.

Synthesis and Crystal Structures of a New μ-Bis(tetradentate) Schiff Base Ligand and its Mononuclear Iron(III) Complex: Iron(III) Induced Imidazolidine Ring Hydrolysis of Binucleating Schiff Base Ligand

The μ-bis(tetradentate) ligand, [C27H26Cl3N4O4], H3L′, 1,3-bis[N-(5-chloro-2-hydroxybenzylidene)- 2-aminoethyl]-2-(5-chloro-2-hydroxyphenyl)imidazolidine and its mononuclear iron(III) complex, [Fe(L)](ClO4), L = N, N′-bis(5-chloro-2-hydroxybenzylidene)-triethylenetetramine have been synthesized and their crystal structures determined. Minimum energy conformations of the ligand were calculated (MOPAC, AM1) as a function of two torsion angles and the results compared with optimized crystal structure. The ligand (H3L′) reacts with Fe(ClO4)2 · 6H2O in aqueous methanol to form the mononuclear [Fe(L)](ClO4) complex with the imidazolidine ring cleaved by hydrolysis. The complex has an N4O2 donor atom set forming a distorted octahedral coordination geometry around the metal atom as established from a crystal structure determination. The terminal oxygen donor atoms occupy cis positions, and the remaining four nitrogen atoms (two cis amine and two trans imine) complete the coordination sphere.

New binuclear metal chelate complexes of copper(II) chloride and acetate with bis(hetarylhydrazones) obtained by condensation of 2,6-diformyl-4-R-phenols with 2-hydrazinoquinoline

Bis(hetarylhydrazones) synthesized for the first time were used as ligands to obtain a number of binuclear copper(II) chelates from copper acetate and chloride. The physicochemical and, first of all, magnetic properties of the resulting complexes were studied. Analysis of the calculated exchange parameters showed that they differ significantly from those obtained previously for analogous binuclear metal chelates containing the identical set of donor atoms. Possible reasons for the differences in the magnetochemical behavior of the binuclear complexes obtained are discussed.