Tetradentate Chelating Research Articles

Studies on the synthesis, spectra, catalytic and antibacterial activities of binuclear ruthenium(II) complexes

A new series of stable binuclear ruthenium(II) carbonyl complexes of the general formula [{RuX(CO)(EPh 3) 2} 2L] (where X = H or Cl; E = P or As and L = dibasic tetradentate diacetyl resorcinol (H 2-DAR)) have been synthesised by reacting ruthenium(II) starting complexes [RuHX(CO)(EPh 3) 3] (where X = H or Cl; E = P or As) and 4,6-diacetylresorcinol (H 2-DAR) ligand in benzene medium. The structure of the new binuclear ruthenium(II) carbonyl complexes was established using elemental analysis, spectra (FT-IR, UV–vis and 1H NMR), electrochemical and thermal studies. In these reactions, the 4,6-diacetylresorcinol (H 2-DAR) ligand behaves as a binegative tetradentate chelating ligand coordinating through O,O atoms of both the carbonyl and phenolic C–O groups by replacing a molecule of PPh 3/AsPh 3 and a hydride ion from the starting complexes. Further, all these complexes were also employed as new catalysts for the oxidation of primary and secondary alcohols in the presence of N-methylmorpholine-N-oxide (NMO) as a more viable co-oxidant. The free ligand and their metal complexes have also been screened for their antibacterial activity against the growth of gram +ve and gram −ve bacterial cultures.

Novel 99mTc ‘4 + 1’ peptide conjugates: Tuning the biodistribution by variation of coligands

A sophisticated coligand strategy is presented for peptide-derived radioconjugates based on 99mTc ‘4 + 1’ mixed-ligand complexes. The new pharmacologically active coligands are assessed for 99mTc-labeling of the RGD-peptide cyclo(Arg-Gly-Asp- d-Tyr-Lys) which is an established vehicle to target α vβ 3 integrins playing a crucial part in tumor pathogenesis. Complexes of the general formula [ 99mTc(NS 3R)X] were synthesized and evaluated, in which Tc(III) is coordinated by NS 3R, a derivative of the tetradentate chelator 2,2′,2″-nitrilotriethanethiol (NS 3), and by X, a monodentate binding isocyanide bearing the biomolecule. The novel tetradentate chelators (NS 3R = NS 3crown, NS 3en, NS 3(COOH) 3) constitute NS 3 with a crown ether, an amine or a tricarboxylic acid as pharmacological modifiers. The isocyanides (X = L2-RGD, L2-Lys) contained the linker isocyanobutanoic acid (L2) coupled to N 6-Lys of the RGD-peptide and additionally to a single Lys. The lipophilicity (distribution coefficient log D O / W , pH = 7.4) of the RGD-containing radiotracers decreased in the order of the coligands NS 3crown (−1.7 ± 0.1), NS 3en (−2.7 ± 0.1) and NS 3(COOH) 3 (−3.3 ± 0.1). In the same order of the coligands, the biodistribution of the series [ 99mTc(NS 3R)(L2-RGD)] in normal rats showed a decrease of hepatobiliary and an increase of urinary excretion. The ratio of specifically to unspecifically uptaken activity (sum of surface bound and internalized activity) in α vβ 3 integrin-expressing M21 cells was in the range of approximately 4–5 and comparable for all [ 99mTc(NS 3R)(L2-RGD)] tracers. The biodistribution of [ 99mTc(NS 3en)(L2-RGD)] in ν/ν mice bearing M21 and M21L (control) tumor xenografts exhibited a specific tumor uptake with a low target-background ratio. The metabolic stability of the [ 99mTc(NS 3R)(L2-RGD)] tracers in normal rats was high, since 75–87% of the radioactivity in the plasma extract was assigned to the injected radiotracers 60 min after intravenous application in a rat. The hypothetical metabolites [ 99mTc(NS 3R)(L2-Lys)] were not found. These results demonstrate a considerable improvement of in vivo properties of 99mTc ‘4 + 1’ peptide conjugates and open up the possibility of applying the labeling approach for further radiodiagnostic peptides.

Efficient preparation of 99mTc(III) ‘4+1’ mixed-ligand complexes for peptide labeling with high specific activity

An improved labeling procedure for peptides attached to organometallic (99m)Tc(III) '4+1' mixed-ligand complexes in which the radiometal is coordinated by a tripodal tetradentate chelator 2,2',2''-nitrilotriethanethiol (NS(3)) and a monodentate isocyanide ligand is presented. The labeling procedure was evaluated by the synthesis of [(99m)Tc(NS(3))(L2-RGD)]. The containing radiopharmaceutically interesting RGD-peptide cyclo[Arg-Gly-Asp-D-Tyr-Lys] was modified with 4-isocyanobutanoic acid (L2) as linker conjugated to N(6)-Lys to get the monodentate ligand L2-RGD. The structural identity of the (99m)Tc-conjugate was confirmed by comparison to a Re reference compound. The Tc- and Re-conjugates had matching retention times under identical HPLC conditions. The (99m)Tc-labeling was performed in a novel one-step procedure using the eluate of a (99)Mo/(99m)Tc generator, NS(3), the isocyanide modified peptide, SnCl(2), Na(2)EDTA, mannitol and ascorbic acid in the reaction mixture. Using optimized reagents it is possible to label 50nmol peptide with (99m)Tc within 60min at room temperature with a radiochemical yield higher than 95% and a specific activity of approximately 20GBq/micromol.

Chelation and Bridging of Zinc(II) by Tetradentate N2S2Donor Ligands: Similarities and Differences to Copper(I) and Copper(II) Systems

AbstractReaction of zinc(II) triflate with neutral bis(benzimidazole‐thioether) ligands yielded the complexes [Zn(bmbdh)(H2O)(η1‐SO3CF3)](SO3CF3) (1) with tetradentate chelating 1,6‐bis(N‐methylbenzimidazol‐2‐yl)‐2,5‐dithiahexane (bmbdh), and dinuclear {[Zn(μ‐bmdhp)(H2O)(η1‐SO3CF3)]2}(SO3CF3)2(2) with bis‐bidentate bridging 1,7‐bis(N‐methylbenzimidazol‐2‐yl)‐2,6‐dithiaheptane (bmdhp). In both cases, the oxygen donor ligands aqua and triflate adoptcispositions at the approximately octahedrally configured Zn2+ions. The hexacoordination is facilitated by rather long Zn–S(thioether) distances in contrast to the typically shorter Zn–S(thiolate) bonds from classical zinc finger proteins or their models. With the shorter central (CH2)nchain (n= 2) the ligand bmbdh exhibits facial coordination of the two NSS donor sets in1, similar as in [Cu(bmdhp)]+ions with lower coordinate copper(I) but in contrast to the related [CuII(bmdhp)(H2O)](ClO4)2with the longer chain ligand bmdhp. In2, the bis‐bidentate coordination of bmdhp ligands to each Zn2+ion occurs in an all‐cisfashion for the pairs of N, S and O donor atoms, in contrast to related [CuI(mmb)2](BF4), mmb = 1‐methyl‐2‐methylthiomethyl‐1H‐benzimidazole.

Classical tetradentate chelation and novel bis(bidentate) congregation motifs of a neutral N-donor Schiff base in dinuclear cadmium(II) complexes: Synthesis, structure and luminescence behaviour

Abstract Three dinuclear compounds [Cd2(bpbd)3](Y)4 (Y = ClO 4 - , 1; Y = PF 6 - , 2) and [Cd(bpbd)(Cl)]2(PF6)2 (3) [bpbd = N,N′-(bis(pyridine-2-yl)benzylidene)-1,4-butanediamine] are prepared and characterized. Structures of 1 and 3 are solved by X-ray study that reveals interesting variation in ligational motifs of bpbd from classical tetradentate chelation binding a single metal ion to bis(bidentate) congregation encapsulating two cadmium(II) centers. In the crystalline state, the dinuclear units of 1 pack through cooperative face-to-face π⋯π interactions and C–H⋯O hydrogen bonds whereas those of 3 are engaged in edge-to-face C–H⋯π interactions and C–H⋯F and C–H⋯Cl hydrogen bonds each leading to 2D sheet structure. High-energy intraligand 1(π–π∗) fluorescence at room temperature in DMF solutions and intraligand 3(π–π∗) phosphorescence in glassy solutions (DMF at 77 K) are seen in all the complexes.

Synthesis, characterization and in vitro evaluation of new oxorhenium- and oxotechnetium-CCK4 derivatives as molecular imaging agents for CCK2-receptor targeting

The goal of this study is to design new 99mTc-radiolabelled shortened CCK derivatives that might be suitable for the molecular imaging of cholecystokinin-2 receptors (CCK2-R), these receptors being over-expressed in a number of neuroendocrine tumors such as medullary thyroid cancer and small-cell lung cancer. For this purpose, we designed several modified CCK4 analogs bearing an ON2S tetradentate chelating agent at the N-terminus, the CCK4 sequence representing the minimal peptide sequence that presents nanomolar affinity and activity towards the CCK2-R. Four peptide conjugates of general formula (Trt)SN2OPh–(X)n–CCK4 (X=β-alanine or 6-aminohexanoic acid spacers; n=0, 2, 4) and their oxorhenium peptide conjugates have been synthesized and characterized. In vitro evaluation of these compounds showed a close relationship between the nature and the length of the spacer and the corresponding binding affinity values. The most promising oxorhenium complex 5-Re exhibited potent CCK2-receptor agonist properties in promoting the production of inositol phosphate in COS-7 cells (EC50=5.17nM). Preliminary 99mTc-radiolabelling studies with peptide conjugates 3 or 5 led exclusively to the corresponding 99mTcO-complexes 3-Tc and 5-Tc, which exhibited high resistance towards an excess of cysteine and satisfactory stabilities in human serum. To conclude, the promising in vitro characteristics of compounds 5-Re, 5-Tc illustrate the feasibility to develop stable radiolabelled shortened CCK4 derivatives with a nanomolar CCK2-R affinity.

Aryl-Containing Chelates and Amine Debenzylation to Afford 1,3-Di-2-pyridyl-2-azaallyl (smif): Structures of {κ-C,N,Npy2-(2-pyridylmethyl)2N(CH2(4-tBu-phenyl-2-yl))}FeBr and (smif)CrN(TMS)2

Aryl-bromide ligand precursors have been prepared with the potential to afford tetradentate chelates (2-pyridylmethyl)(3-x)N(CH(2)-2-Aryl)(x) (x = 1, 2) containing metal-aryl linkages that promise to impart stronger fields about first row transition metals. Oxidative addition to Ni(COD)(2) afforded two diamagnetic Ni(II) complexes, {kappa-C,N,N(py)-(2-pyridylmethyl)N(CH(2)(4-(t)Bu-phenyl-2-yl))(CH(2)(4-(t)Bu-phenyl-2-Br))}NiBr (1-Ni) and {(kappa-C,N,N(py)(2)-(2-pyridylmethyl)(2)N(CH(2)(4-(t)Bu-phenyl-2-yl))}NiBr (2-Ni) in 96% and 67% yield, respectively. Extending these synthetic efforts to iron provided {kappa-C,N,N(py)(2)-(2-pyridylmethyl)(2)N(CH(2)(4-(t)Bu-phenyl-2-yl))}FeBr (2-Fe, X-ray) in 91% yield via reduction of an adduct, {kappa-N,N(py)(2)-(2-pyridylmethyl)(2)N(CH(2)(4-(t)Bu-phenyl-2-Br))}FeBr(2) (3-Fe). 5-Coordinate 2-Fe possessed a pseudo-tbp structure, and SQUID magnetometry showed it to be S = 2 with significant zero field splitting (ZFS). 2-Fe was initially prepared via oxidative addition to Fe{N(TMS)(2)}(2)(THF) upon disproportionation to "Fe(0)" and 2 Fe{N(TMS)(2)}(3), but when this approach was attempted with Cr{N(TMS)(2)}(2)(THF)(2), the azaallyl complex {kappa-N,N(py)(2)-1,3-dipyridyl-2-azaallyl}CrN(TMS)(2) ((smif)CrN(TMS)(2), 4-Cr, X-ray), formed instead (>50%) via amine debenzylation. An alternative route consisting of addition of 1,3-di-2-pyridyl-2-azapropene to Cr{N(TMS)(2)}(2)(THF)(2) afforded 4-Cr in 74% yield. Pseudo-square planar 4-Cr was also S = 2 (SQUID) with marked ZFS. The dipyridylazaallyl ligand "smif" imparts a remarkable optical density to 4-Cr via intraligand bands at 675 nm (epsilon approximately 15,000 M(-1)cm(-1)) and 396 nm (epsilon approximately 27,000 M(-1)cm(-1)). The effective fields of the chelate complexes are discussed, and a comparison of smif to isoelectronic NHC ligands is given.

High catalytic activities in the norbornene polymerization with neutral palladium complexes containing N4-type tetradentate chelating ligands

Norbornene polymerization catalyzed by new Pd(II) complexes bearing N4-type tetradentate ligands obtained from the reaction between a 6-methyl-2-picolinic acid or picolinic acid and appropriate diamines has been studied. A class of new palladium complexes, [Pd(X 1X 2bpb)] and [Pd(X 1X 2-6-Me 2bpb)] (X 1 = Me, X 2 = Me ( 1 and 4); X 1 = H, X 2 = H ( 2 and 5); X 1 = H, X 2 = NO 2 ( 3 and 6); bpb = N,N′-( o-phenylene)bis(pyridine-2-carboxamidate); 6-Me 2bpb = N,N′-( o-phenylene)bis(6-methylpyridine-2-carbox-amidate)) were synthesized and characterized. The molecular structure of Pd complex 5 was determined by X-ray crystallography, showing distorted square planar configurations. Using modified methylaluminoxanes (MMAO) as an activator, the palladium complexes exhibited high catalytic activities for the polymerization of norbornene. The catalytic activities up to 4.0 × 10 6 g of PNBEs/mol Pd·h and M w up to 8.34 × 10 5 g/mol with PDI < 2.53 were observed. Amorphous polynorbornenes (PNBEs) were obtained with good solubility in halogenated aromatic solvents. Interestingly, the structural modification with the methyl groups of pyridyl rings and the strong electron-withdrawing substituents induced improvement in solubility, thermal stability and catalytic activity. FT-IR, 1H, and 13C NMR analyses of the polymers suggest that the catalytic polymerization occurs via vinyl addition mechanism.

Supramolecular networks constructed from mono- and bi-nuclear lanthanide complexes with 1,2-phenylenedioxydiacetic acid

[Tb2(1,2-pdoa)3 · 6H2O] · H2O (1) and [La(1,2-pdoa)(1,2-H2pdoa)(OH) · H2O] · 5H2O (2) (1,2-H2pdoa = 1,2-phenylenedioxydiacetic acid) have been synthesized and structurally characterized by single crystal X-ray diffraction. Complex 1 is a binuclear molecule in which one 1,2-pdoa ligand is a tetradentate bridge linking two Tb3+ ions, the other two 1,2-pdoa ligands bond Tb13+ and Tb1A3+ via tetradentate chelating coordination. Tb3+ is nine-coordinate by six oxygens of 1,2-pdoa and three waters. Complex 2 is mono-nuclear with La3+ ten-coordinate by eight oxygens of two 1,2-pdoa, one hydroxide and one water. 1,2-Pdoa is tetradentate chelating with La3+ ion. The packing diagrams of 1 and 2 show supramolecular networks via H-bonds. The fluorescence spectrum of 1 shows characteristic emission of Tb3+ with 5D4 → 7Fj (j = 6–3) transitions.

Microwave assisted synthesis, spectroscopic and antibacterial studies of titanocene chelates of Schiff bases derived from 3-substituted-4-amino-5-hydrazino-1,2,4-triazoles

The reactions of bis(cyclopentadienyl)titanium(IV) chloride with a new series of Schiff bases (LH2), derived by condensing 3-(phenyl/2-chlorophenyl/4-nitrophenyl)-4-amino-5-hydrazino-1,2,4-triazoles with salicylaldehyde or 2-hydroxyacetophenone, have been studied both by conventional stirring method and also by using microwave technology. The products of type [(η5-C5H5)2Ti(L)] have been isolated in both cases. Tentative structural conclusions are drawn for the reaction products based upon physico-chemical and spectroscopic methods. The ligands behave as dibasic, tetradentate chelating agents and a six-coordinated structure have been assigned to these derivatives. Studies were conducted to assess the growth inhibiting potential of the free Schiff bases and their complexes against various bacterial strains.

Template synthesis and characterization of oxovanadium(IV) complexes with tetraaza macrocyclic ligands and their activity on potato virus X

The oxovanadium(IV) complexes (I) of the type [VO(L)]SO4 have been prepared using an in-situ method of synthesis with ligands derived from di-2-thienylethanedione with 1,2-diaminobenzene or 2,3-diaminopyridine. These parent complexes have been further reacted with μ-diketones to yield macrocyclic complexes (II) of types [VO(mac)]SO4 (where mac = macrocyclic ligands derived by condensation of amino group of parent complex with μ-diketones), wherein the VO2+ cation acts as a template. Tentative structures of these complexes have been proposed on the basis of elemental analysis, electrical conductance, magnetic moments and spectral (infrared, electronic and electron spin resonance) data. The oxovanadium(IV) complexes are five coordinated wherein the tetraaza macrocyclic ligands act as tetradentate chelating agents. All the complexes are found to inhibit the infectivity of potato virus X, when checked using the test plant Chenopodium amaranticolor.

Organotin(IV) oxo -homoscorpionate: preparation, spectroscopic characterization and antimicrobial properties

An oxo-homoscorpionate ligand [potassium bis(phthalato)borate] (KL) was prepared by solid state reaction of potassium borohydride and phthalic acid (1 : 3 molar ratio). The ligand is uninegative and can be a bi, tri and tetra-dentate chelating agent. The di- and tri-organotin(IV) complexes of this ligand are formed by replacement of chloride in R2SnCl2 and (R = methyl, butyl and R′ = phenyl and butyl). The ligand and all its complexes were characterized by elemental analyses and spectral studies (IR, 1H, 13C, 119Sn NMR and ESI mass spectra). Spectroscopic data reveal that all the complexes are hexacoordinate; the diorganotin complexes have trans octahedral geometry while the triorganotin complexes are distorted octahedral geometry. The toxicity of these organotin(IV) derivatives on selected microbes was considered. The compounds exhibit antibacterial (Bacillus anthracius and Escherichia coli) and antifungal (Candida albicans and Penicillium italicum) activities in vitro. The ligand shows less toxicity towards the microorganisms and its toxicity significantly increased after complexation with organotin(IV). Triorganotin derivatives (R3SnL) of the ligand are more effective compared to diorganotin derivatives (R2SnL2).

Synthesis and characterization of a chiral Schiff base containing α- d-altropyranoside unit and its zinc(II) complex

A novel chiral Schiff base containing α- d-altropyranoside unit, methyl 4,6- O-benzylidene-3-deoxy-3-(2-salicylideneaminoethylamino)-α- d-altropyranoside ( 3, Me-Alt-NNOH), has been prepared. Treatment of zinc chloride with 3 in the presence of triethylamine afforded a five-coordinated zinc(II) complex [Zn(Me-Alt-NNO)Cl] ( 4). Both 3 and 4 have been characterized by infrared, 1H NMR, 13C NMR, MS, and elemental analysis. The crystal structure of 4 has been determined by X-ray diffraction. Crystal structure analysis shows that the complex 4 exits a distorted triangular bipyramid geometry and the Schiff base motif acts as a uninegatively charged tetradentate chelating agent. The methoxy and amino groups cis-chelate to the zinc atom and the pyrano-ring is in an ideal 4 C 1 chair conformation.

Spectral and electro-chemical characterization of transition metal complexes of a modified [N6] macrocycle. A mimic to cyclic hexapeptide

The 16-membered modified [N6] macrocylic ligand (L), a mimic to cyclic, hexapeptide is reacted with MCl2 and MCl3 resulting in complexes with stoichiometrices [MLCl2] (M = Cr, Mn, Co, Ni, Cu), [MLCl3] (M = Pt, Pd) and [MLCl2]Cl (M = Fe, Ru). Its reactions with the precursors [M(Ph3P)2Cl2] (M = Co, Ni, Pt, Pd) follow a ligand displacement path affording the final products which do not contain coordinated Ph3P. Complexes have been characterized from results of elemental analyses, conductometric, magnetic susceptibility, i.r. and u.v.–vis (ligand field) spectral studies. Magnetic susceptibility and ligand field spectral data are consistent with a hexacoordinate geometry for Cr2+, Mn2+, Fe3+, Co2+, Ni2+ and Cu2+ and four coordinate square-planar geometry for Pt2+ and Pd2+. Molecular orbital computations using CSChem ultra MOPAC software for an optimized minimum energy plot of the structure shows that the ligand binds metal ions as a tetradentate (N,N,N,N) chelating agent. Cyclic voltammetric studies indicate formation of stable reversible or quasi-reversible redox couples in solutions, which corroborates a kinetic stability of these complexes in their variable oxidation states.

Nickel(II) and copper(II) complexes with chiral bis-α-thiooxime, the derivative of natural terpenoid (+)-3-carene: Synthesis and structures

Chiral bis-α-thiooxime (H2L1), the derivative of the natural monoterpenoid (+)-3-carene, was synthesized and used to prepare paramagnetic complexes of the composition M(H2L1)Cl2 (M=Ni, Cu). The crystal structures of [Ni(H2L1)Cl2] (I) and [Cu(H2L1)Cl2] (II) were determined by X-ray diffraction analysis. Crystals I and II consist of mononuclear acentric molecules. The Ni2+ ion in a molecule of complex I coordinates two N atoms and two S atoms of a tetradentate chelating ligand (the H2L1 molecule) and two Cl atoms. The NiCl2N2S2 coordination core forms octahedron compressed along the apical N atoms. In a molecule of complex II, the Cu2+ ion coordinates two S atoms and the N atom of a tridentate chelating H2L1 ligand and two Cl atoms. The CuCl2NS2 coordination core forms a trigonal bipyramid.

Hydrolytic cleavage of Schiff bases by [RuCl 2(DMSO) 4

New hexa-coordinated Ru(II) complexes of the type [RuCl 2(DMSO) 2(diamine)] (diamine = o-phenylenediamine and ethylenediamine) have been prepared by reacting cis-[RuCl 2(DMSO) 4] with Schiff bases (H 2sal-en, 1; H 2nap-en, 2; H 2sal- o-pdn, 3; H 2nap- o-pdn, 4) in a 1:1 ratio. The ligands, which were expected to act as tetradentate (N 2O 2) chelates under the normal reaction conditions, were found to undergo hydrolytic cleavage to form the diamine and the corresponding aldehyde. All the complexes have been characterized by analytical and spectroscopic (IR, electronic and 1H NMR) data. Single-crystal X-ray analysis of the complex [RuCl 2(DMSO) 2( o-pndn)] revealed that the coordination environment around the ruthenium metal consists of a N 2S 2Cl 2 octahedron.

Synthesis and properties of copper(II) complexes with a chiral dioxatetraazamacrocyclic ligand based on a natural monoterpene, (+)-3-carene

The copper(II) compounds [CuL](NO3)2 · H2O (I), [CuL](ClO4)2 · H2O (II), CuLCl2 · 3H2O (III), and CuLBr2 · 4H2O (IV), where L is a chiral dioxatetraazamacrocyclic ligand based on the natural monoterpene (+)-3-carene, have been synthesized. According to IR and EPR spectroscopy, L acts as a tetradentate chelating ligand coordinated through the N atoms of the NH and C=N groups. The NO 3 − anions in I and the ClO 4 − anions in II are outer-sphere. I and II have a planar coordination core CuN4, III has a CuN4ClO coordination core, and IV has a CuN4Br2 coordination core.

Extraction Behavior of Copper(II) Cation Using Sulfonamide-type Polyanionic Chelators Having Ethylene Bridges

To research possible use of sulfonamide-type polyanionic chelators having ethylene bridges as extractants, extraction (cation-exchange) behavior of Cu2+ with several chelators was investigated. Tripod sulfonamide-type chelator, tris [2- (p-toluenesulfonamido) ethyl] amine, showed higher extractability than its monosulfonamide analog, 2- (p-toluenesulfonamido) ethylamine. Although the former can be generally considered as trianionic tetradentate chelator, on extraction of Cu2+, it acted as dianionic tridentate ligand and formed neutral 1: 1 complex having one coordinating H2O. The formed complex seems to be stabilized by chelate effect of the ligand and shielding effect of the coordinating sulfonamido group.

Precatalysts Involved in Copper-Catalyzed Arylations of Nucleophiles

In spite of the importance of ligand/copper-catalyzed arylations of nucleophiles in organic chemistry, their mechanism and in particular the interactions between ligands and copper have not been well understood until now. In this work, we synthesized a precatalyst involving one of the best ligands of the literature, the tetradentate chelator 1, and characterized it at the solid state and in solution (NMR, electrochemistry, X-ray crystallography). Thanks to the information collected, we now have a better understanding of the role of the ligand (influence on the solubility of copper and on its electrochemical properties) and the role of the solvent and can propose a description of the various reactions taking place at the early stages of copper-catalyzed arylations of nucleophiles. We thus showed that, in acetonitrile, the association of tetradentate ligand and copper(I) salts leads to the formation of an insoluble dimeric complex, behaving like a copper(I) reservoir. A very small part of the latter is soluble, but its structure is lost to the benefit of a monomeric form. This monomer, in which the copper center is at the +I oxidation level and made electron-rich by the ligand, is the only species present at the beginning of the catalytic process.

Synthesis and Characterization of Mononuclear Octahedral Fe(III) Complex Containing a Biomimetic Tripodal Ligand, N-(Benzimidazol-2-ylmethyl)iminodiacetic Acid

The mononuclear iron complex 1, <TEX>$Fe^{III}$</TEX>(Hbida)Cl(<TEX>$H_2O$</TEX>), was synthesized using a tripodal tetradentate ligand, N-(benzimidazol-2-ylmethyl)iminodiacetic acid (H3bida), which has two carboxylate groups, one benzimida- zoyl group, and one tertiary amine where it serves as a tetradentate chelating ligand for the octahedral Fe(III) ion. The four equatorial positions of the octahedral complex are occupied by two monodentate carboxylates, a benzimidazole nitrogen, and an oxygen of a water molecule. One of the axial positions is occupied by an apical nitrogen of the Hbida and the other by a chloride anion. The mononuclear octahedral complex 1 mimics the geometry of the key intermediate structure of the catalytic reaction cycle proposed for the FeSODs, which is a distorted octahedral geometry with three histidyl imidazoles, an aspartyl carboxylate, a superoxide anion, and a water molecule. The redox potential of complex 1, <TEX>$E_{1/2}$</TEX> is -0.11V vs. Ag/AgCl (0.12 V vs. NHE), which is slightly lower than those reported for the most FeSODs. The magnetic susceptibility of complex 1 at room temperature is 5.83 <TEX>$\mu$</TEX>B which is close to that of the spin only value, 5.92 <TEX>$\mu$</TEX>B of high-spin d5 Fe(III).