Coordination network solids based on Cu(II) coordination compounds with 1,4-bis-(1,2,4-triazol-1-yl)-butane as a flexible alkyl spacer ligand: Synthesis, characterization and X-ray structures

The coordination chemistry of three selected copper(II) salts with the flexible ligand 1,4-bis(1,2,4-triazol-1-yl)butane (abbreviated as btb) is described. This ligand acts as a bidentate ligand, bridging copper(II) ions, thereby generating polymers in 2D and 3D network solids.     

Cytotoxic activity, X-ray crystal structures and spectroscopic characterization of cobalt(II), copper(II) and zinc(II) coordination compounds with 2-substituted benzimidazoles

Herein we present the synthesis, structural and spectroscopic characterization of coordination compounds of cobalt(II), copper(II) and zinc(II) with 2-methylbenzimidazole (2mbz), 2-phenylbenzimidazole (2phbz), 2-chlorobenzimidazole (2cbz), 2-benzimidazolecarbamate (2cmbz) and 2-guanidinobenzimidazole (2gbz). Their cytotoxic activity was evaluated using human cancer cell lines, PC3 (prostate), MCF-7 (breast), HCT-15 (colon), HeLa (cervic-uterine), SKLU-1 (lung) and U373 (glioblastoma), showing that the zinc(II) and copper(II) compounds [Zn(2mbz)₂Cl₂]·0.5H₂O, [Zn(2cmbz)₂Cl₂]·EtOH, [Cu(2cmbz)Br₂]·0.7H₂O and [Cu(2gbz)Br₂] had significant cytotoxic activity. The isostructural cobalt(II) complexes showed not significant activity. The cytotoxic activity is related to the presence of halides in the coordination sphere of the metal ion. Recuperation experiments with HeLa cells, showed that the cells recuperated after removing the copper(II) compounds and, on the contrary, the cells treated with the zinc(II) compounds did not. These results indicate that the mode of action of the coordination compounds is different.     

Direct synthesis and crystal structures of new heteropolynuclear complexes containing aminoalcohol ligands: From heterobi- (Co/Zn) to heterotrimetallic (Cu/Co/Zn) compounds

Three novel heterometallic complexes [Co₂Zn₂(Dea)₂(HDea)₂(NCS)₄] · 4dmso (1), [CuCoZn₂(Dea)₃Cl₃(HOMe)] · MeOH (2) and [CuCoZn(Me₂Ea)₄(NCS)₂(OAc)] (3) have been prepared from zerovalent zinc, cobalt thiocyanate and a dimethylsulfoxide solution of diethanolamine (1), zerovalent copper, cobalt chloride, zinc chloride and a methanol solution of diethanolamine (2), zerovalent copper, cobalt thiocyanate, zinc acetate and an acetonitrile solution of 2-(dimethylamino)ethanol in air (3) [H₂Dea is diethanolamine and HMe₂Ea is 2-(dimethylamino)ethanol]. The structures of all complexes were determined by single crystal X-ray analysis. Compound 1 consists of a centrosymmetric molecule based on a Co₂Zn₂O₆ core. In contrast to 1, the coordination cores of the structures of the heterotrimetallic complexes, CuCoZn₂O₆ (for 2) and CuCoZnO₄ (for 3) have no inversion centre. Both 2 and 3 possess a similar CuCoZnO₄ fragment that can be viewed as an incomplete distorted cube.     

Transition metal-saccharide chemistry: d-glucose complexes of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II)

Metal complexes of d-glucose (d-Glc) from large cation containing dibromo-dichloro salts of dipositive metals [NEt₄]₂[MBr₂Cl₂] (M = Mn, Co, Ni, Cu and Zn) and the disodium salt of glucose were synthesized from a MeOH:MeCN mixture. The complexes were characterized by UV-vis absorption, circular dichroism, IR and proton magnetic resonance spectroscopies, and by elemental analysis, and were found to be Na[M(d-Glc)(OMe)Cl]. Cyclic voltammetric studies of these complexes, in the acidic to neutral pH range, indicated no dissociation, even in highly acidic conditions.This paper is dedicated to Professor Richard H. Holm (Harvard University) on the occasion of his 60th birthday.     

Variations in the coordination environment of Co, Cu and Zn complexes prepared from a tridentate (imino)pyridine ligand and their structural comparisons

The variations in the coordination environment of Co(II), Cu(II) and Zn(II) complexes with the neutral, tridentate ligand bis[1-(cyclohexylimino)ethyl]pyridine (BCIP) are reported. Analogous syntheses were carried out utilizing either the M(BF₄)₂ · xH₂O or MCl₂ · xH₂O metal salts (where M = Co(II), Cu(II) or Zn(II)) with one equivalent of BCIP. When the hydrated metal starting material was used, cationic, octahedral complexes of the type [M(BCIP)₂]²⁺ were isolated as the tetrafluoroborate salt (4, 5). Conversely, when the hydrated chloride metal salt was used as the starting material, only neutral, pentacoordinate [M(BCIP)Cl₂] complexes (1-3) formed. All complexes were characterized by X-ray diffraction studies. The three complexes that are five coordinate have distortions due mainly to the pyridine di-imine bite angle. The [Cu(BCIP)Cl₂] (2) also exhibits deviations in the Cu(II)-Cl bond distances with values of 2.4242(9) and 2.2505(9) Å, which are not seen in the analogous Zn(II) and Co(II) structures. Similarly, the two six coordinate complexes (5, 6) are also altered by the ligand frame bite angle giving rise to distorted octahedral geometries in each complex. The [Cu(BCIP)₂](BF₄)₂ (6) also exhibits Cu(II)-N_imine bond lengths that are on average 0.14 Å longer than those found in the analogous 5 coordinate complex, [Cu(BCIP)Cl₂]. In addition to X-ray analysis, all complexes were also characterized by UV/Vis and IR spectroscopy with ¹H NMR spectroscopy being used for the analysis of the Zn(II) analogue (3).     

Synthesis, structures, and luminescence properties of mixed ligand Cd(II) and Zn(II) coordination compounds mediated by 1,2-bis(4-pyridyl)ethane

Interaction of cadmium(II) or zinc(II) acetate with 1,2-bis(4-pyridyl)ethane (bpe) in the presence of dioxime(1,2-cyclohexanedionedioxime = NioxH₂ or diphenylglyoxime = dpgH₂) resulted in three complexes with the compositions [Cd₂(CH₃COO)₄(NioxH₂)₂(bpe)(H₂O)₂] (1), [Cd(CH₃COO)₂(bpe)(H₂O)]_n (2) and [Zn(CH₃COO)₂(NioxH₂)(bpe)(H₂O)]_n (3), which were characterized by single-crystal X-ray diffraction, elemental analysis, IR, and luminescence spectroscopy. Dioxime-containing binuclear molecule 1 and 1D linear polymer 3 possess moderate luminescence properties, while the dioxime-free 1D polymer 2 demonstrates strong blue luminescence.     

Syntheses, crystal structures and properties of Cd(II) and Co(II) complexes with 4,4′-dipyridylsulfide ligand

Two new coordination polymers [Cd(dps)₂Cl₂] (1) and [Co(dps)₂(H₂O)₂]·(abs)₂(H₂O)₂ (2) (dps = 4, 4′-dipyridylsulfide, Habs = 4-amino benzenesulfonic acid) have been synthesized under similar conditions and characterized by elemental analysis, fluorescence spectra and single crystal X-ray diffraction. Compound 1 displays a dps-bridged 2D puckered, grid-like layer, which is further linked by C-H?Cl hydrogen bonds to form a 3D supramolecular architecture. Compound 2 shows a dps-bridged double-stranded chain structure, which is extended by N-H?O and O-H?O hydrogen bonds generating a 3D network. Solid-state fluorescence results reveal that both complexes can emit strong emission bands, at 467 nm and 518 nm for 1 and 344 nm for 2, respectively. Magnetic measurements show that there are weak antiferromagnetic interactions between the adjacent Co(II) ions in 2.     

Preparation, separation and characterisation of two regioisomers of a N-hydroxyalkylpyridylpyrazole ligand: A structural investigation of their coordination to Pd(II), Pt(II) and Zn(II) centres

The reaction of the β-diketone 1-phenyl-3-(pyridyn-2-yl)propane-1,3-dione, and the monosubstituted hydrazine 2-hydroxyethylhydrazine has been investigated. Two regioisomers were identified, 2-(3-phenyl-5-(pyridyn-2-yl)-1H-pyrazol-1-yl)ethanol (pzol.1) and 2-(5-phenyl-3-(pyridyn-2-yl)-1H-pyrazol-1-yl)ethanol (pzol.2) in 57:43 ratio. The separation of the regioisomers was done by silica column chromatography using ethyl acetate as eluent.Palladium(II) and platinum(II), [MCl₂(pzol.1)₂], [MCl₂(pzol.2)], and zinc(II), [ZnCl₂(pzol.1)], [ZnCl₂(pzol.2)] complexes were synthesised and characterised. The crystals and molecular structures of [PdCl₂(pzol.2)]·H₂O and [ZnCl₂(pzol.2)] were solved by X-ray diffraction, and consist of mononuclear complexes. In complex [PdCl₂(pzol.2)]·H₂O, the Pd(II) centre has a typical square planar geometry, with a slight tetrahedral distortion. The tetra-coordinated atom is bonded to one pyridinic nitrogen, one pyrazolic nitrogen and two chlorine atoms in cis disposition. The pzol.2 ligand acts as a bidentate chelate forming a five-membered metallocycle ring. In complex [ZnCl₂(pzol.2)], the Zn(II) is five-coordinated with two Zn-N bonds (Zn-N_pz and Zn-N_py), one Zn-OH bond and two Zn-Cl bonds. The coordination geometry is intermediate between a trigonal bipyramid and a square pyramid. In this complex, the ligand pzol.2 is tridentated and forms two metallocycle rings.     

Synthesis and characterization of [H2NC(S)NP(S)(OiPr)2] complexes of Co(II), Ni(II), Zn(II) and Cd(II)

Reaction of the potassium salt of the N-thiophosphorylthiourea H₂NC(S)NHP(S)(OiPr)₂ (HL) with Co(II), Ni(II), Zn(II) and Cd(II) cations in aqueous EtOH leads to the chelate complexes [ML₂] all showing a 1,5-S,S′-coordination formed by the CS and PS sulfur atoms of two deprotonated ligands L⁻. The structures of the resulting compounds were studied by IR, UV-Vis, ¹H, ³¹P{¹H} NMR spectroscopy and microanalysis. The metal center is found in a tetrahedral environment in [CoL₂], [ZnL₂] and [CdL₂]. According to NMR and UV-Vis spectroscopy the metal cation of [NiL₂] exhibits square planar coordination geometry in CH₂Cl₂, CHCl₃ and C₆H₆, while tetrahedral geometry is observed in acetone, DMSO and DMF. Regardless of the solvent used for the crystallization of [NiL₂], the molecular structure in the solid is always square planar as was confirmed by XRD of single crystals and magnetic measurements of the polycrystalline material. The magnetic and photoluminescent properties of all complexes are also reported.     

Syntheses, characterization and antimicrobial activity of the first complexes of Zn(II), Cd(II) and Co(II) with N-benzyloxycarbonylglycine: X-ray crystal structure of the polymeric Cd(II) complex

For the first time, complexes of Zn(II), Cd(II) and Co(II) (1-3) with N-benzyloxycarbonylglycine have been synthesized and characterized. The complexes adopt tetrahedral, pentagonal-bipyramidal and octahedral geometry, respectively. The structure of the polymeric cadmium complex was resolved by single crystal X-ray analysis. The cadmium ion has a distorted pentagonal-bipyramidal coordination formed by two water molecules and two N-benzyloxycarbonylglycinato ligands (N-Boc) coordinated in different fashions, one as bidentate and the second connecting three cadmium atoms. In a rather complicated 2D supramolecular structure, the phenyl rings interact mutually exclusively by the CH?π interactions.Investigation of the antimicrobial activity of the obtained complexes and N-benzyloxycarbonylglycine revealed that the ligand does not inhibit the growth of Candida albicans, whereas the newly synthesized complexes suppress the growth of this human fungal pathogen.     

Study of the coordination behaviour of (3,5-diphenyl-1H-pyrazol-1-yl)ethanol against Pd(II), Zn(II) and Cu(II)

A new easily synthetic route with a 96% yield of ligand 2-(3,5-diphenyl-1H-pyrazol-1-yl)ethanol (L) is obtained. The reactivity of L against Pd(II), Zn(II) and Cu(II) leads to [PdCl₂(L)₂] (1), [ZnCl₂(L)] (2) and [CuCl(L′)]₂ (3) (L′ is the ligand L without alcoholic proton), respectively. According to the different geometries imposed by the metallic centre and the capability of L to present various coordination links, it has been obtained complexes with square planar (1 and 3) or tetrahedral (2) geometry and different nuclearity: monomeric (1 and 2) or dimeric (3). Complete characterisation by analytical and spectroscopic methods, resolution of L and 1-3 by single-crystal X-ray diffraction and magnetic studies for complex 3 are presented.     

Synthesis, crystal structures, and characterization of three mercury(II) halides inorganic-organic hybrid compounds with 1,4-diazabicyclo[2.2.2]octane ligand

Three new hybrid mercury halides compounds with 1,4-diazabicyclo[2.2.2]octan (DABCO) ligand, namely, Hg₂(μ₂-I)₂I₃(H-DABCO) (1), {[Hg₅(μ₂-I)₆I₄(μ₂-DABCO)₂][Hg₂I₄(μ₂-DABCO)]}_n (2) and {[Hg₂(μ₂-Br)₂Br₂(μ₂-DABCO)]·2(HgBr₂)}_n (3), have been synthesized. Compound 1 crystallizes in a P2₁ chiral space group and is a discrete dinuclear mercury(II) compound with inorganic Hg₂(μ₂-I)₂I₃ unit. Compound 2 consists of 1D neutral [Hg₅(μ₂-I)₆I₄(μ₂-DABCO)₂]_n broad polymeric array containing uncommon inorganic decanuclear [Hg₁₀(μ₂-I)₁₂I₈]_n clusters, and neutral guest Hg₂(μ₂-DABCO)I₄ molecules. Compound 3 is composed of a neutral [Hg₂(μ₂-Br)₂Br₂(μ₂-DABCO)]_n polymeric chain and guest inorganic HgBr₂ molecules. In all cases, the halide ions participate in the framework connectivity as bridging ligands. A variety of coordination geometries displayed by the metal sites such as linear, trigonal and tetrahedral are observed in these compounds. Of the three compounds, only compound 3 displays photoluminescence with a emission maximum at 528 nm upon excitation at 468 nm. In addition, these compounds are characterized by IR, elemental analysis, and thermogravimetric analysis.     

Crystal structures and catalytic activities of Zn(II) compounds containing 1,3-bis(4-pyridyl)propane

The structures of new compounds containing Zn(II) ions and bpp (1,3-bis(4-pyridyl)propane) ligands have been determined. The coordinating halides (Br⁻ or Cl⁻) produce one-dimensional compounds 6 and 7, and intra- and inter-chain CH?X (X = Br or I) interactions play roles for building crystal structures with the flexible bpp ligands. The non-coordinating anions do not produce hydroxyl bridged zinc cations or polymeric compounds, and produce only a monomeric complex 4 containing four bpp ligands and two water ligands. Previously reported polymeric compounds 1 and 2 containing hydroxyl-bridged zinc cations [Zn₂OH] were found to carry out the catalytic transesterification of a range of esters with methanol at room temperature under the mild conditions, whereas the rest of compounds did not catalyze the transesterification reactions at all. In addition, the catalysts 1 and 2 have shown even better catalytic activity than zinc salts Zn(NO₃)₂ and Zn(OTf)₂.     

Equilibrium and structural studies on Co(II), Ni(II), Cu(II) and Zn(II) complexes with N-(2-benzimidazolyl)methyliminodiacetic acid: crystal structures of Ni(II) and Cu(II) complexes

Combined pH-metric, UV-Vis, ¹H NMR and EPR spectral investigations on the complex formation of M(II) ions (M=Co, Ni, Cu and Zn) with N-(2-benzimidazolyl)methyliminodiacetic acid (H₂bzimida, hereafter H₂L) in aqueous solution at a fixed ionic strength, I=10⁻¹ mol dm⁻³, at 25 ± 1 °C indicate the formation of M(L), M(H₋₁L)⁻ and M₂(H₋₁L)⁺ complexes. Proton-ligand and metal-ligand constants and the complex formation equilibria have been elucidated. Solid complexes, [M(L)(H₂O)₂] · nH₂O (n=1 for M = Co and Zn, n=2 for M = Ni) and {Cu (μ-L) · 4H₂O}_n, have been isolated and characterized by elemental analysis, spectral, conductance and magnetic measurements and thermal studies. Structures of [Ni(L)(H₂O)₂] · 2H₂O and {Cu(μ-L) · 4H₂O}_n have been determined by single crystal X-ray diffraction. The nickel(II) complex exists in a distorted octahedral environment in which the metal ion is coordinated by the two carboxylate O atoms, the amino-N atom of the iminodiacetate moiety and the pyridine type N-atom of the benzimidazole moiety. Two aqua O atoms function as fifth and sixth donor atoms. The copper(II) complex is made up of interpenetrating polymeric chains of antiferromagnetically coupled Cu(II) ions linked by carboxylato bridges in syn-anti (apical-equatorial) bonding mode and stabilized via interchain hydrogen bonds and π-π stacking interactions.     

Synthesis and structures of pyridinecarboxylate-containing zinc(II) complexes in dien and tren system

Four new zinc(II) complexes [Zn(dien)(μ-nic)]₂(BPh₄)₂·2CH₃OH (1), {[Zn(dien)(isonic)]BPh₄}_n (2), [Zn(tren)(nic)]BPh₄ (3) and [Zn(tren)(isonic)]BPh₄ (4) (dien/tren = diethylenetriamine/triethylenetriamine, nic/isonic = nicotinate/isonicotinate anion) were synthesized and structurally characterized by IR, ¹H NMR and single crystal X-ray diffraction. In the zinc(II) complexes of dien, both nicotinate and isonicotinate connect the zinc(II) ions via N,O-bis-monodentate mode. Complex 1 contains a centrosymmetric dinuclear unit bridged by two nicotinate anions in anti-parallel way. Complex 2 is characterized by an infinite one-dimensional zigzag chain bridged by isonicotinate anion in an end-to-end mode. The Zn···Zn distance is 6.782 for 1 and 8.805 Å for 2. While in the complexes of tren, both 3 and 4 are mononuclear complexes with nicotinate and isonicotinate coordinated to zinc(II) ion through only one oxygen atom of their carboxylate groups. The zinc(II) ions in all of the four complexes are in a distorted trigonal bipyramidal geometry. Complex 3 forms a dinuclear unit and complex 4 forms an infinite 2D sheet structure through intermolecular H-bonds. In all of the crystal lattices, the counterions act to balance the electronic charge at the same time to construct different 3D structures through noncovalent interactions such as C-H···π, N-H···π and van der Waals interactions.     

Syntheses, characterization and X-ray crystal structures of a mono- and a penta-nuclear nickel(II) complex with oximato Schiff base ligands

A mononuclear octahedral nickel(II) complex [Ni(HL¹)₂](SCN)₂ (1) and an unusual penta-nuclear complex [{(NiL²)(μ-SCN)}₄Ni(NCS)₂]·2CH₃CN (2) where HL¹ = 3-(2-aminoethylimino)butan-2-one oxime and HL² = 3-(hydroxyimino)butan-2-ylidene)amino)propylimino)butan-2-one oxime have been prepared and characterized by X-ray crystallography. The mono-condensed ligand, HL¹, was prepared by the 1:1 condensation of the 1,2-diaminoethane with diacetylmonoxime in methanol under high dilution. Complex 1 is found to be a mer isomer and the amine hydrogen atoms are involved in extensive hydrogen bonding with the thiocyanate anions. The dicondensed ligand, HL², was prepared by the 1:2 condensation of the 1,3-diaminopropane with diacetylmonoxime in methanol. The central nickel(II) in 2 is coordinated by six nitrogen atoms of six thiocyanate groups, four of which utilize their sulphur atoms to connect four NiL² moieties to form a penta-nuclear complex and it is unique in the sense that this is the first thiocyanato bridged penta-nuclear nickel(II) compound with Schiff base ligands.     

Two new end-to-end single dicyanamide bridged Cu(II) complexes with Schiff base ligands: Structural, electrochemical and magnetic properties

The synthesis, crystal structures and magnetic properties of two different copper(II) complexes of formula [Cu(L¹)(dca)]_n · nClO₄ (1) and [Cu(L²)]₂(dca)(ClO₄) (2) [L¹ = N,N-dimethylethylene-N′-(pyridine-2-carbaldiiminato), HL² = N,N-dimethylethylene-N′-salicylaldiiminato, dca = dicyanamide anion] are described. Spectroscopic and electrochemical properties have also been discussed. A one-dimensional chain structure with single, symmetrical, μ_1,5-dca bridges is found in compound 1. The copper atom in 1 has a square pyramidal geometry. A tridentate Schiff base ligand, having NNN donor sites, and one nitrogen atom from dca occupy the basal plane. N(18) of a neighbouring unit occupies the apical site. The Schiff base used in compound 2 is a tridentate anion with NNO donor sites, which changes the structure in a dinuclear unit of copper atoms bridged by single end-to-end dicyanamide ion. The environment around copper in 2 is square planar. Magnetic susceptibility measurements for 1 and 2 reveal the occurrence of weak antiferromagnetic interaction through the dca ligand.     

Synthesis and crystal structures of dimeric W(Mo)/Cu/S and polymeric W/Cu/S neutral clusters with flexible 1,4-bis(3,5-dimethylpyrazol-1-yl)butane as a linker ligand

Reactions of [MES₃]²⁻ (M = W, Mo; E = S, O) with CuCl and bbd [1,4-bis(3,5-dimethylpyrazol-1-yl)butane] in DMF affords two new double nest-shape clusters, [WOS₃Cu₃Cl(μ-bbd)]₂ (1) and [MoOS₃Cu₃Cl(μ-bbd)]₂ (2) and a novel one-dimensional polymeric complex [WS₄Cu₂(μ-bbd)]_n (4). The complexes have been characterized by elemental analysis, IR and UV-Vis spectroscopy, and single-crystal X-ray diffraction. In the core structure of the isostructural dimeric [MOS₃Cu₃Cl(μ-bbd)]₂ clusters, the copper atoms have a distorted trigonal planar geometry, with CuS₂N and CuS₂Cl core environments. The [MOS₃Cu₃] fragments are interconnected by a pair of flexible μ-bbd ligands via their nitrogen donor atoms to form a centrosymmetric macrocyclic twin-nest-shaped cluster in which the two fragments are also linked by direct secondary Cu…Cl interactions. Complex 4 represents the first example of a polymeric heterothiometallic cluster, interconnected by bbd ligands, to be structurally characterized by X-ray crystallography. In the repeat unit of 4, the skeleton of the cluster core [CuS₂WS₂Cu] has an essentially linear Cu…W…Cu arrangement. The W atom retains the tetrahedral geometry of the parent [WS₄]²⁻ anion. These cluster cores are linked by bbd bridges having alternately two different conformations to construct a zigzag structure. Complexes [MoS₄Cu₃Cl(bbd)_0.5]₂ (3) and [WS₄Cu₄(NCS)₂(bbd)₂]_n (5) have been synthesized and characterized by elemental analysis, IR and UV-Vis spectroscopy, but we have been unable to grow suitable crystals of 3 and 5 for X-ray analysis.     

Synthesis, characterization, and anion selectivity of copper(II) complexes with a tetradentate Schiff base ligand

A new mononuclear Cu(II) complex, [CuL(ClO₄)₂] (1) has been derived from symmetrical tetradentate di-Schiff base, N,N′-bis-(1-pyridin-2-yl-ethylidene)-propane-1,3-diamine (L) and characterized by X-ray crystallography.The copper atom assumes a tetragonally distorted octahedral geometry with two perchlorate oxygens coordinated very weakly in the axial positions.Reactions of 1 with sodium azide, ammonium thiocyanate or sodium nitrite solution yielded compounds [CuL(N₃)]ClO₄ (2), [CuL(SCN)]ClO₄ (3) or [CuL(NO₂)]ClO₄ (4), respectively, all of which have been characterized by X-ray analysis.The geometries of the penta-coordinated copper(II) in complexes 2-4 are intermediate between square pyramid and trigonal bipyramid (tbp) having the Addition parameters (τ) 0.47, 0.45 and 0.58, respectively.In complex 4, the nitrite ion is coordinated as a chelating ligand and essentially both the O atoms of the nitrite occupy one axial site.Complex 1 shows distinct preference for the anion in the order in forming the complexes 2-4 when treated with a mixture. Electrochemical electron transfer study reveals Cu^IICu^I reduction in acetonitrile solution.     

Comparative study of the influence of the metal centres: Fe(III), Cu(II) and Zn(II), on the ring opening and oxidative dehydrogenation reactions occurring in a coordinated imidazolidine ligand

Condensation of 2-pyridinecarboxaldehyde and 1,9-bis-(2′-pyridyl)-2,5,8-triazanonane, L¹, yields 1-[3-aza-4-(2-pyridyl)butyl]-2-(2-pyridyl)-3-[(2-pyridyl)methyl]imidazolidine, L², as proven by NMR solution spectra. When L² is reacted with Fe(III) in different alcohols, an imidazolidine ring opening and an oxidative dehydrogenation reaction occur resulting in new complexes of the type: [Fe^IIL^n′]²⁺. Compound 1 with a coordinated L^3′ ligand was obtained in n-propanol as a solvent. Compounds 2, 3 and 4 were obtained with L^4′, L^5′ and L^6′ when iso-propanol, n-butanol and iso-butanol were used as solvent, respectively. The structures for 1, 2, 3 and 4 were determined by NMR solution spectra and additionally by X-ray crystallography in the case of the n-butoxy derivative 3. When Cu(II) was used, the hexadentate ligand L² undergoes also an imidazolidine ring opening reaction on complex formation, however, now generating the well-known pentadentate ligand L¹ that is coordinated to the metal ion, 7. Evidence is again provided by the corresponding X-ray structure. With Zn(II) the initial structure of L² is maintained and in this case L² functions as a tetradentate, 5, or bis-tridentate ligand, 6, depending on whether the stoichiometric ratio M:L was 1:1 or 2:1, respectively. This has been proven by a solid-state X-ray structure analysis as well as by NMR solution spectra. The ring opening reaction in the presence of Fe(III) can be explained as a result of a higher Lewis acidity of this metal centre, which decreases the electronic density on the nitrogen atom of the imidazolidinic cycle, thus weakening the nitrogen-carbon bond, favouring the nucleophilic attack on the carbon atom by alcohols and producing a more stable hexacoordinated species. Electrochemical evidence is provided in order to support this reaction mechanism.