Synthesis and structures of dimeric iron(III)-Oxo and -imido complexes containing intramolecular hydrogen bonds

Hydrogen bonding networks proximal to metal centers are emerging as a viable means for controlling secondary coordination spheres. This has led to the regulation of reactivity and isolation of complexes with new structural motifs. We have used the tridenate ligand bis[(N′-tert-butylureido)-N-ethyl]-N-methylaminato ([H₂1]²⁻) that contains two hydrogen bond donors to examine the oxidation of the Fe^II-acetate complex, [Fe^IIH₂1(η²-OAc)]⁻ with dioxygen, amine N-oxides, and xylyl azide. A complex with Fe^III-O-Fe^III core results from the oxidation with dioxygen and amine N-oxides, in which the oxo ligand is involved in hydrogen bonding to the [H₂1]²⁻ ligand. A distinctly different hydrogen bonding network was found in Fe^III dimer isolated from the reaction with the xylyl azide: a rare Fe^III-N(R)-Fe^III core was observed that does not have hydrogen bonds to the bridging nitrogen atom. The intramolecular H-bond networks within these dimers appear to adjust to the presence of the bridging species and rearrange to its size and electron density.     

Adenine complexes with divalent 3d metal chlorides

Upon refluxing 2:1 mixtures of adenine (adH) and divalent 3d metal chloride hydrates in a 7:3 (v/v) mixture of ethanol-triethyl orthoformate for several days, partial substitution of ad^? for Cl^? ligands occurs, and solid complexes of the M(ad)Cl· 2H₂0 (M = Mn, Zn), Fe₂(ad)(adH)₂Cl₃·2H₂O, M(ad)- (adH)Cl·H₂O (M = Co, Cu) and Ni₂(ad)₃Cl·6H₂O types are eventually isolated [1]. It is probably of interest that during analogous previous synthetic work, involving interaction of ligand and salt in refluxing ethanol, no substitution reactions between Cl^? and ad^? took place, and MCl₂ adducts with neutral adH were reportedly obtained. Characterization studies suggest that the new complexes reported are linear chainlike polymeric species, involving single adenine bridges between adjacent M²⁺ ions. Terminal chloro, adenine and aqua ligands complete the coordination around each metal ion. The new Ni²⁺ complex is hexacoordinated, whilst the rest of the complexes are pentacoordinated. Most likely binding sites are considered to be N(9) for terminal unidentate and N(7), N(9) for bridging bidentate adenine [1].     

Risedronate metal complexes potentially active against Chagas disease

In the search for new metal-based drugs for the treatment of Chagas disease, the most widespread Latin American parasitic disease, novel complexes of the bioactive ligand risedronate (Ris, (1-hydroxy-1-phosphono-2-pyridin-3-yl-ethyl)phosphonate), [M^II(Ris)₂]·4H₂O, where M═Cu, Co, Mn and Ni, and [Ni^II(Ris)₂(H₂O)₂]·H₂O were synthesized and characterized by using analytical measurements, thermogravimetric analyses, cyclic voltammetry and infrared and Raman spectroscopies. Crystal structures of [Cu^II(Ris)₂]·4H₂O and [Ni^II(Ris)₂(H₂O)₂]·H₂O were solved by single crystal X-ray diffraction methods. The complexes, as well as the free ligand, were evaluated in vitro against epimastigotes and intracellular amastigotes of the parasite Trypanosoma cruzi, causative agent of Chagas disease. Results demonstrated that the coordination of risedronate to different metal ions improved the antiproliferative effect against T. cruzi, exhibiting growth inhibition values against the intracellular amastigotes ranging the low micromolar levels. In addition, this strong activity could be related to high inhibition of farnesyl diphosphate synthase enzyme. On the other hand, protein interaction studies showed that all the complexes strongly interact with albumin thus providing a suitable means of transporting them to tissues in vivo.     

Synthesis and chemical characterization of Cu, Ni and Zn complexes of 3,5-bis(2′-pyridyl)-1,2,4-oxadiazole and 3-(2′-pyridyl)5-(phenyl)-1,2,4-oxadiazole ligands

The synthesis and structural characterization of Ni^II, Cu^II and Zn^II complexes of two chelating 1,2,4-oxadiazole ligands, namely 3,5-bis(2′-pyridyl)-1,2,4-oxadiazole (bipyOXA) and 3-(2′-pyridyl)5-(phenyl)-1,2,4-oxadiazole (pyOXA), is here reported. The formed hexacoordinated metal complexes are [M(bipyOXA)₂(H₂O)₂](ClO₄)₂ and [M(pyOXA)₂(ClO₄)₂], respectively (M = Ni, Cu, Zn). X-ray crystallography, ¹H and ¹³C NMR spectroscopy and C, N, H elemental analysis data concord in attributing them an octahedral coordination geometry. The two coordinated pyOXA ligands assume a trans coplanar disposition, while the two bipyOXA ligands are not. The latter result is a possible consequence of the formation of H-bonds between the coordinated water molecules and the nitrogen atom of the pyridine in position 5 of the oxadiazole ring. The expected splitting of the d metal orbitals in an octahedral ligand field explains the observed paramagnetism of the d⁸ and d⁹ electron configuration of the nickel(II) and copper(II) complexes, respectively, as determined by the broadening of their NMR spectra.     

Highly flexible O,O′,N ligands and their Fe, Ni, Cu and Zn complexes

Three new chiral ligands bearing an O,O′,N donor set (O_methoxyO_hydroxyN_pyridine) were synthesised and coordinated to Fe^III, Fe^II, Ni^II, Cu^II and Zn^II to yield complexes with the general formula [M(OON)Cl_x]_y. While the pyridine N and the hydroxy O atoms coordinate strongly to all applied metal ions, the methoxy donor seems not to be involved in coordination, although some evidence for a weak interaction between O_Me and the Zn^II were found in NMR spectra. In the bidentate O′,N coordination mode the new ligands exhibit several coordination geometries as analysed in the solid compounds by XRD, EXAFS and EPR and in solution by UV-Vis absorption, cyclic voltammetry, EXAFS, EPR or NMR spectroscopy.     

Nickel(II) and iron(II) mononuclear complexes with 1-methylimidazole-2-aldoximate: New building units for molecule-assembled magnetic materials

A new class of mononuclear metal complexes with 1-methylimidazole-2-aldoximate (miao) has been synthesized and characterized: trans-Ni^II(Cl)₂(Hmiao)₂ (1), trans-Ni^II(miao)₂(py)₂ (2), NO-trans-Ni^II(miao)₂(phen) (3), and NO-trans-Fe^II(miao)₂(phen) (4). The crystal structures of 2, 3, and 4 have been determined by single-crystal X-ray crystallography. Compound 1 having the protonated miao ligand (i.e., Hmiao) is a precursor for synthesizing 2 and 3. Compound 2 is an octahedral Ni^II complex surrounded by two miao bidentate ligands and two monodentate ligands of pyridine in a trans-arrangement. Compound 3 is a cis-type octahedral Ni^II complex with two miao ligands and a bidentate ligand of 1,10-phenanthroline, in which the ligand arrangement around Ni^II center is found in an NO-trans form. Compound 4 is an isostructural Fe^II derivative of 3. Compounds 1, 2, and 3 exhibit paramagnetic nature with an S = 1 spin and a positive zero-field splitting, among which it for 3 is overlapped with intermolecular ferromagnetic interaction (zJ/k_B = +0.16 K). Compound 4 is diamagnetic due to the existence of low-spin Fe^II ion.     

Synthesis, characterization, thermal behavior, and DNA-cleaving studies of cyano-bridged nickel(II)-copper(II) complexes of 4-(pyridin-2-ylazenyl)resorcinol

We present here the syntheses of a mononuclear Cu^II complex and two polynuclear Cu^II Ni^II complexes of the azenyl ligand, 4‐(pyridin‐2‐ylazenyl)resorcinol (HL; 1). The reaction of HL ( 1 ) and copper(II) perchlorate with KCN gave a mononuclear complex [CuL(CN)] ( 4 ). Using 4 , one pentanuclear complex, [{CuL(NC)}₄Ni](ClO₄)₂ ( 5 ) and one trinuclear complex, [{CuL(CN)}₂NiL]ClO₄ ( 6 ), were prepared and characterized by elemental analyses, magnetic susceptibility, molar conductance, IR, and thermal analysis. Stoichiometric and spectral results of the mononuclear Cu^II complex indicated that the metal/ligand/CN ratio was 1 : 1 : 1, and the ligand behaved as a tridentate ligand forming neutral metal chelates through the pyridinyl and azenyl N‐, and resorcinol O‐atom. The interaction between the compounds (the ligand 1 , its Ni^II and Cu^II complexes without CN, i.e., 2 and 3 , and its complexes with CN, 4 – 6 ) and DNA has also been investigated by agarose gel electrophoresis. The pentanuclear Cu₄Ni complex ( 5 ) with H₂O₂ as a co‐oxidant exhibited the strongest DNA‐cleaving activity.     

Porous Co and Ni coordination polymers based on 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol for various solvent inclusion

The reactions of 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol (Hpyt) with typical octahedral metal ions (Co^II and Ni^II) under different conditions generate a series of porous coordination polymers with the general formula of {[M(pyt)₂(H₂O)₂] · (solvents)}_n. In all these complexes, the versatile pyt anionic ligand behaves as the thioamide isomer with the μ-N_py,N_oxa binding fashion, as confirmed by the X-ray crystallographic studies. Generally, the pyt linkers connect the metal centers to afford uniform 2-D grid-like host coordination frameworks, which stack in a parallel manner to engender the final 3-D crystalline lattices with 1-D open channels. Especially for Co^II, different reactive routes and media lead to the productions of four analogous compounds with the inclusion of varied guest solvates, in one of which unusual water clusters are observed.     

Synthesis, crystal structures, and molecular hyperpolarizabilities of a new Schiff base ligand, and its copper(II), nickel(II), and cobalt(II) metal complexes

A new ligand (HL) obtained from the Schiff base condensation of 4-(diethylamino)salicylaldehyde with 4-nitroaniline is reported, with its nickel(II), copper(II), and cobalt(II) complexes. The crystal structures are reported for the four derivatives. While, Ni^IIL₂ and Cu^IIL₂ are centrosymmetric molecules, Co^IIL₂ exhibits a pseudo-tetrahedral molecular structure. The quadratic hyperpolarizabilities (β) of HL and Co^IIL₂, measured by electric field induced second harmonic (EFISH) technique, are equal to 66 and 110 × 10⁻³⁰ cm⁵ esu⁻¹, respectively. Beside a geometric effect (pseudo-T_d symmetry), the coordination of the metal center provides an intrinsic enhancement of the NLO response. In addition, an enhancement of the thermal stability of about 60° is found upon metal complexation.     

Xanthine complexes with 3d metal perchlorates

Complexes of xanthine (xnH) with 3d metal perchlorates were prepared by refluxing mixtures of ligand and metal salt in ethyl acetate-triethyl orthoformate. In all cases, partial substitution of anionic xn⁻ for ClO₄⁻ groups occurs, and the solid complexes isolated also contain invariably two neutral xnH ligands per metal ion, viz. Cr(xn)₂(xnH)₂ClO₄, Fe(xn)₂(xnH)₂ClO₄·H₂O, M(xn)(xnH)₂ClO₄·H₂O (M = Fe, Co, Ni) and M(xn)(xnH)₂ClO₄· 2H₂O (M = Mn, Zn). The new complexes are generally hexacoordinated and appear to be linear chainlike polymeric species characterized by a (-Mxn-)_n single-bridged backbone. Four terminal ligands per metal ion, including two xnH groups in all cases, complete its inner coordination sphere; the remaining two terminal ligands differ from complex to complex as follows: M = Cr³⁺ xn⁻, -OClO₃; Fe³⁺ xn⁻, H₂O; Fe²⁺, Co²⁺, Ni²⁺OClO₃, H₂O; Mn²⁺, Zn²⁺ two aqua ligands. Probable binding sites of bidentate bridging xn⁻ and unidentate terminal xnH and xn⁻ are discussed.     

Complex formation and stability of westiellamide derivatives with copper(II)

The Cu^II coordination chemistry of three synthetic analogues of westiellamide (H₃L^wa) with an [18]azacrown-6 macrocyclic structure and imidazole (H₃L¹), oxazole (H₃L²), or thiazole (H₃L³) heterocyclic donors in addition to the peptide groups, is reported. The N_heterocycle–N_peptide–N_heterocycle binding sites are highly preorganized for the coordination to Cu^II ions. The stability constants of mono- and dinuclear Cu^II complexes of H₃L¹, H₃L², and H₃L³, obtained by isothermal titration microcalorimetry, are reported. EPR and NMR spectroscopy as well as electrospray ionization mass spectrometry (ESI-MS) were used to characterize the complexes formed in solution. The stabilities of the mononuclear and dinuclear Cu^II complexes of the three ligands are in the range of 10⁵ M⁻¹, but there are subtle differences; specifically the oxazole-derived ligand has, in contrast to the other two macrocycles, a negative formation entropy for coordination to the first Cu^II ion and a higher stability for complexation to a second Cu^II center in comparison with the first Cu^II center (cooperativity). Differences between the three ligands are also apparent in terms of the formation mechanism. With the oxazole-based ligand H₃L², NMR spectroscopy, EPR spectroscopy, and ESI-MS indicate the formation of a ligand–Cu^II 2:1 intermediate, and this may explain the differences in the formation entropy as well as the cooperativity.     

Hydrothermal synthesis, crystal structures and properties of new Fe, Co, Ni, and Zn complexes with 6-quinolinecarboxylate: Interplay of coordinative and noncovalent interactions

Reactions of Fe^II, Co^II, Ni^II, and Zn^II salts with 6-quinolinecarboxylic acid (HL) under the hydrothermal conditions afford three monomeric complexes [M(L)₂(H₂O)₄] (M = Fe^II for 1, Co^II for 2, and Ni^II for 3) and a 1-D polymeric species {[Zn(L)₂(H₂O)] · H₂O}_n (4). The crystal structures of the ligand HL and these four complexes have been determined by using the X-ray single-crystal diffraction technique. The results suggest that complexes 1-3 are isostructural, displaying novel 3-D pillar-layered networks through multiple intermolecular hydrogen bonds, whereas in coordination polymer 4, the 1-D comb-like coordination chains are extended to generate a hydrogen-bonded layer, which is further reinforced via aromatic stacking interactions. Solid-state properties such as thermal stability and fluorescence emission of the polymeric Zn^II complex 4 have also been investigated.     

Azo-containing pyridine amide ligand. A six-coordinate nickel(II) complex and its one-electron oxidized species: Structure and properties

A new potentially tridentate ligand HL¹¹ consisting of 2-pyridinecarboxamide unit and azo functionality has been used, in its deprotonated form, to prepare a nickel(II) complex which has been structurally characterized. The ligand L¹¹(−) affords a bis-complex [Ni^II(L¹¹)₂] (1). In 1, the two L¹¹(−) ligands bind to the Ni^II center in a mer configuration. The relative orientations within the pairs of pyridyl-N, deprotonated amido-N, and azo-N atoms are cis, trans, and cis, respectively. The Ni^IIN₂(pyridyl)N′₂(amide)N″₂(azo) coordination environment is severely distorted from ideal octahedral geometry. The Ni-N_am (am = amide) bond lengths are the shortest and the Ni-N_azo bond lengths are the longest. Complex 1 exhibits a quasireversible Ni^III/Ni^II redox process. Moreover, the complex displays two ligand-centered (azo group) quasireversible redox processes. Spectroscopic (absorption and EPR) properties have been studied on coulometrically-generated nickel(III) species. To understand the nature of metal-ligand bonding interactions Density Functional Theory (DFT) calculations have been performed on 1 at the B3LYP level of theory. Calculations have also been done for closely related nickel(II) complexes of deprotonated pyridine amide ligands and comparative discussion has been made using observed results.     

Exchange of isonitrile ligands in the complex [Mo(O)Cl(CNMe)4] by the tetraphos ligand prP4: Stereochemical influences on the reaction course

Reaction of [Mo(O)Cl(CNMe)₄]⁺ with the linear tetraphos ligand meso and rac prP₄ leads to a mixture of [Mo(O)Cl(κ⁴-meso-prP₄)]⁺ and [Mo(O)Cl(CNMe)(κ³-rac-prP₄)]⁺ which are identified by X-ray structural analysis and/or ³¹P NMR spectroscopy. In the meso κ⁴-product both of the phenyl groups of the central phosphorus atoms are oriented towards the oxo ligand whereas in the rac κ³-product one of these phenyl groups is oriented to the oxo and the other to the chloro ligand. The origin of the different coordination modes lies in the different steric demands of the oxo and chloro ligands. The influences of the steric interactions are enhanced by the fact that exchange of the fourth isonitrile is difficult. This hypothesis is supported by the preparation of the complex [Mo(O)Cl(CNMe)(dpepp)]PF₆ whose isonitrile ligand is inert towards exchange by monophosphines, even under drastic conditions.     

Conformational flexibility of 2,6-bis(pyrazol-1-ylmethyl)pyridine (L) in [(L)Co(H2O)3]Cl2 and [(L)Ni(H2O)2Cl]Cl·H2O. Molecular structures and non-covalent interactions

Using a non-planar tridentate ligand 2,6-bis(pyrazol-1-ylmethyl)pyridine (L⁵) two new coordination complexes [(L⁵)Co^II(H₂O)₃]Cl₂ (1) and [(L⁵)Ni^II(H₂O)₂Cl]Cl·H₂O (2) have been synthesized and structurally characterized. Complex 1 has N₃O₃ distorted octahedral environment around Co^II with coordination by L⁵ (two pyrazole and a pyridine nitrogen in a facial mode) and three water molecules. Complex 2 has N₃O₂Cl distorted octahedral geometry around Ni^II with meridional L⁵ coordination, two water molecules, and a Cl⁻ ion. Analysis of the crystal packing diagram reveals the involvement of solvent (water as metal-coordinated and as solvent of crystallization) and counteranion (Cl⁻) to play significant roles in generating 1D chains, involving O-H···Cl, and O-H···O interactions.     

Synthesis, structure, and properties of monomeric Fe(II), Co(II), and Ni(II) complexes of neutral N-(aryl)-2-pyridinecarboxamides

We have prepared and structurally characterized six-coordinate Fe(II), Co(II), and Ni(II) complexes of types [M^II(HL¹)₂(H₂O)₂][ClO₄]₂ (M = Fe, 1; Co, 3; and Ni, 5) and [M^II(HL²)₃][ClO₄]₂ · MeCN (M = Fe, 2 and Co, 4) of bidentate pyridine amide ligands, N-(phenyl)-2-pyridinecarboxamide (HL¹) and N-(4-methylphenyl)-2-pyridinecarboxamide (HL²). The metal centers in bis(ligand)-diaqua complexes 1, 3 and 5 are coordinated by two pyridyl N and two amide O atoms from two HL¹ ligands and six-coordination is completed by coordination of two water molecules. The complexes are isomorphous and possess trans-octahedral geometry. The metal centers in isomorphous tris(ligand) complexes 2 and 4 are coordinated by three pyridyl N and three amide O atoms from three HL² ligands. The relative dispositions of the pyridine N and amide O atoms reveal that the pseudo-octahedral geometry have the meridional stereochemistry. To the best of our knowledge, this work provides the first examples of structurally characterized six-coordinate iron(II) complexes in which the coordination is solely by neutral pyridine amide ligands providing pyridine N and amide O donor atoms, with or without water coordination. Careful analyses of structural parameters of 1-5 along with that reported in the literature [M^II(HL¹)₂(H₂O)₂][ClO₄]₂ (M = Cu and Zn) and [Co^III(L²)₃] have allowed us to arrive at a number of structural correlations/generalizations. The complexes are uniformly high-spin. Spectroscopic (IR and UV/Vis) and redox properties of the complexes have also been investigated.     

Unusual ligand design: A platinum(II) mediated [2+2] photocycloaddition followed by a nickel(II) induced methoxyactivation

The novel dimer of the composition [Pt₂Cl₄(μ-(κP¹:κP²-o-MeO-trans-dppen))₂] (1) (o-MeO-trans-dppen = 1,2-(bis(o-methoxyphenyl)phosphanyl)ethylene) has been prepared and characterized by a single crystal X-ray structure analysis, NMR spectroscopy, mass spectrometry and elemental analysis. This latter compound undergoes a [2+2] photocycloaddition reaction yielding the tetraphosphane all-trans-1,2,3,4-tetrakis(di(o-methoxyphenyl)phosphanyl)cyclobutane (o-MeO-dppcb). The X-ray structure of the dimeric Ni(II) complex that contains the latter ligand, of the formula [Ni₂Cl₄(μ-(κP¹:κP²:κP³:κP⁴-o-MeO-dppcb))] (2) reveals that the apical coordination sites of both square pyramidal Ni(II) coordination spheres are occupied by methoxy-oxygen atoms of the ligand. As a consequence, this dimeric Ni(II) complex 2 is prone to a thermally induced regio- and diastereoselective metal-assisted methoxy-group cleavage. The stepwise formed new mono- and bis-phenolate complexes [Ni₂Cl₃(μ-(κO¹,κP¹:κP²:κP³:κP⁴-o-MeO-O-dppcb))] (3) and [Ni₂Cl₂(μ-(κO¹,κP¹:κP²:κO²,κP³:κP⁴-o-MeO-O,O′-dppcb))] (4), respectively, contain the novel chiral tetraphosphane ligands all-trans-1,2,3-tris((di-o-methoxyphenyl)phosphano)-4-((o-methoxy-phenyl)(o-phenolate)phosphano)cyclobutane (o-MeO-O-dppcb) and all-trans-1,2-bis((di-o-methoxyphenyl)phosphano)-3,4-bis((o-methoxyphenyl)(o-phenolate)phosphano)cyclobutane (o-MeO-O,O′-dppcb). Compounds 3 and 4 have been synthesized independently and are also fully characterized by both single crystal X-ray structure analyses, NMR spectroscopy, mass spectrometry and elemental analyses. The conversion of 2 into 3 and then further into 4 has been followed by a variable-temperature ³¹P{¹H} NMR experiment with compound 2 in DMF-d⁷, revealing that the cleavage of the second methoxy group is kinetically disfavoured. This is in agreement with the X-ray structure analysis of 3, indicating the lack of any methoxy-oxygen atom coordination that could easily induce a further methoxy-group cleavage. o-MeO-O-dppcb and o-MeO-O,O′-dppcb are rare P-stereogenic tetraphosphine ligands and contribute to the synthetic field of new κ³-P,P,O-coordinating phosphanylphenolate ligands that are believed to be important for the SHOP process (SHOP, Shell Higher Olefin Process).     

Phloroglucinol-bridged trinuclear complexes with three paramagnetic octahedral nickel(II) ions: Syntheses, crystal structures, and magnetic properties

Three new C₃-symmetric tritopic ligands with a central phloroglucinol bridging unit have been synthesized and characterized. The ligands are accessible through Schiff-base condensation of 2,4,6-triformylphloroglucinol with 2-aminomethylpyridine (H₃tfpg-ampy), N,N-bis(pyridin-2-ylmethyl)-ethylenediamine (H₃tfpg-unspenp), and benzhydrazide (H₆tfpg-bhy). These ligands differ in nature and number of the donor atoms within the resulting binding pockets. Based on these ligands the synthesis of the first trinuclear phloroglucinol-bridged nickel(II) complexes with three octahedrally coordinated nickel centers is reported. The ligands H₃tfpg-ampy and H₆tfpg-bhy, which provide tridentate binding pockets, react with nickel(II) perchlorate in the presence of bis(pyridin-2-ylethyl)-amine (bpea) as an additional tridentate capping ligand leading to the formation of the trinuclear complexes [Ni₃(tfpg-ampy)(bpea)₃](ClO₄)₃ and [Ni₃(tfpg-bhy)(bpea)₃](ClO₄)₃, respectively. Due to the pentadentate binding pocket in ligand H₃tfpg-unspenp, no additional coligand is needed and a water molecule occupies the sixth coordination site at the nickel(II) ion resulting in the complex [Ni₃(tfpg-unspenp)(H₂O)₃](ClO₄)₃. Temperature-dependent magnetic measurements reveal overall weak antiferromagnetic exchange interactions within the trinuclear complex together with a rather strong zero-field splitting (ZFS) for the nickel(II) ions. The observed isotropic coupling constants for the three complexes are in the range of 0.14 < − J < 0.37 cm⁻¹, whereas for the zero-field splitting parameter ∣D∣ values between 1.8 and 5.5 cm⁻¹ are found. This is indicative for competitive spin-polarization and superexchange mechanisms, with the latter prevailing the interaction between the nickel(II) ions through the meta-phenylene-linkage for the complexes reported.     

Coordination complexes of 4-methylimidazole with Zn<Superscript>II</Superscript> and Cu<Superscript>II</Superscript> in gas phase and in water: a DFT study

A quantum chemistry study of mononuclear metal coordination with four 4-methylimidazole ligands (4-MeIm) was investigated. The four complexes [Cu(4-MeIm)₄]²⁺, [Cu(4-MeIm)₄, H₂O]²⁺, [Zn(4-MeIm)₄]²⁺ and [Zn(4-MeIm)₄, H₂O]²⁺ were studied with particular attention to the Nπ or Nτ possible coordinations of the 4-MeIm ring with the metals, using different DFT methods. The results suggest that the Nτ coordination of 4-MeIm ring to Zn^II or Cu^II is more favorable whatever the level of calculation. In contrast, the addition of one water molecule in the first coordination sphere of the metal ions provides five-coordinated complexes showing no Nπ or Nτ preferences. There is good agreement between the DFT-calculated structure and those available experimentally. When metal ions are four-fold coordinated, they adopt a tetrahedral geometry. When Cu^II and Zn^II are five-fold coordinated, highly symmetric structures or intermediate structures are calculated. Similar energies are calculated for different structures, suggesting flat potential energy surfaces. The addition of implicit solvent modifies the calculated first coordination sphere, especially for [Cu(4-MeIm)₄, H₂O]²⁺ structures. The QTAIM and ELF topological analyses of the interaction between Cu^II and the neutral ligands, clearly indicate a dative bonding with a strong ionic character.     

Spectroscopic and structural characterizations of water-soluble peroxo complexes of niobium(V) with N-containing heterocyclic ligands

New water-soluble heteroleptic peroxo complexes of niobium^V have been prepared with N-containing heterocyclic ligands. The compounds correspond to the general formula (gu)_x[Nb(O₂)₃(L)] · nH₂O (gu = guanidinium). Three different ligands (L) have been used: picolinic acid (Hpic), picolinic acid N-oxide (HpicO) and pyrazine 2,5-dicarboxylic acid (2,5-H₂pzdc). The Nb^V complexes have been characterized on the basis of elemental and thermal analysis as well as by IR and ESI-mass spectrometry. The crystal structure of the peroxo complex with the picolinato N-oxide ligand, (gu)₂[Nb(O₂)₃(picO)] (II) has been determined, showing an eightfold-coordinated Nb atom surrounded by three peroxide groups and a picO ligand bound in a bidentate mode via the carboxylato and the N-oxide groups. The coordination polyhedron around the metal is a triangular dodecahedron.