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.     

Synthesis, structures and magnetic properties of nickel(II), manganic(II) and zinc(II) complexes containing pyridyl-substituted nitronyl nitroxide and tris(2-benzimidazolymethyl)amine

Three new magnetic compounds were synthesized by using 2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NIT2Py) and tris(2-benzimidazolymethyl)amine (NTB) as ligands. The structures and magnetic properties of the complexes with formula [Ni(NIT2Py)(NTB)](ClO₄)₂(CH₃OH) 1, [Mn(NIT2Py)(NTB)](ClO₄)₂2 and {[Zn(NIT2Py)₂(CH₃OH)₂](ClO₄)₂}{[Zn(NTB)(H₂O)](ClO₄)₂} 3 were characterized. Compounds 1 and 2 both have [M(NIT2Py)(NTB)] structural units, where the metal ion is in an octahedral environment bound to one NIT2Py through one pyridyl nitrogen atom and one nitroxide oxygen atom. However, compound 3, the chelating zinc ion has two crystallographically independent molecules in the asymmetric unit: one is six coordinated octahedral structure [Zn(NIT2Py)₂(CH₃OH)₂](ClO₄)₂, and the other one is five coordinated pyramidal structure [Zn(NTB)(H₂O)](ClO₄)₂. The magnetic behaviors of these compounds indicate that both the nickel ion and the manganese ion are antiferromagnetically coupled with the NIT2Py ligand with a coupling constant of −19.44 and −0.37 cm⁻¹, respectively, whereas two NIT2Py ligands in compound 3 are ferromagnetically coupled with a coupling constant of 19.1 cm⁻¹.     

Coordination chemistry of two new tetraaza Ni(II) and Cu(II) macrocyclic and two new Co(II) and Zn(II) macroacyclic Schiff base complexes containing piperazine moiety: X-ray crystal structures of Ni(II) and Zn(II) complexes

A new potentially tetradentate (N₄) Schiff base ligand (L), 1,9,12,20-tetraazatetracyclo[18.2.2.02,7.014,19]tetracosa-2(7),3,5,8,12,14(19),15,17-octaene containing a piperazine moiety is described. Macrocyclic Schiff base complexes, [NiL](ClO₄)₂ (1) and [CuL](ClO₄)₂ (2) have been obtained from equimolar amounts of ligand (L) with nickel(II) and copper(II) metal ions. While the equilibrium reaction in the presence of cobalt(II) and zinc(II) metal ions with ligand L in a 1:1 molar ratio yielded the open-chain Schiff base complexes, [CoL′](ClO₄)₂ (3) and [ZnL′](ClO₄)₂ (4) containing two terminal primary amino groups. The ligand L′ is 1,4-bis(2-(2-aminoethyliminomethyl)phenyl)piperazine. The crystal structures of (1) and (4) have been also determined by X-ray diffraction. It was shown that the Ni(II) is coordinated to the ligand L by two nitrogen atoms of piperazine group and two nitrogen atoms of the imine groups, in a slightly distorted square-planar geometry. Also single crystal X-ray analysis of (4) confirmed a distorted octahedral arrangement in the vicinity of Zn atom with N₆ donor set. The spectroscopic characterization of all complexes is consistent with their crystal structures.     

Synthesis, characterisation and crystal structures of a few coordination complexes of nickel(II), cobalt(III) and zinc(II) with N′-[(2-pyridyl)methylene]salicyloylhydrazone Schiff base

Four new coordination complexes, Ni^II(L)₂ (1), [Co^III(L)₂]ClO₄ (2), [Zn(HL)(L)]ClO₄ · H₂O (3) and [Zn(L)₂][Zn(L)(HL)]ClO₄ · 7H₂O (4) (where L is a monoanion of a Schiff base ligand, N′-[(2-pyridyl)methylene]salicyloylhydrazone (HL) with NNO tridentate donor set), have been synthesised and systematically characterised by elemental analysis, spectroscopic studies and room temperature magnetic susceptibility measurements. Single crystal X-ray diffraction analysis reveals that 1 is a neutral complex, while 2-4 are cationic complexes. Among them, 4 is a rare type of cationic complex with two molecules in the asymmetric unit. The ligand chelates the metal centre with two nitrogen atoms from the pyridine and imino moieties and one oxygen atom coming from its enolic counterpart. All the reported complexes show distorted octahedral geometry around the metal centres, with the two metal-N (imino) bonds being significantly shorter than the two metal-N (Py) bonds.     

Conformational control of spin state in iron(II) tripodal imidazole complexes

Three new iron(II) N₆ tripodal complexes provide information on the role of ligand conformation on spin crossover behavior. The ligands (generated in situ) are the Schiff base condensate of tris(2-aminoethyl)amine (tren) with three equivalents of 4-methyl-5-imidazolecarboxaldehyde, H₃(1), and the condensates of tris(2-aminoethyl)methylammonium ion (N(Me)tren⁺) with three equivalents of 4-methyl-5-imidazolecarboxaldehyde, N(Me)H₃(1)⁺, or with 2-imidazole carboxaldehyde, N(Me)H₃(3)⁺. The structures of [FeH₃(1)](ClO₄)₂, [FeN(Me)H₃(1)](ClO₄)₃ and [FeN(Me)H₃(1)](ClO₄)₃ are reported. The central tren nitrogen atom in these complexes exhibits three different geometries, pyramidal with the nitrogen pointed toward the iron (“N in”, Fe-N distance of 3.050 Å), planar (Fe-N distance of 3.527 Å), and pyramidal with the nitrogen pointed away from the iron atom (“N out”, Fe-N distance of 3.921 Å). With iron(II) the “N in” geometry is high spin while the planar and “N out” geometries are low spin. [FeH₃(1)](ClO₄)₂ exhibits spin crossover behavior between room temperature and 77 K as determined by Mössbauer spectroscopy and also exhibits a conformational change from “N in” to planar over this same temperature range. The structures of [FeN(Me)H₃(1)](ClO₄)₃ and [FeN(Me)H₃ (3)](ClO₄)₃ are locked into the “N out” geometry due to the quaternary nitrogen atom and are low spin even at room temperature. The LS planar and “N out” conformations place a strain on the bond angles of the aliphatic arms of the ligand, which are more pronounced in the “N out” case. The HS “N in” geometry lacks this strain.     

Tuning of redox potential and visible absorption band of ruthenium(II) complexes of (benzimidazolyl) derivatives: Synthesis, characterization, spectroscopic and redox properties, X-ray structures and DFT calculations

Six ruthenium(II) complexes have been prepared using the tridentate ligands 2,6-bis(benzimidazolyl) pyridine and bis(2-benzimidazolyl methyl) amine and having 2,2′-bipyridine, 2,2′:6′,2″-terpyridine, PPh₃, MeCN and chloride as coligands. The crystal structures of three of the complexes trans-[Ru(bbpH₂)(PPh₃)₂(CH₃CN)](ClO₄)₂ · 2H₂O (2), [Ru(bbpH₂)(bpy)Cl]ClO₄ (3) and [Ru(bbpH₂)(terpy)](ClO₄)₂ (4) are also reported. The complexes show visible region absorption at 402-517 nm, indicating that it is possible to tune the visible region absorption by varying the ancillary ligand. Luminescence behavior of the complexes has been studied both at RT and at liquid nitrogen temperature (LNT). Luminescence of the complexes is found to be insensitive to the presence of dioxygen. Two of the complexes [Ru(bbpH₂)(bpy)Cl]ClO₄ (3) and [Ru(bbpH₂)(terpy)](ClO₄)₂ (4) show RT emission in the NIR region, having lifetime, quantum yield and radiative constant values suitable for their application as NIR emitter in the solid state devices. The DFT calculations on these two complexes indicate that the metal t_2g electrons are appreciably delocalized over the ligand backbone.     

Oligonuclear nickel(II) complexes sustained by intermolecular hydrogen-bonding

Reaction of Ni(OAc)₂ with the symmetric `end-off' compartmental proligand 2,6-[N,N^′-bis(2-hydroxy-phenylmethyl)-N,N^′-bis(2-pyridylmethyl)aminomethyl]-4-methylphenol (H₃L) in the presence of NaPF₆ has been found to generate a homotetranuclear nickel(II) complex [(Ni₄HL)(L)(OAc)₂(H₂O)₂(HOAc)₂]PF₆. The crystal structure of the complex reveals that the complex is donor asymmetric and that the extended supra-ligand periphery is maintained by a tight hydrogen-bond between two pendant phenol/phenoxy groups of adjacent ligands and by further tight hydrogen-bonds between coordinated acetic acid molecules and the remaining pendant phenols of the ligand, generating a double acid salt of the type [CH₃COO?H?LH?L?H?OOCCH₃]⁵⁻. Reaction of H₃L with Ni(OAc)₂ and NaClO₄ in methanol gave the complex [Ni₂(HL)(OAc)₂(OH₂)₂][ClO₄]. The structure was determined by X-ray diffraction and showed that the complex exists as a dimer promoted by intermolecular hydrogen-bonding.     

Cyclic voltammetric and infrared spectral studies on the interaction of Ni(II) dithiocarbamates with triphenylphosphine and the crystal and molecular structure of diethyldithiocarbamatobis(triphenylphosphine)nickel(II)perchlo-rate, [Ni(dedtc)(PPh3)2]ClO4

Ni(II) dithiocarbamates (Ni(dtc)₂) with various substituents on dtc were allowed to react with triphenylphosphine (PPh₃). Mixed ligand complexes of the general formulae Ni(dtc)Cl(PPh₃) and [Ni(dtc)(PPh₃)₂]ClO₄ were prepared. The complexes were analysed by high resolution IR spectra. Comparison of the ν(C–N) frequencies of different complexes viz., Ni(dtc)₂, Ni(dtc)Cl(PPh₃) and [Ni(dtc)(PPh₃)₂]ClO₄, showed the following order of decreasing v(C–N) values: [Ni(dtc)(PPh₃)₂]⁺> Ni(dtc)Cl(PPh₃)> Ni(dtc)₂. The observation showed the extent of contribution of the thiouride form in describing the structure of the complexes. The higher the contribution, larger is the value of ν(C–N). Cyclic voltammetric studies on the complexes showed the one electron reduction potentials to decrease in the following order: Ni(dtc)Cl(PPh₃)>Ni(dtc)₂> [Ni(dtc)(PPh₃)₂]⁺. The observations are explained with the nature of the substituents on the dtc moiety and other ligands present around Ni(II). Crystal structure of [Ni(dedtc) (PPh₃)₂]ClO₄ (dedtc = diethyldithiocarbamate) was determined to study the effect of the introduction of PPh₃ in place of Cl in the Ni(dtc)Cl(PPh₃) complex. The complex is planar with NiS₂P₂ chromophore. The NiS distances are 2.190(2) and 2.239(2) Å and the NiP distances are 2.230(2) and 2.200(2) Å. The asymmetry in the NiS and NiP distances is ascribed to the steric effect due to bulky PPh₃. The structural aspects are compared with those of the Ni(dtc)Cl(PPh₃) complex.     

A novel trinuclear nickel(II) complex of an unsymmetrical tetradentate ligand involving bridging oxime and acetylacetone functions

A novel trinuclear nickel(II) complex, [Ni₃(L)₂(H₂O)₂](ClO₄)₂, where L is a bridging unsymmetrical tetradentate ligand, involving o-phenylenediamine, diacetyl monoxime and acetylacetone (H₂L = 4-[2-(3-hydroxy-1-methyl-but-2-enylideneamino)-phenylimino]-pentan-2-one oxime) has been synthesized and characterized structurally. In the complex, an octahedral Ni(II) centre is held in the middle by two square planar units with the aid of oxime and ketonic bridges.     

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.     

Heterobinuclear copper(II)-nickel(II) complexes of macrocyclic oxamide with diamines and tetraazacyclam as blocking ligands: synthesis, crystal structure and magnetic properties

Three novel oxamido-bridged heterobinuclear copper(II)-nickel(II) complexes derived from macrocylic oxamido compound with diamines and tetraazacyclam as blocking ligands were synthesized and characterized by IR, ESR and electronic spectra. Their formula is [Cu(L)Ni(en)₂](ClO₄)₂·0.5C₂H₅OH·H₂O (1), [Cu(L)Ni(tmd)₂](ClO₄)₂·4H₂O (2) and [Cu(L)Ni(rac-cth)](ClO₄)₂·CH₃OH (3), where L=1,4,8,11-tetraazacyclotradecanne-2,3-dione, en=1,2-diaminoethande, tmd=1,3-diaminopropane and rac-cth is rac-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane. The crystal structures of the three complexes have been determined. The structures consist of binuclear units in which the copper(II) ion is in a square-planar environment and linked to the nickel(II) ion via the exo-cis oxygen atoms of the oxamido macrocyclic ligand, with Cu?Ni separations of 5.311 (1), 5.420 (2) and 5.307 Å (3), respectively. The temperature dependence of the magnetic susceptibility for 1, 2 and 3 was analyzed by means of the Hamiltonian ?=−2J?_Ni?_Cu, leading to J=−52.8, −45.7 and −56.9 cm⁻¹ for 1, 2 and 3, respectively.     

Synthesis and characterization of novel unsymmetrical macrobicyclic tricompartmental mono and binuclear nickel(II) complexes: Electrochemical and kinetic studies of phosphate hydrolysis

A series of macrobicyclic mono and binuclear nickel(II) complexes of type [NiL](ClO₄) and [Ni₂L](ClO₄)₂, where L is macroyclic ligand derived from the precursor compound 3,4:10,11-dibenzo-1,13[N,N′-bis{(3-formyl-2-hydroxy-5-methyl)benzyl}diaza]-5,9-dioxocyclopentadecane, have been synthesized in order to examine electrochemical and catalytic studies on the basis of macrocyclic ring size. The macrocycle consists of three dissimilar compartments arising from ether oxygen, tertiary nitrogen and imine nitrogen atoms. Electrochemical studies have shown that the mononuclear nickel(II) complexes undergo quasireversible single step one electron reduction and oxidation and binuclear nickel(II) complexes undergo two quasireversible one electron reduction and oxidation. The EPR silent nature is ascribed to Ni(II) state and all the nickel(II) complexes have square planar geometry and are diamagnetic in nature. The complexes were subjected to hydrolysis of 4-nitrophenyl phosphate and the catalytic activities of the complexes are found to increase with macrocyclic ring size of the complexes. As the macrocyclic ring size of the complexes increases, the spectral, electrochemical and catalytic studies of the complexes show remarkable variation due to distortion in the geometry around the nickel(II) centre.     

Equilibrium and structural studies of copper and nickel complexes with a pentadentate ligand providing amide, amine and pyridyl nitrogen donors. Crystal structure of the dinuclear nickel(II) complex

A novel potentially pentadentate ligand 1,7-bis(2-pyridylcarbamoyl)-1,4,7-triazaheptane (pycdien) has been synthesised as its trihydrochloride salt; its protonation constants and the stability constants of the copper(II) and nickel(II) chelates have been determined by potentiometric and spectrophotometric techniques. Amide groups deprotonation allows the formation of [M(pycdienH₋₁)]⁺ and [M(pycdienH₋₂)] (M = Cu or Ni) species in all cases, whereas the detection of the complexes of the neutral or the protonated forms of the ligand depends on the nature of the metal ions. In the solid state, mononuclear complexes {[Cu(pycdienH)](ClO₄)₂Cl (1) and [Cu(pycdien)](ClO₄)₂ (2)} and dinuclear compounds {[Cu₂(pycdienH₋₂)₂] · H₂O (3) and [Ni₂(pycdienH₋₂)₂(OH₂)₂] · 6H₂O (4)} have been synthesised and characterised by spectroscopic techniques. X-ray crystal determination shows that the dimeric [Ni₂(pycdienH₋₂)₂ (OH₂)₂] · 6H₂O (4) crystallizes in the triclinic space group , and each nickel(II) center resides in a octahedral geometry generated by the N_pyridyl, N_amido, N_amine portion of one ligand and one pyridine-2-carboxamido end of the other. The hexacoordination of each Ni²⁺ ion is achieved by a water molecule. The complexes obtained with the deprotonated form of the ligand imply the coordination through the nitrogen atoms of the amidic groups.     

Alteration of molecular conformations and spectral properties for nickel(II), zinc(II) and copper(II) complexes having 3,8-di(thiophen-2′,2′′-yl)-1,10-phenanthroline ligands

Four transition metal complexes of 3,8-di(thiophen-2′,2″-yl)-1,10-phenanthroline (dtphen), formulated as [Ni(dtphen)₂(H₂O)₂]·(ClO₄)₂ (1), [Zn(dtphen)₂(H₂O)]·(ClO₄)₂ (2) [Cu(dtphen)₂(H₂O)]·(ClO₄)₂ (3), [Cu(dtphen)(phen)₂]·(ClO₄)₂ (4) (phen = 1,10-phenanthroline) with different metal-to-ligand ratios, were synthesized and characterized herein. The X-ray single-crystal diffraction studies of 1-4 exhibit that different molecular configurations for the dtphen ligand can be observed where the side thiophene rings adopt the trans/trans, trans/cis, trans/disorder and cis/cis conformations relative to the central 1,10-phenanthroline unit in different compounds. Fluorescence emission spectra of 1-4 in methanol show that the fluorescence emission of 2 is much stronger than the other three metal complexes, which is mainly due to its full d¹⁰ electronic configuration of Zn(II) ion.     

Synthesis, structures, and magnetic properties of dicopper(II) and dinickel(II) complexes with a new bis(macrocycle), 7,7-(2-hydoxypropane-1,3-diyl)bis{3,7,11,17-tetraazabicyclo[11.3.1]- heptadeca-1(17),13,15-triene}

A new bis(macrocycle) ligand, 7,7^′-(2-hydoxypropane-1,3-diyl)-bis{3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene} (HL), and its dicopper(II) ([Cu₂(HL)Cl₂](NO₃)₂ · 4H₂O (4a), [Cu₂(HL)I₂]I₂ · H₂O (4b)) and dinickel(II) ([Ni₂(L)(OH₂)](ClO₄)₃ (5a), [Ni₂(L)(OH₂)]I₃ · 2H₂O (5b), [Ni₂(L)N₃](N₃)₂ · 7H₂O (5c)) complexes have been synthesized. The alkoxide bridged face-to-face structure of the dinickel(II) complex 5c has been revealed by X-ray crystallography, as well as the “half-opened clamshell” form of the bis(macrocyclic) dicopper(II) complex 4b. Variable temperature magnetic susceptibility studies have indicated that there exists intramolecular antiferromagnetic coupling (J=−33.8 cm⁻¹ (5a), −32.5 cm⁻¹ (5b), and −29.7 cm⁻¹ (5c)) between the two nickel(II) ions in the nickel(II) complexes.     

Cobalt(II), nickel(II), copper(II) and zinc(II) complexes with an open-chain pentadentate nitrogen ligand

The open-chain, potentially, pentadentate, ligan 1,11-bis(dimethylamino)-3,6,9-trimethyl-3,6,9,-triazaundecane (Me₇tetren) forms a series of metal complexes having the general formula [M(Me₇tetren)]Y₂ (Y = 1, M = Co, Ni; Y = ClO₄, M = Co, Ni, Cu, Zn). On the basis of their physical properties, it is suggested that all these compounds contains isostructural five-coordinate [M(Me₇tetren)]²⁺ cations, the ligand acting as pentadentate. These complexes react in solution with thiocyanate ion to give mono- and, with exception of copper(II), di-thiocyanato five- and six-co-ordinate derivatives. Mono-thiocyanato derivatives of cobalt(II), nickel(II) and zinc(II) have been isolated as tetraphenylborate salts. Cobalt(II) and nickel (II) di-thiocyanato derivatives have been also isolated. Results are discussed in terms of the steric requirements of the ligand and electronic properties of the metal ions.     

Imidazole derivatives of binuclear copper (II) and nickel (II) complexes incorporating bis(1,4,7-triazacyclononan-1-yl) ligands

A series of new binuclear copper (II) and nickel (II) complexes of the macrocyclic ligands bis(1,4,7-triazacyclononan-1-yl)butane (L^but) and bis(1,4,7-triazacyclononan-1-yl)-m-xylene (L^mx) have been synthesized: [Cu₂L^butBr₄] (1), [Cu₂L^but(imidazole)₂Br₂](ClO₄)₂ (2), [Cu₂L^mx(μ-OH)(imidazole)₂](ClO₄)₃ (3), [Cu₂L^but(imidazole)₄](ClO₄)₄ · H₂O (4), [Cu₂L^mx(imidazole)₄](ClO₄)₄ (5), [Ni₂ L^but(H₂O)₆](ClO₄)₄ · 2H₂O (6), [Ni₂L^but(imidazole)₆](ClO₄)₄ · 2H₂O (7) and [Ni₂L^mx (imidazole)₄(H₂O)₂](ClO₄)₄ · 3H₂O (8). Complexes 1, 2, 7 and 8 have been characterized by single crystal X-ray studies. In each of the complexes, the two tridentate 1,4,7-triazacyclononane rings of the ligand facially coordinate to separate metal centres. The distorted square-pyramidal coordination sphere of the copper (II) centres is completed by bromide anions in the case of 1 and/or monodentate imidazole ligands in complexes 2, 4 and 5. Complex 3 has been formulated as a monohydroxo-bridged complex featuring two terminal imidazole ligands. Complexes 6-8 feature distorted octahedral nickel (II) centres with water and/or monodentate imidazole ligands occupying the remaining coordination sites. Within the crystal structures, the ligands adopt trans conformations, with the two metal binding compartments widely separated, perhaps as a consequence of electrostatic repulsion between the cationic metal centres. The imidazole-bearing complexes may be viewed as simple models for the coordinative interaction of the binuclear complexes of bis (tacn) ligands with protein molecules bearing multiple surface-exposed histidine residues.     

Synthesis, characterization and supramolecular structures of two Ni(II) complexes with potentially heptadentate ligand N,N-bis(2-hydroxybenzyl)-1,3-bis[(2-aminoethyl)amino]-2-propanol

A potentially heptadentate ligand H₃L (N,N^′-bis(2-hydroxybenzyl)-1,3-bis[(2-aminoethyl)amino]-2-propanol) and its two Ni(II) complexes, [Ni(H₂L)H₂O](H₂O)₃ClO₄ (1) and [Ni(H₂L)(H₂O)](H₂O)Cl (2) were prepared and characterized. X-ray structural analyses indicate that complex 1 has a distorted octahedral coordination geometry, with four amine N atoms of H₂L⁻ defining the equatorial plane, one aqua O atom and one phenoxo O atom of the ligand occupying two axial positions, respectively. The Ni(II) center of 2 has coordination geometry similar to that of 1. IR and electronic spectra of 1 and 2 are in agreement with their crystal structural features. Approximately along the ab plane, 2D supramolecular structure of 1 is assembled through multiple hydrogen bonds between hydroxy groups of the ligands, coordinated and crystal lattice H₂O and π-π stacking interactions between adjacent phenyl rings of the ligands, while for that of 2, probably along the a axis, 1D chain structure is also formed by multiple hydrogen bonds, but lack of π-π stacking interactions.     

Synthesis and characterization of Cu(II) and Ni(II) complexes of a tripodal ligand containing imidazoles

Two complexes of the formula [MH₃L](ClO₄)₂ [M = Cu(II) (1), Ni(II) (2)] have been prepared by the reaction of M(ClO₄)₂ · 6H₂O with the ligand (H₃L) formed by the Schiff base condensation of tris(2-aminoethyl)amine (tren) with three molar equivalents of 4-methyl-5-imidazolecarboxaldehyde and structurally and magnetically characterized. The structures of 1 and 2 are isomorphous with each other and with the iron(II) complex of H₃L which has been reported previously. The ligand, while potentially heptadentate, forms six coordinate complexes with both metal centers forming three M-N_imine and three M-N_imidazole bonds. The tren central N atom is at a nonbonded distance from M of 3.261 Å for 1 and 3.329 Å for 2. The neutral complex CuHL 3 was prepared by reaction of H₃L with Cu(OCH₃)₂ and the ionic complex Na[NiL] 4 was prepared by deprotonation of 2 with aqueous sodium hydroxide. Magnetic measurements of 1-3 are consistent with the spin-only values expected for S = 1/2 (d⁹, Cu(II)) and S = 1 (d⁸, Ni (II)) systems.