Pentadentate Schiff Base Ligand Research Articles

{μ-2-[(3-Amino-2,2-dimethylpropyl)iminomethyl]-6-methoxyphenolato-1:2κ5O1,O6:N,N′,O1}{2-[(3-amino-2,2-dimethylpropyl)iminomethyl]-6-methoxyphenolato-1κ3N,N′,O1}-μ-azido-1:2κ2N:N-azido-2κN-methanol-2κO-dinickel(II)

Two distinct coordination geometries are found in the binuclear title complex, [Ni2(C13H19N2O2)2(N3)2(CH3OH)], as one Schiff base ligand is penta­dentate, coordinating via the anti­cipated oxide O, imine N and amine N atoms (as for the second, tridentate, ligand) but the oxide O is bridging and coordination also occurs through the meth­oxy O atom. The NiII atoms are linked by a μ2-oxide atom and one end of a μ2-azide ligand, forming an Ni2ON core. The coordination geometry for the NiII atom coordinated by the tridentate ligand is completed by the meth­oxy O atom derived from the penta­dentate ligand, with the resulting N3O3 donor set defining a fac octa­hedron. The second NiII atom has its cis-octa­hedral N4O2 coordination geometry completed by the imine N and amine N atoms of the penta­dentate Schiff base ligand, a terminally coordinated azide N and a methanol O atom. The arrangement is stabilized by an intra­molecular hydrogen bond between the methanol H and the oxide O atom. Linear supra­molecular chains along the a axis are formed in the crystal packing whereby two amine H atoms from different amine atoms hydrogen bond to the terminal N atom of the monodentate azide ligand.

Two complexes of Eu3+ ion with pentadentate Schiff-base ligands exhibiting red photoluminescence

Two luminescent europium complexes, Eu(DAPBH)2(NO3)3·2H2O (1) and Eu(DAPBH)(CH3CH2OH)(SCN)3·H2O (2) (where DAPBH=(1,1′-(pyridine-2,6-diyl)bis(ethan-1-yl-1-ylidene))dibenzohydrazide, have be synthesized and structurally characterized. The Eu3+ ion is 10-coordinated with six N atoms and four O atoms from two pentadentate DAPBH ligands to form a distorted bicapped tetragonal antiprism geometry and the nitrate counter ions are not coordinated to Eu3+ ion in 1, while the 9-coordinate Eu3+ ion is bound to the N atoms of three monodentate thiocyanates (counter ions), three N atoms and two O atoms of a pentadentate DAPBH ligand as well as to the O atom of an ethanol molecule with a distorted monocapped tetragonal antiprism coordination environment in 2. Two complexes exhibit a red luminescence of Eu3+ ion, stemming from the intra-4f transitions, which is contributed to that (1) the coordination sphere of Eu3+ ion completed by pentadentate DAPBH ligands or counter anions effectively protect the excited state of Eu3+ ion from being quenched by water molecules in 1 and 2; (2) there should be a better match between the pentadentate DAPBH ligand triplet excited state and europium 5D0 excited state.

Isolation of four new CoII/CoIII and NiII complexes with a pentadentate Schiff base ligand: syntheses, structural descriptions and magnetic studies

In this paper we report the temperature and pH dependent syntheses and systematic characterization of four new Co(II)/Co(III) and Ni(II) complexes with a pentadentate Schiff base ligand H(3)L obtained by condensing 1,3,-diaminopropan-2-ol with 2-hydroxyacetophenone in 1:2 molar ratio. The room temperature syntheses involving Co(II) and Ni(II) nitrates and the ligand H(3)L lead to the isolation of the dinuclear species [Co(2)L(2)(H(2)O)] (1), and the mononuclear complex [Ni(LH)] (3), respectively, whereas refluxing at basic pH leads to the tetranuclear complexes, [Co(II)(2)Co(III)(2)L(2)(μ(3)-OMe)(2)(NO(3))(H(2)O)(2)]NO(3)·2(H(2)O) (2), and [Ni(4)L(2)(μ(3)-OMe)(2)(H(2)O)(2)]·2H(2)O (4). 1 is found to be a simple mono alkoxo-bridged Co(III) dinuclear species, whereas 2 and 4 are both rhomb-like tetrameric complexes with double oxo bridges and μ(3)-methoxo bridges, derived from the methanol solvent, in an open dicubane arrangement. Moreover 2 shows six coordinate ordered Co(II) and Co(III) ions and 4 has both six- and five-coordinate Ni(II) centers. Compound 3 is assigned a tentative mononuclear structure based on IR, UV-Vis spectroscopic, (1)H-NMR and ESI mass study results and is supposed to have one Ni(II) center coordinated with a ligand fragment in square planar geometry. The variable temperature magnetic susceptibility study for 2 and 4 is performed which indicate for both 2 and 4 the presence of intracluster dominant ferromagnetic interactions.

Tuning of spin crossover behaviour in iron(iii) complexes involving pentadentate Schiff bases and pseudohalides

Investigations on a series of eight novel mononuclear iron(III) Schiff base complexes with the general formula [Fe(L(5))(L(1))]·S (where H(2)L(5) = pentadentate Schiff-base ligand, L(1) = a pseudohalido ligand, and S is a solvent molecule) are reported. Several different aromatic 2-hydroxyaldehyde derivatives were used in combination with a non-symmetrical triamine 1,6-diamino-4-azahexane to synthesize the H(2)L(5) Schiff base ligands. The consecutive reaction with iron(III) chloride resulted in the preparation of the [Fe(L(5))Cl] precursor complexes which were left to react with a wide range of the L(1) pseudohalido ligands. The low-spin compounds were prepared using the cyanido ligand: [Fe(3m-salpet)(CN)]·CH(3)OH (1a), [Fe(3e-salpet)(CN)]·H(2)O (1b), while the high-spin compounds were obtained by the reaction of the pseudohalido (other than cyanido) ligands with the [Fe(L(5))Cl] complex arising from salicylaldehyde derivatives: [Fe(3Bu5Me-salpet)(NCS)] (2a), [Fe(3m-salpet)(NCO)]·CH(3)OH (2b) and [Fe(3m-salpet)(N(3))] (2c). The compounds exhibiting spin-crossover phenomena were prepared only when L(5) arose from 2-hydroxy-1-naphthaldehyde (H(2)L(5) = H(2)napet): [Fe(napet)(NCS)]·CH(3)CN (3a, T(1/2) = 151 K), [Fe(napet)(NCSe)]·CH(3)CN (3b, T(1/2) = 170 K), [Fe(napet)(NCO)] (3c, T(1/2) = 155 K) and [Fe(napet)(N(3))], which, moreover, exhibits thermal hysteresis (3d, T(1/2)↑ = 122 K, T(1/2)↓ = 117 K). These compounds are the first examples of octahedral iron(III) spin-crossover compounds with the coordinated pseudohalides. We report the structure and magnetic properties of these complexes. The magnetic data of all the compounds were analysed using the spin Hamiltonian formalism including the ZFS term and in the case of spin-crossover, the Ising-like model was also applied.

Molecular Structure and Electrochemical Behavior of Uranyl(VI) Complex with Pentadentate Schiff Base Ligand: Prevention of Uranyl(V) Cation−Cation Interaction by Fully Chelating Equatorial Coordination Sites

The U(VI) complex with a pentadentate Schiff base ligand (N,N'-disalicylidenediethylenetriaminate = saldien(2-)) was prepared as a starting material of a potentially stable U(V) complex without any possibility of U(V)O(2)(+)...U(V)O(2)(+) cation-cation interaction and was found in three different crystal phases. Two of them had the same composition of U(VI)O(2)(saldien) x DMSO in orthorhombic and monoclinic systems (DMSO = dimethyl sulfoxide, 1a and 1c, respectively). The DMSO molecule in both 1a and 1c does not show any coordination to U(VI)O(2)(saldien), but it is just present as a solvent in the crystal structures. The other isolated crystals consisted only of U(VI)O(2)(saldien) without incorporation of solvent molecules (1b, orthorhombic). A different conformation of the coordinated saldien(2-) in 1c from those in 1a and 1b was observed. The conformers exchange each other in a solution through a flipping motion of the phenyl rings. The pentagonal equatorial coordination of U(VI)O(2)(saldien) remains unchanged even in strongly Lewis-basic solvents, DMSO and N,N-dimethylformamide. Cyclic voltammetry of U(VI)O(2)(saldien) in DMSO showed a quasireversible redox reaction without any successive reactions. The electron stoichiometry determined by the UV-vis-NIR spectroelectrochemical technique is close to 1, indicating that the reduction product of U(VI)O(2)(saldien) is [U(V)O(2)(saldien)](-), which is stable in DMSO. The standard redox potential of [U(V)O(2)(saldien)](-)/U(VI)O(2)(saldien) in DMSO is -1.584 V vs Fc/Fc(+). This U(V) complex shows the characteristic absorption bands due to f-f transitions in its 5f(1) configuration and charge-transfer from the axial oxygen to U(5+).

Syntheses, structures, and photochemistry of manganese nitrosyls derived from designed Schiff base ligands: potential NO donors that can be activated by near-infrared light.

Two manganese nitrosyls, namely, [Mn(SBPy(3))(NO)](ClO(4))(2) (1) and [Mn(SBPy(2)Q)(NO)](ClO(4))(2) (2), have been synthesized by using designed pentadentate Schiff base ligands N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine (SBPy(3)) and N,N-bis(2-pyridyl methyl)amine-N-ethyl-2-quinoline-2-aldimine (SBPy(2)Q). Reaction of NO(g) with [Mn(SBPy(3))(MeOH)](ClO(4))(2) and [Mn(SBPy(2)Q)(EtOH)](ClO(4))(2) in MeCN affords 1 and 2, respectively, in good yields. Narrow-width peaks in the (1)H NMR spectra and strong nu(NO) at 1773 cm(-1) (of 1) and 1759 cm(-1) (of 2) confirm a strongly coupled {low-spin Mn(II)-NO*}formulation for both these {Mn-NO}(6) nitrosyls. In MeCN, 1 exhibits two strong absorption bands with lambda(max) at 500 and 720 nm. These bands red shift to 550 and 785 nm in case of 2 because of substitution of the pyridyl-imine moiety of SBPy(3) with quinolyl-imine moiety in the SBPy(2)Q ligand frame. Exposure of solutions 1 and 2 to near-infrared (NIR) light (780 nm, 5 mW) results in rapid bleaching of the orange and fuchsia solutions, and free NO is detected in the solutions by an NO-sensitive electrode. The high quantum yield values (Phi) of 1 (0.580 +/- 0.010, lambda(irr) = 550 nm, MeCN) and 2 (0.434 +/- 0.010, lambda(irr) = 550 nm, MeCN) and in particular their sensitivity to NIR light of 800-950 nm range strongly suggest that these designed manganese nitrosyls could be used as NIR light-triggered NO donors.

High-spin Schiff-base dinuclear iron(III) complexes bridged by N-oxide ligands

Synthesis and structure of dinuclear complexes [{Fe III(L 5)}b{Fe III(L 5)}](BPh 4) 2, where L 5 is a pentadentate Schiff-base ligand, b is a bidentate N-oxide bridging ligand based on bipyridine, is reported. Magnetic behavior is investigated in terms of the magnetic susceptibility, magnetization, and Mössbauer spectroscopy revealing that the complexes are high-spin over the whole temperature region.

Functionalization of Gold Nanoparticles by Iron(III) Complexes Derived from Schiff Base Ligands

AbstractA series of iron(III) complexes derived from a variety of pentadentate Schiff base ligands, namely salten [H2salten = bis(3‐salicylideneaminopropyl)amine] derivatives, have been synthesized and further used to stabilize and functionalize gold nanoparticles (Au‐NPs). The iron complexes have been specifically designed to interact with the Au‐NPs through an ambidentate thiocyanate ligand or through a pendant thiol group carried by the salten derivative ligand. The gold nanocomposites have been synthesized either by direct reduction of AuIII in the presence of iron complexes or by coordination chemistry on prefunctionalized gold nanoparticles; in the latter case Au‐NPs bearing pendant empty salten cages have been treated with FeIII to generate the iron complexes. Steric and electrostatic repulsions have been tuned to obtain stable colloidal solutions or to induce the precipitation of the gold nanocomposites. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Photoswitching of a heptanuclear FeII-FeIII complex — A case of multifunctional magnetic materials

AbstractA heptanuclear mixed-valence complex of the type [{FeIIIL5(NC)}6FeII] has been synthesised and characterised by analytical methods (L5 stands for a pentadentate Schiff-base ligand). Light of different wavelengths was applied inside a SQUID magnetometer at low temperatures and various magnetic fields in order to investigate the response function (magnetisation) depending on temperature, magnetic field, and wavelength. The irradiation with various wavelengths caused a significant decrease of the bulk magnetisation in the present case. In the applied field, photoswitching is a fast and well reproducible process.

New dimanganese(III) complexes of pentadentate (N 2O 3) Schiff base ligands with the [Mn 2(μ-OAc)(μ-OR) 2] 3+ core: Synthesis, characterization and mechanistic studies of H 2O 2 disproportionation

Two new diMn III complexes [ Mn 2 III L 1 ( μ - AcO ) ( μ - MeO ) ( methanol ) 2 ] Br ( 1) and [ Mn 2 III L 2 ( μ - AcO ) ( μ - MeO ) ( methanol ) ( ClO 4 ) ] ( 2) (L 1H 3 = 1,5-bis(2-hydroxybenzophenylideneamino)pentan-3-ol; L 2H 3 = 1,5-bis(2-hydroxynaphtylideneamino)pentan-3-ol) were synthesized and structurally characterized. Structural studies evidence that these complexes have a bis(μ-alkoxo)(μ-carboxylato) triply bridged diMn III core in the solid state and in solution, with two substitution-labile sites – one on each Mn ion – in cis-position. The two complexes show catalytic activity toward disproportionation of H 2O 2, with saturation kinetics on [H 2O 2], in methanol and dimethyl formamide at 25 °C. Spectroscopic monitoring of the H 2O 2 disproportionation reaction suggests that (i) complexes 1 and 2 dismutate H 2O 2 by a mechanism involving redox cycling between Mn 2 III and Mn 2 IV , (ii) the complexes retain the dinuclearity during catalysis, (iii) the active form of the catalyst contains bound acetate, and (iv) protons favors the formation of inactive Mn II species. Comparison to other dimanganese complexes of the same family shows that the rate of catalase reaction is not critically dependent on the redox potential of the catalyst, that substitution of phenolate by naphtolate in the Schiff base ligand favors formation of the catalyst-substrate adduct, and that, in the non-protic solvent, the bulkier substituent at the imine proton position hampers the binding to the substrate.

Syntheses and structures of two Zn(II) complexes with the pentadentate Schiff-base ligands

Two structurally divergent zinc complexes [Zn · H 2L 1 · Cl 2] · 0.5CH 3OH ( 1) and [Zn 2 · L 2 · Cl 2 · (CH 3OH) 2] of the newly designed pentadentate H 2N 3O 2 Schiff bases H 2L 1 and H 2L 2 have been synthesized and structurally characterized. 1 has one five-coordinated Zn(II) ion in a distorted square-pyramid while 2 contains one novel seven-coordinate ZnN 3O 4 core in a pentagonal bipyramid polyhedron and one ZnO 2Cl 2 core in a trigonal pyramidal geometry. The coordinated units in 1 and 2 are both linked by hydrogen bonds forming infinite one-dimensional zigzag chains.

Magnetostructural Studies on Ferromagnetically Coupled Copper(II) Cubanes of Schiff‐Base Ligands

Three cubane copper(II) clusters, namely [Cu(4)(HL')4] (1), [Cu4L2(OH)2] (2), and [Cu4L2(OMe)2] (3), of two pentadentate Schiff-base ligands N,N'-(2-hydroxypropane-1,3-diyl)bis(acetylacetoneimine) (H3L') and N,N'-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) (H3L), are prepared, structurally characterized by X-ray crystallography, and their variable-temperature magnetic properties studied. Complex 1 has a metal-to-ligand stoichiometry of 1:1 and it crystallizes in the cubic space group P43n with a structure that consists of a tetranuclear core with metal centers linked by a mu(3)-alkoxo oxygen atom to form a cubic arrangement of the metal and oxygen atoms. Each ligand displays a tridentate binding mode which means that a total of eight pendant binding sites remain per cubane molecule. Complexes [Cu4L2(OH)2] (2) and [Cu4L2(OMe)2] (3) crystallize in the orthorhombic space group Pccn and have a cubane structure that is formed by the self-assembly of two {Cu2L}+ units. The variable-temperature magnetic susceptibility data in the range 300-18 K show ferromagnetic exchange interactions in the complexes. Along with the ferromagnetic exchange pathway, there is also a weak antiferromagnetic exchange between the copper centers. The theoretical fitting of the magnetic data gives the following parameters: J1 = 38.5 and J2 = -18 cm(-1) for 1 with a triplet (S = 1) ground state and quintet (S = 2) lowest excited state; J1 = 14.7 and J2 = -18.4 cm(-1) for 2 with a triplet ground state and singlet (S = 0) lowest excited state; and J1 = 33.3 and J2 = -15.6 cm(-1) for 3 with a triplet ground state and quintet lowest excited state, where J1 and J2 are two different exchange pathways in the cubane {Cu4O4} core. The crystal structures of 2 * 6 H2O and 3 * 2 H2O * THF show the presence of channels containing the lattice solvent molecules.

Synthesis and Electrochemical Properties of Oxovanadium Complexes with a Pentadentate Schiff Base Ligand

Abstract Oxovanadium(IV) and (V) complexes with a pentadentate Schiff base ligand, [VIVO(HL)]·H2O {H3L: N,N′-bis(3-ethoxysalicylidene)-2-hydroxy-1,3-propanediamine} and [VVO(L)]·CH3CN, were prepared by the reaction of the Schiff base ligand with [VIVO(acac)2] (H2acac: acetylacetone) in CH3CN under an argon atmosphere and in air, respectively. IR data of the complexes indicate that [VIVO(HL)] and [VVO(L)] have polymeric (···V=O···V=O···) and monomeric structures, respectively, in the solid state. An X-ray crystal structure analysis of [VVO(L)]·CH3CN was carried out. The complex has a distorted octahedral structure with one of the imine nitrogen atoms trans to the oxo atom. The cyclic voltammogram of [VVO(L)] in CH2Cl2 shows a quasi-reversible V(V)/V(IV) redox couple with E1/2 = −0.74 V vs Ag/Ag+. On the other hand, the cyclic voltammogram of [VIVO(HL)] in CH2Cl2 shows two irreversible oxidation waves at +0.23 and +0.33 V with a scan rate 100 mV/s. The cyclic voltammogram of [VIVO(HL)] in DMSO shows a quasi-reversible V(V)/V(IV) redox couple with E1/2 = −0.05 V. Electrochemcal data indicate that [VIVO(HL)] form dimeric species in a weakly Lewis basic solvent by coodination of the alcohol group to the vanadium atom of the adjacent molecule.

Dicopper(II) Schiff base aminobenzoates with discrete molecular and 1D-chain polymeric structures

Dicopper(II) complexes [Cu 2L(O 2CC 6H 4- o-NH 2)] ( 1) and [Cu 2L(O 2CC 6H 4- p-NH 2)] ( 2) are prepared from a reaction of [Cu 2L(O 2CMe)] with the corresponding aminobenzoic acid and a base in MeOH, where L is a trianionic pentadentate Schiff base ligand N, N′-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine). The complexes are structurally characterized by X-ray crystallography. The crystal structures show the presence of an asymmetrically dibridged {Cu 2 II(μ-OR)(μ-O 2CR)} core where the endogenous mono-atomic bridging alkoxo group is derived from the Schiff base and the aminobenzoate ligand displays syn–syn bridging mode. The Cu⋯Cu separation and Cu–OR–Cu bond angle in 1 and 2 are 3.472(1) Å, 131.02(18)° and 3.511(1) Å, 132.20(17)°, respectively. The o-aminobenzoate complex is discrete dimeric nature. The pendant o-amino group is involved in an intramolecular hydrogen bonding interaction with one carboxylate oxygen atom. The crystal structure of the p-aminobenzoate species 2 shows the formation of a one-dimensional polymeric chain resulting from the axial binding of the pendant p-amino group to one copper center belonging to another dimeric unit. Magnetic studies show that both the complexes are antiferromagnetic in nature giving a singlet–triplet energy separation of −130 and −150 cm −1 for 1 and 2, respectively. The 1D chain complex 2 magnetically behaves like the discrete dimeric complex 1.

Synthesis, crystal structure and magnetic properties of quasi-linear tetranuclear copper(II) Schiff base complexes formed by covalent linkage of asymmetrically dibridged dicopper(II) units

Alkoxo-phenoxo bridged tetranuclear copper(II) complexes [Cu4L2(O2CC6H4-p-OH)2] (1) and [Cu4L2(O2CC6H4-o-OH)2] (2) containing pentadentate Schiff base ligand N,N′-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) (H3L) are prepared and structurally characterized. Crystal structures of the complexes show the covalent linkage between two {Cu2L(O2CR)}(R = C6H4-p-OH, C6H4-o-OH) units through the phenoxo atoms of the Schiff base ligand showing axial/equatorial bonding modes. The Cu(1)–O(2)–Cu(2) alkoxo bridge angle is 131° in 1 and 2. The pendant ortho- and para- OH groups of the three-atom bridging carboxylate ligands show no apparent bonding interactions with the metal or other group(s). The complexes show a d–d band near 635 nm in CH2Cl2. Variable temperature magnetic susceptibility measurements in the temperature range 300–18 K show antiferromagnetically coupled spin system. A theoretical fit of the magnetic data using exchange parameters J1 and J2 for the intradimer and interdimer units of the quasi-linear tetrameric core gave values as: J1=−132,J2=−72 cm−1 for 1 and J1=−167,J2=−67 cm−1 for 2.

Relation between Magnetic, Spectroscopic and Structural Properties of Binuclear Copper(II) Complexes of Pentadentate Schiff-base Ligand, Semi-empirical and ab-initio Calculations

The synthesis and characterization of [Cu2(L1)(3,5 prz)] (L1=1,3-Bis(2-hydroxy-3,5-chlorosalicylideneamino) propan-2-ol) 1 and of [Cu2(L2)(3,5 prz)] (L2=1,3-Bis(2-hydroxy-bromosalicylideneamino) propan-2-ol) 2 are reported. The compounds were studied by elemental analysis, infrared and electronic spectra. The structure of the Cu2(L1)(3,5 prz)] complex was determined by x-ray diffraction. The magnetochemical characteristics of these compounds were determined by temperaturedependent magnetic susceptibility measurements, revealing their antiferromagnetic coupling. The superexchange coupling constants are 210 cm−1 for 1 and 440 cm−1 for 2. The difference in the magnitude of the coupling constants was explained by the metal-ligand orbital overlaps and confirmed by ab-initio restricted Hartree-Fock (RHF) calculations. In order to determine the nature of the frontier orbitals, Extended Hückel Molecular Orbital (EHMO) calculations are also reported.

NMR studies on a palladium(II) complex containing an incompletely coordinated facultative pentadentate Schiff base ligand

AbstractPalladium(II) dichloride reacts with 1,10‐bis(2‐pyrrolyl)‐2,5,9‐triaza‐1,9‐decadiene to give a [Pd(C15H20N5)]Cl complex in which the ligand is four‐coordinated, leaving one pyrrole group dangling. By using COSY, gHSQC, gHMBC connectivities and NOE experiments it has been concluded that one linkage isomer exists in DMSO solution, in spite of the fact that different sets of N atoms of potentially pentadentate ligand might be involved in coordination, and that the three chelate rings in the complex cation are arranged in a sequence: five‐membered, six‐membered, five‐membered which is different from that (5–5–6) found by x‐ray studies on the related [Ni(C15H20N5)]Cl compound. NMR studies allowed an unambiguous assignment of all 1H and 13C NMR resonances for the complex. Results of x‐ray structural analysis of [Pd(C15H20N5)](CH3COO)H2O supported the five‐membered, six‐membered, five‐membered ring sequence in the [Pd(C15H20N5)]+ complex cation and show an E (trans) orientation of the dangling pyrrole group with respect to the metal center. Copyright © 2002 John Wiley & Sons, Ltd.

Synthesis, crystal structure and magnetic behavior of two new binuclear complexes bridged by a pentadentate ligand: [Ni 2L 2(NCS) 2](ClO 4) 2 and [Ni 2L 2(NCO) 2](ClO 4) 2 [L=pentadentate ligand

Two new binuclear nickel(II) complexes [Ni 2L 2(NCS) 2](ClO 4) 2 ( 1) and [Ni 2L 2(NCO) 2](ClO 4) 2 ( 2), where L is a pentadentate Schiff base ligand derived from the reaction of 2 mol of 2-benzoyl pyridine and 1 mol of diethylene triamine, have been prepared and characterized. The structures of complexes 1 and 2 have been studied by single crystal X-ray diffraction analysis. The structure analysis reveals that each nickel(II) center is coordinated in a distorted octahedral (NiN 6) fashion consisting of three nitrogen atoms from one and two nitrogen atoms from other pentadentate ligand and the last one from terminal thiocyanato/cyanato ion. The variable temperature magnetic susceptibility of both compounds 1 and 2 have been studied and found a very weak magnetic interaction between two nickel(II) centers consistent with the structures (Ni⋯Ni distance of 5.809(1) Å for 1 and 5.695(1) Å for 2 with no suitable bridge or linkage to propagate magnetic interaction).

Spontaneous reduction of a low-spin Fe(III) complex of a neutral pentadentate N(5) Schiff base ligand to the corresponding Fe(II) species in acetonitrile.

The iron complexes of a designed pentadentate Schiff base ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine (SBPy(3)) have been synthesized. The low-spin mononuclear Fe(III) complex [(SBPy(3))Fe(DMF)](ClO(4))(3) (2), though stable in the solid state, is spontaneously reduced to the corresponding Fe(II) species [(SBPy(3))Fe(MeCN)](2+) in MeCN. Fe(II) complex [(SBPy(3))Fe(MeCN)](BF(4))(2) (3) has been isolated independently and characterized by crystallography. Electrochemical studies indicate that SBPy(3), like other pentadentate polypyridine ligands, stabilizes the Fe(II) center to a great extent (E(1/2) = 1.01 V vs SCE in MeCN). This fact is responsible for the ready reduction of 2. It is evident that such reactivity has brought complications in the syntheses of iron complexes of polypyridine ligands reported in previous accounts. Very low solubility of 2 in MeOH has allowed isolation of analytically pure 2 in the present work. Storage of dilute methanolic solution of 2 results in the formation of the mu-oxo Fe(III) dimer [(SBPy(3))FeOFe(SBPy(3))](ClO(4))(4) (5), the structure of which has also been determined. Fe(II) complex 3 reacts with CN(-) to afford cyanide adduct [(SBPy(3))Fe(CN)](BF(4)) (4) but does not exhibit any reactivity toward NO. The azomethine moiety (CH=N-py) of 2 is rapidly oxidized by H(2)O(2) to a pyridine-2-carboxamido (C(=O)-N-py) unit and affords [(PaPy(3))Fe(MeCN)](ClO(4))(2) (1), a complex previously reported by us.

Asymmetric Schiff Base (N 2 O 3 ) Complexes as Ligands Towards Mn(II), Fe(III) and Co(II), Synthesis and Characterization

Heterobi- and tri-nuclear complexes [LMM'Cl] and [(LM) 2 M'](M=Ni or Cu and M'=Mn, Fe or Co) have been synthesised. The heteronuclear complexes were prepared by stepwise reactions using two mononuclear Ni(II) and Cu(II) complexes of the general formula [HLM]·1/2H 2 O, as ligands towards the metal ions, Mn(II), Fe(III) and Co(II). The asymmetrical pentadentate (N 2 O 3 ) Schiff-base ligands used were prepared by condensing acetoacetylphenol and ethylenediamine, molar ratio 1 1, to yield a half-unit compound which was further condensed with either salicylaldehyde or naphthaldehyde to yield the ligands H 3 L 1 and H 3 L 2 which possess two dissimilar coordination sites, an inner four-coordinate N 2 O 2 donor set and an outer three-coordinated O 2 O set. 1 H NMR and IR spectra indicate that the Ni(II) and Cu(II) ions are bonded to the inner N 2 O 2 sites of the ligands leaving their outer O 2 O sites vacant for further coordination. Different types of products were obtained according to the type of metal ion. These products differ in stoichiometry according to the type of ligand in the parent compound. Electronic spectra and magnetic moments indicate that the structures of the parent Ni(II) and Cu(II) complexes are square-planar while the geometry around Fe(III), Mn(II) and Co(II) in their products are octahedral as elucidated from IR, UV-visible, ESR, 1 H NMR, mass spectrometry and magnetic moments.