Presence Of Weak Antiferromagnetic Coupling Research Articles

New heteroleptic l-argininato copper(II) complex with bromide and N, N’– heterocyclic ligands – synthesis, supramolecular, spectroscopic, magnetic and theoretical investigations

A new heteroleptic [CuBr(l−Arg)(bpy)]Br·2H2O (1) (l−Arg=l−Arginine, bpy=2,2’−bipyridine) complex has been synthesized and its molecular and supramolecular structure aspects have been discussed based on the X−ray single crystal structure and Hirshfeld analyses. In addition, the properties of complex 1 have been elucidated using different spectroscopic methods (FT−IR / FIR, Raman, NIR−Vis−UV, Q− / X− band EPR) and magnetic measurements. In this complex, the bromide anion occupies the apex of a slightly distorted square pyramid (τ = 0.12). The [CuBr(l−Arg)(bpy)]+ complex units exhibit a ribbon−like organization, with Cu(II) metal centers located at the corners of triangles where the shortest Cu∙∙∙Cu distance is 5.579 Å. The aromatic moieties of bpy are aligned nearly parallel leading to π···π interactions. Both, non−bonded and coordinated, Br− ions act as hydrogen−bond acceptors for guanidinium groups. Hirshfeld analysis indicated the importance of the Br∙∙∙H (17.9 − 18.4 %), O∙∙∙H (14.9−15.2 %), and H∙∙∙H (42.3−43.1 %) interactions in the molecular packing of the studied complex. Atoms in molecules (AIM) calculations indicated mainly the closed shell character for the Cu−N, Cu−O and Cu−Br coordination interactions. The G value calculated using the Q−band EPR spectra parameters g|| = 2.235 and g⊥= 2.05 is 4.7, what indicating negligible coupling and that local tetragonal axes are aligned parallel. A negative value of Weiss temperature θCW = -0.32 K indicates the presence of weak antiferromagnetic coupling between the neighboring Cu(II) ions mediated through hydrogen bonds and π-π stacking of aromatic moieties of bpy. The value of gav= 2.113 obtained from Q−band EPR spectra is consistent with that corresponding to the Curie constant (C = 0.4138 emu K mol−1), i.e.g = 2.101, and that obtained from the best fitting to the Brillouin function: g = 2.139.

Dysprosium‐Radical Based Single‐Molecule Magnet Containing a Seven Coordinate Dy(III) Ion: A System for ab initio Study

AbstractSingle–ion magnets (SIMs) possessing a Dy(III) ion have recently attracted huge scientific attention. Herein, we report on a Dy(III)‐radical single–molecule magnet containing a seven‐coordinate Dy(III) ion with an O3N3Cl donor set. The frequency–dependent slow relaxation of magnetization has been observed below 8 K. DC magnetic studies have shown the presence of weak antiferromagnetic coupling between the Kramers ion (Dy(III), J = 15/2) and the ligand‐centered radical electron (S = 1/2) which was confirmed by multifunctional CASSCF calculations.

Heterometallic Chain Compounds of Tetrakis(µ-carboxylato)diruthenium and Tetracyanidoaurate

Heterometallic complexes of tetrakis(µ-carboxylato)diruthenium(II,III) with tetracyanidoaurate(III) [Ru2(RCOO)4Au(CN)4]n (R = CH3 (1), C2H5 (2), i-C3H7 (3), and t-C4H9 (4)) were synthesized and characterized by C,H,N-elemental analysis and infrared spectroscopy and diffuse reflectance spectroscopy. The molecular structures were determined by a single-crystal X-ray diffraction method. A polymeric arrangement with the Ru2(RCOO)4+ units alternately linked by Au(CN)4− units is formed in these complexes. The trans-bridging mode of the Au(CN)4− unit for connecting the two Ru2(RCOO)4+ units was observed for 1 and 4, while the cis-bridging mode of the Au(CN)4− unit was observed for 2 and 3. Magnetic susceptibility data with variable temperature were modeled with a zero-field splitting model (D = 75 cm−1) and the presence of weak antiferromagnetic coupling between the RuIIRuIII units (zJ = −0.15~−0.10 cm−1) was estimated. N2-adsorption isotherms showed Type II curves with SBET of 0.728–2.91 m2 g−1.

Heterometallic 3d-4f Complexes as Air-Stable Molecular Precursors in Low Temperature Syntheses of Stoichiometric Rare-Earth Orthoferrite Powders.

Four 3d-4f hetero-polymetallic complexes [Fe2Ln2((OCH2)3CR)2(O2CtBu)6(H2O)4] (where Ln = La (1 and 2) and Gd (3 and 4); and R = Me (1 and 3) and Et (2 and 4)) are synthesized and analyzed using elemental analysis, Fourier transform infrared spectroscopy, thermogravimetric analysis, and SQUID magnetometry. Crystal structures are obtained for both methyl derivatives and show that the complexes are isostructural and adopt a defective dicubane topology. The four heavy metals are connected with two alkoxide bridges. These four precursors are used as single-source precursors to prepare rare-earth orthoferrite pervoskites of the form LnFeO3. Thermal decomposition in a ceramic boat in a tube furnace gives orthorhombic LnFeO3 powders using optimized temperatures and decomposition times: LaFeO3 formed at 650 °C over 30 min, whereas GdFeO3 formed at 750 °C over 18 h. These materials are structurally characterized using powder X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray map spectroscopy, and SQUID magnetometry. EDX spectroscopy mapping reveals a homogeneous spatial distribution of elements for all four materials consistent with LnFeO3. Magnetic measurements on complexes 1-4 confirm the presence of weak antiferromagnetic coupling between the central Fe(III) ions of the clusters and negligible ferromagnetic interaction with peripheral Gd(III) ions in 3 and 4. Zero-field-cooled and field-cooled measurements of magnetization of LaFeO3 and GdFeO3 in the solid-state suggest that both materials are ferromagnetic, and both materials show open magnetic hysteresis loops at 5 and 300 K, with Msat higher than previously reported for these nanomaterials. We conclude that this is a new and facile low temperature route to these important magnetic materials that is potentially universal, limited only by what metals can be programmed into the precursor complexes.

Dysprosium Chlorocyanoanilate-Based 2D-Layered Coordination Polymers.

A series of two-dimensional (2D)-layered coordination polymers (CPs) based on the heterosubstituted anilate ligand ClCNAn2- derived from 3-chloro-6-cyano-2,5-dihydroxybenzoquinone and DyIII are reported. By changes in the synthetic methods (layering technique, solvothermal or conventional one-pot reactions) and conditions (solvent, concentration, etc.), different types of 2D extended networks could be prepared and structurally characterized. Compounds 1 and 1', two polymorphs with the formula [Dy2(ClCNAn)3(DMSO)6]n·(H2O)x [x = 7 (1), 0 (1')], were prepared by a conventional one-pot reaction and recrystallized at different concentrations. Compound 2, formulated as [Dy2(ClCNAn)3(DMF)6]n, was prepared by a layering technique, while compound 3, formulated as {(Me2NH2)2[Dy2(ClCNAn)4(H2O)2]·(DMF)2·(H2O)5}n, was obtained by a solvothermal method. Compounds 1 and 2 are neutral 2D CPs of the ClCNAn2- ligand and DyIII ions, while 3 presents 2D anionic layers of [Dy2(ClCNAn)4(H2O)2]2- alternating with cationic layers of Me2NH2+ ions. These compounds show very diverse networks, with compound 1 forming 2D (8,3) and (4,3) topology with eight- and four-membered rings with square cavities, 1' and 2, respectively, a 2D (6,3) topology with six-membered rings (a rectangular cavity for 1' and a regular hexagonal cavity for 2), and 3 a 2D (4,4) topology with distorted square cavities. In this respect, 1 and 1' represent the first examples of polymorphism in the family of anilate-based CPs. Thermal analysis measurements (differential scanning calorimetry and thermogravimetry) show an exothermic polymorphic transformation from the kinetically stable 1' phase to the thermodynamically stable phase 1. The magnetic behavior of 1-3 very likely indicates depopulation of the mJ levels, while the presence of weak antiferromagnetic coupling between the DyIII centers mediated by the anilate bridge cannot be excluded.

A zig-zag end-to-end azido bridged MnIII 1-D coordination polymer: Spectral elucidation, magnetism, redox study and biological activity

A zig-zag one-dimensional Mn(III) coordination polymer, {[C6H4(O)CHN(CH2)2NCH(O)C6H4]Mn(N3)}n (1) has been synthesized reacting a manganese salt with the Schiff base precursor H2L, obtained condensing salicyldehyde and ethylenediamine. The compound has been characterized by IR, UV–Vis and EPR spectroscopy. Single crystal X-ray diffraction studies have revealed that the Mn atom possesses a distorted octahedral environment and the metals connected by end-to-end azido ligands lead to the formation of 1-D zig-zag molecular chains. Temperature dependent magnetic studies have shown the presence of weak anti-ferromagnetic coupling among the Mn centers. Cyclic voltammetry has evidenced the presence of a reversible redox couple for the Mn3+/Mn2+ system. Both the ligand and complex 1 exhibit anti-mycobacterial activity and considerable efficacy on Mycobacterium tuberculosis H37Rv ATCC 27294 and M. tuberculosis H37Ra ATCC 25177 strains. The cytotoxicity study on different human cancer cell lines (Caco 2, MCF7 and A549) suggests that complex 1 has potential anticancer properties.

Hydrazone‐Ligand‐Based Homodinuclear Lanthanide Complexes: Synthesis, Structure, and Magnetism

A series of dinuclear lanthanide complexes, [Dy2(LH)2(µ2‐Piv‐κ2O,O′)2(NO3‐κ2O,O′)2] (1), [Tb2(LH)2(µ2‐Piv‐κ2O,O′)2(NO3‐κ2O,O′)2] (2), [Gd2(LH)2(µ2‐Piv‐κ2O,O′)2(NO3‐κ2O,O′)2] (3), and [Ho2(LH)2(µ2‐Piv‐κ2O,O′)2(NO3‐κ2O,O′)2] (4) have been synthesized by the reaction of Ln(NO3)3·nH2O with the multidentate hydrazone‐based Schiff‐base ligand, N′‐(2‐hydroxy‐3‐methoxybenzylidene)acetohydrazide (LH2) in the presence of pivalic acid. X‐ray crystallography data reveals that 1–4 are isostructural, neutral, and contain two mono‐deprotonated ligands, two pivalates, and two chelated nitrates. Within the dinuclear assembly, the two lanthanides are bridged by two phenolate and two pivalate oxygens. Each of the two lanthanide ions is nine‐coordinated and possesses a distorted monocapped square‐antiprism geometry. Detailed magnetochemical analysis revealed the presence of weak antiferromagnetic coupling at low temperature for all complexes. The DyIII analogue displayed SMM behavior in the absence of a dc field with two relaxation domains in which the thermally activated domain exhibits an energy barrier of 40 K and relaxation time of 6.5 × 10–5 s.

Three-dimensional two-fold interpenetrated CrIII–GdIIIheterometallic framework as an attractive cryogenic magnetorefrigerant

The hydrothermal reaction of CrO3, Gd2O3 and iminodiacetic acid (H2IDA) yields a three-dimensional (3D) two-fold interpenetrated complex [CrGd(IDA)2(C2O4)]∞ (1). The oxalate anions were generated in situ under hydrothermal conditions. Magnetic studies suggest the presence of weak antiferromagnetic coupling between CrIII and GdIII ions, and negligible GdIII⋯GdIII and CrIII⋯CrIII couplings. Complex 1 exhibits a large magnetocaloric effect with − ΔSmaxm = 39.86 J kg−1 K−1 (the mass aspect) and 93.69 mJ cm−3 K−1 (the volumetric aspect), making it an attractive refrigerant for low-temperature applications.

Homometallic FeIII4 and Heterometallic {FeIII4LnIII2} (Ln = Dy, Tb) Complexes – Syntheses, Structures, and Magnetic Properties

AbstractThe sequential reaction of the multisite coordination ligand 2‐methoxy‐6‐{[2‐(2‐hydroxyethylamino)ethylimino]methyl}phenol (LH2) with Fe(ClO4)2·4H2O or a combination of Fe(ClO4)2·4H2O and Ln(NO3)3·5H2O [DyIII and TbIII] in the presence of tetramethylammonium hydroxide and pivalic acid (pivH) under ambient conditions afforded a homometallic tetranuclear FeIII complex and heterometallic hexanuclear FeIII/LnIII complexes, namely, [Fe4(μ3‐O)2(L)4]·3CH3CN·2CH3OH (1), [Fe4Dy2(μ3‐O)2(L)2(O2CCMe3)10(MeOH)]·2CH3CN (2), and [Fe4Tb2(μ3‐O)2(L)2(O2CCMe3)10(HO2CCMe3)]·H2O (3). Compounds 1–3 crystallize in the monoclinic system in space group P21/n (Z = 4). The homometallic tetranuclear derivative 1 possesses three contiguous four‐membered Fe2O2 rings in a ladderlike configuration. Each of the iron centers is hexacoordinate (2N, 4O) in a distorted octahedral geometry. On the other hand, 2 and 3 contain two symmetrically related Fe2Ln units that are joined to each other through oxygen atoms from the ethoxide side arms of the ligand L2–; the two lanthanide ions are located at the opposite ends of the hexanuclear core. In 2 and 3, the lanthanide centers are eight‐ (distorted trigonal dodecahedron) and nine‐coordinate (distorted monocapped square antiprism), respectively, and the iron centers are hexacoordinate (distorted octahedral). Magnetic studies on 2 and 3 reveal the presence of weak antiferromagnetic coupling between the FeIII and LnIII ions.

Thickness Dependent Structure and Magnetic Properties in <sup>57</sup>Fe/Ti/Co Multilayers

Structural and magnetic properties of57Fe/Ti/Co multilayers (MLs), prepared by Ion beam sputtering on Si substrate have been studied as a function of trilayers (TLs) thickness and modulation periods. X-ray reflectivity (XRR) shows the successful growth of high-quality layered structures along with up to 3rdorder Bragg peaks with distinct Kiessig oscillation. X-ray diffraction (XRD) and Mössbauer studies evidence the formation of different phases mainly at the interfaces. Soft magnetic properties with high saturation values (~800emu/cm3) and high anisotropy field (~20kOe) are observed in the room temperature magnetization curve. The in plane value of coercivities are 18Oe and 60Oe for the 12 and 24 TLs respectively while in the out-of plane direction it increases to nearly 8 and 12 times respectively. The 12 TLs sample shows the presence of two fold anisotropy whereas the 24 TLs sample shows an isotropic behaviour. Low temperature DC magnetization studies evidence the presence of weak antiferromagnetic coupling while the Kelly Henkel plots for these samples show that the dominant exchange interactions between the grains are ferromagnetic.

Formation of two drastically different MOFs based on Mn(ii)–benzoate and pyrazine with a change in seasonal temperature: structural analysis and magnetic study

Two new stoichiometrically and structurally very different Mn(II) coordination compounds {[Mn(pyz)(C6H5COO)2](C2H5OH)}n (1) and [Mn5(pyz)(C6H5COO)10(H2O)2]n (2) (where pyz = pyrazine) have been synthesized by the reaction of Mn(C6H5COO)2·4H2O and pyz in 1 : 1 molar ratio in different seasons of the year. The structural analyses reveal that complex 1, which is synthesized in summer when the temperature remains around 28–40 °C, features a 1D molecular ladder formed by joining of syn–syn benzoate-bridged dimeric Mn(II) units by pyz. Complex 2, which is obtained in winter (temperature around 10–20 °C), is an unprecedented 2D rectangular sheet motif based on a 1D column constructed from syn–syn benzoate bridged repetitive Mn5 units which are further interconnected by the pyz spacer. The compounds are interconvertible in solution: when an ethanol solution of 1 is allowed to evaporate slowly in winter or in a refrigerator (5–6 °C) it transformed into 2 whereas crystallization of 2 from ethanol medium in summer (28–40 °C) yields 1. Variable-temperature (1.8–300 K) magnetic susceptibility measurements showed presence of weak antiferromagnetic coupling in 1 that has been fitted with the van Vleck equation. The interaction in benzoate-bridged Mn2 unit was characterized to be J = −0.586(1) cm−1, where the spin-Hamiltonian is defined as H = −2JS1S2. The interaction in 2 was also characterized with the modified van Vleck equation to be weakly antiferromagnetic.

Lanthanide-pyridyl-2,5-dicarboxylate N-oxide frameworks with rutile topology

Four novel lanthanide-organic frameworks Ln(2,5-pdco)(CH3COO)(H2O) [Ln = La (1), Eu (2), Gd (3), Tb (4); 2,5-H2pdco = pyridyl-2,5-dicarboxylic acid N-oxide] have been synthesized and characterized. The coordination networks are similarly constructed viaLn2(μ-CH3COO)2 dinuclear secondary building units (SBUs), which are further linked by six 3-connected 2,5-pdco tectons to result in a novel 3D (3,6)-connected framework with rutile topology. Compound 2 shows the characteristic red emissions of Eu3+ at room temperature with the life time of 0.42(1) ms and emission quantum yield of 32.1%. The variable-temperature magnetic susceptibility studies indicate the presence of weak antiferromagnetic coupling between Gd3+ in 3 while weak ferromagnetic like one between Tb3+ in 4 through chelating-bridging acetate anions, despite their similar crystal structures.

Divalent metal 1,3-phenylenediacetate coordination polymers with rigid or flexible dipyridyl tethers: Chains, layers, and interpenetrated networks

Hydrothermal synthesis has afforded four divalent metal 1,3-phenylenediacetate (1,3-phda) coordination polymers containing different dipyridyl-type ligands. {[Cu(1,3-phda)(dpa)(H 2O)]·H 2O} n ( 1, dpa = 4,4′-dipyridylamine) exhibits a simple 2-D (4,4) rhomboid grid structure. {[Co(1,3-phda)(bpy)]·1.5H 2O} n ( 2, bpy = 4,4′-bipyridine) also possesses a (4,4) layer structure, but with syn– syn bridged {Co 2(OCO) 2} dimeric kernels serving as 4-connected nodes. {[Co(H 2O) 4(3-bpmpH 2)](1,3-phda) 2·8H 2O} n ( 3, 3-bpmp = bis(3-pyridylmethyl)piperazine) manifests cationic 1-D [Co(H 2O) 4(3-bpmpH 2)] n 4 n + chains linked into higher dimensionality by unligated 1,3-phda anions and curled tetrameric water molecule units. {[Ni(1,3-phda)(4-bpmp)(H 2O) 2]·2H 2O} n ( 4, 4-bpmp = bis(4-pyridylmethyl)piperazine) has an underlying twofold interpenetrated 6 58 ( cds) 3-D network topology. Variable temperature magnetic susceptibility studies revealed the presence of weak antiferromagnetic coupling and zero-field splitting ( J = −1.65(4) cm −1 and D = 30.9(7) cm −1 with g = 2.20(1)) within the {Co 2(OCO) 2} dimers in 2.

Synthesis, structure and magnetic properties of a pair of copper dicarboxylate/dipyridylamine coordination polymers with a non-interpenetrated CdSO 4 topology

Hydrothermal synthesis has afforded a pair of divalent copper coordination polymers containing the kinked and hydrogen-bonding capable imine 4,4′-dipyridylamine (dpa) and aromatic dicarboxylates, {[Cu(iph)(dpa)]·0.5H 2O} n ( 1, iph = isophthalate) and [Cu(tdc)(dpa)] n ( 2, tdc = 2,5-thiophenedicarboxylate). Compounds 1 and 2 contain orthogonally disposed parallel sets of 1-D [Cu(iph)] n and [Cu(tdc)] n chains, respectively, containing dicarboxylate-bridged dinuclear {CuOCO} 2 units. The chain motifs are joined by tethering dpa ligands to construct uncommon non-interpenetrated 3-D CdSO 4 lattices (6 58 topology) in both cases. Variable temperature magnetic studies show the presence of weak antiferromagnetic coupling within the {CuOCO} 2 dimers in both complexes, with J = −2.66(3) and −1.68(5) cm −1 for 1 and 2, respectively.

A new mixed ligand coordination polymer of Mn(II): structural aspect and cryomagnetic study

A new 1-D polymeric chain complex [Mn(pydc)(1,10-phen)]n· nH2O (pydc = pyridine-2,3-dicarboxylate, 1,10-phen = 1,10-phenanthroline) has been synthesised and characterised by elemental analysis, FT-IR spectrum, thermal analysis and variable temperature magnetic susceptibility studies. Single crystal X-ray diffraction study reveals that the central Mn(II) ion is in a distorted octahedral coordination geometry, and is coordinated to pydc and 1,10-phen. The complex shows interesting hydrogen bond modes involving the dicarboxylates and lattice water molecules. The presence of weak antiferromagnetic coupling with J = −0.72 cm−1 for the complex has been concluded from the cryomagnetic susceptibility studies.

Porous coordination polymer of copper(II) assembled from mixed organic ligands pyridine-2,4-dicarboxylic acid and trans-1,2-bis(4-pyridyl)ethylene: Synthesis, crystal structure and magnetic study

A 3-D porous coordination polymer of Cu(II) has been synthesized using two different organic bridging ligands, pyridine-2,4-dicarboxylate (pydc) and trans-1,2-bis(4-pyridyl)ethylene (bpe). Here, pydc and bpe connect two adjacent Cu(II) centers, resulting in a rectangular grid-like 2-D sheet with grid dimensions 10.058 × 15.258 Å. The channels are formed when 2-D networks running along the b-axis which are filled by solvent water molecules. The stability of porous networks, after removal of guest water molecules is confirmed by thermogravimetric analysis and X-ray powder diffraction technique. The low temperature magnetic study has been carried out, which indicates the presence of weak antiferromagnetic coupling with J = −0.59 cm −1.

High-spin metal–cyanide clusters: species incorporating [Mn(salen)] + complexes as a source of anisotropy

The use of N, N′-ethylenebis(salycylideneiminato) (salen) complexes of Mn III in assembling high-spin metal–cyanide coordination clusters with significant magnetic anisotropy is demonstrated. The reaction of [Mn(salen)(H 2O) 2] +with [Cr(CN) 6] 3− in aqueous solution generates {Cr[CNMn(salen)(H 2O)] 6}[Cr(CN) 6]·6H 2O ( 1), a previously reported compound featuring a heptanuclear cluster with a distorted octahedral geometry. A fit to the variable-temperature magnetic susceptibility data for 1 revealed the presence of weak antiferromagnetic coupling of J=−2.5 cm −1 within the cluster, giving rise to an S=21/2 ground state. In addition, variable-field magnetization data collected at low temperatures revealed the presence of magnetic anisotropy in the ground state, with the best fit yielding zero-field splitting parameters of D=+0.19 cm −1 and |E|=0.036 cm −1. A reaction intended to produce a direct analogue of 1 by employing [Fe(CN) 6] 3− in place of [Cr(CN) 6] 3− instead gave an unusually complex compound of formula {Fe(CN) 4[CNMn(salen)(MeOH)] 2}{[Mn(salen)(H 2O)] 2}[Mn(salen)(H 2O)(MeOH)] 2[Fe(CN) 6]·4H 2O ( 2). The crystal structure and magnetic properties of this compound are reported.

Synthesis, electrochemical and e.p.r. spectral studies of binuclear copper(II) complexes with new pentadentate L-proline-based binucleating ligands

The synthesis, reduction, optical and e.p.r. spectral properties of a series of new binuclear copper(II) complexes, containing bridging moieties (OH−, MeCO2−, NO2−, and N3−), with new proline-based binuclear pentadentate Mannich base ligands is described. The ligands are: 2,6-bis[(prolin-1-yl)methyl]4-bromophenol [H3L1], 2,6-bis[(prolin-1-yl)methyl]4-t-butylphenol [H3L2] and 2,6-bis[(prolin-1-yl)methyl]4-methoxyphenol [H3L3]. The exogenous bridging complexes thus prepared were hydroxo: [Cu2L1(OH)(H2O)2] · H2O (1a), [Cu2L2(OH)(H2O)2] · H2O (1b), [Cu2L3(OH)(H2O)2] · H2O (1c), acetato [Cu2L1(OAc)] · H2O (2a), [Cu2L2(OAc)] · H2O (2b), [Cu2L3(OAc)] · H2O (2c), nitrito [Cu2L1(NO2)(H2O)2] · H2O (3a), [Cu2L2(NO2)(H2O)2] · H2O (3b), [Cu2L3(NO2)(H2O)2] · H2O (3c) and azido [Cu2L1(N3)(H2O)2] · H2O (4a), [Cu2L2(N3)(H2O)2] · H2O (4b) and [Cu2L3(N3)(H2O)2] · H2O (4c). The complexes were characterized by elemental analysis and by spectroscopy. They exhibit resolved copper hyperfine e.p.r. spectra at room temperature, indicating the presence of weak antiferromagnetic coupling between the copper atoms. The strength of the antiferromagnetic coupling lies in the order: NO2 ≅ N3 ≥ OH ≥ OAc. Cyclic voltammetry revealed the presence of two redox couples CuIICuII ⇌ CuIICuI ⇌ CuICuI. The conproportionality constant Kcon for the mixed valent CuIICuI species for all the complexes have been determined electrochemically.

Exogenous bridging and nonbridging in Cu(II) complexes of Mannich base ligands: Synthesis and physical properties

Preparation of pentadentate ligands L1, L2, L3 and L4, where L1 = 4-chloro-3-methyl-2[(prolin-1-yl)methyl]-6-[N-phenyl piperazin-1-yl)methyl]phenol, L2 = 4-ethyl-2-[(prolin-1-yl)methyl]-6-[(N-phenyl piperazin-1-yl)methyl]phenol, L3 = 4-chloro-3-methyl-2-[(prolin-1-yl)methyl]-6-[N-methyl piperazin-1-yl]methyl phenol, L4 = 4-methoxy-2-[(prolin-1-yl)methyl]-6-[(N-phenyl piperazin-1-yl)methyl]phenol is described together with that of the corresponding Cu(II) complexes with various bridging motifs like OH, OAc and NO2. The complexes are characterized by elemental analysis, electrochemical and electron paramagnetic spectral studies. Redox properties of the complexes in acetonitrile are highly quasireversible due to the chemical or/and stereochemical changes subsequent to electron transfer. The complexes show resolved copper hyperfine EPR at room temperature, indicating the presence of weak antiferromagnetic coupling between the copper atoms. Strengths of the antiferromagnetic interactions are in the order NO2>OAc>OH.

Influence of metal–metal interaction on magnetic and redox properties in different series of heterodinuclear copper(II), nickel(II), and zinc(II) complexes

Different series of heterodinuclear complexes, viz.,|CuNiL](ClO4)2•nH2O,[CuZnL](ClO4)2•nH2O,[NiCuL](ClO4)2•nH2O, and [NiZnL](ClO4)2•nH2O complexes where L is a dinucleating Schiff base ligand derived from the condensation of 1 mole of 2-hydroxy-5-methylbenzene-1,3-dicarbaldehyde, 1 mole of 1,3-diaminopropane, and 2 moles of ortho- or para-substituted aniline, have been synthesized and investigated. The μeff values of the CuNiL and NiCuL series of complexes suggest presence of weak antiferromagnetic coupling in them. In MeCN and DMF the CuNiL and NiCuL series of complexes exhibit two reversible one-electron redox couples at two different potentials. The large separation between the two redox couples is attributed to the antiferromagnetic interaction between the metal centres in the complexes. In the NiCuL series of complexes, the observed shift of 100–150 mV in E1 to more negative values in complexes with potential donor groups such as OMe, OEt, SMe, etc. in the ortho position of the phenyl group of L is explained on the basis of weak bonding interactions of these groups with the copper centre. The CuNiL series of complexes do not exhibit such shift in E1 values since there is no significant change in the square planar geometry of the copper centre. The CuZnL and NiZnL series of complexes exhibit only single one-electron redox couple as could be anticipated.