Electrocatalytic Behavior of an Amide Functionalized Mn(II) Coordination Polymer on ORR, OER and HER.

The new 3D coordination polymer (CP) [Mn(L)(HCOO)]n (Mn-CP) [L = 4-(pyridin-4-ylcarbamoyl)benzoate] was synthesised via a hydrothermal reaction using the pyridyl amide functionalized benzoic acid HL. It was characterized by elemental, FT-IR spectroscopy, single-crystal and powder X-ray diffraction (PXRD) analyses. Its structural features were disclosed by single-crystal X-ray diffraction analysis, which revealed a 3D structure with the monoclinic space group P21/c. Its performance as an electrocatalyst for oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions was tested in both acidic (0.5 M H2SO4) and alkaline (0.1 M KOH) media. A distinct reduction peak was observed at 0.53 V vs. RHE in 0.1 M KOH, which corresponds to the oxygen reduction, thus clearly demonstrating the material's activity for the ORR. Tafel analysis revealed a Tafel slope of 101 mV dec-1 with mixed kinetics of 2e- and 4e- pathways indicated by the Koutecky-Levich analysis. Conversely, the ORR peak was not present in 0.5 M H2SO4 indicating no activity of Mn-CP for this reaction in acidic media. In addition, Mn-CP demonstrated a noteworthy activity toward OER and HER in acidic media, in contrast to what was observed in 0.1 M KOH.

Acceptorless Dehydrogenation of Alcohols and Transfer Hydrogenation of Aldehydes in Water Using a Single Bifunctional, Bistate, Water-soluble Ruthenium Catalyst.

A water-soluble bifunctional, bistate ruthenium catalyst has been developed using the 8-aminoquinoline ligand which responds to an acid-base stimuli and is catalytically active for two complementary reactions. The "Ru-amino state" is highly active for catalytic transfer hydrogenation of various aldehydes using formic acid as the hydrogen source resulting in a range of primary alcohols. On the other hand, the "Ru-amido state" is active for the acceptorless dehydrogenation of alcohols to yield carboxylate salts or ketones. The reactions can be carried out in air without an inert atmosphere and the products can be purified by simple extraction without the use of any chromatographic techniques. A range of functional groups which include electron-rich and deficient (hetero)arenes and alkenes, alkynes, halides, esters, cyano, and nitro groups, are all well tolerated, indicating excellent chemoselectivity for transfer hydrogenation.

Shielding Effect of Nanomicelles: Stable and Catalytically Active Oxidizable Pd(0) Nanoparticle Catalyst Compatible for Cross-Couplings of Water-Sensitive Acid Chlorides in Water.

Under the shielding effect of nanomicelles, a sustainable micellar technology for the design and convenient synthesis of ligand-free oxidizable ultrasmall Pd(0) nanoparticles (NPs) and their subsequent catalytic exploration for couplings of water-sensitive acid chlorides in water is reported. A proline-derived amphiphile, PS-750-M, plays a crucial role in stabilizing these NPs, preventing their aggregation and oxidation state changes. These NPs were characterized using 13C nuclear magnetic resonance (NMR), infrared (IR), and surface-enhanced Raman scattering (SERS) spectroscopy to evaluate the carbonyl interactions of PS-750-M with Pd. High-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDX) studies were performed to reveal the morphology, particle size distribution, and chemical composition, whereas X-ray photoelectron spectroscopy (XPS) measurements unveiled the oxidation state of the metal. In the cross-couplings of water-sensitive acid chlorides with boronic acids, the micelle's shielding effect and boronic acids plays a vital role in preventing unwanted side reactions, including the hydrolysis of acid chlorides under basic pH. This approach is scalable and the applications are showcased in multigram scale reactions.

A Mixed Valence CoIICoIII2 Field-Supported Single Molecule Magnet: Solvent-Dependent Structural Variation.

One-pot reaction of the Schiff base N,N'-ethylene bis(salicylaldimine) (H2L), CoCl2.6H2O, and [Ph2SnCl2] in acetone produces the mixed valence CoIICoIII2 compound [CoIICoIII2(µ-L)2(Ph)2(µ-Cl)2]·(CH3)2CO·H2O (1). Our recent study already revealed that the same reaction mixtures in methanol or ethanol produced a heterometallic SnIVCoIII (2) or monometallic CoIII complex (3), respectively. Comparison of these organometallic systems shows that the 2,1-intermetallic Ph shift occurs in any of those solvents, but their relevant structural features (mononuclear, dinuclear-heterometallic, and trinuclear mixed valence) are solvent dependent. Geometrical structural rotation is also discussed among the related organometallic CoIICoIII2 systems. The AC magnetic susceptibility measurements indicate that 1 is a single molecule magnet (SMM), exhibiting a field-induced slow magnetic relaxation with two modes. The relaxation time for the low-frequency channel is as slow as τ~0.6 s at T = 2.0 K and BDC = 1.0 T.

NaCl as Catalyst and Water as Solvent: Highly E-Selective Olefination of Methyl Substituted N-Heteroarenes with Benzyl Amines and Alcohols.

Oxidative coupling of benzylamines and alcohols with methyl substituted N-heteroarenes such as quinolines and quinoxalines has been achieved using chloride, a sea abundant anion as the catalyst for practical synthesis of a wide range of E-disubstituted olefins in aqueous medium. Detailed mechanistic studies and control experiments were carried out to deduce the reaction mechanism which indicated that in situ formed ClO2- is the active form of the catalyst. We have successfully carried out a 1 g scale reaction using this methodology, and five pharmaceutically relevant conjugated olefins were also synthesized by this method in moderate to good yields.

Catalytic Oxidation of Alcohols and Amines to Value-Added Chemicals using Water as the Solvent.

Designing reactions in aqueous media has been one of the major challenges in modern organic synthesis, especially to avoid the use of large amounts of organic solvents whose disposal is a matter of grave concern from an environmental perspective. The oxidation of alcohols and amines is an essential and important step in the synthesis of many valuable products including polymers and pharmaceuticals. In recent times, there has been a surge in the use of water as a solvent in many organic reactions. This review focuses specifically on the oxidation reactions of alcohols and amines carried out in water media using transition metal catalysts, metal-free catalysts and photocatalysts.

Oxidative Coupling of Benzylamines with Indoles in Aqueous Medium to Realize Bis-(Indolyl)Methanes Using a Water-Soluble Cobalt Catalyst and Air as the Oxidant.

Oxidative coupling of benzylamines with indoles to bis-(indolyl)methanes (BIMs) was achieved using an inexpensive water-soluble cobalt complex, Co(bpb). The catalyst was found to be quite effective when air was used as the oxidant and water as solvent under mild reaction conditions. Aromatic amines were successfully converted to the corresponding BIMs with yields up to 85 %. We have successfully carried out a 1 gram scale reaction, and few pharmaceutically relevant compounds were also synthesised by this method in good yields. Control experiments have also been carried out to understand the possible reaction mechanism.

Aerobic Oxidation of Primary Amines to Imines in Water using a Cobalt Complex as Recyclable Catalyst under Mild Conditions.

Oxidative coupling of primary amines to imines has been achieved by using a water soluble cobalt complex as catalyst and air as the oxidant at near ambient conditions. Aromatic, heteroaromatic and aliphatic amines were successfully converted to the corresponding imines with yields of up to 96 %. A 20 gram scale reaction for the synthesis of imine from benzylamine in good yield is also demonstrated with this method. The catalyst has been found to be reusable with up to five cycles. It is highly efficient, gives a turnover number (TON) of up to 128, and shows chemoselectivity with the only byproducts being water and ammonia. Control experiments and mechanistic studies indicate that the CoII /CoIII catalytic cycle is responsible for these oxidative transformations. Some of the reactive intermediates of this reaction have also been isolated and structurally characterized.

Synthesis of unsymmetrical multi-aroyl derivatives of ferrocene using palladium catalysed oxidative C-H aroylation.

A palladium catalysed methodology for mono-selective oxidative aroylation of ferrocene has been achieved. 10 mol% of Pd(OAc)2 was used as the catalyst for this radical aroylation reaction using O-methyl-oxime as the directing group. tert-Butyl hydroperoxide was used as an oxidant and both aldehydes and alcohols were used as acyl-equivalents. The protocol has been extended to aryl, heteroaryl and sterically hindered aroylating agents. Acidic-hydrolysis with 6(N) HCl of the directing group resulted in unsymmetrically substituted ketone derivatives of ferrocene, which have not been achieved otherwise.

Flexibility and lability of a phenyl ligand in hetero-organometallic 3d metal-Sn(iv) compounds and their catalytic activity in Baeyer-Villiger oxidation of cyclohexanone.

The single compartmental Schiff base N,N'-ethylenebis(salicylaldimine) (H2L) and [SnPh2Cl2] were utilized to synthesize heterobimetallic 3d metal-Sn complexes, the CoIIISnIV compound [{SnPhCl2}(1κO2N2,2κO2-µ-L)(µ-OMe){CoPh}] (1), the NiIISnIV compound [{SnPh2Cl2}(1κO2N2,2κO2-µ-L)Ni] (2) and the CuIISnIV compound [{SnPh2Cl2}(1κO2N2,2κO2-µ-L)Cu] (3). Attempting to prepare the ethoxido bridged compound analogous to 1 (in ethanol) gives the phenylcobalt(iii) complex [Co(κO2N2)Ph(H2O)] (1A). Single crystal X-ray structure analyses reveal that 1 is derived from an intermetallic (Sn to Co) phenyl shift and that 1A is a transmetallated product; in compounds 2 and 3, the phenyl groups remain coordinated to SnIV but one of the π rings interacts with the 3d-metal. Thus, while systems 1 and 1A show the lability of the phenyl ligand, 2 and 3 reveal its flexible nature. Theoretical DFT calculations demonstrate that the conceivable Ph group shift occurs in the oxidized CoIII intermediate [{SnIVPh2Cl2}(κO2N2-µ-L){CoIII(MeO)}] (5) rather than in the corresponding CoII species [{SnIVPh2Cl2}(κO2N2-µ-L){CoII(MeOH)}] (4). Their catalytic studies in the Baeyer-Villiger oxidation of cyclohexanone into ε-caprolactone with two different oxidants reveal that the sacrificial aldehyde method (with dioxygen/benzaldehyde) is better than that with aqueous H2O2 (30%). The effects of various reaction parameters such as solvent, catalyst amount, temperature, time and heating method were studied allowing the achievement of yields up to 83% with 89% selectivity.

Unfolding biological properties of a versatile dicopper(II) precursor and its two mononuclear copper(II) derivatives.

Synthesis, inter-conversions and biological study of the dichloro bridged dicopper(II) compound [CuLCl]2 (1) and its two mononuclear derivatives [CuLCl(H2O)]·H2O (2) and [CuLCl(py)] (3) (HL=2-(2-pyridylmethyleneamino)benzenesulfonic acid) are described. The dimeric compound 1 collapses into monomers 2 and 3 in the presence of coordinating solvents, water and pyridine, respectively, and 1 is regenerated upon simple stirring of 2 or 3 in methanol. The reactions of 1 with neutral (present study) and charged (earlier studies) ligands result in monomeric and multimeric compounds, respectively, attesting that it is a versatile dicopper(II) precursor. The anticancer activity of these copper complexes (1-3) was screened against lung (A-549) and breast (MDA-MB-231) human cancer cell lines. The IC50 (half maximal inhibitory concentration) value for one (3) of the compounds suggests preferential cytotoxicity against breast cancer MDA-MB-231 cell line. Furthermore, the IC50 value obtained for complex 3 is found to be almost two-fold times cytotoxic than the standard drug cisplatin. In addition, the underlying possible mechanism of its apoptosis-inducing efficacy in MDA-MB-231 cells has been rationalized by using flow cytometry (FACS) and Hoechst 33342/propidium iodide (PI) fluorescence staining. The stimulation of apoptotic induction for complex 3 has further been affirmed by reactive oxygen species (ROS) generation and mitochondrial aggregations studies.

N-HO and N-HCl supported 1D chains of heterobimetallic CuII/NiII-SnIV cocrystals.

The Schiff base H2L1 [N,N'-ethylenebis(3-methoxysalicylaldimine)] or H2L2 [N,N'-ethylenebis(3-ethoxysalicylaldimine)] was reacted with MCl2·xH2O and SnCl4·5H2O to afford the supramolecular heterobimetallic systems (H2ED)2+·2[ML]·[SnCl6]2- [M = Cu, L = L1 (1), L = L2 (2); M = Ni, L = L1 (3), L = L2 (4); ED = 1,2-ethylenediamine], whose structures were established by single crystal X-ray analyses. Each structure includes different entities, viz. a mononuclear [CuL]/[NiL] neutral complex (coformer), a hexachlorostannate dianion [SnCl6]2-, a 1,2-ethylenediammonium dication (H2ED2+) and, only in 2 and 4, a methanol molecule. Based on the work of Grothe et al. (Cryst. Growth Des., 2016, 16, 3237-3243), compounds 1 and 3 are cocrystal salts, 2 and 4 are cocrystal salt solvates. The ionic pairs (H2ED)2+·[SnCl6]2- in 1-4 are encapsulated by the Cu- or Ni-complexes, and stabilized by N-HO and one N-HCl bond interactions leading to infinite 1D chains. The antimicrobial studies of 1-4 against yeasts (C. albicans and S. cerevisiae) and Gram-positive (S. aureus and E. faecalis) and -negative bacteria (P. aeruginosa and E. coli) indicate that the Ni2Sn systems (3 and 4) are more active than the analogous Cu2Sn ones (1 and 2).

Syntheses and crystal structures of benzene-sulfonate and -carboxylate copper polymers and their application in the oxidation of cyclohexane in ionic liquid under mild conditions.

The syntheses, crystal structures and catalytic activities of the polymers derived from 2-(2-pyridylmethyleneamino)benzenesulfonic acid (HL), viz. [CuL(H2tma)]n (1) and [{Cu2L2(H2pma)}·(8H2O)]n (2) [H3tma = benzene-1,3,5-tricarboxylic (trimesic) acid and H4pma = benzene-1,2,4,5-tetracarboxylic (pyromellitic) acid], are presented. Despite the comparable combinations and compositions of ligands (sulfonate and carboxylate) in these two polymers the bridging moiety in 1 is sulfonate while in 2 it is carboxylate. Complexes 1 and 2 act as catalysts in the peroxidative oxidation of cyclohexane under mild conditions using either the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] or acetonitrile as the solvent. The ionic liquid medium leads to increases in the yields and in the turnover numbers, achieved in shorter reaction times in comparison with those when using the conventional acetonitrile solvent. A simple recycling of the catalysts in the ionic liquid medium is achieved without loss of activity and selectivity.

Sulfonated Schiff base Sn(IV) complexes as potential anticancer agents.

Syntheses, crystal structures and biological activities of the diphenoxo-bridged diorgano dinuclear Sn(IV) compounds [Sn(Et)2(HL)(H2O)]2 (1) and [Sn(n-Bu)2(HL)(H2O)]2 (2) derived from the Schiff base 2-[(2,3-dihydroxyphenyl)methylideneamino]benzenesulfonic acid trihydrate (H3L·3H2O) are described. The monoprotonated form (HL2-) of the Schiff base behaves as O,O'-bidentate ligand, chelating the metal by the two phenoxo oxygen atoms. The hexacoordinated metal centres in 1 and 2 are bridged by a phenoxo oxygen and the remaining coordination positions are fulfilled by the other phenoxo oxygen, two organic groups (ethyl for 1 and n-butyl for 2) and a water molecule. A two dimensional zigzag sheet in 1 and three dimensional polymeric networks in H3L·3H2O and 2 are stabilized by a number of non-covalent, H-bonding and π⋯π stacking interactions. The DNA binding activities of these complexes have been studied by UV-vis and fluorescence spectroscopies. Their antiproliferative efficacies have been evaluated on A-549, HeLa and MDA-MB-231 cancer cell lines by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. IC50 values (1.35±0.23, 2.43±0.54 and 1.74±0.04µM for 2) are indicative of a substantial cytotoxicity of 2, mainly towards the A-549 lung cancer cell line. The greater antiproliferative efficacy of 2has further been studied by fluorescence activated cell sorting (FACS) and nuclear morphology by Hoechst/propidium iodide (PI) double staining method. The possible mode of the apoptotic pathway for 2has been substantiated by the reactive oxygen species (ROS) generation studies.

Bis-phenoxido and bis-acetato bridged heteronuclear {Co(III)Dy(III)} single molecule magnets with two slow relaxation branches.

Two bis-µ-phenoxido-bis-µ-acetato heterobimetallic {Co(III)Dy(III)} complexes and , formulated as [Co(III)Dy(III)L(µ-OAc)2(NO3)2] derived from the comparable hexadentate Schiff bases N,N'-ethylenebis(3-ethoxysalicylaldimine) and N,N'-ethylenebis(3-methoxysalicylaldimine) were synthesized and X-ray structure analysis confirms their nearly identical structures. These are the first examples of bis(µ-phenoxido)-bis(µ-carboxylato) {Co(III)Dy(III)} systems. The AC susceptibility measurements show that both complexes exhibit a field-induced slow magnetic relaxation with two relaxation branches. While the high-frequency process spans the usual range of the relaxation time for analogous single molecule magnets (τ0 ∼ 10(-7) s), the low-frequency branch is as slow as τ ∼ 0.1 s at T = 1.9 K and B = 0.2 T.

Synthesis, structure and catalytic application of lead(II) complexes in cyanosilylation reactions.

Hydrothermal reactions of a lead(II) salt with 3-aminopyrazine-2-carboxylic acid (HL1) gave rise to a series of lead(II) coordination compounds (1­6) having zero, one, two and three dimensional structures. X-ray diffraction structural analyses reveal that complexes [Pb(L1)2]2 (1) and [Pb(L1)(OCHNH2)(η-OCHO)]2 (2) possess dinuclear structures, containing a centre of symmetry. Complexes [Pb2(L1)4(NHMeCHO)2]n (3) and [Pb2(L1)4]n·(H2O)n·(2.5DMF)n (4) have 1D chain like structures, and [Pb5(L1)7(η-NO3)(µ-HCOO)(η-HCOO)]n·(DMF)n·(MeOH)n (5) shows a 2D sheet like structure constructed by the [Pb5O5(HCOO)] cluster and 3-aminopyrazine-2-carboxylate anions. The hydrothermal reaction of lead(II) nitrate with HL1 in DMF led to in situ formation of 3,3'-(methylenebis(azanediyl))bis(pyrazine-2-carboxylic acid) [H2L2] which produces the 3D framework [Pb2(L2)2]n·(2DMF)n·(H2O)n (6). The L1(−) and L2(2−) ligands bind the metal cations by means of a pyrazine N-atom and one, or both, carboxylate O-atoms. The carboxylate group of L1(−) presents a diversity of coordination modes, viz., monodentate (1 and 3), bridging µ2 (3) and bridging µ3 (4), monodentate bridging µ2 (1, 2, 4, 5 and 6) and bridging chelate µ2 (5). The carboxylate moiety of L2(2−) in 6 binds the metal in a bridging µ2 fashion. The Pb(II) ions display coordination numbers from 5 to 8 with hemi- or holodirected coordination environments. The Pb(II) complexes act as heterogeneous catalysts for the cyanosilylation reaction, at 15 °C, of different aldehydes with trimethylsilyl cyanide (TMSCN) and can be recycled at least three times without losing activity.

Syntheses, structures, magnetic properties, and density functional theory magneto-structural correlations of bis(µ-phenoxo) and bis(µ-phenoxo)-µ-acetate/bis(µ-phenoxo)-bis(µ-acetate) dinuclear Fe(III)Ni(II) compounds.

The bis(µ-phenoxo) Fe(III)Ni(II) compound [Fe(III)(N3)2LNi(II)(H2O)(CH3CN)](ClO4) (1) and the bis(µ-phenoxo)-µ-acetate/bis(µ-phenoxo)-bis(µ-acetate) Fe(III)Ni(II) compound {[Fe(III)(OAc)LNi(II)(H2O)(µ-OAc)](0.6)·[Fe(III)LNi(II)(µ-OAc)2](0.4)}(ClO4)·1.1H2O (2) have been synthesized from the Robson type tetraiminodiphenol macrocyclic ligand H2L, which is the [2 + 2] condensation product of 4-methyl-2,6-diformylphenol and 2,2'-dimethyl-1,3-diaminopropane. Single-crystal X-ray structures of both compounds have been determined. The cationic part of the dinuclear compound 2 is a cocrystal of the two species [Fe(III)(OAc)LNi(II)(H2O)(µ-OAc)](+) (2A) and [Fe(III)LNi(II)(µ-OAc)2](+) (2B) with weights of 60% of the former and 40% of the latter. While 2A is a triply bridged bis(µ-phenoxo)-µ-acetate system, 2B is a quadruply bridged bis(µ-phenoxo)-bis(µ-acetate) system. Variable-temperature (2-300 K) magnetic studies reveal antiferromagnetic interaction in 1 and ferromagnetic interaction in 2 with J values of -3.14 and 7.36 cm(-1), respectively (H = -2JS1·S2). Broken-symmetry density functional calculations of exchange interaction have been performed on complexes 1 and 2 and also on previously published related compounds, providing good numerical estimates of J values in comparison to experiments. The electronic origin of the difference in magnetic behavior of 1 and 2 has been well understood from MO analyses and computed overlap integrals of BS empty orbitals. The role of acetate and thus its complementarity/countercomplementarity effect on the magnetic properties of diphenoxo-bridged Fe(III)Ni(II) compounds have been determined on computing J values of model compounds by replacing bridging acetate and nonbridging acetate ligand(s) by water ligands in the model compounds derived from 2A,B. The DFT calculations have also been extended to develop several magneto-structural correlations in these types of complexes, and the correlations focus on the role of Fe-O-Ni bridge angle, average Fe/Ni-O bridge distance, Fe-O-Ni-O dihedral angle, and out-of-plane shift of the phenoxo group.

Magnetic exchange interactions and magneto-structural correlations in heterobridged µ-phenoxo-µ(1,1)-azide dinickel(II) compounds: a combined experimental and theoretical exploration.

This investigation presents the syntheses, crystal structures, magnetic properties, and density functional theoretical modeling of magnetic behavior of two heterobridged µ-phenoxo-µ(1,1)-azido dinickel(II) compounds [Ni(II)(2)(L(1))(2)(µ(1,1)-N(3))(N(3))(H(2)O)]·CH(3)CH(2)OH (1) and [Ni(II)(2)(L(2))(2)(µ(1,1)-N(3))(CH(3)CN)(H(2)O)](ClO(4))·H(2)O·CH(3)CN (2), where HL(1) and HL(2) are the [1+1] condensation products of 3-methoxysalicylaldehyde and 1-(2-aminoethyl)-piperidine (for HL(1))/4-(2-aminoethyl)-morpholine (for HL(2)), along with density functional theoretical magneto-structural correlations of µ-phenoxo-µ(1,1)-azido dinickel(II) systems. Compounds 1 and 2 crystallize in orthorhombic (space group Pbca) and monoclinic (space group P2(1)/c) systems, respectively. The coordination environments of both metal centers are distorted octahedral. The variable-temperature (2-300 K) magnetic susceptibilities at 0.7 T of both compounds have been measured. The interaction between the metal centers is moderately ferromagnetic; J = 16.6 cm(-1), g = 2.2, and D = -7.3 cm(-1) for 1 and J = 16.92 cm(-1), g = 2.2, and D(Ni1) = D(Ni2) = -6.41 cm(-1) for 2. Broken symmetry density functional calculations of exchange interaction have been performed on complexes 1 and 2 and provide a good numerical estimate of J values (15.8 cm(-1) for 1 and 15.35 cm(-1) for 2) compared to experiments. The role of Ni-N bond length asymmetry on the magnetic coupling has been noted by comparing the structures and J values of complexes 1 and 2 together with previously published dimers 3 (Eur. J. Inorg. Chem. 2009, 4982), 4 (Inorg. Chem. 2004, 43, 2427), and 5 (Dalton Trans. 2008, 6539). Our extensive DFT calculations reveal an important clue to the mechanism of coupling where the orientation of the magnetic orbitals seems to differ with asymmetry in the Ni-N bond lengths. This difference in orientation leads to a large change in the overlap integral between the magnetic orbitals and thus the magnetic coupling. DFT calculations have also been extended to develop several magneto-structural correlations in this type of complexes and the correlation aim to focus on the asymmetry of the Ni-N bond lengths reveal that the asymmetry plays a proactive role in governing the magnitude of the coupling. From a completely symmetric Ni-N bond length, two behaviors have been noted: with a decrease in bond length there is an increase in the ferromagnetic coupling, while an increase in the bond lengths leads to a decrease in ferromagnetic interaction. The later correlation is supported by experiments. The magnetic properties of 1, 2, and three previously reported related compounds have been discussed in light of the structural parameters and also in light of the theoretical correlations determined here.

Slow magnetic relaxation and electron delocalization in an S = 9/2 iron(II∕III) complex with two crystallographically inequivalent iron sites.

The magnetic, electronic, and Mössbauer spectral properties of [Fe(2)L(µ-OAc)(2)]ClO(4), 1, where L is the dianion of the tetraimino-diphenolate macrocyclic ligand, H(2)L, indicate that 1 is a class III mixed valence iron(II∕III) complex with an electron that is fully delocalized between two crystallographically inequivalent iron sites to yield a [Fe(2)](V) cationic configuration with a S(t) = 9∕2 ground state. Fits of the dc magnetic susceptibility between 2 and 300 K and of the isofield variable-temperature magnetization of 1 yield an isotropic magnetic exchange parameter, J, of -32(2) cm(-1) for an electron transfer parameter, B, of 950 cm(-1), a zero-field uniaxial D(9∕2) parameter of -0.9(1) cm(-1), and g = 1.95(5). In agreement with the presence of uniaxial magnetic anisotropy, ac susceptibility measurements reveal that 1 is a single-molecule magnet at low temperature with a single molecule magnetic effective relaxation barrier, U(eff), of 9.8 cm(-1). At 5.25 K the Mössbauer spectra of 1 exhibit two spectral components, assigned to the two crystallographically inequivalent iron sites with a static effective hyperfine field; as the temperature increases from 7 to 310 K, the spectra exhibit increasingly rapid relaxation of the hyperfine field on the iron-57 Larmor precession time of 5 × 10(-8) s. A fit of the temperature dependence of the average effective hyperfine field yields |D(9∕2)| = 0.9 cm(-1). An Arrhenius plot of the logarithm of the relaxation frequency between 5 and 85 K yields a relaxation barrier of 17 cm(-1).

Magneto-structural correlation studies and theoretical calculations of a unique family of single end-to-end azide-bridged Ni(II)4 cyclic clusters.

The work in this paper aims to portray a complete structural, magnetic, and theoretical description of two original end-to-end (EE) µ(1,3)-azide-bridged, cyclic tetranuclear Ni(II) clusters, [{Ni(II)(L(1))(µ(1,3)-N(3))(H(2)O)}(4)] (1) and [{Ni(II)(L(2))(µ(1,3)-N(3))(H(2)O)}(4)] (2), where the ligands used to achieve these species, HL(1) and HL(2), are the tridentate Schiff base ligands obtained from [1 + 1] condensations of salicylaldehyde with 1-(2-aminoethyl)-piperidine and 4-(2-aminoethyl)-morpholine, respectively. The title compounds, 1 and 2, crystallize in a monoclinic P2(1) space group. Overall, both species can be described in a similar way; where all Ni(II) centers within each molecule are hexacoordinated and bound to [L(1)](-) or [L(2)](-) through the phenoxo oxygen, imine nitrogen, and piperidine/morpholine nitrogen atoms of the corresponding ligand. The remaining coordination sites are satisfied by one molecule of H(2)O and two nitrogen atoms from N(3)(-) anions. The latest act as bridges between Ni(II) ions, and eventually, only four azido groups are linked to the same number of Ni(II) centers resulting in the formation of cyclic Ni(II)(4) systems. Interestingly, compounds 1 and 2 are the two sole examples of tetranuclear clusters generated exclusively by EE azide-bridging ligands to date. All the N(azide)-Ni-N(azide) moieties are almost linear in 1 and 2 indicating trans arrangement of the azido ligand. Variable-temperature (2-300 K) magnetic susceptibilities of 1 and 2 have been measured under magnetic fields of 0.04 T (from 2 to 30 K) and 0.7 T (from 30 to 300 K), and magneto-structural correlations have been performed. Despite the presence of both ferromagnetic and antiferromagnetic interactions in both compounds, significant differences have been observed in their magnetic behaviors directly related to the arrangement of the bridging azido ligands. Hence, compound 1 has an overall moderate antiferromagnetic behavior due to the presence of an exchange pathway with an unprecedented Ni-N···N-Ni torsion angle close to 0°, meanwhile complex 2 exhibits a predominant ferromagnetic behavior, with torsion angles between 50 and 90°. Density functional theory calculations have been performed to provide more insight into the magnetic nature of this new family of Ni(II)-azido complexes and also to corroborate the fitting of the data.