Construction of Polynuclear Lanthanide (Ln = Dy(III), Tb(III), and Nd(III)) Cage Complexes Using Pyridine-Pyrazole-Based Ligands: Versatile Molecular Topologies and SMM Behavior.

Employment of two different pyridyl-pyrazolyl-based ligands afforded three octanuclear lanthanide(III) (Ln = Dy, Tb) cage compounds and one hexanuclear neodymium(III) coordination cage, exhibiting versatile molecular architectures including a butterfly core. Relatively less common semirigid pyridyl-pyrazolyl-based asymmetric ligand systems show an interesting trend of forming polynuclear lanthanide cage complexes with different coordination environments around the metal centers. It is noteworthy here that construction of lanthanide complex itself is a challenging task in a ligand system as soft N-donor rich as pyridyl-pyrazol. We report herein some lanthanide complexes using ligand containing only one or two O-donors compare to five N-coordinating sites. The resultant multinuclear lanthanide complexes show interesting magnetic and spectroscopic features originating from different spatial arrangements of the metal ions. Alternating current (ac) susceptibility measurements of the two dysprosium complexes display frequency- and temperature-dependent out-of-phase signals in zero and 0.5 T direct current field, a typical characteristic feature of single-molecule magnet (SMM) behavior, indicating different energy reversal barriers due to different molecular topologies. Another aspect of this work is the occurrence of the not-so-common SMM behavior of the terbium complex, further confirmed by ac susceptibility measurement.

A star-shaped heteronuclear Cr(III)Mn(II)3 species and its precise electronic and magnetic structure: spin frustration studied by X-ray spectroscopic, magnetic, and theoretical methods.

Molecular magnets incorporate transition-metal ions with organic groups providing a bridge to mediate magnetic exchange interactions between the ions. Among them are star-shaped molecules in which antiferromagnetic couplings between the central and peripheral atoms are predominantly present. Those configurations lead to an appreciable spin moment in the nonfrustrated ground state. In spite of its topologically simple magnetic structure, the [Cr(III)Mn(II)(3) (PyA)(6)Cl(3)] (CrMn(3)) molecule, in which PyA represents the monoanion of syn-pyridine-2-aldoxime, exhibits nontrivial magnetic properties, which emerge from the combined action of single-ion anisotropy and frustration. In the present work, we elucidate the underlying electronic and magnetic properties of the heteronuclear, spin-frustrated CrMn(3) molecule by applying X-ray magnetic circular dichroism (XMCD), as well as magnetization measurements in high magnetic fields, density functional theory, and ligand-field multiplet calculations. Quantum-model calculations based on a Heisenberg Hamiltonian augmented with local anisotropic terms enable us not only to improve the accuracy of the exchange interactions but also to determine the dominant local anisotropies. A discussion of the various spin Hamiltonian parameters not only leads to a validation of our element selective transition metal L edge XMCD spin moments at a magnetic field of 5 T and a temperature of 5 K but also allows us to monitor an interesting effect of anisotropy and frustration of the manganese and chromium ions.

The trinity of polyoxometalates: connecting {M12} Keggin and {M18} Dawson clusters to triangles.

Two topologically equivalent triangular POM aggregates based upon the Keggin () and Dawson () cluster species, with distinctive double {W-O-M} intra-molecular linkages involving Mn(III) and Co(III) ions are presented; both these compounds demonstrate electrocatalytic activity.

Synthesis, structural and magnetochemical studies of iron phosphonate cages based on {Fe3O}7+ core.

We report the synthesis, structures, and magnetic properties of twelve iron(III) phosphonate cages: [Fe(4)(mu(3)-O)Cl(PhCO(2))(3)(PhPO(3))(3)(py)(5)] 1, [Fe(4)(mu(3)-O)((t)BuCO(2))(4)(C(10)H(17)PO(3))(3)(py)(4)] 2 (C(10)H(17)PO(3)H(2) = camphylphosphonic acid), [Fe(7)(mu(3)-O)(2)(PhPO(3))(4)(MeCO(2))(9)(py)(6)] 3, [Fe(7)(mu(3)-O)(2)(PhPO(3))(4)(PhCO(2))(9)(py)(6)] 4, [Fe(7)(mu(3)-O)(2)((t)BuPO(3))(4)((t)BuCO(2))(8)(py)(8)](NO(3)) 5, [Fe(7)(mu(3)-O)(2)(PhPO(3))(4)(MeCO(2))(8)(py)(8)] 6, [Fe(9)(mu(3)-O)(2)(mu(2)-OH)(PhPO(3))(6)((t)BuCO(2))(10)(MeCN)(H(2)O)(5)] 7, [Fe(9)(mu(3)-O)(2)(mu(2)-OH)(C(10)H(17)PO(3))(6)(PhCO(2))(10)(H(2)O)(6)] 8, [Fe(6)(mu(3)-O)(2)(O(2))((t)BuCO(2))(8)(PhPO(3))(2)(H(2)O)(2)] 9, [Fe(6)(mu(3)-O)(2)(O(2))((t)BuCO(2))(8)(C(10)H(17)PO(3))(2)(H(2)O)(2)] 10, [Fe(6)(mu(3)-O)(2)(O(2))((t)BuCO(2))(8)((t)BuPO(3))(2)(py)(2)] 11, and [Fe(14)(mu(3)-O)(4)(O(2))(2)(PhPO(3))(8)((t)BuCO(2))(12)(H(2)O)(12)](NO(3))(2) 12. The results have allowed us to compare the magnetic exchange found with magneto-structural correlations found previously for iron-oxo cages.

Example of two novel thiocyanato bridged copper (II) complexes derived from substituted thiosemicarbazone ligand: structural elucidation, DNA/albumin binding, biological profile analysis, and molecular docking study.

Two novel copper (II) substituted thiosemicarbazone Schiff base complexes [Cu(L1)(µ-SCN)]n(NO3)2 (1) and [Cu2(µ-SCN)(SCN)(L2)2](NO3) (2) have been synthesized by condensing substituted thiosemicarbazides like 4-methyl-3-thiosemicarbazide or 4-ethyl-3-thiosemicarbazide with 2-acetylpyridine. Both the metal complexes 1 and 2 are characterized using different spectroscopic techniques like IR, UV-Vis, ESR spectroscopy followed by elemental analysis, cyclic voltammetric measurement and single crystal X-ray structure analysis. X-ray crystal structure analysis reveal that complex 1 is polymeric while complex 2 is dimeric in nature. The coordination geometry around Cu(II) are square pyramidal in which thiosemicarbazone Schiff base ligand coordinate to the central Cu(II) atom in tridentate fashion. The prominent interaction patterns of 1 and 2 with CT-DNA were examined by employing electronic absorption and emission spectral titrations, cyclic voltammetry and viscosity measurements. All the results show that CT-DNA binds with both copper (II) complexes 1 and 2. Furthermore, protein binding ability in vitro of complexes 1 and 2 with both BSA and HSA were carried out using multispectroscopic techniques and a static quenching pattern was observed in both cases. Molecular docking study was employed to ascertain the exact mechanism of action of 1 and 2 with DNA and protein molecules (BSA and HSA). In vitro cytotoxicity activity of complexes 1 and 2 toward AGS and A549 was evaluated using MTT assay which demonstrates that both complexes 1 and 2 have superior prospectus to act as anticancer agents. Communicated by Ramaswamy H. Sarma.

A one-pot synthesis of a paramagnetic high-nuclearity nickel(II) cluster: an octadecanuclear Ni(II)16Na(I)2 metal aggregate.

An unusual Ni(II)(16)Na(I)(2) cluster which features formate as a bridging clamp between two octanuclear nickel cages is reported; preliminary magnetic studies exhibit paramagnetic low-lying states resulting from dominating antiferromagnetic interactions between the nickel(II) centers.

Hexanuclear copper(II) cage with {Cu3O···H···OCu3} core supported by a dicompartmental oxime ligand with m-xylyl spacer: synthesis, molecular structure and magnetic studies.

A new dicompartmental dioxime ligand (H(2)L) with m-xylyl spacer between the donor sites has been synthesised by Schiff-base condensation of α,α'-diamino-m-xylene and diacetyl monooxime. The ligand reacts with copper(ii) salts giving rise to hexanuclear tricationic copper(II) cage complexes [Cu(II)(6)(µ(3)-O···H···O-µ(3))L(3)(H(2)O)(6)]X(3) (X = BF(4), 1a; X = ClO(4), 1b). The complexes have been characterised by different analytical and spectroscopic techniques and confirmed the hexanuclear structure even in solution. Single crystal X-ray diffraction studies of both the complexes revealed a very similar core structure with three dicompartmental ligands supporting two triangular Cu(3)O cores that share a proton, located on their common threefold axis and involved in a strong hydrogen bond interaction (O···O distance of 2.517(2) Å). Two Cu(3)O units do not superimpose but are staggered and disposed with the formation of a helicate structure. However both the enantiomers are present in the centrosymmetric space group. The facing Cu(3)-planes in 1a are separated at a distance of 3.476 Å. The temperature dependence of the magnetic behaviour of the hexanuclear complex 1a clearly indicates an overall antiferromagnetic exchange interaction between the spin carriers in the cage having two Cu(3)O subunits and leaves a single unpaired electron in each triangular unit. The unpaired electrons in the two Cu(3)O units interact antiferromagnetically through hydrogen bonding giving rise to an overall singlet-spin ground state.

A nonanuclear copper(II) cluster incorporating endogenous alkoxo- and exogenous hydroxo bridges containing [Cu3(micro3-OH)] cores.

A remarkable nonanuclear copper(II) complex, , with the composition [Cu9(L)4(micro3-OH)4(MeOH)2](ClO4)2, in which L is a pentadentate trianionic Schiff-base ligand with N2O3 donor atoms, has been characterized by analytical, structural and magnetochemical methods. The structure consists of four dinuclear [Cu2L]+ units linked covalently to a central copper atom by four micro3-OH, thus yielding the nonacopper core. Variable-temperature (2-290 K) magnetic susceptibility data of , have been analyzed by a "three-J" exchange coupling model (H=-2JSi.Sj), with the simulated coupling constants J'1=-189.1 cm(-1), J'2=-22.7 cm(-1), J'3=-45.7 cm(-1). A rationale for the different strength of antiferromagnetic interactions based on a comparison with those reported in the literature is forwarded.

Synthesis, structural characterisation and magnetic studies of polymetallic iron phosphonate cages.

Four new polymetallic iron(III) phosphonate cages have been made and structurally characterised. These are an octanuclear cage [Fe(8)O(3)(OH)(2)(O(2)C(t)Bu)(11)(PhCH(2)PO(3))(3)(py)(3)], a decanuclear cage [Fe(10)O(2)(OH)(8)(O(2)C(t)Bu)(10)(PhCH(2)PO(3))(4)(pip)(2)], a heterometallic cage [Fe(6)Li(5)(mu(3)-O)(2)((t)BuPO(3))(6)(O(2)C(t)Bu)(8)(MeOH)(2)(Py)(4)] and a tridecanuclear cage [Et(3)NH](2)[Fe(13)(mu(3)-O)(3)(mu(2)-OH)(7)((t)BuPO(3))(7)(Me(3)CCO(2))(14)(H(2)O)] (pip = piperidine, py = pyridine). Magnetic studies of the first three compounds show anti-ferromagnetic exchange between the iron(III) centers leading to diamagnetic ground states for the homometallic cages. For the heterometallic cage, the six Fe(III) centers are arranged in two triangles, and each triangle has an S = 1/2 spin ground state.

cis-Tach based pentadecadentate ligands as building blocks in the synthesis of Fe III and Pd II coordination clusters.

A dodecanuclear Fe(III) complex, consisting of two {Fe(6)} units coupled across a single hydroxy-bridge and a cage-like Pd(II) trimer were synthesised using the highly branched, pentadecadentate chelating ligand cis,cis-1,3,5-cyclohexanetriamine-N,N,N',N',N'',N''-hexaacetic acid (H(6)hact).

An oximate-based hexanuclear mixed-valence Mn(III)4Mn(II)2 edge-sharing bitetrahedral core with an St = 5 spin ground state.

The synthesis, structures and magnetic properties of two hexanuclear Mn6 clusters are reported: Mn6(mu4-O)2(dapdo)2(dapdoH)4(mu2-OH)2](ClO4)(2).6MeCN (1.6MeCN) and [Mn6(mu4-O)2(dapdo)2(dapdoH)4(mu2-OCH3)2](ClO4)(2).2Et2O (2.2Et2O) [dapdo2- is the dianion of 2,6-diacetylpyridine dioxime and dapdoH- is the monoanion of the aforesaid dioxime ligand]. Both complexes are mixed-valent with two Mn(II) and four Mn(III) atoms disposed in an edge-sharing bitetrahedral core. Both complexes 1 and 2 display the same [Mn(III)4Mn(II)2(mu4-O)2(mu2-OR)2]10+ core in which R = H for 1 and R = Me for 2. The [Mn(III)4Mn(II)2] core is rather uncommon compared to the reported [Mn(III)2Mn(II)4] core in the literature. DC magnetic susceptibility measurements on 1 and 2 reveal the presence of competing exchange interactions resulting in an St = 5 ground spin state. The magnetic behavior of the compounds indicates antiferromagnetic coupling between the manganese(III) centers, whereas the coupling between the manganese(III) and manganese(II) is weakly antiferromagnetic or ferromagnetic depending on the bridging environments. Finally the interaction between the manganese(II) centers from the two fused tetrahedra is weakly ferromagnetic in nature stabilizing St = 5 ground spin state in compounds 1 and 2.

Star-shaped molecule of MnII4O6 core with an St=10 high-spin state. A theoretical and experimental study with XPS, XMCD, and other magnetic methods.

We report a comprehensive study of the electronic and magnetic properties of a star-shaped molecule comprising a MnII4O6 core. One feature of this compound is weak magnetic coupling constants compared to other similar polyoxo compounds. This leads to complicated low-lying magnetic states in which the ground state is not well separated from the upper-lying states, yielding a high-spin molecule with a giant magnetic moment of up to 20 microB/formula unit. We apply X-ray diffraction and magnetometry as well as other X-ray spectroscopic techniques, namely, X-ray photoelectron spectroscopy, X-ray magnetic circular dichroism, and X-ray emission spectroscopy. We compare our experimental results with ab initio electronic band structure calculations as well as the localized electronic structure around the Mn2+ ions with charge-transfer multiplet calculations.

Rational synthesis of a heterotetranuclear CrIII2CuII2 species using a metaloximate as a building block. A magnetostructural study.

The reaction of [Cu(DapdoH(2))2](2+) ions with [Cr(III)L(MeOH)2Br](2+) ions (L = 1,4,7-trimethyl-1,4,7-triazacyclononane; DapdoH(2) = 2,6-diacetylpyridine dioxime) produced in situ in methanol, yielded, in the presence of triethylamine, the complex [L(2)Cr(III)(2)(Dapdo)2(OH)2Cu(II)(2)Br(2)](ClO(4))2. It has been characterized on the basis of elemental analysis, IR spectroscopy and variable-temperature (2-298 K) magnetic susceptibility measurements. The molecular structure established by X-ray diffraction consists of a [Cr(III)(2)(micro-NO)4(micro-OH)2Cu(II)(2)](4+)-core, which can be considered as two edge-sharing triangular CrCu(2)-units. The variable-field variable-temperature magnetic measurements revealed a ground state of S(t) = 2 with uncommon antiferromagnetic exchange interactions between the chromium(III) and copper(II) centers: J(A) = -79 +/- 2 cm(-1), J(B) = -17 +/- 1.7 cm(-1), where J(A) represents the interactions through a combination of oximate (>N-O-) and a hydroxo-bridging, while J(B) is the exchange through only a two-atom oximate (>N-O-) group.

Tridentate facial ligation of tris(pyridine-2-aldoximato)nickel(II) and tris(imidazole-2-aldoximato)nickel(II) To generate NiIIFeIIINiII, MnIIINiII, NiIINiII, and ZnIINiII and the electrooxidized MnIVNiII, NiIINiIII, and ZnIINiIII species: a magnetostructural, electrochemical, and EPR spectroscopic study.

Eight hetero- and homometal complexes 1-6, containing the metal centers Ni(II)Fe(III)Ni(II) (1), Mn(III)Ni(II) (2), Ni(II)Ni(II) (3a-c and 4), Zn(II)Ni(II) (5), and Zn(II)Zn(II) (6), are described. The tridentate ligation property of the metal complexes tris(pyridine-2-aldoximato)nickel(II) and tris(1-methylimidazole-2-aldoximato)nickel(II) with three facially disposed pendent oxime O atoms has been utilized to generate the said complexes. Complex 1 contains metal centers in a linear arrangement, as is revealed by X-ray diffraction. Complexes were characterized by various physical methods including cyclic voltammetry (CV), variable-temperature (2-290 K) magnetic susceptibility, electron paramagnetic resonance (EPR) measurements, and X-ray diffraction methods. Binuclear complexes 2-6 are isostructural in the sense that they all contain a metal ion in a distorted octahedral environment MN(3)O(3) and a second six-coordinated Ni(II) ion in a trigonally distorted octahedral NiN(6) geometry. Complexes 1-4 display antiferromagnetic exchange coupling of the neighboring metal centers. The order of the strength of exchange coupling in the isostructural Ni(II)2 complexes, 3a-c, and 4, demonstrates the effects of the remote substituents on the spin coupling. The electrochemical measurements CV and square wave voltammograms (SQW) reveal two reversible metal-centered oxidations, which have been assigned to the Ni center ligated to the oxime N atoms, unless a Mn ion is present. Complex 2, Mn(III)Ni(II), exhibits a reduction of Mn(III) to Mn(II) and two subsequent oxidations of Mn(III) and Ni(II) to the corresponding higher states. These assignments of the redox processes have been complemented by the X-band EPR measurements. That the electrooxidized species [3a]+, [3b]+, [3c]+, and [4]+ contain the localized mixed-valent NiIINiIII system resulting from the spin coupling, a spin quartet ground state, S(t) = 3/2, has been confirmed by the X-band EPR measurements.

A spin-frustrated star-shaped heterotetranuclear CrIIIMnII3 species and its magnetic and HF-EPR measurements.

A tetranuclear complex [Cr(III)Mn(II)(3)(PyA)(6)Cl(3)] 1 containing pyridine-2-aldoximato monoanion, PyA, has been structurally and magnetochemically characterized. The compound is a rare example of a tetranuclear star-shaped metal topology containing pyridine-2-aldoximato ligands. Static magnetic studies have demonstrated very weak antiferromagnetic exchange interactions between the paramagnetic centers (S(Cr) = 3/2 and S(Mn) = 5/2) resulting in closely spaced low-lying levels, which undergo splitting and crossing. Preliminary high-field EPR measurements (20 < nu < 388 GHz) indicate the presence of zero-field splitting D of the order of 0.7 cm(-1).

Ferromagnetic vs. antiferromagnetic coupling in bis(mu2-1,1-azido)dinickel(II) complexes with syn- and anti-conformations of the end-on azide bridges.

Reaction of a 1 : 1 : 1 molar ratio of NiCl(2), NaN(3) and H(2)L, a tetradentate ligand N-(2-pyridyl)methyl)-N,N-bis(2'-hydroxy-3',5'-dimethylbenzyl)amine in methanol in presence of Et(3)N results in a turquoise precipitate, which affords deep green crystals of [Ni(2)(HL)(2)(N(3))(2)]*1.5CH(2)Cl(2) (1) and [Ni(2)(HL)(2)(N(3))(2)]*H(2)O (2) upon crystallization from CH(2)Cl(2)-MeOH or THF-MeOH, respectively. Both complexes reveal distorted octahedral NiN(4)O(2) coordination environments around the Ni(ii) centers with bis(micro-1,1-azido) bridging ligands. Complex 1 displays an unprecedented small Ni-N-Ni bridge angle of 90.4 degrees , whereas 2 contains the said angle of av. 98.7 degrees lying in the usual range observed for other comparable structures. The variable-temperature magnetic susceptibility together with the variable-field, variable-temperature (VTVH) magnetization measurements discern different ground states of S(t) = 0 for 1 and S(t) = 2 for 2. The magnetic behaviours of these compounds are discussed in the context of the known bis(end-on azido) bridging dinickel(II) complexes.

Deliberate synthesis for magnetostructural study of linear tetranuclear complexes BIIIMnIIMnIIBIII, MnIIIMnIIMnIIMnIII, MnIVMnIIMnIIMnIV, FeIIIMnIIMnIIFeIII, and CrIIIMnIIMnIICrIII. Influence of terminal ions on the exchange coupling.

As part of an ongoing effort to deliberate synthesis of polynuclear heterometal complexes, we are exploring synthetic routes to high-nuclearity complexes using "metal oximates" as building blocks. Series of tetranuclear linear complex ions of the general types M(A)M(B)M(B)M(A), where M(A) is a trivalent or tetravalent metal ion and M(B) is a divalent metal ion, e.g., Mn(II), have been synthesized by using the dimetal(II) anionic cores, [(M(II)(B))(2)(dfmp)(3)](5)(-) as a bridging ligand for the terminal LM(A) fragments where H(3)dfmp is a dinucleating phenol-oxime ligand, 2,6-diformyl-4-methylphenol oxime, and L denotes a facially coordinating cyclic tridentate amine, 1,4,7-trimethyl-1,4,7-triazacyclononane. The following combinations are reported here, B(III)Mn(II)Mn(II)B(III) (1), Mn(III)Mn(II)Mn(II)Mn(III) (2), Mn(IV)Mn(II)Mn(II)Mn(IV) (3), Fe(III)Mn(II)Mn(II)Fe(III) (4), and Cr(III)Mn(II)Mn(II)Cr(III) (5). The compounds have been characterized spectroscopically and by magnetic susceptibility measurements in the temperature range 2.0-290 K at different field strengths. Complexes 1-4 have also been structurally characterized by single-crystal X-ray diffraction techniques at 100 K. The magnetic behaviors of the compounds indicate weak antiferromagnetic coupling between the manganese(II) centers in the central trisphenoxo-bridged dimanganese(II) core, whereas the coupling between the terminal M(A) and its neighboring Mn(II) center varies and is weak ferromagnetic or antiferromagnetic. The relative interaction intensity in such a series of complexes is discussed. Finally, a profound influence of the charge on the terminal metal ions on the strength of the exchange coupling in the central dimanganese(II) core has been observed and discussed in relation to the covalency of the metal-ligand bonding.

Photoinduced intramolecular proton transfer of phenol-containing ligands and their zinc complexes.

N,N'-Bis(salicylidene)hydrazine (L(I)), a bis-2-hydroxybenzene-type ligand H2L, its tert-butyl derivative (L(II)), and the corresponding Zn2+ complexes of the type Zn2(LH)2L (Zn-I and Zn-II) were synthesized. The molecular structure of Zn-II was determined by X-ray crystallography at -170 degrees C. The photoreactions of the four compounds in solution were studied by time-resolved UV-vis spectroscopy using nanosecond laser pulses. A weak but strongly Stokes shifted fluorescence signal of the ligands L(I) or L(II) is suggested to be due to excited-state intramolecular proton transfer (ESIPT) from the phenolic hydroxy group to the nitrogen of the methine bond in analogy to the fast enol --> keto tautomerization of other 2-hydroxybenzenes. A transient with the maximum at 480 nm, bleaching at 370 nm, and a lifetime of 0.01-0.3 ms is attributed to the trans-keto tautomer, formed via internal conversion. The decay occurs via trans --> cis isomerization and proton back-transfer to the enol form. Quenching by water indicates a proton-catalyzed reaction. To account for similar fluorescence and transient properties in the cases of the Zn2+ complexes, a photoinduced tautomerism at one of the two free phenolic hydroxy groups is proposed. The rapid ESIPT followed by a relatively slow relaxation process is reversible.