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Five neutral bis(μ-phenoxido)dicopper(II) complexes, [Cu2(LMe,Me,Me)2] (1), [Cu2(LMe,Me,Et)2]·CH2Cl2 (2), [Cu2(Li-Pr,i-Pr,i-Pr)2]·2H2O (3), [Cu2(Lt-Bu,Me,i-Pr)2] (4), and [Cu2(Lt-Bu,t-Bu,i-Pr)2]·H2O (5) have been synthesized and characterized by single-crystal X-ray diffraction analyses, magnetic studies, and density functional theory (DFT) calculations, in which the ligands [H2LMe,Me,Me = N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N′,N′-dimethylethylene-1,2-diamine, H2LMe,Me,Et = N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N′,N′-dimethylethylene-1,2-diamine, H2Li-Pr,i-Pr,i-Pr = N,N-bis(2-hydroxy-3,5-diisopropylbenzyl)-N′,N′-diisopropylethylene-1,2-diamine, H2Lt-Bu,Me,i-Pr = N,N-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-N′,N′-diisopropylethylene-1,2-diamine, and H2Lt-Bu,t-Bu,i-Pr = N,N-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N′,N′-diisopropylethylene-1,2-diamine] contain the same [O,N,N,O]-donor atoms combination but differ in substituents at phenol rings and at an amino nitrogen atom. The effect of these remote substituents on the nature of exchange coupling interactions (ferromagnetic vs antiferromagnetic) between the copper(II) ions has been investigated. The average Cu–O–Cu angle, Cu–O–Cu–O torsion angle, and Cu···Cu separation in 1–5 are varied systematically by these remote ligand substituents in the range 98.6–83.3°, 26.0–46.5°, and 2.982–2.633 Å, respectively. As a result, the intramolecular spin–spin coupling in these complexes are changing gradually from a strong antiferromagnetic (J = −395 cm–1, where Ĥ = −JŜ1Ŝ2) to a moderate ferromagnetic (J = +53.2 cm–1) regime. The crossover angle at which the magnetic interaction changes from antiferromagnetic to ferromagnetic (J = 0) is determined to be ca. 87° for this series of dicopper(II) complexes. DFT calculations support the experimentally determined crossover angle and disclose various magneto–structural correlations in the series 1–5.

Bis-bidentate ligand H4LB featuring two o-amidophenol noninnocent units was used to synthesize novel binuclear complexes [(LR)MIII(•LB•)MIII(LR)](ClO4)2, M = Fe (1) and Co (2, 3), with HLR (R = CH3, Cl) being the facially coordinating tetradentate coligands. Upon the synthesis, the fully reduced amidophenolate form of the ligand (LB)4– becomes oxidized, resulting in the formation of a rare example of a biradical (•LB•)2– bridge connecting two metal ions, as supported by X-ray crystallography. The electronic structures of the complexes have been probed by Mossbauer spectroscopy, magnetic susceptibility measurements, and electron paramagnetic resonance (EPR) spectroscopy. Species 1 contains two high-spin Fe(III) ions (S = 5/2) each coupled strongly antiferromagnetically (|J| > 150 cm–1; Ĥ = −2JŜ1Ŝ2) with a semiquinone π-radical (S = 1/2) form of the bridging (•LB•)2– ligand. The effective S = 2 spins of each [Fe(III)+R●] monomeric unit are then weakly ferromagnetically coupled with J = +0.22 cm–1. Species 2...

The molecular charge transfer complexes using 9-methyl anthracene (9MA) and pyrene as donors and pyromellitic diimide (PMDI) as acceptor have been characterized by crystal structure analysis and photophysical studies. A π-π stacking interaction is observed in case of the 1:1 PMDI-Pyrene CT complex whereas the 1:1 PMDI-9MA CT complex exhibits selective stacking structure of the acceptor and donor in an interlock manner. The optimized structures from DFT calculations of PMDI-9MA and PMDI-Pyrene CT complexes in gas phase support the results. The photophysical studies of these complexes have been compared with the CT complexes of PMDA with four donors Naphthalene, Anthracene, Methyl Anthracene and Pyrene and the CT complexes of NTCDA with Methyl Anthracene and Pyrene. All the PMDA CT complexes exhibit blue shift of the charge transfer absorption and the corresponding emission band with increasing polarity of the solvents. This unusual solvent effect has been ascribed to specific interaction of the acceptor with the solvent. The red shift of the CT band in the solid state for the complexes compared to that observed in solvents support the contention of specific interaction. The binding constants for all the CT complexes are found to be of the order of 103 and the interplanar spacing of the stacked complexes is found to be around 3.3 A. Time resolved emissions of all the complexes reveal two different conformations in solution. The average lifetime of fluorescence in transparent KBr pellet is observed to be longer than that in solution. The structural properties, the spectral data and the geometry of the complexes in the excited states evaluated by TDDFT and DFT methods match well with the experimental results. These studies will help understand π-π interaction in supramolecular systems and in various biological interactions.

Oxidase activities of a μ-hydroxidodimanganese(III) system involving a series of tetradentate capping ligands H2LR1,R2 with a pair of phenolate arms have been investigated in the presence of 3,5-di-tert-butylcatechol (H2DBC) as a coligand cum-reductant. The reaction follows two distinctly different paths, decided by the substituent combinations (R1 and R2) present in the capping ligand. With the ligands H2Lt-Bu,t-Bu and H2Lt-Bu,OMe, the products obtained are semiquinonato compounds [MnIII(Lt-Bu,t-Bu)(DBSQ)]·2CH3OH (1) and [MnIII(Lt-Bu,OMe)(DBSQ)]·CH3OH (2), respectively. In the process, molecular oxygen is reduced by two electrons to generate H2O2 in the solution, as confirmed by iodometric detection. With the rest of the ligands, viz., H2LMe,Me, H2Lt-Bu,Me, H2LMe,t-Bu, and H2LCl,Cl, the products initially obtained are believed to be highly reactive quinonato compounds [MnIII(LR1,R2)(DBQ)]+, which undergo a domino reaction with the solvent methanol to generate products of composition [MnIII(LR1,R2)(BMOD)]...

A new family of [3 + 3] hexanuclear 3d-4f complexes [(μ

Heterobimetallic compounds [L'OV(V)(μ-O)M(III)L]n (n = 1, M = Mn, 1-5; n = 2, M = Fe, 6 and 7) containing a discrete unsupported V(V)-O-M(III) bridge have been synthesized through a targeted synthesis route. In the V-O-Mn-type complexes, the vanadium(V) centers have a square-pyramidal geometry, completed by a dithiocarbazate-based tridentate Schiff-base ligand (H2L'), while the manganese(III) centers have either a square-pyramidal (1 and 3) or an octahedral (2 and 5) geometry, made up of a Salen-type tetradentate ligand (H2L) as established by X-ray diffraction analysis. The V-O-Mn bridge angle in these compounds varies systematically from 155.3° to 128.1° in going from 1 to 5 while the corresponding dihedral angle between the basal planes around the metal centers changes from 86.82° to 20.92°, respectively. The V-O-Fe-type complexes (6 and 7) are tetranuclear, in which the two dinuclear V(μ-O)Fe units are connected together by apical iron(III)-aryl oxide interactions, forming a dimeric structure with a pair of Fe-O-Fe bridges. The X-ray data also confirm the V═O → M canonical form to contribute predominantly on the overall V-O-M bridge structure. The molecules in solution also retain their heterobinuclear composition, as established by electrospray ionization mass spectrometry and (51)V NMR spectroscopy. Electrochemically, these complexes are quite interesting; the manganese(III) complexes (1-5) display three successive reductions (processes I-III), each with a monoelectron stoichiometry. Process I is due to a Mn(III)/Mn(II) reduction (E1/2 ranges between -0.32 and -0.05 V), process II is a ligand-based reduction, and process III (E1/2 = ∼1.80 V) owes its origin to a V(V)O/V(IV)O reduction; all potentials are versus Ag/AgCl. The iron(III) compounds (6 and 7), on the other hand, show at least four irreversible processes, appearing at Epc = -0.20, -1.0, -1.58, and -1.68 V in compound 6 (processes IV-VII), together with a reversible process (process VIII) at E1/2 = -1.80 V (ΔEp = 80 mV). While the first two of these are due to Fe(III)/Fe(II) reductions at the two iron(III) centers of these tetranuclear cores, the reversible reduction at a more negative potential (ca. -1.80 V) is due to a V(V)O/V(IV)O-based electron transfer.

The tridentate dithiocarbazate-based Schiff base ligands H2L (S-methyl-3-((5-X-2-hydroxyphenyl)methyl)dithiocarbazate, X = H, L = L1; X = NO2, L = L2) react with [VIVO(acac)2] in acetonitrile solution under refluxing condition in the presence of an organic base or ammonium ion (added in moderate excess) as co-ligand to generate cis-dioxidovanadium(V) complexes viz. (NH4)[VVO2L2(H2O)] 1, (gunH)[VVO2L1] 2, (gunH)[VVO2L2] 3 and (cytH)[VVO2L1](cyt) 4 (gun = guanidine and cyt = cytosine). These compounds have oligomeric structures in the solid state as confirmed by X-ray crystallographic investigation, involving an alternating array of anionic [LVO2]− species and the BH+ counterion held together by Coulombic interactions and strong hydrogen bonding. These compounds are stable in methanol solution. In aprotic solvents of higher donor numbers, viz. CH3CN, DMF, or DMSO, these compounds undergo photo-induced transformation when exposed to visible light. The product obtained appears to be a μ-oxidodivanadium(IV/V) mixed-oxidation compound (BH)[LOVV-(μ-O)-VIVOL] as established by EPR, electronic spectroscopy in the NIR region, and dynamic 1H NMR spectroscopic studies.

Four tetranuclear complexes involving both homo- and hetero-metal combinations, viz. [Zn(II)(2)L(2)(μ(4)-PhCOO)(2)Zn(II)(2)(hfac)(2)] (1), [Cd(II)(2)L(2)(μ(4)-PhCOO)(2)Cd(II)(2)(hfac)(2)] (2), [Zn(II)(2)L(2)(μ(4)-PhCOO)(2)Tb(III)(2)(hfac)(4)] (3), and [Cd(II)(2)L(2)(μ(4)-PhCOO)(2)Tb(III)(2)(hfac)(4)] (4) have been prepared following a single-pot synthesis protocol using N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine (H(2)L) as a primary ligand. Both benzoate and hexafluoroacetylacetonate (hfac(-)), used here as ancillary ligands, play crucial roles in generating a tetranuclear core with high thermodynamic stability. Oxygen atoms of each carboxylate moiety bind all the four metal centers together in a rare η(2):η(2):μ(4)-bridging mode as confirmed by X-ray crystallography. In the homo-metallic complexes (1 and 2), the metal centers are all lying in a square plane, each occupying a corner, and remain connected together by oxygen bridges forming octagonal metallacrowns. These structures remain intact in solution as confirmed by (1)H NMR spectroscopy and photoluminescent studies. In the hetero-metal complexes (3 and 4), the metal centers are arrayed in alternate positions of the tetranuclear core. The Tb(III) centers have eight coordinate bi-capped trigonal prismatic coordination environments with different degrees of distortions. The all oxygen O(8) core surrounding each Tb(III) center is devoid of solvent molecules that make fluorescent emission from these molecules (3 and 4) quite interesting. The hfac(-)-based (1)(π-π*) emissions observed in 1 and 2 are quenched in 3 and 4. These sensitized Tb(III) emissions [(5)D(4)→(7)F(j); j = 6, 5, 4, and 3] are influenced by the local environments surrounding the 4f-metal center. The lifetime for the luminescence decay of 3 ((5)D(4)→(7)F(5) transition) is about 1.5 times longer than that of 4 in all the solvents studied at 298 K.

A new family of [2 × 2] tetranuclear 3d-4f heterometallic complexes have been synthesized. These are [Zn(2)Dy(2)L(2)(μ(3)-OH)(2)(μ(4)-OH)(dbm)(2)(MeOH)(2)](NO(3))·2H(2)O·MeOH (3), [Ni(2)Dy(2)L(2)(μ(3)-OH)(2)(μ(4)-OH)(dbm)(2)(MeOH)(2)](NO(3))·MeOH (4), [Ni(2)La(2)L(2)(μ(3)-OH)(2)(μ(4)-OH)(dbm)(2)(MeOH)(2)](ClO(4))·H(2)O·2MeOH (5), [Ni(2)Tb(2)L(2)(μ(3)-OH)(2)(μ(4)-OH)(dbm)(2) (MeOH)(2)](NO(3))·MeOH (6), and [Ni(2)Gd(2)L(2)(μ(3)-OH)(2)(μ(4)-OH)(dbm)(2)(MeOH)(2)](NO(3))·MeOH (7), [H(2)L = N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine and Hdbm = dibenzoylmethane] obtained through a single-pot synthesis using [Zn(HL)(dbm)] (for 3)/[Ni(HL)(dbm)]·2CH(3)OH (for 4, 5, 6, and 7) as 3d-metal ion precursors. Single-crystal X-ray diffraction analysis and electrospray ionization (ESI) mass spectroscopy have been used to establish their identities. Compounds are isostructural, in which the metal ions are all connected together by a bridging hydroxido ligand in a rare μ(4)-mode. In complexes 3-7, the metal ions are antiferromagnetically coupled. Taking a cue from the results of 3 and 5, precise estimations have been made for the antiferromagnetic Ni···Ni (J(Ni) = -50 cm(-1)), Ni···Gd (J(NiGd) = -4.65 cm(-1)), and Gd···Gd (J(Gd) = -0.02 cm(-1)) exchange interactions in 7, involving the gadolinium(III) ions. The Zn(II)(2)Dy(III)(2) compound 3 has shown the tail of an out-of-phase signal in alternating current (AC) susceptibility measurement, indicative of slow relaxation of magnetization. Interestingly, the Ni(II)(2)Dy(III)(2) compound 4 in which both the participating metal ions possess large single ion anisotropy, has failed to show up any slow magnetic relaxation.

This brief review deals with the development of a general protocol for the synthesis of μ-oxido divanadium(V) compounds [LOV V -(μ-O)-V V O(Salen)] (L = L 1 –L 5 ) ( 1 – 5 ) incorporating coordination asymmetry. One of the vanadium centers in these compounds has an octahedral environment, completed by tetradentate Salen ligand, while the other center has a square pyramidal geometry, made up of tridentate biprotic Schiff-base ligands (H 2 L 1–5 ) with ONO ( 1 – 3 ) and ONS ( 4 , 5 ) type donor combinations. Single crystal X-ray diffraction, ESI-MS, and multi-nuclear NMR ( 1 H and 51 V) spectroscopy have been used extensively for the characterization of these compounds. The V 2 O 3 core in these compounds, save 3 , has a rare type of twist-angular structure. The V(1)⋯V(2) separations (3.7921(7)–3.3084(6) A) are by far the largest in these compounds compared to their peers containing a V 2 O 3 core. The molecules retain their unsymmetrical binuclear structures also in solution as established by NMR spectroscopy. The mixed-oxidation compound (ImH)[L 4 OV IV -(μ-O)-V V OL 5 ] 7 containing two dissimilar ligands has a V 2 O 3 core with a syn -angular structure and exhibits crystallographically imposed mirror symmetry due to static disorder. In solution of donor solvents, this angular core structure changes into a linear one ( anti -linear) by accepting solvents in to the vacant coordination site of the metal centers. Finally, the protocol for the synthesis of heterobimetallic compounds with vanadium(V) and Re(VII) combination flanked by a single μ-oxido bridge has been developed in which the precursor complexes [V IV OL 6,7 ] (H 2 L 6,7 are Salen type of ligands) are allowed to oxidize aerially in the presence of added perrhenate anion. The oxidized [V V OL 6,7 ] + species hold the ReO 4 − anion in the vacant coordination site of the metal ion, trans to the terminal oxido group, thus generating the V V –O–Re VII moiety in the heterobimetallic compounds ( 9 and 10 ). Both X-ray crystallography and 1 H NMR spectroscopy have been used to establish the identities of these compounds. In compound 9 , the Re(1)–O(11)–V(1) bridge angle is barely linear (170.2(3)°) with a Re⋯V separation of 3.9647(9) A. The redox behavior of 9 and 10 are quite interesting, each undergoing two reductions both in the positive potential range at E 1/2 = 0.59 and 0.16 V vs. Ag/AgCl reference and have single-electron stoichiometry, confirmed by constant potential coulometry.

An unprecedented single crystal-to-single crystal transformation occurs when a binuclear oxovanadium(V) compound [V(V)(2)O(2)(L)(2)] 1 involving 2,6-bis(hydroxymethyl)-p-cresol (H(3)L) as a bridging ligand is exposed simultaneously to white light and aerial oxygen to generate an oligomeric compound [V(IV)(2)O(2)(L*)(2)] 2 (H(2)L* is 3-hydroxymethyl-5-methylsalicylaldehyde). Each vanadium(V) center in 1 is reduced to vanadium(IV) in 2 at the expense of a two-electron alcohol-to-aldehyde oxidation in the coordinated ligand. The additional electron being released is possibly consumed by molecular oxygen to generate hydrogen peroxide.

A family of trinuclear oximato complexes [(M(II)L)(3)(mu(3)-O)]ClO(4) (M = Ni, 1-3; Pd, 4 and 5) (HL = 2-alkylamino-3-oximobutane) involving a discrete [M(3)(mu(3)-O)](4+) core have been synthesized in moderate to high yields by a simple one-pot reaction. The products were characterized by ESI-mass and (1)H NMR spectroscopy as well as by single-crystal X-ray diffraction analysis of representative compounds viz., 1, 2, and 4. The oximato oxygen atoms from the ligands and the central mu(3)-O atom connect the metal centers, forming an inverse metallacrown topology in these complexes. In the isostructural nickel compounds (1, 2), the metal centers are situated at the vertices of an equilateral triangle with its centroid position being occupied by the mu(3)-O atom; the Ni-O-Ni angles vary in the range 119.0(2)-120.2(2) degrees . In the palladium complex 4, the geometry of the Pd(3)O core is better described as a regular trigonal pyramid with the metal centers and the mu(3)-O atom occupying the apexes; the Pd-O-Pd angles are close to 109 degrees . The coordination square planes around the individual palladium centers bend appreciably from each other (dihedral angles vary in the range 28.62-34.53 degrees ), providing more of a bowl shape compared to the overall metallacrown topology that remains virtually planar in the nickel complexes. The mu(3)-oxygen atom in 4 is displaced by 0.687 A from the center of the triangular plane with corners occupied by the Pd(II) ions. The protons of the metallacrown peripheral rings in 4 and 5 are more deshielded compared to their nickel(II) counterparts, as revealed from their (1)H NMR spectra in dichloromethane-d(2) solution.

Four multiring heterocyclic ligands with benzimidazole (L1 and L3) and benzothiazole nuclei (L2 and L4) are reported. Their silver(I) complexes involving a variety of anions (both organic and inorganic) have been prepared by the process of self-assembly and structurally characterized by single-crystal X-ray diffraction analyses. Discrete metallocyclic complexes [Ag(L3)(X)]2 (X = NO3−, 3a; cis-HOOCCH═CHCOO−, 3b; 0.5SiF62-, 3c) and [Ag(L4)(Y)]2 (Y = NO3−, 4a; CF3SO3−, 4b) have been formed with the ligands L3 and L4, respectively, where the pyridine nitrogen atom N1 is in the 3-position as against the coordination polymers [Ag(L1)(H2O)(NO3)]n, 1a, [Ag(L1)(CF3COO)]n, 1b and {2[Ag(L2)2(ClO4)].0.5.C2H5OH}n 2a, and [Ag(L2)2(cis-HOOCCH═CHCOO)]n, 2b, with the ligands L1 and L2, respectively, in which the N1 atom occupies the 4-position in the pyridine ring. In addition to the primary ligands (L1−L4), the counteranions also have a dominant influence on the overall structures of these compounds. Secondary bonding in...

Heterobimetallic complexes of a vanadium(V) and rhenium(VII) combination connected by a mu-oxido bridge [LVO(mu-O)ReO 3].H 2O [H 2L = N, N'-ethylene bis(salicylideneimine) (H 2salen) and its methoxy derivative] ( 1, 2) are reported. The compounds have been prepared by a single-pot synthesis in which the precursor [V (IV)OL] complexes are allowed to be oxidized aerially in the presence of added perrhenate. The oxidized [V (V)OL] (+) species accommodate the ReO 4 (-) anion in their vacant coordination site, trans to the terminal oxido group, providing the complexes 1 and 2. The later generates a binuclear oxovanadium(V) compound [H 2en][(TBC)VO(mu-TBC) 2OV(TBC)].5H 2O ( 3) when treated with tetrabromocatechol. Single crystal X-ray diffraction analysis and (1)H NMR spectroscopy have been used to establish their identities. In compound 2, the Re(1)-O(11)-V(1) bridge angle is barely linear [170.2(3) degrees ] with a Re...V separation of 3.9647(9) A. The redox behavior of 1 and 2 are quite interesting, each undergoing two reductions both in the positive potential range at E 1/2 = 0.59 (process I) and E 1/2 = 0.16 V (process II) versus Ag/AgCl reference (corresponding potentials are 0.59 and 0.18 V for 2). Process I has a single-electron stoichiometry involving the [VO(salen)] part of the complexes as established by combined coulometry-Electron Paramagnetic Resonance (EPR) experiments which provide an eight-line isotropic EPR pattern at room temperature ( = 1.967; = 87 x 10 (-4) cm (-1)), characteristic of an unpaired electron being coupled to a vanadium nuclear spin ( (51)V, I = 7/2). The almost linear V-O-Re bridge in 1 and 2 allows this unpaired electron to interact effectively with the neighboring Re nuclear spin, leading to familiar " two-line pattern" superhyperfine coupling ( A ( (185,187)Re) = 20.7 x 10 (-4) cm (-1)). Process II, on the other hand, is based on a Re(VII/VI) electron transfer as confirmed by differential pulse and normal pulse voltammetric experiments.

Dinuclear oxovanadium(V) compounds [LV(V)O(mu-OH)OV (V)L](PF6) [H2L = N,N'-tert-ethylene bis(salicylideneimine) (H 2Salen) and its derivatives] ( 1- 3) have been obtained by aerial oxidation of V (IV)OL precursors in THF in the presence of added NH 4PF 6. The oxidized vanadium(V) probably extracts an OH (-) ligand from the residual moisture in the solvent and is retained as an unsupported hydroxo-bridge between the metal centers of these compounds as confirmed by single-crystal X-ray diffraction analyses. The molecules of 1- 3 have centrosymmetric structures with each vanadium center having a distorted octahedral geometry. The bridging OH (-) group is located trans to the terminal VO t bond. The latter exerts strong trans labilizing influence to set the participating vanadium centers apart by about 4.1 A. These separations are by far the largest (e.g., V...V#, 4.131 A in 1) among all binuclear compounds containing an unsupported hydroxo bridge reported to date. The complexes retain their identity also in solution as established by (1)H NMR spectroscopy. Electrochemically, the behaviors of 1-3 are quite interesting as studied by cyclic voltammetry in acetonitrile, each undergoing three (except 3) nearly reversible metal-based reductions, all in the positive potential range (e.g., at E (1/2) = 0.57, 0.39, and 0.04 V versus Ag/AgCl reference for 1) as indicated by steady state voltammetry. The electrode process at 0.39 V appears to involve a single-step two-electron transfer as revealed from the normal and differential pulse voltammetric data and probably includes a combination of V(V)-V(IV) V(III)-V(IV) mixed oxidation states. Compounds 1-3 thus provide a unique example of divanadium compounds in which the metal centers are linked by an unsupported hydroxo-bridge.

A new family of pentanuclear 3d-4f heterometal complexes of general composition [Ln(III)2(M(II)L)3(μ3-O)3H](ClO4)·xH2O (1-5) [Ln = Nd, M = Zn, 1; Nd, Ni, 2; Nd, Cu, 3; Gd, Cu, 4; Tb, Cu, 5] have been synthesized in moderate yields (50-60%) following a self-assembly reaction involving the hexadentate phenol-based ligand, viz., N,N-bis(2-hydroxy-3-methoxy-5-methylbenzyl)-N('),N(')-diethylethylenediamine (H2L). Single-crystal X-ray diffraction analyses have been used to characterize these complexes. The compounds are all isostructural, having a 3-fold axis of symmetry that passes through the 4f metal centers. The [M(II)L] units in these complexes are acting as bis-bidentate metalloligands and, together with μ3-oxido bridging ligands, complete the slightly distorted monocapped square antiprismatic nine-coordination environment around the 4f metal centers. The cationic complexes also contain a H(+) ion that occupies the central position at the 3-fold axis. Magnetic properties of the copper(II) complexes (3-5) show a changeover from antiferromagnetic in 3 to ferromagnetic 3d-4f interactions in 4 and 5. For the isotropic Cu(II)-Gd(III) compound 4, the simulation of magnetic data provides very weak Cu-Gd (J1 = 0.57 cm(-1)) and Gd-Gd exchange constants (J2 = 0.14 cm(-1)). Compound 4 is the only member of this triad, showing a tail of an out-of-phase signal in the ac susceptibility measurement. A large-spin ground state (S = 17/2) and a negative value of D (-0.12 cm(-1)) result in a very small barrier (8 cm(-1)) for this compound. Among the three Nd(III)2M(II)3 (M = Zn(II), Ni(II), and Cu(II)) complexes, only the Zn(II) analogue (1) displays an NIR luminescence due to the (4)F(3/2) → (4)I(11/2) transition in Nd(III) when excited at 290 nm. The rest of the compounds do not show such Nd(III)/Tb(III)-based emission. The paramagnetic Cu(II) and Ni(II) ions quench the fluorescence in 2-5 and thereby lower the population of the triplet state.

A new series of nonoxido vanadium(IV) compounds [VL2] (L = L(1)-L(3)) (1-3) have been synthesized using dithiocarbazate-based tridentate Schiff-base ligands H2L(1)-H2L(3), containing an appended phenol ring with a tert-butyl substitution at the 2-position. The compounds are characterized by X-ray diffraction analysis (1, 3), IR, UV-vis, EPR spectroscopy, and electrochemical methods. These are nonoxido V(IV) complexes that reveal a rare distorted trigonal prismatic arrangement around the "bare" vanadium centers. Concerning the ligand isomerism, the structure of 1 and 3 can be described as intermediate between mer and sym-fac isomers. DFT methods were used to predict the geometry, g and (51)V A tensors, electronic structure, and electronic absorption spectrum of compounds 1-3. Hyperfine coupling constants measured in the EPR spectra can be reproduced satisfactorily at the level of theory PBE0/VTZ, whereas the wavelength and intensity of the absorptions in the UV-vis spectra at the level CAM-B3LYP/gen, where "gen" is a general basis set obtained using 6-31+g(d) for S and 6-31g for all the other elements. The results suggest that the electronic structure of 1-3 can be described in terms of a mixing among V-dxy, V-dxz, and V-dyz orbitals in the singly occupied molecular orbital (SOMO), which causes a significant lowering of the absolute value of the (51)V hyperfine coupling constant along the x-axis. The cyclic voltammograms of these compounds in dichloroethane solution display three one-electron processes, two in the cathodic and one in the anodic potential range. Process A (E1/2 = +1.06 V) is due to the quasi-reversible V(IV/V) oxidation while process B at E1/2 = -0.085 V is due to the quasi-reversible V(IV/III) reduction, and the third one (process C) at a more negative potential E1/2 = -1.66 V is due to a ligand centered reduction, all potentials being measured vs Ag/AgCl reference.

Two cis-dioxomolybdenum(VI) complexes [MoO2L] (L: L 1, 2 and L: L 2, 3) in a phenol-based sterically encumbered N2O2 ligand environment have been synthesized, and their crystallographic characterizations are reported. The orange crystals of 2 are monoclinic, space group P21/a with unit cell dimensions as a=16.2407(17) A, b=7.2857(8) A, c=18.400(2) A, β=98.002(9)°, Z=4, and d cal=1.486 g cm−3. The light orange crystals of 3, however, are orthorhombic, space group, Pbcn, with unit cell dimensions a=8.3110(12) A, b=12.637(3) A, c=34.673(5) A, Z=4, and d cal=1.187 g cm−3. The structures were refined by a full-matrix least-squares procedure on F 2 to a final R=0.046 (0.055 for 3) using 4944 (3677) all independent data. In both the cases, the Mo atom exists in a distorted octahedral geometry defined by a N2O4 donor set, which features a cis-Mo(–O)2 and a trans-Mo(OPh)2 arrangement. Compound 2 undergoes a quasireversible one-electron reduction at −1.3 V vs Ag/AgCl reference due to MoVIO2/MoVO2 electron transfer and thus providing a rare example of steric solution to the comproportionation–dimerization problem encountered frequently in the development of valid biomimetic models for the active sites of oxomolybdenum enzymes.

The anionic cis-dioxovanadium (V) complex species LVO 2 - of tridentate biprotic dithiocarbazate-based ligands H2L (S-methyl-3-((5-R-2-hydroxyphenyl)methyl)dithiocabazate, R = H, L = L1; and R = Br, L = L2) can bind alkali metal ions. The products [LVO2M(H2O)n] (M = Na+, L = L1,1; L = L2, 2 and M = K+, L = L1,3) have extended chain structures in the solid state, stabilized by strong hydrogen bonding and Coulombic interactions as revealed from X-ray crystallography. The LVO 2 - moieties here behave like analogues of carboxylate groups and display interesting variations in their binding patterns. It appears thatl is a single stranded helicate with LVO 2 - units forming the strands which surround the labile sodium ions occupying positions on the axis. The compounds are stable in water and methanol as solvents, while in aprotic solvents of higher donor strengths, viz. CH3CN, DMF and DMSO, they undergo photo-induced reduction when exposed to visible light, yielding green solutions from their initial yellow colour. The putative product is a mixed-oxidation (μ-oxo)divanadium (IV/V) species as revealed from EPR, electronic spectroscopy, dynamic1H NMR, and redox studies.

S-Methyl 3-((2-hydroxyphenyl)methyl)dithiocarbazate (H(2)L(1)) and its bromo derivative (H(2)L(2)), which are traditionally biprotic tridentate (ONS) ligands, behave in an unprecedented manner when allowed to react with [VO(acac)(2)] under an oxidative environment in acetonitrile-water medium containing a catalytic amount of alkali metal ion. The products obtained are oxovanadium(V) compounds [VOL(L(cyclic))] (L = L(1), 1a, and L(2), 1b) that contain one molecule of ligand which undergoes metal-induced cyclization to form a thiadiazole ring. Compound 1a crystallizes in the triclinic space group P(-)1 with a = 9.1830(9) A, b = 9.4165(12) A, c = 12.700(2) A, alpha = 100.988(8)(o), beta = 100.195(7)(o), gamma = 78.774(8)(o), V = 1046.3(2) A(3), and Z = 2. With cobalt(III), however, the products [CoL(HL)].H(2)O (L = L(1), 2a, and L(2), 2b) have hydrogen-bonded dimeric structures with each ligand virtually carrying 1.5 units of negative charge as confirmed by X-ray crystal structure analysis of 2a. It also crystallizes in triclinic space group P(-)1 with a = 12.0842(8) A, b = 13.5251(9) A, c = 14.1960(10) A, alpha = 78.122(6)(o), beta = 73.888(6)(o), gamma = 78.255(6)(o), V = 2154.7(3) A(3), and Z = 4. In solution, 2a is a symmetric molecule as indicated by (1)H NMR, involving a characteristic hydrogen-bonded O-H-O broad feature in the downfield (at 14.5 ppm) connecting both monoprotonated (LH(-)) and deprotonated (L(2-)) forms of the ligand--a situation somewhat analogous to the classic H-F-H case as observed in bifluoride ion.

A new family of 3d-4f heterometal 2 × 2 complexes [Co(II)2(L)2(PhCOO)2Ln(III)2(hfac)4] (1-5) (Ln = Gd (compound 1), Tb (compound 2), Dy (compound 3), Ho (compound 4), and La (compound 5)) have been synthesized in moderate yields (48-63%) following a single-pot protocol using stoichiometric amounts (1:1 mol ratio) of [Co(II)(H2L)(PhCOO)2] (H2L = N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine) as a metalloligand and [Ln(III)(hfac)3(H2O)2] (Hhfac = hexafluoroacetylacetone) as a lanthanide precursor compound. Also reported with this series is the Zn-Dy analog [Zn(II)2(L)2(PhCOO)2Dy(III)2(hfac)4] 6 to help us in understanding the magnetic properties of these compounds. The compounds 1-6 are isostructural. Both hexafluoroacetylacetonate and benzoate play crucial roles in these structures as coligands in generating a tetranuclear core of high thermodynamic stability through a self-assembly process. The metal centers are arranged alternately at the four corners of this rhombic core, and the carboxylato oxygen atoms of each benzoate moiety bind all of the four metal centers of this core in a rare μ4-η(2):η(2) bridging mode as confirmed by X-ray crystallography. The magnetic susceptibility and magnetization data confirm a paramagnetic behavior, and no remnant magnetization exists in any of these compounds at vanishing magnetic field. The metal centers are coupled in an antiferromagnetic manner in these compounds. The [Co(II)2Dy(III)2] compound exhibits a slow magnetic relaxation below 6 K, as proven by the AC susceptibility measurements; the activation energy reads U/kB = 8.8 K (τ0 = 2.0 × 10(-7) s) at BDC = 0, and U/kB = 7.8 K (τ0 = 3.9 × 10(-7) s) at BDC = 0.1 T. The [Zn(II)2Dy(III)2] compound also behaves as a single-molecule magnet with U/kB = 47.9 K and τ0 = 2.75 × 10(-7) s.

Cobalt-(II) and -(III) complexes of pentadentate N4S ligands based on methyl 2-aminocyclopent-1-ene-1-carbodithioate with appended pyrazolyl groups (3,5-Me2C3HN2CH2)2NCH(R)CH2NHC5H6C(S)SCH3 (R = H, Hmmecd; CH3, Hmmpcd) have been prepared and characterised by IR, 1H NMR and electronic spectroscopy. Two of these compounds have also structurally been characterised by X-ray single crystal diffraction analyses. Cobalt(II) in [Co(mmpcd)]ClO4, 1, shows a five-coordinate, trigonal bipyramidal geometry while its cobalt(III) counterpart, [Co(mmpcd)Cl]ClO4, 2, reveals a six-coordinated distorted octahedral structure by the inclusion of a chloride ligand in its equatorial plane. In dmf or acetonitrile solution, 1 can bind dioxygen reversibly as indicated by EPR spectra recorded at cryogenic temperatures. Metal–dioxygen binding in 1 appears to be weak, possibly due to its trigonal bipyramidal structure and the presence of a sulfur donor in the ligand framework. Electronic spectra of the cobalt(III) complexes show two LMCT bands in the near UV region, tentatively assigned to S→CoIII charge transfer.

In methanol, the complexation reaction of palladium(II) with pentadentate N4S ligand having flexible pyrazolyl arms, appears to proceed through two parallel paths, one of which is thermodynamically controlled, generating product 1, while the other is kinetically driven to obtain product 2 in comparable yields. Structural anomeric effect plays a crucial role in the formation of 2.

Reaction of [V(IV)OL(1)(Im)] (H(2)L(1) = S-methyl-3-((2-hydroxyphenyl)methyl)dithiocarbazate) with [V(V)OL(OCH(3))] allows isolation of (ImH)[L(1)OV-(mgr;-O)-VOL] complexes 2 (H(2)L = H(2)L(2) = S-methyl-3-((5-bromo-2-hydroxyphenyl)methyl)dithiocarbazate) and 3 (H(2)L = H(2)L(1)), one of which (2) has ligand asymmetry not previously known in this type of complex. In the solid state, (ImH)[L(1)OV-(mgr;-O)-VOL(2)] (2) provides an example of a divanadium(IV,V) compound with a syn angular [V(2)O(3)](3+) core structure that exhibits crystallographically imposed mirror symmetry due to static disorder. Crystals of 2 are orthorhombic, space group Pnma, with a = 10.740(2) Å, b = 18.912(4) Å, c = 17.163(4) Å, and Z = 4. In toluene at room temperature, both 2 and 3 have 8-line EPR spectra, characteristic of trapped-valence structure. When acetonitrile is added to these solutions, the spectra reveal 15-line features with asymmetric distortions that smooth out with the lowering of temperature. This probably has its origin in a solvent-dependent equilibrium involving two magnetically inequivalent structural forms of the divanadium(IV,V) compound, with syn angular and anti linear structures of the [V(2)O(3)](3+) core. Variable temperatures (298-220 K) (51)V NMR spectroscopic studies in solution also support this view. In acetonitrile, both 2 and 3 exhibit an intervalence transfer band in the near-IR region at ca. 970 nm (epsilon, 1600 and 1480 M(-)(1) cm(-)(1) for 2 and 3, respectively) and they undergo one-electron reversible oxidation at ca. 0.40 V (vs SCE) due to the V(IV)V(V)/V(V)V(V) couple.

Pentadentate N4S ligands based on methyl 2-aminocyclopent-1-ene-1-dithiocarboxylate with flexible pyrazolyl arms (Me2pzCH2)2NC2H3RNHC5H6CSSCH3 (R = H, Hmmecd and R = CH3, Hmmpcd) undergo a nickel(II)-induced alcoholysis reaction through the activation of a saturated C−N bond linkage. The products obtained are square-planar complexes 1−4 containing a modified ligand structure possessing an N3S donor set and a pendant arm that holds the alkoxy group provided by the solvent. [Ni(N3S)-CH2OMe]ClO4 1 crystallizes in the triclinic space group P1 with a = 10.4886(5) A, b = 10.706(1) A, c = 11.487(1) A, α = 108.784(4)°, β = 108.887(6)°, γ = 95.139(6)°, V = 1128.0(4) A3, and Z = 2; while [Ni(N3S)‘-CH2 OPrn]ClO4 4 has the monoclinic space group P21/n with a = 8.875(2) A, b = 18.629(2) A, c = 15.399(2) A, β = 91.37(2)°, V = 2545(1) A3, and Z = 4 per unit cell. Complexes 1−4 with acyclic ligand environments have interesting electrochemical behavior in acetonitrile, involving a reversible Ni(II)/Ni(I) reduction, E1/2 ...

In the presence of some hydroxylated compounds as substrates, complexations of nickel(II) with the pentadentate N4S ligand methyl 2-{2-bis[(3,5-dimethylpyrazol-1-ylmethyl)amino]propylamino}cyclopent-1-ene-1-carbodithioate (Hmmpcd) having a pair of flexible pyrazolyl arms proceeded through unusual reaction paths involving C–N bond cleavage process. For example, the product 1 obtained in methanol as substrate is a square planar compound with a modified ligand structure that includes a methoxy group provided by the solvent. With an acidic substrate viz. phenol the product is also a square planar compound 2 with one of the pendant arms of Hmmpcd being replaced by a hydrogen atom. On the other hand, in the presence of a basic substrate viz. ethanolamine the ligand undergoes more extensive breakdown to generate a binuclear complex 3 with a bridging pyrazolyl group. These complexes 1–3 were characterised by X-ray crystallography and 1H NMR spectroscopy. Compound 1 displayed a reversible one-electron NiII → NiI reduction at E½ ≈ –1.0 V vs. Ag–AgCl reference as established by cyclic voltammetry and combined coulometry/EPR experiments. Such reversibility of electron transfer is unusual for a square-planar nickel(II) compound in an acyclic ligand environment. Stabilisation of the reduced nickel(I) species by methoxy group co-ordination provides a probable explanation for this reversible electron transfer.

Isovalent mu-oxo divanadium(V) compounds [L1VO(mu-O)VO(salen)] (1) and its bromo derivative [L2VO(mu-O)VO(salen)].CH3CN (2) (both H2L1 and H2L2 are tridentate dithiocarbazate-based ONS ligands) with ligands providing donor set and coordination number asymmetry in tandem have been synthesized for the first time; confirmations in favor of these unsymmetrical molecular structures have come from single-crystal X-ray diffraction analysis, as well as from NMR (both 1H and 51V) spectroscopy.

The mononuclear oxovanadium complexes [VVOL(OR)] (R: Me, 1a; Et, 1b; Pri, 1c) and [VIVOLB] (B: Im, 2a; BzIm, 2b) of the tridentate ONS donor ligand S-methyl 3-((2-hydroxyphenyl)methyl)dithiocarbazate (H2L) are reported. The novel mixed-valence (μ-oxo)divanadium(IV,V) complex (BzImH)[(LVO)2O] (3) was synthesized using 1a and 2b as precursors. All of the complexes were characterized by various physical techniques, including 1H NMR, EPR, and cyclic voltammetry. The X-ray structures of 1a and 3 were determined. Crystals of 1a are triclinic, of space group P1, with a = 11.045(1) A, b = 13.429(4) A, c = 9.611(1) A, α = 100.00(1)°, β = 114.33(1)°, γ = 81.00(1)°, and Z = 4. The asymmetric unit of 1a contains two independent molecules with minor conformational differences. Crystals of 3 are monoclinic, of space group P21/n, with a = 10.10(1) A, b = 20.594(9) A, c = 14.679(8) A, β = 95.28(6)°, and Z = 4. 3 is a bent molecule, with a V−O−V bridge angle of 144.0(6)°. It has a trapped-valence structure in the solid...

In order to study the reactivity pattern of Mo−Ot bonds associated with anionic sulfur ligands, precursor complexes MoO2L·D (H2L = S-methyl 3-(2-hydroxyphenyl)methylenedithiocarbazate; D = CH3OH (1), H2O (2)) were synthesized. Complex 2 crystallizes in the orthorhombic space group P212121, with a = 6.079(1) A, b = 11.638(2) A, c = 17.325(2) A, V = 1225.7(4) A3, and Z = 4. In its reaction with PhNHOH, 1 forms a seven-coordinate oxaziridine compound [MoO(η2-ONPh)L·CH3OH] (3) by oxo-rearrangement (elimination−substitution) without a change in the molybdenum oxidation state. The crystal data for 3 are a = 9.573(3) A, b = 9.859(2) A, c = 10.604(3) A, α = 95.90(2)°, β = 95.81(2)°, γ = 112.12(2)°, V = 911.5(4) A3, Z = 2, and triclinic space group P1. In contrast to that of the precursor compound 1 (Mo−Ot = 1.700(4) A), the terminal Mo−Ot distance in 3 (1.668(2) A) is typical of Mo−O bonds of order 3, which drives the formation of an apparently unstable three-membered metallacycle via spectator oxo stabilization...

Reaction of [VO(acac)2] with S-methyl 3-(2-hydroxyphenyl) methylenedithiocarbazate (H2L1) and its nitro- (H2L2) and bromo derivatives (H2L3) as tridentate heterodonor (ONS)2− ligands offers a basic VO(ONS) core. In the presence of bidentate (N,N′) donor ancillar ligands, various mono- and dinuclear oxovanadium(IV) complexes [VOL(2,2′-bipy)] (1–3), [VOL(Phen)] (4, 5) and [LOV(μ-4,4′-bipy)VOL] (4, 5) and [LOV(μ-4,4′-bipy)VOL] (6–8) were obtained. The compounds have been characterized by EPR, electronic and FAB mass spectroscopic studies. The X-ray crystal structure of a representative compound [VOL2(2,2′-bipy)] (2) is reported and shows the presence of the tridentate ONS ligand occupying the meridional sites leading to distorted octahedral geometry. The nitrogen donor, occupying an apical position has a trans-labilizing influence, resulting in elongation of the VO terminal bond. Magnetic susceptibility and EPR spectral studies indicate non-interacting nature (vbJ vb ≈ 0) of the vanadium(IV) unpaired electrons in the dinuclear complexes (6–8).

The photoluminescence characteristics of the [Cd(bipy)3][PF6]2 complex are reported. A moderately large quantum yield (φ) of 1.07 × 10−2 is exhibited in acetonitrile solution at 298 K. Crystallography shows the dication to have a distorted octahedral geometry and the crystal structure to be stabilized by CH···π and CH···F interactions. Copyright © 2005 John Wiley & Sons, Ltd.

A new family of trinuclear homo- and heterometal complexes with a triangular [V(V)O-MO-V(V)O] (M = V(IV), 1 and 2; Re(V), 3] all-oxido-metal core have been synthesized following a single-pot protocol using compartmental Schiff-base ligands, N,N'-bis(3-hydroxysalicylidene)-diiminoalkanes/arene (H4L(1)-H4L(3)). The upper compartment of these ligands with N2O2 donor combination (Salen-type) contains either a V(IV) or a Re(V) center, while the lower compartment with O4 donor set accommodates two V(V) centers, stabilized by a terminal and a couple of bridging methoxido ligands. The compounds have been characterized by single-crystal X-ray diffraction analyses, which reveal octahedral geometry for all three metal centers in 1-3. Compound 1 crystallizes in a monoclinic space group P2(1)/c, while both 2 and 3 have more symmetric structures with orthorhombic space group Pnma that renders the vanadium(V) centers in these compounds exactly identical. In DMF solution, compound 1 displays an 8-line EPR at room temperature with and values of 1.972 and 86.61 × 10(-4) cm(-1), respectively. High-resolution X-ray photoelectron spectrum (XPS) of this compound shows a couple of bands at 515.14 and 522.14 eV due to vanadium 2p(3/2) and 2p(1/2) electrons in the oxidation states +5 and +4, respectively. All of these, together with bond valence sum (BVS) calculation, confirm the trapped-valence nature of mixed-oxidation in compounds 1 and 2. Electrochemically, compound 1 undergoes two one-electron oxidations at E(1/2) = 0.52 and 0.83 V vs Ag/AgCl reference. While the former is due to a metal-based V(IV/V) oxidation, the latter one at higher potential is most likely due to a ligand-based process involving one of the catecholate centers. A larger cavity size in the upper compartment of the ligand H4L(3) is spacious enough to accommodate Re(V) with larger size to generate a rare type of all-oxido heterotrimetallic compound (3) as established by X-ray crystallography.

9 pages; International audience; Homo- and heterotrinuclear complexes [LNi{M(Ln)}2](ClO4)2 H2O involving NiIIMII 2 nonlinear cores (M = Ni, Cu, and Pd) (1-6) have been synthesized by a single-pot reaction when the oximato metal complexes [MLn(H2O)]ClO4 (HLn are tridentate oxime ligands), prepared in situ in methanol are allowed to react with the precursor nickel(II) complex [LNi(H2O)2] (H2L = N,N0-dimethyl-N,N0-bis(2-hydroxy-3,5- dimethylbenzyl)ethylenediamine). Single crystal X-ray diffraction analysis, and ESI-MS spectroscopy have been used to establish their identities which involve an octahedral Ni(II) site flunked by two metal-oximate moieties, each in a square planar environment. The electronic and molecular structures of these compounds are interesting due to a synergistic bonding mechanism operative through the deprotonated oxime and the phenolate oxygen atoms via the metal centers........

Four tetranuclear complexes involving both homo- and hetero-metal combinations, viz. [Zn(II)(2)L(2)(μ(4)-PhCOO)(2)Zn(II)(2)(hfac)(2)] (1), [Cd(II)(2)L(2)(μ(4)-PhCOO)(2)Cd(II)(2)(hfac)(2)] (2), [Zn(II)(2)L(2)(μ(4)-PhCOO)(2)Tb(III)(2)(hfac)(4)] (3), and [Cd(II)(2)L(2)(μ(4)-PhCOO)(2)Tb(III)(2)(hfac)(4)] (4) have been prepared following a single-pot synthesis protocol using N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine (H(2)L) as a primary ligand. Both benzoate and hexafluoroacetylacetonate (hfac(-)), used here as ancillary ligands, play crucial roles in generating a tetranuclear core with high thermodynamic stability. Oxygen atoms of each carboxylate moiety bind all the four metal centers together in a rare η(2):η(2):μ(4)-bridging mode as confirmed by X-ray crystallography. In the homo-metallic complexes (1 and 2), the metal centers are all lying in a square plane, each occupying a corner, and remain connected together by oxygen bridges forming octagonal metallacrowns. These structures remain intact in solution as confirmed by (1)H NMR spectroscopy and photoluminescent studies. In the hetero-metal complexes (3 and 4), the metal centers are arrayed in alternate positions of the tetranuclear core. The Tb(III) centers have eight coordinate bi-capped trigonal prismatic coordination environments with different degrees of distortions. The all oxygen O(8) core surrounding each Tb(III) center is devoid of solvent molecules that make fluorescent emission from these molecules (3 and 4) quite interesting. The hfac(-)-based (1)(π-π*) emissions observed in 1 and 2 are quenched in 3 and 4. These sensitized Tb(III) emissions [(5)D(4)→(7)F(j); j = 6, 5, 4, and 3] are influenced by the local environments surrounding the 4f-metal center. The lifetime for the luminescence decay of 3 ((5)D(4)→(7)F(5) transition) is about 1.5 times longer than that of 4 in all the solvents studied at 298 K.

Heterobimetallic compounds [L(1)OV(V)═O→Fe(metsalophen)(H(2)O)] (1) and [L(2)OV(V)═O→Fe(metsalophen)(H(2)O)]CH(3)CN (2), where H(2)L(1) and H(2)L(2) are tridentate dithiocarbazate-based Schiff base ligands, containing a discrete V(V)-μ-O-Fe(III) angular core have been synthesized for the first time through a targeted synthesis route: confirmation in favor of such a heterobimetallic core structure has come from single-crystal X-ray diffraction analysis and electrospray ionization mass spectrometry.

The protonated form [H(2)(L)](CF(3)SO(3))(2) (1) of a new redox-active bis-bidentate nitrogenous heterocyclic ligand, viz., 3,3'-dipyridin-2-yl[1,1']bi[imidazo[1,5-a]pyridinyl] (L), and its zinc(II) and cadmium(II) complexes (2 and 3) have been synthesized and characterized by single-crystal X-ray diffraction analysis. In the solid state, both 2 and 3 have triple-stranded helical structures involving ligands that experience twisting and bending to the extent needed by the stereoelectronic demand of the central metal ion. The metal centers in the zinc(II) complex [Zn(2)(L)(3)](ClO(4))(4) (2) are equivalent, each having a distorted octahedral geometry, flattened along the C(3) axis with a Zn1···Zn1# separation of 4.8655(13) Å. The cadmium complex [Cd(2)(L)(3)(H(2)O)](ClO(4))(4) (3), on the other hand, has a rare type of helical structure, showing coordination asymmetry around the metal centers with a drastically reduced Cd1···Cd2 separation of 4.070 Å. The coordination environment around Cd1 is a distorted pentagonal bipyramid involving a N(6)O donor set with the oxygen atom coming from a coordinated water, leaving the remaining metal center Cd2 with a distorted octahedral geometry. The structures of 2 and 3 also involve anion-π- and CH-π-type noncovalent interactions that play dominant roles in shaping the extended structures of these molecules in the solid state. In solution, these compounds exhibit strong fluxional behavior, making the individual ligand strands indistinguishable from one another, as revealed from their (1)H NMR spectra, which also provide indications about these molecules retaining their helical structures in solution. Electrochemically, these compounds are quite interesting, undergoing ligand-based oxidations in two successive one-electron steps at E(1/2) of ca. 0.65 and 0.90 V versus a Ag/AgCl (3 M NaCl) reference. These molecules are all efficient emitters in the red and blue regions because of ligand-based π*-π fluorescent emissions, tuned appropriately by the attached Lewis acid centers.

Valence tautomeric compounds involving nondixolene-type ligands are rare. The triple-helicate copper(II) complex [Cu(II)(2)(L)(3)](ClO(4))(4)·3CH(3)CN (1) containing a redox-active N-heterocyclic ligand (L) has been prepared and displays VT equilibrium in solution, as established by electronic spectroscopy, electron paramagnetic resonance spectroscopy, and cyclic and differential pulse voltammetry carried out at variable temperatures. The process involves intramolecular transfer of an electron from one of the L ligands to a copper(II) center, leading to the oxidation of L to an L(•+) radical with concomitant reduction of the Cu(II) center to Cu(I), as shown by the equilibrium [Cu(II)Cu(I)L(•+)L(2)](4+) ⇄ [Cu(II)(2)L(3)](4+).

A square-pyramidal geometry is found about the copper(II) centre in the title complex, [CuCl(C6H4NO2)(C7H7NO2)] or [Cu(2-pic)(2-picMe)Cl], a product of CuII-induced methan­olysis of 1,2-di(2-pyridyl)-1,2-di­hydroxy­ethyl­ene. Each of the 2-pic and 2-picMe ligands chelates the copper centre via N and and one of their O-donor atoms. In the case of 2-picMe, the donor atoms span basal and axial positions via the N and O atoms, respectively. Weak secondary Cu⋯Cl interactions link mol­ecules of the complex in the crystal into centrosymmetric dimeric aggregates.

10 pages; International audience; A trinuclear oximato complex, [(NiHL1)3( μ3-O)]ClO4 (1), with inverse metallacrown 9-MC-3 topology has been synthesized using a Schiff-base ligand (H2L1) formed by condensation of ethanolamine (Hea) and diacetylmonoxime (Hdamo). The diamagnetic compound has been characterized by electrospray ionization mass spectrometry as well as by single-crystal X-ray diffraction analysis. In the solid state, the alcoholic OH group in this molecule stays away from coordination. Surprisingly in a similar chemical reaction, when intact Hea and Hdamo have been used as ligands instead of their Schiff-base forms, the product obtained is a 12-MC-4-type metallacrown, (Et3NH)[Ni4(damo)4(Hea)2(ea)2](ClO4)3 (2), with a larger cavity size needed to accommodate a pair of hydrogen-bonded (O-H3 3 3 O)- anions. Unlike in 1, the alcoholic OH groups in 2 take part in metal coordination. Compound 2 on being refluxed with lithium hydroxide inmethanol is converted to 1 in almost quantitative yield. This appears to be a novel reaction type, leading to contraction of a metallacrown ring size. A family of 12-MC-4 Ni4 metallacrowns in inverse topology, viz., [Ni4(damo)4(H2dea)2(Hdea)2](ClO4)2 3 2H2O (3), [Ni4(dpko)4(Hea)2(ea)2](ClO4)2 3 4H2O (4), and [Ni4(mpko)4(Hmea)2(mea)2](ClO4)2 (5), have been synthesized following a methodology similar to that adopted for 2, using different combinations of free oximes [viz., dipyridylketonoxime (Hdpko) and methylpyridylketonoxime (Hmpko)] and amin alcohols [viz., diethanolamine (H2dea), and N-methylethanolamine (Hmea)]. Crystal and molecular structures of 3-5 have been reported, each involving either a quasi (in 3) or a perfect (in 4 and 5) square plane (S4 symmetry) with four octahedral Ni centers occupying the corners, and serve as a backbone of puckered metallacrown rings that accommodate a pair of hydrogenbonded (O-H3 3 3 O)- anions. Antiferromagnetic interactions within the [Ni4] core [J/kB ≈ -20 to -27 K based on the following spin Hamiltonian: H=-2J(S1 3 S2þ S2 3 S3þ S3 3 S4þ S4 3 S1)] lead to an ST=0 ground state for these complexes. Introduction Metallacrowns are a type of metallacycle that can be considered as inorganic analogues of crown ethers because of the similarities they have both in structure and in function.1 These compounds usually have a combination of a transition-metal ion and a heteroatom (usually nitrogen)

Manganese(II/III) complexes of a phenol-based tetradentate ligand L(2-) [H(2)L = N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)-ethylenediamine], namely, [Mn(4)(L)(2)(PhCOO)(6)] (1), [Mn(3)(L)(2)(CH(3)CH(2)COO)(2)(OMe)(2)].H(2)O (2), and [Mn(L){(CH(3))(3)CCOO}(CH(3)OH)].CH(3)OH (3), have been synthesized. The basicity and steric congestion provided by the carboxylate moiety used as an ancillary ligand have profound influence on tuning the nuclearity of these compounds. Results of X-ray crystallography, electronic spectroscopy, and variable-temperature (1.8-300 K) magnetic measurements have been used to characterize these compounds. Complex 1 has a very interesting centrosymmetric structure that involves two crystallographically equivalent binuclear [Mn(II)-Mn(III)] units, connected together by a pair of syn-anti bridging benzoates to generate a "dimer of dimers" structural motif. Compound 2 with propionate as the ancillary ligand, on the other hand, has a nearly linear Mn(III)-Mn(II)-Mn(III) core with antiferromagnetically coupled (J = -0.13 cm(-1)) metal centers. Compound 1 has an S(T) = 9 spin ground state with ferromagneticlly coupled metal centers (J(wb)= 2.8(1) and J(bb) = 0.09(2) cm(-1)) that failed to function as a single molecule magnet due to the presence of low-lying excited states with smaller spin values and a weak magnetic anisotropy. The electron paramagnetic resonance spectrum of 1 in the frozen solution (12 K) displays two signals in the g = 2 and g = 4 regions, each split into six lines due to (55)Mn (I = 5/2) superhyperfine couplings. The use of bulky pivalate as a replacement for benzoate provides enough steric bulk to generate a mononuclear species [Mn(L){(CH(3))(3)CCOO}(CH(3)OH)].CH(3)OH (3). The lone manganese(III) center in this compound has an octahedral geometry, completed by the tetradentate ligand L(2-) together with an axially coordinated methanol molecule and a monodentate pivalate. The latter two are connected by a hydrogen bond, thus stabilizing the monodentate carboxylate moiety. Redox behaviors (CV) of 1 and 3 are grossly similar, each undergoing a quasi-reversible reduction process at E(1/2) = -0.03 and -0.11 V, respectively, versus a Ag/AgCl reference.

3 pages; International audience; In basic media, the coordinated semiquinone radical in the spincoupled [(bipy)ClVIVO(TCSQ)] 1 (HTCSQ ) tetrachlorosemiquinone) undergoes nucleophilic ipso substitution (OH- for Cl-) to generate the chloranilate anion (CA2-) that bridges the vanadium(IV) centers, forming a binuclear compound [(bipy)ClVIVO(CA)OVIVCl(bipy)] 2.

Eight-co-ordinated mixed-ligand complexes [MoIV(S2CNR2)n(acda)4–n]1a–1d and [MoV(S2CNR2)n(acda)4–n]Br 2a–2d(n= 2 or 3) containing predominantly σ-donor S2CNR2(R = Et or Pr) and π-acceptor acda (2-aminocyclopent-1-ene-1-carbodithioate) ligands have been synthesised and characterised by UV/VIS, magnetic, ESR and electrochemical studies. For the lower-energy ligand-to-metal charge-transfer bands of compounds 1a–1d the observed spectral intensity showed an almost linear increment with the number of co-ordinated acda ligands. The molybdenum(V) compounds 2a–2d are ESR active, and their frozen solutions (140 K) give rise to axial spectra with grossly identical features (gII≈ 1.983, AII≈ 52.5 × 10–4 cm–1: g⊥≈ 1.984, A⊥≈ 21.5 × 10–4 cm–1) consistent with the electron being localised in a metal-centred orbital of nearly constant composition for the entire series of compounds. Cyclic voltammetry indicates the existence of three molybdenum oxidation states [MonS8] with n=III, IV and V. The E½ values (vs. saturated calomel electrode) are –0.40 and –0.18 V for the MoIII→ MoIV and MoIV→ MoV couples respectively and independent of whether the starting compound is a molybdenum(IV), 1a, 1c, or a molybdenum(V) species 2a–2d. Of the two types of ligand systems present in [Mo(S2CNR2)n(acda)4–n]m(m= 0 or +1), acda has been found to have a profound influence in controlling the redox and charge-transfer properties.

A general protocol for the synthesis of micro-oxo divanadium(V) compounds [LOV(micro-O)VO(Salen)] (1-5) incorporating coordination asymmetry has been developed for the first time. One of the vanadium centers in these compounds has an octahedral environment, completed by tetradentate Salen ligand, while the remaining center has square pyramidal geometry, made up of tridentate biprotic Schiff-base ligands (L2-) with ONO (1-3) and ONS (4, 5) type donor combinations. Single crystal X-ray diffraction analysis, ESI-MS, and NMR (both 1H and 51V) spectroscopy have been used extensively to establish their identities. The V(1)-O(6)-V(2) bridge angle in these compounds, save 3, lie in a narrow range (166.20(9)-157.79(16) degrees) with the V2O3 core having a rare type of twist-angular structure, somewhat intermediate between the regular anti-linear and the syn-angular modes. For 3, however, the bridge angle is sufficiently smaller 117.92(8) degrees that it forces the V2O3 core to adopt an anti-angular geometry. The V(1)...V(2) separations in these molecules (3.7921(7)-3.3084(6) A) are by far the largest compared to their peers containing a V2O3 core. The molecules retain the binuclear structures also in solution as confirmed by NMR spectroscopy. Their redox behaviors appear quite interesting, each undergoing a one-electron reduction in the positive potential range (E1/2, 0.42-0.45 V vs Ag/AgCl) to generate a trapped-valence mixed-oxidation products [LVVO-(micro-O)-OVIV(salen)]1-, confirmed by combined coulometry-EPR experiments. The bent V-O-V bridge in these molecules probably prevents the symmetry-constrained vanadium d xy orbitals, containing the unpaired electron, to overlap effectively via the ppi orbitals of the bridging oxygen atom, thus accounting for the trapped-valence situation in this case.

The tetra- and binuclear heterometallic complexes of nickel(II)-vanadium(IV/V) combinations involving a phenol-based primary ligand, viz., N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine (H2L1), are reported in this work. Carboxylates and beta-diketonates have been used as ancillary ligands to obtain the tetranuclear complexes [Ni(II)(2)V(V)(2)(RCOO)(2)(L(1))(2)O(4)] (R = Ph, 1; R = Me(3)C, 2) and the binuclear types [(beta-diket)Ni(II)L(1)V(IV)O(beta-diket)] (3 and 4), respectively. X-ray crystallography shows that the tetranuclear complexes are constructed about an unprecedented heterometallic eight-membered Ni(2)V(2)O(4) core in which the (L(1))(2)- ligands are bound to the Ni center in a N(2)O(2) mode and simultaneously bridge a V atom via the phenoxide O atoms. The cis-N(2)O(4) coordination geometry for Ni is completed by an O atom derived from the bridging carboxylate ligand and an oxo O atom. The latter two atoms, along with a terminal oxide group, complete the O5 square-pyramidal coordination geometry for V. Each of the dinuclear compounds, [(acac)Ni(II)L(1)V(IV)O(acac)] (3) and [(dbm)Ni(II)L(1)V(IV)O(dbm)] (4) [Hdbm = dibenzoylmethane], also features a tetradentate (L(1))(2)- ligand, Ni in an octahedral cis-N(2)O(4) coordination geometry, and V in an O(5) square-pyramidal geometry. In 3 and 4, the bridges between the Ni and V atoms are provided by the (L(1))(2)- ligand. The Ni...V separations in the structures lie in the narrow range of 2.9222(4) A (3) to 2.9637(5) A (4). The paramagnetic Ni centers (S = 1) in 1 and 2 are widely separated (Ni...Ni separations are 5.423 and 5.403 A) by the double V(V)O(4) bridge that leads to weak antiferromagnetic interactions (J = -3.6 and -3.9 cm-1) and thus an ST = 0 ground state for these systems. In 3 and 4, the interactions between paramagnetic centers (Ni(II) and V(IV)) are also antiferromagnetic (J = -8.9 and -10.0 cm-1), leading to an S(T) = 1/2 ground state. Compound 4 undergoes two one-electron redox processes at E(1/2) = +0.66 and -1.34 V vs Ag/AgCl reference due to a V(IV/V) oxidation and a Ni(II)/I reduction, respectively, as indicated by cyclic and differential pulse voltammetry.

Copper(II) complexes (1-3) of a sterically constrained phenol-based tetradentate N(2)O(2) ligand 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)piperazine (H(2)L) have been reported. The associated anions of the copper(II) ion precursors have profound influence on the stoichiometry of the products. Thus, with perchlorate ion, the product is a binuclear compound [Cu(2)L(2)] (1), while with coordinating anions viz. Cl(-) and N(3)(-), the products [Cu(3)L(2)Cl(2)(H(2)O)].1/2H(2)L (2) and [Cu(3)L(2)(N(3))(2)(CH(3)OH)].4H(2)O (3) have triangulo trinuclear composition. The syntheses, X-ray structures, and spectroscopic and magnetic properties of these complexes are described. Compound 1 has a noncentrosymmetric structure with a rectangular Cu(2)(OPh)(2) core. It appears to be a rare example of a phenolato-bridged Cu(II) dimer exhibiting ferromagnetic interactions (J = 0.93 cm(-)(1)), a behavior in agreement with the theoretical predictions but seldom observed experimentally. In compounds 2 and 3, the copper centers are triangularly disposed, and the molecules have a shape much like that of a butterfly. The terminal copper centers Cu(1) and Cu(2) in 2 and 3 have distorted square pyramidal geometry, connected to each other by a bridging chloro- (in 2) or azido ligand (in 3) in "end to end" fashion. The central copper center (Cu(3) in 2 and Cu in 3) in both the compounds has distorted square planar geometry. The separations between the metal centers, viz. Cu(1)...Cu(2), Cu(2)...Cu(3), and Cu(3)...Cu(1), are 4.826, 3.214, and 3.244 A, respectively, in 2. The corresponding distances in 3 are 5.590, 3.178, and 3.485 A, respectively. The overall magnetic behaviors in 2 and 3 are consistent with antiferromagnetic interactions between the spin centers. In 3, the exchange couplings between the terminal and central copper centers J(Cu(1))(-)(Cu) and J(Cu(2))(-)(Cu) appear to be equal (-234 cm(-)(1)), resulting in an S = (1)/(2) ground state at temperatures near or below 77 K.

Heterobinuclear copper(II)-zinc(II) complexes and their homobinuclear dicopper(II) counterparts (1-4) of two asymmetric ligands (H2L1 and H2L2), based on 2-aminocyclopent-1-ene-1-dithiocarboxylate, are reported. The ligands are capable of providing both donor set and coordination number asymmetry in tandem. Metal centers in these complexes are connected by a micro-alkoxo and a bridging pyrazolate moiety, as confirmed by X-ray structure analyses of 1, 3, and 4. The Cu(1) site in the dicopper complex (1) is square planar and so are the copper sites in the Cu-Zn complexes 3 and 4. The pentacoordinated Zn sites in the latter complexes have distorted TBP geometry (tau = 0.74), while the corresponding Cu site in 1 has a highly distorted square pyramidal structure (tau = 0.54). The Cu...Zn separations in 3 and 4 are 3.3782 and 3.3403 angstroms, respectively, while the Cu...Cu distance in 1 is 3.3687 angstroms. The dicopper complexes are EPR silent at 77 K, in which the copper(II) centers are coupled by strong antiferromagnetic coupling (J = ca. -290 cm(-1)) as confirmed by variable-temperature (4-300 K) magnetic measurements. These compounds (1 and 2) undergo two one-electron reductions and a single step two-electron oxidation at ca. -0.26, -1.40, and 1.0 V vs Ag/AgCl reference, respectively, as indicated by cyclic and differential pulse voltammetry done at subambient temperatures. EPR spectra of 3 and 4 display axial anisotropy at 77 K with the gperpendicular region being split into multiple lines due to N-superhyperfine coupling (AN = 15.3 x 10(-4) cm(-1)). The observed trend in the spin-Hamiltonian parameters, gparallelgperpendicular2.04 and |Aperpendicular||Aparallel| approximately (120-150) x 10(-4) cm(-1), indicates a d(x2-y2)-based ground state with tetragonal site symmetry for the Cu(II) center in these molecules.

Three new flexidentate 5-substituted salicylaldimino Schiff base ligands (L1-OH-L3-OH) based on 1-(2-aminoethyl)piperazine (X=H, L1-OH; X=NO2, L2-OH; and X=Br, L3-OH) and their nickel(II) complexes (1a, 1b, 2, and 3) have been reported. The piperazinyl arm of these ligands can in principle have both boat and chair conformations that allow the ligands to bind the Ni(II) center in an ambidentate manner, forming square-planar and/or octahedral complexes. The nature of substitution in the salicylaldehyde aromatic ring and the type of associated anion in the complexes have profound influences on the coordination geometry of the isolated products. With the parent ligand L1-OH, the product obtained is either a planar red compound [Ni(L1-O)]+, isolated as tetraphenylborate salt (1a), or an octahedral green compound [Ni(L1-NH)(H2O)3](2+), isolated with sulfate anion (1b); both have been crystallographically characterized. In aqueous solution, both these planar (S=0) and octahedral (S=1) forms are in equilibrium that has been followed in the temperature range 298-338 K by 1H NMR technique using the protocol of Evans's method. The large exothermicity of the equilibrium process [Ni(L1-O)]+ + 3H2O + H+ [Ni(L1-NH)(H2O)3](2+) (DeltaH degrees=-46 +/- 0.2 kJ mol(-1) and DeltaS degrees=-133 +/- 5 J K(-1) mol(-1)) reflects formation of three new Ni-OH2 bonds in going from planar to the octahedral species. With the 5-nitro derivative ligand L2-OH, the sole product is an octahedral compound 2, isolated as a sulfate salt while with the bromo derivative ligand L3-OH, the exclusive product is a planar molecule 3 with associated tetraphenylborate anion. Both 2 and 3 have been structurally characterized by X-ray diffraction analysis.