Tetranuclear homo- (Zn(II)4 and Cd(II)4) and hetero-metal (Zn(II)2Tb(III)2 and Cd(II)2Tb(III)2) complexes with a pair of carboxylate ligands in a rare η2:η2:µ4-bridging mode: syntheses, structures and emission properties.

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.

Single crystal-to-single crystal irreversible transformation from a discrete vanadium(V)-alcoholate to an aldehydic-vanadium(IV) oligomer.

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.

Tri- and tetranuclear nickel(II) inverse metallacrown complexes involving oximato oxygen linkers: role of the guest anion (oxo versus alkoxo) in controlling the size of the ring topology.

A trinuclear oximato complex, [(NiHL(1))(3)(µ(3)-O)]ClO(4) (1), with inverse metallacrown 9-MC-3 topology has been synthesized using a Schiff-base ligand (H(2)L(1)) 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, (Et(3)NH)[Ni(4)(damo)(4)(Hea)(2)(ea)(2)](ClO(4))(3) (2), with a larger cavity size needed to accommodate a pair of hydrogen-bonded (O-H···O)(-) anions. Unlike in 1, the alcoholic OH groups in 2 take part in metal coordination. Compound 2 on being refluxed with lithium hydroxide in methanol 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 Ni(4) metallacrowns in inverse topology, viz., [Ni(4)(damo)(4)(H(2)dea)(2)(Hdea)(2)](ClO(4))(2)·2H(2)O (3), [Ni(4)(dpko)(4)(Hea)(2)(ea)(2)](ClO(4))(2)·4H(2)O (4), and [Ni(4)(mpko)(4)(Hmea)(2)(mea)(2)](ClO(4))(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 amino alcohols [viz., diethanolamine (H(2)dea), 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 (S(4) symmetry) with four octahedral Ni centers occupying the corners, and serve as a backbone of puckered metallacrown rings that accommodate a pair of hydrogen-bonded (O-H···O)(-) anions. Antiferromagnetic interactions within the [Ni(4)] core [J/k(B) ≈ -20 to -27 K based on the following spin Hamiltonian: H = -2J(S(1)·S(2) + S(2)·S(3) + S(3)·S(4) + S(4)·S(1))] lead to an S(T) = 0 ground state for these complexes.

Building metallacrown topology around a discrete [M(3)(mu(3)-O)] (M = Ni(II) and Pd(II)) core using oximato oxygen linkers: synthesis, structures, and spectroscopic characterization of a new family of compounds with an inverse-9-MC-3 motif.

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.

Coordination asymmetry in divanadium(V) compounds containing a V2O3 core: synthesis, characterization, and redox 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.

Reporting a new class of divanadium(V) compounds connected by an unsupported hydroxo bridge.

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.