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 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.

Four multiring heterocyclic ligands with benzimidazole (L 1 and L 3 ) and benzothiazole nuclei (L 2 and L 4 ) 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) have been formed with the ligands L 3 and L 4 , respectively, where the pyridine nitrogen atom N1 is in the 3-position as against the coordination polymers, and [Ag(L 2 ) 2 (cis-HOOCCHdCHCOO)] n , 2b, with the ligands L 1 and L 2 , respectively, in which the N1 atom occupies the 4-position in the pyridine ring. In addition to the primary ligands (L 1 -L 4 ), the counteranions also have a dominant influence on the overall structures of these compounds. Secondary bonding interactions, namely, hydrogen bonding, π 3 3 3 π-stacking, and C-H 3 3 3 π interactions, are also proven effective in shaping the dimensionalities of the solid state structures. Thus, a zigzag chain structure of 1a mediated by a nitrate anion generates a more complicated double layer structure in 1b where trifluoroacetate has replaced nitrate as the counterion. Discrete 12-membered metallocycles in 3a-3c generate 2-D arrays of flat (3a) and undulating topologies (3b and 3c), depending upon the type of their associated anions. Metallocycles 4a and 4b have less complicated structures compared to those of 3a-3c because of the replacement of NH by S in the heterocyclic ring, thereby reducing the hydrogen-bonding potential in the primary ligand in going from L 3 to L 4 . In the solid state, the complexes show enhanced phosphorescence at 77 K with triplet lifetime in the range of 0.5-0.8 s, much shorter than those for the free ligands (2.3 -3.3 s) because of increased spin-orbit coupling introduced by the coordinated Ag þ ion. This heavy-atom effect also has a causative influence in shortening the fluorescence lifetimes of these compounds.