Coordination asymmetry in μ-oxido divanadium complexes: Development of synthetic protocols

This brief review deals with the development of a general protocol for the synthesis of mu-oxido divanadium(V) compounds [LOVV-(mu-O)-(VO)-O-V(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 (H2L1-5) with ONO (1-3) and ONS (4.5) type donor combinations. Single crystal X-ray diffraction, ESI-MS, and multi-nuclear NMR (H-1 and V-51) spectroscopy have been used extensively for the characterization of these compounds. The V2O3 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)angstrom) are by far the largest in these compounds compared to their peers containing a V2O3 core. The molecules retain their unsymmetrical binuclear structures also in solution as established by NMR spectroscopy. The mixed-oxidation compound (ImH)[(LOVIV-)-O-4(-mu O)-(VOL5)-O-V] 7 containing two dissimilar ligands has a V2O3 core with a synangular 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 mu,-oxido bridge has been developed in which the precursor complexes [(VOL6.7)-O-IV](H2L6.7 are Salen type of ligands) are allowed to oxidize aerially in the presence of added perrhenate anion. The oxidized [(VOL6.7)-O-V](+) species hold the ReO4- 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 H-1 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)degrees) with a Re ... V separation of 3.9647(9) angstrom. 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 0.16V vs. Ag/AgCl reference and have single-electron stoichiometry, confirmed by constant potential coulometry. (C) 2011 Elsevier B.V. All rights reserved.