Interpretation of the multiple vanadium-oxygen bonds in the central VO(eta2-O2)+ group. Synthesis, structure, supramolecular interactions and DFT studies for complexes with 2,2'-bipyridine, 1,10-phenanthroline, pyrazinato(1-) and pyrazinamide ligands

Neutral peroxovanadium(v) complexes, VO(O2)(pca)(bpy)] (1), VO(O2)(pca)(phen)] (2) and VO(O2)(pic)(pcaa)(H2O)].H2O(3), were synthesized 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), pyrazinecarboxamide (pcaa), 2-pyrazinecarboxylic (Hpca) and picolinic (Hpic) acids]. Their X-ray single crystal analysis revealed a distorted pentagonal bipyramidal geometry in all complex molecules. The four "free" coordination sites of the vanadium atoms of the VO(eta2-O2)+ moieties in 1 and 2 are occupied by the donor atoms of two bidentate heteroligands. The supramolecular structures of 1 and 2 are exclusively constructed by intermolecular C--H(ar)...O hydrogen bonds dH(H...O): 2.292-2.708 A (1), and 2.260-2.720 A (2)]. In addition, the structures are stabilized by parallel off-set pi-pi interactions between the bpy rings resp. non-parallel off-set interactions between the phen rings centroid distances: 3.7000(1) A (1), 3.9781(2) and 3.6757(2) A (2)]. In the molecular structure of 3, pcaa is coordinated in an equatorial position of the bipyramid via the nitrogen atom of the pyrazine ring, while the aqua ligand is in the apical position. The disordered crystal water molecules are located in 1D channels oriented along the a axis. The intermolecular C-H(ar)...O hydrogen bonds in 3 were found within the dH(H..O) range 2.409-2.669 A. The pic ligands are off-set pi-pi stacked, with centroid distances: 3.6725(3) and 3.8323(3) A. The DFT orbital calculations and NBO analysis for the VO(eta2-O2)+ group gave evidence for a triple Vtriple bond]O bond, and showed that the observed cis arrangement of the oxo and peroxo ligands results from the direct interaction between them. Experimental and calculated UV-Vis and IR spectral data are presented.