Add time:08/03/2019         Source:sciencedirect.com

From the V2O5H2O2H2dipic–nica/pa/phen–H2O/H2OCH3CN reaction systems (H2dipic = dipicolinic acid, nica = nicotinamide, pa = picolinamide, phen = 1,10-phenanthroline), three monoperoxidovanadium(V) complexes have been synthesized and structurally characterized: (Hpa)[VO(O2)(dipic)(H2O)]⋅H2O (1), (Hnica)[VO(O2)(dipic)(H2O)] (2) and (Hphen)[VO(O2)(dipic)(H2O)]⋅H2O (3). The organic counterions in 1–3 are protonated on the aromatic nitrogen atoms, whereas the dipicolinato(2-) (=dipic) ligand adopts a tridentate chelating coordination mode in all three complexes. The NOO donor atoms of dipic occupy the three equatorial positions of the characteristic distorted pentagonal bipyramid around the central vanadium atom. The aqua ligand is located in the apical position trans to the short VO(oxido) bond. In addition to electrostatic cation–anion interactions, the supramolecular architecture of the title complexes is formed by: (i) a network of DH⋯O (D = N, O and C) hydrogen bonds, (ii) π–π interactions between offset pyridine rings of dipic (in 1 and 2) as well as between the rings of Hpa+, Hnica+ or Hphen+, (iii) anion–π interactions (in 1 and 3) between the oxygen atoms of the COO− group and rings of the dipic ligands, and (iv) the rarely recognized lone pair–π interaction between the carboxamide oxygen atoms of Hpa+ and pyridine rings in 1, as well as between the oxygen atoms from crystal water molecules and the pyridine rings of Hphen+ in 3. The anion–π and lone pair–π interactions were studied more in detail by DFT. A common feature for both these interactions was the lack of significant covalent contributions to the attraction between the respective partners. 51V NMR spectra of the acidic aqueous solutions of 1–3 showed that the structure of the complex anion is for 2 and 3 maintained even after dissolution (single shift δV = −597 ppm), whereas 1 partially decomposes with formation of monoperoxidovanadium(V) species: [VO(O2)(H2O)(pa)]+ (δV = −582 ppm) and [VO(O2)(H2O)y]+ (δV = −539 ppm).

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