Divanadium(V) and Trapped Valence Linear Tetravanadium(IV,V,V,IV) Complexes

Abstract

Keywords: Vanadium / Mixed-valent compounds / EPR spectroscopy / Redox chemistry / Localized valenceIn an acetonitrile/water mixture, reactions of the N, NЈ-bis(diacetyl)hydrazine (H 2 diah), bis(acetylacetonato)oxidovanadium(IV) [VO(acac) 2 ] and monodentate N-coordinating heterocycles (hc) in a 1:2:2 mol ratio provide yellow divanadium(V) complexes of formula [(hc)O 2 V(µ-diah)VO 2 (hc)] (1, hc = imidazole; 2, hc = pyrazole; 3, hc = 3,5-dimethyl pyrazole). On the other hand, in the same solvent mixture reactions of the same reagents in a 1:4:2 mol ratio produce green linear tetravana-(4, hc = imidazole; 5, hc = pyrazole; 6, hc = 3,5-dimethyl pyrazole). The complexes 1-6 have been characterized by elemental analysis, magnetic susceptibility, and various spectroscopic and electrochemical measurements. The X-ray crystal structures of 1, 3 and 6 have been determined. In all three structures, the diazine ligand diah 2-is in trans configuration. Metal-centred bond parameters are consistent with the localized electronic structure IntroductionThe chemistry of multinuclear oxido-bridged vanadium complexes has been a subject of continuous interest over the past two decades because of the biochemical importance of these species and their potential use as therapeutic agents and oxidation catalysts.[1] We have been working on oxidovanadium(IV/V) complexes with acid hydrazide based Schiff bases for some time.[2] Recently we reported a coordinatively unsymmetrical complex (variation in the ligands bonded to the two metal centres) of the mixed valence syn-{OV(µ-O)VO} 3+ core that is of trapped valence character in the solid state as well as in solution at ambient temperature.[2h] Before this, there was a solitary report on a complex with the same core, where the two tridentate ligands attached to the two metal centres differ only in the substituent on the aromatic fragment, [3l] and there were very few examples of the {OV(µ-O)VO} 3+ with a valence localized electronic structure in the fluid state. [3g,3i,3l] Compared to dinuclear species, tetranuclear complexes composed of two {OV(µ-O)VO} 3+ units are extremely rare. To the best of our knowledge only three structurally characterized complexes [a]

Structural and electron paramagnetic resonance (EPR) characterization of novel vanadium(V/IV) complexes with hydroquinonate-iminodiacetate ligands exhibiting “noninnocent” activity

“…7. Each of the anisotropic spectra are consisted from 16 lines, indicating that the complex is valence-localized, which is in accordance with the 0.21 Å difference of the bridging oxygen-vanadium bond lengths in the V 2 O 3 3+ core [44]. The best fitting of the experimental spectra with the simulation gave at pH = 2.0 g ⊥ = 1.978, g = 1.936, A ⊥ = -69.7 × 10 -4 cm -1 and A = -178.8 × 10 -4 cm -1 and for the spectra at pH = 8.9 g ⊥ = 1.976, g = 1.939, A ⊥ = -57.7 × 10 -4 cm -1 , and A = -168.7 × 10 -4 cm -1 .…”

Aerial Oxidation of a VIV–Iminopyridine Hydroquinonate Complex: A Trap for the VIV–Semiquinonate Radical Intermediate

“…One interesting feature of the crystal structure of 4 is the V V –O hydroquinonate and V V –O bridged bond lengths, which are shorter [from 1.846(5) to 1.852(5) and 1.714(5) to 1.769(5) Å, respectively] than the respective V IV bond lengths [from 1.912(4) to 1.923(6) and 1.850(5) to 1.909(5) Å, respectively], indicating that the spins in this mixed-valent complex are localized. These localized structures have been observed for dinuclear mixed-valent V V /V IV species containing an asymmetric bent V–O–V bridge (angle less than 160°) and one or two π-donor ligands at the equatorial xy plane (the short terminal VO bond defines the z axis). − However, σ-donor atoms at the equatorial plane result in the formation of mixed-valent V V /V IV complexes with delocalized valences containing symmetric and linear V–O–V bridges. − The hexanuclear complex 4 has V–O–V angles larger than 160° (∼166°), even though the valences are localized at the vanadium centers. Presumably, steric interactions in the triangular structure do not permit further bending of the V–O–V bridges.…”

Supramolecular Isomerism of Coordination Compounds Based on the in Situ Formed [V2O3]4+ Core: Discrete and Chain-like Architectures

“…[32] However, complexes with asymmetric coordination environments are not so numerous, with few known examples where vanadium ions assume, as is the case here, different coordination geometries. [20,[34][35][36][37][38] Such systems have stimulated considerable attention since they can be considered as models for the construction of heterometallic complexes. It should be noted that a solvate of the tetranuclear complex 3 has been previously reported.…”