Vanadium(V) Complexes with Substituted 1,5-bis(2-hydroxybenzaldehyde)carbohydrazones and Their Use As Catalyst Precursors in Oxidation of Cyclohexane

Author(s)
Diana Dragancea, Natalia Talmaci, Sergiu Shova, Ghenadie Novitchi, Denisa Darvasiova, Peter Rapta, Martin Breza, Mathea Sophia Galanski, Jozef Kozisek, Nuno M. R. Martins, Luisa M. D. R. S. Martins, Armando J. L. Pombeiro, Vladimir B. Arion
Abstract

Six dinuclear vanadium(V) complexes have been synthesized: NH

4[(VO

2)

2(

HLH)] (NH

4[1]), NH

4[(VO

2)

2(

t-BuLH)] (NH

4[2]), NH

4[(VO

2)

2(

CILH)] (NH

4[3]), [(VO

2)(VO)(

HLH)(CH

3O)] (4), [(VO

2)(VO)(

t-BuLH)(C

2H

5O)] (5), and [(VO

2)(VO)(

CILH)(CH

3O)- (CH

3OH/H

2O)] (6) (where

HLH

4 = 1,5-bis(2-hydroxybenzaldehyde)- carbohydrazone,

t-BuLH

4 = 1, 5 - bis(3,5- di-tert-butyl- 2- hydroxybenzaldehyde)carbohydrazone, and

CILH

4 = 1,5-bis(3,5-dichloro- 2-hydroxybenzaldehyde)carbohydrazone). The structures of NH

4[1] and 4-6 have been determined by X-ray diffraction (XRD) analysis. In all complexes, the triply deprotonated ligand accommodates two V ions, using two different binding sites ONN and ONO separated by a diazine unit -N-N-. In two pockets of NH

4[1], two identical VO

2

+ entities are present, whereas, in those of 4-6, two different VO

2

+ and VO

3+ are bound. The highest oxidation state of V ions was corroborated by X-ray data, indicating the presence of alkoxido ligand bound to VO

3+ in 4-6, charge density measurements on 4, magnetic susceptibility, NMR spectroscopy, spectroelectrochemistry, and density functional theory (DFT) calculations. All four complexes characterized by XRD form dimeric associates in the solid state, which, however, do not remain intact in solution. Compounds NH

4[1], NH

4[2], and 4-6 were applied as alternative selective homogeneous catalysts for the industrially significant oxidation of cyclohexane to cyclohexanol and cyclohexanone. The peroxidative (with tert-butyl hydroperoxide, TBHP) oxidation of cyclohexane was performed under solvent-free and additive-free conditions and under low-power microwave (MW) irradiation. Cyclohexanol and cyclohexanone were the only products obtained (high selectivity), after 1.5 h of MW irradiation. Theoretical calculations suggest a key mechanistic role played by the carbohydrazone ligand, which can undergo reduction, instead of the metal itself, to form an active reduced form of the catalyst.

Organisation(s)
NMR Centre, Department of Inorganic Chemistry, Core Facility Crystal Structure Analysis
External organisation(s)
Academy of Sciences of the Republic of Moldova, Romanian Academy, Centre National De La Recherche Scientifique (CNRS), Slovak University of Technology in Bratislava, Universidade de Lisboa, Instituto Politécnico de Lisboa
Journal
Inorganic Chemistry
Volume
55
Pages
9187-9203
No. of pages
17
ISSN
0020-1669
DOI
https://doi.org/10.1021/acs.inorgchem.6b01011
Publication date
09-2016
Peer reviewed
Yes
Austrian Fields of Science 2012
104003 Inorganic chemistry
Keywords
ASJC Scopus subject areas
Inorganic Chemistry, Physical and Theoretical Chemistry
Portal url
https://ucrisportal.univie.ac.at/en/publications/3f1f87d7-4db7-476c-90fd-7f6b3540751d