Liquid-crystalline oxovanadium(IV) complexes accessed from bidentate [N, O] donor salicylaldimine Schiff-base ligands

Abstract

A series of bis[4-(n-alkoxy)-N-(4′-R-phenyl)salicylideneiminato]oxovanadium(IV) complexes (n = 6, 10, 14, 16, 18 and R = C₃H₇) were prepared and their mesogenic properties were investigated. The mesomorphic behaviors of the compounds were studied by polarized optical microscopy and differential scanning calorimetry. Ligands display SmA/SmC and unexpected nematic mesophases. The complexes bearing longer alkoxy carbon chain (n = 10, 14, 16, and 18) showed both monotropic or enantiotropic transitions with smectic A and high ordered smectic E phases. However, the complex with shorter carbon chain length (n = 6) showed monotropic transition with an unprecedented nematic (N) phase. A density functional theory study was carried out using DMol3 at BLYP/DNP level to obtain a stable optimized structure. A square-pyramidal geometry for the vanadyl complexes has been suggested. A ν_V=O stretching value of ∼970 cm⁻¹ corroborated absence of any V = O ··· V = O interactions. Cyclic voltammetry revealed a quasireversible one-electron response at 0.61 V for the VO(IV)–VO(V) redox couple. Variable temperature magnetic susceptibility measurements of the vanadyl complexes suggested absence of any exchange interactions among the vanadyl spin centers.

Keywords:

• Vanadium
• Metallomesogen
• Schiff bases
• DFT

Acknowledgments

The authors thank SAIF, NEHU, and CDRI, Lucknow for analytical and spectral data. GD acknowledges the financial support from DST and UGC, Government of India. Dr R.C. Deka, Tezpur University, India, is acknowledged for computational facility.