Tetradentate Schiff base-oxovanadium(IV) complexes: structures and reactivities in the solid state

This article describes the crystal structures of several vanadium complexes containing tetradentate Schiff base ligands, and their properties and reactivities in the solid state. [VO{sal-(R,R)-stien}] (H(2)sal-(R,R)-stien = N ,N'-disalicylidene-(R,R)-1,2-diphenyl-1,2-ethanediamine) crystallized in two different forms, green (from dichloromethane and chloroform) and orange (from acetonitrile). X-ray structure analysis revealed that the green form contains mononuclear square-pyramidal molecules of the complex, whereas the orange form has a polynuclear linear chain structure. The green crystals turn orange when heated at 120 C for a few minutes (thermochromism). Both forms are vapochromic, the orange crystals turning green on exposure to chloroform vapor, and the green crystals turning orange on exposure to acetonitrile vapor. The color of the orange complex changes to green on grinding (mechanochromism). [VO{3-EtOsal-(R,R)-2,4-ptn}] (H(2)3-EtOsal-(R,R)-2,4-ptn = N ,N'-di-3-ethoxysalicylidene-(R,R)-2,4-pentanediamine) also crystallizes in two different forms, green and orange. The polymeric orange crystals turn into the monomeric green form upon heating at 170 degreesC for 10 min. The mechanism of this conversion was studied using X-ray structure analysis and thermal analysis. Thermal isomerization in the solid state between a pair of diastereomers, I and II, of the oxovanadium(IV) complex with an unsymmetrical tetradentate Schiff base ligand, [VO{3-EtOsal,sal-(R,R)-chxn}] (H(2)3-EtOsal,sal(R,R)-chxn = N-salicylidene-N'-3-ethoxysalicylidene-(R,R)-1,2-cyclohexanediamine), was studied at 195 degreesC. The two diastereomers were separated using column chromatography, and each crystallized in two different colors: green (monomeric) and orange (polymeric). The orange complexes, I (orange) and II (orange), turned green on heating at 195 degreesC for a few minutes. All four complexes, I (green), II (green), I (orange), and II (orange), undergo isomerization at 195 degreesC to yield an equilibrium mixture, I:II,: 1:1. On heating [VO(3-EtOsal-meso-stien)] (H(2)3-EtOsal-meso-stien = N,N'-di-3-ethoxysalicylidene-(R,S)(S,R)-1,2-diphenyl-1,2-ethanediamine) at 190 degreesC for 8 h, dehydrogenation took place at the two benzylic carbon atom sites to form a C = C double bond in the five-membered N-N chelate ring moiety. A vanadium(V) species formed on heating the starting complex in the air will be the oxidant of the thermal dehydrogenation reaction. (C) 2002 Elsevier Science B.V. All rights reserved.