Synthesis and structure of vanadate esters of glycerol and propane-1,3-diol

Abstract

Complexes of the type [VO(L)(H₂gl)] and [VO(L)(Hpd)] have been synthesized in excellent yields by reacting bis(acetylacetonato)oxovanadium(IV) with H₂L in the presence of excess glycerol (H₃gl) and propane-1,3-diol (H₂pd) in methanol–acetone (hydroxyphenylmethylenehydrazones of 4-hydroxy-4-phenylbut-3-en-2-one, salicylaldehyde and 2-hydroxynaphthaldehyde are respectively abbreviated as H₂L¹, H₂L² and H2_L³ and generally as H₂L). The crystal structures of [VO(L²)(H₂gl)] and [VO(L²)(Hpd)] have revealed tridentate ONO co-ordination by [L²]^2– while [H₂gl]^– and [Hpd]^– form five- and six-membered V(O,O) chelate rings respectively. In the distorted octahedral VO₅N co-ordination sphere the V–O (alkoxide) bond is 0.5–0.6 Å shorter than the V–O (alcoholic) bond which lies trans to the oxo oxygen atom. In [VO(L²)(Hpd)] the lattice consists of dimers held together by O · · · N hydrogen bonding between the OH group of [Hpd]^– in one molecule and the unco-ordinated [L²]^2– nitrogen in an adjacent molecule. In the [VO(L²)(H₂gl)] lattice such dimers self-assemble into an infinite pattern via additional O · · · O hydrogen bonding between the unco-ordinated OH group of [H₂gl]– of one molecule with the co-ordinated alkoxidic oxygen of the adjacent molecule. In [VO(L)(H₂gl)] both the metal site and the [H₂gl]– ligand are chiral and the two equally abundant diastereoisomers are present in solution (¹H NMR). The ⁵¹V chemical shifts are diagnostic of the alkoxidic chelate ring size: the shifts of [VO(L)(H₂gl)] being ≈30 ppm downfield from that of [VO(L)(Hpd)]. The complexes have low VO³⁺–VO²⁺ reduction potentials.