A Family of Vanadate Esters of Monoionized and Diionized Aromatic 1,2-Diols: Synthesis, Structure, and Redox Activity

The concerned diols (general abbreviation, H2L) are catechol (H2L1) and its 3,5-But2 derivative (H2L2). Esters of the
type VO(xsal)(HL), 2, are obtained by reacting H2L with VO(xsal)(H2O) or VO(xsal)(OMe)(HOMe), where xsal2- is
the diionized salicylaldimine of glycine (x = g), l-alanine (x = a), or l-valine (x = v). The reaction of VO(acac)2
with H2L and the salicylaldimine (Hpsal) of 2-picolylamine has furnished VO(psal)(L), 3. In the structures of VO(gsal)(HL1), 2a, and VO(vsal)(HL2), 2f, the HL- ligand is O,O-chelated, the phenolic oxygen lying trans to the oxo
oxygen atom. The xsal2- coligand has a folded structure and the conformation of 2f is exclusively endo. In both
2a and 2f the phenolic oxygen atom is strongly hydrogen bonded (O···O, 2.60 Å) to a carboxylic oxygen atom of
a neighboring molecule. In VO(psal)(L2)·H2O, 3b, the diionized diol is O,O-chelated to the metal and the water
molecule is hydrogen bonded to a phenoxidic oxygen atom (O···O, 2.84 Å). The C−O and C−C distances in the
V(diol) fragment reveal that 2 is a pure catecholate and 3 is a catecholate−semiquinonate hybrid. In solution each
ester gives rise to a single 51V NMR signal (no diastereoisomers), which generally shifts downfield with a decrease
in the ester LMCT band energy. The V(V)/V(IV) and catecholate−semiquinonate reduction potentials lie near −0.75
and 0.35, and 1.10 and 0.70 V vs SCE for 2 and 3, respectively. Molecular oxygen reacts smoothly with 2 quantitatively
furnishing the corresponding o-quinone, and in the presence of H2L the reaction becomes catalytic. In contrast,
type 3 esters are inert to oxygen. The initial binding of O2 to 2 is proposed to occur via hydrogen bonding with
chelated HL-.

所涉及的二醇（diols，通用缩写为H₂L）为儿茶酚（catechol，H₂L¹）及其3,5-二叔丁基衍生物（H₂L²）。通式为VO(xsal)(HL)的酯类化合物2，可通过H₂L与VO(xsal)(H₂O)或VO(xsal)(OMe)(HOMe)反应制得，其中xsal²⁻为甘氨酸（x = g）、L-丙氨酸（x = a）或L-缬氨酸（x = v）的二电离水杨醛亚胺（salicylaldimine）配体。二(乙酰丙酮)氧钒(IV) (VO(acac)₂)与H₂L以及2-吡啶甲胺的水杨醛亚胺（Hpsal）反应，得到了VO(psal)(L)即化合物3。

在VO(gsal)(HL¹)（2a）与VO(vsal)(HL²)（2f）的晶体结构中，HL⁻配体以O,O-螯合模式配位，其酚氧原子与氧钒基的氧原子呈反式构型。xsal²⁻辅助配体具有折叠结构，且2f的构象仅为内型（endo）。在2a和2f中，酚氧原子均与相邻分子的一个羧酸氧原子形成强氢键（O···O键长为2.60 Å）。

在水合物VO(psal)(L²)·H₂O即3b中，二电离的二醇配体以O,O-螯合方式与金属中心配位，水分子与一个酚氧阴离子形成氢键（O···O键长为2.84 Å）。V(二醇)片段中的C−O与C−C键长数据表明，化合物2为典型儿茶酚酸酯（catecholate）配合物，而化合物3则为儿茶酚酸酯-半醌（semiquinonate）杂化配合物。

在溶液中，每种酯类化合物均会产生单一的⁵¹V核磁共振（NMR）信号（未检出非对映异构体（diastereoisomers）），且其化学位移通常会随着配体到金属电荷转移跃迁（ligand-to-metal charge transfer, LMCT）能带能量的降低而向低场移动。相较于饱和甘汞电极（SCE），化合物2与3的V(V)/V(IV)以及儿茶酚酸酯-半醌还原电位分别约为−0.75 V、0.35 V与1.10 V、0.70 V。

分子氧可与化合物2平稳发生定量反应，生成对应的邻醌（o-quinone）；若体系中存在额外的H₂L，则该反应可实现催化循环。与之相反，类型3的酯类化合物对氧气表现出惰性。研究认为，O₂与2的初始结合是通过与螯合的HL⁻配体形成氢键实现的。