Centre for Catalysis Research and Innovation, Department of Chemistry, University of Ottawa, 10 Marie Curie Street BSC 431, K1N 6N5ON Ottawa, Ontario, Canada.
Chemistry. 2010 Jul 19;16(27):8167-80. doi: 10.1002/chem.201000679.
The previously unreported equilibrium in aqueous solution between the V(IV)O square pyramidal and trans octahedral form with an axial water molecule for a number of bidentate ligands with (N,O) and (O,O) donor sets (6-methylpicolinic (6-mepicH) and 6-methyl-2,3-pyridinedicarboxylic (6-me-2,3-pdcH(2)) acids, dipyridin-2-ylmethanol (Hdpmo) and 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone (Hdhp)) has been demonstrated by the combined application of EPR spectroscopy and DFT methods. The EPR spectra suggest that, with increasing ionic strength, the equilibrium is shifted towards the formation of the pentacoordinated species and values of K approximately 4.0 and 7.0 for the systems containing 6-methyl-2,3-pyridinedicarboxylic acid and dipyridin-2-ylmethanol were measured. DFT calculations performed with Gaussian 03 and ORCA software predict the (51)V anisotropic hyperfine coupling constant along the z axis (A(z)), which can be used to demonstrate the presence of an axially bound ligand trans to the V=O bond. The results suggest that an axial donor (charged or not) can lower |A(z)|, in contrast to what was previously believed on the basis of the "additivity rule", and this explains the anomalous behaviour of the V(IV)O complexes formed by N-{2-[(2-pyridylmethylene)amino]phenyl}pyridine-2-carboxamide (Hcapca) and several amidrazone derivatives. The decrease in |A(z)| for the axial binding of a solvent molecule is mainly a result of the reduction of |A(iso)| and this was also observed when the solid [VO(6-methylpicolinato)(2)] was dissolved in DMSO or DMF. The variations in the structural (V=O, V-O and V-N distances, O-V-O and N-V-N angles, and the trigonality index tau) and spectroscopic (|A(z)|, |A(iso)| and nu(V=O)) properties as a function of the axial V-OH(2) distance (R) are also presented. Finally, the electronic structures of the penta- and hexacoordinated complexes are discussed.
此前，通过结合使用电子顺磁共振（EPR）光谱和密度泛函理论（DFT）方法，证明了一系列具有（N，O）和（O，O）供体基团的双齿配体（6-甲基吡啶甲酸（6-mepicH）和 6-甲基-2,3-吡啶二甲酸（6-me-2,3-pdcH(2)）酸、二吡啶-2-基甲醇（Hdpmo）和 1,2-二甲基-3-羟基-4(1H)-吡啶酮（Hdhp））与 V(IV)O 之间存在未报告的平衡，其中 V(IV)O 呈正方锥和反式八面体形式，轴向配体为水分子。EPR 光谱表明，随着离子强度的增加，平衡向五配位物种的形成方向移动，并且对于含有 6-甲基-2,3-吡啶二甲酸和二吡啶-2-基甲醇的体系，测定的 K 值约为 4.0 和 7.0。使用 Gaussian 03 和 ORCA 软件进行的 DFT 计算预测了沿 z 轴的（51）V 各向异性超精细耦合常数（A(z)），可用于证明轴向配体位于 V=O 键的反式位置。结果表明，与基于“加和规则”之前的观点相反，轴向供体（带电荷或不带电荷）可以降低|A(z)|，这解释了由 N-{2-[(2-吡啶基亚甲基)氨基]苯基}吡啶-2-甲酰胺（Hcapca）和几种脒嗪衍生物形成的 V(IV)O 配合物的异常行为。轴向配体溶剂分子的|A(z)|降低主要是由于|A(iso)|的降低，当固态[VO(6-甲基吡啶甲酸盐)(2)]溶解在 DMSO 或 DMF 中时也观察到了这种情况。还呈现了结构（V=O、V-O 和 V-N 距离、O-V-O 和 N-V-N 角度以及三角指数 tau）和光谱（|A(z)|、|A(iso)|和 nu(V=O)）性质随轴向 V-OH(2)距离（R）的变化。最后，讨论了五配位和六配位配合物的电子结构。
