Understanding the orientation and dynamic motion of planar heterocyclic N-donor ligands by exploiting the symmetry properties of mixed-ligand mu-oxorhenium(V) dinuclear complexes.
Understanding the orientation and dynamic motion of planar heterocyclic N-donor ligands by exploiting the symmetry properties of mixed-ligand mu-oxorhenium(V) dinuclear complexes.
复制标题
通过利用混合配体 mu-氧铼 (V) 双核配合物的对称性，了解平面杂环 N 供体配体的方向和动态运动。
DOI：
10.1021/ic990858t
复制
发表时间：
2000
影响因子：
4.6
通讯作者：
Marzilli,LG
中科院分区：
文献类型：
--
作者：
Alessio,E;Zangrando,E;Iengo,E;Macchi,M;Marzilli,PA;Marzilli,LG
Factors influencing the orientation and dynamic motions of planar N-donor heterocyclic ligands (L) are of interest since such features have broad relevance in metallobiochemistry [Marzilli, L. G.; Marzilli, P. A.; Alessio, E.Pure Appl. Chem.1998,70, 961−968]. We found that μ-oxorhenium(V) dinuclear complexes [ReOCl2LsLt]−O−[ReOCl2LsLt] bearing either symmetrical (L = py = pyridine; 3,5-lut = 3,5-lutidine) or lopsided (L = Me3Bzm = 1,5,6-trimethylbenzimidazole)cisL ligands are particularly useful for studying these factors. NMR data showed that terminal (Lt) and stacked (Ls) ligands were exchanged by ∼180° rotation about the Re−O−Re bond system. Such exchange occurred, however, between degenerate chiral conformers. Here we report a combined X-ray structural and solution NMR investigation of theAA+CC(racemic) andAC(meso) forms of two mixed-ligand μ-oxorhenium dimers that bear one lopsided and one symmetrical ligand on each Re atom, namely, Re2O3Cl4(py)2(Me3Bzm)2(1racand1meso) and Re2O3Cl4(3,5-lut)2(Me3Bzm)2(2racand2meso). The presence of two differentcisL ligands in1and2breaks the local symmetry at each Re atom, so that, in the racemic dimers, the exchange of terminal and stacked ligands leads to nondegenerate conformers. Overall, NMR data showed that the unsymmetrical dimers1and2undergo two dynamic processes contemporaneously, namely, 180° rotation about the Re−N(py or 3,5-lut) bond and coupled rotation about the Re−O−Re/Re−N bonds. Both processes reach the slow exchange limit below −80 °C. Rotation of py in1occurs faster than that of 3,5-lut in2; this difference is attributed to the higher steric demands of 3,5-lut compared to py. For both dimers NMR data provided compelling evidence of the preferred conformers in solution, including ligand orientations. The low-Tsolution structure of1mesoand2mesois chiral, the same as that found in the solid state for2meso, where the Me3Bzm on one Re atom is stacked with the 3,5-lut on the other Re atom. The remaining Me3Bzm and 3,5-lut, one on each Re atom, are both terminal. In solution the coupled Re−O−Re/Re−N rotations interconvert the two halves of eachmesodimer to yield the same overall stable chiral conformation. For theracemicdimers, however, this process does not interconvert one enantiomer into the other, but instead interconverts two rotamers,R1andR2, each of which is chiral. We found that, in the case of both1racand2rac, the conformer with stacking symmetrical ligands (R1) is roughly 1 order of magnitude more stable than that with stacking Me3Bzm ligands (R2). Moreover, the solution conformation ofR1is the same as that found in the solid state of1rac. Solution- and solid-state data indicate that the key interaction favoring the observed conformations is very likely the electrostatic attraction between the δ+H2 atoms on the Me3Bzm ligands and the negative O and Cl groups in the core of the dimers. Finally, for bothmesoandracemicdimers we were also able to elucidate the preferred pathways of the coupled dynamic motions and establish that, very likely, the two halves of the dimers swing back and forth by ∼130° through theantieclipsed form.
