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 [ReOCl2(L)(L ')]-O-[ReOCl2(L)(L ')]: A combined X-ray structural and dynamic NMR investigation

Factors influencing the orientation and dynamic motions of planar N-donor h eterocyclic ligands (L) are of interest since such features have broad rele vance in metallobiochemistry [Marzilli, L. G.; Marzilli, P. A.; Alessio, E. Pure Appl. Chem. 1998, 70, 961-968]. We found that mu-oxorhenium(V) dinucl ear complexes [(ReOCl2LLt)-L-s]-O-[(ReOCl2LLt)-L-s] bearing either symmetri cal (L = py = pyridine; 3,5-lut = 3,5-lutidine) or lopsided (L = Me(3)Bzm = 1,5,6-trimethylbenzimidazole) cis L ligands are particularly useful for st udying these factors. NMR data showed that terminal (L-t) and stacked (L-s) ligands were exchanged by similar to 180 degrees rotation about the Re-O-R e bond system. Such exchange occurred, however, between degenerate chiral c onformers. Here we report a combined X-ray structural and solution NMR inve stigation of the AA + CC (racemic) and AC (meso) forms of two mixed-ligand mu-oxorhenium dimers that bear one lopsided and one symmetrical ligand on e ach Re atom, namely, Re2O3Cl4(py)(2)(Me(3)Bzm)(2) (1rac and 1meso) and Re2O 3Cl4(3,5-lut)(2)(Me(3)Bzm)(2) (2rac and 2meso). The presence of two differe nt cis L ligands in 1 and 2 breaks the local symmetry at each Re atom, so t hat, in the racemic dimers, the exchange of terminal and stacked ligands le ads to nondegenerate conformers. Overall, NMR data showed that the unsymmet rical dimers 1 and 2 undergo two dynamic processes contemporaneously, namel y, similar to 180 degrees rotation about the Re-N(py or 3,5-lut) bond and c oupled rotation about the Re-O-Re/Re-N bonds. Both processes reach the slow exchange limit below -80 degrees C. Rotation of py in 1 occurs faster than that of 3,5-lut in 2; this difference is attributed to the higher steric d emands of 3,5-lut compared to py. For both dimers NMR data provided compell ing evidence of the preferred conformers in solution, including ligand orie ntations. The low-T solution structure of 1meso and 2meso is chiral, the sa me as that found in the solid state for 2meso, where the Me(3)Bzm on one Re atom is stacked with the 3,5-lut on the other Re atom. The remaining Me(3) Bzm and 3,5-lut, one on each Re atom, are both terminal. In solution the co upled Re-O-Re/Re-N rotations interconvert the two halves of each meso dimer to yield the same overall stable chiral conformation. For the racemic dime rs, however, this process does not interconvert one enantiomer into the oth er, but instead interconverts two rotamers, R1 and R2, each of which is chi ral. We found that, in the case of both 1rac and 2rac, the conformer with s tacking symmetrical ligands (R1) is roughly 1 order of magnitude more stabl e than that with stacking Me3Bzm ligands (R2). Moreover; the solution confo rmation of R1 is the same as that found in the solid state of 1rac. Solutio n- and solid-state data indicate that the key interaction favoring the obse rved conformations is very likely the electrostatic attraction between the delta(+) H2 atoms on the Me(3)Bzm ligands and the negative O and Cl groups in the core of the dimers. Finally, for both meso and racemic dimers we wer e also able to elucidate the preferred pathways of the coupled dynamic moti ons and establish that, very likely, the two halves of the dimers swing bac k and forth by similar to 130 degrees through the anti eclipsed form.