Dynamic Pathways for Fluxional Molecules Defined Using Exchange-NOE Peaks

In examining NMR methods to assess orientation and fluxional motions of planar N-donor heterocyclic coordinated ligands (L's) in solution, we introduce the use of exchange-NOE NMR data as a powerful method for defining (i) the extent of L rotation about the metal-N bond, (ii) the direction of L rotation, and (iii) even the halves of <i>C</i><sub>2</sub>-symmetrical L's that interchange during dynamic processes. The full value of the approach depends on a strategy in which the complexes studied are chiral and similar except that one has a lopsided L (Me<sub>3</sub>Bzm = 1,5,6-trimethylbenzimidazole in [Re<sub>2</sub>O<sub>3</sub>Cl<sub>4</sub>(Me<sub>3</sub>Bzm)<sub>4</sub>] (<b>1</b>)) and the other has a <i>C</i><sub>2</sub>-symmetrical L (3,5-lut = 3,5-lutidine in [Re<sub>2</sub>O<sub>3</sub>Cl<sub>4</sub>(3,5-lut)<sub>4</sub>] (<b>2</b>)). Each Re is part of a nearly linear ORe−O−ReO grouping and has a “terminal” L (L<sup>t</sup>) and a stacked L (L<sup>s</sup>). The fluxional inversion of the two chiral dimers involves rotations of ∼180° about the Re−O−Re bonds and of ∼90° about all four Re−L bonds. The exchange-NOE data for <b>2 </b>show that the half of L<sup>t</sup> away from the dimer center interchanges with the half of L<sup>s</sup> close to the center, with the L plane rotating past the ORe−O bonds, not the N−Re−Cl bonds. Thus, the exchange-NOE data help to establish the direction of L rotation. In <b>1</b>, Me<sub>3</sub>Bzm<sup>t</sup> has the rare head-to-head (HH) orientation with respect to the partner. This partner Me<sub>3</sub>Bzm<sup>s</sup> stacks with the Me<sub>3</sub>Bzm<sup>s</sup> from the other Re in the common head-to-tail (HT) orientation. Compelling evidence that the predominant solution conformer of <b>1</b> has the HH,HT,HH structure includes unusual chemical shift dispersions and a strong interligand NOE cross-peak. This is the only case in which <i>cis,bis</i> imidazole-ring-ligated untethered ligands have been found to be predominantly HH in solution. This predominance can be attributed to the electrostatic attraction of the δ<sup>+</sup> N<sub>2</sub>C proton for the negative core of the molecule (bridging O, <i>cis</i> Cl on same Re, and two <i>cis</i> Cl's on the other Re).