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 C 2 -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 3 Bzm = 1,5,6-trimethylbenzimidazole in [Re 2 O 3 Cl 4 (Me 3 Bzm) 4 ] ( 1 )) and the other has a C 2 -symmetrical L (3,5-lut = 3,5-lutidine in [Re 2 O 3 Cl 4 (3,5-lut) 4 ] ( 2 )). Each Re is part of a nearly linear O Re−O−Re O grouping and has a “terminal” L (L t ) and a stacked L (L s ). 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 2 show that the half of L t away from the dimer center interchanges with the half of L s 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 1, Me 3 Bzm t has the rare head-to-head (HH) orientation with respect to the partner. This partner Me 3 Bzm s stacks with the Me 3 Bzm s from the other Re in the common head-to-tail (HT) orientation. Compelling evidence that the predominant solution conformer of 1 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 cis,bis 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 + N 2 C proton for the negative core of the molecule (bridging O, cis Cl on same Re, and two cis Cl's on the other Re).