| Abstract
| - In examining NMR methods to assess orientation andfluxional motions of planar N-donor heterocycliccoordinated ligands (L's) in solution, we introduce the use ofexchange-NOE NMR data as a powerful method fordefining (i) the extent of L rotation about the metal-N bond, (ii) thedirection of L rotation, and (iii) even the halvesof C2-symmetrical L's that interchange duringdynamic processes. The full value of the approach depends onastrategy in which the complexes studied are chiral and similar exceptthat one has a lopsided L (Me3Bzm =1,5,6-trimethylbenzimidazole in[Re2O3Cl4(Me3Bzm)4](1)) and the other has aC2-symmetrical L (3,5-lut =3,5-lutidinein[Re2O3Cl4(3,5-lut)4](2)). Each Re is part of a nearly linearORe−O−ReO grouping and has a “terminal” L(Lt) and a stacked L (Ls). The fluxionalinversion of the two chiral dimers involves rotations of ∼180°about theRe−O−Re bonds and of ∼90° about all four Re−L bonds. Theexchange-NOE data for 2 show that the half ofLtaway from the dimer center interchanges with the half of Lsclose to the center, with the L plane rotating past theORe−O bonds, not the N−Re−Cl bonds. Thus, theexchange-NOE data help to establish the direction of Lrotation. In 1, Me3Bzmthas the rare head-to-head (HH) orientation with respect to the partner.This partner Me3Bzms stacks with the Me3Bzmsfrom the other Re in the common head-to-tail (HT) orientation.Compelling evidencethat the predominant solution conformer of 1 has theHH,HT,HH structure includes unusual chemical shiftdispersionsand a strong interligand NOE cross-peak. This is the only case inwhich cis,bis imidazole-ring-ligateduntetheredligands have been found to be predominantly HH in solution. Thispredominance can be attributed to the electrostaticattraction of the δ+ N2C proton for thenegative core of the molecule (bridging O, cis Cl on sameRe, and two cisCl's on the other Re).
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