Electronic Control of Metal-Centered Chirality in η5:κS-Indenyl-Sulfanyl and -Sulfinyl Rhodacycles of 2-Phenylpyridine

Cyclometalation of 2-phenylpyridine with the eta 5:kappa S-indenyl-sulfanyl and -sulfinyl rhodium dichloride complexes rac-8, (R-p ,R-S )-9, and (R-p ,S-S )-9 in the presence of AgSbF6 provided the rhodacyclic complexes 10, 11, and 12, respectively, in yields ranging from 72% to 92%. The complexes were obtained as mixtures of chiral-at-metal epimers, with ratios of the epimer with the phenyl ligand anti to the indenyl benzo-ring [(R-p ,S-Rh )] to the epimer with the phenyl ligand syn to the benzo-ring [(R-p ,R-Rh )] of 3:2, 10:1, and 2:5 for complexes 10, 11, and 12, respectively, establishing that sulfoxide chirality has a significant role in stereocontrol of metal-centered chirality in the initially formed complexes. Equilibration of the rhodacyclic complexes takes place at 120 degrees C in 1,1,2,2-tetrachloroethane-d(2) (C2D2Cl4) solution or at 60 degrees C in C2D2Cl4 solution in the presence of catalytic 4-ethylpyridine. Kinetic studies demonstrate that under the former conditions, epimerization is catalyzed by adventitious water and that epimerization occurs through an associative activation mechanism. The thermodynamic ratios for the SRh /RRh epimers of the complexes were >100:1, >100:1, and 25:1 for complexes 10, 11, and 12, respectively, consistent with a structural indenyl effect favoring the epimer where the stronger structural trans effect phenyl ligand is anti to the indenyl benzo-ring. This is supported by 13C NMR and X-ray diffraction studies which show greater slip-fold distortions toward eta(3)-coordination of the indenyl ligand in the S-Rh epimers of the rhodacyclic complexes.