Chemical Exchange Saturation Transfer (CEST) as a Tool to Measure Ligand Flexibility of Chelating Chiral Di-N-heterocyclic Carbene Complexes

A series of eight [(diNHC)Rh(CO)(2)][OTf] complexes, 1'-R, 2'-R, and 3'-R (where diNHC = DEAM, trans-9,10-dihydro-9,10-ethancianthracene-11,12-diylmethane-diyl carbene, and DEA, trans-9,10-dihydro-9,10-ethanopthracene-11,12-diyl carbene ligands; R = R-methylphenylmethane (R-CHMePh), diphenylmethane (diPh), benzyl (Bn), methyl (Me), isopropyl (Pr-t), and o-methylbenzyl (o-MeBn); carbene heterocycle = imidazol-2-ylidene, and benzimidazol-2-ylidene), are synthesized and characterized. The purpose for the synthesis of these complexes was to examine the relationship between carbene N-heterocycle, R-group substitution, and metallocycle ring size (DEAM = 9, DEA = 7 atom ring) on ligand flexibility. The complexes were examined by chemical exchange saturation transfer to measure the rate of conformational isomerization. The results indicate that flexibility is not significantly altered by choice of carbene heterocycle and R group. However, the constrained DEA diNHC derivative does not undergo conformational isomerization.