The para-aminobiphenyl compound [(η6-C6H5)(C6H4-4-NH2)]Cr(CO)3 (1) has an arene–phenyl dihedral angle of 38.01(6)°, as determined by single-crystal X-ray crystallography, and 34.7(11)°, as determined by DFT calculations. It undergoes haptotropic rearrangement at 140 °C in solution to form [(η6-C6H4-4-NH2)(C6H5)]Cr(CO)3 (2), even though previous reports have suggested that such rearrangements should not be observed in compounds with arene–phenyl dihedral angles greater than 22°. NMR analysis gave a rate constant of k = 5.0 × 10−5 s−1 for the rearrangement of 1 to 2. The ortho-substituted analog [(η6-C6H5)(C6H4-2-NH2)]Cr(CO)3 (3) has an arene–phenyl dihedral angle of 67.70(7)°, as determined by single-crystal X-ray crystallography, and 51.9(10)°, as determined by DFT calculations. Surprisingly, even though it displays a more extreme canting of arene rings, 3 rearranges to [(η6-C6H4-2-NH2)(C6H5)]Cr(CO)3 (4) at 140 °C in solution with a rate constant of k = 2.6 × 10−4 s−1. This approximately five-fold rate enhancement likely results from the ortho-amino group providing intramolecular stabilization for intermediates formed during the rearrangement.
