Switchable Haptotropic Migrations of Tricarbonylchromium Complexes of 1,2-Dihydro-2-phenyl-1,2-azaborine

1,2-Dihydro-2-phenyl-1,2-azaborine (5) reacts with Cr(CO)3(CH3CN)3 in THF at 50 °C to form the Cr(CO)3 complex 7 in which the chromium is η6-bound to the heterocyclic ring. Heating 7 in THF to 101 °C causes it to isomerize to complex 8, in which the chromium is η6-bound to the phenyl ring. The Cr(CO)3 group of 8 may be switched back to the heterocyclic ring by base conversion to the anion 9, followed by thermal isomerization to anion 10. Protonation of 10 re-forms 7. The X-ray structure of 8 shows that the uncoordinated 1,2-dihydro-1,2 azaborine ring is aromatic.