Coordination vs. oxidative rearrangement of photochromic spiropyrans upon reacting with FeIIICl3 and FeIIIBr3

Abstract

Photochromic 1,3,3-trimethylindolino-β-naphthopyrylospiran (TMI–NPS) and 1,3,3-trimethylindolinonaphthospirooxazine (TMI–NSO) spiropyrans transform into the open merocyanine (MC) form, which exhibits a red-violet color upon reacting with one equivalent of anhydrous Fe^IIICl₃ or Fe^IIIBr₃ in o-dichlorobenzene. Slow mixing with n-hexane precipitates black crystals of the coordination complexes: (TMI–NPS)·Fe^IIICl₃ (1), (TMI–NPS)·Fe^IIIBr₃·2C₆H₄Cl₂ (2) and (TMI–NSO)·FeX₃·0.5C₆H₄Cl₂ (where X = Cl for 3 and Br for 4). The oxygen atoms of the MC form of spiropyrans coordinate to Fe^III forming short Fe–O bonds of 1.841(2) for 1, 1.839(2) for 2, and 1.888(2) Å length for 4. Complexes manifest intense absorption bands in the visible range, and the spectra are different for the two spiropyrans. Iron(III) ions have high (S = 5/2) spin states in 1 and 2, and complexes 2–4 manifest broad isotropic EPR signals with g = 2.05–2.11. An asymmetric EPR signal with the main component at g = 3.44 (4.2 K) is observed for 1. According to DFT, strong charge transfer from ligands to electron accepting Fe^III ions and large dipole moments are evaluated in the complexes. The intense color of the (TMI–NSO)·Fe^IIIBr₃ complex disappears when it reacts with 5,6,11,12,17,18-hexaazatrinaphthylene (HATNA). This decoloration indicates the formation of a colorless closed form of TMI–NPS and (HATNA)·Fe^IIIX₃. Slow mixing with n-hexane precipitates crystals of (C₂₂H₁₉N₂O⁺)(Fe^IIIBr₄⁻) (5) salt containing nearly planar C₂₂H₁₉N₂O⁺ cations (1,3,3-trimethyl-2-(naphtho[1,2-d]oxazol-2yl-3H-indol-1-ium cations) obtained as a result of the oxidative rearrangement and deprotonation of TMI–NSO. These cations manifest intense bands at 471 and 497 nm. They are diamagnetic and only an isotropic EPR signal with g = 2.15–2.16 is observed from the Fe^IIIBr₄⁻ anions. The properties of the complexes and possible routes to the formation of 5 are discussed based on DFT calculations.