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Coordination vs. oxidative rearrangement of photochromic spiropyrans upon reacting with FeIIICl3 and FeIIIBr3
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 FeIIICl3 or FeIIIBr3 in o-dichlorobenzene. Slow mixing with n-hexane precipitates black crystals of the coordination complexes: (TMI–NPS)·FeIIICl3 (1), (TMI–NPS)·FeIIIBr3·2C6H4Cl2 (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 FeIII 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 FeIII ions and large dipole moments are evaluated in the complexes. The intense color of the (TMI–NSO)·FeIIIBr3 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)·FeIIIX3. Slow mixing with n-hexane precipitates crystals of (C22H19N2O+)(FeIIIBr4−) (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 FeIIIBr4− anions. The properties of the complexes and possible routes to the formation of 5 are discussed based on DFT calculations.