Unique 1∶2 adduct formation of meso-tetraarylporphyrins and meso-tetraalkylporphyrins with BF3: a spectroscopic and ab initio study

Abstract

The interaction of a series of free base meso-tetraarylporphyrins (arylpor) and meso-tetraalkylporphyrins (alkylpor) with BF₃·Et₂O, with different molar ratios (<1∶1 to >1∶2) in chloroform, immediately and exclusively yielded the 1∶2 adducts (BF₃)₂por. The close spectral correlation between the corresponding (BF₃)₂por and (CF₃COOH)₂por were suggestive of similar saddled porphyrin core structures with BF₃ molecules coordinated to the two pyrrolenine nitrogen donors, and simultaneously hydrogen bonded to the pyrrole NH groups of the porphyrin macrocycle from above and below the plane of the porphyrins. The complexation of various arylpor and alkylpor with BF₃ and their protonation with CF₃COOH caused red shifts of the Soret bands (3 to ∼30 nm). The interaction of arylpor (except H₂tmp) and also H₂t(tert-Bu)p with BF₃·OEt₂ and CF₃COOH demonstrated red shifts of the Q(0,0) bands (5.4 to 40 nm). In contrast, reactions of the alkylpor (alkyl = Me, Et, n-Pr, n-Bu) and H₂tmp with BF₃ or CF₃COOH displayed blue shifts of the Q(0,0) bands (−13.5 to −31.8 nm). The observed differences in the Q(0,0) bands shifts for the complexation of arylpor versus alkylpor are presumably related to the relative co-planarity of the meso-aryl groups with the porphyrin core, and the possible π-interactions in the former. It is noteworthy that while the UV-vis spectrum of H₄t(tert-Bu)p²⁺ was very sensitive to excess amounts of CF₃COOH, the UV-vis spectrum of (BF₃)₂H₂t(tert-Bu)p showed no changes in the presence of additional BF₃·Et₂O. The ¹H and ¹³C NMR spectra of the 1∶2 adducts demonstrated a general correspondence with those of the related protonated porphyrins. However, the pyrrole NH signals of the (BF₃)₂por were upfield shifted to an unusual extent as compared to those of the diprotonated H₂por²⁺ species. This effect presumably is due to the weaker NH hydrogen bonding of the 1∶2 molecular complexes compared to the protonated porphyrins. It was also observed that, in contrast to the gradual upfield shifts of the NH signals of H₂por²⁺ with increasing CF₃COOH concentration, the NH signals of (BF₃)₂por complexes remained fixed and independent of BF₃·OEt₂ concentration. ¹⁹F and ¹¹B NMR spectra of various (BF₃)₂por showed upfield shifts of both ¹¹B and ¹⁹F signals relative to those of BF₃·OEt₂. The observed larger upfield shifts of ¹¹B (−6.48 to −6.71 ppm) signals than those of ¹⁹F (−3.53 to −4.20 ppm), apparently reflect direct coordination of the B atoms to the nitrogen donors and their closer proximity to the porphyrin core. The results of ab initio calculations illustrated that in (BF₃)₂H₂tpp the two BF₃ molecules are coordinated to the pyrrolenine nitrogen donors and are hydrogen-bonded to the pyrrole NH groups. Also calculations indicated that the addition of a BF₃ molecule to the 1∶1 species, BF₃H₂tpp, is more favorable (2.4 kcal mol⁻¹) than its coordination to H₂tpp, and the 1∶2 molecular complex is more stable (14.5 kcal mol⁻¹) than the 1∶1 adduct. A mechanism is proposed to explain the absence of the 1∶1 adduct and the observed symmetric NMR spectra of the pyrrole rings and fluorines in (BF₃)₂por.