1H and 13C NMR detection of the carbocations or zwitterions from rhodamine B base, a fluoran-based black color former, trityl benzoate, and methoxy-substituted trityl chlorides in the presence of alkali metal or alkaline earth metal perchlorates in acetonitrile solution

The cleavage of C-O bonds in two fluoran-based dyes by the addition of alkali metal (M+ = Li+, Na+) and alkaline earth metal (M2+ = Mg2+, Ba2+) perchlorates in acetonitrile solution was examined by means of UV-visible absorption and H-1 and C-13 NMR spectroscopy at room temperature. Although the H-1 NMR signals of the xanthene part of Rhodamine B base (3',6'-bis(diethylamino)fluoran) were shifted toward lower fields, those of the isobenzofuran (or the benzoate) part were not shifted much upon the addition of M+ and M2+. The appearance of C-13 signals at 163.6 and 162.4 ppm (vs TMS) in the presence of Li+ and Ba2+, respectively, confirmed the formation of the zwitterions from Rhodamine B base. The assignments of the H-1 and C-13 signals were performed by the HMBC method. The colored zwitterion of a practical black color former, 2'-anilino-6'-dibutylamino-3'-methylspiro[3H-isobenzofuran-1, 9'-(9H)xanthene]-3-one, was produced by the addition of M+ and M2+. The C-13 NMR signals at 162.2 and 159.8 ppm in the presence of 2.0 mol dm(-3) Mg(ClO4)(2) and Ba(ClO4)(2), respectively, gave conclusive evidence of the formation of the sp(2)-hybrid carbon center for the black color former. The interaction between the metal ions and the zwitterions from the fluoran-based dyes decreased as Mg2+ > Ba2+ > Li+ > Na+. The 1:1 complex formation constants with the metal ions for both dyes were evaluated based on the UV-visible absorption data. The smaller formation constants of the black color former, compared with those of Rhodamine B base, indicate some more difficulty in the cleavage of the gamma-lactone ring, which is probably based on the asymmetric structure of the compound, i.e, only a single dibutylamono-group at the 3'- and 6'-positions. The evidence for the formation of both the trityl cation and the benzoate ion was obtained by the H-1 NMR signals from trityl benzoate in the presence of Mg(ClO4)(2) in CD3CN containing a small amount of CF3SO3D. The C-Cl bond cleavage of 4-methoxy, 4,4'-dimethoxy, and 4,4',4-trimethoxytrityl chlorides, through the chemical interaction between Ba2+ and Cl- in acetonitrile soluition, was justified by our C-13 NMR results.