Observation of a stable water-soluble lithium porphyrin

The deprotonated form of the water-soluble porphyrin 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin, H2OBT(4-N-MePyP4+, has a strong affinity for lithium and forms a stable lithium complex in basic aqueous solution. The fully deprotonated form, OBT(4-N-MePyP2+, is present above pH 10 (pK(a3) = 6.5 +/- 0.1 and pK(a4) = 10.2 +/- 0.1). Lithium ions bind to this porphyrin in aqueous solution in a 1:1 stoichiometry with a binding constant of (9.6 +/- 0.5) x 10(2) M(-1) (0.1 M KOH, 298 K). The apparent binding constant for Li+ is reduced in the presence of a large excess of Na+, and a binding constant for Na+ of 1.0 +/- 0.3 M(-1) was obtained. The Li+ porphyrin in D2O shows a Li-7 NMR signal at -10.25 ppm vs 1.1 M LiCl, and the Line widths show exchange occurs at 299 K, in contrast to earlier lithium porphyrins, which exchange more slowly. The lithium-binding behavior and Li-7 NMR spectra for 5,10, 15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin, HxT(4-N-MePy)P-(2+x+) (x = 0-2) are also reported.