Conformational Landscapes of Metal(II) Porphyrinato, Chlorinato, and Morpholinochlorinato Complexes.

Abstract

The macrocycle conformation of [meso-tetraarylporphyrinato]metal complexes is metal-dependent. Furthermore, hydroporphyrins and some of their analogues are known to be more conformationally flexible than the parent porphyrins, but the extent to which this is reflected in their metal-dependent conformations was much less studied. meso-Tetraarylmorpholinochlorins are intrinsically nonplanar chlorin analogues in which the five-membered pyrroline moiety was replaced by a six-membered morpholine moiety. The metal complexes (M = Ni2+, Cu2+, Zn2+, Pd2+, Ag2+) of meso-aryl-2,3-dimethoxychlorins and meso-arylmorpholinochlorins were prepared. Their conformations were determined using X-ray crystal structure diffractometry and compared against those of their free bases, as well as against the conformations of the corresponding metalloporphyrins. Out-of-plane displacement plots visualized and quantified the conformational changes upon stepwise conversion of a pyrrole moiety to a dimethoxypyrroline moiety and to a dialkoxymorpholine moiety, respectively. The generally nonplanar macrocycle conformations were found to be central-metal-dependent, with the smaller ions showing more nonplanar conformations and with the metallomorpholinochlorins generally showing a much larger conformational range than the corresponding metallochlorins, which, in turn, were more nonplanar than the corresponding porphyrins. This attests to the larger conformational flexibility of the morpholinochlorin macrocycle compared to that of a chlorin or even a porphyrin macrocycle. The degree of nonplanarity affects the electronic structure of the metal complexes, as can also be seen in a comparison of their UV-vis spectra. We thus further define the conformational and electronic effects governing pyrrole-modified porphyrins.