posted on 2017-06-26, 15:19authored byMeenakshi Sharma, Alexander L. Ticho, Lalith Samankumara, Matthias Zeller, Christian Brückner
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.