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 = Ni, Cu, Zn, Pd, Ag) 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.

Chirogenesis in Zinc Porphyrins: Theoretical Evaluation of Electronic Transitions, Controlling Structural Factors and Axial Ligation

“…[ 98 , 139 , 147 ] Apparently, the out of plane deformations weaken the macrocycle π‐electronic system through reducing the overlap of neighboring p z orbitals, [138] and hence may generate chirality in achiral porphyrins. Therefore, in this particular case the structural changes are strongly correlated with the intrinsic rotational strength[ 42 , 58 , 117 ] and are also a main factor responsible for the low energy shift of B‐band. [ 55 , 147 ] However, Pfeffer et al.…”

“…Synthetic porphyrins also found a broad application in catalysis, [4–6] optical and chemo sensors, [7–15] light harvesting, [5,16–18] medicine, [19–22] supramolecular systems, [1,23–27] electronic devices, [11,28,29] etc. Specific natural and supramolecular chiral environment may result in chirogenic processes in porphyrin chromophores, which can be detected by various spectroscopic methods such as X‐ray, [30–33] IR, [31,34] Raman, [32] NMR, [31,32,35,36] circular polarized luminescence, [30,37,38] vibrational, [34,39,40] electronic (ECD), [9,19,41–44] and magnetic [34,45,46] circular dichroism.…”

Crystal structure of 2,3-dimethoxy-meso-tetrakis(pentafluorophenyl)morpholinochlorin methylene chloride 0.44-solvate

“…= 0.323 Å ) (Shelnutt et al, 1998) than in 1b (r.m.s. = 0.276 Å ; Sharma et al, 2017). While the tripyrrolic portion of 1d is significantly more ruffled than the corresponding section of 1b, the morpholine moieties are, except for the position of the ring oxygen, rather similar.…”

“…This formal replacement is achieved by a stepwise oxygen insertion into a porphyrin using a so-called 'breaking and mending' strategy (Brü ckner, 2016). As a consequence of the atom insertion, morpholinochlorins are non-planar (McCarthy et al, 2003;Brü ckner et al, 2011;Sharma et al, 2017). The twisted (ruffled) conformation of helimeric chirality of the morpholinochlorins was found to be affected by the size and number of alkoxy substituents, the presence of covalent links between the morpholine unit and the flanking aryl group, and the presence and type of central metal (Daniell & Brü ckner, 2004;Brü ckner et al, 2011;Sharma et al, 2017).…”

“…As a consequence of the atom insertion, morpholinochlorins are non-planar (McCarthy et al, 2003;Brü ckner et al, 2011;Sharma et al, 2017). The twisted (ruffled) conformation of helimeric chirality of the morpholinochlorins was found to be affected by the size and number of alkoxy substituents, the presence of covalent links between the morpholine unit and the flanking aryl group, and the presence and type of central metal (Daniell & Brü ckner, 2004;Brü ckner et al, 2011;Sharma et al, 2017). Porphyrinoids containing two morpholine moieties are known (Daniell & Brü ckner, 2004;Guberman-Pfeffer et al, 2017), as well as other porphyrinoids containing morpholine building blocks (Lara et al, 2005;Samankumara et al, 2015;Akhigbe et al, 2016).…”