1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

3 Discussion and Results
first reduction potential of a standard tetraarylporphyrin. As one could have expected, the
shift should be doubled compared to the one in 53 in respect to non-annulated systems. But
that is not at all the case, since also here, the positioning of the exocycles turns out decisive.
The lowest shifts are observed for 3 & 17 bis-annulated systems 67 and 69, higher ones for
71 (3 & 7 bis-annulated) and the highest for 68 and 70 being annulated in diagonal fashion (3
& 13). Additionally, not only the reduction potentials themselves are shifted but also their
difference ΔE½ Red is affected. While in 67, 69 and 71 the Δ value is approx. 0.36 V, in 68 and
70 only half the value (0.19 V and 0.18 V) is found. Therewith it can be nicely proven, that
the setup of analog systems of different symmetries provides a nice tool to fine-tune their
electronic properties due to effects on the shapes and coefficients of their frontier orbitals.
Table 22. Half-wave potentials E½ given in V versus ferrocene, E(Fc/Fc + ) = +0.53 V, obtained
from measurements in CH2Cl2 solution (c = 10 -3 M). For the exact experimental setup see
paragraph 6.1, p. 139.
104
Compound E½ Red2 [V] E½ Red1 [V] E½ Ox1 [V] E½ Ox2 [V]
67 -1.15 -0.79 +1.10 +1.38
68 -0.86 -0.67 +1.07 +1.31
69 -1.16 -0.79 +1.10 +1.38
70 -0.85 -0.67 +1.08 +1.31
71 -1.04 -0.69 +1.05 +1.31
53 96 -1.23 -0.95 +1.02 +1.25
15 a,112
-1.45 -1.14 +1.05 +1.35
a literature data on 5,10,15,20-tetraphenylporphyrin (see Scheme 6, p. 7) a
3.2.7.7 Preferential Formation of Certain BCKPs during Synthesis
After having discussed the properties of the setup BCKPs, the remainder question, why some
of those are preferentially formed during synthesis, shall be clarified. As already mentioned,
the 3 & 17 bis-annulation appears favored over the corresponding double ring closure in 3 &
13 fashion by approximately 4 : 1. This could be generally due to electronic and/or steric
reasons. Assuming, both annulations take place in a successive manner, the distortional
effect of the first exocycle on shape and coefficients of the frontier orbital to be attacked has
to be taken into account. A further effect could arise from the paramagnetic metal center by
spin delocalization effects as copper(II) complexes were used. Also, the steric impact of the
first cyclization could provide a preference as one pyrrolic unit might be deviated from