ic301835c_si_001.pdf (2.53 MB)
Free Base and Metal Complexes of 5,15-Diaza-10,20-dimesitylporphyrins: Synthesis, Structures, Optical and Electrochemical Properties, and Aromaticities
journal contribution
posted on 2012-12-03, 00:00 authored by Yoshihiro Matano, Tarou Shibano, Haruyuki Nakano, Yoshifumi Kimura, Hiroshi ImahoriThe synthesis, structures, optical and electrochemical
properties, and aromaticity of a series of 5,15-diaza-10,20-dimesitylporphyrins
(M–DAP; M = Pb, H2, Ni, Pd, Pt, Zn; mesityl = 2,4,6-trimethylphenyl)
are reported. Treatment of mesityl-substituted bis(5,5′-dibromodipyrrin)
with sodium azide in the presence of lead(II) acetylacetonate afforded
Pb–DAP, which was quantitatively converted to H2–DAP by acidolysis. The free base H2–DAP
reacted with palladium(II), platinum(II), and zinc(II) salts to give
Pd–DAP, Pt–DAP, and Zn–DAP, respectively. The
crystal structures, optical and electrochemical properties, and aromaticities
of these β-unsubstituted M–DAPs were comprehensively
investigated by X-ray crystallography, UV–vis absorption/fluorescence
spectroscopy, nanosecond flash photolysis, cyclic and differential
pulse voltammetry, NMR spectroscopy, and density functional theory
calculations. The obtained data show that replacement of the 5- and
15-methine carbons with nitrogen atoms alters the intrinsic properties
of the porphyrin 18π system as follows: (i) the coordination
spheres at the N4 core become contracted while keeping
high planarity; (ii) the Q bands are red shifted and largely intensified;
(iii) the electron-accepting ability is enhanced, whereas the electron-donating
ability is reduced; (iv) the radiative decay rates from the S1 state are enhanced; and (v) the aromaticity of the 18π
circuit is slightly reduced in terms of both geometric and magnetic
criteria. These optical and electrochemical properties of M–DAPs
stem from their characteristic frontier orbitals; two HOMOs and two
LUMOs are nondegenerate as a result of the incorporation of the electronegative
nitrogen atoms at the two meso positions. In addition, the group 10
metals incorporated at the core finely tune the fundamental properties
of DAP π systems through inductive effects as well as dπ–pπ
antibonding orbital interactions; the HOMO–LUMO gaps of the
group 10 metal complexes increase in the order Ni–DAP <
Pd–DAP < Pt–DAP.