Free Base and Metal Complexes of 5,15-Diaza-10,20-dimesitylporphyrins: Synthesis, Structures, Optical and Electrochemical Properties, and Aromaticities

The 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.