Redox switchable 19π and 18π 5,10,20-triaryl-5,15-diazaporphyrinoid–nickel(II) complexes

Abstract

The synthesis and optical, electrochemical, and magnetic properties of nickel(II) complexes of 5,10,20-triaryl-5,15-diazaporphyrin (TriADAP) are reported. Metal-templated cyclization of unsymmetrically substituted nickel(II)–bis(1-amino-9-chloro-5-mesityldipyrrin; mesityl = 2,4,6-trimethylphenyl) complexes afforded the corresponding TriADAPs or 5-aryl-15-benzyl-10,20-dimesityl-5,15-diazaporphyrin, depending on the combination of base and solvent. The latter macrocycle was converted to TriADAP by deprotection of the [Formula: see text]-benzyl group through Pd/C-promoted hydrogenation. TriADAPs were isolated in both 18[Formula: see text] (cation) and 19[Formula: see text] (neutral) forms. The interconversion between these two oxidation states resulted in a distinct change in the optical properties of the DAP [Formula: see text]-system. NMR spectroscopy of the 18[Formula: see text] TriADAP cations showed that they had aromatic character, whereas EPR spectroscopy of the 19[Formula: see text] TriADAP showed a highly delocalized electron spin of the [Formula: see text]-radical. The para substituents of the [Formula: see text]-aryl groups of TriADAPs had a small but distinct impact on their HOMO and LUMO energies. The change in the net charge of one electron was directly reflected in the redox properties of the DAP ring; TriADAP was more easily reduced and less easily oxidized than DAP. The difference in the net charge was also reflected by the shielding of the pyrrolic [Formula: see text]-protons observed in the [Formula: see text]H NMR spectra. The present results confirm that TriADAP is a highly promising framework for constructing a new class of azaporphyrin-based materials with 18[Formula: see text]–19[Formula: see text] redox-switchable optical and magnetic properties.

Synthesis and properties of redox‐switchable zinc complexes of 10,15,20‐triaryl‐15‐aza‐5‐oxaporphyrin

“…The redox potentials of TriAAOP were considerably shifted to the positive side (Δ E = 0.34‐0.36 V), compared with the corresponding potentials of TADAP. The observed results are in good accordance with the theoretically calculated results for their models (Figure ); the HOMO of 7‐m ( E = −4.56 eV) and LUMO of 9‐m ( E = −5.49 eV) are energetically stabilized compared with the corresponding orbitals of P0‐m ( E = −4.26 eV) and P2‐m ( E = −5.16 eV), respectively, at the same level of theory . These molecular orbitals have relatively large orbital coefficients at the meso ‐heteroatoms (vide supra) and therefore are markedly stabilized upon introducing the highly electronegative oxygen atom.…”

“…As shown in Figure 5, TriAAOP exhibited two separate and reversible redox processes at −0.35 and +0.28 V (vs ferrocene/ferrocenium; Fc/Fc + ), which were attributed to , respectively, at the same level of theory. [25] These molecular orbitals have relatively large orbital coefficients at the meso-heteroatoms (vide supra) and therefore are markedly stabilized upon introducing the highly electronegative oxygen atom.…”

“…Recently, we reported a new approach to synthesize neutral 20π and 19π porphyrins by replacing meso ‐methine units of a porphyrin ring with nitrene units, which was based on the concept of macrocyclic π‐conjugation involving the unshared electrons in the p‐orbitals of meso ‐nitrogen atoms . As expected, 20π and 19π derivatives of 5,10,15,20‐tetraaryl‐5,15‐diazaporphyrin (TADAP) and 5,10,20‐triaryl‐5,15‐diazaporphyrin (TriADAP) were isolated in the neutral or cationic form. Most importantly, these novel diazaporphyrin derivatives have very flat π‐planes and are extremely air‐stable compared with the isoelectronic 20π porphyrin dianions and 19π porphyrin radical anions.…”

Platforms for Stable Carbon‐Centered Radicals

“…In 2018, Matanosg roup reported that 19p neutral radicals (38R)b ased on 5,10,20triaryl-5,15-diazaporphyrins were quite stable. [57] Importantly, incorporation of one N-arylated amine-nitrogen unit altered the net charge of the p circuits of 5,15-diazaporphyrins, thereby making the 19p radical species stable enough to be isolated.…”

Site‐Selective N‐Methylation of 5,15‐Diazaporphyrins: Reactive Cationic Porphyrinoids that Provide Isoporphyrin Analogues

“…The presence of the meso ‐nitrogen atoms renders the redox properties of the 5,15‐diazaporphyrins flexible, which enables the isolation of stable 18π, 19π, and 20π species . In addition, cationic 5,15‐diazaporphyrin Ni II complexes with aryl groups at the meso ‐nitrogen atom have been prepared through one‐electron oxidation of 19π diazaporphyrin radicals …”