Si(IV) complexes of 10,20-diaryl-5,15-diazaporphyrin and 5,10,20-triaryl-5,15-diazaporphyrinoids: Evaluation of charge effect on the optical, redox, and magnetic properties

The central metals in group 14 metalloporphyrins significantly influence the optical, redox, and magnetic properties of the porphyrin π-electron systems. This study reports the first examples of Si(IV) complexes of 10,20-diaryl-5,15-diazaporphyrin (Ar₂DAP) and 5,10,20-triaryl-5,15-diazaporphyrin (Ar₃DAP), designed to explore the effects of meso-nitrogen atoms and net charge on these fundamental properties of the porphyrin ring. Reacting the free base of Ar₂DAP with HSiCl₃, followed by metathesis of axial ligands, yielded the corresponding Si(IV) complexes (Ar₂DAPSiX₂; X = OH, F). Copper-catalyzed N-phenylation of Ar₂DAPSiF₂ and subsequent redox reactions produced 18π- and 19π-electron derivatives of Ar₃DAPSiF₂. Nuclear magnetic resonance spectroscopy, cyclic voltammetry, and density functional theory calculations revealed that the diatropic ring-current effects derived from the aromatic 18π-electron systems of Ar₂DAPSiX₂ and Ar₃DAPSiF₂ strongly depend on the net charge and dipole moment of the DAP ligand. All Si(IV) complexes exhibited fluorescence, with Stokes shifts of 120–310 cm⁻¹. Measurements of fluorescence quantum yields and lifetimes indicated that N-phenylation primarily reduced the nonradiative decay rate constant. These findings provided insights into the influence of net charge on the aromatic character, as well as the optical and redox properties, of six-coordinate Si(IV) complexes of DAP derivatives.