Nickel(II) and copper(II) complexes of β-unsubstituted 5,15-diazaporphyrins and pyridazine-fused diazacorrinoids: Metal-template syntheses and peripheral functionalizations

The present paper reports the first comprehensive study on the synthesis, structures, optical and electrochemical properties, and peripheral functionalizations of nickel(II) and copper(II) complexes of β-unsubstituted 5,15-diazaporphyrins (M-DAP; M=Ni, Cu) and pyridazine-fused diazacorrinoids (Ni-DACX; X=N, O). These two classes of compounds were constructed starting from mesityldipyrromethane by a metal-template method. Ni-DAP and Cu-DAP were prepared in high yields by the reaction of the respective metal-bis(dibromodipyrrin) complexes with NaN ₃-CuX (X=I, Br), whereas Ni-DACN and Ni-DACO were formed as predominant products by the reaction with NaN ₃. In both cases, the metal centers change their geometry from tetrahedral to square planar during the aza-annulation; X-ray crystallographic analyses of M-DAPs showed highly planar diazaporphyrin π planes. The Q band of Ni-DAP was redshifted and intensified compared with that of a nickel-porphyrin reference, due to the involvement of electronegative nitrogen atoms at the meso positions. It was found that the peripheral bromination of Ni-DAP and Ni-DACO occurred regioselectively to afford Ni-DAP-Br ₄ and Ni-DACO-Br, respectively. These brominated derivatives underwent Stille reactions with tributyl(phenyl)stannane to give the corresponding phenylated derivatives, Ni-DAP-Ph ₄ and Ni-DACO-Ph. On the basis of the absorption spectra and X-ray analysis, it has been concluded that the attached phenyl groups efficiently conjugate with the diazaporphyrin π system. The present results unambiguously corroborate that the β-unsubstituted DAPs and DACXs are promising platforms for the development of a new class of π-conjugated azaporphyrin-based materials.