Diphenylphosphine-Oxide-Fused and Diphenylphosphine-Fused Porphyrins: Synthesis, Tunable Electronic Properties, and Formation of Cofacial Dimers.

Diphenylphosphine-oxide-fused Ni ^II porphyrin 8 was synthesized from 3,5,7-trichloroporphyrin 5 via a reaction sequence of nucleophilic aromatic substitution with lithium diphenylphosphide, oxidation with H ₂ O ₂ , and palladium-catalyzed intramolecular cyclization. Reduction of 8 with HSiCl ₃ gave diphenylphosphine-fused Ni ^II porphyrin 9. The embedded P=O and P moieties serve as a strong electron-accepting and electron-donating group to perturb the optical and electrochemical properties of the Ni ^II porphyrin. Ni ^II porphyrin 9 is diamagnetic with a low-spin Ni ^II center in solution but becomes paramagnetic with a five-coordinated Ni ^II center with high-spin (S=1) state in the solid state. Diphenylphosphine-oxide-fused Zn ^II porphyrin 10 was also synthesized and shown to form a face-to-face dimer with mutual O-Zn bonds in the crystal and in nonpolar and moderately polar solvents. The dimerization of 10 in CDCl ₃ has been revealed to be an entropy-driven process with a large entropy gain (ΔS _D =207 J K ^-1  mol ^-1 ).
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