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 (Δ S D =207 J K −1  mol −1 ).