Remote site photosubstitution in metalloporphyrin-rhenium tricarbonylbipyridine assemblies: photo-reactions of molecules with very short lived excited states

The synthesis is reported of a series of metalloporphyrins (and the corresponding free-base porphyrin), mono-meso-substituted with a bipyridyl group via an amide link at the 4-position of one phenyl group: [Re(CO)(3)(Pic) Bpy-MTPP][OTf], whereM= Mg, Zn, Pd or 2H, Pic = 3-picoline, Bpy = 2,2'-bipyridine, TPP = tetraphenylporphyrin. The photochemical reactions of the assemblies with the sacrificial electron donor triethylamine have been investigated by IR spectroscopy and compared to the behaviour of analogues of the type Bpy-MTPP without rhenium. Selective long-wavelength irradiation of the metalloporphyrin unit in the presence of excess picoline leads to reduction at the rhenium bipyridine centre. In the absence of 3-picoline, the latter is not reduced, but substituted by added halide or by the THF solvent. Mechanistic analysis highlights the differences between the zinc and magnesium chelate on the one hand and the palladium porphyrin on the other. The free-base assembly, [Re(CO)(3)(Pic) Bpy-H2TPP][OTf] is unreactive. The zinc and magnesium porphyrin assemblies initially coordinate Et3N before undergoing photo-induced inner-sphere electron transfer from the triethylamine to form a charge-shifted excited state of the assembly. In contrast, the palladium-based dyad reacts via outer-sphere reductive quenching of a porphyrin-based excited state. The substitution products are postulated to form by a mechanism involving an electron-transfer chain.