Synthesis and Photochemistry of Amido-Linked, Peripherally-Molybdenated Tetraphenylporphyrins

Abstract

The para, meta, and ortho peripherally-molybdenated tetraphenylporphyrin complexes [5-{[Mo(NO)Tp*Cl]NHC6H4}-10,15,20-Ph3porphH2] [1−3, respectively; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate; Ph3porphH2 = triphenylporphyrin] have been prepared by reaction of the corresponding 5-(aminophenyl)-10,15,20-triphenylporphyrin derivative with [Mo(NO)Tp*Cl2]. Cyclic voltammetry shows that most of these complexes undergo two oxidation processes (associated with the porphyrin) and three reduction processes (two associated with the porphyrin, and one with the molybdenum fragment). The redox potentials of the molybdenum fragment are little influenced by the presence of the macrocyclic ring, and vice versa. The ΔG° values for charge separation in 1−3 are close to 0 eV. Photochemical measurements have been made on 1−3, and they were all found to undergo photoinduced intramolecular electron transfer from the excited singlet state of the porphyrin macrocycle to the molybdenum moiety, yielding charge-separated states with lifetimes of 300−340 ps. Further studies, using electrochemical methods and EPR spectroscopy, have been made to investigate the nature of the reduced molybdenum species. The molecular structure of 3 was determined crystallographically: C59H52N12BClMoO, monoclinic, space group P21/c, a = 13.678(6), b = 16.650(2), and c = 26.555(6) Å, β = 91.56(3)°, Z = 4. It was shown that the Mo−NH (aniline) distance is relatively short [1.928(16) Å], consistent with pπ → dπ donation. The Mo−porphyrin centroid distance is 6.75 Å.