C–H bond activation with Ge-oxyl complex generated by photoinduced-electron-transfer of di(hydroxo)porphyrin GeIV complex

Abstract

Visible-light irradiation of MeOH solution containing di(hydroxo)tetraphenylporphyrin atogermanium(IV) complex ( tppGe ( OH )2; 1a), cumene, and Fe 3+ ion ( Fe ( NO 3)3) as an electron acceptor gave cumyl alcohol as an oxidative product along with Fe 2+ ion as a reductive product. The quantum yield (Φox) and turn over frequency (TOF) for the formation of cumyl alcohol was 0.033 and 111.1 h-1, respectively. The addition of KOH aqueous solution (1 mM) into reaction solution led to an increase of Φox to 0.047. The free energy change (ΔG) with electron transfer from excited triplet state (31a*) to Fe 3+ was estimated as a large negative value (-1.37 eV). Furthermore, in 1a-photosensitized oxidation of MeOH in the presence of K 2 PtCl 6 as an electron acceptor, formaldehyde ( HCHO ) was formed in Φox = 0.034 and TOF = 120.0 h-1. The isotope effect for the formation of HCHO (Φox(H)/Φox(D) = 5.04) was observed when MeOH -d4 was employed as an substrate. Both formation of cumyl alcohol and formaldehyde was not observed at all in the case of photosensitized reactions by tppGe ( OMe )2 (1b) having two axial methoxo ligands. These findings indicate the photosensitized reaction was initiated by photoinduced electron transfer from 31a* to Fe 3+ or K 2 PtCl 6 to generate porphyrin radical cation (1a+•), which underwent a proton dissociation of hydroxo axial ligand to give tpp ( OH ) Ge - O • ( Ge -oxyl complex) as key intermediate. The Ge -oxyl complex oxidized substrates through a hydrogen-atom abstraction. It is strongly suggested that 1a can act as a good sensitizer for being able to activate C – H bond of organic compounds.