SUPRAMOLECULAR PHOTOCHEMISTRY OF THE [(ETA(5)-C5H4CH2CH2NH3+)2MO2(CO)6][PF6-]2 COMPLEX - CHEMICAL RAMIFICATIONS OF A TENTACLE LIGAND COVALENTLY BONDED NEARBY TO A REACTIVE METAL CENTER

The photochemistry of the [(CpCH2CH2NH3+)2Mo2(CO)6][PF6-]2 Complex is described. Because of the tentacle group on the Cp ligand, the photochemistry of the complex is considerably different from that of the Cp2Mo2(CO)6 parent complex. Thus, visible irradiation of [(CpCH2CH2NH3+)2Mo2(CO)6][PF6-]2 in THF in the absence of added ligands leads to metal-metal bond disproportionation. For comparison, no net reaction occurs when the (MeCp)2Mo2(CO)6 dimer is irradiated in the presence of an excess of [C6H5CH2CH2NH3+][PF6-]. Mechanistic work established that the different reactivities can be ascribed to a supramolecular effect of the tentacle ligand; specifically, the tentacle ligand inhibits a back-reaction of the initial photochemical products in the case of the [(CpCH2CH2NH3+)2Mo2(CO)6][PF6-]2 complex. The following pathway is proposed: Irradiation of either dimer complex leads to photolysis of the metal-metal bond. The photogenerated 17-electron complexes then react with THF (the solvent) to form 19-electron adducts; disproportionation then follows, likely by the standard chain pathway previously reported. The initial disproportionation products are CpxMo(CO)3(THF)+ and CpxMo(CO)3- (Cp(x) = CpCH2CH2NH3+, MeCp). In the case of the (MeCp)2Mo2(CO)6 dimer, a facile back-reaction of these products ensues, and the result is no net reaction. In the case of the tentacle dimer, proton transfer yields [(CpCH2CH2NH3+)Mo(CO)3H] and [(CpCH2CH2NH2)-Mo(CO)3THF+, and then facile coordination of the free amine effectively blocks the back-reaction by forming [(CpCH2CH2NH2)Mo(CO)3]+. This cooperative reactivity between the tentacle ligands and the unstable initial products of disproportionation to yield stable products is an example of supramolecular reactivity. Reaction of the free amine with a CO group also yields the carbamoyl complex [(CpCH2CH2NHCO)Mo(CO)2PPh3].