Photochemistry and Emission of the Dinuclear Complexes (CO)5MnRe(CO)3(L) (L = 2,2'-Bipyrimidine, 2,3-Bis(2-pyridyl)pyrazine) and Bridged Trinuclear Complexes (CO)5MnRe(CO)3(L)Re(Br)(CO)3 and (CO)5MnRe(CO)3(BPYM)W(CO)4: Effect of the Remote Metal Center on the Photodissociation of the ...

Article abstract of DOI:10.1021/ic00125a016

Photochemical and emission properties of the dinuclear (CO)5MnRe(CO)3(L) and trinuclear (CO)5MnRe(CO)3(L)Re(Br)(CO)3 and (CO)5MnRe(CO)3(BPYM)W(CO)4 complexes (L = 2,2'-bipyrimidine (BPYM), 2,3-bis(2-pyridyl)pyrazine(DPP)) are described. All these compounds undergo photochemical homolysis of the Mn-Re bond upon excitation into their MLCT absorption band(s) in the visible spectral region. Mn(Cl)(CO)5 and Re(Cl)(CO)3(L) or Re(Cl)(CO)3(L)Re(Br)(CO)3 are formed in chlorinated solvents (CH2Cl2, CCl4) from the former two types of complexes, respectively. In THF, photolysis produces Mn2(CO)10, together with [Re(CO)3(L)].bul., [Re(CO)3(L)Re(Br)(CO)3].bul., or [Re(CO)3(BPYM)W(CO)4].bul. radicals, respectively, which presumably contain also a coordinated THF molecule. Photoreactions of the dinuclear complexes occur with high quantum yields (0.36 for BPYM and 0.54 for DPP), which are independent of the temperature and of the excitation wavelength. The attachment of the Re(Br)(CO)3 group to the potentially bridging ligand L in (CO)5MnRe(CO)3(L) to form the L-bridged trinuclear species strongly influences the excited state dynamics involved in the photochemistry. Thus, the photochemical quantum yields of the trinuclear complexes are both temperature and excitation wavelength dependent. The apparent activation energy, together with the overall quantum yield, decreases upon changing the excitation from the high- to the low-energy MLCT absorption band. The Mn-Re bond homolysis is about 6 times more efficient for bridging DPP than for bridging BPYM. The dinuclear complexes exhibit, in a 2-MeTHF glass at 80 K, an emission from thermally unequilibrated states, whereas double emission, extending into the near-IR spectral region, was observed for (CO)5MnRe(CO)3(DPP)Re(Br)(CO)3. Its BPYM analogue is nonemissive. To account for this complex photobehavior, an excited state diagram and a qualitative dynamics model are proposed. The reaction is assumed to occur from a (3)σπ(*) state that is nonradiatively populated from the higher MLCT state(s). The main effects of the attachment of the Re(Br)(CO)3 group, which is responsible for the changed photochemical behavior, are the profound stabilization of the π(*) LUMO of the bridging ligand L and the introduction of another MLCT excited state into the trinuclear molecule.