Complete family of mono-, bi-, and trinuclear Re^I(CO) ₃Cl complexes of the bridging polypyridyl ligand 2,3,8,9,14,15- hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene: Syn/anti isomer separation, characterization, and photophysics

The syn and anti isomers of the bi- and trinuclear Re(CO)₃Cl complexes of 2,3,8,9,14,15-hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene (HATN-Me₆) are reported. The isomers are characterized by ¹H NMR spectroscopy and X-ray crystallography. The formation of the binuclear complex from the reaction of HATN-Me₆ with 2 equiv of Re(CO)₅Cl in chloroform results in a 1:1 ratio of the syn and anti isomers. However, synthesis of the trinuclear complex from the reaction of HATN-Me₆ with 3 equiv of Re(CO)₅Cl in chloroform produces only the anti isomer. syn-{(Re(CO)₃Cl)₃(μ-HATN-Me ₆)} can be synthesized by reacting 1 equiv of Re(CO)₅Cl with syn-{(Re(CO)₃Cl)₂(μ-HATN-Me₆)} in refluxing toluene. The product is isolated by subsequent chromatography. The X-ray crystal structures of syn-{(Re(CO)₃Cl)₂(μ-HATN- Me₆)} and anti-{(Re(CO)₃Cl)₃(μ-HATN-Me ₆)} are presented both showing severe distortions of the HATN ligand unit and intermolecular π stacking. The complexes show intense absorptions in the visible region, comprising strong π → π* and metal-to-ligand charge-transfer (MLCT) transitions, which are modeled using time-dependent density functional theory (TD-DFT). The energy of the MLCT absorption decreases from mono- to bi- to trinuclear complexes. The first reduction potentials of the complexes become more positive upon binding of subsequent Re(CO)₃Cl fragments, consistent with changes in the energy of the MLCT bands and lowering of the energy of relevant lowest unoccupied molecular orbitals, and this is supported by TD-DFT. The nature of the excited states of all of the complexes is also studied using both resonance Raman and picosecond time-resolved IR spectroscopy, where it is shown that MLCT excitation results in the oxidation of one rhenium center. The patterns of the shifts in the carbonyl bands upon excitation reveal that the MLCT state is localized on one rhenium center on the IR time scale.