A Pd_4Br_4 macrocycle trapped by cocrystallization from a highly dynamic equilibrium of η~3-cyclo-heptatrienide complexes

摘要

In the coordination chemistry of palladium, dimers bridged via halides are a common motif. Higher oligomers, however, are still rare. We report the structure of an alternating eight-membered [Pd_4Br_4]~4- ring framed by cycloheptatrienide ligands, which was obtained by cocrystallization of dimers and tetramers of the complex salt bromido{η~3-[3-(2,6-diisopropylphenyl)imidazolium-1-yl]cyclohepta-trienido}palladium(II) tetrafluoroborate, namely bis[di-μ-bromido-bis({η~3-[3-(2,6-diisopropylphenyl)imidazolium-1-yl]cycloheptatrienido}palladium(II))] cyclo-tetra-μ-bromido-tetrakis({η~3 -[3-(2,6-diisopropylphenyl)imidazolium-1-yl]cyclo-heptatrienido}palladium(II)) octakis(tetrafluoroborate) dichloromethane octa-solvate, [Pd_4Br_4(C_22H_26N_2)_4][Pd_2Br_2(C_22H_26N_2)_2]_2(BF_4)_8.8CH_2Cl_2. These dimers and tetramers form a highly dynamic equilibrium in solution which was studied by low-temperature NMR spectroscopy. In the light of the presented results, tetrameric PdII species can be assumed to co-exist as a second species in many cases where by current knowledge only a dimeric compound would be expected.