Dendritic arrays of [Re₆(μ₃-Se)₈] ²⁺ core-containing clusters: Exploratory synthesis and electrochemical studies

The reaction between the previously reported site-differentiated cluster solvate [Re₆(μ₃-Se)₈-(PEt₃) ₅(MeCN)](SbF₆)₂ (1) with pyridyl-based ditopic ligands 4,4′-trimethylenedipyridine (2), 1,2-bis(4-pyridyl)-ethane (3), and (E)-1,2-bis(4-pyridyl)ethene (4) afforded cluster complexes of the general formula [Re₆(μ₃-Se)₈(PEt₃) ₅(L)](SbF₆)₂ (5-7), where L represents one of the pyridyl-based ligands. Reacting these cluster complex-based ligands with the fully solvated cluster complex [Re₆(μ₃-Se) ₈(MeCN)₆](SbF₆)₂ (8) produced dendritic arrays of the general formula {Re₆(μ₃-Se) ₈[Re₆(μ₃-Se)₈(PEt ₃)₅(L)]₆}(SbF₆)₁₄ (9-11), each featuring six circumjacent [Re₆(μ₃-Se) ₈(PEt₃)₅]^2- units bridged to a [Re₆(μ₃-Se)₈]2-core cluster by the pyridyl-based ligands. Electrochemical studies using a thin-layer electrochemical cell revealed cluster-based redox events in these cluster arrays. For 9 (L = 2), one reversible oxidation event corresponding to the removal of 7 electrons was observed, indicating noninteraction or extremely weak interactions between the clusters. For 10 (L = 3), two poorly resolved oxidation waves were found. For 11 (L = 4), two reversible oxidation events, corresponding respectively to the removal of 1 and 6 electrons, were observed with the 1-electron oxidation event occurring at a potential 150 mV more positive than the 6-electron oxidation. These electrochemical studies suggest intercluster coupling in 11 via through-bond electronic delocalization, which is consistent with electronic spectroscopic studies of this same molecule.