Tricarbonylrhenium(I) Complexes of Dinucleating Redox-Active Pincer Ligands

Two homobimetallic tricarbonylrhenium­(I) complexes of new redox-active dinucleating pincer ligands have been prepared to assess the impact of spacer size on the first oxidation potentials with respect to mononucleating analogues and on intramolecular electronic communication. The new pincer ligands feature two tridentate NNN- sites each composed of two pyrazolyl flanking donors and a diarylamido anchor that are either directly linked (to form a central benzidene core, H₂(L1)) or linked via a para-phenylene group (to form a para-terphenyldiamine core, H₂(L2)). The bimetallic complexes of the deprotonated ligands, [fac-Re­(CO)₃]₂(μ-L1), 1, and [fac-Re­(CO)₃]₂(μ-L2), 2, were fully characterized in solution and the solid state including by single-crystal X-ray diffraction for 1. The electrochemical properties of each depended strongly on solvent and electrolyte. Complex 1 exhibits two one-electron oxidations in all electrolyte-containing solutions but with separations between first and second oxidation potentials, ΔE_1/2, between 119 and 316 mV depending on conditions. On the other hand, cyclic voltammetry of 2 showed one two-electron oxidation in DMF with NBu₄PF₆ as an electrolyte but two one-electron oxidations with a maximal separation in ΔE_1/2 of 96 mV in CH₂Cl₂ with NBu₄B­(C₆F₅)₄ as an electrolyte. The oxidized complexes 1ⁿ⁺ and 2ⁿ⁺ (n = 1, 2) were prepared by chemical oxidation and were studied spectroscopically (UV–vis/NIR, EPR). The mono-oxidized complex 1⁺ behaves as a Robin–Day Class III species, while 2⁺ is a Robin–Day Class II species that shows thermal valence trapping at 77 K by EPR spectroscopy. As suggested from theoretical studies using DFT methods, the oxidized complexes maintain considerable ligand radical character, so their electronic structures can be formulated as (CO)₃Re^I(μ-Lⁿ⁺)­Re^I(CO)₃ (n = 1 or 2).