Tuning the Electrochemistry of Re26+ Species with Divergent Bicyclic Guanidinate Ligands and by Modification of Axial π Interactions

Abstract

Four Re(2)(6+) paddlewheel compounds with equatorial bicyclic guanidinate ligands and two monodentate anions in axial positions show a large change in the metal-metal distance that depends on the bite angle of the ligands and whether there are pi interactions between the dimetal unit and the axial ligands. These processes are accompanied by significant changes in the redox behavior. The two pairs of compounds that have been synthesized are Re(2)(tbn)(4)Cl(2), 1, and Re(2)(tbn)(4)(SO(3)CF(3))(2), 2, as well as Re(2)(tbo)(4)Cl(2), 3, and Re(2)(tbo)(4)(SO(3)CF(3))(2), 4, where tbn is the anion of a bicyclic guanidinate with six- and five-membered rings (1,5,7-triazabicyclo[4.3.0]non-6-ene) and tbo is an analogous species with two five-membered rings (the anion of 1,4,6-triazabicyclo[3.3.0]oct-4-ene). For both 1 and 2 as well as for 3 and 4, the metal-metal distances are shorter for the triflate species than for the chloride analogues because of the π interactions of the Cl with the π bonds of the triply bonded Re(2)(6+) cores compounded by a small but symmetry allowed interaction between the antisymmetric combination of the filled σp orbitals of the chlorine atom and the empty σ* orbital of the metal atoms. In addition there is a significant increase in the Re-Re distance from that in the six/five tbn-membered ring to the five/five-membered tbo species. Electrochemical measurements show two redox processes for each set of compounds corresponding to the uncommon Re(2)(6+) → Re(2)(7+) and Re(2)(7+) → Re(2)(8+) processes, which are strongly affected by the bite angle of the guanidinate ligand as well as the ability of the axial ligands to interact with the π orbitals of the dirhenium unit. For 1 and 3, the first redox couples are at 0.146 and 0.487 V, respectively, while for 2 and 4 these are at 0.430 and 0.698 V, respectively.