Bimetallic Iron(3+) Spin-Crossover Complexes Containing a 2,2′-Bithienyl Bridging bis-QsalH Ligand

We describe the synthesis of a new 3,3′-diethynyl-2,2′-bithienyl bridging bis-QsalH ligand (5), and the preparation of four bimetallic iron(3+) complexes containing 5 with Cl− (6), SCN− (7), PF6− (8), and ClO4− (9) counteranions. We show with variable temperature magnetic susceptibility, Mössbauer, and electron paramagnetic resonance (EPR) spectroscopy that each complex undergoes a spin-crossover in the solid state. In all four complexes, we observe very gradual and incomplete S = 5/2, 5/2 to S = 1/2, 1/2 spin-crossover processes, with three of the four complexes exhibiting nearly identical magnetic properties. We investigated the electronic properties of the complexes by cyclic and differential pulse voltammetry, and attempted electropolymerization reactions with acetonitrile solutions of the complexes, which were not successful. Each complex features a single iron(3+) reduction wave at approximately −0.7 V (versus ferrocene), and the oxidation of the 2,2′-bithienyl substituent occurs at +1.1 V. These materials represent a new structural paradigm for the study of rare bimetallic iron(3+) spin-crossover complexes.