Dimanganese and Diiron Complexes of a Binucleating Cyclam Ligand: Four-Electron, Reversible Oxidation Chemistry at High Potentials

Abstract

The reaction of a binucleating biscyclam ligand cyclam_2^iPrO [where cyclam_2^iPrO = (1,3-bis[1,4,8,11-tetraazacyclododecane]-2-hydroxypropane] with Mn(CF_3SO_3)_2 or Fe(CF_3SO_3)_2•2MeCN gives [(cyclam_2^iPrO)Mn_2(μ-CF_3SO_3)](CF_3SO_3)_2 (4) and [(cyclam_2^iPrO)Fe_2(μ-CF_3SO_3)](CF_3SO_3)_2 (6), respectively. [(cyclam_2^iPrO)Mn_2(μ-N_3)](CF_3SO_3)_2 (5) is obtained by the reaction of 4 with NaN_3. Single-crystal X-ray structural characterization indicates that in each of the bimetallic complexes the two metal centers are facially coordinated by a cyclam ligand and bridged by the isopropoxide linker of the ligand in addition to a triflate counteranion. Upon replacement of the triflate bridge with the single-atom bridge of an end-bound azide ligand in 5, the Mn—Mn distance decreases by 0.38 Å. All of the complexes are high-spin and colorless and were characterized by magnetic susceptibility measurements, electron paramagnetic resonance spectroscopy, and electrochemical methods. Magnetic susceptibility measurements indicate that 4 and 6 are weakly antiferromagnetically coupled while 5 is weakly ferromagnetically coupled. Cyclic voltammetry measurements indicate that the hard donor amine ligands impart high oxidation potentials to the metal centers and that four-electron redox activity can be accessed with a narrow potential range of 0.72 V. Upon inclusion of water in the cyclic voltammetry experiment, the oxidative waves shift to higher potentials, which is consistent with water binding the manganese centers. The diiron complex 6 displays four one-electron redox couples, of which the final two are irreversible. Inclusion of water in the cyclic voltammetry measurement for compound 6 resulted in two sets of shifted peaks, which suggests that two molecules of water bind the diiron core. In accordance with the observed reversibility of the electrochemical results, the dimanganese complex is more efficient than the diiron complex for mediating O-atom transfer to organic substrates and is an excellent hydrogen peroxide disproportionation catalyst, with the reaction proceeding for over 20 000 turnovers.

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