Heterotrinuclear Oxo-Bridged Fe^IIIORu^IVOFe^III Complexes of Ruthenium Porphyrin and Borylated Low-Spin Iron Dioximes¹

The synthesis, structure, spectra, and reactivity of diamagnetic linear chain heterotrinuclear oxo-bridged complexes L−FeIII N4−O−RuIV(TPP‘)−O−FeIIIN4−L are described where TPP‘ is tetrakis(4-methoxyphenylporphyrinate), N4 is one of the borylated bis(dioximate) macrocycles, N4 = ((DMG)BF2)2 in 1, ((DMG)BPh2)2 in 2, and ((DPG)BF2)2 in 3, and L is a monodentate terminal ligand, L = BuNH2, CH3CN, Py, 1-MeIm, (BuO)3P, etc. Crystal data for 3-(BuNH2)2 = [(BuNH2)Fe((DPG)BF2)2−O]2Ru(TPP‘) monoclinic space group P21/n, a = 16.845 Å, b = 27.184 Å, c = 24.593 Å, β = 90.41°, Fe−O = 1.79(1) Å, Ru−O = 1.80(1) Å, Fe−O−Ru = 175°. Large porphyrin and phenyl ring current shifts permit ready characterization of the materials by 1H NMR. A red-shifted Soret band at 433 nm and two charge transfer transitions in the 650−850 nm region are observed in the visible spectra. Clean reduction of 1, 2, and 3-(CH3CN)2 with 4-tert-butylcatechol is observed giving Fe(II) and Ru(II) products. The kinetics of reduction parallel those of low-spin (μ-oxo)diiron FeN4 systems showing a 1/[CH3CN] dependence, but the rates are 103−105 times slower. The RuIV is proposed to stabilize the μ-oxo−Fe bonds toward reductive cleavage via delocalization of the oxo π electrons onto the ruthenium. Electrochemical data for the heterotrinuclear Fe−O−Ru−O−Fe systems are compared with those for the binuclear Fe−O−Fe systems. The HOMO is stabilized by 120 mV over the corresponding Fe−O−Fe system.