Synthesis of μ2-Oxo-Bridged Iron(III) Tetraphenylporphyrin-Spacer-Nitroxide Dimers and their Structural and Dynamics Characterization by using EPR and MD Simulations

Iron(III) porphyrins have the propensity to form mu(2)-oxo-dimers, the structures of which resemble two wheels on an axle. Whereas their crystal structure is known, their solution structure and internal dynamics is not. In the present work, the structure and dynamics of such dimers were studied by means of electron paramagnetic resonance (EPR) spectroscopy and quantum chemistry based molecular dynamics (MD) simulations by using the semiempirical tight-binding method (GFN-xTB). To enable EPR investigation of the dimers, a nitroxide was attached to each of the tetraphenylporphyrin cores through a linear and a bent linker. The inter-nitroxide distance distributions within the dimers were determined by continuous-wave (cw)-EPR and pulsed electron-electron double resonance (PELDOR or DEER) experiments and, with the help of MD, interpreted in terms of the rotation of the porphyrin planes with respect to each other around the Fe-O-Fe axis. It was found that such rotation is restricted to the four registers defined by the phenyl substituents. Within the registers, the rotation angle swings between 30 degrees and 60 degrees in the proximal and between 125 degrees and 145 degrees in the distal register. With EPR, all four angles were found to be equally populated, whereas the 30 degrees and 145 degrees angles are strongly favored to the expense of the 60 degrees and 125 degrees angles in the MD simulation. In either case, the internal dynamics of these dimers thus resemble the motion of a step motor.