X-RAY-DIFFRACTION STUDY OF THE CORRELATION BETWEEN ELECTROSTATIC POTENTIAL AND K-ABSORPTION EDGE ENERGY IN A BIS(MU-OXO) MN(III)-MN(IV) DIMER

The mixed-valence compound bis(mu-oxo)tetrakis(2,2'-bipyridine)dimanganese(III,IV) is a small molecule model compound for the oxygen-evolving center, an enzyme which converts water into dioxygen in photosystem II in the photosynthesis of green plants and algae. Crystallographic data: M(r) 1077.47 g/mol, monoclinic, P2(1)/c, Z = 4, a = 13.584(2) Angstrom, b = 14.058(4) Angstrom, c = 23.622(5) Angstrom, beta = 105.25(2)degrees, V = 4352(4) Angstrom(3) at T = 9 K. A quantitative estimate for the valence contrast, i.e., the difference in electrostatic potential of the two Mn atoms in this compound, has been established by two different experimental methods which both involve single-crystal X-ray diffraction. A resonant synchrotron X-ray diffraction experiment examining the Bragg scattering for energies of the incoming beam close to the Mn absorption edge at 6539 keV (similar to 1.9 Angstrom wavelength) shows a 3.7 eV difference in ionization energies of the first K-shell electron for Mn(IV) and Mn(III), respectively. The Mn(TV) edge has the higher energy. Low-temperature (9 K) X-ray diffraction data allowed a multipolar refinement of the electron density distribution, The most remarkable feature of the deformation density maps is a large accumulation of density at the mu-oxygen atoms inside the planar 4-membered Mn(III)-O-Mn(IV)-O ring. From the electron density, the electrostatic potential at each of the manganese nuclei has been calculated, first using a direct space summation and, second, a Fourier summation combined with an atomic cluster calculation. When corrected for the energy term, which originates from relaxation of the remaining electrons upon removal of one electron, the two calculations give estimates of valence shifts of the ionization energy of 4.0 and 2.9 eV, respectively. These values closely bracket the value of 3.7 eV obtained in the resonant synchrotron X-ray diffraction experiment, and they are in good accordance with the result, 3,2 eV, of an all-electron ab-initio model calculation. It is to the authors' knowledge the first attempt to compare values of shifts in core-ionization energies as found from the two different types of diffraction experiments.