Spin hamiltonian models for artificial and native water splitting systems revealed by hybrid DFT calculations. Oxygen activation by high-valent Mn and Ru ions

Electronic and spin states of putative ruthenium (Ru)-quinine(Q) complex(1) mononuclear[Ru(trpy)(3,5-t-Bu ₂Q)(OH ₂)(trpy = 2,2':6',2′-terpyridine, 3,5-di-tert-butyl-1,2-benzoquinone)(2) and binuclear [Ru ₂(btpyan)(3,6-di-Bu ₂Q) ₂(OH ₂)](SbF ₆) ₂ (btpyan = 1,8-bis(2,2':6',2′- terpyrid-4'-yl)anthracene,3,6-t-Bu ₂Q = 3,6-di-tert-butyl-1,2- benzoquinone) (3) were investigated by broken-symmetry (BS) hybrid density functional (DFT) methods. BS DFT computations for 3 have elucidated that the closed-shell structure (3b): Ru(II)-quinone (Q) complex is less stable than the open-shell structure (3bb) consisted of Ru(III) and semiquinone fragments. These computations have also elucidated eight different electronic and spin structures of tetraradical intermediates generated in the course of water splitting reaction. The Heisenberg spin Hamiltonian model has been derived to elucidate common theoretical pictures for these systems. Effective exchange interactions for four spin systems have been determined using total energies of the 15 different DFT functionals. The natural orbital (NO) analysis of these BS DFT solutions have also been performed to elucidate the NOs and their occupation numbers that are useful for lucid understanding of the nature of chemical bonds of these Ru complexes (1, 2, and 3). Implications of the computational results are discussed in relation to proposed reaction mechanisms of water splitting reaction in artificial photosynthesis systems, similarity between artificial and native water splitting systems, and elements science of water splitting reaction.