Theoretical studies on effective spin interactions, spin alignments and macroscopic spin tunneling in polynuclear manganese and related complexes and their mesoscopic clusters

ABSTRACT:Hybrid density functional theory (HDFT) and post Hartree-Fock CCSD(T) methods are applied to elucidate the binding energies and the optimized MOO distances of transition metal oxides: MO (M ϭ Cr, Mn, Fe, Co, Ni, Cu). The HDFT method can reproduce the CCSD(T) results, in agreement with the experimental ones. The nature of the manganese-oxygen bonds in the Mn(VI)OO, Mn(IV)OO porphyrin (PP), and Mn(V)OO PP systems are examined in relation to possible mechanisms of oxygen evolution from H 2 O 2 and H 2 O in native and non-native manganese complexes. It is found that the radical character of the high-valent (PP)Mn(V)OO bond is remarkable, showing the strong potential to generate molecular oxygen because of its high reactivity. The electronic structure and magnetic property of -oxo-bridged manganese porphyrin dimer (PPMn(III)OMn(III)PP) are investigated for further discussion of structure and reactivity of PPMn(X)O (X ϭ II-IV). The potential curve for release of molecular oxygen from PPMn(II)O 2 is also examined to show weak affinity of O 2 in the Mn complex where the oxidation number (X) of Mn is low. Implications of the computational results are also discussed in relation to oxygen evolution reactions.

supporting

confidence: 89%

The electronic structure and magnetic property of metal‐oxo, porphyrin manganese‐oxo, and μ‐oxo‐bridged manganese porphyrin dimer

Koizumi

Shoji

Nishiyama

et al. 2004

“…However, such structural modification entails a breakdown of the closed-shell picture. Indeed, the BS method does not suffer the closed-shell constraint if the HOMO-LUMO energy gap for Mn(V)¼ ¼O becomes small, leading to the HOMO-LUMO mixing that affords the broken-symmetry (BS) orbitals for the up-and down-spins [76][77][78]. This labile (soft) pair is chemically regarded as a metal singlet diradical (:) Mn(IV)¼ ¼O (;) [53]: the formal valence of the Mn ion becomes IV because the oxygen site is regarded as oxygen radical anion instead of oxygen dianion.…”

Section: Computational Methods and Homo-lumo Mixingmentioning

confidence: 99%

“…Thus the HOMO-LUMO mixing entails more or less metal diradical character of high-valence manganese-oxo species [53][54][55][76][77][78].…”

Section: Computational Methods and Homo-lumo Mixingmentioning

confidence: 99%

Possible mechanisms of water splitting reaction based on proton and electron release pathways revealed for CaMn₄O₅ cluster of PSII refined to 1.9 Å X‐ray resolution

Saitô

Yamanaka

Kanda

et al. 2011

Recently, Umena et al. have revealed the X-ray diffraction structure of the CaMn 4 O 5 cluster in the oxygen evolving complex (OEC) of photosystem II (PSII) refined to 1.9 Å resolution. Their X-ray structure has first elucidated hydrogen-bonding networks and proton release pathways at OEC of PSII. Here, several working hypotheses Additional Supporting Information may be found in the online version of this article Correspondence to: S. Yamanaka; e-mail: syama@chem.sci.osaka-u.ac.jp Contract grant sponsor: Grants-in-Aid for Scientific Research; Contract grant numbers: 19750046, 19350070, 18350008, 23550016 (heuristic principles) for water splitting reaction are derived from their X-ray structure for theoretical modeling. These hypotheses suggest how water can be oxidized at OEC of PSII: namely possible reaction mechanisms for the reaction. To confirm them, we have also performed broken-symmetry (BS) UB3LYP calculations for active site models based on their XRD structure. The bond lengths of formal Mn(V)AO with labile dp-pp bonds are optimized to clarify possible roles of the species that are often introduced as a key intermediate in the catalytic (Kok) cycle for water splitting reaction at OEC of PSII. Location of the transition structure for the oxygen-oxygen (OAO) bond formation is also performed by the energy optimization technique. The natural orbital (NO) analysis of the UB3LYP solutions has been performed to obtain the natural molecular orbitals and their occupation numbers that have been useful for classification of localized d-electrons, labile chemical bonds and closed-shell (valence) orbitals. The localized d-electrons characterized by the NO analysis are the origins for the magnetism revealed by ENDOR and other magnetic experiments. On the other hand, the nature of labile (soft) dp-pp bonds responsible for the OAO bond formation has been investigated on the basis of chemical indices such as effective bond order (b), diradical character (y), and spin density (Q) indices that are calculated using the orbital overlap between broken-symmetry orbitals. These chemical indices have been calculated for the transition structure of the OAO bond formation at OEC of PSII. Implications of present computational results are discussed in relation to the derived hypotheses and available accumulated experimental results.

“…This means that multinuclear manganese complexes are crucial for a smooth OAO bond formation without a spin cross over. As shown previously, several guiding principles for structure and reactivity of manganese oxides have been obtained by the previous BS DFT calculations of these species [26,34,[41][42][43]. These principles (A-F) summarized in Supporting Information can be used for the water splitting reaction in the OEC.…”

Section: One-electron and Electron-pair Transfer Mechanisms For Binucmentioning

confidence: 85%

“…Under this approximation, the J values by our approximate spin projection procedure are given by the simple equation [34,[41][42][43]:…”

Section: One-electron and Electron-pair Transfer Mechanisms For Binucmentioning

confidence: 99%

Theory of chemical bonds in metalloenzymes. XVII. Symmetry breaking in manganese cluster structures and chameleonic mechanisms for the OO bond formation of water splitting reaction

Saitô

Kanda

Isobe

et al. 2011

Symmetry breaking in cluster structures of manganese oxides by doping of Ca(II) ion is examined in relation to chameleonic mechanisms of water splitting reaction. The orbital and spin correlation diagrams have been depicted to clarify oneelectron transfer and electron-pair transfer mechanisms for the reaction. The spin-polarized molecular orbital models have been applied to elucidate correspondence between magnetic-coupling mode and reaction mechanism of the oxygen-oxygen (OAO) bond formation and oxygen evolution catalyzed by multicenter Ca(II) manganese oxides and related systems. The present UB3LYP calculations followed by the natural orbital analyses have been performed to elucidate electronic structures of the key intermediates and the transition state structure for the OAO bond formation. The results indicate that the reaction proceeds through the continuous diradicaloid mechanism without discreet free radical fragments and/or electron-pair transfer mechanism induced by symmetry breaking with Ca(II) in the pure low-spin singlet state. ible with local singlet and triplet diradical-coupling mechanisms for the OAO bond formation in the low-and high-spin states, respectively. Thus, magnetic (exchange) coupling modes in the oxygen evolution complex are directly related to the local singlet and triplet diradical mechanisms as in the case of soluble methane monooxygenase.