Four- and three-coordinate planar iron(II) complexes supported by bulky organosilyl ligands.

The ligand exchange reaction of (THF)2Fe[Si(SiMe3)3]2 with 2 equivalents of an N-heterocyclic carbene (NHC) led to the formation of a square-planar iron(II) complex with trans-oriented -Si(SiMe3)3 ligands. Conversely, the introduction of a cis-coordinate bidentate organosilyl ligand instead of -Si(SiMe3)3 resulted in the formation of a square planar iron(II) complex supported by a cis-coordinate bidentate organosilyl ligand. A three-coordinate planar iron(II) bis(silyl) complex was also synthesized using a cis-coordinate bidentate organosilyl ligand and a cyclic (alkyl)(amino)carbene auxiliary ligand. Investigation of the catalytic performance of these complexes in the hydrosilylation of acetophenone revealed that the square-planar iron(II) complex with trans-oriented -Si(SiMe3)3 ligands exhibits superior reactivity relative to its tetrahedral precursor.

Role of redox-inactive metals in controlling the redox potential of heterometallic manganese-oxido clusters.

Photosystem II (PSII) contains Ca2+, which is essential to the oxygen-evolving activity of the catalytic Mn4CaO5 complex. Replacement of Ca2+ with other redox-inactive metals results in a loss/decrease of oxygen-evolving activity. To investigate the role of Ca2+ in this catalytic reaction, we investigate artificial Mn3[M]O2 clusters redox-inactive metals  [M] ([M] = Mg2+, Ca2+, Zn2+, Sr2+, and Y3+), which were synthesized by Tsui et al. (Nat Chem 5:293, 2013). The experimentally measured redox potentials (Em) of these clusters are best described by the energy of their highest occupied molecular orbitals. Quantum chemical calculations showed that the valence of metals predominantly affects Em(MnIII/IV), whereas the ionic radius of metals affects Em(MnIII/IV) only slightly.

pKa of the ligand water molecules in the oxygen-evolving Mn4CaO5 cluster in photosystem II.

Release of the protons from the substrate water molecules is prerequisite for O2 evolution in photosystem II (PSII). Proton-releasing water molecules with low pKa values at the catalytic moiety can be the substrate water molecules. In some studies, one of the ligand water molecules, W2, is regarded as OH-. However, the PSII crystal structure shows neither proton acceptor nor proton-transfer pathway for W2, which is not consistent with the assumption of W2 = OH-. Here we report the pKa values of the four ligand water molecules, W1 and W2 at Mn4 and W3 and W4 at Ca2+, of the Mn4CaO5 cluster. pKa(W1) ≈ pKa(W2) << pKa(W3) ≈ pKa(W4) in the Mn4CaO5 cluster in water. However, pKa(W1) ≈ pKa(D1-Asp61) << pKa(W2) in the PSII protein environment. These results suggest that in PSII, deprotonation of W2 is energetically disfavored as far as W1 exists.