| Abstract
| - Reaction of the Mn4O46+ “cubane” core complex, Mn4O4L6 (1) (L = diphenylphosphinate, Ph2PO2-), with ahydrogen atom donor, phenothiazine (pzH), forms the dehydrated cluster Mn4O2L6 (2), which has lost two μ-oxobridges by reduction to water (H2O). The formation of 2 was established by electrospray mass spectrometry,whereas FTIR spectroscopy confirmed the release of water molecules into solution during the reduction of 1.UV−vis and EPR spectroscopies established the stoichiometry and chemical form of the pzH product by showingthe production of 4 equiv of the neutral pz· radical. By contrast, the irreversible decomposition of 1 to individualMn(II) ions occurs if the reduction is performed using electrons provided by various proton-lacking reductants,such as cobaltocene or electrochemical reduction. Thus, cubane 1 undergoes coupled four-electron/four-protonreduction with the release of two water molecules, a reaction formally analogous to the reverse sequence of thesteps that occur during photosynthetic water oxidation leading to O2 evolution. 1H NMR of solutions of 2 revealthat all six of the phosphinate ligands exhibit paramagnetic broadening, due to coordination to Mn ions, and aremagnetically equivalent. A symmetrical core structure is thus indicated. We hypothesize that this structure isproduced by the dynamic averaging of phosphinato ligand coordination or exchange of μ-oxos between vacantμ-oxo sites. The paramagnetic 1H NMR of water molecules in solution shows that they are able to freely exchangewith water molecules that are bound to the Mn ion(s) in 2, and this exchange can be inhibited by the addition ofcoordinating anions, such as chloride. Thus, 2 possesses open or labile coordination sites for water and anions,in contrast to solutions of 1, which reveal no evidence for water coordination. Complex 2 exhibits greaterparamagnetism than that of 1, as seen by 1H NMR, and it possesses a broad (440 G wide) EPR absorption,centered at g = 2, that follows a Curie−Weiss temperature dependence (10−40 K) and is visible only at lowtemperatures, compared to EPR-silent 1. Its comparison to a spin-integration standard reveals that 2 contains 2equiv of Mn(II), which is in agreement with the formal oxidation state of 2Mn(II)2Mn(III) that was derived fromthe titration. The EPR and NMR data for 2 are consistent with a loss of two of the intermanganese spin-exchangecoupling pathways, versus 1, which results in two “wingtip” Mn(II) S = 5/2 spins that are essentially magneticallyuncoupled from the diamagnetic Mn2O2 base. Bond-enthalpy data, which show that O2 evolution via the reaction1 → 2 + O2, is strongly favored thermodynamically but is not observed in the ground state due to an activationbarrier, are included. This activation barrier is hypothesized to arise, in part, from the constraining effect of thefacially bridging phosphinate ligands.
- The Mn4O46+ “cubane” core complex, Mn4O4L6 (L = Ph2PO2-), 1 undergoes coupled four-electron/four-proton reduction to form Mn4O2L6 by releasing two water molecules, a reaction formally analogous to steps during photosynthetic water oxidation/O2 evolution. Bond enthalpy data indicate that the O2 evolution reaction, 1 → 2 + O2, is favored thermodynamically but is not observed in the ground state owing to an activation barrier.
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