Transition from Hydrogen Atom to Hydride Abstraction by Mn4O4(O2PPh2)6 versus [Mn4O4(O2PPh2)6]+: O−H Bond Dissociation Energies and the Formation of Mn4O3(OH)(O2PPh2)6

Abstract

Synthesis, characterization, and reactions of the novel manganese-oxo cubane complex [Mn(4)O(4)(O(2)PPh(2))(6)](ClO(4)), 1+ (ClO(4)(-)), are described. Cation 1+ is composed of the [Mn(4)O(4)](7+) core surrounded by six bidentate phosphinate ligands. The proton-coupled electron transfer (pcet) reactions of phenothiazine (pzH), the cation radical (pzH(.+)(ClO(4)(-)), and the neutral pz* radical with 1+ are reported and compared to Mn(4)O(4)(O(2)PPh(2))(6) (1). Compound 1+ (ClO(4)(-)) reacts with excess pzH via four sequential reduction steps that transfer a total of five electrons and four protons to 1+. This reaction forms the doubly dehydrated manganese cluster Mn(4)O(2)(O(2)PPh(2))(6) (2) and two water molecules derived from the corner oxygen atoms. The first pcet step forms the novel complex Mn(4)O(3)(OH)(O(2)PPh(2))(6) (1H) and 1 equiv of the pz+ cation by net hydride transfer from pzH. Spectroscopic characterization of isolated 1H is reported. Reduction of 1 by pzH or a series of para-substituted phenols also produces 1H via net H atom transfer. A lower limit to the homolytic bond dissociation energy (BDE) (1H --> 1 + H) was estimated to be >94 kcal/mol using solution phase BDEs for pzH and para-substituted phenols. The heterolytic BDE was estimated for the hydride transfer reaction 1H --> 1+ + H(-) (BDE approximately 127 kcal/mol). These comparisons reveal the O-H bond in 1H to be among the strongest of any Mn-hydroxo complex measured thus far. In three successive H atom transfer steps, 1H abstracts three hydrogen atoms from three pzH molecules to form complex 2. Complex 2 is shown to be identical to the "pinned butterfly" cluster produced by the reaction of 1 with pzH (Ruettinger, W. F.; Dismukes, G. C. Inorg. Chem. 2000, 39, 1021-1027). The Mn oxidation states in 2 are formally Mn(4)(2II,2III), and no further reduction occurs in excess pzH. By contrast, outer-sphere electron-only reductants such as cobaltacene reduce both 1+ and 1 to the all Mn(II) oxidation level and cause cluster fragmentation. The reaction of pzH(.+) with 1+ produces 1H and the pz+ cation by net hydrogen atom transfer, and terminates at 1 equiv of pzH(.+) with no further reaction at excess. By contrast, pz* does not react with 1+ at all, indicating that reduction of 1+ by electron transfer to form pz+ does not occur without a proton (pcet to 1+ is thermodynamically required). Experimental free energy changes are shown to account for these pcet reactions and the absence of electron transfer for any of the phenothiazine series. Hydrogen atom abstraction from substrates by 1 versus hydride abstraction by 1(+ )()illustrates the transition to two-electron one-proton pcet chemistry in the [Mn(4)O(4)](7+) core that is understood on the basis of free energy consideration. This transition provides a concrete example of the predicted lowest-energy pathway for the oxidation of two water molecules to H(2)O(2) as an intermediate within the photosynthetic water-oxidizing enzyme (vs sequential one-electron/proton steps). The implications for the mechanism of photosynthetic water splitting are discussed.