Synthesis and Structural Characterization of a Series of Mn^IIIOR Complexes, Including a Water-Soluble Mn^IIIOH That Promotes Aerobic Hydrogen-Atom Transfer

Abstract

Hydrogen atom transfer reactions (HAT) are a class of proton-coupled electron transfer (PCET) reactions used in biology to promote substrate oxidation. The driving force for such reactions depend on both the oxidation potential of the catalyst and the pKa of the proton acceptor site. Both high-valent transition-metal oxo M(IV)=O (M= Fe, Mn) and lower-valent transition-metal hydroxo compounds M(III)–OH (M= Fe, Mn) have been shown to promote these reactions. Herein we describe the synthesis, structure and reactivity properties of a series of Mn(III)-OR compounds (R= pNO2Ph(5), Ph(6), Me(7), H(8)), some of which abstract H-atoms. The Mn(III)-OH complex 8 is water-soluble and represents a rare example of a stable mononuclear Mn(III)-OH. In water, the redox potential of 8 was found to be pH-dependent and the Pourbaix (Ep,c vs pH) diagram has a slope (52 mV/pH) that is indicative of the transfer a single proton with each electron (ie, PCET). The two compounds with the lowest oxidation potential, hydroxide and methoxide-bound 7 and 8 are found to oxidize TEMPOH, whereas the compounds with the highest oxidation potential, phenol-ligated 5 and 6, are shown to be unreactive. Hydroxide-bound 8 reacts with TEMPOH an order of magnitude faster than methoxide-bound 7. Kinetic data (kH/kD= 3.1 (8), kH/kD= 2.1 (7)) are consistent with concerted H-atom abstraction. The reactive species 8 can be aerobically regenerated in H2O, and at least 10 turnovers can be achieved without significant degradation of the “catalyst”. The linear correlation between redox potential and pH, obtained from the Pourbaix diagram, was used to calculate the BDFE= 74.0±0.5 kcal/mol for Mn(II)-OH2 in water, and in MeCN its BDFE was estimated to be (70.1 kcal/mol). The reduced protonated derivative of 8, [MnII(SMe2N4(tren))(H2O)]+ (9), was estimated to have a pKa of 21.2 in MeCN. The ability (7) and inability (5 and 6) of the other members of the series to abstract a H-atom from TEMPOH was used to estimate either an upper or lower limit to the Mn(II)-O(H)R pKa based on their experimentally determined redox potentials. The trend in pKa (21.2(R=H) > 16.2(R=Me) > 13.5(R=Ph) > 12.2(R=pNO2Ph)) is shown to oppose that of oxidation potential Ep,c (−220(R= pNO2Ph) > −300(R= Ph) > −410(R= Me) > −600(R= H) mV vs Fc+/0) for this particular series.

Water-Soluble Fe(II)–H2O Complex with a Weak O–H Bond Transfers a Hydrogen Atom via an Observable Monomeric Fe(III)–OH

“…Exploring this as an alternative route to 7 would support the assumption that the aqueous PCET reaction of Figure 3 generates an Fe(III)−OH species analogous to Mn(III)−OH ( 2 ). 54 Dioxygen addition to Fe(II)−H 2 O ( 1 ) in MeCN was, in fact, found to afford a binuclear oxo-bridged species, {[Fe III (O Me2 N 4 (tren))] 2 -(µ-O)} 2+ ( 8 ), the structure of which was verified by X-ray crystallography (Figure 6). Monooxo-bridged 8 can also be synthesized via the addition of PhIO to 1 in MeCN at −40 °C.…”

“…Structurally analogous [Mn III (S Me2 N 4 (tren))(OH)] + ( 2 ) also displays PCET in aqueous solution. 54 The product of proton-coupled oxidation of 1 in H 2 O would be hydroxo-ligated [Fe III (O Me2 N 4 (tren))-(OH)] + ( 7 , Scheme 1). Although we could not isolate 7 , it is reproducibly generated in situ in protic solvents (wet MeOH and H 2 O), as demonstrated by the presence of a cathodic wave, associated with Fe(III) → Fe(II) conversion, in the cyclic voltammogram of 1 ( i a / i c = 1.0) in H 2 O (Figure 4) as well as the reproducible observation of an intense EPR signal ( g = 8.79, 5.16, 4.23; wet MeOH/EtOH glass; Figure 5) that accounts for 96% of the iron.…”

“…The lower redox potential of 1 relative to 2 indicates that Fe(III)−OH ( 7 ) should be less oxidizing and therefore less capable of abstracting H atoms from substrates than Mn(III)−OH ( 2 ). 54 Conversely, this indicates that Fe(II)−H 2 O ( 1 ) contains weaker O−H bonds relative to Mn(II)−H 2 O ( 5 ) (vide infra) and should thus be capable of acting as a H-atom donor.…”

“…The manganese analogue of Fe(III)−OH ( 7 ), Mn(III)−OH ( 2 ), 54 was obtained previously via the proton-induced cleavage of bimetallic oxo-bridged {[Mn III (S Me2 N 4 (tren))] 2 -(µ-O)} 2+ ( 3 ) in H 2 O. 78 Oxo-bridged 3 was obtained via O 2 addition to the corresponding Mn(II) precursor [Mn II (S Me2 N 4 (tren))] + ( 4 ).…”

Dioxygen Activation and Catalytic Reduction to Hydrogen Peroxide by a Thiolate-Bridged Dimanganese(II) Complex with a Pendant Thiol

“…In the case of the dinuclear μ-oxo thiolate Mn III complexes described by Kovacs et al, concomitant addition of ROH is required to release water. 75 In our system, the release of water is driven by the tendency of the complex to form μ-S bridges.…”