Advanced Paramagnetic Resonance Studies on Manganese and Iron Corroles with a Formal d4 Electron Count.

Metallocorroles wherein the metal ion is MnIII and formally FeIV are studied here using field- and frequency-domain electron paramagnetic resonance techniques. The MnIII corrole, Mn(tpfc) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole trianion), exhibits the following S = 2 zero-field splitting (zfs) parameters: D = -2.67(1) cm-1, |E| = 0.023(5) cm-1. This result and those for other MnIII tetrapyrroles indicate that when D ≈ - 2.5 ± 0.5 cm-1 for 4- or 5-coordinate and D ≈ - 3.5 ± 0.5 cm-1 for 6-coordinate complexes, the ground state description is [MnIII(Cor3-)]0 or [MnIII(P2-)]+ (Cor = corrole, P = porphyrin). The situation for formally FeIV corroles is more complicated, and it has been shown that for Fe(Cor)X, when X = Ph (phenyl), the ground state is a spin triplet best described by [FeIV(Cor3-)]+, but when X = halide, the ground state corresponds to [FeIII(Cor•2-)]+, wherein an intermediate spin (S = 3/2) FeIII is antiferromagnetically coupled to a corrole radical dianion (S = 1/2) to also give an S = 1 ground state. These two valence isomers can be distinguished by their zfs parameters, as determined here for Fe(tpc)X, X = Ph, Cl (tpc = 5,10,15-triphenylcorrole trianion). The complex with axial phenyl gives D = 21.1(2) cm-1, while that with axial chloride gives D = 14.6(1) cm-1. The D value for Fe(tpc)Ph is in rough agreement with the range of values reported for other FeIV complexes. In contrast, the D value for Fe(tpc)Cl is inconsistent with an FeIV description and represents a different type of iron center. Computational studies corroborate the zfs for the two types of iron corrole complexes. Thus, the zfs of metallocorroles can be diagnostic as to the electronic structure of a formally high oxidation state metallocorrole, and by extension to metalloporphyrins, although such studies have yet to be performed.

Spectroscopic Characterization of Platinum(IV) Terpyridyl Complexes.

Pt(tpy)X3+ [X = Cl (1), Br (2); tpy = 2,2':6',2″-terpyridine] salts were prepared by the oxidative addition of Pt(tpy)X+ with X2 as originally reported by Morgan and Burstall in 1934. The complexes have been fully characterized by 1H NMR spectroscopy, elemental analysis, mass spectrometry, and X-ray crystallography. The electronic structures of 1 and 2 were investigated using absorption and emission spectroscopy, and the accumulated data are consistent with stabilization of the singlet ligand-centered and potentially singlet ligand field/singlet ligand-to-metal charge-transfer states for 1 and 2 compared to those for Pt(tpy)Cl+ (3) and Pt(tpy)Br+ (4). The changes in the lowest-energy-absorbing state result in drastic differences in the emission behavior among 1-4. Specifically, 1 emits from a lowest-energy state that appears to have triplet ligand field/triplet ligand-to-metal charge-transfer character, whereas 2 exhibits no appreciable emission between 400 and 800 nm.

Valence Tautomerization of High-Valent Manganese(V)-Oxo Corrole Induced by Protonation of the Oxo Ligand.

The addition of an organic acid to the manganese(V)-oxo corrole complex (tpfc)Mn(V)(O) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole) induces valence tautomerization resulting in the formation of (tpfc(+•))Mn(IV)(OH) in acetonitrile at 298 K. The corrole radical cation manganese(IV) hydroxo complex has been fully characterized by EPR, (1)H NMR, and UV-vis spectroscopy. The reactivity of the valence tautomer (tpfc(+•))Mn(IV)(OH) is compared to that of (tpfc)Mn(V)(O) in three reaction types: hydrogen atom transfer (HAT), electron transfer (ET), and oxygen atom transfer (OAT). (tpfc(+•))Mn(IV)(OH) shows a dramatic 5 orders of magnitude enhancement in the rate of ET but surprisingly does not undergo OAT with PhSMe. The high-valent (tpfc)Mn(V)(O) complex is moderately more reactive toward HAT with substituted phenol and shows superior activity in OAT.

Activationless electron self-exchange of high-valent oxo and imido complexes of chromium corroles.

Rate constants of electron self-exchange of high-valent oxo and imido complexes of chromium(V/IV) corrole have been determined in acetonitrile and toluene at various temperatures by electron paramagnetic resonance (EPR) line width variation of the EPR spectra. The observed activation enthalpies (ΔHobs(⧧)) of electron self-exchange of chromium(V)-oxo and -imido corrole with the corresponding chromium(IV) complexes are zero in toluene, whereas the ΔHobs(⧧) values are slightly positive in acetonitrile. Such activationless electron self-exchange transfer resulted in extremely fast electron-transfer reactions of chromium(V)-oxo and -imido corrole in sharp contrast with slow electron-transfer reactions of other high-valent metal-oxo and -imido complexes.

High-valent chromium-oxo complex acting as an efficient catalyst precursor for selective two-electron reduction of dioxygen by a ferrocene derivative.

Efficient catalytic two-electron reduction of dioxygen (O2) by octamethylferrocene (Me8Fc) produced hydrogen peroxide (H2O2) using a high-valent chromium(V)-oxo corrole complex, [(tpfc)Cr(V)(O)] (tpfc = tris(pentafluorophenyl)corrole) as a catalyst precursor in the presence of trifluoroacetic acid (TFA) in acetonitrile (MeCN). The facile two-electron reduction of [(tpfc)Cr(V)(O)] by 2 equiv of Me8Fc in the presence of excess TFA produced the corresponding chromium(III) corrole [(tpfc)Cr(III)(OH2)] via fast electron transfer from Me8Fc to [(tpfc)Cr(V)(O)] followed by double protonation of [(tpfc)Cr(IV)(O)](-) and facile second-electron transfer from Me8Fc. The rate-determining step in the catalytic two-electron reduction of O2 by Me8Fc in the presence of excess TFA is inner-sphere electron transfer from [(tpfc)Cr(III)(OH2)] to O2 to produce the chromium(IV) superoxo species [(tpfc)Cr(IV)(O2(•-))], followed by fast proton-coupled electron transfer reduction of [(tpfc)Cr(IV)(O2(•-))] by Me8Fc to yield H2O2, accompanied by regeneration of [(tpfc)Cr(III)(OH2)]. Thus, although the catalytic two-electron reduction of O2 by Me8Fc was started by [(tpfc)Cr(V)(O)], no regeneration of [(tpfc)Cr(V)(O)] was observed in the presence of excess TFA, regardless of the tetragonal chromium complex being to the left of the oxo wall. In the presence of a stoichiometric amount of TFA, however, disproportionation of [(tfpc)Cr(IV)(O)](-) occurred via the protonated species [(tpfc)Cr(IV)(OH)] to produce [(tpfc)Cr(III)(OH2)] and [(tpfc)Cr(V)(O)].

Non-heme manganese catalysts for on-demand production of chlorine dioxide in water and under mild conditions.

Two non-heme manganese complexes are used in the catalytic formation of chlorine dioxide from chlorite under ambient temperature at pH 5.00. The catalysts afford up to 1000 turnovers per hour and remain highly active in subsequent additions of chlorite. Kinetic and spectroscopic studies revealed a Mn(III)(OH) species as the dominant form under catalytic conditions. A Mn(III)(µ-O)Mn(IV) dinuclear species was observed by EPR spectroscopy, supporting the involvement of a putative Mn(IV)(O) species. First-order kinetic dependence on the manganese catalyst precludes the dinuclear species as the active form of the catalyst. Quantitative kinetic modeling enabled the deduction of a mechanism that accounts for all experimental observations. The chlorine dioxide producing cycle involves formation of a putative Mn(IV)(O), which undergoes PCET (proton coupled electron-transfer) reaction with chlorite to afford chlorine dioxide. The ClO2 product can be efficiently removed from the aqueous reaction mixture via purging with an inert gas, allowing for the preparation of pure chlorine dioxide for on-site use and further production of chlorine dioxide.