Oxygen-atom transfer reactivity of axially ligated Mn(V)-oxo complexes: evidence for enhanced electrophilic and nucleophilic pathways (original) (raw)
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Dramatic Influence of an Anionic Donor on the Oxygen-Atom Transfer Reactivity of a Mn V -Oxo Complex
Chemistry - A European Journal, 2014
Addition of an anionic donor to an Mn V (O) porphyrinoid complex causes a dramatic increase in 2-electron oxygen-atom-transfer (OAT) chemistry. The 6-coordinate [Mn V (O)(TBP 8 Cz)(CN)] À was generated from addition of Bu 4 N + CN À to the 5-coordinate Mn V (O) precursor. The cyanide-ligated complex was characterized for the first time by Mn K-edge X-ray absorption spectroscopy (XAS) and gives MnÀO = 1.53 , MnÀCN = 2.21 . In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN À complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e À -reduced Mn III (CN) À complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24 000-fold versus the same reaction for the parent 5-coordinate complex. An Eyring analysis gives DH°= 14 kcal mol À1 , DS°= À10 cal mol À1 K À1 . Computational studies fully support the structures, spin states, and relative reactivity of the 5-and 6-coordinate Mn V (O) complexes.
Inorganic Chemistry, 2012
The synthesis, structural, and spectroscopic characterization of four new coordinatively unsaturated mononuclear thiolate-ligated manganese(II) complexes ([Mn II (S Me2 N 4 (6-Me-DPEN))](BF 4 ) (1), [Mn II (S Me2 N 4 (6-Me-DPPN))](BPh 4 )·MeCN (3), [Mn I I (S M e 2 N 4 (2-QuinoPN))](PF 6 )·MeCN·Et 2 O (4), and [Mn II (S Me2 N 4 (6-H-DPEN)(MeOH)](BPh 4 ) (5)) is described, along with their magnetic, redox, and reactivity properties. These complexes are structurally related to recently reported [Mn II (S Me2 N 4 (2-QuinoEN))](PF 6 ) (2) (Coggins, M. K.; Kovacs, J. A. J. Am. Chem. Soc. 2011, 133, 12470). Dioxygen addition to complexes 1−5 is shown to result in the formation of five new rare examples of Mn(III) dimers containing a single, unsupported oxo bridge: [2 ·2MeCN . Labeling studies show that the oxo atom is derived from 18 O 2 . Ligand modifications, involving either the insertion of a methylene into the backbone or the placement of an ortho substituent on the N-heterocyclic amine, are shown to noticeably modulate the magnetic and reactivity properties. Fits to solid-state magnetic susceptibility data show that the Mn(III) ions of μ-oxo dimers 6−10 are moderately antiferromagnetically coupled, with coupling constants (2J) that fall within the expected range. Metastable intermediates, which ultimately convert to μ-oxo bridged 6 and 7, are observed in low-temperature reactions between 1 and 2 and dioxygen. Complexes 3−5, on the other hand, do not form observable intermediates, thus illustrating the effect that relatively minor ligand modifications have upon the stability of metastable dioxygen-derived species.
Equatorial Ligand Perturbations Influence the Reactivity of Manganese(IV)-Oxo Complexes
Angewandte Chemie (International ed. in English), 2017
Manganese(IV)-oxo complexes are often invoked as intermediates in Mn-catalyzed C-H bond activation reactions. While many synthetic Mn(IV) -oxo species are mild oxidants, other members of this class can attack strong C-H bonds. The basis for these reactivity differences is not well understood. Here we describe a series of Mn(IV) -oxo complexes with N5 pentadentate ligands that modulate the equatorial ligand field of the Mn(IV) center, as assessed by electronic absorption, electron paramagnetic resonance, and Mn K-edge X-ray absorption methods. Kinetic experiments show dramatic rate variations in hydrogen-atom and oxygen-atom transfer reactions, with faster rates corresponding to weaker equatorial ligand fields. For these Mn(IV) -oxo complexes, the rate enhancements are correlated with both 1) the energy of a low-lying (4) E excited state, which has been postulated to be involved in a two-state reactivity model, and 2) the Mn(III/IV) reduction potentials.
Inorganic Chemistry, 2019
The oxomanganese(IV) complex [(dpaq)Mn IV (O)] +-M n+ (1-M n+ , M n+ = redox-inactive metal ion, H-dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-ylacetamide), generated in the reaction of the precursor hydroxomanganese(III) complex 1 with iodosylbenzene (PhIO) in the presence of redox-inactive metal triflates, has recently been reported. Herein the generation of the same oxomanganese(IV) species from 1 using various combinations of protic acids and oxidants at 293 K is reported. The reaction of 1 with triflic acid and the one-electron-oxidizing agent [Ru III (bpy) 3 ] 3+ leads to the formation of the oxomanganese(IV) complex. The putative species has been identified as a mononuclear high-spin (S = 3/2) nonheme oxomanganese(IV) complex (1-O) on the basis of mass spectrometry, Raman spectroscopy, EPR spectroscopy, and DFT studies. The optical absorption spectrum is well reproduced by theoretical calculations on an S = 3/2 ground spin state of the complex. Isotope labeling studies confirm that the oxygen atom in the oxomanganese(IV) complex originates from the Mn III − OH precursor and not from water. A mechanistic investigation reveals an initial protonation step forming the Mn III −OH 2 complex, which then undergoes one-electron oxidation and subsequent deprotonations to form the oxomanganese(IV) transient, avoiding the requirements of either oxo-transfer agents or redox-inactive metal ions. The Mn IV −oxo complex cleaves the C−H bonds of xanthene (k 2 = 5.5 M −1 s −1), 9,10-DHA (k 2 = 3.9 M −1 s −1), 1,4-CHD (k 2 = 0.25 M −1 s −1), and fluorene (k 2 = 0.11 M −1 s −1) at 293 K. The electrophilic character of the nonheme Mn IV −oxo complex is demonstrated by a large negative ρ value of 2.5 in the oxidation of para-substituted thioanisoles. The complex emerges as the "most reactive" among the existing Mn IV/V −oxo complexes bearing anionic ligands.
Angewandte Chemie International Edition, 2009
Manganese(III) peroxo complexes are postulated as reactive intermediates in the reactions of Mn-containing enzymes such as manganese superoxide dismutase (Mn-SOD), catalase, and the oxygen-evolving complex (OEC) of photosystem II. [1] In biomimetic studies, a number of manganese(III)-O 2 complexes have been synthesized and characterized with various spectroscopic techniques. [2] X-ray crystal structures of heme and nonheme Mn III -peroxo complexes were also reported, such as a side-on peroxo manganese(III) porphyrin complex [Mn III (TPP)(O 2 )] À (TPP = meso-tetraphenylporphyrin) and two monomeric side-on peroxo manganese(III) complexes bearing nonheme ligands. [2c,d] The manganese(III)-peroxo complexes have shown reactivity in oxidative nucleophilic reactions with substrates such as acyl halides, aldehydes, and electron-deficient olefins. [2c, 4, 5] Axial ligands play key roles in dioxygen activation by metalloenzymes and model compounds. [6] For example, reactivities of high-valent iron-oxo intermediates in heme enzymes and iron porphyrin models are markedly affected by axial ligands trans to the iron-oxo group in electrophilic oxidation reactions. [7] Very recently, the axial ligand effect was also demonstrated in electrophilic oxidation reactions by nonheme iron(IV) and ruthenium(IV) oxo complexes. [8] In contrast, the axial ligand effect has rarely been investigated in oxidative nucleophilic reactions of metal peroxo complexes. Herein, we report the synthesis and X-ray crystal structure of a manganese(III)-peroxo complex bearing a 13-membered macrocyclic ligand, [Mn III (13-TMC)(O 2 )] + (1; 13-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclotridecane). The Xray crystal structure of 1 shows the binding of a peroxo ligand in a side-on h 2 fashion. We also report for the first time a remarkable axial ligand effect on the reactivity of the Mn IIIperoxo complex in oxidative nucleophilic reactions.
Inorganica Chimica Acta, 2006
Chemical oxidation in acetonitrile of the previously reported phenolato-bridged binuclear Mn(II) complex [(mL)MnMn(mL)] 2+ (1), where mLH is pentadentate N,N 0 -bis-(2-pyridylmethyl)-N-(2-hydroxybenzyl)-N 0 -methyl-ethane-1,2-diamine ligand [C. Hureau, et al., Chem. Eur. J. 2004, 10, 1998-2010 using iodosylbenzene PhIO (dissolved in methanol) is described. The addition of one to four equivalents of PhIO per Mn ion leads to the transient formation of the mono-l-oxo binuclear Mn 2 (III,III) complex [(mL)Mn(l-O)Mn(mL)] 2+ (2), previously studied. After addition of five equivalents of PhIO per Mn ion, the mononuclear Mn(III) species [(mL)Mn(OMe)] + (3) is quantitatively generated. The UV-Vis spectrum of 3 displays a broad band at 456 nm (e = 1000 L mol À1 cm À1 ) attributed to phenolato to Mn(III) charge transfer transition. Complex 3 exhibits a reversible oxidation wave at E 1/2 = 0.68 V versus SCE, and the mononuclear Mn(IV) complex [(mL)Mn(OMe)] 2+ (3 ox ) can thus be generated by exhaustive electrolysis at 1.0 V versus SCE. The 9.4 GHz EPR spectrum of complex 3 ox shows a strong transition near g = 4 consistent with a rhombically distorted S = 3/2 system with a zero-field splitting dominating the Zeeman effect. UV-Vis spectrum displays a large phenolato to Mn(IV) charge transfer transition at 670 nm (e = 2450 L mol À1 cm À1 ).
Generation of high-valent oxomanganese complexes through controlled removal of protons and electrons from low-valent congeners is a crucial step toward the synthesis of functional analogues of the native oxygen evolving complex (OEC). In-depth studies of the water oxidation activity of such biomimetic compounds help to understand the mechanism of O―O bond formation presumably occurring at the last step of the photosynthetic cycle. Scarce reports of reactive high-valent oxomanganese complexes underscores the impetus for the present work, wherein we report the electrochemical generation of the non-heme oxomanganese(IV) species, [(dpaq)MnIV(O)]+ (2), through a proton-coupled electron transfer (PCET) process from the hydroxomanganese complex [(dpaq)MnIII(OH)]ClO4 (1). Controlled potential spectroelectrochemical studies of 1 in wet acetonitrile at 1.45 V vs. NHE revealed quantitative formation of 2 within 10 min. The high-valent oxomanganese(IV) transient exhibited remarkable stability a...
1996
MnCI2-4H20 reacts with benzohydroxamic acid (BHAH2) in aqueous/non-aqueous solution (alkaline) forming manganese(IV) complexes of the type (PPh4) z [Mn(BHA)3 ].3H20. Its IR data suggest the hydroximato mode of coordination (O, O). This complex in frozen solution exhibits axial spectra with strong zero field splitting giving rise to a g ~ 4 signal which is more intense than that for g ~ 2. The solid, however, shows rhombic spectra. A probable reason for this difference is assigned. Trifunctional (ONS) Schiff bases HzL 1 and H2L z (5-Rsalicylaldehyde thiosemicarbazone and 5-R-salicyl aldehyde-4-phenyl thiosemicarbazone, respectively; R = H, Me or Br) furnish Mn(IV) complexes of the type [MnL2](L = L 1 or L2), whereas H2 L3 and H2 L4 (H2 L3= S-methyl 3-(5-R-2hydroxyphenyl) methylene dithiocarbazate; H2 L4= S-benzyl 3-(5-R-2-hydroxyphenyl) methylene dithiocarbazate; R = H, Me or Br) afford manganese(III) complexes of the type [MnL(O2CMe)] or [MnL(acac)] (acac = acetylacetonate, L= L 3 or L4), when treated with Mn n (O2CMe)2"4H20 or [Mnm(acac)3] in ethanol medium in air. The EPR spectra of the manganese(IV) complexes in frozen dimethyl formamide (dmf)-methanol solution show weak and strong signals at (9) ~ 4-0 and ,~ 2.0 respectively, implying a small zero field splitting. The (g) ~ 2.0 shows hyperfine (55 Mn) as well as forbidden lines. Cyclic voltammograms of these complexes scanned in DMF show reversible and quasi-reversible Mn~V-Mn H~ couple, the E°98 values of which are significantly affected by the electronic effects of the R-substituents in the salicyl phenyl ring and those attached to the carbon atom bound to the thiolate functionality of the ligands. The Hammett av values of the R-substituents are linearly correlated with the E~98 values. Keywords. Mn(III) and Mn(IV) complexes; hydroximato manganate(IV) complex; EPR of Mn(IV) complexes; ONS donor Schiffbase ligands; electrochemistry of Mn-(IV) and -(III) complexes. Seela J L, Huffman J C and Christou G 1985