MnII Complexes Research Articles

Crystal structure of a seven-coordinate manganese(II) complex with tris-(pyridin-2-ylmeth-yl)amine (TMPA).

Structural analysis of (acetato-κ2O,O')(methanol-κO)[tris-(pyridin-2-ylmeth-yl)amine-κ4N,N',N'',N''']manganese(II) tetraphenyl-borate, [Mn(C2H3O2)(C18H18N4)(CH3OH)](C24H20B) or [Mn(TMPA)(Ac)(CH3OH)]BPh4 [TMPA = tris-(pyridin-2-ylmeth-yl)amine, Ac = acetate, BPh4 = tetra-phenyl-borate] by single-crystal X-ray diffraction reveals a complex cation with tetra-dentate coordination of the tripodal TMPA ligand, bidentate coordination of the Ac ligand and monodentate coordination of the methanol ligand to a single MnII center, balanced in charge by the presence of a tetra-phenyl-borate anion. The MnII complex has a distorted penta-gonal-bipyramidal geometry, in which the central amine nitro-gen and two pyridyl N atoms of the TMPA ligand, and two oxygen atoms of the acetate ligand occupy positions in the penta-gonal plane, while the third pyridyl nitro-gen of TMPA and the oxygen from the methanol ligand occupy the axial positions. Within the complex, the acetate O atoms participate in weak C-H⋯O hydrogen-bonding inter-actions with neighboring pyridyl moieties. In the crystal, complexes form dimers by pairs of O-H⋯O hydrogen bonds between the coordinated methanol of one complex and an acetate oxygen of the other, and weak π-stacking inter-actions between pyridine rings. Separate dimers then undergo additional π-stacking inter-actions between the pyridine rings of one moiety and either the pyridine or phenyl rings of another moiety that further stabilize the crystal.

Three Polyhydroxyl-Bridged Defective Dicubane Tetranuclear MnIII Complexes: Synthesis, Crystal Structures, and Spectroscopic Properties

Three polyhydroxyl-bridged tetranuclear MnIII complexes [Mn4(L1a)2(μ3-OMe)2(μ2-OMe)2(MeOH)2] (1), [Mn4(L2a)2(μ3-OMe)2(μ2-OMe)2(H2O)2] (2), and [Mn4(L3a)2(μ3-OMe)2(μ2-OMe)2(H2O)2] (3) derived from Mnn+-promoted reactivity of Schiff base ligands (HL1 = 1-(4-{[(E)-3,5-dichlorine-2-hydroxybenzylidene]amino}phenyl)ethanone O-benzyloxime, HL2 = 1-(4-{[(E)-3-bromine-5-chloro-2-hydroxybenzylidene]amino}phenyl) ethanone O-benzyloxime, and HL3 = 1-(4-{[(E)-3,5-dibromine-2-hydroxybenzylidene]amino}phenyl)ethanone O-benzyloxime) have been synthesized and characterized. In the MnIII complexes 1, 2, and 3, the newly formed ligands (L1a)4−, (L2a)4−, and (L3a)4− are derived from the chemoselective cleavage of the C=N bond in the original Schiff base ligands HL1, HL2, and HL3 to form corresponding halogenated salicylaldehyde, 3,5-dichlorosalicylaldehyde, 3-bromine-5-chlorosalicylaldehyde, and 3,5-dibrominesalicylaldehyde, respectively. Then, the further addition of acetone to two halogenated salicylaldehyde molecules in situ α,α double aldol reaction promoted by Mnn+ ions in the presence of base to give the new ligands ((Lna)4−. X-ray crystallographic analyses of the MnIII complexes 1, 2, and 3 show that the three complexes are all tetranuclear structure and crystallizes in the triclinic system, space group P-1. The four MnIII ions and bridging alkoxido groups are arranged in a face-shared dicubane-like core with two missing vertices. In the three MnIII complexes, the asymmetric unit contains two kinds of different MnIII ions (Mn1 and Mn2), where the MnIII ions are all hexacoordinated with slightly distorted octahedral geometries. Simultaneously in the synthesis of multinuclear Mnn+ complexes above, we explored the crystal structure, spatial configuration, and spectroscopic properties of the multinuclear MnIII complexes with different halogen substituents.

Synthesis of MnII and CuII complexes including azobenzene and its application to mediators of laccase for biofuel cells

Synthesis of MnII and CuII complexes including azobenzene and its application to mediators of laccase for biofuel cells

Spin Coupling and Magnetic Anisotropy in 1D Complexes with Manganese(III) Units and Carboxylate Bridges – Synthesis, Analysis, Calculations, and Models

We report the synthesis, structure and magnetic properties of a series of MnIII complexes with N,N′‐ethylenebis(5‐methoxysalicylideneiminate) (5MeO‐salen) and acetate ligands, namely, [Mn2(5MeO‐salen)2(CH3COO)]X [X = ClO4– (1), PF6– (2), CF3SO3– (3)]. The [Mn(5MeO‐salen)]+ cations form oxo‐bridged dimers, the further assembly of which by acetate groups leads to zigzag chains. There are two alternating types of dimeric {Mn2O2} units, each of which is intrinsically ferromagnetic or anisotropic but mutually coupled antiferromagnetically along the chain. Magnetostructural analyses were performed with the help of ab initio calculations to elucidate the subtle effects of local magnetic anisotropy combined with exchange effects. A qualitative mechanism for the exchange coupling is advanced on the basis of the ab initio calculations to explain the ferromagnetism inside the phenoxo‐bridged dimeric units and the antiferromagnetic behaviour of the acetate bridge.

In vitro antimicrobial activity evaluation of newly synthesized furochromone Schiff base complexes

A series of VOII, NiII, CuII, CoII, MnII, CdII and ZnII complexes of new Schiff base ligand resulting form 4-aminoantipyrine and Furobenzopyrone were prepared. The structure of newly synthesized compounds (1-8) was proven by studying their elemental analyses, spectral data (IR, NMR, UV-vis), molar conductivity and magnetic susceptibility. In addition the in vitro antimicrobial activi-ty of all compounds was examined. The results cleared that nickel and zinc complexes exhibited promising activity compared with standard drug (chloramphenicol).

Two isomorphous azide/formate Mn(ii) coordination polymers show spin-canted antiferromagnetism only in the formate system

We reported two isomorphous azide/formate MnII complexes: the azide complex had normal antiferromagnetic coupling, while a canted antiferromagnet with a Néel temperature of TN = 38.5 K was obtained in the formate system.

MnIII-Peroxo adduct supported by a new tetradentate ligand shows acid-sensitive aldehyde deformylation reactivity.

The new tetradentate L7BQ ligand (L7BQ = 1,4-di(quinoline-8-yl)-1,4-diazepane) has been synthesized and shown to support MnII and MnIII-peroxo complexes. X-ray crystallography of the [MnII(L7BQ)(OTf)2] complex shows a monomeric MnII center with the L7BQ ligand providing four donor nitrogen atoms in the equatorial field, with two triflate ions bound in the axial positions. When this species is treated with H2O2 and Et3N at -40 °C, a MnIII-peroxo adduct, [MnIII(O2)(L7BQ)]+ is formed. The formation of this new intermediate is supported by a variety of spectroscopic techniques, including electronic absorption, Mn K-edge X-ray absorption and electron paramagnetic resonance methods. Evaluation of extended X-ray absorption fine structure data for [MnIII(O2)(L7BQ)]+ resolved Mn-O bond distances of 1.85 Å, which are on the short end of those previously reported for crystallographically characterized MnIII-peroxo adducts. An analysis of the X-ray pre-edge region of [MnIII(O2)(L7BQ)]+ revealed a large pre-edge area of 20.8 units. Time-dependent density functional theory computations indicate that the pre-edge intensity is due to Mn 4p-3d mixing caused by geometric distortions from centrosymmetry induced by both the peroxo and L7BQ ligands. The reactivity of [MnIII(O2)(L7BQ)]+ towards aldehydes was assessed through reaction with cyclohexanecarboxaldehyde and 2-phenylpropionaldehyde. From these experiments, it was determined that [MnIII(O2)(L7BQ)]+ only reacts with aldehydes in the presence of acid. Specifically, the addition of cyclohexanecarboxylic acid to [MnIII(O2)(L7BQ)]+ converts the MnIII-peroxo adduct to a new intermediate that could be responsible for the observed aldehyde deformylation activity. These observations underscore the challenges in identifying the reactive metal species in aldehyde deformylation reactions.

Oxoiron(IV) complexes as synthons for the assembly of heterobimetallic centers such as the Fe/Mn active site of Class Ic ribonucleotide reductases.

Nonheme oxoiron(IV) complexes can serve as synthons for generating heterobimetallic oxo-bridged dimetal complexes by reaction with divalent metal complexes. The formation of FeIII-O-CrIII and FeIII-O-MnIII complexes is described herein. The latter complexes may serve as models for the FeIII-X-MnIII active sites of an emerging class of Fe/Mn enzymes represented by the Class 1c ribonucleotide reductase from Chlamydia trachomatis and the R2-like ligand-binding oxidase (R2lox) found in Mycobacterium tuberculosis. These synthetic complexes have been characterized by UV-Vis, resonance Raman, and X-ray absorption spectroscopy, as well as electrospray mass spectrometry. The FeIII-O-CrIII complexes exhibit a three-band UV-Vis pattern that differs from the simpler features associated with FeIII-O-FeIII complexes. The positions of these features are modulated by the nature of the supporting polydentate ligand on the iron center, and their bands intensify dramatically in two examples upon the binding of an axial cyanate or thiocyanate ligand trans to the oxo bridge. In contrast, the FeIII-O-MnIII complexes resemble FeIII-O-FeIII complexes more closely. Resonance Raman characterization of the FeIII-O-MIII complexes reveals an 18O-sensitive vibration in the range of 760-890cm-1. This feature has been assigned to the asymmetric FeIII-O-MIII stretching mode and correlates reasonably with the Fe-O bond distance determined by EXAFS analysis. The likely binding of an acetate as a bridging ligand to the FeIII-O-MnIII complex 12 lays the foundation for further efforts to model the heterobimetallic active sites of Fe/Mn enzymes.

Dimeric and Trimeric Supramolecular Systems Formed by Donor‒Acceptor Interactions of ZnII, MnIII, and SnIV Porphyrin Complexes

The paper presents the methods of synthesis of dimeric and trimeric compounds formed by donor–acceptor interactions of ZnII, MnIII, and SnIV porphyrin complexes. The structure of the porphyrin ensembles was determined by 1H NMR spectroscopy. The extra coordination properties of the synthesized dimeric and trimeric supramolecular systems in relation to nitrogen-containing compounds were studied and the stability constants of the resulting porphyrin extra complexes were determined.

Evidence for involvement of cationic intermediate in epoxidation of chiral allylic alcohols and unfunctionalised alkenes catalysed by MnIII(quinazolinone) complexes

A pair of MnIII(quinazolinone) complexes was prepared and evaluated in the catalytic epoxidation of the chiral allylic alcohols and unfunctionalised alkenes with iodosylbenzene. Epoxidation of chiral allylic alcohols with 1,3-allylic strain proceeded chemo- and diastereoselectively to give threo-epoxy alcohol (up to 99% d.r). Epoxidation of unfunctionalised alkenes by the proposed catalyst system proceeded nonselectively, as evidenced by the formation of isomerization and rearrangement products in the epoxidation of (Z)-stilbene. A three-step pathway involving a cationic intermediate is proposed for the formation of isomerization and rearrangement products. The fact that only products resulting from the cationic intermediate were detected in the oxidation of a mechanistic probe, (2-methoxy-3-vinylcyclopropyl)benzene, with MnIII(quinazolinone) and PhIO substantiated the cationic oxygen transfer mechanism.

Mn K-edge X-ray absorption studies of mononuclear Mn(III)-hydroxo complexes.

Mn K-edge X-ray absorption spectroscopy experiments were performed on the solid- and solution-phase samples of [MnII(dpaqR)](OTf) (R=H, Me) and [MnIII(OH)(dpaqR)](OTf). The extended X-ray absorption fine structure (EXAFS) data show distinct differences between the MnII and MnIII-OH complexes, with fits providing metric parameters in excellent agreement with values from X-ray crystallography and density functional theory (DFT) computations. Evaluation of the EXAFS data for solid-phase [MnIII(OH)(dpaq)](OTf) resolved a short Mn-OH bond distance of 1.79Å; however, the short trans-amide nitrogen bond of the supporting ligand precluded the resolution of the Mn-OH bond distance in the corresponding solution-phase sample and for both [MnIII(OH)(dpaqMe)](OTf) samples. The edge energy also increases by approximately 2eV from the MnII to the MnIII-OH complexes. Experimental pre-edge analysis shows the MnII complexes to have pre-edge areas comparable to the MnIII-OH complexes, despite the presence of the relatively short Mn-OH distance. Time-dependent density functional theory (TD-DFT) computations illustrate that Mn 3d-4p mixing, a primary contributor to pre-edge intensities, decreases by~0.3% from the MnII to MnIII-OH complexes, which accounts for the very similar pre-edge areas. Collectively, this work shows that combined EXAFS and XANES analysis has great potential for identification of reactive MnIII-OH intermediates, such as those proposed in enzyme active sites.

Synthesis, Crystal Structure and Magnetic Properties of [Mn((1R, 2R)‐Salcy)N3/NCS] Complexes: Solvent Dependent Crystallization of Monomers, Chains and Dimers.

AbstractFive new chiral MnIII complexes [Mn((1R, 2R)‐Salcy)N3]∞⋅CH3CN (1), [Mn((R,R)‐Salcy)N3]∞ (2), [Mn((R,R)‐Salcy)N3] (4), [Mn((R,R)‐Salcy)(H2O)N3]⋅C4H8O (5) and [Mn((R,R)‐Salcy)NCS] (6) have been synthesized and characterized by X‐ray structural analysis and magnetic measurements (Salcy=N,N′‐bis(salicylidene)‐1,2‐diaminocyclohexane}. The four azido complexes have been obtained from the previously known [Mn((R,R)‐Salcy)(CH3OH)N3] (3) by crystallization from various solvents. Compounds 1 and 2 are trans‐μ‐1,3‐azide bridged coordination polymers. While 1 forms helical chains, the chain in 2 is formed via translation of a dimeric helicate repeat unit. Compound 3 is a methanol coordinated mononuclear azido complex which forms a 2D‐network through H‐bonding. Compounds 4 and 6 are phenoxo‐bridged dimers without inversion center. Compound 5 is an aqua coordinated mononuclear azido complex, which forms a 1D‐chain through O−H⋅⋅⋅N hydrogen bonding involving axial ligands. In all six compounds MnIII exhibits a characteristic Jahn−Teller elongation. The magnetic coupling in 1 and 2 leads to antiferromagnetic interaction along the chain (2 J=−11.6 cm−1 for 1 and –11.9 cm−1 for 2) through the μ‐1,3‐azide bridge. Compound 3 is weakly antiferromagnetic due to hydrogen bonding interactions in the lattice. The phenoxo dimers (4 and 6) do not have measurable exchange interaction, the low temperature susceptibility being influenced mainly by zero‐field splitting.

Facile total synthesis of lysicamine and the anticancer activities of the RuII, RhIII, MnII and ZnII complexes of lysicamine.

Lysicamine is a natural oxoaporphine alkaloid, which isolated from traditional Chinese medicine (TCM) herbs and has been shown to possess cytotoxicity to hepatocarcinoma cell lines. Reports on its antitumor activity are scarce because lysicamine occurs in plants at a low content. In this work, we demonstrate a facile concise total synthesis of lysicamine from simple raw materials under mild reaction conditions, and the preparation of the Ru(II), Rh(III), Mn(II) and Zn(II) complexes 1–4 of lysicamine (LY). All the compounds were fully characterized by elemental analysis, IR, ESI-MS, 1H and 13C NMR, as well as single-crystal X-ray diffraction analysis. Compared with the free ligand LY, complexes 2 and 3 exhibited superior in vitro cytotoxicity against HepG2 and NCI-H460. Mechanistic studies indicated that 2 and 3 blocked the cell cycle in the S phase by decreasing of cyclins A2/B1/D1/E1, CDK 2/6, and PCNA levels and increasing levels of p21, p27, p53 and CDC25A proteins. In addition, 2 and 3 induced cell apoptosis via both the caspase-dependent mitochondrial pathway and the death receptor pathway. in vivo study showed that 2 inhibited HepG2 tumor growth at 1/3 maximum tolerated dose (MTD) and had a better safety profile than cisplatin.

Novel CDTA-based, Bifunctional Chelators for Stable and Inert MnII Complexation: Synthesis and Physicochemical Characterization.

In the search for MnII MR and PET/MR imaging agents with optimal balance between thermodynamic stability, kinetic inertness, and relaxivity, two novel bifunctional MnII chelators (BFMnCs) based on CDTA (trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid) were synthesized. A six-step synthesis, involving the buildup of a functionalized trans-1,2-diaminocyclohexane core, provided CuAAC-reactive 6a and 6b bearing an alkyne or azide substituent on the cyclohexane ring, respectively (CuAAC = CuI-catalyzed azide-alkyne 1,3-dipolar cycloaddition). Thermodynamic, kinetic, and relaxometric studies were performed with 4-HET-CDTA (8a) as a "model chelator," synthesized in two steps from 6a. The protonation constants revealed that 8a is slightly less basic than CDTA and forms a MnII complex of marginally lower thermodynamic stability (log KMnL = 13.80 vs 14.32, respectively), while the conditional stability constant is almost identical for both chelates (pMn = 8.62 vs 8.68, respectively). Kinetic assessment of the CuII-mediated transmetalation of [Mn(4-HET-CDTA)]2- showed that proton-assisted complex dissociation is slightly slower than for [Mn(CDTA)]2- (k1 = 297 vs 400 M-1 s-1, respectively). Importantly, the dissociation half-life near physiological conditions (pH 7.4, 25 °C) underlined that [Mn(4-HET-CDTA)]2- is ∼35% more inert (t1/2 = 16.2 vs 12.1 h, respectively). Those findings may be accounted for by a combination of reduced basicity and increased rigidity of the ligand. Analysis of the 17O NMR and 1H NMRD data attributed the high relaxivity of [Mn(4-HET-CDTA)]2- (r1 = 4.56 mM-1 s-1 vs 3.65 mM-1 s-1 for [Mn(CDTA)]2-; 20 MHz, 25 °C) to slower rotational dynamics (τR298 = 105 ps). Additionally, the fast water exchange of the complex correlates well with the value reported for [Mn(CDTA)]2- (kex298 = 17.6 × 107 vs 14.0 × 107 s-1, respectively). Given the exquisite compromise between thermodynamic stability, kinetic inertness, and relaxivity achieved by [Mn(4-HET-CDTA)]2-, appropriately designed CuAAC-conjugates of 6a/6b are promising precursors for the preparation of targeted, bioresponsive, or high relaxivity manganese-based PET/MR tracers (52g/55 MnII) and MR contrast agents (MnII).

Synthesis, crystal structures and magnetic properties of cyanide-bridged heterobimetallic trinuclear Cr 2 III MnII complexes based on the cis-dicyanidemetalate [Cr(2,2′-bipy)2(CN)2]ClO4 building block

A structurally characterized cis-dicyanidemetalate [Cr(2,2′-bipy)2(CN)2]ClO4 (1) building block and two mononuclear seven-coordinate macrocyclic manganese(II) complexes have been employed to assemble two cyanide-bridged heterometallic CrIII–MnIII complexes: {[Mn(L1)][Cr(2,2′-bipy)2(CN)2]2}{ClO4}4·H2O (2) and {[Mn(L2)][Cr(2,2′-bipy)2(CN)2]2}{ClO4}4·2H2O (3) (L1 = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene, L2 = 2,13-dimethyl-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene). Single-crystal X-ray diffraction analysis shows that 2 and 3 have similar cyanide-bridged cationic trinuclear Cr2Mn structures with free ClO4 − as balancing anions, in which the cyanide precursor acts as a monodentate ligand to connect the macrocyclic manganese(II) units. The coordination geometry of the manganese(II) centers in the two complexes is slightly distorted pentagonal-bipyramidal, with two cyanide nitrogen atoms at the trans-positions and N5 or N3O2 donor sets in the equatorial plane provided by the macrocyclic ligand. Investigation of the magnetic properties reveals antiferromagnetic magnetic coupling between the cyanide-bridged Cr(III) and Mn(II) centers. A best-fit analysis of the magnetic susceptibility leads to the magnetic coupling constants J = −1.15 and −1.08 cm−1 for complexes 2 and 3, respectively.

Multi-Electron Oxidation of Anthracene Derivatives by Nonheme Manganese(IV)-Oxo Complexes.

Six-electron oxidation of anthracene to anthraquinone by a nonheme MnIV -oxo complex, [(Bn-TPEN)MnIV (O)]2+ , proceeds through a rate-determining electron transfer from anthracene to [(Bn-TPEN)MnIV (O)]2+ , followed by subsequent fast oxidation reactions to give anthraquinone. The reduced MnII complex ([(Bn-TPEN)MnII ]2+ ) is oxidized by [(Bn-TPEN)MnIV (O)]2+ rapidly to produce the μ-oxo dimer ([(Bn-TPEN)MnIII -O-MnIII (Bn-TPEN)]4+ ). The oxygen atoms of the anthraquinone product were found to derive from the manganese-oxo species by the 18 O-labelling experiments. In the presence of Sc3+ ion, formation of an anthracene radical cation was directly detected in the electron transfer from anthracene to a Sc3+ ion-bound MnIV (O) complex, [(Bn-TPEN)MnIV (O)-(Sc(OTf)3 )2 ]2+ , followed by subsequent further oxidation to yield anthraquinone. When anthracene was replaced by 9,10-dimethylanthracene, electron transfer from 9,10-dimethylanthracene to [(Bn-TPEN)MnIV (O)-(Sc(OTf)3 )2 ]2+ occurred rapidly to produce stable 9,10-dimethylanthracene radical cation. The driving force dependence of the rate constants of electron transfer from the anthracene derivatives to [(Bn-TPEN)MnIV (O)]2+ and [(Bn-TPEN)MnIV (O)-(Sc(OTf)3 )2 ]2+ was well-evaluated in light of the Marcus theory of electron transfer.

Synthesis, Structure, and Magnetic Properties of a Mononuclear Chiral (Acetato)bis(aminoalkoxido)manganese(III) Complex

A new synthetic procedure was developed to obtain a novel mononuclear chiral square‐pyramidal (acetato)bis(aminoalkoxido)MnIII complex from a heterometallic ZnII/MnII system with a crucial role for O2 in the process. The resulting compound was characterized spectroscopically and crystallographically, and its magnetic properties were determined. The MnIII ion exhibits magnetic anisotropy with a zero‐field splitting D parameter equal to –2.95 cm–1.

The first coordination compounds of OP[NC4H8O]3 phosphoric triamide ligand: structural study and Hirshfeld surface analysis of SnIV and MnII complexes

The first X-ray crystal structures of coordination compounds of OP[NC4H8O]3 phosphoric triamide (L) are investigated in Cl2(CH3)2Sn(trans-L)2 (1) and [Mn(H2O)4(trans-L)2]Cl2·2H2O (2) as models of molecular and salt complexes for Hirshfeld surface (HS)-based analysis. The crystal packing of 1 includes weak interactions, while in the salt complex 2, a 2-D aggregate, along the (001) plane, is mediated by normal O–H⋯Cl and O–H⋯O hydrogen bonds. In the Hirshfeld study, the crystal cohesions of 1 and 2 are recognized via H⋯H, O⋯H/H⋯O, and Cl⋯H/H⋯Cl contacts. Among these interactions, hydrogen bonds O–H⋯Cl occur in the salt structure of 2, as well as some weaker hydrogen interactions as C–H⋯O (1 and 2), C–H⋯Cl (1), and O–H⋯O (2). The full fingerprint plots have nearly symmetric shapes for two independent molecules of 1, while an asymmetric shape appears for the cationic component of 2. To extract more detailed information on close intermolecular contacts, the molecular surface of the previously reported structure L was also mapped. The structure 2 is the first monomeric octahedral Mn(II)–phosphoric triamide complex reported so far. Furthermore, the HS analysis of 2 is the first such study on a cation–anion complex structure including phosphoric triamide ligand.

One-Pot Synthesis of a bis-Pocket Corrole through a 14-fold Bromination Reaction.

A one‐pot protocol, effecting 14‐fold bromination with elemental bromine, has afforded copper β‐octabromo‐meso‐tris(2,6‐dibromo‐3,5‐dimethoxyphenyl)corrole, a new bis‐pocket metallocorrole. The Cu complex underwent smooth demetalation under reductive conditions, affording the free corrole ligand, which in turn could be readily complexed to MnIII and AuIII. A single‐crystal X‐ray structure was obtained for the MnIII complex.

Switching between Inner- and Outer-Sphere PCET Mechanisms of Small-Molecule Activation: Superoxide Dismutation and Oxygen/Superoxide Reduction Reactivity Deriving from the Same Manganese Complex.

Readily exchangeable water molecules are commonly found in the active sites of oxidoreductases, yet the overwhelming majority of studies on small-molecule mimics of these enzymes entirely ignores the contribution of water to the reactivity. Studies of how these enzymes can continue to function in spite of the presence of highly oxidizing species are likewise limited. The mononuclear MnII complex with the potentially hexadentate ligand N-(2-hydroxy-5-methylbenzyl)-N,N',N'-tris(2-pyridinylmethyl)-1,2-ethanediamine (LOH) was previously found to act as both a H2O2-responsive MRI contrast agent and a mimic of superoxide dismutase (SOD). Here, we studied this complex in aqueous solutions at different pH values in order to determine its (i) acid-base equilibria, (ii) coordination equilibria, (iii) substitution lability and operative mechanisms for water exchange, (iv) redox behavior and ability to participate in proton-coupled electron transfer (PCET) reactions, (v) SOD activity and reductive activity toward both oxygen and superoxide, and (vi) mechanism for its transformation into the binuclear MnII complex with (H)OL-LOH and its hydroxylated derivatives. The conclusions drawn from potentiometric titrations, low-temperature mass spectrometry, temperature- and pressure-dependent 17O NMR spectroscopy, electrochemistry, stopped-flow kinetic analyses, and EPR measurements were supported by the structural characterization and quantum chemical analysis of proposed intermediate species. These comprehensive studies enabled us to determine how transiently bound water molecules impact the rate and mechanism of SOD catalysis. Metal-bound water molecules facilitate the PCET necessary for outer-sphere SOD activity. The absence of the water ligand, conversely, enables the inner-sphere reduction of both superoxide and dioxygen. The LOH complex maintains its SOD activity in the presence of •OH and MnIV-oxo species by channeling these oxidants toward the synthesis of a functionally equivalent binuclear MnII species.