Comparison of Nonheme Manganese- and Iron-Containing Flavone Synthase Mimics

Heme and nonheme-type flavone synthase enzymes, FS I and FS II are responsible for the synthesis of flavones, which play an important role in various biological processes, and have a wide range of biomedicinal properties including antitumor, antimalarial, and antioxidant activities. To get more insight into the mechanism of this curious enzyme reaction, nonheme structural and functional models were carried out by the use of mononuclear iron, [Fe^II(CDA-BPA*)]²⁺ (6) [CDA-BPA = N,N,N’,N’-tetrakis-(2-pyridylmethyl)-cyclohexanediamine], [Fe^II(CDA-BQA*)]²⁺ (5) [CDA-BQA = N,N,N’,N’-tetrakis-(2-quinolilmethyl)-cyclohexanediamine], [Fe^II(Bn-TPEN)(CH₃CN)]²⁺ (3) [Bn-TPEN = N-benzyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-diaminoethane], [Fe^IV(O)(Bn-TPEN)]²⁺ (9), and manganese, [Mn^II(N4Py*)(CH₃CN)]²⁺ (2) [N4Py* = N,N-bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethylamine)], [Mn^II(Bn-TPEN)(CH₃CN)]²⁺ (4) complexes as catalysts, where the possible reactive intermediates, high-valent Fe^IV(O) and Mn^IV(O) are known and well characterised. The results of the catalytic and stoichiometric reactions showed that the ligand framework and the nature of the metal cofactor significantly influenced the reactivity of the catalyst and its intermediate. Comparing the reactions of [Fe^IV(O)(Bn-TPEN)]²⁺ (9) and [Mn^IV(O)(Bn-TPEN)]²⁺ (10) towards flavanone under the same conditions, a 3.5-fold difference in reaction rate was observed in favor of iron, and this value is three orders of magnitude higher than was observed for the previously published [Fe^IV(O)(N2Py2Q*)]²⁺ [N,N-bis(2-quinolylmethyl)-1,2-di(2-pyridyl)ethylamine] species.     

Tuning the reactivity of mononuclear nonheme manganese(iv)-oxo complexes by triflic acid

Triflic acid (HOTf)-bound nonheme Mn(iv)-oxo complexes, [(L)Mn^IV(O)]²⁺–(HOTf)₂ (L = N4Py and Bn-TPEN; N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine and Bn-TPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine), were synthesized by adding HOTf to the solutions of the [(L)Mn^IV(O)]²⁺ complexes and were characterized by various spectroscopies. The one-electron reduction potentials of the Mn^IV(O) complexes exhibited a significant positive shift upon binding of HOTf. The driving force dependences of electron transfer (ET) from electron donors to the Mn^IV(O) and Mn^IV(O)–(HOTf)₂ complexes were examined and evaluated in light of the Marcus theory of ET to determine the reorganization energies of ET. The smaller reorganization energies and much more positive reduction potentials of the [(L)Mn^IV(O)]²⁺–(HOTf)₂ complexes resulted in greatly enhanced oxidation capacity towards one-electron reductants and para-X-substituted-thioanisoles. The reactivities of the Mn(iv)-oxo complexes were markedly enhanced by binding of HOTf, such as a 6.4 × 10⁵-fold increase in the oxygen atom transfer (OAT) reaction (i.e., sulfoxidation). Such a remarkable acceleration in the OAT reaction results from the enhancement of ET from para-X-substituted-thioanisoles to the Mn^IV(O) complexes as revealed by the unified ET driving force dependence of the rate constants of OAT and ET reactions of [(L)Mn^IV(O)]²⁺–(HOTf)₂. In contrast, deceleration was observed in the rate of H-atom transfer (HAT) reaction of [(L)Mn^IV(O)]²⁺–(HOTf)₂ complexes with 1,4-cyclohexadiene as compared with those of the [(L)Mn^IV(O)]²⁺ complexes. Thus, the binding of two HOTf molecules to the Mn^IV(O) moiety resulted in remarkable acceleration of the ET rate when the ET is thermodynamically feasible. When the ET reaction is highly endergonic, the rate of the HAT reaction is decelerated due to the steric effect of the counter anion of HOTf.     

Efficient Olefin Epoxidation by Robust Re4 Cluster‐Supported MnIII Complexes with Peracids: Evidence of Simultaneous Operation of Multiple Active Oxidant Species,MnVO,MnIVO,and MnIII?OOC(O)R

Two new tetranuclear chalcocyanide cluster complexes, [{Mn(saloph)H₂O}₄Re₄Q₄(CN)₁₂]?4 CH₃OH? 8 H₂O (saloph=N,N′‐o‐phenylenebis(salicylidenaminato), Q=Se ( 1 ‐Se), Te ( 2 ‐Te)), have been synthesized by the diffusion of a methanolic solution of [PPh₄]₄[Re₄Q₄(CN)₁₂] into a methanolic solution of [Mn(saloph)]⁺. The structure of 2 ‐Te has been determined by X‐ray crystallography. These rhenium cluster‐supported [Mn^III(saloph)] complexes have been found to efficiently catalyze a wide range of olefin epoxidations under mild experimental conditions in the presence of meta‐chloroperbenzoic acid (mCPBA). Olefin epoxidation by these catalysts is proposed to involve the multiple active oxidants Mn^V?O, Mn^IV?O, and Mn^III? OOC(O)R. Evidence in support of this interpretation has been derived from reactivity and Hammett studies, H₂¹⁸O‐exchange experiments, and the use of peroxyphenylacetic acid as a mechanistic probe. Moreover, it has been observed that the participation of Mn^V?O, Mn^IV?O, and Mn^III? OOC(O)R can be controlled by changing the substrate concentration. This mechanism provides the greatest congruity with related oxidation reactions that employ certain Mn complexes as catalysts.     

Intramolecular Gas‐Phase Reactions of Synthetic Nonheme Oxoiron(IV) Ions: Proximity and Spin‐State Reactivity Rules

The intramolecular gas‐phase reactivity of four oxoiron(IV) complexes supported by tetradentate N₄ ligands ( L ) has been studied by means of tandem mass spectrometry measurements in which the gas‐phase ions [Fe^IV(O)( L )(OTf)]⁺ (OTf=trifluoromethanesulfonate) and [Fe^IV(O)( L )]²⁺ were isolated and then allowed to fragment by collision‐induced decay (CID). CID fragmentation of cations derived from oxoiron(IV) complexes of 1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane (tmc) and N,N′‐bis(2‐pyridylmethyl)‐1,5‐diazacyclooctane ( L ⁸Py₂) afforded the same predominant products irrespective of whether they were hexacoordinate or pentacoordinate. These products resulted from the loss of water by dehydrogenation of ethylene or propylene linkers on the tetradentate ligand. In contrast, CID fragmentation of ions derived from oxoiron(IV) complexes of linear tetradentate ligands N,N′‐bis(2‐pyridylmethyl)‐1,2‐diaminoethane (bpmen) and N,N′‐bis(2‐pyridylmethyl)‐1,3‐diaminopropane (bpmpn) showed predominant oxidative N‐dealkylation for the hexacoordinate [Fe^IV(O)( L )(OTf)]⁺ cations and predominant dehydrogenation of the diaminoethane/propane backbone for the pentacoordinate [Fe^IV(O)( L )]²⁺ cations. DFT calculations on [Fe^IV(O)(bpmen)] ions showed that the experimentally observed preference for oxidative N‐dealkylation versus dehydrogenation of the diaminoethane linker for the hexa‐ and pentacoordinate ions, respectively, is dictated by the proximity of the target C? H bond to the oxoiron(IV) moiety and the reactive spin state. Therefore, there must be a difference in ligand topology between the two ions. More importantly, despite the constraints on the geometries of the TS that prohibit the usual upright σ trajectory and prevent optimal σ_CH–σ* overlap, all the reactions still proceed preferentially on the quintet (S=2) state surface, which increases the number of exchange interactions in the d block of iron and leads thereby to exchange enhanced reactivity (EER). As such, EER is responsible for the dominance of the S=2 reactions for both hexa‐ and pentacoordinate complexes.     

Spectroscopic,Electrochemical and Computational Characterisation of Ru Species Involved in Catalytic Water Oxidation: Evidence for a [RuV(O)(Py2Metacn)] Intermediate

A new family of ruthenium complexes based on the N‐pentadentate ligand Py₂^Metacn (N‐methyl‐N′,N′′‐bis(2‐picolyl)‐1,4,7‐triazacyclononane) has been synthesised and its catalytic activity has been studied in the water‐oxidation (WO) reaction. We have used chemical oxidants (ceric ammonium nitrate and NaIO₄) to generate the WO intermediates [Ru^II(OH₂)(Py₂^Metacn)]²⁺, [Ru^III(OH₂)(Py₂^Metacn)]³⁺, [Ru^III(OH)(Py₂^Metacn)]²⁺ and [Ru^IV(O)(Py₂^Metacn)]²⁺, which have been characterised spectroscopically. Their relative redox and pH stability in water has been studied by using UV/Vis and NMR spectroscopies, HRMS and spectroelectrochemistry. [Ru^IV(O)(Py₂^Metacn)]²⁺ has a long half‐life (>48 h) in water. The catalytic cycle of WO has been elucidated by using kinetic, spectroscopic, ¹⁸O‐labelling and theoretical studies, and the conclusion is that the rate‐determining step is a single‐site water nucleophilic attack on a metal‐oxo species. Moreover, [Ru^IV(O)(Py₂^Metacn)]²⁺ is proposed to be the resting state under catalytic conditions. By monitoring Ce^IV consumption, we found that the O₂ evolution rate is redox‐controlled and independent of the initial concentration of Ce^IV. Based on these facts, we propose herein that [Ru^IV(O)(Py₂^Metacn)]²⁺ is oxidised to [Ru^V(O)(Py₂^Metacn)]²⁺ prior to attack by a water molecule to give [Ru^III(OOH)(Py₂^Metacn)]²⁺. Finally, it is shown that the difference in WO reactivity between the homologous iron and ruthenium [M(OH₂)(Py₂^Metacn)]²⁺ (M=Ru, Fe) complexes is due to the difference in the redox stability of the key M^V(O) intermediate. These results contribute to a better understanding of the WO mechanism and the differences between iron and ruthenium complexes in WO reactions.     

Kinetic and Product Study of the S-oxidation vs HAT Chemoselectivity in Reactions Promoted by Nonheme Iron(IV)-oxo Complex/NHPI Mediator System

The chemoselectivity between S-oxidation and hydrogen atom transfer (HAT) from C−H bonds has been investigated in the oxidations of a series of aryl sulfides, alkyl aromatic compounds and benzylic alcohols promoted by the iron(IV)-oxo complex [(N4Py)Fe^IV(O)]²⁺ (N4Py: N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)-methylamine) either alone or in the presence of the N-hydroxyphthalimide (NHPI) mediator via kinetic and product studies. Kinetic analyses indicate a generally higher reactivity of [(N4Py)Fe^IV(O)]²⁺ for S-oxidation process while HAT is favored in the reactions promoted by phthalimide-N-oxyl radical (PINO) deriving from NHPI oxidation. Product analysis in intermolecular competitive oxidations confirms the kinetic results with sulfoxides obtained as major products in the oxidation promoted by [(N4Py)Fe^IV(O)]²⁺. Conversely, when NHPI is employed as a mediator, significant differences in terms of chemoselectivity are observed, and HAT-derived products are obtained in higher yields which translate into an inversion of selectivity in the case of the substrates containing activated C−H bonds like diphenylmethane, triphenylmethane and benzylic alcohols. A similar change of chemoselectivity is also observed in the oxidation of aromatic substrates containing both a sulfur atom and α to OH benzylic C−H bonds, with the sulfoxide product more abundant in the absence of NHPI and carbonyl products prevailing with the [(N4Py)Fe^IV(O)]²⁺/NHPI system.     

First MnIII complexes with tetradentate (N2O2) Schiff bases and tricyanomethanide: synthesis,crystal structure,and magnetic properties

The first Mn^III complexes with Schiff bases and tricyanomethanide-anion were synthesized: [Mn(salen)C(CN)₃(H₂O)] (1), [Mn(5-Brsalen)C(CN)₃(H₂O)] (2), [Mn(salpn)C(CN)₃(H₂O)] (3), [Mn(3-MeOsalen)C(CN)₃(H₂O)] (4), [Mn(5-Brsalen)(MeOH)(H₂O)][C(CN)₃] (5), and [Mn(3-MeOsalpn)(H₂O)₂][C(CN)₃] (6), where SalenH₂ is N,N′-bis(salicylidene)ethylenediamine, 5-BrsalenH₂ is N,N′-bis(5-bromosalicylidene)ethylenediamine, SalpnH₂ is N,N′-bis-(salicylidene)-1,3-diaminopropane, 3-MeOsalenH₂ is N,N′-bis(3-methoxysalicylidene)-ethylenediamine, 3-MeOsalpnH₂ — N,N′-bis(3-methoxysalicylidene)-1,3-diaminopropane. The tricyanomethanide anion in complexes 1–4 acts as a the terminal ligand, whereas in complexes 5 and 6 tricyanomethanide is not coordinated by Mn^III and acts as an out-of-sphere counterion. The structures of complexes 1–4 are characterized by the formation of dimers due to hydrogen bonds between the water molecules and oxygen atoms of the Schiff bases. The Mn...Mn distances inside the dimers are 4.69–5.41 Å. Complex 6 has a zigzag chain structure consisting of the [Mn(3-MeOsalpn)(H₂O)₂]⁺ cations bound by double bridging aqua ligands. The study of the magnetic properties of complexes 1, 3, 4, and 6 showed the existence of antiferromagnetic interactions between the Mn^III ions through the system of hydrogen bonds.     

Synthesis,spectroscopic and X-ray crystallographic properties of manganese compounds of keto- and enol-coordinated di-2-pyridyl ketone benzoyl hydrazone (dpkbh). Reactions of [Mn(CO)5Br] with dpkbh

Reactions between [Mn(CO)₅Br] and dpkbh in low boiling solvents in air gave fac-[Mn^I(CO)₃(κ²-N_py,N_im-dpkbh)Br]·H₂O, [Mn^IIBr₂(κ³-N_py,N_im,O-dpkbh)], and [Mn^II(κ³-N_py,N_im,O-dpkbh-H)₂]·0.5H₂O (N_im = imine nitrogen and N_py = pyridyl nitrogen). Crystallization of fac-[Mn^I(CO)₃(κ²-N_py,N_im-dpkbh)Br]·H₂O from dmso or CH₃CN produced dark red crystals of [Mn^II(κ³-N_py,N_im,O-dpkbh-H)₂]·nX (X = dmso, n = 1 and X = H₂O, n = 0.22). This is in contrast to the reaction of [Re(CO)₅Cl] with dpkbh in refluxing toluene to form fac-[Re^I(CO)₃(κ²-,N_py,N_py-dpkbh)Cl] which can be crystallized from CH₃CN, dmso or dmf to form fac-[Re^I(CO)₃(κ²-,N_py,N_py-dpkbh)Cl]·nX (X = CH₃CN, n = 0 and solvate = dmso or dmf, n = 1). Infrared spectral measurements are consistent with keto coordination of dpkbh to Mn(I) in fac-[Mn^I(CO)₃(κ²-N_py,N_im-dpkbh)Br]·H₂O and Mn(II) in [Mn^IIBr₂(κ³-N_py,N_im,O-dpkbh)] plus enol coordination of the amide-deprotonated dpkbh, to the Mn(II) center in [Mn^II(κ³-N_py,N_im,O-dpkbh-H)₂]·0.5H₂O. Electronic absorption spectral measurements in non-aqueous solvents indicate sensitivity of fac-[Mn^I(CO)₃(κ²-N_py,N_im-dpkbh)Br]·H₂O and [Mn^II(κ³-N_py,N_im,O-dpkbh-H)₂]·0.5H₂O to changes in their outer-shell environments. X-ray crystallographic analyses elucidated the identities of [Mn^IIBr₂(κ³-N_py,N_im,O-dpkbh)] and [Mn^II(κ³-N_py,N_im,O-dpkbh-H)₂]·nX and divulged weaker coordination of [dpkbh] to Mn(II) in [Mn^IIBr₂(κ³-N_py,N_im,O-dpkbh)] and stronger coordination of [dpkbh-H]^? to Mn(II) in [Mn^II(κ³-N_py,N_im,O-dpkbh-H)₂]·0.22H₂O. Low-temperature X-ray structural analyses were employed to account for the disorder in the structure of [Mn^II(κ³-N_py,N_im,O-dpkbh-H)₂] and the short NH bond distance observed in the structure of [Mn^IIBr₂(κ³-N_py,N_im,O-dpkbh)]. A PLATON Squeeze treatment was invoked to account for the fractional occupancy of lattice water in the structure of [Mn^II(κ³-N_py,N_im,O-dpkbh-H)₂].     

Binary and ternary complexes involving manganese(II), 2,2’-bipyridine and halide or thiocyanate ions inN,N-dimethylformamide

Ternary complexation involving the manganese(II) ion, 2,2’-bipyridine (bipy), and halide (chloride, bromide) or pseudohalide (thiocyanate) ions has been studied by precise titration calorimetry inN,N -dimethylformamide (DMF) at 298K. All the titration curves are explained well in terms of formation of mononuclear complexes of the type [MnX_m(bipy)_n](^2-m) + (X = CI, Br or SCN), and the formation of [MnCl(bipy)]⁺, [MnCl₂(bipy)], [MnCl(bipy)₂]⁺ and [MnCl₂(bipy)₂] has been established in the chloride system, [MnBr(bipy)]⁺, [MnBr₂(bipy)], [MnBr(bipy)₂]⁺ in the bromide system, and [Mn(NCS)(bipy)]⁺, [Mn(NCS)₂(bipy)], [Mn(NCS)₃(bipy)]^-, [Mn(NCS)(bipy)₂]⁺, and [Mn(NCS)₂(bipy)₂] in the thiocyanate system. The data were analyzed on the basis of the thermodynamic parameters for the binary Mn^lIbipy and Mn^II-X (X = Cl, Br and SCN) systems, the latter being determined in previous work. The formation constants, reaction enthalpies, and entropies of the ternary complexes were extracted. The thermodynamic parameters thus obtained are discussed in comparison with those of the corresponding systems of other transition metal(II) ions.     

Synthesis and spectral studies on heterobimetallic complexes of manganese and ruthenium derived from N-(2-hydroxynaphthalen-1-yl)methylenebenzoylhydrazide

The complex [Mn^IV(napbh)₂] (napbhH₂ = N-(2-hydroxynaphthalen-1-yl)methylenebenzoylhydrazide) reacts with activated ruthenium(III) chloride in methanol in 1 : 1.2 molar ratio under reflux, giving heterobimetallic complexes, [Mn^IV(napbh)₂Ru^IIICl₃(H₂O)] · [Ru^III(napbhH)Cl₂(H₂O)] reacts with Mn(OAc)₂·4H₂O in methanol in 1 : 1.2 molar ratio under reflux to give [Ru^III(napbhH)Cl₂(H₂O)Mn^II(OAc)₂]. Replacement of aquo in these heterobimetallic complexes has been observed when the reactions are carried out in the presence of pyridine (py), 3-picoline (3-pic), or 4-picoline (4-pic). The molar conductances for these complexes in DMF indicates 1 : 1 electrolytes. Magnetic moment values suggest that these heterobimetallic complexes contain Mn^IV and Ru^III or Ru^III and Mn^II in the same structural unit. Electronic spectral studies suggest six coordinate metal ions. IR spectra reveal that the napbhH₂ ligand coordinates in its enol form to Mn^IV and bridges to Ru^III and in the keto form to Ru^III and bridging to Mn^II.     

Synthesis,characterization, reactivity,and electrochemical studies of manganese(IV) complexes of bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone

Manganese(IV) complexes [Mn^IV(npah)(H₂O)₂] (1) and [Mn^IV(npah)(A)₂]?·?nH₂O (where A?=?py (2), 2-pic (3), 3-pic (4), 4-pic (5)) and Mn^IV(npah)(NN)] (NN?=?bpy (6) and phen (7)) have been synthesized from bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone in methanol. The composition of the complexes has been established by elemental analyses. Complex 3 has been characterized by mass spectral data also. Structural assessment of the complexes has been based on data from molar conductance, magnetic moment, electronic, electron paramagnetic resonance, and infrared (IR) spectral studies. Molar conductances of the complexes in DMSO suggest non-electrolytes. Magnetic moment and EPR studies suggest +4 oxidation state for manganese in these complexes. Electronic spectral studies suggest six-coordinate octahedral geometry around the metal ions. IR spectra reveal that H₄npah coordinates to the metal in enol form. Reaction of the complexes with benzyl alcohol and SO₂ has been investigated. Cyclic voltammetric studies of the complexes have also been carried out.     

Facile generation of platinum(IV) compounds with mixed labile moieties. Hydrogen peroxide oxidation of platinum(II) to platinum(IV) compounds

Hydrogen peroxide oxidation of platinum(II) compounds containing labile groups such as Cl, OH, and alkene moieties has been carried out and the products characterized. The reactions of [Pt^II (X)₂ (N–N)] (X = Cl, OH, X₂ = isopropylidenemalorate (ipm); N–N 2,2-dimethyl-1,3-propanediamine [(dmpda), N-isopropyl-1,3-propanediamine (ippda)] with hydrogen peroxide in an appropriate solvent at room temperature affords [Pt^IV (OH)(Y)(X)₂(N–N)] (Y = OH, OCH₃). The crystal structures of [Pt^IV(OH)(OCH₃)(Cl)₂(dmpda)]·2H₂O (P-1 bar, a = 6.339(2) Å , b = 9.861(1) Å, c = 11.561(1) Å, a = 92.078(9)°, β = 104.78(1)°, γ=100.54(1)°, V = 684.3(2) ų, Z = 2R = 0.0503) and [Pt^IV(OH)₂(ipm)(ippda)]·3H₂O (C 2/c, a = 27.275(6) Å, b=6.954(2) Å, c = 22.331(4) Å, β = 118.30(2)°, V = 3729(2) ų, Z = 8, R = 0.0345) have been solved and refined. The local geometry around the platinum(IV) atom approximates to a typical octahedral arrangement with two added groups (OH and OCH₃; OH and OH) in a transposition. The platinum(IV) compounds with potential labile moieties may be important intermediate species for further reactions.     

Unraveling the Mechanism of Water Oxidation Catalyzed by Nonheme Iron Complexes

Density functional theory (DFT) is employed to: 1) propose a viable catalytic cycle consistent with our experimental results for the mechanism of chemically driven (Ce^IV) O₂ generation from water, mediated by nonheme iron complexes; and 2) to unravel the role of the ligand on the nonheme iron catalyst in the water oxidation reaction activity. To this end, the key features of the water oxidation catalytic cycle for the highly active complexes [Fe(OTf)₂(Pytacn)] (Pytacn: 1‐(2′‐pyridylmethyl)‐4,7‐dimethyl‐1,4,7‐triazacyclononane; OTf: CF₃SO₃^?) ( 1 ) and [Fe(OTf)₂(mep)] (mep: N,N′‐bis(2‐pyridylmethyl)‐N,N′‐dimethyl ethane‐1,2‐diamine) ( 2 ) as well as for the catalytically inactive [Fe(OTf)₂(tmc)] (tmc: N,N′,N′′,N′′′‐tetramethylcyclam) ( 3 ) and [Fe(NCCH₃)(^MePy₂CH‐tacn)](OTf)₂ (^MePy₂CH‐tacn: N‐(dipyridin‐2‐yl)methyl)‐N′,N′′‐dimethyl‐1,4,7‐triazacyclononane) ( 4 ) were analyzed. The DFT computed catalytic cycle establishes that the resting state under catalytic conditions is a [Fe^IV(O)(OH₂)(LN₄)]²⁺ species (in which LN₄=Pytacn or mep) and the rate‐determining step is the O?O bond‐formation event. This is nicely supported by the remarkable agreement between the experimental (ΔG^≠=17.6±1.6 kcal mol^?1) and theoretical (ΔG^≠=18.9 kcal mol^?1) activation parameters obtained for complex 1 . The O?O bond formation is performed by an iron(V) intermediate [Fe^V(O)(OH)(LN₄)]²⁺ containing a cis‐Fe^V(O)(OH) unit. Under catalytic conditions (Ce^IV, pH 0.8) the high oxidation state Fe^V is only thermodynamically accessible through a proton‐coupled electron‐transfer (PCET) process from the cis‐[Fe^IV(O)(OH₂)(LN₄)]²⁺ resting state. Formation of the [Fe^V(O)(LN₄)]³⁺ species is thermodynamically inaccessible for complexes 3 and 4 . Our results also show that the cis‐labile coordinative sites in iron complexes have a beneficial key role in the O?O bond‐formation process. This is due to the cis‐OH ligand in the cis‐Fe^V(O)(OH) intermediate that can act as internal base, accepting a proton concomitant to the O?O bond‐formation reaction. Interplay between redox potentials to achieve the high oxidation state (Fe^V?O) and the activation energy barrier for the following O?O bond formation appears to be feasible through manipulation of the coordination environment of the iron site. This control may have a crucial role in the future development of water oxidation catalysts based on iron.     

Cyanide-bridged FeIII–MnIII bimetallic chain derived from [Fe(bipy)(CN)4]−: synthesis,structure and magnetic characteristics

A chain-like compound of [Mn(salpn)][Fe(bipy)(CN)₄] (1) (salpn = N,N′-propylenebis(salicylideneiminato)dianion; bipy = 2,2′-bipyridine), assembled from building blocks of [Fe(bipy)(CN)₄]^? and [Mn(salpn)]⁺, has been characterized by elemental analyses, ICP, IR, thermoanalysis, single crystal X-ray structure analysis and magnetic measurements. In 1, each [Fe(bipy)(CN)₄]^? anion coordinates with two [Mn^III(salpn)]⁺ cations via two trans-CN^? groups, and each [Mn^III(salpn)]⁺ cation is axially coordinated by two [Fe(bipy)(CN)₄]^? ions, resulting in a straight 1-D chain. The chains stack via aromatic π–π-type interactions. Magnetic studies reveal the presence of weak antiferromagnetic interactions between adjacent Fe^III and Mn^III ions through cyanide-bridges.     

Dramatic Influence of an Anionic Donor on the Oxygen‐Atom Transfer Reactivity of a MnV–Oxo Complex

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)(TBP₈Cz)(CN)]^? was generated from addition of Bu₄N⁺CN^? 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 Mn?O=1.53 Å, Mn?CN=2.21 Å. In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN^? complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e^?‐reduced Mn^III(CN)^? 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 ΔH^≠=14 kcal mol^?1, ΔS^≠=?10 cal mol^?1 K^?1. Computational studies fully support the structures, spin states, and relative reactivity of the 5‐ and 6‐coordinate Mn^V(O) complexes.     

Manganese(III)-carboxylate binding: Chemistry and structure of a hydrated pyridinedicarboxylate

The reaction of Mn(CH₃COO)₃ 2H₂O with the carboxyl-rich ligand pyridine-2,6-dicarboxylic acid (H₂L) in methanol affords a high-spin (S = 2) hydratedbis-complex. Structure determination has revealed the solid to be [Mn^III(H₂ L)(L)] [Mn^IIIL₂] 5H₂ O: space group P−1;Z = 2;a = 7.527(3)?³,b= 14.260(4)?,c = 16.080(6)?,α = 91.08(3)°,β = 103.58(3)°,γ= 105.41(3)° andV= 1611.2(10)?³. Each ligand is planar and is bonded in the tridentate O₂N fashion. The MnO₄N₂ coordination spheres show large distortions from octahedral symmetry. The lattice is stabilised by an extensive network of O…O hydrogen-bonding involving water molecules and carboxyl functions. Upon dissolution in water, protic redistribution occurs and the complex acts as the mono-basic acid Mn(HL)(L) (pK, 4.3 ±0.05). The deprotonated complex displays high metal reduction potentials: Mn^IVL₂-Mn^IIIL ₂ ⁻ , 1.05V; Mn^IIIL ₂ ⁻ Mn^IIL ₂ ²⁻ -, 0.28V vs. SCE     

A New Thiolate-Bound Dimanganese Cluster as a Structural and Functional Model for Class Ib Ribonucleotide Reductases

In class Ib ribonucleotide reductases (RNRs) a dimanganese(II) cluster activates superoxide (O₂⋅⁻) rather than dioxygen (O₂), to access a high valent Mn^III−O₂−Mn^IV species, responsible for the oxidation of tyrosine to tyrosyl radical. In a biomimetic approach, we report the synthesis of a thiolate-bound dimanganese complex [Mn^II₂(BPMT)(OAc)₂](ClO)₄ (BPMT=(2,6-bis{[bis(2-pyridylmethyl)amino]methyl}-4-methylthiophenolate) ( 1 ) and its reaction with O₂⋅⁻ to form a [(BPMT)MnO₂Mn]²⁺ complex 2 . Resonance Raman investigation revealed the presence of an O−O bond in 2 , while EPR analysis displayed a 16-line S_t=1/2 signal at g=2 typically associated with a Mn^IIIMn^IV core, as detected in class Ib RNRs. Unlike all other previously reported Mn−O₂−Mn complexes, generated by O₂⋅⁻ activation at Mn₂ centers, 2 proved to be a capable electrophilic oxidant in aldehyde deformylation and phenol oxidation reactions, rendering it one of the best structural and functional models for class Ib RNRs.     

3‐Rhoda‐1,2‐diazacyclopentanes: A Series of Novel Metallacycle Complexes Derived From CN Functionalization of Ethylene

Rh‐containing metallacycles, [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NR)₂‐]Cl; TPA=N,N,N,N‐tris(2‐pyridylmethyl)amine have been accessed through treatment of the Rh^I ethylene complex, [(TPA)Rh(η²‐CH₂CH₂)]Cl ([ 1 ]Cl) with substituted diazenes. We show this methodology to be tolerant of electron‐deficient azo compounds including azo diesters (RCO₂N?NCO₂R; R=Et [ 3 ]Cl, R=iPr [ 4 ]Cl, R=tBu [ 5 ]Cl, and R=Bn [ 6 ]Cl) and a cyclic azo diamide: 4‐phenyl‐1,2,4‐triazole‐3,5‐dione (PTAD), [ 7 ]Cl. The latter complex features two ortho‐fused ring systems and constitutes the first 3‐rhoda‐1,2‐diazabicyclo[3.3.0]octane. Preliminary evidence suggests that these complexes result from N–N coordination followed by insertion of ethylene into a [Rh]?N bond. In terms of reactivity, [ 3 ]Cl and [ 4 ]Cl successfully undergo ring‐opening using p‐toluenesulfonic acid, affording the Rh chlorides, [(TPA)Rh^III(Cl)(κ¹‐(C)‐CH₂CH₂(NCO₂R)(NHCO₂R)]OTs; [ 13 ]OTs and [ 14 ]OTs. Deprotection of [ 5 ]Cl using trifluoroacetic acid was also found to give an ethyl substituted, end‐on coordinated diazene [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NH)₂‐]⁺ [ 16 ]Cl, a hitherto unreported motif. Treatment of [ 16 ]Cl with acetyl chloride resulted in the bisacetylated adduct [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NAc)₂‐]⁺, [ 17 ]Cl. Treatment of [ 1 ]Cl with AcN?NAc did not give the Rh?N insertion product, but instead the N,O‐chelated complex [(TPA)Rh^I ( κ²‐(O,N)‐CH₃(CO)(NH)(N?C(CH₃)(OCH?CH₂))]Cl [ 23 ]Cl, presumably through insertion of ethylene into a [Rh]?O bond.     

A Highly Oxidizing and Isolable Oxoruthenium(V) Complex [RuV(N4O)(O)]2+: Electronic Structure,Redox Properties,and Oxidation Reactions Investigated by DFT Calculations

The electronic structure and redox properties of the highly oxidizing, isolable Ru^V?O complex [Ru^V(N₄O)(O)]²⁺, its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8 kcal mol^?1, respectively. The overall free energy activation barrier (ΔG^≠=25.5 kcal mol^?1) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [Ru^III(N₄O)(HOCl)]²⁺ (ΔG=15.0 kcal mol^?1) and the substitution of HOCl by a water molecule (ΔG^≠=10.5 kcal mol^?1). For water oxidation, nucleophilic attack on Ru^V?O by water, leading to O? O bond formation, has a free energy barrier of 24.0 kcal mol^?1, the major component of which comes from the cleavage of the H? OH bond of water. Intermolecular self‐coupling of two molecules of [Ru^V(N₄O)(O)]²⁺ leads to the [(N₄O)Ru^IV? O₂? Ru^III(N₄O)]⁴⁺ complex with a calculated free energy barrier of 12.0 kcal mol^?1.