Controlling the Reactivity of a Metal-Hydroxo Adduct with a Hydrogen Bond

Structural and electrochemical properties of mononuclear copper(II) complexes with pentadentate ethylenediamine-based ligands with pyridine/quinoline/isoquinoline/quinoxaline binding sites

N-Monosubstituted ethylenediamine derivatives with three methylene-tethered aromatic groups ((ArCH2)2NCH2CH2N(R)CH2Ar (R-ArArAr), where Ar = 2-pyridyl, 2-quinolyl, 1- and 3-isoquinolyl and 2-quinoxalyl; R = methyl, benzyl and phenyl) were utilized as pentadentate ligands for copper(II) complexation. Fifteen mononuclear copper(II) complexes were synthesized and exhibit differences in cyclic voltammetry, absorption spectroscopy and solid state geometries, depending on the aromatic group (Ar) and the substituent on the aliphatic nitrogen atom (R) of the ligand. Compared with the pyridine and isoquinoline complexes, the quinoline and quinoxaline derivatives exhibit distinct Cu(II)/Cu(I) redox potentials and d-d transition absorption wavelengths. Similarly, the phenyl derivatives are different from their methyl and benzyl counterparts. These characteristic trends are discussed in relation to the square-pyramidal/trigonal-bipyramidal structure of the complexes which is perturbed by the location of quinoline moieties in the penta- or hexacoordinate complexes with Jahn-Teller distortion. In addition, the results are compared to the copper(II) complexes with pyridine/quinoline mixed ligands, Ph-Ar1Ar2Ar3 ((Ar1CH2)(Ar2CH2)NCH2CH2N(Ph)CH2Ar3).

Synthesis, characterization and reactivity of a Mn(III)-hydroxido complex as a biomimetic model for lipoxygenase

Manganese hydroxido (Mn-OH) complexes supported by a tripodal N,N',N″-[nitrilotris(ethane-2,1-diyl)]tris(P,P-diphenylphosphinic amido) ([poat]3-) ligand have been synthesized and characterized by spectroscopic techniques including UV-vis and electron paramagnetic resonance (EPR) spectroscopies. X-ray diffraction (XRD) methods were used to confirm the solid-state molecular structures of {Na2[MnIIpoat(OH)]}2 and {Na[MnIIIpoat(OH)]}2 as clusters that are linked by the electrostatic interactions between the sodium counterions and the oxygen atom of the ligated hydroxido unit and the phosphinic (P=O) amide groups of [poat]3-. Both clusters feature two independent monoanionic fragments in which each contains a trigonal bipyramidal Mn center that is comprised of three equatorial deprotonated amide nitrogen atoms, an apical tertiary amine, and an axial hydroxido ligand. XRD analyses of {Na[MnIIIpoat(OH)]}2 also showed an intramolecular hydrogen bonding interaction between the MnIII-OH unit and P=O group of [poat]3-. Crystalline {Na[MnIIIpoat(OH)]}2 remains as clusters with Na+---O interactions in solution and is unreactive toward external substrates. However, conductivity studies indicated that [MnIIIpoat(OH)]- generated in situ is monomeric and reactivity studies found that it is capable of cleaving C-H bonds, illustrating the importance of solution-phase speciation and its direct effect on chemical reactivity. Synopsis: Manganese-hydroxido complexes were synthesized to study the influence of H-bonds in the secondary coordination sphere and their effects on the oxidative cleavage of substrates containing C-H bonds.

Controlling Redox Potential of a Manganese(III)-Bis(hydroxo) Complex through Protonation and the Hydrogen-Atom Transfer Reactivity

A series of mononuclear manganese(III)-hydroxo and -aqua complexes, [MnIII(TBDAP)(OH)2]+ (1), [MnIII(TBDAP)(OH)(OH2)]2+ (2) and [MnIII(TBDAP)(OH2)2]3+ (3), were prepared from a manganese(II) precursor and confirmed using various methods including X-ray crystallography. Thermodynamic analysis showed that protonation from hydroxo to aqua species resulted in increased redox potentials (E1/2) in the order of 1 (-0.15 V) < 2 (0.56 V) < 3 (1.11 V), while pKa values exhibited a reverse trend in the order of 3 (3.87) < 2 (11.84). Employing the Bordwell Equation, the O-H bond dissociation free energies (BDFE) of [MnII(TBDAP)(OH)(OH2)]+ and [MnII(TBDAP)(OH2)2]2+, related to the driving force of 1 and 2 in hydrogen atom transfer (HAT), were determined as 75.3 and 77.3 kcal mol-1, respectively. It was found that the thermodynamic driving force of 2 in HAT becomes greater than that of 1 as the redox potential of 2 increases through protonation from 1 to 2. Kinetic studies on electrophilic reactions using a variety of substrates revealed that 1 is only weakly reactive with O-H bonds, whereas 2 can activate aliphatic C-H bonds in addition to O-H bonds. The reaction rates increased by 1.4 × 104-fold for the O-H bonds by 2 over 1, which was explained by the difference in BDFE and the tunneling effect. Furthermore, 3, possessing the highest redox potential value, was found to undergo an aromatic C-H bond activation reaction under mild conditions. These results provide valuable insights into enhancing electrophilic reactivity by modulating the redox potential of manganese(III)-hydroxo and -aqua complexes through protonation.

Thirty years with three-dimensional structures of lipoxygenases

The X-ray crystal structures of soybean lipoxygenase (LOX) and rabbit 15-LOX were reported in the 1990s. Subsequent 3D structures demonstrated a conserved U-like shape of the substrate cavities as reviewed here. The 8-LOX:arachidonic acid (AA) complex showed AA bound to the substrate cavity carboxylate-out with C10 at 3.4 Å from the iron metal center. A recent cryo-electron microscopy (EM) analysis of the 12-LOX:AA complex illustrated AA in the same position as in the 8-LOX:AA complex. The 15- and 12-LOX complexes with isoenzyme-specific inhibitors/substrate mimics confirmed the U-fold. 5-LOX oxidizes AA to leukotriene A4, the first step in biosynthesis of mediators of asthma. The X-ray structure showed that the entrance to the substrate cavity was closed to AA by Phe and Tyr residues of a partly unfolded α2-helix. Recent X-ray analysis revealed that soaking with inhibitors shifted the short α2-helix to a long and continuous, which opened the substrate cavity. The α2-helix also adopted two conformations in 15-LOX. 12-LOX dimers consisted of one closed and one open subunit with an elongated α2-helix. 13C-ENDOR-MD computations of the 9-MnLOX:linoleate complex showed carboxylate-out position with C11 placed 3.4 ± 0.1 Å from the catalytic water. 3D structures have provided a solid ground for future research.

Synthetic Advances for Mechanistic Insights: Metal-Oxygen Intermediates with a Macrocyclic Pyridinophane System

ConspectusMetalloenzymes, which are proteins containing earth-abundant transition-metal ions as cofactors in the active site, generate various metal-oxygen intermediates via activating a dioxygen molecule (O2) to mediate vital metabolic functions, such as the oxidative metabolism of xenobiotics and the biotransformation of naturally occurring molecules. By replicating the active sites of metalloenzymes, many bioinorganic chemists have studied the geometric and electronic properties and reactivities of model complexes to understand the nature of enzymatic intermediates and develop bioinspired metal catalysts. Among the reported model complexes, nonporphyrinic macrocyclic ligands are the predominant coordination system widely used in stabilizing and isolating diverse metal-oxygen intermediates, which allows us to extensively investigate the physicochemical characteristics of the analogs of reactive intermediates of metalloenzymes. In particular, it has been reported that the ring size of the macrocyclic ligands, defined by the number of atoms in the macrocyclic ring, drastically affects the identity of the metal-oxygen intermediate. Thus, systematic modification of the macrocyclic ligands has been a great subject being examined in various inorganic fields.In this Account, we describe synthetic advances of a macrocyclic ligand system by introducing pyridine donors into a 12-membered tetraazamacrocyclic ligand (12-TMC) that initially has 4 amine donors. Interestingly, the backbone of the pyridinophane ligand with 2 pyridine and 2 amine donors in a 12-membered ring is shown to be much more folded than in other macrocyclic ligands, thereby allowing the axial and equatorial donors to separately control the electronic structure of metal complexes. Then, we looked over independent electronic and steric effects on metal-oxygen species with thorough physicochemical analysis. The NiIII-peroxo complexes exhibit nucleophilic reactivity dependent on the steric hindrance of the second coordination sphere. Furthermore, the C-H bond strength of the second coordination sphere has also been an important factor in determining the stability of MnIV-bis(hydroxo) intermediates. Electronic tuning on CoIII-hydroperoxo intermediates results in a trend between the electron-donating abilities of para-substituents on pyridine in the pyridinophane ligand and electrophilic reactivities, from which mechanistic insights into the metal-hydroperoxo species have been gained. Importantly, the metal-oxygen intermediates supported by the pyridinophane ligand system have revealed quite challenging chemical reactions, including dioxygenase-like nitrile activation by CoIII-peroxo intermediates and the oxidation of aldehyde and aromatic compounds by manganese-oxygen intermediates. Based on the fine substitution of donors, we have addressed that those novel reactions originated from the unique framework of the pyridinophane system incorporating spin-crossover behavior and high redox potentials of the metal-oxygen intermediates. These results will be valuable for the structure-activity relationship of metal-oxygen intermediates, giving a better understanding on the enzymatic coordination system where amino acid ligands vary for specific chemical reactions.

Iron and manganese lipoxygenases of plant pathogenic fungi and their role in biosynthesis of jasmonates

Lipoxygenases (LOX) contain catalytic iron (FeLOX), but fungi also produce LOX with catalytic manganese (MnLOX). In this review, the 3D structures and properties of fungal LOX are compared and contrasted along with their associations with pathogenicity. The 3D structures and properties of two MnLOX (Magnaporthe oryzae, Geaumannomyces graminis) and the catalysis of five additional MnLOX have provided information on the metal center, substrate binding, oxygenation, tentative O₂ channels, and biosynthesis of exclusive hydroperoxides. In addition, the genomes of other plant pathogens also code for putative MnLOX. Crystals of the 13S-FeLOX of Fusarium graminearum revealed an unusual altered geometry of the Fe ligands between mono- and dimeric structures, influenced by a wrapping sequence extension near the C-terminal of the dimers. In plants, the enzymes involved in jasmonate synthesis are well documented whereas the fungal pathway is yet to be fully elucidated. Conversion of deuterium-labeled 18:3n-3, 18:2n-6, and their 13S-hydroperoxides to jasmonates established 13S-FeLOX of F. oxysporum in the biosynthesis, while subsequent enzymes lacked sequence homologues in plants. The Rice-blast (M. oryzae) and the Take-all (G. graminis) fungi secrete MnLOX to support infection, invasive hyphal growth, and cell membrane oxidation, contributing to their devastating impact on world production of rice and wheat.

Recent advances in the hybridization of cellulose and carbon nanomaterials: Interactions, structural design, functional tailoring, and applications

Due to the good biocompatibility and flexibility of cellulose and the excellent optical, electronic, as well as mechanical properties of carbon nanomaterials (CNMs), cellulose/CNM hybrid materials have been widely synthesized and used in energy storage, sensors, adsorption, biomedicine, and many other fields. In this review, we present recent advances (2016-current) in the design, structural design, functional tailoring and various applications of cellulose/CNM hybrid materials. For this aim, first the interactions between cellulose and CNMs for promoting the formation of cellulose/CNM materials are analyzed, and then the hybridization between cellulose with various CNMs for tailoring the structures and functions of hybrid materials is introduced. Further, abundant applications of cellulose/CNM hybrid materials in various fields are presented and discussed. This comprehensive review will be helpful for readers to understand the functional design and facile synthesis of cellulose-based nanocomposites, and to promote the high-performance utilization and sustainability of biomass materials in the future.

Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn(III)-hydroxo and Mn(III)-aqua complexes

Hydrogen atom transfer (HAT) of metal-oxygen intermediates such as metal-oxo, -hydroxo and -superoxo species have so far been studied extensively. However, HAT reactions of metal-aqua complexes have yet to be...

Bioinspired mononuclear Mn complexes for O2 activation and biologically relevant reactions

A general interest in harnessing the oxidizing power of dioxygen (O₂) continues to motivate research efforts on bioinspired and biomimetic complexes to better understand how metalloenzymes mediate these reactions. The ubiquity of Fe- and Cu-based enzymes attracts significant attention and has resulted in many noteworthy developments for abiotic systems interested in direct O₂ reduction and small molecule activation. However, despite the existence of Mn-based metalloenzymes with important O₂-dependent activity, there has been comparatively less focus on the development of these analogues relative to Fe- and Cu-systems. In this Perspective, we summarize important contributions to the development of bioinspired mononuclear Mn complexes for O₂ activation and studies on their reactivity, emphasizing important design parameters in the primary and secondary coordination spheres and outlining mechanistic trends.

Mimicking Elementary Reactions of Manganese Lipoxygenase Using Mn-hydroxo and Mn-alkylperoxo Complexes

Manganese lipoxygenase (MnLOX) is an enzyme that converts polyunsaturated fatty acids to alkyl hydroperoxides. In proposed mechanisms for this enzyme, the transfer of a hydrogen atom from a substrate C-H bond to an active-site Mn^III-hydroxo center initiates substrate oxidation. In some proposed mechanisms, the active-site Mn^III-hydroxo complex is regenerated by the reaction of a Mn^III-alkylperoxo intermediate with water by a ligand substitution reaction. In a recent study, we described a pair of Mn^III-hydroxo and Mn^III-alkylperoxo complexes supported by the same amide-containing pentadentate ligand (^6Medpaq). In this present work, we describe the reaction of the Mn^III-hydroxo unit in C-H and O-H bond oxidation processes, thus mimicking one of the elementary reactions of the MnLOX enzyme. An analysis of kinetic data shows that the Mn^III-hydroxo complex [Mn^III(OH)(^6Medpaq)]⁺ oxidizes TEMPOH (2,2′-6,6′-tetramethylpiperidine-1-ol) faster than the majority of previously reported Mn^III-hydroxo complexes. Using a combination of cyclic voltammetry and electronic structure computations, we demonstrate that the weak Mn^III-N(pyridine) bonds lead to a higher Mn^III/II reduction potential, increasing the driving force for substrate oxidation reactions and accounting for the faster reaction rate. In addition, we demonstrate that the Mn^III-alkylperoxo complex [Mn^III(OO^tBu)(^6Medpaq)]⁺ reacts with water to obtain the corresponding Mn^III-hydroxo species, thus mimicking the ligand substitution step proposed for MnLOX.