Novel Iron(III) Complexes of Sterically Hindered 4N Ligands: Regioselectivity in Biomimetic Extradiol Cleavage of Catechols

Rapid atmospheric carbon dioxide fixation by nickel(II) complexes: meridionally coordinated diazepane-based 3N ligands facilitate fixation

Octahedral complexes of the type [Ni(L)(H2O)3](ClO4)2 (1 and 2), where L is the tridentate 3N ligand 4-methyl-1-(pyrid-2-ylmethyl)-1,4-diazacycloheptane (L1, 1), or 4-methyl-1-(N-methylimidazolyl)-1,4-diazacycloheptane (L2, 2), have been isolated and characterized using elemental analysis, ESI-MS and electronic absorption spectroscopy. The DFT optimized structures of 1 and 2 reveal that the tridentate 3N ligands are coordinated meridionally constituting a distorted octahedral coordination geometry around nickel(ii). In methanol solution, the complexes, upon treatment with triethylamine, generate the reactive red colored low-spin square planar Ni-OH intermediate [Ni(L1/L2)(OH)]+ (1a and 2a), as characterized by ESI-MS and electronic absorption spectroscopy, and energy minimized structures. The latter when exposed to the atmosphere rapidly absorbs atmospheric CO2 to produce the carbonate bridged dinickel(ii) complexes [Ni2(L1/L2)2(μ-CO3)(H2O)2](ClO4)2 (3 and 4), as characterized by elemental analysis and the IR spectral feature (∼1608 cm-1) characteristic of bridging carbonate. The single crystal X-ray structure of 3 reveals the presence of a dinickel(ii) core bridged by a carbonate anion in a symmetric mode. Both the Ni(ii) centers are identical to each other with each Ni(ii) possessing a distorted octahedral coordination geometry constituted by a meridionally coordinated 3N ligand, a carbonate ion and a water molecule. The decay kinetics of the red intermediates generated by 1 (kobs, 7.7 ± 0.1 × 10-5 s-1) and 2 (kobs, 5.8 ± 0.3 × 10-4 s-1) in basic methanol solution with atmospheric CO2 has been determined by absorption spectroscopy. DFT studies illustrate that meridional coordination of the 3N ligand and the electron-releasing imidazole ring as in 2 facilitate fixation of CO2. The carbonate complex 3 efficiently catalyzes the conversion of styrene oxide into cyclic carbonate by absorbing atmospheric and pure CO2 with excellent selectivity.

Biological Inspirations: Iron Complexes Mimicking the Catechol Dioxygenases

Within the broad group of Fe non-heme oxidases, our attention was focused on the catechol 1,2- and 2,3-dioxygenases, which catalyze the oxidative cleavage of aromatic rings. A large group of Fe complexes with N/O ligands, ranging from N₃ to N₂O₂S, was developed to mimic the activity of these enzymes. The Fe complexes discussed in this work can mimic the intradiol/extradiol catechol dioxygenase reaction mechanism. Electronic effects of the substituents in the ligand affect the Lewis acidity of the Fe center, increasing the ability to activate dioxygen and enhancing the catalytic activity of the discussed biomimetic complexes. The ligand architecture, the geometric isomers of the complexes, and the substituent steric effects significantly affect the ability to bind the substrate in a monodentate and bidentate manner. The substrate binding mode determines the preferred mechanism and, consequently, the main conversion products. The preferred mechanism of action can also be affected by the solvents and their ability to form the stable complexes with the Fe center. The electrostatic interactions of micellar media, similar to SDS, also control the intradiol/extradiol mechanisms of the catechol conversion by discussed biomimetics.

Recent advances in the synthesis of piperazine based ligands and metal complexes and their applications

The piperazine scaffold is a privileged structure frequently found in biologically active compounds. Piperazine nucleus is found in many marketed drugs in the realm of antidepressants (amoxapine), antipsychotics (bifeprunox), antihistamines (cyclizine and oxatomide), antifungals (itraconazole), antibiotics (ciprofloxacin), etc. This is one of the reasons why piperazine based compounds are gaining prominence in today's research. In addition to the ring carbons, substitution in the nitrogen atom of piperazine not only creates potential drug molecules but also makes it unique with versatile binding possibilities with metal ions. Piperazine ring-based compounds find their application in biological systems with antihistamine, anticancer, antimicrobial and antioxidant properties. They have also been successfully used in the field of catalysis and metal organic frameworks (MOFs). The present review focuses on the synthesis and application of different piperazine derivatives and their metal complexes having diverse applications.

Ligand Taxonomy for Bioinorganic Modeling of Dioxygen-Activating Non-Heme Iron Enzymes

Novel functions emerge from novel structures. To develop efficient catalytic systems for challenging chemical transformations, chemists often seek inspirations from enzymatic catalysis. A large number of iron complexes supported by nitrogen-rich multidentate ligands have thus been developed to mimic oxo-transfer reactivity of dioxygen-activating metalloenzymes. Such efforts have significantly advanced our understanding of the reaction mechanisms by trapping key intermediates and elucidating their geometric and electronic properties. Critical to the success of this biomimetic approach is the design and synthesis of elaborate ligand systems to balance the thermodynamic stability, structural adaptability, and chemical reactivity. In this Concept article, representative design strategies for biomimetic atom-transfer chemistry are discussed from the perspectives of "ligand builders". Emphasis is placed on how the primary coordination sphere is constructed, and how it can be elaborated further by rational design for desired functions.

Single-step benzene hydroxylation by cobalt(ii) catalysts via a cobalt(iii)-hydroperoxo intermediate

Cobalt(ii) complexes reported as efficient and selective catalysts for single-step phenol formation from benzene using H₂O₂. The catalysis proceeds likely via cobalt(iii)-hydroperoxo species.

Bioinspired models for an unusual 3-histidine motif of diketone dioxygenase enzyme

Bioinspired models for contrasting the electronic nature of neutral tris-histidine with the anionic 2-histidine-1-carboxylate facial motif and their subsequent impact on catalysis are reported. Herewith, iron(ii) complexes [Fe(L)(CH₃CN)₃](SO₃CF₃)₂1-3 of tris(2-pyridyl)-based ligands (L) have been synthesized and characterized as accurate structural models for the neutral 3-histidine triad of the enzyme diketone dioxygenase (DKDO). The molecular structure of one of the complexes exhibits octahedral coordination geometry and Fe-N11_py bond lengths [1.952(4) to 1.959(4) Å] close to the Fe-N_His bond distances (1.98 Å) of the 3-His triad in the resting state of the enzyme, as obtained by EXAFS studies. The diketonate substrate-adduct complexes [Fe(L)(acac^R)](SO₃CF₃) (R = Me, Ph) of 1-3 have been obtained using Na(acac^R) in acetonitrile. The Fe^2+/3+ redox potentials of the complexes (1.05 to 1.2 V vs. Fc/Fc⁺) and their substrate adducts (1.02 to 1.19 V vs. Fc/Fc⁺) appeared at almost the same redox barrier. All diketonate adducts exhibit two Fe(ii) → acac MLCT bands around 338 to 348 and 430 to 490 nm. Exposure of these adducts to O₂ results in the decay of both MLCT bands with a rate of (k_O2) 5.37 to 9.41 × 10^-3 M^-1 s^-1. The k_O2 values were concomitantly accelerated 20 to 50 fold by the addition of H⁺ (acetic acid), which nicely models the rate enhancement in the enzyme kinetics by the glutamate residue (Glu98). The oxygenation of the phenyl-substituted adducts yielded benzoin and benzoic acid (40% to 71%) as cleavage products in the presence of H⁺ ions. Isotope-labeling experiments using ¹⁸O₂ showed 47% incorporation of ¹⁸O in benzoic acid, which reveals that the oxygen originates from dioxygen. Thus, the present model complexes exhibit very similar chemical surroundings to the active site of DKDO and mimic its functions elegantly. On the basis of these results, the C-C bond cleavage reaction mechanism is discussed.

A Structural and Functional Model for the Tris-Histidine Motif in Cysteine Dioxygenase

The iron(II) complexes [Fe(L)(MeCN)₃ ](SO₃ CF₃ )₂ (L are two derivatives of tris(2-pyridyl)-based ligands) have been synthesized as models for cysteine dioxygenase (CDO). The molecular structure of one of the complexes exhibits octahedral coordination geometry and the Fe-N_py bond lengths [1.953(4)-1.972(4) Å] are similar to those in the Cys-bound Fe^II -CDO; Fe-N_His : 1.893-2.199 Å. The iron(II) centers of the model complexes exhibit relatively high Fe^III/II redox potentials (E_1/2 =0.988-1.380 V vs. ferrocene/ferrocenium electrode, Fc/Fc⁺ ), within the range for O₂ activation and typical for the corresponding nonheme iron enzymes. The reaction of in situ generated [Fe(L)(MeCN)(SPh)]⁺ with excess O₂ in acetonitrile (MeCN) yields selectively the doubly oxygenated phenylsulfinic acid product. Isotopic labeling studies using ¹⁸ O₂ confirm the incorporation of both oxygen atoms of O₂ into the product. Kinetic and preliminary DFT studies reveal the involvement of an Fe^III peroxido intermediate with a rhombic S= 1 / 2 Fe^III center (687-696 nm; g≈2.46-2.48, 2.13-2.15, 1.92-1.94), similar to the spectroscopic signature of the low-spin Cys-bound Fe^III CDO (650 nm, g≈2.47, 2.29, 1.90). The proposed Fe^III peroxido intermediates have been trapped, and the O-O stretching frequencies are in the expected range (approximately 920 and 820 cm^-1 for the alkyl- and hydroperoxido species, respectively). The model complexes have a structure similar to that of the enzyme and structural aspects as well as the reactivity are discussed.

One step phenol synthesis from benzene catalysed by nickel(ii) complexes

Nickel(ii)complexes of N₄-ligands are reported as efficient catalysts for direct benzene hydroxylation via bis(μ-oxo)dinickel(iii) intermediate species. The exclusive phenol formation is achieved with a yield of 41%.

Nickel(ii) complexes of a 3N ligand as a model for diketone cleaving unusual nickel(ii)-dioxygenase enzymes

Diketone substrate bound nickel(ii) complexes of 2,6-bis(1-methylbenzimidazolyl)pyridine have been synthesized and characterized as relevant active site models for unusual diketone cleaving Ni(ii)-dependent enzymes Ni-ARD and DKDO. The average Ni-N_py/benzim bond distances (2.050-2.107 Å) of model complexes are almost identical to the Ni-N_His bond distances of Ni^II-ARD (2.02-2.19 Å). The reaction of these adducts with dioxygen exhibited C-C cleavage with the rate of k_O2, 5.24-73.71 × 10^-3 M^-1 s^-1. The phenyl substituted adduct regioselectively elicits 52% of benzoic acid as the major product.

Novel nickel(ii) complexes of sterically modified linear N4 ligands: effect of ligand stereoelectronic factors and solvent of coordination on nickel(ii) spin-state and catalytic alkane hydroxylation

The donor atom type and diazacyclo backbone of the ligands and solvent of coordination dictate the Ni(ii) spin state (4, LS; 1–3, 5, HS) and catalytic activity of complexes.

Activation of Dioxygen by Iron and Manganese Complexes: A Heme and Nonheme Perspective

The rational design of well-defined, first-row transition metal complexes that can activate dioxygen has been a challenging goal for the synthetic inorganic chemist. The activation of O2 is important in part because of its central role in the functioning of metalloenzymes, which utilize O2 to perform a number of challenging reactions including the highly selective oxidation of various substrates. There is also great interest in utilizing O2, an abundant and environmentally benign oxidant, in synthetic catalytic oxidation systems. This Perspective brings together recent examples of biomimetic Fe and Mn complexes that can activate O2 in heme or nonheme-type ligand environments. The use of oxidants such as hypervalent iodine (e.g., ArIO), peracids (e.g., m-CPBA), peroxides (e.g., H2O2) or even superoxide is a popular choice for accessing well-characterized metal-superoxo, metal-peroxo, or metal-oxo species, but the instances of biomimetic Fe/Mn complexes that react with dioxygen to yield such observable metal-oxygen species are surprisingly few. This Perspective focuses on mononuclear Fe and Mn complexes that exhibit reactivity with O2 and lead to spectroscopically observable metal-oxygen species, and/or oxidize biologically relevant substrates. Analysis of these examples reveals that solvent, spin state, redox potential, external co-reductants, and ligand architecture can all play important roles in the O2 activation process.

Unravelling the Molecular Origin of the Regiospecificity in Extradiol Catechol Dioxygenases

Many factors have been suggested to control the selectivity for extradiol or intradiol cleavage in catechol dioxygenases. The varied selectivity of model complexes and the ability to force an extradiol enzyme to do intradiol cleavage indicate that the problem may be complex. In this paper we focus on the regiospecificity of the proximal extradiol dioxygenase, homoprotocatechuate 2,3-dioxygenase (HPCD), for which considerable advances have been made in our understanding of the mechanism from an experimental and computational standpoint. Two key steps in the reaction mechanism were investigated: (1) attack of the substrate by the superoxide moiety and (2) attack of the substrate by the oxyl radical generated by O-O bond cleavage. The selectivity at both steps was investigated through a systematic study of the role of the substrate and the first and second coordination spheres. For the isolated native substrate, intradiol cleavage is calculated to be both kinetically and thermodynamically favored, therefore nature must use the enzyme environment to reverse this preference. Two second sphere residues were found to play key roles in controlling the regiospecificity of the reaction: Tyr257 and His200. Tyr257 controls the selectivity by modulating the electronic structure of the substrate, while His200 controls selectivity through steric effects and by preventing alternative pathways to intradiol cleavage.

Manganese(ii) complexes of tetradentate 4N ligands with diazepane backbones for catalytic olefin epoxidation: effect of nucleophilicity of peroxo complexes on reactivity

Mononuclear Mn(ii) complexes of linear 4N ligands have been studied as catalysts for deformylation of aldehydes and epoxidation (H₂O₂) of unfunctionalised olefins to understand the role of ligand stereoelectronic factors.

Manganese(II) semiquinonato and manganese(III) catecholato complexes with tridentate ligand: modeling the substrate-binding state of manganese-dependent catechol dioxygenase and reactivity with molecular oxygen

Catecholate catwalk: Monomeric manganese(III) catecholato and manganese(II) semiquinonato complexes as the substrate-binding model of catechol dioxygenase have been synthesized and structurally characterized. The semiquinonato complex reacted with molecular oxygen to give ring-cleaved products and benzoquinone in the catalytic condition.

Iron(III) complexes with meridional ligands as functional models of intradiol-cleaving catechol dioxygenases

Six dichloroiron(III) complexes of 1,3-bis(2'-arylimino)isoindoline (BAIH) with various N-donor aryl groups have been characterized by spectroscopy (infrared, UV-vis), electrochemistry (cyclic voltammetry), microanalysis, and in two cases X-ray crystallography. The structurally characterized Fe(III)Cl(2)(L(n)) complexes (n = 3, L(3) = 1,3-bis(2'-thiazolylimino)isoindoline and n = 5, L(5) = 1,3-bis(4-methyl-2'-piridylimino)isoindoline) are five-coordinate, trigonal bipyramidal with the isoindoline ligands occupying the two axial and one equatorial positions meridionally. These compounds served as precursors for catechol dioxygenase models that were formed in solution upon addition of 3,5-di-tert-butylcatechol (H(2)DBC) and excess triethylamine. These adducts react with dioxygen in N,N-dimethylformamide, and the analysis of the products by chromatography and mass spectrometry showed high intradiol over extradiol selectivity (the intradiol/extradiol product ratios varied between 46.5 and 6.5). Kinetic measurements were performed by following the change in the intensity of the catecholate to iron ligand-to-metal charge transfer (LMCT) band, the energy of which is influenced by the isoindolinate-ligand (827-960 nm). In combination with electrochemical investigations the kinetic studies revealed an inverse trend between reaction rates and oxidation potentials associated with the coordinated DBC(2-). On the basis of these results, a substrate activation mechanism is suggested for this system in which the geometry of the peroxide-bridged intermediate may be of key importance in regioselectivity.

Synthesis, structure, spectra and reactivity of iron(III) complexes of imidazole and pyrazole containing ligands as functional models for catechol dioxygenases

A series of new 1 : 1 iron(iii) complexes of the type [Fe()Cl(3)], where is a tridentate 3N donor ligand, has been isolated and studied as functional models for catechol dioxygenases. The ligands (1-methyl-1H-imidazol-2-ylmethyl)pyrid-2-ylmethyl-amine (), N,N-dimethyl-N'-(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine () and N-(1-methyl-1H-imidazol-2-ylmethyl)-N'-phenylethane-1,2-diamine () are linear while the ligands tris(1-pyrazolyl)methane (), tris(3,5-dimethyl-1-pyrazolyl)methane () and tris(3-iso-propylpyrazolyl)methane () are tripodal ones. All the complexes have been characterized by spectral and electrochemical methods. The X-ray crystal structure of the dinuclear catecholate adduct [Fe()(TCC)](2)O, where TCC(2-) is a tetrachlorocatecholate dianion, has been successfully determined. In this complex both the iron(iii) atoms are bridged by a mu-oxo group and each iron(iii) center possesses a distorted octahedral coordination geometry in which the ligand is facially coordinated and the remaining coordination sites are occupied by the TCC(2-) dianion. Spectral studies suggest that addition of a base like Et(3)N induces the mononuclear complex species [Fe()(TCC)Cl] to dimerize forming a mu-oxo-bridged complex. The spectral and electrochemical properties of the catecholate adducts of the complexes generated in situ reveal that a systematic variation in the ligand donor atom type significantly influences the Lewis acidity of the iron(iii) center and hence the interaction of the complexes with simple and substituted catechols. The 3,5-di-tert-butylcatecholate (DBC(2-)) adducts of the type [Fe()(DBC)Cl], where is a linear tridentate ligand (), undergo mainly oxidative intradiol cleavage of the catechol in the presence of dioxygen. Also, the extradiol-to-intradiol product selectivity (E : I) is enhanced upon removal of the coordinated chloride ion in these adducts to obtain [Fe()(DBC)(Sol)](+) and upon incorporating coordinated N-methylimidazolyl nitrogen in them. In contrast to the iron(iii) complexes of imidazole-based ligands, those of the tripodal pyrazole-based ligands yield major amounts of the oxidized product benzoquinone and small amounts of both intra- and extradiol products. One of the pyrazole arms coordinated in the equatorial plane of these sterically constrained complexes is substituted by a solvent molecule upon adduct formation with DBC(2-), which encourages molecular oxygen to attack this site leading to benzoquinone formation. The DBSQ/DBC(2-) redox potentials of both the imidazole- and pyrazole-based complexes fall in the narrow range of -0.186 to -0.214 V supporting this proposal.