The Conversion of Superoxide to Hydroperoxide on Cobalt(III) Depends on the Structural and Electronic Properties of Azole-Based Chelating Ligands.

Conversion from superoxide (O2-) to hydroperoxide (OOH-) on the metal center of oxygenases and oxidases is recognized to be a key step to generating an active species for substrate oxidation. In this study, reactivity of cobalt(III)-superoxido complexes supported by facially-capping tridentate tris(3,5-dimethyl-4-X-pyrazolyl)hydroborate ([HB(pzMe2,X)3]-; TpMe2,X) and bidentate bis(1-methyl-imidazolyl)methylborate ([B(ImN-Me)2Me(Y)]-; LY) ligands toward H-atom donating reagent (2-hydroxy-2-azaadamantane; AZADOL) has been explored. The oxygenation of the cobalt(II) precursors give the corresponding cobalt(III)-superoxido complexes, and the following reaction with AZADOL yield the hydroperoxido species as has been characterized by spectroscopy (UV-vis, resonance Raman, EPR). The reaction of the cobalt(III)-superoxido species and a reducing reagent ([CoII(C5H5)2]; cobaltocene) with proton (trifluoroacetic acid; TFA) also yields the corresponding cobalt(III)-hydroperoxido species. Kinetic analyses of the formation rates of the cobalt(III)-hydroperoxido complexes reveal that second-order rate constants depend on the structural and electronic properties of the cobalt-supporting chelating ligands. An electron-withdrawing ligand opposite to the superoxide accelerates the hydrogen atom transfer (HAT) reaction from AZADOL due to an increase in the electrophilicity of the superoxide ligand. Shielding the cobalt center by the alkyl group on the boron center of bis(imidazolyl)borate ligands hinders the approaching of AZADOL to the superoxide, although the steric effect is insignificant.

Tuning the O2 Binding Affinity of Cobalt(II) Centers by Changing the Structural and Electronic Properties of the Distal Substituents on Azole-Based Chelating Ligands.

The effects of the substituents on the chelating ligands located in the secondary coordination sphere on the O2 affinity of cobalt(II) centers have been explored. The combination of facially capping tridentate tris(pyrazolyl)borates (= TpMe2,4R) and bidentate bis(imidazolyl)borates (= [B(Im N-Me)2MeX]- ; LX) yields square-pyramidal cobalt(II) complexes. The structural properties of the substituent groups X attached to the boron center of LX affect the arrangement of X in the resulting cobalt(II) complexes [CoII(TpMe2,4R)(LX)]. When the boron-attached moiety of X is a relatively bulky sp3-CH2Y group (i.e., X:Y = Me:H and nBu: nPr), the alkyl group X faces the cobalt center, whereas for isopropoxy (O iPr) and phenyl (Ph) groups, of which the boron-attached atoms are a less hindered oxygen atom and a planer sp2-carbon, respectively, the X group is arranged away from the cobalt center. This flexible behavior of LX is reflected in the O2 affinity of the cobalt(II) center, which depends on the extent to which the complex sphere is shielded by the ligands. The dependence of the cobalt(II) oxidation potential on the X substituent of LX is inconsistent with the O2 affinity. On the other hand, the electronic properties of R, which is attached to the fourth position of the pyrazolyl rings in the rigid TpMe2,4R ligand, are reflected in the electrochemical properties and O2 affinity of the cobalt center.

Cobalt(II) Complexes with N,N,N-Scorpionates and Bidentate Ligands: Comparison of Hydrotris(3,5-dimethylpyrazol-1-yl)borate Tp* vs. Phenyltris(4,4-dimethyloxazolin-2-yl)borate ToM to Control the Structural Properties and Reactivities of Cobalt Centers.

Scorpionate ligands Tp* (hydrotris(3,5-dimethylpyrazol-1-yl)borate) and ToM (tris(4,4-dimethyloxazolin-2-yl)phenylborate) complexes of cobalt(II) with bidentate ligands were synthesized. Both Tp* and ToM coordinate to cobalt(II) in a tridentate fashion when the bidentate ligand is the less hindered acetylacetonate. In crystal structures, the geometry of cobalt(II) supported by the N3O2 donor set in the Tp* complex is a square-pyramid, whereas that in the ToM complex is close to a trigonal-bipyramid. Both Tp*- and ToM-acac complexes exhibit solvatochromic behavior, although the changing structural equilibria of these complexes in MeCN are quite different. In the bis(1-methylimidazol-2-yl)methylphenylborate (LPh) complexes, Tp* retains the tridentate (к³) mode, whereas ToM functions as the bidentate (к²) ligand, giving the tetrahedral cobalt(II) complex. The bowl-shaped cavity derived from the six methyl groups on ToM lead to susceptibility to the bulkiness of the opposite bidentate ligand. The entitled scorpionate compounds mediate hydrocarbon oxidation with organic peroxides. Allylic oxidation of cyclohexene occurs mainly on the reaction with tert-butyl hydroperoxide (TBHP), although the catalytic efficiency of the scorpionate ligand complexes is lower than that of Co(OAc)2 and Co(acac)2. On cyclohexane oxidation with meta-chloroperbenzoic acid (mCPBA), both ToM and Tp* complexes function as catalysts for hydroxylation. The higher electron-donating ToM complexes show faster initial reaction rates compared to the corresponding Tp* complexes.

Characterization of Mononuclear Non-heme Iron(III)-Superoxo Complex with a Five-Azole Ligand Set.

Reaction of O2 with a high-spin mononuclear iron(II) complex supported by a five-azole donor set yields the corresponding mononuclear non-heme iron(III)-superoxo species, which was characterized by UV/Vis spectroscopy and resonance Raman spectroscopy. (1)H NMR analysis reveals diamagnetic nature of the superoxo complex arising from antiferromagnetic coupling between the spins on the low-spin iron(III) and superoxide. This superoxo species reacts with H-atom donating reagents to give a low-spin iron(III)-hydroperoxo species showing characteristic UV/Vis, resonance Raman, and EPR spectra.

Structural characterization and oxidation reactivity of a nickel(II) acylperoxo complex.

The nickel(II)-acylperoxo complex [Ni(Tp(CF3Me))(κ(2)-mCPBA)] (1(CF3Me)) [Tp(CF3Me) = hydrotris(3-trifluoromethyl-5-methylpyrazolyl)borate, mCPBA = m-chloroperbenzoate] was isolated and fully characterized. The electrophilic oxygenation ability of 1(CF3Me) toward sulfides and olefins was confirmed. The Michaelis-Menten-type behavior of thioanisole oxygenation indicates the existence of a pre-equilibrium of substrate association in the reaction. In addition, 1(CF3Me) retains H-atom abstraction ability for hydrocarbons with activated methylene C-H bonds (e.g., fluorene). The oxidations of styrenes and these readily oxidizable hydrocarbons follow second-order kinetics, first-order each with respect to 1(CF3Me) and substrate. The lack of clear acceleration in the decay of 1(CF3Me) in the presence of substrates with high C-H bond dissociation energies (e.g., cyclohexane) suggests that another reaction pathway contributes through the O-O-cleaved intermediate.

Selective alkane hydroxylation and alkene epoxidation using H2O2 and Fe(II) catalysts electrostatically attached to a fluorinated surface.

Fe(II) complexes with pentadentate ligands, including N-heterocyclic carbene moieties, were prepared and electrostatically attached onto the perfluorinated surface of a mesoporous aluminosilicate. The heterogeneous catalysts were applied to the catalytic oxidation of cyclohexane and cyclohexene using H2O2 as an oxidant in CH3CN, demonstrating high performance and selectivity in alkane hydroxylation and cyclohexene epoxidation.

Thermosalient effect of a naphthalene diimide and tetrachlorocobaltate hybrid and changes of color and magnetic properties by ammonia vapor.

An organic-inorganic hybrid metal halide (OIMH), namely the electron-deficient naphthalene diimide (NDI) and [CoCl4]2- hybrid (1), showed potential as a sensor for ammonia and amines, in addition to magnetic changes upon vapochromism. Crystal 1 exhibited thermosalient behavior such as leaping and movement, at around 130 °C, which could be explained to be associated with the removal of water molecules from the crystal lattice as shown by TGA and DSC. Compound 1 changed from green to black within 5 minutes when exposed to ammonia vapor, which was attributed to the radical formation in the NDI moiety as evidenced by ESR, and this phenomenon was preserved even when other mono- and di-alkylamines were applied. The exposure of 1 to ammonia resulted in a subsequent color alteration, progressing from black to a gradually dark orange after one day (1_NH3_1 day). This transformation was concomitant with the formation of [Co(NH3)6]3+ from [CoCl4]2-, leading to a modification of the magnetic properties from paramagnetic Co(II) (S = 3/2) to diamagnetic Co(III) (S = 0). Based on these findings, compound 1 represents the first example of an OIMH that exhibits thermosalient behaviour, color change, and magnetic conversion upon exposure to ammonia.

Development of a novel scorpionate ligand with 6-methylpyridine and comparison of the structural and electronic properties of nickel(II) complexes with related tris(azolyl)borates.

A novel anionic tridentate borate ligand with a 6-methylpyridyl donor, TpyMe, has been synthesized. Comparison of the molecular structures and reactivities of nickel(II)-bromido complexes with tris(azolyl)borate ligands composed of pyridyl, pyrazolyl, or oxazolinyl donors indicates the characteristic sterically demanding nature and strong electron donating ability of TpyMe.

Dioxygen activation and substrate oxygenation by a p-nitrothiophenolatonickel complex: unique effects of an acetonitrile solvent and the p-nitro group of the ligand.

The nickel(II) complex [Ni(Tp(Me2)) (SC(6)H(4)NO(2))] [1a; Tp(Me2) = hydrotris(3,5-dimethylpyrazol-1-yl)borate] reacts with O(2) to form the ligand oxygenation product ArSO(2)(-) in MeCN, and also 1a catalyzes the oxygenation of external substrates such as triphenylphosphine. The reactivity may correlate to the unique quinoid-like resonance structure of the thiophenolate ligand. The structure is stabilized by a p-nitro group and induced by coordination of MeCN.

Efficient alkane hydroxylation catalysis of nickel(ii) complexes with oxazoline donor containing tripodal tetradentate ligands.

Tris(oxazolynylmethyl)amine TOAR (where R denotes the substituent groups on the fourth position of the oxazoline rings) complexes of nickel(ii) have been synthesized as catalyst precursors for alkane oxidation with meta-chloroperoxybenzoic acid (m-CPBA). The molecular structures of acetato, nitrato, meta-chlorobenzoato and chlorido complexes with TOAMe2 have been determined using X-ray crystallography. The bulkiness of the substituent groups R affects the coordination environment of the nickel(ii) centers, as has been demonstrated by comparison of the molecular structures of chlorido complexes with TOAMe2 and TOAtBu. The nickel(ii)-acetato complex with TOAMe2 is an efficient catalyst precursor compared with the tris(pyridylmethyl)amine (TPA) analogue. Oxazolynyl donors' strong σ-electron donating ability will enhance the catalytic activity. Catalytic reaction rates and substrate oxidizing position selectivity are controlled by the structural properties of the R of TOAR. Reaction of the acetato complex with TOAMe2 and m-CPBA yields the corresponding acylperoxido species, which can be detected using spectroscopy. Kinetic studies of the decay process of the acylperoxido species suggest that the acylperoxido species is a precursor of an active species for alkane oxidation.

Heteroleptic cobalt(iii) acetylacetonato complexes with N-heterocyclic carbine-donating scorpionate ligands: synthesis, structural characterization and catalysis.

Exposure of O2 to a reaction mixture containing bis(acac)cobalt(ii), a facially capping tris(N-heterocyclic carbene)borate ligand and 1-methylimidazole yields a heteroleptic cobalt(iii) complex with acac, 1-methylimidazole and tris(NHC)borate ligands. meta-Chloroperbenzoic acid is efficiently activated by this heteroleptic complex to catalytically oxidize cyclohexane at ambient temperature.

Immobilization of a Boron Center-Functionalized Scorpionate Ligand on Mesoporous Silica Supports for Heterogeneous Tp-Based Catalysts.

To develop novel immobilized metallocomplex catalysts, allyltris(3-trifluoromethylpyrazol-1-yl)borate (allyl-TpCF3) was synthesized. A boron-attached allyl group reacts with thiol to afford the desired mesoporous silica-immobilized TpCF3. Cobalt(II) is an efficient probe for estimating the structures of the immobilized metallocomplexes. The structures of the formed cobalt(II) complexes and their catalytic activity depended on the density of the organic thiol groups and on the state of the remaining sulfur donors on the supports.

A pseudotetrahedral nickel(II) complex with a tridentate oxazoline-based scorpionate ligand: chlorido[tris(4,4-dimethyloxazolin-2-yl)phenylborato]nickel(II).

Poly(pyrazol-1-yl)borates have been utilized extensively in coordination compounds due to their high affinity toward cationic metal ions on the basis of electrostatic interactions derived from the mononegatively charged boron centre. The original poly(pyrazol-1-yl)borates, christened `scorpionates', were pioneered by the late Professor Swiatoslaw Trofimenko and have expanded to include various borate ligands with N-, P-, O-, S-, Se- and C-donors. Scorpionate ligands with boron-carbon bonds, rather than the normal boron-nitrogen bonds, have been developed and in these new types of scorpionate ligands, amines and azoles, such as pyridines, imidazoles and oxazolines, have been employed as N-donors instead of pyrazoles. Furthermore, a variety of bis- and tris(oxazolinyl)borate ligands, including chiral ones, have been developed. Tris(oxazolin-2-yl)borates work as facially capping tridentate chelating ligands in the same way as tris(pyrazol-1-yl)borates. In the title compound, [Ni(C21H29BN3O3)Cl], the NiII ion is coordinated by three N atoms from the facially capping tridentate chelating tris(4,4-dimethyloxazolin-2-yl)phenylborate ligand and a chloride ligand in a highly distorted tetrahedral geometry. The Ni-Cl bond length [2.1851 (5) Å] is comparable to those found in a previously reported tris(3,5-dimethylpyrazol-1-yl)hydroborate derivative [2.1955 (18) and 2.150 (2) Å]. The molecular structure deviates from C3v symmetry due to the structural flexibility of the tris(4,4-dimethyloxazolin-2-yl)phenylborate ligand.

Synthesis and Characterization of the Benzoylformato Ferrous Complexes with the Hindered Tris(pyrazolyl)borate Ligand as a Structural Model for Mononuclear Non-Heme Iron Enzymes.

By using a hindered tripodal ligand, hydrotris(3-tert-butyl-5-isopropylpyrazol-1-yl)borate HB(3-tBu-5-iPrpz)(3), a series of monomeric ferrous complexes having acetate, hydroxide, and benzoylformate ligands were synthesized. Reaction of KHB(3-tBu-5-iPrpz)(3) with anhydrous Fe(OAc)(2) yielded acetato complexes Fe(OAc)[HB(3-tBu-5-iPrpz)(3)] (1) and Fe(OAc)[HB(3-tBu-5-iPrpz)(3)](3-iPr-5-tBupzH) (2). A hydroxo complex Fe(OH)[HB(3-tBu-5-iPrpz)(3)] (3) was prepared by the treatment of 1 or 2 with aqueous NaOH. The geometry of Fe(II) in 3 is a slightly distorted tetrahedron as determined by X-ray crystallography. The hydroxo complex 3 reacted with benzoylformic acid to give the benzoylformato complex Fe(O(2)CC(O)Ph)[HB(3-tBu-5-iPrpz)(3)] (4), which showed thermochromism which depended on the coordination geometry of the benzoylformate ligand. The Fe(II) ion in the colorless form of 4 isolated at 4 degrees C is coordinated by a tetrahedral N(3)O(1) ligand donor set including the unidentate benzoylformato ligand. On the other hand, the bluish purple form of 4 isolated at -20 degrees C has a five-coordinate trigonal bipyramidal Fe(II) center. The benzoylformate ligand in this bluish purple form works as a chelate ligand through coordination of the unidentate carboxylate oxygen atom as well as the ketonic oxygen atom. A benzoylformato complex containing an additional pyrazole, Fe(O(2)CC(O)Ph)[HB(3-tBu-5-iPrpz)(3)](3-iPr-5-tBupzH) (5), was obtained by the reaction of 3 with benzoylformic acid in the presence of 3-tert-butyl-5-isopropylpyrazole. The iron atom in 5 is coordinated by an N(4)O(1) ligand donor set with trigonal bipyramidal geometry. A hydrogen-bonding interaction between the carboxylate oxygen atom and the additional pyrazole's NH proton in 5 is suggested from the short distance between O(carboxylate) and N(pyrazole) observed in the X-ray structure and the absence of the nuNH vibration in the IR spectrum.

Alkane oxidation by an immobilized nickel complex catalyst: structural and reactivity differences induced by surface-ligand density on mesoporous silica.

Immobilized nickel catalysts SBA*-L-x/Ni (L = bis(2-pyridylmethyl)(1H-1,2,3-triazol-4-ylmethyl)amine) with various ligand densities (L content (x) = 0.5, 1, 2, 4 mol % Si) have been prepared from azidopropyl-functionalized mesoporous silicas SBA-N3-x. Related homogeneous ligand L(tBu) and its Ni(II) complexes, [Ni(L(tBu))(OAc)2(H2O)] (L(tBu)/Ni) and [Ni(L(tBu))2]BF4 (2 L(tBu)/Ni), have been synthesized. The L/Ni ratio (0.9-1.7:1) in SBA*-L-x/Ni suggests the formation of an inert [NiL2] site on the surface at higher ligand loadings. SBA*-L-x/Ni has been applied to the catalytic oxidation of cyclohexane with m-chloroperbenzoic acid (mCPBA). The catalyst with the lowest loading shows high activity in its initial use as the homogeneous L(tBu)/Ni catalyst, with some metal leaching. As the ligand loading increases, the activity and Ni leaching are suppressed. The importance of site-density control for the development of immobilized catalysts has been demonstrated.

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.

Characterization of nickel(II)-acylperoxo species relevant to catalytic alkane hydroxylation by nickel complex with mCPBA.

Nickel complexes with hydrotris(pyrazolyl)borate ( = Tp(R)) ligands catalyze alkane oxidation with organic peroxide meta-Cl-C(6)H(4)C([double bond, length as m-dash]O)OOH ( = mCPBA). The electronic and steric hindrance properties of Tp(R) affect the catalyses. The complex with an electron-withdrawing group containing a less-hindered ligand, that is, Tp(Me2,Br), exhibits higher alcohol selectivity. Higher selectivity for secondary over tertiary alcohols upon oxidation of methylcyclohexane indicates that the oxygen atom transfer reaction proceeds within the coordination sphere of the nickel centers. A reaction of the catalyst precursor, dinuclear nickel(ii)-bis(µ-hydroxo) complexes, with mCPBA yields the corresponding nickel(ii)-acylperoxo species, as have been characterized by spectroscopy. Thermal decomposition of the nickel(ii)-acylperoxo species in CH(2)Cl(2) yields the corresponding nickel(ii)-chlorido complexes through Cl atom abstraction. Employment of the brominated ligand increases the thermal stability of the acylperoxo species. Kinetic isotope effects observed on decay of the nickel(ii)-acylperoxo species indicate concerted O-O breaking of the nickel-bound acylperoxide and H-abstraction from the solvent molecule.