Shiro Hikichi

Structural Characterization and Intramolecular Aliphatic C−H Oxidation Ability of MIII(μ-O)2MIII Complexes of Ni and Co with the Hydrotris(3,5-dialkyl-4-X-pyrazolyl)borate Ligands TpMe2,X (X=Me, H, Br) and TpiPr2

Reaction of the dinuclear M(II)-bis(mu-hydroxo) complexes of nickel and cobalt, [(M(II)(TpR)]2(mu-OH)2] (M = Ni; 3Ni M = Co: 3Co), with one equivalent of H2O2 yields the corresponding M(III)-bis(mu-oxo) complexes, [[M(III)(TpR)]2-(mu-O)2] (M=Ni; 2Ni, M=Co: 2Co). The employment of a series of TpMe2,X (TpMe2,X = hydrotris(3,5-dimethyl-4-X-1-pyrazolyl)borate; X = Me, H, Br) as a metal supporting ligand makes it possible to isolate and structurally characterize the thermally unstable M(III)-bis-(mu-oxo) complexes 2Ni and 2Co. Both the starting (3Ni and 3Co) and resulting complexes (2Ni and 2Co) contain five-coordinate metal centers with a slightly distorted square-pyramidal geometry. Characteristic features of the nickel complexes 2Ni, such as the two intense absorptions around 400 and 300 nm in the UV-visible spectra and the apparent diamagnetism, are very similar to those of the previously reported bis(mu-oxo) species of Cu(III) and Ni(III) with ligands other than TpR, whereas the spectroscopic properties of the cobalt complexes 2Co (i.e., paramagnetically shifted NMR signals and a single intense absorption appearing at 350 nm) are clearly distinct from those of the isostructural nickel compounds 2Ni. Thermal decomposition of 2Ni and 2Co results in oxidation of the inner saturated hydrocarbyl substituents of the TpR ligand. Large kH/kD values obtained from the first-order decomposition rates of the TpMe3 and Tp(CD3)2,Me derivatives of 2 evidently indicate that the rate-determining step is an hydrogen abstraction from the primary C-H bond of the methyl substituents. mediated by the M(III)2-(mu-O)2 species. The nickel complex 2Ni shows reactivity about 10(3) times greater than that of the cobalt analogue 2Co. The oxidation ability of the M(III)(mu-O)2M(III) core should be affected by the hindered TpR ligand system, which can stabilize the +2 oxidation state of the metal centers.

New aspects of the cobalt-dioxygen complex chemistry opened by hydrotris(pyrazoly)borate ligands (TpR): unique properties of TpRCo-dioxygen complexes

Abstract Recent advances in the chemistry of cobalt-dioxygen and related complexes supported by hydrotris(pyrazolyl)borate ligands (TpR) are reviewed. The unique properties of inorganic and organometallic TpRCo complexes clearly indicate that hindered TpR ligands carrying alkyl substituents on the pyrazolyl groups can stabilize the coordinatively unsaturated, low valent cobalt species, and that the reactivity of the coordinatively unsaturated species is influenced by steric hindrance of TpR. The unusual low-valent metal-peroxo and high-valent metal-oxo species such as CoII-superoxo, -alkylperoxo and dinuclear CoIII-bis(μ-oxo) complexes are characterized. The high-valent metal-oxo species, [TpRCoIII]2(μ-O)2, are capable of abstracting the H atom from the alkyl groups proximal to the bimetallic bis(μ-oxo) core. In the hydrotris(3,5-diisopropyl-1-pyrazolyl)borate ligand system (TpPr2i), oxygenation of the proximal isopropyl substituents on TpPr2i is mediated by the CoIII2-(μ-O)2 and CoIIue5f8OOX (X=alkyl, H) species.

CONVERSION OF MOLECULAR OXYGEN INTO A HYDROPEROXO SPECIES BY RING-OPENING PROTONATION OF A CYCLIC ETA 2-PEROXO INTERMEDIATE : CHARACTERIZATION OF THE ETA 2-PEROXO AND HYDROPEROXO COMPLEXES

A transition metal-hydroperoxo species is formed by the oxygenation of a low-valent rhodium precursor followed by a protonation of the resulting eta(2)-O(2) ligand; the latter process is assisted by an intramolecular hydrogen-bonding interaction (see scheme). This process is the first structural evidence for an effective method for the activation of molecular oxygen as postulated for the cytochrome P-450 system.

Characterization of a dinuclear Mn(II) tri(μ-carboxylato) complex with the hindered hydrotris(3,5-diisopropyl-1-pyrazolyl)borate (=TpiPr2) ligand: intramolecular hydrogen bonding interaction between the protonated TpiPr2 and Mn-coordinating carboxylate ligands

Abstract A dinuclear Mn(II) di(μ-hydroxo) complex having hydrotris(3,5-diisopropyl-1-pyrazolyl)borate (=TpiPr2) reacted with benzoic acid to yield a dinuclear Mn(II) tri(μ-carboxylato) complex, TpiPr2Mn-(μ-OBz)3-Mn(TpiPr2H). X-ray crystallography reveals the unsymmetrical coordination environments for the manganese centers. One of the two TpiPr2 ligands, which bound to the five-coordinated Mn center, is protonated by the action of the third carboxylic acid and the resulting non-Mn-binding N–H moiety forms an intramolecular hydrogen bond with the oxygen donor of a carboxylate ligand. Steric congestion in the bimetallic core results in the large separation of the manganese centers bridged by the syn-anti carboxylate ligand.

An approach to the O2 activating mononuclear non-heme Fe enzymes: structural characterization of Fe(II)–acetato complex and formation of alkylperoxoiron(III) species with the highly hindered hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate

Abstract Structural characterization of an Fe(II)–acetato complex and attempts to synthesize mononuclear Fe(III) dioxygen complexes bearing the highly sterically demanding TptBu,iPr (=hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate) ligand have been investigated. X-ray crystallography reveals that the acetato complex consists of the distorted square pyramidal Fe(II) center as found for the previously reported O2-reactive TpiPr2 derivative. In contrast to the less hindered TpiPr2, (=hydrotris(3,5-diisopropyl-1-pyrazolyl)borate) complexes, oxidative addition of O2 to the coordinatively unsaturated Fe(II) centers of the acetato and a hydroxo complexes with TptBu,iPr has never been observed in any conditions. Reaction of the ferrous hydroxo complex with ROOH (R=H, alkyl) results in the formation of the thermally unstable intermediates. Especially, the Fe(III)-alkylperoxo complex is characterized by UV–Vis, ESR and resonance Raman spectroscopy. The extremely bulky TptBu,iPr ligand hinders the approach of the exogenous O2 molecule to the Fe(II) centers but stabilizes the unstable Fe(III)alkylperoxo intermediate enough to be detected.

Umwandlung von molekularem Sauerstoff in eine Hydroperoxoverbindung über eine Protonierungs‐induzierte Ringöffnung eines cyclischen η2‐Peroxo‐Intermediats: Charakterisierung der η2‐Peroxo‐ und Hydroperoxokomplexe

EineUbergangsmetall-Hydroperoxoverbindung bildet sich durch Sauerstoffanlagerung an ein niedervalentes Rhodium-Edukt, der eine Protonierung des resultierenden η2-O2-Liganden folgt, wobei dieser Vorgang durch eine intramolekulare Wechselwirkung uber Wasserstoffbrucken unterstutzt wird (siehe Schema). Dieser Prozes ist der erste strukturelle Nachweis einer wirksamen Methode zur Aktivierung von molekularem Sauerstoff, wie sie fur Cytochrom P-450 postuliert wurde.

An entry to half-sandwich complexes of first row transition metals (Ni, Co, Mn) containing the Kläui tripodal ligand, LOMe: synthesis and crystal structures of oligomeric, [LOMeM(μ-X)(L)]n (n = 2, 4), and monomeric complexes, LOMeM(L2)(X)

A series of half-sandwich complexes of Ni, Co and Mn containing the Klaui tripodal ligand LOMe [κ3-(η5-C5H5)Co{P(O)(OCH3)2}3] are prepared by reaction of the labile precursors, LOMeM(L)(κn-NO3) [Mxa0=xa0Ni, Co; Lxa0=xa0acetone (1, nxa0=xa02), PPh3 (2, nxa0=xa02), κ2-bipy (8, nxa0=xa01), κ2-tmeda (9, nxa0=xa01)] and [LOMeMn(py)(μ-NO3)]2 (15), with anionic N- and O-nucleophiles. Oligomeric complexes are formed from complexes 1, 2, and 15via elimination of the κ1-ligands (L). Reactions with NaN3 and sodium aryloxide (NaOAr) afford the tetrameric μ3-azido complexes, [LOMeM(μ3-N3)]4 (3), and the dimeric μ-aryloxo complexes, [LOMeM(μ-OAr)(X)]2 (5, 6: Arxa0=xa02,6-Me2-C6H3, 2,4,6-Me3-C6H2, 2- and 4-NO2-C6H4, C6F5; Xxa0=xa0none, ROH), respectively, but the sandwich complexes (LOMe)2M result from reactions with other O-nucleophiles such as NaOH, NaOR, and NaOAr [Arxa0=xa0C6H5, 4-MeC6H4, 2,6-X2-C6H3 (Xxa0=xa0F, Cl)]. Mononuclear complexes LOMeM(κ2-L)(κ1-NO3) 8 and 9 are obtained by treatment of 1 or 2 with bidentate ligands and further converted to LOMeM(κ2-L)-X-type azido (10, 11), halo (12), aryloxo (13) and MeCN-coordinated cationic complexes (14).

Novel monoanionic tripodal ligands: methylbis(1-methylimidazol-2-yl)(pyrazol-1-yl)borate (= [MeB(ImN-Me)2(PzR)]−). Synthesis and structural characterization of their nickel complexes and carboxylate shift from nickel to boron

Novel monoanionic tripodal ligands, [MeB(ImN-Me)2(PzR)]− (=xa0methylbis(1-methylimidazol-2-yl)(pyrazol-1-yl)borate), are synthesized; their mode of coordination to a nickel center depends on the steric congestion around the boron centers.

Interaction of hydroperoxopalladium complexes, (TpR)(py)Pd–OOH, with hydroxo-nickel and -cobalt complexes, [(μ-OH)(MTpR′)]2(M = Ni, Co), leading to oxidative dehydrogenation of the saturated hydrocarbyl moiety in the ancillary ligand (TpiPr2)

Treatment of hydroperoxopalladium complexes, (TpR)(py)Pd–OOH, with hydroxonickel complexes, [(μ-OH)NiTpR′]2, when either TpR or TpR′ is TpiPr2, results in dehydrogenation of an isopropyl group of the TpiPr2 ligand to give heterobimetallic di-μ-hydroxo complexes bearing the 3-isopropenyl-substituted Tp ligand [HB(pziPr2)2(pz3-isopropenyl-5-iPr). Similar dehydrogenation is observed for the reaction with the hydroxocobalt complex bearing the TpiPr2 ligand. The dehydrogenated products are characterized by spectroscopic and crystallographic methods and a mechanism involving a heterobimetallic μ-peroxo intermediate formed via dehydrative condensation has been proposed for the oxidative dehydrogenation.

Synthesis and characterization of enolato-cobalt and -nickel complexes bearing TpiPr2 ligand, [TpiPr2M(XCHY)]n [TpiPr2=hydrotris(3,5-diisopropylpyrazolyl)borato; n=1, 2], obtained from hydroxometal complexes, (μ-OH)2(MTpiPr2)2, and active methylene compounds, CH2XY

Abstract Reaction of hydroxo-cobalt and -nickel complexes, (TpiPr2M)2(μ-OH)2 (1) [M=Co (1Co), Ni (1Ni); TpiPr2: hydrotris(3,5-diisopropylpyrazolyl)borato], with active methylene compounds, CH2XY (2) [X/Y=CN/COPh (2a); CN/CN (2b); COCH2CMe2CH2CO (2d: dimedone); COMe/COOMe (2e); COOMe/COOMe (2f)], in the presence of a drying agent (e.g. Na2SO4) results in the dehydrative condensation to give N/O-bound enolato complexes with monomeric chelated structure TpiPr2 Mue5f8XCHY (3e and 3f), or dimeric cyclic structure (TpiPr2M)2(μ-XCHY)2 (3a, 3b, and 3d). In contrast, treatment of 1 with methyl cyanoacetate (2c) gives the κ2-carboxylato complex, TpiPr2M(κ2-OOCCH2CN) (4c), via hydrolysis of the ester moiety by the action of the OH functional group in 1. The reaction pathway (enolato formation vs. ester hydrolysis) is a combined result of various factors including the acidity of the CH moiety in 2, rigidity of the enolato skeleton, and relative thermodynamic stability of possible structures. Molecular structures of 3bCo-2py, 3eCo-MeCN, 3fNi and 4cNi determined by X-ray crystallography reveal the formation the N/O-bound structure, where the negative charge is widely delocalized over the enolato functional group. No isomerization to C-bound enolate, TpRM-CHXY, is observed, even when a less sterically hindered ligand, TpMe2, is employed. Reaction of 3aCo and 3aNi with benzaldehyde carried out as a preliminary study of the reactivity of the obtained enolato complexes 3 affords the metallacyclic products TpiPr2C oue5f8O-C(Ph)-C(CN-CoTpiPr2)-CHPh-C(CN)-C(Ph)-O (5) (characterized by X-ray crystallography as a 2MeCN adduct) and TpiPr2N iue5f8O-C(Ph)-C(CN-H)-CHPh-C(CN)-C(Ph)-O (6), respectively, by way of double condensation of PhCHO with two enolato functional groups.