Showing papers by "Shiro Hikichi published in 2002"

Inorganic chemistry based on Tp ligands: From dioxygen complexes to organometallic systems

Abstract: Our recent research on transition metal complexes containing hydrotris(pyrazolyl)borato ligands (TpR), which ranges from bioinorganic chemistry of dioxygen complexes to organometallic systems, is reviewed. The TpR ligand turns out to be versatile in a wide variety of inorganic studies, because the coordination properties (both steric and electronic) can be finely tuned in a systematic manner by choosing appropriate substituents on the pyrazolyl rings to serve the purpose of the study (e.g. stabilization, activation, functionalization). For first row transition metal complexes, TpR serves as a tetrahedral enforcer to form a coordinatively unsaturated tetrahedral species, which is interconverted with five- and six-coordinate structures via coordination/dissociation of donor(s). In addition to the traditional view of TpR as a ligand system isoelectronic with cyclopentadienyls (η5-C5R5), the TpR ligand is now recognized as a unique ancillary ligand suitable for bioinorganic studies, because the tripodal ligan...

Synthesis and Characterization of a Series of (Hydroperoxo)-, (Alkylperoxo)-, and (μ-Peroxo)palladium Complexes Containing the Hydrotris(3,5-diisopropylpyrazolyl)borato Ligand (TpiPr2): (TpiPr2)(py)Pd−OO−X [X = H, t-Bu, Pd(TpiPr2)(py)] and (TpiPr2)(py)Pd−(μ-κ1:κ2-OO)−PdTpiPr2

Abstract: Dehydrative condensation of the hydroxopalladium complex (TpiPr2)(py)Pd−OH (1) with hydroperoxides (XOOH: X = H, t-Bu) produces the corresponding (hydroperoxo)-, (TpiPr2)(py)Pd−OOH (2a), and (tert-butylperoxo)palladium complexes, (TpiPr2)(py)Pd−OOBut (3). Treatment of 2a with PPh3 results in concomitant ligand displacement giving (TpiPr2)(Ph3P)Pd−OOH (2b) and oxygenation of PPh3 giving OPPh3. Further condensation between 1 and 2a gives the μ-κ1:κ1-peroxo complex (TpiPr2)(py)Pd−OO−Pd(TpiPr2)(py) (4), while condensation between the bis(μ-hydroxo)dipalladium complex (PdTpiPr2)2(μ-OH)2 (7) with 2a affords the unsymmetrical μ-κ1:κ2-peroxo complex (TpiPr2)(py)Pd−OO−PdTpiPr2 (5). These peroxopalladium complexes 2−5 have been fully characterized by a combination of spectroscopic and crystallographic analyses, which lead to description of the O−O moieties in these complexes as peroxide (O22-) with sp3-hybridized oxygen atoms. The OOH moiety in 2b is found to interact with the noncoordinated nitrogen atom of the p...

Highly labile cationic tris-acetonitrile complexes, [TpRM(NCMe)3]OTf (M = Ni, Co; TpR: hydrotrispyrazolylborato, R = Ph, Me and iPr2): versatile precursors for TpR-containing nickel and cobalt complexes

Abstract: The cationic tris-MeCN adducts of TpRM fragments, [TpRM(NCMe)3]OTf [TpR = hydrotrispyrazolylborato, R = 3-Ph-5-Me (1), 3,5-iPr2 (1′); M = Ni, Co], were prepared by chloride abstraction from the corresponding chloro complexes, TpRM–Cl (2), with AgOTf in acetonitrile. The MeCN ligands in 1 and 1′ are labile enough to be readily converted to a variety of adducts, [TpRM(L)n]OTf, via treatment with N- and P-donors [NC(CH2)2CN, p-NCC6H4CN, pyridine, o-bipy, p-bipy, Ph2P(CH2)mPPh2 (m = 2, 3)].

Selective Synthesis, Characterization, and Configurational Flexibility of the Coordinatively Unsaturated Metal Center of Half-Sandwich Type Complexes with the Less-Hindered Hydrotris(3, 5-dimethyl-4-X-1-pyrazolyl)borate Ligands [TpMe2,XMII(κ2-O,O′-L)] (M = Ni, Co; L = NO3, OAc; X = Me, H, Br)

Abstract: The selective synthesis of half-sandwich type complexes of Ni(II) and Co(II) with the less-hindered hydrotris(3,5-dimethyl-4-X-1-pyrazolyl)borate [X = Me (TpMe3; a series), H (TpMe2; b series), and Br (TpMe2,Br; c series)] ligands, [TpMe2,XMII(κ2-O,O′-L)] [L = NO3 (2), OAc (3)], has been achieved by dropwise addition of a THF solution of NaTpMe2,X to an excess amount of ML2 dissolved in MeOH. The solid state structures of the nitrato complexes 2 have been characterized by X-ray crystallography as their solvated form, 2·solv. (solv. = MeCN, MeOH), in which the metal centers have the six-coordinated octahedral geometry. Solvates are readily dissociated from the metal center upon dissolution in non-coordinationg solvents, such as CH2Cl2 and toluene, as well as drying of a solid sample to give the non-solvated five-coordinated species 2. The crystal structure of the non-solvated TpMe2Ni derivative 3bNi consists of a highly distorted trigonal bipyramidal nickel center.

Di- and tri-valent chlorochromium complexes bearing hydrotris(3,5-diisopropylpyrazolyl)borato ligand (TpiPr2), TpiPr2CrCln(L) (n = 1, 2) and [TpiPr2Cr(μ-Cl)]2: flexible coordination behavior of the TpiPr2 ligand and reduction features of Cr(II) species

Abstract: Divalent chlorochromium complexes bearing the TpiPr2 ligand [hydrotris(3,5-diisopropylpyrazolyl)borato], TpiPr2CrIICl(L) [L = 3,5-diisopropylpyrazole (1a), py (1b), bipy (2)] and TpiPr2CrII(μ-Cl)]2 (3), were prepared by treatment of CrCl2 with a TpiPr2 salt in the presence (1,2)/absence (3) of a donor (L). Reaction of 1 with benzyl chloride results in quantitative conversion to the corresponding trivalent dichlorochromium species, TpiPr2CrIIICl2(L) (6), while reaction with CH2Cl2 gives the methylene-inserted product, TpiPr2CrIIICl(CH2-L) 7, via a carbene intermediate, which is supported by cyclopropanation of styrene with CH2Cl2 in the presence of 3. The Cr(II) complex 1b induces reductive coupling of benzaldehyde to give a 3,4-diphenyl-2,5-dioxachromacyclopent-3-ene, TpiPr2Cr[κ2-O–C(Ph)C(Ph)–O] 8. Steric tuning effects of the TpiPr2 ligand leads to a variety of coordination geometries including five-coordinate square-pyramidal and trigonal-bipyramidal structures and a six-coordinate octahedral structure as revealed by crystallographic characterization.

Reactivity Control of a Dinuclear Xenophilic Species, Tp#Ni–RuCp(CO)2, via Spin Crossover across a Metal-metal Bond [Tp#: Hydrotris(4-bromo-3,5-dimethylpyrazolyl)borato]

Abstract: A high-spin, triplet xenophilic complex, Tp#Ni-RuCp(CO)2, readily reacted with 2e-donors (L) via spin crossover at Ni associated with transposition of the resultant coordinatively unsaturated site from Ni to Ru to give diamagnetic adducts, Tp#Ni-RuCp(μ-CO)2(L).