Shigeki Kuwata

Hydrosulfido complexes of transition metals

Abstract Interest in hydrosulfido (SH) complexes stems from their relevance to metalloenzymes and metal sulfide catalysts for industrial hydrodesulfurization. This review provides an overview of the chemistry of hydrosulfido complexes of transition metals. Synthetic methods of hydrosulfido complexes will first be classified, followed by the description of their spectroscopic properties and structural features. Significant emphasis is also placed on the reactivities of hydrosulfido complexes including nucleophilic reactions, oxidative coupling, and sulfido cluster formation.

β‐Protic Pyrazole and N‐Heterocyclic Carbene Complexes: Synthesis, Properties, and Metal–Ligand Cooperative Bifunctional Catalysis

This Minireview provides an overview of the chemistry of pyrazole and N-heterocyclic carbene (NHC) complexes bearing an NH group at the β-position to the metal. The synthesis and structures as well as the Brønsted acidic nature of the β-NH group are described in detail. These complexes are attractive candidates for novel metal-ligand cooperative bifunctional catalysts, which would benefit highly effective molecular and energy transformations.

N–N Bond Cleavage of Hydrazines with a Multiproton-Responsive Pincer-Type Iron Complex

N-N bond cleavage of hydrazines on transition metals is of considerable importance in understanding the mechanism of biological nitrogen fixation under ambient conditions. We found that a metal-ligand-bifunctional complex of iron with a pincer-type ligand bearing two proton-responsive pyrazole arms catalyzes the disproportionation of hydrazine into ammonia and dinitrogen. The NH groups in the pyrazole ligands and hydrazines are crucial for the reaction, which most likely occurs through multiple and bidirectional proton-coupled electron transfer between the iron complex and hydrazine. The multiproton-responsive pincer-type ligand also stabilizes the intermediate diazene complex through a hydrogen-bonding network, as revealed by structural characterization of a κ(1)N-phenylhydrazine complex.

H-H and N-H bond cleavage of dihydrogen and ammonia with a bifunctional parent imido (NH)-bridged diiridium complex.

Hydrogenation and protonation of parent imido complexes have attracted much attention in relation to industrial and biological nitrogen fixation. The present study reports the structure and properties of the highly unsaturated diiridium parent imido complex [(Cp*Ir)(2)(μ(2)-H)(μ(2)-NH)](+) derived from deprotonation of a parent amido complex. Because of the Lewis acid-Brønsted base bifunctional nature of the metal-NH bond, the parent imido complex promotes heterolysis of H(2) and deprotonative N-H cleavage of ammonia to afford the corresponding parent amido complexes under mild conditions.

Hydrogen-and Oxygen-Driven Interconversion between Imido-Bridged Dirhodium(III) and Amido-Bridged Dirhodium(II) Complexes

The reaction of [Cp*RhCl(2)](2) (Cp* = eta(5)-C(5)(CH(3))(5)) with 2 equiv of p-toluenesulfonamide in the presence of KOH resulted in the formation of the sulfonylimido-bridged dirhodium(III) complex [(Cp*Rh)(2)(mu-NTs)(2)] (1a; Ts = SO(2)C(6)H(4)CH(3)-p). The imido complex 1a reacted with hydrogen donors such as H(2) and 2-propanol to give the sulfonylamido-bridged dirhodium(II) complex [(Cp*Rh)(2)(mu-NHTs)(2)] (2). Treatment of the (amido)rhodium(II) complex 2 with O(2) regenerated the (imido)rhodium(III) complex 1a. Complex 1a also underwent reversible protonation to afford the cationic amido- and imido-bridged dirhodium(III) complex [(Cp*Rh)(2)(mu-NHTs)(mu-NTs)](+) (4), which further reacted with H(2) or 2-propanol to give the (hydrido)bis(amido)dirhodium(III) complex [(Cp*Rh)(2)(mu-H)(mu-NHTs)(2)](+) (5). On the basis of DFT calculations and experimental results using 4 and 5, the reaction of 1a with H(2) proved to proceed via heterolytic cleavage of H(2) assisted by the sulfonyl oxygen atom followed by proton migration from the metal center. Furthermore, the redox interconversion between 1a and 2 was applied to catalytic aerobic oxidation of H(2) and an alcohol by using 1a as a well-defined dinuclear catalyst. The iridium complex [(Cp*Ir)(2)(mu-NTs)(2)] (1b) as well as a rhodium complex [Cp*RhCl(2)](2) without bridging imido ligands did not catalyze these aerobic oxidation reactions.

Synthesis, Structures, and Reactivities of Pincer-Type Ruthenium Complexes Bearing Two Proton-Responsive Pyrazole Arms

A new metal-ligand bifunctional, pincer-type ruthenium complex [RuCl(L1-H(2))(PPh(3))(2)]Cl (1; L1-H(2)=2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine) featuring two proton-delivering pyrazole arms has been synthesized. Complex 1, derived from [RuCl(2)(PPh(3))(3)] with L1-H(2), underwent reversible deprotonation with potassium carbonate to afford the pyrazolato-pyrazole complex [RuCl(L1-H)(PPh(3))(2)] (2). Further deprotonation of 1 and 2 with potassium hexamethyldisilazide in methanol resulted in the formation of the bis(pyrazolato) complex [Ru(L1)(MeOH)(PPh(3))(2)] (3). Complex 3 smoothly reacted with dioxygen and dinitrogen to give the side-on peroxo complex [Ru(L1)(O(2))(PPh(3))(2)] (4) and end-on dinitrogen complex [Ru(L1)(N(2))(PPh(3))(2)] (5), respectively. On the other hand, the reaction of [RuCl(2)(PPh(3))(3)] with less hindered 2,6-di(1H-pyrazol-3-yl)pyridine (L3-H(2)) led to the formation of the dinuclear complex [{RuCl(2)(PPh(3))(2)}(2)(μ(2)-L3-H(2))(2)] (6), in which the pyrazole-based ligand adopted a tautomeric form different from L1-H(2) in 1 and the central pyridine remained uncoordinated. The detailed structures of 1-6 were determined by X-ray crystallography.

Intramolecular 1,3-Dipolar Cycloaddition of Nitrile N-Oxide Accompanied by Dearomatization

Intramolecular 1,3-dipolar cycloaddition of 2-phenoxybenzonitrile N-oxides to benzene rings, accompanied by dearomatization, formed the corresponding isoxazolines in high yields. The X-ray single-crystal structure analysis revealed that the reaction formed the cis-adduct as a single isomer. The substituents on the benzene rings markedly affected the reaction rate, yield, and structure of the final product.

Unsymmetrical Pincer‐Type Ruthenium Complex Containing β‐Protic Pyrazole and N‐Heterocyclic Carbene Arms: Comparison of Brønsted Acidity of NH Groups in Second Coordination Sphere

A reaction of a 2-(imidazol-1-yl)methyl-6-(pyrazol-3-yl)pyridine with [RuCl2 (PPh3 )3 ] resulted in tautomerization of the imidazole unit to afford the unsymmetrical pincer-type ruthenium complex 2 containing a protic pyrazole and N-heterocyclic carbene (NHC) arms. Deprotonation of 2 with one equivalent of a base led to the formation of the NHC-pyrazolato complex 3, indicating that the protic NHC arm is less acidic. When 2 was treated with two equivalents of a base under H2 or in 2-propanol, the hydrido complex 4 containing protic NHC and pyrazolato groups was obtained through metal-ligand cooperation.

Aerobic oxidation with bifunctional molecular catalysts

A new series of half-sandwich group 8 and 9 metal complexes bearing a metal/NH bifunctional moiety were synthesized from benzylic amines. The isolable Ir amide complexes serve as effective catalysts for aerobic oxidative transformation of secondary and primary alcohols into the corresponding ketones and esters under mild conditions. The aerobic oxidative kinetic resolution of racemic secondary alcohols with chiral bifunctional Ir catalysts was found to proceed smoothly under mild conditions with high selectivity. A novel imido-bridged dirhodium complex, which may be regarded as a dinuclear variant of the bifunctional mononuclear amide complexes, also proved to promote aerobic oxidation of a secondary alcohol and H2.

Cyclodextrin-Based Size-Complementary [3]Rotaxanes: Selective Synthesis and Specific Dissociation

α-Cyclodextrin (CD)-based size-complementary [3]rotaxanes with alkylene axles were prepared in one-pot by end-capping reactions with aryl isocyanates in water. The selective formation of [3]rotaxane with a head-to-head regularity was indicated by the X-ray structural analyses. Thermal degradation of the [3]rotaxanes bearing appropriate end groups proceeded by stepwise dissociation to yield not only the original components but also [2]rotaxanes. From the kinetic profiles of the deslippage, it turned out that the maximum yield of [2]rotaxane was estimated to be 94 %. Thermodynamic studies and NOESY analyses of such rotaxanes revealed that [2]rotaxanes are specially stabilized, and that the dissociation capability of the [3]rotaxanes to the components can be adjusted by controlling the structure of the end groups, direction of the CD groups, and length of the alkylene axle.