Sumio Shinoda

Rh2(OAc)4-PPh3 as a catalyst for the liquid-phase dehydrogenation of 2-propanol

The most active homogeneous catalyst hitherto known for the selective dehydrogenation of 2-propanol has been found to form from Rh2(OAc)4 by adding PPh3 in situ, although Rh2(OAc)4 itself possesses no catalytic activity. From the reaction solution a complex was isolated, 31P(1H) NMR of which showed that coordination of PPh3 occurs not only to the axial but also to the equatorial positions of the Rh-Rh axis. When this complex is used as a catalyst with PPh3 and acetic acid present, no induction period was observed, which appears for the catalyst generated in situ. Drastic changes in catalytic properties occurred by replacing Rh2(OAc)4 with Rh2(OCOCF3)4 or PPh3 with P(OPh)3, which indicate prospects of tailor-making the catalyst.

Mechanistic study on dehydrogenation of methanol with [RuCl2(PR3)3]-type catalyst in homogeneous solutions

Abstract Catalytic dehydrogenation of methanol has been investigated in homogeneous solutions with a series of Ru(II) complexes, [RuCl2(P(p-C6H4X)3)3] (X = H (1), Me (2), F (3), OMe (4)) and [RuCl2(PMePh2)3] (5). In the gas phase, hydrogen was formed selectively (> 99.5%), and formaldehyde, methylal (formaldehyde dimethyl acetal) and methyl formate were found in the liquid phase with satisfactory stoichiometry to the formed hydrogen. The reaction was retarded by the extra addition of free phosphine, suggesting the presence of pre-equilibrium dissociation of phosphine ligand. Kinetic analyses from this viewpoint explained well the dependence of rate on the concentration of catalyst (saturation curve). The order of evaluated pre-equilibrium constant (1 ≈ 2 > 5) is in accord with the general idea that the dissociation of phosphine ligand is controlled principally by steric bulk of ligands. The order of rate (3 > 1 > 2 > 4) for 1–4, possessing the same cone angle of phosphine ligand, correlated clearly with basicity of phosphines. The results are interpreted in terms of the mechanism of rate-determining β-hydrogen abstraction in the Ru-OCH3 intermediate.

Photocatalytic dehydrogenation of methanol in the liquid phase with cis-Rh2Cl2(CO)2(dpm)2 and Pd2Cl2(dpm)2 complex catalysts

Abstract The liquid-phase dehydrogenation of methanol has proved to be possible at a moderate rate through the use of the soluble complex catalysts cis -Rh 2 Cl 2 (CO) 2 (dpm) 2 and Pd 2 Cl 2 (dpm) 2 under photoirradiation conditions (high-pressure mercury lamp), the addition of acetone accelerating the rate quite noticeably. Quantitative analysis of the reaction products (dihydrogen, formaldehyde, formaldehyde dimethyl acetal, ethylene glycol, 2,3-dimethyl2, 3-butanediol, 2-propanol, 2-methyl-1,2-propanediol) showed that mass balance for hydrogen was established satisfactorily among them. In comparison to the acetone-photosensitized reaction of methanol, the observed product distribution could be characterized as follows: (1) the gas-phase product is almost pure dihydrogen (in comparison to being mainly methane) and (2) the yield of formaldehyde and formaldehyde dimethyl acetal is greatly increased (in comparison to the products being mainly 2-methy 1-1,2propanediol and ethylene glycol).

The mechanisms of photocatalytic dehydrogenation of methanol in the liquid phase with cis-[Rh2Cl2(CO)2(dpm)2] complex catalyst

Abstract The nature and mechanisms of the photocatalysis of soluble cis-[Rh2-Cl2(CO)2(dpm)2] complex for the dehydrogenation of methanol have been examined with and without the presence of added acetone, by determining the rate of the reaction, product distribution and quantum efficiency. In the absence of acetone, the products were purely formaldehyde (and its dimethyl acetal) and dihydrogen in the wavelength range of 260 – 440 nm, although quantum efficiency was rather low (0.1 – 7 × 10−3). Addition of acetone accelerated the rate of dihydrogen evolution (still > 95% of the gas-phase components), whereas formaldehyde, formaldehyde dimethyl acetal, ethylene glycol, 2,3-dimethyl-2,3-butanediol, 2-propanol and 2-methyl-1,2-propanediol were found in the liquid phase with a satisfactory mass balance for hydrogen. In comparison with the purely acetone-photosensitized reaction of methanol, the rhodium catalyst facilitates the formation of formaldehyde (and its dimethyl acetal) over all the wavelengths studied (260 – 440 nm). In the wavelength range where acetone is a strong absorber of photons (⩽312 nm), the rhodium catalyst is considered to convert ·CH2OH radicals into formaldehyde and dihydrogen. The enhanced formaldehyde formation in the wavelength range where acetone is a very poor absorber of photons (⩾338 nm) suggests participation of a possible rhodium-acetone complex in the photoexcitation processes. The fact that the ratio of the amounts of products [ethylene glycol] [2,3-dimethyl-2,3-butanediol] /[2-methyl-1,2-propanediol]2, is constant independent of the wavelength, indicates that these three products are formed from the homogeneous and heterogeneous coupling of ·CH2OH and ·C(CH3)2(OH) radicals irrespective of the presence of the rhodium catalyst.

Complex formation of platinum(II) and rhodium(III) ions with aminated silica surfaces as studied by c-13 nmr spectroscopy

Abstract Complex formation of Pt(II) and Rh(III) ions with aminated silica surfaces (NH 2 CH 2 CH 2 CH 2 Si≡, NH 2 CH 2 CH 2 NHCH 2 CH 2 CH 2 Si≡) has been analyzed by C-13 NMR spectroscopy in both the suspended state (in water; ordinary high-resolution NMR spectrometer with low-power 1 H decoupling) and the dry state (with CP-MAS techniques). New peaks assignable to Pt(II)-coordinated species appeared for the diamine silica in contact with aqueous K 2 PtCl 4 solutions, where bis-chelate formation was suggested on the basis that the peaks of uncoordinated species were converted completely to those of coordinated ones at the [Pt]/[ligand] ratio of 0.5; bis-chelate structure was also indicated for RhCl 3 /diamine silica by diffuse reflectance spectra. While coordinated species did not give high-resolution signals in other cases, they were found detectable with CP-MAS techniques, although the peaks were not fully resolved at 25 MHz. The implications of the associated protonation shifts of uncoordinated ligands are also discussed.

Synthesis of acetic acid from methanol alone by homogeneous metal complex catalyst. Part II1. Mechanistic study on methyl acetate formation from methanol alone by [(η5-C5H5)(PPh3)2RuX] (X=Cl, SnF3) complex catalyst

Abstract Catalytic activity of [( η 5 -C 5 H 5 ) (PPh 3 ) 2 RuX] (X=Cl ( 1 ), SnF 3 ( 2 )) was investigated for the highly selective formation of methyl acetate with methanol used as the sole source. Complex 2 was found to be more catalytically active than the singlemetallic complex 1 . For both complexes, the initial reaction rate was first order with respect to the catalyst concentration, and a saturation curve was obtained for dependence on the reactant concentration. Extra addition of Cl − ion considerably slowed the reaction with 1 taken as catalyst, and an almost linear relationship was obtained between the reciprocal of initial rate and the concentration of added Cl − ion. This fact indicates the presence of pre-equilibrium to form catalyst-reactant complex through the substitution of Cl − ligand. In the case of 2 , extra addition of PPh 3 gave a similar effect on the rate, allowing the same type of kinetic analysis. The role of Sn(II) ligand is considered in line with the mechanism that satisfies the rate equations derived from these kinetic results.

Hydrogen bonding in silica-bonded amino groups as probed by carbon-13 spin-lattice relaxation times

Abstract Carbon-13 spin-lattice relaxation data showed that internal mobility of silica-bonded amino groups became greater upon protonation, which suggests the motional restriction in the unprotonated state due to hydrogen bonding.

Photocatalysis of trans-[RhCl(CO)(PPh3)2] under MLCT irradiation for 2-propanol dehydrogenation

Abstract Exclusive evolution of hydrogen without carbon monoxide was observed for the liquid-phase dehydrogenation of 2-propanol with trans -[RhCl(CO)(PPh 3 ) 2 ] under MLCT irradiation, suggesting the catalytic role of the photogenerated [RhCl(PPh 3 ) 2 ] species before its rapid replenishment with CO ligand. A quantum efficiency exceeding unity (1.6) was obtained at 364 nm and 108 °C in refluxing solution mixed with diglyme. Homogeneous photocatalysis in the visible region was discussed from the standpoint of energy storing.

Comparative study on catalytic oxidation of ethylene by palladium(ii) in aqueous and acetic acid solutions

Abstract A contrasted feature was found for the catalytic oxidation of ethyIene by palladium(II) in aqueous and acetic acid solutions. The identical distribution of six kinds of deuterio-products, obtained from both ethylene-1,1-d2 and trans-ethylene-1,2-d2 in the latter solution, was ascertained in the deuterium decoupled PMR spectra, suggesting that the redox-decomposition for the yielding of vinyl acetate is preceded by an equilibrium process between the 2- and 1-acetoxyethylpalladium(II) σ-complexes without any intermolecular hydrogen exchange. In the Wacker reaction, however, different ratios of CH2DCDO/CHD2CHO were confirmed for these two kinds of ethylene-d2, although a reversible hydrogen-shift mechanism via a vinyl alcohol π-complex had been proposed. On the basis of kinetic deuterium isotope effects, the mechanisms of hydroxypalladation and hydrogen-shift processes in the Wacker reaction are discussed.

COMMUNICATION: CATALYSIS OF METHYL ACETATE FORMATION FROM METHANOL ALONE BY (η5-C5H5)(PPh3)2RuX (X = Cl, SnCl3, SnF3): HIGH ACTIVITY FOR THE SnF3 COMPLEX

The authors have recently shown that the Ru(II)-Sn(II) bimetallic complex can catalyze the unprecedented one-step formation of acetic acid (or methyl acetate) with methanol used as the sole source. It was suggested that the reaction consists of sequential processes of methanol {r_arrow} formaldehyde (methyl){r_arrow}methyl formate {r_arrow} acetic acid (methyl acetate). While the Ru(II) complexes capable of catalyzing the dehydrogenation of methanol into methyl formate are known, this catalyst system is unique because of its extra ability to isomerize methyl formate to acetic acid without a CO atmosphere (usually high pressure) or an iodide promoter (often corrosive to reaction apparatus). In this communication, the authors examine the cyclopentadienyl bis(triphenylphosphine) ruthenium(II) auxilliary in view of its well defined geometry and configurational stability, and demonstrate that combination with the SnF{sub 3} ligand gives quite high catalytic ability compared to the conventional SnCl{sub 3} ligand. 12 refs., 1 fig.Abstract We have recently shown1,2 that the Ru(II)-Sn(II) bimetallic complex can catalyze the unprecedented one-step formation of acetic acid (or methyl acetate) with methanol used as the sole source. It was suggested that the reaction consists of sequential processes of methanol → formaldehyde (methylal) → methyl formate → acetic acid (methyl acetate). While the Ru(II) complexes capable of catalyzing the dehydrogenation of methanol into methyl formate are known,3–5 this catalyst system is unique because of its extra ability to isomerize methyl formate to acetic acid without a CO atmosphere (usually high pressure) or an iodide promoter (often corrosive to reaction apparatus).6 In this communication, we examine the cyclopentadienyl bis(triphenylphosphine) ruthenium(II) auxilliary in view of its well-defined geometry and configurational stability,7 and demonstrate that combination with the SnF3 − ligand8 gives quite high catalytic ability compared to the conventional9 SnCl3 − ligand.