Hiroaki Itagaki

Transition metal homogeneous catalysis for liquidphase dehydrogenation of methanol

Abstract Transition metal homogeneous catalysts active for liquid-phase dehydrogenation of methanol to yield formaldehyde and dihydrogen under refluxing conditions have been found, in addition to those active only under photoirradiation. From the mixture of [Ru 2 (OAc) 4 Cl] and PR 3 dissolved in methanol, the reactive complex of [Ru(OAc)Cl(PPh 3 ) 3 ] or [Ru(OAc)Cl(PEtPh 2 ) 3 ] was isolated and characterized. The addition of acetic acid enhanced the initial rate, suggesting the proton-hydride reaction scheme. The catalyst complex, [Ru(OAc)Cl(PEtPh 2 ) 3 ], was deactivated gradually, with the bidendate OAc ligand being displaced with H and CO. The mechanism of the C—H cleavage of a methoxy ligand to give HCHO and an intermediate hydride complex, occurring on these cis -disposed sites, and the physico-chemical properties of metal complexes appropriate to methanol dehydrogenation are discussed.

Dehydrogenation of methanol in the liquid phase with a homogeneous ruthenium complex catalyst

Catalytic dehydrogenation of methanol occurs in solution by use of a homogeneous ruthenium complex,[Ru2(OAc)4Cl]–t-phosphine, or mononuclear ruthenium(II) complexes, which can be isolated from the reaction solution as catalysts.

Photocatalysis of cyclooctane dehydrogenation by the a-frame dinuclear rhodium complex [Rh2(µ-S)(CO)2(dppm)2][dppm = bis(diphenylphosphino)methane]

Visible-light irradiation at the longest-wavelength absorption band (λmax= 475 nm) of the A-frame dinuclear complex [Rh2(µ-S)(CO)2(dppm)2][dppm = bis(diphenylphosphino)methane] caused catalytic dehydrogenation of cyclooctane [initial turnover frequency = 32.8 h–1, total turnover number = 27.3 (per complex)]. The absorption band effective for photocatalysis was assigned to a metal-to-ligand charge transfer (m.l.c.t.) by extended Huckel molecular orbital (EHMO) calculations. The m.l.c.t. transition energy of [Rh2(µ-S)(CO)2(dppm)2] was lower than that of the photocatalytically active mononuclear complex [RhCl(CO)(PPh3)2](λmax= 364 nm), the origin of which was investigated in detail in terms of the Rh–Rh and Rh–S interactions.

Wavelength dependence of photocatalytic cyclohexane dehydrogenation by carbonyl- and thiocarbonyl-(chloro)phosphine rhodium(I) complexes

Photocatalytic dehydrogenation of cyclohexane using [RhCl(CS)(PPh3)2] was closely related to photodissociation of the CS ligand, where the effective absorption band was not of the lowest energy but the second lowest, in contrast to the CO analogue. The estimated turnover number of [RhCl(PPh3)2] in the dehydrogenation cycle was 3.8 on the assumption that [RhCl(CS)(PPh3)2] is photodecomposed irreversibly. The wavelength dependence of photocatalysis by these complexes is well interpreted in terms of the molecular-orbital energy levels and orbital interactions.