Metal−ligand cooperation, in which metal and ligand participate in bond cleavage and formation, is gathering great attention in recent years. In contrast to the classical bond cleavage by active metal centers with spectator ligands, metal−ligand cooperation has enabled unprecedented reactivities. Especially, metal−ligand cooperative H−H bond cleavage has been extensively studied and applied to various catalysts. On the other hand, there are substantial efforts to expand the scope of the bond to be cleaved other than the H−H bond. This review summarizes the recent progress in the metal−ligand cooperative cleavages of Si−H, B−H, and C−H bonds and their catalytic applications.

For valorization of biomass, the conversion of lignin to deoxygenated bulk aromatic compounds is an emerging subject of interest. Because aromatic rings are susceptible to metal-catalysed hydrogenation, the selective hydrogenolysis of carbon-oxygen bonds still remains a great challenge. Herein we report direct and selective hydrogenolysis of sp 2 C-OH bonds in substituted phenols and naphthols catalysed by hydroxycyclopentadienyl iridium complexes. The corresponding arenes were obtained in up to 99% yields, indicating the possible production of arenes from lignin-derived bio-oils. Furthermore, the same catalysts were applied to the unprecedented selective hydrogenolysis of the sp 3 C-O bonds in aryl methyl ethers. Thus, the hydrodeoxygenation of vanillylacetone, a lignin model compound, afforded alkylbenzenes as the major products via triple deoxygenation.

Palladium-catalyzed coordination-insertion copolymerization of ethylene with acrylonitrile (AN) proceeded only by using phosphine-sulfonate (P-SO(3)) as a ligand among the neutral and anionic ligands we examined, those are phosphine-sulfonate (P-SO(3)), diphosphine (P-P), and imine-phenolate (N-O). In order to answer a question that is unique for P-SO(3), theoretical and experimental studies were carried out for the three catalyst systems. By comparing P-SO(3) and P-P, it was elucidated that (i) the π-acrylonitrile complex [(L-L')PdPr(π-AN)] is less stable than the corresponding σ-complex [(L-L')PdPr(σ-AN)] in both the phosphine-sulfonato complex (L-L' = P-SO(3)) and the diphosphine complex (L-L' = P-P) and (ii) the energetic difference between the π-complex and the σ-complex is smaller in the P-SO(3) complexes than in the P-P complexes. Thus, the energies of the transition states for both AN insertion and its subsequent ethylene insertion relative to the most stable species [(L-L')PdPr(σ-AN)] are lower for P-SO(3) than for P-P. The results nicely explain the difference between these two types of ligands. That is, ethylene insertion subsequent to AN insertion was detected for P-SO(3), while aggregate formation was reported for cationic [(L-L)Pd(CHCNCH(2)CH(3))] complex. Aggregate formation with the cationic complex can be considered as a result of the retarded ethylene insertion to [(L-L)Pd(CHCNCH(2)CH(3))]. In contrast, theoretical comparison between P-SO(3) and N-O did not show a significant energetic difference in both AN insertion and its subsequent ethylene insertion, implying that ethylene/AN copolymerization might be possible. However, our experiment using [(N-O)PdMe(lutidine)] complex revealed that β-hydride elimination terminated the ethylene oligomerization and, more importantly, that the resulting Pd-H species lead to formation of free N-OH and Pd(0) particles. The β-hydride elimination process was further studied theoretically to clarify the difference between the two anionic ligands, P-SO(3) and N-O.

Unprecedented direct acceptorless dehydrogenation of C-C single bonds adjacent to functional groups to form α,β-unsaturated compounds has been accomplished by using a new class of group 9 metal complexes. Metal-ligand cooperation operated by the hydroxycyclopentadienyl ligand was proposed to play a major role in the catalytic transformation.

Half-sandwich Cp*RhIII complexes (Cp* = η5-1,2,3,4,5-pentamethylcyclopentadienyl) supported by 2,2′-bipyridine or 4,4′-di-tert-butyl-2,2′-bipyridine catalyze dehydrogenation of dimethylamine–borane (Me2NH·BH3) to produce H2 and dimethylamino–borane dimer (Me2NBH2)2 with turnovers of 2200. The IrIII analogues, on the other hand, display dramatically poorer catalytic activity. Mechanistic inferences drawn from stoichiometric reactions and DFT calculations suggest noninnocent involvement of the Cp* moiety as a proton shuttle.

Hydroformylation, a reaction that adds carbon monoxide and dihydrogen across an unsaturated carbon-carbon multiple bond, has been widely employed in the chemical industry since its discovery in 1938. In contrast, the reverse reaction, retro-hydroformylation, has seldom been studied. The retro-hydroformylation reaction of an aldehyde into an alkene and synthesis gas (a mixture of carbon monoxide and dihydrogen) in the presence of a cyclopentadienyl iridium catalyst is now reported. Aliphatic aldehydes were converted into the corresponding alkenes in up to 91% yield with concomitant release of carbon monoxide and dihydrogen. Mechanistic control experiments indicated that the reaction proceeds by retro-hydroformylation and not by a sequential decarbonylation-dehydrogenation or dehydrogenation-decarbonylation process.

Novel PC II P-Ir I monochloride complexes (1-Cl and 2-Cl) bearing a phosphine-carbene-phosphine pincer type ligand were synthesized. Reactions of 1-Cl with hexachloroethane, hydrogen chloride, and lithium triethylborohydride under a dihydrogen atmosphere afforded PC II P-Ir III trichloride (1-Cl 3 ), hydride dichloride (1-HCl 2 ), and trihydride (1-H 3 ) complexes, respectively. The strong electron-donating ability of carbene in PC II P-Ir complexes was confirmed by X-ray crystallography and DFT calculations. Moreover, in complex 1-Cl, strong π backdonation from the iridium center to the carbene carbon was observed. Hydrogenation of CO 2 with triethanolamine catalyzed by PC II P-Ir complexes was investigated. The novel PC II P-Ir complex 1-Cl exhibited a longer lifetime in comparison to the PNP-Ir III complex 3-H 3 : the turnover number of 1-Cl is significantly higher than that of 3-H 3 (in 46 h, 1-Cl 230000 and 3-H 3 54000).

Methoxy-substituted PNP–iridium(III) complexes and pyrazine-based PNP–iridium(III) complexes were developed and used to hydrogenate carbon dioxide in the presence of triethanolamine as a base. The methoxy-substituted PNP–hydridodichloridoiridium complex (C-HCl2) showed the highest turnover number, 160000; this is the highest value ever reported with an organic base in an aqueous medium. The reactivities of these complexes, derived from their ligand modification, were further studied. The results were as follows. (i) The pyrazine-based PNP–trihydridoiridium complex undergoes release of dihydrogen to afford dihydridoamido complex, possibly because of easy dearomatization of the pyrazine ring. This process was reversible, i.e., B-H2amido can readily be converted back to B-H3 on exposure to dihydrogen. (ii) The p-methoxy-substituted dihydridochlorido complex showed facile disproportionation of the chloride anion on the iridium center; this is attributed to the electron-donating nature of the methoxypyridine backbone.