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.