Pincer-Type Ligand-Assisted Catalysis and Small-Molecule Activation by non-VSEPR Main-Group Compounds.

In 2005, a facile dihydrogen activation was reported by the Power group using an alkyne analog of germanium [ArGe≡GeAr; Ar=2,6-Trip2 -C6 H3 (Trip=2,4,6-i Pr3 -C6 H2 )]. After that, a significant progress has been made in the activation of various small molecules by main-group compounds, and a variety of stoichiometric and catalytic processes have been formulated using the p-block elements. In this regard, compounds containing low-valent main-group elements with a frontier orbitals of relatively small energy gaps or compounds forming frustrated Lewis pair (FLP) became quite successful. In spite of these promising stoichiometric and catalytic transformations, redox-cycling catalysts based on main-group elements remain extremely rare. Recently, it has been observed that pincer type ligands supported geometry constrained main-group compounds are capable of acting as redox catalysts similar to those of the transition metals. In this review, we focus on the synthesis and the structural aspects of the geometry constrained main-group compounds using pincer ligands. Emphasis has been placed on their applications on catalytic activity and small molecules activation.