Organometallic Pincer Chemistry

During the past 40 years, the monoanionic, tridentate ligand platform that has been named “Pincer” has established itself as a privileged ligand in a variety of research and application areas. Exciting discoveries with NCN and PCP-pincer metal complexes in the late 1970s created a firm basis for the tremendous development of the field. Some of the basic findings are summarized with emphasis on the organometallic aspects of the ECE-pincer metal system.

Pincer complexes are generally viewed as stable compounds in which the pincer ligand framework remains unchanged during stoichiometric and catalytic reactions. However, there are now several cases in which the pincer ligand itself undergoes transformations that result in extraordinary reactivity of the metal complex and formation of unusual species. In the current chapter, we review our work on “noninnocent” reactivity modes of various PCP and PCN-type pincer ligands of Rh, Ir, Ru, Os, Pd, and Pt. Participation of the arene ring in the reactivity of PCP type complexes has led to formation of unprecedented quinonoid complexes, including complexes in which the pincer ligand adopts structures of quinone methides, thio-quinone methides, xylylenes, methylene arenium, and oxo-arenium compounds. In addition, pincer systems can collapse and be regenerated under redox conditions, and reduction can lead to a ring-localized radical anion complex. The generation of C–H agostic arene PCP complexes has led to new insights regarding the C–H bond activation process, and the effect of CO ligands on it.

A survey of the electronic and steric characteristics of anionic (^RPCP, ^RPOCOP, ^RPNCNP) and neutral (^RPNP, ^RPONOP, ^RPNNNP) phosphine terdentate ligands is presented. A review of the scope of syntheses, particularly in regard to the variation of phosphine PR₂ substituents, is followed by a general discussion of pincer steric variation, terdentate ligand conformations, and coordination geometries. Buried volume parameters, %V_bur, provide a measure of overall steric influence, while the concepts of cis and trans ligand void space and PR₂ steric influence are used to more completely define asymmetric steric influence. Pincer arm conformations are categorized in terms of C₂ twist, C_s “gull wing,” and asymmetric bending, which accommodate a range of non-meridional P–M–P bending geometries. For 5-coordinate metal complexes, geometries can vary between two square pyramidal and two trigonal bipyramidal geometrical extremes and are distinguished using subtending angle parameters α, β, and γ. A key feature to note is that pincer ligands often assume non-meridional geometries with P–M–P angles as small as 107°. DFT studies on P–Ir–P bending energetics for (^RPCP)Ir(L) (L = CO, NH₃) with a wide range of R substituents show that destabilization from bending is primarily steric in origin and is particularly significant (+12 to 30 kcal mol⁻¹) for ^tBuPCP systems. Pincer electronic effects are surveyed using comparative IR, electrochemical, and binding affinity data. While R electronic effects are significant, particularly for our R = CF₃ systems, DFT calculations of ΔG _diss(Ir–CO) for (^RPCP)Ir(CO)₂ systems show that reduced Ir–CO binding correlates mostly with steric destabilization in non-meridional tBu-substituted pincer systems.

Recent achievements in the chemistry of selected group 13–15 elements containing pincer type coordinating ligands are summarized. This chapter covers chemistry of heavier elements Ga, In, Tl from the group 13; Ge, Sn, Pb—especially low valent compounds with formal oxidation state +II or +I and their reactivity from the group 14; and As, Sb, Bi from the group 15. Only the classical pincer type ligands containing nitrogen or oxygen atoms as in-built donor functionalities are considered, i.e., 2,6-(R₂NCH₂)₂C₆H₃ (L ^N1 ), 2,6-(ArN=C(CH₃))₂C₆H₃ (L ^N2 ), 2,6-(ROCH₂)C₆H₃ (L ^{O1, 2} , R = Me, t-Bu), 2-(Me₂NCH₂)-6-(ROCH₂)C₆H₃ (L ^NO1,2 , R = Me, t-Bu), 2,6-((RO)₂P(O))₂-4-(t-Bu)C₆H₃ (L ^PO ). The influence of the present pincer ligands on the structures of the compounds as well as their key role in the stabilization of these compounds is discussed.

Application of pincer complexes in catalytic applications is a rapidly expanding field in organic synthesis. This chapter is mainly focused on selective formation of carbon–carbon, carbon–nitrogen, and carbon–metal (C–B, C–Si, and S–Sn) bonds, as well as transfer hydrogenation reactions. The described pincer-complex catalyzed processes are more efficient and more selective than the corresponding transformations catalyzed by metal salts and added ligands. Some of the described pincer-complex catalyzed reactions are not amenable by traditional metal catalysts at all. It has been demonstrated that the superiority of pincer-complex catalysts over the traditional ones is based on the high stability and well-defined structure and stoichiometry of these species. These properties of pincer complexes allow a rational design of active and highly selective catalysts.

Amine boranes have received attention as attractive materials for the chemical storage of hydrogen. Thermal dehydrogenation of these materials is possible, but catalyzed dehydrogenation would allow for greater control of the rate and extent of H₂ release. Recent work has shown that metal complexes bearing pincer ligands are competent catalysts for amine borane dehydrogenation.

Carbometalated pincer complexes represent a family of powerful compounds having tremendous number of manifold applications in organometallic chemistry, synthesis, catalysis, material science, and bioinorganic chemistry. This chapter reviews the recent developments in the chemistry and catalytic applications of PC(sp ³)P transition metal pincer complexes stressing their singular reactivity that stem from the unique electronic properties and topology.

Pincer complexes have been shown to be widely useful and versatile in organic synthesis and catalysis, but they have also been employed in a wide range of intriguing physical and materials applications. The capacity for selective and directional metal–ligand coordination offers a tool for construction and function. Pincer complexes have been successfully used as a means for templating macrocycles, assembling metallodendrimers, functionalizing surfaces, and functionalizing polymer chains. Furthermore, pincer complexes have proven to be useful mechanistic probes of polymer physical processes, linear polymers, networks, cross-linked brushes, films, and gels. Recently, the activity of pincer complexes has extended into the realm of mechanochemistry, with applications in fundamental mechanistic studies as well as mechanocatalysis. This chapter explores the use of pincer metal–ligand coordination events in these various physical and materials contexts.