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Über dieses Buch

Gerard van Koten: The Mono-anionic ECE-Pincer Ligand - a Versatile Privileged Ligand Platform: General Considerations.- Elena Poverenov, David Milstein: Non-Innocent Behavior of PCP and PCN Pincer Ligands of Late Metal Complexes.- Dean M. Roddick: Tuning of PCP Pincer Ligand Electronic and Steric Properties.- Gemma R. Freeman, J. A. Gareth Williams: Metal Complexes of Pincer Ligands: Excited States, Photochemistry, and Luminescence.- Davit Zargarian, Annie Castonguay, Denis M. Spasyuk: ECE-Type Pincer Complexes of Nickel.- Roman Jambor and Libor Dostál: The Chemistry of Pincer Complexes of 13 - 15 Main Group Elements.- Kálmán J. Szabo: Pincer Complexes as Catalysts in Organic Chemistry.- Jun-ichi Ito and Hisao Nishiyama: Optically Active Bis(oxazolinyl)phenyl Metal Complexes as Multi-potent Catalysts.- Anthony St. John, Karen I. Goldberg, and D. Michael Heinekey: Pincer Complexes as Catalysts for Amine Borane Dehydrogenation.- Dmitri Gelman and Ronit Romm: PC(sp3)P Transition Metal Pincer Complexes: Properties and Catalytic Applications.- Jennifer Hawk and Steve Craig: Physical Applications of Pincer Complexes.



The Monoanionic ECE-Pincer Ligand: A Versatile Privileged Ligand Platform—General Considerations

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.
Gerard van Koten

Noninnocent Behavior of PCP and PCN Pincer Ligands of Late Metal Complexes

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.
Elena Poverenov, David Milstein

Tuning of PCP Pincer Ligand Electronic and Steric Properties

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, %Vbur, 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, Cs “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.
Dean M. Roddick

Metal Complexes of Pincer Ligands: Excited States, Photochemistry, and Luminescence

Pincer ligands of the form ECE that incorporate N-heterocyclic lateral units, E, are terdentate analogues of NC-cyclometallating ligands such as 2-phenylpyridine. They are able to form a variety of highly luminescent complexes with platinum(II) and iridium(III). They can also be thought of as cyclometallating analogues of the NNN-coordinating ligand terpyridine. The introduction of the carbon atom can impart significant changes on the nature and energy of the electronic excited states, as is evident for complexes of ruthenium(II), where a shift in the absorption to low energy in the metallated systems has sparked interest in applications for dye-sensitised solar cells. Investigations in these areas over the past decade are reviewed. We also consider related complexes in which the lateral coordinating units are aliphatic N donors, phosphines, or sulphur ligands. The lack of good π-accepting units in such compounds tends to lead to weaker ligand fields and hence to low-energy metal-centred states. These states dictate much of the excited-state chemistry and compromise the efficiency of luminescence at room temperature.
Gemma R. Freeman, J. A. Gareth Williams

ECE-Type Pincer Complexes of Nickel

Pincer complexes of transition metals have demonstrated valuable catalytic reactivities and desirable properties as functional materials. Much more is known about pincer complexes of noble metals, but the pincer chemistry of nonprecious, 3d metals is poised for rapid growth over the next decade. This chapter presents a literature survey of nickel complexes based on tridentate ECE-type pincer ligands featuring a meridional coordination of the central metal atom through two dative E → Ni interactions and a covalent C–Ni linkage. The discussion is focused on the synthesis, characterization, and reactivities of complexes featuring both symmetrical and unsymmetrical ligands, ECE and ECE′. The material is organized into various sections according to the type of donor moiety E and E′ (phosphine, amine, phosphinite, phosphinimine, thioether, and N-heterocyclic carbene) and the hydrocarbyl linker (aromatic or aliphatic). Where possible, the presentation reflects the chronological order of the developments in this field of study. The review concludes with an overview of the current state of the chemistry of (ECE)Ni complexes and offers some predictions on the future prospects of this field.
Davit Zargarian, Annie Castonguay, Denis M. Spasyuk

The Chemistry of Pincer Complexes of 13–15 Main Group Elements

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.
Roman Jambor, Libor Dostál

Pincer Complexes as Catalysts in Organic Chemistry

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.
Kálmán J. Szabó

Optically Active Bis(oxazolinyl)phenyl Metal Complexes as Multipotent Catalysts

The bis(oxazolinyl)phenyl (abbreviated as phebox) ligand, which consists of two chiral oxazolines and a benzene backbone, is one of the most useful scaffolds for the construction of chiral NCN pincer complexes. The resulting phebox metal complexes can effectively deliver a C 2-symmetric environment around the active metal center. Currently, various transition metals in the range of early to late metals have been introduced by cyclometalation and transmetalation reactions to construct phebox metal complexes. Among them, the phebox Rh and Ru complexes have been extensively investigated as molecular catalysts for asymmetric catalytic reactions. High efficiency and selectivity were obtained in functionalization reactions, such as hydrosilylation, conjugate reduction, and borylation, and carbon–carbon bond formation reactions, such as reductive aldol reaction and alkynylation. The phebox complex is a good representative multipotent catalyst.
Graphical Abstract
Jun-ichi Ito, Hisao Nishiyama

Pincer Complexes as Catalysts for Amine Borane Dehydrogenation

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.
Anthony St. John, Karen I. Goldberg, D. Michael Heinekey

PC(sp 3)P Transition Metal Pincer Complexes: Properties and Catalytic Applications

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.
Dmitri Gelman, Ronit Romm

Physical and Materials Applications of Pincer Complexes

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.
Jennifer L. Hawk, Stephen L. Craig


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