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Elsevier AMS Ch19-N53138 Job code: CPC 5-2-2007 4:45 p.m. Page:441 Trimsize:165×240 MM CHAPTER 19 01 02 03 Perspective and prospects for pincer ligand chemistry 04 05 06 07 08 09 William D. Jones 10 11 12 Department of Chemistry, University of Rochester, Rochester, NY 14627, USA 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 Inorganic chemistry has undergone a tremendous transformation during the past century, beginning with the development and understanding of coordination chemistry through the work of Alfred Werner. These pioneering studies established the importance of ligands in dictating the properties of a central metal ion and demonstrated the rich diversity possible with a limited number of simple, unidentate ligands. The discovery of ferrocene in 1951 by Kealy and Pauson extended the horizons of inorganic chemistry to include covalent metal−carbon bonds, and in the following decades, hundreds of new cyclopentadienyl complexes appeared in the literature. Paralleling these synthetic achievements were a host of new chemical reaction pathways as the field of organometallic chemistry blossomed into existence. Phosphine donor ligands and their chelates appeared on the scene, and efficient new catalysts such as Wilkinson’s catalyst showed unequaled activity and usefulness for the transformation of organic materials. More recently, the cyclopentadienyl analog trispyrazolylborate appeared and once again hundreds of new compounds were synthesized. In the midst of these developments, the specific desire for a meridional tridentate ligand became apparent and the pincer ligand was born out of the early work of Shaw and van Koten. This book is dedicated to presentation of the wide variety of tridentate pincer ligands that have been developed, the chemistry that they enable to take place at a metal center, and the catalytic reactions they can perform. The range of types of pincer ligands is indeed quite broad. These ligands favor meridional coordination, although facial binding is occasionally observed. The pincers appear most often with a central aryl ring possessing two ortho-substituted arms containing donor ligands. The central attachment is typically directly through a carbon of the aryl ring, and the pendant arms can be found with C, N, P, O, S, or Se donor atoms. Other architectures have also been prepared, including pincers with a central pyridine or phosphinine ring, a central N-heterocyclic carbene, a phospha-barrelene, a cycloheptatriene, an anthracene, or a simple divalent carbene carbon. The use of a central diarylimido donor gives rise to a more rigid ‘meridional enforcer’ geometry for the pincer. Groups on the pendant arms vary from phosphines, amines, thiols, ethers, and selinides to phosphine-sulfides, iminophosphoranes, and N-heterocyclic carbenes. S−C−S pincers have also been made from bis-diphenylphosphineomethane by conversion to the bis-sulfide and removal of the central hydrogens. Chiral pincer ligands have The Chemistry of Pincer Compounds Edited by David Morales-Morales et al. Basal Fonts:Times Margins:Top:13 MM Gutter:20 MM © 2007 Elsevier B.V. All rights reserved. Font Size:10/12 pt Text Width:125 MM Depth:47 Lines Elsevier AMS Ch19-N53138 Job code: CPC 5-2-2007 4:45 p.m. 442 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 Page:442 Trimsize:165×240 MM W.D. Jones also been prepared using chiral oxazoline arms attached to a central pyridine (Pybox) or phenyl (Phebox) ring. Chiral-disubstituted tetrahydropyroles have also been employed to introduce chirality in the arms. These ligands impose a C2 symmetry at the metal center, allowing for investigations of chiral induction in reactive substrates. One interesting class of pincers can be prepared by chloropalladation of difunctional alkynes, giving N−C−S and N−C−O derivatives in which there are a combination of hard and soft donor atoms. Pincer ligands have been supported on montmorillonite and bentonite clays, as well as on dendrimers to allow facile recovery of the metal. The chemistry of pincer-containing compounds demonstrates a tremendous breadth in their applications. The tridentate P−C−P and N−C−N pincers force allyl ligands to be 1 , which allows them to be transferred as nucleophiles to acyl derivatives. Palladium complexes with these ligands catalyze the transfer of allyl from tin to benzaldehydes or sulfonimides under mild conditions. These catalysts also transfer allyl from trifluoroallylborates to sulfonimine substrates. In addition, the palladium pincer complexes also serve as catalysts for the synthesis of allylboronic acids, allyltrifluoroborates, and allylstannanes, all of which are useful reagents in allyl transfer reactions. These pincers also catalyze stannylation and selenation of propargyl halides. A large number of pincer complexes have been reported to be efficient catalysts for the Heck addition of olefins to aryl halides. Many of these pincer complexs have been shown to serve as controlled-release precursors of either a low-ligated Pd(0) species or Pd nanoparticle, as evidenced by poisoning of the activity with liquid mercury or polyvinylpyridine. Other systems appear to keep the metal pincer intact during catalysis, being resistant to poisoning or metal leaching. As a consequence, care must be taken in any newly developed system to carefully characterize the nature of the active catalyst. Fortunately, a number of tests are available (kinetic, three-phase, and poisoning) to sort out if a catalyst is truly homogeneous or if a tethered pincer catalyst stays bound to the substrate. In the absence of such tests, any mechanistic proposals should be regarded with due caution. While some pincer ligand complexes have shown the ability to readily release the metal from the tridentate environment, other pincer ligands of the C−N−C type pyridine(NHC)2 have shown high stability which has potential applications in radiopharmaceutical imaging applications. But an even more important application of these highly stabile pincer complexes is in the catalytic dehydrogenation of alkanes. Wilkinson’s catalyst, RhCl(PPh3 3 , has been known for decades to be an excellent homogeneous catalyst for alkene hydrogenation, a process that is exothermic by >20 kcal/mol. Since Wilkinson’s catalyst is a catalyst, this complex must be capable of accelerating both the forward and the reverse reactions. That is, if one could allow Wilkinson’s catalyst to interact with an alkane and then remove the small amount of dihydrogen that would be formed at equilibrium, then an alkane could be converted to an alkene (+H2 , at least in theory. In practice, at the high temperatures required for such a reaction to occur and to drive off hydrogen at an appreciable rate, Wilkinson’s catalyst decomposes via P−C cleavage, although early experiments by Mary L. Deem indicated that the possibility of this dehydrogenation existed, but were largely unnoticed. It was only with the development of P−C−P pincer complexes of rhodium and iridium that this goal was ultimately achieved. The groups of Goldman and Jensen both produced strong evidence that both transfer dehydrogenation and catalytic dehydrogenation were possible and that the reason for this possibility was the robust nature of the pincer catalysts Basal Fonts:Times Margins:Top:13 MM Gutter:20 MM Font Size:10/12 pt Text Width:125 MM Depth:47 Lines Elsevier AMS Ch19-N53138 Job code: CPC 5-2-2007 4:45 p.m. Page:443 Trimsize:165×240 MM Perspective and prospects for pincer ligand chemistry 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 443 with these metals. Variation of the metal, arm linkage, and phosphine alkyl groups led to the development of catalysts that produced alkene (+ dihydrogen) from alkane in concentrations approaching 0.5 M upon reflux in open systems! While terminal olefins are the kinetic products in these dehydrogenations, competitive isomerization to give internal olefins is unavoidable with these catalysts. Perhaps of even more interest is the recent observation that the olefins produced in this reaction can undergo metathesis with Schrock alkylidenes to give re-distributated olefins which in turn are re-hydrogenated back to the saturated hydrocarbons, resulting in an overall ‘alkane metathesis’ process. This discovery has huge potential impact on the petroleum industry, as it will enable redistribution of alkanes from light and heavy hydrocarbon fractions to the more valuable mid-range diesel fractions. All of this chemistry results from the robust thermal stability of pincer ligands on transition metal catalysts. Where are the developments for pincer chemistry going in the future? While prediction of specific applications are speculative at best, it is clear that the pincer ligand will have a strong impact on future research. The variability in the ligand structure is immense. One has control over donor/acceptor ability at both the central and adjacent side-arm positions. One has control over influences on the steric environment surrounding over 50% of the metal. One can influence electronics by adjustment of the donor ligand set in the meridonial coordination environment. The ligand synthesis is relatively straightforward and, importantly, can be designed to be resistant to undesirable chemical side reactions. The important property of high thermal stability for some of the pincer ligand subset is an important accomplishment that should not be underestimated. One of the greatest drawbacks of organometallic catalysts is its lack of tolerance to elevated temperatures. Processes such as P−C cleavage of phosphines, undesirable ligand dissociation, and irreversible ligand oxidative addition are well documented with many catalysts. Some of the pincer systems developed to date appear to avoid these pitfalls, perhaps due to their restrictive environments, at least until much higher temperatures, so it appears as if the development of higher temperature catalytic processes appears to be one area where future growth can be anticipated. Furthermore, the nature of the meridional coordination of pincers ligands, and along with this their ability to enforce a stereo-specific environment above and below the ML3 plane, offers a strong opportunity to capitalize upon the potential for chiral synthesis. The ability of the chiral ligand BINAP and its derivatives has demonstrated the potential for such chelating C2 -symmetric ligands, and chiral pincer ligands offer a similar, if not improved, opportunity for the development of chiral catalysts since the ‘businessend’ of the catalyst is even closer to the stereo-directing environment. As seen in this treatment, chiral enhancements can be observed in Diels-Alder and aldol reactions, dipolar additions to aldehydes, Michael additions, and reductions, leading to appreciable ee’s in the products. It is noteworthy that control of stereodirection in the metallocene single-site catalysts (C2 vs Cs has allowed for the efficient stereo-specific polymerization of propylene to give isotactic or syndiotactic polymer. Future developments in this area with pincer complexes are likely, with the ultimate discovery of a ligand with BINAP-type control appearing in the near future. In summary, tridentate pincer ligands have shown a long period of development and an explosive period of exploitation. The future for further enhancements using this ligand scaffold is bright and lucrative. The ability to produce such a broad spectrum of ligand properties is unparalleled with other ligand systems. Pincer ligands are here to provide an anchor for the future of organometallic chemistry. Basal Fonts:Times Margins:Top:13 MM Gutter:20 MM Font Size:10/12 pt Text Width:125 MM Depth:47 Lines AU1 Elsevier AMS Ch19-N53138 Job code: CPC 5-2-2007 4:45 p.m. Page:444 Trimsize:165×240 MM 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 Basal Fonts:Times Margins:Top:13 MM Gutter:20 MM Font Size:10/12 pt Text Width:125 MM Depth:47 Lines Elsevier AMS 01 query19 Job code: CPC 5-2-2007 4:45 p.m. Page:1 Trimsize:165×240 MM Chapter No: 19 02 03 04 05 Query No Query AU1 Please check whether ‘re-distributated’ can be changed to ‘re-distributed’. 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 Basal Fonts:Times Margins:Top:13 MM Gutter:20 MM Font Size:10/12 pt Text Width:125 MM Depth:47 Lines