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CHAPTER 19
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Perspective and prospects
for pincer ligand chemistry
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William D. Jones
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Department of Chemistry, University of Rochester, Rochester, NY 14627, USA
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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.
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Elsevier AMS
Ch19-N53138
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442
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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
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Perspective and prospects for pincer ligand chemistry
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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.
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Elsevier AMS
Ch19-N53138
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