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[[File:Iridium PCP pincer complex 3D skeletal.png|thumbnail|320x290px|3D image of an [[iridium]] diphosphine pincer complex.]]
In [[chemistry]], a '''transition metal pincer complex''' is a type of  coordination complex with a pincer ligand.  Pincer ligands are [[chelation|chelating agent]]s that binds tightly to three adjacent coplanar sites in a meridional configuration.<ref>The Chemistry of Pincer Compounds; Morales-Morales, D.; Jensen, C., Eds.; Elsevier Science: Amsterdam, 2007. {{ISBN|0444531386}}</ref><ref>{{Cite book|title=The Chemistry of Pincer Compounds|isbn=978-0-444-53138-4|last=David Morales-Morales|first=Craig Jensen|publisher=Elsevier|year=2007}}</ref>  The inflexibility of the pincer-metal interaction confers high thermal stability to the resulting complexes.  This stability is in part ascribed to the constrained geometry of the pincer, which inhibits [[Metallocycle|cyclometallation]] of the organic substituents on the donor sites at each end.  In the absence of this effect, cyclometallation is often a significant deactivation process for complexes, in particular limiting their ability to effect [[C-H bond activation]].  The organic substituents also define a hydrophobic pocket around the reactive coordination site.  [[Stoichiometric]] and [[catalytic]] applications of pincer complexes have been studied at an accelerating pace since the mid-1970s.  Most pincer ligands contain [[phosphines]].<ref name=Jens>Jensen, C. M., "Iridium PCP pincer complexes: highly active and robust catalysts for novel homogeneous aliphatic dehydrogenations", Chemical Communications, 1999, 2443–2449. {{doi|10.1039/a903573g}}.</ref>  Reactions of metal-pincer complexes are localized at three sites perpendicular to the plane of the pincer ligand, although in some cases one arm is [[hemilability|hemi-labile]] and an additional coordination site is generated transiently.  Early examples of pincer ligands (not called such originally) were anionic with a carbanion as the central donor site and flanking phosphine donors; these compounds are referred to as PCP pincers.

==Scope of pincer ligands==
Although the most common class of pincer ligands features PCP donor sets, variations have been developed where the phosphines are replaced by thioethers and tertiary amines.  Many pincer ligands also feature nitrogenous donors at the central coordinating group position (see figure), such as [[pyridine]]s.<ref>Gunanathan, C.; Ben-David, Y. and Milstein, D., "Direct Synthesis of Amides from Alcohols and Amines with Liberation of H<sub>2</sub>", Science, 2007, 317, 790-792.{{doi|10.1126/science.1145295}}.</ref>

[[File:MilsteinCat.png|thumb|center|380 px|Reaction of H<sub>2</sub> with a "Milstein catalyst" used for the dehydrocoupling of [[alcohols]] and [[amines]].]]

An easily prepared pincer ligand is [[POCOP]]. Many tridentate ligands types occupy three contiguous, coplanar coordination sites.  The most famous such ligand is [[terpyridine]] (“terpy”). Terpy and its relatives lack the steric bulk of the two terminal donor sites found in traditional pincer ligands.

Metal pincer complexes are often prepared through [[carbon-hydrogen bond activation|C-H bond activation]].<ref name="three">{{Cite journal|doi=10.1021/cr1002112|pmid=21087012|title=Catalysis by Palladium Pincer Complexes|journal=Chemical Reviews|volume=111|issue=3|pages=2048–76|year=2011|last1=Selander|first1=Nicklas|last2=j. Szabó|first2=Kálmán|doi-access=free}}</ref><ref name=Canty06>{{Cite journal | last1 = Canty | first1 = A. J. | last2 = Rodemann | first2 = T. | last3 = Skelton | first3 = B. W. | last4 = White | first4 = A. H. | title = Access to Alkynylpalladium(IV) and -Platinum(IV) Species, Including Triorgano(diphosphine)metal(IV) Complexes and the Structural Study of an Alkynyl(pincer)platinum(IV) Complex, Pt(O2CArF)I(C⋮CSiMe3)(NCN) (ArF= 4-CF3C6H4, NCN = \2,6-(dimethylaminomethyl)phenyl-N,C,N]-) | doi = 10.1021/om0601495 | journal = Organometallics | volume = 25 | issue = 16 | pages = 3996 | year = 2006 | url = https://figshare.com/articles/journal_contribution/22859450 }}</ref>

Ni(II) N,N,N [[pincer complex]]es are active in [[Kumada coupling|Kumada]], [[Sonogashira coupling|Sonogashira]], and [[Suzuki-Miyaura coupling|Suzuki-Miyaura]] coupling reactions with unactivated alkyl halides.<ref>{{Cite journal |last1=Csok |first1=Zsolt |last2=Vechorkin |first2=Oleg |last3=Harkins |first3=Seth B. |last4=Scopelliti |first4=Rosario |last5=Hu |first5=Xile |date=2008-07-01 |title=Nickel Complexes of a Pincer NN2 Ligand: Multiple Carbon−Chloride Activation of CH2Cl2 and CHCl3 Leads to Selective Carbon−Carbon Bond Formation  |journal=Journal of the American Chemical Society |volume=130 |issue=26 |pages=8156–8157 |doi=10.1021/ja8025938 |pmid=18528995 }}</ref><ref>{{Cite journal |last1=Di Franco |first1=Thomas |last2=Stojanovic |first2=Marko |last3=Keller |first3=Sébastien Carlos |last4=Scopelliti |first4=Rosario |last5=Hu |first5=Xile |date=2016-11-01 |title=A Structure–Activity Study of Nickel NNN Pincer Complexes for Alkyl-Alkyl Kumada and Suzuki–Miyaura Coupling Reactions  |journal=Helvetica Chimica Acta |language=en |volume=99 |issue=11 |pages=830–847 |doi=10.1002/hlca.201600165 |url=http://infoscience.epfl.ch/record/225084 }}</ref> 
[[File:Hu N,N,N pincer.tif|center|394x394px|thumb|N,N,N-pincer complexes]]

== Types of pincer ligands ==
The pincer ligand is most often an anionic, two-electron donor to the metal centre. It consists of a rigid, [[Plane (geometry)|planar]] backbone usually consisting of aryl frameworks and has two neutral, two-electron donor groups at the meta-positions. The general formula for pincer ligands is 2,6-(ER<sub>2</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub> – abbreviated ECE – where E is the two-electron donor and C is the ipso-carbon of the aromatic backbone (e.g. PCP – two [[phosphine]] donors).<ref>{{Cite journal|doi=10.1002/1521-3773(20011015)40:20<3726::AID-ANIE3726>3.0.CO;2-D|title=Enantioselective Organocatalysis|journal=Angewandte Chemie International Edition|volume=40|issue=20|pages=3726|year=2001|last1=Dalko|first1=Peter I.|last2=Moisan|first2=Lionel|pmid=11668532 }}</ref> Due to the firm [[tridentate]] coordination mode, it allows the metal complexes to exhibit high thermal stability as well as air-stability.<ref name="three"/> It also implies that a reduced number of coordination sites are available for reactivity, which often limits the number of undesirable products formed in the reaction due to ligand exchange, as this process is suppressed.

There are various types of pincer ligands that are used in [[transition metal]] [[catalysis]]. Often, they have the same two-electron donor flanking the metal centre, but this is not a requirement.
[[File:Examples pincer complexes.jpg|500x410px|thumbnail|center|Examples of transition metal pincer complexes]]
The most common pincer ligand designs are PCP, NCN, PCN, SCS, and PNO. Other elements that have been employed at different positions in the ligand are [[boron]], [[arsenic]], [[silicon]], and even [[selenium]].

By altering the properties of the pincer ligands, it is possible to significantly alter the chemistry at the metal centre. Changing the hardness/softness of the donor, using [[deactivating groups|electron-withdrawing groups]] (EWGs) in the backbone, and the altering the steric constraints of the ligands are all methods used to tune the reactivity at the metal centre.

== Synthesis ==
The synthesis of the ligands often involves the reaction between 1,3-dibromoethylbenzene with a secondary phosphine followed by deprotonation of the quaternary phosphorus intermediates to generate the ligand.<ref>{{Cite journal|doi=10.1021/om100325v|title=Mechanisms of the CO2Insertion into (PCP) Palladium Allyl and Methyl σ-Bonds. A Kinetic and Computational Study|journal=Organometallics|volume=29|issue=16|pages=3521|year=2010|last1=Johnson|first1=Magnus T.|last2=Johansson|first2=Roger|last3=Kondrashov|first3=Mikhail V.|last4=Steyl|first4=Gideon|last5=Ahlquist|first5=MåRten S. G.|last6=Roodt|first6=Andreas|last7=Wendt|first7=Ola F.}}</ref>

[[File:Pincer Ligand Synthesis.png|500x410px|center|Synthesis of a [[diphosphines|diphosphine]] pincer ligand]]

To generate the metal complex, two common routes are employed. One is a simple oxidative addition of the ipso-C-X bond where X = Br, I to a metal centre, often a M(0) (M = Pd, Mo, Fe, Ru, Ni, Pt) though other metal complexes with higher oxidation states available can also be used (e.g. Rh(COD)Cl<sub>2</sub>).<ref>{{Cite journal | last1 = Benito-Garagorri | first1 = D. | last2 = Kirchner | first2 = K. | doi = 10.1021/ar700129q | title = Modularly Designed Transition Metal PNP and PCP Pincer Complexes based on Aminophosphines: Synthesis and Catalytic Applications | journal = Accounts of Chemical Research | volume = 41 | issue = 2 | pages = 201–213 | year = 2008 | pmid =  18211031}}</ref><ref>{{Cite journal | last1 = Weng | first1 = W. | last2 = Guo | first2 = C. | last3 = Çelenligil-Çetin | first3 = R. | last4 = Foxman | first4 = B. M. | last5 = Ozerov | first5 = O. V. | title = Skeletal change in the PNP pincer ligand leads to a highly regioselective alkyne dimerization catalyst | doi = 10.1039/B511148J | pmid = 16372104 | journal = Chemical Communications | issue = 2 | pages = 197–199 | year = 2006}}</ref>

The other significant method of metal introduction is through [[carbon-hydrogen bond activation|C-H bond activation]].,<ref name=three /> The major difference is that the metal used in this method is already in a higher oxidation state (e.g. PdCl<sub>2</sub> – Pd(II) species). However, these reactions have been found to proceed much more efficiently by employing metal complexes with weakly-bound ligands (e.g. Pd(BF<sub>4</sub>)<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub> or Pd(OTf)<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub> where OTf = F<sub>3</sub>CO<sub>2</sub>SO<sup>−</sup>).<ref name=Canty06/>

== Role in catalysis ==
The potential value of pincer ligands in catalysis has been investigated, although no process has been commercialized. Aspirational applications are motivated by the  high thermal stability and rigidity.  Disadvantages include the cost of the ligands.

=== Suzuki-Miyaura coupling ===
[[File:Suzuki Mechanism Pd Cycle.png|thumbnail|right|The general mechanism for the [[Suzuki reaction]]]]
Pincer complexes have been shown to catalyse [[Suzuki reaction|Suzuki-Miyaura]] coupling reactions, a versatile carbon-carbon bond forming reaction.

Typical Suzuki coupling employ Pd(0) catalysts with [[monodentate]] tertiary phosphine ligands (e.g. Pd(PPh<sub>3</sub>)<sub>4</sub>). It is a very selective method to couple aryl substituents together, but requires elevated temperatures.<ref>{{Cite book|title=Organotransition Metal Chemistry: From Bonding to Catalysis|isbn=978-1-891389-53-5|last=John|first=Hartwig|publisher=University Science Books|year=2010}}</ref>

Using PCP pincer-palladium catalysts, aryl-aryl couplings can be achieved with turnover numbers (TONs) upwards of 900,000 and high yields.<ref name=three /> Additionally, other groups have found that very low catalyst loadings can be achieved with asymmetric palladium pincer complexes. Catalyst loadings of 0.0001&nbsp;mol % have been found to have TONs upwards of 190,000 and upper limit TONs can reach 1,100,000.

=== Sonogashira coupling ===
[[File:Sonogashira Cross-Coupling Reaction.svg|center|A general [[Sonogashira reaction]] involving an alkyne and aryl or vinyl halide or pseudohalide]]
Sonogashira coupling has found widespread use in coupling aryl halides with alkynes. TONs upwards of 2,000,000 and low catalyst loadings of 0.005&nbsp;mol % can be achieved with PNP-based catalysts.<ref>{{Cite journal | last1 = Bolliger | first1 = J. L. | last2 = Frech | first2 = C. M. | doi = 10.1002/adsc.200900112 | title = Highly Convenient, Clean, Fast, and Reliable Sonogashira Coupling Reactions Promoted by Aminophosphine-Based Pincer Complexes of Palladium Performed under Additive- and Amine-Free Reaction Conditions | journal = Advanced Synthesis & Catalysis | volume = 351 | issue = 6 | pages = 891 | year = 2009 }}</ref>

=== Dehydrogenation of alkanes ===
Alkanes undergo [[dehydrogenation]] at high temperatures. Typically this conversion is promoted heterogeneously because typically homogeneous catalysts do not survive the required temperatures (~200&nbsp;°C) The corresponding conversion can be catalyzed homogeneously by pincer catalysts, which are sufficiently thermally robust.  Proof of concept was established in 1996 by Jensen and co-workers.  They reported that an iridium and rhodium pincer complex catalyze the dehydrogenation of cyclooctane with a turnover frequency of 12 min<sup>−1</sup> at 200&nbsp;°C. They found that the dehydrogenation was performed at a rate two orders of magnitude greater than those previously reported.<ref name=eleven />  The iridium pincer complex was also found to exhibit higher activity than the rhodium complex.  This rate difference may be due to the availability of the Ir(V) oxidation state which allows stronger Ir-C and Ir-H bonds.<ref name=eleven>{{Cite journal | last1 = Gupta | first1 = M. | last2 = Hagen | first2 = C. | last3 = Flesher | first3 = R. J. | last4 = Kaska | first4 = W. C. | last5 = Jensen | first5 = C. M. | title = A highly active alkane dehydrogenation catalyst: Stabilization of dihydrido rhodium and iridium complexes by a PCP pincer ligand | doi = 10.1039/CC9960002083 | journal = Chemical Communications | issue = 17 | pages = 2083–2084 | year = 1996 }}</ref>

[[File:Iridium Pincer Catalysis.png|thumbnail|right|[[Iridium]] pincer complex catalyzing the dehydrogenation of [[cyclooctane]] to [[cyclooctene]]]]

The homogeneously catalyzed process can be coupled to other reactions such as alkene metathesis.  Such tandem reactions have not been demonstrated with heterogeneous catalysts.<ref>{{cite journal|doi=10.1021/ar3000713|title=Alkane Metathesis by Tandem Alkane-Dehydrogenation–Olefin-Metathesis Catalysis and Related Chemistry|year=2012|last1=Haibach|first1=Michael C.|last2=Kundu|first2=Sabuj|last3=Brookhart|first3=Maurice|last4=Goldman|first4=Alan S.|journal=Accounts of Chemical Research|volume=45|issue=6|pages=947–958|pmid=22584036}}</ref>
<ref>{{Cite journal | last1 = Choi | first1 = J. | last2 = MacArthur | first2 = A. H. R. | last3 = Brookhart | first3 = M. | last4 = Goldman | first4 = A. S. | title = Dehydrogenation and Related Reactions Catalyzed by Iridium Pincer Complexes | doi = 10.1021/cr1003503 | journal = Chemical Reviews | volume = 111 | issue = 3 | pages = 1761–1779 | year = 2011 | pmid =  21391566}}</ref>

==History==
The original work on PCP ligands arose from studies of the Pt(II) complexes derived from long-chain ditertiary phosphines, species of the type R<sub>2</sub>P(CH<sub>2</sub>)<sub>n</sub>PR<sub>2</sub> where n >4 and R = [[tert-butyl]].  Platinum metalates one methylene group with release of HCl, giving species such as PtCl(R<sub>2</sub>P(CH<sub>2</sub>)<sub>2</sub>CH(CH<sub>2</sub>)<sub>2</sub>PR<sub>2</sub>).<ref name=Jens/>

Pincer complexes catalyze the dehydrogenation of alkanes.  Early reports described the dehydrogenation of cyclooctane by an Ir pincer complex with a turnover frequency of 12 min<sup>−1</sup> at 200&nbsp;°C. The complexes are thermally stable at such temperatures for days.<ref name=eleven/>

== See also ==
* [[Carbene]]
* [[Coupling reaction]]
* [[Heck reaction]]
* [[Kumada coupling]]

==References==
<references />

[[Category:Coordination chemistry]]