The monoanionic pincer–metal platform: a scaffold for meeting challenges in catalysis and materials research

In 1976, Shaw and Moulton reported on a pincer–nickel compound with the nickel atom held in place by a monoanionic, terdentate PCP ligand.¹ At that time these authors could not have imagined that 35 years later a Dalton Transactions themed issue would be devoted to papers describing novel developments in organometallic chemistry featuring the pincer platform as a crucial factor determining reactivity and selectivity of the pincer complexes involved. The Guest Editors of this issue feel honoured to have been invited to organize this themed issue on Pincer Metal Chemistry and are grateful that so many authors, by submitting their newest achievements, accepted the opportunity to showcase their fascination for using this “privileged” ligand platform (see Fig. 1) and share their views concerning the current status of the field and its rapid development.

Fig. 1 General representation of the potentially terdentate Pincer ligand and its σ-M–Z bonding to monoanionic Z.²

When pincer–metal chemistry started to get off the ground the greater part of the applied pincer ligands comprised an ortho, ortho-disubstituted, monoanionic aryl ring (or corresponding alkyl chain), covalently bound via a central M–C σ bond to the metal center and two ortho-substituents each bearing a hetereoatom E complementing the terdentate bonding of the pincer ligand with two E–M dative bonds. Interestingly, Shaw's compound contained t-Bu₂PCH₂ groupings as ortho-substituents and these so-called² PCP-nickel complexes appeared to be the first representatives of a rich chemistry of PCP-pincer-metal complexes (with Kaska and Milstein as early players). In general, these complexes appeared directly accessible through regioselective C–H activation of the corresponding arenes with a suitable metal salt. Independently, in the early 1970s the first examples of complexes with an NCN-pincer scaffold (van Koten et al.) were developed for which preparation generally needs a lithiation-transmetallation protocol.³ Also SCS-pincer-metal complexes (Shaw 1980), again accessible via C–H activation, belonged to early examples of ECE-pincer-metal complexes.⁴ It is of interest to note that initially Shaw selected his PCP-pincer ligand because it was expected to act as a trans-spanning ligand while van Koten et al. selected the NCN-pincer platform for its anticipated rigid tridentate, mer-NCN ligand arrangement and the expectation was that this would severely influence metal reactivity involving changes of the formal oxidation state of the metal. However, reality was much more exciting! It is now generally recognized that the pincer platform has great flexibility and consequently shows great versatility with respect to its binding to the metal. In addition to the expected mer-terdentate ECE-coordination, numerous examples of pincer–metal complexes featuring either monodentate C-, bidentate EC- or even fac-ECE bonding (in particular for NCN-pincer TM complexes) have been reported.⁵ This shows that the pincer platform has hemilabile coordination properties that, for example, can be advantageous when the ECE-pincer metal compound is active as a homogeneous catalyst. Notably, structural studies of ECE-pincer metal compounds have shown that the actual bonding mode depends on the nature of the heteroatom E of the ECE-pincer ligand, the nature of the other ligands and anions in the metal coordination sphere and the actual electronic state of the metal center, i.e. in the ECE-pincer platform, ample interplay between the anionic ligand and the covalently bonded metal center occurs.

Exploration of the properties and reactivity patterns of the ECE-pincer metal complexes revealed that in a number of cases these organometallic compounds are unprecedented in their robustness towards thermal decomposition (e.g., through homolytic M–C bond cleavage), or stable under strongly acidic or basic reaction conditions. Furthermore, it has been discovered that ECE-pincer compounds are excellent homogeneous catalysts for a wide variety of key organic reactions, have unique reactivity for the cleavage of unreactive carbon–carbon and carbon–heteroatom bonds, can be incorporated into polymer networks, can reversibly bind small molecules (providing sensor properties) and can be used as building blocks in supramolecular chemistry. Some relevant reviews can be found in ref. 5.

The chemistry presented in the various contributions to this themed issue shows exciting uses of the pincer ligand platform in a number of diverse areas. It ranges from the synthesis of pincer–metal compounds involving novel methods (synthesis in the solid state, assembling the pincer platform in situ), or by exploring pincer ligands with different neutral donor atoms in the ortho-substituents providing hemilabile coordination properties to the resulting ECE’ donor set. Furthermore, pincer ligands in which the monoanionic C-ipso donor atom has been replaced with a heteroatom giving rise to a pincer-platform with non-innocent ligand character are presented. Several papers describe the synthesis of new pincer–metal derivatives for e.g. biological applications and applications in the field of organic synthesis including dehydrogenation of alcohols, the synthesis of heterocyclic compounds, the racemization of chiral alcohols and as catalysts in polymer chemistry. Earlier studies showed that the pincer–metal platform is particularly suited to affect the redox potential of the metal center (e.g. Ni^II/Ni^III, Pd^II/Pd^IV)^5b and exciting further examples of this property and its consequences in catalysis are reported in this issue.

The content of this themed issue clearly testifies to the great potential of the monoanionic, potentially terdentate EZE’-pincer platform (Z is the monoanionic atom directly interacting with the metal center) as a “privileged” ligand platform (see Fig. 1) in fundamental organometallic/inorganic chemistry and in the fields of materials science and catalysis.

As a last note it is interesting to remark that with the fac-coordination mode the 6-electron pincer ligand platform seemingly is able to mimic the η⁵-binding mode of the cyclopentadienyl anion, which is a well-established privileged ligand with a proven potential in a wide range of applications. Most importantly, the pincer platform is highly complementary to the cyclopentadienyl ligand. The chemistry presented in this themed issue illustrates the great flexibility of the pincer platform providing ample opportunity to design and vary the ligand architecture with regards to both its stereochemistry, set of neutral anionic donor atoms in the EZE’-sequence, featuring innocent vs. non-innocent behavior and hemilability. These properties provide unique opportunities for the exploration of the use of the pincer platform in a wide range of materials, catalytic and bio(medical) applications. Finally, the pincer platform has ample possibilities for being tethered to nano-sized structures or for (multi) attachment to functional polymers thus providing materials with specified properties in which the pincer–metal site(s) play(s) a defined role.

The Guest Editors would like to emphasize that this themed issue can only serve to further illustrate the potentiality of the pincer platform as well as the richness of the chemistry that this privileged ligand can display. By no means does this issue have the scope to show all of the exciting developments that presently are ongoing. However, the Guest Editors hope that the work reported can be an inspiration for others.

References

1. C. J. Moulton and B. L. Shaw, J. Chem. Soc., Dalton Trans., 1976, 1020.
2. First time coining of the word “pincer”: G. van Koten, Pure Appl. Chem., 1989, 61, 1681.
3. G. van Koten, J. T. B. H. Jastrzebski, J. G. Noltes, A. L. Spek and J. C. Schoone, J. Organomet. Chem., 1978, 148, 233; G. van Koten, K. Timmer, J. G. Noltes and A. L. Spek, J. Chem. Soc., Chem. Commun., 1978, 250.
4. J. Errington, W. S. McDonald and B. L. Shaw, J. Chem. Soc., Dalton Trans., 1980, 2312.
5. (a) The Chemistry of Pincer Compounds, ed. D. Morales-Morales and C. M. Jensen, Elsevier, Amsterdam, 2007; (b) M. Albrecht and G. van Koten, Angew. Chem., Int. Ed., 2001, 40, 3750; (c) M. E. van der Boom and D. Milstein, Chem. Rev., 2003, 103, 1759; (d) W. Leis, H. A. Mayer and W. C. Kaska, Coord. Chem. Rev., 2008, 252, 1787; (e) J. Choi, A. H. R. MacArthur and M. Brookhart, Chem. Rev., 2011, 111, 1761; (f) N. Selander and K. J. Szabo, Chem. Rev., 2011, 111, 2048.

Footnotes

† http://www.uu.nl/science/occ

‡ http://www.gerardvankoten.nl

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