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Pincer Ligands and Complexes

Complexes with NNS Type Ligands of Hydrogenation

Strem offers a novel class of ligands and  transition metal complexes for hydrogenation reactions. These materials, developed at Los Alamos National Laboratory, are based on the NNS scaffold with pincer type ligands containing a central NH functionality.[1] (See Figure 1)

Figure 1

Figure 1: The general formula of NNS type of ligands

These donor NNS ligands, which contain two nitrogen and a sulfur atom,  are based on aliphatic amines.  These three donor atoms are linked by two or three C spacers.

In most of these compounds, with the exception of 77-0560, the central nitrogen is a secondary amine.  It has an N-H functionality which is integral to the performance of the catalysts in hydrogenation applications.  This N−H functional group stabilizes the key transition state via hydrogen bonding.[2-3]

The second amine, pendant on the central nitrogen, is a tertiary aliphatic amine.  This group provides a hemilabile moiety as a weak electron donor within the catalysts.  The sulfur functionality exists as a thioether. 

These compounds are capable of acting as bidentate, tridentate or tetradentate ligands depending on the metal. Iridium complexes form an additional carbon-iridium bond by ortho-metallation on the phenyl ring. The hydride atom in these iridium complexes originates from the same phenyl group.

In ruthenium compounds the ligands act as tridentate species, affording octahedral complexes.  These are true pincer compounds and can be either mer,  fac, or both.

Figure 2

Figure 2: Ru and Ir complexes with NNS ligands

Figure 2 shows these commercially available air-stable Ru and Ir catalysts with NNS coordination. An important application of these complexes is demonstrated by the selective hydrogenation of methyl trifluoroacetate for the important synthon trifluoroacetaldehyde methyl hemiacetal.  Accomplished under basic and relatively mild conditions (35−40 °C, 25 bar H2), the reaction displays reasonable turnover numbers:

Figure 3

For more detailed information about the catalytic activity of these items, and many more, please review the corresponding item specific technical notes found on our website.



  1.        Organometallics, 2015, 34, 4464.
  2.        J. Am. Chem. Soc., 2017, 139, 1245.
  3.        ACS Catal., 2017, 7, 6635.


Products mentioned in this blog can be found on our literature sheet:

 Catalysts & Ligands for Ruthenium/Iridium Hydrogenation


Additional Resources:

Pincer Ligands & Pincer Complexes

Milstein Catalysts



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