Another recent release from Strem Chemicals, Inc. offers a selected array of pincer ligands and pincer compounds. Many of these types of compounds were developed by David Milstein who is a director at the Weizmann institute of Science in Israel where he is the director at the Kimmel Centre for Molecular Design and the Israel Matz Professional Chair of Organic Chemistry.
Strem currently offers five of these compounds. Milstein’s compounds are distinguished due to the non-innocent behaviour of the ligands. The dearomatisation / aromatisation processes of the ligand, in conjunction with the metal centre, leads to useful catalytic reactions.
A typical example of this technology is “PNN” ligand, 15-00731, 2. CAS 863971-66-8 Reaction of this pincer ligand with a readily available ruthenium complex (44-0085) CAS 16971-33-8 leads to the compound (44-0081) CAS 863971-62-4 where the tridentate ligand remains intact and forms a pincer complex with the metal centre. Treatment of this precatalyst with base, however, leads to the abstraction of a chloride anion from the metal centre and a proton from one “arm” of the PNN ligand. The result is dearomatisation of the pyridine core and the formation of active catalyst (44-0091) CAS 863971-63-5 This system was shown to be highly effective in the formation of esters from alcohols and the formation of amides from alcohols and amines3. The only by product in these reactions was hydrogen.
A similar pincer compound from Milstein is based on the acridine scaffold. In the ruthenium complex, 44-05254, 5, CAS 1101230-25-4 the previously planar ligand is bent at the middle aryl ring. The catalyst shows reactivity in neutral and basic medium.
1. J. Am. Chem. Soc, 2005, 127,10840; J. Am. Chem. Soc, 2005, 127,12429.
2. Angew. Chem. Int. Ed., 2006, 45,1113.
3. Science 2007, 317, 790.
4. J. Am. Chem. Soc., 2009, 131, 3146.
5. Angew. Chem. Int. Ed., 2008, 47, 8661.