Hydroxy- and Mercaptopyridine Pincer Platinum and Palladium Complexes Generated by Silver-Free Halide Abstraction

A silver-free route has been employed for the synthesis of a number of Pd and Pt complexes supported by an
NCN “pincer” ligand (NCN = [2,6-(Me2NCH2)2C6H3]-) via halide abstraction. This was achieved by the use of o-,
m-, and p-hydroxypyridines or o- and p-mercaptopyridines in basic ethanol solutions. The acidic OH or SH protons
are removed to generate the formally anionic ligands. X-ray crystal structure determination of these complexes
shows that the bonding of the substituted pyridine ligands occurs exclusively through the pyridine N for
hydroxypyridines and exclusively through the thiol S in mercaptopyridines. In the hydroxypyridine case, the ortho
and para isomers tautomerize to generate pyridone structures with a covalent M−N bond, whereas the
m-hydroxypyridine complex, which is unable to tautomerize, is found to still be bound through the pyridine N but
in a zwitterionic structure. The mercaptopyridine complexes are the first reported examples of pincer-ligated Pd
and Pt thiolate species. The pyridine ligands can be quantitatively exchanged for chloride by reaction with excess
NH4Cl, a potentially useful chemical switch for ligation/decomplexation of this ligand type.