Structurally Diverse Diazafluorene-Ligated Palladium(II) Complexes and Their Implications for Aerobic Oxidation Reactions

4,5-Diazafluoren-9-one
(DAF) has been identified as a highly effective
ligand in a number of Pd-catalyzed oxidation reactions, but the mechanistic
basis for its utility has not been elucidated. Here, we present the
complex coordination chemistry of DAF and palladium­(II) carboxylate
salts. Multiple complexes among an equilibrating mixture of species
have been characterized by 1H and 15N NMR spectroscopy
and X-ray crystallography. These complexes include monomeric and dimeric
PdII species, with monodentate (κ1), bidentate
(κ2), and bridging (μ:κ1:κ1) DAF coordination modes. Titration studies of DAF and Pd­(OAc)2 reveal the formation of two dimeric DAF/Pd­(OAc)2 complexes at low [DAF] and four monomeric species at higher [DAF].
The dimeric complexes feature two bridging acetate ligands together
with either a bridging or nonbridging (κ1) DAF ligand
coordinated to each PdII center. The monomeric structures
consist of three isomeric Pd­(κ1-DAF)2(OAc)2 complexes, together with Pd­(κ2-DAF)­(OAc)2 in which the DAF exhibits a traditional bidentate coordination
mode. Replacing DAF with the structurally related, but more-electron-rich
derivative 9,9-dimethyl-4,5-diazafluorene (Me2DAF) simplifies
the equilibrium mixture to two complexes: a dimeric species in which
the Me2DAF bridges the two Pd centers and a monomeric species
with a traditional κ2-Me2DAF coordination
mode. The use of DAF in combination with other carboxylate ligands
(CF3CO2– or tBuCO2–) also results in a simplified
collection of equilibrating PdII–DAF complexes.
Collectively, the results highlight the ability of DAF to equilibrate
rapidly among multiple coordination modes, and provide valuable insights
into the utility of DAF as a ligand in Pd-catalyzed oxidation reactions.