C–H Bond Activation by a Palladium(II) Thioether Complex: Formation of the Bis(nitromethanate) Complex [Pd(9S3)(CH2NO2)2

Abstract

Palladium(II) acetate reacts with 1 equiv of 1,4,7-trithiacyclononane (9S3) at room temperature to produce the neutral complex [Pd(9S3)(OAc)2] (1; OAc– = CH3COO–) as an analytically pure yellow solid, which has been characterized using 1H and 13C NMR spectroscopy and single-crystal X-ray diffraction. The crystal structure of 1 shows the first example of an exodentate third sulfur of the 9S3 ligand in a Pd(II) complex. Complex 1 reacts with nitromethane at room temperature in methanol to produce [Pd(9S3)(CH2NO2)2] (2) and acetic acid, as confirmed by 1H NMR and 13C NMR spectroscopy. Moreover, complex 2 has been characterized by single-crystal X-ray diffraction. Its crystal structure is the first example of any transition-metal complex containing two nitromethanate (anionic nitromethane) ligands. The complex shows the more typical elongated-square-pyramidal structure and [S2C2 + S1] coordination with one long Pd–S axial interaction at 2.823(2) Å and two C-bound nitromethanate ligands. Interestingly, each nitromethanate ligand is in a different coordination environment and varies in their trans-directing abilities. Reactivity studies suggest that the complexation behavior of the 9S3 ligand and the Pd(II) center as well as the σ-donor ability of the leaving group play key roles in the C–H activation of the nitromethane. Two related metal complexes, [Pd(dppe)(OAc)2] (dppe = 1,2-bis(diphenylphosphino)ethane) and [Pd(9S3)(CF3COO)2], were synthesized, but neither of these react in a similar fashion to form a nitromethanate complex. Also, the reaction of 1 with nitrobenzene and nitrocyclopentane was studied, but these nitro-organics do not undergo C–H bond activation like nitromethane.