C‒X vs C‒H activation for the synthesis of the cyclometalated complexes [Pd(YPhbpy)X] (HPhbpy = ۶-phenyl-۲,۲’-bipyridine; X/Y = (pseudo)halides)

The organometallic Pd(II) complexes [Pd(Phbpy)X] (X = Cl, Br, or I) containing the tridentate C^N^N cyclometalating ligand ۶-(phen-۲-ide)-۲,۲’-bipyridine (–Phbpy) were synthesised through oxidative addition using the protoligands X‒Phbpy (X = Cl, Br, and I) and [Pd۲(dba)۳] tris(dibenzylideneacetone)dipalladium(۰) in yields ranging from ۲۳ to ۵۱%. Further complexes [Pd(YPhbpy)Cl] resulted from C‒H palladation of the protoligand derivatives Y‒Phbpy with Y = F, Cl, Br, H, HO, MeO, and triflate) with K۲PdCl۴ in yields ranging from ۵۲ to ۹۸%. All protoligands and Pd(II) complexes were fully characterised using mass spectrometry (MS), nuclear magnetic resonance (NMR) spectroscopy, and single crystal X-ray diffraction (XRD) for Y = F, MeO. The complexes were studied in detail using electrochemical (cyclic voltammetry) and spectroelectrochemical (UV-vis absorption) methods and UV-vis absorption spectroscopy. Relative shifts in the potentials of the ligand centred electrochemical reductions in the range ‒۱.۷ to ‒۲.۷ (vs. ferrocene/ferrocenium) or the Pd‒X centred oxidations around +۱ V are in excellent agreement with variations in the density functional theory (DFT) calculated highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) constitutions. Long-wavelength absorption maxima attributable to metal(d)-to-ligand(π*) charge transfer transition observed in the range ۳۵۰ to ۵۵۰ nm were successfully modelled using time-dependent methods (TD-DFT) showing small contributions from triplet states.

Keywords:

Palladium , cyclometalation , C‒X activation , C‒H activation , Spectroelectrochemistry