C‒X vs C‒H activation for the synthesis of the cyclometalated complexes [Pd(YPhbpy)X] (HPhbpy = 6-phenyl-2,2’-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 6-(phen-2-ide)-2,2’-bipyridine (–Phbpy) were synthesised through oxidative addition using the protoligands X‒Phbpy (X = Cl, Br, and I) and [Pd2(dba)3] tris(dibenzylideneacetone)dipalladium(0) in yields ranging from 23 to 51%. 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 K2PdCl4 in yields ranging from 52 to 98%. 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 ‒1.7 to ‒2.7 (vs. ferrocene/ferrocenium) or the Pd‒X centred oxidations around +1 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 350 to 550 nm were successfully modelled using time-dependent methods (TD-DFT) showing small contributions from triplet states.