The [Pd(bipy)]2+ “merry-go-round”: Insights into the lability of the Pd–N bond

Two tripods (1 and 2) featuring pyrimidinyl pendant arms have been synthesized from 5-(1H-pyrazol-3-yl)-pyrimidine (5) and 1,3,5-tribromomethylbenzene derivatives. Reaction with three equivalents of [Pd(bipy)](NO3)2 to form a macrotricycle closed by palladium coordination unexpectedly afforded the mononuclear species [Pd(1)(bipy)]2+ and [Pd(2)(bipy)]2+. These complexes show fluxional behavior on the 1H NMR timescale, the [Pd(bipy)]2+ fragment hopping between the pyrimidinyl coordinating moieties. The ΔGc‡’s estimated by the coalescence method are temperature independent, which means that ΔSc‡ = 0. This indicates that the “merry-go-round” process of [Pd(bipy)]2+ occurs intramolecularly, presumably by nucleophilic attack of the free pyrimidinyl arm to the bound Pd2+ center. This phenomenon permits to quantify the lability of the Pd–N coordination bond, the average ΔGc‡ ≈ ΔHc‡ value being 70 kJ mol–1.