Synthesis, structure and dynamics of NHC-based palladium macrocycles

Abstract

A series of macrocyclic CNC pincer pro-ligands based on bis(imidazolium)lutidine salts with octa-, deca- and dodecamethylene spacers have been prepared and their coordination chemistry investigated. Using a Ag₂O based transmetallation strategy, cationic palladium(II) chloride complexes [PdCl{CNC–(CH₂)_n}][BAr^F₄] (n = 8, 10, 12; Ar^F = 3,5-C₆H₃(CF₃)₂) were prepared and fully characterised in solution, by NMR spectroscopy and ESI-MS, and in the solid-state, by X-ray crystallography. The smaller macrocyclic complexes (n = 8 and 10) exhibit dynamic behaviour in solution, involving ring flipping of the alkyl spacer across the Pd–Cl bond, which was interrogated by variable temperature NMR spectroscopy. In the solid-state, distorted coordination geometries are observed with the spacer skewed to one side of the Pd–Cl bond. In contrast, a static C₂ symmetric structure is observed for the dodecamethylene based macrocycle. For comparison, palladium(II) fluoride analogues [PdF{CNC–(CH₂)_n}][BAr^F₄] (n = 8, 10, 12) were also prepared and their solution and solid-state structures contrasted with those of the chlorides. Notably, these complexes exhibit very low frequency ¹⁹F chemical shifts (ca. −400 ppm) and the presence of C–H⋯F interactions (^2hJ_FC coupling observed by ¹³C NMR spectroscopy). The dynamic behaviour of the fluoride complexes is largely consistent with the smaller ancillary ligand; [PdF{CNC–(CH₂)₈}][BAr^F₄] exceptionally shows C_2v time averaged symmetry in solution at room temperature (CD₂Cl₂, 500 MHz) as a consequence of dual fluxional processes of the pincer backbone and alkyl spacer.