Synthesis, structure and dynamics of NHC-based palladium macrocycles

Rhiann E Andrew,Adrian B Chaplin

doi:10.1039/c3dt52578c

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(F)4] (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(F)4] (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 (19)F chemical shifts (ca. -400 ppm) and the presence of C-H···F interactions ((2h)J(FC) coupling observed by (13)C NMR spectroscopy). The dynamic behaviour of the fluoride complexes is largely consistent with the smaller ancillary ligand; [PdF{CNC-(CH2)8}][BAr(F)4] 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.

Highlights

N-heterocyclic carbenes (NHCs) have quickly emerged as a powerful class of carbon-based ligand in organometallic chemistry and catalysis.[1]
2a: following the general procedure using 2,6-bis(bromomethyl)pyridine (0.79 g, 2.98 mmol) and 1a (0.73 g, 2.98 mmol), the product was obtained as an off-white foam
4c: following the general procedure using AgF (0.022 g, 0.173 mmol) and 3c (0.050 g, 0.035 mmol) – stirred for 1 hour, the product was obtained as an oil that was washed with pentane and dried to give a white foam (0.022 g, 45%)

Summary

Introduction

N-heterocyclic carbenes (NHCs) have quickly emerged as a powerful class of carbon-based ligand in organometallic chemistry and catalysis.[1]. 2a: following the general procedure using 2,6-bis(bromomethyl)pyridine (0.79 g, 2.98 mmol) and 1a (0.73 g, 2.98 mmol), the product was obtained as an off-white foam.

Results

Conclusion