Modular synthesis of PNP pincer ligands starting from lutidine

The double lithiation of 2,6-dimethylpyridine (lutidine) has been optimised for efficient P-C bond formation by reaction with easily accessible chlorophosphines (R2PCl) to access a range of symmetrically and unsymmetrically substituted pyridine-based PNP pincer ligands. In-situ transmetalation to magnesium prior to R2PCl addition allows the utilisation of base-sensitive chlorophosphines that are incompatible with the di-lithiated intermediate. Protection of the crude PNPs with elemental selenium permits facile isolation, purification, and storage of the pro-ligands, while allowing quantification of their donor properties by NMR and XRD analysis. Six novel (R)2PNP(R)2 selenides with a range of diverse substituents (R = tBu, iPr, Cy, Me, Ph, Fur) have been synthesised and fully characterised, and their deprotection with (TMS)3SiH is demonstrated. Omission of selenium oxidation and air-free aqueous workup gives straightforward access to free PNPs in high purity and yield.