A New Synthetic Pathway to the Second and Third Generation of Superbasic Bisphosphazene Proton Sponges: The Run for the Best Chelating Ligand for a Proton

We
present the up to now strongest chelating neutral pincer ligand for
the simplest electrophile of chemistry, the proton. Two novel bisphosphazene
proton sponges, 1,8-bis­(tris­pyrroli­dino­phospha­zenyl)­naphtha­lene
(TPPN) and its higher homologue P2-TPPN, were obtained
via a Staudinger reaction and investigated concerning their structural
features and basic properties by experimental and computational means.
They exhibit experimental pKBH+ values in acetonitrile of 32.3 and 42.1, respectively, exceeding
the existing basicitiy record for proton sponges by more than 10 orders
of magnitude. We show that Schwesinger’s concept of homologization
of phosphazene bases and Alder’s concept of proton chelation
in a constrained geometry regime of basic centers can be combined
in the design of highly basic nonionic superbases of pincer type.