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

Abstract

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 pK(BH)(+) 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.