Protonation Equilibria and Intrinsic 31P NMR Chemical Shifts of the Stepwise Protonated Species of Diphosphate and Imidodiphosphate: A Unified Structural Interpretation

Abstract The acid–base equilibria and intrinsic 31P NMR chemical shifts of diphosphate and imidodiphosphate ions were investigated in aqueous solution. Stepwise protonation constants were determined by potentiometric titration and analysis of the pH-dependent 31P NMR chemical shifts, and the intrinsic shifts of individual protonation species by nonlinear least-squares analysis. Imidodiphosphate exhibited consistently higher protonation constants than diphosphate, confirming the greater Lewis basicity imparted by the bridging imino group. The intrinsic shifts were systematically compared with previously reported data for short-chain and cyclic phosphate and imidophosphate ions. Despite substantial differences in ligand skeletons and overall charge, they were governed primarily by whether the phosphorus nucleus belongs to a terminal or internal phosphate group and by the nature and number of adjacent bridging atoms. Each adjacent imino nitrogen atom produces an approximately 10 ppm downfield shift. Tautomerism involving proton transfer between imino nitrogen and phosphoryl oxygen atoms further influences the electronic environments of phosphorus nuclei and their intrinsic shifts. Minor effects from remote bridging atoms were also identified, particularly in cyclic ions. These findings provide a unified framework for interpreting intrinsic 31P NMR chemical shifts and understanding the protonation and coordination behavior of phosphorus–oxygen and phosphorus–nitrogen ligands.