Anion recognition with hydrogen-bonding cyclodiphosphazanes.

Abstract

Modular cyclodiphosph(V)azanes are synthesised and their affinity for chloride and actetate anions were compared to those of a bisaryl urea derivative (1). The diamidocyclodiphosph(V)azanes cis-[{ArNHP(O)(μ-tBu)}2 ] [Ar=Ph (2) and Ar=m-(CF3 )2 Ph (3)] were synthesised by reaction of [{ClP(μ-NtBu)}2 ] (4) with the respective anilines and subsequent oxidation with H2 O2 . Phosphazanes 2 and 3 were obtained as the cis isomers and were characterised by multinuclear NMR spectroscopy, FTIR spectroscopy, HRMS and single-crystal X-ray diffraction. The cyclodiphosphazanes 2 and 3 readily co-crystallise with donor solvents such as MeOH, EtOH and DMSO through bidentate hydrogen bonding, as shown in the X-ray analyses. Cyclodiphosphazane 3 showed a remarkably high affinity (log[K]=5.42) for chloride compared with the bisaryl urea derivative 1 (log[K]=4.25). The affinities for acetate (AcO(-) ) are in the same range (3: log[K]=6.72, 1: log[K]=6.91). Cyclodiphosphazane 2, which does not contain CF3 groups, exhibits weaker binding to chloride (log[K]=3.95) and acetate (log[K]=4.49). DFT computations and X-ray analyses indicate that a squaramide-like hydrogen-bond directionality and Cα H interactions account for the efficiency of 3 as an anion receptor. The Cα H groups stabilise the Z,Z-3 conformation, which is necessary for bidentate hydrogen bonding, as well as coordinating with the anion.