A Sterically Hindered Phosphonic Acid with a Hydrogen‐Bonded Cage Structure: [4‐ tert ‐Bu‐2,6‐Mes 2 ‐C 6 H 2 P(O)(OH) 2 ·H 2 O] 4

The synthesis and molecular structure of the novel phosphonic acid 4‐ tert ‐Bu‐2,6‐Mes 2 ‐C 6 H 2 P(O)(OH) 2 ( 1 ) is reported. Compound 1 crystallizes in form of its monohydrate as a hydrogen‐bonded cluster ( 1·H 2 O ) 4 comprizing four phosphonic acid molecules (O···O 2.383(3)‐3.006(4) Å). Additionally, sterically hindered terphenyl‐substituted phosphorus compounds of the type 4‐ tert ‐Bu‐2,6‐Mes 2 ‐C 6 H 2 PR(O)(OH) ( 5 , R = H; 7 , R = O 2 CC 6 H 4 ‐3‐Cl; 9 , R = OEt) were prepared, which all show dimeric hydrogen‐bonded structures with O···O distances in the range 2.489(2)–2.519(3) Å. Attempts at oxidizing 5 using H 2 O 2 , KMnO 4 , O 3 , or Me 3 NO in order to give 1 failed. Crystallization of 5 in the presence of Me 3 NO gave the novel hydrogen bonded aggregate 4‐ tert ‐Bu‐2,6‐Mes 2 ‐C 6 H 2 PH(O)(OH)·ONMe 3 ( 6 ) showing an O–H···O distance of 2.560(4) Å.