Sodiated β‐Diphosphonate Carbanions: Characterization of the Tetrameric Cubane and the Hexameric Ladder Complexes [{(iPrO)2P(O)}2CHNa]4 and [{(EtO)2P(O)}2CHNa]6

Abstract

AbstractReaction of the methylene diphosphonates [(RO)2P(O)]2CH2, where R = iPr or Et, with the metalating reagents NaH, nBuNa or the superbasic mixture tBuONa/nBuLi leads to monodeprotonation, and formation of the complexes [{(iPrO)2P(O)}2CHNa]4 (1) and [{(EtO)2P(O)}2CHNa]6 (2). Single‐crystal X‐ray diffraction studies revealed that 1 is tetrameric in the solid state, with a central Na4O4 cubane core. Each metal in 1 is rendered pentacoordinate by binding to three μ3‐O phosphoryl bridges within the cubane, a chelating phosphoryl unit, and also a single iPrO group. In contrast, the structure of 2 is hexameric, and is composed of an unusual six‐rung Na6O6 ladder core. Three distinct sodium environments as well as μ1‐, μ2‐ and μ3‐O phosphoryl units are present within the ladder framework. No Na−C interactions are present in either 1 or 2. All of the six‐membered NaOPCPO chelate rings display localized P=O double bonds and partial multiple bonding on the P−C−P linkages (mean P−O: 1.49 Å, and mean P−C: 1.69 Å). Ab initio molecular orbital calculations (HF/6−31G*) on model complexes indicate that the hexameric arrangement found for 2 is thermodynamically more stable than the cubane structure of 1, in the absence of steric effects. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)