Structures of fluorobenzylphosphonic acid isomers and their calcium salts

Kenneth J. Langley, Philip J. Squattrito, Fabrizio Adani, Enzo Montoneri

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33 Scopus citations


The ortho-, meta-and para-substituted isomers of fluorobenzylphosphonic acid have been synthesized. The crystal structures of the ortho and meta isomers have been determined by single crystal X-ray methods. 2-FC6H4CH2PO3H2: monoclinic, space group P21/c, Z=4, a=8.567(1), b=10.052(2), c=9.178(2) Å, β=96.04(1)°, V= 786.0(2) Å3, R(F)=0.032 for 1080 observations (I>3σ(I)) and 141 variables; 3-FC6H4CH2PO3H2: orthorhombic, space group Pbca, Z=8, a=7.808(3), b=31.483(7), c=6.969(2) Å, V=1713(2) Å3, R(F)=0.061 for 795 observations (I>3σ(I)) and 110 variables. Both molecules crystallize in anhydrous form with the acidic protons located on the phosphonate oxygen atoms hydrogen-bonded to the oxygen atoms of neighboring molecules. The 3-fluoro molecules pack in double layers with the polar PO3H2 groups directed to the center of the layer so that the organic portions of adjacent layers are in contact. The 2-fluoro molecules pack head-to-tail in wavy layers, an arrangement that is about 10% denser than that of the 3-fluoro isomer. The calcium monohydrogen salts of the ortho-and para-substituted acids have also been prepared and structurally characterized. Ca(2-FC6H4CH2PO3H) 2·3H2O: orthorhombic, space group Pca21, Z=4, 0=9.194(5), b=7.165(4), c=30.526(4) Å, V=2011(2) Å3, R(F)=0.064 for 1278 observations (I>3σ(I)) and 247 variables; Ca(4-FC6H4CH2PO3H) 2·3H2O: monoclinic, space group C2/c, Z=4, a=31.671(6), b=7.109(7), c=9.173(4) Å,3=98.60(4)°, V=2042(2) Å3, R(F)=0.033 for 1294 observations (I>3σ(I)) and 155 variables. The two compounds have very similar structures containing double layers of phosphonate anions held together by the calcium ions which bond directly to the PO3H groups. The Ca ions are in pentagonal bipyramidal environments of four phosphonate oxygen atoms and three water molecules. The major difference between the structures is in how the layers stack along the long axis.

Original languageEnglish
Pages (from-to)77-85
Number of pages9
JournalInorganica Chimica Acta
Issue number1
StatePublished - Dec 1 1996


  • Calcium salts
  • Crystal structures
  • Fluorobenzytphosphonic acid salts
  • Isomer salts


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