Synthesis, spectral characterization and crystal structures of organophosphonic diamides: pyramidal nitrogen centers and hydrogen bonding in [PhP(O)((NHBu)-Bu-t)(2)], [PhP( O)(NHDipp)(2)](Dipp=2,6-(Pr2C6H3)-Pr-i) and [(BuP)-Bu-t(O)((NHPr)-Pr-i)(2)]
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Organophosphonic diamides of general formula [(RP)-P-1(O)(NHR2)(2)] (1-8; R-1=Ph, Cy or Bu-t and R-2=Cy, Pr-i, Bu-t or Dipp) have been prepared by the addition of P, P'-dichloro(alkyl/aryl)phosphine oxide, (RP)-P-1(O)Cl-2, to a solution of primary amine in toluene. The synthesis of PhP(O)((NHBu)-Bu-t)(2) (1) in high yield was achieved by slow oxidation of PhP((NHBu)-Bu-t)(2) in air over several days. All new compounds have been characterized by elemental analysis and by IR, EI mass and NMR (H-1 and P-31) spectroscopy. The molecular structures of [PhP(O)((NHBu)-Bu-t)(2)] (1), [PhPO(NHDipp)(2)] (2) and [(BuP)-Bu-t(O)((NHPr)-Pr-i)(2)] (7) have been determined by single crystal X-ray diffraction studies. The amido nitrogen atoms in 1, 2 and 7 show considerable deviation from the expected trigonal-planar geometry. The observed dihedral angles and the orientation of the nitrogen lone pair ( l. p.) in these compounds point to the role of l.p.(N)-->sigma*(P-X) type negative hyperconjugative interactions in P-N multiple bonding. Compounds 1 and 7 form interesting polymeric structures in the solid state with the aid of N-H...O=P hydrogen bonding interactions while the presence of the very bulky Dipp substituent on nitrogen in 2 prevents the participation of the N-H protons in hydrogen bonding with phosphoryl oxygen atoms.
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