Synthesis, spectral characterization and crystal structures of organophosphonic diamides: pyramidal nitrogen centers and hydrogen bonding in [PhP(O)(NHtBu)2], [PhP(O)(NHDipp)2] (Dipp = 2,6-iPr2C6H3) and [tBuP(O)(NHiPr)2]

Abstract

Organophosphonic diamides of general formula [R¹P(O)(NHR²)₂] (1–8; R¹ = Ph, Cy or ^tBu and R² = Cy, ⁱPr, ^tBu or Dipp) have been prepared by the addition of P,P'-dichloro(alkyl/aryl)phosphine oxide, R¹P(O)Cl₂, to a solution of primary amine in toluene. The synthesis of PhP(O)(NH^tBu)₂ (1) in high yield was achieved by slow oxidation of PhP(NH^tBu)₂ in air over several days. All new compounds have been characterized by elemental analysis and by IR, EI mass and NMR (¹H and ³¹P) spectroscopy. The molecular structures of [PhP(O)(NH^tBu)₂] (1), [PhPO(NHDipp)₂] (2) and [^tBuP(O)(NHⁱPr)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) → σ*(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⋯OP 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.