Carbenes as Electron-Pair Donors for P⋅⋅⋅C Pnicogen Bonds.

Abstract

Ab initio MP2/aug'-cc-pVTZ calculations were performed on the P⋅⋅⋅C pnicogen-bonded complexes of the singlet carbene molecules C(NH2 )2 , C(OH)2 , and cyclic C(OCH)2 [OHC] with H2 XP molecules, with X=F, Cl, NC, OH, CH3 , CN, CCH, and H. The H2 XP:C(NH2 )2 and H2 XP:C(OH)2 complexes have Cs symmetry and two different structures: one in which the symmetry plane of the complex and the local symmetry plane of the carbene are non-coplanar, and the other in which they are coplanar. The non-coplanar H2 XP:C(NH2 )2 and H2 XP:C(OH)2 complexes arise only when X is one of the more electronegative substituents. Coplanar H2 XP:C(NH2 )2 complexes form when X is one of the more electropositive substituents, whereas coplanar H2 XP:C(OH)2 complexes exist for all X. H2 XP:C(NH2 )2 and H2 XP:C(OH)2 are stabilized by covalent P-C bonds or P⋅⋅⋅C pnicogen bonds, but co-planar H2 (CH3 )P:C(OH)2 and H3 P:C(OH)2 are stabilized by O-H⋅⋅⋅P hydrogen bonds. The H2 XP:OHC complexes have non-coplanar structures that are also stabilized by P-C covalent bonds or pnicogen bonds. The H2 (CH3 )P:OHC and H3 P:OHC complexes in which the symmetry plane of the complex and the local symmetry plane of the carbene are perpendicular are stabilized by P⋅⋅⋅π bonds with P acting as the electron-pair donor to the OHC π system. The H2 XP:C(NH2 )2 , H2 XP:C(OH)2 , and H2 XP:OHC complexes are described in terms of their binding energies, charge-transfer energies, electron density properties, and equation-of-motion coupled cluster singles and doubles spin-spin coupling constants.