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(NH ) , C(OH) , and cyclic C(OCH) [OHC] with H XP molecules, with X=F, Cl, NC, OH, CH , CN, CCH, and H. The H XP:C(NH ) and H XP:C(OH) complexes have C 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 H XP:C(NH ) and H XP:C(OH) complexes arise only when X is one of the more electronegative substituents. Coplanar H XP:C(NH ) complexes form when X is one of the more electropositive substituents, whereas coplanar H XP:C(OH) complexes exist for all X. H XP:C(NH ) and H XP:C(OH) are stabilized by covalent P-C bonds or P⋅⋅⋅C pnicogen bonds, but co-planar H (CH )P:C(OH) and H P:C(OH) are stabilized by O-H⋅⋅⋅P hydrogen bonds. The H XP:OHC complexes have non-coplanar structures that are also stabilized by P-C covalent bonds or pnicogen bonds. The H (CH )P:OHC and H 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 H XP:C(NH ) , H XP:C(OH) , and H 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.

Stretching the P–C Bond. Variations on Carbenes and Phosphanes

“…Further type of complexes with somewhat extended P–C interatomic distance can also be optimized in case of X: Cl and F. These E′ structures (Figure b) not only exhibit extended P–C bond distances (2.041–3.025 Å with the stabilized carbenes: 1–8 ) compared to the E “see-saw” adduct, but they form a typical trigonal bipyramidal arrangement, with the carbene and one P–X bond aligned axially. The assembly is held together by the recently intensively discussed pnictogen interaction, ,, which is interpreted as (i) the electrostatic attraction between the positively charged region (σ-hole) next to the phosphorus lone pair and negative charge at the carbene lone pair and (ii) the donation of the carbene lone pair to the σ* orbital of the axially aligned P–X bond (see on Figure b). − The resulting structures were discussed in detail in case of the adducts between highly stabilized carbenes (as imidazol-2-ylidene) and PH 2 X (X: halogen). , This interaction is weaker than in case of the T-shaped PX 3 because the LUMO is at higher energy (in case of PCl 3 as much as by 3.81 eVsee Figure ). However, because the T-shaped transition state itself is by about 50 kcal/mol higher energy as was mentioned above, the E and E′ structures have comparable stability.…”

“…38−40,44−48 These structures exhibit a "see-saw" arrangement at phosphorus. 38−40 Further interactions between carbenes and phosphanes have also been studied computationally, 49,50 with specific emphasis on weak pnictogen interactions. 51−53 According to these calculations, carbene− phosphane distances between ∼2.0 and 3.3 Å were reported for different stabilized singlet carbenes (including the NHC imidazol-ylidene and its oxygen analogue NOC) and various substituents at phosphorus.…”

Carbene triel bonds between TrR3 (Tr = B, Al) and N‐heterocyclic carbenes

“…It was demonstrated earlier that derivatives of (H 3 P=N) 2 Si are powerful neutral superbases. [59] The CT in pnicogen bonding with silylene (>0.3e) [60] is much larger than that in pnicogen bonding with carbene (<0.1e), [61] leading to the supposition that CT is enhanced when the C atom of the carbene is changed to Si.…”

Complexes H2CO:PXH2and HCO2H : PXH2for X=NC, F, Cl, CN, OH, CCH, CH3, and H: Pnicogen Bonds and Hydrogen Bonds

“…Subsequent papers reported structures, binding energies, and spin-spin coupling constants of heterodimers in a variety of pnicogen-bonded binary complexes. [4,5,6,7,8,9,10,11] During the same year in which Hey-Hawkins and Kirchner published their seminal paper, Scheiner et al also published a study of the complexes between PFH 2 and a series of Lewis bases. [12,13,14] Another author who has contributed significantly to this field is Esrafili.…”