Complexes H2 CO:PXH2 and HCO2 H : PXH2 for X=NC, F, Cl, CN, OH, CCH, CH3 , and H: Pnicogen Bonds and Hydrogen Bonds

Properties and Stabilities of Cyclic and Open Chains of Halogen Bonds

Open and cyclic chains from two to eight units of ICl and IF are constructed and examined by density functional theory (DFT) calculations. These chains contain either I···I or I···X halogen bonds (XBs) where X refers to Cl or F. The closed rings are more stable than the open chains due to the presence of an additional XB and enhanced cooperativity. This pattern is true even for most trimers where there is sizable geometric distortion in the rings. I···F rings are generally more stable than the corresponding I···I cycles as the I···F bond is stronger than I···I even in the simple dimer. However, I···I rings are comparable in energy to I···Cl. It is possible to construct I···I rings of at least as large as eight units, which are held together exclusively by XBs. On the other hand, the maximum possible size of I···X rings is 6. Red shifts are observed in the I-X stretching frequency bands, which magnify as the chain, both cyclic and open, grows longer. The NMR chemical shielding of the I atoms increases for I···I chains but diminishes when I···Cl bonds are present.

On the Ability of Nitrogen to Serve as an Electron Acceptor in a Pnicogen Bond

Whereas pnicogen atoms like P and As have been shown repeatedly to act as electron acceptors in pnicogen bonds, the same is not true of the more electronegative first-row N atom. Quantum calculations assess whether N can serve in this capacity in such bonds and under what conditions. There is a positive π-hole belt that surrounds the central N atom in the linear arrangement of NNNF, NNN-CN, and NNO, which can engage a NH₃ base to form a pnicogen bond with binding energy between 3 and 5 kcal/mol. Within the context of a planar arrangement, the π-hole above the N in NO₂OF, N(CN)₃, and CF₃NO₂ is also capable of forming a pnicogen bond, the strongest of which amounts to 11 kcal/mol with NMe₃ as base. In their pyramidal geometry, NF₃ and N(NO₂)₃ engage with a base through the σ-hole on the central N, with variable binding energies between 2 and 9 kcal/mol. AIM and NBO provide somewhat different interpretations of the secondary interactions that occur in some of these complexes.

Primary Phosphines and Phosphine Oxides with a Stereogenic Carbon Center Adjacent to the Phosphorus Atom: Synthesis and Anti-Markovnikov Radical Addition to Alkenes

Organophosphorus compounds with stereogenic phosphorus and carbon atoms have received increasing attention. In this regards, primary phosphines with a stereogenic carbon atom adjacent to the phosphorus atom were synthesized by the reduction in phosphonates and phosphonoselenoates with a binaphthyl group. Their oxidized products, i.e., phosphine oxides with a stereogenic tetrasubstituted carbon atom, were found to undergo BEt3-mediated radical addition to cyclohexene to give P-stereogenic secondary phosphine oxides with a diastereoselectivity of 91:9. The products were characterized by ordinary analytical methods, such as Fourier transform infrared spectroscopy; 1H, 13C, and 31P NMR spectroscopies; and mass spectroscopy. Computational studies on the phosphorus-centered radical species and the obtained product implied that the thermodynamically stable radical and the adduct may be formed as a major diastereomer. The radical addition to a range of alkenes took place in an anti-Markovnikov fashion to give P-stereogenic secondary phosphine oxides. A variety of functional groups in the alkenes were tolerated under the reaction conditions to afford secondary phosphine oxides in moderate yields. Primary phosphines with an alkenyl group, which were generated in situ, underwent intramolecular cyclization to give five- and six-membered cyclic phosphines in high yields after protection by BH3.

Influence of N-Base and O-Base Hybridization on Triel Bonds

The complexes of TrR₃ (Tr = B and Al; R = H, F, Cl, and Br) with three N-bases (NH₃, CH₂NH, and HCN) and three O-bases (CH₃OH, H₂CO, and CO) are utilized to explore the hybridization effect of N and O atoms on the strength, properties, and nature of the triel bond. The sp-hybridized O and N atoms form the weakest triel bond, followed by the sp²-hybridized O atom or the sp³-hybridized N atom, and the sp³-hybridized O atom or the sp²-hybridized N atom engages in the strongest triel bond. The hybridization effect is also related to the substituent of TrR₃. Most complexes are dominated by electrostatic, with increasing polarization contribution from sp to sp² to sp³. Although the CO oxygen engages in a weaker triel bond, its carbon atom is a better electron donor and the interaction energy even amounts to -37 kcal/mol in the BH₃ complex.