Effects of carbon chain substituents on the P⋯N noncovalent bond

Strongly bound noncovalent (SO3)n:H2CO complexes (n = 1, 2)

SO₃and H₂CO dimers and trimers are held together by S⋯O chalcogen bonds, supplemented by weaker CH⋯O and/or O⋯C bonds.

Complexes containing CO2and SO2. Mixed dimers, trimers and tetramers

Two stable minima for the 1 : 1 heterodimer of CO₂ : SO₂, both bound by about 2 kcal mol⁻¹. Binding is dominated by charge transfer from O lone pairs of SO₂to CO π* antibonding orbitals.

Intramolecular pnicogen interactions in phosphorus and arsenic analogues of proton sponges

A computational study of the intramolecular pnicogen bond in 1,8-bis-substituted naphthalene derivatives (ZXH and ZX₂ with Z = P, As and X = H, F, Cl, and Br), structurally related to proton sponges, has been carried out.

Properties of complexes H2C═(X)P:PXH2, for X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2: P···P pnicogen bonding at σ-holes and π-holes

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on complexes H2C═(X)P:PXH2, for X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2. Three sets of complexes have been found on the potential surfaces. Conformation A complexes have A-P···P-A approaching linearity, with A the atom of X directly bonded to P. Conformation B complexes have A-P···P linear, but the P···P═C orientation of H2C═PX may differ significantly from linearity. Conformation C complexes are unique, since the pnicogen bond involves π-electron donation and acceptance by H2C═PX. The order of binding energies of the three conformations of H2C═(X)P:PXH2 is C > A > B, with two exceptions. Although the binding energies of conformation C complexes tend to be greater than the corresponding conformation A complexes, intermolecular distances in conformation C tend to be longer than those in conformation A. Charge transfer stabilizes H2C═(X)P:PXH2 complexes. The preferred direction of charge transfer is from H2C═PX to PXH2. In conformations A and B, charge transfer occurs from a P lone pair on one molecule to an antibonding σ* orbital on the other. However, in conformation C, charge transfer occurs from the π orbital of H2C═PX to the σ*P-A orbital of PXH2, and from the lone pair on P of PXH2 through the π-hole to the π*P═C orbital of H2C═PX. Changes in charges on P upon complexation do not correlate with changes in (31)P chemical shieldings. Computed EOM-CCSD spin-spin coupling constants correlate with P-P distances. At each distance, the ordering of (1p)J(P-P) is A > B > C. Binding energies and spin-spin coupling constants of conformation A complexes of (PH2X)2, H2C═(X)P:PXH2, and (H2C═PX)2 with A-P···P-A approaching linearity have been compared. For complexes with the more electronegative substituents, binding energies are ordered (PH2X)2 > H2C═(X)P:PXH2 > (H2C═PX)2, while the order is reversed for complexes formed from the more electropositive substituents. A plot of ΔE(PH2X)2/ΔE(H2C═PX)2 versus ΔE[H2C═(X)P:PXH2]/ΔE(H2C═PX)2 indicates that there is a systematic relationship among the stabilities of these complexes. Complexes (PH2X)2 tend to have larger spin-spin coupling constants and shorter P-P distances than H2C═(X)P:PXH2, which in turn have larger coupling constants and shorter P-P distances than (H2C═PX)2, although there is some overlap. Complexes having similar P-P distances have similar values of (1p)J(P-P).

Pnicogen bonded complexes of PO2X (X = F, Cl) with nitrogen bases

An ab initio MP2/aug'-cc-pVTZ study has been carried out on complexes formed between PO2X (X = F and Cl) as the Lewis acids and a series of nitrogen bases ZN, including NH3, H2C═NH, NH2F, NP, NCH, NCF, NF3, and N2. Binding energies of these complexes vary from -10 to -150 kJ/mol, and P-N distances from 1.88 to 2.72 Å. Complexes ZN:PO2F have stronger P(...)N bonds and shorter P-N distances than the corresponding complexes ZN:PO2Cl. Charge transfer from the N lone pair through the π-hole to the P-X and P-O σ* orbitals leads to stabilization of these complexes, although charge-transfer energies can be evaluated only for complexes with binding energies less than -71 kJ/mol. Complexation of PO2X with the strongest bases leads to P···N bonds with a significant degree of covalency, and P-N distances that approach the P-N distances in the molecules PO2NC and PO2NH2. In these complexes, the PO2X molecules distort from planarity. Changes in (31)P absolute chemical shieldings upon complexation do not correlate with changes in charges on P, although they do correlate with the binding energies of the complexes. EOM-CCSD spin-spin coupling constants (1p)J(P-N) are dominated by the Fermi-contact term, which is an excellent approximation to total J. (1p)J(P-N) values are small at long distances, increase as the distance decreases, but then decrease at short P-N distances. At the shortest distances, values of (1p)J(P-N) approach (1)J(P-N) for the molecules PO2NC and PO2NH2.

Complexes between dihydrogen and amine, phosphine, and arsine derivatives. Hydrogen bond versus pnictogen interaction

A theoretical study of the complexes between dihydrogen, H2, and a series of amine, phosphine, and arsine derivatives (ZH3 and ZH2X, with Z = N, P, or As and X = F, Cl, CN, or CH3) has been carried out using ab initio methods (MP2/aug-cc-pVTZ). Three energetic minima configurations have been characterized for each case with the H2 molecule in the proximity of the pnictogen atom (Z). In configuration A, the σ-electrons of H2 interact with σ-hole region of the pnictogen atom generated by the of X-Z bond. These complexes can be ascribed as pnictogen bonded. In configuration C, the lone electron pair of Z acts as the Lewis base, and H2 plays the role of the Lewis acid. Finally, configuration B presents a variety of noncovalent interactions depending on the binary complex considered. The atoms-in-molecules theory (AIM), natural bond orbitals (NBO) method as well as the density functional theory-symmetry adapted perturbation theory (DFT-SAPT) approach were used in this study to deepen the nature of the interactions considered.

Intramolecular pnicogen interactions in PHF-(CH2)(n)-PHF (n=2-6) systems

A computational study of the intramolecular pnicogen bond in PHF-(CH2)n-PHF (n=2-6) systems was carried out. For each compound, two different conformations, (R,R) and (R,S), were considered on the basis of the chirality of the phosphine groups. The characteristics of the closed conformers, in which the pnicogen interaction occurs, were compared with those of the extended conformer. In several cases, the closed conformations are more stable than the extended conformations. The calculated interaction energies of the pnicogen contact, by means of isodesmic reactions, provide values between -3.4 and -26.0 kJ mol(-1). Atoms in molecules and electron localization function analysis of the electron density showed that the systems in the closed conformations with short P···P distances have a partial covalent character in this interaction. The calculated absolute chemical shieldings of the P atoms showed an exponential relationship with the P···P distance. In addition, a search in the Cambridge crystallographic database was carried out to detect those compounds with a potential intramolecular pnicogen bond in the solid phase.

The pnicogen bond: its relation to hydrogen, halogen, and other noncovalent bonds

Among a wide range of noncovalent interactions, hydrogen (H) bonds are well known for their specific roles in various chemical and biological phenomena. When describing conventional hydrogen bonding, researchers use the notation AH···D (where A refers to the electron acceptor and D to the donor). However, the AH molecule engaged in a AH···D H-bond can also be pivoted around by roughly 180°, resulting in a HA···D arrangement. Even without the H atom in a bridging position, this arrangement can be attractive, as explained in this Account. The electron density donated by D transfers into a AH σ* antibonding orbital in either case: the lobe of the σ* orbital near the H atom in the H-bonding AH···D geometry, or the lobe proximate to the A atom in the HA···D case. A favorable electrostatic interaction energy between the two molecules supplements this charge transfer. When A belongs to the pnictide family of elements, which include phosphorus, arsenic, antimony, and bismuth, this type of interaction is called a pnicogen bond. This bonding interaction is somewhat analogous to the chalcogen and halogen bonds that arise when A is an element in group 16 or 17, respectively, of the periodic table. Electronegative substitutions, such as a F for a H atom opposite the electron donor atom, strengthen the pnicogen bond. For example, the binding energy in FH(2)P···NH(3) greatly exceeds that of the paradigmatic H-bonding water dimer. Surprisingly, di- or tri-halogenation does not produce any additional stabilization, in marked contrast to H-bonds. Chalcogen and halogen bonds show similar strength to the pnicogen bond for a given electron-withdrawing substituent. This insensitivity to the electron-acceptor atom distinguishes these interactions from H-bonds, in which energy depends strongly upon the identity of the proton-donor atom. As with H-bonds, pnicogen bonds can extract electron density from the lone pairs of atoms on the partner molecule, such as N, O, and S. The π systems of carbon chains can donate electron density in pnicogen bonds. Indeed, the strength of A···π pnicogen bonds exceeds that of H-bonds even when using strong proton donors such as water with the same π system. H-bonds typically have a high propensity for a linear AH···D arrangement, but pnicogen bonds show an even greater degree of anisotropy. Distortions of pnicogen bonds away from their preferred geometry cause a more rapid loss of stability than in H-bonds. Although often observed in dimers in the gas phase, pnicogen bonds also serve as the glue in larger aggregates, and researchers have found them in a number of diffraction studies of crystals.