Pnicogen-bonded Complexes Research Articles

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

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate H2 CO : PXH2 pnicogen-bonded complexes and HCO2 H : PXH2 complexes that are stabilized by pnicogen bonds and hydrogen bonds, with X=NC, F, Cl, CN, OH, CCH, CH3 , and H. The binding energies of these complexes exhibit a second-order dependence on the O-P distance. DFT-SAPT binding energies correlate linearly with MP2 binding energies. The HCO2 H : PXH2 complexes are stabilized by both a pnicogen bond and a hydrogen bond, resulting in greater binding energies for the HCO2 H : PXH2 complexes compared to H2 CO : PXH2 . Neither the O-P distance across the pnicogen bond nor the O-P distance across the hydrogen bond correlates with the binding energies of these complexes. The nonlinearity of the hydrogen bonds suggests that they are relatively weak bonds, except for complexes in which the substituent X is either CH3 or H. The pnicogen bond is the more important stabilizing interaction in the HCO2 H : PXH2 complexes except when the substituent X is a more electropositive group. EOM-CCSD spin-spin coupling constants 1p J(O-P) across pnicogen bonds in H2 CO:PXH2 and HCO2 H : PXH2 complexes increase as the O-P distance decreases, and exhibit a second order dependence on that distance. There is no correlation between 2h J(O-P) and the O-P distance across the hydrogen bond in the HCO2 H : PXH2 complexes. 2h J(O-P) coupling constants for complexes with X=CH3 and H have much greater absolute values than anticipated from their O-P distances.

Synergistic and antagonistic interplay between tetrel bond and pnicogen bond in complexes involving ring compounds.

The binary and ternary complexes composed of GeH3F, AsH2F and ring compounds (benzene, borazine and cyclopentadienyl anion) have been studied by theoretical calculations to understand the interplay between the tetrel bond and pnicogen bond interactions. The bonding strength of intermolecular interactions in these complexes is analyzed by means of atoms in molecules (AIM), natural bond orbital (NBO) and noncovalent interaction (NCI) index methods. The binary tetrel-bonded and pnicogen-bonded complexes can be classified as an n-type or π-type complex according to the orbital interactions involved in the complexes. Three binding modes can be distinguished according to the interplay between interactions for the ternary complexes. The binding mode A is characterized by the interplay between π-type tetrel bond and n-type pnicogen bond; binding mode B is characterized by the interplay between π-type pnicogen bond and n-type tetrel bond, and binding mode C is characterized by the interplay between π-type tetrel bond and π-type pnicogen bond. The binding modes A and B exhibit the synergistic interplay effect, while the antagonistic effect is reflected in mode C. The synergistic effect in binding modes A and B is stronger than antagonistic effect in mode C, and the synergistic effect in binding mode B is stronger than that in mode A.

Theoretical and conceptual DFT study of pnicogen- and halogen-bonded complexes of PH2X---BrCl.

The pnicogen and halogen bonding interactions in the PH2X---BrCl(X = H, F, OH, OCH3 and CH3) complexes have been studied at the MP2/aug-cc-pVTZ level. Analysis of interaction energies shows that the pnicogen-bonded structures are less stable than the corresponding halogen-bonded structures. The pnicogen and halogen bonds were also studied by conceptual DFT reactivity indices. Noncovalent interaction (NCI) and SAPT analysis reveals that the dispersion interactions dominate the pnicogen-bonded complexes of PH2X---BrCl in nature, while the halogen-bonded complexes are dominantly electrostatic energy. Graphical abstract It is found that the local softness s+ or s-on the basic center P of PH2X is related to the interaction energies (ΔECP) of halogen- or pnicogen-bonded complexes.

Pnicogen bonds in complexes with CO and CS: differentiating properties

ABSTRACTAb initio MP2/aug’-cc-pVTZ calculations have been performed on pnicogen-bonded complexes with CO and CS as electron-pair donors to PH2X, for X = F, NC, OH, CN, CCH, and H. CO:PH2X and OC:PH2X complexes are stabilised by traditional pnicogen bonds. CS is an electron-pair donor through its in-plane π system to four PH2X molecules. It forms C··· P phosphorus-shared bonds with some ion-pair character with PH2F, PH2(OH-Z), and PH2(OH-E), and traditional pnicogen bonds with all PH2X except PH2F. C-O and C-S stretching frequencies are blue-shifted for C···P pnicogen bonds, and red-shifted for O···P and S···P bonds. EOM-CCSD spin-spin coupling constants 1pJ(P-C) for OC:PH2X and 1pJ(P-O) for CO:PH2X are characteristic of complexes stabilised by traditional pnicogen bonds. Coupling constants 1pJ(P-C) as a function of the P-C distance for SC:PH2X illustrate the evolution of the C···P pnicogen bond. They increase as the P-C distance decreases in complexes with traditional bonds, reach a maximum for SC:PH2OH transition structures as the P-C distance further decreases and the bonds gain phosphorus-shared character, and then change sign and continue to decrease as the P-C distance further decreases and the phosphorus-shared pnicogen bonds gain ion-pair character. They approach the values of 1J(P-C) for the cation (H2PCS)+.

Competition between tetrel bond and pnicogen bond in complexes of TX3-ZX2 and NH3

The complexes formed between TX3-ZX2 (T = C, Si, Ge; Z = P, As, Sb; X = F, Cl) and NH3 were studied at the MP2/aug-cc-pVTZ(PP) level. For each TX3-ZX2, two types of complex were obtained. For CX3-ZX2, NH3 is inclined to approach the σ-hole on the Z atom, forming a pnicogen bond. For TX3-ZX2 (T = Si and Ge), however, the base favors engaging in a tetrel bond with the σ-hole on the T atom although the corresponding pnicogen-bonded complex is also stable. When NH3 approaches the CX3 terminal of CX3-ZX2, weak interactions are observed that may be classified as van der Waals interactions. The relative stability of both types of complexes is not affected by the substituent X. The tetrel bond is very strong and the largest interaction energy is up to -144kJmol-1. Dispersion is dominant in the weak van der Waals complexes, while tetrel- and pnicogen-bonded complexes are dominated by electrostatic interactions, with comparable contributions from polarization.

Exploring the pnicogen bond non-covalent interactions in 4-XPhNH2:PFnH3-n complexes (n = 1–3, X = H, F, CN, CHO, NH2, CH3, NO2 and OCH3)

Quantum chemical calculations were carried out to explore the effects of substituents in para position of aniline as well as phosphine on the strength of interactions in pnicogen-bonded (PB) complexes of the 4-XPhNH2:PFnH3-n (n=1–3 and X=H, F, CN, CHO, NH2, CH3, NO2 and OCH3). The P⋯N pnicogen bond strength in these complexes was examined at M06-2X/6–311++G(d,p), MP2/6–311++G(d,p), MP2/cc-pVDZ, CCSD/cc-pVDZ and MP2/aug-cc-pVTZ levels of theory. The dispersion corrected M06-2X-GD3, B2PLYP-GD3 and mPW2PLYP-GD2 functionals were also used to calculate the binding energies. The changes in the pnicogen bond strength due to the change of the substituents were well monitored by the changes in the interaction energy, structural parameters, natural charge, charge transfer, electron density topology, NMR chemical shielding and spin-spin coupling constants. The BSSE-corrected BEs change from 14.5 to 21.4kJmol−1 for the 4-XPhNH2:PF3, 21.5 to 30.5kJmol−1 for the 4-XPhNH2:PF2H and 24.2 to 33.1kJmol−1 for the XPhNH2:PFH2 complex at MP2/aug-cc-pVTZ level and 12.5 to 18.9kJmol−1 for the 4-XPhNH2:PF3, 17.4 to 26.5kJmol−1 for the 4-XPhNH2:PF2H and 18.9 to 26.5kJmol−1 for the XPhNH2:PFH2 complexes at MP2/6–311++G(d,p) level. For each PFnH3-n monomer, results demonstrated that the strength of the pnicogen bonds increases by introducimg electron-donating substituents in 4-XPhNH2 molecule, whereas a reverse situation was found upon introducing electron-accepting substituents. Natural bond orbital analysis confirmed that the charge transfer takes place from 4-XPhNH2 to PFnH3-n. Electron density properties based on atoms in molecules theory were also utilized to characterize the pnicogen bonds.

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

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.

Super-pnicogen bonding in the radical anion of the fluorophosphine dimer

The LUMO of the pnicogen-bonded fluoro-phosphine dimer has PP bonding character. Radical anion and dianion form relatively strong pnicogen bonds with some covalent character where however the dianion turns out to be a second order transition state. The binding energy of (FPH2)2- is 30.4kcal/mol (CCSD(T)/aug-cc-pVTZ; CASPT2(5,8): 30.7kcal/mol) and the bond strength order measured with the local PP bond stretching force constant increases from 0.055 for the neutral dimer to 0.187 thus revealing that the stabilization of the radical anion is to a large extend a result of one-electron six-center delocalization. Pnicogen-bonded complexes have a stabilizing electron affinity.

An ab initio study on the nature of σ-hole interactions in pnicogen-bonded complexes with carbene as an electron donor

ABSTRACTA theoretical study of the complexes formed between ZH2X (Z = P, As, Sb, Bi; X = F, Cl, Br, CN, NC, OH, NH2) and an N-heterocyclic carbene (imidazol-2-ylidene) is carried out by means of ab initio calculations. According to molecular electrostatic potential analysis, it is inferred that the divalent C atom of the carbene can act as a Lewis base with the pnicogen atom Z of ZH2X. The pnicogen bond distances (Z–C) are in the range of 2.050–2.911 for these complexes. While the Z−X bonds are longer than the corresponding Z−C bonds in the X = Cl and Br complexes, most of the Z−X bonds are short enough to suggest that they should be considered as covalent bonds which have lost some degree of covalency. For a given Z, the ZH2Br forms the strongest complex, followed by ZH2Cl and ZH2F. On the other hand, the binding energy in the halogenated ZH2X complexes follows the reverse ranking expected based on the values of the σ-hole of the isolated ZH2X monomers. The nature of the pnicogen bond interaction in these complexes is analysed by quantum theory of atoms in molecules (QTAIM) and natural bond orbital methods. According to QTAIM analysis, a partially covalent character can be attributed to the pnicogen bonds studied here.

Using (FH)2 and (FH)3 to Bridge the σ-Hole and the Lone Pair at P in Complexes with H2 XP, for X=CH3 , OH, H, CCH, F, Cl, NC, and CN.

Ab initio MP2/aug'-cc-pVTZ calculations are used to investigate the binary complexes H2 XP:HF, the ternary complexes H2 XP:(FH)2 , and the quaternary complexes H2 XP:(FH)3 , for X=CH3 , OH, H, CCH, F, Cl, NC, and CN. Hydrogen-bonded (HB) binary complexes are formed between all H2 XP molecules and FH, but only H2 FP, H2 ClP, and H2 (NC)P form pnicogen-bonded (ZB) complexes with FH. Ternary complexes with (FH)2 are stabilized by F-H⋅⋅⋅P and F-H⋅⋅⋅F hydrogen bonds and F⋅⋅⋅P pnicogen bonds, except for H2 (CH3 )P:(FH)2 and H3 P:(FH)2 , which do not have pnicogen bonds. All quaternary complexes H2 XP:(FH)3 are stabilized by both F-H⋅⋅⋅P and F-H⋅⋅⋅F hydrogen bonds and P⋅⋅⋅F pnicogen bonds. Thus, (FH)2 with two exceptions, and (FH)3 can bridge the σ-hole and the lone pair at P in these complexes. The binding energies of H2 XP:(FH)3 complexes are significantly greater than the binding energies of H2 XP:(FH)2 complexes, and nonadditivities are synergistic in both series. Charge transfer occurs across all intermolecular bonds from the lone-pair donor atom to an antibonding σ* orbital of the acceptor molecule, and stabilizes these complexes. Charge-transfer energies across the pnicogen bond correlate with the intermolecular P-F distance, while charge-transfer energies across F-H⋅⋅⋅P and F-H⋅⋅⋅F hydrogen bonds correlate with the distance between the lone-pair donor atom and the hydrogen-bonded H atom. In binary and quaternary complexes, charge transfer energies also correlate with the distance between the electron-donor atom and the hydrogen-bonded F atom. EOM-CCSD spin-spin coupling constants (2h) J(F-P) across F-H⋅⋅⋅P hydrogen bonds, and (1p) J(P-F) across pnicogen bonds in binary, ternary, and quaternary complexes exhibit strong correlations with the corresponding intermolecular distances. Hydrogen bonds are better transmitters of F-P coupling data than pnicogen bonds, despite the longer F⋅⋅⋅P distances in F-H⋅⋅⋅P hydrogen bonds compared to P⋅⋅⋅F pnicogen bonds. There is a correlation between the two bond coupling constants (2h) J(F-F) in the quaternary complexes and the corresponding intermolecular distances, but not in the ternary complexes, a reflection of the distorted geometries of the bridging dimers in ternary complexes.

H2XP:OH2 Complexes: Hydrogen vs. Pnicogen Bonds

A search of the Cambridge Structural Database (CSD) was carried out for phosphine-water and arsine-water complexes in which water is either the proton donor in hydrogen-bonded complexes, or the electron-pair donor in pnicogen-bonded complexes. The range of experimental P-O distances in the phosphine complexes is consistent with the results of ab initio MP2/aug’-cc-pVTZ calculations carried out on complexes H2XP:OH2, for X = NC, F, Cl, CN, OH, CCH, H, and CH3. Only hydrogen-bonded complexes are found on the H2(CH3)P:HOH and H3P:HOH potential surfaces, while only pnicogen-bonded complexes exist on H2(NC)P:OH2, H2FP:OH2, H2(CN)P:OH2, and H2(OH)P:OH2 surfaces. Both hydrogen-bonded and pnicogen-bonded complexes are found on the H2ClP:OH2 and H2(CCH)P:OH2 surfaces, with the pnicogen-bonded complexes more stable than the corresponding hydrogen-bonded complexes. The more electronegative substituents prefer to form pnicogen-bonded complexes, while the more electropositive substituents form hydrogen-bonded complexes. The H2XP:OH2 complexes are characterized in terms of their structures, binding energies, charge-transfer energies, and spin-spin coupling constants 2hJ(O-P), 1hJ(H-P), and 1J(O-H) across hydrogen bonds, and 1pJ(P-O) across pnicogen bonds.

Influence of the protonation of pyridine nitrogen on pnicogen bonding: competition and cooperativity.

Ab initio MP2/aug-cc-pVTZ calculations were performed to investigate the pnicogen-bonded complexes of PyZX2 (Py = pyridine, Z = P and As, X = H and F) and their protonated analogues. The selected Lewis bases include H2S, PH3, H2O, NH3, and H2CO. The relative stability of pnicogen-bonded complexes is related to the nature of PyZX2 and bases. When the nitrogen atom of the pyridine ring in PyZX2 is protonated, the protonated complexes are more stabilized than their neutral counterparts, with the interaction energies increased by 8.5-34.6 kJ mol(-1) and the binding distances shortened by 0.050-0.574 Å. Protonation strengthens the pnicogen bond, from a weak interaction to one of moderate strength. In the neutral complexes of PyZX2 and H2O, the formation of a N···H-O hydrogen bond is favorable compared to the pnicogen bond. Such inclination is more prominent in the complexes of protonated PyZX2 and NH3. In H2O···PyZX2···H2O, pnicogen bonding is strengthened by hydrogen bonding due to positive synergistic effects; however, in NH3···H(+)-PyZX2···NH3, pnicogen bonding is weakened by hydrogen bonding due to negative synergistic effects.

Unusual acid-base properties of the P4 molecule in hydrogen-, halogen-, and pnicogen-bonded complexes.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate hydrogen bonding, halogen bonding, and pnicogen bonding involving tetrahedral P4 and the FH, ClH, and FCl molecules. P4 has three unique interaction sites: at a vertex (designated the P1 atom); at an edge (the P2-P3 bond); and at the P2-P3-P4 face. The uniqueness of molecular P4 is its ability to act as an electron donor and an electron acceptor at the same site, except for the P2-P3 bond, which is only an electron donor. FCl and FH form five different complexes with P4, but ClH forms only three. The type of complex formed and its binding energy depend on both the interaction site of molecular P4 and the interacting molecule. For all complexes with FH, ClH, and FCl, the binding energies at a given site with the P4 molecule acting as the base are greater than the binding energies when P4 is the acid. Thus, P4 is a better electron donor than an electron acceptor. Charge-transfer interactions and EOM-CCSD spin-spin coupling constants across hydrogen, halogen, and pnicogen bonds are reported for all of the P4 complexes. Relative to 1J(Pi-Pj) in molecular P4, 1J(P1-P2) coupling constants decrease in absolute value and 1J(P2-P3) coupling constants increase in pnicogen-bonded complexes and the complex with FCl that has a PF halogen bond. Absolute values of 1J(P1-P2) increase and those of 1J(P2-P3) decrease in hydrogen-bonded complexes and complexes with PCl halogen bonds. 1J(P1-P2) and 1J(P2-P3) exhibit a single linear correlation with the corresponding Pi-Pj distances.

Properties of cationic pnicogen-bonded complexes F4-nHnP+:N-base with H–P···N linear and n = 1–4

ABSTRACTAb initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the pnicogen-bonded complexes F4-nHnP+:N-base, for n = 1–4, each with a linear or nearly linear Hax–P···N alignment. The sp3-hybridised nitrogen bases include NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3, and the sp bases are NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2. Binding energies increase as the P–N distance decreases, with an exponential curve showing this relationship when complexes with sp3 and sp hybridised bases are treated separately. However, the correlations are not as good as they are for the complexes F4-nHnP+:N-base for n = 0–3 with F–P···N linear. Different patterns are observed for the change in the binding energies of complexes with a particular base as the number of F atoms in the acid changes. Thus, the particular acid–base pair is a factor in determining the binding energies of these complexes.Three different charge-transfer interactions stabilise these complexes, namely Nlp→σ*P–Hax, Nlp→σ*P–Feq, and Nlp→σ*P–Heq. Unlike the corresponding complexes with F–P···N linear, Nlp→σ*P–Hax is not always the dominant charge-transfer interaction, since Nlp→σ*P–Feq is greater in some complexes. Nlp→σ*P–Heq makes the smallest contribution to the total charge-transfer energy. The total charge-transfer energies of all complexes increase exponentially as the P–N distance decreases in a manner very similar to that observed for the series of complexes with F–P···N linear.Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin–spin coupling constants 1pJ(P–N) across the pnicogen bond vary with the P–N distance, but different patterns are observed which depend on the nature of the acid, and for some acids, on the hybridisation of the nitrogen base. 1pJ(P–N) values for complexes of F3HP+ initially increase as the P–N distance decreases, reach a maximum, and then decrease with decreasing P–N distance as the P···N bond acquires increased covalent character. 1pJ(P–N) for complexes with H–P···N linear and those with F–P···N linear exhibit similar distance dependencies depending on the number of F atoms in equatorial positions and the hybridisation of the base. Complexation may increase, decrease, or leave the P–Hax distance unchanged, but 1J(P–Hax) always decreases relative to the corresponding isolated ion. Decreasing 1J(P–Hax) can be related to decreasing intermolecular P–N distance.

Properties of Cationic Pnicogen-Bonded Complexes F(4-n)H(n)P(+):N-Base with F-P···N Linear and n = 0-3.

Ab initio MP2/aug'-cc-pVTZ calculations were performed to investigate the pnicogen-bonded complexes F(4-n)H(n)P(+):N-base, for n = 0-3, each with a linear or nearly linear F-P···N alignment. The nitrogen bases include the sp(3) bases NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3 and the sp bases NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2. The binding energies vary between -20 and -180 kJ·mol(-1), while the P-N distances vary from 1.89 to 3.01 Å. In each series of complexes, binding energies decrease exponentially as the P-N distance increases, provided that complexes with sp(3) and sp hybridized bases are treated separately. Different patterns are observed for the change in the binding energies of complexes with a particular base as the number of F atoms in the acid changes. Thus, the particular acid-base pair is a factor in determining the binding energies of these complexes. Three different charge-transfer interactions stabilize these complexes. These arise from the nitrogen lone pair to the σ*P-F(ax), σ*P-F(eq), and σ*P-H(eq) orbitals. The dominant single charge-transfer energy in all complexes is N(lp) → σ*P-F(ax). However, since there are three N(lp) → σ*P-F(eq) charge-transfer interactions in complexes with F4P(+) and two in complexes with F3HP(+), the sum of the N(lp) → σ*P-F(eq) charge-transfer energies is greater than the N(lp) → σ*P-F(ax) charge-transfer energies in the former complexes, and similar to the N(lp) → σ*P-F(ax) energies in the latter. The total charge-transfer energies of all complexes decrease exponentially as the P-N distance increases. Coupling constants (1p)J(P-N) across the pnicogen bond vary with the P-N distance, but different patterns are observed for complexes with F4P(+) and complexes of the sp(3) bases with F3HP(+). These initially increase as the P-N distance decreases, reach a maximum, and then decrease with decreasing P-N distance as the P···N bond acquires increased covalent character. For the remaining complexes, (1p)J(P-N) increases with decreasing P-N distance. Complexation increases the P-F(ax) distance and (1)J(P-F(ax)) relative to the corresponding isolated ion. (1)J(P-F(ax)) correlates quadratically with the P-N distance.

A comprehensive analysis of P···π pnicogen bonds: substitution effects and comparison with Br···π halogen bonds.

Ab initio calculations were carried out in a systematic investigation of P···π pnicogen-bonded complexes XH2P···C2H2/C2H4 and FH2P···C2R2/C2R4 for X = H, CH3, OH, CN, Br, Cl, NO2, F, and R = F, CH3, as well as corresponding Br···π halogen-bonded complexes XBr···C2H2. Both the electron-withdrawing and electron-donating substituents in the electron acceptor have enhancing effects on the strength of P···π interactions. The electron-donating group in the electron donor leads to strengthening while the electron-withdrawing group leads to weakening of P···π interactions. The studied P···π and Br···π interactions are similar and are typically "closed-shell" non-covalent character in nature. Moreover, analyses of natural bond orbital and density difference of molecular formation indicated that charge transfer and polarization also play important roles in P···π interactions. The substituents have direct effects on the molecular electrostatic potential, and the charge transfer amount and extent of polarization of the P···π interaction are also specific to each substituent.

P···N pnicogen bonds in cationic complexes of F₄P⁺ and F₃HP⁺ with nitrogen bases.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on cationic pnicogen-bonded complexes F4P(+):N-base and F3HP(+):N-base, with linear F(ax)-P···N and H(ax)-P···N, respectively. The bases include the sp(3)-hybridized nitrogen bases NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3, and the sp bases NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2. The binding energies of these complexes span a wide range, from -15 to -180 kJ mol(-1), as do the P-N distances, which vary from 1.89 to 3.11 Å. There is a gap in the P-N distances between 2.25 and 2.53 Å in which no complexes are found. Thus, the equilibrium complexes may be classified as inner or outer complexes based on the value of the P-N distance. Inner complexes have P···N bonds with varying degrees of covalent character, whereas outer complexes are stabilized by intermolecular P···N bonds with little or no covalency. Charge-transfer stabilizes these pnicogen-bonded complexes. For complexes F4P(+):N-base, the dominant charge-transfer interaction is from the lone pair on N to the σ*P-F(ax) orbital. In addition, there are three other charge-transfer interactions from the lone pair on N to the σ*P-F(eq) orbitals, which taken together, are more stabilizing than the interaction involving σ*P-F(ax). In contrast, the dominant charge-transfer interaction for complexes F3HP(+):N-base is from the lone pair on N to the σ*P-F(eq) orbitals. Computed EOM-CCSD Fermi-contact terms are excellent approximations to the total spin-spin coupling constants (1p)J(P-N) and (1)J(P-H(ax)), but are poor approximations to (1)J(P-F(ax)). (1p)J(P-N) values increase with decreasing P-N distance, approach a maximum, and then decrease and change sign as the P-N distance further decreases and the pnicogen bond acquires increased covalency. (1)J(P-F(ax)) values for F4P(+):N-base complexes increase with decreasing distance. Although the P-H(ax) distance changes very little in complexes F3HP(+):N-base, patterns exist which suggest that changes in (1)J(P-H(ax)) reflect the hybridization of the nitrogen base and whether the complex is an inner or outer complex.

Pnicogen-bonded complexes H(n)F(5-n)P:N-base, for n = 0-5.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on the pnicogen-bonded complexes H(n)F(5-n)P:N-base, for n = 0-5 and nitrogen bases NC(-), NCLi, NP, NCH, and NCF. The structures of these complexes have either C(4v) or C(2v) symmetry with one exception. P-N distances and interaction energies vary dramatically in these complexes, while F(ax)-P-F(eq) angles in complexes with PF5 vary from 91° at short P-N distances to 100° at long distances. The value of this angle approaches the F(ax)-P-F(eq) angle of 102° computed for the Berry pseudorotation transition structure which interconverts axial and equatorial F atoms of PF5. The computed distances and F(ax)-P-F(eq) angles in complexes F5P:N-base are consistent with experimental CSD data. For a fixed acid, interaction energies decrease in the order NC(-) > NCLi > NP > NCH > NCF. In contrast, for a fixed base, there is no single pattern for the variations in distances and interaction energies as a function of the acid. This suggests that there are multiple factors that influence these properties. The dominant factor appears to be the number of F atoms in equatorial positions, and then a linear F(ax)-P···N rather than H(ax)-P···N alignment. The acids may be grouped into pairs (PF5, PHF4) with four equatorial F atoms, then (PH4F, PH2F3) with F(ax)-P···N linear, and then (PH3F2 and PH5) with H(ax)-P···N linear. The electron-donating ability of the base is also a factor in determining the structures and interaction energies of these complexes. Charge transfer from the N lone pair to the σ* P-A(ax) orbital stabilizes H(n)F(5-n)P:N-base complexes, with A(ax) either F(ax) or H(ax). The total charge-transfer energies correlate with the interaction energies of these complexes. Spin-spin coupling constants (1p)J(P-N) for (PF5, PHF4) complexes with nitrogen bases are negative with the strongest bases NC(-) and NCLi but positive for the remaining bases. Complexes of (PH4F, PH2F3) with these same two strong bases and H4FP:NP have positive (1p)J(P-N) values but negative values for the remaining bases. (PH5, PH3F2) have negative values of (1p)J(P-N) only for complexes with NC(-). Values of (1)J(P-F(ax)) and (1)J(P-H(ax)) correlate with the P-F(ax) and P-H(ax) distances, respectively.

Pnicogen-bonded anionic complexes.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the pnicogen-bonded complexes H2YP:X(-), for X,Y = Cl, NC, F, CCH, and CH3. Of the 36 possible complexes, only 21 are unique equilibrium structures. All substituents form (H2XPX)(-) complexes with symmetric X-P-X bonds. The P-A ion-molecule pnicogen bonds in these and some additional complexes have partial covalent character, while some P-A' covalent bonds have partial ion-molecule character. A and A' are the atoms of X and Y, respectively, which are directly bonded to P. Complexes with these types of bonds include the symmetric complexes (H2XPX)(-), H2(CH3)P:F(-), H2(CCH)P:F(-), H2FP:NC(-), H2FP:Cl(-), H2FP:CN(-), and H2(NC)P:Cl(-). Charge transfer from A to the P-A' σ* orbital stabilizes H2YP:X(-) complexes and leads to a reduction of the negative charge on X. For fixed X, the smallest negative charge occurs in the symmetric complex. Then, for a given X, the order of decreasing negative charge with respect to Y is CH3 > CCH > CN (bonded through C) > F > NC (bonded through N) > Cl, which is also the order of decreasing P-A distance. EOM-CCSD spin-spin coupling constants (1p)J(P-A) differentiate between shorter ion-molecule pnicogen bonds with partial covalent character and longer P···A ion-molecule pnicogen bonds. Similarly, coupling constants (1)J(P-A') differentiate between longer covalent P-A' bonds with partial ion-molecule character and shorter P-A' covalent bonds.

Influence of Substituent Effects on the Formation of P···Cl Pnicogen Bonds or Halogen Bonds

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out in search of equilibrium structures with P···Cl pnicogen bonds or halogen bonds on the potential energy surfaces H2FP:ClY for Y = F, NC, Cl, CN, CCH, CH3, and H. Three different types of halogen-bonded complexes with traditional, chlorine-shared, and ion-pair bonds have been identified. Two different pnicogen-bonded complexes have also been found on these surfaces. The most electronegative substituents F and NC form only halogen-bonded complexes, while the most electropositive substituents CH3 and H form only pnicogen-bonded complexes. The halogen-bonded complexes involving the less electronegative groups Cl and CN are more stable than the corresponding pnicogen-bonded complexes, while the pnicogen-bonded complexes with CCH are more stable than the corresponding halogen-bonded complex. Traditional halogen-bonded complexes are stabilized by charge transfer from the P lone pair to the Cl-A σ* orbital, where A is the atom of Y directly bonded to Cl. Charge transfer from the Cl lone pair to the P-F σ* orbital stabilizes pnicogen-bonded complexes. As a result, the H2FP unit becomes positively charged in halogen-bonded complexes and negatively charged in pnicogen-bonded complexes. Spin-spin coupling constants (1X)J(P-Cl) for complexes with traditional halogen bonds increase with decreasing P-Cl distance, reach a maximum value for complexes with chlorine-shared halogen bonds, and then decrease and change sign when the bond is an ion-pair bond. (1p)J(P-Cl) coupling constants across pnicogen bonds tend to increase with decreasing P-Cl distance.