Triel Bonds between BH3/C5H4BX and M(MDA)2 (X = H, CN, F, CH3, NH2; M = Ni, Pd, Pt, MDA = Enolated Malondialdehyde) and Group 10 Transition Metal Electron Donors.

A systematic theoretical study was conducted on the triel bonds (TrB) within the BH3∙∙∙M(MDA)2 and C5H4BX∙∙∙M(MDA)2 (M = Ni, Pd, Pt, X = H, CN, F, CH3, NH2, MDA = enolated malondialdehyde) complexes, with BH3 and C5H4BX acting as the electron acceptors and the square-coordinated M(MDA)2 acting as the electron donor. The interaction energies of these systems range between -4.71 and -33.18 kcal/mol. The larger the transition metal center M, the greater the enhancement of the TrB, with σ-hole TrBs found to be stronger than π-hole TrBs. In the σ-hole TrB complex, an electron-withdrawing substituent on the C opposite to the B atom enhances the TrB, while an electron-donating substituent has little effect on the strength of TrB in the Pd and Pt complexes but enhances the TrB in the Ni-containing complexes. The van der Waals interaction plays an important role in stabilizing these binary systems, and its contribution diminishes with increasing M size. The orbital effect within these systems is largely due to charge transfer from the dz2 orbital of M into the empty pz orbital of B.

Triel Bonds with Au Atoms as Electron Donors.

The novel triel bonds of BX3 (X=H, F, Cl, Br, and I) and C5 H5 B as electron acceptors and AuR2 (R=Cl and CH3 ) as an electron donor were explored. The triel bond is a primary driving force for most complexes, while the contribution from a halogen-chlorine interaction in BX3 -AuCl2 (X=Cl, Br, and I) and an iodine-Au interaction in BI3 -Au(CH3 )3 is also very important. Interestingly, the positively charged Au atom of AuCl2 can attractively bind with the holes of BX3 and C5 H5 B. The interaction energy lies in the range of 1 and 80 kcal/mol, in the order X=F

The Tetrel Bonds of Hypervalent Halogen Compounds.

The tetrel bond between PhXF2Y(TF3) (T = C and Si; X = Cl, Br, and I; Y = F and Cl) and the electron donor MCN (M = Li and Na) was investigated at the M06-2X/aug-cc-pVDZ level of theory. As the electronegativity of the halogen atom X increases, the strength of the tetrel bond also increases, but as the electronegativity of the halogen atom Y increases, the strength of the tetrel bond decreases. The magnitude of the interaction energy in most -CF3 complexes was found to be less than 10 kcal/mol, but to exceed 11 kcal/mol for PhClF2Cl(CF3)⋯NCNa. The tetrel bond is greatly enhanced when the -SiF3 group interacts with LiCN or NaCN, with the largest interaction energy approaching 100 kcal/mol and displaying a covalent Si⋯N interaction. Along with this enhancement, the Si⋯N distance was found to be less than the X-Si bond length, the -SiF3 group to be closer to the N atom, and in most -SiF3 systems, the X-Si-F angle to be less than 90°; the -SiF3 group therefore undergoes inversion and complete transfer in some systems.

Interactions in Model Ionic Dyads and Triads Containing Tetrel Atoms.

The interactions in model ionic YTX3···Z (Y = NC, F, Cl, Br; X = F, Cl, Br, Z = F-, Cl-, Br-, Li+) dyads containing the tetrel atoms, T = C, Si, Ge, were studied using ab initio computational methods, including an energy decomposition analysis, which found that the YTX3 molecules were stabilized by both anions (via tetrel bonding) and cations (via polarization). For the tetrel-bonded dyads, both the electrostatic and polarization forces make comparable contributions to the binding in the C-containing dyads, whereas, electrostatic forces are by far the largest contributor to the binding in the Si- and Ge-containing analogues. Model metastable Li+···NCTCl3···F- (T = C, Si, Ge) triads were found to be lower in energy than the combined energy of the Li+ + NCTCl3 + F- fragments. The pair energies and cooperative energies for these highly polar triads were also computed and discussed.

A Computational Study of Chalcogen-containing H2 X YF and (CH3 )2 X YF (X=O, S, Se; Y=F, Cl, H) and Pnicogen-containing H3 X' YF and (CH3 )3 X' YF (X'=N, P, As) Complexes.

A computational study was undertaken for the model complexes H2 X…YF and (CH3 )2 X…YF (X=O, S, Se; Y=F, Cl, H), and H3 X'…YF and (CH3 )3 X'…YF (X'=N, P, As), at the MP2/6-311++G(d,p) level of theory. For H2 X…YF and H3 X'…YF, noncovalent interactions dominate the binding in order of increasing YF dipole moment, except for H3 As…F2 , and possibly H3 As…ClF. However, for the methyl-substituted complexes (CH3 )2 X…YF and (CH3 )3 X'…YF the binding is especially strong for the complexes containing F2 , implying significant chemical bonding between the interacting molecules. The relative stability of these complexes can be rationalized by the difference in the electronegativity of the X or X' and Y atoms.

The effect of anions on noncovalent interactions in model clusters of chalcogen-containing (CH3)2X (X = O, S, Se) molecules.

A computational study of F-(CH3)2XYF (X = O, S, Se; Y = F, H) triads, with F- bound to the protons of the two methyl groups, found significant enhancement of the XY interactions relative to the neutral (CH3)2XYF dyads, more so for the XF than the XH interaction. A subsequent model study of anionic F-(CH3)2OYZ (YZ = HF, CH3F, SiH3F, GeH3F, NF3, OF2, F2, ClF) triads revealed a similar enhancement of all OY noncovalent interactions. The relative stabilities and associated properties of the OY σ-hole interactions were rationalized using the electronegativity difference between O and Y. Very strongly bound Z-(CH3)2X-F2 (Z = F, Cl, Br; X = S, Se) clusters were also predicted and involve interactions between the anion Z-, Lewis base (CH3)2X and two F atoms sandwiching the central X atom to form a nearly linear F-X-F axis.

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.

Comparative computational study of model halogen-bonded complexes of FKrCl.

Quantum chemical calculations for the FKrCl molecule at various levels of theory were performed and suggest that this molecule is metastable and may be amenable to experimental synthesis under cryogenic conditions. The FKrCl molecule forms weak halogen-bonded complexes FKrCl···Y with small molecules like FH and H2O and its computed properties were compared with those for analogous complexes of its precursor, FCl, and its rare gas hydride counterpart, FKrH. The cooperative effect of additional noncovalent interactions introduced at the F atom in the FKrCl···Y dimer (to give Z···FKrCl···Y trimers) showed a general strengthening of the intermolecular interactions in the order halogen bond < hydrogen bond < beryllium bond < lithium bond.

Variation of sigma-hole magnitude with M valence electron population in MX(n)Y(4-n) molecules (n = 1-4; M = C, Si, Ge; X, Y = F, Cl, Br).

Sigma holes are described as electron-deficient regions on atoms, particularly along the extension of covalent bonds, due to non-uniform electron density distribution on the surface of these atoms. A computational study of MX(n)Y(4-n) molecules (n = 1-4; M = C, Si, Ge; X, Y = F, Cl, Br) was undertaken and it is shown that the relative sigma hole potentials on M due to X-M and Y-M can be adequately explained in terms of the variation in the valence electron population of the central M atom. A model is proposed for the depletion of the M valence electron population which explains the trends in sigma hole strengths, especially those that cannot be accounted for solely on the basis of relative electronegativities.

The effect of atomic ions on model σ-hole bonded complexes of AH3Y (A = C, Si, Ge; Y = F, Cl, Br).

A computational study of ionic X···AH3-Y complexes (X = F(-), Cl(-), Br(-), Li(+), Be(2+); A = C, Si, Ge; Y = F, Cl, Br) predicted optimized structures which are held together by a combination of attractive forces, including ion-dipole and ion-σ-hole electrostatic interactions, and polarization forces. The trends (with variation in the halogen Y) for selected properties were rationalized by considering the electron density shifts due to the ion's electric field. Although it has been found previously that the trends for binding energies in neutral complexes follow the sigma-hole strength, the present study found that the dependence on the dipole polarizability of the A-Y bond can explain the trends for binding energies in these more strongly bound ionic complexes.

Cooperative effects in novel LiF∕HF⋯LiF⋯XF (X = F, Cl, Br) clusters.

Highly stable trimeric clusters of general formula LiF∕HF⋯LiF⋯XF (X = F, Cl, Br) are predicted computationally. These clusters involve a LiF⋯XF dyad, with both the positively charged Li and negatively charged F atom of LiF non-covalently bonded to the X atom of XF. A third molecule (LiF or HF) is complexed to this dyad via ionic-type F⋯Li and Li(H)⋯F interactions to form a substantially stronger cluster.

Cooperative effects of hydrogen, lithium and halogen bonding on F-H/LiOH2 complexes.

A comparative computational study of the cooperative effect of hydrogen-, lithium- and halogen-bonding on model F-H and F-Li complexes with H2O was undertaken at the MP2/6-311++G(d,p) level of theory. The general trend of increased attraction or a positive cooperative effect on introduction of a third molecule to the F-H/LiOH2 dimer shows that lithium bonding has the greatest effect, followed by hydrogen bonding and then halogen bonding. The computed three-body nonadditive energy has a more substantial contribution to the interaction energy of the cyclic trimers than to the open linear trimer clusters.

Communication: An unusual halogen-bonding motif: the LiBr···BrF dimer as a model system.

A stable complex, LiBr···BrF, is predicted in which the negative Br atom of LiBr is anchored to the Br atom of BrF by a halogen bond, while the positively charged Li atom interacts with the lone pair electron density on the Br atom of BrF in a direction roughly perpendicular to the halogen bond. As far as we are aware, this is the first reported instance of an atom of one diatomic molecule (Br of BrF) being bonded to two different, oppositely charged atoms (Li and Br) of another diatomic molecule (LiBr). Other less stable dimers of LiBr and BrF were predicted and compared with this novel complex.

Cooperative effects of noncovalent bonds to the Br atom of halogen-bonded H3N...BrZ and HCN...BrZ (Z = F, Br) complexes.

A series of complexes formed between halogen-bonded H(3)N/HCN...BrZ (Z = Br, F) dimers and H(3)N/HCN...BrZ...XY (XY = HF, ClF, BeH(2), LiF) trimers were investigated at the MP2 and B3LYP levels of theory using a 6-31++G(d,p) basis set. Optimized structures, interaction energies, and other properties of interest were obtained. The addition of XY to the H(3)N/HCN...BrZ dyad leads to enhanced intermolecular binding with respect to the isolated monomers. This enhanced binding receives contributions from the electrostatic and inductive forces between the constituent pairs, with, in some instances, substantial three-body non-additive contributions to the binding energy. It was found that the XY = LiF interaction causes the greatest distortion of the H(3)N/HCN...BrZ halogen bond from the preferred linear orientation and also provides the strongest binding energy via the nonadditive energy.

A comparative computational study of FKrCCH...Y, FCCKrH...Y, and FCCH...Y complexes (Y = BF, CO, N2, OH2, OH(CH3), O(CH3)2).

The structural and spectroscopic changes in complexes of FCCKrH...Y and FKrCCH...Y (Y = BF, CO, N(2), OH(2), OH(CH(3)), O(CH(3))(2)) were computed at the MP2∕6-31++G(d,p) level of theory and compared with the corresponding properties for FCCH...Y. The computed bond length changes and frequency shifts on complexation were rationalized by comparing with a perturbation model, which gives quantitative agreement with the standard ab initio results. A recently proposed model also gives a reasonable qualitative account of the observed trends in these complexes.

Cooperative hydrogen bonding in trimers involving HCN and HBO.

A computational study of the cooperative effect of hydrogen bonding in linear trimers comprised of HCN and HBO molecules was undertaken at the MP2/6-311++G(2d,2p) level of theory. It was found that the third molecule leads to enhancement of the binding relative to that of the dimer species comprising these monomers. HBO is a better proton acceptor and a weaker proton donor than HCN.

A theoretical study of hydrogen- and lithium-bonded complexes of F-H∕Li and Cl-H∕Li with NF3, NH3, and NH2(CH3).

Hydrogen- and lithium-bonded complexes of A-H∕Li (A = F, Cl) with the amine analogues NF(3), NH(3), and NH(2)(CH(3)) were studied at the MP2∕6-311++G(d,p) level of theory. Bond extensions and redshifts were obtained for the H-bonded complexes, while bond extensions and blueshifts were obtained for the Li-bonded species. The variation of these and other properties with the basicity of the amines was investigated and rationalized by comparing the ab initio results with predictions from a model derived from perturbation theory.

Halogen and hydrogen bonding to the Br atom in complexes of FBr.

A computational study predicts a number of stable, unusual halogen- and hydrogen-bonded complexes involving FBr, NCH, and FH. Starting from the linear halogen-bonded FBr...NCH dimer, increasingly more stable complexes are obtained by the successive hydrogen bonding of one to three FH molecules to the lone pairs on the Br atom of FBr to form a trimer, tetramer, and pentamer. A hexamer is obtained from the pentamer by the bonding of FH to the F atom of FBr. The combined halogen and hydrogen bonding gives rise to a large computed zero-point corrected binding energy of 98 kJ/mol for the hexamer at the MP2/6-31++G(d,p) level of theory.

A comparative computational study of hydrogen and lithium-bonded complexes.

A computational study of hydrogen-bonded complexes of F(3)CH and C1H and of lithium-bonded complexes of F(3)CLi and CILi, with small molecules such as N(2) and H(2)O was undertaken at the MP2/6-311++G(d,p) level of theory. Bond extensions and redshifts were obtained for the Cl[Single Bond]H bond in the ClH complexes, while bond contractions and blueshifts were obtained for the C[Single Bond]H bond in the F(3)CH complexes. By contrast, bond extensions and blueshifts were obtained for all of the lithium-bonded species. These results were rationalized using a model derived from perturbation theory.

Cooperative and diminutive hydrogen bonding in Y...HCN...HCN and NCH...Y...HCN trimers (Y=BF,CO,N2).

A computational study of the cooperative effect of hydrogen bonding in Y...HCN...HCN and its diminutive effect in NCH...Y...HCN (Y=BF,CO,N2) linear complexes relative to the Y...HCN dimer was undertaken at the MP2/6-311++G(2d,2p) level of theory. It was found that the additional hydrogen bond in Y...HCN...HCN leads to an enhanced Y...HCN dissociation energy, extended H-C bond length, and larger redshift of the H-C stretch relative to Y...HCN, while opposite features are observed in NCH...Y...HCN. The cooperativity is diminished as the hardness of the Y atom directly bonded to the HCN molecule increases. A particularly interesting result is that the small bond contraction and blueshift associated with the H-C bond in BF...HCN is converted to a small bond extension and redshift on the formation of the BF...HCN...HCN trimer.