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Azines As Electron-Pair Donors to CO2 for N···C Tetrel Bonds

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Azines As Electron-Pair Donors to CO2 for N···C Tetrel Bonds This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Article Cite This: J. Phys. Chem. A 2017, 121, 8017-8025 pubs.acs.org/JPCA ··· Azines as Electron-Pair Donors to CO2 for N C Tetrel Bonds Published as part of The Journal of Physical Chemistry virtual special issue “Manuel Yań̃ez and Otilia Mó Festschrift”. § § ‡ Ibon Alkorta,*, JoséElguero,*, and Janet E. Del Bene*, § Instituto de Química Medica,́ Consejo Superior de Investigaciones Cientificas, Juan de la Cierva, 3, E-28006 Madrid, Spain ‡ Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555, United States *S Supporting Information ABSTRACT: Ab initio MP2/aug′-cc-pVTZ calculations were performed to investigate tetrel-bonded complexes formed between CO2 and the aromatic bases pyridine, the diazines, triazines, tetrazines, and pentazine. Of the 23 unique equilibrium azine:CO2 complexes, 14 have planar structures in which a single nitrogen atom is an electron-pair donor to the carbon of the CO2 molecule, and 9 have perpendicular structures in which two adjacent nitrogen atoms donate electrons to CO2, with bond formation occurring along an N−N bond. The binding energies of these complexes vary from 13 to 20 kJ mol−1 and decrease as the number of nitrogen atoms in the ring increases. For a given base, planar structures have larger binding energies than perpendicular structures. The binding energies of the planar complexes also tend to increase as the distance across the tetrel bond decreases. Charge transfer in the planar pyridine:CO2 complex occurs from the N lone pair to a virtual nonbonding orbital of the π* − CO2 carbon atom. In the remaining planar complexes, charge transfer occurs from an N lone pair to the remote in-plane C O − π* − − Downloaded via CSIC on June 19, 2018 at 07:28:39 (UTC). orbital. In perpendicular complexes, charge transfer occurs from an N N bond to the adjacent O C O orbital of CO2. 13 Decreases in the bending frequency of the CO2 molecule and in the C chemical shielding of the C atom of CO2 upon complex formation are larger in planar structures compared to perpendicular structures. EOM-CCSD spin−spin coupling constants 1tJ(N−C) for complexes with planar structures are very small but still correlate with the N−C distance across the tetrel bond. See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ■ INTRODUCTION complexes, the carbon atom interacts with the nitrogen atom of − The field of intermolecular interactions has expanded from the NCH and NH3 at N C distances of 2.998 and 2.9875 Å, ’ 1 respectively. More recently, the structures of the pyridine:CO2 hydrogen bond described in detail in Pimentel s classic book fl to other types of intermolecular interactions. These include and 3,5-di uoropyridine:CO2 complexes have been re- − 20,21 halogen bonds involving group 17 atoms as the acids,2 4 ported. In both cases, the complexes have planar C2v − − chalcogen bonds for group 16,5 8 pnicogen bonds for group symmetry, with N C distances of 2.7977 and 2.8245 Å, − − − 15,9 11 and tetrel bonds for group 14.12 14 Recently, Legon respectively. It is interesting that the N C distances in the and Resnati et al. emphasized the similarities among these pyridine complexes are shorter than they are in the complexes bonds and suggested that they should be considered as arising with NCH and NH3. when a σ-hole15,16 or a π-hole associated with an E atom in one Previously, we investigated tetrel bonds involving the cation (H CPH )+ as the electron-pair acceptor,22 the anions F− molecular entity interacts with a nucleophilic region such as a 2 − 2 π and Cl as the electron donors to substituted methanes,23 pair of nonbonding or electrons in another, or the same, fi molecular entity.17 complexes formed by electron-de cient and electron-rich Complexes that have CO2 as the Lewis acid have been known for many years. In 1984, Klemperer’s group used Received: August 25, 2017 microwave spectroscopy to determine the gas-phase structures Revised: September 25, 2017 18 19 of the CO2/NCH and CO2/NH3 complexes. In these Published: September 25, 2017 © 2017 American Chemical Society 8017 DOI: 10.1021/acs.jpca.7b08505 J. Phys. Chem. A 2017, 121, 8017−8025 The Journal of Physical Chemistry A Article carbon atoms,24 and uncharged systems in which carbenes are negative of the reaction energy for the formation of the ··· − the electron donors for C C tetrel bonds and C C covalent complex from the corresponding azine and CO2. bonds.25,26 We now expand our studies of tetrel bonds to Molecular electrostatic potentials (MEPs) were evaluated for include complexes in which the azines are the electron-pair the 11 azine bases with the DAMQT program.35 The electron σ donors to CO2 through the -hole at C. The azines include density properties at bond critical points (BCPs) of azine:CO2 pyridine, three diazines, three triazines, three tetrazines, and complexes were analyzed using the Atoms in Molecules (AIM) − pentazine and are shown in Scheme 1. In this paper we present methodology36 39 employing the AIMAll40 program. The topological analysis of the electron density produces the Scheme 1. Azines molecular graph of each complex. This graph identifies the location of electron density features of interest, including the electron density (ρ) maxima associated with the various nuclei, and saddle points that correspond to BCPs. The zero gradient line that connects a BCP with two nuclei is the bond path. The electron density at the intermolecular nitrogen−carbon bond ρ ∇2ρ critical point ( BCP), the Laplacian ( BCP) at that point, and the total energy density (HBCP) were also evaluated. The Natural Bond Orbital (NBO) method41 was used to obtain the stabilizing charge-transfer interactions in complexes using the NBO-6 program.42 Since MP2 orbitals are nonexistent, charge-transfer interactions were computed using the B3LYP functional with the aug′-cc-pVTZ basis set at the MP2/aug′-cc-pVTZ complex geometries. This allows for the inclusion of at least some electron correlation effects. NMR absolute chemical shieldings were evaluated at MP2/ aug′-cc-pVTZ with the Gauge-Invariant Atomic Orbital (GIAO) method43,44 as implemented in the Gaussian-09 program. Equation of motion coupled cluster singles and doubles (EOM-CCSD) spin−spin coupling constants were evaluated in the configuration interaction (CI)-like approx- and discuss the structures and binding energies of the imation45,46 with all electrons correlated. For these calculations, azine:CO complexes, their charge-transfer energies, bonding 2 the Ahlrichs47 qzp basis set was placed on 13C, 15N, and 17O, parameters, and spectroscopic data including O−C−O bending and the Dunning cc-pVDZ basis set32 was placed on 1H atoms. frequencies, 13C NMR chemical shieldings, and NMR spin− Fermi contact terms were evaluated for all complexes. spin coupling constants 1tJ(N−C). Whenever possible, total coupling constants were also evaluated ■ METHODS as the sum of the paramagnetic spin orbit (PSO), diamagnetic spin orbit (DSO), Fermi contact (FC), and spin dipole (SD) The structures of the isolated azine monomers and CO2 and of terms using ACES II48 on the HPC cluster Oakley at the Ohio the binary complexes azine:CO were optimized at second- 2 − Supercomputer Center. order Møller−Plesset perturbation theory (MP2)27 30 with the aug′-cc-pVTZ basis set.31 This basis set was derived from the Dunning aug-cc-pVTZ basis set32,33 by removing diffuse ■ RESULTS AND DISCUSSION functions from H atoms. Frequencies were computed to Azine Monomers. The structures, total energies, and establish that these optimized structures correspond to molecular graphs of the 11 planar azines are given in Table S1 equilibrium structures on their potential surfaces and to of the Supporting Information. The molecular electrostatic evaluate changes in C−O stretching frequencies upon complex potential (MEP) isosurfaces of 1,2- 1,3-, and 1,4-diazine are formation. Optimization and frequency calculations were illustrated in Figure 1, and values of the MEP minima for the performed using the Gaussian 09 program.34 The binding azines are reported in Table 1. Each minimum is associated energies (−ΔE) of the complexes were computed as the with a nitrogen lone pair of electrons. The largest negative Figure 1. Molecular electrostatic potential isosurfaces of 1,2- 1,3-, and 1,4-diazine. Blue and red regions correspond to values of +0.06 and −0.06 au, respectively. 8018 DOI: 10.1021/acs.jpca.7b08505 J. Phys. Chem. A 2017, 121, 8017−8025 The Journal of Physical Chemistry A Article Table 1. Values of Unique MEP Minima (au) for the Azines Table 2. Binding Energies (−ΔE) and Charge-Transfer −1 − − Energies (CT, kJ mol ), Intermolecular N C7 Distances pyridine 0.094 (N1) (R, Å), and Descriptions of Equilibrium Azine:CO − a 2 1,2-diazine 0.094 (N1) Complexes 1,3-diazine −0.079 (N1) − −Δ − b,c 1,4-diazine 0.078 (N1) azine:CO2 E R(N C7) CT description 1,2,3-triazine −0.076 (N1) −0.088 (N2) d pyr(1) 20.0 2.724 43.2 C2v planar 1,2,4-triazine −0.078 (N1) −0.078 (N2) −0.061(N4) − di12(1 2) 18.9 2.906 12.4 C2v perpendicular 1,3,5-triazine −0.065 (N1) (1) 18.7 2.791 8.0 Cs planar 1,2,3,4-tetrazine −0.058 (N1) −0.070 (N2) di13(1) 18.0 2.787 13.5 Cs planar 1,2,3,5-tetrazine −0.059 (N1) −0.070 (N2) −0.044 (N5) di14(1) 18.3 2.752 14.6 Cs planar 1,2,4,5-tetrazine −0.059 (N1) tri123(1) 17.0 2.850 10.0 Cs planar pentazine −0.039 (N1) −0.051(N2) −0.050 (N3) − (1 2) 16.8 2.970; 2,909 6.1 Cs perpendicular tri124(1) 17.3 2.828 10.0 C planar − s MEP values of 0.094 au are found for pyridine and 1,2- (1−2) 16.9 2.923;2.927 7.0 C perpendicular − s diazine, and the smallest value of 0.039 au belongs to (4) 16.3 2.828 10.6 Cs planar pentazine.
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