Azines as Electron-Pair Donors to CO2


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Azines as Electron-Pair Donors to CO for N C Tetrel Bonds Ibon Alkorta, José Elguero, and Janet E. Del Bene J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08505 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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The Journal of Physical Chemistry

Azines as Electron-Pair Donors to CO2 for N…C Tetrel Bonds

Ibon Alkorta,§ José Elguero,§ and Janet E. Del Bene‡

§



Instituto de Química Médica (IQM-CSIC), Juan de la Cierva, 3, E-28006 Madrid, Spain Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555 USA

Corresponding authors:

I.A: E-mail: [email protected] (+34 915622900). J.E.: E-mail: [email protected] (+34 915622900). J.E.D.B.: E-mail: [email protected] (+1 330-609-5593).

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ABSTRACT: Ab initio MP2/aug’-cc-pVTZ calculations have been carried out 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 CO2 carbon atom. In the remaining planar complexes, charge-transfer occurs from an N lone pair to the remote inplane π*C-O orbital. In perpendicular complexes, charge transfer occurs from an N-N bond to the adjacent π*O-C-O orbital of CO2. Decreases in the bending frequency of the CO2 molecule and in the

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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

1t

constants J(N-C) for complexes with planar structures are very small but still correlate with the N-C distance across the tetrel bond.

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INTRODUCTION The field of intermolecular interactions has expanded from the hydrogen bond described in detail in Pimentel’s classic book,1 to other types of intermolecular interactions. These include halogen bonds involving group 17 atoms as the acids,2,3,4 chalcogen bonds for group 16,5,6,7,8 pnicogen bonds for group 15,9,10,11 and tetrel bonds for group 14.12,13,14 Recently, Legon and Resnati, et al. emphasized the similarities among these bonds, and suggested that they should be considered as arising when a σ-hole15,16 or a π-hole associated with an E atom in one molecular entity interacts with a nucleophilic region such as a pair of nonbonding or π electrons in another, or the same, molecular entity.17 Complexes that have CO2 as the Lewis acid have been known for many years. In 1984, Klemperer’s group used microwave spectroscopy to determine the gas-phase structures of the CO2:NCH18 and CO2:NH319 complexes. In these complexes, the carbon atom interacts with the nitrogen atom of NCH and NH3 at N-C distances of 2.998 and 2.9875 Å, respectively. More recently, the structures of the pyridine:CO2 and 3,5-difluropyridine:CO2 complexes have been reported.20,21 In both cases, the complexes have planar C2v symmetry, with N-C distances of 2.7977 and 2.8245 Å, respectively. It is interesting that the N-C distances in the pyridine complexes are shorter than they are in the complexes with NCH and NH3. Previously, we have investigated tetrel bonds involving the cation (H2C=PH2)+ as the electron-pair acceptor,22 the anions F– and Cl– as the electron donors to substituted methanes,23 complexes formed by electron-deficient and electron-rich carbon atoms,24 and uncharged systems in which carbenes are the electron donors for C…C tetrel bonds and C-C covalent bonds.25,26 We have now expanded our studies of tetrel bonds to include complexes in which the azines are the electron-pair donors to CO2 through the σ-hole at C. The azines include pyridine, three diazines, three triazines, three tetrazines, and pentazine, and are shown in Scheme 1. In this paper we present and discuss the structures and binding energies of the azine:CO2 complexes, their chargetransfer energies, bonding parameters, and spectroscopic data including O-C-O bending frequencies, 13C NMR chemical shieldings, and NMR spin-spin coupling constants 1tJ(N-C).

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Scheme 1. The azines

METHODS The structures of the isolated azine monomers and CO2, and of the binary complexes azine:CO2 were optimized at second-order Møller-Plesset perturbation theory (MP2) 27,28,29,30 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 functions from H atoms. Frequencies were computed to establish that these optimized structures correspond to equilibrium structures on their potential surfaces, and to evaluate changes in C-O stretching frequencies upon complex formation. Optimization and frequency calculations were performed using the Gaussian 09 program.34 The binding energies (–∆E) of the complexes were computed as the negative of the reaction energy for the formation of the complex from the corresponding azine and CO2. Molecular electrostatic potentials (MEPs) have been evaluated for the eleven azine bases with the DAMQT program.35 The electron density properties at bond critical points (BCPs) of azine:CO2 complexes have been analyzed using the Atoms in Molecules (AIM) methodology36,37,38,39 employing the AIMAll40 program. The topological analysis of the electron density produces the molecular graph of each complex. This graph identifies the location of 4

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electron density features of interest, including the electron density (ρ) maxima associated with the various nuclei, and saddle points which correspond to bond critical points (BCPs). The zero gradient line which connects a BCP with two nuclei is the bond path. The electron density at the intermolecular nitrogen-carbon bond critical point (ρBCP), the Laplacian (∇2ρBCP) at that point, and the total energy density (HBCP) have also been evaluated. The Natural Bond Orbital (NBO) method41 has been used to obtain the stabilizing chargetransfer interactions in complexes using the NBO-6 program.42 Since MP2 orbitals are nonexistent, charge-transfer interactions have been 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 have been 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 CI (configuration interaction)-like approximation45,46 with all electrons correlated. For these calculations, the Ahlrichs47 qzp basis set was placed on 13

C, 15N, and 17O, and the Dunning cc-pVDZ basis set32 was placed on 1H atoms. Fermi contact

terms were evaluated for all complexes. Whenever possible, total coupling constants were also evaluated as the sum of the paramagnetic spin orbit (PSO), diamagnetic spin orbit (DSO), Fermi contact (FC), and spin dipole (SD) terms using ACES II48 on the HPC cluster Oakley at the Ohio Supercomputer Center.

RESULTS AND DISCUSSION Azine Monomers The structures, total energies, and molecular graphs of the eleven planar azines are given in Table S1 of the Supporting Information. The molecular electrostatic potential (MEP) isosurfaces of 1,2- 1,3-, and 1,4-diazine are illustrated in Fig. 1, and values of the MEP minima for the azines are reported in Table 1. Each minimum is associated with a nitrogen lone pair of electrons. The largest negative MEP values of –0.094 au are found for pyridine and 1,2-diazine, and the smallest value of –0.039 au belongs to pentazine. For those azines that have three sequential nitrogen atoms, the largest absolute value is found for the central nitrogen. Moreover, 5

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in molecules that have nitrogen-nitrogen bonds, the region of negative electrostatic potential extends across the N-N bond, as illustrated in Fig. 1 for 1,2-diazine. The values of the MEP minima tend to decrease as the number of nitrogen atoms in the ring increases, as can be seen in Fig. S1 of the Supporting Information. How well the MEP values correlate with the binding energies of the azine:CO2 complexes will be discussed below.

Fig. 1. The 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.

Table 1. Values of unique MEP minima (au) for the azines pyridine –0.094 (N1) 1,2–diazine –0.094 (N1) 1,3–diazine –0.079 (N1) 1,4–diazine –0.078 (N1) 1,2,3–triazine –0.076 (N1) –0.088 (N2) 1,2,4–triazine –0.078 (N1) –0.078 (N2) –0.061(N4) 1,3,5–triazine –0.065 (N1) 1,2,3,4–tetrazine –0.058 (N1) –0.070 (N2) 1,2,3,5–tetrazine –0.059 (N1) –0.070 (N2) –0.044 (N5) 1,2,4,5–tetrazine –0.059 (N1) pentazine –0.039 (N1) –0.051(N2) –0.050 (N3) Azine:CO2 complexes Structures and Binding Energies.

To facilitate discussion of the azine:CO2 complexes,

nitrogen and carbon atoms in the azine rings have been numbered 1 - 6 according to the standard numbering system for these aromatic bases. C7 is the carbon atom of CO2, O8 is the oxygen 6

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atom that approaches closer to a C-H bond of the azine in planar complexes with Cs symmetry, and O9 is the remote oxygen. The different isomers of a particular azine are abbreviated as tri123, tri124, and tri135, for example, to indicate 1,2,3-, 1,2,4-, and 1,3,5-triazine, respectively. For a particular isomer of an azine, different isomers of the complex may exist because stable complexes may form at different ring sites. Thus, there are four isomers of 1,2,4-triazine:CO2 which are identified as tri124(1), tri124(1-2), tri124(4), and tri124(2). A single number in parentheses identifies the nitrogen atom which is involved in the tetrel bond. Two numbers in parentheses indicate that the interaction with CO2 occurs at a particular N-N bond. Fig. 2 illustrates the complexes tri124(1-2) and tri124(4). tri124(1-2) has a perpendicular structure in which the CO2 molecule is perpendicular to the symmetry plane of the triazine at the N1-N2 bond, with the CO2 molecule slightly closer to N1. tri124(4) has a planar structure with bond formation at N4.

tri124 (1-2)

tri124(4)

Fig. 2. Two isomers of 1,2,4-triazine with CO2 illustrating the planar and perpendicular structures, and the numbering system Table S2 of the Supporting Information reports the structures, total energies, and molecular graphs of all of the unique azine:CO2 complexes, and Table 2 presents their binding energies, intermolecular N-C7 distances, symmetries, and structures. Planar structures have the CO2 molecule in the symmetry plane of the azine, while perpendicular structures have the CO2 molecule perpendicular to that plane. There are four complexes that have C2v symmetry, namely, planar pyr(1) and three perpendicular structures in which the CO2 molecule is perpendicular to 7

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an N-N bond, namely, di12(1-2), tet1234(2-3), and tet1245(1-2). The remaining complexes have Cs symmetry, 13 with planar structures and 6 with perpendicular structures.

Based on the

symmetry of the azine, it might have been expected that planar 1,4-diazine with bond formation at N1, 1,3,5-triazine at N1, and 1,2,3,5-tetrazine at N5 might also have planar C2v structures, but they do not. The molecular graphs of Table S2 indicate that in each of these, the CO2 molecule is tilted and interacts with the adjacent C-H bond of the ring. The absence of this secondary interaction may account for the fact that there are no azine:CO2 planar complexes that have bond formation at a nitrogen atom that is bonded to two other nitrogen atoms in the ring, even though MEP minima are found at each of these N atoms.

Moreover, there are no equilibrium

perpendicular complexes that form at a single nitrogen atom. This implies that the approach of the CO2 molecule polarizes the lone pairs on adjacent nitrogens to the extent that a single equilibrium complex is found with formation of the tetrel bond somewhere along the N-N bond. The binding energies of the azine:CO2 complexes are given in Table 2. The complex binding energies are 20.0 kJ.mol–1 for pyr(1), vary from 18.0 to 18.9 kJ.mol–1 for the diazines, 15.7 to 17.3 kJ.mol–1 for the triazines, 14.1 to 15.9 for the tetrazines, and 12.7 to 13.6 kJ.mol–1 for the pentazine complexes. The binding energies of complexes with different numbers of nitrogen atoms in the ring overlap in only one case, as tet1234(1) has a binding energy that is 0.2 kJ.mol–1 greater than tri135(1). Thus, the overall trend is for these binding energies to decrease as the number of nitrogen atoms in the ring increases, as shown in Fig. 3. The correlation coefficient of the linear trendline is 0.943. This trend is similar to the trend for the MEP minima as a function of the number of nitrogens in the ring, as illustrated in Fig. S1 of the Supporting Information.

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Table 2. Binding energies (–∆E) and charge-transfer energies (CT, kJ.mol–1), intermolecular N-C7 distances (R, Å), and descriptions of equilibrium azine:CO2 complexesa azine:CO2 –∆E R(N-C7) CTb,c Description d pyr(1) 20.0 2.724 43.2 C2v planar di12(1-2) 2.906 C2v perpendicular 18.9 12.4 (1) 18.7 2.791 8.0 Cs planar di13(1) 18.0 2.787 13.5 Cs planar di14(1) 18.3 2.752 14.6 Cs planar tri123(1) 17.0 2.850 10.0 Cs planar Cs perpendicular (1-2) 2.970; 2,909 6.1 16.8 tri124(1) 17.3 2.828 10.0 Cs planar (1-2) 2.923;2.927 Cs perpendicular 16.9 7.0 (4) 16.3 2.828 10.6 Cs planar (2) 16.0 2.844 9.6 Cs planar tri135(1) 15.7 2.807 11.3 Cs planar tet1234(1) 15.9 2.887 7.5 Cs planar (1-2) 2.985;2,932 Cs perpendicular 14.9 5.7 (2-3) 2.972 C2v perpendicular 14.7 5.2 tet1235(1-2) 2.998;2.930 Cs perpendicular 14.7 5.5 (1) 14.6 2.905 6.9 Cs planar (5) 14.1 2.872 8.6 Cs planar e tet1245(1) 14.8 2.888 7.3 Cs planar e (1-2) 14.8 2.950 C2v perpendicular 5.9 pent(1) 13.6 2.951 5.4 Cs planar (2-3) 2.986;3.006 Cs perpendicular 12.8 4.5 (1-2) 3.015;2.952 Cs perpendicular 12.7 4.7 a) Data in bold refer to complexes with perpendicular structures. b) See text for a description of the nature of the charge-transfer interactions. c) The total charge-transfer energy. In the perpendicular complexes, charge transfer occurs from two N atoms. d) Charge-transfer from the N lone pair to a virtual non-bonding orbital on C7. e) The two tetrel-bonded complexes on this surface accidentally have the same binding energy.

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–∆E & CT, kJ.mol–1

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Number of N atoms in the ring Fig. 3. Binding energies and charge-transfer energies of the azine:CO2 complexes versus the number of nitrogen atoms in the ring

Given that the equilibrium complexes have planar and perpendicular structures, it is not surprising that the binding energies do not show a strong correlation with the N-C7 distance, as evident from Fig. 4. The exponential trendline for the planar complexes has a correlation coefficient of 0.820. The points for the perpendicular structures are usually found at longer NC7 distances compared to the planar structures, although there is some overlap. The longer distances in the perpendicular complexes arise because C7 is positioned somewhere in between the two nitrogen atoms which are involved in the formation of the tetrel bond. Nevertheless, that the binding energies of these complexes depend on the number of N atoms in the ring and the structure of the complex is evident from Fig. 4. In addition, the binding energies of planar complexes depend on the intermolecular N-C7 distance. That distance in the pyridine:CO2 complex is 2.724 Å, compared to an experimental value of 2.798±0.006 Å.20 It should be noted, however, that the computed distance is the distance at equilibrium (Re), while the experimental distance is the distance in the ground vibrational state (Ro), which should be longer than Re.

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–∆E , kJ.mol–1

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R(N-C7), Å Fig. 4. The binding energies of the azine:CO2 planar and perpendicular complexes versus the NC7 distance. The trendline refers to complexes with planar structures. It should also be noted that the ordering of complex binding energies does not always follow the ordering of MEP minima. For example, atoms N2 and N4 in 1,2,4-triazine have MEP minima values of –0.078 and –0.061 au, respectively, but the planar complex tri124(4) is 1.0 kJ.mol–1 more stable than tri124(2). tet1234(1-2) and tet1234(2-3) complexes have perpendicular structures and binding energies of 14.9 and 14.7 kJ.mol–1, respectively. However, N1 and N2 in 1,2,3,4-tetrazine have MEP values of –0.058 and –0.070 au, respectively, while N2 and N3 have MEP values of –0.070 au. MEP values do not take into account the polarizing effect of the CO2 molecule on the azine electron density, nor do they account for other secondary interactions which may arise in the complex.

Bonding Analyses. Charge-transfer interactions stabilize azine:CO2 complexes. Charge-transfer in planar pyr(1) is most unusual insofar as it occurs from the N atom to an unfilled nonbonding orbital on C7. The charge-transfer interaction involving these two orbitals is illustrated in Fig. S2 of the Supporting Information. In planar complexes with Cs symmetry, charge-transfer occurs from the N lone pair to the remote in-plane π*C7-O9 orbital. This interaction is also

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illustrated for di12(1) in Fig. S2. In perpendicular complexes, charge transfer occurs from the two N atoms to the adjacent π*O-C-O orbital of CO2. NBO charge transfer energies are also reported in Table 2. These range from 4.5 kJ.mol–1 in pent(2-3) to 14.6 kJ.mol–1 in di14(1), not including the large charge-transfer energy of 43.2 kJ.mol–1 for the Nlp→σC7 interaction in pyr(1). Fig. 3 provides a plot of the charge-transfer energies against the number of N atoms in the ring, which shows that charge transfer tends to decrease as the number of N atoms in the ring increases. The scatter seen for a given number of nitrogen atoms is a consequence of the existence of isomers, and of their different structures. The point for the charge-transfer energy of py(1) in Fig. 3 lies far from the trendline for the remaining complexes, a reflection of the nature of the electron-acceptor orbital of CO2. The charge-transfer data support the structural data which indicate that the azine:CO2 complexes are stabilized by tetrel bonds. Table S3 provides values of the electron densities at N-C7 bond critical points (ρBCP), the Laplacians (∇2ρBCP) at those points, and the total energy densities (HBCP) obtained from the NBO analyses. All of these electron density properties have very small values. Thus, ρBCP does not exceed 0.02 au, ∇2ρBCP is less than 0.06 au, and HBCP is always positive, but less than 0.002 au. These data indicate that all of the intermolecular N…C7 bonds are traditional tetrel bonds.

Spectroscopic Data. How do the IR frequencies of the CO2 molecule and the

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C7 chemical

shieldings change upon complexation, and what are the values of N-C7 coupling constants across tetrel bonds? What correlations exist between the IR and NMR data and other properties of these complexes? Do IR and NMR data distinguish between planar and perpendicular structures? The answers to these questions can be found in the data of Table 3 and the accompanying figures. The C-O symmetric and asymmetric stretching frequencies are coupled with frequencies of the azine rings, which does not allow for direct analysis.

However, the O-C-O bending

frequencies do provide some useful information. Upon complex formation, the O-C-O bending frequency decreases by 9 to 37 cm–1. What is most interesting is that the decrease is related to both the type of complex and to the number of N atoms in the ring. This can be seen from the plots of Fig. 5. For a given number of nitrogen atoms, the frequency decrease for planar structures is always greater than that for perpendicular structures. For the planar diazines, the 12

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decrease is between 28 and 30 cm-1 while for di(1-2) it is 17 cm–1. Planar triazine complexes exhibit a decrease between 22 and 24 cm-1, while perpendicular structures have a decrease of 14 and 16 cm–1. The O-C-O bending frequency decreases by 21 cm–1 in tet1234(1) and by 17 cm–1 in the remaining planar tetrazine structures, and by 10 to 12 cm–1 in the perpendicular structures. The frequency in planar pent(1) decreases by 13 cm–1, and by 9 cm–1 in the perpendicular structures. The best-fit trendline in Fig. 5 for the planar structures is an exponential curve with a correlation coefficient of 0.977. The best fit for the perpendicular structures is a second-order polynomial with a correlation coefficient of 0.923. The change in the IR stretching frequency upon complexation should make it possible to differentiate experimentally between azine:CO2 complexes that have planar and perpendicular structures.

Table 3. Decrease in the harmonic CO2 bending frequency (∆ν, cm–1) and the 13C chemical shielding (∆δ13C7, ppm) of CO2, and FC terms and total 1tJ(N-C7), (Hz) for azine:CO2 complexes azine:CO2 ∆νb FC(N-C7)c,d ∆δ13C7 of CO2e pyr(1) -37.2 0.8 (0.8) –0.93 di12(1-2) -16.9 0.1 (0.1) –0.72 (1) -29.6 0.6 –0.90 di13(1) -28.5 0.5 –0.91 di14(1) -30.4 0.6 (0.6) –0.84 tri123(1) -22.8 0.4 –0.91 (1-2) -15.7 0.1 –0.71 tri124(1) -23.7 0.5 –0.82 (1-2) -13.7 0.1 –0.62 (4) -22.3 0.4 –0.82 (2) -23.1 0.4 –0.83 tri135(1) -22.8 0.5 (0.4) –0.82 tet1234(1) -20.6 0.3 –0.81 (1-2) -11.2 0.1 –0.61 (2-3) -10.2 0.1 (0.1) –0.70 tet1235(1-2) -11.9 0.1 –0.61 (1) -17.1 0.3 –0.81 (5) -17.2 0.3 –0.75 tet1245(1) -17.2 0.3 –0.73 (1-2) -11.0 0.1 (0.1) –0.50 pent(1) -13.0 0.2 –0.72 (2-3) -9.4 0.1 –0.59 (1-2) -9.1 0.1 –0.50 13

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a) Data in bold refer to perpendicular complexes. b) Isolated CO2 has an O-C-O bending frequency of 659.0 cm–1. c) Data in parentheses are total 1tJ(N-C7) values. d) In perpendicular complexes, the FC term for the other N-C7 coupling constant is either 0.0 or 0.1 Hz. e) The δ13C7 chemical shielding of isolated CO2 is 69.57 ppm.

–δυ, cm–1

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Number of N atoms Fig. 5. Decrease in the O-C-O bending frequency versus the number of N atoms in the ring for azine:CO2 complexes Complexation produces a small decrease in the 13C7 chemical shielding of CO2 of –0.50 to –0.93 ppm. For a fixed number of nitrogen atoms, the decrease is greater in complexes that have planar structures. The chemical shieldings also tend to decrease as the number of nitrogen atoms in the azines increase, as illustrated in Fig. 6. The linear trendlines in Fig. 6 are based on the average values of the

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C7 chemical shielding for a fixed number of N atoms.

The

correlation coefficients for the planar and perpendicular structures are 0.986 and 0.998, respectively.

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∆δ13C7, ppm

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Number of N atoms Fig. 6. Decrease in the 13C7 chemical shielding versus the number of N atoms in the ring

Table 3 also presents the FC terms for N-C7 coupling across the tetrel bonds in azine:CO2 complexes, and values of the total coupling constant 1tJ(N-C7) for a few of these. Although the number of comparisons is limited, the FC terms are within 0.1 Hz of the total coupling constant 1t

J(N-C7), and will be used to approximate 1tJ(N-C7). These coupling constants are very small

with values between 0.2 and 0.8 Hz for the planar structures, and either 0.0 or 0.1 Hz for the perpendicular structures. Despite the small values for the planar structures, Fig. 7 illustrates that a second-order correlation exists between 1tJ(N-C7) and the N-C7 distance, with a correlation coefficient of 0.941. It is also important to note that these coupling constants tend to decrease as the number of N atoms in the rings decrease, consistent with the behavior of other properties of these complexes. Although these coupling constants are very small, it would seem that they could be used as an experimental tool to differentiate between azine:CO2 complexes with planar and perpendicular structures.

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R(N-C7), Å Fig. 7.

1t

J(N-C7) versus the N-C7 distance for planar azine:CO2 complexes

CONCLUSIONS

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to investigate tetrel-bonded complexes formed between CO2 and the azines pyridine, the diazines, triazines, tetrazines, and pentazine. The results of these calculations support the following statements. 1. From the eleven nitrogen bases, 23 unique equilibrium azine:CO2 complexes have been found on the potential surfaces. 14 of these have planar structures in which a single nitrogen atom is an electron-pair donor to the carbon atom of the CO2 molecule, and 9 have perpendicular structures in which two adjacent nitrogen atoms are the electron donors, with bond formation occurring somewhere along the N-N bond. The intermolecular bonds are formed as the azine donates a pair of electron to CO2 through its σ-hole, and are traditional tetrel bonds. 2. 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. Planar complexes with Cs symmetry are also stabilized by a secondary interaction between the adjacent oxygen with a C-H bond of the azine. 16

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The binding energies of the planar complexes tend to increase as the distance across the tetrel bond decreases. 3. Charge transfer stabilizes azine:CO2 complexes. In the planar C2v complex with pyridine, charge transfer occurs from the N lone pair to a virtual nonbonding orbital of the carbon atom of CO2. In the remaining planar complexes, charge-transfer occurs from an N lone pair to the remote in-plane π*C-O orbital. In perpendicular complexes, charge transfer is from an N-N bond to the adjacent π*O-C-O orbital of CO2. 4. The bending frequency of the CO2 molecule decreases upon complex formation. For a given azine, this decrease is larger in the planar complexes than in the perpendicular complexes. 5. The NMR δ13C7 chemical shieldings exhibit a small decrease upon complex formation. This decrease is greater in complexes with planar structures. 6. EOM-CCSD spin-spin coupling constants for complexes with planar structures are very small at less than 1.0 Hz, but still show a good correlation with the N-C7 distance. Coupling constants for perpendicular structures are even smaller, with values of 0.0 or 0.1 Hz.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:xxx, and includes: plot of MEP minima for azines versus the number of N atoms in the ring; structures, total energies, and molecular graphs of azine monomers and azine:CO2 complexes; depiction of orbitals involved in charge transfer interactions in the pyridine:CO2 complex and the planar complex of 1,2-diazine with CO2; electron densities, Laplacians, and total energy densities at intermolecular bond critical points.

AUTHOR INFORMATION Corresponding Authors * I.A. [email protected] (+34 915622900) ORCID: 0000-0001-6876-6211 *J.E. [email protected] (+34 915622900) ORCID: 0000-0002-9213-6858 *J.E.D.B. [email protected] (+1 330-609-5593) ORCID: 0000-0002-9037-2822

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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was carried out with financial support from the Ministerio de Economía y Competitividad (Project No. CTQ201235513-C0202) and Comunidad Autónoma de Madrid (S2013/MIT2841, Fotocarbon). Thanks are also given to the Ohio Supercomputer Center and CTI (CSIC) for their continued computational support.

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