Activation of Small Molecules (H2, CO2, N2O, CH4, and C6H6) by a

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Article Cite This: ACS Omega 2018, 3, 17199−17211

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Activation of Small Molecules (H2, CO2, N2O, CH4, and C6H6) by a Porphyrinoid-Based Dimagnesium(I) Complex, an Electride Ranajit Saha† and Pratim K. Chattaraj*,†,‡ †

Department of Chemistry and Center for Theoretical Studies, Indian Institute of Technology Kharagpur, Kharagpur 721302, India Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400076, India

‡

ACS Omega 2018.3:17199-17211. Downloaded from pubs.acs.org by 193.56.65.102 on 12/18/18. For personal use only.

S Supporting Information *

ABSTRACT: A density functional theory-based computation has been carried out to reveal the geometrical and electronic structures of Mg2EP (1), where EP is an extended (3.1.3.1) porphyrinoid system. EP is a 22 π electronic system and is aromatic in nature. Here, we have studied the thermodynamic and kinetic stabilities of EP2−-supported Mg22+ ion. The nature of bonding has been studied using natural bond orbital and atoms in molecule schemes. The presence of a covalent Mg(I)−Mg(I) σ-bond in Mg2EP is confirmed. The occurrence of a non-nuclear attractor (NNA) with large electron population, negative Laplacian of electron density at NNA, and presence of an electron localization function basin along with large nonlinear optical properties prompt us to classify Mg2EP as the first porphyrinoid-based organic electride. Further five small molecules, viz., dihydrogen (H2), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and benzene (C6H6), are found to be activated by the electron density between the two Mg atoms in Mg2EP.

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INTRODUCTION Magnesium (Mg) at its +1 oxidation state in a stable Mg−Mg bond has been detected in HMgMgH by spectroscopic technique at low-temperature gas matrices.1,2 However, a complete laboratory synthesis of stable Mg(I)−Mg(I) bonded complexes was reported by Green et al. in 2007.3 This work reported the syntheses of {Mg(DippPriso)}2 (DippPriso = [(DippN)2CNiPr2], Dipp = 2,6-diisopropylphenyl, iPr = isopropyl) and {Mg(DippNacnac)}2 (DippNacnac = [(Dipp)NC(Me)]2CH) complexes, where a Mg(I)−Mg(I) σ-bond is confirmed by computational studies. After this work, stabilizing Mg atoms at the +1 oxidation state became a state-of-the-art research arena for chemists. The common synthetic strategy involved is to design and synthesize bulky ligands that can stabilize Mg22+ entity in its cavity. A large number of complexes with stable Mg(I)−Mg(I) σ-bond have been synthesized by experimentalists, and computational studies have been carried out by theoreticians to predict such stable complexes.4−6 Porphyrins are one of the important classes of macrocycles, where four pyrrole units are connected alternately through methine (CH−) linkers.7 The 18 conjugated π-electrons ((4n + 2) π-electrons; n = 4) present in the porphyrins make it highly aromatic.8−11 The deprotonated free base porphyrins can form complexes with metal atoms; they are known as metalloporphyrins, which are fundamental units of a number of biomolecules without which life on this earth cannot survive.12,13 The core of the porphyrin has been extended and stretched using bridging atoms.14−17 These modified porphyrinoid cores have strong red-shifted UV−vis absorption © 2018 American Chemical Society

that has potential applications in photochemistry as photosensitizers.18,19 The stretched porphyrinoid cores can be used as ligands for the generation of unusual metalloderivatives.20 In 1990, [22]octaethylporphyrin(3.1.3.1) has been synthesized where the inner ring of the porphyrin unit has been extended by four carbon atoms.16,18 This extended porphyrin has 22 πelectrons (follows Hückel’s (4n + 2) rule, n = 5) and hence is aromatic in nature.17 Here, the extended (3.1.3.1) porphyrin (without any substitution, denoted EP), H2EP, has been considered as a ligand that can stabilize Mg22+ ion in Mg2EP as shown in Scheme 1a. The binding modes of the “Nacnac” ligand can be mimicked by the EP2−; see Scheme 1b. In the {Mg(Nacnac)}2 complex, the two ligands are not connected and oriented almost perpendicular to each other.3 However, the EP2− is planar and cyclic. So, it will be interesting to study the Mg2EP complex and explore its nature of bonding and other properties. Electrides are a class of ionic compounds that have trapped electrons as anions. These loosely bound electrons in electrides result in high nonlinear optical (NLO) properties21,22 and accordingly are useful as in reversible H2 storage material,23 in electron emitter,24 and in activation of small molecules.25,26 In 1983, Dye et al. synthesized the first crystalline organic electride, Cs+(18-crown-6)2e−,27 which influenced them to Received: October 30, 2018 Accepted: November 30, 2018 Published: December 13, 2018 17199

DOI: 10.1021/acsomega.8b03006 ACS Omega 2018, 3, 17199−17211

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Scheme 1. Schematic Representations of (a) the Stabilization Path for the Mg2EP Complex and (b) Structural Similarity between the Nacnac and EP2− Ligands

synthesize a series of such compounds.28 In 2005, Na+(tri-pipaza-2.2.2)e− was synthesized by Redko et al.29 The first synthesized crystalline inorganic electride is [Ca24Al28O64]4+· (4e−), where the unit cell vacancies hold the trapped electrons.30 The stability and hence the practical applications of electrides depend on their sensitivity toward air and temperature.31 Density functional theory (DFT)-based computations have been performed and thermodynamic parameters have been calculated to check the stability of the Mg2EP complex. Complete bonding analyses have been performed with the help of natural bond orbital (NBO) and atoms in molecules (AIM) schemes. The kinetic stability of the complex has also been studied using the ab initio molecular dynamics. The bowl inversion of the central “Mg2N4H2” unit has been observed. Furthermore, the electride property of Mg2EP has been elucidated. Small molecules like H2, CO2, N2O, CH4, and C6H6 are found to be activated followed by dissociation of H− H, C−O, N−O, aliphatic C−H, and aromatic C−H bonds, respectively, by the Mg2EP. Stable complexes are formed with bridging ligands between the two Mg atoms.

at 1 atm pressure. We have simulated the Mg2EP system over a total time length of 1 ps with 2 fs time interval. We have used the Gaussian 16 program package for the calculations.39 The natural population analysis40 is carried out to get natural charges on the atoms, and the Wiberg bond indices (WBIs)41 are computed to obtain the bond order between any atomic pair using the natural bond orbital (NBO)42 scheme at the M06-2X-D3/6-311+G(d,p) level of theory. We have also compared the NBO results of Mg2EP at the M06-2X-D3 level with that obtained at the MP243/Aug-cc-pVTZ44,45/Aug-ccpVDZ//M06-2X-D3/6-311+G(d,p) level of theory (denoted MP2/aV(D+T)Z), where Aug-cc-pVTZ is used for the Mg and N atoms and Aug-cc-pVDZ basis sets are used for the C and H atoms, respectively. The NBO analysis has been carried out using NBO 3.146 as implemented in Gaussian 16. The topological descriptors of the electron density47 have been computed at DFT and MP2 levels with the help of Multiwfn software.48 Basin analyses based on the electron density (ρ(r)) and the electron localization function (ELF)49,50 have been performed. The localization index (λ) and delocalization index (δ) based on the ELF basin have been computed.51−54 It is termed as “basin” following these references. Average linear polarizability (α̅ ), first hyperpolarizability (β), and second hyperpolarizability (γ∥) were calculated at the B3LYP/6-31+G(d)/6-31+G level, where the 6-31+G(d) basis set is used for Mg atoms and the 6-31+G basis set is used for all other atoms. We have used the following equations to calculate α̅ , β, and γ∥55,56

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COMPUTATIONAL DETAILS The geometry optimizations in this study have been performed at the M06-2X-D3/6-311+G(d,p)32−34 level of theory. Frequency calculations are also carried out at the same computational level to ensure that the optimized structures are at the minima on their respective potential energy surfaces. The M06-2X functional is well known for its wide and accurate predictive applications involving main group thermochemistry.35 The dynamical behavior of the Mg2EP has been studied using an atom-centered density matrix propagation (ADMP)36−38 technique. In this ADMP simulation, we have evolved the optimized Mg2EP at 298 and 500 K temperatures

α̅ = 17200

1 3

∑

αii

i=x ,y,z DOI: 10.1021/acsomega.8b03006 ACS Omega 2018, 3, 17199−17211

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Figure 1. Optimized geometry of Mg2EP at the minimum energy (1) and at the transition state (TS1−1) computed at M06-2X-D3/6-311+G(d,p) and B3LYP/6-311++(d,p) levels. The unit of the bond lengths is in Å. The values without and within parenthesis are computed at M06-2X-D3/6311+G(d,p) and B3LYP/6-311++(d,p) levels, respectively.

ij yz j z β = jjj ∑ βi2zzz jj zz ki=x ,y ,z {

1/2

γP =

1 15

∑

1 , where βi = 3

∑

thermochemical stability of 1 at 298 K temperature and 1 atm pressure. The Gibbs’ free energy change (ΔG1) for the formation of 1 is −83.9 kcal/mol, which indicates that the formation of 1 is a thermochemically favorable process (see Scheme 1a). Further, the dissociation of 1 into Mg22+ and EP2− fragments is thermochemically nonspontaneous as ΔG2 is 515.9 kcal/mol. A planar transition state (TS1−1) has been identified and optimized corresponding to the inversion of the two Mg atoms with respect to the N4-plane in 1. The point group symmetry of TS1−1 is C2h. The Mg−Mg distance in TS1−1 is 2.652 Å at M06-2X-D3 and 2.664 Å at B3LYP levels. The imaginary frequency is 148i (137i) cm−1 at the M06-2X-D3 (B3LYP) level. All four Mg−N bond lengths are equal to each other, and it is 2.026 (2.039) Å at the M06-2X-D3 (B3LYP) level. Geometry 1 goes through this TS1−1, and the Mg2N4H2 unit gets inverted, that is, the two Mg atoms (and two H atoms of the opposite side of the N4-plane) go to the other side of the N4-plane through the planar TS1−1 akin to that in NH3 inversion. The Gibbs’ free energy barrier (ΔG) for the inversion of the Mg2N4H2 unit is found to be 5.8 (5.2) kcal/ mol at the M06-2X-D3 (B3LYP) level. Figure 2a represents a complete “bell-shaped” IRC path that connects the two conformers of 1 and TS1−1. The ab initio molecular dynamics study has been performed to reveal the dynamical behavior of 1. The optimized Mg2EP has been simulated to evolve using the ADMP method over 2 ps of time at 298 and 500 K temperatures. The time evolution of the total energy at both the temperatures is shown in Figure 2b. The total energy at 500 K temperature is higher than that at 298 K, as expected. The oscillations in the energy curves have been observed at both the temperatures. This may be due to an increase in the nuclear kinetic energy changes arising out of the structural deformations of 1. The changes in the Mg− Mg (Δr(Mg−Mg), Å) and Mg−N (Δr(Mg−N), Å) bond lengths with respect to their optimized values in 1 (i.e., geometry at t = 0) against time have been plotted. To keep

(βijj + βjij + βjji)

j=x ,y,z

(γiijj + γijij + γijji)

i,j=x ,y,z

The activation of small molecules followed by dissociation using the Mg2EP has been studied at the M06-2X-D3/6311+G(d,p) level at the gas phase. The intermediates and the transition states (TS) are optimized along with the frequency calculations. The complete intrinsic reaction coordinate (IRC) calculations taking each TS have been carried out connecting the respective minima. The Gibbs’ free energy changes have been calculated, and the complete reaction profiles have been generated. The NBO analyses at each minimum and TS have been performed to get the natural charge and WBI values.

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RESULTS AND DISCUSSION Structure and Energetics. The minimum energy geometry of Mg2EP (1) at the M06-2X-D3/6-311+G(d,p) level has been presented in Figure 1. The optimized geometry of 1 is nonplanar and found to be in the C2v point group of symmetry. The Mg−Mg bond length in 1 is 2.787 Å. Pyykkö et al.57 have predicted the Mg−Mg single bond length to be 2.780 Å, which is in good agreement with our computed value. The four Mg−N bond lengths are equal in 1, and it is 2.062 Å. We have also optimized the geometry of 1 at the B3LYP/6311++G(d,p) level. The rMg−Mg and rMg−N are found to be 2.792 and 2.075 Å, respectively. Thus, we can say that the result is not an artifact of using a particular computational level. The four N atoms of the EP2− ligand in 1 form a rectangle (denoted N4-plane), and the two Mg atoms are above that plane and in the same side of this N4-plane. The distance between the center of two Mg atoms and the center of the N4-plane is 0.661 Å at M06-2X-D3 and 0.672 Å at B3LYP levels. Scheme 1 has been considered to find out the 17201


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Figure 2. (a) IRC plot corresponding to the inversion of 1 through TS1−1. The plot of (b) total energy against time, (c) change in the rMg−Mg and rMg−N bond lengths with respect to the optimized 1 (t = 0) against time, and (d) dMg and dH against time at 298 and 500 K.

Nature of Bonding in 1. Molecular Orbital (MO) Diagram. Molecular orbital analysis has been carried out on 1 at the M06-2X-D3/6-311+G(d,p) level. The shapes and energies of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have been presented in Figure 4. The shape of the LUMO represents a πorbital, distributed over the EP2− ligand. The HOMO − 1 represents the Mg−Mg σ bond. The 3s and 3p atomic orbitals on each Mg atom are the major contributors, and they contribute 26.0 and 15.4%, respectively, toward the formation of the Mg−Mg σ bond. The contribution from other atoms toward HOMO is less than 5%. The energy difference between the HOMO and the LUMO is 3.18 eV. Natural Bond Orbital (NBO) Analysis. The natural charge on the Mg and N atoms and the Wiberg bond index (WBI) values for the Mg−Mg and Mg−N bonds in 1 at the M06-2XD3/6-311+G(d,p) level are presented in Table 1. The natural charges on each of the Mg and N atoms are 1.01 and −0.80 |e|, respectively. The WBI value for the Mg−Mg bond is 0.647, which suggests the presence of a single bond between the two Mg atoms. The WBI value for the Mg−N bonds is 0.081 in Mg2EP, suggesting that the interaction between the Mg and N atoms is noncovalent in nature. The NBO results at the MP2 level also support the same

track of the dynamics, two Mg atoms and four N atoms have been marked as shown in Figure S1a. The plots in Figures 2c and S1b show that Mg−Mg and four Mg−N bond lengths fluctuate around their optimized values all throughout the simulations at both the temperatures, but they do not disintegrate. These two observations reveal the kinetic stability of the Mg2EP. The inversion of the central Mg2N4H2 unit at 500 K may be clearly understood from the plots in Figure 2d. Here, we have calculated the dMg (dMg = distances between the center of the two Mg atoms and the center of the four N atoms, with proper sign) and dH (dH = distances between the center of the two H atoms and the center of the four N atoms, with proper sign) at both the temperatures throughout the overall simulation and plotted it against time. The plot clearly shows an inversion of the dMg and dH, which indicates the inversion of the two Mg and two H atoms at 500 K, but it is not the case at 298 K. The nuclear kinetic energy is not enough at 298 K to flip the Mg2N4H2 unit. The snapshots of the geometries at different time points at both of the temperatures have been presented in Figure 3. Movie files of the ADMP results at 298 and 500 K have also been presented in the Supporting Information, which clearly portray the inversion dynamics and the role played by the nuclear kinetic energy therein. 17202


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Figure 3. Snapshots at different time steps of 1 at (a) 298 K and (b) 500 K.

Figure 4. Relevant molecular orbitals of 1 generated at the M06-2X-D3/6-311+G(d,p) level of theory. The values in the parentheses are energies of the corresponding orbitals in eV.

Table 1. Natural Charges (q, |e|) on Mg and N Atoms; Wiberg Bond Indices (WBIs) for the Mg−Mg and Mg−N Bonds in 1a complex

q (Mg)

Mg2EP

1.01 (1.01)

q (N) −0.80 (−0.88)

WBI (Mg− Mg)

WBI (Mg−N)

0.647 (0.709)

0.081 (0.067)

of 1 have been computed at both MP2 and DFT levels and are presented in Table 2. The molecular graph of the Mg2EP is shown in Figure 5a. The molecular graph at the MP2 and DFT methods are the same. A non-nuclear attractor (NNA; (3, −3)type critical point) has been located close to the middle of the two Mg atoms. The presence of NNA in dimeric Mg(I) complexes has been confirmed experimentally.58 The electron density (ρ(rc)) at this NNA is 0.034 au at the MP2 level. The Laplacian of electron density (∇2ρ(rc)) at the NNA is −0.043 au. The negative ∇2ρ(rc) indicates the electron accumulation between the two Mg atoms. The population of the NNA basin is 1.03 electrons with 46% of localization. Two NNA−Mg bond paths are found, connecting the NNA and the two Mg atoms. The ρ(rc) values at the NNA−Mg bond critical points (BCPs) are 0.032 au and the ∇2ρ(rc) values are 0.009 au. The energy density (H(rc)) values are negative. The DFT-level calculations have predicted almost similar results. Figure 5b,c shows a distinct region between the two Mg atoms where electron density is accumulated. An ELF basin between the two Mg atoms, C(NNA), is identified (see Figure 5d,e) with a population of 1.78 electron in it with 61% localization at MP2/

a

The values without and within parenthesis represent the results obtained at M06-2X-D3/6-311+G(d,p) and MP2/av(D+T)z levels, respectively.

arguments. The Mg−Mg bond orbital has been generated at the MP2 level and is shown in Figure S2. The occupancy of Mg−Mg bond orbital is 1.79 |e|. The atomic orbital contributions from the two Mg atoms are equal, indicating that the Mg−Mg bond is nonpolar in nature. The contributions from the 3s and 3p atomic orbitals are 92.1 and 5.9%, respectively, toward the Mg−Mg bond orbital, whereas the 3d orbital contributes only 1.6%. Electron Density Analysis. Different topological parameters based on the electron density at relevant critical points (CPs) 17203


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Table 2. Electron Density (ρ(rc)), Laplacian of Electron Density (∇2ρ(rc)), Kinetic Energy Density (G(rc)), Potential Energy Density (V(rc)), Total Energy Density (H(rc)), Basin Population (N(pop)), Localization Index (LI), Percentage of Localization Index (% LI), and Delocalization Index (δ) at Different Critical Points (CP) of Mg2EPa type

ρ(rc)

∇2ρ(rc)

G(rc)

V(rc)

H(rc)

N(pop)

LI

% LI

NNA

(3, −3)

(3, −1)

Mg−N

(3, −1)

−0.012 {−0.011} [−0.011] −0.020 {−0.016} [−0.016] −0.006 {−0.005} [−0.005] −0.068 {−0.060} [−0.056]

−0.011 {−0.010} [−0.010] −0.009 {−0.009} [−0.009] −0.001 {−0.001} [−0.001] 0.004 {0.008} [0.008]

46 {30} [31]

NNA−H

0.001 {0.001} [0.001] 0.011 {0.007} [0.007] 0.005 {0.004} [0.004] 0.072 {0.067} [0.064]

0.47 {0.20} [0.22]

(3, −1)

−0.043 {−0.034} [−0.036] 0.009 {−0.009} [−0.010] 0.013 {0.013} [0.012] 0.305 {0.300} [0.285]

1.03 {0.68} [0.71]

NNA−Mg

0.034 {0.030} [0.031] 0.032 {0.030} [0.031] 0.015 {0.013} [0.014] 0.047 {0.045} [0.044]

CP

δ

0.35 {0.31} [0.32] 0.07 {0.04} [0.05] 0.16 {0.20} [0.20]

a

The values without parentheses, within second brackets, and within square brackets indicate the values calculated at MP2/aV(D+T)Z, B3LYP/6311++G(d,p), and M06-2X-D3/6-311+G(d,p) levels, respectively.

Figure 5. Topological analysis of 1. (a) Molecular graph. (b) Three-dimensional (3D) surface plot of ∇2ρ(r); isosurface value = −0.001; (c) twodimensional (2D) plot of ∇2ρ(rc); the blue and green lines indicate ∇2ρ(r) > 0 and ∇2ρ(rc) < 0 regions, respectively. (d) Three-dimensional (3D) surface plot of ELF; isosurface value = 0.800. (e) Two-dimensional (2D) plot of ELF.

Low δ value at the NNA−H BCP further supports this argument. Nonlinear Optical (NLO) Property of 1. Average linear polarizability (α̅ ), first hyperpolarizability (β), and second hyperpolarizability (γ∥) have been calculated for 1 and are presented in Table S1. The α̅ , β, and γ∥ values are 493.7, 3.62 × 102, and 1.1 × 105 au, respectively. Further, we have

aV(D+T)Z level of theory. On the basis of these results, we can say that there is a considerable amount of degree of covalency in the bond between the two Mg atoms. The positive ∇2ρ(rc) and H(rc) and low δ values indicate that the Mg−N interactions are noncovalent in nature. Two NNA−H bond paths have been found. The low ρ(rc) and positive ∇2ρ(rc) and H(rc) values suggest that the interaction is weak. 17204


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compared the α̅ , β, and γ∥ values of 1 with the corresponding previously reported values for Li@Calix, Na@Calix, Li@ B10H14, Li2(TCNQ), and Na2(TCNQ).56 The results show that Mg2EP has the largest α̅ value among all of them. However, the β and γ∥ values of 1 are smaller as compared to those in the other systems, albeit sufficiently high. Thus, 1 can be treated as an NLO material. Electride Property of 1. Classification of a molecule as an electride is based on several properties, viz., large nonlinear optical properties,50,56,59,60 presence of a non-nuclear attractor (NNA)50 of the electron density, electron localization function (ELF) basins,50,61−64 and noncovalent index (NCI),62,64,65 which have to be computed by reliable and sophisticated methods. The AIM results have confirmed the presence of an NNA with a negative ∇2ρ(r) value, with a large electron population in it (see Table 2). The ELF analysis showed that the C(NNA) has 1.78 electron population with 61% localization in it. The high polarizability and first and second hyper polarizabilities also suggest the NLO behavior of 1. The green region in the NCI plot (see Figure S3) shows the electron localization between the two Mg atoms in 1. All these results strongly establish that 1 can be classified as an organic electride, and it is the first electride having a porphyrinoid moiety. Activation of Small Molecules. Activation and dissociation of small gas molecules, especially greenhouse gases like carbon dioxide (CO2) and nitrous oxide (N2O), have been considered to be the need of the hour. This provides a twopronged benefit, viz., it can remove the hazardous global warming gases and it provides chemical precursors to various chemical reactions. Activation of the C−H bonds in organic molecules is one of the most challenging jobs for chemists. Finding cheap, selective, and efficient catalysts that can activate and eventually dissociate both aliphatic and aromatic C−H bonds has been considered to be a great challenge. Electrides are found to activate small molecules like CO2 and N2, and they have been used for reversible storage of molecular dihydrogen (H2).23,66 Here, we have studied the activation followed by dissociation of five small molecules, viz., H−H bond in H2, C−O bond in CO2, N−O bond in N2O, and C−H bond in methane (CH4) and benzene (C6H6) molecules using Mg2EP, as shown in Scheme 2. Mechanistic Investigation of H2 Reactivity. Direct hydrogenation of dimeric Mg(I) complexes has not been achieved

experimentally till now.67 The kinetic stability of Mg(I) dimer is suggested as the possible reason for the inertness of these complexes.68 Computations at the B3LYP/6-31G(d) level have shown that the hydrogenation of guanidinate-supported Mg22+ complexes producing the corresponding bridged hydrido (μ-H) complexes are exothermic in nature (ΔH ≈ −24 kcal/mol). 69 An indirect hydrogenation process for the Mg(I) dimers, using aluminum(III) hydride has been achieved.70 Complex 1 has been subjected to hydrogenation, which leads to the product Mg2EP(μ-H)2 (denoted 4, in Figure 6). The hydrogenation process is exothermic by ΔH =

Figure 6. Relative free energy (in kcal/mol) profile for H2 activation by 1 calculated at the M06-2X-D3/6-311+G(d,p) level of theory. The sum of the Gibbs’ free energies of 1 and H2 (−1544.39913162 au) is considered as 0.0 kcal/mol.

−33.4 kcal/mol and exergonic by ΔG = −25.1 kcal/mol. Thus, the hydrogenation process is thermochemically spontaneous. The hydrogenation of 1 is found to be a stepwise process, and the complete reaction path is shown in Figure 6. At the early step of the reaction, 1 forms a prereaction complex, 2, with one H2 molecule. Then, it generates a transition state, TS2−3, where the Gibbs’ free energy barrier is 31.8 kcal/mol with respect to 2. The H−H bond length in the free H2 molecule is 0.740 Å (see Figure S4), which is increased by 0.006 Å in 2, which further increased to 1.124 Å in TS2−3 (see Table S2). Then, the structure TS2−3 moves to the intermediate 3, where the H−H bond is completely dissociated (rH−H = 2.940 Å). One H atom is bonded to two Mg atoms, forming a hydrido bridge, whereas another is only bound to one Mg atom in 3. Structure 3 is stabilized by −1.4 kcal/mol of Gibbs’ free energy as compared to that of 1 and one H2 molecule. But, it then reorganizes itself toward a more stable geometry, 4. The bridged H atom moves inward to the N4plane. Another TS3−4 is formed, which has 2.9 kcal/mol more free energy than 3. The structural parameters of 3 and TS3−4 are almost similar, except the bent of the μ-H. The bent of the μ-H in 3 is outward, which becomes inward in TS3−4. At last, it forms the final product 4 with a total gain in free energy of −25.1 kcal/mol. The HOMO of TS2−3 corresponds to the interaction between the Mg−Mg σ-bond orbital of 1 and the σ*-orbital of the H2 molecule (see Figure S5). The activation of the H2 molecule is indicated by the elongated H−H bond length in TS2−3. The charges on the H atoms of H2 molecules are −0.36 and −0.48 |e| (see Table S2), making a total of −0.84 |e| electronic charge on the H2 molecule in TS2−3. This indicates a transfer of electron density from 1 to the H2 molecule. The WBIH−H values are 1.000 in free H2, which is

Scheme 2. Schematic Representation of the Reactions of H2, CO2, N2O, CH4, and C6H6 Molecules with 1

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Although 6 is a stable complex, it dissociates into 9 and CO via the intermediates 7 and 8 and transition states TS6−7 and TS7−8. Transition state TS6−7 leads to the formation of 7 from 6. Transition state TS7−8 has been optimized, which has 6.1 kcal/mol more free energy than that of 7. Finally, TS7−8 leads to 8, which is an adduct of complex 9 and CO. This 9 is an oxo (μ-O) bridged complex, which has one Mg−O−Mg moiety. Further, compound 9 grabs another CO2 molecule and forms adduct 10. Then, 10 forms TS10−11, which finally leads to carbonate complex 11. Mechanistic Investigation of N 2O Reactivity. N2O comprises only 6% of the total greenhouse gases and has 296 times of the global warming potential (GWP) as compared to that of CO2 (81% of the total greenhouse gases).75 N2O has 121 years of atmospheric lifetime, which is also much higher than that of CO2, viz., 30−95 years.76,77 N2O is thermodynamically stable and is often used as an oxygen transfer agent in organometallic chemistry.78−80 Here in this study, we have used N2O to react with 1, which ends up in forming an oxobridged complex 9. The complete reaction energy profile along with the TSs and intermediates are depicted in Figure 8. First,

decreased to 0.294 in TS2−3; this indicates the weakening of the H−H bond by the population of the σ*-orbital of the H2 molecule. In 3, the H2 molecule is completely dissociated, and WBIH−H is 0.008 only. Structure 4 has two hydrides bridged between the two Mg atoms. The Mg−H bond lengths are 1.855 and 1.850 Å. The Mg−Mg distance in 4 is 2.665 Å, which is smaller than that in 1. Mechanistic Investigation of CO2 Reactivity. Previous studies have shown that LMgMgL type of complexes can effectively activate and dissociate CO2, forming an oxo-bridged (μ-O) complex, which finally gives a carbonate complex.71−73 System 1 effectively activate and split CO2 and form an oxobridged (μ-O) complex (9) and CO. Then, 9 further bind to another CO2 molecule to finally produce a carbonate complex 11. The complete reaction path is presented in Figure 7. One

Figure 7. Relative free energy (in kcal/mol) profile for CO2 activation by 1 calculated at the M06-2X-D3/6-311+G(d,p) level of theory. The sum of the Gibbs’ free energies of 1 and CO2 (−1731.80561006 au) is considered as 0.0 kcal/mol.

CO2 molecule first binds to 1 and forms a complex 5; the ΔG for this adduct formation is only 0.1 kcal/mol. Then, TS5−6 is formed with a gain in the free energy value of 5.8 kcal/mol. A similar type of TS having almost an equal energy barrier has been identified earlier, where the Mg atoms were stabilized using tripodal diimine-enolate ligands.72 The C−O bonds are elongated to have bond lengths of 1.189 and 1.157 Å, and the ∠OCO became bent to 161.7°. The TS5−6 transforms to a symmetric intermediate 6, which is stabilized by −30.6 kcal/mol. The activation of CO2 requires a transformation from a linear to a bent geometry.74 The ∠OCO is 125.1°, and the rC−O is 1.269 Å. The bent ∠OCO indicates the presence of activated CO2 in TS5−6 and 6. The HOMO of TS5−6 corresponds to the interaction between the Mg−Mg σ-bond orbital (HOMO) of the Mg2EP and the LUMO of the CO2 molecule (see Figure S5). The total charge on the CO2 fragment in TS5−6 is −0.67 |e|. Thus, the flow of electron density from 1 to the CO2 molecule is confirmed. The total charge in the CO2 fragment is −1.56 |e| in 6. The main contribution to HOMO − 2 of 6 came from HOMO of CO22− (see Figure S6). The WBIMg−Mg value is decreased to 0.517 in TS5−6 and to 0.016 in 6 as compared to that in 1. Thus, no Mg−Mg bond is present in 6. The low WBIC−O values suggest the weakening of the C−O bonds in TS5−6 and 6 (see Table S2).

Figure 8. Relative free energy (in kcal/mol) profile for N2O activation by 1 calculated at the M06-2X-D3/6-311+G(d,p) level of theory. The sum of the Gibbs’ free energies of 1 and N2O (−1727.87256059 au) is considered as 0.0 kcal/mol.

N2O binds to 1 and forms a prereaction complex 12. The ΔG for the formation of 12 is 0.3 kcal/mol. A transition state TS12−13 has been found to have 10.2 kcal/mol higher energy as compared to 12. The linear N2O becomes bent in TS12−13. The ∠O−N−N is 161.7°, and the N−O and N−N bond lengths in TS12−13 are increased by 0.076 and 0.009 Å, respectively, as compared to that in free N2O. The inspection of the HOMO of TS12−13 reveals the interaction between the HOMO of 1 with the LUMO of N2O (see Figure S5). The total charge of N2O is −0.26 |e|, which suggests that the direction of the flow of the electron density is 1 to N2O. The TS12−13 then transforms into intermediate 13, which is stabilized by −35.4 kcal/mol of free energy. The N−O and N−N bond lengths are 1.370 and 1.219 Å, respectively, and the ∠O−N−N is 121.0° in 13. The total charge of N2O is −1.64 |e| in 13. The HOMO − 2 of 13 looks like the HOMO of N2O2−, indicating the presence of N2O2− fragment (see Figure S6). At last, 13 gains 14.8 kcal/mol energy and forms TS13−14, which leads to the removal of one N2 molecule in the 17206


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free energy is −18.3 kcal/mol, indicating the spontaneity of the complete process, and it is the driving force for the reaction. Mechanistic Investigation of Benzene Reactivity. Activation of an aromatic C−H bond is one of the tough challenges for organic chemists. We have considered the C−H bond of benzene to get activated using 1. The complete reaction profile for the C−H bond activation is shown in Figure 10. Benzene

reaction. The N−O bond length in TS13−14 is elongated as compared to that in 13, whereas the N−N bond length is decreased in TS13−14. The van der Waals complex 14 has been identified; here, the oxo-bridged 9 and N2 molecule are bound very loosely. The total stabilization energy of 14 is −122.1 kcal/mol. At last, 14 removes the N2 molecule, and complex 9 is found. Mechanistic Investigation of CH4 Reactivity. Methane, the smallest member of the alkane family, has 84 times more GWP than that of CO2. The complete reaction path for CH4 activation is shown in Figure 9, which looks like the H2

Figure 10. Relative free energy (in kcal/mol) profile for C6H6 activation by 1 calculated at the M06-2X-D3/6-311+G(d,p) level of theory. The sum of the Gibbs’ free energies of 1 and C6H6 (−1775.42920640 au) is considered as 0.0 kcal/mol. Figure 9. Relative free energy (in kcal/mol) profile for CH4 activation by 1 calculated at the M06-2X-D3/6-311+G(d,p) level of theory. The sum of the Gibbs’ free energies of 1 and CH4 (−1583.72749242 au) is considered as 0.0 kcal/mol.

forms a prereaction complex, 18. The ΔG for the formation of 18 is −3.7 kcal/mol. Benzene is bound to 1 with weak van der Walls interaction between them. This complex 18 then undergoes a structural change and forms TS18−19 with a gain of 39.9 kcal/mol free energy as compared to 18. The HOMO of TS18−19 corresponds to the interaction between the HOMO of 1 and the LUMO of C6H6. The LUMO of C6H6 receives electron density from HOMO of 1 (see Figure S5). The charges on the H atom and C atom near the Mg atoms are −0.27 and −0.66 |e|, respectively, in TS18−19 (see Table S2). There is no considerable change in the charge distribution of the rest of the benzene molecule (q(rest-C5H5) = −0.04 |e|) as compared to that in the free C6H6. The total charge of C6H6 is −0.97 |e|, which suggests the flow of electron density from 1 to C6H6. The TS18−19 is transformed to 19, which is 36 kcal/mol lower in free energy as compared to TS18−19. The geometry of 19 is almost similar to that of 16. One H atom forms a bridge between the two Mg atoms, and the C6H5 unit is bound to only one Mg atom. After this point, the μ-H starts to move inward, which gives TS19−20, with a gain in 2.5 kcal/mol in free energy as compared to 19. Finally, the μ-H went to the opposite side of the N4-plane, and the final complex Mg2EP(μC6H5)(μ-H), 20, is formed; 20 has one bridging H atom and one bridging C6H5 group. The total free energy change for the whole process is −22.1 kcal/mol. Smallest Mg−Mg Distance. Another important characteristic of 4, 17, and 20 is that these geometries correspond to the smallest Mg···Mg distance without having any bond between them. We have considered the Mg···Mg distances in some known bridged [Mg2] complexes, viz., Mg2H4 (21), Mg2F4 (22), {(MesNacnac)Mg(μ-H)}2 (23),70 {(DippNacnac)Mg(μ-H)} 2 (24), 8 2 ( D i p p Nacnac)Mg(μ-H)(μ-I)Mg-

activation reaction profile. The free energy change for the formation of the prereaction complex, 15, is 2.4 kcal/mol. A transition state TS15−16 has been formed with a gain of 41.1 kcal/mol energy as compared to 15. Here, we would like to mention that recently high energy barriers for C−H bond activation in CH4 using transition-metal complexes have been reported.81 The C−H bond length in free CH4 is 1.089 Å, and the WBIC−H is 0.958. The C−H bond in TS15−16 is elongated to 1.593 Å with a decrease in the WBIC−H value to 0.245 (see Table S2). Thus, the C−H bond is activated here. The HOMO of the TS15−16 corresponds to the interaction between the HOMO of 1 and LUMO of CH4 (see Figure S5). The charge distribution shows that only the C atom and H atom corresponding to the elongated C−H bond differs as compared to that in the free CH4. The H atom acquires negative charge on it, and the charge on the C atom becomes more negative as compared to the free CH4. The total charge on CH4 is −0.94 | e|, indicating a charge donation from 1 to CH4. The TS15−16 transforms to the intermediate 16. Here, the C−H bond under consideration is dissociated. The H atom formed a bridge between the two Mg atoms, and the CH3 group is connected to one Mg atom. Then, 16 reorganizes to its final geometry Mg2EP(μ-CH3)(μ-H), 17, through the transition state TS16−17. The geometrical and electronic parameters of the intermediate 16 and TS16−17 are almost the same except for the orientation of the bridged H atom. The outward orientation of μ-H becomes inward in TS16−17. At last, complex 17 has been formed with one μ-H and one μ-CH3 bond. The total gain in 17207


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Figure 11. Plot of ∇2ρ(r) for 4, 17, and 20 calculated at the M06-2X-D3/6-311+G(d,p) level of theory. The blue and green lines indicate ∇2ρ(rc) > 0 and ∇2ρ(rc) < 0 regions, respectively.

(MesNacnac) (25),83 (MesNacnac)2Mg2(μ-H)(μ-OSiMe2H) (26),70 and Na2Mg2(NiPr2)4(μ-H)2(toluene) (27)84 (see Figure S7), and compared the Mg···Mg distances with that of 4, 17, and 20 in Table S3. The comparison reveals that 17 has the smallest Mg···Mg distance and that in 4 and 20 are comparable to 17. The plots of ∇2ρ(r) of 4, 17, and 20 along the Mg···Mg are presented in Figure 11. There is no bond path and no bond critical point along the Mg···Mg axes in these complexes. The smallest distance is not achieved by any form of chemical bond between the two Mg atoms. The force applied by the captive porphyrinoid ring is a possible reason behind this small Mg···Mg distance in 4, 17, and 20.

the activation of H2, CH4, and C6H6 by Mg2EP are fairly high to be used for practical purposes. The current work provides us with a recipe toward a transition-metal-free catalytic process. Future modification of the ligand as well as the use of different metals might lower the activation barrier further and might be used as a better catalyst.

SUMMARY AND CONCLUSIONS The extended 3.1.3.1 [22]porphyrinoid system, H2EP, can stabilize the Mg22+ ion. Gas-phase calculations within the DFT framework have shown that this bimetallic Mg2EP complex is thermochemically and kinetically stable. The complex possesses one Mg−Mg σ-bond, which is formed mainly by the 3s and 3p orbitals on the Mg atoms. The AIM analysis revealed the presence of an NNA near the midpoint of the two Mg atoms. The electron population at the NNA is high, and the ∇2ρ(r) < 0 at NNA. An ELF basin has been identified with 61% localization. The computed average linear polarizability (α̅ ), first hyperpolarizability (β), and second hyperpolarizability (γ∥) for Mg2EP are 493.7, 3.62 × 102, and 1.1 × 105 au, respectively. Thus, Mg2EP can be used as a nonlinear optical material. The properties determined by the AIM and NLO analyses suggest that the Mg2EP can be considered as an organic electride material. Furthermore, five small molecules, viz., H2, CO2, N2O, CH4, and C6H6, are shown to get activated using Mg2EP. The H−H bond gets cleaved and finally forms a complex with two hydrido bridges. The inert aliphatic (CH4) and aromatic (C6H6) C−H bond activations can be achieved by the Mg2EP complex. Further, the activated C−H bond can be broken, and it may form one μ-H and one μ-CH3 or μ-C6H5 bonded complex. Thus, Mg2EP has the potential to be used as a catalyst for C−H bond activation. The global warming gases like CO2 and N2O are effectively split to CO and N2, respectively. Thus, Mg2EP is an excellent choice for the activation of these pollutant gases. The free energy barriers for

Representation of atomic numbers in 1 as used in the ADMP study, plots of the fluctuations of the Mg-N bonds in 1 against time, Mg−Mg natural bond orbital, plot of NCI isosurface, geometrical parameters for the small molecules, frontier molecular orbitals of the TSs, the NLO properties, structural parameters of the intermediates and TSs and the coordinates of the geometries (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03006.

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ADMP simulations (AVI) (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pratim K. Chattaraj: 0000-0002-5650-7666 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.K.C. would like to thank DST, New Delhi, for the J. C. Bose National Fellowship, grant number SR/S2/JCB-09/2009. R.S. thanks UGC, New Delhi, for his Senior Research Fellowship. The departmental computational facility “Kangsavati”, at the Department of Chemistry, IIT Kharagpur, is also acknowledged. 17208


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for radical ion pair salts M2̇+TCNQ̇ − (M = Li, Na, K). Phys. Chem. Chem. Phys. 2009, 11, 402−408. (23) Kitano, M.; Inoue, Y.; Yamazaki, Y.; Hayashi, F.; Kanbara, S.; Matsuishi, S.; Yokoyama, T.; Kim, S.-W.; Hara, M.; Hosono, H. Ammonia synthesis using a stable electride as an electron donor and reversible hydrogen store. Nat. Chem. 2012, 4, 934−940. (24) Toda, Y.; Yanagi, H.; Ikenaga, E.; Kim, J. J.; Kobata, M.; Ueda, S.; Kamiya, T.; Hirano, M.; Kobayashi, K.; Hosono, H. Work Function of a Room-Temperature, Stable Electride [Ca24Al28O64]4+(e−)4. Adv. Mater. 2007, 19, 3564−3569. (25) Buchammagari, H.; Toda, Y.; Hirano, M.; Hosono, H.; Takeuchi, D.; Osakada, K. Room Temperature-Stable Electride as a Synthetic Organic Reagent: Application to Pinacol Coupling Reaction in Aqueous Media. Org. Lett. 2007, 9, 4287−4289. (26) Yanagi, H.; Kuroda, T.; Kim, K.-B.; Toda, Y.; Kamiya, T.; Hosono, H. Electron injection barriers between air-stable electride with low work function, C12A7:e−, and pentacene, C60 and copper phthalocyanine. J. Mater. Chem. 2012, 22, 4278−4281. (27) Ellaboudy, A.; Dye, J. L.; Smith, P. B. Cesium 18-crown-6 compounds. A crystalline ceside and a crystalline electride. J. Am. Chem. Soc. 1983, 105, 6490−6491. (28) Dye, J. L. Electrides: Early Examples of Quantum Confinement. Acc. Chem. Res. 2009, 42, 1564−1572. (29) Redko, M. Y.; Jackson, J. E.; Huang, R. H.; Dye, J. L. Design and Synthesis of a Thermally Stable Organic Electride. J. Am. Chem. Soc. 2005, 127, 12416−12422. (30) Matsuishi, S.; Toda, Y.; Miyakawa, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Tanaka, I.; Hosono, H. High-density electron anions in a nanoporous single crystal: [Ca24Al28O64]4+(4e−). Science 2003, 301, 626−629. (31) Sun, W.-M.; Li, X.-H.; Li, Y.; Ni, B.-L.; Chen, J.-H.; Li, C.-Y.; Wu, D.; Li, Z.-R. Theoretical Study of the Substituent Effects on the Nonlinear Optical Properties of a Room-Temperature-Stable Organic Electride. ChemPhysChem 2016, 17, 3907−3915. (32) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Selfconsistent molecular orbital methods. XX. A basis set for correlated wave functions. J. Chem. Phys. 1980, 72, 650−654. (33) McLean, A. D.; Chandler, G. S. Contracted Gaussian basis sets for molecular calculations. I. Second row atoms, Z = 11−18. J. Chem. Phys. 1980, 72, 5639−5648. (34) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. V. R. Efficient diffuse function-augmented basis sets for anion calculations. III.* The 3-21+G basis set for first-row elements, Li− F. J. Comput. Chem. 1983, 4, 294−301. (35) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215−241. (36) Schlegel, H. B.; Millam, J. M.; Iyengar, S. S.; Voth, G. A.; Daniels, A. D.; Scuseria, G. E.; Frisch, M. J. Ab initio molecular dynamics: Propagating the density matrix with Gaussian orbitals. J. Chem. Phys. 2001, 114, 9758−9763. (37) Iyengar, S. S.; Schlegel, H. B.; Millam, J. M.; Voth, G. A.; Scuseria, G. E.; Frisch, M. J. Ab initio molecular dynamics: Propagating the density matrix with Gaussian orbitals. II. Generalizations based on mass-weighting, idempotency, energy conservation and choice of initial conditions. J. Chem. Phys. 2001, 115, 10291− 10302. (38) Schlegel, H. B.; Iyengar, S. S.; Li, X.; Millam, J. M.; Voth, G. A.; Scuseria, G. E.; Frisch, M. J. Ab initio molecular dynamics: Propagating the density matrix with Gaussian orbitals. III. Comparison with Born-Oppenheimer dynamics. J. Chem. Phys. 2002, 117, 8694−8704. (39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.;

REFERENCES

(1) Wang, X.; Andrews, L. Infrared Spectra of Magnesium Hydride Molecules, Complexes, and Solid Magnesium Dihydride. J. Phys. Chem. A 2004, 108, 11511−11520. (2) Tague, T. J., Jr.; Andrews, L. Pulsed Laser Evaporated Magnesium Atom Reactions with Hydrogen: Infrared Spectra of Five Magnesium Hydride Molecules. J. Phys. Chem. 1994, 98, 8611− 8616. (3) Green, S. P.; Jones, C.; Stasch, A. Stable magnesium(I) compounds with Mg-Mg bonds. Science 2007, 318, 1754−1757. (4) Bonyhady, S. J.; Jones, C.; Nembenna, S.; Stasch, A.; Edwards, A. J.; McIntyre, G. J. β-Diketiminate-Stabilized Magnesium(I) Dimers and Magnesium(II) Hydride Complexes: Synthesis, Characterization, Adduct Formation, and Reactivity Studies. Chem. - Eur. J. 2010, 16, 938−955. (5) Green, S. P.; Jones, C.; Stasch, A. Stable adducts of a dimeric magnesium(I) compound. Angew. Chem., Int. Ed. 2008, 47, 9079− 9083. (6) Stasch, A.; Jones, C. Stable dimeric magnesium(I) compounds: from chemical landmarks to versatile reagents. Dalton Trans. 2011, 40, 5659−5672. (7) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000. (8) Sondheimer, F.; Wolovsky, R.; Amiel, Y. Unsaturated Macrocyclic Compounds. XXIII. The Synthesis of the Fully Conjugated Macrocyclic Polyenes Cycloöctadecanonaene ([18]Annulene), Cyclotetracosadodecaene ([24]Annulene), and Cyclotriacontapentadecaene ([30]Annulene). J. Am. Chem. Soc. 1962, 84, 274−284. (9) Vogel, E. The porphyrins from the ‘annulene chemist’s’ perspective. Pure Appl. Chem. 1993, 65, 143−152. (10) Lash, T. D.; Jones, S. A.; Ferrence, G. M. Synthesis and Characterization of Tetraphenyl-21,23-dideazaporphyrin: The Best Evidence Yet That Porphyrins Really Are the [18]Annulenes of Nature. J. Am. Chem. Soc. 2010, 132, 12786−12787. (11) Lash, T. D. Origin of aromatic character in porphyrinoid systems. J. Porphyrins Phthalocyanines 2011, 15, 1093−1115. (12) Ferreira, G. C. Handbook of Porphyrin Science: With Applications to Chemistry, Physics, Materials Science, Engineering, Biology and Medicine. In Chlorophyll, Photosynthesis and Bio-inspired Energy; World Scientific Publishing Company, 2014; Vol. 28. (13) Haaland, A. Molecular structure and bonding in the 3d metallocenes. Acc. Chem. Res. 1979, 12, 415−422. (14) Berger, R. A.; LeGoff, E. The synthesis of a 22π-electron tetrapyrrolic macrocycle, [1,3,1,3] platyrin. Tetrahedron Lett. 1978, 19, 4225−4228. (15) Franck, B.; Nonn, A. Novel Porphyrinoids for Chemistry and Medicine by Biomimetic Syntheses. Angew. Chem., Int. Ed. 1995, 34, 1795−1811. (16) König, H.; Eickmeier, C.; Möller, M.; Rodewald, U.; Franck, B. Synthesis of a Bisvinylogous Octaethylporphyrin. Angew. Chem., Int. Ed. 1990, 29, 1393−1395. (17) Gosmann, M.; Franck, B. Synthesis of a Fourfold Enlarged Porphyrin with an Extremely Large, Diamagnetic Ring-Current Effect. Angew. Chem., Int. Ed. 1986, 25, 1100−1101. (18) Beckmann, S.; Wessel, T.; Franck, B.; Hönle, W.; Borrmann, H.; von Schnering, H.-G. [22]Coproporphyrin II for Photodynamic Therapy. Angew. Chem., Int. Ed. 1990, 29, 1395−1397. (19) Bonnett, R. Photosensitizers of the porphyrin and phthalocyanine series for photodynamic therapy. Chem. Soc. Rev. 1995, 24, 19− 33. (20) Sessler, J. L.; Gebauer, A.; Weghorn, S. J. Expanded Porphyrins. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 2, pp 55−124. (21) Chen, W.; Li, Z.-R.; Wu, D.; Li, Y.; Sun, C.-C.; Gu, F. L. The structure and the large nonlinear optical properties of Li@Calix[4]pyrrole. J. Am. Chem. Soc. 2005, 127, 10977−10981. (22) Li, Z.-J.; Wang, F.-F.; Li, Z.-R.; Xu, H.-L.; Huang, X.-R.; Wu, D.; Chen, W.; Yu, G.-T.; Gu, F. L.; Aoki, Y. Large static first and second hyperpolarizabilities dominated by excess electron transition 17209


ACS Omega

Article

(60) Wang, Y.-F.; Li, Z.-R.; Wu, D.; Sun, C.-C; Gu, F.-L. Excess electron is trapped in a large single molecular cage C60F60. J. Comput. Chem. 2010, 31, 195−203. (61) Lee, K.; Kim, S. W.; Toda, Y.; Matsuishi, S.; Hosono, H. Dicalcium nitride as a two-dimensional electride with an anionic electron layer. Nature 2013, 494, 336−340. (62) Dale, S. G.; Otero-de-la-Roza, A.; Johnson, E. R. Densityfunctional description of electrides. Phys. Chem. Chem. Phys. 2014, 16, 14584−14593. (63) Marqués, M.; Ackland, G. J.; Lundegaard, L. F.; Stinton, G.; Nelmes, R. J.; McMahon, M. I.; Contreras-García, J. Potassium under Pressure: A Pseudobinary Ionic Compound. Phys. Rev. Lett. 2009, 103, No. 115501. (64) Dale, S. G.; Johnson, E. R. Theoretical Descriptors of Electrides. J. Phys. Chem. A 2018, DOI: 10.1021/acs.jpca.8b08548. (65) Johnson, E. R.; Keinan, S.; Mori-Sanchez, P.; Contreras-Garcia, J.; Cohen, A. J.; Yang, W. Revealing Noncovalent Interactions. J. Am. Chem. Soc. 2010, 132, 6498−6506. (66) Toda, Y.; Hirayama, H.; Kuganathan, N.; Torrisi, A.; Sushko, P. V.; Hosono, H. Activation and splitting of carbon dioxide on the surface of an inorganic electride material. Nat. Commun. 2013, 4, No. 2378. (67) Chu, T.; Nikonov, G. I. Oxidative Addition and Reductive Elimination at Main-Group Element Centers. Chem. Rev. 2018, 118, 3608−3680. (68) Bogdanović, B.; Ritter, A.; Spliethoff, B. Active MgH2Mg Systems for Reversible Chemical Energy Storage. Angew. Chem., Int. Ed. 1990, 29, 223−234. (69) Datta, A. How Stable are the Mg−Mg Bonds in Magnesium (I) Compounds toward Hydrogenation? J. Phys. Chem. C 2008, 112, 18727−18729. (70) Bonyhady, S. J.; Collis, D.; Frenking, G.; Holzmann, N.; Jones, C.; Stasch, A. Synthesis of a stable adduct of dialane(4) (Al2H4) via hydrogenation of a magnesium(I) dimer. Nat. Chem. 2010, 2, 865− 869. (71) Lalrempuia, R.; Stasch, A.; Jones, C. The reductive disproportionation of CO2 using a magnesium(I) complex: analogies with low valent f-block chemistry. Chem. Sci. 2013, 4, 4383−4388. (72) Kefalidis, C. E.; Jonesb, C.; Maron, L. Mechanistic insights from theory on the reduction of CO2, N2O, and SO2 molecules using tripodal diimine-enolate substituted magnesium(I) dimers. Dalton Trans. 2016, 45, 14789−14800. (73) Kefalidis, C. E.; Stasch, A.; Jones, C.; Maron, L. On the mechanism of the reaction of a magnesium(I) complex with CO2: a concerted type of pathway. Chem. Commun. 2014, 50, 12318−12321. (74) Zhao, T.; Wang, Q.; Jena, P. Rational design of super-alkalis and their role in CO2 activation. Nanoscale 2017, 9, 4891−4897. (75) Atmospheric Chemistry and Greenhouse Gases Online, 2018. https://www.ipcc.ch/ipccreports/tar/wg1/pdf/TAR-04.PDF (accessed Oct, 2018). (76) Anthropogenic and Natural Radiative Forcing Online, 2018. http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_ Chapter08_FINAL.pdf (accessed Oct, 2018). (77) Jacobson, M. Z. Correction to ‘‘Control of fossil-fuel particulate black carbon and organic matter, possibly the most effective method of slowing global warming’’. J. Geophys. Res. 2005, 110, No. 4410. (78) Lee, D.-H.; Mondal, B.; Karlin, K. D. Nitrogen Monoxide and Nitrous Oxide Binding and Reduction. In Activation of Small Molecules: Organometallic and Bioinorganic Perspectives; Tolman, W. B., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2006; pp 66−72. (79) Vaughan, G. A.; Hillhouse, G. L.; Lum, R. T.; Buchwald, S. L.; Rheingold, A. L. Oxygen-atom transfer from nitrous oxide. Synthesis and structure of a zirconocene oxametallacyclobutene complex. J. Am. Chem. Soc. 1988, 110, 7215−7217. (80) Robinson, R., Jr.; Shaw, M. F.; Stranger, R.; Yates, B. F. Theoretical study of the mechanism for the sequential N−O and N− N bond cleavage within N2O adducts of N-heterocyclic carbenes by a vanadium(III) complex. Dalton Trans. 2016, 45, 1047−1054.

Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 16; Gaussian, Inc.: Wallingford CT, 2016. (40) Reed, A. E.; Weinstock, R. B.; Weinhold, F. Natural population analysis. J. Chem. Phys. 1985, 83, 735−746. (41) Wiberg, K. B. Application of the pople-santry-segal CNDO method to the cyclopropylcarbinyl and cyclobutyl cation and to bicyclobutane. Tetrahedron 1968, 24, 1083−1096. (42) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Intermolecular interactions from a natural bond orbital, donor-acceptor viewpoint. Chem. Rev. 1988, 88, 899−926. (43) Møller, C.; Plesset, M. S. Note on an Approximation Treatment for Many-Electron Systems. Phys. Rev. 1934, 46, 618−622. (44) Dunning, T. H., Jr. Gaussian basis sets for use in correlated molecular calculations. I. The atoms boron through neon and hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (45) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. Electron affinities of the first-row atoms revisited. Systematic basis sets and wave functions. J. Chem. Phys. 1992, 96, 6796−6806. (46) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO 3.1. QCPE Bulletin, 1990; Vol. 10, p 58. (47) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford University Press: Oxford, U.K., 1990. (48) Lu, T.; Chen, F. W. Multiwfn: a multifunctional wavefunction analyzer. J. Comput. Chem. 2012, 33, 580−592. (49) Becke, A. D.; Edgecombe, K. E. A simple measure of electron localization in atomic and molecular systems. J. Chem. Phys. 1990, 92, 5397−5403. (50) Postils, V.; Garcia-Borràs, M.; Solà, M.; Luis, J. M.; Matito, E. On the existence and characterization of molecular electrides. Chem. Commun. 2015, 51, 4865−4868. (51) Matito, E.; Solà, M. The role of electronic delocalization in transition metal complexes from the electron localization function and the quantum theory of atoms in molecules viewpoints. Coord. Chem. Rev. 2009, 253, 647−665. (52) Matito, E.; Duran, M.; Solà, M. The aromatic fluctuation index (FLU): A new aromaticity index based on electron delocalization. J. Chem. Phys. 2005, 122, No. 014109. (53) Feixas, F.; Matito, E.; Duran, M.; Solà, M.; Silvi, B. Electron Localization Function at the Correlated Level: A Natural Orbital Formulation. J. Chem. Theory Comput. 2010, 6, 2736−2742. (54) Matito, E.; Solà, M.; Salvador, P.; Duran, M. Electron sharing indexes at the correlated level. Application to aromaticity calculations. Faraday Discuss. 2007, 135, 325−345. (55) Bishop, D. M.; Norman, P. Calculations of Dynamic Hyperpolarizabilities for Small and Medium-Sized Molecules. In Handbook of Advanced Electronic and Photonic Materials; Nalwa, H. S., Ed.; Academic: San Diego, CA, 2001; Vol. 9, pp 1−62. (56) Garcia-Borràs, M.; Solà, M.; Luis, J. M.; Kirtman, B. Electronic and Vibrational Nonlinear Optical Properties of Five Representative Electrides. J. Chem. Theory Comput. 2012, 8, 2688−2697. (57) Pyykkö, P.; Atsumi, M. Molecular Single-Bond Covalent Radii for Elements 1−118. Chem. Eur. J. 2009, 15, 186−197. (58) Platts, J. A.; Overgaard, J.; Jones, C.; Iversen, B. B.; Stasch, A. First Experimental Characterization of a Non-nuclear Attractor in a Dimeric Magnesium(I) Compound. J. Phys. Chem. A 2011, 115, 194− 200. (59) Chen, W.; Li, Z.-R.; Wu, D.; Li, Y.; Sun, C.-C.; Gu, F. L. The Structure and the Large Nonlinear Optical Properties of Li@ Calix[4]pyrrole. J. Am. Chem. Soc. 2005, 127, 10977−10981. 17210


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(81) Najafian, A.; Cundari, T. R. C−H Activation of Methane by Nickel−Methoxide Complexes: A Density Functional Theory Study. Organometallics 2018, 37, 3111−3121. (82) Green, S. P.; Jones, C.; Stasch, A. Stable adducts of a dimeric magnesium(I) compound. Angew. Chem., Int. Ed. 2008, 47, 9079− 9083. (83) Bonyhady, S. J.; Jones, C.; Nembenna, S.; Stasch, A.; Edwards, A. J.; McIntyre, G. J. β-Diketiminate-Stabilized Magnesium(I) Dimers and Magnesium(II) Hydride Complexes: Synthesis, Characterization, Adduct Formation, and Reactivity Studies. Chem. - Eur. J. 2010, 16, 938−955. (84) Gallagher, D. J.; Henderson, K. W.; Kennedy, A. R.; O’Hara, C. T.; Mulvey, R. E.; Rowlingsa, R. B. Hydride encapsulation in s-block metal inverse crown chemistry. Chem. Commun. 2002, 0, 376−377.

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