Electronic Interactions in Iminophosphorane Superbase Complexes

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Electronic Interactions in Iminophosphorane Superbase Complexes with Carbon Dioxide Francesca Ingrosso, and Manuel F. Ruiz-Lopez J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11853 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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The Journal of Physical Chemistry

Electronic Interactions in Iminophosphorane Superbase Complexes with Carbon Dioxide Francesca Ingrosso1,2* and Manuel F. Ruiz-López1,2* 1

SRSMC, University of Lorraine, BP 70239, 54506 Vandoeuvre-lès-Nancy, France.

2

CNRS, UMR 7565, BP 70239, 54506 Vandoeuvre-lès-Nancy, France.

ACS Paragon Plus Environment

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ABSTRACT

Iminophosphoranes or phosphazenes are an important class of compounds with increasing use in synthetic organic chemistry as neutral organic superbases exhibiting low nucleophilicity. Their electronic structure and therefore their properties strongly depend on substitution but there have been very few theoretical studies devoted to this topic, and more specifically to the formation of electron donor-acceptor complexes of iminophosphoranes with electrophiles. In this work, we have investigated the interaction with carbon dioxide at different ab initio levels. Carbon dioxide usually behaves as a Lewis acid and the reaction with iminiphosphoranes has been described as a non-conventional aza-Wittig process leading to isocyanates. The reaction can be conducted in supercritical CO2 conditions (carbon dioxide acts as both solvent and reactant), which is a promising strategy in the context of green chemistry. Our calculations have been carried out at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level for model systems and at the M06-2X/6611+G(d,p) level for a larger species used in experiments. The electronic interactions and the interaction energies are analyzed and discussed in detail using the natural bond orbital method. Proton affinities and gas-phase basicities are provided as well.


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Introduction Molecules containing nitrogen-phosphorus bonds have attracted attention for the last several decades. A pioneer theoretical study on this chemical family was carried out about 25 years ago by Yañez, Mó and co-workers.1 Iminophosphoranes, in particular, display a versatile reactivity and are common compounds used in organic synthesis. Iminophosphoranes with the general structure RN=PR’3 were first described by Staudinger et Meyer in 1919.2 These compounds are known under several names, iminophosphorane, phosphinimine, phosphazene, phosphinimide, among others. They are obtained through the Staudinger reaction in which an azide reacts with a phosphorous derivative, in general a phosphine, to yield the corresponding ylide. In the Staudinger reaction, the ylide is converted into amines through hydrolysis, but ylides can also react with carbonyl compounds to yield imines. The later is the so-called aza-Wittig reaction (for a review of this reaction see Palacios et al3). This reaction can be exploited for the design of chemical processes that respect the principles of Green Chemistry, such as for instance atom economy and the use of sustainable solvents. Indeed, in a series of papers,4-6 Marsura and co-workers showed that organic carbonyl compounds in the aza-Wittig reaction can be replaced by carbon dioxide yielding an isocyanate (reaction 1), which after reaction with an amine leads to urea derivatives. Following similar procedures but changing the amine by a different nucleophile, most of the carbonic acid functions are accessible in one step synthesis.

RN = PR′ + CO → O = PR′ + RN = C = O

(1)


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The unconventional aza-Wittig reaction (1) is extremely interesting since: 1) it represents a safer route to isocyanate and ureas because it does not need the use of hazardous compounds such as phosgene, which is still largely employed by the pharmaceutical and chemical industry, and 2) the reaction can be carried out in supercritical CO2 solvent (scCO2),5 which is considered as one of the most promising media to be used in the context of Green Chemistry. Since in scCO2, carbon dioxide is both solvent and reactant, the reaction is quite efficient in this medium. It is interesting to mention also that use of iminophosphorane combined with imidazolium-based ionic liquids has been studied as a possible medium for efficient and reversible capture of CO2 (see for instance 7). Although some other examples of reactivity studies in scCO2 are present in the literature,8 9 10 11 4 12 13 14

very little is known about the reaction mechanisms in the presence of this peculiar solvent

and only a few theoretical studies have addressed this question so far.15 16 17 18 On the other hand, it is apparent that significant improvement on the technological applications of scCO2 is obtained when intermolecular interactions are described and understood at a molecular level. This is particularly true in the case of CO2-philicity, a concept that was developed to describe the weak interactions between some specific functional groups, such as for instance the fluorine atom19 20 21

or the carbonyl group,22 and CO2, which make it possible to design polar molecules that are

soluble in this apolar medium. To this aim, computational approaches provide key information on the nature of the solute-solvent interactions.23 24 Study of the interaction between iminophosphoranes and carbon dioxide is important from the point of view of electronic structure calculations as well. First, iminophosphoranes display a remarkable proton affinity and are a well-known example of organic neutral, low-nucleophilic superbases. This property can be explained by the fact that they are 1,2-dipolar compounds,


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which may behave as zwitterions having the structure RN − P R′ . Thus, the charge on N atom is expected to be quite large and accordingly these molecules possess an extremely high basicity. For instance, the pK of some triaryliminophosphorane in MeCN were measured25 (22.7-25) and shown to be much higher than that of triethylamine (18.82), and comparable to that of guanidine (23.56) and other superbases. Besides, the Schwesinger base,26 a special type of iminophosphorane, is one of the most powerful superbases today (pK = 42.6 in acetonitrile). The strong basicity of phosphorus imines and ylides in gas phase has been confirmed by ab initio and semiempirical calculations27 and by recent experiments.28 Iminophosphoranes can also behave as electron donor ligands for the formation of inorganic complexes, in which the lone pair on the nitrogen atom is responsible of the donor character of the interaction.29 Finally, they may exhibit some minor acceptor character related to π antibonding orbitals.29 In the case of azaWittig reaction between iminophosphoranes with aldehydes, ab initio calculations have shown that these molecular orbitals are involved into a [2+2] cycloaddition leading to a stable intermediate, further evolving into the products containing a double nitrogen-carbon bond.30 Other calculations for simple iminophosphorane and aldehyde derivatives have been reported in the literature.31-35 Secondly, CO2 usually behaves as an electron acceptor (EA) or Lewis acid. For instance, the interaction between compounds containing fluorine atoms or carbonyl groups and CO2 were described in the literature as Lewis acid (LA) - Lewis base (LB) interactions in which CO2 plays the role of EA.36 37 Nonetheless, CO2 can behave as an electron donor (ED) or Lewis base in some cases, for instance when involved in a H-bond interaction. In addition, we have previously shown that, according to very high-level ab initio calculations, CO2 may interact with π-systems through a cooperative donor-acceptor interaction, which may be significantly stronger than the


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conventional LA-LB one. We studied a series of carbonyl derivatives and found that ureas and carbamides, in particular lead to very stable complexes, with a stabilization energy that can reach up to 7 kcal/mol for some 1:1 complexes.38 39 The results thus provided an explanation of why molecules bearing carbonyl groups, such as polycarbonates, display high solubility in scCO2. A review on theoretical studies of solubility in scCO2 has been reported recently.24 Considering the above comments, it appears interesting to carry out a theoretical study of the interaction between low nucleophilic iminophosphorane superbases and carbonyl dioxide, which has received little attention in the literature despite the potential interest of the reaction to fix carbon in synthetic organic chemistry. In this paper, we focus on the electronic properties and energetics of some complexes trying to shed some light on the iminophosphorane – CO2 interaction mechanism. The aza-Wittig reaction leading in fine to the phosphine oxide and isocyanate formation will be studied in forthcoming work. Preliminary calculations for this process show that a four member intermediate comprising the N and P atoms of the base and one O and the C atom of CO2 is formed, much like in the reaction with carbonyl compounds.30 From this point of view, studying the geometrical and electronic structure of CO2⋅⋅⋅iminophosphorane complexes is particularly relevant.

Computational Details Two model complexes, CO2⋅⋅⋅HN=CH2 and CO2⋅⋅⋅CH3N=PH3, were first studied for comparison with the target complex involving the triphenylphosphine derivative RN=PPh3 considered in the experiments,4 R=CH3 in the present work. The model system CH3N=PH3 was chosen instead of HN=PH3 for convenience in the comparison. The study was conducted in gas phase (isolated molecules).


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Computations with the model systems were carried out as follows. The geometries were optimized by means of the MØller-Plesset method at the second order of perturbation (MP2)40 with the aug-cc-pVTZ41 basis set. Single-point CCSD(T) calculations using the same basis set were then carried out to estimate the interaction energies. Since for the target system CO2 ⋅⋅⋅ CH3N=PPh3 this theoretical level is not affordable, we also carried out calculations (geometry optimization and interaction energies) using Density Functional Theory (DFT) based methods, specifically the M06-2X functional42 and the 6-311+G(d,p) basis set.43 The comparison between the CCSD(T), MP2 and M06-2X results led us to the conclusion that the DFT method provides reasonably accurate interaction energies, and therefore only this approach was used for the larger system. Frequency calculations were carried out at the MP2 and M06-2X levels after geometry optimization to verify the nature of stationary points and to estimate the zero-point energy correction (ZPE), as well as thermal contributions to the thermodynamic quantities needed to calculate proton affinities and gas phase basicities (see below). Besides, basis set superposition errors were estimated using the counterpoise method.44 The Natural Bond Orbital (NBO) method45-46 was used to analyze the electronic interactions between the two interacting species. The proton affinity (PA) and gas phase basicity (GB) of the iminophosphoranes and HN=CH2 at 298K have been computed for comparison. They are defined, respectively, as the negative of the enthalpy and free energy of the protonation process in the gas phase: B + H ⇌ BH

(2)

PA = −ΔH

(3)


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GB = −ΔG

(4)

These quantities were only calculated at the M06-2X/6-311+G(d,p) level. All the quantum mechanical calculations were done with Gaussian09.47

Results and discussion

Model complexes Previous theoretical studies at B3LYP/6-311+G(d,p) level have shown that the basicity of imines and iminophosphoranes is very different.27 Calculations in this work at the M06-2X/6311+G(d,p) level led to the proton affinities and gas phase basicities reported in Table 1. As shown, the proton affinity of the model iminophosphorane CH3N=PH3 is about 30 kcal/mol higher that for the model imine HN=CH2, and similar trends are obtained for the gas phase basicities. In the following, we focus on their nucleophilicity through the study of their complexes with a weak Lewis acid such as CO2. Table 1. Proton affinity (PA) and gas-phase basicity (GB) of the studied compounds in kcal/mol at the M06-2X/6-311+G(d,p) level. PA

GB

HN=CH2

206.1

198.6

CH3N=PH3

236.6

229.6

CH3N=PPh3

259.9

251.6


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Possible formation of an ED-EA complex between CO2 and methanimine HN=CH2 and between CO2 and CH3N=PH3 was therefore explored. Inspection of the potential energy surface followed by full geometry optimization at MP2/aug-cc-pVTZ and M06-2X/6-311+G(d,p) levels led to the structures reported in Figure 1. As shown, the two theoretical methods lead to similar structures and similar trends, although systematically M06-2X predicts somewhat shorter intermolecular distances. It is worth noting that in the case of the CO2⋅⋅⋅CH3N=PH3 complex, the M06-2X optimized structure displays an imaginary frequency. Several attempts were made to eliminate such a frequency, including optimization starting from a non-symmetric structure and/or decreasing the convergence threshold, but all attempts were unfruitful. Nevertheless, the frequency absolute value is very small (12 cm-1) and we then concluded that the optimized structure can be considered a good approximation for the energy minimum, possibly in a flat potential energy surface including some anharmonic character in at least one dimension. In both cases, the CO2 molecule lies in the plane of the X-N=Y bonds and the complex displays a short intermolecular C⋅⋅⋅N distance, as well as one or two weak hydrogen-bonds between the oxygen atoms in CO2 and hydrogen atoms in the partner unit. Such a structure is indeed expected for the imine because in an ED-EA complex, the non-bonding n orbital on the imine nitrogen must interact with one π* orbital on CO2. The computations show, in addition, that a similar structure is obtained in the case of the iminophosphorane. In this case, two hydrogen-bonds are predicted with the CH3 and PH3 subunits, the shortest one corresponding to the later group, which is consistent with the fact that it carries a larger positive charge (see below). When the two complexes are compared, it is found that the C⋅⋅⋅N distance is significantly shorter in the iminophosphorane despite its larger steric hindrance. Clearly, nucleophilicity of the nitrogen atom is enhanced upon substituting the CH2 group by the PH3 one.


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Figure 1. Optimized geometries for the model complexes CO2 ⋅⋅HN=CH2 and CO2⋅⋅⋅CH3N=PH3. Values of internal parameters at the MP2/aug-cc-pVTZ and M06-2X/6-311+G(d,p) (in parenthesis) levels. δ holds for P=N-C(CH3)-C(CO2) or C(CH2)=N-H(NH)-C(CO2) dihedral angles.

Let us now discuss the interaction energies. They are summarized in Table 2. In the case of the imine, the formation energy is -2.44 kcal/mol while in the case of the iminophosphorane, it amounts -4.66 kcal/mol (CCSD(T) level with ZPE and BSSE corrections). Values at MP2 level are similar while energies at M06-2X level are slightly larger in absolute value (as expected from the shorter interatomic distances) but maintain the same trend. The different nucleophilicity between HN=CH2 and CH3N=PH3 is confirmed by these results, which provide reference values with which complexes of CO2 with larger iminophosphoranes can be compared. It can be rationalized by considering the value of the HOMO energies in the monomers, a usual nucleophilicity index in DFT,48 which are -9.3 eV and -7.4 eV, respectively. Further analysis will be presented below.


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Table 2. Computed interaction energies for the different complexes studied in this work (in kcal/mol). Values in italics include ZPE corrections. CCSD(T)a

CO2⋅⋅⋅HN=CH2

CO2⋅⋅⋅CH3N=PH3

CO2⋅⋅⋅CH3N=PPh

MP2 b

M06-2X c

d

e

d

e

d

e

-3.69

-3.25

-3.99

-3.53

-5.06

-4.80

-2.88

-2.44

-3.18

-2.72

-4.19

-3.93

-6.05

-5.33

-5.95

-5.20

-8.24

-7.30

-5.38

-4.66

-5.28

-4.53

-7.36

-6.42

-6.68

-5.84

-6.02

-5.18

3

a) b) c) d) e)

CCSD(T)/aug-ccPVTZ//MP2/aug-ccPVTZ calculations (ZPE is obtained at the MP2 level) MP2/aug-ccPVTZ calculations M06-2X/6-311+G(d,p) calculations Without BSSE corrections Including counterpoise corrections for BSSE

It is interesting to extend the comparison to other computations in the literature reported for EDEA complexes with CO2. Compared to amines, for instance, which has been widely investigated for the purpose of carbon dioxide capture, the CH3N=PH3 iminophosphorane displays larger interaction energy with CO2 and a shorter intermolecular distance.49-50 Jorgensen et al,50 in particular, studied the complexes with ammonia, methylamine and dimethylamine. According to their results,50 the C⋅⋅⋅N distance amounts 3.0-2.98 Å at the B3LYP/aug-cc-pVTZ level and the interaction energy (calculated as the difference in formation enthalpies at 298K) around -1 kcal/mol in the three cases (at the same theoretical level). These values change a little when other methods are used for the optimization of the geometry or the calculation of the energy but even at the largest level considered in reference50 the interaction energies are significantly smaller, in absolute value, than the values reported here for the iminophosphorane complex (the largest


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interaction energy was found for dimethylamine using a modified composite ccCA method in which the B97D dispersion-corrected functional is used for geometry optimization, amounting 3.0 kcal/mol, for details see50). Note also that some amines, like diethanolamine or DEA (a common compound used for CO2 capture), displayed larger interaction energies,50 although in that case the complexes do not involve nitrogen-carbon ED-EA interactions but the formation of strong hydrogen-bonds between the –OH or –NH groups and CO2. Hence, they cannot be directly compared with the systems studied in our work. The iminophosphorane-CO2 interaction energy in Table 2 is also larger than the values reported for conventional complexes with carbonyl derivatives,38 though it is comparable to non-conventional complexes with amides or ureas.39 The comparisons above are a bit intricate because of the use of a variety of computational approaches in the different works. Overall, nevertheless, iminophosphoranes appear as good CO2-philes and should have a good solubility in scCO2, which is a key property to develop their chemistry in that medium.

Complex with CH3N=PPh3 Geometry optimization of the CO2⋅⋅⋅CH3N=PPh3 complex at the M06-2X/6-311+G(d,p) level led to the structure reported in Figure 2, which includes some significant structural parameters. The complex displays a C⋅⋅⋅N intermolecular distance of 2.66 Å, which is a little larger than the distance in the model iminophosphorane compound at the same theoretical level (see Figure 1), but which remains consistent with the formation of an ED-EA complex. As with the model system, calculations predict two relative short intermolecular O⋅⋅⋅H distances. Apart from a longer C⋅⋅⋅N distance, the main difference with respect to the model compound is the fact that in


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the present case, the intermolecular interaction does not take place in the X-N=Y plane, as indicated by the value of the torsional angle δ (151.9°). This result points at a different interaction mode, which will be discussed below using NBO analysis. The interaction energy of the complex evaluated at the M06-2X theoretical level, -5.18 kcal/mol (with BSSE and ZPE corrections) is larger than for the model imine, but smaller than for the model iminophosphorane. This result might be surprising considering the relative proton affinities and basicities of CH3N=PH3 and CH3N=PPh3 reported in Table 1. As expected, the basicity of CH3N=PPh3 is much higher, with a value (251.6 kcal/mol) falling in the range of superbasicity. However, CH3N=PPh3 is sterically hindered and this is a usual factor explaining the low nucleophilicity of iminophosphorane superbases. Our results for the two compounds follows the expected trend from this common interpretation: in the model iminophosphorane CH3N=PH3 the steric hindrance is smaller and hence the nucleophilicty is larger. But in order to get a deeper insight into the nucleophilicity difference, we have carried out NBO calculations, which are presented hereafter.


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Figure 2. Two different views of the optimized geometry of the CO2⋅⋅⋅CH3N=PPh3 complex at the M06-2X/6-311+G(d,p) level. δ holds for the P=N-C(CH3)-C(CO2) dihedral angle.

Natural Bond Orbital (NBO) analysis As discussed above, the structure predicted for the complexes, particularly the short C⋅⋅⋅N distances obtained for the optimized geometries, suggests an ED-EA interaction type between the partners. To check this assumption and to get a more detailed description of the electronic terms involved, we have carried out NBO calculations using in all cases the results at the M062X/6-311+G(d,p) level. The results for the three studied complexes are summarized in Table 3 and Figures 3 and 4. Table 3 contains values for some net charges, dipole moment and charge transfer, as well as for second-order energies obtained in the perturbation theory analysis of the Fock matrix in the NBO basis. Figure 3 contains the relevant Wiberg bond orders while Figure 4 displays the molecular orbitals that are involved in the main ED-EA interactions.


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A first remark can be done for the nature of the N=P bonds. Compared to N=C bonds, N=P bonds exhibit a much lower bond order and a much higher polarity. Actually, N=C bond orders are close to the formal value 2, while N=P bond orders are significantly smaller, lying in the range 1.2-1.3. In addition, the atomic net charge and dipole moment values gathered in Table 3 show that N=P bonds are highly polarized suggesting that the iminophosphoranes can partially be described by a zwitterionic structure RN − P R′. Furthermore, under formation of the complexes with CO2, the N=P bond order decreases and simultaneously bond polarity increases. A more detailed analysis on the N=P bond in the simplest iminophosphorane compound HN=PH3 can be found in reference.31 Looking now at the intermolecular N⋅⋅⋅C bond order, the calculated value for the iminophosphorane-CO2 complexes is substantially larger than for the imine, illustrating the stronger interaction in the first case. This bond order indeed follows the order of the interaction energy with CO2, namely: HN=CH2 < CH3N=PPh3 < CH3N=PH3, and likewise the opposite order of the interatomic distance. The electron charge transfer to CO2 is significant in the three cases, and is notably larger than (for instance) in complexes with CO2-philic carbonyl compounds,38 specially for CH3N=PH3, although there is not clear correlation with the interaction energy or N⋅⋅⋅C distance. A possible explanation for this finding will be discussed below.

Table 3. Electronic properties of the studied complexes at the M06-2X/6-311+G(d,p) level from NBO analysis. Net atomic charges in the N=Y bonds (qN and qC,P, in au), dipole moment (µ, in D), net electron charge transfer to CO2 (∆q, in au) and main second-order energy from


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perturbation theory analysis of the Fock matrix in NBO basis energy (E(2), in kcal/mol). In the case of CO2⋅⋅⋅CH3N=PPh3,, two orbital interactions provide significant contributions to the stabilization of the complex (the corresponding E(2) values are noted (a) and (b), see the text for further explanations). Values in parenthesis refer to the isolated monomers.

CO2⋅⋅⋅HN=CH2

CO2⋅⋅⋅CH3N=PH3

qN

qY (Y=C,P)

µ

∆q

E(2)

-0.6268

-0.0158

2.653

0.0094

2.81

(-0.5905)

(-0.0318)

(2.201)

-1.1353

+1.1229

3.654

0.0207

7.41

(-1.0929)

(+1.1166)

(3.039)

+1.8232

4.269

0.0081

(a) 6.53

(+1.8084)

(3.362)

CO2⋅⋅⋅CH3N=PPh3 -1.1557 (-1.1180)

(b) 3.65

Figure 3. Wiberg bond orders for the three studied complexes at M06-2X/6-311+G(d,p) level. Values in parenthesis refer to the isolated monomers.


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The main second-order energy contributions of the intermolecular donor-acceptor orbital interactions (see Table 3) correlate well with the total interaction energies in Table 2, although the values for the iminophosphoranes are much larger than in the imine case. Besides, in the analysis of these energies, one observes only one strong stabilizing component for the model compounds HN=CH2 and CH3N=PH3. It corresponds to the interaction between a lone pair on the N atom (ED) and an unoccupied anti-bonding π* orbital on CO2 (EA) shown in Figure 4. The other intermolecular components found in the NBO analysis are quite small and for simplicity they are not discussed here. Let us just mention, as an example, than the component associated to the weak hydrogen-bond in the imine complex CO2⋅⋅⋅HN=CH2 is 0.10 kcal/mol. The corresponding orbitals are shown in Figure 5. A strong nπ* interaction is also present in the case of CH3N=PPh3 (noted (a) in Table 3 and Figure 4), but in this system further stabilization is provided by a second contribution (noted (b)). This fact points towards a different interaction mechanism with respect to the model iminophosphorane and two factors can explain this finding. The first one is connected to steric interactions with the phenyl groups, which may be responsible for an out-of-plane positioning of CO2 (out-of-plane here means that the CO2 axis does not lye on the X-N=Y plane). Indeed, as mentioned above, the complex displays a P=N-C(CH3)-C(CO2) dihedral angle close to 152°, significantly different from the 180° dihedral angle predicted for the model complex. Therefore, n and π orbitals in the C-N=P fragment are not formally orthogonal; they can mix together and both can interact with the π* anti-bonding orbital in CO2. The second factor is connected to the intrinsic electronic structure of the iminophosphorane. We have shown before that the nitrogenphosphorous bond in CH3N=PPh3 monomer displays both a larger polarity and a lower bond order than in CH3N=PH3, indicating a higher zwitterionic and a smaller double-bond characters.


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In the limit of a C − N − P mesomeric structure, two lone pairs would be present on the N atom, which would adopt sp3 symmetry under the interaction with CO2 and would force out of plane geometries. The two factors can play a role but since they are interrelated it is not straightforward to separate their effect. Clearly, however, the first one contributes to lessening the nucleophilicity of the system, while the second one contributes to increase its basicity.

Figure 4. Molecular orbitals involved in the main electron donor–acceptor interaction in the complexes with CO2 according to NBO analysis at M06-2X/6-311+G(d,p) level. In the case of CH3N=PPh3, two orbital interactions provide significant contributions to the stabilization of the complex with CO2 (they are noted (a) and (b)). See Table 3 for the corresponding second-order energy contributions E(2).


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Figure 5. Molecular orbitals involved in the weak hydrogen-bond interaction in the CO2⋅⋅⋅HN=CH2 complex according to NBO analysis at M06-2X/6-311+G(d,p) level.

Conclusions The calculations reported in this paper show that the basicity and nucleophilicity of iminophosphoranes derivatives strongly depend on substitution. For the two systems studied in this work, CH3N=PR3 (R=H, Ph), the substitution of H by Ph on the phosphorous atom makes the gas phase basicity increase by as much as 22 kcal/mol, which makes the basicity of the larger compound to lye on the superbasicity region (251.6 kcal/mol). However, the same modification decreases the nucleophilicity of the iminophosphorane, and the interaction energy with the Lewis acid CO2 lowers from -6.42 kcal/mol to -5.18 kcal/mol (including ZPE and BSSE corrections). The different effect of substitution on basicity and nucleophilicity is usually explained in terms of steric hindrance. Our results confirm this interpretation but they also put in evidence an electronic effect related to the higher zwitterionic character of the larger iminophosphorane. It implies a decrease of the N=P double-bond character in the monomer together with a modification of the orbital hybridization on nitrogen under the interaction with the electrophile, with raising sp3 character. Both steric and electronic effects are interrelated and their relative importance is not easy to assess. Interestingly, solvation effects would not influence too much


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the steric factors while they might play an important role on the polarity of the N=P bond. Hence, by studying the nucleophilicity of different iminophosphoranes in different media, further insights should be obtained, and this is a possible direction for future work. Our study also shows that iminophosphoranes, compared to imine compounds, display much higher basicity, as expected. Their nucleophilicity is slightly higher too, as shown by the complexation energies and HOMO values.

Corresponding Authors Francesca Ingrosso: [email protected] Manuel Ruiz-López: [email protected] ACKNOWLEDGMENT The authors are grateful to the French CINES (project lct2550) for providing computational resources, and to the University of Lorraine and the French CNRS for financial support.

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Figure 1 236x113mm (150 x 150 DPI)


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Figure 2 255x109mm (150 x 150 DPI)



Figure 3 257x99mm (150 x 150 DPI)


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Electronic Interactions in Iminophosphorane Superbase Complexes

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