Complexes between Dihydrogen and Amine, Phosphine, and Arsine

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Complexes Between Dihydrogen and Amine, Phosphine and Arsine Derivatives. Hydrogen Bond vs. Pnictogen Interaction Slawomir J. Grabowski, Ibon Alkorta, and José Elguero J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp4016933 • Publication Date (Web): 21 Mar 2013 Downloaded from http://pubs.acs.org on March 27, 2013

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The Journal of Physical Chemistry

Complexes between dihydrogen and amine, phosphine and arsine derivatives. Hydrogen bond vs. pnictogen interaction Sławomir J. Grabowski,a,b* Ibon Alkortac* and José Elgueroc a

Kimika Fakultatea, Euskal Herriko Unibertsitatea, PK 1072, 20080, Donostia, Spain

b

IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain c

Instituto de Química Médica (CSIC) Juan de la Cierva, 3 E-28006 Madrid, Spain

*Authors to whom correspondence should be addressed: [email protected] (SJG) and [email protected] (IA)

Abstract A theoretical study of the complexes between dihydrogen, H2, and a series of amine, phosphine and arsine derivatives (ZH3 and ZH2X, with Z = N, P or As and X = F, Cl, CN or CH3) has been carried out using ab initio methods (MP2/aug-cc-pVTZ). Three energetic minima configurations have been characterized for each case with the H2 molecule in the proximity of the pnictogen atom (Z). In configuration A, the σ-electrons of H2 interact with σ-hole region of the pnictogen atom generated by the of X-Z bond. These complexes can be ascribed as pnictogen bonded. In configuration C the lone electron pair of Z acts as the Lewis base and H2 plays the role of the Lewis acid. Finally, configuration B presents a variety of non-covalent interactions depending on the binary complex considered. The Atoms in Molecules theory (AIM), Natural Bond Orbitals (NBO) method as well as the DFT-SAPT approach were used in this study to deepen the nature of the interactions considered.

Key words: pnictogen interactions, non-covalent interaction, hydrogen bond, DFTSAPT approach, Atoms in Molecules theory, Natural Bond Orbitals method, molecular hydrogen.

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Introduction The number of studies on various hydrogen storage materials have increased significantly during the last decades.1,2 In parallel, the knowledge of the interaction between molecular hydrogen in different environment 3 is of great important in order to allow to predict which compounds may be potentially used as hydrogen storage materials. Among those studies the search on candidates for storage materials,4 the utilization of hydrogen as the energy source for fuel-cell vehicles,5 the catalytic enhancement of hydrogen storage,6 the hydrogen adsorption in metal-organic frameworks (MOF)

7,8

and nanotubes 9 or fullerenes

10

as novel materials for hydrogen

storage have been considered in the literature. On the other hand theoretical studies have been carried out on the molecular hydrogen clusters,11 on the possibility to use organic frameworks in the H2 storage12 and on the importance of the different components of the energy in the binding energy of hydrogen.13 The effect of the external electric field on the equilibria between dihydrogen bonded complexes and the generation of H2 has been considered theoretically.14 It has already been described that the molecular hydrogen may act both as Lewis base and as Lewis acid.15 In the first case the σ-bond of H2 acts as the Lewis base group. Several theoretical studies describe the possibility that the σ-bond of H2 acts as hydrogen bond acceptor, for example in the T-shaped H2···HF complex.16 The calculated binding energies of T-shaped complexes such as F-H···H2, HCCH···H2, H3NH+···H2 and H2OH+···H2 calculated at MP2/6-311++G(3df,3pd) computational level and corrected for BSSE amount to 4.6, 1.3, 10.9 and 23.0 kJ mol–1, respectively.17 In the same way the theoretical study of H3XH+···H2 complexes (X = N, P, As, Sb, Bi) and the related, more complicated clusters, XH4+···(H2)5 indicated the existence of X-H···σ hydrogen bonds. 18 In general, these complexes show binding energies, 8.4 kJ mol–1 or smaller, an indication that the σ-electrons of H2 molecule are a rather weak Lewis base. However, in a few cases the binding energies are not negligible as in the H2OH+···H2 complex, where the binding energy is greater, for instance, than the one in the water dimer where the typical O-H···O hydrogen bond exists. Other examples where H···σ interactions have been described are H2···HRgY complexes (Rg = Ar, Kr; Y = F, Cl, CN)19 and H2···HX complexes (X = -CCH, -CCLi, -CCF, -CN, -NC, -OH, -F and -Cl).20 It is worth mentioning that the σ-electrons of H2 may act as the Lewis base for other weak interactions in addition to hydrogen bonds. For example, the T-shaped F-Cl···H2 complex was analyzed where the ClF and H2 molecules are linked through the halogen 2 Environment ACS Paragon Plus

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bond21 or cation···H2 interactions, which are much stronger than the X-H···σ neutral hydrogen bonds.17 There are numerous studies where the σ-electrons of H2 interact with metal centres,3 as the theoretical studies on the series of metal cation dihydrogen (M+ – H2) complexes,22 the experimental evidences of the Cu2+···H2 interactions23 or the more general study on the metal – dihydrogen interactions.24 There are also interesting interactions between metal hydrides and dihydrogen. The negatively charged H-atom centers of the hydride interact with the electron deficient region of H2 providing M-H–δ···+δH-H-δ linear complexes15 which can be classified as dihydrogen bonds.25 The positively charged metal atoms of metal hydrides may also act as the Lewis acid centers and interact with σ-electrons of molecular hydrogen.15 Even the H2(σ-electrons)···+δM-H–δ ···+δH-H–δ complexes of the metal hydrides were analyzed where both kinds of the interactions mentioned above exist.15 Among the newly described interaction, the pnictogen or pnicogen interaction has been recognized as a potential important type of intermolecular contact. It is defined as an intra- or inter-molecular interaction where an atom from group 15 (N, P and As) acts as a Lewis acid.26 It was described the existence of such interaction in the crystal structures of racemic 1-dichlorophosphanyl-2-N,N-dimethylaminomethylferrocene, racemic

1-dichlorophosphanyl-2-N,N-dimethylaminomethyl-3-triphenylsilylferrocene

and (S)-N,N-dimethyl-1-[(R)-2-(dichlorophosphanyl)ferrocenyl]ethylamine where short N···P intramolecular distances were detected and attributed to weak N → P dative bonds.27 A short intramolecular P···P distance found in the crystal structure of 1,2(diphenylphosphino)-1,2-dicarba-closo-dodecaborane28 was confirmed to be attractive using B3LYP/6-31G(d) calculations and NBO analysis (P lone pair → σPC* donoracceptor stabilizing interaction). The possibility that the N, P and As atoms can act as Lewis acid centers may be explained in terms of the σ-hole concept29,30,31 which was firstly introduced to explain the nature of halogen bond29 and next it was extended for the other non-covalent interactions.30,31 In short, numerous non-covalent interactions may be treated, in terms of the σ-hole concept as contacts between the opposite charged Lewis acid and Lewis base centers. For pnictogen interactions, parts of the atomic sphere of the atoms of group 15 possess positive electrostatic potential and other parts, negative potential. Hence at the same time N, P and As centers may act as the Lewis acid and as the Lewis base. For example, the calculated FH2As···AsH2F dimer showed a binding energy of 17.2 kJ mol–1 at MP2/6-311++G(3df,2p)//B3PW91/6-311G(3df,2p) level.31 In this complex, a positive region on the extension of each F-As bond interacts 3 Environment ACS Paragon Plus


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with the negative potential on the other As atom. In such a way this complex may be treated as linked by two equivalent electrostatic interactions.31 Similarly, the (PHFX)2, X = F, Cl, CN, CH3, NC, homodimers were analyzed further in detail.32 In addition, a number of theoretical studies have treated this interaction33 and it has been compared with the other non-covalent interactions such as hydrogen and halogen bonds.34 The goal of this study is to analyze the complexes of dihydrogen with amine, phosphine and arsine derivatives. Such a choice is related with interesting characteristics of the pnictogen bond which may exist between dihydrogen and the species mentioned above. Besides, the two components of the interaction, dihydrogen and pnictogen derivatives, present a dual character as Lewis acid and as Lewis base. Thus it seems to be interesting to know what types of interactions link ZH3 species (Z = N, P, As) and their derivatives, ZH2X, with dihydrogen.

Computational details The geometry of the monomers and complexes studied in this article has been fully optimized at the second order Møller-Plesset perturbation theory (MP2),35 and the Dunning type aug-cc-pVTZ basis set.36 Frequency calculations have been carried out at the same computational level to confirm that the obtained structures correspond to energetic minima. The calculations were performed using the Gaussian09 program.37 The binding energies were calculated as differences between the energy of the complex and the sum of energies of monomers optimized separately. Hence the effect of the deformation connected with the complexation was included in the binding energies. The binding energies were corrected for the basis set superposition error (BSSE) by the counterpoise method.38 The corrected binding energies and BSSE corrections are summarized in the Supporting Information (SI) material. The electron density of the systems considered here has been analyzed within the Atoms in Molecules (AIM)39 methodology with the AIMAll program.40 The intermolecular bond critical points (BCPs) have been located and their electron density properties has been analyzed. The Natural Bond Orbitals (NBO) method 41 was applied to analyze interactions occurring in the complexes considered. The orbital interaction between occupied and empty orbital of different molecules has often been considered as one of the main characteristics of a number of interactions as hydrogen bond, halogen bond, etc.41


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In the complexes studied here, two different situations can be found: i) H2 acts as Lewis acid (electron acceptor) and ii) H2 acts as Lewis base (electron donor). In the first case, the pnictogen derivative is the Lewis base and the interaction is connected with the maximum nZ → σHH* overlap. Where nZ designates the lone electron pair of the pnictogen derivative and σHH* is an antibonding orbital of the H2 molecule. The nZ → σHH* interaction is calculated as the second-order perturbation theory energy (Eq. 1): ∆E (nZ → σHH*) = –2 〈nZFσHH*〉2 / [ε(σHH*) – ε(nZ)]

Eq. 1

〈nZFσHH*〉 designates the Fock matrix element and [ε(σHH*) – ε(nZB)] is the orbital energy difference. If the σ-electrons of H2 act as the Lewis base and the pnictogen atom (N, P or As) is the Lewis acid center, then the σHH → σZX(H)* interaction may be considered as the most important one connected with the electron charge transfer (Eq. 2), in such case the complex is linked through the pnictogen bond. ∆E (σHH → σZX(H)*) = –2 〈σHHFσZX(H)*〉2/[ε(σZX(H)*) – ε(σHH)]

Eq. 2

This is in line with previous studies28 where the n(P lone pair) → σPC* donoracceptor interaction was proposed for P···P contacts existing in the crystal structure and also with the very recent study where for P···N, P···P and N···N pnictogen bonds, nZ → σZH(F)* interactions were found.26 The NBO5.0 program,42 incorporated in the Gamess set of codes,43 was used to calculate the energies mentioned above as well as the atomic charges. Density Functional Theory – Symmetry Adapted Perturbation Theory (DFT-SAPT) calculations were carried out to consider the energy of interaction as a sum of the following terms: electrostatic (Eel), exchange (Eex), induction (Eind), dispersion (Edisp) energies and the δ(HF) which is the Hartree-Fock correction that includes higher order induction and exchange corrections not included in the other terms. The DFT-SAPT calculations44 were carried out using PBE0/aug-cc-pVTZ computational level.45 Asymptotic correction was included in the calculations using the experimental ionization potentials of the molecules when available46 while in the rest of the cases it has been calculated at MP2/aug-cc-pVTZ computational level. All of these calculations have been performed 5 Environment ACS Paragon Plus


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using the MOLPRO program.47 MP2/aug-cc-pVTZ absolute chemical shieldings have been calculated within the GIAO approximation.48

Results and Discussion This section has been divided in five parts. In the first one, the properties of the isolated pnictogen derivatives have been discussed. In the second one the geometry, energies and AIM analysis of the complexes have been treated. The third part considers the NBO analysis. The fourth and fifth parts analyze the DFT-SAPT and chemical shielding results.

Isolated pnictogen derivatives The pnictogen derivatives are presented in Scheme 1. They are ordered based on the atomic number of the pnictogen atom involved in the interaction (N, P or As) and the electronegativity of the selected derivatives (X, electronegativity increases from CH3 to F). The calculated and experimental geometries of these compounds have been gathered in Table 1.

Scheme 1. The pnictogen derivatives analyzed in this study.


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The molecular electrostatic potentials (MEP) on 0.001 au electron density isosurfaces (as suggested by Bader et al.49) have been characterized for the molecules considered here (Table 2 and Fig. 1). In general, the MEP around the pnictogen atoms in the molecules studied presents a negative region (VS,min) associated to the lone pair and up to three positive regions (VS,max) associated to the σ-hole30 of the X group and H atoms (Table 2). In the NH3 and NH2F molecules only the presence of the lone pair minima is detected since the positively charged hydrogen atoms mask the σ holes´ positive regions. In the PH3 and AsH3, the negative MEP region along the 3-fold axis of symmetry of the molecule is accompanied by three maxima associated to the σ-hole of the hydrogen atoms. The electronegativity of the substituent clearly affect, increasing the positive values of the σ-hole of the X group and H atom, as well as reducing in absolute value those of the lone pairs. A special case is the PH2CN molecule that can present resonance forms (Scheme 2) that can affect the general trends observed.

Scheme 2. Resonance forms of the PH2CN molecule. The dihydrogen molecule is characterized by the negative electrostatic potential corresponding to σ-electrons (VS,min = –0.005 au) and two positive regions potentials (VS,max = 0.018 au) symmetrical situated at the ends of the H2 molecule which correspond to the H-atoms' positions. The values of the MEP for H2 molecule may suggest that in general it is much weaker Lewis acid and Lewis base than the pnictogen species considered here, and additionally that this molecule is characterized by much stronger acidic properties than the basic ones.

Geometries, energies and AIM analysis Based on the MEP characteristics of the pnictogen derivatives, for each molecule, three complexes with H2 have been considered, two in which the H2 molecule is located in 7 Environment ACS Paragon Plus


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the extension of the X-Z and H-Z bonds in the proximity of Z, X-Z···H2 and H-Z···H2 (A and B in Scheme 3, respectively). For these cases the pnictogen atom could act as the Lewis acid centre while the H2 molecule acts as the Lewis base. In addition, the H2 molecule has been located in the region where the lone pair of Z is approximately situated (C in Scheme 3). In this configuration, Z is the potential Lewis base center and the dihydrogen the Lewis acid one.

Scheme 3. Three configurations of ZH3(ZH2X)…H2 complexes. The interaction energy, and interatomic distance of the complexes analyzed here are gathered in Table 3 and the molecular graph of the complexes in Table 4. Complexes in configuration A and B are found to be minima for all the compounds, with the exception of NH3 where it reverts towards configuration C. These complexes show Cs and C1 symmetries for all the configurations A and B, respectively. Three different arrangements are found for the complexes in configuration C. In the case of the parent compounds (ZH3), the H2 molecule aligns with the C3 symmetry axes interacting with the lone pair of ZH3. In the fluorinated and methylphosphine derivatives, the H2 molecules tend to move towards the X group. Even for the C configuration of AsH2F:H2 there is the F···H bond path indicating that the fluorine atom is the Lewis base center and not the lone pair of arsenic. Finally in the PH2Cl and PH2CN molecules no minimum in such configuration was found. The situation is much more complex for B configurations (see Table 4). Initially such configurations were considered as model interactions between the σ-electrons of


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dihydrogen and the positive electrostatic potential situated roughly along the extension of the H-Z bond. However, because of the existence of the competitive positive Hatoms' centers and in some cases the strong Lewis base centers of the X groups (for example, the F-substituents) the optimizations led to the other links which are rather not attributed to pnictogen bonds. In fact, the AIM analysis of the complexes in configuration B (Table 4) shows intermolecular bond paths between the hydrogen atom of H2 and atoms of the X groups but never between with the Z atoms, with the exception of the PH3:H2 and AsH3:H2 where configurations A and B are the same. The strongest intermolecular interactions correspond to the PH2F:H2 (A) and AsH2F:H2 (A) complexes, being the binding energies equal to 5.1 and 6.8 kJ mol–1, respectively. Configuration A is the most stable in six of the cases. Exceptions are the NH3:H2 complex where only configuration C is found to be a minimum, PH2CN:H2 where configuration B is more stable that A and PH2CH3:H2 where configuration C is more stable than A and B. In all cases the configuration A shows the shortest Z···H interatomic distance of the three configurations considered. The characteristics of BCPs corresponding to all bond paths of the studied systems are summarized in Supplementary Information (SI). The bond paths linking the pnictogen atoms and the H2 ones, present values of the ρBCP in the range between 0.0097 and 0.0029 au, which corresponds to interactions slightly stronger than the vdW ones (0.002 au). The values of the Laplacian at the BCP are small and positive (between 0.031 and 0.008 au) as indication that the complexes are within the regime of closed shell interactions. Exponential relationships are found for the ρBCP and Laplacian vs. the interatomic distances between the P and H atoms with square correlations coefficients of 0.95 in both cases, in agreement with previous reports that indicate that this is a general feature of weak interactions.50 The AIM results show that the complexes in A configuration are linked through δ

δ

the pnictogen bond, the configurations C through the Z– ···+ H-H–δ hydrogen bond and for the B configurations the pnictogen compound acts as the Lewis base or as the Lewis acid. The reason why the pnictogen bond exists for the A and not for the B configurations is connected with the fact that the electrostatic positive potential is greater along the extension of X-Z bond than along the extension of the H-Z bond if X is the electronegative substituent.30 The electron density at the BCP corresponding to the intermolecular link is often treated as a descriptor of the strength of the interaction;



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especially for the hydrogen bonded systems such relation was often observed.51 A linear correlation is found between the electron density at Z···H BCP (see SI) and the binding energy (Table 3) with correlation coefficient of 0.98 and 0.96 for the complexes in A configuration and for complexes in configuration C, respectively.

NBO analysis It was stated in previous studies that the pnictogen bond is characterized by the n → σZH(X)

*

interaction connected with the electron charge transfer from the Lewis base to the

Lewis acid (pnictogen derivative).26,27,28 On the other hand, if the lone pair of the pnictogen compound plays the role of the Lewis base, then the nZ → σ* interaction should be the most important one. This is connected with the redistribution of the electron charge as a result of the complexation; the σ* antibonding orbital concerns here to any bond of the Lewis acid interacting with the Z center. It was pointed out in the previous section that the nZ → σHH* interaction (Eq. 2) may exist for the C configurations and the σHH → σZX(H)* interaction if the pnictogen bond exists, i.e. for the A interactions. The AIM results exclude B-configurations as potentially linked through the pnictogen bond. It seems that the charge transfer interactions in the complexes analyzed could clarify the picture. These interactions are collected in Table 5. For all the complexes in A configurations, the nH2 →σZX* interaction exists which corresponds to the presence of a positive electrostatic potential region along the extension of the Z-X bond in the ZH2X molecules. This is in line with the AIM findings presented earlier where a bond path linking the Z atom with the H2 is observed for all the complexes in A configurations as indication of the existence of the pnictogen bond. In the three stronger complexes in configuration A (PH2F:H2, PH2Cl:H2 and AsH2F:H2) the nH2 →σZX* interactions are the most important ones with differences larger than 2 kJ mol–1 than other interactions. In contrast, the weakest complexes in this configuration show that the nZ →σH2* charge transfer is more important than the nH2 →σZX* one. The dominant charge transfer stabilization interaction in the complexes in configuration B corresponds to the nX →σH2* in agreement with the intermolecular bond paths observed in the AIM analysis. The situation is quite clear for the C configurations; all interactions are connected with the Lewis acid properties of the dihydrogen. And for all C complexes the nZ →


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σHH* interaction is more important than the nX → σHH*, when latter one is observed. The interactions where the H2 molecule acts as the Lewis base are not observed for C configurations, with the exception of the AsH2F:H2 complex. In this complex, the most important intermolecular charge transfer corresponds to the nF → σH2* interaction which amounts 6.2 kJ mol–1, while the nAs → σHH* corresponds to an stabilization of 1.0 kJ mol–1. In addition, a weak interaction between nH2 →σAsF* that stabilized by 0.5 kJ mol–1 is detected. However, it was pointed out previously that for this complex a H···F bond path exists (see Table 4) and it was not possible to find a minimum energy configuration where the H2 molecule remains attached to the As-center. Linear correlations are found between the total charge transfer energies in each complex vs. its corresponding binding energies when the complexes are separated in the three configurations, A, B and C (Fig. 2). It is worth mentioning that the non-covalent interaction is usually associated with the electron charge transfer from the Lewis base sub-unit of the complex to the Lewis acid sub-unit.52 Hence, the charge of H2 molecule in the complex should be the decisive indicator if the dihydrogen acts as the Lewis base or as the Lewis acid. However, the interactions analyzed here are not strong and the changes associated with the complexation are often negligible. The NBO and AIM integrated charges of H2 molecule in complexes are presented in the Supplementary Information. The range of charge for dihydrogen, for the sample of complexes analyzed here, is from –0.006 to +0.005 au for the NBO charges while for the AIM charges it is from –0.007 to +0.004 au. Thus, there are only slight changes of the H2 molecule charge as an effect of the complexation. However, for both approaches, NBO and AIM, there are positive charges of H2 for the A configurations with the strongest interactions (bolded in Table 5). It means that H2 acts here as the Lewis base. For example, for the AsH2F:H2 complex the NBO charge of dihydrogen is equal to 0.005 au while the AIM charge amounts 0.001 au.

DFT-SAPT analysis The interaction energy terms calculated with the DFT-SAPT methodology have been gathered in Table 6. The most important term corresponds to the repulsive exchange term, Eex, except in the PH2CH3:H2 (B) complex where the dispersion term, Ed, shows a larger absolute value. Regarding the attractive terms, the most important one



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corresponds to the dispersion, Ed, which accounts for 30-42% of all the sum of the attractive interaction term. The only exception is the NH3:H2 (C) complex where the electrostatic term is larger in absolute value. The second most important term is the electrostatic one which corresponds to 7-34% of all the attractive terms. Finally, the least important term is the induction which accounts only for 1-6% of the attractive terms. The first order Heitler-London interaction energy, which corresponds to the sum of the electrostatic interactions (Eel) and exchange repulsion (Eex), is positive. This means that the exchange repulsion outweighs the electrostatic attraction and the stabilization of the complexes came from higher order attractive interactions. In this case, the most important second order term is the dispersion. One should also note that the low values of the induction energy correspond to the small charges of dihydrogen in the complexes. These results contrast with those of strong hydrogen bonds where the induction energy term is the most important connected with the redistribution of the electron charge upon complexation.53 It seems that the DFT-SAPT results (Table 6) are not in agreement with the NBO results (Table 3) where interactions associated with the electron charge transfer are more noticeable than the induction energy. However, it is worth mentioning that the meaning of the DFT-SAPT interaction energy terms is different than it is for the NBO method. Besides it is well known that NBO-like methods provide values of the charge transfer energy as large as 60%.54

NMR nuclear shieldings Table 7 presents variation of the

15

N,

31

P and

33

As chemical shielding upon

complexation. The largest changes correspond to the most strongly bound complexes, i.e. for PH2F:H2 and AsH2F:H2 complexes such changes amount 11.95 and 35.53 ppm, respectively what corresponds to the binding energies of 5.1 and 6.8 kJ mol–1. However the relationship between the change in chemical shielding upon complexation and the binding energy was not found for the collection of complexes considered here. The reason is that all changes due to the complexation are rather negligible; the redistribution of the electron charge is not significant and this is reflected in the low value of the induction energy and also with the slight changes in chemical shieldings. In the case of two complexes mentioned earlier, PH2F:H2 and AsH2F:H2, the AIM and


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NBO results show that there is no doubt that they are linked through the pnictogen bond.

Conclusions The complexes between dihydrogen, H2, and a series of amine, phosphine and arsine derivatives have been analyzed on the basis of ab initio results, and other tools such as Atoms in Molecules (AIM) theory, Natural Bond Orbitals (NBO) method as well as DFT-SAPT approach and the analysis of the variation of the chemical shielding upon complexation. The results show that the molecular hydrogen may act as the Lewis acid and as the Lewis base. This is related with its molecular electrostatic potential which is positive in the regions neighboring with the positions of H-atoms and negative in the region of the σ H-H bond. The pnictogen Z atoms in the ZH3 compounds and in their derivatives ZH2X are characterized by the regions of positive and negative electrostatic potential. Therefore, various intermolecular Lewis acid – Lewis base interactions exist for the complexes analyzed here. It was found that for the A configurations of complexes, the pnictogen bond exist, especially there are evidences of such an interaction for the most strongly bound complexes of PH2F:H2, PH2Cl:H2 and AsH2F:H2. For the C configurations, the Z atom of ZH3 and ZH2X acts as the Lewis base and in the corresponding complexes the dihydrogen plays the role of the Lewis acid. The pnictogen bond is a new kind of interaction, which is detectable for complexes of dihydrogen: previously this interaction was not considered as responsible for the stability of complexes with molecular hydrogen.

Acknowledgments We thank the Ministerio de Ciencia e Innovación (Project No. CTQ2009-13129-C02-02 and CTQ2012-35513-C02-02), and the Comunidad Autónoma de Madrid (Project MADRISOLAR2, ref. S2009/PPQ-1533) for continuing support. Thanks are given to the CESGA and CTI (CSIC) for an allocation of computer time. Financial support comes from Eusko Jaurlaritza (GIC 07/85 IT-330-07) and the Spanish Office for Scientific Research (CTQ2011-27374) – (SJG). Technical and human support provided by Informatikako Zerbitzu Orokora - Servicio General de Informática de la Universidad del Pais Vasco (SGI/IZO-SGIker UPV/EHU), Ministerio de Ciencia e Innovación (MICINN), Gobierno Vasco Eusko Jaurlanitza (GV/EJ), European Social Fund (ESF) is 13 Environment ACS Paragon Plus


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gratefully acknowledged (SJG).

Supporting Information BSSE corrected interaction energies, electron density properties at the BCPs, NBO and AIM charges of all the complexes studied in this article. This material is available free of charge via the Internet at http://pubs.acs.org.


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Table 1. Calculated geometries (Å, º) of the pnictogen derivatives at MP2/aug-cc-pVTZ computational level. The experimental values are given when available in parenthesis.

NH2F

Z-F

Z-H

H-Z-H

H-Z-X

1.424 (1.4329)55

1.018 (1.0225) 1.012 (1.012)56 1.416 (1.415) 1.413 (1.411) 1.411 (1.424) 1.412 (1.421)55 1.413 (1.423) 1.506 1.499 (1.511)55

105.1 (101.08)

97.7 (97.8) 96.6 (96.4) 94.7 (95.7)

101.4 (106.27) 106.8 (106.67) 92.2 (92.0) 92.6 (92.8) 94.2 (93.9)

97.7

(93.3) 93.5

NH3 PH2F PH2Cl PH2CN

1.622 (1.602)57 2.075 (2.063)55 1.790 (1.787)58

PH3 PH2CH3 AsH2F AsH3

1.858 (1.858)55 1.751

95.5


91.4 92.5 (92.083)

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Table 2. Molecular electrostatic potential (au) stationary points on the 0.001 au electron density isosurface around the pnictogen atoms calculated at the MP2/aug-cc-pVTZ computational level.

VS,max X σ hole

VS,max H σ hole

PH2F PH2Cl PH2CN PH3 PH2CH3

0.060 0.053 0.058

0.029 0.025 0.042 0.020 0.019

–0.019 –0.015 –0.003 –0.025 –0.034

AsH2F AsH3

0.069

0.032 0.022

–0.009 –0.021

NH2F NH3

0.013

VS,min Lone pair –0.045 –0.059



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Table 3. Energetic (Ebin – binding energy, in kJ mol–1) and geometric results (the shortest H···Z distance is presented, in Å) for the complexes calculated at the MP2/augcc-pVTZ computational level.

Complex NH2F:H2 NH2F:H2 NH2F:H2

Interaction type A B C

Ebin (kJ mol–1) –3.43 –3.41 –2.77

Shortest Z···H distance 2.652 3.038 2.786

NH3:H2

C

–3.05

2.829

PH2F:H2 PH2F:H2 PH2F:H2

A B C

–5.07 –2.65 –1.51

2.805 3.293 3.339

PH2Cl:H2 PH2Cl:H2

A B

–3.78 –3.14

2.975 3.312

PH2CN:H2 PH2CN:H2

A B

–2.92 –3.67

3.154 3.347

PH3:H2 PH3:H2

A C

–1.85 –1.62

3.244 3.462

PH2CH3:H2 PH2CH3:H2 PH2CH3:H2

A B C

–2.14 –2.03 –2.76

3.138 3.586 3.225

AsH2F:H2 AsH2F:H2 AsH2F:H2

A B C

–6.81 –4.34 –3.60

2.814 3.147 3.307

AsH3:H2 AsH3:H2

A C

–2.78 –2.34

3.048 3.331


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Table 4. Molecular graphs of the complexes considered here, big circles correspond to attractors, small ones to the bond critical points, the bond paths (solid and broken lines are also presented. A

B

NH2F:H2

NH3:H2

PH2F:H2

PH2Cl:H2


C


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PH2CN:H2

PH3:H2

PH2CH3:H2

AsH2F:H2

AsH3:H2


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Table 5. Orbitalic charge transfer energies (kJ mol–1) obtained using the NBO method, with values above 0.5 kJ mol–1.

Configuration

Interaction type H2 as the Lewis base H2 as the Lewis acid nH2 →σZX* nH2 →σZH* nZ→σH2* nX→σH2* 1.7 2.3 0.8 3.1 5.0 0.6

Complex NH2F:H2 NH2F:H2 NH2F:H2

A B C

NH3:H2

C


A B C

6.2

PH2Cl:H2 PH2Cl:H2

A B

4.0

PH2CN:H2 PH2CN:H2

A B

2.3 0.6

PH3:H2 PH3:H2

A C

0.9

1.1 3.2


A B C

1.1

2.3


A B C

9.6 0.8 0.5

AsH3:H2 AsH3:H2

A C

1.5

7.8 2.6 2.9 2.8 1.3 3.2 0.8 0.6

1.0 3.0 1.9 0.5


1.0 1.9 4.6

5.2 6.2


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Page 26 of 31

Table 6. Interaction energy terms (kJ mol–1) calculated with the DFT-SAPT methodology. Complex NH2F:H2 NH2F:H2 NH2F:H2

A B C

Eel –3.63 –3.88 –2.71

Eex 5.89 6.30 4.32

Ei –0.49 –0.62 –0.40

Ed –4.18 –4.22 –3.16

δHF –0.54 –0.46 –0.34

Total E –2.94 –2.89 –2.28

NH3:H2

C

–4.20

5.73

–0.82

–3.00

–0.57

–2.85


A B C

–5.88 –2.29 –0.82

12.31 4.44 2.46

–1.26 –0.20 –0.11

–7.51 –3.62 –2.37

–1.57 –0.23 –0.29

–3.92 –1.90 –1.13

PH2Cl:H2 PH2Cl:H2

A B

–3.73 –2.35

8.02 5.40

–0.69 –0.26

–5.72 –4.66

–0.92 –0.41

–3.04 –2.29

PH2CN:H2 PH2CN:H2

A B

–2.33 –2.89

4.78 5.68

–0.46 –0.44

–4.04 –4.88

–0.41 –0.44

–2.46 –2.97

PH3:H2 PH3:H2

A C

–1.28 –1.51

3.17 2.60

–0.13 –0.16

–3.04 –2.09

–0.27 –0.34

–1.56 –1.51


A B C

–1.58 –0.96 –2.27

3.85 3.58 4.82

–0.18 –0.13 –0.23

–3.51 –3.89 –4.27

–0.45 –0.29 –0.53

–1.87 –1.68 –2.48


A B C

–7.51 –3.71 –3.10

15.80 7.52 6.29

–1.93 –0.45 –0.32

–8.62 –4.96 –4.10

–1.89 –0.44 –0.40

–4.15 –2.04 –1.63

AsH3:H2 AsH3:H2

A C

–0.69 –1.99

4.86 4.37

–0.29 –0.21

–4.08 –2.89

–0.47 –0.64

–0.67 –1.36


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Table 7. Variation of the 15N, 31P and 33As chemical shielding (ppm) upon complexation

A –0.80

B 0.08

C 0.80 –1.68

PH2F:H2 PH2Cl:H2 PH2CN:H2 PH3:H2 PH2CH3:H2

11.95 0.09 –3.48 –4.46 –4.22

0.01 –1.62 –2.99

1.17

AsH2F:H2 AsH3:H2

35.53 –8.20

0.11

NH2F:H2 NH3:H2

2.42


–0.92 1.44 2.80 –1.33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure caption

Fig. 1 Calculated electrostatic potential on the 0.001 au electron density isosurface of (a) AsH3, (b) AsH2F and (c) H2. Color ranges from red (negative potential) to blue (positive potential). Fig. 2. Binding Energy (kJ mol–1) vs. the intermolecular charge transfer energy (kJ mol –1

). Two outliers (PH2CN:H2, conf. B and PH2CH3:H2, conf. C) have not been included

in the figure and the correlations.


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(a)

(b)

(c)

Fig. 1 Grabowski, Alkorta, Elguero



Conf. A Conf. B Conf. C

12

E charge transfer

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Page 30 of 31

8

4

0 -8

-6

-4

Binding Energy

Fig. 2 Grabowski, Alkorta, Elguero


-2

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TOC


Complexes between Dihydrogen and Amine, Phosphine, and Arsine

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