PÂ·Â·Â·N Pnicogen Bonds in Cationic Complexes of F4P+ and F3HP+ with

Article pubs.acs.org/JPCA

P···N Pnicogen Bonds in Cationic Complexes of F4P+ and F3HP+ with Nitrogen Bases Janet E. Del Bene,*,‡ Ibon Alkorta,*,§ and José Elguero§ ‡

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

§

S Supporting Information *

ABSTRACT: Ab initio MP2/aug’-cc-pVTZ calculations have been carried out on cationic pnicogen-bonded complexes F4P+:N-base and F3HP+:N-base, with linear Fax−P···N and Hax-P···N, respectively. The bases include the sp3-hybridized nitrogen bases NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3, and the sp bases NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2. The binding energies of these complexes span a wide range, from −15 to −180 kJ mol−1, as do the P−N distances, which vary from 1.89 to 3.11 Å. There is a gap in the P−N distances between 2.25 and 2.53 Å in which no complexes are found. Thus, the equilibrium complexes may be classified as inner or outer complexes based on the value of the P−N distance. Inner complexes have P···N bonds with varying degrees of covalent character, whereas outer complexes are stabilized by intermolecular P···N bonds with little or no covalency. Charge-transfer stabilizes these pnicogenbonded complexes. For complexes F4P+:N-base, the dominant charge-transfer interaction is from the lone pair on N to the σ*P− Fax orbital. In addition, there are three other charge-transfer interactions from the lone pair on N to the σ*P−Feq orbitals, which taken together, are more stabilizing than the interaction involving σ*P−Fax. In contrast, the dominant charge-transfer interaction for complexes F3HP+:N-base is from the lone pair on N to the σ*P−Feq orbitals. Computed EOM-CCSD Fermi-contact terms are excellent approximations to the total spin−spin coupling constants 1pJ(P−N) and 1J(P−Hax), but are poor approximations to 1 J(P−Fax). 1pJ(P−N) values increase with decreasing P−N distance, approach a maximum, and then decrease and change sign as the P−N distance further decreases and the pnicogen bond acquires increased covalency. 1J(P−Fax) values for F4P+:N-base complexes increase with decreasing distance. Although the P−Hax distance changes very little in complexes F3HP+:N-base, patterns exist which suggest that changes in 1J(P−Hax) reflect the hybridization of the nitrogen base and whether the complex is an inner or outer complex.

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The F4P + ion has been investigated theoretically by Reed and Schleyer.7 ICR3 and IR6 studies of trifluorophosphonium cation have also been reported. This ion was prepared as F3HP+·SbF6−·HF by protonation of F3P with HF/F5Sb in anhydrous HF. The structure and stability of this ion have been investigated theoretically using ab initio methods.8 The X-ray crystal structure and Raman spectrum have been measured and interpreted with the aid of ab initio MP2 and CCD calculations.9,10 The NMR spectra of both F4P+ and F3HP+ have been reported by Minkwitz and Liedke.11 The experimental values for F4P+:Sb3F16− (in SO2ClF at 195 K), are 31P = −33.5 ppm; 1 J(P−F) = 1203.2 Hz. Those for F3HP+:Sb2F11− (in SO2 at 205 K) are 1H 9.68, 19F −73.3, 31P 21.2 ppm; 1J(P−H) = 1187.7 and 1J(P−F) = 1279.8, 2J(F−H) = 76.7 Hz. Kaupp et al.12 have calculated the 31P chemical shifts of F4P+. Our interest in complexes of the cations F4P+ and F3HP+ follows from our recent investigation of pnicogen bonding in

INTRODUCTION This paper examines P···N pnicogen bonds formed by two important but not widely investigated phosphonium cations: perfluorophosphonium F4P+ and trifluorophosphonium F3HP+. The first one is better known, and is related to phosphorus pentafluoride, F5P, by the reaction F4 P+ + F− → F5P

In fact, some authors suggest that F5P should be written as P+(F5)−, or (F4P)+F−.1 This cation has been studied in the gas phase by ion cyclotron resonance spectroscopy (ICR)2 in the reaction

and has also been characterized by Raman spectroscopy.2−4 In addition, complexes of F4P+ with N2, 2,2′-bipyridine, and 1,10phenanthroline have been characterized by 19F NMR,5 and the vibrational spectrum of a complex of this cation with the anion hexadecafluorotriantimonate has been reported by Minkwitz.6 © XXXX American Chemical Society

Received: January 29, 2015 Revised: March 3, 2015

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DOI: 10.1021/acs.jpca.5b00944 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A anionic complexes H2YP:X−, for X,Y = Cl, NC (bonded at both C and N), F, CCH, and CH3.13 To our knowledge, the only previous theoretical study of pnicogen-bonding interactions involving phosphorus cations was reported by Grabowski, who investigated complexes of F4P+, F3HP+, and H4P+ with the nitrogen bases NCH and NCLi.13 We have now carried out a systematic investigation of P···N pnicogen bonds in complexes formed between F4P+ and F3HP+ and 18 different nitrogen bases. The structures, binding energies, bonding properties, and charge-transfer energies of these complexes have been determined. In addition, spin−spin coupling constants 1pJ(P− N) for all complexes, 1J(P−Fax) for complexes with F4P+, and 1 J(P−Hax) for complexes with F3HP+ have been computed, with Fax and Hax the F and H atoms which occupy axial positions in the complexes. It is the purpose of this paper to report the results of this study.

ACES II36 on the IBM Cluster 1350 (Glenn) at the Ohio Supercomputer Center.

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RESULTS AND DISCUSSION The optimized structures of the isolated F4P+ and F3HP+ monomers have Td and C3v symmetry, respectively. The calculated P−F distances are 1.492 and 1.502 Å, respectively, and the P−H distance in F3HP+ is 1.383 Å. The molecular electrostatic potentials (MEPs) on the 0.001 au electron density isosurfaces exhibit four identical σ-holes on the F4P+ surface, and 3 identical σ-holes and a fourth σ-hole on the F3HP+ surface, as illustrated in Figure 1. The σ-holes have

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METHODS The structures of the isolated monomers and the complexes F4P+:N-base and F3HP+:N-base for the sp3-hybridized bases NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3, and the sp bases NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2 were optimized at second-order Møller−Plesset perturbation theory (MP2)15−18 with the aug′cc-pVTZ basis set.19 This basis set is derived from the Dunning aug-cc-pVTZ basis set20,21 by removing diffuse functions from hydrogen atoms. Frequencies were computed to establish that the optimized structures correspond to equilibrium structures on their potential surfaces. The binding energies of these complexes were computed as the difference between the total energy of the complex and the sum of the energies of the optimized monomers. All calculations were performed using the Gaussian 09 program.22 The electron densities of the complexes have been analyzed using the atoms in molecules (AIM) methodology23−26 employing the AIMAll27 program. The topological analysis of the electron density produces the molecular graph of each complex. This graph identifies the location of electron density features of interest, including the electron density (ρ) maxima associated with the various nuclei, saddle points which correspond to bond critical points (BCPs), and ring critical points which indicate a minimum electron density within a ring. The zero gradient line which connects a BCP with two nuclei is the bond path. The electron density at the BCP (ρBCP), the Laplacian of the electron density at the BCP (∇2ρBCP), and the total energy density at the BCP (HBCP) are additional useful quantities for characterizing interactions.28 In addition, the natural bond order (NBO)29 method has been used to analyze the stabilizing charge-transfer interactions using the NBO-6 program.30 Since MP2 orbitals are nonexistent, the chargetransfer interactions have been computed using the B3LYP functional31,32 with the aug′-cc-pVTZ basis set at the MP2/ aug′-cc-pVTZ complex geometries, so that at least some electron correlations effects could be included. Spin−spin coupling constants were evaluated using the equation-of-motion coupled cluster singles and doubles (EOMCCSD) method in the CI(configuration interaction)-like approximation,33,34 with all electrons correlated. For these calculations, the Ahlrichs35 qzp basis set was placed on 13C, 15 N, 17O, and 19F, and the qz2p basis set on 31P, 35Cl, and the 1 H atom which occupies the axial position in complexes with F3HP+. The EOM-CCSD calculations were performed using

Figure 1. MEPs on the 0.001 au electron density isosurfaces of F4P+ and F3HP+. The color code ranges between red (0.18 au) and blue (0.30 au). The locations of the σ-holes are indicated with black dots and their values are given in au.

values of 0.302 au on the F4P+ surface, and 0.306 and 0.281 au on the F3HP+ surface. On the latter surface, the three larger σholes at P are associated with extensions of the P−F bonds, while the 0.281 au hole is found on an extension of the P−H bond. Structures and Binding Energies of Complexes. The complexes F4P+:N-base and F3HP+:N-base have either C3v or Cs symmetry, with Fax−P···N and Hax−P···N, respectively, linear or approaching linearity. The structures, total energies, and molecular graphs of these complexes are reported in Table S1 of the Supporting Information, and the complexes F4P+:NCH and F3HP+:NF3 are illustrated in Figure 2. The binding

Figure 2. Complexes F4P+:NCH and F3HP+:NF3, illustrating the linear alignment of the axial F and H atoms with the P···N bond, designated Fax−P···N and Hax−P···N, respectively.

energies, intermolecular P−N distances, axial P−Xax distances, and Xax−P−N angles for complexes F4P+:N-base and F3HP+:Nbase are given in Tables 1 and 2, respectively, with Xax equal to Fax in Table 1 and Hax in Table 2. From these tables it can be seen that the P−N distances span an unusually large range, from 1.886 to 3.009 Å in complexes of F4P+ with NCNH2 and N2, respectively, and from 1.905 to 3.107 Å in complexes of these same bases with F3HP+. As expected, there is also a large variation in binding energies, from −19.7 to −180.7 kJ mol−1 for complexes with F4P+, and from −15.0 to −152.4 kJ mol−1 for complexes with F3HP+. For a fixed base, the binding energy B


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The Journal of Physical Chemistry A Table 1. Binding Energies (ΔE, kJ mol−1), P−N and Axial P−Fax Distances (R, Å), and Fax−P−Feq angles (∠, deg) for Complexes F4P+:N-Basea sp3 bases

ΔE

R(P−N)

R(P−Fax)b

∠Fax−P−Feqb

NH3 NClH2 NFH2 NCl2H NCl3 NFCl2 NF2H NF2Cl NF3 sp bases

−180.7 −141.5 −122.7 −109.2 −87.1 −68.1 −61.1 −40.6 −19.7 ΔE

1.914 1.950 1.970 2.015 2.106 2.136 2.098 2.245 2.913 R(P−N)

1.541 1.539 1.533 1.535 1.531 1.526 1.524 1.517 1.496 R(P−Fax)b

95 95 96 96 97 98 99 101 108 ∠Fax−P−Feqb

NCNH2 NCCH3 NP NCOH NCCl NCH NCF NCCN N2

−165.6 −147.8 −142.2 −133.1 −112.2 −99.1 −85.0 −58.7 −24.3

1.886 1.924 1.917 1.938 1.965 2.044 2.108 2.637 3.009

1.545 1.540 1.541 1.539 1.536 1.530 1.526 1.503 1.495

95 96 95 96 96 98 99 106 108

no complexes are found. This gap falls between P−N distances of 2.25 and 2.53 Å. Complexes with P−N distances shorter than 2.25 Å will be referred to as inner complexes, and those with P−N distances longer than 2.53 Å as outer complexes. This gap and the energy variation of these complexes as a function of the P−N distance are illustrated in Figure 3. The correlation coefficients R2 for the trendlines in Figure 3 are 0.912 and 0.858 for the inner and outer complexes, respectively. The trendlines show that these energies as a function of distance follow two different curves for inner and outer complexes. It should also be noted in Figure 3 that some outer complexes with sp3-hybridized N-bases have greater binding energies at longer P−N distances than some of the inner complexes with sp-hybridized bases at shorter P−N distances. It is also possible to distinguish inner and outer complexes based on the Aax−P-Feq angles, with Aax equal to Fax for complexes with F4P+, and Hax for complexes with F3HP+. The data of Tables 1 and 2 indicate that the inner complexes have Aax−P−Feq angles less than 100°, except for F4P+:NF2Cl which has a value of 101°. In contrast, values for the outer complexes are near the tetrahedral angle, varying from 105 to 110°. The smaller values of this angle for the inner complexes tend to reduce repulsive interactions between nitrogen and the equatorial F atoms. NBO Results. It is well-known that charge-transfer plays an important role in the stabilization of pnicogen-bonded complexes, and such is certainly the case for these cationic complexes. The total amount of electron transfer and the charge-transfer energies are reported in Table 3 for the complexes F4P+:N-base. However, all of the inner complexes of F4P+ except for those with NF2Cl and NFCl2 are treated by the NBO program as single molecules, so no charge-transfer energies are reported in Table 3. For the two complexes with NF2Cl and NFCl2 and the outer complexes with NF3, NCCN, and N2, charge transfer from the N lone pair to the antibonding P−F ax orbital is the largest individual charge-transfer interaction. The charge-transfer energies for the two inner complexes are much greater than the charge-transfer energies from the same N lone pair to the antibonding N−Feq orbital. However, since there are three N−Feq antibonding orbitals, the total charge-transfer Nlp → σ*P−Feq is greater than Nlp → σ*P−Fax. Figure 4 depicts the orbitals on the 0.001 au isosurface which are involved in the charge-transfer interactions Nlp → σ*P−Fax and Nlp → σ*P−Feq in the F4P+:NCCN complex. Table 4 reports the total amount of electron transfer and the charge-transfer energies for complexes with F3HP+. Chargetransfer energies have been computed for the three inner complexes involving the nitrogen bases NCl3, NF2H, and NFCl2. In contrast to the complexes with F4P+, the larger single charge-transfer energy in all complexes with F3HP+ is the Nlp → σ*P−Feq interaction rather than Nlp → σ*P−Hax There is an excellent exponential correlation between the Nlp → σ*P−Xax charge-transfer energy and the P−N distance, as illustrated in Figure 5. The trendlines have correlation coefficients of 0.995 and 0.988 for complexes with F4P+ and F3HP+, respectively. AIM Results. The electron densities at bond critical points (ρBCP), the Laplacians of the electron densities (∇2ρBCP), and the total energy densities (HBCP) are reported in Table S2 of the Supporting Information. Values of ρBCP are significantly greater for the inner complexes relative to the outer. These densities are exponentially related to the P−N distance, with

a

Bases that form outer complexes are written in italics. bThe P−Fax distance is 1.492 Å and the Fax−P−Feq angle is 109.5° in F4P+.

Table 2. Binding energies (ΔE, kJ mol−1), P−N and Axial P−Hax Distances (R, Å), and Hax−P−Feq angles (∠, deg) for complexes F3HP+:N-basea sp3 bases

ΔE

R(P−N)

R(P−Hax)b

∠Hax−P−Feqb


−152.4 −113.9 −97.3 −82.2 −61.3 −45.3 −39.9 −31.0 −15.0 ΔE

1.923 1.967 1.987 2.042 2.172 2.249 2.177 2.872 3.050 R(P−N)

1.383 1.382 1.382 1.382 1.381 1.381 1.380 1.382 1.382 R(P−Hax)b

95 96 97 97 99 101 101 108 109 ∠Hax−P−Feqb


−133.4 −117.5 −112.3 −103.5 −81.6 −78.0 −70.2 −48.7 −20.1

1.905 1.956 1.939 1.981 2.536 2.657 2.723 2.824 3.107

1.383 1.382 1.382 1.382 1.380 1.379 1.381 1.381 1.382

96 97 96 97 105 106 107 108 110

a

Bases that form outer complexes are written in italics. bThe P−Hax distance is 1.383 Å and the Hax−P−Feq angle is 110.5° in F3HP+.

with F4P+ and F−P···N linear is always greater than the binding energy with F3HP+ with H−F···N linear. This is not surprising since it has been observed previously that complexes in which F−P···P or F−P···N approach linearity are more stable than corresponding complexes with H−P···P or H−P···N linear.37−39 Although there is a large range of P−N distances in the cationic complexes, a closer examination of Tables 1 and 2 indicates that there is also a gap in the P−N distances in which C


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The Journal of Physical Chemistry A

Figure 3. Binding energies versus the P−N distance for inner and outer complexes of F4P+ and F3HP+.

Table 3. Total Charge Transfer (au) from the N-Base and Charge-Transfer Energies (Nlp → σ*P−Fax and Nlp → σ*P−Feq, kJ mol−1) for Complexes F4P+:N-Basea sp3 bases

a

charge transfer

Nlp → σ*P−Fax

Nlp → σ*P−Feq

total Nlp → σ*P−Feq

56.3

2× 38.5, 40.8

117.8


0.348 0.315 0.307 0.271 0.216 0.193 0.230 0.141 0.016 charge transfer

41.1 5.2 Nlp → σ*P−Fax

2 × 28.1, 27.2 3 × 2.5 Nlp → σ*P−Feq

83.4 7.5 total Nlp → σ*P−Feq


0.281 0.256 0.277 0.249 0.230 0.191 0.160 0.020 0.003

12.7 3.1

3 × 6.5 3 × 1.3

19.5 3.9

Bases that form outer complexes are written in italics.

ionic bond at short distances,25 it is difficult to classify bonds at short distances as having either ionic or covalent character, since the P−N distances and binding energies of sp3 and sp inner complexes are similar. In contrast, the energy densities are always negative for the inner complexes, varying between −0.03 and −0.11 au, and are either slightly negative or positive for the outer complexes. Figure S1 of the Supporting Information illustrates the smooth variation of the energy density with P−N distance for complexes with sp3 and sp nitrogen bases. Spin−Spin Coupling Constants. The components of spin−spin coupling constants 1pJ(P−N) across the pnicogen bond, and of the one-bond coupling constants 1J(P−Fax) for complexes F 4 P + :N-base and 1 J(P−H ax ) for complexes F3HP+:N-base, are reported in Tables S3, S4, and S5, respectively, of the Supporting Information. Because of the size of some of these complexes, it was possible to compute

Figure 4. Depiction of the orbitals involved in the Nlp → σ*P−Fax and Nlp → σ*P−Feq charge-transfer interactions in F4P+:NCCN.

correlation coefficients of 0.999 for complexes with sp3 and sp hybridized bases.40−48 The Laplacians of the electron densities are always positive for the outer complexes, negative for the inner complexes with the sp3 hybridized nitrogen bases except NH3, but positive for the inner complexes with the sp bases. Since it has been noted previously that a positive value of the Laplacian may indicate an D


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Table 4. Total Charge Transfer (au) from the N-Base and Charge-Transfer Energies (Nlp → σ*P−Hax and Nlp → σ*P−Feq, kJ mol−1) from the N Lone Pair of the Base to F3HP+a sp3 bases

a

charge transfer

Nlp → σ*P−Hax

Nlp → σ*P−Feq

total Nlp → σ*P−Feq

3 × 48.1 2 × 36.7, 38.5 2 × 56.4, 53.1 3 × 4.3 3 × 2.5 Nlp → σ*P−Feq

144.3 111.9 165.9 12.9 7.5 total Nlp → σ*P−Feq


0.350 0.311 0.302 0.258 0.184 0.141 0.195 0.015 0.012 charge transfer

28.8 21.8 35.7 2.8 1.8 Nlp → σ*P−Hax


0.279 0.249 0.271 0.236 0.039 0.022 0.017 0.008 0.002

11.4 7.2 5.7 3.7 1.2

3 3 3 3 3

× × × × ×

15.7 9.7 7.5 5.0 1.5

47.1 29.1 22.5 15.0 4.5

Bases that form outer complexes are written in italics.

of the FC terms are designated as 1pJ(P−F) and 1J(P−Hax). However, only total 1J(P−Fax) values are employed for analysis. 1p J(P−N). Tables 5 and 6 present 1pJ(P−N) values for complexes F4P+:N-base and F3HP+:N-base, and these values are Table 5. Coupling Constants 1pJ(P−F) and 1J(P−Fax) (Hz) and Corresponding P−N and P−Fax Distances (Å) for Complexes F4P+:N-Base

Figure 5. Nlp → σ*P−Fax and Nlp → σ*P−Hax charge-transfer energies versus the P−N distance in complexes F4P+:N-base and F3HP+:N-base.

PF4+

total J for only six of the complexes with and nine with F3HP+. As evident from Table S3, the Fermi-contact term is by far the dominant contributor to 1pJ(P−N) for these. Moreover the small contributions from the PSO and SD terms are of opposite sign and partially cancel, with the result that the FC term approximates 1pJ(P−F) to about 1 Hz as illustrated in Figure S2 of the Supporting Information. The FC term for 1 J(P−Hax) is very large and dominant, while the PSO, DSO, and SD contributions are small and tend to cancel. Thus, the FC term approximates 1J(P−Hax) to within 1 Hz out of more than 850 Hz. Finally, the data of Table S5 indicate that although the negative FC term is the dominant contributor to 1 J(P−Fax), the PSO term also makes a large negative contribution while the SD term makes a smaller positive contribution. As a result, the FC term is not a good approximation to total 1J(P−Fax). In the section below, values

a

sp3 bases

R(P−N)

NH3 NClH2 NH2F NCl2H NF2H NF3 NCNH2 NCCH3 NP NCOH NCCl NCH NCF NCCN N2 PF4+

1.914 1.950 1.970 2.015 2.098 2.913 1.886 1.924 1.917 1.938 1.965 2.044 2.108 2.637 3.009

J(P−N)a

1p

R(P−F)

−25.8 −23.2 −19.0 −19.9 −6.1 2.5 −35.7 −22.8 −15.2 −19.6 −12.2 0.9 8.6 6.4 1.5

Approximated by the FC term. bTotal value of 1203.2 Hz from ref 11.

1p

1

J(P−Fax)b,c

1.541

−1043.4

1.533

−1068.3

1.541

−1025.8

1.530 1.526

−1058.1 −1068.8

1.495 1.492

−1277.6 −1318.9

J(P−Fax). cExperimental

plotted against the P−N distance in Figure 6. The gap in intermolecular distances between 2.25 and 2.53 Å is evident. Figure 6 clearly illustrates the differences between 1pJ(P−N) values for inner and outer complexes. All 1pJ(P−N) values are positive for outer complexes, independent of whether the nitrogen base is sp or sp3 hybridized. With only three exceptions which are found at the longer P−N distances for inner complexes, 1pJ(P−N) values for inner complexes are negative. The data in Figure 6 have been fitted by a trendline E


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The Journal of Physical Chemistry A Table 6. Coupling Constants 1pJ(P−F) and 1J(P−Hax) (Hz) and Corresponding P−N and P−Hax Distances (Å) for Complexes F3HP+:N-Base J(P−N)a

R

1.923 1.967 1.987 2.042 2.177 3.050 R(P−N)

−21.7 −18.1 −14.1 −13.6 3.4 1.9 1p J(P−N)a

1.383 1.382 1.382 1.382 1.380 1.382 R

917.6 915.5 915.3 908.3 906.2 947.0 1 J(P−Hax)a,b

1.905 1.956 1.939 1.981 2.536 2.657 2.723 2.824 3.107

−26.3 −11.9 −7.6 −5.9 11.5 7.9 6.8 4.5 1.3

1.383 1.382 1.382 1.382 1.380 1.380 1.381 1.381 1.382 1.383

879.8 877.5 885.1 875.6 897.2 907.5 914.0 924.1 943.9 956.0

sp3 bases

R(P−N)

NH3 NH2Cl NH2F NCl2H NHF2 NF3 sp bases NCNH2 NCCH3 NP NCOH NCCl NCH NCF NCCN N2 F3HP+

1p

are reported for six complexes in Table 5 and the ion F4P+, along with the corresponding P−Fax distances. Complex formation always leads to an increase in the P−Fax distance and a decrease in the absolute value of 1J(P−Fax) relative to F4P+. The P−Fax distance of 1.492 Å in the ion increases slightly to 1.495 Å in the outer complex F4P+:N2, while the value of −1319 Hz for 1J(P−Fax) in the cation decreases to −1278 Hz in this complex. These values are consistent with the data reported in ref 11. A more dramatic decrease is found for the five inner complexes, for which the values of 1J(P−Fax) vary from −1026 to −1069 Hz as the P−Fax distance increases to between 1.526 and 1.541 Å. Figure 7 provides plots of 1J(P−

J(P−Hax)a,b

1

a Approximated by the FC term. bExperimental value of 1187.7 Hz from ref 11.

Figure 7. 1J(P−Fax) and the corresponding FC terms versus the P−Fax distance in complexes F4P+:N-base. The points at the shortest distance are those of the cation F4P+.

Fax) for the six complexes and F4P+ and of all of the FC terms versus the P−Fax distance. This plot illustrates the difference between values of total J and the FC terms, as well as the similarities in the characteristics of the trendlines for these two quantities. The correlation coefficients R2 are 0.993 and 0.985, respectively. 1 J(P−Hax). The one-bond coupling constants 1J(P−Hax) and the P−Hax distances are reported in Table 6. The P−Hax distance in the cation F3HP+ and the complexes F3HP+:N-base varies only slightly between 1.380 and 1.383 Å. However, 1J(P− Hax) always decreases upon complexation, from its value of 956 Hz in the cation to between 878 and 947 Hz in the complexes. These values are less than but consistent with the value reported in ref 11. Although the distance changes are small, it is possible to observe some patterns in the variation of 1J(P−Hax) with distance by subdividing the complexes according to the hybridization of N and whether the pnicogen bond occurs in an inner or outer complex, as illustrated in Figure 8. Values for 1 J(P−Hax) for the five outer complexes with sp-hybridized nitrogen bases and the one outer complex with an sp3 base appear to increase linearly with increasing distance, approaching the value of 1J(P−Hax) for the cation. There are five inner complexes with sp3-hybridized nitrogen bases which span the range of P−Hax distances. However, 1J(P−Hax) for these complexes show little dependence on distance, with values between 906 and 918 Hz. Finally, 1J(P−Hax) values for the four inner complexes with sp-hybridized nitrogen bases decrease significantly, ranging between 876 and 885 Hz. Thus, formation of these inner complexes with sp-hybridized nitrogen bases has

Figure 6. 1pJ(P−N) versus the P−N distance for complexes F4P+:Nbase and F3HP+:N-base. All points were used to determine a best-fit 4th-order trendline, except the single point at 2.36 Å which refers to a reference nonequilibrium complex.

which indicates that 1pJ(P−N) for outer complexes increases as the P−N distance decreases. Once the gap is crossed, then 1p J(P−N) decreases with decreasing distance, and becomes negative. At short distances, 1pJ(P−N) decreases rapidly. In Figure 6 there is a point in the gap which belongs to a constrained complex F3HP+:NF2H optimized with the P−N distance fixed at 2.36 Å. 1pJ(P−N) for this complex at this distance has a value of 8.5 Hz, and this point is positioned reasonably in the plot. Decreasing values of 1pJ(P−N) with decreasing P−N distance have been observed previously, and attributed to the changing nature of the P···N bond as it acquires increased covalency.49,50 1 J(P−Fax). As noted above, the one-bond coupling constants 1 J(P−Fax) for complexes F4P+:N-base are not well-approximated by the FC terms. Therefore, only total 1J(P−Fax) values F


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Figure 8. 1J(P−Hax) versus the P−Hax distance in complexes F3HP+:N-base.

5 EOM-CCSD Fermi-contact terms are excellent approximations to total 1pJ(P−N) and 1J(P−Hax), but are poor approximations to 1J(P−Fax). 1pJ(P−N) values increase with decreasing P−N distance, approach a maximum, and then decrease and change sign as the P−N distance further decreases. This behavior is characteristic of intermolecular pnicogen bonds at long distances, which assume increased covalent character as the P−N distance decreases. 1J(P−Fax) values for F4P+:N-base complexes increase with decreasing P−Fax distance. The P−Hax distance changes very little in complexes F3HP+:N-base, but patterns exist which suggest that changes in 1J(P− Hax) reflect the hybridization of the nitrogen base and whether the complex is an inner or outer complex.

ASSOCIATED CONTENT

* Supporting Information S

Geometries, total energies, and molecular graphs of complexes F4P+:N-base and F3HP+:N-base; values of electron densities, Laplacians, and energy densities at P···N bond critical points; components of spin−spin coupling constants; and full refs23 and36. This material is available free of charge via the Internet at http://pubs.acs.org.

1

the greatest effect on J(P−Hax) relative to the cation, as clearly evident from Figure 8.

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CONCLUSIONS Ab initio MP2/aug’-cc-pVTZ calculations have been carried out on cationic pnicogen-bonded complexes F4P+:N-base and F3HP+:N-base with linear Fax−P···N and Hax−P···N, respectively. The bases include the sp3-hybridized bases NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3, and the sp bases NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2. The results of these calculations support the following statements. 1 The binding energies of these complexes span a wide range. For a given base, the binding energy with F4P+ is greater than that with F3HP+. The P−N distances also span a wide range, but there is a gap of about 0.3 Å in the P−N distance in which no complexes are found. Thus, these pnicogen-bonded complexes may be classified as inner and outer complexes based on the value of this distance. Some outer complexes with sp3-hybridized bases have greater binding energies at longer P−N distances than some of the inner complexes with sphybridized bases at shorter P−N distances. 2 Inner and outer complexes may also be distinguished based on the value of the Xax−P-Feq angle, with Xax equal to Fax for complexes with F4P+, and Hax for F3HP+. Values of these approach the tetrahedral angle for the outer complexes, but are reduced for the inner ones. 3 Inner complexes have P···N bonds with varying degrees of covalent character, whereas outer complexes are stabilized by intermolecular P···N bonds with little or no covalency. 4 Charge-transfer stabilizes these pnicogen-bonded complexes. Charge-transfer interactions could be computed for outer complexes and a few of the inner complexes. For complexes F4P+:N-base, the dominant chargetransfer interaction is from the lone pair on N to the σ*P−Fax orbital. In addition, there are three other charge-transfer interactions from the lone pair on N to the σ*P−Feq orbitals, which taken together, are more stabilizing than the interaction involving σ*P−Fax. In contrast, the dominant charge-transfer interaction for complexes F3HP+:N-base is from the lone pair on N to the σ*P−Feq orbitals.

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AUTHOR INFORMATION

Corresponding Authors

*(J.E.D.B.) E-mail: [email protected]. Telphone: +1 330-6095593. *(I.A.) E-mail: [email protected]. Telephone: +34 915622900. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out with financial support from the Ministerio de Economiá y Competitividad (Project No. CTQ2012-35513-C02-02) and Comunidad Autónoma de Madrid (Project FOTOCARBON, ref S2013/MIT-2841). Thanks are also given to the Ohio Supercomputer Center and CTI (CSIC) for their continued computational support.

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PÂ·Â·Â·N Pnicogen Bonds in Cationic Complexes of F4P+ and F3HP+ with

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