1H-Phosphepine-benzene phosphine valence tautomerizations: Impacts of substituents at ab initio and DFT levels

Journal of Molecular Structure: THEOCHEM 865 (2008) 73–78

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1H-Phosphepine-benzene phosphine valence tautomerizations: Impacts of substituents at ab initio and DFT levels M.Z. Kassaee *, A. Cheshmehkani, S.M. Musavi, M. Majdi, E. Motamedi Department of Chemistry, Tarbiat Modares University, Pole Gish, P.O. Box 14155-175, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 23 February 2008 Received in revised form 21 June 2008 Accepted 24 June 2008 Available online 5 July 2008 Keywords: Benzene phosphine 1H-Phosphepine Valence tautomerization Substituent effect Ab initio DFT NICS

a b s t r a c t The main goal of this work is to find a more stable seven-membered ring 1H-phosphepine connected to its bicyclic benzene phosphine valence tautomer after passing through a nonplanar transition state. This challenge is met by probing the impacts of substituents (X = H, F, Cl, Br, CN, Me, CF3, NH2, and OMe) on the umbrella inversions of 1H-phosphepines, as well as their effects on two series of equilibria consisting of 2-X-benzene phosphines 3-X-1H-phosphepines (series A) and 3-X-benzene phosphines 4-X1H-phosphepines (series B), all in the more stable endo form of pyramidal phosphorus atom. Among 17 tautomerization systems, scrutinized at B3LYP/6-311G* level, only the 3-methoxybenzene phosphine 4-methoxy-1H-phosphepine system shows equilibrium shift to the right. In contrast to the tautomerizations, umbrella inversions of 1H-phosphepines are not much sensitive to the effects of substituents placed around their carbon skeletons. The magnetic (NICS) and structural criteria indicate partial aromaticity of 1H-phosphepines and high anti-aromaticity for their planar inversion transition states. Some justifications based on NBO data and calculated DEHOMO–LUMO are presented. Various inconsistencies occur between the ab initio and DFT data, making the equilibrium constants (Keqs) sensitive to the methods of calculation along the substituents effects. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Perhaps due to the importance of switching compounds in biological systems, a growing interest is witnessed on the valence bond tautomerization systems involving 7-Y-bicyclo[4.1.0]hepta2,4-dienes Y-cycloheptatrienes, along with the conformational ring inversions of the latter species [1–8] (Fig. 1). Equilibria appear to shift towards either left or right depending on the choice of Y. The right shifts are reported for tautomerization systems involving: Y = B (boranorcaradiene borepin) [9], Y = C (norcaradiene

cycloheptatriene) [10,11], Y = N (benzene imine 1H-azepine) [12], and Y = O (benzene oxide oxepin) [13]. In contrast, the left shifts are observed for tautomerization systems involving: Y = S (benzene sulfide thiepin) [14], and Y = P (benzene phosphine (1H) 1H-phosphepine (2H)) [15] systems. Hence, for X = H, the equilibria tend to shift in favor of monocyclic systems for tautomerizations containing the second row elements: B, C, N, and O [9–13]. In contrast, systems containing the third row elements (S, and P) show predominance of their corresponding bicyclic valence tautomers [14,15]. Again, these observations are mostly for unsubstituted species (X = H), while perturbation of the above equilibria via specific substituents, around the carbon skeleton (X 6¼ H), await * Corresponding author. Tel.: +98 912 1000392; fax: +98 21 88006544. E-mail address: [email protected] (M.Z. Kassaee). 0166-1280/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2008.06.025

further research. The electronic and steric effects of substituents are noticed for the conversions of the 2-(X)-benzene imines to their corresponding 3-(X)-1H-azepines [5]. Also, small effects of substituents are observed on thiepine benzene sulfide equilibria, as well as conformational inversions of thiepines [14]. Interestingly, a trimethylene bridge constricts the carbon atoms at the 1,6positions and freezes the norcaradiene form [16,17]. Nevertheless, not much is known about the benzene phosphine (1H) 1H-phosphepine (2H) tautomerization, compared to the other heteropine analogues. A recent theoretical study on the 1H 2H equilibrium predicts 4.2 kcal mol1 energy preferences for 1H over 2H [15]. The B86-P88/TZP calculated barrier for 2H 1H valence tautomerization is found to be 10.6 kcal mol1. Transition metal coordination at phosphorus stabilizes 2H over 1H because the distant CAC bond is weakened by r, p-interactions. Moreover, a 1,5-sigmatropic shift relates the more favorable 1H with the 15.5 kcal mol1 less stable 7-phosphanorbornadiene 3H (Fig. 2). While 1H, 2H, and 3H are not known experimentally, the oxide [18] of 2H and its 2,7-dialkylsubstituted [19], as well as its annelated derivatives [20] are known without structural details. In addition, a strained derivative of 3H has been isolated [21]. To date no report has appeared on the effects of substituent on inversion of 1H-phosphepines or their tautomerization to benzene phosphines. In order to find a rather more stable 1H-phosphepine, compared to its valence tautomer, the impacts of substituents

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M.Z. Kassaee et al. / Journal of Molecular Structure: THEOCHEM 865 (2008) 73–78

Fig. 1. Possible valence tautomerizations of 2-X, and 3-X substituted 7-Y-bicyclo[4.1.0]hepta-2,4-dienes (1X, and 10X , respectively) to 3-X, and 4-X substituted Y-cycloheptatrienes (2X, and 20X , respectively), along with the conformational ring inversions of 2X, and 20X , as well as the rearrangements of 1X, and 10X to 2-X-7-Ynorbornadienes (3X), where Y = P, B [9], C [10,11], N [12], O [13], and S [14]; and X = H, F, Cl, Br, CN, Me, CF3, NH2, and OMe.

Fig. 2. Relative energy barriers (kcal mol1) for benzene phosphine (1H) the valence tautomerization transition state (TSt) 1H-phosphepine (2H), calculated at B3LYP/ 6-311G* level. In addition to energetic of the umbrella inversion of 2H to the 2H, through the inversion transition state (TSinv), as well as the rearrangement of 1H to 7-phosphanorbornadiene (3H), through the rearrangement transition state (TSr).

(X = H, F, Cl, Br, CN, Me, CF3, NH2, and OMe) are probed on two series of equilibria consisting of 2-X-benzene phosphines (1X) 3-X1H-phosphepines (2X) (series A), and 3-X-benzene phosphines (10X ) 4-X-1H-phosphepines (20X ) (series B) (Fig. 1). In addition, the conformation inversion and aromatic characters of 1H-phosphepines 2X, and 20X are addressed.

2. Computational methods All calculations are carried out using the Gaussian 98 suite of programs [22]. Structures are optimized at the HF/6-311+G* [23], MP2/6-311G* [24] and B3LYP/6-311G* [25,26] levels of theory. The nature of the optimized structures is characterized by fre-

M.Z. Kassaee et al. / Journal of Molecular Structure: THEOCHEM 865 (2008) 73–78

quency calculations with HF and B3LYP methods (i.e., number of imaginary frequencies: 0 for minima, 1 for transition states, etc.). Unfortunately, in most cases the frequency calculations at MP2 level crashed due to disk space limitations. Nucleus independent chemical shift NICS calculations [27] are performed using the gauge independent atomic orbital method (GIAO/B3LYP/6-311G*) [28]. The ghost atom (Bq) is placed at the ring center. Evidently, the geometrical center of the ring’s heavy atoms practically serves as the most easily defined reference point. This is referred to as the standard NICS location (NICS (0)) and is used along with that 1 Å above the plane of ring (NICS (1)). NICS (1) is recommended to be a better measure of the p-electron delocalization, compared to NICS (0) due to the lack of local r shielding [29]. The reliability of various density functional theory (DFT) models is already evaluated for the study of bond dissociation energies, heats of formation, and geometrical parameters [30,31]. Among all DFT methods, B3LYP often gives geometries and vibration frequencies which are closest to those obtained from the MP2 method. Thus, B3LYP with the 6-311G* basis set is employed as the method of choice in this work [32]. Also, the NBO population analysis on optimized structures is accomplished at the B3LYP/6-311G* level [33].

3. Results and discussion Our main goal is to probe the effects of specific substituents (X) in shifting two equilibrium series including 2-X-benzene phosphines (1X) 3-X-1H-phosphepines (2X) (series A), and 3-X-benzene phosphines (10X ) 4-X-1H-phosphepines ð20X ) (series B), at HF/6-311+G*, MP2/6-311G* and B3LYP/6-311G* levels of theory, where X = H, F, Cl, Br, CN, Me, CF3, NH2, and OMe (Table 1, Fig. 1). This is in addition to our study of the effects of substituents (X) on 1H-phosphepine aromaticity, shown by calculated NICS values (Table 2). Finally, we will discuss the effects of substituents on the umbrella inversions of 2X and 20X species, at the B3LYP/6-311G* level. Evidently, valence tautomerizations as well as ring inversions are affected by geometrical parameters. Hence, complete geometry optimizations are performed on the scrutinized 1X, 10X , 2X, and 20X molecules which have C1 symmetry (Supporting information). The effects of substituent, aromaticity, and structural parameters on the tautomerizations and inversions will be presented under the following two subtitles. 3.1. Valence tautomerizations Valence tautomerization shifts to the left sides of the equilibria (1X 2X, and 10X 20 X ), occur in favor of 1X and 10X tautomers, with average enthalpy (DH1x ? 2x, and DH10 x ? 20 x) of 4 and 2 kcal mol1, for series A and B, respectively, when X = F, Cl, Br, CN, Me, CF3, and NH2, at B3LYP/6-311G* level (Table 1). Also for X = OMe, tautomerization right shift occurs in favor of 20X , in series B, with DH10 OMe ? 20 OMe = 1.26 kcal mol1. Nevertheless, in most cases gas phase activation energies of forward reactions (E6¼ t1x ! 2x , and E6¼ 0 0 Þ are higher than those of their corresponding reverse t1 x ! 2 x 6¼ transformations (E6¼ t2x ! 1x , and Et20 x ! 10 x , respectively), at HF/6311+G*, MP2/6-311G*, and B3LYP/6-311G* levels of theory (Fig. 1, Table 1). We estimate the E6¼ t for conversion of 1H to 2H to be 15.66 kcal mol1, while for the reverse process (2H to 1H) it turns out to be 13.16 kcal mol1(Fig. 2). This is consistent with the energetic preference for 1H (over 2H) with a rather similar energy barrier reported by Lammertsma et al. [15] Also, preference for 1H is consistent with the reported shifts towards substituted benzene sulfides in their valence tautomerizations to the corresponding thiepines [14]. However, the observed left shift towards 1H, in its equilibrium with 2H, does not seem energetically low enough to

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suggest a rapid inter-conversion of the isomers in the gas phase, at room temperature reminding that in the liquid phase the situation may change. Compared to X = H, other substituents attached at position 2 in series A, increase the calculated E6¼ t in going from 1X to 2X (Table 1). The left shift, which is observed in favor of 1H (over 2H), appears in contrast to the reported right shifted equilibria including: boranorcaradiene borepin[9], norcaradiene

cycloheptatriene [10,11], benzene imine 1H-azepine [12], and benzene oxide oxepin [13]. Therefore, valence tautomerizations involving the second row elements (B, C, N, and O), tend to right shift towards their corresponding cycloheptatriene valence tautomers while systems containing the third row elements (S, and P), show left shift towards their bicyclic tautomers (Fig. 1). In general, results obtained via time consuming MP2/6-311G* level are much closer to the B3LYP/6-311G* data than those of HF/6-311+G*. Specifically, the activation energies obtained at HF are about 8–9 kcal mol1 higher than those of B3LYP and MP2 suggesting a significant effect imposed by the choice of the calculation methods. Using data of activation energies and rate constants (Kr), we have calculated the corresponding equilibrium constants (Keq)1, for series A (1X 2X), and B ð10X 20X Þ (Tables 1 and 3). Nearly all Keqs calculated at HF are higher than those obtained at B3LYP. Comparing the two series, calculated Keqs for series A are smaller than their corresponding ones in series B. Also, except for X = OMe in series B, all Keqs are less than those in A, at B3LYP/6-311G* level. In general, most of the thermodynamic data are in favor of bicyclic 1X, and 10X over monocyclic 2X, and 20X in their binary equilibrium systems. Comparing species within each series, per se, is instructive. At B3LYP level, the Keqs for X = F, Cl, and Br are 5- to 10-fold higher than those with X 6¼ halogens, in series A; while Keq for X = OMe is about 6-fold higher than species with X 6¼ OMe, in series B (Table 3). Remembering that, it is also possible and maybe easier to estimate the equilibrium constants at 298 K by the standard expression DG0 = RT lnK, where DG0 is the free energy difference between reactants and products [34,35]. The high pyramidality suggests a large phosphorus inversion barrier and a pronounced s character for the lone pair (lp) at P atom and presumably high nucleophilic character for phosphirane rings in all 1X, and 10X species. On the other hand, in monocyclic species 2X, and 20X the lp on the phosphorus has almost equal s and p characters, while bonding orbitals to phosphorus have predominantly p character (Tables 4 and 5). Moreover, in the bicyclic species 1X, and 10X bonds to phosphorus have clearly higher p characters than those of corresponding monocyclic 2X, and 20X (Tables 4 and 5). This is consistent with the Walsh model for three-membered rings [36]. Nevertheless, the d orbital participations are generally negligible. Comparisons of HOMO–LUMO energy gaps (DEHOMO–LUMO) pertaining to 1X, 10X , 2X, and 20X are constructive. The calculated DEHOMO–LUMO for most 1X species are higher than those of their corresponding 2X monocyclic species by about 2–9 kcal mol1 (Table 4). This suggests higher stabilities for most 1X species over their corresponding 2X valence tautomers. In contrast, for X = CF3, and CN, in series A, 2X species have larger DEHOMO–LUMO than their corresponding 1X. One may justify the above phenomena, for the electron withdrawing substituents by considering a direct resonance between CN and the lone pair on P which translates into the lowering of the corresponding HOMO. Also, higher electron withdrawing effects of CF3 may disperse the lone pair electron density of P, causing the lowering of the corresponding HOMO. Similarly, the DEHOMO–LUMO of 10X is higher than those of 20X by about 2–4 kcal mol1, in series B, for all substituents other than 1 G = H – TS = (E + RT) – TS = [(E0 + Evib + Erot + Etrans) + RT] – TS, where: E0 = Eelec + ZPE. According to the transition state theory, the rate constant (kr), is calculated as DG6¼ shown by the following equation: kr ¼ kBhT K 6¼ ¼ kBhT e RT .

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Table 1 Changes of enthalpy (DH1x ? 2x, and DH10 x ? 20 x) for two valence tautomerizations series (A, and B, respectively), along with their corresponding activation energies (forward: 6¼ 6¼ 6¼ 1 E6¼ , at B3LYP/6-311G*, MP2/6-311G*, and HF/6-311+G* levels of theory t1x ! 2x , Et10 x ! 20 x ; and reverse: Et2x ! 1x , Et20 x ! 10 x ), in kcal mol Substituent(X)

Valence tautomerizations Series A 2-X-benzene phosphines (1X) = 3-X-phosphepines (2X) E6¼ t1x ! 2x

DH1x ? 2x

H F Cl Br CN Me CF3 NH2 OMe

Series B 3-X-benzene phosphines (10X ) = 4-X-phosphepines (20X )

E6¼ t2x ! 1x

E6¼ t10 x ! 20 x

DH10 x ! 20 x

E6¼ t20 x ! 10 x

B3LYP

HF

B3LYP

MP2

HF

B3LYP

MP2

HF

B3LYP

HF

B3LYP

MP2

HF

B3LYP

2.51 3.33 3.60 3.20 4.62 4.54 3.95 6.23 4.81

0.13 0.21 0.84 0.62 1.63 1.61 1.22 2.30 1.61

15.66 18.07 17.07 16.44 15.94 18.51 16.80 18.86 21.35

15.09 18.05 16.87 16.13 15.13 17.71 16.17 17.70 20.83

24.47 26.99 26.03 25.41 24.31 27.54 25.90 – 30.09

13.16 14.73 13.43 13.24 11.32 13.97 12.85 12.63 16.53

8.25 10.32 8.55 8.18 6.71 9.02 7.93 7.84 12.06

24.60 26.77 25.19 24.80 22.68 25.93 24.68 – 28.48

2.51 1.43 2.29 2.24 3.68 3.25 3.62 1.24 1.26

0.13 0.79 0.24 0.24 1.36 0.65 1.52 1.10 2.65

15.66 15.52 15.69 15.69 15.38 16.99 16.15 16.81 15.00

15.09 – 14.84 14.67 14.50 15.92 15.52 15.94 14.47

24.47 24.30 24.70 24.63 24.01 26.09 25.53 25.39 –

13.16 15.50 13.43 13.43 11.70 13.74 12.52 15.57 16.26

MP2 8.25 – 8.45 8.31 6.90 8.93 7.80 11.21 11.64

HF 24.60 25.09 24.46 24.39 22.65 25.44 24.01 26.48 –

Table 2 The NICS (total) values (ppm) at the ring centers, NICS(0), and 1 Å above the plane of the rings, NICS(1), for 1H-phosphepines (2X, and 20X ), and their corresponding planar 0 inversion transition states (TSinv, and TS inv, respectively), using different substituents (X), for two series A, and B, respectively, at GIAO-B3LYP/6-311G*//B3LYP/6-311G* Substituent (X)

Series A

Series B

2X

H F Cl Br CN Me CF3 NH2 OMe

20X

TSinv

NICS(0)

NICS(1)

NICS(0)

NICS(1)

NICS(0)

NICS(1)

NICS(0)

NICS(1)

0.37 0.68 0.79 0.92 0.05 0.32 0.26 0.99 3.72

2.90 2.68 3.44 3.87 3.08 3.16 3.05 3.16 5.23

10.20 9.48 9.67 9.69 9.82 9.39 9.97 9.77 9.37

7.95 7.11 7.12 9.17 7.00 7.41 6.99 7.30 6.85

0.37 1.87 1.17 1.00 0.28 0.46 0.26 1.68 2.29

2.90 3.17 2.97 2.88 2.56 2.91 2.65 3.37 4.26

10.20 8.63 9.21 – 9.78 9.69 9.93 4.09 8.98

7.95 6.71 6.92 – 7.09 6.83 9.33 5.49 6.76

Table 3 Equilibrium constants (Keq, and K 0eq Þ of two valence tautomerizations series (A and B, respectively), for different substituents (X), at B3LYP/6-311G*, and HF/6-311+G* levels of theory Substituent (X)

H F Cl Br CN Me CF3 NH2 OMe

TS 0inv

K 0eq (series B)

Keq (series A) B3LYP

HF

B3LYP

HF

0.03 0.01 5.2  1003 0.01 6.05  1004 1.74  1003 1.74  1003 7.27  1005 6.05  1004

2.60 1.71 0.55 0.79 0.14 0.15 0.27 – 0.15

0.03 0.12 0.42 0.046 1.74  1003 5.02  1003 5.02  1003 0.34 2.88

2.60 8.31 1.41 1.38 0.35 0.71 0.35 8.30 123.72

X = OMe, and NH2, where DEHOMO–LUMO of the corresponding 20X are somewhat higher than those of 10X (Table 5). On the other hand, for most electron donating substituents one may suggest the possibility of lowering the LUMO of 2X, and 20X through direct resonance between X and phosphorus empty d orbitals. Nevertheless, no such interaction between P and X can be attributed to any 1X or 10X species. The above results are confirmed by structural parameters of the valence bond tautomerization transition state complexes (TSt, and TS0t ), and their corresponding minima: 1X, 10X , 2X, and 20X , calculated at B3LYP/6-311G*, and MP2/6-311G* levels of theory (Supporting information). All bicyclic species (1X and 10X ) appear non-planar for showing dihedral angles of P7C1C2C3 = 72–78° between their three- and six-membered rings. The small degrees of nonplanarity

(2.5–5°) for the six-membered rings, encountered in 1X and 10X species, are measured through their dihedral angles C1C2C3C4. Similarly, the monocyclic species (2X and 20X ) appear rather nonplanar with extremely shallow boat conformations and dihedral angle ranges of P1C2C3C4 = 4–8° and C2C3C4C5 = 33–39°. The three-membered phosphirane ring with an intra-cyclic C6P7C1 angle of about 46° is obviously very strained, whereas the intra-cyclic C6P7C1 angles of all 1X and 10X remain remarkably constant. The internal P7-C1 and C1AC6 bond lengths vary in the narrow ranges of 1.90-1.93 Å and 1.50-1.51 Å, respectively. The C2P1C7 angles of 2X, and 20X species lie in the range of 97.0–98.9°, with diminishing of the ring strain compared to 1X, and 10 X . There are two sets of variables which change dramatically during the course of the valence tautomerization. The first variables are the C1AC6 bond lengths in 1X and 10X which turn into C2AC7 bonds in the corresponding 2X and 20X , respectively (Fig. 1). The second set of variables is the angles that three-membered rings make with the corresponding benzenoid rings in 1X, and 10X . For instance, the C1AC6 bond length is 1.50 Å in 1H, which becomes 2.76 Å in 2H, after passing through TSt with C2AC7 bond length of 2.04 Å. The calculated dipole moments for 1X, 10 X , 2X, and 20X as well as their transition states are also instructive (Table 6). Except for X = Me in series A, nearly all 1X species have rather larger dipole moments than their corresponding 2X. The transition state dipole moments are larger and are much closer to those of 1X, and 10X than the corresponding 2X and 20X (This is except for X = NH2 in series A and X = Cl, Me, and OMe, in series B). One may remember that from a simple Onsager point of view, if two species are in equilibrium and one has a larger dipole moment, then changing to a more polar solvent will shift the equilibrium towards the more polar molecule [37]. Nevertheless, in most cases

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Table 4 DEHOMO–LUMO (kcal mol1) for species involved in series A (1X, and 2X); along with hybridizations of their P atoms associated to r bonds and lone pairs (lp), calculated at B3LYP/ 6-311G* Substituent (X)

Species

H

Hybridization (series A)

1H 2H 1F 2F 1Cl 2Cl 1Br 2Br 1CN 2CN 1Me 2Me 1CF3 2CF3 1NH2 2NH2 1OMe 2OMe

F Cl Br CN Me CF3 NH2 OMe

DEHOMO–LUMO

rP-C2

rP-C7

rP-H

P(lp)

s1p10.22d0.06 s1p4.76d0.04 s1P10.44d0.07 s1p4.88d0.05 s1p10.02d0.06 s1p4.87d0.05 s1p9.91d0.06 s1p4.91d0.05 s1p9.75d0.06 s1p4.86d0.05 s1p10.11d0.06 s1p4.65d0.04 s1p9.76d0.06 s1p4.81d0.05 s1p10.13d0.06 s1p4.53d0.04 s1p10.09d0.06 s1p4.61d0.04

s1p10.23d0.06 s1p4.76 d0.04 s1p10.25d0.06 s1p 4.82d0.05 s1p10.40d0.06 s1p 4.85d0.05 s1p10.38d0.06 s1 p4.85d0.05 s1p10.62d0.07 s1p 4.79d0.05 s1p10.40d0.06 s1p 4.78d0.05 s1p10.50d0.07 s1p 4.77d0.04 s1p10.56d0.06 s1p 4.90d0.05 s1p10.36d0.06 s1p 4.86d0.05

s1p6.98d 0.07 s1p7.00d0.07 s1p6.95d 0.07 s1p6.96d0.07 s1p6.97d 0.07 s1p6.99d0.07 s1p6.98d 0.07 s1p7.00d0.07 s1p6.95d 0.07 s1p6.93d0.07 s1p6.95d 0.07 s1p7.00d0.07 s1p6.97d 0.07 s1p6.95d0.07 s1p7.03d 0.07 s1p6.96d0.07 s1p6.94d 0.07 s1p7.00d0.07

s1p0.42d0.00 s1p0.87d0.00 s1p0.42d0.00 s1p0.85d0.00 s1p0.42d0.00 s1p0.85d0.00 s1p0.43d0.00 s1p0.84d0.0 s1p0.43d0.00 s1p0.86d0.00 s1p0.42d0.00 s1p0.88d0.00 s1p0.43d0.00 s1p0.86d0.00 s1p0.42d0.0 s1p0.88d0.00 s1p0.42d0.00 s1p0.87d0.00

101.54 99.26 103.82 95.56 100.01 97.56 99.30 97.03 91.01 96.80 101.81 100.50 99.56 100.26 98.18 90.40 103.12 96.40

Table 5 DEHOMO–LUMO (kcal mol1) for species involved in series B (10X , and 20X ); along with hybridizations of their P atoms associated to r bonds and lone pairs (lp), calculated at B3LYP/ 6-311G* Substituent (X)

Species

Hybridization (series B)

10H 20H 10F 20F 10Cl 20Cl 10Br 20Br 10CN 20CN 10Me 20Me 10CF3 20CF3 10NH2 20NH2 10OMe 20OMe

H F Cl Br CN Me CF3 NH2 OMe

DEHOMO–LUMO

rP-C2

rP-C7

rP-H

P-lp

s1p10.22d0.06 s1p4.76d0.04 s1p9.94d0.06 s1p4.85d0.05 s1p10.24d0.06 s1p4.82d0.05 s1p10.27d0.06 s1p4.81d0.05 s1p10.94d0.07 s1p4.84d0.05 s1p10.18d0.06 s1p4.75d0.04 s1p10.56d0.07 s1p4.81d0.05 s1p10.05d0.06 s1p4.81d0.05 s1p10.11d0.06 s1p4.78d0.05

s1p10.23d0.06 s1p4.76d0.04 s1p10.61d0.07 s1p4.74d0.04 s1p10.34d0.06 s1p4.79d0.05 s1p10.29d0.06 s1p4.81d0.05 s1p9.77d0.06 s1p 4.86d0.05 s1p10.32d0.06 s1p4.72d0.04 s1p9.92d0.06 s1p4.84d0.05 s1p10.84d0.07 s1p4.60d0.04 s1p10.95d0.07 s1p4.65d0.04

s1p6.98d 0.07 s1p7.00d0.07 s1p6.93d0.07 s1p6.93d0.07 s1p6.98d0.07 s1p6.95d0.07 s1p6.97d0.07 s1p6.95d0.07 s1p7.02d0.07 s1p6.94d0.07 s1p6.96d0.07 s1p7.00d0.07 s1p7.01d0.07 s1p6.96d0.07 s1p6.94d0.07 s1p6.97d0.07 s1p6.91d0.07 s1p6.97d0.07

s1p0.42d0.00 s1p0.87d0.00 s1p0.42d0.00 s1p0.86d0.00 s1p0.86d0.00 s1p0.42d0.00 s1p0.42d0.00 s1p0.86d0.00 s1p0.89d0.00 s1p0.85d0.00 s1p0.42d0.00 s1p0.87d0.00 s1p0.85d0.00 s1p0.42d0.00 s1p0.42d0.00 s1p0.88d0.00 s1p0.42d0.00 s1p0.88d0.00

Table 6 Dipole moments (Debye) for 2-X-, and 3-X-benzene phosphines (1X, and 10X , respectively), 3-X-, 4-X-1H-phosphepines (2X, and 20X , respectively), valence tautomerization transition states (TSt, and TS0t ), and inversion transition states (TSinv, and TS0inv ), for different substituents (X), at B3LYP/6-311G* level Substituent (X)

Dipole moment (D) Series A

H F Cl Br CN Me CF3 NH2 OMe

Series B

1X

TStot

TSinv

2X

10X

TS0tot

TS0inv

20X

1.35 1.86 2.17 2.05 4.70 1.41 3.02 2.96 1.90

1.42 1.99 2.40 2.30 4.88 1.49 3.06 1.53 2.16

1.27 1.98 2.28 2.17 4.37 1.48 2.59 2.02 1.06

1.28 1.72 2.03 1.92 4.26 1.48 2.55 1.97 1.62

1.35 2.18 2.52 2.39 5.06 1.39 3.33 2.87 2.20

1.42 2.68 3.21 3.11 5.80 1.21 3.99 2.28 1.52

1.27 0.70 1.16 2.34 3.77 1.53 2.05 1.81 2.51

1.28 2.04 2.45 2.34 4.83 1.31 3.09 1.48 1.74

of this study there is not considerable difference in dipole moments of two species in the equilibrium and it is doubtful that solvent effects will matter.

101.54 99.26 101.06 98.80 100.16 99.42 99.74 99.30 95.00 91.84 102.22 102.05 102.05 98.59 95.19 97.68 92.90 100.65

3.2. 1H-Phosphepine ring inversions Conformational ring inversions of 2X and 20 X are affected by substituents and aromaticity (Fig. 1). Before entering discussion, two structural key points should be taken into account. (i) Like 1H-azepine and benzene imine [5], phosphepine and benzene phosphine with pyramidal phosphorus can accept two conformations in which the H on P is pointing towards or away from the ring (exo and endo, respectively). Evidently, 2H and 20 H are not mirror images of each other and are thus not guaranteed to have the same energy. However, our B3LYP calculations denote rather small energy differences between all endo and exo forms of phosphepines and benzene phosphines in the favor of endo conformers (i.e. DEendo–exo of 0.62 kcal mol1 for 1H and 0.32 kcal mol1 for 2H). (ii) In the course of inversion of seven-membered rings by the umbrella motion, the endo conformers of phosphepines convert the exo conformers and vice versa each with different barriers (remembering that phosphorus inversion in the phosphine appear slow and would be higher in energy than the umbrella movement). Nevertheless, all inversion transition states (TSinv, and TS0inv ) show negative force constants. Planar geometries are encountered

78

M.Z. Kassaee et al. / Journal of Molecular Structure: THEOCHEM 865 (2008) 73–78

Table 7 Calculated inversion activation energies (Einv 6¼ and Einv 6¼0 ,) for two series (A and B, respectively), in kcal mol1, using different substituents (X), at B3LYP/6-311G* Substituent (X)

H F Cl Br CN Me CF3 NH2 OMe

Activation energy Einv 6¼ (series A)

Einv 6¼0 (series B)

5.24 4.76 5.98 6.37 6.20 6.06 6.03 7.04 5.60

5.24 5.13 6.07 – 5.53 5.90 5.80 5.08 5.06

structural criteria indicate partial aromaticity of 1H-phosphepines 2X and 20X and high anti-aromaticity for the planar inversion transition states. Umbrella inversions of 2x and 20X , while susceptible to the aromaticity, are not much sensitive to the effects of substituents. Thus, regardless of the X employed, an energy barrier of 5– 7 kcal mol1 at room temperature is required for these inversions, at our method of choice, B3LYP/6-311G* level. Acknowledgements We thank A. Aghaee, M. Ghambarian, H. Aref-Rad, and S. Soleimani for many stimulating and helpful discussions. Appendix A. Supplementary data

for every TSinv, and TS0inv , except where X = Br in series B. Both (4n + 2) p-electron Hückel (homoaromaticity) or (4n) p-electron Möbius arrays are probable for the seven-membered 2X and 20X and their planar inversion transition states: TSinv, and TS0inv , respectively. Despite the C1 symmetry of 2X and 20X , which is suggested by their geometrical parameters (Supporting information), most of these species are moderately aromatic with 2.56 to 5.23 ppm, NICS (1) values, calculated at B3LYP/6-311G* (Table 2). For comparison, the well-known six p-electron Hückel-aromatic tropylium cation has NICS (1) of 8.2 ppm [30]. Besides NICS, ‘‘bond length localizations” DrCAC of 0.104–0.128 Å, typically used as a structural aromaticity criterion [3], confirm the partial aromaticity of 1Hphosphepines 2X and 20X . Excluding 2F as an exception, possibly due to the small size of F, all the substituents (X) fairly increase the aromaticity of 2X (in series A). This effect becomes less important for 20X (in series B) where substituents are further away from P. Based on NICS values, the greatest impact on enhancing the aromaticity of seven-membered 2X and 20X is for X = OMe. Conversely, 1Hphosphepines constrained to planarity in TSinv, and TS0inv appear anti-aromatic (paratropic) with considerable bond length localization (DrCAC = 0.129–0.134 Å) and the high positive NICS values (6.71 to 9.33 ppm). Again, most substituents decrease the anti-aromaticity encountered in TSinv, and TS0inv . The few inconsistencies in calculated NICS values are likely due to the local effects [38]. The B3LYP/6-311G* calculated free activation energy of inversion 0 1 (Einv 6¼ , or E6¼ at 298 K (Table 2). A posinv ) for X = H is 5.24 kcal mol 0 sible source of strain for TSinv, and TSinv is the fact that some bond angles undergo a change of 10° or more in going from the boat conformer to the planar form, increasing the ring strain. However, substituents have no considerable effects on the calculated inversion activation energies (Table 7). Aromatic characters in 1H-phosphepines 2X and 20X along with concurrent anti-aromaticity in the planar transition states (TSinv, and TS0inv ) for conformational inversions are consistent with the corresponding energetic results (Table 1). 4. Conclusions Equilibrium constants (Keqs) are reported for tautomerizations of 2(X)- and 3(X)-benzene phosphines (1X, and 10X ) to their corresponding valence isomers 3(X)- and 4(X)-1H-phosphepines (2X and 20X , respectively), all in the more stable endo form of pyramidal phosphorus atom. The Keqs appear sensitive to the substituents (X) placed around the carbon skeletons, aromaticity, and to some extent dependent on the methods of calculations employed. At B3LYP level, the Keqs for X = F, Cl, and Br are 5- to 10-fold higher than those with X 6¼ halogens, in 1X 2X equilibria (series A), while Keq for X = OMe is about 6-fold higher than species with X 6¼ OMe, in 10X 20X equilibria (series B). Novel shifts toward 2X and 20X valence tautomers are demonstrated by X = H, at HF/6-311+G* level. Further shifts towards 20X are shown by X = F and OMe at the same level (as well as X = OMe, at B3LYP/6-311G* level). The magnetic (NICS) and

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