Electron accepting abilities of arylborane derivatives: Effect of fluorine substituents

Chemical Physics 356 (2009) 171–176

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Electron accepting abilities of arylborane derivatives: Effect of fluorine substituents Bin Peng a, Qian-Shu Li a,b,*, Yaoming Xie c, R. Bruce King a,c,*, Henry F. Schaefer III c a b c

Center for Computational Quantum Chemistry, South China Normal University, Guangzhou 510631, China Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, China Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, GA 30602, USA

a r t i c l e

i n f o

Article history: Received 3 October 2008 Accepted 14 October 2008 Available online 6 November 2008 Keywords: Boron Electron affinity Arylboranes Fluorine

a b s t r a c t The structures of neutral monomeric RnBH3n and the radical anions RnBH3–n–• (R = C6H5 and C6F5) have been optimized using density functional theory (B3LYP and BP86 methods) in order to evaluate their neutral-anion energy separations as indicated by their adiabatic electron affinities (EAad), vertical electron affinities (EAvert), and vertical detachment energies (VDEs). Substitution of hydrogen by C6H5 or substitution of C6H5 by C6F5 is seen to increase greatly the neutral-anion energy separations. The B–C bond rotation barriers in the neutral C6X5BH2 derivatives (X = H, F) at 8–11 kcal/mol are lower than those in the corresponding anions at 15–21 kcal/mol. The highest neutral-anion energy separations are found in (C6F5)3B, consistent with its strong Lewis acidity relating to its use as a cocatalyst in olefin polymerization systems. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction The chemistry of arylboron compounds dates back to the synthesis of (C6H5)3B by Krause and Nitsche in 1922 by the reaction of C6H5MgBr with BF3 [1]. Triphenylborane and related triarylboranes were subsequently found to have significant electron affinities as indicated initially by their reactions with sodium to give  radical anions or dianions [2]. The resulting radical anions R3 B 2 and dianions R3B are isoelectronic with the stable triarylmethyl free radicals R3C and triaryl carbanions R3C, respectively. Replacing the hydrogens in the aryl groups of arylboron derivatives with fluorine atoms is expected to increase significantly their electron affinities through the inductive effect of the fluorine substituents. The most extreme effects are expected by the complete substitution of hydrogen with fluorine. In this connection (C6F5)3B was first prepared by Massey et al. [3] in 1963 by the reaction of C6F5Li with BCl3 at low temperatures. Subsequently, the strong Lewis acidity and electron accepting ability of (C6F5)3B and related compounds have led to their extensive applications as cocatalysts in olefin polymerization processes [4–8]. In addition, the high electron accepting ability of triarylboranes makes them of interest in other applications including non-linear optics [9,10], organic light emitting diodes [11–13], and anion sensors [14–19]. Recently an approach was developed to enhance the electron

* Corresponding authors. Address: Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, GA 30602, USA. Tel.: +1 706 542 1901; fax: +1 706 542 9454 (R. Bruce King). E-mail address: [email protected] (R. Bruce King). 0301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2008.10.050

accepting ability of triarylboranes by p-conjugation with 2,2bipyridyl followed by chelating the bipyridyl nitrogen to transition metals such as copper [20]. The theoretical study reported in this paper uses density functional theory (DFT) to explore systematically the effects on the electron affinity of boranes upon substitution of hydrogen with phenyl and substitution of phenyl with pentafluorophenyl. In this study the structures of neutral monomeric RnBH3n and the radical –• anions RnBH3–n (R = C6H5 and C6F5) are first optimized. Information on the optimized structures is then used to determine the electron affinities of the neutral derivatives. Since the objective of this work was to compare the predicted electron affinities of mononuclear borane derivatives, only the mononuclear RnBH3n boranes were investigated in this work rather than the dimeric R2nB2H42n(lH)2 boranes with structures analogous to the known diborane B2H4(l-H)2. 2. Theoretical methods In this paper, two DFT methods were used. The first method uses the B3LYP functional, which combines Becke’s three parameter hybrid exchange functional with the Lee–Yang–Parr correlation functional [21,22]. The second DFT method uses the BP86 functional, which combines Becke’s 1988 exchange functional (B) with Perdew’s 1986 gradient corrected correlation functional method (P86) [23,24]. DFT methods can be very useful, especially for anionic systems of size (C6F5)3B and larger. However, for smaller molecules, convergent quantum mechanical methods, such as coupled cluster theory, are far more reliable when used with larger basis sets [25].

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The basis sets used in this work are standard double-f plus polarization (DZP) basis sets augmented with diffuse functions, labeled as DZP++. For boron, carbon and fluorine, the DZP++ basis set comprises the Huzinaga–Dunning [26,27] contracted double-f Gaussian basis set with one set of five pure d angular momentum polarization functions and a set of sp diffuse functions. The exponents of the polarization functions are ad(B) = 0.70, ad(C) = 0.75, and ad(F) = 1.00. The exponents of the diffuse functions are as(B) = 0.0288, ap(B) = 0.0225, as(C) = 0.0430, ap(C) = 0.0363, as(F) = 0.1049, and ap(F) = 0.0826. For hydrogen, the DZP++ basis set adds one set of p polarization functions with ap(H) = 0.75 to one s diffuse function with as(H) = 0.0441. The geometries of all structures were fully optimized using the DZP++ B3LYP and DZP++ BP86 methods. Vibrational frequencies were determined by evaluating analytically the second derivatives of the energy with respect to the nuclear coordinates. The corresponding infrared intensities were also evaluated analytically. All of the computations were carried out with the Gaussian 03 [28] and Gaussian 94 [29] programs, exercising the fine grid option (75 radial shells, 302 angular points) for evaluating integrals numerically. The tight (108 hartree) designation was the default for the self-consistent field (SCF) convergence. 3. Results

Table 1 Relative energies in eV (or in kcal/mol in parentheses) for the C6H5BH2 structures. Structure

Electronic state

Point group

B3LYP

BP86

2na 2nb

1

C2v C2v

0.00 0.45 (10.3)

0.00 0.44 (10.1)

A1 A1

1

Fig. 3. Structures found for C6 H5 BH 2 (bond distances in angstroms).

Table 2 Relative energies in eV (or in kcal/mol in parentheses) for the C6 H5 BH 2 structures. Structure

Electronic state

Point group

B3LYP

BP86

2aa 2ab

2

C2v C2v

0.00 0.72 (16.5)

0.00 0.68 (15.8)

B1 B2

2

3.1. BH3 and BH 3 Borane (BH3) is predicted to exhibit a D3h planar structure (1na in Fig. 1) in a singlet ground state 1A10 with a B–H bond distance of 1.196 Å (B3LYP) or 1.205 Å (BP86) in reasonable accord with the experimental result of 1.185 Å by Kawaguchi [30]. For the corresponding radical anion BH 3 , the predicted global minimum is the D3h structure 1aa (Fig. 1). Its electronic ground state is 2A200 . The B–H bond distance is predicted to be 1.211 Å (B3LYP) or 1.222 Å (BP86). 3.2. C6H5BH2 and C 6 H5 BH 2 Two structures have been investigated for neutral C6H5BH2 (Fig. 2 and Table 1). The global minimum is predicted to be the planar C2v structure 2na with an electronic ground state of 1A1 sym-

metry and a predicted B–C bond distance of 1.537 Å (B3LYP) or 1.541 Å (BP86). The orthogonal or twisted non-planar structure 2nb is predicted to lie 10 kcal/mol in energy above 2na. Structure 2nb is predicted to have an imaginary vibrational frequency of 359i cm1 (B3LYP) or 362i cm1 (BP86). Following the corresponding normal mode leads to rotation of the BH2 group about the B–C axis to give 2na. Two structures have been investigated for the C6 H5 BH 2 anion (Fig. 3 and Table 2). The global minimum is again predicted to be a C2v planar structure 2aa in its 2B1 electronic ground state with a predicted B–C distance is 1.521 Å (B3LYP) or 1.531 Å (BP86). The orthogonal or twisted C6 H5 BH 2 structure 2ab is predicted to lie above 2aa by 16 kcal/mol. Structure 2ab has one imaginary harmonic vibrational frequency of 466i cm1 (B3LYP). Following the corresponding normal mode leads to rotation of the BH2 group about the B–C axis to give 2aa. 3.3. (C6H5)2BH and (C6H5)2BH–

Fig. 1. The global minimum structures of BH3 and BH 3 (bond distances in angstroms).

Fig. 2. Structures predicted for C6H5BH2 (bond distances in angstroms).

Three structures were found for the singlet electronic ground state of (C6H5)2BH (Fig. 4 and Table 3). The global minimum is predicted to be the C2 structure 3na with a 1A electronic state and a predicted B–C distance of 1.554 Å (B3LYP) or 1.558 Å (BP86). In 3na the phenyl rings form 21 ± 1° angles with the BX3 coordination plane. The C2v structure 3nb of (C6H5)2BH lies slightly higher in energy at 0.9 kcal/mol above 3na. The B–C distance in 3nb is predicted to be 1.557 Å (B3LYP) or 1.561 Å (BP86). In 3nb both phenyl rings are coplanar with the BX3 coordination plane. Structure 3nb has one very small imaginary harmonic vibrational frequency of 22i cm1 (B3LYP) or 27i cm1 (BP86). Following the corresponding normal mode leads to structure 3na. The Cs structure 3nc of (C6H5)2BH is predicted to lie 4 kcal/mol in energy above the global minimum 3na with an electronic state of 1A0 . In 3nc one of the phenyl rings is coplanar with the BX3 coordination plane whereas the other phenyl ring is orthogonal to the BX3 coordination plane. Structure 3nc of (C6H5)2BH has one imaginary harmonic vibrational frequency of 61i cm1 (B3LYP) or 64i cm1 (BP86). Following the corresponding normal mode leads

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Fig. 4. Structures of (C6H5)2BH (bond distances in angstroms).

Table 3 Relative energies in eV (or in kcal/mol in parentheses) for the (C6H5)2BH structures.

Table 4 Relative energies in eV (or in kcal/mol in parentheses) for the (C6H5)2BH structures.

Structure

Electronic state

Point group

B3LYP

BP86

Structure

Electronic state

Point group

B3LYP

BP86

3na 3nb 3nc

1

C2 C2v Cs

0.00 0.04 (0.9) 0.18 (4.2)

0.00 0.04 (0.9) 0.18 (4.2)

3aa 3ab

2

C2v Cs

0.00 0.34 (7.8)

0.00 0.35 (8.2)

A 1 A1 1 0 A

B1 2 00 A

to rotation of the phenyl groups about the B–C axis to give again 3na. Two structures of the ground state radical anion (C6H5)2BH were obtained (Fig. 5 and Table 4). The coplanar C2v structure 3aa is predicted to be the global minimum with a 2B1 electronic state and B–C distances of 1.549 Å (B3LYP) or 1.554 Å (BP86). The Cs structure 3ab with a 2A00 electronic state is predicted to lie 8 kcal/mol above 3aa. In 3ab one of the phenyl rings is coplanar with the boron valence plane whereas the other phenyl ring is orthogonal to the BX3 coordination plane. Structure 3ab has one imaginary vibrational frequency of 72i cm1 (B3LYP) or 94i cm1 (BP86). Following the corresponding normal mode leads to structure 3aa. Fig. 6. Structures of (C6H5)3B and (C6H5)3B (bond distances in angstroms).

3.4. (C6H5)3B and (C6H5)3B For (C6H5)3B, the global minimum 4na (Fig. 6) is found to be a D3 structure with a 1A1 electronic ground state. The three phenyl groups are tilted by 34° (B3LYP) or 33° (BP86) with respect to the BR3 coordination plane. This is in agreement with the experimental [31] phenyl tilt values of 28–35°. The B–C bond distance in 4na is predicted to be 1.568 Å (B3LYP) or 1.572 Å (BP86), which

is close to the experimental values of 1.571–1.589 Å. For the radical anion (C6H5)3B, the D3 structure 4aa with a 2A2 electronic ground state is predicted to be the global minimum. The B–C bond distance in 4aa is predicted to be 1.572 Å (B3LYP) or 1.575 Å (BP86). Both BP86 and B3LYP methods predict the three phenyl groups to be tilted by 29° with respect to the boron valence plane.

Fig. 5. Structures of (C6H5)2BH (bond distances in angstroms).

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3.5. C6F5BH2 and C6F5BH2– For C6F5BH2, two C2v structures with 1A1 electronic states are found (Fig. 7 and Table 5). The planar structure 5na (Fig. 7) is predicted to be the global minimum with a B–C bond distance of 1.545 Å (B3LYP) or 1.547 Å (BP86). The orthogonal or twisted C6F5BH2 structure 5nb (Fig. 5) is predicted to lie 8 kcal/mol above 5na with a B–C bond distance of 1.572 Å (B3LYP) or 1.575 Å (BP86). Structure 5nb is a transition state with an imaginary harmonic vibrational frequency of 308i cm1 (B3LYP) or 335i cm1 (BP86). Following the corresponding normal mode leads to rotation about the C–B bond of the pentafluorophenyl group to give 5na.

Two structures were found for the radical anion C6 F5 BH 2 (Fig. 8 and Table 6). The global minimum structure 5aa is an essentially planar Cs structure with a 2A0 electronic ground state and a predicted B–C bond distance of 1.517 Å (B3LYP) or 1.527 Å (BP86). In 5aa the fluorine atom in the para position relative to the boron atom is tilted by 7.4° (B3LYP) or 9.9° (BP86) with respect to the BX3 coordination plane (Fig. 8). The orthogonal or twisted C2v structure for C6 F5 BH 2 , namely 5ab (Fig. 8 and Table 6), lies 20.4 kcal/mol (B3LYP) or 19.0 kcal/mol (BP86) in energy above the global minimum 5aa. The electronic state of 5ab is of 2B2 symmetry and the predicted B–C bond distance is 1.600 Å (B3LYP or BP86). For 5ab a large imaginary vibrational frequency of 669i cm1 is predicted by B3LYP, while BP86 predicts two imaginary vibrational frequencies, namely 712i cm1 and 86i cm1. Following the corresponding normal mode leads to 5aa. 3.6. (C6F5)2BH and (C6F5)2BH

Fig. 7. Structures of C6F5BH2 (bond distances in angstroms).

Table 5 Relative energies in eV (or in kcal/mol in parentheses) for the C6F5BH2 structures. Structure

Electronic state

Point group

B3LYP

BP86

5na 5nb

1

C2v C2v

0.0 0.35 (8.1)

0.0 0.38 (8.8)

A1 1 A1

Table 6 Relative energies in eV (or in kcal/mol in parentheses) for the C6 F5 BH 2 structures. Structure

Electronic state

Point group

B3LYP

BP86

5aa 5ab

2

Cs C2v

0.0 0.89 (20.4)

0.0 0.82 (19.0)

A0 2 B2

Two equilibrium structures were found for (C6F5)2BH (Fig. 9 and Table 7). The C2 structure 6na is predicted to be the 1A symmetry global minimum with a predicted B–C bond distance of 1.555 Å (B3LYP) or 1.557 Å (BP86). Both of the pentafluorophenyl rings in 6na are twisted by angles of 31° (B3LYP or BP86) relative to the BX3 coordination plane. The planar C2v structure 6nb for (C6F5)2BH is predicted to lie in energy above the global minimum by 8.4 kcal/mol (B3LYP) or 7.9 kcal/mol (BP86) with an electronic state of 1A1. Structure 6nb is predicted to be a transition state, with a small imaginary vibrational frequency of 27i cm1 (B3LYP) or 28i cm1 (BP86). Following the corresponding normal mode leads to 6na. Two structures were found for (C6F5)2BH (Fig. 10 and Table 8). The global minimum is predicted to be the C2 structure 6aa with 2A symmetry and predicted B–C bond distances of 1.549 Å (B3LYP) or 1.555 Å (BP86). Both of the pentafluorophenyl rings in 6aa are twisted by angles of 28° (B3LYP or BP86) relative to the BX3 coordination plane. The planar C2v structure 6ab of (C6F5)2BH lies in energy above the global minimum 6aa by 7 kcal/mol with a predicted B–C bond distance of 1.562 Å (B3LYP) or 1.567 Å (BP86). Structure 6ab is predicted to be a transition state with a small imaginary vibrational frequency of 22i cm1 (B3LYP) or

Fig. 8. Structures of C6 F5 BH 2 (bond distances in angstroms).

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Fig. 9. Structures of (C6F5)2BH (bond distances in angstroms).

Table 7 Relative energies in eV (or in kcal/mol in parentheses) for the (C6F5)2BH structures.

Table 8 Relative Energies in eV (or in kcal/mol in parentheses) for the (C6F5)2BH structures.

Structure

Electronic state

Structure

Electronic state

Point group

B3LYP

BP86

6na 6nb

1

6aa 6ab

2

C2 C2v

0.0 0.30 (7.0)

0.0 0.29 (6.7)

1

A A1

Point group C2 C2v

B3LYP 0.0 0.37 (8.4)

BP86 0.0 0.34 (7.9)

B B1

2

23i cm1 (BP86). Following the corresponding normal mode leads to 6aa. 3.7. (C6F5)3B and (C6F5)3B The structure predicted for (C6F5)3B is the D3 structure 7na with B–C bond lengths of 1.568 Å (B3LYP) or 1.571 Å (BP86) (Fig. 11). Each of the pentafluorophenyl rings in 7na is tilted by 40° (B3LYP or BP86) relative to the BR3 coordination plane. The most intense predicted ring vibrational frequencies for (C6F5)3B of 1667, 1546, 1502, and 1495 cm1 (B3LYP) correspond rather closely to the most intense experimentally determined [32] frequencies of 1647, 1523, and 1481 cm1. For the radical anion (C6F5)3B the predicted structure 7aa also has D3 symmetry with an electronic state symmetry of 2A2 and a B–C bond distance is 1.571 Å (B3LYP) or 1.574 Å (BP86). In 7aa each of the pentafluorophenyl rings is tilted by 35° (B3LYP) or 36° (BP86) relative to the BR3 coordination plane.

Fig. 11. Structures of (C6F5)3B and (C6F5)3B (bond distances in angstroms).

but not the orthogonally twisted structures. Such pp–pp interactions lead to partial B–C double bonding resulting in shorter B–C bond distances.

4. Discussion 4.1. Boron–carbon bond lengths

4.2. Electron affinities

Table 9 compares the arylboron B–C bond distances for the arylboron structures discussed in this paper. In general the B–C distances for the C6X5BH2 derivatives (X=H, F) are predicted to be shorter for the structures where the BX3 plane is coplanar with the plane of the aryl ring than for the corresponding twisted structures in which the BX3 is orthogonal to the plane of the aryl ring, i.e. a 90° angle between the BX3 and aryl planes. This is reasonable since pp–pp interactions between the empty boron p orbital and the filled p orbitals of the aromatic ring are possible in the planar

Three forms of the neutral-anion energy separations are determined as differences in total energies for a given molecule, namely the adiabatic electron affinity EAad, the vertical electron affinity EAvert, and the vertical detachment energy VDE defined by the following:

EAad ¼ Eðoptimized neutralÞ  Eðoptimized anionÞ

ð1Þ

EAvert ¼ Eðoptimized neutralÞ  Eðanion at optimized neutral geometryÞ

ð2Þ

VDE ¼ Eðneutral at optimized anion geometryÞ  Eðoptimized anionÞ

ð3Þ

Fig. 10. Structures of (C6F5)2BH (bond distances in angstroms).

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Table 9 B–C distances in Å for all structures. Structures

B3LYP

BP86

Angle between the BX3 and aryl planes, degrees

2na 2nb 2aa 2ab 3na 3nb 3nc 3aa 3ab 4na 4aa 5na 5nb 5aa 5ab 6na 6nb 6aa 6ab 7na 7aa

1.537 1.557 1.521 1.594 1.554 1.557 1.568, 1.545 1.549 1.596, 1.518 1.568 1.572 1.545 1.572 1.517 1.600 1.555 1.569 1.549 1.562 1.568 1.571

1.541 1.559 1.531 1.595 1.558 1.561 1.571, 1.550 1.554 1.597, 1.529 1.572 1.575 1.547 1.575 1.527 1.600 1.557 1.570 1.555 1.567 1.571 1.574

Coplanar (0) Twisted (90) Coplanar (0) Twisted (90) Twisted (21 ± 1) Coplanar (0) Twisted (0, 91) Coplanar (0) Twisted (0, 90) Twisted (34 ± 1) Twisted (29) Coplanar (0) Twisted (90) Coplanar (1) Twisted (90) Twisted (31) Coplanar (0) Twisted (28) Coplanar (0) Twisted (40) Twisted (36)

group to an RBH2 derivative. Among the compounds studied in this work the lowest neutral-anion separations are found in BH3 with EAad, EAvet, and VDE values, respectively, of 0.11, 0.10, and 0.11 eV (B3LYP) or 0.32, 0.31, and 0.33 eV (BP86) and the highest neutral-anion separations are found in (C6F5)3B with EAad, EAvet, and VDE values, respectively, of 2.46, 2.34, and 2.57 eV (B3LYP) or 2.52, 2.42, and 2.62 eV (BP86). These calculations are thus consistent with the strong Lewis acidity of (C6F5)3B and indicate huge substituent effects in determining boron electron affinities. Acknowledgments We are indebted to the 111 Project (B07012) in China and the U. S. National Science Foundation (Grants CHE-0749868, and CHE0716718) for support of this research. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.chemphys.2008.10.050. References

Table 10 Adiabatic electron affinities (EAad), zero-point corrected EAad (EAzpve ad ),vertical electron affinities (EAvert) and vertical detachment energies (VDE) for the boranes RnBH3n (R = C6H5 and C6F5) in eV. EAad

EAzpve ad

EAvert

VDE

BH3

B3LYP BP86

0.11 0.32

0.17 0.38

0.10 0.31

0.11 0.33

C6H5BH2

B3LYP BP86

0.47 0.68

0.57 0.78

0.40 0.62

0.54 0.74

(C6H5)2BH

B3LYP BP86

0.74 0.98

0.84 1.07

0.65 0.89

0.84 1.07

(C6H5)3B

B3LYP BP86

0.78 1.04

0.89 1.15

0.70 0.97

0.86 1.11

C6F5BH2

B3LYP BP86

1.37 1.49

1.47 1.59

1.24 1.36

1.64 1.82

(C6F5)2BH

B3LYP BP86

2.00 2.09

2.09 2.19

1.89 1.99

2.11 2.20

(C6F5)3B

B3LYP BP86

2.46 2.52

2.54 2.60

2.34 2.42

2.57 2.62

Table 10 summarizes these data for the boranes of interest in this work. The data in Table 10 indicate that substitution of a phenyl group for hydrogen is seen to increase any of these neutral-anion energy separations by major amounts. Furthermore, complete fluorination of a phenyl group, i.e. going from a C6H5 derivative to the corresponding C6F5 derivative, is also seen to increase any of these neutral-anion energy separations. The effect on the neutral-anion energy separation of adding a third C6H5 or C6F5 group to a R2BH derivative is less than the effect of adding a second C6H5 or C6F5

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