J Mol Model (2014) 20:2213 DOI 10.1007/s00894-014-2213-9

ORIGINAL PAPER

Computational investigations on the electronic and structural properties of the unsaturated silylenoid HP=SiLiF Yuhua Qi & Jing Ma & Chongjuan Xu & Bing Geng & Maoxia He

Received: 25 December 2013 / Accepted: 16 March 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract The structures of unsaturated silylenoid HP=SiLiF were studied by density functional theory at the B3LYP/6311+G(d,P) level. Four equilibrium structures, the threemembered ring (1), the four-membered ring (2), the “classical” silane (3), and the linear (4) structures, were located. Their energies are in the order of 4>3>1>2. To exploit the stability of HP=SiLiF, the insertions reaction of 2 and HP=Si into C-Cl have been investigated, respectively. The results show that the insertion of HP=Si is more favorable. To compare with the saturated silylenoid, the insertion reaction of H2SiLiF was also investigated. The calculations indicate that the insertion of HP=SiLiF (2) is more favorable. The unsaturated siylenoid HP=SiLiF has similar reaction characters to saturated silylenoid H2SiLiF and silylene HP=Si. Keywords DFT . Insertion reactions . Unsaturated silylenoids

Introduction Similar to carbenoids, silylenoids are complexes formed between free silylenes and inorganic salts, which can be donated as R2SiMX (M: alkali metal, X: halogen). Silylenoids were first postulated as intermediates in the reduction of dihalosilanes with alkali metals to synthesize polysilanes [1, 2]. They are one of the most often occurring key intermediates Y. Qi (*) : J. Ma : C. Xu : B. Geng : M. He Key Laboratory of Fluorine Chemistry and Chemical Materials of Shandong Province, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, People’s Republic of China e-mail: [email protected] M. He Shandong University, Environmental Research Institute, Jinan 250100, People’s Republic of China

in reactions of organosilium compounds [3, 4]. In recent decades, the synthesis and chemistry of silylenoids have attracted attention from the viewpoints of both fundamental and applied chemistry. Many studies have been carried out both theoretically and experimentally. Clark et al. [5] have carried out the first theoretical study on the simplest silylenoid H2SiLiF, and now many silylenoids such as H2SiMX (M=Li, Na, K; X=F, Cl, Br), and R2SiLiF (R=CH3, CH2CH3) [6–13] have been systematically investigated. The pentacoordinate [14, 15] silylenoids have also been well studied theoretically. In 1995, Tamao et al. [16] experimentally detected the existence of silylenoid Ph2SiLi(OBu-t) for the first time. Most recently, Lee et al. [17] reported the syntheses of stable halosilylenoids (Tsi)X2SiLi (Tsi=C(SiMe3)3, X=Br, Cl) at room temperature. As to the unsaturated silylenoids, HN=SiLiF [18] and H2C=SiLiF [14] have already been studied theoretically. To perform systematic theoretical studies on unsaturated silylenoids, HP=SiLiF are penetrated in the present paper. Through this work, we hope (i) to provide the first computational study of geometries, electronic structures, and the stabilities of the unsaturated HP=SiLiF, (ii) to discuss the main factors contributing to the stabilities of unsaturated silylenoid HP=SiLiF, (iii) to analyze the difference between the properties of the saturated and the unsaturated silylenoids.

Theoretical methods Optimized geometries and energies for the stationary points were obtained using density functional theory at the B3LYP/6311+G(d,p) level [19–22]. The corresponding harmonic vibration frequency calculations at the same level were carried out in order to verify whether the stationary points are local minima (no imaginary frequencies) or saddle points (one imaginary frequency). Based on the B3LYP/6-311+G(d,p)

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potential of HP=Si, which are situated on the P atom (negative), and Si atom (positive), respectively. For LiF, there are two extrema in the molecular electrostatic potential, located on the F atoms (negative) and the Li atom (positive), respectively. So the calculated electrostatic potentials indicate that the Si and P atoms in HP=Si, F and Li atoms in LiF, are active atoms in interacting with other molecules. When HP=Si associates with LiF, four equilibrium structures, 1, 2, 3, and 4, of silylenoid HP=SiLiF are found. Their B3LYP/6-311+ G(d,p) optimized geometries are shown in Fig. 1. The total and relative energies (relative to the sum of energies of silylene HP=Si and LiF) are listed in Table 1. Unless otherwise noted, the energies given in the text are those determined at the B3LYP/6-311+G(d,p) level and included vibrational ZPE (without scale) corrections.

optimized geometries, natural bond orbital (NBO) [23–25] analyses were then used to study the nature of different interactions between atoms and groups. The reaction paths were examined by intrinsic reaction coordinate (IRC) [26] calculations. Gaussian 03 [27] series of programs were employed in all calculations.

Results and discussion Isomers of silylenoid HP=SiLiF The most stable structure of silylene HP=Si is a singlet. The singlet state is 171.2 kJ mol−1 more stable than the triplet state. Thus the singlet state is discussed in the following study. Figure 1 shows the structure of HP=Si. Natural bond analyses show that Si atoms are sp hybridized, in which one sp hybrid orbital forms σ bond with phosphorus atom and two electrons of Si occupy the other sp orbital. As for the two vertical p orbitals of Si, one forms π bonding with the corresponding p orbital of the phosphorus atom and the other p orbital is an unoccupied orbital. So the silylene HP=Si has electrophilicity in the empty p orbital and nucleophilicity in the occupied sp orbital. The molecular electrostatic potentials of HP=Si and LiF are shown in Fig. 2. There are two extrema in the electrostatic Fig. 1 The B3LYP/6-311+ G(d,p) geometries of HP=Si and HP=SiLiF, where the bond length and bond angel are in angstrom and degree, respectively. Values in parentheses are the natural charges

Three-membered ring structure 1 When silylene HP=Si and LiF approach each other, the F atom of LiF closes to the empty p orbital on Si atom of HP=Si, and the sp occupied orbital on Si atom attacks the empty 2s orbital of Li atom simultaneously. As a result, 1 is formed. These electrostatic interactions between Si and LiF make 1 stable. Its energy is 113.3 kJ mol−1 lower than the sum of energies of silylene HP=Si and LiF. There is a three-membered ring of F,

H (-0.045)

(-0.765) F

1.491

1.735 Li

1.854

p 61.9

2.063

P (-0.366)

Si (0.412)

sp

(-0.414) 105.8 2.099 Si P

2.452

(0.306)

85.5

1.435

HP=Si(in singlet)

44.9

(0.887)

H (-0.015)

1 (0.885)

(-0.078) H

Li (0.783)

1.771

Li

F (-0.761)

2.471

LiFSiP=27.6

P

1.815

1.450

97.1 P

2.109

1.423 H (-0.002)

Si (0.615)

(-0.660)

128.9 95.7

98.3 2.196

2.451

(-0.456)

Si (0.358) 116.1

1.665 F (-0.683)

3

2 H (-0.030) 1.496 P (-0.235)

FLiSiP=-144.8 62.0 2.045

1.583

1.593 Si (0.297)

4

2.818

Li (0.920)

F (-0.952)

Li (0.857)

LiF

F (-0.857)

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Fig. 2 Molecular electrostatic potiential of HP=Si and LiF at the B3LY/6-311+G(d,p) level. The most positive potential is blue, while the most negative is red

LiF

Si, and Li atoms in 1, where Si-Li, Si-F, and F-Li distances are 2.452, 1.854, and 1.735Ǻ, respectively. NBO analyses indicate that the main part of HOMO in 1 is situated on the Si atom (occupied number: 0.289), and the main LUMO is localized on the Li atom (occupied number: 0.552). Therefore, the HP=Si moiety in 1 has nucleophilicity in chemical reactions, which would be similar to free silylene HP=Si. Compared with that of silylene HP=Si, the positive charge of Si atom decreases by 0.106. It is reasonable to say that the nucleophilicity of the HP=Si moiety in 1 is enhanced relative to that of silylene HP=Si. Four-membered ring structure 2 The electrons of F atom of LiF donation toward the empty p orbital of Si atom of silylene HP=Si and the electrons transfer from P to Li result the formation of structure 2. As displayed in Fig. 1, complex 2 is a four-membered ring structure. In 2, Li atom interacts not only with the F atom but also with P and H atoms. In fact, 2 is a complex ion pair [HP=Si-F]−Li+. The stability of 2 is enhanced through these electron actions. The energy of complex 2 is −147.1 kJ mol−1 relative to those of monomers, which is the most stable one among the available complexes. 2 may be experimentally detectable and is the structure in which HP=SiLiF exists. NBO analyses indicates that the main part of HOMO in 2 is localized on the Si atom (occupied number: 0.313), and the main LUMO is also on the Si atom (occupied number: 0.462). Table 1 Relative (kJ mol−1) energies of various structures at the B3LYP/ 6-311+G(d,p) level Molecules

ZPE

E + ZPE

HP=Si (in singlet) HP=Si (in triplet) HP=Si (in singlet) + LiF 1 2 3 4

0.00759 0.00760 0.00963 0.01271 0.01297 0.01273 0.01042

−631.43084 (0.0) −631.36564 (171.2) −738.89701 (0.0) −738.94017 (−113.3) −738.95305 (−147.1) −738.92589 (−75.8) −738.90458 (−19.9)

HP=Si

‘Classical’silane structure 3 Complex 3 is a plain structure. In 3, Si atom is sp2 hybridized. The Si-F and Si-Li distances are shorter than those in 1, 2, and 4 structures, indicating that the interactions between the Si and F, Li atoms are strong. 3 is not a complex of silylene HP=Si and LiF but a silane compound. 3 is a ‘classical’ silane. The relative energy of 3 is −75.8 kJ mol−1. Linear structure 4 Complex 4 is formed by donating of the occupied sp electrons on Si atom of silylene HP=Si to the positive Li atom of LiF. In 4, the Si-Li distance is 2.818Ǻ and the four atoms, P, Si, Li, F, are nearly in the same line. Compared with those of 1 and 2, the Si-Li distance is the longest. This indicates that the interaction between silylene HP=Si and LiF in 4 is weakest in the three isomers. The relative energy of 4 is −19.9 kJ mol−1. Insertion reactions into C-Cl To gain a further understanding of the stability of unsaturated silylenoids, the insertion reaction of silylenoid HP=SiLiF with CH3Cl has been investigated. In order to compare the character of unsaturated silylenoids with saturated silylenoids, the Table 2 Relative (kJ mol−1 ) energies for insertions of silylene, silylenoids into CH3Cl at the B3LYP/6-311+G(d,p) level Molecules

ZPE

E + ZPE

HP=Si (in singlet) + CH3Cl TS1 Pro1 2+ CH3Cl TS2 Pro2 H2SiLiF +CH3Cl Pre3 TS3 Pro3

0.04524 0.04531 0.04779 0.05062 0.04935 0.06532 0.05656 0.05736 0.05615 0.05929

−1,131.544924 (0.0) −1,131.48846(148.2) −1,131.63627(−239.8) −1,239.06714 (0.0) −1,239.00750 (156.6) −1,239.17818 (−291.5) −898.26662 (0.0) −898.26663 (0.0) −898.19742 (181.7) −898.35308 (−227.0)

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H 1.434 P

H 2.166

Si

2.078

95.2 2.810

P

1.871

132.0

Si 108.6

2.371

HPSiCl=171.6

are adopted. We also studied the insertion reaction of silylene HP=Si into C-Cl to compare with the results of silylenoid insertions. The total and relative energies (relative to the energy of reactants in the same reaction) are listed in Table 2. The structures of the stationary points along the reaction path are displayed in Figs. 3, 4, 5, and 6. Table 3 lists the natural charges of atoms and groups in the insertion stationaries.

CH3

2.073 PSiCCl=180

H3C

Cl

2.441

Insertion of silylene HP=Si into C-Cl

Cl

TS1

Pro1 The singlet silylene HP=Si has an sp lone-pair orbital and a perpendicularly empty p-orbital on Si atom. When HP=Si approaches CH3Cl, the Cl atom of CH3Cl acts on the porbital of Si atom, and the positive CH3 group of CH3Cl simultaneously interacts with the sp orbital on the Si atom. As the reaction proceeds, the Si atom becomes sp2 hybridized, and the reaction reaches the transition state TS1. Figure 3

Fig. 3 The B3LYP/6-311+G(d,p) geometries (in Å and (˚)) for some stationary points in the insertion reaction of HP=Si with CH3Cl

insertion of H2SiLiF into C-Cl was also researched. For the convenience of comparison, the most stable structure 2 of HP=SiLiF and the three-membered ring structure of H2SiLiF Fig. 4 The B3LYP/6-311+ G(d,p) geometries (in Å and (˚)) for some stationary points in the insertion reaction of HP=SiLiF with CH3Cl

Li

1.795

Li

1.832

F 2.435

F 2.447

1.812

1.755 P

H

2.227

101.0

Si Cl 2.415

LiFSiP=5.0

H

2.179

P

2.774

Si 2.092

LiFSiP=5.5

2.335 CH3

1.865 Cl 3.215

CH3

Pro2

TS2

Fig. 5 Energy (E) and bond distance (r) vs. reaction coordinate (S) in the insertion reaction of HP=SiLiF with CH3Cl at the B3LYP/6-311+G(d,p) level

103.9

E/a.u.

r/angstorm 3.4

Si-C

-1239.06

E 3.2

-1239.08

3.0

Si-Cl

2.8

-1239.10

2.6 -1239.12 2.4

Si -P

-1239.14

2.2 2.0

-1239.16

Si-F C-Cl

1.8

-1239.18 -8

-6

-4

-2

0 1/2

2

S/amu -bohr

4

6

8

J Mol Model (2014) 20:2213 CH3

1.807

Page 5 of 6, 2213 F

1.854

1.740

Cl

Li

CH3Cl

2.392

H H

Si 26.7

3.483

F H

Cl

H Si 1.508

2.390

Li

4.014

1.843

1.749

1.806 C H3

H2SiLiF

Pre1

F 1.928

1.690 Li

2.508

F

H Si 53.2

1.645

H

2.112

H

H

Li

Si

2.961

2.614

2.753

97.9

1.924

2.194

Cl H 3C

Cl H3C

2.407

3.112

TS3

Pro3

Fig. 6 The B3LYP/6-311+G(d,p) geometries (in Å and (˚)) for the stationary points in the insertion reaction of H2SiLiF with CH3Cl

shows that the Si-Cl and Si-C distances in TS1 are 2.371 and 2.810 Å, respectively. The breaking C-Cl distance is 2.441 Å, 0.634 Å longer than that in CH3Cl. The activation barrier for this insertion is 148.2 kJ mol−1. After getting over the transition state TS1, Si-Cl and Si-C bonds are formed with the breaking of the C-Cl bond. Then the product silane Pro1 can be obtained. As shown in Table 2, the insertion reaction of silylene HP=Si into C-Cl is highly exothermic by 239.8 kJ mol−1. Insertion of silylenoid HP=SiLiF (2) into C-Cl Figure 4 shows that the insertion process of HP=SiLiF (2) is similar to that of silylene HP=Si. The sp electrons of Si atom are partially donated into the CH3 group of CH3Cl, meanwhile, the Cl atom with the lone pair electrons attacks the positive Si atom. As a result, the transition state TS2 forms. Compared with those of 2 and CH3Cl, the positive charges of Si (0.615) atom in TS2 increase by 0.137, whereas the electron charges of CH3Cl moiety in TS2 decrease to −0.102. The insertion

reaction path was also fully confirmed by the IRC computations (Fig. 5). It is obvious that the bond lengths, Si-C, Si-Cl, and C-Cl, change strongly in the course of the reaction. The Si-C and Si-Cl distances rapidly shorten from reactant side and arrive to 2.774, 2.415 Å in TS2, respectively. Figure 5 shows that the C-Cl bond lengthens and the equilibrium bond length of C-Cl was broken at about s=−4.0 (amu)1/2 bohr. The activation barrier for the reaction is 156.6 kJ mol−1, which is 8.4 kJ mol−1 higher than that of the silylene HP=Si insertion reaction. After getting over the transition state TS2, Pro2 are gradually formed with the LiF moiety leaving from the Si atom. Pro2 adopts a tetra-coordinate conformation on the silicon center. The results show that the insertion of silylenoid HP=SiLiF into C-Cl resembles the silylene HP=Si insertion into C-Cl, but the later is more favorable. It is indicated silylenoid HP=SiLiF is more stable than the corresponding silylene. This may help to suggest further synthetic applications. Insertion reaction of silylenoid H2SiLiF into C-Cl Figure 6 indicates the insertion reaction process of silylenoid H2SiLiF into C-Cl is similar to the insertions of silylene HP=Si and silylenoid HP=SiLiF into C-Cl. The initial formation of the precursor complex Pre3 is facilitated by the interaction between the p orbital on Si and the negative Cl atom. Two electronic donation effects contribute to the proceeding of the insertion reaction. One is the donation of the electrons of Cl into the p orbital on the Si atom. The other is the donation of the σ electrons on the Si atom to the positive CH3 group. These interactions make the reaction intermediate Pro3 formed via the transition state TS3. The activation barrier for this insertion is 181.7 kJ mol−1, which is 25.1 kJ mol−1 higher than that for HP=SiLiF insertion into C-Cl. Comparing the three insertion reactions, it is obvious that the insertion process and reaction mechanisms are

Table 3 The NBO charges of atoms and groups in the insertion stationaries at the B3LYP/6-311++G(d,p) level

Si P Cl C CH3 F Li

HP=Si

TS1

Pro1

1

TS2

Pro2

0.411 −0.366

0.394 −0.308 −0.339 −0.361 0.267

1.003 −0.291 −0.350 −1.105 −0.358

0.615 −0.660

0.752 −0.742 −0.314 −0.409 0.212 −0.758 0.855

1.337 −0.718 −0.384 −1.119 −0.392 −0.721 0.882

−0.761 0.885

H2SiLiF

Pre3

TS3

Pro3

0.306

0.284

0.482

1.128

−0.746 0.856

−0.067 −0.539 0.082 −0.754 0.856

−0.423 −0.427 −0.164 −0.775 0.885

−0.491 −1.106 −0.449 −0.832 0.960

CH3Cl

−0.079 −0.537 0.079

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similar. These calculation results show that HP=SiLiF has similar reaction characters to silylene HP=Si, and silylenoid H2SiLiF.

Conclusions 1. Silylenoid HP=SiLiF has four isomers, the three-membered ring (1), the four-membered ring (2), the “classical” silane (3), and the linear (4) structures. Their energies are in the order of 4>3>1>2. 2. The insertion reactions of silylenoid HP=SiLiF (2) into C-Cl is similar to that of silylene HP=Si into the C-Cl bond. The activation barrier for the former is 8.4 kJ mol−1 higher than that for the latter. Silylenoid HP=SiLiF is more stable than silylene HP=Si. 3. The energy barrier of HP=SiLiF insertion into C-Cl is 25.1 kJ mol−1 lower than that of H2SiLiF, showing that the unsaturated silylenoid is less stable. Acknowledgments The project was supported by Key Laboratory of Fluorine Chemistry and Chemical Materials of Shandong Province.

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