ARTICLE pubs.acs.org/Organometallics
Theoretical Description of Triplet Silylenes Evolved from H2SidSi Mohammad R. Momeni and Farnaz A. Shakib* Department of Chemistry, Shahr_e Qods Branch, Islamic Azad University, Tehran 37515-374, Iran
bS Supporting Information ABSTRACT: Computations describe the dependence of the H2MdSi triplet electronic structure on the R-substituent. Whereas silylidenes H2CdSi and H2SidSi benefit from a π1p1 triplet state, the electronegative nitrogen of HNdSi prefers an n1p1 triplet. CCSD(T) and B3LYP calculations predict R2SidSi triplet silylenes are stabilized by π-donor/σ-acceptor R substituents which compensate for electron deficiency in the singly occupied π orbital of the π(1)p(1) triplet state. (NH2)2SidSi, (OH)2SidSi, F2SidSi, (NH2)HSidSi, and (OH)HSidSi all are triplet ground states. In particular, (NH2)2SidSi and (OH)2SidSi have singlet triplet energy gaps (ΔES-T = ET ES) of 10.2 and 10.3 kcal/mol, respectively. More practical results are achieved via cyclization of (NH2)2SidSi, which eliminates the probability of rearrangement. Unsaturation of the resulting cyclic structure to give (NHCHCHNH)SidSi leads to a more favorable triplet silylene with a ΔES-T value of 19.6 kcal/mol. Similar to the common approach of bulky substitution in the synthesis of singlet Arduengo-type N-heterocyclic silylenes, triplet (NRCH2CH2NR)SidSi and (NRCHCHNR)SidSi could be experimentally achievable.
’ INTRODUCTION The concept of finding triplet ground state silylenes has been the subject of many experimental and theoretical studies.1 Why? The answer is in the low ability of heavier group 14 elements to form hybrid orbitals. They prefer the (ns)2(np)2 valence electron configuration, which leads to the divalent singlet ground state H2M (M = Si, Ge, Sn, Pb).2 While methylene (H2C) has a triplet ground state with a singlettriplet energy separation (ΔES-T = ET ES) of 9.0 kcal/ mol,3 the ground state of silylene (H2Si) is a singlet that lies 19 23 kcal/mol lower than its corresponding triplet state.4 Theoretical studies guided chemists toward the ideal synthesis of “triplet silylenes” by means of bulky5 and electropositive6 substituents. In 1999, Apeloig et al. calculated a ΔES-T value of 7.1 kcal/mol in favor of the triplet state for [(t-Bu)3Si]2Si and confidently stated that “[(t-Bu)3Si]2Si has a triplet ground state multiplicity”.7 In 2001, Gaspar et al. reported characterization of a triplet silylene with the [(t-Bu)3Si][(i-Pr)3Si]Si formula8 and offered some evidence for its participation in some reactions through the triplet state. However, Yoshida and Tamaoki believed that this molecule was a singlet ground state silylene with a small ΔES-T value.9 They thought that only [(t-Bu)3Si]2Si would be a viable triplet species, a molecule that was finally synthesized in 2003 by Sekiguchi et al., followed by the synthesis of its alkali-metal derivatives.10,11 In addition to these studies on R2Si silylenes, the unsaturated framework was not neglected.1,12 The simplest unsaturated silylene, i.e. silylidene (H2CdSi), was the subject of several theoretical studies for years1315 and was found as the global minimum on the SiCH2 potential energy surface. However, there were limited experimental data until 1992, when Bengali and Leopold carried out a groundbreaking photoelectron study of SiCH2 and SiCD2.16 The anion photoelectron spectra displayed activity in four vibrational r 2011 American Chemical Society
modes of the neutral species, yielding the first experimentally determined vibrational frequencies for H2CdSi and D2CdSi. Later, the calculations of Sherrill and Schaefer at the TZ2Pf CCSD(T) level confirmed their assignments and brought theory and experiment into satisfactory agreement.17 In 1992, Harper et al. used laser-induced fluorescence (LIF) spectroscopy and ab initio calculations to determine the ground state molecular structure of H2CdSi.18 This singlet silylidene now offers an interesting opportunity to devise novel triplet silylenes. This paper intends to shed some light on the effect of different R and β substituents on determining the electronic states of H2CdSibased silylenes. Furthermore, it will be shown that appropriate substitutions and structural improvements decrease the ΔES-T value of silylidene enough to bring the resulting triplet silylenes to the light of experimental synthesis.
’ COMPUTATIONAL METHODS All calculations are carried out using the Gaussian98 package.19 Full geometry optimizations were performed with QCISD20,21 and B3LYP22,23 methods using the standard 6-31G(d) and 6-31+G(d) Pople basis sets, respectively.24 These levels were found to be the best choice for minimizing the significant spin contaminations anticipated for openshell species examined here.25 Improved energetic results were obtained by single-point calculations at higher levels of theory, including CCSD(T)/6-311++G(d,p)26 and B3LYP/AUG-cc-pVTZ27 based on QCISD/ 6-31G(d) and B3LYP/6-31+G(d) geometries, respectively. A rather good consistency was observed between the trends obtained at the Received: July 19, 2011 Published: August 25, 2011 5027
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Table 1. Electronic States and Principal Geometrical Parameters of r-Substituted Species at QCISD and B3LYP (in Parentheses) along with Their ΔES-T Values at the CCSD(T) and B3LYP (in Parentheses) Levels and NBO Atomic Charges at the B3LYP Levela HX, Å
XSi, Å
ΔES-T, kcal/mol
C2v (C2v)
1.094 (1.094)
1.726 (1.721)
C2v (C2v) C2v (C2v)
1.094 (1.095) 1.494 (1.490)
1.877 (1.887) 2.215 (2.213)
A (3A00 )
C2v (C2v)
1.498 (1.496)
2.292 (2.306)
SG (1A0 )
C∞v (C∞v)
1.008 (1.006)
1.565 (1.562)
Cs (Cs)
1.027 (1.025)
1.731 (1.728)
species
state
H2CdSi
s
H2SidSi
t s t
HNdSi
s t
a
1 0
1 0
A ( A)
3 00
A (3A00 ) 1 0 1 0 A ( A) 3 00 1
3 0
A (3A0 )
PG
qH
qX
qSi
+37.3 (+35.3)
+0.236
1.310
+0.839
+9.3 (+6.6)
+0.165 0.122
0.762 0.022
+0.431 +0.266
+0.165
+0.326
+0.049
+0.388
1.410
+1.021
+0.336
0.968
+0.632
+81.1 (+76.8)
See the Computational Methods for basis sets employed.
Figure 1. The two possible triplet states for silylidenes, emphasizing the R-substituent effect. CCSD(T) and B3LYP levels; however, we use CCSD(T) results within the text. Harmonic vibrational frequencies were computed at the same level of optimization in order to characterize the stationary points as minima, representing equilibrium structures, and to evaluate zero-point vibrational energies. Also, the NBO28 population analysis on optimized structures was accomplished at the B3LYP/AUG-cc-pVTZ level.
’ RESULTS AND DISCUSSION r-Substituents. It was shown by Worthington and Cramer that there are two symmetry-distinct triplet states of similar energy in vinylidene (H2CdC).29 One is the result of the promotion of an electron from the carbene σ (sp) orbital to its p orbital, while the other corresponds to the promotion of an electron from the CdC π orbital to the vacant p orbital. Silylidene (H2CdSi) is expected to behave differently, due to the low ability of silicon to form hybrid orbitals that exert a 3s and a 3p orbital on the silylene center. As a result, the energy gap between the occupied and vacant orbitals of a typical silylene is larger than that of a carbene. What would be the resulting effect on the electronic structure of the triplet silylidene? On the basis of our optimized geometries (Table 1), the only observable difference between the 1A0 singlet and 3A00 triplet states of H2CdSi is the length of the CSi bond. It is 1.726 Å in the singlet state, which is very similar to a typical double H2CdSiH2 bond of 1.720 Å, optimized at the same level. Interestingly, this bond is lengthened to 1.877 Å in the triplet state, very close to a typical single CSi bond of 1.888 Å. An NBO analysis can shed more light on this difference from an electronic viewpoint.30 For the singlet H2CdSi, the computed natural Lewis structure (NLS) has a CSi σ and a CSi π NBO, in addition to a lone pair and a vacant NBO on Si. On the other hand, the predicted NLS for the corresponding triplet state again includes a doubly occupied CSi σ NBO but a semioccupied CSi π with an occupancy number of 0.997e. While the 3s lone pair of Si remains unchanged, the occupancy number of the 3p orbital of Si increases to 0.961e. Hence, it seems that the π1p1 triplet silylidene (Figure 1) is preferred to its n1p1 opponent, due to the high energy gap between the 3s lone pair (n) and vacant 3p of Si. Accordingly, the related frontier molecular orbitals show that the π orbital is higher in energy than the 3s orbital (Figure S1, Supporting Information). The
NBO analysis also shows that the +0.839 formal charge of the singlet Si decreases to +0.431 in the triplet as a result of electron density transfer from the R-carbon. Practically, one finds the singlet silylidene to be much more preferred to the corresponding triplet, leading to a ΔES-T value of +37.3 kcal/mol in favor of the former. This results from the higher electronegativity of carbon relative to silicon (Pauling electronegativities (EN): C, 2.5; Si, 1.8), which obstructs the electron density transfer from the CdSi bonding π orbital to the Si vacant px orbital. Replacing the R-carbon with electropositive silicon affects the electronic structure of the triplet state. The optimized SiSi bond lengths of the singlet and triplet states of H2Si(2)dSi(1) are 2.215 and 2.292 Å, respectively (compare to the typical double and single SiSi bonds of 2.187 and 2.340 Å, respectively). On the basis of the NBO analysis, the predicted structure for the triplet state of H2Si(2)dSi(1) is not a classic NLS, as one finds a doubly occupied Si(2)Si(1) σ NBO, a lone pair on Si(1), and two semioccupied Si(2)Si(1) π NBOs (with occupancy numbers of 0.980e and 0.966e). One of these π NBOs originates from the overlap of the Si(1) px orbital with a hybrid pd0.5 orbital of Si(2). It has, however, been well-established that d orbitals on Si do not take part in the electronic structure of its compounds, and when they are augmented to the basis set, they serve as polarization functions. Hence, this NBO is actually a p orbital on Si(1) which is slightly polarized toward Si(2), not a symmetric π bond. This matter was checked via NBO calculations at another level with fewer d orbitals, MP2/6-31+G(d), and the 3A00 π1p1 electronic structure of the triplet H2SidSi (Figure 1) was established. In order to verify that this is indeed the lowest energy triplet electronic structure, the 3A0 n1p1 structure was computed and found to be 4.5 (5.2) kcal/mol less stable than 3A00 . The 3A0 n1p1 H2CdSi structure was also 18.4 (22.4) kcal/mol less stable than 3A00 π 1p1 H2CdSi. Generally, R-silicon is a triplet state stabilizer, due to its electropositive character, which easily permits the promotion of an electron from a π orbital. This effect is quantified in terms of the isodesmic reaction (1), which shows that R-silicon stabilizes the triplet state by 17.4 kcal/mol while it destabilizes the singlet by 10.5 kcal/mol. As a result, the ΔES-T value decreases from +37.3 kcal/mol for H2CdSi to +9.3 kcal/mol for H2SidSi. H2 CdSi : þ H2 SidSiH2 f H2 SidSi : þ H2 CdSiH2
ð1Þ
ΔES ¼ þ 10:5 ð þ 8:1Þ kcal=mol ΔET ¼ 17:4 ð 20:6Þ kcal=mol Opposite to the electropositive silicon, the electronegative nitrogen could differently affect the electronic properties. The interesting geometrical feature about HNdSi is the linear singlet state with an angle of 180.00°, a symmetry of C∞v, and a NSi bond length of 1.565 Å (Table 1), which lies within the 5028
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Table 2. Electronic States and Principal Geometrical Parameters of Monosubstituted (X)HSidSi Species at the QCISD and B3LYP Levels (in Parentheses) along with Their ΔES-T Values at the CCSD(T) and B3LYP Levels (in Parentheses) and NBO Atomic Charges at the B3LYP Levela state
PG
XSi, Å
SiSi, Å
ΔES-T, kcal/mol
qX
qSi
1
+7.1 (+4.6)
1.051
+0.296
+0.203
A (3A)
1.114
+0.620
0.005
1.275
+0.561
+0.087
1.249
+0.770
0.100 +0.101 0.054
species (H3C)HSi=Si (H2N)HSi=Si (HO)HSidSi FHSi=Si
s
Cs (Cs)
1.899 (1.901)
2.215 (2.215)
t
3
Cs (Cs)
1.892 (1.896)
2.287 (2.301)
s
1
Cs (Cs)
1.720 (1.725)
2.232 (2.242)
t
3
Cs (Cs)
1.708 (1.718)
2.290 (2.297)
Cs (Cs) Cs (Cs)
1.673 (1.675) 1.664 (1.670)
2.226 (2.235) 2.276 (2.287)
1.4 (3.2)
1.085 1.086
+0.680 +0.877
Cs (Cs)
1.626 (1.637)
2.225 (2.235)
+0.3 (1.8)
0.649
+0.709
+0.131
Cs (Cs)
1.621 (1.636)
2.270 (2.286)
0.659
+0.911
0.026
s t s t
a
qSi:
1
A ( A) A (1A) A (3A)
1 0
A (1A0 ) A (3A00 )
3 00 1 0
A (1A0 )
3 00
A (3A00 )
5.1 (5.9)
See the Computational Methods for basis sets employed.
ranges for triple and double NSi bonds of 1.458 and 1.646 Å, respectively. This is consistent with the NBO analysis, which shows one σ and two π NSi bonds, all with an occupancy number of 1.998e. Hence, the R-nitrogen adopts sp hybridization to maximize the overlap between its lone pair and the vacant Si px, leading to the singlet stabilization (32.8 kcal/mol based on the isodesmic reaction (2)). Both HN and NSi bonds of the triplet HNdSi are longer than those of the singlet state, 1.027 and 1.731 Å vs 1.008 and 1.565 Å, respectively. The NBO analysis shows that the 3A0 HNdSi conserves two paired electrons in the NdSi π orbital, while each of the 3s and 3px orbitals of Si are semioccupied (with occupancy numbers of 0.999e and 0.978e, respectively). This n1p1 triplet state (Figure 1) is the result of the high electronegativity of R-nitrogen, which does not permit the excitation of an electron from the π bond, leading to 11.1 kcal/ mol destabilization of the triplet state. H2 CdSi : þ HNdSiH2 f HNdSi : þ H2 CdSiH2
Table 3. Calculated ΔE Values for the β Effects of Monosubstituted (Left Columns, Reaction 3) and Disubstituted (Right Columns, Reaction 4) Species on the Singlet (s) and Triplet (t) States, Separatelya ð3Þ
H2 SidSi : þ X2 SidSiH2 f X2 SidSi : þ H2 SidSiH2
ð4Þ
ΔE, kcal/mol species
ð2Þ
ΔES ¼ 32:8 ð30:4Þ kcal=mol ΔET ¼ þ 11:1 ðþ11:1Þ kcal=mol a
CCSD(T) B3LYP
ΔE, kcal/mol species
CCSD(T) B3LYP
(H3C)HSidSi s
+0.3
+0.3 (CH3)2SidSi s
+0.9
+1.0
t (H2N)HSidSi s
2.0 +5.5
1.7 t +5.1 (NH2)2SidSi s
3.6 +7.9
2.7 +10.4 7.4
t
8.9
7.5
t
11.7
(HO)HSidSi s
+3.4
+3.1 (OH)2SidSi s
+5.4
+7.2
t
7.4
6.6
t
14.3
10.7
FHSidSi
Mono β-Substituent. It was shown that electropositive
R-silicon stabilizes the triplet state of H2SidSi, leading to a decreased ΔES-T value. The probability of further triplet stabilization is investigated by the effect of β-substituents, including CH3, NH2, OH, and F. One methyl substitution does not substantially alter the geometrical parameters of the parent H2SidSi. Both singlet and triplet states of (H3C)HSidSi are molecules having Cs symmetry and a shorter SiSi bond for the singlet (2.215 vs 2.287 Å, respectively; Table 2). As for the parent H2SidSi, the stability of the singlet state compared to the triplet is confirmed through the +7.1 kcal/mol ΔES-T value in favor of the former. However, isodesmic reaction (3) (Table 3) shows that methyl substitution stabilizes the triplet state by 2.0 kcal/mol, while it does not have a considerable effect on the singlet (+0.3 kcal/mol). The inductive effect of the substituents on the semioccupied px orbital of carbene was supposed as the strongest triplet state stabilizing interaction on vinylidene.29 Such an explanation is applicable to (H3C)HSi(2)dSi(1), where a pSi(1)fσ*CSi(2) interaction creates 6.9 kcal/ mol stabilization energy for the triplet state. Similarly, a pSi(1)fσ*HSi(2) interaction causes 6.5 kcal/mol stabilization energy. Because of these interactions, the electron occupancy number of px decreases to 0.892e.
H2 SidSi : þ XHSidSiH2 f XHSidSi : þ H2 SidSiH2
s
+2.8
+2.3 F2SidSi
s
+6.4
+6.1
t
6.3
6.2
t
9.0
7.3
See the Computational Methods for basis sets employed.
Substituting one of the hydrogens on H2SidSi with an amino group generates (H2N)HSidSi which, interestingly, is a triplet ground state lying 5.1 kcal/mol lower than its corresponding singlet state (Table 2). It is worthy of mentioning that a β substituent of any kind does not alter the π1p1 electronic structure of the main body (see Table S3 in the Supporting Information). For singlet (H2N)HSi(2)dSi(1), the computed NLS contains a Si(2)Si(1) σ, a Si(2)Si(1) π, and one NSi(2) σ orbital. A 3s lone pair is on Si(1) with an occupancy number of 1.976e, a 2p lone pair on N with an occupancy number of 1.868e, and a vacant p NBO on Si(1). The semioccupied Si(2)Si(1) π of 3A triplet (H2N)HSi(2)dSi(1) is capable of interacting with the parallel lone pair on nitrogen. Because of this interaction, the occupancy number of the nitrogen lone pair is reduced to 0.936e and a π NSi(2) bond is formed with an occupancy number of 0.996e. This is observable in terms of the donoracceptor interaction of LPNfLP*Si(2), which creates 122.7 kcal/mol stabilization energy for the triplet (H2N)HSidSi. Worthington and Cramer have stated that substituent effects on vinylidene multiplet splittings are dominated by σ-electron-withdrawing 5029
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Table 4. Electronic States and Principal Geometrical Parameters of Disubstituted (X)2SidSi Species at the QCISD and B3LYP Levels (in Parentheses) along with Their ΔES-T Values at the CCSD(T) and B3LYP Levels(in Parentheses) and NBO Atomic Charges at the B3LYP Levela state
PG
XSi, Å
SiSi:, Å
ΔES-T, kcal/mol
qX
qSi
1
4.8 (2.9)
1.069
+0.607
+0.145
A (3A)
1.126
+0.885
0.044
species (H3C)2SidSi (H2N)2SidSi (HO)2SidSi F2SidSi a
s
1
C2v (C2v)
1.899 (1.903)
2.218 (2.220)
t
3
C2v (C2v)
1.893 (1.899)
2.288 (2.303)
s
1
C2 (C2)
1.723 (1.726)
2.237 (2.248)
t
3
C2v (C2v)
1.707 (1.717)
2.319 (2.313)
s t
1
C2 (C2) C2v (C2v)
1.670 (1.670) 1.649 (1.657)
2.225 (2.240) 2.310 (2.311)
s
1
C2v (C2v)
1.610 (1.622)
2.236 (2.253)
t
3
C2v (C2v)
1.605 (1.621)
2.297 (2.304)
A ( A) A (1A) A2 (3A2) A (1A) A (3A00 )
3 00
A1 (1A1) A2 (3A2)
10.2 (11.2)
qSi:
1.300
+1.025
0.008
1.300
+1.217
0.206
10.3 (11.3)
1.102 1.088
+1.210 +1.368
+0.001 0.199
6.1 (6.8)
0.646
+1.278
+0.015
0.655
+1.410
0.099
See Computational Methods for basis sets employed.
interactions that could stabilize the carbene p orbital preferentially.29 The stabilizing pSi(1)fσ*HSi(2) interaction is also detectable in the triplet (H2N)HSidSi, but with a very small energy of 3.9 kcal/mol. On the other hand, there is not a pSi(1)fσ*NSi(2) interaction. Hence, in contrast to Worthington and Cramer’s findings on the stabilization of vinylidenes, our results show that in the case of silylidenes β substituents affect the triplet state through π-donating interactions. The geometries visualize these delocalizations with shorter NSi and longer SiSi bonds of the triplet state in comparison to the corresponding bonds of the singlet state (1.708 and 2.297 Å vs 1.720 and 2.242 Å, respectively). However, the NSi bond length of the singlet (H2N)HSidSi (1.720 Å) is close to a typical double NdSi bond of 1.646 Å. This short bond length intensifies the electronic repulsion between the electrons of the π orbital and nitrogen lone pair, resulting in the destabilization of the singlet state. The isodesmic reaction (3) (Table 3) interprets all these discussions in terms of 8.9 kcal/mol stabilization of the triplet silylene and 5.5 kcal/mol destabilization of the singlet state under the effect of β-amino substitution. On the basis of the obtained results, a triplet ground state is also anticipated for (HO)HSidSi. However, due to the higher electronegativity of oxygen (EN: O, 3.5; N, 3.0) one may not expect the same triplet stabilization as for the amino group. The isodesmic reaction (3) completely confirms this expectation, where it shows 7.4 kcal/mol stabilization of the triplet (HO)HSidSi, 1.5 kcal/mol less than that exerted by an amino group (Table 3). Indeed, the predicted NLS for both singlet and triplet states of (HO)HSidSi are similar to those of (H2N)HSidSi and the oxygen atom has two lone pairs, a sp2 and a p. The occupied p NBO is the donor in the LPO f LP*Si(2) interaction of the triplet (HO)HSidSi which creates a stabilization energy of 92.1 kcal/ mol, less than that of the triplet (H2N)HSidSi. Simultaneously, the hydroxy group has a less destabilizing effect on the singlet state than the amino group (3.4 vs 5.5 kcal/mol, isodesmic reaction (3); Table 3). Both of these effects lead to the ΔES-T value of 1.4 kcal/mol for (HO)HSidSi, which is again in favor of the triplet state. Subsequently, the triplet state of FHSidSi is stabilized by 6.3 kcal/mol compared to the triplet state of the parent H2SidSi, while the corresponding singlet state is destabilized by 2.8 kcal/mol (both effects are lower than those of amino and hydroxy) (Table 3). The LPFfLP*Si(2) interaction creates 77.5 kcal/mol stabilization energy for the triplet state of FHSidSi (lower than those for amino and hydroxy groups). Although a fluoro group is a triplet state stabilizer, it cannot reverse the order of the stability of the singlet and triplet states
(ΔES-T = +0.3 kcal/mol). One may find it interesting that while fluoro substitution on vinylidene (FHC(2)dC(1)) creates 28.7 kcal/mol stabilization of the 3A00 electronic structure through pC(1)fσ*FC(2),29 such an interaction is absent in the case of FHSidSi. It is worthy of mentioning that although LPXfLP*Si(2) (X = N, O, F) stabilization energies are outstandingly large values, indicating the limitations of the NBO localization procedure, the important matter is the decreasing trend of stabilizations from amino to fluoro substituents according to their electronegativities, not the absolute values, and this intention is completely achievable through NBO calculations. Two Identical β Substituents. In order to achieve triplet silylenes with higher stabilities, we introduced two identical substituents to the structure simultaneously. The geometrical features and singlettriplet gaps of the resulted structures are summarized in Table 4. Generally, the geometrical parameters are similar to those of monosubstituted silylenes, despite their higher symmetries. The general trend is the increase of the absolute values of the ΔES-Ts for all of the silylenes. On the basis of the isodesmic reaction (4) (Table 3), two amino substituents exert 11.7 kcal/mol stabilization for the triplet silylene, while that of one amino group was 8.9 kcal/mol. In contrast, two hydroxy groups stabilize the triplet state by 14.3 kcal/mol, which is about twice that of one hydroxy group and 2.6 kcal/mol higher than that for two amino groups. It seems that while the two hydroxy groups cooperate in triplet silylene stabilization, the two amino groups compete with each other. In a comparison between hydroxy and amino groups it is relevant that the lower electronegativity of nitrogen atoms gives them a higher tendency for conjugation with the double bond and hence the two amino groups do not permit each other to show the highest degree of triplet stabilization individually. On the other hand, the higher electronegativity of oxygen atoms lessens the degree of this obstruction. The next molecule is F2SidSi, which was previously shown as the global minimum on the Si2F2 potential energy surface.31 Its ΔES-T value satisfactorily decreases to 6.1 kcal/ mol, owing to 9.0 kcal/mol triplet state stabilization and 6.4 kcal/ mol singlet destabilization. Developing the Structural Features. The probability of rearrangement of acyclic triplet silylenes may obstruct their experimental synthesis. Cyclization to strain-free five-membered structures seems a desirable strategy to stabilize them and minimize the probability of rearrangement. Our (NH2)2SidSi and (OH)2SidSi are the most stable triplet silylenes which can be cyclized. Cyclization of (NH2)2SidSi conserves two substitution 5030
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Organometallics
Figure 2. Optimized geometrical parameters of the triplet (singlet) saturated/unsaturated cyclic silylenes at the QCISD level as well as their ΔES-T values (in kcal/mol) at the CCSD(T) (B3LYP) level.
sites on nitrogens, which makes the cyclic silylene practically accessible by means of providing steric hindrance (Figure 2). The lengths of NSi bonds of both singlet and triplet states of (NHCH2CH2NH)Si(2)dSi(1) are longer than those of the acyclic (NH2)2SidSi (1.717 vs 1.707 Å for the triplet and 1.728 vs 1.723 Å for the singlet states). The main geometrical feature of cyclization is the narrowing of the NSiN angle from 127.48° to 91.79° in the triplet state and from 126.97° to 91.90° in the singlet state. This structural difference lowers the tendency of the two nitrogens of the triplet state to donate electron density to Si(2). Despite the absence of a stabilizing LPNfLP*Si(2) interaction, (NHCH2CH2NH)SidSi benefits from a ΔES-T value of 14.4 kcal/mol, 4.2 kcal/mol more than that of its acyclic analogue in favor of the triplet state. Indeed, similar to the stabilizing interactions inferred by Worthington and Cramer about vinylidenes,29 here one encounters two πSi(1)Si(2)fσ*N(3)Si(2) and πSi(1)Si(2)fσ*N(4)Si(2) interactions, each creating 23.2 kcal/mol stabilizing energy for the triplet state. Simultaneously, there are two σN(3)Si(2)fπ*Si(1)Si(2) and σN(4)Si(2)fπ*Si(1)Si(2) interactions, each stabilizing the triplet state by 23.1 kcal/mol. The possibility of a further decrease in ΔES-T has been investigated through unsaturation of the five-membered ring. As depicted in Figure 2, the NSi bonds of both triplet and singlet states of (NHCHCHNH)SidSi are longer than the corresponding bonds of the saturated structure. This geometry shows the tendencies of nitrogens to conjugate with the double bond, which reduces the negative charges on the nitrogens from 1.079 in (NHCH2CH2NH)SidSi to 0.975 in (NHCHCHNH)SidSi. Subsequently, the stabilizing energy of the πSi(1)Si(2)fσ*N(3)Si(2) and πSi(1)Si(2)fσ*N(4)Si(2) interactions increase to 29.4 kcal/mol and those of σN(3)Si(2)fπ*Si(1)Si(2) and σN(4)Si(2)fπ*Si(1)Si(2) increase to 28.9 kcal/mol. The overall effect of unsaturation of the five-membered ring is the greater decrease of ΔES-T to 19.6 kcal/mol in favor of the triplet state. Now, it seems that, via a strategy similar to the common approach of bulky substitution on the synthesis of singlet Arduengo-type Nheterocyclic silylenes, the future synthesis of triplet (NRCH2CH2NR)SidSi and (NRCHCHNR)SidSi silylenes is expectable.
’ CONCLUSION Singlet ground state H2CdSi ,which is the global minimum on the SiCH2 potential energy surface, was chosen as the base for studying possible triplet silylenes in an unsaturated structure. Investigating the R effect leads to the ΔES-T values of +37.3, +9.3,
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and +81.1 kcal/mol for H2CdSi, H2SidSi, and HN=Si, respectively, at the CCSD(T)/6-311++G(d,p)//QCISD/6-31G(d) level. Electropositive R-silicon easily permits the promotion of an electron from the π orbital, leading to 17.4 kcal/mol triplet stabilization, on the basis of our isodesmic reaction. Replacing one hydrogen atom of H2SidSi with CH3, NH2, OH, and F reveals that π-donor/σ-acceptor amino, hydroxy, and fluoro groups decrease the ΔES-T value to 5.1, 1.4 and +0.3 kcal/ mol, respectively, due to stabilization/destabilization of triplet/ singlet states of the silylenes. Substitution of both hydrogens with two identical groups increases this effect. The triplet states of (H2N)2SidSi, (HO)2SidSi, and F2SidSi lie 10.2, 10.3, and 6.1 kcal/mol lower than their corresponding singlet states. It is indicated that cyclization and unsaturation not only eliminate the probability of rearrangement but also decrease the ΔES-T values that would bring these triplet silylenes to the light of experimental synthesis. (NHCH2CH2NH)SidSi has a ΔES-T value of 14.4 kcal/mol, which decreases further to 19.6 kcal/mol in (NHCHCHNH)SidSi. Nitrogens provide suitable substitution sites for the steric protection of the triplet states of (NRCH2CH2NR)SidSi and (NRCHCHNR)SidSi, and hence they should be synthetically accessible.
’ ASSOCIATED CONTENT Supporting Information. Text, figures, and tables giving the full ref 19 frontier molecular orbitals of H2CdSi, AIM analyses of R- and β-substituted silylenes, relative energies of different electronic states of β-substituted silylenes, and Cartesian coordinates for all calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*Cell: +98 9143163892. Tel/fax: +982146896519. E-mail: fshakib@ yahoo.com.
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