Article pubs.acs.org/Organometallics
Silicon Tetrahydroborate and Silylene Dihydroborate with Interelement B−H−Si and BSi Bonds Jie Zhao, Bing Xu, Wenjie Yu, and Xuefeng Wang* Shanghai Key Lab of Chemical Assessment and Sustainability, Department of Chemistry, Tongji University, Shanghai 200092, China S Supporting Information *
ABSTRACT: Reactions of laser-ablated boron atoms with SiH4 molecules gave silylene dihydroborate (H2BSiH2) in excess argon, which rearranged to silicon tetrahydroborate [Si(μ-H)2BH2] upon 300−350 nm irradiation. These reaction products were identified by functional group infrared frequencies, isotopic shifts, and comparison to infrared frequencies calculated by using density functional theory. It is found that H2BSiH2 has planar doublet ground state with electron-deficient B−Si double-bond character. Bonding analysis for Si(μ-H)2BH2 shows that 3c−2e B−H−Si bond is formed by accepting electrons donated from B−H σ bond.
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INTRODUCTION Boron, the only nonmetal in group 13, is electron-deficient, which favors it to participate in multicenter bonding in species such as deltahedralboranes,1 pure boron clusters,2−5 and in a two-dimensional boron lattice known as α sheet to avoid sp2 hybridization and trigonal structure at the boron atoms.6 The chemical reactivity of B atom with inorganic and organic molecules is a fascinating subject of research from the experimental and theoretical viewpoint. A series of works have been reported on the reactions of boron atoms with hydrocarbons. For example, co-deposition of laser-ablated boron atoms with methane in argon matrix produced several new organoborane species such as H2BCH2, HBCH2, HBCH, and HBCBH.7,8 However, to our knowledge, no work about the reaction of boron atoms with hydrosilanes was reported. Compared with C−H bonds, the oxidative addition of Si−H bond is quite facile due to the weaker Si−H bond. In organic and organometallic chemistry, silyboranes have received much attention because the electronegativity difference of boron and silicon allows for chemoselective activation of Si−B bond and because the distinct reactivity of the boryl and silylgroups enables their sequential introduction into a carbon skeleton with high levels of regiocontrol.9−13 A variety of silylboranes have been synthesized since the first silylboranes prepared in early 1960s.14 The most commonly employed method for preparing silylboranes involves reaction of silyl anions with boron eletrophiles.15 Hartwig et al. reported direct silane borylation catalyzed by iridium;16 Shirobokov et al. demonstrate activation of Si−H bonds on a single borane center and unveiled a mechanism.17 Here we report the infrared spectra of silicon tetrahydroborate Si(μ-H)2BH2 and silylene dihydroborate H2BSiH2, which were produced from the reactions of B atoms with SiH4. Matrix isolation technique is employed to study the reaction and the reaction intermediate, and product could be isolated in our experiment and detected by in situ infrared © XXXX American Chemical Society
spectroscopy. Combined with theoretical calculations, the reaction mechanism and the bond natures of the B−Si bond have been explored.
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RESULTS
Experiments were done using different SiH4 concentrations ranging from 0.1 to 1.5% in argon with different laser energies. Infrared spectra for the reactions of laser-ablated (typically 20 mJ/pulse) nB (natural boron: 80.4% 11B + 19.6% 10B), 10B, and 11 B with 0.5% SiH4 molecules in selected regions are shown in Figures 1−3, respectively. Figure 4 presents the infrared spectra for the reactions of laser-ablated 11B and 10B with SiD4 molecules, and the product absorptions are summarized in Table 1.The stepwise annealing and photolysis behavior of the product absorptions combined with the isotopic shift were employed for product identification. In the annealing process, the heat system was stopped immediately after reaching the desired temperature. The sample was then exposed to a radiation from a high-pressure mercury lamp (175 W) with the aid of filters to permit only selected wavelengths to pass. New bands observed in our 10B + SiH4 experiments were grouped into two sets of bands based on their stepwise annealing and photolysis behavior. One set of bands (labeled m in figures) at 2591.8, 2510.6, 1138.9, 956.5, 815.5, and 744.6 cm−1 appeared after co-deposition, increased on annealing but decreased on 300−350 nm photolysis, and increased again following annealing, indicating these bands are due to a molecule that is thermodynamically favored. Another set of bands (labeled h in figures) at 2551.4, 2474.7, 1927.7, 1449.2, 1444.6, and 1131.5 cm−1 were very weak after co-deposition but enhanced greatly on 300−350 nm photolysis and slightly decreased following annealing. All the bands sharply decreased on full arc photolysis. Other weak absorptions observed in our Received: May 25, 2016
A
DOI: 10.1021/acs.organomet.6b00368 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Figure 1. Infrared spectra from the laser-ablated enriched 10B atoms reactions with SiH4 in excess argon: (a) 10B + 0.5% SiH4 in argon co-deposited for 1 h, (b) after annealing to 25 K, (c) after 300−350 nm irradiation, (d) after annealing to 25 K, and (e) after annealing to 30 K.
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Table 1. Infrared Absorptions (cm−1) from Co-Deposition of Laser-Ablated Boron Atoms with Silane in Excess Argon SiH4 10
B
2591.8 2551.4 2510.6 2474.7 1927.7 1449.2 1444.6 1138.9 1131.5 956.5 815.5 744.6 a
SiD4 11
B
2578.1 2538.3 2502.9 2469.6 1926.3 1447.8 1443.0 1131.5 1125.8 956.5 807.1 738.4
10
B
1969.0 1933.3 1860.7 1815.9 1400.5 1069.3 1084.6 895.4 850.9 699.8 642.1 588.4
11
B
intensa
assignment
m w m w m s vw w w m w m
H2BSiH2 Si(μ-H)2BH2 H2BSiH2 Si(μ-H)2BH2 Si(μ-H)2BH2 Si(μ-H)2BH2 Si(μ-H)2BH2 H2BSiH2 Si(μ-H)2BH2 H2BSiH2 H2BSiH2 H2BSiH2
1949.2 1915.5 1844.1 1807.2 1398.1 1066.9 1081.6 910.2 842.0 699.5 633.4 582.4
DISCUSSION
H2BSiH2. As shown in Figure 1 in the reaction of laserablated 10B atoms with silane, six bands at 2591.8, 2510.6, 1138.9, 956.5, 815.5, and 744.6 cm−1 (labeled m) tracked together in the whole experimental process, appearing on codeposition, increasing by nearly 2-fold upon annealing to 25 K, sharply decreasing following 300−350 nm photolysis, increasing about 20% again on annealing to 25 K and doubling following annealing to 30 K. The experiment was repeated but with 11B in place of 10B, and these bands appeared at 2578.1, 2502.9, 1131.5, 956.5, 807.1, and 738.4 cm−1, respectively. Excepting the 956.5 cm−1 band, the bands show boron isotopic shifts (Figure 2). n B + SiH4 bands at 2591.8, 2510.6 cm−1 (10B + SiH4) and 2578.1, 2502.9 cm−1 (11B + SiH4) show large enough boron isotopic shifts to give 1:4 doublet, which verifies a single B atom involved in these vibrations (Figure 3). The deuterium counterparts (Figure 4) of this group of bands shift to 1969.0, 1860.7, 910.2, 699.8, 642.1, and 588.4 cm−1 with 10B + SiD4 and 1949.2, 1844.1, 895.4, 699.5, 633.4, and 582.4 cm−1 with 11B + SiD4, respectively. The 2578.1 and 2502.9 cm−1 bands (11B + SiH4) shifted to 1949.2 and 1844.1 cm−1 in the 11B + SiD4 experiments, defining the H/D isotopic ratio of 1.3226 and 1.3572 (Table 4), respectively, which are due to typical symmetric and antisymmetric stretching vibrations of terminal BH2 group.7,8 With 10B + SiH4 ,the B−H stretching modes were found at 2591.8 and 2510.6 cm−1, which show similar H/D ratios at 1.3163 and 1.3493 and 10B/11B ratios at 1.0053 and 1.0031. For absorptions at 1131.5 and 807.1 cm−1 (11B + SiH4) and 1138.9 and 815.5 cm−1 (10B + SiH4), the slightly larger 10B/11B (1.0065 and 1.0104) and smaller H/D (1.2637 and 1.2742)
Key: s, strong; m, medium; w, weak; v, very.
experiments also appeared in the experiments of SiH4 with other metal atoms, which are due to precursor fragments and reactive species such as SiH, SiH2, Si2H2, and Si2H6 that have been reported previously.18 The absorptions due to impurities such as H2O, CO2, and CO in the experiments can never be eliminated but can be reduced at most. DFT calculations based on products of boron insertion into the Si−H bond were performed. Two stable isomers, namely, H2BSiH2 and Si(μ-H)2BH2, were obtained, and calculated frequencies and absorbance intensities of these molecules are listed in Tables 2 and 3. B
DOI: 10.1021/acs.organomet.6b00368 Organometallics XXXX, XXX, XXX−XXX
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Table 2. Calculated Vibrational Frequencies (cm−1) and Intensities (km/mol, in Parentheses) of H2BSiH2 at the B3LYP/aug-ccpVTZ Level 10
B/H
2698.5(71) 2600.9(81) 2242.5(93) 2223.3(50) 1166.3(20) 956.9(65) 820.1(12) 755.6(21) 722.3(6) 414.3(0) 408.2(10) 371.0(2)
11
B/H
2682.8(71) 2595.2(79) 2242.5(93) 2223.2(50) 1158.0(19) 956.9(65) 811.1(12) 748.9(20) 703.6(7) 414.3(0) 408.1(10) 370.5(2)
10
B/D
2029.5(32) 1888.8(52) 1621.7(54) 1591.4(26) 912.2(16) 699.6(26) 647.6(5) 596.9(15) 636.9(9) 293.1(0) 292.4(5) 280.3(2)
11
B/D
2007.9(33) 1879.4(49) 1621.7(54) 1591.3(26) 895.7(15) 699.4(27) 636.0(6) 590.3(15) 628.6(9) 293.1(0) 292.4(5) 280.0(2)
description B−H asym str B−H sym str Si−H asym str Si−H sym str BH2 bend SiH2 bend BH2 wag H2BSiH2 def H2B-SiH2 str H2BSiH2 def H2BSiH2 def SiH2 wag
Table 3. Calculated Vibrational Frequencies (cm−1) and Intensities (km/mol, in Parentheses) of Si(μ-H)2BH2 at the B3LYP/ aug-cc-pVTZ Levela 10
B/H
2662.0(73) 2568.4(90) 2026.2(282) 1880.0(1) 1521.7(417) 1519.8(19) 1151.4(84) 955.7(0) 903.4(1) 833.1(0) 548.3(45) 165.7(2) a
11
B/H
2646.7(72) 2563.2(86) 2024.2(280) 1873.6(1) 1520.4(418) 1517(19) 1145.4(90) 955.7(0) 896.1(1) 825.8(0) 533.9(41) 165.6(2)
10
B/D
2001.0(41) 1862.0(68) 1448.3(148) 1385.5(0) 1094.7(215) 1101.1(10) 860.2(14) 686.4(0) 676.0(0) 640.1(0) 516.1(49) 119.3(1)
11
B/D
1979.9(40) 1853.3(64) 1445.6(147) 1374.8(0) 1092.7(217) 1098.3(10) 852.1(18) 679.0(0) 676.0(0) 632.8(0) 506.4(46) 119.3(1)
description B−Ht asym str B−Ht sym str B−Hb sym str B−Hb asym str Si−Hb sym str Si−Hb asym str B(Ht)2 bend Si(μ-H)2BH2 def Si(μ-H)2BH2 def Si(μ-H)2BH2 def Si(μ-H)2BH2 def Si(μ-H)2BH2 def
Hb = bridging H atom, Ht = terminal H atom.
Figure 2. Infrared spectra from the laser-ablated enriched 11B atoms reactions with SiH4 in excess argon: (a) 11B + 0.5% SiH4 in argon co-deposited for 1 h, (b) after annealing to 25 K, (c) after 300−350 nm irradiation, and (d) after annealing to 30 K.
C
DOI: 10.1021/acs.organomet.6b00368 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Figure 3. Infrared spectra from the laser-ablated natural B atoms (80% 11B + 20% 10B) reactions with SiH4 in excess argon: (a) natural B + 0.5% SiH4 in argon co-deposited for 1 h, (b) after annealing to 25 K, and (c) after 300−350 nm irradiation.
Figure 4. Infrared spectra from the laser-ablated boron atoms reactions with SiD4 in excess argon: (a) 11B + 0.5% SiD4 in argon co-deposited for 1 h, (b) after annealing to 25 K, (c) after 300−350 nm irradiation, (d) 10B + 0.5% SiD4 in argon co-deposited for 1 h, (e) after annealing to 25 K, and (f) after 300−350 nm irradiation.
D
DOI: 10.1021/acs.organomet.6b00368 Organometallics XXXX, XXX, XXX−XXX
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Table 4. Comparisons between Experimental and Calculated Vibrational Frequencies (cm−1) at the B3LYP/aug-cc-pVTZ Level and Isotopic Frequency Ratios of H211BSiH2 and Si(μ-H)211BH2a 10
freq mode
a
calcd
obsd
B−H asym str B−H sym str BH2 bending SiH2 bending BH2 waging H2BSiH2 def
2682.8 2595.2 1158.0 956.9 811.1 748.9
2578.1 2502.9 1131.5 956.5 807.1 738.4
B−Ht asym str B−Ht sym str B−Hb sym str Si−Hb sym str Si−Hb asym str B(Ht)2 bend
2646.7 2563.2 2024.2 1520.4 1517.0 1145.4
2538.3 2469.6 1926.3 1447.8 1443.0 1125.8
calcd H2BSiH2 1.0059 1.0022 1.0072 1.0000 1.0111 1.0089 Si(μ-H)2BH2 1.0058 1.0020 1.0010 1.0009 1.0018 1.0052
B/11B
H/D obsd
calcd
obsd
1.0053 1.0031 1.0065 1.0000 1.0104 1.0084
1.3361 1.3809 1.2928 1.3682 1.2753 1.2687
1.3226 1.3572 1.2637 1.3674 1.2742 1.2679
1.0052 1.0021 1.0007 1.0010 1.0011 1.0051
1.3368 1.3830 1.4002 1.3914 1.3812 1.3442
1.3251 1.3665 1.3778 1.3570 1.3341 1.3371
Hb = bridging H atom, Ht = terminal H atom.
Table 5. Electronic Structure of H2BSiH2 And Si(μ-H)2BH2 Calculated at B3LYP Level
ratios for these modes listed in Table 4 match experimental values very well. The terminal SiH2 antisymmetric and symmetric stretching vibrations of H2BSiH2 are predicted frequencies of 2242.5 and 2223.2 cm−1, which are very close to the Si−H stretching vibrations of precursor SiH4 (predicted at 2237.1 and 2225.4 cm−1 with the same method and basis set). Efforts have been made to identify the Si−H stretching vibrations of H2BSiH2 by reducing the concentration of SiH4 and increasing the amount of B atoms in the matrix, but since the intensities of Si−H stretching modes are so strong, we never have a chance to observe the covered bands. Fortunately, the terminal SiH2 bending vibration is observed at 956.5 cm−1 (site at 947.6 cm−1), which gives help to the assignment of SiH2 subunit. The calculated terminal SiH2 bending vibration frequency (956.9 cm−1) and predicted H/ D frequency ratio (1.3682) are in excellent agreement with the observed values (frequency: 956.5 cm−1, ratio: 1.3674). Si(μ-H)2BH2. In 10B + SiH4 experiment (Figure 1), the doublet bands at 2551.4 and 2474.7 cm−1, together with further prominent bands at 1927.7, 1449.2, 1444.6, and 1131.5 cm−1 (labeled h in figures) appeared as weak absorptions after codeposition and showed nearly no change following annealing to 25 K but were enhanced greatly on 300−350 nm photolysis at the expense of the absorptions assigned to H2BSiH2, decreased following annealing to 25 K, and decreased again on annealing
frequency ratios indicate these absorptions are appropriate to the in-plane and out-of-plane bending vibration modes of terminal BH2 group in the molecule.7,8,19 The absorptions at 738.4 cm−1 (11B + SiH4) and 744.6 cm−1 (10B + SiH4) show a 10 B/11B ratio of 1.0084 and H/D ratio of 1.2679. Moreover, the absorption at 956.5 cm−1 shifts to 699.8 cm−1 with SiD4 as reactant, giving H/D isotopic frequency ratio of 1.3674, but showed no 10B/11B shift, indicating that only Si and H are involved in this mode. It is most likely that this absorption arises from the terminal Si−H bending vibrations. Accordingly, this group of bands is assigned to H2BSiH2 molecule. The H2BSiH2 molecule is predicted to have C2v symmetry with 2B1 ground electronic state at the B3LYP/aug-cc-pVTZ level. The optimized geometric parameters are shown in Table 5. The symmetric and antisymmetric stretching vibrations of the BH2 subunit in H211BSiH2 are predicted at 2682.8 and 2595.2 cm−1 (Table 2), which are overestimated by 4.1 and 3.7%, respectively. The in-plane and out-of-plane bending modes for terminal BH2 are predicted as 1158.0 and 811.1 cm−1, which are in very good agreement with the experimental values (1131.5 and 807.1 cm−1). Furthermore, the BH2 rocking mode coupled with SiH2 subunit rocking vibration is calculated as 748.9 cm−1, which is only 10 cm−1 higher than the observed value at 738.4 cm−1. Note that the calculated H/D and 10B/11B E
DOI: 10.1021/acs.organomet.6b00368 Organometallics XXXX, XXX, XXX−XXX
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is predicted as 1873.6 cm−1 with a much lower intensity (1 km/ mol), which was not observed experimentally. The antisymmetric and symmetric stretching modes for bridged Si(Hb)2 subunit (Hb = bridging H atom) coupled with ring deformation were predicted at 1520.4 and 1517.0 cm−1, overestimated by 5% for each mode, which are typical systematic discrepancies with B3LYP functional calculations.22,23 In the reactions of B with SiH4, the B atom inserts into the Si−H bond spontaneously and releases heat of about 61.2 kcal/ mol. The produced hot HBSiH3 molecules rearrange to the most-stable isomer H2BSiH2 (24.0 kcal/mol lower in energy than HBSiH3) immediately, surmounting a reaction barrier of 5.6 kcal/mol. Furthermore, H2BSiH2 rearranged to Si(μH)2BH2 (15.1 kcal/mol higher in energy than H2BSiH2) upon 300−350 nm photolysis. However, in the reaction of Al and Ga with SiH4 in matrix, only the insertion product HMSiH3 and its dissociation product MSiH3 was observed (M = Al, Ga).24,25
to 30 K. In the 11B + SiH4 reactions (Figure 2), all the bands showed redshift and appeared at 2538.3, 2469.6, 1926.3, 1447.8, 1443.0, and 1125.8 cm−1 in turn. The natural boron experiment shows (Figure 3) that bands at 2551.4, 2474.7 cm−1 (10B + SiH4) and 2538.3, 2469.6 cm−1 (11B + SiH4) give a 1:4 doublet with nB + SiH4, which confirms a single B atom involved in these vibrations. The deuterium counterparts of these two bands appeared at 1933.3 and 1815.9 cm−1 in reaction of 10B atoms with SiD4 and 1915.5 and 1807.2 cm−1 in 11 B + SiD4 (Figure 4), which gave typical H/D ratios for B−H stretching vibrations. Two bands are slightly lower than the symmetric and antisymmetric stretching modes of BH2 in H2BSiH2 molecule, which must be associated with the terminal B−H stretching vibration fundamentals for a new species. The absorptions at 1926.3, 1447.8, and 1443.0 cm−1 (11B + SiH4) with corresponding deuterium counterparts at 1398.1, 1066.9, and 1081.6 cm−1 show larger H/D ratios at 1.3778, 1.3570, and 1.3341 but very little boron isotopic shifts, indicating that hydrogen is primarily involved in these modes. Previous N2 matrix isolation investigation concluded that for gallaborane (GaBH6) bridged B−Hb (Hb = bridging H atom) stretching frequency at 1925 cm−1 and Ga−Hb stretching vibrations at 1432 and 1422 cm−1 were found,19 which immediately reminded us that the three frequencies observed at 1926.3, 1447.8, and 1443.0 cm−1 are appropriate for the bridged B−Hb stretching modes and two Si−Hb stretching modes coupled with the ring deformation. In addition, the bands at 1131.5 cm−1 (10B) and 1125.8 cm−1 (11B) exhibit larger boron isotopic shifts and H/D ratios 1.3298 and 1.3371, which are typical H− B−H bending mode for B(Ht)2 (Ht= terminal H atom) subunit. A similar mode has been observed for H2BSiH2 and GaBH6.19 Actually, for bidentate metal tetrahydroborates, two strong symmetric and antisymmetric terminal B−Ht stretches are expected at 2400−2600 cm−1, with a splitting of 40−80 cm−1. In addition, the symmetric and antisymmetric bridging B−Hb stretches coupled with ring deformation are usually observed as overlapped bands in the infrared absorptions near 2000 cm−1. M−Hb symmetric and antisymmetric stretches coupled with ring deformation at 1300−1500 cm−1 and a B(Ht)2 bending vibration at 1050−1150 cm−1 are regularly observed.20,21 Accordingly, this group of absorptions are assigned to the Si(μ-H)2BH2 molecule. The confirmation of assignment for Si(μ-H)2BH2 molecules built on the excellent agreement between observed and calculated frequencies (Table 3) for six fundamental vibrations in three different spectral regions, i.e., terminal B−Ht stretching, bridged B−Hb and Si− Hb stretching modes coupled with ring deformation, and B(Ht)2 bending. B3LYP/aug-cc-pVTZ calculation predicts the Si(μ-H)2BH2 molecule to have C2v symmetry with 2B1 ground electronic state. The symmetric and antisymmetric terminal B−Ht stretching vibrations of Si(μ-H)211BH2 are calculated as 2646.7 and 2563.2 cm−1, which are overestimated only by 4.2 and 3.8%, respectively. The bending vibration of terminal 11 B(Ht)2 is calculated as 1145.4 cm−1 with relative intensity of 90 km/mol, corresponding to the observed absorption at 1125.8 cm−1, overestimated by 1.7%. As listed in Table 4, the calculated deuterium and boron isotopic shifts match experimental values very well. More importantly, the symmetric bridged 11B−Hb stretching mode was predicted as 2024.2 cm−1 with appreciable intensity of 280 km/mol, which is overestimated by 5.1%. The antisymmetric 11B−Hb stretching mode
ℏv
B + SiH4 → HBSiH3 → H 2BSiH 2 XoooY Si(μ‐H)2 BH 2
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heat
BOND CHARACTER The singly occupied molecular orbital (SOMO) in Si(μH)2BH2 is mainly the Si(2px) orbital (perpendicular to Si(μH)2B plane) (Table 5), and the natural population analysis (NPA) charge on Si atom of +0.5 au also suggests that one electron is transferred from Si to BH4 group to make BH4− satisfy the octet rule. The chemical and physical nature of such metal or semimetal tetrahydroborates catches scientific interest not only because it provides routes to the synthetically useful hydrides but also because the structure, bonding, and conformation variability may also be relevant to the chemistry of the isoelectronic CH4 molecule. Among the diverse chemical properties of BH4− is the tendency to form unusual covalent complexes with several transition metals, lanthanides, and actinides,21 in which the B−Ht stretches usually appeared at 2400−2600 cm−1, while B−Hb stretches usually appeared near 2000 cm−1.21,26 The ionic character of complexes tends to lower the B−Ht vibrations and decrease the frequency difference between the B−Ht and B−Hb vibrations.27 For example, NaBH4, good example of ionic species containing the BH4− ligand26 with B−Ht stretches appeared at 2270 cm−1, which is grossly different from that of transition metal and actinide tetrahydroborates. To our knowledge, Si(μ-H)2BH2 is the first semimetal tetrahydroborate observed experimentally despite its existence has been postulated by theoretical calculations.28 Based on the regions where the B−Ht and B− Hb stretches locate, it suggests that Si(μ-H)2BH2 should display structure similar to that of transition metal and actinide tetrahydroborates.20 Electron localization function (ELF)29 analysis characterizes the hydrogen bridged bond B−H−Si as a 3c−2e bond by the identification of two trisynaptic basins V(B,H,Si) with a population of 1.94 e. On the atoms in molecules (AIM)30 picture, the bond critical points between B, Si, and H define the two hydrogen bridges in Si(μ-H)2BH2. As there is no bond critical point between B and Si atoms, the two H-bridges enclose a ring critical point (Table 5). The bond critical point between Si and bridged hydrogen atoms is located in the region with positive Laplacian value, whereas the negative value for the local energy density H(r) = −0.03755 indicates the covalent F
DOI: 10.1021/acs.organomet.6b00368 Organometallics XXXX, XXX, XXX−XXX
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Organometallics bonding character.31 It can also qualitatively be observed from the Laplacian contour map that the B−H bond electron population has a rather distorted shape toward Si atom, which indicates the concentration of electrons in the interaction region of B−H with Si atom, corresponding to the formation of a covalent bond. The natural bond orbital (NBO)32 analysis indicates that the B−H σ bond serves as “dative bond” to empty the 3p orbital of Si atom with second-order perturbation energy E2 = 112.7 kcal/mol and back-donation from lone pair of Si to B−H σ* orbital with E2 = 7.8 kcal/mol. The calculated result shows that the B−H−Si bond is a 3c−2e bond, which is formed by the donation of electrons from the B−H σ bond to empty p orbital of Si. The main group semimetal element Si in Si(μ-H)2BH2 displays properties of transition metals, forming bidentate complexes with BH4− through hydrogen bridge bonds. In the H2BSiH2 molecule, the shape of disynaptic basin V(Si,B) from ELF analysis showed the double-bond character. The integral of electron density in the V(Si,B) basin is 2.79 e. The calculated geometry also showed a shorter bond length dB−Si of H2BSiH2 (1.92 Å) than that of HBSiH3 (1.98 Å). The negative values of energy density (−0.0910 au) and Laplacian of electron density (−0.0612 au) at the bond critical point in the B−Si interaction region are typical for a covalent bond. The existence of this kind of molecules is in contrary to the consensus embodied in the “double-bond rule”, which stated that elements with principal quantum number greater than 2 do not form multiple bonds with themselves or with other elements. In contrast to the other silylene complexes,33−35 the boron atom in H2BSiH2 has no sufficient valence electrons to satisfy a complete double bond, as only three electrons are located in the valence shell orbital of boron atom. The frontier orbital shown in Table 5 suggests the π bond is formed by the interaction of the empty p orbital of boron and the singly occupied p orbital of silicon. Previous theoretical and experimental work found out the importance of the presence of the electron density on 2pz-AOs of boron atoms for the boron hydrides, 36 pure boron clusters, and all-boron graphene.2−6 In these species, the boron atoms are trying to have some electron density on all 2p-AOs and to avoid having one 2p-AO empty. This is in accord with our work finding that the global minimum structure of BSiH4 is the planar silylene dihydroborate H2BSiH2, which is 11.5 kcal/mol more stable than Si(μ-H)2BH2 and 24.0 kcal/mol more stable than HBSiH3. Indeed, in the H2BSiH2 case, the charge transfer from silicon singly occupied 3pz-AO (perpendicular to the molecular plane) to the empty 2pz-AO of boron through π-bonding makes the occupation of the 2pz-AO on boron atoms about 0.3 e, which makes the structure very stable, while in the HBSiH3 case one 2p-AO is empty, which is highly unfavorable.
electronic state molecule. Furthermore, substantial spin density on B at the expense of Si substantiates the π-bonding interaction. Thus, we report here a new class of electrondeficient BSi silylene complex.
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EXPERIMENTAL AND COMPUTATIONAL METHODS
Our experimental method has been described in detail previously.37,38 Laser-ablated natural isotopic boron nB (Aldrich, 80.4% 11B, 19.6% 10 B), enriched 10B (Eagle Pitcher, 93.8% 10B, 6.2% 11B), or enriched 11 B (Eagle Pitcher, 97.5% 11B, 2.5% 10B) was co-deposited with silane in excess argon onto a 4 K CsI window, which was mounted onto cold tip closed-cycle helium refrigerator (Sumitomo Heavy Industrious, Model RDK408D) for 1 h at a rate of 2−4 mmol/h. Typically, 10−30 mJ/pulse laser power was used. SiH4 was purchased from Sdragonchem (Nanjing) Chemical Industry Limited Liability Company (Nanjing, China, chemical purity, ≥99.99%), and SiD4 was purchased from Linde Electronic and Specialty Gases (chemical purity, ≥99.99%, isotopic enrichment, ≥99 atom %). After deposition, infrared spectra were recorded on a Bruker 80 V spectrometer at 0.5 cm−1 resolution between 4000 and 400 cm−1 using a liquid nitrogen cooled broad band MCT detector. Samples were annealed at different temperatures and quickly recooled after reaching the desired temperature, and then more spectra were taken. Selected samples were also subjected to photolysis using a mercury lamp (175 W, without globe) as the light source. Quantum chemical calculations were performed to predict the structures and vibrational frequencies of the observed products using the Gaussian 09 program.39 The Becke three-parameter hybrid functional with the Lee−Yang−Parr correlation corrections (B3LYP) was used.40,41 The aug-cc-pVTZ basis sets were employed for B, Si, and H atoms. Geometries were fully optimized, and vibrational frequencies were calculated with analytical second derivatives. AIM30 and ELF29 analysis were performed with Multiwfn code42 to explore bonding characters.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00368. All computed molecule Cartesian coordinates (XYZ)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (nos. 21373152 and 21371136) and the Ministry of Science and Technology of China (no. 2012YQ220113-7).
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CONCLUSIONS Silane molecules react with boron in solid argon matrix to form silylboron hydride HBSiH3 spontaneously without energy barrier, which may go without energy supply toward a stable silylene dihydroborate H2BSiH2. Upon 300−350 nm photolysis, H2BSiH2 rearranged to Si(μ-H)2BH2, where Si displays properties of transition metals to form donor−acceptor bond with BH4−. H2BSiH2 possesses electron-deficient B−Si double bonds with the computed BSi bond being shorter than a B− Si single bond. Theoretical bond orbital analysis shows that the degenerate π molecular orbital has 68% Si (3p) and 32% B (2p) character and contains a single α-spin electron for this doublet
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REFERENCES
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