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Article Cite This: ACS Omega 2018, 3, 76−85
Triple-Bonded BoronPhosphorus Molecule: Is That Possible? Jia-Syun Lu,† Ming-Chung Yang,† and Ming-Der Su*,†,‡ †
Department of Applied Chemistry, National Chiayi University, Chiayi 60004, Taiwan Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708, Taiwan
‡
S Supporting Information *
ABSTRACT: The effect of substitution on the potential energy surfaces of RBPR (R = H, F, OH, SiH3, and CH3) is studied using density functional theories (M06-2X/Def2-TZVP, B3PW91/Def2TZVP, and B3LYP/LANL2DZ+dp). There is significant theoretical evidence that RBPR compounds with smaller substituents are fleeting intermediates, so they would be difficult to be detected experimentally. These theoretical studies using the M06-2X/Def2-TZVP method demonstrate that only the triply bonded R′BPR′ molecules bearing sterically bulky groups (R′ = Tbt (=C6H2-2,4,6-{CH(SiMe3)2}3), SiMe(SitBu3)2, Ar* (=C6H3-2,6-(C6H2-2,4,6-i-Pr3)2), and SiiPrDis2) are significantly stabilized and can be isolated experimentally. Using the simple valence-electron bonding model and some sophisticated theories, the bonding character of R′BPR′ should be viewed as R′BI PR′. The present theoretical observations indicate that both the electronic and the steric effect of bulkier substituent ligands play a key role in making triply bonded R′BPR′ species synthetically accessible and isolable in a stable form.
I. INTRODUCTION Molecules that do not contain a metal atom but which feature a triple bond are very rare, probably because the nonmetallic elements do not have valence d orbitals. Therefore, it is quite difficult to use nonmetallic atoms to prepare or synthesize triply bonded molecules.1−8 The most famous example of a triply bonded nonmetallic compound is ethyne, HCCH. During the last two decades, heavy homonuclear acetylene analogues, RE14E14R (E14 = Si, Ge, Sn, and Pb), have been successfully isolated and structurally characterized.9−18 Heteronuclear alkyne analogues, RE14E′14R (E′14 = group 14 elements), have also been the subject of many studies in attempts to increase the understanding of the structural features and their high reactivity.19−31 However, the chemistry of the triply bonded RE13E15R (E13 = group 13 atoms and E15 = group 15 atoms) molecules, which are isoelectronic with acetylene, has not been the subject of as much study as the RE14E14R species. In fact, experimental studies of molecules that feature a E13E15 triple bond are few.32−35 Significantly little is known about the structural and electronic properties of these triply bonded RE13E15R systems, but there have been many studies of these new types of acetylene analogues in many disciplines, such as structural chemistry, catalysis, inorganic chemistry, and organometallic chemistry.1−8,32−38 Since the pioneering work of Coates and Livingstone on the preparation of phosphinoborane (Ph2BPPh2) in 1961,39 many new examples of the B−P single bond have been found.40,41 These are reviewed in the literature.42−44 In addition to that of the molecules that feature a B−P single bond, the synthesis of compounds that feature the BP double bond character has attracted the interest of inorganic and organometallic chemists.42−45 Synthesis of these molecules, which have either © 2018 American Chemical Society
the B−P single bond or the BP double bond, is of interest because they are respectively isoelectronic with ethane and ethylene, from the valence-electron viewpoint. It is therefore expected that the structural, physical, and chemical properties of BP species are all similar to those of CC. However, there is no such similarity of reactivity between CC and BP molecules.46 The polarity and the relative weakness of B−P bonds make the BP molecules much more reactive, in additions or substitutions,46 than comparable CC compounds. Although molecules that feature either a B−P single or a BP double bond have been the subject of many experimental and theoretical studies of their structures and reactivity,47−73 compounds that feature a BP triple bond remain elusive. To the authors’ best knowledge, there have been no experimental or theoretical studies on this subject. This is the first theoretical examination to explore the possibility of forming triply bonded RBPR molecules with respect to small and large substituents. For details about the methodology and computational results, see the Supporting Information. The substituent groups that are studied are H, F, OH, SiH3, CH3 and SiiPrDis2, Ar* (=C6H3-2,6-(C6H2-2,4,6-iPr 3 ) 2 ), SiMe(SitBu 3 ) 2 , and Tbt (=C 6 H 2 -2,4,6-{CH(SiMe3)2}3)74−76 (see Scheme 1). The conclusions of this theoretical study will give experimental synthetic chemists a useful insight into a new research area: the BP triply bonded species. Received: October 4, 2017 Accepted: December 18, 2017 Published: January 4, 2018 76
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together to produce a triply bonded RBPR compound in the singlet ground state. One is [R−B]1 + [R−P]1 → [RBPR]1 (path [1]) and the other is [R−B]3 + [R−P]3 → [RBPR]1 (path [2]). Their valence-bond orbital interactions are schematically shown in Figure 1. Figure 1 shows that the pathway that is used to yield RB PR depends on the promotion energies of the fragments. As will be shown in the next section, the present density functional theory (DFT) calculations show that the R−B component has a singlet ground state, whereas the R−P unit has a triplet ground state. Therefore, if the excitation energy from the triplet ground state to the singlet excited state for the R−P unit is smaller than that from the singlet ground state to the triplet excited state for the R−B component, then R−B and R−P follow path [1] to form a triply bonded RBPR molecule. However, if the promotion energy from the singlet ground state to the triplet excited state for the R−B fragment is smaller than that from the triplet ground state to the singlet excited state for the R−P fragment, then R−B and R−P follow path [2] to produce the RBPR species with a BP triple bond. As seen for path [1] in Figure 1, the triple bond in the RB PR compound can be viewed as two donor−acceptor π bonds and one donor−acceptor σ bond. Accordingly, the bonding property of RBPR can be regarded as RB PR. Similarly, as seen for path [2] in Figure 1, the triple bond in the RBPR compound can be described as one customary σ bond, one customary π bond, and one donor−acceptor π bond, so the bonding character of RBPR can be considered as RB PR. Using these theoretical analyses, the bonding properties of the RBPR species are interpreted in the following section.
Scheme 1
II. GENERAL CONSIDERATIONS According to the valence bond theory,77 the most convenient way to understand the chemical bonding natures of the BP triple bond is to partition the RBPR molecule into two units: R−P and R−B (see Figure 1). From the spin viewpoint, there are two reaction routes that combine the two fragments
III. RESULTS AND DISCUSSION III.I. Small Ligands on Substituted RBPR. The small groups, R (=H, F, OH, SiH3, and CH3), are used to examine the effect of substituents on the stability of triply bonded RB PR molecules. Three types of DFT calculations (M06-2X/ Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ +dp) are also used to determine their reliability. Some physical properties and the important geometrical parameters of RB PR are collected in Table 1. From Table 1, the DFT computations anticipate that the BP triple bond lengths (Å) are between 1.713 and 1.777 (M06-2X/Def2-TZVP), 1.714 and 1.781 (B3PW91/Def2TZVP), and 1.725 and 1.793 (B3LYP/LANL2DZ+dp). According to the available experimental data, the bond distances (Å) for the B−P single bond and the BP double bond are 1.94−2.015 and 1.786−1.859,42−46,78 respectively. The bond lengths of the BP triple bonds that are obtained from DFT calculations are shorter than those that are acquired experimentally.42−46,78 Additionally, the DFT computations given in Table 1 indicate that the promotion energy from the singlet ground state to the triplet state for the R−B unit is calculated to be at least 22 kcal/mol. On the other hand, the excitation energy from the triplet ground state to the singlet state for the R−P fragment is estimated to be at least 15 kcal/mol. Namely, the theoretical evidence demonstrates that the formation of the triply bonded RBPR molecule with small ligands should follow path [1], as shown in Figure 1. In other words, the bonding character of a triply bonded RBPR molecule possessing small ligands can be represented as RB PR. Because the covalent radii of B and P are reported to be 82 and 106 pm,79 respectively, the overlapping populations between
Figure 1. Valence-bond bonding paths [1] and [2] for the triply bonded RBPR compound. ΔE1 = E(triplet state for R−P) − E(singlet state for R−P) and ΔE2 = E(triplet state for R−B) − E(singlet state for R−B). 77
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ACS Omega Table 1. Important Geometrical Parametersa R BP (Å)
∠R−P−B (deg)
∠B−P−R (deg)
∠R−P−B−R (deg)
QBb
QPc
ΔEB for R−B (kcal/mol)d
ΔEP for R−P (kcal/mol)e
HOMO−LUMO (kcal/mol)
BE (kcal/mol)f
WBIg
F
OH
H
CH3
SiH3
1.777 (1.781) [1.793] 173.5 (173.1) [174.3] 98.82 (99.24) [96.42] 180.0 (180.0) [180.0] 0.6484 (0.6101) [0.6673] 0.3809 (0.3772) [0.3842] 81.01 (73.60) [73.97] −28.91 (−33.35) [−31.76] 217.2 (210.1) [194.7] 127.2 (120.8) [121.1] 1.868 (1.876) [1.874]
1.770 (1.775) [1.784] 177.2 (176.3) [177.5] 97.56 (98.51) [96.19] 174.1 (178.5) [180.0] 0.5520 (0.5181) [0.5992] 0.2947 (0.2875) [0.2829] 68.97 (62.12) [64.90] −17.53 (−21.29) [−20.24] 219.8 (208.8) [200.3] 123.4 (117.7) [117.4] 1.863 (1.871) [1.862]
1.718 (1.723) [1.736] 176.9 (179.6) [178.9] 72.53 (77.20) [76.40] 180.0 (180.0) [180.0] 0.1535 (0.1664) [0.1097] −0.0975 (−0.0618) [−0.0281] 28.74 (27.65) [25.39] −30.75 (−35.49) [−33.16] 136.8 (128.3) [121.7] 140.7 (145.9) [128.6] 2.231 (2.176) [2.179]
1.726 (1.728) [1.743] 172.1 (173.5) [173.7] 101.9 (104.6) [99.30] 180.0 (178.1) [179.6] 0.4269 (0.4007) [0.4689] −0.0564 (−0.0633) [−0.0386] 38.47 (32.69) [36.79] −26.47 (−30.26) [−29.21] 228.6 (219.0) [215.1] 129.7 (124.8) [121.9] 2.078 (2.047) [2.056]
1.713 (1.714) [1.725] 174.7 (173.3) [175.0] 72.21 (73.04) [76.42] 180.0 (180.0) [179.7] 0.3578 (0.3283) [0.2387] −0.0484 (−0.0561) [−0.1344] 22.33 (21.77) [22.28] −5.80 (−8.67) [−14.46] 156.2 (144.4) [144.9] 139.5 (132.9) [146.0] 2.220 (2.184) [2.189]
a The Wiberg bond index (WBI), the natural charge densities (QP and QB), the highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) energy gaps, the singlet−triplet energy splitting (ΔEB and ΔEP), and the binding energies (BEs) for RBPR using the B3PW91/Def2-TZVP (in round brackets), M06-2X/Def2-TZVP, and B3LYP/LANL2DZ+dp (in square brackets) levels of theory. bThe natural charge density on the boron atom. cThe natural charge density on the phosphorus atom. dΔEB = E(triplet state for R−B) − E(singlet state for R−B). e ΔEP = E(triplet state for R−P) − E(singlet state for R−P). fBE = E(singlet state for R−B) + E(singlet state for R−P) − E(singlet state for RB PR). gThe Wiberg bond index (WBI) for the BP bond: see refs 80, 81.
not stabilize triply bonded RBPR species, sterically bulky ligands (R′), such as SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar*74−76 (Scheme 1), are used to determine the possibility of forming a triply bonded R′BPR′ molecule. Recently, Liptrot and Power demonstrated that London dispersion forces, which are nonvalent interactions between the bulkier ligands, significantly influence the stability and structure of sterically congested inorganic systems.82 Therefore, the M06-2X/Def2TZVP method83 is used in this work to obtain precise results. Similarly to RBPR that are substituted with small ligands, as discussed previously, the 1,2-migration reactions, R′BPR′ → R2′BP: and R′BPR′ → :BPR2′ (Scheme 2), are used to determine the feasibility of producing triply bonded R′B PR′ compounds, using the M06-2X/Def2-TZVP method. Clearly, as seen in Table 2, the M06-2X/Def2-TZVP data demonstrates that because of a large steric effect the doubly bonded isomers (:BPR2′ and R2′BP:) are all respectively higher in energy by at least 89 and 73 kcal/mol than their corresponding triply bonded R′BPR′ molecule. There is strong theoretical evidence that sterically crowded substituents protect the weak BP triple bond.
boron and phosphorus should be small. Indeed, as seen in Table 1, the Wiberg bond index (WBI)80,81 for the BP bond is predicted to be less than 2.2. However, the WBI for the C C bond in acetylene is shown to be 2.99. The 1,2-migration reactions are used to determine the kinetic stability of the RBPR species that are substituted with small ligands at the three DFT levels of theory. The computational results are listed in Figure 2, which shows that the 1,2-shift reactions have two reaction routes: RBPR → TS1 → R2B P: and RBPR → TS2 → :BPR2. From Figure 2, the three DFT calculations all demonstrate that regardless of whether the small substituent groups are electronegative or electropositive, the triply bonded RBPR molecules are neither thermodynamically nor kinetically stable on the 1,2-shift potential energy surfaces. These theoretical observations provide strong evidence that all of the RBPR triply bonded molecules possessing small groups should be unstable and must spontaneously rearrange to other doubly bonded isomers. III.II. Large Ligands on Substituted R′BPR′. Because these theoretical conclusions show that small ligands (R) do 78
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Figure 2. Relative Gibbs free energy surfaces for RBPR (R = H, F, OH, SiH3, and CH3). These energies are calculated in kcal/mol and are calculated at the M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp levels of theory. For details, see the text and Table 1.
Figure 1, the bonding character of a triply bonded R′BPR′ compound featuring bulky ligands can be considered to be R′B PR′. In other words, its BP triple bond consists of one traditional σ bond, one donor−acceptor π bond, and one traditional π bond. However, for the cases of Ar*, our computational values shown in Table 2 indicate that ΔEB′ for the B−R′ fragment (35 kcal/mol) is larger than ΔEP′ for the P−R′ fragment (at least 31 kcal/mol). According to Figure 1, the triply bonded (Ar*)BP(Ar*) molecule can be considered (Ar*) (path [1]). to be (Ar*)BP From Figure 1, it is noteworthy that the lone-pair orbital of the R′−P fragment includes both s and p valence orbitals. This means that the overlap coupling between the lone-pair orbital of R′−P and the pure p orbital of R′−B is feeble. As a result, the bond order of the BP triple bond must not be large. Indeed, the M06-2X/Def2-TZVP results in Table 2 demonstrate that the WBI values for the BP triple bond are
As seen in Table 2, the triple bond lengths for these triply bonded R′BPR′ molecules are in the range of 1.74−2.02 Å, which are somewhat larger than the BP triple bond distances of the substituted RBPR compounds bearing small ligands shown in Table 1. The reason for the longer bond lengths for the R′BPR′ species should be the steric effects. Table 2 shows that the singlet−triplet energy splitting (ΔEB′) for the R′−B unit is at least 25 kcal/mol, but the modulus of ΔEP′ for the R′−P moiety is predicted to be at least 30 kcal/ mol on the basis of the M06-2X/Def2-TZVP method. Namely, it is easier for the R′−B unit to jump from the singlet ground state to the triplet state than for the R′−P monomer, which is promoted from the triplet ground state to the singlet state. Therefore, the two bulkily substituted fragments, R′−B and R′−P, follow path [2] (Figure 1) to yield a triply bonded R′BPR′ molecule. That is, [R′−B]3 + [R′−P]3 → [R′B PR′]1. According to the valence-orbital interactions shown in 79
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is No. 224 (HOMO) orbital, demonstrating that the former donates electrons (0.216e) to the latter mainly through the HOMO orbital. In addition, the largest contributions from a (SiMe(SitBu3)2)−B component to a (SiMe(SitBu3)2)−P component is No. 223 (HOMO-1) orbital, indicating that the latter donates electrons (0.0682e) to the former principally through the HOMO-1 orbital. After considering the electron donations from various orbitals of both (SiMe(SitBu3)2)−P and (SiMe(SitBu3)2)−B, one may obtain the net amount of electron transfer, which is calculated to be negative (−0.226), suggesting that a (SiMe(SitBu3)2)−P unit donates more electrons to a (SiMe(SitBu3)2)−B unit. This conclusion based on the CDA method agrees well with the valence-electron bonding model (Figure 3). Again, the theoretical information indicates that the bonding nature of R′BPR′ can be represented as R′B PR′. The natural bond orbital (NBO)80,81 and natural resonance theory (NRT)85−87 are used to analyze the electronic densities of (SiMe(SitBu3)2)BP(SiMe(SitBu3)2), (Tbt)BP(Tbt), (SiiPrDis2)BP(SiiPrDis2), and (Ar*)BP(Ar*), whose theoretical results are collected in Table 4. On the basis of the WBI values in Table 4, the bond orders for the BP bond are 2.39, 1.96, 2.15, and 1.97 for SiMe(SitBu3)2, Tbt, SiiPrDis2, and Ar*, respectively. This data is similar to the values that are calculated using NRT, of 2.55, 2.47, and 2.27, respectively. This theoretically computed data strongly suggests that steric hindrance shelters the intramolecular rearrangements and gives the central BP bond a greater bond order. The NBO/NRT analyses in Table 4 show that the bulkily substituted R′BPR′ compounds that are studied in this work all have an analogous electronic structure. For instance, the triple bond in (SiMe(SitBu3)2)BP(SiMe(SitBu3)2) is composed of a σ component, which is slightly polarized toward the P element (57%) and two nondegenerate π components that are heavily polarized toward the P center (π⊥, 76% and π∥, 63%).88 Both the σ and π NBO orbitals are equally filled, with approximately 1.98 (σ), 1.92 (π⊥), and 1.93 (π∥) electrons, which is a feature of localized bonds. Figure 3 schematically
Scheme 2
calculated to be 2.388 (SiMe(SitBu3)2), 2.152 (SiiPrDis2), 1.963 (Tbt), and 1.966 (Ar*), respectively. Again, the theoretical findings show that the bulky ligands can effectively protect the central weak BP triple bond and increase its bond order. The charge decomposition analysis (CDA), which was published by Dapprich and Frenking,84 has been utilized in this work to explain the interactions between two components (i.e., (SiMe(SitBu3)2)−B and (SiMe(SitBu3)2)−P) of a (SiMe(SitBu3)2)BP(SiMe(SitBu3)2) molecule. On the basis of the M06-2X/Def2-TZVP method, the computational data are given in Table 3. From Table 3, the largest contribution from a (SiMe(SitBu3)2)−P fragment to a (SiMe(SitBu3)2)−B fragment
Table 2. Bond Lengths (Å), Bond Angles (deg), Singlet−Triplet Energy Splitting (ΔEB′ and ΔEP′), Natural Charge Densities (QB′ and QP′), Binding Energies (BEs), the Wiberg Bond Index (WBI), HOMO−LUMO Energy Gaps, and Some Reaction Enthalpies for R′BPR′ at the M06-2X/Def2-TZVP Level of Theorya R′
SiMe(SitBu3)2
SiiPrDis2
Tbt
Ar*
BP (Å) ∠R′−B−P (deg) ∠B−P−R′ (deg) ∠R′−B−P−R′ (deg) QB′b QP′c ΔEB′ for R′−B (kcal/mol)d ΔEP′ for R′−P (kcal/mol)e HOMO−LUMO (kcal/mol) BE (kcal/mol)f ΔH1 (kcal/mol)g ΔH2 (kcal/mol)g WBIh
1.736 157.2 122.0 174.7 −0.2574 −0.1824 25.92 −33.10 73.76 89.54 73.75 80.53 2.388
2.021 166.0 112.5 165.5 −0.1395 −0.3922 24.86 −37.47 43.44 90.37 86.65 77.67 2.152
2.023 164.4 121.3 168.9 0.2718 0.2260 28.76 −29.74 47.10 85.42 87.89 101.7 1.963
2.021 166.6 123.3 169.5 0.3520 0.2522 34.64 −30.52 41.60 71.43 87.59 88.01 1.966
See also Scheme 2. bThe natural charge density on the boron atom. cThe natural charge density on the phosphorus atom. dΔEB′ (kcal mol−1) = E(triplet state for R′−B) − E(singlet state for R′−B). eΔEP′ (kcal mol−1) = E(triplet state for R′−P) − E(singlet state for R′−P). fBE (kcal mol−1) = E(triplet state for R′−B) + E(triplet state for R′−P) − E(singlet for R′BPR′). gSee Scheme 2. hThe Wiberg bond index (WBI) for the BP bond: see refs 80, 81. a
80
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Table 3. Charge Decomposition Analysis (CDA) Resultsb for the R′BPR′ (R′ = SiMe(SitBu3)2) System Based on M06-2X Orbitalsa
HOMO LUMO sumc
orbital
occupancy
A
B
A−B
M
214 215 216 217 218 219 220 221 222 223 224 225 226
2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 2.000000 0.000000 0.000000 448.000000
0.001584 0.008839 0.002683 0.000235 −0.000147 0.003282 0.004845 −0.000176 0.001401 0.006280 −0.021614 0.000000 0.000000 0.063156
0.051729 0.005614 0.004550 0.004887 0.000075 0.000852 0.002735 0.004588 0.018757 0.068203 −0.039334 0.000000 0.000000 0.289323
−0.050145 0.003225 −0.001867 −0.004653 −0.000222 0.002430 0.002110 −0.004764 −0.017356 −0.061923 0.017720 0.000000 0.000000 −0.226167
−0.047241 −0.026216 −0.017145 −0.009183 −0.004893 −0.021957 −0.003281 −0.012462 −0.048796 −0.111756 −0.026170 0.000000 0.000000 −0.189901
a
The A term shows the number of electrons donated from the R′−B fragment to the R′−P fragment. The B term shows the number of electrons back-donated from the R′−P fragment to the R′−B fragment. The M term shows the number of electrons involved in repulsive polarization. bFor clearness, only the A, B, and M terms are listed for HOMO (No. 224) − 10 ∼ LUMO + 2. cSummation of contributions from all unoccupied and occupied orbitals.
Figure 3. Natural BP π bonding orbitals (π⊥ and π∥ for (i) and (ii), respectively) for (SiMe(SitBu3)2)BP(SiMe(SitBu3)2), compared with path [2] in Figure 1.
shows the two nondegenerate π bonding orbitals (π⊥ and π∥). These are similar to those in path [2], shown in Figure 1.
bonded RBPR species. In terms of these substituted RBPR systems, the following conclusions are drawn. (1) Calculations at three different levels of DFT are in excellent agreement. All three DFT computations demonstrate that regardless of the electronegativity of the substituents that are attached,88 triply bonded RB PR molecules, which bear the small substituents, are unstable and readily undergo intramolecular rearrangement to yield the correspondingly doubly bonded isomers. In other words, the triply bonded RBPR
IV. CONCLUSIONS This study uses theoretical computations to determine the possibility of producing triply bonded RBPR molecules using different types of substituents. The present work provides the first theoretical examination of the effect of substituents on the stability and electronic bonding structures of these triply 81
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Table 4. Natural Bond Orbital (NBO) and Natural Resonance Theory (NRT) Analysis for R′BPR′ Molecules Possessing Bulky Ligands (R′ = SiMe(SitBu3)2, Tbt, SiiPrDis2, and Ar*) at the M06-2X/Def2-TZVP Level of Theorya,b NBO analysis
NRT analysis
R′BPR′
WBI
occupancy
R′=SiMe(SitBu3)2
2.39
σ: 1.98
σ: 0.6538 B (sp1.20) + 0.7567 P (sp0.80)
π⊥: 1.92
π⊥: 0.4861 B (sp17.12) + 0.8739 P (sp9.30)
π∥: 1.93
π∥: 0.6062 B (sp99.99) + 0.7953 P (sp1.00)
σ: 1.93
σ: 0.6141 B (sp2.20) + 0.7892 P (sp1.20)
π⊥: 1.86
π⊥: 0.5402 B (sp2.52) + 0.8415 P (sp4.70)
π∥: 1.94
π∥: 0.5208 B (sp99.99) + 0.8537 P (sp1.00)
σ: 1.92
σ: 0.7076 B (sp0.94) + 0.7066 P (sp11.73)
π⊥: 1.86
π⊥: 0.5382 B (sp63.76) + 0.8428 P (sp68.98)
π∥: 1.77
π∥: 0.5317 B (sp99.99) + 0.7737 P (sp1.00)
σ: 1.93
σ: 0.7171 B (sp0.74) + 0.6969 P (sp12.85)
π⊥: 1.84
π⊥: 0.5842 B (sp99.99) + 0.8516 P (sp99.99)
π∥: 1.76
π∥: 0.5402 B (sp99.99) + 0.8426 P (sp1.00)
R′ = SiiPrDis2
R′ = Tbt
R′ = Ar*
2.15
1.96
1.97
hybridization
molecules possessing the small ligands are so fragile that their experimental detection is unlikely. (2) Only sterically large substituents (R′) significantly stabilize triply bonded R′BPR′ compounds. These theoretical observations demonstrate that these bulky ligands hinder the intramolecular rearrangement and obstruct the intermolecular associations. Accordingly, the present theoretical proof demonstrates that it should be possible to synthesize molecules that feature a BP triple bond possessing both bulkier ligands. (3) On the basis of the valence electron bonding model and some theoretical bonding analyses (i.e., NBO and NRT), it is concluded that the bonding nature of triply bonded R′BPR′ species that feature bulky groups is R′B PR′. Namely, the BP triple bond comprises one donor−acceptor π bond, one conventional σ bond, and one conventional π bond. Experimentalists are encouraged to confirm these theoretical predictions.
■
■
resonance weight
polarization
total/covalent/ionic
42.74% 57.26% 23.63% 76.37% 36.74% 63.26% 37.71% 62.29% 29.19% 70.81% 27.13% 72.87% 50.07% 49.93% 28.97% 71.03% 26.44% 73.56% 51.43% 48.57% 27.48% 72.52% 25.74% 74.26%
2.19/1.52/0.67
B−P: 6.07% BP: 68.87% BP: 25.06%
2.52/1.23/1.29
B−P: 10.18% BP: 73.31% BP: 16.51%
2.12/1.52/0.60
B−P: 7.29% BP: 73.23% BP: 19.48%
2.11/1.57/0.54
B−P: 2.63% BP: 83.80% BP: 13.57%
(B) (P) (B) (P) (B) (P) (B) (P) (B) (P) (B) (P) (B) (P) (B) (P) (B) (P) (B) (P) (B) (P) (B) (P)
B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp levels of theory (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jia-Syun Lu: 0000-0001-6776-0889 Ming-Der Su: 0000-0002-5847-4271 Author Contributions
J.-S.L. and M.-C.Y. conducted all of the theoretical computations and analyzed the results. M.-D.S. supervised the research activities and contributed to the manuscript preparation. Three authors regularly discussed the progress of the research, reviewed the manuscript, and gave approval for the final version. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to the National Center for HighPerformance Computing of Taiwan for generous amounts of computing time, and the Ministry of Science and Technology of Taiwan for the financial support. Special thanks are also due to reviewers 1, 2, and 3 for very helpful suggestions and comments.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01480. The optimized geometries and the absolute energies (in Hartrees) for all of the points on the potential energy surfaces of RBPR (R = F, OH, H, CH3, and SiH3) and R′InSbR′ (R′ = SiMe(SitBu3 ) 2, SiiPrDis 2 , Tbt (=C6H2-2,4,6-{CH(SiMe3)2}3), and Ar* (=C6H3-2,6(C 6 H 2 -2,4,6-i-Pr 3 ) 2 )) at the M06-2X/Def2-TZVP,
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