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The effect of substitution on the potential energy surfaces of RE13≡AsR (E13 = group 13 elements; R = F, OH, H, CH3, and SiH3) is determined using density functional theory (M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp). The computational studies demonstrate that all triply bonded RE13≡AsR species prefer to adopt a bent geometry that is consistent with the valence electron model. The theoretical studies also demonstrate that RE13≡AsR molecules with smaller substituents are kinetically unstable, with respect to the intramolecular rearrangements. However, triply bonded R′E13≡AsR′ species with bulkier substituents (R′ = SiMe(SitBu3)2, SiiPrDis2, and NHC) are found to occupy the lowest minimum on the singlet potential energy surface, and they are both kinetically and thermodynamically stable. That is to say, the electronic and steric effects of bulky substituents play an important role in making molecules that feature an E13≡As triple bond as viable synthetic target.
Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan
Ming‐Chung Yang
Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan
Shih‐Hao Su
Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan
Ming‐Der Su*
Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung, Taiwan
*Address all correspondence to: midesu@mail.ncyu.edu.tw
1. Introduction
In the past two decades, studies that have been performed by many synthetic chemists have successfully synthesized and characterized homonuclear heavy alkyne‐like RE14≡E14R (E14 = Si, Ge, Sn, and Pb) molecules [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]. Recently, heteronuclear ethyne‐like compounds, RC≡E14R, have also been experimentally studied [24, 25, 26] and theoretically predicted [27, 28, 29].
However, from the valence electron viewpoint, RE13≡E15R (E13 = group 13 elements and E15 = group 15 elements) is isoelectronic with the RE14≡E14R species. Therefore, triply bonded RE13≡E15R is the next synthetic challenge. To the best of the authors’ knowledge, only R2BN molecules that contain a B≡N triple bond have been experimentally demonstrated to exist [30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40].
This chapter reports the possible existence of triply bonded RE13≡AsR molecules, from the viewpoint of the effect of substituents, using density functional theories (DFT): M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp for small substituents and B3LYP/LANL2DZ+dp//RHF/3‐21G* for large substituents. It is hoped that this theoretical study will stimulate further research into the synthetic chemistry of triply bonded RE13≡AsR species.
The effect of the electronegativity of six types of small substituents (R = F, OH, H, CH3, and SiH3) on the stability of the triply bonded RE13≡AsR molecules is determined using the three DFT methods. The molecular properties (geometrical parameters, singlet‐triplet energy splitting, natural charge densities, binding energies (BE), and the Wiberg Bond Index (WBI)) are all listed in Tables 1–5. The reaction profiles for the unimolecular rearrangement reactions for the RE13≡AsR compounds are also given in Figures 1–5.
Figure 1.
The relative Gibbs free energies for RB ≡ AsR (R = F, OH, H, CH3, and SiH3). All energies are in kcal/mol and are calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.
Figure 2.
The relative Gibbs free energies for RAl≡AsR (R = F, OH, H, CH3, and SiH3). All energies are in kcal/mol and are calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.
Figure 3.
The relative Gibbs free energies for RGa ≡ AsR (R = F, OH, H, CH3, and SiH3). All energies are in kcal/mol and are calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.
Figure 4.
The relative Gibbs free energies for RIn≡AsR (R = F, OH, H, CH3, and SiH3). All energies are in kcal/mol and are calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.
Figure 5.
The relative Gibbs free energies for RTl≡AsR (R = F, OH, H, CH3, and SiH3). All energies are in kcal/mol and are calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.
R
F
OH
H
CH3
SiH3
B≡As (Å)
1.901
1.892
1.837
1.839
1.814
(1.898)
(1.888)
(1.835)
(1.839)
(1.820)
[1.908]
[1.906]
[1.849]
[1.861]
[1.839]
∠R–B–As (°)
177.2
179.5
178.1
175.1
175.3
(177.8)
(179.5)
(174.6)
(175.1)
(172.4)
[177.0]
[179.1]
[177.5]
[174.3]
[174.8]
∠B–As–R (°)
93.03
92.73
81.22
94.69
68.92
(92.71)
(92.21)
(89.39)
(94.69)
(68.98)
[92.39]
[92.95]
[78.37]
[96.15]
[72.25]
∠R–B–As–R (°)
180.0
179.8
180.0
179.8
148.7
(180.0)
(180.0)
(180.0)
(179.8)
(180.0)
[180.0]
[176.2]
[179.0]
[176.3]
[179.4]
QB1
0.354
0.184
−0.017
−0.007
0.037
(0.262)
(0.108)
(−0.028)
(−0.057)
(0.036)
[0.232]
[0.070]
[−0.106]
[−0.160]
[−0.407]
QAs2
0.243
0.080
−0.152
−0.073
−0.085
(0.255)
(0.097)
(−0.134)
(−0.040)
(−0.017)
[0.238]
[0.086]
[0.034]
[−0.035]
[0.030]
BE (kcal mol−1)3
63.56
56.97
114.7
94.39
79.90
(63.34)
(60.28)
(120.1)
(137.6)
(74.75)
[57.45]
[55.28]
[113.7]
[132.6]
[73.79]
WBI4
1.800
1.830
2.141
2.027
2.204
(1.813)
(1.823)
(2.158)
(2.029)
(2.168)
[1.835]
[1.836]
[2.135]
[2.041]
[2.185]
Table 1.
The main geometrical parameters, the singlet‐triplet energy splitting (ΔEST), the natural charge densities (QB and QAs), the binding energies (BE), and the Wiberg Bond Index (WBI) for RB≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.
The natural charge density on the B atom.
The natural charge density on the As atom.
BE = E(triplet state for R–B) + E(triplet state for R–As) − E(singlet state for RB≡AsR).
The Wiberg bond index (WBI) for the B–As bond: see [45, 46].
R
F
OH
H
CH3
SiH3
Al≡As (Å)
2.327
2.321
2.218
2.253
2.227
(2.325)
(2.323)
(2.221)
(2.256)
(2.236)
[2.355]
[2.358]
[2.269]
[2.285]
[2.292]
∠R–Al–As (°)
178.6
174.4
172.5
172.8
168.4
(179.5)
(174.3)
(172.2)
(172.0)
(167.3)
[178.8]
[173.9]
[177.5]
[171.1]
[173.7]
∠Al–As–R (°)
93.07
91.08
66.95
98.77
91.93
(93.51)
(92.45)
(67.45)
(100.7)
(95.83)
[90.64]
[90.97]
[75.97]
[100.5]
[90.36]
∠R–Al–As–R (°)
180.0
180.0
180.0
174.2
174.7
(179.8)
(178.5)
(179.6)
(176.8)
(175.7)
[180.0]
[179.0]
[178.0]
[174.5]
[176.8]
QAl1
0.555
0.4574
0.2401
0.293
0.291
(0.530)
(0.443)
(0.234)
(0.280)
(0.313)
[0.784]
[0.540]
[0.504]
[0.353]
[0.245]
QAs2
0.158
0.015
−0.276
−0.170
−0.262
(0.142)
(−0.007)
(−0.246)
(−0.156)
(−0.209)
[0.056]
[−0.032]
[−0.209]
[−0.284]
[−0.290]
BE (kcal mol−1)3
33.90
28.23
71.86
56.47
53.22
(38.90)
(31.24)
(77.42)
(60.57)
(54.98)
[33.89]
[25.68]
[69.27]
[52.63]
[67.74]
WBI4
1.532
1.523
1.714
1.649
1.647
(1.567)
(1.553)
(1.742)
(1.679)
(1.675)
[1.557]
[1.545]
[1.714]
[1.690]
[1.550]
Table 2.
The main geometrical parameters, the singlet‐triplet energy splitting (ΔEST), the natural charge densities (QAl and QAs), the binding energies (BE), and the Wiberg Bond Index (WBI) for RAl≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.
The natural charge density on the Al atom.
The natural charge density on the As atom.
BE = E(triplet state for R–Al) + E(triplet state for R–As) − E(singlet state for RAl≡AsR).
The Wiberg bond index (WBI) for the Al–As bond: see [45, 46].
R
F
OH
H
CH3
SiH3
Ga≡As (Å)
2.261
2.339
2.239
2.330
2.243
(2.319)
(2.314)
(2.224)
(2.243)
(2.242)
[2.364]
[2.364]
[2.263]
[2.285]
[2.270
∠R–Ga–As (°)
179.5
173.2
176.2
169.9
168.5
(178.5)
(177.4)
(178.6)
(173.6)
(179.1)
[179.3]
[176.2]
[179.1]
[171.1]
[179.2]
∠Ga–As–R (°)
92.80
93.16
76.00
103.0
93.43
(94.36)
(94.54)
(79.18)
(99.37)
(73.64)
[91.81]
[93.68]
[80.30]
[100.4]
[76.86]
∠R–Ga–As–R (°)
180.0
175.6
179.6
175.7
173.5
(180.0)
(178.1)
(179.1)
(178.4)
(175.6)
[173.1]
[177.4]
[178.2]
[174.5]
[178.1]
QGa1
0.7067
0.592
0.310
0.4451
0.3352
(0.554)
(0.410)
(0.215)
(0.260)
(0.241)
[0.706]
[0.474]
[0.435]
[0.295]
[0.174]
QAs2
0.0899
−0.047
−0.374
−0.256
−0.3697
(0.154)
(0.023)
(−0.262)
(−0.151)
(−0.222)
[0.133]
[0.006]
[−0.184]
[−0.246]
[−0.284]
BE (kcal mol−1)3
28.56
23.82
67.79
53.57
49.26
(30.61)
(25.96)
(71.91)
(58.12)
(51.77)
[27.65]
[90.75]
[65.14]
[50.32]
[62.24]
WBI4
1.476
1.498
1.691
1.648
1.646
(1.486)
(1.503)
(1.717)
(1.652)
(1.596)
[1.487]
[1.495]
[1.707]
[1.668]
[1.615]
Table 3.
The main geometrical parameters, the singlet‐triplet energy splitting (ΔEST), the natural charge densities (QGa and QAs), the binding energies (BE), and the Wiberg Bond Index (WBI) for RGa≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.
The natural charge density on the Ga atom.
The natural charge density on the As atom.
BE = E(triplet state for R–Ga) + E(triplet state for R–As) − E(singlet state for RGa≡AsR).
The Wiberg bond index (WBI) for the Ga–As bond: see [45, 46].
R
F
OH
H
CH3
SiH3
In≡As (Å)
2.511
2.512
2.412
2.431
2.411
(2.495)
(2.497)
(2.399)
(2.418)
(2.404)
[2.535]
[2.546]
[2.432]
[2.459]
[2.444]
∠R–In–As (°)
179.9
178.8
179.3
173.6
170.9
(179.9)
(176.9)
(179.9)
(173.3)
(168.4)
[177.8]
[175.2]
[179.8]
[172.5]
[167.4]
∠In–As–R (°)
92.32
95.31
81.43
99.72
93.85
(93.86)
(96.11)
(82.67)
(100.4)
(99.59)
[91.08]
[94.22]
[82.28]
[100.5]
[102.0]
∠R–In–As–R (°)
180.0
169.3
177.3
174.7
177.1
(180.0)
(166.8)
(175.9)
(173.0)
(177.4)
[180.0]
[163.8]
[179.6]
[179.8]
[178.2]
QIn1
1.288
1.233
1.012
1.144
0.8840
(1.196)
(1.123)
(0.912)
(1.037)
(0.7881)
[1.343]
[1.287]
[1.076]
[1.121]
[0.9682]
QAs2
0.138
0.036
−0.624
−0.388
−0.767
(0.146)
(0.047)
(−0.571)
(−0.335)
(−0.703)
[0.077]
[−0.005]
[−0.591]
[−0.367]
[−0.748]
BE (kcal mol−1)3
22.14
18.30
55.63
53.87
57.82
(19.72)
(20.13)
(60.95)
(50.24)
(57.34)
[24.06]
[16.22]
[57.18]
[53.36]
[54.39]
WBI4
1.536
1.551
1.773
1.719
1.726
(1.546)
(1.554)
(1.798)
(1.738)
(1.749)
[1.572]
[1.562]
[1.780]
[1.729]
[1.710]
Table 4.
The main geometrical parameters, the singlet‐triplet energy splitting (ΔEST), the natural charge densities (QIn and QAs), the binding energies (BE), and the Wiberg Bond Index (WBI) for RIn≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.
The natural charge density on the In atom.
The natural charge density on the As atom.
BE = E(triplet state for R–In) + E(triplet state for R–As) − E(singlet state for RIn≡AsR).
The Wiberg bond index (WBI) for the In–As bond, see [45, 46].
R
F
OH
H
CH3
SiH3
Tl≡As (Å)
2.535
2.531
2.426
2.446
2.431
(2.533)
(2.536)
(2.428)
(2.450)
(2.432)
[2.558]
[2.570]
[2.429]
[2.459]
[2.433]
∠R–Tl–As (°)
179.9
178.2
180.0
176.6
176.5
(179.9)
(175.8)
(179.5)
(175.0)
(173.4)
[179.2]
[177.0]
[179.5]
[173.8]
[177.7]
∠Tl–As–R (°)
91.49
94.88
84.22
97.14
90.08
(93.64)
(96.73)
(84.51)
(99.33)
(93.68)
[92.21]
[96.20]
[84.07]
[99.33]
[89.37]
∠R–Tl–As–R (°)
180.0
175.5
173.0
178.0
179.2
(179.3)
(176.7)
(178.1)
(178.2)
(178.5)
[180.0]
[172.9]
[179.6]
[177.6]
[177.2]
QTl1
0.736
0.640
0.3883
0.482
0.3051
(0.656)
(0.538)
(0.352)
(0.428)
(0.382)
[0.817]
[0.549]
[0.472]
[0.361]
[0.244]
QAs2
0.190
0.035
−0.4169
−0.251
−0.3290
(0.163)
(0.013)
(−0.351)
(−0.208)
(−0.291)
[0.139]
[0.021]
[−0.204]
[−0.273]
[−0.336]
BE (kcal mol−1)3
13.48
10.36
50.28
38.25
29.93
(16.73)
(13.88)
(55.13)
(43.44)
(30.60)
[15.13]
[8.720]
[49.40]
[37.22]
[45.10]
WBI4
1.109
1.148
1.456
1.382
1.409
(1.143)
(1.174)
(1.492)
(1.416)
(1.407)
[1.168]
[1.175]
[1.484]
[1.413]
[1.411]
Table 5.
The main geometrical parameters, the singlet‐triplet energy splitting (ΔEST), the natural charge densities (QTl and QAs), the binding energies (BE), and the Wiberg Bond Index (WBI) for RTl≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.
The natural charge density on the Tl atom.
The natural charge density on the As atom.
BE = E(triplet state for R–Tl) + E(triplet state for R–As) − E(singlet state for RTl≡AsR).
The Wiberg bond index (WBI) for the Tl–As bond, see [45, 46].
From the tables, the three DFT calculations show that the triple bond distances (Å) for B≡As, Al≡As, Ga≡As, In≡As, and Tl≡As are estimated to be 1.835–1.908 (Table 1), 2.218–2.358 (Table 2), 2.239–2.364 (Table 3), 2.404–2.546 (Table 4), and 2.426–2.570 (Table 5). As previously mentioned, no experimental values for these triple bond lengths have been reported, so these computational data are a prediction.
In Tables 1–5, these DFT computations all demonstrate that the triply bonded RE13≡AsR molecules favor a bent structure, rather than a linear structure. This is explained by the bonding model, as shown in Figure 6. Because there is a significant difference between the sizes of the valence s and p atomic orbitals in the As atom, hybrid orbitals between the valence s and p orbitals are not easily formed (the so‐called orbital non‐hybridization effect or the inert s‐pair effect) [41, 42, 43, 44]. Therefore, RE13≡AsR molecules that have a heavier As center are predicted to favor a bent angle ∠E13–As–R (close to 90°). The DFT computational data that are shown in Tables 1–5 confirm this prediction.
In terms of the stability of the RE13≡AsR species, the three DFT computations are used to study the energy surfaces for the RE13≡AsR systems, and the theoretical results are shown in Figures 1–5. These figures show three local minima (i.e., R2E13=As, RE13≡AsR, and E13=AsR2) and two saddle points that connect them. It is seen that regardless of the type of small substituent, triply bonded RE13≡AsR molecules are unstable on the potential energy surfaces, so they easily undergo a 1,2‐migration reaction to produce the most stable doubly bonded isomers. There is strong theoretical evidence that there is no possibility of observing triply bonded RE13≡AsR compounds in transient intermediates or even in a matrix.
Figure 6.
The bonding models (I) and (II) for the triply bonded RE13≡AsR molecule.
3.2. Large ligands on substituted R′E13≡AsR′
Bulky substituents are used to determine the possible existence of triply bonded R′E13≡AsR′ (R′= SiMe(SitBu3)2, SiiPrDis2, and NHC; (Scheme 1)) molecules. The molecular properties, the natural bond orbital (NBO) [45, 46], and the natural resonance theory (NRT) [47, 48, 49] analyses of R′E13≡AsR′ are computed at the B3LYP/LANL2DZ+dp//RHF/3‐21G* level of theory, and the results are shown in Tables 6, 7 (R′B≡AsR′), 8, 9 (R′Al≡AsR′), 10, 11 (R′Ga≡AsR′), 12, 13 (R′In≡AsR′), and 14 and 15 (R′Tl≡AsR′).
Scheme 1.
Three bulky ligands: SiMe(SitBu3)2, SiiPrDis2, and N‐heterocyclic carbine.
R′
SiMe(SitBu3)2
SiiPrDis2
NHC
B≡As (Å)
1.837
1.821
1.819
∠R′–B–As (°)
177.2
172.9
174.5
∠B–As–R′ (°)
128.2
121.6
111.2
∠R′–B–As–R′ (°)
179.6
177.4
171.5
QB1
−0.280
−0.397
−0.205
QAs2
−0.228
−0.134
0.061
ΔEST (kcal mol−1)3
94.42
75.22
83.64
Wiberg BO4
2.327
2.395
2.254
Table 6.
The geometrical parameters, natural charge densities (QB and QAs), Binding Energies (BE), the HOMO‐LUMO Energy Gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′B≡AsR′ at the B3LYP/LANL2DZ+dp//RHF/3‐21G* Level of Theory.
The natural charge density on the central B atom.
The natural charge density on the central As atom.
BE = E (triplet state for B–R′) + E (triplet state for As–R′) − E (singlet state for R′B≡AsR′).
The Wiberg bond index (WBI) for the B–As bond.
5 ΔH1 = E (:B=AsR′2) − E (R′B≡AsR′); see Scheme 2.
6 ΔH2 = E (R′2B=As:) − E (R′B≡AsR′); see Scheme 2.
R′B≡AsR′
WBI
NBO analysis
NRT analysis
Occupancy
hybridization
Polarization
total/covalent/ionic
Resonance weight
R′ = SiMe(SitBu3)2
2.31
σ = 1.98
σ : 0.6627 B (sp1.46) + 0.7489 As (sp1.07)
43.91% (B) 56.09% (As)
2.35/1.66/0.69
B–As: 5.68% B=As: 60.70% B≡As: 33.62%
π = 1.94
π : 0.5941 B (sp1.00) + 0.8044 As (sp99.99)
35.29% (B) 64.71% (As)
R′ = SiiPrDis2
2.27
σ = 1.98
σ : 0.6630 B (sp1.54) + 0.7486 As (sp1.22)
43.96% (B) 56.04% (As)
2.24/1.71/0.53
B–As: 6.04% B=As : 57.2% B=As : 36.74%
π = 1.94
π : 0.5880 B (sp99.99) + 0.8089 As (sp99.99)
34.58% (B) 65.42% (As)
R′ = NHC
2.26
σ = 1.98
σ : 0.6918 B (sp0.90) + 0.7221 As (sp2.66)
47.86% (B) 52.14% (As)
2.23/1.52/0.71
B–As : 7.05% B=As : 69.13% B≡As : 23.82%
π = 1.94
π : 0.5899 B (sp99.99) + 0.8075 As (sp99.99)
34.80% (B) 65.20% (As)
Table 7.
Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/LANL2DZ+dp level of theory for R′B≡AsR′ compounds that have large substituents.
R′
SiMe(SitBu3)2
SiiPrDis2
NHC
Al≡As (Å)
2.285
2.257
2.307
∠R′–Al–As (°)
179.4
176.0
174.5
∠Al–As–R′ (°)
116.4
118.7
113.0
∠R′–Al–As–R′ (°)
176.1
170.6
176.4
QAl1
0.3771
0.3120
0.4392
QAs2
−0.5579
−0.4907
−0.3144
ΔEST (kcal mol−1)3
44.64
54.23
34.53
Wiberg BO4
2.171
2.184
2.185
Table 8.
The geometrical parameters, natural charge densities (QAl and QAs), binding energies (BE), the HOMO‐LUMO energy gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′Al≡AsR′ at the B3LYP/LANL2DZ+dp//RHF/3‐21G* level of theory.
The natural charge density on the central Al atom.
The natural charge density on the central As atom.
BE = E(triplet state for Al–R′) + E(triplet state for As–R′) − E(singlet state for R′Al≡AsR′).
The Wiberg bond index (WBI) for the Al–As bond.
5 ΔH1 = E(:Al=AsR′2) − E(R′Al≡AsR′); see Scheme 2.
6 ΔH2 = E(R′2Al=As:) – E(R#x2032;Al≡AsR′); see Scheme 2.
R′Al≡AsR′
WBI
NBO analysis
NRT analysis
Occupancy
hybridization
Polarization
total/covalent/ionic
Resonance weight
R′ = SiMe(SitBu3)2
2.21
σ = 1.92
σ : 0.5080 Al (sp1.59) + 0.8614 As (sp1.14)
25.81% (Al) 74.19% (As)
2.24/1.66/0.58
Al–As : 6.51% Al=As : 70.32% Al≡As : 23.17%
π = 1.92
π : 0.4437 Al (sp99.99) + 0.8962 As (sp99.99)
19.69% (Al) 80.31% (As)
R′ = SiiPrDis2
2.29
σ = 1.92
σ : 0.4956 Al (sp1.84) + 0.8685 As (sp1.06)
24.57% (Al) 75.43% (As)
2.27/1.73/0.54
Al–As: 4.52% Al≡As : 57.55% Al≡As : 37.93%
π = 1.91
π : 0.4383 Al (sp99.99) + 0.8988 As (sp99.99)
19.21% (Al) 80.79% (As)
R′ = NHC
2.36
σ = 1.87
σ : 0.5834 Al (sp0.99) + 0.8122 As (sp10.87)
34.04% (Al) 65.96% (As)
2.30/1.59/0.71
Al–As : 6.61% Al=As : 74.90% Al=As : 18.49%
π = 1.94
π : 0.4408 Al (sp90.78) + 0.8976 As (sp99.99)
19.43% (Al) 80.57% (As)
Table 9.
Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/LANL2DZ+dp level of theory for R′Al≡AsR′ compounds that have large substituents.
R′
SiMe(SitBu3)2
SiiPrDis2
NHC
Ga≡As (Å)
2.274
2.252
2.316
∠R′–Ga–As (°)
178.8
178.1
171.9
∠Ga–As–R′ (°)
119.3
122.8
114.5
∠R′–Ga–As–R′ (°)
176.6
171.0
176.5
QGa1
0.1760
0.09195
0.2133
QAs2
−0.4683
−0.3978
−0.2257
ΔEST (kcal mol−1)3
40.67
31.52
33.97
Wiberg BO4
2.125
2.174
2.154
Table 10.
The geometrical parameters, natural charge densities (QGa and QAs), binding energies (BE), the HOMO‐LUMO energy gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′Ga≡AsR′ at the B3LYP/LANL2DZ+dp//RHF/3‐21G* level of theory.
The natural charge density on the central Ga atom.
The natural charge density on the central As atom.
BE = E(triplet state for Ga–R′) + E(triplet state for As–R′) − E(singlet state for R′Ga≡AsR′).
The Wiberg bond index (WBI) for the Ga–As bond.
5 ΔH1 = E(:Ga=AsR′2) − E(R′Ga≡AsR′); see Scheme 2.
6 ΔH2 = E(R′2Ga=As:) − E(R′Ga≡AsR′); see Scheme 2.
R′Ga≡AsR′
WBI
NBO analysis
NRT analysis
Occupancy
Hybridization
Polarization
total/covalent/ionic
Resonance weight
R′ = SiMe(SitBu3)2
2.19
σ = 1.90
σ : 0.5320 Ga (sp1.52) + 0.8468 As (sp1.32)
28.30% (Ga) 71.70% (As)
2.27/1.62/0.65
Ga–As : 4.72% Ga=As : 56.61% Ga=As : 38.67%
π = 1.93
π : 0.4467 Ga (sp99.99) + 0.8947 As (sp99.99)
19.95% (Ga) 80.05% (As)
R′ = SiiPrDis2
2.25
σ = 1.91
σ : 0.5386 Ga (sp1.49) + 0.8426 As (sp1.46)
29.01% (Ga) 70.99% (As)
2.31/1.64/0.67
Ga–As : 7.03% Ga=As : 68.13% Ga=As : 24.84%
π = 1.92
π : 0.4392 Ga (sp99.99) + 0.8984 As (sp99.99)
19.29% (Ga) 80.71% (As)
R′ = NHC
2.33
σ = 1.85
σ : 0.6076 Ga (sp0.98) + 0.7942 As (sp12.06)
36.92% (Ga) 63.08% (As)
2.14/1.71/0.43
Ga–As : 7.12% Ga=As : 75.34% Ga=As : 17.54%
π = 1.93
π : 0.4370 Ga (sp82.50) + 0.8995 As (sp99.99)
19.09% (Ga) 80.91% (As)
Table 11.
Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/LANL2DZ+dp level of theory for R′Ga≡AsR′ compounds that have large substituents.
R′
SiMe(SitBu3)2
SiiPrDis2
NHC
In≡As (Å)
2.446
2.430
2.482
∠R′–In–As (°)
155.9
168.4
171.3
∠In–As–R′ (°)
127.8
120.3
110.8
∠R′–In–As–R′ (°)
173.9
162.0
168.3
QIn1
0.874
0.880
1.021
QAs2
−0.783
−0.822
−0.359
ΔEST (kcal mol−1)3
41.5
45.2
35.7
Wiberg BO4
2.174
2.271
2.141
Table 12.
The geometrical parameters, natural charge densities (QIn and QAs), Binding Energies (BE), the HOMO‐LUMO Energy Gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′In≡AsR′ at the B3LYP/LANL2DZ+dp//RHF/3‐21G* Level of Theory.
The natural charge density on the central In atom.
The natural charge density on the central As atom.
BE = E(triplet state for In–R′) + E(triplet state for As–R′) − E(singlet state for R′In≡AsR′).
The Wiberg bond index (WBI) for the In–As bond.
5 ΔH1 = E(:In=AsR′2) – E(R′In≡AsR′); see Scheme 2.
6 ΔH2 = E(R′2In=As:) − E(R′In≡AsR′); see Scheme 2.
R′In≡AsR′
WBI
NBO analysis
NRT analysis
Occupancy
Hybridization
Polarization
total/covalent/ionic
Resonance weight
R′ = SiMe(SitBu3)2
1.50
σ = 1.87
σ : 0.4940 In (sp1.58) + 0.8695 As (sp1.28)
24.41% (In) 75.59% (As)
2.31/1.55/0.76
In–As : 5.78% In=As :55.2 % In=As : 39.0%
π = 1.85
π : 0.4411 In (sp2.80) + 0.8975 As (sp4.33)
19.45% (In) 80.55% (As)
R′ = SiiPrDis2
1.48
σ = 1.87
σ : 0.4854 In (sp1.71) + 0.8743 As (sp1.26)
23.56% (In) 76.44% (As)
2.18/1.62/0.56
In–As : 6.01% In=As : 56.29% In=As : 37.70%
π = 1.83
π : 0.3873 In (sp99.99) + 0.9220 As (sp1.00)
15.00% (In) 85.00% (As)
R′ = NHC
1.33
σ = 1.80
σ : 0.5709 In (sp1.07) + 0.8210 As (sp8.66)
32.60% (In) 67.40% (As)
2.21/1.48/0.73
In–As : 7.72% In=As : 78.30% In=As : 13.98%
π = 1.94
π : 0.4805 In (sp37.19) + 0.8770 As (sp14.95)
23.09% (In) 76.91% (As)
Table 13.
Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/LANL2DZ+dp level of theory for R′In≡AsR′ compounds that have large substituents.
R′
SiMe(SitBu3)2
SiiPrDis2
NHC
Tl≡As (Å)
2.615
2.565
2.653
∠R′–Tl–As (°)
176.9
177.6
178.7
∠Tl–As–R′ (°)
127.7
121.8
108.0
∠R′–Tl–As–R′ (°)
172.2
170.4
175.2
QTl1
0.310
0.246
0.262
QAs2
−0.462
−0.440
−0.313
ΔEST (kcal mol−1)3
45.07
32.71
34.83
Wiberg BO4
2.157
2.214
2.209
Table 14.
The geometrical parameters, natural charge densities (QTl and QAs), Binding Energies (BE), the HOMO‐LUMO Energy Gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′Tl≡AsR′ at the B3LYP/LANL2DZ+dp//RHF/3‐21G* Level of Theory.
The natural charge density on the central Tl atom.
The natural charge density on the central As atom.
BE = E(triplet state for Tl–R′) + E(triplet state for As–R′) − E(singlet state for R′Tl≡AsR′).
The Wiberg bond index (WBI) for the Tl–As bond.
5 ΔH1 = E(:Tl=AsR′2) − E(R′Tl≡AsR′); see Scheme 2.
6 ΔH2 = E(R′2Tl=As:) – E(R′Tl≡AsR′); see Scheme 2.
R′Tl≡AsR′
WBI
NBO analysis
NRT analysis
Occupancy
Hybridization
Polarization
total/covalent/ionic
Resonance weight
R′ = SiMe(SitBu3)2
2.15
σ = 1.74
σ : 0.5404 Tl (sp1.51) + 0.8414 As (sp2.20)
29.20% (Tl) 70.80% (As)
2.24/1.68/0.56
Tl–As : 6.11% Tl=As : 57.27% Tl=As : 36.62%
π = 1.79
π : 0.3968 Tl (sp4.25) + 0.9179 As (sp1.51)
15.74% (Tl) 84.26% (As)
R′ = SiiPrDis2
2.21
σ = 1.90
σ : 0.3627 Tl (sp38.20) + 0.9318 As (sp1.44)
13.16% (Tl) 86.84% (As)
2.16/1.73/0.43
Tl–As : 7.01% Tl=As : 66.48% Tl=As : 26.51%
π = 1.94
π : 0.3315 Tl (sp99.99) + 0.9435 As (sp1.00)
10.99% (Tl) 89.01% (As)
R′ = NHC
2.11
σ = 1.97
σ : 0.7814 Tl (sp0.07) + 0.6240 As (sp52.63)
61.06% (Tl) 38.94% (As)
2.14/1.71/0.43
Tl–As : 6.71% Tl=As : 75.51% Tl=As : 17.78%
π = 1.97
π : 0.4726 Tl (sp57.71) + 0.8812 As (sp17.96)
22.34% (Tl) 77.66% (As)
Table 15.
Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/LANL2DZ+dp level of theory for R′Tl≡AsR′ compounds that have large substituents.
The results in Tables 6–15 allow three conclusions to be drawn.
The calculations that are shown in Tables 6 (B), 8 (Al), 10 (Ga), 12 (In), and 14 (Tl) show that the computed E13≡As triple bond distances (Å) for these bulkily substituted species (R′E13≡AsR′) are estimated to be 1.821–1.837 (B≡As), 2.257–2.307 (Al≡As), 2.252–2.316 (Ga≡As), 2.430–2.482 (In≡As), and 2.565–2.653 (Tl≡As). The values for the WBO that are shown in Tables 6–10 (for bulky ligands) are obviously greater than those that are shown in Tables 1–5 (for smaller ligands). These WBO values show that bulkier substituents increase the bond order for the E13≡As triple bond length.
Similarly to the results for small ligands, the computational results show that R’E13≡AsR’ species that feature large substituents all adopt a bent conformation. This phenomenon is explained by bonding model (II), which is shown in Figure 6.
The NBO values that are shown in Tables 7 (B≡As), 9 (Al≡As), 11 (Ga≡As), 13 (In≡As), and 15 (Tl≡As) show that the acetylene‐like R’E13≡AsR’ compounds feature a weak triple bond. For example, the B3LYP/LANL2DZ+dp data for the NBO analyses of the B≡As π bonding in (SiiPrDis2–B≡As–SiiPrDis2), which shows that NBO(B≡As) = 0.5880(2s2p99.99)B + 0.8089(4s4p1.00)As, provide strong evidence that the predominant bonding interaction between the B–SiiPrDis2 and the As–SiiPrDis2 units results from 2p(B) ← 4p(As) donation, whereby boron’s electron deficiency and π bond polarity are partially balanced by the donation of the arsenic lone pair into the empty boron p orbital to develop a hybrid π bond. The polarization analyses using the NBO model again demonstrate the presence of the B≡As π bonding orbital, 34.58% of which is composed of natural B orbitals and 65.42% of which is natural As orbitals. Table 7 also shows that the B≡As triple bond in (SiiPrDis2–B≡As–SiiPrDis2) has a shorter single bond character (6.04%) and a shorter triple bond character (36.74%), but a greater double bond character (57.2%), because the ionic part of the NRT bond order (0.53) is shorter than its covalent part (1.71). The same theoretical observations are also seen for the other two differently substituted R’B≡AsR’ compounds, as shown in Table 7, and in the data for the other R’E13≡BiR’ compounds that is shown in Tables 9 (Al), 11 (Ga), 13 (In), and 15 (Tl). These computational data demonstrate that these R’E13≡AsR’ molecules have a weak E13≡As triple bond.
This study of the effect of substituents on the possibility of the existence of triply bonded RE13≡AsR allows the following conclusions to be drawn (Scheme 2):
The theoretical observations provide strong evidence that bonding mode (B) is dominant in the triply bonded RE13≡BiR species, because their structures are bent due to electron transfer (denoted by arrows in Figure 1) and the relativistic effect, which increases stability.
The theoretical evidence shows that both the electronic and the steric effects of substituents are crucial to rendering the E13≡As triple bond synthetically accessible. However, this theoretical study shows that these E13≡As triple bonds are weak. They are not as strong as the traditional C≡C triple bond. The results of this theoretical study show that triply bonded R′E13≡AsR′ molecules that feature bulky substituents are more stable because bulky substituents not only protect the central E13≡As triple bond because there is large steric hindrance but also prohibit polymerization reactions.
Scheme 2.
The predicted structure for the triply bonded RE13≡AsR molecules based on the present theoretical computations.
The authors are grateful to the National Center for High‐Performance Computing of Taiwan in providing huge computing resources to facilitate this research. They also thank the Ministry of Science and Technology of Taiwan for the financial support.
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Written By
Jia‐Syun Lu, Ming‐Chung Yang, Shih‐Hao Su and Ming‐Der Su
Open access peer-reviewed chapter
