The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group 13 Elements: Theoretical Designs and Characterizations The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group 13 Elements: Theoretical Designs and Characterizations

The effect of substitution on the potential energy surfaces of RE 13 ≡ AsR (E 13 = group 13 elements; R = F, OH, H, CH 3 , and SiH 3 ) is determined using density functional theory (M06‐2X/Def2‐TZVP,B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp). The computa‐ tional studies demonstrate that all triply bonded RE 13 ≡ AsR species prefer to adopt a bent geometry that is consistent with the valence electron model. The theoretical studies also demonstrate that RE 13 ≡ AsR molecules with smaller substituents are kinetically unstable, with respect to the intramolecular rearrangements. However, triply bonded R′E 13 ≡ AsR′ species with bulkier substituents (R′ = SiMe(Si t Bu 3 ) 2 , Si i PrDis 2 , 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 E 13 ≡ As triple bond as viable synthetic target.


Introduction
In the past two decades, studies that have been performed by many synthetic chemists have successfully synthesized and characterized homonuclear heavy alkyne-like RE 14 ≡E 14 R (E 14 = Si, Ge, Sn, and Pb) molecules . Recently, heteronuclear ethyne-like compounds, RC≡E 14 R, have also been experimentally studied [24,25,26] and theoretically predicted [27,28,29].  (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. Tables 1-5, these DFT computations all demonstrate that the triply bonded RE 13 ≡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, RE 13 ≡AsR molecules that have a heavier As center are predicted to favor a bent angle ∠E 13 -As-R (close to 90°). The DFT computational data that are shown in Tables 1-5 confirm this prediction. 1 The natural charge density on the Al atom.

In
2 The natural charge density on the As atom.
3 BE = E(triplet state for R-Al) + E(triplet state for R-As) − E(singlet state for RAl≡AsR).
2 The natural charge density on the As atom.
3 BE = E(triplet state for R-Ga) + E(triplet state for R-As) − E(singlet state for RGa≡AsR).
2 The natural charge density on the As atom.
3 BE = E(triplet state for R-In) + E(triplet state for R-As) − E(singlet state for RIn≡AsR).
4 The Wiberg bond index (WBI) for the In-As bond, see [45,46]. 1 The natural charge density on the Tl atom.
2 The natural charge density on the As atom.
3 BE = E(triplet state for R-Tl) + E(triplet state for R-As) − E(singlet state for RTl≡AsR).
4 The Wiberg bond index (WBI) for the Tl-As bond, see [45,46].     The results in Tables 6-15 allow three conclusions to be drawn.

2.
Similarly to the results for small ligands, the computational results show that R'E 13 ≡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.

3.
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'E 13 ≡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 (SiiPrDis 2 -B≡As-SiiPrDis 2 ), which shows that NBO(B≡As) = 0.5880(2s2p 99.99 ) B + 0.8089(4s4p 1.00 )As, provide strong evidence that the predominant bonding interaction between the B-SiiPrDis 2 and the As-SiiPrDis 2 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 (SiiPrDis 2 -B≡As-SiiPrDis 2 ) 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'E 13 ≡BiR' compounds that is shown in Tables 9 (Al), 11 (Ga), 13 (In), and 15 (Tl). These computational data demonstrate that these R'E 13 ≡AsR' molecules have a weak E 13 ≡As triple bond.

Conclusion
This study of the effect of substituents on the possibility of the existence of triply bonded RE 13 ≡AsR allows the following conclusions to be drawn (Scheme 2): 1. The theoretical observations provide strong evidence that bonding mode (B) is dominant in the triply bonded RE 13 ≡BiR species, because their structures are bent due to electron transfer (denoted by arrows in Figure 1) and the relativistic effect, which increases stability.

2.
The theoretical evidence shows that both the electronic and the steric effects of substituents are crucial to rendering the E 13 ≡As triple bond synthetically accessible. However, this theoretical study shows that these E 13 ≡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′E 13 ≡AsR′ molecules that feature bulky substituents are more stable because bulky substituents not only protect the central E 13 ≡As triple bond because there is large steric hindrance but also prohibit polymerization reactions.