Theoretical Designs for Organoaluminum C2Al4R4 with Well

Sep 5, 2017 - Our model calculations at the CBS-QB3 level showed that C2Al4H4-01 can split H2 by the cooperation of the Al(I) and Al(III) sites with a...
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Theoretical Designs for Organoaluminum C2Al4R4 with WellSeparated Al(I) and Al(III) Ying-ying Xue,† Jing-jing Sui,† Jing Xu,*,‡ and Yi-hong Ding*,† †

Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China ‡ Department of Chemistry, University of California, Irvine, California 92697-2025, United States S Supporting Information *

ABSTRACT: It is well-known that the chemistry of aluminum is dominated by Al(III) in the +3 oxidation state. Only during the past 2 decades has the chemistry of Al(I) and Al(II) been rapidly developed. However, if Al(I) and Al(III) are combined, the inherently high reactivities of Al(I) and Al(III) mostly result in their coupling with each other or interacting with surrounding elements, which easily results in significant deactivation or quenching of the desired oxidation states, as in the case of reported mixed valent Al-compounds. In this article, we report an unprecedented type of organoaluminum system, C2Al4R4 (R = H, SiH3, Si(C6H5)3, SiiPrDis2, SiMe(SitBu3)2), whose lowest-energy structure, C2Al4R4-01, contains two Al(I) and two Al(III) atoms. The global nature and bonding motif of the parent C2Al4R4-01 (R = H) were supported by an extensive global isomeric search, CBS-QB3 energy calculations, adaptive natural density partitioning, and bond order analysis. Interestingly and in sharp contrast to most organoaluminum species, C2Al4R4-01 is associated with little multicenter bonding. C2Al4R4-01 has a high feasibility of being observed either in the gas or condensed phases (with suitable substitutents). With well-separated Al(I) and Al(III), C2Al4R4-01 (with suitable substitutents) could serve as the first Al/Al frustrated Lewis pair.

1. INTRODUCTION

The +1 oxidation state Al(I) with two empty orbitals and one electron lone pair (B in Scheme 1) will behave similarly to carbene species.9−12 Simple Al(I) species like AlH, AlX (X = F, Cl, Br, I), and Al2O have long been known to exist in the gas phase at high temperature and low pressure.7,13−15 Yet, stable Al(I) compounds remained unknown until 1991. The first crystallized polymeric Al(I) compound, (Cp*Al)4, was reported to stably exist at room temperature.16 Subsequently, more and more stable Al(I) compounds have been reported and have found various important applications such as in catalysis, materials, agrochemicals, and others.7,11,12,17−22 Simply from the electronic features shown in Scheme 1, it can be seen that Al(III) is a Lewis acid and that Al(I) with an electron lone pair can be viewed as a Lewis base. Therefore, when Al(I) and Al(III) are combined, the Al(I) center is prone to undergo direct coordination reactions with the Al(III) center or with other Al(I) center(s).16,23,24 Furthermore, when the Al(I) and Al(III) centers approach each other, they have a high chance of undergoing a disproportionation reaction. For example, R1AlAlR3 with Al(I)/Al(III) (C in Scheme 2) would be transformed into the thermodynamically more stable

Aluminum is the third most abundant element in the Earth’s crust, and its chemistry has long been a hot research topic. Aluminum-containing materials have been used in various fields, e.g., hydrogen storage, semiconductors, ceramics, and so forth.1−4 Most known aluminum compounds have the aluminum atom(s) in a formal oxidation state of +3, which can be attributed to the high stability of Al(III) (see A in Scheme 1) with an empty orbital. In contrast, the chemistry of aluminum(I) has been much less developed due to the low stability of the +1 oxidation state of Al, and only in recent decades has the study of aluminum(I) received much attention.5−8 Scheme 1. Exemplified Al(III) and Al(I) Species

Received: April 20, 2017 Accepted: August 16, 2017 Published: September 5, 2017 © 2017 American Chemical Society

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Scheme 2. Reported Compounds R1Al(I)−Al(III)R3 (C) and R1RAl(II)−Al(II)R2 (D) and Synthesized Compound Cp*Al−Al(C6F5)3 (E)a

a 1

R /R = H, Cl, or CH3.

structure R1RAl−AlR2 with Al(II)/Al(II) (D in Scheme 2) for substituents H, Cl, and CH3.17 As a result, the activities of Al(III)’s Lewis acidity and Al(I)’s Lewis base are both lost; i.e., Al(III) and Al(I) should most probably become deactivated or quenched. Utilizing bulky substituents, a stable compound E with Al(I) and Al(III) (see E in Scheme 2) was synthesized in 2001.23 In E, Al(I) and Al(III) are connected by a dative bond, and the activities of both Al(I) and Al(III) are quenched by the donor−acceptor interaction.17,24 Is It Possible To Obtain Organoaluminum Species with Well-Separated Al(I)/Al(III)? In this article, we report a special molecular type, C2Al4R4, whose low-energy structure 01 has well-separated Al(I) and Al(III). The stability of the cyclic C2Al4 core can be ascribed to the appreciable rigidity of the skeletal C atoms in sp3 hybridization. Moreover, the very weak Al(I)−Al(III) interaction indicates that C2Al4R4-01 can be classified as the first Al/Al frustrated Lewis pair (FLP).2,25,26 Besides gas-phase detection, we show that suitable substitution could very promisingly facilitate the laboratory synthesis of 01 under condensed conditions, e.g., C2Al4R2R′2, where R/R′ = SiH3, R/R′ = Si(C6H5)3, and R = SiH3/R′ = Si(C6H5)3, SiiPrDis2, or SiMe(SitBu3)2.

Figure 1. Isomers (ΔE < 20 kcal/mol) of C2Al4H4 at the CBS-QB3 level. The gray balls represent carbon atoms, and the pink balls represent aluminum atoms. The isomers are labeled 01−22. The values denote the CBS-QB3 relative energies in kcal/mol. For 01 and 02, the bond distances in Å were obtained at the B3LYP/6311G(2d,d,p) level and the values in brackets are at the CCSD(T)/ cc-pVTZ level.

T1 values are smaller than or around 0.02 (see Table S1) (except for bicapped structure 06, with T1 being 0.028). This indicates that, in general, the considered structures have no significant multireference character and our CBS-QB3 calculations based on the single reference functions are reliable. The T1 values of former key structures 01, 02, 03, and 04 are 0.018, 0.016, 0.017, and 0.017, respectively. The type-1 class of C2Al4H4 isomers contains both lowvalent Al(I) and normal valent Al(III), which are well-separated in space. In the quasi-planar C2Al2 ring of 01, 03, and 04, the two bridgehead Al atoms form the two Al(III), whereas the two opposite sp3 carbon atoms are linked by two Al(I) ligands. In contrast to 01, 03, and 04, the only Al(III) is located at the CAlC center and three Al(I) atoms are connected to one sp3 carbon in isomers 05 and 19. Interestingly, 01, 03, and 04 with a lower Al(I)/Al(III) ratio (i.e., 1/1) are energetically more stable than 05 and 19 with a much higher Al(I)/Al(III) ratio (i.e., 3/1). From the distance values of the two Al(III) (2.622, 2.567, and 2.607 Å, respectively) (see Table 1), four-membered ring isomers 01, 03, and 04 seem to have an Al−Al cross bond. However, the small Mayer bond indexes, i.e., 0.06, 0.10 and 0.09, respectively, show that the bridgehead interaction is very weak. The unusually short Al−Al distance in 01, 03, and 04 should be a result of the rigid skeleton effect of the C2Al2 ring. This is quite similar to very recently reported berylliumcontaining clusters, in which the contact of Be−Be is unusually

2. RESULTS AND DISCUSSION Our extensive “skeleton-ligand” cluster-growth calculations resulted in a total of 3442 C2Al4H4 isomers as local minima at the B3LYP/6-31G(d) level. The number of isomers is surprisingly large, indicating both the complexity of hydrogenated carbon−aluminum species and the importance of the global isomeric study. For simplicity, we present the 22 lowenergy isomers that lie within 20 kcal/mol at the eventual CBSQB3 level (see Figure 1). 2.1. Structural and Bonding Features of Low-Lying C2Al4H4 Isomers. In general, on the basis of the skeletal (core) characteristics of the obtained isomers (see Figure 1), they can be sorted into three categories: (1) Al(I)/Al(III): 01 (0.00), 03 (5.48), 04 (7.80), 05 (7.92), and 19 (18.53); (2) cage: 02 (2.79) and 06 (12.03); and (3) planar tetracoordinate carbon (ptC): 07 (12.90), 09 (13.14), 10 (14.99), 14 (17.41), 15 (17.42), 16 (17.82), 17 (17.91), 20 (18.82), 21 (18.83), and 22 (19.12). Note that the value in parentheses denotes the relative energy (in kcal/mol) of each isomer with reference to isomer 01. The T1 diagnostics of Lee and Taylor are often used as a qualitative estimate of the degree of multireference character of a system and the reliability of single reference methods.27 All 5408

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Table 1. Bond Distances R (in Å) and Mayer Bond Indexes (MBI) of the Four-Membered Ring of Isomers 01, 03, and 04

a

Isomers 01, 03, and 04 are at the B3LYP/6-311G(2d,d,p) level.

close yet with little bonding interaction.28−30 Thus, the two bridgehead Al atoms should be viewed as tricoordinate Al(III). The remaining two Al atoms are one-coordinate and could be viewed as Al(I) sites. The structural differences among 01, 03, and 04 lie in the different allocation of two Al(I). The Al(I)− Al(I) distances in 01, 03, and 04 are 3.263, 4.767, and 6.269 Å, respectively, with corresponding Mayer bond order values of 0.14, 0.02, and 0.01. The slight Al(I)−Al(I) interaction in 01 could account for its significantly lower energy than 03 and 04. The chemical bonding nature of the most stable isomer 01 is further characterized in Figure 2 from adaptive natural density

partitioning (AdNDP) analysis, which has been successfully applied in many scientific papers.31−34 Isomer 01 has two aluminum lone pairs on two Al(I) and ten classical localized bonds, i.e., six 2c−2e C−Al σ-bonds, two 2c−2e Al−H σbonds, and two 2c−2e C−H σ-bonds. The occupation number (ON) of the two aluminum lone pairs are both 1.97|e|. The ten 2c−2e σ-bonds have large ON values ranging from 1.83|e| to 2.00|e|. Therefore, 01 possesses little multicenter bonding. Clearly, the bonding of isomer 01 presents a marked contrast to that of most organoaluminum species, which usually have multicenter bonding due to the electron-deficient characteristics of aluminum. The type-2 class of C2Al4H4 isomers contains cage-like 02 and 06, both comprising a cage structure. Yet, the inherent electron-counting rule is different. Isomer 02 can be viewed as an Al-substituted Wade−Mingos cluster C2Al3H4R (here R is Al) with n + 1 skeletal electron pairs (n = 5). Isomer 06 is a C− C bonded hyper-closo C2Al4H4 cluster with n skeletal electron pairs (n = 6). We have also considered the C−C separated hyper-closo C2Al4H4 cluster at the B3LYP/6-311G(2d,d,p) level. However, it has two imaginary frequencies, and upon relaxation, it would lead to an isomer that lies as high as 34.37 kcal/mol at the CBS-QB3 level. It is very interesting to find that among the 22 C2Al4H4 lowlying isomers, the number of ptC isomers amounts to nearly half. This could be partly ascribed to the peculiar features of aluminum, i.e., Al is electropositive (relative to carbon) and has an atomic size that would easily allow for the formation of a ptC structure as a local minimum.35−37 Of particular note, there are two types of carbon atoms in the ptC species. In one type (10 and 20), both carbon atoms feature a ptC nature, namely, a double ptC core (dptC38), whereas in the other type (07, 09, 14, 15, 16, 17, 21, and 22), one carbon is ptC and the other is sp3. Although the parent C2Al4 core is truly the global minimum,38 the present calculations show that hydrogenation can pose a great effect on the isomerism, destabilizing the dptC and ptC forms in C2Al4H4. It is clear that at the composite CBS-QB3 level using B3LYP/6-311G(2d,d,p) geometries, C2Al4H4 has global minimum structure 01 followed by cage isomer 02 by 2.79 kcal/mol. For further refinement, we reoptimized the former

Figure 2. Localized molecular orbitals of C2Al4H4-01 obtained by AdNDP analysis. “ON” denotes the occupation number on the localized orbital. 5409

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Besides, due to the huge structural discrepancy, the conversion of 01 to isomers 02, 03, and 04 should be kinetically hindered. Thus, C2Al4H4-01 can exist in the gas phase. What about the fate of 01 in the condensed phase? With an increased concentration of molecules, the condensation of fragments and the molecule itself (i.e., oligomerization) might take place to lower the energy of the system. We first considered the sequential processes C2Al4H4-01 → C2H4 (ethylene) + 4Al(g) and Al(g) → Al(s). The former reaction is still endothermic by 51.74 kcal/mol at the CBS-QB3 level. The latter reaction is exothermic by 70.17 kcal/mol (derived from the vaporization of condensed Al) when the condensation of Al fragments is considered (https://en.wikipedia.org/wiki/ Enthalpy_of_vaporization#cite_note-GeWang2009-4). Therefore, the overall process C2Al4H4-01 → C2H4 (ethylene) + 4Al(s) has an exothermicity of 18.43 kcal/mol. Second, we computed the reaction heats of the C2Al4H4-01 → 2C2Al4H401 dimerization process by considering the possible interactive sets Al(I)/Al(III) and Al(I)/Al(I). At the B3LYP/6-31+G(d)+D3(BJ)//B3LYP/6-31G(d) level, the dimerization is exothermic by 31.59 (Al(I)/Al(III)) and 25.61 kcal/mol (Al(I)/Al(I)), respectively. This indicates that in thermodynamics, the formation of species 01 bearing the simplest substituent (R = H) is disfavored in the condensed phase. Clearly, to make C2Al4R4-01 feasible in the condensed phase, consideration of non-H substituents is indispensible, which can induce both electronic and steric stabilization. For the four substituents R, we considered both homosubstitution (i.e., C2Al4R4) and heterosubstitution (C2Al4R2R2′). For the former C2Al4H4 isomers within 20 kcal/mol at the CBS-QB3 level, we first evaluated the substitution effect of all H atoms by the simplest silyl group, SiH3 (known as an electropositive or electron-donating group). We found that the ground state nature of 01 can be maintained, and notably the cage-like isomer 02 even becomes less stable than 03 and 04 (both with separated Al(I) and Al(III) as in 01) (see Figure S1). The effectiveness of SiH3 could be ascribed to its electron-donating ability partially stabilizing the Lewis acid site (here, tricoordinate Al) of 01, 03, and 04 as it operates to the triply bonded silicon.44 Further consideration of solvent effects with the presence of benzene (chosen due to its absence of lone pair electrons that can deactivate Al(I) and Al(III)) still supports the lowest-energy nature of 01 and indicates that cage-like 02 again lies higher than 03 and 04 (see Figure S2). For bulky groups (R′), considering the limitation of both CPU time and memory size and considering consistency, we calculated C2Al4R2R′2-01 and C2Al4R2R′2-02 (R/R′ = SiH3, Si(C6H5)3, SiiPrDis2, SiMe(SitBu3)2) at the B3LYP/6-31+G(d)+D3(BJ)// RHF/3-21G(d) level (see Scheme 3). For C2Al4R2R′2 (R/R′ = SiH3, Si(C6H5)3, SiiPrDis2, SiMe(SitBu3)2), 01 is more stable than 02 (see Table 2), manifesting the effectiveness of various silyl groups in stabilizing 01 over 02. What about dimerization in these silyl-substituted 01 compounds? As shown in Table 2, for R/R′ = SiH3/SiH3, SiH3/Si(C6H5)3, SiH3/SiiPrDis2, the dimerization is exothermic at −25.15, −44.44, and −49.40 kcal/mol, respectively, at the B3LYP/6-31+G(d)+D3(BJ)//RHF/3-21G(d) level, indicating that the three sets of silyl groups cannot suppress the dimerization. For R/R′ = Si(C6H5)3/Si(C6H5)3 and SiH3/ SiMe(SitBu3)2, in spite of numerous attempts, we failed to locate any covalently bound minimum dimer of 01 with either Al(I)/Al(I) or Al(I)/Al(III) interaction. This hints that suitable silyl groups with sufficient steric effects might prevent 01 from

low-lying isomers 01 and 02 at the rather costly CCSD(T)/ ccpVTZ level. As shown in Figure 1, the key bond distances of both isomers with both methods agree consistently with each other. 2.2. Chemical Properties of Isomer C2Al4H4-01. The four Al atoms in the lowest-energy isomer C2Al4H4-01 can be classified into two types: two Al(I) and two Al(III). There is almost negligible interaction between Al(I) and Al(III) according to the Mayer bond order values and AdNDP analysis. In this sense, Al(I) and Al(III) can be viewed as being “separated” from each other. Since Al(I) comprises both an electron lone pair and empty orbitals, Al(I) should be associated both with the Lewis base and Lewis acid features. In contrast, Al(III) with one empty orbital should only possess the acid feature. We took frequently used diagnostic molecules, e.g., carbon monoxide (CO),39,40 as an electron donor and the simplest boranes (BR3)41,42 as an electron acceptor to sense the above interesting features of aluminum. CO can couple both with Al(I) and Al(III) with corresponding binding energies of 2.85 and 7.42 kcal/mol at the CBS-QB3 level. The Hsubstituted BR3, i.e., BH3, interacts with Al(I) with a binding energy of 22.53 kcal/mol. Hence, in C2Al4H4-01, Al(I) is dominated by Lewis base character and accompanied by minor Lewis acidity, whereas Al(III) is solely a Lewis acid. To the best of our knowledge, we are not aware of any examples that comprise well-separated Al(I) and Al(III) in the same molecule. Note that in some mixed-valent Al compounds, although the notation Al(I)/Al(III) is still used, the two Al types are in fact deactivated or quenched due to the effective donor−acceptor interaction.17,23,24 Furthermore, the separated Al(I)/Al(III) would face energetic competition from other isomers resulting from the disproportionation reaction. Calculations have shown that the donor−acceptor bonded Al(I) → Al(III) structure R1Al(I) → Al(III)R3 (C in Scheme 2) are energetically less stable than the Al(II)−Al(II) bonded structure R1RAl(II)−Al(II)R2 for some substituents.17 We postulate that the successful holding of well-separated Al(I)/ Al(III) in 01 benefits from the rigid sp3-C within the CAlCAl ring. Interestingly, when the Al(I) and Al(III) sites within 01 were manually put together, the optimization led to isomer 09, which lies at 13.14 kcal/mol at the CBS-QB3 level. Thus, the intramolecular combination of Al(I) and Al(III) within 01 is energetically disfavored. Besides, intuitively, the two Al(I) sites (each with the donor and acceptor feature) might form two sets of dative bonds from one to the other. Yet, the Mayer bond order value and the AdNDP result indicate that there is little interaction between them. The reason could again be ascribed to the rigidity of the tetrahedral carbon, which frustrates the orbital overlap between the lone pair and empty orbitals of both Al(I) atoms. 2.3. Viability of C2Al4R4-01. Being the global minimum structure and difficult to transform into other topological forms, 01 is stable intrinsically, at least in theory. Yet, one should be aware that a quantum chemical minimum location does not mean the existence of a molecule. The generation, detection, and storage of a molecule usually depend on various factors including the synthetic precursor and environment. Our computed atomization energy (AE) of C2Al4H4-01 with respect to 2C + 4Al + 4H (each atom is in its electronic ground state) is 707.07 kcal/mol. By comparison, the experimentally known CH3Al43 has an AE value of 355.42 kcal/mol. The average AE values (divided by the total number of atoms) of C2Al4H4-01 and CH3Al are comparable, i.e., 70.71 and 71.08 kcal/mol. 5410

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SiH3, the Gibbs free energy barrier for H2 splitting is decreased to 29.45 kcal/mol. The barrier is comparable to that of the P/Al FLP (25.44 kcal/mol).50 It is worthy of note that in addition to the above sterically protected strategy of the two Al (I) atoms each with two empty orbitals within C2Al4R4-01 by applying bulky substituents, another alternative stabilizing approach could be to use the Lewis base to bind Al(I) by donating a pair of electrons, as in the case of the reported monomeric aluminum(I) compound [{HC(CMeNAr)2}Al] (Ar = 2,6iPr2C6H3).12 The thermodynamic and kinetic influences of this kind of donor to the Al(I) sites of C2Al4R4-01 deserve to be studied in the future.

Scheme 3. Complex Species of C2Al4R4-01 and C2Al4R4-02 upon Homo- and Heterosubstitutiona

3. CONCLUSIONS Quantum chemical calculations showed that a new organoaluminum series C2Al4R4 with optional R (e.g., H, SiH3, Si(C6H5)3, SiiPrDis2, SiMe(SitBu3)2) might possesses the global isomer 01 with two sets of well-separated Al(I) and Al(III). This is in contrast with known mixed-valent Al compounds in which Al(I) is easily deactivated by Al(III) or converted to Al(II). The unique feature of C2Al4R4-01 allows it to be viewed as the first Al/Al FLP. a

R/R′ = SiH3, Si(C6H5)3, SiiPrDis2, or SiMe(SitBu3)2.

4. THEORETICAL METHODS To study the low-energy isomers of C2Al4R4, we first performed a global isomeric search for the parent molecule C2Al4H4 (i.e., R = H) and subsequently evaluated substitutional effects for relevant structures. For C2Al4H4, it is reasonable to consider it as a “skeleton-ligand” type, i.e., hydrogen atoms are “ligands” around the C2Al4 skeleton (core). To determine the isomeric structures as many compounds as possible, we applied our locally developed “skeleton-ligand cluster-growth” method.51 First, the isomeric search of the small C2Al4 core was undertaken at the B3LYP/6-31G(d) level via our “grid-based comprehensive isomeric strategy”,52 which has been successfully tested in our previous works.54−58 Second, via the “clustergrowth pattern”, each optimized C2Al4 core was saturated by hydrogen atoms one by one in the forms of terminal, bridge, and delta-bonding types. For each hydrogenated structure, B3LYP/6-31G(d) structural and frequency calculations were carried out before the next hydrogenation. The obtained isomers within 30 kcal/mol at the B3LYP/6-31G(d) level were selected for further refinement calculations at the composite CBS-QB3 level.59,60 Note that in CBS-QB3 calculations, the geometries and frequencies were computed at the B3LYP/6311G(2d,d,p) level followed by a series of sophisticated and high-level single-point energy evaluations with the eventual basis set extrapolated to the complete basis set (CBS) limit.

dimerization/oligomerization. Note that the intermolecular Al(I)−Al(III) interaction to form two Al(II) sites takes place via two steps: the first is to form a donor−acceptor complex between Al(I) and Al(III), and the second is to undergo R migration from Al(III) to Al(I). Very large silyl groups can prohibit the first step from taking place, making Al(I) + Al(III) → 2Al(II) unlikely. The well-separated Al(I) and Al(III) within C2Al4H4-01 remind us of another interesting topic: the frustrated Lewis pair (FLP).25,26 An FLP is the combination of a Lewis acid and a Lewis base where the acid and base are either sterically or electronically restricted from interaction with each other. Bulky substituted FLPs have found rich applications in small-molecule activation and splitting (e.g., H2, CO2).45−50 Most of the reported FLPs are heteronuclear, containing P/B, C/B, N/B, or P/Al moieties, and the first homonuclear FLP with B/B was published only very recently (in 2015) by Kinjo and coworkers.53 The presently predicted global C2Al4R4-01 with well-separated group 13 Al(I) and Al(III) surely deserves to be considered as the previously unreported Al/Al FLP and the second homonuclear FLP. Our model calculations at the CBSQB3 level showed that C2Al4H4-01 can split H2 by the cooperation of the Al(I) and Al(III) sites with a Gibbs free energy barrier 33.52 kcal/mol (T = 298 K). With R/R′ = SiH3/

Table 2. Relative Energies (in kcal/mol) of C2Al4R4-01 and C2Al4R4-02 upon Homo- and Heterosubstitution at the CBS-QB3 (for R/R′ = SiH3/SiH3) and B3LYP/6-31+G(d)+D3(BJ)//RHF/3-21G(d) Levels and the Dimerization Energy (in kcal/mol) of C2Al4R4-01 at the B3LYP/6-31+G(d)+D3(BJ)//RHF/3-21G(d) Level R/R′ CBS-QB3 C2Al4R4-01 C2Al4R4-02 C2Al4R4-01 dimerization energy

a

B3LYP/6-31+G(d)+D3(BJ)//RHF/3-21G(d)

SiH3/ SiH3

SiH3/SiH3

Si(C6H5)3/Si(C6H5)3

SiH3/Si(C6H5)3

SiH3/SiiPrDis2

SiH3/SiMe(SitBu3)2

0.00 9.76

0.00 12.87 −25.15a −36.66b

0.00 30.83

0.00 4.39 −44.44a

0.00 8.35 −49.40a

0.00 32.41

Dimerization of C2Al4R4-01 via Al(I)/Al(I) interaction. bDimerization of C2Al4R4-01 via Al(I)/Al(I) and Al(I)/Al(III) interactions. 5411

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The resulting low-lying isomers with close energy were subject to costly CCSD(T)61/cc-pVTZ structural optimization. For the isomers of C2Al4H4 within 20 kcal/mol at CBS-QB3, we calculated the CCSD(T) T1 diagnostics27 at the CCSD(T)/ccpVTZ level and evaluated substitutional effects (with the electronically positive substituent group SiH3) and solvent effects (single-point, in benzene) at the B3LYP/6-311G(2d,d,p) level. For non-hydrogen groups (R/R′ = SiH3, R/R′ = Si(C6H5)3, R = SiH3/R′ = Si(C6H5)3, SiiPrDis, SiMe(SitBu3)2), the RHF/3-21G(d) calculation was used to fully optimize C2Al4R2R′2-01 and C2Al4R2R′2-02 followed by the frequency calculation at the RHF/3-21G(d) level, and to produce better energetics, the stationary points were further computed at the B3LYP/6-31+G(d)+D3(BJ)//RHF/321G(d) level of theory. Here “D3(BJ)” denotes the correction for the dispersion effect by using the D3 version of Grimme’s dispersion with Becke−Johnson damping.62 For all electronic structure calculations, the commercial Gaussian 0363 and Gaussian 0964 package suites were used. To discuss the nature of the bonding of key structures of C2Al4R4 (R = H), Mayer bond order index 65 and the adaptive natural density partitioning (AdNDP) analyses66,67 were carried out at the B3LYP/6-311G(2d,d,p) level.

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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/acsomega.7b00487. T1 values calculated at the CCSD(T)/cc-pVTZ level; structures of SiH3-substituted C2Al4R4 (R = SiH3); Cartesian coordinates and total energies computed of key structures C 2 Al 4 H 4 , C 2 Al 4 R 4 -01, C 2 Al 4 R 4 -02, and C2Al4R4-01 dimers (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.X). *E-mail: [email protected] (Y.-h.D). ORCID

Jing Xu: 0000-0001-5558-6908 Yi-hong Ding: 0000-0001-9598-437X Author Contributions

Y.-y.X. carried out all of the theoretical computations. Y.-y.X., J.X., and Y.-h.D. supervised the research activities and contributed to preparing the manuscript. Y.-y.X., J.-j.S., J.X., and Y.-h.D. regularly discussed the progress of the research, analyzed the results, reviewed the manuscript, and gave approval for the final version. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China (nos. 21273093 and 21473069). The reviewers’ and editor’s invaluable comments and suggestions are greatly appreciated.

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REFERENCES

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