Ti Group Metallocene-Catalyzed Synthesis of 1-Hexene Dimers and Tetramers

1-Hexene transformations in the catalytic systems L2MCl2–XAlBui2 (L = Cp, M = Ti, Zr, Hf; L = Ind, rac-H4C2[THInd]2, M = Zr; X = H, Bu i) and [Cp2ZrH2]2-ClAlR2 activated by MMAO-12, B(C6F5)3, or (Ph3C)[B(C6F5)4] in chlorinated solvents (CH2Cl2, CHCl3, o-Cl2C6H4, ClCH2CH2Cl) were studied. The systems [Cp2ZrH2]2-MMAO-12, [Cp2ZrH2]2-ClAlBui2-MMAO-12, or Cp2ZrCl2-HAlBui2-MMAO-12 (B(C6F5)3) in CH2Cl2 showed the highest activity and selectivity towards the formation of vinylidene head-to-tail alkene dimers. The use of chloroform as a solvent provides further in situ dimer dimerization to give a tetramer yield of up to 89%. A study of the reaction of [Cp2ZrH2]2 or Cp2ZrCl2 with organoaluminum compounds and MMAO-12 by NMR spectroscopy confirmed the formation of Zr,Zr-hydride clusters as key intermediates of the alkene dimerization. The probable structure of the Zr,Zr-hydride clusters and ways of their generation in the catalytic systems were analyzed using a quantum chemical approach (DFT).


Introduction
Alkene dimers and oligomers represent a large class of compounds that are used as comonomers in ethylene polymerization and as raw materials for the production of adhesives, surfactants, fragrances, synthetic lubricating fuel additives, etc. [1][2][3][4][5][6]. The following industrial processes have been successfully developed and implemented for the production of olefin oligomers: (i) oligomerization of ethylene in the presence of triethylaluminum with subsequent oxidation to higher alcohols (Ziegler-Alfol process), (ii) Philips process of ethylene oligo-and polymerization on chromium catalysts, (iii) Ni-catalyzed synthesis of linear α-olefins via ethylene oligomerization (shell higher olefin process (SHOP)), (iv) oligomerization of ethylene to linear C4-C10 olefins by α-select technology (Axens) or α-SABLIN technology (Sabic and Linde) [7]. Currently, some research groups are developing strategies for the production of highly efficient jet and diesel fuels via oligomerization of alkenes (1-butene, 1-hexene) synthesized from renewable plant raw materials [2,8].
Despite significant progress in this area, the mechanism of the dimerization reaction is not fully understood. Earlier, the hypothesis on the key role of the hydride cation [Cp2ZrH] + was put forward [14]. Subsequently, it was postulated that bimetallic hydride complexes L2Zr(µ-H)3AlR2 can act as catalytically active species in the alkene dimerization [21,22]. Recently, we studied the systems Cp2ZrCl2-XAlBu i 2 (X = H, Bu i ) and [Cp2ZrH2]2-ClAlR2 (R = Me, Et, Bu i ) activated by MMAO-12 (modified methylaluminoxane) or organoboron compounds and found new biszirconium hydride intermediates (Cp2ZrH2 . Cp2ZrHCl . ClAlR2), which provide selective formation of head-to-tail dimerization products [19,20,23]. However, the questions on the possible structure of the key intermediates and their formation in the catalytic systems remain open.
Thus, our work was aimed at investigating the effect of transition metal

Transformations of 1-Hexene with Cp2MY2 (M = Ti, Zr, Hf; Y = H, Cl)-XAlBu i 2 (X = H, Bu i ) Catalytic Systems Activated by MMAO-12, B(C6F5)3, or (Ph3C)[B(C6F5)4]
The conditions for the selective synthesis of vinylidene dimers in the presence of catalytic systems Cp2ZrY2 (Y = H, Cl)-XAlBu i 2 (X = H, Bu i )-activator (Scheme 2, catalytic systems A and B), developed in our previous study, were taken as the starting point for the experiment (Table 1, entry 1) [19,20]. Despite significant progress in this area, the mechanism of the dimerization reaction is not fully understood. Earlier, the hypothesis on the key role of the hydride cation [Cp 2 ZrH] + was put forward [14]. Subsequently, it was postulated that bimetallic hydride complexes L 2 Zr(µ-H) 3 AlR 2 can act as catalytically active species in the alkene dimerization [21,22]. Recently, we studied the systems Cp 2 ZrCl 2 -XAlBu i 2 (X = H, Bu i ) and [Cp 2 ZrH 2 ] 2 -ClAlR 2 (R = Me, Et, Bu i ) activated by MMAO-12 (modified methylaluminoxane) or organoboron compounds and found new biszirconium hydride intermediates (Cp 2 ZrH 2 . Cp 2 ZrHCl . ClAlR 2 ), which provide selective formation of head-to-tail dimerization products [19,20,23]. However, the questions on the possible structure of the key intermediates and their formation in the catalytic systems remain open.
Thus, our work was aimed at investigating the effect of transition metal η 5 -complexes and solvents (chlorine-containing solvents) on the activity, chemo-and regioselectivity of the systems L 2 MCl 2 -XAlBu i 2 (L = Cp, M = Ti, Zr, Hf; L = Ind, rac-H 4 C 2 [THInd] 2 , M = Zr; X = H, Bu i ) and [Cp 2 ZrH 2 ] 2 -ClAlR 2 activated by MMAO-12, B(C 6 F 5 ) 3 , or (Ph 3 C)[B(C 6 F 5 ) 4 ] in alkene oligomerization. NMR studies and quantum chemical calculations were applied to establish the possible structure of the hydride intermediates formed in these systems. The results showed that the M,M-type bimetallic hydrides and their action mechanisms are unique for the dimerization reaction. The conditions for the selective synthesis of vinylidene dimers in the presence of catalytic systems Cp 2 ZrY 2 (Y = H, Cl)-XAlBu i 2 (X = H, Bu i )-activator (Scheme 2, catalytic systems A and B), developed in our previous study, were taken as the starting point for the experiment (Table 1, entry 1) [19,20].

Results and Discussion
The reaction proceeds in toluene or benzene at 20-60 • C to give the target products in 81-98% yields within 5 to 150 min, depending on the type of activator (Table 1,  . When the reaction is carried out in chloroform, the relative content of dimers decreases to 92% (entry 4). Under the same conditions, the dimers produced in the first minutes of the reaction serve as substrates for the subsequent dimerization to afford dimers of dimer 7 in 75-79% yields in 180 min (entries 7, 8, Figures S26 and S27). According to 1 H and 13 C NMR data, the structures of these products correspond to those proposed earlier [16,22,[24][25][26]. Dimerization of 5 occurs probably via participation of cationic species formed upon the reaction of CHCl 3 with ClAlR 2 or MMAO-12.
tion (Table 1, entry 2). When the reaction is carried out in chloroform, the relative content of dimers decreases to 92% (entry 4). Under the same conditions, the dimers produced in the first minutes of the reaction serve as substrates for the subsequent dimerization to afford dimers of dimer 7 in 75-79% yields in 180 min (entries 7, 8, Figures S26 and S27). According to 1 H and 13 C NMR data, the structures of these products correspond to those proposed earlier [16,22,[24][25][26]. Dimerization of 5 occurs probably via participation of cationic species formed upon the reaction of CHCl3 with ClAlR2 or MMAO-12.  Under these conditions, the conversion of 1-hexene in 3 h was higher in CH2Cl2 (99%, entry 26), and the reaction predominantly gave tetramer 7 in 69% yield. In contrast, in chloroform, the 1-hexene conversion was 81% in 16 h, and oligomeric products 6, including compound 7, prevailed in the product mixture (entries 24,25).
The catalytic system based on Cp 2 ZrCl 2 , HAlBu i 2, and MMAO-12 also provides vinylidene dimers 5 in chlorinated solvents. In CH 2 Cl 2 and CHCl 3 , the reaction proceeds in 30 min with the conversion of 1-hexene above 99% and the yield of dimers of 98% (entries 28, 34). Increasing 1-hexene initial concentration to 1000 equivalents leads to decreased alkene conversion to 82% and the yield of dimeric product to 80% in CH 2 Cl 2 . Under these conditions, the conversion and the yield of dimers are higher in CHCl 3 than in CH 2 Cl 2 and amount to >99% and 90-98% in 30 min, respectively (entries 32, 35). The highest yield of dimer 5 at the level of 99% is achieved by using a neutral boroncontaining activator B(C 6 F 5 ) 3  Subsequently, the content of dimers decreases to 55%, and the amount of tetramer 7 increases to 33% (entry 50). Using chloroform as a solvent raises the yield of 7 to 65-72% (entries 54, 55).
The activity of Cp 2 TiCl 2 (1a) and Cp 2 HfCl 2 (1c) in the studied systems was lower than that of Cp 2 ZrCl 2 (1b). Indeed, the conversion of 1-hexene in CH 2 Cl 2 at 40 • C was 80% in 60 min of the reaction in the case of the Ti complex ( Table 2, entries 1) and 84% in 120 min for the Hf complex (entries 5). The use of chloroform as a solvent increases substrate conversion to 93% for Cp 2 TiCl 2 and decreases the conversion to 60% for Cp 2 HfCl 2 (entries 3,8). Moreover, these catalysts are significantly inferior in chemo-and regioselectivity to zirconocene dichloride. Thus, in the presence of Cp 2 TiCl 2 , the yield of oligomeric products 6 and tetramer 7, which represent a mixture of regioisomers, increases in total to 89%. The use of Cp 2 HfCl 2 in CH 2 Cl 2 leads to increased the content of vinylidene oligomers up to 28-37%. In chloroform, the yield of regioisomeric oligomers also increases in the first 60 min of the reaction (entry 8). Further, the appearance of products of toluene mono-, di-, and trialkylation with 1-hexene was found (entries 9,10). It should be noted that in the presence of HAlBu i 2 , upon replacing MMAO-12 with boron-containing activators, the systems based on Cp 2 TiCl 2 and Cp 2 HfCl 2 completely lost their activity ( Table 2, entries [11][12][13][14]. The effect of the ligand structure of zirconocenes on the activity of catalytic systems and the reaction route was also studied. For this purpose, we tested Zr η 5 -complexes (C 5 Me 5 ) 2 ZrCl 2 (1d), Ind 2 ZrCl 2 (1e), and rac-H 4 C 2 (THInd) 2 (Table 3, entries 1,2). In this case, 1-hexene was consumed for toluene alkylation (toluene is present in the system as a solvent for MMAO-12). The complex Ind 2 ZrCl 2 showed lower activity than Cp 2 ZrCl 2 ( Table 3, entries 3-7) and shifted the reaction route toward the oligomerization (up to 87%) both in CH 2 Cl 2 and CHCl 3 . Ansa-zirconocene rac-H 4 C 2 (THInd) 2 ZrCl 2 was inactive in CHCl 3 (entry 9); however, it catalyzed alkene oligomerization in CH 2 Cl 2 (1-hexene conversion of >99%, entry 8). Thus, the zirconocene ligand structure significantly affects the course of the reaction, and some tested complexes mainly provided the formation of oligomeric products. CHCl 3 960 0 a regioselectivity is significantly reduced due to double bond migration; b byproducts of toluene mono-(13%), di-(4%) and tri-(2%) alkylation with 1-hexene are formed; c byproducts of toluene mono-(22%), di-(7%), and tri-(4%) alkylation with 1-hexene are formed; d wt % in the reaction mixture, determined by GC-MS of deuterolysis or hydrolysis products (the details of the GC-MS analysis is shown in Supporting Information). CH 2 Cl 2 960 0 a 1-hexene is completely consumed for toluene alkylation; b 6-mers (15%) and 7-mers (9%); c 6-mers (17%) and 7-mers (9%); d 6-mers (3%); e wt % in the reaction mixture, determined by GC-MS of deuterolysis or hydrolysis products (the details of the GC-MS analysis is shown in Supporting Information).

NMR Study of Intermediate Structure in MMAO-12-Activated Systems Cp 2 ZrY 2 (Y = H, Cl)-OAC in Chlorinated Solvents
The effect of chlorinated solvents (CD 2 Cl 2 and CDCl 3 ) on the structure of the intermediates responsible for the alkene dimerization in the catalytic systems [Cp 2 ZrH 2 ] 2 -MMAO-12 was studied by NMR. The addition of MMAO-12 to a solution of [Cp 2 ZrH 2 ] 2 in CD 2 Cl 2 gives rise to a triplet at −5.95 ppm (J = 17.0 Hz) in the 1 H NMR spectra (Figure 1c, Figures S21-S23). In the COSY HH spectrum, the triplet correlates with the signal at −0.47 ppm, which is superimposed with the region of resonance lines of methyl group protons of MMAO-12 (−0.82-0.32 ppm). The spectra also exhibited signals for the Cp rings at 6.16 ppm (108.4 ppm in 13 C NMR spectra), 6.39 ppm (112.8 ppm in 13 C NMR spectra), and 6.61 ppm (116.2 ppm in 13 C NMR spectra). The NOESY spectra showed the cross-peaks of the Cp-ring signal at 6.16 ppm with the upfield signals at −5.95 ppm and −0.47 ppm. Relying on these results and the data from previous studies [19,20], these signals were assigned to the biszirconium trihydride complex 9. The signals at 6.61 ppm and 6.39 ppm correspond to the Cp rings of Cp 2 ZrCl 2 and Cp 2 ZrMeCl (11), respectively [27]. A similar picture is observed in the 1 H and 13 C NMR spectra of Cp2ZrCl2-HAlBu i 2 in CD2Cl2 ( Figures S5 and S6). When MMAO-12 is added to the system, the signals of the dimeric complex 10 also disappear, the intensity of the signals of the trihydride 8 decreases, and additional triplets of 9·MAO appear in the upfield region at −6.21 ÷ −5.92 ppm (Figures S7 and S8).
The reaction of [Cp2ZrH2]2 with MMAO-12 in either CDCl3 or CD2Cl2 gives complexes Cp2ZrCl2 (δCp 6.76 ppm) and Cp2ZrMeCl (δCp 6.58 ppm) (Figure 2c, Figures S19 and  S20). Moreover, the 1 H NMR spectrum showed a significant broadening of the signals of the hydride atom at −6.12 ppm and Cp rings of the heavy adduct 9·MAO, which disappeared from the spectra due to the precipitation of the heavy fraction to the bottom of the NMR tube.
In the reaction of Cp2ZrCl2 with HAlBu i 2 (1:4) in CDCl3, only the Zr,Al-trihydride complex 8 was identified (Figure 2a, Figures S1 and S2). The signals of complexes 9 and 9·MAO appeared in the 1 H NMR spectra after the addition of MMAO-12 and 1-hexene to the system (Figure 2b, Figures S3 and S4). The formation of complexes 8-10 was observed in the reaction of zirconocene dihydride with ClAlBu i 2 taken in a 1:2 ratio in CDCl3 (Figure S9). The addition of MMAO-12 to this system results in the disappearance of signals of complex 10, broadening of complex 8 signals, and the appearance of an additional broadened multiplet at −6.03 ppm, corresponding to the heavy adduct 9·MAO ( Figure  S11). NMR monitoring of the system's reaction with 1-hexene showed the consumption of 9·MAO adduct and the accumulation of the vinylidene dimer ( Figures S24 and S25). As shown in Scheme 3, the system [Cp 2 ZrH 2 ] 2 -MMAO-12 in CD 2 Cl 2 produces complex 9 via in situ formation of Cp 2 ZrHCl and Cp 2 ZrCl 2 by the reaction of zirconocene dihydride with a chlorine-containing solvent [28]. Residual AlMe 3 present in the MMAO-12 solution reacts with Cp 2 ZrCl 2 to give methyl chloride complex 11 and ClAlMe 2 . The reaction of Cp 2 ZrHCl with the starting Cp 2 ZrH 2 and ClAlMe 2 makes it possible to selectively obtain complex 9 and then high-molecular-weight associates with MMAO-12 (9·MAO), which are active in the alkene dimerization [19,20]. ppm and −0.47 ppm. Relying on these results and the data from previous studies [19,20], these signals were assigned to the biszirconium trihydride complex 9. The signals at 6.61 ppm and 6.39 ppm correspond to the Cp rings of Cp2ZrCl2 and Cp2ZrMeCl (11), respectively [27]. As shown in Scheme 3, the system [Cp2ZrH2]2-MMAO-12 in CD2Cl2 produces complex 9 via in situ formation of Cp2ZrHCl and Cp2ZrCl2 by the reaction of zirconocene dihydride with a chlorine-containing solvent [28]. Residual AlMe3 present in the MMAO-12 solution reacts with Cp2ZrCl2 to give methyl chloride complex 11 and ClAl-Me2. The reaction of Cp2ZrHCl with the starting Cp2ZrH2 and ClAlMe2 makes it possible to selectively obtain complex 9 and then high-molecular-weight associates with MMAO-12 (9·MAO), which are active in the alkene dimerization [19,20].  The 1 H and 13 C NMR spectra of the system [Cp 2 ZrH 2 ] 2 -ClAlBu i 2 (Figure 1a, Figures S14 and S15) in CD 2 Cl 2 exhibited signals of previously described intermediates [19,23,28,29]: complex 8 (broadened signals of hydride atoms at δ H −0.86 and −1.91 ppm), dimeric complex 10 (broadened signals of hydride atoms at δ H −1.42 and −2.38 ppm), and biszirconium trihydride complex 9 (doublet signal at δ H −0.70 ppm ( 2 J = 17.0 Hz) and triplet at −5.87 ppm). The addition of MMAO-12 to the equilibrium mixture of the complexes leads to the vanishing of signals of complex 10 from the 1 H NMR spectrum and the appearance of an additional triplet at −6.01 ppm attributable to the adduct 9·MAO (Figure 1b, Figures S16-S18).
A similar picture is observed in the 1 H and 13 C NMR spectra of Cp 2 ZrCl 2 -HAlBu i 2 in CD 2 Cl 2 ( Figures S5 and S6). When MMAO-12 is added to the system, the signals of the dimeric complex 10 also disappear, the intensity of the signals of the trihydride 8 decreases, and additional triplets of 9·MAO appear in the upfield region at −6.21 ÷ −5.92 ppm (Figures S7 and S8).
The reaction of [Cp 2 ZrH 2 ] 2 with MMAO-12 in either CDCl 3 or CD 2 Cl 2 gives complexes Cp 2 ZrCl 2 (δ Cp 6.76 ppm) and Cp 2 ZrMeCl (δ Cp 6.58 ppm) (Figure 2c, Figures S19 and S20). Moreover, the 1 H NMR spectrum showed a significant broadening of the signals of the hydride atom at −6.12 ppm and Cp rings of the heavy adduct 9·MAO, which disappeared from the spectra due to the precipitation of the heavy fraction to the bottom of the NMR tube. Thus, biszirconium complex 9 is readily formed in the systems Cp2ZrCl2-HAlBu i 2, [Cp2ZrH2]2-MMAO-12, and [Cp2ZrH2]2-ClAlBu i 2 both in CD2Cl2 and in CDCl3. This intermediate reacts with methylaluminoxane to give a heavy adduct, which selectively provides vinylidene dimers of 1-hexene.

DFT Study of the Structure of Biszirconium Complex 9
To refine the structure of complex 9, probable structures of cyclic isomers 9a-9d were optimized using the PBE/3ζ quantum chemical method [30][31][32]. Their structure is in line with the obtained NMR spectral data on the ratio of the signal intensities of the constituent moieties and the symmetry of the complex (Scheme 4). As follows from Scheme 4, the structures of the complexes significantly differ from one another. For example, in complex 9c, all three hydrides are located between the zirconium atoms and form a trihydride bridge, while in molecule 9a, there is only one bridging H atom. If the optimized structures of the two most energetically favorable hydride complexes 9a and 9c (Figure 3) In the reaction of Cp 2 ZrCl 2 with HAlBu i 2 (1:4) in CDCl 3 , only the Zr,Al-trihydride complex 8 was identified (Figure 2a, Figures S1 and S2). The signals of complexes 9 and 9·MAO appeared in the 1 H NMR spectra after the addition of MMAO-12 and 1-hexene to the system (Figure 2b, Figures S3 and S4). The formation of complexes 8-10 was observed in the reaction of zirconocene dihydride with ClAlBu i 2 taken in a 1:2 ratio in CDCl 3 ( Figure S9). The addition of MMAO-12 to this system results in the disappearance of signals of complex 10, broadening of complex 8 signals, and the appearance of an additional broadened multiplet at −6.03 ppm, corresponding to the heavy adduct 9·MAO ( Figure S11). NMR monitoring of the system's reaction with 1-hexene showed the consumption of 9·MAO adduct and the accumulation of the vinylidene dimer ( Figures S24 and S25).

DFT Study of the Structure of Biszirconium Complex 9
To refine the structure of complex 9, probable structures of cyclic isomers 9a-9d were optimized using the PBE/3ζ quantum chemical method [30][31][32]. Their structure is in line with the obtained NMR spectral data on the ratio of the signal intensities of the constituent moieties and the symmetry of the complex (Scheme 4). As follows from Scheme 4, the structures of the complexes significantly differ from one another. For example, in complex 9c, all three hydrides are located between the zirconium atoms and form a trihydride bridge, while in molecule 9a, there is only one bridging H atom. If the optimized structures of the two most energetically favorable hydride complexes 9a and 9c (Figure 3 In isomer 9c, all three hydrogen atoms are inside the biszirconium cage. Meanwhile, the lengths of bridging Zr-H bonds are also increased compared to those in Cp2ZrHCl, but are not equivalent to each other: dZr1-H1 = dZr2-H1 = 2.01 Å, dZr1-H2 = dZr2-H3 = 1.97 Å, dZr1-H3 = dZr2-H2 = 1.99 Å. Thus, one hydrogen atom is equidistant from both zirconium atoms, while the other two H atoms, forming an "inner" bridge, are characterized by some displacement towards one of the zirconium atoms. It is quite obvious that structural differences also determine energy differences (Table 4, Table S1).  The most thermodynamically stable complex is 9a, in which the chlorine atoms are part of the Zr-Cl-Al moieties. Isomer 9b is higher in energy by 3.3 kcal/mol. The presence of bulky Cl atoms in the inner bridge of the biszirconium cage of structure 9d makes this complex least thermodynamically stable relative to other compounds.
To identify the structure of the complex observed by NMR spectroscopy, we compared theoretical and experimental NMR chemical shifts of hydride atoms in each of them (Table 5). It was found that in complex 9a, the hydrogen atom of the Zr-H-Zr moiety is significantly shielded (δH 1 = −4.2 ppm), while two other hydride atoms experience a de-shielding effect (δH 2 = δH 3 = 2.9 ppm). Thus, the difference in chemical shifts between the considered hydrogen atoms is ∆δ = 7.1 ppm, which is in good agreement with the experimental data. A similar trend is also observed for complex 9c, in which one  Thus, biszirconium complex 9 is readily formed in the systems Cp2ZrCl2-HAlBu i 2, [Cp2ZrH2]2-MMAO-12, and [Cp2ZrH2]2-ClAlBu i 2 both in CD2Cl2 and in CDCl3. This intermediate reacts with methylaluminoxane to give a heavy adduct, which selectively provides vinylidene dimers of 1-hexene.

DFT Study of the Structure of Biszirconium Complex 9
To refine the structure of complex 9, probable structures of cyclic isomers 9a-9d were optimized using the PBE/3ζ quantum chemical method [30][31][32]. Their structure is in line with the obtained NMR spectral data on the ratio of the signal intensities of the constituent moieties and the symmetry of the complex (Scheme 4). As follows from Scheme 4, the structures of the complexes significantly differ from one another. For example, in complex 9c, all three hydrides are located between the zirconium atoms and form a trihydride bridge, while in molecule 9a, there is only one bridging H atom. If the optimized structures of the two most energetically favorable hydride complexes 9a and 9c (Figure 3) are considered in detail, the length of the Zr-H bond varies, which may cause differences in the reactivity of the studied complexes. Thus, the lengths of both Zr-H bonds in the Zr-H-Zr moiety of complex 9a is 2.09 Å, while the lengths of two terminal Zr-H bonds are dZr1-H = dZr2-H = 1.83 Å. It is comparable with the Zr-H bond length in Cp2ZrHCl calculated by the same method (dZr-H = 1.84 Å).  In isomer 9c, all three hydrogen atoms are inside the biszirconium cage. Meanwhile, the lengths of bridging Zr-H bonds are also increased compared to those in Cp 2 ZrHCl, but are not equivalent to each other: d Zr1-H1 = d Zr2-H1 = 2.01 Å, d Zr1-H2 = d Zr2-H3 = 1.97 Å, d Zr1-H3 = d Zr2-H2 = 1.99 Å. Thus, one hydrogen atom is equidistant from both zirconium atoms, while the other two H atoms, forming an "inner" bridge, are characterized by some displacement towards one of the zirconium atoms. It is quite obvious that structural differences also determine energy differences (Table 4, Table S1). The most thermodynamically stable complex is 9a, in which the chlorine atoms are part of the Zr-Cl-Al moieties. Isomer 9b is higher in energy by 3.3 kcal/mol. The presence of bulky Cl atoms in the inner bridge of the biszirconium cage of structure 9d makes this complex least thermodynamically stable relative to other compounds.
To identify the structure of the complex observed by NMR spectroscopy, we compared theoretical and experimental NMR chemical shifts of hydride atoms in each of them (Table 5). It was found that in complex 9a, the hydrogen atom of the Zr-H-Zr moiety is significantly shielded (δH 1 = −4.2 ppm), while two other hydride atoms experience a de-shielding effect (δH 2 = δH 3 = 2.9 ppm). Thus, the difference in chemical shifts between the considered hydrogen atoms is ∆δ = 7.1 ppm, which is in good agreement with the experimental data. A similar trend is also observed for complex 9c, in which one of the hydride atoms of the inner Zr-H-Zr bridge should be in the upfield region of the 1 H NMR spectrum. As follows from Table 5, the calculated NMR data for complexes 9b and 9d are in poor agreement with the NMR spectral parameters observed. It should be noted for comparison that the calculated chemical shifts of the hydride atoms of the open structure 9e proposed earlier [23] also do not agree well with the NMR experiment. As a result of a comprehensive analysis of chemical shifts and relative Gibbs energy, the complex 9a was proposed as the most probable structure.

Computational Details
DFT calculations were carried out in Priroda-06 program [32,35]. Geometry optimization, vibrational frequency analysis, and calculation of absolute chemical shielding, entropy, and thermodynamic corrections to the total energy of the compounds were carried out using the Perdew−Burke−Ernzerhof (PBE) functional [30]. PBE functional was used in combination with a 3ζ basis set [31]. The electronic configurations of the molecular systems were described by the orbital basis sets of contracted Gaussian-type functions of size (5s1p)/[3s1p] for H, (11s6p2d)/[6s3p2d] for C, (15s11p2d)/[10s6p2d] for Al and Cl, and (20s16p11d)/[14s11p7d] for Zr, which were used in combination with the densityfitting basis sets of uncontracted Gaussian-type functions of size (5s2p) for H, (10s3p3d1f) for C, (14s3p3d1f1g) for Al and Cl, and (22s5p5d4f4g) for Zr. No symmetry of internal coordinate constraints was applied during optimizations. Thermodynamic parameters were determined at 298.15. Normal-mode vibrational frequency analysis was performed to confirm minima structures. Computations of absolute chemical shielding, σ, were carried out in the GIAO approach [36,37]. Chemical shifts were calculated as δ = σ TMS − σ, where σ TMS was the calculated shielding constant of tetramethylsilane. For comparison, structural and energetic parameters of complexes 9a-e were calculated using Gaussian 09 [38] at the PBE0 level of theory [39] employing the def2-TZVP basis set [40,41] with and without Grimme's D3(0) empirical dispersion correction (GD3) [42] (Tables S2 and S3). Moreover, the method was successfully employed to calculate chemical shifts of Au hydrides [43]. Calculated chemical shifts obtained for 9a by two methods was comparable (PBE0/def2-TZVP: δH 1 = −3.5 ppm, δH 2 = 3.1 ppm, δH 3 = 3.1 ppm). Therefore, the PBE/3ζ method was used for the calculation of the chemical shifts of other complexes. As follows from Table S3 (Supporting Information), using the GD3 correction does not lead to significant changes in the energy of the studied complexes. Furthermore, we carried out the study on the solvent effect (CH 2 Cl 2 and CHCl 3 ) using the conductor-like polarizable continuum model (CPCM) [44,45]. The data indicated minor energy changes (Supporting Information, Tables S4 and S5).
The program ChemCraft [46] was used to visualize obtained quantum chemical data. The energy at 0 K, the ZPVE correction, the enthalpy, the Gibbs free energy in gas, the temperature multiplied by the entropy, and Cartesian coordinates for all optimized structures are given in the Supporting Information.

Conclusions
As a result of studying the catalytic transformations of 1-hexene under the action of Ti group metallocenes, organoaluminum compounds, and activators MMAO-12, B(C 6 F 5 ) 3 or (Ph 3 C)[B(C 6 F 5 ) 4 ] in chlorinated solvents, it was found that systems based on Zr complexes [Cp 2 ZrH 2 ] 2 -ClAlBu i 2 -MMAO-12, Cp 2 ZrCl 2 -HAlBu i 2 -MMAO-12 in CH 2 Cl 2 selectively afford dimeric products in high yields. The use of CHCl 3 as the solvent facilitates the formation of non-classical tetramers of 1-hexene, the products of dimer dimerization.
The probable structure of the key biszirconium hydride complex was proposed based on a comparison of experimental and theoretical NMR data and estimation of the thermo-dynamical stability of the complexes. These studies are necessary to further understand the mechanism of key intermediate activation by methylaluminoxane or organoboron compounds for selective alkene dimerization.