Catalytic Systems Based on Cp2ZrX2 (X = Cl, H), Organoaluminum Compounds and Perfluorophenylboranes: Role of Zr,Zr- and Zr,Al-Hydride Intermediates in Alkene Dimerization and Oligomerization

The activity and chemoselectivity of the Cp2ZrCl2-XAlBui2 (X = H, Bui) and [Cp2ZrH2]2-ClAlEt2 catalytic systems activated by (Ph3C)[B(C6F5)4] or B(C6F5)3 were studied in reactions with 1-hexene. The activation of the systems by B(C6F5)3 resulted in the selective formation of head-to-tail alkene dimers in up to 93% yields. NMR studies of the reactions of Zr complexes with organoaluminum compounds (OACs) and boron activators showed the formation of Zr,Zr- and Zr,Al-hydride intermediates, for which diffusion coefficients, hydrodynamic radii, and volumes were estimated using the diffusion ordered spectroscopy DOSY. Bis-zirconium hydride clusters of type x[Cp2ZrH2∙Cp2ZrHCl∙ClAlR2]∙yRnAl(C6F5)3−n were found to be the key intermediates of alkene dimerization, whereas cationic Zr,Al-hydrides led to the formation of oligomers.

In continuation of this research, we studied the activation of Zr,Zr-and Zr,Al-hydride complexes, formed in the Cp 2 ZrCl 2 -XAlBu i 2 (X = H, Bu i ) and [Cp 2 ZrH 2 ] 2 -ClAlEt 2 systems, by organoboron compounds (Ph 3 C)[B(C 6 F 5 ) 4 ] or B(C 6 F 5 ) 3 and the ability of the corresponding adducts to act as active intermediates in the alkene di-and oligomerization. The L 2 ZrCl 2 -XAlBu i 2 (X = H, Bu i ) and [Cp 2 ZrH 2 ] 2 -ClAlR 2 bimetallic systems perform alkene hydrometalation providing zirconium and aluminum alkyls [48,49]. The addition of (Ph 3 C)[B(C 6 F 5 ) 4 ] or B(C 6 F 5 ) 3 to the Cp 2 ZrCl 2 -XAlBu i 2 system (X = H, Bu i ) (X = H, Bu i ) (catalytic system A) gives alkene dimers (4) and oligomers (5) (Scheme 3, Table 1). The product yield and chemoselectivity markedly depend on the reagent ratio and reaction temperature.  Table 1, Entry 1). In the mixture of oligomeric products, dimer 4 predominates (41% yield). The increase of the reaction temperature to 60 • C leads to higher monomer conversion (at the level of 95%) and maximizes the percentage of oligomeric products (  Entry 7) also leads to a higher oligomer yield. In all experiments, the yield of hydroalumination product (hexane) after the hydrolysis was no more than 1%. It should be noted that, in almost all cases, the dimerization product significantly pre-   (Table 1, Entry 7) also leads to a higher oligomer yield. In all experiments, the yield of hydroalumination product (hexane) after the hydrolysis was no more than 1%. It should be noted that, in almost all cases, the dimerization product significantly prevailed, which may be a consequence of different pathways to dimers and oligomers in these catalytic systems.

Results and Discussion
The reaction pathway can be completely shifted towards the formation of dimers by using zirconocene hydride. Thus, in the course of studying the properties of catalytic system B, [Cp 2 ZrH 2 ] 2 -ClAlEt 2 -(Ph 3 C)[B(C 6 F 5 ) 4 ], in the reaction with 1-hexene, it was found that the activity of the system significantly depends on the initial reagent ratio.  In order to identify the intermediates responsible for alkene dimerization and oligomerization, we further studied the structure and activity of hydride complexes formed in the 3 ) systems by means of NMR spectroscopy. Our investigation of the reaction of Cp 2 ZrCl 2 with HAlBu i 2 at low content of organoaluminum compound (OAC) and [Zr]:[Al] = 1: (3)(4)(5) showed the existence of a Zr,Zr-hydride intermediate 7 [47], along with the well-known Zr,Al-hydride clusters 6 [41,44,50] and 8 [41,42] (Figure 1a, Scheme 4, Table 2). As we have previously demonstrated [46], an equilibrium mixture of complexes 6-8 is also formed upon the interaction of [Cp 2 ZrH 2 ] 2 with ClAlEt 2 taken in 1: 3 ratio. In this case, a larger relative amount of complex 7 is observed ( Figure 2a). This complex, in fact, is the product of replacing one hydride atom in the zirconocene dihydride dimer with chlorine. Therefore, it can be considered as a complex formed in the reaction of the Cp 2 ZrH 2 monomer with Cp 2 ZrHCl. The dialkylaluminum hydride, produced as a result of the chloride/hydride exchange, reacts with Cp 2 ZrH 2 and ClAlEt 2 , providing trihydride complex 6. equilibrium mixture of complexes 6-8 is also formed upon the interaction of [Cp2ZrH2]2 with ClAlEt2 taken in 1: 3 ratio. In this case, a larger relative amount of complex 7 is observed ( Figure 2a). This complex, in fact, is the product of replacing one hydride atom in the zirconocene dihydride dimer with chlorine. Therefore, it can be considered as a complex formed in the reaction of the Cp2ZrH2 monomer with Cp2ZrHCl. The dialkylaluminum hydride, produced as a result of the chloride/hydride exchange, reacts with Cp2ZrH2 and ClAlEt2, providing trihydride complex 6.   The 1 Н NMR spectrum of complex 7a exhibits a distinct triplet signal at δH −6.53 ppm, which was assigned to the hydride atom of the Zr-H-Zr bridge. This signal correlates with a doublet at δH −1.39 ppm in the COSY HH spectrum. The observed signals correspond to the AX2 spin system, denoting a symmetrical arrangement of hydride ligands in the molecule. For these hydrides, the geminal constants 2 J = 17 Hz of the hydrides in 7 were greater than that of analogous atoms in 6 or 8 ( 2 J = 4-8 Hz) [41,42,44,50]. The sharp multiplet signals of hydride atoms and the absence of their exchange in the exchange spectroscopy (EXSY) spectra in the case of 7 are consequences of higher stability of the structure, The 1 H NMR spectrum of complex 7a exhibits a distinct triplet signal at δ H −6.53 ppm, which was assigned to the hydride atom of the Zr-H-Zr bridge. This signal correlates with a doublet at δ H −1.39 ppm in the COSY HH spectrum. The observed signals correspond to the AX 2 spin system, denoting a symmetrical arrangement of hydride ligands in the molecule. For these hydrides, the geminal constants 2 J = 17 Hz of the hydrides in 7 were greater than that of analogous atoms in 6 or 8 ( 2 J = 4-8 Hz) [41,42,44,50]. The sharp multiplet signals of hydride atoms and the absence of their exchange in the exchange spectroscopy (EXSY) spectra in the case of 7 are consequences of higher stability of the structure, whereas complexes 6 and 8 could be involved in the dynamic processes [44]. The intensity ratio being 1 (Zr-H): 2 (Zr-H): 20 (Cp) indicates the presence of two ZrCp 2 moieties in the molecule. A study of the reaction of complex 7a with B(C 6 F 5 ) 3 ( Figure S15, Supplementary Materials) shows that the adducts 9 and 10 contain one diethylaluminum chloride molecule. Indeed, the 1 H NMR spectrum exhibits quartet signals of CH 2 groups of ethyl substituents at Al in the range of −0.2 to -0.3 ppm, the intensity of which corresponds to one AlEt 2 moiety. The signals correlate both with high-field hydride signals and with a low-field signal of the cyclopentadienyl group in the NOESY spectra ( Figure S13, Supplementary Materials). Moreover, HMBC spectra show cross-peaks between the doublet of hydrides at δ H −1.39 ppm and the signal of α-carbon atom of the AlCH 2 group at δ C 3.3 ppm ( Figure S16). Therefore, we can conclude that complexes 7, 9, and 10 contain one ClAlR 2 molecule per bis-zirconium core. This is in agreement with the structure of the adducts formed in the presence of methylaluminoxane (MAO) [47]. Thus, as follows from NMR data and our preliminary quantum-chemical calculations, complex 7 probably has a cyclic structure shown in Scheme 4. The 1 Н NMR spectrum of complex 7a exhibits a distinct triplet signal at δH −6.53 ppm, which was assigned to the hydride atom of the Zr-H-Zr bridge. This signal correlates with a doublet at δH −1.39 ppm in the COSY HH spectrum. The observed signals correspond to the AX2 spin system, denoting a symmetrical arrangement of hydride ligands in the molecule. For these hydrides, the geminal constants 2 J = 17 Hz of the hydrides in 7 were greater than that of analogous atoms in 6 or 8 ( 2 J = 4-8 Hz) [41,42,44,50]. The sharp multiplet sig- The addition of (Ph 3 C)[B(C 6 F 5 ) 4 ] to the Cp 2 ZrCl 2 -HAlBu i 2 system brings about changes in the NMR spectra (Figure 1b-d). First, the signals of the dimeric complex 8 disappear. Second, the signals of the hydride atoms of complex 6 begin to split, which may be attributable to the formation of cationic species similar to those described previously [11,26,27,39]. Third, new high-field doublet and triplet signals corresponding to adducts 9 and 10 arise. A similar behavior of the complexes is observed in the [Cp 2 ZrH 2 ] 2 -ClAlEt 2 system upon the addition of (Ph 3 C)[B(C 6 F 5 ) 4 ] or B(C 6 F 5 ) 3 (Figure 2 and Figure S15). The diffusion coefficients of adducts 9 and 10 are much lower than those of complex 7 (Table 2, Figure 3). During the reaction, as in the case of methylaluminoxane [46,47], the formation of a heavy fraction is observed at the bottom of the NMR tube. The NOESY spectra of adduct 10 ( Figure S13, Supplementary Materials) show a negative NOE effect inherent in large molecules [51,52]. Since the lineshape of the hydride atom signals of complexes 7, 9, and 10 does not change, it can be concluded that the bis-zirconium core in complex 7, which is neutral, is preserved after the addition of a boron activator. Catalysts 2021, 11, x FOR PEER REVIEW 10 of 17 The hydrodynamic volumes (Vh) of complexes as spherical particles were calculated according to Equation (2) as follows:

NMR Study of Hydride
Adduct 10 reacts with 1-hexene to give dimer 4 ( Figure 4, Scheme 6). It should be noted that complexes 7 and 9 were inactive towards the alkene. Thus, complex 7 activated by an organoboron compound is the intermediate that selectively affords dimers in the studied systems. The same activity towards the alkene was observed for the adducts of complex 7 with MAO [47].
The trihydride complex 6, mainly formed both in system A and in system B with an excess of OAC, reacts with (Ph3C)[B(C6F5)4] to give a mixture of intermediates, which give rise to signals in the −6.6 to −0.1 ppm range for hydride atoms (Figures 1a-c, 2c and 5b) and which are in intermolecular exchange, as follows from the EXSY spectra ( Figure S22 ratio and the order of reagent mixing. The observed 1 H NMR signal splitting is probably caused by the hydrogen atom removal with the formation of Ph3CH and cationic complexes structurally similar to those described previously [11,26,27,39]. The addition of 1hexene to this system leads to the appearance of oligomers as soon as in the 5th minute of the reaction (Figure 5c, Scheme 6).  (Figures 2e and S15). This fact can be explained as follows. Compound (Ph3C)[B(C6F5)4], which is initially present in the system, quickly reacts with HAlR2, generated in the [Cp2ZrH2]2−ClAlEt2 system (Scheme 4), to give RnAl(C6F5)3−n (Scheme 5). The latter is coordinated to complex 7, resulting from the reaction of Cp2ZrH2 with Cp2ZrHCl and ClAlEt2. A low concentration of HAlR2 does not allow complex 6 to be formed. An estimation of the hydrodynamic radii and volumes of complexes 6-10 through the diffusion coefficients determined using the diffusion ordered spectroscopy (DOSY) is shown in A change in the order of reagent mixing, namely, the addition of ClAlEt 2 and then [Cp 2 ZrH 2 ] 2 to the activator, provides an increase in the content of Zr,Zr-hydride clusters 9 and 10 ( Figure 2e and Figure S15). This fact can be explained as follows. Compound (Ph 3 C)[B(C 6 F 5 ) 4 ], which is initially present in the system, quickly reacts with HAlR 2 , gener-ated in the [Cp 2 ZrH 2 ] 2 −ClAlEt 2 system (Scheme 4), to give R n Al(C 6 F 5 ) 3−n (Scheme 5). The latter is coordinated to complex 7, resulting from the reaction of Cp 2 ZrH 2 with Cp 2 ZrHCl and ClAlEt 2 . A low concentration of HAlR 2 does not allow complex 6 to be formed.
An estimation of the hydrodynamic radii and volumes of complexes 6-10 through the diffusion coefficients determined using the diffusion ordered spectroscopy (DOSY) is shown in Table 2. The hydrodynamic radii R h were calculated using the modified Stokes-Einstein equation, Equation (1) [55,56], as follows: where R h is the hydrodynamic radius of the solute, R solv is the hydrodynamic radius of the solvent (d 6 -benzene; the van der Waals radius is 2.7 Å), k is the Boltzmann constant, T is the temperature, D t is the diffusion coefficient, and η is the viscosity of the solution.
The hydrodynamic volumes (V h ) of complexes as spherical particles were calculated according to Equation (2) as follows: Adduct 10 reacts with 1-hexene to give dimer 4 ( Figure 4, Scheme 6). It should be noted that complexes 7 and 9 were inactive towards the alkene. Thus, complex 7 activated by an organoboron compound is the intermediate that selectively affords dimers in the studied systems. The same activity towards the alkene was observed for the adducts of complex 7 with MAO [47].    Thus, the Cp2ZrCl2-XAlBu i 2 (X = H, Bu i ) and [Cp2ZrH2]2-ClAlEt2 systems activated by (Ph3C)[B(C6F5)4] or B(C6F5)3 are able to accomplish selective dimerization and oligomerization of alkenes to give head-to-tail vinylidene products, similarly to systems based on Zr complexes and МАО [3,[6][7][8]13,[57][58][59]. Among the post-metallocene Zr and Hf complexes with [ONNO]-type amino-bis(phenolate) ligands activated by neutral B(C6F5)3, the best selectivity to 1-hexene dimers (up to 97%) was found to hafnium catalysts [60]. Nevertheless, data on the direct participation of metal hydrides in chemo-and regioselective dimerization of alkenes are limited and are mainly concerned with Sc [61], Y [62], Ta [63], and Ru [64] complexes. The trihydride complex 6, mainly formed both in system A and in system B with an excess of OAC, reacts with (Ph 3 C)[B(C 6 F 5 ) 4 ] to give a mixture of intermediates, which give rise to signals in the −6.6 to −0.1 ppm range for hydride atoms (Figure 1a-c, Figures 2c  and 5b) and which are in intermolecular exchange, as follows from the EXSY spectra ( Figure S22, Supplementary Materials). The composition of the mixture depends on the [Zr]:[Al]:[B] ratio and the order of reagent mixing. The observed 1 H NMR signal splitting is probably caused by the hydrogen atom removal with the formation of Ph 3 CH and cationic complexes structurally similar to those described previously [11,26,27,39]. The addition of 1-hexene to this system leads to the appearance of oligomers as soon as in the 5th minute of the reaction (Figure 5c, Scheme 6).  The bis-zirconium hydride complexes we discovered, in combination with either an МАО activator [46,47] or organoboron compounds ((Ph3C)[B(C6F5)4] or B(C6F5)3), provide the selective formation of dimers in reactions with alkenes. It is evident that the first step is hydrometalation of the alkene (Scheme 7). The addition of a second alkene molecule and subsequent β-H elimination to give dimers could occur with the involvement of one or two metal sites. Similar two-site catalysis is known for alkene polymerization in the presence of group 4 metal complexes [65]. Nevertheless, certain issues remain unanswered: how the bis-zirconium structures are activated; what is the principle of formation Thus, the Cp 2 ZrCl 2 -XAlBu i 2 (X = H, Bu i ) and [Cp 2 ZrH 2 ] 2 -ClAlEt 2 systems activated by (Ph 3 C)[B(C 6 F 5 ) 4 ] or B(C 6 F 5 ) 3 are able to accomplish selective dimerization and oligomerization of alkenes to give head-to-tail vinylidene products, similarly to systems based on Zr complexes and MAO [3,[6][7][8]13,[57][58][59]. Among the post-metallocene Zr and Hf complexes with [ONNO]-type amino-bis(phenolate) ligands activated by neutral B(C 6 F 5 ) 3 , the best selectivity to 1-hexene dimers (up to 97%) was found to hafnium catalysts [60]. Nevertheless, data on the direct participation of metal hydrides in chemo-and regioselective dimerization of alkenes are limited and are mainly concerned with Sc [61], Y [62], Ta [63], and Ru [64] complexes.
The bis-zirconium hydride complexes we discovered, in combination with either an MAO activator [46,47] or organoboron compounds ((Ph 3 C)[B(C 6 F 5 ) 4 ] or B(C 6 F 5 ) 3 ), provide the selective formation of dimers in reactions with alkenes. It is evident that the first step is hydrometalation of the alkene (Scheme 7). The addition of a second alkene molecule and subsequent β-H elimination to give dimers could occur with the involvement of one or two metal sites. Similar two-site catalysis is known for alkene polymerization in the presence of group 4 metal complexes [65]. Nevertheless, certain issues remain unanswered: how the bis-zirconium structures are activated; what is the principle of formation of the heavy fraction that is active towards alkenes; and how dimerization of alkenes with the participation of these adducts takes place. Further development of our study would be concerned with the determination of the structure of active sites and the mechanism of alkene dimerization under the action of bimetallic complexes. The bis-zirconium hydride complexes we discovered, in combination with either an МАО activator [46,47] or organoboron compounds ((Ph3C)[B(C6F5)4] or B(C6F5)3), provide the selective formation of dimers in reactions with alkenes. It is evident that the first step is hydrometalation of the alkene (Scheme 7). The addition of a second alkene molecule and subsequent β-H elimination to give dimers could occur with the involvement of one or two metal sites. Similar two-site catalysis is known for alkene polymerization in the presence of group 4 metal complexes [65]. Nevertheless, certain issues remain unanswered: how the bis-zirconium structures are activated; what is the principle of formation of the heavy fraction that is active towards alkenes; and how dimerization of alkenes with the participation of these adducts takes place. Further development of our study would be concerned with the determination of the structure of active sites and the mechanism of alkene dimerization under the action of bimetallic complexes. Scheme 7. Proposed mechanism: one or two Zr reaction centers? Scheme 7. Proposed mechanism: one or two Zr reaction centers?
The product composition was determined using a gas chromatograph mass spectrometer GCMS-QP2010 Ultra (Shimadzu, Tokyo, Japan) equipped with the GC-2010 Plus chromatograph (Shimadzu, Tokyo, Japan), TD-20 thermal desorber (Shimadzu, Tokyo, Japan), and an ultrafast quadrupole mass-selective detector (Shimadzu, Tokyo, Japan). Details on the GC-MS analysis of dimers and oligomers are given in the Supplementary Materials.
CAUTION: The pyrophoric nature of aluminum hydrides and aluminum alkyls require special safety precautions in their handling.  Method A. The NMR tube was filled with 0.05 mmol (11.2 mg) of [Cp 2 ZrH 2 ] 2 (2) and 0.5 mL of benzene-d 6 under argon. Then 0.15 mmol (18.6 mg) of ClAlEt 2 was added dropwise at 0 • C. The mixture was stirred and the formation of complexes 6-8 was monitored by NMR at room temperature. After addition of 0.025 mmol (22.8 mg) of (Ph 3 C)[B(C 6 F 5 ) 4 ] or B(C 6 F 5 ) 3 (12.8 mg) a division of the reaction media into two fractions was observed. Method B. The NMR tube was filled with 0.025 mmol (22.8 mg) of (Ph 3 C)[B(C 6 F 5 ) 4 ] or B(C 6 F 5 ) 3 (12.8 mg), 0.15 mmol (18.6 mg) of ClAlEt 2 , and 0.5 mL of benzene-d 6 under argon.
Further 0.05 mmol (11.2 mg) of [Cp 2 ZrH 2 ] 2 were added at 0 • C. The mixture was stirred and studied by NMR at room temperature.

Conclusions
In summary, we found the conditions for the synthesis of alkene dimers or oligomers in the Cp 2 ZrCl 2 -XAlBu i 2 (X = H, Bu i ) and [Cp 2 ZrH 2 ] 2 -ClAlEt 2 catalytic systems activated by (Ph 3 C)[B(C 6 F 5 ) 4 ] or B(C 6 F 5 ) 3 . NMR studies showed that adducts with the bis-zirconium core of type x[Cp 2 ZrH 2 ·Cp 2 ZrHCl·ClAlR 2 ]· yR n Al(C 6 F 5 ) 3-n are key intermediates of the alkene dimerization. The oligomerization pathway in these systems is realized due to the existence of cationic Zr,Al-hydride species formed in the reaction of the complex Cp 2 Zr(µ-H) 3 (AlBu i 2 ) 2 (µ-Cl) with boron activators. Therefore, it is relevant to continue studies of the reaction mechanism in order to identify the activation principle of bis-zirconium complexes by organoboron compounds and understand the necessity of participation of bimetallic sites for selective dimerization of alkenes.