Catalytic Systems Based on Cp2zrx2 (X = Cl, H), Organoaluminum

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Article 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

Lyudmila V. Parfenova 1,* , Pavel V. Kovyazin 1, Almira Kh. Bikmeeva 1 and Eldar R. Palatov 2

1 Institute of Petrochemistry and Catalysis of Russian Academy of Sciences, Prospekt Oktyabrya, 141, 450075 Ufa, Russia; [email protected] (P.V.K.); [email protected] (A.K.B.) 2 Bashkir State University, st. Zaki Validi, 32, 450076 Ufa, Russia; [email protected] * Correspondence: [email protected]; Tel.: +7-347-284-3527

i i Abstract: The activity and chemoselectivity of the Cp2ZrCl2-XAlBu 2 (X = H, Bu ) 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[Cp ZrH ·Cp ZrHCl·ClAlR ]·yRnAl(C F ) − were found to be the key intermediates of alkene 2 2 2 2 6 5 3 n dimerization, whereas cationic Zr,Al-hydrides led to the formation of oligomers. Citation: Parfenova, L.V.; Kovyazin, P.V.; Bikmeeva, A.K.; Palatov, E.R. Keywords: zirconocene; metal hydrides; alkene dimerization; nuclear magnetic resonance

Catalytic Systems Based on Cp2ZrX2 (X = Cl, H), Organoaluminum Compounds and Perfluorophenylboranes: Role of 1. Introduction Zr,Zr- and Zr,Al-Hydride Among the catalytic methods for alkene di-, oligo-, and polymerization, approaches Intermediates in Alkene Dimerization that use Ziegler–Natta-type catalytic systems, based on metallocenes or post-metallocenes, and Oligomerization. Catalysts 2021, 11, 39. https://doi.org/10.3390/ typically Group 4 transition metal complexes, in combination with organoaluminum catal11010039 or organoboron activators, have great potential for development [1–5]. Apart from the + − known action of alkyl cations [(L2MAlk) X ] as polymer chain growth centers, the idea + − Received: 10 December 2020 of the participation of [(L2MH) X ]-type hydride complexes as catalytic active species Accepted: 27 December 2020 in the reactions of alkene di-, oligo-, and polymerization has been repeatedly pointed Published: 31 December 2020 out [2,4,6–15]. This assumption follows both from the structure of the reaction products (the presence of vinylidene terminal groups, arising upon chain termination via β-H Publisher’s Note: MDPI stays neu- elimination, which generates intermediates with M–H bond) and from the fact that the tral with regard to jurisdictional clai- efficiency of catalytic systems increases upon the introduction of hydride-generating ad- i ms in published maps and institutio- ditives such as AlBu 3 [13,16–25]. Moreover, there are several studies on the synthesis nal affiliations. and identification of catalytically active hydride Zr,B-complexes (Scheme1). For exam- + − + − ple, complexes [Cp’2ZrH] [MeB(C6F5)3] and [Cp’2ZrH] [HB(C6F5)3] were found to be active in the ethylene and propylene polymerization [26,27]. Additionally, hydride complexes [Cp0 Zr H ][B(C F R) ] (R = F, SiPri ), highly active initiators for the isobutene Copyright: © 2020 by the authors. Li- 4 2 3 6 4 4 3 censee MDPI, Basel, Switzerland. homopolymerization and isobutene–isoprene copolymerization, were described [28]. This article is an open access article A number of Zr borohydride complexes were prepared by Piers et al. by the reaction distributed under the terms and con- of alkylzirconocenes with HB(C6F5)2 [29–31]. The complexes were found to be active in ditions of the Creative Commons At- ethylene polymerization provided that additional alkylation with MAO takes place [30]. tribution (CC BY) license (https:// Zr,B-hydride complexes were also obtained in the reaction of rac-ethylenebis(4,5,6,7- i creativecommons.org/licenses/by/ tetrahydro-1-indenyl)]zirconium difluoride with HAlBu 2 and B(C6F5)3 [32,33]. The acti- i 4.0/). vation of hafnocenes with AlBu 3/(Ph3C)[B(C6F5)4] yielded hydrido-bridged species of

Catalysts 2021, 11, 39. https://doi.org/10.3390/catal11010039 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 39 2 of 16

i + i + the type [LHf(µ-H)2AlBu 2] or [LHf(µ-H)2Al(H)Bu ] , which were more active in alkene polymerizations than methyl-bridged heterobinuclear intermediates [LHf(µ-Me)2Al(µ- Me)2][MeMAO] and [LHf(µ-Me)2Al(µ-Me)2][B(C6F5)4][34]. Conversely, the formation of a + + – similar intermediate, [Cp2Zr(µ-H)2AlMe2] , observed in the [Cp2Zr(µ-Me)2AlMe2] [B(C6F5)4] -1-alkene system by electrospray ionization mass spectrometry (ESI-MS), is interpreted as a pathway for catalyst deactivation [35]. The reaction of zirconocenes with excess i AlBu 3 and boron activators gave an oily product that contained hydride complexes [LZr(µ- i H)(µ-C4H7)-AlBu 2][B(C6F5)4][36,37], which were proposed as the thermodynamic sink of the catalyst in alkene polymerization [37]. As a result of studying the polymerization reactions in the presence of dialkyl ansa-complexes [Me2C(Cp)IndMMe2] (M = Zr; Hf), activated with B(C6F5)3, hydride intermediates Me2C(Cp)IndMMe(µ-H)B(C6F5)3, which were unreactive towards monomer insertion and, therefore, represented dormant states, were found [38]. Modiﬁcation of Zr,Al-hydride complexes Cp’2ZrH3AlH2 (Cp’ = C5Me5, C5H4SiMe3) by B(C6F5)3 was accompanied by the formation of di- or polynuclear metal- Catalysts 2020, 10, x FOR PEER REVIEW 2 of 17 locenium ion pairs containing terminal and bridging counteranions HB(C6F5)3, appearing due to hydride abstraction, which showed the ability to polymerize ethylene [39]. Het- found to be active in the ethylene and propylene polymerization [26,27].i Ad- i erometallic complexes formed in the reactions of L2ZrCl2 with XAlBu 2 (X = H, Cl, Bu )[40] ditionally, hydride complexes [Cp′4Zr2H3][B(C6F4R)4] (R = F, SiPri3), highly ac- being transformed into the cationic species by [Ph C][B(C F ) )] also become capable of tive initiators for the isobutene homopolymerization3 and isobutene–isoprene6 5 4 alkene polymerization [10,11]. copolymerization, were described [28].

+ - [Cp'2ZrH] [HB(C6F5)3] b + - a [Cp'2ZrH] [MeB(C6F5)3]

- t [B(C6F4R)4] - Cp' = Me5C5, Bu 2C5H3 [B(C6F4R)4] c b 5 T. Marks, 1992, 1994 Cp' = η -C5H3SiMe3 i R = F, SiPr 3

a M. Bochmann, 1999 - -

W. E. Piers, 1995-1998 R = Cl, Me, Ph

- [B(C6F5)4]

5 L = Ph2C[(fluorenyl)(η -C5H4)] M = Zr; Hf P. Arndt; U. Rosenthal, 2003, 2006 C. Gotz, 2002 Ab. Al-Humydi; Sc. Collins, 2005

- - - [B(C6F5)4] [B(C6F5)4] [B(C6F5)4]

M= Zr; Hf 5 2 2 2 η 5 4 5 L = Me Si(1-indenyl) ; Me C[(fluorenyl)( -C H )]; L = EBI, EBTHI, Me2C(1-indenyl)2, Me4C2(η -C5H4)2, 5 2 η 5 4 5 Ph C[(fluorenyl)( -C H )]; 2 η 5 4 2 2 2 5 3 2 [Me Si( -C H ) , [(Me Si) (C H ) , C2H4[(fluorenyl)(5,6-C3H6-2-MeInd)] 5 5 n 5 5 η -C5H5, η -C5H4 Bu, η -C5H4SiMe3, η -1,2-Me2C5H3 H.-H. Brintzinger, 2011 K.P. Bryliakov, M. Bochmann, 2008

- [B(C6F5)4] 2[HB(C6F5)3]

n H. S. Zijlstra, S. Collins and J. S. McIndoe, 2020 L = CpMe5; BuCp; Me3SiCp R. Gonzalez-Hernandez, Sc. Collins, 2006

SchemeScheme 1. 1.Examples Examples of of Zr(Hf),B-hydride Zr(Hf),B-hydride complexes. complexes. OurA number recent of study Zr borohydride was concerned complexes with the were structure, prepared dynamics, by Piers et andal. by reactivity of i hydridethe reaction complexes of alkylzirconocenes generated in with the followingHB(C6F5)2 [29–31]. systems: The L2 complexesZrCl2-XAlBu were2 (X = H, Cl, i i Bufound)[41 –to45 be] and active [Cp in2ZrH ethylene2]2-ClAlR polymeri2 (R =zation Et, Bu provided)-methylaluminoxane that additional (MMAO-12) alkyl- [46,47] ation with МАО takes place [30]. Zr,B-hydride complexes were also obtained in the reaction of rac-ethylenebis(4,5,6,7-tetrahydro-1-indenyl)]zirconium difluoride with HAlBui2 and B(C6F5)3 [32,33]. The activation of hafnocenes with AlBui3/(Ph3C)[B(С6F5)4] yielded hydrido-bridged species of the type [LHf(µ-H)2AlBui2]+ or [LHf(µ-H)2Al(H)Bui]+, which were more active in alkene polymerizations than methyl-bridged heterobinuclear intermediates [LHf(µ-Me)2Al(µ-Me)2][MeMAO] and [LHf(µ-Me)2Al(µ-Me)2][B(C6F5)4] [34]. Conversely, the formation of a similar intermediate, [Cp2Zr(µ-H)2AlMe2]+, observed in the [Cp2Zr(µ-Me)2AlMe2]+[B(C6F5)4]--1-alkene system by electrospray ionization mass spectrometry (ESI-MS), is interpreted as a pathway for catalyst deactivation [35]. The reaction of zirconocenes with excess AlBui3 and boron activators gave an oily product that contained hydride complexes [LZr(µ-H)(µ-C4H7)-AlBui2][B(C6F5)4] [36,37], which were proposed as the thermodynamic sink of the catalyst in alkene polymerization [37]. As a result of studying the polymerization reactions in the presence of dialkyl ansa-complexes [Me2C(Cp)IndMMe2] (M = Zr; Hf), activated with B(C6F5)3, hydride in-

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termediates Me2C(Cp)IndMMe(µ-H)B(C6F5)3, which were unreactive towards monomer insertion and, therefore, represented dormant states, were found [38]. Modification of Zr,Al-hydride complexes Cp’2ZrH3AlH2 (Cp’ = C5Me5, C5H4SiMe3) by B(C6F5)3 was accompanied by the formation of di- or polynuclear metallocenium ion pairs containing terminal and bridging counteranions HB(C6F5)3, appearing due to hydride abstraction, which showed the ability to polymerize ethylene [39]. Heterometallic complexes formed in the reactions of L2ZrCl2 with XAlBui2 (X = H, Cl, Bui) [40] being transformed into the cationic species by [Ph3C][B(C6F5)4)] also become capable of alkene polymerization [10,11]. Our recent study was concerned with the structure, dynamics, and reac- Catalysts 2021, 11, 39 3 of 16 tivity of hydride complexes generated in the following systems: L2ZrCl2-XAl- Bui2 (X = H, Cl, Bui) [41–45] and [Cp2ZrH2]2-ClAlR2 (R = Et, Bui)-methylaluminoxane (MMAO-12) [46,47] (Scheme 2). Among the hydride complexes, novel Zr,Zr-structures(Scheme2). Among were theidentified hydride and complexes, their high novel affinity Zr,Zr-structures to MAO with were the identifiedabil- and their ityhigh to give affinity x[Cp to2 MAOZrH2∙Cp with2ZrHCl the ability∙ClAlR to2]∙yMAO-type give x[Cp2ZrH adducts2·Cp2ZrHCl was ·shownClAlR 2]·yMAO-type [46].adducts Moreover, was shownthe adducts [46]. Moreover,were found the to adductsbe reactive were towards found toalkenes be reactive to spe- towards alkenes cificallyto specifically give dimerization give dimerization products products in high yields in high [47]. yields [47].

AlR2 AlR2 H AlR2 H H H Cl Cl H H L2 Zr H L2 Zr AlR2 L2 Zr AlR2 Cl Cl H AlR2 H H AlR2 AlR2 [41-47] [43-45] [43-45]

L L Cp Cp H H H Zr Zr H H Zr Zr H H Cl (X)AlR AlR3 R2Al(X) L L 2 R3Al Cp Cp [41-47] [46]

Scheme 2. Structures of Zr,Al-hydride complexes studied in Refs. [41–47]. Scheme 2. Structures of Zr,Al-hydride complexes studied in Refs. [41–47]. In continuation of this research, we studied the activation of Zr,Zr- and Zr,Al-hydride In continuation of this research, we studiedi the activationi of Zr,Zr- and complexes, formed in the Cp2ZrCl2 –XAlBu 2 (X = H, Bu ) and [Cp2ZrH2]2 –ClAlEt2 Zr,Al-hydride complexes, formed in the Cp2ZrCl2 –XAlBui2 (X = H, Bui) and systems, by organoboron compounds (Ph3C)[B(C6F5)4] or B(C6F5)3 and the ability of the [Cp2ZrH2]2 –ClAlEt2 systems, by organoboron compounds (Ph3C)[B(С6F5)4] or corresponding adducts to act as active intermediates in the alkene di- and oligomerization. B(С6F5)3 and the ability of the corresponding adducts to act as active interme- diates2. Results in the alkene and Discussion di- and oligomerization. 2.1. Study of 1-Hexene Transformations in Cp ZrCl -XAlBui (X = H, Bui) and 2. Results and Discussion 2 2 2 [Cp2ZrH2]2-ClAlEt2 Catalytic Systems Modiﬁed with Boron Activators 2.1. Study of 1-Hexene Transformationsi ini Cp2ZrCl2-XAlBui2 (X = H, Bui) and The L2ZrCl2-XAlBu 2 (X = H, Bu ) and [Cp2ZrH2]2-ClAlR2 bimetallic systems perform [Cpalkene2ZrH2] hydrometalation2-ClAlEt2 Catalytic providing Systems Modifie zirconiumd with and Boron aluminum Activators alkyls [48,49]. The addition i i i i i ofThe (Ph3 C)[B(CL2ZrCl62-XAlBuF5)4] or2 B(C (X 6=F H,5)3 Buto the) and Cp [Cp2ZrCl2ZrH2 -XAlBu2]2-ClAlR2 system2 bimetallic (X = H,sys- Bu ) (X = H, Bu ) tems(catalytic perform system alkeneA )hydrometalation gives alkene dimers providing (4) and zirconium oligomers and (5) aluminum (Scheme3, Table1 ). The alkylsproduct [48,49]. yield The and addition chemoselectivity of (Ph3C)[B(C markedly6F5)4] or depend B(C6F5) on3 to the the reagent Cp2ZrCl ratio2 - and reaction XAlButemperature.i2 system (X For = example,H, Bui) (X the = H, reaction Bui) (catalytic carried system out at [Zr]:[Al]:[B]:[1-alkene]A) gives alkene ratio of dimers1:16:1:1000 (4) and and oligomers a temperature (5) (Scheme of 40 ◦ C3, forTable 90min 1). The results product in 60% yield conversion and of 1-hexene chemoselectivity(Table1 , Entry 1).markedly In the mixture depend of on oligomeric the reagent products, ratio and dimer reaction4 predominates temper- (41% yield). ◦ ature.The For increase example, of the the reaction reaction temperature carried out to at 60 [Zr]:[Al]:[B]:[1-alkene]C leads to higher monomer ratio of conversion (at Catalysts 2020, 10, x FOR PEER REVIEW1:16:1:1000the level ofand 95%) a temperature and maximizes of 40 the °C percentagefor 90 min ofresults oligomeric in 60% products conversion4 of 17 (Table 1, Entry 2). of 1-hexene (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 Cat.monomer system A conversionor B (at the level of 95%) and maximizes the percentage of oligomeric20-60°C products (Table 1, Entry 2). 3 n 4 5 n=1-5 i Cat. system A: Cp2ZrCl2 (1):HAlBu2:[Ph3C][B(C6F5)4] or B(C6F5)3:1-alkene = (1-4):(16-25):1:1000 Cat. system B: Cp2ZrH2 (2):ClAlEt2:[Ph3C][B(C6F5)4] or B(C6F5)3:1-alkene = (1-4):(4-5):1:400

SchemeScheme 3. 1-Hexene dimerization andand oligomerizationoligomerizationunder under the the action action of of catalytic catalytic systems A or B. systems A or B.

An increase in Cp2ZrCl2 and HAlBui2 content up to [Zr]:[Al]:[B]:[alkene] = 4:16:1:1000 increases the conversion of the substrate to 90% and the yield of dimers up to 67% (Table 1, Entry 3). As in the previous case, the relative amount of oligomeric products grows with the rise of the reaction temperature (Table 1, Entry 4). The replacement of HAlBui2 with AlBui3 (Table 1, Entry 5) or increase in the HAlBui2 concentration (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. 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, [Cp2ZrH2]2-ClAlEt2-(Ph3C)[B(C6F5)4], in the reaction with 1-hexene, it was found that the activity of the system significantly depends on the initial reagent ratio. For example, when [Zr]:[Al]:[B]:[1- alkene] = 1:3:1:400, the degree of alkene conversion is 30% in 150 min at 60 °C (Table 1, Entry 9). An increase in the (Ph3C)[B(C6F5)4] content to [Zr]:[B] = 1:(3– 10) leads to a complete loss of catalytic system activity. An increase in [Cp2ZrH2]2 and ClAlEt2 concentration in system B substantially elevates the conversion of the substrate. At the ratio [Zr]:[Al]:[B]:[1-alkene] = 4:8:1:400 and a temperature of 40 °C, 81% of dimers with vinylidene double bond are formed in 150 min of the reaction (Table 1, Entry 10). The replacement of (Ph3C)[B(C6F5)4] by B(C6F5)3 results in a more selective formation of dimerization products in up to 93% yield in the presence of either Cp2ZrCl2 or [Cp2ZrH2]2 (Table 1, Entries 6,11). 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 Cp2ZrCl2-HAlBui2-(Ph3C)[B(PhF5)4] (B(C6F5)3) and [Cp2ZrH2]2-ClAlEt2-(Ph3C)[B(C6F5)4] (B(C6F5)3) systems by means of NMR spectroscopy.

Catalysts 2021, 11, 39 4 of 16

Table 1. Catalytic activity and chemoselectivity of systems A and B in the reaction with 1-hexene.

Catalytic Systems A or B Product Composition, % ◦ Time, Alkene Entry [Zr]:[Al]:[B]:[1-alkene] T, C 5 Zr Complex a Organoboron Activator min Conversion, % OAC 4 n = 1 n = 2 n = 3 n = 4 n = 5 1 40 90 60 41 10 4 2 2 1 i (Ph C)[B(C F ) ] 2 HAlBu 2 3 6 5 4 1:16:1:1000 60 90 95 29 16 15 13 12 9

i 3 HAlBu 2 (Ph3C)[B(C6F5)4] 40 90 90 67 10 5 3 2 2 i 4 HAlBu 2 (Ph3C)[B(C6F5)4] 60 90 97 52 14 13 8 7 3 i 4:16:1:1000 5 Cp2ZrCl2 AlBu 3 (Ph3C)[B(C6F5)4] 40 90 95 38 14 16 10 9 7 i 6 HAlBu 2 B(C6F5)3 40 60 > 99 93 3 - - - - i 7 HAlBu 2 (Ph3C)[B(C6F5)4] 4:25:1:1000 40 90 99 52 17 11 9 6 3 i 8 HAlBu 2 (Ph3C)[B(C6F5)4] 4:16:1:400 40 90 >99 59 13 10 7 5 5

9 ClAlEt2 (Ph3C)[B(C6F5)4] 1:3:1:400 60 150 30 28 - - - - -

10[Cp2ZrH2]2 ClAlEt (Ph C)[B(C F ) ] 40 150 81 81 - - - - - 2 3 6 5 4 4:8:1:400 11 ClAlEt2 B(C6F5)3 40 90 91 86 - - - - - a OAC—organoaluminum compound. Catalysts 2021, 11, 39 5 of 16

i An increase in Cp2ZrCl2 and HAlBu 2 content up to [Zr]:[Al]:[B]:[alkene] = 4:16:1:1000 increases the conversion of the substrate to 90% and the yield of dimers up to 67% (Table1, Entry 3). As in the previous case, the relative amount of oligomeric products grows with i the rise of the reaction temperature (Table1, Entry 4). The replacement of HAlBu 2 with i i AlBu 3 (Table1, Entry 5) or increase in the HAlBu 2 concentration (Table1, 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. 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, [Cp2ZrH2]2-ClAlEt2-(Ph3C)[B(C6F5)4], in the reaction with 1-hexene, it was found that the activity of the system signiﬁcantly depends on the initial reagent ratio. For example, when [Zr]:[Al]:[B]:[1-alkene] = 1:3:1:400, the degree of alkene conversion is 30% in ◦ 150 min at 60 C (Table1, Entry 9). An increase in the (Ph 3C)[B(C6F5)4] content to [Zr]:[B] = 1:(3–10) leads to a complete loss of catalytic system activity. An increase in [Cp2ZrH2]2 and ClAlEt2 concentration in system B substantially elevates the conversion of the substrate. At the ratio [Zr]:[Al]:[B]:[1-alkene] = 4:8:1:400 and a temperature of 40 ◦C, 81% of dimers with vinylidene double bond are formed in 150 min of the reaction (Table1, Entry 10). The replacement of (Ph3C)[B(C6F5)4] by B(C6F5)3 results in a more selective formation of dimerization products in up to 93% yield in the presence of either Cp2ZrCl2 or [Cp2ZrH2]2 (Table1, Entries 6,11). 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 i Cp2ZrCl2-HAlBu 2-(Ph3C)[B(PhF5)4] (B(C6F5)3) and [Cp2ZrH2]2-ClAlEt2-(Ph3C)[B(C6F5)4] (B(C6F5)3) systems by means of NMR spectroscopy.

2.2. NMR Study of Hydride Intermediate Structure in the Cp2ZrY2 (Y = H, Cl)-OAC Systems Activated by (Ph3C)[B(C6F5)4] or B(C6F5)3 i Our investigation of the reaction of Cp2ZrCl2 with HAlBu 2 at low content of organoaluminum compound (OAC) and [Zr]:[Al] = 1:(3–5) 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] (Figure1a, Scheme4, Table2). As we have previously demonstrated [ 46], an 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 (Figure2a). 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.

Table 2. 1H and 13C NMR (δ, ppm, 400.13 MHz (1H), 100.62 (13C), T = 298 K), diffusion coefﬁcients, hydrodynamic radii, and volumes of complexes 6–10 obtained by the diffusion ordered spectroscopy DOSY.

D , V , Complex δ Cp δ Cp δ Zr-H δ AlR t R ,Å h H C H H 10−10 m2 s−1 h Å3 5.61 −1.09 (br.t, 6.3 Hz, 1H) 0.32 (m) 6a a 104.7 9.3 4.2 319 (s, 10H) −2.28 (br.d, 6.3 Hz, 2H) 1.25 (m) 5.48 −1.39 (d, 17.0 Hz, 2H) 0.17 (q, 8.1 Hz, 4H) 7a a 107.6 7.5 4.9 505 (s, 20H) −6.53 (t, 17.0 Hz, 1H) 1.35 (t, q, 8.1 Hz, 4H) 5.73 −1.66 (br.s, 1H) 0.32 (m) 8a a 107.2 8.3 4.6 403 (s, 10H) −2.63 (br.s, 1H) 1.25 (m) 5.30 −1.51 (d, 16.8 Hz, 2H) 0.14 (q, 8.1 Hz, 4H) 9a b 107.7 5.0 6.9 1344 (s, 20H) −6.62 (t, 16.8 Hz, 1H) 1.33 (t, q, 8.1 Hz, 4H) 5.06 −1.72 (br.d, 16.8 Hz, 2H) −0.11 (br.q, 7.2 Hz, 4H) 10 b 107.5 1.8 17.2 21236 (s, 20H) −6.87 (br.t, 16.8 Hz, 1H) 0.88–1.01 (m) a b Complexes are obtained in the [Cp2ZrH2]2-ClAlEt2, d8-toluene system. Complexes are obtained in the [Cp2ZrH2]2-ClAlEt2-(Ph3C)[ B(C6F5)4], d6-benzene system. Catalysts 2021, 11, x FOR PEER REVIEW 6 of 17

2.2. NMR Study of Hydride Intermediate Structure in the Cp2ZrY2 (Y = H, Cl)-ОАС Systems Catalysts 2021, 11, x FOR PEER REVIEWActivated by (Ph3C)[B(C6F5)4] or B(C6F5)3 7 of 17

Our investigation of the reaction of Cp2ZrCl2 with HAlBui2 at low content of organoaluminum compound (OAC) and [Zr]:[Al] = 1:(3–5) 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 [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 com- Catalysts 2021, 11, 39 plex formed in the reaction of the Cp2ZrH2 monomer with Cp2ZrHCl. The dialkylalumi-6 of 16 num hydride, produced as a result of the chloride/hydride exchange, reacts with Cp2ZrH2 and ClAlEt2, providing trihydride complex 6.

Figure 2. 1H NMR of the [Cp2ZrH2]2-ClAlEt2-(Ph3C)[ B(C6F5)4] system in С6D6 (T= 299 K): (a) [Zr]:[Al]:[B]Figure 1. 11H = NMR 1:4:0; of (b the) [Zr]:[Al]:[B] Cp2ZrCl2 -HAlBu = 1:4:0.15;i2 -(Ph i(c)3 C)[B(C[Zr]:[Al]:[B]6F5)4] system= 1:4:0.2, in upper С6D6 (T=layer; 299 ( dK):) [Zr]:[Al]:[B] (a) Figure 1. H NMR of the Cp2ZrCl2 -HAlBu 2 -(Ph3C)[B(C6F5)4] system in C6D6 (T= 299 K): (a) = 1:4:0.2, heavy fraction; and (e) [Zr]:[Al]:[B] = 1:3:0.5. The order of mixing the reagents is [Zr]:[Al]:[B][Zr]:[Al]:[B] = = 1:5:0; 1:5:0; ( b(b)) [Zr]:[Al]:[B] [Zr]:[Al]:[B] = = 1:3:0.25; 1:3:0.25; ( c()c) [Zr]:[Al]:[B] [Zr]:[Al]:[B] = = 1:3:0.45; 1:3:0.45; and and (d ()d [Zr]:[Al]:[B]) [Zr]:[Al]:[B] = 1:3:0.5.= i (Ph1:3:0.5.3C)[B(C The6 Forder5)4]–ClAlEt of mixing2–[Cp th2ZrHe reagents2]2. in the experiments (b–d) isi HAlBu 2–(Ph3C)[B(C6F5)4]– The order of mixing the reagents in the experiments (b–d) is HAlBu 2–(Ph3C)[B(C6F5)4]–Cp2ZrCl2. Cp2ZrCl2.

i 3-5 eq. HAlBu 2 Cp2 Cp2 Cp2ZrCl2 Cp2

6a,b 7a,b 3 eq. ClAlR2 R = Et (a), Bu (b) R = Et

ClAlR2 8a,b [Cp2ZrH2]2 Cp2ZrH2 Cp2ZrHCl R2AlH

Cp2ZrH2 Cp2ZrHCl ClAlR2 7

Scheme 4. Reaction of Cp ZrCl and [Cp ZrH ] with organoaluminum compounds (OACs). Scheme 4. Reaction of Cp2ZrCl2 and [Cp22ZrH2]22 with2 organoaluminum compounds (OACs). 1 The 1НH NMR spectrum ofof complexcomplex7a 7a exhibitsexhibitsa adistinct distinct triplet triplet signal signal at atδ δHH− −6.53 ppm, whichwhich waswas assignedassigned toto thethe hydridehydride atomatom ofof thethe Zr-H-Zr bridge.bridge. This signal correlates withwith δ − a doublet at δH −1.391.39 ppm ppm in in the the COSY COSY HH HH spectrum. spectrum. The The observed observed signals signals correspond correspond to tothe the AX AX2 spin2 spin system, system, denoting denoting a symmetrical a symmetrical arrangement arrangement of hydride of hydride ligands ligands in the in mol- the 2 molecule.ecule. For these For these hydrides, hydrides, the geminal the geminal constants constants 2J = 17 HzJ = 17of the Hz hydrides of the hydrides in 7 were in greater7 were 2 greaterthan that than of thatanalogous of analogous atoms atomsin 6 or in 86 (2orJ =8 4–8( J = Hz) 4–8 [41,42,44,50]. Hz) [41,42,44 ,The50]. sharp The sharp multiplet multiplet sig- signalsnals of ofhydride hydride atoms atoms and and the the absence absence of of their their exchange exchange in in the exchange spectroscopyspectroscopy (EXSY)(EXSY) spectraspectra in in the the case case of 7ofare 7 consequencesare consequences of higher of higher stability stability of the structure, 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 ZrCp2 moieties in the molecule. A study of the reaction of complex 7a with B(C F ) (Figure S15, Supplementary 6 5 3 Materials) shows that the adducts 9 and 10 contain one diethylaluminum chloride molecule. 1 Indeed, the H NMR spectrum exhibits quartet signals of CH2 groups of ethyl substituents at Al in the range of −0.2 to –0.3 ppm, the intensity of which corresponds to one AlEt2 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 δH −1.39 ppm and the signal of α-carbon atom of the AlCH2 group at δC 3.3 ppm Catalysts 2021, 11, 39 7 of 16

(Figure S16). Therefore, we can conclude that complexes 7, 9, and 10 contain one ClAlR2 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 Catalysts 2021, 11, x FOR PEER REVIEW 7 of 17 data and our preliminary quantum-chemical calculations, complex 7 probably has a cyclic structure shown in Scheme4.

Figure 2. 11H NMR of the [Cp2ZrH2]2-ClAlEt2-(Ph3C)[ B(C6F5)4] system in С6D6 (T= 299 K): (a) Figure 2. H NMR of the [Cp2ZrH2]2-ClAlEt2-(Ph3C)[B(C6F5)4] system in C6D6 (T= 299 K): (a) [Zr]:[Al]:[B][Zr]:[Al]:[B] = = 1:4:0;1:4:0; ((bb)) [Zr]:[Al]:[B][Zr]:[Al]:[B] = = 1:4:0.15;1:4:0.15; ((cc)) [Zr]:[Al]:[B][Zr]:[Al]:[B] == 1:4:0.2,1:4:0.2, upperupper layer;layer; ((dd)) [Zr]:[Al]:[B][Zr]:[Al]:[B] = 1:4:0.2, heavy fraction; and (e) [Zr]:[Al]:[B] = 1:3:0.5. The order of mixing the reagents is = 1:4:0.2, heavy fraction; and (e) [Zr]:[Al]:[B] = 1:3:0.5. The order of mixing the reagents is (Ph3C)[B(C6F5)4]–ClAlEt2–[Cp2ZrH2]2. (Ph3C)[B(C6F5)4]–ClAlEt2–[Cp2ZrH2]2.

i i The addition3-5 of eq. (Ph HAlBu3C)[B(C2 6F5)4] to the Cp2ZrCl2-HAlBu 2 system brings about Cp2 Cp2 changesCp in2ZrCl the2 NMR spectra (Figure1b–d). First, the signals of the dimeric complex Cp2 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 pre- 6a,b 7a,b viously [11,26,27,393 ].eq. Third, ClAlR2 new high-ﬁeld doublet and triplet signals corresponding to adducts 9 and 10 arise. A similar behaviorR = ofEt ( thea), Bu complexes (b) is observed in the [Cp ZrH ] - R = Et 2 2 2 ClAlEt2 system upon the addition of (Ph3C)[B(C6F5)4] or B(C6F5)3 (Figure2 and Figure S15). The diffusion coefﬁcients of adducts 9 and 10 are much lower than those of complex 7 (Table2, Figure3). During the reaction, as in the case of methylaluminoxane [ 46,47], the ClAlR2 8a,b formation[Cp2ZrH2]2 of a heavyCp fraction2ZrH2 Cp is2 observedZrHCl R2AlH at the bottom of the NMR tube. The NOESY spectra of adduct 10 (Figure S13, Supplementary Materials) show a negative NOE effect Cp2ZrH2 Cp2ZrHCl ClAlR2 7 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 complexScheme 4.7 ,Reaction which is of neutral, Cp2ZrCl is2 and preserved [Cp2ZrH after2]2 with the organoaluminum addition of a boron compounds activator. (OACs).

The 1Н 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 2J = 17 Hz of the hydrides in 7 were greater than that of analogous atoms in 6 or 8 (2J = 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,

Catalysts 2021, 11, 39 8 of 16 Catalysts 2021, 11, x FOR PEER REVIEW 10 of 17

Catalysts 2021, 11, x FOR PEER REVIEW 9 of 17

HAlBui2 with (Ph3C)[B(C6F5)4] showed the appearance of Ph3CH (δH 5.40 ppm), Bui- Figure 3. DOSY of thei [Cp2ZrH2]2-ClAlEt2-B(C6F5)3 (1:3:0.5) system in С6D6 (T = 297.1 K), heavy FigurenAl(C 3.6F5DOSY)3−n and of Bu then [CpB(C26ZrHF5)3−2n] 2within-ClAlEt 52 -B(Cmin6 (FiguresF5)3 (1:3:0.5) S5 an systemd S6, inSupplementary C6D6 (T = 297.1 Materials). K), heavy phase.Thephase latter. (BuinAl(C6F5)3−n and BuimB(C6F5)3−m) are generated as a result of [Al- Bui2]+[B(C6F5)4]− degradation (Scheme 5) [53,54]. The coordination of complex 7 with the organoaluminumWeThe assume hydrodynamic that compound7 interacts volumes Bu withinAl(C (V perfluorophenylaluminumh) 6ofF5 )complexes3−n, which gives as spherical adducts (Schemeparticles 9 and 105 were),, whichfollows calculated may from betheaccording generated cross-correlation to via Equation the reaction of (2)doublet as of follows: (Ph signal3C)[B(C of the6F 5hydride)4] with atom dialkylaluminum at −1.51 ppm hydride,with the formedsignal of in the [Cp2ZrH2]2-ClAlEt2 system (Scheme4). Indeed, NMR monitoring1 19 of the reac- o-F atoms of the perfluorophenyl group in OAC3𝑉 at −120.78 ppm in the H- F HMBC spec- tion of HAlBui with (Ph C)[B(C F ) ] showed the appearance of Ph CH (δ 5.40 ppm), tra (Figure S8,2 Supplementary3 Materials).6 5 𝑅4 = 3 H (2) Bui Al(C F ) and Bui B(C F ) within4𝜋 5 min (Figures S5 and S6, Supplementary n A change6 5 3− nin the ordern of6 reagent5 3−n mixing, namely, the addition of ClAlEt2 and then Materials).Adduct The 10 latter reacts (Bu withi Al(C 1-hexeneF ) andto give Bui dimerB(C F4 )(Figure) are 4, generated Scheme 6). as It a resultshould of be [Cp2ZrH2]2 to the activator,n provides6 5 3− ann increasem in the6 5 content3−m of Zr,Zr-hydride clusters 9 i + − [AlBuandnoted 102] that [B(C(Figures complexes6F5)4 ] 2edegradation and7 and S15). 9 were (SchemeThis inactive fact5)[ towardscan53, 54be]. Thetheexplained alkene. coordination Thus,as follows. ofcomplex complex Compound 7 activated7 with i theby organoaluminum an organoboron compoundcompound Buis thenAl(C intermediateF ) − , which that givesselectively adducts affords9 and dimers10, follows in the (Ph3C)[B(C6F5)4], which is initially present6 in5 the3 n system, quickly reacts with HAlR2, gen- fromstudied the cross-correlationsystems. The same of doubletactivity towards signal of the the alkene hydride was atom observed at −1.51 for ppm the adducts with the of erated in the [Cp2ZrH2]2−ClAlEt2 system (Scheme 4), to give RnAl(C6F5)3−n (Scheme 5). The signalcomplex of o -F7 with atoms MAO of the [47]. perfluorophenyl group in OAC at −120.78 ppm in the 1H-19F latter is coordinated to complex 7, resulting from the reaction of Cp2ZrH2 with Cp2ZrHCl HMBCThe spectra trihydride (Figure complex S8, Supplementary 6, mainly formed Materials). both in system A and in system B with an and ClAlEt2. A low concentration of HAlR2 does not allow complex 6 to be formed. excess of OAC, reacts with (Ph3C)[B(C6F5)4] to give a mixture of intermediates, which give

rise toi signals in the −6.6 to −0.1 ppm range for hydridei + atoms- (Figures 1a–c, 2c and 5b) HAlBu 2 (Ph3C)[B(C6F5)4] Ph3CH [AlBu 2] [B(C6F5)4] and which are in intermolecular exchange, as follows from the EXSY spectra (Figure S22,

Supplementaryi + - Materials). Thei composition of the mixturei depends on thei [Zr]:[Al]:[B] [AlBu 2] [B(C6F5)4] Bu 2AlC6F5 B(C6F5)3 Bu nAl(C6F5)3-n Bu mB(C6F5)3-m ratio and the order of reagent mixing. The observed 1H 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 1- Cp2 Cp2 Cp2 Cp2 hexene to this system leads to the appearance of oligomers as soon as in the 5th minute of i i ⋅ yBu nAl(C6F5)3-n the reaction (Figure 5c,Bu nSchemeAl(C6F5)3-n 6). x

7 9, 10

SchemeScheme 5. 5.Possible Possible way way for for the the formation formation of of adducts adducts9 9and and10 10. .

AAn change estimation in the of order the hydrodynamic of reagent mixing, radii namely, and volumes the addition of complexes of ClAlEt 6–210and through then [Cpthe2 ZrHdiffusion2]2 to coefficients the activator, determined provides anusing increase the diffusion in the content ordered of Zr,Zr-hydridespectroscopy (DOSY) clusters is 9shownand 10 in(Figure Table 22.e The and hydrodynamic Figure S15). This radii fact Rh canwere be calculated explained using as follows. the modified Compound Stokes– (PhEinstein3C)[B(C equation,6F5)4], which Equation is initially (1) [55,56], present as in follows: the system, quickly reacts with HAlR2, gener- 𝑅 . 𝑘𝑇 1 + 0.695 𝑅 (1) 𝐷 = 6𝜋𝜂𝑅

Catalysts 2021, 11, 39 9 of 16

ated in the [Cp2ZrH2]2−ClAlEt2 system (Scheme4), to give R nAl(C6F5)3−n (Scheme5 ). 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 Table2. The hydrodynamic radii Rh were calculated using the modiﬁed Stokes–Einstein equation, Equation (1) [55,56], as follows:

2.234 kT 1 + 0.695 Rsolv Rh Dt = (1) Catalysts 2021, 11, x FOR PEER REVIEW 6πηRh 11 of 17

where Rh is the hydrodynamic radius of the solute, Rsolv is the hydrodynamic radius of the solvent (d6-benzene; the van der Waals radius is 2.7 Å), k is the Boltzmann constant, T is the temperature, Dt is the diffusion coefﬁcient, and η is the viscosity of the solution. Cp2 Cp2 The hydrodynamic volumes (Vh) of complexes as spherical particles were calculated x ⋅ according to EquationyR (2)nAl(C as6F follows:5)3-n r 4 3 3Vh 10 Rh = (2) (Ph3C)[B(C6F5)4], 4π Adduct 10 reacts with 1-hexene to give dimer 4 (Figure4, Scheme6). It should be Cp noted that2 complexes 7 and 9 were inactive towards the alkene. Thus, complex 7 activated n by an organoboron6a,b compound is the intermediate that selectively affords dimers in the 5 n=1-5 studied systems.R= Et (a), Bu The (b) same activity towards the alkene was observed for the adducts of complex 7 with MAO [47]. Scheme 6. Reaction of complex 6 and adduct 10 with 1-hexene.

Figure 4. NMR monitoring of the reaction of complexes 7, 9, and 10 with 1-hexene in d6-benzene Figure 4. NMR monitoring of the reaction of complexes 7, 9, and 10 with 1-hexene in d6-benzene (in- (intensity of upfield signals is increased): (a) [Cp2ZrH2]2-ClAlEt2-(Ph3C)[B(PhF5)4] system, tensity of upﬁeld signals is increased): (a) [Cp2ZrH2]2-ClAlEt2-(Ph3C)[B(PhF5)4] system, [Zr]:[Al]:[B] [Zr]:[Al]:[B] = 1:3:0.5; (b) [Zr]:[Al]:[B]:[1-alkene] = 1:3:0.5:3.5, 5 min; and (c) [Zr]:[Al]:[B]:[1-alkene] = = 1:3:0.5; (b) [Zr]:[Al]:[B]:[1-alkene] = 1:3:0.5:3.5, 5 min; and (c) [Zr]:[Al]:[B]:[1-alkene] = 1:3:0.5:3.5, 1:3:0.5:3.5, 10 min. 10 min.

Catalysts 2021, 11, 39 10 of 16 Catalysts 2021, 11, x FOR PEER REVIEW 11 of 17

Cp2 Cp2

x ⋅ yR nAl(C6F5)3-n

10 (Ph3C)[B(C6F5)4],

Cp2 n 6a,b 5 n=1-5 R= Et (a), Bu (b) Scheme 6. Reaction of complex 6 and adduct 10 with 1-hexene. Scheme 6. Reaction of complex 6 and adduct 10 with 1-hexene.

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 (Figure1a–c, Figures2c and5b) 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 1H 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 Catalysts 2021, 11, x FOR PEER REVIEW1-hexene to this system leads to the appearance of oligomers as soon as in the 5th minute12 of 17 of the reaction (Figure5c, Scheme6).

Figure 4. NMR monitoring of the reaction of complexes 7, 9, and 10 with 1-hexene in d6-benzene (intensity of upfield signals is increased): (a) [Cp2ZrH2]2-ClAlEt2-(Ph3C)[B(PhF5)4] system, [Zr]:[Al]:[B] = 1:3:0.5; (b) [Zr]:[Al]:[B]:[1-alkene] = 1:3:0.5:3.5, 5 min; and (c) [Zr]:[Al]:[B]:[1-alkene] = 1:3:0.5:3.5, 10 min.

Thus, the Cp2ZrCl2-XAlBui2 (X = H, Bui) 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 [3,6–8,13,57–59]. Among the post-metallocene Zr and Hf complexes with [ONNO]-type amino-bis(phenolate) ligands activated by neutral B(C6F5)3, the bestFigure selectivity 5. The formation to 1-hexene of cationic dimers complexes (up to 97%) in the was Cp2 ZrClfound2-HAlBu to hafniumii2-(Ph3C)[B(C catalysts6F5)4] [60].system Nev- Figure 5. The formation of cationic complexes in the Cp2ZrCl2-HAlBu 2-(Ph3C)[B(C6F5)4] system and their reactivity towards 1-hexene (intensity of upfield signals is increased): (a) Cp2ZrCl2-HAl- andertheless, their reactivity data on towards the direct 1-hexene participation (intensity of of upﬁeld metal signals hydrides is increased): in chemo- (a) CpandZrCl regioselective-HAlBui i i 2 2 2 dimerizationBu 2 system, [Zr]:[Al] of alkenes = 1:5; are (b) limitedCp2ZrCl and2-HAlBui are mainly2-(Ph3C)[B(C concerned6F5)4] system, with [Zr]:[Al]:[B]Sc [61], Y [62], = 1:5:0.5; Ta [63], system, [Zr]:[Al] = 1:5; (b) Cp2ZrCl2-HAlBu 2-(Ph3C)[B(C6F5)4] system, [Zr]:[Al]:[B] = 1:5:0.5; and (c) and (c) Cp2ZrCl2–HAlBui2–(Ph3C)[B(C6F5)4] system after the 1-hexene addition. and Ru [64] complexes.i Cp2ZrCl2–HAlBu 2–(Ph3C)[B(C6F5)4] system after the 1-hexene addition.

The bis-zirconium hydridei complexesi we discovered, in combination with either an Thus, the Cp2ZrCl2-XAlBu 2 (X = H, Bu ) and [Cp2ZrH2]2-ClAlEt2 systems activated МАО activator [46,47] or organoboron compounds ((Ph3C)[B(C6F5)4] or B(C6F5)3), provide by (Ph3C)[B(C6F5)4] or B(C6F5)3 are able to accomplish selective dimerization and oligomer- izationthe selective of alkenes formation to give head-to-tail of dimers in vinylidene reactions products,with alkenes. similarly It is evident to systems that based the first on Zrstep 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 unan-

swered: 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.

Cp2 Cp2

x ⋅ y R nAl(C6F5)3-n

10 Cp2 Cp2 Cp2 Cp2 Zr Zr Zr Zr -(10)

Cp2 Cp2 Cp2 Cp2 carbometalation Zr Zr Zr Zr β-H elimination

Cp2 Cp2 Zr Zr Cp2 Cp2 hydrometalation Zr Zr -(10)

Scheme 7. Proposed mechanism: one or two Zr reaction centers?

Catalysts 2021, 11, x FOR PEER REVIEW 12 of 17

Catalysts 2021, 11, 39 11 of 16

complexes and MAO [3,6–8,13,57–59]. Among the post-metallocene Zr and Hf complexes with [ONNO]-type amino-bis(phenolate) ligands activated by neutral B(C6F5)3, the best Figure 5. The formation of cationic complexes in the Cp2ZrCl2-HAlBui2-(Ph3C)[B(C6F5)4] system selectivity to 1-hexene dimers (up to 97%) was found to hafnium catalysts [60]. Never- and their reactivity towards 1-hexene (intensity of upfield signals is increased): (a) Cp2ZrCl2-HAl- theless, data on the direct participation of metal hydrides in chemo- and regioselective Bui2 system, [Zr]:[Al] = 1:5; (b) Cp2ZrCl2-HAlBui2-(Ph3C)[B(C6F5)4] system, [Zr]:[Al]:[B] = 1:5:0.5; dimerizationand (c) Cp2ZrCl of2–HAlBu alkenesi2 are–(Ph limited3C)[B(C6 andF5)4] aresystem mainly after concernedthe 1-hexene with addition. Sc [61 ], Y [62], Ta [63], and Ru [64] complexes. TheThe bis-zirconium bis-zirconium hydride hydride complexes complexes we we discovered, discovered, in in combination combination with with either either an an MAOМАО activator activator [46 [46,47],47] or or organoboron organoboron compounds compounds ((Ph ((Ph3C)[B(C3C)[B(C6F56F)45])4 or] or B(C B(C6F65F)53)),3), provide provide thethe selective selective formation formation of of dimers dimers in in reactions reactions with with alkenes. alkenes. It It is is evident evident that that the the ﬁrst first step step isis hydrometalation hydrometalation of of the the alkene alkene (Scheme (Scheme7). 7). The The addition addition of of a seconda second alkene alkene molecule molecule andand subsequent subsequentβ β-H-H elimination elimination to to give give dimers dimers could could occur occur with with the the involvement involvement of of one one oror two two metal metal sites. sites. Similar Similar two-site two-site catalysis catalysis is is known known for for alkene alkene polymerization polymerization in in the the presencepresence of of group group 4 metal 4 metal complexes complexes [65]. [65]. Nevertheless, Nevertheless, certain certain issues remainissues remain unanswered: unan- howswered: the bis-zirconiumhow the bis-zirconium structures structures are activated; are ac whattivated; is thewhat principle is the principle of formation of formation of the heavyof the fractionheavy fraction that is activethat is towardsactive towards alkenes; alkenes; and how and dimerization how dimerization of alkenes of alkenes with with the participation of these adducts takes place. Further development of our study would be the participation of these adducts takes place. Further development of our study would concerned with the determination of the structure of active sites and the mechanism of be concerned with the determination of the structure of active sites and the mechanism of alkene dimerization under the action of bimetallic complexes. alkene dimerization under the action of bimetallic complexes.

Cp2 Cp2

x ⋅ y R nAl(C6F5)3-n

10 Cp2 Cp2 Cp2 Cp2 Zr Zr Zr Zr -(10)

Cp2 Cp2 Cp2 Cp2 carbometalation Zr Zr Zr Zr β-H elimination

Cp2 Cp2 Zr Zr Cp2 Cp2 hydrometalation Zr Zr -(10)

SchemeScheme 7. 7.Proposed Proposed mechanism: mechanism: one one or or two two Zr Zr reaction reaction centers? centers?

3. Materials and Methods 1D (1H, 13C, and 19F) and 2D (COSY HH, HSQC, HMBC, NOESY) NMR spectra were recorded on spectrometer Bruker AVANCE-400 (400.13 MHz (1H), 100.62 MHz (13C), 376.44 (19F)) (Bruker, Rheinstetten, Germany) using standard Bruker pulse sequences. Toluene- 19 d8 and benzene-d6 were used as the solvents and the internal standards. F spectra were referenced externally to CFCl3. 1D and 2D DOSY spectra were obtained using the ledbpgp2s pulse sequence (LED with bipolar gradient pulse pair, 2 spoil gradients) with acquisition parameters δ = 1 ms, ∆ = 0.1–0.2 s. The spectra were recorded at 296–298 K and temperature stabilization accuracy within 0.1 K. For the calibration of gradient strength the −9 2 self-diffusion coefficient of HDO in D2O was measured at 298 K (1.902·10 m /s) [66,67]. The diffusion coefficients were defined using NMR relaxation Guide implemented into the Bruker TopSpin program v.3.2 following the known procedure (see, for example, Refs. [68,69]). 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. Catalysts 2021, 11, 39 12 of 16

3.1. General Procedures All operations for organometallic compounds were carried out under argon according to the Schlenk technique. Complex 1 was synthesized from ZrCl4 (99.5%, Merck, Darmstadt, Germany) according to a known procedure [70]. The zirconocene dihydride [Cp2ZrH2]2 (2) was obtained from (1) as described previously [41,42,50]. Commercially available i i ClAlEt2 (97%, Strem, Newburyport, MA, USA), HAlBu 2 (99%, Merck), AlBu 3 (95%, Strem), (Ph3C)[B(C6F5)4] (97%, Abcr, Karlsruhe, Germany), B(C6F5)3 (95%, Merck), and terminal alkene 1-hexene (97%, Fisher Scientiﬁc, Pittsburgh, Pennsylvania, U.S.) were used. i The solvents (toluene, benzene) were dried over AlBu 3 and distilled immediately prior to use. CAUTION: The pyrophoric nature of aluminum hydrides and aluminum alkyls require special safety precautions in their handling.

i i 3.2. Reaction of Cp2ZrCl2 with HAlBu 2 (AlBu 3), (Ph3C)[B(C6F5)4] (B(C6F5)3), and 1-hexne A ﬂask equipped with a magnetic stirrer and ﬁlled with argon was loaded with 0.022– i 0.088 mmol (6.3–25.7 mg) of Cp2ZrCl2, 0.062–0.352 mmol (0.011–0.063 mL) of HAlBu 2 i or 0.14 mmol (0.035 mL) of AlBu 3, 0.022 mmol of (Ph3C)[B(C6F5)4] (20 mg) or B(C6F5)3 (11 mg), and 0.22–22 mmol (0.03–2.7 mL) of 1-hexene. The reaction was carried out with stirring at temperatures 40 or 60 ◦C. The samples (0.1 mL) were syringed into tubes under argon after 15, 30, 60, 90, 120, and 150 min of the reaction; then the samples were immediately decomposed with 10% HCl at 0 ◦C. Products were extracted with benzene, and the organic layer was dried over Na2SO4. The yields of dimers and oligomers were determined by GC/MS.

3.3. Reaction of [Cp2ZrH2]2 with ClAlEt2, (Ph3C)[B(C6F5)4] (B(C6F5)3), and 1-Hexene A ﬂask equipped with a magnetic stirrer and ﬁlled with argon was loaded with 0.088 mmol (20 mg) of [Cp2ZrH2]2 (all ratios are given relative to monomer), 0.176 mmol (0.022 mL) of ClAlEt2, 0.022 mmol of (Ph3C)[B(C6F5)4] (20 mg) or B(C6F5)3 (11 mg), and 8.8 mmol (1.1 mL) of 1-hexene. The reaction was carried out with stirring at temperatures 40 or 60 ◦C. The samples (0.1 mL) were syringed into tubes under argon after 15, 20, 30, 60, 90, 120, and 150 min of the reaction; then the samples were immediately decomposed with 10% HCl at 0 ◦C. Products were extracted with benzene, and the organic layer was dried over Na2SO4. The yields of dimers and oligomers were determined by GC/MS.

i 3.4. NMR Study of the Reaction of Cp2ZrCl2 with HAlBu 2, (Ph3C)[B(C6F5)4], and 1-Hexene

Method A. The NMR tube was ﬁlled with 0.05–0.1 mmol (14.6–29.2 mg) of Cp2ZrCl2 i and 0.5 mL of benzene-d6 under argon. Then 0.15–0.3 mmol (0.027–0.054 mL) of HAlBu 2 was added at room temperature. The mixture was stirred and the formation of complexes 6–8 was monitored by NMR. Further 0.025 mmol (22.8 mg) of (Ph3C)[B(C6F5)4] was added, and the mixture was analyzed by NMR. Method B. The NMR tube was ﬁlled with i 0.025 mmol (22.8 mg) of (Ph3C)[B(C6F5)4], 0.15–0.3 mmol (0.027–0.054 mL) of HAlBu 2, and 0.5 mL of benzene-d6 under argon at room temperature. The mixture was stirred and 0.05–0.1 mmol (14.6–29.2 mg) of Cp2ZrCl2 was added. The formation of complexes was monitored by NMR.

3.5. NMR study of the Reaction of [Cp2ZrH2]2 with ClAlEt2 and (Ph3C)[B(C6F5)4] (B(C6F5)3)

Method A. The NMR tube was ﬁlled with 0.05 mmol (11.2 mg) of [Cp2ZrH2]2 (2) and 0.5 mL of benzene-d6 under argon. Then 0.15 mmol (18.6 mg) of ClAlEt2 was added drop- wise 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 (Ph3C)[B(C6F5)4] or B(C6F5)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 (Ph3C)[B(C6F5)4] or B(C6F5)3 (12.8 mg), 0.15 mmol (18.6 mg) of ClAlEt2, and 0.5 mL of benzene-d6 under argon. Catalysts 2021, 11, 39 13 of 16

◦ Further 0.05 mmol (11.2 mg) of [Cp2ZrH2]2 were added at 0 C. The mixture was stirred and studied by NMR at room temperature.

4. Conclusions In summary, we found the conditions for the synthesis of alkene dimers or oligomers i i in the Cp2ZrCl2-XAlBu 2 (X = H, Bu ) and [Cp2ZrH2]2-ClAlEt2 catalytic systems activated by (Ph3C)[B(C6F5)4] or B(C6F5)3. NMR studies showed that adducts with the bis-zirconium core of type x[Cp2ZrH2·Cp2ZrHCl·ClAlR2]· yRnAl(C6F5)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 Cp2Zr(µ- i H)3(AlBu 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.

Supplementary Materials: The following are available online at https://www.mdpi.com/2073-434 4/11/1/39/s1. Author Contributions: Conceptualization, L.V.P.; methodology, P.V.K.; validation, L.V.P., P.V.K. and A.K.B.; formal analysis, P.V.K.; investigation, P.V.K., A.K.B. and E.R.P.; data curation, L.V.P. and P.V.K.; writing—original draft preparation, P.V.K.; writing—review and editing, L.V.P.; visualization, L.V.P.; supervision, L.V.P.; project administration, L.V.P.; funding acquisition, L.V.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Russian Foundation for Basic Research, grant number 18-03-01159a. The structural studies were carried out on unique equipment at the “Agidel” Collective Usage Center (Ufa Federal Research Center, Russian Academy of Sciences, Ufa, Russian Federation) with the financial support of the Russian Ministry of Education and Science (project No. 2019-05-595- 000-058). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article or supplementary material. Acknowledgments: Part of the research was carried out in accordance with federal program № AAAA-A19-119022290004-8. Conflicts of Interest: The authors declare no conflict of interest.

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