catalysts 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 prod- ucts (the presence of vinylidene terminal groups, arising upon chain termination via b-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]. c + - [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 al- kene 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 elec- trospray 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-com- plexes [Me2C(Cp)IndMMe2] (M = Zr; Hf), activated with B(C6F5)3, hydride in- Catalysts 2020, 10, x FOR PEER REVIEW 3 of 17 termediates Me2C(Cp)IndMMe(µ-H)B(C6F5)3, which were unreactive to- wards 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 coun- teranions HB(C6F5)3, appearing due to hydride abstraction, which showed the ability to polymerize ethylene [39].