Cationic Rare‐Earth Polyhydrido Complexes: Synthesis, Structure, and Catalytic Activity for the cis‐1,4‐Selective Polymerization of 1,3‐Cyclohexadiene

Abstract

Metal hydrides are fundamental components in a wide range of stoichiometric and catalytic reactions. Their importance in modern inorganic and organic chemistry cannot be overemphasized. Rare-earth (Group 3 and lanthanide) hydrides are among the most active metal-hydride complexes. Together with their alkyl analogues, metal-hydride complexes of the rare-earth metals have occupied an especially important position in the development of the organometallic chemistry of the rare-earth elements. Generally, cationic complexes differ in their structure and reactivity from their neutral analogues. However, although a large number of metal-hydride complexes of the rare-earth metals have been synthesized and structurally characterized, [1] cationic complexes of this type have not been reported previously.We recently reported the synthesis and hydrogenation reactions of a new class of polynuclear rare-earth polyhydrido complexes exemplified by [Y 4 (C 5 Me 4 SiMe 3 ) 4 H 8 (thf) n ] (1 a: n = 0, 1 b: n = 1, 1 c: n = 2). [2,3] In view of the unique reactivity of these hydride clusters [2b-e,g] and the excellent olefinpolymerization activity of the related cationic rare-earthalkyl complexes, [4,5] we became interested in the cationic hydrido species generated from these rare-earth-hydride clusters.Herein, we report the synthesis, structural characterization, and olefin-polymerization catalysis of the cationic hydride complexes obtained from 1 a-c (Scheme 1) and related rare-earth-hydride clusters. These cationic polyhydrido complexes not only are the first cationic rare-earthhydride complexes but also show excellent regio-and stereoselectivity for the polymerization of 1,3-cyclohexadiene (CHD), which afforded, for the first time, soluble crystalline cis-1,4-linked poly(CHD) (1,4 selectivity: 100 %; cis selectivity: 99 %). For comparison, the reaction of the neutral hydride cluster, 1 b, with CHD is also described. This reaction leads to the formation of a structurally well-defined CHD insertion product instead of the polymerization of CHD. Various metal catalysts and initiators were reported previously for the polymerization of CHD, but most yielded a mixture of 1,4-and 1,2-poly(CHD) [6][7][8][9][10] or insoluble polymers, [10,11] and none was reported to produce pure soluble crystalline cis-1,4-linked poly(CHD).Reaction of the thf-free octahydrido yttrium cluster 1 a ] with one equivalent of [Ph 3 C][B(C 6 F 5 ) 4 ] in chlorobenzene or toluene at 25 8C gave the cationic heptahydrido complex 2 a and Ph 3 CH (Scheme 1). In contrast to the neutral complex 1 a, which shows a good solubility in most organic solvents, the cationic complex 2 a was only slightly soluble in benzene and toluene, and almost insoluble in hexane. Recrystallization of 2 a from chlorobenzene/hexane afforded colorless single Scheme 1. Synthesis of cationic yttrium-hydride clusters.

Rare Earth Arene-Bridged Complexes Obtained by Reduction of Organometallic Precursors

“…In (Tardif et al, 2003), which represented the first examples of structurally well-defined, salt-free lanthanide polyhydrido clusters (Hou et al, 2001). These clusters and their derivatives could break strong bonds such as the C-O bond in CO (Shima and Hou, 2006) and CO2 (Tardif et al, 2004) and showed interesting polymerization activity (Li et al, 2006). A related trinuclear titanium polyhydride was found to be able to induce dinitrogen cleavage followed by partial hydrogenation in the presence of dihydrogen (Shima et al, 2013).…”

Synthesis and Reactions of Polynuclear Polyhydrido Rare Earth Metal Complexes Containing “(C5Me4SiMe3)LnH2” Units: A New Frontier in Rare Earth Metal Hydride Chemistry

“…The overall structure of the mono(THF)-containing polyhydride complexes 2-4 is almost the same as that of the THF-free Sc complex 1. In the case of the analogous complexes of larger metals such as Gd (5), [9] Dy (6), [9] and Ho (7), [9] an agostic interaction between the metal center and a methyl group of an adjacent C 5 is also observed, probably due to the greater steric unsaturation of these larger metal centers (Scheme 2). [9,10] These results show that the fine structures of the tetranuclear octahydride complexes are, to some extent, dependent on the ionic radius of the metal centers.…”

“…Hydrogenolysis of the bis(trimethylsilylmethyl) complexes of larger rare earth metals such as Y, Er, and Tm in toluene affords the corresponding tetranuclear octahydride complexes [{(C 5 Me 4 SiMe 3 )Ln(µ-H) 2 } 4 (THF)] (2: Ln = Y; [8] 3: Ln = Er; [9] 4: Ln = Tm [9] ) where one of the four metal centers in these complexes bears an additional THF ligand (Scheme 2). The overall structure of the mono(THF)-containing polyhydride complexes 2-4 is almost the same as that of the THF-free Sc complex 1.…”

Facile, Efficient Copolymerization of Ethylene with Norbornene-Containing Dienes Promoted by Single Site Non-Metallocene Oxovanadium(V) Catalytic System

“…The synthesis of well-defined polyolefin with reactive pendent vinyl group, which can be easily converted into functional groups by further chemical transformations, has drawn much attention over recent decades [1][2][3][4][5][6][7][8][9][10]. Among the various dienes successfully incorporated into the linear polyethylene (PE) or polypropylene (PP) main chain, the unsymmetrical dienes, such as p-(3-butenyl) styrene and dicyclopentadiene (DCPD), are very promising because of their two double bonds usually showing different selectivity toward coordinative incorporation and subsequent chemical transformation, respectively.…”