Dynamic NMR Studies of Cationic Bis(2-phenylindenyl)zirconium Pyridyl Complexes:  Evidence for syn Conformers in Solution

Abstract

The conformationally dynamic unbridged metallocene (2-PhInd) 2 ZrMe 2 (1) was activated with trispentafluorophenylborane (B(C 6 F 5 ) 3 , B2) or trityl tetrakispentafluorophenylborate ("trityl borate", [ 2b), respectively. Activation parameters for ion-pair separation were determined by line-shape analysis (2a: ∆H q ips ) 20 ( 1 kcal/mol, ∆S q ips ) 17 ( 4 eu; 2b: ∆H q ips ) 15 ( 2 kcal/mol, ∆S q ips ) 13 ( 7 eu). For 2a, a much slower B(C 6 F 5 ) 3 dissociation-reassociation process was also observed (∆G q 83°C ) 18.9 ( 0.1 kcal/ mol). Both 2a and 2b were treated with a series of o-substituted pyridines, and the behavior of the resulting zirconocenium-pyridyl complexes (3a-8b) was studied by 1 H, 13 C, and 19 F NMR over the temperature range -100 to 100 °C. Below room temperature, 1 H NMR NOESY spectra revealed signals characteristic of a C s -symmetric syn conformation.

Novel Synthetic Strategy for Developing an Isospecific Unbridged Metallocene System for Propylene Polymerization

“…The polymerization results presented in Table 3 are in good agreement with this hypothesis, and 3 and 4/MAO indeed afford much increased amounts of highly isotactic polypropylene even at high temperature ( Table 3, entries 18-23). Most remarkably, the SMe-substituted 5 that produces negligible amounts of isotactic polypropylene under atmospheric monomer pressure also gives appreciable amounts of a highly isotactic portion ( Table 3, entries [24][25][26]. Consistent with this result is the high T m value of all the polypropylenes, and the T m values are still greater than that from EBIZr, indicating higher isotacticity, as similarly observed under atmospheric conditions.…”

“…After this significant finding, the “oscillating” [(2‐phenylindenyl) 2 ZrCl 2 ] that produces isotactic–atactic, block polypropylene was also reported by Waymouth and co‐workers 17. The key concept of these papers was to impart isospecific stereocontrol to unbridged metallocene catalysts by introducing hindered ligand rotation 15–26. Although several reports showed that the microstructure of the produced polypropylene strongly depends on the steric and electronic effects of the substituents, as well as on the ion‐pair structure of a cationic active center and a counter anion,16, 20, 23, 25 in most cases, the unbridged metallocenes afforded atactic polypropylene due to rapid ligand rotation 19.…”

“…ligand rotation. [15][16][17][18][19][20][21][22][23][24][25][26] Although several reports showed that the microstructure of the produced polypropylene strongly depends on the steric and electronic effects of the substituents, as well as on the ion-pair structure of a cationic active center and a counter anion, [16,20,23,25] in most cases, the unbridged metallocenes afforded atactic polypropylene due to rapid ligand rotation. [19,27] When we consider the relative ease of synthesis of unbridged metallocenes compared to that of ansa-metallocenes, it is still of great interest to develop unbridged metallocene catalytic systems capable of inducing stereospecific polymerization of propylene.…”

C–H Functionalization of Azines

“…A variety of studies using group 3 and 4 metals on the ortho -metalation of pyridine have been reported since the 1980s (Sc, , Y, − lanthanides, , Ti, − Zr, − ). Jordan and co-workers carried out pioneering work on the cationic zirconium metallocene catalyzed reaction of propene and 2-methylpyridine (Scheme ).…”

1H and 13C NMR Studies of Cationic Intermediates Formed upon Activation of “Oscillating” Catalyst (2-PhInd)2ZrCl2 with MAO, MMAO, and AlMe3/[CPh3]+[B(C6F5)4]-

“…The so-called “oscillating” catalysts (bis(2-Ar-indenyl)zirconocenes), which produce elastomeric polypropylenes, are an intriguing class of olefin polymerization catalysts. − Apparently, formation of stereoblock elastomeric plastics is a result of the dynamic character of the active species, the cation of general formula [(2-Ar-indenyl) 2 ZrP] + (Ar = aryl; P = polymeryl). − According to the hypothesis of Waymouth, the active species “oscillates” between a “ rac -like” (isotactic-selective) and a “ meso -like” (nonstereoselective) conformation at a frequency intermediate between those of monomer insertion and chain transfer. ,, Alternatively, Busico has proposed that the achiral meso -isomer can be disregarded as a source of the atactic stereosequences, and the generation of isotactic and atactic stereosequences is a result of the interconversion of chiral rac -isomers at a rate competitive with propylene insertion . For one of the rac -isomers, the rate of indenyl ring rotation is faster than propylene insertion (atactic block is formed); for another rac -isomer, this rotation is slower than propylene insertion (isotactic block is formed).…”