Regarding the Stability of d⁰ Monocyclopentadienyl Zirconium Acetamidinate Complexes Bearing Alkyl Substituents with β-Hydrogens

Abstract

The monocyclopentadienyl zirconium acetamidinate complexes, (eta(5)-C(5)Me(5))Zr[N(R(1))C(Me)N(R(2))]R(3)R(4) (1-8), have been shown to be remarkably resistant to beta-hydrogen eliminations/abstractions, including the tert-butyl derivative, 3 (R(1) = R(2) = Cy, R(3) = t-Bu, R(4) = Cl), which resists both decomposition and isomerization in solution to temperatures of at least 100 degrees C. Further, two striking examples of an apparent preference for alternative hydrogen-atom abstractions in which complexes 1 and 7/8 that bear isomeric dibutyl substituents are transformed at elevated temperatures to complexes 9 and 10/11 that contain the isomeric butadiene and trimethylenemethane (TMM) C(4) fragments, respectively, are presented. These results serve to not only introduce a new ligand environment for zirconium in which beta-hydrogen elimination/abstraction processes are substantially retarded, but they further document the availability of alternative low-energy hydrogen abstraction pathways for group 4 alkyl complexes.

Evaluation of (η5‐C5Me5)Hf(R)2[N(Et)C(Me)N(t‐Bu)] (R=Me and i‐Bu) for the Stereospecific Living and Degenerative Transfer Living Ziegler–Natta Polymerization of α‐Olefins

“…asc.wiley-vch.de ture of 3b as determined by single crystal X-ray analysis, and once again, as with 3a, the bond lengths involving the hafnium metal center in this complex are shorter than those in the previously reported analogous zirconium complex, Cp*Zr(i-Bu) 2 [N(Et)C(Me)N(t-Bu)] (2b). [9] Thus, in 3b, the hafnium-nitrogen bond lengths are Hf(1) À N(1) 2.3298(1) and Hf(1) À N(2) 2.2003 (12) , whereas in the zirconium complex 2b, the bond lengths are 2.2471(16) and 2.3113(15) , respectively. The Cp* fragment is also closer to the metal center in 3b vs. 2b as evidenced by the two metal-centroid distances of 2.2301 (7) [3c, d,g,4a,10] revealed some important similarities and differences.…”

Insight into cis‐to‐trans Olefin Isomerisation Catalysed by Group 4 and 6 Cyclopentadienyl Compounds

“…Both C and D need higher concentrations of catalysts 4 and 5 than their terminal counterparts A or B. As expected from their relative stabilities, internal olefins were converted, to some extent, into (2) provides an example of a stable β-H containing Zr-alkyl derivative [6,8,12] and has been proposed to model the transient Zr-alkyl intermediates present in the isomerisation processes achieved by analogous cyclopentadienyl Zr hydride derivatives 5 or 6. [8] In order to explore the ability of 2 to give β-H elimination reactions, the zirconacycle compound was also tested in the isomerisation reaction of A in [D 6 ]benzene.…”

“…Yield: 0.25 g (53 %). C 12 13 C NMR (plus APT, 100 MHz, C 6 D 6 ): δ = 221.5 (-, CO), 103.6, 106.8 (both -, Cp ipso : C-Me), 87.9 (+, Cp: C-H), 11.6,13.4 (both +, ppm.…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…68 Other group 4 systems studied for alkene polymerisation include titanium() iminoimidazolidides, 69 such as 6, hafnium() pyrrolide-imines, 70,71 cyclopenta[b]pyridine, 72 and triphosphorus macrocycles. 73 The stability of zirconium() Cp acetamidinates bearing alkyl substituents 74 and the direct observation of structural isomerisation of an insertion product has been investigated by NMR spectroscopy. 75 A kinetic study of β-methyl elimination in [Hf(η 5 -C 5 Me 5 ) 2 (µ-MeB(C 6 F 5 ) 2 )R] (where R = Me and CH 2 CMe 3 ) 76 and C-H activation of the aromatic substituents of Cp ligands coordinated to a {Ti(CH 2 Ph) 3 } moiety, 77 and competitive metathesis and C-H activation from reaction between AlMe 3 and [TiMe 2 (NP t Bu 3 ) 2 ] have been reported.…”