A New Type of Doubly Silylamido-Bridged Cyclopentadienyl Group 4 Metal Complexes

Abstract

Dedicated to Professor Rafael UsoÂn on the occasion of his 75th birthdayThe isolation of metal complexes containing the h 1 -amidosilyl-h 5 -cyclopentadienyl ligand, which provides more acidic metal centers with a more open coordination site than the ansa-dicyclopentadienyl compounds, has proved particularly useful. [1] These structural features are responsible for their catalytic ability to copolymerize long-chain a-olefins. [2] Numerous contributions in this field have benefited from the research into the use of variously substituted ligands, [3] the characterization of the active cationic species, [4] the development of new industrial applications, [5] and the reactivity of the metal-coordinated amidosilyl moiety. [6] Few examples of doubly bridged h 5 -cyclopentadienyl-di(h 1ligand) metal compounds have been reported. [7] We have now extended our studies in this field by synthesizing new substituted cyclopentadienyl ligands that can form more than one amidosilyl bridge, that is di(h-amidosilyl)-h 5 -cyclopentadienyl ligands, and characterizing and studying the reactivity of their Group 4 metal complexes.The di(amidosilyl)cyclopentadiene C 5 H 4 [SiMe 2 (NHtBu)] 2 (1) was synthesized as a yellow oil from 1,1-(SiMe 2 Cl) 2 C 5 H 4 [8] and LiNHtBu in 12 h at room temperature using THF as solvent. Selective deprotonation of the more acidic cyclopentadiene proton and only one of the amino protons of 1 by using [Zr(NMe 2 ) 4 ] afforded the monosilylamido derivative 2 as a light-brown oil (Scheme 1). A C 6 D 6 solution of 2 was heated to 110 8C to give the disilylamido compound 3 with evolution of NHMe 2 . However this reaction was reversed when the mixture was cooled to room temperature, preventing the isolation of 3. Similar deprotonation of 1 by using the tetrabenzyl compound [Zr(CH 2 Ph) 4 ], gave a mixture of the brown monosilylamido 4, identified by 1 H NMR spectroscopy, and the disilylamido derivative 5 at room temperature (Scheme 1), while 5 was the unique reaction product at Scheme 1. Synthesis of neutral bridged h 5 -cyclopentadienyl mono-and di(h-amidosilyl)titanium and -zirconium complexes and their cationic species.temperatures higher than 65 8C. The deprotonating capacity of complex 5 facilitated the irreversible transformation of 2 into 3 by elimination of the NHMe 2 resulting from that transformation. A similar reaction of 1 with [Ti(CH 2 Ph) 4 ] gave the monosilylamido complex 6 as a red solid, which could only be transformed into the disilylamido derivative 7 by refluxing a solution of the compound in toluene.All the new compounds were characterized by elemental analyses, and the 1 H and 13 C NMR spectra were consistent (see Supporting Information) with the asymmetry of the monosilylamido compounds 2, 4, and 6, and with the presence of a plane of symmetry in the disilylamido compounds 3, 5, and 7 (Scheme 1). The chemical shifts of the tBu tertiary carbon atoms were observed between d 55.8 (3) and d 61.5 (6) for bridged, and between d 49.6 (2) and d 49.8 (6) for unbridged amidosilyl groups; t...