Abstract
Metal hydride complexes find widespread application in catalysis and their properties are often understood on the basis of the available crystal structures. However, some catalytically relevant metal hydrides are only spontaneously formed in situ, cannot be isolated in large quantities or crystallised and their structure is therefore ill defined. One such example is the paramagnetic Ti(iii)-hydride involved in homogeneous Ziegler–Natta catalysis, formed upon activation of CpTi(iv)Cl3 with modified methylalumoxane (MMAO). In this contribution, through a combined use of electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopies we identify the nature of the ligands, their bonding interaction and the extent of the spin distribution. From the data, an atomistic and electronic model is proposed, which supports the presence of a self-assembled ion pair between a cationic terminal Ti-hydride and an aluminate anion, with a hydrodynamic radius of ca. 16 Å.
Highlights
Transition metal hydrides play crucial roles in many catalytic processes of importance in biology, materials science and industrial chemistry
Through a combined use of electron paramagnetic resonance (EPR), electronnuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopies we identify the nature of the ligands, their bonding interaction and the extent of the spin distribution
The formation of a number of open-shell Ti(III) species upon MAO/modified methylalumoxane (MMAO) activation was demonstrated through conventional continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy in solution
Summary
Butadiene copolymers where the role of the Ti(III) species as active catalysts has been rmly assessed. The formation of a number of open-shell Ti(III) species upon MAO/MMAO activation was demonstrated through conventional continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy in solution. quantitative 1H NMR analysis of the syndiotactic polystyrene produced under quasi living polymerization conditions (polydispersity index (PDI) 1⁄4 1.3–1.6) by these catalysts demonstrated that Ti(III)-H species are involved in the initiation reaction and that a stationary concentration ($20% of the paramagnetic fraction) is produced under polymerization conditions through a reversible insertion–elimination of the growing polymer chain.. Pulsed EPR offers a whole arsenal of speci c experiments (ENDOR, ESEEM, HYSCORE) capable of investigating the local coordination environment of paramagnetic species, affording resolution comparable to that of advanced NMR techniques In this contribution we employ X- and Q-band EPR spectroscopy to elucidate the electronic and geometrical structure of Ti(III) species obtained by reaction of CpTiCl3 with MMAO. We provide compelling evidence for a terminal Ti-H bond and for the presence of a 27Al in the second coordination sphere of the Ti(III) cationic complex This is the rst detailed structural description of the metal hydride formed in situ upon reductive activation of a titanium half-metallocene precursor, which is stabilized by the formation of an ion pair with the alumoxane moiety
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