Experimental and Theoretical Studies of Olefin Insertion for ansa-Niobocene and ansa-Tantalocene Ethylene Hydride Complexes

Abstract

Using dynamic NMR methods the rates of hydrogen exchange following intramolecular ethylene insertion into the metal-hydride bond have been measured for the following group 5 ansa-metallocene complexes:The singly bridged ansa-niobocenes exchange up to 3 orders of magnitude faster than unbridged complexes. However, the doubly bridged ansa-tantalocene complex exchanges at a rate comparable to that previously reported for (η 5 -C 5 Me 5 ) 2 Ta(CH 2 CH 2 )H and much slower than a singly bridged complex, [Me 2 Si(η 5 -C 5 -Me 4 ) 2 ]Ta(CH 2 CH 2 )H. These "ansa-effects" were investigated by DFT calculations on model complexes. The computed exchange pathway showed the presence of an agostic ethyl intermediate. The calculated barriers for hydrogen exchange of model unbridged, singly bridged, and doubly bridged niobocenes correlate with the experimental results.

Alkyl–η2-alkene niobocene and tantalocene complexes with the allyldimethylsilyl–η5-cyclopentadienyl ligand: synthesis, NMR studies and DFT calculations

“…The structure of various hydrido-alkene niobocene and tantalocene complexes and the kinetics of their insertion reactions have been extensively studied [5][6][7] and the behaviour of ansametallocenes in comparison with their non-bridged systems has been reported recently. 8 Related chloro-alkene group 5 metal(III) compounds have occasionally been singularly isolated by reduction of dichlorometallocenes in the presence of olefin, 9 but relatively few alkylalkene compounds have been reported. Ligand promoted olefin insertion into the niobium-hydrido bond of permethylniobocene [NbCp* 2 (olefin)H] complexes to give the ligand-trapped alkyl compounds has been reported only for small ligands (CO, CNMe), 6d whereas the related niobocene complexes have been shown to afford either ligand-trapped alkyl compounds, 5a,6b or the olefin substitution hydrido products.…”

Synthesis and Reactivity of New Niobocene Hydride-Stibine and Hydride-Stilbene Complexes. X-ray Crystal Structure of [Nb(η5-C5H4SiMe3)2(H)(trans-η2-C,C-PhCH═CHPh)]

“…This is the first example of a hydride olefin niobocene complex having been prepared by the reaction of a niobocene trihydride with a sterically demanding olefin. − Some years ago some of us reported the synthesis of new hydride olefin complexes, namely, [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (H)( trans -η 2 -C,C-ROOC-C(H)C(H)-COOR)] (R = Me, t Bu) by reaction of 2 with activated alkynes such as ROOC–CC–COOR (R = Me, t Bu) in an insertion process of the carbon–carbon triple bond into the Nb–H bond …”

“…Moreover, after the preparation of the first hydride-olefin niobocene [Nb(η 5 -C 5 H 5 ) 2 (H)(η 2 -C 2 H 4 )] by Tebbe and Parshall, two alternative processes, namely, the reaction of metallocene dihalides and trihydrides (M = Nb, Ta) with various alkylmagnesium halides and olefins, respectively, have been employed to prepare this class of complex. Bercaw and Green and co-workers have investigated in depth the mechanism for the formation of these compounds. A few years later, several of us reported an investigation into this kind of complex that improved our understanding of the mechanistic and reactivity behavior .…”

12 Vanadium, niobium and tantalum

“…DFT calculations on model complexes showed the presence of an agostic ethyl intermediate, and the calculated barriers correlated well with the experimental results. 39 As a comparison to the known [TaH 2 (OSiBu t 3 ) 3 ] system, DFT calculations were performed on the alkene insertion and b-elimination characteristics of the model complex [TaH 2 (OH) 3 ]. It was found that alkene insertion takes place either endo (between two hydrides), or exo (to the side of the hydrides), with the exo mode favoured by charge polarised alkenes such as vinyl halides and ethers.…”