Niobium‐ and Tantalum‐Based Ethylene Polymerisation Catalysts Bearing Methylene‐ or Dimethyleneoxa‐Bridged Calixarene Ligands

Treatment of p ‐ tert ‐butylcalix[6]areneH 6 (H 6 t Bu‐L) or p ‐ tert ‐butylcalix[8]areneH 8 (H 8 t Bu‐L 1 ) with [MCl 5 ] (M=Nb, Ta) in refluxing toluene or dichloromethane affords, after work‐up, the complexes [{M(NCMe)Cl 2 } 2 ( t Bu‐L)] (M=Nb ( 1 ), Ta ( 2 )) and [(MCl 2 ) 2 ( t Bu‐L 1 H 2 )] (M=Nb ( 4 ), Ta ( 5 )), respectively. Complex 1 , as well as [{Nb 2 (μ‐O) 2 (μ‐Cl)( t Bu‐LH)} 2 ] ( 3 ), is also available from [NbOCl 3 ] and H 6 t Bu‐L. Reaction of [MOCl 3 ] (M=Nb, Ta) with Li 3 ( t Bu‐L 2 ) in diethyl ether, where H 3 t Bu‐L 2 is p ‐ tert ‐butylhexahomotrioxacalix[3]areneH 3 , affords, after work‐up, the trimeric complexes [{M( t Bu‐L 2 )(μ‐O)} 3 ] (M=Nb ( 6 ), Ta ( 7 )). The behaviour of 1 to 7 (not 3 ), as well as the known complexes [{(MCl) p ‐ tert ‐butylcalix[4]arene} 2 ] (M=Nb ( 8 ), Ta ( 9 )) and [(MCl 2 ) p ‐ tert ‐butylcalix[4]arene(OMe)] (M=Nb ( 10 ), Ta ( 11 )), as pro‐catalysts for the polymerisation of ethylene has been investigated. In the presence of dimethyl (or diethyl)aluminium chloride, methylaluminoxane or trimethylaluminium, these niobium and tantalum procatalysts are all active (<35 g mmol −1  h −1  bar −1 ), for the polymerisation of ethylene affording high‐molecular‐weight linear polyethylene. The dimethyleneoxa‐bridged systems (derived from 6 and 7 ) are more active (84 and 46 g mmol −1  h −1  bar −1 , respectively) than the methylene‐bridged systems. The molecular structures of 1 – 6 and 10 (acetonitrile solvate) are reported.