β‐Diketiminate‐supported magnesium alkyl: synthesis, structure and application as co‐catalyst for polymerizations mediated by a lanthanum half‐sandwich complex

Mg( n ‐Bu){η 2 ‐HC[C(Me)NMes] 2 } ( 2 ) (Mes = mesityl, 2,4,6‐Me 3 C 6 H 2 ), a new β‐diketiminate‐supported magnesium alkyl, has been synthesized and structurally characterized. The X‐ray analysis of the lanthanum half‐sandwich complex Cp*La(BH 4 ) 2 (THF) 2 ( 1 ) (Cp* = pentamethylcyclopentadienyl; THF = tetrahydrofuran) is also reported. Complex 2 has been assessed as both alkylating agent and chain transfer agent for the lanthanum‐catalysed polymerization and coordinative chain transfer polymerization of isoprene and styrene using 1 as the pre‐catalyst. The results are compared with those for n ‐butylethylmagnesium (BEM) which is traditionally used for this purpose. The 1,4‐ trans stereospecific polymerization of isoprene shows a more controlled character using 2 versus BEM, and higher activities are observed for the chain transfer polymerization of styrene when 2 is used as chain transfer agent. The activity is in turn lower than that observed using BEM when 1 equiv. of magnesium compound is used for the polymerization of styrene. The combination of 1 , 2 and Al(i‐Bu) 3 leads finally to a 1,4‐ trans stereoselective coordinative chain transfer polymerization of isoprene, in a similar way to BEM. Copyright © 2015 John Wiley & Sons, Ltd.