Synthesis and rac-lactide ring-opening polymerisation studies of new alkaline earth tetrahydroborate complexes

Reaction of K[HC(C(Me)NAr')(2)] (Ar' = 2,6-(C6H3Pr2)-Pr-i) with Mg(BH4)(2) afforded the pseudo four-coordinate tetrahydroborate complex Mg{HC(C(Me)NAr')(2)}(BH4)(THF) (1). The corresponding reaction of Ca-(BH4)(2)(THF)(2) or Sr(BH4)(2)(THF)(2) gave the pseudo five-coordinate analogues M{HC(C(Me)NAr')(2)}(BH4)-(THF)(2) (M = Ca (2) or Sr (3)). All three compounds 1-3 have been structurally characterised. According to X-ray crystallography and IR spectroscopy all possess kappa(3)-bound BH4 ligands in the solid state. This coordination mode is also maintained in THF solution for 2 and 3, whereas complex 1 appears to form a bis (THF) complex containing a kappa(2)-bound BH4. Reaction of 1 with K[HC(P(Ph-2)NAr')(2)](THF)(2) formed Mg{HC-(P(Ph-2)NAr')(2)}(BH4)(THF)(2) (4) possessing a kappa(3)-bound BH4 ligand in both the solid state and solution. Compounds 1, 2 and 4 are highly active for the ring-opening polymerisation of epsilon-caprolactone forming dihydroxytelechelic PCL. Compound 1 is also extremely active for the ROP of rac-lactide forming highly heterotactic PLA with good agreement between predicted and measured M-n, in accord with previous studies of alkoxide and amide initiators based on this metal and ligand class. Compounds 2 and 4 were less productive and gave PLA with poorer control of M-n and negligible heterotactic enrichment. MALDI-ToF MS analysis of the PLA formed with all three catalysts showed a mixture of both -CH(Me)CHO and -CH(Me)CH2OH termini arising from the M-BH4 initiating groups.