Heteroleptic Silylamido Phenolate Complexes of Calcium and the Larger Alkaline Earth Metals: β‐Agostic Ae⋅⋅⋅SiH Stabilization and Activity in the Ring‐Opening Polymerization of L ‐Lactide

The factors governing the stability and the reactivity towards cyclic esters of heteroleptic complexes of the large alkaline earth metals (Ae) have been probed. The synthesis and stability of a family of heteroleptic silylamido and alkoxide complexes of calcium [{LO i }CaNu(thf) n ] supported by mono‐anionic amino ether phenolate ligands ( i =1, {LO 1 } − =4‐( tert ‐butyl)‐2,6‐bis(morpholinomethyl)phenolate, Nu − =N(SiMe 2 H) 2 − , n =0, 4 ; i =2, {LO 2 } − =2,4‐di‐ tert ‐butyl‐6‐{[2‐(methoxymethyl)pyrrolidin‐1‐yl]methyl}phenolate, Nu − =N(SiMe 2 H) 2 − , n =0, 5 ; i =4, {LO 4 } − =2‐{[bis(2‐methoxyethyl)amino]methyl}‐4,6‐di‐ tert ‐butylphenolate, Nu − =N(SiMe 2 H) 2 − , n =1, 6 ; Nu − =HCCCH 2 O − , n =0, 7 ) and those of the related [{LO 3 }AeN(SiMe 2 H) 2 ] ({LO 3 } − =2‐[(1,4,7,10‐tetraoxa‐13‐azacyclopentadecan‐13‐yl)methyl]‐4,6‐di‐ tert ‐butylphenolate Ae=Ca, 1 ; Sr, 2 ; Ba, 3 ) have been investigated. The molecular structures of 1 , 2 , [( 4 ) 2 ], 6 , and [( 7 ) 2 ] have been determined by X‐ray diffraction. These highlight Ae⋅⋅⋅HSi internal β‐agostic interactions, which play a key role in the stabilization of [{LO i }AeN(SiMe 2 H) 2 ] complexes against ligand redistribution reactions, in contrast to regular [{LO i }AeN(SiMe 3 ) 2 ]. Pulse‐gradient spin‐echo (PGSE) NMR measurements showed that 1 , 4 , 6 , and 7 are monomeric in solution. Complexes 1 – 7 mediate the ring‐opening polymerization (ROP) of L ‐lactide highly efficiently, converting up to 5000 equivalents of monomer at 25 °C in a controlled fashion. In the immortal ROP performed with up to 100 equivalents of exogenous 9‐anthracenylmethanol or benzyl or propargyl alcohols as a transfer agent, the activity of the catalyst increased with the size of the metal ( 1 < 2 < 3 ). For Ca‐based complexes, the enhanced electron‐donating ability of the ancillary ligand favored catalyst activity ( 1 > 6 > 4 ≈ 5 ). The nature of the alcohol had little effect over the activity of the binary catalyst system 1 /ROH; in all cases, both the control and end‐group fidelity were excellent. In the living ROP of L ‐LA, the HCCCH 2 O − initiating group (as in 7 ) proved superior to N(SiMe 2 H) 2 − or N(SiMe 3 ) 2 − (as in 6 or [{LO 4 }CaN(SiMe 3 ) 2 ] ( B ), respectively).