Heavier alkaline earth and heterobimetallic s-block “ate” complexes of a di(amido)siloxane ligand: solid-state structure and dynamic solution-phase behaviour†

Abstract

The diverse solid-state structures and solution-phase dynamics of both neutral and heterometallic s-block “ate” complexes of the heavier alkaline earth metals (Ae; Ca–Ba) supported by a chelating and flexible di(amido)siloxane ligand ([^NON-DippL]²⁻ = [O(SiMe₂NDipp)₂]²⁻) are described, enabling comparison with those of closely related di(amido) ligands based on either flexible aliphatic or rigid xanthene-based backbones. Three dimeric alkaline earth complexes [(^NON-DippL)Ae]₂ (Ae = Ca (2), Sr (3) and Ba (4)) which feature a κ³-N,O,N′-κ¹-N′-tridentate coordination mode were prepared from protonolysis reactions between ^NON-DippLH₂ with (Ae = Ca, Sr and Ba); N′′ = [N(SiMe₃)₂]⁻. In tetrahydrofuran, these complexes were readily converted into the monomeric adducts [(^NON-DippL)Ae(thf)_n] (n = 2, Ae = Ca (5); n = 3, Ae = Sr (6) and Ba (7)). Heterometallic Ae/K amide “ate” complexes were afforded through two routes: reaction of previously reported [(^NON-DippL)Mg]₂ (1) with two equivalents of KN′′ at elevated temperatures resulted in [(^NNO-DippL)Mg(μ-N′′)K]_n (8; ^NNO-DippL = [OSiMe₂NDippSiMe₂NDipp]²⁻), whereas the equimolar reaction of ^NON-DippLH₂ with led to [(^NON-DippL)Ae(μ-N′′)K]_n (Ae = Ca (9), Sr (10) and Ba (11)). Complexes 8–11 exist as one-dimensional coordination polymers propagated by K⁺–aryl π-facial interactions in the solid-state. The mixed amide/siloxide “NNO” ligand in 8 results from a 1,3-silyl retro-Brook rearrangement of the original di(amido)siloxane ligand, while the larger Ae²⁺ congeners readily accommodate the coordination of KN′′ with the di(amido)siloxane ligand retaining a κ³-N,O,N′-tridentate motif in 9–11. Finally, the solution-phase behaviour of 8–11 in both toluene and thf were investigated indicating the reversible dissociation of KN′′ from 9–11 and the thermodynamic parameters of this process were elucidated.