Heteroleptic Heavier Alkaline Earth Metal Amide Complexes Stabilized by a Superbulky β-Diketiminate Ligand

Abstract

Heteroleptic alkaline earth metal (Ae = Ca, Sr, Ba) amide complexes with the superbulky β-diketiminate ligand DIPePBDI (CH­[C­(Me)­N-DIPeP]2, DIPeP = 2,6-di-iso-pentylphenyl) have been prepared by direct deprotonation of DIPePBDI-H with either AeN′′2 or AeN′′2·(THF)2 (N′′ = N­(SiMe3)2). Despite long reaction times of 5–14 days, this convenient one-step synthetic method has the major advantage that metal-pure products are obtained in generally quantitative yields. All (DIPePBDI)­AeN′′ and (DIPePBDI)­AeN′′·THF complexes are monomeric and stabilized by agostic metal···Me3Si and metal···iso-pentyl interactions. They are highly soluble in toluene and indefinitely stable toward ligand scrambling, even after 2 weeks at 140 °C. The same series with the smaller DIPPBDI ligand (CH­[C­(Me)­N-DIPP]2, DIPP = 2,6-di-iso-propylphenyl) could, except for Ca, also be prepared by direct ligand deprotonation. The (DIPPBDI)­CaN′′ and (DIPPBDI)­CaN′′·THF complexes are stable toward ligand exchange up to 110 °C. Whereas THF-free (DIPPBDI)­SrN′′ and (DIPPBDI)­BaN′′ decompose at 50 and 20 °C, respectively, their THF adducts were found to be stable up to 60 °C. This is, however, strongly dependent on complex purity. Slight hydrolysis or contamination with KN′′ accelerates ligand scrambling. Therefore, partial hydrolysis and salt metathesis routes that involve KN′′ should be avoided when synthesizing heteroleptic complexes of the heavier Ae metals.

Stabilization of Heteroleptic Heavier Alkaline Earth Metal Complexes with an Encapsulating Dipyrromethene Ligand

“…Although ( DIPP BDI)AeN′′ complexes may seem NMR‐pure, contamination with KN′′ can often not be noticed using NMR as an analytical method . We demonstrated that slight contamination with KN′′ leads to significantly lower stabilities and faster ligand exchange . This is likely due to the formation of intermediate ate complexes: ( DIPP BDI)AeN′′ + KN′′ → [( DIPP BDI)AeN′′ 2 – ] [K + ] .…”

“…The importance of the anagostic Ae··MeSi interaction, which is quantified by the discrepancy in the Ae-N-Si angles, decreases from Mg to Ba. Compared to the same series of ( DIPP BDI)AeN′′ complexes (Ae = Mg, [22] Ca, [23] Sr, [14] Ba [14] ), the Ae-N distances in the DPM complexes are consistently 0.01-0.08 Å longer. This fits with our earlier observations for DIPP DPM zinc complexes for which it could be shown that the DPM ligand is clearly weaker bound to the metal than the corresponding BDI ligand.…”

“…Schlenk equilibria are clearly promoted by the presence of K-containing impurities. [14] Although ( DIPP BDI)AeN′′ complexes may seem NMR-pure, contamination with KN′′ can often not be noticed using NMR as an analytical method. [26] We demonstrated that slight contamination with KN′′ leads to significantly lower stabilities and faster ligand exchange.…”

“…[13] Most recently, we introduced a superbulky -diketiminate ligand, DIPeP BDI (II), which was shown to stabilize a range of heavier Ae amide complexes ( DIPeP BDI)AeN(SiMe 3 ) 2 (Ae = Ca, Sr, Ba). [14] The heavy Ba complex was found to be extraordinarily stable towards ligand exchange: a solution of ( DIPeP BDI)BaN(SiMe 3 ) 2 in toluene did not decompose even after heating for two weeks at 140°C.…”

Barium‐Catalysed Dehydrocoupling of Hydrosilanes and Borinic Acids: A Mechanistic Insight

“…The synthesis of an heteroleptic boryloxide/aminoether‐phenolate [{LO NO4 }BaOB{CH(SiMe 3 ) 2 } 2 ], was successful, although this complex is not sufficiently stable in solution to be implemented in catalysis. Still, the choice of different ancillary ligands can of course be envisaged, for instance bulky β‐diketiminates or aminoether‐functionalised iminoanilides and amidinates . In line with other AE‐mediated reactions, it is possible that the resulting heteroleptic barium complexes will exhibit greater reaction rates than the homoleptic complexes used here.…”