Calcium and Magnesium Bis(β-diketiminate) Complexes: Impact of the Alkylene Bridge on Schlenk-Type Rearrangements

Abstract

Dinuclear heteroleptic alkaline-earth-metal complexes are interesting synthetic targets because the close proximity of two metals allows for cooperative effects. However, these complexes are also prone to undergoing Schlenk-type rearrangements, affording less-active homoleptic complexes. Here we present the metalation of bis­(β-diketimine) ligands possessing flexible bridging groups, i.e., 1,2-ethylene, 1,3-propylene, and trans-1,2-cyclohexylene, using calcium and magnesium precursors. Four mononuclear homoleptic calcium complexes were obtained, highlighting the pronounced tendency of calcium to undergo Schlenk-like redistributions. In the case of magnesium, however, the bridging group plays a crucial role, yielding seven dinuclear heteroleptic complexes but also one mononuclear and one dinuclear homoleptic complexes. In addition, a trinuclear mixed heteroleptic–homoleptic magnesium complex, which is a rare example of an intermediate of the Schlenk equilibrium, was isolated.

Atom-economic access to cationic magnesium complexes

“…Single crystal X-ray analysis revealed these to be the charge-separated complex [Mg·6THF] 2+ 2[AlPh 4 ] − ( 1a , Fig. 1) which we believe is a minor Schlenk equilibrium 13 product of the targeted complex 1 . However, its anionic moiety unequivocally confirms that ligand (phenyl) transfer from Mg to Al has occurred as intended, supporting the 27 Al NMR observations.…”

Appropriation of group II metals: synthesis and characterisation of the first alkaline earth metal supported transition metal carbonite complexes

“…However, monitoring the reaction of I Na with CaI 2 by NMR spectroscopy it was observed that two species are present in solution (see ESI,† Section S3). The same product mixture was also generated when reacting isolated 2 at room temperature with an excess of CaI 2 indicating that 2 is participating in a kind of Schlenk equilibrium 9 (see Scheme 2). Indeed the minor product was identified as 2 while for the main product a composition of [CaI(THF) 4 (L t Bu NiCO 2 )], 2′ , is suggested (see below).…”

Synthesis and Characterization of Magnesium-Hydride Complexes with Alkylene-Bridged Bis(β-diketiminate) Ligands

“…While bis(β-diketiminate) calcium-hydride complexes have not yet been reported, the Harder group was able to access dimeric tetranuclear C but also a larger octanuclear magnesium-hydride cluster. The absence of the former so far is most likely due to the pronounced tendency of calcium to undergo Schlenk-type rearrangements associated with the formation of homoleptic complexes, which is even more pronounced in the case of ditopic ligands because of entropic effects …”

“…Later on, it was shown that mononuclear three-coordinate magnesium-hydride complexes are also stable without an additional coligand, i.e., when using a sterically demanding β-diketiminate . Besides the utilization of β-diketimines, ditopic bis­(β-diketimine)­s have also been applied in alkaline-earth metal chemistry . While bis­(β-diketiminate) calcium-hydride complexes have not yet been reported, the Harder group was able to access dimeric tetranuclear C but also a larger octanuclear magnesium-hydride cluster.…”