Magnesium 4, 5, and 6 coordinate complexes with ligands bound via sp or sp2 hybridized atoms

Abstract

As a metallic element of high natural abundance, magnesium finds a wide range of uses in both stoichiometric and more recently catalytic applications. This often takes advantage of the basic or nucleophilic properties of its compounds or their ability to co-complex with other organometallic compounds. However, the homoleptic chemistry of Mg(II) is heavily skewed towards alkyl and aryl ligands bound via sp² and sp³ hybridized atoms. Here, we report our combined NMR spectroscopic and X-ray crystallographic study into much rarer alternative THF solvates of homoleptic magnesium complexes using ligands which bind via sp (alkynyl) and sp² (imido) atoms. Specifically, we exploit the high acidity of terminal alkynes and diphenylacetonitrile to prepare tetra-solvated distorted octahedral complexes Mg(C≡CC₆H₄R-p)₂(THF)₄ (R = Me, CF₃) and Mg(N=C=CPh₂)₂(THF)₄ starting from the commercial alkyl reagent MgBu₂. Adopting a similar deprotonative strategy using benzophenoneimine Ph₂C=NH affords the heteroleptic trinuclear complex Mg₃(N=CPh₂)₄nBu₂(THF)₂ with distorted tetrahedral Mg centres. Related imidomagnesium halide complexes can be accessed either by deprotonation of the imine with a Grignard reagent, or nucleophilic addition of a Grignard reagent to a nitrile, but these unusual five-coordinate complexes are unresponsive to a 1,4-dioxane induced Schlenk equilibrium shift. Employing a higher reflux temperature switching from a THF to a toluene medium permits access to the THF solvate of a homoleptic imido complex which also possesses a trinuclear constitution in Mg₃(N=CPh₂)₆(THF)₂.