Synthesis and structures of molecular beryllium Grignard analogues featuring terminal and bridging pseudohalides

Abstract

The carbene-stabilised beryllium Grignards [(CAAC)BeBrR] (R = CAACH 1a, Dur 1b; CAAC/H = 1-(2,6-diisopropylphenyl)-2,2,4,4-tetramethylpyrrolidin-2-yl/idene; Dur = 2,3,5,6-tetramethylphenyl) undergo salt metathesis with various pseudohalide salt precursors. Whereas with [NaNCS] the thiocyanato Grignards [(CAAC)Be(NCS)R] (R = CAACH 2a, Dur 2b) are obtained selectively, salt metatheses with [Na(OCP)(dioxane)_2.3] and [K(OCN)] are fraught with side reactions, in particular scrambling of both neutral and anionic ligands, leading to complex product mixtures, from which the first examples of beryllium phosphaethynolate Grignards [(thf)₂(CAACH)Be(OCP)] (3) and [(CAAC)Be(OCP)R] (R = CAACH 4a, Dur 4b), as well as the isocyanate-bridged hexamer [(CAAC)BrBe(1,3-μ-OCN)]₆ (7) were determined as the main products. The complexity of possible side reactions is seen in complex 5, a byproduct of the salt metathesis of 1b with [Na(OCP)(dioxane)_2.3], which hints at radical redox processes, OCP homocoupling, OCP coupling with CAAC, as well as OCP insertion into the Be–R bond. Finally, the unstable, tetrameric cyano-bridged beryllium Grignard [(thf)(CAACH)Be(1,2-μ-CN)] (8) was obtained by salt metathesis of 1a with [Na/KSeCN] alongside one equiv. CAACSe. The new complexes were characterised by heteronuclear NMR and IR spectroscopy, as well X-ray crystallography.