Pre-inverse-crowns: synthetic, structural and reactivity studies of alkali metal magnesiates primed for inverse crown formation

Abstract

Two new alkali metal monoalkyl-bisamido magnesiates, the potassium compound [KMg(TMP)₂ⁿBu] and its sodium congener [NaMg(TMP)₂ⁿBu] have been synthesised in crystalline form (TMP = 2,2,6,6-tetramethylpiperidide). Devoid of solvating ligands and possessing excellent solubility in hydrocarbon solvents, these compounds open up a new gateway for the synthesis of inverse crowns. X-ray crystallography established that [KMg(TMP)₂ⁿBu] exists in three polymorphic forms, namely a helical polymer with an infinite KNMgN chain, a hexamer with a 24-atom (KNMgN)₆ ring having endo-disposed alkyl substituents, and a tetramer with a 16-atom (KNMgN)₄ ring also having endo-disposed alkyl substituents. Proving their validity as pre-inverse-crowns, both magnesiates react with benzene and toluene to generate known inverse crowns in syntheses much improved from the original, supporting the idea that the metallations take place via a template effect. [KMg(TMP)₂ⁿBu] reacts with naphthalene to generate the new inverse crown [KMg(TMP)₂(2–C₁₀H₇)]₆, the molecular structure of which shows a 24-atom (KNMgN)₆ host ring with six naphthalene guest anions regioselectively magnesiated at the 2-position. An alternative unprecedented 1,4-dimagnesiation of naphthalene was accomplished via [NaMg(TMP)₂ⁿBu] and its NaTMP co-complex “[NaMg(TMP)₂ⁿBu]·NaTMP”, manifested in [{Na₄Mg₂(TMP)₄(2,2,6-trimethyl-1,2,3,4-tetrahydropyridide)₂}(1,4-C₁₀H₆)]. Adding to its novelty, this 12-atom (NaNNaNMgN)₂ inverse crown structure contains two demethylated TMP ligands as well as four intact ones. Reactivity studies show that the naphthalen-ide and -di-ide inverse crowns can be regioselectively iodinated to 2-iodo and 1,4-diiodonaphthalene respectively.