Synthesis and reactivity of alkaline-earth stannanide complexes by hydride-mediated distannane metathesis and organostannane dehydrogenation

Abstract

The synthesis of heteroleptic complexes with calcium– and magnesium–tin bonds is described. The dimeric β-diketiminato calcium hydride complex, [(BDI)Ca(μ-H)]₂ (I^Ca) reacts with Ph₃Sn–SnPh₃ to provide the previously reported μ₂-H bridged calcium stannanide dimer, [(BDI)₂Ca₂(SnPh₃)(μ-H)] (3). Computational assessment of this reaction supports a mechanism involving a hypervalent stannate intermediate formed by nucleophilic attack of hydride on the distannane. Monomeric calcium stannanides, [(BDI)Ca(SnPh₃)·OPPh₃] (8·OPPh₃) and [(BDI)Ca(SnPh₃)·TMTHF] (8·TMTHF, TMTHF = 2,2,5,5-tetramethyltetrahydrofuran) were obtained from I^Ca and Ph₃Sn–SnPh₃, after addition OPPh₃ or TMTHF. Both complexes were also synthesised by deprotonation of Ph₃SnH by I^Ca in the presence of the Lewis base. The calcium and magnesium THF adducts, [(BDI)Ca(SnPh₃)·THF₂] (8·THF₂) and [(BDI)Mg(SnPh₃)·THF] (9·THF), were similarly prepared from [(BDI)Ca(μ-H)·(THF)]₂ (I^Ca·THF₂) or [(BDI)Mg(μ-H)]₂ (I^Mg) and Ph₃SnH. An excess of THF or TMTHF was essential in order to obtain 8·TMTHF, 8·THF₂ and 9·THF in high yields whilst avoiding redistribution of the phenyl-tin ligand. The resulting Ae–Sn complexes were used as a source of [Ph₃Sn]⁻ in salt metathesis, to provide the known tristannane Ph₃Sn–Sn(t-Bu)₂–SnPh₃ (11). Nucleophilic addition or insertion with N,N′-di-iso-propylcarbodiimide provided the stannyl-amidinate complexes, [(BDI)Mg{(iPrN)₂CSnPh₃}] (12) and [(BDI)Ca{(iPrN)₂CSnPh₃}·L] (13·TMTHF, 13·THF, L = TMTHF, THF). The reactions and products were monitored and characterised by multinuclear NMR spectroscopy, whilst for compounds 8, 9, 12, and 13·THF, the X-ray crystal structures are presented and discussed.