Cyclooctatetraenyl calcium and strontium amido complexes

Abstract

This paper reports the synthesis and structural characterisation of a series of cyclooctatetraenyl (C₈H₈) complexes of calcium and strontium, most of them containing the bis(trimethylsilyl)amido ligand. Mixing MI₂ (M = Ca, Sr), KN(SiMe₃)₂ and K₂(C₈H₈) in thf in a 2 : 2 : 1 ratio gave the inverse sandwich amido complexes [[{(Me₃Si)₂N}M(thf)x]₂(μ-C₈H₈)] (M = Ca, x = 1; M = Sr, x = 2) (1-Ca, 1-Sr) in fair to good yields. From a 1 : 1 : 1 ratio, the mixed potassium calcium complex [[{(Me₃Si)₂N}Ca(thf)](μ-C₈H₈)K] (2) was obtained in good yield. The reaction of 2 with a 1 : 1 mixture of CaI₂ and KN(SiMe₃)₂ yielded 1-Ca confirming that the reaction could be carried out stepwise. Attempts at making heterobimetallic calcium strontium amido complexes from the reaction of 2 with Sr{N(SiMe₃)₂}₂ led to redistribution reactions which afforded the potassium strontium complex [[K{(Me₃Si)₂N}₂Sr]₂(μ-C₈H₈)] (3) among other species. Complex 3 was more conveniently synthesised in fair yield starting from a 2 : 4 : 1 molar mixture of SrI₂, KN(SiMe₃)₂ and K₂(C₈H₈), respectively. Treatment of 2 with PhCCH in benzene yielded an insoluble complex tentatively formulated as the polymeric complex [CaK(CCPh)(C₈H₈)(thf)_x]_n (4). Upon dissolution in thf, 4 underwent a redistribution reaction yielding the X-ray characterised [{Ca(μ-C₈H₈)₂}K₂(thf)₂] (5) and a homoleptic calcium alkynyl complex [Ca(CCPh)₂(thf)_x]. Several compounds were characterised by X-ray diffraction as discrete or polymeric structures where Ca⋯Cπ interactions were identified in some cases. Despite identified drawbacks, such as the lability of the coordinated thf ligands and facile redistribution reactions, this study represents the first synthetic approach to cyclooctatetraenyl heavy alkaline-earth metal amido complexes.