Tethered cationic alkaline earth – olefin complexes

Abstract

The aminofluoroalcohol N,N,N-(CH₂CHCH₂CH₂–),(CH₃OCH₂CH₂–),(HOC(CF₃)₂CH₂–)N ({RO^F}H) possessing both a methoxy and an olefin dangling side arms enables the preparation of the heteroleptic charge neutral alkaline earth complexes [{μ²-RO^F}AeN(SiMe₂H)₂]₂ (Ae = Ca, 1; Sr, 2). These O-bridged dinuclear complexes were characterised by NMR spectroscopy and X-ray diffraction crystallography. XRD analysis of 1 and 2 showed that both complexes are stabilised by intramolecular Ae⋯F–C and β-agostic Ae⋯H–Si secondary interactions, and that the olefin does not bind to the alkaline earths while the methoxy side-arm does. The discrete ion pairs [({μ²-RO^F}Ae·(Et₂O)₂)₂]²⁺·2[H₂N{B(C₆F₅)₃}₂]⁻ (Ae = Ca, 3a; Sr, 4a) were synthesised upon treatment of the parent complexes 1 and 2 by [H·(Et₂O)₂]⁺.[H₂N{B(C₆F₅)₃}₂]⁻. NMR spectroscopy showed the presence of two coordinated Et₂O molecules on each metal in these complexes. The water adduct [({μ²-RO^F}Sr·(H₂O))₂]²⁺·2[H₂N{B(C₆F₅)₃}₂]⁻ (4b), presumably derived from 4a upon adventitious introduction of moisture during recrystallisation, was characterised by crystallographic methods. The dication [({μ²-RO^F}Sr·(H₂O))₂]²⁺ exists as an O-bridged dinuclear species featuring very strong intramolecular Sr⋯F–C interactions in the range 2.788(5)–2.997(6) Å, and no interaction with the weakly coordinating anion. Perhaps more importantly, the tethered olefins are coordinated onto the metal cations, with short Sr⋯Cπ interactions in the range 3.066(10)–3.092(10) Å; this represents the first example of a Sr-olefin cationic complex. Hence, cationisation of the charge neutral precursor to generate the discrete ion pair increases electron deficiency at the metal centre, resulting in the first observation of the binding of an olefin onto a well-defined cationic heavy alkaline earth. On the other hand, our attempts to produce Ae-olefin cations devoid of coordinated Lewis base with this strategy have met no success so far.