Bismuth–lithium bonding in the ion pairs: LiBiL2, where L = a porphyrin or a salen ligand

Abstract

From the reaction between BiCl₃ (1 equiv.) and LiN(SiMe₃)₂ (4 equiv.) and LH₂ (2 equiv.), where L = a tetraphenylporphyrin, TPP, an octaethylporphyrin, OEP and phsalen in THF the title compounds have been obtained I LiBiTPP₂, II LiBiOEP₂, and IIIA LiBi(phsalen)₂ and IIIB LiBi(phsalen)₂·THF. Crystals grown from CH₂Cl₂–hexanes are colored; I (green), II (red-purple) and IIIA and IIIB (red-orange). The molecular structures of compound I, IIIA and IIIB were determined by single-crystal X-ray crystallography and are shown to have short Li⋯Bi bonds of distance 2.8 Å involving the LiL⁻BiL⁺. Compound IIIA shows a slipped structure involving Li to two oxygens and a Li⋯Bi distance of 3.1 Å. Compounds IIIA and IIIB undergo a rapid reversible exchange in toluene-d₈ at 90 °C. The MALDI-MS yields weak molecular ions due to LiBiL₂^+/− with more intense ions due to BiL⁺ and LiL⁻ in the positive and negative modes. The short Li⁺ to Bi³⁺ distances are comparable to those seen in Li⋯Bi compounds, such a LiBiR₂, and are comparable to those seen by Pyykkö (P. Pyykkö, J. Phys. Chem. A, 2015, 119, 2326; P. Pyykkö and M. Atsumi, Chem. – Eur. J., 2009, 15, 186; P. Pyykkö and M. Atsumi, Chem. – Eur. J., 2009, 15, 12770) for Li–Bi bonds. These can be seen to be involving Bi6s6p hybrid lone-pairs to Li⁺ atoms. The lithium bis(bistrimethylsilyl)amide (2 equiv.) and phsalenH₂ in THF gave a compound having Li₂L·2THF, IV. Crystallographically compound IV contains two Li⁺ atoms, one coordinated to five atoms LiO₂N₂·THF. and the other being coordinated to three atoms, LiO₂·THF. By ⁷Li and ¹H NMR both lithium atoms share an equivalent environment.