Crown ether complexation of scandium(III). Formation and spectroscopic characterisation of ScCl2+–oxacrown species and crystal structures of two scandium dibenzocrowns

Abstract

The reaction systems ScCl₃–SbCl₅–MeCN–crown ether(L), where L = 6,7,9,10,17,18,20,21 -octa-hydrodibenzo[b,k][1,4,7,10,13,16] hexaoxacyclooctadecine (dibenzo-18-crown-6), 6,7,9,10,12,13,20,21,23,24,26,27 -dodecahydrodibenzo [b,n][1,4,7,10,13,16,19,22] octaoxacyclotetracosine (dibenzo-24-crown-8) and 6,7,9,10,12,13,15,16,23,24,26,27,29,30,32,33-hexadecahydrodibenzo [b,q][1,4,7,10,13,16,19,22,25,28] decaoxacyclotriacontine (dibenzo-30-crown-10), have been investigated. Halide abstraction generates the ScCl₂⁺ cation in situ which can be stabilised by macrocyclic complexation with the appropriate cyclic ether. The compound [ScCl₂(dibenzo-18-crown-6)][SbCl₆] was isolated as a white solid and characterised as the hexachloroantimonate (V) salt from microanalytical and spectroscopic (IR, ¹H and ¹³C NMR) data. Entrapment of a trans-ScCl₂ unit within the ring cavity involving five-co-ordination of the ring oxygen atoms is seen as the most likely arrangement. Removal of a further chloride ion from this compound, effected by the addition of SbCl₅(three equivalents), provides [ScCl(dibenzo-18-crown-6)(MeCN)][SbCl₆]₂ as established by microanalytical and spectroscopic data, and similar cavity entrapment of a trans-ScCl(MeCN) unit has been discussed. Solution studies, as monitored by ¹H NMR spectroscopy, indicate that complete removal of chloride ion from [ScCl(dibenzo-18-crown-6)(MeCN)][SbCl₆]₂ can be effected but only with a heavy excess (10 equivalents) of SbCl₅. The compound [ScCl₂(dibenzo-24-crown-8)(H₂O)][SbCl₆]·2MeCN 1 was isolated as red crystals and structurally characterised by X-ray diffraction studies. Crystals are monoclinic, space group P2₁/n, Z= 4 and R 0.0638. The structure consists of [ScCl₂(dibenzo-24-crown-8)(H₂O)]⁺ cations and [SbCl₆]^– anions with two solvent (MeCN) molecules trapped in the lattice. For the cation the Sc^III co-ordination geometry is essentially pentagonal bipyramidal involving two axial chlorine atoms [Sc–Cl 2.387(4) and 2.397(4)Å, Cl–Sc–Cl 173.8(2)°] and five equatorial oxygen atoms comprising four from the crown ether with Sc–O distances in the range 2.184(7)–2.297(7)(mean 2.25 Å) and one from a co-ordinated water molecule [Sc–O_water 2.130(8)Å]. The threaded ScCl₂⁺ unit is located within the ring cavity but in an ‘off-centre’ position. Intracavity hydrogen bonding of the type Sc–OH₂⋯ O_ring is present with O_water⋯ O_ring 2.65 and 2.75 Å. The compound [ScCl₂(dibenzo-30-crown-10)(H₂O)₂][SbCl₆]· MeCN·H₂O 2 was isolated as yellow needle crystals and characterised crystallographically as the hexachloroantimonate (V) salt. Crystals are triclinic, space group P, Z= 2 and R= 0.0639. There are two solvent molecules (MeCN and H₂O) trapped in the lattice. In the [ScCl₂(dibenzo-30-crown-10)(H₂O)₂]⁺ cations the Sc^III ion is seven-co-ordinate, involving bonds to three oxygens from the crown ether, two chlorine atoms and two water molecules, and shows distorted pentagonal-bipyramidal geometry. The trans-ScCl₂ unit [Sc–Cl 2.411 (3) and 2.419(3)Å, Cl–Sc–Cl 169.3(2)°] is located in an annexe section of the ring with Sc–O_ring bond distances ranging between 2.208(8) and 2.280(8)(mean 2.25 Å), Sc–O_water 2.148(9) and 2.160(8)Å. Both of the co-ordinated water molecules are involved in a complex pattern of intracavity hydrogen bonding of the type Sc–OH₂⋯ O_ring with one molecule forming three O_water⋯ O_ring close contracts in the range 2.83–2.98 Å and the other forming four such contracts in the range 2.75–3.00 Å.