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CROWN-ETHER COMPLEXATION OF SCANDIUM(III) - FORMATION AND SPECTROSCOPIC CHARACTERIZATION OF SCCL2+-OXACROWN SPECIES AND CRYSTAL-STRUCTURES OF 2 SCANDIUM DIBENZOCROWNS

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Abstract

The reaction systems ScCl3-SbCl5-MeCN-crown ether(L), where L = 6,7,9,10,17,18,20,21-octahydrodibenzo [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, H-1 and C-13 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], 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) dan 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 P2(1)/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) Angstrom, Cl-Sc-Cl 173.8(2)degrees] 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 Angstrom) and one from a co-ordinated water molecule [Sc-O-water 2.130(8) Angstrom]. 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 Angstrom. 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 ($) over bar 1, 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 pentagonalbipyramidal geometry. The trans-ScCl2 unit [Sc-Cl 2.411(3) and 2.419(3) Angstrom, 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 Angstrom), Sc-O-water 2.148(9) and 2. 160(8) Angstrom. Both of the co-ordinated 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 contacts in the range 2.83-2.98 Angstrom and the other forming four such contacts in the range 2.75-3.00 Angstrom

Topics: QD
Publisher: ROYAL SOC CHEMISTRY
OAI identifier: oai:wrap.warwick.ac.uk:19974
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