Synthesis of the scandium chloride hydrates ScCl₃·3H₂O and Sc₂Cl₄(OH)₂·12H₂O and their characterisation by X-ray diffraction, ⁴⁵Sc NMR spectroscopy and DFT calculations

Abstract

The compounds ScCl3·3H2O (SCTH) and [{Sc(H2O)5(μ-OH)}2]Cl4·2H2O (SCOH), have been synthesised and characterised by single-crystal XRD, 45Sc NMR spectroscopy and DFT calculations, with the crystal structure of SCTH reported here for the first time. From 45Sc NMR measurements under static and MAS conditions, both chemical shift and quadrupolar coupling parameters have been determined. The quadrupolar coupling constants χ for the octahedrally coordinated scandium sites in SCTH are 2.0 ± 0.1 MHz for Sc(1) and 3.81 ± 0.05 MHz for Sc(2). For SCOH, where the hepta-coordination of the single scandium site constitutes a less symmetric electronic environment, 14.68 ± 0.05 MHz was found. DFT calculations for the static SCTH structure consistently overestimate the quadrupolar coupling constants, indicating the possible presence of crystal water dynamics on the NMR time scale.

On the singularity of scandium

“…This has consequences in terms of the extent of hydrolysis of the cations in aqueous solution, this hydrolysis in turn leading to different forms of the dimeric first product of such reaction. As earlier noted, the dimeric [(H 2 O) 5 Sc(m-OH) 2 Sc(H 2 O) 5 ] 4+ ion has been well characterised, both in the solid state by X-ray diffraction 13,[16][17][18][19][20][21] and in solution by large-angle X-ray scattering (LAXS). 29 Cr(III) behaves similarly in forming the [(H 2 O) 4 Cr(m-OH) 2 Cr(OH 2 ) 4 ] 4+ ion, characterised as its p-toluenesulfonate salt, 90,91 although where ammonia is a co-ligand, the basic and acid ''rhodo'' ions, [Cr(NH 3 ) 5 OCr(NH 3 ) 5 ] 4+ and [Cr(NH 3 ) 5 (OH)Cr(NH 3 ) 5 ] 5+ , involve a single oxygen bridge, a feature seemingly favoured by CrÁ Á ÁCr interactions within the dimer.…”

“…30 also independently determined later by others. 19 Here, H-bonding between coordinated water and coordinated chloride once again generates a threedimensional H-bond polymer (Fig. 9).…”

“…In 1989, this ion was shown (as the benzene sulfonate salt) to contain seven coordinate scandium, with a mean Sc-OH 2 distance of 2.19(4) Å. 16 This cation was subsequently isolated with other counter-ions as [(H 2 O) 5 Sc(m-OH) 2 Sc(H 2 O) 5 ]X 4 (X = Cl, [17][18][19] Br 20,21 and I 21 ). The presence of seven coordinate scandium in all these species is suggestive of the nature of the parent aqua ion, although as is discussed later, there is ample evidence for metal ion solvates in general that species isolated in the solid state are not necessarily those most abundant in solution.…”

“…1 The hydroxido-bridged dimer and its local H-bond interactions (dashed lines) in the crystal of its chloride. 19 Sc = blue, O = red, Cl = green; Hatoms are shown as small spheres of an arbitrary 0.1 Å radius. The structural figure was prepared, as for other such figures in this article, from cifs deposited in the Cambridge Structural Database using CrystalMaker.…”

“…Fig.9The cation-anion pair of 'free' ScCl 3 Á3H 2 O19,30 and a portion of the H-bond (dashed lines) network showing one of the modes of linking of the two species.…”