Hexahalometallate salts of trivalent scandium, yttrium and lanthanum: cation–anion association in the solid state and in solution †

The hexahalide salts, [N n Bu 4 ] 3 [LaCl 6 ], [BMPYRR] 3 [LaCl 6 ] (BMPYRR = 1-butyl-1-methylpyrrolidinium), [EMIM] 3 [MX 6 ] (EMIM = 1-ethyl-3-methylimidazolium; M = La, X = Cl, Br, I; M = Sc, Y, Ce, X = Cl) and [EDMIM] 3 [MX 6 ] (EDMIM = 1-ethyl-2,3-dimethylimidazolium; M = Y, X = Cl; M = La, X = Cl, I) have been prepared and X-ray crystal structures determined for several of them, with a view to probing the eﬀect of varying the trivalent metal ion, the halide and the counter-cation on the structures adopted in the solid state. The crystal structures of the EMIM and EDMIM salts show extensive H-bonding between the halide ligands and organic cations; based upon the H-bonding distances, this appears to be strongest for the [EMIM] 3 [MCl 6 ] salts, becoming progressively weaker for heavier metal ion or halide. In terms of the cations, changing from EMIM to EDMIM also reduces the strength of the H-bonding. The strength of the cation–anion pairing in solution has also been probed in solution via NMR spectroscopy where possible ( 45 Sc, 89 Y and 189 La) and, for the EMIM salts, via the shift of d (H2) relative to [EMIM]Cl at a standard concentration. The trends observed in solution mirror those determined in the solid state.

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Accepted Manuscript NJC www.rsc.org/njc Introduction In the course of our work on electrodeposition of metals and semiconductors from unusual low polarity solvents, such as CH 2 Cl 2 1 and supercritical CH 2 F 2 , 2 we have established that halometallates can often offer both the solubility and extent of dissociation into discrete cations and anions to be highly effective as reagents for delivering the elements of choice. Taking germanium as an example, we have shown that the cation, halide and anion charge all play an important role in determining the extent of cation-halogermanate anion association both in the solids and in solution. 3 While we have often used quaternary ammonium salts for this purpose, recent work on the electrodeposition of elemental germanium from supercritical CH 2 F 2 has established that using the 1ethyl-3-methylimidazolium (EMIM) cation leads to improved quality of the electrodeposited germanium, 4 and hence this and other substituted imidazolium cation salts, which are also used extensively for electrodeposition from ionic liquids, 5 have become of greater importance in this area of work.
In order to gain a better understanding of the cation-anion association in halometallate salts, we describe here a study of the [MX 6 ] 3− trianions incorporating a range of trivalent metal ions from Group 3 and the lanthanides, in the anticipation that the trianionic charge would give rise to significant cation-anion association both in the solid state and in low polarity solvents. We report the synthesis of a series of halometallate anion salts of scandium, yttrium and several lanthanides. All complexes have been characterised by NMR spectroscopy ( 1 H, 45 Sc, 89 Y and 139 La as appropriate), IR spectroscopy, elemental analysis and, in several cases, single-crystal X-ray diffraction.
To gain further insight into the cation-anion pairing in these species, we have explored the effects that systematic changes in the counter-cation, metal ion and halide co-ligands have on the solid-state structures adopted and the spectroscopic behaviour of these halometallate salts in MeCN (and where possible, CH 2 6 ] had previously been synthesised and crystallographically characterised. 6,7 Crystals of the bromide ( Figure 1) and iodide ( Figure 2)  are the only other structurally authenticated complexes. Here it should be noted that in our experience, the criteria used for assigning the strength of hydrogen bonds 13 are not always applicable where metal complexes with heavy halogens are acting as acceptors. Other important factors such as crystal packing effects and π-bonding interactions can have a significant effect on the donoracceptor distances and donor-hydrogen-acceptor (DHA) angles which are used to classify 'strong', 'moderate' or 'weak' hydrogen bonds. Typically, these DHA interactions are quantified via the crystallographically determined D⋅⋅⋅A (C⋅⋅⋅X) distances. The radii of Brand especially Ialso have an effect on the C⋅⋅⋅X bond lengths.
Nonetheless, for a given halide, the shortest C···X distances for the [EMIM] 3 [LaX 6 ] series of complexes (Table   1) occur where the NCN proton (H2) acts as the donor, consistent with it being the most acidic proton on the imidazolium ring. The C···X distances where the backbone NCCN protons (H4/H5) act as donors are typically 0.1-0.2 Å longer, implying weaker hydrogen bonding. In all cases the DHA angles are also generally greater than 130°, hence we assign these as 'moderate' hydrogen bonds. Varying the metal:     through C13 ( Figure 5).

Spectroscopic characterisation and comparisons:
In keeping with our previous observations for [GeCl 6 ] 2anions, 3 the chemical shift of the H2 proton in the 1 H NMR spectrum was used to indicate the strength of the hydrogen bonding in solution (Table 4).
From the NMR data it is apparent that the chemical shift of the signal associated with the H2 proton shifts to low For the [MX 6 ] 3− salts, 45 Sc, 89 Y and 139 La NMR spectra were also obtained (