Martin J. D.
Champion
,
William
Levason
,
David
Pugh
and
Gillian
Reid
*
School of Chemistry, University of Southampton, Southampton SO17 1BJ, UK. E-mail: G.Reid@soton.ac.uk
First published on 20th June 2016
The hexahalide salts, [NnBu4]3[LaCl6], [BMPYRR]3[LaCl6] (BMPYRR = 1-butyl-1-methylpyrrolidinium), [EMIM]3[MX6] (EMIM = 1-ethyl-3-methylimidazolium; M = La, X = Cl, Br, I; M = Sc, Y, Ce, X = Cl) and [EDMIM]3[MX6] (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[MCl6] 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 (45Sc, 89Y and 189La) and, for the EMIM salts, via the shift of δ(H2) relative to [EMIM]Cl at a standard concentration. The trends observed in solution mirror those determined in the solid state.
In order to gain a better understanding of the cation–anion association in halometallate salts, we describe here a study of the [MX6]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 (1H, 45Sc, 89Y and 139La 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, CH2Cl2) solution.
A small number of earlier studies report structural data on halometallate salts of the Ln(III) ions. The examples most pertinent to the present study are [EMIM]3[LnCl6] (Ln = La, Pr, Nd, Sm, Eu) grown from hydrated LnCl3 in [EMIM]Cl at 110 °C,6,7 [MPPYRR]3[NdI6] (MPPYRR = 1-methyl-1-propylpyrrolidinium),8 and [SEt3]3[LnI6] (Ln = Nd, Sm), obtained from reaction of LnI2 with the ionic liquid [SEt3][NTf2] (NTf2 = bis-(trifluoromethanesulfonyl)imide).9 A very recent study has described the structures of a series of [BMIM]3[CeX6] salts (BMIM = 1-butyl-3-methylimidazolium), also obtained directly from CeX3 in the [BMIM]X ionic liquids, which are reported to exhibit intense Ce3+ based 5d–4f-centred emission.10
[EMIM]3[LaCl6] and [EMIM]3[LaBr6] are isomorphous and isostructural, whereas [EMIM]3[LaI6] crystallises with a different unit cell, presumably reflecting the increased steric requirements of I− over Br− and Cl−. Nonetheless, in each case the [LaX6]3− anion is octahedrally coordinated with six halides comprising the primary coordination sphere. Two crystallographically independent [LaX6]3− anions were observed in the solid state with different arrangements of hydrogen bonding cations, including bifurcated hydrogen bonding, accounting for the difference. Most of the hydrogen bonding occurs through the acidic NCN proton of the imidazolium ring but interactions through the NCCN backbone protons (which are less acidic) were also observed, resulting in an extended structure in the solid state (Fig. 2(b)).
Here it should be noted that in our experience, the criteria used for assigning the strength of hydrogen bonds13 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 donor–acceptor 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 Br− and especially I− also have an effect on the C⋯X bond lengths. Nonetheless, for a given halide, the shortest C⋯X distances for the [EMIM]3[LaX6] 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.
[EMIM]3[LaCl6]a | [EMIM]3[LaBr6] | [EMIM]3[LaI6] | ||||
---|---|---|---|---|---|---|
Length (Å) | Angle (°) | Length (Å) | Angle (°) | Length (Å) | Angle (°) | |
a Data from ref. 7. | ||||||
C⋯X (H2) | 3.557(8) | 152.2 | 3.579(12) | 134.2 | 3.942(5) | 139.8 |
3.479(6) | 131.6 | 3.659(10) | 153.8 | 3.758(5) | 134.0 | |
3.393(4) | 120.2 | 3.765(17) | 134.7 | 3.864(5) | 134.9 | |
3.612(4) | 150.2 | 3.825(5) | 160.4 | |||
C⋯X (H4/5) | 3.635(5) | 150.3 | 3.723(12) | 121.6 | 3.974(5) | 136.7 |
3.614(6) | 148.4 | 3.683(11) | 139.8 | 3.972(5) | 113.5 | |
3.692(5) | 146.7 | 3.744(12) | 118.3 | 3.796(5) | 143.2 | |
3.632(5) | 145.9 | 3.755(11) | 146.8 | 3.894(5) | 119.0 | |
3.564(6) | 137.5 | 3.708(12) | 145.1 | 3.830(5) | 124.0 | |
3.693(12) | 153.7 | 4.210(5) | 172.5 | |||
3.708(16) | 143.9 |
The Sc and Y complexes are isostructural and isomorphous, with a slight increase in cell parameters owing to the increase in metal ionic radius (Sc = 0.745 Å, Y = 0.90 Å for 6-coordinate complexes).14 The known lanthanide examples are structurally different, but within the series (including La, Eu, Gd, Nd, Pr and Sm analogues) they are all isomorphous and isostructural7 and the cell parameters for [EMIM]3[CeCl6] fit well with this trend.
In the solid state all the metal centres are six-coordinate regular octahedra. There is only one metal environment in the asymmetric unit of [EMIM]3[ScCl6] and [EMIM]3[YCl6] and the hydrogen bonding interactions are very similar (Table 2). Despite the decreased charge:radius ratio for Y over Sc, the hydrogen bonds through H4/5 are identical within experimental error. However, there is a small increase in the hydrogen bond lengths involving H2 for [EMIM]3[YCl6] compared to [EMIM]3[ScCl6]. These are notably shorter than the equivalent bonds in [EMIM]3[LaCl6], continuing the same trend. Trifurcated hydrogen bonding is present in [EMIM]3[CeCl6] through C13 (Fig. 5).
[EMIM]3[ScCl6] | [EMIM]3[YCl6] | [EMIM]3[CeCl6] | ||||
---|---|---|---|---|---|---|
Length (Å) | Angle (°) | Length (Å) | Angle (°) | Length (Å) | Angle (°) | |
C⋯X (H2) | 3.506(3) | 149.5 | 3.547(4) | 146.6 | 3.547(7) | 150.8 |
3.496(3) | 131.3 | 3.531(5) | 130.0 | 3.425(7) | 132.4 | |
3.407(3) | 138.1 | 3.422(5) | 135.9 | 3.333(7) | 117.1 | |
3.604(3) | 135.2 | 3.650(4) | 134.6 | 3.569(8) | 141.9 | |
3.548(3) | 146.8 | 3.588(5) | 145.0 | |||
C⋯X (H4/5) | 3.586(3) | 166.7 | 3.584(5) | 165.0 | 3.568(7) | 150.4 |
3.692(3) | 152.5 | 3.695(4) | 153.1 | 3.583(7) | 143.2 | |
3.604(3) | 135.6 | 3.604(5) | 135.8 | 3.622(8) | 142.7 | |
3.434(3) | 131.7 | 3.459(4) | 128.6 | 3.542(8) | 147.2 | |
3.383(3) | 135.1 | 3.365(5) | 136.2 | 3.512(7) | 139.7 |
Structural characterisation of [EDMIM]3[LaCl6] revealed that the [LaCl6]3− trianion was stabilised by hydrogen bonding through five associated [EDMIM]+ cations. The C⋯Cl distances were slightly shorter than the corresponding C⋯Cl distances for [EMIM]3[LaCl6], suggesting that hydrogen bonding through the backbone protons may be slightly stronger when no H2 proton is present. However, 1H NMR data (below) show that the chemical shifts for H4/5 are consistent with complete dissociation of the molecule in CD3CN solution. The La–Cl bond lengths are comparable to those observed for [EMIM]3[LaCl6].
A comparison between the [EMIM]+ and [EDMIM]+ cations can also be drawn for the [YCl6]3− and [LaI6]3− salts. The structures of [EDMIM]3[YCl6] and [EDMIM]3[LaI6] are shown in Fig. 7 and 8, respectively.
[EDMIM]3[YCl6] and [EDMIM]3[LaCl6] are isomorphous and isostructural, whereas [EMIM]3[YCl6] and [EMIM]3[LaCl6] crystallise in different space groups. This suggests that the hydrogen bonding interactions play a greater role in the crystal packing arrangements for the [EMIM]+ salts. The C⋯Cl (H4/H5) distances (Table 3) are not notably different between [EMIM]3[YCl6] and [EDMIM]3[YCl6].
[EDMIM]3[YCl6] | [EDMIM]3[LaCl6] | [EDMIM]3[LaI6] | ||||
---|---|---|---|---|---|---|
Length (Å) | Angle (°) | Length (Å) | Angle (°) | Length (Å) | Angle (°) | |
C⋯X (H4/5) | 3.542(2) | 142.4 | 3.580(2) | 149.6 | 3.948(3) | 118.8 |
3.693(2) | 123.8 | 3.515(2) | 136.6 | 3.956(3) | 118.5 | |
3.388(2) | 129.8 | 3.521(2) | 148.9 | 3.933(3) | 139.2 | |
3.482(2) | 145.1 | 3.686(2) | 122.5 | 3.775(3) | 125.2 | |
3.520(2) | 149.9 | 3.455(2) | 125.8 | 3.883(3) | 124.3 | |
3.544(2) | 150.2 | 3.601(2) | 112.0 | 3.892(3) | 124.8 | |
3.577(2) | 122.6 | 3.524(2) | 143.0 | 3.883(3) | 115.3 |
For [EDMIM]3[LaI6] two different [LaX6]3− environments are apparent, with one iodine acting as an acceptor to five different donor hydrogens. However, the C⋯I distances (Table 3) are not notably different between the EMIM and EDMIM compounds.
Compound | δ(1H)a/ppm | |
---|---|---|
N–CH–N | Δ | |
a 6.8 mM solutions in CD3CN. b Change in shift compared with [EMIM]Cl at the same concentration in CD3CN. | ||
[EMIM]3[LaCl6] | 9.27 | +0.45 |
[EMIM]3[LaBr6] | 8.93 | +0.11 |
[EMIM]3[LaI6] | 8.61 | −0.21 |
[EMIM]3[CeCl6] | 9.77 | +0.95 |
[EMIM]3[YCl6] | 9.26 | +0.44 |
[EMIM]3[ScCl6] | 9.41 | +0.59 |
[EMIM]Cl | 8.82 | — |
[EMIM]Br | 8.84 | — |
From the NMR data it is apparent that the chemical shift of the signal associated with the H2 proton shifts to low frequency for the heavier halides. This correlates with decreasing strength of the solution phase hydrogen bonding interactions upon changing from Cl to I. The chemical shifts of the less acidic H4/5 protons are not affected by changing the halide, suggesting that the salts are completely dissociated in solution. This is consistent with the trend observed for [EMIM][GeX3] (X = Cl, Br, I).3
For the [MX6]3− salts, 45Sc, 89Y and 139La NMR spectra were also obtained (Table 5). All complexes were soluble in MeCN at approximate 10–15 mM concentration. The [NnBu4]+ salt was soluble at a similar concentration in CH2Cl2, whereas the [EMIM]+, [BMPYRR]+ and [EDMIM]+ salts were poorly soluble in CH2Cl2. For the [LaCl6]3− salts a small high frequency shift in δ139La occurs as the cation changes along the series: [NnBu4]+ → [BMPYRR]+ → [EDMIM]+ → [EMIM]+ in both MeCN and CH2Cl2 solution, but given the large 139La NMR chemical shift range, these differences may not be significant. The 139La NMR resonance shifts to high frequency as the halide becomes heavier; the [LaI6]3− system showing a very broad resonance at ∼1400 ppm (W1/2 ∼ 12000 Hz). The yttrium salts show no 89Y NMR resonance at room temperature in MeCN, presumably due to chloride exchange. However, cooling to 233 K reveals a sharp singlet for each compound.
Salt | δ (ppm) | W 1/2 (Hz) | δ (ppm) | W 1/2 (Hz) |
---|---|---|---|---|
MeCN | CH2Cl2 | |||
a Spectrum recorded at 233 K. b n.o. = not observed due to low solubility. | ||||
[NnBu4]3[LaCl6] | 846 | 800 | 872 | 1200 |
[BMPYRR]3[LaCl6] | 855 | 500 | 880 | 300 |
[EDMIM]3[LaCl6] | 864 | 600 | n.o.b | — |
[EMIM]3[LaCl6] | 867 | 400 | 892 | 300 |
[EMIM]3[LaBr6] | 1095 | 1200 | n.o.b | — |
[EMIM]3[LaI6] | ∼1400 | 12![]() |
n.o.b | — |
[EMIM]3[YCl6]a | 407 | — | — | — |
[EDMIM]3[YCl6]a | 401 | — | — | — |
[EMIM]3[ScCl6] | 254 | 170 | — | — |
Substitution of the H2 proton with a Me group in the EDMIM cation has a significant influence on the cation–anion interactions in the solid state. The structure of [EDMIM]3[LaCl6] revealed five associated [EDMIM]+ cations and the C⋯Cl distances involving H4/H5 were shorter than those for [EMIM]3[LaCl6].
The solution 1H NMR trends indicate that the hydrogen bonding to the EMIM (H2) cations becomes weaker as the halide becomes heavier, while the EDMIM salts appear to be completely dissociated in solution.
Compound | [EMIM]3[ScCl6] | [EMIM]3[YCl6] | [EMIM]3[CeCl6] | [EMIM]3[LaBr6] |
---|---|---|---|---|
a Common items: wavelength (Mo-Kα) = 0.71073 Å; θ(max) = 27.5°. b R 1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑w(Fo2 − Fc2)2/∑wFo4]1/2. | ||||
Formula | C18H33Cl6N6Sc | C18H33Cl6N6Y | C18H33CeCl6N6 | C18H33Br6LaN6 |
M/g mol−1 | 591.16 | 635.11 | 686.32 | 951.87 |
Temp./K | 150(2) | 100(2) | 100(2) | 100(2) |
Crystal system | Orthorhombic | Orthorhombic | Monoclinic | Monoclinic |
Space group (no.) | Pna21 (33) | Pna21 (33) | P21/c (14) | P21/c (14) |
a/Å | 19.411(2) | 19.580(2) | 15.492(3) | 15.925(5) |
b/Å | 13.059(1) | 13.036(2) | 12.549(2) | 12.787(4) |
c/Å | 11.046(1) | 11.166(1) | 14.708(3) | 15.252(5) |
α/° | 90 | 90 | 90 | 90 |
β/° | 90 | 90 | 90.482(3) | 90.588(5) |
γ/° | 90 | 90 | 90 | 90 |
U/Å3 | 2800.0(4) | 2849.9(5) | 2859.2(9) | 3106(2) |
Z | 4 | 4 | 4 | 4 |
μ(Mo-Kα) mm−1 | 0.853 | 2.626 | 2.170 | 9.111 |
F(000) | 1224 | 1296 | 1372 | 1800 |
Total reflections | 15![]() |
14![]() |
12![]() |
14![]() |
Unique reflections | 5189 | 6374 | 5811 | 6318 |
R int | 0.027 | 0.037 | 0.122 | 0.092 |
Goodness-of-fit on F2 | 1.023 | 0.796 | 0.962 | 1.145 |
R 1 [Io > 2σ(Io)] | 0.026 | 0.028 | 0.063 | 0.082 |
R 1 (all data) | 0.029 | 0.043 | 0.090 | 0.116 |
wR2b [Io > 2σ(Io)] | 0.063 | 0.045 | 0.167 | 0.134 |
wR2 (all data) | 0.064 | 0.048 | 0.187 | 0.148 |
Compound | [EMIM]3[LaI6] | [EDMIM]3[YCl6] | [EDMIM]3[LaCl6] | [EDMIM]3[LaI6] |
---|---|---|---|---|
Formula | C18H33I6LaN6 | C21H39Cl6N6Y | C21H39Cl6LaN6 | C21H39I6LaN6 |
M/g mol−1 | 1233.81 | 677.19 | 727.19 | 1275.89 |
Temp./K | 120(2) | 100(2) | 100(2) | 100(2) |
Crystal system | Monoclinic | Monoclinic | Monoclinic | Triclinic |
Space group (no.) | P21/c (14) | P21/n (14) | P21/n (14) |
P![]() |
a/Å | 18.803(7) | 9.9170(8) | 10.027(2) | 10.347(2) |
b/Å | 11.133(2) | 16.274(1) | 16.306(2) | 11.075(3) |
c/Å | 17.017(2) | 19.028(2) | 19.133(3) | 16.577(3) |
α/° | 90 | 90 | 90 | 92.594(3) |
β/° | 102.162(7) | 99.973(1) | 99.265(3) | 90.255(3) |
γ/° | 90 | 90 | 90 | 106.930(4) |
U/Å3 | 3482(1) | 3024.6(4) | 3087.4(9) | 1815.1(7) |
Z | 4 | 4 | 4 | 2 |
μ(Mo-Kα) mm−1 | 6.562 | 2.480 | 1.924 | 6.299 |
F(000) | 2232 | 1392 | 1464 | 1164 |
Total reflections | 34![]() |
16![]() |
26![]() |
16![]() |
Unique reflections | 7929 | 6878 | 7055 | 8311 |
R int | 0.059 | 0.027 | 0.020 | 0.028 |
Goodness-of-fit on F2 | 1.073 | 1.028 | 1.078 | 0.984 |
R 1 [Io > 2σ(Io)] | 0.031 | 0.028 | 0.023 | 0.019 |
R 1 (all data) | 0.036 | 0.039 | 0.026 | 0.021 |
wR2b [Io > 2σ(Io)] | 0.075 | 0.064 | 0.055 | 0.041 |
wR2 (all data) | 0.078 | 0.067 | 0.057 | 0.042 |
Structure solution and refinement were straightforward using WinGX and software packages within,16 except as detailed below. Although Q-peaks corresponding to the location of protons were observed in the Fourier difference map, hydrogen atoms were placed in geometrically assigned positions with C–H distances of 0.95 Å (CH), 0.98 Å (CH3) or 0.99 Å (CH2) and refined using a riding model, with Uiso(H) = 1.2Ueq(C) (CH, CH2) or 1.5Ueq(C) (CH3). Mercury17and enCIFer18 were used to prepare material for publication.
All the crystals of [EMIM]3[CeCl6] looked at in this study were multiply twinned and all attempts at multi-component twin integration failed. A satisfactory dataset was obtained by integrating on just the major component and ignoring the minor overlaps from the other domains. This caused a couple of large Q-peaks to appear close to the cerium atoms (3.2 e Å−3 at ∼1.04 Å distance from Ce) as well as two large holes (−5.9 e Å−3 at ∼0.84 Å from Ce). This explains the level A and B CheckCIF errors PLAT971_ALERT_2_A (holes) and PLAT971_ALERT_2_B (Q-peaks). The crystals of [EMIM]3[LaBr6] were all also multi-component twins and treated in a similar fashion.
Footnote |
† CCDC 1472564 ([EMIM]3[ScCl6]), 1472565 ([EMIM]3[YCl6]), 1472566 ([EMIM]3[CeCl6]), 1472567 ([EMIM]3[LaBr6]), 1472568 ([EMIM]3[LaI6]), 1472569 ([EDMIM]3[YCl6]), 1472570 ([EDMIM]3[LaCl6]) and 1472571 ([EDMIM]3[LaI6]). For crystallographic data in CIF or other electronic format see DOI: 10.1039/c6nj01068g |
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