Unexpected Halide Transfer: Aluminium and the Lanthanoids

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Unexpected Halide Transfer: Complex Reorganisation Between Aluminium and the Lanthanoids.

  • Glen B. Deacon, David J. Evans and Peter C. Junk.*

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[Pr(MeCN)9][AlCl4]3.MeCN undergoes reorganisation upon the addition of an ether. In the case of recrystallisation from tetrahydrofuran, the ionic nature is lost, whereas the addition of crown ether gives reorganisation, whilst maintaining ionic character.

Isolation of homoleptic ionic trivalent lanthanoid complexes, under non aqueous conditions, has been investigated using nitrogen based ligand systems 1-5. The interest surrounding these homoleptic complexes is attributed to their potential catalytic properties 3,5. Under non aqueous conditions, the use of highly labile ligands, such as solvent molecules, presents the possibility of exposing the metal centre, hence providing a site for catalysis and thus can be considered to be ‘near naked’ 3. Complexes involving Ln3+ ions, that can be considered ‘near naked’ have to date been restricted to complexes such as [Ln(MeCN)n]3+ , with anions such as AsF6 and AlCl4 3-5.

With this in mind, we have investigated the ability to access homoleptic ‘near naked’ Ln3+ complexes with tetrahydrofuran (thf) ligands. Currently, no such complexes have been reported for the smaller trivalent species unlike the larger divalent species, for which there is precedent viz. [Sm(thf)7][BPh4]2 6. Exploitation of the coordination abilities of crown ether has been investigated with the isolation of [ScCl2(18-crown-6)][FeCl4]. Via Sc n.m.r it has been shown that [ScCl(thf)(18-crown-6)][FeCl4]2 and then subsequently [Sc(thf)2(18-crown-6)][FeCl4]3 can be synthesised even though it has not been structurally characterised. With this in mind it should therefore be possible to isolate similar adducts in MeCN.

Results and Discussion

Homoleptic acetonitrile Ln3+ complexes can be obtained via two pathways viz equations 1 and 2 3. It was our intention to extend this chemistry to involve homoleptic Ln3+ complexes with ether ligands in place of MeCN. In reactions analogous to equations 1 and 2 with thf in place of MeCN, we found to our surprise [LnCl3(thf)2]n (Ln = Pr, Nd) was the sole Ln complex isolatable. This suggests that the complex is formed by a concerted process whereby AlCl4 binds to Ln3+ releasing AlCl3, allowing binding of another AlCl4 and so on until complete halide transfer to Ln3+ occurs yielding LnCl3(thf)n (equations 3, 4). Similarly, addition of 18-crown-6 to [Pr(MeCN)9][AlCl4]3 resulted in reorganisation to [(PrCl(–Cl)(18-crown-6))2][AlCl4]2.2(MeCN) (1) .

Isolation of 1 illustrates there is an equilibrium in solution involving [Pr(MeCN)9][AlCl4]3.(MeCN). Conductivity measurements show a 1:3 electrolyte 7.This is in contrast to that previously reported for the Sm complex by Hu and supported by Bünzli for which a 1:2 is electrolyte is reported 4,8. We believe that the complex [Ln(MeCN)9][AlCl4]3.(MeCN) undergoes rearrangement in solution ranging from a 1:3 down to a 1:2 electrolyte (equation 5).

This change in coordination environment of the lanthanoid metal establishes the pathway to halide transfer involving a transient species related to that shown in Figure 1. Structural motifs similar to this have been observed for several lanthanoid complexes including [Sm(ή6-C6Me6)(AlCl4)3] .toluene 9,10. The reaction is completed by the substitution of MeCN by the crown ether and cleavage of the bridging Al–Cl bonds in a similar fashion to that observed for reactions involving thf.

Complex 1 has a nine coordinate Pr centre that is bound to all six oxygens of the crown ether. The Pr is also bound to one terminal and a bridging chloride, and dimerises through an inversion centre. There is a distinct change in bond lengths between the terminal (Pr-Clter 2.715(2)Å) and bridging chlorides (Pr-Clbr 2.839(2) and 2.858(2)Å) as would be expected with similar changes identified in [PrCl(-Cl)(tetraethyleneglycol)]2 11. The distances for Pr-Ocrown range from 2.572(4) – 2.590(7)Å, following the same trends in the related cation [(DyCl(–Cl)(dibenzo18-crown-6))2][(DyCl3(–Cl)(MeCN))2] 12, albeit with a lengthening of Ln-O in line with increased ionic radius between Dy and Pr.

The crown ethers adopt a saddle type morphology with the metal residing in almost the centre of the cavity made by the O1, O3, O4, O6 (0.601Å) plane and the O2, O5 (0.491Å) plane. The crown ether collapses to accommodate the smaller size of the Pr3+ which is evident in the planes derived by the oxygen atoms of the crown. The angle between plane 1 (O1, O2, O5, O6) and plane 2 (O2, O3, O4, O5) is 125.71o showing this slight closure to ensure that the oxygen atoms are all bound. This closure of the crown ether is observed for all the Ln3+ 18-crown-6 complexes in which the angle closes from 129.74o in complex [LaCl3(18-crown-6)] 13 through to 68.95o in [Lu(CH2(SiCH3))2(18-crown-6)][(CH2(SiCH3))B(C6H5)3].C2H4Cl2 14 owing to the reduction in size of the ionic radius of the Ln centre.

Notes and references

All reactions were carried out under dry nitrogen using dry box and standard Schlenk techniques. Solvents were dried by distillation from sodium wire/benzophenone (thf) or CaH/P2O5 (MeCN). IR and far IR data were obtained as described previously 15. Metal analyses were carried out by complexiometric EDTA titration with the addition of 5% sulphosalicylic acid to mask Al 16. Anhydrous AlCl3, LnCl3, and 18-crown-6 were supplied by Sigma Aldrich. AlCl3 was freshly sublimed prior to use. Conductivity measurements were carried out on a Crison Conductimeter 522 (serial no; 3807), using a locally manufactured air-sensitive cell. The complex [Pr(MeCN)9] [AlCl4]3 was made using previous published methods 3 and conductivity measurements were carried out as mentioned above (367.97 S cm2 mol-1 1.097 x 10-3 mol dm-3, MeCN).

1: Method A: [Pr(MeCN)9][AlCl4]3. MeCN (0.20g, 0.19 mmol) and 18-crown-6 (0.20g, 0.57 mmol), was dissolved in MeCN (30 ml). The solution was stirred and heated to near boiling to assist dissolution. The resulting green solution was then filtered and reduced in-vacuo. The solution was then cooled at -30oC yielding small green crystals. (0.21 g (81%)). m.p. 170oC(dec), C28H54Al2Cl12N2O12Pr2; calcd. Pr 10.27; found Pr 10.68%. I.r absorption (Nujol): cm-1. Unit cell collection confirms the same product as via method A.

Method B: A mixture of PrCl3 (0.10 g, 0.40 mmol), AlCl3 (0.16 g, 1.20 mmol) and 18-crown-6 (0.29g, 0.83 mmol), was dissolved in MeCN (30 ml). The solution was stirred and heated to near boiling to assist dissolution. The resulting green solution was then filtered and reduced in-vacuo. The solution was then cooled at -30oC yielding small green crystals. (0.44 g (87%)). m.p. 170oC(dec), C28H54Al2Cl12N2O12Pr2; calcd. Pr 10.27; found Pr 10.42% I.r absorption (Nujol): 2291w, 2253s, 1644w, 1353s, 1291s, 1248s, 1082s, 1034s, 966s, 925w, 878w, 837s, 802w cm-1. 27Al nmr: 104 ppm(AlCl4)


X-ray data for complex 1 was collected on a Nonius Kappa CCD, MoK radiation, = 0.71073 Å, T = 123(2)K. The structure was solved and refined using the programs SHELXS-97 17 and SHELXL-97 18 respectively. The program X-Seed 19 was used as an interface to the SHELX programs, and to prepare the figures. 1: [(Pr(Cl2)(C12H24O6))2][AlCl4]2.2(C2H3N): C28H54Al2Cl12N2O12Pr2, M = 1371.91, green prismatic, 0.40 ï‚´ 0.40 ï‚´ 0.30 mm, monoclinic, space group P21/n (No. 14), a = 12.377(3), b = 15.356(3), c = 14.387(3) Å, = 107.97(3)°, V = 2601.0(9) Å3, Z = 2, Dc = 1.752 g/cm3, F000 = 1360, Nonius Kappa CCD, MoK radiation, = 0.71073 Å, T = 123(2)K, 2max = 56.6º, 20600 reflections collected, 6215 unique (Rint = 0.0864). Final GooF = 1.022, R1 = 0.0478, wR2 = 0.1052, R indices based on 4182 reflections with I >2sigma(I) (refinement on F2), 263 parameters, 0 restraints. Lp and absorption corrections applied, = 2.551 mm-1.

Fig. 2 The structure of the cation [{PrCl(-Cl)(18C6)}2]2+. Hydrogen atoms omitted for clarity. Thermal ellipsoids shown at 35%. Coordination environment of the atom Pr(1) with applicable bond lengths (Å) and angles(o). Symmetry transformations used to generate equivalent atoms: -x+1,-y+1,-z+1. Pr(1) – O(1), O(2), O(3), O(4), O(5), O(6), Cl(1), Cl(2), Cl(2)’, 2.572(4), 2.579(4), 2.574(3), 2.590(4), 2.588(4), 2.587(6), 2.715(2), 2.839(2), 2.858(2). Cl(1)-Pr(1)-Cl(2), Cl(2)’, 144.30(4), 143.18(4), Cl(2) – Pr – Cl(2)’,72.52(4).

[Ln(MeCN)9][AlCl4]3  [Ln(MeCN)n(AlCl4)][AlCl4] + 2AlCl3 + (9-n)MeCN


Fig.1 Proposed cation structure observed prior to ether coordination and subsequent cleavage of Al– Cl bonds.

[LnCl3(MeCN)n]+ mthf  [LnCl3 (thf)m] + (n)MeCN


2Ln + 6AlCl3 + 3 C2Cl6 + nMeCN  2[Ln(MeCN)n][AlCl4]3 + 3C2Cl4


LnCl3 + AlCl3 + nMeCN  [Ln(MeCN)n][AlCl4]3


[Ln(MeCN)9][AlCl4]3  [LnCl3(MeCN)n] + 3AlCl3 (9-n)MeCN


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