Divalent ytterbium complexes with crown and heterocrown ethers

Abstract

Several unusual complexes of ytterbium(II) stabilised by coordination to mixed O₄X₂-donor macrocyclic ligands (X = O, NH, S or Se) are described. Distorted 8-coordination is evident from the crystal structure of the neutral [YbI₂([18]aneO₄Se₂)], forming the first reported example of Yb(II)-selenoether coordination.

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The chemistry of the lanthanides (4f elements) is dominated by the 3+ oxidation state,¹ and apart from the fluorides and oxides of Pr⁴⁺ and Tb⁴⁺, only cerium forms an extensive range of compounds in the 4+ state.^1–3 For many years lower oxidation states were limited to some divalent samarium, europium and ytterbium complexes, and compounds such as LaI₂ or GdS, which were formulated as “metallic” Ln(III) species with the extra electron delocalised in a conduction band.² Early uses of Eu(II) as a one electron reductant were followed by the development of a similar role for SmI₂ and [(Cp*)₂Sm(thf)₂].⁴ Recent work aimed at developing suites of lanthanide(II) complexes with varying reactivities for use in organic reductions, couplings, polymerisation and dinitrogen activation has led to new organometallic complexes of all the M(II) (except for Pm) as [K(macrocycle)][Ln(C₅H₄SiMe₃)₃] (macrocycle = 18-crown-6 or 222-crypt),§ although samarium(II) reagents remain the most widely used in organic chemistry.⁵

We recently reported studies of Ln(III) complexes of oxa-thia, oxa-selena macrocycles and crown ethers, focused on La and Lu complexes, including X-ray structural characterisation of rare examples of Ln-thioether and the first examples of Ln-selenoether coordination.⁶ The metal iodides were used for much of this work,⁶ since for both the lanthanide and some early transition metal complexes⁷ the metal iodides are more soluble than lighter halide analogues in modestly polar organic solvents, presumably due to lower lattice energies, which facilitates synthesis and solution studies. Here we report studies of similar macrocyclic ligand complexes of ytterbium(II) iodide. The f¹⁴ Yb(II) permits direct comparison with the isoelectronic Lu(III) complexes,⁶ as well as with recently reported complexes of these ligands with Ca and Sr,⁸ whilst its diamagnetism aids solution NMR studies.

Reaction of anhydrous YbI₂ with the ligands 18-crown-6, [18]aneO₄S₂, [18]aneO₄Se₂ and [18]aneO₄N₂ in anhydrous MeCN gave modest yields of white to pale yellow species, [YbI₂(ligand)] (Scheme 1); the formulation was confirmed by microanalysis.

Scheme 1

The IR spectra showed the presence of the ligand and absence of MeCN (and H₂O). The complexes are moderately soluble in anhydrous MeCN, and very readily hydrolysed and oxidised by exposure to air. Very small crystals of [YbI₂([18]aneO₄Se₂)] were obtained after storing an MeCN solution of the complex in a freezer (−18 °C).

The complex has crystallographic two-fold symmetry and is isomorphous with [CaI₂([18]aneO₄Se₂)],⁸ based upon hexadentate coordination of the macrocycle, with two mutually cis iodide ligands completing the distorted 8-coordination at Yb(II) (Fig. 1). The bond lengths and angles are also very similar to the Ca(II) species. This is as expected given their similar ionic radii (Yb²⁺ = 1.16 Å, Ca²⁺ = 1.14 Å).⁹ The neutral [YbI₂([18]aneO₄Se₂)] molecule is also structurally very similar to the cation in the isoelectronic [LuI₂([18]aneO₄Se₂)]I·2MeCN,⁶ albeit with slightly shorter corresponding bonds (by ∼0.1–0.15 Å) in the latter, reflecting the smaller radius resulting from the higher charge (Lu³⁺ ionic radius 1.00 Å). In the Yb(II) complex, the Se–M–Se angle is some 13° wider than in the Lu(III) analogue, and the I–M–I angle ∼5° wider. The larger La³⁺ ion (ionic radius = 1.22 Å)⁹ forms nine-coordinate [LaI₃(macrocycle)] with these hexadentate macrocycles.⁶

Fig. 1 View of the structure of [YbI₂([18]aneO₄Se₂)] showing the atom labelling scheme. The displacement ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Symmetry operation: i = −x, y, 1/2 − z. Selected bond lengths (Å) and angles (°): Yb1–I1 = 3.1699(5), Yb1–Se1 = 3.1036(7), Yb1–O1 = 2.624(3), Yb1–O2 = 2.472(3), O1–Yb1–O1ⁱ = 117.98(14), O1–Yb1–O2 = 76.42(10), O1ⁱ–Yb1–Se1 = 65.00(7), O2ⁱ–Yb1–Se1 = 70.00(7), Se¹–Yb1–Se1ⁱ = 152.86(2), I1–Yb1–I1ⁱ = 100.488(19).

Although we were unable to obtain diffraction-quality crystals of [YbI₂(18-crown-6)], [YbI₂([18]aneO₄S₂)] or [YbI₂([18]aneO₄N₂)], on the basis of microanalytical and spectroscopic data it seems highly probable that they also contain eight-coordinate Yb(II).

The complexes are very poorly soluble in CH₂Cl₂, however, they are moderately soluble in MeCN, affording yellow solutions. ¹H NMR spectra of [YbI₂([18]aneO₄S₂)] and [YbI₂([18]aneO₄Se₂)] in anhydrous CD₃CN show broad OCH₂ and S(Se)CH₂ resonances, with smaller coordination shifts than observed in the corresponding Lu(III) cations.⁶ We were unable to observe a ⁷⁷Se NMR resonance from [YbI₂([18]aneO₄Se₂)], contrasting with [LuI₂([18]aneO₄Se₂)]I which exhibited a sharp resonance with a low frequency coordination shift (Δ) of −32 ppm. The NMR data are consistent with the macrocycles exhibiting dynamic processes in solution and weaker binding to the Yb(II), compared with the isoelectronic Lu(III) analogues. In 10⁻³ mol dm⁻³ solutions in anhydrous MeCN, the complexes have high molar conductivities, in the range characteristic of 1 : 2 electrolytes,¹⁰ which suggests that like the Ln(III) analogues,⁶ the iodide ligands are displaced by MeCN in dilute solution, although MeCN is not present in the solid complexes isolated from this solvent.

Cyclic voltammetry experiments were performed to probe the redox responses from these complexes. Owing to their poor solubility in non-coordinating solvents such as CH₂Cl₂, electrochemistry experiments were initially attempted using 0.1 mol dm⁻³ [NⁿBu₄]I in anhydrous MeCN as supporting electrolyte. However, the presence of excess iodide significantly suppressed the solubility of the complexes. Therefore, subsequent experiments used 0.1 mol dm⁻³ [NⁿBu₄][BF₄]–MeCN as supporting electrolyte. Under these conditions the complexes each show voltammetric peaks attributed to the electro-oxidation of free iodide. The relative peak heights indicated the liberation of one iodide ion per Yb(II) complex. No other peaks associated with Yb(II)/(III) redox couples were evident under these conditions over the potential range −2 to +1.5 V.

Experimental

Details of the spectroscopic and electrochemical measurements and X-ray crystallography are given in the ESI.‡

Synthesis

[YbI₂(18-crown-6)]. YbI₂ (0.075 g, 0.176 mmol) was dissolved in acetonitrile (10 mL). 18-crown-6 (0.047 g, 0.176 mmol) dissolved in acetonitrile (10 mL) was then added and the reaction mixture was stirred for ca. 10 min. The solvent was removed in vacuo and the residual very pale yellow solid was washed with hexane (10 mL) and dried in vacuo. Yield 0.102 g, 84%. Required for C₁₂H₂₄I₂O₆Yb (691.2): C, 20.85.19; H, 3.50. Found C, 21.02; H, 3.39%. ¹H NMR (CD₃CN, 300 K): δ 3.80 (s) ppm. Λ_M (MeCN 10⁻³ mol dm⁻³) = 189 Ω⁻¹ cm² mol⁻¹.

[YbI₂([18]aneO₄S₂)]. YbI₂ (0.050 g, 0.117 mmol) was dissolved in acetonitrile (10 mL). [18]aneO₄S₂ (0.035 g, 0.117 mmol) also dissolved in acetonitrile (10 mL) was then added and the reaction mixture was stirred for ca. 10 min. After reducing the solvent volume by a quarter in vacuo, toluene (20 mL) was added. This led to precipitation of a small amount of off-white precipitate. The flask was then placed in the freezer for ca. 2 h which increased the amount of precipitate. This was collected by filtration and dried in vacuo. Yield: 0.030 g, 36%. Required for C₁₂H₂₄I₂O₄S₂Yb (723.3): C, 19.93; H, 3.34. Found C, 19.82; H, 3.26%. ¹H NMR (CD₃CN, 300 K): δ 3.80 (br s, [16H] OCH₂), 2.80 (br s, [8H], SCH₂CH₂O) ppm. Λ_M (MeCN 10⁻³ mol dm⁻³) = 220 Ω⁻¹ cm² mol⁻¹.

[YbI₂([18]aneO₄Se₂)]. Made similarly to the [18]aneO₄S₂ complex above, using YbI₂ (0.050 g, 0.117 mmol) and [18]aneO₄Se₂ (0.046 g, 0.117 mmol). Pale yellow solid. Yield: 0.030 g, 32%. Required for C₁₂H₂₄I₂O₄Se₂Yb (817.1): C, 17.64; H, 2.96. Found C, 17.49; H, 2.85%. ¹H NMR (CD₃CN, 300 K): δ 3.83 (br t, J = 6 Hz, [8H], SeCH₂CH₂O), 3.72 (s, [8H], OCH₂CH₂O), 2.84 (t, J = 6 Hz, [8H], SeCH₂CH₂O) ppm. ⁷⁷Se NMR (CH₃CN, 295 K and 230 K): not observed. Crystals were obtained by storing an MeCN solution of the complex for several weeks in a freezer at −18 °C. Λ_M (MeCN 10⁻³ mol dm⁻³) = 255 Ω⁻¹ cm² mol⁻¹.

[YbI₂([18]aneO₄N₂)]. YbI₂ (0.050 g, 0.117 mmol) was dissolved in acetonitrile (10 mL). [18]aneO₄N₂ (0.031 g, 0.117 mmol) was also dissolved in acetonitrile (10 mL) and added to the YbI₂ solution. The reaction mixture was stirred for ca. 10 min. by which time a small amount of pale yellow precipitate had formed. The solvent volume was reduced by 50% in vacuo, increasing the amount of precipitate. This was then collected by filtration and dried in vacuo. Yield: 0.044 g, 54%. Required for C₁₂H₂₆I₂N₂O₄Yb (689.2): C, 20.91; H, 3.80, N, 4.06. Found: C, 20.87; H, 3.73; N, 4.13%. IR (Nujol mull): 3220 cm⁻¹ νNH. ¹H NMR (CD₃CN, 300 K): δ 3.78 (v.br m, [8H] OCH₂), 3.59 (v.br m, [8H] OCH₂CH₂N), 2.95 (v.br s, [4H] CH₂N), 2.68 (v.br s, [4H], CH₂N), 2.40 (v.br s, [2H], NH) ppm. Λ_M (MeCN 10⁻³ mol dm⁻³) = 225 Ω⁻¹ cm² mol⁻¹.

Conclusions

The synthesis, spectroscopic and structural characterisation of rare heterocrown complexes of a divalent lanthanide, specifically Yb(II), has been achieved. Coordination of soft thioether and selenoether functions to Yb(II) has been demonstrated. The properties of these complexes are intermediate between those of Ln(III) and Ca(II)/Sr(II) analogues. More generally, although the coordination of the soft donors is clearly aided by the presence of O-donors in the macrocycle, the study suggests that a wider range of coordination complexes derived from divalent lanthanides with neutral ligands, including other soft Lewis bases, should be accessible with judicious choice of solvent, reaction conditions and anion.

Acknowledgements

We thank EPSRC for support (EP/1033394/1). The SCFED Project (http://www.scfed.net) is a multidisciplinary collaboration of British universities investigating the fundamental and applied aspects of supercritical fluids.

Notes and references

1. S. Cotton, in Comprehensive Coordination Chemistry II, ed. J. A. McCleverty and T. J. Meyer, Elsevier, Oxford, 2004, vol. 4, p. 93.
2. D. A. Johnson, Adv. Inorg. Chem. Radiochem., 1977, 20, 1.
3. N. A. Piro, J. R. Robinson, P. J. Walsh and E. J. Schelter, Coord. Chem. Rev., 2014, 260, 21.
4. (a) W. J. Evans, Coord. Chem. Rev., 2000, 206–207, 263; (b) M. N. Bochkarev, Coord. Chem. Rev., 2004, 248, 835.
5. (a) M. Szostak and D. J. Procter, Angew. Chem., Int. Ed., 2012, 51, 9238; (b) M. R. MacDonald, J. E. Bates, J. W. Ziller, P. Furche and W. J. Evans, J. Am. Chem. Soc., 2013, 135, 9857.
6. M. J. D. Champion, P. Farina, W. Levason and G. Reid, Dalton Trans., 2013, 42, 13179.
7. (a) W. Levason, M. L. Matthews, B. Patel, G. Reid and M. Webster, Dalton Trans., 2004, 3305; (b) S. D. Reid, A. L. Hector, W. Levason, G. Reid, B. J. Waller and M. Webster, Dalton Trans., 2007, 4769.
8. P. Farina, W. Levason and G. Reid, Dalton Trans., 2013, 42, 89.
9. F. A. Cotton, G. Wilkinson, C. A. Murillo and M. Bochmann, Advanced Inorganic Chemistry, Wiley, New York, 6th edn, 1999.
10. 1 : 1 Electrolytes have Λ_M (MeCN 10⁻³ mol dm⁻³) ∼120–160 and 1 : 2 electrolytes ∼220–300 Ω⁻¹ cm² mol⁻¹. W. J. Geary, Coord. Chem. Rev., 1971, 6, 81.

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Footnotes

† Dedicated to the memory of Professor Ken Wade FRS in recognition of his many contributions to inorganic chemistry.

‡ Electronic supplementary information (ESI) available: Further experimental and crystallographic details for the structure of [YbI₂([18]aneO₄Se₂)]. CCDC 1024072. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt03462g

§ [18]aneO₄S₂ = 1,4,10,13-tetraoxa-7,16-dithiacyclooctadecane, [18]aneO₄Se₂ = 1,4,10,13-tetraoxa-7,16-diselenacyclooctadecane, [18]aneN₂O₄ = 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane, 18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane.

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