Gallyl lanthanide complexes containing unsupported Ln–Ga (Ln = Sm, Eu, Yb or Tm) bonds

Abstract

A series of bis(gallyl) lanthanide(II) complexes, [LnII{Ga[(ArNCH)₂]}₂(tmeda)₂] (Ln = Sm, Eu or Yb; Ar = C₆H₃Prⁱ₂-2,6), and a gallyl thulium(III) compound, [Tm^III{Ga[(ArNCH)₂]}{(ArNCH)₂}(tmeda)], have been prepared; the first structural studies of Ga–Tm and Ga–Sm bonded complexes are described.

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The chemistry of complexes containing metal–metal bonds is of much importance, both from fundamental and applications standpoints.¹ With this in mind, in recent years we have been systematically examining the coordination and further chemistry of the anionic gallium(I) heterocycle, [:Ga(ArNCH)₂]⁻1,² which is a valence isoelectronic analogue of the important N-heterocyclic carbene (NHC) class of ligand. The majority of complexes derived from this ligand have been with p-³ or d-block⁴ metal fragments and these have highlighted similarities, but also differences, between 1 and both NHCs and cyclic boryl ligands. In 2006, this work was extended to group 2 with the preparation of compounds 2 and 3,⁵ which exhibited the first examples of gallium-group 2 bonds in molecular species. The next year, and in collaboration with Arnold et al., the first gallium–lanthanide bonded complex, 4, was prepared and shown by X-ray crystallographic and theoretical studies to have an unsupported, polar-covalent Nd–Ga bond.⁶ Wiecko and Roesky later reported the only other gallium–lanthanide complexes, [Cp*₂Eu(GaCp*)₂] and [Cp*₂Yb(GaCp*)(THF)] (Cp* = C₅Me₅⁻),^7,8 though unlike 4, these contain weak Ga^I→ Ln^II donor–acceptor bonds.

There is no known further chemistry for Ln–Ga bonded compounds. For this to advance with any pace, it seemed that the development of systems containing polar-covalent lanthanide(II)–gallium bonds would be necessary. Once such compounds are accessed, it would be of considerable interest to compare their redox chemistry with that of widely explored lanthanide(II) reducing agents such as SmI₂⁹ and SmCp*₂.¹⁰ Here we detail preliminary results towards this end, namely the synthesis of three bis(gallyl) lanthanide(II) compounds and a related gallyl thulium(III) complex. X-Ray crystallographic studies on these compounds have led to the first structural elucidations of Ga–Sm and Ga–Tm bonds.

Early in this study, attempts were made to prepare complexes of the type [Ln{Ga[(ArNCH)₂]}₂(THF)_n] (Ln = Sm, Eu or Yb) by reduction of the digallane(4), [{Ga^II[(ArNCH)₂]}₂], or the paramagnetic gallium(III) halide complex, [GaI₂{(ArNCH)₂}], with the elemental lanthanide in THF. Although this methodology was successfully used to prepare the group 2 complexes, 2 and 3, no reaction occurred with the lanthanide metals. Salt elimination reactions of [K(tmeda)][Ga{(ArNCH)₂}] with half an equivalent of LnI₂ in THF were then investigated, but all led to intractable mixtures of products. However, when the reactions were repeated in the presence of an excess of tmeda, the bis(gallyl) lanthanide complexes, 5 (dark green), 6 (orange) and 7 (red–orange), were reproducibly obtained in low to moderate isolated yields (Scheme 1).†

Scheme 1 Reagents and conditions: i, LnI₂(THF)_n, toluene/tmeda, −KI; ii, TmI₂(THF)₅, toluene/tmeda, −KI, −Ga_(s).

After Eu²⁺, Yb²⁺ and Sm²⁺, Tm²⁺ is the next most stable lanthanide dication (E° Tm³⁺/Tm²⁺ = −2.22 V) and has a rapidly emerging molecular chemistry.¹¹ Given the thermal stability of 5–7, and the previously demonstrated reducing nature of the gallium heterocycle, [:Ga{(ArNCH)₂}]⁻,^3,4 it was thought possible that the thulium analogue of 5–7 might be a stable entity at room temperature. However, a comparable reaction with TmI₂ reproducibly led to a low isolated yield of the red thulium(III)–gallyl complex, 8, the coordination sphere of which is completed by a molecule of tmeda and a doubly reduced diazabutadiene ligand. It seems likely that the intermediate in the reaction that gave 8 is the target thulium(II) complex, [Tm{Ga[(ArNCH)₂]}₂(tmeda)₂], which was subject to an intramolecular reduction of one gallyl ligand by the thulium centre, leading to elimination of gallium metal.

Little useful information could be obtained from the ¹H NMR spectra of 5, 6 and 8 due their paramagnetic nature.¹² The spectrum of 7 is, however, more informative and displays a major set of resonances that is consistent with its solid state structure (vide infra). It also exhibits a more complex, minor set of resonances which we believe corresponds to the cis-isomer of the compound (major isomer : minor isomer ratio is ca. 85 : 15). The most compelling evidence for this proposal is that there are two chemically inequivalent sets of backbone protons for the gallyl ligands which resonate as an AB spin system. The fact that these protons are inequivalent suggests that the bulky heterocyclic ligands of the complex are “interlocked” and cannot rotate freely with respect to each other. Very similar spectra have been observed for square planar transition metal complexes, e.g. cis-[Pd{Ga[(ArNCH)₂]}₂(tmeda)].^4a That the two isomers of 7 exist in equilibrium in solution was seemingly confirmed by dissolving several crystallographically authenticated samples of the trans-isomer of the compound in C₆D₆ which, in each case, led to spectra corresponding to identical isomeric mixtures. In addition, only trans-7 could be crystallised from these solutions, suggesting this is the thermodynamically favoured isomer. The low solubility of 7 in aromatic solvents at low temperature precluded a variable temperature NMR study of the equilibrium between the isomers. Furthermore, no signals were observed in the ¹⁷¹Yb NMR spectrum of 7, presumably because of significant peak broadening, arising from the coordination of the Yb atom of both isomers of 7 by two quadrupolar Ga centres (⁶⁹Ga, 60% abundant, I = 3/2; ⁷¹Ga, 40% abundant, I = 3/2). The apparent isomerisation of 7 in solution is of some interest as observations of geometric isomerism in lanthanide complexes are rare, though not unknown.¹³

Compounds 5–7 were crystallographically characterised and found to be isostructural.‡ As a result, only the molecular structure of 5 is depicted in Fig. 1, though relevant geometric parameters for the other two compounds are included in the caption. Each compound has a distorted lanthanide octahedral geometry with the gallyl ligands trans- to each other (cf.3). The Ln–Ga distances in the compounds follow the expected trend (Sm–Ga ∼ Eu–Ga > Yb–Ga) based on the effective ionic radii for six-coordinate Ln²⁺ cations (Sm²⁺ 1.19 Å, Eu²⁺ 1.17 Å, Yb²⁺ 1.02 Å).¹⁴ In addition, given the similarity between the ionic radii of Yb²⁺ and Ca²⁺ (1.00 Å; six-coordinate¹⁴), it is of note that the Ca–Ga bonds in 3 (3.1587(6) Å) are only slightly shorter than the Yb–Ga bonds of 7. Although the Ln–Ga bonds in 5–7 are almost certainly very polar, they should possess some covalent character based on prior theoretical studies on 2–4.^5,6 The Ln–Ga separations of the compounds in this study are, however, significantly greater than sums of the covalent radii for the atom pairs (Sm–Ga 3.20 Å, Eu–Ga 3.20 Å or Yb–Ga 3.09 Å).¹⁵ Although there are no known Sm–Ga bonded complexes to compare with, the Eu–Ga and Yb–Ga bonds in 6 and 7 are, not surprisingly, shorter than those in the adducts, [Cp*₂Eu(GaCp*)₂] (3.320 Å mean) and [Cp*₂Yb(GaCp*)(THF)] (3.2872(4) Å).⁷

Fig. 1 Molecular structure of 5 (20% thermal ellipsoids; hydrogen atoms omitted). Symmetry operation: ′−x, −y, −z. Selected bond lengths (Å) and angles (°) for 5: Ga–Sm 3.3124(9), Sm–N 2.724 (mean), Ga–N 1.931 (mean); N–Ga–N 83.66(8); 6: Ga–Eu 3.3124(11), Eu–N 2.677 (mean), Ga–N 1.934 (mean); N–Ga–N 83.57(18); 7: Ga–Yb 3.226 (mean), Yb–N 2.596 (mean), Ga–N 1.939 (mean); N–Ga–N 83.74 (mean).

The molecular structure of 8 is shown in Fig. 2 and exhibits the first example of a structurally characterised Tm–Ga bond in a molecular compound. The doubly reduced diazabutadiene ligand is coordinated to the thulium centre in what can be described as a slipped η⁴-mode, as has been seen in related complexes, e.g. [Yb{(ArNCH)₂}(C₅Me₅)(THF)].^16,17 This gives rise to a heavily distorted trigonal bipyramidal coordination geometry (not taking into account the alkene–Tm interaction) with Ga(1) and N(5) in the axial positions. The Ln–Ga distance in the compound is markedly shorter than those in 4–7, and unlike those compounds, is well within the sum of the covalent radii for the two metals (3.12 Å).¹⁵

Fig. 2 Molecular structure of 8 (20% thermal ellipsoids; hydrogen atoms, isopropyl groups and a second crystallographically independent molecule omitted). Selected bond lengths (Å) and angles (°): Tm(1)–Ga(1) 2.9742(16), Tm(1)–N(3) 2.163(10), Tm(1)–N(4) 2.171(8), Tm(1)–N(5) 2.481(9), Tm(1)–N(6) 2.512(10), Tm(1)–C(27) 2.547(11), Tm(1)–C(28) 2.563(10), N(3)–C(27) 1.452(15), N(4)–C(28) 1.409(14), C(27)–C(28) 1.344(16); N(3)–Tm(1)–N(4) 85.1(4), N(3)–Tm(1)–N(5) 92.2(4), N(4)–Tm(1)–N(5) 94.4(3), N(3)–Tm(1)–N(6) 128.3(4), N(4)–Tm(1)–N(6) 142.9(4), N(5)–Tm(1)–N(6) 71.4(3), N(3)–Tm(1)–Ga(1) 109.6(3), N(4)–Tm(1)–Ga(1) 97.6(2), N(5)–Tm(1)–Ga(1) 155.9(2), N(6)–Tm(1)–Ga(1) 86.7(2).

In conclusion, a series of stable bis(gallyl) lanthanide(II) complexes have been prepared. In addition, an analogous bis(gallyl) thulium(II) complex has been implicated as an intermediate in the formation of a gallyl thulium(III) complex. Crystallographic studies on these complexes have given rise to the first structurally characterised GaTm or GaSm bonds. We are currently examining the use of 5–7 as reductants towards a range of unsaturated substrates and will report on this in due course.

We gratefully acknowledge financial support from the Australian Research Council (fellowships for CJ and AS), and the EPSRC Mass Spectrometry Service, Swansea.

Notes and references

1. F. A. Cotton, C. A. Murillo and R. A. Walton, Multiple Bonds Between Metal Atoms, 3rd edn, Springer, New York, 2005.
2. R. J. Baker, R. D. Farley, C. Jones, M. Kloth and D. M. Murphy, J. Chem. Soc., Dalton Trans., 2002, 3844.
3. See, for example: (a) S. P. Green, C. Jones, K. -A. Lippert, D. P. Mills and A. Stasch, Inorg. Chem., 2006, 45, 7242; (b) R. J. Baker, C. Jones, D. P. Mills, D. M. Murphy, E. Hey-Hawkins and R. Wolf, Dalton Trans., 2006, 64; (c) R. J. Baker, C. Jones and M. Kloth, Dalton Trans., 2005, 2106; (d) R. J. Baker, C. Jones, M. Kloth and J. A. Platts, Angew. Chem., Int. Ed., 2003, 43, 2660.
4. See, for example: (a) C. Jones, D. P. Mills, R. P. Rose and A. Stasch, Dalton Trans., 2008, 4395; (b) C. Jones, R. P. Rose and A. Stasch, Dalton Trans., 2007, 2997; (c) S. P. Green, C. Jones, D. P. Mills and A. Stasch, Organometallics, 2007, 26, 3424; (d) R. J. Baker, C. Jones and J. A. Platts, J. Am. Chem. Soc., 2003, 125, 10534.
5. C. Jones, D. P. Mills, J. A. Platts and R. P. Rose, Inorg. Chem., 2006, 45, 3146.
6. P. L. Arnold, S. T. Liddle, J. McMaster, C. Jones and D. P. Mills, J. Am. Chem. Soc., 2007, 129, 5360.
7. M. Wiecko and P. W. Roesky, Organometallics, 2007, 26, 4846.
8. N.B. Two Al–lanthanide and one Al–actinide donor–acceptor complexes have also been reported: (a) M. T. Gamer, P. W. Roesky, S. N. Konchenko, P. Nava and R. Ahlrichs, Angew. Chem., Int. Ed., 2006, 45, 4447; (b) S. G. Minasian, J. L. Krinsky, V. A. Williams and J. Arnold, J. Am. Chem. Soc., 2008, 130, 10086.
9. G. A. Molander and C. R. Harris, Chem. Rev., 1996, 96, 307 , and references therein.
10. W. J. Evans, J. Organomet. Chem., 2002, 652, 61 , and references therein.
11. M. N. Bochkarev, Coord. Chem. Rev., 2004, 248, 835.
12. The solution state magnetic moments (Evans method, C₆D₆, 298 K) for 5 (3.3 μ_B), 6 (7.3 μ_B) and 8 (7.0 μ_B), all lie in the expected ranges: W. J. Evans and M. A. Hozbor, J. Organomet. Chem., 1987, 326, 299.
13. S. Beaini, G. B. Deacon, E. E. Delbridge, P. C. Junk, B. W. Skelton and A. H. White, Eur. J. Inorg. Chem., 2008, 4586.
14. R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751 ; N.B. the value for Sm²⁺ was obtained by extrapolation.
15. B. Cordero, V. Gomez, A. E. Platero-Prats, M. Reves, J. Echeverria, E. Cremades, F. Barragan and S. Alvarez, Dalton Trans., 2008, 2832.
16. A. A. Trifonov, I. A. Borovkov, E. A. Fedorova, G. K. Fukin, J. Larionova, N. O. Druzhkov and V. K. Cherkasov, Chem. Eur. J., 2007, 13, 4981.
17. A related complex, [{Ce^III[(ArNCH)₂](tmeda)(μ-I)}₂], was obtained in very low yield from the reaction of [CeI₃(THF)₄] with [K(tmeda)][:Ga{(ArNCH)₂}]. See ESI† for details of its crystal structure.

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Footnotes

† Electronic supplementary information (ESI) available: Full synthetic and spectroscopic details; ORTEP diagrams for 6 and 7, and full crystallographic information for [{Ce^III[(ArNCH)₂](tmeda)(μ-I)}₂]. CCDC 705275–705279. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b817933f

‡ Crystal data for 5: C₆₄H₁₀₄Ga₂N₈Sm, M = 1275.34, monoclinic, space groupC2/c, a = 29.344(6), b = 13.855(3), c = 21.633(4) Å, β = 131.48(3)°, V = 6589(2) ų, Z = 4, D_c = 1.286 g cm⁻³, F(000) = 2672, μ(Mo-Kα) = 1.734 mm⁻¹, 123(2) K, 9610 unique reflections [R(int) = 0.0226], R (on F) = 0.0312, wR (on F²) = 0.0802 (I > 2σI).6: C₆₄H₁₀₄EuGa₂N₈, M = 1276.95, monoclinic, space groupC2/c, a = 29.408(6), b = 14.089(3), c = 21.746(4) Å, β = 131.73(3)°, V = 6724(2) ų, Z = 4, D_c = 1.261 g cm⁻³, F(000) = 2676, μ(Mo-Kα) = 1.758 mm⁻¹, 150(2) K, 6552 unique reflections [R(int) = 0.0425], R (on F) = 0.0553, wR (on F²) = 0.1474 (I > 2σI).7: C₆₄H₁₀₄Ga₂N₈Yb, M = 1298.03, monoclinic, space groupP2₁/c, a = 21.866(4), b = 14.060(3), c = 21.602(4) Å, β = 95.47(3)°, V = 6611(2) ų, Z = 4, D_c = 1.304 g cm⁻³, F(000) = 2704, μ(Mo-Kα) = 2.254 mm⁻¹, 123(2) K, 13377 unique reflections [R(int) 0.0442], R (on F) = 0.0459, wR (on F²) = 0.01029 (I > 2σI).8·Et₂O: C₆₂H₉₈GaN₆OTm, M = 1182.11, triclinic, space groupP1̄, a = 16.791(3), b = 19.556(4), c = 20.300(4) Å, α = 90.45(3), β = 99.45(3), γ = 90.31°, V = 6575(2) ų, Z = 4 (two crystallographically independent molecules are present in the asymmetric unit; there are no significant geometric differences between them), D_c = 1.194 g cm⁻³, F(000) = 2480, μ(Mo-Kα) = 1.790 mm, 123(2) K, 22752 unique reflections [R(int) 0.0715], R (on F) = 0.0853, wR (on F²) = 0.2011 (I > 2σI).

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