Arene complexation of Sm, Eu, Tm and Yb atoms: a variable temperature spectroscopic investigation

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Abstract

Codeposition of a monatomic lanthanide vapour (Sm, Eu, Tm or Yb) and tri-tert-butylbenzene C6H3tBu3-1,3,5 onto a cold (77 K) surface affords matrices that contain zerovalent bis(η-arene)lanthanide complexes of the form [Ln(η6-C6H3tBu3-1,3,5)2] as formed in macroscale co-condensation reactions using metal vapour synthetic (MVS) techniques. The reproduction of the experiments in the 77 K matrix allows the detailed characterization of thermally unstable members of the series that were previously not possible. The bis(arene) sandwich complexes [Ln(η6-C6H3tBu3-1,3,5)2] (Ln=Sm, Eu) are stable at low temperatures, but may not be the only products of the cocondensation reactions, while the analogues [Ln(η6-C6H3tBu3-1,3,5)2] (Ln=Tm or Yb) cannot be made at liquid nitrogen temperatures. By replacement of tri-tert-butylbenzene with N- and P-substituted heteroarenes NC5H2tBu3-2,4,6 and PC5H2tBu3-2,4,6, the relative stabilities of these zerovalent complexes have been determined. Lanthanum vapour was also cocondensed with the arenes using MVS techniques, and the crude product extracted directly from the machine was analysed in solution at −78 °C. There is very little difference in thermal stability between carbocyclic and heteroaromatic sandwich derivatives of an individual metal.

Thermally unstable zerovalent lanthanide complexes [Ln(η6-C6H3tBu3-1,3,5)2] (Sm, Eu, Tm or Yb) have been studied in frozen matrices at and above 77 K. Analogous N- and P-substituted arene sandwiches have also been made. The matrix experiments allow a detailed study of thermally unstable members of the series that were previously not possible in macroscale metal vapour synthesis (MVS) experiments.

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Introduction

A number of unusual f-element complexes in unprecedented, formally low-valent, oxidation states have been reported recently, including divalent lanthanum, lanthanide, thorium and uranium cations in the +1 and +2 oxidation states [1], [1](a), [1](b), [1](c), [1](d), [1](e), [1](f), [1](g), [1](h). Many of these show electrostatic interactions with aromatic ligand groups such as five-membered ring heterocycles, or commonly, arene solvent molecules. These reduced electropositive metal complexes show fascinating reductive chemistry, and have opened a new area of chemistry for these electropositive metals [2], [2](a), [2](b), [2](c), [2](d), [2](e). We have previously used the technique of metal vapour synthesis (MVS) to generate bis(η6-arene) f-element complexes, in which the spectroscopic and magnetic properties of the complexes support the formulation of the zerovalent oxidation state of the lanthanide [3]. A metal vapour formally provides the simplest source of zerovalent lanthanide ‘starting material’ for the synthesis of molecular zero- or low-oxidation state lanthanide organometallic complexes [4]. Kinetic stabilisation of the complexes using bulky ligands, e.g. 1,3,5-tri-tert-butylbenzene (Bz*) allows the isolation of a range of complexes such as [Gd(η6-C6H3tBu3-1,3,5)2] (1), for which a significant metal arene bond enthalpy has been measured [5], [5](a), [5](b), [5](c). However, the largest metals, La and Ce, and those with a high one-electron f→d promotion energy, including Sm, Eu, Tm and Yb, did not give complexes that were isolable at −78 °C—the lowest convenient temperatures at which products can be extracted from the MVS apparatus [6] (Fig. 1).

In order to understand further the factors affecting the stability of these complexes, we have used specially developed cryostats to study the low-temperature syntheses of the sandwich arene complexes. In this paper we report the results of a spectroscopic investigation of the formation and decomposition of different arene sandwich complexes of the four metals Sm, Tm, Eu and Yb. We have also prepared the unstable La complex and analysed the crude product at low temperatures. The study of the analogous lanthanide complexes of the heteroatom-substituted arenes 2,4,6-tri-tert-butylpyridine (NBz*) and 2,4,6,-tri-tert-butylphosphorin (PBz*), has also been undertaken, to investigate the influence of the heteroatom on complex stability (Fig. 2).

Section snippets

Experimental

All metals were used as purchased except lanthanum which was degassed before use (99.9%). The ligands 1,3,5-tri-tert-butylbenzene, 2,4,6-tri-tert-butylpyridine, and 2,4,6-tri-tert-butylphosphorin were synthesised according to literature procedures and were recrystallised from pentane, then sublimed (10−3 mbar) before use.[7], [7](a), [7](b), [7](c), [7](d) In the synthesis of PBz* the final step differed from that described in the literature: Under an inert atmosphere, a stirred yellow solution

Cocondensation experiments

Macroscale cocondensation experiments were used to study lanthanum complexes, and a mixture of macro and microscale cocondensation experiments were used to study samarium, europium, thulium and ytterbium complex formation and stability.

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Both macroscale and microscale co-condensation reactions were carried out for Sm and Bz* or NBz*. From the physical and spectroscopic properties we infer that the thermally unstable π-complexes formed in the matrix are the same as those formed in the macroscale

Conclusions

The less thermally stable members of the family of zerovalent arene–lanthanide complexes have proven amenable to study by matrix isolation techniques. However, it is not possible to make all members of the series of zerovalent bis(arene) or bis(heteroarene) lanthanides at 77 K. The large, early metals form complexes which are characterised as bis(η-arene) complexes, but incorporation of the heteroatom is insufficient to stabilise the La complex at room temperature. The change from carbocyclic

Acknowledgements

This work was financially supported by INTAS grant 94-4299 and the EPSRC. We thank Dr Andrea Sella for helpful discussions.

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