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

Codeposition of a monatomic lanthanide vapour (Sm, Eu, Tm or Yb) and tri-tert-butylbenzene C6H3–tBu3-1,3,5 onto a cold (77 K) surface affords matrices that contain zerovalent bis(η-arene)lanthanide complexes of the form [Ln(η6-C6H3–tBu3-1,3,5)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-C6H3–tBu3-1,3,5)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-C6H3–tBu3-1,3,5)2] (Ln=Tm or Yb) cannot be made at liquid nitrogen temperatures. By replacement of tri-tert-butylbenzene with N- and P-substituted heteroarenes NC5H2–tBu3-2,4,6 and PC5H2–tBu3-2,4,6, 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.