The Study of Lanthanides for Organometallic and Separations Chemistry

Part 1. The effective use of f-element complexes as catalysts for reactions such as hydroamination and hydrophosphination has been demonstrated in many recent reports. Unfortunately, the homoleptic starting materials underpinning this chemistry are generally derived from only a handful of ligands. That is, of the homoleptic lanthanide complexes that exist, the majority make use of alkylsilane (-CH₂SiMe₃), silylamide (-N(SiMe₃)₂), or benzyl (-CH₂C₆H₅) derivatives.

The paucity of tri-alkyl lanthanide complexes can be attributed to the required extended coordination sphere and high Lewis acidity or electrophilicity of these metals. While these properties make lanthanum alkyl complexes very reactive, in turn they also make their synthesis and manipulation quite challenging. Our efforts have focused on the development of unexplored homoleptic tri-alkyl rare-earth metal complexes utilizing simple ligands that fulfill both the electronic and steric requirements of these metal centers. Herein, we report our findings of a new class of homoleptic tri-alkyl rare-earth metal complexes using alpha-metallated N,N-dimethylbenzylamine ligands as stable benzyl ligand derivatives to form lanthanide complexes free of coordinating solvent.

We have also expanded the reactivity scope of these homoleptic lanthanide complexes to include stoichiometric insertion and catalytic hydrophosphination reactions involving heterocumulenes. The tris alpha-metallated N,N-dimethylbenzylamine lanthanum and yttrium complexes [¿¿-La(DMBA)₃, ¿¿-Y(DMBA)₃] have proven to be capable starting materials that undergo triple insertion reactions with a variety of carbodiimide ligands. In addition ¿¿-La(DMBA)₃ demonstrated excellent catalytic activity for the room temperature hydrophosphination of a wide array of heterocumulenes.

Part 2. Another current focus of the Schmidt group involves development of a class of lower rim functionalized calix[4]arenes containing ligands that have been previously shown to selectively separate lanthanides. These calix[4]arenes will be attached to a solid support through an alkyl linker at the methylene position and separation will be based primarily on the ionic radius of the metal cation.

In summary, we report the synthesis and derivatization of a series of 2-(¿¿-(alkyl))tetramethoxy-p-tert butylcalix[4]arenes. These calix[4]arenes represent intermediates in a multi-step synthesis for the design of lower rim modified calix[4]arenes that will be evaluated for the separation of lanthanide ions with the future goal of attachment to a solid support.