Lanthanide Heteroatom-Stabilised Alkylidenes: A New Approach to Multiply Bonded Lanthanide Chemistry

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One of the most exciting and challenging areas of contemporary lanthanide chemistry (lanthanides are the 14 elements at the bottom of the periodic table) is the isolation of lanthanide alkylidenes (compounds which feature a carbon atom doubly bonded to a lanthanide, Ln=C), and only a very limited number are known. Therefore, enduring fundamental questions regarding the stability, structure, bonding, and reactivity of lanthanide alkylidenes remain. Such species are scarce because they lack sufficiently good orbital interactions to stabilise the Ln=C double bond in the way numerous transition metals can. However, if heteroatoms (phosphorus or silicon) are appended to the alkylidene they can help stabilise the electrons in the Ln=C double bond. This First Grant will build on preliminary results the applicant has obtained, which exploits alkylidene heteroatom stabilisation, and utilises his expertise in lanthanide chemistry, to effect a significant and rapid expansion of this field to address the new chemistry of lanthanide alkylidenes. New lanthanide alkylidenes will be subjected to a detailed structural and spectroscopic interrogation, which will be combined with theoretical computational calculations to comprehensively establish their stability, structure and bonding. The reactivity patterns of these complexes will be screened in reactions which parallel transition metal alkylidene reactions (cycloadditions, Wittig-type). Importantly, the applicants preliminary results include a system which exhibits not only a formal Ln=C double bond, but also a reactive Ln-C single bond. This places two very reactive fragments in close proximity at the same metal centre, and presents a unique opportunity to exploit the synergy resulting from them being harnessed together. We will use this synergy to attempt access to previously unknown structural motifs, such as the first terminal lanthanide imides (Ln=N double bond) and the first lanthanide tetrylenes (Ln=Si and Ln=Ge double bonds). These will be highly interesting inorganic versions of unsaturated organic olefins and allenes (C=C, C=C=C), and given the extensive use of transition metal alkylidenes and imides by academia and industry for the synthesis of fine chemicals, pharmaceutical drugs, and commodity plastic polymers, they promise novel new reactivities.