Cyaphido Complexes of the Rare-Earth Metals and Their Tetramerization

The synthesis and isolation of crystalline rare-earth (RE) metallocene complexes bearing the cyaphide (CP⁻) ligand is reported. These species were synthesized through salt metathesis reactions of [RE(Cpttt)₂Cl] (Cpttt = 1,2,4-tris(tert-butyl)cyclopentadienyl) with the cyaphide transfer reagent [Mg(DippNacNac)(CP)]₂ (DippNacNac = CH{C(CH₃)N(Dipp)}₂; Dipp = 2,6-di(iso-propyl)phenyl). The ionic radius of the RE ions was found to play a pivotal role in the solution-phase speciation of such compounds, with smaller ions favoring stable monomeric complexes [RE(Cpttt)₂(CP)] (RE = Y, Sm, Lu), while the larger ions were found to slowly tetramerize to afford [{RE(Cpttt)₂}₄(μ₄-C₄P₄)] (RE = La-Nd). The monometallic [RE(Cpttt)₂(CP)] compounds represent the first examples of terminal κ¹-cyaphide ions in the coordination spheres of RE elements. The oligomers, [{RE(Cpttt)₂}₄(μ₄-C₄P₄)], contain an unprecedented [P═C═C-P═P-C═C═P]⁴⁻ motif, the formation of which proceeds via the monomeric [RE(Cpttt)₂(CP)] intermediates (observed by multielement NMR and IR spectroscopy for La-Nd, and by single-crystal X-ray diffraction for Pr). The mechanism for this unique tetramerization reaction was probed computationally, providing insights into how such oligomerization reactions may be used to control the polymerization of cyaphide compounds to target specific oligomers or extended solids, such as carbon phosphide.