Differences in the cyclometalation reactivity of bisphosphinimine-supported organo-rare earth complexes

Abstract

The pyrrole-based ligand N,N′-((1H-pyrrole-2,5-diyl)bis(diphenylphosphoranylylidene))bis(4-isopropylaniline) (HL_B) can be deprotonated and coordinated to yttrium and samarium ions upon reaction with their respective trialkyl precursors. In the case of yttrium, the resulting complex [L_BY(CH₂SiMe₃)₂] (1) is a Lewis base-free monomer that is remarkably resistant to cyclometalation. Conversely, the analogous samarium complex [L_BSm(CH₂SiMe₃)₂] is dramatically more reactive and undergoes rapid orthometalation of one phosphinimine aryl substituent, generating an unusual 4-membered azasamaracyclic THF adduct [κ⁴-L_BSm(CH₂SiMe₃)(THF)₂] (2). This species undergoes further transformation in solution to generate a new dinuclear species that features unique carbon and nitrogen bridging units [κ¹:κ²:μ²-L_BSm(THF)]₂ (3). Alternatively, if 2 is intercepted by a second equivalent of HL_B, the doubly-ligated samarium complex [(κ⁴-L_B)L_BSm] (4) forms.