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

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(B)Y(CH2SiMe3)2] (1) is a Lewis base-free monomer that is remarkably resistant to cyclometalation. Conversely, the analogous samarium complex [L(B)Sm(CH2SiMe3)2] is dramatically more reactive and undergoes rapid orthometalation of one phosphinimine aryl substituent, generating an unusual 4-membered azasamaracyclic THF adduct [κ(4)-L(B)Sm(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 [κ(1):κ(2):μ(2)-L(B)Sm(THF)]2 (3). Alternatively, if 2 is intercepted by a second equivalent of HL(B), the doubly-ligated samarium complex [(κ(4)-L(B))L(B)Sm] (4) forms.