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