Differences in the cyclometalation reactivity of bisphosphinimine-supported organo-rare earth complexesElectronic supplementary information (ESI) available: Atomic coordinates, interatomic distances and angles, anisotropic thermal parameters, and hydrogen parameters for 1, 2, 3, 4, and 5. CCDC 991840-991844 for 1-5. For crystallographic data in CIF format see DOI: 10.1039/c4dt00863d

The pyrrole-based ligand N , N ′-((1 H -pyrrole-2,5-diyl)bis(diphenylphosphoranylylidene))bis(4-isopropylaniline) (H L 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(CH 2 SiMe 3 ) 2 ] ( 1 ) is a Lewis base-free monomer that is remarkably resistant to cyclometalation. Conversely, the analogous samarium complex [ L B Sm(CH 2 SiMe 3 ) 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(CH 2 SiMe 3 )(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 H L B , the doubly-ligated samarium complex [(κ 4 - L B ) L B Sm] ( 4 ) forms. A novel yttrium complex [L n Y(CH 2 SiMe 3 ) 2 ] is resistant to cyclometalation, while samarium variants undergo C-H activation, forming unique cyclometalated motifs.