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(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.

Insights into the decomposition pathway of a lutetium alkylamido complex via intramolecular C–H bond activation

“…In solution complex 3 exhibits spectra consistent with Cs symmetry on the NMR timescale and is thus unlikely to be a metallocycle complex resulting from a single P-or N-aryl Csp2-H bond activation, which we have previously observed with our bis(phosphinimine)carbazole ligand scaffolds (vide supra). [11][12][13] Notably, a weak, but diagnostic resonance at δ 198.0 in the 13 C{ 1 H} NMR spectrum is consistent with previously observed Csp2 atoms bound to lutetium, [12][13][14] implying that complex 2 decomposes via ortho-metalation of one of the phenyl rings of the triphenylmethylamido ligand to afford cyclometalated complex 3 LLu(κ 2-N,C -(NHCPh2(C6H4))), (Scheme 2). Due to the similarity of the diffusion coefficient (4.17(2)  10 -10 m 2 •s -1 ) for complex 3, obtained via a DOSY NMR experiment, to that of complexes 2 (5.62(2)  10 -10 m 2 •s -1 ) and 4 (8.71(2)  10 -10 m 2 •s -1 ), we have ruled out the possibility that intermediate 3 exists as a dinuclear species in solution.…”

“…Previously, we reported a thermally stable dialkyl lutetium complex which was supported by a bis(phosphinimine)pyrrole ligand, 10 This complex has been shown to resist thermal decomposition (80 °C) unlike our previously utilized bis(phosphinimine)carbazole framework. 11,12 Rare earth complexes supported by the carbazole scaffold underwent spontaneous self-destructive cyclometalation at the P-and N-bound groups of the phosphinimine donors. As such, we have chosen to employ our pyrrole-based ligand in a strategic attempt to generate a robust Lu=N functionality.…”

Yttrium-Alkyl Complexes Supported by a Ferrocene-Based Phosphinimine Ligand

“…17,[35][36][43][44][45][46][47][48][49][50] On the other hand, reports of structurally characterized alkyl or aryl rare earth complexes bearing an imine functionality in the backbone are rare, [51][52] the majority being represented by phosphinimine ligands. [53][54][55][56][57][58][59][60] In our lab, the synthesis of an electrophilic metal alkyl complex supported by a Schiff base ferrocene ligand would continually fail. Although a few examples of alkyl yttrium complexes supported by a ligand containing an imine group exist, 53,61 in general, attempts to isolate such complexes resulted in the migration of the alkyl ligand to the imine group.…”

N-Containing Ligands as Universal Coordination Environments for Rare-Earth Alkyl and Bis(alkyl) Complexes

“…Lutetium bis(alkyl) derivatives (CzPNAr)Lu(CH 2 SiMe 3 ) 2 (Ar = Ph (496), C 6 H 4 iPr-4 (497)) were prepared by the reactions of Lu(CH 2 SiMe 3 ) 3 OPEN, 2018, 1 (1 The use of bis(phosphinimine)pyrrole allowed one to avoid side processes of ortho-metallation. Solvent-free bis(alkyl) complexes [2,5-(Ph 2 P=NC 6 H 4 iPr-4) 2 C 4 H 2 N]Ln(CH 2 SiMe 3 ) 2 (Ln = Sc (502), Er (503), Lu (504) [181], Y (505) [182]) were synthesized by the reactions of Ln(CH 2 SiMe 3 ) 3 (THF) 2 (Ln = Sc, Er, Lu, Y) with 2,5-(Ph 2 P=NC 6 H 4 iPr-4) 2 C 4 H 2 NH in high yields (Fig. 40).…”

“…Lutetium complex 504 demonstrated high stability in solution. No traces of decomposition were observed even upon heating at 60 °C for 4.5 h. An attempt to synthesize a similar compound of rare-earth element with the larger ionic radius (Sm 3+ ) afforded cyclometalled adduct 507 (Scheme 72) [182]. The authors noted that bis(alkyl) complex 506 is extremely unstable and rapidly undergoes cyclometallation at one of the aryl substituents on the nitrogen atom to form four-membered azamacrocycle κ 4 The reactions of N,N,N-tridentate 2,5-bis((pyrrolidine-1yl)methylene)-1H-pyrrole and 2,5-bis((piperidine)methylene)-1H-pyrrole with Ln(CH 2 SiMe 3 ) 3 (THF) 2 (Ln = Sc, Y, Lu) led to bis(alkyl) complexes [2,5-(C 4 H 8 NCH 2 ) 2 C 4 H 2 N]Ln(CH 2 SiMe 3 ) 2 (THF) n (Ln = Sc, n = 0 (511); Ln = Y, n = 1 (512); Ln = Lu, n = 1 (513)) and [2,5-(C 5 H 10 NCH 2 ) 2 C 4 H 2 N]Sc(CH 2 SiMe 3 ) 2 (514) in moderate and good yields ( Fig.…”