Y, Lu, and Gd complexes of NCO/NCS pincer ligands: synthesis, characterization, and catalysis in the cis-1,4-selective polymerization of isoprene.

A series of NCO/NCS pincer precursors, 3-(Ar(2)OCH2)-2-Br-(Ar(1)N=CH)C6H3 (((Ar1)NCO(Ar2))Br, 3a-d) and 3-(2,6-Me2C6H3SCH2)-2-Br-(Ar(1)N=CH)C6H3 (((Ar1)NCS(Me))Br, 4a and 4b) were synthesized and characterized. The reactions of [(Ar1)NCO(Ar2)]Br/[(Ar1)NCS(Me)]Br with nBuLi and the subsequent addition of the rare-earth-metal chlorides afforded their corresponding rare-earth-metal-pincer complexes, that is, [((Ar1)NCO(Ar2))YCl2(thf)2] (5a-d), [((Ar1)NCO(Ar2))LuCl2(thf)2] (6a, 6d), [((Ar1)NCO(Ar2))GdCl2(thf)2] (7), [{((Ar1)NCS(Me))Y(μ-Cl)}2{(μ-Cl)Li(thf)2(μ-Cl)}2] (8, 9), and [{((Ar1)NCS(Me))Gd(μ-Cl)}2{(μ-Cl)Li(thf)2(μ-Cl)}2] (10, 11). These diamagnetic complexes were characterized by (1)H and (13)C NMR spectroscopy and the molecular structures of compounds 5a, 6a, 7, and 10 were well-established by X-ray diffraction analysis. In compounds 5a, 6a, and 7, all of the metal centers adopted distorted pentagonal bipyramidal geometries with the NCO donors and two oxygen atoms from the coordinated THF molecules in equatorial positions and the two chlorine atoms in apical positions. Complex 10 is a dimer in which the two equal moieties are linked by two chlorine atoms and two Cl-Li-Cl bridges. In each part, the gadolinium atom adopts a distorted pentagonal bipyramidal geometry. Activated with alkylaluminum and borate, the gadolinium and yttrium complexes showed various activities towards the polymerization of isoprene, thereby affording highly cis-1,4-selective polyisoprene, whilst the NCO-lutetium complexes were inert under the same conditions.