C5Me5/ER-ligated samarium(II) complexes with the neutral "C5Me5M" ligand (ER = OAr, SAr, NRR ', or PHAr; M = K or Na): A unique catalytic system for polymerization and block-copolymerization of styrene and ethylene

Reactions of (C5Me5)(2)Sm(THF)(2) with 1 equiv of K(ER) in THF gave in high yields the Sm(II) complexes [(C5Me5)Sm(THF)(m)(ER)(mu -C5Me5)K(THF)(n)](in finity) (m = 0 or 1; n = 1 or 2; ER = (OC6H2Bu2)-Bu-t-2,6-Me-4 (la), (OC6H3 Pr2)-Pr-i-2,6 (Ib), (SC6H2Pr3)-Pr-i-2,4,6 (Ic), (NHC6H2Bu3)-Bu-t-2,4,6 (Id) , or N(SiMe3)(2) (le)), in which the "(C5Me5)K(THF)(n)" unit acts as a neut ral coordination ligand bonding to the Sm(II) center with the "C5Me5" part. The similar reaction of (C5Me5)(2)Yb(THF)(2) with 1 equiv of KN(SiMe3)(2) yielded the corresponding ro(II) complex [(C5Me5)Yb(N(SiMe3)(2)) (mu -C5Me3 )K(THF)(2)](infinity) (If) in 90% yield. These complexes all adopt a simila r polymeric structure via "intermolecular" interactions between the K atom and a C5Me5 ligand. The analogous reaction of (C5Me5)(2)Sm(THF)(2) with 1 e quiv of KPHAr afforded [(C5Me5)Sm(THF)(mu -PHAr)K-(C5Me5)(THF)](infinity) ( Ig, Ar = (C6H2Bu3)-Bu-t-2,4,6), in which the "C5Me5K" unit is bonded to the phosphide site with its K atom. The reaction of the silylene-linked bis(te tramethylcyclopentadienyl) samarium(n) complex Me2Si(C5Me4)(2)Sm(THF)(2) wi th 1 equiv of KOAr in THF yielded [Me2Si(C5Me4)(mu -C5Me4)K(THF)(n)Sm(OAr)] (infinity) (Ar = (C6H2Bu2)-Bu-t-2,6-Me-4 (Ih, n = 2) or (C6H3Bu2)-Bu-t-2,6 (li, n = 1)), which can be viewed as a C5Me4/OAr-ligated Sm(II) species coo rdinated by the silylene-linked, neutral "C5Me4K'' ligand. The use of NaN(S iMe3)(2) in place of KN(SiMe3)(2) in the reactions with (C5Me5)(2)Ln(THF)(2 ) afforded the "C5Me5Na(THF)(3)"-coordinated, "monomeric" Ln(II) complexes (C5Me5)Ln(N(SiMe3)(2))(mu -C5Me5)Na(THF)(3) (Ln = Sm (1j) or YB (1k)). Reac tions of the polymeric complexes 1a,e with 2 equiv of HMPA (per Sm) in THF yielded the corresponding HMPA-coordinated, monomeric Sm(II) complexes (C5M e5)Sm(ER)(HMPA)(2) (ER = (OC6H2Bu2)-Bu-t-2,6-Me-4 (2a), N(SiM3)(2) (2e)), T his type of C5Me5/ER-ligated Sm(II) complexes, particularly 1a-c, showed un ique reactivity toward styrene and ethylene, which can not only polymerize styrene and ethylene but also copolymerize them into block styrene-ethylene copolymers under the presence of both monomers. The less reducing Yb(II) complex If or the silylene-linked cyclopentadienyl Sm(II) complex Ih did not show an activity for the polymerization of ethyl ene under the same conditions, suggesting that the polymerization reaction in the present systems is initiated by dissociation of the neutral "C5Me5M" ligand (M = K or Na) from the Sm(II) center, followed one-electron transfe r from the resultant C5Me5/ER-ligated Sm(II) species to an incoming monomer . As a leaving group, "C5Me5K" seemed more suitable than "C5Me5Na". Among t he ER ligands, the thiolate ligand (SC6H2Pr3)-Pr-i-2,4,6 (Ic) showed the hi ghest selectivity for the block copolymerization of styrene and ethylene, w hile the aryloxide (OC6H2Bu2)-Bu-t-2,6-Me-4 (la) and the silylamide N(SiMe3 )(2) (le) gave the highest activity for the polymerization of ethylene and that of styrene, respectively. Possible mechanisms for the polymerization a nd copolymerization reactions are proposed.