The isolable bis­(tetramethylaluminate) lanthanide complexes [2-R2,6-XC6H4NC­(tBu)­(2,6-R1 2C6H3)]­Ln­(AlMe4)2 (R1 = R2 = X = Me, Ln = La (1); R1 = R2 = X = Me, Ln = Nd (2); R1 = iPr, R2 = H, X = OMe, Ln = La (3); R1 = iPr, R2 = H, X = OMe, Ln = Nd (4)) coordinated by bi- and tridentate amidinate ligands were synthesized by the reactions of the amidines 2,6-Me2C6H3NC­(tBu)­NH-2,6-Me2C6H3 and 2,6-iPr2C6H3NC­(tBu)­NH-2-MeOC6H4 with Ln­(AlMe4)3 (Ln = La, Nd) and were isolated in 81, 75, 86, and 74% yields, respectively. X-ray analyses revealed that complexes 3 and 4 feature intramolecular coordination of an OMe group to the metal ions. Both AlMe4 fragments in complexes 3 and 4 coordinate to the metal centers in a κ2 mode, while in Nd complex 2 one AlMe4 group adopts a κ2 and the second a κ3-coordination mode. Complexes 1–4 were evaluated as precatalysts for isoprene polymerization. The ternary catalytic systems 1–4/[Ph3C]­[B­(C6F5)4],[PhNHMe2]­[B­(C6F5)4]/nAlR3 (AlR3 = AlMe3, n = 10; AliBu2H, n = 1; AliBu3, n = 10) enable 82–100% conversions of 1000 equiv of monomer at 25 °C within 5–60 min. cis-1,4-Selectivity of the ternary catalytic systems proved to be affected by the type of ligand, the borate, the AlR3 cocatalyst, and the nature of the metallo center and varied in the range of 34.4–91.8%. The binary systems 1–4/[Ph3C]­[B­(C6F5)4],[PhNHMe2]­[B­(C6F5)4] also catalyze isoprene polymerization and afford polyisoprenes containing 96.2–99.0% of cis-1,4 units in polymer chains. The stereoselectivity of binary systems containing complex 1 dramatically depends on the type of borate: 66.8% of trans-1,4 for [Ph3C]­[B­(C6F5)4] and 87.8% of cis-1,4 for [PhNHMe2]­[B­(C6F5)4].

A series of bis­(tetramethylaluminate) lanthanide complexes {2-[Ph2PX]­C6H4NC­(tBu)­N­(2,6-Me2C6H3)}­Ln­(AlMe4)2 (X = O, Ln = La (1); X = O, Ln = Nd (2); X = O, Ln = Sm (3); X = S, Ln = La (4); X = S, Ln = Nd (5)) coordinated by tridentate amidinate ligands bearing a hard (PO) or soft (PS) donor group were synthesized in 82, 71, 54, 77, and 69% yields, respectively. An X-ray analysis revealed that complexes 1–5 feature an intramolecular coordination of the Ph2PX group to the metal ion and both AlMe4 fragments coordinate to the metal center in a κ2 fashion. The binary systems containing Nd complexes 2 and 5 in combination with [Ph3C]­[B­(C6F5)4] (TB) or [PhNHMe2]­[B­(C6F5)4] (HNB) have high catalytic activity in isoprene polymerization. The activity of 5 containing a soft Lewis base (PS) in the presence of both TB and HNB was much higher in comparison to that of 2 bearing a hard PO group. For the ternary systems 1–5/TB,HNB/nAlR3 (AliBu3, n = 10; AliBu2H, n = 1; AlMe3, n = 10; toluene, room temperature) record reaction rates were observed for neodymium complex 5: in combination with TB, HNB, and AliBu3 it enables quantitative polymerization of 1000 equiv of monomer within 5 min. The effect of the nature of the pendant donor group (hard PO vs soft PS) on the catalytic activity of the ternary systems is clearly manifested only in the case of neodymium complexes 2 and 5: the systems based on complex 5 have much higher activity in comparison to 2. The systems 4/TB,HNB/10AliBu3 and 5/TB/10AliBu3 provide the best stereoselectivities and enable formation of polyisoprene with 73.9–77.4% content of cis-1,4-units. These systems allow for the synthesis of polyisoprenes with a rather narrow polydispersity (M w/M n = 1.17–1.83).

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