Bis(tetramethylaluminate) Lanthanide Complexes Supported by Amidinate Ligands with a Pendant Ph2P═X (X = O, S) Group: Application in Isoprene Polymerization

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 (Mw/Mn = 1.17–1.83).