Bis(alkyl) rare-earth complexes supported by a new tridentate amidinate ligand with a pendant diphenylphosphine oxide group. Synthesis, structures and catalytic activity in isoprene polymerizationElectronic supplementary information (ESI) available. CCDC 1041025-1041028. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt02570b

A new tridentate amidine 2-[Ph 2 P(O)]C 6 H 4 NHC( t Bu)&z.dbd;N(2,6-Me 2 C 6 H 3 ) ( 1 ) bearing a side chain pendant Ph 2 P&z.dbd;O group was synthesized and proved to be a suitable ligand for coordination to rare-earths ions. Bis(alkyl) complexes {2-[Ph 2 P(O)]C 6 H 4 NC( t Bu)N(2,6-Me 2 C 6 H 3 )}Ln(CH 2 SiMe 3 ) 2 (THF) n (Ln = Y, n = 1 ( 3 ), Ln = Er, n = 1 ( 4 ), Ln = Lu, n = 0 ( 5 )) were prepared using alkane elimination reactions of 1 and Ln(CH 2 SiMe 3 ) 3 (THF) 2 (Ln = Y, Er, Lu) in hexane and were isolated in 50, 70 and 75% yields respectively. The X-ray studies revealed that complexes 2-5 feature intramolecular coordination of P&z.dbd;O groups to metal ions. The lutetium complex 5 proved to be rather stable: at 20 °C its half-life time is 1155 h, while for the yttrium analogue the half-life time was found to be 63 h. Complexes 3-5 were evaluated as precatalysts for isoprene polymerization. The systems Ln/borate/AliBu 3 (Ln = 3-5 , borate = [PhNHMe 2 ][B(C 6 F 5 ) 4 ], [Ph 3 C][B(C 6 F 5 ) 4 ]) turned out to be highly efficient in isoprene polymerization and enable complete conversion of 1000-10 000 equivalents of monomer into polymer at 20 °C within 0.5-2.5 h affording polyisoprenes with a very high content of 1,4- cis units (up to 96.6%) and from narrow (1.49) to moderate (3.54) polydispersities. A comparative study of catalytic performance of the related bis(alkyl) yttrium complexes supported by amidinate ligands of different denticities and structures [ t BuC( N -2,6iPr 2 C 6 H 4 ) 2 ] − , [ t BuC( N -2,6-iPr 2 C 6 H 4 )( N -2-MeOC 6 H 4 )] − and {2-[Ph 2 P(O)]C 6 H 4 NC( t Bu)N(2,6-Me 2 C 6 H 3 )} − demonstrated that the introduction of a pendant donor group (2-MeOC 6 H 4 or Ph 2 P(O)) into a side chain of amidinate scaffolds results in a significant increase in catalytic activity. The amidinate ligand bearing a Ph 2 P(O)-group provides a high isoprene polymerization rate along with excellent control over regio- and stereoselectivities and allows us to obtain polyisoprenes with a reasonable molecular weight distribution. The introduction of a pendant Ph 2 P(O) group into an amidinate ligand resulted in high 1,4- cis selectivity (96.6%) while maintaining very high activity.