Rare-Earth-Metal Complexes Bearing Phosphazene Ancillary Ligands: Structures and Catalysis toward Highly Trans-1,4-Selective (Co)Polymerizations of Conjugated Dienes

The bis-arylated phosphazene compounds [HN­(PPh2NAr)2] (Ar = phenyl (HL1), 2,6-dimethylphenyl
(HL2), 2,6-diisopropylphenyl (HL3)) and the
imidodiphosphinate
compound HN­(PPh2O)2 (HL4) have been
prepared via the Staudinger reaction. Treatment of the neutral compounds
HL1, HL2, and HL3 with Ln­(CH2SiMe3)3(THF)2 (Ln = Sc, Y,
Lu) generated the solvent-free bis­(alkyl) complexes L1Ln­(CH2SiMe3)2 (Ln = Sc (1a),
Y (1b), Lu (1c)), L2Sc­(CH2SiMe3)2 (2a), L3Y­(CH2SiMe3)2 (3b),
and L3Lu­(CH2SiMe3)2 (3c), respectively. The reaction between HL4 and
Y­(CH2SiMe3)3(THF)2 gave
the rare zwitterionic complex 4b. Lithiation of the ligand
HL1 by nBuLi followed by a metathesis
reaction with Nd­(BH4)3(THF)3 afforded
the corresponding complex L1Nd­(BH4)2(THF)2 (5). Complexes 1 upon
incorporation of [Ph3C]­[B­(C6F5)4] and AliBu3 led to ternary systems
that initiated isoprene polymerization with high activities, among
which complex 1a was the first example of a scandium
catalytic precursor providing trans-1,4-selectivity (90.0%), while
the lutetium analogue 1c had medium trans-1,4-selectivity
(54.3%) and the yttrium complex 1b exhibited high cis-1,4-selectivity
(76.3%). The ternary system based on the zwitterion 4b displayed the highest activity for the isoprene polymerization among
these complexes and gave cis-1,4-regularity-enriched polyisoprene
(70.6%). Highly stereospecific homopolymerizations of isoprene (trans-1,4-content:
97.0%) and butadiene (trans-1,4-content: 94.0%) were achieved by using
the borohydrido complex 5 upon the activation of dibutylmagnesium.
The copolymerization of isoprene and butadiene with 1a/[Ph3C]­[B­(C6F5)4/AliBu3] gave randomly arranged trans-1,4-regulated
polybutadiene and polyisoprene sequences. The kinetics study displayed
competitive polymerization rates of rBD = 2.89 and rIP = 0.41. The thermal behaviors
of the (co)­polymers were investigated.