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Issue 37, 2015
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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 polymerization

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Abstract

A new tridentate amidine 2-[Ph2P(O)]C6H4NHC(tBu)[double bond, length as m-dash]N(2,6-Me2C6H3) (1) bearing a side chain pendant Ph2P[double bond, length as m-dash]O group was synthesized and proved to be a suitable ligand for coordination to rare-earths ions. Bis(alkyl) complexes {2-[Ph2P(O)]C6H4NC(tBu)N(2,6-Me2C6H3)}Ln(CH2SiMe3)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(CH2SiMe3)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[double bond, length as m-dash]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/AliBu3 (Ln = 3–5, borate = [PhNHMe2][B(C6F5)4], [Ph3C][B(C6F5)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 [tBuC(N-2,6iPr2C6H4)2], [tBuC(N-2,6-iPr2C6H4)(N-2-MeOC6H4)] and {2-[Ph2P(O)]C6H4NC(tBu)N(2,6-Me2C6H3)} demonstrated that the introduction of a pendant donor group (2-MeOC6H4 or Ph2P(O)) into a side chain of amidinate scaffolds results in a significant increase in catalytic activity. The amidinate ligand bearing a Ph2P(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.

Graphical abstract: 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 polymerization

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The article was received on 07 Jul 2015, accepted on 17 Aug 2015 and first published on 18 Aug 2015


Article type: Paper
DOI: 10.1039/C5DT02570B
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Citation: Dalton Trans., 2015,44, 16465-16474

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    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 polymerization

    A. O. Tolpygin, T. A. Glukhova, A. V. Cherkasov, G. K. Fukin, D. V. Aleksanyan, D. Cui and A. A. Trifonov, Dalton Trans., 2015, 44, 16465
    DOI: 10.1039/C5DT02570B

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