Tridentate CCC-Pincer Bis(carbene)-Ligated Rare-Earth Metal Dibromides. Synthesis and Characterization

The first xylene-bridged bis(N-heterocyclic carbene) (bis(NHC))-ligated CCC-pincer rare-earth metal dibromides (PBNHC)LnBr2(THF) (PBNHC = 2,6-(2,4,6-Me3C6H2NCHCHNCCH2)2C6H3; 1: Ln = Sc; 2: Ln = Lu; 3: Ln = Sm) were prepared by in situ treatment of a THF suspension of 2,6-bis(1-mesitylimidazolium methyl)-1-bromobenzene dibromides ((PBNHC-Br)·2HBr) and lanthanide trichlorides (LnCl3) with dropwise addition of nBuLi at room temperature. The overall molecular structure of these complexes is an isostructrual monomer of a THF solvate. The monoanionic xylene-bridged bis(NHC)s bond to the central metal as a tridentate CCC-pincer moiety in a κC:κC:κC′ mode, which, in combination with the two trans-located bromo units, generates a twisted tetragonal-bipyramidal geometry

Tridentate CCC-Pincer Bis(carbene)-Ligated Rare-Earth Metal Dibromides. Synthesis and Characterization

The first xylene-bridged bis(N-heterocyclic carbene) (bis(NHC))-ligated CCC-pincer rare-earth metal dibromides (PBNHC)LnBr2(THF) (PBNHC = 2,6-(2,4,6-Me3C6H2NCHCHNCCH2)2C6H3; 1: Ln = Sc; 2: Ln = Lu; 3: Ln = Sm) were prepared by in situ treatment of a THF suspension of 2,6-bis(1-mesitylimidazolium methyl)-1-bromobenzene dibromides ((PBNHC-Br)·2HBr) and lanthanide trichlorides (LnCl3) with dropwise addition of nBuLi at room temperature. The overall molecular structure of these complexes is an isostructrual monomer of a THF solvate. The monoanionic xylene-bridged bis(NHC)s bond to the central metal as a tridentate CCC-pincer moiety in a κC:κC:κC′ mode, which, in combination with the two trans-located bromo units, generates a twisted tetragonal-bipyramidal geometry

CCC-Pincer Bis(carbene) Lanthanide Dibromides. Catalysis on Highly <i>cis</i>-1,4-Selective Polymerization of Isoprene and Active Species

A series of CCC-pincer 2,6-xylenyl bis(carbene)-ligated rare-earth metal dibromides (PBNHC)LnBr2(THF) ((PBNHC) = 2,6-(2,4,6-Me3C6H2NCHCHNCCH2)2C6H3; Ln = Sc (1), Y (2), La (3), Nd (4), Sm (5), Gd (6), Dy (7), Ho (8), Tm (9), Lu (10)) have been synthesized. Upon activation with AlR3 (R = Me, Et, iBu) and [Ph3C]+[B(C6F5)4]−, complexes 2, 4, 6, 7, and 8 exhibited high activity and cis-1,4 selectivity (99.6%, 25 °C) toward the polymerization of isoprene, although complexes 1, 3, 5, 9, and 10 were inert. The selectivity was not affected by the nature of the central metal and AlR3 and was maintained at elevated temperatures up to 80 °C (97.4%). The yttrium hydrido aluminate cation [(PBNHC)Y(μ-H)2AliBu2]+ was identified as the active species according to NMR spectroscopic analysis

CCC-Pincer Bis(carbene) Lanthanide Dibromides. Catalysis on Highly <i>cis</i>-1,4-Selective Polymerization of Isoprene and Active Species

A series of CCC-pincer 2,6-xylenyl bis(carbene)-ligated rare-earth metal dibromides (PBNHC)LnBr2(THF) ((PBNHC) = 2,6-(2,4,6-Me3C6H2NCHCHNCCH2)2C6H3; Ln = Sc (1), Y (2), La (3), Nd (4), Sm (5), Gd (6), Dy (7), Ho (8), Tm (9), Lu (10)) have been synthesized. Upon activation with AlR3 (R = Me, Et, iBu) and [Ph3C]+[B(C6F5)4]−, complexes 2, 4, 6, 7, and 8 exhibited high activity and cis-1,4 selectivity (99.6%, 25 °C) toward the polymerization of isoprene, although complexes 1, 3, 5, 9, and 10 were inert. The selectivity was not affected by the nature of the central metal and AlR3 and was maintained at elevated temperatures up to 80 °C (97.4%). The yttrium hydrido aluminate cation [(PBNHC)Y(μ-H)2AliBu2]+ was identified as the active species according to NMR spectroscopic analysis

Highly 3,4-Selective Living Polymerization of Isoprene with Rare Earth Metal Fluorenyl N-Heterocyclic Carbene Precursors

Fluorenyl modified N-heterocyclic carbene ligated rare earth metal bis(alkyl) complexes, (Flu-NHC)Ln(CH2SiMe3)2 (Flu-NHC = (C13H8CH2CH2(NCHCCHN)C6H2Me3-2,4,6); Ln = Sc (1a); Ln = Y (1b); Ln = Ho (1c); Ln = Lu (1d)), were synthesized and fully characterized by NMR and X-ray diffraction analyses. Complexes 1b−d with the activation of AliBu3 and [Ph3C][B(C6F5)4] exhibited high activity, medium syndio- but remarkably high 3,4-regio-selectivity, and the unprecedented livingness for the polymerization of isoprene. Such distinguished catalytic performances could be maintained under various monomer-to-initiator ratios (500−5000) and broad polymerization temperatures (25−80 °C). The resultant polymers are crystalline, having syndiotactically enriched (racemic enchainment triad rr = 50%, pentad rrrr = 30%) 3,4-regulated (99%) microstructure and high glass-transition temperatures (40−49 °C). In contrast, complex 1a was almost inert, while complexes bearing indenyl modified N-heterocyclic carbene moiety, (Ind-NHC)Ln(CH2SiMe3)2 (Ind-NHC = C9H6CH2CH2(NCHCCHN)C6H2Me3-2,4,6; Ln = Sc (2a); Ln = Y (2b); Ln = Ho (2c); Ln = Lu (2d)), also showed low activity or selectivity. These differences in catalytic performance could be attributed mainly to the electronics and spacial sterics of the metal center of these precursors

Highly 3,4-Selective Living Polymerization of Isoprene with Rare Earth Metal Fluorenyl N-Heterocyclic Carbene Precursors

Fluorenyl modified N-heterocyclic carbene ligated rare earth metal bis(alkyl) complexes, (Flu-NHC)Ln(CH2SiMe3)2 (Flu-NHC = (C13H8CH2CH2(NCHCCHN)C6H2Me3-2,4,6); Ln = Sc (1a); Ln = Y (1b); Ln = Ho (1c); Ln = Lu (1d)), were synthesized and fully characterized by NMR and X-ray diffraction analyses. Complexes 1b−d with the activation of AliBu3 and [Ph3C][B(C6F5)4] exhibited high activity, medium syndio- but remarkably high 3,4-regio-selectivity, and the unprecedented livingness for the polymerization of isoprene. Such distinguished catalytic performances could be maintained under various monomer-to-initiator ratios (500−5000) and broad polymerization temperatures (25−80 °C). The resultant polymers are crystalline, having syndiotactically enriched (racemic enchainment triad rr = 50%, pentad rrrr = 30%) 3,4-regulated (99%) microstructure and high glass-transition temperatures (40−49 °C). In contrast, complex 1a was almost inert, while complexes bearing indenyl modified N-heterocyclic carbene moiety, (Ind-NHC)Ln(CH2SiMe3)2 (Ind-NHC = C9H6CH2CH2(NCHCCHN)C6H2Me3-2,4,6; Ln = Sc (2a); Ln = Y (2b); Ln = Ho (2c); Ln = Lu (2d)), also showed low activity or selectivity. These differences in catalytic performance could be attributed mainly to the electronics and spacial sterics of the metal center of these precursors

Highly 3,4-Selective Living Polymerization of Isoprene with Rare Earth Metal Fluorenyl N-Heterocyclic Carbene Precursors

Fluorenyl modified N-heterocyclic carbene ligated rare earth metal bis(alkyl) complexes, (Flu-NHC)Ln(CH2SiMe3)2 (Flu-NHC = (C13H8CH2CH2(NCHCCHN)C6H2Me3-2,4,6); Ln = Sc (1a); Ln = Y (1b); Ln = Ho (1c); Ln = Lu (1d)), were synthesized and fully characterized by NMR and X-ray diffraction analyses. Complexes 1b−d with the activation of AliBu3 and [Ph3C][B(C6F5)4] exhibited high activity, medium syndio- but remarkably high 3,4-regio-selectivity, and the unprecedented livingness for the polymerization of isoprene. Such distinguished catalytic performances could be maintained under various monomer-to-initiator ratios (500−5000) and broad polymerization temperatures (25−80 °C). The resultant polymers are crystalline, having syndiotactically enriched (racemic enchainment triad rr = 50%, pentad rrrr = 30%) 3,4-regulated (99%) microstructure and high glass-transition temperatures (40−49 °C). In contrast, complex 1a was almost inert, while complexes bearing indenyl modified N-heterocyclic carbene moiety, (Ind-NHC)Ln(CH2SiMe3)2 (Ind-NHC = C9H6CH2CH2(NCHCCHN)C6H2Me3-2,4,6; Ln = Sc (2a); Ln = Y (2b); Ln = Ho (2c); Ln = Lu (2d)), also showed low activity or selectivity. These differences in catalytic performance could be attributed mainly to the electronics and spacial sterics of the metal center of these precursors

Highly 3,4-Selective Living Polymerization of Isoprene with Rare Earth Metal Fluorenyl N-Heterocyclic Carbene Precursors

Fluorenyl modified N-heterocyclic carbene ligated rare earth metal bis(alkyl) complexes, (Flu-NHC)Ln(CH2SiMe3)2 (Flu-NHC = (C13H8CH2CH2(NCHCCHN)C6H2Me3-2,4,6); Ln = Sc (1a); Ln = Y (1b); Ln = Ho (1c); Ln = Lu (1d)), were synthesized and fully characterized by NMR and X-ray diffraction analyses. Complexes 1b−d with the activation of AliBu3 and [Ph3C][B(C6F5)4] exhibited high activity, medium syndio- but remarkably high 3,4-regio-selectivity, and the unprecedented livingness for the polymerization of isoprene. Such distinguished catalytic performances could be maintained under various monomer-to-initiator ratios (500−5000) and broad polymerization temperatures (25−80 °C). The resultant polymers are crystalline, having syndiotactically enriched (racemic enchainment triad rr = 50%, pentad rrrr = 30%) 3,4-regulated (99%) microstructure and high glass-transition temperatures (40−49 °C). In contrast, complex 1a was almost inert, while complexes bearing indenyl modified N-heterocyclic carbene moiety, (Ind-NHC)Ln(CH2SiMe3)2 (Ind-NHC = C9H6CH2CH2(NCHCCHN)C6H2Me3-2,4,6; Ln = Sc (2a); Ln = Y (2b); Ln = Ho (2c); Ln = Lu (2d)), also showed low activity or selectivity. These differences in catalytic performance could be attributed mainly to the electronics and spacial sterics of the metal center of these precursors

Highly 3,4-Selective Living Polymerization of Isoprene with Rare Earth Metal Fluorenyl N-Heterocyclic Carbene Precursors

Fluorenyl modified N-heterocyclic carbene ligated rare earth metal bis(alkyl) complexes, (Flu-NHC)Ln(CH2SiMe3)2 (Flu-NHC = (C13H8CH2CH2(NCHCCHN)C6H2Me3-2,4,6); Ln = Sc (1a); Ln = Y (1b); Ln = Ho (1c); Ln = Lu (1d)), were synthesized and fully characterized by NMR and X-ray diffraction analyses. Complexes 1b−d with the activation of AliBu3 and [Ph3C][B(C6F5)4] exhibited high activity, medium syndio- but remarkably high 3,4-regio-selectivity, and the unprecedented livingness for the polymerization of isoprene. Such distinguished catalytic performances could be maintained under various monomer-to-initiator ratios (500−5000) and broad polymerization temperatures (25−80 °C). The resultant polymers are crystalline, having syndiotactically enriched (racemic enchainment triad rr = 50%, pentad rrrr = 30%) 3,4-regulated (99%) microstructure and high glass-transition temperatures (40−49 °C). In contrast, complex 1a was almost inert, while complexes bearing indenyl modified N-heterocyclic carbene moiety, (Ind-NHC)Ln(CH2SiMe3)2 (Ind-NHC = C9H6CH2CH2(NCHCCHN)C6H2Me3-2,4,6; Ln = Sc (2a); Ln = Y (2b); Ln = Ho (2c); Ln = Lu (2d)), also showed low activity or selectivity. These differences in catalytic performance could be attributed mainly to the electronics and spacial sterics of the metal center of these precursors

Pyrrolide-Supported Lanthanide Alkyl Complexes. Influence of Ligands on Molecular Structure and Catalytic Activity toward Isoprene Polymerization

The N,N-bidentate ligands 2-{(N-2,6-R)iminomethyl)}pyrrole (HL1, R = dimethylphenyl; HL2, R = diisopropylphenyl) have been prepared. HL1 reacted readily with 1 equiv of lanthanide tris(alkyl)s, Ln(CH2SiMe3)3(THF)2, affording lanthanide bis(alkyl) complexes L1Ln(CH2SiMe3)2(THF)n (1a, Ln = Lu, n = 2; 1b, Ln = Sc, n = 1) via alkane elimination. Reaction of the bulky ligand HL2 with 1 equiv of Ln(CH2SiMe3)3(THF)2 gave the bis(pyrrolylaldiminato) lanthanide mono(alkyl) complexes L22Ln(CH2SiMe3)(THF) (2a, Ln = Lu; 2b, Ln = Sc), selectively. The N,N-bidentate ligand HL3, 2-dimethylaminomethylpyrrole, reacted with Ln(CH2SiMe3)3(THF)2, generating bimetallic bis(alkyl) complexes of central symmetry (3a, Ln = Y; 3b, Ln = Lu; 3c, Ln = Sc). Treatment of the N,N,N,N-tetradentate ligand H2L4, 2,2‘-bis(2,2-dimethylpropyldiimino)methylpyrrole, with equimolar Lu(CH2SiMe3)3(THF)2 afforded a C2-symmetric binuclear complex (4). Complexes 3a, 3b, 3c, and 4 represent rare examples of THF-free binuclear lanthanide bis(alkyl) complexes supported by non-cyclopentadienyl ligands. All complexes have been tested as initiators for the polymerization of isoprene in the presence of AlEt3 and [Ph3C][B(C6F5)4]. Complexes 1a, 1b, and 3a show activity, and 1b is the most active initiator, whereas 2a, 2b, 3b, 3c, and 4 are inert. The microstructure of the resultant polyisoprene has a cis-1,4 or trans-1,4 configuration depending on the initiator applied