Heterogenization of Lanthanum and Neodymium Monophosphacyclopentadienyl Bis(tetramethylaluminate) Complexes onto Periodic Mesoporous Silica SBA-15

Abstract

International audienceThe monophosphacyclopentadienyl bis(tetramethylaluminate) lanthanide complexes (η5-PC4Me4)Ln[(μ-Me)2AlMe2]2 and [η5-PC4Me2(SiMe3)2]Ln[(μ-Me)2AlMe2]2 (Ln = La, Nd) have been immobilized onto mesoporous silica SBA-15, which was dehydroxylated at 500 °C. Major reaction pathways comprise methane elimination, that is, silanolysis of Ln(μ-Me)Al moieties with surface silanol groups, and trimethylaluminum separation resulting from donor-induced tetramethylaluminate cleavage. The formation of bis- and monosiloxy surface species is discussed involving transient [(η5-PC4Me4)Ln[(μ-OSi≡)(μ-Me)AlMe2]x[(μ-Me)2AlMe2]2-x] and [{η5-PC4Me2(SiMe3)2}Ln[(μ-OSi≡)(μ-Me)AlMe2]x[(μ-Me)2AlMe2]2-x] (Ln = La, Nd, x = 1, 2) as well as more stable entities such as [(η5-PC4Me2R2)(≡SiO)Ln[(μ-Me)2AlMe2]] and/or [(η5-PC4Me2R2)[(μ-OSi≡)(μ-Me)AlMe2]Ln(Me)] and [(η5-PC4Me2R2)(≡SiO)Ln[(μ-OSi≡)(μ-Me)AlMe2]] and/or [(≡SiO)2Ln(η5-PC4Me2R2)]. Moreover, surface alumination via released trimethylaluminum and the formation of [(≡SiO)3-yAlMey] (with y = 1, 2) surface sites is observed. The organometallic/inorganic hybrid materials (η5-PC4Me2R2)Ln(AlMe4)2@SBA-15-500 have been characterized by DRIFT and solid-state NMR spectroscopy, elemental analysis, and nitrogen physisorption. The equimolar reaction of complex (η5-PC4Me4)Nd[(μ-Me)2AlMe2]2 with tris(tert-butoxy)silanol (HOSi(OtBu)3) produces the siloxide complex (η5-PC4Me4)Nd[{μ-OSi(OtBu)3}(μ-Me)AlMe2][(μ-Me)2AlMe2], which was crystallographically authenticated as a model of a potential monophosphacyclopentadienyl neodymium surface species. All of the mesoporous hybrid materials are moderately active initiators for isoprene polymerization, producing 1,4-cis polyisoprene PI (>99%), which is in contrast to the case for the borate-activated molecular precursors (η5-PC4Me2R2)Ln(AlMe4)2 giving preferentially 1,4-trans PI. The surface organometallic chemistry as well as the polymerization performance markedly depend on the thermal pretreatment of the SBA-15 silica, as shown for the corresponding hybrid materials obtained from SBA-15-200 and SBA-15-700

Silica‐Grafted Neodymium Catalysts for the Production of Ultrahigh‐Molecular‐Weight cis‐1,4‐Polyisoprene

“…Carbon and aluminum elemental analyses (Table S1, Supporting Information) gave on average C/Nd and Al/Nd ratios of 9.5±0.3 and 2.9±0.2, respectively, for the hybrid materials Nd(AlMe 4 ) 3 @PMS ‐500 ( 1 Nd – 4 a Nd ). These data are consistent with an irreversible chemical grafting of the molecular alkylaluminate complexes as corroborated by the observed methane evolution through 1 H NMR spectroscopy …”

“…In accordance with our previous results on half‐sandwich‐complex‐derived hybrid materials (η 5 ‐PC 4 Me 2 R 2 )Ln(AlMe 4 ) 2 @SBA‐15 ‐ T (Ln=La, Nd; R=Me, SiMe 3 ; T =200, 500, 700 °C), the combined characterizations confirmed the occurrence of a multifunctional surface reaction . The grafting of complex Nd(AlMe 4 ) 3 did not lead to the selective formation of the mono‐ and bipodal [Ln[(μ‐OSi)(μ‐Me)AlMe 2 ] x [(μ‐Me) 2 AlMe 2 ] 3− x ] and [Ln[(μ‐OSi)(μ‐Me)AlMe 2 ] x [(μ‐Me) 2 AlMe 2 ] 3− x ] 1 Nd, x a – 4 a Nd, x a ( x= 1,2) surface complexes, but possibly to a mixture of major 1 Nd, 1 b ‐ ‐ 4 a Nd, 1 b , 1 Nd, 1 c – 4 a Nd, 1 c and 1 Nd, 2 b – 4 a Nd, 2 b (Figure ) and minor 1 Nd, 1 d – 4 a Nd, 1 d and 1 Nd, 2 c – 4 a Nd, 2 c surface species (Scheme S1, Supporting Information).…”

“…These observations differed greatly from our previous investigations of homogeneous binary systems in which Ln(AlMe 4 ) 3 (Ln=Y, La, Nd) were converted by Et 2 AlCl into polymeric [LnClMe 2 ] n . We hypothesize that owing to site isolation an intermolecular rearrangement of the surface species present on 1 Al/Nd – 5 Al/Nd is hampered and, therefore, the formation of Ln−Me moieties through the release of AlMe 3 is unfavorable . Similar NMR spectra patterns have been observed if 1 equivalent of Et 2 AlCl was added onto hybrid material La(AlMe 4 ) 3 @MCM‐4 ‐500 1 La leading to 1 La/Al , supporting our hypothesis (Figure S17, Supporting Information).…”

“…These data are consistent with an irreversible chemicalg rafting of the molecular alkylaluminate complexesa sc orroborated by the observed methane evolution through 1 HNMR spectroscopy. [21] Owing to the paramagnetic Nd 3 + metal center,t he solidstate 1 HNMR spectrum (Figure S12, Supporting Information) of hybrid material 1 Nd showed an intensea nd broad resonance centereda tÀ1.89 ppm. This signale xhibits two well-resolved shoulders at approximately À1.66 and À2.21 ppm which are attributedt od istinct methyl groups as previously detected for the model complex [(m-OSi(OtBu) 3 (m-Me)AlMe 2 )Nd((m-Me) 2 AlMe 2 ) 2 ].…”

CO2 Hydrogenation to Formate with Immobilized Ru‐Catalysts Based on Hybrid Organo‐Silica Mesostructured Materials

“…Finally, Ru was incorporated, by first reacting the imidazolium with LiHMDS in the presence of LiBr,, followed by the addition of RuHCl(CO)(PPh 3 ) 3 as the molecular precursor. After ligand functionalization under click conditions and Me 3 Si‐passivation on the material, the surface area dropped from 706 to 451 m 2 g −1 , which is likely due to the surface modification with Me 3 Si groups and post‐functionalization of the propyl azido groups with the very large molecular structure . Introduction of the Ru precursor further caused the surface area of the material to drop to 312 m 2 g −1 but the porous vermicular structure remained intact (Figure b).…”