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

The 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–y
AlMey] (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.