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]_<i>x</i>[(μ-Me)₂AlMe₂]_2–<i>x</i>] and [{η⁵-PC₄Me₂(SiMe₃)₂}­Ln­[(μ-OSi)­(μ-Me)­AlMe₂]_<i>x</i>[(μ-Me)₂AlMe₂]_2–<i>x</i>] (Ln = La, Nd, <i>x</i> = 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–<i>y</i>AlMe_<i>y</i>] (with <i>y</i> = 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(<i>tert</i>-butoxy)­silanol (HOSi­(O<i>t</i>Bu)₃) produces the siloxide complex (η⁵-PC₄Me₄)­Nd­[{μ-OSi­(O<i>t</i>Bu)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-<i>cis</i> 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-<i>trans</i> 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

Synthesis and Characterization of Bidentate Rare-Earth Iminophosphorane <i>o</i>-Aryl Complexes and Their Behavior As Catalysts for the Polymerization of 1,3-Butadiene

<i>O</i>-Aryllithium complexes are easily prepared from stable aminophosphonium salts, and their coordination to rare-earth metals was studied. The ligand to metal ratio in the formed complexes was shown to depend exclusively on the substituent on the nitrogen atom of the ligand. Aryllithium derivatives <b>3a</b> and <b>3b</b>, exhibiting bulky groups (SiMe₃ and ^<i>t</i>Bu, respectively), gave monocoordinated yttrium complexes <b>4a-</b>Y and <b>4b-</b>Y. On the other hand, with aryllithium <b>3a</b>, possessing an <i></i>isopropyl at nitrogen, complexes of Y^III, Nd^III, and Gd^III with a 2:1 ligand to metal ratio could be obtained. Finally with less hindered ligands such as <b>6c</b>, featuring an <i>n</i>-butyl substituent, triscoordinated Y, Nd, and La complexes were accessible. X-ray crystal structures have been obtained with all three stoichiometries. These complexes were employed as catalyst precursors for 1,3-butadiene polymerization using various activators. Yttrium complexes were found ineffective, but some neodymium complexes achieved highly selective polymerization of 1,3-butadiene, giving up to 95% of 1,4-<i>cis</i>-polybutadiene albeit with mild activity

Ligand Influence on the Redox Chemistry of Organosamarium Complexes: Experimental and Theoretical Studies of the Reactions of (C₅Me₅)₂Sm(THF)₂ and (C₄Me₄P)₂Sm with Pyridine and Acridine

The reactions of the samarium­(II) complexes Tmp₂Sm (Tmp = 2,3,4,5-tetramethyl-1<i>H</i>-phosphol-1-yl) and Cp*₂Sm­(THF)₂ (Cp* = 1,2,3,4,5-tetramethyl-2,4-cyclopentadien-1-yl) with pyridine were found to be different, despite the fact that the Cp* and Tmp π-ligands are similar in size. With Tmp₂Sm, a simple adduct, Tmp₂Sm­(pyridine)₂ is isolated, while with Cp*₂Sm­(THF)₂ pyridine is dimerized with concomitant oxidation of samarium to form [Cp*₂Sm­(C₅H₅N)]₂[μ-(NC₅H₅–C₅H₅N)]. However, reaction of Tmp₂Sm with acridine, a better π-acceptor than pyridine, did result in acridine dimerization and the isolation of [Tmp₂Sm]₂[μ-(NC₁₃H₉–C₁₃H₉N)]. DFT calculations on the model structures of Tmp₂Sm and Cp*₂Sm, and on the single electron transfer step from Sm to pyridine and acridine in these ligand environments, confirmed that, even though the Sm−π-ligand bonds are mostly ionic, the different electronic properties of the Tmp ligand versus that of Cp are responsible for the difference in reactivity of Tmp₂Sm and Cp*₂Sm

Ligand Influence on the Redox Chemistry of Organosamarium Complexes: Experimental and Theoretical Studies of the Reactions of (C₅Me₅)₂Sm(THF)₂ and (C₄Me₄P)₂Sm with Pyridine and Acridine

The reactions of the samarium­(II) complexes Tmp₂Sm (Tmp = 2,3,4,5-tetramethyl-1<i>H</i>-phosphol-1-yl) and Cp*₂Sm­(THF)₂ (Cp* = 1,2,3,4,5-tetramethyl-2,4-cyclopentadien-1-yl) with pyridine were found to be different, despite the fact that the Cp* and Tmp π-ligands are similar in size. With Tmp₂Sm, a simple adduct, Tmp₂Sm­(pyridine)₂ is isolated, while with Cp*₂Sm­(THF)₂ pyridine is dimerized with concomitant oxidation of samarium to form [Cp*₂Sm­(C₅H₅N)]₂[μ-(NC₅H₅–C₅H₅N)]. However, reaction of Tmp₂Sm with acridine, a better π-acceptor than pyridine, did result in acridine dimerization and the isolation of [Tmp₂Sm]₂[μ-(NC₁₃H₉–C₁₃H₉N)]. DFT calculations on the model structures of Tmp₂Sm and Cp*₂Sm, and on the single electron transfer step from Sm to pyridine and acridine in these ligand environments, confirmed that, even though the Sm−π-ligand bonds are mostly ionic, the different electronic properties of the Tmp ligand versus that of Cp are responsible for the difference in reactivity of Tmp₂Sm and Cp*₂Sm

To Bend or Not To Bend: Experimental and Computational Studies of Structural Preference in Ln(Tp^iPr₂)₂ (Ln = Sm, Tm)

The synthesis and characterization of Ln­(Tp^iPr2)₂ (Ln = Sm, <b>3Sm</b>; Tm, <b>3Tm</b>) are reported. While the simple ¹H NMR spectra of the compounds indicate a symmetrical solution structure, with equivalent pyrazolyl groups, the solid-state structure revealed an unexpected, “bent sandwich-like” geometry. By contrast, the structure of the less sterically congested Tm­(Tp^Me2,4Et)₂ (<b>4</b>) adopts the expected symmetrical structure with a linear B–Tm–B arrangement. Computational studies to investigate the origin of the unexpected bent structure of the former compounds indicate that steric repulsion between the isopropyl groups forces the Tp ligands apart and permits the development of unusual interligand C–H···N hydrogen-bonding interactions that help stabilize the structure. These results find support in the similar geometry of the Tm­(III) analogue [Tm­(Tp^iPr2)₂]­I, <b>3Tm</b>^<b>+</b>, and confirm that the low symmetry is not the result of a metal–ligand interaction. The relevance of these results to the general question of the coordination geometry of MX₂ and M­(C₅R₅)₂ (M = heavy alkaline earth and Ln­(II), X = halide, and C₅R₅ = bulky persubstituted cyclopentadienyl) complexes and the importance of secondary H-bonding and nonbonding interactions on the structure are highlighted