Cäcilia Maichle-Mössmer

The synthesis of alkali-metal-free compounds [LnMe3]n is scrutinized using two distinct protocols. The donor-promoted cleavage of homoleptic tetramethylaluminate complexes Ln(AlMe4)3 proves rare-earth metal (Ln) size-dependent and gives access to [LnMe3]n (small to midsized Ln(III)) and [LnMe3-x(AlMe4)x(OEt2)y] (large Ln(III)). Both compounds can be converted to homoleptic tetramethylgallates Ln(GaMe4)3 by addition of trimethylgallium. Crystallization attempts of [LnMe3-x(AlMe4)x(OEt2)y] via extended donor-solvent treatment produced structural snapshots of a diverse set of clusters featuring different stages of methyl group degradation. New homometallic coordination motifs are detected for carbides [(C4−)(Ln3+)4] (Ln = La, Nd) and [(C4−)(Gd3+)6] as well as methylidyne [(CH3−)(Nd3+)5]. Applying the salt-metathesis protocol (Me3TACN)LnCl3(THF)x (x = 0, 1)/MeLi the discrete monomeric 6-coordinate (Me3TACN)LnMe3 (Ln = Dy, Er, Tm) and 7-coordinate (Me3TACN)LnMe3(THF) (Ln = La, Ce) can be obtained (Me3TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane). Incomplete Cl/CH3 exchange to afford mixed methyl/chloride complexes (Me3TACN)LnMe3-xClx was observed for praseodymium, neodymium, and samarium. Crystal structures of most of the new complexes have been obtained and are discussed along with the findings from NMR studies.

The impact of the group 13 metals aluminum, gallium, and indium on isoprene polymerization was investigated for binary catalyst systems composed of the homoleptic tetramethyltrielate rare-earth-metal compounds Ln(EMe4)3 (Ln = La, Nd, Y; E = Al, Ga, In) and the alkylated metal chlorides R2ECl (E = Al, Ga, In; R = Me, Et) at varying nLn: nCl ratios. The formation of the active species is controlled by a favorable association of the precatalyst Ln(EMe4)3 with the cocatalyst R2ECl and concomitant halogenation of the rare-earth-metal center. Group 13 metal alkyl species also have a stabilizing/mediating effect on the active catalyst because a very weak association can lead to temperature-dependent decomposition reactions. A significant “indium effect” was detected for the small yttrium center. The ternary mixture of [YMe3]n/InMe3/Et2InCl (nY:nIn:nCl = 1:3:2) fabricated cis-1,4 polyisoprene (PI) selectively (99%), exceeding the selectivities of Y(AlMe4)3 (47% cis-1,4 PI) and Y(GaMe4)3 (85% cis-1,4 PI) by far. Note that Y(InMe4)3 is not isolable. The importance of balancing the association/dissociation of the active species with the group 13 metal was also revealed when the precatalyst Ln(EMe4)3 was combined with the borate cocatalysts [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4]. The binary catalyst system Nd(InMe4)3/B(C6F5)3 produced PI with a remarkably high cis-1,4 selectivity of 92%, compared to Nd(AlMe4)3/B(C6F5)3 (57% cis-1,4 PI). This study demonstrates that the group 13 metal alkyl has a significant stereoregulating effect, most likely driven by the degree of association with the active rare-earth-metal center.

Recent advances have shown that light metal pyrazolate complexes not only achieve high CO₂ uptake but are also able to convert epoxides and CO₂ to cyclic carbonates catalytically. Surface organometallic chemistry (SOMC) combines reactive metal complexes with the durability and robustness of a support material to form environmentally even more benign materials for CO₂ capture and conversion. In this study, light metal pyrazolates with a variety of oxidation states and ionic radii were grafted onto mesoporous silica SBA-15₅₀₀ affording the hybrid materials [Mg(pz^tBu2)₂]₂@SBA-15₅₀₀, Al(pz^tBu2)₃@SBA-15₅₀₀, Ti^+IV(pz^Me2)₄@SBA-15₅₀₀, and Ti^+III(pz^tBu2)₃@SBA-15₅₀₀. The hybrid materials were characterized via N₂ physisorption, elemental analysis, ICP/OES, DRIFTS, and solid-state NMR spectroscopy, suggesting successful grafting with monometallic surface species and revealing a CO₂ uptake of up to 11 wt%. In addition, Ti^+IV(pz^Me2)[OSi(OtBu)₃]₃ was synthesized as a model complex for surface species likely present for Ti^+IV(pz^Me2)₄@SBA-15₅₀₀. Complex Ti^+IV(pz^Me2)[OSi(OtBu)₃]₃ is also able to insert CO₂ under the formation of the carbamate complex Ti^+IV(CO₂·pz^Me2)[OSi(OtBu)₃]₃, emulating material CO₂@Ti^+IV(pz^Me2)₄@SBA-15₅₀₀. All hybrid materials under study are active catalysts in the cycloaddition of epoxides with CO₂ to form cyclic carbonates. The magnesium hybrid material [Mg(pz^tBu2)₂]₂@SBA-15₅₀₀ exceeds its homogeneous congener, featuring high conversion even for bulkier epoxides along with a desirable reusability.

A series of sandwich cerium azolates (pyrazolates, triazolates, tetrazolates) has been synthesized via salt-metathesis (cerous precursor: [Cp*₂CeCl₂K(thf)]_n; Cp* = C₅Me₅) and protonolysis protocols (cerous precursors: Cp*₂Ce[N(SiHMe₂)₂] or Cp*₂Ce[N(SiMe₃)₂]) and their carboxophilicity has been probed. The sandwich and half-sandwich complexes Cp*₂Ce(pz^Me,Me) and Cp*Ce(pz^Me,Me)₂(thf)₂ show exhaustive CO₂ insertion into the pyrazolato moieties, which leads to bis(carbamato)-bridged dimeric complexes. Complex Cp*₂Ce(pz^Ph,Ph), featuring the sterically more demanding and less nucleophilic diphenylpyrazolato ligand inserts only one molecule CO₂ per two metal centers, forming the asymmetrically bridged complex Cp*₂Ce(μ-pz^Ph,Ph·CO₂)CeCp*₂(pz^Ph,Ph). The triazolato derivatives Cp*₂Ce(tz^Me,Me)(dmap) and Cp*₂Ce(tz^Ph,Ph) indicate CO₂ insertion only for the DMAP-free complex. Utilizing the nitrogen-richer 5-phenyltetrazole resulted in the formation of the trimer [Cp*₂Ce(tet^Ph)]₃, which does not insert CO₂ at ambient temperature. The absorption and electrochemical properties of the compounds were investigated and their catalytic activity in the cycloaddition of CO₂ and propylene oxide was examined.

[This corrects the article DOI: 10.1021/acs.organomet.5c00195.].

A straightforward synthesis of Cp*₂Sc(AlMe₄) (Cp* = C₅Me₅) applying Cp*₂ScCl(thf) and LiAlMe₄/AlMe₃ is described. Donor-assisted trimethyltriel exchange gives access to Cp*₂Sc(EMe₄) (E = Ga, In), which represent the first scandium tetramethylgallate and tetramethylindate complexes. Thermal treatment of Cp*₂Sc(EMe₄) (E = Ga, In) in benzene leads to the isolation of methylidene clusters Cp*₆Sc₄E₈(CH₂)₁₂Me₆ (E = Ga, In). This methyl group deprotonation differs from the benzene activation previously observed for Cp*₂Y(EMe₄) (E = Al, Ga). Treatment of Cp*₂Sc(EMe₄) (E = Al, Ga) with excess GaMe₃ in benzene at elevated temperatures generated Ga₈(CH₂)₁₂, following the known reactivities of Cp*₂Ln(GaMe₄) (Ln = Y, Lu) with GaMe₃. Analogous reactions of scandocene complexes Cp*₂Sc(EMe₄) (E = Al, In) with excess InMe₃ at elevated temperatures did not yield the putative homoleptic indium methylidene. Instead, the methylidene cluster Cp*₄Sc₄In₈(CH₂)₁₂Me₈ was isolated, which features half-sandwich scandium fragments exclusively. A reaction of InMe₃ and HCp* at high temperatures yielded Me₂InCp*InMe₃, which might incorporate the missing Cp* ligands in the cluster formations. Rare-earth-metal compounds were analyzed by SC-XRD, ICP-OES and elemental analysis. Compounds Cp*₂Sc(EMe₄) (E = Ga, In) and [Cp*₂Sc(ClAlMe₃)]₂ were additionally analyzed by ¹H, ¹³C{¹H}, ⁴⁵Sc NMR as well as ¹H and ⁴⁵Sc variable temperature NMR studies.