Alpha-metalated N,N-dimethylbenzylamine rare-earth metal complexes and their catalytic applications

This perspective summarizes our group's extensive research in the realm of organometallic lanthanide complexes, while also placing the catalytic reactions supported by these species within the context of known lanthanide catalysis worldwide, with a specific focus on phosphorus-based catalytic reactions such as intermolecular hydrophosphination and hydrophosphinylation. α-Metalated N,N-dimethylbenzylamine ligands have been utilized to generate homoleptic lanthanide complexes, which have subsequently proven to be highly active lanthanum-based catalysts. The main goal of our research program has been to enhance the catalytic efficiency of lanthanum-based complexes, which began with initial successes in the stoichiometric synthesis of organometallic lanthanide complexes and utilization of these species in catalytic hydrophosphination reactions. Not only have these species supported traditional lanthanide catalysis, such as the hydrophosphination of heterocumulenes like carbodiimides, isocyanates, and isothiocyanates, but they have also been effective for a plethora of catalytic reactions tested thus far, including the hydrophosphinylation and hydrophosphorylation of nitriles, hydrophosphination and hydrophosphinylation of alkynes and alkenes, and the heterodehydrocoupling of silanes and amines. Each of these catalytic transformations is meritorious in its own right, offering new synthetic routes to generate organic scaffolds with enhanced functionality while concurrently minimizing both waste generation and energy consumption. Objectives: We aim for the research summary presented herein to inspire and encourage other researchers to investigate f-element based stoichiometric and catalytic reactions. Our efforts in this field began with the recognition that potassium salts of benzyldimethylamine preferred deprotonation at the α-position, rather than the ortho-position, and we wondered if this regiochemistry would be retained in the formation of lanthanide complexes. The pursuit of this simple idea led first to a series of structurally fascinating homoleptic organometallic lanthanide complexes with surprisingly good stability. Fundamental studies of the protonolysis chemistry of these complexes ultimately revealed highly versatile lanthanide-based precatalysts that have propelled a catalytic investigation spanning more than a decade. We anticipate that this summative perspective will animate the synthetic as well as biological communities to consider La(DMBA)3-based catalytic methods in the synthesis of functionalized organic scaffolds as an atom-economic, convenient, and efficient methodology. Ultimately, we envision our work making a positive impact on the advancement of novel chemical transformations and contributing to progress in various fields of science and technology.

Electrophilic Hydrosilylation of Electron-Rich Alkenes Derived from Enamines

The low-electron count, air-stable, platinum complexes [Pt(ItBu')(ItBu)][BArF] (C1) (ItBu=1,3-di-tert-butylimidazol-2-ylidene), [Pt(SiPh)3(ItBuiPr)2][BArF] (C2) (ItBuiPr=1-tert-butyl-3-iso-propylimidazol-2-ylidene), [Pt(SiPh)3(ItBuMe)2][BArF] (C3), [Pt(GePh3)(ItBuiPr)2][BArF] (C4), [Pt(GePh)3(ItBuMe)2][BArF] (C5) and [Pt(GeEt)3(ItBuMe)2][BArF] (C6) (ItBuMe=1-tert-butyl-3-methylimidazol-2-ylidene) are efficient catalysts (particularly the germyl derivatives) in both the silylative dehydrocoupling and hydrosilylation of electron rich alkenes derived from enamines. The steric hindrance exerted by the NHC ligand plays an important role in the selectivity of the reaction. Thus, bulky ligands are selective towards the silylative dehydrocoupling process whereas less sterically hindered promote the selective hydrosilylation reaction. The latter is, in addition, regioselective towards the β-carbon atom of both internal and terminal enamines, leading to β-aminosilanes. Moreover, the syn stereochemistry of the amino and silyl groups implies an anti Si-H bond addition across the double bond. All these facts point to a mechanistic picture that, according to experimental and computational studies, involves a non-classical hydrosilylation process through an outer-sphere mechanism in which a formal nucleophilic addition of the enamine to the silicon atom of a platinum σ-SiH complex is the key step. This is in sharp contrast with the classical Chalk-Harrod mechanism prevalent in platinum chemistry.

Sustainable preparation of aminosilane monomers, oligomers, and polymers through Si-N dehydrocoupling catalysis

This article covers historical and recent efforts to catalyse the dehydrocoupling of amines and silanes, a direct method for Si-N bond formation that offers hydrogen as a byproduct. In some applications, this transformation can be used as a sustainable replacement for traditional aminosilane synthesis, which demands corrosive chlorosilanes while generating one equivalent of ammonium salt waste for each Si-N bond that is formed. These advantages have driven the development of Si-N dehydrocoupling catalysts that span the periodic table, affording mechanistic insight that has led to advances in efficiency and selectivity. Given the divergence in precursors being used, characterization methods being relied on, and applications being targeted, this article highlights the formation of monomeric aminosilanes separately from oligomeric and polymeric aminosilanes. A recent study that allowed for the manganese catalysed synthesis of perhydropolysilazane and commercial chemical vapor deposition precursors is featured, and key opportunities for advancing the field of Si-N dehydrocoupling catalysis are discussed.

Silicon-nitrogen bond formation via dealkynative coupling of amines with bis(trimethylsilyl)acetylene mediated by KHMDS

The catalytic synthesis of silylamines mediated by s- and p-block catalysts is largely underdeveloped. Herein, commercially available potassium bis(trimethylsilyl)amide serves as an efficient alternative to transition metal complexes. N-H/Si-C dealkynative coupling was achieved by means of user-friendly main-group catalysis with ample substrate scope and high chemoselectivity.

N-Silylamines in catalysis: synthesis and reactivity

A summary of the catalytic synthesis and reactivity of N-silylated amines is presented. Dehydrocoupling of amines with silanes, hydrosilylation of imines and dealkenylative coupling of amines with vinylsilanes are three ways to achieve their catalytic syntheses. The resultant N-silylamines serve as substrates in a variety of reactions, including C-N and C-C bond forming reactions, and are preferred in transformations because of the facile Si-N hydrolytic cleavage to reveal free amine products upon reaction completion. This review highlights the distinct electronic properties of N-silyl amines, N-silyl imines and N-silyl enamines that result in complementary reactivity to that of parent non-silyl variants.

Activation of Si–H and B–H bonds by Lewis acidic transition metals and p-block elements: same, but different

In this Perspective we discuss the ability of transition metal complexes to activate and cleave the Si–H and B–H bonds of hydrosilanes and hydroboranes (tri- and tetra-coordinated) in an electrophilic manner, avoiding the need for the metal centre to undergo two-electron processes (oxidative addition/reductive elimination). A formal polarization of E–H bonds (E = Si, B) upon their coordination to the metal centre to form σ-EH complexes (with coordination modes η¹ or η²) favors this type of bond activation that can lead to reactivities involving the formation of transient silylium and borenium/boronium cations similar to those proposed in silylation and borylation processes catalysed by boron and aluminium Lewis acids. We compare the reactivity of transition metal complexes and boron/aluminium Lewis acids through a series of catalytic reactions in which pieces of evidence suggest mechanisms involving electrophilic reaction pathways.

In this Perspective we compare the ability of transition metals and p-block Lewis acids to activate electrophilically hydrosilanes and hydroboranes. The mechanistic similarities and dissimilarities in different catalytic transformations are analyzed.

Coordinatively Unsaturated Amidotitanocene Cations with Inverted σ and π Bond Strengths: Controlled Release of Aminyl Radicals and Hydrogenation/Dehydrogenation Catalysis

Cationic amidotitanocene complexes [Cp₂ Ti(NPhAr)][B(C₆ F₅ )₄ ] (Cp=η⁵ -C₅ H₅ ; Ar=phenyl (1 a), p-tolyl (1 b), p-anisyl (1 c)) were isolated. The bonding situation was studied by DFT (Density Functional Theory) using EDA-NOCV (Energy Decomposition Analysis with Natural Orbitals for Chemical Valence). The polar Ti-N bond in 1 a-c features an unusual inversion of σ and π bond strengths responsible for the balance between stability and reactivity in these coordinatively unsaturated species. In solution, 1 a-c undergo photolytic Ti-N cleavage to release Ti(III) species and aminyl radicals ⋅NPhAr. Reaction of 1 b with H₃ BNHMe₂ results in fast homolytic Ti-N cleavage to give [Cp₂ Ti(H₃ BNHMe₂ )][B(C₆ F₅ )₄ ] (3). 1 a-c are highly active precatalysts in olefin hydrogenation and silanes/amines cross-dehydrogenative coupling, whilst 3 efficiently catalyzes amine-borane dehydrogenation. The mechanism of olefin hydrogenation was studied by DFT and the cooperative H₂ activation key step was disclosed using the Activation Strain Model (ASM).

Highly Efficient Synthesis of Poly(silylether)s: Access to Degradable Polymers from Renewable Resources

The design of new materials with tunable properties and intrinsic recyclability, derived from biomass under mild conditions, stands as a gold standard in polymer chemistry. Reported herein are platinum complexes which catalyze the formation of poly(silylether)s (PSEs) at low catalyst loadings. These polymers are directly obtained from dual-functional biobased building blocks such as 5-hydroxymethylfurfural (HMF) or vanillin, coupled with various dihydrosilanes. Access to different types of copolymer architectures (statistical or alternating) is highlighted by several synthetic strategies. The materials obtained were then characterized as low T_g materials (ranging from -60 to 29 °C), stable upon heating (T_-5% up to 301 °C) and resistant towards uncatalyzed methanolysis. Additionally, quantitative chemical recycling of several PSEs could be triggered by acid-catalyzed hydrolysis or methanolysis. These results emphasize the interest of biobased poly(silylether)s as sustainable materials with high recycling potential.

Silicon-Nitrogen Bond Formation via Heterodehydrocoupling and Catalytic N-Silylation

Silicon-nitrogen bond formation is an important subfield in main group chemistry, and catalysis is an attractive route for efficient, selective formation of these bonds. Indeed, heterodehydrocoupling and N-silylation offer facile methods for the synthesis of small molecules through the coupling of primary, secondary, and tertiary silanes with N-containing substrates such as amines, carbazoles, indoles, and pyrroles. However, the reactivity of these catalytic systems is far from uniform, and critical issues are often encountered with product selectivity, conversions, substrate scope, catalyst activation, and in some instances, competing side reactions. Herein, a catalogue of catalysts and their reactivity for Si-N heterodehydrocoupling and N-silylation are reported.

Syntheses of Dianionic α-Iminopyridine Rare-Earth Metal Complexes and Their Catalytic Acitivities toward Dehydrogenative Coupling of Amines with Hydrosilanes

Reactions of [(Me₃Si)₂N]₃RE(μ-Cl)Li(THF)₃ with aminomethylene-substituted pyridine 2-[O(CH₂CH₂)₂NCH₂CH₂NCH₂]C₅H₄N (1) gave the dianionic α-iminopyridine rare-earth metal amido complexes {μ-η²:σ¹:κ¹:κ¹-2-[O(CH₂CH₂)₂NCH₂CH₂NCH]C₅H₄N}₂RE₂[N(SiMe₃)₂]₂ (RE = Y(2a), La(2b), Pr(2c), Nd(2d), Sm(2e), Dy(2f), Er(2g), and Lu (2h)). However, reaction of [(Me₃Si)₂N]₃Y(μ-Cl)Li(THF)₃ with pyridin-2-ylmethyl-substituted amines such as 2-(RNHCH₂)C₅H₄N (R = ^tBu (3a) and 2,6-ⁱPr₂Ph (3b)) or benzyl-substituted amine O(CH₂CH₂)₂NCH₂CH₂NHCH₂C₆H₅ (5) afforded the corresponding yttrium complexes containing monoanionic ligands [2-(RNCH₂)C₅H₄N]₂YN(SiMe₃)₂ (R = ^tBu (4a) and 2,6-ⁱPr₂Ph (4b)) or [O(CH₂CH₂)₂NCH₂CH₂NCH₂C₆H₅][(Me₃Si)₂N)]Y(μ-Cl)(μ-η³-O(CH₂CH₂)₂NCH₂CH₂NCH₂C₆H₅)Li(THF) (6). Dianionic α-iminopyridine rare-earth metal amido complexes showed high catalytic activities for the dehydrogenation coupling reaction of hydrosilanes and amines providing a variety of silylamines in high yields.

Enhancing the catalytic properties of well-defined electrophilic platinum complexes

Platinum complexes have been often considered as the least reactive of the group 10 triad metals. Slow kinetics are behind this lack of reactivity but, still, some industrially relevant catalytic process are dominated by platinum compounds and sometimes different selectivities can be found in comparison to Ni or Pd. Nevertheless, during the last years, it has been reported that the catalytic behaviour of well-defined platinum derivatives can be improved through a judicious choice of their electronic and steric properties, leading to highly electrophilic or low-electron count platinum systems. In this feature article, we highlight some catalytic processes in which well-defined electrophilic platinum complexes or coordinatively unsaturated systems play an important role in their catalytic activity.