Organoaluminum Hydroxides Supported by β-Diketiminato Ligands: Synthesis, Structural Characterization, and Reactions

Main group bimetallic partnerships for cooperative catalysis

Over the past decade s-block metal catalysis has undergone a transformation from being an esoteric curiosity to a well-established and consolidated field towards sustainable synthesis. Earth-abundant metals such as Ca, Mg, and Al have shown eye-opening catalytic performances in key catalytic processes such as hydrosilylation, hydroamination or alkene polymerization. In parallel to these studies, s-block mixed-metal reagents have also been attracting widespread interest from scientists. These bimetallic reagents effect many cornerstone organic transformations, often providing enhanced reactivities and better chemo- and regioselectivities than conventional monometallic reagents. Despite a significant number of synthetic advances to date, most efforts have focused primarily on stoichiometric transformations. Merging these two exciting areas of research, this Perspective Article provides an overview on the emerging concept of s/p-block cooperative catalysis. Showcasing recent contributions from several research groups across the world, the untapped potential that these systems can offer in catalytic transformations is discussed with special emphasis placed on how synergistic effects can operate and the special roles played by each metal in these transformations. Advancing the understanding of the ground rules of s-block cooperative catalysis, the application of these bimetalic systems in a critical selection of catalytic transformations encompassing hydroamination, cyclisation, hydroboration to C-C bond forming processes are presented as well as their uses in important polymerization reactions.

(μ-Di-tert-butyl-silanediolato)bis-[bis-(η5-cyclo-penta-dien-yl)methyl-zirconium]

The reaction of t-Bu2Si(OH)2 with two equivalents of Cp2Zr(CH3)2 produces the title t-Bu2SiO2-siloxide bridged dimer, [Zr2(CH3)2(C5H5)4(C8H18O2Si)] or [Cp2Zr(CH3)]2[μ-t-Bu2SiO2] (1), where one methyl group is retained per zirconium atom. The same product is obtained at room temperature even when equimolar ratios of the silanediol and Cp2Zr(CH3)2 are used. Attempts to thermally eliminate methane and produce a bridging methyl-ene complex resulted in decomposition. The crystal structure of 1 displays typical Zr-CH3 and Zr-O distances but the Si-O distance [1.628 (2) Å] and O-Si-O angle [110.86 (15)°] are among the largest observed in this family of compounds suggesting steric crowding between the t-Bu substituents of the silicon atom and the cyclo-penta-dienyl groups. The silicon atom lies on a crystallographic twofold axis and both Cp rings are disordered over two orientations of equal occupancy.

Aluminum Complexes with Redox-Active Formazanate Ligand: Synthesis, Characterization, and Reduction Chemistry

The synthesis of aluminum complexes with redox-active formazanate ligands is described. Salt metathesis using AlCl₃ was shown to form a five-coordinate complex with two formazanate ligands, whereas organometallic aluminum starting materials yield tetrahedral mono(formazanate) aluminum compounds. The aluminum diphenyl derivative was successfully converted to the iodide complex (formazanate)AlI₂, and a comparison of spectroscopic/structural data for these new complexes is provided. Characterization by cyclic voltammetry is supplemented by chemical reduction to demonstrate that ligand-based redox reactions are accessible in these compounds. The possibility to obtain a formazanate aluminum(I) carbenoid species by two-electron reduction was examined by experimental and computational studies, which highlight the potential impact of the nitrogen-rich formazanate ligand on the electronic structure of compounds with this ligand.

The synthesis of a series of aluminum complexes with redox-active formazanate ligands is described and crystallographic, spectroscopic, and voltammetric characterization data are presented. The reduction chemistry of these newly synthesized complexes has been explored and the results are supported by a computational (DFT) study.

Hydrolysis of organometallic and metal–amide precursors: synthesis routes to oxo-bridged heterometallic complexes, metal-oxo clusters and metal oxide nanoparticles

Organometallic and metal amide reagents react with –OH groups to generate metal–oxygen connectivity, yielding metal-oxo heterobimetallics, clusters and nanoparticles.

Reaction of sterically encumbered phenols, TEMPO-H, and organocarbonyl insertion reactions with L-AlH2 (L = HC(MeCNDipp)2, Dipp = 2,6-diisopropylphenyl)

A bulky aluminum dihydride reacts with R–OH and organocarbonyls to give a variety of products; including OC insertion into the Al–H bond.

Insight into Oxide-Bridged Heterobimetallic Al/Zr Olefin Polymerization Catalysts

Reaction of (TBBP)AlMe⋅THF with [Cp*₂ Zr(Me)OH] gave [(TBBP)Al(THF)-O-Zr(Me)Cp*₂ ] (TBBP=3,3',5,5'-tetra-tBu-2,2'-biphenolato). Reaction of [DIPPnacnacAl(Me)-O-Zr(Me)Cp₂ ] with [PhMe₂ NH]⁺ [B(C₆ F₅ )₄ ]^- gave a cationic Al/Zr complex that could be structurally characterized as its THF adduct [(DIPPnacnac)Al(Me)-O-Zr(THF)Cp₂ ]⁺ [B(C₆ F₅ )₄ ]^- (DIPPnacnac=HC[(Me)C=N(2,6-iPr₂ -C₆ H₃ )]₂ ). The first complex polymerizes ethene in the presence of an alkylaluminum scavenger but in the absence of methylalumoxane (MAO). The adduct cation is inactive under these conditions. Theoretical calculations show very high energy barriers (ΔG=40-47 kcal mol^-1 ) for ethene insertion with a bridged AlOZr catalyst. This is due to an unfavorable six-membered-ring transition state, in which the methyl group bridges the metal and ethene with an obtuse metal-Me-C angle that prevents synchronized bond-breaking and making. A more-likely pathway is dissociation of the Al-O-Zr complex into an aluminate and the active polymerization catalyst [Cp*₂ ZrMe]⁺ .

Assembling heterometals through oxygen: an efficient way to design homogeneous catalysts

Assembling a molecule containing two metal centers with entirely different chemical properties remains a synthetic challenge. One of the major motivations for this chemistry is its ability to catalyze various organic transformations. The proximity between two different metals in a heterometallic complex allows more pronounced chemical communication between the metals and often leads to the modification of the fundamental properties of the individual metal atoms through the well-known cooperative interaction. Although various types of heterometallic systems are known, the M-O-M' framework is particularly important because it brings the metals into close proximity with each other. In this Account, we describe several suitable synthetic routes for the assembly of heterometals of entirely different chemical properties through an oxygen atom. The new synthetic strategies for the construction of heterobimetallic complexes take advantage of unprecedented syntheses of a number of hydroxide precursors of the type LMR(OH) [L = CH{N(Ar)(CMe)}(2), Ar = 2,6-iPr(2)C(6)H(3); M = Al, Ga, or Ge; R = alkyl, aryl, or lone pair of electrons], [LSr(mu-OH)](2).(THF)(3), and Cp*(2)ZrMe(OH). We used the Brønsted acidic character of the proton in the M(O-H), Sr(O-H), or Zr(O-H) moiety, to build a new class of heterobimetallic complexes based on M-O-M' motif. This synthetic strategy assembles a main group element with another main group element, a transition metal, or a lanthanide metal. This synthetic development provides access to a new class of heterobimetallic complexes through oxygen bridging. In many cases these complexes prove to be excellent candidates for polymerization of monomers including epsilon-caprolactone, ethylene, and styrene. Some of these catalysts bear a chemically grafted methylalumoxane (MAO) unit in the backbone of an active metal center, which led to efficient ethylene polymerizations at an unusually low MAO concentration. We attribute this reactivity both to the presence of a chemically grafted (Me)Al-O backbone in the active catalysts (a part of externally added cocatalyst, MAO) and to the enhanced Lewis acidity from the bridging oxygen at the active metal center. In addition, we have demonstrated the development of heterometallic systems having two catalytically active centers. Such structures could aid in the development of a catalytic system bearing two active centers with different chemistries.