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Expanding the Scope of Aluminum Chemistry with Noninnocent Ligands
Abstract
Conspectus
Aluminum is the most abundant metal in the earth’s crust at 8%, and it is also widely available domestically in many countries worldwide, which ensures a stable supply chain. To further the applications of aluminum (Al), such as in catalysis and electronic and energy storage materials, there has been significant interest in the synthesis and characterization of new Al coordination compounds that can support electron transfer (ET) and proton transfer (PT) chemistry. This has been achieved using redox and chemically noninnocent ligands (NILs) combined with the highly stable M(III) oxidation state of Al and in some cases the heavier group 13 ions, Ga and In.
When ligands participate in redox chemistry or facilitate the breaking or making of new bonds, they are often termed redox or chemically noninnocent, respectively. Al(III) in particular supports rich ligand-based redox chemistry because it is so redox inert and will support the ligand across many charge and protonation states without entering into the reaction chemistry. To a lesser extent, we have reported on the heavier group 13 elements Ga and In, and this chemistry will also be included in this Account, where available.
This Account is arranged into two technical sections, which are (1) Structures of Al–NIL complexes and (2) Reactivity of Al–NIL complexes. Highlights of the research work include reversible redox chemistry that has been enabled by ligand design to shut down radical coupling pathways and to prevent loss of H2 from unsaturated ligand sites. These reversible redox properties have in turn enabled the characterization of Class III electron delocalization through Al when two NIL are bound to the Al(III) in different charge states. Characterization of the metalloaromatic character of square planar Al and Ga complexes has been achieved, and characterization of the delocalized electronic structures has provided a model within which to understand and predict the ET and PT chemistry of the NIL group 13 compounds. The capacity of Al–NIL complexes to perform ET and PT has been employed in reactions that use ET or PT reactivity only or in reactions where coupled ET/PT affords hydride transfer chemistry. As an example, ligand-based PT reactions initiate metal–ligand cooperative bond activation pathways for catalysis: this includes acceptorless dehydrogenation of formic acid and anilines and transfer hydrogenation chemistry. In a complementary approach, ligand based ET/PT chemistry has been used in the study of dihydropyridinate (DHP–) chemistry where it was shown that N-coordination of group 13 ions lowers kinetic barriers to DHP– formation. Taken together, the discussion presented herein illustrates that the NIL chemistry of Al(III), and also of Ga(III) and In(III) holds promise for further developments in catalysis and energy storage.
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Many chemicals known today are partially or fully aromatic, since a ring framework experiences additional stabilization through the delocalization of π‐electrons. While aromatic rings with equal numbers of π‐electrons and ring atoms such as benzene are particularly stable, those with the minimally required two π‐electrons are very rare and yet remain limited to three‐ and four‐membered rings if not stabilized in the coordination sphere of heavy metals. Here we report the facile synthesis of a dipotassium cyclopentagallene, a unique example of a five‐membered aromatic ring stabilized by only two π‐electrons. Single‐crystal X‐ray diffraction revealed a planar Ga5 ring with almost equal gallium–gallium bond lengths, which together with computational and spectroscopic data confirm its aromatic character. Our results prove that aromatic stabilization goes far beyond what has previously been assumed as minimum π‐electron count in a five‐atom ring fragment.
Molecular design ultimately furnishes improvements in performance over time, and this has been the case for Rh‐ and Ir‐based molecular catalysts currently used in transfer hydrogenation (TH) reactions for fine chemical synthesis. In this report, we describe a molecular pincer ligand Al catalyst for TH, (I2P²⁻)Al(THF)Cl (I2P=diiminopyridine; THF=tetrahydrofuran). The mechanism for TH is initiated by two successive Al‐ligand cooperative bond activations of the O−H bonds in two molecules of isopropanol (ⁱPrOH) to afford six‐coordinate (H2I2P)Al(OⁱPr)2Cl. Stoichiometric chemical reactions and kinetic experiments suggest an ordered transition state, supported by polar solvents, for concerted hydride transfer from ⁱPrO⁻ to substrate. Metal‐ligand cooperative hydrogen bonding in a cyclic transition state is a likely support for the concerted hydride transfer event. The available data does not support involvement of an intermediate Al‐hydride in the TH. Proof‐of‐principle reactions including the conversion of isopropanol and benzophenone to acetone and diphenylmethanol with 90 % conversion in 1 h are described. The analogous hydride compound, (I2P²⁻)Al(THF)H, also cleaves the O−H bond in ⁱPrOH to afford (HI2P⁻)Al(OⁱPr)H and (HI2P⁻)Al(OⁱPr)2, but no activity for catalytic TH was observed.
The development of alternative energy sources is the utmost priority of developing society. Unlike many prior homogeneous electrocatalysts that rely on a change in the oxidation state of the metal center and/or electrochemically active ligand, here we report the synthesis and structural characterization of a bimetallic zinc selenolate complex consisting of a redox silent zinc metal ion and a tridentate ligand that catalyzes the reduction of protons into hydrogen gas electrochemically and displays one of the highest reported TOF for a homogeneous TM-metal free ligand centered HER catalyst, 509 s-1. The current-voltage analysis confirms the onset overpotential of 0.86 V vs. Ag/AgCl for the HER process. Constant potential electrolysis (CPE) has been carried out to study the bulk electrolysis of our developed protocol, which reveals that the bimetallic zinc selenolate catalyst is stable under cathodic as well as anodic potentials and generates hydrogen gas with a faradaic efficiency of 75%. Preliminary studies on the heterogeneous catalyst were conducted by depositing the bimetallic zinc selenolate catalyst on the electrode surface.
This review highlights recent advances in the research of organoaluminum complexes in catalysis. Many fundamental transformations can be carried out by aluminum catalysts, such as hydroboration, hydroamination, copolymerization and CO2 insertion reactions, hydrosilylation, and hydroacetylenation of carbodiimides, which make use of the abundant reserves of aluminum in the earth‘s crust to accomplish the transformation. Recent research in the field has shown that these systems have extraordinary catalytic activity and are inexpensive, abundant, and environmentally friendly.
Ligand-based mixed valent (MV) complexes of Al(III) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I2P) have been prepared. The MV states containing EW groups are both assigned as Class II/III, and those with ED functional groups are Class III and Class II/III in the (I2P⁻)(I2P²⁻)Al and [(I2P²⁻)(I2P³⁻)Al]²⁻ charge states, respectively. No abrupt changes in delocalization are observed with ED and EW groups and from this we infer that ligand and metal valence p-orbitals are well-matched in energy and the absence of LMCT and MLCT bands supports the delocalized electronic structures. The MV ligand charge states (I2P⁻)(I2P²⁻)Al and [(I2P²⁻)(I2P³⁻)Al]²⁻ show intervalence charge transfer (IVCT) transitions in the regions 6850–7740 and 7410–9780 cm⁻¹, respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I2P²⁻)(I2P³⁻)Al]²⁻ complexes containing –PhNMe2 or –PhF5 substituents. Minor localization of charge in [(I2P²⁻)(I2P³⁻)Al]²⁻ was observed when –PhOMe substituents are included.
Most p-block metal amides irreversibly react to metal alkoxides when subjected to alcohols, rendering reversible transformations with OH-substrates a challenging task. Herein, we describe how the combination of a Lewis acidic square-planar-coordinated aluminum(III) center with metal-ligand cooperativity leverages unconventional reactivity toward protic substrates. The calix[4]pyrrolato aluminate performs OH-bond activation of primary, secondary, and tertiary aliphatic and aromatic alcohols, that can be fully reversed under reduced pressure. The products exhibit a new form of metal-ligand cooperative amphoterism and undergo counterintuitive substitution reactions of a polar covalent Al-O bond by a dative Al-N bond. A comprehensive mechanistic picture of all processes is buttressed by isolation of intermediates, spectroscopy, and computation. This study delineates how structural constraints can twist thermodynamics for seemingly simple addition reactions and invert common trends in bond energies.
Metal‐ligand cooperativity (MLC) had a tremendous impact on d‐block metal‐mediated bond activation and homogeneous catalysis. Is this concept translatable to the elements of the p‐block? Are there analogies already at hand? In this review, we describe contributions in which a p‐block element (group 13–15) and its ligand (surrounding molecular framework) operate synergistically in a substrate activation or a catalytic cycle. This activity is termed element‐ligand cooperativity (ELC), in correspondence to MLC. After the concepts of p‐block low‐valent states and frustrated Lewis pairs mimicking the ambiphilic reactivity of transition metals, the spatial proximity of nucleophilic and electrophilic reaction sites and a small HOMO‐LUMO gap in a p‐block ELC complex might offer yet another approach. Selected examples shall illustrate common reactivities of ELC, disclose conceptual analogies with d‐block MLC, and outline shortcomings in this field.
Metal‐ligand cooperativity (MLC) had a remarkable impact on transition metal chemistry and catalysis. By use of the calix[4]pyrrolato aluminate, [1]⁻, which features a square‐planar AlIII, we transfer this concept into the p‐block and fully elucidate its mechanisms by experiment and theory. Complementary to transition metal‐based MLC (aromatization upon substrate binding), substrate binding in [1]⁻ occurs by dearomatization of the ligand. The aluminate trapps carbonyls by the formation of C−C and Al−O bonds, but the products maintain full reversibility and outstanding dynamic exchange rates. Remarkably, the C−C bonds can be formed or cleaved by the addition or removal of lithium cations, permitting unprecedented control over the system's constitutional state. Moreover, the metal‐ligand cooperative substrate interaction allows to twist the kinetics of catalytic hydroboration reactions in a unique sense. Ultimately, this work describes the evolution of an anti‐van't Hoff/Le Bel species from their being as a structural curiosity to their application as a reagent and catalyst.
Aside from simple Lewis acid-base chemistry, the reaction chemistry of aluminacyclopentadienes, which are commonly referred to as aluminoles or simply alumoles, remains relatively underdeveloped. To date, few attempts to extend their inherent insertion and cycloaddition reactivity to the construction of stable aluminum-containing heterocycles have been reported. Herein, we demonstrate the selective ring expansion of a cyclopentadienyl-substituted alumole with a series of organic azides to form unsaturated six-membered AlN heterocycles. Depending on the substituent on the azide, the reaction proceeds either with or without loss of dinitrogen, leading to incorporation of only the “NR” unit of the azide or the entire azo substituent into the periphery of the heterocycle. A deeper understanding of these ring expansion reactions is reached through computational studies, illustrating deviations in the mechanism as a function of the organic azide employed.
Over the past decades, β-diketiminate ligands were widely used in coordination chemistry and are capable of stabilizing various metal complexes in multiple oxidation states. Recently, the chemistry of aluminum and gallium in their +1 oxidation state has rapidly emerged. NacNacM(I) (M = Al, Ga, NacNac = β-diketiminate ligand) exhibits a two coordinate metal center comparable with singlet carbene-like species. The metal center also possesses a formally vacant p-orbital. In this article we present an overview of the last 10 years for aluminum(I) and gallium(I) stabilized β-diketiminate ligands that have been widely explored in bond breaking and forming species.
Salen ligands are a class of Schiff bases simply obtained through condensation of two molecules of a hydroxyl-substituted aryl aldehyde with an achiral or chiral diamine. The prototype salen, or N,N′-bis(salicylidene)ethylenediamine has a long history, as it was first reported in 1889, and immediately, some of its metal complexes were also described. Now, the salen ligands are a class of N,N,O,O tetradentate Schiff bases capable of coordinating many metal ions. The geometry and the stereogenic group inserted in the diamine backbone or aryl aldehyde backbone have been utilized in the past to efficiently transmit chiral information in a variety of different reactions. In this review we will summarize the important and recent achievements obtained in stereocontrolled reactions in which Al(salen) metal complexes are employed. Several other reviews devoted to the general applications and synthesis of chromium and other metal salens have already been published.
Electrochemical regeneration of organic hydrides is often hindered by the rapid dimerization of organic radicals produced as the first intermediates of these electrochemical transformations. In this work, we utilize proton-coupled electron transfer to outcompete the undersired dimerization and achieve successful hydride regenerations of two groups of organic hydrides – acridines and benzimidazoles. This work provides an analysis of the critical factors that control the regeneration pathways of organic hydrides.
The series of pentacoordinated bis‐o‐iminobenzosemiquinonato complexes with general formula (imSQ)2MR where imSQ – is a radical anion of 4,6‐di‐tert‐butyl‐N‐(2,6‐di‐isopropylphenyl)‐o‐iminobenzoquinone (M = Al, Ga, In; R = Akl, Hal, N3, NCS) has been expanded through synthesis of six new (imSQ)2MR complexes [M = In, R = Me (1), Br (5), Cl (6); M = Ga, R = OPh (16), NCO (17); M = Al, R = NCO (23)]. In all complexes (imSQ)2MR except for R = Alk the strong antiferromagnetic exchange coupling between organic radicals, conditioned by the presence of the pathway for superexchange, takes place. In the (imSQ)2GaR complexes the value of magnetic exchange interaction (J) was found to depend linearly on the distance between metal center and inorganic apical substituent: the shortening of Ga–R distance in the row Ga–I > Ga–Br > Ga–Cl > Ga–NCS > Ga–OPh is accompanied by the increasing of J value from –68.6 to –139.3 cm–1. For Al and In analogues complexes such dependence has not been observed. The pathway for superexchange in pentacoordinated (imSQ)2MR complexes was found to be broken after complex coordination geometry changeover to hexacoordinated one, which was clearly demonstrated by the example of pyridine molecule coordination to the (imSQ)2InI (4).
Boron and aluminum are lighter Group 13 elements, found in daily life commodities, and considered environmentally benign. Nevertheless, they markedly differ in their elemental properties (e.g., metal character, atomic radius). The use of Lewis acidic complexes of boron and aluminum for methods of bond activation and catalysis (e.g., hydrogenation of unsaturated substrates, polymerization of olefins and epoxides) is quickly expanding. The introduction of cationic charge may boost the metalloid‐centered Lewis acidity and allow for its fine‐tuning particularly with regard to preference for “hard” or “soft” Lewis bases (i.e., substrates). Especially the isolation of low‐coordinate cations (number of ligand atoms smaller than four) demands elaborate techniques of thermodynamic and kinetic stabilization (i.e., electronic saturation and steric shielding) by a ligand system. Furthermore, the properties of the solvent and the counteranion must be considered with care. Here, selected examples of boron and aluminum cations are described.
The present study first describes the reactivity of low valent Al(II) and Ga(II) complexes of the type (dpp-bian)M–M(dpp-bian) (1, M = Al; 2, Ga; dpp-bian2- = 1,2-bis-(2,6-iPr2-C6H3)-acenaphthenequinonediamido) with cyclic esters/carbonates such as ε-caprolactone (CL) and trimethylene carbonate (TMC). CL and TMC both readily coordinates to the Al(II) species 1 to form the corresponding bis-adducts (dpp-bian)Al(L)–(L)Al(dpp-bian) (3, L = CL; 4, L = TMC), which were structurally characterized confirming that the Al(II)–Al(II) dimetallic backbone retains its integrity in the presence such cyclic polar substrates. In contrast, the less Lewis acidic Ga(II) analogue 2 shows no reaction in the presence of stoichiometric amount of CL and TMC at room temperature. In combination with BnOH, the dinuclear Al(II) species 1 revealed to be an extremely active Al(II) initiator for the controlled ROP of CL at room temperature, outperforming all its Al(III) congeners thus far reported. Detailed DFT studies on the ROP mechanism are consistent with a process occurring thanks to metallic cooperativity between the two Al(II) proximal (since directly bonded) metal centers in 1, which undoubtedly favors the ROP process through bimetallic activation and thus rationalize the unusually high CL ROP activity at room temperature.
Metal-free hydrides are of increasing research interest due to their roles in recent scientific advances in catalysis, such as hydrogen activation with frustrated Lewis pairs and electrocatalytic CO2 reduction with pyridinium and other aromatic cations. The structural design of hydrides for specific applications necessitates the correct description of their thermodynamic and kinetic prowess using reliable parameters – thermodynamic hydricity (ΔGH⁻) and nucleophilicity (N). This review summarizes reported experimental and calculated hydricity values for more than 200 metal-free hydride donors, including carbon-, boron-, nitrogen- and silicon-based hydrides. We describe different experimental and computational methods used to obtain these thermodynamic and kinetic parameters. Furthermore, tabulated data on metal-free hydrides are discussed in terms of structure–property relationships, relevance to catalysis and contemporary limitations for replacing transition-metal hydride catalysts. Finally, several selected applications of metal-free hydrides in catalysis are described, including photosynthetic CO2 reduction and hydrogen activation with frustrated Lewis pairs.
Transition-metal-based molecular complexes are a class of catalyst materials for electrochemical CO2 reduction to CO that can be rationally designed to deliver high catalytic performance. One common mechanistic feature of these electrocatalysts developed thus far is an electrogenerated reduced metal center associated with catalytic CO2 reduction. Here we report a heterogenized zinc-porphyrin complex (zinc(II) 5,10,15,20-tetramesitylporphyrin) as an electrocatalyst that delivers a turnover frequency as high as 14.4 site(-1) s(-1) and a Faradaic efficiency as high as 95% for CO2 electroreduction to CO at -1.7 V vs the standard hydrogen electrode in an organic/water mixed electrolyte. While the Zn center is critical to the observed catalysis, in situ and operando X-ray absorption spectroscopic studies reveal that it is redox-innocent throughout the potential range. Cyclic voltammetry indicates that the porphyrin ligand may act as a redox mediator. Chemical reduction of the zinc-porphyrin complex further confirms that the reduction is ligand-based and the reduced species can react with CO2. This represents the first example of a transition-metal complex for CO2 electroreduction catalysis with its metal center being redox-innocent under working conditions.
The imidazolin-2-imino group is an N-heterocyclic imino functionality that derives from the class of compounds known as guanidines. The exocyclic nitrogen atom preferably bonds to electrophiles and its electron-donating character is markedly enhanced by efficient delocalization of cationic charge density into the five-membered imidazoline ring. Thus, this imino group is an excellent choice for thermodynamic stabilization of electron-deficient species. Due to the variety of available imidazoline-based precursors to this ligand, its steric demand can be tailored to meet the requirements for kinetic stabilization of otherwise highly reactive species. Consequently, it does not come as a surprise that the imidazolin-2-iminato ligand has found widespread applications in transition-metal chemistry to furnish pincer complexes or "pogo stick" type compounds. In comparison, the field of main-group metal compounds of this ligand is still in its infancy; however, it has received growing attention in recent years. A considerable number of electron-poor main-group element species have been described today which are stabilized by N-heterocyclic iminato ligands. These include low-valent metal cations and species that are marked by formerly unknown bonding modes. In this article we provide an overview on the present chemistry of main-group element compounds of the imidazolin-2-iminato ligand, as well as selected examples for the related imidazolidin- and benzimidazolin-2-imino system.
The reaction of (dpp-bian)AlEt(Et2O) () (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) with 4,4'-dimethoxybenzophenone leads to the replacement of Et2O with the ketone resulting complex (dpp-bian)AlEt[O[double bond, length as m-dash]C(4-MeOC6H4)2] (). The reaction of compound with 2,6-di-tert-butyl-4-methylphenol (1 : 1) or Ph2NH (1 : 1) affords the addition products [dpp-bian(H)]AlEt[O(2,6-tBu2-4-MeC6H2)] () and [dpp-bian(H)]AlEt(NPh2) () correspondingly. Treatment of complex with 0.5 molar equivalents of H2O gives ethylalumoxane {[dpp-bian(H)]AlEt}2(μ-O) (). Newly prepared complexes have been characterized by NMR and IR spectroscopy, and the molecular structures of , , and have been determined by single crystal X-ray analysis.
N-alkylation and N-metallation of pyridine are explored herein to understand how metal-ligand complexes can model NAD+ redox chemistry. Syntheses of substituted dipyrazolylpyridine (pz2P) compounds (pz2P)Me+ (1+) and (pz2P)GaCl2+ (2+) are reported, and compared with (pz2P)AlCl2(THF)+ and transition element pz2P complexes from previous reports. Cyclic voltammetry measurements of cationic 1+ and 2+ show irreversible reduction events ∼900 mV anodic those for neutral pz2P complexes of divalent metals. We proposed that N-metallation using Group 13 ions of 3+ charge provides an electrochemical model for N-alkylated pyridyls like NAD+.
Primary phosphido complexes of aluminum(III) are rare, particularly those that are not supported by interactions with Lewis bases or stabilizing cations. Here we report two new examples of unsupported primary phosphido complexes of Al(III). The ancillary ligand on Al is a pincer ligand, diiminopyridine (denoted as I2P). Solid-state structures show distorted tetrahedral geometry about Al and single bond character in the Al−P bond. Near infra-red spectra display low energy absorption bands near 1050 nm that are consistent with pincer ligand – Al charge transfer transitions and metalloaromatic character in the “(I2P²⁻)Al” fragment of the molecules. The phosphido ligands lie out of the I2P ligand plane by up to 15° and this is consistent with our previous reports where π-donor halide ligands occupy the fourth coordination site on Al.
While the implication of the aromaticity concept has been dramatically expanded to date since its emergence in 1865, the classical [4n+2]/4n-electron counting protocol still plays an essential role in evaluating the aromatic nature of compounds. Over the last few decades, a variety of heavier heterocycles featuring the formal [4n+2] π-electron arrangements have been developed, which allows for assessing their aromatic nature. In this review, we present recent developments of the [4n+2]-electron systems of heavier heterocycles involving group 13-15 elements. The synthesis, spectroscopic data, structural parameters, computational data, and reactivity are introduced.
The selective formation of the 1,4-dihydropyridine isomer of NAD(P)H is mirrored by the selective formation of 1,4-dihydropyridinate ligand-metal complexes in synthetic systems. Here we demonstrate that ligand conjugation can be used to promote selective 1,3-dihydropyridinate formation. This represents an advance toward controlling and tuning the selectivity in dihydropyridinate formation chemistry. The reaction of (I2P2-)Al(THF)Cl [1; I2P = bis(imino)pyridine; THF = tetrahydrofuran] with the one-electron oxidant (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) afforded (I2P-)Al(TEMPO)Cl (2), which can be reduced with sodium to the twice-reduced ligand complex (I2P2-)Al(TEMPO) (3). Compounds 2 and 3 serve as precursors for high-yielding and selective routes to an aluminum-supported 1,3-dihydropyridinate complex via the reaction of 2 with 3 equiv of potassium metal or the reaction of 3 with KH.
The chemistry of organoaluminum hydroxides and related chalcogen derivatives is a topic of interest in view of their applications in material science and catalysis. The main interest of organoaluminum hydroxides is due to the tremendous importance of alumoxanes as catalysts or co-catalysts in the polymerization process, and these organoaluminum hydroxides are generally as the intermediates in the hydrolysis of alkyl aluminum compounds. The Brønsted acidic character of the Al-(OH) moiety determines the advantage in building up a new class of heterometallic compounds. Moreover, the heavier chalcogen derivatives with terminal EH (E = S, Se, or Te) groups generally require flexible and innovative synthetic strategies. Therefore, the successful synthesis of organoaluminum hydroxides, thiols, selenols, and tellurols results in kinds of interesting heterobimetallic compounds. The heterobimetallic molecules exhibit high activity in polymerization catalysis and supply more insight into the intramolecular architecture of heterogeneous catalysts. Herein, we summarize the development of organic ligand-supported aluminum hydroxides and the corresponding chalcogen derivatives especially stabilized by β-diketiminate ligand in the past two decades, as well as their applications as building blocks for heterobimetallic compounds consisting of Al-E-M (E = chalcogen elements) skeletons.
Neutral heavier group 13 metals aluminum, gallium, and indium have been utilized as Lewis acid catalysts in various organic transformations ranging from classical organic reactions to polymerization reactions. The introduction of cationic charge can enhance the Lewis acidity of metal centers and allow cationic group 13 complexes to be excellent catalysts in Lewis acid catalysis, including most of the transformations achieved with neutral group 13 complexes. While cationic aluminum complexes have been investigated extensively in catalysis, there is a more recent push to explore the catalytic reactivities of cationic gallium and indium complexes. The field of cationic group 13 complexes has been expanding with discrete cationic complexes supported by purposely designed ligands. This review aims to provide an overview of what has been done to date and ideas of what can be possibly done from now in the growing field of cationic group 13 complexes as catalysts.
Redox flow batteries (RFB's) operate by storing electrons on soluble molecular anolytes and catholytes, and large increases in the energy density of RFB's could be achieved if multiple electrons could be stored in each molecular analyte. Here, we report an organoaluminum analyte, [(I2P-)2Al]+, in which four electrons can be stored on organic ligands, and for which charging and discharging cycles performed in a symmetric nonaqueous RFB configuration remain stable for over 100 cycles at 70% state of charge and 97% Coulombic efficiency (I2P is a bis(imino)pyridine ligand). The stability of the analyte is promoted by the kinetic inertness of the anolyte to trace water in solvents and by the redox inertness of the Al(III) ion to the applied cur-rent. The solubility of the analyte was optimized by exchanging the counter anion for trifluoromethane sulfonate (triflate) and the cell was further optimized using graphite rods as electrodes which, in comparison with glassy carbon and reticulated vitreous carbon, eliminated deposition of analyte on the electrode. Proof-of principle experiments performed with an asym-metric NRFB configuration further demonstrate that up to four electrons can be stored in the cell with no degradation of the analyte over multiple cycles that show 96% Coulombic efficiency.
Syntheses of square planar (SP) coordination complexes of gallium(III) are reported herein. Using the pyridine diimine ligand (PDI), we prepared both (PDI2-)GaH (4) and (PDI2-)GaCl (5), which were spectroscopically and structurally characterized. Reduction of PDI using Na metal afforded "Na2PDI", which reacts with in situ-prepared "GaHCl2" or GaCl3 to afford the SP 4 and 5. The planar geometry of these and previously reported SP Al(III) complexes is attributed to energetic stabilization derived from a ring-current effect, or metalloaromaticity. Typically, aromaticity in metal-containing ring systems can be difficult to characterize or confirm experimentally. An experimental approach employing proton NMR spectroscopy and described here provided an estimate of a downfield chemical shift promoted by a small ring-current associated with metalloaromaticity. Near infrared spectroscopic analyses display ligand-metal charge transfer bands which support the assignment of aromaticity. The SP complexes (PDI2-)AlH (1), (PDI2-)AlCl (2), (PDI2-)AlI (3), 4, and 5 are all discussed in this report, using aromaticity as a model for their electronic structure and reactivity properties.
The phosphaaluminirenes HC[(CMe)(NDipp)]2Al[C(R)=P] (Dipp = 2,6-i-Pr2C6H3, R = tBu or adamantyl) 2 and 3, featur-ing an unsaturated three-membered AlCP ring, have been synthesized as crystalline solids via a [1 + 2] cycloaddition reaction of the aluminium(I) complex HC[(CMe)(NDipp)]2Al (1) with phosphaalkynes. Computational investigations infer three-centered 2π-electron aromaticity of the AlCP rings. Compound 3 is readily protonated by tBuOH to induce a ring opening σ-bond metathesis, giving an alumina-substituted P-hydrogeno phosphaalkene 4. Remarkably, the high strain of the AlCP ring of 3 allows for facile ring enlargement in reactions with CyNC, bis(diisopropylamino) cyclopro-penylidene (BAC), elemental Se, Ph2CO, PhCH=CHCOPh, and PhCN at room temperature. These furnish a series of unprecedented main group heterocycles 5-10 with the C=P unsaturated bonds remaining intact. The mechanisms are considered in the light of thorough density functional theory (DFT) calculations.
Water stable organic mixed valence (MV) compounds have been prepared by reaction of reduced bis(imino)pyridine ligands (I2P) with the trichloride salts of Al, Ga, and In. Coordination of two tridentate ligands to each ion affords octahedral complexes that are accessible with five ligand charge states: [(I2P0)(I2P-)M]2+, [(I2P-)2M]+, (I2P-)(I2P2-)M, [(I2P2-)2M]-, [(I2P2-)(I2P3-)M]2-, and for M = Al only, [(I2P3-)2M]3-. In solid-state structures the anionic members of the redox series are stabilized by intercalation of Na+ cations within the ligands. The MV members of the redox series, (I2P-)(I2P2-)M and [(I2P2-)(I2P3-)M]2-, show characteristic intervalence transitions, in the near-infrared region between 6800 - 7400 and 7800 – 9000 cm-1, respectively. Cyclic voltammetry (CV), NIR spectroscopic, and X-ray structural studies support the assignment of Class II for compounds [(I2P2-)(I2P3-)M]2- and Class III for M = Al and Ga in (I2P-)(I2P2-)M. All compounds containing a singly reduced I2P- ligand exhibit a sharp, low energy transition in the region 5100 – 5600 cm-1 that corresponds to a * - * transition. CV studies demonstrate that the electron transfer events in each of the redox series, Al, Ga, and In span 2.2, 1.4 and 1.2 V, respectively.
The zinc(II) complex of diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-hydrazonepyridine), ZnL1 (1), was prepared and evaluated as a precatalyst for the hydrogen evolution reaction (HER) under homogeneous conditions in acetonitrile. Complex 1 is protonated on the noncoordinating nitrogen of the hydrazonepyridine moiety to yield the active catalyst Zn(HL1)OAc (2) upon addition of acetic acid. Addition of methyl iodide to 1 yields the corresponding methylated derivative ZnL2I (3). In solution, partial dissociation of the coordinated iodide yields the cationic derivative 3'. Complexes 1-3 were characterized by 1H NMR, FT-IR, and UV-visible spectroscopies. The solid-state structures of 2 and 3 were determined by single crystal X-ray diffraction. HER studies conducted in acetonitrile with acetic acid as the proton source yield a turnover frequency (TOF) of 7700 s-1 for solutions of 1 at an overpotential of 1.27 V and a TOF of 6700 s-1 for solutions of 3 at an overpotential of 0.56 V. For both complexes, the required potential for catalysis, Ecat/2, is larger than the thermodynamic reduction potential, E1/2, indicative of a kinetic barrier attributed to intramolecular proton rearrangement. The effect is larger for solutions of 1 (+440 mV) than for solutions of 3 (+160 mV). Controlled potential coulometry studies were used to determine faradaic efficiencies of 71 and 89% for solutions of 1 and 3, respectively. For both catalysts, extensive cycling of potential under catalytic conditions results in the deposition of a film on the glassy carbon electrode surface that is active as an HER catalyst. Analysis of the film of 3 by X-ray photoelectron spectroscopy indicates the complex remains intact upon deposition. A proposed ligand-centered HER mechanism with 1 as a precatalyst to 2 is supported computationally using density functional theory (DFT). All catalytic intermediates in the mechanism were structurally and energetically characterized with the DFT/B3LYP/6-311g(d,p) in solution phase using a polarizable continuum model (PCM). The thermodynamic feasibility of the mechanism is supported by calculation of equilibrium constants or reduction potentials for each proposed step.
A large number of industrially relevant enzymes depend upon dihydronicotinamide adenine dinucleotide (NADH) and dihydronicotinamide adenine dinucleotide phosphate (NADPH) cofactors, which are too expensive to be added in stoichiometric amounts. Existing NAD(P)H-recycling systems suffer from low activity, or the generation of side products. This review focuses on NAD(P)H cofactor regeneration catalyzed by transition metal complexes such as rhodium, ruthenium and iridium complexes using cheap reducing agents such as hydrogen (H2) and ethanol, which have attracted increasing attention as sustainable energy carriers. The catalytic mechanisms for the regioselective reduction of NAD(P)+ are discussed with emphasis on identification of catalytically active intermediates such as transition metal hydride complexes. Applications of NAD(P)H-recycling systems to develop artificial photosynthesis are also discussed.
Reduction of (ArL)Co(II)Br (ArL = 5-mesityl-1,9-(2,4,6-Ph3C6H2)dipyrrin) with potassium graphite afforded the novel Co(I) synthon (ArL)Co. Treatment of (ArL)Co with a stoichiometric amount of various alkyl azides (N3R) furnished three-coordinate Co(III) alkyl imidos (ArL)Co(NR) as confirmed by single crystal X-ray diffraction (R: CMe2Bu, CMe2(CH2)2CHMe2). The exclusive formation of four-coordinate cobalt tetrazido complexes (ArL)Co(κ2-N4R2) was observed upon addition of excess azide, inhibiting any subsequent C–H amination. However, when a weak C–H bond is appended to the imido moie-ty, as in the case of (4-azido-4-methylpentyl)benzene, intramolecular C–H amination kinetically outcompetes formation of the corresponding tetrazene species to generate 2,2-dimethyl-5-phenylpyrrolidine in a catalytic fashion without requir-ing product sequestration. The imido (ArL)Co(NAd) exists in equilibrium in the presence of pyridine with a four-coordinate, cobalt imido (ArL)Co(NAd)(py) (Ka = 8.04 M-1) as determined by 1H NMR titration experiments. Kinetic stud-ies revealed that pyridine binding slows down the formation of the tetrazido complex by blocking azide coordination to the CoIII imido. Further, (ArL)Co(NR’)(py) displays enhanced C−H amination reactivity compared to the pyridine-free complex enabling higher catalytic turnover numbers under milder conditions. The mechanism of C–H amination was probed via kinetic isotope effect (KIE) experiments [kH/kD = 10.2(9)] and initial rate analysis with para-substituted azides suggesting a two-step radical pathway. Lastly, the enhanced reactivity of (ArL)Co(NR’)(py) can be correlated to a higher spin state population, resulting in a decreased crystal field due to a geometry change upon pyridine coordination.
N-aryl and N-alkyl substituted pyridine-2,6-diimines (pdi) are useful tridentate ligands that are redox-active and can coordinate to main group elements, transition metal ions, lanthanides and actinides. The neutral (pdi)⁰ ligand can accept up to four electrons generating a monoanion (pdi[rad])¹⁻ π-radical, singlet or triplet dianion (pdi)²⁻ or (pdi[rad][rad])²⁻, a trianionic π-radical (pdi[rad])³⁻, and a singlet tetraanion (pdi)⁴⁻. Upon this stepwise reduction the four C–N bond distances (Cpy–Npy and Cimine–Nimine) increase and at the same time the two Cpy–Cimine bond distances decrease. We show here that the single structural parameter Δ = [(d2 + d2′)/2 − (d1 + d1′ + d3 + d3′)/4] varies in a linear fashion with increasing reduction of the (pdi⁰)-ligand. Δ represents therefore a powerful structural parameter for the determination of the oxidation level of this ligand in a given complex provided the central metal ion does not exhibit significant π-backdonation effects (M → pdi⁰) as found in compounds with a neutral ligand and where M is a 2nd or 3rd row transition metal ion with dⁿ configuration and n ≥ 6.
This paper reports the first example of selective dearomatization of popular terpyridine (tpy) ligands via alkylation at various positions of the central pyridine ring (2′/6′ vs 3′/5′ and 4′) mediated by AlIII. Isolable zwitterionic Meisenheimer Al complexes, obtained as a result of 3′/5′-alkylation, were identified as efficient precatalysts for the selective hydroboration of C=O and C≡C functionalities. Turnover numbers (TONs) up to ~1000 place the corresponding complexes in the category of the most efficient Al catalysts reported to date for the title reaction.
A series of aluminum(III) complexes supported by the tridentate bis(enol)amine ligand (ONO, 1,1′-azanediylbis(3,3-dimethylbutan-2-one)) in two protonation states have been synthesized and characterized structurally. Reaction of AlCl3 with singly deprotonated H2ONO⁻ afforded pseudo-octahedral [(H2ONO⁻)2Al][AlCl4] (1). AlCl3 was also reacted with doubly deprotonated HONO²⁻ to afford the five-coordinate, pseudotrigonal bipyramidal complex (HONO²⁻)AlCl(THF) (2). The reaction of complex 2 with HCl yielded complex 1, which demonstrates reversible protonation of the ligand backbone. Synthesis of an Al(III) complex of ONO³⁻ was not achieved using a variety of Lewis bases. An attempt to deprotonate complex 2 with tBuOK yielded [(HONO²⁻)Al](μ-tBuO) (3), with no change to the ligand protonation state.
The roles of Al(III) and Ga(III) complexes in the homogeneous catalysis of oxidation/reduction reactions is reviewed. Free metal salts and discrete complexes have primarily been used to catalyze hydride transfer between organic molecules, the epoxidation of alkenes, and the activation of allylic and aliphatic C–H bonds. The complexation of more highly coordinating ligands can improve the chemo-, regio-, and/or stereoselectivity of the reactions catalyzed by the free Al(III) and Ga(III) salts. The Al(III) and Ga(III) metal centers can accelerate the reactions by activating one or both of the redox-active starting materials and/or bringing the reagents together to orient them in a closer approximation of the reaction's transition state. Complexation of a redox-active ligand can enable Al(III) and Ga(III) complexes to gain or lose electrons directly. The reactivities of complexes with non-innocent ligands can approximate those of transition metal complexes in that they can be converted into metal-based oxidants and reductants.
In the last two decades, tremendous progress has been made towards the development of compounds with s- and p-block elements. β-Diketiminate ligands have been widely used in coordination chemistry due to their versatile tunability in both electronic and steric properties. Aluminum complexes stabilized by β-diketiminate ligands have been widely explored in stoichiometric studies to activate small molecules. This review article highlights the recent progress of aluminum complexes supported by β-diketiminate ligands and Cp∗(pentamethylcyclopentadienyl) substituents with univalent aluminum as reagents as well as aluminum hydrides in reactions.
Since the latter quarter of the twentieth century, main group chemistry has undergone significant advances. Power's timely review in 2010, highlighted the inherent differences between the lighter and heavier main group elements, and that the heavier analogues resemble transition metals as shown by their reactivity towards small molecules. In this concept article, we present an overview of the last 10 years since Power's seminal review, and the progress made towards catalytic application. This examines the use of low oxidation state and/or low coordinate group 13 and 14 complexes towards small molecule activation (oxidative addition step in a redox based cycle) and how ligand design plays a crucial role in influencing subsequent reactivity. The challenge in these redox based catalytic cycles still centres on the main group complexes' ability to undergo reductive elimination, however considerable progress in this field has been reported via reversible oxidative addition reactions. Within the last 5 years the first examples of well‐defined low valent main group catalysts have begun to emerge, representing a bright future ahead for main group chemistry.
The present contribution highlights the most representative and emerging developments reported since 2012 on the use of well-defined group 13 metal species in homogeneous catalysis. Apart from their use in polymerization catalysis, group 13 metal catalysts have primarily been developed for the functionalization of polar/unsaturated small molecules, with most reactions involving C-C, C-O or C-N double or triple bonds functionalization (most frequently (C6F5)3Al and low-coordinate Al cations). The exploitation of group 13 catalysts for CO2 functionalization chemistry has made remarkable advances over the past five years, including the development of Al- and Ga-based complexes for CO2 hydrosilylation/hydroboration. Highly effective Al catalysts for the production of cyclic carbonates via CO2/epoxide coupling are also discussed. The emerging use of simple Group 13 metal Lewis pairs for the controlled polymerization of polar monomers is also reviewed.
Redox-active ligands bring electron- and proton-transfer reactions to main-group coordination chemistry. In this Forum Article, we demonstrate how ligand pKa values can be used in the design of a reaction mechanism for a ligand-based electron- and proton-transfer pathway, where the ligand retains a negative charge and enables dihydrogen evolution. A bis(pyrazolyl)pyridine ligand, (iPr)Pz2P, reacts with 2 equiv of AlCl3 to afford [((iPr)Pz2P)AlCl2(THF)][AlCl4] (1). A reaction involving two-electron reduction and single-ligand protonation of 1 affords [((iPr)HPz2P(-))AlCl2] (2), where each of the electron- and proton-transfer events is ligand-centered. Protonation of 2 would formally close a catalytic cycle for dihydrogen production. At -1.26 V versus SCE, in a 0.3 M Bu4NPF6/tetrahydrofuran solution with salicylic acid or (HNEt3)(+) as the source of H(+), 1 produced dihydrogen electrocatalytically, according to cyclic voltammetry and controlled potential electrolysis experiments. The mechanism for the reaction is most likely two electron-transfer steps followed by two chemical steps based on the available reactivity information. A comparison of this work with our previously reported aluminum complexes of the phenyl-substituted bis(imino)pyridine system ((Ph)I2P) reveals that the pKa values of the N-donor atoms in (iPr)Pz2P are lower, which facilitates reduction before ligand protonation. In contrast, the (Ph)I2P ligand complexes of aluminum are protonated twice before reduction liberates dihydrogen.
The past decades have witnessed staggering progress in the chemistry of compounds with s- and p-block elements. Aluminum compounds, especially soluble aluminum hydrides, received wide explorations due to their high reactivity towards protonic reagents and unsaturated compounds containing multiple bonds such as C=O, C=NR, C≡N, and C≡C. Recent studies suggest that reactions employed aluminum hydrides usually occurred via deprotonation or hydroalumination, which exhibit great perspective in main group catalysis. These stoichiometric reactions often act as the initial step during the overall catalytic cycle. Appropriate ligands at the central Al atom are important for the activation of the substrates and the regeneration of the active catalytic molecules. In this review, we focus on the activation of carbonyl compounds, alkenes, and alkynes using soluble aluminum hydrides based on the previous stoichiometric reactions. Different mechanisms were proposed to explain the driving force for the turnover of the catalytic cycle in dehydrocoupling, hydroboration, and hydrosilylation. Moreover, aluminum hydrides stabilized by tridentate ligands, which function in the dehydrocoupling of benzylamine and dehydrogenation of formic acid, are also included in this review.
The unusual square-planar (SP) structure of four-coordinate Al(III)-complexes with the phenyl-substituted tridentate bis(imino)pyridine ((Ph)I2P) ligand has been studied by a combination of density functional theory (DFT) calculations, frontier molecule orbital (FMO), nucleus-independent chemical shift (NICS) and strain/interaction analyses. The calculations disclose that: (i) the aromaticity shifting from the pyridine ring to the alumina-imidazolate metallacycle, and (ii) the small strain energy imposed on the ligand backbone are two favourable factors for driving the overall SP coordination of the Al(III) ion. Meanwhile, the calculations reveal that the SP coordination of the Al(III) ion is also influenced by the fourth ligand that is bonded to the Al(III) ion. The less steric demanding ligand could ensure the stronger aromaticity of the alumina-imidazolate metallacycle and small strain energy on the ligand backbone. Additionally, the calculations predict that Ga(III) and In(III) ions favour the SP geometry in the analogue (Ph)I2P-ligated complexes, while B(III) and Tl(III) ions are reluctant to adopt the SP coordination with the (Ph)I2P ligand. Overall, all of these findings would be helpful for the synthesis of more unknown low-valent main-group metal complexes bearing the bis(imino)pyridine ligand.
The activation of O-H bonds in alcohol substrates is the initial step in acceptor-less catalytic dehydrogenation of alcohols. At room temperature, the bis(imino)pyridine-ligated aluminum hydride compound, denoted as ((I2P2-)-I-Ph)AlH (1), activates O-H bonds in alcohols through a metal-ligand cooperative pathway to afford the phenmdde and benzyloxide complexes with protonated ligand: ((HI2P1-)-H-Ph)Al(OR)H, where R = Ph, Bn. Thermochemical measurements indicate that the amido nitrogen of the protonated ligand in ((HI2P1-)-H-Ph)Al(OPh)H is far more basic (pK(a) = 36-45) than the analogous proton for the previously reported ((HI2P1-)-H-Ph)Al(NHPh)H (pK(a) = 10-14), and this is consistent with reactivity we observe, where ((HI2P1-)-H-Ph)Al(OPh)H complexes do not intramolecularly liberate H-2. The inability of ((HI2P1-)-H-Ph)Al(OR)H to release H-2 upon heating precludes access to a four-coordinate Al center and results in an inability of 1 to dehydrogenate benzyl alcohol to benzaldehyde. These observations also lend further support to the mechanism for benzylamine dehydrogenation that we have previously proposed and provide insights for future catalyst design using metalligand cooperative pathways with Al.
Environmentally sustainable hydrogen‐evolving electrocatalysts are key in a renewable fuel economy, and ligand‐based proton and electron transfer could circumvent the need for precious metal ions in electrocatalytic H2 production. Herein, we show that electrocatalytic generation of H2 by a redox‐active ligand complex of Al3+ occurs at −1.16 V vs. SCE (500 mV overpotential). GermanZwei in einem: Protonen‐ und Elektronentransfer durch einen Al3+‐Komplex mit redoxaktivem Iminopyridinliganden fördern die elektrokatalytische H2‐Entwicklung. Das Al3+‐Zentrum bringt das Reduktionspotential des Liganden in einen zugänglichen Bereich für die Protonenbildung bei niedriger Überspannung. Der Mechanismus umfasst zwei Protonierungen am Liganden und eine Zwei‐Elektronen‐Reduktion zur H2‐Freisetzung (siehe Bild).
The hydrogenation of alkenes is one of the most impactful reactions catalyzed by homogeneous transition metal complexes finding application in the pharmaceutical, agrochemical, and commodity chemical industries. For decades, catalyst technology has relied on precious metal catalysts supported by strong field ligands to enable highly predictable two-electron redox chemistry that constitutes key bond breaking and forming steps during turnover. Alternative catalysts based on earth abundant transition metals such as iron and cobalt not only offer potential environmental and economic advantages but also provide an opportunity to explore catalysis in a new chemical space. The kinetically and thermodynamically accessible oxidation and spin states may enable new mechanistic pathways, unique substrate scope, or altogether new reactivity. This Account describes my group's efforts over the past decade to develop iron and cobalt catalysts for alkene hydrogenation. Particular emphasis is devoted to the interplay of the electronic structure of the base metal compounds and their catalytic performance. First generation, aryl-substituted pyridine(diimine) iron dinitrogen catalysts exhibited high turnover frequencies at low catalyst loadings and hydrogen pressures for the hydrogenation of unactivated terminal and disubstituted alkenes. Exploration of structure-reactivity relationships established smaller aryl substituents and more electron donating ligands resulted in improved performance. Second generation iron and cobalt catalysts where the imine donors were replaced by N-heterocyclic carbenes resulted in dramatically improved activity and enabled hydrogenation of more challenging unactivated, tri- and tetrasubstituted alkenes. Optimized cobalt catalysts have been discovered that are among the most active homogeneous hydrogenation catalysts known. Synthesis of enantiopure, C1 symmetric pyridine(diimine) cobalt complexes have enabled rare examples of highly enantioselective hydrogenation of a family of substituted styrene derivatives. Because improved hydrogenation performance was observed with more electron rich supporting ligands, phosphine cobalt(II) dialkyl complexes were synthesized and found to be active for the diastereoselective hydrogenation of various substituted alkenes. Notably, this class of catalysts was activated by hydroxyl functionality, representing a significant advance in the functional group tolerance of base metal hydrogenation catalysts. Through collaboration with Merck, enantioselective variants of these catalysts were discovered by high throughput experimentation. Catalysts for the hydrogenation of functionalized and essentially unfunctionalized alkenes have been discovered using this approach. Development of reliable, readily accessible cobalt precursors facilitated catalyst discovery and may, along with lessons learned from electronic structure studies, provide fundamental design principles for catalysis with earth abundant transition metals beyond alkene hydrogenation.
Diphenylacetylene reacts with aluminum complex (dpp-bian)AlEt(Et2O) (1) (dpp-bian - 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) in the absence of any solvent at 110-130 degrees C under vacuum to give compound [dpp-bian(PhC=CPh)]AlEt (2). The reaction of methylvinylketone with complex 1 easily proceeds at ambient temperature in Et2O and results in the formation of compound [dpp-bian(CH2-CH=C(Me)-O)]AlEt (3). Both reactions proceed via addition of unsaturated organic substrate across Al-N-C bond sequence in complex 1. Complexes 2 and 3 have been characterized by IR and H-1 NMR spectroscopy. Molecular structures of 2 and 3 have been determined by single-crystal X-ray analysis. Complex 2 was found to be catalyst for the reaction between phenylacetylene and diphenylamine. A full conversion of the reagents was achieved with 5 mol% of complex 2 in benzene in 140 h at 110 degrees C resulting N-phenyl-2-(1-phenylvinyl)aniline (4a) as the major product (87%).
Herein, we report that molecular aluminium complexes of the bis(imino) pyridine ligand, ((I2P2-)-I-Ph) Al(THF) X, X = H (1), CH3 (2), promote selective dehydrogenation of formic acid to H-2 and CO2 with an initial turnover frequency of 5200 turnovers per hour. Low-temperature reactions show that reaction of 1 with HCOOH affords a complex that is protonated three times: twice on the (I2P2-)-I-Ph ligand and once to liberate H-2 or CH4 from the Al-hydride or Al-methyl, respectively. We demonstrate that in the absence of protons, insertion of CO2 into the Al-hydride bond of 1 is facile and produces an Al-formate. Upon addition of protons, liberation of CO2 from the Al-formate complex affords an Al-hydride. Deuterium labelling studies and the solvent dependence of the reaction indicate that outer sphere beta-hydride abstraction supported by metal-ligand cooperative hydrogen bonding is a likely mechanism for the C-H bond cleavage.
Non-Innocent ligand complexes of aluminum are described in this Concept article, beginning with a discussion of their synthesis, and then structural and electronic characterization. The main focus concerns the ability of the ligands in these complexes to mediate proton transfer reactions. As examples, aluminum-ligand cooperation in the activation of polar bonds is described, as is the importance of hydrogen bonding to stabilization of a transition state for β-hydride abstraction. Taken together these reactions enable catalytic processes such as the dehydrogenation of formic acid.
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The synthesis of two four-coordinate and square planar (SP) complexes of aluminum(III) is presented. Reaction of a phenyl-substituted bis(imino)pyridine ligand that is reduced by two electrons, Na2(PhI2P2−), with AlCl3 afforded five-coordinate [(PhI2P2−)Al(THF)Cl] (1). Square-planar [(PhI2P2−)AlCl] (2) was obtained by performing the same reaction in diethyl ether followed by lyphilization of 2 from benzene. The four-coordinate geometry index for 2, τ4, is 0.22, where 0 would be a perfectly square-planar molecule. The analogous aluminum hydride complex, [(PhI2P2−)AlH] (3), is also square-planar, and was characterized crystallographically and has τ4=0.13. Both 2 and 3 are Lewis acidic and bind 2,6-lutidine.