Synthesis and Reactivity of Discrete Europium(II) Hydride Complexes.

The bulky ß-diketiminate ligand frameworks [BDIDCHP]- and [BDIDipp/Ar]- (BDI=[HC{C(Me)2N-Dipp/Ar}2]- (Dipp=2,6-diisopropylphenyl (Dipp); Ar=2,6-dicyclohexylphyenyl (DCHP) or 2,4,6-tricyclohexylphyenyl (TCHP)) have been developed for the kinetic stabilisation of the first europium (II) hydride complexes, [(BDIDCHP)Eu(µ-H)]2, [(BDIDipp/DCHP)Eu(µ-H)]2 and [(BDIDipp/TCHP)Eu(µ-H)]2, respectively. These complexes represent the first step beyond the current lanthanide(II) hydrides that are all based on ytterbium. Tuning the steric profile of ß-diketiminate ligands from a symmetrical to unsymmetrical disposition, enhanced solubility and stability in the solution-state. This provides the first opportunity to study the structure and bonding of these novel Eu(II) hydride complexes crystallographically, spectroscopically and computationally, with their preliminary reactivity investigated.

Reductive Coupling of a Diazoalkane Derivative Promoted by a Potassium Aluminyl and Elimination of Dinitrogen to Generate a Reactive Aluminium Ketimide.

The reaction of 9-diazo-9H-fluorene (fluN2 ) with the potassium aluminyl K[Al(NON)] ([NON]2- =[O(SiMe2 NDipp)2 ]2- , Dipp=2,6-iPr2 C6 H3 ) affords K[Al(NON)(κN1 ,N3 -{(fluN2 )2 })] (1). Structural analysis shows a near planar 1,4-di(9H-fluoren-9-ylidene)tetraazadiide ligand that chelates to the aluminium. The thermally induced elimination of dinitrogen from 1 affords the neutral aluminium ketimide complex, Al(NON)(N=flu)(THF) (2) and the 1,2-di(9H-fluoren-9-yl)diazene dianion as the potassium salt, [K2 (THF)3 ][fluN=Nflu] (3). The reaction of 2 with N,N'-diisopropylcarbodiimide (iPrN=C=NiPr) affords the aluminium guanidinate complex, Al(NON){N(iPr)C(N=CMe2 )N(CHflu)} (4), showing a rare example of reactivity at a metal ketimide ligand. Density functional theory (DFT) calculations have been used to examine the bonding in the newly formed [(fluN2 )2 ]2- ligand in 1 and the ketimide bonding in 2. The mechanism leading to the formation of 4 has also been studied using this technique.

Potassium Aluminyl Promoted Carbonylation of Ethene.

The potassium aluminyl [K{Al(NONDipp )}]2 ([NONDipp ]2- =[O{SiMe2 NDipp}2 ]2- , Dipp=2,6-iPr2 C6 H3 ) activates ethene towards carbonylation with CO under mild conditions. An isolated bis-aluminacyclopropane compound reacted with CO via carbonylation of an Al-C bond, followed by an intramolecular hydrogen shift to form K2 [Al(NONDipp )(µ-CH2 CH=CO-1κ2 C1,3 -2κO)Al(NONDipp )Et]. Restricting the chemistry to a mono-aluminium system allowed isolation of [Al(NONDipp )(CH2 CH2 CO-κ2 C1,3 )]- , which undergoes thermal isomerisation to form the [Al(NONDipp )(CH2 CH=CHO-κ2 C,O)]- anion. DFT calculations highlight the stabilising influence of incorporated benzene at multiple steps in the reaction pathways.

Ytterbium (II) Hydride as a Powerful Multielectron Reductant.

A dimeric ß-diketiminato ytterbium(II) hydride affects both the two-electron aromatization of 1,3,5,7-cyclooctatetraene (COT) and the more challenging two-electron reduction of polyaromatic hydrocarbons, including naphthalene (E0 =-2.60 V). Confirmed by Density Functional Theory calculations, these reactions proceed via consecutive polarized Yb-H/C=C insertion and deprotonation steps to provide the respective ytterbium (II) inverse sandwich complexes and hydrogen gas. These observations highlight the ability of a simple ytterbium(II) hydride to act as a powerful two-electron reductant at room temperature without the necessity of an external electron to initiate the reaction and avoiding radicaloid intermediates.

Oxidative Addition of Hydridic, Protic, and Nonpolar E-H Bonds (E = Si, P, N, or O) to an Aluminyl Anion.

The aluminyl anion K[Al(NONDipp)] {NONDipp = [O(SiMe2NDipp)2]2-; Dipp = 2,6-iPr2C6H3} engages in oxidative additions with the E-H (E = Si, P, N, or O) bonds of phenylsilane (PhSiH3), mesityl phosphane (MesPH2; Mes = 2,4,6-Me3C6H2), 2,6-di-iso-propylaniline (DippNH2), and 2,6-di-tert-butyl-4-methylphenol (ArOH). The resulting (hydrido)aluminate salts are formed regardless of the E-H bond polarity. All of the products were characterized by nuclear magnetic resonance and infrared spectroscopic techniques and single-crystal X-ray diffraction. This study highlights the versatility of aluminyl anions to activate hydridic, acidic, and (essentially) nonpolar E-H bonds.

Controlling Al-M Interactions in Group 1 Metal Aluminyls (M = Li, Na, and K). Facile Conversion of Dimers to Monomeric and Separated Ion Pairs.

The aluminyl compounds [M{Al(NONDipp)}]2 (NONDipp = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3), which exist as contacted dimeric pairs in both the solution and solid states, have been converted to monomeric ion pairs and separated ion pairs for each of the group 1 metals, M = Li, Na, and K. The monomeric ion pairs contain discrete, highly polarized Al-M bonds between the aluminum and the group 1 metal and have been isolated with monodentate (THF, M = Li and Na) or bidentate (TMEDA, M = Li, Na, and K) ligands at M. The separated ion pairs comprise group 1 cations that are encapsulated by polydentate ligands, rendering the aluminyl anion, [Al(NONDipp)]- "naked". For M = Li, this structure type was isolated as the [Li(TMEDA)2]+ salt directly from a solution of the corresponding contacted dimeric pair in neat TMEDA, while the polydentate [2.2.2]cryptand ligand was used to generate the separated ion pairs for the heavier group 1 metals M = Na and K. This work shows that starting from the corresponding contacted dimeric pairs, the extent of the Al-M interaction in these aluminyl systems can be readily controlled with appropriate chelating reagents.

Dihydrogen Activation by Lithium- and Sodium-Aluminyls.

To date, aluminyl anions have been exclusively isolated as their potassium salts. We report herein the synthesis of the lithium and sodium aluminyls, M2 [Al(NONDipp )]2 (M=Li, Na. NONDipp =[O(SiMe2 NDipp)2 ]2- ; Dipp=2,6-iPr2 C6 H3 ). Both compounds crystallize from non-coordinating solvent as "slipped" contacted dimeric pairs with strong M⋅⋅⋅π(aryl) interactions. Isolation from Et2 O solution affords the monomeric ion pairs (NONDipp )Al-M(Et2 O)2 , which contain discrete Al-Li and Al-Na bonds. The ability of the full series of Li, Na and K aluminyls to activate dihydrogen is reported.

The "Metallo"-Diels-Alder Reactions: Examining the Metalloid Behavior of Germanimines.

Addition of MesN3 (Mes=2,4,6-Me3 C6 H2 ) to germylene [(NONtBu )Ge] (NONtBu =O(SiMe2 NtBu)2 ) (1) gives germanimine, [(NONtBu )Ge=NMes] (2). Compound 2 behaves as a metalloid, showing reactivity reminiscent of both transition metal-imido complexes, undergoing [2+2] addition with heterocumulenes and protic sources, as well as an activated diene, undergoing a [4+2] cycloaddition, or "metallo"-Diels-Alder, reaction. In the latter case, the diene includes the Ge=N bond and π-system of the Mes substituent, which is reactive towards dienophiles including benzaldehyde, benzophenone, styrene, and phenylacetylene.

Carbon-Carbon Bond Forming Reactions Promoted by Aluminyl and Alumoxane Anions: Introducing the Ethenetetraolate Ligand.

[K{Al(NONDipp )}]2 (NONDipp =[O(SiMe2 NDipp)2 ]2- , Dipp=2,6-iPr2 C6 H3 ) reacts with CS2 to afford the trithiocarbonate species [K(OEt2 )][Al(NONDipp )(CS3 )] 1 or the ethenetetrathiolate complex, [K{Al(NONDipp )(S2 C)}]2 [3]2 . The dimeric alumoxane [K{Al(NONDipp )(O)}]2 reacts with carbon monoxide to afford the oxygen analogue of 3, [K{Al(NONDipp )(O2 C)}]2 [4]2 containing the hitherto unknown ethenetetraolate ligand, [C2 O4 ]4- .

Isoelectronic Aluminium Analogues of Carbonyl and Dioxirane Moieties.

We report the anion [Al(NONAr )(Se)]- (NONAr =[O(SiMe2 NAr)2 ]2- , Ar=2,6-iPr2 C6 H3 ), which is an isoelectronic Group 13 metal analogue of the carbonyl group containing an aluminium-selenium multiple bond. It was synthesized in a single step from the reaction of the aluminyl anion [Al(NONAr )]- with elemental selenium. Spectroscopic, crystallographic, and computational analysis confirmed multiple bonding between aluminium and selenium. Addition of a second equivalent of selenium afforded the diselenirane, [Al(NONAr )(κ2 -Se2 )]- , which is an isoelectronic analogue of the dioxirane group.

Aluminium-Mediated Carbon Dioxide Reduction by an Isolated Monoalumoxane Anion.

The deoxygenative conversion of carbon dioxide to carbon monoxide is promoted by the aluminyl anion [Al(NONAr )]- (NONAr =[O(SiMe2 NAr)2 ]2- , Ar=2,6-iPr2 C6 H3 ). The reaction proceeds via the isolable monoalumoxane anion [Al(NONAr )(O)]- , containing a terminal aluminum-oxygen bond. This species reacts with a second equivalent of carbon dioxide to afford the carbonate [Al(NONAr )(CO3 )]- , and with nitrous oxide to generate the hyponitrite anion, [Al(NONAr )(κ2 O,O'-N2 O2 )]- .

Reduction vs. Addition: The Reaction of an Aluminyl Anion with 1,3,5,7-Cyclooctatetraene.

The potassium aluminyl complex K[Al(NONAr )] (NON=NONAr =[O(SiMe2 NAr)2 ]2- , Ar=2,6-iPr2 C6 H3 ) reacts with 1,3,5,7-cyclooctatetraene (COT) to give K[Al(NONAr )(COT)]. The COT-ligand is present in the asymmetric unit as a planar µ2 -η2 :η8 -bridge between Al and K, with additional K⋅⋅⋅π-aryl interactions to neighboring molecules that generate a helical chain. DFT calculations indicate significant aromatic character, consistent with reduction to [COT]2- . Addition of 18-crown-6 causes a rearrangement of the C8 -carbocycle to form the isomeric 9-aluminabicyclo[4.2.1]nona-2,4,7-triene anion.

Indyllithium and the Indyl Anion [InL]- : Heavy Analogues of N-Heterocyclic Carbenes.

Reduction of the indate complex In(NONAr )(µ-Cl)2 Li(OEt2 )2 (NONAr =[O(SiMe2 NAr)2 ]2- ; Ar=2,6-iPr2 C6 H3 ) with sodium generates the InII diindane species [In(NONAr )]2 . Further reduction with a mixture of potassium and [2.2.2]crypt affords the InI N-heterocyclic indyl anion [In(NONAr )]- , which crystallizes with a non-contacted [K([2.2.2]crypt)]+ cation. The indyl anion can also be isolated as the indyllithium compound In(NONAr )(Li{THF}3 ), which contains an In-Li bond. Density functional theory calculations show that the HOMO of the indyl anion is a metal-centred lone pair, and preliminary reactivity studies confirm its nucleophilic behaviour.

Alkaline Earth-Centered CO Homologation, Reduction, and Amine Carbonylation.

Reactions of ß-diketiminato magnesium and calcium hydrides with 1 atm of CO result in a reductive coupling process to produce the corresponding derivatives of the cis-ethenediolate dianion. Computational (DFT) analysis of this process mediated by Ca, Sr, and Ba highlights a common mechanism and a facility for the reaction that is enhanced by increasing alkaline earth atomic weight. Reaction of CO with PhSiH3 in the presence of the magnesium or calcium hydrides results in catalytic reduction to methylsilane and methylene silyl ether products, respectively. These reactions are proposed to ensue via the interception of initially formed group 2 formyl intermediates, an inference which is confirmed by a DFT analysis of the magnesium-centered reaction. The computational results identify the rate-determining process, requiring traversal of a 33.9 kcal mol-1 barrier, as a Mg-H/C-O σ-bond metathesis reaction, associated with the ultimate cleavage of the C-O bond. The carbonylation reactivity is extended to a variety of magnesium and calcium amides. With primary amido complexes, which for calcium include a derivative of the parent [NH2]- anion, CO insertion is facile and ensues with subsequent nitrogen-to-carbon migration of hydrogen to yield a variety of dinuclear and, in one case, trinuclear formamidate species. The generation of initial carbenic carbamoyl intermediates is strongly implicated through the isolation of the CO insertion product of a magnesium N-methylanilide derivative. These observations are reinforced by a DFT analysis of the calcium-centered reaction with aniline, which confirms the exothermicity of the formamidate formation (ΔH = -67.7 kcal mol-1). Stoichiometric reduction of the resultant magnesium and calcium formamidates with pinacolborane results in the synthesis of the corresponding N-borylated methylamines. This takes place via a sequence of reactions initiated through the generation of amidatohydridoborate intermediates and a cascade of reactivity that is analogous to that previously reported for the deoxygenative hydroboration of organic isocyanates catalyzed by the same magnesium hydride precatalyst. Although a sequence of amine formylation and deoxygenation may be readily envisaged for the catalytic utilization of CO as a C1 source in the production of methylamines, our observations demonstrate that competitive amine-borane dehydrocoupling is too facile under the conditions of 1 atm of CO employed.

Alkaline-Earth Derivatives of the Reactive [HB(C6F5)3]- Anion.

The ß-diketiminato magnesium amidoboranes [HC{(Me)CNDipp}2Mg(NMe2BH2NMe2·BH3)] and [HC{(t-Bu)CNDipp}2Mg(NMe2·BH3)] are readily converted to the corresponding derivatives of the [HB(C6F5)3]- anion by treatment with B(C6F5)3. The bis(borohydride) derivatives of the heaviest alkaline-earth elements, strontium and barium, may be similarly synthesized by reaction of strontium or barium dimethylamidoboranes and B(C6F5)3 and by metathesis reactions of either SrI2 or BaI2 and 2 molar equiv of K(HB(C6F5)3). The strontium and barium compounds have been fully characterized in solution and in the solid state as the respective tris(diethyl ether) and tetrakis(tetrahydrofuran) adducts. The magnesium compound [HC{(Me)CNDipp}2Mg(HB(C6F5)3)] has been applied to the catalytic hydroboration of i-PrN═C═N-i-Pr with HBpin. In contrast to carbodiimide hydroboration catalyzed by the corresponding ß-diketiminato magnesium hydride, which results in the exclusive production of the monoborylated amidine, use of the [HB(C6F5)3]- derivative provides the product of bis-borylation, the aminal H2C(N{Bpin}i-Pr)2, under mild conditions. A series of stoichiometric reactions highlight that, while this reactivity is likely to be primarily magnesium mediated, B(C6F5)3 plays a vital role both in the delivery of reactive hydride and through the Lewis acid activation of the heteroallene substrate and various reactive intermediates.

Alkaline-Earth-Promoted CO Homologation and Reductive Catalysis.

Reaction between a ß-diketiminato magnesium hydride and carbon monoxide results in the isolation of a dimeric cis-enediolate species through the reductive coupling of two CO molecules. Under catalytic conditions with PhSiH3 , an observable magnesium formyl species may be intercepted for the mild reductive cleavage of the CO triple bond.

Controlled reductive C-C coupling of isocyanides promoted by an aluminyl anion.

We report the reaction of the potassium aluminyl, K[Al(NON)] ([NON]2- = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3) with a series of isocyanide substrates (R-NC). In the case of tBu-NC, degradation of the isocyanide was observed generating an isomeric mixture of the corresponding aluminium cyanido-κC and -κN compounds, K[Al(NON)(H)(CN)]/K[Al(NON)(H)(NC)]. The reaction with 2,6-dimethylphenyl isocyanide (Dmp-NC), gave a C3-homologation product, which in addition to C-C bond formation showed dearomatisation of one of the aromatic substituents. In contrast, using adamantyl isocyanide Ad-NC allowed both the C2- and C3-homologation products to be isolated, allowing a degree of control to be exercised over the chain growth process. These data also show that the reaction proceeds through a stepwise addition, supported in this study by the synthesis of the mixed [(Ad-NC)2(Dmp-NC)]2- product. Computational analysis of the bonding within the homologised products confirm a high degree of multiple bond character in the exocyclic ketenimine units of the C2- and C3-products. In addition, the mechanism of chain growth was investigated, identifying different possible pathways leading to the observed products, and highlighting the importance of the potassium cation in formation of the initial C2-chain.

Extending chain growth beyond C1 → C4 in CO homologation: aluminyl promoted formation of the [C5O5]5- ligand.

(NONDipp)Al-K(TMEDA)2 (NONDipp = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3), containing an Al-K bond, activates and reductively couples cabon monoxide gas to form the [C4O4]4- ligand. This oxocarbon anion is thermally isomerised in the presence of CO and TMEDA. In contrast, the dimeric potassium aluminyl [K{Al(NONDipp)}]2 yields an aluminium complex containing the hitherto unknown [C5O5]5- ligand.

Carbon-chalcogen bond formation initiated by [Al(NONDipp)(E)]- anions containing Al-E{16} (E{16} = S, Se) multiple bonds.

Multiply-bonded main group metal compounds are of interest as a new class of reactive species able to activate and functionalize a wide range of substrates. The aluminium sulfido compound K[Al(NONDipp)(S)] (NONDipp = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3), completing the series of [Al(NONDipp)(E)]- anions containing Al-E{16} multiple bonds (E{16} = O, S, Se, Te), was accessed via desulfurisation of K[Al(NONDipp)(S4)] using triphenylphosphane. The crystal structure showed a tetrameric aggregate joined by multiple K⋯S and K⋯π(arene) interactions that were disrupted by the addition of 2.2.2-cryptand to form the separated ion pair, [K(2.2.2-crypt)][Al(NONDipp)(S)]. Analysis of the anion using density functional theory (DFT) confirmed multiple-bond character in the Al-S group. The reaction of the sulfido and selenido anions K[Al(NONDipp)(E)] (E = S, Se) with CO2 afforded K[Al(NONDipp)(κ2 E,O-EC{O}O)] containing the thio- and seleno-carbonate groups respectively, consistent with a [2 + 2]-cycloaddition reaction and C-E bond formation. An analogous cycloaddition reaction took place with benzophenone affording compounds containing the diphenylsulfido- and diphenylselenido-methanolate ligands, [κ2 E,O-EC{O}Ph2]2-. In contrast, when K[Al(NONDipp)(E)] (E = S, Se) was reacted with benzaldehyde, two equivalents of substrate were incorporated into the product accompanied by formation of a second C-E bond and complete cleavage of the Al-E{16} bonds. The products contained the hitherto unknown κ2 O,O-thio- and κ2 O,O-seleno-bis(phenylmethanolate) ligands, which were exclusively isolated as the cis-stereoisomers. The mechanisms of these cycloaddition reactions were investigated using DFT methods.

Isolating elusive 'Al(µ-O)M' intermediates in CO2 reduction by bimetallic Al-M complexes (M = Zn, Mg).

The reaction of compounds containing Al-Mg and Al-Zn bonds with N2O enabled isolation of the corresponding Al(µ-O)M complexes. Electronic structure analysis identified largely ionic Al-O and O-M bonds, featuring an anionic µ-oxo centre. Reaction with CO2 confirmed that these species correspond to the proposed intermediates in the formation of µ-carbonate compounds.