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.

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.

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.

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.

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.

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.

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.

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.

Hydroarylation of olefins catalysed by a dimeric ytterbium(II) alkyl.

Although the nucleophilic alkylation of aromatics has recently been achieved with a variety of potent main group reagents, all of this reactivity is limited to a stoichiometric regime. We now report that the ytterbium(II) hydride, [BDIDippYbH]2 (BDIDipp = CH[C(CH3)NDipp]2, Dipp = 2,6-diisopropylphenyl), reacts with ethene and propene to provide the ytterbium(II) n-alkyls, [BDIDippYbR]2 (R = Et or Pr), both of which alkylate benzene at room temperature. Density functional theory (DFT) calculations indicate that this latter process operates through the nucleophilic (SN2) displacement of hydride, while the resultant regeneration of [BDIDippYbH]2 facilitates further reaction with ethene or propene and enables the direct catalytic (anti-Markovnikov) hydroarylation of both alkenes with a benzene C-H bond.

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.

Double insertion of CO2 into an Al-Te multiple bond.

We report the [Al(NONDipp)(Te)(THF)]- anion containing a terminal aluminium telluride bond. DFT calculations confirm appreciable Al-Te multiple bond character and reaction with CO2 proceeds via a double insertion to afford the previously unknown tellurodicarbonate ligand.

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- .

Synthesis and reactivity of a terminal aluminium-imide bond.

The reaction of the potassium aluminyl K[Al(NONDipp)] (NONDipp = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3) with an organic azide generates the aluminium imide complex, K[Al(NONDipp)(NMes)] (Mes = mesityl = 2,4,6-Me3C6H2). DFT calculations indicate the Al-Nimide interaction is polarized but has appreciable multiple-bond character. This is demonstrated experimentally by the reaction with carbon dioxide, giving a rare example of a main group carbamate dianion via a [2+2] cycloaddition reaction.

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.

Reactions of In-Zn bonds with organic azides: products that result from hetero- and homo-bimetallic behaviour.

The indyl anion, K[In(NONDipp)] (NONDipp = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3) reacts with group 12 compounds M(BDIR)Cl (M = Zn, Cd; BDI = [HC{C(Me)NR}2]-, R = 2,4,6-Me3C6H2 (Mes), Dipp) to afford the heterobimetallic compounds (NONDipp)In-M(BDIR) that contain the first In-Zn and In-Cd bonds. The reactivity of the In-Zn bonds towards organic azides, R'N3 (R' = Mes, Dipp, Ph) was investigated. (NONDipp)In-Zn(BDIMes) reduces MesN3via an isolable triazenide intermediate to generate the bridging imido compound, (NONDipp)In-(µ-NMes)-Zn(BDIMes). Similar reactivity is noted from early-late heterobimetallic complexes. Under the same conditions, PhN3 reacts to afford a product that contains a bridging tetraazenide ligand, which is formed from the formal (2 + 3)-cycloaddition of second azide to an indium-imido bond. However, increasing the bulk of the BDI-ligand in (NONDipp)In-Zn(BDIDipp) leads to reductive coupling of PhN3 to give the hexazene complex. This mode of reactivity is reminiscent of the reductive behaviour of homobimetallic compounds.

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 )]- .

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.

Reduction of organic azides by indyl-anions. Isolation and reactivity studies of indium-nitrogen multiple bonds.

The synthesis of a new potassium-indyl complex, K[In(NONAr)] (NONAr = [O(SiMe2NAr)2]2-, Ar = 2,6-iPr2C6H3) and its reactivity with organic azides RN3 is reported. When R = 2,6-bis(diphenylmethyl)-4- t Bu-phenyl, a dianionic alkyl-amide ligand is formed via C-H activation across a transient In-Nimide bond. Reducing the size of the R-group to 2,4,6-trimethylphenyl (mesityl, Mes) enables oxidation of the indium and elimination of dinitrogen to afford the imide species, K[In(NONAr)(NMes)]. The anion contains a short In-Nimide bond, shown computationally to contain appreciable multiple bond character. Reaction of isolated imides with an additional equivalent of azide (R = Mes, SiMe3) generates tetrazenido-indium compounds K[In(NONAr){κ-N,N'-N4(Mes)(R)-1,4}], shown by X-ray crystallography to contain planar InN4 heterocycles in the anion.