Alkali Metal Dihydropyridines in Transfer Hydrogenation Catalysis of Imines: Amide Basicity versus Hydride Surrogacy.

Catalytic reduction of a representative set of imines, both aldimines and ketimines, to amines has been studied using transfer hydrogenation from 1,4-dicyclohexadiene. Unusually, this has been achieved using s-block pre-catalysts, namely 1-metallo-2-tert-butyl-1,2-dihydropyridines, 2-tBuC5 H5 NM, M(tBuDHP), where M=Li-Cs. Reactions have been monitored in C6 D6 and tetrahydrofuran-d8 (THF-d8 ). A definite trend is observed in catalyst efficiency with the heavier alkali metal tBuDHPs outperforming the lighter congeners. In general, Cs(tBuDHP) is the optimal pre-catalyst with, in the best cases, reactions producing quantitative yields of amines in minutes at room temperature using 5 mol % catalyst. Supporting the experimental study, Density Functional Theory (DFT) calculations have also been carried out which reveal that Cs has a pathway with a significantly lower rate determining step than the Li congener. In the postulated initiation pathways DHP can act as either a base or as a surrogate hydride.

Methodologies for Operando ATR-IR Spectroscopy of Magnesium Battery Electrolytes.

We explore the suitability of operando attenuated total reflection infrared (ATR-IR) spectroscopy methodologies for the study of organoaluminate electrolytes for Mg battery applications. The "all-phenyl complex" in tetrahydrofuran (THF), with the molecular structure [Mg2Cl3·6THF]+[AlPh4]-, is used as an exemplar electrolyte to compare two different spectroelectrochemical cell configurations. In one case, a Pt gauze is used as a working electrode, while in the second case, a thin (∼10 nm) Pt film working electrode is deposited directly on the surface of the ATR crystal. Spectroscopic measurements indicate substantial differences in the ATR-IR response for the two configurations, reflecting the different spatial arrangements of the working electrode with respect to the ATR sampling volume. The relative merits and potential pitfalls associated with the two approaches are discussed.

Hydrocarbon Soluble Alkali-Metal-Aluminium Hydride Surrog[ATES].

A series of group 1 hydrocarbon-soluble donor free aluminates [AM(t BuDHP)(TMP)Al(i Bu)2 ] (AM=Li, Na, K, Rb) have been synthesised by combining an alkali metal dihydropyridyl unit [(2-t BuC5 H5 N)AM)] containing a surrogate hydride (sp3 C-H) with [(i Bu)2 Al(TMP)]. These aluminates have been characterised by X-ray crystallography and NMR spectroscopy. While the lithium aluminate forms a monomer, the heavier alkali metal aluminates exist as polymeric chains propagated by non-covalent interactions between the alkali metal cations and the alkyldihydropyridyl units. Solvates [(THF)Li(t BuDHP)(TMP)Al(i Bu)2 ] and [(TMEDA)Na(t BuDHP)(TMP)Al(i Bu)2 ] have also been crystallographically characterised. Theoretical calculations show how the dispersion forces tend to increase on moving from Li to Rb, as opposed to the electrostatic forces of stabilization, which are orders of magnitude more significant. Having unique structural features, these bimetallic compounds can be considered as starting points for exploring unique reactivity trends as alkali-metal-aluminium hydride surrog[ATES].

Sigma/pi Bonding Preferences of Solvated Alkali-Metal Cations to Ditopic Arylmethyl Anions.

Arylmethyl anions allow alkali-metals to bind in a σ-fashion to the lateral carbanionic centre or a π-fashion to the aryl ring or in between these extremities, with the trend towards π bonding increasing on descending group 1. Here we review known alkali metal structures of diphenylmethane, fluorene, 2-benzylpyridine and 4-benzylpyridine. Next, we synthesise Li, Na, K monomers of these diarylmethyls using polydentate donors PMDETA or Me6 TREN to remove competing oligomerizing interactions, studying the effect that two aromatic rings has on negative charge (de)localisation via NMR, X-ray crystallographic and DFT studies. Diphenylmethyl and fluorenyl anions maintain C(H)-M interactions regardless of alkali-metal, although the adjacent arene carbons engage in interactions with larger alkali-metals. Introducing a nitrogen atom into the ring (at the 2- or 4-position) encourages relocalisation of negative charge away from the deprotonated carbon and onto nitrogen. Phenyl(2-pyridyl)methyl moves from an enamide formation at one extremity (lithium) to an aza-allyl formation at the other extremity (potassium), while C- or N-coordination modes become energetically viable for Na and K phenyl(4-pyridyl)methyl complexes.

Alkali-Metal-Mediated Synergistic Effects in Polar Main Group Organometallic Chemistry.

The development of synthetic chemistry since the early 1900s owes much to the service of organolithium reagents. Brilliant bases (e.g., deprotonating C-H bonds), nucleophiles (e.g., adding to unsaturated molecules), and transfer agents (e.g., delivering ligands to other metals), these versatile virtuosi and to a lesser extent the organic derivatives of the other common alkali metals sodium and potassium have proved indispensable in both academia and technology. Today these monometallic compounds are still utilized widely in synthetic campaigns, but in recent years they have been joined by an assortment of bimetallic formulations that also contain an alkali metal but in company with another metal. These bimetallic formulations often exhibit unique chemistry that can be interpreted in terms of synergistic effects, for which the alkali metal is essential, though it is often the second metal that performs the synthetic transformation. Here, this "alkali-metal-mediated" chemistry is surveyed focusing mainly on bimetallic formulations containing two alkali metals or an alkali metal paired with magnesium, calcium, zinc, aluminum, or gallium. In this International Year of the Periodic Table (IYPT), we ponder whether a Pairiodic Table of Element Pairs will emerge in the future.

Donor-influenced Structure-Activity Correlations in Stoichiometric and Catalytic Reactions of Lithium Monoamido-Monohydrido-Dialkylaluminates.

A series of heteroleptic monoamido-monohydrido-dialkylaluminate complexes of general formula [iBu2 AlTMPHLi⋅donor] were synthesized and characterised in solution and in the solid state. Applying these complexes in catalytic hydroboration reactions with representative aldehydes and ketones reveals that all are competent, however a definite donor substituent effect is discernible. The bifunctional nature of the complexes is also probed by assessing their performance in metallation of a triazole and phenylacetylene and addition across pyrazine. These results lead to an example of phenylacetylene hydroboration, which likely proceeds via deprotonation, rather than insertion as observed with the aldehydes and ketones. Collectively, the results emphasise that reactivity is strongly influenced by both the mixed-metal constitution and mixed-ligand constitution of the new aluminates.

1-Alkali-metal-2-alkyl-1,2-dihydropyridines: Soluble Hydride Surrogates for Catalytic Dehydrogenative Coupling and Hydroboration Applications.

Equipped with excellent hydrocarbon solubility, the lithium hydride surrogate 1-lithium-2-tert-butyl-1,2-dihydropyridine (1tLi) functions as a precatalyst to convert Me2 NH⋅BH3 to [NMe2 BH2 ]2 (89 % conversion) under competitive conditions (2.5 mol %, 60 h, 80 °C, toluene solvent) to that of previously reported LiN(SiMe3 )2 . Sodium and potassium dihydropyridine congeners produce similar high yields of [NMe2 BH2 ]2 but require longer times. Switching the solvent to pyridine induces a remarkable change in the dehydrocoupling product ratio, with (NMe2 )2 BH favoured over [NMe2 BH2 ]2 (e.g., 94 %:2 % for 1tLi). Demonstrating its versatility, precatalyst 1tLi was also successful in promoting hydroboration reactions between pinacolborane and a selection of aldehydes and ketones. Most reactions gave near quantitative conversion to the hydroborated products in 15 minutes, though sterically demanding carbonyl substrates require longer times. The mechanisms of these rare examples of Group 1 metal-catalysed processes are discussed.

Lithium Dihydropyridine Dehydrogenation Catalysis: A Group†1 Approach to the Cyclization of Diamine Boranes.

In reactions restricted previously to a ruthenium catalyst, a 1-lithium-2-alkyl-1,2-dihydropyridine complex is shown to be a competitive alternative dehydrogenation catalyst for the transformation of diamine boranes into cyclic 1,3,2-diazaborolidines, which can in turn be smoothly arylated in good yields. This study established the conditions and solvent dependence of the catalysis through NMR monitoring, with mechanistic insight provided by NMR (including DOSY) experiments and X-ray crystallographic studies of several model lithio intermediates.

Developing lithium chemistry of 1,2-dihydropyridines: from kinetic intermediates to isolable characterized compounds.

Generally considered kinetic intermediates in addition reactions of alkyllithiums to pyridine, 1-lithio-2-alkyl-1,2-dihydropyridines have been rarely isolated or characterized. This study develops their "isolated" chemistry. By a unique stoichiometric (that is, 1:1, alkyllithium/pyridine ratios) synthetic approach using tridentate donors we show it is possible to stabilize and hence crystallize monomeric complexes where alkyl is tert-butyl. Theoretical calculations probing the donor-free parent tert-butyl species reveal 12 energetically similar stereoisomers in two distinct cyclotrimeric (LiN)3 conformations. NMR spectroscopy studies (including DOSY spectra) and thermal volatility analysis compare new sec-butyl and iso-butyl isomers showing the former is a hexane soluble efficient hydrolithiation agent converting benzophenone to lithium diphenylmethoxide. Emphasizing the criticalness of stoichiometry, reaction of nBuLi/Me6 TREN with two equivalents of pyridine results in non-alkylated 1-lithio-1,4-dihydropyridine⋅Me6 TREN and 2-n-butylpyridine, implying mechanistically the kinetic 1,2-n-butyl intermediate hydrolithiates the second pyridine.

Adding a Structural Context to the Deprotometalation and Trans-Metal Trapping Chemistry of Phenyl-Substituted Benzotriazole.

Organometallic bases are becoming increasingly complex, because mixing components can lead to bases superior to single-component bases. To better understand this superiority, it is useful to study metalated intermediate structures prior to quenching. This study is on 1-phenyl-1H-benzotriazole, which was previously deprotonated by an in situ ZnCl2 ⋅TMEDA/LiTMP (TMEDA=N,N,N',N'-tetramethylethylenediamine; TMP=2,2,6,6-tetramethylpiperidide) mixture and then iodinated. Herein, reaction with LiTMP exposes the deficiency of the single-component base as the crystalline product obtained was [{4-R-1-(2-lithiophenyl)-1H-benzotriazole⋅3THF}2 ], [R=2-C6 H4 (Ph)NLi], in which ring opening of benzotriazole and N2 extrusion had occurred. Supporting lithiation by adding iBu2 Al(TMP) induces trans-metal trapping, in which C-Li bonds transform into C-Al bonds to stabilise the metalated intermediate. X-ray diffraction studies revealed homodimeric [(4-R'-1-phenyl-1H-benzotriazole)2 ], [R'=(iBu)2 Al(µ-TMP)Li], and its heterodimeric isomer [(4-R'-1-phenyl-1H-benzotriazole){2-R'-1-phenyl-1H-benzotriazole}], whose structure and slow conformational dynamics were probed by solution NMR spectroscopy.

Application of Bis(amido)alkyl Magnesiates toward the Synthesis of Molecular Rubidium and Cesium Hydrido-magnesiates.

Rubidium and cesium are the least studied naturally occurring s-block metals in organometallic chemistry but are in plentiful supply from a sustainability viewpoint as highlighted in the periodic table of natural elements published by the European Chemical Society. This underdevelopment reflects the phenomenal success of organometallic compounds of lithium, sodium, and potassium, but interest in heavier congeners has started to grow. Here, the synthesis and structures of rubidium and cesium bis(amido)alkyl magnesiates [(AM)MgN'2alkyl]∞, where N' is the simple heteroamide -N(SiMe3)(Dipp), and alkyl is nBu or CH2SiMe3, are reported. More stable than their nBu analogues, the reactivities of the CH2SiMe3 magnesiates toward 1,4-cyclohexadiene are revealed. Though both reactions produce target hydrido-magnesiates [(AM)MgN'2H]2 in crystalline form amenable to X-ray diffraction study, the cesium compound could only be formed in a trace quantity. These studies showed that the bulk of the -N(SiMe3)(Dipp) ligand was sufficient to restrict both compounds to dimeric structures. Bearing some resemblance to inverse crown complexes, each structure has [(AM)(N)(Mg)(N)]2 ring cores but differ in having no AM-N bonds, instead Rb and Cs complete the rings by engaging in multihapto interactions with Dipp π-clouds. Moreover, their hydride ions occupy µ3-(AM)2Mg environments, compared to µ2-Mg2 environments in inverse crowns.

Synthesis, characterisation, and catalytic application of a soluble molecular carrier of sodium hydride activated by a substituted 4-(dimethylamino)pyridine.

Recently main group compounds have stepped into the territory of precious transition metal compounds with respect to utility in the homogeneous catalysis of fundamentally important organic transformations. Inspired by the need to promote more sustainability in chemistry because of their greater abundance in nature, this change of direction is surprising since main group metals generally do not possess the same breadth of reactivity as precious transition metals. Here, we introduce the dihydropyridylsodium compound, Na-1,2-tBu-DH(DMAP), and its monomeric variant [Na-1,2-tBu-DH(DMAP)]·Me6TREN, and demonstrate their effectiveness in transfer hydrogenation catalysis of the representative alkene 1,1-diphenylethylene to the alkane 1,1-diphenylethane using 1,4-cyclohexadiene as hydrogen source [DMAP = 4-dimethylaminopyridine; Me6TREN = tris(N,N-dimethyl-2-aminoethyl)amine]. Sodium is appealing because of its high abundance in the earth's crust and oceans, but organosodium compounds have been rarely used in homogeneous catalysis. The success of the dihydropyridylsodium compounds can be attributed to their high solubility and reactivity in organic solvents.

Concealed cyclotrimeric polymorph of lithium 2,2,6,6-tetramethylpiperidide unconcealed: X-ray crystallographic and NMR spectroscopic studies.

Lithium 2,2,6,6-tetramethylpiperidide (LiTMP), one of the most important polar organometallic reagents both in its own right and as a key component of ate compositions, has long been known for its classic cyclotetrameric (LiTMP)4 solid-state structure. Made by a new approach through transmetalation of Zn(TMP)2 with tBuLi in n-hexane solution, a crystalline polymorph of LiTMP has been uncovered. X-ray crystallographic studies at 123(2) K revealed this polymorph crystallises in the hexagonal space group P63 /m and exhibited a discrete cyclotrimeric (C3h ) structure with a strictly planar (LiN)3 ring containing three symmetrically equivalent TMP chair-shaped ligands. The molecular structure of (LiTMP)4 was redetermined at 123(2) K, because its original crystallographic characterisation was done at ambient temperature. This improved redetermination confirmed a monoclinic C2/c space group with the planar (LiN)4 ring possessing pseudo (non-crystallographic) C4h symmetry. Investigation of both metalation and transmetalation routes to LiTMP under different conditions established that polymorph formation did not depend on the route employed but rather the temperature of crystallisation. Low-temperature (freezer at -35 °C) cooling of the reaction solution favoured (LiTMP)3 ; whereas high-temperature (bench) storage favoured (LiTMP)4 . Routine (1) H and (13) C NMR spectroscopic studies in a variety of solvents showed that (LiTMP)3 and (LiTMP)4 exist in equilibrium, whereas (1) H DOSY NMR studies gave diffusion coefficient results consistent with their relative sizes.

Monomerizing alkali-metal 3,5-dimethylbenzyl salts with tris(N, N-dimethyl-2-aminoethyl)amine (Me6TREN): structural and bonding implications.

The series of alkali-metal (Li, Na, K) complexes of the substituted benzyl anion 3,5-dimethylbenzyl (Me2C6H3CH2(-)) derived from 1,3,5-trimethylbenzene (mesitylene) have been coerced into monomeric forms by supporting them with the tripodal tetradentate Lewis donor tris(N,N-dimethyl-2-aminoethyl)amine, [N(CH2CH2NMe2)3, Me6TREN]. Molecular structure analysis by X-ray crystallography establishes that the cation-anion interaction varies as a function of the alkali-metal, with the carbanion binding to lithium mainly in a σ fashion, to potassium mainly in a π fashion, with the interaction toward sodium being intermediate between these two extremes. This distinction is due to the heavier alkali-metal forcing and using the delocalization of negative charge into the aromatic ring to gain a higher coordination number in accordance with its size. Me6TREN binds the metal in a η(4) mode at all times. This coordination isomerism is shown by multinuclear NMR spectroscopy to also extend to the structures in solution and is further supported by density functional theory (DFT) calculations on model systems. A Me6TREN stabilized benzyl potassium complex has been used to prepare a mixed-metal ate complex by a cocomplexation reaction with tBu2Zn, with the benzyl ligand acting as an unusual ditopic σ/π bridging ligand between the two metals, and with the small zinc atom relocalizing the negative charge back on to the lateral CH2 arm to give a complex best described as a contacted ion pair potassium zincate.

Synthetically important alkali-metal utility amides: lithium, sodium, and potassium hexamethyldisilazides, diisopropylamides, and tetramethylpiperidides.

Most synthetic chemists will have at some point utilized a sterically demanding secondary amide (R2 N(-) ). The three most important examples, lithium 1,1,1,3,3,3-hexamethyldisilazide (LiHMDS), lithium diisopropylamide (LiDA), and lithium 2,2,6,6-tetramethylpiperidide (LiTMP)-the "utility amides"-have long been indispensible particularly for lithiation (Li-H exchange) reactions. Like organolithium compounds, they exhibit aggregation phenomena and strong Lewis acidity, and thus appear in distinct forms depending on the solvents employed. The structural chemistry of these compounds as well as their sodium and potassium congeners are described in the absence or in the presence of the most synthetically significant donor solvents tetrahydrofuran (THF) and N,N,N',N'-tetramethylethylenediamine (TMEDA) or closely related solvents. Examples of hetero-alkali-metal amides, an increasingly important composition because of the recent escalation of interest in mixed-metal synergic effects, are also included.

Atom-economic access to cationic magnesium complexes.

Cationic alkaline-earth complexes attract interest for their enhanced Lewis acidity and reactivity compared with their neutral counterparts. Synthetic protocols to these complexes generally utilize expensive specialized reagents in reactions generating multiple by-products. We have studied a simple ligand transfer approach to these complexes using (NacNac)MgR and ER3 (NacNac = ß-diketiminate anion; E = group 13 element; R = aryl/amido anion) which demonstrates high atom economy, opening up the ability to target these species in a more sustainable manner. The success of this methodology is dependent on the identity of the group 13 element with the heavier elements facilitating faster ligand exchange. Furthermore, while this reaction is successful with aromatic ligands such as phenyl and pyrrolyl, the secondary amide piperidide (pip) fails to transfer, which we attribute to the stronger 3-centre-4-electron dimerization interaction of Al2(pip)6.

catena-Poly[sodium-µ2-(N,N,N',N'-tetra-methyl-ethane-1,2-diamine)-κ(2) N:N'-sodium-bis-[µ2-bis-(trimethyl-sil-yl)aza-nido-κ(2) N:N]].

The title compound, [Na2(C6H18NSi2)2(C6H16N2)] n , was found to consist of dimeric [Na(NSiMe3)2] units with crystallographically imposed centrosymmetry based upon four-membered NaNNaN rings. The dimers are bridged by N,N,N',N'-tetra-methyl-ethylenediamine ligands, which act in an unusual extended non-chelating coordination mode. This gives a one-dimensional coordination polymer that extends parallel to the a-axis direction.

Dizincation of a 2-substituted thiophene: constructing a cage with a [16]crown-4 zincocyclic core.

Metal detector: a bowl-shaped nanomolecule (see picture; S yellow, C gray, Zn blue) containing an unprecedented 16-atom [ZnC(3)](4) "anti-crown" ring has been unearthed by isolating a dizincated 2-substituted thiophene intermediate that would normally be hidden in tandem functionalization methodology.

Atom-efficient synthesis of a benchmark electrolyte for magnesium battery applications.

The benchmark magnesium electrolyte, [Mg2Cl3]+ [AlPh4]-, can be prepared in a 100% atom-economic fashion by a ligand exchange reaction between AlCl3 and two molar equivalents of MgPh2. NMR and vibrational spectroscopy indicate that the reported approach results in a simpler ionic composition than the more widely adopted synthesis route of combining PhMgCl with AlCl3. Electrochemical performance has been validated by polarisation tests and cyclic voltammetry, which demonstrate excellent stability of electrolytes produced via this atom-efficient approach.

Main group multiple C-H/N-H bond activation of a diamine and isolation of a molecular dilithium zincate hydride: experimental and DFT evidence for alkali metal-zinc synergistic effects.

The surprising transformation of the saturated diamine (iPr)NHCH(2)CH(2)NH(iPr) to the unsaturated diazaethene [(iPr)NCH═CHN(iPr)](2-) via the synergic mixture nBuM, (tBu)(2)Zn and TMEDA (where M = Li, Na; TMEDA = N,N,N',N'-tetramethylethylenediamine) has been investigated by multinuclear NMR spectroscopic studies and DFT calculations. Several pertinent intermediary and related compounds (TMEDA)Li[(iPr)NCH(2)CH(2)NH(iPr)]Zn(tBu)(2) (3), (TMEDA)Li[(iPr)NCH(2)CH(2)CH(2)N(iPr)]Zn(tBu) (5), {(THF)Li[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)}(2) (6), and {(TMEDA)Na[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)}(2) (11), characterized by single-crystal X-ray diffraction, are discussed in relation to their role in the formation of (TMEDA)M[(iPr)NCH═CHN(iPr)]Zn(tBu) (M = Li, 1; Na, 10). In addition, the dilithio zincate molecular hydride [(TMEDA)Li](2)[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)H 7 has been synthesized from the reaction of (TMEDA)Li[(iPr)NCH(2)CH(2)NH(iPr)]Zn(tBu)(2)3 with nBuLi(TMEDA) and also characterized by both X-ray crystallographic and NMR spectroscopic studies. The retention of the Li-H bond of 7 in solution was confirmed by (7)Li-(1)H HSQC experiments. Also, the (7)Li NMR spectrum of 7 in C(6)D(6) solution allowed for the rare observation of a scalar (1)J(Li-H) coupling constant of 13.3 Hz. Possible mechanisms for the transformation from diamine to diazaethene, a process involving the formal breakage of four bonds, have been determined computationally using density functional theory. The dominant mechanism, starting from (TMEDA)Li[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu) (4), involves the formation of a hydride intermediate and leads directly to the observed diazaethene product. In addition the existence of 7 in equilibrium with 4 through the dynamic association and dissociation of a (TMEDA)LiH ligand, also provides a secondary mechanism for the formation of the diazaethene. The two reaction pathways (i.e., starting from 4 or 7) are quite distinct and provide excellent examples in which the two distinct metals in the system are able to interact synergically to catalyze this otherwise challenging transformation.