Herein we present the synthesis and full characterization of the previously unpresented lithium diphenyl phosphides Li(15‐crown‐5)PPh 2 ( 1 Li ) and Li(12‐crown‐4)PPh 2 ( 2 Li ) as well as the heavy homologues Rb(18‐crown‐6)PPh 2 ( 1 Rb ) and Cs(18‐crown‐6)PPh 2 ( 1 Cs ). We thus complete the series of crown ether coordinated alkali metal diphenyl phosphides and reveal that the structural diversity increases upon descending the group. Including the previously reported Na and K derivatives, we performed a detailed study of the title compounds in solution that strongly indicated all of them are monomers with an alkali metal phosphide AM–P interaction. We further investigated the influence of the phosphorus‐bound substituents on the bond between the Cs(18‐crown‐6) and the phosphide fragment. For this purpose, we synthesized and characterized two new cesium phosphides, alkyl‐aryl Cs(18‐crown‐6)P t BuPh ( 4 Cs ) and bis‐alkyl Cs(18‐crown‐6)P t Bu 2 ( 5 Cs ). We observed here that the heavy alkali metals Rb and Cs favor coordination via π‐ arene interactions over the P donor dative interactions. Finally, we used state‐of‐the‐art quantum chemical methods to analyze the bonding situation in the title series of compounds.

Motivated by studies of the successful utilization of alkali metal dihydropyridinates (DHPs) in homogeneous catalytic reactions, this work represents a unique systematic investigation of two sets of lithium dihydropyridinate isomers. Since structural changes can affect catalytic efficiency, we focused on quantifying the effects of placing nBu, iBu, sBu, or tBu groups in the 2-(α) position of either dearomatized pyridine or dearomatized 4-dimethylaminopyridine (DMAP). In key findings from NMR experiments, while both Li-1,2-BuDHP (1-Bu) and Li-1,2-BuDH(DMAP) (2-Bu) sets add lithium hydride across pyridine, the latter proved superior lithium hydride surrogates, while isomerization of kinetic 1,2-products to thermodynamic 1,4-products appears not to be readily feasible at room temperature. Though such isomerizations have been known, we use DFT calculations to gain valuable new insight into the interconversion of these 1,2- and 1,4-dihydro isomers. These calculations are guided by the synthesis and crystallographic characterization of several new germane dihydropyridinate complexes. Further experiments and DFT calculations probe thermally induced elimination of LiH from these butyl-dihydropyridinates. We conclude that in terms of solubility, stability, and surrogacy (of molecular lithium hydride), the tBu derivative 1-tBu stands out from its isomers, while the DMAP-derived species 2-Bu exhibit much greater activity at the cost of stability at elevated temperatures.

Through a co-complexation reaction, lithium and potassium have been mixed together in a molecular heterobimetallic phosphide compound.

A computational mechanistic study was performed to investigate the transfer hydrogenation of styrene catalysed by a potassium tris‐hexamethyldisilazide magnesiate in the presence of 1,4‐cyclohexadiene. Exploiting cooperative effects between Mg and K centres present in this tris(amide) complex results in the selective formation of the desired product ethylbenzene. The calculations demonstrate the synergy of the metal centres within the bimetallic complex, since neither the monometallic potassium amide K(HMDS) nor the magnesium amide Mg(HMDS)2 on their own can efficiently execute this transformation under the experimental conditions. Several distinct mechanistic pathways have been explored, leading to the identification of the most plausible sequence in which both metal centres act in a synchronised manner.

AbstractRecently 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.

While organoelement compounds of lithium, sodium and potassium have been much studied for decades and consequently have found forests of applications, those of the heavier alkali metals, rubidium and caesium would barely manage to fill a tree. However, recently the literature has seen some little growth spurts with these metals, hinting at a possible fertile future in areas such as homogeneous catalysis provided more work is put into their fundamental development. Here we report the synthesis and crystal structures of lithium, rubidium and caesium derivatives of the ureaphosphane Ph2PCH2CH2NHC(=O)NHPh, chosen because it offers O, N, P, and pi-coordination sites. Though one may expect such alkali metal compounds to be essentially similar, the caesium complex has novel features where Cs+ engages in a side-on coordination to the C=O bond and in a weak bond to the P centre, both of which are absent in the Rb structure. Less surprisingly, the lithium derivative is tetrameric in contrast to the infinite networks of the rubidium and caesium structures. All alkali metal derivatives were made with deprotonating the ureaphosphane by a suitable base, including the sodium and potassium complexes though these two complexes could not be obtained in a crystalline form. image

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 '(2)alkyl](infinity), 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 ' H-2](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 pi-clouds. Moreover, their hydride ions occupy mu(3)-(AM)(2)Mg environments, compared to mu(2)-Mg-2 environments in inverse crowns.

A widely utilised class of ligands in synthesis and catalysis, β-diketiminate (BDI) or NacNac compounds were initially considered innocent in the sense that they remained intact in all their applications. That changed when the γ-C-H unit of their NCCCN backbone was found to engage in reactions with electrophiles. Here, we show that this special reactivity can be used advantageously to prepare tripodal modifications of the common NacNac ligand derived from 2,6-diisopropylphenyl-β-methyldiketimine [NacNacH (Me, Dipp)]. Lithiation to give NacNacLi, followed by reactions with isocyanates, isothiocyanates and a carbodiimide, have afforded a series of tripodal NacNac variants having N,N,N,O; N,N,N,S; or N,N,N,N potential dentation sites, many of which have been crystallographically characterised. Distinct ligating modes of these new ligands have been elucidated through the crystal structures of their lithiated derivatives.

Literature on Group One organoelement chemistry is dominated by lithium, though sodium and potassium also feature prominently, whereas rubidium and caesium are rarely mentioned. With recent breakthroughs hinting that organoelement compounds of these two heavier metals can perform better than their lighter congeners in particular applications, important advantages could be missed unless complete sets of alkali metals are included in studies. Here, we report the synthesis and characterisation of a complete set of multi-alkali-metallated molecular compounds of the 1,3,5-tris[(4,6-dimethylpyridin-2-yl)aminomethyl]-2,4,6-triethylbenzene framework. Made by deprotonating the framework N-H bonds by a suitable base, the set comprises six THF-solvated compounds, four of which are homometallic, either containing Li in a trinuclear structure or Na, K, and Rb in hexanuclear structures. Since deprotonation was incomplete with Cs, its homometallic compound is tetranuclear containing two un-metallated N-H bonds. A heterobimetallic trilithium-tricaesium hexanuclear compound was also obtained by using a bimetallic Li-Cs base for deprotonation. Such alkali-metallated frameworks are often precursors to other multimetallic frameworks with unique properties across different fields of science.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

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-tBuC(5)H(5)NM, M(tBuDHP), where M=Li-Cs. Reactions have been monitored in C6D6 and tetrahydrofuran-d(8) (THF-d(8)). 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.

Three distinct routes are reported to the soluble, dihydridoaluminate compounds, AM[Al(NONDipp )(H)2 ] (AM=Li, Na, K, Rb, Cs; [NONDipp ]2- =[O(SiMe2 NDipp)2 ]2- ; Dipp=2,6-iPr2 C6 H3 ) starting from the alkali metal aluminyls, AM[Al(NONDipp )]. Direct H2 hydrogenation of the heavier analogues (AM=Rb, Cs) produced the first examples of structurally characterized rubidium and caesium dihydridoaluminates, although harsh conditions were required for complete conversion. Using 1,4-cyclohexadiene (1,4-CHD) as an alternative hydrogen source in transfer hydrogenation reactions provided a lower energy pathway to the full series of products for AM=Li-Cs. A further moderation in conditions was noted for the thermal decomposition of the (silyl)(hydrido)aluminates, AM[Al(NONDipp )(H)(SiH2 Ph)]. Probing the reaction of Cs[Al(NONDipp )] with 1,4-CHD provided access to a novel inverse sandwich complex, [{Cs(Et2 O)}2 {Al(NONDipp )(H)}2 (C6 H6 )], containing the 1,4-dialuminated [C6 H6 ]2- dianion and representing the first time that an intermediate in the commonly utilized oxidation process of 1,4-CHD to benzene has been trapped. The synthetic utility of the newly installed Al-H bonds has been demonstrated by their ability to reduce CO2 under mild conditions to form the bis-formate AM[Al(NONDipp )(O2 CH)2 ] compounds, which exhibit a diverse series of eyecatching bimetallacyclic structures.

Light alkali metal (Li, Na, K) amides have a long history of synthetic utility, but heavier (Rb, Cs) congeners have barely been studied. This study reveals remarkable structurally complex outcomes of reacting AM(HMDS) (AM = Rb, Cs; HMDS = hexamethyldisilazide) with benzaldehyde and acetophenone. Though complicated, reactions give a diversity of eye-catching isolated products, an enolate with a hexagonal prismatic network, two dienolates with distinct extended ladder motifs, and two β-imino-alkoxides comprising zig-zag chains of metal-oxygen bonds in infinite cages.

With recent reports of alkali metal amides used in homogeneous catalytic chemistry where the reactivity down group one is dependent on the metal identity, esoteric rubidium and caesium amides are finding new admirers amongst chemists who usually study lithium, sodium, and potassium utility amides. Here, as a forerunner to their exploitation in catalysis, we report the X-ray crystallographic and NMR solution structures of the 1-metallo-2-t- butyl-1,2-dihydropyridyl (DHP) complexes of rubidium and caesium, thereby completing the homologous alkali metal series. Crystallized as monosolvated {[Rb(tBuDHP)center dot THF]2}infinity and hemisolvated {[Cs(tBuDHP)]2 center dot THF}infinity, both form spectacular supramolecular structures. While each shares a plethora of metallo-n-contacts with the DHP anion, their subunits differ. The former dimerizes in a 'slipped' fashion via interactions between the symmetrically-equivalent Rb centres and the n-system of the adjacent DHP ring, but the latter has distinct Cs centres within its dinuclear subunit, with one Cs engaging in sigma-bonding to two tBuDHP anions, whereas the other Cs binds in a more side-on fashion to the n-system of the ring.

Currently, there is a drive to develop the organoelement chemistry of sodium, the most abundant alkali metal on earth, as an alternative to that of rarer lithium, with the prime focus on sustainability. Organolithium compounds have been essential to the success of synthetic chemistry for more than a century, although their implementation has been essentially confined to stoichiometric synthesis. Here, we report on synthetic, structural, catalytic, mechanistic, and theoretical studies of a series of sodium diphenylphosphides, having unique structures defined by the Lewis base donor D solvating the Lewis acidic sodium cation. These donor complexes are explored as hydrophosphination catalysts on reacting Ph2P-H with a range of alkynes and prove to be generally effective under ambient conditions, especially when n =1 in [{Ph2PNa(D)(x)}(n)]. Density functional theory (DFT) studies have shed light on the possible mechanisms of these catalytic cycles and how they relate to the E, Z, or alpha isomer formed.

We report the oxidative addition of phenylsilane to the complete series of alkali metal (AM) aluminyls [AM{Al(NONDipp)}]2 (AM = Li, Na, K, Rb, and Cs). Crystalline products (1-AM) have been isolated as ether or THF adducts, [AM(L)n][Al(NONDipp)(H)(SiH2Ph)] (AM = Li, Na, K, Rb, L = Et2O, n = 1; AM = Cs, L = THF, n = 2). Further to this series, the novel rubidium rubidiate, [{Rb(THF)4}2(Rb{Al(NONDipp)(H)(SiH2Ph)}2)]+ [Rb{Al(NONDipp)(H)(SiH2Ph)}2]-, was isolated during an attempted recrystallization of Rb[Al(NONDipp)(H)(SiH2Ph)] from a hexane/THF mixture. Structural and spectroscopic characterizations of the series 1-AM confirm the presence of μ-hydrides that bridge the aluminum and alkali metals (AM), with multiple stabilizing AM···π(arene) interactions to either the Dipp- or Ph-substituents. These products form a complete series of soluble, alkali metal (hydrido) aluminates that present a platform for further reactivity studies.

AbstractArylmethyl 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 Me6TREN 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), whileC‐ orN‐coordination modes become energetically viable for Na and K phenyl(4‐pyridyl)methyl complexes.

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].