Potassium Salts of 2,5-Bis(trimethylsilyl)-Germolide: Switching between Aromatic and Non-Aromatic States

The reduction of a 1-mesityl-2,5-bis-trimethylsilylchlorogermole 8 with KC₈ is reported. While the reaction with one equivalent of KC₈ gave the dimer with a Ge−Ge bond 10 , excess of KC₈ (four equivalents) resulted in the formation of the potassium salt of the germole dianion, 11 with reductive cleavage of the Ge−C bond. Careful reduction with two equivalents of KC₈ in THF provided the potassium salt of the planar germolide 5 . Its solid-state structure revealed contact ion pairs with the potassium ion η⁵-coordinated to the germacyclopentadienide ring. The molecular structure of the anion indicates a high degree of cyclic electron delocalization, in agreement with results from DFT calculations. Separation of the ion pair by complexation of the potassium ions with 18-crown-6 triggers the isomerization to germolide 6 , which is characterized by a pyramidal coordination sphere of the germanium atom and a localized diene structure. The isomers 5 and 6 represent a rare example for a structurally manifested switch between a non-aromatic and an aromatic state induced by an external stimulus, in this case the complexation of the counter cation.     

Saccharified Uranyl Ions: Self-Assembly of UO22+ into Trinuclear Anionic Complexes by the Coordination of Glucosamine-Derived Schiff Bases

The reaction of UO₂(OAc)₂ ⋅ 2H₂O with the biologically inspired ligand 2-salicylidene glucosamine (H₂ L¹ ) results in the formation of the anionic trinuclear uranyl complex [(UO₂)₃(μ₃-O)( L¹ )₃]²⁻ ( 1 ²⁻), which was isolated in good yield as its Cs-salt, [Cs]₂ 1 . Recrystallization of [Cs]₂ 1 in the presence of 18-crown-6 led to formation of a neutral ion pair of type [M(18-crown-6)]₂ 1 , which was also obtained for the alkali metal ions Rb⁺ and K⁺ (M=Cs, Rb, K). The related ligand, 2-(2-hydroxy-1-naphthylidene) glucosamine (H₂ L² ) in a similar procedure with Cs⁺ gave the corresponding complex [Cs(18-crown-6)]₂[(UO₂)₃(μ₃-O)( L² )₃ ([Cs(18-crown-6)]₂ 2 ). From X-ray investigations, the [(UO₂)₃O( Lⁿ )₃]²⁻ anion (n=1, 2) in each complex is a discrete trinuclear uranyl species that coordinates to the alkali metal ion via three uranyl oxygen atoms. The coordination behavior of H₂ L¹ and H₂ L² towards UO₂²⁺ was investigated by NMR, UV/Vis spectroscopy and mass spectrometry, revealing the in situ formation of the 1 ²⁻ and 2 ²⁻dianions in solution.     

The Sulfur Rich Fluorothiophosphate Dianions [S5P2F2]2− and [S3PF]2− : Cluster and Chelation Control of P-S Heterolysis

The sulfur rich difluoropentathiodiphosphate dianion [S₅P₂F₂]²⁻, from fluoride addition to P₄S₁₀, has a somewhat checkered history and proves to be the main product of the reaction in acetonitrile. Its optimized synthesis, and structural characterization, as either a tetraphenylphosphonium or a tetrapropylammonium salt, [NⁿPr₄]₂[S₅P₂F₂] allows for the first coordination chemistry for this dianion. Reactions of [S₅P₂F₂]²⁻ with d¹⁰ metal ions of zinc(II), and cadmium(II), and d⁹ copper(II) resulted in a surprising diverse array of binding modes and structural motifs. In addition to the simple bis-chelate coordination of [S₅P₂F₂]²⁻ with zinc, cleavage of the P−S bond resulted in complexes with the unusual [S₃PF]²⁻ fluorotrithiophosphate dianion. This was observed in two cluster complexes: a trinuclear cadmium complex with mixed [S₅P₂F₂]²⁻/[S₃PF]²⁻ ligands, [Cd₃(S₅P₂F₂)₃(S₃PF)₂]⁴⁻ as well as an octanuclear copper cluster, [Cu₈(S₃PF)₆]⁴⁻ which form rapidly at room temperature. These new metal/sulfur/ligand clusters are of relevance to understanding multimetal binding to metallothionines, and to potential capping strategies for the condensed nanoparticulate cadmium chalcogenide semiconductors CdS and CdSe.     

Electrochemically Driven Swinging of a Nitrobenzyl Pendant Arm in a Nickel Scorpionand Complex

A radical anion −NO₂^.− is formed upon an electrochemically reversible one-electron reduction of the square-planar Ni^II complex of N-nitrobenzylcyclam. The −NO₂^.− group goes to occupy an axial position of the metal ion, thus establishing a significant electronic interaction with the metal center. In particular, the ESR spectrum supports the occurrence of an electron transfer from −NO₂^.− to the metal, which therefore presents a significant Ni^I character. On re-oxidation, the nitrobenzyl side chain detaches and the Ni^II complex is restored, providing an example of a fully reversible redox driven intramolecular motion.     

Bis(4-benzhydryl-benzoxazol-2-yl)methane – from a Bulky NacNac Alternative to a Trianion in Alkali Metal Complexes

A novel sterically demanding bis(4-benzhydryl-benzoxazol-2-yl)methane ligand 6 (^4−BzhH2BoxCH₂) was gained in a straightforward six-step synthesis. Starting from this ligand monomeric [M(^4-BzhH2BoxCH)] (M=Na ( 7 ), K ( 8₁ )) and dimeric [{M(^4-BzhH2BoxCH)}₂] (M=K ( 8₂ ), Rb ( 9 ), Cs ( 10 )) alkali metal complexes were synthesised by deprotonation. Abstraction of the potassium ion of 8 by reaction with 18-crown-6 resulted in the solvent separated ion pair [{(THF)₂K@(18-crown-6)}{bis(4-benzhydryl-benzoxazol-2-yl)methanide}] ( 11 ), including the energetically favoured monoanionic (E,E)-(^4-BzhH2BoxCH) ligand. Further reaction of ^4−BzhH2BoxCH₂ with three equivalents KH and two equivalents 18-crown-6 yielded polymeric [{(THF)₂K@(18-crown-6)}{K@(18-crown-6)K(^4-BzhBoxCH)}]_n (n→∞) ( 12 ) containing a trianionic ligand. The neutral ligand and herein reported alkali complexes were characterised by single X-ray analyses identifying the latter as a promising precursor for low-valent main group complexes.     

Double Deprotonation of CH3CN by an Iron-Aluminium Complex**

Herein we present the first double deprotonation of acetonitrile (CH₃CN) using two equivalents of a bimetallic iron-aluminium complex. The products of this reaction contain an exceeding simple yet rare [CHCN]²⁻ dianion moiety that bridges two metal fragments. DFT calculations suggest that the bonding to the metal centres occurs through heavily polarised covalent interactions. Mechanistic studies reveal the intermediacy of a monomeric [CH₂CN]⁻ complex, which has been characterised in situ. Our findings provide an important example in which a bimetallic metal complex achieves a new type of reactivity not previously encountered with monometallic counterparts.^{[1, 2]} The isolation of a [CHCN]²⁻ dianion through simple deprotonation of CH₃CN also offers the possibility of establishing a broader chemistry of this motif.     

Structural and Electronic Effects of Stepwise Reduction of a Tetraaryl[3]Cumulene

The chemical reduction of a [3]cumulene ([3]TrTol) has been explored using alkali metals. Mono‐ and doubly reduced forms of [3]TrTol were isolated as solvent‐separated ion pairs with {Na(18‐crown‐6)THF₂}⁺ and {K(18‐crown‐6)THF₂}⁺ counterions and crystallographically characterized. This allowed analysis of structural parameters of the “naked” anions of [3]TrTol without interference from metal binding. The dianion of [3]TrTol was also isolated as a contact‐ion complex with {Cs(18‐crown‐6)}⁺ cations, thereby adding the effect of metal coordination to the core. Structural comparisons of anions to the neutral molecule, [3]TrTol, outline monotonic increases in bond‐length alternation (BLA) upon stepwise reduction. The greatest BLA value is found for the contact‐ion complex, which shows an alternating sequence of short and long carbon–carbon bonds, consistent with the structure of an alkyne. In contrast to studies on tetraphenyl[3]cumulene, the cumulenic framework of [3]TrTol remains planar in all the derivatives.     

The 9H-9-Borafluorene Dianion: A Surrogate for Elusive Diarylboryl Anion Nucleophiles

Double reduction of the THF adduct of 9H-9-borafluorene ( 1 ⋅THF) with excess alkali metal affords the dianion salts M₂[ 1 ] in essentially quantitative yields (M=Li–K). Even though the added charge is stabilized through π delocalization, [ 1 ]²⁻ acts as a formal boron nucleophile toward organoboron ( 1 ⋅THF) and tetrel halide electrophiles (MeCl, Et₃SiCl, Me₃SnCl) to form B−B/C/Si/Sn bonds. The substrate dependence of open-shell versus closed-shell pathways has been investigated.     

Hydrogen-Bonded Polyrotaxane Cation Structure in Nickel Dithiolate Anion Radical Salts: Ferromagnetic and Semiconducting Behavior Associated with Structural Phase Transition

The pseudo-polyrotaxane structure of [(H-bpy⁺)- (DB-24-crown-8)]_∞ (H-bpy⁺ = monoprotonated 4,4-bipyridinium; DB-24-crown-8 = dibenzo-24-crown-8) has been incorporated into the anion radical salt [Ni(dmit)₂]⁻ (dmit²⁻ = 1,3-dithiole-2-thione-4,5-dithiolate). (H-bpy⁺)(DB-24-crown-8)[Ni(dmit)₂]⁻ crystallized as two polymorphs, crystals 1 and 2 . Crystal 1 was found to have a lower density and looser packing structure in which H-bpy⁺ forms a one-dimensional hydrogen-bonding chain that passes though the crown ether ring of DB-24-crown-8. DB-24-crown-8 adopts a U-shaped conformation in which two phenylene rings sandwich one of the pyridyl rings of H-bpy⁺ to stabilize the structure. The [Ni(dmit)₂]⁻ anions are arranged in a layer parallel to the (10) plane with uniform side-by-side interactions. A structural phase transition was observed at 235 K, accompanied by ordering of the polyrotaxane structure. In crystal 1 , at 173 K, H-bpy⁺ is twisted around the central C−C bond, which perturbs the arrangement of [Ni(dmit)₂]⁻ through short C−H⋅⋅⋅S contacts. As a result, the semiconducting behavior, with an activation energy of 0.21 eV, becomes insulating below 235 K. The crystal exhibits ferromagnetic interactions because of the weak side-by-side interactions between [Ni(dmit)₂]⁻ anions. Crystal 2 has a similar pseudo-polyrotaxane structure but showed no phase transition. This suggests that the looser crystal packing in crystal 1 induces the structural change of the pseudo-polyrotaxane, perturbing the electron system of [Ni(dmit)₂]⁻.     

Ambient Water-Stable Dianionic Electron Donors: Intramolecular Noncovalent Conduits Assist Charge Delocalization

Electron-rich π-conjugated dianions are known to be ambient unstable and their stabilization in ambient water is yet to be realized. We report the first example of an exceptionally stable naphthalenediimide-based dianion in ambient and hot water, forming one of the most stable redox-active dianion. The half-life (t_1/2) of dianion ( 1 a²⁻ ) is more than four months in ambient water. The dianionic state was confirmed by X-ray crystallography and by various spectroscopic methods. The noncovalent electronic conduits introduced for the first time in dianions, embrace n_O→π*_C≡N interactions and aid in delocalizing the dianionic charge as validated from theoretical studies. The dianions harness strong NIR absorption and electron donor ability to organic acceptors and metal ions, which make them suitable for potential green energy applications.     

Radical Anions,Radical-Anion Salts,and Anionic Complexes of 2,1,3-Benzochalcogenadiazoles

By means of cyclic voltammetry (CV) and DFT calculations, it was found that the electron-acceptor ability of 2,1,3-benzochalcogenadiazoles 1 – 3 (chalcogen: S, Se, and Te, respectively) increases with increasing atomic number of the chalcogen. This trend is nontrivial, since it contradicts the electronegativity and atomic electron affinity of the chalcogens. In contrast to radical anions (RAs) [ 1 ]^.− and [ 2 ]^.−, RA [ 3 ]^.− was not detected by EPR spectroscopy under CV conditions. Chemical reduction of 1 – 3 was performed and new thermally stable RA salts [K(THF)]⁺[ 2 ]^.− ( 8 ) and [K(18-crown-6)]⁺[ 2 ]^.− ( 9 ) were isolated in addition to known salt [K(THF)]⁺[ 1 ]^.− ( 7 ). On contact with air, RAs [ 1 ]^.− and [ 2 ]^.− underwent fast decomposition in solution with the formation of anions [ECN]⁻, which were isolated in the form of salts [K(18-crown-6)]⁺[ECN]⁻ ( 10 , E=S; 11 , E=Se). In the case of 3 , RA [ 3 ]^.− was detected by EPR spectroscopy as the first representative of tellurium–nitrogen π-heterocyclic RAs but not isolated. Instead, salt [K(18-crown-6)]⁺₂[ 3 -Te₂]²⁻ ( 12 ) featuring a new anionic complex with coordinate Te−Te bond was obtained. On contact with air, salt 12 transformed into salt [K(18-crown-6)]⁺₂[ 3 -Te₄- 3 ]²⁻ ( 13 ) containing an anionic complex with two coordinate Te−Te bonds. The structures of 8 – 13 were confirmed by XRD, and the nature of the Te−Te coordinate bond in [ 3 -Te₂]²⁻ and [ 3 -Te₄- 3 ]²⁻ was studied by DFT calculations and QTAIM analysis.     

Manipulating Electric Double Layer Adsorption for Stable Solid-Electrolyte Interphase in 2.3 Ah Zn-Pouch Cells

Constructing a reliable solid-electrolyte interphase (SEI) is imperative for enabling highly reversible zinc metal (Zn⁰) electrodes. Contrary to conventional “bulk solvation” mechanism, we found the SEI structure is dominated by electric double layer (EDL) adsorption. We manipulate the EDL adsorption and Zn²⁺ solvation with ether additives (i.e. 15-crown-5, 12-crown-4, and triglyme). The 12-crown-4 with medium adsorption on EDL leads to a layer-structured SEI with inner inorganic ZnF_x/ZnS_x and outer organic C−O−C components. This structure endows SEI with high rigidness and strong toughness enabling the 100 cm² Zn||Zn pouch cell to exhibit a cumulative capacity of 4250 mAh cm⁻² at areal-capacity of 10 mAh cm⁻². More importantly, a 2.3 Ah Zn||Zn_0.25V₂O₅⋅n H₂O pouch cell delivers a recorded energy density of 104 Wh L_cell⁻¹ and runs for >70 days under the harsh conditions of low negative/positive electrode ratio (2.2 : 1), lean electrolyte (8 g Ah⁻¹), and high-areal-capacity (≈13 mAh cm⁻²).     

Side arm participation in lariat ether carboxylate-alkali metal cation complexes in solution

Lariat ether carboxylic acids of structure CECH₂OCH₂C₆H₄-2-CO₂H with crown ether (CE) ring sizes of 12-crown-4, 15-crown-5 and 18-crown-6 are prepared and converted into alkali metal-lariat ether carboxylate complexes. Absorptions for the diastereotopic benzylic protons in the ¹H NMR spectra of the complexes in CDCl₃ are utilized to probe the extent of side arm interaction with the crown ether-complexed metal ion as a function of the crown ether ring size and identity of the alkali metal cation.     

Potentiometric investigations of the equilibria between caffeic acid and copper(II), zinc(II), iron(II) and hydrogen ions in aqueous solution

Interactions in aqueous solution of caffeate with copper(II), zinc(II), iron(II) and iron(III) have been investigated. Virtually instantaneous and complete reduction of iron(III) was observed. Glass electrode potentiometry was used to determine the speciation and corresponding formation constants of caffeate with each of the other three metal ions named. Conditions were: temperature, 25°C; ionic strength, 0.100 mol dm⁻³ with respect to chloride. Values obtained for the logarithms of the stepwise protonation constants of the singly protonated dianion of caffeate (L²⁻) are 8.72 and 4.41. The titration data carried out in the presence of the three metal(II) ions can be explained by postulating the major complexes: LCu, logβ₁₁₀=6.02; LCuH₋⁻₁, logβ_11-1=0.25; L₃Cu₂H₋₃⁻⁵, logβ_32-3=0.97; LZnH₋₁⁻, logβ_11-1 = −3.03; L₃ZnH₋₂⁶⁻, logβ_31-2 = −5.51; LFe, logβ₁₁₀ = 3.86; LFeH₋₁⁻, logβ_11-1 = −3.83; L₃FeH₋₂⁶⁻, logβ_31-2 = −6.14, together with a variety of minor species. Complexation in the major species involves, predominantly, chelation by the catecholic site of caffeate whereas coordination to the carboxylate group together with catechol-type chelation featured amongst the minor species. The tendency of copper(II) to form oligonuclear complexes is evident. A single dinuclear iron(II) complex was also found amongst the minor species.     

Poly[[di­aqua­barium(II)]‐μ5‐m‐phenylenedioxydiacetato]: a three‐dimensional barium(II) coordination polymer

In the title coordination polymer, [Ba(1,3‐BDOA)(H₂O)₂]_n (where 1,3‐BDOA²⁻ is the m‐phenylenedioxydiacetate dianion, C₁₀H₈O₆), each Ba^II ion is ten‐coordinated by six carboxyl O atoms and two ether O atoms from different 1,3‐BDOA²⁻ ligands, as well as by two water mol­ecules, thus defining a dodeca­hedron. The Ba^II atoms are covalently linked by 1,3‐BDOA²⁻ ligands in different crystallographic directions, giving rise to a three‐dimensional open framework. The crystal structure is further stabilized by hydrogen bonds and π–π stacking inter­actions. In previously studied polymers of this type, the 1,3‐BDOA²⁻ ligand showed multiple bridging modes with metal ions, including bi‐, tri‐, tetra‐ and hexa­dentate. The title Ba^II complex represents a novel three‐dimensional coordination polymer constructed by octa­dentate bridging 1,3‐BDOA²⁻ ligands.     

Trivalent Rare-Earth Metal Amide Complexes as Catalysts for the Hydrosilylation of Benzophenone Derivatives with HN(SiHMe2)2 by Amine-Exchange Reaction

The rare-earth metal complexes Ln( L¹ )[N(SiHMe₂)₂](thf) (Ln=La, Ce, Y; L¹ =N,N′′-bis(pentafluorophenyl)diethylenetriamine dianion) were synthesized by treating Ln[N(SiHMe₂)₂]₃(thf)₂ with L¹ H₂. The lanthanum and cerium derivatives are active catalysts for the hydrosilylation of benzophenone derivatives with HN(SiHMe₂)₂. An amine-exchange reaction was revealed as a key step of the catalytic cycle, in which Ln−Si−H β-agostic interactions are proposed to promote insertion of the carbonyl moiety into the Si−H bond.     

Synthesis,characterization, and photochemistry of the supramolecules formed from chromium(III) thiocyanato anion associated with crown ethers

A series of anionic chromium(III) thiocyanato complexes with metal crown ether cations have been prepared and characterized. These complexes have the form [Crown-M]₂⁺[Cr(NCS)₅(H₂O)]²⁻ and [Crown-M]₃⁺[Cr(NCS)₆]³⁻, where M=Na⁺, K⁺, or NH₄⁺ and crown represents the crown ether. The crown ethers are 15-crown-5, B-15-crown-5, 18-crown-6, DB-18-crown-6, and DB-24-crown-8, where B- and DB- stand for benzo- and dibenzo-, respectively. The complexes are stable for at least 20 h in the dark in dimethylformamide(DMF) or in acetonitrile, and they release thiocyanate slowly, k=(0.71–2.67)×10⁻⁹ mol/(L s) in acetonitrile in the dark. Photoanation of thiocyanate was observed for the complexes in DMF and in acetonitrile. The quantum yields of thiocyanate release in DMF and in acetonitrile are reported. The quantum yields were in the range 0.05 to 0.52 mol einstein⁻¹ and were solvent and wavelength dependent. In general, larger quantum yields were observed in DMF than in acetonitrile. The photoreaction mechanism is discussed.     

A molecular mechanics study of the selectivity of crown ethers for metal ions on the basis of their size

A molecular mechanics (MM) analysis is carried out on complexes of crown ethers CH₂(OCH₂CH₂)_nCH₂O, 12-crown-4 (n=3), 15-crown-5 (n=4), 18-crown-6 (n=5), 24-crown-8 (n=7), and 30-crown-11 (n=9) to determine the nature of the selectivity shown by these ligands for metal ions on the basis of metal ion size. The MM program used is SYBYL, and M-O bonds are represented using a covalent model, i.e. the M-O bonds are modelled with ideal M-O bond lengths and force constants. The previously used technique of calculating strain energy as a function of M-O bond length is used for all the complexes, and also the complexes of the non-macrocyclic polyethylene glycol analogues. It is concluded that the crown ethers fall into three groups with regard to selectivity for metal ions. Group one consists of the smaller macrocycles such as 12-crown-4 and 15-crown-5, where metal ions generally are too large to enter the cavity of the macrocycle, and the metal ion is coordinated lying outside the plane of the donor atoms of the ligand. Here factors that control selectivity are the same as in non-macrocyclic ligands, chiefly the size of the chelate ring. Group 2 contains only 18-crown-6 of the ligands studied here. 18-Crown-6 complexes have three important conformers, one of which, theD _3d , shows sharp size match selectivity, preferring metal ions with M-O bond lengths of about 2.9 . The other two conformers are adopted by metal ions too small for theD _3d conformer, and are more flexible, exerting little size-match selectivity. These other two conformers are of higher energy than theD _3d conformer for metal ions with M-O bond lengths greater than 2.55 . Thus, a genuine size match selectivity is found for K⁺ with 18-crown-6. With an ideal M-O bond length of 2.88 , K⁺ fits the cavity of theD _3d conformer of 18-crown-6 very closely. The third group consists of very large macrocycles such as 24-crown-8 and 30-crown-10. These enfold the metal ion in extremely folded conformations, but may, as does 30-crown-10, exert considerable selectivity for metal ions on the basis of their size by virtue of the conformation resulting in a set of torsional angles in the ring atoms of the macrocycle which confer considerable rigidity on the ligand.     

Group II Metal Complexes of the Germylidendiide Dianion Radical and Germylidenide Anion

The two‐electron reduction of a Group 14‐element(I) complex [RË?] (E=Ge, R=supporting ligand) to form a novel low‐valent dianion radical with the composition [RË:]^{. 2?} is reported. The reaction of [LGeCl] ( 1 , L=2,6‐(CH?NAr)₂C₆H₃, Ar=2,6‐iPr₂C₆H₃) with excess calcium in THF at room temperature afforded the germylidenediide dianion radical complex [LGe]^{. 2?}?Ca(THF)₃²⁺ ( 2 ). The reaction proceeds through the formation of the germanium(I) radical [LGe?], which then undergoes a two‐electron reduction with calcium to form 2 . EPR spectroscopy, X‐ray crystallography, and theoretical studies show that the germanium center in 2 has two lone pairs of electrons and the radical is delocalized over the germanium‐containing heterocycle. In contrast, the magnesium derivative of the germylidendiide dianion radical is unstable and undergoes dimerization with concurrent dearomatization to form the germylidenide anion complex [C₆H₃‐2‐{C(H)?NAr}Ge‐Mg‐6‐{C(H)‐NAr}]₂ ( 3 ).     

A Polarographic Study on Some Complexes of Pb2+ and Cd2+ with Macrocyclic Polyethers (Crown Ethers)

Abstract

Complexation constants of Pb²⁺ and Cd²⁺ nitrates with five crown compounds (18-crown-6, dicyclohexyl-18-crown-6, benzo-15-crown-5, dibenzo-24-crown-8 and 12-crown-4), have been determined by d.c. and a.c. polarographic measurements in aqueous medium using 0.1 M HNO₃ as supporting electrolyte. The complexes of lead with 18-crown-6 and dicyclohexyl-18-crown-6 are very stable which may be attributed to the partially covalent bonds formed by this metal ion.