Magnesium–halobenzene bonding: mapping the halogen sigma-hole with a Lewis-acidic complex

Complexes of the Lewis base-free cations (^MeBDI)Mg⁺ and (^tBuBDI)Mg⁺ with Ph–X ligands (X = F, Cl, Br, I) have been studied (^MeBDI = HC[C(Me)N-DIPP]₂ and ^tBuBDI = HC[C(tBu)N-DIPP]₂; DIPP = 2,6-diisopropylphenyl). For the smaller β-diketiminate ligand (^MeBDI) only complexes with PhF could be isolated. Heavier Ph–X ligands could not compete with bonding of Mg to the weakly coordinating anion B(C₆F₅)₄⁻. For the cations with the bulkier ^tBuBDI ligand, the full series of halobenzene complexes was structurally characterized. Crystal structures show that the Mg⋯X–Ph angle strongly decreases with the size of X: F 139.1°, Cl 101.4°, Br 97.7°, I 95.1°. This trend, which is supported by DFT calculations, can be explained with the σ-hole which increases from F to I. Charge calculation and Atoms-In-Molecules analyses show that Mg⋯F–Ph bonding originates from electrostatic attraction between Mg²⁺ and the very polar C^δ+–F^δ− bond. For the heavier halobenzenes, polarization of the halogen atom becomes increasingly important (Cl < Br < I). Complexation with Mg leads in all cases to significant Ph–X bond activation and elongation. This unusual coordination of halogenated species to early main group metals is therefore relevant to C–X bond breaking.

Complexes of a highly Lewis acidic Mg cation and the full series of Ph–X (X = F, Cl, Br, I) have been structurally characterized. The Mg⋯X–Ph angle decreases with halogen size on account of the growing halogen σ-hole.     

Ga+-catalyzed hydrosilylation? About the surprising system Ga+/HSiR3/olefin,proof of oxidation with subvalent Ga+ and silylium catalysis with perfluoroalkoxyaluminate anions

Already 1 mol% of subvalent [Ga(PhF)₂]⁺[pf]⁻ ([pf]⁻ = [Al(OR^F)₄]⁻, R^F = C(CF₃)₃) initiates the hydrosilylation of olefinic double bonds under mild conditions. Reactions with HSiMe₃ and HSiEt₃ as substrates efficiently yield anti-Markovnikov and anti-addition products, while bulkier substrates such as HSiⁱPr₃ are less reactive. Investigating the underlying mechanism by gas chromatography and STEM analysis, we unexpectedly found that H₂ and metallic Ga⁰ formed. Without the addition of olefins, the formation of R₃Si–F–Al(OR^F)₃ (R = alkyl), a typical degradation product of the [pf]⁻ anion in the presence of a small silylium ion, was observed. Electrochemical analysis revealed a surprisingly high oxidation potential of univalent [Ga(PhF)₂]⁺[pf]⁻ in weakly coordinating, but polar ortho-difluorobenzene of E_1/2(Ga⁺/Ga⁰; oDFB) = +0.26–0.37 V vs. Fc⁺/Fc (depending on the scan rate). Apparently, subvalent Ga⁺, mainly known as a reductant, initially oxidizes the silane and generates a highly electrophilic, silane-supported, silylium ion representing the actual catalyst. Consequently, the [Ga(PhF)₂]⁺[pf]⁻/HSiEt₃ system also hydrodefluorinates C(sp³)–F bonds in 1-fluoroadamantane, 1-fluorobutane and PhCF₃ at room temperature. In addition, both catalytic reactions may be initiated using only 0.2 mol% of [Ph₃C]⁺[pf]⁻ as a silylium ion-generating initiator. These results indicate that silylium ion catalysis is possible with the straightforward accessible weakly coordinating [pf]⁻ anion. Apparently, the kinetics of hydrosilylation and hydrodefluorination are faster than that of anion degradation under ambient conditions. These findings open up new windows for main group catalysis.

Nobler than expected: subvalent [Ga(PhF)₂][pf] ([pf]⁻ = [Al{OC(CF₃)₃}₄]⁻) oxidizes hydrosilanes to silylium ions, allowing for catalytic hydrosilylation and hydrodefluorination and suggesting that silylium catalysis is possible with the [pf]− anion.     

Trapped interfacial redox introduces reversibility in the oxygen reduction reaction in a non-aqueous Ca2+ electrolyte

Electrochemical investigations of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have been conducted in a Ca²⁺-containing dimethyl sulfoxide electrolyte. While the ORR appears irreversible, the introduction of a tetrabutylammonium perchlorate (TBAClO₄) co-salt in excess concentrations results in the gradual appearance of a quasi-reversible OER process. Combining the results of systematic cyclic voltammetry investigations, the degree of reversibility depends on the ion pair competition between Ca²⁺ and TBA⁺ cations to interact with generated superoxide (O₂⁻). When TBA⁺ is in larger concentrations, and large reductive overpotentials are applied, a quasi-reversible OER peak emerges with repeated cycling (characteristic of formulations without Ca²⁺ cations). In situ Raman microscopy and rotating ring-disc electrode (RRDE) experiments revealed more about the nature of species formed at the electrode surface and indicated the progressive evolution of a charge storage mechanism based upon trapped interfacial redox. The first electrochemical step involves generation of O₂⁻, followed primarily by partial passivation of the surface by Ca_xO_y product formation (the dominant initial reaction). Once this product matrix develops, the subsequent formation of TBA⁺--O₂⁻ is contained within the Ca_xO_y product interlayer at the electrode surface and, consequently, undergoes a facile oxidation reaction to regenerate O₂.

An interlayer product of oxygen reduction with Ca²⁺/TBA⁺ yields a quasi-reversible oxygen evolution reaction by inducing a trapped interfacial redox process.     

Anion mediated,tunable isoguanosine self-assemblies: decoding the conformation influence and solvent effects

Systematic investigations were performed with various substituted groups at C8 purine and ribose. A series of isoG analogs, C8-phenyl substituted isoG were synthesized and applied for Cs⁺ coordination. The structural proximity between purine and ribose limited pentaplex formation for C8-phenyl substituted isoG derivatives. Based on this observation, deoxy isoG derivative with modification on ribose (tert-butyldimethylsilyl ether) was applied to assemble with the Cs⁺ cation. Critical solvent (CDCl₃ and CD₃CN) and anion (BPh₄⁻, BARF⁻, and PF₆⁻) effects were revealed, leading to the controllable formation of various stable isoG pentaplexes, including singly charged decamer, doubly charged decamer, and 15-mer, etc. Finally, the X-ray crystal structure of [isoG₂₀Cs₃]³⁺(BARF⁻)₃ was successfully obtained, which is the first example of multiple-layer deoxy isoG binding with the Cs⁺ cation, providing solid evidence of this new isoG ionophore beyond two-layer sandwich self-assembly.

The first example of multiple-layer deoxy isoG self-assembly was characterized by X-ray crystal structure. Critical solvent and anion effects were revealed, leading to the controllable formation of various stable isoG assemblies.     

Delocalization tunable by ligand substitution in [L2Al]n− complexes highlights a mechanism for strong electronic coupling

Ligand-based mixed valent (MV) complexes of Al(iii) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I₂P) have been prepared. The MV states containing EW groups are both assigned as Class II/III, and those with ED functional groups are Class III and Class II/III in the (I₂P⁻)(I₂P²⁻)Al and [(I₂P²⁻)(I₂P³⁻)Al]²⁻ charge states, respectively. No abrupt changes in delocalization are observed with ED and EW groups and from this we infer that ligand and metal valence p-orbitals are well-matched in energy and the absence of LMCT and MLCT bands supports the delocalized electronic structures. The MV ligand charge states (I₂P⁻)(I₂P²⁻)Al and [(I₂P²⁻)(I₂P³⁻)Al]²⁻ show intervalence charge transfer (IVCT) transitions in the regions 6850–7740 and 7410–9780 cm⁻¹, respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I₂P²⁻)(I₂P³⁻)Al]²⁻ complexes containing –PhNMe₂ or –PhF₅ substituents. Minor localization of charge in [(I₂P²⁻)(I₂P³⁻)Al]²⁻ was observed when –PhOMe substituents are included.

Organo-aluminum mixed-valent complexes combine properties of both organic and transition element mixed-valent compounds. This supports delocalized electronic structures that are structurally and electronically tunable.     

“Anti-electrostatic” halogen bonding in solution

Halogen-bonded (XB) complexes between halide anions and a cyclopropenylium-based anionic XB donor were characterized in solution for the first time. Spontaneous formation of such complexes confirms that halogen bonding is sufficiently strong to overcome electrostatic repulsion between two anions. The formation constants of such “anti-electrostatic” associations are comparable to those formed by halides with neutral halogenated electrophiles. However, while the latter usually show charge-transfer absorption bands, the UV-Vis spectra of the anion–anion complexes examined herein are determined by the electronic excitations within the XB donor. The identification of XB anion–anion complexes substantially extends the range of the feasible XB systems, and it provides vital information for the discussion of the nature of this interaction.

Spontaneous formation of “anti-electrostatic” complexes in solution demonstrates that halogen bonding can be sufficiently strong to overcome anion–anion repulsion when the latter is attenuated by the polar medium.

Halogen bonding (XB) is an attractive interaction between a Lewis base (LB) and a halogenated compound, exhibiting an electrophilic region on the halogen atom.¹ It is most commonly related to electrostatic interaction between an electron-rich species (XB acceptor) and an area of positive electrostatic potential (σ-holes) on the surface of the halogen substituent in the electrophilic molecule (XB donor).² Provided that mutual polarization of the interacting species is taken into account, the σ-hole model explains geometric features and the variation of stabilities of XB associations, especially in the series of relatively weak complexes.³ Based on the definition of halogen bonding and its electrostatic interpretation, this interaction is expected to involve either cationic or neutral XB donors. Electrostatic interaction of anionic halogenated species with electron-rich XB acceptors, however, seems to be repulsive, especially if the latter are also anionic. Yet, computational analyses predicted that halogen bonding between ions of like charges, called “anti-electrostatic” halogen bonding (AEXB),⁴ can possibly be formed^5–12 and the first examples of AEXB complexes formed by different anions, i.e. halide anions and the anionic iodinated bis(dicyanomethylene)cyclopropanide derivatives 1 (see Scheme 1) or the anionic tetraiodo-p-benzoquinone radical, were characterized recently in the solid state.^13,14 The identification of such complexes substantially extends the range of feasible XB systems, and it provides vital information for the discussion of the nature of this interaction. Computational results, however, significantly depend on the used methods and applied media (gas phase vs. polar environment and solvation models) and the solid state arrangements of the XB species might be affected by crystal forces and/or counterions. Unambiguous confirmation of the stability of the halogen-bonded anion–anion complexes and verification of their thermodynamic characteristics thus requires experimental characterization of the spontaneous formation of such associations in solution. Still, while the solution-phase complexes formed by hydrogen bonding between two anionic species were reported previously,^15–17 there is currently no example of “anti-electrostatic” XB in solution.Open in a separate windowScheme 1Structures of the XB donor 1 and its hydrogen-substituted analogue 2.To examine halogen bonding between two anions in solution, we turn to the interaction between halides and 1,2-bis(dicyanomethylene)-3-iodo-cyclopropanide 1 (Scheme 1).^‡ Even though this compound features a cationic cyclopropenylium core, it is overall anionic, and calculations have demonstrated that its electrostatic potential is universally negative across its entire surface.¹³ The solution of 1 (with tris(dimethylamino)cyclopropenium (TDA) as counterion) in acetonitrile is characterized by an absorption band at 288 nm with ε = 2.3 × 10⁴ M⁻¹ cm⁻¹ (Fig. 1). As LB, we first applied iodide anions taken as a salt with n-tetrabutylammonium counter-ion, Bu₄NI. This salt does not show absorption bands above 290 nm, but its addition to a solution of 1 led to a rise of absorption in the 290–350 nm range (Fig. 1). Subtraction of the absorption of the individual components from that of their mixture produced a differential spectrum which shows a maximum at about 301 nm (insert in Fig. 1). At constant concentration of the XB donor (1) and constant ionic strength, the intensity of the absorption in the range of 280–300 nm (and hence differential absorbance, ΔAbs) rises with increasing iodide concentration (Fig. S1 in the ESI^†). This suggests that the interaction of iodide with 1 results in the formation of the [1, I⁻]-complex which shows a higher absorptivity in this spectral range (eqn (1)):1 + X⁻ ⇌ [1, X⁻]1Open in a separate windowFig. 1Spectra of acetonitrile solutions with constant concentration of 1 (0.60 mM) and various concentrations of Bu₄NI (6.0, 13, 32, 49, 75, 115 and 250 mM, solid lines from the bottom to the top). The dashed lines show spectra of the individual solutions 1 (c = 0.60 mM, red line) and Bu₄NI (c = 250 mM, blue line). The ionic strength was maintained using Bu₄NPF₆. Insert: Differential spectra of the solutions obtained by subtraction of the absorption of the individual components from the spectra of their corresponding mixtures.To clarify the mode of interaction between 1 and iodide in the complex, we also performed analogous measurements with the hydrogen-substituted compound 2 (see Scheme 1). The addition of iodide to a solution of 2 in acetonitrile did not increase the absorption in the 280–300 nm spectral range. Instead, some decrease of the absorption band intensity of 2 with the increase of concentration of I⁻ anions was observed (Fig. S2 in the ESI^†). Such changes are related to a blue shift of this band resulting from the hydrogen bonding between 2 and iodide (formation of hydrogen-bonded [2, I⁻] complex is corroborated by the observation of the small shift of the NMR signal of the proton of 2 to the higher ppm values in the presence of I⁻ anions, see Fig. S3 in the ESI^†).^§ Furthermore, since H-compound 2 should be at least as suitable as XB donor 1 to form anion–π complexes with the halide, this finding (as well as solid-state and computational data^¶) rules out that any increase in absorption in this region observed with the I-compound 1 may be due to this alternative interaction.Likewise, the addition of NBu₄I to a solution of TDA cations taken as a salt with Cl⁻ anions did not result in an increase in the relevant region. Hence, we could also rule out anion–π interactions with the TDA counter-ions as source of the observed changes, which is in line with previous reports on the electron-rich nature of TDA.¹⁸All these observations (supported by the computational analysis, vide infra) indicate that the [1, I⁻] complex (eqn (1)) is formed via halogen bonding of I⁻ with iodine substituents in 1. The changes in the intensities of the differential absorption ΔAbs as a function of the iodide concentration (with constant concentration of XB donor (1) as well as constant ionic strength) are well-modelled by the 1 : 1 binding isotherm (Fig. S1 in the ESI^†). The fit of the absorption data produced a formation constant of K = 15 M⁻¹ for the [1, I⁻] complex (Table 1).^|| The overlap with the absorption of the individual XB donor hindered the accurate evaluation of the position and intensity of the absorption band of the corresponding complex which is formed upon LB-addition to 1. As such, the values of Δλ_max shown in Table 1 represent a wavelength of the largest difference in the absorptivity of the [1, I⁻] complex and individual anion 1, and Δε reflects the difference of their absorptivity at this point (see the ESI^† for the details of calculations).Equilibrium constants and spectral characteristics of the complexes of 1 with halide anions X⁻

Anion–Anion Interactions in Aerogen-Bonded Complexes. Influence of Solvent Environment

Ab initio calculations are applied to the question as to whether a AeX₅⁻ anion (Ae = Kr, Xe) can engage in a stable complex with another anion: F⁻, Cl⁻, or CN⁻. The latter approaches the central Ae atom from above the molecular plane, along its C₅ axis. While the electrostatic repulsion between the two anions prevents their association in the gas phase, immersion of the system in a polar medium allows dimerization to proceed. The aerogen bond is a weak one, with binding energies less than 2 kcal/mol, even in highly polar aqueous solvent. The complexes are metastable in the less polar solvents THF and DMF, with dissociation opposed by a small energy barrier.     

Facilitated inversion complicates the stereodynamics of an SN2 reaction at nitrogen center

Bimolecular nucleophilic substitution (S_N2) reactions at carbon center are well known to proceed with the stereospecific Walden-inversion mechanism. Reaction dynamics simulations on a newly developed high-level ab initio analytical potential energy surface for the F⁻ + NH₂Cl nitrogen-centered S_N2 and proton-transfer reactions reveal a hydrogen-bond-formation-induced multiple-inversion mechanism undermining the stereospecificity of the N-centered S_N2 channel. Unlike the analogous F⁻ + CH₃Cl S_N2 reaction, F⁻ + NH₂Cl → Cl⁻ + NH₂F is indirect, producing a significant amount of NH₂F with retention, as well as inverted NH₂Cl during the timescale within the unperturbed NH₂Cl molecule gets inverted with only low probability, showing the important role of facilitated inversions via an FH…NHCl⁻-like transition state. Proton transfer leading to HF + NHCl⁻ is more direct and becomes the dominant product channel at higher collision energies.

Multiple-inversion, the analogue of the double-inversion pathway recently revealed for S_N2@C, is the key mechanism in S_N2 at N center undermining stereospecificity.     

High-performance all-solid-state electrolyte for sodium batteries enabled by the interaction between the anion in salt and Na3SbS4

All-solid-state sodium batteries with poly(ethylene oxide) (PEO)-based electrolytes have shown great promise for large-scale energy storage applications. However, the reported PEO-based electrolytes still suffer from a low Na⁺ transference number and poor ionic conductivity, which mainly result from the simultaneous migration of Na⁺ and anions, the high crystallinity of PEO, and the low concentration of free Na⁺. Here, we report a high-performance PEO-based all-solid-state electrolyte for sodium batteries by introducing Na₃SbS₄ to interact with the TFSI⁻ anion in the salt and decrease the crystallinity of PEO. The optimal PEO/NaTFSI/Na₃SbS₄ electrolyte exhibits a remarkably enhanced Na⁺ transference number (0.49) and a high ionic conductivity of 1.33 × 10⁻⁴ S cm⁻¹ at 45 °C. Moreover, we found that the electrolyte can largely alleviate Na⁺ depletion near the electrode surface in symmetric cells and, thus, contributes to stable and dendrite-free Na plating/stripping for 500 h. Furthermore, all-solid-state Na batteries with a 3,4,9,10-perylenetetracarboxylic dianhydride cathode exhibit a high capacity retention of 84% after 200 cycles and superior rate performance (up to 10C). Our work develops an effective way to realize a high-performance all-solid-state electrolyte for sodium batteries.

A high-performance all-solid-state PEO/NaTFSI/Na₃SbS₄ electrolyte for sodium batteries is realized owing to the electrostatic interaction between TFSI⁻ in the salt and Na₃SbS₄, which immobilizes TFSI⁻ anions and promotes the dissociation of NaTFSI.     

Au3-to-Ag3 coordinate-covalent bonding and other supramolecular interactions with covalent bonding strength

An efficient strategy for designing charge-transfer complexes using coinage metal cyclic trinuclear complexes (CTCs) is described herein. Due to opposite quadrupolar electrostatic contributions from metal ions and ligand substituents, [Au(μ-Pz-(i-C₃H₇)₂)]₃·[Ag(μ-Tz-(n-C₃F₇)₂)]₃ (Pz = pyrazolate, Tz = triazolate) has been obtained and its structure verified by single crystal X-ray diffraction – representing the 1^st crystallographically-verified stacked adduct of monovalent coinage metal CTCs. Abundant supramolecular interactions with aggregate covalent bonding strength arise from a combination of M–M′ (Au → Ag), metal–π, π–π interactions and hydrogen bonding in this charge-transfer complex, according to density functional theory analyses, yielding a computed binding energy of 66 kcal mol⁻¹ between the two trimer moieties – a large value for intermolecular interactions between adjacent d¹⁰ centres (nearly doubling the value for a recently-claimed Au(i) → Cu(i) polar-covalent bond: Proc. Natl. Acad. Sci. U.S.A., 2017, 114, E5042) – which becomes 87 kcal mol⁻¹ with benzene stacking. Surprisingly, DFT analysis suggests that: (a) some other literature precedents should have attained a stacked product akin to the one herein, with similar or even higher binding energy; and (b) a high overall intertrimer bonding energy by inferior electrostatic assistance, underscoring genuine orbital overlap between M and M′ frontier molecular orbitals in such polar-covalent M–M′ bonds in this family of molecules. The Au → Ag bonding is reminiscent of classical Werner-type coordinate-covalent bonds such as H₃N: → Ag in [Ag(NH₃)₂]⁺, as demonstrated herein quantitatively. Solid-state and molecular modeling illustrate electron flow from the π-basic gold trimer to the π-acidic silver trimer with augmented contributions from ligand-to-ligand’ (LL′CT) and metal-to-ligand (MLCT) charge transfer.

A stacked Ag₃–Au₃ bonded (66 kcal mol⁻¹) complex obtained crystallographically exhibits charge-transfer characteristics arising from multiple cooperative supramolecular interactions.     

Trends in trigonal prismatic Ln-[1]ferrocenophane complexes and discovery of a Ho3+ single-molecule magnet

Lanthanide metallocenophanes are an intriguing class of organometallic complexes that feature rare six-coordinate trigonal prismatic coordination environments of 4f elements with close intramolecular proximity to transition metal ions. Herein, we present a systematic study of the structural and magnetic properties of the ferrocenophanes, [LnFc₃(THF)₂Li₂]⁻, of the late trivalent lanthanide ions (Ln = Gd (1), Ho (2), Er (3), Tm (4), Yb (5), Lu (6)). One major structural trend within this class of complexes is the increasing diferrocenyl (Fc²⁻) average twist angle with decreasing ionic radius (r_ion) of the central Ln ion, resulting in the largest average Fc²⁻ twist angles for the Lu³⁺ compound 6. Such high sensitivity of the twist angle to changes in r_ion is unique to the here presented ferrocenophane complexes and likely due to the large trigonal plane separation enforced by the ligand (>3.2 Å). This geometry also allows the non-Kramers ion Ho³⁺ to exhibit slow magnetic relaxation in the absence of applied dc fields, rendering compound 2 a rare example of a Ho-based single-molecule magnet (SMM) with barriers to magnetization reversal (U) of 110–131 cm⁻¹. In contrast, compounds featuring Ln ions with prolate electron density (3–5) don''t show slow magnetization dynamics under the same conditions. The observed trends in magnetic properties of 2–5 are supported by state-of-the-art ab initio calculations. Finally, the magneto-structural relationship of the trigonal prismatic Ho-[1]ferrocenophane motif was further investigated by axial ligand (THF in 2) exchange to yield [HoFc₃(THF*)₂Li₂]⁻ (2-THF*) and [HoFc₃(py)₂Li₂]⁻ (2-py) motifs. We find that larger average Fc²⁻ twist angles (in 2-THF* and 2-py as compared to in 2) result in faster magnetic relaxation times at a given temperature.

Lanthanide ferrocenophanes are an intriguing class of organometallic complexes that feature rare six-coordinate trigonal prismatic coordination environments of 4f elements with close intramolecular proximity to iron ions.     

Solvate sponge crystals of (DMF)3NaClO4: reversible pressure/temperature controlled juicing in a melt/press-castable sodium-ion conductor

A new type of crystalline solid, termed “solvate sponge crystal”, is presented, and the chemical basis of its properties are explained for a melt- and press-castable solid sodium ion conductor. X-ray crystallography and atomistic simulations reveal details of atomic interactions and clustering in (DMF)₃NaClO₄ and (DMF)₂NaClO₄ (DMF = N-N′-dimethylformamide). External pressure or heating results in reversible expulsion of liquid DMF from (DMF)₃NaClO₄ to generate (DMF)₂NaClO₄. The process reverses upon the release of pressure or cooling. Simulations reveal the mechanism of crystal “juicing,” as well as melting. In particular, cation–solvent clusters form a chain of octahedrally coordinated Na⁺–DMF networks, which have perchlorate ions present in a separate sublattice space in 3 : 1 stoichiometry. Upon heating and/or pressing, the Na⁺⋯DMF chains break and the replacement of a DMF molecule with a ClO₄⁻ anion per Na⁺ ion leads to the conversion of the 3 : 1 stoichiometry to a 2 : 1 stoichiometry. The simulations reveal the anisotropic nature of pressure induced stoichiometric conversion. The results provide molecular level understanding of a solvate sponge crystal with novel and desirable physical castability properties for device fabrication.

Stimuli-responsive “solvate-sponge”-(DMF)₃NaClO₄ exhibits linear chains of DMF–Na⁺ ions with ClO₄⁻ anions in the interstitial space. At increased pressure or temperature, DMF is expelled (reversibly), resulting in a new stoichiometry-(DMF)₂NaClO₄.     

Cationic molybdenum oxo alkylidenes stabilized by N-heterocyclic carbenes: from molecular systems to efficient supported metathesis catalysts

Cationic d⁰ group 6 olefin metathesis catalysts have been recently shown to display in most instances superior activity in comparison to their neutral congeners. Furthermore, their catalytic performance is greatly improved upon immobilization on silica. In this context, we have developed the new family of molecular cationic molybdenum oxo alkylidene complexes stabilized by N-heterocyclic carbenes of the general formula [Mo(O)(CHCMe₃)(IMes)(OR)[X⁻]] (IMes = 1,3-dimesitylimidazol-2-ylidene; R = 1,3-dimesityl-C₆H₃, C₆F₅; X⁻ = B(3,5-(CF₃)₂C₆H₃)₄⁻, B(Ar^F)₄, tetrakis(perfluoro-t-butoxy)aluminate (PFTA)). Immobilization of [Mo(O)(CHCMe₃)(IMes)(O-1,3-dimesityl-C₆H₃)⁺B(Ar^F)₄⁻] on silica via surface organometallic chemistry yields an active alkene metathesis catalyst that shows the highest productivity towards terminal olefins amongst all existing molybdenum oxo alkylidene catalysts.

The first cationic molybdenum oxo complexes were synthesized and immobilized on partially dehydroxylated silica. Vastly enhanced catalytic activity for terminal olefins was found compared to their neutral congeners.     

Semiquinone radical-bridged M2 (M = Fe,Co, Ni) complexes with strong magnetic exchange giving rise to slow magnetic relaxation

The use of radical bridging ligands to facilitate strong magnetic exchange between paramagnetic metal centers represents a key step toward the realization of single-molecule magnets with high operating temperatures. Moreover, bridging ligands that allow the incorporation of high-anisotropy metal ions are particularly advantageous. Toward these ends, we report the synthesis and detailed characterization of the dinuclear hydroquinone-bridged complexes [(Me₆tren)₂M^II₂(C₆H₄O₂²⁻)]²⁺ (Me₆tren = tris(2-dimethylaminoethyl)amine; M = Fe, Co, Ni) and their one-electron-oxidized, semiquinone-bridged analogues [(Me₆tren)₂M^II₂(C₆H₄O₂⁻˙)]³⁺. Single-crystal X-ray diffraction shows that the Me₆tren ligand restrains the metal centers in a trigonal bipyramidal geometry, and coordination of the bridging hydro- or semiquinone ligand results in a parallel alignment of the three-fold axes. We quantify the p-benzosemiquinone–transition metal magnetic exchange coupling for the first time and find that the nickel(ii) complex exhibits a substantial J < −600 cm⁻¹, resulting in a well-isolated S = 3/2 ground state even as high as 300 K. The iron and cobalt complexes feature metal–semiquinone exchange constants of J = −144(1) and −252(2) cm⁻¹, respectively, which are substantially larger in magnitude than those reported for related bis(bidentate) semiquinoid complexes. Finally, the semiquinone-bridged cobalt and nickel complexes exhibit field-induced slow magnetic relaxation, with relaxation barriers of U_eff = 22 and 46 cm⁻¹, respectively. Remarkably, the Orbach relaxation observed for the Ni complex is in stark contrast to the fast processes that dominate relaxation in related mononuclear Ni^II complexes, thus demonstrating that strong magnetic coupling can engender slow magnetic relaxation.

A semiquinone radical bridging two trigonal bipyramidal metal centers facilitates strong magnetic exchange and single-molecule magnet behavior.     

Reactivity of cyano- and isothiocyanatoborylenes: metal coordination,one-electron oxidation and boron-centred Brønsted basicity

Doubly base-stabilised cyano- and isothiocyanatoborylenes of the form LL′BY (L = CAAC = cyclic alkyl(amino)carbene; L′ = NHC = N-heterocyclic carbene; Y = CN, NCS) coordinate to group 6 carbonyl complexes via the terminal donor of the pseudohalide substituent and undergo facile and fully reversible one-electron oxidation to the corresponding boryl radical cations [LL′BY]˙⁺. Furthermore, calculations show that the borylenes have very similar proton affinities, both to each other and to NHC superbases. However, while the protonation of LL′B(CN) with PhSH yielding [LL′BH(CN)⁺][PhS⁻] is fully reversible, that of LL′B(NCS) is rendered irreversible by a subsequent B-to-C_CAAC hydrogen shift and nucleophilic attack of PhS⁻ at boron.

Borylenes of the form (CAAC)(NHC)BY (Y = CN, NCS; CAAC = cyclic alkyl(amino)carbene; NHC = N-heterocyclic carbene) coordinate to group 6 carbonyl complexes via Y, and show reversible boron-centered Brønsted basicity and one-electron oxidation.     

Dehydrogenation of iron amido-borane and resaturation of the imino-borane complex

We report on the first isolation and structural characterization of an iron phosphinoimino-borane complex Cp*Fe(η²-H₂B NC₆H₄PPh₂) by dehydrogenation of iron amido-borane precursor Cp*Fe(η¹-H₃B–NHC₆H₄PPh₂). Significantly, regeneration of the amido-borane complex has been realized by protonation of the iron(ii) imino-borane to the amino-borane intermediate [Cp*Fe(η²-H₂B–NHC₆H₄PPh₂)]⁺ followed by hydride transfer. These new iron species are efficient catalysts for 1,2-selective transfer hydrogenation of quinolines with ammonia borane.

Dehydrogenation of an amido-borane iron complex provides an imino-borane complex. Regeneration of the amido-borane precursor was achieved by protonation of the imino-borane followed by hydride transfer to the amino-borane intermediate.

Because of relevance to H₂ storage^1–10 and hydrogenation catalysis,^11–15 metal amine-borane complexes^16–18 and their dehydrogenated forms, such as amino-boranes^20–22 and imino-boranes⁴ are arising as a significant family in organometallic chemistry. In transition metal-catalyzed dehydrocoupling of amine-boranes and related transfer hydrogenations, the interactions between the metal and the borane fragment are essential to dehydrogenation and the consequent transformations.^16–20 Specifically, amino-borane complexes containing a M–H₂B NR₂ moiety are the primary dehydrogenated species and are often identified as a resting point in the catalysis (Scheme 1a).^20–22 Management of reversible dehydrogenation–regeneration reactions on a M–BH₂ NR₂ platform could provide a strategy with which to design efficient catalysts capable of operating sustainable syntheses.Open in a separate windowScheme 1Schematic representation of metal-based amine-borane dehydrogenation.Wider exploration of metal amino-borane chemistry is challenging since M–H₂B NH₂ species are very reactive toward H₂ release. In 2010, Aldridge et al. reported the isolation of [(IMes)₂Rh(H)₂(η²-H₂B NR₂)] and [(IMes)₂Ir(H)₂(η²-H₂B NR₂)] from the metal-catalyzed dehydrogenation of R₂HN·BH₃.^21a At the same time, Alcaraz and Sabo-Etienne reported the preparation of (PCy₃)₂Ru(H)₂(η²-H₂B NH_nMe_2−n) (n = 0–2) complexes^22a by the dehydrogenation of amine-boranes with the corresponding ruthenium precursors. Subsequently, a straightforward synthesis of Ru, Rh, and Ir amino-borane complexes by reaction of H₂B NR₂ (R = iPr or Cy) with the bis(hydrogen) complexes of M(H)₂(η²-H₂)₂(PCy₃)₂ or [CpRu(PR₃)₂]⁺ fragments was developed.^21b,22b Turculet et al. have shown that the ruthenium-alkoxide complex is able to activate H₃B·NHR₂ producing hydrido ruthenium complex.²³ Notably, Weller and Macgregor found that dehydrocoupling of ammonia-borane by [Ph₂P(CH₂)₃PPh₂Rh(η⁶-C₆H₅F)] affords a μ-amino-borane bimetallic Rh complex, in which the simplest H₂B NH₂ moiety is trapped on a rhodium dimer.^20aAlthough iron-catalyzed dehydrocoupling of amine-boranes has attracted great interest,^24–29 iron amine-borane complexes, their dehydrogenated derivatives, and especially the catalysis relevant to organic synthesis are largely unexplored. Recently, Kirchner et al. reported a pincer-type iron complex generated by protonation of the borohydride iron complex (PNP)Fe(H)(η²-BH₄) with ammonium salts.³⁰ Inspired by earlier research on M–H₂B NR₂ chemistry, we intended to establish the reversible conversions of amino-borane complexes and their dehydrogenated forms in a synthetic piano-stool iron system. Herein, we report dehydrogenation of iron amido-borane complex Cp*Fe(η¹-H₃B–NHC₆H₄PPh₂) (2) (Cp* = Me₅C₅⁻) to the imino-borane complex Cp*Fe(η²-H₂B NC₆H₄PPh₂) (3), and resaturation of the imino-borane by stepwise protonation and hydride transfer (Scheme 1b). This new class of iron species is capable of catalyzing 1,2-selective transfer hydrogenation of quinolines with H₃N·BH₃.To synthesize the iron amido-borane complex, a new monomer, the iron tetrahydridoborate precursor Cp*Fe(η²-BH₄)(NCMe) (1), was prepared in situ by the reaction of [Cp*Fe(NCMe)₃]PF₆ with Bu₄NBH₄ in acetonitrile at room temperature for 5 min. Such ferrous borohydrides have been documented only rarely,³¹ since they are prone to form polynuclear iron borate clusters.^32,33 The ¹¹B NMR spectrum of the reaction solution shows a quintet at δ 15.4 (J_BH = 88 Hz) for the BH₄⁻ ligand of 1, and this stands in contrast to the signal at δ −32.0 observed for Bu₄NBH₄. Upon storing the reaction mixture at −30 °C overnight, single crystals suitable for X-ray diffraction were obtained. Crystallographic analysis confirmed the structure of 1 as a piano-stool iron tetrahydridoborate compound (ESI, Fig. S1^†).Addition of phosphinoamine ligand 1,2-Ph₂PC₆H₄NH₂ to a solution of 1 in acetonitrile caused an instantaneous color change from deep blue to dark brown (Scheme 2). ESI-MS studies indicated the production of the iron amido-borane compound (2) with m/z = 481.1793 (calcd m/z = 481.1770), which was isolated in 87% yield. NMR spectra showed a boron resonance at δ −17.5, and a phosphorus resonance at δ 85.9. The ¹H NMR spectrum exhibits a characteristic hydride signal at δ −13.98, which is assigned to the bridging hydride Fe–H–B. Owing to exchange between the hydrogen atoms at the boron,³⁴ the terminal B–H resonances in the ¹H NMR spectrum are very broad and are obscured by the distinct Cp* signals. To assign the B–H hydride signals, the deuterated compound Cp*Fe(D₃B–NHC₆H₄PPh₂) (d-2) was synthesized from Cp*Fe(BD₄)(NCMe). In addition to the Fe–D–B signal at δ −13.98, the ²H NMR spectrum of d-2 displayed discrete peaks at δ 2.23 and 0.19 for the terminal B–D hydrides (Fig. 1).Open in a separate windowFig. 1 ²H NMR spectra for dehydrogenation of d-2 to d-3.Open in a separate windowScheme 2Synthetic route to imino-borane complex.When a C₆H₆ solution of 2 was held at 50 °C for 6 h the dehydrogenated imino-borane compound (3) was produced in 92% yield. The ESI-MS spectrum of 3 has a strong peak at m/z 479.1626 (calcd m/z = 479.1637) which can be compared to the peak at m/z = 481.1793 for 2. The isotopic distributions match well with the calculated values (see Fig. S3^†). GC analysis shows that the reaction produced H₂ nearly quantitatively (see Fig. S4^†). In solution, the ³¹P NMR spectrum of 3 displays a sharp signal at δ 71.9, in contrast to the peak at δ 85.9 for 2. The ¹¹B resonance shifts significantly, from δ −17.5 for 2 to δ 42.7 for 3 (Fig. S16^†), and is particularly diagnostic of a three-coordinate boron atom.^21,35 This result indicates the B N double bond character in the dehydrogenated form of the amido-borane complex. In the ¹H NMR spectrum, the Fe–H–B signal was observed at δ −17.91 with the integral of 2H, and no characteristic signal for a terminal B–H hydride was found. To confirm the formation of an imino-borane compound, the hydrogen decoupling was also carried out with compound d-2 and monitored by ²H NMR spectra. Only a deuterium signal was observed at δ −17.91 for Fe–D–B, indicating the formation of d-3 (Fig. 1). When the dehydrogenation was conducted in a J-Young tube in C₆D₆, a characteristic triplet corresponding to HD appeared at δ 4.43 (J_HD = 45 Hz) in the ¹H NMR spectrum (Fig. S18^†).³⁶The structures of 2 and 3 were verified by X-ray crystallographic analysis (Fig. 2). Consistent with NMR spectroscopic analysis, the BH₃ moiety in 2 is stabilized by one of the B–H bonds binding at the Fe–NH unit to form an Fe–H–B–N four-membered metallacycle. This metal–ligand cooperative binding mode increased the B–H bond length in the bridging B–H(1) bond to 1.362 Å vs. 1.129 Å and 1.121 Å for the two terminal B–H bonds. The B–N bond length of 1.545(3) Å in 2 is slightly shorter than that in H₃B·NH₃ (d_B–N = 1.58(2) Å).³⁷ Crystallographic analysis of 3 confirmed an imino-borane complex with a Cp*Fe(η²-H₂B NC₆H₄PPh₂) framework. After dehydrogenation of 2, striking structural changes were observed. The N atom has been become detached from Fe, while the BH₂ fragment acts as a bis(σ-borane) ligand coordinated to the metal center.^21–23 The B–N bond distance of 1.455(5) Å in 3 is shorter by 0.09 Å than that in 2, and is close to that reported for the cyclic trimer borazine (1.4355(21) Å).³⁸ Combined with the NMR results, the B–N bond length in 3 suggests some double bond character.^21,22 As the imino-borane fragment is tethered in the coordination sphere, the boron center adopts a quasi-tetrahedral geometry, and the B–N bond appears to be partially sp³ hybridized. Dehydrogenation of the amido-borane complex also caused the decrease of the Fe⋯B distances from 2.223(3) Å to 2.026(4) Å which is shorter than the sum of the covalent radii of Fe and B atom (2.16 Å), indicating that the borane and the metal are bonded.Open in a separate windowFig. 2Solid-sate structure (50% probability thermal ellipsoids) of (a) complex 2 and (b) 3. For clarity, hydrogen atoms of Cp* and phenyl rings are omitted.Notably, the amido-borane compound 2 can be regenerated by stepwise protonation of 3 and transfer of a hydride (Scheme 3). Complex 3 reacts readily with H(Et₂O)₂BAr^F₄ in C₆H₅F. The reaction solution was analyzed by ESI-MS spectroscopy, which showed an ionic peak at m/z = 480.1726 (calcd m/z = 480.1715), suggesting the formation of [3H]⁺. Alternatively, the reaction of complex 2 with H(Et₂O)₂BAr^F₄ unambiguously provides [3H]⁺ and produces H₂. X-ray crystallographic analysis reveals that the resulting cationic complex [3H]⁺ exhibits a similar framework to its imino-borane precursor (3). The BH₂ moiety retains a binding mode of the bis(σ-BH₂) fashion (Fig. 3). In contrast, the B–N distance in [3H]⁺ (1.586(6) Å) is extended by 0.13 Å and the [3H]⁺ framework becomes much less compact than that of 3. Probably due to the fluxional structure of the seven-membered Fe–P–C–C–N–B(H) ring, the solution of [3H][BAr^F₄] gives broad ¹H NMR resonances even at −60 °C. The phosphorus resonance arose at δ 72.0 as a singlet when the solution sample was cooled to −40 °C (Fig. S20 and S21^†).Open in a separate windowFig. 3Solid-state structures of (a) complex [3H]⁺ and (b) [3H(PPh₃)]⁺. For clarity, counterion [BAr^F₄]⁻, hydrogen atoms of Cp* and phenyl rings have been omitted.Open in a separate windowScheme 3Conversions of iron imino-borane, amino-borane and amido-borane complexes.In [3H]⁺, the boron is coordinatively unsaturated, as manifested by its interaction with a σ-donor. For instance, treatment of 2 with [HPPh₃][BAr^F₄] (pK^MeCN_a = 7.6)³⁹ provides a Ph₃P-stabilized borane complex, [3H(PPh₃)]⁺ (m/z = 742.2620, calcd m/z = 742.2626). The ¹H NMR spectrum of [3H(PPh₃)]⁺ exhibits an NH resonance at δ 4.68, suggesting that protonation occurred at the N site. The distinctive upfield hydride signal for Fe–H–B is observed at δ −15.58. In the ³¹P NMR spectrum, two phosphorus signals at δ 78.90 and −1.26 correspond to the Fe–P and the B–P resonances, respectively. The ¹¹B signal at δ −13.72 indicates a tetracoordinated boron, which is further confirmed by crystallographic analysis of [3H(PPh₃)]⁺ (Fig. 3). In the solid-sate structure, a Ph₃P molecule is bound to the B center (d_B–P = 1.982(4) Å), leading to the formation of a new Fe–H–B–N four-membered metallacycle. As a amido-borane complex, [3H(PPh₃)]⁺ has a B–N bond length of 1.527(5) Å, somewhat shorter than 1.545(3) Å in 2.After attaching a proton at the N atom, we subsequently explored restoration of the original borane moiety. Treatment of freshly prepared [3H][BAr^F₄] in fluorobenzene with catecholborane-NEt₃ adduct (δ_B = 10.56, J_HB = 142.4 Hz)⁴⁰ results in the regeneration of 2, as evidenced by the NMR spectra (Fig. S29 and S30^†). The ¹H NMR spectrum of the reaction mixture displays a characteristic hydride signal at −13.97 ppm, indicating the recovery of the iron amido-borane complex. On the other side, concomitant formation of the borenium ion (δ_B = 13.86) was also observed in the ¹¹B NMR spectrum, which agrees with the hydride transfer from the organohydride reagent to [3H]⁺. It was interesting that the ion [3H]⁺ is stable towards 5,6-dihydrophenanthridine and Hantszch ester. These results indicate that the hydride-donating ability (ΔG_H⁻) of 2 is in the range of 55–59 kcal mol⁻¹.⁴¹ The reactive nature of the hydride in 2 was demonstrated by the reaction with [HPPh₃][BAr^F₄], which produces [3H(PPh₃)]⁺ and releases H₂ (Scheme 3).1The metal amine-borane complexes and their dehydrogenated derivatives are implicated throughout the catalytic cycle of amine-borane dehydrogenation. We found both the iron complexes 2 and 3 are efficient catalysts for H₃N·BH₃ dehydrogenation at room temperature. In the presence of 1 mol% catalyst, a THF solution of H₃N·BH₃ (1.0 mmol) generates about 2.2 equivalent of H₂ within 6 h based on GC quantification (Fig. S33^†). More importantly, such catalytic dehydrocoupling systems allow for selective transfer hydrogenation of quinolines to dihydroquinolines, which are valuable synthons leading to many bio-active compounds.⁴² For instance, addition of methyl-6-quinolineacetate (4) to the catalytic system containing one equiv. of H₃N·BH₃ and 1 mol% of 3 gave 1,2-dihydro-methyl-6-quinolineacetate (5) in excellent yield within 6 h (eqn (1)). The outcome of this reaction was unaffected by switching the catalyst from 3 to 2, or by use of excess reducing agent or by an increase in the reaction temperature (Table S1^†).     

Crystal Structure and Magnetic Properties of Trinuclear Transition Metal Complexes (MnII,CoII, NiII and CuII) with Bridging Sulfonate-Functionalized 1,2,4-Triazole Derivatives

Here we present the synthesis, structure and magnetic properties of complexes of general formula (Mn)(Me₂NH₂)₄][Mn₃(μ-L)₆(H₂O)₆] and (Me₂NH₂)₆[M₃(μ-L)₆(H₂O)₆] (M = Co^II, Ni^II and Cu^II); L⁻² = 4-(1,2,4-triazol-4-yl) ethanedisulfonate). The trinuclear polyanions were isolated as dimethylammonium salts, and their crystal structures determined by single crystal and powder X-ray diffraction data. The polyanionic part of these salts have the same molecular structure, which consists of a linear array of metal(II) ions linked by triple N1-N2-triazole bridges. In turn, the composition and crystal packing of the Mn^II salt differs from the rest of the complexes (with six dimethyl ammonia as countercations) in containing one Mn⁺² and four dimethyl ammonia as countercations. Magnetic data indicate dominant intramolecular antiferromagnetic interactions stabilizing a paramagnetic ground state. Susceptibility data have been successfully modeled with a simple isotropic Hamiltonian for a centrosymmetric linear trimer, H = −2J (S₁S₂ + S₂S₃) with super-exchange parameters J = −0.4 K for Mn^II, −7.5 K for Ni^II and −45 K for Cu^II complex. The magnetic properties of these complexes and their easy processing opens unique possibilities for their incorporation as magnetic molecular probes into such hybrid materials as magnetic/conducting multifunctional materials or as dopant for organic conducting polymers.     

Cerium(iv) complexes with guanidinate ligands: intense colors and anomalous electronic structures

A series of cerium(iv) mixed-ligand guanidinate–amide complexes, {[(Me₃Si)₂NC(NⁱPr)₂]_xCe^IV[N(SiMe₃)₂]_3−x}⁺ (x = 0–3), was prepared by chemical oxidation of the corresponding cerium(iii) complexes, where x = 1 and 2 represent novel complexes. The Ce(iv) complexes exhibited a range of intense colors, including red, black, cyan, and green. Notably, increasing the number of the guanidinate ligands from zero to three resulted in significant redshift of the absorption bands from 503 nm (2.48 eV) to 785 nm (1.58 eV) in THF. X-ray absorption near edge structure (XANES) spectra indicated increasing f occupancy (n_f) with more guanidinate ligands, and revealed the multiconfigurational ground states for all Ce(iv) complexes. Cyclic voltammetry experiments demonstrated less stabilization of the Ce(iv) oxidation state with more guanidinate ligands. Moreover, the Ce(iv) tris(guanidinate) complex exhibited temperature independent paramagnetism (TIP) arising from the small energy gap between the ground- and excited states with considerable magnetic moments. Computational analysis suggested that the origin of the low energy absorption bands was a charge transfer between guanidinate π orbitals that were close in energy to the unoccupied Ce 4f orbitals. However, the incorporation of sterically hindered guanidinate ligands inhibited optimal overlaps between Ce 5d and ligand N 2p orbitals. As a result, there was an overall decrease of ligand-to-metal donation and a less stabilized Ce(iv) oxidation state, while at the same time, more of the donated electron density ended up in the 4f shell. The results indicate that incorporating guanidinate ligands into Ce(iv) complexes gives rise to intense charge transfer bands and noteworthy electronic structures, providing insights into the stabilization of tetravalent lanthanide oxidation states.

A series of cerium(iv) mixed-ligand guanidinate-amide complexes, {[(Me₃Si)₂NC(NⁱPr)₂]_xCe^IV[N(SiMe₃)₂]_3−x}+ (x = 0−3), was prepared by chemical oxidation and studied spectroscopically and computationally, revealing trends in 4f/5d orbital occupancies.