Recent developments in cyclopentadienyl-alkalimetal chemistry

Revisiting the origin of the bending in group 2 metallocenes AeCp2 (Ae = Be-Ba)

Metallocenes are well-established compounds in organometallic chemistry, and can exhibit either a coplanar structure or a bent structure according to the nature of the metal center (E) and the cyclopentadienyl ligands (Cp). Herein, we re-examine the chemical bonding to underline the origins of the geometry and stability observed experimentally. To this end, we have analysed a series of group 2 metallocenes [Ae(C₅R₅)₂] (Ae = Be-Ba and R = H, Me, F, Cl, Br, and I) with a combination of computational methods, namely energy decomposition analysis (EDA), polarizability model (PM), and dispersion interaction densities (DIDs). Although the metal-ligand bonding nature is mainly an electrostatic interaction (65-78%), the covalent character is not negligible (33-22%). Notably, the heavier the metal center, the stronger the d-orbital interaction with a 50% contribution to the total covalent interaction. The dispersion interaction between the Cp ligands counts only for 1% of the interaction. Despite that orbital contributions become stronger for heavier metals, they never represent the energy main term. Instead, given the electrostatic nature of the metallocene bonds, we propose a model based on polarizability, which faithfully predicts the bending angle. Although dispersion interactions have a fair contribution to strengthen the bending angle, the polarizability plays a major role.

Porphyrinatonickel(II)-Cyclopentene and Porphyrinatonickel(II)-Cyclopentadiene Hybrids: Zirconacyclopentadiene-Mediated Syntheses, Structures, and Mechanistic Study

The reaction of meso-formyl Ni(II) porphyrin 1 with zirconacyclopentadiene 2 in the presence of AlCl₃ afforded four products 3, 4, 5, and 6 with a total yield of over 85%. The structures of these compounds are well-characterized by ¹H NMR an d¹³C NMR spectroscopy, HRMS, and X-ray single-crystal diffraction. The mechanism is proposed mainly on the basis of isotopic labeling experiments, which showed that a Friedel-Crafts-type reaction and β-H shift may be critical during the formation of 5 and 6.

Alkali metal influences in aluminyl complexes

The previously reported potassium aluminyl complex [(BDI-H)Al^-K⁺]₂ was converted in Li⁺ or Na⁺ salts by a salt metathesis reaction with Li(BPh₄) or Na(BPh₄), respectively; BDI-H = dianionic [(DIPP)N-C(Me)C(H)-C(CH₂)-N(DIPP)^2-] and DIPP = 2,6-diisopropylphenyl. The Rb and Cs aluminyl complexes were obtained by reaction of (BDI)Al with RbC₈ or CsC₈; BDI = HC[C(Me)N(DIPP)]₂. Crystal structures of two monomers, (BDI-H)Al^-Li⁺·(Et₂O)₂ and (BDI-H)Al^-Na⁺·(Et₂O)(TMEDA), and four dimers [(BDI-H)Al^-M⁺]₂ (M = Li, Na, Rb, Cs) are discussed. Lewis base-free dimers [(BDI-H)Al^-M⁺]₂ crystallize either as slipped dimers (Li⁺, Na⁺) in which each Al center features only one Al-M contact or as a symmetric dimer (K⁺, Rb⁺, Cs⁺) in which the cation bridges both Al centers. The dimer with the largest cation (Cs⁺) shows Cs⋯CH₂C interactions between dimers, resulting in a coordination polymer. AIM and charge analysis reveal highly ionic Al-M bonds with strong polarization of the Al lone-pair towards the smaller cation Li⁺ and Na⁺. The Al-M bonds become weaker from Li to Cs. Calculated dimerization energies suggest that in apolar solvents only complexes with the heavier metals Rb and Cs may be in a monomer-dimer equilibrium. This is confirmed by DOSY measurements in benzene. Dimeric aluminyl complexes with heavier alkali metals (K-Cs) react with benzene to give a double C-H activation in para-positions.

Bulking up CpBIG: A Penta-Terphenyl Cyclopentadienyl Ligand

The modification of cyclopentadienyl ligands with carefully selected substituents is a widely used strategy for tuning their steric and electronic properties. We describe the synthesis of an extremely bulky penta-terphenyl cyclopentadienyl ligand (Cp^T5) by arylation of cyclopentadiene. Deprotonation reactions with various group 1 metals and bases afforded a complete series of alkali metal salts MCp^T5 (M = Li–Cs). The compounds were isolated as solvate-free salts, which were characterized by multinuclear nuclear magnetic resonance spectroscopy, ultraviolet–visible spectroscopy, and elemental analysis. Single-crystal X-ray diffraction studies of LiCp^T5, NaCp^T5 (crystallized as a solvate with one tetrahydrofuran molecule per formula unit), and KCp^T5 revealed the formation of metallocene-like sandwich structures in the solid state.

One-Step Synthesis and Schlenk-Type Equilibrium of Cyclopentadienylmagnesium Bromides

In the in situ Grignard metalation method (iGMM), the addition of bromoethane to a suspension of magnesium turnings and cyclopentadienes [C5 H6 (HCp), C5 H5 -Si(iPr)3 (HCpTIPS )] in diethyl ether smoothly yields heteroleptic [(Et2 O)Mg(CpR )(μ-Br)]2 (CpR =Cp (1), CpTIPS (2)). The Schlenk equilibrium of 2 in toluene leads to ligand exchange and formation of homoleptic [Mg(CpR )2 ] (3) and [(Et2 O)MgBr(μ-Br)]2 (4). Interfering solvation and aggregation as well as ligand redistribution equilibria hamper a quantitative elucidation of thermodynamic data for the Schlenk equilibrium of 2 in toluene. In ethereal solvents, mononuclear species [(Et2 O)2 Mg(CpTIPS )Br] (2'), [(Et2 O)n Mg(CpTIPS )2 ] (3'), and [(Et2 O)2 MgBr2 ] (4') coexist. Larger coordination numbers can be realized with cyclic ethers like tetrahydropyran allowing crystallization of [(thp)4 MgBr2 ] (5). The interpretation of the temperature-dependency of the Schlenk equilibrium constant in diethyl ether gives a reaction enthalpy ΔH and reaction entropy ΔS of -11.5 kJ mol-1 and 60 J mol-1 , respectively.

The Bis(η6 -benzene)lithium Cation: A Fundamental Main-Group Organometallic Species

The synthesis and characterization of the bis(η⁶ -benzene)lithium cation, the benzene metallocene of the lightest metal, is reported. The boron compound FmesBCl₂ [Fmes: 2,4,6-tris(trifluoromethyl)phenyl] reacted with three molar equivalents of the lithio-acetylene reagent Li-C≡C-Fmxyl [Fmxyl: 3,5-bis(trifluoromethyl)phenyl]. Subsequent crystallization from benzene gave the [bis(η⁶ -benzene)Li]⁺ cation with the [{FmesB(-C≡C-Fmxyl)₃ }₂ Li]^- anion. This parent [(arene)₂ Li]⁺ cation shows an eclipsed arrangement of the pair of benzene ligands at the central lithium cation with uniform carbon-lithium bond lengths. The corresponding [(η⁶ -toluene)₂ Li]⁺ and [(η⁶ -durene)₂ Li]⁺ containing salts were similarly prepared. The bis(arene)lithium cations were characterized by X-ray diffraction, by solid-state ⁷ Li MAS NMR spectroscopy and their bonding features were analyzed by DFT calculations.

Diphosphanylmetallocenes of Main-Group Elements

Several 1,1'-diphosphanyl-substituted metallocenes of magnesium (magnesocenes) were synthesized, structurally characterized, and their reactivity and coordination chemistry were investigated. Transmetalation of these magnesocenes gives access to group 14 metallocenes (tetrelocenes), as well as to group 15 stibonocenes. These s- and p-block metallocenes represent a novel class of bis(phosphanyl) ligands, exhibiting Lewis-amphiphilic character. Their coordination chemistry towards different transition-metal and main-group fragments was investigated and different complexes are presented.

Solution structures of alkali metal cyclopentadienides in THF estimated by ECC-DOSY NMR-spectroscopy (incl. software)

In this paper we present the aggregational motifs of the widely used alkali-metal cyclopentadienides (CpLi, CpNa, CpK, CpRb, CpCs) in THF-d₈ solution estimated by ECC-DOSY NMR spectroscopy. They form monomeric contact ion pairs (CIPs) in THF-d₈ solution, whereas in NH₃ solvent-separated ion pairs (SSIPs) are observed. The applicability of ECC-DOSY is further advanced by introducing ECC-MW estimation software.

Heterobimetallic Pd-K carbene complexes via one-electron reductions of palladium radical carbenes

Heterobimetallic Pd-K carbenes featuring Pd-C_carbene-K moieties were synthesized via an unprecedented sequential substitution/reduction reaction from a radical precursor, [{PC˙(sp²)P} ^tBuPdI] ([PC(sp²)P] ^tBu = bis[2-(di-iso-propylphosphino)-4-tert-butylphenyl]methylene). Polymeric structures were observed in the solid state for the heterobimetallic compounds that can be interrupted in the presence of a donor solvent.

Remarkable stability of tetrabenzo[a,c,g,i]fluorenyl ammonium salts in water: syntheses, reactions, and crystal structures

Deprotonation of 8bH-tetrabenzo[a,c,g,i]fluorene (8bH-Tbf), an extremely large benzannulated cyclopentadienyl derivative, by [NR3R'][OH] (R = (n)Bu, Et; R' = (n)Bu, Et, Bn) leads to a series of Tbf ammonium salts of the type [NR3R'][Tbf]. These cyclopentadienide analogs were smoothly isolated from a hydrous medium and structurally characterized by single X-ray diffraction. Subsequent reactions demonstrate the high potential of derivatization by converting the presented [Tbf] anions into functionalized Tbf compounds.

Progress in Polyarsolyl Chemistry

The synthesis of heteroatom analogues of the cyclopentadienyl anion Cp^- is a fascinating and challenging field of research. The replacement of methine moieties by phosphorus is well investigated for the synthesis of mono-, tri- and pentaphospholyl ligands. On the other hand, arsenic derivatives are rare and 1,2,4-triarsolyl and tetraarsolyl salts are unknown. Herein, we report on the synthesis of Cs[E₃ C₂ (trip)₂ ] (1 a: E=P; 1 b: E=As; trip=2,4,6-triisopropylphenyl) and Cs[E₄ C(trip)] (2 a: E=P; 2 b: E=As). Compound 1 b represents the first 1,2,4-triarsolyl and 2 b the first tetraarsolyl anion. All salts are obtained in one-pot syntheses using E(SiMe₃ )₃ , 2,4,6-triisopropylbenzoyl chloride and CsF. The products 1 a⋅2 C₄ H₈ O₂ , 2 a⋅Et₂ O and 2 b⋅3 C₄ H₈ O₂ were characterized by X-ray structural analysis, which revealed planar heterocycles. Nucleus-independent chemical shifts (NICS) confirmed the aromaticity of these anions. Notably, compound 2 a⋅Et₂ O is only the second tetraphospholyl ligand which is structurally characterized.

Structure and conformation of the medium-sized chlorophosphazene rings

Medium-sized cyclic oligomeric phosphazenes [PCl2N]m (where m = 5-9) that were prepared from the reaction of PCl5 and NH4Cl in refluxing chlorobenzene have been isolated by a combination of sublimation/extraction and column chromatography from the predominant products [PCl2N]3 and [PCl2N]4. The medium-sized rings [PCl2N]m have been characterized by electrospray ionization-mass spectroscopy (ESI-MS), their (31)P chemical shifts have been reassigned, and their T1 relaxation times have been obtained. Crystallographic data has been recollected for [PCl2N]5, and the crystal structures of [PCl2N]6, and [PCl2N]8 are reported. Halogen-bonding interactions were observed in all the crystal structures of cyclic [PCl2N]m (m = 3-5, 6, 8). The crystal structures of [P(OPh)2N]7 and [P(OPh)2N]8, which are derivatives of the respective [PCl2N]m, are also reported. Comparisons of the intermolecular forces and torsion angles of [PCl2N]8 and [P(OPh)2N]8 with those of three other octameric rings are described. The comparisons show that chlorophosphazenes should not be considered prototypical, in terms of solid-state structure, because of the strong influence of halogen bonding.

Unravelling the mechanism of the asymmetric hydrogenation of acetophenone by [RuX2(diphosphine)(1,2-diamine)] catalysts

The mechanism of catalytic hydrogenation of acetophenone by the chiral complex trans-[RuCl2{(S)-binap}{(S,S)-dpen}] and KO-t-C4H9 in propan-2-ol is revised on the basis of DFT computations carried out in dielectric continuum and the most recent experimental observations. The results of these collective studies suggest that neither a six-membered pericyclic transition state nor any multibond concerted transition states are involved. Instead, a hydride moiety is transferred in an outer-sphere manner to afford an ion-pair, and the corresponding transition state is both enantio- and rate-determining. Heterolytic dihydrogen cleavage proceeds neither by a (two-bond) concerted, four-membered transition state, nor by a (three-bond) concerted, six-membered transition state mediated by a solvent molecule. Instead, cleavage of the H-H bond is achieved via deprotonation of the η(2)-H2 ligand within a cationic Ru complex by the chiral conjugate base of (R)-1-phenylethanol. Thus, protonation of the generated (R)-1-phenylethoxide anion originates from the η(2)-H2 ligand of the cationic Ru complex and not from NH protons of a neutral Ru trans-dihydride complex, as initially suggested within the framework of a metal-ligand bifunctional mechanism. Detailed computational analysis reveals that the 16e(-) Ru amido complex [RuH{(S)-binap}{(S,S)-HN(CHPh)2NH2}] and the 18e(-) Ru alkoxo complex trans-[RuH{OCH(CH3)(R)}{(S)-binap}{(S,S)-dpen}] (R = CH3 or C6H5) are not intermediates within the catalytic cycle, but rather are off-loop species. The accelerative effect of KO-t-C4H9 is explained by the reversible formation of the potassium amidato complexes trans-[RuH2{(S)-binap}{(S,S)-N(K)H(CHPh)2NH2}] or trans-[RuH2{(S)-binap}{(S,S)-N(K)H(CHPh)2NH(K)}]. The three-dimensional (3D) cavity observed within these molecules results in a chiral pocket stabilized via several different noncovalent interactions, including neutral and ionic hydrogen bonding, cation-π interactions, and π-π stacking interactions. Cooperatively, these interactions modify the catalyst structure, in turn lowering the relative activation barrier of hydride transfer by ~1-2 kcal mol(-1) and the following H-H bond cleavage by ~10 kcal mol(-1), respectively. A combined computational study and analysis of recent experimental data of the reaction pool results in new mechanistic insight into the catalytic cycle for hydrogenation of acetophenone by Noyori's catalyst, in the presence or absence of KO-t-C4H9.

Synthesis and NMR-study of 1-trimethylsilyl substituted silole anion [Ph₄C₄Si(SiMe₃)]⁻•[Li]⁺ and 3-silolenide 2,5-carbodianions {[Ph₄C₄Si(n-Bu)₂]⁻²•2[Li]⁺, [Ph₄C₄Si(t-Bu)₂]⁻²•2[Li]⁺} via silole dianion [Ph₄C₄Si]⁻²•2[Li]⁺

1-Trimethylsilyl, 1-R (R = Me, Et, i-Bu)-2,3,4,5-tetraphenyl-1-silacyclopentadiene [[Ph₄C₄Si(SiMe₃)]R] are synthesized from the reaction of 1-trimethylsilyl,1-lithio-2,3,4,5-tetraphenyl-1-silacyclopentadienide anion [[Ph₄C₄Si(SiMe₃]⁻•[Li]⁺ (3) with methyl iodide, ethyl iodide, and i-butyl bromide. The versatile intermediate 3 is prepared by hemisilylation of the silole dianion [Ph₄C₄Si]⁻²•2[Li]⁺ (2) with trimethylsilyl chloride and characterized by ¹H-, ¹³C-, and ²⁹Si-NMR spectroscopy. 1,1-bis(R)-2,3,4,5-tetraphenyl-1-silacyclopentadiene [Ph₄C₄SiR₂] {R = n-Bu (7); t-Bu (8)} are synthesized from the reaction of 2 with n-butyl bromide and t-butyl bromide. Reduction of 7 and 8 with lithium under sonication gives the respective 3-silolenide 2,5-carbodianions {[Ph₄C₄Si(n-Bu)₂]⁻²•2[Li]⁺ (10) and {[Ph₄C₄Si(t-Bu)₂]⁻²•2[Li]⁺ (11)}, which are characterized by ¹H-, ¹³C-, and ²⁹Si-NMR spectroscopy. Polarization of phenyl groups in 3 is compared with those of silole anion/dianion, germole anion/dianion, and 3-silolenide 2,5-carbodianions 10 and 11.