Cyclooctatetraenyl-komplexe der frühen übergangsmetalle und lanthanoide: IV. Strukturchemie des anionisichen sandwich-komplexes [Ce(COT)2]−

The cerocene(III) derivatives [Li(THF)₄][Ce(COT)₂] (1) and (THF)₃Na(μ-COT)Ce(COT) (2) have been prepared and characterized structurally by an X-ray diffraction study (COT  η⁸-cycloocta-tetraenyl(2−)). The molecular structures differ significantly, depending on the nature of the alkali metal cation. In the solid state, compound 1 consists of separated ion pairs. In 2 a cyclooctatetraenyl ligand bridges cerium and sodium to give a linear (COT)Ce(μ-COT)Na arrangement.     

Bicyclo[6.3.0]undecapentaenyl Anion: The Next Higher Homolog of the Indenyl Anion with Exceptionally Large Ion‐Pairing Effects on its Tropicity

The title anion 1 was generated as a fairly thermally stable species in tetrahydrofuran (THF) and dimethylsulfoxide (DMSO) by the action of several bases (sodium hydride, potassium hydride, lithium diisopropylamide, and lithium hexamethyldisilazide) with appropriate bicyclo[6.3.0]undecapentaenes. Variable‐temperature ¹H NMR spectra of 1? Li⁺ in [D₈]THF reveal that the anion exhibits exceptionally large ion‐pairing effects; proton chemical shifts vary by more than 1 ppm as a function of ion‐pairing conditions. Thus, anion 1 , in a contact ion pair (Li⁺ at ambient temperature in THF), behaves as an aromatic cyclopentadienyl anion that is perturbed only slightly by the electronic effects of a paramagnetic cyclooctatetraene (COT), whereas 1 in a separated ion pair (Li⁺ at low temperatures in THF or at ambient temperature in DMSO) behaves as an overall paratropic species with a 12 π‐electron periphery. ¹³C NMR spectroscopy indicates no major skeletal rearrangement and only small variations of the electron density. The variable tropicity of 1 can be ascribed to small conformational changes of the molecule. In addition to its unusual, tunable tropicity, anion 1 can also serve as a versatile building block for the synthesis of cyclopentanoid conjugated systems fused to a fully unsaturated eight‐membered ring. A theoretical calculation predicts that the 10‐position of 1 should have the highest electron density. In agreement with this prediction, the reactions of 1 with electrophiles occur predominantly at the 10‐position. The corresponding ferrocene, two fulvenes, two diazo derivatives, and a COT‐fused azulene were obtained by the reactions of 1 with appropriate electrophiles.     

A convenient synthesis of (η-C8H8)UI2(THF)2 (THF = tetrahydrofuran), a precursor to monocyclooctatetraenyl uranium complexes

(COT)₂U (COT = η-C₈H₈) reacts in tetrahydrofuran (THF) with I₂ to give the monocyclooctatetraenyl compound (COT)UI₂(THF)₂ (I) which is transformed into (COT)UI₂(HMPA)₂ (II) upon addition of 2 equiv. of hexamethylphosphoramide. Treatment of I with Kacac (acac = MeCOCHCOMe), KC₅Me₅ and LiCH₂SiMe₃ give (COT)U(acac)₂ (III), (COT)(C₅Me₅)UI (IV) and [(COT)U(CH₂SiMe₃)₃]-[Li(THF)₃] (V), respectively.     

The dinuclear scandium(III) cyclooctatetraenyl chloride complex di‐μ‐chlorido‐bis[(η8‐cyclooctatetraene)(tetrahydrofuran‐κO)scandium(III)]

Only a few cyclooctatetraene dianion (COT) π‐complexes of lanthanides have been crystallographically characterized. This first single‐crystal X‐ray diffraction characterization of a scandium(III) COT chloride complex, namely di‐μ‐chlorido‐bis[(η⁸‐cyclooctatetraene)(tetrahydrofuran‐κO )scandium(III)], [Sc₂(C₈H₈)₂Cl₂(C₄H₈O)₂] or [Sc(COT)Cl(THF)]₂ (THF is tetrahydrofuran), (1), reveals a dimeric molecular structure with symmetric chloride bridges [average Sc—Cl = 2.5972 (7) Å] and a η⁸‐bound COT ligand. The COT ring is planar, with an average C—C bond length of 1.399 (3) Å. The Sc—C bond lengths range from 2.417 (2) to 2.438 (2) Å [average 2.427 (2) Å]. Direct comparison of (1) with the known lanthanide (Ln) analogues (La, Ce, Pr, Nd, and Sm) illustrates the effect of metal‐ion (M ) size on molecular structure. Overall, the M —Cl, M —O, and M —C bond lengths in (1) are the shortest in the series. In addition, only one THF molecule completes the coordination environment of the small Sc^III ion, in contrast to the previously reported dinuclear Ln–COT–Cl complexes, which all have two bound THF molecules per metal atom.     

Cyclooctatetraenyl-komplexe der frühen Übergangsmetalle und lanthanoide: V. Synthese und struktur von monocyclooctatetraenyl-komplexen des dreiwertigen Titans

[(COT)Ti(μ-Cl)(THF)]₂ (1) reacts with K[HBpz₃] or K[HB(3,5-Me₂pz)₃] to give the new monocyclooctatetraenyl half-sandwich complexes (COT)Ti[HBpz₃] (2) and (COT)Ti[HB(3,5-Me₂pz)₃] (3) respectively, as dark green, air-sensitive solids (COT  η⁸-cyclooctatetraenyl(2-)). The molecular structure of 2 has been determined by an X-ray diffraction study. The monomeric organotitanium(III) complexes (COT)Ti[PhC(NSiMe₃) ₂](THF) (4), (COT)Ti[MeOC₆H₄C(NSiMe₃)₂](THF) (5) and (COT)Ti[Ph₂P (NSiMe₃)₂] (6) have been prepared by treatment of [(COT)Ti(μ-Cl)(THF)]₂ (1) with the corresponding heteroallylic ligands.     

Pentamethylcyclopentadienyl-übergangsmetall-komplexe: X. Halbsandwichkomplexe des Fe und Ni als reaktive zwischenprodukte

Reaction of NiX₂·DME (X = Cl, Br; DME = 1,2-Dimethoxyethane) with Cp′Li (Cp′ = η⁵-C₅Me₅) in THF at ?10°C yields as intermediates dimeric halogeno complexes [Cp′NiX]₂ (I) as shown by mass spectroscopy. 1 reacts with neutral and anionic donor ligands viz. PPh₃ to Cp′Ni(PPh₃)X, 1,5-COD to [Cp′NiCOD]⁺, CpNa to CpCp′Ni and with COTLi₂ to (Cp′Ni)₂COT (COT = cyclooctatetraene). Analogously the reaction product from FeBr₂·DME and Cp′Li at ?80°C in THF is converted by CpNa to CpCp′Fe and by CO to Cp′Fe(CO)₂Br.     

DFT study of the effect of sigma-ligands on the structure of ester enolates in THF, as models of the active center in the anionic polymerization of methyl methacrylate.

A Density Functional Theory (DFT) study was carried out on structures of the lithium ester enolate of methyl isobutyrate (MIB-Li) in THF solution, in the presence of TMEDA, dimethoxyethane (DME), crown ether 12-crown-4, and cryptand-2,1,1, as electron donor ligands (sigma-ligands). Both specific solvation with THF and/or ligand molecules and nonspecific solvation by the solvent continuum were taken into account. The possibility of ligand-separated ion pair formation was analyzed for each of the ligands, including THF alone. In most cases peripherally solvated dimers are the most stable species. Only in the presence of cryptand-2,1,1 was a ligand-separated triple ion pair, (MIB-Li-MIB)(-)(THF)(2),Li(2,1,1)(1)(+), shown to be comparable in stability to the THF-solvated dimer, (MIB-Li)(2)(THF)(4). These results are in agreement with experimental NMR data on the structure of MIB-Li in the presence of DME, 12-crown-4, and cryptand-2,1,1. An upfield shift of the (13)C NMR signal of the alpha-carbon of MIB-Li observed in the presence of cryptand-2,1,1, originally attributed to a ligand-separated monomer, MIB(-),Li(2,1,1)(+), was well reproduced by Hartree--Fock calculated NMR shifts for the predicted ligand-separated triple ion pair.     

Synthese und Charakterisierung von Fluorenylgallaten und Fluorenylindaten

Synthesis and Characterization of Fluorenyl Gallates and Fluorenyl Indates GaCl₃ reacts with Fluorenyllithium (LiFl) in the ratio 1:4 in Et₂O to [Li(THF)₄][GaFl₄] ( 1 ). The addition of DME (1,2-dimethoxyethane) to solutions of 1 in THF leads to [Li(DME)₃][GaFl₄] ( 2 ) under replacement of THF molecules by DME molecules in the coordination sphere of the Li⁺ ions. Treatment of InCl with LiFl in Et₂O and recrystallization from THF gives [Li(THF)₄][ClInFl₃] ( 3 ), which is formed by an disproportionation reaction. 3 can also be obtained by the reaction of InCl with FlZnCl/LiCl in Et₂O and recrystallization from THF. 1 and 2 crystallize from THF and THF/DME as [Li(THF)₄][GaFl₄] · THF ( 1 · THF) and [Li(DME)₃][GaFl₄] · THF ( 2 · THF), respectively. Crystalline 3 is isolated from the reaction of InCl and FlZnCl/LiCl, while the reaction mixture of InCl and LiFl gives after recrystallization in THF 3 · 1,5 THF. The gallate ions in 1 and 2 differ mainly in the position of the fluorenyl ligands. The unit cells of 3 and 3 · 1,5 THF contain two crystallographic unique ion pairs of [Li(THF)₄][ClInFl₃].     

Density functional theory study of redox pairs: 2. Influence of solvation and ion-pair formation on the redox behavior of cyclooctatetraene and nitrobenzene

A study of the electrochemical behavior of cyclooctatetraene (COT) and nitrobenzene with Density Functional Theory and the conductor like solvation model (COSMO) is reported. The two-electron reduction of the tub-shaped COT molecule is accompanied by a structural change to a planar structure of D(4)(h)() symmetry in the first electron addition step, and to a fully aromatic structure of D(8)(h)() symmetry in the second electron addition step. Theoretical models are examined that are aimed at understanding the electrolyte- and solvent-dependent redox behavior of COT, in which a single 2e(-) redox wave is observed with KI electrolyte in liquid ammonia solution (DeltaDeltaE(disp) = [E(-2) - E(-1)] - [E(-1) - E(0)] < 0, inverted potential), while two 1e(-) redox waves are observed (DeltaDeltaE(disp) > 0) with NR(4)(+)X(-) (R = butyl, propyl; X(-) = perchlorate) electrolyte in dimethylformamide solution. In all cases, the computed reaction energy profiles are in fair agreement with the experimental reduction potentials. A chemically intuitive theoretical square scheme method of energy partitioning is introduced to analyze in detail the effects of structural changes and ion-pair formation on the relative energies of the redox species. The structural relaxation energy for conversion of tub-COT to planar-COT is mainly apportioned to the first reduction step, and is therefore a positive contribution to DeltaDeltaE(disp). The effect of the structural change on the disproportionation energy for COT is counteracted by the substantially more positive reduction potential for planar-(COT)(-1) in comparison to tub-(COT)(-1). Ion pairing of alkali metal counterions with the anionic reduction products gives rise to a negative contribution to DeltaDeltaE(disp) because the second ion-pairing step is more exothermic than the first, and the reduction of [KA] (A = COT, NB) is more exothermic than the reduction of A(-1). For COT, this negative energy differential term as a result of ion pairing predicts the experimentally observed inversion in the two 1e(-) potentials (DeltaDeltaE(disp) < 0). Nitrobenzene is treated with the same computational protocol to provide a system for comparison that is not complicated by the major structural change that influences the COT energy profile.     

7Li, 31P, and 1H pulsed gradient spin-echo (PGSE) diffusion NMR spectroscopy and ion pairing: on the temperature dependence of the ion pairing in Li(CPh3), fluorenyllithium, and Li[N(SiMe3)2] amongst other salts

7Li, 31P, and 1H variable-temperature pulsed gradient spin-echo (PGSE) diffusion methods have been used to study ion pairing and aggregation states for a range of lithium salts such as lithium halides, lithium carbanions, and a lithium amide in THF solutions. For trityllithium (2) and fluorenyllithium (9), it is shown that ion pairing is favored at 299 K but the ions are well separated at 155 K. For 2-lithio-1,3-dithiane (13) and lithium hexamethyldisilazane (LiHMDS 16), low-temperature data show that the ions remain together. For the dithio anion 13, a mononuclear species has been established, whereas for the lithium amide 16, the PGSE results allow two different aggregation states to be readily recognized. For the lithium halides LiX (X = Br, Cl, I) in THF, the 7Li PGSE data show that all three salts can be described as well-separated ions at ambient temperature. The solid state structure of trityllithium (2) is described and reveals a solvent-separated ion pair formed by a [Li(thf)4]+ ion and a bare triphenylmethide anion.     

Cyclooctatetraenyl-komplexe der frühen übergangsmetalle und lanthanoide: III. Cyclooctatetraenyl-lanthanoidtriflate und -iodide: Neue ausgangsmaterialien für die organolanthanoid-chemie

The dimeric complexes [(COT)Ln(μ-O₃SCF₃)(THF)₂]₂ (Ln  Ce (1), Pr (2), Nd (3), Sm (4)) are easily prepared by treatment of anhydrous lanthanide(III) triflates with equimolar amounts of K₂COT (COT = η⁸-cyclooctatetraenyl(2−)). The reaction of lanthanide triiodides with K₂COT affords monomeric complexes of the type (COT)Ln(I)(THF)₃ (Ln  Nd (5), Sm (6)). Due to their increased solubility in polar organic solvents these new precursors offer preparative advantages over the previously used chloro derivatives [(COT)Ln(μ-Cl)(THF)₂]₂. The molecular structures of 3 and 5 have been determined by X-ray diffraction.     

Organogermanium(II) Hydrides as a Source of Highly Soluble LiH

The reactions of monomeric C,N-chelated organogermanium(II) hydride L(H)Ge ⋅ BH₃ with organolithium salts RLi yielded lithium hydrogermanatoborates (Li(THF)₂{BH₃[L(H)GeR]})₂. Compound (Li(THF)₂{BH₃[L(H)GePh]})₂ was used as a source of LiH for the reduction of organic C=O or C=N bonds in nonpolar solvents accompanied by the elimination of a neutral complex L(Ph)Ge ⋅ BH₃. The interaction of (Li(THF)₂{BH₃[L(H)GePh]})₂ with the polar C=O bond was further investigated by computational studies revealing a plausible geometry of a pre-reactive intermediate. The experimental and theoretical studies suggest that, although the Li atom of (Li(THF)₂{BH₃[L(H)GePh]})₂ coordinates the C=O bond, the GeH fragment is the active species in the reduction reaction. Finally, benzaldehyde was reduced by a mixture of L(H)Ge ⋅ BH₃ with PhLi in nonpolar solvents.     

A Structurally Characterized Organometallic Plutonium(IV) Complex

The blood‐red plutonocene complex Pu(1,3‐COT′′)(1,4‐COT′′) ( 4 ; COT′′=η⁸‐bis(trimethylsilyl)cyclooctatetraenyl) has been synthesized by oxidation of the anionic sandwich complex Li[Pu(1,4‐COT′′)₂] ( 3 ) with anhydrous cobalt(II) chloride. The first crystal structure determination of an organoplutonium(IV) complex revealed an asymmetric sandwich structure for 4 where one COT′′ ring is 1,3‐substituted while the other retains the original 1,4‐substitution pattern. The electronic structure of 4 has been elucidated by a computational study, revealing a probable cause for the unexpected silyl group migration.     

Ion equilibria in THF solutions of models of “living” methacrylonitrile oligomers with Li+ counterion

The electro-chemical characteristics of isobutyronitrile lithium (P₁Li) and 2-lithio-4-cyano-2,4-dimethylpentanenitrile (P₂Li) are studied. The dissociation constants K of P₁Li and P₂Li are of the order of 10^?11M and show that in THF mainly contact ion pairs exist. The lower value of K for P²Li is an indication for intramolecular complexation between the counterion and the cyano group, next to the active center. Both compounds tend to form ion triples. The results obtained show that in THF the nitrile group of the second monomer unit participates both in intra- and intermolecular interactions with the counterion.     

NMR and DFT Studies with a Doubly Labelled 15N/6Li S-Trifluoromethyl Sulfoximine Reveal Why a Directed ortho-Lithiation Requires an Excess of n-BuLi

This work shows why it is imperious to use an excess of butyllithium for a directed ortho-lithiation of a trifluoromethyl sulfoximine. The analysis of mixtures of n-BuLi and sulfoximine 1 in THF-d₈ using {¹H, ⁶Li, ¹³C, ¹⁵N, ¹⁹F} NMR experiments at low temperatures reveal that a first deprotonation occurs that leads to dimeric and tetrameric N-lithiated sulfoximine (93 : 7). Using an excess n-BuLi (5 equivalents), the second deprotonation on the ortho-position of the aromatic occurs. Six species were observed and characterized on the way. It includes three aggregates involving a sulfoximine: i) a [dilithiated sulfoximine/(n-BuLi)] dimer solvated by four molecules of THF ( Agg2 , 39 %); ii) a [dilithiated sulfoximine/(n-BuLi)₃] tetramer solvated by six molecules of THF ( Agg3 , 39 %); iii) a [dilithiated sulfoximine/(n-BuOLi)₃] tetramer solvated by four molecules of THF ( Agg1 , 22 %). A DFT study afforded optimized solvated structures for all these aggregates, fully consistent with the NMR data.     

Intramolecular chelation as a factor in the stereochemistry of anionic vinyl polymerization

The methylation (CH₃I) of 1,3-bis(2-pyridyl)butyllithium in THF at −78°C is highly meso-selective (>98%) but the selectivity decreases with increasing cation-size or -coordination. The reaction of 1,3-bis(2-pyridyl)butyllithium with other electrophiles such as i-C₃H₇Br, PhCH₂Cl, Me₃SiCl (CD₃)₂CO and 4-vinylpyridine is also stereoselective under these conditions giving meso-like products. On the other hand, addition of 2-vinylpyridine is only slightly selective (64%) and this is consistent with the 65% meso content of P2VP formed by polymerization in the presence of Li ion. The chemistry of the above reactions is rationalized by intramolecular coordination of Li or other cations by the penultimate 2-pyridyl group and this is supported by equilibria involving proton abstraction of 1,3-bis(2-pyridyl)butane and similar compounds by bases having Li, Na or K counterions. It is shown that the tendency for intramolecular chelation is highest for Li and lowest for K ion. Temperature dependence of the above equilibrium shows that intramolecular chelation of Li and Na ions is exothermic (−1.4 and −1.3 kcal respectively) whereas the AH for K ion is very small (−0.5 kcal). Entropies of chelation are slightly positive for Li (0.8 e.u.) and negative for Na and K ions (-2.60 and −0.40 e.u. respectively). The lack of stereoregular polymerization of 2-VP in the presence of Li ion is most likely due to the requirement that the Li ion of the newly formed 2-pyridyl anion is coordinated with the 2-pyridyl group of the previous asymmetric center. Thus it would appear that intramolecular coordination of metal ion by penultimate 2-pyridine does not necessarily lead to isotactic-polymerization.     

Vicinal 2H/1H Isotope Shifts for 6Li-NMR and the Aggregation Behavior of Phenyllithium

Vicinal deuterium-induced ⁶Li-NMR isotope shifts have been found for (D₅)phenyllithium. These shifts are to low field, and their magnitude varies from 7 to 19 ppb with different solvents and donor molecules. On the basis of the isotopic fingerprints observed in the ⁶Li-NMR spectra of C₆H₅⁶Li/C₆D₅⁶Li mixtures, the aggregation behaviour of phenyllithium under different solvent conditions and in the presence of amines as donor ligands (TMEDA, PMDTA) was studied. Tetramers and dimers are observed in Et₂O, dimers as well as momomers in Et₂O/TMEDA and in THF, and momomers in THF/PMDTA. The hitherto unidentified species in Et₂O/TMEDA was shown to be a monomer.     

Isolation and Characterization,Including by X‐ray Crystallography,of Contact and Solvent‐Separated Ion Pairs of Silenyl Lithium Species

Reaction of bromoacylsilane 1 (pink solution) with tBu₂MeSiLi (3.5 equiv) in a 4:1 hexane:THF solvent mixture at ?78 °C to room temperature yields the solvent separated ion pair (SSIP) of silenyl lithium E‐[(tBuMe₂Si)(tBu₂MeSi)C=Si(SiMetBu₂)]^? [Li?4THF]⁺ 2 a (green–blue solution). Removal of the solvent and addition of benzene converts 2 a into the corresponding contact ion pair (CIP) 2 b (violet–red solution) with two THF molecules bonded to the lithium atom. The 2 a ? 2 b interconversion is reversible upon THF? benzene solvent change. Both 2 a and 2 b were characterized by X‐ray crystallography, NMR and UV/Vis spectroscopy, and theoretical calculations. The degree of dissociation of the Si?Li bond has a large effect on the visible spectrum (and thus color) and on the silenylic ²⁹Si NMR chemical shift, but a small effect on the molecular structure. This is the first report of the X‐ray molecular structure of both the SSIP and the CIP of any R₂E=E′RM species (E=C, Si; E′=C, Si; M=metal).     

Isolation and Characterization,Including by X‐ray Crystallography,of Contact and Solvent‐Separated Ion Pairs of Silenyl Lithium Species

Reaction of bromoacylsilane 1 (pink solution) with tBu₂MeSiLi (3.5 equiv) in a 4:1 hexane:THF solvent mixture at −78 °C to room temperature yields the solvent separated ion pair (SSIP) of silenyl lithium E‐[(tBuMe₂Si)(tBu₂MeSi)C=Si(SiMetBu₂)]⁻ [Li⋅4THF]⁺ 2 a (green–blue solution). Removal of the solvent and addition of benzene converts 2 a into the corresponding contact ion pair (CIP) 2 b (violet–red solution) with two THF molecules bonded to the lithium atom. The 2 a ⇌ 2 b interconversion is reversible upon THF⇌ benzene solvent change. Both 2 a and 2 b were characterized by X‐ray crystallography, NMR and UV/Vis spectroscopy, and theoretical calculations. The degree of dissociation of the Si−Li bond has a large effect on the visible spectrum (and thus color) and on the silenylic ²⁹Si NMR chemical shift, but a small effect on the molecular structure. This is the first report of the X‐ray molecular structure of both the SSIP and the CIP of any R₂E=E′RM species (E=C, Si; E′=C, Si; M=metal).     

A 7LI NMR spectral investigation of group III and IV metalates in ether solvents

The ⁷Li chemical shifts of Et₂O, THF and DME solutions of the metalates LiBMe₄, LiAlMe₄, LiGaMe₄, and LiTlMe₄, are reported. These data are correlated with values from the literature. The observed changes in ⁷Li chemical shift are discussed in terms of solvation of the lithium ion and ion pair formation in solution. The ⁷Li chemical shift of LiSnMe₃ in THF is also reported and a brief discussion of the ⁷Li chemical shifts of LiMPh₃ (M = C, Si, Ge, Sn, Pb) is presented.