Iron Acyl Research Articles

Hydroxylation of Unactivated C(sp3)-H Bonds with m-Chloroperbenzoic Acid Catalyzed by an Iron(III) Complex Supported by a Trianionic Planar Tetradentate Ligand.

Hydroxylation of cyclohexane with m-chloroperbenzoic acid was examined in the presence of an iron(III) complex supported by a trianionic planar tetradentate ligand. The present reaction system shows a high turnover number of 2750 with a high product selectivity of alcohol (93%). The turnover frequency was 0.51 s-1, and the second-order rate constant (k) for the C-H bond activation of cyclohexane was 1.08 M-1 s-1, which is one of the highest values among the iron complexes in the oxidation of cyclohexane so far reported. The present catalytic system can be adapted to the hydroxylation of substrates having only primary C-H bonds such as 2,2,3,3-tetramethylbutane as well as gaseous alkanes such as butane, propane, and ethane. The involvement of an iron(III) acyl peroxido complex as the reactive species was suggested by spectroscopic measurements of the reaction solution.

Iron Hydride Detection and Intramolecular Hydride Transfer in a Synthetic Model of Mono-Iron Hydrogenase with a CNS Chelate.

We report the identification and reactivity of an iron hydride species in a synthetic model complex of monoiron hydrogenase. The hydride complex is derived from a phosphine-free CNS chelate that includes a Fe-C(NH)(═O) bond (carbamoyl) as a mimic of the active site iron acyl. The reaction of [((O═)C(HN)N(py)S(Me))Fe(CO)2(Br)] (1) with NaHBEt3 generates the iron hydride intermediate [((O═)C(HN)N(py)S(Me))Fe(H)(CO)2] (2; δFe-H = -5.08 ppm). Above -40 °C, the hydride species extrudes CH3S(-) via intramolecular hydride transfer, which is stoichiometrically trapped in the structurally characterized dimer μ2-(CH3S)2-[((O═)C(HN)N(Ph))Fe(CO)2]2 (3). Alternately, when activated by base ((t)BuOK), 1 undergoes desulfurization to form a cyclometalated species, [((O═)C(NH)NC(Ph))Fe(CO)2] (5); derivatization of 5 with PPh3 affords the structurally characterized species [((O═)C(NH)NC)Fe(CO)(PPh3)2] (6), indicating complex 6 as the common intermediate along each pathway of desulfurization.

Synthesis and reactivity of (pyrazol-1-yl)acyl iron complexes

Treatment of 1-methyl-3,5-dialkylpyrazoles with n-BuLi, and subsequently with iron carbonyl and iodine yielded (pyrazol-1-yl)acyl iron complexes CH2(CO) (3,5-R2Pz)Fe(CO)3I (R = Me or Pri, Pz = pyrazol-1-yl). Reaction of CH2(CO) (3,5-Me2Pz)Fe(CO)3I with PhSNa gave a dimeric complex [CH2(CO) (3,5-Me2Pz)Fe(CO)2(SPh)]2, while similar reactions of CH2(CO) (3,5-R2Pz)Fe(CO)3I with PySNa (Py = 2-pyridyl) gave mononuclear complexes CH2(CO) (3,5-R2Pz)Fe(CO)2(SPy), which exhibited an isomerization in solution. Treatment of the dimeric complex with PPh3 at room temperature resulted in the decomposition of the starting material. Furthermore, this dimeric complex readily underwent reductive elimination to generate CH2(COSPh) (3,5-Me2Pz) when heated at relatively low temperature, and thermal decomposition reaction to give PhSSPh in refluxing toluene solution. Reaction of mononuclear complexes with PPh3 caused one carbonyl to be replaced by phosphine ligand to give complexes CH2(CO) (3,5-R2Pz)Fe(CO) (PPh3) (SPy). All these acyl iron complexes were fully characterized by IR and NMR spectroscopy, and their structures were unambiguously determined by X-ray crystallography.

Theoretical study of iron acyl complexes modeling the active site of [Fe]-hydrogenase: Solvation effects play a significant role

In this work, density functional theory (DFT) calculations were carried out to study the role of solvation effects on the reaction of diiron dithiolate complex with CO to form [Fe]-hydrogenase model complex. In the gas phase, the energy barrier of the first transition state TS1 species is ca. 6.1kcal/mol higher than the second transition state TS2 species. However, when the solvation effects were included, the energy order was reversed, i.e., the energy barrier of TS1 falls ca. 1.2kcal/mol lower than TS2, indicating that the insertion of the second CO to iron is the rate-determining step in the whole transformation process. The initial insertion of the CO plays an important role in increasing the reaction barrier of the binding of a second CO, which prevented the second step transformation. Thus, the solvation effects play a significant role in determining the reaction mechanism. In addition, the energy of PC species is lower than RC species, demonstrating that this transformation is a significantly exothermic process.

Diastereoselective conjugate additions to alkoxycarbene cations of the iron chiral auxiliary [(η5-C5H5)Fe(CO)(PPh3)[dbnd]C(OMe)CH[dbnd]CHR

The conjugate additions of alkyllithium reagents to alkoxycarbene cations of the iron chiral auxiliary [(η5-C5H5)Fe(CO)(PPh3)C(OMe)CHCHR]+ proceeded with moderate to excellent diastereoselectivity. The resultant methoxyvinyl complexes were transformed into the corresponding iron acyl complexes following a sequence of protonation and demethylation. The overall transformation yields iron acyl complexes of opposite relative configuration to those which result from the conjugate addition of the same nucleophile to the corresponding α,β-unsaturated iron acyl complex.

Synthesis of acyl iron complexes based on bis(heteroaryl)methanes

Treatment of the lithium salts, derived from the lithiation of bis(pyrazol-1-yl)methanes, bis(1-methylimidazol-2-yl)methane [CH2(mim)2] and 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine [CH2(3,5-Me2Pz)(Py)] using n-BuLi at low temperature, with iron carbonyl and iodine resulted in significantly different acyl iron derivatives. Reaction of bis(pyrazol-1-yl)methanes with n-BuLi, and subsequently with iron carbonyl and iodine yielded bis(pyrazol-1-yl)acyl iron complexes, while similar reaction of CH2(mim)2 gave valeryl iron complex CH2(mim)2Fe(CO)2(COBu-n)I, which was also obtained by the sequential treatment of iron carbonyl with n-BuLi, CH2(mim)2 and iodine. Additionally, the consecutive reaction of CH2(3,5-Me2Pz)(Py) with n-BuLi, iron carbonyl and iodine yielded (3,5-dimethylpyrazol-1-yl)(2-pyridyl)acyl iron complex. The corresponding valeryl iron complex CH2(3,5-Me2Pz)(Py)Fe(CO)2(COBu-n)I was obtained when iron carbonyl was reacted with n-BuLi, and subsequently with CH2(3,5-Me2Pz)(Py) and iodine. However, analogous valeryl iron complexes supported by bis(pyrazol-1-yl)methanes could not be formed by these methods. The different electron-donating ability of these bis(heteroaryl)methanes and the distinct nucleophilicity of the bis(heteroaryl)methyl carbanions were possibly responsible for the diversification of reactivities. All these acyl iron complexes were fully characterized by IR and NMR spectroscopy, and their structures were unambiguously determined by X-ray crystallography.

ChemInform Abstract: [Fe]‐Hydrogenase and Models That Contain Iron—Acyl Ligation

AbstractReview: 39 refs.

Fe]‐Hydrogenase and Models that Contain IronAcyl Ligation

[Fe]-hydrogenase is a newly characterized type of hydrogenase. This enzyme heterolytically splits hydrogen in the presence of a natural substrate. The active site of the enzyme contains a mono-iron complex with intriguing iron-acyl ligation. Several groups have recently developed iron-acyl complexes as synthetic models of [Fe]-hydrogenase. This Focus Review summarizes the studies of this enzyme and its model compounds, with an emphasis on our own research in this area.

Reactions of Iron Acyl Chalcogenide Clusters with Electrophiles: Reactivity Comparison and Theoretical Calculations

AbstractThe reactions of two types of EFe3‐based acyl clusters [EFe3(CO)9C(O)C(H)CCH2)]− (E = S, 1; Se, 2; Te, 3) with different electrophiles have been systematically studied and compared. When the sulfur‐substituted allylcarbonyl Fe3‐cluster [Fe3(CO)9{μ3‐η2:η1:η3‐C(O)CHC(S)CH2}]− (1) was treated with CH3COCl in CH2Cl2, the Fischer‐type acyloxy carbene cluster [Fe3(CO)9{μ3‐η2:η4:η1‐C(OCOCH3)‐C(H)C(S)CH2}] (4) was obtained, along with the previously reported SC(O)C(CH3)=CH bridged Fe2‐complex [Fe2(CO)6{μ‐η2:η3‐SC(O)C(CH3)CH}] (5) and [HSFe3(CO)9]−. Similar acetylation of the allylcarbonyl SeFe3‐cluster [(μ3‐Se)Fe3(CO)9(μ3‐η1:η1:η3‐C(O)C(H)CCH2)]− (2) produced the Fe2‐carbene complex [Fe2(CO)6{μ‐η2:η3‐C(OCOCH3)C(H)C(Se)CH2}] (6) and the propyne‐bridged SeFe3‐complex [(μ3‐Se)Fe3(CO)9{μ3‐η1:η2:η1‐C(CH3)=C(H)}] (7) as well as [HSeFe3(CO)9]− and [Se2Fe3(CO)9]. However, when the analogous TeFe3‐based allylcarbonyl cluster [(μ3‐Te)Fe3(CO)9(μ3‐η1:η1:η3‐C(O)C(H)CCH2)]− (3) reacted with CH3COCl, only the fragmentation product [Te2Fe3(CO)9] was formed. Further acidification of cluster 1 with HBF4 proceeded to form complex 5 and [HSFe3(CO)9]−. Similar acidification of 2 formed the propyne‐capped complex 7, however, the protonation of 3 led to the formation of [Te2Fe3(CO)9]. Contrast to the reported reactions of 1 and 3, the reaction of 2 with [Cu(CH3CN)4][BF4] formed complex 7 and a new C(O)CH=C(Se)(CH3) bridged Fe2‐complex [Fe2(CO)6{μ‐η2:η3‐C(O)CHC(Se)(CH3)}] (8). The nature and formation of new carbene complexes 4 and 6 were further elucidated with the aid of molecular orbital calculations at the B3PW91 level of density functional theory.

Carbonylation of alkyl halides with [Fe(CO)3(NO)]−: in silico identification of a common intermediate

The mechanism of carbonylation of alkyl halides using [Fe(CO)3(NO)](-) has been studied using density functional theory (DFT). Our results suggest an SN2 mechanism for the alkylation event followed by a well-defined oxidative addition and alkyl migration. An experimentally elusive common intermediate [Fe(CO)2NO(Ac)] has been identified in two isomers and reacted in silico with a number of ligands (CO, PH3 and PPh3) to give the corresponding iron acyl complexes. Pathways between the two isomers, including direct involvement of a solvent molecule (THF) or iodide, have been elucidated.

Evidence for acyl–iron ligation in the active site of [Fe]-hydrogenase provided by mass spectrometry and infrared spectroscopy

[Fe]-hydrogenase catalyzes the reversible heterolytic cleavage of H(2) and stereo-specific hydride transfer to the substrate methenyltetrahydromethanopterin in methanogenic archaea. This enzyme contains a unique iron guanylylpyridinol (FeGP) cofactor as a prosthetic group. It has recently been proposed-on the basis of crystal structural analyses of the [Fe]-hydrogenase holoenzyme-that the FeGP cofactor contains an acyl-iron ligation, the first one reported in a biological system. We report here that the cofactor can be reversibly extracted with acids; its exact mass has been determined by electrospray ionization Fourier transform ion cyclotron resonance mass-spectrometry. The measured mass of the intact cofactor and its gas-phase fragments are consistent with the proposed structure. The mass of the light decomposition products of the cofactor support the presence of acyl-iron ligation. Attenuated total reflection infrared spectroscopy of the FeGP cofactor revealed a band near wave number 1700 cm(-1), which was assigned to the C=O (double bond) stretching mode of the acyl-iron ligand.

The Hieber Anion [Fe(CO)3(NO)

(A) Allylic Substitutions with C-Nucleophiles: Building on early work by Roustan et al. and Xu et al. an efficient allylic substitution using carbon-centered nucleophiles was developed. In a first account the beneficial effects of Ph3P were shown. ipso Substitution was observed and a σ-allyl-iron intermediate was suggested.4a A second-generation protocol allowed for the selective formation of either linear or branched products based on the choice of ligand. The t-Bu-substituted NHC ligand gave access to branched selectively, whereas the aryl-substituted NHC ligand gave access to the linear product preferentially. (B) Allylic Substitutions with N-Nucleophiles: The allylic amination of allyl carbonates could be carried out by using catalytic amounts of Bu4N[Fe(CO)3(NO)]. The addition of piperidine HCl proved to be crucial for the reaction to take place when less substituted carbonates were used. ipso Substitution and retention of the stereochemistry was observed in all cases. (C) Allylic Substitutions with S-Nucleophiles: The allylic sulfonation of allyl carbonates was reported using catalytic amounts of Bu4N[Fe(CO)3(NO)] and sodium sulfinates as nucleophiles. Consistent with previous observations ipso substitution and retention of the stereochemistry was observed. Recently an allylic sulfenylation has been reported. (D) Allylic Substitutions with O-Nucleophiles: The O-allylation of phenols was reported using allyl carbonates, which upon treatment with catalytic amounts of Ph3P and Bu4N[Fe(CO)3(NO)] forms allyl phenyl ethers. Interestingly, this reaction was suggested to proceed via a π-allyl mechanism, based on the regiochemical outcome of this reaction. (E) C1-C16 Fragment of Ionomycin: Kocienski and co-workers employed Bu 4N[Fe(CO)3(NO)] in the preparation of the C1-C16 fragment of the polyketide natural product ionomycin. In a stoichiometric reaction of the enantiopure mesylate with the Hieber anion a configurationally stable iron allyl complex is formed as a 4:1 mixture of diastereoisomers. Separation via crystallization and subsequent reaction with an organocuprate gives rise to the desired advanced intermediate with retention of the stereochemistry based on the mesylate. (F) Carbonylation of Primary and Secondary Alkyl Halides: Davies et al. reported the carbonylation of alkyl halides with stoichiometric amounts of Bu4N[Fe(CO)3(NO)] in the presence of a carbon monoxide atmosphere. Initial alkylation takes place at the iron nucleophile followed by CO insertion to give an iron acyl species which upon substitution by methoxide gives the product.10 It has to be noted that potential β-hydride elimination does not take place. When MeOH was replaced by the hydride source NaHB(OMe)3 the corresponding alcohols were obtained. (G) Transesterification: The Bu4N[Fe(CO)3(NO)] catalyzed transesterification was recently reported. Alcohols were readily acylated using vinyl acetate in nhexane at elevated temperature. Further, molecular sieves had to be employed as the reaction exhibited a severe sensitivity to towards moisture. Phenyl esters or activated methyl esters also proved to be good acyl donors in this reaction. Mechanistically the reaction proceeds via a putative iron acyl intermediate. (H) Thioesterification: The transesterification was developed into a protocol for the preparation of thioesters from p-chlorophenyl esters in the presence of Bu4N[Fe(CO)3(NO)]. In the course of this study this reaction was applied to a native chemical ligation affording dipeptides via intermediate thioesters. (I) Hydrosilylation of Aldehydes and Ketones: Treatment of the appropriate carbonyl compounds with polymethylhydrosiloxane (PMHS), a hydride source, in the presence of as little as 1 mol% Bu4N[Fe(CO) 3(NO)] results in excellent yields of the alcohols after basic workup. Inclusion of the phosphine ligand PCy3 had a profound effect on the yield.

Iron Acyl Thiolato Carbonyls: Structural Models for the Active Site of the [Fe]-Hydrogenase (Hmd)

Phosphine-modified thioester derivatives are shown to serve as efficient precursors to phosphine-stabilized ferrous acyl thiolato carbonyls, which replicate key structural features of the active site of the hydrogenase Hmd. The reaction of Ph(2)PC(6)H(4)C(O)SPh and sources of Fe(0) generates both Fe(SPh)(Ph(2)PC(6)H(4)CO)(CO)(3) (1) and the diferrous diacyl Fe(2)(SPh)(2)(CO)(3)(Ph(2)PC(6)H(4)CO)(2), which carbonylates to give 1. For the extremely bulky arylthioester Ph(2)PC(6)H(4)C(O)SC(6)H(3)-2,6-(2,4,6-trimethylphenyl)(2), oxidative addition is arrested and the Fe(0) adduct of the phosphine is obtained. Complex 1 reacts with cyanide to give Et(4)N[Fe(SPh)(Ph(2)PC(6)H(4)CO)(CN)(CO)(2)] (Et(4)N[2]). (13)C and (31)P NMR spectra indicate that substitution is stereospecific and cis to P. The IR spectrum of [2](-) in ν(CN) and ν(CO) regions very closely matches that for Hmd(CN). XANES and EXAFS measurements also indicate close structural and electronic similarity of Et(4)N[2] to the active site of wild-type Hmd. Complex 1 also stereospecifically forms a derivative with TsCH(2)NC, but the adduct is more labile than Et(4)N[2]. Tricarbonyl 1 was found to reversibly protonate to give a thermally labile derivative, IR measurements of which indicate that the acyl and thiolate ligands are probably not protonated in Hmd.

ChemInform Abstract: Asymmetric Synthesis of β-Lactams and Pseudopeptides via Stereoselective Conjugate Additions of Lithium (α-Methylbenzyl)allylamide to α,β-Unsaturated Iron Acyl Complexes.

Lithium (α-methylbenzyl)allylamide 2 undergoes stereoselective conjugate additions to α,β-unsaturated iron acyl complexes 3a–c to afford β-amino iron acyl adducts 5a–c and 6a–c. These adducts may be deallylated smoothly using palladium(0) catalysis providing the corresponding homochiral secondary amines 7a–c which, upon oxidative decomplexation with bromine or N-bromosuccinimide, undergo direct β-lactam ring formation. The diastereoselectivity for the conjugate addition to 3a may be further improved by the use of magnesium amide 10. Oxidative decomplexation of one of the β-amino iron acyl adducts 5b in the presence of α-amino esters provides pseudopeptide fragments comprising an α-amino acid coupled to a β-amino acid.

Synthesis and Reactivity of Iron Acyl Complexes Modeling the Active Site of [Fe]-Hydrogenase

A [Fe(II)Fe(II)] dithiolate complex containing acyl and carbonyl ligands was synthesized. The diiron complex reacted with phosphine, cyanide, isocyanide, and CO to give monomeric Fe(II) complexes reproducing the first coordination sphere of the active site of [Fe]-hydrogenase (H(2)-forming methylene-tetrahydromethanopterin dehydrogenase, Hmd) in various states. All the acyl and carbonyl carbons in the diiron complex underwent facile isotopic exchange with (13)CO via monomeric Fe tricarbonyl intermediates.

Stereoselective conjugate addition reactions of lithium amides to α,β-unsaturated chiral iron acyl complexes [(η 5-C 5H 5)Fe(CO)(PPh 3)(COCH [dbnd]CHR)

Conjugate addition of achiral lithium dimethylamide to the chiral iron cinnamoyl complexes ( S, E)- and ( S, Z)-[(η 5-C 5H 5)Fe(CO)(PPh 3)(COCH CHPh)] proceeds with high diastereoselectivity, with this protocol being used to establish unambiguously the absolute configuration of Winterstein’s acid (3- N, N-dimethylamino-3-phenylpropanoic acid) as ( R). The highly diastereoselective conjugate addition of lithium N-benzyl- N-trimethylsilylamide to a range of α,β-unsaturated iron acyl complexes, followed by in-situ elaboration of the derived enolate by either alkylation or aldol reactions is also demonstrated, facilitating the stereoselective synthesis of both cis- and trans-β-lactams. This methodology has been used to effect the formal asymmetric syntheses of (±)-olivanic acid and (±)-thienamycin. Addition of chiral lithium amides derived from primary and secondary amines to the iron crotonyl complex [(η 5-C 5H 5)Fe(CO)(PPh 3)(COCH CHMe)] indicates that lithium N-α-methylbenzylamide shows low levels of enantiorecognition, while lithium N-3,4-dimethoxybenzyl- N-α-methylbenzylamide and lithium N-benzyl- N-α-methylbenzylamide show high levels of enantiodiscrimination. The high level of observed enantiorecognition was used to facilitate a kinetic resolution of ( RS)-[(η 5-C 5H 5)Fe(CO)(PPh 3)(COCH CHMe)] with homochiral lithium ( R)- N-3,4-dimethoxybenzyl- N-α-methylbenzylamide. Further mechanistic studies show that conjugate additions of ( RS)-lithium N-benzyl- N-α-methylbenzylamide to either the ( RS)- or homochiral iron crotonyl complex show 2:1 stoicheiometry, while homochiral lithium N-benzyl- N-α-methylbenzylamide shows 1:1 stoicheiometry.

Asymmetric synthesis of tetracyclic substructures of Strychnos indole alkaloids

The addition of the enolate of methyl 1-methyl-2-indoleacetate 1 and lithium 2-(lithiomethyl)indole-1-carboxylate 5 to pyridines and N-alkylpyridinium salts bearing a chiral auxiliary at the 3-position (tolylsulfinyl, acyl iron complexes, bornane-10,2-sultam), with subsequent acid cyclization of the resulting dihydropyridines, is investigated.

Metal complexes of anionic 3-borane-1-alkylimidazol-2-ylidene derivatives

Addition of BH 3·thf to 1-alkylimidazoles (alkyl=methyl, butyl) and 1-methylbenzimidazole leads to BH 3 adducts, which are deprotonated by BuLi to yield the organolithium compounds (L)Li +( 1b– d) −. In the solid state (thf)Li + 1b − is dimeric. The acyl–iron complexes (thf) 3Li +( 3b, d) − are formed from (thf)Li +( 1b, d) − and Fe(CO) 5. (L)Li +( 1a– c) − react with [CpFe(CO) 2X], however, the only complex obtained is [CpFe(CO) 2 1a] (5a). The analogous reaction of (L)Li + 1a − with the pentadienyl complex [(C 7H 11)Fe(CO) 2Br] yields the corresponding iron compound 6a. Their compositions follow from spectroscopic data. Treatment of Cp 2TiCl with (L)Li + 1a − leads to [Cp 2Ti 1a] ( 7a), which could not be oxidized with PbCl 2 to give the corresponding Ti(IV) complex. The compounds [Li(py) 4] + 9a − and [Li(L) 4] +( 10b– d) − are obtained when (L)Li + 1 − are reacted with VCl 3 and ScCl 3. The X-ray structure analysis of the vanadium complex reveals a distorted tetrahedron of the anion [V( 1a) 4] − with two smaller and four larger CVC angles. The scandium compound [Li(dme) 2 + 10c −] has a different structure: the distorted tetrahedron of the anion [Sc( 1c) 4] − contains two larger (140.2 and 142.9°) and four smaller CScC angles (93.9–98.7°). This arrangement allows the formation of four bridging BHSc 3c,2e bonds to give an eight-fold coordination. The anion 10c − is formally a 16e complex.

Asymmetric synthesis of β-lactams and pseudopeptides via stereoselective conjugate additions of lithium (α-methylbenzyl)allylamide to α,β-unsaturated iron acyl complexes

Lithium (α-methylbenzyl)allylamide 2 undergoes stereoselective conjugate additions to α,β-unsaturated iron acyl complexes 3a–c to afford β-amino iron acyl adducts 5a–c and 6a–c. These adducts may be deallylated smoothly using palladium(0) catalysis providing the corresponding homochiral secondary amines 7a–c which, upon oxidative decomplexation with bromine or N-bromosuccinimide, undergo direct β-lactam ring formation. The diastereoselectivity for the conjugate addition to 3a may be further improved by the use of magnesium amide 10. Oxidative decomplexation of one of the β-amino iron acyl adducts 5b in the presence of α-amino esters provides pseudopeptide fragments comprising an α-amino acid coupled to a β-amino acid.

Borane-Substituted Imidazol-2-ylidenes: Syntheses, Structures, and Reactivity⋆

Reactions of the Lewis acids BH3 and BEt3 with trimethylimidazole (1) lead to the borane adducts 2a and 2b. Deprotonation of 2a with n-butyllithium results in the formation of the novel N-borane-substituted imidazol-2-ylidene anion 3a– whereas deprotonated 2b rearranges unexpectedly to the anionic compound 3b–. This can be transformed into the carbene–borane adduct 4 by methylation. The reaction of 3a– with [Mn(CO)5Br] yields the carbene complex 5. Surprisingly, 3a– attacks Fe(CO)5 at a carbon atom which leads to the iron acyl complex anion 6–. The compositions of the products follow from spectroscopic and analytical data and from X-ray structure analyses for Li(thp)+3a–, Li(thf)2+3b– and Li(thp)3+6–.