The Bis(pentafluoroethyl)germylene Trimethylphosphane Adduct (C2F5)2Ge⋅PMe3: Characterization,Ligand Properties,and Reactivity

The synthesis of the germylene phosphane adduct (C₂F₅)₂Ge?PMe₃ is described. Starting from (C₂F₅)₃GeH in an excess of PMe₃, heating was applied, whereupon reductive elimination of C₂F₅H occurred. The molecular structure was ascertained by X‐ray diffraction and compared with information obtained by quantum chemical methods. The ligand properties were derived by studying the IR spectrum of the nickel(0) complex [Ni(CO)₃{Ge(C₂F₅)₂(PMe₃)}] in the CO region. (C₂F₅)₂Ge?PMe₃ turned out to be a π‐accepting ligand comparable to PMe₃, in terms of Tolman's electronic parameter. Furthermore a [2+4] cycloaddition reaction with 2,3‐dimethyl‐1,3‐butadiene, and σ‐bond insertion reactions were recorded. Activation of the C?Cl bond in dichloromethane gives rise to the formation of the phosphonium ylide complex [(C₂F₅)₂Cl₂Ge‐CH₂PMe₃], which was fully characterized by X‐ray diffraction.     

Fundamental Differences between Group 8 Metals: Unexpected Oxidation State Preferences and Mechanisms in Ruthenium Borylene Complex Formation

The reaction of the salts K[Ru(CO)₃(PMe₃)(SiR₃)] (R=Me, Et) with Br₂BDur or Cl₂BDur (Dur=2,3,5,6‐Me₄C₆H) leads to both boryl and borylene complexes of divalent ruthenium, the former through simple salt elimination and the latter through subsequent CO loss and 1,2‐halide shift. The balance of products can be altered by varying the reaction conditions; boryl complexes can be favored by the addition of CO, and borylene complexes by removal of CO under vacuum. All of these products are in competition with the corresponding (aryl)(halo)(trialkylsilyl)borane, a reductive elimination product. The Ru^II borylene products and the mechanisms that form them are distinctly different from the analogous reactions with iron, which lead to low‐valent borylene complexes, highlighting fundamental differences in oxidation state preferences between iron and ruthenium.     

ÜbergangsmetallCarbin-Komplexe: LXXXVI. Synthese mono-, bis-, und tris-Trimethylphosphan-substituierter Diethylaminocarbin-Komplexe des Wolframs

The reaction of trans-I(CO)₄WCNEt₂ (I) with a slight excess of PMe₃ results in the replacement of one carbonyl group to give mer-I(CO)₃(PMe₃)WCNEt₂ (II). Complex II reacts at room temperature with additional PMe₃ under CO replacement to give a mixture of cis- and trans-dicarbonyl-I(CO)₂(PMe₃)₂WCNEt₂ (III, IV). Complexes III and IV, which can be separated by column chromatography, isomerize slowly at room temperature, the thermodynamic equilibrium favouring the more stable trans complex IV. The cis isomer III can be obtained from I(CO)₂py₂WCNEt₂ (V) and PMe₃. Another CO ligand can be eliminated from III or IV by an excess of PMe₃ in boiling hexane and gives mer-I(CO)(PMe₃)₃WCNEt₂ (VI). Moreover complex VI can be prepared by oxidative decarbonylation from III or IV by iodine and subsequent reduction of the intermediate, an isolable, seven-coordinated carbyne complex formulated as (I)₃(CO)(PMe₃)₂WCNEt₂ (VII), by two equivalents of PMe₃.     

Heterobi‐ and ‐trinuclear Complexes with an Amino(ethynyl)carbene as Bridging Ligand

The sequential reaction of the amino(trimethylsilyl)carbene complex [(CO)₅W=C(NH₂)C≡CSiMe₃] ( 1 ) with nBuLi and [I‐Fe(CO)₂Cp] affords the C(carbene)‐N bridged heterobinuclear complex [(CO)₅W=C{NHFe(CO)₂Cp}C≡CSiMe₃] ( 2 ). Desilylation of 1 is achieved by treatment with KF in THF/MeOH. From the reaction of the resulting complex [(CO)₅W=C(NH₂)C≡CH] ( 3 ) with nBuLi and [I‐Fe(CO)₂Cp] two binuclear WFe compounds in a ratio of approximately 1:1 are obtained: the C(carbene)‐C≡C bridged complex 4 and the C(carbene)‐N bridged complex 5 . Repetition of the deprotonation/metallation sequence yields the trinuclear WFe₂ complex 6 . One Fe(CO)₂Cp fragment in 6 is bonded to the amino group and the other one to the terminal carbon atom of the ethynyl substituent. The analogous reaction of 3 with nBuLi and [Br‐Ni(PMe₂Ph)₂Mes] gives a ca. 1:1 mixture of two heterobinuclear complexes ( 7 and 8 ). Complex 7 is bridged by the C(carbene)‐C≡C and complex 8 by the C(carbene)‐N fragment. Subsequent reaction of 7 with BuLi and [Br‐Ni(PMe₂Ph)₂Mes] finally affords the trinuclear WNi₂ complex 9 related to 6 . The solid‐state structure of 2 is established by an X‐ray diffraction analysis. The spectroscopic data of the bi‐ and trinuclear complexes indicate electronic communication between the metal centers through the bridging group.     

Isocyanide insertion reaction in alkylcomplexes of iron: A dihaptoiminoacyl derivative of iron (II)

The dihaptoiminoacyl complex [Fe(CO)₂ (PMe₃)₂(η²CMeNCMe₃)]⁺ I^? was obtained by reaction of [Fe(CO)₂(PMe₃)₂ MeI] and tertbutylisocyanide. The structure of the complex was determined by an X-ray structure analysis.     

Coupling of Titanacyclopentadienes with a Cp Ligand and Elimination of One Substituent

Titanacyclopentadienes, prepared from [Cp₂TiBu₂] and either two equivalents of an alkyne or a diyne, were treated with PMe₃ (3 equiv) at 50 °C for 3 h and then with azobenzene at room temperature for 12 h to give 4,5,6‐trisubstituted indene derivatives with the loss of one substituent in good yields. This reaction contrasts sharply with our previously reported reaction for the formation of 4,5,6,7‐tetrasubstituted indene derivatives without the loss of substituents by the treatment of titanacyclopentadienes with azobenzene without PMe₃. ¹³C NMR spectroscopy of the product derived from a ¹³C‐enriched complex revealed that the five carbon atoms originating from a Cp ligand were arranged linearly in the trisubstituted indene derivatives, in contrast to the 4,5,6,7‐tetrasubsituted indene derivatives, in which the corresponding five carbon atoms are arranged in a ring.     

Effect of Cyanide Ligands on the Electronic Structure of [FeFe] Hydrogenase Active‐Site Model Complexes with an Azadithiolate Cofactor

A detailed characterization of a close synthetic model of the [2 Fe]_H subcluster in the [FeFe] hydrogenase active site is presented. It contains the full primary coordination sphere of the CO‐inhibited oxidized state of the enzyme including the CN^? ligands and the azadithiolate (adt) bridge, [((μ‐S? CH₂)₂NR)Fe₂(CO)₄(CN)₂]^2?, R=CH₂CH₂SCH₃. The electronic structure of the model complex in its Fe^IFe^II state was investigated by means of density functional theory (DFT) calculations and Fourier transform infrared (FTIR) spectroscopy. By using a combination of continuous‐wave (CW) electron paramagnetic resonance (EPR) and hyperfine sublevel correlation (HYSCORE) experiments as well as DFT calculations, it is shown that, for this complex, the spin density is delocalized over both iron atoms. Interestingly, we found that the nitrogen hyperfine coupling, which represents the interaction between the unpaired electron and the nitrogen at the dithiolate bridge, is slightly larger than that in the analogous complex in which the CN^? ligands are replaced with PMe₃ ligands. This reveals, first, that the CN^?/PMe₃ ligands coordinated to the iron core are electronically coupled to the amine in the adt bridge. Second, the CN^? ligands in this complex are somewhat stronger σ‐donor ligands than the PMe₃ ligand, and thereby enable more spin density to be transferred from the Fe core to the adt unit, which might in turn affect the reactivity of the bridging amine.     

Synthesis and Characterization of Iron,Cobalt, and Nickel [PNP] Pincer Amido Complexes by N–H Activation

The N–H bond activation product [PNP]‐Fe^I(PMe₃)₂ ( 2 ) was obtained at room temperature by the reaction of diphosphinito [PNP] pincer ligand ((Ph₂P(C₆H₄))₂NH ( 1 )) with Fe(PMe₃)₄. Treatment of 1 with Co(PMe₃)₄, CoCl(PMe₃)₃ and CoMe(PMe₃)₄ afforded the same N–H bond activation product [PNP]‐Co^I(PMe₃)₂ ( 3 ). In order to have a better understanding of the mechanism of formation of 3 , in situ IR and ¹H NMR spectroscopic investigations were conducted.The reaction of 1 with Ni(PMe₃)₄ afforded the ligand replacement complex 4 while a [PNP]‐Ni^IIMe complex 5 was obtained via deprotonation through the reaction of 1 with NiMe₂(PMe₃)₃. The molecular structures of 2 – 4 were confirmed by X‐ray diffraction analysis.     

Rhenium–Germanium Triple Bonds: Syntheses and Reactions of the Germylidyne Complexes mer‐[X2(PMe3)3ReGe?R] (X=Cl,I, H; R=m‐terphenyl)

A general approach to the first compounds that contain rhenium–germanium triple and double bonds is reported. Heating [ReCl(PMe₃)₅] ( 1 ) with the arylgermanium(II) chloride GeCl(C₆H₃‐2,6‐Trip₂) ( 2 ; Trip=2,4,6‐triisopropylphenyl) results in the germylidyne complex mer‐[Cl₂(PMe₃)₃Re?Ge? C₆H₃‐2,6‐Trip₂] ( 4 ) upon PMe₃ elimination. An equilibrium that is dependent on the PMe₃ concentration exists between complexes 1 and 4 . Removal of the volatile PMe₃ shifts the equilibrium towards complex 4 , whereas treatment of 4 with an excess of PMe₃ gives a 1:1 mixture of 1 and the PMe₃ adduct of 2 , GeCl(C₆H₃‐2,6‐Trip₂)(PMe₃) ( 2 ‐PMe₃). Adduct 2 ‐PMe₃ can be selectively obtained by addition of PMe₃ to chlorogermylidene 2 . The NMR spectroscopic data for 2 ‐PMe₃ indicate an equilibrium between 2 ‐PMe₃ and its dissociation products, 2 and PMe₃, which is shifted far towards the adduct site at ambient temperature. NMR spectroscopic monitoring of the reaction of complex 1 with 2 and the reaction of complex 4 with PMe₃ revealed the formation of two key intermediates, which were identified to be the chlorogermylidene complexes cis/trans‐[Cl(PMe₃)₄Re?Ge(Cl)C₆H₃‐2,6‐Trip₂] (cis/trans‐ 3 ) by using NMR spectroscopy. Labile chlorogermylidene complexes cis/trans‐ 3 can be also generated from trans‐[Cl(PMe₃)₄Re?Ge? C₆H₃‐2,6‐Trip₂]BPh₄ ( 9 ) and (nBu₄N)Cl at low temperature, and decompose at ambient temperature to give a mixture of complexes 1 and 4 . Complex 4 reacts with LiI to give the diiodido derivative mer‐[I₂(PMe₃)₃Re?Ge? C₆H₃‐2,6‐Trip₂] ( 5 ), which undergoes a metathetical iodide/hydride exchange with Na(BEt₃H) to give the dihydrido germylidyne complex mer‐[H₂(PMe₃)₃Re?Ge? C₆H₃‐2,6‐Trip₂] ( 6 ). Carbonylation of 4 induces a chloride migration from rhenium to the germanium atom to afford the chlorogermylidene complex mer‐[Cl(CO)(PMe₃)₃Re?Ge(Cl)C₆H₃‐2,6‐Trip₂] ( 7 ). Similarly, MeNC converts complex 4 into the methylisocyanide analogue mer‐[Cl(MeNC)(PMe₃)₃Re?Ge(Cl)C₆H₃‐2,6‐Trip₂] ( 8 ). Chloride abstraction from 4 by NaBPh₄ in the presence of PMe₃ gives the cationic germylidyne complex trans‐[Cl(PMe₃)₄Re?Ge? C₆H₃‐2,6‐Trip₂]BPh₄ ( 9 ). Heating complex 4 with cis‐[Mo(PMe₃)₄(N₂)₂] induces a germylidyne ligand transfer from rhenium to molybdenum to afford the germylidyne complex trans‐[Cl(PMe₃)₄Mo?Ge? C₆H₃‐2,6‐Trip₂] ( 10 ). All new compounds were fully characterized and their molecular structures studied by X‐ray crystallography, which led to the first experimentally determined Re? Ge triple‐ and double‐bond lengths.     

Deprotonation of Coordinated Phosphanes in a Rhenium Complex: CC Coupling with Diimine Coligands

The reaction of fac‐[Re(bipy)(CO)₃(PMe₃)][OTf] (bipy=2,2′‐bipyridine) with KN(SiMe₃)₂ affords two neutral products: cis,trans‐[Re(bipy)(CO)₂(CN)(PMe₃)], and a thermally unstable compound, which features a new C?C bond between a P‐bonded methylene group (from methyl group deprotonation) and the C6 position of bipy. The solid‐state structures of more stable 1,2‐bis[(2,6‐diisopropylphenyl)imino]acenaphthene analogs, resulting from the deprotonation of PMe₃, PPhMe₂, and PPh₂Me ligands, are determined by X‐ray diffraction.     

Exploring possible reaction pathways for the o‐atom transfer reactions to unsaturated substrates catalyzed by a [Ni‐NO2] ↔ [Ni‐NO] redox couple using DFT methods

The (nitro)(N‐methyldithiocarbamato)(trimethylphospane)nickel(II), [Ni(NO₂)(S₂CNHMe)(PMe₃)] complex catalyses efficiently the O‐atom transfer reactions to CO and acetylene. Energetically feasible sequence of elementary steps involved in the catalytic cycle of the air oxidation of CO and acetylene are proposed promoted by the Ni(NO₂)(S₂CNHMe)(PMe₃)] ↔ Ni(NO₂)(S₂CNHMe)(PMe₃) redox couple using DFT methods both in vacuum and dichloromethane solutions. The catalytic air oxidation of HC≡CH involves formation of a five‐member metallacycle intermediate, via a [3 + 2] cyclo‐addition reaction of HC≡CH to the Ni‐N = O moiety of the Ni(NO₂)(S₂CNHMe)(PMe₃)] complex, followed by a β H‐atom migration toward the C_α carbon atom of the coordinated acetylene and release of the oxidation product (ketene). The geometric and energetic reaction profile for the reversible [Ni( ‐NO₂)(S₂CNHMe)(PMe₃)] [Ni( ‐ONO)(S₂CNHMe)(PMe₃)] linkage isomerization has also been modeled by DFT calculations. © 2017 Wiley Periodicals, Inc.     

Neue Rhodium(I)‐Komplexe mit Donor/Akzeptor‐Chelatliganden

Alternative Ligands. XXXVI. Novel Rhodium(I) Complexes with Donor/Acceptor Chelating Ligands In order to generate metal base/Lewis‐acid interactions in rhodium(I) phosphane complexes the binuclear complex [Rh(CO)₂Cl]₂ was reacted in benzene with dipod ligands of the type R₂M′(OCH₂PMe₂)_x(CH₂CH₂PMe₂)_2–x (R = F, Me; M′ = Si, Ge; x = 0–2) using the Ziegler dilution principle with the aim to produce mononuclear compounds in which with formation of five‐membered chelate rings in principle Rh → M′ contacts are possible. The reactions of ligands 1 – 7 (Table 1) with [Rh(CO)₂Cl]₂ proceed under CO elimination and, in spite of large turnovers, lead to a variety of products 8 – 14 (Table 1), in case of 11 , 13 and 14 accompanied by degradation of the corresponding ligands. Intact ligands are present in the 16‐membered rings of the binuclear complexes 8 – 10 and 12 , for which, due to the molecular structure, Rh → M′ interactions can be excluded. In the reaction of Me₂Si(OCH₂PMe₂)₂ ( 4 ) with [Rh(CO)₂Cl]₂ the unusual binuclear system 11 with a central Rh₂O₂ four‐membered ring and two RhO(SiMe₂OCH₂PMe₂) six‐membered rings is formed. Small amounts of the mononuclear compounds Rh(CO)Cl(Me₂PCH₂OH)₂ ( 13 ) and Rh(CO)Cl₃(Me₂PCH₂OH)₂ ( 14 ), respectively, are obtained in crystalline form from the reaction mixtures of [Rh(CO)₂Cl]₂ with Me₂Ge(OCH₂PMe₂)(CH₂CH₂PMe₂) ( 6 ) or Me₂Ge(OCH₂PMe₂)₂ ( 7 ). The new complexes were characterized by analytic (C, H), spectroscopic (NMR, IR, MS) and, except for 12 , by single crystal structural analyses.     

Preparation of Ru(CO)3(PMe3)MeI and its acetyl derivatives

[Ru(CO)₄PMe₃] reacts with MeI to give fac-[Ru(CO)₃(PMe₃)(Me)I]. The latter reacts with PMe₃ to give a mixture of the three isomers of cis-bis(trimethylphosphine)-cis-dicarbonyl acetyl iodide [Ru(CO)₂(PMe₃)₂(COMe)I]. Decarbonylation of the mixture gives only the trans-bis(trimethylphosphine)-cis-dicarbonyl methyl iodide complex [Ru(CO)₂(PMe₃)₂MeI], which was also prepared by oxidative addition of MeI to [Ru(CO)₃(PMe₃)₂].     

Synthesis and reactivity of a novel hydridocobalt(III) complex containing trimethylphosphine and thiophenolato ligands

The novel hydridocobalt(III) complex [mer-Co(H)(SPh)₂(PMe₃)₃] (1) was prepared by reaction of thiophenol with [Co(PMe₃)₃Cl], [Co(PMe₃)₄] and [Co(PMe₃)₄Me]. A dinuclear cobalt dithiophenolato complex [Co(PMe₃)₂(SPh)]₂ (2) was obtained from the reaction of thiophenol with [Co(PMe₃)₄Me]. Reaction of 1 with iodomethane afforded complex [Co(PMe₃)₃(I)₂] (3). Reaction of complex 2 with carbon monoxide gave a mononuclear dicarbonyl cobalt(I) complex [Co(PMe₃)₃(CO)₂(SPh)] (4). The crystal structures of 1-4 were determined by X-ray diffraction. Formation mechanism of 1 is discussed.     

Komplexe mit kohlenstoffsulfiden und -seleniden als liganden: IV. Ein- und zweikernige rhodium—CS2-komplexe und kristallstruktur eines RhSCSC-heterocyclus

The complexes C₅H₅Rh(PMe₃)CS₂(II) and C₅H₅Rh(PMe₂Ph)CS₂(III) are formed in excellent yields in the reaction of C₅H₅Rh(C₂H₄)PR₃(PR₃ = PMe₃, PMe₂Ph) with CS₂ in benzene. The CS₂ ligand in II and III is dihapto-bonded and at least in III is rigid. II reacts with Cr(CO)₅THF and C₅H₅Mn(CO)₂THF to give the binuclear complexes C₅H₅(PMe₃)Rh(SCS)Cr(CO)₅ (IV) and C₅H₅- (PMe₃)Rh(SCS)Mn(CO)₂C₅H₅ (V) in which the CS₂ molecule bridges two different metal atoms. In the reaction of C₅H₅Rh(C₂H₄)PMe₃ and CS₂ under certain conditions a second product of C₅H₅Rh(PMe₃)C₂S₄ (VI) is formed. The cyrstal structure shows that in this complex a five-membered RhSCSC heterocyclic ring is present.     

Synthesis of Mixed Silylene–Carbene Chelate Ligands from N‐Heterocyclic Silylcarbenes Mediated by Nickel

The Ni^II‐mediated tautomerization of the N‐heterocyclic hydrosilylcarbene L²Si(H)(CH₂)NHC 1 , where L²=CH(C?CH₂)(CMe)(NAr)₂, Ar=2,6‐iPr₂C₆H₃; NHC=3,4,5‐trimethylimidazol‐2‐yliden‐6‐yl, leads to the first N‐heterocyclic silylene (NHSi)–carbene (NHC) chelate ligand in the dibromo nickel(II) complex [L¹Si:(CH₂)(NHC)NiBr₂] 2 (L¹=CH(MeC?NAr)₂). Reduction of 2 with KC₈ in the presence of PMe₃ as an auxiliary ligand afforded, depending on the reaction time, the N‐heterocyclic silyl–NHC bromo Ni^II complex [L²Si(CH₂)NHCNiBr(PMe₃)] 3 and the unique Ni⁰ complex [η²(Si‐H){L²Si(H)(CH₂)NHC}Ni(PMe₃)₂] 4 featuring an agostic Si? H→Ni bonding interaction. When 1,2‐bis(dimethylphosphino)ethane (DMPE) was employed as an exogenous ligand, the first NHSi–NHC chelate‐ligand‐stabilized Ni⁰ complex [L¹Si:(CH₂)NHCNi(dmpe)] 5 could be isolated. Moreover, the dicarbonyl Ni⁰ complex 6 , [L¹Si:(CH₂)NHCNi(CO)₂], is easily accessible by the reduction of 2 with K(BHEt₃) under a CO atmosphere. The complexes were spectroscopically and structurally characterized. Furthermore, complex 2 can serve as an efficient precatalyst for Kumada–Corriu‐type cross‐coupling reactions.     

Iron pyrrole‐based PNP pincer ligand complexes as catalyst precursors

The structure of a pincer ligand consists of a backbone and two `arms' which typically contain a P or N atom. They are tridentate ligands that coordinate to a metal center in a meridional configuration. A series of three iron complexes containing the pyrrole‐based PNP pincer ligand 2,5‐bis[(diisopropylphosphanyl)methyl]pyrrolide (PN^pyrP) has been synthesized. These complexes are possible precursors to new iron catalysts. {2,5‐Bis[(diisopropylphosphanyl)methyl]pyrrolido‐κ³P ,N ,P ′}carbonylchlorido(trimethylphosphane‐κP )iron(II), [Fe(C₁₈H₃₄NP₂)Cl(C₃H₉P)(CO)] or [Fe(PN^pyrP)Cl(PMe₃)(CO)], (I), has a slightly distorted octahedral geometry, with the Cl and CO ligands occupying the apical positions. {2,5‐Bis[(diisopropylphosphanyl)methyl]pyrrolido‐κ³P ,N ,P ′}chlorido(pyridine‐κN )iron(II), [Fe(C₁₈H₃₄NP₂)Cl(C₅H₅N)] or [Fe(PN^pyrP)Cl(py)] (py is pyridine), (II), is a five‐coordinate square‐pyramidal complex, with the pyridine ligand in the apical position. {2,5‐Bis[(diisopropylphosphanyl)methyl]pyrrolido‐κ³P ,N ,P ′}dicarbonylchloridoiron(II), [Fe(C₁₈H₃₄NP₂)Cl(CO)₂] or [Fe(PN^pyrP)Cl(CO)₂], (III), is structurally similar to (I), but with the PMe₃ ligand replaced by a second carbonyl ligand from the reaction of (II) with CO. The two carbonyl ligands are in a cis configuration, and there is positional disorder of the chloride and trans carbonyl ligands.     

Transition‐Metal Complexes [(PMe3)2Cl2M(E)] and [(PMe3)2(CO)2M(E)] with Naked Group 14 Atoms (E=C–Sn) as Ligands; Part 1: Parent Compounds

The equilibrium geometries and bond dissociation energies of 16‐valence‐electron(VE) complexes [(PMe₃)₂Cl₂M(E)] and 18‐VE complexes [(PMe₃)₂(CO)₂M(E)] with M=Fe, Ru, Os and E=C, Si, Ge, Sn were calculated by using density functional theory at the BP86/TZ2P level. The nature of the M? E bond was analyzed with the NBO charge decomposition analysis and the EDA energy‐decomposition analysis. The theoretical results predict that the heavier Group 14 complexes [(PMe₃)₂Cl₂M(E)] and [(PMe₃)₂(CO)₂M(E)] with E=Si, Ge, Sn have C_2v equilibrium geometries in which the PMe₃ ligands are in the axial positions. The complexes have strong M? E bonds which are slightly stronger in the 16‐VE species 1ME than in the 18‐VE complexes 2ME . The calculated bond dissociation energies show that the M? E bonds become weaker in both series in the order C>Si>Ge>Sn; the bond strength increases in the order Fe<Ru<Os for 1ME , whereas a U‐shaped trend Ru<Os<Fe is found for 2ME . The M? E bonding analysis suggests that the 16‐VE complexes 1ME have two electron‐sharing bonds with σ and π symmetry and one donor–acceptor π bond like the carbon complex. Thus, the bonding situation is intermediate between a typical Fischer complex and a Schrock complex. In contrast, the 18‐VE complexes 2ME have donor–acceptor bonds, as suggested by the Dewar–Chatt–Duncanson model, with one M←E σ donor bond and two M→E π‐acceptor bonds, which are not degenerate. The shape of the frontier orbitals reveals that the HOMO?2 σ MO and the LUMO and LUMO+1 π* MOs of 1ME are very similar to the frontier orbitals of CO.     

Theoretical Elucidation of the Mechanism of Cleavage of the Aromatic C?C Bond in Quinoxaline by a Tungsten‐Based Complex [W(PMe3)4(η2‐CH2PMe2)H]

The aromatic C? C bond cleavage by a tungsten complex reported recently by Sattler and Parkin 15 offers fresh opportunities for the functionalization of organic molecules. The mechanism of such a process has not yet been determined, which appeals to computational assistance to understand how the unstrained C? C bond is activated at the molecular level. 16 , 17 In this work, by performing density functional theory calculations, we studied various possible mechanisms of cleavage of the aromatic C? C bond in quinoxaline (QoxH) by the W‐based complex [W(PMe₃)₄(η²‐CH₂PMe₂)H]. The calculated results show that the mechanism proposed by Sattler and Parkin involves an overall barrier of as high as 42.0 kcal mol^?1 and thus does not seem to be consistent with the experimental observation. Alternatively, an improved mechanism has been presented in detail, which involves the removal and recoordination of a second PMe₃ ligand on the tungsten center. In our new mechanism, it is proposed that the C? C cleavage occurs prior to the second C? H bond addition, in contrast to Sattler and Parkin’s mechanism in which the C? C bond is broken after the second C? H bond addition. We find that the rate‐determining step of the reaction is the ring‐opening process of the tungsten complex with an activation barrier of 28.5 kcal mol^?1 after the first PMe₃ ligand dissociation from the metal center. The mono‐hydrido species is located as the global minimum on the potential‐energy surface, which is in agreement with the experimental observation for this species. The present theoretical results provide new insight into the mechanism of the remarkable C? C bond cleavage.     

Catalytic Michael/Ring‐Closure Reaction of α,β‐Unsaturated Pyrazoleamides with Amidomalonates: Asymmetric Synthesis of (−)‐Paroxetine

A highly enantioselective tandem Michael/ring‐closure reaction of α,β‐unsaturated pyrazoleamides and amidomalonates has been accomplished in the presence of a chiral N,N′‐dioxide–Yb(OTf)₃ complex (Tf: trifluoromethanesulfonyl) to give various substituted chiral glutarimides with high yields and diastereo‐ and enantioselectivities. Moreover, this methodology could be used for gram‐scale manipulation and was successfully applied to the synthesis of (?)‐paroxetine. Further nonlinear and HRMS studies revealed that the real catalytically active species was a monomeric L ‐PMe₂ –Yb³⁺ complex. A plausible transition state was proposed to explain the origin of the asymmetric induction.