Organometallics最新文献

For decades, transition-metal catalysts based on noble metals have proven to be highly efficient for a wide range of organic transformations. However, due to their low abundance and highly volatile price the use of such metals is now critical for the economy. Thus, the use of more sustainable alternatives is mandatory and the more abundant first-row transition metals are becoming interesting challengers. Among them low-valent 3d transition metals appeared as the best candidates, as they are intrinsically more reactive than high-valent ones. However, this reactivity implies that these complexes are often generated in situ, putting further away the aspect of atom economy and low waste. To circumvent this and to get more insight on the mechanism of reaction, the concept of well-defined complexes has emerged. In this area, due to its historic development and the trouble associated with the preparation of other well-defined 3d transition-metal complexes, cobalt appears as an important player. In this review, after a definition of what is a low-valent complex and the presentation of the concept of a well-defined (pre)catalyst also known as the “single-component” strategy we will report the syntheses and applications in catalysis of low-valent well-defined cobalt complexes classified by their oxidation state and ligand environment.

TMS-substituted alkynes are versatile building blocks in organic synthesis. Traditional synthesis involves alkyne deprotonation and the reaction with TMSCl. Recently, TMS-acetylene has become an increasingly inexpensive bulk chemical, offering an attractive alternative to accessing TMS-substituted alkynes, especially when the alkyne is expensive or not commercially available. However, this route has been established with carcinogenic HMPA as a cosolvent. In this work, we disclose optimized conditions utilizing DMPU as a substitute for HMPA.

Reported herein are the synthesis and characterization of mono- and bis-alkenyl Co^III(TIM) (TIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) complexes along with products containing a 1-aza-2-cobalt-cyclopropane unit. The trans-[Co(TIM)(C(CH₂)Ar)Cl]⁺-type (Ar = phenyl (Ph, 1a), −C₆H₄-4-^tBu (1b), and −C₆F₅ (1c)), trans-[Co(TIM)(C(CH₂)Ar)₂]⁺-type (Ar = Ph (2a) and −C₆F₅ (2c)), and trans-[Co(TIM″)(C(CH₃)Ar)Cl]⁺-type complexes (Ar = Ph (3a) and −C₆H₄-4-^tBu (3b); TIM″ is the resultant derivative of TIM) were prepared from the reaction between trans-[Co(TIM)Cl₂]PF₆ and the corresponding terminal aryl alkyne in the presence of NaBH₄. Molecular structures of 1a/c, 2a/c, and 3a were established via single-crystal X-ray diffraction studies. Characterization using ¹H NMR further confirmed the occurrence of either an alkenyl (1 and 2) or 1-aza-2-cobalt-cyclopropane (3). The absorption spectra of 1 and 3 reveal differences in the optical HOMO–LUMO gaps with well-defined d–d bands at 477 and 512 nm for 1a and 3a, respectively. Cyclic voltammograms of 1 and 2 consist of an irreversible oxidation and an irreversible reduction characteristic of the Co^III center, while those of 3 display two reduction events. Density functional theory calculations were performed to investigate the bonding and electronic structures of 1a–3a. Natural bonding orbital calculations on 3a suggest significant stabilization in donor → acceptor resonance-type interactions afforded by the 1-aza-2-cobalt-cyclopropane unit.

Phenoxy-imine NON pro-ligands H^R,DippL [1-OH-2,6-(HC═NDipp)-4-R-C₆H₂, where R = H, Me, or ^tBu] were deprotonated using KH to afford the corresponding potassium salts ^R,DippLK·thf_x [R = H (1·thf_x), Me (2·thf_x), and ^tBu (3·thf_x)]. The addition of crown ether (18-c-6) to these salts allowed for the structures of the resulting adducts to be elucidated in the solid state: [1(18-c-6)]_n, [2(18-c-6)]_n, and [3(18-c-6)(thf)]. The derivatives with the smaller para-substituents were found to be 1D coordination polymers stabilized by unusual non-covalent interactions between the diisopropyl-methyl groups and the potassium center. Heteroleptic calcium complexes ^R,DippLCaI(thf)₃ [R = H (4), Me (5), and ^tBu (6)] were prepared by the salt metathesis reaction of 1–3·thf_x with CaI₂. Complexes 4–6 were evaluated as initiators for the ring-opening polymerization of lactide monomers and were all found to be active; the addition of benzyl alcohol resulted in large rate increases, e.g., ∼12-fold difference for 6 (0.70 vs 0.06 h^–1). The propagation rate constants were found to lie within the range 88–135 M^–1 h^–1. Variation of co-initiator concentration revealed only a fractional dependence; this agrees with the other experimental observations, which suggest that the heteroleptic catalysts work via a “ligand-assisted, activated monomer” mechanism.

A series of six-coordinate, idealized octahedral phenoxyimine (FI)–cobalt(III) dimethyl bis(trimethylphosphine) complexes was synthesized and characterized by NMR spectroscopy and single-crystal X-ray diffraction. The thermal stability of the parent Ph-FI complex was evaluated at 60 °C in benzene-d₆, and a 13(1):1 ratio of ethane to methane was observed. The major detectable cobalt product of this reaction was the bis(chelate) cobalt derivative (Ph-FI)₂Co formed by disproportionation of the (FI)cobalt(I) product following ethane reductive elimination. Addition of excess PMe₃ inhibited C(sp³)–C(sp³) reductive elimination, consistent with phosphine dissociation preceding C–C bond formation from a five-coordinate (FI)cobalt(III) dimethyl intermediate. The reductive elimination of substituted (R-FI)cobalt(III) dimethyl bis(trimethylphosphine) compounds was evaluated in acetonitrile-d₃, where ligands bearing electron-donating aniline substituents underwent reductive elimination the fastest and electron-withdrawing substituents the slowest. These data support a buildup of positive charge in the rate-limiting step, consistent with the formation of a more electropositive five-coordinate cobalt center prior to rate-limiting C–C reductive elimination. Attempted synthesis of a cobalt(III) dimethyl complex bearing a sterically hindered FI ligand with a tert-butyl substituent ortho to the phenol led exclusively to the corresponding bis(chelate) cobalt derivative, whose formation was rationalized by steric destabilization of pre–reductive elimination intermediates.

The reason for the discrepancy in reaction rate in hydrogenation reactions using cyclopentadienone iron complexes as catalysts, depending on the absence or presence of a negative charge-tag (sulfonate or phosphonate), was investigated experimentally. Based on NMR and kinetic experiments, the direct binding of the charge-tag to the active site of the catalyst and electric field effects influencing transition state energies could be excluded. Preactivation of the catalysts as the monoacetonitrile dicarbonyl complexes was found to be superior to the in situ activation of the tricarbonyl complex with trimethylamine oxide with an up to 32-fold rate enhancement. CO ligand removal from the tricarbonyl iron complexes with Me₃NO was found to be disfavored in protic solvents compared to polar, aprotic solvents. Micelle formation was observed for the negatively charge-tagged complexes, with critical micelle concentrations in the range of 1.75–17 mM depending on the alkali metal counterion. The presence of the tertiary amine moiety in the charge-tagged catalyst was found to be responsible for the decrease in reaction rate. The presence of micelles was found to increase the reaction rate compared to noncharged complexes bearing a tertiary amine group.

While alkyne metathesis reactions involving d⁰ carbynes (alkylidynes) are well documented, those involving well-defined non-d⁰ carbynes are still rare. This work reports the synthesis and alkyne metathesis activity of d² Re(V) carbyne complexes supported by a [LXL]-type monoanionic [PNP]-pincer ligand, a [LLX]-type monoanionic [PNO]-pincer ligand, and a [ON^OH]-bidentate ligand derived from the Schiff base 2-[(2-hydroxyphenyl)iminomethyl]phenol. Treatment of Re(≡CCH₂Ph)Cl₂(PMePh₂)₃ (1) with bis(2-diphenylphosphino-4-tolyl)amine (PNHP) and 2-{(2-diphenylphosphino-phenyl)iminomethyl}phenol (PNOH) in the presence of NEt₃ produced the pincer complexes Re(≡CCH₂Ph)Cl(PMePh₂)(PNP) (2) and Re(≡CCH₂Ph)Cl(PMePh₂)(PNO) (3), respectively. The Schiff base 2-[(2-hydroxyphenyl)iminomethyl]phenol (HON^OH) reacted with Re(≡CCH₂Ph)Cl₂(PMePh₂)₃ (1) to give Schiff base complex Re(≡CCH₂Ph)Cl(PMePh₂)(ON^OH) (4) (ON^OH = o-O-C₆H₄–CH═N-(o-C₆H₄OH)) bearing a [ON^OH]-bidentate ligand. The [PNP]- and [PNO]-pincer complexes are catalytically active for homometathesis of neat 1-methoxy-4-(1-propyn-1-yl)benzene at 150 °C, while the complex bearing the [ON^OH]-bidentate Schiff base ligand is catalytically inactive under similar conditions. The energy profiles for metathesis reactions of model pincer rhenium alkyne-carbyne complexes have been calculated with DFT methods.