Synthetic Organometallic Chemistry Research Articles

The H-cluster of [FeFe] Hydrogenases: Its Enzymatic Synthesis and Parallel Inorganic Semisynthesis.

ConspectusNature's prototypical hydrogen-forming catalysts─hydrogenases─have attracted much attention because they catalyze hydrogen evolution at near zero overpotential and ambient conditions. Beyond any possible applications in the energy sphere, the hydrogenases feature complicated active sites, which implies novel biosynthetic pathways. In terms of the variety of cofactors, the [FeFe]-hydrogenase is among the most complex.For more than a decade, we have worked on the biosynthesis of the active site of [FeFe] hydrogenases. This site, the H-cluster, is a six-iron ensemble consisting of a [4Fe-4S]H cluster linked to a [2Fe]H cluster that is coordinated to CO, cyanide, and a unique organic azadithiolate ligand. Many years ago, three enzymes, namely, HydG, HydE, and HydF, were shown to be required for the biosynthesis and the in vitro maturation of [FeFe] hydrogenases. The structures of the maturases were determined crystallographically, but still little progress was made on the biosynthetic pathway. As described in this Account, the elucidation of the biosynthetic pathway began in earnest with the identification of a molecular iron-cysteinate complex produced within HydG.In this Account, we present our most recent progress toward the molecular mechanism of [2Fe]H biosynthesis using a collaborative approach involving cell-free biosynthesis, isotope and element-sensitive spectroscopies, as well as inorganic synthesis of purported biosynthetic intermediates. Our study starts from the radical SAM enzyme HydG that lyses tyrosine into CO and cyanide and forms an Fe(CO)2(CN)-containing species. Crystallographic identification of a unique auxiliary 5Fe-4S cluster in HydG leads to a proposed catalytic cycle in which a free cysteine-chelated "dangler" Fe serves as the platform for the stepwise formation of a [4Fe-4S][Fe(CO)(CN)(cysteinate)] intermediate, which releases the [Fe(CO)2(CN)(cysteinate)] product, Complex B. Since Complex B is unstable, we applied synthetic organometallic chemistry to make an analogue, syn-B, and showed that it fully replaces HydG in the in vitro maturation of the H-cluster. Syn-B serves as the substrate for the next radical SAM enzyme HydE, where the low-spin Fe(II) center is activated by 5'-dAdo• to form an adenosylated Fe(I) intermediate. We propose that this Fe(I) species strips the carbon backbone and dimerizes in HydE to form a [Fe2(SH)2(CO)4(CN)2]2- product. This mechanistic scenario is supported by the use of a synthetic version of this dimer complex, syn-dimer, which allows for the formation of active hydrogenase with only the HydF maturase. Further application of this semisynthesis strategy shows that an [Fe2(SCH2NH2)2(CO)4(CN)2]2- complex can activate the apo hydrogenase, marking it as the last biosynthetic intermediate en route to the H-cluster. This combined enzymatic and semisynthetic approach greatly accelerates our understanding of H-cluster biosynthesis. We anticipate additional mechanistic details regarding H-cluster biosynthesis to be gleaned, and this methodology may be further applied in the study of other complex metallocofactors.

Spin-phonon coupling and magnetic relaxation in single-molecule magnets.

Electron-phonon coupling is important in many physical phenomena, e.g. photosynthesis, catalysis and quantum information processing, but its impacts are difficult to grasp on the microscopic level. One area attracting wide interest is that of single-molecule magnets, which is motivated by searching for the ultimate limit in the miniaturisation of binary data storage media. The utility of a molecule to store magnetic information is quantified by the timescale of its magnetic reversal processes, also known as magnetic relaxation, which is limited by spin-phonon coupling. Several recent accomplishments of synthetic organometallic chemistry have led to the observation of molecular magnetic memory effects at temperatures above that of liquid nitrogen. These discoveries have highlighted how far chemical design strategies for maximising magnetic anisotropy have come, but have also highlighted the need to characterise the complex interplay between phonons and molecular spin states. The crucial step is to make a link between magnetic relaxation and chemical motifs, and so be able to produce design criteria to extend molecular magnetic memory. The basic physics associated with spin-phonon coupling and magnetic relaxation was outlined in the early 20th century using perturbation theory, and has more recently been recast in the form of a general open quantum systems formalism and tackled with different levels of approximations. It is the purpose of this Tutorial Review to introduce the topics of phonons, molecular spin-phonon coupling, and magnetic relaxation, and to outline the relevant theories in connection with both the traditional perturbative texts and the more modern open quantum systems methods.

Leibniz‐Institute for Catalysis (LIKAT): From Basic Research to Practical Applications

Leibniz‐Institute for Catalysis (LIKAT): From Basic Research to Practical Applications

Synthesis, Structure, and Catalysis of Palladium Complexes Bearing a Group 13 Metalloligand: Remarkable Effect of an Aluminum-Metalloligand in Hydrosilylation of CO2.

Efficient synthesis and catalysis of a series of palladium complexes having a group 13 metalloligand (Al, Ga, In) are reported utilizing 6,6″-bis(phosphino)terpyridine as a new scaffold for Pd-E bonds (E = Al, Ga, In). Systematic investigation revealed unique characteristics of the Al-metalloligand in both structure and reactivity, which exhibited the highest catalytic activity for hydrosilylation of CO2 ever reported (TOF = 19 300 h-1). This study demonstrated fine-tuning of catalyst activity by the precisely designed metalloligand is a promising approach for new catalyst development in synthetic organometallic chemistry.

Room temperature transmetallation from tris(pentafluorophenyl)borane to cyclometallated iridium(iii).

Transmetallation reactions involving organoboron reagents and transition metals are legion in synthetic organometallic chemistry and homogeneous catalysis. Triarylboranes (BAr3) have been observed to participate in transmetallation reactions with many transition metals, typically following abstraction of an alkyl (R-) or hydride ligand by the Lewis acidic borane. Here, an unusual transmetallation strategy is described where an aryl group from a borane replaces a weakly coordinated PF6- ligand. The precursors Ir(C^N)2(CNAr)(FPF5) (C^N = cyclometallating ligand, CNAr = aryl isocyanide) react smoothly with B(C6F5)3 to give complexes of the type Ir(C^N)2(CNAr)(C6F5), a previously unobserved structure type featuring an unchelated aryl ligand. The reaction tolerates a variety of C^N ligands and a range of electronically and sterically varied aryl isocyanide ancillary ligands. A total of six complexes of this type are described, two of which are characterized by single-crystal X-ray diffraction. All but one of the complexes luminesces at room temperature, with the emission wavelength dependent on the C^N ligand.

ChemInform Abstract: Recent Advances in Transition Metal‐Catalyzed C—H Bond Functionalization of Ferrocene Derivatives

AbstractReview: 54 refs.

ChemInform Abstract: Open‐Shell Organometallics: Reactivity at the Ligand

AbstractReview: 109 refs.

Pentafluorophenyl copper: aggregation and complexation phenomena, photoluminescence properties, and applications as reagent in organometallic synthesis

Recent advances in the chemistry of pentafluorophenyl copper are discussed, including the observation of strongly luminescent adducts with pyridine, the first successful structural characterization of an organocopper-arene complex, and the complexation with electron-rich transition metal complexes such as ferrocene derivatives. In addition, new applications in synthetic organometallic chemistry are discussed, which include the discovery of tin/copper exchange reactions for the preparation of organocopper compounds that are otherwise not readily accessible, and the selective transfer of the C(6)F(5) groups to boron to form catalytically active and electronically interesting organoboron polymers.

Single Electron Transfer Reactions in the Synthetic Organometallic Chemistry of First‐Row Transition Metals.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Single Electron Transfer Reactions in the Synthetic Organometallic Chemistry of First-Row Transition Metals

The discovery of new applications in small-molecule activation, olefin polymerization, and organic synthesis has stimulated interest in well-defined, paramagnetic, first-row transition-metal organometallic compounds. Single-electron-transfer reactions have proved to be a useful tool in the targeted synthesis of such complexes. In this review, several examples from the recent literature are used to illustrate this strategy. The case studies include β-diketiminato complexes of Cr(II) and Cr(III), three-coordinate Ni(II) species, and β-diketiminato complexes of V(III), V(IV), and V(V).

23 Magnetism

This report begins with a celebration of the 300th Anniversary of the synthesis of Prussian Blue. The synthesis of this material marked a turning point in chemical science and the birth of synthetic organometallic chemistry. 300 years on, studies of Prussian Blue and its relatives still represent an important frontier of scientific research. They display many of the properties that captivate current research into magnetism. Some particular aspects of this are briefly reviewed before this report continues with the wider literature on magnetism. In this way the research into the Prussian Blues is used to create a context for the research which follows. At the same time the more general review creates its own context, one which celebrates the wealth of science that is so well demonstrated by the Prussian Blues.

The Versatility of Lithium Reagents in Synthetic Organometallic Chemistry: Their differing reactions with [Cp*Mo(NO)(CH2SiMe3)2

Abstract[Cp*Mo(NO)(CH2SiMe3)2] exhibits three principal types of reaction with the various lithium reagents investigated during this study, namely: regioselective deprotonation, reduction, and addition. Deprotonation of the reactant, achieved by treatment with lithium amide reagents, leads ultimately to the formation of the alkylidene „ate”︁ complex [Cp*Mo(NO)(CH2SiMe3)2(CHSiMe3)]2‐[Li2(thf)3] (1). While LiN(SiMe3)2 effects this conversion directly with no detectable intermediates, reaction with 1 equiv of LDA in THF for 15 min deprotonates the Cp* ligand to form the lithium salt of the „tucked‐in”︁ ate complex [(η5,η1‐C5Me4CH2)Mo(NO)(CH2SiMe3)2]‐[Li(thf)3] (2) in 40% isolated yield. Complex 2 slowly converts to the thermodynamically more stable 1 when left as a THF or C6D6 solution at ambient temperature for 48 h. Reaction of the dialkyl starting material with either tBuLi or PhLi leads to the production of the alkylidene complex 1 in irreproducible yields (10–50% NMR; not isolable). A kinetic analysis of the reaction of [Cp*Mo(NO)(CH2SiMe3)2] with LiN(SiMe3)2 indicated that the reaction was first‐order in both the lithium and molybdenum reagents, and the activation parameters of ΔH* = 7.3∓1.0 kcal mol−1 and ΔS* = − 34∓3 e.u. suggest an associative process. Treatment of the neutral dialkyl with 1 equiv of LiPPh2 in THF results in a one‐electron reduction and production of {[Cp*Mo(NO)(CH2SiMe3)2[Li2(thf)3] (3). If left in solution, the 17e− dialkyl anion 3 is converted to the 18e− alkylidene anion 1 by the Ph2P–PPh2 coproduct, which effects the requisite hydrogen‐atom abstraction. Finally, addition of a sterically undemanding alkyllithium reagent such as MeLi to the 16e− dialkyl reactant leads to the formation of the 18e− trialkyl anionic complex [[Cp*Mo(NO)(CH2SiMe3)2(CHSiMe3)]2‐[Li2(thf)3](Me)][Li(thf)3] (4). Warming of 4 in a C6D6 solution results in loss of methane and production of 1. In most cases, the chemistry exhibited by the Mo system is duplicated by the analogous W congener, [Cp*Mo(NO)(CH2SiMe3)2(CHSiMe3)]2‐[Li2(thf)3]. The solid‐state molecular structures of complexes 1 and 4 have been established by single‐crystal X‐ray crystallographic analyses.

Solid State Structures of Cyclopentadienyllithium, -sodium, and -potassium. Determination by High-Resolution Powder Diffraction

The structures of MCp (M = Li, Na, K; Cp = cyclopentadienyl), some of the most important precursors in synthetic organometallic chemistry, have been investigated. All three compounds form polymeric “multidecker structures”. In LiCp and NaCp the metal atoms form a “string of pearls” whereas the potassium atoms in KCp are arranged in a zigzag chain that interacts with neighboring chains.

ChemInform Abstract: Multipurpose Vacuum Line Designs for Use in Synthetic Organometallic Chemistry

ChemInform Abstract: Multipurpose Vacuum Line Designs for Use in Synthetic Organometallic Chemistry

ChemInform Abstract: On the Mechanism of Higher Alcohol Formation Over Metal Oxide Catalysts.

AbstractA new mechanism for the synthesis of alcohols from synthesis gas is proposed based on recent observations from surface spectroscopy, catalysis, and synthetic organometallic chemistry.

On the mechanism of higher alcohol formation over metal oxide catalysts: I. A rationale for branching in the synthesis of higher alcohols from syngas

A new mechanism for the synthesis of alcohols from synthesis gas is proposed based on recent observations from surface spectroscopy, catalysis, and synthetic organometallic chemistry. Stepwise transfer of hydrogens to coordinated CO and chain growth by CO insertion provide the primary pathway for the construction of higher alcohols over metal oxide catalysts. The insertion of a carbon monoxide into a surface-bound aldehyde is proposed as the primary carbon-carbon bond forming step in the chain growth. A competing carbon-carbon bond-forming step is the reaction of a surface n 3 -enolate with a surface alkoxide. This condensation reaction is critical for providing product distributions deviating from those predicted by simple polymerization schemes. A novel rationale is proposed to explain the selectivity to branched products. The relative stabilities of the enolate precursors to the various alcohols and the relative rates of the 1,2-shift reactions of methyl and hydrogen are the rate-controlling mechanistic features which regulate selective formation of branched higher alcohol products. The mechanism is related to the dehydration of secondary alcohols to 1-olefins catalyzed by basic metal oxides.