Developing more efficient catalysts remains one of the primary targets of organometallic chemists. To accelerate reaching this goal, effective molecular descriptors and visualization tools can represent a remarkable aid. Here, we present a Web application for analyzing the catalytic pocket of metal complexes using topographic steric maps as a general and unbiased descriptor that is suitable for every class of catalysts. To show the broad applicability of our approach, we first compared the steric map of a series of transition metal complexes presenting popular mono-, di-, and tetracoordinated ligands and three classic zirconocenes. This comparative analysis highlighted similarities and differences between totally unrelated ligands. Then, we focused on a recently developed Fe(II) catalyst that is active in the asymmetric transfer hydrogenation of ketones and imines. Finally, we expand the scope of these tools to rationalize the inversion of enantioselectivity in enzymatic catalysis, achieved by point mutation of three amino acids of mononuclear p-hydroxymandelate synthase.
Tables of H-1 and C-13 NMR chemical shifts have been compiled for common organic compounds often used as reagents or found as products or contaminants in deuterated organic solvents. Building upon the work of Gottlieb, Kotlyar, and Nudelman in the Journal of Organic Chemistry, signals for common impurities are now reported in additional NMR solvents (tetrahydrofuran-d(8), toluene-d(8), dichloromethane-d(2), chlorobenzene-d(5), and 2,2,2-trifluoroethanol-d(3)) which are frequently used in organometallic laboratories. Chemical shifts for other organics which are often used as reagents or internal standards or are found as products in organometallic chemistry are also reported for all the listed solvents.
This review provides an introduction into the fascinating area of organometallic anticancer compounds. Although the subject dates back many years, it has witnessed considerable growth only in the past decade. A brief overview of the subject together with recent pertinent examples is provided. The properties of organometallic compounds that lend themselves to medical applications, the main current approaches used, and possible avenues for future research are identified.
Single-molecule magnets (SMMs) display slow relaxation of the magnetization, purely of molecular origin, in the absence of an applied magnetic field. This review summarizes the important role played by organometallic chemistry in the recent development of SMMs. The broad applicability of organometallic synthesis has led to a series of organometallic SMMs containing transition metals, lanthanides, or actinides, with several examples accounting for some of the most fascinating low-temperature magnetism. The review has two main aims. The first aim is to provide organometallic chemists with an introduction to one of the most exciting areas of modern molecular magnetism and, in particular, to highlight how organometallic chemistry has allowed the field to evolve in new directions. The second aim is more of a clarion call: organometallic chemistry still has hugely underexploited potential in the development of single-molecule magnets, and it is reasonable to expect that different synthetic approaches will lead to new and unusual magnetic phenomena. By using this review as an entry point for studying the literature in more detail, hopefully more organometallic chemists will consider directing their synthetic repertoire toward the design and realization of new, and possibly improved, single-molecule magnets.
A brief account of the major developments of palladium-catalyzed cross-coupling during the last two decades is highlighted chronologically, with an emphasis on the personal experiences of the corresponding author. Important contributions from both academia and industry, which have been vital to the accelerated growth of this area, are presented. The developments of new classes of ligands and the switch from in situ to preformed catalysts tailored to address the challenges in cross-coupling are reviewed, reflecting an evolution in continued growth.
The C-N cross-coupling chemistry intensely developed since the late 1990s has supplied synthesists with an overwhelming number of methods to effectively combine carbon and nitrogen residues. This new chemistry relies on complexes of mainly two metals, copper and palladium, used as catalysts or stoichiometric agents. The development of new methods has revealed both similarities and differences in the principles used for the design of new catalytic systems and analysis of their reactivity and selectivity. The discussion of cross-coupling chemistry of these two metals can be performed within a common mechanistic paradigm, helping to elucidate the key factors governing the behavior of the transition-metal complexes involved.
This personal account summarizes our work, beginning with the discovery of the first stable carbene in 1988 up until the recent isolation of mesoionic carbenes. It explains why we have moved our focus from acyclic to cyclic carbenes and shows that these stable species are not limited to the role of ligand for transition metals but that they are also powerful agents for the activation of small molecules, and for the stabilization of highly reactive diamagnetic and paramagnetic species.
Among the transition metals, copper based catalyst systems enable the widest range of N-containing reagents in C-H amination to allow for the direct incorporation of versatile N-based functionalities via ubiquitous C H bonds. In addition to nitrene-based approaches involving sulfonyliminoiodinanes: (PhI=NSO2R), diverse non-nitrene protocols have been developed that allow for the direct use of organic amides, nitrosoarenes, and hydroxylamines, strained heterocycles such as oxaziridines, acetonitrile, secondary sulfonylamines, and even alkylamines and arylamines. Synthetic, Mechanistic, and theoretical studies reveal discrete copper nitrenes [Cu]=NR and copper amides [Cu]-NHR to be key reactive intermediates in C-H amination. Copper catalyzed sp(3) C-H amination is reviewed connecting catalytic reactivity patterns with likely copper intermediates wherever possible, with the goal to stimulate the further development Of C H functionalization reactions With copper which possess significant sustainability advantages over other Contemporary approaches involving noble metals.
Terminal 1,2-dialkynylarenes undergo an unexpected cyclization hydroarylation reaction toward naphthalene derivatives in benzene as the solvent. The regioselectivity of the reaction can be controlled by careful catalyst tuning. Also, the preparation of a bench-stable cationic amine complex or simple heterogenization of the catalyst on neutral aluminum oxide, which enables efficient catalyst recycling, was possible. Intensive mechanistic investigations were undertaken, giving new insights into the so-far underestimated role of acetylides in gold chemistry. The gold plays a fascinating dual role serving to both catalyze the reaction and activate the substrate by Au-C-sigma bond formation. Evidence of gem-diaurated compounds playing an important part for gold catalysis is also reported.
From the core of the earth to the core of the archetypal organometallic compound ferrocene, iron and its compounds contribute to create many aspects of our reality: the earth's magnetism, hemoglobin's respiratory function, and a myriad of life-essential biochemical reactions. This review deals with synthetic iron compounds from the literature, in particular ferrocene and derivatives, which are potential new anticancer agents. Cytotoxic ferrocenes are among the most promising metal-based drugs for cancer chemotherapy. There is a vast range of reported compounds, herein classified into three categories, according to their chemical nature and supramolecular organization. The first comprises ferrocenium salts and ferrocene derivatives, from simple functionalized ferrocenes to elaborate iron-based mimics of organic drugs, the second includes heterometallic complexes (with two or more metal centers), in which ferrocene has the role of an ancillary ligand, and the third comprises cytotoxic ferrocenes associated with carrier systems, namely aqueous-soluble polymers, multilayer micelles, and cyclodextrins.
This account describes recent progress (> 2006) in the synthesis and structural characterization of isolable N-heterocyclic silylenes (NHSi's) and their fascinating reactivities with respect to an emergent topic in main-group chemistry: metal-free small-molecule activation. Since the seminal discovery of stable N-heterocyclic silicon analogues of nucleophilic Wanzlick-Arduengo-type carbenes in 1994, new types of NHSi's have emerged with unique electronic features and strikingly different reactivities. Among them, the first zwitterionic (ylide-like) silylene LSi: (L = CH-[(C=CH2)CMe][NAr](2); Ar = 2,6-Pr'2C6H3) and unprecedented N-heterocyclic bis(silylenes) with amidinate ligands and Si(I)-Si(I) bonds were synthesized. Their striking electronic structures open new doorways to metal-free activation of C H, C X, Si X, E H (E = group IS, group 16 elements), P-P, E-O (E = C, N), and E E bonds (E = O, S, Se, Te).
A new method for the assessment of the pi-acceptor strength of N-heterocyclic carbenes is presented. The Se-77 chemical shifts of the easily available selenium carbene adducts 1.Se-7.Se correlate with the pi-acceptor character of the respective carbenes. The observed delta(Se-77) values cover a range of almost 800 ppm, with increasing pi-acidity leading to a downfield shift of the signal.
The electro- and spectroelectrochemical behavior of diverse (multi)ferrocenyl five-membered heterocyclic compounds, including furan, thiophene, pyrroles, phospholes, etc. is discussed, giving a close insight into the electron transfer processes of these organometallic compounds in their mixed-valence state, whereby electronic and structural modification of the heterocyclic connecting units and/or of the redox-active ferrocenyl termini directly influences the electron transfer properties. In addition, the structural features of these compounds can be correlated with their electrochemical behavior, allowing calculation of the effective electron transfer distance within a specific series of molecules. Tendencies in molecular wire molecules based on bi-, ter-, quarter-, quinque-, and sexithiophene connecting building blocks and the appropriate pyrrole derivatives are discussed as well. The consequences of introducing an additional redox-active transition- metal building block, such as a titanocene or a zirconocene moiety, respectively, into the heterocyclic ring, on the electrochemical behavior of the resulting five-membered heterocycles are also highlighted.
The palladium-catalyzed carbonylation of aryl and vinyl halides was first described more than 30 years ago by Richard Heck. However, limitations in the conditions originally described have meant that this reaction has achieved less prominence than the coupling reaction that also bears his name. Nevertheless, the attractiveness of this chemistry for forming carbonyl derivatives has led many researchers over the intervening years to attempt to increase the scope of the reaction beyond the originally described bromide, iodide, and triflate substrates, with conditions suited to large-scale application (particularly low pressure). To a large degree, this has now been achieved. This review describes the progress made regarding the selection of catalysts and conditions for this set of reactions, including illustrations from our own research. Reactions can now be carried out with aryl chlorides and tosylates bearing a range of substituents affecting both the electronic and steric properties of the substrate. The complexities of the reaction, represented by the interplay of catalyst, CO pressure, temperature, base, and solvent, make parallel screening desirable for optimization studies.
The PNP-ligated iridium(III) trihydride complex 1 exhibited the highest catalytic activity for hydrogenation of carbon dioxide in aqueous KOH. The catalytic hydrogenation can be tuned to be a reversible process with the same catalyst at the expense of the activity, when triethanolamine was used as a base. Theoretical studies on the hydrogenation of carbon dioxide using DFT calculations suggested two competing reaction pathways: either the deprotonative dearomatization step or the hydrogenolysis step as the rate-determining step. The results nicely explain our experimental observations that the catalytic cycle is dependent on both the strength of the base and hydrogen pressure.
This review is a cautionary note against the often purported direct relation between the half-wave potential splitting Delta E-1/2 (or Delta E degrees) for stepwise, consecutive electron transfer from systems featuring two or more identical redox sites and the true electronic coupling H-AB and charge or spin distribution in the ground state of intermittently formed mixed-valent (MV) systems. Several examples where these different quantities go in parallel are contrasted with other examples where this is not the case. Different kinds of such "non-conformist" behavior are outlined with the aid of representative examples. These include cases of fairly strong electronic couplings and large degrees of ground-state delocalization despite small values of Delta E-1/2 sometimes just above the statistical limit or even below that as well as examples for just the opposite behavior of no detectable electronic coupling despite appreciable electrochemical half-wave potential splitting. The crucial roles of the nominal bridges that interconnect the individual redox sites and of the environment (solvent, supporting electrolyte) in determining Delta E-1/2 and H-AB are emphasized. We also seek to provide some guidelines for the practitioner as to how to discriminate between these various types of behaviors and how to determine the strength of the electronic coupling between the redox sites.
The current state of the art and perspectives of homogeneous and heterogeneous catalysis are discussed for C-C and C-heteroatom bond formation in organic synthesis. The relationship between catalyst centers represented by a single metal atom and by multiple metal atoms is considered for reactions taking place in solution. The influence of leaching and catalyst evolution in the liquid phase on the activity, selectivity, and stability of the catalyst is highlighted from a mechanistic point of view. Metal nanoparticle and "nanosalt" types of catalysts are compared for constructing new C-C and C-heteroatom bonds.