Biometallorganische Chemie: Ein faszinierendes Forschungsgebiet

Bioorganometallic Chemistry is a new research area in which organometallic compounds are coupled with biomolecules (sugars, peptides, DNA and its constituents, steroides, vitamines, enzymes). In a narrow sense such organometallic complexes occur in nature (vitamin B₁₂), have a biological function (e.g. nickel enzymes in bacterias) or are of potential medical use (as novel drug or as marker for biomolecules). In a wider sense bioorganometallic chemistry includes simply metal complexes which besides organometallic ligands (e.g. CO, π‐hydrocarbon) have a biomolecule as ligand (e.g. with interesting structures, for catalysis).     

Synthesis and structures of cadmium(II) complexes of a series of multinucleating N/S donor ligands

The coordination chemistry of a series of bis-bidentate ligands with cadmium(II) ions has been investigated. The ligands, containing two N,S-donor chelating (pyrazolyl/thioether) fragments, have afforded complexes of a variety of structural types (dinuclear M₂L₂ ‘mesocate’ complexes, a one-dimensional chain coordination polymer and a simple mononuclear complex) according to whether the bis-bidentate ligands act as bridges spanning two metal ions, or a tetradentate chelate to a single metal ion. The p-phenylene and m-biphenyl spaced ligands L¹ and L³ form dinuclear M₂L₂ complexes where the ligands are arranged in a ‘side-by-side’ fashion. In contrast the m-phenylene spaced ligand L² forms a one-dimensional coordination polymer where the ligands adopt a highly folded conformation. The 1,8-naphthalene spaced ligand L⁴ adopts a tetradendate chelating mode and affords a simple mononuclear complex.     

Mixed Ligands from Group 15/16 Elements: Interface of Solid-State and Molecular Chemistry

A wide range of chemical compounds is spanned by heteroatomic ligands from Group 15/16 elements, which in Nature extend from AsS (in realgar) to [AsS₂⁻]_∞ (in sulfosalt minerals). The stabilization of labile molecules or those that do not exist in the free state by incorporation into transition metal complexes like [Cp^*₂Fe₂(AsSe)₂] ( 1 ) and the oligomerization of molecular units under the influence of metal ions or complexes to form solids or hybrid clusters with inorganic cores and peripheral organometallic ligands are the main subjects of this article. Cp*=C₅Me₅.     

Molecular Organometallic Chemistry on Surfaces: Reactivity of Metal Carbonyls on Metal Oxides

Metal carbonyls react on metal oxide surfaces to give a wide range of structures analogous to those of known compounds. The reactions leading to formation of surface-bound metal carbonyls are explained by known molecular organometallic chemistry and the functional group chemistry of the surfaces. The reaction classes include formation of acid-base adducts as the oxygen of a carbonyl group donates an electron pair to a Lewis acidic center; nucleophilic attack at CO ligands by basic surface hydroxyl groups or O^2? ions; ion-pair formation by deprotonation of hydrido carbonyls to give carbonylate ions; interaction of bifunctional complexes with surface acid-base pair sites such as [Mg^2⊕O^2?]; and oxidative addition of surface hydroxyl groups to metal clusters. The reactions of surface-bound organometallic species include redox condensation and cluster formation on basic surfaces (paralleling the reactions in basic solution) as well as oxidation of mononuclear metal complexes and oxidative fragmentation of metal clusters by reaction with surface hydroxyl groups. Most supported metal carbonyls are unstable at high temperatures, but some, including osmium carbonyl cluster anions on the basic MgO surface, are strongly stabilized in the presence of CO and are precursors of catalysts for CO hydrogenation at 550 K.     

Functional ligands and complexes for new structures, homogeneous catalysts and nanomaterials

In this account, we focus on results from our laboratory to illustrate recent developments in various fields of organometallic chemistry. Studies on hemilabile P,N donor ligands and on the ion-pair behaviour of cationic Pd(II) complexes have led to the full characterization of complexes with η¹-allyl ligands. This still rare bonding mode for the allyl ligand in palladium chemistry allows facile insertion of CO into the Pd-C σ-bond, in contrast to the situation in related η³-allyl Pd(II) complexes. In order to develop new homogeneous catalysts for the selective dimerization and oligomerization of ethylene, a range of Ni(II) complexes have been prepared with new chelating P,N ligands where P represents a phosphine, phosphinite or phosphonite donor group and N a pyridine or oxazoline moiety. Finally, we shall examine bottom-up approaches to the formation of new nanomaterials of magnetic or catalytic interest by covalent anchoring of metal complexes and clusters into mesoporous materials using functional phosphine or alkyne ligands containing an alkoxysilyl group.     

Ion/Molekülreaktionen negativ geladener KomplexeSchiffscher Basen mit potentiellen Ligandmolekülen in der Gasphase

Ion/molecule reactions of four coordinateSchiff base complexes under negative ion chemical ionization conditions have been studied. The complex metal ions consisted of cobalt(II), nickel(II) and copper(II).Schiff base ligands with different donor strengths were employed. The gas mixtures used contained 90% methane and 10% of the gases O₂, NO or CO. The spectra showed intense molecular negative ions, formed by secondary electron capture processes. Secondary ions were formed via ion/molecule reactions between the parent molecular negative ion and added gas molecules to giveMLX ^–,X=O₂, NO, CO;L=Schiff base ligand,M=Co(II) or Ni(II). Consistent with former investigations, secondary ion formation was not found for the copper compounds. Influence of the central metal ion as well as the ligand donor strength on the ion/molecule reactions are discussed. From the results obtained a mechanism of the secondary ion formation is suggested.

     

The Coordination Chemistry and Organometallic Chemistry of Tridentate Oxygen Ligands with π-Donor Properties

Metal complexes are capable of accomplishing almost anything, provided they contain the proper metal/ligand combinations. A host of essential biochemical transformations—but also a great many industrially significant reactions—occur within the coordination spheres of metal ions. For instance, the particular arrangement of ligands in the zinc-containing enzyme carboanhydrase is responsible for an acceleration in the hydration of CO₂ by a factor of 10⁹. It is the ligands that determine whether an iron atom will transfer molecular oxygen, as in the case of hemoglobin, or electrons, as with the cytochromes. By varying the ligands it is possible to establish in advance whether a metal ion in the presence of synthesis gas will cause an olefin to be hydrogenated or hydroformylated. Stated more generally, it is the ligands that stabilize the particular oxidation states of a metal and determine how substrate molecules will be coordinated and undergo reaction. The synthesis of new ligands that confer specific reactivity on metal ions is thus an important challenge for the coordination chemist. The following article describes organometallic compounds of the type [CpCo{P(O)R′R″}₃]^?, which have developed from an extremely unreactive laboratory curiosity into versatile oxygen-containing ligands whose steric and electronic properties promise a series of interesting applications.     

The Coordination Chemistry of Multidentate Isocyanide Ligands

In spite of the excellent ligation properties of isocyanides, until a few years ago there was only a small number of known multidentate ligands of this type. One of the reasons for this lack of interest, when compared to monodentate isocyanides, was the linear arrangement of the M? C?N? R group, which usually inhibits the formation of mononuclear chelate complexes and leads to the formation of multinuclear or polymeric metal complexes. In these, the multidentate ligand acts in a monodentate fashion towards each metal atom. Only recently has a series of polyisocyanides with large ligand backbones been synthesized successfully. Bidentate isocyanides can bridge two metal atoms or react to give chelates with only one metal center. Tripodal ligands form mono- or binuclear complexes, in which the largest organometallic rings observed to date occur (up to 36 atoms). This class of ligands promises to be interesting for the synthesis of stable, diagnostically important technetium complexes of the type [Tc(CNR) ₆ ]⁺. There also appear to be applications for tripodal isocyanides in catalysis. A facial, chiral Cr(CNR^*)₃ unit might be able to catalyze the hydrogenation or isomerization of prochiral double bonds. It is even possible to bind triisocyanides with suitable backbones to carbonyl trimetal clusters, thereby stabilizing them, or making selective cluster formation possible. Coordinated isocyanides can be transformed readily into carbene ligands, which, in the future, could lead to complexes with polycarbene ligation.     

High-Fidelity,Narcissistic Self-Sorting in the Synthesis of Organometallic Assemblies from Poly-NHC Ligands

Highly selective, narcissistic self-sorting has been observed in the one-pot synthesis of three organometallic molecular cylinders of type [M₃{L-(NHC)₃}₂](PF₆)₃ (M=Ag⁺, Au⁺; L=1,3,5-benzene, triphenylamine, or 1,3,5-triphenylbenzene) from L-(NHC)₃ and silver(I) or gold(I) ions. The molecular cylinders contain only one type of tris-NHC ligand with no crossover products detectable. Transmetalation of the tris-NHC ligands from Ag⁺ to Au⁺ in a one-pot reaction with retention of the supramolecular structures is also demonstrated. High-fidelity self-sorting was also observed in the one-pot reaction of benzene-bridged tris-NHC and tetrakis-NHC ligands with Ag₂O. This study for the first time extends narcissistic self-sorting in metal–ligand interactions from Werner-type complexes to organometallic derivatives.     

Syntheses and Characterization of the First Cycloheptatrienyl Transition-Metal Complexes with a M-CF3 Bond

The organometallic chemistry of metal complexes with organocyclic ligands of higher than five hapticity is much more lacking than the chemistry of metal complexes with η⁵-cyclopentadienyl ligands, which has been explored in considerable depth, resulting in novel advances. The main reason for this is stability. In particular, reports indicate that (η⁷-C₇H₇)ML_n complexes are considerably less stable than analogous (η⁵-C₅H₅)ML_n. In perfluoroalkyl metal chemistry, there is currently no reported (η⁷-C₇H₇)ML_n derivative, whereas a number of alkylated ones are known and important conclusions have been drawn about their stability. Responding to this void, and using Morrison’s trifluoromethylating reagent, the present study reports the synthesis and characterization of the first cycloheptatrienyl molybdenum complexes bearing the trifluoromethyl moiety; (η⁷-C₇H₇)Mo(CO)₂CF₃ (I), and (η⁷-C₇H₇)Mo(CO)(PMe₃)CF₃ (II) and discusses their low thermal instability.     

Phosphoryl Complexes of Some Metal(II) Trifluoromethanesulfonates

Cobalt(II), nickel(II), copper(II), and zinc(II) trifluoromethanesulfonates form complexes with the phosphoryl ligands hexamethylphosphoric triamide, nonamethyl imidodiphosphoric tetramide, trimorpholinophosphine oxide, tributylphosphine oxide, and triphenylphosphine oxide. The compounds have been prepared by a substitution reaction using trialkyl orthoformates as dehydrating agents and were investigated with the aid of infrared and ligand-field spectroscopy. In all compounds the ligands coordinate via the phosphoryl oxygen atoms. In some complexes the trifluoromethanesulfonate anions are (semi-)coordinated to the metal ions. The coordination around the metal ions was found to be tetrahedral, square pyramidal, or octahedral depending on the particular combination of metal ion and ligand. In its coordination behaviour the CF₃SO₃^? ion resembles the perrhenate ion.     

Influence of O/S/Se Exchange on the Stability of 1,3,4-Selana(Thia/Oxa)Diazole-Palladium Complexes as Studied by Electrospray Ionization Mass Spectrometry

Abstract

The complexes of 2-phenyl-1,3,4-selena(thia/oxa)diazole with a palladium cation were studied by using electrospray ionization mass spectrometry. Palladium chloride was used as a source of palladium cations. The complexes of ligand:metal stoichiometry of 3:1 (ions [L₃+PdCl]⁺) were formed for selenadiazoles and thiadiazoles. Quantum mechanical calculations performed indicated that ligand molecules are attached to palladium cation by the N-4 atom. The fragment ions formed [L₂–H+Pd]⁺ may be regarded as organometallic species. Selenadiazoles were found more prone to form the palladium complexes than thiadiazoles. Oxadiazoles did not yield the respective palladium complexes. For comparison, the nickel cation was also included in the study but only 1:1, and less abundant 2:1 complexes were observed. Exchange of selenium into oxygen does not affect the abilities of the ligands to form nickel complexes.     

Fluorinated Isocyanides—More than Ligands with Unusual Properties

The substitution of hydrogen by fluorine in organic compounds usually results in drastic changes in their properties. For isocyanides, for which fluorinated examples have only recently become available in preparative quantities, this substitution leads to a significantly increased reactivity and a tendency to polymerize, which, on one hand, makes their handling more difficult. On the other hand, this high reactivity makes the fluorinated isocyanides useful building blocks for the synthesis of compounds like N-trimethylformamide. Energetically favorable π^* orbitals bestow excellent π-acceptor properties towards low-valent transition metal complexes, especially on the ligand trifluoromethyl isocyanide. The pronounced tendency of this ligand to bridge two metal atoms enables the formation of structural types that are not accessible with other π-acceptor ligands. Thus it was possible to prepare [(Os₃(CO)₁₁(μ₂-CNCF₃)₂] (a) derivative of the hypothetical [Os(CO)₁₃]) which may be considered as a model for an associative mechanism of ligand substitution at carbonyl clusters. In contrast to the well-studied chemistry of trifluoromethyl isocyanide, that of the few other known fluorinated isocyanides is only now receiving attention. In particular the only recently synthesized trifluorovinyl isocyanide promises a rich chemistry as a result of its difunctionality.     

A mass spectrometry and DFT study of pyrithione complexes with transition metals in the gas phase

2‐Mercaptopyridine N ‐oxide (pyrithione, PTOH) along with several transition metal ions forms coordination compounds displaying notable biological activities. Gas‐phase complexes formed between pyrithione and manganese (II), cobalt (II), nickel (II), copper (II), and zinc (II) were investigated by infusion in the electrospray source of a quadrupole‐time of flight mass spectrometer. Remarkably, positive ion mode spectra displayed the singly charged metal adduct ion [C₁₀H₈MN₂O₂S₂]²⁺ ([M(PTO)₂]^+• or [M(DPTO)]^+•), where DPTO is dipyrithione, 2,2′‐dithiobis(pyridine N ‐oxide), among the most abundant peaks, implying a change in the oxidation state of whether the metal ion or the ligands. In addition, doubly charged ions were recognized as metal adduct ions containing DPTO ligands, [M(DPTO)_n]²⁺. Generation of [M(PTO)₂]^+• / [M(DPTO)]^+• could be traced by CID of [M(DPTO)₂]²⁺, by observation of the sequential losses of a charged (PTO⁺) and a radical (PTO^•) deprotonated pyrithione ligand. The fragmentation pathways of [M(PTO)₂]^+• / [M(DPTO)]^+• were compared among the different metal ions, and some common features were noticed. Density functional theory (DFT) calculations were employed to study the structures of the observed adduct ions, and especially, to decide in the adduct ion [M(PTO)₂]^+• / [M(DPTO)]^+• whether the ligands are 2 deprotonated pyrithiones or a single dipyrithione as well as the oxidation state of the metal ion in the complex. Characterization of gas‐phase pyrithione metal ion complexes becomes important, especially taking into account the presence of a redox‐active ligand in the complexes, because redox state changes that produce new species can have a marked effect on the overall toxicological/biological response elicited by the metal system.     

Efficient Direct Nitrosylation of α-Diimine Rhenium Tricarbonyl Complexes to Structurally Nearly Identical Higher Charge Congeners Activable towards Photo-CO Release

The reaction of rhenium α-diimine (N-N) tricarbonyl complexes with nitrosonium tetrafluoroborate yields the corresponding dicarbonyl-nitrosyl [Re(CO)₂(NO)(N-N)X]⁺ species (where X = halide). The complexes, accessible in a single step in good yield, are structurally nearly identical higher charge congeners of the tricarbonyl molecules. Substitution chemistry aimed at the realization of equivalent dicationic species (intended for applications as potential antimicrobial agents), revealed that the reactivity of metal ion in [Re(CO)₂(NO)(N-N)X]⁺ is that of a hard Re acid, probably due to the stronger π-acceptor properties of NO⁺ as compared to those of CO. The metal ion thus shows great affinity for π-basic ligands, which are consequently difficult to replace by, e.g., σ-donor or weak π-acids like pyridine. Attempts of direct nitrosylation of α-diimine fac-[Re(CO)₃]⁺ complexes bearing π-basic OR-type ligands gave the [Re(CO)₂(NO)(N-N)(BF₄)][BF₄] salt as the only product in good yield, featuring a stable Re-FBF₃ bond. The solid state crystal structure of nearly all molecules presented could be elucidated. A fundamental consequence of the chemistry of [Re(CO)₂(NO)(N-N)X]⁺ complexes, it that the same can be photo-activated towards CO release and represent an entirely new class of photoCORMs.     

Untersuchung der Komplexierung von Na+, K+, Ca2+ und Ba2+ mit einigen elektrisch neutralen Liganden durch Messung von 13C-chemischen Verschiebungen und Spin-Gitter-Relaxationszeiten

Investigation of the complexing of Na⁺, K⁺, Ca²⁺ and Ba²⁺ with some uncharged ligands by ¹³C-chemical shift and spin-lattice relaxation time measurements The influence of Na⁺, K⁺, Ca²⁺ and Ba²⁺ ions on ¹³C chemical shifts and on spin-lattice relaxation times of some electrically neutral ion carriers was investigated. In the solvents CD₃CN and CD₃OD and in presence of an excess of metal ions ligand 4 (see the Scheme) forms complexes of 1:1 stoichiometry. All four oxygen atoms of the ligand as well as solvent molecules take part in the coordination. In CDCl₃ as solvent, for all ions investigated except sodium, only 1:2 complexes (metal/ligand) were observed with 4 . Sodium ions form both 1:1 and 1:2 complexes in this solvent. In the 1:2 complexes of the investigated monovalent ions only one, in those of the divalent ions both amide carbonyl groups of ligand 4 take part in the coordination.     

Untersuchungen zur oxidation des dischwefel-liganden in (η5-C5Me5)M(CO)2S2 (M = Mn,Re)

The halfsandwich compounds (Cp^*M(CO)₂S₂ (M = Mn or Re) are oxidized by 3-chloro-perbenzoic acid at the disulfur ligand to give disulfur monoxide complexes, Cp^*M(CO)₂S₂O¹ Further oxidation leads to Cp^*M(CO)₂S₂O₂. A comparison of the carbonyl stretching frequencies, ν(CO), and derived force constants, k(CO), indicates that sulfur-containing ligands (S₂, SO, SO₂, S₂O, S₂O₂) are stronger electron-withdrawing groups than the ligand cation monoxide, CO. The acceptor capacity increases in the order S₂ < S₂O < S₂O₂.     

Redox-Switchable Behavior of Transition-Metal Complexes Supported by Amino-Decorated N-Heterocyclic Carbenes

The coordination chemistry of the N-heterocyclic carbene ligand IMes^(NMe2)2, derived from the well-known IMes ligand by substitution of the carbenic heterocycle with two dimethylamino groups, was investigated with d⁶ [Mn(I), Fe(II)], d⁸ [Rh(I)], and d¹⁰ [Cu(I)] transition-metal centers. The redox behavior of the resulting organometallic complexes was studied through a combined experimental/theoretical study, involving electrochemistry, EPR spectroscopy, and DFT calculations. While the complexes [CuCl(IMes^(NMe2)2)], [RhCl(COD)(IMes^(NMe2)2)], and [FeCp(CO)₂ (IMes^(NMe2)2)](BF₄) exhibit two oxidation waves, the first oxidation wave is fully reversible but only for the first complex the second oxidation wave is reversible. The mono-oxidation event for these complexes occurs on the NHC ligand, with a spin density mainly located on the diaminoethylene NHC-backbone, and has a dramatic effect on the donating properties of the NHC ligand. Conversely, as the Mn(I) center in the complex [MnCp(CO)₂ ((IMes^(NMe2)2)] is easily oxidizable, the latter complex is first oxidized on the metal center to form the corresponding cationic Mn(II) complex, and the NHC ligand is oxidized in a second reversible oxidation wave.     

Substituierte halogencarbonylmetallate des chroms,Molybdäns und Wolframs : II. Photochemische phosphin-abspaltung,ein neuer weg zu metall (VI A) carbonylderivaten

Irradiation of bis(phosphine) tetracarbonyl complexes L₂M(CO)₄ (M = Cr, Mo, W) in the presence of donor ligands (amine, nitrile, halide ion) leads, via loss of one phosphine ligand, to neutral (LL′M(CO)₄) or ionic ([LM(CO)₄X]^?) metal carbonyl compounds. The use of this reaction as the first step in a general synthesis of unsymmetrically disubstituted derivatives of Group VIA hexacarbonyls is discussed.