Christian Ganter

Chiral organometallic half-sandwich complexes with defined metal configuration

Chiral organometallic half-sandwich complexes with stereogenic metal atoms are close relatives of chiral organic compounds with stereogenic carbon atoms. Similarities and differences between these two classes of compounds are outlined. Some representative metal complexes are discussed in an introductory section followed by a more detailed treatment of the available strategies to control the metal configuration by means of chiral auxiliaries. Special sections are devoted to the discussion of the configurational stability of chiral-at-metal complexes and their applications in stoichiometric and catalytic stereoselective reactions.

Expanding the Chemistry of Cationic N-Heterocyclic Carbenes: Alternative Synthesis, Reactivity, and Coordination Chemistry

A new synthetic route to complexes of the cationic N-heterocyclic carbene ligand 2 has been developed by the attachment of a cationic pentamethylcyclopentadienylruthenium ([RuCp*](+)) fragment to a metal-coordinated benzimidazol-2-ylidene ligand. The coordination chemistry and the steric and electronic properties of the cationic carbene were investigated in detail by experimental and theoretical methods. X-ray structures of three carbene-metal complexes were determined. The cationic ligand 2 is a poorer overall electron donor relative to the related neutral carbene, which is evident from cyclic voltammetry (CV) and IR measurements.

ENANTIOMERICALLY PURE PHOSPHAFERROCENES WITH PLANAR CHIRALITY

Abstract Enantiomerically pure 2-formyl-3,4-dimethylphosphaferrocene 1 was prepared straightforwardly by resolution of rac-1 via column chromatography on silica of diastereomeric aminals formed of 1 and (R),(R)-1,2-di(N-methylamino)cyclohexane2. Both enantiomers (S)- 1 and (R)- 1 are obtained in 94% yield with ee>99%. The amine 2 is recycled in 89% yield. The absolute configuration of (S)- 1 has been determined by X-ray crystallography.

Hybrid Ligands with N‐Heterocyclic Carbene and Chiral Phosphaferrocene Components

N-Heterocyclic carbenes (NHCs) possessing one or two 3,4-dimethylphosphaferrocenyl substituents and either methylene or ethylene alkyl bridges have been prepared. These carbenes turned out to be remarkably stable and were characterized by NMR methods and partly by mass spectrometry. Their molybdenum and ruthenium complexes were examined in order to determine the electronic properties and the coordination behaviour of these chiral PC- and PCP-chelate ligands, which combine a NHC unit as a strong sigma-donor with pi-accepting phosphaferrocene moieties. Crystal structures of one ligand precursor and of three complexes have been determined.

Tuning the electronic properties of an N-heterocyclic carbene by charge and mesomeric effects

N-Methylation of a pyridoimidazolium salt and subsequent deprotonation afford a cationic NHC ligand. The spectroscopic characteristics of its Rh and Ir metal complexes reveal the reduced donor and enforced π-acceptor behaviour of this ligand system.

Weakly-coordinated stable platinum nanocrystals

Stable platinum nanocrystals with small diameters (1.0–2.3 nm), that are stable for a long time, with narrow size distributions were easily and reproducibly prepared without any additional stabilizers in glycol, glycerol, the ionic liquids (ILs) 1-n-butyl-3-methyl-imidazolium tetrafluoroborate [BMIm][BF4] and N-butyl-N-trimethyl-ammonium bis(trifluoromethylsulfonyl)imide [N4111][NTf2] or diphenylmethane (CH2Ph2) by thermal, photolytic or microwave assisted decomposition of the organometallic precursor methylcyclopentadienyl-trimethylplatinum(IV), (MeCp)PtMe3. Decomposition of the easily dispensible, air and moisture stable organometallic Pt(IV) precursor (MeCp)PtMe3 leads to well defined, small, crystalline and longterm stable Pt-nanoparticle (Pt-NP) dispersions without any additional surface-capping ligands. The Pt-NP/IL dispersion was shown to be a highly active catalyst (TOF 96 000 h−1 at 0.0125 mol% Pt and quantitative conversion) for the biphasic hydrosilylation of phenylacetylene with triethylsilane, to the distal and proximal products triethyl(2- and1-phenylvinyl)silane.

Fulvene-Like Cationic Phosphaferrocene Species as Synthetically Valuable Intermediates: Preparative and Mechanistic Aspects of the Diastereoselective Formation ofα-Phosphanyl-Substituted 2-Ethylphosphaferrocenes

The slow rate of isomerization of cationic phosphaferrocene species (E)-1 (easily prepared from the parent alcohol) to the more stable (Z)-1 provides selective access to both possible diastereomeric products by addition of nucleophiles from a direction trans to the CpFe moiety. Configurational assignment of the species involved is made on the basis of NMR data and has been confirmed by X-ray structural analysis.

Tri- and tetra-nuclear µ-alkyne clusters from [Ru2(µ-CO)(µ-C2R2)(η-C5H5)2](R = Ph or CF3)

The unsaturated RuRu double-bonded complexes [Ru2(µ-CO)(µ-C2R2)(η-C5H5)2](R = Ph 1a or CF31b) react at room temperature with [Fe2(CO)9] to produce the new mixed-metal trinuclear complexes [Ru2Fe(CO)3(µ-CO)(µ3-CO)(µ3-C2R2)(η-C5H5)2](R = Ph 2 or CF33) in good yield. Similarly, complexes 1a and 1b react with [Ru(CO)4(C2H4)] at room temperature to afford the analogous triruthenium complexes [Ru3(CO)3(µ-CO)(µ3-CO)(µ3-C2R2)(η-C5H5)2](R = Ph 4 or CF35). The new complexes exist in two geometric forms which can be regarded as ‘rotamers’, differing only in the metal atom to which the alkyne ligand is π bound. The structure of complex 2 has been determined by X-ray diffraction and shows a triangular metal core with a triply bridging alkyne ligand bonding parallel to the (C5H5)Ru–Ru(C5H5) edge, π bound to the Fe(CO)3 unit, giving the molecule a plane of symmetry. The major isomers of 3 and 4 and the minor isomer of 5 also have this structure. An X-ray diffraction study on the major isomer of the latter, 5b, revealed that this adopts the other geometric form which has the alkyne π bound to a Ru(C5H5) unit, with no plane of symmetry. The rotation of the alkyne relative to the {M(C5H5)}2M(CO)3 triangle is accompanied by a shift of the µ-CO ligand to bridge a {M(C5H5)}M(CO)3 edge of the M3 triangle, i.e. as in the other isomer, the edge opposite the π-bound metal is bridged. A similar structure is observed for the minor isomers of 3 and 4. Fluxional interconversion was observed for the isomers of 4 at elevated temperatures. Reaction of 1a with [Co2(CO)8] at room temperature gives the tetranuclear cluster [Ru2Co2(CO)4(µ3-CO)2(µ4-C2Ph2)(η-C5H5)2]6 as one of the products. An X-ray diffraction study showed that this 60-electron complex has a closo-Co2Ru2C2 octahedral core with each CoRu2 face capped by a µ3-CO ligand. Complex 6 can also be regarded as a ‘butterfly’ type structure with the Co atoms at the wing-tips and the diphenylethyne ligand bridging all four metal atoms.

Phosphaferrocenes Containing the Chiral Pinene-Fused Cyclopentadienyl Ligand PCp

Abstract The chiral PCp-substituted phosphaferrocene 7 was prepared from the dimeric iron carbonyl complex [PCpFe(CO)2]2 (5) and t-butylphosphole (PCp=pinene-fused cyclopentadienyl). In 7, the PCp ligand is coordinated to the iron atom via its exo-side as shown by X-ray crystallography. Formylation of the sandwich complex 7 leads to a mixture of the diastereomeric aldehydes 10a,b in an approximate ratio of 2:1. As a side-product of the synthesis of 7, the homoleptic ferrocene (PCp)2Fe (8) was obtained by a ring ligand transfer reaction. Complex 8 is formed as a single C2-symmetrical isomer.

First N‐Heterocyclic Carbenes Relying on the Triazolone Structural Motif: Syntheses, Modifications and Reactivity

4-Phenylsemicarbazide and 1,5-diphenylcarbazide are suitable starting materials for the syntheses of N-heterocyclic carbene (NHC) compounds with new backbone structures. In the first case, cyclisation and subsequent methylation leads to a cationic precursor whose deprotonation affords the triazolon-ylidene 2, which was converted to the corresponding sulfur and selenium adducts and a range of metal complexes. In contrast, cyclisation of diphenylcarbazide affords a neutral betain-type NHC-precursor 7, which is not in equilibrium with its carbene tautomer 7a. Precursor 7 can either be deprotonated to give the anionic NHC 8 or methylated at the N or O atom of the backbone resulting in two isomeric cationic species 16 and 20. Deprotonation of the latter two provides neutral NHC compounds with a carboxamide or carboximidate backbone, respectively. The ligand properties of the new NHC compounds were evaluated by IR and (77) Se NMR spectroscopy. Tolman electronic parameter (TEP) values range from 2050 to 2063 cm(-1) with the anionic NHC 8 being the best overall donor.