Cyclopalladation in the Periphery of a NHC Ligand as the Crucial Step in the Synthesis of Highly Active Suzuki-Miyaura Cross-Coupling Catalysts.

Starting from 2,4-dichloropyrimidine, 4-(2-dialkylamino)pyrimidinyl functionalized mesitylimidazolium chlorides are accessible in a five-step reaction sequence. Two routes leading to palladium NHC complexes derived from these ligands have been worked out: By transmetalation with the corresponding NHC-AgCl complexes, C,N-coordinated palladium(II) complexes can be obtained. Treatment of palladium dichloride with the imidazolium salts in pyridine and in the presence of K2 CO3 gives cyclometalated and thus C,C-coordinated compounds. The reactivities of all these compounds were investigated in detail as well as their performance in the catalytic Suzuki-Miyaura cross-coupling reaction. It turned out that the C,C-coordinated derivatives exhibit high catalytic activities in the coupling of arylboronic acids with aryl chlorides, which is consistent with the generally accepted mechanistic ideas on substrate activation.

Exploring the Gas-Phase Activation and Reactivity of a Ruthenium Transfer Hydrogenation Catalyst by Experiment and Theory in Concert.

This study elucidates structures, activation barriers, and the gas-phase reactivity of cationic ruthenium transfer hydrogenation catalysts of the structural type [(η6-cym)RuX(pympyr)]+. In these complexes, the central ruthenium(+II) ion is coordinated to an η6-bound p-cymene (η6-cym), a bidentate 2-R-4-(2-pyridinyl)pyrimidine ligand (pympyr) with R = NH2 or N(CH3)2, and an anion X = I-, Br-, Cl-, or CF3SO3-. We present infrared multiple-photon dissociation (IR-MPD) spectra of precursors (before HCl loss) and of activated complexes (after HCl loss), which elucidates C-H activation as the key step in the activation mechanism. A resonant two-color IR-MPD scheme serves to record several otherwise "dark" bands and enhances the validity of spectral assignments. We show that collision-induced dissociation (CID)-derived activation energies of the [(η6-cym)RuX(pympyr)]+ (R = N(CH3)2) complexes depend crucially on the anion X. The obtained activation energies for the HX loss correlate well with quantum chemical activation barriers and are in line with the HSAB concept. We further elucidate the reaction of the activated complexes with D2 under single-collision conditions. Quantum mechanical simulations substantiate that the resulting species represent analogues for hydrido intermediates formed after abstraction of H+ and H- from isopropanol, as postulated for the catalytic cycle of transfer hydrogenation by us before.

Infrared spectroscopy of N2 adsorption on size selected cobalt cluster cations in isolation.

We report IR active N2 stretching frequencies in isolated and size selected cobalt cluster nitrogen adsorbate complexes, [Con(N2)1](+) as recorded by virtue of InfraRed Photon Dissociation (IRPD) spectroscopy. The observed frequencies of the [Con(N2)1](+) complexes (n = 8-17) are significantly redshifted (2180 to 2290 cm(-1)) with respect to the IR inactive vibrations of free N2 (2359 cm(-1)). These bands are assigned to a µ1 head-on type of coordination of the N2 to the cobalt cluster surface, revealing remarkable cluster size dependent features to interpret.

Multistate-Mediated Rearrangements and FeCl2 Elimination in Dinuclear FePd Complexes.

Mass spectrometric, spectroscopic, and computational characterization of a novel bifunctional iron-palladium complex proves a change of coordination upon solvation. Collisional excitation reveals FeCl2 and HCl elimination in a solvent-modulated competition. Hereby, syn and anti isomers, identified by theoretical calculations, favor and disfavor FeCl2 elimination, respectively. The FeCl2 elimination likely proceeds by chlorido and Cp ligand exchange among the metallic centers in a concerted, ballet-like manner. A multitude of stationary points were identified along the computed multistep reaction coordinates of the three conceivable spin states. The quintet state shows a static Jahn-Teller type relaxation by a tilt away of the Cp ligand at the iron center. The direct singlet-quintet spin crossover is an unprecedented assumption, leaving behind the triplet state as a spectator without involvement. The FeCl2 elimination would decrease catalytic activity. It is kinetically hindered within a range of applicable temperatures in conceivable technical applications.

Two-color delay dependent IR probing of torsional isomerization in a [AgL1L2]⁺ complex.

Two-color infrared multiple photon dissociation (2c-IR-MPD) spectroscopy with delayed pulses indicates a torsional isomerization in a "ligand-metal-chelate" complex [AgL1L2](+). Ab initio calculations reveal the torsional barrier as well as the change in vibrational frequencies and IR intensities along the isomerization pathway. The current approach bears prospects for further elucidation of competing interactions within naked or microsolvated complexes in gas phase coordination chemistry.

Heterobimetallic cuprates consisting of a redox-switchable, silicon-based metalloligand: synthesis, structures, and electronic properties.

A series of bimetallic silyl halido cuprates consisting of the new tripodal silicon-based metalloligand [κ(3)N-Si(3,5-Me2pz)3Mo(CO)3](-) is presented (pz = pyrazolyl). This metalloligand is straightforwardly accessible by reacting the ambidentate ligand tris(3,5-dimethylpyrazolyl)silanide ({Si(3,5-Me2pz)3}(-)) with [Mo(CO)3(η(6)-toluene)]. The compound features a fac-coordinated tripodal chelating ligand and an outward pointing, "free" pyramidal silyl donor, which is easily accessible for a secondary coordination to other metal centers. Several bimetallic silyl halido cuprates of the general formula [CuX{µ-κ(1)Si:κ(3)N-Si(3,5-Me2pz)3Mo(CO)3}](-) (X = Cl, Br, I) have been synthesized. The electronic and structural properties of these complexes were probed in detail by X-ray diffraction analysis, electrospray mass spectrometry, infrared-induced multiphoton dissociation studies, cyclic voltammetry, spectroelectrochemistry, gas-phase photoelectron spectroscopy, as well as UV/Vis and fluorescence spectroscopy. The heterobimetallic complexes contain linear two-coordinate copper(I) entities with the shortest silicon-copper distances reported so far. Oxidation of the anionic complexes in methylene chloride and acetonitrile solutions at E(1/2)(0( = -0.60 and -0.44 V (vs. ferrocene/ferrocenium (Fc/Fc(+))), respectively, shows substantial reversibility. Based on various results obtained from different characterization methods, as well as density functional theory calculations, these oxidation events were attributed to the Mo(0)/Mo(I) redox couple.

Mechanistic Studies on Ruthenium(II)-Catalyzed Base-Free Transfer Hydrogenation Triggered by Roll-Over Cyclometalation.

The synthesis of 2-substituted pyridine-pyrimidine ligands and their complexation with arene ruthenium(II) chloride moieties is reported. Depending on the electronic and steric influences of the ligand, the catalysts undergo CH activation by roll-over cyclometalation. This process opens up the route to the catalytic transfer hydrogenation of ketones with isopropanol as the hydrogen source under base-free and mild conditions. Barriers related to the roll-over cyclometalation process can be determined experimentally by collision-induced dissociation ESI mass spectrometry. They are supported by DFT calculations and allow the classification of the ligands according to their electronic and steric properties, which is also in accordance with critical bond parameters derived from X-ray structure data. DFT calculations furthermore reveal that the formation of a ruthenium(II) hydrido species is plausible through ß-hydride elimination from isopropanol.