Torsten Beweries

Five-membered metallacycles of titanium and zirconium – attractive compounds for organometallic chemistry and catalysis

In these days a renaissance of metallacycles as an increasingly important class of organometallic compounds for synthetic and catalytic applications is evident, making such very attractive for a plethora of investigations. Titanocene and zirconocene bis(trimethylsilyl)acetylene complexes, regarded as three-membered metallacycles (1-metallacyclopropenes), present a rich chemistry towards unsaturated molecules. By elimination of the alkyne these complexes form by reaction with unsaturated compounds five-membered titana- and zirconacycles, all of which are relevant to stoichiometric and catalytic C-C coupling and cleavage reactions of unsaturated molecules.

Energetics of Halogen Bonding of Group 10 Metal Fluoride Complexes

A study is presented of the thermodynamics of the halogen-bonding interaction of C(6)F(5)I with a series of structurally similar group 10 metal fluoride complexes trans-[Ni(F)(2-C(5)NF(4))(PCy(3))(2)] (2), trans-[Pd(F)(4-C(5)NF(4))(PCy(3))(2)] (3), trans-[Pt(F){2-C(5)NF(2)H(CF(3))}(PR(3))(2)] (4a, R = Cy; 4bR = iPr) and trans-[Ni(F){2-C(5)NF(2)H(CF(3))}(PCy(3))(2)] (5a) in toluene solution. (19)F NMR titration experiments are used to determine binding constants, enthalpies and entropies of these interactions (2.4 ≤ K(300) ≤ 5.2; -25 ≤ ΔH(o) ≤ -16 kJ mol(-1); -73 ≤ ΔS(o) ≤ -49 J K(-1) mol(-1)). The data for -ΔH(o) for the halogen bonding follow a trend Ni < Pd < Pt. The fluoropyridyl ligand is shown to have a negligible influence on the thermodynamic data, but the influence of the phosphine ligand is significant. We also show that the value of the spin-spin coupling constant J(PtF) increases substantially with adduct formation. X-ray crystallographic data for Ni complexes 5a and 5c are compared to previously published data for a platinum analogue. We show by experiment and computation that the difference between Pt-X and Ni-X (X = F, C, P) bond lengths is greatest for X = F, consistent with F(2pπ)-Pt(5dπ) repulsive interactions. DFT calculations on the metal fluoride complexes show the very negative electrostatic potential around the fluoride. Calculations of the enthalpy of adduct formation show energies of -18.8 and -22.8 kJ mol(-1) for Ni and Pt complexes of types 5 and 4, respectively, in excellent agreement with experiment.

Reactions of group 4 metallocene alkyne complexes with carbodiimides: experimental and theoretical studies of the structure and bonding of five-membered hetero-metallacycloallenes.

The reaction of the low-valent metallocene(II) sources Cp(2)Ti(η(2)-Me(3)SiC(2)SiMe(3)) (7) and Cp(2)Zr(py)(η(2)-Me(3)SiC(2)SiMe(3)) (11, Cp = η(5)-cyclopentadienyl, py = pyridine) with carbodiimides RN═C═NR (R = Cy, i-Pr, p-Tol) leads to the formation of five membered hetero-metallacycloallenes Cp(2)M{Me(3)SiC═C═C[N(SiMe(3))(R)]-N(R)} (9M-R) (M = Ti, R = i-Pr; M = Zr, R = Cy, i-Pr, p-Tol). Elimination of the alkyne (as the hitherto known reactivity of titanocene and zirconocene alkyne complexes would suggest) was not observed. The molecular structures of the obtained complexes were confirmed by X-ray studies. Moreover, the structure and bonding of the complexes 9Zr-Cy and 9Zr-p-Tol was investigated by DFT calculations.

Recent advances in the chemistry of heterometallacycles of group 4 metallocenes

This perspective gives an overview of recent developments in the chemistry of heterometallacycles of group 4 metallocenes. This class of compounds has been known for several decades now, however, in recent years significant efforts were made in order to further explore the structural scope of these complexes and their potential in catalytic applications. This overview shows some special heterometallacycles which could be of great interest for theoretical and synthetic chemists, owing to their unusual structural and bonding parameters as well as their applicability in a wide range of synthetic and catalytic transformations.

Catalytic and Kinetic Studies of the Dehydrogenation of Dimethylamine Borane with an iPr Substituted Titanocene Catalyst

In recent years, amine borane adducts have been widely discussed as promising candidates for the storage of hydrogen, owing mainly to the high gravimetric capacity (ideally up to 19.6 % in ammonia borane) and the facile release of the energy carrier under mild conditions. A significant amount of work has been reported towards the catalytic dehydrogenation of such compounds using transition metal complexes, as well as in the efficient regeneration of spent fuels. However, some of the main drawbacks for general applicability remain, such as the production of amines and boranes using environmentally benign processes and the fact that most amine borane adducts are solid and thus cannot serve as a liquid “fuel” without dilution and loss of hydrogen storage capacity. In this context, Baker et al. reported on the use of amine borane fuel blends, which remain liquid throughout the catalytic process. Moreover, addition of an ionic liquid hindered the formation of insoluble dehydrogenation products such as linear poly(aminoborane). The use of early transition metal complexes, especially of group 4 metals for the catalytic dehydrogenation of dimethylamine borane (1) was demonstrated by Manners and Chirik and co-workers for titanocene and zirconocene compounds. A study of dimethylamine borane dehydrogenation using cationic zirconocene-phosphinoaryloxide complexes was published by Wass et al. and revealed high activities (TOF up to 600 h ) and reaction times of only several minutes. Very recently, our group reported on the efficient dehydrogenation of dimethylamine borane using well-defined metallocene bis(trimethylsilyl)acetylene complexes of the type [Cp’2M(L)(h Me3SiC2SiMe3)] [M = Ti, Cp’= cyclopentadienyl (Cp) or pentamethylcyclopentadienyl (Cp*), no L; M = Zr, Cp’= Cp, L = pyridine; M = Zr, Cp’= Cp*, no L] , which eliminate the alkyne under mild conditions and thus generate the catalytically active 14 electron fragment “Cp2M”. [7] It became evident that complexes bearing Cp ligands are active whereas fully methylated Cp* species are completely inactive for dehydrogenation reactions. However, a full study of the influence of steric and electronic parameters of substituents at the Cp ligands is not available to date. The titanocene bis(trimethylsilyl)acetylene complex [(h-iPrC5H4)2Ti(h -Me3SiC2SiMe3)] (2) appeared to be worth investigating for the catalytic dehydrogenation of dimethylamine borane (Scheme 1), as it possesses the iPr group as a single sterically demanding yet moderately electron donating substituent. Complex 2 was first described by our group several years ago and reported to be isolable as a dark yellow oily liquid.

Metallacycles: Breaking the rules

Chemists have long been interested in synthesizing compounds that push the boundaries of conventional molecular structure. Incorporating metal centres into the ring unit of highly strained and unsaturated cyclic molecules can help reduce strain — a tactic that has now been used to render a previously inaccessible metallapentalyne isolable.

Photoassisted TiO Activation in a Decamethyltitanocene Dihydroxido Complex: Insights into the Elemental Steps of Water Splitting†

One of the major challenges for mankind concerns power supply. Renewable forms of energy are being investigated intensively as an alternative to the fossil resources commonly used nowadays. As all types of such renewable energies (except geothermal and tidal power) originate in the solar radiation, direct utilization of this ubiquitous sustainable energy source appears to be most reasonable. Apart from photovoltaic electricity production, for example, by Gr tzel cells, the conversion of sunlight into chemical energy is a very promising research topic. After the discovery of the Honda–Fujishima effect, which describes the photoassisted generation of dihydrogen and dioxygen from water using a TiO2/Pt electrode array, [3]

An Intermolecular Heterobimetallic system for Photocatalytic Water Reduction

Teamwork: A new intermolecular heterobimetallic system for photocatalytic water reduction, consisting of a photosensitizer of the type [Ru(bpy)(2)(L)](PF(6))(2) (L=bidentate ligand), a dichloro palladium complex PdCl(2)(L) serving as the water reduction catalyst, and triethyl amine as electron donor, is presented. Variations of the ligand as well as of the palladium source results in a significant improvement of the performance of the catalyst system.

The Contrasting Character of Early and Late Transition Metal Fluorides as Hydrogen Bond Acceptors

The association constants and enthalpies for the binding of hydrogen bond donors to group 10 transition metal complexes featuring a single fluoride ligand (trans-[Ni(F)(2-C5NF4)(PR3)2], R = Et 1a, Cy 1b, trans-[Pd(F)(4-C5NF4)(PCy3)2] 2, trans-[Pt(F){2-C5NF2H(CF3)}(PCy3)2] 3 and of group 4 difluorides (Cp2MF2, M = Ti 4a, Zr 5a, Hf 6a; Cp*2MF2, M = Ti 4b, Zr 5b, Hf 6b) are reported. These measurements allow placement of these fluoride ligands on the scales of organic H-bond acceptor strength. The H-bond acceptor capability β (Hunter scale) for the group 10 metal fluorides is far greater (1a 12.1, 1b 9.7, 2 11.6, 3 11.0) than that for group 4 metal fluorides (4a 5.8, 5a 4.7, 6a 4.7, 4b 6.9, 5b 5.6, 6b 5.4), demonstrating that the group 10 fluorides are comparable to the strongest organic H-bond acceptors, such as Me3NO, whereas group 4 fluorides fall in the same range as N-bases aniline through pyridine. Additionally, the measurement of the binding enthalpy of 4-fluorophenol to 1a in carbon tetrachloride (-23.5 ± 0.3 kJ mol(-1)) interlocks our study with Laurences scale of H-bond basicity of organic molecules. The much greater polarity of group 10 metal fluorides than that of the group 4 metal fluorides is consistent with the importance of pπ-dπ bonding in the latter. The polarity of the group 10 metal fluorides indicates their potential as building blocks for hydrogen-bonded assemblies. The synthesis of trans-[Ni(F){2-C5NF3(NH2)}(PEt3)2], which exhibits an extended chain structure assembled by hydrogen bonds between the amine and metal-fluoride groups, confirms this hypothesis.

Design of BODIPY Dyes as Photosensitisers in Multicomponent Catalyst Systems for Light-Driven Hydrogen Production

A study of visible-light-driven hydrogen production using a multicomponent system consisting of different boron dipyrromethene (BODIPY) dyes, triethylamine and [{Pd(PPh3)Cl2}2] from THF/water mixtures is presented. A trio of meso-mesityl BODIPY dyes display the best activities and long-term stabilities of more than ten days with the 2,6-diiodo derivative showing the best performance.