Hansjörg Grützmacher

Molecular catalysts for hydrogen production from alcohols

An industrially applicable catalytic methodology for dihydrogen formation from a proton source remains at the forefront of all efforts to replace the present fossil fuel economy by a hydrogen economy. This review tries to summarize the achievements which have been made with molecular organometallic complexes as catalysts for the dehydrogenation of alcohols. Biology uses NAD+ as a metal-free hydrogen acceptor which converts with the help of enzymes (alcohol dehydrogenase, aldehyde dehydrogenase) alcohols in carbonyl compounds, NADH, and protons. In the regeneration of NADH to NAD+, electrons are stored in electron transfer enzymes (ferredoxines) which are subsequently used to reduce protons to hydrogen with the help of hydrogenases or nitrogenases which ensures a very low overpotential for the reduction. Man-made organometallic complexes are rather primitive with respect to this complex machinery but use some principles from biology as guide lines. Classical complexes like rhodium or ruthenium phosphane complexes achieve at best a few thousands of turn over frequencies (TOFs). Established reactions like oxidative addition of the hydroxyl group of the substrate to the metal centre, β-hydrogen elimination from the α-CH group of the coordinated alcohol, product dissociation, and reductive elimination of hydrogen are involved in the proposed catalytic cycles. Complexes which show metal–ligand cooperativity show a significantly better performance with respect to turn over frequencies (conversion rate = activity) and turn over numbers (number of product molecules per catalyst molecule = efficiency). In these catalytic systems, the alcohol substrate is converted with the help of active centres in the ligand backbone which participate directly and reversibly in the transformation of the substrate. Present results indicate that dehydrogenative coupling reactions of the type, R–CH2–OH + XH → RCOX + 2H2, proceed especially well and can be applied to a wide range of substrates including multiple dehydrogenative couplings leading to polyesters or polyamides. In photocatalytic conversions, alcohols can be deoxygenated to hydrocarbons, CO, and H2 which should be further explored in the future. New developments consist of the construction of organometallic fuel cells (OMFCs) where the anode is composed of molecular catalysts embedded into a conducting support material. Here no free hydrogen is evolved but it is directly converted to electric current and protons according to H2 → 2H+ + 2e. The review focuses on the catalysis with organometallic complexes but lists some selected results obtained with heterogeneous catalytic systems for comparison.

Synthesis and Characterization of Terminal [Re(XCO)(CO)2(triphos)] (X=N, P): Isocyanate versus Phosphaethynolate Complexes

The terminal rhenium(I) phosphaethynolate complex [Re(PCO)(CO)(2)(triphos)] has been prepared in a salt metathesis reaction from Na(OCP) and [Re(OTf)(CO)(2)(triphos)]. The analogous isocyanato complex [Re(NCO)(CO)(2)(triphos)] has been likewise prepared for comparison. The structure of both complexes was elucidated by X-ray diffraction studies. While the isocyanato complex is linear, the phosphaethynolate complex is strongly bent around the pnictogen center. Computations including natural bond orbital (NBO) theory, natural resonance theory (NRT), and natural population analysis (NPA) indicate that the isocyanato complex can be viewed as a classic Werner-type complex, that is, with an electrostatic interaction between the Re(I) and the NCO group. The phosphaethynolate complex [Re(P=C=O)(CO)(2)(triphos)] is best described as a metallaphosphaketene with a Re(I)-phosphorus bond of highly covalent character.

Redox‐Triggered Reversible Interconversion of a Monocyclic and a Bicyclic Phosphorus Heterocycle

Molecules which change their structures significantly and reversibly upon an oxidation or reduction process have potential as future components of smart materials. A prerequisite for such an application is that the molecules should undergo the redox-coupled transformation within a reasonable electrochemical window and lock into stable redox states. Sodium phosphaethynolate reacts with two equivalents of dicyclohexylcarbodiimide (DCC) to yield an anionic, imino-functionalized 1,3,5-diazaphosphinane [3u2009a](-). The oxidation of this anion with elemental iodine causes an intramolecular rearrangement reaction to give a bicyclic 1,3,2-diazaphospholenium cation [6](+). This umpolung of electronic properties from non-aromatic to highly aromatic is reversible, and the cation [6](+) is reduced with elemental magnesium to reform the 1,3,5-diazaphosphinanide anion [3u2009a](-). Theoretical calculations suggest that phosphinidene species are involved in the rearrangement processes.

Elucidating the Thermal Decomposition of Dimethyl Methylphosphonate by Vacuum Ultraviolet (VUV) Photoionization: Pathways to the PO Radical, a Key Species in Flame‐Retardant Mechanisms

The production of phosphoryl species (PO, PO2, HOPO) is believed to be of great importance for efficient flame-retardant action in the gas phase. We present a detailed investigation of the thermal decomposition of dimethyl methylphosphonate (DMMP) probed by vacuum ultraviolet (VUV) synchrotron radiation and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. This technique provides a snapshot of the thermolysis process and direct evidence of how the reactive phosphoryl species are generated during heat exposure. One of the key findings of this work is that only PO is formed in high concentration upon DMMP decomposition, whereas PO2 is absent. It can be concluded that the formation of PO2 needs an oxidative environment, which is typically the case in a real flame. Based on the identification of products such as methanol, formaldehyde, and PO, as well as the intermediates O=P-CH3, H2C=P-OH, and H2C=P(=O)H, supported by quantum chemical calculations, we were able to describe the predominant pathways that lead to active phosphoryl species during the thermal decomposition of DMMP.

Low-valent iron(i) amido olefin complexes as promotors for dehydrogenation reactions.

Fe(I) compounds including hydrogenases show remarkable properties and reactivities. Several iron(I) complexes have been established in stoichiometric reactions as model compounds for N2 or CO2 activation. The development of well-defined iron(I) complexes for catalytic transformations remains a challenge. The few examples include cross-coupling reactions, hydrogenations of terminal olefins, and azide functionalizations. Here the syntheses and properties of bimetallic complexes [MFe(I) (trop2 dae)(solv)] (M=Na, solv=3u2009thf; M=Li, solv=2u2009Et2 O; trop=5H-dibenzo[a,d]cyclo-hepten-5-yl, dae=(N-CH2 -CH2 -N) with a d(7) Fe low-spin valence-electron configuration are reported. Both compounds promote the dehydrogenation of N,N-dimethylaminoborane, and the former is a precatalyst for the dehydrogenative alcoholysis of silanes. No indications for heterogeneous catalyses were found. High activities and complete conversions were observed particularly with [NaFe(I) (trop2 dae)(thf)3 ].

Simple One-Pot Syntheses of Water-Soluble Bis(acyl)phosphane Oxide Photoinitiators and Their Application in Surfactant-Free Emulsion Polymerization

The sodium salt of the new bis(mesitoyl)phosphinic acid (BAPO-OH) can be prepared in a very efficient one-pot synthesis. It is well soluble in water and hydrolytically stable for at least several weeks. Remarkably, it acts as an initiating agent for the surfactant-free emulsion polymerization (SFEP) of styrene to yield monodisperse, spherical nanoparticles. Time-resolved electron paramagnetic resonance (TR-EPR) and chemically induced electron polarisation (CIDEP) indicate preliminary mechanistic insights.

N-Heterocyclic Carbenes as Promotors for the Rearrangement of Phosphaketenes to Phosphaheteroallenes: A Case Study for OCP to OPC Constitutional Isomerism.

The concept of isomerism is essential to chemistry and allows defining molecules with an identical composition but different connectivity (bonds) between their atoms (constitutional isomers) and/or a different arrangement in space (stereoisomers). The reaction of phosphanyl ketenes, (NHP)-P=C=O (NHP=N-heterocyclic phosphenium) with N-heterocyclic carbenes (NHCs) leads to phosphaheteroallenes (NHP)-O-P=C=NHC in which the PCO unit has been isomerized to OPC. Based on the isolation of several intermediates and DFT calculations, a mechanism for this fundamental isomerisation process is proposed.

Snowballing Radical Generation Leads to Ultrahigh Molecular Weight Polymers

Styrene is the classical monomer obeying zero-one kinetics in radical emulsion polymerization. Accordingly, particles that are less than 100 nm in diameter contain either one or no growing radical(s). We describe a unique photoinitiated polymerization reaction accelerated by snowballing radical generation in a continuous flow reactor. Even in comparison to classical emulsion polymerization, these unprecedented snowballing reactions are rapid and high-yielding, with each particle simultaneously containing more than one growing radical. This is a consequence of photoinitiator incorporation into the nascent polymer backbone and repeated radical generation upon photo-irradiation.

Improvement in the efficiency of an OrganoMetallic Fuel Cell by tuning the molecular architecture of the anode electrocatalyst and the nature of the carbon support

The electrooxidation of ethanol to acetate is achieved with Rh(I) diolefin amine complexes of the general formula [Rh(Y)(trop2NH)(L)] (L = PPh3, P(4-n-BuPh)3; Y = triflate, acetate; Bu = butyl) in direct alcohol fuel cells that have the peculiarity of containing a molecular anode electrocatalyst and, hence, are denoted as OrganoMetallic Fuel Cells (OMFCs). Changing the carbon black support from Vulcan XC-72 (Cv) to Ketjenblack EC 600JD (Ck) and/or the axial phosphane to produce non crystalline complexes has been found to remarkably change the electrochemical properties of the organorhodium catalysts, especially in terms of specific activity and durability. An in-depth study has shown that either Ck or P(4-n-butylPh)3 favour the formation of an amorphous Rh-acetato phase on the electrode, leading to a much more efficient and recyclable catalyst as compared to a crystalline Rh-acetate complex which is formed on Cv with PPh3 as the ligand. The ameliorating effect of the amorphous phase has been ascribed to its higher number of surface complex molecules as compared to the crystalline phase. A specific activity as high as 10u2006000 A gRh−1 has been found in a half cell, which is the highest value ever reported for ethanol electrooxidation.

Energy and Chemicals from the Selective Electrooxidation of Renewable Diols by Organometallic Fuel Cells

Organometallic fuel cells catalyze the selective electrooxidation of renewable diols, simultaneously providing high power densities and chemicals of industrial importance. It is shown that the unique organometallic complex [Rh(OTf)(trop2NH)(PPh3)] employed as molecular active site in an anode of an OMFC selectively oxidizes a number of renewable diols, such as ethylene glycol , 1,2-propanediol (1,2-P), 1,3-propanediol (1,3-P), and 1,4-butanediol (1,4-B) to their corresponding mono-carboxylates. The electrochemical performance of this molecular catalyst is discussed, with the aim to achieve cogeneration of electricity and valuable chemicals in a highly selective electrooxidation from diol precursors.