Valentin Radtke

Absolute Brønsted Acidities and pH Scales in Ionic Liquids

Although receiving large interest over the last years, some fundamental aspects of Brønsted acidity in ionic liquids (ILs) have up to now been insufficiently highlighted. In this work, standard states, activity, and activity coefficient definitions for IL solvent systems were developed from general thermodynamic considerations and then extended to a general mixed solvent standard state. By using the bromide/bromoaluminate systems as representative ILs, formulae for thermodynamically consistent pH scales for ILs with simple (Br(-) ) and complex ([Aln Br3n+1 ](-) ) anions were derived on the basis of the chemical potential of the proton. Supported by quantum chemical [ccsd(t)/MP2/DFT/COSMO-RS] calculations, Gibbs solvation energies of the proton were calculated, which allowed the ILs to be ranked in absolute acidity, that is, pHabs or μabs (H(+) , IL), and additionally allowed their acidity to be compared with molecular Brønsted acid systems. It was shown that bromoaluminate ILs are suited for reaching superacidic conditions. The complexity of autoprotolysis processes in C6 MIM(+) [AlBr4 ](-) (C6 MIM=1-hexyl-3-methylimidazolium) with or without the addition of basic (i.e. Br(-) ) or acidic (AlBr3 and/or HBr) solutes was examined in detail by model calculations, and they indicated a large thermodynamic influence of small deviations from the exact stoichiometric composition.

[Ni(cod)2][Al(ORF)4], a source for naked nickel(I) chemistry

The straightforward synthesis of the cationic, purely organometallic Ni(I) salt [Ni(cod)2](+)[Al(OR(F))4](-) was realized through a reaction between [Ni(cod)2] and Ag[Al(OR(F))4] (cod = 1,5-cyclooctadiene). Crystal-structure analysis and EPR, XANES, and cyclic voltammetry studies confirmed the presence of a homoleptic Ni(I) olefin complex. Weak interactions between the metal center, the ligands, and the anion provide a good starting material for further cationic Ni(I) complexes.

Electroless, Electrolytic and Galvanic Copper Deposition with the Scanning Electrochemical Microscope (SECM)

Summary The Scanning Electrochemical Microscope (SECM) can be used with different techniques of microstructured copper deposition. A first approach involves the electrolytic copper deposition on noble metals, whereby copper ions are released from a complex by a suitable tip reaction and then reduced on the polarised conducting surface to form a copper microstructure. The second approach is very similar to the first, but does not involve polarising the substrate. It generates a tip-induced microgalvanic cell, the positive electromotoric force of which is constituted by two electrochemical reactions at different areas of the substrate. Finally the electroless copper deposition is performed on nonconducting surfaces like glass or semiconducting surfaces like silicon. This involves locally reducing a suitable precursor film whose surface has been previously immobilised.

Scanning Electrochemical Microscopy as a Versatile Tool for Modifying Surfaces

Abstract Micromodification of surfaces is the introduction of well-defined, directly local changes in the chemical or physical properties of such surfaces. This includes etching processes, deposition of materials and chemical modification of the surface material. As a scanning probe technique, the scanning electrochemical microscope (SECM) is predestined to generate chemical changes locally. Our intention is to demonstrate the effect of local chemical changes on a surface. For this purpose, we first recapitulate the work of our group on micromodification and then deal with galvanic deposition of metals, electroless deposition of conducting polymers and the use of etching to introduce organic features onto an inorganic surface.

Metal Deposition by Inducing a Microgalvanic Cell with the Scanning Electrochemical Microscope (SECM)

A new mechanism of metal deposition on conducting surfaces is presented. But in contrast to former procedures where metal ions were deposited by electrolysis on a conducting cathodically polarised surface, now the deposition occurs without any additional electrochemical energy even if the substrate metal is more noble than the deposited metal. This phenomenon can be explained by the formation of a microgalvanic cell as a consequence of the tip reaction. The process is similar to the effects of a local element well known in corrosion science. The latter one is an undesirable effect due to surface impurities whereas the former one can be targeted on generating microstructures. In this paper, we will show that the mechanism of deposition is congenerous to that one of the positive feedback, since both may occur on a nonpolarised conducting surface. Both are “tip-induced” electrochemical cells with a positive electromotive force.

Basic Remarks on Acidity

This Review provides a unified view on Brønsted acidity. For this purpose, a brief overview of the concepts acidity, acid strengths, and pH value is given, including problems, proposed solutions, and the use of the pHabs /pHabsH2O scale as a unifying concept. Thereafter, some examples of the accessibility and application of unified pHabs values are given. The Review is rounded off with the analogy of acid-base chemistry to redox chemistry with the introduction of the unified redox scale peabs . The combination of pHabs and peabs values in the protoelectric potential map (PPM), as elaborated in ongoing studies on the thermochemistry of single ions, provides a means to classify and to compare all possible acid-base/redox reactions in a medium-independent and, thus, unified fashion.

Synthesis, Characterisation and Reactions of Truly Cationic NiI-Phosphine Complexes

The recently published purely metallo-organic NiI salt [Ni(cod)2 ][Al(ORF )4 ] (1, cod=1,5-cyclooctadiene, RF =C(CF3 )3 ) provides a starting point for a new synthesis strategy leading to NiI phosphine complexes, replacing cod ligands by phosphines. Clearly visible colour changes indicate reactions within minutes, while quantum chemical calculations (PBE0-D3(BJ)/def2-TZVPP) approve exergonic reaction enthalpies in all performed ligand exchange reactions. Hence, [Ni(dppp)2 ][Al(ORF )4 ] (2, dppp=1,3-bis(diphenylphosphino)propane), [Ni(dppe)2 ][Al(ORF )4 ] (3, dppe=1,3-bis(diphenyl-phosphino)ethane), three-coordinate [Ni(PPh3 )3 ][Al(ORF )4 ] (4) and a remarkable two-coordinate NiI phosphine complex [Ni(PtBu3 )2 ][Al(ORF )4 ] (5) were characterised by single crystal X-ray structure analysis. EPR studies were performed, confirming a nickel d9 -configuration in complexes 2, 4 and 5. This result is supported by additional magnetization measurements of 4 and 5. Further investigations by cyclic voltammetry indicate relatively high oxidation potentials for these NiI compounds between 0.7 and 1.7 V versus Fc/Fc+ . Screening reactions with O2 and CO gave first insights on the reaction behaviour of the NiI phosphine complexes towards small molecules with formation of mixed phosphine-CO-NiI complexes and oxidation processes yielding new NiI and/or NiII derivatives. Moreover, 4 reacted with CH2 Cl2 at RT to give a dimeric NiII ylide complex (4 c). As CH2 Cl2 is a rather stable alkyl halide with relatively high C-Cl bond energies, 4 appears to be a suitable reagent for more general C-Cl bond activation reactions.