Gold carbonyl cations and beyond: homoleptic gold(I) complexes with P4 and P4S3 and the half-sandwich cation [Au(C6H6)(CO)]. Chem Commun (Camb) 2024;60:5403-5406. [PMID: 38682872 DOI: 10.1039/d4cc01374c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Oxidation of Au0 with the synergistic Ag+/0.5 I2 system in the commercial organic solvent 1,2,3,4-tetrafluorobenzene led to the perfluoroalkoxyaluminate salt of the [Au(CO)2]+ cation known from superacid chemistry. This [Au(CO)2]+ salt proved to be an excellent 'naked' Au+-synthon yielding complex salts with [Au(η2-P4)2]+, [Au(η1-P4S3)2]+ and half-sandwich [Au(η2-C6H6)(CO)]+ cation.

The Unified Redox Scale for All Solvents: Consistency and Gibbs Transfer Energies of Electrolytes from their Constituent Single Ions

We have devised the unified redox scale Eabs H2O , which is valid for all solvents. The necessary single ion Gibbs transfer energy between two different solvents, which only can be determined with extra-thermodynamic assumptions so far, must clearly satisfy two essential conditions: First, the sum of the independent cation and anion values must give the Gibbs transfer energy of the salt they form. The latter is an observable and measurable without extra-thermodynamic assumptions. Second, the values must be consistent for different solvent combinations. With this work, potentiometric measurements on silver ions and on chloride ions show that both conditions are fulfilled using a salt bridge filled with the ionic liquid [N2225 ][NTf2 ]: if compared to the values resulting from known pKL values, the silver and chloride single ion magnitudes combine within a uncertainty of 1.5 kJ mol-1 to the directly measurable transfer magnitudes of the salt AgCl from water to the solvents acetonitrile, propylene carbonate, dimethylformamide, ethanol, and methanol. The resulting values are used to further develop the consistent unified redox potential scale Eabs H2O that now allows to assess and compare redox potentials in and over six different solvents. We elaborate on its implications.

[Ni(CO)4]+ as NiI Synthon: Synthesis and Characterization of NiI Half-Sandwich and Sandwich Cations

The stable, easily accessible salt [Ni(CO)4]+[F{Al(ORF)3}2]- (RF = C(CF3)3) was used as a NiI synthon to generate the novel half-sandwich complexes [Ni(arene)(CO)2]+ (arene = C6H6, o-dfb = 1,2-F2C6H4). By irreversible removal of CO from the equilibrium, even the rather endergonic reaction to a [Ni(o-dfb)2]+ salt was successful (DrG°(solv) = +78 kJ mol-1). The latter displays an unprecedented slipped η3,η3-sandwich structure and is the ultimate synthon to NiI-chemistry.

Measurements and Utilization of Consistent Gibbs Energies of Transfer of Single Ions: Towards a Unified Redox Potential Scale for All Solvents

Utilizing the “ideal” ionic liquid salt bridge to measure Gibbs energies of transfer of silver ions between the solvents water, acetonitrile, propylene carbonate and dimethylformamide results in a consistent data set with a precision of 0.6 kJ mol⁻¹ over 87 measurements in 10 half‐cells. This forms the basis for a coherent experimental thermodynamic framework of ion solvation chemistry. In addition, we define the solvent independent peabsH2O ‐ and the EabsH2O values that account for the electronating potential of any redox system similar to the pHabsH2O value of a medium that accounts for its protonating potential. This EabsH2O scale is thermodynamically well‐defined enabling a straightforward comparison of the redox potentials (reducities) of all media with respect to the aqueous redox potential scale, hence unifying all conventional solvents′ redox potential scales. Thus, using the Gibbs energy of transfer of the silver ion published herein, one can convert and unify all hitherto published redox potentials measured, for example, against ferrocene, to the EabsH2O scale.

A unified pH scale for all solvents: part I – intention and reasoning (IUPAC Technical Report)

The definition of pH, its measurement and standard buffers, is well developed in aqueous solutions. Its definition in solvents other than water has been elaborated for a couple of solvents and their mixtures with water. However, the definition of a universal pH scale spanning all solvents and phases, not to mention standard procedures of measurement, is still a largely uncharted territory. UnipHied is a European collaboration and has the goal of putting the theoretical concept of an earlier introduced (2010) unified pHabs scale on a metrologically well-founded basis into practice. The pHabs scale enables the comparability of acidity between different phases. This article draws the connection of the concepts of unified acidity and secondary pH measurement.

The Inverted Philosopher’s Stone: how to turn silver to a base metal

Metals often are classified as “noble” or “base”—characterizing their reduction potential as one of the most important chemical properties. We show that metals are only as noble as allowed by their environment, i.e. this is a relative term, and the “frame of reference” simply is the solvent in which the redox system is present. We prove that silver is a prime example for a noble metal that forfeits its noble character in the simple ionic liquid HMIM Br (1-hexyl-3-methylimidazolium bromide) as an example for such a solvent.

Influence of the cofactor structure on the photophysical processes initiating signal transduction in a phototropin-derived LOV domain

Due to their biological importance, the photochemistry of blue-light photoreceptor proteins has been studied extensively over the last few decades. Most blue-light photoreceptors, such as cryptochromes and phototropins, utilize flavin chromophores as their cofactors. After irradiation with light, the chromophore undergoes electron transfer with nearby redox-active amino-acid residues within the protein, whereby this first step of signal transduction may be initiated either from the flavin's excited singlet or triplet state. Despite the collective effort of theoreticians and experimentalists to characterize and understand the photochemistry of flavoproteins, the mechanistic details of the excited state processes initiating signal transduction are yet to be revealed. Here, we use a light-oxygen-voltage-sensing domain from Avena sativa phototropin to get additional insight into the excited state photochemistry of flavoproteins. The influence of structural variations of the cofactor flavin mononucleotide (FMN) is explored by varying the methyl substitution pattern in positions 7 and 8 of the flavin core. The photophysical properties of the FMN derivatives, in the absence and presence of the protein environment, are investigated by UV-vis absorption, fluorescence, and electron paramagnetic resonance spectroscopies as well as cyclic voltammetry. The comparison of the properties of the modified flavin cofactors with those of FMN shows that the rates of the different excited state reactions, and therefore also the singlet/triplet yields, can be modulated substantially by only minor structural modifications of the flavin core.

Isolation of a stable pyridine radical anion

For almost 150 years, pyridine radical anions have been described as elusive transient species that cannot be isolated due to dimerization and/or subsequent decomposition reactions. In this work the first example of a stable pyridine radical anion is presented.

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 pH_abs /pHabsH2O scale as a unifying concept. Thereafter, some examples of the accessibility and application of unified pH_abs 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 pe_abs . The combination of pH_abs and pe_abs 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.

The Ideal Ionic Liquid Salt Bridge for Direct Determination of Gibbs Energies of Transfer of Single Ions, Part II: Evaluation of the Role of Ion Solvation and Ion Mobilities

An important intermediate goal to evaluate our concept for the assumption-free determination of single-ion Gibbs transfer energies Δ_tr G°(i, S₁ →S₂ ) is presented. We executed the crucial steps a) and b) of the methodology, described in Part I of this treatise, exemplarily for Ag⁺ and Cl^- with S₁ being water and S₂ being acetonitrile. The experiments showed that virtually all parts of the liquid junction potentials (LJPs) at both ends of a salt bridge cancel, if the bridge electrolyte is an "ideal" ionic liquid, that is, one with nearly identical diffusion of anion and cation. This ideality holds for [N₂₂₂₅ ]⁺ [NTf₂ ]^- in the pure IL, but also in water and acetonitrile solution. Electromotive force measurements of solvation cells between S₁ and S₂ demonstrated Nernstian behavior for Ag⁺ concentration cells and constant like cell potentials for solutions with five tested Ag⁺ counterions.

The Ideal Ionic Liquid Salt Bridge for the Direct Determination of Gibbs Energies of Transfer of Single Ions, Part I: The Concept

Described is a procedure for the thermodynamically rigorous, experimental determination of the Gibbs energy of transfer of single ions between solvents. The method is based on potential difference measurements between two electrochemical half cells with different solvents connected by an ideal ionic liquid salt bridge (ILSB). Discussed are the specific requirements for the IL with regard to the procedure, thus ensuring that the liquid junction potentials (LJP) at both ends of the ILSB are mostly canceled. The remaining parts of the LJPs can be determined by separate electromotive force measurements. No extra-thermodynamic assumptions are necessary for this procedure. The accuracy of the measurements depends, amongst others, on the ideality of the IL used, as shown in our companion paper Part II.

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

The recently published purely metallo-organic Ni^I salt [Ni(cod)₂ ][Al(OR^F )₄ ] (1, cod=1,5-cyclooctadiene, R^F =C(CF₃ )₃ ) provides a starting point for a new synthesis strategy leading to Ni^I 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(OR^F )₄ ] (2, dppp=1,3-bis(diphenylphosphino)propane), [Ni(dppe)₂ ][Al(OR^F )₄ ] (3, dppe=1,3-bis(diphenyl-phosphino)ethane), three-coordinate [Ni(PPh₃ )₃ ][Al(OR^F )₄ ] (4) and a remarkable two-coordinate Ni^I phosphine complex [Ni(PtBu₃ )₂ ][Al(OR^F )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 Ni^I 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 Ni^I phosphine complexes towards small molecules with formation of mixed phosphine-CO-Ni^I complexes and oxidation processes yielding new Ni^I and/or Ni^II derivatives. Moreover, 4 reacted with CH₂ Cl₂ at RT to give a dimeric Ni^II 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.

[Ni(cod)2][Al(OR(F))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)

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.

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.

Scanning Electrochemical Microscopy as a Versatile Tool for Modifying Surfaces

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.