Michael Linseis

Oligonuclear Ferrocene Amides: Mixed-Valent Peptides and Potential Redox-Switchable Foldamers

Trinuclear ferrocene tris-amides were synthesized from an Fmoc- or Boc-protected ferrocene amino acid, and hydrogen-bonded zigzag conformations were determined by NMR spectroscopy, molecular modelling, and X-ray diffraction. In these ordered secondary structures orientation of the individual amide dipole moments approximately in the same direction results in a macrodipole moment similar to that of α-helices composed of α-amino acids. Unlike ordinary α-amino acids, the building blocks in these ferrocene amides with defined secondary structure can be sequentially oxidized to mono-, di-, and trications. Singly and doubly charged mixed-valent cations were probed experimentally by Vis/NIR, paramagnetic ¹H NMR and Mössbauer spectroscopy and investigated theoretically by DFT calculations. According to the appearance of intervalence charge transfer (IVCT) bands in solution, the ferrocene/ferrocenium amides are described as Robin-Day class II mixed-valent systems. Mössbauer spectroscopy indicates trapped valences in the solid state. The secondary structure of trinuclear ferrocene tris-amides remains intact (coiled form) upon oxidation to mono- and dications according to DFT calculations, while oxidation to the trication should break the intramolecular hydrogen bonding and unfold the ferrocene peptide (uncoiled form).

The Complexed Triphosphaallyl Radical, Cation, and Anion Family

Radically complex: The photolytic reaction of [Cp*P{W(CO)(5)}(2)] (Cp* = C(5)Me(5)) with a diphosphene produces, via a radical intermediate, an air-stable complexed triphosphaallyl radical, in which the unpaired electron is evenly distributed over both terminal P atoms. Oxidation of the radical leads to a triphosphaallyl cation, which is only stable at low temperatures in solution, whereas the stable triphosphaallyl anion is formed by reduction (see picture, Mes* = 2,4,6-tri-tert-butylphenyl).

Redox-Active Tetraruthenium Macrocycles Built from 1,4-Divinylphenylene-Bridged Diruthenium Complexes

Metallamacrocylic tetraruthenium complexes were generated by treatment of 1,4-divinylphenylene-bridged diruthenium complexes with functionalized 1,3-benzene dicarboxylic acids and characterized by HR ESI-MS and multinuclear NMR spectroscopy. Every divinylphenylene diruthenium subunit is oxidized in two consecutive one-electron steps with half-wave potential splittings in the range of 250 to 330 mV. Additional, smaller redox-splittings between the +/2+ and 0/+ and the 3+/4+ and 2+/3+ redox processes, corresponding to the first and the second oxidations of every divinylphenylene diruthenium entity, are due to electrostatic effects. The lack of electronic coupling through bond or through space is explained by the nodal properties of the relevant molecular orbitals and the lateral side-by-side arrangement of the divinylphenylene linkers. The polyelectrochromic behavior of the divinylphenylene diruthenium precursors is retained and even amplified in these metallamacrocyclic structures. EPR studies down to T=4 K indicate that the dications 1-H(2+) and 1-OBu(2+) are paramagnetic. The dications and the tetracation of macrocycle 3-H display intense (dications) or weak (3-H(4+) ) EPR signals. Quantum chemical calculations indicate that the four most stable conformers of the macrocycles are largely devoid of strain. Bond parameters, energies as well as charge and spin density distributions of model macrocycle 5-H(Me) were calculated for the different charge and spin states.

Macrocyclic Triruthenium Complexes Having Electronically Coupled Mixed-Valent States

5-Ethynyl-2-furancarboxylic acid and 3-ethynylbenzoic acid self-assemble with [HRu(CO)Cl(PiPr3 )2 ] to form macrocyclic C3 -symmetric triangular triruthenium alkenyl complexes [{Ru(CO)(PiPr3 )2 (CH=CHArCOO)}3 ] (Ar=C6 H4 : 1-B, Ar=C4 H2 O: 1-F), which were characterized by multinuclear NMR spectroscopy, high-resolution ESI mass spectrometry, and, in the case of 1-B, by X-ray crystallography. Electrochemical studies indicate that the macrocycles are oxidized in three consecutive one-electron steps. The mixed-valent states obtained by electrochemical or chemical oxidation show signs of valence delocalization, which makes these complexes rare examples of molecule-based conductive loops with through-bond charge delocalization.

Directing Energy Transfer in Panchromatic Platinum Complexes for Dual Vis–Near-IR or Dual Visible Emission from σ-Bonded BODIPY Dyes

We report on the platinum complexes trans-Pt(BODIPY)(8-ethynyl-BODIPY)(PEt3)2 (EtBPtB) and trans-Pt(BODIPY)(4-ethynyl-1,8-naphthalimide)(PR3)2 (R = Et, EtNIPtB-1; R = Ph, EtNIPtB-2), which all contain two different dye ligands that are connected to the platinum atom by a direct σ bond. The molecular structures of all complexes were established by X-ray crystallography and show that the different dye ligands are in either a coplanar or an orthogonal arrangement. π-stacking and several CH···F and short CH···π interactions involving protons at the phosphine substituents lead to interesting packing motifs in the crystal. The complexes feature several strong absorptions (ε = 3.2 × 105-5.5 × 105 M-1 cm-1) that cover the regime from 350 to 480 nm (EtNIPtB-1 and EtNIPtB-2) or from 350 to 580 nm (EtBPtB). Besides the typical absorption bands of both kinds of attached dyes, they also feature an intense band near 400-420 nm, which is assigned by time-dependent density functional theory calculations to a higher-energy transition within the ethynyl-BODIPY (EtB) ligand or to charge transfer between the BODIPY (B) and naphthalimide (NI) chromophores. All complexes show dual fluorescence and phosphorescence emission from either the B (EtNIPtB-1 and EtNIPtB-2) or EtB (EtBPtB) ligand with a maximum phosphorescence quantum yield of 41% for EtNIPtB-1. The latter seems to be the highest reported value for room temperature phosphorescence from a BODIPY dye. The complete quenching of the emission from the chromophore absorbing at the higher energy and the appearance of the corresponding absorption bands in the fluorescence and phosphorescence excitation spectra indicate complete and rapid energy transfer to the chromophore with the lower-energy excited state, i.e., EtNI → B in EtNIPtB-1 and EtNIPtB-2 and B → EtB in EtBPtB. The latter process was further investigated by transient absorption spectroscopy, indicating that energy transfer is complete within 0.6 ns. EtNIPtB-1 catalyzes the photooxidation of 1,5-dihydroxynaphthalene with photogenerated 1O2 to Juglone at a much faster rate than methylene blue but with only modest quantum yields of 37% and with the onset of photodegradation after 60 min.