Electronic structure of metallocenes

1,3,5-Triferrocenyl-2,4,6-tris(ethynylferrocenyl)-benzene--a new member of the family of multiferrocenyl-functionalized cyclic systems

The consecutive synthesis of 1,3,5-triferrocenyl-2,4,6-tris(ethynylferrocenyl)benzene (6c) is described using 1,3,5-Cl3-2,4,6-I3-C6 (2) as starting compound. Subsequent Sonogashira C,C cross-coupling of 2 with FcC≡CH (3) in the molar ratio of 1:4 afforded solely 1,3,5-Cl3-2,4,6-(FcC≡C)3-C6 (4c) (Fc = Fe(η(5)-C5H4)(η(5)-C5H5)). However, when 2 is reacted with 3 in a 1:3 ratio a mixture of 1,3,5-Cl3-2-(FcC≡C)-4,6-I2-C6 (4a) and 1,3,5-Cl3-2,4-(FcC≡C)2-6-I-C6 (4b) is obtained. Negishi C,C cross-coupling of 4c with FcZnCl (5) in the presence of catalytic amounts of [Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2 gave 1,3-Cl2-5-Fc-2,4,6-(FcC≡C)3-C6 (6a), 1-Cl-3,5-Fc2-2,4,6-(FcC≡C)3-C6 (6b) and 1,3,5-Fc3-2,4,6-(FcC≡C)3-C6 (6c) of which 6b is the main product. Column chromatography allowed the separation of these organometallic species. The structures of 4a,b and 6a in the solid state were determined by single crystal X-ray diffractometry showing a π–π interacting dimer (4b) and a complex π–π pattern for 6a. The electrochemical properties of 4a–c and 6a–c were studied by cyclic voltammetry (=CV) and square wave voltammetry (=SWV). It was found that the FcC≡C-substituted benzenes 4a–c show only one reversible redox event, indicating a simultaneous oxidation of all ferrocenyl units, whereby 4c is most difficult to oxidise (4a, E°′1 = 190, ΔEp = 71; 4b, E°′1 = 195, ΔEp = 59; 4c, E°′1 = 390, ΔEp = 59 mV). In case of 4c, the oxidation states 4c(n+) (n = 2, 3) are destabilised by the partial negative charge of the electronegative chlorine atoms, which compensates the repulsive electrostatic Fc+–Fc+ interactions with attractive electrostatic Fc+–Cl(δ−) interactions. When ferrocenyl units are directly attached to the benzene C6 core, organometallic 6a shows three, 6b five and 6c six separated reversible waves highlighting that the Fc units can separately be oxidised. UV-Vis/NIR spectroscopy allowed to determine IVCT absorptions (=Inter Valence Charge Transfer) for 6c(n+) (n = 1, 2) (n = 1: νmax = 7860 cm(−1), εmax = 405 L mol(−1) cm(−1), Δν1/2 = 7070 cm(−1); n = 2: νmax = 9070 cm(−1), εmax = 620 L mol(−1) cm(−1), Δν1/2 = 8010 cm(−1)) classifying these mixed-valent species as weakly coupled class II systems according to Robin and Day, while for 6a,b only LMCT transitions (=ligand to metal charge transfer) could be detected.

Tris(triazole) tripodal receptors as selective probes for citrate anion recognition and multichannel transition and heavy metal cation sensing

The preparation and binding properties towards citrate anions and cations of two three-armed triazole based receptors are described.

Nitrogen-rich multinuclear ferrocenophanes as multichannel chemosensor molecules for transition and heavy-metal cations

[m.n] Multinuclear ferrocenophanes prepared by aza-Wittig reaction of bisiminophosphoranes derived from 1,1′-diazidoferrocene and isophthaladelhyde or 2,5-diformylthiophene, behave as efficient electrochemical and chromogenic chemosensor molecules for Zn²⁺, Pb²⁺, and Hg²⁺ metal cations. Whereas the OSWV of receptor 3, bearing two m-phenylene units in the bridges, display one oxidation peak, receptor 4 incorporating two thiophene rings in the bridges, exhibits two well-separated oxidation peaks. In both receptors only the addition of Zn²⁺, Pb²⁺, and Hg²⁺ metal cations induced a remarkable anodic shift of ferrocene/ferrocenium redox couple. Likewise, in the absorption spectra of these receptors the low energy band is red-shifted by Δλ = 165 − 209 nm, and these changes promoted a significant color changes which could be used for the naked eye detection of these metal cations. The coordination modes for two representative cases were unveiled by DFT calculations that show an unsual coordination in the [4₂Pb]²⁺ complex with the Pb²⁺ cation in a distorted cubic N₄S₄ donor cage.

A simplified Tamm-Dancoff density functional approach for the electronic excitation spectra of very large molecules

Two approximations in the Tamm-Dancoff density functional theory approach (TDA-DFT) to electronically excited states are proposed which allow routine computations for electronic ultraviolet (UV)- or circular dichroism (CD) spectra of molecules with 500-1000 atoms. Speed-ups compared to conventional time-dependent DFT (TD-DFT) treatments of about two to three orders of magnitude in the excited state part at only minor loss of accuracy are obtained. The method termed sTDA ("s" for simplified) employs atom-centered Löwdin-monopole based two-electron repulsion integrals with the asymptotically correct 1∕R behavior and perturbative single excitation configuration selection. It is formulated generally for any standard global hybrid density functional with given Fock-exchange mixing parameter ax. The method performs well for two standard benchmark sets of vertical singlet-singlet excitations for values of ax in the range 0.2-0.6. The mean absolute deviations from reference data are only 0.2-0.3 eV and similar to those from standard TD-DFT. In three cases (two dyes and one polypeptide), good mutual agreement between the electronic spectra (up to 10-11 eV excitation energy) from the sTDA method and those from TD(A)-DFT is obtained. The computed UV- and CD-spectra of a few typical systems (e.g., C60, two transition metal complexes, [7]helicene, polyalanine, a supramolecular aggregate with 483 atoms and about 7000 basis functions) compare well with corresponding experimental data. The method is proposed together with medium-sized double- or triple-zeta type atomic-orbital basis sets as a quantum chemical tool to investigate the spectra of huge molecular systems at a reliable DFT level.

Synthesis and Properties of Bridgehead-Functionalized Permethylbicyclo[2.2.2]octasilanes

A series of previously unknown bridgehead-functionalized bicyclo[2.2.2]octasilanes, Me3Si-Si8Me12-X, X-Si8Me12-X, and X-Si8Me12-Y [X, Y = -SiMe n Ph3-n (n = 1, 2) (2, 3, 10), -SiMe2Fc (Fc = ferrocenyl) (4, 11, 13, 14), -COR (R = Me, tBu) (6, 7, 12), COOMe (8), COOH (9)], have been prepared by the reaction of the silanides Me3Si-Si8Me12-K+ or K+-Si8Me12-K+ with proper electrophiles and fully characterized. The molecular structures of 2, 3, 4, 6, 8, 9, 10, and 13 as determined by single-crystal X-ray diffraction analysis exhibit a slightly twisted structure of the bicyclooctasilane cage. Endocyclic bond lengths, bond angles, and dihedral angles are not influenced considerably by the substituents attached to the bridgehead silicon atoms. Due to σ(SiSi)/π(aryl) conjugation, a 20-30 nm bathochromic shift of the longest wavelength UV absorption band relative to Me3Si-Si8Me12-SiMe3 (1) is evident in the UV absorption spectra of the phenyl and ferrocenyl derivatives. Otherwise, UV absorption data do not support the assumption of aryl/aryl or aryl/C=O interaction via the σ(SiSi) bicyclooctasilane framework.