Electronic structure of metallocenes

Highly selective mercury(II) cations detection in mixed-aqueous media by a ferrocene-based fluorescent receptor

A new chemosensor molecule 3 based on a ferrocene-imidazophenanthrophenazine dyad effectively recognizes Hg(2+) in an aqueous environment through three different channels. Upon recognition, an anodic shift of the ferrocene-ferrocenium oxidation potential (ΔE(1/2) = 240 mV) and a progressive red shift (Δλ = 17 nm) of the low energy band in its absorption spectrum is produced. The emission spectrum of 3 in an aqueous environment, CH(3)CN-EtOH-H(2)O (65:25:10), and conducted at pH = 7.4 (20 × 10(-3) M HEPES) (Φ = 0.003), is perturbed after addition of Hg(2+) cations and an intense and structureless red shift emission band at 494 nm (Δλ = 92 nm) appeared along with an increase of the intensity of the emission band (CHEF = 77), the quantum yield (Φ = 0.054) resulted in a 18-fold increase. The combined (1)H NMR data of the complex and the theoretical calculations suggest the proposed bridging coordination mode.

Aminoferrocene-based prodrugs activated by reactive oxygen species

Cancer cells generally generate higher amounts of reactive oxygen species than normal cells. On the basis of this difference, prodrugs have been developed (e.g., hydroxyferrocifen), which remain inactive in normal cells, but become activated in cancer cells. In this work we describe novel aminoferrocene-based prodrugs, which, in contrast to hydroxyferrocifen, after activation form not only quinone methides (QMs), but also catalysts (iron or ferrocenium ions). The released products act in a concerted fashion. In particular, QMs alkylate glutathione, thereby inhibiting the antioxidative system of the cell, whereas the iron species induce catalytic generation of hydroxyl radicals. Since the catalysts are formed as products of the activation reaction, it proceeds autocatalytically. The most potent prodrug described here is toxic toward cancer cells (human promyelocytic leukemia (HL-60), IC(50) = 9 μM, and human glioblastoma-astrocytoma (U373), IC(50) = 25 μM), but not toxic (up to 100 μM) toward representative nonmalignant cells (fibroblasts).

Ferrocene based chemosensor for Cu2+--a dual channel signaling system

A new ferrocene based molecule behaves as a dual channel signaling chemosensor for Cu(2+) over other metal ions. The perturbations in the absorption pattern and electrochemical behavior of the chemosensor are presented. These have been proposed to be caused by the interaction of Cu(2+) with the d-electrons and the electron rich π-system of the ferrocenyl derivative. The sensing event is manifested by a high energy shift in the ligand centered π-π* absorption and appearance of a new redox wave at more positive potential, in addition to Fe(II)/Fe(III) couple wave (two wave electrochemical behavior).

Synthesis, UV/vis spectra and electrochemical characterisation of arylthio and styryl substituted ferrocenes

Two series of substituted ferrocenes were synthesised using either the Horner-Wadsworth-Emmons reaction or monolithiation of ferrocene. The series consist of arylthio- and styryl-ferrocenes with different substituents in the para position of the aryl rings of the systems. The electronic communication was investigated by comparing the substituent effects in absorption spectroscopy and in cyclic voltammetry. A small substituent effect was found in the electronic transitions of the styryl substituted ferrocenes. The oxidation of the ferrocene derivatives showed clear substituent effects as illustrated by the linear Hammett plots. The effect was shown to be an order of magnitude larger in the arylthio-systems than in the styryl systems. It is suggested that the reason behind the large effect is a direct sulfur-iron orbital overlap.

Developing click reactions of a 3-ferrocenyl-2H-azaphosphirene complex

Atom efficient reactions of 3-ferrocenyl-2H-azaphosphirene complex 1 using a newly developed acid/base ring expansion protocol are presented. Consecutive reaction of 1 in the presence of dimethyl cyanamide with triflic acid and triethylamine yielded the selective formation of 2H-1,4,2-diazaphosphole complex 3. The same protocol using acetone, benzaldehyde and ferrocenyl aldehyde afforded 1,3,5-oxazaphosphol-3-ene complexes 4, 5a,b and 6a,b, the latter as mixtures of diastereomers, which could be separated. Apart from NMR, IR and UV/vis spectroscopic data, the single-crystal X-ray structures of complexes 3, 4 and 6a are discussed.

Synthesis, electronic structure, and reactivity of strained nickel-, palladium-, and platinum-bridged [1]ferrocenophanes

The group 10 bis(phosphine)metalla[1]ferrocenophanes, [{Fe(η(5)-C(5)H(4))(2)}M(Pn-Bu(3))(2)] [M = Ni (4a), Pd (4b), and Pt (4c)], have been prepared by the reaction of Li(2)[Fe(η(5)-C(5)H(4))(2)]·tmeda (5, tmeda = N,N,N',N'-tetramethylethylenediamine) with trans-[MCl(2)(Pn-Bu(3))(2)] [M = Ni (trans-6a) and Pd (trans-6b)] and cis-[PtCl(2)(Pn-Bu(3))(2)] (cis-6c), respectively. Single crystal X-ray diffraction revealed highly tilted, strained structures as characterized by α angles of 28.4° (4a), 24.5° (4b), and 25.2° (4c) and a distorted square planar environment for the group 10 metal center. UV/visible spectroscopy and cyclic voltammetry indicated that all three compounds had smaller HOMO-LUMO gaps and were more electron-rich in nature than ferrocene and other comparable [1]ferrocenophanes. DFT calculations suggested that these differences were principally due to the electron-releasing nature of the M(Pn-Bu(3))(2) metal-ligand fragments. Attempts to induce thermal or anionic ring-opening polymerization of 4a-c were unsuccessful and were complicated by, for example, competing ligand dissociation processes or unfavorable chain propagation. In contrast, these species all reacted rapidly with acids effecting clean extrusion of the bis(phosphine)metal fragment. Carbon monoxide inserted cleanly into one of the palladium-carbon bonds of 4b to afford the ring-expanded, acylated product [{Fe(η(5)-C(5)H(4))(η(5)-C(5)H(4))(CO)}Pd(Pn-Bu(3))(2)] (10). The nickel analogue 4a, however, afforded [Ni(CO)(2)(Pn-Bu(3))(2)] whereas the platinum-bridged complex 4c was inert. Remarkably, all compounds 4a-c were readily oxidized by elemental sulfur to afford the [5,5']bicyclopentadienylidene (pentafulvalene) complexes [{η(4):η(0)-C(5)H(4)(C(5)H(4))}M(Pn-Bu(3))(2)] [M = Ni (11a)] and [(η(2)-C(10)H(8))M(Pn-Bu(3))(2)] [M = Pd (11b) and Pt (11c)] by a formal 4-electron oxidation of the carbocyclic ligands. Compounds 11b and 11c represent the first examples of [5,5']bicyclopentadienylidene as a neutral η(2)-ligand. The relative energies of η(2)-coordination with respect to that of η(4):η(0) bonding were investigated for 11a-c by DFT calculations.

Heterobimetallic metal-complex assemblies constructed from the flexible arm-like ligand 1,1'-bis[(3-pyridylamino)carbonyl]ferrocene: structural versatility in the solid state

The bidentate ferrocenyl sandwich molecule 1,1'-bis[(3-pyridylamino)carbonyl]ferrocene (3-BPFA) has been employed as an organometallic ligand in reactions with a series of transition metal salts to construct heterobimetallic architectures. X-ray crystallographic characterization reveals that the crystal packing of free ligand 3-BPFA induces spontaneous resolution of helical chains via intermolecular hydrogen bonds. By combining the flexibility from the arm-like molecule (3-BPFA) with the variation of the coordination property from different metal ions and/or the different counteranions, five different types of architectures are prepared: one octahedral coordination cage (copper(II) complex 1); two discrete pseudocapsules for combination of chlorine anions (nickel(II) complex 2 and cobalt(II) complex 3); two dimers with metal-metal interactions (silver(I) complexes 4 and 5); one macrocyclic complex (mercury(II) complex 6); and five two-dimensional mixed-metal-organic frameworks (M'-MOFs) (zinc(II), cadmium(II), and mercury(II) complexes 7-11). The structures of all complexes are characterized in detail by IR, elementary analysis, and single-crystal X-ray diffraction analysis. The factors inducing the structure variation among the complexes are discussed by taking account of the coordination geometry of different metal ions, the span angle between the two "arms", and the coordination mode of the 3-BPFA ligand.

Intermetallic communication in titanium(IV) ferrocenyldiketonates

A tetradentate bis(ferrocenyldiketonate) ligand, Fc(2)BobH(2), is prepared via Claisen condensation of acetylferrocene and 2,2'-biphenyldiacetyl chloride, and is metalated with titanium(IV) isopropoxide to give (Fc(2)Bob)Ti(O(i)Pr)(2) in good yield. The isopropoxide groups are replaced with di(4-nitrophenyl)phosphate groups on treatment with the corresponding acid, and with chlorides on treatment with trimethylsilyl chloride. Metathesis with catechol leads to the bis(o-hydroxyphenoxide) complex rather than the chelating catecholate complex. Hydrolysis selectively gives the mu-oxo trimer (Delta,Delta,Delta)/(Lambda,Lambda,Lambda)-{(Fc(2)Bob)Ti(mu-O)}(3). The solid-state structures of the mu-oxo trimer and the bis(o-hydroxyphenoxide) complex show that the ferrocene substituents are oriented proximal to the biphenyl backbone rather than pointed out toward the exogenous groups. The complexes show dramatic changes in color depending on the bound anions, ranging from the red isopropoxide (lambda(max) = 489 nm) to the green bis(di(4-nitrophenyl)phosphate) (lambda(max) = 653 nm). The oxidation potentials of the ferrocenes show modest shifts based on the titanium environment, but the redox potentials of the two ferrocenes are never separated by more than 60 mV. These results and those of density-functional theory (DFT) calculations indicate that the titanium interacts principally with the lowest unoccupied molecular orbital (LUMO) of the ferrocenyldiketonate and very little with its highest occupied molecular orbital (HOMO).

Intra- and intermolecular charge transfer: twin themes and simultaneous competing transitions involving ferrocenes

Electronic absorption spectra of ferrocene, ferrocenecarboxylaldehyde, butylferrocene, and 1,1'-diacetylferrocene in pure organic polar and non-polar solvents, pure halocarbon solvents and in several hexane-halocarbon solvent mixtures have been recorded. The investigated ferrocenes have shown several intra-molecular electronic transitions of the types pi-pi*, n-pi*, and d-d*. On using protonic solvents (HA) each of the ferocenes (Fc) acquires a proton exported from the solvent to form a complex with the formula [FcH](+)[A](-). However, on using halocarbon solvents each of the ferrocenes has shown an intermolecular charge-transfer-to-solvent (CTTS) which is characterized by the appearance of new absorption spectral band(s) for each ferrocene-halocarbon solvent interaction. Formation constants (K(CT)) and molar absorption coefficients (epsilon(CT)) of these interactions have been determined and discussed. The study has indicated that the observed different transitions are dependent upon the number and nature of the substituents involved in the ferrocenes.