Electrochemical Parametrization in Sandwich Complexes of the First Row Transition Metals

Modeling ligand electrochemical parameters by repulsion-corrected eigenvalues

Ligand electrochemical parameters, E_L , more commonly known as Lever parameters, have played a major research role in understanding redox processes involved in inorganic electrochemistry, enzymatic reactions, catalysis, solar cells, biochemistry, and materials science. Despite their broad usefulness, Lever parameters are not well understood at a first-principles level. Using density functional theory, we demonstrate in this contribution that a ligand's Lever parameter is fundamentally related to the ligand's ability to alter the eigenvalue of the electroactive spin-orbital in an octahedral transition metal complex. Our analysis furthers a first-principles understanding of the nature of Lever parameters.

Ionization Energy and Reduction Potential in Ferrocene Derivatives: Comparison of Hybrid and Pure DFT Functionals

Hybrid density functionals have been regularly applied in state-of-the-art computational models for predicting reduction potentials. Benchmark calculations of the absolute reduction potential of ferricenium/ferrocene couple, the IUPAC-proposed reference in nonaqueous solution, include the B3LYP/6-31G(d)/LanL2TZf protocol. We used this procedure to calculate ionization energies and reduction potentials for a comprehensive set of ferrocene derivatives. The protocol works very well for a number of derivatives. However, a significant discrepancy (>1 V) between experimental and calculated data was detected for selected cases. Three variables were assessed to detect an origin of the observed failure: density functional, basis set, and solvation model. It comes out that the Hartree-Fock exchange fraction in hybrid-DFT methods is the main source of the error. The accidental errors were observed for other hybrid models like PBE0, BHandHLYP, and M06-2X. Therefore, hybrid DFT methods should be used with caution, or pure functionals (BLYP or M06L) may be used instead.

Electrochemical, Spectroelectrochemical, Mößbauer, and EPR Spectroscopic Studies on Ferrocenyl-Substituted Tritylium Dyes

Four monoferrocenyl tritylium derivatives with donor-substituted (OMe, NMe₂ ) aryl rings are reported, along with their spectroscopic and electrochemical properties. All the complexes show a one-electron reduction and a quasi-reversible ferrocene oxidation at a very positive potential. Small quadrupole splittings, ΔE_Q , in Mößbauer spectra agree with highly electron-deficient ferrocenes. Comparison of the experimental half-wave potentials for ferrocene oxidation, E_1/2 (Fc/Fc⁺ ), with those estimated from established correlations of E_1/2 (Fc/Fc⁺ ) with ΔE_Q indicates that the E_1/2 values of the anisyl-substituted congeners FcOMe⁺ and FcMeOMe⁺ are affected by Coulombic repulsion between the positive charges at the Fe ion and the neighboring methylium site. Electronic spectra are recorded and interpreted with the aid of quantum chemical calculations. UV/Vis spectroelectrochemical measurements as well as chemical reduction provide insight into the redox-induced color changes upon ferrocene oxidation or upon reduction to the neutral trityl radicals. The neutral radicals reversibly form EPR-silent dimers. This process is studied by temperature-dependent EPR spectroscopy, and thermodynamic data for their dimerization are determined. Experimental and quantum chemical data suggest that the dimers assume classical hexaarylethane structures as opposed to normal or "offset" Jacobson-Nauta-type structures.

Polysubstituted ferrocenes as tunable redox mediators

A series of four ferrocenyl ester compounds, 1-methoxycarbonyl- (1), 1,1’-bis(methoxycarbonyl)- (2), 1,1’,3-tris(methoxycarbonyl)- (3) and 1,1’,3,3’-tetrakis(methoxycarbonyl)ferrocene (4), has been studied with respect to their potential use as redox mediators. The impact of the number and position of ester groups present in 1–4 on the electrochemical potential E_1/2 is correlated with the sum of Hammett constants. The 1/1⁺–4/4⁺ redox couples are chemically stable under the conditions of electrolysis as demonstrated by IR and UV–vis spectroelectrochemical methods. The energies of the C=O stretching vibrations of the ester moieties and the energies of the UV–vis absorptions of 1–4 and 1⁺–4⁺ correlate with the number of ester groups. Paramagnetic ¹H NMR redox titration experiments give access to the chemical shifts of 1⁺–4⁺ and underline the fast electron self-exchange of the ferrocene/ferrocenium redox couples, required for rapid redox mediation in organic electrosynthesis.

Anodic electrochemistry of mono- and dinuclear aminophenylferrocene and diphenylaminoferrocene complexes

Two related three-membered series of nonlinear aminophenylferrocene and diphenylaminoferrocene complexes were prepared and characterized by 1H and 13C NMR spectroscopy. The first series consists of 4-(diphenylamino)phenylferrocene (TPA-Fc, 1a), its dimethoxy-substituted tetraphenylphenylenediamine derivative (M2TPPD-Fc, 1c), and the triphenylamine-bridged bis(ferrocenyl) complex (Fc-TPA-Fc, 1b). The second series involves bis(4-methoxyphenyl)aminoferrocene (M2DPA-Fc, 1d), 4-methoxyphenylaminoferrocene (MPA-Fc) with N-phenyl-appended terminal TPA (1e), and the corresponding bis(MPA-Fc) complex with bridging TPA (1f). The structure of complex 1d was further confirmed by single crystal X-ray diffraction. Combined investigations, based on anodic voltammetry, UV-vis-NIR spectroelectrochemistry and density functional theory (DFT) calculations, were conducted to illustrate the influence of the integration of multiple redox-active components on the sequential oxidation of these complexes. The first anodic steps in 1a-1f are localized preferentially on the ferrocenyl units, followed by oxidation of the TPA or TPPD moieties (absent in 1d). Irreversible oxidation of the ferrocene-appended strong donor DPA/MPA units in 1d-1f terminates the anodic series. The one-electron oxidation of the triphenylamine-bridged diferrocenyl (1b) and bis(phenylaminoferrocenyl) (1f) complexes triggers their facile redox disproportionation to dicationic bis(ferrocenium) products.

Ferrocenyl-sulfonium ionic liquids - synthesis, characterization and electrochemistry

New ferrocenylsulfonium cation based ionic liquids were prepared by direct alkylation of the corresponding ferrocenyl-based thioethers with N-alkylbis(trifluoromethanesulfonyl)imides (R'TFSI). This convenient direct access to organometallic sulfonium bis(trifluoromethanesulfonyl)imide (TFSI) salts without the need for ion exchange was chosen in order to obtain highly pure and reversibly redox active room temperature ILs in many cases. In other cases the anion cation interaction in the solid state was studied by XRD analyses. Moreover a diferrocenylmethylsulfonium tetrafluoroborate with two redox active centers was synthesized. The redox chemistry of these sulfonium salts was investigated via cyclic voltammetry. Furthermore, UV-Vis spectra and thermoanalytical data are discussed. The electron-withdrawing sulfonium group is directly bonded to the ferrocenyl unit, therefore this cationic group influences the potential of these ionic liquids in a more pronounced way than being anchored to the ferrocenyl unit via an organic spacer. With their low absorbance in the visible light and reversible, tunable redox potential, these room temperature ILs open perspectives as redox mediators in dye sensitized solar cells (DSSCs), as redox electrolytes in supercapacitors or as overcharge protection additives in batteries.

Redox-Active N-Heterocyclic Germylenes and Stannylenes with a Ferrocene-1,1'-diyl Backbone

We describe ferrocene-based N-heterocyclic germylenes and stannylenes of the type [Fe{(η⁵ -C₅ H₄ )NR}₂ E:] (1 RE; E=Ge, Sn; R=neopentyl (Np), mesityl (Mes), trimethylsilyl (TMS)), which constitute the first examples of redox-functionalised N-heterocyclic tetrylenes (NHTs). These compounds are thermally stable and were structurally characterised by means of X-ray diffraction studies, except for the neopentyl-substituted stannylene 1 NpSn, the decomposition of which afforded the aminoiminoferrocene [fc(NHCH₂ tBu)(N=CHtBu)] (2) and the spiro tin(IV) compound (1 Np)₂ Sn (3). DFT calculations show that the HOMO of the NHTs of our study is localised on the ferrocenylene backbone. A one-electron oxidation process affords ions of the type 1 RE^+. . In contrast to the NHC system 1 RC, the localised ferrocenium-type nature of the oxidised form does not compromise the fundamental tetrylene character of 1 RE^+. .

A chelating bis(aminophenol) ligand bridged by a 1,1′-ferrocene-bis(para-phenylene) linker

The 1,1′-ferrocene-bis(p-phenylene) group serves as a structural linker connecting two iminosemiquinones to palladium without significant electronic interactions between the ferrocene and the palladium complex.

A Stable Planar-Chiral N-Heterocyclic Carbene with a 1,1'-Ferrocenediyl Backbone

This paper focuses on the stable, ferrocene-based N-heterocyclic carbene (NHC) rac-[Fe{(η(5)-t-BuC5H3)NpN}2C:] (A'-Np, Np = neopentyl), which is planar-chiral due to the two tert-butyl substituents in 3,3'-positions. A'-Np was synthesized in nine steps starting from 1,1'-di-tert-butylferrocene (1), the first step being its 3,3'-dilithiation to afford rac-[Fe(η(5)-t-BuC5H3Li)2] (rac-fc'Li2, 2). The structures of rac-fc'(SiMe3)2 (3), rac-fc'Br2 (4), rac-fc'(N3)2 (5), and the immediate carbene precursor [A'-NpH]BF4 were determined by single-crystal X-ray diffraction (XRD). The chemical properties of A'-Np were found to be very similar to those of its tert-butyl-free congener A-Np, both being ambiphilic NHCs with rather high calculated HOMO energies (ca. -4.0 eV) and low singlet-triplet gaps (ca. 35 kcal/mol). A Tolman electronic parameter value of 2050 cm(-1) was derived from IR data of cis-[RhCl(A'-Np)(CO)2], indicating the high donicity of A'-Np as a ligand. Consistent with its ambiphilic nature, A'-Np was found to react readily with carbon monoxide, affording the betainic enolate (A'-Np)2CO as four stereoisomers, viz. (RpRp-A'-Np)═C(O(-))(RpRp-A'-Np(+)), (SpSp-A'-Np)═C(O(-))(SpSp-A'-Np(+)), (RpRp-A'-Np)═C(O(-))(SpSp-A'-Np(+)), and (SpSp-A'-Np)═C(O(-))(RpRp-A'-Np(+)). The former two isomers were structurally characterized as a racemic compound by single-crystal XRD. A'-Np was found to react swiftly with dichloromethane, affording the addition product A'-NpH-CHCl2 in a reaction that is unprecedented for diaminocarbenes. A-NpH-CHCl2 was obtained analogously. Both compounds were structurally characterized by single-crystal XRD. An electrochemical investigation of A'-Np by cyclic and square wave voltammetry revealed a reversible oxidation of the carbene at a half-wave potential of -0.310 vs ferrocene/ferrocenium (THF/NBu4PF6). The electrochemical data previously published for A-Np were identified to be incorrect, since unnoticed hydrolysis of the NHC had taken place, affording A-Np(H2O). The hydrolysis products of A-Np and A'-Np were found to be reversibly oxidized at half-wave potentials of -0.418 and -0.437 V, respectively.

Ruthenium(II) bipyridine complexes bearing new keto-enol azoimine ligands: synthesis, structure, electrochemistry and DFT calculations

The novel azoimine ligand, Ph-NH-N=C(COCH3)-NHPh(C≡CH) (H2L), was synthesized and its molecular structure was determined by X-ray crystallography. Catalytic hydration of the terminal acetylene of H2L in the presence of RuCl3·3H2O in ethanol at reflux temperature yielded a ketone (L1=Ph-N=N-C(COCH3)=N-Ph(COCH3) and an enol (L2=Ph-N=N-C(COCH3)=N-PhC(OH)=CH2) by Markovnikov addition of water. Two mixed-ligand ruthenium complexes having general formula, trans-[Ru(bpy)(Y)Cl2] (1-2) (where Y=L1 (1) and Y=L2 (2), bpy is 2.2'-bipyrdine) were achieved by the stepwise addition of equimolar amounts of (H2L) and bpy ligands to RuCl3·3H2O in absolute ethanol. Theses complexes were characterized by elemental analyses and spectroscopic (IR, UV-Vis, and NMR (1D (1)H NMR, (13)C NMR, (DEPT-135), (DEPT-90), 2D (1)H-(1)H and (13)C-(1)H correlation (HMQC) spectroscopy)). The two complexes exhibit a quasi-reversible one electron Ru(II)/Ru(III) oxidation couple at 604 mV vs. ferrocene/ferrocenium (Cp2Fe(0/+)) couple along with one electron ligand reduction at -1010 mV. The crystal structure of complex 1 showed that the bidentate ligand L1 coordinates to Ru(II) by the azo- and imine-nitrogen donor atoms. The complex adopts a distorted trans octahedral coordination geometry of chloride ligands. The electronic spectra of 1 and 1+ in dichloromethane have been modeled by time-dependent density functional theory (TD-DFT).

Ruthenium(II) bipyridine complexes bearing quinoline-azoimine (NN'N″) tridentate ligands: synthesis, spectral characterization, electrochemical properties and single-crystal X-ray structure analysis

Four octahedral ruthenium(II) azoimine-quinoline complexes having the general molecular formula [Ru(II)(L-Y)(bpy)Cl](PF6) {L-Y=YC6H4N=NC(COCH3)=NC9H6N, Y=H (1), CH3 (2), Br (3), NO2 (4) and bpy=2,2'-bipyrdine} were synthesized. The azoimine-quinoline based ligands behave as NN'N″ tridentate donors and coordinated to ruthenium via azo-N', imine-N' and quinolone-N″ nitrogen atoms. The composition of the complexes has been established by elemental analysis, spectral methods (FT-IR, electronic, (1)H NMR, UV/Vis and electrochemical (cyclic voltammetry) techniques. The crystal structure of complex 1 is reported. The Ru(II) oxidation state is greatly stabilized by the novel tridentate ligands, showing Ru(III/II) couples ranging from 0.93-1.27 V vs. Cp2Fe/Cp2Fe(+). The absorption spectrum of 1 in dichloromethane was modeled by time-dependent density functional theory (TD-DFT).

Ferrocenyl-phosphonium ionic liquids - synthesis, characterisation and electrochemistry

New unsymmetrically substituted ferrocenyl-phosphonium ionic liquids (ILs) [FcPR2R']NTf2 are synthesized by two or three step syntheses starting from ferrocene, Fc = (C5H5)Fe(C5H4); R = Me, (n)Bu, (n)Hex, Ph; R' = Me, (n)Pr, (n)Bu, Ph; NTf2 = N(SO2CF3)2. The selective synthesis of alkyl phosphines FcPR2via a Friedel-Crafts phosphorylation is highlighted as an alternative for the standard protocol commonly used for ferrocenyl arylphosphines involving lithiation of FcH followed by phosphorylation. The influence of the P-substituents on thermal stability, electrochemical potential, chemical shift, and UV-Vis absorption behavior of the ILs is studied. The phosphonium group acts both as an ionic tag and as an electron-withdrawing substituent directly bound at the Cp-ring position. Therefore the title compounds are attractive for further studies to use them as tunable redox mediators for (photo)electrochemical devices such as dye sensitized solar cells (DSSCs) or redox flow batteries.

Rational design of a redox-labeled chiral target for an enantioselective aptamer-based electrochemical binding assay

A series of redox-labeled L-tyrosinamide (L-Tym) derivatives was prepared and the nature of the functional group and the chain length of the spacer were systematically varied in a step-by-step affinity optimization process of the tracer for the L-Tym aptamer. The choice of the labeling position on L-Tym proved to be crucial for the molecular recognition event, which could be monitored by cyclic voltammetry and is based on the different diffusion rates of free and bound targets in solution. From this screening approach an efficient electroactive tracer emerged. Comparable dissociation constants Kd were obtained for the unlabeled and labeled targets in direct or competitive binding assays. The enantiomeric tracer was prepared and its enantioselective recognition by the corresponding anti-D-Tym aptamer was demonstrated. The access to both enantiomeric tracer molecules opens the door for the development of one-pot determination of the enantiomeric excess when using different labels with well-separated redox potentials for each enantiomer.

A career in phthalocyanine chemistry – the Linstead Award 2002

A stroll through memory lane highlighting the author's contributions to phthalocyanines, including synthetic aspects especially of binuclear species, manganese oxygen breathing chemistry, mixed valence species, solution and surface electrochemistry, electrocatalysis, two-electron concerted processes, charge transfer spectroscopy, ligand electrochemical parameter applications and phthalocyanine design.