Winter, Rainer F.
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Electronic communication in oligonuclear ferrocene complexes with anionic four-coordinate boron bridges
The di- and trinuclear ferrocene species Li[Fc-BPh2-Fc] (Li[9]) and Li2[Fc-BPh2-fc-BPh2-Fc] (Li2[10]) have been investigated with regard to their electrochemical properties and the degree of intervalence charge-transfer after partial oxidation. Li[9] shows two distinct one-electron redox waves for its chemically equivalent ferrocenyl substituents in the cyclic voltammogram (E1/2 = −0.38 V, −0.64 V; vs. FcH/FcH+). The corresponding values of Li2[10] are E1/2 = −0.45 V (two-electron process) and −1.18 V. All these redox events are reversible at r. t. on the time scale of cyclic voltammetry. X-ray crystallography on the mixed-valent FeII2FeIII complex Li(12-c-4)2[10] reveals the centroid–centroid distance between the cyclopentadienyl rings of each of the terminal ferrocenyl substituents (3.329 Å) to be significantly smaller than in the central 1,1′-ferrocenediyl fragment (3.420 Å). This points towards a charge-localized structure (on the time scale of X-ray crystallography) with the central iron atom being in the FeIII state. Mößbauer spectroscopic measurements on Li(12-c-4)2[10] lend further support to this interpretation. Spectroelectrochemical measurements on Li[9] and Li2[10] in the wavelength range between 300–2800 nm do not show bands interpretable as intervalence charge-transfer absorptions for the mixed-valent states. All data accumulated so far lead to the conclusion that electronic interaction between the individual Fe atoms in Li[9] and Li2[10] occurs via a through-space pathway and/or is electrostatic in nature.
Synthesis, solid state structure and spectro-electrochemistry of ferrocene-ethynyl phosphine and phosphine oxide transition metal complexes
The synthesis of ferrocene-ethynyl phosphine platinum dichloride complexes based on (FcCtriple bond; length of mdashC)nPh3−nP (1a, n = 1; 1b, n = 2; 1c, n = 3; Fc = ferrocenyl, (η5-C5H5)(η5-C5H4)Fe) is described. Air-oxidation of 1c afforded (FcCtriple bond; length of mdashC)3Pdouble bond; length as m-dashO (6). Treatment of 1a–1c with [(PhCtriple bond; length of mdashN)2PtCl2] (2) or [(tht)AuCl] (tht = tetrahydrothiophene) (7), respectively, gave the heterometallic transition complexes cis-[((FcCtriple bond; length of mdashC)nPh3−nP)2PtCl2] (3a, n = 1; 3b, n = 2; 3c, n = 3) or [((FcCtriple bond; length of mdashC)nPPh3−n)AuCl] (8a, n = 1; 8b, n = 2). Further treatment of these molecules with HCtriple bond; length of mdashCMc (4a, Mc = Fc; 4b, Mc = Rc = (η5-C5H5)(η5-C5H4)Ru) in the presence of [CuI] produced trans-[((FcCtriple bond; length of mdashC)Ph2P)2Pt(Ctriple bond; length of mdashCFc)2] (5) (reaction of 3a with 4a) and [(FcCtriple bond; length of mdashC)nPh3−nPAuCtriple bond; length of mdashCMc] (n = 1: 9a, Mc = Fc; 9b, Mc = Rc; n = 2: 11a, Mc = Fc; 11b, Mc = Rc) (reaction of 4a, 4b with 8a, 8b), respectively.
The structures of 3a, 5, 6, 8, 9a, and 9b in the solid state were established by single-crystal X-ray structure analysis. The main characteristic features of these molecules are the linear phosphorus–gold–acetylide arrangements, the tetra-coordination at phosphorus and the square-planar surrounding at platinum.
The electrochemical and spectro-electrochemical behavior of complexes 5, 8a, 9a, 9b and [(Ph3P)AuCtriple bond; length of mdashCFc] was investigated in the UV/Vis/NIR. Near IR bands that are likely associated with charge transfer from the ((FcCtriple bond; length of mdashC)Ph2P)2Pt or the ((FcCtriple bond; length of mdashC)nPh3−nP)Au (n = 0, 1) moieties appear upon oxidation of the σ-bonded ferrocene-ethynyl groups. These bands undergo a (stepwise) blue shift as ferrocene-ethynyl substituents on the phosphine coligands are oxidized.
Six-Membered N-Heterocyclic Carbenes with a 1,1′-Ferrocenediyl Backbone : Bulky Ligands with Strong Electron-Donor Capacity and Unusual Non-Innocent Character
Abstract
The stable, crystalline N-heterocyclic diaminocarbene fc[N(CH2tBu)-C-N(CH2tBu)] (2d, fc = 1,1′-ferrocenediyl) was prepared by deprotonation of its formamidinium precursor fc[N(CH2tBu)-CH-N(CH2tBu)][BF4] (1d) and used for the preparation of the 16 valence electron complexes [Mo(2d)(CO)4], [RhCl(2d)(cod)] (cod = 1,5-cyclooctadiene) and [RhCl(2d)(CO)2]. 1d, 2d and [RhCl(2d)(cod)] were structurally characterised by single-crystal X-ray diffraction studies. The electrochemical properties of 2d, its 2-adamantyl analogue 2c, its complex [RhCl(2d)(CO)2] and of the precursors 1d and 1,1′-bis(neopentylamino)ferrocene were investigated by electrochemistry. The carbenes are easily oxidised to the corresponding radical cation, whose persistent nature is unprecedented in the chemistry of N-heterocyclic carbenes. The spin density is located at the Fe atom and the carbene C atom according to the results of EPR spectroscopic studies and DFT calculations.
The synthesis and electrochemistry of 2,5-dimethylazaferrocenes with heteroaryl bridges
We report on the synthesis of complexes having two equivalent redox active 2,5-dimethylazaferrocenyl entities connected by heteroaryl (heteroaryl = thiophenyl, bithiophenyl and pyridyl) bridges. The new compounds have been investigated by various electrochemical techniques including cyclic voltammetry (CV), differential pulse voltammetry (DPV) and square wave voltammetry (SW) and were found to exhibit two consecutive reversible or partially reversible one-electron oxidations. Comproportionation constants (Kc) calculated from ΔE1/2 values indicate that the thermodynamic stability of their monoxidized forms exceeds those of analogous ferrocenes. In this paper we also report the X-ray crystal structure and UV–Vis spectroelectrochemistry of parent 2,5-dimethylazaferrocene.
Electron Transfer Across Multiple Hydrogen Bonds: The Case of Ureapyrimidinedione-Substituted Vinyl Ruthenium and Osmium Complexes
Ruthenium and osmium complexes 2a,b and 3a,b featuring the N-4,6-dioxo-5,5-dibutyl- or the N-4,6-dioxo-5,5-di-(2-propenyl)-1,4,5,6-tetrahydropyrimidin-2-yl-N′(4-ethenylphenyl)-urea ligand dimerize by a self-complementary quadruply hydrogen-bonding donor/donor/acceptor/acceptor (DDAA) motif. We provide evidence that the dimeric structures are maintained in nonpolar solvents and in 0.1 M NBu4PF6/CH2Cl2 supporting electrolyte solution. All complexes are reversibly oxidized in two consecutive two-electron oxidations (ΔE1/2 ≈ 500 mV) without any discernible potential splitting for the oxidation of the individual hydrogen-bridged redox active moieties. IR and UV/vis/NIR spectroelectrochemistry show a one-step conversion of the neutral to the dication without any discernible features of an intermediate monooxidized radical cation. Oxidation-induced IR changes of the NH and CO groups that are involved in hydrogen bonding are restricted to the styryl-bonded urea NH function. IR band assignments are aided by quantum chemical calculations. Our experimental findings clearly show that, at least in the present systems, the ureapyrimidinedione (Upy) DDAA hydrogen-bonding motif does not support electron transfer. The apparent reason is that neither of the hydrogen-bonding functionalities contributes to the occupied frontier levels. This results in nearly degenerate pairs of MOs representing the in-phase and out-of-phase combinations of the individual monomeric building blocks.
DFT modeling of Spectral and Redox Properties of Di-and Tetranuclear Ruthenium Transition Metal Complexes with Bridging Ligands
The electronic structures of di-and tetranuclear transition metal complexes with bridging ligands (tetracyanoethene, tetracyano-p-quinodimethane, divinylphenylene and tetrakis(4-styryl)ethene) were calculated by density functional (DFT) method. DFT method was used for calculations of IR frequencies in different oxidation states and EPR parameters of radical ions. The observed electronic transitions of closed shell systems were assigned by TD DFT. The different aspects of bridge mediated metal-metal interaction are discussed. ©2009 American Institute of Physics
The Synthesis, Structure, and FTIR Spectroelectrochemistry of W(CO)5 Complexes of 4-Oxo-4-(2,5-dimethylazaferrocen-1′-yl)butanoic and 5-Oxo-5-(2,5-dimethylazaferrocen-1′-yl)pentanoic Acids
With the aim of developing new IR-detectable metal–carbonyl tracers for the amino function, we have synthesized W(CO)5 complexes of 4-oxo-4-(2,5-dimethylazaferrocen-1′-yl)butanoic acid (2) and 5-oxo-5-(2,5-dimethylazaferrocen-1′-yl)pentanoic acid (3) by AlCl3-catalyzed Friedel–Crafts reaction of W(CO)5–2,5-dimethylazaferrocene (1) with succinic or glutaric anhydride. Complexes 2 and 3 are thermally stable and display sharp, intense absorption bands of tungsten-coordinated CO ligands at ca. 1923 cm–1. In the crystalline state, molecules of 2 and 3 are stabilized by a network of intra- and intermolecular hydrogen bonds, as shown by single-crystal X-ray structure analysis. Complex 2 was transformed into the corresponding N-succinimidyl ester 4. Its utility toward labeling of amino acids was tested in its reaction with glycine methyl ester. Corresponding glycine amide 5 was obtained in 82 % yield and is an air/thermally stable bioconjugate exhibiting intense sharp absorption bands of the W–CO reporting group at ca. 1923 cm–1. Cyclic voltammetry of 1, 2, 3, and acetyl derivative 6 shows the presence of two redox events in each case. The first redox couple is ascribed as an azaferrocene-centered oxidation–reduction, whereas the second, irreversible process at higher potential originates from a W(CO)5-centered oxidation. FTIR spectroelectrochemistry allowed us to monitor the spectroscopic changes accompanying the 1/1·+, 2/2·+, and 6/6·+ redox transformations. Significant W–CO absorption band shifts were recorded in the course of these experiments.
The Complexed Triphosphaallyl Radical, Cation, and Anion Family
Radically complex: The photolytic reaction of [Cp*P{W(CO)5}2] (Cp*=C5Me5) 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).
Fulvalenediyl-bridged heterobimetallic complexes consisting of sandwich and half-sandwich compounds with early–late transition metals
A straightforward synthesis methodology for the preparation of heterobimetallic [(η5-C5H5)(η5-C5H4-C5Me4)M] (3a, M = Fe; 3b, M = Ru) and [(η5-C5H5)((μ-η5:η5-C5H4-C5Me4)TiCl3)M] (4a, M = Fe; 4b, M = Ru) in which early and late transition metals are connected by a fulvalenediyl bridge is reported.
The structures of molecules 3b and 4a in the solid state are discussed. Most noteworthy in 4a is the exo arrangement of the iron and titanium atoms coordinated by the fulvalenediyl unit which itself is twisted with a dihedral angle between the joined cyclopentadienyl rings of 19.33(9)°. Electrochemical, UV/Vis/NIR spectroscopic and spectroelectrochemical experiments on 4a and Cp*TiCl3, for comparison, provide evidence for some transfer of electronic information between the conjoined ferrocene and half-sandwich titanocene trichloride subunits of 4a. Evidence comes from systematic potential shifts and the presence of a fairly intense Fe → Ti charge-transfer absorption band that vanishes upon oxidation and reduction of 4a.
Charge Delocalization in a Heterobimetallic Ferrocene−(Vinyl)Ru(CO)Cl(PiPr3)2 System
Ru(CH═CHFc)Cl(CO)(PiPr3)2 (Fc = ferrocenyl, (η5-C5H4)Fe(η5-C5H5)), 1, has been prepared by hydroruthenation of ethynylferrocene and characterized by NMR, IR, ESI-MS, and Moessbauer spectroscopy and by X-ray crystallography. Complex 1 features conjoined ferrocene and (vinyl)ruthenium redox sites and undergoes two consecutive reversible oxidations. Pure samples of crystalline, monooxidized 1•+ have been prepared by chemical oxidation of 1 with the ferrocenium ion. Structural comparison with 1 reveals an increase of Fe−C and Fe−Cpcentr. bond lengths and ring tilting of the Cp decks, as is typical of ferrocenium ions, but also a discernible lengthening of the Ru−C(CO) and Ru−P bonds and a shortening of the Ru−C(vinyl) bond upon oxidation. This supports the general idea of charge delocalization over both redox sites in 1•+. Band shifts of the charge-sensitive IR labels (ν(CO) for Ru, ν(C−H, Cp) for Fc), the rather small g-anisotropy in the ESR spectrum of 1•+, and the results of quantum chemical calculations indicate that in solution the positive charge partly resides on the vinyl ruthenium moiety. Comparison of IR shifts in the solid state and in solution and the quadrupole splitting in the Moessbauer spectrum of powdered 1•+ point to a larger extent of charge localization on the ferrocenyl site in solid samples. This is probably due to CH···F hydrogen bonding interactions between the cyclopentadienyl hydrogen atoms of the radical cations and the PF6− counterions. Monooxidized 1•+ displays low-energy electronic absorption bands at 1370 and 2150 nm. According to quantum chemical calculations, the underlying transitions are largely localized on the ferrocene part of the molecule with only little charge transfer into the vinyl ruthenium subunit. The second oxidation is more biased toward the (vinyl)ruthenium site.