Cobalt Sandwich Complexes Research Articles

Crystal Structures of Cobalt Sandwich Complexes in the η5-Cyclopentadienyl/η4-Cyclobutadiene and η5-Cyclopentadienyl/η4-Cyclopentadienone Families

The crystal structures of six cobalt sandwich complexes—four in the η5-cyclopentadienyl/η4-cyclobutadiene family, two from the related η5-cyclopentadienyl/η4-cyclopentadienone family—are presented and discussed alongside related structural precedents. In each of the complexes an undecorated cyclopentadienyl ligand is present whereas the partner cyclobutadiene or cyclopentadienone ligand bears four identical aromatic substituents (phenyl, p-tolyl, p-fluorophenyl, 2-thienyl or 2,2′-bithienyl). This range of substituents allows for detailed structural comparison between members of the families of sandwich complexes presented here, and with previously reported structures. Crystal structures of members of the η5-cyclopentadienyl/η4-tetraarylcyclobutadiene (left) and η5-cyclopentadienyl/η5-cyclopentadienone (right) families of sandwich complexes are presented.

Enantiopure Planar Chiral and Chiral-at-Metal Iridacycles Derived from Bulky Cobalt Sandwich Complexes

Reaction of (η5-(S)-2-(4-methylethyl)oxazolinylcyclopentadienyl)-(η4-tetraphenylcyclobutadiene)cobalt with [IrCp*Cl2]2 in acetonitrile with KPF6 and KOt-Bu resulted in S,Sp,SIr and S,Rp,RIr configured acetonitrile and Cp* coordinated cationic iridacycles (d.r. up to 4.8:1 – kinetic control), the planar chiral configuration dictating the configuration of the pseudo-tetrahedral iridium-based stereogenic center. Addition of water to the cycloiridation reaction resulted in an increase in yield (up to 78%) at the cost of diastereoselectivity. Use of the corresponding substrate containing a t-Bu rather than an i-Pr substituted oxazoline gave exclusively the S,Sp,SIr diastereoisomer, and under the same conditions, (S)-2-ferrocenyl-4-(1,1-dimethylethyl)oxazoline cycloiridated to give only the S,Sp,SIr diastereoisomer. Substitution reactions of the title complexes at iridium proceeded with retention of configuration, a computational study revealing the proposed coordinatively unsaturated intermediate of a dissocia...

Living ROMP Synthesis and Redox Properties of Triblock Metallocopolymers Containing Side‐Chain Iron and Cobalt Sandwich Complexes

AbstractMetalloblock copolymers are intensively researched because of their multifunctionalities and potential applications in biomedicine and materials science. In addition, metal fragments with redox stabilities are of interest toward conductors, sensors, and batteries. Here, Grubb’s third‐generation metathesis catalyst is efficiently used in ring‐opening metathesis polymerization (ROMP) reactions leading to the synthesis of two triblock metallopolymers in which each block contains a redox‐reversible iron or cobalt sandwich site. For each metallocopolymer, among six possibilities, only one involving a precise order of introduction of blocks is efficient with the completion of 25 units per block. Cyclic voltammetry measurements show the respective ability of the metal centers to exchange electrons with the electrode, with some electron transfer paths being marred by the positive charges and ligand bulk. Overall, these systems provide a cascade of chemically reversible electron transfers, their number being reasonably estimated using the empirical Bard–Anson model within approximately 20% of the number of redox sites determined by 1H NMR.

Cover Feature: Stereoselective and Stereospecific Reactions of Cobalt Sandwich Complexes: Synthesis of a New Class of Single Enantiomer Bulky Planar Chiral P−N and P−P Ligands (Chem. Eur. J. 17/2018)

Cover Feature: Stereoselective and Stereospecific Reactions of Cobalt Sandwich Complexes: Synthesis of a New Class of Single Enantiomer Bulky Planar Chiral P−N and P−P Ligands (Chem. Eur. J. 17/2018)

Stereoselective and Stereospecific Reactions of Cobalt Sandwich Complexes: Synthesis of a New Class of Single Enantiomer Bulky Planar Chiral P-N and P-P Ligands.

Starting from (η5 -acetylcyclopentadienyl)(η4 -tetraphenylcyclobutadiene)cobalt(I), highly enantioselective (99 % ee) (S)-CBS catalysed ketone reduction followed by stereospecific alcohol-azide exchange, azide reduction and dimethyllation gave (R)-(η5 -α-N,N-dimethylaminoethylcyclopentadienyl)(η4 -tetraphenylcyclobutadiene) cobalt(I) (Arthurs' amine). This underwent highly diastereoselective cyclopalladation to give di-μ-acetate-bis-(R)-[(η5 -(Sp )-2-(α-N,N-dimethylaminoethyl)cyclopentadienyl, 1-C, N)(η4 -tetraphenylcyclobutadiene)cobalt(I)]dipalladium, and highly diastereoselective lithiation to give (R)-(η5 -(Sp )-1-(α-N,N-dimethylaminoethyl)-2-(diphenylphosphino)cyclopentadienyl)(η4 -tetraphenylcyclobutadiene)cobalt(I) (PPCA) following the addition as electrophile of chlorodiphenylphosphine. This PN-ligand was converted into (R)-(η5 -(Sp )-1-(α-dicyclohexylphosphinoethyl)-2-(diphenylphosphino)cyclopentadienyl)(η4 -tetraphenylcyclobutadiene)cobalt(I), a PP-ligand (Rossiphos), by stereospecific amine-phosphine exchange using HPCy2 . These air-stable P-N and P-P complexes are the first examples of a new class of bulky planar chiral ligands for application in asymmetric catalysis.

Diblock Polyelectrolytic Copolymers Containing Cationic Iron and Cobalt Sandwich Complexes: Living ROMP Synthesis and Redox Properties.

Diblock metallopolymer polyelectrolytes containing the two redox-robust cationic sandwich units [CoCp'Cp](+) and [FeCp'(η(6)-C6 Me6)](+) (Cp = η(5)-C5 H5; Cp' = η(5)-C5H4-) as hexafluorophosphate ([PF6](-)) salts are synthesized by ring-opening metathesis polymerization using Grubbs' third generation catalyst. Their electrochemical properties show full chemical and electrochemical reversibilities allowing fine determination of the copolymer molecular weight using Bard-Anson's electrochemical method by cyclic voltammetry.

ChemInform Abstract: Enantioselective Synthesis and Application to the Allylic Imidate Rearrangement of Amine‐Coordinated Palladacycle Catalysts of Cobalt Sandwich Complexes.

AbstractThe enantioselective palladation of amine substrates followed by enantioselective transcyclopalladation enables the synthesis of ferrocene‐based phosphapalladacycles with good enantiomeric excess.

Enantioselective synthesis and application to the allylic imidate rearrangement of amine-coordinated palladacycle catalysts of cobalt sandwich complexes.

The reaction of (η5-(N,N-dimethylaminomethyl)cyclopentadien-yl)(η4-tetraphenylcyclobutadiene)cobalt with sodium tetrachloropalladate and (R)-N-acetylphenylalanine gave planar chiral palladacycle di-μ-chloridebis[(η5-(Sp)-2-(N,N-dimethylaminomethyl)cyclopentadienyl,1-C,3′-N)(η4-tetraphenylcyclobutadiene)cobalt]dipalladium [(Sp)-Me2-CAP-Cl] in 92 % ee and 64 % yield. Enantiopurity (>98 % ee) was achieved by purification of the monomeric (R)-proline adducts and conversion back to the chloride dimer. Treatment with AgOAc gave (Sp)-Me2-CAP-OAc which was applied to asymmetric transcyclopalladation (up to 78 % ee). The (R)-N-acetylphenylalanine mediated palladation methodology was applicable also to the corresponding N,N-diethyl (82 % ee, 39 % yield) and pyrrolidinyl (>98 % ee, 43 % yield) cobalt sandwich complexes. A combination of 5 mol % of the latter [(Sp)-Pyrr-CAP-Cl] and AgNO3 (3.8 equiv) is a catalyst for the allylic imidate rearrangement of an (E)-N-aryltrifluoroacetimidate (up to 83 % ee), and this catalyst system is also applicable to the rearrangement of a range of (E)-trichloroacetimidates (up to 99 % ee). This asymmetric efficiency combined with the simplicity of catalyst synthesis provides accessible solutions to the generation of non-racemic allylic amine derivatives.

Syntheses, Structures and Electronic Properties of Zwitterionic Iron(II) and Cobalt(II) Complexes Featuring Ambidentate Tris(pyrazolyl)methanide Ligands

By deprotonation of the corresponding tris(pyrazolyl)methane ((Me)Tpm) precursor complexes [M((Me)Tpm)(2)][(OTf)(2)] (1 and 2), the zwitterionic tris(pyrazolyl)methanide iron(II) (3) and cobalt(II) (4) "sandwich" complexes of the general formula [M((Me)Tpmd)(2)] are easily accessible. The structurally characterised complexes 3 and 4 are the first such homoleptic 3d transition metal species to feature two "naked", outward pointing pyramidal carbanions. Density functional theory calculations show metal-centred HOMOs and LUMOs with the destabilised carbanion orbitals in the energy region of the filled transition-metal-centred frontier orbitals. The electronic structures of these complexes have been investigated in detail by various spectroscopic techniques such as NMR, EPR, SQUID, Mössbauer, etc. Both complexes adopt a high-spin (HS) configuration at room temperature in solution and in the solid state. A thermally induced high-spin to low-spin (HS-LS) transition is observed for the iron(II) complex 3. The HS-LS transition temperature of 3 in solution differs from that in the solid state, which in turn strongly depends on the amount of solvent molecules in the crystal lattice. Electrochemical studies on the corresponding cobalt(II) complex 4 provide evidence for a HS-Co(II) <==> LS-Co(III) transition upon oxidation, which was confirmed by preliminary synthetic oxidation reactions. Overall, it can be concluded that the related kappa(3)N-donor ligands tris(pyrazolyl)hydroborates (Tp(R)) and (R)Tpmd ligands have similar bonding properties and that the metal cations experience more or less the same ligand environment.

Electron-reservoir complexes and other redox-robust reagents: functions and applications

The functions and applications of electron-reservoir iron and cobalt sandwich complexes are reviewed with emphasis of the author’s work using the 19-electron FeI complexes [(η5-C5R5)Fe(η6-C6Me6)], R = H or Me, the most electron-rich neutral molecules known to date and the 17-electron FeIII complex [(η5-C5Me5)Fe(η6-C6Me6)][SbCl6]2, the strongest known organometallic oxidant. It is shown that the electron-reservoir concept involving steric protection of a central redox site and the resulting redox robustness allows to carry out a variety of functions such as references for the measurement of redox potentials, many useful stoichiometric and catalytic redox reactions, sensing of anions and cations and entry into the area of molecular electronics also including the nanoscale.

(η 4-Tetraarylcyclobutadiene)(η 5-formylcyclopentadienyl)cobalt(I) complexes: Facilities to finetune the electron-donating capability in dipolar organometallics

Five different (η 4-tetraarylcyclobutadiene)(η 5-formylcyclopentadienyl)cobalt(I) complexes ( 1a– 1e) were synthesized in reasonable yields in a one-pot reaction of CoCl(PPh 3) 3, formylcyclopentadienyl sodium and the appropriate diarylethyne. The aryl groups of the ethyne were modified by various para-substituents X (X = Cl, H, Me, OMe, NMe 2), which were intended to alter the redox potentials of the synthesized cobalt sandwich complexes. A cyclic voltammetry study revealed a linear dependence of the first oxidation potential to the Hammett parameter σ p. X-ray structure analyses performed for two complexes (X = Me and NMe 2) demonstrate only subtle changes in the solid state structure despite the large differences in electrochemical properties. A theoretical analysis by the density functional theory method has been performed on the geometries and electronic structures of the complex (η 4-cyclobutadiene)(η 5-cyclopentadienyl)Co(I), its cation and dication.

Metallocenes as references for the determination of redox potentials by cyclic voltammetry — Permethylated iron and cobalt sandwich complexes, inhibition by polyamine dendrimers, and the role of hydroxy-containing ferrocenes

Measurement of the redox potentials (E1/2) of ferrocene (1), decamethylferrocene (2), decamethylcobaltocene (3), and [FeICp*(η6-C6Me6)] (4) (Cp* = η5-C5Me5) in DMF, CH3CN, THF, DME, CH2Cl2, and DMSO were recorded vs. the saturated calomel electrode (SCE), showing that the E1/2 value of 1 was more solvent dependent than those of 2–4, and that the difference in E1/2 values among 2–4 was solvent independent, unlike those found between 1 and 2–4. This shows that 2–4 are much more reliable internal references for determining redox potentials than 1, but using data from this article, one can convert the literature values reported vs. 1 into values vs. 2, 3, or 4. The electrochemistry of 1 and 2 is all the more inhibited by polyamine dendrimers as the dendrimer generation is higher. On the other hand, ferrocenes or other iron sandwich compounds containing hydroxy groups can reestablish reversible cyclic voltammograms. It is shown, using a tetra(ferrocenylmethylamine), that this phenomenon is due to hydrogen bonding between the amido and phenol groups rather than bonding of hydroxy groups to the electrode surface.Key words: ferrocene, reference, electrochemistry, metallocene, sandwich complex, iron, dendrimer, passivation, hydrogen bond.

Electrochemical behaviour of cobalta-dicarbollide sandwich complexes with different capping units

The redox aptitude of a series of cobalt(III) or cobalt(I) sandwich complexes bearing a charge compensated dicarbollide ligand ([9-L-7,8-C2B9H10]−) as a constant unit and different counterparts (varying from classical [7,8-C2B9H11]2− to charge-compensated [9-L-7,8-C2B9H10]− dicarbollides, from cyclopentadienyl [C5R5]− (R = Me, H) to cyclobutadiene [C4Me4]0 ligands) has been studied. All the Co(III) complexes display the reversible sequence Co(III)/Co(II)/Co(I). In contrast, the Co(I) complexes (namely, those capped by tetramethylcyclobutadiene) accede reversibly only to the Co(II) oxidation state, the passage to Co(III) being irreversible. When possible, the Co(II) intermediates have been characterized by EPR spectroscopy. The molecular structures of the monocation [Co(η-9-SMe2-7,8-C2B9H10)2]+ in its DD/LL and meso diastereomeric forms as well as that of heteroleptic (η-7,8-C2B9H11)Co(η-9-SMe2-7,8-C2B9H10) have been obtained by single-crystal diffraction.

Coordination studies of polycondensed π-perimeters: mono-, di- and tri-nuclear cobalt sandwich complexes of tetracene, benzo[ a]pyrene, triphenylene, benzo[ b]triphenylene and perylene

By means of arene ligand exchange of the triple deckers [{(η 5-R 1–5C 5)Co} 2-μ-(η 4:η 4-toluene)] (R 1–5=Me, R 1–3= tert-butyl) at room temperature a number of mono-, di-, and tri-nuclear sandwich complexes of the polycondensed aromatics polycyclic aromatic hydrocarbons (PAHs) tetracene, benzo[ a]pyrene, perylene, triphenylene and benzo[ a]triphenylene have been synthesized. For triphenylene and benzo[ b]triphenylene however, a threefold coordination of [(η 5-C 5Me 5)Co] fragments in the periphery of these PAHs is found, tetracene allows for a twofold coordination of two [(η 5-C 5Me 5)Co] fragments in the ring periphery leaving a central naphthalene unit of this PAH uncomplexed. Perylene serves as a twofold coordination site for [(η 5-1,2,4 tri- tert-butyl-C 5H 2)Co] fragments which are coordinated in each of the two naphthalene fragments of this π-perimeter.

Azaborolinyl-komplexe XIV. Oxidationsreaktionen an bis(η5-1,2-azaborolinyl)cobalt-sandwichkomplexen

Abstract The oxidation of the crystalline paramagnetic bis(a-alkyl(-trimethylsilyl)-2-methyl-η 5 -1,2-azaborolinyl)cobalt sandwich complexes (I) (alkyl = CH 3 , C(CH 3 ) 3 ) with iodine vapour leads to the diamagnetic cobalticinium analogous 18-electron cations in II. They are isolated as tri- and penta-iodide mixtures. The polyiodide ions I 3 − and I 5 − do not exert a distinct influence on the protons of the cations as is shown by the 1 H NMR spectra. Pure compounds are obtained as PF 6 − salts III by oxidation with the ferricinium cation. According to the 1 H NMR spectra the cations in II and III consist of isomers as the starting complexes, differing from each other by the clockwise (B13-N/B13-N) or anti-clockwise (B13-N/N13-B) orientation of the rings. The synthesis and X-ray structure analysis of the so far unknown bis(1,2-dimethyl-ψ 5 -1,2-azaborolinyl)cobalt (Ia) are described.

Azaborolinyl‐Iron and ‐Cobalt Sandwich Complexes by Metal Atom Synthesis

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1982, 50. DOI:10.1002/anie.198200500

Electrochemistry of transition metal π-complexes. IV. Oxidation of cyclopentadienyl diolefine and cyclobutadiene cobalt complexes

Electrochemical oxidation of cobalt sandwich complexes CoCpL, where Cp is η 5-cyclopentadienyl and L an η 4-diolefine or cyclobutadiene ligand, was investigated by means of cyclic voltammetry and polarography. Oxidation is reversible or nearly so for compounds where L is a cyclic diolefine (I–III) or a cyclobutadiene (VI) and for the methylated complexesCo(C 5Me 5)(C 5Me 5R) (R = Me, Ph) (VII) and irreversible for CoCp(C 5H 5Ph) (IV) and the complexes V where L is a divinylborane.