Bucky ferrocene and ruthenocene: serendipity and discoveries

Substituted fullerenes as a promising capping ligand towards stabilization of exohedral Dy(III) based single-ion magnets: a theoretical study

Organometallic dysprosocenium-based molecular magnets are the forefront runners in offering giant magnetic anisotropy and blocking temperatures close to the boiling point of liquid nitrogen. Attaining linearity in the organometallic dysprosocenium complexes is the key to generating giant magnetic anisotropy and blocking barriers. In the present study, we have unravelled the coordination ability of the substituted fullerene (C55X5)- (where X = CCH3, B, and N) generated by fencing around the five-membered ring of fullerene towards stabilizing a new family of exohedral dysprosium organometallic complexes showcasing giant magnetic anisotropy and blockade barriers. Eight exohedral mononuclear dysprosium organometallic complexes, namely [Dy(η5-C55X5)(η4-C4H4)] (1), [Dy(η5-C55X5)(η5-Cp)]+ (2), [Dy(η5-C55X5)(η5-Cp*)]+ (3), [Dy(η5-C55X5)(η6-C6H6)]2+ (4), [Dy(η5-C55X5)(η8-C8H8)] (5), [Dy(η5-C55X5)2]+ (6) (where X = CCH3), [Dy(η5-C55B5)2]+ (7) and [Dy(η5-C55N5)2]+ (8), were studied using scalar relativistic density functional theory (SR-DFT) and the complete active space self-consistent field (CASSCF) methodology to shed light on the structure, stability, bonding and single-ion magnetic properties. SR-DFT calculations predict complexes 1-8 to be highly stable, with a strictly linear geometry around the Dy(III) ion in complexes 6-8. Energy Decomposition Analysis (EDA) predicts the following order for interaction energy (ΔEint value): 5 > 1 > 2 ≈ 3 > 6 > 7 > 8 > 4, with sizable 4f-ligand covalency in all the complexes. CASSCF calculations on complexes 1-8 predict stabilization of mJ |±15/2〉 as the ground state for all the complexes except for 5, with the following trend in the Ucal values: 6 (1573 cm-1) ≈ 3 (1569 cm-1) > 1 (1538 cm-1) > 8 (1347 cm-1) > 2 (1305 cm-1) > 7 (1284 cm-1) > 4 (1125 cm-1) > 5 (108 cm-1). Ab initio ligand field theory (AILFT) analysis provides a rationale for Ucal ordering, where π-type 4f-ligand interactions in complexes 1-4 and 6-8 offer giant barrier height while the large (C8H8)2- rings generate δ-type interaction in 5, which diminishes the axiality in the ligand field. Our detailed finding suggests that the exohedral organometallic dysprosocenium complexes are more linear compared to bent [DyCp*2]+ cations and display a giant barrier height exceeding 1500 cm-1 with negligible quantum tunnelling of magnetization (QTM) - a new approach to design highly anisotropic dysprosium organometallic complexes.

Time-resolved imaging and analysis of the electron beam-induced formation of an open-cage metallo-azafullerene

The visualization of single-molecule reactions provides crucial insights into chemical processes, and the ability to do so has grown with the advances in high-resolution transmission electron microscopy. There is currently a limited mechanistic understanding of chemical reactions under the electron beam. However, such reactions may enable synthetic methodologies that cannot be accessed by traditional organic chemistry methods. Here we demonstrate the synthetic use of the electron beam, by in-depth single-molecule, atomic-resolution, time-resolved transmission electron microscopy studies, in inducing the formation of a doubly holed fullerene-porphyrin cage structure from a well-defined benzoporphyrin precursor deposited on graphene. Through real-time imaging, we analyse the hybrid's ability to host up to two Pb atoms, and subsequently probe the dynamics of the Pb-Pb binding motif in this exotic metallo-organic cage structure. Through simulation, we conclude that the secondary electrons, which accumulate in the periphery of the irradiated area, can also initiate chemical reactions. Consequently, designing advanced carbon nanostructures by electron-beam lithography will depend on the understanding and limitations of molecular radiation chemistry.

A computational study on the nature, strength and cooperativity of bonds in [M(η5–C60Me5)(CO)n] and [M(η5–Cp)(CO)n] (n = 3, M = Mn(i), Tc(i), Re(i); n = 2, M = Co(i), Rh(i), Ir(i)) complexes

It is shown that, due to cooperativity versus anticooperativity of bonds, the total interaction energy of a complex, having weaker metal–ligand bonds, can be comparable to or even larger than that of a similar complex having stronger bonds.

Synthesis of a sumanenyl hafnocene complex

A convex-bound sumanenyl hafnocene complex was selectively synthesized, and the catalytic activity of sumanenyl metallocenes was demonstrated in the carboalumination reaction.

Hybrid materials based on ruthenium and fullerene assemblies

This review provides a detailed overview about the synthesis, properties and applications of all ruthenium-fullerene compounds reported within the last 25 years. The incorporation of ruthenium centers into fullerene compounds by organometallic, covalent or non-covalent bonds has led to a broad range of useful hybrid materials. By this approach novel compounds could be generated that feature the electron-donating and electron-accepting character of ruthenium complexes and fullerenes, respectively. Intramolecular interactions between both units could result in new, combined properties that were studied in the spotlight of emerging applications, such as photovoltaics or catalysis.

Regioselective Synthesis and Characterization of Multinuclear Convex-Bound Ruthenium-[n]Cycloparaphenylene (n = 5 and 6) Complexes

Mono- and multinuclear complexes of ruthenium and [n]cycloparaphenylene (CPP, n = 5 and 6) were synthesized in excellent yields through ligand exchange of the cationic complex [(Cp)Ru(CH3CN)3](PF6) with CPP. In the multinuclear complexes, ruthenium selectively coordinated to alternate paraphenylene units to give bis- and tris-coordinated Ru complexes for [5] and [6]CPPs, respectively. Single-crystal X-ray analysis revealed the Ru was coordinated with η(6)-hapticity on the convex surface of CPP.

Synthesis, crystal structure, and photoluminescence studies of a ruthenocenyl-decorated Sn/S cluster

Recently, we reported on ferrocenyl-decorated Sn/S clusters; herein, we present the extension of our investigations by attachment of ruthenocenyl units to an according cluster skeleton. The latter was realized upon improvement of the synthesis of acetylruthenocene, its conversion to a hydrazone derivative, and the subsequent reaction with a keto-functionalized Sn/S precursor complex. The report comprises the crystal structures of acetylruthenocene and the ruthenocenyl-terminated Sn/S cluster [(R(Rc)Sn)4Sn2S10] (R(Rc) = CMe2CH2C(Me)═N-N═C(Me)Rc), as well as the discussion of the electrochemical properties of the latter and its behavior during time-resolved photoluminescence investigations.

Electrochemistry of redox-active self-assembled monolayers

Redox-active self-assembled monolayers (SAMs) provide an excellent platform for investigating electron transfer kinetics. Using a well-defined bridge, a redox center can be positioned at a fixed distance from the electrode and electron transfer kinetics probed using a variety of electrochemical techniques. Cyclic voltammetry, AC voltammetry, electrochemical impedance spectroscopy, and chronoamperometry are most commonly used to determine the rate of electron transfer of redox-activated SAMs. A variety of redox species have been attached to SAMs, and include transition metal complexes (e.g., ferrocene, ruthenium pentaammine, osmium bisbipyridine, metal clusters) and organic molecules (e.g., galvinol, C(60)). SAMs offer an ideal environment to study the outer-sphere interactions of redox species. The composition and integrity of the monolayer and the electrode material influence the electron transfer kinetics and can be investigated using electrochemical methods. Theoretical models have been developed for investigating SAM structure. This review discusses methods and monolayer compositions for electrochemical measurements of redox-active SAMs.

Chiral Ruthenium–Allenylidene Complexes That Bear a Fullerene Cyclopentadienyl Ligand: Synthesis, Characterization, and Remote Chirality Transfer

Ruthenium complexes that bear both a fullerene and an allenylidene ligand, [Ru(C60Me5)((R)-prophos)=C=C=CR1R2]PF6 (prophos = 1,2-bis(diphenylphosphanyl)propane), were prepared by the reaction of [Ru(C60Me5)Cl((R)-prophos)] and a propargyl alcohol in better than 90% yields, and characterized by 1H, 13C, and 31P NMR, IR, and UV/Vis/NIR spectroscopy and MS. Cyclic voltammograms of these complexes showed one reversible or irreversible reduction wave due to the allenylidene part, and two reversible reduction waves due to the fullerene core. Nucleophilic addition of RMgBr or RLi proceeded regioselectively at the distal carbon atom of the allenylidene array. The reaction took place with a 60:40-95:5 level of diastereoselectivity with respect to the original chirality in the (R)-prophos ligand, which is located six atoms away from the electrophilic carbon center.

Synthesis and Structural, Electrochemical, and Stacking Properties of Conical Molecules Possessing Buckyferrocene on the Apex

A series of conical molecules featuring a [60]fullerene/ferrocene hybrid and five aralkyl side chains (Fe[C60{C6H4-(OCO-C6H3-(OR)2-3,4)-4}5]Cp) have been synthesized and examined for their structural and electrochemical properties as well as their ability to form supramolecular structures in crystals and liquid crystals. When the R group on the side is a methyl group, the compound forms crystals in which the dipolar conical molecules are stacked head-to-tail to form a columnar structure. When the R group is as long as a C18H38 group, the compound forms liquid crystals. Oxidation of the liquid crystalline compound by an aminium salt [(4-BrC6H4)3N][SbCl6] produces the corresponding paramagnetic Fe(III) compound that also exhibits liquid crystalline properties.

Sharing Orbitals: Ultrafast Excited State Deactivations with Different Outcomes in Bucky Ferrocenes and Ruthenocenes

We report on the singlet ground and singlet/triplet excited-state features of a series of bucky ferrocenes, bucky ruthenocenes, and respective reference compounds. In the bucky ferrocene conjugates, intimate contacts between the fullerenes and ferrocenes result in appreciable ground-state interactions-suggesting a substantial shift of charge density from the electron donor (i.e., ferrocene) to the electron acceptor (i.e., fullerene). In contrast, no prominent charge-transfer features were observed for the bucky ruthenocene conjugates. An arsenal of experimental techniques, ranging from fluorescence (i.e., steady state and time-resolved) and pump probe experiments (i.e., femtosecond and nanoseconds) to pulse radiolysis, were employed to examine excited-state interactions. In the excited states, bucky ferrocene conjugates are dominated by rapid charge separation reactions (0.8 +/- 0.1 ps) to yield metastable radical ion pairs. The radical ion pair lifetimes vary between 27 and 39 ps. No charge separation was, however, found in the corresponding bucky ruthenocence. Instead, an intrinsically faster excited-state deactivation (approximately 200 ps) evolves from the heavier ruthenium center-relative to iron. This effect is further augmented by the unfavorably shifted oxidation potential in ruthenocene of about 0.61 V, which in ruthenocene (-deltaG(ET) = -0.26 eV), in contrast to ferrocene (-deltaG(ET) = 0.35 eV), renders charge separation thermodynamically unfeasible.

Synthesis and Electrochemistry of Double-Decker Buckyferrocenes

D5d- and C5v-symmetric double-decker buckyferrocene Fe2(C60R10)Cp2 (R10 = Me5Ph5 and Me10) represents a new type of diiron complexes featuring conjugative connection of two ferrocene groups by a hoop-shaped [10]cyclophenacene. The compounds exhibit two-electron oxidation and two-electron reduction behavior, generating dicationic and dianionic species. The two iron atoms interact with each other through the pi-conjugation of the cyclophenacene, as revealed by differential pulse voltammetric analysis of the D5d Fe2(C60Me10)Cp2.

Regio- and Diastereoselective Synthesis of Bis- and Tetrakisadducts of C70 by Directed Remote Functionalization Using Tröger Base Tethers

Double Bingel cyclopropanation of C70 with bismalonates featuring Tröger base derivatives as chiral spacers afforded bisadducts with almost perfect regio- and stereoselectivity. The excellent directing property of these rigidly folded spacers in the remote functionalization of the higher fullerene was further highlighted by the selective formation of a product with a novel bisaddition pattern involving the C(7)-C(22) and C(33)-C(34) bonds of C70. Enantiomerically pure bisadducts of C70 were prepared by highly diastereoselective transformations of bismalonates incorporating optically pure Tröger base tethers. The absolute configuration of these bisadducts was established by comparison of circular dichroism (CD) spectra with data reported in the literature. For the first time, optically active tetrakisadducts of a fullerene were prepared by two sequential chiral-spacer-controlled remote functionalizations.

Convergent Synthesis of a Polyfunctionalized Fullerene by Regioselective Five-Fold Addition of a Functionalized Organocopper Reagent to C60

[reaction: see text] A method for one-step preparation of polyfunctionalized fullerene derivatives by regioselective penta-addition of an organocopper reagent is described. A functionalized aryl iodide is first converted to the corresponding Grignard reagent and then to a copper reagent and finally is allowed to react with C(60). The method allows introduction of five functional groups to the C(60) skeleton in a convergent manner. The shuttlecock-like molecules crystallize into a columnar packing structure.

A theoretical study of fullerene–ferrocene hybrids

Using density functional theory within the generalized gradient approximation, I analyzed the electronic structure of a C(60)-ferrocene hybrid [= C(60) (*) FeCp] around HOMO in comparison with that of ferrocene, where C(60) (*) and Cp denote C(60)(CH(3))(5) and a cyclopentadienyl ring. HOMO-LUMO gap is significantly smaller than that of ferrocene because of the intervention of pi(C(60) (*)) states below LUMO. In addition, geometrical and electronic structures of N@C(60) (*) FeCp are also investigated. I find that there are two isomers with the energy difference of 0.13 eV. In one of the two, the encased nitrogen atom is located at the center of the fullerene cage. The Fe atom is eta(5)-coordinated to both Cp and R*, where R* is a five-membered ring of C(60) (*) cage. On the other hand, the atom is coordinated to R* with eta(4)-hapticity, and the nitrogen atom is bonded to a carbon atom of the R* ring in the other isomer. Upon the isomerization between the two isomers, there occurs a partial transfer of spin density between the nitrogen and Fe atoms as well as the creation and breaking of a C-N bond.

Coordination chemistry of buckybowls: from corannulene to a hemifullerene

This work highlights the progress made in coordination chemistry of transition-metal centers to open geodesic polyaromatic hydrocarbons that map onto the surface of C60, the family of compounds known as buckybowls or 'fullerene fragments'. In particular, an overview of our recent gas-phase coordination studies of several bowl-shaped polyarenes toward the dinuclear metal complex, [Rh2(O2CCF3)4], is given. Selected buckybowls include corannulene (C20H10) and two of its derivatives, namely dibenzo[a,g]corannulene (C28H14) and 1,3,5,7,9-penta-tert-butylcorannulene (C40H50), as well as a hemifullerene (C30H12). This study has resulted in the first X-ray structural characterization of buckybowl coordination complexes and has revealed eta2-rim coordination preferences of open geodesic polyarenes in rhodium(ii) binding reactions.