Search

The oxidative cleaving of a rhenium-rhenium bond by bromine in binuclear Fischer carbene complexes proves to be an effective method to prepare mononuclear bromido-carbene complexes. The reaction of mono- and dilithiated thieno[2,3-b]thiophene (2,3-b-TTH 2 ) and thieno[3,2-b]thiophene (3,2-b-TTH 2 ) with [Re 2 (CO) 10 ] affords dirhenium nonacarbonyl ethoxycarbene complexes, [Re 2 (CO) 9 {C(OEt)2,3-b-TTH}] (1a) and [Re 2 (CO) 9 {C(OEt)3,2-b-TTH}] (1b), and the tetrarhenium bis(ethoxycarbene) complexes from the dilithiated thiophene substrates, [Re 2 (CO) 9 -μ-{C(OEt)-2,3-b-TT-C(OEt)}Re 2 (CO) 9 ] (2a) and [Re 2 (CO) 9 -μ-{C(OEt)-3,2-b-TT-C(OEt)}Re 2 (CO) 9 ] (2b) featuring bridging thiophene linkers. Rhenium-rhenium bond cleavage by bromine of the monocarbene complexes yielded the scarce class of monorhenium bromido-carbene complexes, cis-[Re(CO) 4 {C(OEt)2,3-b-TTH}Br] (3a) and cis-[Re(CO) 4 {C(OEt)3,2-b-TTH}Br] (3b), while the corresponding reaction of the biscarbene tetrarhenium carbonyl complex of thieno[2,3-b]thiophene afforded the cleaving of both metal-metal bonds to give the novel dirhenium biscarbene dibromido complex with a thienothiophene spacer, [Re(CO) 4 Br-μ-{C(OEt)-2,3-b-TT-C(OEt)}Re(CO) 4 Br] (4a). A new indirect aminolysis route is described to prepare the chlorido dimethylaminocarbene complex 5acis-[Re(CO) 4 {C(NMe 2 )2,3-b-TTH}Cl], with unexpected cleavage of the Re-Re bond. Single crystal X-ray diffraction studies were performed on 1a, 2a, 3a, 5a, 1b and 3b. Spectroscopic and electrochemical methods are employed to investigate the electronic effect of the different conjugation pathways in the different thienothiophenyl carbene substituents, and the replacement of the rhenium pentacarbonyl fragment with a bromido ligand.

The second law of thermodynamics is well known for determining the direction of spontaneous processes in the laboratory, life and the universe. It is therefore often called the arrow of time. Less often discussed but just as important is the effect of kinetic barriers which intercept equilibration and preserve highly ordered, high energy non-equilibrium states. Examples of such states are many modern materials produced intentionally for technological applications. Furthermore, all living organisms fuelled directly by photosynthesis and those fuelled indirectly by living on high energy nutrition represent preserved non-equilibrium states. The formation of these states represents the local reversal of the arrow of time which only seemingly violates the second law. It has been known since the seminal work of Prigogine that the stabilisation of these states inevitably requires the dissipation of energy in the form of waste heat. It is this feature of waste heat dissipation following the input of energy that drives all processes occurring at a non-zero rate. Photosynthesis, replication of living organisms, self-assembly, crystal shape engineering and distillation have this principle in common with the well-known Carnot cycle in the heat engine. Drawing on this analogy, we subsume these essential and often sophisticated driven processes under the term machinery of life.

For the first time, a C(60) derivative (1) and two different lanthanum metallofullerene derivatives, La@C(82)Py(2) and La(2)@C(80)Py (3), that feature a pyridyl group as a coordination site for transition-metal ions have been synthesized and integrated as electron acceptors into coordinative electron-donor/electron-acceptor hybrids. Zinc tetraphenylporphyrin (ZnP) served as an excited-state electron donor in this respect. Our investigations, by means of steady-state and time-resolved photophysical techniques found that electron transfer governs the excited-state deactivation in all of these systems, namely 1/ZnP, 2/ZnP, and 3/ZnP, whereas, in the ground state, notable electronic interactions are lacking. Variation of the electron-accepting fullerene or metallofullerene moieties provides the incentive for fine-tuning the binding constants, the charge-separation kinetics, and the charge-recombination kinetics. To this end, the binding constants, which ranged from log K(assoc) =3.94-4.38, are dominated by axial coordination, with minor contributions from the orbital overlap of the curved and planar π systems. The charge-separation and charge-recombination kinetics, which are in the order of 10(10) and 10(8) s(-1) , relate to the reduction potential of the fullerene and metallofullerenes, respectively., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Two stable electron donor-acceptor conjugates, that is, 3 and 5b, employing La(2)@I(h)-C(80) and Sc(3)N@I(h)-C(80), on one hand, and zinc tetraphenylporphyrin, on the other hand, have been prepared via [1+2] cycloaddition reactions of a diazo precursor. Combined studies of crystallography and NMR suggest a common (6,6)-open addition pattern of 3 and 5b. Still, subtly different conformations, that is, a restricted and a comparatively more flexible topography, emerge for 3 and 5b, respectively. In line with this aforementioned difference are the electrochemical assays, which imply appreciably stronger I(h)-C(80)/ZnP interactions in 3 when compared to those in 5b. Density functional calculations reveal significant attractions between the two entities of these conjugates, as well as their separately localized HOMOs and LUMOs. The geometrical conformations and LUMO distributions of 3 and 5b, at our applied computational level, are slightly varied with their different endohedral clusters. The clusters also exert different impact on the excited state reactivity of the conjugates. For example, 3 undergoes, upon photoexcitation, a fast charge separation process and yields a radical ion pair, whose nature, namely, (La(2)@C(80))(•-)-(ZnP)(•+)) versus (La(2)@C(80))(•+)-(ZnP)(•-)), varies with solvent polarity. 5b, on the other hand, afforded the same (Sc(3)N@C(80))(•-)-(ZnP)(•+)) radical ion pair regardless of the solvent., (© 2011 American Chemical Society)

We report for the first time the versatile Ce(2)@I(h)-C(80) building block toward synthesizing a novel electron donor-acceptor conjugate, Ce(2)@I(h)-C(80)-ZnP (1). A systematic investigation of the charge transfer chemistry documents a reductive charge transfer (i.e., formation of (Ce(2)@I(h)-C(80))(*-)-(ZnP)(*+)) in nonpolar media (i.e., toluene/THF), while an oxidative charge transfer (i.e., formation of (Ce(2)@I(h)-C(80))(*+)-(ZnP)(*-)) dominates in polar media (i.e., benzonitrile/DMF). Reduction of the [Ce(2)](6+) cluster, which is highly localized and collinearly arranged with respect to the quaternary bridge carbon, is sufficiently exothermic in all solvents. Notably weak is the electronic coupling between the [Ce(2)](6+) cluster and the electron-donating ZnP. The oxidation of C(80)(6-) and the simultaneous reduction of ZnP, on the other hand, necessitate solvent stabilization. In such a case, the strongly exothermic (Ce(2)@I(h)-C(80))(*-)-(ZnP)(*+) radical ion pair state formation is compensated within the framework of a nonadiabatic charge transfer by a C(80)(6-)/ZnP electronic matrix element, as the sum of good overlap and short distance, that exceeds that for [Ce(2)](6+)/ZnP.

Stable donor-acceptor conjugates (2, 3) involving an endohedral metallofullerene, La(2)@I(h)-C(80), and pi-extended tetrathiafulvalene (exTTF) have been synthesized by highly regioselective 1,3-dipolar cycloadditions of exTTF-containing azomethine ylides to the endofullerene, yielding exclusively [5,6] metallofulleropyrrolidines with C(1) symmetry in high yields (68-77%). The cyclic voltammograms (CVs) of the conjugates reveal the redox active character of the system due to the presence of both donor and acceptor groups, that is, exTTF and La(2)@I(h)-C(80), respectively. Furthermore, the electrochemically reversible character of the endofullerene confirms the presence of the [5,6] adduct. Despite the relatively close proximity between the exTTF and the endohedral metallofullerene (EMF), only a weak electronic interaction was observed in the ground state, as evidenced by absorption spectroscopy and CV measurements of 2 and 3. On the other hand, in the excited state the fast formation of a radical ion-pair state (i.e., 6.0 x 10(10) s(-1)), that is, the reduction of the electron accepting La(2)@C(80) and the oxidation of exTTF, evolves with lifetimes as long as several ns (3.0 x 10(8) s(-1)) in toluene. Transient absorption spectroscopy experiments confirmed these observations.

A pentapod deca(aryl)[60]fullerene, C(60)(C(6)H(4)CO(2)H)(5)(C(6)H(4)Fc)(5)Me(2) (4; Fc = ferrocenyl), bearing five carboxylic acid and five ferrocenyl groups was synthesized through top and bottom functionalization of [60]fullerene by means of copper-mediated penta-addition reactions. For electrochemical measurements (i.e., E(ox) = 0.08 V, five-electron oxidation of the ferrocenyl groups; E(red) = -1.89 and -2.28 V for the fullerene part vs Fc/Fc(+)), we used an ester-protected compound, C(60)(C(6)H(4)CO(2)Et)(5)(C(6)H(4)Fc)(5)Me(2) (2), and 4 was probed by performing femtosecond flash photolysis experiments in a variety of organic solvents. Importantly, the formation of a radical ion pair state was corroborated with lifetimes of up to 333 ps in toluene. In complementary studies, penta(carboxylic acid)-penta(ferrocenyl) compound 4 was deposited on indium-tin oxide (ITO) electrodes with a surface coverage (i.e., 0.14 nmol/cm(2)) that corresponded to a unique bilayer structure. Decisive for the bilayer motif is the presence of five ferrocenyl groups, which are assembled with a merry-go-round-shaped arrangement on the [60]fullerene. The novel 4/ITO photoelectrode gave rise to a cathodic photocurrent with a 12% quantum yield in the presence of methyl viologen, whereas an anodic photocurrent was generated in the presence of ascorbic acid for a C(60)(C(6)H(4)CO(2)H)(5)(C(6)H(5))(5)Me(2) (5)/ITO photoelectrode. Photophysical investigations revealed that the difference in photocurrent, that is, cathodic versus anodic photocurrents, is related to the nature of the excited state feature in 4 (i.e., charge separated state) and 5 (i.e., triplet excited state). The unique molecular architecture of 4, in combination with its remarkable donor-acceptor properties, validates the use of the pentapod deca(aryl)[60]fullerene in photoelectrochemically active molecular devices.