Reactions of Cp*2Yb(THF)2 with TCNQ and TCNB. A Novel Type of Reactivity of TCNQ toward Metallocenes. The First Example of the π-Dimer [TCNB2]2−

Assembly of a trapped valent CeIII/IV-TCNQ complex through metal-ligand redox cooperativity

Complex (Cp3CeIV)2(TCNQ)(CeIIICp3)2 (1) was prepared by reducing neutral TCNQ0 (tetracyanoquinodimethane) with Cp3Ce(THF). Two types of cerium centres with a dianionic TCNQ2- moiety are present in 1, wherein each of the four cyano-groups are bound by a cation. Formation of this trapped-valent organocerium compound 1 is facilitated by metal-ligand redox cooperativity. Characterization of 1 was carried out using structural-, magnetometry-, and IR-spectroscopic analyses. Photophysical studies on this compound reveal CeIII luminescence, and opens up avenues for promising multifunctional, mixed-valent lanthanide materials.

Divalent metallocenes of the lanthanides - a guideline to properties and reactivity

Since the discovery in the early 1980s, the soluble divalent metallocenes of lanthanides have become a steadily growing field in organometallic chemistry. The predominant part of the investigation has been performed with samarium, europium, and ytterbium, whereas only a few reports dealing with other rare earth elements were disclosed. Reactions of these metallocenes can be divided into two major categories: (1) formation of Lewis acid-base complexes, in which the oxidation state remains +II; and (2) single electron transfer (SET) reductions with the ultimate formation of Ln(III) complexes. Due to the increasing reducing character from Eu(II) over Yb(II) to Sm(II), the plethora of literature concerning redox reactions revolves around the metallocenes of Sm and Yb. In addition, a few reactivity studies on Nd(II), Dy(II) and mainly Tm(II) metallocenes were published. These compounds are even stronger reducing agents but significantly more difficult to handle. In most cases, the metals are ligated by the versatile pentamethylcyclopentadienyl ligand: (C₅Me₅). Other cyclopentadienyl ligands are fully covered but only discussed in detail, if the ligand causes differences in synthesis or reactivity. Thus, the focus lays on three compounds: [(C₅Me₅)₂Sm], [(C₅Me₅)₂Eu] and [(C₅Me₅)₂Yb] and their solvates. We discuss the synthesis and physical properties of divalent lanthanide metallocenes first, followed by an overview of the reactivity rendering the full potential of these versatile reactants.

Formation of Long, Multicenter π-[TCNE]22- Dimers in Solution: Solvation and Stability Assessed through Molecular Dynamics Simulations

Purely organic radical ions dimerize in solution at low temperature, forming long, multicenter bonds, despite the metastability of the isolated dimers. Here, we present the first computational study of these π-dimers in solution, with explicit consideration of solvent molecules and finite temperature effects. By means of force-field and ab initio molecular dynamics and free energy simulations, the structure and stability of π-[TCNE]₂^2- (TCNE=tetracyanoethylene) dimers in dichloromethane have been evaluated. Although the dimers dissociate at room temperature, they are stable at 175 K and their structure is similar to the one in the solid state, with a cofacial arrangement of the radicals at an interplanar separation of approximately 3.0 Å. The π-[TCNE]₂^2- dimers form dissociated ion pairs with the NBu₄⁺ counterions, and their first solvation shell comprises approximately 20 CH₂ Cl₂ molecules. Among them, the eight molecules distributed along the equatorial plane of the dimer play a key role in stabilizing the dimer through bridging C-H⋅⋅⋅N contacts. The calculated free energy of dimerization of TCNE^.- in solution at 175 K is -5.5 kcal mol^-1 . These results provide the first quantitative model describing the pairing of radical ions in solution, and demonstrate the key role of solvation forces on the dimerization process.

Variable coordination of redox-active TCNB in discrete and polymeric ferrocenylcopper(I) complexes: structures and spectroelectrochemical behaviour

1,2,4,5-Tetracyanobenzene (TCNB) was reacted with [Cu(dppf)(CH3CN)2](BF4) and [Cu(dchpf)(CH3CN)](BF4), dppf = 1,1'-bis(diphenylphosphino)ferrocene and dchpf = 1,1'-bis(dicyclohexylphosphino)ferrocene, to produce a heterotetranuclear metallamacrocycle 1, {[Cu(dppf)(μ-TCNB)](BF4)}2, and a heterooctanuclear complex 2, [{Cu(dchpf)}4(μ4-TCNB)](BF4)4. Complex 1 is the first example of a structurally characterised discrete transition metal complex of TCNB. Upon crystallisation attempts, compound 2 formed the structurally identified coordination polymer 3, {[Cu(dchpf)(μ-TCNB)]2(BF4)2}n. Structural and spectroscopic analyses confirmed the redox-innocent behaviour of TCNB in 1, 2 and 3. However, the soluble compounds 1 and 2 could be oxidised and reduced spectroelectrochemically (UV-vis, IR, and EPR). The oxidation occurs invariably at the ferrocene sites without notable splitting of redox potentials. Reduction involves the TCNB bridging ligands to produce radical complexes. As a variably bridging acceptor component of supramolecular structures the TCNB ligand thus adopts an intermediate position between the highly electron transfer-active TCNE, TCNQ and TCNQF4 systems and the numerous redox-innocent bridges.