α,ω-Diferrocenyl Cumulene Molecular Wires. Synthesis, Spectroscopy, Structure, and Electrochemistry

Abstract

Cumulene sp-carbon molecular wires C 2 [Fc(Ph)CdC(Ph)Fc] up to C 7 [Fc(Ph)CdCdCd CdCdCdC(Ph)Fc] endcapped by two electroactive ferrocenyl groups are presented in this report. Synthetically, ferrocenyl cumulenes can be built-up by a modular strategy using C 1 synthon ferrocenyl(phenyl)ketone as starting material with various acetylenic/propargylic/ homopropargylic C 2 -C 5 reagents, taking into account and exploiting the efficient stabilization of an electron-deficient carbenium center by an adjacent ferrocenyl moiety. With increasing cumulene chain length the reactivity of cumulenes increases considerably, indicating steric protection as the main requirement for bulk stability. Even cumulenes C 2 , C 4 , and C 6 are conjugated "molecular wires" effecting electronic communication between the terminal ferrocenyl substituents, whereas odd cumulenes C 3 , C 5 , and C 7 are nonconjugated and electronically decoupled due to their orthogonal terminal π-systems. Electrochemically, separate redox waves can be detected up to a C 6 cumulene spacer, but the electronic communication between the endcapping redox-active ferrocenyl substituents decreases with increasing cumulene length.

The Synthesis and Properties of Bimetallic d-^|^pi; Electron Systems Containing Metallocenyl Substituents of Fe or Ru, and ^|^pi;-Electron Spacers of Heavier Main Group Elements from Group 14 or 15

“…It should be noted that the oxidation processes for both Fc moieties were observed as two separated steps (ΔE E 1 E 2 ) with ΔE 0.55 (3b), 0.42 (4), and 0.48 V (5), respectively. These ΔE values are considerably larger than those observed for Fc N N Fc (ΔE 0.21 V), 1a (E) Fc(Ph)-C C(Ph)Fc (ΔE 0.18 V), 24 (E) Tip(Fc)Si Si(Fc)Tip (ΔE 0.19 V), 15,16 and (E) Tip(Fc)Ge Ge(Fc)Tip (ΔE 0.38 V), 19 suggesting a signi cantly increased and effective ability for conjugation of the heavier group 15 element π spacers relative to those of second row elements, and even heavier group 14 elements. Notably, the ΔE of diphosphene 3b was found to be a the largest of the 1,2 bis(ferrocenyl)dipnictenes.…”

“…27 The electrochemical properties of the respective metallocenes should then also be re ected in the characteristic redox properties of 1,2 bis(metallocenyl)disilenes 1 and 1 when considering them as d π electron systems. The separation of the half wave oxidation potentials were measured as ΔE 1/2 0.19 V (1) and ΔE 1/2 0.20 V (1 ), which is comparable to the carbon analogue (E) Ph(Fc)C C(Fc)Ph (21) (ΔE 1/2 0.18 V), 24 indicating a signi cant coupling between the two metallocenyl moieties in 1 and 1 via the Si Si π bond. Interestingly, the two ruthenocenyl moieties in disilene 1 were found to be able to communicate electrochemically through the Si Si π electrons, which is in stark contrast to the corresponding carbon analogue (E) Me(Rc)C C(Rc)Me (22).…”

“…The separation of the half wave oxidation potentials (ΔE 1/2 ) is indicative of the magnitude of electronic interaction between two redox centers. 26 The separation issigni cantly larger in 2 (ΔE 1/2 0.44 V) than in disilene 1 (ΔE 1/2 0.19 V) 15,16 and (E) Ph(Fc)C C(Fc)Ph (21, ΔE 1/2 0.18 V), 24 re ecting the increased degree of d π electron interaction in 2 relative to its silicon (1) and carbon (21) analogues. With respect to objective (i), the electrochemical analysis indicated that digermene 2 is a very effective d π electron system, exhibiting in comparison to disilene 1 and ole n 21 a stable, multi step redox behavior.…”

Synthese und Struktur von Tetraarylcumulenen: Charakterisierung der Bindungslängenalternanz in Abhängigkeit der Moleküllänge

“…Es wurde jedoch schnell ersichtlich, dass die Di(tertbutyl)phenyl-Gruppen (R = tBu 2 C 6 H 3 ) keine ausreichende Stabilisierung der [7] …”

“…[2d-f, 3] Moleküle mit einem sp-hybridisierten Kohlenstoffgerüst kçnnen in ihrer Grundstruktur einerseits den Polyinen (alternierende Einfach-und Dreifachbindungen), andererseits den Cumulenen (kumulierte Doppelbindungen) entsprechen. Die Chemie der Polyine ist soweit fortgeschritten, dass Verbindungen mit bis zu 44 aufeinanderfolgenden Kohlenstoffatomen (22 Acetyleneinheiten) synthetisiert werden kçnnen.[4] Ihre physikalischen und optoelektronischen Eigenschaften wurden ausgiebig in zahlreichen Studien untersucht.[5] Umso überraschender erscheint die Tatsache, dass die Chemie der Cumulene seit den frühen Arbeiten [6,7] von Kuhn [8,9] und Bohlmann [10,11] im Grunde keine Beachtung fand. Somit verbleiben viele unbeantwortete Fragen bezüg-lich der physikalischen Eigenschaften dieser faszinierenden Klasse linearer Moleküle.…”