Substituent Influence on Charge Transfer Interactions in α,α′-Diferrocenylthiophenes

Abstract

Within a series of α,α′-diferrocenylthiophenes, the influence of electron-withdrawing substituents at the ferrocenyl moieties and electron-donating functionalities at the thiophene bridge on the electronic behavior was studied. The synthesis of the appropriate 2,5-fc 2 -c C 4 H 2 S and 2,5-fc 2 -3,4- CHO (3,7), CN (4, 8)) was carried out using the Negishi C,C cross-coupling protocol to effect the coupling of the thiophene and the ferrocenyl moieties. The corresponding diferrocenylthiophenes were characterized spectroscopically. Within this context, the structural properties of 2−5, [5][B(C 6 F 5 ) 4 ], 6, and 8 in the solid state were investigated by single-crystal X-ray diffraction studies. Electrochemical investigations of thiophenes 2−8 demonstrated an increasing redox separation and hence thermodynamic stability of the corresponding mixed-valent species in the series 1 < 3 < 5 < 4 ≈ 2 < 6 < 7 < 8. Moreover, UV/vis/near-IR and infrared spectroelectrochemical studies verify these observations. These studies further reveal a valence trapped situation in corresponding mixed-valent species and allow a class II classification according to Robin and Day.

Ferrocenes Bridged by Ethylenediamino Thiophene: Varying Charge Transfer Properties in a Series of 3,4-Di-N-substituted 2,5-Diferrocenyl Thiophenes

“…94,110,112 However, the observation of a couple of NIR absorptions close together, as typical for metalcontaining mixed-valent species, makes such a spectra analysis difficult (vide supra). 30,37,105,106,108,109,113 Furthermore, due to the complex capture of the effective charge transfer distance r ab , the geometrical distance of the interacting redox centers is (Table 2) roughly verify the tendency of the increasing electronic interaction in the series of thiophenes 1−4 and in comparison with 2,5-diferrocenyl thiophene as well as 2,5-DiFcEDOT. As an alternative, the investigation of the solvatochromic behavior of IVCT absorptions in a series of mixed-valent compounds is also a useful tool to evaluate the magnitude of the electronic coupling, since the corresponding transitions lose solvent dependency with an increasing electronic coupling.…”

“…The electronic properties of these compounds were investigated by voltammetric experiments in combination with UV−vis−NIR spectroelectrochemical measurements. 30,37,56 ■ RESULTS AND DISCUSSION Synthesis and Characterization. The preparation of thiophene 1 as starting material for molecules 2−4 ( Figure 1) could be performed in a Pd-promoted Suzuki−Miyaura C,C cross-coupling reaction of 2 equiv of ferroceneboronic acid with 1 equiv of 2,5-dibromo-3,4-dinitrothiophene 12,57 in absolute toluene, using K 3 PO 4 as base.…”

“…A similar "resonance effect" was previously reported for (3,5-bis(trifluoromethyl)-phenyl)ferrocenyl-substituted thiophene derivatives and is wellknown for ferrocenyl-functionalized N-aryl pyrroles in comparison with their N-alkyl analogues. 28,33,37 Within this context, an amino thiophene bridge with its dramatically lower oxidation potential (vs thiophene) should have a significant effect on the K C value of the corresponding mixed-valent diferrocenyl heterocycle. 33,37,86 However, for diamino thiophene 2, only a Fc/Fc + redox separation of ΔE°′ = 336 mV was found electrochemical results very similar to those for 2,5-DiFcEDOT (Table 1 and Figure 5).…”

“…28,33,37 Within this context, an amino thiophene bridge with its dramatically lower oxidation potential (vs thiophene) should have a significant effect on the K C value of the corresponding mixed-valent diferrocenyl heterocycle. 33,37,86 However, for diamino thiophene 2, only a Fc/Fc + redox separation of ΔE°′ = 336 mV was found electrochemical results very similar to those for 2,5-DiFcEDOT (Table 1 and Figure 5). 37 These observations suggest an electrosteric repulsion of the N,N-dimethylamino groups on the thiophene ring and thus a nonideal interaction of their nonbonding electron density with the thiophene π system.…”

“…35,102,103 Such a NIR absorption behavior is typical for very weak coupled class II systems, 30,31,35,50,97 since an observed IVCT absorption with a bandwidth at half height (Δν1 /2 ) significantly larger than the theoretical prediction (Δν1 /2 (theor)) in combination with a solvatochromic shift (Δνm ax (IVCT)) 900 cm −1 larger as found for 2,5-diferrocenyl thiophene itself supports this classification and verifies the electrochemical results (vide supra; Table 2 and Figure 6). 37 Upon generation of 1 2+ , the IVCT absorption band decreases, while the MLCT around 600 nm undergoes a formal bathochromic and hypochromic shift ( Figure 6). Otherwise, UV−vis−NIR spectroelectrochemical investigations during the reduction of thiophene 1 reveal a hypsochromic shift of the latter transition together with the appearance of two additional absorptions around 11000 and 18000 cm −1 , probably caused by ligand to metal charge transfer (LMCT, dinitro thiophene to ferrocenyl) interactions in 1 − ( Figure S9 in the Supporting Information).…”