The Intramolecular Electronic Interaction Regulated by Five-membered Heterocycle Spacer

The spacer in diferrocenyl derivatives has signicant effects on intramolecular electronic interaction properties. In this work, nine diferrocenyl ve-membered heterocyclic molecules are synthesized as models to investigate the effect of intramolecular electron-transfer properties systematically, including 2,5-diferrocenyl-1-phenyl-pyrrole (1), 2,5-diferrocenylfuran (2), 2,5-diferrocenylthiophene (3), 2,5-diferrocenyl-1H-imidazole (4), 2,5-diferrocenyloxazole (5), 2,5-diferrocenylthiazole (6), 2,5-diferrocenyl-1,3,4-triazole (7), 2,5-diferrocenyl-1,3,4-oxadiazole (8) and 2,5-diferrocenyl-1,3,4-thiadiazole (9). The molecules were prepared in cyclization reaction and characterized by Elemental analysis, FT-IR, MS and NMR. Moreover, the molecular structures of 2,5-diferrocenylthiazole and 2,5-diferrocenyl-1,3,4-oxadiazole were determined by the single crystal X-ray diffraction. The intramolecular electronic interactions were investigated through cyclic voltammetry in combination with density functional theory (DFT) calculations. The results revealed that the electronic interaction decreased with the increase of heteroatoms in central heterocycle spacer, and the electron-transfer property could be regulated by regulate central heterocycle spacer species.


Introduction
The ferrocene (Fc) is widely used in investigating the intramolecular electron transfers due to its redox activity and electrochemical stability, and the research on intramolecular electron transfers could provide simple and convenient for designing prospective ferrocene-containing molecular wires and conducting polymers [1][2][3][4] . Generally, two or more ferrocenyl units assembled through various conducting spacers to construct molecular models for studying intramolecular electron transfers 5 , the various kind spacers involving of the atom 6 , alkenyl 7 , alkynyl 8 and various aromatic rings 9 . When one of ferrocene unit occur electrochemical oxidize, an intramolecular donor-acceptor system is formed, the charge may be transfer to the other ferrocene units through the spacers, and the spacers' types 10 , structures 11 and chemical properties 12 can in uence the electron transfers capability. For example, when a single C atom spacer 13 or a single N atom spacer 14 link two-ferrocene units, the charge transfer capability of the C atom is better than that of the N atom. In addition, diferrocenyl derivatives containing π-conjugate spacers such as the alkenyl 15 and alkynyl 8 , the electronic interaction capability of the alkenyl is better than that of the alkynyl.
These electronic interaction properties can be associated with the atomic outer boundaries, ionization degree, and hybridization structure factors [16][17][18] . However, when diferrocenyl derivatives containing aromatic rings spacers, the charge transfers are in uenced by various complex factors, including metalmetal center distance 19 , position of substituent 20 , center ring geometric properties 21 , topology 22 and aromatic properties 23,24 , which in uencing factor is the primary factor of intramolecular electron transfer is still an academic debate. In order to further explore this problem, more and more researchers are engaged in numerous heterocycle spacers' studies, especially the ve-membered aromatic heterocycle spacers 25,26 , because these spacers involves π-conjugate structure and heteroatom, which can provide changing chemical structure for studying.
In our research group previous works, the spacers' charge density, properties of substituent and coplanarity of block connections were identi ed as key factors of intramolecular electron-transfers 27-30 . In recent work, the single-heteroatom-ve-membered-heterocycle bridged diferrocenyl derivatives were examined, which exhibited the charge transfer highly depend on the short-range heteroatom bridge 31,32 , but this research had not systematically accomplished. One of inadequacies was the charge transfer properties on the central ve-membered heterocycle spacer containing two and more heteroatoms were still unknown.
To reply the above-mentioned problem, a series of 2,5-diferrocenyl imidazole, oxazole, thiazole, triazole, oxadiazole and thiadiazole derivatives were synthesized as model molecules in this paper (Figure 1), and the intramolecular electron-transfers processes of these model molecules were systematically studied through cyclic voltammetry (CV) and density functional theory (DFT) calculations. The result revealed that the species and the numbers of heteroatom in central spacers could regulate intramolecular electronic transfer processes.

Results And Discussion
The molecular structures and the corresponding data of 1-3 are referenced form our academic paper 31 .
The 4-9 were con rmed by FT-IR, 1 H-NMR, 13 C-NMR, Elemental analysis, and MS (corresponding data are given in synthetic methods). The molecular structure of 6 and 8 were determined by single-crystal X-ray diffraction and shown in Figure S1 and Figure S2. Crystal data and relevant structural parameters enumerate in Table S1. The selected bond lengths, the selected angles and the selected torsion angles list in Table S2 and Table S3. The molecular structure of 9 is referenced form Heinrich Lang published academic paper 33 .

Electrochemistry
The redox potential of compounds 1-9 is determined through cyclic voltammetry, and the electrochemical data is listed in Table 1. The compounds 1-6, 8 and 9 display two redox well-resolved waves in the range of 0-0.8 V (Figure 2-4), which are assigned to the two FeII/FeIII redox couples, from the Ipa/Ipc (≈1) values of each couple, it can be concluded that the redox processes are electrochemical reversible oneelectron-transfer processes 31 . The 7 as an exceptional case display one redox well-resolved wave ( Figure   2), and it is electrochemical reversible two-electron-transfer process 32 .
The rst oxidation potentials (Ea 1 ) of ferrocenyl unit in 1-9 increase, with the increase of heteroatoms in central ve-membered heterocycle spacer ( Table 1). The Ea 1 of 1 is 58 mV and the Ea 1 of 4 is 138 mV, the Ea 1 of 7 even increase to 344 mV, with the central heterocycle form pyrrole change into imidazole and triazole. The Ea 1 of 2, 5 and 8 are 184 mV, 328 mV and 321 mV, respectively, the central heterocycle form furan change into oxazole and oxadiazole. The Ea 1 of 3, 6 and 9 are 217 mV, 304 mV and 308 mV, respectively, the central spacer form thiophene change into thiazole and thiadiazole. The π-conjugation effects of 3, 6 and 9 were investigated through comparing the dihedral angles to explore the Ea 1 changing trend. The two dihedral angles formed by cyclopentadiene planes to the central thiophene plane both are 8.93° of 3. The two dihedral angles formed by cyclopentadiene planes to the central thiazole plane of 6 are different, the one formed by plane S1-C6-N1-C7-C8 to C1-C2-C3-C4-C5 is 3.52°, the other formed by plane S1-C6-N1-C7-C8 to C9-C10-C16-C17-C18 is 5.30°. The two dihedral angles formed by cyclopentadiene planes to the central thiadiazole plane both are 17.08° of 9 ( Figure S3). In theory, the smaller dihedral angle indicate more coplanarity between the cyclopentadienyl planes and the central plane, which suggests there are stronger π-conjugation effects with each other, and the Ea 1 should be lower 32 . Hence, the π-conjugation effect order is 6 > 3 > 9 indicate by dihedral angles, the order of Ea 1 should be 6 (Ea 1 =304 mV) < 3 (Ea 1 =217 mV) < 9 (Ea 1 =308 mV), but this result was disagreement with our experiment. Therefore, the embedded N atoms in 6 and 9 debase charge density of central heterocycle with its stronger electron withdrawing effects, and block the charge transfer between two ferrocenyl units.
The electronic communication effects of 1-9 were discussed through comparing the oxidation potential differences (∆E) with two ferrocenyl units. The ∆E of 1 is 315 mV, the ∆E of 4 is 210 mV. However, the 7 has only one redox wave, of which ∆E cannot be measured. The ∆E of 2 is 161 mV, the ∆E of 5 is 130 mV and the ∆E of 8 is 88 mV. Moreover, the ∆E of 3 is 139 mV, the ∆E of 6 is 124 mV and the ∆E of 9 is 86 mV (Table 1), respectively. Clearly, the values of ∆E are trending downward obviously, which exhibit the embedded N atoms in central spacers have debased electronic communication between two ferrocenyl units.
The Natural Bond Orbital charge (NBO charge) population of heteroatom studies con rmed that electronic communication was highly affected by shorter heteroatom-linked bridge ( Table 2 (Table S4). Hence, the electronic communication capacity was highly depend on shorter heteroatom-linked bridge NBO charge, this conclusion was consistent with our previous work 31 , and the shorter heteroatom-linked bridge NBO charges was in uenced by the molecules structure of central heterocyclic. This work revealed that the electrochemical interaction receded with increase the heteroatom number in central heterocycle spacer, and the electronic interaction could be arti cial regulated by modi ed molecular structure of central heterocycle spacer.
In conclusion, nine 2,5-diferrocenyl ve-membered derivatives have been discussed deeply as the systematic electrochemical model. The results revealed electronic interaction had non-signi cant relationship with the distance of bimetal, π-conjugation effect and aromaticity of the central heterocyclic.
The N atoms located in the 3,4-position of center heterocyclic could severe regulate the electronic transfer between two ferrocenyl units, and the intramolecular electronic interaction receded with the increase of heteroatom in centre heterocycle spacer.

Materials
All operations were carried out in an atmosphere of puri ed argon. All solvents were dried and distilled according to standard procedures. The reactions were monitored by thin-layer chromatography (TLC). 2bromoacetyl-ferrocene 40 , 1,2-bis(ferrocenecarbonyl)hydrazine 41,42 , ferrocenecarbonyl-hydrazine 43 , N-(Ferrocenecarbonylmethyl)ferrocenecarboxamide 44 , ferroceneamidine 45,46 and cyanoferrocene 47 were prepared according to literature methods, and the solvents were commercially available. The 1-3 synthetic method have been report in our previous work 31 . The 4-9 were synthesized through cyclization reaction and the schematic preparation procedure of compounds ( Figure 6) and characterization data are  for 24 h, and then ferrocenecarbonyl-hydrazine (244.2 mg, 1.0 mmol) was added to reactor stirred at 75 °C for 24 h. The reaction solution was poured into crushed ice, then the solvent and the precipitate were extracted by ethyl acetate (25 ml×5), and then the organic phase was removed in vacuo. And the residue was subjected to chromatographic separation on silica gel column (2.0×20 cm) using a mixture of dichloromethane/petroleum ether (1/1, v/v) to elute the product at room temperature. The rst yellow band was unreacted cyanoferrocene. 2,5-diferrocenyl-1,3,4-oxadiazole (8): 1,2-bis(ferrocenecarbonyl)hydrazine (228.1 mg, 0.5 mmol) and pyridine (20 ml) were added to a shrek reactor in atmosphere of pure argon stirred at 0 °C for 0.5 h, then POCl 3 (0.5 ml, 5.0 mmol) was added to reactor stirred at room temperature (20 °C) for 12 h. The reaction solution was poured into crushed ice, then the solvent and the precipitate were extracted by ethyl acetate (20 ml×3), and then the organic phase was removed in vacuo. And the residue was subjected to chromatographic separation on silica gel column (2.0×20 cm) using a mixture of dichloromethane/petroleum ether (1/1, v/v) to elute the product at room temperature. The rst orange band was compound 8 (155.4  2,5-diferrocenyl-1,3,4-thiadiazole (9): 1,2-bis(ferrocenecarbonyl)hydrazine (228.0 mg, 0.5 mmol), lawessons reagent (244.3 mg, 0.6 mmol) and THF (20 ml) were added to a shrek reactor under atmosphere of pure argon stirred at 80 °C for 12 h. The reaction solution was cooled to room temperature, then the solvent was removed in vacuo, and the residue was subjected to chromatographic separation on silica gel column (2.0×20 cm) using a mixture of dichloromethane/petroleum ether (1/1, v/v) to elute the product at room temperature.. The rst orange band was compound 9 (189.0 mg). The single crystal of 9 was obtained through recrystallizing from hexane/dichloromethane (4/1, v/v) at low temperature (-15 °C).