Bridge Conformational Effects On The Electronic Coupling And Electron Transfer In The Covalently Bonded Dimolybdenum D-B-A Systems

In order to further explore the imparts of bridge conformation on the electronic coupling in donor?D?-bridge?B?-acceptor?A?system,in this study,covalently bonded[Mo₂]complex units and large aromatic bridging ligands have been used to create[Mo₂]-B-[Mo₂]experimental models.By changing the chelating atoms from oxygen?O?to sulfur?S?of the bridging ligands,three compounds with similar molecular compositions and structures,but varied torsion angles???between the chelating ring and the ligand plane have been synthesized.Meanwhile,introduction of S atoms to the molecules affects significantly the electron donating or accepting ability of the[Mo2]complex units.The factors that affect the electronic coupling are evaluated experimentally and theoretically;it is found that the bridge conformation plays a dominant role in determining the electronic coupling and electron transfer.Three novel dimolybdenum dimers involved in this study are[Mo₂?DAniF?₃]₂(9,10-O₂CC₁₄H₈CO₂),[Mo₂?DAniF?₃]₂(9,10-OSCC₁₄H₈COS)and[Mo₂?DAniF?₃]₂(9,10-S₂CC₁₄H₈CS₂),?DAniF=N,N?-di?p-anisyl?formamidinate?,denoted as[OO-?9,10-anth?-OO],[OS-?9,10-anth?-OS]and[SS-?9,10-anth?-SS],which are synthesized by assembling two quadruply bonded complex[Mo₂?DAniF?₃]⁺units with 9,10-anthracenyl dicarboxylate or its thiolated bridging ligandThe obtained compounds have been fully characterized by X-ray crystal diffraction,electron paramagnetic resonance spectroscopy?EPR?,electrochemistry and UV-VIS-NIR spectroscopy.These compounds feature a large deviation of the central anthracene ring from the plane defined by the Mo-Mo bond vectors with increasing torsion angle?⁵0-76°?as the chelating atoms of the bridging ligand vary from O to S.Electrochemical measurements of these compounds exhibits one redox couple with the potentials in the range of 76-92 mV?AgCl/Ag?.The mixed-valence complexes,generated by one-electron oxidation of the neutral compounds using ferrocenium hexafluorophosphate,show one peak at g?1.94 in the EPR spectra,indicating that the odd electron is localized in the Mo2 center.Importantly,the mixed-valence complexes[OO-?9,10-anth?-OO]⁺,[OS-?9,10-anth?-OS]⁺and[SS-?9,10-anth?-SS]⁺do not exhibit characteristic intervalence charge transfer absorption in the near-IR spectra,in contrast to the phenylene and naphthalene bridged analogues.From these results,the anthrancene bridged Mo2 dimers should be assigned to Class I in Robin-Day's classification,for which the odd electron is localized on one of the Mo2 centers.In comparison between the Mo₂ dimers with the three different bridges?phenelyene,naphthalene and anthrancene?it is found that the torsion angles between the Mo₂ units and bridge control the transition from electron localized Class I to delocalized III.DFT calculations for the three series show that the energy gaps HOMO-LUMO(?E_H-L)increase and HOMO-HOMO-1(?E_H-H-1)decrease as the torsion angles?increase.Linear correlations between the energy gaps and cos²?are established.Both experimental and theoretical results indicate that the smallest torsion angles in the phenylene bridged series enhance the electron transfer,while in the anthrancene bridged series,with the largest torsion angles,the electronic coupling between the two[Mo₂]units are suppressed.Therefore,bridge conformational variation through molecular design controls the orbital interactions between the Mo₂ center and the bridge,consequently,tuning the electronic coupling between the Mo₂ centers.Therefore,this study verifies the through-bond superexchange formalism on electronic coupling and electron transfer.