Syntheses And Electron Coupling Properties Of Saturated Ligand Bridged Dimolybdenum Complexes

Electron transfer is an important scientific issue in multiple disciplines including chemistry,physics and biology.As the models for study of electron transfer,the mixed-valence D-B-A molecules have been extensively studied,with a focus on conjugated bridging D-B-A molecules.The study also has covered mixed-valence D-B-A systems involving?-?weak interactions or hydrogen-bonded bridging.However,there are few reports on the electron coupling and electron transfer in D-B-A molecules with saturated bridges.In order to systematically study the electronic coupling and electron transfer in saturated bridge ligand linked D-B-A systems,a series of D-B-A compounds with dimolybdenum units as electron donor?D?and acceptor?A?and saturated organic moiety as the bridge have been designed and synthesized.In this dissertation,the impacts of the bridge conjugation,ancillary groups of the Mo₂ center and effective distance of electrons on the metal?Mo₂?-metal?Mo₂?interactions have been studied by electrochemistry and spectroscopy.In the second chapter of this dissertation,1,4-cyclohexanedicarboxylate bridged dimolybdenum dimers[{Mo₂?ArNCHNAr?₃}₂(?-O₂CC₆H₁₀CO₂)]?Ar=p-XC₆H₄,X=N?CH₃?₂,OCH₃,CH₃,CH?CH₃?₂,F,Cl,OCF₃?have been synthesized in order to examine the mixed-valence properties and electronic coupling through the non-conjugated bridge.The molecular structural of these complexes are confirmed by X-ray defraction in solid state and NMR spectroscopy in solution.Importantly,the MV complexes,generated by one-electron oxidation with ferrocenium hexafluorophosphate,do not exhibit characteristic metal to metal?MM?or intervalence?IV?charge transfer?CT?absorptions in the near-IR spectra,in contrast to the analogues with?conjugated bridges.Therefore,the Mo₂ dimers with?bridge should be assigned to the Class I in Robin-Day's scheme.Cyclic voltammograms of the complexes show small separation of half-wave redox potential(?E_1/2),in comparison with the conjugated analogues with similar Mo₂???Mo₂ distances,indicates that the saturated bridge greatly weakens the electronic interaction between two Mo₂units.Notably,the?E_1/2 decreases with increasing electron-withdrawing ability of the substituent on the ancillary ligands,indicating that electronic coupling of this series is affected not only by the degree of conjugated bridges,but also by the properties of the remote ancillary groups.In the third chapter,in order to systematically study the effect of electron transfer distances on the electron coupling and electron transfer in saturated bridge D-B-A systems,malonate,andsuccinatebridgeddimolybdenumdimers[{Mo₂?ArNCHNAr?₃}₂??-O₂CCH₂CO₂?]and[{Mo₂?ArNCHNAr?₃}₂??-O₂CC₂H₄CO₂?]?Ar=p-XC₆H₄,X=OCH₃,CH₃,CH?CH₃?₂,Cl?have been synthesized and structurally characterized.Comparing the malonate,succinate,and 1,4-cyclohexanedicarboxylate series,it is found that the?E_1/2 decreased with increasing r_ab,indicating that the electronic coupling effect is gradually weakened as the effective electron transfer distance increases.These results indicate that the metal-metal distance play a dominant role in determining the magnitude of?35?E_1/2 due to the electrostatic interaction and the resonant effects are esstentially eliminated in these systems.The non-conjugated 1,4-cyclohexanedicarboxylate bridged Mo₂ complexes are compared with the previously reported conjugated terephthalate bridged analogues in order to evaluate the contributions of the two different effects,electrostatic interaction and electronic resonance,on the electronic coupling and electron transfer between D and A.Spectroscopic data and DFT calculations show consistently that the electronic coupling in the non-conjugated dimolybdenum dimers is negligible and the odd electron is fully localized in the mixed-valence system.In the conjugated bridged dimolybdenum dimers mainly the electron coupling and electron transfer are realized by the through-bond mechanism.Therefore,the study validates the superexchange formoism.Finally,the synthesis and characterization of various?-bridged D-B-A compounds in this study will provide new and important experimental models for the photoinduced electron transfer,in which the through-bond pathway is suppressed.Different from other systems,the replacement of conjugated bridges with saturated bridges eliminates the singlet MLCT states of in the photoinduced electron transfer systems,so that we can only observe and study the donor-acceptor electron transfer kinetics from the MMCT state.Therefore,the synthesized?-briged Mo₂ dimers and obtained understanding will be employed for the study of quantum inference effects on electron transfer.