| For the purpose of the design and construction of thiolate-bridged bimetallic biomimetic system,a series of novel thiolate-bridged dicobalt complexes with low-valent and lowcoordinate metal centers were precisely synthesized through the reasonable modulation of the steric effect of auxiliary ligands in this dissertation.Meanwhile,the unique reactivity of these dicobalt complexes toward small molecule activation,such as dihydrogen,and catalytic conversion was investigated in detail,which provides the significant theoretical guidance for developing the biomimetic cooperative bimetallic catalysts based on the earth-abundant metals at the molecular level.Firstly,a series of thiolate-bridged dicobalt complexes featuring potential reactive sites were constructed through the introduction of bulky thiolate ligands.Through treatment of the bromine-bridged dicobalt complexes[Cp(?)Co(μ-Br)2CoCp(?)](Cp(?)=Cp’=η5-C5H2tBu3,Cp*=η5-C5Me5)with different equivalents of mercaptan salts,the {Co2S}-type monothiolate-bridged dicobalt complexes[Cp(?)Co(μ-SR)(μ-Br)CoCp(?)](R=Ad,Fc)and {Co2S2}-type dithiolatebridged dicobalt complexes[Cp(?)Co(μ-SR)2CoCp(?)]were synthesized,respectively,which provides a new reaction platform for the further activation and catalytic transformation of small molecules.Secondly,the reactivity toward both dihydrogen activation and proton reduction to H2 evolution was explored by using the low-valent monothiolate-bridged dicobalt complexes.Specifically,the adamantyl monothiolate-bridged dicobalt precursor 1 underwent successive one-electron reduction to generate rare low-valent and low-coordinate dicobalt complexes[Cp’CoⅠ(μ-SAd)CoⅡCp’](9)and[Cp’CoⅠ(μ-SAd)CoⅠCp’]-(11).Therein,CoⅠCoⅡ complex 9 was capable of triggering the homolytic cleavage of dihydrogen to afford a dicobalt dihydridebrigded complex[Cp’Co(μ-SAd)(H)2CoCp’](12),which can further realize the hydrogen atom transfer to form a dicobalt monohydride-bridged complex[Cp’Co(μ-SAd)(H)CoCp’](13).Moreover,CoⅠCoⅠ complex 11 facilely underwent successive CCEE reaction pathway([CoⅠCoⅠ]-→[CoⅡ(μ-H)CoⅡ]→[CoⅢ(μ-H)2CoⅢ]+→[CoⅡ(μ-H)2CoⅢ]→[CoⅠCoⅠ]-)to accomplish a transformation cycle for proton reduction to dihydrogen.Thirdly,the first example of thiolate-bridged dicobalt nitride was synthesized by adopting the low-coordinate dicobalt complex as a reaction precursor,and its stepwise reductive reacticity toward ammonia formation was also examined.The ligand exchange reaction of monothiolate-bridged dicobalt complex 5 with sodium azide resulted in a stable thiolatebridged dicobalt nitride[Cp*Co(μ-SAd)(μ-N)CoCp*](15),which was readily protonated to give an imido-bridged dicobalt complex[Cp*Co(μ-SAd)(μ-NH)CoCp*][BPh4](16[BPh4]).Furthermore,imide 16[BPh4]was capable of undergoing the proton coupled electron transfer to produce dicobalt amide[Cp*Co(μ-SAd)(NH2)CoCp*](17).Eventually,in the presence of proton acid,ammonia release from amide 17 was realized along with the regeneration of precursor 5’.This system represents the first example that realizes the functional simulation of the late stage(N3-→*N3-→NH2-→NH2-→NH3)during the nitrogen fixation process mediated by dicobalt centers.More interestingly,nitride 15 can directly react with H2 under mild conditions to afford amide 17.Studies in computational chemistry indicated that the heterolytic cleavage of H2 was promoted by the cooperation of cobalt center and nitride ligand.Finally,adopting the similar synthetic strategy,a series of stable {Fe2S2}-type thiolatebridged diiron complexes[Cp’Fe(μ-SR)2FeCp’](18,R=Et;19,R=iPr)and[Cp’Fe(μ-η2:η2pdt)FeCp’](20)were synthesized by choosing a diiron complex[Cp’Fe(μ-I)2FeCp’]featuring sterically demanding auxiliary Cp’ ligand as a precursor.Systematic studies on the temperaturevariable solid-state magnetic susceptibility measurement and zero-field 57Fe Mossbauer spectroscopy at different temperatures were conducted to probe their uncommon electronic structures.Notably,complex 19 features unique temperature-dependent phase-transition and spin-crossover characteristics,which provides a new opportunity for the design and development of the temperature-controlled magnetic memory devices.Moreover,all these diiron complexes were capable of serving as good catalysts for the reductive silylation of N2. |
