Half-Sandiwich Molybdenum-Sulfur Complexes And Molybdenum-Iron Complexes:Synthesis And Reactivity

As an abundant metal,molybdenum is an essential element in the catalytic center of many enzyme,such as nitrogenase,[Mo]-formic acid dehydrogenase,and[MoCu]-carbon monoxide dehydrogenase.Numerous applicable Mo catalysts have emerged in recent years,for instance,sulfur removal from petroleum by molybdenum sulfide catalysts;selective oxidation of alcohols to form aldehydes by molybdenum-oxo catalysts;R.R.Schrock’s high-oxidation-state molybdenum carbenes in olefin metathesis catalysis.However,the research progress of Mo catalysis is relatively slow due to the various valence states of Mo（0,Ⅰ,Ⅱ,Ⅲ,Ⅳ,Ⅴ,Ⅵ）and the synthetic difficulty in related complexes.In this thesis,a series of novel Mo-S complexes Cp^*（1,2-Ph²C₆H₄S）MoL（L=CO,MeCN,N₂,alkyne etc.）were synthesized based on"Metal-ligand Cooperation" and "Metal-metal Cooperation" strategy.The catalytic transfer hydrogenation of nitriles to primary amines was established.We also found that[Cp^*Fe（μ-Cl）]₂ reacted with Mo-N₂ complex to cleavage the N≡N bond,resulting in the formation of the Mo-Fe bimetallic complex Cp^*（1,2-Ph²C₆H₄S）Mo（μ-NH）FeCp^*;Interestingly,Mo and Fe centers are also able to work synergistically to activate C≡C bonds in alkynes.The results are presented as follows:Ⅰ.A half-sandwich molybdenum（Ⅱ）thiolate complex with a η2-MeCN ligand,Cp^*Mo（1,2-Ph²PC₆H₄S）（η2-NCMe）（1（η2-NCMe））is synthesized.The complex can react with BH₃ and cleavage the B-H bond by Mo-S cooperation to generate the MonⅡ-H active species Cp^*（1,2-Ph²PC₆H₄SBH₂）MoH.In the presence of NH₃,nitriles are selectively reduced to primary amines.We used 1（η2-NCMe）as the catalyst and NH₃·BH₃ as the hydrogen source to catalyze transfer hydrogenation of nitriles at room temperature,producing primary amines with high efficiency and selectivity.It is worth noting that the cracking of B-H bond of BH₃ is very challenging.The present work also demonstrates a new method of activating BH₃ and stabilizing the parent H₂B fragment in the form of HM-L（BH₂）.Ⅱ.The MeCN in 1（η-NCMe）complex can be replaced by N₂ and converted into molybdenum-nitrogen complex 1（N₂）2.The reaction of 1（N₂）2 with[Cp^*Fe（μ-Cl）]₂ cleaves the N-N bond to form Cp^*（1,2-Ph²C₆H₄S）Mo（μ-NH）FeCp^*complex.Deuterium isotopic labeling experimental shows that the reaction may be intermediated by a N-bridged Mo-N-Fe complex Cp^*（1,2-Ph²C₆H₄S）Mo（μ-N）FeCp^*.It is highly reactive and can extract H atoms from solvent THF molecules.For example,when the reaction is carried out in THF-d8,the product is Cp^*（1,2-Ph²C₆H₄S）Mo（μ-ND）FeCp^*.The Mo-N（H）-Fe complex was characterized in detail by NMR spectroscopy,ESI-MS and XRD.It should be noted that it is the first example of N₂ cleavage by Mo-Fe cooperativity.Ⅲ.The MeCN ligand in 1（12-NCMe）is replaced by RC≡CH（R=p-OCH₃Ph or Me₃Si）and converted into Mo（Ⅱ）complex 1（η2-HC≡CR）with side-on coordination mode of alkynes.1（η2-HC≡CR）can continue to react with[Cp^*Fe（μ-Cl）]₂ to generate Mo-Fe alkyne complex Cp^*（1,2-Ph²C₆H₄S）Mo（μ-η2:η2-HC≡CR）FeCp^*[2(μ-η2:η2-CH≡CR]+.X-ray diffraction shows that alkynes are bound to Mo-Fe in a side-on mode.The bond lengths of the C≡C triple bonds in[2（p-η2:η2-CH≡CPh-p-OCH₃）]+or[2（μ-η2:η2-CH≡CTMS）]+are 1.344（6）A and 1.40（2）A,respectively,which are about 0.15 longer than the bond lengths of the corresponding 4-methoxyphenylacetylene and trimethylsilyl acetylene,and close to C=C double bond length.The activation of C≡C triple bond by Mo-Fe bimetallic cooperativity provides more understanding the mechanism of bimetallic cooperation.