| Summary: | 碩士 === 國立清華大學 === 化學系 === 102 === The low-coordinate and low-valent quintuple bonded dimolybdenum complex Mo2[μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (1) has two Mo-Mo δ bonds, two Mo-Mo π bonds and one Mo-Mo σ bond, so we wondered that if complex 1 is chemically similar to alkynes, particularly, the coordinating ability. Accordingly, we treated complex 1 with 1 equiv of Co2(CO)8 and we isolated two products, (OC)3Co(μ-κ2-CO){κ1-Mo[(OC)3Co(μ-κ2-CO)]Mo(CO)[μ-κ2-HC(N-2,6-iPr2C6H3)2]2} (2) and [(OC)3Co(μ-κ2-CO)][μ-κ2-HC(N-2,6-ίPr2C6H3)2]Mo(μ-CO)Co(μ-CO)(κ1-CO)Mo [μ-κ2-HC(N-2,6-iPr2C6H3)2] (3). Complex 2 is a kinetic product, while 3 is a thermodynamic product. Attempts to isolate the targeted complex Mo2Co2(CO)6[μ-κ2-HC(N-2,6-iPr2C6H3)2]2, which is conceptually similar to Co2(CO)6(μ-κ2-alkyne), have failed.
The reaction of 1 with 1 equiv W(CO)5(THF) produced (CO)4W[κ2-Mo2[μ-κ2-HC(N-2,6-iPr2C6H3)2]2] (4), in which 1 donates four electrons to the W(CO)4 fragment to make it an 18-electron species. Furthermore, treatment of 1 with Fe2(CO)9 led to the isolation of the complex [(OC)3Fe](μ-κ2-CO)MoMo(CO)[μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (5), where one Mo atom has a double bond to the Fe atom. To synthesize a heterotrinuclear FeMo2 complex similar to 4, we then use the quadruple bonded dimolybdenum complex Mo2[μ-κ2-PhB(N-2,6-iPr2C6H3)2]2 (6) to react with Fe2(CO)9. To our delight, we obtained the targeted compound ((OC)4Fe){κ2-Mo2[μ-κ2-PhB(N-2,6-iPr2C6H3)2]2} (7), where the bonding between two Mo atoms could have a quadruple bond character due to back π donation from Fe to the LUMO (δ character) of 6.
It turns out that the Mo centers of the heterotrinuclear complex 5 are electron deficient. Treatment of complex 5 with 2 equiv of benzonitrile generated [(OC)3Fe](μ-κ2-CO)Mo(κ1-NCPh)Mo(CO)(κ1-NCPh)[μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (8). Two nitriles are simply two-electron σ donors, and the nitrile functionalities of PhCN are not activated judged by their bond lengths. When a bulky nitrile 2,4,6-trimethylbenzonitrile and an electron deficient benzoyl cyanide was employed to react with 5, two structurally similar compounds [(OC)3Fe](μ-κ2-CO)Mo(κ1-NCR)Mo(CO)[μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (R = 2,4,6-Me3C6H2 (9), C(O)C6H5 (10)). On the basis of the reactions above, we surmise that the C-N triple bond of nitriles cannot be activated by complex 5. We then treated 1 with a more electron-donating substrate dimethylcyanamide and isolated the complex Mo2[μ-κ2-NCNMe2]2[μ-κ3-N(Me2)CNC(NMe2)N][μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (11), in which a new guanidinate ligand was formed by two molecules of dimethylcyanamide via a C-N coupling.
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