Small Molecule Activation with Dinuclear Cobalt Complexes based on the Two-In-One Pincer Ligands

Abstract

Dinuclear complexes based on this two-in-one pincer ligand scaffold have exhibited fascinating magnetic properties and promise high reactivity in catalysis and small molecule activation. In this thesis, the main objective is to accomplish small molecule activation using dinuclear cobalt complexes as precursors and isolate important intermediate complexes for better understanding the possible mechanisms of small molecule activation. More specifically, this thesis work can be divided into five parts: N2 fixation and catalytic conversion of N2 (Chapters 2 and 3), the formation of diazene complexes and reduction of azobenzene (Chapter 4), CO2 reduction and CO reactions (Chapter 5) and hydrosilane reactions (Chapter 6). Chapter 2 describes the synthesis and characterization of a series of cobalt complexes bearing the two-in-one pincer ligand, including dicobalt and tetracobalt dinitrogen complexes. Moreover, a novel reversible conversion between the tetracobalt dinitrogen complex 6 and the triflate-bridged complex 7 was investigated. By employing these cobalt complexes as catalysts, the catalytic silylation of dinitrogen into N(SiMe3)3 has been explored. To avoid the C-H activation on the ligand, a new methyl-modified ligand HLMe has been exploited in Chapter 3. However, the reduction of dicobalt dinitrogen complex 10 with strong reductant did not lead to N2 activation but the dissociation of one cobalt(0) atom. To synthesize a series of M(N2HxR) complexes as potential intermediates in the N2 fixation process, in Chapter 4, hydrazine, methylhydrazine and 1,2-dimethylhydrazine have been employed to react with dicobalt dinitrogen complex 2 and after the disproportionation of hydrazine, methylhydrazine or 1,2-dimethylhydrazine, the reactions form diazene-, methyldiazene- or 1,2-dimethyldiazene-bridged complexes. It is worth noting that the employment of azobenzene with complex 2 leads to two electron transfer from the metal centers to azobenzene. In Chapter 5, CO2 reduction mediated by the tetracobalt(I) dinitrogen complex 6 was investigated and the formation of a cobalt(II) carbonate complex and a dicobalt(I) dicarbonyl complex indicate CO2 reductive disproportionation. In addition, direct CO reactions gave rise to the dicarbonyl complex and a tetracarbonyl complex. Chapter 6 presents the reactions of silanes with the tetracobalt dinitrogen complex 6. The addition of diphenylsilane or methylphenylsilane into THF solution of complex 6 results in the formation of a mixed-valent CoI/II diphenylsilyl σ–complex 20 or a mixed-valent CoI/II methylphenylsilyl σ–complex 21. Interestingly, phenylsilane (4.0 eq.) reacts with complex 6 and generates a mixed-valent CoI/II disilyl complex [L2Co4(µ-SiH2SiH2)](OTf)2 22 by redistribution. Moreover, the reaction of phenylsilane (6.0 eq.) and complex 6 formed a tetracobalt tetrasilyl complex [L2Co4(µ-PhSi4H3](OTf)2 23.