基于轴向配位键调控“电子存储”固氮机制的理论研究

The direct conversion of atmospheric dinitrogen to ammonia under ambient condition has been attracting much attention for political, environmental, and sustainable concerns. However, it is still a challenging and long-standing goal for human-beings to design a high efficiency catalytic nitrogen cycle. The landmark work on well-defined Fe-based complex with tris(phosphino)alkyl(BPiPr3) ligand featuring axial dative Fe→B bond has established a modestly effective molecular catalyst for N2-to-NH3 conversion by addition of protons and electrons at low temperatures. The variation of oxidation state of Fe along the reaction path correlates with that of the dative Fe→B bonding. The redox-flexibility of the reverse-dative Fe→B bonding helps to provide an electron reservoir that buffers and stabilizes the evolution of Fe oxidation state, which is essential for forming the key intermediates of N2 activation. The system provides a platform to investigate the mimic enzymatic process of nitrogen fixation on a molecular level. As to the realization of nitrogen fixation, it is of great importance to construct the reasonable coordination environment to regulate “electron reservoir”. In this project, comprehensive theoretical studies using density functional theory (DFT), ab initio wavefunction theory (WFT)and ab initio molecular dynamics (AIMD) will be carried out to unveil the characteristic features of the axial dative bond in the complete catalytic cycle of N2-to-NH3 conversion by complexes, as well as to find a patterned variation of dative bond and formal oxidation state of metal center. The hemilabile axial bonding between a transition metal (TM) center and an ancillary p-block, d-block, as well as f-block atom will be built for facilitating N2 to NH3 conversion, and thus reveal the crucial role of the flexible axial dative bond as an electron reservoir to stabilize the intermediates toward NH3 production. Our work will provide insights for understanding and optimizing homogeneous and surface single-atom catalysts with dative ligands for efficient dinitrogen fixation. The applicant has rich experience in the theoretical research of nitrogen fixation. The completion of this project will be of great scientific significance to the promote the development of ammonia synthesis under ambient condition.

将大气资源丰富的氮气在温和条件下转化为氨是一条极具潜力的发展途径，但构建低能耗、高效率的催化剂驱动绿色固氮循环依然是人类面临的巨大挑战。在首例铁基配合物高效仿生固氮的(TPB)Fe体系中，轴向Fe→B配位键具有灵活的氧化还原特性，在催化固氮中发挥了重要作用。合理构建活性位点的配位环境调节“电子存储”对实现固氮至关重要。本项目拟通过量子化学的计算从分子层面探讨仿生固氮催化剂的电子结构和催化机理，研究轴向配位键在固氮过程中的作用规律。基于热力学、动力学以及成键分析，筛选构筑过渡金属与p区、d区、f区特征原子轴向成键的固氮配合物，揭示d-p、d-d、d-df轨道相互作用调控“电子存储”的催化固氮过程中金属价态的变化规律和影响因素，优化设计轴向配位键调控“电子存储”的固氮催化剂。申请人在固氮领域的理论研究经验丰富，这一项目的完成将对推动温和条件下合成氨的催化过程具有重要的科学意义。

在温和条件下高效低成本地将大气中含量丰富的氮气转化为氨是世界科技前沿的热点、难点领域，设计驱动绿色固氮循环的催化剂具有重要的科学意义和社会价值。本项目通过理论计算手段深入探讨活化双氮合成氨催化剂的结构调控方式，理解基于配位轨道相互作用调控固氮的微观机制和影响因素，明确特殊配位键型调控电子缓冲能力在催化固氮的重要作用。研究工作开展分为三个方向，分别是协同固氮作用机制的探讨、金属-金属相互作用对催化反应调控规律的挖掘以及具有“电子调控”作用新材料的理论探索。基于对“过渡金属位点+锚定原子”催化剂的固氮催化性能与电子结构的关系分析，得到以下结论：第一，由于固氮催化是多步进行的强还原过程，催化位点和锚定原子之间成键的灵活性是催化活性的关键，催化位点和锚定原子之间成键要能够及时响应反应的变化；第二，锚定原子获得电子以及释放电子的能力是另外一个重要因素，锚定原子在催化过程中既可以直接参与电子过程，同时也可以通过反应位点与锚定位点的成键来缓冲各自氧化态的变化；第三，反应位点的d轨道需要有足够高的能级和氮气的反键轨道形成反馈键，从而影响催化固氮的能力。此外，本项目将金属-金属相互作用的探讨范畴从均相分子体系拓展到多相复合体系，将“电子调控”的催化作用特征推广至多步还原的反应作用过程。广泛开展与实验合作，探讨新型材料的电子结构或传统材料与底物结合的不同方式，探索成键组合模式以及“电子存储”的微观作用机制，对具有“电子调控”作用的材料进行一定拓展。