Development of novel dinuclear molybdenum complexes with long-lived excited states for the photochemical N2 activation

该项目的总体目标是合理开发用于金属结合二氮的光驱动活化的催化剂。为此，主要目标是在Mo-N=N-Mo二聚体上建立长寿命的MLCT激发态，由此产生光化学N2活化。近几十年来，世界人口的增长加上粮食需求的增加，使氮肥的生产和使用变得更加重要。因此，通过HABER-BOSCH工艺生产工业氨是最重要的大规模工业工艺之一，它消耗了世界能源供应的12%。除此之外，在定义明确的过渡金属配合物上合成固氮作为将N2转化为NH3的进一步方法，在过去的几年里有了很大的发展。在合成固氮领域，只有少数表征的分子体系能够光化学或光驱动N2活化。对于这些分子化合物中的一些，热N2分裂的机制已经被详细地了解了。然而，光驱动N2活化的工作原理仍然知之甚少。因此，如果光驱动N2分裂最终能够以催化方式进行，这将具有非常重要的社会经济意义，并且在更基本的层面上，在N2桥接二聚体上光活性激发态的发展将为光驱动N2活化和一般无机光化学领域开辟新的可能性。最近，CHIRIK和SCHOLES描述了一种Mo-N=N-Mo二聚体，其光物理性质首次直接揭示了可能适合N2分裂的MLCT状态。这种分子化合物只有很短的激发态寿命，因此，光化学反应无法与3MLCT状态的快速非辐射失活相竞争。因此，本项目旨在开发新的n2桥接Mo-N=N-Mo二聚体，其配位环境结合了更理想的八面体配位几何形状和更强的配体场，从而产生长寿命的3MLCT激发态。这些新化合物，包括各种不同组合的赤道螯合和轴向单齿配体，将被合成和充分表征。此外，将使用时间分辨紫外-可见和红外光谱进行目标配合物的激发态研究和光驱动n2活化研究。此外，本项目的所有研究都将得到计算研究的支持。最后，考虑到牺牲供体对还原性3MLCT的猝灭作用，将研究具有足够长激发态的系统，这是开发光催化剂的关键一步，用于光驱动N2生成NH3。

The overall goal of this proposed project is the rational development of catalysts for the light-driven activation of metal-bound dinitrogen. To this end, the primary goal is the establishment of long-lived MLCT excited states on Mo-N=N-Mo dimers, from which photochemical N2 activation will occur. The growing world population combined with an increase in the demand of food have led to greater importance in the production and use of nitrogenous fertilizers in recent decades. Therefore, industrial ammonia production via the HABER-BOSCH process, which consumes 1 2% of the world’s energy supply, is one of the most important large-scale industrial processes. Besides this, the synthetic nitrogen fixation on well-defined transition metal complexes as a further way to convert N2 to NH3 has developed considerably in the last years. In the field of synthetic nitrogen fixation, only a handful of characterized molecular systems are capable of photochemical or light-driven N2 activation. For some of these molecular compounds the mechanisms of thermal N2 splitting is understood in much detail. Whereas, the operating principles of light-driven N2 activation are still poorly understood. Therefore, if light-driven N2 splitting could ultimately be performed in catalytic fashion, this would have very important socio-economic implications and on a more fundamental level, the development of photoactive excited states on N2-bridged dimers will open up new possibilities in the area of light-driven N2 activation and inorganic photochemistry in general. Recently, CHIRIK and SCHOLES described a Mo-N=N-Mo dimer, and its photophysical properties provided the first direct insight into MLCT states that could be suitable for N2 splitting. This molecular compound exhibits only a very short excited-state lifetime, and therefore, photochemical reactions cannot compete with the rapid nonradiative deactivation of the 3MLCT state. Consequently, this proposed project aims to develop new N2-bridged Mo-N=N-Mo dimers with a coordination environment that combines a more idealized octahedral coordination geometry with a stronger ligand field resulting in long-lived 3MLCT excited states. These new compounds, including a variety of different combinations of equatorial chelate and axial monodentate ligands, will be synthesized and fully characterized. Furthermore, the excited states investigations and the light-driven N2-activation studies of the target complex will be performed using time-resolved UV-Vis and IR spectroscopy. In addition, all investigations in this project will be supported by computational studies. Finally, systems with long-lived enough excited states will be investigated in view of reductive 3MLCT quenching by sacrificial donors, which represents the key step towards the development of a photocatalyst for the light-driven formation of NH3 from N2.