Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States.
Inorg Chem. 2021 Jan 18;60(2):1220-1227. doi: 10.1021/acs.inorgchem.0c03354. Epub 2021 Jan 7.
Terminal dinitrogen complexes of iron ligated by tripodal, tetradentate P ligands (X = B, C, Si) have previously been shown to mediate catalytic N-to-NH conversion (NRR) with external proton and electron sources. From this set of compounds, the tris(phosphino)borane (P) system is most active under all conditions canvassed thus far. To further probe the effects of the apical Lewis acidic atom on structure, bonding, and NRR activity, Fe-N complexes supported by analogous group 13 tris(phosphino)alane (P) and tris(phosphino)gallane (P) ligands are synthesized. The series of PFe-N compounds (X = B, Al, Ga) possess similar electronic structures, degrees of N activation, and geometric flexibility as determined from spectroscopic, structural, electrochemical, and computational (DFT) studies. However, treatment of [Na(12-crown-4)][PFe-N] (X = Al, Ga) with excess acid/reductant in the form of HBAr/KC generates only 2.5 ± 0.1 and 2.7 ± 0.2 equiv of NH per Fe, respectively. Similarly, the use of [HNPh][OTf]/Cp*Co leads to the production of 4.1 ± 0.9 (X = Al) and 3.6 ± 0.3 (X = Ga) equiv of NH. Preliminary reactivity studies confirming PFe framework stability under pseudocatalytic conditions suggest that a greater selectivity for hydrogen evolution versus NRR may be responsible for the attenuated yields of NH observed for PFe and PFe relative to PFe.
以前已经证明，铁与三脚架、四齿 P 配体（X = B、C、Si）配位的末端二氮复合物可以在外部质子和电子源的存在下介导 N 到 NH 的催化转化（NRR）。在这组化合物中，三（膦）硼烷（P）体系在迄今为止研究的所有条件下都是最活跃的。为了进一步探究顶端路易斯酸性原子对结构、键合和 NRR 活性的影响，合成了由类似的第 13 族三（膦）铝烷（P）和三（膦）镓烷（P）配体支持的 Fe-N 配合物。系列 PFe-N 化合物（X = B、Al、Ga）具有相似的电子结构、N 活化程度和几何灵活性，这是通过光谱、结构、电化学和计算（DFT）研究确定的。然而，用过量的酸/还原剂 HBAr/KC 处理[Na(12-crown-4)][PFe-N]（X = Al、Ga）仅生成 2.5 ± 0.1 和 2.7 ± 0.2 当量的 NH/Fe，分别。同样，使用[HNPh][OTf]/Cp*Co 导致生成 4.1 ± 0.9（X = Al）和 3.6 ± 0.3（X = Ga）当量的 NH。初步的反应性研究证实了 PFe 框架在拟催化条件下的稳定性，这表明与 PFe 和 PFe 相比，PFe 对氢逸出而非 NRR 的选择性更高，这可能是观察到的 NH 产率降低的原因。
