Weakening the N–H Bonds of NH3 Ligands: Triple Hydrogen-Atom Abstraction to Form a Chromium(V) Nitride

Weakening and cleaving N–H bonds is crucial for
improving
molecular ammonia (NH3) oxidation catalysts. We report
the synthesis and H-atom-abstraction reaction of bis­(ammonia)­chromium
porphyrin complexes Cr­(TPP)­(NH3)2 and Cr­(TMP)­(NH3)2 (TPP = 5,10,15,20-tetraphenyl-meso-porphyrin and TMP = 5,10,15,20-tetramesityl-meso-porphyrin) using bulky aryloxyl radicals. The triple H-atom-abstraction
reaction results in the formation of CrV(por)­(N),
with the nitride derived from NH3, as indicated by UV–vis
and IR and single-crystal structural determination of Cr­(TPP)­(N).
Subsequent oxidation of this chromium­(V) nitrido complex results in
the formation of CrIII(por), with scission of the CrN
bond. Computational analysis illustrates the progression from CrII to CrV and evaluates the energetics of abstracting
H atoms from CrII-NH3 to generate CrVN. The formation and isolation of CrV(por)­(N)
illustrates the stability of these species and the need to chemically
activate the nitride ligand for atom transfer or N–N coupling
reactivity.

削弱并断裂N-H键对于提升分子氨（NH3）氧化催化剂性能至关重要。本研究报告了使用体积庞大的芳基氧自由基合成了双（氨）铬叶啉复合物Cr-(TPP)-(NH3)2和Cr-(TMP)-(NH3)2（TPP代表5,10,15,20-四苯基均三甲氧基叶啉，TMP代表5,10,15,20-四甲苯基均三甲氧基叶啉），并对其H原子的抽取反应进行了研究。三重H原子抽取反应导致CrV(por)-(N)的形成，其中由NH3衍生的氮化物，经紫外-可见光谱和红外光谱以及Cr-(TPP)-(N)的单晶结构分析证实。随后对这种铬(V)氮化物复合物进行氧化处理，导致CrIII(por)的形成，Cr-N键断裂。计算分析描绘了从CrII到CrV的转化过程，并评估了从CrII-NH3抽取H原子以生成CrV-N的能态。CrV(por)-(N)的形成与分离揭示了此类物种的稳定性以及化学活化氮化物配体以实现原子转移或N-N偶联反应活性的必要性。