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The Synthesis, Characterization and Reactivity of a Series of Ruthenium N-triphosPh Complexes
Published on: April 10, 2015
Ammonia Oxidation with a Ruthenium Polypyridyl Complex: Mechanism-Guided Approach to Low Overpotential
Maximilian Seiß1, Elisa Dickgießer1, Sebastian Dechert1
1University of Göttingen, Institute of Inorganic Chemistry, Tammannstr. 4, 37077 Göttingen, Germany.
Abstract:
The use of ammonia as a potential zero-carbon fuel has attracted high interest in the quest for novel sustainable energy carriers. This requires a deep understanding of the principles of the ammonia oxidation reaction (AOR) to guide the development of efficient catalysts. Here, we report the complex [RuII(TPA)-(NH3)2]-(PF6)2 (1-(PF 6 ) 2 ; TPA = tris-(2-pyridylmethyl)-amine), and we demonstrate that its 1e- oxidation in MeCN is partially ligand centered and finally produces [RuII(TPA)-(MeCN)-(NH3)]-(PF6)2 (2-(PF 6 ) 2 ). Deprotonation of 1 3+ (pK A = 12.6) is derived as the rate-determining step, and bimolecular N-N coupling of the key intermediate [Ru-(TPA)-(NH3)-(NH2)]2+ (4 2+ ) is proposed based on 15N labeling experiments and electrochemical studies. 1-(PF 6 ) 2 is then investigated as a homogeneous AOR electrocatalyst, and the formation of N2 and H2 is confirmed during electrolysis at a moderate operating potential. DFT computations established a complete PCET map to evaluate potential reaction pathways and, in combination with spectroscopic and electrochemical studies as well as product analysis using 15N labeling, allowed us to propose a catalytic cycle where N-N bond formation occurs via intermolecular coupling (I2M) at an early amido stage (4 2+ ) of the PCET sequence. Circumventing higher metal oxidation states gives rise to a relatively low AOR overpotential of 0.96 V for 1 2+ . Complex 2 2+ is identified as an important intermediate that is catalytically competent and initiates a second cycle, while [RuII(TPA)-(MeCN)2]2+ (XI 2+ ) represents an off-cycle product leading to gradual loss of catalytic activity.
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