Low‐Energy Pathways Found for the NH₃ Activation and H₂ Elimination by the Werner‐Type Complexes M(NH₃)₄⁺ (M=Fe, Ru and Os).

Abstract

The interactions of NH3 with the transition metal complexes M(NH3)n+ (M=Fe, Ru, Os; n=0−4) were investigated theoretically with the aim of analyzing the factors that determine their reactivity patterns and product distributions. The reactions of ammonia with the low‐coordination complexes of the three metallic ions (n=1−3) lead only to the addition product M(NH3)n+1+. However, once the highest‐coordinated complex M(NH3)4+ is reached, each of the reactions evolves to metal‐inserted species through low energy channels. For osmium, the hydrogen abstraction from an additional NH3 molecule by the inserted intermediate H−Os(NH3)4‐NH2+ leads to H2 elimination products along a favorable pathway. The facile breaking of the strong N−H bond of ammonia by these Werner‐type complexes is by far the most striking finding emerging from this study. Hopefully, this could inspire future contributions in the areas of chemical and bioinorganic catalysis.

Exploring the potential of the ammine complexes M(NH3)n+ (M = Zr, Re) to activate NH3

“…In a previous contribution, we studied the interaction of NH 3 with Os + . 27 The pattern followed by that reaction is similar to that discussed before for the reaction of NH 3 with Zr + . The NH 3 molecules sequentially add to Os + until the maximum coordination complex Os(NH 3 ) 4 + is obtained.…”

“…Thus, this reaction could proceed to the formation of metal-inserted species through an energetically favorable two-step radical reaction scheme such as that discussed previously for the Zr + reaction. In fact, according to the results obtained for this interaction (not shown herein in the figures and tables), this reaction would end when the products Re(NH 3 ) 3 (NH 2 ) 2 + + H 2 were attained (similarly to the osmium reaction reported in ref. 27).…”

“…This complex readily abstracts a hydrogen atom from an NH 3 molecule to yield the radical fragments H–Os(NH 3 ) 4 + + NH 2 , through which the inserted intermediate Os(NH 3 ) 4 (H)(NH 2 ) and the H 2 elimination products can be obtained. 27 Despite rhenium and osmium being next to each other in the periodic table, the first shows a very different reaction pattern, as, for the reaction of Re + with ammonia, only the adduct Re(NH 3 ) + is observed. With the aim to gain an additional insight into the role played by the NH 3 ligands in this kind of reactions, we explored the interaction of NH 3 with the structure Re(NH 3 ) 3 + .…”

“…Recently, we studied the interactions of NH 3 with the metal–ammine complexes M(NH 3 ) n + (Fe + , Ru + and Os + ; n = 1–4). 27 According to the results obtained from that study, metal–ligand interactions play a crucial role in determining the potential of the metal center toward the N–H bond activation. In fact, for each of the investigated metallic ions, energetically viable channels for NH 3 activation were found only for the reaction of this molecule with the highest coordination complex M(NH 3 ) n (max) + .…”

“…In fact, for each of the investigated metallic ions, energetically viable channels for NH 3 activation were found only for the reaction of this molecule with the highest coordination complex M(NH 3 ) n (max) + . 27 While the results obtained from that study allowed us to analyze comparatively the factors that determine the different product distributions observed for the reactions of ammonia with the group-8 transition ions, other intriguing aspects emerging from the experimental studies reported in ref. 21 remain still unexplored.…”