Catalytic reduction of dinitrogen to ammonia at a single molybdenum center

Abstract

Since our discovery of the catalytic reduction of dinitrogen to ammonia at a single molybdenum center, we have embarked on a variety of studies designed to further understand this complex reaction cycle. These include studies of both individual reaction steps and of ligand variations. An important step in the reaction sequence is exchange of ammonia for dinitrogen in neutral molybdenum(III) compounds. We have found that this exchange reaction is first order in dinitrogen and relatively fast (complete in <1 h) at 1 atm of dinitrogen. Variations of the terphenyl substituents in the triamidoamine ligand demonstrate that the original ligand is not unique in its ability to yield successful catalysts. However, complexes that contain sterically less demanding ligands fail to catalyze formation of ammonia from dinitrogen; it is proposed as a consequence of a base-catalyzed decomposition of a diazenido (Mo-NANH) intermediate.

Die katalytische Reduktion von Distickstoff zu Ammoniak mit Molybdän: Theorie und Experiment

“…[31,32] Wir wendeten uns dann Molybdänkomplexen mit dem [HIPTN 3 N] 3À -Liganden zu (HIPT = 3,5-(2,4,6-iPr 3 C 6 H 2 ) 2 C 6 H 3 , Hexaisopropylterphenyl; Schema 1), [33,34] Für eine hypothetische "lineare" Reduktion von N 2 wurden Intermediate vorgeschlagen (Schema 2), von denen die folgenden acht synthetisiert und kristallographisch und mit anderen Verfahren charakterisiert wurden: [33][34][35][36] 3À leicht modifiziert waren, [40,41] und neuere Befunde bei der Reduktion von Distickstoff zusammengefasst. [42][43][44] Die eingesetzte Säure beeinflusst den Erfolg der Reduktion sehr stark. [42] Auch mit einem schwächeren Reduktionsmittel (CoCp 2 ) verläuft die Reaktion noch katalytisch, die Ausbeute an Ammoniak ist dann aber nur etwa halb so groß.…”

“…[45][46][47][48][49][50][51][52][53][54][55] [51] Es gibt Hinweise auf eine Beschleunigung der Zersetzung von Mo-N=NH durch Lutidiniumionen, [34] + neutrale Komplexe MoL zu synthetisieren (L = THF oder 2,6-Lutidin), lieferten nur Mo(N 2 ). [42] Selbst bei CVUntersuchungen in Fluorbenzol war Mo(THF) nicht nachweisbar, weil es sofort zu Mo(N 2 ) weiterreagierte. Die CVDaten deuten darauf hin, dass die Umsetzung von Mo(THF) zu Mo(N 2 ) drei oder mehr Größenordnungen schneller verläuft als die Reaktion von Mo(NH 3 ) zu Mo(N 2 ).…”

A Biomass-Based Model to Estimate the Plausibility of Exoplanet Biosignature Gases

“…More efficient catalysts are known, which can catalyze the formation of ammonia from elemental nitrogen and hydrogen gases at 500 K and standard pressure (Yue et al 2006;Avanier et al 2007). Others catalyze the formation of ammonia from nitrogen gas and a proton (Yandulov & Schrock 2003;Shilov 2003;Weare et al 2006) or from an activated nitrogen molecule and hydrogen gas (Nishibayashi et al 1998), in water at room temperature and pressure. The final step of combining a room temperature nitrogen reduction catalyst with a room temperature hydrogen splitting catalyst has not been achieved in the lab but is believed to be a realistic goal (Weare et al 2006).…”

“…Others catalyze the formation of ammonia from nitrogen gas and a proton (Yandulov & Schrock 2003;Shilov 2003;Weare et al 2006) or from an activated nitrogen molecule and hydrogen gas (Nishibayashi et al 1998), in water at room temperature and pressure. The final step of combining a room temperature nitrogen reduction catalyst with a room temperature hydrogen splitting catalyst has not been achieved in the lab but is believed to be a realistic goal (Weare et al 2006). Such a combined catalyst would make NH 3 from H 2 and N 2 and would generate energy (heat) from the reaction.…”

Proton and Electron Transfer to a Polymer‐Supported Nitrido Molybdenum(VI) Complex

“…[1,[3][4][5]7,8] Even full catalytic cycles with molybdenum have been devised by Schrock et al with triamidoamine π-donor ligands (Scheme 1, A) [11] and by Nishibayashi et al with PNP-type pincer ligands (Scheme 1, B). [12] Complexes of the type trans-Mo 0 (N 2 ) 2 L 4 (L = phosphane) were investigated by Chatt, and they produce ammonia upon protonation with mineral acids, [1,2,13] whereas the analogous polymer-immobilized complexes [14] (characterized by the IR signature of end-on-coordinated N 2 ligands and gel-phase 31 P NMR spectroscopy) [15,16] fail to release ammonia.…”