Ammonia Activation, H₂ Evolution and Nitride Formation from a Molybdenum Complex with a Chemically and Redox Noninnocent Ligand

Abstract

Treatment of the bis(imino)pyridine molybdenum η-benzene complex (PDI)Mo(η-CH) (PDI, 2,6-(2,6-iPrCHN═CMe)CHN) with NH resulted in coordination induced haptotropic rearrangement of the arene to form (PDI)Mo(NH)(η-CH). Analogous η-ethylene and η-cyclohexene complexes were also synthesized, and the latter was crystallographically characterized. All three compounds undergo loss of the η-coordinated ligand followed by N-H bond activation, bis(imino)pyridine modification, and H loss. A dual ammonia activation approach has been discovered whereby reversible M-L cooperativity and coordination induced bond weakening likely contribute to dihydrogen formation. Significantly, the weakened N-H bonds in (PDI)Mo(NH)(η-CH) enabled hydrogen atom abstraction and synthesis of a terminal nitride from coordinated ammonia, a key step in NH oxidation.

Catalytic Ammonia Oxidation to Dinitrogen by Hydrogen Atom Abstraction

“…This reaction was then exposed to 1atm of naturally abundant 14 NH 3 .A fter the addition of 14 NH 3 ,t he GC-MS analysis revealed the formation of 14 N 15 N(m/z = 29) Figure 2, plot B; present in % 5times the amount of naturally abundant 14 N 15 N(natural abundance 0.7 %);with asubstantial amount of 14 N 2 also having been generated in the reaction. Analysis of this crossover experiment by 15 NN MR spectroscopy further corroborates the formation of 14 N 15 Ngas.Rather than asharp singlet resonance at dÀ70.7 in the 15 NN MR spectrum for 15 N 2 ,as ignificantly broadened 15 Nsignal was observed at dÀ72.4, as expected due to the S = 1nuclear spin of the 14 Nnucleus of 14 N 15 Ngas. Analysis of this crossover experiment by 15 NN MR spectroscopy further corroborates the formation of 14 N 15 Ngas.Rather than asharp singlet resonance at dÀ70.7 in the 15 NN MR spectrum for 15 N 2 ,as ignificantly broadened 15 Nsignal was observed at dÀ72.4, as expected due to the S = 1nuclear spin of the 14 Nnucleus of 14 N 15 Ngas.…”

“…We recently demonstrated that using t Bu 3 ArOC [25] led to the cleavage of all three NÀHb onds of 15 …”

Formation and Rearrangement of Reduced Diiminepyridine Complexes of Zr and Hf

“…There do not appear to be any documented examples of straightforward hydride transfer to a DIP imine carbon, but more complicated sequences resulting in imine reduction have been reported. Starting from a low‐valent (DIP)Mo complex, the group of Chirik reported transfer of hydrogen from coordinated ammonia to an imine carbon , . The same group in earlier work found that thermolysis of (DIP)Co(N 3 ) led to imine reduction via C–H activation .…”

“…Starting from a low-valent (DIP)Mo complex, the group of Chirik reported transfer of hydrogen from coordinated ammonia to an imine carbon. [22,52] The same group in earlier work found that thermolysis of (DIP)Co(N 3 ) led to imine reduction via C-H activation. [53] The reverse reaction, loss of hydrogen with formation of an imine donor functionality, has also been observed.…”

“…An unexpected ligand isomerization to an amido/enamido form was observed for [( Et DIP)ZrCl 2 ], [( Et DIP)-HfMe 2 ] and [( Et DIP)TiMe 2 ], and a mechanism is proposed based on DFT calculations. less attention, although complexes with many heavier metals have been reported: La and Nd, [10,13] Zr, [14] U, [5,6,[15][16][17][18][19][20] Mo, [21,22] Re, [23] Ru, [24][25][26][27][28][29][30] Os, [31] Rh [30,[32][33][34] and Pd. [35] Non-innocence of the DIP ligand is an issue also with these heavier elements, although -judging from the literature -there is a higher tendency with these metals to engage in 2etransfer (or classical backdonation) rather than metal-to-ligand single electron transfer.…”