N‐Heterocyclic Phosphenium Complex of Manganese: Synthesis and Catalytic Activity in Ammonia Borane Dehydrogenation

Abstract

A neutral N-heterocyclic phosphenium complex of manganese was synthesised by a metathesis approach and characterised by IR, NMR, and XRD studies. The short P-Mn distance suggests a substantial metal-ligand double bond character. Reaction with a hydride produced an anionic phosphine complex, which was also characterised by IR and NMR spectroscopy and, after anion exchange, a single-crystal XRD study. Protonation of the anion occurs at the metal to yield a neutral phosphine metal carbonyl hydride, which releases dihydrogen upon irradiation with UV light. These reactions confirm the electrophilic nature of the phosphenium ligand and suggest that the title complex might undergo reactions displaying metal-ligand cooperativity. Surprisingly, reaction with ammonia borane (AB) did not proceed under transfer hydrogenation of the Mn=P double bond but through the catalytic dehydrogenation of AB. The phosphenium complex behaves here as a class II catalyst, which dehydrogenates AB to NH BH that was trapped with cyclohexene. Computational model studies led to the identification of two possible catalytic cycles, which differ in the regioselectivity of the initial AB activation step. In one case, the activation proceeds as cooperative transfer hydrogenation of the Mn=P bond, whereas in the other case a H /H pair is transferred to the phosphorus atom and a nitrogen atom of the phosphenium unit, resulting in a ligand-centred reaction in which the metal fragment acts merely as stabilising substituent. Unexpectedly, this pathway, which constitutes a new reaction mode for phosphenium complexes, seems to be better in accord with experimental findings on the course of the catalysis.

Addition of H2 Across a Cobalt–Phosphorus Bond

“…In the 1 HNMR spectrum, adoublet of doublet of triplets appears at À11 ppm with coupling constants of 78 Hz, 61 Hz, and 36 Hz, respectively,which is diagnostic of ametal hydride.The signal for the P NHP -H proton was difficult to locate in the 1 HNMR spectrum of 4 as aresult of overlap with aromatic resonances; however, a 31 P-decoupled 1 HNMR spectrum revealed the disappearance of adoublet around 8ppm, half of the NHP-H signal, as the signal collapsed into abroad singlet at 7.6 ppm ( Figure S6). [33] To the best of our knowledge this is the first reported instance of H 2 addition across af irst-row metalphosphorus bond. [39] In addition, broad and relatively weak n P-H and n Co-H stretches are observed at 2136 cm À1 and 1893 cm À1 in the solution IR spectrum of 4 (cf.…”

“…[21][22][23][24] Related examples of metalligand cooperativity involving phosphorus are far less common. [33] In fact, there are only af ew examples of H 2 addition across aM ÀPb ond. (uNHP Dipp -H)Mn(H)(CO) 3 could be generated through sequential addition of ah ydride and ap roton to (uNHP Dipp )Mn(CO) 4 ,and H 2 elimination could be promoted by photolysis;however, direct H 2 addition to (uNHP Dipp )Mn-(CO) 4 was not possible.…”

“…(uNHP Dipp -H)Mn(H)(CO) 3 could be generated through sequential addition of ah ydride and ap roton to (uNHP Dipp )Mn(CO) 4 ,and H 2 elimination could be promoted by photolysis;however, direct H 2 addition to (uNHP Dipp )Mn-(CO) 4 was not possible. [33] In fact, there are only af ew examples of H 2 addition across aM ÀPb ond. [28][29][30][31][32] Extension of the metal-ligand cooperativity observed for amide/amine systems to phosphide/phosphines poses as ignificant challenge,b ut may result in more reactive systems since P À H bonds are generally weaker and more acidic than N À Hbonds, and because metal phosphides are more nucleophilic than their metal amide congeners.…”

Stereoselective Transfer Semi‐Hydrogenation of Alkynes to E‐Olefins with N‐Heterocyclic Silylene–Manganese Catalysts

“…In addition to hydrogen storage, AB was successfully employed for transfer hydrogenation of various unsaturated organic substrates . This includes transfer semi‐hydrogenations of alkynes with Ni, Co, Au, and Cu complexes as precatalysts; Mn complexes, however, have merely been employed as precatalysts for H 2 evolution from AB, but not yet in transfer hydrogenation.…”

Me3SiBr/InCl3 catalyzed allylation of alcohols: Identifying the combined Lewis structure and investigating the reaction mechanism

“…All the geometries including reactants, intermediates, transition states, and products were obtained at ωB97XD/def2‐SVP theory level . ωB97XD is range‐separated functional, which can obtain satisfactory accuracy for thermochemistry, kinetics, and noncovalent interactions and be used for reaction mechanism research with def2‐SVP basis set . The calculation used solvent model SMD, and hexane was taken as the solvent .…”