Room‐Temperature Functionalization of N₂ to Borylamine at a Molybdenum Complex

Abstract

Intermolecular,s tepwise functionalization by BH bonds of a( triphosphine)Mo IV -nitrido complex generated by N 2 splitting is reported. The imido-hydride and di-hydrideamido Mo IV complexes have been isolated and characterized. Addition of PinBH to the [Mo(H) 2 (N(BPin) 2 )] + complex at room temperature results in the liberation of borylamines from the metal center.N 2 functionalization using homogeneous metal complexes under mild conditions is one of the most challenging problems facing chemists. [1] Twos trategies have been followed during the past decades.Inanature-like approach, N 2 coordinated to ametal center is functionalized in astepwise manner using an external source of electrons and electrophiles. [2] Most impressively,s everal Mo, [3] Fe, [4] and Co [5] complexes have proven efficient catalysts for NH 3 and N(SiMe 3 ) 3 synthesis,with TON (turn over number) up to circa 415 and 220, respectively. However,the use of strong electrophiles and reducing agents efficiently leads to electrophile homocoupling as ac ompetitive reaction. On the other hand, the Haber-Bosch uses H 2 as both the N 2 functionalizing and reducing species,t hereby potentially avoiding any side reaction. [6] In turn, very large activation energy is required to split N 2 at the surface of the heterogeneous catalyst prior to functionalization. Few, carefully designed molecular complexes have been shown to split N 2 under mild conditions, [7] thus providing synthetic pathways via functionalization of the resulting nitrido complex (Scheme 1, top). Elegant N 2 transformations to nitrile derivatives were reported by Cummins (with Mo) [8] and Schneider (with Re). [9] Unfortunately,both strategies rely on the use of strong electrophiles (acylc hloride/silyltriflate and ethyl triflate,r espectively) which are incompatible with the N 2 reduction step.With the ultimate goal of developing catalytic N 2 fixation, the functionalizing reagent needs to react selectively with the high-valent nitrido complex rather than with the reduced metal center.W eh ave recently shown that Si À Hb onds of silanes react selectively toward a( PP 2 )Mo IVnitrido complex and not with the corresponding Mo I fragment. While silylamine formation was achieved by the double SiÀHb ond functionalization of the MoNi ntermediate, [10] this process is limited to its intramolecular version, and requires the use of bis-silane in excess and prolonged heating. Fort he present study,w ereasoned that the strength of N À B bond would be astrong driving force and that the Lewis acid character of boranes could provide alow energy pathway for MoNfunctionalization. We report herein the unprecedented direct functionalization of N 2 by PinBH (Pinacol borane) that yields borylamines (PinB) x NH (3Àx) in nearly quantitative yields using ah omogeneous (PP 2 )Mo fragment (Scheme 1, bottom). Mechanistic investigations demonstrated the intermediacyo ft he Mo Na sw ell as three successive intermolecular 1,2-B À Ha dditions across Mo À Nb onds.Our study started with the known nitrido complex 2 [(...

Dinitrogen Reduction to Ammonium at Rhenium Utilizing Light and Proton-Coupled Electron Transfer

“…By isolating several intermediates along the sequence from N2 to NH3, experimental studies and DFT calculations provide mechanistic insight. With N2 splitting sequences recently having been proposed for homogeneous catalysts, 13,15,35,36 insight into the individual steps along the N2 splitting pathway from N2 to NH3 may have broad implications.…”

Metal‐Ligand Cooperative Synthesis of Benzonitrile by Electrochemical Reduction and Photolytic Splitting of Dinitrogen

“…[4] Thed irect synthesis of compounds other than NH 3 from N 2 remains af ormidable challenge.C atalytic protocols are only known for trisilylamine. [5] Nitriles, [6][7][8] isocyanates, [9] silylamines, [10,11] and borylamines [12] have been synthesized within stoichiometric,c yclic reaction sequences,w hich allow for evaluating strategies to offset the extremely strong NNbond (225 kcal mol À1 ), enable EÀN(E= C, Si, B) bond formation and deliver six reduction equivalents.A ll reported "synthetic cycles" proceed through initial N 2 cleavage into nitride complexes. Subsequent nitrogen transfer typically requires strong electrophiles like alkyl triflates.T he thermochemistry of N 2 splitting must therefore be tuned to avoid nitride overstabilization and enable functionalization with reagents that are more compatible with reductive conditions.…”

Highly Selective Hydroboration of Carbonyls by a Manganese Catalyst: Insight into the Reaction Mechanism

“…The two peaks in 11 B NMR were likely due to the reaction between HBpin and Mn-1, and the 24.1 ppm peak was tentatively assigned to boryl amine N(Bpin) 3 . 75 The other peak remained unidentified. A few other possibilities, such as (Bpin) 2 NH, 70 (Bpin) 2 , O(Bpin) 2 , HOBpin, 76 and B 2 Pin 3 , 77 seemed less likely.…”