Catalytic Nitrogen-to-Ammonia Conversion by Osmium and Ruthenium Complexes

Abstract

Despite the critical role ruthenium and osmium complexes have played in the development of transition metal dinitrogen chemistry, they have not been previously shown to mediate catalytic N2-to-NH3 conversion (N2RR), nor have M-NxHy complexes been derived from protonation of M-N2 precursors (M = Ru, Os). To help delineate key factors for N2RR catalysis, we report on isostructural tris(phosphino)silyl Ru and Os complexes that mediate catalytic N2RR, and compare their activities with an isostructural iron complex. The Os system is most active, and is demonstrated to liberate more than 120 equiv NH3 per Os center in a single batch experiment using Cp*2Co and [H2NPh2][OTf] as the reductant and acid source. Isostructural Ru and Fe complexes generate very little NH3 under the same conditions. Protonation of an anionic Os-N2− state affords a structurally characterized Os=NNH2+ hydrazido species that itself mediates NH3 generation, suggesting it is a plausible intermediate of the catalysis. Os-hydride species are characterized that form during catalysis as inactive species.

Influence of a Metal Substrate on Small‐Molecule Activation Mediated by a Surface‐Adsorbed Complex

“…A particularly challenging case is nitrogen fixation, as it involves protonation and reduction of the most inert molecule of the universe, N 2 . Inspired by the biological process of nitrogen fixation, a number of molybdenum, iron, and other transition metal complexes have been synthesized that catalyze the conversion of N 2 to NH 3 in homogeneous solution . Besides proton donors such as HBAr F 4 [Ar F =3,5‐bis(trifluoromethyl)phenyl] and lutidinium salts, these processes require the addition of reducing agents (e.g., decamethylchromocene or KC 8 ) …”

“…[9] Inspired by the biological process of nitrogen fixation, [10] an umber of molybdenum, iron, and other transitionm etal complexes have been synthesized that catalyze the conversionofN 2 to NH 3 in homogeneous solution. [2,3,5,[11][12][13][14][15][16][17][18][19][20] Besides proton donors such as HBAr F 4 [Ar F = 3,5-bis(trifluoromethyl)phenyl] and lutidinium salts, these processes require the addition of reducinga gents (e.g.,decamethylchromoceneorK C 8 ). [2,11,12,15] In the past years, significant effort has been directed towardst he attachment of catalysts activei nH ER, OER, or ORR to metal or semiconductor electrodes with the goal of performing the processes in an electrocatalytic fashion.…”

Catalytic Reduction of Molecular Dinitrogen to Ammonia and Hydrazine Using Vanadium Complexes

“…Based on intensive study of the preparation of various transition-metal dinitrogen complexes and their detailed reactivity, [1] Schrock and co-workers discovered adirect and catalytic transformation of molecular dinitrogen into ammonia under ambient reaction conditions using am ononuclear molybdenum dinitrogen complex as ac atalyst. [3,4] After the seminal report of more-effective molybdenumcatalyzed dinitrogen reduction under ambient reaction conditions, [5] iron-, [6][7][8] cobalt-, [9,10] osmium-, [11] and ruthenium [11] catalyzed dinitrogen reductions under mild reaction conditions have been developed, where av ery low reaction temperature such as À78 8 8Ci sn ecessary to avoid the direct reaction of reducing reagents with proton sources.T he produced amounts of ammonia and hydrazine depend on the nature of the metals,l igands,a nd solvents.H owever, successful examples have been limited to the use of mid-tolate transition-metal complexes as catalysts.Early-transitionmetal-catalyzed dinitrogen reduction under mild reaction conditions has not been reported to date. [3,4] After the seminal report of more-effective molybdenumcatalyzed dinitrogen reduction under ambient reaction conditions, [5] iron-, [6][7][8] cobalt-, [9,10] osmium-, [11] and ruthenium [11] catalyzed dinitrogen reductions under mild reaction conditions have been developed, where av ery low reaction temperature such as À78 8 8Ci sn ecessary to avoid the direct reaction of reducing reagents with proton sources.T he produced amounts of ammonia and hydrazine depend on the nature of the metals,l igands,a nd solvents.H owever, successful examples have been limited to the use of mid-tolate transition-metal complexes as catalysts.Early-transitionmetal-catalyzed dinitrogen reduction under mild reaction conditions has not been reported to date.…”

“…[2] Since the report of the first successful example of molybdenum-catalyzed dinitrogen reduction, [2] some successful examples of catalytic dinitrogen reduction systems using other mid-to-late transition-metal dinitrogen complexes as catalysts have been reported by other research groups. [3,4] After the seminal report of more-effective molybdenumcatalyzed dinitrogen reduction under ambient reaction conditions, [5] iron-, [6][7][8] cobalt-, [9,10] osmium-, [11] and ruthenium [11] catalyzed dinitrogen reductions under mild reaction conditions have been developed, where av ery low reaction temperature such as À78 8 8Ci sn ecessary to avoid the direct reaction of reducing reagents with proton sources.T he produced amounts of ammonia and hydrazine depend on the nature of the metals,l igands,a nd solvents.H owever, successful examples have been limited to the use of mid-tolate transition-metal complexes as catalysts.Early-transitionmetal-catalyzed dinitrogen reduction under mild reaction conditions has not been reported to date.In sharp contrast to the Haber-Bosch process,which takes place under harsh reaction conditions, [12] biological nitrogen fixation proceeds smoothly under ambient reaction condi-tions.D etailed investigations have revealed that the FeMo nitrogenase has as ulfur-bridged polynuclear complex containing molybdenum, iron, and carbon atoms although the exact reaction mechanism is not yet understood. [13,14] Arecent study disclosed that the molecular structure of the FeV nitrogenase is quite similar to that of the FeMo nitrogenase.…”

Catalytic Reactivity of Molybdenum–Trihalide Complexes Bearing PCP‐Type Pincer Ligands

“…[8][9][10][11] In 2003, Schrock and ac o-worker reported the first catalytic conversion of dinitrogeni nto ammonia with am ononuclear molybdenum-dinitrogen complex under ambientr eactionc onditions. [12][13][14][15] Since the first report using am olybdenum-dinitrogen complex, other research groups have succeeded in catalytic conversion of dinitrogen into ammonia or hydrazine under mild reactionc onditions such as À78 8Ce mploying iron [16][17][18][19][20][21][22][23][24][25] -, cobalt [26] -, ruthenium [27] -, osmium [27] -, vanadium [28] -, and titanium [29] -dinitrogen complexes.…”