Reversible Dihydrogen Activation and Hydride Transfer by a Uranium Nitride Complex

Abstract

Cleavage of dihydrogen is an important step in the industrial and enzymatic transformation of N 2 into ammonia. The reversible cleavage of dihydrogen was achieved under mild conditions (room temperature and 1atmosphere of H 2 ) by the molecular uranium nitride complex, [Cs{U(OSi-(O t Bu) 3 ) 3 } 2 (m-N)] 1, leading to ar are hydride-imide bridged diuranium(IV) complex, [Cs{U(OSi(O t Bu) 3 ) 3 } 2 (m-H)(m-NH)], 2 that slowlyr eleases H 2 under vacuum. This complex is highly reactive and quickly transfers hydride to acetonitrile and carbon dioxide at room temperature,a ffording the ketimide-and formate-bridged U IV species [Cs{U(OSi-(O t Bu) 3 ) 3 } 2 (m-NH)(m-CH 3 CHN)], 3 and [Cs{U(OSi-(O t Bu) 3 ) 3 } 2 (m-HCOO)(m-NHCOO)], 4.

Recent Advances in the Chemistry of Metal Carbamates

“…Formation of exotic polyanionic carbamato ligands was observed by addition of CO 2 to a nitrido-bridged diuranium(IV) complex [ 225 , 226 ], and to zirconocene and hafnocene dinitrogen complexes [ 227 , 228 ] ( Scheme 21 ). These systems are prone to double CO 2 insertion, either on the same or on different nitrogen atoms [ 229 ].…”

Terminal uranium(V)-nitride hydrogenations involving direct addition or Frustrated Lewis Pair mechanisms

“…In contrast, the reactivity of 1 with H 2 , the mechanisms of which sharply diverge with or without BMes 3 , is surprising and notable because the 5f 1 uranium(V) ion in 1 is high oxidation state and cannot be considered to be electron-rich nor lowcoordinate. Indeed, the only example of any molecular uraniumnitride reacting with H 2 is the diuranium(IV)-nitride-cesium complex [Cs{U(OSi[OBu t ] 3 ) 3 } 2 (μ-N)]; 47 here, the product is the bridging parent imido-hydride complex [Cs{U(OSi[OBu t ] 3 ) 3 } 2 (μ-NH)(μ-H)] and this transformation is enabled by the bridging, polar nature of the nitride and polymetallic cooperativity effects. However, that chemistry stops at the imido-hydride stage, or reverts to nitride and H 2 , and does not proceed to the H-atom 1,1-migratory insertion stage to give an amide.…”

“…However, more remarkable is that fact that the FLP component is actually not mandatory for hydrogenolysis of the U≡N triple bond to occur, though its absence does slow the reaction significantly demonstrating the facilitating role of the FLP mechanism since the main origin of this impediment is that formally a proton has to evolve from a hydride. In the absence of an FLP mechanism H 2 undergoes a direct 1,2-addition across the U V ≡N triple bond to give a H−U V =N−H unit that is reminiscent of the aforementioned reactivity of [Cs{U(OSi[OBu t ] 3 ) 3 } 2 (μ-N)] 47 when their terminal vs bridging natures, respectively, are taken into account. The reactivity of 1 is also reminiscent of aspects of recently reported mechanistic studies of the reaction of uranium(III) with water 66 , and it is germane to note that concerted two-electron redox chemistry at uranium remains a relatively rare phenomena 29,36,67 with one-electron processes dominating.…”

“…Both react with the small molecules CO, CO 2 , and CS 2 40,41 , but since only the protonolysis of 1 with H 2 O to give NH 3 had been previously examined 36 the ability of 1 and 2 to react with H 2 has remained an open question. Indeed, the study of molecular uranium-nitride reactivity remains in its infancy 36,37,[40][41][42][43][44][45][46][47][48] , and only very recently the diuranium(IV)-nitride-cesium complex [Cs {U(OSi[OBu t ] 3 ) 3 } 2 (μ-N)] was reported to reversibly react with H 2 to give the diuranium-imido-hydride complex [Cs{U(OSi [OBu t ] 3 ) 3 } 2 (μ-NH)(μ-H)] 47 . Bridging nitrides tend to be more reactive than terminal ones, so whilst this nitride hydrogenolysis is enabled by the bridging nature of the nitride and polymetallic cooperativity effects 47 , we wondered whether H 2 activation by 1 or 2 might still be accessible, given prior protonation studies 36 , since this would realise the first terminal f-block-nitride hydrogenolyses.…”

“…Indeed, the study of molecular uranium-nitride reactivity remains in its infancy 36,37,[40][41][42][43][44][45][46][47][48] , and only very recently the diuranium(IV)-nitride-cesium complex [Cs {U(OSi[OBu t ] 3 ) 3 } 2 (μ-N)] was reported to reversibly react with H 2 to give the diuranium-imido-hydride complex [Cs{U(OSi [OBu t ] 3 ) 3 } 2 (μ-NH)(μ-H)] 47 . Bridging nitrides tend to be more reactive than terminal ones, so whilst this nitride hydrogenolysis is enabled by the bridging nature of the nitride and polymetallic cooperativity effects 47 , we wondered whether H 2 activation by 1 or 2 might still be accessible, given prior protonation studies 36 , since this would realise the first terminal f-block-nitride hydrogenolyses. Further motivation to study this fundamental reaction stems from the fact that bridging and terminal uranium-nitride reactivity with H 2 is implicated in Haber Bosch NH 3 synthesis when uranium is used as the catalyst 49 , and uranium-nitrides have been proposed as accident tolerant fuels (ATFs) for nuclear fission, but likely reactivity with H 2 formed from radiolysis under extreme conditions or when stored as spent fuel remains poorly understood.…”