Nitrogen activation and cleavage by a multimetallic uranium complex

Multimetallic-multielectron cooperativity plays a key role in the metal-mediated cleavage of N2 to nitrides (N3−). In particular, low-valent uranium complexes coupled with strong alkali metal reducing agents can lead to N2 cleavage, but often, it is ambiguous how many electrons are transferred from the uranium centers to cleave N2. Herein, we designed new dinuclear uranium nitride complexes presenting a combination of electronically diverse ancillary ligands to promote the multielectron transformation of N2. Two heteroleptic diuranium nitride complexes, [K{UIV(OSi(OtBu)3)(N(SiMe3)2)2}2(μ-N)] (1) and [Cs{UIV(OSi(OtBu)3)2(N(SiMe3)2)}2(μ-N)] (3-Cs), containing different combinations of OSi(OtBu)3 and N(SiMe3)2 ancillary ligands, were synthesized. We found that both complexes could be reduced to their U(iii)/U(iv) analogues, and the complex, [K2{UIV/III(OSi(OtBu)3)2(N(SiMe3)2)}2(μ-N)] (6-K), could be further reduced to a putative U(iii)/U(iii) species that is capable of promoting the 4e− reduction of N2, yielding the N24−complex [K3{UV(OSi(OtBu)3)2(N(SiMe3)2)}2(μ-N)(μ-η2:η2-N2)], 7. Parallel N2 reduction pathways were also identified, leading to the isolation of N2 cleavage products, [K3{UVI(OSi(OtBu)3)2(N(SiMe3)2)( 
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 N)}(μ-N)2{UV(OSi(OtBu)3)2(N(SiMe3)2)}]2, 8, and [K4{(OSi(OtBu)3)2UV)(N)}(μ-NH)(μ-κ2:C,N-CH2SiMe2NSiMe3)-{UV(OSi(OtBu)3)2][K(N(SiMe3)2]2, 9. These complexes provide the first example of N2 cleavage to nitride by a uranium complex in the absence of reducing alkali metals.


Introduction
The search for catalysts capable of converting dinitrogen into ammonia, or higher-value organic products under mild lowenergy conditions, continues to attract intensive research on dinitrogen binding and cleavage by molecular compounds of d and f block elements. 1 Uranium nitrides were shown to be highly active catalysts in the conversion of dinitrogen (N 2 ) to ammonia (NH 3 ), before equally active iron oxides were adopted as catalysts in the industrial production of NH 3 . 2 This has generated signicant interest towards the N 2 chemistry of uranium, 1a,b,1d,3a-l leading to the discovery of complexes capable of stoichiometrically converting N 2 to NH 3 , 1b,d,3j and in one system, catalytically converting N 2 to amine (6.4 equiv.). 1d However, the reaction of dinitrogen with well characterized molecular U(III) complexes, in most cases resulted in reversible binding, 3b-f or in the 2e À reduction to N 2 2À , with electrons being provided by two 1e À transfers from two mononuclear complexes. 3a,g In contrast, the few reported cases of 4e À reduction of N 2 1d or N 2 cleavage to nitride, 3d,k,3l usually required a combination of U(III) or U(IV) complexes and highly reducing alkali ions (K or Cs), leaving ambiguity on the number of electrons transferred by the uranium centers.
Recently, we identied a molecular dinuclear nitride U(III) complex (complex C prepared from the reduction of the U(IV)/ U(IV) analogue A, Scheme 1) that effects the 4e À reduction of N 2 to N 2 4À (complex D), which can be further functionalized by protons (H + ) or CO, without addition of highly reducing alkali ions. In this dinuclear system, each uranium center is capable of transferring two electrons to N 2 . 1b Although multielectron transfer reactions are not common in uranium chemistry, reports of metal based two-electron transfer reactions have been rapidly increasing. 4,5 In contrast, redox reactivity involving more than two electrons has been so far limited to complexes of uranium with redox-active ligands. 6 However, our recent report of the rst unambiguous example of metal-based 4e À transfer by a U(II) complex, 7 suggests that 3e À transfer reactions may also be possible. Specically, the 3e À transfer in dinuclear U(III) complexes is a pre-requisite to affect N 2 cleavage without addition of reducing agents.

Synthesis of heteroleptic U(IV) nitride complexes
In order to probe ligand effects on N 2 binding, we sought to synthesize a series of heteroleptic uranium complexes with OSi(O t Bu) 3 and N(SiMe 3 ) 2 ancillary ligands in various ratios.
First, we attempted to incorporate a higher number of OSi(O t Bu) 3 ligands within the previously reported mixed-ligand nitride system, [Na(DME) 3 ] (I), 9a by ligand protonolysis with 3.0 equiv. of HOSi(O t Bu) 3 (see ESI †). Analysis of the reaction mixture by 1 H NMR spectroscopy indicated unreacted starting materials ( Fig. S1 †), suggesting that protonolysis of the U-N(SiMe 3 ) 2 bonds by HOSi(O t Bu) 3 cannot occur in these U(IV) systems.
Therefore, we next investigated the addition of KOSi(O t Bu) 3 (2.0 equiv. total) to the cationic, all-N(SiMe 3 ) 2 complex, [{U IV (N(SiMe 3 ) 2 ) 2 (THF)} 2 (m-N)], E. Addition of 2.0 equiv. of KOSi(O t Bu) 3 to a solution of E in THF at room temperature, resulted in the immediate formation of a pink-brown solution and precipitation of a white solid. Large orange crystals of Fig. 1a), were obtained from a concentrated hexane solution at À40 C in 82% yield. The 1 H NMR spectrum of isolated complex 1 in d 8 -toluene displays two resonances at d À0.10 and À3.81 ppm, corresponding to the OSi(O t Bu) 3 and N(SiMe 3 ) 2 signals, respectively (Fig. S8 †). The solid-state molecular structure of complex 1 was determined by X-ray diffraction studies, and shows the presence of an ion pair consisting of one K cation and the [{U IV (OSi(O t Bu) 3 )(N(SiMe 3 ) 2 ) 2 } 2 (m-N)] anion. The two uranium(IV) ions in 1 are bridged by a nitride (N 3À ), and each are bound by one OSi(O t Bu) 3 and two N(SiMe 3 ) 2 ligands, resulting in a mixed-ligand framework. The K cation binds to the bridging nitride ligand (2.953(3)Å) and to four oxygens of the two OSi(O t Bu) 3 ligands. The U-N-U bond angle (162.5(1) ) and distances (2.071(2), 2.077(2)Å;  (14) ; 2.055(3)Å) 9b and G (179.0(1) ; 2.076(5)Å). 9a Next, we investigated the synthesis of a heteroleptic complex with a different combination of OSi(O t Bu) 3 (four) and N(SiMe 3 ) 2 (two) ligands, to probe the effect of ancillary ligands in N 2 binding and activation.
Isolation of the heteroleptic complex "[U III (OSi(O t Bu) 3 ) 2 (-N(SiMe 3 ) 2 )]" in analytically pure form proved unsuccessful, leading to ligand scrambling products (see ESI † for details), and therefore further reactivity studies were carried out using the in situ generated heteroleptic U(III) complex.
Addition of 0.5 equiv. CsN 3 to an Et 2 O solution of in situ generated heteroleptic complex "[U III (OSi(O t Bu) 3 ) 2 (N(SiMe 3 ) 2 )]" at À40 C for four days, resulted in the formation of a light brown-pink solution. Analysis of the reaction mixture by 1 H NMR spectroscopy in  Fig. 1b), were obtained from a concentrated Et 2 O solution at À40 C in 56% yield. The solid-state molecular structure of complex 3-Cs was determined by X-ray diffraction studies, and shows the presence of an ion pair consisting of one Cs cation and the [{U IV (OSi(O t Bu) 3 ) 2 (N(SiMe 3 ) 2 )} 2 (m-N)] anion. The two uranium(IV) ions in 3-Cs are bridged by a nitride (N 3À ), and each ion is bound by two OSi(O t Bu) 3 and one N(SiMe 3 ) 2 ligand, resulting in a mixed-ligand framework. This altogether differs from complex 1 in the OSi(O t Bu) 3 /N(SiMe 3 ) 2 ligand ratio (e.g. 2 : 4 in 1 vs. 4 : 2 in 3-Cs). The Cs cation binds the bridging nitride (3.063(3)Å) and four oxygens from two OSi(O t Bu) 3 ligands. The U-N-U bond angle (156.9(2) ) is slightly smaller than 1, but has similar U-N bond distances (2.064(3), 2.074(3)Å; Table 1).
Complex 3-Cs is stable in toluene at À40 C for one month and in THF at À80 C for up to 8 hours. However, the product begins to decompose in THF aer 1 hour at À40 C, and is fully consumed aer 2 hours at room temperature with a solution color change from pink to yellow (Fig. S14 †). From this mixture, yellow crystals were isolated at À40 C in concentrated n-hexane   (17) - Fig. 2). The solid-state molecular structure of 3b-Cs suggests that decomposition involves 1,2-addition of a C-H bond of N(SiMe 3 ) 2 across the uranium-nitride bond. This is accompanied by an intramolecular ligand rearrangement, resulting in three OSi(O t Bu) 3 ligands binding one U(IV) ion, whereas, one OSi(O t Bu) 3 , one N(SiMe 3 ) 2 , and a m-k 2 -N,C-CH 2 SiMe 2 NSiMe 3 ligand binds the second U(IV). The two uranium(IV) ions in 3b-Cs are bridged by two dianionic ligands, namely, an imido (NH 2À ) with a U-N-U bond angle (119.7(4) ) signicantly bent compared to 3-Cs, and a m-k 2 -N,C-CH 2 SiMe 2 NSiMe 3 ligand.
The isolation of complex 4 from the reduction of 1 with excess KC 8 , suggests that further reduction to the U(III)/U(III) analogue is not accessible in this ligand environment. This is comparable with the results of the previously reported reduction of complex E, which also led to the U(III)/U(IV) nitride, [{U III/ IV ((N(SiMe 3 ) 2 ) 2 (THF)} 2 (m-N)] (F, Scheme 1). 9b This also shows that replacement of two N(SiMe 3 ) 2 ligands in the all-N(SiMe 3 ) 2 complex, [U IV (N(SiMe 3 ) 2 ) 3 } 2 (m-N)] (G), by two OSi(O t Bu) 3 ligands, allows for the reduction of one uranium center, yielding the U(III)/U(IV) complex, which was unattainable in the reduction of the all-amide complex, G (Scheme 1). So far, nitride-bridged diuranium(III) complexes have only been obtained by reduction of the all-OSi(O t Bu) 3 analogue, complex A. 4c The addition of two OSi(O t Bu) 3 ligands to complex E also results in a higher stability of the U(III)/U(IV) complex 4, when compared to the previously reported U(III)/U(IV) nitride (F)which readily undergoes 1,2 addition of the C-H bond of a N(SiMe 3 ) 2 ligand across the uranium-nitride at À40 C. 9b Whereas, complex 4 is relatively stable over 24 hours in THF at À40 C (Fig. S18 †). The increased stability of 4 is likely due to the presence of the K + cation capping the nitride.
Notably, when the K cation is removed from the core by addition of 2.0 equiv. of 2.2.2-cryptand to 4 in d 8 -THF at À40 C, 1,2-addition of the C-H bond across the nitride occurs readily. Analysis of the reaction mixture by 1 H NMR spectroscopy shows the consumption of 4 and the appearance of [K(2.2.2-cryptand)] (Fig. S32 †). Dark black crystals of the U(III)/U(IV) cyclometalated imido complex, [((Me 3 Si) , 3b-Cs, with thermal ellipsoids at drawn at the 50% probability level. Hydrogen atoms and methyl groups on the OSi(O t Bu) 3 ligands have been omitted for clarity.
a THF/hexane mixture at À40 C in 74% yield, and characterized by X-ray diffraction studies (see ESI †).
Next, we investigated the reduction of complex 3-Cs. Addition of 1.0 or 10.0 equiv. of KC 8 to solutions of 3-Cs in d 8 -THF at À40 C, led to signicant differences in reactivity. Notably, the reaction with 1.0 equiv. of KC 8 led to a brown-red solution and the appearance of new resonances and unreacted 3-Cs (a, Fig. S35 †). Crystals of [K 2 {U IV/III (OSi(O t Bu) 3 ) 2 (N(SiMe 3 ) 2 )} 2 (m-N)] (6-K) (Scheme 3) were isolated from a concentrated n-hexane solution at À40 C and characterized by XRD analysis. Attempts to isolate analytically pure 6-K from this pathway proved unsuccessful due to the contaminant precipitation of 3-Cs.
Alternatively, when the reduction of 3-Cs is carried out in the presence of 10.0 equiv. KC 8 in THF at À40 C, a dark purple solution is obtained. Analysis of the reaction mixture by variable temperature 1 H NMR spectroscopy indicated full consumption of complex 3-Cs, and formation of new resonances. (Fig. S39 and S42 †). The same species was also obtained by reduction of isolated 6-K with KC 8 in THF (Fig. S43 †). This suggests that further reduction of 6-K yields a putative U(III)/U(III) nitride, "[K 3 {U III (OSi(O t Bu) 3 ) 2 (N(SiMe 3 ) 2 )} 2 (m-N)]" (X), analogous to the one previously observed in homoleptic nitride systems (C; Scheme 1). 1b,15 Attempts to isolate single crystals or analytically Scheme 3 Reactivity of complexes (a) 1 and 4 and (b) 3-Cs, 6-K, 6-Cs, X, and 7. The -O t Bu and -(SiMe 3 ) 2 substituents on complexes 6-K, 6-Cs, 7, 8, and 9 were omitted for clarity. pure material from the reaction mixture proved unsuccessful, most likely due to the highly reactive nature of the formed species, which results in the formation of 6-K in toluene solution at À40 C aer 24 hours (Fig. S41 †). These results indicate that depending on the nature of the solvent (Et 2 O or THF), reduction with excess KC 8 can either lead to the formation of a U(III)/U(IV) or U(III)/U(III) species, highlighting the importance in tuning the reaction conditions in order to reach low-valent species.
Next, to probe the role of the cation in the synthesis and stability of the proposed heteroleptic U(III)/U(III) species, X, we also performed the reduction of 3-Cs with 10.0 equiv. CsC 8 in THF at À40 C. In comparison to the KC 8 reactivity, this reaction instead resulted in a red-brown solution (Fig. S37 †). Light orange crystals suitable for XRD analysis were obtained from a concentrated hexane solution at À40 C in 78% yield, and characterized as the Cs analogue of 6-K, [Cs 2 {U III/ IV (OSi(O t Bu) 3 ) 2 (N(SiMe 3 ) 2 )} 2 (m-N)] (6-Cs) (Scheme 3, Fig. 4b).
The 2.081(12)-2.099(12)Å). 15 These results indicate that with excess CsC 8 , we cannot produce the analogous Cs 3 U(III)-N-U(III) species. This contrasts the previous studies for the reduction of the all-OSi(O t Bu) 3 analogue, [Cs{U III (OSi(O t Bu) 3 ) 3 } 2 (m-N)] (A), with KC 8 and CsC 8 , where in both systems, the respective K 3 U(III)-N-U(III) and Cs 3 U(III)-N-U(III) species were obtained. The unreactive nature of the U(III)-N-U(IV) species, with excess CsC 8 , can be explained by a predicted, more negative reduction potential in comparison to 6-K, due to the replacement of the K cation by Cs in the second coordination sphere. Notably, the stronger K-O bonds (vs. Cs-O bonds) in the secondary coordination sphere help drive the reaction to the U(III)/U(III) state. However, the redox potentials for these species could not be obtained due to the rapid decomposition in THF solution at room temperature (3-Cs / 3b-Cs).

Reactivity of heteroleptic nitrides
Now with the U(III)/U(IV) complexes 4, 6-K, and 6-Cs in hand, we sought to probe their reactivity toward N 2 activation and cleavage. However, in all systems, the U(III)/U(IV) precursors showed no reactivity with 1 atm N 2 in toluene or THF solutions, resulting in unreacted starting materials as evidenced by 1 H NMR studies (see ESI †).
In contrast, addition of 1 atm N 2 to the in situ generated U(III)/U(III) complex X, in toluene solution at À100 C, led to an unambiguous color change from dark purple to light brown. Analysis of the reaction mixture by 1 H NMR spectroscopy at variable temperature displays new resonances and the consumption of complex X (Fig. S48 †). Dark brown crystals suitable for XRD analysis were obtained from the toluene reaction mixture at À40 C aer 24 hours, and characterized as the ligands support all three K cations. This contrasts the previously reported all-OSi(O t Bu) 3 U(V)/U(V) nitride-hydrazido complex (D, Scheme 1), in which the three K cations are bound to six OSi(O t Bu) 3 ligands and one K is bound to the N 2 4À moiety, while two K atoms are bound to the bridging nitride. 1b The U-N nitride bond distances (2.101 (15), 2.137(13)Å) ( Table 2) are consistent with the previously reported complex D (2.069(2)Å), 1b and U(V)/ U(V) bis-nitride complexes (2.022(5), 2.101(6)Å). 16 The U 2 (mh 2 :h 2 -N 2 ) moiety features U-N bond distances which range between (2.178(15)-2.318(15)Å), consistent with U-N single bonds. 17 The value of the N^N bond length (1.54(2)Å) corresponds to a highly-activated N 2 , that has undergone a 4e À reduction to a hydrazido moiety (N 2 4À ), and is elongated compared to previously reported U-(N 2 ) 0 (1.109(7)-1.120(14)Å) and U-(N 2 ) 2À (1.124(12)-1.232(10)Å) moieties. 3b,3c,3e,3f,3g Additionally, this value is comparable to the observed N-N bond length in complex D (1.521 (18)

Å). 1b
To date, only one other example of a dinuclear U(III)/U(III) complex capable of reducing N 2 to N 2 4À has been reported (D), 1b the homoleptic-OSi(O t Bu) 3 analogue of X. Next, variable-temperature magnetic and EPR data were measured on isolated complex 7 in order to conrm the presence of two U(V) (5f 1 ) ions, consistent with the 4e À reduction of N 2 . The magnetic susceptibility (c M ) of complex 7 shows a paramagnetic behaviour with two magnetically independent U ions (purple, Fig. S65a †). This is in contrast to the previously reported complex D, which displays antiferromagnetic coupling at 50 K. The solid-state magnetic moment of 7 at 250 K was found to be 1.76 m B per uranium, (data were collected between 2-250 K to avoid thermal decomposition of the product). This value is lower than the theoretical value expected for a free U(V) ion (2.54 m B ; 2 F 5/2 ground term), 18 however it is in the range of other reported bimetallic U(V) systems (1.35-3.32 m B ). 4b,19a,b,19c-e,10a The m eff for complex 7 shows a steady decrease when the temperature is lowered, reaching 1.23 m B at 50 K, and then more rapidly to reach a value of 0.32 m B at 2 K (blue, Fig. S65a †). Although this temperature prole is typical for U(V), the value at 2 K is rather low. However, this behavior has been previously observed for the dimeric U(V) bridging bis-oxo complex, [{(( Ad ArO) 3 N)U} 2 (m-O) 2 ], 20 in which they report a similar c M plot and absence of antiferromagnetic coupling, altogether combined with a low m eff value at 2 K (0.36 m B per U). This was attributed to a potential antiferromagnetic interaction that was not observable, leading to the unexpected low temperature value. Additionally, the X-band (9.4 GHz) EPR spectrum of complex 7 shows an intense signal at 6 K that was t to an axial set of signals (g 1 ¼ 2.740; g 2 ¼ 2.008; g 3 ¼ 1.797), further supporting the U(V) oxidation state (Fig. S64 †). 4a,4f,19b,21 Interestingly, separation and crystallization of the supernatant from four independent reactions of complex X with N 2 , aer removal of complex 7, led to the identication of two N 2 4Àcleavage products, characterized by XRD analysis as a formal (8) (Fig. 6). Over four trials, complex 8 was isolated once, while complex 9 was isolated on three occasions. Each crystallization of the supernatant always led to a mixture of species, in which the only other product that could be crystallographically identied was the K analogue of complex 3-Cs, [K{U IV (OSi(O t Bu) 3 ) 2 (N(SiMe 3 ) 2 )} 2 (m-N)] (3-K) (Fig. S62 †). Overall preventing the further characterization of these N 2 cleavage products. However, these crystal structures provide important insight into N 2 cleavage and reactivity.
The solid-state molecular structure of complex 8 shows the presence of a symmetric tetrameric structure, comprising of two dimers in a diamond-shaped core, with two K atoms at the apical positions, capping the terminal and bridging nitride moieties (Fig. S63 †). Due to the imposed crystal symmetry, identical bond metrics are found for each dimeric unit. U1 is bound by two OSi(O t Bu) 3 and one N(SiMe 3 ) 2 ligands.  (Table 2). This is consistent with the presence of U^N multiple bonds and singly bound U-N, respectively, as previously observed in U(VI) bisnitride bridged complexes. 22 Two nitrides (N2, N3), bind the U2 ion in a slightly bent trans conguration (167.59 (17) ), with different bond distances for U2-N3 and U2-N2 (1.784(4) and 1.927(3)Å). These parameters can be compared to those of the two previously reported linear trans-bis-nitride U(VI) complexes. In the complex, [K 6 -{(OSi(O t Bu) 3 ) 2 U IV } 3 -{(OSi(O t Bu) 3 ) 2 U VI }(m 4 -N) 3 (m 3 -N)(m 3 -O) 2 ], the N^U cen^N bond distances are also slightly asymmetric (1.963(8) and 1.902(8)Å) and the bond angle is linear (179.07 (11) ). 3k In contrast, in the complex [(NH 3 ) 8 U IV -(m-N)-U VI (NH 3 ) 5 -(m-N)-U IV (NH 3 ) 8 ] 8+ , the N^U cen^N bond distances are identical (1.853(3) and 1.854(3)Å) and the bond angle is linear (179.07 (11) ) probably due to the absence of coordinating cations. 23 The terminal nitride has a bond distance of 1.784(4)Å, consistent with the bond distances found in the only two previously reported terminal U(VI) nitrides (1.769(2), 1.799(7)Å) 24 and shorter than those found in alkali-capped (1.846(3)-1.929(6)) 25 and borane-capped U(V) nitrides (1.880(4), 1.916(4)Å). 26 The solid-state molecular structure of complex 9 displays the presence of a dimeric imido cyclometallated anion, where the two uranium ions are bridged by three anionic ligands, namely a nitride (N 3À ), an imido (NH 2À ), and a m-k 2 -N,C-CH 2 SiMe 2 -NSiMe 3 ligand. The presence of an imido group, arising from the 1,2 addition of a C-H bond of the N(SiMe 3 ) 2 ligand across the U-N-U bond, was conrmed by 1 H NMR spectroscopy of the reaction mixture, showing resonances of the imido and methylene protons at d 627.91 ppm and d À138.25 and À153.55 ppm, respectively. In complex 9, one U-bound N(SiMe 3 ) 2 ligand is lost, and two KN(SiMe 3 ) 2 are bound in the second coordination sphere. Two nitrides bind the U2 ion in a trans conguration (170.0(6) ) and the U(V) bond distances for the terminal and bridging nitride are 1.847(13)Å and 1.916(13)Å, respectively. The bis-imido motif R-N]U V ]N-R, an analogue of the UO 2 2+ cation, has been previously observed by Boncella and coworkers. 27 However, such an analogous bis-nitride motif, N^U^N, has not been previously observed for U(V). The terminal nitride (1.847(13)Å) is elongated compared to the terminal K-capped U(VI) nitride in complex 8 (1.784(4)Å; Table  2), and is comparable to that of the previously reported alkalicapped 24 and borane-capped U(V) nitrides. 26 The bridging U-N-U (U1-N1: 2.323(13), U2-N1: 1.916(13)Å) and U-NH-U (U1-N2: 2.218 (14), U2-N2: 2.318(14)Å) 14 bond distances are consistent with the presence of a U-N single and a short U^N multiple bond (similar to complex 8), and a bridging imido, respectively. Complexes 8 and 9 provide the rst example of N 2 cleavage promoted by a low-valent U(III) complex without addition of alkali reducing agents. We recently reported N 2 cleavage in the all-OSi , to yield a tetranitride cluster. 3k However, this reactivity was only observed upon reduction with KC 8 .
Once isolated, the diuranium(V)-N 2 4À complex 7 is stable in toluene solution at À40 C for one month, indicating that the formation of the nitride complexes, 8 and 9, must arise from parallel redox pathways. The formal oxidation state of complex 8, indicates that its formation requires the 1e À reduction of complex 7 by an external reducing agent. This would result in  (b) have been depicted as wire frames for clarity.
the overall 2e À reduction and cleavage of the N 2 4À single bond, where an additional 1e À is provided by the U(V)/U(VI) oxidation. Overall, complex 8 is the product of an unprecedented case of 3e À transfer from a U(III) center. Whereas the formation of complex 9 most likely involves a 2e À reduction of the N 2 4À moiety by an external reducing agent. Formation of these unusual species suggests a high redox exibility of the uranium centers in the mixed-ligand environment, provided by the combination of OSi(O t Bu) 3 and N(SiMe 3 ) 3 ligands.
A potential source for the additional 1 and 2e À required to form complexes 8 and 9, respectively, is the U(III)/U(III) nitride complex, X. One could envision that the reduction of complex 7, promoted by the highly reducing U(III)/U(III) complex X, occurs in parallel with the binding of N 2 .
To conrm such a pathway, we sought out the reduction of 7 with the U(III)/U(III) complex X. Addition of 1.0 equiv. (2e À ) of in situ generated complex X in d 8 -toluene to 7 at À80 C, resulted in an unambiguous colour change from dark purple to brown with precipitation of orange crystals. Analysis of the reaction mixture by variable temperature 1 H NMR spectroscopy, led to the complete consumption of X and formation of new resonances, including the imido (NH; d 627.91 ppm) and methylene protons (d À138.25 and À153.55 ppm), attributed to complex 9 (Fig. S50 †). Quantitative 1 H NMR studies of the reaction mixture demonstrated that complex 9 is formed in 51% yield (Fig. S51 †). Crystallization of the reaction mixture from a concentrated toluene solution at À40 C led to a mixture of pale green, orange, and colorless crystals. The pale green and orange crystals were identiable as complexes 9 and 3-K, respectively. This indicates that complex 7 undergoes a 2e À reduction promoted by the oxidation of the U(III)/U(III) complex to U(IV)/U(IV). Attempts to isolate analytically pure 9 from this route proved unsuccessful due to the contaminant precipitation of 3-K.
In order to probe the reactivity of the U 2 (N 2 4À ) moiety in complex 7, we next investigated the reactivity with acid (H + ). Addition of HCl in Et 2 O to isolated complex 7, led to the formation of a broad, ill-dened triplet at d 7.41 ppm in the 1 H NMR spectrum, possibly consistent with 14 NH 4 Cl (Fig. S57a †). However, we have previously shown that uranium bridging nitride complexes react readily with strong acids to form NH 3 . 14 Therefore, to determine if the NH 4 Cl was derived from the (N 3À ) and (N 2   4À ) ligands, the analogous experiment with 15 N 2 -labelled complex, 7-14/15 N, was also carried out. HCl in Et 2 O addition to 7-14/15 N led to a comparable spectrum as 7 (Fig. S57b †), making the assignment to 15 NH 4 Cl or 14 NH 4 Cl ambiguous. Broad resonances are not common in NH 4 Cl quantication experiments, and are most likely due to our ligand system. Indeed, addition of HCl in Et 2 O to neat KN(SiMe 3 ) 2 , also led to a broad singlet at d 7.35 ppm in the 1 H NMR spectrum (Fig. S55 †), directly overlapping with the resonance for 14/15 NH 4 Cl at d 7.41 ppm. Altogether, this rendered NH 3 quantication difficult, yet comparable yields were obtained for the nitride precursor 3-Cs and complexes 7 and 7-14/15 N (see ESI †), suggesting that protonation of the N 2 4À does not result in NH 3 formation. However, addition of HCl in Et 2 O to the reaction mixture obtained aer reacting complex X and 7, led to the formation of a broad, ill-dened signal at d 7.41 ppm in the 1 H NMR spectrum assigned to 14 NH 4 Cl (Fig. S59 †). Although ammonia quantication is difficult in this system, the number of equivalents of NH 3 increased by about $1.7, compared to the amount obtained from the addition of acid to isolated complex 7.
This indicates that complex 7 undergoes a 2e À reduction and N 2 4À cleavage to a tris-nitride complex, promoted via the U(III)/ U(III) reducing agent, X.

Conclusions
Herein, we have identied rational routes for the synthesis of heteroleptic nitride bridged diuranium(IV) complexes containing OSi(O t Bu) 3 and N(SiMe 3 ) 3 ligands in varying ratios. The differences in the electronic and steric properties of these ligands allow for the tuning of redox properties and reactivity of these diuranium complexes. The reactivity of the heteroleptic complexes [K{U IV (OSi(O t Bu) 3 )(N(SiMe 3 ) 2 ) 2 } 2 (m-N)] (1) and [Cs {U IV (OSi(O t Bu) 3 ) 2 (N(SiMe 3 ) 2 )} 2 (m-N)] (3-Cs), differs signicantly from the previously reported all-OSi(O t Bu) 3 and all-N(SiMe 3 ) 2 analogues. Notably, the reduction of one of the U(IV) centers can be performed for 1 and 3-Cs to yield the nitride bridged U(III)/ U(IV) complexes, 4, 6-K, and 6-Cs, while reduction of the anionic all-amide complex G was unattainable, probably due the higher electron-rich character of the N(SiMe 3 ) 3 ligand compared to OSi(O t Bu) 3 . The presence of two additional OSi(O t Bu) 3 ligands in the U(III)/U(IV) complex 4, compared to the cationic all-amide complex E, results in an increased stability towards the 1,2 addition of the C-H bond of a N(SiMe 3 ) 2 ligand across the uranium-nitride, 9b most likely due to the siloxide-bound K cation capping the nitride. Removal of the K cation by addition of 2.2.2-cryptand, increases nitride reactivity towards C-H activation, affording the imido cyclometallated complex 5. The U(III)/U(IV) complexes 4 and 6-Cs cannot be reduced further, most likely due to the very electron-rich character of the U. However, reduction of the second U(IV) center is possible for complex 6-K, but is highly dependent upon the reduction reaction conditions. The putative U(III)/U(III) heteroleptic nitride complex X, is signicantly more reactive than the only other example of the all-OSi(O t Bu) 3 U(III)/U(III) nitride complex, A. Similar to complex A, X reacts with N 2 and promotes the 4e À reduction to yield the U(V)/U(V)-N 2 4À complex 7. N 2 cleavage products were also identied from the reaction of X and N 2 , and were crystallographically characterized. The tris-nitride complex 8 is generated through the overall 6e À cleavage of the bound N 2 moiety, promoted by a 5e À transfer from the two U(III) centers to form a U(V)/U(VI) complex, with an additional 1e À arising from an external source. In complex 9, cleavage of N 2 has also occurred to yield a U(V)/U(V) nitride, with 2e À arising from an external source. We found that addition of 7 to X results in the formation of complexes 9 and 3-K, indicating that the diuranium(III) precursor, X, provides 1 or 2e À to further reduce the bound N 2 4À moiety in 7. These results demonstrate that by tuning the ancillary ligands, the reducing power and the redox exibility of the uranium center can be altered leading to the 3e À transfer from U(III) to N 2 . This work has demonstrated the rst example of N 2 cleavage by a uranium complex in the absence of alkali metals. Further tuning of the ancillary ligands in diuranium nitride complexes, should allow better control of these transformations.

Data availability
All data associted were included in the supporting information.

Author contributions
M. K. carried out the synthetic experiments and analyzed the experimental data. F. F.-T. and R. S. carried out the X-ray single crystal structure analyses. I. V. collected the variabletemperature magnetic data. M. M. originated the central idea, coordinated the work, and analyzed the experimental data. The manuscript was written through contributions of all authors.

Conflicts of interest
The authors declare no conict of interest.