Lewis Acid-Induced Dinitrogen Cleavage in an Anionic Side-on End-on Bound Dinitrogen Diniobium Hydride Complex

The side-on end-on dinitrogen hydride complex [{Na(dme)}2{(O3)Nb}2(μ-η1:η2-N2)(μ-H)2] (3-Na, [O3]3− = [(3,5-tBu2-2-O-C6H2)3CH]3−) was observed to undergo facile elimination of H2 and cleavage of the N–N bond in the presence of 9-borabicyclo[3.3.1]nonane (9-BBN), AlMe3, and ZnMe2. Treatment of 3-Na with 9-BBN and ZnMe2 afforded the nitride complex [{K(dme)2}2{(O3)Nb}2(μ-N)2] (2-Na). The reaction of 3-Na with AlMe3 afforded [{Na(dme)}2{(O3)AlMe}2(NbMe2)2(μ-N)2] (5). The nitride complex 2-Na was treated with 9-BBN and AlMe3 to form [{Na(dme)}2{(O3)Nb}(μ-NH)(μ-NBC8H14){Nb(O3C)}] (4) and 5, respectively. Complex 2-Na, 4, and 5 were structurally characterized.


Introduction
Activation of dinitrogen under mild conditions has been of long-standing interest in chemistry owing to its industrial and biological significance. In this context, since the discovery of the first dinitrogen complex [Ru(N 2 )(NH 3 ) 5 ] 2+ in 1965 [1], a number of dinitrogen complexes have been synthesized and characterized [2][3][4]. A variety of coordination modes have been found in dinitrogen complexes to date, and dinitrogen complexes show diverse reactivity patterns [5][6][7][8][9][10]. With dinuclear complexes, the side-on end-on coordination mode produces substantial polarization of the coordinated dinitrogen [10][11][12][13], and seminal works from Fryzuk have demonstrated that complexes of this type display a rich functionalization chemistry [14]. However, only a limited number of side-on end-on dinitrogen complexes are known [11,[15][16][17]. Therefore, this coordination mode becomes one of synthetic targets in dinitrogen chemistry.
The reactions of Lewis acids with dinitrogen complexes provide suitable reference points to evaluate the reactivity of coordinated N 2 toward electrophiles. The reactions of mononuclear dinitrogen complexes with group 13 Lewis acids are known and frequently proceed at the β-N atom to form stable adducts [18][19][20][21][22]. Addition of group 13 Lewis acids to side-on end-on dinitrogen hydride complexes has also been reported to occur at the β-N atom. For example, Fryzuk -2,7,9,9-tetramethyl-9H-acridin-10-ide) was also reported to initially form the Lewis acid-base adducts with B(C 6 F 5 ) 3 and AlMe 3 [16].
We  3 CH] 3− ) and the counterion dependence of their reactivity toward N 2 [24]. The potassium derivative 1-K readily reacted with N 2 via the reductive elimination of two equivalents of H 2 to yield the nitride bridged complex [{K(thf) 2 } 2 {(O 3 )Nb} 2 (µ-N) 2 ] (2-K) [25]. For the lithium and sodium derivatives, the reactions with N 2 did not result in the cleavage of the N 2 triple bond but led to the elimination elimination of two equivalents of H2 to yield the nitride bridged complex [{K(thf)2}2{(O3)Nb}2(μ-N)2] (2-K) [25]. For the lithium and sodium derivatives, the reactions with N2 did not result in the cleavage of the N2 triple bond but led to the elimination of only one equivalent of H2 and the formation of the corresponding side-on end-on dinitrogen hydride complexes. The lithium salt of the side-on end-on complex was unstable and converted to the end-on bridging dinitrogen complex with concomitant loss of the birding hydride ligands, whereas the sodium salt [{Na(dme)}2{(O3)Nb}2(μ-η 1 :η 2 -N2)(μ-H)2] (3-Na) was stable enough to use as a starting material for further reactivity studies.
The complex 3-Na is the first example of an anionic side-on end-on dinitrogen complex, and the anionic charge is expected to enhance the nucleophilicity of the β-N atom. We preliminary demonstrated that the reaction of 3-Na with Me3SiCl occurred at the β-N atom of the coordinated N2 unit [25]. Therefore, we were interested in the reactions with other Lewis acids to investigate the nucleophilicity of the N2 unit in the anionic complex 3-Na. Here we report the reactions of the side-on end-on dinitrogen hydride complex 3-Na with ZnMe2, AlMe3, and 9-borabicyclo[3.3.1]nonane (9-BBN).
Reactions of mono-and dinuclear dinitrogen complexes with hydroboranes were reported to result in B-N bond formation due to the considerable strength of a B-N bond [27][28][29][30]. In the reaction with 2-Na, 9-BBN was observed to induce H2 elimination and N-Scheme 1. Reaction of 3-Na with 9-BBN.
Reactions of mono-and dinuclear dinitrogen complexes with hydroboranes were reported to result in B-N bond formation due to the considerable strength of a B-N bond [27][28][29][30]. In the reaction with 2-Na, 9- [31,32]. The resulting boryl-N 2 complex subsequently underwent H 2 reductive elimination, N-N bond cleavage, and ligand arrangement, yielding the borylimide-nitride complex. This difference is presumably due to the great tendency of 2-Na to reductively eliminate H 2 .
On the NMR tube scale, no reaction was observed at room temperature between a 1:1 mixture of 2-Na and 9-BBN in C6D6. Heating at 80˚C for 3 days led to the consumption of all of the 9-BBN and the formation of [{Na(dme)}2{(O3)Nb}(μ-NH)(μ-NBC8H14){Nb(O3C)}] (4) (Scheme 1). However, a small amount of 2-Na remained unreacted because 9-BBN partially decomposed during the reaction. Therefore, further addition of 9-BBN gave complete conversion to 4. The reaction was scaled up using excess 9-BBN to give 4 in a 62% isolated yield. The X-ray crystal structure of 4 ( Figure 2) confirmed its composition and revealed that the product was the result of an N-B and N-H bond formation and a [O3] methine C-H bond scission. The 9-BBN-induced formation of 2-Na from 3-Na is closely related to the reaction of [{(PNP)Ti} 2 (µ-η 1 :η 2 -N 2 )(η-H) 2 ] with pinacol borane (HBpin), wherein the binding of the borane center to the coordinated N 2 unit was proposed to trigger reductive elimination of H 2 and N-N bond cleavage [16]. In addition, the nitride bridged titanium dimer was shown to react with HBpin, yielding the borylimide complex. Mézailles also reported that the N 2 -derived nitride complex [(PPP)Mo(N)I] (PPP = PhP(CH 2 CH 2 PCy 2 ) 2 ) reacted with HBpin to afford borylamines via stepwise functionalization at the nitride group [33]. Therefore, it was of interest to investigate the possibility of hydroboration of the bridging nitride ligands in 2-Na.
On the NMR tube scale, no reaction was observed at room temperature between a 1:1 mixture of 2-Na and 9-BBN in C 6   The two Nb atoms were bridged by a parent imide and a 9-BBN-bound nitride ligand, with bond lengths being Nb1-N1 = 1.855(2), Nb1-N2 = 1.942(2), Nb2-N1 = 2.202(2), and Nb2-N2 = 2.111(2) Å. A 9-BBN unit was attached to a nitride group and exhibited a B1-H1-Nb1 agnostic interaction. The B1-N1 bond length of 1.541(4) Å was in the range expected for an N→B dative bond [34,35]. These geometrical parameters are consistent with the bonding sequence shown in Scheme 1. The central Nb 2 N 2 core of 4 is structurally related to the motif formed via the hydroboration of coordinated N 2 in [{(NPN)Ta} 2 (µ-η 1 :η 2 -N 2 )(µ-H) 2 ] [31,32]. Attempts to observe and isolate any intermediates in the conversion of 2-Na to 4 were unsuccessful, preventing elucidation of the mechanism of the reaction. The source of the NH proton remains to be determined.
The 1 H NMR spectrum of 4 indicates a highly asymmetric compound, with twelve inequivalent tert-butyl singlets and one methine singlet (5.82 ppm). The N-H resonance was observed at 10.6 ppm and split into a doublet ( 1 J NH = 72.2 Hz) when the 15 N-labeled complex was used. The B-H resonance could not be clearly assigned due to overlap with resonances from the BC 8 H 14 group. The 11 B NMR spectrum contained a resonance at 50.3 ppm. The 15 N NMR spectrum of the 15 N-labeled complex displayed 374 and 445 ppm, which were shifted upfield in comparison to that of 2-Na-15 N at 680 ppm. The resonance at 374 ppm was assigned to the NH ligand, but the large quadruple moment of the 93 Nb nucleus prevented observation of the N-H scalar coupling.

Reactions with ZnMe 2 and AlMe 3
The borane 9-BBN mediates the conversion of 3-Na to 2-Na by behaving as a Lewis acid. Prior to reductive elimination of H 2 , the reaction may proceed through a putative borane adduct. The exact role of 9-BBN is unclear, because the interaction may occur at the coordinated N 2 unit, the phenoxide groups, or the hydride ligands. Aiming to isolate acid-base adducts and confirm where the interaction occurs, we studied the behavior of the side-on end-on N 2 complex 3-Na with other Lewis acids, ZnMe 2 and AlMe 3 (Scheme 2). However, we found that these Lewis acids also promote N-N bond cleavage of 3-Na. Addition of ZnMe2 to 3-Na in toluene at room temperature resulted in an immediate color change from dark green to brown and the formation of a mixture of products. The products contained 2-Na according to 1 H NMR spectroscopy, but they proved to be in  Addition of ZnMe 2 to 3-Na in toluene at room temperature resulted in an immediate color change from dark green to brown and the formation of a mixture of products. The products contained 2-Na according to 1 H NMR spectroscopy, but they proved to be inseparable upon workup. In contrast to the reaction with 9-BBN, ZnMe 2 was consumed during the reaction. The formation of CH 4 was observed but attempts to identify any product containing Zn were unsuccessful.
When two equivalents of AlMe 3 were added to a solution of 2-Na in toluene, the reaction mixture immediately turned brown with the concomitant liberation of H 2 . Workup of the reaction mixture led to the isolation of [{Na(dme)} 2 {(O 3 )AlMe} 2 (NbMe 2 ) 2 (µ-N) 2 ] (5) as a brown powder in 69%. The complex 5 was also prepared by the reaction of 3-Na with two equivalents of AlMe 3 in toluene at room temperature. This suggests that the reaction of 3-Na with AlMe 3 initially yields the nitride complex 2-Na in a manner analogous to the reaction with 9-BBN. The nitride complex 2-Na would then form the Lewis acid-base adducts with AlMe 3 , which would undergo further arrangements to produce 5. The potassium salt was also prepared by the analogous reaction of 2-K with AlMe 3 .
The X-ray crystal structure of 5 ( Figure 3) revealed that the molecule lies on a crystallographic inversion center in the middle of the Nb 2 N 2 core. Each niobium atom adopts a trigonal bipyramidal geometry, with two methyl groups, two nitride groups, and one phenoxide group of the [O 3 ] ligand. The complex 5 contains two Al atoms, each of which is tetrahedrally coordinated by two phenoxide groups of the [O 3 ] ligand, one methyl group, and one nitride group. The Nb 2 N 2 Al 2 core closely approaches planarity, imparting a trigonal plane geometry to each nitrogen atom. Two solvated sodium cations are retained as a part of a cluster structure, each being bridged to the Nb 2 Al core by two methyl groups and one phenoxide group. The Nb-N bond lengths of 1.8719(13) and 2.0870(13) Å are comparable with those of 2-Na. The Al-N bond length of 1.8980(14) Å is longer than those for Al-N(amide) bonds [36,37] and shorter than those for Al-N dative bonds [38,39].

Conclusions
We demonstrated that the side-on end-on dinitrogen hydride complex, 3-Na, readily underwent reductive elimination of H2 in the presence of Lewis acids such as 9-BBN, AlMe3, and ZnMe2, followed by N-N bond cleavage to produce 2-Na. The reactions are proposed to proceed via initial formation of Lewis acid adducts with 3-Na at the β-N atom of the coordinated dinitrogen, taking into account the nucleophilicity of the β-N atom of the side-on end-on N2 unit and the silylation of the β-N atom by the reaction of 3-Na with

Conclusions
We demonstrated that the side-on end-on dinitrogen hydride complex, 3-Na, readily underwent reductive elimination of H 2 in the presence of Lewis acids such as 9-BBN, AlMe 3 , and ZnMe 2 , followed by N-N bond cleavage to produce 2-Na. The reactions are proposed to proceed via initial formation of Lewis acid adducts with 3-Na at the β-N atom of the coordinated dinitrogen, taking into account the nucleophilicity of the β-N atom of the side-on end-on N 2 unit and the silylation of the β-N atom by the reaction of 3-Na with Me 3 SiCl [24]. However, attempts to observe and isolate any adducts failed, because the Lewis acid adducts of 3-Na are quite prone to reductive elimination of H 2 . When other Lewis acids such as HBpin and B(C 6 F 5 ) 3 were used, the reactions yielded intractable mixtures. The resulting nitride complex, 2-Na, was found to further react with 9-BBN and AlMe 3 to produce 4 and 5, respectively. Further work will focus on the functionalization of the coordinated N 2 unit in 3-Na.

General Information
All manipulations were carried out using standard Schlenk techniques or in a glovebox under an atmosphere of argon. Anhydrous hexane, pentane, and toluene were dried by passage through two columns of activated alumina and a Q-5 column, while anhydrous THF, Et 2 O, and DME were dried by passage through two columns of activated alumina. Anhydrous deuterated benzene (benzene-d 6 ) was dried and degassed over a potassium mirror prior to use. NMR spectra were recorded on a JEOL ECX-500 spectrometer (JEOL Ltd., Tokyo, Japan). 1 H NMR spectra were reported with reference to solvent resonances of C 6 D 6 residual protons (δ = 7.16 ppm). 13 C NMR spectra were referenced to solvent (peaks δ = 128.06 (C 6 D 6 ) ppm). 11 B chemical shifts were referenced to BF 3 •OEt 2 (neat at 0.0 ppm) as an external standard. 15 N chemical shifts were referenced to 90% formamide in dimethyl sulfoxide-d 6 (112.7 ppm with respect to NH 3 at 0.0 ppm) as an external standard. Complexes 2-K and 3-Na were prepared following the literature procedure [23,24]. Elemental analyses (C, H, and N) were carried out on an Elementar vario MICRO Cube (Elementar, Frankfurt, Germany). IR spectra were recorded on a JASCO VIR-200 spectrometer (JASCO, Tokyo, Japan). Spectra and structures are available in Supplementary Materials.

Synthesis of 2-Na-15 N
The 15 N-labeled analogue was prepared in a manner identical to that used for 2-Na, except for using the 15 N-labeled precursor 3-Na-15 N. 15 N NMR (C 6 H 6 , 50.7 MHz): δ 679.1.

Synthesis of 4
To a solution of 3-Na (42.4 mg, 25.0 µmol) in toluene (6 mL) was added 9-BBN (12.2 mg, 100 µmol). The solution was stirred at room temperature overnight and was then allowed to warm to 80 • C. The color of the solution turned from green to brown to orange. After stirring for 2 days, the volatile materials were removed under vacuum. The residue was washed with hexane, leaving 4 as a yellow powder (62%, 28.0 mg). 1

Synthesis of 5
To a solution of 3-Na (104 mg, 61.5 µmol) in toluene (4 mL) was added a solution of AlMe 3 (1.05 M in hexane, 117 µL, 123 µmol) at room temperature. The solution turned brown instantly and was stirred overnight. The volatile materials were removed under vacuum, and then the residue was washed with hexane. The product 5 was isolated as an orange solid (76.8 mg, 41.8 µmol, 69%). 1

X-ray Crystallography
Single crystals were immersed in immersion oil on a micromount and transferred to a Rigaku Varimax with a Saturn system or a Rigaku XtaLAB Synergy-DW system equipped with a Rigaku GNNP low temperature device (Tokyo, Japan). Data were collected under a cold nitrogen stream at 123 K or 173 K using graphite-monochromated MoKα (λ = 0.71073 Å) or CuKα (λ = 1.54184 Å) radiation. Equivalent reflections were merged, and the images were processed with the CrysAlis Pro software 1.171.42.64a (Rigaku Oxford Diffraction, Japan). Empirical absorption corrections were applied. All structures were solved by direct method using SHELXT [40] and refined by full-matrix least-squares method on F 2 for all data using SHELXL [41] with the Olex2 program [42]. All hydrogen atoms were placed at their geometrically calculated positions. For 4, the hydrogen atoms of the NH group and the BH group were located in the Fourier map and refined isotropically. For 2-Na and 4, some residual electron density was difficult to model, the program SQUEEZE [43] was used to remove the contribution of the electron density in the solvent region from the intensity data. For 2-Na, one tert-butyl group was disordered. A void space contains 175 electrons per unit cell, which could be attributed to distorted pentane molecules (one molecule in the asymmetric unit). A large residual peak in the final difference map was located in the Nb 2 N 2 core. This peak is believed to be due to a small amount of twinning in the crystal. For 4, two tert-butyl groups and one DME molecule were disordered. For 4, a void space contains 1013 electrons per unit cell, which could be attributed to distorted pentane molecules (three molecules in the asymmetric unit). For 5, one benzene molecule was disordered.