Heterometallic Mg−Ba Hydride Clusters in Hydrogenation Catalysis

Reaction of a MgN“2/BaN”2 mixture (N“=N(SiMe3)2) with PhSiH3 gave three unique heterometallic Mg/Ba hydride clusters: Mg5Ba4H11N”7 ⋅ (benzene)2 (1), Mg4Ba7H13N“9 ⋅ (toluene)2 (2) and Mg7Ba12H26N”12 (3). Product formation is controlled by the Mg/Ba ratio and temperature. Crystal structures are described. While 3 is fully insoluble, clusters 1 and 2 retain their structures in aromatic solvents. DFT calculations and AIM analyses indicate highly ionic bonding with Mg−H and Ba−H bond paths. Also unusual H−⋅⋅⋅H− bond paths are observed. Catalytic hydrogenation with MgN“2, BaN”2 and the mixture MgN“2/BaN”2 has been studied. Whereas MgN“2 is only active in imine hydrogenation, alkene and alkyne hydrogenation needs the presence of Ba. The catalytic activity of the MgN”2/BaN“2 mixture lies in general between that of its individual components and strong cooperative effects are not evident.


Introduction
Since the isolation of the first defined Ca hydride complex, [1] the focus in alkaline earth (Ae) metal hydride chemistry has been on the isolation of defined, metal pure complexes. At present, molecular Ae metal hydride complexes throughout group 2 are known. [2][3][4][5] Going down the group, increasing dynamics and ligand exchange reactions make it more challenging to isolate stable Ae metal hydride complexes. After a report on the first Mg hydride complex, [6] various Mg hydride complexes have been fully characterized. [4] While there are also ample examples for Ca hydride complexes, [5] well-defined heavier Sr [7][8][9][10][11][12][13] and Ba [7,14,15] hydride complexes are few and remain a curiosity.
Most recently, Okuda and co-workers presented a mixed Ca/ Sr hydride complex (Figure 1, F). [13] The complex appears to be the first fully characterized example of a two alkaline earth metal hydride blend that has been thoroughly studied..
We recently demonstrated that stabilization of well-defined Ae metal hydrides not necessarily needs large bulky ligands. In combination with multidentate amine ligands, the much smaller (Me 3 Si) 2 N anion, abbreviated in here as N", was able to stabilize a variety of Ca and Sr hydride complexes, also in solution. [12] For Ba we could even abstain from stabilizing multidentate ligands. The heptamer (HBaN") 7 crystallized in a reasonable yield of 51 % and is stable in aromatic solvents. [15] Herein we extend this self-assembly concept to heterobimetallic Ae metal hydrides. We focused hereby on Mg 2 + , a hard Lewis acid, and Ba 2 + , a soft metal cation. The rationale behind this choice is that the hard-soft combination may be advantageous in hydrogenation catalysis, with the softer metal binding the substrate and the harder metal delivering the hydride. Similar observations have been made for Li/Al hydride catalysts. [53][54][55] We herein present synthesis and structures of well-defined Mg/ Ba hydride clusters, which could be seen as potential intermediates during hydrogenation catalysis, and explore the effect of Mg/Ba metal-mixing in hydrogenation catalysis.

Syntheses
Monitoring the reaction of MgN" 2 and BaN" 2 in an approximately 1/1 ratio with PhSiH 3 (20°C, C 6 D 6 ) by 1 H NMR spectroscopy showed immediate conversion of the N" anions to PhSiH 2 N". During the reaction some precipitate was formed but this redissolved within minutes upon stirring. Concentrating the solution led to colorless crystals which were identified as Mg 5 Ba 4 H 11 N" 7 · (benzene) 2 (1, yield: 46 %); Scheme 2.
The 1 H NMR spectrum of a similar reaction of MgN" 2 /BaN" 2 in a 1/2 ratio with PhSiH 3 (20°C, C 6 D 6 ) showed apart from cluster 1 a second species. Removal of 1 by extensive washing with pentane and subsequent crystallization from benzene afforded colorless crystals which were identified as Mg 4 Ba 7 H 13 N" 9 · (benzene) 2 (34 % yield). The crystal quality of this product is poor and high quality crystals were obtained by recrystallization from toluene to give Mg 4 Ba 7 H 13 N" 9 · (toluene) 2 (2). Although the metal/hydride ratio in both clusters 1 and 2 is more or less similar (1: Ae/H = 0.82, 2: Ae/H = 0.85), the second cluster is clearly richer in Ba (1: Mg/Ba = 1.25, 2: Mg/Ba = 0.57).
In an attempt to optimize reaction conditions in favor for the larger cluster, the reaction was performed at higher temperatures. Reaction of MgN" 2 /BaN" 2 in a circa 1/2 ratio with PhSiH 3 (C 6 D 6 ) at room temperature showed 1 H NMR signals for 1 and 2. Prolonged heating did not affect the cluster 1, which seems thermally quite stable, but led to gradual disappearance of 2. Some formation of BaN" 2 seems to occur along with the appearance of trace compounds (see ESI, Figure S15-S16). However, after 12 hours of heating, crystals of a highly insoluble product could be isolated from the hot reaction mixture. This product was identified as Mg 7 Ba 12 H 26 N" 12 (3), a cluster which like 2 is rich in Ba (Mg/Ba = 0.58) but clearly less rich in hydrides (Ae/H = 0.73). Total crystalline yield could be increased when hexane was used as solvent (70°C, 12 h). However, elemental analysis was found not fully consistent with calculated values of 3. Multiple assessments of single crystals obtained from the reaction mixture using X-ray diffraction each confirmed the identity of 3, but with bulk purity in question, we refrain from giving a numerical yield.

Crystal structures
Complex Mg 5 Ba 4 H 11 N" 7 · (benzene) 2 (1) crystallizes as a cluster (Figure 1a) in which the Mg 5 Ba 2 -core could be best described as a pentagonal bipyramid with five equatorial Mg centers, each  bound to a N" ligand, and two axial Ba centers which are both capped by benzene ( Figure 1b). The ten triangular Mg 2 Ba-faces each contain a μ 3 -bridging hydride and the remaining hydride is located in the center of the pyramid and is best described as an interstitial hydride. [56,57] The two remaining N"Ba + cations bridge between two N"À Mg units, breaking the five-fold symmetry of the cluster. Alternatively, the complex could be thought build up from a central (C 6 H 6 )BaÀ HÀ Ba(C 6 H 6 ) 3 + spindle surrounded by a neutral (MgH 2 ) 5 -belt in which each Mg 2 + cation carries a N" À ligand from which two out of five are bridged by N"Ba + (Figure 1c). Although the complex does not have crystallographic symmetry, it is approximately mirror symmetric (C s ). All hydride ligands were localized and freely refined.
The largest cluster, Mg 7 Ba 12 H 26 N" 12 (3), is highly symmetric. It crystallizes in the trigonal spacegroup R-3, featuring a threefold inversion axis. The core of the cluster consists of a (MgH 6 ) 4À octahedron ( Figure 3a). This unit with perfect HÀ MgÀ H angles of 90°and 180°is enclosed by a (Ba 12 H 20 ) 4 + icosahedron consisting of 12 Ba 2 + ions and 20 triangular Ba 3 faces that each contain a μ 3 -H À ligand (Figure 3b). The twelve Ba 2 + ions are adequately shielded by bonding to 6 MgN" 2 units, each bridging between a pair of Ba 2 + ions ( Figure 3c). The formula for this cluster might therefore also be reformulated as The heterometallic Mg/Ba hydride clusters 1-3 are the first of their kind. During our investigation we never found  precipitation of metal-pure metal hydride species, indicating that the interplay of the hard Lewis acidic Mg 2 + ion and soft Ba 2 + has a positive influence on the stability of mixed Ae metal hydride clusters.

Solution NMR studies
Complex Mg 5 Ba 4 H 11 N" 7 · (benzene) 2 (1) is sparingly soluble in benzene-d 6 . At room temperature three hydride singlet signals could be identified at 3.41, 3.36 and 3.29 ppm with an intensity ratio of 4 : 4 : 2, respectively. These chemical shifts fit for typical Mg hydride complexes which, with some exceptions, are generally in the 3.0-4.5 ppm range. [4] They do not fit for Ba hydride complexes which feature hydride resonances at much lower field (range: 7.9-10.4 ppm). [7,14,15] Their ratio of 4 : 4 : 2 fits well with the C s -symmetry of the cluster (Figure 1c). The fourth hydride signal, observed at 1.35 ppm, is assigned to the interstitial hydride ligand which is the most symmetric and least disturbed hydride ligand. Although this signal does not fit the general range for Mg or Ba hydride signals, its chemical shift is comparable to that for the interstitial hydride in the larger cluster [(para) 3 Mg 8 H 10 ] (para = a dianionic para-phenylenebridged bis(β-diketiminate) ligand) .
[60] The central hydride ligand in this complex is linearly surrounded by two Mg 2 + ions and shows a 1 H NMR resonance at 0.56 ppm. Although this value is considerably upfield from the d ¼3.0-4.6 ppm values reported for the a-MgH 2 phase, it fits for the for the β-MgH 2 phase (d = 0.9 ppm). [61,62] The chemical shifts for the hydride ligands in 1 are clearly dominated by the Mg 2 + cations and not by Ba 2 + . High temperature NMR studies showed no coalescence of the hydride signals up to 80°C. At higher temperatures decomposition by formation of HD is observed which was attributed to deprotonation of toluene-d 8 by the H À ligand, a process that already slowly starts at 50°C. The lack of coalescence at higher temperature demonstrates that the cluster 1 is very robust. It also shows that the symmetry-breaking BaN" + cations, attached to the outside of the cluster, cannot wander freely over the cluster surface The complexes Mg 4 Ba 7 H 13 N" 9 · (solvent) 2 (2, solvent = benzene or toluene) are, once crystallized, poorly soluble in aromatic solvents. The 1 H NMR in benzene-d 6 shows five hydride signals at 5.45, 5.44, 4.65, 4.42 and 3.82 ppm in a ratio of 4 : 4 : 2 : 2 : 1. In agreement with the mixed-metal character of the cluster, these chemical shift values are too high for a typical MgÀ H resonance and too low in comparison with typical BaÀ H resonances. A two-dimensional 1 H, 1 H-COSY spectrum (Figure S10) revealed correlation between the two signals of highest intensity (4) and the smallest signal (1). While the latter high-field signal at 3.82 ppm is assigned to the interstitial hydride, the two most intense hydride signals with the highest chemical shifts (5.44 and 5.45 ppm) are assigned to the eight hydrides sitting on Ba 2 Mg-faces (which are different due to the symmetry breaking BaN 3 " À anions and η 6 -toluene ligands). The two remaining hydride resonances at 4.42 and 4.65 ppm, both with intensity 2, are assigned to hydrides on the four Mg 2 Bafaces. It should be noted that chemical shifts for hydrides on the Ba-rich Ba 2 Mg-faces are higher than those for hydrides at Mg-rich Mg 2 Ba-faces. This is in agreement with the heavy metal effect which causes hydrides with close metal contacts to shift downfield (the heavier the metal, the larger the downfield shift). [63,64] Theoretical considerations Complexes 1-3 have been analyzed by DFT methods at the B3PW91/def2tzvp level using core potentials at Ba. For simplicity the (Me 3 Si) 2 N À anions have been replaced by Me 2 N À anions (abbreviated as N*). These model clusters, which are indicated as 1*, 2* and 3*, resemble the experimentally determined crystal structures reasonably well (Table S2). The NPA charges for Mg, Ba and the hydride ligands in 1*-3* are all comparable and in a narrow range: Mg + 1.58/ + 1.63, Ba + 1.70/ + 1.79 and H À 0.80/À 0.88 (Table S3). These values indicate highly ionic bonds. The NPA charges for Mg in 1*-3* compare well to those in recently reported Mg hydride dimer complexes stabilized by the (Me 3 Si) 2 N À anion and neutral Lewis bases: Mg + 1.63/ + 1.67. [65] Charges on the hydrides in the latter Mg hydride dimer (H -0.79) are at the lower end of those in 1*-3*. The NPA charges for Ba in 1*-3* compare well to those in the related (HBaN") 7 cluster (F in Scheme 1) which vary from + 1.68 to + 1.77. Charges on the hydrides in (HBaN") 7 range from À 0.83 to À 0.86 and are at the higher boundary of those in 1*-3*.
Atoms-in-molecules (AIM) analysis of cluster 1* shows bond paths between the interstitial hydride and the two axial Ba 2 + ions with an electron density in the bond-critical-points (bcp's) of 0.162 e · Å À 3 . These BaÀ H bonds are much stronger than those between Ba and the five outer hydrides in the periphery of the cluster (0.132-0.136 e · Å À 3 ). Bonding between the interstitial hydride and the five Mg 2 + ions in the equatorial plane is indicated by bcp's but is much weaker (0.103 e · Å À 3 ) due to long MgÀ H distances (vide supra). Electron densities in bcp's on MgÀ H bonds within the (MgH 2 ) 5 belt are up to twice as high (0.158-0.209 e · Å À 3 ). These values are in favor of describing 1* as a central (C 6 H 6 )BaÀ HÀ Ba(C 6 H 6 ) 3 + spindle surrounded by a neutral (MgH 2 ) 5 -belt. The most pronounced bcp's (0.232 e Å À 3 ) were found between the two barate BaN* 3 À motifs and the hydride core Mg 5 H 10 belt.

Catalytic hydrogenation
Cooperative effects between metal complexes from the first and second main groups are widespread in literature. [38][39][40][41] The combination KH/AeN" 2 (Ae=Mg, Ca) was recently put forward as a hydrogenation catalyst for activated (conjugated) alkenes, terminal unactivated alkenes or cyclic internal alkenes. [67] Screening all alkali metals, mixtures of MN" (M=Li, Na, K, Rb, Cs) and MgN" 2 were shown to be catalysts for transferring hydrogen from 1,4-cyclohexadiene to either Ph 2 C=CH 2 or Ph(H) C=CH 2 . [37] It is notable that each component alone was not active in this catalytic transformation. Herein we present first studies on mixed Ae metal hydride catalysts in which we combine a hard metal (Mg) with a soft metal (Ba).
For evaluation of possible cooperative effects between Mg and Ba, catalytic alkene hydrogenation was not only studied with the MgN" 2 /BaN" 2 mixture but also with the metal-pure precursors MgN" 2 and BaN" 2 alone. Reaction conditions were adapted from previously established hydrogenation protocols using the AeN" 2 precursors (120°C, 6 bar H 2 ). [20,21,23] In order to observe activity for the least active catalyst MgN" 2 , high catalyst loadings of 20 mol% were used. In order to compare results, for the MgN" 2 /BaN" 2 mixture 10 mol% of each has been used which equals 20 mol% total metal content. Although for consistency the metal-pure BaN" 2 catalyst was also used in 20 mol% quantity in most cases, it must be realized that for this highly active catalyst the loadings can be reduced significantly. [20,21,23] Part of the catalytic runs were repeated with the mixed Mg/Ba cluster 1 in which case catalyst loadings of 1.8 mol% were used. Considering the cluster contains 11 hydrides, this converts to circa 20 mol% based on hydride content. Table 2 shows that MgN" 2 is generally fully inactive in alkene hydrogenation. A similar observation was made for transfer hydrogenation of alkenes. [37] Also in alkyne hydrogenation no activity was found (entry 6) but, as we reported previously, [20] MgN" 2 is catalytically active in imine hydrogenation (entries [7][8][9][10][11][12]. In contrast to the poor activity of MgN" 2 in alkene hydrogenation, the mixed-metal system MgN" 2 /BaN" 2 can reduce terminal alkenes like 1-hexene (entry 1), however, with low activity. Precatalyst BaN" 2 alone showed under similar conditions full conversion of 1-hexene but considerable isomerization to 2-hexene was found as a side-reaction. [21] As internal alkenes are much harder to reduce than terminal alkenes, no further hydrogenation to n-hexane was observed. The double bonds in vinyltrimethylsilane and 4-vinyl-cyclohexene cannot be isomerized and show higher conversions (entries 2-3). The reluctance  to hydrogenate internal alkenes is supported by the selective hydrogenation of the vinyl group in 4-vinyl-cyclohexene. Exceptions are the double bonds in norbornadiene which are activated for reduction but also give nortricyclene as a sideproduct (entry 4). Like for BaN" 2 , conversion of p-Cl-styrene is accompanied by nucleophilic aromatic substitution (entry 5) but diphenylacetylene could be fully reduced by the MgN" 2 / BaN" 2 system (entry 6). Various imines (aldimine or ketimine) could be hydrogenated with the MgN" 2 /BaN" 2 combination (entries 7-12).
It seems that the presence of Ba is essential for alkene and alkyne hydrogenation whereas for imine hydrogenation MgN" 2 already showed good activity. This may be explained by activation of the imine substrate by Mg···N(R)=C(H)R' coordination, a complex in which a hard Mg cation combines with a hard N Lewis base. The fact that MgN" 2 alone does not perform in alkene hydrogenation is likely related to the lower ionicity of the MgÀ H bond (cf. the BaÀ H bond) and its inherently lower reactivity. Additionally, it also could be due to poor coordination of soft alkene ligands to hard Mg 2 + cations. Activation of unsaturated substrates by Ae metal ions has been shown essential in catalysis. [68,69] Isolation of unsupported Mg···alkene (or alkyne and arene) complexes so far has only be achieved with highly Lewis acidic cationic Mg complexes which are free of stabilizing Lewis bases. [70][71][72][73][74][75][76] It has been found that the heavier softer Ae metals Ca, Sr and Ba are superior in activating soft unsaturated substrates. [11,29,33] This is also supported by the crystal structures of 1 and 2 in which the arene ligands coordinate exclusively to the larger Ba metal and not to Mg and explains why the presence of Ba is crucial for alkene and alkyne activation. [a] MgN" 2 /BaN" 2 ratio 1 : 1. In this case 10 mol% of each, MgN" 2 and BaN" 2 , was used. The total metal amount is 20 mol%, [b] 10 mol% catalyst loading, [21] [c] 10 mol% catalyst loading @ 0.5 h reaction time. [21] ChemCatChem Full Papers doi.org/10.1002/cctc.202101071 With few exceptions the activity of the mixed-metal catalyst generally lies between that of its individual components. These results demonstrate that, although there are certainly differences in activity between the three catalysts, mixing Mg and Ba does not have an additional advantage over using Ba alone. In order to understand the nature of the mixed catalyst MgN" 2 / BaN" 2 , a solution of this combination in C 6 D 6 was pressurized with H 2 (6 bar) and heated to 80°C (6 h). While a similar reaction of CaN" 2 with H 2 led to formation of undefined Ca x H y N" z clusters with MW's up to 7500 g · mol À 1 , [20] the MgN" 2 /BaN" 2 mixture reacted with H 2 to give primarily Mg 5 Ba 4 H 11 N" 7 · (benzene) 2 (1) together with minor quantities of unknown hydride complexes, indicated by 1 H NMR signals in the 5.0-5.3 ppm region (Figure S42-43). The formation of 1 was further confirmed by its crystallization from this solution. The clusters 1-3 are therefore valid models for intermediates that could form during catalytic hydrogenation with MgN" 2 /BaN" 2 . Indeed, cluster 1 has been successfully used as a hydrogenation catalyst ( Table 2). In all cases the catalytic activity of 1 is comparable to that of the MgN" 2 /BaN" 2 mixture, supporting the idea that these mixed metal Mg/Ba hydride clusters are the catalytically active species.

Conclusions
Reaction of a mixture of MgN" 2 and BaN" 2 with PhSiH 3 resulted in the formation of three unique heterometallic Mg/Ba hydride clusters Mg 5 Ba 4 H 11 N" 7 · (benzene) 2 (1), Mg 4 Ba 7 H 13 N" 9 · (solvent) 2 (2) (solvent = benzene or toluene) and Mg 7 Ba 12 H 26 N" 12 (3). The educt composition influences the Mg/Ba ratio in the metal hydride clusters which become richer in Ba with lower MgN" 2 / BaN" 2 ratios. Cluster composition can also be influenced by temperature. While cluster 1 is thermally stable, cluster 2 is thermolabile and forms at higher temperatures the larger Mg/ Ba hydride cluster Mg 7 Ba 12 H 26 N" 12 (3) which is completely insoluble in aromatic solvents. These clusters, which can all be prepared phase-pure, are among the first examples of wellcharacterized Ae heterobimetallic hydrides. Under no circumstances were homometallic clusters isolated, indicating that the mixing of Mg 2 + and Ba 2 + cations has a positive influence on cluster stability.
While the insoluble cluster 3 could not be studied in solution, the solid state structures of 1 and 2 are retained when dissolved in aromatic solvents. The high symmetry of the Mg x Ba y H z cores found in the solid state structures of 1 and 2 is broken by coordination of BaN" units in the periphery. This makes the hydride ligands in the core inequivalent, leading to several hydride resonances in the 1 H NMR spectra. It is surprising that the clusters 1 and 2 do not show dynamical ligand exchange processes that are typical for s-block metal compounds, also not at higher temperature (+ 80°C). The chemical shifts for the hydride ligands in 1 and 2 range from 1.35 to 5.45 ppm, and should be regarded more typical for MgÀ H resonances (0.55-6.78 ppm) than for BaÀ H resonances (7.9-10.4 ppm). DFT calculation on model system for 1-3 reproduced their structures reasonably well. AIM analysis showed bcp's between the metals and hydride ligands. The electron density on the MgÀ H bond paths is generally higher than that on BaÀ H bond paths which is due to higher covalency in the MgÀ H bond. This is confirmed by calculation of the NPA charges which are higher for Ba 2 + when compared to Mg 2 + . All clusters also show unusual H À ···H À bond paths but the electron densities in their bcp's are generally low (0.05-0.14 e · Å À 3 ).
The activity of the mixture MgN" 2 /BaN" 2 in hydrogenation catalysis was compared to that of the metal pure catalysts MgN" 2 and BaN" 2 . A solution of the MgN" 2 /BaN" 2 mixture in benzene reacts with H 2 to give primarily Mg 5 Ba 4 H 11 N" 7 · (benzene) 2 (1). The MgN" 2 (pre)catalyst is not active in alkene or alkyne hydrogenation but is effective in imine hydrogenation. The MgN" 2 /BaN" 2 mixture was found to be active in alkene, alkyne and imine hydrogenation, however, the activities are in general lower or equal to that of BaN" 2 . This clearly shows that, although imine hydrogenation can also be performed with Mg-based catalysts, for alkene and alkyne hydrogenation the presence of Ba is crucial. This may be explained by the HSAB concept in which hard Lewis acids (Mg 2 + ) bind and activate hard Lewis bases (imines) whereas soft Lewis bases (alkenes and alkynes) need the softer Ba 2 + for activation. A combination of hard (Mg 2 + ) and soft (Ba 2 + ) does, at least in catalytic hydrogenation, not provide additional benefits. This is mainly due to the very high performance of metal-pure Ba catalysts.
GC-MS measurements were performed on a Thermo Scientic™ Trace™ 1310 gas chromatography system (carrier gas Helium) with detection by a Thermo Scientic™ ISQ™ LT Single Quadrupole mass spectrometer. A Phenomenex® Zebron TM ZB-5 GC column of the dimensions 0.25 mm × 30 m with a film thickness of 0.25 μm or a Thermo Scientific™ TraceGOLD™ TG-5SilMS GC Column of the dimensions 0.25 mm × 30 m with a film thickness of 0.25 μm was used. The samples (1 μL) were injected with an Instant Connect-SSL Module in the split mode (injector temperature: 280°C). Temperature programs were started at 40°C followed by heating ramps, optimized for the separation problem, until 280°. Baseline separation of each analyte was achieved by choosing the different temperature programs. The molecular identities were confirmed by comparison with entries in the NIST/EPA/NIH mass spectral library (v2.2, built June 10 2014) or by comparing to authentic samples.
Synthesis of Mg 5 Ba 4 H 11 N" 7 · (benzene) 2 (1). A mixture of BaN" 2 (50.0 mg, 0.11 mmol) and MgN" 2 (47.1 mg, 0.14 mmol) was suspended in 1.5 mL of benzene (the chosen Mg/Ba ratio represents that in 1). The mixture was shortly heated to reflux to dissolve all solids. PhSiH 3 (32.5 mg, 0.30 mmol, 37.0 μL) was added followed by vigorous shaking of the solution. Colorless crystals formed over the next two days. The crystals were isolated and dried at 60°C under vacuum to afford 25 mg of pure Mg 5 Ba 4 H 11 N" 7 · (benzene) 2 (25 mg, 0.013 mmol, 46 % based on BaN" 2 ). Yield was calculated on the basis that four eq. of BaN" 2 are needed to form one eq. of Mg 5 Ba 4 H 11 N" 7 · (benzene) 2 complex. 1  Synthesis of Mg 4 Ba 7 H 13 N" 9 · (benzene) 2 (2). A mixture of BaN" 2 (232 mg, 0.506 mmol) and MgN" 2 (100 mg, 0.290 mmol) was suspended in 3 mL of benzene (the chosen Mg/Ba ratio represents that in 2). The mixture was shortly heated to reflux to dissolve all solids. PhSiH 3 (102 mg, 0.943 mmol, 116 μL) was diluted in benzene (1.8 mL) and added dropwise to the stirring solution. The brownish solution was stirred for 1.5 h at room temperature. The formation of a white precipitate was observed. The reaction mixture was allowed to settle and the solution was decanted. The remaining white solid was washed with pentane (3 × 1.5 mL) and dried under vacuum at 60°C to obtain 66 mg (0.025 mmol) of pure Mg 4 Ba 7 H 13 N" 9 · (benzene) 2 in 34 % yield, based on BaN" 2 . Yield was calculated on the basis that seven eq. of BaN" 2 are needed to form one eq. of Mg 4 Ba 7 H 13 N" 9 · (benzene) 2 complex. The toluene adduct could be isolated by repeating the procedure in toluene. Clear colorless crystals of Mg 4 Ba 7 H 13 N" 9 · (toluene) 2 × toluene were isolated from a saturated toluene solution after one week at room temperature. The compound is poorly soluble in benzene-d 6 (3). A mixture of BaN" 2 (60.0 mg, 0.131 mmol) and MgN" 2 (26.4 mg, 0.076 mmol) was suspended in 1.8 mL of hexane in a 20 mL vial (the chosen Mg/Ba ratio represents that in 3). The mixture was shortly heated to reflux to dissolve all solids. PhSiH 3 (30.7 mg, 0.284 mmol, 35 μL) was diluted in hexane (200 μL) and added dropwise to the stirring solution. The solution turned yellowish and was transferred to an NMR tube. The reaction mixture was heated to 70°C for 12 h to obtain clear, colorless crystals. The crystals (11.4 mg) were washed with hexane (3 × 1 mL) and dried shortly under vacuum. The complex shows no solubility in cyclohexane-d 12 , hot benzene-d 6 , benzene-d 6 /THF-d 8 mixture or THF-d 8  Catalytic hydrogenation with MgN" 2 and BaN" 2 mixtures. The catalyst, MgN" 2 (0.022 mmol, 20 mol%), BaN" 2 (0.022 mmol, 20 mol%) or the mixture MgN" 2 /BaN" 2 (0.011 mmol of each), was dissolved in dry toluene (1.0 ml) in a steel autoclave (15 ml) under an atmosphere of nitrogen and the substrate (0.110 mmol) was added to the solution. The tightly sealed autoclave was applied with hydrogen pressure (6 bar), stirred and quickly heated to the desired temperature (120°C) in a heating block. The conversion after the given time was determined by subsequent GC-MS analysis of the quenched reaction mixture.
Catalytic hydrogenation with Mg 5 Ba 4 H 11 N" 7 · (benzene) 2 (1). Complex 1 (10.0 mg, 5.1 μmol, 1.8 mol%) was dissolved in dry toluene (1.0 ml) in a steel autoclave (15 ml) under an atmosphere of pure nitrogen and the substrate (0.28 mmol) was added to the solution. The tightly sealed autoclave was applied with hydrogen pressure (6 bar), stirred and heated to the desired temperature (120°C) quickly in a heating block. The conversion after the given time was determined by subsequent GC-MS analysis of the quenched reaction mixture. [NOTE: Complex concentration appears to be low at 1.8 mol%, however, normalized to 11 hydrides per complex, the total concentration of hydrides would be � 20 mol%] Supporting Information 1 H, 13 C, 29 Si NMR, DOSY and variable temperature NMR spectra for complexes 1 and 2, ATR-IR-spectra are provided for 1-3. 1 H NMR monitoring of catalytic reactions, crystallographic details including ORTEP plots, details for the DFT calculations including XYZ-files.