Coordination of LiH Molecules to Mo≣Mo Bonds: Experimental and Computational Studies on Mo2LiH2, Mo2Li2H4, and Mo6Li9H18 Clusters.

The reactions of LiAlH4 as the source of LiH with complexes that contain (H)Mo≣Mo and (H)Mo≣Mo(H) cores stabilized by the coordination of bulky AdDipp2 ligands result in the respective coordination of one and two molecules of (thf)LiH, with the generation of complexes exhibiting one and two HLi(thf)H ligands extending across the Mo≣Mo bond (AdDipp2 = HC(NDipp)2; Dipp = 2,6-iPr2C6H3; thf = tetrahydrofuran, C4H8O). A theoretical study reveals the formation of Mo-H-Li three-center-two-electron bonds, supplemented by the coordination of the Mo≣Mo bond to the Li ion. Attempts to construct a [Mo2{HLi(thf)H}3(AdDipp2)] molecular architecture led to spontaneous trimerization and the formation of a chiral, hydride-rich Mo6Li9H18 supramolecular organization that is robust enough to withstand the substitution of lithium-solvating molecules of tetrahydrofuran by pyridine or 4-dimethylaminopyridine.

Introduction

Along with noble gas helium, hydrogen and lithium are the simplest, lightest elements and the only ones that existed in the young universe.[1] Helium hydride, HeH+, is a molecule of astrophysical importance,[2] whereas LiH is the lightest metal hydride and arouses considerable interest due to its many applications.[3−5] In the gas phase, molecules of LiH exist as a result of the overlap of the singly occupied H 1s and Li 2s atomic orbitals,[6] with an experimentally determined interatomic distance of ca. 1.60 Å.[7] In the solid state, LiH adopts a cubic NaCl-type structure, characterized by long Li···H contacts of approximately 2.04 Å.[3]

Molecular hydrides of the s-block elements have been intensively investigated in recent years. For group 2 metals, new, uncommon structures and a diversity of useful applications in hydrometalation, hydrogenation, and other reactions have been uncovered, thanks in no small part to the use of sterically encumbered auxiliary ligands.[3,8−19] Progress for the alkali metals has been more limited, although with notable exceptions. These include Stasch’s hydrocarbon-soluble lithium hydride cluster [{(DippNPPh2)Li}4(LiH)4], containing a (LiH)4 central cube (Dipp = 2,6-Pr2C6H3),[20] as well as the generation by Mulvey and co-workers of hexane-soluble lithium hydride transfer reagents.[21,22] Of particular relevance is the synthesis of the dilithiozincate hydride [(tmed)Li]2[{PrNCH2CH2N(Pr)}Zn(Bu)H] that retains the Li–H bond in solution and undergoes the dynamic association and dissociation of (tmed)LiH.[21] Also noteworthy are reports on hydride encapsulation by molecular alkali metal clusters,[23] the structural characterization of the LiH and LiOBu agglomerate Li33H17(OBu)16,[24] and the synthesis of a (LiH)4 cube coordinated to three bis(amido)alane units.[25]

Transition-metal complexes allegedly containing coordinated monomeric molecules of LiH are sparse. There are, however, some reports describing M–H–Li systems where a degree of covalent bonding within the bridging bond can be proposed on the basis of the observation of one-bond 1H–6,7Li NMR coupling constants.[21,26−35] Despite the scarcity of complexes of this type, it is conceivable that, like other main-group metal–hydrogen bonds (e.g., Mg–H, Al–H, and Zn–H),[36−44] a molecule of lithium hydride might bind to a transition-metal fragment through its Li–H bond, assisted by an interaction with an adjoining ligand that could compensate for the unsaturation of the lithium coordination shell and further heighten the σ-donor strength of the polar Li–H bond.

In this context, we envisioned that quadruply bonded hydride central units [Mo2(H)] (n = 1, 2) could be utilized to build the target molecular architectures. As represented in structure A of Figure , such dimolybdenum dihydride units possess strong hydride character[45] and feature Mo–Mo separations of close to 2.10 Å, with Mo–H vectors nearly perpendicular to the Mo–Mo bond.[45] Here, we discuss the synthesis and structure of complexes 3·thf and 4·thf (Figure ) that contain one and two formally monoanionic, bridging H–Li(thf)–H ligands, respectively, spanning the Mo≣Mo bond. We also study the unexpected formation of a unique, hydrocarbon-soluble, hydride-rich Mo6Li9H18 cluster, 5·thf, formally resulting from the trimerization of unobserved monomer [Mo2{μ-HLi(thf)H}3(μ-AdDipp2)], with the loss of a molecule of tetrahydrofuran. Throughout this article, three-center–two-electron (3c–2e) interactions implicating Mo–H and Li–H bonds are represented with the aid of the half-arrow formalism proposed by Green, Green, and Parkin.[46]

Figure 1

Simplified representations of the structures of complexes 3·thf-5·thf. The three structural types originate from [Mo2(H)] cores by the incorporation of one, two, or three molecules of (thf)LiH (n = 1, complex 3·thf; n = 2, A and complex 4·thf; when n = 3, the unobserved monomer trimerizes to complex 5·thf with the loss of a molecule of tetrahydrofuran). In the structure of 5·thf, symmetry-related lithium atoms share the same color.

Results and Discussion

In recent work, we showed that tetrahydrofuran adduct [Mo2(H)2(μ-AdDipp2)2(thf)2] (1·thf, where AdDipp2 = HC(NDipp)2 and Dipp = 2,6-PrC6H3) is a convenient source of unsaturated dihydride [Mo2(H)2(μ-AdDipp2)2] containing a trans-(H)Mo≣Mo(H) core (structure A in Figure ).[45] The Mo2(H)2 functionality of this complex was engendered by hydrogenolysis of the Mo–C bonds of the [(Me)Mo≣Mo(Me)] homologue,[47] a method that continues to be a main vehicle for the synthesis of transition-metal hydrides. Searching for a related monohydride [(H)Mo≣Mo] core, we carried out the two-step transformation shown in Scheme a. Low-temperature alkylation of [Mo2(μ-O2CH)2(μ-AdDipp2)2] with equimolar amounts of LiEt yielded an ethyl-formate intermediate that was reacted in situ with H2 and converted to the hydride-formate product, 2·thf (Scheme ), in good isolated yields (ca. 70%). The coordinated tetrahydrofuran molecule of 2·thf is highly labile, and it was readily replaced by Lewis bases such as 4-dimethylaminopyridine (dmap), 1,3,4,5-tetramethylimidazol-2-ylidene (IMe4), and PMe3, giving complexes 2·L (Scheme a, top). Similarly, the use of LiAlH4 as a source of LiH permitted the isolation of complex 3·thf that was obtained as a yellow solid in yields of around 60%. This reaction was not, however, simple and also produced related derivative 4·thf, along with minute amounts of a tetrahydroaluminate complex to be described elsewhere. Complex 3·thf possesses a H–Mo≣Mo–H–Li(thf) chelate moiety resulting from the substitution of the coordinated tetrahydrofuran of 2·thf by a molecule of (thf)LiH, with the formation of a σ-Li–H complex, that becomes stabilized by the concomitant formation of a 3c–2e Mo–H⇀Li bond involving the adjacent Mo–H terminus.

Scheme 1

Synthesis of Hydride Complexes with Mo≣Mo Bonds

(a) Compounds 2·thf, 2·L, and 3·thf. (b and c) Direct synthesis of complexes 3·thf and 4·thf. [Mo≣Mo] is an abbreviation for Mo2(μ-AdDipp2)2. L is 4-dimethylaminopyridine (dmap), 1,3,4,5-tetramethylimidazol-2-ylidene, (IMe4), and PMe3.

Next, 1·thf was utilized as a source of the [Mo2(H)2] center (Scheme b). Mixing a tetrahydrofuran solution of this complex with a solution of LiAlH4 in the same solvent caused the immediate precipitation of a bright-yellow solid that was identified as dilithium tetrahydride dimolybdenum complex [Mo2{μ-HLi(thf)H}2(μ-AdDipp2)2] (4·thf), that is, as a Mo2Li2H4 cluster. As drawn in Scheme b, the compound contains two trans-[μ-HLi(thf)H] ligands that extend across the Mo≣Mo bond. Thus, it can be related to 3·thf by means of formal replacement of the bridging formate of the latter by a second μ-Li(thf)H2– three-atom chelating ligand. In agreement with this rationale, complexes 3·thf and 4·thf were generated in high yields (70–85%) by the more direct method summarized in Scheme c, based on the reaction of readily available [Mo2(μ-O2CH)2(μ-AdDipp2)2] with LiAlH4, under appropriate conditions.

Complexes 2·L, 3·thf, and 4·thf were characterized with the aid of microanalytical, spectroscopic, and X-ray data and were additionally studied by computational methods. For molecules of 2·L, the proposed structure is based on IR and NMR data and was unmistakably confirmed by X-ray crystallography for 2·IMe (Figure S1). Regarding complexes 3·thf and 4·thf, their hydride signals were not readily apparent in the IR spectra, possibly because of the Mo–H–Li bridging character, so the unambiguous identification of the three-atom HLiH chains in 3·thf and 4·thf owes much to the 1H and 7Li NMR experiments developed. Surprisingly more soluble in benzene and toluene than in tetrahydrofuran, the H atoms of the HLi(thf)H ligand in 3·thf resonate at δ 4.33 (C6D6), appearing as a partially resolved multiplet due to coupling to the 7Li (92.6%, I = 3/2) and 6Li (7.4%, I = 1) nuclei. As can be seen in Figure , this signal becomes a singlet in the 1H{7Li} NMR spectrum. Moreover, the 4.33 multiplet is absent in the 1H NMR spectrum of the DLiD isotopologue of 3·thf, prepared by the reaction of [Mo2(μ-O2CH)2(μ-AdDipp2)2][48] with LiAlD4. The 7Li{1H} NMR spectrum is a somewhat broad singlet at 3.6 ppm that transforms into a 1:2:1 triplet in the proton-coupled 7Li NMR experiment, with a one-bond 7Li–1H coupling constant of 16 Hz.

Figure 2

From bottom to top, 1H, 1H{7Li}, 7Li{1H}, and 7Li NMR spectra of the HLiH moieties of complexes 3·thf (left) and 4·thf (right). 1H resonances at lower frequency relative to Mo–H–Li are due to methine protons of the AdDipp2 ligands or to tetrahydrofuran.

Complex 4·thf is only scarcely soluble in common solvents such as benzene, toluene, and tetrahydrofuran, but it is just sufficiently soluble in C6H5F for NMR studies. Pertinent NMR data are also included in Figure . In particular, the Mo2LiH2 moieties exhibit comparable 1J(7Li–1H) couplings of 17 Hz. These observations categorically demonstrate the existence of HLiH entities coordinated to the Mo–Mo quadruple bond in the 3·thf and 4·thf molecules. Besides, they attest without a doubt to the fact that, although probably mainly Coulombic in nature (vide infra), the Mo–H–Li–H–Mo bonding interactions involve a considerable degree of covalency, that is, of substantial electron density shared among the molybdenum, hydrogen, and lithium valence orbitals. It is pertinent to remark that the observation of scalar coupling in lithium hydride complexes is rare, to the point that few 1J(6,7Li–1H) values can be found in the literature.[21,29−35] Previously observed couplings range from approximately 6 to 15 Hz such that the 16 to 17 Hz values found for 3·thf and 4·thf are among the highest thus far reported. For the LiH molecule, a 1J(7Li–1H) coupling constant of 159 Hz has been calculated.[49]

Complexes 3·thf and 4·thf possess good thermal stability. Their C6D6 and C6D5F solutions appear to be stable for 1 day at room temperature, though decomposition occurs upon heating at 70 °C for 3 to 4 h. In the solid state, decomposition is apparent only at T ≥ 150 °C. The two compounds behave as soluble LiH carriers, particularly 4·thf, which is the more reactive of the two. For instance, complex 4·thf reacted with Ph2C(O) to give the expected alkoxide Ph2C(H)(OLi).[20,22] Their molecular structures were investigated by X-ray crystallography and optimized by means of DFT calculations. Owing to poor crystal properties, the data collected for the former do not permit a rigorous structural discussion, particularly with respect to what concerns the geometric parameters of H atoms. Nonetheless, they allow us to define beyond any doubt the connectivity represented in Figure S2. Figure contains an ORTEP representation of the molecules of 4·thf, along with selected metrics. A more complete set of bond distances is collected in Table , which contains both experimental and computational data. When this manuscript was being prepared, there was no precedent for a structural motif of this kind in the Cambridge Structural Database (CSD).[50] The two bridging H–Li(thf)–H and AdDipp2 ligands of complex 4·thf occupy mutually trans positions, originating a typical paddle-wheel structure[51−53] around a Mo–Mo quadruple bond of length 2.1006(7) Å. The discrepancy observed between the experimental and calculated Mo–H and Li–H distances collected in Table is most likely due to the incertitude in the localization by X-ray diffraction of hydride ligands bound to a heavy atom such as molybdenum. The computed distances are 1.85 and 1.78 Å, respectively. The first is almost coincident with the average Mo–H–Mo bond lengths determined by neutron diffraction,[54] while the second is somewhat longer than the 1.60 Å value measured for the molecule of LiH in the gas phase but significantly shorter than the interatomic separation of 2.04 Å found for this hydride in the solid state.[3] Regarding the Mo–Li distances, the experimental values of 2.91(2) and 2.97(2) Å are indistinguishable within experimental error, whereas in the optimized structure this slight asymmetry vanishes, leading to a separation of ca. 2.97 Å. For comparison, the sum of the covalent radii of the atoms is 2.82 Å.[55]

Figure 3

Solid-state structure of 4·thf. Some atoms have been omitted for clarity. Thermal ellipsoids are shown at 50%. Selected bond distances (Å) and a bond angle (deg): Mo1–Li1, 2.91(2) and 2.97(2); Mo1–Mo1, 2.1006(7); Mo1–N1, 2.10(1); Mo1–N2, 2.20(1); Li1–O1, 1.86(2) Å; and N1–Mo1–Li1, 92.1(5).

Table 1

Selected Experimental and Computational Bond Distances (Å) for Complexes 4·thf and 5·thfa

	4·thf		5·thf
	calcd	exp	calcd	exp
Mo–Mo	2.134	2.1006(7)	2.14–2.15	2.10 (av.)
Mo–H	1.852	2.05	1.83–1.84 (Mo–Hcent)	1.67–2.04
Mo–Li	2.971	2.97(2)	3.21–3.25 (Mo–Li9)	3.15–3.24
	2.968	2.91(2)
Li–H	1.787	1.74	1.97–2.07 (Li9–Hcent)	1.81–2.09
	1.784	1.85

For the latter complex, the Li7/8–Li9 distances are 2.44 and 2.46 (calcd), 2.45 and 2.50 Å (exp), while corresponding values for the Li7–Li9–Li8 angle are 176.5 and 176.3°.

We have carried out geometry optimization and an NBO analysis of chemical bonding within the Mo–H–Li–H–Mo rings of 4·thf. For simplicity, we describe here the comparable results obtained for monolithiated species 4·thf′, whose structure (Figure b) finds precedent in that of methyl complex analog [Mo2{μ-MeLi(thf)Me}(μ-Me)(μ-AdDipp2)2].[47,56] The energy for the dissociation of 4·thf′ to (thf)Li–H and dihydride [Mo2(H)2(μ-AdDipp2)2] given by our calculations is 27.9 kcal/mol, while the dissociation of two molecules of (thf)Li–H from 4·thf is 55.1 kcal/mol. The NBO analysis discloses four orbitals that are responsible for the σ component, two π components, and one δ component of the quadruple Mo–Mo bond (Figure a). In addition, we find that the dx2–y2 orbitals, not involved in Mo–Mo bonding, form spd hybrids directed toward the hydrides[47] and combine with s(H) orbitals to form the two Mo–H bonds (one of which is shown in Figure a).

Figure 4

(a) Four NBO orbitals corresponding to the σ component, two π components, and one δ component of the quadruple Mo≣Mo bond in 4·thf′ and one of the Mo–H σ bonding orbitals composed by the δ(Mo–Mo)-type x2–y2 and the hydride 1s orbitals. (b) Coordinate orientation and composition of the central fragment of the molecule shown in the orbital plots. (c) Some representative interactions between donor (white and red) and acceptor (light blue and pink) natural orbitals in 4·thf′.

The NBO approach results in limited participation of the lithium atomic orbitals in occupied MOs. However, this does not mean that its interactions with the hydrides and the molybdenum atoms are strictly ionic, since the calculated charge on Li is +0.67, indicative of a non-negligible covalent contribution. The reduced charge of the lithium “ion” is thus associated, in addition to thf → Li donation, with two sets of donor–acceptor interactions: (i) donation from σ(Mo–H) to Li and (ii) donation from the components of the Mo≣Mo bond to Li (Figure c).

From the energy point of view, there are two sets of dominant interactions (Figure a) that imply donations from the σ(Mo–H) and σ(Mo’–H) bonds to both s(Li) and p(Li) and from the σ component of the Mo≣Mo bond to the atomic orbitals of Li. In the first set, we find donation from σ(Mo–H) to both s(Li) and p(Li), which are responsible for 84% of the interaction energy. Among the second set of interactions, donation from σ(Mo–Mo) to s(Li) (Figure c) makes a significant contribution of 12%; smaller contributions come from the donations of δ(Mo–Mo) to p(Li) and of σ(Mo–Mo) to p(Li), while almost negligible contributions appear for π(Mo–Mo) and for p and s(Li).

Figure 5

(a) Relative energy contributions of natural orbital donor–acceptor interactions between [Mo2(H)2(μ-H)(μ-AdDipp2)2]− and (thf)Li+ fragments in 4·thf′. (b) Share of the Li valence electron density at each of its atomic orbitals, resulting from σ(Mo–H) → Li, Mo≣Mo → Li, and thf → Li donor–acceptor interactions.

As a result of all of these donor–acceptor interactions from the Mo2H2 moiety to the lithium ion, the distribution of the 0.33 valence electron held by the Li atomic orbitals (Figure b) reflects the major role played by the 2s and 2p Li AOs as acceptors. The high population of the lithium p orbital compared to its minor acceptor role toward the Mo≣Mo group is undoubtedly due to the donation from its thf ligand. Finally, the lowest atomic orbital population in p results from the interesting donation from the δ(Mo≣Mo) bonding orbital (Figure c).

We can therefore conclude that the stability of the Mo–H–Li–H–Mo ring results mainly from the formation of two 3c–2e Mo–H–Li bonds, supplemented by σ coordination of the Mo≣Mo bond to the Li atom. The latter bonding component is consistent with a short distance between Li and the Mo≣Mo centroid of 2.77 Å (Mo–Li = 2.97 Å), to be compared with a covalent radii sum of 2.82 Å.[55] Although of lesser quantitative importance, the existence of non-negligible electron donation from the bonding π and δ(Mo≣Mo) orbitals is worth being stressed. The fact that the calculated dissociation energy of 4·thf′ into (thf)Li–H and dihydride [Mo2(H)2(μ-AdDipp2)2] is 27.9 kcal/mol, smaller than the sum of NBO interaction energies shown in Figure a (98.7 kcal/mol), is explained by the high energy required to deform the (thf)Li–H group from linear in the free molecule to a highly bent (120°) geometry in 4·thf′ as well as to modify the second coordination sphere of the Mo atoms to make room for the Li–thf moiety.

Having successfully built Mo2LiH2 and Mo2Li2H4 platforms based on Mo≣Mo bonds coordinated to one and two H–Li(thf)–H units, respectively, our next goal was to explore the possibility of reaching a Mo2Li3H6 organization in a purported [Mo2{HLi(thf)H}3(μ-AdDipp2)] complex. To this end and taking into account the successful synthesis of complexes 3·thf and 4·thf by the procedure shown in Scheme c, we prepared tris(acetate) precursor [Mo2(μ-O2CMe)3(μ-AdDipp2)] and performed its reaction with an excess of LiAlH4. Although the above Mo2Li3H6 complex could not be observed, the transformation led to complex 5·thf, identified as a polymetallic hydride cluster Mo6Li9H18 (Scheme ), that probably results from spontaneous trimerization of the targeted Mo2Li3H6 monomer, with the loss of a molecule of tetrahydrofuran. The reaction was, however, complex and gave in addition compound [Mo2(μ-O2CMe)2(μ-AdDipp2)2] through an undisclosed reaction path. Like the bis(formate) analogue (Scheme c), the latter may react further with LiAlH4, justifying that isolated yields of 5·thf are about 25%. Complex 5·thf is very air-sensitive and decomposes instantly in the presence of oxygen and water, both in solution and in the solid state. Under strict anaerobic conditions, solutions in tetrahydrofuran or aromatic hydrocarbons remain unchanged at 25 °C for at least 24 h, although decomposition is fast above 50 °C.

Scheme 2

Formation of Hexamolybdenum Nonalithium Dodecaoctahydride Cluster 5·thf

The new supramolecular entity can be understood as a triangular array of [Mo2(μ-AdDipp2)]3+ components[51,57,58] connected by a [Li9H18]9– linker in a fairly robust manner. The Li-coordinated molecules of tetrahydrofuran were readily substituted by pyridine and 4-dimethylaminopyridine, giving complexes 5·py and 5·dmap without the alteration of the molecular skeleton. Notwithstanding the foregoing, complex 5·thf acted as an efficient source of LiH in the hydrolithiation of Ph2C(O) to give Ph2C(H)(OLi).[20,22] Somewhat unexpectedly, solutions of 5·thf decomposed gradually upon stirring at room temperature under an atmosphere of H2, generating LiAdDipp2 as a byproduct. Dideuterium acted similarly and showed that H/D exchange took place, as attested to by NMR detection of HD along with H2. The H2-promoted cluster breakup was not investigated any further. Nevertheless, it seems plausible that H2 may disrupt the cluster structure by displacing LiH molecules from the [Li9H18]9– linker, eliminating LiAdDipp2. As an extension of these studies, various attempts were made to produce an alleged {Mo2(H)8[Li(thf)]4} complex (i.e., the hydride analogue of known methyl compound {Mo2(CH3)8[Li(OEt2)]4}).[59] As detailed in the SI, all essayed trials were unsuccessful.

The room-temperature 1H NMR spectra of complexes 5·L in C6D6 or thf-d8 solution show two septets and four doublets for the 12 isopropyl groups of the amidinate spectator ligands, in accordance with the proposed D3 molecular symmetry (details in Supporting Information). The 18 H atoms that make up the [Li9H18] linker are expected to give rise to three resonances of equal relative intensity. Whereas for 5·thf one of these signals seems to be hidden underneath other resonances, the three are clearly observed for complex 5·py with chemical shifts of 2.04, 5.21, and 5.41 ppm. They appear as broad multiplets, but while the 2.04 peak becomes a singlet in the 1H{7Li} NMR spectrum, the other two are converted to doublets with 2JHH = 4 Hz. The 7Li NMR spectrum contains three resonances centered at 5.4, 4.7, and 2.7 ppm, with relative intensities approaching roughly 6:2:1, once more in agreement with the proposed structure.

The molecular structure of complex 5·thf was determined by X-ray crystallography (Figure ) and computational studies. Since the calculated and experimental structures are very similar (Table ), all of the features that are discussed here based on the X-ray data also apply to the optimized geometry. The whole cluster is built up by successive concentric groups around a central Li3 unit (Figure , left) formed by Li7, Li9, and Li8 with a nearly linear arrangement (176(1)°) and distances of 2.50(2) and 2.45(2) Å, which are slightly shorter than twice the lithium covalent radius (2.56 Å).[55] We have been unable to locate a solid-state or gas-phase structure in which such a Li3 rod is present. The only Li3 group whose structure we are aware of appears in the crystal structure of Li3[IrD6], with Li–Li distances of 2.58 and 2.76 Å and a Li–Li–Li angle of 75.7°.[60] The first concentric group around the central axis is composed of six Hcent atoms that provide a nearly octahedral coordination sphere to the central Li9 atom (Figure , right) and act as bridging atoms with the terminal atoms of the Li3 rod, with Li–H separations in the 1.70–2.20 Å interval. These hydrides are connected to the molybdenum atoms of the three Mo2 units that constitute the second concentric ring, with the shape of a slightly twisted trigonal prism and Mo–H distances in the range of 1.67–2.05 Å. The Mo–Mo bond length of 2.1020(7) Å is coherent with 4-fold bonding.[51]

Figure 6

Solid-state structure of 5·thf as determined by X-ray diffraction. Thermal ellipsoids are shown at 30%. Hydrogen atoms (except the hydride ligands) are omitted for clarity, as are thf molecules. Selected bond lengths (Å) and a bond angle (deg): Mo–Mo, 2.10 av.; Mo–Li9, 3.20 av.; Mo–N, 2.13 av.; Li9–Li7, 2.45(2); Li9–Li8, 2.50(2); Li7–Li9–Li8, 176.3(9).

Figure 7

Li9 polyhedron present in the molecules of complex 5·thf (left) and the distribution of H and Li atoms in the vicinity of central lithium atom Li9 (right).

Leaving aside the Li atoms, an ionic description of the cluster leaves us with three [Mo2(μ-AdDipp2)H6]3– blocks, in which each Mo atom bears two cis hydrides and one trans hydride relative to the N atoms of the μ-AdDipp2 ligand. The latter have just been described as forming an H6 octahedron around the inner Li3 rod and being bonded to the three Mo2 units as well. The 12 cis hydrides can be described as distorted trigonal prisms, one with the trigonal faces roughly at the height of the external atoms of the central Li3 rod, or H6ext group, and the other with the trigonal faces very close to the central Li9 atom, or H6int. Finally, the six peripheral Li atoms form another trigonal prism (Figure , left) with one of the triangular faces (Li1, Li3, and Li5) rotated ca. 13° relative to the other (Li2, Li4, and Li6). Those Li atoms form three Li2 dumbbells with Li···Li distances of 2.83–2.90 Å and are supported by hydride bridges to neighboring Li and Mo atoms, with Li–H separations in the range of 1.74–2.29 Å (section 5 in the Supporting Information).

Conclusions

We have demonstrated that a monomeric molecule of LiH can bind to the unsaturated molybdenum atom of [(H)Mo≣Mo] entities by means of a 3c–2e Mo–H⇀Li interaction combined with a σ-Li–H⇀Mo bond. [Mo2{μ-HLi(thf)H}] skeletons containing five-membered H–Mo≣Mo–H–Li rings have been constructed in this manner for n = 1 and 2. When n = 3, trimerization of the purported [Mo2{μ-HLi(thf)H}3(μ-AdDipp2)] monomer occurs spontaneously, leading to a hydride-rich Mo6Li9H18 supramolecular organization that features an uncommon linear Li3 group around which are organized Mo6, Li6, and two H6 polyhedra with shapes intermediate between an octahedron and compressed trigonal prisms.