Synthesis and hydrogen storage properties of novel manganese hydrides

Abstract

This disclosure relates to novel manganese hydrides, processes for their preparation, and their use in hydrogen storage applications. The disclosure also relates to processes for preparing manganese dialkyl compounds having high purity, and their use in the preparation of manganese hydrides having enhance hydrogen storage capacity.

Claims

1. A process for preparing a manganese alkyl compound of the formula (III):
MnR.sub.xY.sub.yL.sub.n(III) wherein R is an organic group; L is a Lewis base; n is 0 to about 1; Y, if present, is an alkali metal, an alkaline earth metal, or any combination thereof; x is about 1.8 to about 2.2; and y is 0 to about 0.2; the process comprising reacting a compound of the formula MnX.sub.2(dioxane).sub.z with an alkyl alkali metal compound, an alkyl alkaline earth metal compound, or any combination thereof; wherein z is about 1 to about 2; and X is halide.

2. The process according to claim 1, wherein the process comprises reacting a compound of the formula MnX.sub.2(dioxane).sub.z with a compound of the formula RMgX.sup.1, R.sub.2Mg, or a combination thereof; wherein X and X.sup.1 are, independently, halide.

3. The process according to claim 2, wherein X and X.sup.1 are both Cl.

4. The process according to claim 1, wherein L is selected from the group consisting of an organic solvent, water, H.sub.2S, an amine, a phosphine, a sulfide, and combinations thereof.

5. The process according to claim 1, wherein L is selected from the group consisting of diethylether, THF, dioxane, and combinations thereof.

6. The process according to claim 1, wherein n is selected from the group consisting of (i) 0 to about 0.8, (ii) 0 to about 0.6, (iii) 0 to about 0.5, (iv) 0 to about 0.4, (v) 0 to about 0.2, (vi) 0 to about 0.1, (vii) 0 to about 0.05 and (viii) 0 to about 0.01.

7. The process according to claim 1, wherein y is 0.

8. The process according to claim 1, wherein the reaction is performed in diethylether.

9. The process according to claim 1, wherein the process comprises adding (4-z) molar equivalents of dioxane.

10. The process according to claim 1, wherein the MnX.sub.2(dioxane).sub.z is prepared by reacting MnX.sub.2 with 4 molar equivalents of dioxane.

11. The process according to claim 10, wherein the MnX.sub.2(dioxane).sub.z is not isolated before reaction with a compound of the formula RMgX.sup.1, R.sub.2Mg, or a combination thereof.

12. The process according to claim 1, wherein R does not contain a hydrogen substituent.

13. The process according to claim 1, wherein R is mesityl, neopentyl, trimethylsilylmethyl or benzyl.

14. The process according to claim 1, wherein the compound of formula (III) is free or substantially free of organic residue and/or metal ions other than manganese.

15. The process according to claim 1, further comprising hydrogenating the compound of formula (III).

16. The process according to claim 15, wherein the compound of formula (III) is subjected to about 10 or more hydrogen adsorption-desorption cycles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts an embodiment of a storage system useful in the present invention.

(2) FIG. 2 depicts an embodiment of the storage system attached to a hydrogen fuel cell.

(3) FIG. 3 depicts an IR spectrum of manganese hydride sample Mnd-100.

(4) FIG. 4 depicts an X-ray powder diffraction (XRPD) pattern of manganese hydride sample Mnd-150-H.sub.2-4 hrs.

(5) FIG. 5 depicts the valence region of an X-ray photoelectron spectroscopy (XPS) spectrum for manganese hydride sample Mnd-150-H.sub.2-4 hrs.

(6) FIG. 6 depicts the differential thermal analysis (DTA) (bottom trace) and thermogravimetric analysis (TGA) (top trace) spectra for manganese hydride sample Mnd-150-H.sub.2-4 hrs.

(7) FIG. 7 depicts a hydrogen adsorption-desorption isotherm at 298 K (150 bar) for manganese hydride sample Mnd-100.

(8) FIG. 8 depicts the life cycle hydrogen adsorption (wt. %) for 53 absorption-desorption cycles at 121 bar H.sub.2 for manganese hydride sample Mnd-150-H.sub.2-4 hrs.

(9) FIG. 9 depicts the life cycle hydrogen adsorption (wt. %) for 53 absorption-desorption cycles at the pressure maximum of each absorption-desorption cycle for manganese hydride sample Mnd-150-H.sub.2-4 hrs.

(10) FIG. 10 depicts a hydrogen adsorption-desorption isotherm at 298 K for manganese hydride sample Mnd-150-H.sub.2-4 hrs.

(11) FIG. 11 depicts the IR spectrum of manganese hydride sample Mnd-150-H.sub.2-4 hrs following 53 absorption-desorption cycles at 120-150 bar H.sub.2.

(12) FIG. 12 depicts a nitrogen adsorption-desorption isotherm for manganese hydride sample Mnd-150-H.sub.2-4 hrs following 53 absorption-desorption cycles at 120-150 bar H.sub.2.

(13) FIG. 13 depicts a ET plot for manganese hydride sample Mnd-150-H.sub.2-4 hrs following 53 absorption-desorption cycles at 120-150 bar H.sub.2.

DETAILED DESCRIPTION OF THE INVENTION

(14) Definitions

(15) Unless defined otherwise, all technical and scientific terms used herein generally have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

(16) The term comprising is open ended and, in connection with a composition, refers to the elements recited. The term comprising as used in connection with the compositions described herein can alternatively cover compositions consisting essentially of or consisting of the recited components.

(17) The term coordinate as used here is not limited to a specific type of interaction between a metal center and hydrogen. For example, in one embodiment, the interaction between a metal center and hydrogen is a Kubas interaction.

(18) The term Kubas interaction refers to hydrogen bound in a non-dissociative manner as a dihydrogen molecule to a transition metal center. In a Kubas interaction, free d-electrons of a metal centre interact with hydrogen. Specifically, where the metal centre has a low coordination number, the dihydrogen shares both of its -bonding electrons with the metal centre, and the metal centre back donates electrons by overlap of its symmetry d-orbital with the empty antibonding * empty orbital of the dihydrogen. This results in a lengthening of the HH bond (without rupture) and a shift to a lower wavenumber for the HH resonance (see, e.g. J. Am. Chem. Soc., 119, 9179-9190, 1997).

(19) Without wishing to be bound by theory, the inventor theorizes that one or more H.sub.2 molecules interact with the manganese centers by Kubas interactions to form maganese hydrides of the formula MnH.sub.x in which x is about 4, about 6, about 8 or about 10. However, bimolecular and/or free radical processes may also occur leading to maganese hydrides of the formula MnH.sub.x in which x is about 3, about 5 about 7 or about 9.

(20) The term substantially free as used here in means containing less than about 2 wt %, such as less than about 1 wt %, less than about 0.5 wt %, less than about 0.1 wt %, less than about 0.05 wt %, less than about 0.01 wt %, less than about 0.005 wt % or less than about 0.001 wt % of a specified element or compound.

(21) The term organic group refers to any carbon containing group that may be present in a metal alkyl of Formula (III), or in a metal hydride of Formulas (I) and (II) following hydrogenation of the metal alkyl of Formula (III). For example, the organic group may be a solvent used in the formation of the metal alkyl or metal hydride that has not been fully removed during the synthesis process (e.g., diethyl ether). Another example of an organic ligand may be a ligand (e.g., trimethylsilylmethyl, mesityl, neopentyl or benzyl) that is not fully removed from the metal center during formation of the metal hydride. The organic ligand may also be a compound (e.g., a protic compound, such as methanol) that is added to the metal hydride in order to increase microporosity of the metal hydride structure (e.g., by forming bridging methoxide ligands within the structure), thereby facilitating H.sub.2 moving in and out of the metal hydride.

(22) As used herein, the term alkyl refers to a straight or branched chain saturated hydrocarbon moiety. In one embodiment, the alkyl group is a straight chain saturated hydrocarbon. Unless otherwise specified, the alkyl or alkylene group contains from 1 to 24 carbon atoms. Representative saturated straight chain alkyl groups include, e.g., methyl, ethyl, n-propyl, n-butyl, n-pentyl, and n-hexyl. Representative saturated branched alkyl groups include, e.g., isopropyl, sec-butyl, isobutyl, tert-butyl, neopentyl, and isopentyl. In a preferred embodiment, an alkyl group does not contain a -hydrogen substituent.

(23) As used herein, the term aryl refers to an aromatic hydrocarbon (mono- or multi-cyclic) having from 6 to 24 carbon atoms (e.g., phenyl, naphthyl), bound to the metal center via a metal-carbon bond.

(24) As used herein, the term arylalkyl refers to an alkyl-aryl group, wherein alkyl and aryl are as defined herein (e.g., benzyl).

(25) As used herein, the term heteroararyl refers to an aromatic group (mono- or multi-cyclic) having from 5 to 24 carbon atoms, additionally containing one or more N, S or O atoms.

(26) One of ordinary skill in the art will readily understand that a metal hydride having a non-integral stoichiometry, such as MnH.sub.4.2, refers to a material having manganese atoms coordinated with varying amounts of hydrogen (e.g., an average of 9 parts MnH.sub.4 to 1 part MnH.sub.6). Additionally, any metal hydride defined herein having an integral stoichiometry of metal to hydride ligand (e.g., MnH.sub.x) is intended to also cover a metal hydride sample having an overall mixed stoichiometry of MnH(.sub.x0.2 to x+0.2) (e.g., MnH.sub.3.8-4.2 or MnH.sub.5.8-6.2, for MnH.sub.4, MnH.sub.6, respectively).

(27) Metal Hydrides

(28) In one embodiment, any of the metal hydrides described herein has a BET surface area of less than about 5 m.sup.2/g, such as less than about 4 m.sup.2/g, such as less than about 3 m.sup.2/g, less than about 2 m.sup.2/g or less than about 1.5 m.sup.2/g, such as about 1.4 m.sup.2/g.

(29) In another embodiment, the metal hydride described herein has a BET surface area of about 2 m.sup.2/g or greater, such as about 5 m.sup.2/g or greater, about 7.5 m.sup.2/g or greater, about 10 m.sup.2/g or greater, about 25 m.sup.2/g or greater, about 50 m.sup.2/g or greater, about 75 m.sup.2/g or greater, about 100 m.sup.2/g or greater, about 150 m.sup.2/g or greater, about 200 m.sup.2/g or greater, about 250 m.sup.2/g or greater, about 275 m.sup.2/g or greater, about 300 m.sup.2/g or greater, about 350 m.sup.2/g or greater, about 400 m.sup.2/g or greater, about 450 m.sup.2/g or greater or about 500 m.sup.2/g or greater.

(30) In other embodiments, the BET surface area is from about 2 m.sup.2/g to about 1000 m.sup.2/g, such as from about 10 m.sup.2/g to about 750 m.sup.2/g, from about 50 m.sup.2/g to about 500 m.sup.2/g, from about 100 m.sup.2/g to about 500 m.sup.2/g, from about 250 m.sup.2/g to about 500 m.sup.2/g, from about 300 m.sup.2/g to about 500 m.sup.2/g, or from about 300 m.sup.2/g to about 400 m.sup.2/g.

(31) In one embodiment, the metal hydride is of Formula (I) and x is about 1.5 to about 10. In another embodiment, x is about 2 to about 10. In another embodiment, x is about 4 to about 10. In another embodiment, x is about 6 to about 10. In another embodiment, x is about 8 to about 10. In another embodiment, x is about 1.8 to about 2.2, e.g., about 2. In a further embodiment, x is about 3.8 to about 4.2, e.g., about 4. In a further embodiment, x is about 5.8 to about 6.2, e.g., about 6. In a further embodiment, x is about 7.8 to about 8.2, e.g., about 8. In a further embodiment, x is about 9.8 to about 10.2, e.g., about 10. In a further embodiment, x is about 2.8 to about 3.2, e.g., about 3. In a further embodiment, x is about 4.8 to about 5.2, e.g., about 5. In a further embodiment, x is about 6.8 to about 7.2, e.g., about 7. In a further embodiment, x is about 8.8 to about 9.2, e.g., about 9.

(32) In one embodiment, the metal hydride is of Formula (II) and x is about 1.8 to about 2.2. In another embodiment, x is about 1.9 to about 2.1. In another embodiment, x is about 2. In another embodiment, x is about 0.1 to about 2.2.

(33) In one embodiment, the metal hydrides described herein are amorphous or substantially amorphous (e.g., with little (e.g., nanoscopic order) or no long range order in the position of the atoms in the hydride structure). In one embodiment, the metal hydrides described herein are in the form of a gel.

(34) In one embodiment, the metal hydrides described herein are mesoporous (e.g., have a pore diameter between about 2 and about 50 nm). In another embodiment, the metal hydrides described herein are microporous (e.g., have a pore diameter less than about 2 nm, such as less than about 1 nm).

(35) In further embodiments, the metal hydrides described herein exhibit a gravimetric hydrogen absorption at least about 2%, at least about 3%, at least about 4%, at least about 5%, at least about 6%, at least about 7%, at least about 8%, at least about 9%, at least about 10%, at least about 11%, at least about 12%, at least about 13% or at least about 14%, e.g., in an amount up to about 14%, such as from about 2.0% to about 14.0%, from about 8.0% to about 12.0%, or about 3.5%, about 7.0%, about 10.5%, about 14%) based upon 100% total weight of the metal hydride without molecular hydrogen stored in it.

(36) In one embodiment of the metal hydrides described herein, when x is greater than about 2 (e.g., greater than about 3.8), the material is at a pressure of about 10 bar or more of hydrogen (e.g., at about 15 bar, at about 20 bar, at about 25 bar, at about 30 bar, at about 40 bar, at about 50 bar, at about 75 bar, at about85, about 100, about 125 or about 150 bar of hydrogen, or higher).

(37) Synthesis of Dialkyl Manganese Compounds and Manganese Hydrides

(38) The inventor has surprisingly found that transition metal dialkyl complexes (e.g., manganese dialkyl complexes, such as bis(neopentyl)manganese) may be prepared in high yield and high purity via a one step reaction of a metal halide dioxane complex (e.g., MnCl.sub.2(dioxane).sub.1.3-1.5) with an alkyl Grignard (e.g., (neopentyl)magnesium chloride), a dialkyl magnesium reagent (e.g., bis(neopentyl)magnesium), or, a combination thereof, in an ether solvent (e.g., diethyl ether). In one embodiment, the reaction is conducted in the presence of excess dioxane. For example, MnCl.sub.2(dioxane).sub.1.3-1.5 may be prepared by reaction of MgCl.sub.2 with an excess (e.g., about 4 equivalents) of dioxane. The excess dioxane may be present during reaction of the MnCl.sub.2(dioxane).sub.1.3-1.5 with the Grignard and/or dialkyl magnesium reagent.

(39) Hydrogenation of the resulting metal dialkyl complexes (such as bis(neopentyl)manganese) affords transition metal hydride frameworks, such as compounds of Formula (I) and (II) that interact with hydrogen to form solid state hydrides (such as the hydrides MnH.sub.x, wherein x is about 2.8 to about 10 (or about 3.8 to about 10), e.g., x is about 2.8 to about 3.2, about 3.8 to about 4.2, about 4.8 to about 5.2, about 5.8 to about 6.2, about 6.8 to about 7.2, about 7.8 to about 8.2, about 8.2 to about 9.2 or about 9.8 to about 10.2, e.g., MnH.sub.3, MnH.sub.4, MnH.sub.5, MnH.sub.6, MnH.sub.7, MnH.sub.8, MnH.sub.9 or MnH.sub.10 (for further example, x is about 3.8 to about 4.2, about 5.8 to about 6.2, about 7.8 to about 8.2, or about 9.8 to about 10.2, such as, e.g., MnH.sub.4, MnH.sub.6, MnH.sub.8, MnH.sub.10) and can reversibly release hydrogen, thereby acting as materials for hydrogen storage.

(40) Typically, organomanganese (II) compounds may be prepared by trans-metalation of a manganese dihalide with an organolithium or organomagnesium reagent (see, e.g., Cahiezet et al., J. Chem. Rev., 109, 1434-1476, 2009). To prepare bis(alkyl)manganese (II) compounds on an industrial scale, MnCl.sub.2 is typically used as it is cheaper than both MnBr.sub.2 and MnI.sub.2. However, MnCl.sub.2 is the least soluble of the three dihalides in organic solvents and reacts slowly with Grignard reagents.

(41) The synthesis of bis(mesityl)manganese has previously been reported in 50% yield by reaction of MnCl.sub.2 with mesityl magnesiumbromide in tetrahydrofuran (THF) (see, e.g., Gambarotta et al., J. Chem. Soc., Chem. Commun., 1128, 1983; Fischer et al., J. Organomet. Chem., 694, 1107-1111, 2009). The THF solvent helps to solubilise the MnCl.sub.2, which aids the reaction with mesityl magnesiumbromide. The present inventor, however, has found that THF coordinates to the dialkyl manganese product, thereby making it difficult to isolate the base free dialkyl product. Coordinated THF carries through to the final manganese hydride product prepared from bis(mesityl)manganese prepared according to Gambarotta, adversely affecting the hydrogen adsorption properties (i.e., gravimetric H.sub.2 wt % performance).

(42) Chloro-Grignard reagents are preferred Grignard reagents for industrial scale synthesis, due their lower cost compared to corresponding bromo-Grignard reagent. In both cases, however, removal of MgX.sub.2 (X=Cl, Br) from the final manganese dialkyl product is difficult because of the solubility of these magnesium halide salts in ether and the formation of magnesium etherates. However, if the reaction mixture containing the dialkyl manganese product, the magnesium halide (e.g., MgCl.sub.2) and diethyl ether is heated in vacuo in order to remove the ether and form free magnesium halide, this leads to complexation of the Lewis acidic magnesium halide to the dialkyl manganese product, rendering the organometallic insoluble in hydrocarbon solvents. Because of these problems, dioxane may be used to remove magnesium halide from the final product by forming an insoluble bis(dioxane) adduct of the magnesium halide. For this reason, addition of dioxane after completion of the dialkylation reaction is desirable. If more than 2 equivalents of dioxane are used, however, the dioxane, like THF, can coordinate to the final manganese dialkyl product, and is difficult to remove.

(43) Organomagnesium reagents of the type (alkyl).sub.2Mg react faster than the corresponding Grignard reagent. For example, the synthesis of bis[(trimethylsily)methyl]manganese(II) was reported in ca. 80% yield by reaction of MnCl.sub.2 with bis(trimethylsilylmethyl)magnesium in diethylether after three days stirring at 298 K (see, e.g., Alberola et al., Organometallics, 28, 2112-2118, 2009). Preparation of (alkyl).sub.2Mg or Mg(aryl).sub.2 reagents, however, requires manipulation of the Schlenk equilibrium by adding dioxane to the alkyl or aryl magnesium halide (e.g., RMgX wherein R=alkyl or aryl and X=Cl, Br, I) in a prior step, which causes precipitation of magnesium halide, leaving the (alkyl).sub.2Mg reagent in solution. This adds an undesirable and costly step to any synthetic procedure carried out on an industrial scale, and does not remove the magnesium halide by-product from the final reaction mixture, making it necessary to add dioxane a second time.

(44) Bis(neopentyl)manganese and bis[(trimethylsilyl)methyl]manganese(II) have also been synthesized by Wilkinson by reacting a 1:1 mixture of the Grignard and (alkyl).sub.2Mg reagent in diethyl ether with MnCl.sub.2 in 30-50% yield (see, e.g., Andersen et al., J. Chem. Soc. Dalton Trans., 2204-2211, 1976; Andersen et al., J. Chem. Soc., Chem. Commun., 1807-1808, 1985).

(45) In order to circumvent the problems observed using Grignard (RMgX) reagents, more reactive alkyl lithium compounds (RLi) have also been used. This is unattractive, not only in terms of the higher cost of RLi over RMgX, but also because of the slow reaction of RLi with MnX.sub.2 (X=Cl, Br) due to (i) the insolubility of these halides in ether, and (ii) the preferred formation of the ate complexes R.sub.3MnLi and R.sub.4MnLi.sub.2, which when hydrogenated downstream, lead to poorly performing hydrogen storage materials due to contamination by lithium and lithium etherates.

(46) To overcome the low solubility of MnBr.sub.2 in ether, Cahiez has reported that MnBr.sub.2 may be solubilized with LiBr to form the complex, MnBr.sub.2:LiBr (see, e.g., Cahiez et al., Tetrahedron Letters 30, 3545-3546, 1989). When following this procedure, however, the present inventor found that use of organolithium reagents led, in each case, to formation of the species R.sub.2MnBrLi. It was not possible to remove the coordinated Li halide from the manganese alkyl, thereby leading to poorly performing hydrogen storage materials. MnI.sub.2 reacts more rapidly with organolithium reagents because of its greater solubility in ether, but there are problems with LiI contaminating the final product due to the high solubility of LiI in ether and the solubility of LiI ether complexes in petroleum (see, e.g., Cahiez et al., J. Chem. Rev., 109, 1434-1476, 2009; Cahiez et al., Synthesis, 130-133, 1977; Cahiez et al., Synthesis, 37-40, 1984; Bartlett et al., Organometallics, 7, 1801-1806, 1988). Again, LiI is thus carried forward into the hydrogenated metal hydride product.

(47) In order to take advantage of (i) the affinity of dioxane for Magnesium halides and (ii) the use of dioxane to accelerate reactions of Grignard reagents by forming more reactive (alkyl).sub.2Mg or Mg(aryl).sub.2 species, while (iii) overcoming the insolubility of MnCl.sub.2 in ether, the present inventor has employed a manganese dioxane complex (e.g., MnCl.sub.2(dioxane).sub.1.3-1.5) for the first time as a starting material in the synthesis of dialkyl manganese complexes.

(48) In the present inventor's adaptation of Wilkinson's synthesis (see, e.g., Andersen et al., J. Chem. Soc. Dalton Trans., 2204-2211, 1976; Andersen et al., J. Chem. Soc., Chem. Commun., 1807-1808, 1985), the soluble manganese dioxane complex reacts smoothly in ether to form the manganese bisalkyl product quantitatively and in high purity, with the insoluble magnesium chloride dioxane complex as the only other product. This is easily removed by filtration. In one step, dioxane therefore acts to (i) help solublize MnCl.sub.2 in diethyl ether, (ii) drive the reaction to completion by precipitating out the magnesium salts, (iii) form more reactive R.sub.2Mg, and (iv) reduce the amount of unwanted magnesium salts in the final product.

(49) Without wishing to be bound by theory, the present inventor believes that the dioxane preferentially forms an insoluble complex with MgCl.sub.2 over MnCl.sub.2, thereby helping to drive the reaction to completion and remove the Magnesium salts from solution (see, e.g., Fowles et al., J. Chem. Soc. A, 1842-1846, 1968). For example, MnCl.sub.2 may be reacted with 4 equivalents of dioxane followed by 2 equivalents of neopentyl magnesium chloride. The reaction mixture may be filtered and washed (e.g., with diethyl ether) and the filtrate removed in vacuo down to afford a quantitative yield of magnesium free (or substantially free) bis(neopentyl)Mn. This provides a simple one-step route to synthesizing dialkyl manganese complexes in high yield that are free (or substantially free) of impurities without the need for multiple extractions and purification procedures. The present inventor has found that that hydrogenation of Mn(alkyl).sub.2 or Mn(aryl).sub.2 complexes prepared according to the present invention affords metal hydrides that exhibit significantly better hydrogen storage capability when compared to metal hydrides prepared from Mn(alkyl).sub.2 or Mn(aryl).sub.2 complexes contaminated with lithium halides, solvents such as THF, magnesium halides (and solvates thereof such as magnesium halide etherates), MnR.sub.3.sup. and MnR.sub.4.sup.2.

(50) Thus, in one embodiment, the present invention relates to a process for preparing a manganese alkyl compound of the formula (III)
MnR.sub.xY.sub.yL.sub.n(III)
wherein

(51) R is an organic group (e.g., an organic alkyl group without a -hydrogen substituent, such as mesityl, neopentyl, trimethylsilylmethyl or benzyl);

(52) L is a Lewis base (e.g., an organic solvent (such as an ether solvent, e.g. Et.sub.2O, dioxane, THF), water, H.sub.2S, an amine, a phosphine, a sulfide, and combinations thereof);

(53) n is 0 to about 1 (e.g., about 0 to about 0.8, 0 to about 0.6, 0 to about 0.5, 0 to about 0.4, 0 to about 0.2, about 0 to about 0.1, about 0 to about 0.05 or about 0 to about 0.01);

(54) Y is a metal (other than manganese) (e.g., Y, if present, is an alkali metal (e.g., Na, K or Li), an alkaline earth metal (e.g., Mg, Ca or Be), or any combination thereof);

(55) x is about 1.8 to about 2.2 (e.g., about 1.9 to about 2.1, about 1.95 to about 2.05, or about 2); and

(56) y is 0 to about 0.2 (e.g., 0 to about 0.1, 0 to about 0.05 or 0 to about 0.01).

(57) The process comprises reacting a compound of the formula MnX.sub.2(dioxane).sub.z (wherein z is about 1 to about 2, such as about 1.3 to about 1.5) with an alkyl magnesium reagent of the formula RMgX.sup.1, R.sub.2Mg, or a combination thereof (wherein X and X.sup.1 are each, independently, halide, e.g., Cl, Br or I, preferably Cl) in an organic solvent (e.g., an organic solvent comprising ether, such as Et.sub.2O).

(58) In one embodiment, the MnX.sub.2 dioxane complex is formed by reacting MnX.sub.2 with an excess (such as about 4 equivalents) of dioxane to afford MnX.sub.2(dioxane).sub.z.

(59) Preferred organic groups are neopentyl and mesityl. These groups are symmetrical and can easily be chlorinated to afford one product (e.g., neopentyl chloride). The neopentyl group contains no beta hydrogen, which is preferred in order to form a stable dialkyl manganese complex, and is relatively small and thus able to diffuse out of a solid structure more readily than larger alkyl groups. Trimethylsilylmethyl can also be used as the organic ligand.

(60) In another embodiment, the present invention relates to a process for preparing a manganese hydride according to any of the embodiments described herein (e.g., a manganese hydride suitable for use in hydrogen storage). The process comprises (i) hydrogenating an dialkyl manganese compound (e.g., a compound of formula (III)); (ii) applying a vacuum to the product of step (i); and optionally, (iii) hydrogenating the product obtained in step (ii); and (iv) applying a vacuum to the product of step (iii).

(61) In another embodiment, the process further comprises (v), subjecting the product of step (iv) to one or more (such as about 5 or more, about 10 or more, about 20 or more, about 30 or more, about 40 or more or about 50 or more) additional hydrogen adsorption-desorption cycles, for example, at between about 120 and 150 bar H.sub.2.

(62) In another embodiment, the present invention relates to a method of storing hydrogen comprising providing a metal hydride according to any of the embodiments described herein (e.g., a metal hydride of formula (I) or (II)), adding hydrogen to the metal hydride, and allowing the hydrogen to coordinate to the metal hydride. The storing of hydrogen may be carried out in a storage system.

(63) One embodiment of a storage system suitable for hydrogen storage is a pressure vessel. For example, the pressure vessel may hold the metal hydride of the present invention at a temperature of up to 200 C., e.g., from about 100 to about 150 C., from about 50 to about 0 C., from about 25 to about 0 C., from about 0 to about 150 C., from about 0 to about 50 C., from about 10 to about 30 C. or from about 20 to about 25 C. In one embodiment, the storage system is substantially free of oxygen.

(64) Hydrogen may be added to the storage system (e.g., a pressure vessel) and stored using the metal hydrides of the present invention. In one embodiment, no heating is required when adding hydrogen to the pressure vessel for storage.

(65) The amount of hydrogen that can be stored by the metal hydrides of the present invention is proportional to the pressure in the storage system. For example, at higher pressures, more hydrogen can be stored by the metal hydrides of the present invention. The pressure in the storage system may be increased by adding hydrogen to the storage system. Without wishing to be bound by any particular theory, the inventor theorizes that as the pressure is increased, the number of Kubas interactions per metal centre may increase. For example, when the metal hydride is a manganese hydride such as MnH.sub.2, one hydrogen molecule coordinated to the manganese (e.g., by a Kubas interaction) affords MnH.sub.4. Two hydrogen molecules coordinated to the manganese (e.g., by Kubas interactions) affords MnH.sub.6. Three hydrogen molecules coordinated to the manganese (e.g., by Kubas interactions) affords MnH.sub.8. Four hydrogen molecules coordinated to the manganese (e.g., by Kubas interactions) affords MnH.sub.10. As noted above, however, this process will appear continuous in the bulk state, resulting in the formation of a bulk material containing metal hydrides having a mixture of coordinated hydrogen molecules, and, therefore, an overall non-integer stoichiometry of manganese to hydrogen. Furthermore it may be possible (e.g., via a free radical and/or bimolecular process) to form molecular species of the formula MnH.sub.3, MnH.sub.5, MnH.sub.7 and MnH.sub.9.

(66) In further embodiments, any of the metal hydrides described herein optionally contain one or more additional metals (e.g., a metal other than manganese). For example, the metal hydride may contain one or more additional metals selected from sodium, potassium, aluminum, beryllium, boron, calcium, lithium, magnesium and combinations thereof. In an alternate embodiment, the metal hydride may contain one or more additional metals (e.g., a metal other than manganese) wherein the one or more additional metals is a period 4, 5, 6, 7, 8, 9, 10, 11 and/or 12 transition metal, or a lanthanide, that forms a hydride upon treatment with hydrogen. For example, the metal hydride may contain one or more additional metals selected from zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, and combinations thereof. In one embodiment, any of the metal hydrides described herein may optionally contain one or more additional period 4, period 5 or period 6 transition metals. In another embodiment, the metal hydride may contain one or more additional metals selected from iron, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, mercury, and combinations thereof. The one or more additional metals may be present in an amount of about 50 wt. % or less, about 40 wt. % or less, about 30 wt. % or less, about 25 wt. % or less, about 20 wt % or less, about 10 wt % or less, about 5 wt % or less, about 1 wt % or less, about 0.75 wt % or less, about 0.5 wt % or less, about 0.25 wt % or less, about 0.1 wt % or less, about 0.05 wt % or less or about 0.01 wt % or less. In one embodiment, the metal hydrides described herein contain no additional metal (e.g., no metal other than manganese).

(67) The hydrogen pressure in the system may be increased using a compressor, such as a gas compressor, which pumps hydrogen into the system. Preferably, the hydrogen pressure in the system is increased to about 30 atm or more. For example, the hydrogen pressure in the system may be increased to from about 30 atm to about 500 atm, from about 50 atm to about 200 atm, or from about 75 atm to about 100 atm.

(68) The system preferably has a temperature of (or operates at) up to 200 C., such as about 200 C. to 150 C. (e.g., about 100 C. to 150 C.), about 200 C. to 100 C., about 0 C. to 50 C., about 10 C. to 30 C., or about 20 C. to 25 C. In one embodiment, the system has a temperature (or operates at) about 25 C. to about 50 C. The system is preferably free of oxygen to prevent the oxidation of metal in the system. In one embodiment, the method of storing and releasing hydrogen in a system of the present invention may be carried out without adding heat to and/or cooling the system. In another embodiment, the method of storing and releasing hydrogen in a system of the present invention may be carried out by adding heat to and/or cooling the system.

(69) In a further embodiment, the hydrogen is released from the storage system. For example, this may be accomplished by reducing the pressure of hydrogen in the system. In one embodiment, no heating is required in order to release the hydrogen from the metal hydride. For example, a valve in the storage system may be opened to allow hydrogen gas to escape from the system, thus decreasing the pressure in the storage system. In one embodiment, about 100% of the stored hydrogen is released. In additional embodiments, greater than about 50%, greater than about 55%, greater than about 60%, greater than about 70%, greater than about 75%, greater than about 80%, greater than about 90%, greater than about 95%, greater than about 97.5%, greater than about 99% or greater than about 99.5% of the hydrogen is released. The step of releasing the hydrogen pressure in the system may be carried out by allowing hydrogen gas to escape from the system, thus decreasing the hydrogen pressure. For instance, the step of releasing the hydrogen pressure may decrease the hydrogen pressure in the system to 100 atm or less (such as to 50 atm or less, 30 atm or less, or 20 atm or less). In another embodiment, the hydrogen is released from the storage system by increasing the temperature of the system.

(70) Hydrogen may be added or released from the system at any point throughout the entire pressure gradient of the system without any adverse effects to the storage capacity of the system. In certain embodiments, hydrogen may be added or released from the system any number of times without any adverse effect to the storage capacity of the system. For example, the system can be filled with hydrogen and emptied of hydrogen at least 100, such as at least 200, at least 500, at least 1000 or at least 1500 times without a significant decrease in the storage capacity of the system.

(71) In one embodiment, the storage system (e.g. pressure vessel) is a fuel tank in a vehicle, such as a truck or automobile.

(72) FIG. 1 depicts an embodiment of a storage system useful in the present invention. FIG. 2 depicts an embodiment of the storage system attached to a hydrogen fuel cell. The system 10 comprises a tank body 12 which is made of a material that is impermeable to hydrogen gas, thus preventing undesired leaking of the hydrogen gas out of the tank body 12. For example, the tank body 12 is made of metal, such as, e.g., steel or aluminum. Alternatively, the tank body 12 is made of a composite material, such as a composite of fiberglass and aramid. In another embodiment, the tank body 12 is made of a carbon fiber with a liner. The liner may be a polymer liner, such as a thermoplastic liner or a metal liner, such as a steel liner or an aluminum liner.

(73) The metal hydride 14 of the present invention is present inside the tank body 12. In FIG. 1, the metal hydride 14 is in a gel form. The metal hydride 14 may partially fill or totally fill the tank body 12. In certain embodiments, the metal hydride may be present as a coating on a support or in pellet form, depending upon the requirements for pressure drops in the tank body. In additional embodiments, the metal hydride may be present in admixture with other compounds (such as a binder) which enhance the structural integrity and other properties of the coating or the pellet.

(74) A first passage 16 leads to a first opening 18 in the wall of the tank body 12. A first valve controls the flow of hydrogen gas through the first opening 18.

(75) A second passage 22 extends from a second opening 24 in the wall of the tank body 12. A second valve 26 controls the flow of hydrogen gas through the second opening 24.

(76) The first valve 20 and the second valve 26 can be any type of valve that controls the flow of hydrogen gas through the first opening 18 and the second opening 24, respectively. For example, the first valve 20 and the second valve 26 can be ball valves or gate valves.

(77) In one embodiment, hydrogen is added to the system 10 as follows. A gas compressor 32 pumps hydrogen gas into the first passage 16. The first valve 20 is opened to allow the hydrogen gas to flow through the first opening 18 and into the tank body 12.

(78) A passage tube 28 is in gaseous communication with the first opening 18 and extends into the interior of the tank body 12. The passage tube 28 facilitates the distribution of the hydrogen gas to the metal hydride 14. In one embodiment, the passage tube 28 is made of a material that is permeable to the hydrogen gas. This allows the hydrogen gas to pass through the wall of the passage tube 28 and into contact with the metal hydride 14. The passage tube is also preferably made of a material that is impermeable to the metal hydride 14, thus preventing the metal hydride 14 from entering into the interior of the passage tube 28. The passage tube 28 preferably opens into the interior of the tank body 12. The opening of the passage tube 28 is preferably covered with a filter 30 which prevents the metal hydride 14 from entering into the interior of the passage tube 28.

(79) When the compressor 32 pumps hydrogen gas into the tank body 12, there is an increase of the hydrogen pressure inside the tank body 12. When the hydrogen pressure inside the tank body is increased, the metal hydride 14 is able to coordinate with a greater amount of hydrogen. Preferably, the increase in pressure causes an increase in the number of Kubas interactions per metal centre in the metal hydride 14. After the desired amount of hydrogen has been added to the system, the valve 20 is closed.

(80) When desired, hydrogen may be released from the system 10 as follows. The second valve 26 is opened, which allows hydrogen gas to flow out of the tank body 12 through the second opening 24. When hydrogen gas flows out of the tank body through the second opening 24, there is a decrease in pressure inside the tank body 12. When the pressure is decreased inside the tank body 12, the metal hydride 14 releases hydrogen. For example, the decrease in pressure may cause a decrease in the number of Kubas interactions per metal centre of the metal hydride 14.

(81) Hydrogen that is released by the metal hydride 14 can flow out of the tank body 12 through the second opening 24. As shown in FIG. 2, the hydrogen can flow through the second passage 22 to a fuel cell 36. The fuel cell 36 preferably uses hydrogen as a fuel and oxygen as an oxidant to produce electricity. Typically, a filter is present at the second opening 24 in order to prevent loss of particulates downstream.

(82) In an alternative embodiment, the storage system of the present invention comprises a storage tank with a single opening. In this embodiment, hydrogen flows both into and out of the storage tank through the single opening. A valve is used to control the flow of hydrogen through the opening. Since the enthalpies of H.sub.2 binding are moderate to thermodynamically neutral and binding may be controlled by pressure, the tank may not need an exotic heat management system for most applications, unlike many prior hydrogen storage systems.

(83) In one embodiment, the system is portable. As such, the system can be transported to a filling station to be filled with hydrogen. After being filled with hydrogen, the system can then be transported to a site where the hydrogen energy is to be used. Applications for this system include, but are not limited to, vehicles, airplanes, homes, buildings, and barbeques.

EXAMPLES

(84) The present invention will now be further described by way of the following non-limiting examples. In applying the disclosure of these examples, it should be kept clearly in mind that the examples are merely illustrative of the present invention and should not be construed as limiting the scope of the invention in any way as many variations and equivalents that are encompassed by the present invention will become apparent to those skilled in the art upon reading the present disclosure.

(85) All chemicals were purchased from Sigma-Aldrich and used without further purification except for dry 40-60 C. petroleum ether, diethyl ether and toluene, which were purified via an MBraun Solvent Purification System which was dispensed inside the MBraun glovebox.

(86) MnCl.sub.2(dioxane).sub.1 was prepared according to Fowles et al., J. Chem. Soc. A, 1842-1846, 1968.

(87) Nitrogen adsorption and desorption data were collected at 77K on a Micromeritics ASAP 2010.

(88) Infrared spectroscopy was conducted on a Perkin Elmer Spectrum RX1 using KBr discs. Approximately 5 mg of sample was ground with 200 mg IR grade, oven dried KBr and compressed to form a disc.

(89) Hydrogen adsorption isotherms were obtained using a computer controlled gas sorption Sieverts apparatus manufactured by Hy-Energy. High purity hydrogen (Grade 6, 99.9999% purity) purchased from Air Liquide was used. Stainless steel spacers were added to the sample holder along with the material to reduce excess void space. The void space of the sample was calculated by performing a helium volume calibration at 298K using 3 each adsorption and desorption points (total of 6), with outlying values discarded and rerun. Excess hydrogen storage measurements on a 200 mg standard AX-21 sample (0.65 wt. % at 70 bar and 298 K) were performed and ensure correct functioning of the instrument and to ensure the accuracy of the isotherms. The reported gravimetric hydrogen storage capacity of Carbon-AX21 is 0.3 wt % at 35 bar (Bernard et al., Assessment of Hydrogen Storage on Different Carbons, IEA Report, 2001). This corresponds to 0.6 wt % at 70 bar which gives an error of 0.07 wt % ((0.65-0.6)100/70) at 100 bar H.sub.2 with a 200 mg sample size. This sample size was chosen such that the absolute amount adsorbed was equivalent to that in our manganese hydride hydrogen storage experiments (ca. 1 mmol H.sub.2) to eliminate systematic error, since the instrument measures total mols hydrogen adsorbed and then converts it to wt %.

(90) True volumetric adsorption is defined as the amount of hydrogen adsorbed on or in a given volume of the solid portion of the sample. This may be calculated from the excess storage data and the skeletal density, thereby allowing a comparison between volumetric adsorption values of the solid phase alone from one sample to another without having to correct for the different textural void space in each material.

Example 1

Manganese Hydride Samples

(91) Synthesis

(92) 1) Synthesis of MnCl.sub.2(dioxane).sub.1.3-1.5

(93) Dioxane (50 mL) was added to MnCl.sub.2 (5.8 g, 46.7 mmol) and the mixture was stirred at room temperature overnight. The excess dioxane was removed in vacuo at 10.sup.3 torr and the resulting solid was dried in vacuo (10.sup.3 ton) for 12 hours to afford MnCl.sub.2(dioxane).sub.1.3-1.5 in quantitative yield. The Infra-red (IR) spectrum for the product exhibits CH stretches at 2800 cm.sup.1 and a characteristic coordinated ether CO stretch between 1000 and 1100 cm.sup.1.

(94) 2) Synthesis of bis(neopentyl)manganese 240.30 for 1.3-213.95 for 1.0

(95) MnCl.sub.2(dioxane).sub.1 (10.0 g, 46.7 mmol) or MnCl.sub.2(dioxane).sub.1.3 (11.22 g, 46.7 mmol) was suspended in 200 mL diethylether and enough dioxane was added to make the total amount of dioxane present to be four molar equivalents relative to MnCl.sub.2 (12 mL, 140.1 mmol for MnCl.sub.2(dioxane).sub.1; 10.8 mL, 126.1 mmol for MnCl.sub.2(dioxane).sub.1.3) (or alternatively, MnCl.sub.2 (5.8 g, 46.7 mmol) is stirred at 60 C. with 4 equivalents of dioxane (16 mL, 187 mmol) overnight, followed by cooling to room temperature and addition of 200 mL diethyl ether).

(96) Neopentyl magnesium chloride (93.4 mmol) in 200 mL diethyl ether was then added dropwise and the resulting solution was stirred for 24 hours to give a pale orange suspension. The resulting white solid was then removed by filtration and washed with ether (250 mL) and the combined orange filtrates were concentrated in vacuo (10.sup.3 torr) to afford bis(neopentyl) manganese in 70% yield. Extraction of the resulting product into hexane followed by filtration and concentration in vacuo afforded bis(neopentyl)manganese in 50% yield that could be used without further purification.

(97) The yield after hexane extraction for Mesityl.sub.2Mn synthesized from MesitylMgBr using the above procedure above was 30%. The yield after extraction into toluene and filtration for bis(trimethylsilylmethyl)manganese synthesized from trimethylsilylMgCl using the above procedure was 90%. It should be noted that these compounds are exceedingly air sensitive and that handling in the glove box even at 1 ppm O.sub.2 in a solvated form for several hours can lead to oxidation, resulting in the appearance of a green color followed by a brick red color. See, e.g., Wilkinson et al., J. Chem. Soc. Dalton Trans., 2204-2211, 1976. This oxidation may be followed by IR as oxidation leads to strong stretches observed at 800-1200 cm.sup.1 which are not present in the pure dialkyl. Care should be taken to avoid contamination with any coordinating solvents such as THF, water, amines, etc.

(98) 3) Alternate Synthesis of bis(neopentyl)manganese

(99) MnCl.sub.2 (10 g, 79.5 mmol) was stirred in 200 mL of diethyl ether to afford a pale pink suspension. To this a 1:1 mixture of neopentyl magnesium chloride and bis(neopentyl)magnesium dissolved in diethyl ether (total alkyl content 159 mmol) (see, e.g., Andersen et al., J. Chem. Soc. Dalton Trans., 2204-2211, 1976) was added dropwise and the reaction was stirred for 24 hours at room temperature. All volatiles were removed in vacuo (10.sup.3 torr). The resulting solid was then extracted with warm (40 C.) petroleum ether and filtered to afford a white precipitate and a red-brown filtrate. Cooling the filtrate afforded red-brown crystals which were recrystallized twice from petroleum ether (40 C.) to remove magnesium salts affording bis(neopentyl) manganese (4.7 g, 30% yield).

(100) 4) Preparation of Manganese (II) Hydride

(101) Bis(neopentyl)manganese (75 mg, 0.38 mmol) was stirred in 100 mL of petroleum ether to afford a red-brown solution. The solution was transferred to a stainless steel PARR hydrogenation vessel, which was charged with 100 bar of H.sub.2. The mixture was stirred for 24 hours at room temperature and then for a further 48 hours at 100 bar H.sub.2 and 100 C. The resulting mixture was filtered and the resulting black precipitate was dried at 100 C. in vacuo (10.sup.3 torr) for 4 hours to afford a black air-moisture sensitive solid (Mnd-100) (42.8 mg). The material was further hydrogenated in the solid state using the PCT-Pro for four hours at 85 bar H.sub.2 and 150 C. Following this, the sample was evacuated at 100 C. for two hours and then allowed to cool to room temperature to afford a black solid (Mnd-150-H.sub.2-4 hrs) (21.5 mg).

(102) Sample Characterization

(103) The Infra-red (IR) spectrum for sample Mnd-100 is shown in FIG. 3. Three CH stretches are observed at 2852, 2925 and 2960 cm.sup.1, which correspond to a neopentyl ligand which has not been fully cleaved from the precursor compound via hydrogenolysis. A coordinated ether (CO) stretch is observed at 1000-1100 cm.sup.1 and MnH stretches may be assigned at 1470 cm.sup.1 and 1580-1630 cm.sup.1.

(104) The X-ray powder diffraction (XRPD) pattern of manganese hydride sample Mnd-150-H.sub.2-4 hrs is shown in FIG. 4. As can be seen from FIG. 4, sample Mnd-150-H.sub.2-4 hrs is amorphous. The large peak at approximately 5-6 2 is due to an amorphous mesostructure of the glass and/or the amorphous mesostructure of the sample. The minor peaks at approximately 27 and 34 2 may be assigned to the the glass capilliary.

(105) The the X-ray photoelectron spectroscopy (XPS) spectrum (2P3/2 region) for manganese hydride sample Mnd-150-H.sub.2-4 hrs is shown in FIG. 5. The major emission at 641.9 eV is consistent with Mn (II). The smaller emissions at 643.89 and 646.11 eV may be attributed to higher manganese oxidation states. The XPS is consistent with the sample containing no elemental manganese metal.

(106) FIG. 6 depicts the differential thermal analysis (DTA) (top trace) and thermogravimetric analysis (TGA) (bottom trace) spectra for manganese hydride sample Mnd-150-H.sub.2-4 hrs. The sample retained 97.26% of its mass after heating to 684 C.

(107) Hydrogen Adsorption-Desorption Studies

(108) A PCT (Pressure-Composition-Temperature) hydrogen adsorption-desorption measurement was performed on sample Mnd-100. Sample Mnd-100 (42.8 mg) reached 3 wt. % hydrogen storage (150 bar) at 298 K The hydrogen adsorption-desorption isotherm for sample Mnd-100 is shown in FIG. 7. As can be seen from FIG. 7, there is a slight hysteresis between the adsorption and desorption isotherms.

(109) Subsequently, sample Mnd-100 was hydrogenated in the solid state using the PCT-Pro instrument at 85 bar H.sub.2 and 150 C. for four hours. After evacuation for two hours at 100 C. and cooling to room temperature, the resulting sample (Mnd-150-H.sub.2-4 hrs) was retested. The sample was not reweighed before measurement. However, after running a PCT life cycle (53 cycles) the sample was reweighed and the resulting isotherms adjusted accordingly. The performance improved with cycling even without scaling the weight, indicating that further loss of hydrocarbon with cycling positively affected performance before loss of weight was compensated for. Thus, the starting value of the raw data before scaling was 3.5 wt % at 150 bar before weight correction, and approximately 5 wt % at 120 bar on cycle 53. Since further weight was lost during cycling, the values at the beginning scaled to the final weight may not be accurate, and the best indication of performance will be the average over the last ten cycles. The sample lost 21.3 mg (50%) of its weight over 53 cycles at 120-150 bar (see FIGS. 8 and 9) due to hydrogenolysis of hydrocarbon ligands from the material. As a result of this further loss of hydrocarbon from the material, the hydrogen storage performance significantly improved, reaching 10 wt. % at 120 bar. The accuracy of this value was established by comparing the results to those of a blank AX-21 sample absorbing the same mmol of H.sub.2 at the same pressure.

(110) The PCT life cycle measurement was run for 53 adsorption and desorption cycles in total with a ten-minute evacuation of the sample between each cycle. At the start of the experiment, the pressure maximum of each cycle was set to 150 bar but due to consumption of the test gas the pressure maximum for each cycle had to be reduced. The wt. % of H.sub.2 adsorbed by the sample at 121 bar in each cycle is shown below in FIG. 8. The wt. % at the pressure maximum of each cycle is shown in FIG. 9.

(111) The fluctuations in cycling may be due to error associated with the random noise in the pressure transducer visible due to the small sample size.

(112) The H.sub.2 storage performance of the material did not decay over the course of the 53 cycles, which is an important property for commercialisation of hydrogen storage materials for vehicle applications.

(113) FIG. 10 shows the PCT hydrogen adsorption-desorption isotherm recorded after the 53 life cycle measurements, showing that there is some hysteresis between adsorption and desorption.

(114) After solid state hydrogenation of sample Mnd-100 at 150 C. and 85 bar H.sub.2 for four hours to afford sample Mnd-150-H.sub.2-4 hrs and subsequent exposure to 53 cycles of adsorption and desorption at 120-150 bar, the IR spectrum was recorded to determine the extent of loss of hydrocarbon. As can be seen from FIG. 11, the stretches at 1630 cm.sup.1 and 1460 cm.sup.1 assigned to MnH have increased in intensity as neopentyl ligands have been replaced by hydride ligands. There has also been a reduction in the intensity and number of CH stretches. The low intensity CH stretch at approx. 2800 cm.sup.1 arises from residual hydrocarbon remaining in the framework of the material. The presence of this low intensity CH and the fact that saturation was not reached at 120 bar implies that higher capacity may be possible. This is further supported by the observation that saturation was not reached in the isotherm shown in FIG. 10.

(115) The nitrogen adsorption-desorption isotherm (77K) for sample Mnd-150-H.sub.2-4 hrs following 53 cycles of adsorption and desorption at 120-150 bar H.sub.2 is shown in FIG. 12. The material after cycling exhibits a low BET surface area of 1.4 m.sup.2/g. The N.sub.2 adsorption-desorption isotherm does decrease into negative values, due to the small amount of sample used for this measurement. This indicates that hydrogen is able to diffuse through the structure without pores visible on the nitrogen adsorption size regime. Such diffusion of hydrogen is common in amorphous materials, and often occurs without significant volume change (hydrogen diffusion through amorphous Pd films, for example). This would be advantageous for any hydrogen storage system because it means that volume changes would not have to be taken into account.

(116) FIG. 13 shows the BET plot of sample Mnd-150-H.sub.2-4 hrs after 55 cycles of adsorption and desorption at 120-150 bar H.sub.2.

(117) Theoretically, a hydrogen adsorption of 10.5 wt % would correspond to formation of MnH.sub.8, 7.0 wt % would correspond to formation of MnH.sub.6 and 3.5 wt % would correspond to formation of MnH.sub.4. However, as discussed above, the metal hydrides described herein are not strictly stoichiometric. A hydrogen adsorption of 14 wt % would correspond to formation of MnH.sub.10. While the existence and structure of MnH.sub.10 may seem surprising from a solid state chemistry perspective, rhenium, the congener of Mn, forms Re(VII)H.sub.7 and Na.sub.2Re(VII)H.sub.9 (see, e.g., Parker et al., Inorg. Chem., 45, 10951-10957, 2006; Abrahams et al., Inorg. Chem., 3, 558-567, 1964) and it is typical for the first row transition metal to prefer a lower oxidation state. As these rhenium compounds are stable Re(VII) species, metastable manganese analogues in which the hydrides could exist predominantly as Kubas ligands to preserve the preferred lower oxidation state of manganese, may be envisaged.

(118) Furthermore, on the basis of (R.sub.3P).sub.2Ru(II)H.sub.6 (see, e.g., Grellier et al., Angewandte Chemie Int. Ed., 46, 2613-2615, 2007), a compound with two axial phosphines, two hydrides, and two Kubas H.sub.2 ligands, the structure of MnH.sub.10 can be rationalized by replacing each phosphine with a Kubas H.sub.2. Withoput being bound by theory, this suggests that the solid amorphous polymeric MnH.sub.2 described herein may be effectively solvated by Kubas H.sub.2 units at high pressure to reversibly form Mn(H.sub.2).sub.n (n=2, 3, 4, 5). The expected density of MnH.sub.2 would be close to CaH.sub.2 (2 g/cc) which translates into 280 Kg/m.sup.3 volumetric density for MnH.sub.10, four times the ultimate U.S. Department of Energy target in this category.

(119) The present invention is not to be limited in scope by the specific embodiments described herein. Indeed, various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing description and the accompanying figures. Such modifications are intended to fall within the scope of the appended claims.

(120) Patents, patent applications, publications, product descriptions, and protocols are cited throughout this application, the disclosures of which are incorporated herein by reference in their entireties for all purposes.