Jan Spielmann

Hydrogen Storage in Magnesium Hydride: The Molecular Approach†

Safe and convenient storage of hydrogen is one of the nearfuture challenges. For mobile applications there are strict volume and weight limitations, and these limitations have steered investigations in the direction of compact, solid, lightweight main-group hydrides. Whereas ammonia–borane (NH3BH3) is a nontoxic, nonflammable, H2-releasing solid with a record hydrogen density of 19.8 wt%, it releases hydrogen in an irreversible process. Metal hydrides such as MgH2 are less rich in hydrogen (7.7 wt%) but advantageously display reversible hydrogen release and uptake: MgH2QMg+ H2. [3] Although bulk MgH2 seems an ideal candidate for reversible hydrogen storage, it is plagued by high thermodynamic stability, which translates into relatively high hydrogen desorption temperatures and slow release and uptake kinetics. The kinetics can be improved drastically by doping the magnesium hydride with transition metals and by ball milling or surface modifications. The high hydrogen release temperature (over 300 8C), however, is due to unfavorable thermodynamic parameters (DH= 74.4(3) kJmol ; DS= 135.1(2) Jmol K ), which originate from the enormous lattice energy for [MgH 2]1 (DH= 2718 kJmol ) relative to that of bulk Mg (DH= 147 kJmol ). Although thermodynamic values are intrinsic to the system, recent theoretical calculations demonstrate that for very small (MgH2)n clusters (n< 19), the enthalpy of decomposition sharply reduces with cluster size. Downsizing the particles has a dramatic effect on the stability of saltlike (MgH2)n but much less on that of the metal clusters Mgn. For a Mg9H18 cluster of approximately 0.9 nm diameter a desorption enthalpy of 63 kJmol 1 was calculated, from which a decomposition temperature of about 200 8C can be estimated. At the extreme limit, molecular MgH2 is calculated to be unstable even towards decomposition into its elements (DH= 5.5 kJmol ). The sharp decrease of stability for (MgH2)n clusters with n< 19 can be understood by the rapid increase in surface/volume ratios: surface atoms have a lower coordination number and are loosely bound. It is of interest to note that only clusters with n 19 ( 1.3 nm) have a core with the typical a-MgH2 rutile geometry (six-coordinate Mg and three-coordinate H). Apparently this is the critical size from which clusters start to show bulk behavior. These insights led to increased research activity on the syntheses of MgH2 nanoparticles, either by special ballmilling techniques or by incorporation into confined spaces. Thus nanoparticles in the range of 1–10 nm have been reported. Hydrogen elimination studies indeed show a small reduction of DH and H2 desorption temperatures, but dramatic effects can only be expected for particles smaller than 1 nm. Production of magnesium hydride particles in the subnanometer range would benefit from a molecular “bottomup” approach. We recently reported a simple synthesis protocol for the first soluble calcium hydride complex 1 by the silane route [Eq. (1)], and Jones et al. reported the

Synthesis and structure of a magnesium-amidoborane complex and its role in catalytic formation of a new bis-aminoborane ligand.

A synthetic route to a magnesium-amidoborane complex and its role in the catalytic conversion of a substituted ammonia-borane RNH(2)BH(3) into HB(NHR)(2) is discussed.

Thermal Decomposition of Mono‐ and Bimetallic Magnesium Amidoborane Complexes

Complexes of the type [(DIPPnacnac)MgNH(R)BH(3)] have been prepared (DIPPnacnac=CH{(CMe)(2,6-iPr(2)C(6)H(3)N)}(2)). The following substituents R have been used: H, Me, iPr, DIPP (DIPP=2,6-diisopropylphenyl). Complexes [(DIPPnac- nac)MgNH(2)BH(3)].THF, [{(DIPPnac- nac)MgNH(iPr)BH(3)}(2)] and [(DIPPnacnac)MgNH(DIPP)BH(3)] were structurally characterised. The Mg amidoborane complexes decompose at a significantly higher temperature (90-110 degrees C) than the corresponding Ca amidoborane complexes (20-110 degrees C). The complexes with the smaller R substituents (H, Me) gave a mixture of decomposition products of which one could be structurally characterised as [{(DIPPnacnac)Mg}(2)(H(3)B-NMe-BH-NMe)].THF. [{(DIPP- nacnac)MgNH(iPr)BH(3)}(2)] cleanly decomposed to [(DIPPnacnac)MgH], which was characterised as a dimeric THF adduct. The amidoborane complex with the larger DIPP-substituent decomposed into a borylamide complex [(DIPPnacnac)MgN(DIPP)BH(2)], which was structurally characterised as its THF adduct. Bimetallic Mg amidoborane complexes decompose at lower temperatures (60-90 degrees C) and show a different decomposition pathway. The dinuclear Mg amidoborane complexes presented here are based on DIPPnacnac units that are either directly coupled through N-N bonding (abbreviated NN) or through a 2,6-pyridylene bridge (abbreviated PYR). Crystal structures of [PYR-{Mg(nBu)}(2)], [PYR-{MgNH(iPr)BH(3)}(2)], [NN-{MgNH(iPr)BH(3)}(2)]THF and the decomposition products [PYR-Mg(2)(iPrN-BH-iPrN-BH(3))] and [NN-Mg(2)(iPrN-BH-iPrN-BH(3))].THF are presented. The following conclusions can be drawn from these studies: i) The first step in the decomposition of a metal amidoborane complex is beta-hydride elimination, which results in formation of a metal hydride complex and R(H)N=BH(2), ii) depending on the nature of the metal, the metal hydride is either stable and can be isolated or it reacts further, iii) amidoborane anions with small R substituents decompose into the dianionic species (RN-BH-RN-BH(3))(2-), whereas large substituents result in formation of the borylamide RN==BH(2)(-) and iv) enforced proximity of two Mg amidoborane units results in decomposition at a significantly lower temperature and cleanly follows the BNBN pathway.

Hydrogen Elimination in Bulky Calcium Amidoborane Complexes: Isolation of a Calcium Borylamide Complex

The decomposition route of N-substituted calcium amidoborane complexes depends on the steric bulk of the substituent. Relatively small substituents (R = H, Me, iPr) gave dehydrogenation to dimeric complexes with a bridging RN-BH-N(R)-BH(3)(2-) ion, whereas the larger 2,6-di-isopropylphenyl substituent resulted in formation of a complex with a borylamide anion: H(2)B(R)N(-). This product is a promising species for (catalytic) regeneration with H(2), i.e., hydrogenation to the amidoborane anion H(3)B(R)NH(-).

Convenient synthesis and crystal structure of a monomeric zinc hydride complex with a three-coordinate metal center.

Convenient syntheses for a beta-diketiminate zinc hydride complex, that crystallizes as a monomer with a three-coordinate Zn atom, have been developed.

Well-Defined Molecular Magnesium Hydride Clusters : Relationship between Size and Hydrogen-Elimination Temperature

A new tetranuclear magnesium hydride cluster, [{NN-(MgH)2}2], which was based on a N-N-coupled bis-β-diketiminate ligand (NN(2-)), was obtained from the reaction of [{NN-(MgnBu)2}2] with PhSiH3. Its crystal structure reveals an almost-tetrahedral arrangement of Mg atoms and two different sets of hydride ions, which give rise to a coupling in the NMR spectrum (J = 8.5 Hz). To shed light on the relationship between the cluster size and H2 release, the thermal decomposition of [{NN-(MgH)2}2] and two closely related systems that were based on similar ligands, that is, an octanuclear magnesium hydride cluster and a dimeric magnesium hydride species, have been investigated in detail. A lowering of the H2-desorption temperature with decreasing cluster size is observed, in line with previously reported theoretical predictions on (MgH2)n model systems. Deuterium-labeling studies further demonstrate that the released H2 solely originates from the oxidative coupling of two hydride ligands and not from other hydrogen sources, such as the β-diketiminate ligands. Analysis of the DFT-computed electron density in [{NN-(MgH)2}2] reveals a counterintuitive interaction between two formally closed-shell H(-) ligands that are separated by 3.106 Å. This weak interaction could play an important role in H2 desorption. Although the molecular product after H2 release could not be characterized experimentally, DFT calculations on the proposed decomposition product, that is, the low-valence tetranuclear Mg(I) cluster [(NN-Mg2)2], predict a structure with two almost-parallel, localized Mg-Mg bonds. As in a previously reported β-diketiminate Mg(I) dimer, the Mg-Mg bond is not characterized by a bond critical point, but instead displays a local maximum of electron density midway between the atoms, that is, a non-nuclear attractor (NNA). Interestingly, both of the NNAs in [(NN-Mg2)2] are connected through a bond path that suggests that there is bonding between all four Mg(I) atoms.

High-Magnesian Calcite Mesocrystals: A Coordination Chemistry Approach

While biogenic calcites frequently contain appreciable levels of magnesium, the pathways leading to such high concentrations remain unclear. The production of high-magnesian calcites in vitro is highly challenging, because Mg-free aragonite, rather than calcite, is the favored product in the presence of strongly hydrated Mg(2+) ions. While nature may overcome this problem by forming a Mg-rich amorphous precursor, which directly transforms to calcite without dissolution, high Mg(2+)/Ca(2+) ratios are required synthetically to precipitate high-magnesian calcite from solution. Indeed, it is difficult to synthesize amorphous calcium carbonate (ACC) containing high levels of Mg, and the Mg is typically not preserved in the calcite product as the transformation occurs via a dissolution-reprecipitation route. We here present a novel synthetic method, which employs a strategy based on biogenic systems, to generate high-magnesian calcite. Mg-containing ACC is produced in a nonaqueous environment by reacting a mixture of Ca and Mg coordination complexes with CO(2). Control over the Mg incorporation is simply obtained by the ratio of the starting materials. Subsequent crystallization at reduced water activities in an organic solvent/water mixture precludes dissolution and reprecipitation and yields high-magnesian calcite mesocrystals with Mg contents as high as 53 mol %. This is in direct contrast with the polycrystalline materials generally observed when magnesian calcite is formed synthetically. Our findings give insight into the possible mechanisms of formation of biogenic high-magnesian calcites and indicate that precise control over the water activity may be a key element.

Solid-state and Solution Studies on a β-Diketiminate Zinc Hydride Complex

[MesnacnacZn(μ-H)]2 (1) was synthesized by reaction of MesnacnacZnI with either an equimolar amount of KNH(iPr)BH3 or an excess of NaH and characterized by multinuclear NMR and IR spectroscopy as well as X-ray diffraction. Two polymorphs of 1 were found and their structures determined on single crystals Graphical Abstract Solid-state and Solution Studies on a β-Diketiminate Zinc Hydride Complex

Binuclear magnesium amidoborane complexes : characterization of a trinuclear thermal decomposition product

The bis-β-diketimine with a meta-phenylene bridge (META-H(2): DIPPN(H)CMeCHCMeN-C(6)H(4)-NCMeCHCMeN(H)DIPP; DIPP = 2,6-iPr-C(6)H(3)) reacted with two equivalents of nBu(2)Mg to give the bis-β-diketiminate complex META-(MgnBu)(2). The latter binuclear magnesium complex was converted to META-[MgNH(iPr)BH(3)](2) by reaction with H(2)N(iPr)BH(3). The thermal decomposition of this binuclear iPr-substituted magnesium amidoborane complex has been investigated. In benzene it starts to eliminate H(2) at 90 °C. Two decomposition products could be obtained by fractional crystallization of the residue. The first product is the trinuclear magnesium complex META-Mg(3)[iPrNB(H)N(iPr)BH(3)](2) and the second product is (META-Mg)(2). These products have been formed by ligand exchange reactions of the expected complex META-Mg(2)[iPrNB(H)N(iPr)BH(3)] and were characterized by single crystal X-ray diffraction. The central Mg(2+) ion in META-Mg(3)[iPrNB(H)N(iPr)BH(3)](2) is not connected to the ligand system and its coordination geometry could be representative of that in a solid-state magnesium salt containing the RNB(H)N(R)BH(3)(2-) ion.

Unprecedented reactivity of an aluminium hydride complex with ArNH2BH3: nucleophilic substitution versus deprotonation

Reaction of DIPPnacnacAlH(2) with DIPPNH(2)BH(3) did not give the anticipated deprotonation but nucleophilic substitution at B was observed instead. The product DIPPnacnacAl(BH(4))(2) was isolated and structurally characterized. Nucleophilic displacement at B might play a role in mechanistic pathways related to metal amidoborane complexes.