Sjoerd Harder

Novel Calcium Half-Sandwich Complexes for the Living and Stereoselective Polymerization of Styrene

Tackling tacticity: The first well-defined heteroleptic benzylcalcium complex initiates the living polymerization of styrene. Chain-end control results in a polymer enriched in syndiotactic sequences. Stereo errors arise from fast inversion of the chiral carbanionic chain end. Increasing the styrene concentration accelerates the insertion and leads to a considerable reduction of the stereo errors.

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.

Molecular early main group metal hydrides: synthetic challenge, structures and applications

Within the general area of early main group metal chemistry, the controlled synthesis of well-defined metal hydride complexes is a rapidly developing research field. As group 1 and 2 metal complexes are generally highly dynamic and lattice energies for their [MH](∞) and [MH(2)](∞) salts are high, the synthesis of well-defined soluble hydride complexes is an obvious challenge. Access to molecular early main group metal hydrides, however, is rewarding: these hydrocarbon-soluble metal hydrides are highly reactive, have found use in early main group metal catalysis and are potentially also valuable molecular model systems for polar metal hydrides as a hydrogen storage material. The article focusses specifically on alkali and alkaline-earth metal hydride complexes and discusses the synthetic challenge, molecular structures, reactivity and applications.

α-Methyl-benzylcalcium complexes: syntheses, structures and reactivity

Abstracta-Methyl-benzylcalcium complexes were prepared analogue to a-Me 3 Si-benzylcalcium complexes for which procedures werereported earlier. The crystal structures of homoleptic bis(2-Me 2 N-a-Me-benzyl)calcium / (THF) 2 and heteroleptic (9-Me 3 Si-fluorenyl)(2-Me 2 N-a-Me-benzyl)calcium / (THF) were determined. For both compounds only one of the two diastereomerscrystallized. Barriers for inversion of the chiral benzylic carbon were estimated by variable temperature NMR spectroscopy. The a-methyl-benzylcalcium compounds are less stable and show a higher reactivity and faster initiation of styrene polymerization than theanalogue a-Me 3 Si-benzylcalcium complexes. Intramolecular C / H activation in a heteroleptic a-methyl-benzylcalcium complex wasobserved and the product, a calcium complex with a dianionic alkyl/fluorenyl ansa-ligand, was characterized by crystal structuredetermination.# 2003 Elsevier Science B.V. All rights reserved. Keywords: Calcium; Carbanion; Polystyrene; C / H activation

Methandiide Complexes (R2CM2) of the Heavier Alkali Metals (M=Potassium, Rubidium, Cesium): Reaching the Limit?†

Size does matter: Whereas geminal bimetallic bis(amidophosphorano)methandiide complexes of the heavy alkali metals K and Rb are relatively stable, that of Cs, the largest and most electropositive of the alkali metals, decomposes to form a cyclic product, which cocrystallizes with benzylcesium.

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.