Mg-Mg Bond Research Articles

Low Valent Magnesium Chemistry with a Super Bulky β-Diketiminate Ligand.

The steric bulk of the well-known DIPP BDI ligand (CH[C(CH3 )N-DIPP]2 , DIPP=2,6-diisopropylphenyl) was increased by replacing isopropyl for isopentyl groups. This very bulky DIPeP BDI ligand could not stabilize the radical species (DIPeP BDI)Mg. : reduction of (DIPeP BDI)MgI with Na gave (DIPeP BDI)2 Mg2 with a rather long Mg-Mg bond of 3.0513(8) Å. Addition of TMEDA prior to reduction gave complex (DIPeP BDI)2 Mg2 (C6 H6 ), which could also be obtained as its THF adduct. It is speculated that combination of a bulky spectator ligand and TMEDA prevents dimerization of the intermediate MgI radical, which then reacts with the benzene solvent. Complex (DIPeP BDI)2 Mg2 (C6 H6 ), which formally contains the anti-aromatic anion C6 H6 2- , reacted with tBuOH as a Brønsted base to 1,3- and 1,4-cyclohexadiene and with H2 as a two electron donor to (DIPeP BDI)2 Mg2 H2 and C6 H6 . It also reductively cleaved the C-F bond in fluorobenzene and gave (DIPeP BDI)MgPh, (DIPeP BDI)MgF, and C6 H6 .

A combined experimental and computational study on the reaction of fluoroarenes with Mg-Mg, Mg-Zn, Mg-Al and Al-Zn bonds.

Through a combined experimental and computational (DFT) approach, the reaction mechanism of the addition of fluoroarenes to Mg-Mg bonds has been determined as a concerted SNAr-like pathway in which one Mg centre acts as a nucleophile and the other an electrophile. The experimentally determined Gibbs activation energy for the addition of C6F6 to a Mg-Mg bond of a molecular complex, ΔG‡298 K(experiment) = 21.3 kcal mol-1 is modelled by DFT with the ωB97X functional, ΔG‡298 K(DFT) = 25.7 kcal mol-1. The transition state for C-F activation involves a polarisation of the Mg-Mg bond and significant negative charge localisation on the fluoroarene moiety. This transition state is augmented by stabilising closed-shell Mg···F ortho interactions that, in combination with the known trends in C-F and C-M bond strengths in fluoroarenes, provide an explanation for the experimentally determined preference for C-F bond activation to occur at sites flanked by ortho-fluorine atoms. The effect of modification of both the ligand coordination sphere and the nature and polarity of the M-M bond (M = Mg, Zn, Al) on C-F activation has been investigated. A series of highly novel β-diketiminate stabilised complexes containing Zn-Mg, Zn-Zn-Zn, Zn-Al and Mg-Al bonds has been prepared, including the first crystallographic characterisation of a Mg-Al bond. Reactions of these new M-M containing complexes with perfluoroarenes were conducted and modelled by DFT. C-F bond activation is dictated by the steric accessibility, and not the polarity, of the M-M bond. The more open coordination complexes lead to enhanced Mg···F ortho interactions which in turn lower the energy of the transition states for C-F bond activation.

Reversible Insertion of a C═C Bond into Magnesium(I) Dimers: Generation of Highly Active 1,2-Dimagnesioethane Compounds.

The insertion of 1,1-diphenylethylene into the Mg-Mg bond of two magnesium(I) dimers, [(ArNacnac)Mg-]2 (Ar = C6H2Me3-2,4,6 (Mes); C6H3Et2-2,6 (Dep)), yielding 1,2-dimagnesioethane products, [{(ArNacnac)Mg}2(μ-CH2CPh2)], is described. These reactions are readily reversible at room temperature and thus represent the first examples of room-temperature reversible redox processes for s-block metal complexes. The 1,2-dimagnesioethane products are highly activated magnesium alkyls and show unprecedented, uncatalyzed reactivity toward H2, CO, and ethylene. Computational studies have investigated the mechanisms of all presented reaction types.

Hydrogen dissociation and incorporation on Mg17Al12(100) surface: A density functional theory study

Hydrogen adsorption, dissociation, and penetration on (in) Mg17Al12 (100) surface are studied extensively by DFT total-energy calculations. The adsorption geometries, dissociation barriers, various diffusion pathways, penetrative processes, and electronic structures were investigated. Results show that the atomic and molecular hydrogen forms prefer to be adsorbed on the Mg3-Mg3 bridge sites (C sites). Hydrogen molecules are dissociated on the surface with the minimum barrier energy of 0.63eV. There are two stages in the process of hydrogen incorporation, which are hydrogen diffusion on the surface and the penetration from the surface into the subsurface. Two possible pathways of atomic hydrogen penetration from surface into subsurface are found. The calculations of electronic structures show that the hybridization between the s orbital of H and the s orbitals of Mg is major. The Mg-Mg bond on the outmost surface is shortened from 4.48Å to 3.30Å after the hydrogen adsorption on C sites, showing the strong interaction between Mg and H atoms.

Addition of Carbon-Fluorine Bonds to a Mg(I)-Mg(I) Bond: An Equivalent of Grignard Formation in Solution.

Addition of the carbon–fluorine bond of a series of perfluorinated and polyfluorinated arenes across the Mg–Mg bond of a simple coordination complex proceeds rapidly in solution. The reaction results in the formation of a new carbon–magnesium bond and a new fluorine–magnesium bond and is analogous to Grignard formation in homogeneous solution.

Hydrogen storage and thermal transport properties of pelletized porous Mg-2 wt.% multiwall carbon nanotubes and Mg-2 wt.% graphite composites

We synthesized pelletized porous composites of Mg admixed with 2 wt.% of either multiwall carbon nanotubes or graphite. The composites were prepared by high energy ball-milling of Mg powder with carbonaceous additives, followed by uniaxial compression and sintering in hydrogen environment under mechanical constraint. The correlations between ball-milling conditions, composite microstructure, hydrogenation kinetics, and thermal conductivity of the pellets were established. The presence and condition of carbon additives controls the morphology of Mg particles and, consequently, the mechanical stability of the pellet upon hydrogenation cycling. The best combination of hydrogen desorption kinetics, thermal conductivity, and mechanical stability was obtained for the pellets synthesized from the mixture of Mg with 2 wt.% of carbon nanotubes processed by 4 h of co-milling. The milling transformed carbon nanotubes into carbon nano-particles/nano-onions. These carbonaceous species promote metal nucleation from the hydride phase and allow formation of Mg-Mg bonds between the Mg particles contributing to mechanical stability of the pellet.

NdNiMg5, a New Magnesium-Rich Phase with an Unusual Structural Type

The new intermetallic NdNiMg5 was discovered during the study of the Mg-rich part of the Mg-Nd-Ni system. It was synthetized by melting of the constituent elements in a sealed tantalum tube with subsequent annealing. Its structure was determined by X-ray diffraction on a single crystal. Crystal data: orthorhombic system, Cmcm, Z = 4, a = 4.4799(2) Å, b = 9.9827(3) Å, c = 13.7854(10) Å, d(calc) = 3.49 g·cm(-3). Its structure is made of infinite layers of Mg atoms that form blocks stacked along the c axis. These blocks, with a close-packed array of Mg atoms, are separated by infinite NiNd layers and connected through short Mg-Mg bonds. In the NiNd layer, the Ni and Nd atoms form an ordered graphite-type network. Antiferromagnetic ordering is observed with T(N) = 12 K, and the effective magnetic moment μeff is equal to 3.89(1) μB.

Study of the electrochemical deposition of Mg in the atomic level: Why it prefers the non-dendritic morphology

We report a Density Functional Theory (DFT) study on the electrochemical deposition process of Mg. We studied the impact of the thermodynamics and the kinetics to the morphologies of the deposited phases. Our analysis showed that the free energy difference between high dimensional and low dimensional phases was higher for Mg than for Li as a result of the stronger MgMg bond strength. On the other hand, the migration barriers of Li and Mg showed close values. These findings indicated that the electrochemically deposited Mg essentially prefers to form crystalline layers in comparison with Li due to the intrinsic thermodynamic properties. It suggested that Mg anode provides not only high energy densities, but also the potential to avoid the issues caused by the growth of dendrites.

Mg2H2: New insight on the Mg–Mg bonding and spectroscopic study

The six dimensional potential energy surface of the electronic ground state X̃(1)Σ(g)(+) of Mg(2)H(2) has been generated by the coupled-cluster approach with single, double and perturbative triple excitations [CCSD(T)] combined with the aug-cc-pCVTZ basis set for Mg atoms and the aug-cc-pVTZ basis set for the H atoms. The analytical representation of this surface was used in variational calculations of the rovibrational energies of Mg(2)H(2), Mg(2)D(2), and HMg(2)D for J = 0 and 1. For Mg(2)H(2), the rotational constant B(0) is computed to be 0.1438 cm(-1), and the fundamental anharmonic wavenumbers are calculated to be ν(1) = 1527.3 cm(-1) (Σ(g)(+)), ν(2) = 275.3 cm(-1) (Σ(g)(+)), ν(3) = 1503.6 cm(-1) (Σ(u)(+)), ν(4) = 312.9 cm(-1) (Π(g)), and ν(5) = 256.5 cm(-1) (Π(u)). In addition, the electronic ground states of Mg(2)H, MgH(2), Mg(2), and MgH have been investigated in order to compute the bonding energies of Mg(2)H(2) and to explain the strength of the Mg-Mg bond in this tetra-atomic molecule. The nature of the low-lying excited states of Mg(2)H(2) is also studied.

Solid Solution Softening Mechanisms in Mg-Ca Alloy

In the present work, electronic structures of pure Mg, Mg-Ca and Mg-Al alloys were investigated by first-principle calculations. The charge density between the solute and solvent atoms in the Mg-Ca alloy was lower than that in the Mg-Al alloy. The local charge density of states (LDOS) of Mg atom in the Mg-Ca alloy was almost the same as the LDOS of the Mg atom in the pure Mg, suggesting that the solute Ca atoms had little effect on the Mg-Mg bonding in the Mg-Ca alloy. On the other hand, there is more and deeper valley in the LDOS of the Mg atom in the Mg-Al alloy, compared with that in the Mg-Ca alloy, suggesting that the solute Al atom may increase the covalency of the Mg-Mg bonds. A series of the calculations indicates that Ca atoms enhance the formation of double kinks rather than hinder the movement of dislocations, resulting in solid solution softening of Mg-Ca alloy. [doi:10.2320/matertrans.M2011109]

First Experimental Characterization of a Non-nuclear Attractor in a Dimeric Magnesium(I) Compound

High-resolution X-ray diffraction data, coupled with theoretical calculations, are used to demonstrate the presence of a non-nuclear local maximum in the electron density of a dimeric Mg(I) molecule. This is the first time such a non-nuclear attractor (NNA) has been observed in a stable molecular species. Multipole modeling of the Mg(I) centers requires use of expansion/contraction (κ) coefficients taken from density functional theory (DFT), since accurate scattering factors for Mg(I) are not available. The model developed accurately accounts for the electron density in the Mg-Mg region and is in excellent agreement with directly calculated DFT data. Within the quantum theory of atoms in molecules (QTAIM), this molecule is not bound by a Mg-Mg bond but rather by two Mg-"pseudo-atom" bonds. The NNA is associated with a large region of negative Laplacian in the Mg-Mg internuclear region and arises from the overlap of 3s orbitals in this long, nonpolar "bond". The pseudoatomic basin associated with the NNA contains 0.8 electrons, which are highly delocalized and hence weakly bound. Possible implications of this unusual electronic structure for the chemistry of such molecules, including their use as excellent reducing agents, are discussed.

Stable Magnesium(I) Compounds with Mg-Mg Bonds

The chemistry of the group 2 metals (beryllium, magnesium, calcium, strontium, and barium) is dominated by the +2 oxidation state. Here, we report the reductions of two magnesium(II) iodide complexes with potassium metal in toluene, leading to thermally stable magnesium(I) compounds, (L)MgMg(L) (where L is [(Ar)NC(NPri2)N(Ar)]- or {[(Ar)NC(Me)]2CH}-, Ar is 2,6-diisopropylphenyl, Me is methyl, and Pri is isopropyl) in moderate yields. The results of x-ray crystallographic and theoretical studies are consistent with central Mg2+(2) units that have single, covalent magnesium-magnesium bonding interactions with 2.8508 +/- 0.0012 (standard deviation) and 2.8457 +/- 0.0008 angstrom bond lengths, respectively, and predominantly ionic interactions with the anionic ligands (L).

Density functional theory study on the structure and properties of MgmBn(m=1,2;n=1—4) clusters

Possible geometrical structures and relative stability of MgmBn(m=1,2;n=1—4) clusters are studied by using the hybrid density functional theory(B3LYP) with 6-31G basis sets. For the most stable isomers of MgmBn(m=1,2;n=1—4) clusters, the electronic structure, vibrational properties, bond properties, ionization potential, polarizability and hyperpolarizability are analyzed. The calculated results show that most of the optimized MgmBn(m=1,2;n=1—4) clusters have planar structure, the B-B and B-Mg bonds are coexisting in the cluster, but the Mg-Mg bonds are infrequent in the clusters. The bond length of B-B is about 0.153—0.182nm and that of Mg-B is 0.221—0.231nm. The outcome of population analysis suggests that there is an electronegative center in the cluster, the other B and Mg atoms with positive charge are located at apexes of the cluster.

Structural studies of the Mg/Si(111) interface formation

Structural aspects of the formation of the Mg/Si(111) interface at room temperature were investigated by surface EXAFS at the Mg K threshold. Bond lengths and the symmetry of the adsorption sites were determined as a function of Mg coverage. The Mg atoms are found to adsorb on fourfold sites above second-layer Si atoms (T 4)it, with bond distances to first- and second-layer Si atoms equal to 2.60 ± 0.04 Å and 2.47 ± 0.04 Å, respectively. While in the initial stage of adsorption, no adsorbate-induced LEED pattern is observed, the Mg atoms on T 4it positions give rise to a (√3 × √3)R30° superstructure at a coverage of 1 3 ML. Further Mg deposition preserves the LEED pattern, while the polarization-dependent SEXAFS spectra reflect the presence of additional Mg atoms at off-centered T sites midway between two Mg atoms at T 4it sites. At coverages above 1 ML Mg, the LEED pattern disappears, and a contraction of the Mg-Mg bond distance to the bulk value is found.