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Low valent magnesium compounds

A number of stable low valent magnesium compounds containing a metal-metal, Mg-Mg, bond, where magnesium exhibits the formal oxidation state of +1 are known. These compounds generally have the formula L₂Mg₂, where L represents a bulky ligand.

The preparation of the first compounds made involved the reduction of Mg^II iodine complexes with potassium metal and the bulky ligands were:

Both examples have the formula L₂Mg₂, where L represents the bulky anionic bidentate ligand.

History

The first examples of these stable magnesium(I) compounds were reported in 2007. The chemistry of Mg is dominated by the +2 oxidation state and prior to 2007 only examples of crystalline compounds with short Mg-Mg distances that may indicate an Mg-Mg bond were known, such as the ternary metal hydrides Mg₂RuH₄, Mg₃RuH₃, and Mg₄IrH₅ and magnesium diboride, Calculations had also indicated the stability of the Mg₂²⁺ cation

Before the first stable low valent Mg compound was discovered, many metal complexes with the metal oxidation state of 1 were reported in the form of LM^IM^IL, where M is the metal and L is a bulky ligand. In 2004, evidence for the first Zn(I) complex was presented by the crystal structure of (η⁵-C₅H₅)ZnZn(η⁵-C₅H₅), a decamethyldizincocene. The two metal centers are presumed to behave similarly to other group 12 elements such as Hg₂²⁺ or Cd₂²⁺. A year later, the synthesis and structure of a stable Cr(I) complex ArCrCrAr with a quintuple bond between two Cr(I) centers was reported, where the ligand Ar is C₆H₃-2,6(C₆H₃-2,6-Prⁱ₂)₂Prⁱ.

The discovery of such compounds influenced computational chemists to explore the possibility of LM^IM^IL compounds with s-block metals as M. Xie et al. studied dimetallocene of group 2 metals and found by density functional theory (DFT) calculations that CpMgMgCp adopts a D_5h structure, and the Mg-Mg bond distance is 2.77 Å. This work was expanded later to study multimetallocenes (CpM_nCp where n = 2-5) including magnesocenes. Westerhausen et al. reported a theoretical study of diaryl dicalcium(I) compound, where the Ca-Ca bond is suggested by natural bond orbital (NBO) analysis to be dominated by 4s orbital interaction with a minor p orbital interaction, and analogous Mg compounds can be expected to behave in a similar way. Taken together, these studies suggested that the synthesis of stable group 2 M^I, and more specifically, Mg(I) compounds should be possible under the appropriate conditions.

Subsequently, evidence for Mg(I)-Mg(I) bonds was reported. In an experiment where laser-ablated Mg metals was allowed to react with H₂, various forms of magnesium hydrides including Mg₂H₂ were detected by infrared (IR) spectroscopy. Additionally, it was shown by computations that the formation of Grignard reagent may proceed through CH₃Mg₂X, where X is a halide. CH₃Mg₂X is predicted to have comparable stability to CH₃MgX, and the Mg-Mg bond strong. A stable PhMg₄X cluster compound has been studied by mass spectrometry, but its structure remains unknown.

Structure of the first stable Mg(I) compound, prepared by Green et al.

In 2007, Green and coworkers reported a seminal work on the first stable Mg(I) compounds. These compounds are in the form of LMg^IMg^IL where L is either [(Ar)NC(NPrⁱ₂)N(Ar)]^– (priso) or {[(Ar)NC(Me)]₂CH}^– (nacnac).

• a guanidinate, "priso", [(Ar)NC(NPrⁱ₂)N(Ar)]⁻ where Ar = 2,6-diisopropylphenyl and Prⁱ = isopropyl
• a ketiminate, "nacnac", {[(Ar)NC(Me)]₂CH}⁻,- where Ar = 2,6-diisopropylphenyl and Me = methyl

Both examples have the formula L₂Mg₂, where L represents the bulky anionic bidentate ligand. Bulky ligands L were used to protect the compounds from disproportionation. These compounds were synthesized by reduction of [(Priso)Mg(μ-I)₂Mg(OEt₂)(Priso)] or [MgI(OEt₂)(Nacnac)] by excess potassium in toluene. Proposed structure is consistent with X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy, and mass spectrometry (MS).

Since then, a number of Mg(I) dimer compounds containing a Mg-Mg bond has been synthesized, and theoretical studies have been accelerated. 

Since then a variety of stable Mg(I) compounds have been prepared, some melting over 200 °C, some colorless, others colored, but all involving very bulky ligands. Also complexes of the LMgMgL with monodentate ligands have been prepared and in these the coordination of the Mg atom increases from three to four. The magnesium(I) dimers have proved to be useful reducing agents, for example in the preparation of tin(I) compounds.

Bonding

[Summary sentence] The bonding of the hypothetical binuclear magnesocene has been investigated by DFT and NBO calculations. Mg-Mg distance is 2.77 Å, indicating interaction between the two metal centers. The Wiberg Mg-Mg bond index is 0.912, agreeing with the bond distance. On the other hand, the Mg-C bond index is 0.069, close to none, which can be explained by ionic interaction. In fact, the natural atomic charge of Mg is 0.957, close to +1.

Green and coworkers’ report on stable LMgMgL (L = priso or nacnac) provided experimental data on the structure of low valent Mg compounds. Based on the crystallographic data, the Mg-Mg bond distance is 2.85 Å, which is slightly longer than the sum of covalent radii (2.72 Å), but shorter than the sum of elemental Mg radii (3.20 Å). The length of the Mg-N bond is in the typical range for such bond in other complexes. The Mg-Mg bond is found to be 93.2% s character by NBO, which is expected since Mg is an s-block element. Similar to the magnesocene example described above, the Mg-N bond is best described as ionic, since the natural charge on Mg is +0.82 and N is -0.97. Thus, the binuclear Mg can be described as a Mg₂²⁺ cluster, analogous to the group 12 Hg₂²⁺ (present in mercury(I) chloride) and Cd₂²⁺ ions (present in cadmium(I) tetrachloroaluminate).

There has been quantum theory of atoms in molecules (QTAIM) analysis on the bonding properties of Mg-Mg, which revealed a presence of a non-nuclear attractor between two Mg atoms, shown by local maximum of electron density at a bond. Thus, it was proposed that the more accurate model of Mg-Mg bond is two Mg-“pseudo-atom” bonds. The Laplacian, 3s orbital overlap, on the right shows a negative Laplacian value between Mg atoms. There has also been a topological analysis of electron localization function (ELF) of Mg-Mg bonds in N-coordinated Mg ions. The basin population indicates a single covalent bond for Mg-Mg. In agreement with previous studies, NNA and pseudo-basin was found in the Mg-Mg bond. Large polarity index indicates a dative bond.

Reactivity

LMgMgL (L = priso or nacnac) were air and moisture sensitive, although thermally stable, decomposing at over 300 °C. Although one might expect these compounds to be reducing due to the unusual oxidation state of Mg, there was no reaction H₂ or N₂. There was no evidence of THF coordination at Mg. When CyN=C=NCy (Cy = cyclohexyl) was added to the [(nacnac)Mg]₂ compound in toluene, carbodiimide was inserted between Mg-Mg bond, forming a magnesium magnesoamidinate complex.

In many areas of synthetic chemistry, Mg(I) compounds have shown reactivity as two-center, two-electron reducing agents. For example , binuclear Mg(I) compounds can reduce NHC-stabilized GeCl₂ to a singlet germanium(0) dimer (:Ge=Ge:) with two dative NHC ligands. Mg(I) compounds can also selectively reduce unsaturated organic substrates such as PhN=NPh and cyclooctatetraene by two electrons. The organic substrate becomes coordinated between two Mg atoms, and this reactivity is not seen with analogous Mg(II) hydride molecules. The selective reduction under mild conditions suggests that Mg(I) can potentially be an alternative to reductands for organic and organometallic substrates such as Sm(II).

β-dikitiminate-stabilized Mg(I) dimers were shown to be hydrogenated when reacted when NHC-stabilized aluminum(III) hydride molecules. The reaction can also be reversed by reacting the hydrogenated Mg compound with potassium metal. The implications of this result is two-fold:

1. Mg(I) compound can potentially be used to store H₂ for solar fuel applications, and this reaction could shed light on the mechanism of Mg-H bond formation
2. It provides a new synthetic route to Al₂H₄ (dialane), which has only been observed in solid hydrogen matrices.

An overview of synthesis and types of Mg(I) compounds and their reactivity up to 2011 can be found in a recent review by Stasch et al.