Magnesium(I) dimer

A magnesium(I) dimer is a molecular compound containing a magnesium to magnesium bond (Mg-Mg), giving the metal an apparent +1 oxidation state. Alkaline earth metals are commonly found in the +2-oxidation state, such as magnesium. The M2+ are considered as redox-inert, meaning that the +2 state is significant. However, recent advancements in main group chemistry have yielded low-valent magnesium(I) dimers, also given as Mg(I), with the first compound being reported in 2007. They can be generally represented as LMg-MgL, with L being a monoanionic ligand. For example, β-diketiminate, commonly referred to as Nacnac, is a useful chelate regarding these complexes. By tuning the ligand, the thermodynamics of the complex change. For instance, the ability to add substituents onto Nacnac can contribute to the steric bulk, which can affect reactivity and stability. As their discovery has grown, so has their usefulness. They are employed in organic and inorganic reduction reactions. It is soluble in a hydrocarbon solvent, like toluene, stoichiometric, selective, and safe.

Discovery

Role of zinc The first zinc(I) dimer was isolated in 2004, with more being synthesized in subsequent years. The chemical similarities between magnesium and zinc led researchers to believe that a Mg(I) dimer could then be achieved. With a calculated stability of Mg—Mg bonded dimers, a synthesis route was needed.

Initials calculations and techniques S-block compounds with low oxidation states can be short lived. There are various techniques available for use. However, the generation and detection of these molecules rely on frozen inert gas matrices, low pressures, high temperatures in the gas phase, or a combination of these. This can then be combined with theoretical studies to gain more information regarding the complex. Matrix isolation techniques were carried out for gaining spectroscopic insight on how the Mg(I) dimer may behave. By heating magnesium diboride, MgB2, at 700 °C with a pressure of 0.1 mbar, and passing HCl gas over it several products are formed, such as magnesium chloride, MgCl. The generation of •MgCl and subsequent compounds from the reaction then underwent further study. At 10 K, the solution was combined with an inert gas, undergoing IR and Raman spectroscopic techniques, combined with density functional theory (DFT) calculations. This showed the monomeric and dimeric Mg(I) halides, •MgCl and ClMgMgCl, a linear molecule. While these studies were useful in gaining more insight on the Mg-Mg bond characteristics, it failed to yield a stable Mg(I) dimer in ambient conditions.

Synthesis

Precursors The stability of the Mg-Mg bond needed to be dealt with. Researchers began to investigate sterically demanding guanidinates and amidinates. Their stabilizing abilities in low-oxidation state chemistry was attractive since it allowed for other low-valent main group complexes to be achieved. This research also allowed for the first stable dimer Mg(I) dimer, [{(Priso)Mg}2]. Potassium reduction of heteroleptic Mg(II) iodide precursor complexes were then carried out. The ligands guanidinato and, β-diketiminato Mg(II) iodide etherate complexes can be prepared from free NH ligands and methyl magnesium iodide in diethyl ether. An example of the synthesis of the precursor synthesis can be shown below. An additional precursor synthesis is shown, needed for [{(tBuNacnac)Mg}2], which can be explained in the section below.

Mg(I) dimer species

Reducing the species and its related precursors with sodium or potassium have given dimeric magnesium(I) compounds such as [{(Priso)Mg}2] and other compounds with substituted versions of β-diketiminato. These compounds, with a general formula of [{(ArNacnac)Mg}2]. However, as the size of the substituent on Nacnac decreased, the difficulty to isolate a magnesium(I) dimer increased. This can be shown by phenol, where only a Mg(II) dimer was gained, given by [(PhNacnac)2Mg]. For a bulkier analogue such as [{(tBuNacnac)Mg}2] a different synthesis route was carried out. Dibutyl magnesium and iodine were chosen since the free β -diketimine, tBuNacnacH has a different reactivity.This is due to tBuNacnacH not reacting with the Grignard reagent shown above. Instead, it can be heated with dibutylmagnesium and become deprotonated.

For the reactant, the was stabilized by utilizing a bulkier, or more sterically demanding, N-ligand. This reaction is carried out through potassium reduction of the α-diimine, MeDipDAB and Mg(II) chloride in tetrahydrofuran (THF). It can be noted that MeDipDAB can be shown by the chemical formula as [(DipNCMe)2]). The shown Mg(I) complexes are all thermally stable. Some can even tolerate temperatures up to 300 °C. They also range in colors from colorless to orange. As these compounds are investigated further, the dimers have been found to be kinetically stabilized by multiple β-diketiminate derivatives, a guanidinate, a diiminophosphinate, an enediamide, and several diimine-enolates.

Bond properties The Mg(I) dimer formula, LMgMgL, has undergone multiple theoretical investigations regarding the bonds. Furthermore, L, a monoanionic ligand, can also include halides, hydrogen, small alkyl groups, aryl groups, cyclopentadienyl with respective derivatives and chelating monoanionic nitrogen ligands. Mg—Mg bonded molecules underwent the primary investigation, with the bond length found to be 2.76-2.89 Å. Additionally, the bond dissociation energy was found to be between 45 and 48 kcal mol−1. Specifically, for ClMgMgCl, it was found to be 47.1 kcal mol−1.

s and p-orbital overlap The Mg-Mg bond for a neutral magnesium(I) dimer has shown to be significantly sigma-bonding. This arises from the s-orbital overlap of the two metals. The bonding interaction that occurs may be connected to the highest occupied molecular orbital (HOMO), giving the highest energy bond of the molecule. This can be reflected through the Wiberg Bond Index (WBI). The sigma single bond gives a WBI value of 0.9, having 90% s-character. Further theoretical investigations have proved that this does not hold for every complex. There can be notable p-orbital contribution to the Mg—Mg, with it being determined to be 55% in some complexes as the charge changes. There were also findings regarding the lowest unoccupied molecular orbital (LUMO). For example, bonding character was also discovered in nearly degenerate LUMO and LUMO+1, with a HOMO–LUMO gap of 93 kcal mol−1.

Potential applications

… excerpt ends here. Continue reading the full article.