Germanium(II) hydrides, also called germylene hydrides, are a class of Group 14 compounds consisting of low-valent germanium and a terminal hydride. They are also typically stabilized by an electron donor-acceptor interaction between the germanium atom and a large, bulky ligand.
Synthesis
The first stable monomeric germylene hydride was reported by Roesky et al. in 2001. Initial attempts to synthesize the compound involved treating a β-diketiminato germylene chloride precursor, [{ HC-(CMeNAr)2}GeCl] (where Ar = 2,6-iPr2C6H), with LiAlH4, though this proved unsuccessful, with the formation of an aluminum dihydride species instead. However, use of the weaker reducing agent NaBH4 resulted in the formation of a germylene hydride with a borane adduct. They found that the Ge-H bond was inert in this adduct, and so the borane adduct was able to be selectively removed at room temperature by a PMe3 scavenger, resulting in the desired terminal germylene hydride:
When the β-diketiminato germylene chloride was treated with the alane-amine adduct AlH3•NMe3 in toluene at -4 °C, the solution underwent a color change from yellow to orange-red as the volatile trimethylamine was removed from the solution, and the resulting power was identified as the stable terminal germylene hydride with 60% yield, thus affording the first direct synthetic route:
The synthesis and isolation of two digermanium germanium(II) hydrides have also been reported. One of these compounds was formed by the addition of L-selectride to a bulky germanium(II) chloride in ether, while the other was formed by the addition of L-selectride to the same germanium(II) chloride in toluene, followed by filtration and the dropwise addition of PMe3.
The first stable acyclic germylene hydride was formed from the dissociation of a hydrido-digermene. The initial monomeric germylene chloride was synthesized by the reaction of a bulky lithium amide ligand and GeCl2•dioxane. Although synthesized from multiple synthetic pathways, reaction of the germylene chloride with L-selectride in toluene at -80 °C most directly gives the orange crystalline hydrido-digermene with 52% yield. This digermene is thought to be in equilibrium with a two-coordinate hydrido-germylene, which can be isolated upon addition of DMAP (dimethylaminopyridine) at 20 °C, giving pale yellow crystals of the three-coordinate germylene hydride in 27% yield.
Similar to the germylene hydrides, the first known example of a germyliumylidene hydride (a germylene hydride cation) was isolated in 2014. This compound can be formed from a two step process, starting with the reaction of potassium bis(NHC)-borate with GeCl2•dioxane to yield a zwitterionic germyliumylidene chloride. The Cl/H exchange can then be undergone via reaction with to give the germylene hydride cation in 91% yield.
The germylene hydride cation was also further reacted with the trityl cation, , as a hydride scavenger, which resulted in the formation of an adduct with the three-coordinate germylene hydride cation acting a donor and a two-coordinate Ge(II) dication as an electron acceptor.
Structure and bonding
The β-diketiminato germylene hydride reported Roesky et al. crystallizes in the P21/n space group as two isostructural molecules per unit. X-ray crystallographic analysis of the orange-red crystals showed that the germanium atom is tetrahedrally coordinated by the hydrogen atom, the β-diketiminato ligand, and the germanium lone pair. The Ge-N bond length is reported to be 1.989 Å and the Ge-H bond displays an absorption at 1733 cm−1, corresponding to a stretching mode.
Atoms-in-molecules and electron localization function analysis
In an atoms-in-molecules analysis of GeH, 3 critical points (2 attractors and a bond critical point) were found. Computed natural bond order and atoms-in-molecules charges both showed a positive charge on germanium and a negative charge on the hydrogen, indicating a significant charge transfer to hydrogen and generating a Ge+H− polarization. Similar results were found for Ge2H, with a positive charge on both germanium atoms and a negative charge on the hydrogen. However, there were 3 bond critical points found, as well as 1 ring critical point.