Digermyne

Digermynes are a class of compounds that are regarded as the heavier digermanium analogues of alkynes. The parent member of this entire class is H-Ge≡Ge-H, which has only been characterized computationally, but has revealed key features of the whole class. Because of the large interatomic repulsion between two Ge atoms, only kinetically stabilized digermyne molecules can be synthesized and characterized by utilizing bulky protecting groups and appropriate synthetic methods, for example, reductive coupling of germanium(II) halides.

The bonding between two Ge atoms in digermyne is different from C≡C bond in alkynes, which results in the trans-bent structure of digermyne. Trans-bent structure is quite common in heavier Group 14 element analogues of alkynes. The second order Jahn-Teller (SOJT) effect of digermynes gives rise to slipped π-bond and large molecular geometrical distortion.

Because of the multibonded feature of digermynes and the large interatomic repulsion of two Ge atoms, which therefore leads to the long germanium-germanium distance, digermynes are very reactive and can undergo different kinds of reactions, such as [2+1] and [2+2] cycloaddition reaction with different kinds of unsaturated molecules, [4+1] cycloaddition with 1,3-dimethyl-1,3-butadiene, addition reaction of alcohols and water, and act as π-electron donor to undergo coordination reaction with silver ion.

Although many computational studies have calculated the structures and energies of the parent molecule HGeGeH and digermynes with organic substitutes, they can be only synthesized and isolated upon the protection of bulky R groups. It has been proven that the synthetic strategy that reducing proper precursor, usually germanium(II) halides with bulky protection groups, by strong reductants is powerful for synthesizing digermynes.

The first stable digermyne 2,6-Dipp₂H₃C₆GeGeC₆H₃-2,6-Dipp₂ (Ar¹GeGeAr¹, Dipp = 2,6-diisopropylphenyl) was synthesized and characterized by Philip P. Power and co-workers in 2002. Reductive coupling of bulky 2,6-Dipp₂-C₆H₃ (Ar¹) group protected Ge(II) monochloride (Ge(Cl)Ar¹) under the treatment of potassium in tetrahydrofuran (THF) or benzene gave the formation of Ar¹GeGeAr¹. The core structure C1-Ge1-Ge2-C2 has a centrosymmetric trans-bent feature, with the C1-Ge1-Ge2 angle of 128.67(8)° and a considerably short distance of 2.2850(6) Å between two Ge atoms. It has a good conjugation between two terphenyl rings and C1-Ge1-Ge2-C2 plain because of the nearly zero torsion angle (0.4°) presented. Similar molecule, designated Ar²GeGeAr² has been calculated before the characterization of Ar¹GeGeAr¹, with the optimized trans-bent core structure protected by even more crowded 2,6-Trip₂C₆H₂ (Ar², Trip = 2,4,6-triisopropylphenyl) groups. The trans bending in Ar²GeGeAr² (123.2°) is comparable with Ar¹GeGeAr¹, and the Ge-Ge distance of 2.277 Å also differs little from that of Ar¹GeGeAr¹. Ar²GeGeAr² was obtained using the same reduction method and afforded the structure similar to the calculated one and Ar¹GeGeAr¹.

Similar synthetic method was used to facilitate the synthesis of a digermyne LGeGeL with a Ge-Ge single bond. Instead of taking advantage of bulky ligands with carbon as coordinating atom, nitrogen-based protecting group L (L = N(Si(CH₃)₃)(Ar³)) was used. The bond angles of N-Ge-Ge are 100.09(6)°, which are much more distorted than Ar¹GeGeAr¹ and Ar²GeGeAr².

Sterically crowded trans-dibromodigermylene, which is protected by 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl (Bbt) groups, can be reduced by two equivalent of potassium graphite (KC₈) in benzene at room temperature to give the birth to corresponding digermyne BbtGe≡GeBbt.

The most obvious difference between alkynes and digermynes, and also other heavier alkyne analogues, is the molecular geometry, which is linear in alkynes, but trans-bent in all heavier alkyne analogues. This huge difference in molecular geometry is resulted from the difference between carbon-carbon triple bond and the bonding of two group 14 heavier atoms, for example germanium atoms. Heavier group 14 elements have much larger covalent radii than carbon. For example, the single and triple bond radii of carbon are 75 Å and 60 Å respectively, while the single and triple bond radii of germanium are 121 Å and 114 Å respectively, which are approximately 50% longer. The triple-bond system REER of group 14 elements can be viewed as the interaction between either two quartet ER fragments or two doublet ER fragments. The former case corresponds to the planar structure, while the latter one represents the trans-bent structure. The quartet ER fragments are lower in energy than doublet one only when E is carbon, which is to say for heavier group 14 elements, the trans-bent structure is more energetically favored than planar structure. For example, HGe and PhGe fragments of HGeGeH and PhGeGePh are 44.2 and 44.1 kcal/mol more stable in energy than the quartet states respectively, under the calculation level of B3PW91/6-311+G(2df) (for Ge), 6-31G(d) (for C, H). The criterion of a trans-bent structure can be given by CGMT model. Therefore, the bonding between two Ge atoms in digermynes can be described as donor-acceptor interactions using valence bond models.