Reductive coupling of germanium(II) halides

The crystal structure of Ar¹GeGeAr¹.

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.^[5] 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.^[4] 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^[6] and afforded the structure similar to the calculated one and Ar¹GeGeAr¹.

The synthesis of digermyne 2,6-Dipp₂H₃C₆GeGeC₆H₃-2,6-Dipp₂ (Ar¹GeGeAr¹)

Similar synthetic method was used to facilitate the synthesis of a digermyne LGeGeL with a Ge-Ge single bond.^[7] 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.

The synthesis of LGeGeL.

Reduction of digermylene

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.^[8]

The synthesis of BbtGeGeBbt.

Valence bond models

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.^[9] 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).^[10] The criterion of a trans-bent structure can be given by CGMT model.^[11] Therefore, the bonding between two Ge atoms in digermynes can be described as donor-acceptor interactions using valence bond models.

The quartet and doublet structure of RGe fragments

It can be seen from the bonding representations that Ge atoms are either linked by one σ-bond and two donor-acceptor bonds (from a filled sp hybrid orbital to an empty p orbital) or one σ-bond and one π-bond with a resonating lone pair or two radicals on each Ge atoms. According to the resonance structures, one of the two Ge atoms bears partial positive charge and is electron deficient, the other Ge atom has an electron lone pair and is able to donate some electron density.

Donor-acceptor bonds and resonance structures of digermynes

The abnormal bond angles and the single-bond feature of LGeGeL can be rationalized by the electron donating character of N atom, which leads to the formation of N p(π)→Ge (empty p orbital) interaction. Therefore, the donor-acceptor bonds between two Ge atoms are weakened and are more like non-bonding electron lone pairs. It has been suggested that the bond order of Ge-Ge bond is to some degree affected by the electronic properties of bulky protecting groups.^[8]

Molecular orbital (MO) treatment

Second order Jahn-Teller effect of the bonding between two Ge atoms in digermynes leads to the geometrical distortion of digermynes.

In a molecular orbital (MO) description, the geometrical distortion (trans-bent structure) of digermynes is the consequence of the second order Jahn-Teller (SOJT) effect, which is the symmetry allowed interaction between filled bonding MO (generally the HOMO in digermynes) and empty nonbonding or antibonding MOs (usually the latter one) that are close in energy and can lead to large molecular distortion. If constraining the digermyne molecule in D_∞h point group, two Ge atoms will form one low-lying σ-bonding orbital, two degenerate π-bonding orbitals and π-antibonding orbitals, and one high-lying σ-antibonding orbital, which are the same as alkynes. SOJT mixing of in-plain π-bonding orbital (π_x, HOMO) and σ-antibonding orbital, which have the same b_u symmetry in the trans-bent C_2h point group, give rise to a slipped π-bond with significant non-bonded electron lone pair character which is lower in energy, as well as a σ-antibonding orbital with higher energy. This second order mixing of MOs leads to the molecular distortion in geometry from linear D_∞h to trans-bent C_2h. The mixing of σ-bonding orbital and in-plain π-antibonding orbital (π_x^*, LUMO) is also symmetrically (both are in b_g symmetry) and energetically allowed. Noticing that large SOJT effect occurs between two orbitals with energy difference of 2 eV or even larger, for example, 4 eV, the extent of mixing of orbitals is neglectable in alkynes, but is maximized in heavier elements, like Ge atoms in digermynes.^[12]

Cycloaddition

Because of the multibonded feature of digermynes and the large interatomic repulsion of two Ge atoms, digermynes can undergo cycloaddition reactions with alkenes and alkynes, such as ethylene and acetylene.

[2+1] and [2+2] Cycloaddition

Digermynes are able to react with a variety of unsaturated small molecules, including alkynes, alkenes, PhN=NPh, isocyanides, and azides, due to their relatively weak Ge-Ge bonds. It has been proven that there are two types of reaction modes when BbtGeGeBbt is treated with ethylene, which both undergo a [1+2] cycloaddition reaction at first to afford a germirane-substituted germylene intermediate. Ge atom of the germirane substituent then easily inserts into one of the Ge-C bonds of germylene to generate 1,2-digermacyclobutene, which has been illustrated both experimentally^[13] and computationally.^[13][14] In the case that the pressure of ethylene is about 1 atm, the 1,2-digermacyclobutene further reacts with one equivalent of ethylene through the same cycloaddition reaction to afford a digermane with two four-membered Ge₂C₂ rings, while the digermane with two three-membered GeC₂ rings is obtained with higher pressure of ethylene. It has been suggested that the former one is thermodynamically stable product, whereas the latter one is only kinetically stable.

The reaction between BbtGeGeBbt and ethylene.

Similarly, [2+2] cycloaddition reactions take place between digermynes and alkynes, for example PhC≡CPh, leading to the formation of 1,2-digermacyclobutadiene.^[15]

[4+1] Cycloaddition: diradical character of digermynes

[4+1] cycloaddition reaction of Ar²GeGeAr² with 1,3-dimethyl-1,3-butadiene.

Different from alkynes which undergo [4+2] cycloaddition reaction with 2,3-dimethyl-1,3-butadiene to give 1,4-cyclohexadiene derivatives, digermynes undergo [4+1] cycloaddition reaction because of the presence of the exceedingly reactive diradical character, which can be seen in valence orbital models. In the case of Ar²GeGeAr², it reacts with 2,3-dimethyl-1,3-butadiene to afford unusual germane derivative. The reaction begins between each of the radical center and 2,3-dimethyl-1,3-butadiene first, which give rise to the formation of digermane with two germacyclopent-3-ene rings through [4+1] cycloaddition. The increased steric repulsion of two GeC₄ rings leads to the homolytic cleavage of the Ge-Ge single bond which then produces the final germane by 1,4-addition reaction with additional equivalent of the 2,3-dimethyl-1,3-butadiene.^[6][15] The breaking of the Ge-Ge bond is not seen when BbtGeGeBbt reacts with 2,3-dimethyl-1,3-butadiene, which only gives rise to digermane.^[8]

Addition reaction of alcohols and water: multiple-bond character of digermynes

BbtGeGeBbt has been proven to be able to undergo addition reaction with alcohols such as methanol and water to generate 1,1-dimethoxydigermane and 1,1-dihydroxydigermane, respectively, which demonstrate the multiple-bond character of digermynes.^[8]

Addition reactions of BbtGeGeBbt with methanol and water

Coordination reaction

The quarlitative molecular orbital diagram of the interaction between Ar¹GeGeAr¹ and Ag⁺.

Noticing that the a_u(π) bonding orbital in digermynes has the ability to act as the π-electron donor, Ar¹GeGeAr¹ can react with AgSbF₆ to form [AgAr¹GeGeAr¹]⁺SbF₆⁻ at -40 °C.^[16] [AgAr¹GeGeAr¹]⁺ has a C₂ axis through the silver atom which is perpandicular to the CGeGeC plain and the midpoint of the Ge-Ge bond. The silver atom is coordinated by two Ge atoms and two arenes from Dipp groups of the bulky protecting groups. The bond between GeGe moiety and Ag⁺ is dominated by the interaction between the HOMO of Ar¹GeGeAr¹ and 5s and 5p orbitals of Ag⁺, which claims the σ-character of the interaction, while the π-character can be explained by the relative weak interaction of Ag 4d_xz orbital with π* orbital (LUMO+1). Therefore, it has been suggested that [AgAr¹GeGeAr¹]⁺ is a hybrid of π-complex and a metallacyclopropene-like σ-complex.