Germanium(II) dicationic complexes

Ge(II) dicationic complexes refer to coordination compounds of germanium with a +2 formal oxidation state, and a +2 charge on the overall complex. In some of these coordination complexes, the coordination is strongly ionic, localizing a +2 charge on Ge, while in others the bonding is more covalent, delocalizing the cationic charge away from Ge. Examples of dicationic Ge(II) complexes are much rarer than monocationic Ge(II) complexes, often requiring the use of bulky ligands to shield the germanium center. Dicationic complexes of Ge(II) have been isolated with bulky isocyanide and carbene ligands. Much more weakly coordinated Germanium (II) dications have been isolated as complexes with polyether ligands, such as crown ethers and [2.2.2]cryptand. Crown ethers and cryptands are typically known for their ability to bind metal cations, however these ligands have also been employed in stabilizing low-valent cations of heavier p-block elements. A Ge2+ ion's valence shell consists of a filled valence s orbital but empty valence p orbitals, giving rise to atypical bonding in these complexes. Germanium is a metalloid of the carbon group, typically forming compounds with mainly covalent bonding, contrasting with the dative bonding observed in these coordination complexes.

History In 2007, a Ge(II) based dication was reported by Rupar, Staroverov, Ragogna and Baines in which a Ge(II) unit is coordinated by three bulky N-heterocyclic carbene ligands. Later in 2008, Rupar, Staroverov and Baines isolated a weakly coordinate Ge(II) dication using cryptand[2.2.2], also the first example of a non-metallic mononuclear dication complexed with a cryptand. In this report, a Ge(II) cation is encapsulated within [2.2.2]cryptand with two triflate counter ions. The crystal structure of this Ge cryptand[2.2.2] (CF3SO3)2 salt reveals a lack of coordination between the encapsulated Ge(II) cation and the triflate anions. Since these reports, similar cationic Ge(II) complexes have been prepared employing crown ethers, azamacrocycles, and bulky isocyanide ligands.

Synthesis In the preparation of Ge(II) cationic complexes, triflate is often chosen as a counter anion as it is relatively weakly coordinating. GeCl2•dioxane is often used as a starting material, as it is a convenient source of Ge(II).

Ge(II) cryptand[2.2.2] The Ge(II) cryptand[2.2.2] complex was prepared by the addition of cryptand to a solution of N-heterocyclic carbene stabilized GeCl(CF3SO3) in tetrahydrofuran. The products obtained from this reaction are summarized below. The germanium cryptand salt precipitated from solution as a white powder, and the identity was established using proton NMR and crystal X-ray diffraction. The carbene stabilized germanium chloride side products (structures given below) were identified in solution after the reaction.

Ge(II) crown ethers Ge(II) cationic species have been isolated with several crown ether ligands, including [12]crown-4, [15]crown-5, and [18]crown-6. Rupar et al. reported the synthesis of various germanium crown ethers employing GeCl2•dioxane as the source of Ge(II). Trimethylsilyl trifluoromethanesulfonate (Me3SiOTf) was used to displace chloride ligands with a more weakly associating triflate ligand. The resulting germanium crown ether complexes can adopt different geometries and cation charges depending on the size of the crown ether and the nature of the anionic ligand, summarized in the figure below. Only the Ge complex with [12]crown-4 is able to fully exclude counter anions from coordinating to Ge to give a dicationic complex. The larger crown ethers do not form sandwich complexes with Ge, and leave room for an anion to associate with the encapsulated Ge. These complexes were characterized with NMR, X-ray crystallography, Raman spectroscopy, and mass spectrometry.

Ge(II) carbene complex The Ge(II) carbene stabilized dication reported by Rupar et al. was prepared by treating GeCl2•dioxane with an N-heterocyclic carbene (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) to give the GeCl2 carbene complex. Upon treatment with trimethylsilyl iodide and excess carbene, the dicationic complex consisting of three carbene ligands to one Ge atom was formed.

Ge(II) 2,6-dimethylphenyl isocyanide complex A Ge(II) dication stabilized by 4 isocyanide ligands was prepared by mixing GeCl2•dioxane and 2,6-dimethylphenyl isocyanide in toluene (scheme given below). Three molecules of GeCl2 are required per four molecules of the isocyanide ligand, as the counter anion is GeCl3−. This complex was crystallized from toluene, and was characterized by X-ray crystallography and NMR spectroscopy.

Structure and bonding The geometry of these Ge(II) complexes is not adequately described by VSEPR theory due to the nature of the lone pair on Ge(II). VSEPR theory is used to predict geometric distortions about atoms with nonbonding electrons (lone pairs), but in some cases heavier main group elements can violate VSEPR theory, displaying a stereochemically inactive or "spherically symmetric" lone pair, deemed the inert-pair effect. Ge(II) complexes can possess stereochemically active or inactive lone pairs, depending on the ligand. To further assess the nature of the electronic structure of Ge(II) dicationic complexes, natural bond orbital (NBO) computational analysis is often employed.

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