Germanium(II) dicationic complexes

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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. [1] Dicationic complexes of Ge(II) have been isolated with bulky isocyanide and carbene ligands. [2] [3] 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. [4] 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.

Contents

Structure of a Ge complex with [12]crown-4, from X-ray crystal structure Ge 12-crown-4.png
Structure of a Ge complex with [12]crown-4, from X-ray crystal structure

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. [6] [7] In this report, a Ge(II) cation is encapsulated within [2.2.2]cryptand with two triflate counter ions. [6] 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. [3] [4] [5] [8] [9]

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). [10]

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. [6] 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. [6] The carbene stabilized germanium chloride side products (structures given below) were identified in solution after the reaction. [6]

Ge(II) cryptand synthesis reported in Rupar, P. A.; Staroverov, V. N.; Baines, K. M. Science 2008, 322, 1360-1363. Ge Cryptand Synthesis.png
Ge(II) cryptand synthesis reported in Rupar, P. A.; Staroverov, V. N.; Baines, K. M. Science 2008, 322, 1360–1363.

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). [5] 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. [5]

The preparation of various Ge(II) crown ether complexes Ge crown ethers.png
The preparation of various Ge(II) crown ether complexes

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. [2]

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. [3]

Preparation of the Ge(II) 2,6-dimethylphenyl isocyanide complex Ge(II) isocyanide synthesis.png
Preparation of the Ge(II) 2,6-dimethylphenyl isocyanide complex

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. [11] 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. [12]

Visualization of the Ge(II) lone pair in cryptand[2.2.2], based on NBO analysis Ge Cryptand lone pair.png
Visualization of the Ge(II) lone pair in cryptand[2.2.2], based on NBO analysis

Cryptand and crown ethers

The bonding in such Ge(II) polyether complexes is believed to be mainly ionic in character, differing from the expected mainly covalent character typical of most germanium compounds. This lack of a covalent interaction is exemplified in the relatively long Ge-O distances observed in crystal structures of Ge crown ether and Ge cryptand complexes. Ge-O covalent single bonds are expected to be approximately 1.8 Å in length. [13] The crystal structure of the Ge(II) cryptand[2.2.2] complex reveals a much longer Ge-O distance of 2.49 Å, [6] similarly the Ge-O distances range from 2.38-2.49 Å in the Ge(II) ([12]crown-4)2 sandwich complex. [5] For the Ge(II) cryptand[2.2.2] complex, NBO analysis reveals the Ge(II) cation does not participate in any covalent bonding and that the lone pair on the Ge(II) resides in a purely s orbital, indicating a stereochemically inactive lone pair. [6] This lone pair orbital of Ge(II) within cryptand[2.2.2] is depicted to the right. In the Ge(II) crown ether complexes presented above, only the sandwich complex with [12]crown-4 clearly bears a stereochemically inactive lone pair, suggested by the high symmetry of the complex. [5] The Ge(II) complexes with [15]crown-5, and [18]crown-6 show geometric distortions likely due to the activity of the Ge(II) lone pair.

Carbenes and isocyanides

The bonding in Ge(II) dications stabilized by carbenes and isocyanides is believed to be more covalent in nature compared with the bonding in the polyether complexes. Furthermore, the positive charge in these complexes can be quite delocalized. [2] [3]

In the Ge(II) carbene dication complex reported by Rupar et al., the Ge-C bonds are 2.07 Å in length, only marginally longer than expected Ge-C bond lengths. [2] [13] This suggests that the Ge-carbene interaction is not dative, but more covalent in nature. Limiting resonance forms for the Ge(II) carbene dication can be drawn (shown below), with the Ge(II) bearing the full +2 charge, or with the carbenes forming covalent bonds to the Ge center giving each ligand a +1 charge and the Ge a -1 charge. Natural population analysis, a computational technique associated with NBO assigns a charge of +0.64 to the Ge atom, indicating that charge delocalization is significant, and that the structure is best described as an intermediate between the two limiting representations. [2] This compound adopts a pyramidal geometry, with a stereochemically active lone pair on Ge.

Possible representations of the Ge(II) carbene complex Ge carbene representations.png
Possible representations of the Ge(II) carbene complex


Visualization of the partially filled p orbital of the Ge(II) isocyanide dication Ge isocyanide HOMO.png
Visualization of the partially filled p orbital of the Ge(II) isocyanide dication

Similar to the Ge(II) carbene complex, the Ge-C bond lengths in the Ge(II) (2,6-dimethylphenyl isocyanide)3 structure range between 2.03-2.07 Å, typical for expected Ge-C bonds. [3] The ligands adopt a distorted tetrahedral structure about the germanium center in the crystal structure. [3] NBO analysis of the Ge(II) isocyanide dication reveals a partially filled Ge p orbital as a frontier orbital of this complex, depicted to the right. The nature of the frontier orbitals change upon consideration of the GeCl3− counter anions in the NBO analysis. The NBO analysis also reveals a charge of +0.74 on Ge, with some positive charge delocalized on the isocyanide ligands. [3] Geometry optimizations for both singlet and triplet electron configurations were performed for this complex, and the singlet was found to be favored by 48.6 kcal/mol. [3]

Reactivity

The weakly coordinated Ge(II) cations are Lewis acids. Due to this weak coordination, such Ge(II) crown ether complexes could be useful for the preparation of other germanium compounds. Bandyopadhyay et al. have investigated the reactivity of a GeOTf+ [15]crown-5 complex, and found that the weakly coordinating triflate could be exchanged for H2O or NH3. [9] Addition of water to a solution of GeOTf+ [15]crown-5 in dichloromethane results in the formation of the dicationic water complex, as depicted in the figure below. This water adduct was isolated and the structure was determined by X-ray crystallography, making it the first characterized Ge(II)-water adduct. [9] Further addition of bulk water to this complex results in decomposition. [9]

Displacement of a coordinating triflate ion with water Ge 15-crown-5 water.png
Displacement of a coordinating triflate ion with water

Upon treatment with base, this water adduct [Ge[15]crown-5·OH2]2+can be deprotonated to give the hydroxide adduct [Ge[15]crown-5·OH]+. [9] Upon deprotonation to give the hydroxide adduct, the Ge-O bond becomes shorter and stronger. NBO analysis identifies the H2O-Ge[15]crown-5 interaction as a donor-acceptor interaction, while the HO-Ge[15]crown-5 interaction is identified as a polar single bond. [9] This reactivity presents a potential strategy for the preparation of new Ge complexes.

The empty p orbitals of Ge(II) dications make them potential π-acceptors for transition metal complexes. Intriguingly, dicationic Ge(II) complexes have been shown to act as ligands for Au(I) and Ag(I). [14] Raut and Majumdar report the use of a bis(α-iminopyridine) ligand to prepare a Ge(II) dicationic complex that coordinates to the electron rich Au(I) or Ag(I) metal centers. [14] The bonding in such complexes is best described by σ-donation of the Ge(II) lone pair to the transition metal, and π-back donation from the filled transition metal d orbitals to the vacant Ge(II) p orbitals. This unusual activity for Ge(II) is under investigation for possible applications in catalysis. [14]

See also

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In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bond and is sometimes called an unshared pair or non-bonding pair. Lone pairs are found in the outermost electron shell of atoms. They can be identified by using a Lewis structure. Electron pairs are therefore considered lone pairs if two electrons are paired but are not used in chemical bonding. Thus, the number of electrons in lone pairs plus the number of electrons in bonds equals the number of valence electrons around an atom.

<span class="mw-page-title-main">VSEPR theory</span> Model for predicting molecular geometry

Valence shell electron pair repulsion (VSEPR) theory, is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm theory after its two main developers, Ronald Gillespie and Ronald Nyholm.

<span class="mw-page-title-main">Germanium dichloride</span> Chemical compound

Germanium dichloride is a chemical compound of germanium and chlorine with the formula GeCl2. It is a yellow solid. Germanium dichloride is an example of a compound featuring germanium in the +2 oxidation state.

Carbene analogs in chemistry are carbenes with the carbon atom replaced by another chemical element. Just as regular carbenes they appear in chemical reactions as reactive intermediates and with special precautions they can be stabilized and isolated as chemical compounds. Carbenes have some practical utility in organic synthesis but carbene analogs are mostly laboratory curiosities only investigated in academia. Carbene analogs are known for elements of group 13, group 14, group 15 and group 16.

<span class="mw-page-title-main">Germylene</span> Class of germanium (II) compounds

Germylenes are a class of germanium(II) compounds with the general formula :GeR2. They are heavier carbene analogs. However, unlike carbenes, whose ground state can be either singlet or triplet depending on the substituents, germylenes have exclusively a singlet ground state. Unprotected carbene analogs, including germylenes, has a dimerization nature. Free germylenes can be isolated under the stabilization of steric hindrance or electron donation. The synthesis of first stable free dialkyl germylene was reported by Jutzi, et al in 1991.

<span class="mw-page-title-main">Borylene</span>

A borylene is the boron analogue of a carbene. The general structure is R-B: with R an organic moiety and B a boron atom with two unshared electrons. Borylenes are of academic interest in organoboron chemistry. A singlet ground state is predominant with boron having two vacant sp2 orbitals and one doubly occupied one. With just one additional substituent the boron is more electron deficient than the carbon atom in a carbene. For this reason stable borylenes are more uncommon than stable carbenes. Some borylenes such as boron monofluoride (BF) and boron monohydride (BH) the parent compound also known simply as borylene, have been detected in microwave spectroscopy and may exist in stars. Other borylenes exist as reactive intermediates and can only be inferred by chemical trapping.

<span class="mw-page-title-main">Digermyne</span> Class of chemical compounds

Digermynes are a class of compounds that are regarded as the heavier digermanium analogues of alkynes. The parent member of this entire class is HGeGeH, 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.

<span class="mw-page-title-main">Decamethylsilicocene</span> Chemical Compound

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<i>N</i>-heterocyclic silylene Chemical compound

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References

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