Decamethyldizincocene

Unidentified until recently, the compound decamethyldizincocene [Zn2(η5 – C5Me5)2(Figure 1)] has drawn much attention to metallocene chemistry. This compound is unique in that it is the first stable binuclear metallocene identified that exhibits an essentially linear molecular geometry, with cyclopentadienyl rings at right angles to the metal-metal bond axis and a pair of metal atoms bonded to each other without the support of bridging ligands.

Background
The ability of metals to form heteronuclear or homonuclear metal-metal bonds varies throughout the periodic table, especially among the group 12 elements. Mercury, for instance, readily forms [M-M]2+ units while the elements cadmium and zinc almost exclusively form singular M2+ ions. Therefore, the isolation of decamethyldizincocene, which contains a [Zn-Zn]2+ unit, is highly unusual, especially considering that it is not stabilized by bridging ligands. The synthesis of decamethyldizincocene has led to further investigation into the creation of similar binuclear metallocenes.

Discovery
Decamethyldizincocene was first isolated in 2004 by Resa and coworkers as an unexpected product of the reaction between decamethylzincocene (Zn(C5Me5)2) and diethyl zinc (ZnEt2) conducted in diethyl ether in an oxygen-free argon environment (Figure 2).

The predicted product of this reaction was the half-sandwich complex (η5-C5Me5)ZnEt because the analogous reaction of zincocene (Zn(C5H5)2) with diethyl zinc (ZnEt2) was previously reported to have led to the formation of only (η5-C5Me5)ZnEt.

Therefore, the stabilizing effect of the methyl groups on the cyclopentadienyl rings is of great importance in the formation of decamethydizincocene. The use of ZnEt2 as a reactant is also of particular significance. For instance, when this same reaction is conducted using ZnMe2 in place of ZnEt2, only the half-sandwich compound (η5-C5Me5)ZnMe is obtained.

It has also been discovered that Zn(C6H5)2 can be utilized in place of ZnEt2; however, the use of Zn(C6H5)2 is less favorable because of the low solubility of the diphenyl zinc in ether. Both (η5-C5Me5)ZnEt and decamethyldizincocene are produced from the reaction between Zn(η5-C5Me5)2 and ZnEt2 in varying amounts depending on the conditions under which the reaction is conducted. The reaction can be optimized to obtain one of the resultant compounds as the major product by using different reaction media and thermal conditions while implementing low-temperature 1H-NMR monitoring. For instance, if this reaction is conducted in pentane at -40º C, (η5-C5Me5)ZnEt is the sole product of the reaction; conversely, if the reaction is conducted in diethyl ether at -10º C, (Zn2(η5 – C5Me5)2) is the major product.

Unpredictability of formation
The formation of decamethyldizincocene is, however, rather unpredictable. Several duplications of this reaction (under conditions that favor the formation of decamethyldizincocene) have inexplicably led to the formation of only the half-sandwich complex (η5-C5Me5)ZnEt.

The formation of the products (η5-C5Me5)ZnEt and Zn2(η5 - C5Me5)2 occurs via separate, competitive reaction pathways and, therefore, the two products do not interconvert when left to react over extended periods of time.

The formation of the half-sandwich complex is believed to occur via hydrocarbyl-bridged intermediates. The exact reaction route followed in the formation of the unexpected product decamethyldizincocene is, however, still very uncertain. Previously it was hypothesized that the creation of decamethyldizincocene occurred through the decomposition of diethylzinc, whose decomposition products would have had the capability of reducing decamethylzincocene to decamethyldizincocene.

However, it is now believed that the formation of decamethyldizincocene occurs via a radical reaction involving the combination of two (η5-C5Me5)Zn• radicals.

New methods of synthesis
Recently (2006), a new method of synthesizing decamethyldizincocene has been developed by Resa and coworkers. This new method involves the use of KH in THF to reduce decamethylzincocene to decamethyldizincocene (Figure 3). This reaction requires approximately 2-3 hours to complete and occurs at an optimal temperature of -20º C. Other reductants such as K, Na, or CaH2 may be used as well in the reduction of decamethylzincocene to decamethyldizincocene. This procedure has proved to be more efficient than the previous means of synthesis and has led to higher yields of the desired product. Extending this procedure to the creation of other dizincocenes has yielded some new and interesting compounds. For instance, Zn2(η5-C5Me4Et)2 has been synthesized using the same procedure described above using Zn(C5Me4Et)2 in place of Zn(C5Me5)2.

Properties
Some of the physical and chemical properties of decamethyldizincocene are worth noting. Decamethyldizincocene is a colorless crystalline solid. It burns spontaneously in the presence of oxygen and reacts with water but is stable at room temperature (although it is commonly stored at -20º C) under argon or in a vacuum sealed tube. Decamethyldizincocene is especially soluble in diethyl ether, benzene, pentane, or tetrahydrofuran.

Due to the high reactivity of decamethyldizincocene with water and oxygen, all solvents used in the synthesis of this compound are degassed and dried before use. This complex does not react in the presence of PMe3, PPh3, NEt3, or pyridine, although some decomposition of zinc occurs (most likely a result of trace amounts of water) and even though decamethyldizincocene reacts with water and oxygen, no reaction occurs in the presence of H2, CO2, or CO. This compound decomposes at 110º C and sublimes at 70º C.

Structure
Various methods have been employed in order to determine the structure of decamethyldizincocene, including x-ray diffraction, 1H NMR, and mass spectrometry. Through x-ray diffraction methods it has been found that the zinc atoms are sandwiched between two parallel C5Me5 rings whose planes are perpendicular to the metal-metal bond axis. The separation between the two ring planes is approximately 6.40 Å. The C5Me5 rings are in an eclipsed conformation with the methyl substituents bent slightly outward (away from the central metal atoms) at angles of 3 to 6 degrees.

In mononuclear metallocenes the bending of substituents attached to the rings serves to prevent steric hindrance; however, the radius of a methyl group is only 2.0 Å and therefore the bending in decamethyldizincocene does not serve this purpose since the distance between the two rings is much greater than this value. It is believed that in the case of decamethyldizincocene the bending of the methyl groups attached to the cyclopentadienyl ligands is preferential because it concentrates the electron density away from the central, positively charged metal atoms. The separation between each Zn atom and the center of the cyclopentadienyl ring attached to it is approximately 2.04 Å and the Zn-C(ring) distances range from 2.27 to 2.30 Å. The Zn-Zn bond distance is 2.035 Å, which indicates considerably strong bonding between the two zinc atoms. This can be compared to the known [Hg-Hg]2+ bond length of 2.5 to 2.7 Å. Two separate types of structures for dimetallocenes have been hypothesized including a coaxial structure (which is the structure of decamethyldizincocene) and a perpendicular structure in which the metal-metal bond axis is parallel to the plane of the cyclopentadienyl ligands (which is predicted to be the structure for dicuprocenes). The compound addressed in this paper is essentially linear with Zn-Zn bond angles of approximately 177º:

Crystalline structure
Also of interest, the molecular packing of decamethyldizincocene forms a crystalline arrangement in which each molecule is encircled by four other molecules with all of the bond axes parallel to one another and the neighboring C5Me5 rings arranged in a manner that resembles gears.

Absence of bridging ligands
1H NMR and mass spectrometry studies have been useful in proving that decamethyldizincocene does not include bridging ligands. This study is important considering that the complex previously hypothesized to be Co2(η5-C5Me5)2 was later found using 1H NMR and mass spectrometry data to be supported by three bridging hydrogens. The 1H NMR of decamethyldizincocene shows only one signal at δ 2.02 due to the hydrogens attached to the methyl groups on the cyclopentadienyl ligands.

Considering that complexes that contain bridging hydride ligands generally have an NMR peak for the hydrides, this finding indicates that decamethyldizincocene does not contain bridging hydrogens. The mass spectra of decamethyldizincocene also provide support for the structure of this complex without additional supportive hydride atoms. The molecular ion of this compound occurs at 398.0927 m/e, which fits with the mass of decamethyldizincocene (398.0930) almost perfectly, indicating that there are no extra hydrogen atoms present.

Electronic structure and bonding characteristics
The electronic structure and bonding characteristics of decamethyldizincocene are also of interest. Decamethyldizincocene has an accumulation of electron density between the two zinc atoms, which indicates bonding. This bond has a predicted dissociation energy of 62 kcal*mol-1 and is approximately as strong as those found among metal-halide bonds. NBO (Natural Bond Order) analysis has indicated that sigma bonding occurs between the 4s orbitals of the central metal atoms with a bonding orbital occupancy of 1.9445.9 Using fragment molecular orbital analysis (FMOA) it has been found that there is one principal molecular orbital that participates in the Zn-Zn bonding with approximately 88% bonding character concentrated between the Zn atoms.

Future work
The discovery of decamethyldizincocene has prompted organometallic chemists to attempt the creation of similar binuclear metallocene complexes. Decamethyldizincocene is unique because of its linear geometry and strong metal-metal bond in the absence of stabilizing bridging ligands. Although no heteronuclear complex has been synthesized that is similar to decamethyldizincocene, calculations suggest the possible existence of a metallocene compound similar to decamethyldizincocene with a Zn-Cd bond, although as of yet this bond is unknown. Currently, chemists are focusing on the bonding modes of decamethyldizincocene in order to understand more about its unique and unprecedented structure.