Decamethyldizincocene
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Decamethyldizincocene is an
organozinc compound Organozinc chemistry is the study of the physical properties, synthesis, and reactions of organozinc compounds, which are organometallic compounds that contain carbon (C) to zinc (Zn) chemical bonds.The Chemistry of Organozinc Compounds' (Patai S ...
with the formula n2(η5–C5Me5)2 It is the first and an unusual example of a compound with a Zn-Zn bond. Decamethyldizincocene is a colorless
crystalline solid A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...
that burns spontaneously in the presence of oxygen and reacts with water. It is stable at room temperature and especially soluble in
diethyl ether Diethyl ether, or simply ether, is an organic compound with the chemical formula , sometimes abbreviated as . It is a colourless, highly Volatility (chemistry), volatile, sweet-smelling ("ethereal odour"), extremely flammable liquid. It belongs ...
,
benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
,
pentane Pentane is an organic compound with the chemical formula, formula C5H12—that is, an alkane with five carbon atoms. The term may refer to any of three structural isomerism, structural isomers, or to a mixture of them: in the IUPAC nomenclature, h ...
, or
tetrahydrofuran Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
.


Synthesis

The ability of metals to form
heteronuclear A heteronuclear molecule is a molecule composed of atoms of more than one chemical element. For example, a molecule of water (H2O) is heteronuclear because it has atoms of two different elements, hydrogen (H) and oxygen (O). Similarly, a heter ...
or
homonuclear In chemistry, homonuclear molecules, or elemental molecules, or homonuclear species, are molecules composed of only one element. Homonuclear molecules may consist of various numbers of atoms. The size of the molecule an element can form depends ...
metal-metal bonds varies throughout the
periodic table The periodic table, also known as the periodic table of the elements, is an ordered arrangement of the chemical elements into rows (" periods") and columns (" groups"). It is an icon of chemistry and is widely used in physics and other s ...
. Among the group 12 elements, mercury readily forms -Msup>2+ units whereas the elements
cadmium Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...
and
zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...
form fewer examples of such species. Decamethyldizincocene was reported in 2004 by Carmona and coworkers as an unexpected product of the
reaction Reaction may refer to a process or to a response to an action, event, or exposure. Physics and chemistry *Chemical reaction *Nuclear reaction *Reaction (physics), as defined by Newton's third law * Chain reaction (disambiguation) Biology and ...
between decamethylzincocene (Zn(C5Me5)2) and diethylzinc (ZnEt2). ] : 2 (η5-C5Me5)2Zn + Et2Zn → (η5-C5Me5)2Zn2 + 2 (η5-C5Me5)ZnEt + hydrocarbon(s) The analogous reaction of zincocene (Zn(C5H5)2) with diethylzinc gives (η5-C5H5)ZnEt. Therefore, the stabilizing effect of the
methyl group In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated a ...
s on the cyclopentadienyl rings is of great importance in the formation of decamethydizincocene. The use of ZnEt2 as a reactant is of particular significance. The organozinc precursor is important. Diphenylzinc (Zn(C6H5)2), despite its lower solubility, can be utilized in place of ZnEt2. On the other hand, ZnMe2 gives only the half-sandwich compound η5-C5Me5)ZnMe Both (η5-C5Me5)ZnEt and decamethyldizincocene are produced from the reaction between Zn(η5-C5Me5)2 and ZnEt2. The relative amounts depend on reaction conditions, which can be optimized to favor one or the other. For instance, if this reaction is conducted in
pentane Pentane is an organic compound with the chemical formula, formula C5H12—that is, an alkane with five carbon atoms. The term may refer to any of three structural isomerism, structural isomers, or to a mixture of them: in the IUPAC nomenclature, h ...
at -40 Â°C, (η5-C5Me5)ZnEt is the sole product. Conversely, if the reaction is conducted in diethyl ether at -10 Â°C, (Zn2(η5-C5Me5)2) is the major product.


Unpredictability of synthesis

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 occur 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 In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may b ...
-bridged intermediates. The reaction mechanism is, however, 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 A free-radical reaction is any chemical reaction involving free radicals. This reaction type is abundant in organic reactions. Two pioneering studies into free radical reactions have been the discovery of the triphenylmethyl radical by Moses Gomb ...
involving the combination of two (η5-C5Me5)Zn• radicals. In a new more efficient and more general route to decamethyldizincocene, potassium hydride is used to reduce decamethylzincocene to decamethyldizincocene. Other reductants such as K, Na, or CaH2 may be used as well in the reduction of decamethylzincocene to decamethyldizincocene.


Properties

Decamethyldizincocene does not react with
Lewis base A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
s such PMe3, PPh3, NEt3, or
pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom . It is a highly flammable, weak ...
nor does it react with H2, CO2, or CO. This compound appears to be indefinitely stable at room temperature (however storage at -20 Â°C is advised) and sublimes near 70 Â°C under vacuum. In the presence of water or
tert-butyl alcohol ''tert''-Butyl alcohol is the simplest tertiary alcohol, with a formula of (CH3)3COH (sometimes represented as ''t''-BuOH). Its isomers are 1-butanol, isobutanol, and butan-2-ol. ''tert''-Butyl alcohol is a colorless solid, which melts near ...
, this compound
disproportionates In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation state. The reverse of disproportionatio ...
to zinc metal and zinc hydroxide or zinc ''tert''-butoxide.


Structure

Various methods have been employed in order to determine the structure of decamethyldizincocene, including
x-ray diffraction X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...
, 1H NMR, and
mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...
. 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 Ã…. The C5Me5 rings are in an
eclipsed conformation In chemistry an eclipsed conformation is a conformation in which two substituents X and Y on adjacent atoms A, B are in closest proximity, implying that the torsion angle X–A–B–Y is 0°. Such a conformation can exist in any open chain, ...
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
substituent In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...
s attached to the rings serves to prevent
steric hindrance Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is generally a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivi ...
; however, the radius of a
methyl group In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated a ...
is only 2.0 Ã… 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 Electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial variables and is typical ...
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 Ã… and the Zn-C(ring) distances range from 2.27 to 2.30 Ã…. The Zn-Zn bond distance is 2.305 Ã…, which indicates considerably strong bonding between the two zinc atoms. This can be compared to the known g-Hgsup>2+
bond length In molecular geometry, bond length or bond distance is defined as the average distance between Atomic nucleus, nuclei of two chemical bond, bonded atoms in a molecule. It is a Transferability (chemistry), transferable property of a bond between at ...
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 177°:


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.


Electronic structure and bonding characteristics

Decamethyldizincocene has an accumulation of
electron density Electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial variables and is typical ...
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 In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fl ...
bonds. NBO (Natural Bond Order) analysis has indicated that
sigma bond In chemistry, sigma bonds (σ bonds) or sigma overlap are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals along the internuclear axis. Sigma bonding is most simply defined for diat ...
ing 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 In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding ...
that participates in the Zn-Zn bonding with approximately 88% bonding character concentrated between the Zn atoms.


References

{{Zinc compounds Metallocenes Organozinc compounds Pentamethylcyclopentadienyl complexes