In what is being called "a landmark discovery in the chemistry of zinc," researchers in Spain have prepared and characterized the first stable molecular compound containing a zinc-zinc bond [Science, 305, 1136 (2004)].
The discovery was unexpected, says Ernesto Carmona, a professor of inorganic chemistry at the University of Seville. Working in his lab, graduate student Irene Resa reacted Zn(C5Me5)2 (where Me = methyl) with diethylzinc (ZnEt2), intending to make Zn(5-C5Me5)Et. Indeed, this product was obtained, but in addition, there was an unanticipated side product that turned out to be Zn2(5-C5Me5)2.
X-ray crystallography of this latter compound--decamethyldizincocene--revealed it to consist of a Zn22+ unit sandwiched between two C5Me5 rings. The X-ray studies were carried out by Enrique Gutierrez-Puebla and Angeles Monge of the Materials Science Institute of Madrid.
By optimizing the reaction conditions, Resa found a way to produce the dizincocene as the major product. "Subsequently, we have developed a more direct route" to it that will be reported later, Carmona tells C&EN.
The Seville chemists report that decamethyldizincocene is "exceedingly reactive toward oxygen and moisture," but when kept away from these substances, "it appears to be indefinitely stable at room temperature."
A dicobalt analog, Co2(5-C5Me5)2, was reported in 1991 by another group, but later was shown to contain two cobalt atoms that, rather than being directly bonded, are instead bridged by three hydride ligands. Furthermore, in 2001, a German group announced the first compound having two zinc atoms that are bridged by two hydrogen atoms.
To rule out the possibility that hydride bridges could be masquerading as a Zn–Zn bond in their compound, the Spanish team members have amassed a significant body of evidence to support their claim.
In a Science commentary on the work, chemistry professor Gerard Parkin of Columbia University notes that it is "experimentally very difficult to disprove the presence of a bridging hydride ligand (especially by X-ray diffraction), but excellent evidence is provided by the high-resolution mass spectrum," which is consistent with a Zn–Zn bond free of bridging hydrides. Structural and chemical evidence detailed by Carmona and coworkers also argues against a hydride formulation, according to Parkin.
Carmona points out that the oxidation state of zinc in the dizincocene is +1, in marked contrast to the +2 oxidation state observed "almost invariably" for zinc in other compounds. However, Parkin notes that the +1 oxidation state is merely a consequence of the fact that each zinc atom is bonded to another zinc atom. "Despite the +1 oxidation state," he writes, "the valence of zinc is two: Zinc has used both of its valence electrons in bonding to the [C5Me5] ligand and the other zinc atom."
Nevertheless, Parkin continues, the Zn–Zn bond "illustrates that the molecular chemistry of zinc can still yield surprises. The next frontier for zinc chemistry will be the isolation of a simple molecular compound that features a bona fide monovalent zinc center."