Rhodocene
Rhodocene is a chemical compound with the formula. Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has covalent rhodium–carbon bonds. The radical is found above or when trapped by cooling to liquid nitrogen temperatures. At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.
The history of organometallic chemistry includes the 19th-century discoveries of Zeise's salt and nickel tetracarbonyl. These compounds posed a challenge to chemists as the compounds did not fit with existing chemical bonding models. A further challenge arose with the discovery of ferrocene, the iron analogue of rhodocene and the first of the class of compounds now known as metallocenes. Ferrocene was found to be unusually chemically stable, as were analogous chemical structures including rhodocenium, the unipositive cation of rhodocene and its cobalt and iridium counterparts. The study of organometallic species including these ultimately led to the development of new bonding models that explained their formation and stability. Work on sandwich compounds, including the rhodocenium-rhodocene system, earned Geoffrey Wilkinson and Ernst Otto Fischer the 1973 Nobel Prize for Chemistry.
Owing to their stability and relative ease of preparation, rhodocenium salts are the usual starting material for preparing rhodocene and substituted rhodocenes, all of which are unstable. The original synthesis used a cyclopentadienyl anion and trisrhodium; numerous other approaches have since been reported, including gas-phase redox transmetalation and using half-sandwich precursors. Octaphenylrhodocene was the first substituted rhodocene to be isolated at room temperature, though it decomposes rapidly in air. X-ray crystallography confirmed that octaphenylrhodocene has a sandwich structure with a staggered conformation. Unlike cobaltocene, which has become a useful one-electron reducing agent in research, no rhodocene derivative yet discovered is stable enough for such applications.
Biomedical researchers have examined the applications of rhodium compounds and their derivatives in medicine and reported one potential application for a rhodocene derivative as a radiopharmaceutical to treat small cancers. Rhodocene derivatives are used to synthesise linked metallocenes so that metal–metal interactions can be studied; potential applications of these derivatives include molecular electronics and research into the mechanisms of catalysis.
History
Discoveries in organometallic chemistry have led to important insights into chemical bonding. Zeise's salt,, was reported in 1831 and Mond's discovery of nickel tetracarbonyl occurred in 1888. Each contained a bond between a metal centre and small molecule, ethylene in the case of Zeise's salt and carbon monoxide in the case of nickel tetracarbonyl. The space-filling model of the anion of Zeise's salt shows direct bonding between the platinum metal centre and the carbon atoms of the ethylene ligand; such metal–carbon bonds are the defining characteristic of organometallic species. Bonding models were unable to explain the nature of such metal–alkene bonds until the Dewar–Chatt–Duncanson model was proposed in the 1950s. The original formulation covered only metal–alkene bonds but the model was expanded over time to cover systems like metal carbonyls where π backbonding is important.Ferrocene,, was first synthesised in 1951 during an attempt to prepare the fulvalene by oxidative dimerization of cyclopentadiene; the resultant product was found to have molecular formula and reported to exhibit "remarkable stability". The discovery sparked substantial interest in the field of organometallic chemistry, in part because the structure proposed by Pauson and Kealy was inconsistent with then-existing bonding models and did not explain its unexpected stability. Consequently, the initial challenge was to definitively determine the structure of ferrocene in the hope that its bonding and properties would then be understood. The sandwich structure was deduced and reported independently by three groups in 1952: Robert Burns Woodward and Geoffrey Wilkinson investigated the reactivity in order to determine the structure and demonstrated that ferrocene undergoes similar reactions to a typical aromatic molecule, Ernst Otto Fischer deduced the sandwich structure and also began synthesising other metallocenes including cobaltocene; Eiland and Pepinsky provided X-ray crystallographic confirmation of the sandwich structure. Applying valence bond theory to ferrocene by considering an centre and two cyclopentadienide anions, which are known to be aromatic according to Hückel's rule and hence highly stable, allowed correct prediction of the geometry of the molecule. Once molecular orbital theory was successfully applied, the reasons for ferrocene's remarkable stability became clear.
The properties of cobaltocene reported by Wilkinson and Fischer demonstrated that the unipositive cobalticinium cation exhibited stability similar to that of ferrocene itself. This observation is not unexpected given that the cobalticinium cation and ferrocene are isoelectronic, although the bonding was not understood at the time. Nevertheless, the observation led Wilkinson and F. Albert Cotton to attempt the synthesis of rhodocenium and iridocenium salts. They reported the synthesis of numerous rhodocenium salts, including those containing the tribromide, perchlorate, and reineckate anions, and found that the addition of dipicrylamine produced a compound of composition. In each case, the rhodocenium cation was found to possess high stability. Wilkinson and Fischer went on to share the 1973 Nobel Prize for Chemistry "for their pioneering work, performed independently, on the chemistry of the organometallic, so called sandwich compounds".
The stability of metallocenes can be directly compared by looking at the reduction potentials of the one-electron reduction of the unipositive cation. The following data are presented relative to the saturated calomel electrode in acetonitrile:
These data clearly indicate the stability of neutral ferrocene and the cobaltocenium and rhodocenium cations. Rhodocene is ca. 500 mV more reducing than cobaltocene, indicating that it is more readily oxidised and hence less stable. An earlier polarographic investigation of rhodocenium perchlorate at neutral pH showed a cathodic wave peak at −1.53 V at the dropping mercury electrode, corresponding to the formation rhodocene in solution, but the researchers were unable to isolate the neutral product from solution. In the same study, attempts to detect iridocene by exposing iridocenium salts to oxidising conditions were unsuccessful even at elevated pH. These data are consistent with rhodocene being highly unstable and may indicate that iridocene is even more unstable still.
Speciation
The 18-electron rule is the equivalent of the octet rule in main group chemistry and provides a useful guide for predicting the stability of organometallic compounds. It predicts that organometallic species "in which the sum of the metal valence electrons plus the electrons donated by the ligand groups total 18 are likely to be stable." This helps to explain the unusually high stability observed for ferrocene and for the cobalticinium and rhodocenium cations – all three species have analogous geometries and are isoelectronic 18-valence electron structures. The instability of rhodocene and cobaltocene is also understandable in terms of the 18-electron rule, in that both are 19-valence electron structures; this explains early difficulties in isolating rhodocene from rhodocenium solutions. The chemistry of rhodocene is dominated by the drive to attain an 18-electron configuration.Rhodocene exists as, a paramagnetic 19-valence electron radical monomer only at or below or above in the gas phase. It is this monomeric form that displays the typical staggered metallocene sandwich structure. At room temperature, the lifetime of the monomeric form in acetonitrile is less than two seconds; and rhodocene forms, a diamagnetic 18-valence electron bridged dimeric ansa-metallocene structure. Electron spin resonance, nuclear magnetic resonance and infrared spectroscopic measurements point to the presence of an equilibrium interconverting the monomeric and dimeric forms. ESR evidence confirms that the monomer possesses a high order axis of symmetry with a mirror plane perpendicular to it as symmetry elements; this experimentally demonstrates that the monomer does possess the typical sandwich structure of a metallocene although the interpretation of the ESR data has been questioned. The decomposition pathway of the monomer has also been studied by mass spectrometry. The dimerisation is a redox process; the dimer is a rhodium species and the monomer has a rhodium centre. Rhodium typically occupies oxidation states +I or +III in its stable compounds.
This dimerisation process has the overall effect of decreasing the electron count around the rhodium centre from 19 to 18. This occurs because the oxidative coupling of the two cyclopentadienyl ligands produces a new ligand with lower hapticity and which donates fewer electrons to the metal centre. The term hapticity is used to indicate the "number of carbon atoms through which binds " to a metal centre and is symbolised as ηn. For example, the ethylene ligand in Zeise's salt is bound to the platinum centre through both carbon atoms, and it hence formally has the formula. The carbonyl ligands in nickel tetracarbonyl are each bound through only a carbon atom and are hence described as monohapto ligands, but η1-notations are typically omitted in formulae. The cyclopentadienyl ligands in many metallocene and half-sandwich compounds are pentahapto ligands, hence the formula for the rhodocene monomer. In the rhodocene dimer, the coupled cyclopentadienyl ligands are 4-electron tetrahapto donors to each rhodium metal centre, in contrast to the 6-electron pentahapto cyclopentadienyl donors. The increased stability of the 18-valence electron rhodium dimer species as compared to the 19-valence electron rhodium monomer likely explains why the monomer is only detected under extreme conditions.
Cotton and Wilkinson demonstrated that the 18-valence electron rhodium rhodocenium cation can be reduced in aqueous solution to the monomeric form; they were unable to isolate the neutral product as not only can it dimerise, the rhodium radical monomer can also spontaneously form the mixed-hapticity stable rhodium species. The differences between rhodocene and this derivative are found in two areas:
- One of the bound cyclopentadienyl ligands has formally gained a hydrogen atom to become cyclopentadiene, which remains bound to the metal centre but now as a 4-electron η4- donor.
- The rhodium metal centre has been reduced to rhodium.
Fischer and co-workers also studied the chemistry of iridocene, the third transition series analogue of rhodocene and cobaltocene, finding the chemistry of rhodocene and iridocene are generally similar. The synthesis of numerous iridocenium salts including the tribromide and hexafluorophosphate have been described. Just as with rhodocene, iridocene dimerises at room temperature but a monomer form can be detected at low temperatures and in gas phase and IR, NMR, and ESR measurements indicate a chemical equilibrium is present and confirm the sandwich structure of the iridocene monomer. The complex, the analogue of rhodocene derivative reported by Fischer, has also been studied and demonstrates properties consistent with a greater degree of π-backbonding in iridium systems than is found in the analogous cobalt or rhodium cases.