Metal aromaticity ormetalloaromaticity is the concept ofaromaticity, found in manyorganic compounds, extended tometals and metal-containing compounds.[1] The first experimental evidence for the existence of aromaticity in metals was found in aluminiumcluster compounds of the typeMAl−
4 where M stands forlithium,sodium or copper.[2] Theseanions can be generated in ahelium gas bylaser vaporization of an aluminium /lithium carbonate composite or a copper or sodium / aluminiumalloy, separated and selected bymass spectrometry and analyzed byphotoelectron spectroscopy. The evidence for aromaticity in these compounds is based on several considerations.Computational chemistry shows that these aluminium clusters consist of a tetranuclearAl2−
4 plane and a counterion at the apex of asquare pyramid. TheAl2−
4 unit is perfectly planar and is not perturbed by the presence of thecounterion or even the presence of two counterions in the neutral compoundM
2Al
4. In addition itsHOMO is calculated to be a doubly occupied delocalized pi system making it obeyHückel's rule. Finally a match exists between the calculated values and the experimental photoelectron values for the energy required to remove the first 4 valence electrons. The first fully metal aromatic compound was a cyclogallane with a Ga32- core discovered by Gregory Robinson in 1995.[3]
D-orbital aromaticity is found in trinucleartungstenW
3O−
9 andmolybdenumMo
3O−
9metal clusters generated by laser vaporization of the pure metals in the presence ofoxygen in a helium stream.[4] In these clusters the three metal centers are bridged by oxygen and each metal has two terminal oxygen atoms. The first signal in the photoelectron spectrum corresponds to the removal of the valence electron with the lowest energy in the anion to the neutralM
3O
9 compound. This energy turns out to be comparable to that of bulktungsten trioxide andmolybdenum trioxide. The photoelectric signal is also broad which suggests a large difference in conformation between the anion and the neutral species.Computational chemistry shows that theM
3O−
9 anions andM
3O2−
9 dianions are ideal hexagons with identical metal-to-metalbond lengths. Tritantalum oxide clusters (Ta3O3−) also are observed to exhibit possible D-orbital aromaticity.[3]
The molecules discussed thus far only exist diluted in the gas phase. A study exploring the properties of a compound formed in water fromsodium molybdate (Na
2MoO
4·2H
2O) andiminodiacetic acid also revealed evidence of aromaticity, but this compound has actually been isolated.X-ray crystallography showed that the sodium atoms are arranged in layers of hexagonal clusters akin topentacenes. The sodium-to-sodiumbond lengths are unusually short (327pm versus 380 pm in elemental sodium) and, like benzene, the ring is planar. In this compound each sodium atom has a distortedoctahedral molecular geometry with coordination to molybdenum atoms and water molecules.[5] The experimental evidence is supported by computedNICS aromaticity values.
