REACTIVE PI-COMPLEXES OF ELECTRON-RICH TRANSITION-METALS .13. (ETA(6)-HEXALKYLBENZENE)(ETA(4)-NAPHTHALENE)IRON COMPLEXES - SYNTHESIS, PROPERTIES, STRUCTURE AND REACTIVITY

(Eta6-Hexaalkylbenzene)(eta4-naphthalene)iron- and (eta6-hexaalkylbenzene)(eta4-1,4-dimethylnaphthalene)iron-complexes (alkyl methyl and ethyl) can be prepared from bis(ethylene)(toluene)iron, the desired naphthalene derivative and three molecules of butyne or hexyne respectively. The products are formed by stoichiometric [2 + 2 + 2]-cyclic additions of the alkynes and a complete exchange of the coordination sphere of the metal below room temperature. The naphthalene ligands are bonded by four carbon atoms only, which causes an electronic separation inside the bicyclus. One can characterize the coordinated pi4-moiety, which behaves like an 1,3-cyclohexadiene ligand, whereas the free pi6-moiety acts like an annelated benzo ring. This causes a folding of the naphthalene along the line Cl-C4 by 34-degrees, away from the iron atom, and significant changes in the NMR spectra. In contrast to other (arene)(alkylnaphthalene)iron-complexes, with toluene or p-Kylene as arene ligands, which decompose around 0-degrees-C, the hexaalkylarenes stabilize this type of compound significantly [T(dec.) for (hexamethylbenzene)(naphthalene)iron ca. 140-degrees-C]. Electrochemical investigations indicate a reversible redox equilibrium of the neutral complexes with the mono- and dications. However, the oxidized species are only stable at low temperatures. Extended Huckel MO-theoretical calculations indicate a structural change for the second oxidation product, since the energy of the pi6:pi6-dictation is lower than that of the eta6:eta4-dictation. Reactivity studies qualify (hexametylbenzene)(naphthalene)iron as a selective catalyst for the cyclic trimerization of ester-substituted alkynes at temperatures as low as -30-degrees-C, but not for butyne or hexyne, not even at elevated temperatures.