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

(Eta6-Hexaalkylbenzene)(eta4-naphthalene)iron- and nzene)(eta4-1,4-dim ethylnaphthalene)iron-complexes (alkyl methyl and ethyl) can be prepar ed from bis(ethylene)(toluene)iron, the desired naphthalene derivative and three molecules of butyne or hexyne respectively. The products ar e 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 a toms 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 a nnelated benzo ring. This causes a folding of the naphthalene along th e line Cl-C4 by 34-degrees, away from the iron atom, and significant c hanges in the NMR spectra. In contrast to other (arene)(alkylnaphthale ne)iron-complexes, with toluene or p-Kylene as arene ligands, which de compose around 0-degrees-C, the hexaalkylarenes stabilize this type of compound significantly [T(dec.) for (hexamethylbenzene)(naphthalene)i ron ca. 140-degrees-C]. Electrochemical investigations indicate a reve rsible redox equilibrium of the neutral complexes with the mono- and d ications. However, the oxidized species are only stable at low tempera tures. Extended Huckel MO-theoretical calculations indicate a structur al change for the second oxidation product, since the energy of the pi 6:pi6-dictation is lower than that of the eta6:eta4-dictation. Reactiv ity 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, n ot even at elevated temperatures.