Organometallic reactivity on a calix[4]arene oxo surface. Synthesis and rearrangement of Zr-C functionalities anchored to a calix[4]arene moiety

This report concerns the formation and the main properties of Zr-C functionalities supported by the O-4 matrix of either the monoalkylated (trianionic) or dialkylated (dianionic) forms of [p-Bu-t-calix[4]arene] (1). The synthesis of the starting materials [p-Bu-t-calix[4]-(OMe)(2)(O)(2)ZrCl2 (4) and [{mu-p-Bu-t-calix[4]-(OMe)(O)(3)}(2) Zr2Cl2] (10) has been performed via the lithiation of the corresponding protic forms, [p-Bu-t-calix[4]-(OMe)(2)(OH)(2)] (2) and [p-Bu-t-calix[4]-(OMe)(OH)(3)] (8), followed by reaction with ZrCl4 .(thf)(2). Complex 4 undergoes a facile base-induced (base = Et3N, Py) demethylation, leading to the monomethoxy derivative [p-Bu-t-calix[4]-(OMe)(O)(3)ZrCl2](-)[MeNEt3](+) (5) and ionization of the Zr-Cl bond to give [p-Bu-t-calix[4]-(OMe)(2)(O)(2)Zr(OTf)(2)] (6). The alkylation of 4 led to the corresponding dialkylaryl derivatives [p-Bu-t-calix[4]-(OMe)(2)(O)(2)ZrR2](R = Me, 12; R = CH2Ph, 13; R = CH2-SiMe3, 14; R = p-MeC6H4, 15]. They undergo a base-induced demethylation and dealkylation illustrated by the obtention of [p-Bu-t-calix[4]-(OMe)(O)(3)Zr(CH2Ph)(Py)] (18), while the thermal decomposition gave a particularly interesting result in the case of 15, with the formation of the corresponding eta(2)-benzyne, [p-Bu-t-calix[4]-(OMe)(2)(O)(2)-Zr(eta(2)-MeC6H3)] (16). Cationic alkyl derivatives of zirconium have been obtained, in the case of 13, using the monoelectronic oxidizing agent [Cp2Fe](+) or a strong Lewis acid such as [B(C6F5)(3)], [p-Bu-t-calix[4]-(OMe)(2)(O)(2)-Zr(CH2Ph)]+BPh4- (19); [p-Bu-t-calix[4]-(OMe)(2)(O)(2)Zr(CH2Ph)(thf)]+BPh4- (20); and [p-Bu-t-calix[4]-(OMe)(2)(O)(2)-Zr(CH2Ph)](+) [B(C6F5)(3)(CH2Ph)](-) (21). The cationic alkyl derivatives undergo easy demethylation by pyridine to give 18. The alkylation of 10 with LiPh led to a dimeric dimetallic aryl compound, [{mu-p-Bu-t-calix[4]-(OMe)(O)(3)}(2)-Zr2Ph2] (22), where the calix[4]arene unit bridges two metal atoms and the methoxy groups are only weakly bound to the metal. The spectator methoxy group binds strongly to the metal in the monomeric eta(2)-iminoncyl [p-Bu-t-calix[4]-(OMe)(O)(3)Zr((BuN)-N-t=CPh)] (23), formed from the migratory insertion of (BuNC)-N-t into the Zr-Ph in 22. The proposed structures have been supported by X-ray analyses carried out on 4, 6, 13, 16, 22, and 23.