Zirconium–Butadiene Bonded over a Planar Set of Oxygens Derived from Calix[4]arene and Its Behavior as a Source of Zirconium(II)

This report deals with the first examples of butadienes bonded to a Zr-O matrix provided by a calix[4]arene skeleton and their reactivity. The reaction of [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}ZrCl2], 2, with Mg(C4H6)(thf)(2) and Mg(Ph2C4H4)(thf)(3) led to [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}Zr(eta(4)-C4H6)], 3, and [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}Zr(eta(4)-Ph2C4H4)] 4. The butadiene fragment exhibits a pi(2),eta(4) bonding mode, as shown by an X-ray analysis on both compounds. Extended Huckel calculations confirmed the energetically preferred s-cis conformation and the pi(2),eta(4) bonding mode vs the sigma(2),pi,eta(4) one. The parent compound 3 of this series behaves, however, both as a source of zirconium(II) in displacement reactions or as a dialkyl derivative of zirconium(IV) in insertion reactions. In the former class of transformations, the reaction of 3 with Ph2CO and PhCOCOPh led to the dioxo metallacycles [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}Zr{-OC(Ph)(2)-C(Ph)(2)O-}], 5, and [{p-Bu-t-calix[4]-(OMe)(2)(O)2}Zr{-OC(Ph)=C(Ph)O-})], 6, respectively. Butadiene was also displaced by diphenylacetylene leading to [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}Zr(eta(2)-Ph2C2)], 7, and by a phenylnitrene source [PhN3] forming the dinuclear phenylimido-bridged complex [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}(2)Zr-2(mu-NPh)(2)], 8. Unlike at room temperature, Ph2CO reacted with 3 at low temperature without displacing the butadiene, but leading, instead, to the stepwise insertion of the ketone to form [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}Zr{CH2CH=CHCH2C(Ph)(2)O}], 10, and [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}Zr{-OC(Ph)(2)CH2CH=CHCH2C(Ph)(2)O-}], 11. Using 2 mol of acetone, [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}Zr{-OC(Me)(2)CH2CH=CHCH2C(Me)(2)O-}], 12, was obtained. MeCN and EtCN inserted in a single Zr-C bond (of a formal sigma(2),pi,eta(4) structure) yielding [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}Zr{-CH2CH=CHCH2C(Me)=N-}], 13, and [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}Zr{-CH2CH=CHCH2C(Et)=N-}], 14. (BuNC)-N-t engaged 3 in a multistep insertion reaction leading to [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}Zr{-(Bu-t)NC(C4H6)C(C=NBut)N(Bu-t)-}], 15, via a pathway which has been observed in the reaction of (BuNC)-N-t with dialkylzirconium calix[4]arene derivatives. The sigma(2) behavior of butadiene in 3 was further verified in the reaction with H2O to give [{p-Bu-t-calix[4]-(OMe)(2)(O)(2)}(2)Zr-2(mu-O)(2)], 9, and butenes.