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 }ZrCl 2 ], 2, with Mg(C 4 H 6 )(thf) 2 and Mg(Ph 2 C 4 H 4 )(thf) 3 led to [{ p- Bu t -calix[4]-(OMe) 2 (O) 2 }Zr(η 4 -C 4 H 6 )], 3, and [{ p- Bu t -calix[4]-(OMe) 2 (O) 2 }Zr(η 4 -Ph 2 C 4 H 4 )], 4 . The butadiene fragment exhibits a π 2,η 4 bonding mode, as shown by an X-ray analysis on both compounds. Extended Hückel calculations confirmed the energetically preferred s - cis conformation and the π 2,η 4 bonding mode vs the σ 2,π,η 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 Ph 2 CO 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(η 2 -Ph 2 C 2 )], 7, and by a phenylnitrene source [PhN 3 ] forming the dinuclear phenylimido-bridged complex [{ p- Bu t -calix[4]-(OMe) 2 (O) 2 } 2 Zr 2 (μ-NPh) 2 ], 8 . Unlike at room temperature, Ph 2 CO 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{CH 2 CH CHCH 2 C(Ph) 2 O}], 10, and [{ p- Bu t -calix[4]-(OMe) 2 (O) 2 }Zr{−OC(Ph) 2 CH 2 CH CHCH 2 C(Ph) 2 O−}], 11 . Using 2 mol of acetone, [{ p- Bu t -calix[4]-(OMe) 2 (O) 2 }Zr{−OC(Me) 2 CH 2 CH CHCH 2 C(Me) 2 O−}], 12, was obtained. MeCN and EtCN inserted in a single Zr−C bond (of a formal σ 2,π,η 4 structure) yielding [{ p- Bu t -calix[4]-(OMe) 2 (O) 2 }Zr{−CH 2 CH CHCH 2 C(Me) N−}], 13, and [{ p- Bu t -calix[4]-(OMe) 2 (O) 2 }Zr{−CH 2 CH CHCH 2 C(Et) N−}], 14 . Bu t NC engaged 3 in a multistep insertion reaction leading to [{ p- Bu t -calix[4]-(OMe) 2 (O) 2 }Zr{−(Bu t )NC(C 4 H 6 )C(C NBu t )N(Bu t )−}], 15, via a pathway which has been observed in the reaction of Bu t NC with dialkylzirconium calix[4]arene derivatives. The σ 2 behavior of butadiene in 3 was further verified in the reaction with H 2 O to give [{ p- Bu t -calix[4]-(OMe) 2 (O) 2 } 2 Zr 2 (μ-O) 2 ], 9, and butenes.