Synthesis, X-ray structures and reactivity of calix[5]arene bismuth(iii) and antimony(iii) complexes

Abstract

A series of calix[5]arene bismuth(III) and antimony(III) mono- and bimetallic complexes were synthesized and fully characterized by NMR, X-ray, IR, mp, UV-Vis and elemental analysis. Reaction of p-tert-butylcalix[5]arene (^tBuC5(H)₅) trianionic salts M′₃·^tBuC5(H)₂ (M′ = Li, Na, K) with MCl₃ (M = Bi, Sb) yielded monometallic complexes [Bi{^tBuC5(H)₂}] 1 and [Sb{^tBuC5(H)₂}] 2, respectively. ¹H NMR spectra of both complexes showed two remaining OH groups available for further reactivity. Alternatively, complexes 1 and 2 can be obtained by reacting ^tBuC5(H)₅ in a 1 : 1 ratio with M(O^tBu)₃, but the yields are lower. When the ^tBuC5(H)₅ lower rim monobenzyl ether [^tBuC5(Bn)(H)₄] is treated in a 1 : 1 ratio with Bi(O^tBu)₃, the monometallic complex [Bi{^tBuC5(Bn)(H)}]₂3 is prepared. If, however, [^tBuC5(Bn)(H)₄] reacts with Sb(NMe₂)₃ or Sb(O^tBu)₃ in a 1 : 2 ratio the production of the bimetallic complex [Sb₂O{^tBuC5(Bn)}] 4 is observed. p-Benzylcalix[5]arene (BnC5(H)₅) reacts with excess Bi(O^tBu)₃ to produce the bimetallic complex [Bi₂O{BnC5(H)}]₂5. ¹H NMR spectra of 5 display patterns characteristic for a cone conformer in solution. Treatment of calix[5]arene [HC5(H)₅] with one equivalent of Bi[N(SiMe₃)₂]₃ or with 0.75 equivalents of Sb(NMe₂)₃ yields bimetallic complexes [Bi₂O{HC5(H)}] 6 and [Sb₂O{HC5(H)}] 7, respectively. The reactivity of monometallic complexes 1 and 2 was tested in order to investigate the availability of their remaining OH groups. Treatment of 1 with Bi(O^tBu)₃ at ambient temperature yields bimetallic complex [Bi₂O{^tBuC5(H)}]₂8 while the reaction of complex 2 with Sb(O^tBu)₃ in a 1 : 1 ratio produces complex [Sb₂O{^tBuC5(H)}] 9. The crystal structures of the monometallic bismuth complexes 1 and 3 display dimeric units with the calixarene ligands in distorted cone and “paco-in” conformations, respectively. Complexes 4, 7 and 9 are all monomeric units, displaying [(RO)₂Sb]₂(μ-O) cores and the calixarene ligands in 1,2-alternate conformation. The dimeric units of bimetallic complexes 5 and 8 contain Bi₄O₂(OR)₈ core structures that force the calixarene ligands to adopt a flattened cone conformation.