Synthesis of bismuth and antimony complexes of the “larger” calix[n]arenes (n = 6–8); from mononuclear to tetranuclear complexes

Abstract

A series of calix[n]arene (n = 6–8) bismuth and antimony complexes were synthesized and fully characterized by NMR, X-ray, IR, UV-Vis and elemental analysis. The monobismuth calix[6]arene complex [Bi{^tBuC6(H)₃}]₂1 was prepared by the reaction of para-tert-butylcalix[6]arene (^tBuC6(H)₆) with one equivalent of Bi[N(SiMe₃)₂]₃. Complex 1 featured a Bi₂(μ-O)₂ central core similar to other bismuth calixarene complexes prepared by our group. Reaction of calix[6]arene (HC6(H)₆) with two equivalents of Bi(O^tBu)₃ yielded different outcomes depending on the reaction solvent. If THF was used, complex [Bi{HC6(H)₃}] 2 was obtained in 72% yield; however, when toluene was used, complexes 2 and [Bi₂{HC6}] 3 were isolated in 23 and 57% yields, respectively. Mononuclear complexes 1 and 2 displayed dimeric structures in the solid state with cone-like conformations for the calixarene ligands. The ¹H NMR spectrum of complex 2 displays patterns for an asymmetric structure with two signals in a 2 : 1 ratio for the unreacted OH groups. Treatment of calix[6]arenes RC6(H)₆ (R = H, ^tBu) with two equivalents of SbR₃ (R = O^tBu, NMe₂) produced dinuclear complexes [Sb₂{HC6}] 4, and [Sb₂{^tBuC6}] 5, respectively. The ¹H NMR spectra for the dinuclear complexes 3, 4, and 5 showed the characteristic calixarene pattern for a 1,2,3-alternate conformer. In the process of recrystallization of complex 4 an unexpected trimetallic complex with composition [Sb₃O₂{HC6(H)}] 4a was obtained in low yield. Treatment of para-tert-butylcalix[7]arene (^tBuC7(H)₇) with two equivalents of Bi(O^tBu)₃ produced the bimetallic complex [Bi₂O{^tBuC7(H)₃}]₂6. Complex 6 contains an overall Bi₄O₂(OAr)₈ core system with a structural resemblance to other bimetallic bismuth calixarene complexes reported by our group. The larger para-benzylcalix[8]arene (BnC8(H)₈) and calix[8]arene (HC8(H)₈) reacted with excess Bi(O^tBu)₃ to produce the tetranuclear complexes [Bi₄O₂{HC8}] 7 and [Bi₄O₂{BnC8}] 8, respectively. The solid state structure of complex 8 featured a dimeric unit with the calixarene ligands in pinched-cone conformation and displaying an overall Bi₈O₄(OAr)₁₆·H₂O core. No metal π-arene interactions were observed.