Coordination chemistry of [2 + 2] Schiff-base macrocycles derived from the dianilines [(2-NH2C6H4)2X] (X = CH2CH2, O): structural studies and ROP capability towards cyclic esters

Abstract

Reaction of the [2 + 2] Schiff-base macrocycles {[2-(OH)-5-(R)-C₆H₂-1,3-(CH)₂][CH₂CH₂(2-C₆H₄N)₂]}₂ (R = Me, L¹H₂; tBu, L²H₂) with FeBr₂ afforded the complexes [FeBr(L¹H₂)]₂[(FeBr₃)₂O]·2MeCN (1·2MeCN), [FeBr(L²H₂)][X] (X = 0.5(FeBr₃)₂O, 2·0.5MeCN, X = Br, 3·5.5MeCN), respectively. Reaction of L²H₂ with [KFe(OtBu)₃(THF)] (formed in situ from FeBr₂ and KOtBu), following work-up, led to the isolation of the complex [Fe(L²)(L²H)]·3MeCN (4·3MeCN), whilst with [CuBr₂] afforded [CuBr(L²H₂)][CuBr₂]·2MeCN (5·2MeCN). Attempts to form mixed Co/Ti species by reaction of [CoBrL²][CoBr₃(NCMe)] with TiCl₄ resulted in [L²H₄][CoBr₄]·2MeCN (6·2MeCN). Use of the related oxy-bridged Schiff-base macrocycles {[2-(OH)-5-(R)-C₆H₂-1,3-(CH)₂][O(2-C₆H₄N)₂]}₂ (R = Me, L³H₂; tBu, L⁴H₂) with CoBr₂ led to the isolation of the complexes [(CoBr)₂(L³)]·2C₃H₆O (7·2C₃H₆O), [Co(NCMe)₂(L⁴H₂)][CoBr₄]·5MeCN (8·5MeCN), [Co(NCMe)₆][CoBr₃(MeCN)]₂·2MeCN (9·2MeCN). For comparative structural/polymerisation studies, the complexes {CoBr(NCMe)L⁵}₂·2MeCN (10·2MeCN) and [Co(NCMe)₂L⁵]₂[CoBr₃(NCMe)]₂ (11), [FeBr(NCMe)L⁵]₂·2MeCN (12·2MeCN) where L⁵H = 2,6-(CHO)₂-4-tBu-C₆H₂OH, as well as the chelate-free salt [Fe(NCMe)₆][FeBr₃OFeBr₃] (13) have been isolated and structurally characterized. The ability of these complexes to act as catalysts for the ring opening polymerisation (ROP) of ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL) was investigated, as well as co-polymerisation of ε-CL with rac-lactide (r-LA) and vice versa.