Mono-, di- and tetra-zinc complexes derived from an amino-benzotriazole phenolate ligand containing a bulkier N-alkyl pendant arm: synthesis, structure and catalysis for ring-opening polymerization of cyclic esters

Zinc complexes constructed from the amino-modified benzotriazole phenol pro-ligand, 2-(2H-benzotriazol-2-yl)-6-((diisopropylamino) methyl)-4-(2,4,4-trimethylpentan-2-yl) phenol ((BTP)-B-C8DIA-H, 1), were synthesized stepwise and structurally characterized. The reaction of (BTP)-B-C8DIA-H (1) with one equivalent of diethyl zinc (ZnEt2) generates a dimeric and four-coordinated zinc complex, [(mu-(BTP)-B-C8DIA)ZnEt](2) (2), which is doubly bridged by two phenolate groups of (BTP)-B-C8DIA ligands. Further reaction of 2 with benzyl alcohol (BnOH) in stoichiometric proportions affords a tetranuclear zinc benzylalkoxide complex [(mu-OBn)((BTP)-B-C8DIA) Zn] 4 (3) that possesses a saddle-shaped core with four mu(2)-bridging benzylalkoxy groups upon four Zn centres. Interestingly, the di-nuclear Zn alkoxide [(mu-OBn)((BTP)-B-C8DIA) Zn(DMAP)] 2 (4) could be prepared by treatment of 3 with a stoichiometric amount of 4-(dimethylamino) pyridine (DMAP). ZnEt2 reacts with two equivalents of 1 in the presence of DMAP (1.0 mol equiv.) to yield a five-coordinated mononuclear zinc complex, [((BTP)-B-C8DIA)(2)Zn(DMAP)] (5). All complexes adopt an N, O-bidentate coordination mode from the phenoxy oxygen atom and benzotriazole nitrogen atom, in which the nitrogen atom of the pendent arm substituent is not coordinated to the zinc centre. Ring-opening polymerization of epsilon-caprolactone and beta-butyrolactone catalysed by 2 and 3 was investigated.