Synthesis and characterization of zinc complexes supported by NHC-based CNN- and CNP-tridentate ligands and their catalysis in the ring-opening polymerization of rac-lactide and ε-caprolactone

A series of zinc complexes bearing NHC-based C,N,N- or C,N,P-tridentate ligands were synthesized and characterized and the catalysis of the complexes toward the ring-opening polymerization of rac-lactide was evaluated. The ligand precursors [2-(CHN(R)(CH)(2)N)C6H4N=P(Pb)(2)CH2(Ph)(2)CH2Py](+) I- (2a, R = Me; 2b, R = iPr) prepared from [2-N3C6H4(N(CH)(2)N(R)CH)](+) I- (R = Me or iPr) and Ph2PCH2Py were successively treated with nBuLi and ZnEt2 to afford corresponding zinc complexes [Zn(Et){2-(CN(R)(CH)(2)N)(C6H4NP(Ph)(2)=CHpy}] (3a, R = Me; 3b, R = iPr). Similar treatment of [2-(CHN(Me)(CH)(2)N)C6H4N P(Ph)(2)CH2PPh2]I-+(-) (4a) (prepared from [2-N3C6H4(N(CH)(2)N(Me)CH)(+) I- and Ph2PCH2PPh2) with nBuLi and then ZnEt2 gave [Zn(Et){2-(CN(Me)(CH)(2)N)C6H4NP(Ph)(2)=CHPPh2)] (5). Treatment of [2-(CHN(R)(CH2N)C6H4N-P(Ph)(2)CH2PPh2]X-+(-) (4a, R = Me, X = l; 4h, R = Bn, X = Br; 4c, R = iPr, X = l) with p-MeC6H4N3 resulted in [2-(CHN(R)(CH)(2)N)C6H4N P(Ph)(2)CH2F(Ph)(2)=N(p-MeC6H4)(+) X-(6a, R = Me, X = l; 6b, R = Bn, X = Br; 6c, R = iPr, X = l). The reaction of 6a with nBuLi and ZnEt2 in sequence produced IZn(Et){2-(CN(Me)(CH)(2)N)C6H4NP(Ph)(2)=C1IP(Ph)(2)=N(p-McC(6)H(4))}1 (7). Whereas the reaction of 6a c with an equimolar amount of ZnEt2 gave [Zn(X){2-(ON(R)(CH)(2))C6H4NP(Ph)(2)=CHP(Ph)(2)=N(p-MeC6H4)}1(8a, R = Me, X = l; 8b, R = Bn, X = Br; 8c, R = iPr, X = l). All new compounds were characterized by H-1, C-13 and P-31 NMR spectroscopy and elemental analyses. The structures of complexes 3a, 5 and 8b were additionally characterized by single crystal X-ray diffraction. In the presence of BnOH complexes 3a, 3b, 5 and 7 catalyze the ring-opening polymerization of rac-lactide at room temperature and 3a, 3b, and 5 exhibited high activity. Complexes 8a c are inactive under the same conditions. Complexes 3a and 5 were also tested for the catalysis in the ROP of epsilon-caprolactone and both showed good catalytic activity. (C) 2015 Elsevier B.V. All rights reserved.