Zirconium and hafnium complexes containing N-alkyl substituted amine biphenolate ligands: coordination chemistry and living ring-opening polymerization catalysis

Abstract

The coordination chemistry of zirconium and hafnium complexes containing the tridentate amine biphenolate ligands [RN(CH₂-2-O-3,5-C₆H₂(tBu)₂)₂]²⁻ ([R-ONO]²⁻; R = tBu (1a), iPr (1b), nPr (1c)) featuring distinct N-alkyl substituents is described. Alcoholysis of Zr(OiPr)₄(HOiPr) or Hf(OiPr)₄(HOiPr) with H₂[1a] in diethyl ether solutions at −35 °C generates the corresponding five-coordinate [1a]M(OiPr)₂ (M = Zr (2a), Hf (3a)) in high isolated yield. Similar reactions employing H₂[1b] produce six-coordinate [1b]M(OiPr)₂(HOiPr) (M = Zr (2b·HOiPr), Hf (3b·HOiPr)) as an isopropanol adduct. Repetitive trituration of 2b·HOiPr and 3b·HOiPr with diethyl ether gives five-coordinate 2b and 3b, respectively. Treatment of M(OiPr)₄(HOiPr) with H₂[1c] under similar conditions affords six-coordinate [1c]M(OiPr)₂(HOiPr) (M = Zr (2c·HOiPr), Hf (3c·HOiPr)), subsequent recrystallization of which from acetonitrile–diethyl ether solutions leads to acetonitrile adducts 2c·MeCN and 3c·MeCN. Reactivity studies of these zirconium and hafnium complexes revealed that they are all active catalysts for ring-opening polymerization of ε-caprolactone. Among them, the N-isopropyl derived complexes are most reactive. Polymerizations catalyzed by 2b, 3b and 3c·MeCN were proved to be living. The X-ray structures of 2a·HOiPr, 2a·MeCN, 2c·HOiPr, 2c·MeCN, and 3c·MeCN are presented.