Synthesis and Characterization of <i>N</i>,<i>N</i>,<i>C</i> and <i>N</i>,<i>N</i>,<i>O</i> Tridentate β-Diketiminato Rare-Earth Metal Alkyl Complexes and Their Catalytic Performances on the Dimerization of Aldehydes or Terminal Alkynes

Abstract

Three β-diketiminato proligands incorporating a thiophene or tetrahydrofuran heterocyclic group, H2L1, H2L2, and HL3 (MeC(NDipp)CHC(Me)N(CH2)n-2-(HCG), HCG = C4H2S, n = 2, L1; n = 1, L2; HCG = C4H7O, n = 1, L3; Dipp = 2,6-iPr2C6H3), have been developed. Unusual N,N,C tridentate β-diketiminato rare-earth metal monoalkyl complexes L1RE(CH2SiMe3)(thf) (RE = Y (1a), Er (1b), Yb (1c), Lu (1d), thf = tetrahydrofuran) and L2Yb(CH2SiMe3)(thf) (2c) were achieved unexpectedly by the reactions of RE(CH2SiMe3)3(thf)2 with H2L1 and H2L2, respectively. In this process, the C–H bond activation of the thiophene ring occurred. In sharp contrast, the treatment of RE(CH2SiMe3)3(thf)2 with a tetrahydrofuran-functionalized β-diketiminato proligand HL3, under the same conditions, gave five-coordinate N,N,O tridentate β-diketiminato rare-earth metal dialkyl complexes L3RE(CH2SiMe3)2 (RE = Y (3a), Er (3b), and Yb (3c)). In addition, their unique catalytic performances have been described. The monoalkyl complexes exhibited high efficiency toward the dimerization of various aldehydes, providing a wide range of carboxylic esters in good to high yields under mild conditions. The dialkyl complexes could promote the head-to-head dimerization of terminal alkynes to afford conjugated 1,4-disubstituted enynes with excellent regio- and stereoselectivity (up to 100% Z-selectivity). More importantly, a rare type of rare-earth metal complexes [{L3RE(μ-C≡CPh)}2(μ-η2:η2-PhCCCCPh)] (RE = Y (6a), Er (6b)) containing two bridging alkynyl groups and a Z-butatrienediyl fragment were isolated and characterized by the reaction of N,N,O tridentate rare-earth metal dialkyls with phenylacetylene, which is unarguably responsible for the outcome of Z-configured enynes during alkyne dimerization.

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