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


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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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