American Chemical Society
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Multiple N−H Bond Activation:  Synthesis and Reactivity of Functionalized Primary Amido Ytterbium Complexes

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posted on 2020-04-03, 05:39 authored by Chengfu Pi, Zhengxing Zhang, Zhen Pang, Jie Zhang, Jun Luo, Zhenxia Chen, Linhong Weng, Xigeng Zhou
A series of new functionalized amido complexes of ytterbium, [Cp2YbNHR]2 (R = 8-quinolyl(Qu) (1a), 2-pyridyl(Py) (1b), 2-aminophenyl (1c), 3-amino-2-pyridyl (1d), and Cp2Yb[NHC6H4(CH2NH2-2)] (1e), have been synthesized by metathesis of Cp2YbCl and the corresponding amido lithium salts. Their reactivity toward carbodiimides has been investigated, in which multiple N−H activation behavior for metal-bound neutral NH2 and anionic nitrogen-containing fragments, a property that is expressed without dissociation from the lanthanide center, is observed. These results provide an alternative mechanistic insight for the metal-mediated mono- and diguanylation of primary amines and elucidate factors that affect the chemo- and regioselectivities of the addition and protonation steps. Reaction of 1a with 2 equiv of RNCNR [R = cyclohexyl (Cy), isopropyl (iPr)] leads to the formal insertion of carbodiimide into the N−H bond of the Yb-bonded amido group to yield Cp2Yb[η12-RNC(NHR)NQu]2 [R = Cy (2a), iPr (2b)]. Interestingly, treatment of 1b with RNCNR affords the unexpected products (Cp2Yb)2[μ-η22-PyNC(NR)2](THF) [R = Cy (3a), iPr (3b)], representing the first example of dianionic guanidinate lanthanide complexes. The reaction of 1c with 2 equiv of RNCNR in THF at room temperature leads to the isolation of the single N−H addition products Cp2Yb[η12-RNC(NHR)NC6H4NH2-2] [R = Cy (4a), iPr (4b)] in satisfied yields, while treatment of 1c with 4 equiv of RNCNR under the same conditions gives the double N−H addition products Cp2Yb[η12-RNC(NHR)NC6H4{NC(NHR)2-2}] [R = Cy (5a), iPr (5b)], via the intraligand proton transfer from chelating NH2 to the guanidinate group of 4 to give new amido intermediates, followed by a second RNCNR insertion into the N−H bond of the resulting amido groups. Complexes 5 can also be obtained by reacting 4 with 1 equiv of RNCNR. The double-addition product Cp2Yb[η12-CyNC(NHCy)NC5H3N{NC(NHCy)2-3}] (6) could be obtained as red crystals from the 1:4 reaction between 1d and CyNCNCy in 63% yield. Interestingly, 6 can be converted into (Cp2Yb)223-(CyN)2CNC5H3N{NC(NHCy)2-3}] (7) under reflux in THF and with liberation of a neutral diguanidine, C5H3N(NC(NHCy)2)-2,3. A preference of the proton transfer to the second carbodiimide insertion into the N−H bond in the formation of 5 and 6 has been confirmed by the fact that treatment of weaker acidic 1e with an excess of carbodiimides in THF/toluene, even with prolonged heating, provides only the monoaddition product Cp2Yb[η21-CyNC(NHCy)NC6H4(CH2NH2-2)] [R = Cy (8a), iPr (8b)]. All complexes were characterized by elemental analysis and spectroscopic properties. The structures of all complexes except 2b, 3b, 4a, and 5b were also determined through X-ray crystal diffraction analysis.