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

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[η1:η2-RNC(NHR)NQu]2 [R = Cy
(2a), iPr (2b)]. Interestingly, treatment of 1b with RNCNR affords the unexpected products (Cp2Yb)2[μ-η2:η2-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[η1:η2-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[η1:η2-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[η1:η2-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)2[η2:η3-(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[η2:η1-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.

一系列新型功能化的铒酰胺复合物[Cp2YbNHR]2（R = 8-喹啉基（Qu），2-吡啶基（Py），2-氨基苯基（1c），3-氨基-2-吡啶基（1d），以及Cp2Yb[NHC6H4(CH2NH2-2)]（1e）]已通过Cp2YbCl与相应的酰胺锂盐的置换反应合成。对这些复合物对亚胺的反应活性进行了研究，观察到金属配位中性氨基和阴离子含氮片段的N-H激活行为，该性质无需从镧系中心解离即可表达。这些结果为金属介导的初级胺的单和二胍化反应提供了另一种机制上的见解，并阐明了影响加成和质子化步骤的化学和区域选择性的因素。1a与2等摩尔的RN=C(NR)（R = 环己基（Cy），异丙基（iPr））反应导致亚胺正式插入到Yb键合酰胺基团的N-H键中，生成Cp2Yb[η1:η2-RNC(NHR)NQu]2 [R = Cy（2a），iPr（2b）]。有趣的是，1b与RN=C(NR)反应得到意外产物（Cp2Yb）2[μ-η2:η2-PyNC(NR)2](THF) [R = Cy（3a），iPr（3b）]，这是第一个双阴离子胍化镧系复合物的例子。1c在室温下与2等摩尔的RN=C(NR)在THF中的反应导致单N-H加成产物Cp2Yb[η1:η2-RNC(NHR)NC6H4NH2-2] [R = Cy（4a），iPr（4b）]以满意的产率被分离出来，而在相同条件下，1c与4等摩尔的RN=C(NR)反应则得到双N-H加成产物Cp2Yb[η1:η2-RNC(NHR)NC6H4{NC(NHR)2-2}] [R = Cy（5a），iPr（5b）]，这是通过内配体质子转移从螯合氨基到4的胍化基团，从而产生新的酰胺中间体，随后是第二个RN=C(NR)插入到生成的酰胺基团的N-H键中。复合物5也可以通过4与1等摩尔的RN=C(NR)反应得到。双加成产物Cp2Yb[η1:η2-CyNC(NHCy)NC5H3N{NC(NHCy)2-3}]（6）可以从1d与CyN=C(NCy)的1:4反应中，以63%的产率得到红色晶体。有趣的是，6可以在THF回流下，并释放中性双胍化物C5H3N(NC(NHCy)2)-2,3的情况下转化为（Cp2Yb）2[η2:η3-(CyN)2CNC5H3N{NC(NHCy)2-3}]（7）。通过实验证实，在形成5和6的过程中，质子转移优先于第二个亚胺插入到N-H键中。所有复合物均通过元素分析和光谱性质进行了表征。除了2b、3b、4a和5b之外的所有复合物的结构也通过X射线晶体衍射分析确定。