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.

一系列新型官能化镱酰胺配合物，即[Cp₂YbNHR]₂（其中Cp为环戊二烯基（Cyclopentadienyl），R分别为8-喹啉基(Qu)（1a）、2-吡啶基(Py)（1b）、2-氨基苯基（1c）、3-氨基-2-吡啶基（1d），以及Cp₂Yb[NHC₆H₄(CH₂NH₂-2)]（1e）），通过二(环戊二烯基)氯化镱（Cp₂YbCl）与相应酰胺锂盐的复分解反应合成得到。研究了该系列配合物与碳二亚胺的反应活性，观测到金属键合的中性NH₂与阴离子含氮片段发生多重N-H键活化行为，该过程无需从镧系金属中心解离即可进行。上述结果为金属介导的伯胺单胍基化与双胍基化反应提供了全新的机理视角，并阐明了影响加成与质子化步骤化学选择性及区域选择性的关键因素。配合物1a与2当量的RN=C=NR[R为环己基(Cy)、异丙基(iPr)]反应时，碳二亚胺可形式上插入Yb键合的酰胺基团的N-H键中，得到Cp₂Yb[RNC(NHR)NQu]₂[R=Cy(2a)、iPr(2b)]。有趣的是，1b与RN=C=NR反应则得到意外产物(Cp₂Yb)₂[μ-η²:η²-PyNC(NR)₂](THF)[R=Cy(3a)、iPr(3b)]，其中THF为四氢呋喃（tetrahydrofuran），这是首例双阴离子胍基镧系配合物。配合物1c与2当量的RN=C=NR在四氢呋喃（THF）中室温反应，以良好收率分离得到单N-H加成产物Cp₂Yb[RNC(NHR)NC₆H₄NH₂-2][R=Cy(4a)、iPr(4b)]；而1c与4当量的RN=C=NR在相同条件下反应，则得到双N-H加成产物Cp₂Yb[RNC(NHR)NC₆H₄{NC(NHR)₂-2}][R=Cy(5a)、iPr(5b)]，反应路径为：配体上螯合的NH₂发生配体内质子转移至中间体4的胍基位点，生成新型酰胺配合物，随后第二个RN=C=NR插入到该酰胺基团的N-H键中。配合物5也可通过4与1当量的RN=C=NR反应制备得到。配合物1d与CyN=C=NCy以1:4的摩尔比反应，可得到红色晶体状的双加成产物Cp₂Yb[CyNC(NHCy)NC₅H₃N{NC(NHCy)₂-3}]（6），收率为63%。有趣的是，6在THF中回流条件下可转化为(Cp₂Yb)₂[η²:η³-(CyN)₂CNC₅H₃N{NC(NHCy)₂-3}]（7），同时释放出中性双胍C₅H₃N(NC(NHCy)₂)-2,3。实验证实，在生成5和6的过程中，质子转移优先发生于第二个碳二亚胺插入N-H键的步骤：将酸性较弱的1e与过量碳二亚胺在THF/甲苯（toluene）混合溶剂中反应，即便延长加热时间，也仅能得到单加成产物Cp₂Yb[CyNC(NHCy)NC₆H₄(CH₂NH₂-2)][R=Cy(8a)、iPr(8b)]。所有配合物均通过元素分析与光谱性质进行了表征，除2b、3b、4a及5b外，其余配合物的结构均通过X射线单晶衍射分析得以确定。