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.