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.