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

A series of new functionalized amido complexes of ytterbium, [Cp 2 YbNHR] 2 (R = 8-quinolyl(Qu) ( 1a ), 2-pyridyl(Py) ( 1b ), 2-aminophenyl ( 1c ), 3-amino- 2 -pyridyl ( 1d ), and Cp 2 Yb[NHC 6 H 4 (CH 2 NH 2 -2)] ( 1e ), have been synthesized by metathesis of Cp 2 YbCl and the corresponding amido lithium salts. Their reactivity toward carbodiimides has been investigated, in which multiple N−H activation behavior for metal-bound neutral NH 2 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 ( i Pr)] leads to the formal insertion of carbodiimide into the N−H bond of the Yb-bonded amido group to yield Cp 2 Yb[η 1:η 2 -RNC(NHR)NQu] 2 [R = Cy ( 2a ), i Pr ( 2b )]. Interestingly, treatment of 1b with RN C NR affords the unexpected products (Cp 2 Yb) 2 [μ-η 2:η 2 -PyNC(NR) 2 ](THF) [R = Cy ( 3a ), i Pr ( 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 Cp 2 Yb[η 1:η 2 -RNC(NHR)NC 6 H 4 NH 2 -2] [R = Cy ( 4a ), i Pr ( 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 Cp 2 Yb[η 1:η 2 -RNC(NHR)NC 6 H 4 {NC(NHR) 2 -2}] [R = Cy ( 5a ), i Pr ( 5b )], via the intraligand proton transfer from chelating NH 2 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 Cp 2 Yb[η 1:η 2 -CyNC(NHCy)NC 5 H 3 N{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 (Cp 2 Yb) 2 [η 2:η 3 -(CyN) 2 CNC 5 H 3 N{NC(NHCy) 2 -3}] ( 7 ) under reflux in THF and with liberation of a neutral diguanidine, C 5 H 3 N(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 Cp 2 Yb[η 2:η 1 -CyNC(NHCy)NC 6 H 4 (CH 2 NH 2 -2)] [R = Cy ( 8a ), i Pr ( 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.