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

Abstract

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 RNdCdNR [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 RNdCdNR 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 RNdCdNR 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 RNdCdNR 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 RNdCdNR insertion into the N-H bond of the resulting amido groups. Complexes 5 can also be obtained by reacting 4 with 1 equiv of RNdCdNR. 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 CyNdCdNCy 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.

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