Unprecedented reaction of bridged bis(guanidinate) lanthanide complexes: sterically induced deprotonation

Reaction of LnCl3 with a lithium salt of bridged bis(guanidinate) bearing a rigid naphthalene linker [Li2{LH2}2Li2] (LH2 = 1,8-C10H6{NC(N(i)Pr)(NH(i)Pr)}2) (1), which was synthesized by the reaction of 1,8-diaminonaphthalene with 2 equiv. of (n)BuLi, followed by treatment with 2 equiv. of N,N'-diisopropylcarbodiimide, yielded the chlorides [LnCl{LH2}] (Ln = Yb (2), Y (3)) in good yields. Metathesis reaction of 2 and 3 with NaN(SiMe3)2 did not afford the corresponding amide complexes, but the deprotonation products of [Ln{LH}]2 (LH = 1,8-C10H6{NC(N(i)Pr)(NH(i)Pr)}{NC(N(i)Pr)2}; Ln = Yb (4), Y (5)) were isolated instead. The LH in 4 and 5 both act as a bridging ligand binding to two metals in a μ-η(1):η(2):η(2) fashion, and the re-arrangement of LH occurred during the reaction. The size of the amido group was found to have a great influence on the outcome of the metathesis reaction. Reaction of 2 with bulky NaNH(C6H3(i)Pr2-2,6) afforded 4 as the only product, whereas the same reaction with less bulky LiNH(C6H4Cl-4) and LiNH(C6H4CH3-4) led to the corresponding amides, [Yb{LH2}(NHC6H4Cl-4)]2 (6) and [Yb{LH2}(NHC6H4CH3-4)]2 (7), respectively. The re-arrangement of ligands was also observed in both cases. A possible pathway for the deprotonation of LH2 was discussed. Molecular structures of 1-7 were determined by X-ray single crystal analysis.