Synthesis, Structures and Reactivity of Lanthanoid(II) Formamidinates of Varying Steric Bulk

New reactive, divalent lanthanoid formamidinates [Yb(Form) 2 (thf) 2 ] (Form=[RNCHNR]; R= o ‐MeC 6 H 4 ( o ‐TolForm; 1 ), 2,6‐Me 2 C 6 H 3 (XylForm; 2 ), 2,4,6‐Me 3 C 6 H 2 (MesForm; 3 ), 2,6‐Et 2 C 6 H 3 (EtForm; 4 ), o ‐PhC 6 H 4 ( o ‐PhPhForm; 5 ), 2,6‐ i Pr 2 C 6 H 3 (DippForm; 6 ), o ‐HC 6 F 4 (TFForm; 7 )) and [Eu(DippForm) 2 (thf) 2 ] ( 8 ) have been prepared by redox transmetallation/protolysis reactions between an excess of a lanthanoid metal, Hg(C 6 F 5 ) 2 and the corresponding formamidine (HForm). X‐ray crystal structures of 2 – 6 and 8 show them to be monomeric with six‐coordinate lanthanoid atoms, chelating N , N ′‐Form ligands and cis ‐thf donors. However, [Yb(TFForm) 2 (thf) 2 ] ( 7 ) crystallizes from THF as [Yb(TFForm) 2 (thf) 3 ] ( 7 a ), in which ytterbium is seven coordinate and the thf ligands are “pseudo‐meridional”. Representative complexes undergo CX (X=F, Cl, Br) activation reactions with perfluorodecalin, hexachloroethane or 1,2‐dichloroethane, and 1‐bromo‐2,3,4,5‐tetrafluorobenzene, giving [Yb(EtForm) 2 F] 2 ( 9) , [Yb( o ‐PhPhForm) 2 F] 2 ( 10) , [Yb( o ‐PhPhForm) 2 Cl(thf) 2 ] ( 11) , [Yb(DippForm) 2 Cl(thf)] ( 12) and [Yb(DippForm) 2 Br(thf)] ( 16) . X‐ray crystallography has shown 9 to be a six‐coordinate, fluoride‐bridged dimer, 12 and 16 to be six‐coordinate monomers with the halide and thf ligands cis to each other, and 11 to have a seven‐coordinate Yb atom with “pseudo‐meridional” unidentate ligands and thf donors cis to each other. The analogous terbium compound [Tb(DippForm) 2 Cl(thf) 2 ] ( 13 ), prepared by metathesis, has a similar structure to 11 . CBr activation also accompanies the redox transmetallation/protolysis reactions between La, Nd or Yb metals, Hg(2‐BrC 6 F 4 ) 2 , and HDippForm, yielding [Ln(DippForm) 2 Br(thf)] complexes (Ln=La ( 14 ), Nd ( 15 ), Yb ( 16 )).