Low coordinate lanthanide(ii) complexes supported by bulky guanidinato and amidinato ligands

Abstract

The preparation of a series of homoleptic, four-coordinate lanthanide(II) complexes, [Ln(Priso)₂] (Ln = Sm, Eu or Yb) incorporating the bulky guanidinate ligand Priso⁻ ([(ArN)₂CNPrⁱ₂]⁻, Ar = 2,6-diisopropylphenyl) is described. X-ray crystallography shows the complexes to be isostructural and to exhibit coordination geometries midway between tetrahedral and planar. Comparisons between the geometries of the complexes and those of the bulkier systems, [Ln(Giso)₂] (Giso⁻ = [(ArN)₂CNCy₂]⁻, Cy = cyclohexyl) are discussed. Attempts to prepare the less hindered amidinate complexes, [Ln(Piso)₂] (Piso⁻ = [(ArN)₂CBu^t]⁻), were not successful, but did give rise to the heteroleptic complex, [(κ¹-N,η⁶-Piso)Sm(THF)(μ-I)₂Sm(κ¹-N,η⁶-Piso)]. Whereas the amidinate ligands in this complex chelate the samarium centre in an κ¹-N,η⁶-Ar-fashion, in the closely related complex, [{(κ²-N,N′-Priso)Yb(THF)(μ-I)}₂], the ytterbium atoms are κ²-N,N′-chelated by the guanidinate ligand. The facility of the planar four-coordinate complex, [Sm(Giso)₂], to act as a one-electron reducing agent towards a variety of unsaturated substrates has also been explored. The complex has been shown to selectively reductively couple CS₂, via a C–S bond formation, to give the samarium(III) dimer, [(Giso)₂Sm(μ-η²-:η²-S₂CSCS)Sm(Giso)₂].