Mono-cyclopentadienyl complexes of lanthanum: synthesis and characterization of anilido derivatives

Use of the bulky cyclopentadienyl ligand [η5-C5H2(SiMe3)3-1,2,4]− (Cp‴) allows for the isolation of monomeric, mono-ring lanthanide species. As previously reported, (Cp‴)K reacts with LaI3(THF)4 (THF=tetrahydrofuran) in THF/pyridine to form the mono-ring complex (Cp‴)LaI2(py)3 (1) (py=pyridine); a minor product of this reaction is the bis-ring species (Cp‴)2LaI(py) (2). The solid state structure of 2 reveals a monomeric compound containing a pseudo-tetrahedral metal center exhibiting no unusual intramolecular contacts. Addition of one equiv of KNHAr (Ar=2,6-iPr2C6H3) to complex 1 in THF generates the mono-anilido compound (Cp‴)LaI(NHAr)(THF)2 (3), which may be converted to the more stable pyridine adduct (Cp‴)LaI(NHAr)(py)2 (4) by the addition of pyridine to 3. An X-ray crystal structure of 3 indicated a trigonal bipyramidal metal center with the anilido group oriented trans to the iodide atom (N1–La1–I1=123.1(3)°). A structural study on the bis-pyridine adduct 4 revealed a similar Cs-symmetric structure with a slightly increased Nanilido–La–I angle of 132.1(2)°. Addition of KNHAr to the di-iodo bipyridine adduct (Cp‴)LaI2(bipy)(py) (5), in which the two iodide atoms are cis-disposed, yields the mono-anilido complex (Cp‴)LaI(NHAr)(bipy)(py) (6) (bipy=2,2′-bipyridine); this compound may also be prepared by the addition of bipy to (Cp‴)LaI(NHAr)(py)2 (4). An X-ray diffraction study shows that the lanthanum center in 6 is octahedrally coordinated by a Cp‴ ring, an iodide, an anilido group, a pyridine molecule and two nitrogens of a bipy molecule. In this case, the anilido moiety and the iodide ligand are arranged in a cis fashion (Nanilido–La–I=111.2(2)°), resulting in a complex with C1 symmetry. Both (Cp‴)LaI(NHAr)(py)2 (4) and (Cp‴)LaI(NHAr)(bipy)(py) (6) are inactive as catalysts for the hydroamination/cyclization of 2-amino-hex-5-ene.