Synthesis and characterization of organolanthanidocene(III) (Ln = La, Ce, Pr, Nd) complexes containing the 1,4-cyclohexa-2,5-dienyl ligand (benzene 1,4-dianion):: Structures of [K([18]-crown-6)][Ln{η5-C5H3(SiMe3)2-1,3}2(C6H6)] [Cp" = η5-C5H3(SiMe3)2-1,3; Ln = La, Ce, Nd]

The new tricyclopentadienyllanthanoid metal(III) complexes [LnCp"(3)] are obtained in good yields from the appropriate anhydrous metal(III) triflate or chloride and 3/2 MgCp"(2) or 3 KCp" in tetrahydrofuran at ambient temperature [Cp" = eta(5)-C5H3(SiMe3)(2)-1,3 and Ln = La (1) or Pr (3)]. Treatment of the appropriate compound 1 or 3, or [CeCp"(3)] (2) or [NdCp"(3)] (4), with 2 equiv of each ([18]-crown-6) and potassium in benzene at ambient temperature affords the red or red-brown (7) crystalline salts [K([18]-crown-6)][Ln{eta(5)-C5H3(SiMe3)(2)-1,3}(2)-(C6H6)][Ln = La (5), Ce (6), Pr (7), Nd (8)], with [K([18]crown-6)][Cp''] as a coproduct. Each of 5-8 is soluble in hot benzene, which allowed the H-1 NMR spectra to be recorded. The salt 5 is diamagnetic, consistent with its containing a lanthanate(III) anion. For 6-8, the H-1 NMR spectral signals of the ([18]-crown-6) moiety are only slightly paramagnetically shifted. Hydrolysis of 5 (or 6-8) yields cyclohexa-1,4-diene. The molecular structures of the isomorphous salts 5, 6, and 8 reveal that each comprises a tight ion pair, a C6H6 ligand bridging the K and Ln atoms. The potassium atom has close contacts to the six crown ether oxygen atoms and the centroids of the 2,3- and 5,6-carbon atoms of the C6H6 ligand. The latter is boat-shaped, the shortest C-Ln distances being those to the 1- and 4-carbon atoms; the endocyclic bond angles at the 1- and 4-carbon atoms are narrower (110.65 +/- 1.15 degrees) than those at the other four (122.4 +/- 1.2 degrees), while the four C-C bonds to the 1- and 4-carbon atoms are longer (1.46 +/- 0.02 Angstrom) than the remaining two (1.350 +/- 0.013 Angstrom). The reaction of [LaCp"(3)] (1) with K and ([18]-crown-6) in benzene, monitored by EPR spectroscopy, reveals the presence of at least four La(II) paramagnetic intermediate species prior to formation of 5.