Mono(cycloheptatrienyl) Tantalum Chemistry: Synthesis and Characterization of New Tantalum Halide, Hydride, and Alkyl Species

The new tantalum(II) complex (η6-C7H8)TaCl2(PMe3)2 (1) was synthesized by the reduction of TaCl5 with n-butyllithium in the presence of PMe3 and cycloheptatriene. Compound 1 adopts a four-legged piano stool structure in which the tantalum center is bound to a η6-cycloheptatriene ring in addition to two chlorides and two phosphine ligands in a transoid arrangement. Treatment of 1 with methyllithium results in a loss of the equivalents of HCl and formation of the η7-cycloheptatrienyl complex (η7-C7H7)TaCl(PMe3)2 (2), whereas treatment of 1 with sodium or sodium borohydride affords small amounts of the η5-cycloheptadienyl complex (η5-C7H9)TaCl2(PMe3)2 (3). Compound 2 adopts a three-legged piano stool structure; the η7-C7H7 ring is fully aromatic and planar. The molecular structure of 3 is similar to that of 1, except for the η5 binding mode of the seven-membered ring. Treatment of the previously described sandwich compound (C5Me5)Ta(C7H7) with allyl bromide affords the tantalum(V) product (C5Me5)Ta(C7H7)Br (4), which reacts with LiAlH4 to give the tantalum(V) hydride (C5Me5)Ta(C7H7)H (5). Compound 4 also reacts with alkylating agents to generate the methyl, allyl, and cyclopropyl complexes (C5Me5)Ta(C7H7)Me (6), (C5Me5)Ta(C7H7)(η1-CH2CH═CH2) (7), and (C5Me5)Ta(C7H7)(c-C3H5) (8). Compounds 4−8 all adopt bent sandwich structures in which the dihedral angle between the two carbocyclic rings is 34.9° for the bromo compound 4, 26.6° for the hydride 5, 33.1° for the methyl compound 6, 34.2° for the allyl compound 7, and 37.5° for the cyclopropyl compound 8. 1H and 13C NMR data are reported for the diamagnetic compounds.