Mono(cycloheptatrienyl) tantalum chemistry: Synthesis and characterization of new tantalum halide, hydride, and alkyl species

The new tantalum(II) complex (η⁶-C₇H ₈)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(PMe ₃)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 (C₅Me ₅)Ta(C₇H₇) with allyl bromide affords the tantalum(V) product (C₅Me₅)Ta(C₇H ₇)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(C₇H ₇)Me (6), (C₅Me₅)Ta(C₇H ₇)(η¹-CH₂CH=CH₂) (7), and (C₅Me₅)Ta(C₇H₇)(C-C ₃H₅) (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.