TANTALUM BOROLLIDE TRICHLORIDE - A VERSATILE ENTRY INTO TANTALUM BOROLLIDE COMPLEXES

The reaction of Li-2[C4H4B-N(CHMe2)(2)]. THF with 2 equiv of AlCl3 and 1 equiv of TaCl5 gives mononuclear [C4H4B-N(CHMe2)(2)]TaCl3 (1) in 47 % yield. Alkylation of 1 with 3 equiv of MeMgCl gives methylated speci es in conjunction wi th the triple decker complex C4H4B-N(CHMe2)(2)]Me 2Ta[mu-C4H4B-N(CHMe2)(2)]TaMe4 (2). Monoalkylation is possible with Li CH(SiMe3)(2) to give [C4H4B -N(CHMe2)(2)]TaCl2-[CH(SiMe3)(2)] (3) whic h contains a Ta-C-alpha-H agostic interaction. Addition of H2NAr (Ar = 2,6-Pr-i(2)-C6H3) and triethylamine to 1 affords [C4H4B-NH(CHMe2)(2)] Ta(NAr)Cl-2 (4). When 2 equiv of acetone are added to 1, the result is [C4H4B-NH(CHMe2)(2)]TaCl3[Me2C(O)CH2C(O)Me] (5). Reaction with LiCp (Cp = C5Me5) gives Cp*[C4H4B -N(CHMe2)(2)]TaCl2 (6). Reduction of 6 w ith Rig under an atmosphere of CO produces Cp-[C4H4B-N(CHMe2)(2)]Ta(C O)(2) (7) which can be protonated with [H(OEt2)(2)][B(C6H3(CF3)(2))(4) ] to form {Cp-[C4H4B-NH(CHMe2)(2)]Ta(CO)(2)}{ [B(C6H3(CF3)(2))(4)]} ( 8). Reaction of 1 with excess LiCp' (Cp' = C5H4Me) affords Cp'(2)[eta( 2)-C4H4B-N(CHMe2)(2)]TaCl (10) in which the borole ligand is eta(2)-bo und. Addition of Li[C5H5B-R] to 1 results in the formation of. [C4H4B- N(CHMe2)(2)][C5H5B-R]TaCl2 (11, R = Ph; 12, R = NMe2). Methylation of 11 affords [C4H4B-N(CHMe2)(2)][C5H5B-Ph]TaMe2 (14), which reacts with H-2 in the presence of PMe3 to give [C4H4B-N(CHMe2)(2)][C5H5B-Ph]Ta(PM e3)(2) (16). For PEt3, the product is [C4H4B-N(CHMe2)(2)][C5H5B-Ph]Ta( H)(2)PEt3 (17). Reduction of 1 in the presence of PMe3 under nitrogen gives {-[C4H4B-N(CHMe2)(2)](Me3P)(2)CITaN}(2) (18). Under an argon atm osphere the reduced product is [C4H4B-N(CHMe2)(2)]Ta(PMe3)(3)Cl (19). Complex 19 reacts with hydrogen to give the asymmetric dinuclear compl ex (H)(PMe3)Cl)mu-H([C4H4B-N(CHMe2)(2)]Ta(PMe3)(2)Cl) (20). The crysta llographic characterization of complexes 1, 3, 4, 5, 7, 10, 11, 12, 16 , 17, 18, 19, and 20 is also presented. These data give important insi ght into the metal-borollide relationship under a variety of ligand en vironments and different oxidation states. They also allow for an esti mation of the contribution from the possible resonance forms.