Tantalum Borollide Trichloride: A Versatile Entry into Tantalum Borollide Complexes

Abstract

The reaction of Li2[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 species in conjunction with the triple decker complex [C4H4B-N(CHMe2)2]Me2Ta[μ-C4H4B-N(CHMe2)2]TaMe4 (2). Monoalkylation is possible with LiCH(SiMe3)2 to give [C4H4B-N(CHMe2)2]TaCl2[CH(SiMe3)2] (3) which contains a Ta−Cα−H agostic interaction. Addition of H2NAr (Ar = 2,6-iPr2-C6H3) and triethylamine to 1 affords [C4H4B-NH(CHMe2)2]Ta(NAr)Cl2 (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 with Mg under an atmosphere of CO produces Cp*[C4H4B-N(CHMe2)2]Ta(CO)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[η2-C4H4B-N(CHMe2)2]TaCl (10) in which the borole ligand is η2-bound. 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 H2 in the presence of PMe3 to give [C4H4B-N(CHMe2)2][C5H5B-Ph]Ta(PMe3)2 (16). For PEt3, the product is [C4H4B-N(CHMe2)2][C5H5B-Ph]Ta(H)2PEt3 (17). Reduction of 1 in the presence of PMe3 under nitrogen gives {[C4H4B-N(CHMe2)2](Me3P)2ClTaN}2 (18). Under an argon atmosphere the reduced product is [C4H4B-N(CHMe2)2]Ta(PMe3)3Cl (19). Complex 19 reacts with hydrogen to give the asymmetric dinuclear complex [([C4H4B-N(CHMe2)2]Ta(H)(PMe3)Cl)μ-H([C4H4B-N(CHMe2)2]Ta(PMe3)2Cl) (20). The crystallographic characterization of complexes 1, 3, 4, 5, 7, 10, 11, 12, 16, 17, 18, 19, and 20 is also presented. These data give important insight into the metal−borollide relationship under a variety of ligand environments and different oxidation states. They also allow for an estimation of the contribution from the possible resonance forms.