Tantalum Borollide Trichloride:  A Versatile Entry into Tantalum Borollide Complexes

The reaction of Li 2 [C 4 H 4 B-N(CHMe 2 ) 2 ]·THF with 2 equiv of AlCl 3 and 1 equiv of TaCl 5 gives mononuclear [C 4 H 4 B-N(CHMe 2 ) 2 ]TaCl 3 ( 1 ) in 47% yield. Alkylation of 1 with 3 equiv of MeMgCl gives methylated species in conjunction with the triple decker complex [C 4 H 4 B-N(CHMe 2 ) 2 ]Me 2 Ta[μ-C 4 H 4 B-N(CHMe 2 ) 2 ]TaMe 4 ( 2 ). Monoalkylation is possible with LiCH(SiMe 3 ) 2 to give [C 4 H 4 B-N(CHMe 2 ) 2 ]TaCl 2 [CH(SiMe 3 ) 2 ] ( 3 ) which contains a Ta−C α −H agostic interaction. Addition of H 2 NAr (Ar = 2,6- i Pr 2 -C 6 H 3 ) and triethylamine to 1 affords [C 4 H 4 B-NH(CHMe 2 ) 2 ]Ta(NAr)Cl 2 ( 4 ). When 2 equiv of acetone are added to 1, the result is [C 4 H 4 B-NH(CHMe 2 ) 2 ]TaCl 3 [Me 2 C(O)CH 2 C(O)Me] ( 5 ). Reaction with LiCp* (Cp* = C 5 Me 5 ) gives Cp*[C 4 H 4 B-N(CHMe 2 ) 2 ]TaCl 2 ( 6 ). Reduction of 6 with Mg under an atmosphere of CO produces Cp*[C 4 H 4 B-N(CHMe 2 ) 2 ]Ta(CO) 2 ( 7 ) which can be protonated with [H(OEt 2 ) 2 ][B(C 6 H 3 (CF 3 ) 2 ) 4 ] to form {Cp*[C 4 H 4 B-NH(CHMe 2 ) 2 ]Ta(CO) 2 }{[B(C 6 H 3 (CF 3 ) 2 ) 4 ]} ( 8 ). Reaction of 1 with excess LiCp‘ (Cp‘ = C 5 H 4 Me) affords Cp‘ 2 [η 2 -C 4 H 4 B-N(CHMe 2 ) 2 ]TaCl ( 10 ) in which the borole ligand is η 2 -bound. Addition of Li[C 5 H 5 B-R] to 1 results in the formation of [C 4 H 4 B-N(CHMe 2 ) 2 ][C 5 H 5 B-R]TaCl 2 ( 11, R = Ph; 12, R = NMe 2 ). Methylation of 11 affords [C 4 H 4 B-N(CHMe 2 ) 2 ][C 5 H 5 B-Ph]TaMe 2 ( 14 ), which reacts with H 2 in the presence of PMe 3 to give [C 4 H 4 B-N(CHMe 2 ) 2 ][C 5 H 5 B-Ph]Ta(PMe 3 ) 2 ( 16 ). For PEt 3, the product is [C 4 H 4 B-N(CHMe 2 ) 2 ][C 5 H 5 B-Ph]Ta(H) 2 PEt 3 ( 17 ). Reduction of 1 in the presence of PMe 3 under nitrogen gives {[C 4 H 4 B-N(CHMe 2 ) 2 ](Me 3 P) 2 ClTaN} 2 ( 18 ). Under an argon atmosphere the reduced product is [C 4 H 4 B-N(CHMe 2 ) 2 ]Ta(PMe 3 ) 3 Cl ( 19 ). Complex 19 reacts with hydrogen to give the asymmetric dinuclear complex [([C 4 H 4 B-N(CHMe 2 ) 2 ]Ta(H)(PMe 3 )Cl)μ-H([C 4 H 4 B-N(CHMe 2 ) 2 ]Ta(PMe 3 ) 2 Cl) ( 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.