1-Aza-1,3-butadiene complexes of tantalum: preparation and alkylation of TaCl 2(η 5-C 5Me 5)(η 4-1-aza-1,3-butadiene)

Abstract

We have synthesized half-metallocene 1-aza-1,3-butadiene (abbr. AD) complexes of tantalum and have demonstrated the unique coordination mode and reactivity of the AD ligand. Treatments of a dimeric complex [TaCl 2Cp*] 2 ( 2) (Cp*=pentamethylcyclopentadienyl) with one equivalent of 1,4-diphenyl-1-aza-1,3-butadiene (abbr. Ph-AD) and 1- p-methoxyphenyl-4-phenyl-1-aza-1,3-butadiene (abbr. p-MeOC 6H 4–AD) afforded purple complexes of the formula TaCl 2Cp*(supine-η 4-AD) ( 3: AD=Ph-AD; 4: AD= p-MeOC 6H 4–AD), whose formulation and supine-η 4-coordination mode with a contribution of η 1-N-η 3-allyl canonical form were elucidated by their combustion analysis and NMR spectroscopy along with the X-ray crystal structure determination for the complex 3. Reaction of 3 with one equivalent of MgMe 2 gave a dimethyl complex TaMe 2Cp*(supine-η 4-Ph-AD) ( 6) in 96% yield, whereas the methylation reaction using MeLi or MeMgX did not afford 6. Benzylation of 3 and 4 using one equivalent of Mg(CH 2Ph) 2 in toluene led to the formation of dibenzyl complexes Ta(CH 2Ph) 2Cp*(η 2-C,N-AD) ( 9: AD=Ph–AD; 11: AD= p-MeOC 6H 4–AD) in modest yield, while reaction of 3 with 0.5 equivalent of Mg(CH 2Ph) 2 gave a monobenzylated complex TaCl(CH 2Ph)Cp*(supine-η 4-Ph–AD) ( 13). When complex 3 reacted with one equivalent of Mg(CH 2Ph) 2 in THF, we obtained a product mixture of 9 and a metallacyclic tantalum–carbene complex 10. Similarly, reaction of 4 with Mg(CH 2Ph) 2 in THF gave 11 and a metallacyclic tantalum–carbene complex 12. The carbene complexes 10 and 12 might be respectively derived from nascent benzylidene species, Ta(CHPh)Cp*(η 4-AD) ( 14: AD=Ph–AD; 15: AD= p-MeOC 6H 4–AD), which may abstract the H 4 proton of the AD ligand.