1-Aza-1,3-butadiene complexes of tantalum: preparation and alkylation of TaCl2(η5-C5Me5)(η4-1-aza-1,3-butadiene)

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 [TaCl2Cp*]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-MeOC6H4–AD) afforded purple complexes of the formula TaCl2Cp*(supine-η4-AD) (3: AD=Ph-AD; 4: AD=p-MeOC6H4–AD), whose formulation and supine-η4-coordination mode with a contribution of η1-N-η3-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 MgMe2 gave a dimethyl complex TaMe2Cp*(supine-η4-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(CH2Ph)2 in toluene led to the formation of dibenzyl complexes Ta(CH2Ph)2Cp*(η2-C,N-AD) (9: AD=Ph–AD; 11: AD=p-MeOC6H4–AD) in modest yield, while reaction of 3 with 0.5 equivalent of Mg(CH2Ph)2 gave a monobenzylated complex TaCl(CH2Ph)Cp*(supine-η4-Ph–AD) (13). When complex 3 reacted with one equivalent of Mg(CH2Ph)2 in THF, we obtained a product mixture of 9 and a metallacyclic tantalum–carbene complex 10. Similarly, reaction of 4 with Mg(CH2Ph)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*(η4-AD) (14: AD=Ph–AD; 15: AD=p-MeOC6H4–AD), which may abstract the H4 proton of the AD ligand.