Synthesis and reactivity of organotantalum complexes containing nitrogen ligation

Abstract

This work examines various aspects of hydrodenitrogenation catalysis. Various tetrahydroquinolinyl (THQ) and indolinyl (DHI) complexes of tantalum were synthesized including: Ta(THQ)ₓCl₅₋ₓ(x = 1 to 5), Ta(DHI) ₓCl₅₋ₓ(THF) (x = 1 and 2). In addition, THQ and DHI complexes containing aryloxide (OAr = 2,6-diisopropylphenoxide) and alkyl complexes were also synthesized: Ta(THQ)(OAr)Cl₃(OEt₂), Ta(THQ)₂(OAr)₂Cl, Ta(THQ)(OAr)R₃Cl (R = Me and Et), Ta(THQ)₂(OAr)₂R (R = Me and Et), Ta(DHI)(OAr)Cl₃(OEt₂), and Ta(DHI)₂(OAr)₂Cl. The effects of sulfur ligation versus oxygen ligation was examined by comparison of the crystal structures of Ta[η²(N,C)-2,4,6-tri-t-butylpyridinel](OAr)₂(OᵗBu) and Ta[η²(N,C)-2,4,6-tri-t-butylpyridine](OAr)₂(SᵗBu). The sulfur ligand donates less π density to the metal center, resulting in a longer Ta-C bond for the pyridine ligand. The arene complexes (η⁶C₆Me₆)Ta(OAr)(SAr')Cl and (η⁶-C₆Me₆)Ta(OAr)(SAr'')Cl were also synthesized. These arene-thiolate complexes do not appear to bind quinoline, unlike the oxygen analogs A number of Ta(η⁴-diene)(OAr)₃ complexes were synthesized (diene = butadiene, isoprene, 2,3-dimethylbutadiene, 1,3-cyclohexadiene) and their reactivity with respect to nitriles examined. Nitriles typically insert into Ta(η⁴-isoprene)(OAr)₃ to form (ArO₃Ta(CH₂CHCMeCH₂CRN) which can then be thermally rearranged to (ArO) ₃Ta(=NCRCH₂CHCMeCH₂) (R = Me, Ph, ᵗBu). Similar reactivity is observed with butadiene and 2,3-dimethylbutadiene diene complexes. The Ta(η⁴-C₄H₆)(OAr)₃ complex is also capable of inserting two nitriles to form (ArO)₃Ta(NCRCH₂CHCHCH₂CRN), whereas Ta(η⁴-1,3-cyclohexadiene)(OAr)₃ only forms the bis-insertion product. Aspects of the formation of the above complexes with specific concentration on the tantalum-nitrogen interactions are presented.