Niobium−Carbon Functionalities Supported by meso-Octaethylporphyrinogen and Derived Macrocycles

This report concerns the organometallic chemistry of niobium based on a macrocyclic ligand. To this purpose, the (meso-octaalkylporphyrinogen)niobium(V) complex [(η⁵:η¹:η⁵:η¹-Et₈N₄)NbCl], 2, has been used as an appropriate starting material. The ionization of the Nb−Cl bond by the use of AgSO₃CF₃ gave a bifunctional acid−base system with an increased acidity of the metal center in [(η⁵:η¹:η¹:η¹-Et₈N₄)Nb(THF)(O₃SCF₃)], 3. The alkylation of 2 with LiMe gave a quite stable Nb(V) organometallic derivative [(η⁵:η¹:η¹:η¹-Et₈N₄)Nb−Me], 4, which undergoes a migratory insertion reaction with Bu^tNC leading to an η²-iminoacyl derivative [(η⁵:η¹:η¹:η¹-Et₈N₄)Nb(Bu^tNC)(η²-C(Me)NBu^t)], 6 (ν_CN, 2217 and 1736 cm^-1). Two steps of the reaction have been identified. An attempt to functionalize the NbO in [(η⁵:η¹:η¹:η¹-Et₈(C₄H₂N)₃(p-MeC₅H₂N)NbO], 7, by the use of LiMe led, on the contrary, to [(η¹:η¹:η¹:η¹-Et₇(CH-Me)(C₄H₂N)₃(p-MeC₅H₂N)NbOLi(THF)₃], derived from the metalation of one of the meso-ethyl groups in 9. While X-ray analysis provided information on the solid state structures of 3, 4, 6, and 9, NMR studies allowed us to establish a relationship between the bonding mode of the porphyrinogen in the solid state and in solution.