Dinitrogen Coordination and Cleavage Promoted by a Vanadium Complex of a σ,π,σ-Donor Ligand

The deprotonation of the tripyrrole MeTPH₂ [MeTPH₂ = 2,5-[(2-pyrrolyl)(C₆H₅)₂C]₂(MeNC₄H₂)], containing one N-methylated pyrrolyl ring, was carried out with 2 equiv of KH. The corresponding dipotassium salt reacted with VCl₃(THF)₃ to afford the complex [(MeTP)VCl(THF)]·THF (1). While the two lateral pyrrolide rings are σ-bonded, the central one is perpendicularly oriented in a sort of π-fashion. However, the bond distances clearly indicated that only the quaternized N atom is forming a bonding contact. Subsequent reduction of 1 with Na yielded the corresponding divalent complex [(MeTP)V(THF)]·(C₇H₈)_0.5 (2) where the central N-methylated ring adopted a more regular π-orientation. When treated with a strong Lewis acid (AlMe₃), THF was extracted from the vanadium coordination sphere, forming the dinuclear dinitrogen complex [(MeTP)V(μ-N₂)]₂·(C₇H₈)_2.9 (3). Reduction of 3 with potassium graphite gave cleavage of dinitrogen, affording the mixed-valent nitride-bridged complex [(MeTP)V(μ-N)]₂·(THF) (4).