Inner-sphere two-electron reduction leads to cleavage and functionalization of coordinated dinitrogen

Abstract

Activation of molecular nitrogen by transition metal complexes is an area of current interest as investigations using the inert N2 molecule to produce higher-value organonitrogen compounds intensify. In an attempt to extend the addition of hydride reagents E-H (where E = BR2, AlR2, and SiR3) to the dinitrogen complex ([NPN]Ta)2(mu-H)2(mu-eta1:eta2-N2) [1; where NPN = (PhNSiMe2CH2)2PPh], the reaction with zirconocene chlorohydride, [Cp2Zr(Cl)H]x, was examined. The crystalline product formed in 35% yield was determined to be ([NP(N)N]Ta)(mu-H)2(mu-N)(Ta[NPN])(ZrCp2) (2) in which the coordinated N2 has been cleaved to form a phosphinimide bridging between Ta and Zr and a triply bridging nitride. The mechanism of this reaction was examined to determine the fate of the chloride and hydride ligands attached to Zr in the starting zirconocene reagent. Using the zirconocene dihydride dimer ([Cp2ZrH2]2), a higher yield of 2 was obtained (76%), and H2 was also observed by 1H NMR spectroscopy. To probe the origin of the eliminated H2, the dideuterated dinitrogen complex ([NPN]Ta)2(mu-D)2(mu-eta1:eta2-N2) (d2-1) was allowed to react with ([Cp2ZrH2]2), which resulted in the formation of ([NP(N)N]Ta)(mu-D)2(mu-N)(Ta[NPN])(ZrCp2), (d2-2), with no evidence of hydrogen for deuterium scrambling between the starting zirconocene dihydride and the ditantalum dinitrogen complex. Studies into the use of preformed Zr(II) and Ti(II) reagents were also performed. The proposed mechanism involves initial adduct formation that facilitates inner-sphere electron transfer to cleave the N-N bond to form a species with bridging nitrides, one of which is transformed by nucleophilic attack of a phosphine donor to generate the observed phosphinimide.

Fig. 1.

Sequence of reactions for the addition of simple main-group hydride reagents E-H to the dinitrogen complex 1.

Fig. 2.

ortep drawing of the solid-state molecular structure of 2 (spheroids at 50% probability; hydrogen and phenyl ring carbon atoms other than ipso omitted for clarity); the hydride ligands were modeled by using xhydex. Selected bond distances (Å) and angles (°), respectively, are: Ta1-Ta2, 2.7007, 6; N1· · · N2, 2.925, 9; P1-N1, 1.595, 9; N1-Ta1, 2.102, 9; Ta1-N2, 2.139, 8; Ta2-N2, 1.935, 9; N1-Zr1, 2.146, 9; N2-Zr1, 2.040, 9; Ta1-N3, 2.054, 9; Ta1-N4, 2.102, 9; Ta2-N5, 2.112, 10; Ta2-N6, 2.119, 9; Ta2-P2, 2.624, 3; Ta1-Zr1, 3.0319, 11; N1-Ta1-N2, 87.2, 3; P1-N1-Ta1, 121.5, 5; N1-Zr1-N2, 88.6, 3; Ta1-N2-Ta2, 82.9, 3; N3-Ta1-N4, 107.0, 4; N5-Ta2-N6, 105.6, 4; P1-N1-Ta1-N2, −176.3, 6; P2-Ta2-Ta1-Zr1, −13.25, 11; P2-Ta2-N2-Ta1, −175.8, 2.

Fig. 3.

Outcome of the reaction of the dinitrogen complex 1 with [Cp₂Zr(Cl)H]_x.

Fig. 4.

Proposed mechanism of the reaction of Cp₂M with 1.