Selective Extraction of N2 from Air by Diarylimine Iron Complexes

Treatment of cis-(Me3P)4FeMe2 with ortho-substituted diarylimines afforded 2 equiv of MeH, PMe3, and {mer-κC,N,C′-(Ar-2-yl)­CH2NCH­(Ar′-2-yl)}­Fe­(PMe3)3 (Ar = 3,4,6-(F)3-C6H, Ar′ = 3,5-(CF3)2-C6H2, 1a; Ar = 3,4,6-(F)3-C6H, Ar′ = 3,4,5-(F)3-C6H, 1b; Ar = 4,5,6-(F)3-C6H, Ar′ = 3,5-(CF3)2-C6H2, 1c; Ar = C6H4, Ar′ = 3-(OMe)-C6H3, 1d; Ar = 4,5,6-(F)3-C6H, Ar′ = 3,6-Me2-C6H3, 1e; Ar = C6H4, Ar′ = 3,6-Me2-C6H2, 1f). Exposure of 1a–f to O2 caused rapid degradation, but substitution of the unique PMe3 with N2 occurred when 1a–f were exposed to air or N2 (1 atm), yielding {mer-κC,N,C′-(Ar-2-yl)­CH2NCH­(Ar′-2-yl)}­Fe­(PMe3)2L (L = N2, 2a–f); CO, CNMe, and N2CPh2 derivatives (L = CO, 3a–d,f; L = CNMe, 8b; L = N2CPh2, 9b) were prepared. Dihydrogen or NH3 binding to {mer-κC,N,C′-(3,4,6-(F)3-C6H-2-yl)­CH2NCH-(3,4,5-(F)3-C6H-2-yl)}­Fe­(PMe3)2 (1b′, S = 1 (calc)) to provide 5b (L = H2) or 6b (L = NH3) was found comparable to that of N2, while PMe3 (1b) and pyridine (L = py, 7b) adducts were unfavorable. Protolytic conditions were modeled using HCCR as weak acids, and trans-{κC,N-(3,4,5-(F)3-C6H2)­CH2NCH­(3,4,6-(F)3-C6H-2-yl)}­Fe­(PMe3)3(CCR) (R = Me, 4b-Me; R = Ph, 4b-Ph) were generated from 1b. Exposure of 1b to N2O or N3SO2tol generated 2b and Me3PO or Me3PN­(SO2)­tol, respectively. Calculations revealed 2b to be thermodynamically and kinetically favored over the calculated Fe­(III) superoxide complex, 3[FeO2], relative to 1b′ + N2 + O2. The correlation of 1b′ + 3O2 to 3[FeO2] is likely to have a relatively high intersystem crossing point (ICP) relative to 1b′ + N2 to 2b, thereby explaining the dinitrogen selectivity.