Nitrogen nuclear magnetic resonance spectroscopy. Part V. Diazirine, diazomethane, and related azo-, diazo-, and tetra-azo-compounds, some comments on dinitrogen as a ligand, and an improved absolute scale for nitrogen shielding

Abstract

Nitrogen-14 n.m.r. measurements of diazirine, diazomethane, diphenyldiazomethane. ethyl diazoacetate, tetra-methyl- and tetraethyl-tetraz-2-ene, and diethyl azodicarboxylate are reported. The diarrsgnetic and paramagnetic shielding of the two nitrogens in diazomethane is compared with that of central and terminal nitrogen in other linear triatomic systems [OCN, CNC (isocyano), NNN, NCN (carbodi-imide), and CCN], and carbon in allene (CCC). The nitrogen shielding is higher in the CH₂N₂ isomers than in unstrained cis- or trans-diazo-compounds, and for diazirine and molecular N₂ it is intermediate between that of the diazomethane nitrogens. These compounds help to define regions in which the absorption of dinitrogen ligands might be expected. The ¹⁵N resonances assigned by Bercaw to edge-on and end-on co-ordinated dinitrogen in [Ti(C₅Me₅)₂(N₂)] are significantly downfield of the region expected for these, and are in (and downfield of) the region of azo-nitrogen (X–NN–X). An absolute scale for nitrogen shielding is appended, based on DeLucia and Gordy's accurate molecular-beam maser measurement of the spin-rotation constants in NH₃, combined with Alei's chemical-shift measurements for ¹⁵NH₃(g) relative to [¹⁵NH₄]⁺(aq).