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
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).