Nitrogen-15 nuclear magnetic resonance relaxation mechanisms in dinitrogen complexes of molybdenum, tungsten, rhenium, and osmium

Longitudinal relaxation times T1 and 15N-{1H} nuclear Overhauser enhancement (n.O.e.) factors at 2.114 and 9.396 T are reported for 15N in dinitrogen complexes of molybdenum, tungsten, rhenium, and osmium, together with some ancillary 31P and 13C n.m.r. measurements. The n.O.e. factors are more negative for 15Nβ than for 15Nα(ligating nitrogen) and nearly all are disadvantageous, because of other mechanisms competing with the dipole–dipole (d.d.) relaxation by protons in the tertiary phosphine coligands and in the tetrahydrofuran solvent. The T1 values and n.O.e. factors at 9.396 T are smaller by a factor of five or so than those at 2.114 T (ca. 30 s). Interpretation of the field dependence in terms of shielding anisotropy (s.a.) relaxation gives rotational correlation times of 40–90 ps, the larger values being for the complexes with bulkier phosphine ligands. At 2.114 T the s.a. and d.d.1H mechanisms are both important, and d.d.15N relaxation (i.e. of 15N by 15N) becomes significant with longer correlation times. In the rhenium complex 15Nα is quite strongly relaxed by coupling to quadrupolar 185,187Re, and d.d. 185,187Re relaxation is significant. The 14N linewidths are consistent with the correlation times otherwise derived, as also are the 31P relaxation times; the 31P relaxation appears to be fully dipolar (d.d.1H).