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

Abstract

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 ¹⁵N_β than for ¹⁵N_α(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 ¹⁵N_α 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).