Central transition nuclear magnetic resonance in the presence of large quadrupole couplings: Cobalt-59 nuclear magnetic resonance of cobaltophthalocyanines

The acquisition of distortion-free solid-state nuclear magnetic resonance ( NMR) powder line shapes from half-integer spin systems possessing large qua drupole couplings is exemplified on cobaltophthalocyanine complexes. The ac quisition of ideal-like static line shapes even for megahertz-wide central transition patterns is discussed, with the aid of spin-echo sequences incor porating short and very weak radio frequency (rf) pulses. Under these condi tions most of the crystallites within the bandwidth of interest are excited with essentially orientation-independent pulse angles, while the acquisiti on of several experiments with varying carrier frequency offsets alleviates the limited bandwidth of the excitation given by the rf pulse lengths. Aft er this approach was tuned with the aid of quantum mechanical calculations and model compounds, it was applied to the study of diamagnetic metal cente rs in hexacoordinated Co(III)phthalocyanines. Solid-state Co-59 NMR spectra were acquired as a function of the external magnetic field on complexes wi th general structure [(L)(2)CoPc]Br, where Pc denotes the phthalocyanine ma crocycle and the axial ligands L were pyridine, methylimidazole, methylpipe ridine, and ammonia. Iterative numerical fittings of these data revealed an isotropic coupling parameters that were larger than those observed in cobal toporphyrin analogues but, which like the latter, deviated from trends trad itionally observed for nonaromatic octahedral cobalt complexes. These syste matic differences observed for the various Co-59 coupling parameters are di scussed.