Solid-State ⁹⁵Mo NMR Study of (Aryldiazenido)- and (Organohydrazido)polyoxomolybdates. Investigation of Model Compounds of Catalytic Molybdenum Environments

The ⁹⁵Mo static powder and magic angle spinning NMR spectra of the central ±1/2 transition of a number of (aryldiazenido)- polyoxomolybdates, (organohydrazido)polyoxomolybdates, and unsubstituted polyoxomolybdates were obtained. The spectra clearly demonstrated the existence of octahedral and tetrahedral sites within these compounds by the contrast between the respective line widths and the dependence under MAS conditions of the resonances from these sites. Variation of ligand coordination was also seen to yield a visible chemical shift to the octahedral molybdenum sites in selected complexes. An interactive graphics curve-fitting program was used to estimate the quadrupolar coupling constants, asymmetry parameters of the electric field gradients, the three principal elements of the shielding tensor, and the Euler angles relating the quadrupole and chemical shielding principal axis systems for the molybdenum sites from the static powder spectra. Assignments of the line shape components was made to the different molybdenum sites present in these species. The method of assignment, based on degree of distortion at the site, was vindicated by MO calculations carried out on a simple [MoO₆]⁶⁻ model species. These MO calculations demonstrate the increase in the magnitude of the electric field gradient (and hence the value of A_oc) was one distorts the octahedral environment by a combination of bond length and angular variations. The effectiveness of the ⁹⁵Mo NMR technique to obtain these data from Mo(VI) compounds was thus demonstrated and has laid a foundation from which future solid-state ⁹⁵Mo NMR studies of biological and catalytic molybdenum systems can benefit by comparison of the results obtained with those obtained for these model compounds.