Multinuclear solid-state magnetic resonance study of oxo-bridged diniobium and quadruply-bonded dimolybdenum carboxylate clusters.

Carboxylate paddlewheels and their oxo-bridged analogues constitute ideal building blocks for the assembly of two- and three-dimensional framework materials. Here, we present a multinuclear (H, C, Nb, Mo) magnetic resonance study of solid samples of NbOCl(OPh) (1), Mo(OCMe) (2), and Mo(OCCHF) (3). High-resolution proton and C CP/MAS NMR spectra provide valuable information on structure and crystal symmetry and on cocrystallized solvent. Nb solid-state NMR spectra of 1 provide quadrupolar coupling constants and chemical shift tensors which are characteristic of the axially asymmetric Nb-O-Nb bridging environment. Mo solid-state NMR spectra of 2 and 3 provide quadrupolar coupling constants and chemical shift tensors which are directly characteristic of the molybdenum-molybdenum quadruple bonds in these compounds. The quadruple bonds are characterized by particularly large Mo chemical shift tensor spans on the order of 5500ppm. Density functional theoretical computations provide good agreement with the Nb and Mo experimental data, with some exceptions noted. This work demonstrates possible NMR approaches to characterize more complex framework materials and provides key insight into the Mo-Mo quadruple bond.