Pb-207 NMR chemical shifts of lead tetracarboxylates

Pb-207 NMR spectroscopy was used to investigate 20 lead(IV) tetracarboxylat es both in solution and in the solid state by cross-polarization magic angl e spinning (CP/MAS) NMR. There is generally good correlation between isotro pic chemical shifts in the solid state and those in solution. Multiple CP/M AS NMR isotropic resonances are observed for three of the compounds and are interpreted in terms of multiple molecules in the crystallographic asymmet ric unit. From an analysis of the range of, and trends in, the NMR chemical shifts, circumstantial evidence is presented for augmentary coordination b y Lewis basic ortho-aromatic substituents of the carboxylate groups. The Pb -207 chemical shift anisotropy (CSA) parameters were extracted from analysi s of the spinning sideband manifolds of the CP/MAS spectra. The CSA spreads are small in comparison with others reported for Pb-207, and all have posi tive skew, in contrast to the typical case for lead(II) compounds. Thirteen of the 20 CSA analyses performed show effectively axial CSA tensors. A sim ple shielding model which rationalizes this typical CSA pattern is presente d, based on an analysis of geometric information derived from the two known single-crystal x-ray structures of lead(IV) carboxylates. Copyright (C) 20 01 John Wiley & Sons, Ltd.