A combined solid-state NMR and X-ray crystallography study of the bromide ion environments in triphenylphosphonium bromides

Abstract

Multinuclear ( 31P and 79/81Br), multifield (9.4, 11.75, and 21.1 T) solid-state nuclear magnetic resonance experiments are performed for seven phosphonium bromides bearing the triphenylphosphonium cation, a molecular scaffold found in many applications in chemistry. This is undertaken to fully characterise their bromine electric field gradient (EFG) tensors, as well as the chemical shift (CS) tensors of both the halogen and the phosphorus nuclei, providing a rare and novel insight into the local electronic environments surrounding them. New crystal structures, obtained from single-crystal X-ray diffraction, are reported for six compounds to aid in the interpretation of the NMR data. Among them is a new structure of BrPPh 4, because the previously reported one was inconsistent with our magnetic resonance data, thereby demonstrating how NMR data of non-standard nuclei can correct or improve X-ray diffraction data. Our results indicate that, despite sizable quadrupolar interactions, 79/81Br magnetic resonance spectroscopy is a powerful characterisation tool that allows for the differentiation between chemically similar bromine sites, as shown through the range in the characteristic NMR parameters. 35/37Cl solid-state NMR data, obtained for an analogous phosphonium chloride sample, provide insight into the relationship between unit cell volume, nuclear quadrupolar coupling constants, and Sternheimer antishielding factors. The experimental findings are complemented by gauge-including projector-augmented wave (GIPAW) DFT calculations, which substantiate our experimentally determined strong dependence of the largest component of the bromine CS tensor, δ 11, on the shortest Br-P distance in the crystal structure, a finding that has possible application in the field of NMR crystallography. This trend is explained in terms of Ramsey's theory on paramagnetic shielding. Overall, this work demonstrates how careful NMR studies of underexploited exotic nuclides, such as 79/81Br, can afford insights into structure and bonding environments in the solid state. Onto exotic nuclides: 79/81Br solid-state NMR measurements and GIPAW (gauge-including projector-augmented wave) DFT computations of 79/81Br quadrupolar coupling constants and chemical shift tensors for a series of triphenylphosphonium bromides demonstrate how careful NMR studies of underexploited exotic nuclides can afford insights into structure and bonding environments in the solid state (see figure). © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.