Structure Elucidation of Lithium Compounds Using ⁷Li Residual Quadrupolar Couplings

Abstract

A new NMR method for the structure elucidation of lithium compounds under solution-like conditions is presented. It is based on the measurement of 7 Li residual quadrupolar couplings (RQCs) in a stretched polystyrene (PS) gel, and comparison to RQCs predicted from crystal or DFTderived model structures in combination with alignment tensors derived from one-bond 1 H, 13 C residual dipolar couplings (RDCs). The method was applied to five lithium model complexes containing monoanionic, bidentate bis(benzoxazole-2-yl)methanide, bis(benzothiazole-2yl)methanide and bis(pyridyl)methanide ligands, of which two are first introduced in this work. In agreement with the crystalline state, four complexes are monomeric with Li coordinated fourfold by two additional THF molecules, whereas in one complex bulky tBu groups only provide space for one additional THF molecule.

The quest for organo-alkali metal monomers: unscrambling the structure–reactivity relationship

“…7 Li residual quadrupolar couplings have themselves also been used to determine aggregation state. 33,34 Heteronuclear Overhauser effect spectroscopy (HOESY) 23 and exchange spectroscopy (EXSY) 26 have also been utilised to study aggregation and exchange. Early research often used cryoscopic measurements (freezing point depression), but the calculated value shows only a statistic average of the degrees of aggregation in solution.…”

Resonance and structural assignment in (car)borane clusters using 11B residual quadrupolar couplings

“…Resonance and structural assignment in (car)borane clusters using 11 Since carboranes were first discovered and classified in the 1960s, they have quickly gained popularity in various fields. 1 The main body is usually derived from polyhedral structures that can be described by the Wade-Mingos rules as closo (closed), nido (nest) and arachno (spider web).…”

“…4 For such highly specialised applications, reliable methods for the structural characterisation are needed, where X-ray crystallography and NMR spectroscopy 5 are certainly the most versatile methods. Both naturally occurring boron isotopes, 10 B (I = 3) and 11 B (I = 3/2), are NMR active, but the heavier isotope 11 B is usually preferred due to its (i) higher natural abundance (ca. 80% vs. ca.…”

“…Simple 11 B spectra provide basic information about the relative number of equivalent boron atoms and coupling to directly bound hydrogen atoms but give limited knowledge about the skeletal structure of the cage. 6 Here, the implementation of homo-and heteronuclear two-dimensional experiments such as 11 B-11 B COSY present further possibilities in determining the arrangement and assignment of boron atoms.…”

“…The quadrupolar nature of the 11 B nucleus can provide valuable information due to its coupling to the electric field gradient (EFG) which is highly sensitive to the closest coordination environment of the boron atom. 8 Quadrupole coupling constants are most commonly derived from standard solid state NMR measurements, but very difficult to extract if more than 1-2 chemically distinct boron atoms are present in the cluster.…”