Combination of crystal-field dependent and independent paramagnetic NMR hyperfine shift analysis methods for investigating the solution structures of inert self-assembled heterodimetallic d–f supramolecular complexes

Abstract

The segmental ligand 2-{6-[N,N-diethylcarbamoyl]-pyridin-2-yl}-1,1′-dimethyl-2′-(5-methylpyridin-2-yl)-5,5′-methylenebis[1H-benzimidazole] (L) produces quantitatively the self-assembled triple-stranded non-covalent head-to-head-to-head podates (HHH)-[LnCo^IIL₃]⁵⁺ (Ln = La to Lu or Y) in acetonitrile. Subsequent selective Co^II/III oxidation gives the related rigid supramolecular complexes (HHH)-[LnCo^IIIL₃]⁶⁺ possessing an inert and diamagnetic pseudo-octahedral cobalt(III) tripod ideally suited for testing and extending paramagnetic NMR hyperfine shift analysis methods in solution for dimetallic complexes. Comparison of structure independent NMR hyperfine shift analysis methods led to the conclusion that only a combination of both crystal-field independent and dependent approaches is suitable for (i) accurately separating contact and pseudo-contact contributions in axial complexes, (ii) rationalising and predicting NMR spectra for (HHH)-[LnCo^IIIL₃]⁶⁺ and (iii) investigating spin delocalisation and isostructurality along the lanthanide series in solution. The extraction of molecular structures in solution from pseudo-contact terms by using linear and non-linear least-squares fits of lanthanide induced shifts (LIS) and field-dependent lanthanide induced relaxation (LIR) effects demonstrates that the crystal structures of the cations (HHH)-[LnCo^IIIL₃]⁶⁺ in [LaCo^III(L)₃][ClO₄]_5.5[OH]_0.5·4CH₃CN·2(H₂O) and [LuCo^III(L)₃][CF₃SO₃]₆·2CH₃CN· H₂O are maintained in acetonitrile, thus confirming the considerable rigidity of these supramolecular assemblies. An extension of this complete NMR approach for the characterisation of (HHH)-[LnCo^II(L)₃]⁵⁺ in which both metal ions are strongly paramagnetic provides identical conclusions for magnetically uncoupled dimetallic systems, thus opening new perspectives for the characterisation of polymetallic d–f supramolecular complexes in solution. The origin of the systematic breaks occurring near the middle of the lanthanide series for classical structure independent hyperfine shift analysis methods is discussed.

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