Ferrocene-like iron bis(dicarbollide), [3-FeIII-(1,2-C2B9H11)2]−. The first experimental and theoretical refinement of a paramagnetic 11B NMR spectrum

Abstract

Nuclear magnetic resonance (NMR) of paramagnetic molecules (pNMR) provides detailed information on the structure and bonding of metallo-organic systems. The physical mechanisms underlying chemical shifts are considerably more complicated in the presence of unpaired electrons than in the case of diamagnetic compounds. We report for the first time a combined first-principles theoretical as well as experimental liquid-state ¹¹B NMR study of a paramagnetic compound, applied on the [3-Fe^III-(1,2-C₂B₉H₁₁)₂]⁻ metallaborane, which is an electronically open-shell structure where the iron centre binds two hemispherical boron–carbon cages. We show that this combined theoretical and experimental analysis constitutes a firm basis for the assignment of experimental ¹¹B NMR chemical shifts in paramagnetic metallaboranes. In the calculations, the roles of the different physical contributions to the pNMR chemical shift are elaborated, and the performance of different popular exchange–correlation functionals of density-functional theory as well as basis sets, are evaluated. A dynamic correction to the calculated shifts via first-principles molecular dynamics simulations is found to be important. Solvent effects on the chemical shifts were computed and found to be of minor significance.