Cationic Bismuth Aminotroponiminates: Charge Controls Redox Properties

Abstract

The behavior of the redox-active aminotroponiminate (ATI) ligand in the coordination sphere of bismuth has been investigated in neutral and cationic compounds, [Bi(ATI)₃ ] and [Bi(ATI)₂ L_n ][A] (L=neutral ligand; n=0, 1; A=counteranion). Their coordination chemistry in solution and in the solid state has been analyzed through (variable-temperature) NMR spectroscopy, line-shape analysis, and single-crystal X-ray diffraction analyses, and their Lewis acidity has been evaluated by using the Gutmann-Beckett method (and modifications thereof). Cyclic voltammetry, in combination with DFT calculations, indicates that switching between ligand- and metal-centered redox events is possible by altering the charge of the compounds from 0 in neutral species to +1 in cationic compounds. This adds important facets to the rich redox chemistry of ATIs and to the redox chemistry of bismuth compounds, which is, so far, largely unexplored.

Keywords: aminotroponiminates; bismuth; cationic species; redox chemistry; redox-active ligands.

Scheme 1

Controlling the properties of open‐shell main‐group compounds: a) through magnetic coupling (dimerization vs. isolable species); b) through the choice of the central atom (reversible electron transfer vs. dimerization). X=Cl, O₃SCF₃; R=2,6‐iPr₂‐C₆H₃.

Scheme 2

Bismuth complexes with redox‐active ligands (F, G) and potentially redox‐active ligands (H). R/R′=tBu/tBu, Ph/mesityl; Dtp=3,5‐tBu₂‐C₆H₃.

Scheme 3

Synthesis of neutral and cationic bismuth ATI complexes.

Figure 1

a) Molecular structure of [Bi(ATI^Ph/iPr)₃] (1) in the solid state. Displacement ellipsoids are shown at the 50 % probability level. Carbon atoms of Ph and iPr groups are shown in the wireframe model and hydrogen atoms are omitted for clarity. Bi1−N1 2.552(4), Bi1−N2 2.384(4), Bi1−N3 2.531(4), Bi1−N4 2.354(4), Bi1−N5 2.581(4), Bi1−N6 2.368(4), N1−C1 1.313(6), N2−C2 1.349(6), N3−C17 1.326(6), N4−C18 1.354(6), N5−C33 1.319(7), N6−C34 1.342(6) Å; N1‐Bi1‐N6 155.87(14), N2‐Bi1‐N3 158.65(13), N4‐Bi1‐N5 154.62(14), N1‐Bi1‐N2 64.89(12), N3‐Bi1‐N4 67.16(13), N5‐Bi1‐N6 64.89(13), N1‐Bi1‐N5 117.64(13)°. b) HOMO‐3 of compound 1 (isovalue=0.05), as determined by DFT calculations. This molecular orbital (MO) shows contributions by an s‐type bismuth atomic orbital that is polarized towards the hemisphere in which the NiPr groups are localized and may be associated with a stereochemically active lone pair (for details, see the Supporting Information).

Figure 2

Molecular structures of compounds 2‐X (a–c), 3‐X (d, e), and 4‐BAr^F (f) in the solid state. Displacement ellipsoids are shown at the 50 % probability level. Carbon atoms, except for those of the ATI backbone, are shown as wireframe. Hydrogen atoms and lattice‐bound solvent molecules are omitted for clarity. Only one of the six crystallographically independent, but chemically identical, formula units is shown in d). Atoms that exceed one formula unit are shown as colorless ellipsoids (d, f). For details, see the Supporting Information.

Figure 3

Cyclic voltammograms of 1 (a) and [Bi(ATI^Ph/iPr)₂][BAr^F] (2‐BAr^F) (b) in THF/0.1 m [N(nBu)₄][PF₆] at a temperature of 23 °C and scan rates in the range of 100–1000 mV s⁻¹.

Figure 4

Structures obtained from geometry optimizations by DFT calculations. LUMOs of 1 (a) and 2‐BAr^F (b; the counteranion is included in the calculation, but omitted in the figure for clarity) at isovalues of 0.03. Spin densities of [Na(thf)₂][Bi(ATI^Ph/iPr)₃] (Na‐1‐rad) (c) and [Bi(ATI^Ph/iPr)₂] (2‐rad) (d) at isovalues of 0.002.