Aminotroponiminates: Impact of the NO2 Functional Group on Coordination, Isomerisation, and Backbone Substitution

Abstract

Aminotroponiminate (ATI) ligands are a versatile class of redox-active and potentially cooperative ligands with a rich coordination chemistry that have consequently found a wide range of applications in synthesis and catalysis. While backbone substitution of these ligands has been investigated in some detail, the impact of electron-withdrawing groups on the coordination chemistry and reactivity of ATIs has been little investigated. We report here Li, Na, and K salts of an ATI ligand with a nitro-substituent in the backbone. It is demonstrated that the NO₂ group actively contributes to the coordination chemistry of these complexes, effectively competing with the N,N-binding pocket as a coordination site. This results in an unprecedented E/Z isomerisation of an ATI imino group and culminates in the isolation of the first "naked" (i. e., without directional bonding to a metal atom) ATI anion. Reactions of sodium ATIs with silver(I) and tritylium salts gave the first N,N-coordinated silver ATI complexes and unprecedented backbone substitution reactions. Analytical techniques applied in this work include multinuclear (VT-)NMR spectroscopy, single-crystal X-ray diffraction analysis, and DFT calculations.

Keywords: alkali metal; aminotroponiminates; electrophilic substitution; isomerisation; non-coordinate anionic ligand.

Figure 1

Substituents at the nitrogen atoms and at the backbone of ATI ligands and their role in various fields of research. EWG=electron‐withdrawing group.

Scheme 1

Synthesis of 3 and 4.

Figure 2

Solid‐state molecular structure of 3. Hydrogen atoms except for H2 are omitted for clarity. Displacement ellipsoids are depicted at the 50 % probability level. Selected bond lengths [Å] and angles [°]: O1−N3 1.243(2), O2−N3 1.240(2), N1−C1 1.296(2), N2−C2 1.323(2), N3−C5 1.450(2), C1−C2 1.507(2), C2−C3 1.400(2), C3−C4 1.385(2), C4−C5 1.371(2), C5−C6 1.415(2), C6−C7 1.357(2), C7−C1 1.448(2), C1−C2−C3 126.9(2), C2−C3−C4 130.9(2), C3−C4−C5 130.2(2), C4−C5−C6 127.7(2), C5−C6−C7 128.6(2), C6−C7−C1 133.1(2), C7−C1−C2 122.6(2), N1−C1−C2−N2 1.15(2).

Scheme 2

a) Synthesis of alkali‐metal complexes 5‐M (with isomers N ‐5‐M, (E,E)‐5‐M, and (E,Z)‐5‐M) and b) relative energies of different isomers according to DFT calculations. L=neutral ligand.

Scheme 3

a) Synthesis of sodium and potassium crown ether ATI complexes 6‐M and 7. b) DFT‐calculated thermodynamic and kinetic parameters for the E/Z isomerisation of compound 7.

Figure 3

Solid‐state structures of alkali‐metal complexes 5‐M, 6‐Na, and 7. Hydrogen atoms are omitted and the carbon atoms of crown ethers are shown as wireframe for clarity. Displacement ellipsoids are depicted at the 50 % probability level.

Scheme 4

Synthesis of silver(I) complex 8 and sodium argentate(I) complexes 9 and 10.

Figure 4

Solid‐state molecular structures of 8, the anionic part in 9, and 10. Hydrogen atoms are omitted and pyridine as well as crown‐ether carbon atoms are depicted as wire model for clarity. Thermal displacement ellipsoids are depicted at the 50 % probability level.

Scheme 5

Syntheses of 3‐CPh₃‐substituted ATI complexes 11 and 12.

Figure 5

Solid‐state molecular structure of 11. Hydrogen atoms are omitted and carbon atoms of the crown ether moiety as well as the CPh₃ substituent are depicted as wire model for clarity. Displacement ellipsoids are depicted at the 50 % probability level. O1−N3 1.2796(3), O2−N3 1.2787(4), N1−C1 1.2901(3), N2−C2 1.2734(3), N3−C5 1.3752(4), C1−C2 1.5203(7), C2−C3 1.4933(3), C3−C4 1.3407(4), C4−C5 1.4459(4), C5−C6 1.4219(5), C6−C7 1.3526(4), C7−C1 1.4540(3), C3−C14 1.5586(4), O1−N3‐O2 118.552(41), O1−N3−C5 120.850(39), O2−N3−C5 120.597(38), N1−C1−C2 114.109(35), N2−C2−C1 124.887(36).88888