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 NO 2 group actively contributes to the coordination chemistry of these complexes, effectively com-peting 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.

Sulfinyl-aminotroponiminates: alkali- (Li, Na, K) and heavy-metal (Bi) complexes

“…The variation of C–C bond lengths in the ligand backbone of ATI complexes has been used as a simple indicator for electron delocalization (related to the aromatic character of the N 2 C 7 moiety in ATI species). This C–C bond lengths alternation in compounds 2-Li-py , 2-K-thf , and 3 (Δ(C–C), 0.06–0.08 Å (excluding C1–C2)) is less pronounced than in related NO 2 -substituted species (Δ(C–C), 0.11–0.13 Å (excluding C1–C2)), 20 giving further evidence for the sulfinyl group modifying the structural parameters of ATI compounds in a more subtle way than the NO 2 moiety.…”

“…46 In contrast, addition of one equivalent of 15-crown-5 to the nitro-substituted ATI complex [Na(NO 2 –ATI iPr/iPr )] afforded the crown-ether adduct [(15-crown-5)Na(NO 2 –ATI iPr/iPr )], in which the alkali metal is not anymore coordinated by the N , N binding pocket, but via the NO 2 functional group. 20…”

“…S16 and S17†), with analogies to previous reports on ATI-bound NO 2 and phenyl groups acting as Lewis bases towards alkali metal ions. 20,43 The distortion of the coordination polyhedra can be traced back to the small bite angle of the ATI ligand ( 2-Li-py : N1–Li1–N2, 76.7(3)°, 2-K-thf : N1–K1–N2, 58.43°). The Li1–N1/2 and Li1–O bond lengths of 2-Li-py (Li–N, 2.03 Å; Li–O 1.96 Å) are within limits of error identical to those of the parent compound [Li–ATI iPr/iPr (thf) 2 ] (Li–N, 1.99–2.00 Å; Li–O, 1.96–1.97 Å).…”

“…1–5 Their characteristic seven-membered cyclic backbone distinguishes them from closely related diazadiene, 6–9 phenylenediamine, 10–12 amidinate 13–16 and β-diketiminate 17,18 ligands, and has recently been subject to chemical modification. 19,20…”

Tetranitro- and tetraamino-dibenzo[18]crown-6-ether derivatives: complexes for alkali metal ions, redox potentials, crystal structures, molecular sorption, and proton conducting behaviours

“…3g). 64,65 The face-to-face arrangement of two O6N1-coordinated [Rb(1)I]•CH 3 CN forms a cage-like + -encapsulated structure. Further increasing the cation size from Rb + to Cs + generated a 1D coordination polymer along the b-axis, where the oxygen atoms of all -NO 2 groups were coordinated to Cs + cations, forming a 1D Cs +coordination polymer (Fig.…”