6,6′-{[Ethane-1,2-diylbis(azaneylylidene)]bis(methaneylylidene)}bis[(1-oxyl-2,2,5,5-tetramethylpyrrolidine-3-carboxy)phenolato] Nickel(II)

Abstract

Conductive polymers with TEMPO pendants are considered as a promising class of energy storage materials. However, the rational design of such materials demands for the minimization of the molar mass of the unit. The utilization of the PROXYL stable radical instead of TEMPO may decrease the weight of the monomeric unit and, thus, improve the capacity of the materials. Herein, we report a NiSalen complex with 3-carboxy-PROXYL pendants, designed to decrease the molar mass of the complex. The resulting product was characterized by ¹H NMR, electrospray high-resolution mass spectrometry and EPR.

1. Introduction

Polymeric nickel complexes of Salen-type Schiff base ligands, poly[Ni(Salen)], are considered as a promising family of energy storage and electrocatalytic materials [1]. Due to their high electrical conductivity and redox capacity, these polymers meet their applications in electrochemical power sources [2,3,4,5,6,7,8], chemical battery protection elements [9] and (photo)elecrocatalysis [10,11]. At the same time, energy storage materials are expected to deliver greater redox capacity, which could be fulfilled by the use of redox polymers, containing isolated redox centers such as nitroxyl radicals [12,13,14,15], quinones [16,17,18,19], ferrocene [20,21] or phenothiazine [22]. Unfortunately, redox polymers exhibit poor electrical conductivity, which hinders their application in energy storage devices.

Of particular interest are the redox-conductive polymers, which combine the benefits of conductive and redox polymers in the single macromolecule. As shown previously, redox-conductive polymers based on a poly[Ni(Salen)] backbone and TEMPO redox pendants provide a robust and versatile platform for the construction of the energy storage materials exhibiting both high power and acceptable energy density [23,24].

To increase further the energy density of these materials, it is necessary to reduce the molar mass of the monomeric unit with the preservation of its functional design. Among the possibilities for the mass reduction, the use of PROXYL redox unit instead of the TEMPO one is proposed. Herein, we report the synthesis of the PROXYL-containing [Ni(Salen)] complex, which is considered as a monomer for the preparation of the high-energy materials for electrochemical power sources. Polymerization of this monomer may afford a cathode material for Li-ion batteries or supercapacitors with theoretical capacity above 100 mAh g⁻¹, which, in combination with ultrafast redox kinetics of nitroxyl radicals, will result in outstanding energy storage properties. The title product, 6,6′-{[ethane-1,2-diylbis(azaneylylidene)]bis(methaneylylidene)}bis[(1-oxyl-2,2,5,5-tetramethylpyrrolidine-3-carboxy)phenolato] nickel(II), was obtained by a two-step procedure. The obtained product was characterized with ¹H NMR, HRMS and EPR.

2. Results

Synthesis of the desired complex 4 was conducted as depicted in Scheme 1. Namely, 3-carboxyPROXYL 1 was brought to reaction with 2,3-dihydroxybenzaldehyde 2 according to the Steglich esterification protocol. Acylation of the 2,3-dihydroxybenzaldehyde proceeded selectively in 3-position, affording the 3-formyl-2-hydroxyphenyl 1-oxyl-2,2,5,5-tetramethylpyrrolidine-3-carboxylate 3 in 89% yield. Due to the presence of paramagnetic center, the ¹H NMR spectrum of compound 3 was obtained in DMSO-d₆, with ascorbic acid as a quenching agent. The proton spectrum contains a set of four methyl singlets of 3-substituted PROXYL in the range of 1.0–1.5 ppm, three signals of the PROXYL periphery around 2 (two signals) and 3 (one signal) ppm, three aromatic signals of 1,2,3-trisubstitued benzene ring in the 7.0–7.6 range and an aldehyde singlet at 10.21 ppm (Figure S1).

The obtained aldehyde 3 was, without further purification, brought to condensation with 1,2-diaminoethane in ethanol. Ligand formed in this reaction was in situ metalated with nickel acetate, affording the target product 4 in 57% yield. ¹H NMR spectrum, recorded with ascorbic acid as a quenching agent, reveals that the product consists of two diastereomers due to the chirality of each 3-carboxyPROXYL fragment. The spectrum contains a multiple set of overlapping singlets at 1.0–1.3 ppm, corresponding to the methyl groups of PROXYL, three signals in aliphatic region from PROXYL periphery, a multiplet of ethylene bridge, partially overlapped with the ascorbic acid signals, three aromatic signals of 1,2,3-trisubstitued benzene rings and two singlets from the imine protons of diastereomeric complexes (Figure S2). In the HRMS spectrum, molecular ion [M + H]⁺ was found (for C₃₄H₄₃N₄NiO₈⁺ calcd. 693.2429 found 693.2419) with a small portion of [M]⁺ ion (Figure S3). The FTIR spectrum contains characteristic peaks of ester at 1751 and imine at 1621 (Figure S4).

To confirm the presence of the free radical centers in the structure of 4, we have acquired the EPR spectrum (Figure 1). The resulting spectrum is a mixed signal due to the movement of the PROXYL-bearing linkers, so that the distance between the two radicals changes over time [24]. In this paper, we do not consider the dynamics of movement and use two-component fit based on three- and five-line spectra, which gives a result that is quite close to experimental spectra. The first component is a five-line structure resulting from hyperfine interaction between electron and two ¹⁴N atoms (S = ½, 2 × I = 1 system) of two PROXYL groups located in close proximity to one another. The second component is a three-line structure which corresponds to hyperfine interaction of electron and one ¹⁴N atom (S = ½, I = 1 system) of an individual PROXYL group located at a greater distance. According to the experimental results we can conclude that in a solution a conformation of compound 4 with shorter distance between free radical centers of PROXYL groups is more populated.

As a result, a first NiSalen-type complex with PROXYL pendants was synthesized in an overall yield of 51% after two steps. The obtained complex may be employed as a monomer for a variety of prominent polymeric functional materials for energy storage, catalysis and sensing. Comparing with the corresponding TEMPO-containing monomer, the molar mass of this complex is 20.8% lower, while the number of the redox centers per monomeric unit is the same, which may significantly improve the energy storage parameters of the materials based thereof.

3. Materials and Methods

3.1. General Consideration

Reagents of “reagent grade” purity were purchased from Sigma–Aldrich (Europe). ¹H spectra were acquired on a Bruker Avance 400 spectrometer (Bruker Analytische Messtechnik GmbH, Rheinstetten, Germany) at 400 MHz in DMSO-d₆. Before NMR analysis, the paramagnetic center of nitroxyl radical residues was reduced in situ by ascorbic acid. The Fourier transform infrared spectra were recorded on Shimadzu IRaffinity-1 FTIR spectrophotometer (Shimadzu Europa GmbH, Kyoto, Japan) in KBr pellets. HRMS spectrum was recorded using electrospray ionization on a Bruker microTOF apparatus (Bruker Analytische Messtechnik GmbH, Rheinstetten, Germany). EPR spectra were recorded at X-band frequencies (~9.4 GHz) using a MiniScope MS-5000 (Freiberg Instruments, Germany) spectrometer. The sample was measured at room temperature in 100 mM toluene solution at 13 dB (5.01 mW power) and 0.05 mT modulation amplitude with 100 kHz modulation frequency. Fitting was performed using Easyspin v.6.0.5 toolbox [25] using the pepper function with isotropic g and A parameters and Lorentzian shape lines as shown in Figure 1.

3.2. Synthesis of 3

To a solution of 3-carboxyPROXYL (150 mg, 0.81 mmol) and DCC (183 mg, 0.89 mmol) in 6 mL of dry DCM, a solution of 2,3-dihydroxybenzaldehyde (134 mg, 0.97 mmol) and DMAP (10 mg, 81 µmol) in 3 mL of dry DCM was added in one portion, and the reaction mixture was stirred at RT for 18 h; reaction progress was monitored by TLC. Then, the dicyclohexylurea was filtered off, and the residue was purified by flash chromatography (SiO2, hexane—EtOAc 20:1 → 2:1) and crystallized from pentane to obtain product 3 as yellow crystals (221 mg, 0.72 mmol, 89%).

M.p. 174–177 °C (pentane). ¹H NMR (400 MHz, DMSO-d₆ + ascorbic acid) δ, ppm: 10.21 (s, 1H), 7.63 (dd, J = 7.9, 1.6 Hz, 1H), 7.37 (dd, J = 7.8, 1.6 Hz, 1H), 7.04 (t, J = 7.8 Hz, 1H), 3.08 (dd, J = 11.9, 8.1 Hz, 1H), 2.15–1.95 (m, 1H), 1.85 (dd, J = 11.9, 8.0 Hz, 1H), 1.33 (s, 3H), 1.13 (s, 3H), 1.10 (s, 3H), 1.08 (s, 3H).

3.3. Synthesis of 4

Aldehyde 1 (48 mg, 0.16 mmol) and 1,2-diaminoethane (4.8 mg, 0.08 mmol) were mixed together in 2.5 mL of ethanol and stirred at RT for 1 h. After completion of the reaction (monitored by TLC), Ni(OAc)₂·4H₂O (19.9 mg, 0.08 mmol) was added to the reaction mixture, which was heated at 70 °C for 3 h in a sealed vial. After disappearance of the ligand spot on TLC, the mixture was cooled in a fridge overnight, centrifuged, washed with 2 mL of EtOH and dried in vacuo. As a result, desired product 4 was obtained as a yellow solid (31 mg, 0.09 mmol, 57%).

M.p. 262–265 °C (dec., EtOH). ¹H NMR (400 MHz, DMSO-d6 + ascorbic acid) δ, ppm: 8.11 (s, 1H), 8.09 (s, 1H), 7.20 (d, J = 8.0 Hz, 2H), 6.93 (ddd, J = 7.6, 6.0, 1.7 Hz, 2H), 6.49 (td, J = 7.7, 2.4 Hz, 2H), 3.45–3.55 (m, 4H), 3.02–2.70 (m, 2H), 2.03 (ddd, J = 13.0, 10.1, 8.3 Hz, 2H), 1.81 (ddd, J = 12.9, 10.3 Hz, 7.9 Hz, 2H), 1.29 (s, 3H), 1.27 (s, 3H), 1.12 (s, 3H), 1.09 (m, 9H), 1.01 (s, 3H), 1.00 (s, 3H). HRMS (ESI) m/z [M + H]⁺ calcd for C₃₄H₄₃N₄NiO₈⁺ 693.2429, found 693.2419. FTIR (KBr) ῦ, cm⁻¹: 2800–3000 (C-H), 1751 (CO₂R) and 1621 (C=N).