6,6′-{[Ethane-1,2-Diylbis(azaneylylidene)]bis(methaneylylidene)}bis[2-(4-Oxy(2,2,6,6-Tetramethylpiperidin-1-Oxyl)butyloxy)phenolato] Cobalt(II)

Abstract

Salen-type complexes with transition metals and corresponding polymers attract great scientific interest due to their outstanding electrochemical properties and catalytic potential. Cobalt complexes of this kind are known as catalysts for a large variety of redox reactions, including oxygen reduction. Herein, we report the preparation of a modified cobalt Salen-type complex, modified by TEMPO pendants linked to the core by flexible butylenedioxy linkers. The resulting product was characterized by electrospray–high-resolution mass spectrometry and Fourier-transform infrared spectroscopy.

1. Introduction

Complexes of cobalt with Salen-type Schiff base ligands, [Co(Salen)] are a well-known class of molecular and macromolecular catalysts. Due to the unique coordination chemistry of cobalt in the N,N,O,O square planar coordination environment, these complexes are able to effectively bind various axial ligands such as dioxygen [1] or carbon dioxide [2]. Also, [Co(Salen)] complexes show excellent catalytic efficiency towards various reduction and oxidation transformations of organic substrates [3,4,5], including enantioselective catalysis [6], and their polymers are considered prominent energy storage materials [7].

TEMPO derivatives are known for their oxidative properties and are widely used in the electrochemical oxidation of alcohol and amines [8] and oxygen reduction reactions [9]. TEMPO-containing metal–Salen complexes and their polymers meet applications in electrochemical energy storage [10,11]. Thus, the synergistic effect of Co and TEMPO metal sites can be expected for TEMPO-containing CoSalen systems. According to previous studies, the nature of the linker, in particular its length and flexibility, may dramatically alter the properties of multiredox molecules [12,13].

Herein, we report the synthesis of the CoSalen molecule with butylenedioxy TEMPO-containing pendants, namely 6,6′-{[ethane-1,2-diylbis(azaneylylidene)]bis(methaneylylidene)}bis[2-(4-oxy(2,2,6,6-tetramethylpiperidin-1-oxyl)butyloxy)phenolato] cobalt(II), using a facile and highly scalable procedure. The obtained product was characterized by high-resolution mass spectrometry (HRMS) and Fourier-transform infrared spectroscopy (FTIR) spectra.

2. Results

The desired product was obtained according to the following scheme: 4-OH-TEMPO was alkylated by an excessive amount of 1,4-dibromobutane, then the resulting bromide 1 was used for the alkylation of the 2,3-dihydroxybenzaldehyde to afford a substituted salicylic aldehyde 2, which was then converted to the corresponding Co complex 3 using the one-pot procedure of condensation with 1,2-diaminoethane followed by metalation with Co(OAc)₂.

Aldehyde 2 was prepared according to the previously reported protocol [14], as depicted in Scheme 1. Briefly, 4-OH-TEMPO was alkylated with an excess of 1,4-dibromobutane using NaOH as a base with the phase-transfer catalysis to obtain product 1, which was used for the alkylation of 2,3-dihydroxybenzaldehyde in DMSO with NaH as a base. The alkylation proceeded selectively on the -OH group in the 3-position, affording the corresponding aldehyde 2 a 76% overall yield. The structure of the product was confirmed by the ¹H NMR spectrum, with ascorbic acid used as a reductive agent to quench the paramagnetic nitroxyl radicals.

Aldehyde 2 was brought to condensation with 1,2-diaminoethane in refluxing ethanol. The metalation of the ligand with cobalt (II) acetate produced the product in low yields (below 30%), but the addition of excess TEA along with the metal salt improved the yield to 89%. Owing to the paramagnetic Co center, the NMR spectra of the product could not be acquired, and thus, the product was characterized by HRMS and FTIR spectroscopy.

The HRMS spectrum of product 3 (Figures S1 and S2) in the positive mode contained the [M]⁺ molecular ion peak (exp. 809.3865, calcd. for C₄₂H₆₂CoN₄O₈⁺: 809.3894). The FTIR spectrum recorded in KBr (Figure S3) contained a strong pair of peaks at 1634 and 1605 cm⁻¹ (C=N) and strong peaks at ca. 1250 and 1100 cm⁻¹, corresponding to the C-N and N-O stretching vibrations. Elemental analysis fits, within the accuracy of the method, were determined with either 3·2H₂O or 3·O₂. According to the literature, 3-alcoxy-substituted CoSalen-type complexes are prone to coordinate H₂O and/or O₂, so we were unable to distinguish these options on the basis of the existing data [15].

To confirm the radical nature of product 3, we obtained the EPR spectrum of this product (Figure 1). The resulting room temperature spectrum was a mixed signal due to the movement of the TEMPO-bearing linkers, so the distance between the two radicals changed over time [11], which led to a change in the intensity ratio of the spectrum. In this paper, we do not consider the dynamics of movement and instead use a two-component fit based on three- and five-line spectra, considering two types of molecules with closely and distantly located TEMPO groups, which gave a result that was quite close to the experimental spectra. The first component is a five-line structure resulting from the hyperfine interaction between an electron and two ¹⁴N atoms (S = ½, 2 × I = 1 system) of two TEMPO groups located in close proximity to one another. The second component is a three-line structure that corresponds to the hyperfine interaction of an electron and one ¹⁴N atom (S = ½, I = 1 system) of an individual TEMPO group located at a greater distance. We did not observe a contribution of ⁵⁹Co to the spectrum structure at room temperature, which was most likely due to the fast relaxation of spins in the Co orbitals. According to the experimental results, we can conclude that in a solution, compound 3, with shorter distances between free radical centers of TEMPO groups, is more populated. An analysis of compound 3 at 10 K and 100 K (Figure 2) in a frozen solution in 1:1 toluene/acetonitrile yielded a poorly resolved spectrum typical of immobilized, densely packed nitroxyl groups without any noticeable cobalt signal.

An analysis of a similar cobalt-based complex ([Co(CH₃OSalEn)], [15]) without the presence of TEMPO groups did not give an EPR signal at room temperature; however, it allowed us to obtain an early EPR signal of cobalt at 200 K (Figure 3a). Since the spectrum is fairly narrow and has no distinct structure, one can conclude that the electron density is delocalized throughout the conjugation system, which significantly reduces the contribution of hyperfine interactions with individual nuclei to the structure of the signal.

For comparison, a series of EPR spectra of [Co(CH₃OSalEn)] (Figure 3b) with additives of TEMPOL were obtained at 100 K (Figure 4). When TEMPOL was added, three distinct (typical for S = ½, I = 1 system) peaks appeared in the spectrum, with clear anisotropy describing TEMPO groups in the frozen state. As the TEMPOL concentration increased, the broader cobalt peak became less noticeable in the overall picture. We assume that material 3 will show similar behavior, complicated by dipole–dipole and exchange interactions of more densely packed TEMPO groups, which leads to a broadening of the spectrum and even greater overlapping of the spectra.

As a result, the first CoSalen-type complex with TEMPO pendants was synthesized at an overall yield of 45% after three steps. The obtained complex may be employed as a monomer for a variety of prominent polymeric functional materials for energy storage, catalysis, and sensing. Due to the long and flexible linkers, the complex is highly soluble in organic solvents. It is noteworthy that heating the complex at 60 °C in vacuo results in a loss of c.a. 3% in the initial weight, which is restored after a short exposition to air or oxygen.

3. Materials and Methods

3.1. General Consideration

Reagents of “reagent grade” purity were purchased from Sigma-Aldrich (Taufkirchen, Germany). The Fourier-transform infrared spectra were recorded on the Shimadzu IRaffinity-1 FTIR spectrophotometer (Shimadzu Europa GmbH, Kyoto, Japan) in KBr pellets. The HRMS spectrum was recorded using electrospray ionization on a Bruker maXis apparatus (Bruker Analytische Messtechnik GmbH, Rheinstetten, Germany). EPR spectra were recorded at X-band frequencies (~9.4 GHz) at room temperature and 100 K using a MiniScope MS-5000 (Freiberg Instruments, Germany) spectrometer with a nitrogen cooling system. EPR spectra at 10–100 K were recorded using a laboratory-built EPR spectrometer: the magnetic field was regulated using a Bruker BH15 field controller and monitored with a Bruker ER 035M NMR Gaussmeter while a Bruker ER 041MR microwave bridge (with an ER 048 R microwave controller) was used for microwave generation (200 mW source) and detection (diode detection). The static magnetic field was modulated at 100 kHz, and lock-in detection was carried out using a Stanford Research SR810 lock-in amplifier in combination with a Wangine WPA-120 audio amplifier. An ESR 910 helium flow cryostat, together with an ITC503 temperature controller (Oxford Instruments, Abingdon, UK), was used for low-temperature measurements. The samples were measured in 10–100 µM toluene or a 1:1 toluene/acetonitrile solution at 10 dB–37 dB (10–0.039 mW power) and 1–0.15 mT modulation amplitudes with 100 kHz modulation frequency. Fitting was performed using the Easyspin v.6.0.5 toolbox [16], as well as 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 compound 2 (215 mg, 0.59 mmol) in 3 mL of EtOH, 1,2-diaminoethane (18 mg, 0.3 mmol) was added. The resulting mixture was stirred under Ar at 60 °C for 48 h. After completion of the reaction, monitored by TLC, a solution of Co(OAc)₂·4H₂O (74 mg, 0.3 mmol) in 4 mL of EtOH was added to the reaction mixture, followed by triethylamine (1 mL). The reaction was further stirred under Ar at 60 °C for 48 h. After the completion of the reaction, monitored by TLC, the reaction mixture was cooled in a fridge and centrifuged before the precipitate was washed twice with 40% aqueous EtOH and dried in vacuo at 50 °C, producing the desired product as a brownish solid (213 mg, 0.26 mmol, 89%). FTIR (KBr) ῦ, cm⁻¹: 2800–3000 (C-H), 1634 and 1605 (C=N), 1250 (C-N), and 1100 (N-O). HRMS (ESI) m/z [M]⁺ calcd for C₄₂H₆₂CoN₄O₈⁺ 809.3894 found 809.3865. CHN: calcd for C₄₂H₆₂CoN₄O₈·O₂: C 59.92%, H 7.42%, N 6.65% and calcd. for C₄₂H₆₂CoN₄O₈·2H₂O: C 59.63%, H 7.86%, N 6.62% found C 59.69%, H 7.63%, N 6.32%.