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Article

Electrochemically Active Copper Complexes with Pyridine-Alkoxide Ligands

by
Christopher K. Webber
,
Erica K. Richardson
,
Diane A. Dickie
and
T. Brent Gunnoe
*
Department of Chemistry, University of Virginia, Charlottesville, Virginia, VA 22904, USA
*
Author to whom correspondence should be addressed.
Inorganics 2024, 12(8), 200; https://doi.org/10.3390/inorganics12080200
Submission received: 12 June 2024 / Revised: 12 July 2024 / Accepted: 17 July 2024 / Published: 24 July 2024
(This article belongs to the Special Issue Feature Papers in Organometallic Chemistry 2024)
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Abstract

:
Pyridine-alkoxide (pyalk) ligands that support transition metals have been studied for their use in electrocatalytic applications. Herein, we used the pyalk proligands diphenyl(pyridin-2-yl)methanol ([H]PhPyalk, L1), 1-(pyren-1-yl)-1-(pyridin-2-yl)ethan-1-ol ([H]PyrPyalk, L2), 1-(pyridine-2-yl)-1-(thiophen-2-yl)ethan-1-ol ([H]ThioPyalk, L3), and 1-(ferrocenyl)-1-(pyridin-2-yl)ethan-1-ol ([H]FePyalk, L4) to synthesize CuII complexes that vary in nuclearity and secondary coordination sphere. Also, the proligand 1-(ferrocenyl)-1-(5-methoxy-pyridin-2-yl)ethan-1-ol ([H]FeOMePyalk, L5) was synthesized with a methoxy substituted pyridine; however, the isolation of a CuII complex ligated by L5 was not possible. Under variable reaction conditions, the pyalk ligands reacted with CuII precursors and formed either mononuclear or dinuclear CuII complexes depending on the amount of ligand added. The resulting complexes were characterized by single crystal X-ray diffraction, elemental analysis, and cyclic voltammetry.

1. Introduction

Ligands with pyridine-alkoxide (pyalk) groups have been demonstrated to stabilize high-oxidation-state metal complexes, including examples of pyalk-transition metal complexes that catalyze oxidative transformations [1,2]. For example, the Brudvig and Crabtree groups, among others, have demonstrated the utility of pyalk-type ligands for oxidative electrocatalytic applications. Examples containing pyalk ligands include piano-stool IrIII–pentamethylcyclopentadienyl complexes, which are active for water oxidation in acidic media, which demonstrates their durability under harsh oxidizing conditions [3,4]. Also, the use of pyalk-type ligands with CuII complexes for both water and ammonia electrocatalytic oxidation has been demonstrated (Figure 1) [5,6,7,8]. Likewise, complexes that incorporate multiple Cu active sites have been shown to benefit electrocatalytic water oxidation [9,10,11,12]. Efforts to synthesize ligands that contain multiple pyalk sites for multinuclear complexes have been studied [13]. More recently, it was demonstrated that a tetranuclear cubane-pyalk structure rearranges into a dinuclear bridging alkoxide structure and slowly decomposes to CuO in alkaline oxidative conditions [14].
The synthesis of pyalk ligands from commercially available ketone-containing compounds potentially allows a variety of secondary coordination sphere functional groups to be incorporated into ligand structures. Many aromatic groups are redox active and can undergo transformations under oxidative conditions. For example, pyrene and thiophene are both known to electropolymerize under oxidative potentials and have been explored for various applications [15,16]. Pyrene has been shown to be amenable toward noncovalent immobilization on electrode surfaces, with multiple pyrene-substituted complexes immobilized on conductive materials being reported for electrocatalytic water oxidation [3,17]. Thiophene-based hybrid polymers have been reported as effective water oxidation electrocatalysts [18]. Ferrocene has been explored for various electrochemical applications due to its stability and the redox flexibility of the FeII/FeIII redox cycle [19]. Examples of ferrocene benefitting electrocatalytic processes through redox mediation have been reported [20].
Herein, we report the synthesis of CuII complexes with various aromatic-group-substituted pyalk ligands. We found that alkoxide-bridged dinuclear CuII complexes are accessible by treating CuII precursors with <1 eq of ligand. Three new pyalk ligand precursors have been characterized by 1H and 13C{1H} NMR spectroscopy, single crystal X-ray diffraction, and elemental analysis. Further, CuII complexes containing these new ligands, as well as two previously reported ligands, were characterized by single crystal X-ray diffraction, elemental analysis, and cyclic voltammetry.

2. Results and Discussion

Synthesis and characterization of ligands. The proligands [H]PhPyalk (L1) and [H]PyrPyalk (L2) were synthesized using the previously reported lithiation reactions of 2-bromopyridine and their respective ketone precursors [3,4]. The proligands [H]ThioPyalk (L3), [H]FePyalk (L4), and [H]FeOMePyalk (L5) were synthesized in a similar method using 2-acetyl thiophene and acetylferrocene as their ketone precursors (Scheme 1). For the synthesis of L5, 2-methyoxy-5-bromopyridine was used as the pyridine precursor (Table 1). The proligand L3 was isolated cleanly as a white solid, and the 1H NMR spectrum of L3 in CDCl3 shows seven resonances in the aromatic region, corresponding to four pyridine and three thiophene protons. The synthesis and purification of L4 and L5 gave reddish-brown solids. The 1H NMR spectra in CDCl3 of L4 and L5 indicated one cyclopentadienyl (Cp) ring on the ferrocenyl moiety that has five equivalent proton resonances. The pyalk-substituted Cp rings of L4 and L5 show four distinct 1H NMR peaks due to the chiral center at the tertiary carbon of the pyalk compound. The 13C{1H} NMR spectra of L4 and L5 further support the structures of L4 and L5 with five distinct carbon peaks, as opposed to three if there were mirror symmetry. Compounds L3-L5 were each isolated in moderate yields and characterized by 1H and 13C{1H} NMR, single crystal X-ray diffraction (Figure 2), and elemental analysis.
Synthesis and Characterization of Complexes. Monocopper complexes Cu(PhPyalk)2 (1a), Cu(PyrPyalk)2 (2a), Cu(ThioPyalk)2 (3a), and Cu(FePyalk)2 (4a) (the label “a” indicates a mononuclear copper complex) were synthesized by treating CuCl2·2H2O with two equivalents of ligand in the presence of base, resulting in mononuclear CuII complexes with two ligands coordinated (Scheme 2a). The ligand L5 did not form a complex with CuII under any conditions attempted. Upon the addition of 1 M KOH/MeOH into methanolic solutions of two equivalents of L1, L2, and L3 with CuCl2·2H2O, the solutions immediately turn purple, and 1a and 2a slowly began forming a pinkish/purple precipitate (3a was more soluble in methanol than 1a or 2a). Solutions of L4 and CuCl2·2H2O began as brown/red solutions that turned yellow upon the addition of base and likewise began precipitating a yellow solid. The solid-state structures of 1a-4a are shown in Figure 3. The solid-state structure of 2a exhibited close aryl–aryl interatomic distances, potentially indicating non-covalent interactions in the solid state between pyrene groups (Figure S31). In the unit cell, the ferrocene group in 4a is likewise interlocked between the ferrocene moieties of another 4a molecule, indicating a potential non-covalent interaction as well (Figure S32). Also, there are interstitial molecules of methanol and water in hydrogen bonding interactions with the basic alkoxides of 4a (Figure S33).
Each of the monocopper complexes 1a–4a exhibit square planar geometries that are typical of CuII [21]. The Cu–N bond distances range from 1.9598(12) Å to 1.980(3) Å, and the Cu–O bond distances range from 1.8832(16) Å to 1.9158(8) Å, which are in close agreement with the bond lengths of similar CuII complexes previously reported by Crabtree and Brudvig (selected bond lengths and angles shown in Table 2) [5,8]. These complexes were paramagnetic and insoluble in most solvents except chlorocarbons and 2,2,2-trifluoroethanol (TFE). Due to their paramagnetic nature, NMR spectra were not useful.
The formation of bis–copper complexes can be accomplished with control of the ligand/Cu stoichiometry. Thus, the bis–copper complexes {CuCl}2(µ-O-PhPyalk)2 (1b), {CuCl}2(µ-O-PyrPyalk)2 (2b), {CuCl}2(µ-O-ThioPyalk)2 (3b), and {CuCl}2(µ-O-FePyalk)2 (4b) (the label “b” indicates dinuclear complexes) were synthesized in the same manner as 1a–4a but with 0.9 equivalent of ligand resulting in a dinuclear alkoxide-bridged complex (Scheme 2b). The reactions occurred similarly to the monocopper complexes; however, in this case, the color changed from blue to dark green upon the slow addition of base into the methanolic solution, and the product precipitated as green solids. The solid-state structures of 1b–4b demonstrate the flexibility of alkoxide moieties to bridge two metal centers (Figure 4-1b,3b,4b; Figure S34-2b). These structures closely resemble recently reported structures of the potential decomposition of CuII pyridine-alkoxide water oxidation catalyst products [14]. Similar to 1a–4a, due to the paramagnetic nature of 1b–4b, NMR spectra were not useful. The local geometry at Cu does not change significantly between monocopper and bis–copper pyalk complexes, as demonstrated by similar Cu–ligand bond lengths (Table 2).
Cyclic voltammetry. The relative oxidative stability of the ligands and complexes were probed via cyclic voltammetry in 2,2,2-trifluoroethanol (TFE). The compound L1 exhibited an irreversible oxidation at Ep,a = 1.64 V vs. Fc/Fc+ (Fc/Fc+ = E1/2 of the FeII/FeIII of ferrocene) (Figure 5a). The pyrene containing ligand L2 displayed a broad irreversible oxidation peak when scanned anodically (Figure 5b). After cycling through the irreversible oxidation multiple times, a reversible peak formed with an E1/2 of 0.43 V vs. Fc/Fc+. This reversible peak was retained after rinsing the working electrode and scanning anodically in a fresh electrolyte solution containing no L2. Pyrene has been demonstrated to form polymers when placed under oxidative potentials due to a coupling reaction between radical pyrenes [15]. The cyclic voltammogram of L3 shows a redox event at Ep,a = 1.44 V vs. Fc/Fc+ (Figure 5c). This oxidation does not result in the formation of a new reversible peak as was observed with L2. Although it is common for thiophene-containing molecules to electropolymerize [16], the steric bulk of the pyalk potentially prevents the polymerization reaction. The compound L4 exhibits an FeII/FeIII E1/2 at 0.08 V vs. Fc/Fc+ that scan rate studies indicate is a freely diffusing process (Figure S7). Upon scanning more anodic of the FeII/FeIII oxidation, an irreversible peak appeared at Ep,a = 1.20 V vs. Fc/Fc+, and the subsequent scan of the ferrocene peak showed the formation of a new reversible peak with an E1/2 = 0.23 V vs. Fc/Fc+, which lies between the FeII/FeIII redox feature of ferrocene and acetylferrocene (Figure 5d). The peak-to-peak separation for this feature is 0.09 V, and the current ratio is 1.23. The compound L5 had oxidative responses similar to L4, with an FeII/FeIII E1/2 = 0.05 V vs. Fc/Fc+ as well as an irreversible peak at Ep,a = 1.42 V vs. Fc/Fc+ and a degradation species E1/2 of 0.20 V vs. Fc/Fc+.
Previously reported mechanisms of tertiary alcohol oxidation include radical mechanisms in which a C–H bond is cleaved. For example, when tert-butanol is oxidized by hydroxyl radicals, a C–H bond is cleaved to form acetone and methyl radical, which can then be oxidized to formaldehyde [22]. Tertiary alcohols also can be oxidized via dehydration mechanisms and the subsequent oxidation of the product. Triphenyl methanol, which bears similarity to the ligands described here, does not contain easily abstracted C–H bonds, which results in the formation of stable peroxide compounds when treated with hydrogen peroxide [23]. The presence of an FeII/FeIII oxidation after the oxidative decomposition of L4 with an E1/2 in between that of ferrocene and acetylferrocene indicates that the methyl substituent is likely lost, and the alcohol is oxidized to a ketone.
The mononuclear complexes 1a and 3a each have a reversible CuII/CuIII event in the oxidative window (E1/2 = 0.87 V and 0.94 V vs. Fc/Fc+, respectively) (Figure 6a). The CuII/CuIII oxidation potentials are similar to the oxidation potentials reported by the Brudvig and Crabtree groups of a similar complex, but direct comparisons cannot be made due to our use of TFE (as opposed to water) as the cyclic voltammetry solvent [5]. When oxidized past the reversible peak at 0.94 V, 3a gives an irreversible peak at Ep,a = 1.61 V vs. Fc/Fc+, and the reversibility of the CuII/CuIII cycle is lost, indicating oxidative decomposition (Figure S24). When scanned to the same potential, complex 1a did not display an irreversible peak, indicating stability under oxidizing conditions. The increase in the stability of 1a compared to 3a may be due to the lack of easily oxidizable methyl C–H bonds in 1a compared to 3a. When scanned to oxidative potentials, 2a shows an irreversible peak that is similar to L2, indicating that 2a likely electropolymerizes (Figure 6b,c). When swept multiple times, the solution of 2a turned from pink to yellow, potentially indicating the complex is protonated by freed protons released during the polymerization reaction (Figure S18). In previous work, it has been demonstrated that electropolymerization can impact pH by releasing free protons into the electrolyte [24]. A reversible peak similar to the peak formed by the electropolymerization of L4 was observed after rinsing of the electrode and scanning in fresh electrolyte solution containing no 2a (Figure 6c). Cyclic voltammetry of 4a shows a typical FeII/FeIII oxidation event at 0.06 V vs. Fc/Fc+ (Figure 6d); however, when scanned to more positive potentials, two irreversible oxidations appeared at Ep,a = 1.12 and Ep,a = 1.41 V vs. Fc/Fc+ (Figure S25). After scanning through the irreversible peaks, a new species forms with an E1/2 = 0.22 V vs. Fc/Fc+, which is similar to the degradation of L4 and L5. The current density of the FeII/FeIII oxidation of 4a is approximately double the current density of the FeII/FeIII oxidation of the ligand L4 at the same scan rates and concentrations, which may indicate that both ferrocenes are oxidized. A summary of the oxidative stability of 1a-4a is included in Table 3.
The mononuclear complexes were studied under reducing conditions to −1.3 V (vs. Fc/Fc+). The cathodic scans of each mononuclear complex were quasi-reversible and led to complicated broad reoxidation peaks that ranged from 0.1–0.29 V vs. Fc/Fc+ depending on the complex used (Figures S17, S19, S23 and S29). Upon scanning anodically after reducing, a broad oxidation wave appears with Ep,a’s ranging from 0.1 to 0.29 V vs. Fc/Fc+. The low reducing potentials required likely result from the donating nature of the pyalk ligand. The complicated nature of the reduction and subsequent reoxidation process may be due to disproportionation or the direct two-electron reduction of CuII to Cu0 [25].
The dinuclear 1b, 3b, and 4b complexes did not show CuII/III oxidations within the stable oxidative window of TFE, indicating that the presence of two pyridine-alkoxide ligands per Cu are necessary to lower CuII/CuIII potentials into observable electrochemical windows. Complex 2b is insoluble in TFE.

3. Experimental Section

Materials, methods, and instrumentation. Reagent-grade chemicals were purchased from Ambeed (Arlington Heights, IL, USA), TCI Chemical (Japan) or Sigma-Aldrich (St. Louis, MO, USA) and used for synthetic procedures without further purification. Standard Schlenk and glovebox techniques were utilized for the synthesis of ligands L1–L5. Glovebox N2 purity was maintain by periodic purges, and O2 and H2O levels were kept below 20 ppm. For NMR experiments, either a Varian (Palo Alto, CA, USA) 600 MHz or a Bruker (Billerica, MA, USA) 800 MHz spectrometer was used. All reported chemical shifts were referenced to residual 1H resonances (1H NMR) or 13C{1H} resonances (13C{1H} NMR) of chloroform-d. 1H NMR: 7.26 ppm, 13C{1H} NMR: 77.2 ppm [26]. Tetrahydrofuran was purified by passage through an alumina oxide column and stored under N2 over 4 Å molecular sieves prior to use. The solvent 2,2,2-trifluoroethanol was purchased from Oakwood Chemical (Estill, SC, USA), refluxed over CaSO4 (purchased from Sigma-Aldrich), and stored in an N2-filled glovebox prior to use. Tetrabutylammonium hexafluorophosphate (TBAPF6) was purchased from a commercial source and used as received (purchased from Sigma-Aldrich). All cyclic voltammetry data were recorded on a CH Instruments (Austin, TX, USA) CHI630E potentiostat. All scans are referenced to the E1/2 of the FeII/FeIII redox event of ferrocene (purchased from Millipore Corporation, Burlington, MA, USA), added at the end of experiments. In the case of overlap between a ligand or complex with the FeII/FeIII of ferrocene, acetylferrocene was used (using our conditions, the E1/2 of the FeII/FeIII oxidation was 0.35 V vs. Fc/Fc+). An Ag/AgCl pseudo-reference electrode stored in a TFE solution containing 0.1 M TBAPF6 was used as an internal reference. All cyclic voltammetry scans were taken of 1 mM solutions of the analyte. The scans shown in the main text were all taken at a scan rate of 100 mV/s. A 3.00 mm (0.0707 cm2) geometric-diameter glassy carbon (GC) working electrode purchased from BASi (West Lafayette, IN, USA) was used. Elemental analyses were performed in-house using a Perkin-Elmer (Waltham, MA, USA) CHNS/O series 2 elemental analyzer. Details of the single-crystal diffraction measurements are given in the SI. Ligands L1 and L2 were synthesized as described previously [3,4]. L4 and 1a were previously reported using a different synthetic pathway [27,28].
[H]ThioPyalk (L3). To a 250 mL Schlenk flask inside an N2-filled glovebox, 1.20 mL (1.99 g, 12.5 mmol) of 2-bromopyridine and ~100 mL THF were added. The flask was then sealed and placed under N2 on a Schlenk line and cooled to −78 °C. Next, 5.5 mL of 2.5 M n-BuLi (solution in hexanes) were added dropwise to the reaction, which resulted in a color change from colorless to yellow and then red. The reaction was stirred at −78 °C for two hours. After two hours, a separate 50 mL round-bottom flask was charged with 2.20 mL (2.56 g, 12.5 mmol) of acetyl thiophene and diluted to ~25 mL with THF inside an N2-filled glovebox and sealed. The acetyl thiophene solution was then added via cannula into the reaction vessel under N2, and the reaction mixture was allowed to return to room temperature overnight while stirring. The next day, the reaction was removed from N2, and 100 mL of H2O were added. The organic layer was then extracted three times with DCM, dried over Na2SO4, and then filtered through a Celite-packed medium-porosity frit. The filtrate was then evaporated, and the resulting brown oil was placed in a freezer at −15 °C. Crystals were crushed and filtered from the oil and washed with 10 mL hexanes (3x) and then dried to form a white powder (0.598 g, 23%). Single crystals suitable for X-ray diffraction were found after the slow evaporation of a solution of benzene and hexanes containing L3. 1H NMR (600 MHz, CDCl3) δ 8.53 (d, 3JHH = 5 Hz, 1H, pyridine proton), 7.69 (dd, 3JHH = 8 Hz, 1H, pyridine proton), 7.38 (d, 3JHH = 8 Hz, 1H, pyridine proton), 7.24–7.18 (m, 2H, thiophene/pyridine proton), 6.98 (m, 1H, thiophene proton), 6.94 (m, 1H, thiophene proton), 6.20 (s, 1H, alkoxyl proton), 1.97 (s, 3H, methyl protons). 13C{1H} NMR (201 MHz, CDCl3) δ 163.8, 153.0, 147.3, 137.4, 126.8, 125.0, 123.6, 122.5, 120.1, 73.9, 30.8. Anal. Calcd. for C11H11NOS: C, 64.36; H, 5.40; N, 6.82. Found: C, 64.20; H, 5.26; N, 6.83.
[H]FePyalk (L4). To a 250 mL Schlenk flask inside an N2-filled glovebox, 2.99 g (18.9 mmol) of 2-bromopyridine and ~100 mL THF were added. The flask was then sealed and placed under N2 on a Schlenk line and cooled to −78 °C. Next, 7.6 mL of 2.5 M n-BuLi (solution in hexanes) were added dropwise to the reaction, and the mixture changed from colorless to yellow and then red. The reaction was then stirred at −78 °C for two hours. After two hours, a separate 100 mL round-bottom flask was charged with 3.98 g (17.5 mmol) of acetylferrocene and ~25 mL THF inside an N2-filled glovebox and sealed. The acetylferrocene solution was then added via cannula, and the mixture was allowed to return to room temperature overnight while stirring. The next day, the reaction was removed from N2, and 150 mL of H2O were added. The organic layer was extracted three times with DCM, dried over Na2SO4, and then filtered through a Celite-packed medium-porosity frit. The solvent was then evaporated by vacuum, and crystals formed from the reddish-brown oil. The crystals were crushed and washed with minimal pentanes and then dried, leaving a red/brown powder as the product in a (3.319 g, 62%). Single crystals suitable for X-ray diffraction were found after the slow evaporation of DCM solutions containing L4. 1H NMR (800 MHz, CDCl3) δ 8.52 (d, J = 5 Hz, 1H, pyridine proton), 7.64 (dd, J = 8, 8 Hz, 1H, pyridine proton), 7.36 (d, J = 8 Hz, 1H, pyridine proton), 7.17 (dd, J = 7, 5 Hz, 1H, pyridine proton), 5.29 (s, 1H, alkoxyl proton), 4.30–4.28 (m, 1H ferrocene proton), 4.18 (s, 5H, ferrocene protons), 4.16–4.14 (m, 1H, ferrocene proton), 4.11–4.09 (m, 1H, ferrocene proton), 4.04–4.02 (m, 1H, ferrocene proton), 1.83 (s, 3H, methyl proton). 13C{1H} NMR (201 MHz, CDCl3) δ 164.9, 147.3, 136.7, 122.0, 119.7, 98.4, 72.6, 68.7, 68.0, 67.6, 66.8, 65.8, 30.3. Anal. Calcd. for C17H17FeNO: C, 66.47; H, 5.58; N, 4.56. Found: C, 66.20; H, 5.47; N, 4.41.
[H]FeOMePyalk (L5). Inside an N2-filled glovebox, to a 250 mL Schlenk flask, 1.16 g (6.15 mmol) of 2-bromo-6-methoxy pyridine and ~100 mL THF were added. The flask was then sealed and placed under N2 on a Schlenk line and cooled to −78 °C. Next, 2.7 mL of 2.5 M n-BuLi (solution in hexanes) were added dropwise to the reaction, and the mixture changed from colorless to yellow and then red. The mixture was then stirred at −78 °C for two hours. After two hours, a separate 100 mL round-bottom flask was charged with 1.41 g (6.16 mmol) acetylferrocene and ~50 mL THF inside an N2-filled glovebox and sealed. The acetylferrocene solution was added via cannula, and the mixture was allowed to return to room temperature overnight while stirring. The next day, the reaction was removed from N2, and 200 mL of DI H2O were added. The organic layer was extracted three times with DCM, dried over Na2SO4, and then filtered through a Celite-packed medium-porosity frit. The filtrate was evaporated, leaving a reddish-brown oil which crystallized upon standing at room temperature. The crystals were then pulverized with a mortar and pestle and washed with minimal pentanes, leaving an orange solid as the product (1.541 g, 74%). Single crystals suitable for X-ray diffraction were found by allowing a solution of DCM containing L5 to slowly evaporate. 1H NMR (800 MHz, CDCl3) δ 7.53 (dd, 3JHH = 8 Hz, 4JHH = 8 Hz 1H, pyridine proton), 6.88 (d, 3JHH = 7 Hz, 1H, pyridine proton), 6.61 (d, 3JHH = 8 Hz, 1H, pyridine proton), 4.92 (s, 1H, alkoxy proton), 4.27–4.24 (m, 1H, ferrocene proton), 4.18 (s, 5H ferrocene proton), 4.16–4.13 (m, 1H ferrocene proton), 4.12–4.08 (m, 2H ferrocene proton), 3.97 (s, 3H methoxy protons), 1.82 (s, 3H, methyl protons). 13C{1H} NMR (201 MHz, CDCl3) δ 163.1, 162.6, 139.5, 112.1, 108.8, 98.1, 72.7, 68.8, 68.0, 67.6, 66.9, 66.0, 53.5, 30.3. Anal. Calcd. for C18H19FeNO2: C, 64.12; H, 5.68; N, 4.15. Found: C, 64.13; H, 5.64; N, 4.11.
Cu(PhPyalk)2 (1a). In a 100 mL round-bottom flask, CuCl2·2H2O (78 mg, 0.456 mmol), L1 (251 mg, 0.961 mmol), and a 2 mL aliquot of 1M KOH/MeOH were stirred in 50 mL MeOH to give a pink precipitate after stirring overnight. The next day, the solvent was decanted off, and the solid was transferred to a vial with fresh MeOH and washed with MeOH, pentanes, and diethyl ether and dried in vacuo to give a pink powder (214 mg, 80%). Crystals suitable for single crystal X-ray diffraction were found from evaporating a solution of chloroform and MeOH containing 1a. Anal. Calcd. for C36H28CuN2O2: C, 74.02; H, 4.83; N, 4.80. Found: C, 73.67; H, 4.74; N, 4.76.
{CuCl}2(µ-O-PhPyalk)2 (1b). In a 100 mL round-bottom flask, CuCl2·2H2O (110 mg, 0.647 mmol), and L1 (152 mg, 0.582 mmol) were stirred in 30 mL of MeOH to give a light blue solution. An aliquot of 0.58 mL of 1 M KOH/MeOH was added, turning the solution dark green and immediately forming a green precipitate. The reaction was stirred overnight at room temperature. The next day, the dark green solid was collected by filtration and dissolved in DCM and filtered through a Celite-packed medium-porosity frit. The filtrate was reduced to ~25 mL and left to crystallize via slow evaporation. The resulting solid was washed with pentanes and then dried in vacuo to give a green powder (178 mg, 85%). Single crystals suitable for X-ray diffraction were formed by vapor diffusing of diethyl ether into solutions of DCM containing 1b. Anal Calcd. for C36H28Cl2Cu2N2O2: C, 60.17; H, 3.93; N, 3.90. Found: C, 60.33; H, 3.98; N, 3.84.
Cu(PyrPyalk)2 (2a). In a 100 mL round-bottom flask, CuCl2·2H2O (121 mg, 0.708 mmol), L2 (481 mg, 1.49 mmol), and a 3 mL aliquot of 1 M KOH/MeOH were stirred in 150 mL MeOH to give a pink precipitate after stirring overnight. The next day, the solvent was evaporated, and the residue was dissolved in DCM and filtered through a Celite-filled medium-porosity frit. The DCM solution was left to evaporate to produce crystals. The crystals were then washed with pentanes and dried in vacuo to give a pink powder (265 mg, 53%). Single crystals suitable for X-ray diffraction were formed by evaporating solutions of DCM containing 2a. Anal. Calcd. for C46H32CuN2O2: C, 78.00; H, 4.55; N, 3.95. Found: C, 77.38; H, 4.55; N, 3.70.
{CuCl}2(µ-O-PyrPyalk)2 (2b). In a 100 mL round-bottom flask, CuCl2·2H2O (118 mg, 0.690 mmol), L2 (200 mg, 0.620 mmol), and a 0.62 mL aliquot of 1M KOH/MeOH were stirred in 100 mL MeOH to give a brown/green precipitate after stirring overnight. The next day, the solvent was decanted, and the residue was collected on a fine-porosity frit. The green solid was then dissolved in DCM and filtered through a Celite-filled medium-porosity frit. The DCM solution was evaporated, and the solid was washed with pentanes and with diethyl ether and then dried in vacuo to give a green powder (123 mg, 47%). Single crystals suitable for X-ray diffraction were formed by evaporating DCM solutions containing 2b. Anal. Calcd. for C46H32Cl2Cu2N2O2: C, 65.56; H, 3.83; N, 3.32. Found: C, 65.16; H, 3.74; N, 3.20.
Cu(ThioPyalk)2 (3a). In a 100 mL round-bottom flask, CuCl2·2H2O (59 mg, 0.344 mmol), L3 (149 mg, 0.728 mmol), and a 0.73 mL aliquot of 1M KOH/MeOH were stirred in 30 mL MeOH to give a purple solution at room temperature overnight. The next day, the solvent was evaporated, and the purple residue was redissolved in DCM and filtered through a Celite-filled medium-porosity frit. The purple solution was then evaporated, and the purple solid was washed with diethyl ether then dried under vacuum (91 mg, 56%). Single crystals suitable for X-ray diffraction were formed by vapor diffusing pentanes into solutions of DCM containing 3a. Anal. Calcd. for C22H20CuN2O2S2: C, 55.97; H, 4.27; N, 5.93. Found: C, 55.35; H, 4.23; N, 5.74.
{CuCl}2(µ-O-ThioPyalk)2 (3b). In a 100 mL round-bottom flask, CuCl2·2H2O (171 mg, 1.00 mmol), L3 (185 mg, 0.902 mmol), and a 0.91 mL aliquot of 1 M KOH/MeOH were stirred in 30 mL MeOH to give a green precipitate after stirring overnight. Next day the, reaction was dried by vacuum, leaving a green residue which was then dissolved in DCM and passed through a Celite-packed medium-porosity frit. The filtrate was then evaporated, leaving a green powder which was washed with diethyl ether and MeOH (181 mg, 66%). Single crystals suitable for X-ray diffraction were formed by vapor diffusing diethyl ether into solutions of DCM containing 3b. Anal. Calcd. for C22H20Cl2Cu2N2O2S2: C, 43.57; H, 3.32; N, 4.62. Found: C, 43.11; H, 3.11; N, 4.45.
Cu(FePyalk)2 (4a). In a 100 mL round-bottom flask, CuCl2·2H2O (27 mg, 0.160 mmol), L4 (100 mg, 0.327 mmol), and a 0.36 mL aliquot of 1M KOH/MeOH were stirred in 100 mL MeOH to give a brown/red precipitate after stirring overnight. The next day, the solvent was evaporated, leaving a yellow powder. The yellow solid was then dissolved in DCM and filtered through a Celite-filled medium-porosity frit. The DCM solution was then reduced to ~2 mL, and a solid was precipitated out with diethyl ether and washed with diethyl ether and dried in vacuo as a yellow powder (60 mg, 56%). Single crystals suitable for X-ray diffraction were formed by slow evaporation of a MeOH/CHCl3 solution containing 4a. Anal. Calcd. for C34H32CuFe2N2O2: C, 60.42; H, 4.77; N, 4.14. Found: C, 60.46; H, 4.75; N, 4.08.
{CuCl}2(µ-O-FePyalk)2 (4b). In a 100 mL round-bottom flask, CuCl2·2H2O (65 mg, 0.384 mmol), L4 (107 mg, 0.350 mmol), and a 0.35 mL aliquot of 1M KOH/MeOH were stirred in 100 mL MeOH to give a green precipitate after stirring overnight. The next day, the solvent was evaporated, leaving a green powder. The green solid was then dissolved in DCM and filtered through a Celite-filled medium-porosity frit. The DCM solution was then reduced to ~2 mL, and a solid was precipitated out with diethyl ether and washed with diethyl ether and dried in vacuo as a green powder (55 mg, 39%). Single crystals suitable for X-ray diffraction were formed by vapor diffusing pentanes into solutions of 1,2-dichloroethane containing 4b. Anal. Calcd. for C34H32Cl2Cu2Fe2N2O2: C, 50.40; H, 3.98; N, 3.46. Found: C, 49.99; H, 4.01; N, 3.44.

4. Summary and Conclusions

We have synthesized and characterized three new pyalk ligands and demonstrated varying coordination modes. Mononuclear CuII structures containing various aryl substituents have been synthesized and characterized crystallographically and electrochemically. Dinuclear CuII structures were synthesized through varying the amount of available ligand, leading to alkoxide bridging structures. The decomposition of the ligands and the mononuclear complexes was studied via cyclic voltammetry in TFE. Each ligand decomposed into new products under high oxidative potentials. L1, L3, L4, and L5 each decomposed irreversibly to new species under oxidative potentials, with L4 and L5 forming a new species that likely contains Fe due to a new reversible oxidation appearing near the typical FeII/FeIII oxidation of ferrocene. L2 electropolymerized under oxidative conditions to form a film on the surface of the electrode. The mononuclear complexes displayed similar responses to the ligands, with the added inclusion of CuII/CuIII oxidations.
Access Codes: CCDC 2352067-2352077 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/structures.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics12080200/s1, Figure S1: 1H NMR of [H]ThioPyalk (L3); Figure S2: 13C{1H} NMR of [H]ThioPyalk (L3); Figure S3: 1H NMR of [H]FePyalk (L4); Figure S4: 13C{1H} NMR of [H]FePyalk (L4); Figure S5: 1H NMR of [H]FeOMePyalk (L5); Figure S6: 13C{1H} NMR of [H]FeOMePyalk (L5); Figure S7: Scan rate study of [H]FePyalk (L4); Figure S8: Peak Current (mA/cm2) vs. Scan Rate (mV/s) for [H]FePyalk (L4); Figure S9: Peak Current (mA/cm2) vs. Square Root of Scan Rate (mV/s).5 for [H]FePyalk (L4); Figure S10: Cyclic voltammogram of [H]FeOMePyalk (L5) depicting oxidative degradation; Figure S11: Scan rate study of [H]FeOMePyalk (L5); Figure S12: Peak Current (mA/cm2) vs. Scan Rate (mV/s) for [H]FeOMePyalk (L5); Figure S13: Peak Current (mA/cm2) vs. Square Root of Scan Rate (mV/s).5 for [H]FeOMePyalk (L5); Figure S14: Scan rate study of Cu(PhPyalk)2 (1a); Figure S15: Peak Current (mA/cm2) vs. Scan Rate (mV/s) of Cu(PhPyalk)2 (1a); Figure S16: Peak Current (mA/cm2) vs. Square Root of Scan Rate (mV/s).5 of Cu(PhPyalk)2 (1a); Figure S17: Wide potential scan of Cu(PhPyalk)2 (1a); Figure S18: Pictures demonstrating the color change exhibited by Cu(PyrPyalk)2 after oxidative degradation; Figure S19: Cathodic scan of Cu(PyrPyalk)2 (2a); Figure S20: Scan rate study of Cu(ThioPyalk)2 (3a); Figure S21: Peak Current (mA/cm2) vs. Scan Rate (mV/s) of Cu(ThioPyalk)2 (3a); Figure S22: Peak Current (mA/cm2) vs. Square Root of Scan Rate (mV/s).5 of Cu(ThioPyalk)2 (3a); Figure S23: Wide potential scan of Cu(ThioPyalk)2 (3a); Figure S24: Oxidative scans of Cu(PhPyalk)2 (1a) and Cu(ThioPyalk)2 (3a); Figure S25: Cyclic voltammogram of Cu(FePyalk)2 (4a) demonstrating oxidative degradation; Figure S26: Scan rate study of Cu(FePyalk)2 (4a); Figure S27: Peak Current (mA/cm2) vs. Scan Rate (mV/s) of Cu(FePyalk)2 (4a); Figure S28: Peak Current (mA/cm2) vs. Square Root of Scan Rate (mV/s).5 of Cu(FePyalk)2 (4a); Figure S29: Wide potential scan of Cu(FePyalk)2 (4a); Figure S30: Cyclic voltammogram overlay of film formed from the electropolymerization of [H]PyrPyalk (L2) and Cu(PyrPyalk)2 (2a); Figure S31: Solid state packing diagram of Cu(PyrPyalk)2 (2a); Figure S32: Solid state packing diagram of Cu(FePyalk)2 (4a); Figure S33: Solid state structure of Cu(FePyalk)2 (4a) demonstrating hydrogen bonding with interstitial solvent; Figure S34: Crystal structure of the 86% major component of {CuCl}2(µ-O-PyrPyalk)2 (2b); Table S1: Crystal structure data table for [H]ThioPyalk (L3), [H]FePyalk (L4) and [H]FeOMePyalk (L5); Table S2: Crystal structure data table for Cu(PhPyalk)2 (1a), Cu(PyrPyalk)2 (2a), Cu(ThioPyalk)2 (3a) and Cu(FePyalk)2 (4a); Table S3: Crystal structure data table for {CuCl}2(µ-O-PhPyalk)2 (1b), {CuCl}2(µ-O-PyrPyalk)2 (2b), {CuCl}2(µ-O-ThioPyalk)2 (3b) and {CuCl}2(µ-O-FePyalk)2 (4b). The Supporting Information (additional experimental procedures and details, NMR spectra, crystal structure data, and additional cyclic voltammetry data) is available free of charge. References [29,30,31,32] are cited in the supplementary materials.

Author Contributions

Conceptualization, T.B.G.; Formal analysis, D.A.D.; Investigation, C.K.W., E.K.R. and D.A.D.; Writing—original draft, C.K.W.; Writing—review & editing, D.A.D. and T.B.G.; Supervision, T.B.G.; Project administration, T.B.G.; Funding acquisition, T.B.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the U.S. National Science Foundation (CHE-2311116).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 12 00200 g001
Figure 1. Depiction of previously reported pyalk complexes used for oxidative electrocatalytic transformations.
Figure 1. Depiction of previously reported pyalk complexes used for oxidative electrocatalytic transformations.
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Scheme 1. Syntheses of L1–L5.
Scheme 1. Syntheses of L1–L5.
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Figure 2. Crystal structures of ligands L3–L5. Ellipsoids drawn at 50% probability. Hydrogen atoms and solvent molecules are removed for clarity.
Figure 2. Crystal structures of ligands L3–L5. Ellipsoids drawn at 50% probability. Hydrogen atoms and solvent molecules are removed for clarity.
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Inorganics 12 00200 sch002
Scheme 2. (a) Synthesis of mononuclear complexes Cu(PhPyalk)2 (1a), Cu(PyrPyalk)2 (2a), Cu(ThioPyalk)2 (3a), and Cu(FePyalk)2 (4a). (b) Synthesis of dinuclear bridged complexes {CuCl}2(µ-O-PhPyalk)2 (1b), {CuCl}2(µ-O-PyrPyalk)2 (2b), {CuCl}2(µ-O-ThioPyalk)2 (3b), and {CuCl}2(µ-O-FePyalk)2 (4b).
Scheme 2. (a) Synthesis of mononuclear complexes Cu(PhPyalk)2 (1a), Cu(PyrPyalk)2 (2a), Cu(ThioPyalk)2 (3a), and Cu(FePyalk)2 (4a). (b) Synthesis of dinuclear bridged complexes {CuCl}2(µ-O-PhPyalk)2 (1b), {CuCl}2(µ-O-PyrPyalk)2 (2b), {CuCl}2(µ-O-ThioPyalk)2 (3b), and {CuCl}2(µ-O-FePyalk)2 (4b).
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Figure 3. Crystal structures of monocopper complexes Cu(PhPyalk)2 (1a), Cu(PyrPyalk)2 (2a), Cu(ThioPyalk)2 (3a), and Cu(FePyalk)2 (4a). Ellipsoids drawn at 50% probability. Hydrogen atoms and solvent hidden for clarity.
Figure 3. Crystal structures of monocopper complexes Cu(PhPyalk)2 (1a), Cu(PyrPyalk)2 (2a), Cu(ThioPyalk)2 (3a), and Cu(FePyalk)2 (4a). Ellipsoids drawn at 50% probability. Hydrogen atoms and solvent hidden for clarity.
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Figure 4. Solid-state structures of bis–copper complexes {CuCl}2(µ-O-PhPyalk)2 (1b), {CuCl}2(µ-O-ThioPyalk)2 (3b), and {CuCl}2(µ-O-FePyalk)2 (4b). Ellipsoids drawn at 50% probability. Hydrogen atoms and solvent are omitted for clarity. The solid-state structure of {CuCl}2(µ-O-PyrPyalk)2 (2b) is shown in the supporting information (Figure S34). The full model disorder refinement for 2b is detailed in the supporting information.
Figure 4. Solid-state structures of bis–copper complexes {CuCl}2(µ-O-PhPyalk)2 (1b), {CuCl}2(µ-O-ThioPyalk)2 (3b), and {CuCl}2(µ-O-FePyalk)2 (4b). Ellipsoids drawn at 50% probability. Hydrogen atoms and solvent are omitted for clarity. The solid-state structure of {CuCl}2(µ-O-PyrPyalk)2 (2b) is shown in the supporting information (Figure S34). The full model disorder refinement for 2b is detailed in the supporting information.
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Figure 5. (a) Oxidative degradation of L1; (b) 10 sweeps of oxidative electropolymerization of L2; (c) oxidative degradation of L3; and (d) oxidative degradation of L4.
Figure 5. (a) Oxidative degradation of L1; (b) 10 sweeps of oxidative electropolymerization of L2; (c) oxidative degradation of L3; and (d) oxidative degradation of L4.
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Figure 6. (a) Comparison of CuII/CuIII oxidations of 1a and 3a; (b) electropolymerization of 2a; (c) reversible peak resulting from electropolymerization of 2a in a fresh solution; (d) oxidation of 4a.
Figure 6. (a) Comparison of CuII/CuIII oxidations of 1a and 3a; (b) electropolymerization of 2a; (c) reversible peak resulting from electropolymerization of 2a in a fresh solution; (d) oxidation of 4a.
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Table 1. Isolated yields of L3–L5.
Table 1. Isolated yields of L3–L5.
LigandYield (%)
L323
L461
L574
Table 2. Table of selected bond distances of Cu(PhPyalk)2 (1a), {CuCl}2(µ-O-PhPyalk)2 (1b), Cu(PyrPyalk)2 (2a), {CuCl}2(µ-O-PyrPyalk)2 (2b), Cu(ThioPyalk)2 (3a), {CuCl}2(µ-O-ThioPyalk)2 (3b), Cu(FePyalk)2 (4a), and {CuCl}2(µ-O-FePyalk)2 (4b).
Table 2. Table of selected bond distances of Cu(PhPyalk)2 (1a), {CuCl}2(µ-O-PhPyalk)2 (1b), Cu(PyrPyalk)2 (2a), {CuCl}2(µ-O-PyrPyalk)2 (2b), Cu(ThioPyalk)2 (3a), {CuCl}2(µ-O-ThioPyalk)2 (3b), Cu(FePyalk)2 (4a), and {CuCl}2(µ-O-FePyalk)2 (4b).
Bond Distances (Å)Bond Angles
ComplexCu–NCu–OCu–Cu 3N-Cu-O 4O-Cu-O
1a1.9722(10)1.9158(8)n/a83.54(4)n/a
1b1.9721(10)1.9199(8)3.0205(5)81.95(4)102.87(4)
2a1.9598(12)1.8923(10)n/a85.08(5)n/a
2b 1,21.970(12)–1.966(12)1.953(9)–1.914(9)3.019(6)81.2(4)–80.8(4)102.7(4)–102.6(4)
3a1.964(2)1.8832(16)n/a85.16(7)n/a
3b1.976(2)1.9135(19)3.0230(7)81.24(8)103.53(9)
4a 11.980(3)–1.966(4)1.903(3)–1.874(3)n/a84.48(13)–83.02(13)n/a
4b 12.0005(16)–1.9942(16)1.9809(13)–1.9179(13)3.0025(4)82.01(6)–80.87(6)98.90(6)–102.19(6)
1 Ranges were provided for bond lengths due to two complexes within the asymmetric unit cell or asymmetry of the complex. 2 Complex 2b was significantly disordered, so only the bond lengths of the major position are shown. 3 Cu–Cu distances are not applicable for 1a–4a since they contain only one Cu. 4 N-Cu-O bond corresponds to the angle between one pyalk ligand and coordinated Cu (i.e., N1-Cu1-O1 or N2-Cu2-O2).
Table 3. Description of relative oxidative stability in 2,2,2-trifluoroethanol.
Table 3. Description of relative oxidative stability in 2,2,2-trifluoroethanol.
ComplexCuII/CuIII (E1/2 vs. Fc/Fc+)Additional Events
1a0.87 VStable within window scanned
2aCovered by electropolymerizationElectropolymerized
3a0.94 VOxidative decomposition at high potentials
4aIrreversibleFormed new species under high potentials
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Webber, C.K.; Richardson, E.K.; Dickie, D.A.; Gunnoe, T.B. Electrochemically Active Copper Complexes with Pyridine-Alkoxide Ligands. Inorganics 2024, 12, 200. https://doi.org/10.3390/inorganics12080200

AMA Style

Webber CK, Richardson EK, Dickie DA, Gunnoe TB. Electrochemically Active Copper Complexes with Pyridine-Alkoxide Ligands. Inorganics. 2024; 12(8):200. https://doi.org/10.3390/inorganics12080200

Chicago/Turabian Style

Webber, Christopher K., Erica K. Richardson, Diane A. Dickie, and T. Brent Gunnoe. 2024. "Electrochemically Active Copper Complexes with Pyridine-Alkoxide Ligands" Inorganics 12, no. 8: 200. https://doi.org/10.3390/inorganics12080200

APA Style

Webber, C. K., Richardson, E. K., Dickie, D. A., & Gunnoe, T. B. (2024). Electrochemically Active Copper Complexes with Pyridine-Alkoxide Ligands. Inorganics, 12(8), 200. https://doi.org/10.3390/inorganics12080200

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