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. Author manuscript; available in PMC: 2024 Mar 28.
Published in final edited form as: Dalton Trans. 2023 Mar 28;52(13):4152–4160. doi: 10.1039/d2dt03727k

Light-induced O2-dependent aliphatic carbon-carbon (C-C) bond cleavage in bipyridine-ligated Co(II) chlorodiketonate complexes

Stephen N Anderson a, Josiah G D Elsberg a, Lisa M Berreau a
PMCID: PMC10426287  NIHMSID: NIHMS1883161  PMID: 36891768

Abstract

Mononuclear bipyridine (bpy)-ligated Co(II) chlorodiketonate complexes [(bpy)2Co(R-PhC(O)C(Cl)C(O)R-Ph)]ClO4 (R = -H (8), -CH3 (9), and -OCH3 (10)), were prepared, characterized and investigated for O2-dependent aliphatic C-C bond cleavage reactivity. Complexes 8-10 have a distorted psuedo-octahedral geometry. 1H NMR spectra of 8-10 in CD3CN show signals for the coordinated diketonate moiety, and signals suggesting ligand exchange reactivity leading to the formation of a small amount of [(bpy)3Co](ClO4)2 (11) in solution. While 8-10 are air stable at room temperature, illumination at 350 nm results in oxidative cleavage reactivity within the diketonate moiety leading to the formation of 1,3-diphenylpropanetrione, benzoic acid, benzoic anhydride, and benzil. Illumination of 8 under 18O2 results in a high level of 18O incorporation (>80%) in the benzoate anion. The product mixture, high level of 18O incorporation, and additional mechanistic studies suggest a reaction sequence wherein light-induced reactivity leads to the formation of a triketone intermediate that undergoes either oxidative C-C bond cleavage or benzoyl migration promoted by a bipyridine-ligated Co(II) or Co(III) fragment.

Graphical Abstract

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Introduction

Oxidative aliphatic carbon-carbon bond cleavage reactions mediated by first-row transition metals involving O2 as the terminal oxidant are of current interest for synthetic applications. Examples involving iron and copper salts are the most common.1,2 Related to such systems are model complexes for metalloenzyme-catalyzed O2-dependent aliphatic C-C bond cleavage reactions. These include synthetic complexes of relevance to O2-dependent flavonol, acireductone and diketone-cleaving enzymes.3–5 It is important to note that flavonols and acireductones exhibit enhanced reactivity with O2 as compared to simple diketones. In fact, there are very few well-characterized first-row metal diketonate complexes that are known to exhibit reactivity with O2 and undergo aliphatic C-C bond cleavage.5,6 To date, a Co(II) diketonate complex which undergoes O2-dependent aliphatic C-C bond cleavage within the diketonate moiety has not been reported.

As an approach for investigating the O2 reactivity of metal diketonate complexes, our laboratory has pursued studies of first-row metal divalent metal chlorodiketonate complexes.6 A mononuclear Ni(II) p-methoxy-substituted chlorodiketonate complex (1, Scheme 1(a)) supported by an aryl-appended tris(2-pyridylmethyl)amine ligand is stable with respect to O2 under ambient conditions. However, oxidative C-C bond cleavage occurs within the diketonate unit upon illumination with UV light (350 nm) under aerobic conditions.6a The products identified included Ni(II) chloride and carboxylate complexes and benzil (Scheme 1(a)). Mechanistic studies suggest that illumination of 1 results in intramolecular redox to produce a diketonate radical and a Ni(I) species which lead to O2 activation and the production of superoxide (O2−). The involvement of superoxide in the reaction pathway is supported by the results of an independent reaction between 1 and KO2 (in the presence 18-crown-6) which also yields anisic acid as the major β-diketonate derived product. Labelling studies involving 18O2 showed 36% incorporation of a labelled oxygen in the carboxylic acid product. The formation of benzil as a minor organic product in the reaction pathway suggests the involvement of a triketone intermediate. In support of this proposal, α-I-β-diketones have been reported to undergo reaction with O2 under illumination to produce 1,3-diphenylpropanetrione, PhC(O)C(O)C(O)Ph).7 Notably, analogs of 1 having p-methyl or p-H chlorodiketonate ligands exhibit significantly lower levels of oxidative C-C bond cleavage reactivity, indicating that the electronic structure of the diketonate ligand has an impact on the photoinduced reactivity in this system.6a

Scheme 1.

Scheme 1

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Examples of O2-reactive divalent metal chlorodiketonate complexes.

A Cu(II) chlorodiketonate complex supported by the same 6-Ph2TPA ligand (5, Scheme 1) exhibits thermal O2-mediated C-C bond cleavage reactivity in the presence of added chloride anion at room temperature.6b–c The oxidative C-C bond cleavage products generated in this reaction are similar to those generated from 1 under illumination conditions, indicating that a change in the metal center enables enhanced thermal reactivity. Exposure of the Cu(II) chlorodiketonate complex [(6-Ph2TPA)Cu(PhC(O)C(Cl)C(O)Ph)]ClO4 (5) to 18O2 results in the formation of benzoic acid containing ~60% of one labelled oxygen atom.6b Under dry conditions, benzoic anhydride is formed and quantitative 18O incorporation is found in a 1,3-diphenylpropanetrione intermediate. We note that a bipyridine-ligated Cu(II) chlorodiketonate complex, [(bpy)Cu(PhC(O)C(Cl)C(O)Ph)]ClO4 (7), is also reactive with O2 under ambient conditions.6c Notably, use of the tetradentate tris(2-pyridylmethyl)amine supporting ligand in a Cu(II) chlorodiketonate complex results in a non-redox, water-dependent retro-Claisen pathway becoming more prevalent for aliphatic C-C cleavage.6d

Examples of Co(III) chloroacetoacetonate complexes supported by amine ligands have been previously reported.8 However, Co(II)-containing chlorodiketonate complexes akin to 1, 5 or 7 have not been previously reported. With an interest in further understanding reactions of first row metal complexes with O2 leading to aliphatic C-C bond cleavage in diketonate ligands, we outline herein the preparation, characterization, and O2 reactivity of a series of Co(II) complexes [(bpy)2Co(R-PhC(O)C(Cl)C(O)Ph-R)]ClO4, R = -H (8), -CH3 (9), and OCH3 (10). These complexes exhibit UV light-induced oxidative aliphatic C-C bond cleavage to give reaction products similar to those produced in the thermal reaction of the Cu(II)-containing 5. The light-driven O2 reactivity of the Co(II) complexes also results in the formation of a triketone intermediate which leads either to oxidative C-C cleavage reactivity or benzoyl migration resulting in benzil formation. Notably, the latter migration chemistry is enhanced when the more highly Lewis acidic Co(III) is formed in the reaction mixture.

Results

Synthesis and Characterization.

The Co(II) complexes 8–10 were prepared by mixing 2,2’bipyridine (2 eq) with Co(ClO4)2·6H2O (1 eq) in CH3CN followed by combining of this solution with an Et2O solution of the lithium salt of a 2-chloro-1,3-diphenyl-1,3-propanedione (Scheme 2) formed in situ. Crude orange solids obtained from these mixtures were recrystallized (8, 10: CH3CN with Et2O diffusion; 9: CH3CN/CH2Cl2 (5:1) with Et2O diffusion) to give crystalline solids in 53–57% yield. Each complex was characterized by X-ray crystallography, elemental analysis, IR, ESI-MS, 1H NMR, and UV-vis spectroscopy.

Scheme 2.

Scheme 2

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Synthesis of Co(II) chlorodiketonate complexes 8-10.

Compound 8 crystallizes with one equivalent of Et2O in the lattice, crystalline 9 contains 0.5 eq. Et2O and 0.5 eq. CH3CN, and 10 contains no solvent in the lattice. Representations of the cationic portions of 8-10 are shown in Fig 1. Details of the data collection and refinement are provided in Table S1. Selected bond distances and angles are given in Tables S2 and S3. Compound 9 crystallizes with two similar molecules (labelled A and B) in the asymmetric unit. The Co(II) center in 8-10 exhibits a pseudo-octahedral geometry with Co-N distances of 2.10–2.15 Å and Co-O distances of 2.02–2.06 Å. These distances are elongated relative to those of previously reported bis-bipyridine or phenanthroline Co(III) diketonate complexes (Co(III)-O, 1.88–1.89 Å, Co(III)-N, 1.93–1.96 Å).9

Fig 1.

Fig 1

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Thermal ellipsoid (50%) representations of the cationic portions of a) 8, b) 9, and c) 10. Only one of two independent cations in the asymmetric unit of 9 is shown. Hydrogen atoms are omitted for clarity.

Each complex gives satisfactory CHN combustion results. The solid-state IR spectra 8-10 (Fig S1–S3) contain features at approximately 1094 and 623 cm−1 consistent with the presence of the perchlorate counterion. ESI-MS spectra of CH3CN solutions of 8-10 (Fig S4–S6) show the presence of the expected molecular ions with appropriate isotope patterns. UV-vis spectra of 8-10 contain a π→π* diketonate transition at 350–360 nm (Fig 2). The π→π* transitions of 8-10 move to higher energy with increasing electron density, which is consistent with stabilization of the HOMO or destabilization of the LUMO. Differences in the molar absorptivity values for the π→π* transitions for 8-10 are similar to those found for Ni(II) chlorodiketonate complexes (R = H (8) < R = -CH3 (9) < R = -OCH3 (10)).6a Magnetic moment measurements performed via the Evans method10 yielded μeff = 4.6–4.8 μB consistent with a high-spin Co(II) in each complex. 1H NMR data collected for 8-10 under paramagnetic conditions display signals for the diketonate aryl protons in the region of 5–25 ppm (Fig S7–S9), with the signal for the ortho protons of the diketonate aryl rings being the most downfield-shifted for each diketonate ligand. We note that the 1H NMR spectrum for each of the analytically pure complexes 8-10 in CD3CN includes signals consistent with the presence of a small amount of [(bpy)3Co](ClO4)2 (11, Fig S10), which was confirmed through independent synthesis, elemental analysis and 1H NMR analysis.11 This indicates that some ligand exchange occurs in solution. Outlined in Scheme 3 is a proposal for the ligand exchange reactivity of 8-10 resulting in the formation of [(bpy)3Co](ClO4)2 (11). The Co(II) bis-diketonate derivatives have not been independently identified. However, other remaining unassigned signals present in the 1H NMR spectra of 8-10 may be associated with these species.

Fig 2.

Fig 2

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Absorption spectral features of 8-10 in acetonitrile.

Scheme 3.

Scheme 3

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Proposed ligand exchange reactivity of 8–10 in CH3CN.

Reactivity Studies with O2.

Orange acetonitrile solutions of 8-10 are generally stable with respect to O2 for ~20 h when kept in the dark (Fig S11–S13). The p-OCH3 chlorodiketonate complex 10 does show some minor degradation (Fig S13) after 20 h. Illumination of acetonitrile solutions of 8-10 under N2 at 350 nm results in no reaction as determined by organic recovery studies for the chlorodiketonate ligand (Fig S14–S16). However, illumination at 350 nm under O2 instead results in a loss in intensity of the π→π* absorption band at ~350 nm for 8 (Fig 3). Similar spectral changes are observed for 9 and 10 (Fig S17 and S18). Using 1H NMR to monitor the illumination reaction of 8 under O2, loss of the Co(II) complex diketonate resonances was identified, along with the appearance of new resonances in the region of 7–9 ppm indicating the formation of non-coordinated aromatic products (Fig S19). We note that the signals for [(bpy)3Co](ClO4)2 (11) remain unchanged in the reaction mixture. Products of the reactions of 8-10 were analyzed by ESI-MS and organic product extraction. For 8, ESI-MS data shows the presence of cationic Co(II) and Co(III) species having a benzoate ligand (e.g., [(bpy)2Co(II)(O2CPh)]+ and [(bpy)2Co(III)(O2CPh)2]+, Fig S20) and Co(III) chloride species such as [(bpy)2Co(III)Cl(O2CPh)]+and [(bpy)2Co(III)Cl2]+). Similar reaction mixtures are observed for 9 and 10 (Fig S21 and S22). Performing the light-induced reaction of 8 in the presence of 18O2 results in >80% incorporation of one 18O atom in [(bpy)2Co(II)(O2CPh)]+ providing evidence that O2-dependent aliphatic C-C bond cleavage has occurred [(bpy)2Co(18OOCPh)]+ (Fig S23). As a control reaction to probe the importance of the diketonate chloride substituent, we generated the unsubstituted analog of 8, [(bpy)2Co(PhC(O)CHC(O)Ph]ClO4 (12; Fig S24 and S25), and examined its reactivity in acetonitrile upon illumination at 350 nm for 20 h under O2 (Fig S26 and Fig S27). Minimal change was observed in the 1H NMR and absorprtion spectral features of 12, indicating no reaction occurred.

Fig 3.

Fig 3

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Absorption spectral changes upon illumination of 8 in O2-purged CH3CN at 350 nm.

The aromatic organic products derived from light-induced reactivity of the diketonate ligand in 8 include 1,3-diphenylpropanetrione, benzoic anhydride and benzil (Fig 4 and Scheme 4). Similar organic product mixtures were found for 9 and 10 (Figures S28 and S29). Isolation of the organic products from the reaction of unsubstituted diketonate complex 12 showed no reaction had taken place (Fig S30) indicating the requirement of the chloride substituent for reactivity.

Fig 4.

Fig 4

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1H NMR spectra of (a) the organic products identified in the light-induced reaction of 8. Standard samples (b-e) show that the oxidation products include 1,3-diphenylpropanetrione, benzil, and benzoic anhydride.

Scheme 4.

Scheme 4

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(top) Overview of reaction products identified in the light-induced reaction of 8 under O2. (bottom) Reactions of 1,3-diphenylpropane trione leading to oxidative aliphatic C-C cleavage products.

The reaction quantum yield (QY) for 8 with illumination at 350 nm in acetonitrile under air is 0.0009(3) as measured using ferrioxalate actinometry. The QYs measured for the reactions of 9 and 10 under identical conditions were 0.00050(1) and 0.00124(4), respectively. For 8-10 and the Ni(II) chlorodiketonate series of complexes previously reported, increasing the electron density within the diketonate via a p-OCH3 substitutent only slightly influenced the quantum yield (e.g, 1, 0.00038(2) vs. 0.0002(1) in [(6-Ph2TPA)Ni(PhC(O)C(Cl)C(O)Ph))]ClO4) 6a

The similarity of the organic product mixtures produced in the light-induced reactions of 8-10 with those found in the thermal O2 reaction of the Cu(II) chlorodiketonate complex 56b,6c (Scheme 1(b)) is noteworthy. In the Cu(II)-mediated reaction, computational studies suggest that hypochlorite may be formed via reaction of the chlorodiketonate with O2. This reaction also results in the formation of 1,3-diphenylpropanetrione via O-O and C-Cl bond cleavage. Hypochlorite is then proposed to act as the oxidant for aliphatic carbon-carbon bond cleavage in 1,3-diphenylpropanetrione leading to the formation of benzoic anhydride or benzoic acid. Benzil formation is proposed to occur via benzoyl migration involving 1,3-diphenylpropanetrione, which is likely facilitated by the Lewis acidic Cu(II) center. With the identified formation of 1,3-diphenylpropanetrione in the light-induced reactions of 8-10, we explored reaction pathways involving this trione and a Co(II)-containing solvate complex in the presence of a possible in-situ formed oxidant (hypochlorite) to probe for aliphatic C-C bond cleavage products. Specifically, we examined the reactivity of a bpy2-ligated Co(II) solvate complex, [(bpy)2Co(CH3CN)2](ClO4)2 (13; Fig S31), with 1,3-diphenylpropanetrione and NaOCl in acetonitrile under various conditions. Treatment of 13 with stoichiometric amounts of 1,3-diphenylpropanetrione and hypochlorite under O2 and stirring for 19 h at ambient temperature resulted in the formation benzil and benzoic acid (Fig S32) with significant unreacted 1,3-diphenylpropanetrione remaining. Notably, purging of an acetonitrile solution of 13 with O2 for 8 h followed by stirring for an additional 19 h results in the solution turning dark red-brown (Fig S33; Fig S34). This solution exhibits 1H NMR features consistent with the formation of a diamagnetic Co(III) species (Fig S35 and Fig S36). Addition of 1,3-diphenylpropanetrione and NaOCl to this solution and stirring for 19 h results in the complete consumption of the 1,3-diphenylpropanetrione with the formation of primarily benzil (Fig S37). These combined results provided evidence that 1,3-diphenylpropanetrione formed in the light induced reactions of 8-10 could undergo reaction with an oxidant (hypochlorite) leading to oxidative cleavage products (e.g., benzoic acid), or can undergo benzoyl migration to give benzil. The relative amounts of these products likely depends on the degree to which the Co(II) center undergoes oxidation to Co(III) in the reaction mixture, as the latter as a better Lewis acid to promote benzoyl migration. With regard to oxidation of the Co(II) center, a control reaction involving treatment of 13 with stoichiometric NaOCl in CD3CN showed that an oxidant such as hypochlorite can directly oxidize the Co(II) center in 13 to Co(III). Changes in the 1H NMR spectrum (Fig S38) suggest the formation of Co(III) species similar to that produced upon extended exposure of 13 to O2.

Discussion

Synthetic systems that enable studies of divalent metal-mediated O2-dependent aliphatic carbon-carbon bond cleavage within a diketonate ligand remain rare.5,6,13 In this regard, studies of the oxidative cleavage of divalent metal chlorodiketonate complexes as a function of the metal ion are a novel approach toward examining the effect of metal ion on O2 activation and aliphatic C-C cleavage.6 As summarized in Scheme 5, at ambient temperature under air, complexes 1, 5 and 8 are stable with respect to O2. It should be noted that the Ni(II) and Co(II) chlorodiketonate complexes 1 and 8 are coordinatively saturated, whereas the Cu(II) complex 5 exhibits a five-coordinate metal center in the solid state. Illumination of 1 and 8 results in reactivity with O2 leading to aliphatic C-C cleavage within the chlorodiketonate unit. The Cu(II) complex undergoes reaction with O2 at room temperature in the presence of a catalytic amount of chloride anion.6

Scheme 5.

Scheme 5

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Triggered O2 reactivity in divalent metal chloro-diketonate complexes.

The product mixtures derived from the light-induced reactions 8-10 under O2 are similar to those found in the thermal reaction of the Cu(II) chlorodiketonate complex 5 (Scheme 1). Specifically, Co(II) and Co(III) benzoate and chloride species were identified, along with 1,3-diphenylpropanetrione, benzil, benzoic anhydride and benzoic acid (Scheme 4). The level of 18O incorporation is significantly higher in the benzoate products generated from 5 and 8 (>60% and 80%, respectively) versus that found in the Ni(II)-containing benzoate product 3 (36%; Scheme 1). This provides evidence for differences in the light-induced reactions pathways of Ni(II)-containing 1 versus Co(II)-containing 8. We hypothesize that this is related to light-induced redox reactivity in the nickel-containing system leading to Ni(I) formation and free diketonate radical. Organic products resulting from radical decomposition and free diketone reactivity were identified in the Ni(II) reaction but were not identified in the reactions of 8-10.6a We note that in the Ni(II) chlorodiketonate systems, variation in the electronic properties of the diketonate ligand impacted the organic product mixture that was generated. For example, the reactivity of 1 (Scheme 1) under O2 was significantly higher than that found for the analog lacking the p-methoxy diketonate substituent. In the Co(II) series 8-10, similar levels of reactivity were identified regardless of the aryl substituent present. This indicates a fundamental difference in the light-driven reaction pathways between Ni(II) and Co(II) chlorodiketonate complexes.

The similarity of the product mixtures for the reactions of 5 and 8 led us to evaluate a possible reaction pathway for the Co(II) complexes 8-10 that is inspired by the thermal reaction of 5. The light-induced reactions of 8-10 lead to the formation of 1,3-diphenylpropanetrione derivatives, which have been identified in the reaction mixtures. We propose that an O2-derived oxidant, potentially hypochlorite, is also formed in these reactions but could not be directly identified. To probe potential subsequent reactivity leading to aliphatic C-C bond cleavage, independent studies were performed to examine the reactivity of 1,3-diphenylpropanetrione with NaOCl in the presence of a Co(II) complex. The results of these investigations show the feasibility of these reactants to lead to the formation of benzoic acid or benzoic anhydride. Additionally, it was determined that oxidation of the Co(II) to Co(III) enhanced benzil formation from 1,3-diphenylpropanetrione. We note that we cannot currently rule out the formation of other intermediate oxidant species in the light-induced reactions of 8-10. For example, the light induced formation of Cl• leading to the formation of ClO2• could potentially provide a similar set of reaction products.

Co(II) complexes have been previously reported that promote oxidative C-C bond cleavage using O2 as the terminal oxidant.16 In mechanistic proposals for some of these systems, cobalt-dioxygen species or oxidants derived therefrom are proposed to be involved in C-C cleavage. For the Co(II) bipyridine-ligated systems described herein the formation of 1,3-diphenylpropanetrione appears to be a key step as this intermediate enables subsequent formation of aliphatic C-C cleavage products resulting from oxidative cleavage and benzoyl migration reactivity. Co(II) and Co(III) species in the reaction mixture likely facilitate benzoyl migration reactivity involving the triketone leading to substantial amounts of benzil formation.

Experimental

General Methods.

All solvents and reagents were purchased from commercial sources and used without prior purification unless otherwise stated. Solvents were dried following previously published procedures and distilled under N2 prior to use.17 All synthetic procedures for Co(II) complexes were performed in a glovebox under a N2 atmosphere unless otherwise noted. The diketone 1,3-(4’,4’-methyl)-phenylpropane-1,3-dione, chlorodiketones 2-chloro-1,3-(4’,4’-R)-phenylpropane-1,3-dione (R = -H, -CH3, -OCH3), and [(bpy)3Co](ClO4)2 (11) were synthesized according to previously published procedures.6a,11

Physical Methods.

1H NMR spectra were collected on a Brüker Advance III HD Ascend-500 spectrometer. Chemical shifts (ppm) are reported relative to the residual solvent peak in CD2HCN (1.94 ppm, quintet). 1H NMR spectra of paramagnetic Co(II) complexes were collected using the following parameters: 300 scans that consist of 8,000 data points over a spectral window of 0–120 ppm, with a 4.96 μs excitation pulse and a 30° pulse angle. FT-IR spectra were collected as KBr pellets using a Shimadzu FTIR-8400 spectrometer. Absorption spectral data was collected using a Hewlett-Packard 8453A diode array spectrometer at ambient temperature. ESI-mass spectral data was collected using a Shimadzu LCMS-2020. High resolution mass spectra were acquired at the Analytical Chemistry Instrumentation Facility at the University of California Riverside. Elemental analyses were performed by Robertson Microlit Laboratories (Ledgewood, NJ) or Atlantic Microlab, Inc. (Norcross, GA). Magnetic moments were determined using the Evans method.10 Quantum yields were determined using ferrioxalate to measure photon flux.18,19

Caution!

Perchlorate salts of metal complexes containing organic ligands are potentially explosive. Only small amounts of materials should be prepared (~50 mg) and these should be handled with great caution.20

[(bpy)2Co(PhC(O)CClC(O)Ph)]ClO4 (8).

A solution of Co(ClO4)2·6H2O (50.0 mg, 0.137 mmol) in CH3CN (~3 mL) was combined with solid 2,2’-bipyridine (bpy) (42.7 mg, 0.273 mmol) at ambient temperature. The resulting mixture was stirred for 30 min. In a separate vial, 2-chloro-1,3-diphenyl-1,3-propanedione (35.3 mg, 0.137 mmol) and lithium bis(trimethylsilyl) amide (22.9 mg, 0.137 mmol) were combined in Et2O (~3 mL) and stirred for 10 min to produce a pale yellow solution. The two solutions were then combined and stirred for an additional four hours. This resulted in the formation of a turbid orange solution. After removing the solvent under reduced pressure, the resulting orange precipitate was washed twice with 10 mL of Et2O and dried under reduced pressure. X-ray quality crystals were grown via Et2O diffusion into an CH3CN solution containing a few drops of CH2Cl2 (53%). ESI-MS: m/z calc. for C35H26N4ClO2Co, 628.1 [M-ClO4]+; found: 628.1. Anal calcd. for C35H26N4Cl2O4Co·O(CH3CH2)2: C, 58.22; H, 4.76; N, 6.96; found: C, 58.32; H, 4.51; N, 7.19. UV-Vis λmax, nm (ε, M−1cm−1): 350 (8800). FT-IR (KBr, cm−1): 1091 (νClO4), 621 (νClO4); μeff = 4.83 μB.

[(bpy)2Co(CH3PhC(O)CClC(O)PhCH3]ClO4 (9).

This complex was prepared and crystalized following the same procedure as 8 (57%). ESI-MS: m/z calculated for C37H30N4ClO2Co 656.1 [M-ClO4]+; found: 656.6; Anal calcd. for C37H30N4Cl2O4Co: C, 58.59; H, 4.25; N, 7.39; found: C, 58.88; H, 4.52; N, 7.62. UV-Vis λmax, nm (ε, M−1cm−1): 353 (11100). FT-IR (KBr, cm−1): 1091 (νClO4), 621 (νClO4); μeff = 4.67 μB.

[(bpy)2Co(CH3OPhC(O)CClC(O)PhOCH3]ClO4 (10).

This complex was prepared following the same procedure as 8 (55%). ESI-MS: m/z calculated for C37H30N4ClO4Co 688.1 [M-ClO4]+; found: 688.3; Anal calcd. For C37H30N4Cl2O8Co·0.15CH2Cl2: C, 55.55; H, 4.05; N, 6.98; found: C, 55.47; H, 3.95; N, 7.03. UV-Vis λmax, nm (ε, M−1cm−1): 360 (13400). FT-IR (KBr, cm−1): 1069 (νClO4), 620 (νClO4); μeff = 4.66 μB.

[(bpy)3Co](ClO4)2 (11).

A solution of 2,2’-bipyridine (bpy) (3 eq) in CH3CN (~3 mL) was combined with solid Co(ClO4)2·6H2O (1 eq) at ambient temperature. The reaction mixture was stirred for 4 h and then brought to dryness under vacuum. The remaining solid was dissolved in CH3CN (1 mL) and this solution was added to excess Et2O (~18 mL). Storing this mixture at room temperature for 24 h resulted in the deposition of a precipitate. The solvent was decanted from this precipitate via pipet. A second precipitation procedure was performed from CH3CN/Et2O and the solid isolated was dried under vacuum. Anal calcd. for C37H30N4Cl2O8Co·H2O: C, 48.41; H, 3.52; N, 11.29; found: C, 48.15; H, 3.45; N, 11.24. A 1H NMR spectrum of 11 in CD3CN matched the signals reported for [(bpy)3Co](PF6)2.11

[(bpy)2Co(PhC(O)CHC(O)Ph)]ClO4 (12).

The complex was prepared and isolated following the same procedure as 8 (30%). ESI-MS: m/z calc. for C35H27N4O2Co, 594.1461 [M-ClO4]+; found: 594.1493; UV-Vis λmax, nm (ε, M−1cm−1): 351 (14230).

[(bpy)2Co(CH3CN)2](ClO4)2 (13).

In dry CH3CN (3 mL) under a N2 atmosphere, [Co(H2O)6](ClO4)2 (50 mg, 1.4 × 10−4 mol) was combined with 2,2’-bipyridine (43 mg, 2.7 × 10−4 mol) and the resulting mixture was stirred for 2 hours at ambient temperature. The solvent was then removed under reduced pressure providing an orange precipitate. This precipitate was dissolved in a minimal amount of CH3CN and was recrystallized by slow diffusion of Et2O. The resulting orange crystals were of sufficient quality for single crystal X-ray crystallography.

Reactivity of in situ generated [(bpy)2Co(CH3CN)2](ClO4)2 (13) with 1,3-diphenylpropanetrione and NaOCl.

Under N2, [Co(H2O)6](ClO4)2 (20 mg, 5.5 × 10−5 mol) and bpy (17 mg, 1.1 × 10−4 mol) were combined in dry acetonitrile (15 mL) and stirred for 2 hours at ambient temperature. The solution was removed the glovebox and combined with 1,3-diphenylpropanetrione (13 mg, 5.5 × 10−5 mol) and NaOCl (3.3 mg, 4.4 × 10−5 mol). Oxygen was then bubbled through the solution for two minutes. The resulting solution was stirred for 19 hours and then an organic extraction was performed. A second solution was prepared similarly but was oxygenated for 8 h prior to the addition of 1,3-diphenylpropanetrione and NaOCl. This longer oxygenation produced a red/black solution suggesting oxidation of the Co(II) center.

X-ray crystallography.

X-ray diffraction data for 8-10 were collected at the University of Montana at 100 K on a Bruker D8 Venture using MoΚα radiation (λ = 0.71073 Å). Data for each complex was corrected for absorption using the SADABS21 area detector correction program. Using Olex222, the structures were solved using the SHELXT23 program with direct methods and refined with the SHELXL24 refinement package using least squares minimization. All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were located from difference Fourier maps, but their final positions were placed in geometrically calculated positions and were refined using a riding model. Isotropic thermal parameters of the placed hydrogen atoms were fixed to 1.2 times the U value of the atoms to which they are attached (1.5 times for the methyl groups). Calculations and refinement of structures were carried out using APEX325, SHELXTL26, and OLEX222 software.

The checkCIF for 9 suggested a center of symmetry element however this is just a pseudo element as the center of symmetry is close but not perfect. To further evaluate this matter, the data were solved in the space group Pnma. This resulting model exhibits poor structural parameters when compared to structures in Cambridge Structural Database using the Cambridge Crystallographic Data Centre program Mogul727 and Mercury8.28 Therefore, we have elected to present the structure in the Pna2(1) setting.

A single crystal of 13 was mounted on glass fiber loop using paratone oil. Data collection was performed using a Rigaku XtaLAB Mini II Diffractometer with MoKα radiation (λ = 0.71073 Å) at 100 K. An absorption correction was done using a Gaussian grid with a 0.5 mm 1D horizontal Gaussian beam correction for the graphite monochromator. The structure was solved using Olex222 with SHELXT23 using intrinsic phasing method and refined using the SHELXL24 program with the least square method. All non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were placed in geometrically calculated positions and further refined using a riding model. Isotropic parameters of all hydrogen atoms were fixed to 1.2 times the U value of the atoms to which they are attached (1.5 times for the methyl groups).

Photoillumination studies.

Solutions of 8–10 and 12 in dry CH3CN under N2 (15 mL, 2.0 mM) were prepared in 30 mL Pyrex test tubes sealed with rubber septa and tape. An absorption spectrum was collected for each sample by removing a 100 μL aliquot and adding it to 2700 μL of CH3CN (7.1 ×10−2 mM). For O2 reactivity studies, the solution was purged with O2 for two minutes before illuminating for twenty hours at 350 nm. Following illumination, another absorption spectrum was collected with that sample being then used for ESI-MS. The remaining reaction solution was concentrated under vacuum. A 1H NMR of the remaining residue was collected in CD3CN. The 1H NMR sample was then added back to the reaction vessel and the solvent was then removed under vacuum. The crude product was dissolved in 1.5 M HCl (15 mL) and stirred for ten minutes before CH2Cl2 (15 mL) was added and stirred for an additional ninety minutes. The solution was transferred to a separation funnel and the organic compounds were collected in the CH2Cl2 layer following three washes of 1.5 M HCl (15 mL). The CH2Cl2 fraction was dried over Na2SO4, filtered, concentrated under vacuum, and dried on vacuum line for two hours before a 1H NMR was collected. The 1H NMR solution was then added back to the reaction vessel and dried for 24 hours before the residue was analyzed using mass spectrometry.

Conclusions

The results presented herein demonstrate that bipyridine-supported Co(II) chlorodiketonate complexes exhibit light-induced oxidative aliphatic C-C bond resulting in products similar to the thermal reaction pathway found for structurally similar Cu(II) complexes. Both involve formation of a 1,3-diphenylpropanetrione intermediate that is susceptible to oxidative cleavage reactivity or benzoyl migration to give benzil.

Supplementary Material

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Acknowledgements

We thank the National Science Foundation (CHE-1664977 to LMB; CHE-1429195 for Brüker Avance III HD 500 MHz NMR; CHE-1828764 for Rigaku Benchtop XtaLAB Mini II X-ray diffractometer). X-ray crystallographic studies of were performed at the University of Montana X-ray diffraction core facility, which is supported by the Center for Biomolecular Structure and Dynamics CoBRE (National Institutes of Health, CoBRE NIGMS P20GM103546). Single crystal X-ray diffraction data were collected using a Bruker D8 Venture (NSF MRI CHE-1337908).

Footnotes

Conflicts of interest

There are no conflicts to declare.

Electronic Supplementary Information (ESI) available: X-ray crystallographic data; Spectroscopic data for characterization; spectra of reaction and product mixtures. See DOI: 10.1039/x0xx00000x

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