Spectroscopic and DFT study of tris (β-diketonato)cobalt(III) complexes

Abstract

Nuclear magnetic resonance measurements confirm that, as expected, the mer isomer of tris(β-diketonato) cobalt(III) complexes is more stable (mer isomers are more stable because they are less affected by steric hindrance and they have lower dipoles). An excess of the mer isomer or only the mer isomer is experimentally observed. The existence of both fac and mer isomers is predicted by density functional theory calculations. At room temperature, the nuclear magnetic resonance peaks of phenyl-containing mer complexes are broad due to the intermediate rotation of bulky phenyl groups resulting in only the average orientation of the non-equivalent protons in the mer complexes. However, upon heating to 55 °C (thus speeding up the rotation) or cooling to −50 °C (slowing down the rotation), this broad peak is resolved into three sets of carbon peaks appearing in the ¹³C NMR spectrum, as well as splitting of the ¹H NMR peaks for the non-equivalent protons in the mer isomer. In solid-state ¹³C NMR the aromatic rings do not rotate, resulting in the splitting of all the ¹³C NMR peaks. Two sets of well-defined peaks are observed for the fluorine-containing compound [Co(CF₃COCHCOCPh)₃] in both the ¹³C and ¹⁹F NMR spectra, indicating that both the fac and mer isomers are present with a ratio of fac:mer = 0.1:1. The X-ray photoelectron spectroscopy measured binding energy of the Co 2p_3/2 photoelectron lines are influenced by the subtle electronic (electron-donating or electron-withdrawing) properties of the R groups on the β-diketonato ligands. Various relationships are established between the binding energy of Co 2p_3/2 and several physical and density functional theory–calculated properties, confirming good electronic communication (the measurable electronic effect that a molecular fragment has on the physical properties of another molecular fragment in the molecule) of the substituent groups through the pseudo aromatic system of the β-diketonato backbones to the metal.

Peak resolution in the NMR is dependent on the rotation speed of the phenyl rings, which in turn is dependent on the temperature; however, the experimental fac/mer isomer ratio is independent of temperature of Co(β-diketonato)₃.

Highlights

• Density functional theory (DFT) calculations show that both fac and mer isomers are possible for Co(β-diketonato)₃, as indeed been experimentally observed.

• At room temperature, nuclear magnetic resonance (NMR) spectra revealed one broad peak representing the average aromatic entities of the non-equivalent protons of the mer isomer.

• High- and low-temperature NMR resulted in three well-defined peaks for the aromatic entities of the mer isomer.

• From relationship between binding energy (BE) of Co 2p_3/2 and other physical properties, BE for unknown compounds can be predicted.

Introduction

β-Diketones (the most well-known type is acetylacetone (Hacac)) can easily coordinate to many metals forming square planar,^1,2 octahedral,^3–5 or eight-coordinated complexes.^6,7 This property of β-diketones is applied in the solvent extraction of metals, for example, cobalt(II) can be extracted almost completely (95%–99.5% by a single extraction) under the correct experimental conditions, forming the very stable tris(acetylacetonate)cobalt(III) complex. ⁸ Many metal-β-diketonato complexes have applications in industry, both as homogeneous and heterogeneous catalysts. For example, [Co(acac)₃] is used as a homogeneous catalyst in many reactions, such as cycloaddition, cyclization, co-dimerization and olefin coupling reactions, organometallic coupling and addition reactions, radical addition to olefins, and oxidation reactions. ⁹ Different tris(β-diketonato)cobalt(III) complexes were tested as catalysts for the hydration of olefins. The yield was dependent on the type of tris(β-diketonato)cobalt(III) complex used, as well as the type of olefin substrate catalyzed, though Co(III)-containing 1,3-substituted β-diketones in many cases performed better than [Co(acac)₃]. ¹⁰ The different electron-donating and electron-withdrawing properties of the substituents R¹ and R² on the β-diketonato ligand, (R¹COCHCOR²)⁻, donate or withdraw electron density via the pseudo aromatic backbone to or from the metal it is coordinated to, thereby influencing the reactivity of the metal. The influence of the electron-donating and electron-withdrawing properties of the substituents R¹ and R² on the electron-richness of the metal can inter alia be measured by redox potential of the metal, which also relates to various experimental properties (pK_a, ⁸ X-ray photoelectron spectroscopy (XPS) binding energies,^11,12 reduction potential of the free β-diketone^13,14), empirical parameters (Hammett constant,^15,16 Lever parameters, ¹⁷ Gordy scale group electronegativities ¹⁸ ), and theoretically calculated energies (frontier orbital energies,^19,20 electron affinity (EA), ionization potential (IP), the Mulliken electronegativity (χ),^21,22 the global electrophilicity index (ω) ²³ ) of the substituents R¹ and R² of the β-diketone R¹COCH₂COR² or the metal-β-diketonato complex. To our knowledge, the binding energies (BE) of the core Co 2p electrons, as measured by XPS, of only one tris(β-diketonato)cobalt(III) complex, the tris(acac)cobalt(III) complex, has been reported. ²⁴ In this contribution, we determine the electronic influence of the substituents R¹ and R² on cobalt(III) in tris(β-diketonato)cobalt(III) complexes, [Co(R¹COCHCOR²)₃], by nuclear magnetic resonance (NMR) and XPS spectroscopy, combined with a theoretical density functional theory (DFT) study of five tris(β-diketonato)cobalt(III) complexes containing different β-diketonato ligands, (R¹COCHCOR²)⁻ with R¹, R² = CH₃, CH₃ (1); CH₃, Ph (2); Ph, Ph (3); CH₃, CF₃ (4); CF₃, Ph (5) (see Figure 1).

Figure 1.

The tris(β-diketonato)cobalt(III) complexes in this study: [Co(acac)₃] (1), [Co(ba)₃] (2), [Co(dbm)₃] (3), [Co(tfaa)₃] (4), and [Co(tfba)₃] (5). Only the Δ stereo isomers for 1–5, and the fac isomers for 2, 4, and 5 are shown (see Figure 2).

Results and discussion

The tris(β-diketonato)cobalt(III) complexes [Co(acac)₃] (1) and [Co(dbm)₃] (3) contain symmetrically substituted β-diketonato ligands, while [Co(ba)₃] (2), [Co(tfaa)₃] (4), and [Co(tfba)₃] (5) contain unsymmetrically substituted β-diketonato ligands. Consequently, depending on the arrangement of the β-diketonato ligands around cobalt, two isomers are possible for each of [Co(ba)₃] (2), [Co(tfaa)₃] (4), and [Co(tfba)₃] (5), namely a fac and a mer isomer. In the fac isomer, the three identical R groups of the β-diketonato ligand (R¹COCHCOR²)⁻ lie near the same octahedral face of the complex, while in the mer isomer the three identical R groups are near the same octahedral meridian through cobalt (see Figure 2(b)). In addition, each symmetrical complex 1 and 3 and each fac and mer isomer of 2, 4, and 5 has a stereoisomer (enantiomer): a Λ (left-handed propeller twist formed by three bidentate ligands) and Δ (right-handed propeller twist) stereoisomer (Figure 2(a)). Statistically, the fac:mer ratio is 1:3 (Figure 2(c)). For complexes containing large R substituents, it is expected that the mer isomers are more stable, since they are less affected by steric hindrance caused between bigger R substituents, when compared to their fac isomers. ²⁵ The mer isomers are also expected to be more stable, due to their lower dipole^26,27 (the solvent used for NMR analysis is CDCl₃). Experimentally, from the literature, both fac and mer tris(β-diketonato)cobalt(III) isomers were characterized, for example, 20% fac [Co(ba)₃] (2) (based on yield), ²⁸ 18% fac [Co(tfaa)₃] (4) (based on NMR measurements),^27,29,30 and both fac and mer [Co(β-diketonato)₃] crystal structures are known. ³¹ For example, mer [Co(ba)₃] (2), ³² mer [Co(tfaa)₃] (4), and fac [Co(tfba)₃] (5) ³² crystal structures have been isolated. ³³

Figure 2.

(a) The Λ and Δ stereoisomers, and (b) the fac and mer isomers of [Co(β-diketonato)₃] complexes, containing an unsymmetrical β-diketonato ligand (R¹COCHCOR²)^– with R¹ ≠ R² and (c) an illustration of the statistical fac: mer = 1:3 distribution. The blue arrows in (a) indicate the right-handed propeller twist of the R¹ and R² groups of the bidentate ligand for the Δ stereoisomer, whereas for the Λ stereoisomer the left-handed propeller twist of the R¹ and R² groups is stipulated. The pink arrows in (c) indicate the spacial orientation of the R¹ groups on different bidentate ligands which results in the statistical 3 (mer):1 (fac) ratio. Note mer-1, mer-2, and mer-3 are identical and only illustrate the statistical population of the arrangement of the ligands around Co. Thus, according to statistics, each fac molecule forms 3 mer molecules.

DFT study

DFT calculations using the B3LYP/6-311G(d,p)/Lanl2dz(Co) theory level showed that the DFT-optimized Λ and Δ isomers of each molecule have the same bond lengths and angles and are equi-energetic. The DFT-calculated free energy of the fac and mer isomers depends on their substituents and the temperature. It was found that the mer isomers are more stable for 4 and 5 (lower free energy than fac), while the fac isomer is more stable for 2 at all temperatures between 50 and 340 K. See Figure 3 for the DFT-calculated fac and mer isomer distribution as a function of temperature. Isomerization between the fac isomer and the mer isomer theoretically occurs with temperature, leading theoretically to mainly the mer isomers for 4 and 5 at higher temperatures.

Figure 3.

Temperature-dependent populations of fac and mer isomers of 2, 4, and 5, as predicted from the B3LYP/6-311G(d,p)/Lanl2dz(Co) calculated free energies in CHCl₃ as an implicit solvent, at the indicated temperatures.

The DFT calculations, in agreement with experimental observations, thus show that both fac and mer isomers are experimentally possible. In addition, it has been experimentally observed that the yield of the minor fac isomer of 4 can be increased with an increase in temperature during synthesis, due to isomerization of the mer isomer to the fac isomer. ³⁴ This observation suggests that the synthetic conditions such as the temperature can determine the fac:mer ratio in an experimental sample.

NMR studies

Using NMR, it is possible to distinguish between a fac and a mer isomer for 2, 4, and 5. Due to the C₃ symmetry of the fac isomer, the protons (C or F) of the three different coordinated β-diketonato ligands are chemically equivalent, leading to one set of NMR peaks for the fac isomer. For example, considering the ¹H NMR spectra for the fac isomers, one sharp signal counting for three protons at ca. 6–7 ppm is expected for the three methine protons (CH) the fac isomers of 2, 4 and 5 each. Similarly, one sharp signal at ca. 2.2–2.5 ppm, counting for nine protons, is expected for the protons of the methyl groups in fac 2 and fac 4; similar observations are expected for the aromatic protons and for the ¹³C and ¹⁹F NMR. However, the mer isomer is unsymmetrical and therefore the different protons (C or F) of the three different coordinated β-diketonato ligands are chemically non-equivalent and will each have their own set of signals. Thus, for example, considering the ¹H NMR spectra for a mer isomer, three sharp signals, each counting for one proton at ca. 6–7 ppm, is expected for the three methine protons (CH) for the mer isomer of 2, 4, and 5. Similarly, three sharp signals at ca. 2.2–2.5 ppm, due to three protons each, are expected for the protons of the methyl groups in mer 2 and mer 4; similar observations are expected for the aromatic protons and for the ¹³C and ¹⁹F NMR.

The liquid-state ¹H and ¹³C NMR spectra of compounds containing symmetrically substituted β-diketonato ligands 1 and 3 exhibited peaks associated with only one isomer, as expected (Figures S1, S2 and S11–S13 in the Supporting Information). For 2 and 4 having an unsymmetrically substituted β-diketonato ligand, only the mer isomers were detected. This assignment is made in correlation with published results showing the splitting of the peaks present in either the mer or the fac isomer.^34,35

Compound 4 displayed 3 well-defined sets of peaks in each of the ¹H, ¹³C, and ¹⁹F NMR spectra at room temperature (Figures 4–6), corresponding to the non-equivalent signals of the different atoms on the β-diketonato ligand. The ¹H NMR spectrum (Figure 4) clearly shows three sharp signals, each counting for one proton at ca. 6 ppm for the three methine protons (CH) of the mer isomer of 4 and three sharp signals at ca. 2.4–2.5 ppm, counting for three protons each for the protons of the methyl groups in mer 4. The ¹⁹F NMR spectrum (Figure 5) shows three sharp signals, counting for three fluorines each at ca. −73 ppm for the three fluorines of each CF₃ group in mer 4.

Figure 4.

¹H NMR spectrum of 4 measured at 25 °C, showing three sharp signals, each counting for 1 proton at ca. 6 ppm for the three methine protons (CH) of the mer isomer of 4 and three sharp signals at ca. 2.4–2.5 ppm, counting for three protons each for the protons of the methyl groups in mer 4.

Figure 5.

¹⁹F NMR spectrum of 4 measured at 25 °C (left) and 55 °C (right), showing three sharp signals, each counting for three F atoms each at ca. −73 ppm for the three F atoms of each CF₃ group in mer 4.

Figure 6.

¹³C NMR spectrum of 4 measured at 25 °C highlighting (1) three sharp signals at ca. 199 ppm for the C of the CO group near CH₃, (2) three quartets at ca. 170 ppm for the C of the CO group near CF₃, that split due to coupling to F, (3) a quartet at 114.4 ppm for the C of the CF₃ group (this splits due to coupling with the three F atoms), (4) three methine (–CH–) carbons are present at ca. 94.5 ppm, (5) three sharp signals at ca. 27 ppm for the C of the methyl CH₃, and (6) three sharp signals at ca. 77 ppm accounting for the solvent peak of CDCl₃.

The ¹³C NMR spectrum of 4 measured at 25 °C highlights (1) three sharp signals at ca. 199 ppm for the C of the CO group near CH₃, (2) three quartets at ca. 170 ppm for the C of the CO group near CF₃, that split due to coupling to F, (3) a quartet at 114.4 ppm for the C of the CF₃ group (this splits due to coupling with the three F atoms), (4) the three methine (–CH–) carbons are present at ca. 94.5 ppm, (5) three sharp signals at ca. 27 ppm for C of the methyl CH₃, and (6) three sharp signals at ca. 77 ppm accounting for the solvent peak of CDCl₃. No peaks for the fac isomer are present in the ¹³C NMR signals of 4. This corresponds well with the DFT calculations, which indicate that temperature-dependent population of the fac isomer of 4 is infinitely small and accordingly only the mer isomers are present at temperatures higher than ca. 290 K. According to the DFT calculations at 223 K, a small amount of the fac isomer is predicted to be present; however, in the ¹H NMR spectrum recorded at −50 °C, the presence of a fac isomer could not be confirmed. However, what is clear from the temperature study is that as the temperature increases the ¹H NMR peaks shift upfield indicating that the increase in temperature leads to slightly better shielding due to the fast rotation of the functional groups (see Figure 4 and Figures S14–S21 in the Supporting Information). For the ¹⁹F NMR spectrum, better peak separation was observed with an upfield shift of signals at 55 °C (Figure 5).

For 2, containing aromatic phenyl groups, broad peaks are observed in the ¹H NMR spectrum (run at 25 °C) due to the relatively slow rotation of the bulky aromatic ring substituent on the β-diketonato ligand and thus resulting in only the average orientation being acquired within the NMR spectrum (see Figure S3 in the Supporting Information). The presence of the mer isomer (in 2) is confirmed by the splitting of the ¹³C NMR peaks (run at 25 °C) belonging to the non-equivalent CO– (180–200 ppm), CH– (94–95 ppm), and CH₃-groups (27–28 ppm), and in correlation with published results (Figure S7 in the Supporting Information). Also, only one set of ¹³C NMR peaks belonging to the aromatic carbons (120–130 ppm) is observed at room temperature. However, upon heating compound 2 to 55 °C within the NMR probe, and thus speeding up the rotation of the aromatic rings, the ¹³C NMR aromatic peaks at 127.5, 128, and 131 ppm split into three sets of aromatic carbon peaks in the ¹³C NMR spectrum (see Figure S8 in the Supporting Information). At 55 °C, the splitting of the ¹H NMR peaks for the non-equivalent protons in the mer isomer is clearer than at 25 °C (see Figure S4 in the Supporting Information). The same was observed when the rotation of the aromatic rings was slowed down by cooling the sample down to −50 °C (see Figures S5 and S9 in the Supporting Information). In the solid state, these aromatic rings do not rotate, and the ring carbons will experience different chemical environments, resulting in the splitting of all the ¹³C NMR peaks in the solid-state NMR of 2 at room temperature (Figure 7 and Figure S10 in the Supporting Information).

Figure 7.

Liquid state at 25 °C (bottom) and solid-state (top) ¹³C NMR spectra of 2.

For both 2 and 4, only the mer isomers were observed at different temperatures, concentrations, and by re-acquiring NMR spectra after 6 days.

Contrasting 2 and 4, the liquid-state ¹H and ¹³C NMR spectra of compound 5, containing the unsymmetrically substituted β-diketonato ligand with the –C₆H₅ and –CF₃ groups, revealed the presence of ca. 9% of the fac isomer.

The ¹⁹F NMR spectrum of compound 5 exhibited four sets of well-defined peaks at room temperature, indicating that both the fac and mer isomers are present in solution with a ratio of fac:mer = 0.1:1. In the ¹H NMR spectrum, one of the CH peaks of the mer isomer overlaps with that of the fac isomer, but by heating the sample to 55 °C, these peaks separate. From the ¹⁹F NMR spectrum, it was observed that different temperatures (heating and cooling), concentrations and by re-acquiring the NMR spectra after 9 days, had no influence on the fac:mer ratio.

Thus, the NMR results of this study show that the fac:mer ratio of a sample of compounds 2, 4, and 5 in solution are experimentally not influenced by concentration, time, and temperature in solution and is here proposed to be a consequence of the experimental conditions during synthesis.

XPS study

XPS studies have been undertaken pertaining to the study of the electronic structure of 1–5 by measuring the binding energies (BE) (in eV) of the core Co 2p electrons of the complexes. It is a powerful method to investigate the local environment of the measured species (in this case Co), which is affected by its formal oxidation state and its neighboring atoms (chemical environment).

When a sample is exposed to the atmosphere, large quantities of adventitious carbon deposit on the sample. The main simulated C 1s photoelectron line of the adventitious carbon (284.8 eV) is used as a reference to correct the position of the photoelectron lines of the elements due to any charging that might have occurred during the measurement.

Figure 8 shows the detailed comparative XPS spectra of the Co 2p_3/2 photoelectron lines of compounds 1–5 (the entire Co 2p area is presented in the Supporting Information). Data that can be extracted from Figure 8 are summarized in Table 1. The peak maxima of the broad Co 2p_3/2 and Co 2p_1/2 envelopes are located between 780.8–781.0 eV and 796.1–796.6 eV, respectively. These binding energies coincide with 1–5 being in the Co(III) oxidation state, according to reported Co 2p_3/2 data for K₃Co(C₂O₄)₃ (781.1 eV) ³⁶ and K₃[Co(CN)₆] (781.2 eV). ³⁷ In accordance with Ivanova et al., ³⁸ the Co 2p_3/2 photoelectron line was simulated with four peaks, one main and three satellite structures. The detailed data of the simulated peaks are summarized in Table S1 in the Supporting Information.

Figure 8.

Detailed XPS spectra of the Co 2p_3/2 region for 1–5.

Table 1.

The accumulative Gordy group electronegativity of the R groups of the β-diketonato ligands, Σχ_R, the accumulative Hammett constants (Σσ_R), the pK_a of the β-diketonato ligand, the DFT-calculated electrophilicity index (ω in eV), the electrochemical potential (μ in eV) and the Mulliken electronegativity (χ in eV), the Lever constants (L_E) as well as E(HOMO) (in eV) and E(LUMO) (in eV), and the DFT-calculated energy of the Co 2p orbital energy, E(Co 2p) (eV) of 1–5.

Complex		ΣχR a	ΣσR b	pKa c	ω d	μ e	χ e	LE d	E(HOMO) e	E(LUMO) e	BE Co 2p3/2	FWHM	ΔBEmain	IΣ	BE F 1s	E(Co 2p) (eV)
1	Co(acac)3	4.68	−0.138	8.95	4.24	−4.25	4.25	−0.08	−6.38	−2.12	780.87	2.77	15.37	0.25		−764.92
2	Co(ba)3	4.55	−0.009	8.70	4.40	−4.26	4.26	−0.06	−6.32	−2.20	780.92	2.90	15.32	0.34		−765.08
3	Co(dbm)3	4.42	0.12	9.35	4.61	−4.29	4.29	−0.04	−6.28	−2.29	780.88	2.67	15.28	0.43		−765.20
4	Co(tfaa)3	5.35	0.361	6.3	6.07	−5.03	5.03	0.03	−7.12	−2.95	781.00	3.76	15.56	0.73	687.6	−766.00
5	Co(tfba)3	5.22	0.49	6.3	6.05	−4.94	4.94	0.05	−6.96	−2.92	780.97	3.15	15.51	0.67	687.4	−766.01

DFT: density functional theory; E(HOMO): energy of the highest occupied molecular orbital; E(LUMO): energy of the lowest unoccupied molecular orbital; BE: binding energy; FWHM: full width at half maximum.

The XPS measured binding energy of the main Co 2p_3/2 photoelectron line (in eV), the full width at half maximum (FWHM in eV) of the main Co 2p_3/2 photoelectron line, the spin-orbit splitting between the Co 2p_1/2 and Co 2p_3/2 photoelectron lines (ΔBE₁ in eV), the sum of the intensity ratio of all the satellite structures and the intensity of the main Co 2p_3/2 photoelectron lines (I_Σ = (I_{Co 2p3/2 sat1} + I_{Co 2p3/2 sat2} + I_{Co 2p3/2 sat3})/(I_{Co 2p3/2 main})), as well as the binding energy of the F 1s photoelectron line (in eV) of 4 and 5.

a

Sum of the R group electronegativities, Σχ_R. χ_CF3 = 3.01, χ_CH3 = 2.34, and χ_C6H5 = 2.21.^2,39 By way of example, for 4, this is calculated as follows: Σχ_R = (χ_CF3) + (χ_CH3) = (3.01) + (2.34) = 5.35.

b

Sum of the Hammett constants of the R-groups, Σσ_R.^15,16

c

pK_a from Starý. ⁸

d

L_E from Lever. ¹⁷

e

DFT-calculated ω_R, μ, χ (χ = −µ (μ is the electrochemical potential), E(Co 2p), E(HOMO), and E(LUMO) data from this study. DFT-calculated values for the fac and mer isomers were the same within 0.02 V, thus the values obtained for the fac isomers are reported here.

The subtle influence of the electronic (electron-donating or electron-withdrawing) properties of the R groups on the β-diketonato ligands on the chemical environment of the Co center can be graphically displayed from the plot of the binding energies of the main Co 2p_3/2 photoelectron lines (BE Co 2p_{3/2 main}) of 1–5 versus the sum of the Gordy group electronegativities of the R groups ¹⁸ on the β-diketonato ligands, Σχ_R, see Figure 9(a). The direct link indicates the susceptibility of the Co 2p_3/2 binding energies toward the different chemical environments of the Co atoms of 1–5. This relationship confirms the good electronic communication between the R group on the β-diketonato ligands via the pseudo aromatic backbone and the core electrons of Co. β-Diketonato ligands with electron-withdrawing groups such as 4 and 5 with (R = CF₃) (having a high Σχ_R) deshield the Co core, which increase the binding energy of the photoelectrons.

Figure 9.

The relationship between the binding energy of the main Co 2p_3/2 photoelectron line (BE Co 2p_{3/2 main}) and (a) the sum of Gordy scale R group electronegativities of the β-diketonato ligands, Σχ_R, of 1–5; (b) the energy of the highest occupied molecular orbital (E(HOMO)); (c) the calculated Mulliken electronegativity, χ, which is a measure of the tendency of an atom or molecule to attract electrons); (d) the global electrophilicity index, ω, which is a measure of the electrophilic power of atoms and molecules; (e) the energy of the lowest unoccupied molecular orbital (E(LUMO)); and (f) the DFT-calculated Co 2p orbital energies.

The energy of the highest occupied molecular orbital (E(HOMO)) and BE Co 2p_{3/2 main} are inversely related to each other (Figure 9(b)). The lower the energy of the HOMO implies that more energy is required to remove an electron occupying the HOMO, that is, to ionize the complex.^20,23 Thus, the more difficult it is to ionize a complex (to remove an electron from a lower energy Co d HOMO), the more difficult it is to emit a photoelectron from the Co 2p orbital (as indicated by the higher BE Co 2p_{3/2 main}) on this metal ion. Additional relationships were established between BE Cu 2p_{3/2 main} and the DFT-calculated properties, that is, the calculated Mulliken electronegativity (χ, a measure of the tendency of an atom or molecule to attract electrons) ²¹ and the global electrophilicity index (ω, a measure of the electrophilic power of atoms and molecules) ²³ confirm the dependence of the BE Co 2p_{3/2 main} on the initial state of (1)–(5), see Figure 9(c) and (d). The electronic effect of the β-diketonato ligands on Co (initial state) is also being reflected by the energy of the LUMO (i.e. related to the EA of the molecules),^20,23 leading to the inverse relationship between (E(LUMO)) and BE Co 2p_{3/2 main}, in support of the dependence of BE Co 2p_{3/2 main} on the initial states of 1–5, before photoionization, see Figure 9(e).

Further relationships between BE Co 2p_{3/2 main} and the pK_a of the β-diketonato ligand, the Hammett constants (Σσ), the DFT-calculated electrochemical potential (μ), and the Lever constants (L_E) can be found in the Supporting Information. The equations and R² values, obtained from the relationships involving BE Co 2p_{3/2 main}, are summarized in Table 2. Using these relationships involving BE Co 2p_{3/2 main}, it is potentially possible to predict the BE of a related complex, for example, Co(hfaa)₃ (hfaa = CF₃COCHCOCF₃) as 781.09 with a mean absolute deviation (MAD) of 0.02.

Table 2.

Relationships obtained between XPS measured binding energies of the main Co 2p_3/2 photoelectron lines (in eV) and the Gordy group electronegativities of the R groups of the β-diketonato ligands, Σχ_R, the accumulative Hammett constants (Σσ_R), the pK_a of the β-diketonato ligand, the DFT-calculated electrophilicity index (ω in eV), the electrochemical potential (μ in eV) and the Mulliken electronegativity (χ in eV), the Lever constants (L_E), as well as the E(HOMO) (in eV) and E(LUMO) (in eV).

Equation	x	Value of x	R2 value	Calculated BE 2p3/2 main for Co(hfaa)3
y = 0.1223x + 780.34	ΣχR	6.02	0.81	781.08
y = 0.1823x + 780.9	ΣσR	0.86	0.70	781.06
y = −0.0356x + 781.21	pKa	4.71	0.90	781.07
y = 0.057x + 780.64	ω	8.43	0.85	781.12
y = 0.1332x + 780.32	χ	5.89	0.87	781.10
y = 0.8692x + 780.95	LE	0.17	0.77	781.10
y = −0.1319x + 780.06	E(HOMO)	−7.94	0.86	781.11
y = −0.1286x + 780.61	E(LUMO)	−3.83	0.86	781.10
Average				781.09
MAD				0.02

XPS: X-ray photoelectron spectroscopy; DFT: density functional theory; E(HOMO): energy of the highest occupied molecular orbital; E(LUMO): energy of the lowest unoccupied molecular orbital; BE: binding energy; MAD: mean absolute deviation.

Predicted BE of the Co 2p_{3/2 main} photoelectron for Co(hfaa)₃ using the tabulated equations.

To further validate the predicted BE Co 2p_{3/2 main} for Co(hfaa)₃ (hfaa = CF₃COCHCOCF₃) as 781.09 (Table 2), the Co 2p orbital energies, that are related to XPS IPs, were determined using DFT, and the values are given in Table 1. The relationship between the XPS measured binding energy of the main Co 2p_3/2 photoelectron line and the DFT-calculated Co 2p orbital energies, as shown in Figure 9(f), is

$${\mathrm{BE}\mathrm{Co}2\mathrm{p}}_{3/2\mathrm{main}}=-0.0993\times \mathrm{E}\left(\mathrm{Co}2\mathrm{p}\right)+704.93$$

Using this equation and the calculated Co 2p orbital energy of −767.11 for Co(hfaa)₃, a value of 781.10 eV is obtained, being inherently the same value as that predicted from the relationships shown in Table 2.

A spin-orbit splitting (ΔBE_main = BE_{Co2p1/2 main} − BE_{Co2p3/2 main}) of 15.3–15.6 eV was observed, depending on the electronic properties of the R group on the β-diketonato ligands (Figure 10(a)). This peak separation (ΔBE_main) can be used to quantify the delocalization of the valence electrons^38,40 and their weakened interaction with the core. Since a smaller separation (ΔBE_main) is associated with a higher degree of delocalization of the unpaired electrons,^36,41 it is implied that Co compounds with more electron-donating groups (lower Σχ_R) display smaller ΔBE_main values, and accordingly have increased delocalization of the 3d valence electrons, see Figure 10(a). The average ΔBE_main of ca. 15.4 eV is characteristic of diamagnetic Co(III) compounds. ³⁸

Figure 10.

(a) The relationship between the spin-orbit splitting of the main Co 2p_3/2 and Co 2p_1/2 photoelectron lines (ΔBE_main = BE Co 2p_1/2 – BE Co 2p_3/2) and Σχ_R, and (b) the relationship between FWHM of the main Co 2p_3/2 photoelectron line and the sum of the β-diketonato ligand Gordy scale R group electronegativities, Σχ_R, of 1–5.

The uncertainty principle predicts that a lifetime of the core-hole created by photoemission will result in a wider full width at half maximum (FWHM). ⁴² The FHWM can thus be used as an indication of the lifetime of a core-hole (since the time of measurement for all the samples were the same). From the trend shown in Figure 10(b), the lifetime of the core-hole created by photoemission is linearly dependent on Σχ_R. Thus, the more the electron density withdrawn from the Co(III) center (e.g. 4), the larger the FWHM and accordingly the shorter the lifetime of the core-hole.

Despite reports stating that diamagnetic Co(III) displays no to weak satellite structures, ³⁸ and that the XPS data of Co(acac)₃ reported in 1975 gives weak satellites, ²⁴ the Co(β-diketonato)₃ complexes 1–5 displayed weak to moderate satellite structures to the higher energy side of the main Co2p_3/2 photoelectron lines. These are attributed to the shake-up process, ⁴³ which involves the excitation of the valence electrons due to the interaction between the emitted photoelectrons and the valence electrons. In general, the intensity ratio of the sum of all the satellite structures to the intensity of the main Co 2p_3/2 photoelectron lines (I_Σ = (I_{Co 2p3/2 sat1} + I_{Co 2p3/2 sat2} + I_{Co 2p3/2 sat3})/(I_{Co 2p3/2 main})) increases with an increase in the binding energy of the main Co 2p_3/2 photoelectron line (BE Co 2p_{3/2 main}). This trend is opposite to that observed for paramagnetic Mn(III) compounds^12,44,45 and could be attributed to the decomposition of the F-containing compounds under X-ray irradiation. ⁴⁶ This effect is exaggerated when the exposure time (to the X-ray beam) is increased, see Figure 11 for 5. Another possible explanation for the opposite trend observed for Co versus Mn is the presence of the Jahn-Teller effect in the Mn complexes,^47,48 which is absent in the Co complexes.

Figure 11.

XPS of the Co 2p photoelectron lines of 5 measured after exposure to the X-ray beam after (top) 60 min and (bottom) 1870 min.

Conclusion

An insight into how the electronic properties of the R groups on the β-diketonato ligands influence its communication with the central cobalt atom may be obtained by critically evaluating the relationships between the XPS measured binding energy of the Co 2p_3/2 photoelectron lines and the sum of Gordy scale R group electronegativities of the β-diketonato ligands, the E(HOMO), E(LUMO), the Mulliken electronegativities, and the global electrophilicity indices. Using these relationships as well as the DFT-calculated Co 2p orbital energies, the binding energy of an unknown tris(β-diketonato) colbalt(III) can be predicted with high accuracy (MAD of 0.02). In addition, from the NMR results, it can be concluded that % of fac/mer isomers in a sample is not influenced by temperature or time when dissolved in solution (thus no dynamic equilibrium exists). It is therefore assumed that the % of fac/mer isomers of a sample is a consequence of the experimental conditions.

Experimental

Materials and methods

Reagents were obtained from Merck and Sigma-Aldrich. Solid reagents employed in preparations were used directly, without further purification. Liquid reactants and solvents were dried and distilled prior to use. Melting points (m.p.) were determined using an Olympus BX51 system microscope, assembled on top of a Linkam THMS600 stage, and connected to a Linkam TMS94 temperature programmer.

Synthesis

Complexes 1–5 were synthesized using reported methods ⁴⁹ with minor changes. ³² CoCO₃ (0.010 mol) was added to acetyl acetone (10 mL) or of the respective β-diketone (0.030 mol) dissolved in ethanol (10 mL) and heated to 95 °C while stirring. 25% Hydrogen peroxide (8 mL) was added dropwise to the suspension over 30 min to oxidize Co(II) to Co(III)

$$\begin{array}{l}{2\mathrm{CoCO}}_3+6†-\mathrm{diketone}+{\mathrm{H}}_2{\mathrm{O}}_2\mathrm{\to }\\ 2[\mathrm{Co}{(\mathrm{†}-\mathrm{diketonato})}_3]+4{\mathrm{H}}_2\mathrm{O}+2{\mathrm{CO}}_2\end{array}$$

Addition of H₂O₂ to the ligand suspension caused vigorous frothing; therefore, the addition of H₂O₂ was done very slowly. The reaction mixture turned dark green as coordination to the oxidized Co(III) occurred. After 40 min, the reaction mixture was cooled on ice and the obtained green precipitate was filtered and washed with excess cold water and then dried in a desiccator at room temperature. The complexes do not dissolve in water; they are, however, slightly soluble in acetone and soluble in methanol, acetonitrile, and most halogenated organic (chloroform) solvents. The different melting points reported in the literature are due to the different crystalline phases and different melting points for the fac and mer isomers. ³⁰

Characterization data for [Co(CH₃COCHCOCH₃)₃], (1) [Co(acac)₃]: Yield 63% dark green solid; m.p. 213 °C (Lit. 210 °C, ⁵⁰ 213 °C, ⁴⁹ 220 °C, 219–220 °C, ³⁰ 220 °C ⁵¹ ); IR ῡ (cm⁻¹): 1580 (CO);^{30,52 1}H NMR (600 MHz, CDCl₃, 25 °C): δ 5.53 (s, 3H, 3 x CH), 2.19 (s, 18H, 6 x CH₃); ¹³C NMR (150 MHz, CDCl₃, 25 °C): δ 189.5 (CO), 97.1 (CH), 26.1 (CH₃).

Characterization data for [Co(C₆H₅COCHCOCH₃)₃], (2) [Co(ba)₃]: Yield 66% dark green solid; m.p. 184.3 °C ³² (Lit 193–197 °C, ⁵⁰ 213–214 °C, ³⁰ 213 °C ⁵¹ ); IR ῡ (cm⁻¹): 1554 (CO), ⁵⁰ 1585 (CO). ³⁰ MS (MALDI-TOF): m/z = 542.11 [M⁺]; ¹H NMR (600 MHz, CDCl₃, 25 °C): δ 7.86 (br s, 6H, 3 x Ph), 7.41 (br s, 3H, 3 x Ph), 7.32 (br s, 6H, 3 x Ph), 6.23 (br s, 3H, 3 x CH), 2.38 (br s, 9H, 3 x CH₃); ¹H NMR (600 MHz, CDCl₃, 55 °C): δ 7.82–7.87 (m, 6H, 3 x Ph), 7.40–7.37 (m, 3H, 3 x Ph), 7.33–7.29 (m, 6H, 3 x Ph), 6.21 (s, 1H, CH), 6.20 (s, 1H, CH), 6.19 (s, 1H, CH), 2.36 (s, 3H, CH₃), 2.35 (s, 3H, CH₃), 2.33 (s, 3H, CH₃); ¹³C NMR (150 MHz, CDCl₃, 25 °C): δ 191.8 (CO), 191.6 (CO), 191.4 (CO), 182.6 (CO), 182.5 (CO), 182.3 (CO), 137.0 (3 x Ph), 131.0 (3 x Ph), 128.0 (6 x Ph), 127.5 (6 x Ph), 94.6 (CH), 94.5 (CH), 94.2 (CH), 27.0 (2 x CH₃), 26.8 (CH₃); ¹³C CP/MAS NMR δ (100 MHz, 25 °C) δ 192.9 (CO), 192.0 (CO), 189.0 (CO), 184.2 (CO), 183.3 (CO), 181.2 (CO), 138.6 (Ph), 136.9 (Ph), 135.0 (Ph), 133.3 (Ph), 131.6 (Ph), 130.4–124.5 (13 x Ph), 93.5 (3 x CH), 26.9 (CH₃), 25.4 (2 x CH₃).

Characterization data for [Co(C₆H₅COCHCOC₆H₅)₃], (3) [Co(dbm)₃]: Yield 72% dark green solid; m.p. 237 °C, ⁵⁰ 252–253 °C; ³⁰ IR ῡ (cm⁻¹): 1545 (CO), ⁵⁰ 1600 (CO); ^{30 1} H NMR (600 MHz, CDCl₃, 25 °C): δ 7.99 (br s, 12H, 6 x Ph), 7.43 (br s, 6H, 6 x Ph), 7.36 (br s, 12H, 6 x Ph), 6.92 (br s, 3H, 3 x CH); ¹³C NMR (150 MHz, CDCl₃, 25 °C): δ 184.5 (6 x CO), 137.6 (6 x Ph), 131.2 (6 x Ph), 128.2 (12 x Ph), 127.7 (12 x Ph), 91.9 (3 x CH).

Characterization data for [Co(CH₃COCHCOCF₃)₃], (4) [Co(tfaa)₃]: Yield 51% dark green solid; m.p. (fac 128-129.5 °C, mer 158-159 °C); ³⁰ IR ῡ (cm⁻¹): 1600 (CO); ^{30 1} H NMR (600 MHz, CDCl₃, 25 °C): δ 6.07 (s, 1H, CH), 6.05 (s, 1H, CH), 6.01 (s, 1H, CH), 2.45 (s, 3H, CH₃), 2.44 (s, 3H, CH₃), 2.40 (s, 3H, CH₃); ¹³C NMR (150 MHz, CDCl₃, 25 °C): δ 199.5 (CO), 199.0 (CO), 198.9 (CO), 170.8 (q, J_C-F = 34.6 Hz, CO), 170.6 (q, J_C-F = 34.8 Hz, CO), 170.1 (q, J_C-F = 34.7 Hz, CO), 114.4 (q, J_C-F = 282.7 Hz, CF₃), 94 (CH), 94.1 (CH),94.3 (CH), 27.8 (CH₃), 27.7 (CH₃), 27.5 (CH₃); ¹⁹F NMR (565 MHz, CDCl₃, 25 °C): δ −73.46, −73.65, −73.66.

Characterization data for [Co(C₆H₅COCHCOCF₃)₃], (5) [Co(tfba)₃]: Yield 13% dark green solid; m.p. 107.6 °C, ³² 192–193 °C; ³⁰ IR ῡ (cm⁻¹): 1600 (CO). ³⁰ MS (MALDI-TOF): m/z = 703.98 [M⁻¹]⁻; ¹H NMR (600 MHz, CDCl₃, 25 °C): δ 8.00–7.88 (m, 6H, 3 x Ph, fac + mer), 7.60–7.54 (m, 3H, 3 x Ph, fac + mer), 7.48–7.40 (m, 6H, 3 x Ph, fac + mer), 6.75 (s, 1H, CH, mer), 6.72 (s, 1H, CH, mer), 6.71 (s, 1H, CH, fac), 6.69 (s, H, CH, mer); ¹³C NMR (150 MHz, CDCl₃, 25 °C): δ 189.9 (2 x CO, mer), 189.7 (CO, fac), 189.6 (CO, mer), 172.6 (q, J_C-F = 35.1 Hz, CO, mer), 172.5 q, J_C-F = 34.6 Hz, CO, fac), 172.2 (q, J_C-F = 34.6 Hz, CO, mer), 172.1 (q, J_C-F = 34.6 Hz, CO, mer), 135.3 (Ph, fac), 135.2 (Ph, mer), 135.1 (Ph, mer), 135.0 (Ph, mer), 133.5 (2 x Ph, mer), 133.4 (Ph, fac), 133.3 (Ph, mer), 128.8 (2 x Ph, mer), 128.7 (2 x Ph, fac), 128.7 (2 x Ph, mer), 128.6 (2 x Ph, mer), 128.2 (2 x Ph, mer), 128.1 (2 x Ph, mer), 128.1 (2 x Ph, mer), 128.0 (2 x Ph, fac), 91.0 (CH, mer), 90.8 (CH, fac), 90.7 (CH, mer), 90.5 (CH, mer); ¹⁹F NMR (565 MHz, CDCl₃, 25 °C): δ −73.41 (fac), −73.48 (mer), −73.52 (mer), −73.58 (mer).

Spectroscopic analysis

Nuclear magnetic resonance spectroscopy

The liquid-state ¹H, ¹³C, and ¹⁹F NMR spectra were recorded at 25.0 °C on a 600 MHz Avance II Bruker spectrometer operating at 600.28, 150.95, and 564.83 MHz for ¹H, ¹³C, and ¹⁹F, respectively. Deuterated chloroform was used as the solvent. The chemical shifts (δ) are reported in parts per million (ppm) and the spectra are referenced relative to the Me₄Si internal standard at 0 ppm for ¹H and ¹³C spectra. Coupling constants (J) are reported in Hz. The solid-state NMR spectra were collected on a 400 MHz Bruker Avance III spectrometer equipped with a 4-mm VTN multinuclear double resonance magic angle spinning probe, operating at 25.0 °C. The ¹³C NMR spectra were recorded at 100.61 MHz, using the cross polarization magic angle spinning (CP/MAS) technique. A rotating speed of 14,000 Hz was used with a contact time of 2 ms, a recycle delay of 5 s, and an acquisition time of 33.9 ms. The sample was packed in a 4-mm zirconia rotor. ¹⁹F NMR chemical shifts are given relative to hexafluorobenzene in CDCl₃ at δ = −164.9 ppm (external reference).

X-ray photoelectron spectroscopy

XPS data were recorded on a PHI 5000 Versaprobe system with a monochromatic Al K X-ray source. Spectra were obtained using the aluminum anode (Al Kα = 1486.6 eV) operating at 50 μm, 12.5 W, and 15 kV energy (97 X-ray beam). The survey scans were recorded at a constant pass energy of 187.85 eV and region scans at pass energies ranging from 29 to 90 eV with the analyzer resolution ⩽0.5 eV. The background pressure was ca. 2 × 10⁻⁸ mbar. Spectra have been corrected to the main line of the adventitious carbon 1s spectrum, which was set to 284.8 eV. The XPS data were analyzed utilizing Multipak version 8.2c computer software, ⁵³ using Gaussian–Lorentz fits (the Gaussian/Lorentz ratios were always >95%).

Matrix-assisted laser desorption ionization time-of-flight

MALDI-TOF spectra were collected using a Bruker Microflex LRF20 in positive mode with 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) as the matrix. A minimum laser power 337 nm was required to observe signals.

Density functional theory calculations

DFT calculations were performed using the B3LYP functional, as implemented in the Gaussian 16 package, ⁵⁴ using the triple-ζ basis set 6-311G(d, p) for all non-metal atoms, and the Lanl2dz basis set ⁵⁵ that corresponds to the Los Alamos ECP plus DZ was used for both the metal electronic core and valence electrons. The B3LYP exchange-correlation functional includes 20% Hartree-Fock exchange and GGA corrections (Becke 88 exchange functional and the Lee, Yang and Parr correlation functional) in addition to LDA (VWN local-density approximation) electron-electron and electron-nuclei energy. ⁵⁶ B3LYP is well established in the literature and is on average the best choice for most systems, especially small organic systems. ⁵⁷ All optimized structures have been verified as minima by performing frequency calculations in order to ensure that no imaginary frequency was present. Calculations were done in chloroform implicit solvent, since NMR spectra were obtained in chloroform solution. For solvent calculations, the integral equation formalism polarizable continuum model (IEFPCM) of solvation to describe the dielectric continuum medium was used.^58,59 The input coordinates for the compounds were constructed using Chemcraft. ⁶⁰ The ratio of the relative population of the fac and mer isomers was determined by the Boltzmann equation at temperature T as indicated in Figure 3

$$\ln \frac{{\mathrm{n}}_{\mathrm{j}}}{{\mathrm{n}}_{\mathrm{i}}}\mathrm{=}-\frac{{\mathrm{(G}}_{\mathrm{j}}{\mathrm{-G}}_{\mathrm{i}}\mathrm{)}}{\mathrm{kT}}$$

where n_i is the number of molecules with free energy G_i (fac or mer in this case), with Boltzmann’s constant, k = 1.38066 × 1023 J K⁻¹.

DFT-calculated Mulliken electronegativity (χ), the electronic chemical potential (μ), and the global electrophilicity index (ω) were calculated as described in our previous studies, ⁶¹ namely: the EA and IP of each of the compounds were determined from the frontier orbital energies (highest occupied molecular orbital = HOMO and lowest unoccupied molecular orbital = LUMO), according to Koopman’s theorem^20,23

$$\mathrm{IP}=-{\mathrm{E}}_{\mathrm{HOMO}}\mathrm{and}\mathrm{EA}=-{\mathrm{E}}_{\mathrm{LUMO}}$$

The Mulliken electronegativity (χ)^21,22 and the global electrophilicity index (ω), defined by Parr and coworkers as the electrophilic power of atoms and molecules, ²³ were calculated for each compound, by application of the following formulae

$$\mathrm{\chi }=\left(\mathrm{IP}+\mathrm{EA}\right)/2\mathrm{and}\mathrm{\omega }={\mathrm{\mu }}^2/\left(2\mathrm{\eta }\right)$$

where μ = −(IP + EA)/2 is the electronic chemical potential (the same as −χ = μ) ⁶² and η = (IP − EA) is the chemical hardness^63,64 of the ground state of the atoms and molecules.

The energies of the Co 2p orbital of the molecules are determined by optimizing the molecules using the ADF program system,^65,66 and the OLYP basis set,^67,68 corrected for dispersion by Grimme’s D3 correction, ⁶⁹ with an all-electron TZ2P basis set.

Supplemental Material

sj-docx-1-chl-10.1177_17475198231184788 – Supplemental material for Spectroscopic and DFT study of tris(β-diketonato)cobalt(III) complexes

Supplemental material, sj-docx-1-chl-10.1177_17475198231184788 for Spectroscopic and DFT study of tris(β-diketonato)cobalt(III) complexes by Linette Twigge, Jeanet Conradie and Elizabeth Erasmus in Journal of Chemical Research