Can Electrochemical Measurements Be Used To Predict X-ray

Publication Date (Web): May 15, 2018. Copyright © 2018 American ... redox potentials now allows BE predictions from simple electrochemical measuremen...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Can Electrochemical Measurements Be Used To Predict X‑ray Photoelectron Spectroscopic Data? The Case of Ferrocenyl-βDiketonato Complexes of Manganese(III) Blenerhassitt E. Buitendach,† Elizabeth Erasmus,† J. W. Niemantsverdriet,‡ and Jannie C. Swarts*,† †

Department of Chemistry, University of the Free State, Bloemfontein 9300, South Africa SynCat@DIFFER, Syngaschem BV, De Zaale 20, 5612 AJ Eindhoven, The Netherlands

‡

S Supporting Information *

ABSTRACT: In order to better understand intramolecular communication between molecular fragments, a series of ferrocene-functionalized β-diketonato manganese(III) complexes, [Mn(FcCOCHCOR)3] with R = CF3, 1, CH3, 2, Ph = C6H5, 3, and Fc = FeII(η5-C5H4)(η5-C5H5), 4, the mixed ligand β-diketonato complex [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)], 5, as well as the acac complex [Mn(CH3COCHCOCH3)3], 6, were subjected to an electrochemical and X-ray photoelectron spectroscopy (XPS) study. The ferrocenyl (FeII) and MnIII redox potentials, E°′, and photoelectron lines were sufficiently resolved in each complex to demonstrate a linear correlation between E°′ and group electronegativities of ligand R groups, χR, or ΣχR, as well as with binding energies of both the Fe 2p3/2 and Mn 2p3/2 photoelectron lines. These relationships are consistent with effective communication between molecular fragments of 1−5. From these relationships, prediction of Mn and Fe core electron binding energies in [Mn(R1COCHCOR2)3] complexes from known manganese and/or ferrocenyl redox potentials are, therefore, now possible. Ligand infrared carbonyl stretching frequencies were successfully related to binding energy as a measure of the energy required for inner-sphere reorganization. In particular it became possible to explain why, upon electrochemical oxidation or photoionization, the ferrocenyl FeII inner-shell of 1−5 needs more energy in complexes with ligands bearing electronwithdrawing (CF3) groups than in ligands bearing electron-donating groups such as ferrocenyl. The XPS determined entity Iratio (the ratio between the intensities of the satellite and main metal 2p3/2 photoelectron lines) is an indication not only of the amount of charge transferred, but also of the degree of inner-sphere reorganization. Just as for binding energy, the quantity Iratio was also found to be related to the energy requirements for the inner-sphere reorganization depicted by the vibrational frequency, vco.

1. INTRODUCTION The stability of ferrocene, the ease by which it can undergo chemically transformations, and the reversible ferrocenyl iron(II/III) redox couple have led to many applications and fundamental studies of new ferrocenyl-containing materials and have caused Astruc to devote an entire review article to the exceptional properties of the family of ferrocene complexes.1 Ferrocene and its derivatives have been studied as nonlinear optical materials,2 asymmetric catalysts,3 high combustion-rate catalysts,4 smoke suppressing agents,5 donors in energy transfer processes,6 and anticancer drugs.7 They are also known to accelerate the rate of oxidative addition reactions8 while decelerating substitution processes.9 Many homometallic β-diketonato complexes are known,10 but heterometallic β-diketonato complexes are relatively rare.11 With respect to heterometallic complexes, Zanello12 previously reported the synthesis and electrochemistry of the Fe3−Mn tetrametallic β-diketonato complex tris(1-ferrocenyl-1,3butanedionato)manganese(III), [Mn(FcCOCHCOCH3)3], 2. Subsequently, we added to the Zanello study by describing the © XXXX American Chemical Society

synthesis and X-ray photoelectron spectroscopic (XPS) properties of the series of ferrocenyl-functionalized β-diketonato manganese(III) complexes, [Mn(FcCOCHCOR)3] with R = CF3, 1, CH3, 2, Ph = C6H5, 3, and Fc = FeII(η5-C5H4)(η5C5H5), 4, as well as the mixed ligand β-diketonato complex [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)], 5, Figure 1.13 Because of the octahedral coordination sphere of MnIII in these complexes, the four unpaired electrons of MnIII in 1−4 result in Jahn−Teller distortions with axial Mn−O bond lengths longer than equatorial Mn−O bond lengths.14 From the crystal structure determination of 2,13 the difference between the largest axial Mn−O and smallest equatorial Mn−O bond length of 2 was found to be 0.039 Å. A refreshingly new relationship was established13 in that group electronegativities of ligand R groups of 1−5, χR,15 or the sum of group electronegativities of β-diketonato pendant side groups, ΣχR, are linearly related to the binding energies of Received: March 20, 2018

A

DOI: 10.1021/acs.inorgchem.8b00745 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

results of an electrochemical study. Successive ferrocenyl formal redox potentials will be shown to be a function of χR or ΣχR. We then highlight the relationship between ferrocenyl redox potentials and XPS data of 1−5, thereby showing inter alia it is possible to predict binding energies from electrochemical measurements.

2. EXPERIMENTAL SECTION 2.1. General. [Mn(FcCOCHCOR)3] complexes 1−4 and [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)], 5, were prepared13 from [Mn3(OAc)6O·3H2O-OAc] (Aldrich) and FcCOCH2COR after care was taken to remove the acetyl ferrocene Aldol self-condensation product,21 FcCOCHC(CH3)Fc, from freshly synthesized22 betadiketones. [Mn(CH3COCHCOCH3)3], 6,23 as well as tetrabutylammonium tetrakispentafluorophenylborate, [N(nBu)4][B(C6F5)4], were prepared as described before.24 For electrochemical experiments, spectrochemical grade dichloromethane (Aldrich) was dried by distillation from calcium hydride and, to remove photochemically generated HCl, passed through basic alumina prior to use. 2.2. Electrochemistry. Cyclic voltammograms (CVs), square wave voltammograms (SWs) and linear sweep voltamograms (LSVs) were recorded using a computer-controlled BAS model 100 B potentiostat. All experiments were performed in a dry cell under an argon atmosphere. Platinum wires were used for the pseudo internal reference electrode as well as the auxiliary electrode, while a glassy carbon working electrode (experimentally determined surface area 7.071 mm2)25 was utilized. Between each set of scans the working electrode was polished on a Buhler polishing mat with 1 μm and then with 1/4 μm diamond paste. All electrode potentials are reported using the potential of the ferrocene/ferrocenium redox couple [FcH/ FcH+] (FcH = Fe(η5-C5H5)2, E°′ = 0.00 V) as reference. Decamethyl ferrocene, Fc*, was used as internal standard26 to prevent signal overlap with ferrocenyls of complexes 1−5; it has a potential of −608 mV versus free ferrocene with ΔE = 72 mV and ipc/ipa = 1 under the conditions employed. Analyte concentrations were ca. 0.5 mM in CH2Cl2, and 0.1 M tetrabutylammonium tetrakispentafluorophenylborate, [N(nBu)4][B(C6F5)4], was used as a supporting electrolyte. Data were exported to a spreadsheet program for adjustment and diagram preparation. 2.3. X-ray Photoelectron Spectroscopy. XPS data of powdered samples mounted on the sample holder by means of carbon tape were recorded on a PHI 5000 Versaprobe system with monochromatic AlKα X-ray source as described before.13,16 2.4. Infrared Spectroscopy. IR spectra were recorded on a Bruker Tensor 27 spectrophotometer with a Bruker Platinum attenuated total reflectance (ATR) accessory (diamond crystal) using OPUS software version 6.5. Powdered samples were dried under a vacuum at 50 °C for 1 h prior to taking measurements.

Figure 1. Top: Facial ( fac) isomers of 1−3 and the mer isomer of 3. Complex 4 has a symmetric ligand and no fac or mer isomer exists. Bottom: The mixed ligand β-diketonato complex 5 and the acac complex 6. Fac isomers possess a symmetry plane shown in blue which passes through one face of the octahedron of the Mn complexes, the vertices of which are occupied by the same substituent of the three unsymmetrical ligands (here the ferrocenyl groups). Mer isomers possess a symmetry plane passing through the Mn metal center which is here formed by the three identical ferrocenyl substituents of the three unsymmetrical ligands.

both the low spin Fe 2p 3/2 and high spin Mn 2p 3/2 photoelectron lines. Erasmus and co-workers have also shown that XPS data can distinguish between meridional (mer) and facial (fac) isomers of 1−3 (Figure 1; 4 and 5 do not possess such isomers)16 and that multiplet splitting as calculated by Gupta and Sen17 may be simulated to the high-spin main Mn 2p3/2 XPS photoelectron lines of 1−5 to reproduce the experimental XPS spectra well. The difference between the binding energies of the main Mn 2p3/2 and Mn 2p1/2 spin−orbit splitting photoelectron envelopes, ΔBE = BEMn 2p1/2 − BEMn 2p3/2, and the sum of βdiketonato ligand Gordy scale R-group electronegativities, ΣχR, were found to be directly proportional to each other. Shake-up and shake-down satellite peaks in the Mn 2p3/2 and Fe 2p3/2 XPS binding energy envelopes resulting from a charge transfer process from ligand to metal could be identified, and the intensity ratio of these peaks (Iratio = (IMn or Fe 2p3/2satel)/ (IMn or Fe 2p3/2main)) also related linearly to ΣχR.16 With respect to the electrochemical properties of ferrocenecontaining heterometallic β-diketonato complexes, better electronic communication between pendent ferrocenyl substituents in octahedral [Al(FcCOCHCOR)3] complexes with R = CF3, CH3, Ph, and Fc) were found18 than in square planar [Cu(FcCOCHCOR)2] complexes,19 and it was possible to quantify ferrocenyl formal redox potentials, E°′, as a function of the sum of group electronegativities, ΣχR of terminal βdiketonato substituents.18 This raised the question of whether it was possible to also relate binding energies of iron and manganese in complexes 1−5 to ferrocenyl redox potentials and whether these redox potentials could be related to χR, or ΣχR.20 In this study we shall highlight electronic communication between ferrocenyl groups in manganese complexes 1−5 from

3. RESULTS AND DISCUSSION 3.1. Electrochemistry. CV, LSV, and SW 27 were conducted on 0.5 mM solutions of manganese(III) complexes 1−6 in dry CH2Cl2 utilizing 0.1 M [(nBu)4N]B(C6F5)4] as supporting electrolyte. Data for cyclic voltammetry experiments are summarized in Table 1, CVs of 5 at different scan rates are shown in Figure 2, while Figure 3 allows comparison of CVs of 1−6 at a scan rate of 100 mV s−1 with each other. CH2Cl2 was used as solvent because it minimizes solvent− compound interactions, while the chosen supporting electrolyte, [(nBu)4N][B(C6F5)4], minimizes ionic interactions of the type (cations)n+···−[B(C6F5)4].28 Zanello12 previously studied [Mn(FcCOCHCOCH3)3], 2, in CH3CN with [(nBu)4N][PF6] as supporting electrolyte. This solvent/electrolyte system did not allow for the resolution of the three individual ferrocenylbased redox processes; only three unresolved 1e− processes (i.e., just one electrochemical wave) at E°′ = 1/2(Epa + Epc) = B

DOI: 10.1021/acs.inorgchem.8b00745 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Cyclic Voltammetry Data at a Glassy Carbon Electrode of 0.5 mM Solutions of 1−5 in CH2Cl2 Containing 0.1 M [(nBu)4N][B(C6F5)4] as Supporting Electrolyte at 20 °Ca wave 2 1 (MnIII/II) F1 F2 F3 2 1 (MnIII/II) F1 F2 F3 3 (MnIV/III)f,g 2 1 (MnIII/II) F1 F2 F3 3 (MnIV/III)f,g 2 1 (MnIII/II)i F1 F2 F3 F4 F5 F6 2 1 (MnIII/II) F1 F2 F3 F4 F5

Epa/V

ΔEp/V

E°′/V

1, [Mn(FcCOCHCOCF3)3] −0.791c 0.086 −0.834 −0.272c 0.187 −0.207j 0.335 0.060 0.285 0.447 0.077 0.403 0.586 0.078 0.547 2, [Mn(FcCOCHCOCH3)3] −1.392c 0.074 −1.355 −0.905c 0.304 −0.753j 0.146 0.097 0.097 0.267 0.118 0.208 0.385 0.088 0.341 1.360 0.076 1.322 3, [Mn(FcCOCHCOPh)3] −1.336c 0.071 −1.300 −0.930c 0.385 −0.738j 0.131 0.076 0.093 0.266 0.105 0.213 0.392 0.081 0.351 1.270 0.076 1.232 4, [Mn(FcCOCHCOFc)3] −1.466c 0.080i −1.426i

ipa/μA

ipc/ipa

4.56d 0.90d 4.73 4.24 4.40

0.98e 1.57e 0.97 0.95 0.94

1.62d 2.57d 4.32 4.19 4.46 1.28

1.00e 0.87e 0.99 1.00 0.94 0.67g

1.37d 2.60d 4.25 4.11 4.38 1.91

1.10e 0.95e 1.00 0.97 0.97 0.56g

0.086d

h

0.087 0.070 0.052 0.14 0.245 0.090 0.200 0.23 0.325 0.080 0.285 0.14 0.437 0.085 0.395 0.23 0.661 0.082 0.620 0.09 0.818 0.076 0.780 0.09 5, [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)] −1.392c 0.079 −1.353 1.85d c j −0.974 0.304 −0.822 1.41 0.115 0.063 0.084 4.07 0.245 0.111 0.189 4.63 0.335 0.065 0.303 4.44 0.475 0.060 0.444 2.59 0.640 0.077 0.602 1.11

Figure 2. Top: Square wave voltammogram (SW) of a 0.5 mM solution of [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)], 5, in CH2Cl2 (20 °C) at 10 Hz in the presence of Fc* = decamethylferrocene as internal standard. Second from above: 100 mV s−1 scan rate cyclic voltammogram (CV) of 5 in the absence of the internal standard, Fc*. Third from above: CVs at scan rates 100 (smallest currents), 200, 300, 400, and 500 mV s−1 in the presence of Fc*. Bottom: Linear sweep voltammograms (LSV) of 5 at 2 mV s−1.

0.77 1.00 0.77 1.00 1.00 1.00

eventual peak resolution is within the same experimental limits for both techniques. Utilizing 5 as example (Figure 2), CV ΔEpeak res = EFn,pa − EF(n‑1),pa with n = 2−5 was 90−165 mV, while for SW voltammetry it was 108−162 mV. As was found for [Al(FcCOCHCOR)3] and [Cu(FcCOCHCOR)2] complexes,18,19 the good resolution between ferrocenyl peaks demonstrates that communication between the ferrocenyl centers exists. Each ferrocenyl wave F1−F6 was involved in the same number of electrons transferred as demonstrated for 5 in Figure 2 with the LSV scan because the peak intensity for each LSV step is equal. That each step represented a one-electron transfer process stems from the CV scans. One-electron electrochemical reversible redox processes are theoretically characterized by CV peak separations of ΔEp = Epa − Epc = 59 mV, while twoelectron transfer processes are characterized by ΔEp = ca. 30 mV.27 Experimentally, for example, for 5, ΔEp was found to be at slow scan rates (100 mV s−1) within the range 63 < ΔEp < 77 mV except for wave F2 which showed ΔEp = 111 mV (Table 1). This outlying value is, however, more the result of poor resolution between Epc of F2 and F3. Applying the same criteria, including accommodation of the difficulties of accurately measuring Epa and Epc values for closely overlapping redox processes,29 complexes 1−4 also exhibited one-electron electrochemical reversible ferrocenyl redox processes, Table 1. Chemically, the ferrocenyl redox processes of 1−5 were also reversible by virtue of ipc/ipa ratios approaching 1, Table 1. The only exceptions were peaks where the CV ipc were poorly identifiable, e.g., F1 and F3 of 4 (due to poor complex solubility) and F5 of 5.

0.77e 0.79e 0.92 0.96 1.00 0.86 0.80

Scan rate = 100 mV s−1. bDue to the proximity and resolution of the ferrocenyl peaks of 4 and 5, the peak current values are at best estimates. cPeak cathodic potential, Epc. dPeak cathodic current, ipc. e ipa/ipc. fWave 3 for 1, 4, and 5, due to the strong electron-withdrawing power of the CF3 and Fc+ species, fell outside the solvent potential window. gEstimated values only due to the close proximity to the edge of the solvent potential window. hipc/ipa could not be determined as ipc could not be measured. iPoor solubility of 4 implies for wave 2, Epa could barely be identified, no ipa value could be determined. For wave 1, no measurements were possible. jBecause of the large ΔEp value of this wave, we prefer to label this quantity the E1/2 potential rather than E°′ because the MnIII/II couple is clearly electrochemically irreversible. a

0.59 V versus a standard calomel electrode (SCE) were observed. They also observed an irreversible MnIII/MnII reduction at E°′ = −0.90 V. By using the CH2Cl2/[(nBu)4N][B(C6F5)4] system, it was possible to observe resolved one-electron ferrocenyl-based redox processes for 1−5 (Figures 2 and 3). Each ferrocenyl redox process is labeled F1, F2, F3···F6. SW voltammetry allowed better peak detection than CV (Figure 2), but the C

DOI: 10.1021/acs.inorgchem.8b00745 Inorg. Chem. XXXX, XXX, XXX−XXX

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mV; ipc/ipa approaches 1) at one of the β-diketonato ligands between potentials −1.355 and −0.834 V at wave 2. Due to poor solubility of 4, no realistic MnIII or ligand reduction processes could be identified (note that currents for 4 were already intensified 6-fold for observational purposes in Figure 3). 3.2. Sequence of Redox Events. Gordy scale group electronegativities, χR, are used as a measure of the electronwithdrawing properties of the R-groups.15 Values22 for χR of the ligand R-groups of this study are as follows: χR(Fc) = 1.87; χR(Ph) = 2.21; χR(CH3) = 2.34; χR(CF3) = 3.01. The oxidized ferrocenyl moiety is almost as electronegative as the CF3 group with χR(Fc+) = 2.82. As the R-group in [Mn(FcCOCHCOR)3] becomes less electron-withdrawing in moving from R = CF3 to Fc in 1−4, the ferrocenyl waves F1−F3 moves systematically to smaller potentials. This can be seen, for example, in wave F1 with E°′ = 0.285, 0.097, 0.093, and 0.052 V for complexes 1 (R = CF3), 2 (R = CH3), 3 (R = Ph), and 4 (R = Fc) respectively. In the same way, the MnIII/MnII redox process moves also to smaller potentials, Table 1. This dependency of potentials on χR values also testifies of good communication between molecular fragments of 1−5. To establish an electrochemical scheme that depicts the sequence of electronic events, cognace of the following may be taken: During the oxidation of each ferrocenyl group of 1−5, mixed-valent intermediates are formed. The difference in group electronegativities of the neutral ferrocenyl group (χFc = 1.87) and that of the positively charged ferrocenium group (χFc+ = 2.82) allows for the electrochemical detection of each of these species in solution, Figure 3. As the complexes become progressively more oxidized, the electron-withdrawing effect of each positively charged ferrocenium group is transmitted via electrostatic field effects18 to other molecular fragments. For example, 5, [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)], contains in the ground state two (FcCOCHCOFc)− ligands and one (FcCOCHCOCH3)− ligand and shows five ferrocenylbased redox processes during oxidation, Figure 2. By taking the group electronegativities of CH3 (2.34), Fc (1.87), and Fc+ (2.82) into account, it can be concluded that a single ferrocenyl group of one of the bisferrocenyl β-diketonato ligands has greater available electron density than the ferrocenyl βdiketonato ligand with the methyl substituent and should be oxidized first. This was also found to be the case in a separate study involving the free β-diketonato ligands.31 Here, under identical electrochemical conditions, the redox couple corresponding to the first ferrocenyl-group in the free ligand, FcCOCH2COFc, was found at E°′ = 0.202 V and that of the ferrocenyl-group on the free ligand, FcCOCH2COCH3, at E°′ = 0.234 V. This suggest oxidation of one Fc substituent of a bisferrocenyl diketonato ligand of complex 5 is easier than of the ferrocenyl group on the methyl-containing β-diketonato ligand and wave F1 at E°′ = 0.084 V of 5 (Figure 2) is assigned to the oxidation of one of the two Fc groups on the bisferrocenyl ligands. The formed ferrocenium moiety is almost as electronegative as the CF3 group (χFc+ = 2.82, χCF3 = 3.01). It withdraws electron-density from the ligand’s remaining Fcgroup as well as the rest of the molecule through the Mncenter. As a result, wave F2 (E°′ = 0.189 V) is assigned to the oxidation of a Fc group of the other bisferrocenyl ligand. Wave F3 (E°′ = 0.303 V) is assigned to the oxidation of Fc in the methyl-substituted ligand, followed by the remaining Fc-groups of the bisferrocenyl ligands F4 (E°′ = 0.444 V) and F5 (E°′ =

Figure 3. Cyclic voltammograms of ca. 0.5 mM solutions of [Mn(FcCOCHCOR) 3] 1−4, [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)], 5 and [Mn(CH3COCHCOCH3)3], 6, at 100 mV s−1 in CH2Cl2 with 0.1 mM [N(nBu)4][B(C6F5)4] at 20 °C. Superimposed on the CV of 4 is an SW of the ferrocenyl region at 10 Hz. The CV of [Mn(CH3COCHCOCH3)3] unambiguously identifies the MnIII/MnII and MnIV/MnIII redox processes.

One-electron reduction of 1−5 at wave 1 (Figure 3) at potentials between −0.2 and −0.9 V versus FcH/FcH+, depending on R-group, is associated with Mn(III) reduction; it is an electrochemically irreversible redox process (187 < ΔEp < 385 mV), although chemical reversibility was reasonable as indicated by ipa/ipc values approaching 1, Table 1. The CVs of complexes 2 and 3 also showed a one-electron MnIII oxidation to MnIV (wave 3) at the edge of the solvent potential window (E°′ = 1.322 and 1.232 V respectively). For 2 and 3 this redox process was electrochemically reversible at slow (100 mV/s) scan rates (ΔEp ca. 76 mV), but peak current ratios, ipc/ipa, could not be measured because of the closeness of this redox process to the edge of the solvent redox window. Wave 3 for 1, 4, and 5 fell outside the redox potential range of the solvent. Complex 6, [Mn(CH3COCHCOCH3)3], also showed the MnIII/II couple (wave 1 at Epc = −0.877 V, ΔEp = 416 mV, ipc = 6.58 ipa/ipc = 0.81) as well as the MnIV/III couple (wave 3 at Epa = 0.59 V, ΔEp = 178 mV, ipa = 6.85, ipc/ipa = 0.99), this time as electrochemically irreversible redox processes at potentials much closer to 0 V (Figure 3) because in this complex, redox processes are independent of the electron-donating effects of a ferrocenyl group (wave 1), or the electron withdrawing effects of a CF3 group or the oxidized ferrocenium group (wave 3). These potentials are in line with those found for other Mn βdiketonato complexes.30 Complexes 1−3 and 5 also showed a one-electron electrochemical and chemical reversible reduction (ΔEp < 90 D

DOI: 10.1021/acs.inorgchem.8b00745 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 0.602 V). Likewise, at wave 2, one of the β-diketonato ligands is reduced. Because the group electronegativity of the CH3 group is larger than that of a ferrocenyl group, the (FcCOCHCOCH3)− ligand is here reduced to (FcCOCHCOCH3)2−. With this background, an electrochemical scheme describing the redox processes of 5 can be written, Scheme 1. In this

Then for 5+: ΣχR = (χFc + χFc ) + (χFc + + χFc ) + (χFc + χCH3 )

Scheme 1. Sequence of Electrochemical Processes Associated with Waves 2, 1, and F1−F5 of Mn(FcCOCHCOFc)2(FcCOCHCOCH3)], 5a

= (1.87 + 1.87) + (2.82 + 1.87) + (1.87 + 2.34) = 12.64

Similarly, a measure of the electron density on 52+ is ΣχR = 13.59. The quantity ΣχR was calculated for all electrochemically observed intermediates for all complexes. Figure 4 shows the

Figure 4. Relationship between the sum of the group electronegativities, ΣχR, and the ferrocenyl-based redox potentials for each compound individually. The numbers in parentheses are simultaneously complex numbers and also equation numbers in the text.

linear relationships between ΣχR and E°′ obtained between each of the ferrocenyl redox processes for each of complexes 1−5 individually. Only eq 4 includes data for waves 4, 5, and 6, while eq 5 also incorporates data from waves 4 and 5. The equations predicting E°′ (in volts) from ΣχR for 1−5 are

a

The superscript number after the square bracket of each complex represents the total charge on the molecule in the particular redox state. Eo′ values are given at a scan rate of 0.1 V s−1. For wave 1, large ΔE values (Table 1) suggests E1/2 as a better symbol than Eo′. ΔEo′ = (Eo′ of wave Fn) − (Eo′ of wave F(n−1)) provides the degree of resolution between ferrocenyl waves.

scheme, the negative charge of the ligand is not shown because of its coordination to MnII or MnIII, but the reduced ligand at wave 2 in Scheme 1 does show the extra negative charge that the reduction process generated. 3.3. Quantification of the Relationship Between ΣχR and E°′. Although it is clear from the χR/E°′ discussion above that relationships between these quantities are possible, it is more convenient to use the quantity “sum of group electronegativities”, ΣχR, with respect to β-diketonato ligands because then one is not restricted to a common pendant side group in a particular ligand, like Fc in (FcCOCHCOR)−. ΣχR values of β-diketonato ligands coordinated to a manganese(III) center may be used to predict ferrocenyl redox potentials. To explain the calculation of ΣχR, consider peak F2 of [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)], 5 (Scheme 1 and Figure 2). Peak F2 represents the couple

For 1 : E o ′ = 0.1379ΣχR − 1.738, R2 = 0.998

(1)

For 2 : E o ′ = 0.1284ΣχR − 1.529, R2 = 0.997

(2)

For 3 : E o ′ = 0.1358ΣχR − 1.572, R2 = 0.998

(3)

For 4 : E o ′ = 0.1507ΣχR − 1.660, R2 = 0.979

(4)

For 5 : E o ′ = 0.1359ΣχR − 1.522, R2 = 0.992

(5)

In an attempt to unify the relationship between E°′ and ΣχR for all observed electrochemical processes of 1−5, Figure 5 was constructed. Data points from waves F4, F5, and F6 of 4 and F4 and F5 of 5 were not used in the fit because those peaks did not fit the trend set by all other ferrocene related electrochemical processes of the complexes. This may be a consequence of the large number of positive charges in close proximity that is generated when 4 and 5 is oxidized the fourth to sixth time. The resulting cations probably induce electrostatic repulsive effects that contribute to this deviation. The data fit eq 6. E o ′ = 0.0875ΣχR − 0.932, R2 = 0.884

(6)

The better R -values of eqs 1−5 indicate the individual fits for each complex are more accurate than the combined predictive fit described by eq 6. 2

E

DOI: 10.1021/acs.inorgchem.8b00745 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

The Fe 2p3/2 photoelectron lines of the ferrocenyl molecular fragments in 1−5 are sharp well-defined curves with a full width at half-maximum (fwhm) of ca. 1.7 eV, as previously described.13 In contrast, the Mn 2p photoelectron lines are broad with a fwhm of ca. 3.86−4.64 eV. The Mn 2p photoelectron lines have been described in detail.16 One distinct feature in both the Fe 2p and Mn 2p photoelectron lines is the presence of shake-up and shake-down photoelectron lines, Figure 7. For the Fe 2p3/2 photoelectron line, the shakedown feature is at a few eV smaller than the main Fe 2p line, while for the Mn 2p photoelectron line, the shake-up feature is at a binding energy of a few eV larger than the main Mn 2p photoelectron lines. These shake-down and shake-up features are due to charge transfer from the ligand to the Mn.16 Since the XPS observations and interpretations are already described in detail,13,16 here only the correlations with electrochemistry will be discussed. 3.5. Relationship between Binding Energy and Redox Potentials. During electrochemical oxidation of the ferrocenyl group of complexes 1−5, electrons are removed from the iron (FeII), which creates a FeIII species, producing a positively charged complex. During an XPS experiment, the sample is irradiated with X-rays which in turn causes emission of a core electron. This photoionization also generates a charged species. A link between core ionization (the binding energies, BE, of the Fe 2p photoelectron peaks) and the electrochemical formal redox potentials (E°′) of the ferrocenyl groups (from electrochemistry) in complexes 1−5 may therefore be expected. Similarly, there should be a relationship between the manganese E1/2 and the BE of the Mn 2p photoelectron lines of complexes 1−5. To confirm this relationship and quantify it so that it can be used to predict XPS binding energies from formal redox potentials, E°′, of the ferrocenyl groups were plotted against BE of the Fe 2p3/2 photoelectron line (Figure 8). This showed that for the first three FeII/FeIII couples (F1, F2, and F3), as E°′ increases (more difficult to oxidize), the binding energy of the Fe 2p3/2 photoelectron line also increases. It was found that it is more difficult to remove a photoelectron by XPS (i.e., larger BE values) from a ferrocenyl group with a larger E°′, for instance, for 1 (R = CF3, χCF3 = 3.01, E°′F1 = 0.285 V, BE = 708.03 eV), than it is for 4 (R = Fc, χFc = 1.87, E°′F1 = 0.052 V, BE = 707.77 eV). This direct proportionality implies that the more difficult it is to lose an electron during electrochemical oxidation, the more difficult it also is to lose an electron during photoemission. The approximately linear relationships obtained for Fe in Figure 8 fits the following equations:

Figure 5. Relationship between the sum of the group electronegativities and the first three ferrocenyl-based redox potentials for compounds 1−5 combined. Labels F1−F3 indicate ferrocenyl oxidation wave F1, F2, and F3 for each complex.

A similar relationship was established for the manganese(II/ III) redox couple using the half-potentials, E1/2, or cathodic peak potentials, Epa. E1/2 rather than E°′ was used because, unlike the ferrocenyl-based redox processes, the manganese couples are not electrochemically reversible. This would imply potentials associated with ipc values represent that potential where kinetically the rate of electron transfer between electrode and Mn centers is the fastest, but it does not necessarily represent true thermodynamic potentials. The relationship broke down when 6 was included in the fit because of the excessive large ΔE value (416 mV).

Figure 6. Relationship between E1/2 and ΣχR for the electrochemical irreversible MnIII/II couple. Complex 6 (red dot) was omitted from the fitting process; complex 4 was not soluble enough to detect a MnIII/II couple.

The equation predicting E1/2 (in V) of the MnIII/II redox couple from ΣχR for 1, 2, 3, and 5 is 2

E1/2 = 0.2163ΣχR − 3.3985, R = 0.9565

BE Fe2p3/2 = 1.0239E o ′F1 + 707.69; R2 = 0.9638

(8)

BE Fe2p3/2 = 1.1486E o ′F2 + 707.52; R2 = 0.9248

(9)

BE Fe2p3/2 = 0.9619E o ′F3 + 707.47; R2 = 0.9740

(10)

(7)

Complex 4 could not be utilized in the fitting process because its solubility was so low that no MnIII/II couple could be detected. By utilizing ΣχR = 11.22 for 4 and utilizing eq 7, a predicted half-potential of −0.971 V for the unidentified MnIII/II couple of 4 could be calculated. 3.4. X-ray Photoelectron Spectroscopy. In previous studies,13,16 the XPS spectra of 1−6 were described in detail. Table 2 summarizes the important binding energies, BE, and Iratio values (the ratio between the intensities of the satellite and main metal 2p 3/2 photoelectron lines = (I M2p3/2satel )/ (IM2p3/2main)) obtained as well as relevant electrochemical and IR vCO data.

Deviations of data points from these linear equations are assigned to a lack of accuracy of BE values; BEs in eV can at most be obtained to two decimal accuracy, whereas E°′ values in V may be obtained to three-decimal accuracy. A correlation of redox potentials of the Mn center of 1−5 with the binding energy of the Mn 2p3/2 photoelectron line (Figure 8, right) also showed that as E1/2 Mn increases (i.e., MnII becomes more difficult to oxidize), the binding energy of the F

DOI: 10.1021/acs.inorgchem.8b00745 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 2. Gordy Group Electronegativity, χR, of the Various R-Groups on β-Diketonato Ligands, the Binding Energies (BE) of the Fitted Fe 2p3/2 and Mn 2p3/2 Peaks of [Mn(FcCOCHCOR)3] Complexes 1−4, [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)], 5, and [Mn(CH3COCHCOCH3)3], 6, Iratio Values As Well As the Formal Redox Potentials of FeIII/II (Peaks Labeled F1, F2, F3) and MnIII/II Couples vs. FcH/FcH+ of Each Complex 1: 2: 3: 4: 5: 6:

comp.: R

χRa

CF3 CH3 C6H5 Fc Fc,Fc,CH3e CH3,CH3f

3.01 2.34 2.21 1.87

vCO1;vCO2 /cm−1 1572; 1542; 1506; 1498; 1498;

1535 1506 1483 1456 1456

Fe 2p3/2 BEb/eV

Mn 2p3/2 BEb/eV

Fe Iratiob,c

Mn Iratiob,d

E1/2 Mng/V

E°′F1/V

E°′F2/V

E°′F3/V

708.03 707.87 707.84 707.77 707.79

641.86 641.53 641.50 641.31 641.41 641.73

0.28 0.16 0.16 0.09 0.11

0.13 0.19 0.18 0.23 0.23

−0.207 −0.753 −0.738

0.285 0.097 0.093 0.052 0.084

0.403 0.208 0.213 0.200 0.189

0.547 0.341 0.351 0.285 0.303

−0.822 −0.877

χFc+ = 2.87; values from ref 22. bValues from refs 13 and 16. cIratio = ratio between the intensities of the satellite and main Fe 2p3/2 photoelectron lines = (IFe 2p3/2satel)/(IFe 2p3/2main). dIratio = ratio between the intensities of the satellite and main Mn 2p3/2 photoelectron lines = (IMn 2p3/2satel)/ (IMn 2p3/2main). eThis R-group combination gives rise to the complex [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)], 5. fThe complex [Mn(CH3COCHCOCH3)3]. gStrictly speaking this is not the thermodynamic quantity E°′ for Mn because ΔE > 180 mV, Table 1. It is only the mid potential between Epc and Epa, E1/2, which is mathematically calculated the same way as E°′ but has a nonthermodynamic meaning. a

Vibrational excitations (a measure of which is the β-diketonato C···O IR vibrations observed in IR spectra, Supporting Information, Figures S1−S6) may act as an indicator of this inner-sphere reorganization. In order to find a link between the inner-sphere reorganization after the removal of a photoelectron and binding energy, a relationship was searched between the two ligand carbonyl stretching frequencies of the ferrocenyl-containing β-diketonato ligands (vCO) and the binding energies of the Fe 2p3/2 and Mn 2p3/2 photoelectron lines (Figure 9). A direct proportionality is evident. Since the wavenumber (which is inversely proportional to light wavelength) is directly proportional to spatial vibrational frequency and photon energy, and by implication also the energy of inner-sphere reorganization, it follows that XPS binding energies should also be directly proportional to the frequency and energy. Therefore, the increase in wavenumber and binding energies of the Fe 2p3/2 and Mn 2p3/2 photoelectron lines is associated with increased energy of inner-sphere reorganization. This implies that 1 (R = CF3) requires more energy for inner-sphere reorganization of it electrons than 4 (R = Fc). This is due to the CF3-group’s electron withdrawing capability (χCF3 = 3.01), pulling electron density toward itself and away from the Fe inner-sphere core. Thus, when an electron is removed from the ferrocenyl FeII inner-shell, either by electrochemical oxidation or photoionization, more energy is needed to reorganize the electrons (shrinking of the core), because the electron density is further away from the Fe core (being distributed toward the electron withdrawing CF3 group). The relationship to predict

Figure 7. XPS’s of the Fe 2p3/2 and Mn 2p areas of 4, showing the shake-down and shake-up photoelectron lines.

Mn 2p3/2 photoelectron line increases as well. This means that when it is more difficult for a MnII center to lose an electron during electrochemical oxidation, it is also more difficult to lose an electron during photoemission from a core Mn orbital. Prediction of binding energies for MnIII complexes from electrochemical data can be made by eq 11, the linear relationship obtained from the Mn data in Figure 8. BEMn2p3/2 = 0.6838E1/2MnIII/II + 642.01; R2 = 0.9769 (11)

3.6. Relationship between Binding Energy and IR Wave Numbers. According to the Franck−Condon principle,32 during an oxidation or reduction process of a molecular fragment (in this case FeII from Fc) or metal-center (in this case MnIII), the complex undergoes inner-sphere reorganization to compensate for the newly formed differently charged species. This inner-sphere reorganization is accompanied by a change in bond lengths or geometry of the coordination shell, and it plays an important role in determining the charge transfer kinetics.

Figure 8. Relationship between (left) ferrocenyl E°′ F1, F2, and F3 values and XPS obtained Fe 2p3/2 binding energies (BEFe 2p3/2), as well as between Mn E1/2 and XPS determined BEMn 2p3/2 values of 1−5 (right). G

DOI: 10.1021/acs.inorgchem.8b00745 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 9. Relationship between the wavenumber of the carbonyl stretching frequency of the carbonyls of the ferrocenyl-containing β-diketonato ligands (vCO) and the Fe 2p3/2 binding energies (BEFe 2p3/2) measured by XPS of 1−5 (left) as well as relationship between the wavenumber (vCO) and the Mn 2p3/2 binding energies measured by XPS of 1−5 (right).

Figure 10. Relationship between the wavenumber of the carbonyl stretching frequencies of the two carbonyls of the ferrocenyl-containing βdiketonato ligands (vCO1 and vCO2) and Iratios for the XPS-obtained Fe 2p3/2 data (left) and the Mn 2p3/2 data (right) of complexes 1−5.

A direct proportionality exists between vCO and Iratio of the Fe 2p3/2 photoelectron lines, while an inversely proportional relationship exists between vCO and the Iratio of the Mn 2p3/2 photoelectron lines. For the iron centers of the electron donating ferrocenyl group of the ligands of 1−5, the directly proportional relationship means that the higher the degree of inner-sphere reorganization, the more energy it requires. This is also dependent on the electronic properties of the other Rgroup. For instance, the FeII in the β-diketonato ligand of 1 (R = CF3, χCF3 = 3.01) undergoes a higher degree of inner-sphere reorganization due to core−shell contraction, because it loses electron density by polarization toward CF3, than the FeII in 4. In 4 (R = Fc, χFc = 1.87) the electron-donating R = Fc substituent will cause the electron shell of the second ferrocenyl group to become larger. After loss of an FeII electron, either by electrochemical oxidation or photoemission, the newly formed FeIII electron shell will undergo inner-sphere reorganization to contract to compensate for the loss of the electron. However, the CF3 groups of the ligands of 1 will oppose this contraction, and it will thus require more inner-sphere reorganizational energy. In contrast, the second Fc group on the ligands of 4 will assist this contraction during either electrochemical oxidation or photoemission, thereby lowering the inner-sphere reorganizational energy. The relationship which is indicative of the degree and ease of inner-sphere reorganization is the link between the wavenumber of the carbonyl group, vCO, and Fe Iratios values; it is given by

the ease of inner-sphere reorganization (wavenumber vs BE) applied to the data of Figure 9 fits the following equations: BE Fe2p3/2 = 0.003vCO1 + 703.31; R2 = 0.9069

(12a)

BE Fe2p3/2 = 0.0029vCO2 + 703.53; R2 = 0.9187

(12b)

BEMn2p3/2 = 0.0059vCO1 + 632.6; R2 = 0.8563

(13a)

BEMn2p3/2 = 0.0058vCO2 + 632.96; R2 = 0.8861

(13b)

R2 values for eqs 12 and 13 were lower than those observed for other relationships in this study and stems from the fact that both binding energies and IR vibrational frequencies cannot be measured with the same accuracy as redox potentials or group electronegativities. 3.7. Relationship between the Ratio of the Intensities of the Charge Transfer and Main Photoelectron Lines and IR Wave Numbers. To further understand this innersphere reorganization, a relationship was established between the wavenumber of the CO vibrations and the Iratios, see Figure 10. Physically, Iratios are determined by determining the ratio of the intensities of the Fe or Mn charge transfer peaks and main Fe or Mn 2p3/2 photoelectron line. For the purpose of this section, CO wave numbers, νCO, is an indicator of the energy required for inner-sphere reorganization, while the quantity Iratio is an indication of the degree of inner-sphere reorganization according to the Born−Oppenheimer approximation within the Franck−Condon principle, which states that the intensities of photoelectron lines are proportional to nuclear coordinates.33 Thus, higher energy photoelectron line intensities are caused by a more significant change in nuclear coordinates (higher degree of inner-sphere reorganization).

vCO1 = 0.0021Iratio,Fe − 2.985; R2 = 0.841

(14a)

vCO2 = 0.0021Iratio,Fe − 2.9031; R2 = 0.8956

(14b)

III

As far as the electron acceptor, Mn , is concerned, an inversely proportional relationship exists between the waveH

DOI: 10.1021/acs.inorgchem.8b00745 Inorg. Chem. XXXX, XXX, XXX−XXX

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number of the vCO and the Iratio of the Mn 2p3/2 photoelectron lines (Figure 10, right). This denotes that for the MnIII, a higher degree of inner-sphere reorganization requires less inner-sphere reorganizational energy. After electrochemical reduction, the gaining of an electron to produce MnII, the electron shell undergoes inner-sphere reorganization to expand. Since the six Fc-groups of 4 (R = Fc, χFc = 1.87) already donate electron density to the Mn core (more than the three Fc groups of 1; for 1 the CF3 group actually withdraws electron density from MnIII), the degree of expansion of the electron shell (innersphere reorganization) after electrochemical reduction will be more for 4 than that for 1 (R = CF3, χCF3 = 3.01) but the energy to do so is less. The relationship which is indicative of the degree and ease of inner-sphere reorganization for the Mn center is the link between vCO and the Iratios and from Figure 10, right, are given by vCO1 = −0.0011Iratio,Mn + 1.8821; R2 = 0.7696

(15a)

vCO2 = −0.0012Iratio,Mn + 1.9116; R2 = 0.8944

(15b)

AUTHOR INFORMATION

Corresponding Author

*Fax: +27 51 4017295. E-mail: [email protected]. ORCID

Jannie C. Swarts: 0000-0003-2608-0371 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the National Research Foundation under Grant 96123, the University of the Free State and Syngaschem BV, The Netherlands, for financial support. Syngaschem BV acknowledges funding from Synfuels China Technology Co. Ltd., Beijing-Huairou, P.R. China.

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REFERENCES

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4. CONCLUSIONS Insight in how molecular fragments in [Mn(FcCOCHCOR)3] complexes with R = CF3 (1), CH3 (2), Ph (3), and Fc (4), as well as the mixed ligand complex [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)] (5) communicate may be obtained from electrochemical, photoemission, and infrared spectroscopic studies. This knowledge is considered important for a complete understanding of this class of compounds. All ferrocenyl-based redox processes of 1−5 were found to be electrochemically reversible. Redox potentials linearly relate to R-group group electronegativities, χR as well as ΣχR, implying all molecular fragments in the [Mn(β-diketonato)3] series of compounds communicate electronic effects from one molecular fragment to another. The linear relationship between binding energies obtained from an X-ray photoelectron spectroscopic study utilizing the ferrocenyl Fe 2p3/2 photoelectron lines (as well as the Mn 2p3/2 photoelectron lines) and the ferrocenyl group (or Mn center) redox potentials allows binding energy prediction for this class of complexes from simple electrochemical measurements. No such relationship has ever before been demonstrated for any manganese complexes. As for electrochemical oxidations, XPS-induced photoionization of the ferrocenyl FeII inner-shell of 1−5 needs more energy in complexes with ligands bearing electron withdrawing (CF3) groups than in ligands bearing more electron-donating groups such as ferrocenyl. Traditionally, the metal Iratio (the ratio between the intensities of the satellite and main metal 2p3/2 photoelectron lines) determined by XPS is an indication of the amount of charge transferred to or from a molecular fraction. Here we have shown, in accordance with the Franck−Condon principle, it can also be used as an indication of the degree of inner-sphere reorganization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00745. IR spectra of 1−6 (PDF) I

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DOI: 10.1021/acs.inorgchem.8b00745 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00745 Inorg. Chem. XXXX, XXX, XXX−XXX