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Intramolecular Electron Transfers Thwart Bistability in a Pentanuclear Iron Complex Eric Gouré,*,†,‡ Bertrand Gerey,†,‡ Martin Clémancey,§,∥ Jacques Pécaut,⊥,# Florian Molton,†,‡ Jean-Marc Latour,∥ Geneviève Blondin,*,∥,∇ and Marie-Noel̈ le Collomb*,†,‡ †
Université Grenoble Alpes, DCM, F-38000 Grenoble, France CNRS, DCM, F-38000 Grenoble, France § Université Grenoble Alpes, LCBM, pmb, F-38000 Grenoble, France ∥ CEA, BIG-LCBM, pmb, F-38000 Grenoble, France ⊥ Université Grenoble Alpes, INAC-SyMMES, F-38000 Grenoble, France # CEA, INAC-SyMMES, Reconnaissance Ionique et Chimie de Coordination, F-38000 Grenoble, France ∇ CNRS UMR 5249, LCBM, pmb, F-38000 Grenoble, France ‡
S Supporting Information *
ABSTRACT: With the intention to investigate the redox properties of polynuclear complexes as previously reported for the pentamanganese complex [{MnII(μ-bpp)3}2MnIIIMnII2(μ3-O)]3+ (23+), we focused on the analogous pentairon complex that was previously isolated as all-ferrous. As Masaoka and co-workers recently published, aerobic synthesis leads to the [{FeII(μbpp)3}2FeIIIFeII2(μ3-O)]3+ complex (13+). This species exhibits in acetonitrile solution four reversible one-electron oxidation waves. Accordingly, the three oxidized species 14+, 15+, and 16+ with a 3FeII2FeIII, 2FeII3FeIII, and 1FeII4FeIII composition, respectively, were generated by bulk electrolysis and isolated. Mössbauer spectroscopy allowed us to determine the spin states of all the iron ions and to unambiguously locate the sites of the successive oxidations. They all occur in the μ3-oxo core except for the 14+ to 15+ process that presents a striking electronic rearrangement, with both metals in axial position being oxidized while the core is reduced to the [FeIIIFeII2(μ3-O)]5+ oxidation level. This strongly differs from the redox behavior of the Mn5 system. The origin of this electronic switch is discussed.
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INTRODUCTION Self-assembly of metal complexes is a powerful method for the construction of discrete and well-defined three-dimensional architectures under thermodynamic control. Many diverse architectures such as helicates, grids, and wheels have been prepared by making use of coordination interactions between multibidentate ligands and 2-, 3-, ..., or 6-coordinate metal ions.1−11 Their interesting structure-specific properties, such as chirality,12−14 magnetic properties,15 multiredox behavior,15−18 and also guest molecule recognition,13,19,20 ensure that this class of molecular compounds continues to attract much attention. More specifically, the tetradentate binucleating 3,5-bis(pyridin-2-yl)pyrazole ligand (Hbpp) and the related Hbpt and H3bbp ligands (Scheme 1A) present the particularity to form pentanuclear bis(triple-helical) complexes by selfassembly of six ligands around five metal ions of various natures. These complexes having a {M3(μ3-O)} (M = Mn, Fe, Zn, Cd)21−24 or {M3(μ3-OH)} (M = Ni, Cu, Zn)25,26 trinuclear central core wrapped by two {M(bpp)3} entities are rare examples of helicates in which the metal ions define the helicate axis.27−29 The helicate arrangement also causes peculiar © XXXX American Chemical Society
magnetic properties. Spin-frustation has been observed for the copper complex 26 while for the iron [{Fe II (μbpt)3}2FeII3(μ3-O)]2+ complex21 it has been shown that the spin state of the two terminal iron(II) species can be tuned by the nature of the counterion. The redox properties of these pentanuclear compounds, although almost unexplored so far,22,30 are another potentially interesting facet of their physicochemical characteristics. Indeed, our group recently demonstrated that the five metal atoms confer to the architecture a multiredox behavior in the case of the [{MnII(μ-bpp)3}2MnIIIMnII2(μ3-O)]3+ complex (23+),22 with the detection of five successive reversible metalcentered MnII/III processes. The first three processes, wellseparated (ΔE1/2 in the range 0.48−0.70 V), were unambiguously assigned to MnIII/MnII processes within the central core, while the last two oxidation processes, close in potential (ΔE1/2 of 0.12 V), concern the oxidation of the two axial MnII ions. Such compounds having multistable redox states are very attractive candidates for the design of molecular switching Received: March 30, 2016
A
DOI: 10.1021/acs.inorgchem.6b00791 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
the same complex evidencing that its highest redox state, 17+, acts as an efficient electrocatalyst for water oxidation.31 Due to the very rich redox behavior and to the robustness of this architecture, we were able to generate by electrochemical oxidation and to isolate the iron cluster in five out of six oxidation states, namely, 12+−16+. UV−vis and Mössbauer spectroscopies proved instrumental in localizing the different redox sites and ultimately revealing an unprecented intramolecular electronic rearrangement in the 14+ → 15+ oxidation. We would like here to comment on this unexpected electronic switch that may be a key property of this system enabling it to oxidize water.
Scheme 1. (A) Selection of Ligands Leading to Polynuclear Helical Complexes (Hbpp = 3,5-Bis(pyridin-2-yl)pyrazole, Hbpt = 3,5-Bis(pyridin-2-yl)triazole, H3bbp = 3,5Bis(benzimidazol-2-yl)pyrazole) and (B) Scheme of the Fe5 Complex 1a
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RESULTS This section briefly summarizes the results we have obtained and points out some minor discrepancies with those presented by Masaoka et al.31 Synthesis and X-ray Structure of 13+. The self-assembled [{FeII(μ-bpp)3}2FeIIIFeII2(μ3-O)]3+ cation, 13+, has been synthesized in high yield by thermal treatment at 170 °C during 3 days of an acetonitrile mixture of Hbpp and Fe(BF4)2· 4H2O in a 6:5 molar ratio in the presence of Bu4NOH under aerobic conditions. This solvothermal approach is similar to that described previously by Oshio’s group for the synthesis of [{FeII(μ-bpt)3}2FeII3(μ3-O)]2+,21 except that in the present case the reaction was performed under air. The X-ray features determined on 1(B(C6F5)4)3·0.5CH3CN·0.5(C2H5)2O are similar to those published for 1(BF4)3 (see Supporting Information Tables S1−S3). All the performed characterizations converged to conclude to the presence in 13+ of two low-spin (LS) FeII ions in axial position, the triangular μ-oxo core containing two high-spin (HS) FeII ions and one HS FeIII ion (see Figures 2A,B and 3B and Table 1). Cyclic Voltammetry. The cyclic voltammogram of complex 13+ in acetonitrile (Figure 1B) displays five redox processes, one reversible reduction wave at E1/2 = −0.46 V and
a
Color code: Fe in dark grey, O in red, N in light blue, and C in light grey.
materials or electron storage devices.16,18 However, the development of new systems along these lines requires that their redox-dependent physical properties be fully understood for a range of different metal ions. In this context, the aim of the present work is to explore the redox properties of other members of the pentanuclear bis(triple-helical) complex’s family. In this work, we focused on the iron derivative, and we synthesized and isolated the mixed-valent [{FeII(μ-bpp)3}2FeIIIFeII2(μ3-O)]3+ (13+) cluster (Scheme 1B and Supporting Information Figure S1), which corresponds to a one-electron higher oxidation state (4FeII1FeIII) compared to that of the previously isolated allferrous analogue (12+).23 While we were ready to submit the manuscript, Masaoka and co-workers published a nice work on
Table 1. Parameters Used To Reproduce the Zero-Field 80 K Powder Mössbauer Spectra of Complexes 12+−16+a 12+ δ1 (mm s−1) ΔEQ1 (mm s−1) %c oxidation and spin states
0 FeIII HS
δ2 (mm s−1) ΔEQ2 (mm s−1) %c oxidation and spin states
0.97 2.33 60 FeII HS
−1
δ3 (mm s ) ΔEQ3 (mm s−1) %c oxidation and spin states
0.43 0.20 40 FeII LS
13+ Doublet 0.46 2.03 20 FeIII HS Doublet 0.95 3.03 40 FeII HS Doublet 0.43 0.26 40 FeII LS Doublet
−1
δ4 (mm s ) ΔEQ4 (mm s−1) %c oxidation and spin states
1
14+
15+
16+
0.40 1.91 40 FeIII HS
0.44 1.95 20 FeIII HS
0.31 1.75 40 FeIII HS
0.93 3.21 20 FeII HS
0.89 3.01 40 FeII HS
1.00 3.14 20 FeII HS
0.21 1.28 40 FeIII LS
0.20 1.05 40 FeIII LS
b
2b
3b 0.43 0.26 40 FeII LS 4b
a Line widths and uncertainties are given in the Supporting Information Table S4. bDoublets 1 and 2 are drawn in Figure 3 as red and dark blue solid lines, respectively. Doublet 3 is shown in light blue for 12+−14+ and doublet 4 in orange for 15+ and 16+. cFixed.
B
DOI: 10.1021/acs.inorgchem.6b00791 Inorg. Chem. XXXX, XXX, XXX−XXX
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chemical signature at the end of the electrolysis. The stability of the 12+, 14+, 15+, and 16+ species has allowed their isolation in solid form from preparative electrolyses (see Experimental Section). These species have been further characterized by Mössbauer and UV−vis spectroscopies. UV−Vis and Mössbauer Characterizations. Figure 2 reproduces the monitoring of the four successive oxidations performed starting from an electrogenerated solution of 12+. Isosbestic points are observed during each redox process indicating a simple transformation. As observed by Masaoka et al.,31 a broad and intense absorption is detected for 12+−14+
Figure 1. Cyclic voltammograms at a Pt electrode (diameter 5 mm) in CH3CN, 0.05 M [Bu4N]ClO4 of (A) after exhaustive reduction at −0.68 V of the (B) solution (formation of 12+), (B) a 0.20 mM solution of 13+, (C) after exhaustive oxidation at +0.32 V of the (B) solution (formation of 14+), (D) after exhaustive oxidation at +0.62 V of the (C) solution (formation of 15+), (E) after exhaustive oxidation at +0.87 V of the (D) solution (formation of 16+); scan rate of 100 mV s−1.
four successive reversible oxidation waves at E1/2 = +0.21, +0.39, +0.79, and +1.19 V (ΔEp = 60 mV for each of them) at a scan rate of 100 mV s−1. Similar redox potentials were determined by Masaoka et al.31 Each of the five reversible redox processes corresponds to the exchange of about one electron per molecule of complex. They are thus assigned to the Fe II4 Fe III/Fe II5 , FeII3FeIII2/FeII4Fe III, Fe II2 Fe III3 /Fe II3FeIII2, FeIIFeIII4/FeII2FeIII3, and FeIII5/FeIIFeIII4 redox couples, respectively. The fully reduced species FeII5 (12+) and three oxidized species FeII3FeIII2 (14+), FeII2FeIII3 (15+), and FeIIFeIII4 (16+) are quantitatively generated by successive electrolyses at E1/2 = −0.68, +0.32, +0.62, and +0.87 V of a solution of 13+ as demonstrated by the recorded cyclic voltammograms (Figure 1A,C−E) and voltamperograms at a rotating disk electrode (see Supporting Information Figure S2). Whereas solutions of 12+, 14+, and 15+ are perfectly stable, 16+ starts to degrade in solution about half an hour after completion of the electrolysis. This time lag allows the cyclovoltammogram and the UV−vis signature to be recorded satisfactorily. By contrast, the fully oxidized FeIII5 species (17+) is definitively not stable in CH3CN with the total disappearance of the pentanuclear electro-
Figure 2. UV−vis absorption monitoring during electrolyses of a 0.20 mM electrogenerated solution of 12+ in CH3CN, 0.05 M [Bu4N]ClO4: (A) oxidation at −0.20 V (12+ converted into 13+); (B) oxidation at +0.32 V (conversion of 13+ into 14+); (C) oxidation at +0.63 V (from 14+ to 15+); (D) oxidation at +0.87 V (from 15+ into 16+), l = 1 mm. C
DOI: 10.1021/acs.inorgchem.6b00791 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry between 400 and 450 nm that can be assigned to LS FeII-toligand charge transfer transitions.32−34 This absorption almost vanished upon oxidation to the 5+ state. This indicates that the two LS FeII ions in axial position have been oxidized. The 14+, 15+, and 16+ species were also generated by Masaoka et al. by electrochemical oxidation as well as by chemical oxidation, and characterized by UV−vis spectroscopy. However, it should be noted that in their study the two oxidation processes of 14+ into 15+ and 15+ into 16+ appear to be incomplete. As a consequence, their spectra of 15+ and 16+ correspond, respectively, to a mixture of 14+ and 15+, and to 15+ (see Figure 2C,D). Figure 3 reproduces the zero-field Mö ssbauer spectra recorded at 80 K on powder samples of 12+−16+. The nuclear parameters used to reproduce the experimental data are listed in Table 1 along with the assignments. Comparison of the spectra of 12+, 13+, and 14+ shows that they are dominated by the central narrow doublet (doublet 3) assigned to the axial LS FeII ions. Conversely, the spectra of 15+ and 16+ present a new doublet (doublet 4) contributing for 40% to the total spectra and the parameters of which (δ4 = 0.20−0.21 mm s−1, ΔEQ4 = 1.05−1.28 mm s−1) are consistent with a LS FeIII ion.35−37 The location of these two LS FeIII ions on the axis of the bipyramid is supported by the small changes of the nuclear parameters of the two other doublets associated with the three other Fe ions (Table 1). Moreover, a perusal of Table 1 reveals that the parameters and the respective 1:2 intensities of doublets 1 and 2 in 15+ are similar to the same parameters in 13+ suggesting that the two forms have the same core composition 2FeII1FeIII. The same is true for 16+ and 14+ taking into account a reversal of the intensities of the two doublets in agreement with the following core configuration 1FeII2FeIII. Consequently the oxidation processes are all located in the triangular μ-oxo core except for the 14+ → 15+ transformation that is more complex. Masaoka and co-workers also reported the Mössbauer spectra for the 13+−15+ species. Whereas the nuclear parameters determined for 13+ and 14+ are similar to ours, those of 15+ differ significantly.31 In their study, the isomer shift for the HS FeIII site in 15+ is abnormally large (0.85 mm s−1), and the quadrupole splitting is surprising low (1.18 mm s−1) compared to the values determined for the same ion in 13+ and 14+ (δ = 0.49 mm s−1, ΔEQ = 1.85 and 1.72 mm s−1, respectively). A close examination of the published Mössbauer spectrum associated with the putative 15+ species reveals that these values are due to the presence of a line at ≈0.25 mm s−1. This line is indeed strongly reminiscent of the absorption due to the two LS FeII ions in 14+, suggesting that the sample that has been investigated is a mixture of 14+ and 15+. This observation is fully consistent with the weaker decrease detected at 400−450 nm by UV−vis spectroscopy (see above). The higher homogeneity of the nuclear parameters of 15+ within the whole series for the different types of Fe sites gives us confidence in the results presented here. To conclude this section, Mössbauer spectroscopy allows the determination of the location of the oxidation processes, in full agreement with the UV−vis data. Whereas all transformations correspond to the oxidation of one HS FeII ion in the Fe3O core into HS FeIII, three Fe sites are involved in the 14+-to-15+ transformation: the two LS FeII ions in axial position are simultaneously oxidized into LS FeIII, and one of the HS FeIII within the equatorial plane of the Fe5-bipyramid is reduced to HS FeII. These changes are summarized in Scheme 2. The
Figure 3. Zero-field 80 K Mössbauer spectra recorded on powder samples of complexes 12+ (A), 13+ (B), 14+ (C), 15+ (D), and 16+ (E). Experimental spectra are shown with hatched marks and simulations as overlaid solid black lines. The different contributions are shown above the theoretical spectra as colored solid lines: Red, dark blue, light blue, and orange traces correspond to doublets 1−4, respectively. See Table 1 for parameter values.
following discussion aims at deciphering the processes underlying the 14+ → 15+ oxidation.
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DISCUSSION Oxidation Affects the Axial Fe Ions. The redox events identified on the Fe5 complex contrast with those observed in the related [{Mn(μ-bpp)3}2Mn3(μ3-O)]3+ complex (23+) in D
DOI: 10.1021/acs.inorgchem.6b00791 Inorg. Chem. XXXX, XXX, XXX−XXX
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Scheme 3. Focus on the 13+ to 16+ Species Organized as in Scheme 2a
Scheme 2. Six Redox States for 1 Organized According to Their Increasing Charge on the Abscissa and on the Increasing Oxidation State of the μ-Oxo Core on the Ordinatea
a
Superscripts indicate the number of oxidizing equivalents stored in the μ-oxo triangular core (T) and on the axis (A) starting from 12+. Wave potentials are indicated in V vs Ag/0.01 M AgNO3. The dashed box around 17+ indicates its instability. LS FeII ion is drawn in light blue, HS FeII in dark blue, LS FeIII in light orange and HS FeIII in red.
a Species T1A1 and T2A1 resulting from the one-electron oxidation in axial position of T1A0 and T2A0 are indicated. The virtual oxidation processes TxA0 → TxA1 and TxA1 → TxA2 (x = 1, 2) are indicated by dashed arrows. K4 and K5 are the equilibrium constants associated to T1A1 ⥂ T2A0 and T2A1 ⥂ T1A2.
two respects.22 First, we have previously shown that, starting from the fully reduced species 22+, the −0.58, +0.13, and +0.61 V redox processes correspond to the three successive MnII to MnIII oxidations within the Mn3(μ3-O) core. Second, once the Mn3(μ3-O) core has been fully oxidized, the oxidation of the axial MnII ions occurs successively at +1.21 and +1.33 V. In the following paragraphs, we will shed light on the basic properties and phenomena that are at the root of these differences. To more clearly locate where the successive oxidations occur, we will label T the triangular μ-oxo unit and A the pseudo-3fold axis of the M5-bipyramidal structure (M = Fe, Mn). The number of oxidizing equivalents stored starting from the fully reduced species 12+ or 22+ will be indicated as superscripts on the T and A units (see Scheme 2). As mentioned above, the first three one-electron transfers in 2 concern the Mn ions within the trinuclear core and are thus associated with the T0A0 → T1A0, T1A0 → T2A0, and T2A0 → T3A0 electron transfers. In the case of 1, whereas the first two electron transfers occur ca. 0.1 V higher (−0.46 and +0.21 V) than for 2, the third one occurs at +0.39 V, a potential substantially lower than expected (ca. 0.7 V) if the same trend was followed. This observation strongly supports that the third redox event in 1 affects the pair of axial LS FeII ions and not the triangular core, in full agreement with the spectroscopic results. To substantiate this conclusion, we searched the literature for the redox potentials of HS MnII and LS FeII ions with the same ligands. Whereas the oxidations of the Fe sites are always observed, those of the analogous Mn complexes are either not detected or located up to 0.4 V higher.38−44 These data support that the oxidation of 14+ at +0.39 V is centered on one axial LS FeII ion. Thermodynamics Explain Why Both Axial Ions Are Oxidized. However, it remains to explain why both axial ions are oxidized during the one-electron conversion of 14+ to 15+ that can be written explicitly T2A0 → T1A2. To interpret this phenomenon, let us decompose the overall transformation in elementary one-electron transfers. Scheme 3 illustrates the two successive oxidations leading from T2A0 to T2A2 through T1A2
as observed experimentally. The above redox potential considerations suggest that the one-electron oxidation of T2A0 should most likely produce T2A1, which is not observed. Hence, it must be envisaged that an intramolecular electron transfer occurs in T2A1 between the axial FeII ion and one FeIII ion of the triangular core: T2A1 → T1A2. Alternatively, it may be envisaged that an equilibrium exists between the two species T2A1 ⥂ T1A2 which is displaced toward T1A2 (see below). To evaluate the feasibility of such an electron transfer, it is necessary to estimate the redox potential of the T2A1/T2A0 couple and build a Frost diagram describing the two possible routes to transform T2A0 into T2A2, via either T1A2 (solid arrows) or T2A1 (dashed arrows) as shown in Scheme 3. The elaboration of a Frost diagram will enable us to compare the thermodynamics of the two pathways once the values of the potentials of the T2A1/T2A0 (E0A1) and T2A2/T2A1 (E0A2) couples have been estimated. The difference in the potentials of two successive oxidations occurring on the axial positions of Fe2 or Mn2 double or triple helical complexes can be estimated from the electrochemical studies reported in the literature. To summarize these data, three situations can be distinguished: (i) when the two Fe centers communicate through three conjugated bridging ligands, a separation of 0.15−0.22 V is observed;38,44 (ii) When a M3(μ3-O) core is also bound by these ligands, the separation is slightly diminished to 0.12−0.15 V;22,30 (iii) When the bridging ligands do not provide an electronic communication and only electrostatic or Coulombic interactions take place, the potential difference is reduced to
