A Hexanuclear Iron(II) Layer with Two Square-Planar FeO4 Units

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A Hexanuclear Iron(II) Layer with Two Square-Planar FeO4 Units Spanned by Tetrasiloxide Ligands: Mimicking of Minerals and Catalysts N. Manicke, S. Hoof, M. Keck, B. Braun-Cula, M. Feist, and C. Limberg* Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany S Supporting Information *

ABSTRACT: A hexanuclear iron(II) siloxide complex has been prepared by reacting an incompletely condensed silsesquioxane first with NaOMe and then with Fe(OTf)2. In the process of product formation, the siloxane framework undergoes a transformation and it was shown that this happens already upon addition of base: Treatment of the ligand precursor with NaOMe leads to a completely condensed silsesquioxane cage with 12 Si atoms that is composed of 2 equiv of the tetrasiloxide ligands found in the product complex. Its iron centers form a twodimensional array reminiscent of the situations found in minerals and two-dimensional oxide films caused by segregation of FeOx and silica. As the hexairon(II) assembly contains two high-spin square-planar FeO4 unitssuggested to represent the active sites in Fe-zeolites, which react with N2O to generate strongly oxidizing sitesit was treated with Me3NO. This led to the oxidation of two of the iron centers to the oxidation state +III and elimination of one iron ion, so that a pentanuclear, mixed valent iron siloxide was formed. All complexes were fully characterized.

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INTRODUCTION From the vast amount of aluminosilicates known from nature, it is rather obvious that Al3+ finds a favorable environment in silicate surroundings. It then appears at first sight quite astonishing that hardly any silicates exist where Fe3+ is incorporated in the framework, although Al3+ and Fe3+ are known to have very similar properties. In a tetrahedral environment, Fe3+ has a somewhat larger ionic radius than Al3+, though, which makes corresponding silicate sites less attractive for Fe3+ and all the less for Fe2+ with an even larger radius (ionic radii for Al3+ in a tetrahedral environment: 0.39 Å, for Fe3+ in a tetrahedral environment: 0.49 Å, and for Fe2+ in a tetrahedral environment: 0.63 Å).1 Hence, in most silicates containing iron ions, these represent the counterions for aluminosilicate frameworks2−4 or they form layers and feature octahedral coordination environments.5,6 Consequently, there are also structural discussions concerning artificial iron-zeolites: Upon impregnation of zeolites with iron ions, these could either be incorporated into the zeolite framework via isomorphous substitution of Si4+ or remain located outside the framework, where they may function as counterions within the zeolite structure, and it is difficult to distinguish between these possibilities analytically, especially in the case of multiple different sites.7 Molecular model compounds can provide valuable information about the spectroscopic signatures that can be expected for certain structural motifs in iron silicates, and the rather interesting oxidation reactions, which are © XXXX American Chemical Society

mediated by Fe-modified zeolites (see below), motivate investigations on molecular iron siloxides, too.7,8 In this context, we have investigated the coordination behavior of polysilanols toward iron(II) precursor compounds a couple of years ago. Deprotonation of a tripodal trisilanol with NaOMe, followed by treatment with iron(II) triflate, led to a ligand transformation and formation of a compound, where two iron centers are found in square-planar donor environments and high-spin configuration (Figure 1),9a a rather unusual situation as typically10due to the large separation of the dx2−y2 orbital from the residual d orbitalssquare-planar complexes with more than 4 d electrons are associated with

Figure 1. A dinuclear FeII siloxide complex I with two edge-sharing high-spin square-planar FeO4 units.9a Received: May 26, 2017

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DOI: 10.1021/acs.inorgchem.7b01347 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

53.90%, H 4.14%; found C 53.67%, H 4.18%. IR (KBr): 4040 (w), 3289 (br, s), 3062 (m), 1967 (vw), 1894 (vw), 1826 (vw), 1775(vw), 1672 (vw), 1626 (vw), 1595 (w), 1483 (vw), 1432 (w), 1382 (vw), 1310 (vw), 1120 (vs), 1103 (vs), 893 (s), 735 (s), 694 (s), 655 (m), 588 (w), 493 (vs) cm−1. ESI-MS (m/z): 1049.08 ([M − H3O+]−, calcd 1049.07), 1092.07 ([M + Na]+, calcd 1092.08), 1108.05 ([M + K]+, calcd 1108.05). Preparation of [Ph6Si6O7(O)4Fe3(H2O)(thf)2(μ3-OH)(OTf)]2, 2. Ph8Si8O10(OH)4 (275 mg, 0.257 mmol) was dissolved in thf (15 mL), resulting in a colorless solution, and NaOMe (56.95 mg, 1.054 mmol, 4.1 equiv) was added. The colorless reaction mixture was stirred for 5 h at room temperature. Afterward, Fe(OTf)2 (301 mg, 0.852 mmol) was added. After stirring overnight, the royal blue reaction mixture was reduced to 50% under vacuum and filtered. All volatile compounds of the filtrate were removed under vacuum, and the remaining blue residue was extracted with cold thf (5 mL, −30 °C). After removing all volatiles from the extract under reduced pressure and drying in vacuum, 2 (303 mg, 0.116 mmol, 90%) could be obtained as a blue powder. Single crystals suitable for X-ray could be grown by overlaying a solution of 2 in thf with n-hexane at room temperature. Elemental Analysis: calcd for C90H98O36S2F6Si12Fe6 (2605.94 g/mol): C 42.79%, H 4.17%, S 2.33%; found C 42.99%, H 4.43%, S 2.33%. IR (KBr): 3558 (w), 3443 (br, w), 3069 (w), 3050 (w), 2974 (w), 2877 (w), 1894 (vw), 1595 (w), 1485 (vw), 1459 (w), 1431 (w), 1374 (w), 1313 (m), 1223 (s), 1133 (vs), 1067 (vs), 1030 (s), 992 (s), 939 (s), 877 (m), 739 (m), 700 (s), 638 (m), 571 (w), 495 (s) cm−1. Preparation of [(Ph6Si6O7(O)4)2Fe5(H2O)(thf)3](MeCN)(μ2-OH)(μ3-OH)2(OTf), 3. To a blue solution of 2 (153 mg, 0.059 mmol) in thf (5 mL), a solution of Me3NO (4.41 mg, 0.059 mmol, 1 equiv) in a mixture of thf and MeCN (1:1, 5 mL) was added at room temperature and stirred for 12 h. Afterward, all volatile compounds were removed under reduced pressure and thf (1 mL) was added to the resulting brown residue. From the filtrate, all volatile compounds were removed again in vacuo. Compound 3 could be obtained as a brown powder (98.75 mg, 0.417 mmol, 71%). Single crystals suitable for X-ray could be grown by overlaying a solution of 3 in thf with n-hexane at room temperature. Elemental Analysis: calcd for C86H92O32S2F3NSi12Fe5 (2350.95 g/mol): C 44.10%, H 3.91%, S 1.35%, N 0.59%; found C 44.52%, H 3.85%, S 1.80%, N 0.17%.

low-spin configurations. Very recently, with a more simple disilanol precursor we have also been able to prepare a mononuclear variant {(thf)2Li}2[Fe{(OSiPh2)2O}2], II.9b Altogether, these findings indeed turned out to be highly relevant to zeolite chemistry, as in 2016, high-spin squareplanar FeIIO4 units were identified as the catalytically active sites (α-Fe) of Fe-modified zeolites that allow for the oxygenation of unactivated hydrocarbons with N2O.11 Having employed a trisilanol and a disilanol as ligand precursors in the past, now we report the results obtained investigating the reaction of an iron(II) precursor with a more complicated incompletely condensed silsesquioxane with four silanol functions.

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out in a glovebox or by means of Schlenk-type techniques involving the use of a dry and oxygen-free argon atmosphere. Solvents were dried by using an MBraun Solvent Purification System SPS. The NMR spectra were recorded on a Bruker AV 400 NMR spectrometer (1H 400.1 MHz, 13 C-{1H} NMR 100.6 MHz, 29Si NMR 79.5 MHz) at 23 °C. The 1H NMR spectrum was calibrated against the residual proton and natural abundance 13C resonances of the deuterated solvent and the 29Si NMR spectrum against a TMS standard. Microanalyses were performed with a HEKAtech Euro EA 3000 elemental analyzer. Infrared (IR) spectra were recorded in the region 4000−400 cm−1 using solid samples prepared as KBr pellets with a Shimadzu FTIR 8400S. HR-ESI-MS measurments were performed with an Agilent Technologies 6210 time-of-flight liquid chromatography (LC)-MS instrument. A thermal gravimetric analyzer STA 409 Skimmer connected to a mass spectrometer QMG 421 (Balzers) was used to investigate the stability of the synthesized complexes. Mössbauer spectra were recorded with a Rivertec MCo7.114 source (57Co in Rh matrix) with an activity of about 1 Bq using a SeeCo MS6 spectrometer. Simulation of the experimental data was performed with the WMOSS4 program version F. Temperature-dependent magnetic susceptibility data were measured from powder samples with a SQUID magnetometer (Quantum Design MPMS-7) in the range from 2.0 to 295 K. The instrument was calibrated with a standard palladium reference sample, error < 2%). Sample holders of quartz with O-ring sealing were used, and the SQUID response curves (raw data) have been corrected for holder and solvent contributions by subtracting the corresponding response curves obtained from separate measurements without sample material. The experimental magnetization data were corrected for underlying diamagnetism by use of tabulated Pascal’s constants. Preparation of Ph8Si8O10(OH)4, 1. The preparation of Ph8Si8O10(OH)4 was achieved via slightly modified literature procedures:12,13 In a round-bottom flask, phenyltrimethoxysilane (15.85 g, 79.93 mmol), sodium hydroxide (2.14 g, 53.38 mmol), and deionized water (1.67 mL, 92.42 mmol) were dissolved in isopropyl alcohol (80 mL). The colorless reaction mixture was refluxed for 4 h and afterward stirred for an additional 12 h at room temperature. All volatile compounds were removed in vacuum, and the resulting white solid was suspended in thf (120 mL). To the colorless suspension, acetic acid (2.35 g, 39.16 mmol) was added and stirred for 1 h at room temperature, resulting in a colorless solution. Afterward, a saturated aqueous solution of NaHCO3 (40 mL) was added for neutralization. The organic layer was removed and washed with deionized water (2 × 20 mL). After drying with MgSO4 and filtration, all volatile compounds were removed in vacuum, providing a colorless powder (4.81 g, 4.50 mmol, 45%) after drying in vacuo. 1H NMR (400.1 MHz, THF-d8): δ 7.59 (dd, 3JH,H = 7.1 Hz, 4JH,H = 0.75 Hz, 8H, Ph-oCH), 7.44−7.12 (m, 32H, Ph-p,mH), 2.63 (br s, 4H, SiOH) ppm. 13C-{1H}NMR (100.6 MHz, THF-d8): δ 133.88 (8C, Ph-oCH), 133.70 (8C, Ph-oCH), 132.22 (4C, Ph-Cq), 131.22 (4C, Ph-Cq), 129.81(8C, PhpCH), 127.23 (8C, Ph-mCH), 127.10 (8C, Ph-mCH) ppm. 29Si NMR (79.5 MHz, THF-d8): δ −79.40 (4Si, SiOH), −68.92 (4Si, Ph2Si2) ppm. Elemental Analysis: calcd for C48H44O14Si8 (1069.53 g/mol): C

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RESULTS AND DISCUSSION Incompletely condensed silsesquioxanes can appear with many possible polyhedral cage sizes and configurations so that they offer a huge variety of different coordination environments as precursors for complex metal siloxides.14 The octaphenylsilsesquioxane tetrasilanol Ph8Si8O10(OH)4 (1) with the general designation Ph-T8(OH)4 (T = trifunctional building block [RSiO1.5], R = Ph) contains four Si(OH) units in a unique double-decker-shaped configuration and is classified as one of the promising starting materials among polyhedral oligomeric silsesquioxanes (Figure 2).15 Bearing in mind the findings described in the Introduction, we were interested in the behavior of 1 as a ligand precursor for iron(II) under the conditions applied before in the case of the di- and trisilanols mentioned above; various interesting structural motifs were conceivable to result. The reaction of

Figure 2. Double-decker-shaped incompletely condensed silsesquioxane Ph-T8(OH)4 (1). B


Article

Inorganic Chemistry the iron(II) precursor [Fe(OTf)2] (OTf = trifluoromethanesulfonate) with 1 after deprotonation with sodium methanolate in thf at r.t. afforded a hexanuclear iron(II)-siloxide compound, which was isolated from thf in high yield as royal blue crystals (Scheme 1). Its molecular structure (Figure 3) was determined by single-crystal X-ray diffraction and the product thus identified as [Ph6Si6O7(O)4Fe3(H2O)(thf)2(μ3-OH)(OTf)]2 (2).

Scheme 2. Transformation of Ligand Precursor 1 during Synthesis of Compound 2

Scheme 1. Synthesis of a Hexanuclear Iron(II) Complex 2 (2: Si = Si−Ph)

molecule, and the bridging OH and triflate units. Coordinative saturation of these Fe centers (Fe2, Fe2′) is achieved by interaction with the oxygen atom (O15) derived from a water molecule. The two Fe atoms Fe3 and Fe3′ have square-planar coordination spheres (Scheme 1, highlighted in blue) formed by three oxygen donors coming from the siloxide ligands and one oxygen atom corresponding to the bridging OH function. Having described the general composition and setup of 2, we will now put the focus on certain structural and spectroscopic features. Transformation of the Siloxide Framework. As mentioned above, the ligand precursor 1 undergoes a transformation according to Scheme 2. The transformation may formally be regarded to include two steps, involving (i) two cyclodehydrations, leading to the fully condensed cubic T8 cage and two molecules of water, which may correspond to those found at the Fe2 and Fe2′ atoms, and (ii) the elimination of a [PhSi−O−SiPh]4+ edge from the T8 cage, with formation of a book-shaped tetrasiloxide (Scheme 2). The elimination of water from incompletely condensed silsesquioxanes in the presence of transition metal or main group compounds as reagents has also been mentioned in the literature,16,17 and also the ability of incompletely condensed silsesquioxanes to rearrange has been observed. For instance, Feher and Budzichowski presumed the skeletal degradation of the silsesquioxane framework in the course of the deprotonation of the trisilanols R-T7(OH)3 (R = c-C5H9, c-C6H11, cC7H13) with NaOtBu.18 Also in recent literature, cagerearrangement of silsesquioxane compounds in the presence of nucleophiles has been discussed. In 2016, Ervithayasuporn and co-workers showed the influence of the nature of the nucleophile on the degree of cage-rearrangement.19 Szafert et al. were able to trap intermediate compounds during the cagerearrangement of a hexahedral cubic T8 cage [RSiO1.5]8 to a heptahedral five-sided prismatic T10 cage [RSiO1.5]10.20 To find out whether the contact with base or with the iron ions (or both) triggers the rearrangement in the case of 1, we have investigated the stability of ligand precursor 1 in contact with NaOMe in the absence of an iron precursor (Scheme 3). After workup of the reaction mixture, formed upon treatment of compound 1 with 4 equiv of sodium methanolate in thf at

Figure 3. Molecular structure of 2 in the crystal showing the atomlabeling scheme. H, C, O, F, S, Si, and Fe drawn at the 50% probability level, peripheral H atoms and peripheral C atoms of the phenyl and thf groups omitted for clarity (H (light gray), C (light blue), O (red), F (green), S (green), Si (gray), Fe (orange)).

The Structure. The single-crystal X-ray analysis revealed that the molecule contains six iron atoms in the oxidation state +II (see below and bond valence sum analyses in the SI), which are spanned by two tetrasiloxide ligands, which do not correspond to deprotonated 1, though. They are derived from 1 through a transformation as shown in Scheme 2. The molecule is centrosymmetric. In the crystal structure of 2, four Fe atoms (Fe1, Fe1′ and Fe2, Fe2′) have an octahedral coordination sphere. Fe1/1′ is surrounded by six oxygen atoms coming from the tetrasiloxide ligands, a thf molecule, a bridging OH unit, and the bridging triflate anion. Fe2/2′ is also attached to six oxygen atoms, which belong to one siloxide ligand, a thf

Scheme 3. Transformation of Ligand Precursor 1 upon Deprotonation with NaOMe (Si = Si−Ph)

C


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Inorganic Chemistry

occurs in the mineral grunerite (Fe7Si8O22(OH)2),25 where the iron atoms are surrounded by six oxygen atoms, some coming from hydroxy groups like what is seen in complex 2. Hence, FeO6 octahedra link and segregate progressively through the mineral classes depending on the silica content.23−25 In summary, both, the results of surface science and our findings, thus suggest that the formation of “in frame” Fe sites in zeolites is thermodynamically unfavorable and manifest the observations made for naturally occurring materials: While octahedrally coordinated iron is commonly found in clay minerals, tetrahedral iron ions substituting Si4+ in silicate frameworks is a rare case. High-Spin Iron(II) Centers. As mentioned above, 2 contains two square-planar iron(II) centers, which are facing each other in a distance of Fe3···Fe3′ = 3.604 Å and deserve special attention in particular with regard to their spin state. As pointed out above, we have observed square-planar FeO4 moieties in siloxides before (Figure 1),9 and they had a highspin state, which, for the reasons mentioned, is a rare situation. It is found in the natural mineral gillespite (BaFeIISi4O10), which is a red pyrosilicate (Figure 6),26 but the first molecular representative was reported only in 2011.10a

room temperature, colorless crystals could be grown from a concentrated thf solution. The crystalline product was identified as the silsesquioxane Ph12Si12O18, which may be regarded as the condensation product of two of the book-shaped tetrasiloxides found in 2 as illustrated in Figure 4. We can thus infer that NaOMe is responsible for the process depicted in Scheme 2.

Figure 4. Molecular structure of a completely condensed silsesquioxane showing the interconnection of two book-shaped Si6 frameworks drawn in blue and red (left: O (red) and Si (gray) drawn at the 50% probability level, H atoms and peripheral C atoms of the phenyl groups omitted for clarity; right: Si = Si−Ph).21

Arrangement of the Iron Atoms. The six iron centers within complex 2 are forming a layer, which is surrounded by two siloxide ligands. Within the layer, the distances of the iron centers are Fe1···Fe2′ 3.173 Å, Fe1···Fe3 3.019 Å, and Fe2··· Fe3 3.069 Å. This result fits nicely to observations made during the preparation of two-dimensional oxide structures: Growing ultrathin Fe-silicate films on a ruthenium support, Shaikhutdinov, Sauer, Freund, and co-workers showed in 2013 that, unlike aluminum, which was distributed uniformly in a bilayer silicate framework, iron does not substitute silicon randomly, but aggregates into ordered FeO−SiO2 layered structures as found in clays and minerals:22 An iron oxide monolayer formed below a monolayer of SiO4 tetrahedra, so that the film was compared with a sheet of dehydroxylated nontronite (Figure 5, right).5

Figure 6. A segment of gillespite (right, Si (gray), Fe (orange), and O (red)).22

The dihedral angle of the slightly distorted square FeO4 units in compound 2 is θ = 32.81°, and it is thus comparable with the angle found for a high-pressure modification of gillespite (21 kbar, θ = 33.72°). The results described above suggest that these two iron(II) centers in 2 are in a high-spin state, too. To get more information, the electronic structure of 2 was investigated. Complex 2 is paramagnetic and gives broad signals in NMR spectra. A SQUID measurement showed a μeff value at room temperature of 9.1 μB, which is somewhat lower than the spinonly value expected for four Fe(II) high-spin S = 2 centers and two planar Fe(II) S = 1 centers (10.58 μB) and much lower than the spin-only value expected for six S = 2 centers (12 μB). However, the considerable positive slope of μeff(T) at 295 K indicates the presence of a wide split spin ladder due to antiferromagnetic spin coupling within the six-spin system which reduces the observed effective moment at this temperature. In summary, the magnetic data appear to be consistent with the interpretation that all of the six iron centers within 2 are in a high-spin state. At very low temperatures, μeff drops continuously to 6.5 μB at 0 K (cf. SI, Figures S8 and S9). An 57Fe Mössbauer spectrum recorded for the hexanuclear iron siloxide 2 is shown in Figure 7, and the fit parameters are given in Table 1. The spectrum was measured at 13 K, and the fits showed two doublets belonging to three different iron(II) species within complex 2.

Figure 5. Schematic side view of the “iron-siloxide” film grown on a ruthenium surface22 (left: Si (yellow), Fe (purple), O (red), Ru (gray)); cut-out of structure determined for crystals of the mineral nontronite5 (right: Si (gray), Fe (orange), O (red), Na (purple)).

The same sort of segregation of iron is observed on the molecular level also in the formation of 2. Also in minerals containing iron in the oxidation state +II, such findings can be made, whichconsidering the even larger ion radius of Fe2+ (0.63 Å) compared to Fe3+ (0.49 Å)is not surprising. One prominent representative in nature is the iron silicate fayalite (Fe2SiO4),23 where the silicon ions are tetrahedrally coordinated to oxygen atoms, whereas iron ions occupy the centers of distorted oxygen octahedra. Another example for octahedrally coordinated iron in silicates is the chain-silicate ferrosilite (Fe2Si2O6).24 It consists of chains of corner-sharing silicate tetrahedra that cross-link, by shared oxygen atoms, and parallel bands of octahedrally coordinated iron ions. A similar situation D


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Inorganic Chemistry

Both, the values of the isomer shift and quadrupole splitting for the square-planar iron(II) centers of complex 2 are quite similar to the values of the mineral gillespite. Thermal Stability. The thermal behavior of the complex 2 was investigated by employing the TA-MS (thermal analysis on line-coupled to a mass spectrometer) method. To allow for comparing TA with ion current (IC) events, Figures 8 and 9

Figure 7. 57Fe Mössbauer spectrum and the fits for complex 2.

Table 1. Fit Parameters for the 57Fe Mössbauer Spectra for Complex 2 iron centers Fe1 Fe1′ Fe2 Fe2′ Fe3 Fe3′

IS (mm/s) 1.31 0.95

QS (mm/s)

spectral contribution (%)a

coordination geometry

2.69

68.3

octahedral

0.59

27.4

square-planar

Figure 8. TA-MS curves of 2 (34.54 mg) in argon with the IC curves for the most intensive mass numbers of thf (m42, m72) and m77 (C6H5+); see below the graphs for a summary of the color coding.

a

The remaining small doublet with an isomer shift of 1.44 mm/s and a quadrupole splitting of 3.52 mm/s can be assigned to an impurity of high-spin iron(II) (presumably [Fe(OTf)2] (4.3%)).

One of these doublets has an isomer shift (IS) of 1.31 mm/s combined with a large quadrupole splitting (QS) ΔEq = 2.69 mm/s and can be assigned to the iron(II) centers with a octahedral coordination sphere.27 The remaining doublet with an isomer shift of 0.95 mm/s and a small quadrupole splitting of 0.59 mm/s is suggested to originate from the square-planar coordinated iron(II) centers Fe3 and Fe3′ as the small QS is characteristic of a high-spin state.28 Such small quadrupole splittings in Mössbauer spectra have been already observed for other synthetic and also natural highspin square-planar Fe(II) systems, for instance, for the dinuclear Fe(II) complex I shown in Figure 1,9a the abovementioned mononuclear complex II,9b a trianionic pincer Fe(II) complex (II) synthesized in 2015 by Veige et al.,29 an αFe(II)-containing zeolite Fe(II)-beta (Fe-BEA),11 and also for the already mentioned mineral gillespite (Table 2); they can be rationalized by the compensation of the electric field gradient caused by the single dz2 β-electron by the lattice contribution arising from the square-planar ligand field (opposite sign).28

Figure 9. TA-MS curves of 2 (34.54 mg) in argon, as already displayed in Figure 8, together with the IC curves for m18 (H2O+), 48 (SO+), 64 (SO2+), and 69 (CF3+)); see below the graphs for a summary of the color coding.

both show the same conventional TA curves (TG, DTG, DTA) measured, however, together with two different sets of the curves for relevant mass numbers (e.g., m/z = 77 (C6H5+) for the phenyl group, abbreviated in the text as m77). The thermal degradation of the iron(II) complex 2 starts at ca. 50 °C with the simultaneous release of predominantly thf and a small amount of water (m18 in Figure 9); it is almost finished at ca. 280−300 °C. Practically no enthalpic effect is expressed in the DTA trace, which indicates a weak bonding strength of thf and water. The following degradation step, however, is clearly exothermic; it represents the decay of the siloxide part of the structure as follows from the qualitatively different curve shape of the corresponding mass numbers (compare m42, 72 with m77). Note that the main degradation step with Tonex (extrapolated onset temperature) of 326 °C is preceded by a prestep that is clearly expressed by the IC maximum for m77 at ca. 240 °C and weakly expressed by the second DTG peak in the low temperature range (Figure 9). As

Table 2. Comparison of Isomer Shifts and Quadrupole Splittings for High-Spin Square-Planar Fe(II)-Containing Compounds iron compound

IS (mm/s)

QS (mm/s)

temperature (K)

complex 2 I (Figure 1)9a II9b III29 Fe-BEA11 gillespite28

0.95 0.91 0.87 0.83 0.89 0.66

0.59 0.37 0.53 0.45 0.55 0.56

13 80 13 4.2 298 80 E


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Inorganic Chemistry the first mass loss in this range up to 300 °C (13.48%) is generated by two substances, we tried to separate the water contribution to the TG step by means of PulseTA (pulse thermal analysis), i.e., by a preceding calibration of the IC intensity of m18 in a distinct TA run of a suitable calibration substance.30 This led to 0.46% for water and 13.03% for thf. These data approximate the values calculated for the two molecules of water (1.38%) and 4 molecules of thf (11.07%) contained within the structure of 2, as described by its molecular formula. The exothermal step of the decomposition of the iron(II) siloxide 2 is a rather complex reaction as characterized by the variety of product fragments such as the phenyl and the CF3 groups. Interestingly enough, SO2 could be clearly detected as well. Reactivity toward an O-Atom Transfer Reagent. Bearing in mind that the α-Fe sites in Fe-zeolites (identified as high-spin square-planar FeO4 moieties)11 react with N2O to generate the ultimate hydrocarbon oxidizing sites, the fact that 2 contains such centers suggested investigations on its behavior toward O atom transfer reagents. The treatment of a solution of 2 in thf with 1 equiv of Me3NO in a thf/MeCN mixture led to a quick color change from blue to green-brown. After workup, the reaction of the hexanuclear complex 2 with Me3NO leads to the formation of the pentanuclear iron complex [[(Ph6Si6O7(O)4)2Fe5(H2O)(THF)3](MeCN)(μ2-OH)(μ3-OH)2(OTf)] (3) (Scheme 4).

Figure 10. Molecular structure of 3 in the crystal showing the atomlabeling scheme; H, C, N, O, F, S, Si, and Fe drawn at the 50% probability level, peripheral H atoms and peripheral C atoms of the phenyl and thf groups omitted for clarity (H (light gray), C (light blue), N (violet), O (red), F (green), S (yellow), Si (gray), Fe (orange)).

and one nitrogen coming from the acetonitrile ligand. These two edge-sharing octahedrons are connected via one triflate unit in a distance of Fe1···Fe2 3.140 Å and are sharing one OH group (O2) and one silanolate oxygen atom. The iron centers Fe3 and Fe4, which likely correspond to the original square-planar iron centers in 2, possess a distorted square pyramidal coordination sphere in compound 3 with a distance Fe3···Fe4 of 3.419 Å, which is 0.185 Å shorter than that in complex 2. They are connected by an OH unit (O1). The iron atom Fe5 has a distorted trigonal bipyramidal coordination geometry with a calculated tau (τ) value of 0.69. The pentanuclear iron siloxide 3 is a mixed-valent complex with three iron centers in the oxidation state +II and two iron centers with the oxidation state +III (for BVS analyses, see the SI). Hence, 2 indeed reacts with an oxygen atom transfer reagent and the two oxidation equivalents of one O atom have oxidized two FeII to FeIII ions. However, it is difficult to reveal more information about the primary reaction step, and all attempts to intercept the species formed therein through performance of the reaction in the presence of substrates like PPh3, thioanisol, styrene, toluene, benzene, cyclooctene, cyclohexadiene, and dihydroanthracene failed.

Scheme 4. Synthesis of the Pentanuclear Complex 3 Starting from the Hexanuclear Iron Siloxide 2 (Si = Si−Ph)

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Brown crystals of 3 could be grown out of a thf solution layered with n-hexane and were investigated by single-crystal Xray diffraction analysis (Figure 10). The ligand system has remained intact and also the principal arrangement of the structure is similar as in 2. 3 differs from 2, though, by missing constituents that can formally be combined to [Fe(OTf)(H2O)(thf)]+, while an acetonitrile ligand attached to one iron center and an hydroxide ligand, which is bridging three iron centers Fe3, Fe4, and Fe5, represent additional components. Two atoms in the structure have a distorted octahedral coordination sphere: one iron atom (Fe2) is surrounded by six oxygen atoms and the other one (Fe1) by five oxygen atoms

CONCLUSION

Altogether, our results show that, as in minerals, iron(II) ions prefer not to reside in tetrahedral sites offered by siloxide ligands but segregate to form layers with octahedral coordination spheres. Again, high-spin square-planar FeO4 units are found within the structure, confirming further that siloxide ligands support this situation, consistent with recent findings made for Fe-zeolites. Finally, an interesting transformation of an incompletely condensed silsesquioxane triggered by base treatment has been found and thus a first synthetic route to the T12 silsesquioxane shown in Scheme 3. F


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Inorganic Chemistry

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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.7b01347. NMR spectra, IR spectra, ESI-MS spectra, SQUID spectrum, crystal and SQUID data (PDF) Accession Codes

CCDC 1552151 and 1552152 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +49 (0) 30 20937382. ORCID

C. Limberg: 0000-0002-0751-1386 Author Contributions

All synthetic work was done by N. Manicke. S. Hoof and B. Braun-Cula have performed the crystallographic measurements and M. Keck performed Mössbauer measurements. M. Feist was responsible for TA-MS measurements. The manuscript was mainly written by N. Manicke and C. Limberg. All authors gave approval to this manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the CRC 1109 funded by the Deutsche Forschungsgemeinschaft and the Humboldt-Universität zu Berlin for financial support. Furthermore, we would like to thank Prof. E. Bill for performing SQUID measurements.

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REFERENCES

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A Hexanuclear Iron(II) Layer with Two Square-Planar FeO4 Units

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