Metal–Ligand Cooperativity Promoting Sulfur Atom Transfer in

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Metal−Ligand Cooperativity Promoting Sulfur Atom Transfer in Ferrous Complexes and Isolation of a Sulfurmethylenephosphorane Adduct Dieter Sorsche,† Matthias E. Miehlich,‡ Eva M. Zolnhofer,‡ Patrick J. Carroll,† Karsten Meyer,*,‡ and Daniel J. Mindiola*,† †

Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Egerlandstraße 1, 91058 Erlangen, Germany

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/27/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: The reaction of elemental sulfur with the cisdivacant octahedral complex [(pyrr2py)Fe(OEt2)] (1; pyrr2py2− = 3,5-tBu2-bis(pyrrolyl)pyridine) yields the iron dimer [(pyrr-1-Spyrrpy)Fe]2 (2; pyrr-1-S-pyrrpy2− = 3,5-(tBu2-pyrrolyl)(1-S3,5-tBu2-pyrrolyl)pyridine) resulting from a pyrr2py2− ligand based S-oxidation of one pyrrole arm. Addition of the phosphorus ylide H2CPPh3 to 1 forms the ylide adduct [(pyrr2py)Fe(CH2PPh3)] (3), which upon reaction with elemental sulfur produces a rare example of a sulfurmethylenephosphorane adduct, [(pyrr2py)Fe(SCH2PPh3)] (4). The sulfur-oxidized pyrrole group of the ligand pyrr-1-S-pyrrpy2− can be reversed, since complex 2 exhibits S atom transferability via the addition of 2 equiv of H2CPPh3 to yield a mixture of compounds 3 and 4. For all complexes reported, the ferrous ion remains S = 2. Complexes 2−4 were characterized by single-crystal X-ray diffraction as well as 1H NMR spectroscopy, solid and solution magnetic studies, and 57Fe Mössbauer spectroscopic measurements.

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INTRODUCTION The direct use of elemental sulfur, S8, as a single sulfur source in organic syntheses is appealing due to its vast abundance and low cost. Moreover, anthropogenic sources, obtained from natural gas or crude oil purification, have resulted in massive amounts of this element being accumulated; hence, emphasizing interest in its synthetic utility in chemical transformations. However, examples of the direct addition of sulfur into organic frameworks are still scarce, and suitable reagents for the transfer of sulfur from its elemental form are of enormous interest.1−15 Several metal complexes that have been shown to play a role in catalytic S atom transfer require the use of more reactive thiiranes as a form of preactivated sulfur, as opposed to S8.1,5,16 Here, we report the ligand-centered reductive activation of elemental sulfur by the four-coordinate iron(II) complex [(pyrr2py)Fe(OEt2)] (1), supported by the pincer ligand [pyrr2py]2− (pyrr2pyH2 = 5,5′-(pyridine-2,6-diyl)bis(2,4-di-tert-butylpyrrol-1-ide), and its subsequent transfer to form a C−S bond with the methylene group of the phosphorus ylide H2CPPh3. Remarkably, all reported species show the iron center to retain its formal oxidation state +2 (S = 2), while the activated sulfur atom is integrated into the organic framework of the pincer ligand [pyrr2py]2− to form the scaffold [pyrr-1-S-pyrrpy]2− (pyrr-1-S-pyrrpy2− = 3,5-di-tert-butyl-2-(6-(3,5-di-tert-butyl-2sulfido-2H-pyrrol-2-yl)pyridin-2-yl)pyrrol-1-ide.17 Release of © XXXX American Chemical Society

the sulfur atom through C−S bond cleavage with H2CPPh3 fully regenerates the reduced [pyrr2py]2− and, hence, proves a pathway by which a sulfur atom is transferred from elemental S8 to the ligand and then to the substrate with the Fe center always remaining +2 and S = 2.

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EXPERIMENTAL DETAILS

General Procedures. Unless otherwise stated, all operations were performed in a MBraun Lab Master single drybox under an atmosphere of purified nitrogen. Benzene, n-pentane, n-hexane, cyclohexane, and toluene were purchased from Fisher Scientific. Solvents were purged with argon for 20 min and dried using a two-column solvent purification system where columns designated for benzene, hexane, pentane, and toluene were packed with Q5 and alumina, respectively. All solvents were stored over 4 Å molecular sieves and Na metal after being transferred to a glovebox. Deuterated benzene (C6D6) was purchased from Cambridge Isotope Laboratories (CIL). C6D6 was dried over elemental potassium and distilled under nitrogen. Celite and 4 Å molecular sieves were activated under vacuum overnight at 200 °C. S8 was purchased from Sigma and brought into a glovebox under an atmosphere of purified nitrogen. S8 was further purified via gentle heating of a stirring, heterogeneous toluene mixture until a homogeneous solution was obtained. Subsequent filtration through a medium-porosity frit containing Celite followed by storage of the Received: June 8, 2018

A

DOI: 10.1021/acs.inorgchem.8b01599 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 1. Overview of All-Ferrous Iron Complexes Reported in This Work, Enabling Activation of Elemental Sulfur through Reversible Ligand Oxidation in Dimer 2

filtrate at −35 °C, overnight, yielded canary yellow crystals of pure S8, which were filtered and dried under reduced pressure. (pyrr2py)Fe(OEt2) (1) was synthesized via previously reported methods.18 H2CPPh3 was prepared according to literature procedures.19 1H NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer. 1 H NMR spectra are reported with reference to residual 1H solvent resonances of C6D6 at 7.16 ppm. Solution magnetic moments were obtained by the Evans method and included diamagnetic corrections calculated from Pascal constants.20,21 Elemental analyses were performed with a FLASH EA 1112 Series CHN analyzer (Thermo Finnigan). Synthesis of [(pyrr-1-S-pyrrpy)Fe]2 (2). To a stirrred red cyclohexane (10 mL) solution of 1 (0.357 g, 0.64 mmol) was added dropwise a toluene (3 mL) solution of S8 (0.0204 g, 0.080 mmol), resulting in a dark red solution that was stirred for 4 h at room temperature. The solution was then filtered through Celite and the solvent removed in vacuo to yield a deep dark red solid. The solid residue was suspended in ∼1 mL of cyclohexane and filtered through a medium-porosity frit, washed two times with 0.5 mL of toluene, and dried under reduced pressure (0.181 g, 0.16 mmol, 51% yield). Single crystals of 2 were grown from storage of a concentrated pentane solution stored at −35 °C for 16 h. 1H NMR (400 MHz, 25 °C, C6D6): δ 67.06 (Δν1/2 = 84 Hz), 59.93 (Δν1/2 = 56 Hz), 42.33 (Δν1/2 = 80 Hz), 28.94 (Δν1/2 = 56 Hz), 16.50 (Δν1/2 = 60 Hz), 11.51 (Δν1/2 = 52 Hz), 5.04 (Δν1/2 = 40 Hz), 4.63 (Δν1/2 = 76 Hz), − 14.07 (Δν1/2 = 68 Hz). μeff = 5.37 μB calculated per Fe center (C6D6, 298 K, Evans method). Anal. Calcd: N, 8.09; C, 67.04; H, 7.95; S, 6.17. Found: N, 8.10; C, 68.02; H, 8.08; S, 5.84. Synthesis of [(pyrr2py)Fe(CH2PPh3)] (3). To a stirred red C6H6 (5 mL) solution of [(pyrr2py)Fe(OEt2)] (0.595 g, 1.06 mmol) was added dropwise at room temperature a C6H6 (3 mL) solution of the ylide H2CPPh3 (0.292 g, 1.06 mmol). Upon addition of the phosphorus ylide solution the mixture initially turned lighter red. After the mixture was stirred for 1 h, an orange slurry formed which was stirred for an additional 1 h. Subsequent removal of volatiles yielded an orange solid, which was washed with pentane. The solid material was isolated on a medium-porosity frit and dried under reduced pressure to yield

complex 3 (0.502 g, 0.66 mmol, 89% yield). Single crystals of 3 were grown from storage of a concentrated benzene solution at 23 °C for 16 h. 1H NMR (400 MHz, 25 °C, C6D6): δ 119.81 (Δν1/2 = 216 Hz), 93.27 (Δν1/2 = 172 Hz), 21.69 (Δν1/2 = 104 Hz), 7.52 (Δν1/2 = 36 Hz), 5.96 (Δν1/2 = 88 Hz), 5.38 (Δν1/2 = 68 Hz), − 7.13 (Δν1/2 = 608 Hz). μeff = 4.76 μB (C6D6, 298 K, Evans method). Multiple attempts to obtain satisfactory elemental analysis failed. Synthesis of [(pyrr2py)Fe(SCH2PPh3)] (4). To a stirred orange solution of 3 (0.321 g, 0.42 mmol) in C6H6 (5 mL) was added dropwise 3 mL of a C6H6 solution of recrystallized S8 (0.0145 g, 0.056 mmol), resulting in a red solution that subsequently formed a red slurry after the mixture was stirred for 30 min at room temperature. The reaction mixture was stirred for an additional 5 h, and subsequent removal of volatiles yielded a red solid that was suspended in pentane. The solid material, 4, was filtered and isolated on a medium-porosity frit and dried under reduced pressure (0.314 g, 0.39 mmol, 94% yield). Single crystals of 4 were grown from storage of a concentrated toluene solution layered with pentane stored at −35 °C for 16 h. 1H NMR (400 MHz, 25 °C, C6D6): δ 117.26 (Δν1/2 = 108 Hz), 96.03 (Δν1/2 = 92 Hz), 21.14 (Δν1/2 = 60 Hz), 10.65 (Δν1/2 = 116 Hz), 4.66 (Δν1/2 = 36 Hz), 3.00 (Δν1/2 = 48 Hz), − 8.29 (Δν1/2 = 216 Hz). μeff = 4.57 μB (C6D6, 298 K, Evans method). Multiple attempts to obtain satisfactory elemental analysis failed. Reaction of 2 with 2 equiv of H2CPPh3. To a yellowish black C6D6 solution of 2 (0.0403 g, 0.036 mmol) was added at room temperature 2 equiv of a yellow C6D6 solution of the phosphorus ylide H2CPPh3 (0.0102 g, 0.036 mmol). An immediate color change to dark red-black was observed, and after 30 min the formation of 3 and 4 was detected via 1H NMR spectroscopy along with some trace amounts of unreacted 2. 1H NMR (400 MHz, 25 °C, C6D6): δ 119.81 (Δν1/2 = 216 Hz), 117.26 (Δν1/2 = 108 Hz), 96.03 (Δν1/2 = 92 Hz), 93.27 (Δν1/2 = 172 Hz), 67.06 (Δν1/2 = 84 Hz), 59.93 (Δν1/2 = 56 Hz), 42.33 (Δν1/2 = 80 Hz), 28.94 (Δν1/2 = 56 Hz), 21.69 (Δν1/2 = 104 Hz), 21.14 (Δν1/2 = 60 Hz), 16.50 (Δν1/2 = 60 Hz), 11.51 (Δν1/2 = 52 Hz), 10.65 (Δν1/2 = 116 Hz), 7.52 (Δν1/2 = 36 Hz), 5.96 (Δν1/2 = 88 Hz), 5.38 (Δν1/2 = 68 Hz), 5.04 (Δν1/2 = 40 Hz), 4.66 (Δν1/2 = 36 Hz), 4.63 (Δν1/2 = 76 Hz), B

DOI: 10.1021/acs.inorgchem.8b01599 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 3.00 (Δν1/2 = 48 Hz), − 8.29 (Δν1/2 = 216 Hz), − 14.07 (Δν1/2 = 68 Hz). Mössbauer Spectroscopic Details. 57Fe Mössbauer spectra were recorded on a WissEl Mössbauer spectrometer (MRG-500) at 77 K in constant acceleration mode. 57Co/Rh was used as the radiation source. The minimum experimental line widths were 0.20 mm s−1. The temperature of the samples was controlled by an MBBC-HE0106 Mössbauer He/N2 cryostat within an accuracy of ±0.3 K. Isomer shifts were determined relative to α-iron at 298 K. 57Fe Mössbauer data were analyzed and simulated using the software “mcal” and “mf” written by E. Bill (mail: [email protected], MPI for Chemical Energy Conversion, Mülheim an der Ruhr). SQUID Magnetization Measurement Details. Magnetic data were collected using a Quantum Design MPMS-XL SQUID magnetometer. Measurements were obtained for a finely ground microcrystalline powder restrained within a polycarbonate gel capsule. Dc susceptibility data were collected in the temperature range from 2 to 300 K, under a dc field of 1 T. The data were corrected for core diamagnetism of the sample estimated using Pascal’s constants.21 Magnetic susceptibility data were analyzed and simulated using the julX program written by E. Bill (mail: [email protected], MPI for Chemical Energy Conversion, Mülheim an der Ruhr).

Figure 1. Variable-temperature SQUID magnetization measurement of a solid sample of complex 2 (χdia = −678.2 × 10−6 cm3 mol−1, 1 T, black trace), and its fit (red trace). The fit was performed by accounting for a paramagnetic impurity of 1.0% (S = 2.5) and the following parameters: S1 = S2 = 2.0, J12 = −31.3 cm−1, g1 = g2 = 2.25, |D1| = |D2| = 37.5 cm−1, TIP = 1554 × 10−6 emu. MB in the SQUID plots describes the temperature were the corresponding Mössbauer spectrum has been measured.

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RESULTS AND DISCUSSION We recently reported the synthesis of the cis-divacant octahedral complex [(pyrr2py)Fe(OEt2)] (1) and its use as a precursor in the preparation of a novel, low-spin Fe(IV)-imido complex confined to a cis-divacant octahedral geometry: namely, [(pyrr2py)FeNAd] (Ad = 1-adamantyl).18 Inspired by the ability of the electron-rich [pyrr2py]2− framework to support low-spin, high-valent iron complexes, we hypothesized that the reactivity of 1 with elemental sulfur could also yield Fe(IV) sulfide species. Accordingly, treatment of 1 with 0.125 equiv of S8 in a cyclohexane/toluene mixture resulted in the formation of a dark red solution. Upon workup of the reaction mixture, the complex [(pyrr-1-S-pyrrpy)Fe]2 (2) was identified as a dark red crystalline material, which was isolated in 51% yield from a concentrated n-pentane solution stored at −35 °C over a period of 12 h (Scheme 1). Surprisingly, formation of either the terminal sulfide [(pyrr2py)FeS] or its respective dimer [(pyrr2py)Fe(μ2-S)]2 complex were not observed. Instead, it was found that the iron center remained in the oxidation state +2 by forming a dimer, in which oxidation of one pyrrole motif in the pincer ligand [pyrr2py] (at the 1-position) had taken place (Scheme 1). In contrast to previous studies, which suggested that the supporting [pyrr2py]2− ligand remained inert toward oxidation, formation of 2 through pyrrole oxidation clearly shows unprecedented ligand-based reactivity.18,22,23 The paramagnetic 1 H NMR spectrum of 2, recorded in C6D6 at 298 K, reveals nine distinct resonances ranging from −14.1 to +67.1 ppm in accordance with an asymmetric ligand in 2 being produced. The paramagnetic nature of 2 was also confirmed by solution magnetic susceptibility measurements, at room temperature in C6D6, revealing a magnetic moment, μeff, of 5.37 μB, a value that is slightly higher than the calculated spin-only value of 4.90 μB expected for a mononuclear S = 2 ferrous complex. Solid-state SQUID magnetization measurements of a microcrystalline sample of 2 (Figure 1) revealed a temperature dependence of the effective magnetic moment, which was determined to be μeff = 5.35 μB per dimer at 300 K. The magnetic moment continuously decreases with lowering the temperature until it levels off at a value of ∼1.00 μB in a range from 20 to 17 K and finally reaches to a value of 0.54 μB at 2 K. The small effective magnetic moment at 2 K is the result of an antiferromagnetically

coupled FeII/FeII high-spin system with a magnetic exchange coupling constant (J) of −31.3 cm−1 (in the exchange Hamiltonian −2JS1S2 with S1 = 2.0 and S2 = 2.0), which is shown in Figure 1. Single crystal X-ray diffraction (XRD) studies unambiguously show formation of the thiolate-bridged dimer 2, where one pyrrole group has been oxidized by sulfur at the 1-position, rendering this carbon center chiral (Figure 2). The complex

Figure 2. Solid state molecular structure of 2·(n-pentane) (CCDC1842884). Thermal ellipsoids are set at 50% probability, labels are shown for all heteroatoms, and solvent (pentane) and hydrogen atoms have been removed for clarity.

exhibits a distorted geometry between tetrahedral and square planar with the iron centers within the Fe2S2 core forming a nearly square geometry (∠(S−Fe−S) = 98.83(3)°, ∠(Fe−S− Fe) = 81.81(3)°). The two planes defined by the chelating angles ∠(N−Fe−N) and ∠(S−Fe−S) are twisted relative to one another by an angle of 44.97° (Figure 3). With respect to Fe2S2 clusters with supporting N2 chelating ligands reported in the literature, this strong distortion from an ideal tetrahedral N2FeS2 geometry likely stems from the structural rigidity imposed by the [NNS]2− ligand.24−27 C

DOI: 10.1021/acs.inorgchem.8b01599 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Distorted-tetrahedral geometry of 2: (left) beige planes defined by the atoms N1, N2, Fe1 and S1, S1′, Fe1, respectively; (right) view along the mean N1, N2, Fe1 and S1, S1′, Fe1 mean planes, depicting an angle of 44.97°.

In accordance with 1H NMR spectroscopy and single-crystal X-ray diffraction studies, the halves of the dimer are related via C2 symmetry. The Fe−S bond lengths (2.4578(8) and 2.3679(8) Å) are indicative of single-bond character and are in accordance with a nearly square Fe2S2 core. The non-oxidized pyrrole and pyridine groups exhibit a slight elongation in the Fe−Npy (2.073(2) and 2.021(2) Å) and Fe−Npyrr (2.017(2) and 1.987(1) Å) bond lengths in comparison to those seen in complex 1. The C−S bond, the locus of oxidation of the ligand in [pyrr2py]2−, is consistent with a C−S single bond (1.842(3) Å); thus, rendering the new [pyrr-1-S-pyrrpy]2− ligand overall dianionic with both metal ion centers remaining ferrous. Oxidation of the pyrrole carbon results in its pyramidalization, rendering the formal pyrrole motif neutral. Consequently, this motif no longer binds to the Fe center (Fe1···N3 (2.621(1) Å). It should be noted that the supporting ligand [pyrr2py]2− has previously been shown to stabilize the low-spin Fe(IV) imido complex (pyrr2py)FeNAd (Ad = 1-adamantyl).17 For the related cobalt complex, however, it was shown that the respective imide reacts via intramolecular C−H insertion of the pincer tert-butyl to form the corresponding amine.23 This kind of reactivity is known for high-valent transition-metal imido complexes bearing aliphatic C−H bonds in the adjacent ligand framework.28 Notably, the observed pyrrole oxidation by sulfur, reported in this work, represents an unusual type of ligand reactivity, as it proceeds via dearomatization of the pyrrole ring rather than by C−H insertion. We decided to probe the reactivity of this system with methylene triphenylphosphine ylide H2CPPh3 as a strong carbon σ-donor reagent. Due to its capability to stabilize reactive metal ligand multiple bonds, it has been speculated that coordination of the phosphorus ylide H2CPPh3 could facilitate the isolation of a terminal Fe(IV) sulfide, in the form of [(pyrr2py)Fe(S)(CH2PPh3)].29,30 Accordingly, we attempted the synthesis of the corresponding ylide derivative of the basic [(pyrr2py)Fe] framework. Addition of 1 equiv of H2CPPh3 to a benzene solution of 1 expectedly resulted in the formation of complex [(pyrr2py)Fe(CH2PPh3)] (3), which was isolated in 89% yield (Scheme 1). The high-spin Fe(II) nature (S = 2) of 3 was confirmed by the Evans method of 1H NMR spectroscopy and solid-state magnetic measurements. The paramagnetic 1H NMR spectrum of 3 displays seven broad but discernible resonances, ranging from −7.1 to 119.8 ppm. A μeff value of 4.76 μB was determined via the method of Evans, recorded in C6D6 at room temperature. Variable-temperature SQUID magnetization measurements (Figure 4) display temperature dependence with

Figure 4. Variable-temperature SQUID magnetization measurements of two independently prepared samples of 3 (χdia = −502 × 10−6 cm3 mol−1, 1 T, black and white scatter) and their fit (red trace). Fit parameters: S = 2, g = 2.15, |D| = 8.90 cm−1, E/D = 0.14, TIP = 310 × 10−6 emu. MB in the SQUID plots describes the temperature were the corresponding Mössbauer spectrum has been measured.

an effective magnetic moment ranging from 2.99 to 5.30 μB over the temperature range 2−300 K, consistent with 3 being a ferrous monomer. The data were fitted with a g factor of 2.15 and a zero-field splitting parameter |D| = 8.90 cm−1 for a given E/D value of 0.14. The solid-state structure of complex 3 (Figure 5) clearly shows formation of a four-coordinate monomeric Fe(II)

Figure 5. ORTEP drawing of 3·2C6H6 (CCDC-1842886). Thermal ellipsoids are set at 50% probability, labels are shown for all heteroatoms, and residual solvent (benzene) and hydrogen atoms have been removed for clarity. D

DOI: 10.1021/acs.inorgchem.8b01599 Inorg. Chem. XXXX, XXX, XXX−XXX

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case for 3, the data were fitted with a g value of 2.11 and a zerofield splitting parameter |D| of 10.5 cm−1 for a given E/D value of 0.16. The solid-state single crystal X-ray structure of 4, shown in Figure 7 (left), shows that the molecule adopts a cis-divacant octahedral geometry and crystallizes in the rhombohedral crystal system with space group R3̅ (see the Supporting Information). As a result, six molecules are assembled around a 3-fold rotation axis along the c axis. Figure 7 (right) shows how all six molecules are assembled around a 3-fold rotation axis and translate onto each other by the 3-fold rotation axis and inversion center (highlighted in the same respective colors red and blue). The Fe−Npy (2.0466(16) Å) and Fe−Npyrr (2.0319(17) and 2.0654(17) Å) distances are both akin to those reported for 1 and 3. Nonplanar coordination of the Fe atom to pyrr2py is also found in 4, but the distance between the plane defined by the three N atoms of the [pyrr2py]2− ligand and Fe is markedly lower than that found in 3: i.e., 0.54 Å versus 0.77 Å. It is also worth noting that the Npy−Fe−S angle (N2−Fe1−S1 = 126.91(5)°) is approximately 5° more obtuse than the Npy−Fe−C angle (N2− Fe1−C30 = 122.04(5)°) found in 3. However, the most notable feature in the solid-state structure of 4 is the presence of an extended sulfur ylide, SCH2PPh3, bound to the Fe(II) center. To our knowledge, this is the first example reported in the literature of an iron complex bearing such a sulfurmethyleneylide ligand. To further corroborate the formal +2 oxidation state in complexes 2−4, we collected zero-field 57Fe Mössbauer spectra (Figure 8). All complexes (2−4) exhibit comparable isomeric shifts δ of 0.83(1), 0.70(1), and 0.77(1) mm s−1, respectively, similar to those for the the Fe(II) precursor 1 (CCDC1001461).18 Quadrupole splitting values ΔEQ of the monomeric complexes 3 and 4 are slightly larger with respect to 1; i.e. 1.72(1) and 1.35(1) mm s−1, respectively, but are in accordance with the retained cis-divacant octahedral geometry around the metal center (Table 1). The dimeric complex 2, however, shows a significantly broadened quadrupole splitting, which is generally observed in sulfide- or thiolate-bridged iron(II) dimers.24,27,33 It is worth noting that the observed ΔEQ value of 3.19(1) mm s−1 compares particularly well with that observed in the thiolate dimer [NEt4]2[Fe2(SEt)6] with a ΔEQ value of 3.25 mm s−1.34 Encouraged by the relatively unhindered nature of the sulfur atoms in dimer 2 (Figure 9) and the ability of sulfur to insert into the Fe−C bond of 3 to form 4, it was conjectured that the addition of the ylide H2CPPh3 to 2 could also result in S atom transfer reaction from the [pyrr-1-S-pyrrpy]2− ligand to the ylide. Gratifyingly, the addition of 2 equiv of ylide to the dimer 2 in C6D6 led to the formation of 4 in approximately 60% yield, according to 1H NMR spectroscopy (Scheme 1). It should be noted that the reaction also results in formation of ∼30% of phosphorus ylide adduct 3 and it is unclear if such a side reaction is accompanied by the deposition of elemental sulfur. S atom transfer to phosphines has been observed previously by Hayton and co-workers via addition of triphenylphosphine, PPh3, to a uranium-based disulfide complex: namely, [K(18crown-6)][U(η2-S2)(NR2)3] (R = SiMe3).35 However, we are not aware of such a process occurring via ligand reactivity and with a cooperative metal−ligand promoted redox process. It is worth noting that the addition of PPh3 to 2 results in no reaction to form the anticipated complex [(pyrr2py)Fe(SPPh3)]. We are now beginning to explore other substrates capable of accepting the sulfur atom from complex 2.

complex that adopts a cis-divacant octahedral geometry that has previously been established for iron complexes supported with the [pyrr2py2]2− ligand.17 The Fe−Npy (2.0392(13) Å) and Fe− Npyrr (2.0281(13) and 2.0475(13) Å) distances in the solid-state structure of 3 are in good agreement with those reported for 1, giving further credence to its expected ferrous nature.12 The position of the Fe(II) ion in 3 is located ∼0.77 Å above the idealized plane defined by the three nitrogen atoms encompassing the pincer ligand. In order to probe the influence of the ylide ligand on sulfur activation, complex 3 was reacted with 1/8 equiv of S8. Rather than stabilizing the hypothesized Fe(IV) sulfide, the reaction proceeds via insertion of sulfur into the Fe−C bond of the phosphorus ylide to give the complex [(pyrr 2 py)Fe(SCH2PPh3)] (4) in 94% yield. Complex 4 represents a rare example of a sulfurmethylenephosphorane ligand bound to iron through the sulfur atom.31,32 In this case, sulfur oxidation of the methylene carbon of the ylide atom occurred rather than oxidation of the iron center. At this point, however, we cannot refute the possibility of a species such as an Fe(IV) sulfido species, [(pyrr2py)Fe(S)(CH2PPh3)], undergoing reductive coupling of the sulfide and ylide to form the sulfurmethylenephosphorane ligand in 4. Another likely possibility involves sulfur oxidation of the ligand pyrr2py followed by S−C coupling with the ylide H2CPPh3. The retention of a ferrous ion in 4 is proposed as a result of the 1H NMR spectroscopic and magnetic data (both in solution and in solid-state phases), being consistent with such species still possessing a high-spin and mononuclear ferrous ion complex, akin to the ferrous monomers 1 and 3. Specifically, the 1H NMR spectrum of 4 displays seven broad resonances, ranging from −8.3 to 117.3 ppm, whereas a μeff value of 4.57 μB was calculated by the Evans method in C6D6 at room temperature. The variable-temperature SQUID magnetization measurements of 4 displayed a temperature dependence similar to that of 3, with an effective magnetic moment varying from 2.88 to 5.47 μB over the temperature range 2−300 K (Figure 6). Notably, the magnetic moment values determined for both compounds 3 and 4 are slightly lower than those expected. Unfortunately, and despite significant efforts, we were not able to obtain sufficiently pure bulk samples, which likely accounts for the observed deviation. Akin to the

Figure 6. Variable-temperature SQUID magnetization measurements of two independently prepared samples of 4 (χdia = −510 × 10−6 cm3 mol−1, 1 T, black and white scatter) and their fit (red trace). Fit parameters: S = 2, g = 2.11, |D| = 10.5 cm−1, E/D = 0.16, TIP = 846 × 10−6 emu. MB in the SQUID plots describes the temperature were the corresponding Mössbauer spectrum has been measured. E

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Figure 7. (left) ORTEP drawing of 4 without additional solvent molecules (solvent disorder was resolved using the SQUEEZE algorithm, CCDC1842885). Thermal ellipsoids are set at 50% probability, labels are shown for all heteroatoms, and hydrogen atoms have been removed for clarity. (right) Packing of six molecules of 4 assembled along a 3-fold rotation axis (c axis).

Figure 8. Zero-field 57Fe Mössbauer spectra for complexes 2 (left), 3 (middle), and 4 (right).

Table 1. 57Fe Mössbauer Parameters for Complexes 1−4 type δ (mm s−1) ΔEQ (mm s−1) Γfwhm (mm s−1)

Attempts to isolate a putative sulfide species using the Lewis

1

2

3

4

basic triphenylphosphine ylide resulted in S atom transfer and

doublet 0.86(1) 1.12(1) 0.28(1)

doublet 0.83(1) 3.19(1) 0.34(1)

doublet 0.70(1) 1.72(1) 0.36(1)

doublet 0.77(1) 1.35(1) 0.39(1)

C−S bond formation, yielding the first example reported in the literature of a complex containing the sulfurmethylenetriphenylphosphine ylide, SCH2PPh3. The ligand-based reactivity observed in 2 suggests that this species could be a viable

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candidate for sulfur (derived from S8) delivery reactions to other

CONCLUSIONS Reactivity studies of the previously reported complex 1 with elemental sulfur revealed unprecedented non-innocent behavior of the supporting [pyrr2py]2− ligand. S atom transfer to yield an oxidized pyrrole fragment results in the dianionic [pyrr-1-Spyrrpy]2− ligand with the Fe center therefore remaining divalent.

small molecules. We are currently exploring such studies using this metal−ligand cooperativity that allows for electrons to be transferred, reversibly, without degradation of the ligand or by disproportionation pathways.

Figure 9. Space-filling model of 2 with hydrogen atoms omitted for clarity with a schematic representation shown on the right. F

DOI: 10.1021/acs.inorgchem.8b01599 Inorg. Chem. XXXX, XXX, XXX−XXX

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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.8b01599. Complete crystallographic and spectral data for 2−4 (PDF) Accession Codes

CCDC 1842884−1842886 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 Authors

*E-mail for K.M.: [email protected]. *E-mail for D.J.M.: [email protected]. ORCID

Karsten Meyer: 0000-0002-7844-2998 Daniel J. Mindiola: 0000-0001-8205-7868 Funding

For funding, we thank the University of Pennsylvania and the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (DEFG02−07ER15893). K.M., M.E,M., and E.M.Z. thank the Friedrich-Alexander University Erlangen-Nürnberg (FAU), and D.J.M. acknowledges some financial support from the Alexander von Humboldt Foundation. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge kind support from Dr. Eckhard Bill at the Max-Planck-Institute for Chemical Energy Conversion in Mülheim an der Ruhr. D.J.M. thanks Kyle T. Smith for stimulating discussions.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01599 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01599 Inorg. Chem. XXXX, XXX, XXX−XXX