A Pyrrole-Based Pincer Ligand Permits Access to Three Oxidation

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A Pyrrole-Based Pincer Ligand Permits Access to Three Oxidation States of Iron in Organometallic Complexes C. Vance Thompson, Hadi D. Arman, and Zachary J. Tonzetich* Department of Chemistry, University of Texas at San Antonio (UTSA), San Antonio, Texas 78249, United States S Supporting Information *

ABSTRACT: Treatment of FeCl2(thf)1.5 with the sodium salt of bis(dicyclohexylphosphinomethyl)pyrrole (NaCyPNP) in the presence of pyridine (py) affords the high-spin, five-coordinate iron(II) complex [FeCl(py)(CyPNP)]. The chloride complex serves as a starting point for a variety of organometallic iron(II) compounds including S = 1 square planar alkyls, [FeR(CyPNP)] (R = Me, Et, Bn, Ph). Treatment of [FeCl(py)(CyPNP)] with NaBEt3H does not generate an analogous square-planar hydride complex, but rather a bimetallic high-spin iron(II) species containing bridging hydride ligands. Analogous transmetalation reactions in the presence of carbon monoxide produced six-coordinate low-spin species [FeR(CO)2(CyPNP)] (R = Et, Ph, and H). Reduction of [FeCl(py)(CyPNP)] under CO afforded the low-spin iron(I) carbonyl complex [Fe(CO)2(CyPNP)]. This species could be further reduced with KC8 to afford the anionic iron(0) complex K[Fe(CO)2(CyPNP)]·thf. The breadth of coordination numbers, spin states, and valencies supported by this pyrrole-based PNP ligand demonstrates the flexibility afforded by this pincer framework.

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INTRODUCTION The organometallic chemistry of iron complexes supported by pincer ligands has enjoyed increased attention as new strategies to exploit iron catalysis have proliferated in recent years.1,2 Hydrofunctionalization reactions remain the most popular application of iron pincer catalysts,3−10 although examples of CO2 activation11−13 and C−C cross-coupling have also been reported.14 The benefits of the pincer framework to stabilize reactive iron centers and permit detailed mechanistic investigation are among the many advantages afforded by this ligand class.15 In this vein, our laboratory has been investigating a new class of PNP-type pincer ligands containing a central pyrrole unit (Chart 1).16−18 The ligand synthesis is compatible

The majority of reported Fe pincer complexes make use of charge neutral ligands that feature central donor moieties based on pyridines and simple amines.24,25 Examples of iron complexes containing anionic pincer ligands are known and include both PCP and PNP variants containing hydrocarbyl and amido donors, respectively.26−38 Despite this precedent, anionic pincers are underdeveloped in the chemistry of iron, and the applications of such compounds to catalysis are few in comparison to their neutral analogues, which have enjoyed a measure of success. Moreover, among the iron complexes of anionic pincer ligands reported to date, the majority concern low-spin iron in the 2+ and 0 oxidation states. The ability to access multiple oxidation and spin states of iron would greatly expand the range of pincer chemistry for this element and provide a foundation for new catalytic methodologies. In this contribution, we demonstrate the ability of the pyrrole-based R PNP pincer to stabilize iron in multiple oxidation and spin states with organometallic coligands.

Chart 1. RPNP Ligand

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Delivery of CyPNP to iron(II) was accomplished through use of the sodium salt Na(CyPNP). In contrast to findings of Yoshizawa and Nishibayashi with the bulkier tBuPNP analogue,23 the CyPNP ligand was unable to support a squareplanar iron(II) chloride complex, and metalation only proceeded in the presence of an additional Lewis base. Consequently, inclusion of pyridine in the reaction of FeCl2(thf)1.5 with Na(CyPNP) produced the desired five-

with a variety of phosphine substituents that to date include phenyl, cyclohexyl, isopropyl, and tert-butyl.19−22 Despite the versatility of this scaffold, the first reports of its chemistry with iron only appeared recently in which the tert-butyl-substituted version was used to support complexes active for catalytic N2 reduction.23 As observed in nickel chemistry, however, changes to the phosphine substituents are expected to play a dramatic role in the resulting chemistry of the metal complexes.19,22 Accordingly, we were very interested in establishing the organometallic iron chemistry of this ligand scaffold with phosphine substituents other than tert-butyl. © XXXX American Chemical Society

RESULTS AND DISCUSSION

Received: February 24, 2017

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Organometallics coordinate iron complex [FeCl(py)(CyPNP)] (1, py = pyridine) in satisfactory yields on scales up to 5 g (eq 1).

challenge in subsequent reactions and therefore did not pursue the compound further. An alternate adduct that proved relevant to the present study was the carbonyl complex [FeCl(CO)2(CyPNP)] (2). Addition of Na(CyPNP) to FeCl2(thf)1.5 under an atmosphere of CO gas produced 2 in good yield (eq 2). The complex was also

Complex 1 is a yellow solid that displays a solution magnetic susceptibility of 5.0(2) μB consistent with high-spin iron(II). This fact makes 1 the first example of a high-spin complex supported by RPNP. The electronic absorption spectrum of the material in benzene displays two ligand field transitions centered at 6020 and 13160 cm −1 (see Supporting Information). The values of these transitions are relatively high in energy for five-coordinate iron(II) and suggest that 1 is likely near the threshold of remaining high spin.39 The 1H NMR spectrum of 1 is diagnostic showing 13 resonances in the range of −20 to +60 ppm. Because of the broad nature of many of the resonances, however, peak assignments were not as straightforward as with other paramagnetic iron(II) complexes of the CyPNP ligand discussed below. Cyclic voltammetry of 1 in thf demonstrated a reversible wave for the FeII/III couple at −410 mV vs Fc/Fc+ (see Supporting Information). Compound 1 readily crystallizes from benzene/pentane mixtures, and the solid-state structure is depicted in Figure 1.

observed to form upon treatment of 1 with CO, although this reaction was slower and lower yielding due to the persistence of an intermediate species that we tentatively assign as a mixed pyridine-CO adduct. Compound 2 displays two strong CO absorptions by IR spectroscopy consistent with the cisdicarbonyl isomer. 1H NMR spectra further confirm the cis nature of the CO ligands, showing inequivalent resonances for the methylene H atoms of the ligand arms. The solid-state structure of 2 appears in the Supporting Information. The geometry about iron(II) is octahedral with bond metrics comparable to those of 1 but slightly contracted due to the lowspin nature of the complex. Treatment of compound 1 with a variety of Grignard reagents produced the iron(II) alkyl species [FeR(CyPNP)] (R = Me (3), Bn (4), and Ph (5), eq 2). Solution magnetic susceptibility measurements on 3 demonstrate an effective magnetic moment of 2.7(2) μB consistent with intermediate spin iron(II) (S = 1). Such results are consistent with those found previously for the tBuPNP analogue.23 1H NMR spectra of 3−5 are notably sharper than those of 1, displaying the expected number of peaks in each instance for time-averaged C2v symmetry (see Supporting Information). In the case of 3 and 4, resonances for the hydrogens atoms of Cα are not observed because of their proximity to the metal center.

Figure 1. Thermal ellipsoid drawing (50%) of the solid-state structure of 1. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe(1)−N(1) = 2.0344(19); Fe(1)−Pavg = 2.5384(8); Fe(1)−Cl(1) = 2.3019(8); Fe(1)−N(2) = 2.1282(18); P(1)−Fe(1)−P(2) = 152.79(2); N(1)−Fe(1)−Cl(1) = 157.67(6); N(1)−Fe(1)−N(2) = 77.59(5).

The solid-state structures of 3 and 5 are similar to one another and display square-planar geometries about iron. Compound 3 is shown in Figure 2 as representative of the pair. Notably, the Fe−N distance is found to contract by nearly 0.1 Å from the value of 2.03 Å found in 1 to 1.94 Å in 3, reflecting the lower coordination number and change in spin state from S = 2 to S = 1. Reaction of 1 with NaBEt3H did not result in an analogous square-planar iron(II) hydride species, [FeH(CyPNP)]. Instead, the bimetallic bridging hydride [Fe2(μ-H)2(CyPNP)2] was obtained (6, eq 4). Compound 6 is sparingly soluble in aromatic solvents such as benzene but dissolves readily in thf. The 1H NMR spectrum of 6 in thf-d8 displays a large number of paramagnetically shifted peaks consistent with a lowering in symmetry (see Supporting Information). Solution susceptibility measurements at 25 °C demonstrate an effective magnetic

The five-coordinate geometry of 1 is best described as a distorted square-pyramid (τ = 0.08).40 Consistent with the high-spin nature of 1, the bond lengths about iron are longer than those observed in all previous RPNP complexes of the 3d metals. Attempts to prepare an analogue of 1 using PPh3 as the Lewis base produced material with NMR features similar to those of the pyridine adduct. However, the putative [FeCl(PPh3)(CyPNP)] complex proved more difficult to isolate than 1. In addition, we expected that removal of PPh3 would pose a B

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Figure 2. Thermal ellipsoid drawing (50%) of the solid-state structure of 3. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe(1)−N(1) = 1.938(3); Fe(1)−Pavg = 2.2454(11); Fe(1)−C(31) = 2.039(4); P(1)−Fe(1)−P(2) = 164.43(4); N(1)−Fe(1)−C(31) = 178.96(13). Figure 3. Thermal ellipsoid drawing (50%) of the solid-state structure of 6. One of two crystallographically independent molecules in the asymmetric unit is shown. Hydrogen atoms, except for H(A) and H(B), and cyclohexyl groups originating from P(1) and P(4) are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe(1)−Fe(2) = 2.4796(7); Fe(1)−H(A) = 1.70(4); Fe(1)−H(B) = 1.56(4); Fe(2)−H(A) = 1.65(4); Fe(2)−H(B) = 1.64(4); Fe(1)− N(1) = 1.996(2); Fe(1)−P(1) = 2.3843(9); Fe(1)−P(3) = 2.3618(9); H(A)−Fe(1)−H(B) = 82(2); P(1)−Fe(1)−P(3) = 111.06(4); N(1)− Fe(1)−P(1) = 84.11(8); N(1)−Fe(1)−P(3) = 114.40(8).

moment of 8.2(2) μB, which is curious given the proximity of the iron atoms (vida infra). The precise nature of the magnetic ground state of 6 and its exchange interactions are not known at present, although the compound represents a rare example of a high-spin iron(II) bridging-hydride species. Other examples include the four-coordinate β-diketiminate complexes prepared by the groups of Holland, Murray, and Limberg.41−44 Such species are of interest due to their role in the chemistry of nitrogenase enzymes.45 The solid-state structure of 6 is depicted in Figure 3. Electron density corresponding to the bridging hydrogen atoms was detected in the difference map and used to refine the positions of H(A) and H(B). The geometry about the iron atoms in 6 is best regarded as distorted trigonal bipyramidal (τavg = 0.95) with the bridging hydride ligands occupying one axial and one equatorial position. The CyPNP ligand has rearranged such that one of the phosphine arms coordinates to a second iron atom. The Fe−Fe distance of 2.45 Å is quite short as expected for a bridging hydride, but the remaining metrics to the CyPNP ligand are in line with a high spin assignment for the iron(II) centers. In contrast to the findings with 1 and NaBEt3H, similar reactions in the presence of CO produced an isolable monometallic iron(II) hydride species (7, eq 5). Compound

absorptions consistent with a cis-carbonyl complex. The solidstate structure of 7 is depicted in the Supporting Information. Analogous reactions of 2 with NaBEt3H failed to produce 7, instead resulting in reduction (vida infra). It therefore appears that the six-coordinate chloride complex 2 is ill-suited for subsequent transmetalation reactions. A preliminary survey of the reactivity of 7 with alkenes did not result in products indicative of olefin insertion. Such a finding further demonstrates that the coordinative saturation about 7 attenuates its reactivity, in this case in migratory insertion processes. Analogous reactions of alkenes with 6 proceeded slowly but yielded a complex mixture of products. Likewise, direct addition of CO to 6 produced only small quantities of 7 after several days as judged by 1H NMR spectroscopy. Efforts to prepare the ethyl complex [FeEt(CyPNP)] generated material with 1H NMR signatures akin those of 3− 5. However, attempted isolation of the compound resulted in 6, suggesting that β-H elimination occurs readily for the complex. Addition of CO to in situ-generated [FeEt(CyPNP)] did result in a stable compound that we formulate as [FeEt(CO)2(CyPNP)] on the basis of NMR, IR, and combustion analysis (8, eq 6). The six-coordinate nature of 8 likely stabilizes the complex against β-H elimination permitting its isolation.

7 is diamagnetic, displaying a triplet (JHP = 49.0 Hz) resonance for the hydride ligand at −7.38 according to 1H NMR spectroscopy. The compound also displays two strong infrared C

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iron(I) dicarbonyl species, although dinuclear analogues feature prominently in the chemistry of [Fe,Fe] hydrogenase models.46 A related Fe(I) dinitrogen complex was recently described as a key species in catalytic N2 reduction. The spin-state of the previously reported dinitrogen complex was also assigned as S = 1/2, although with a larger effective magnetic moment of 3.0(2) μB.23 The solid-state structure of 10 is shown in Figure 5. The geometry about iron is square-pyramidal (τ = 0.08) with a cis

In similar fashion to complex 8, direct addition of CO to isolated 5 produced low-spin [FePh(CO)2(CyPNP)] (9, eq 7).

Figure 5. Thermal ellipsoid drawing (50%) of the solid-state structure of 10. Hydrogen atoms and cocrystallized heptane molecule omitted for clarity. Selected bond distances (Å) and angles (deg): Fe(1)−N(1) = 1.9420(13); Fe(1)-Pavg = 2.2472(4); Fe(1)−C(31) = 1.7722(17); Fe(1)−C(32) = 1.8275(16); P(1)−Fe(1)−P(2) = 159.698(18); N(1)−Fe(1)−C(31) = 154.65(7); C(31)−Fe(1)−C(32) = 97.12(7).

Figure 4. Thermal ellipsoid drawing (50%) of the solid-state structure of 9. Hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles (deg): Fe(1)−N(1) = 1.9666(12); Fe(1)−Pavg = 2.2580(4); Fe(1)−C(31) = 2.0588(14); Fe(1)−C(37) = 1.7891(15); Fe(1)−C(38) = 1.7580(14); N(1)−Fe(1)−C(38) = 179.30(6); C(31)−Fe(1)−C(37) = 176.54(6).

arrangement of the carbonyl ligands. Despite the lower Fe(I) oxidation state, the Fe−CO distances in 10 are longer than those in Fe(II) complexes 2, 7, and 9. The remaining metric parameters about 10 are unremarkable and similar to those of other compounds discussed above. To further characterize compound 10, we recorded its Xband EPR spectrum as a 2-Methf glass at 50 K (Figure 6). As expected, the compound displayed a signal centered near g = 2 consistent with low-spin Fe(I) (S = 1/2).47−50 The signal appears to be axial with rich superhyperfine coupling and is markedly different than that reported for [Fe(N2)(tBuPNP)].23 Given that the electron count in 10 is 17, we were curious as to whether the CyPNP ligand could stabilize an 18-electron iron(0) complex. It was proposed that iron(0) species of the tBu PNP ligand are key intermediates in the N2 reduction pathway.23 Accordingly, we examined the cyclic voltammogram of 10 to characterize its accessible redox events. The CV in THF is displayed in Figure 7. As is evident, 10 demonstrates two reversible one-electron processes at −0.447 and −2.152 V (vs ferrocene/ferrocenium). We attribute these events to the FeI/II and FeI/0 couples, respectively. The stability of the iron(II) dicarbonyl is not surprising given the existence of compound 2. More intriguing is the reversible FeI/0 couple at −2.152 V, which suggests that an iron(0) species should be isolable. Gratifyingly, treatment of 1 under a CO atmosphere with an excess of KC8 (5 equiv) produced the desired iron(0) complex, K[Fe(CO)2(CyPNP)]·thf as orange crystals (11, eq 9). The complex displays infrared absorptions for the CO ligands at 1787 and 1731 cm−1. The 1H NMR spectrum of 11 is

The solid-state structure of 9 is depicted in Figure 4. Much like 2 and 7, the geometry about 9 is octahedral with a cis disposition of the carbonyl ligands. Notably, the Fe−Cipso bond distance in 9 is longer than that in 5 despite the change from S = 1 to S = 0. This bond elongation most likely reflects the increase in coordination number from 4 to 6. Treatment of 2 with NaBEt3H resulted in formation of the iron(I) dicarbonyl complex [Fe(CO)2(CyPNP)] (10, eq 8),

presumably with loss of H2. Compound 10 could also be prepared from 1 by reaction with KC8 under an atmosphere of CO, although this route was lower yielding due to the presence of unreacted 2. The iron(I) complex is low-spin as judged by EPR studies (vida infra) and solution magnetic susceptibility measurements, which demonstrate an effective magnetic moment of 2.0(2) μB. The 1H NMR spectrum of 10 is essentially featureless, displaying several broadened resonances between 0 and 6 ppm. The IR spectrum shows two strong CO absorptions at 1945 and 1877 cm−1 consistent with the lowering in oxidation number (see 2013 and 1954 cm−1 in 2). Complex 10 is a rare example of a mononuclear paramagnetic D

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8. The compound crystallizes as a coordination polymer in the solid state with two different iron environments. The first is

Figure 8. Thermal ellipsoid drawing (50%) of a portion of the solidstate structure of 11. Hydrogen atoms and cyclohexyl carbon atoms omitted for clarity. Selected bond distances (Å) and angles (deg): Fe(1)−N(1) = 2.024(4); Fe(2)−N(2) = 2.018(4); Fe(1)−Pavg = 2.2186(13); Fe(2)−Pavg = 2.2047(14); Fe(1)−C(61) = 1.739(5); Fe(1)−C(62) = 1.708(5); Fe(2)−C(63) = 1.717(5); Fe(2)−C(64) = 1.745(5); C(61)−Fe(1)−C(62) = 110.0(2); P(1)−Fe(1)−P(2) = 155.55(5); P(3)−Fe(2)−P(4) = 157.20(5); C(63)−Fe(2)−C(64) = 110.2(2).

Figure 6. X-band EPR spectrum of 10 in a 2-Methf glass at 50 K. Spectrometer conditions: 9.643 GHz microwave frequncey; 4.0 μW microwave power; 1.0 G modulation amplitude.

best described as distorted square-pyramid (τ = 0.32), whereas the second is further distorted toward a trigonal bipyramid (τ = 0.43). The potassium cations bind to the pyrrolide rings in an η5 fashion with the remaining coordination sites filled by CO and/or thf ligands (see Supporting Information). The Fe−CO bond lengths are shorter than those of all compounds discussed previously as expected due to the presence of increased backbonding in the anion. In conclusion, we have demonstrated here the diversity in oxidation state and coordination geometry of iron complexes supported by the CyPNP ligand. Examples of Fe(II), Fe(I), and Fe(0) complexes in high-, intermediate-, and low-spin states have been synthesized and characterized. Coordination numbers of 4 through 6 are accommodated by the ligand, which suggests that these systems have potential for catalytic transformations such as cross-coupling and hydrofunctionalization. Efforts to realize this catalytic reactivity are currently underway in our laboratory.

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Figure 7. Cyclic voltammogram of 10 at a glassy carbon electrode in THF. The scan rate is 50 mV/s, and the supporting electrolyte is 0.2 M Bu4NPF6.

EXPERIMENTAL SECTION

General Comments. All manipulations were performed under an atmosphere of purified nitrogen gas using a Vacuum Atmospheres glovebox. Tetrahydrofuran, diethyl ether, pentane, and toluene were purified by sparging with argon and passage through two columns packed with 4 Å molecular sieves or activated alumina (THF). 1H NMR spectra were recorded on a Varian spectrometer operating at 500 MHz (1H) in benzene-d6 unless otherwise noted and referenced to the residual protium resonance of the solvent (δ 7.16 ppm). For selected paramagnetic compounds, only peak maxima are listed, and those with fwhm greater than 400 Hz are denoted as broad (br). 31P NMR spectra were referenced automatically using the 2H lock frequency. FT-IR spectra were recorded with a ThermoNicolet iS 10 spectrophotometer in benzene-d6 solution using an airtight liquid transmission cell (Specac OMNI) with KBr windows. UV−vis spectra were recorded on a Cary-60 spectrophotometer in Teflon-capped

broadened at 25 °C in thf-d8 but displays peaks consistent with a diamagnetic complex (see Supporting Information). A portion of the solid-state structure of 11 is displayed in Figure E

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Organometallics quartz cells. X-band EPR spectra were recorded on a Bruker EMX EPR spectrometer in 4 mm o.d. quartz tubes. Cyclic voltammetry was performed in the glovebox at 23 °C on a CH Instruments 620D electrochemical workstation. A 3-electrode setup was employed comprising a 2 mm glassy C disk working electrode, Pt wire auxiliary electrode, and Ag/AgCl quasi-reference electrode. Triply recrystallized Bu4NPF6 was used as the supporting electrolyte. All electrochemical data were referenced internally to the ferrocene/ferrocenium couple at 0.00 V. Magnetic susceptibility measurements were performed in solution using the Evan’s method with reported diamagnetic corrections.51 Elemental analyses were performed by the CENTC facility at the University of Rochester. In each case, recrystallized material was used for combustion analysis. Materials. H(CyPNP),17 FeCl2(thf)1.5,52 and potassium graphite (KC8)53 were prepared according to published procedures. Carbon monoxide gas was obtained from Sigma-Aldrich in a lecture bottle and delivered to NMR samples or reaction mixtures via a syringe needle/ septum. All other reagents were purchased from commercial suppliers and used as received. Crystallography. Crystals suitable for X-ray diffraction were mounted, using Paratone oil, onto a nylon loop. All data were collected at 98(2) K using a Rigaku AFC12/Saturn 724 CCD fitted with Mo Kα radiation (λ = 0.71075 Å). Low-temperature data collection was accomplished with a nitrogen cold stream maintained by an X-Stream low-temperature apparatus. Data collection and unit cell refinement were performed using CrystalClear software.54 Data processing and absorption correction, giving minimum and maximum transmission factors, were accomplished with CrysAlisPro55 and SCALE3 ABSPACK,56 respectively. The structure, using Olex2,57 was solved with the ShelXT58 structure solution program using direct methods and refined (on F2) with the ShelXL59 refinement package using the full-matrix, least-squares techniques. All nonhydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atom positions were determined by geometry and refined by a riding model except for the hydride ligands in compound 6, which were identified in the difference map and refined isotropically. Crystallographic data and refinement parameters for each structure can be found in the Supporting Information. Na(CyPNP). A flask was charged with 1.187 g (2.43 mmol) of H(CyPNP) and 20 mL of toluene. To the pale yellow solution was added 0.449 g (2.45 mmol) of NaHMDS as a solid in one portion. The resulting mixture was allowed to stir overnight at room temperature during which time a white precipitate accumulated. The flask was cooled to −30 °C for 3 h before the precipitate was collected by filtration. The precipitate was washed with copious amounts of pentane to afford 0.890 g (72%) of a white powder. The as-isolated solid was employed in subsequent reactions without further purification. NMR(thf-d8) 1H: δ 5.42 (s, 2H), 2.79 (s, 4H), 1.72 (m, 18H), 1.63 (app d, 4H), 1.51 (m, 4H), 1.21 (m, 14), 1.11 (m, 4H). [FeCl(py)(CyPNP)], 1. A flask was charged with 5.045 g of Na(CyPNP) (9.90 mmol), 1.6 mL (20 mmol) of pyridine, and 50 mL of thf. To the resulting pale yellow solution was added 2.326 g of FeCl2(thf)1.5 (9.90 mmol). The color of the solution immediately changed to dark yellow, and the mixture was allowed to stir for 18 h at room temperature. All volatiles were removed in vacuo leaving a yellow-green residue that was extracted into 20 mL of toluene. The toluene extract was filtered through a pad of Celite and evaporated to dryness. The resulting yellow green solid was washed with Et2O and collected by filtration to afford 4.054 g (62%). Crystals suitable for Xray diffraction were grown by vapor diffusion of pentane into a concentrated benzene solution at room temperature. Mp: 182−183 °C. μeff = 4.7(2) μB. NMR: 1H δ 51.3, 42.0, 34.9 (br), 32.4, 26.0 (br), 11.3 (br), 8.6 (br), 2.3, 1.7, 0.3, −1.6, −5.7 (br), −8.0 (br). Anal. Calcd for C35H55ClFeN2P2: C, 63.98; H, 8.44; N, 4.26. Found: C, 64.10; H, 8.17; N, 4.17. [FeCl(CO)2(CyPNP)], 2. A flask was charged with 0.265 g (0.520 mmol) of Na(CyPNP) and 10 mL of thf. To the resulting pale yellow solution was added 0.122 g (0.519 mmol) of FeCl2(thf)1.5. The solution immediately became orange. To the solution, 1 atm of CO was added, which resulted in a darkening to a red color. The red

solution was allowed to stir overnight at room temperature during which time it lightened to orange. All volatiles were removed in vacuo, and the orange residue was washed with pentane and collected by filtration to afford 0.195 g (59%) of an orange microcrystalline solid. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a concentrated benzene solution. Mp: 193−195 °C. NMR: 1H δ 6.57 (s, 2H), 3.44 (dt, 2H), 3.07 (dt, 2H), 2.94 (m, 2H), 2.33 (app d, 2H), 2.03 (m, 2H), 1.93 (app d, 2H), 1.79 (app d, 2H), 1.68 (app d, 2H), 1.59 (m, 10H), 1.50 (m, 6H), 1.34 (m, 4H), 1.05 (m, 10H). 31P δ 80.7. IR: cm−1 2013 (CO), 1954 (CO). Anal. Calcd for C32H50ClFeNO2P2: C, 60.62; H, 7.95; N, 2.21. Found: C, 60.46; H, 7.89; N, 2.12. [FeMe(CyPNP)], 3. A flask was charged with 0.256 g (0.390 mmol) of [FeCl(py)(CyPNP)] and 10 mL of thf. To the dark yellow solution was added 0.54 mmol MeMgCl dropwise as a 2.4 M solution in thf. The reaction solution immediately became dark purple. The mixture was allowed to stir for 1 h at room temperature before all volatiles were removed in vacuo. The remaining brown residue was extracted into toluene and filtered through a pad of Celite. The resulting solution was evaporated to dryness, and the residue was washed with pentane to afford 0.174 g (80%) of a brown microcrystalline solid. Crystals suitable for X-ray diffraction were grown from a saturated pentane solution at room temperature. Mp: 179−181 °C. μeff = 2.7(2) μB. NMR: 1H δ −1.50 (4H), −2.49 (4H), −6.80 (4H), −6.92 (app d, 4H), −7.27 (app m, 4H), −12.22 (2H), −13.48 (4H), −14.11 (8H), −17.92 (4H), −28.29 (4H), −29.02 (4H), −61.50 (4H). Anal. Calcd for C31H53FeNP2: C, 66.78; H, 9.58; N, 2.51. Found: C, 66.40; H, 9.44; N, 2.45. [FeBn(CyPNP)], 4. A flask was charged with 0.052 g (0.079 mmol) of [FeCl(py)(CyPNP)] and 5 mL of thf. To the dark yellow solution was added 0.11 mmol BnMgCl dropwise as a 1.0 M solution in 2-Methf. The reaction became dark brown immediately and was allowed to stir for 1 h at room temperature. All volatiles were removed in vacuo, and the remaining brown residue was extracted into toluene and filtered through a pad of Celite. The toluene solution was evaporated to dryness, and the residue was washed with pentane to afford 0.014 g (28%) of a brown microcrystalline solid. Mp: 138−140 °C. NMR: 1H δ 50.94 (2H), 25.73 (1H), 8.72 (2H), −2.48 (4H), −3.12 (4H), −8.28 (4H), −8.41 (4H), −8.53 (4H), −10.58 (2H), −13.33 (4H), −13.92 (4H), −14.68 (4H), −22.25 (4H), −28.75 (4H), −34.73 (4H), −58.09 (4H). Anal. Calcd for C37H57FeNP2: C, 70.13; H, 9.07; N, 2.21. Found: C, 68.27; H, 9.17; N, 2.21. Repeated analyses (2×) returned low values for carbon despite repeated crystallization and satisfactory NMR spectra. [FePh(CyPNP)], 5. A flask was charged with 0.262 g (0.399 mmol) of [FeCl(py)(CyPNP)] and 10 mL of thf. To the dark yellow solution was added 0.56 mmol PhMgCl dropwise as a 2.8 M solution in thf. The solution became dark orange immediately. The mixture was allowed to stir for 1 h at room temperature before all volatiles were removed in vacuo. The remaining orange residue was extracted into toluene and filtered through a pad of Celite. The resulting solution was evaporated to dryness, and the residue was washed with pentane to afford 0.167 g (68%) of an orange microcrystalline solid. Crystals suitable for X-ray diffraction were grown from a saturated pentane solution at room temperature. Mp: 196−198 °C. μeff = 2.8(2) μB. NMR: 1H δ 67.66 (2H), 51.82 (1H), 16.84 (2H), −1.26 (d, 4H), −4.75 (4H), −4.94 (app d, 4H), −6.38 (d, 4H), −8.22 (4H), −11.24 (4H), −12.67 (d, 4H), −13.48 (4H), −16.03 (2H), −18.39 (4H), −29.27 (4H), −33.97 (4H), −65.81 (4H). Anal. Calcd for C36H55FeNP2: C, 69.78; H, 8.95; N, 2.26. Found: C, 67.33; H, 8.73; N, 2.26. Repeated analyses (3×) returned low values for carbon despite repeated crystallization and satisfactory NMR spectra. [Fe2(μ-H)2(CyPNP)2], 6. A flask was charged with 0.301 g (0.458 mmol) of [FeCl(py)(CyPNP)] and 10 mL of thf. To the dark yellow solution was added 0.458 mmol of NaEt3BH dropwise as a 1.0 M solution in toluene. The solution immediately became dark brown. The mixture was allowed to stir at room temperature for 1 h after which time all volatiles were removed in vacuo. The resulting brown residue was washed with benzene to afford 0.189 g (38%) of a brown microcrystalline solid. Crystals suitable for X-ray diffraction were F

DOI: 10.1021/acs.organomet.7b00144 Organometallics XXXX, XXX, XXX−XXX

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Organometallics grown by first allowing a vapor diffusion of pentane into a concentrated 2-methyl THF solution and then slowly cooling that solution to −30 °C. Mp: 182−184 °C. μeff = 8.2(2) μB. NMR(thf-d8): 1 H δ 46.1, 37.4 (br), 34.6 (br), 27.7 (br), 24.0, 18.9, 11.5, 11.2, 9.6, 9.0, 7.8, 7.2, 5.5, 3.8, 3.0, 2.8, 2.5, −0.7, −4.7, −6.1 (br), −8.3 (br), −10.7, −16.1 (br), −24.1 (br), −34.1 (br), −42.5 (br), −48.3, −58.5 (br). Anal. Calcd for C60H102Fe2N2P4: C, 66.29; H, 9.46; N, 2.58. Found: C, 66.36; H, 9.38; N, 2.51. [FeH(CO)2(CyPNP)], 7. A flask was charged with 0.112 g (0.170 mmol) of [FeCl(py)(CyPNP)] and 5 mL of toluene. The flask was capped with a septum, and 1 atm of CO was introduced. The dark yellow solution became dark red. To the red solution was added 0.17 mmol of NaEt3BH as a 1.0 M solution in toluene. The solution was allowed to stir for 8 h at room temperature. The mixture was filtered through a pad of Celite to generate a blue solution. All volatiles were removed in vacuo, and the crude product was obtained by crystallization from heptane at −30 °C. The compound precipitated as 0.043 g (42%) of blue crystals. Crystals of the pentane solvate suitable for X-ray diffraction were grown from pentane at room temperature. Mp: 173−175 °C. NMR: 1H δ 6.46 (s, 2H), 3.03 (app dt, 2H), 2.95 (app dt, 2H), 2.06−1.86 (m, 10H), 1.73−1.52 (m, 16H), 1.52−1.34 (m, 8H), 1.13 (m, 10H), −7.38 (t, 1H, 2JHP = 49.0 Hz). 31P δ 100.3. IR: cm−1 1988 (CO), 1931 (CO). Combustion analysis was consistent with the presence of fractional amounts of pentane. Anal. Calcd for C32H51FeNO2P2·1/2C5H12. C, 65.19; H, 9.04; N, 2.20. Found: C, 65.17; H, 9.05; N, 2.06. [FeEt(CO)2(CyPNP)], 8. A flask was charged with 0.099 g (0.15 mmol) of [FeCl(py)(CyPNP)] and 5 mL of thf. To the dark yellow solution was added 0.21 mmol of EtMgCl as a 1.0 M solution in thf. The reaction mixture became dark purple and was allowed to stir at room temperature for 15 min. All volatiles were removed in vacuo, and the purple residue was extracted into toluene and filtered through a pad of Celite into a flask. The flask was capped with a septum, and 1 atm of CO was introduced. The dark purple solution remained purple upon addition of CO. All volatiles were removed in vacuo leaving a dark purple residue. The residue was dissolved in warm heptane and set aside at −30 °C to cool during which time the desired compound precipitated as 0.035 g (37%) of a purple solid. Mp: 153−156 °C. NMR: 1H δ 6.66 (s, 2H), 3.18 (dt, 2H), 3.02 (dt, 2H), 2.90 (q, 2H), 2.18 (app d, 2H), 2.04−1.87 (m, 6H), 1.74−1.36 (m, 22H), 1.09 (m, 8H), 0.99 (m, 4H), 0.93 (t, 3H), 0.71 (app q, 2H). 31P δ 83.2. IR: cm−1 1904 (CO), 1877 (CO). Anal. Calcd for C44H61FeNO2P2: C, 65.07; H, 8.83; N, 2.23. Found: C, 65.29; H, 8.89; N, 2.12. [FePh(CO)2(CyPNP)], 9. A flask was charged with 0.102 g (0.155 mmol) of [FeCl(py)(CyPNP)] and 5 mL of thf. To the dark yellow solution was added 0.216 mmol of PhMgCl as a 1.0 M solution in thf. The reaction mixture became dark orange and was allowed to stir at room temperature for 1 h. All volatiles were removed in vacuo, and the orange residue was extracted into toluene and filtered through a pad of Celite into a flask. The flask was capped with a septum, and 1 atm of CO was introduced. The dark orange solution initially became dark green but soon changed to light brown. All volatiles were removed in vacuo. The light brown residue was washed with pentane to afford 0.079 g (68%) of a tan solid. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a concentrated benzene solution. Mp: 211−214 °C. NMR: 1H δ 8.11 (d, 1H), 7.06 (t, 1H), 7.01 (t, 1H), 6.97 (t, 1H), 6.67 (s, 2H), 6.08 (d, 1H), 3.07 (m, 4H), 2.11 (m, 2H), 2.05 (m, 4H), 1.96 (m, 2H), 1.66 (m, 6H), 1.56 (m, 6H), 1.44 (m, 6H), 1.35 (app t, 2H), 1.14 (m, 6H), 1.04 (m, 4H), 0.91 (m, 2H), 0.77 (m, 4H). 31P δ 84.4. IR: cm−1 1989 (CO), 1927 (CO). [Fe(CO)2(CyPNP)], 10. Method A. A flask was charged with 0.095 g (0.15 mmol) of [FeCl(CO)2(CyPNP)] and 5 mL of toluene. To the orange solution was added 0.15 mmol of NaBEt3H as a 1.0 M solution in toluene. The reaction mixture immediately became dark blue and was allowed to stir for 4 h at room temperature. After this time, the mixture was filtered to remove insoluble sodium salts leaving a blue solution. All volatiles were removed in vacuo, and the blue residue was subjected to crystallization from concentrated pentane, affording 0.060 g (67%) of blue crystals.

Method B. A flask was charged with 0.282 g (0.429 mmol) of [FeCl(py)(CyPNP)] and 10 mL of toluene. The flask was capped with a rubber septum, and 1 atm of CO was introduced resulting in a color change to dark red. To the solution was added 0.058 g (0.43 mmol) of KC8 as a solid in one portion causing an immediate color change to black. The mixture was allowed to stir at room temperature for 8 h during which time it took on a green appearance. All volatiles were removed in vacuo, and the green residue was extracted into pentane. The blue pentane solution was separated from a yellow solid (2) by filtration, and all volatiles were removed in vacuo. This process resulted in 0.118 g (46%) of product. Much of the remaining mass balance corresponds to unreacted 2. Crystals of the heptane solvate suitable for X-ray diffraction were grown by slow cooling of a hot heptane solution at room temperature. Mp: 123−125 °C. μeff = 2.0(2) μB. NMR: 1H δ 4.6 (br), 4.3 (br), −0.4 (br). IR: cm−1 1945 (CO), 1877 (CO). Combustion analysis was consistent with the presence of fractional amounts of heptane. Anal. Calcd for C32H50FeNO2P2·1/ 4C7H16: C, 65.00; H, 8.73; N, 2.25. Found: C, 65.05; H, 8.76; N, 2.02. K[Fe(CO)2(CyPNP)]·thf, 11. A flask was charged with 0.106 g (0.163 mmol) of [FeCl(py)(CyPNP)] and 5 mL of thf. The flask was capped with a rubber septum, and 1 atm of CO was introduced. To the resulting red solution was added 0.109 g (0.806 mmol, 5 equiv) of KC8 as a solid in one portion causing a color change to black. The mixture was allowed to stir at room temperature for 8 h. Filtration of the reaction mixture produced an orange solution that was evaporated to dryness. The remaining red-orange residue was washed with pentane to afford 0.056 g (49%) of a microcrystalline solid. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a concentrated thf solution. Mp: 286−288 °C. NMR(thf-d8): 1H (all peaks broad) δ 5.64 (2H), 2.75 (4H), 2.08 (4H), 1.26 (16), 0.86 (4H). 31 P δ 110.3. IR: cm−1 1787 (CO), 1731 (CO). Anal. Calcd for C36H58FeNO3P2: C, 60.92; H, 8.24; N, 1.97. Found: C, 59.91; H, 8.32; N, 2.27. The low value of carbon is consistent with partial loss of thf from the complex during analysis.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00144. NMR spectra of all complexes; UV−vis−NIR spectrum of 1; cyclic voltammogram of 1; thermal ellipsoid renderings of 2, 5, and 7; ball-and-stick diagram of 11; and tables of crystallographic data and refinement parameters (PDF) Crystallographic information files (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zachary J. Tonzetich: 0000-0001-7010-8007 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Welch Foundation (AX-1772) for financial support of this work. The CENTC Elemental Analysis Facility is supported by NSF (CHE-0650456). Dr. Jacob Przyojski is acknowledged for assistance with experimental procedures, and Mr. Ian Davis is thanked for assistance with EPR measurements.

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00144 Organometallics XXXX, XXX, XXX−XXX