Iron Pincer Complexes Incorporating Bipyridine: A Strategy for

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Iron Pincer Complexes Incorporating Bipyridine: A Strategy for Stabilization of Reactive Species C. Vance Thompson,† Ian Davis,† Jordan A. DeGayner,‡ Hadi D. Arman,† and Zachary J. Tonzetich*,† †

Department of Chemistry, University of Texas at San Antonio (UTSA), San Antonio, Texas 78249, United States Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States

‡

S Supporting Information *

ABSTRACT: Treatment of FeCl2(thf)1.5 with the pincer ligand bis(dicyclohexylphosphinomethyl)pyrrolide (CyPNP) in the presence of 2,2′-bipyridine (bipy) affords the sixcoordinate complex [FeCl(bipy)(CyPNP)]. The chloride complex exhibits spin-crossover behavior and serves as a starting point for a series of bipy-coordinated iron(II) pincer compounds, including the hydride species [FeH(bipy)(CyPNP)]. Chemical reduction of [FeCl(bipy)(CyPNP)] generates the new complex [Fe(bipy)(CyPNP)], which is found to be consistent with a structure featuring iron(I) as opposed to a bipy radical anion. Reactivity studies with the bipy complexes demonstrate that they are more prone to migratory insertion and β-hydrogen elimination reactions than the corresponding dicarbonyl analogues and that in certain instances the bipy ligand can be sequestered by treatment with a Lewis acid. These observations demonstrate that coordination of bipyridine to pincer-ligated iron(II) species provides a means of stabilizing otherwise unstable compounds while permitting a higher degree of reactivity than is possible with carbonyl coligands.

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INTRODUCTION Hydrofunctionalization strategies with earth-abundant transition-metal catalysts often make use of metal hydride species that are capable of activating olefins and other polar unsaturated functionalities through migratory insertion.1−10 In this vein, many successful catalyst designs have employed pincer-ligated metal hydride complexes to carry out olefin and carbonyl hydrofunctionalization.11−25 In the course of our recent investigations of iron complexes containing a pyrrolebased pincer ligand (RPNP),26−30 we observed that an iron(II) hydride species of potential use in catalysis was unstable with respect to formation of the bimetallic compound displayed in Chart 1.31 Incorporation of carbonyl ligands imparted improved stability to the monometallic hydride species as well as a variety of additional compounds of the CyPNP ligand, including a five-coordinate Fe(I) complex. Recognizing the difficulty in removing or labilizing CO ligands to generate more reactive iron compounds, we have now shifted our focus to target six-coordinate pincer complexes of iron with less strongly Chart 1. Bimetallic Iron(II) Hydride Complex of

associating Lewis bases. We hypothesized that the inclusion of such bases would permit access to otherwise unstable complexes and that dissociation of the base would unmask more reactive species. In pursuit of this goal, we have examined the chemistry of iron CyPNP complexes containing 2,2′-bipyridine. Although bipyridine units serve as common design elements in several pincer systems,32,33 the molecule itself has not been used to a great extent in conjunction with pincer ligands.34−37 Our findings demonstrate a considerable difference in reactivity between the bipy systems and those of the related dicarbonyl compounds, highlighting the coordinative flexibility in the case of the former.

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RESULTS AND DISCUSSION Synthesis. Previously, we reported that delivery of the sodium salt of CyPNP to iron(II) requires coordination of an additional Lewis base.31 Pyridine was successful in stabilizing a five-coordinate high-spin species, whereas metalation of CyPNP in the presence of an atmosphere of carbon monoxide resulted in a six-coordinate low-spin compound. Accordingly, 2,2′bipyridine (bipy) was found to stabilize a new six-coordinate iron(II) species when it was employed during metalation with Na(CyPNP) (eq 1). The new complex, [FeCl(bipy)(CyPNP)] (1), was isolated in high yield as a dark green crystalline solid on scales up to 1 g. Alternatively, 1 could be prepared by direct treatment of [FeCl(py)(CyPNP)] with bipy.

Cy

PNP

Received: October 17, 2017

© XXXX American Chemical Society

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DOI: 10.1021/acs.organomet.7b00772 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Crystallization of 1 from thf/pentane mixtures afforded dark green crystals suitable for X-ray diffraction. The solid-state structure of 1 is depicted in Figure 2. Comparison of the bond

Given the relatively strong ligand field in 1, we were surprised to find that the complex displayed a paramagnetically broadened 1H NMR spectrum at room temperature with resonances between 0 and 55 ppm. Solution magnetic susceptibility measurements in thf-d8 revealed an effective magnetic moment of 3.0(2) μB at 298 K. In order to investigate the magnetic ground state of 1 in the solid state, variabletemperature dc magnetic susceptibility measurements were carried out on a polycrystalline sample under applied dc fields of 0.5 and 2 T from 1.8 to 300 K and 1 T from 1.8 to 340 K. The resulting plots of χMT vs T are shown in Figure 1. At 340

Figure 2. Thermal ellipsoid drawing (50%) of the solid-state structure of 1. Hydrogen atoms, cyclohexane carbon atoms, minor components of the disorder, and a cocrystallized pyridine molecule are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe(1)−N(1) = 1.957(3); Fe(1)−N(2) = 1.938(3); Fe(1)−N(3) = 1.964(3); Fe(1)− Pavg. = 2.313(1); Fe(1)−Cl(1) = 2.368(1); P(1)−Fe(1)−P(2) = 159.00(5); N(1)−Fe(1)−N(3) = 174.2(1); N(2)−Fe(1)−Cl(1) = 174.1(1).

metrics with those of high-spin [FeCl(py)(CyPNP)] reveals that the Fe−N(1) and Fe−P distances are significantly contracted despite the larger coordination number (cf. 2.034(2) and 2.538(1) Å, respectively, in [FeCl(py)(CyPNP)]), consistent with a low-spin complex. For comparison, the bond distances in 1 are in line with those found in the low-spin carbonyl species [FeCl(CO)2(CyPNP)] and [FeCl(CO)(PMe3)(iPrPNP)].28,31 Further support for the spin state of 1 was provided by electronic absorption spectroscopy. The UV−vis−NIR spectrum in tetrahydrofuran displays two bands at 14300 and 22400 cm−1 (see the Supporting Information). The energies of these transitions are much higher than those observed for high-spin [FeCl(py)(CyPNP)] (cf. 6020 and 13200 cm−1) and are consistent with low-spin iron(II) complexes bearing polyamine ligands.46,47 Similar to the case for 1, such iron(II) polyamine compounds display spin-crossover behavior and possess magnetic moments that vary as a function of temperature, solvent medium, and ligand substitution.48 Cyclic voltammetry of 1 in thf displays a complex set of irreversible electrode events corresponding to both oxidation and reduction (see the Supporting Information). Additionally, a reversible reduction is present at very low potential (E1/2 = −2.75 V vs Fc/Fc+). Despite the presence of the bipyridine ligand in 1, we favor a description of these redox events as originating predominantly from the metal on the basis of behavior comparable with that of the related iron(I) complex (vide infra). In order to examine the possible redox congeners of 1, we subjected the complex to treatment with 1 equiv of KC8 in thf. The product resulting from this reaction was found to be the iron(I) species [Fe(bipy)(CyPNP)] (2; eq 2). Use of greater than 1 equiv of KC8 likewise gave rise to compound 2. This behavior is in contrast to findings with the related iron(I)

Figure 1. Variable-temperature dc magnetic susceptibility for 1 collected under an applied field of 1 T. Inset: low-temperature variable-temperature dc magnetic susceptibility collected at the indicated applied fields.

K, compound 1 exhibits a χMT value of 2.30 cm3 K mol−1, slightly lower than that expected for a high-spin Fe(II) ion. As the temperature is decreased, the data undergo a rapid decline before reaching a plateau near 100 K with a χMT value of approximately 0.4 cm3 K mol−1. This behavior is consistent with spin crossover where the high-spin state is only partially populated at 340 K and rapidly depopulated upon lowering the temperature. The low-temperature plateau indicates that a small proportion, likely near 10%, of the Fe(II) centers remains trapped in the high-spin state, as has been observed for a number of other spin-crossover complexes.38−44 While the high-temperature behaviors of 1 are identical under the various magnetic fields probed, the low-temperature data reveal a significant field-dependent downturn (see Figure 1, inset, and Figure S1 in the Supporting Information). This is consistent with a magnetically anisotropic, high-spin Fe(II) center and further supports the ∼100 K plateau arising from a trapped high-spin subset rather than a stoichiometric S = 1/2 impurity.45 B


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complex [Fe(CO)2(CyPNP)], which generated an iron(0) species upon treatment with additional amounts of reductant. Compound 2 is a brown solid that displays improved solubility in arene solvents in comparison to 1. The effective magnetic moment in benzene-d6 solution is 2.4(2) μB, consistent with an S = 1/2 paramagnet. The solid-state structure of the compound is shown in Figure 3. Bond metrics about the bipy ligand are

Figure 4. X-band EPR spectrum (black line) of 2 as a 2-Methf glass at 20 K and its simulation (red line).

width (see the Supporting Information for further details of the fit). In contrast to 1, the electrochemical behavior of 2 proved more straightforward. The cyclic voltammogram of 2 in thf (Figure 5) demonstrates two reversible events below −1 V. We assign the first of these events at −1.77 V to the Fe(I)/Fe(II) couple. The low potential for this oxidation is consistent with the conditions required for generation of 2 and reflects the poorer π acidity of the bipy ligand. For example, the analogous

Figure 3. Thermal ellipsoid drawing (50%) of the solid-state structure of one of the two crystallographically independent molecules of 2 in the asymmetric unit. Hydrogen atoms and cyclohexane carbon atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe(1)−N(1) = 1.966(2); Fe(1)−N(2) = 1.964(2); Fe(1)−N(3) = 1.915(2); Fe(1)−P(1) = 2.3228(9); Fe(1)−P(2) = 2.2794(8); P(1)− Fe(1)−P(2) = 144.38(3); N(1)−Fe(1)−N(3) = 174.4(1). See the Supporting Information for bipy bond metrics.

altered slightly from those in 1, indicating increased backbonding into the bipy π* orbitals, but they do not evince substantial ligand-based radical character (see the Supporting Information). We therefore regard 2 as an Fe(I) complex as opposed to an FeII(bipy•−) species. To better establish the nature of the oxidation state in 2, its EPR spectrum was recorded as a 2-Methf glass at 20 K. The spectrum is displayed in Figure 4 and shows an apparent axial signal with slight rhombicity further consistent with an authentic iron(I) species as opposed to an FeII(bipy•−) radical anion. Moreover, the observed g values for 2 of 2.170 and 2.038 compare well with those of the related iron(I) complexes [Fe(CO)2(CyPNP)] and [Fe(CO)2(tBuPNP)].27,31 Unlike the carbonyl complexes, however, hyperfine coupling to the phosphorus atoms is not well-resolved in 2 and is only observed as broadening of the low-field resonance. Though the signal appears axial, adequate simulation requires slight rhombicity with g values of 2.183, 2.177, and 2.038 as well as anisotropic H strain from unresolved hyperfine coupling with values of 158, 5, and 15 MHz, respectively, and 2.5 mT line

Figure 5. Cyclic voltammogram of 2 in thf at a glassy-carbon electrode. The scan rate was 50 mV/s, and the supporting electrolyte was 0.1 M Bu4NPF6. C


Article

Organometallics redox event in the dicarbonyl complex [Fe(CO)2(CyPNP)] is found ca. 1.3 V more positive at −0.45 V. The second reversible event observed for 2 at −2.84 V is assigned to the Fe(I)/Fe(0) couple. The exceedingly low value for this reduction likely accounts for our inability to isolate the corresponding iron(0) complex. With compound 1 in hand, we next focused on the preparation of organometallic species relevant to catalysis. The four-coordinate iron(II) hydride complex of CyPNP is unstable, transforming into the bimetallic species [Fe2(μH)2(μ-CyPNP)2] (Chart 1). A low-spin, six-coordinate hydride complex of CyPNP can be stabilized with two CO ligands, although dissociation of carbonyl to generate an open coordination site for reactivity is not facile. Therefore, the sixcoordinate bipyridine hydride complex provided an attractive target. The reaction of compound 1 with NaEt3BH in thf afforded [FeH(bipy)(P2CyPyr)] (3) in satisfactory yields (eq 3).

Direct alkylation of compound 1 with Grignard reagents failed to provide the analogous six-coordinate alkyls. Instead, treatment of 1 with methyl and phenyl Grignard reagents afforded the previously reported four-coordinate, intermediatespin species [FeR(CyPNP)] (R = Me, Ph). We surmised that magnesium salt byproducts of the alkylation reaction were likely sequestering the bipyridine ligand from the iron center. Use of n-BuLi in place of a Grignard reagent produced a mixture of compounds 2 and 3, presumably through a combination of outer-sphere electron transfer to give 2 and direct alkylation to give the putative butyl species accompanied by β-hydrogen elimination to form 3. Despite this mixture, both products retain the bipy ligand in contrast to reactions with Grignard, further implicating capture of Mg2+ by bipyridine. Fortunately, treatment of the four-coordinate species [FeMe(CyPNP)] with 1 equiv of bipyrdine generated the desired alkyl complex [FeMe(bipy)(CyPNP)] in satisfactory yield (eq 4).

Compound 4 is low spin in analogy to the related carbonyl complex [FeMe(CO)2(CyPNP)]. Low-quality X-ray diffraction data were obtained on crystals of 4·bipy and appears in the Supporting Information. The appearance of additional bipyridine in the unit cell most likely arises from dissociation during crystal growth. We attribute the increased lability of the bipy ligand in 4 versus 3 to the increased steric congestion about the metal, a situation that is exacerbated in the phenyl complex. Analogous reactions with [FePh(CyPNP)] and bipyridine did not afford the corresponding six-coordinate complex as the sole product but resulted in an equilibrium mixture (eq 5).

Compound 3 is a diamagnetic brown solid that displays a chemical shift for the hydride ligand at −11.51 ppm with a coupling to the phosphorus atoms of 75 Hz. The solid-state structure (Figure 6) displays an octahedral geometry about iron with the hydride ligand in a trans position with respect to one of the bipy nitrogen atoms, similar to the case for 1. Electron density for the hydride ligand was detected in the difference map and refined isotropically to give an Fe−H distance of 1.46(8) Å.

Reactivity Studies. To assess the chemistry of the bipy complexes in comparison to their dicarbonyl analogues, we first examined the reaction of the hydride compounds with CO2. Treatment of [FeH(CO)2(CyPNP)] with 1 atm of CO2 in benzene-d6 resulted in no reactivity after several hours, as judged by NMR spectroscopy. In contrast, similar treatment of 3 with CO2 led to immediate consumption of the hydride and formation of an insoluble material we assign as a formate complex. Due to the intractability of the putative formate, we have not been able to obtain definitive characterization data. However, the IR spectrum of the solid material as a KBr pellet demonstrates a strong vibrational band at 1594 cm−1 along with peaks attributable to the CyPNP ligand consistent with the proposal of a formate complex (see the Supporting Information).12,49,50 Unexpectedly, the attempted independent synthesis of the iron(II) formate complex via reaction of 1 with TlO2CH led to isolation of the hydride complex 3. We interpret this result as shown in Scheme 1. Dissociation of bipy from 3 permits reversible migratory insertion of CO2 into the

Figure 6. Thermal ellipsoid drawing (50%) of the solid-state structure of 3. Hydrogen atoms except for Fe−H are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe(1)−H = 1.46(8); Fe(1)−N(1) = 1.980(4); Fe(1)−N(2) = 1.963(3); Fe(1)−N(3) = 1.924(4); Fe(1)−P(1) = 2.2262(9); P(1)−Fe(1)−P(1A) = 157.69(5); N(1)−Fe(1)−N(3) = 171.21(15). D


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Organometallics

(vide infra) by treatment with BH3·thf were also unsuccessful.51 To demonstrate sequestration of the bipy ligand by Lewis acids, we next turned to compound 4, reasoning that the result of bipy loss would provide a stable complex in the form of [FeMe(CyPNP)]. Indeed, treatment of a thf solution of complex 4 with MgCl2 resulted in smooth formation of [FeMe(CyPNP)], as judged by 1H NMR (eq 6).

Scheme 1. CO2 Insertion with [FeH(bipy)(CyPNP)]

Fe−H bond. Under an atmosphere of CO2, this equilibrium is driven forward, resulting in precipitation of an iron formate complex. Upon treatment of 1 with stoichiometric formate, however, the resulting equilibrium lies toward the hydride complex such that β-H elimination produces 3 and CO2. In order to further test the propensity for bipy dissociation and β-hydride elimination, we next considered compounds with ethyl ligands. As noted previously, the dicarbonyl ethyl complex [FeEt(CO)2(CyPNP)] is stable with respect to β-hydride elimination.31 We therefore sought to prepare the bipy analogue to examine whether or not it shows similar stability. Metalation of [FeCl(py)(CyPNP)] with EtMgCl in thf at low temperature afforded metastable [FeEt(CyPNP)], which can be observed by NMR but undergoes β-hydride elimination upon standing at room temperature.31 Rapid workup to remove the magnesium salts and addition of bipyridine afforded a purple solution that we assign as [FeEt(bipy)(CyPNP)] in analogy to the methyl analogue 4. Upon warming of the solution to room temperature for NMR analysis, the solution changed from purple to brown and the 1H NMR spectrum revealed the hydride complex 3 along with ethylene (Scheme 2 and see the

During the course of preparing compound 3, we found that NaBH4 was also capable of generating the hydride from 1. Borohydride is typically eschewed in metal hydride syntheses due to its weaker hydride donor ability.52,53 In fact, reaction of NaBH4 with [FeCl(py)(CyPNP)], which contains a pyridine ligand instead of bipyridine, did not result in formation of a hydride but rather the borohydride complex 5 (eq 7). We have

not obtained crystals of 5 suitable for X-ray diffraction but believe the compound to be an intermediate-spin complex (S = 1) possessing a κ2-BH4 ligand on the basis of spectroscopic characterization and analogy to other pincer systems.54,55 Treatment of the iron borohydride complex 5 with bipy afforded hydride 3 cleanly, as judged by 1H and 31P NMR spectroscopy (eq 8). This reaction demonstrates the probable

Scheme 2. β-H Elimination from [FeEt(bipy)(CyPNP)]

reaction pathway for formation of 3 from 1 and NaBH4. Initial dissociation of bipy 1 is followed by salt metathesis of the chloride ligand to generate 5. Reassociation of bipy results in loss of BH3 and formation of 3.

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Supporting Information). Therefore, unlike the case for [FeEt(CO)2(CyPNP)], reversible dissociation of the ancillary ligand in [FeEt(bipy)(CyPNP)] must occur readily to allow βhydride elimination to proceed. Furthermore, subsequent reassociation of bipy traps the hydride species as 3, circumventing formation of the bimetallic hydride (Chart 1), which otherwise forms in the case of [FeEt(CyPNP)] decomposition. Attempts to intentionally sequester the bipy ligand in 3 by treatment with the Lewis acid MgBr2·Et2O, resulted in an immediate reaction but did not lead to formation of a welldefined hydride species. NMR spectra of the resulting products indicated a mixture of paramagnetic species that do not include the bimetallic hydride shown in Chart 1. Efforts to trap a putative square-planar hydride as the borohydride complex

CONCLUSIONS In conclusion, we have demonstrated a new class of iron pincer complexes incorporating a 2,2′-bipyridine ligand. The complexes retain many favorable attributes of related coordinatively saturated systems such as stability and diamagnetism but offer the added advantage of greater reactivity because of the possibility of bipy dissociation. In the case of the hydride complex 3, the presence of the bipy ligand prevents aggregation to a bimetallic hydride yet permits reactivity with carbon dioxide that is not observed with the dicarbonyl derivative. Likewise, the reduced ligand-field strength of the bipy ligand versus carbonyl also gives rise to spin-crossover behavior in the case of compound 1. It is envisioned that the use of the bipy E


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Organometallics

model except for the hydride ligand in compounds 3, which was identified in the difference map and refined isotropically. Crystallographic data and refinement parameters for each structure can be found in the Supporting Information. [FeCl(bipy)(CyPNP)] (1). A round-bottom flask was charged with 0.989 g of Na(CyPNP) (1.94 mmol), 0.318 g of 2,2′-bipyridine (2.04 mmol), and 25 mL of THF. To the resulting pale yellow solution was added 0.456 g of FeCl2(thf)1.5 (1.94 mmol). The solution immediately became dark green and was stirred for 2 h at 23 °C. The dark green reaction mixture was filtered through a pad of Celite and evaporated to dryness. The remaining residue was washed with pentane and collected by filtration to afford 1.303 g (91%) of dark green solid. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a concentrated THF solution at room temperature. Mp: 228−230 °C. μeff = 3.0(2) μB. 1H NMR (300 MHz, thf-d8): δ 52.7 (br), 34.4, 25.8, 20.7, 18.8, 18.3, 17.7, 17.4, 14.5, 9.8, 6.2, 4.9, 4.5, 4.0, 3.8, 2.4. Anal. Calcd for C40H58ClFeN3P2: C, 65.44; H, 7.96; N, 5.72. Found: C, 64.96; H, 8.03; N, 5.51. [Fe(bipy)(CyPNP)] (2). A scintillation vial was charged with 0.099 g of 1 (0.14 mmol) and 5 mL of THF. To the dark green solution was added 0.017 g of KC8 (0.14 mmol) in one portion. The mixture immediately became dark purple with an apparent precipitate. The mixture was stirred for 1 h at 23 °C, after which time all volatiles were removed in vacuo. The resulting dark residue was extracted into toluene and filtered through a pad of Celite to give a deep purple solution. The purple solution was evaporated to dryness and then washed with pentane to generate a brown solid. The brown solid was collected by filtration to afford 0.047 g (50%) of the desired compound. Crystals suitable for X-ray diffraction were grown from a saturated heptane solution at room temperature. Mp: 199−201 °C. 1H NMR (300 MHz, C6D6): δ 41.8, 15.4, 9.8, 5.9, 5.7, 4.4, 3.9, 1.0, 0.5, −2.6, −13.5. μeff = 2.4(2) μB. Anal. Calcd for C40H58FeN3P2: C, 68.76; H, 8.37; N, 6.01. Found: C, 67.95; H, 8.30: N, 5.77. [FeH(bipy)(CyPNP)] (3). A round-bottom flask was charged with 0.564 g of 1 (0.767 mmol) and 20 mL of THF. To the dark green solution was added 800 μL of a 1.0 M NaBEt3H solution in toluene (0.80 mmol). The solution immediately became dark brown/bronze and was stirred at 23 °C for 12 h. All volatiles were removed in vacuo, and the resulting brown residue was extracted into 15 mL of toluene. The extract was filtered through a pad of Celite and then evaporated to dryness. The crude residue was washed with pentane and collected by filtration to afford 0.438 g (82%) of a brown solid. Crystals suitable for X-ray diffraction were grown from a saturated heptane solution at room temperature. Mp: 219−221 °C. 1H NMR (500 MHz, C6D6): δ 9.72 (d, 1H, bipy), 8.06 (d, 1H, bipy), 7.47 (d, 1H, bipy), 7.30 (d, 1H, bipy), 6.95 (t, 1H, bipy), 6.82 (t, 1H, bipy), 6.76 (t, 1H, bipy), 6.73 (s, 2H, pyr-CH), 6.41 (t, 1H, bipy), 3.11 (app d, 2H, CH2), 2.94 (app dt, 2H, CH2), 2.34 (app d, 2H, Cy), 2.05 (app q, 2H, Cy), 1.94 (m, 6H, Cy), 1.78 (app t, 4H, Cy), 1.55 (app q, 2H, Cy), 1.37 (overlapping m, 14H, Cy), 1.02 (app d, 2H, Cy), 0.81 (app d, 2H, Cy), 0.66 (m, 6H, Cy), 0.22 (m, 4H, Cy), −11.51 (t, JHP = 75 Hz, FeH). 31P NMR (C6D6): δ 81.2. Anal. Calcd for C40H59FeN3P2: C, 68.66; H, 8.50; N, 6.01. Found: C, 68.07; H, 8.51; N, 6.00. [Fe(BH4)(CyPNP)] (4). A flask was charged with 0.230 g (0.350 mmol) of [FeCl(py)(CyPNP)] and 10 mL of DME. To the dark yellow solution was added 0.014 g (0.37 mmol) of NaBH4. The mixture was stirred for 12 h, resulting in a dark brown solution with a white precipitate. All volatiles were removed in vacuo, and the remaining brown residue was extracted in toluene and filtered through a pad of Celite. The filtered solution was evaporated to dryness, and the residue was washed with pentane to afford 0.127 g (65%) of a brown microcrystalline solid. Mp: 180−183 °C dec. μeff = 3.3(2) μB. IR (KBr, cm−1): 2405, 2367 (terminal νBH) 2021, 1940 (bridging νBH). 1H NMR (300 MHz, toluene-d8): δ 0.29 (4H), 2.36 (app d, 4H), 5.68 (app d, 4H), 6.50 (app br q, 4H), 7.72 (app q, 4H), 12.02 (4H), 12.88 (app d, 4H), 15.40 (4H), 20.55 (4H), 23.97 (4H), 24.60 (4H), 33.67 (2H, pyr-CH), 72.48 (4H). Anal. Calcd for C30H54BFeNP2: C, 64.65; H, 9.77; N, 2.51. Found: C, 64.87; H, 9.78; N, 2.20. [FeMe(bipy)(CyPNP)] (5). A scintillation vial was charged with 0.056 g of [FeMe(CyPNP)] (0.10 mmol) and ∼5 mL of toluene. To

ligand may be a successful strategy in attenuating decomposition pathways of catalytically relevant intermediates by trapping low-coordinate species as dormant states that are capable of re-entering the catalytic cycle upon bipy dissociation. Such a strategy is under current investigation in our laboratory.

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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). Benzene, benzene-d6, n-heptane, and 2-methyltetrahydrofuran were dried over sodium and vacuum-distilled prior to use. 1H NMR spectra were recorded on a Varian spectrometer operating at 300 or 500 MHz (1H) and referenced to the residual protium resonance of the solvent (δ 7.16 ppm for benzene-d6; 2.09 ppm for toluene-d8; 1.58 ppm for thfd8). For selected paramagnetic compounds, only peak maxima are listed. 31P NMR spectra were recorded at 202 MHz and referenced automatically using the 2H lock frequency. FT-IR spectra were recorded with a ThermoNicolet iS 10 spectrophotometer in benzened6 solution using an airtight liquid transmission cell (Specac OMNI) with KBr windows. UV−vis−NIR spectra were recorded on a Cary-60 (UV−vis) or Cary-5000 (UV−vis−NIR) spectrophotometer in Teflon-capped quartz cells. The X-band EPR spectrum was recorded on a Bruker E560 spectrometer in a 4 mm o.d. quartz tube at 20 K, with 2 μW microwave power, 100 kHz modulation frequency, and 0.6 mT modulation amplitude. Temperature control was maintained with an Oxford Mercury iTC cryogen-free cryostat. EPR simulation was performed with the Matlab plug-in EasySpin.56 Solid-state magnetic measurements of 1 were performed on a polycrystalline sample sealed in a 2 mL polyethylene bag under a N2 atmosphere. All data were collected using a Quantum Design MPMS-XL SQUID magnetometer from 1.8 to 340 K at applied dc fields ranging from 0.5 to +2 T. dc Susceptibility data were corrected for diamagnetic contributions from the sample holder, and for the core diamagnetism of each sample was estimated using Pascal’s constants.57 Solution magnetic susceptibility measurements were performed using the Evans method with reported diamagnetic corrections.57 Cyclic voltammetry was performed in the glovebox at 23 °C on a CH Instruments 620D electrochemical workstation. A three-electrode setup was employed comprising a 2 mm glassy-carbon-disk working electrode, Pt-wire auxiliary electrode, and a 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. Elemental analyses were performed by the CENTC facility at the University of Rochester. In each case, recrystallized material was used for combustion analysis. Materials. FeCl 2(thf)1.5,58 Na(CyPNP), [FeCl(py)(CyPNP)], [FeMe(CyPNP)], [FePh(CyPNP)],31 and potassium graphite (KC8)59 were prepared according to published procedures. Carbon dioxide gas was obtained from Airgas and delivered to 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.60 Data processing and absorption correction, giving minimum and maximum transmission factors, were accomplished with CrysAlisPro61 and SCALE3 ABSPACK,62 respectively. The structure, using Olex2,63 was solved with the ShelXT64 structure solution program using direct methods and refined (on F2) with the ShelXL65 refinement package using fullmatrix least-squares techniques. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atom positions were determined by geometry and refined by a riding F


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Organometallics the yellow solution was added 0.016 g (0.10 mmol) of 2,2′-bipyridine. The solution immediately became dark purple and was stirred for 12 h at 23 °C. All volatiles were removed in vacuo, and the purple residue was washed with pentane and collected by filtration to afford 0.041 g (57%) of a purple solid. Crystals suitable for X-ray diffraction were grown by slow cooling of a saturated Et2O solution at −30 °C. Mp: 143−146 °C. 1H NMR (300 MHz, toluene-d8): δ 9.71 (d, 1H, bipy), 8.26 (d, 1H, bipy), 7.38 (d, 1H, bipy), 7.30 (d, 1H, bipy), 6.97 (t, 1H, bipy), 6.86 (t, 1H, bipy), 6.80 (s, 2H, pyr-CH), 6.79 (t, 1H, bipy), 6.65 (t, 1H, bipy), 3.03 (app t, 4H, CH2), 2.32 (app dd, 2H, Cy), 1.71 (app dd, 4H, Cy), 1.58 (app m, 8H, Cy), 1.42 (app m, 8H, Cy), 1.30 (app t, 4H, Cy), 1.22 (t, JHP = 9.1 Hz, 3H, Me), 1.13 (app m, 4H, Cy), 0.93 (app t, 2H, Cy), 0.74 (app m, 6H, Cy), 0.40 (app m, 4H, Cy), −0.05 (app d, 2H, Cy). 31P NMR (toluene-d8): δ 50.0. Anal. Calcd for C40H59FeN3P2: C, 68.99; H, 8.61; N, 5.89. Found: C, 68.25; H, 8.46; N, 5.55. [FePh(bipy)(CyPNP)] (6). This compound was observed in situ by treatment of a solution of [FePh(CyPNP)] with 1 equiv of bipy. 1H NMR (300 MHz, C6D6): δ 10.18 (d, 1H, bipy), 8.88 (d, 1H, bipy), 8.33 (d, 1H, bipy), 7.52 (t, 1H, Ph), 7.34 (d, 1H, bipy), 6.92 (t, 1H, bipy), 6.87 (s, 2H, pyr-CH), 6.86 (t, 1H, bipy), 6.68 (t, 2H, Ph), 6.63 (t, 1H, bipy), 6.52 (t, bipy), 6.47 (d, 2H, Ph), 3.04 (app dt, 2H, CH2), 2.76 (app dt, 2H, CH2), 2.40 (app t, 2H, Cy), 1.53 (m, 12H, Cy), 1.39 (m, 2H, Cy), 1.20 (m, 8H, Cy), 1.00 (app t, 4H, Cy), 0.88 (app d, 2H, Cy), 0.77 (m, 8H, Cy), 0.26 (app q, 2H, Cy), 0.04 (app d, 2H, Cy), −1.29 (app d, 2H, Cy); 31P NMR (toluene-d8): δ 51.9. General Procedure for NMR Experiments. NMR-scale experiments with compounds 1−5 were conducted as follows. In a scintillation vial or small flask, iron pincer compound (20−50 μmol) was dissolved in 3 mL of the reaction solvent and subjected to the reagent in question (CO2, bipy, MgCl2, etc.). In the case of CO2, ∼1 atm of the gas was introduced via needle/septum. The reaction mixtures were evacuated to dryness and extracted into benzene-d6 for direct NMR analysis. In the case of reactions performed directly in benzene-d6, no additional workup was carried out.

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the National Science Foundation (DMR-1351959). NMR instrumentation at UTSA is supported by grant CHE1625963 from the NSF. The CENTC Elemental Analysis Facility is supported by the NSF (CHE-0650456). We thank Dr. Aimin Liu for use of the EPR spectrometer.

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(1) Chakraborty, S.; Guan, H. Dalton Trans. 2010, 39, 7427−7436. (2) Obligacion, J. V.; Chirik, P. J. J. Am. Chem. Soc. 2013, 135, 19107−19110. (3) Chirik, P. J. Acc. Chem. Res. 2015, 48, 1687−1695. (4) Morris, R. H. Acc. Chem. Res. 2015, 48, 1494−1502. (5) Bleith, T.; Wadepohl, H.; Gade, L. H. J. Am. Chem. Soc. 2015, 137, 2456−2459. (6) Friedfeld, M. R.; Shevlin, M.; Margulieux, G. W.; Campeau, L.-C.; Chirik, P. J. J. Am. Chem. Soc. 2016, 138, 3314−3324. (7) Sun, J.; Deng, L. ACS Catal. 2016, 6, 290−300. (8) Wenz, J.; Wadepohl, H.; Gade, L. H. Chem. Commun. 2017, 53, 4308−4311. (9) Liu, Y.; Deng, L. J. Am. Chem. Soc. 2017, 139, 1798−1801. (10) Du, X. Y.; Huang, Z. ACS Catal. 2017, 7, 1227−1243. (11) Benito-Garagorri, D.; Kirchner, K. Acc. Chem. Res. 2008, 41, 201−213. (12) Langer, R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J. W.; BenDavid, Y.; Milstein, D. Angew. Chem., Int. Ed. 2011, 50, 9948−9952. (13) Semproni, S. P.; Milsmann, C.; Chirik, P. J. J. Am. Chem. Soc. 2014, 136, 9211−9224. (14) Scheuermann, M. L.; Semproni, S. P.; Pappas, I.; Chirik, P. J. Inorg. Chem. 2014, 53, 9463−9465. (15) Chakraborty, S.; Lagaditis, P. O.; Förster, M.; Bielinski, E. A.; Hazari, N.; Holthausen, M. C.; Jones, W. D.; Schneider, S. ACS Catal. 2014, 4, 3994−4003. (16) Zell, T.; Milstein, D. Acc. Chem. Res. 2015, 48, 1979−1994. (17) Chakraborty, S.; Bhattacharya, P.; Dai, H.; Guan, H. Acc. Chem. Res. 2015, 48, 1995−2003. (18) Murugesan, S.; Kirchner, K. Dalton Trans. 2016, 45, 416−439. (19) Bleith, T.; Gade, L. H. J. Am. Chem. Soc. 2016, 138, 4972−4983. (20) Bauer, G.; Hu, X. Inorg. Chem. Front. 2016, 3, 741−765. (21) Tokmic, K.; Markus, C. R.; Zhu, L.; Fout, A. R. J. Am. Chem. Soc. 2016, 138, 11907−11913. (22) Tokmic, K.; Fout, A. R. J. Am. Chem. Soc. 2016, 138, 13700− 13705. (23) Spentzos, A. Z.; Barnes, C. L.; Bernskoetter, W. H. Inorg. Chem. 2016, 55, 8225−8233. (24) Ibrahim, A. D.; Entsminger, S. W.; Zhu, L.; Fout, A. R. ACS Catal. 2016, 6, 3589−3593. (25) Vasilenko, V.; Blasius, C. K.; Wadepohl, H.; Gade, L. H. Angew. Chem., Int. Ed. 2017, 56, 8393−8397. (26) Kuriyama, S.; Arashiba, K.; Nakajima, K.; Matsuo, Y.; Tanaka, H.; Ishii, K.; Yoshizawa, K.; Nishibayashi, Y. Nat. Commun. 2016, 7, 12181. (27) Ehrlich, N.; Kreye, M.; Baabe, D.; Schweyen, P.; Freytag, M.; Jones, P. G.; Walter, M. D. Inorg. Chem. 2017, 56, 8415−8422. (28) Holland, A. M.; Oliver, A. G.; Iluc, V. M. Acta Crystallogr., Sect. C: Struct. Chem. 2017, 73, 569−574. (29) Nakajima, K.; Kato, T.; Nishibayashi, Y. Org. Lett. 2017, 19, 4323−4326. (30) Ehrlich, N.; Baabe, D.; Freytag, M.; Jones, P. G.; Walter, M. D. Polyhedron 2017, DOI: 10.1016/j.poly.2017.08.024. (31) Thompson, C. V.; Arman, H. D.; Tonzetich, Z. J. Organometallics 2017, 36, 1795−1802. (32) The Chemistry of Pincer Compounds; Morales-Morales, D., Jensen, C. G. M., Eds.; Elsevier: Amsterdam, 2007. (33) Pincer and Pincer-Type Complexes; Szabó, K., Wendt, O. F., Eds.; Wiley-VCH: Weinheim, Germany, 2014. (34) Naziruddin, A. R.; Huang, Z.-J.; Lai, W.-C.; Lin, W.-J.; Hwang, W.-S. Dalton Trans. 2013, 42, 13161−13171.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00772. 1 H NMR spectra, UV−vis−NIR spectrum of 1−3, cyclic voltammogram of 1, thermal ellipsoid renderings of 1−3, ball-and-stick diagram of 4, EPR simulations for 2, and tables of crystallographic data and refinement parameters (PDF) Accession Codes

CCDC 1580569−1580571 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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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail for Z.J.T.: [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. J.D. is supported by a grant from G


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Organometallics (35) Chung, L.-H.; Cho, K.-S.; England, J.; Chan, S.-C.; Wieghardt, K.; Wong, C.-Y. Inorg. Chem. 2013, 52, 9885−9896. (36) Wong, C.-Y.; Lai, L.-M.; Pat, P.-K.; Chung, L.-H. Organometallics 2010, 29, 2533−2539. (37) Bacciu, D.; Chen, C.-H.; Surawatanawong, P.; Foxman, B. M.; Ozerov, O. V. Inorg. Chem. 2010, 49, 5328−5334. (38) Ksenofontov, V.; Levchenko, G.; Spiering, H.; Gütlich, P.; Létard, J. F.; Bouhedja, Y.; Kahn, O. Chem. Phys. Lett. 1998, 294, 545− 553. (39) Moliner, N.; Gaspar, A. B.; Muñoz, M. C.; Niel, V.; Cano, J.; Real, J. A. Inorg. Chem. 2001, 40, 3986−3991. (40) Ksenofontov, V.; Gaspar, A. B.; Real, J. A.; Gütlich, P. J. Phys. Chem. B 2001, 105, 12266−12271. (41) Stassen, A. F.; Grunert, M.; Dova, E.; Müller, M.; Weinberger, P.; Wiesinger, G.; Schenk, H.; Linert, W.; Haasnoot; Jaap, G.; Reedijk, J. Eur. J. Inorg. Chem. 2003, 2003, 2273−2282. (42) Money, V. A.; Carbonera, C.; Elhaïk, J.; Halcrow, M. A.; Howard, J. A. K.; Létard, J.-F. Chem. - Eur. J. 2007, 13, 5503−5514. (43) Paradis, N.; Chastanet, G.; Varret, F.; Létard, J.-F. Eur. J. Inorg. Chem. 2013, 2013, 968−974. (44) Park, J. G.; Jeon, I.-R.; Harris, T. D. Inorg. Chem. 2015, 54, 359− 369. (45) Khan, O. Molecular Magnetism; Wiley-VCH: New York, 1993. (46) Martin, L. L.; Martin, R. L.; Murray, K. S.; Sargeson, A. M. Inorg. Chem. 1990, 29, 1387−1394. (47) Börzel, H.; Comba, P.; Pritzkow, H.; Sickmüller, A. F. Inorg. Chem. 1998, 37, 3853−3857. (48) Blakesley, D. W.; Payne, S. C.; Hagen, K. S. Inorg. Chem. 2000, 39, 1979−1989. (49) Bielinski, E. A.; Lagaditis, P. O.; Zhang, Y.; Mercado, B. Q.; Wurtele, C.; Bernskoetter, W. H.; Hazari, N.; Schneider, S. J. Am. Chem. Soc. 2014, 136, 10234−10237. (50) Zell, T.; Butschke, B.; Ben-David, Y.; Milstein, D. Chem. - Eur. J. 2013, 19, 8068−8072. (51) Chakraborty, S.; Brennessel, W. W.; Jones, W. D. J. Am. Chem. Soc. 2014, 136, 8564−8567. (52) Heiden, Z. M.; Lathem, A. P. Organometallics 2015, 34, 1818− 1827. (53) Eberhardt, N. A.; Guan, H. Chem. Rev. 2016, 116, 8373−8426. (54) Chakraborty, S.; Zhang, J.; Patel, Y. J.; Krause, J. A.; Guan, H. Inorg. Chem. 2013, 52, 37−47. (55) Murugesan, S.; Stöger, B.; Weil, M.; Veiros, L. F.; Kirchner, K. Organometallics 2015, 34, 1364−1372. (56) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42−55. (57) Bain, G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532−536. (58) Morrow, J. R.; Astruc, D. Bull. Soc. Chim. Fr. 1992, 129, 319− 328. (59) Schwindt, M. A.; Lejon, T.; Hegedus, L. S. Organometallics 1990, 9, 2814−2819. (60) CrystalClear User’s Manual; Rigaku/MSC: The Woodlands, TX, 2011. (61) CrysAlisPro; Rigaku Corporation, The Woodlands, TX, 2015. (62) SCALE3 ABSPACK; Oxford Diffraction Ltd., Oxford, U.K., 2005. (63) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. J. Appl. Crystallogr. 2009, 42, 339−341. (64) Sheldrick, G. Acta Crystallogr., Sect. A: Found. Adv. 2015, 71, 3− 8. (65) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122.

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Iron Pincer Complexes Incorporating Bipyridine: A Strategy for

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