Complexes of Iron(II) and Iron(III) Containing Aryl-Substituted N

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Complexes of Iron(II) and Iron(III) Containing Aryl-Substituted N-Heterocyclic Carbene Ligands Jacob A. Przyojski, Hadi D. Arman, and Zachary J. Tonzetich* Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78249, United States S Supporting Information *

ABSTRACT: Iron(II) and iron(III) complexes containing the 1,3-dimesitylimidazole-2-ylidene (IMes) and 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene (IPr) ligands have been prepared and characterized. Four-coordinate iron(II) carbene species are sensitive to the steric bulk of the carbene ligand and can adopt both monomeric and halide-bridged dimeric structures with 2:1 and 1:1 carbene to iron stoichiometries, respectively. Iron(III) carbene complexes bind a single carbene ligand, which is easily exchanged in solution. The structural and solution phase characterization of all compounds is reported, and their relevance to iron-catalyzed C−C coupling reactions is discussed.

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INTRODUCTION Catalysts based on iron are becoming increasingly popular as costeffective and environmentally compatible alternatives to traditional precious metal systems for a number of important chemical transformations.1−5 Simple iron salts, with or without coligands, have been used to great effect as catalysts for a variety of reactions including C−H hydroxylation,6−14 hydrogenation,15−24 hydrosilylation,25−34 allylic substitution,35−37 and C−C coupling.38−51 The nature of the catalyst mixtures in many of these reactions is as yet unidentified, but examples containing well-defined iron precatalysts are beginning to shed light on possible catalytic mechanisms.17,21,29,52−55 One promising class of iron complexes with potential catalytic applications is that based on N-heterocyclic carbene (NHC) ligands. The strong σ-donor properties of these ligands coupled with their easily modified steric properties make them an obvious choice as supporting ligands in catalysis.56 However, reports of NHC-iron complexes57 are relatively scarce in comparison with other 3d metals such as Ni and Cu.58 Recent work with iron-NHC complexes has demonstrated their utility in biomimetic applications59−66 and catalytic methodology.26,27,67−71 In most of these examples, the NHC ligand is paired with a strong-field ligand such as CO, NO, or cyclopentadienyl, giving rise to low-spin, diamagnetic species. Chelating variants of NHC ligands, some incorporating non-NHC donors, have also been employed to stabilize iron in a variety of oxidation states.71−75 Despite the growing prevalence of NHC ligands in iron chemistry, examples of simple monodentate NHC complexes of iron(II) remain few.76−83 Unlike the compounds mentioned above, complexes of this type reported to date are typically paramagnetic, even those that contain alkyl ligands.78 It is these types of simple adducts that are most relevant to a variety of catalytic methodologies utilizing simple iron salts and NHC coligands. Therefore, information concerning their structures and reactivity is of great importance to a fuller understanding of © 2012 American Chemical Society

the catalytic reactions they effect. In particular, aryl-substituted NHC ligands have found the greatest use as coligands in C−C coupling reactions.38,43,84 The basic chemistry of these species has been mostly neglected in favor of alkyl-substituted NHCs and those incorporating chelating motifs. Additionally, the chemistry of simple iron-NHC complexes has been almost exclusively relegated to the 2+ oxidation state. We have therefore chosen to focus on the fundamental chemistry of iron(II) and iron(III) complexes containing the 1,3-dimesitylimidazol-2ylidene (IMes) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2ylidene (IPr) ligands, as well as a variant containing chlorine atoms in the backbone (Chart 1).85 Our findings indicate notable differences between the chemistry of reported iron complexes containing alkyl-substituted carbene ligands and the compounds described herein. These results therefore have important implications to the mechanism of C−C coupling reactions catalyzed by iron-NHC systems.

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RESULTS AND DISCUSSION Iron(II) Complexes. The synthesis of divalent iron carbene complexes was accomplished by direct reaction of the carbene ligand with FeCl2(THF)1.5 in THF solvent (Scheme 1). Several recent reports have detailed alternative routes to various iron(II) carbene complexes such as protonolysis of basic bis(trimethylsilyl)amide ligands by imidazolium precursors.79,86,87 We have found that direct reaction of the carbene ligand with the readily available FeCl2(THF)1.5 is both high yielding and straightforward in the case of both IMes and IPr, avoiding complications encountered with protonolysis reactions such as salt formation.77 Direct reaction of the carbenes with iron also allows for precise control of binding stoichiometries. For example, Received: February 9, 2012 Published: March 20, 2012 3264

dx.doi.org/10.1021/om300106u | Organometallics 2012, 31, 3264−3271

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Chart 1. Carbene Ligands Used in This Study

Scheme 1

Figure 1. Thermal ellipsoid drawings (50%) of [FeCl2(IMes)2] (left) and [Fe2Cl2(μ-Cl)2(IMes)2] (right). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) for [FeCl2(IMes)2]: Fe(1)−C(1) = 2.139(3), Fe(1)−C(22) = 2.157(3), Fe(1)−Cl(1) = 2.310(1), Fe(1)−Cl(2) = 2.292(1), C(1)−Fe(1)−C(22) = 125.2(1), Cl(1)−Fe(1)−Cl(2) = 106.66(4), C(1)−Fe(1)−Cl(2) = 99.51(8), C(22)−Fe(1)− Cl(1) = 94.82(8); for [Fe2Cl2(μ-Cl)2(IMes)2]: Fe(1)−C(1) = 2.089(4), Fe(1)−Cl(1) = 2.365(1), Fe(1)−Cl(1A) = 2.384(1), Fe(1)−Cl(2) = 2.231(1), C(1)−Fe(1)−Cl(2) = 117.05(12), Cl(1)−Fe(1)−Cl(1A) = 92.50(4), Fe(1)−Cl(1)−Fe(1A) = 87.50(4).

reaction of FeCl2(THF)1.5 with two equivalents of IMes afforded the highly distorted pseudotetrahedral complex [FeCl2(IMes)2],

whereas reaction with one equivalent of IMes yielded the chloride-bridged dimer [Fe2Cl2(μ-Cl)2(IMes)2]. We note that, 3265


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previously, [FeCl2(IMes)2] could not be prepared by protonolysis of [Fe{N(SiMe3)2}2] with IMes·HCl, affording instead the three-coordinate amide species [FeCl(IMes){N(SiMe3)2}].79 Both the monomeric [FeCl2(IMes)2] and dimeric [Fe2Cl2 (μ-Cl)2(IMes)2] give rise to characteristic 1H NMR spectra at 20 °C (see Supporting Information(SI)). The dimer can be easily cleaved by addition of a second equivalent of IMes, as judged by NMR spectroscopy. Magnetic susceptibility measurements in solution at 20 °C gave an effective magnetic moment of 5.2(1) μB for [FeCl2(IMes)2], consistent with a high-spin Fe(II) center. This magnetic moment agrees well with those observed for other divalent iron(II) carbene complexes78,81,82 and several four-coordinate iron(II) phosphine complexes.88,89 In contrast to the monomeric species, the dimeric [Fe2Cl2 (μ-Cl)2(IMes)2] displayed a solution magnetic moment of 6.5(1) μB, less than expected for two noninteracting high-spin iron(II) centers, suggesting a degree of antiferromagnetic coupling within the dimer. This suggestion is further supported by the 1H NMR spectrum of the dimer, which displays a smaller dispersion of chemical shift values compared with the monomeric species (see the (SI)). The solid-state structures of [FeCl2(IMes)2] and [Fe2Cl2 (μ-Cl)2(IMes)2] are displayed in Figure 1 (see SI for crystallographic details). Both species display pseudotetrahedral coordination geometries about iron, although the monomer is heavily distorted. The Ccarbene−Fe−Ccarbene bond angle of [FeCl2(IMes)2] is enlarged to ca. 125° to accommodate the steric bulk of the IMes ligands. This angle differs from that in other reported four-coordinate iron(II) carbene complexes (cf. 103.3° in the related [FeCl2(SIPr*)2], SIPr* = 1,3-bis (2-isopropylphenyl)-4,5-dihydroimidazol-2-ylidene79) and suggests that the steric bulk of the IMes ligand is near the upper limit that still permits binding of two carbene ligands to iron(II). The Fe−Ccarbene bond distances of 2.129(3) and 2.157(3) Å are slightly larger than similar compounds of this type but do not display a significant elongation.78,79 In contrast to [FeCl2(IMes)2], the less congested structure of the dimeric [Fe2Cl2(μ-Cl)2(IMes)2] displays no significant distortion from tetrahedral geometry. The dimer crystallizes on an inversion center, consistent with a preference for the isomer having C2h point symmetry (carbene ligands on opposite sides of the Fe2Cl2 plane). The Fe2Cl2 core is nearly perfectly symmetric, with a total area of 5.69 Å2 (Figure 1). Unlike the IMes ligand, the IPr ligand is too bulky to afford monomeric complexes, and all attempts to prepare such species resulted in formation of the dimeric species [Fe2Cl2(μ-Cl)2 (IPr)2] (Scheme 1). No evidence for an equilibrium involving a 2:1 IPr to Fe complex was observed when a second equivalent of IPr was added to [Fe2Cl2(μ-Cl)2(IPr)2] in benzene-d6, as judged by NMR spectroscopy. This finding is significant considering that several reports concerning catalytic C−C coupling make use of the IPr ligand in 2:1 stoichiometries with iron(II) salts.38,43 Our results indicate that species of the type “[FeX2(IPr)2]” (X = halide or alkyl) are unlikely to persist during a catalytic cycle, although we cannot say at this time whether 2:1 IPr to iron stoichiometries may be important for stabilizing low-valent species. The solid-state structure of [Fe2Cl2(μ-Cl)2(IPr)2] is displayed in Figure 2. As with the IMes analogue, the dimeric IPr complex crystallizes on an inversion center and displays a relatively undistorted tetrahedral geometry about iron. The Fe2Cl2 core area of 5.60 Å2 is comparable to the IMes analogue, as are the Fe−Ccarbene and Fe−Cl bond distances (Figure 2).

Figure 2. Thermal ellipsoid drawing (50%) of [Fe2Cl2(μ-Cl)2(IPr)2]. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe(1)−C(1) = 2.090(2), Fe(1)−Cl(1) = 2.2419(7), Fe(1)−Cl(2) = 2.3473(8), Fe(1)−Cl(2A) = 2.3863(7), C(1)−Fe(1)− Cl(1) = 111.66(5), Cl(2)−Fe(1)−Cl(2A) = 95.00(2), Fe(1)−Cl(2)− Fe(1A) = 85.00(2). 1 H NMR spectra of [Fe2Cl2(μ-Cl)2(IPr)2] display six resonances, consistent with time-averaged C2h symmetry in solution. Variable-temperature NMR measurements (see SI) up to 80 °C demonstrate no significant change, consistent with the integrity of the dimer being maintained at elevated temperatures. Addition of THF to [Fe2Cl2(μ-Cl)2(IPr)2] resulted in no change to the NMR spectrum, also consistent with the compound maintaining its dimeric structure in the presence of coordinating solvents. Stronger bases, such as N-methylimidazole (MeIm), did lead to complete dissociation of the dimer and formation of a species formulated as [FeCl2(IPr)(MeIm)], as judged by NMR spectroscopy (see SI). Of note is the appearance, after addition of MeIm, of an additional downfield resonance near 25 ppm corresponding to the vinylic protons of the NHC ligand, which is present in [Fe2Cl2(IMes)2] but absent in all dimeric species. To ascertain whether the nature of the halide ligand influences the structure of the dimeric iron carbene complexes, the bromide complex was prepared. Addition of IPr to a solution of anhydrous FeBr2 in THF afforded the corresponding bromide-bridged dimeric species [Fe2Br2(μ-Br)2(IPr)2] as a yellow solid, albeit in lower yield (35%) than the chloride complexes (60% to 80%). Solutions of the compound in benzene-d6 displayed a magnetic moment of 6.3(1) μB, consistent with the other dimeric species discussed above. 1H NMR features of [Fe2Br2(μ-Br)2(IPr)2] were similar to those of the chloride analogue, as was the solid-state structure (see SI), which also displayed crystallographically required inversion symmetry. The Fe−Ccarbene bond distance of 2.101(2) Å is longer than that of the chloride analogue even though the Fe2Br2 core is larger (6.31 Å2). However, the presence of a cocrystallized benzene molecule that resides in between the two carbene ligands of the dimer may exert some influence on the bond metrics in the solid state. In addition to varying the halide ligand, we also investigated the effects of an electronically modified carbene ligand, IPrCl, which contains chlorine atoms on the imidazolylidene backbone. Reaction of IPrCl with FeCl2(THF)1.5 proceeded smoothly in

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THF to afford [Fe2Cl2(μ-Cl)2(IPrCl)2]. The 1H NMR spectrum of this species is similar to that of the IPr analogue except for the absence of a resonance due to the vinylic protons. The solid-state structure (see SI) displayed an Fe−Ccarbene bond length of 2.097(2) Å, only slightly elongated from that in the IPr analogue. The Fe2Cl2 core displayed greater asymmetry than the IPr analogue, although the total area of the core (5.64 Å2) was nearly identical. Thus, it appears that changes to the backbone of the carbene ligand have little effect on the structure of the dimers. Reactions catalyzed by iron carbene complexes have been proposed to involve some form of redox chemistry.1,90−92 We therefore chose to examine the cyclic voltammetry of the iron(II) compounds in order to identify the FeII/FeIII couple and determine if any reasonably accessible reduction events take place since low-valent iron species have been implicated in a variety of coupling reactions.40,93 The CV of [FeCl2(IMes)2] in THF is displayed in Figure 3. The compound shows a

Scheme 2

gave an effective magnetic moment of 5.8(1) μB, consistent with a high-spin iron(III) center. In agreement with this fact, NMR spectra of each of the compounds were essentially featureless, displaying only very broadened resonances. The solid-state structures of both [FeCl3(IPr)] and [FeCl3(IPrCl)] were determined by X-ray crystallography and are shown in Figure 4. Both complexes have three-legged piano stool type structures with the carbene ligand arranged perpendicular to one of the Fe−Cl bonds. The Fe−Ccarbene bond lengths are similar to those of the iron(II) complexes and show a slight dependence on the carbene ligand (cf. 2.090(2) vs 2.122(2) Å for IPr and IPrCl complexes, respectively). As expected for iron(III), the Fe−Cl bond lengths are significantly shorter in both complexes when compared with the iron(II) complexes above. The carbene ligand in both [FeCl3(IPr)] and [FeCl3(IPrCl)] appears to be easily displaced in solution. Addition of excess IPr to [FeCl3(IPr)] in benzene-d6 results in a time-averaged spectrum indicating rapid exchange of the free and bound carbene ligand. In addition, CV experiments of [FeCl3(IPr)] in THF demonstrate two quasi-reversible redox events that we propose originate from [FeCl3(IPr)] and [FeCl3(THF)x] (Figure 5a). Identical experiments in CH2Cl2 displayed only a single reversible redox process (Figure 5b) centered at −0.41 V (vs Fc/ Fc+), consistent with this proposal. Despite the relative ease in which the carbene ligand can be displaced, [FeCl3(IPr)] appeared relatively stable toward ambient conditions. Storage of the compound in the solid state under air for several days led to no noticeable visible decomposition, and NMR spectra of solids stored in this matter showed little to no change. This observation contrasts those with iron(II), where immediate oxidation was found to take place upon exposure to air. Preliminary Alkylation Experiments. Reactions of [FeCl2(IMes)2] and [Fe2Cl2(μ-Cl)2(IPr)2] with various Grignard reagents (MeMgCl, EtMgCl, and PhMgCl) in several ethereal and aromatic solvents were investigated with the goal of isolating divalent iron alkyls. In all cases, dark brown mixtures formed upon addition of the alkyl magnesium reagent from which no iron(II) alkyls could be isolated. Even when the reactions were performed at low temperatures, formation of brown mixtures was evident.94,95 When methyllithium was employed in place of MeMgBr, formation of a brown mixture was not observed; however, color changes to red and green took place and no species assignable to [Fe(CH3)2(IMes)2] could be detected. If the iron(III) complex [FeCl3(IPr)] was used in place of iron(II), addition of Grignard led to immediate reduction to iron(II), as judged by a color change from orange to colorless, followed by subsequent formation of the same brown mixtures. In each case, the products of Grignard homocoupling were not observed in significant quantities. These results contrast those obtained recently using alkyl-substituted iron carbene complexes where a

Figure 3. Cyclic voltammogram of [FeCl2(IMes)2] at a glassy carbon electrode in THF (scan rate 100 mV/s, supporting electrolyte 0.2 M Bu4NPF6).

reversible one-electron oxidation at −0.54 V (vs ferrocene/ ferrocenium) indicative of formation of the putative iron(III) complex [FeCl2(IMes)2]+. No other quasi-reversible events could be detected within the solvent window. Similar CV measurements of the dimeric species [Fe2Cl2(μ-Cl)2(IPr)2] displayed a host of irreversible redox events, again demonstrating a degree of electronic communication between the iron centers (see SI). Iron(III) Complexes. Reversible oxidation of [FeCl2(IMes)2] prompted us to examine the possibility of preparing iron(III) carbene complexes. Iron(III) complexes are typically employed as precatalysts in C−C coupling reactions and have the added benefit of greater stability under ambient conditions. In addition, iron(III) complexes containing simple monodentate NHC ligands are nearly unknown and therefore of fundamental interest in expanding the chemistry of iron NHCs.68 Reaction of anhydrous FeCl3 with one equivalent of IPr or IPrCl in THF or toluene afforded the iron(III) carbenes as orange crystalline solids (Scheme 2). Similar reactions with IMes and FeCl3 gave orange powders with very limited solubility in arene solvents, which rendered purification difficult. As with iron(II), a 2:1 IPr to iron(III) complex could not be prepared, and attempts to prepare five-coordinate species of the type “[FeCl3(IPr)2]” resulted instead in isolation of [FeCl3(IPr)]. Solution magnetic susceptibility measurements of [FeCl3(IPr)] 3267


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Figure 4. Thermal ellipsoid drawings (50%) of one of the crystallographically independent [FeCl3(IPr)] molecules in the unit cell (left) and [FeCl3(IPrCl)] (right). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) for [FeCl3(IPr)]: Fe(2)−C(28) = 2.091(2), Fe(2)−Cl(4) = 2.1852(7), Fe(2)−Cl(5) = 2.1813(8), Fe(2)−Cl(6) = 2.1894(7), C(28)−Fe(2)−Cl(4) = 110.47(6), C(28)−Fe(2)−Cl(5) = 105.79(6), Cl(4)−Fe(2)−Cl(5) = 110.40(3), Cl(4)−Fe(2)−Cl(6) = 108.59(3); for [FeCl3(IPrCl)]: Fe(1)−C(1) = 2.122(2), Fe(1)−Cl(3) = 2.1568(8), Fe(1)−Cl(4) = 2.1670(8), Fe(1)−Cl(5) = 2.1796 (8), C(1)−Fe(1)−Cl(3) = 108.47(6), C(1)−Fe(2)−Cl(5) = 110.14(6), Cl(3)−Fe(1)−Cl(5) = 109.02(4), Cl(3)−Fe(1)−Cl(4) = 109.55(4).

Figure 5. Cyclic voltammogram of [FeCl3(IPr)] at a glassy carbon electrode: (a) in THF with 0.2 M Bu4NPF6 as supporting electrolyte; (b) in CH2Cl2 with 0.1 M Bu4NPF6 as supporting electrolyte.

complexes, demonstrate moderate stability toward ambient conditions. The aggregation state of the iron(II) complexes is highly dependent on both stoichiometry and the steric requirements of the carbene ligands, allowing access to both monomeric and dimeric species. Reversible iron(II/III) redox chemistry was observed for the monomeric iron complexes, demonstrating that compounds based on either oxidation state may lead to effective precatalysts in cross-coupling reactions. Preliminary attempts to prepare alkylated iron(II) complexes resulted in formation of unidentified mixtures presumably containing reduced iron. Future work will examine the reduction of aryl-substituted NHC iron complexes with magnesium reagents and the complexities of the alkylation reactions.

series of three- and four-coordinate divalent iron dialkyl complexes could be prepared and isolated.78 The reason for the difficulties encountered in our system may be due to more facile reduction of iron(II) brought about by the less electrondonating aryl-substituted carbene ligands. The fact that we cannot observe iron(II) alkyl species with aryl-substituted NHC ligands argues against their direct involvement in ironcatalyzed C−C coupling reactions in favor of as yet unidentified compounds of low-valent iron.

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CONCLUSIONS Several iron(II) and iron(III) complexes of commonly employed aryl-substituted N-heterocyclic carbene ligands have been prepared and characterized. The compounds are straightforward to synthesize and, in the case of iron(III) 3268


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separation of a solid. The solid was collected by filtration, washed with pentane, and dried in vacuo to give 0.477 g (81%) of an off-white microcrystalline powder. Crystals suitable for X-ray diffraction were grown from a saturated benzene/pentane solution of the complex at 23 °C. 1H NMR (C6D6): δ 0.81 (4H, CH), 0.49 (24H, o-CH3), −0.48 (8H, m-H), −1.32 (12H, p-CH3). μeff (Evans, C6D6): 6.5(1) μB. Anal. Calcd for C42H48Cl4Fe2N4: C, 58.50; H, 5.61; N, 6.50. Found: C, 57.40; H, 5.38, N, 6.36. [Fe2Cl2(μ-Cl)2(IPr)2]. A 100 mL round-bottom flask was charged with 0.303 g (1.30 mmol) of FeCl2(THF)1.5 and 20 mL of THF. To the stirring solution was added 0.500 g (1.29 mmol) of IPr as a solid in one portion. The resulting colorless solution was allowed to stir at 23 °C for 1 h. All volatiles were removed in vacuo, and the solid residue was extracted into ∼10 mL of warm toluene. After filtration, the toluene solution was set aside at −35 °C for 24 h, during which time a white solid precipitated. The precipitate was collected by decanting the mother liquor, washed with pentane, and dried in vacuo, to afford 0.379 g (57% yield) of an off-white microcrystalline solid. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a saturated benzene solution. 1H NMR (C6D6): δ 8.55 (24H, CH3), 1.51 (4H, CH), 0.46 (8H, CHMe2), −2.20 (8H, m-H), −2.89 (24H, CH3), −3.68 (4H, p-H). μeff (Evans, C6D6): 7.4(1) μB. Anal. Calcd for C54H72Cl4Fe2N4: C, 62.93; H, 7.04; N, 5.44. Found: C, 60.95; H, 6.99; N, 5.71. [FeBr2(μ-Br)2(IPr)2]. A flask was charged with 0.2803 g (1.30 mmol) of anhydrous FeBr2 and 25 mL of THF. To the mixture was added 0.5004 g (1.29 mmol) of IPr, and the resulting solution was allowed to stir at 23 °C for 40 min. All volatiles were removed in vacuo, and the residue was extracted into 12 mL of warm toluene, filtered, then concentrated in vacuo to ∼5 mL. The solution was set aside at −35 °C for 18 h, during which time a yellow solid precipitated. The solid was collected by filtration, washed with pentane, and dried in vacuo to give 0.2783 g (36% yield) of a yellow microcrystalline solid. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a concentrated benzene solution. 1H NMR (C6D6): δ 7.32 (24H, CH3), 6.76 (4H,  CH), 4.50 (8H, CHMe2), −0.58 (8H, m-H), −1.49 (24H, CH3), −2.38 (4H, p-H). μeff (Evans, C6D6): 6.3(1) μB. Anal. Calcd for C54H72Br4Fe2N4: C, 53.67; H, 6.01; N, 4.64. Found: C, 52.96; H, 5.75; N, 4.68. [Fe2Cl2(μ-Cl)2(IPrCl)2]. To a flask containing 5 mL of toluene were added 0.0541 g (0.230 mmol) of FeCl2(THF)1.5 and 0.1051 g (0.231 mmol) of IPrCl. The pale yellow solution was allowed to stir for 1.5 h at 23 °C, after which time all volatiles were removed in vacuo. The resulting pale yellow residue was dissolved in ∼3 mL of hot toluene, filtered through glass filter paper, and set aside at −35 °C for 18 h, during which time a precipitate formed. The precipitate was collected by decanting the mother liquor, washed with pentane, and dried in vacuo to give 0.0955 g (60% yield) of pale yellow microcrystals in two crops. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a saturated benzene solution. 1 H NMR (C6D6): δ 9.14 (24H, CH3), −1.17 (8H, CHMe2), −1.31 (8H, m-H), −2.82 (28H, CH3 + p-H), −2.89 (24H, CH3), −3.68 (4H, p-H). μeff (Evans, C 6D6): 7.5(1) μB. Anal. Calcd for C54H68Cl8Fe2N4: C, 55.51; H, 5.87; N, 4.79. Found: C, 55.85; H, 5.99; N, 4.03. [FeCl3(IPr)]. A flask was charged with 0.1669 g (0.430 mmol) of IPr and 8 mL of THF. To the resulting solution was added 0.0691 g (0.426 mmol) of anhydrous FeCl3 as a solid in one portion. The solution immediately turned orange and was allowed to stir at 23 °C for 30 min. All volatiles were removed in vacuo, and the residue was treated with 5 mL of toluene, which caused separation of an orange precipitate. The precipitate was collected by filtration and washed with pentane, affording 0.124 mg (52% yield) of an orange powder. Subsequent recrystallization from THF/pentane afforded orange rods. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a saturated benzene solution. μeff (Evans, C6D6): 5.8(1) μB. CV (CH2Cl2): −0.41 V (FeII/III). UV−vis (toluene) λmax, nm (ε, M−1 cm−1): 330 (8600). Anal. Calcd for C27H36Cl3FeN2: C, 58.88; H, 6.59; N, 5.09. Found: C, 57.54; H, 6.61; N, 4.67. [FeCl3(IPrCl)]. A flask was charged with 0.165 g (0.362 mmol) of IPrCl and 10 mL of toluene. The solution was stirred until all solids

EXPERIMENTAL SECTION

General Comments. All manipulations, unless otherwise noted, were performed under an atmosphere of nitrogen gas using standard Schlenk technique or in a Vacuum Atmospheres glovebox under an atmosphere of purified nitrogen. Tetrahydrofuran, diethyl ether, methylene chloride, pentane, and toluene were purified by sparging with argon and passage through two columns packed with 4 Å molecular sieves. Benzene, benzene-d6, and toluene-d8 were dried over sodium ketyl, then vacuum-distilled. All solvents were stored in the glovebox over 4 Å molecular sieves prior to use. 1H NMR spectra were recorded on a Varian INOVA spectrometer operating at 500 MHz and referenced to the residual proton peak of the solvent. UV−vis spectra were recorded on a Cary-60 spectrophotometer in airtight Tefloncapped quartz cells. Cyclic voltammetry measurements were performed in a single-compartment cell under a nitrogen atmosphere at 23 °C using a CH Instruments 620D electrochemical workstation. A three-electrode setup was employed comprising a glassy carbon working electrode (2.0 mm), platinum 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. Solution magnetic susceptibility measurements were determined by the Evans method without a solvent correction using reported diamagnetic corrections.96 Elemental analyses were performed by Midwest Microlab, LLC in Indianapolis, IN, USA. For nearly all the compounds, analyses for H and N were highly satisfactory, but analyses for C repeatedly yielded values lower than expected, most likely due to formation of carbides, as has been observed in other NHC systems.97,98 Materials. Carbene ligands IMes, IPr, and IPrCl were prepared according to the literature procedure.85 FeCl2(THF)1.5 and anhydrous FeBr2 were prepared by the methods of Astruc and Winter, respectively.99,100 All other reagents were purchased from commercial vendors and used as received. X-ray Data Collection and Structure Solution Refinement. Crystals suitable for X-ray diffraction were mounted in Paratone oil onto a glass fiber and frozen under a nitrogen cold stream. The data were collected at 98(2) K using a Rigaku AFC12/Saturn 724 CCD fitted with Mo Kα radiation (λ = 0.71073 Å). Data collection and unit cell refinement were performed using Crystal Clear software.101 Data processing and absorption correction, giving minimum and maximum transmission factors, were accomplished with Crystal Clear and ABSCOR,102 respectively. All structures were solved by direct methods and refined on F2 using full-matrix, least-squares techniques with SHELXL-97.103,104 All non-hydrogen atoms were refined with anisotropic displacement parameters. All carbon-bound hydrogen atom positions were determined by geometry and refined by a riding model. [FeCl2(IMes)2]. A flask was charged with 1.201 g (3.95 mmol) of IMes and 30 mL of THF. To this solution was added 0.454 g (1.93 mmol) of FeCl2(THF)1.5. The mixture was allowed to stir for 60 min at 23 °C, during which time the mixture became a homogeneous pale pink solution. All volatiles were removed in vacuo, and the residue was extracted into 30 mL of warm toluene and filtered. The toluene was removed in vacuo, and the residue treated with pentane, causing separation of a solid. The solid material was collected by filtration, washed with pentane, and dried in vacuo to afford 1.228 g (86% yield) of a cream-colored microcrystalline powder. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a saturated benzene solution of the complex. 1H NMR (C6D6): δ 28.13 (4H, CH), 5.66 (8H, m-H), 3.77 (24H, o-CH3), 2.22 (12H, p-CH3). μeff (Evans, C6D6): 5.2(1) μB. CV (THF): −0.54 V (FeII/III). Anal. Calcd for C42H48Cl2FeN4: C, 68.58; H, 6.58; N, 7.62. Found: C, 67.52; H, 6.50; N, 7.49. [FeCl2(μ-Cl)2(IMes)2]. A flask was charged with 0.320 g (1.36 mmol) of FeCl2(THF)1.5 and 10 mL of THF. The mixture was allowed to stir at 23 °C until the iron starting material had dissolved. To this solution was added 0.416 g (1.37 mmol) of IMes, and the resulting colorless solution was allowed to stir at 23 °C for 20 min. All volatiles were removed in vacuo, and the residue was treated with ∼5 mL of toluene, causing 3269


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were dissolved, at which point 0.0590 g (0.362 mol) of FeCl3 was added as a solid in one portion. The mixture was stirred at 23 °C for 1 h. All volatiles were removed in vacuo, and the solid residue was extracted into benzene and filtered. The benzene was removed in vacuo, yielding 0.154 g (69% yield) of an orange solid. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a saturated benzene solution. UV−vis (toluene) λmax, nm (ε, M−1 cm−1): 337 (5000), 408 (sh). Anal. Calcd for C27H34Cl5FeN2: C, 52.33; H, 5.53; N, 4.52. Found: C, 50.61; H, 5.60; N, 4.12.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures, spectra, and crystallographic data, as well as the corresponding CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by startup funding from the University of Texas at San Antonio and by grants from the Welch Foundation (AX-1772 to Z.J.T. and AX-0026 to J.A.P.).

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REFERENCES

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Complexes of Iron(II) and Iron(III) Containing Aryl-Substituted N

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