N-Heterocyclic Carbene Complexes of Three- and Four-Coordinate Fe

Article pubs.acs.org/Organometallics

N‑Heterocyclic Carbene Complexes of Three- and Four-Coordinate Fe(I) Yasuhiro Ohki,*,†,‡ Ryoko Hoshino,† and Kazuyuki Tatsumi*,† †

Department of Chemistry, Graduate School of Science, and Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan ‡ PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: N-heterocyclic carbene complexes of three- and fourcoordinate Fe(I), [Fe(LR)4][PF6] (LR = 1,3-R2-4,5-dimethylimidazol-2ylidene, R = Me (2), Et (3), iPr (4)) and [Fe(LMes)2(THF)][PF6] (5) (LMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), were synthesized from successive reactions of [Fe(toluene)2][PF6]2 with 1 equiv of KC8 and LR (4 equiv for R = Me, Et, iPr; 2 equiv for R = Mes). The coordination geometry of the iron atom in these complexes varies depending on the nature of the R group in LR: a tetrahedral geometry was observed for 2, a square-planar one for 3 and 4, and a three-coordinate T-shaped one for 5. In solution, 4 releases LiPr ligand(s), while the LR ligands of the other Fe(I) complexes remain firmly bound. Tetrahedral 2 and T-shaped 5 contain a high-spin (S = 3/2) Fe(I) center, whereas squareplanar 3 and 4 contain Fe(I) in the low-spin state (S = 1/2).

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INTRODUCTION

the +I oxidation state is relatively scarce for organoiron complexes in general.5 However, Fe(I) species have recently been proposed as reactive intermediates in cross-coupling reactions.6 Moreover, Fe(I) complexes supported by multidentate ligands have been successfully employed in the activation of small molecules such as N2 and organoazides,7 whereas various thiolate-bridged dinuclear Fe(I)−Fe(I) complexes8,9 have been studied as model compounds for the active site in [FeFe] hydrogenase, which catalyzes the production and consumption of H2 under physiological conditions.10 In this context, even some NHC complexes of Fe(I) have been employed. For example, an Fe(I) complex supported by a tripodal carbene−amine ligand was able to activate the N−Si bond of N3SiMe3, and subsequent photoirradiation generated a terminal nitride complex.7g Concomitant with this N−Si bond cleavage, oxidation of Fe(I) to Fe(II) occurred, suggesting that mononuclear Fe(I) complexes may often be susceptible to oneelectron oxidation. In fact, an Fe(I)−NHC species generated in situ was proposed to transfer an electron to styrene oxide, thus leading to ring opening of the epoxide moiety and coupling with olefins.11 In addition to the redox-coupled reactions of Fe(I)−NHC complexes, a recent magnetic study of the linear Fe(I) complex [Fe{C(SiMe3)3}2]− 12 and the quest for lowcoordinate Fe(I)−NHC complexes have generated considerable interest in the synthesis and properties of two-coordinate NHC complexes of Fe(I).3 In this report, we present a new

N-heterocyclic carbene (NHC) complexes of iron are an emerging class of organometallic compounds, especially because of their applications in homogeneous catalysis. The wide range of reactions that have been accomplished with iron−NHC complexes, such as C−C bond formation and C−H bond activation reactions as well as catalytic reductions of carbonyl compounds, has been summarized in recent reviews.1 Our groups have contributed to this field by reporting several Fe(0), Fe(II), Fe(III), and formal Fe(IV) complexes with NHC ligands (Figure 1).2 To date, NHC complexes of Fe(I) are still less common than those of iron in other oxidation states,3,4 as

Special Issue: Organometallics in Asia Received: December 26, 2015

Figure 1. NHC complexes of Fe(0), Fe(II), Fe(III), and Fe(IV). © XXXX American Chemical Society

A

DOI: 10.1021/acs.organomet.5b01025 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

(sIMes)2(THF)][BPh4] (sIMes = 1,3-bis(2,4,6trimethylphenyl)imidazolin-2-ylidene) containing a saturated carbene ring was recently synthesized by Deng and co-workers from the sequential reaction of (tmeda)FeCl2 dimer with sIMes, KC8, and NaBPh4.3e Because of the d7 configuration of the iron atom in 2−5, broad and paramagnetically shifted resonances were observed in their 1H NMR spectra in THF-d8. In addition to the paramagnetic signals, resonances consistent with free LiPr were observed in the 1H NMR spectrum of 4, while comparable signals did not appear in the spectra of 2, 3, and 5. This may be attributed to the prevalent steric congestion in 4, which probably leads to reversible dissociation of LiPr in solution (vide infra). The molecular structures of 2−5 were determined by singlecrystal X-ray diffraction analysis (Figure 3 and Table 1). The results revealed that the coordination geometry of the iron center varies depending on the nature of LR, i.e., tetrahedral for 2 (R = Me), square-planar for 3 and 4 (R = Et, iPr), and Tshaped for 5 (R = Mes). The iron center in 2 is slightly distorted from an ideal tetrahedral geometry, with C−Fe−C angles ranging from 99.7(4)° to 115.4(4)°. The Fe−C bond lengths of tetrahedral 2 (2.001(10)−2.077(11) Å) are comparable to those of the previously reported Fe(0)−NHC complex (LEt)2Fe(divinyltetramethyldisiloxane) (2.109(8) and 2.046(9) Å)15 but longer than those of the half-sandwich-type Fe(0)−NHC complexes (arene)Fe(C^C) (arene = benzene/ toluene, C^C = bis{N-(2,6-diisopropylphenyl)imidazol-2ylidene}methylene) (1.9190(19)−1.921(3) Å).16 Square-planar 3 (1.947(4)−1.972(4) Å) and 4 (1.9925(17) and 2.0035(18) Å) exhibit shorter Fe−C bond lengths relative to 2, and the steric congestion imposed by the bulky iPr groups induces elongated Fe−C bonds in 4 compared with 3. The squareplanar NHC arrangement in 3 and 4 results in an axial orientation of the substituents on nitrogen (the R groups) in LR with respect to the iron center, thus reducing the steric congestion between the R groups more efficiently relative to the tetrahedral geometry in 2. The five-membered rings of LR in 3 and 4 are arranged in a windmill-like fashion with pseudo-D4 symmetry, revealing torsion angles of 61.30−62.29° (LEt) and 57.61−59.04° (LiPr) with respect to the FeC4 coordination plane. The two LMes ligands in three-coordinate 5 are located in almost ideal trans positions with respect to each other, comprising a C−Fe−C angle of 161.27(13)°. One molecule of THF occupies a coordination site at the iron center perpendicular to the C−Fe−C axis, resulting in O−Fe−C angles of 96.87(12) and 101.85(11)°. These metrical parameters are comparable to those of [Fe(sIMes)2(THF)]+ (C−Fe−C, 162.9(1)°; O−Fe−C, 95.0(1) and 102.1(1)°),3e whereas its Fe−C distances (1.989(5) and 1.792(5) Å) are shorter than those in 5 (2.016(3) and 2.001(3) Å). In order to minimize the steric congestion between the two LMes ligands, the two imidazol-2-ylidene rings form a torsional angle of 112.09(13)°. Addition of LiPr to [Fe(η6-mesitylene)2]2+. The choice of iron precursor is an important parameter in the synthesis of NHC complexes of Fe(I). Even though the sandwich-type complex [Fe(η6-mesitylene)2][PF6]2 reacted with KC8 and LR (R = Me, Et, iPr) to afford 2−4 in smaller amounts, their isolation was hampered by the formation of byproducts. One of the byproducts obtained from the reaction of [Fe(η6mesitylene)2][PF6]2 with KC8 and LiPr was found to precipitate as orange crystals. Under a microscope, some of these orange crystals were manually separated from the green crystals of 4

class of mononuclear three- and four-coordinate NHC complexes of Fe(I).

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RESULTS AND DISCUSSION Synthesis and Solid-State Structures of NHC Complexes of Fe(I). The sandwich-type Fe(II) complex [Fe(η6toluene)2][PF6]2 (1)13 was used as a precursor to generate discrete Fe(I) species. The NHC ligands used in this study (LR) are shown in Figure 2 (LMe = 1,3,4,5-tetramethylimidazol-

Figure 2. NHC ligands used in this study.

2-ylidene, LEt = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene, LiPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, LMes = 1,3bis(2,4,6-trimethylphenyl)imidazol-2-ylidene).14 Treatment of 1 with 1 equiv of KC8 and 4 equiv of LR (R = Me, Et, iPr) at −30 °C in tetrahydrofuran (THF) resulted in the formation of the homoleptic four-coordinate Fe(I) complexes [Fe(LR)4][PF6] (2, R = Me; 3, R = Et; 4, R = iPr; Scheme 1). The color Scheme 1

of the reaction products was found to vary depending on the substituents on nitrogen in the NHCs. While 2 is orange (λ = 336 nm), 3 and 4 are dark green (λ = 702, 477, 412, 344 nm) and green (λ = 338 nm), respectively. An analogous reaction in the presence of 2 equiv of LMes provided the three-coordinate Fe(I) complex [Fe(LMes)2(THF)][PF6] (5) as dark-red crystals (λ max = 381 nm). The closely related complex [FeB


Article

Organometallics

Figure 3. Structures of the cationic parts of [Fe(LMe)4][PF6] (2), [Fe(LEt)4][PF6] (3), [Fe(LiPr)4][PF6] (4), and [Fe(LMes)2(THF)][PF6] (5) with atomic displacement parameters set at 50% probability. Only selected atoms are labeled, and all of the hydrogen atoms as well as the [PF6]− counterions have been omitted for clarity. Selected bond lengths and angles are listed in Table 1.

Table 1. Selected Bond Distances and Angles in 2−5 Fe−Y

Y−Fe−Z

complex

length (Å)

Y

angle (deg)

Y, Z

2

2.075(10) 2.077(11) 2.001(10) 2.028(10)

C1 C2 C3 C4

3

1.947(4) 1.961(4) 1.953(4) 1.972(4)

C1 C2 C3 C4

4

1.9925(17) 2.0035(18)

C1 C2

5

2.016(3) 2.001(3) 2.182(3)

C1 C2 O

113.7(4) 99.7(4) 112.6(4) 115.3(5) 100.9(4) 115.4(4) 91.25(16) 89.79(16) 89.41(16) 89.56(16) 178.90(16) 178.81(14) 89.98(7) 90.21(7) 179.23(7) 161.27(13) 101.85(11) 96.87(12)

C1, C2 C1, C3 C1, C4 C2, C3 C2, C4 C3, C4 C1, C2 C1, C4 C2, C3 C3, C4 C1, C3 C2, C4 C1, C1* C1, C2* C1, C2 C1, C2 O, C1 O, C2

Figure 4. Molecular structure of 6 with atomic displacement parameters set at 50% probability. Only selected atoms are labeled, and all of the hydrogen atoms as well as the [PF6]− counterion have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe−C5 2.087(3), Fe−C6 2.020(3), Fe−C7 2.029(3), Fe−C8 2.094(3), C1−C2 1.508(4), C2−C3 1.522(4), C2−C8 1.525(4), C3−C4 1.332(5), C3−C5 1.456(4), C1−N1 1.352(4), C1−N2 1.350(4), C2−C8−C7 120.9(3), C3−C5−C6 124.0(3).

For the synthesis of 6, the use of KC8 was not necessarily required. The reaction of [Fe(η6-mesitylene)2][PF6]2 with 2 equiv of LiPr afforded 6 as orange crystals in 35% yield (Scheme 2). As a byproduct, this reaction provided the corresponding

and other solid reaction products and analyzed by single-crystal X-ray diffraction. The observed iron complex 6 (Figure 4) is best described as a sandwich-type iron complex with two sixmembered rings derived from the mesitylene ligands. One of these rings remains attached as an η6 ligand, but the other is bound to iron in an η4 fashion via four carbon atoms. An LiPr carbene ligand is attached to one of the remaining carbon atoms of the six-membered ring, forming a C1−C2 single bond with a bond length of 1.508(4) Å. This C1−C2 bond formation results in the generation of an imidazolium cation, and the C1− N bonds in the N-heterocycle in 6 (1.350(4), 1.352(4) Å) are slightly longer than those in the imidazolium salts [HLiPr][X] (X = Cl, Br, I) (1.32(1)−1.337(3) Å)17 and comparable to those in Fe−LiPr complexes (1.333(7)−1.383(2) Å).2a,18 The localization of the cationic charge on the N-heterocycle suggests that the iron center should be assigned as Fe(0), resulting in a total of 18 electrons considering one η6-arene and one η4-diene ligand. In the η4-bound six-membered ring, three C−C single bonds can be found (C2−C3, 1.522(4) Å; C2−C8, 1.525(4) Å; C3−C5, 1.456(4) Å), commensurate with the loss of aromaticity in the ring. The short C3−C4 distance (1.332(5) Å) is indicative of considerable double-bond character, and thus, one of the C4−H bonds should have been cleaved during the formation of 6.

Scheme 2

imidazolium salt [HLiPr][PF6], which was not isolated but was identified crystallographically (see the Supporting Information). Complex 6 is diamagnetic, and its 1H NMR spectrum in CD3CN exhibits η6-mesitylene signals at 2.18 ppm (9H) and 4.90 ppm (3H) as well as methyl signals associated with the isopropyl groups at 1.13, 1.31, 1.48, and 1.60 ppm. The formation of 6 involves the formation of a C−C bond between LiPr and mesitylene, probably via a nucleophilic C


Article

Organometallics addition of the former to the latter. Nucleophilic addition of MeLi to [Fe(η6-C6Me6)2][PF6]2 has been reported to generate the η5-cyclohexadienyl complex [(η5-C6Me7)Fe(η6-C6Me6)][PF6], which further reacts with another equivalent of MeLi to furnish Fe(η6- C6Me6)(η4-C6Me8), which is an Fe(0) η6-arene/ η4-diene complex analogous to 6.19 In a similar manner, the nucleophilic addition of LiPr to mesitylene in [Fe(η6mesitylene)2][PF6]2 could initially afford an η6-mesitylene η5C6Me3H3-LiPr complex. Subsequent deprotonation of one of the methyl groups in η5-C6Me3H3-LiPr by a second equivalent of LiPr would then lead to the observed formation of 6 and [HLiPr][PF6] (Scheme 3). Scheme 3. Possible Reaction Pathway for the Formation of 6

Electronic Properties of 2−5. Complexes 2−5 contain iron in the +I oxidation state, i.e., with a d7 configuration. Accordingly, both low-spin (S = 1/2) and high-spin (S = 3/2) states are possible for these complexes. The electron paramagnetic resonance (EPR) spectra of 2−4 were measured in THF and in the solid state, whereas the spectrum of 5 was measured only in THF (Figure 5).20 In the solid state, squareplanar 3 and 4 showed nearly axial signals, and the observed g values (g = 2.710, 2.635, and 1.935 for 3; g = 2.752 and 1.911 for 4) were indicative of the presence of a low-spin state. Even though the S = 1/2 signals of 3 and 4 appear at lower field than that of a free electron (ge = 2.0023), analogous axial signals have been observed for the square-planar Rh(II) complex transRh{P(TMP)3}2(CNtBu)2 (TMP = 2,4,6-trimethoxyphenyl) at g = 2.45 and 1.9621 as well as for the square-planar Co(II)− NHC complex [Co(L Et ) 4 ]2+ at g = 2.95 and 2.02. 22 Interestingly, the EPR spectrum of 4 in THF gave rise to an additional high-spin signal at g = 4.356, 3.935, and 1.993, while the solution spectrum of 3 is consistent with that in solid state. An analogous high-spin signal was observed for the threecoordinate complex 5 in THF at g = 4.593, 3.787, and 1.978. The high-spin signals of 4 and 5 are comparable, and this similarity indicates that the S = 3/2 signal in the solution spectrum of 4 arises from a three-coordinate complex. [Fe(LiPr)3]+ and [Fe(LiPr)2(THF)]+ could be feasibly envisioned as potential candidates for such a three-coordinate complex, as they should be generated easily via the release of LiPr ligand(s) on account of the steric congestion imposed by LiPr. The dissociation of LiPr in solution is supported by the observation of free LiPr in the 1H NMR spectrum of 4 in THFd8. Relevant dissociation behavior of a bulky NHC ligand has

Figure 5. EPR spectra of 2−5 in THF solution (left) and in the solid state (right).

been reported for a three-coordinate Fe(II) complex.23 Tetrahedral 2 is expected to be in the high-spin state (S = 3 /2), and indeed its EPR spectrum in the solid state exhibited an S = 3/2 signal at g = 4.756 and 2.008. However, the EPR spectrum of 2 in solution showed an additional sharp signal at g = 2.081. The appearance of this signal is consistent with the presence of an S = 1/2 species, which could be the square-planar isomer of 2. The cyclic voltammograms of 2−5 were measured in THF at room temperature in the presence of [nBu4N][PF6] as the supporting electrolyte, and the potentials are referenced against Ag/Ag+ (Figure 6). Complex 2 showed an irreversible anodic wave at Epa = −1.80 V for the oxidation of Fe(I) to Fe(II). A corresponding cathodic wave for the reduction of Fe(II) to Fe(I) was found at Epc = −1.92 V when 1 equiv of LMe was added to the solution. This behavior indicates facile dissociation of LMe from [Fe(LMe)4]2+ ([2]+) to generate [Fe(LMe)3]2+, which readily incorporates an additional LMe to regenerate [2]+. The cyclic voltammogram of square-planar 3 exhibited a quasireversible Fe(I)/Fe(II) redox couple at E1/2 = −1.83 V. On the other hand, irreversible anodic and cathodic waves were observed for 4 at Epa = −1.42 V and Epc = −2.18 V, respectively. The reduction wave of 4 at Epc = −2.18 V appeared after scanning the positive region, and therefore, this D


Organometallics

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Article

EXPERIMENTAL SECTION

General Procedures. All of the reactions were carried out under a dinitrogen atmosphere using either Schlenk line or glovebox techniques. Toluene, diethyl ether, and THF were purified by the method described by Grubbs,24 in which solvents are passed over columns containing activated alumina and a supported copper catalyst supplied by Hansen & Co. Ltd. UV−vis spectra were measured on a JASCO V560 spectrometer at room temperature. Prior to use, the deuterated solvents THF-d8 and CD3CN were distilled from sodium and CaH2, respectively. 1H and 13C{1H} NMR spectra were recorded on a JEOL ECA-600 spectrometer. 1H NMR signals were referenced to the residual proton peaks of the deuterated solvent. 13C NMR signals were referenced to the carbon signals of the deuterated solvents. The EPR spectra of 2−5 were recorded on a Bruker EMXplus spectrometer at X-band frequencies. Cyclic voltammetry measurements were performed in a single-compartment cell under a dinitrogen atmosphere at room temperature using a BSA-660B electrochemical analyzer. A three-electrode setup was employed, comprising a glassy carbon working electrode, a platinum wire auxiliary electrode, and a Ag/AgNO3 quasi-reference electrode with 0.2 M [nBu4N][PF6] (TCI Co., Ltd.) as the supporting electrolyte. The supporting electrolyte was used after recrystallization from THF. Magnetic susceptibilities in solution were determined by the Evans method using THF-d8/tetramethylsilane solutions. Elemental analyses were carried out on an Elementar Vario MICRO cube instrument, for which crystalline samples were sealed in tin capsules under an atmosphere of dinitrogen. Mössbauer spectra were not measured in this study, partly because of the lack of adequate facilities and instrumentation and partly because of the lack of a corresponding license for the use of γ rays. [Fe(η6-toluene)2][PF6]2 (1)13 and Nheterocyclic carbenes14 were prepared according to literature procedures. Other chemicals were purchased from common commercial sources and used without purification. [Fe(LMe)4][PF6] (2). A suspension of KC8 (137 mg, 1.01 mmol) in THF (10 mL) was added dropwise to a suspension of [Fe(η6toluene)2][PF6]2 (1) (409 mg, 0.77 mmol) in THF (10 mL) at −30 °C. The resulting mixture was allowed to gradually warm to room temperature over 30 min, affording a dark suspension containing black and white solids. After dropwise addition of a solution of LMe (384 mg, 3.09 mmol) in THF (8 mL), the mixture was stirred for 4 h. The reddish-orange solution was separated from the insoluble materials by centrifugation and evaporated to dryness under reduced pressure. The resultant solid was washed with toluene (2 × 10 mL) and extracted with THF (15 mL). The solvent was removed under reduced pressure to afford 2 as an orange powder (219 mg, 31% yield). Single crystals suitable for X-ray diffraction analysis were obtained by recrystallization from THF at −40 °C. 1H NMR (THF-d8): δ 19.1 (w1/2 = 50 Hz), −19.4 (w1/2 = 880 Hz). EPR (X-band, microwave 1.0 mW, 9 K, THF): two sets of signals at g = 3.886 as well as at g = 2.081 and 2.002. EPR (X-band, microwave 1.0 mW, 8 K, solid): g = 4.189 and 2.008. Cyclic voltammogram (2.9 mM in THF): Epa = −1.80 V vs Ag/Ag+. Magnetic susceptibility (THF-d8): μeff = 4.35μB (295 K), 4.38μB (253 K), 4.33μB (223 K). UV−vis (THF): 336 nm (ε = 1800 M−1 cm−1). Anal. Calcd for C28H48N8FePF6: C, 48.21; H, 6.94; N, 16.06. Found: C, 47.87; H, 6.82; N, 15.90. [Fe(LEt)4][PF6] (3). The synthetic procedure for 3 was analogous to that for 2. A dark-green solid was obtained by sequential treatment of a suspension of [Fe(η6-toluene)2][PF6]2 (1) (409 mg, 0.77 mmol) in THF (10 mL) with a suspension of KC8 (122 mg, 0.91 mmol) in THF (10 mL) and a solution of LEt (549 mg, 3.61 mmol) THF (10 mL) followed by centrifugation and evaporation. The obtained dark-green solid was washed with toluene (2 × 10 mL) and extracted with THF (15 mL). The solution was concentrated to ca. 7 mL and stored at −30 °C to afford 3 (344 mg, 47% yield) as dark-green crystals. 1H NMR (THF-d8): δ 8.7 (w1/2 = 190 Hz), 1.5 (w1/2 = 70 Hz), −14.2 (w1/2 = 420 Hz). EPR (X-band, microwave 1.0 mW, 8 K, THF): g = 2.712, 2.631, and 1.930. EPR (X-band, microwave 1.0 mW, 8 K, solid): g = 2.710, 2.635, and 1.935. Cyclic voltammogram (3.6 mM in THF): E1/2 = −1.83 V vs Ag/Ag+. Magnetic susceptibility (THF-d8): μeff =

Figure 6. Cyclic voltammograms of 2−5 in THF with 0.2 M [nBu4N][PF6] as the supporting electrolyte: (a) 2; (b) 2 + LMe (1 equiv); (c) 3; (d) 4; (e) 5.

wave can be ascribed to the reduction process of Fe(II) to Fe(I). As discussed in the previous sections, on the basis of the 1 H NMR and EPR spectra, 4 is probably susceptible to ligand dissociation in solution, and the observed irreversibility of the waves in the cyclic voltammogram of 4 should accordingly be associated with the reversible dissociation of LiPr. In the cyclic voltammogram of 5, a quasi-reversible redox couple was observed at E1/2 = −1.10 V (Fe(I)/Fe(II)), the potential of which is shifted to more positive values relative to those of fourcoordinate complexes 2−4. Complex 5 exhibited another redox couple at E1/2 = −2.08 V, which indicates that 5 may be reduced to Fe(0) on the cyclic voltammetry time scale.

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CONCLUSION This work provides a convenient synthetic route to a series of mononuclear Fe(I) complexes bearing NHC ligands. The coordination number and geometry of the iron center in these Fe(I) complexes vary depending on the substituents on nitrogen (the R groups) in the NHC ligands: tetrahedral for R = Me, square-planar for R = Et and iPr, and T-shaped threecoordinate for R = Mes, which contains an additional THF ligand. The square-planar Fe(I) complexes (R = Et, iPr) are in the low-spin state (S = 1/2), while the tetrahedral (R = Me) and three-coordinate (R = Mes) complexes are in the high-spin state (S = 3/2). LiPr ligand(s) dissociate from square-planar 4 (R = iPr) in solution, thus generating a three-coordinate species, while tetrahedral 2 (R = Me) possibly generates a low-spin (S = 1 /2) species in solution. In light of the suggested importance of Fe(I) species in cross-coupling reactions,6 these new Fe(I) complexes should provide the opportunity to examine their catalytic applications. E


Article

Organometallics Table 2. Crystallographic Data and Refinement Summary for 2−6 and [HLiPr][PF6] formula fw T (K) cryst. syst. space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g/cm3) R1a wR2b GOFc

2

3

4

5

6

[HLiPr][PF6]

C28H48F6FeN8P 697.55 173 orthorhombic P21212 (No. 18) 19.915(9) 19.915(9) 8.807(4)

C48H64F6FeN8O3P 1001.90 173 triclinic P1̅ (No. 2) 11.329(3) 12.795(4) 19.630(5) 97.530(4) 90.594(4) 102.621(14) 2750.5(14) 2 1.210 0.0858 0.1962 1.011

C50H84F6FeN8P 998.06 173 monoclinic C2/c (No. 15) 22.410(6) 11.528(3) 20.720(5)

C50H56F6FeN4O2P 945.83 173 triclinic P1̅ (No. 2) 11.622(3) 13.425(3) 16.377(5) 95.890(5) 98.930(4) 97.354(5) 2483.8(11) 2 1.265 0.0768 0.2481 1.076

C29H43F6FeN2P 620.48 173 monoclinic P21/n (No. 14) 12.460(3) 13.164(4) 18.513(2)

C11H21F6N2P 326.26 173 orthorhombic Pnma (No. 62) 11.922(3) 14.727(4) 8.545(2)

3493(3) 4 1.326 0.0798 0.2278 1.012

91.487(4) 5351(3) 4 1.261 0.0594 0.1713 1.050

100.418(4) 2986.7(12) 4 1.380 0.0689 0.2104 1.112

1500.2(7) 4 1.444 0.0751 0.2199 1.085

a R1 = ∑||Fo| − |Fc||/∑|Fo| (I > 2σ(I)). bwR2 = [∑w(|Fo| − |Fc|)2/∑wFo2]1/2 (all reflections). cGOF = [∑w(|Fo| − |Fc|)2/(No − Nv)]1/2, where No is the number of observations and Nv is the number of variables.

Hz), −27.4 (w1/2 = 120 Hz). EPR (X-band, microwave 1.0 mW, 10 K, THF): g = 4.593, 3.787, and 1.978. Cyclic voltammogram (3.2 mM in THF): E1/2 = −1.11 V vs Ag/Ag+ for Fe(II)/Fe(I) and −2.07 V vs Ag/ Ag+ for Fe(I)/Fe(0). Magnetic susceptibility (THF-d8): μeff = 5.55μB (295 K), 5.96μB (253 K), 5.88μB (223 K). UV−vis (THF): 381 nm (sh, ε = 1300 M−1 cm−1). Anal. Calcd for C46H56N4OFePF6: C, 62.66; H, 6.40; N, 6.35. Found: C, 62.39; H, 6.43; N, 6.13. Complex 6. A solution of LiPr (295 mg, 1.64 mmol) in THF (10 mL) was added to a suspension of [Fe(η6-mesitylene)2][PF6]2 (480 mg, 0.82 mmol) in THF (10 mL) at −30 °C. The reaction mixture was allowed to warm to room temperature and was then stirred overnight. The resultant reddish-orange suspension was centrifuged to separate the insoluble materials containing the imidazolium salt [HLiPr][PF6] and parts of precipitated 6, and then the solution was evaporated to dryness. The resultant solid was washed with toluene (10 mL) and Et2O (10 mL) and then extracted with THF (20 mL). This solution was concentrated to ca. 5 mL, layered with Et2O, and stored for several weeks at −40 °C to afford 6 (179 mg, 35% yield) as reddish-orange crystals. 1H NMR (CD3CN): δ 1.01 (s, 3H), 1.13 (d, 3H, J = 6.9 Hz), 1.31 (d, 3H, J = 7.1 Hz), 1.48 (d, 3H, J = 6.7 Hz), 1.56 (s, 3H), 1.60 (d, 3H, J = 6.9 Hz), 2.08 (s, 3H), 2.18 (s, 9H), 2.23 (s, 3H, J = 4.6 Hz), 2.67 (s, 1H), 3.78 (s, 1H), 3.94 (m, 1H), 4.25 (s, 1H), 4.36 (s, 1H), 4.47 (m, 1H), 4.66 (s, 1H), 4.90 (s, 3H). 13C NMR (CD3CN): δ 8.5, 10.6, 10.8, 19.5, 19.6, 20.7, 22.6, 22.7, 23.8, 45.2, 50.0, 51.2, 51.4, 55.5, 58.5, 77.2, 86.5, 87.6, 96.4, 98.7, 127.3, 127.6, 144.7, 149.0. Anal. Calcd for C29H43N2FePF6: C, 56.14; H, 6.99; N, 4.52. Found: C, 55.84; H, 6.68; N, 4.63. X-ray Diffraction Analysis. The crystal data and refinement parameters for 2−6 are summarized in Table 2. Single crystals were coated with oil (immersion oil type B, code 1248, Cargille Laboratories, Inc.) and mounted on loops (CryoLoop), and diffraction data were collected at −100 °C under a cold dinitrogen stream on a Rigaku RA-Micro7 diffractometer equipped with a Saturn70 CCD detector using graphite-monochromatized Mo Kα radiation (λ = 0.710690 Å). Six preliminary data frames were measured at 0.5° increments of ω in order to assess the crystal quality and preliminary unit cell parameters. Additionally, intensity images were also measured at 0.5° intervals of ω. The frame data were integrated using the CrystalClear program package, and the data sets were corrected for absorption using the REQAB program. Calculations were performed with the CrystalStructure program package. All of the structures were solved by direct methods and refined by full-matrix least-squares. An anisotropic refinement was applied to all of the atoms except for disordered atoms and hydrogen atoms, which were located at calculated positions. Complex 2 crystallizes in the form of merohedral

2.76μB (295 K), 2.13μB (253 K), 1.82μB (223 K). UV−vis (THF): 702 nm (ε = 1600 M−1 cm−1), 477 nm (ε = 2900 M−1 cm−1), 412 nm (ε = 6400 M−1 cm−1), 344 nm (ε = 8200 M−1 cm−1). Anal. Calcd for C36H64N8FePF6: C, 53.40; H, 7.97; N, 13.84. Found: C, 53.79; H, 7.86; N, 13.51. [Fe(LiPr)4][PF6] (4). The synthetic procedure for 4 was analogous to that for 2. A green solid was obtained from the sequential treatment of a suspension of [Fe(η6-toluene)2][PF6]2 (1) (458 mg, 0.86 mmol) in THF (10 mL) with a suspension of KC8 (120 mg, 0.89 mmol) in THF (10 mL) and a solution of LiPr (630 mg, 3.49 mmol) in THF (10 mL) followed by centrifugation and evaporation. The green solid was washed with toluene (2 × 10 mL) and extracted with THF (15 mL). The solution was concentrated to ca. 7 mL and stored at −30 °C to afford 4 (434 mg, 54% yield) as green crystals. Single crystals suitable for X-ray diffraction analysis were obtained by recrystallization from THF at −30 °C in the presence of some drops of C6H5F. 1H NMR (THF-d8): δ 27.3 (w1/2 = 30 Hz), 27.1 (w1/2 = 40 Hz), 11.9 (w1/2 = 150 Hz), 11.4 (w1/2 = 140 Hz), −6.3 (w1/2 = 70 Hz), −12.0 (w1/2 = 2600 Hz). Signals for free LiPr were also observed at δ 4.19, 2.06, and 1.40. In the presence of an equimolar amount of Si(SiMe3)4 (5.5 mg) as the internal standard, the 297 K 1H NMR spectrum of a portion of a solution of 4 (15.8 mg) in THF-d8 (1.45 mL) exhibited ca. 1 equiv of free LiPr. EPR (X-band, microwave 1.0 mW, 8 K, THF): two sets of signals at g = 2.747 and 1.898 as well as at g = 4.356, 3.935, and 1.993. EPR (X-band, microwave 1.0 mW, 8 K, solid): g = 2.752 and 1.911. Cyclic voltammogram (3.1 mM in THF): Epa = −1.42 V vs Ag/Ag+, Epc = −2.18 V vs Ag/Ag+. Magnetic susceptibility (THF-d8): μeff = 3.84μB (295 K). UV−vis (THF): 338 nm (ε = 2400 M−1 cm−1). Anal. Calcd for C44H80N8FePF6: C, 57.32; H, 8.75; N, 12.15. Found: C, 57.06; H, 8.35; N, 11.15. Satisfactory values for the elemental analysis of 4 could not be obtained. Compared with the theoretical values, crystals of 4 consistently produced lower C, H, N values, possibly because of thermal instability arising from either dissociation of LiPr at higher temperatures or incomplete combustion. [Fe(LMes)2(THF)][PF6] (5). The synthetic procedure for 5 was analogous to that of 2, except for the amount of NHC ligand employed. A deep-red solid was obtained from the sequential treatment of a suspension of [Fe(η6-toluene)2][PF6]2 (1) (472 mg, 0.89 mmol) in THF (10 mL) with a suspension of KC8 (123 mg, 0.91 mmol) in THF (10 mL) and a solution of LMes (603 mg, 1.77 mmol) in THF (10 mL) followed by centrifugation and evaporation. The thus-obtained residue was washed with toluene (2 × 10 mL) and extracted with THF (20 mL). The solution was concentrated to ca. 7 mL and stored at −40 °C to afford 5 (445 mg, 57% yield) as deep-red crystals. 1H NMR (THF-d8): δ 1.1 (w1/2 = 600 Hz), 0.21 (w1/2 = 100 F


Article

Organometallics

I.; Chantarojsiri, T.; Dong, Y.; Tilley, T. D. J. Am. Chem. Soc. 2015, 137, 6366−6372. (4) The highest oxidation state reported for iron in iron−NHC complexes is +V, which has been accomplished by the use of a scorpionate NHC ligand. See: Scepaniak, J. J.; Vogel, C. S.; Khusniyarov, M. M.; Heinemann, F. W.; Meyer, K.; Smith, J. M. Science 2011, 331, 1049−1052. (5) (a) Compounds of Group 8; Bruce, M., Ed.; Comprehensive Organometallic Chemistry III, Vol. 6; Crabtree, R. H., Mingos, M. P., Eds. in Chief; Elsevier: Amsterdam, 2007. (b) Knö lker, H.-J. Organoiron Chemistry. In Organometallics in Synthesis: Third Manual; John Wiley & Sons: Hoboken, NJ, 2013. (6) (a) Smith, R. S.; Kochi, J. K. J. Org. Chem. 1976, 41, 502−509. (b) Kleimark, J.; Hedström, A.; Larsson, P.−F.; Johansson, C.; Norrby, P.−O. ChemCatChem 2009, 1, 152−161. (c) Hedström, A.; Bollmann, U.; Bravidor, J.; Norrby, P.−O. Chem. - Eur. J. 2011, 17, 11991−11993. (d) Adams, C. J.; Bedford, R. B.; Carter, E.; Gower, N. J.; Haddow, M. F.; Harvey, J. N.; Huwe, M.; Cartes, M. A.; Mansell, S. M.; Mendoza, C.; Murphy, D. M.; Neeve, E. C.; Nunn, J. J. Am. Chem. Soc. 2012, 134, 10333−10336. (e) Bedford, R. B.; Carter, E.; Cogswell, P. M.; Gower, N. J.; Haddow, M. F.; Harvey, J. N.; Murphy, D. M.; Neeve, E. C.; Nunn, J. Angew. Chem., Int. Ed. 2013, 52, 1285−1288. (f) Bedford, R. B.; Brenner, P. B.; Carter, E.; Carvell, T. W.; Cogswell, P. M.; Gallagher, T.; Harvey, J. N.; Murphy, D. M.; Neeve, E. C.; Nunn, J.; Pye, D. R. Chem. - Eur. J. 2014, 20, 7935−7938. (g) Jin, M.; Adak, L.; Nakamura, M. J. Am. Chem. Soc. 2015, 137, 7128−7134. (7) For examples, see: (a) Smith, J. M.; Lachicotte, R. J.; Pittard, K. A.; Cundari, T. R.; Lukat-Rodgers, G.; Rodgers, K. R.; Holland, P. L. J. Am. Chem. Soc. 2001, 123, 9222−9223. (b) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 322−323. (c) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 10782−10783. (d) Smith, J. M.; Sadique, A. R.; Cundari, T. R.; Rodgers, K. R.; Lukat-Rodgers, G.; Lachicotte, R. J.; Flaschenriem, C. J.; Vela, J.; Holland, P. L. J. Am. Chem. Soc. 2006, 128, 756−769. (e) Mankad, N. P.; Whited, M. T.; Peters, J. C. Angew. Chem., Int. Ed. 2007, 46, 5768−5771. (f) Sadique, A. R.; Brennessel, W. W.; Holland, P. L. Inorg. Chem. 2008, 47, 784− 786. (g) Vogel, C.; Heinemann, F. W.; Sutter, J.; Anthon, C.; Meyer, K. Angew. Chem., Int. Ed. 2008, 47, 2681−2684. (h) Cowley, R. E.; DeYonker, N. J.; Eckert, N. A.; Cundari, T. R.; DeBeer, S.; Bill, E.; Ottenwaelder, X.; Flaschenriem, C.; Holland, P. L. Inorg. Chem. 2010, 49, 6172−6187. (i) Lee, Y.; Kinney, R. A.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 16366−16369. (j) Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84−87. (k) Dugan, T. R.; Bill, E.; MacLeod, K. C.; Christian, G. J.; Cowley, R. E.; Brennessel, W. W.; Ye, S.; Neese, F.; Holland, P. L. J. Am. Chem. Soc. 2012, 134, 20352− 20364. (l) Creutz, S. E.; Peters, J. C. J. Am. Chem. Soc. 2014, 136, 1105−1115. (8) (a) Darensbourg, M. Y.; Lyon, E. J.; Smee, J. J. Coord. Chem. Rev. 2000, 206−207, 533−561. (b) Liu, X.; Ibrahim, S. K.; Tard, C.; Pickett, C. J. Coord. Chem. Rev. 2005, 249, 1641−1652. (c) Heinekey, D. M. J. Organomet. Chem. 2009, 694, 2671−2680. (d) Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245−2274. (e) Gloaguen, F.; Rauchfuss, T. B. Chem. Soc. Rev. 2009, 38, 100−109. (f) Barton, B. E.; Olsen, M. T.; Rauchfuss, T. B. Curr. Opin. Biotechnol. 2010, 21, 292− 297. (g) Wang, N.; Wang, M.; Chen, L.; Sun, L. Dalton Trans. 2013, 42, 12059−12071. (9) For [FeFe] hydrogenase model compounds with NHCs, see: (a) Capon, J.−F.; El Hassnaoui, S.; Gloaguen, F.; Schollhammer, P.; Talarmin, J. Organometallics 2005, 24, 2020−2022. (b) Tye, J. W.; Lee, J.; Wang, H.−W.; Mejia-Rodriguez, R.; Reibenspies, J. H.; Hall, M. B.; Darensbourg, M. Y. Inorg. Chem. 2005, 44, 5550−5552. (c) Morvan, D.; Capon, J.−F.; Gloaguen, F.; Le Goff, A.; Marchivie, M.; Michaud, F.; Schollhammer, P.; Talarmin, J.; Yaouanc, J.−J.; Pichon, R.; Kervarec, N. Organometallics 2007, 26, 2042−2052. (d) Duan, L.; Wang, M.; Li, P.; Na, Y.; Wang, N.; Sun, L. Dalton Trans. 2007, 1277− 1283. (e) Jiang, S.; Liu, J.; Shi, Y.; Wang, Z.; Akermark, B.; Sun, L. Polyhedron 2007, 26, 1499−1504. (f) Song, L.−C.; Luo, X.; Wang, Y.− Z.; Gai, B.; Hu, Q.−M. J. Organomet. Chem. 2009, 694, 103−112. (g) Morvan, D.; Capon, J.−F.; Gloaguen, F.; Pétillon, F. Y.;

twin crystals. Therefore, an orthorhombic space group (P21212, No. 18) was assigned, where the equal unit cell lengths a and b lead to a tetrahedral cell. In the SHELXL refinement, a twin law of (0, 1, 0, 1, 0, 0, 0, 0, −1) was applied, and the BASF parameter was refined to 0.434(2).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b01025. Solid-state structure of [HLiPr][PF6] and UV−vis and 1H NMR spectra of 2−5 (PDF) Crystallographic data for 2−6 and [HLiPr][PF6] (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.O.). *E-mail: [email protected] (K.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the PRESTO Program of the Japan Science and Technology Agency (JST) and Grants-in-Aid for Scientific Research (23000007, 23685015) as well as a Grant-in-Aid for Scientific Research on Innovative Areas (15H00936) from the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). The authors thank Prof. Hiroshi Nakajima (Osaka City University) for helpful discussions on the EPR spectra and gratefully acknowledge Dr. Kimiko Hasegawa (Rigaku Corporation) and Prof. Tsuyoshi Matsumoto (Nagoya University) for fruitful advice regarding solutions for the solid-state structure of 2 and Tsukasa Murayama and Keiya Aoyagi (Nagoya University) for technical assistance.

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REFERENCES

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H


N-Heterocyclic Carbene Complexes of Three- and Four-Coordinate Fe

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