Dinuclear Iron(0) Complexes of N-Heterocyclic Carbenes

(9, 10) On the other hand, NHC complexes of low-valent iron have been ..... Calcd for C21H24N2FeCl3: C, 54.05; H, 5.18; N, 6.00. .... −40 °C. 1H NM...
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Dinuclear Iron(0) Complexes of N‑Heterocyclic Carbenes Takayoshi Hashimoto,† Ryoko Hoshino, Tsubasa Hatanaka,‡ Yasuhiro Ohki,* 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 S Supporting Information *

ABSTRACT: The synthesis, structures, and reactivity of dinuclear Fe0 complexes of N-heterocyclic carbenes (NHCs, denoted as L) are reported. The NHC adducts of ferric chloride (L)FeCl3 were prepared from the reactions of FeCl3 with L in toluene. The reduction of (L)FeCl3 with KC8 resulted in the formation of the dinuclear Fe0 complexes Fe 2 {μ-η 1 (C):η 6 (arene)-L} 2 (2a, L = 1,3-bis(2,4,6trimethylphenyl)imidazol-2-ylidene (IMes); 2b, L = 1,3bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr)), in which NHC ligands bridge two iron atoms using one of the arene rings as an η6 ligand. Their magnetic properties are different: 2a is paramagnetic and 2b is diamagnetic. The dinuclear complexes 2a,b serve as precursors for monomeric (NHC)Fe0 species, and treatment of 2a,b with 1 atm of CO led to the formation of (L)Fe(CO)4. Complex 2a was found to react with 1-azidoadamantane, giving rise to the dinuclear tetrazene complex (IMes)Fe(μ-NAd)2Fe(AdNNNNAd) (4).

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include Fe2−carbonyl−dithiolate complexes of NHC ligands mimicking the active site of [FeFe]-hydrogenase,11 NHC adducts of Fe(COT)2 (COT = cyclooctatetraene),12 FeI and Fe0 complexes having a tridentate or a tetradentate ligand containing NHC groups,13 and carbonyl/NHC complexes of FeI and Fe0.14 In our continuing study on organoiron complexes,15 we have synthesized half-sandwich FeII complexes of NHCs, which activate the C−H bonds of heteroarenes and benzene.15b,c Recently we have also reported FeII−dichloride and FeII−dimethyl complexes of NHCs, which serve as good catalyst precursors for the hydrosilylation of ketones.15e,f In this study, we have examined reactive NHC complexes of lowvalent iron. Herein we report a new class of dinuclear Fe0 complexes having bulky NHCs and their reactions with CO and 1-azidoadamantane.

INTRODUCTION Low-valent transition-metal complexes have been of interest because of their utility in the activation of small molecules. For example, low-coordinate complexes of low-valent iron (FeI or lower) have been shown to reduce N2, leading to substantial weakening or breaking of the NN bond.1 Some 16-electron Fe0−phosphine complexes, such as Fe(PMe3)4 and Fe(dmpe)2 (dmpe = bis(dimethylphosphino)ethane), have been found to promote intra- and intermolecular C−H bond activation.2 Lowvalent iron is probably also available in nature. [FeFe]hydrogenase, which catalyzes biological H2 production and consumption, has a dinuclear iron−thiolate moiety in the active site.3 This dinuclear unit is relevant to the known FeI−FeI complexes (CO)3Fe(μ-SR)2Fe(CO)3,4 and the FeI−FeI and FeII−FeI states have been suggested to be involved in the catalytic cycle.5 One of the FeII−FeI models of [FeFe]hydrogenase carrying a redox-functional unit, [(CO)2(FcP*)Fe{μ-(SCH2)2NBn}Fe(dppv)(CO)]+ (FcP* = (C5Me5)Fe(C5Me4CH2PEt2), dppv = cis-bis(diphenylphosphino)ethene), was found to undergo both proton reduction and H2 activation, reproducing the enzymatic reactions.6 Notably, good πacceptor ligands such as CO and phosphines have been commonly employed for the stabilization of low-valent iron complexes. N-heterocyclic carbenes (NHCs) have received considerable attention as ligands for transition metals.7 They are used as 2edonor ligands like phosphines; however, their stronger σdonating and weaker π-accepting abilities make NHCs unique.8 NHCs often make strong bonds with transition metals and facilitate the synthesis of iron complexes, particularly in the FeII state.9,10 On the other hand, NHC complexes of low-valent iron have been relatively less explored. Previous examples © XXXX American Chemical Society

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RESULTS AND DISCUSSION Synthesis and Structures of (NHC)FeCl3. The synthesis of (NHC)FeCl3 was recently reported by Tonzetich et al.,16 and in situ generation of (NHC)FeX3 (X = F, Cl) for some catalytic reactions has been reported by others.17 With a slight modification of the procedure by Tonzetich et al., we also prepared a series of NHC adducts of FeCl3. Treatment of anhydrous FeCl3 with 1 equiv of NHCs in toluene afforded (L)FeCl3 (1a, L = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2ylidene (IMes); 1b, L = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr); 1c, L = 1,3-bis[(3,5-dimethylphenyl)methyl]imidazol-2-ylidene (IBnMe2); 1d, L = 1,3-diisopropyl4,5-dimethylimidazol-2-ylidene (IiPr)), in good to excellent Received: October 25, 2013

A

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Synthesis, Structures, and Properties of Dinuclear Fe0−NHC Complexes. Reduction of (IMes)FeCl3 (1a) with 3.1 equiv of KC8 in THF afforded the dinuclear Fe0−NHC complex Fe2{μ-η1(C):η6(mesityl)-IMes}2 (2a), which was isolated as dark brown crystals in 39% yield. An analogous complex containing IPr, Fe2{μ-η1(C):η6(iPr2C6H3)-IPr}2 (2b), was similarly synthesized as dark brown crystals in 22% yield (Scheme 2). The products from the reactions of 1c,d with KC8 remain uncharacterized.

isolated yields (72−97%) (Scheme 1). Complexes 1a−d, including the known complex 1b,16 were all characterized by Scheme 1

Scheme 2

elemental analysis, 1H NMR, and X-ray diffraction studies. These complexes are paramagnetic, and their 1H NMR spectra exhibited broad signals in the ranges of 50.8−2.4 ppm (1a), 53.0−2.1 ppm (1b), 53.0−2.5 ppm (1c), and 85.2−4.5 ppm (1d). The molecular structures of 1a−d were determined by X-ray diffraction and are shown in Figure 1 for 1a,c and Figures S1

Figure 2. Molecular structure of Fe2{μ-η1(C):η6(mesityl)-IMes}2 (2a) with thermal ellipsoids at the 50% probability level. Selected bond distances (Å) and angle (deg): Fe−Fe*, 2.6214(6); Fe−C1, 1.9792(18); Fe−C(η6-arene), 2.0202(19)−2.235(3); C1−Fe−Fe*, 89.08(7).

Figure 1. Molecular structures of (IMes)FeCl3 (1a) (top) and (IBnMe2)FeCl3 (1c) (bottom) with thermal ellipsoids at the 50% probability level. Selected bond distances (Å) and angles (deg) for 1a: Fe−C1, 2.091(4); Fe−Cl, 2.1770(13)−2.1827(12), 2.1800 (av); C1− Fe−Cl, 106.48(10)−110.53(10), 108.99 (av); Cl−Fe−Cl, 109.44(5)− 110.37(5), 109.95 (av). Selected bond distances (Å) and angles (deg) for 1c: Fe−C1, 2.081(3); Fe−Cl, 2.1710(11)−2.1881(11), 2.1772 (av); C1−Fe−Cl, 105.62(8)−114.96(10), 109.54 (av); Cl−Fe−Cl, 108.57(5)−109.82(5), 109.37 (av).

and S2 for 1b,d (Supporting Information). The iron atoms in complexes 1a−d are in a nearly ideal tetrahedral geometry with one NHC ligand and three chlorides, and the C−Fe−Cl and Cl−Fe−Cl angles range from 103.72(7) to 114.96(10)°. The Fe−C(carbene) distances are 2.081(3)−2.093(3) Å, which are comparable to those reported for tetrahedral FeIII−NHC complexes (1.909(4)−2.174(2) Å).16,18

Figure 3. Molecular structure of Fe2{μ-η1(C):η6(iPr2C6H3)-IPr}2 (2b) with thermal ellipsoids at the 50% probability level. Selected bond distances (Å) and angle (deg): Fe−Fe*, 2.5832(5); Fe−C1, 1.975(2); Fe−C(η6-arene), 2.0256(19)−2.203(3); C1−Fe−Fe*, 88.77(6). B

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As shown in Figures 2 and 3, X-ray diffraction studies for 2a,b reveal their unique dinuclear structures, each consisting of two Fe0 centers and NHC ligands. The iron center is coordinated by the carbene carbon and one of the arene rings of another NHC ligand in an η6 fashion. An analogous dinuclear structure with a μ-η1(C):η6(arene) coordination mode of NHC was found in the Ni0 complex Ni2{μη1(C):η6(iPr2C6H3)-IPr}2.19 Each nickel atom of this dinuclear complex possesses 18 electrons, and there is no Ni−Ni bonding interaction (Ni···Ni, 3.568(2) Å). In contrast, the iron atoms in 2a,b have 16 electrons, and the Fe−Fe distances are relatively short: 2.6214(6) Å (2a) and 2.5832(5) Å (2b). The Fe−C(η6arene) distances of 2a (2.0202(19)−2.235(3) Å) and 2b (2.0250(19)−2.208(3) Å) are comparable to reported Fe0− C(η6-arene) distances (1.990−2.238 Å).20 Interestingly, the magnetic properties of 2a,b are distinctively different. Complex 2b is diamagnetic, and the 1H NMR spectrum exhibited sharp signals in the range 7.79−0.66 ppm.21 In contrast, complex 2a is paramagnetic, and its 1H NMR in C6D6 exhibited broad signals in the range from +17.8 to −9.47 ppm.21 The solution magnetic moment of 2a in C6D6 obtained from the Evans method22 was μeff = 4.4 μB at 297.7 K. The temperature dependence of magnetization of 2a was also measured at 2−300 K (Figure 4). The effective magnetic

Figure 5. Possible d-orbital interactions in 2a,b.

Figure 6. Structures of 2a (left) and 2b (right), highlighting the planes defined by iron atoms, carbene carbons, and the centroids of ironbound aromatic rings.

is 36.34(9)°. The C−Fe−Fe*−C* torsion angle of 2a is 156.34(6)°. As can be seen from Figure 6 (left, top and bottom), the Fe−Fe* vector of 2a is out of both xy planes, due to the distortion between two Fe(IMes) units. Such distortion in 2a possibly results in less efficient interactions of dx2−y2 and dyz orbitals between two iron centers (Figure 5 right), leading to the S = 2 state. On the other hand, the orbital interactions are efficient in 2b, allowing the S = 0 state. The distortion between two Fe(IMes) units of 2a could be due to steric congestion between the mesityl groups, as the 4-methyl group of the η6-mesityl moiety is near the noncoordinating mesityl moiety of another IMes ligand. The cyclic voltammograms for 2a,b were measured in THF at room temperature using [nBu4N][PF6] as an electrolyte. As shown in Figure 7, a quasi-reverse redox couple was observed at E1/2 = −1.50 V (2a) and −1.38 V (2b) vs Ag/Ag+. Bulk electrolysis suggested that these redox couples are 2e processes,

Figure 4. Temperature dependence of magnetization of 2a in the solid state. Inset: the inverse molar magnetic susceptibility of 2a.

moment of 2a is nearly constant (μeff = 5.14−5.27 μB) in the temperature range 50−300 K. As shown in the inset of Figure 4, a good fit to Curie−Weiss behavior was obtained for 50−300 K, exhibiting a linear dependence of the inverse of the magnetic susceptibility as a function of temperature. The observation of Curie−Weiss behavior at 50−300 K indicates the paramagnetic nature in this temperature range. The slope of the Curie−Weiss plot gives a Curie constant of 3.31 mol emu−1 K−1. The effective magnetic moment of 2a calculated from the Curie constant is μeff = 5.15 μB, which is close to the spin-only value for an S = 2 state (4.90 μB). The significant difference in magnetic properties of 2a,b can be attributed to the extent of d-orbital interactions between two iron centers. The d orbitals which can be used for the Fe−Fe interaction in 2a,b are dx2−y2 and dyz, as shown in Figure 5 (left), where the x axis lies along the Fe−C(carbene) bond and the xy plane is defined by Fe, C(carbene), and the centroid of the iron-bound η6-arene ring. The xy planes are highlighted in red and blue in Figure 6 (top). The two xy planes in 2b are almost coplanar with an interplane angle of 0.40(8)°, while that of 2a

Figure 7. Cyclic voltammograms of 2a (left) and 2b (right) in THF with 0.2 M [nBu4N][PF6] as supporting electrolyte. C

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probably between the Fe0Fe0 and FeIFeI states. The appearance of the 2e process may be due to the instability of the mixedvalence Fe0FeI state. The easier oxidation of 2a in comparison to that of 2b is in agreement with the aforementioned less efficient d-orbital interactions in 2a leading to the higher-lying d electrons. Attempts to synthesize the oxidized forms of 2a,b by treatment with 1 or 2 equiv of [Cp2Fe](PF6) have been unsuccessful, giving uncharacterizable brown solids. Reactions of Dinuclear Fe0−NHC Complexes. (a). CO. While the iron centers in complexes 2a,b are efficiently stabilized by the μ-η1(C):η6(arene)-NHC ligands, these complexes can be seen as a dimeric form of (NHC)Fe0. Indeed, 2a,b were found to split into monomeric species in the presence of CO. When a toluene solution of 2a or 2b was exposed to 1 atm of CO at room temperature, the color of the solutions changed from dark brown to yellow, forming the monomeric 18-electron complexes (L)Fe(CO)4 (3a, L = IMes; 3b, L = IPr) in 83% (3a) and 85% (3b) yields, respectively (Scheme 3). The same iron complexes were recently prepared

Figure 8. Molecular structure of (IMes)Fe(CO)4 (3a) with thermal ellipsoids at the 50% probability level. Selected bond distances (Å) and angles (deg): Fe−C1, 1.9945(18); Fe−C2, 1.780(2); Fe−C3, 1.798(3); Fe−C4, 1.794(3); Fe−C5, 1.792(3); C1−Fe−C2, 174.72(9); C1−Fe−C3, 95.21(9); C1−Fe−C4, 84.50(9); C1−Fe− C5, 95.89(8); C3−Fe−C4, 121.11(11); C3−Fe−C5, 119.52(10); C4−Fe−C5, 119.06(10).

infrared spectra in hexane, 3a,b exhibit four CO bands at 2041, 1960, 1935, 1921 cm‑1 (3a) and 2042, 1962, 1936, 1923 cm‑1 (3b), respectively. (b). 1-Azidoadamantane. Some low-valent iron complexes have been shown to activate organoazides (N3R) to provide iron−imido complexes.13b,18,23,24 For example, Peters et al. have reported the reaction of the FeI complex [PhBP3]Fe(PPh3) ([PhBP3] = [PhB(CH2PPh2)3]−) with p-tolyl azide, where the FeIII−imido complex [PhBP3]Fe(N-p-tolyl) was obtained.23a Smith et al. have synthesized a relevant FeIII− imido complex stabilized by a tris(carbene)−borate ligand, LMesFe(NAd) (LMes = phenyltris(1-mesitylimidazol-2-ylidene)borate)),18b from the reaction of the FeI complex LMesFe(η2C8H14) with 1-azidoadamantane (AdN3). Treatment of a THF solution of 2a with 4 equiv of 1azidoadamantane resulted in the formation of a dinuclear iron− imido complex having a tetrazene (RNNNNR) ligand, (η2N4Ad2)Fe(μ-NAd)2Fe(IMes) (4), in 30% yield (Scheme 4). Complex 4 is paramagnetic, and its 1H NMR spectrum exhibited broad signals in the range from +22.5 to −12.7 ppm. The magnetic susceptibility of 4 in the solid state (Figure S5, Supporting Information) reveals that the effective magnetic moment gradually decreases as the temperature is lowered, from 4.76 μB at 300 K to 3.61 μB at 50 K.

Scheme 3

Scheme 4

from the reactions of Fe3(CO)12 with imidazolium salts or of Fe(CO)5 with L,14f,g and analogous iron complexes with the different NHC ligands IMe (1,3-dimethylimidazol-2-ylidene) and SIMe (1,3-dimethylimidazolin-2-ylidene) have also been reported.14 (IMe)Fe(CO)414a was synthesized via deprotonation of the imidazolium salt IMe·HI with the hydride complex K{FeH(CO)4}, while (SIMe)Fe(CO)414c was prepared from the reaction of Fe(CO)5 with (SIMe)2. The molecular structures of 3a,b were determined by X-ray crystallography and are shown in Figure 8 and Figure S3 (Supporting Information). The coordination geometry of iron in 3a,b is slightly distorted trigonal bipyramidal, with an NHC ligand occupying the apical position. The C(carbene)−Fe− Cap(carbonyl) angles (3a, 174.72(9)°; 3b, 170.67(9)°) are slightly narrowed from the ideal trigonal-bipyramidal geometry. The Fe−C(carbene) distances of 1.9945(18) Å (3a) and 2.0049(17) Å (3b) are similar to that of (IMe)Fe(CO)4 (2.007(5) Å).14b In the 13C{1H} NMR spectra of 3a,b, a single carbonyl resonance was observed at δ 217.2 (3a) and δ 215.5 (3b), suggesting that Berry pseudorotation exchanges apical and equatorial positions within the NMR time scale. In the D

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FeII centers, (IiPr)Fe(μ2-NDipp)2Fe(IiPr)18d (Dipp = 2,6diisopropylphenyl; Fe−C(carbene) = 2.068(3) Å and Fe− N(imido) = 1.867(2) Å). In contrast, the Fe2−N(tetrazene) distances (1.8287(17) and 1.8321(15) Å) are clearly shorter than those reported for tetrahedral FeII complexes having a tetrazene ligand (2.0089(7) and 1.9556(8) Å).26 The Fe2− N(imido) distances (1.7936(15) and 1.7849(17) Å) are also shorter than the Fe1−N(imido) distances (1.8175(17) and 1.8200(15) Å). Thus, the oxidation states of the iron centers would be Fe1II−Fe2IV. Scheme 5 illustrates a possible reaction pathway for the formation of 4. The reaction of 2a with 2 equiv of AdN3 would

The molecular structure of 4 was determined by X-ray analysis (Figure 9). The iron centers, imido nitrogen atoms,

Scheme 5

Figure 9. Molecular structure of (IMes)Fe(μ-NAd)2Fe(η2-N4Ad2) (4) with thermal ellipsoids at the 50% probability level. Selected bond distances (Å) and angles (deg): Fe1−Fe2, 2.3906(5); Fe1−C1, 2.0252(19); Fe1−N1, 1.8175(17); Fe1−N2, 1.8200(15); Fe2−N1, 1.7936(15); Fe2−N2, 1.7849(17); Fe2−N3, 1.8287(17); Fe2−N6, 1.8321(15); N3−N4, 1.351(2); N4−N5, 1.289(3); N5−N6, 1.354(3); C1−Fe1−N1, 131.49(8); C1−Fe1−N2, 132.44(8); N1−Fe1−N2, 95.95(7); N1−Fe2−N2, 98.07(7); N1−Fe2−N3, 121.29(8); N3− Fe2−N6, 79.00(7); N2−Fe2−N6, 120.24(8).

and the carbene carbon are coplanar, with the largest deviation of 0.082(2) Å at the carbene carbon. The Fe−Fe distance of 2.3906(5) Å is shorter than the Fe−Fe separation found in metallic iron (2.48 Å)25 and suggests the presence of an Fe−Fe bonding interaction. While Fe1 is trigonal planar, Fe2 is tetrahedral with two bridging imido ligands and a bidentate 1,4bis(adamantyl)tetrazene (Ad2N4) ligand. The five-membered metallacycle consisting of Ad2N4 and Fe is planar, with the largest deviation of 0.015(2) Å for N6. The two Fe2−N bond distances in the metallacycle are 1.8287(17) Å (Fe2−N3) and 1.8321(15) Å (Fe2−N6). The two N−N bonds neighboring the Fe2−N bonds are 1.351(2) Å (N3−N4) and 1.354(3) Å (N5−N6), and the N4−N5 distance of 1.289(3) Å is shorter than the N3−N4 and N5−N6 distances. The trend of these N−N distances found in the Ad2N4 tetrazene unit is indicative of the dianionic form shown in Figure 10A, with two N−N

lead to the formation of the dinuclear FeII−imido intermediate A, which is an analogue of (IiPr)Fe(μ-NDipp)2Fe(IiPr).18d The reaction of A with AdN3 would follow, if dissociation of one of the IMes ligands from A occurs, due to the possible steric repulsion between the μ-NAd ligands and the mesityl groups of IMes ligands.27 The resultant intermediate B has a terminal imido ligand on a low-coordinate iron center, and the Fe NAd moiety of the tentative species B may further take up AdN3 to undergo [3 + 2] cyclization in a manner analogous to the “click chemistry” between alkynes and organoazides, to provide the tetrazene moiety of 4.28 Concluding Remarks. The first homoleptic NHC complexes of Fe0, Fe2{μ-η1(C):η6(mesityl)-IMes}2 (2a) and Fe2{μ-η1(C):η6(iPr2C6H3)-IPr}2 (2b), were synthesized from the reduction of FeIII−chloride precursors. The Fe0 centers in 2a,b are stabilized by a unique μ-η1(C):η6(arene) coordination mode of the NHC. A quasi-reversible 2e redox event between the Fe0Fe0 and FeIFeI states was observed for both 2a and 2b. The dimeric complexes 2a,b are reactive, and they were found to serve as precursors for a monomeric (NHC)Fe0 moiety in reactions with CO. Complex 2a was also found to activate 1azidoadamantane, giving rise to the dinuclear iron−imido complex 4 having a tetrazene ligand.

Figure 10. Electronic configurations for a bidentate tetrazene ligand.

single bonds (N3−N4 and N5−N6) and an NN double bond (N4N5). Other possible configurations for the fivemembered metallacycle shown in Figure 10 include the monoanionic radical form (B) and the neutral form (C),26 whereas these forms do not fit with the trend of N−N distances in the Ad2N4 ligand of 4. The presence of a dianionic Ad2N4 ligand and two μ-NAd ligands in the dinuclear iron complex 4 leads to the FeIIIFeIII or FeIIFeIV oxidation state. The bond distances around Fe1, Fe1−C(carbene) (2.0252(19) Å) and Fe1−N(imido) (1.8175(17) and 1.8200(15) Å), are close to those of a dinuclear μ-imido complex having trigonal-planar E

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mL × 3) to afford a yellow powder of 1b (1.99 g, 87%). Single crystals suitable for crystallography were obtained from an Et2O solution at −40 °C. 1H NMR (600 MHz, C6D6): δ 53.0 (2H), 8.01 (6H), 5.50 (4H), 2.11 (24H). Anal. Calcd for C27H36N2FeCl3: C, 58.88; H, 6.59; N, 5.09. Found: C, 58.88; H, 6.57; N, 5.03. Synthesis of (IBnMe2)FeCl3 (1c). A toluene (10 mL) solution of IBnMe2 (180 mg, 0.60 mmol) was added to a toluene (30 mL) suspension of FeCl3 (97 mg, 0.60 mmol), and the mixture was stirred overnight at room temperature. After centrifugation, the orange solution was evaporated until dryness, and the residue was washed with hexane (10 mL × 3) to afford a yellow powder of 1c (200 mg, 72%). Single crystals suitable for crystallography were obtained from a toluene solution at −40 °C. 1H NMR (600 MHz, C6D6): δ 53.0 (2H), 9.55 (2H), 6.76 (4H), 3.38 (4H), 2.53 (12H). Anal. Calcd for C21H24N2FeCl3: C, 54.05; H, 5.18; N, 6.00. Found: C, 53.74; H, 5.19; N, 6.06. Synthesis of (IiPr)FeCl3 (1d). A toluene (30 mL) solution of IiPr (1.62 g, 8.97 mmol) was added to a toluene (30 mL) suspension of FeCl3 (1.46 g, 8.97 mmol), and the mixture was stirred overnight at room temperature. After centrifugation, the purple solution was evaporated under reduced pressure, and the residue was washed with hexane (30 mL × 3) to afford a dark purple powder of 1d (3.00 g, 97%). Single crystals suitable for crystallography were obtained from a toluene solution at −40 °C. 1H NMR (600 MHz, C6D6): δ 85.2 (6H), 15.4 (12H), 4.45 (2H). Anal. Calcd for C11H20N2FeCl3: C, 38.58; H, 5.89; N, 8.18. Found: C, 38.12; H, 6.16; N, 8.20. Synthesis of Fe2{μ-η1(C):η6(mesityl)-IMes}2 (2a).

EXPERIMENTAL SECTION

General Procedures. All reactions were carried out using standard Schlenk techniques and a glovebox under a nitrogen or an argon atmosphere. Toluene, diethyl ether, THF, and hexane were purified by the method of Grubbs,29 where the solvents were passed over columns of activated alumina and a supported copper catalyst supplied by Hansen & Co. Ltd. Other solvents were degassed and distilled from sodium benzophenone ketyl. Deuterated solvents, C6D6 and d8-THF, were dried by sodium and distilled prior to use. The 1H and 13C{1H} NMR spectra were recorded on a JEOL ECA-600 instrument. The signals were referenced to the residual peak of the deuterated solvent. Solution magnetic susceptibility was determined by the Evans method, using C6D6/hexamethylbenzene or d8-THF/tetramethylsilane solutions.22 The solid-state magnetic susceptibility was measured using a Quantum Design MPMS-XL SQUID-type magnetometer, and the samples were sealed in quartz tubes. UV−vis spectra were measured on a JASCO V560 spectrometer. Cyclic voltammetry measurements were performed in a single-compartment cell under a nitrogen atmosphere at 23 °C using a BSA-660B electrochemical analyzer. A three-electrode setup was employed, comprising of a glassy-carbon working electrode, platinum-wire auxiliary electrode, and Ag/AgCl quasi-reference electrode with 0.2 M [Bu4N][PF6] as the supporting electrolyte. The bulk electrolysis of 2a,b was performed at a potential of −1.2 V (2a) or −1.0 V (2b) vs Ag/Ag+ with THF (3−5 mL) solutions of Fe(0) complexes (10−20 mg) in the presence of 0.2 M [Bu4N][PF6] as the supporting electrolyte. Elemental analyses were performed on a LECO CHNS-932 microanalyzer, where the samples were sealed into silver or tin capsules in a glovebox. Purification of complex 5 was performed on an LC908 preparative HPLC (Japan Analytical Industry) eluted with toluene under a nitrogen atmosphere. The two HPLC columns (20 mm × 600 cm each) were filled with BioBeads S-X1 (Bio-Rad laboratories). X-ray diffraction data were collected on a Rigaku AFC8 or a Rigaku RA-Micro7 instrument equipped with a CCD area detector by using graphite-monochromated Mo Kα radiation. N-heterocyclic carbenes (IMes, IPr, and IiPr)30 and KC831 were prepared according to literature procedures. Preparation of IBnMe2. KOtBu (1.409 g, 12.6 mmol) was added to a THF (50 mL) solution of imidazole (0.855 g, 12.6 mmol) at room temperature. The resulting white suspension was then stirred for 3 h before addition of a THF (50 mL) solution of 3,5-dimethylbenzyl bromide (5.00 g, 25.1 mmol) at 0 °C. The mixture was stirred for 3 h at room temperature and then filtered to remove KBr. The solvent was removed under reduced pressure and the residue washed with diethyl ether (2 × 20 mL). The crude imidazolium salt IBnMe2·HBr was obtained as a white solid (4.53 g, 93%). KOtBu (0.22 g, 1.96 mmol) was added to a THF (30 mL) suspension of IBnMe2·HBr (0.70 g, 1.82 mmol), and the resulting mixture was stirred overnight. After centrifugation, the solvent was removed under reduced pressure, and the residue was extracted with toluene (15 mL). The orange solution was slowly evaporated to dryness. The resultant solid was washed with hexane (10 mL) to afford IBnMe2 (0.35 g, 63%) as a light yellow powder. 1H NMR (600 MHz, C6D6): δ 6.86 (s, 2H), 6.70 (s, 1H), 6.48 (s, 1H), 5.14 (s, 4H), 2.05 (s, 12H). Synthesis of (IMes)FeCl3 (1a). A toluene (40 mL) solution of IMes (0.938 g, 3.08 mmol) was added to a toluene (100 mL) suspension of FeCl3 (0.500 g, 3.08 mmol), and the mixture was stirred overnight at room temperature. After centrifugation, the solvent was removed under reduced pressure, and the resultant solid was washed with hexane (20 mL × 3) and then Et2O (20 mL × 3) to afford an orange powder of 1a (1.39 g, 97%). Single crystals suitable for crystallography were obtained by diffusion of hexane into the THF extract. 1H NMR (600 MHz, C6D6): δ 50.8 (2H), 4.28 (12H), 2.80 (6H), 2.36 (4H). Anal. Calcd for C21H24N2FeCl3: C, 54.05; H, 5.18; N, 6.00. Found: C, 54.08; H, 5.15; N, 5.96. Synthesis of (IPr)FeCl3 (1b).16 A toluene (50 mL) solution of IPr (1.555 g, 4.00 mmol) was added to a toluene (100 mL) suspension of FeCl3 (0.650 g, 4.01 mmol), and the mixture was stirred overnight at room temperature. After centrifugation, the orange solution was evaporated until dryness, and the residue was washed with hexane (30

Complex 1a (300 mg, 0.64 mmol) and KC8 (280 mg, 2.07 mmol) were charged into a flask, and the mixture was cooled to −197 °C. THF (30 mL) was condensed onto the mixture using a vacuumtransfer technique. The mixture was gradually warmed to room temperature and stirred overnight. After centrifugation, the dark red solution was evaporated until dryness, and the residue was washed with hexane (10 mL × 3) and extracted with Et2O (10 mL). After filtration, the solution was cooled at −40 °C to afford dark brown crystals of 2a (90 mg, 39%). Single crystals suitable for crystallography were obtained by diffusion of hexane into the THF extract at −40 °C. 1 H NMR (600 MHz, C6D6): δ 17.8 (4H, b), 7.68 (4H, g), 7.66 (2H, d or e), 2.83 (12H, f), 2.77 (2H, d or e) 2.62 (6H, h), −1.96 (12H, c), −9.47 (6H, a). Solution magnetic susceptibility (C6D6, 297.7 K): μeff = 4.4 μB. Cyclic voltammogram (2 mM in THF): E1/2 = −1.50 V (vs Ag/ Ag+). UV (THF, room temperature): λmax 379 nm (ε 15000), 471 nm (sh, ε 6600). Anal. Calcd for C42H48N2Fe2: C, 70.01; H, 6.71; N, 7.77. Found: C, 69.93; H, 6.77; N, 7.79. Synthesis of Fe2{μ-η1(C):η6(iPr2C6H3)-IPr}2 (2b).

Complex 1b (300 mg, 0.54 mmol) and KC8 (225 mg, 1.66 mmol) were charged into a flask, and the mixture was cooled to −197 °C. THF (40 mL) was condensed onto the mixture using a vacuumtransfer technique. The mixture was gradually warmed to room temperature and stirred overnight. After centrifugation, the dark red solution was evaporated until dryness, and the residue was washed with hexane (10 mL × 3) and Et2O (10 mL) and then extracted with THF (3 mL). After filtration, diffusion of hexane into the filtrate at −40 °C afforded dark brown crystals of 2b (54 mg, 22%). Single F

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Article

crystals suitable for crystallography were obtained from an Et2O solution at −40 °C. 1H NMR (600 MHz, C6D6): δ 7.79 (d, 4H, k), 7.73 (t, 2H, l), 7.34 (s, 2H, f or g), 7.08 (s, 2H, f or g), 5.57 (s, 2H, a), 4.77 (s, 4H, b or d), 4.69 (s, 4H, b or d), 1.73 (d, 12H, h or j), 1.60 (d, 16H, h or j, (i), 0.89 (s, 12H, c or e), 0.66 (s, 12H, c or e). 13C{1H} NMR (150 MHz, d8-THF): δ 149.2, 142.2, 129.5, 128.3, 124.3, 120.4, 104.5, 86.0, 81.9, 75.4, 31.3, 29.6, 25.4, 23.0, carbene carbon signal could not be detected. Cyclic voltammogram (2 mM in THF): E1/2 = −1.38 V (vs Ag/Ag+). UV (THF, room temperature): λmax 387 nm (ε 17000), 491 nm (sh, ε 4900). Anal. Calcd for C54H72N2Fe2: C, 72.97; H, 8.16; N, 6.30. Found: C, 72.65; H, 8.11; N, 6.25. Synthesis of (IMes)Fe(CO)4 (3a). A toluene (15 mL) solution of 2a (30 mg, 0.042 mmol) was exposed to 1 atm of CO at −78 °C. The mixture was gradually warmed to room temperature and stirred overnight. The solvent was removed under reduced pressure, and the residue was extracted with hexane (10 mL). After filtration, the yellow solution was evaporated until dryness, which gave 3a (31 mg, 83%) as a light yellow powder. Single crystals suitable for crystallography were obtained from a toluene solution at −40 °C. 1H NMR (600 MHz, C6D6): δ 6.77 (s, 4H, CH of m-Mes), 6.13 (s, 2H, CH of CN2C2H2), 2.10 (s, 6H, CH3 of p-Mes), 2.03 (s, 12H, CH3 of o-Mes). 13C NMR (150 MHz, C6D6): δ 217.2, 189.9, 139.5, 137.2, 136.0, 129.7, 124.3, 21.6, 18.1. IR (hexane, cm−1): ν(CO) 2041 (m), 1960 (m), 1935 (s), 1921 (s). Anal. Calcd for C25H24N2O4Fe: C, 63.57; H, 5.12; N, 5.93. Found: C, 63.16; H, 5.40; N, 5.62. Synthesis of (IPr)Fe(CO)4 (3b). A toluene (15 mL) solution of 2b (30 mg, 0.034 mmol) was exposed to 1 atm of CO at −78 °C. The mixture was gradually warmed to room temperature and stirred overnight. The solvent was removed under reduced pressure, and the resultant solid was extracted with hexane (10 mL). After filtration, the yellow solution was evaporated until dryness, which gave 3b (32 mg, 85%) as a light yellow powder. Single crystals suitable for crystallography were obtained from a hexane solution at −40 °C. 1H NMR (600 MHz, C6D6): δ 7.27 (t, 2H, CH of p-Ar), 7.12 (d, 4H, CH of m-Ar), 6.57 (s, 2H, CH of CN2C2H2), 2.71 (sept, 4H, CH of o-Ar), 1.41 (d, 12H, CHCH3 of o-Ar), 0.99 (d, 12H, CHCH3 of o-Ar). 13C NMR (150 MHz, C6D6): δ 215.5, 191.3, 146.3, 136.7, 130.6, 125.4, 124.3, 28.6, 25.6, 22.4. IR (hexane, cm−1): ν(CO) 2042 (m), 1962 (m), 1936 (s), 1923 (s). Anal. Calcd for C31H36N2O4Fe: C, 66.91; H, 6.52; N, 5.03. Found: C, 66.90; H, 6.58; N, 5.07. Synthesis of (IMes)Fe(μ-NAd)2Fe(η2-N4Ad2) (4). A THF (10 mL) solution of N3Ad (150 mg, 0.85 mmol) was added to a THF (20 mL) solution of 2a (150 mg, 0.21 mmol) and the mixture stirred overnight at room temperature. The solvent was removed under reduced pressure, and the resultant brown solid was washed with hexane (3 mL × 3) and extracted with toluene (8 mL). After filtration, the brown filtrate was evaporated until dryness. The residue was again dissolved in toluene (2.5 mL), and the solution was injected into a preparative size-exclusive HPLC under N2 and was eluted with toluene at a flow rate of 4.0 mL/min. The elution time for 4 was 58−66 min. The solution was evaporated to dryness to give 4 (65 mg, 30%) as a brownish green solid. Single crystals suitable for crystallography were obtained from a THF solution at −40 °C. 1H NMR (600 MHz, d8THF): δ 22.5, 15.0, 10.2, 6.1, 2.8, −1.5, −1.9, −7.0, −12.7. UV (THF, room temperature): three shoulders were observed at 430, 560, and 730 nm. Anal. Calcd for C61H84N8Fe2: C, 70.38; H, 8.13; N, 10.76. Found: C, 70.36; H, 8.52; N, 10.67. X-ray Crystal Structure Determination. Crystal data and refinement parameters for 1a−d, 2a,b, 3a,b, and 4 are summarized in Table 1. Single crystals were coated with oil (Immersion Oil, type B: Code 1248, Cargille Laboratories, Inc.) and mounted on loops. Diffraction data were collected at −100, −150, or −160 °C under a cold nitrogen stream on a Rigaku RA-Micro7 equipped with a Saturn70 CCD detector, using graphite-monochromated Mo Kα radiation (λ = 0.710690 Å). Six preliminary data frames were measured at 0.5° increments of ω, to assess the crystal quality and preliminary unit cell parameters. The 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 a REQAB program. The calculations were performed with the

Table 1. Crystal Data and Summary of Refinement for Complexes 1a−d, 2a,b, 3a,b, and 4 1a

1b

1c

formula fw temp (°C) 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

C21H24Cl3FeN2 466.64 −100 orthorhombic Pca21 (No. 29) 16.358(2) 16.636(2) 17.004(2)

C27H36Cl3FeN2 550.80 −100 monoclinic P21/c (No. 14) 18.235(2) 16.233(2) 20.160(3)

4627.4(9) 8 1.340 0.0367 0.0812 1.003 1d

5816.8(11) 8 1.258 0.0478 0.0837 1.005 2a

C21H24Cl3FeN2 466.64 −100 triclinic P1̅ (No. 2) 8.474(2) 9.880(3) 14.618(3) 74.080(13) 73.54(2) 75.082(12) 1107.0(5) 2 1.400 0.0407 0.0838 1.002 2b

formula fw temp (°C) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalcd (g/cm3) R1a wR2b GOFc

C11H20Cl3FeN2 342.50 −100 orthorhombic Pbca (No. 61) 16.021(2) 11.689(2) 17.185(3)

C42H48Fe2N4 720.56 −100 monoclinic C2/c (No. 15) 21.810(5) 8.318(2) 21.827(4) 109.513(3) 3732.4(13) 4 1.282 0.0371 0.0696 1.004 3b

C54H72Fe2N4 888.88 −150 monoclinic P21/n (No. 14) 16.732(2) 14.9184(11) 21.021(2) 110.9691(11) 4899.7(7) 4 1.205 0.0376 0.0788 1.004 4·2C4H8O

C31H36FeN2O4 556.48 −100 monoclinic P21/c (No. 14) 17.345(5) 9.287(3) 18.211(5) 98.590(5) 2901(2) 4 1.274 0.0408 0.0819 1.008

C69H100Fe2N8O2 1185.30 −160 monoclinic P21/n (No. 14) 14.106(2) 23.925(4) 19.691(3) 110.488(2) 6225(2) 4 1.265 0.0448 0.0901 1.005

formula fw temp (°C) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalcd (g/cm3) R1a wR2b GOFc

102.897(2)

3218.4(7) 8 1.414 0.0409 0.0724 1.004 3a C25H24FeN2O4 472.32 −100 monoclinic P21/c (No. 14) 18.420(3) 16.442(3) 7.640(2) 99.650(3) 2281.2(7) 4 1.375 0.0386 0.0813 1.005

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 = number of observations, Nv = number of variables). a

CrystalStructure program package. All structures were solved by direct methods and refined by full-matrix least-squares. Anisotropic refinement was applied to all non-hydrogen atoms except for disordered atoms (refined isotropically), and all hydrogen atoms were placed at calculated positions. In the asymmetric units of 1a,b, there are two crystallographically independent molecules. Additional data are available as Supporting Information. G

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Gallagher, M. K.; Cowie, M.; Hames, B. W.; Fackler, J. P.; Mazany, A. M. Organometallics 1987, 6, 283. (d) Kramer, A.; Lingnau, R.; Lorenz, I.-P.; Mayer, H. A. Chem. Ber. 1990, 123, 1821. (e) Cody, G. D.; Boctor, N. Z.; Filley, T. R.; Hazen, R. M.; Scott, J. H.; Sharma, A., Jr. Science 2000, 289, 1337. (f) Volkers, P. I.; Boyke, C. A.; Chen, J.; Rauchfuss, T. B.; Whaley, C. M.; Wilson, S. R.; Yao, H. Inorg. Chem. 2008, 47, 7002. (5) (a) Fontecilla-Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y. Chem. Rev. 2007, 107, 4273. (b) Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245. (6) Camara, J. M.; Rauchfuss, T. B. Nat. Chem. 2012, 4, 26. (7) (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39. (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (b) Hu, X.; Tang, Y.; Gantzel, P.; Meyer, K. Organometallics 2003, 22, 612. (c) Cavallo, L.; Correa, A.; Costabile, C.; Jacobsen, H. J. Organomet. Chem. 2005, 690, 5407. (d) Nolan, S. P. N-Heterocyclic Carbenes in Synthesis; Wiley-VCH: Weinheim, Germany, 2006. (e) Glorius, F. N-Heterocyclic Carbenes in Transition-Metal Catalysis; Springer: New York, 2007; Topics in Organometallic Chemistry 21. (f) Mata, J. A.; Poyatos, M.; Peris, E. Coord. Chem. Rev. 2007, 251, 841. (g) Dı ́ez-Gonzalez, S.; Nolan, S. P. Coord. Chem. Rev. 2007, 251, 874. (h) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768. (i) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122. (j) Würtz, S.; Glorius, F. Acc. Chem. Res. 2008, 41, 1523. (k) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L. Coord. Chem. Rev. 2009, 253, 687. (l) Dı ́ez-González, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612. (n) Gusev, D. G. Organometallics 2009, 28, 6458. (m) Clavier, H.; Nolan, S. P. Chem. Commun. 2010, 841. (o) Dröge, T.; Glorius, F. Angew. Chem., Int. Ed. 2010, 122, 7094. (8) The π-accepting properties of NHC ligands have been suggested for group 11 metal complexes: (a) Hu, X.; Tang, Y.-J.; Gantzel, P.; Meyer, K. Organometallics 2003, 22, 612−614. (b) Hu, X.; CastroRodriguez, I.; Olsen, K.; Meyer, K. Organometallics 2004, 23, 755− 764. (9) Reviews of NHC−iron complexes: (a) Ingleson, M. J.; Layfield, R. A. Chem. Commun. 2012, 48, 3579. (b) Bézier, D.; Sortais, J.-B.; Darcel, C. Adv. Synth. Catal. 2013, 355, 19. (10) Selected examples: (a) Louie, J.; Grubbs, R. H. Chem. Commun. 2000, 1479. (b) McGuinness, D. S.; Gibson, V. C.; Steed, J. W. Organometallics 2004, 23, 6288. (c) Deng, L.; Holm, R. H. J. Am. Chem. Soc. 2008, 130, 9878. (d) César, V.; Lugan, N.; Lavigne, G. J. Am. Chem. Soc. 2008, 130, 11286. (e) Liu, B.; Xia, Q.; Chen, W. Angew. Chem., Int. Ed. 2009, 48, 5513. (f) Gao, H.-H.; Yan, C.-H.; Tao, X.-P.; Xia, Y.; Sun, H.-M.; Shen, Q.; Zhang, Y. Organometallics 2010, 29, 4189. (g) Xiang, L.; Xiao, J.; Deng, L. Organometallics 2011, 30, 2018. (h) Zlatogorsgy, S.; Muryn, C. A.; Tuna, F.; Evans, D. J.; Ingleson, M. J. Organometallics 2011, 30, 4974. (i) Cramer, S. A.; Jenkins, D. M. J. Am. Chem. Soc. 2011, 133, 19342. (j) Hess, J. L.; Hsieh, C.; Brothers, S. M.; Hall, M. B.; Darensbourg, M. Y. J. Am. Chem. Soc. 2011, 133, 20426. (k) Danopoulos, A. A.; Braunstein, P.; Stylianides, N.; Wesolek, M. Organometallics 2011, 30, 6514. (l) Meyer, S.; Orben, C. M.; Demeshko, S.; Dechert, S.; Meyer, F. Organometallics 2011, 30, 6692. (m) Layfield, R. A.; McDouall, J. J.; Scheer, M.; Schwarzmaier, C.; Tuna, F. Chem. Commun. 2011, 10623. (n) Wang, X.; Mo, Z.; Xiao, J.; Deng, L. Inorg. Chem. 2013, 52, 59−65. (o) César, V.; Castro, L. C. M.; Dombray, T.; Sortais, J.-B.; Darcel, C.; Labat, S.; Miqueu, K.; Sotiropoulos, J.-M.; Brousses, R.; Lugan, N.; Lavigne, G. Organometallics 2013, 32, 4643−4655. (p) Evans, J.; Bickelhaupt, F. M.; Layfield, R. A. J. Am. Chem. Soc. 2013, 135, 13338−13341. (11) (a) Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 2007, 129, 7008. (b) Duan, L.; Wang, M.; Li, P.; Na, Y.; Wang, N.; Sun, L. Dalton Trans. 2007, 1277. (c) Song, L.-C.; Luo, X.; Wang, Y.-Z.; Gai, B.; Hu, Q.-M. J. Organomet. Chem. 2009, 694, 103. (12) (a) Lavallo, V.; Grubbs, R. H. Science 2009, 326, 559. (b) Lavallo, V.; El-Batta, A.; Bertrand, G.; Grubbs, R. H. Angew. Chem., Int. Ed. 2011, 50, 268.

ASSOCIATED CONTENT

S Supporting Information *

Figures giving magnetic susceptibility measurements for complexes 2b and 4 and ORTEP diagrams for complexes 1b,d and 3b and CIF files giving crystallgraphic data for complexes 1a−d, 2a,b, 3a,b, and 4. 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] (Y.O.); [email protected]. nagoya-u.ac.jp (K.T.). Present Addresses †

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5-2, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. ‡ Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 5600043, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by Grants-in-Aid for Scientific Research (Nos. 23000007, 23685015, 25105725, 25109522) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. We are grateful to Prof. Kunio Awaga, Prof. Michio Matsushita, Prof. Hirofumi Yoshikawa, and Dr. Yoshiaki Syuku (Nagoya Univeristy) for aiding us with a SQUID-type magnetometer. We also thank Prof. Kohei Tamao, Prof. Tsukasa Matsuo, and Dr. Mikinao Ito (RIKEN and Kinki University) for their advise on HPLC purification.

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REFERENCES

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Organometallics

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dx.doi.org/10.1021/om401039z | Organometallics XXXX, XXX, XXX−XXX