Iron(II) Complexes Featuring Bidentate N-Heterocyclic Carbene–Silyl

Communication pubs.acs.org/Organometallics

Iron(II) Complexes Featuring Bidentate N‑Heterocyclic Carbene−Silyl Ligands: Synthesis and Characterization Zhenwu Ouyang and Liang Deng* State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai, People’s Republic of China 200032 S Supporting Information *

ABSTRACT: Two silyl donor containing N-heterocyclic carbene iron(II) complexes, [(CSi)Fe(IMes′)] (CSi = bidentate silyl donor containing N-heterocyclic carbene ligand, IMes′ = cyclometalated IMes ligand, IMes = 1,3-dimesitylimidazol-2-ylidene) have been synthesized by sequential ironmediated benzylic C−H bond activation and silylation reactions starting from [(IMes)2FeCl2]. Single-crystal X-ray diffraction studies revealed that the [(CSi)Fe(IMes′)] complexes have seesaw-type FeSiC3 cores. Solution magnetic susceptibility measurements, UV−vis−near-IR spectra, and bond distance data from X-ray diffraction studies corroborate an intermediate spin state, S = 1, for these unique iron(II) complexes. DFT calculations revealed that the bidentate CSi chelates are essentially σdonating ligands in the iron(II) compounds.

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reactive toward silanes,6 which might prevent the direct synthesis of free NHC−silane ligands, we continued our efforts on the sequential metal-mediated C−H bond activation and silylation protocol with other transition-metal−NHC systems. Herein, we wish to communicate the synthesis of iron(II) compounds featuring bidentate CSi ligands by this method, as well as their structural and spectroscopic characterization. The synthetic route for CSi ligand containing iron(II) complexes consists of two steps: iron-mediated benzylic C−H bond activation and silylation of the resulting cyclometalated NHC−iron(II) complex. The first step was achieved through reacting [(IMes)2FeCl2]7 with excess sodium amalgam in THF in an open vessel under a dinitrogen atmosphere, from which the cyclometalated NHC−iron(II) compound [(IMes′)2Fe] (1) was isolated in 54% yield as red crystals (Scheme 1).8 A single-crystal

ilyl donor containing ligands are famous for their capability in stabilizing coordinatively unsaturated transition-metal species,1,2 with which they have proved useful for transitionmetal-mediated small-molecule and inert bond activation, as well as catalysis.3 As they have been explored for decades, a great variety of such ligands1b,4 with phosphine,1b,4a−c,e,f,i,j pyridine,4d and thioether4g,h as the ancillary donors (Chart 1) have been Chart 1. Representatives of Silyl Donor Containing Ligands

Scheme 1. Preparation of [(IMes′)2Fe]

developed and successfully applied in transition-metal chemistry. In addition to these, we recently constructed a bidentate Nheterocyclic carbene−silyl ligand scaffold (denoted as CSi in Chart 1) using a sequential cobalt-mediated C−H bond activation and silylation protocol.5 The coexistence of two strong electron-donating donors, silyl anion and NHC, in the CSi ligands, in combination with their monoanionic nature made them unique and potentially useful for transition-metal catalysis. As a preliminary exploration on this end, we have found that the cobalt complexes (CSi)Co(IMes′) can catalyze the hydrosilylation of 1-octene with phenylsilane with very fast initial rates, high turnover numbers (up to 15000), and high selectivity.5 The successful synthesis of the cobalt(II) complexes raised the question of the accessibility of other CSi ligand containing metal complexes. Noting that N-heterocyclic carbenes (NHCs) are © 2013 American Chemical Society

X-ray diffraction study revealed that the molecular structure of 1 is similar to that of its cobalt congener [(IMes′)2Co],9 in which the iron(II) center adopts an unusual seesaw geometry (Figure 1) with the C(carbene)−Fe−-C(carbene) and C(benzyl)−Fe− C(benzyl) angles being 177.3(1) and 148.4(1)°, respectively. The Fe−C(carbene) and Fe−C(benzyl) bond distances (1.926(2) and 2.066(2) Å) are comparable to the corresponding Received: October 23, 2013 Published: December 11, 2013 7268

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resembles the reactions of Meyer’s tripodal NHC-supported iron(II) complexes with sodium amalgam,16 might proceed by the pathway shown in Figure s1 (Supporting Information). Twoelectron reduction of [(IMes)2FeCl2] could initially afford the two-coordinate iron(0) intermediate (IMes)2Fe (A), which then converts to the iron(II) hydride intermediate (IMes)Fe(IMes′)(H) (B) via intramolecular C−H bond oxidative addition. Intermediate B could subsequently convert to 1 by a second C− H bond activation reaction after eliminating H2, probably via a σbond metathesis mechanism. Both types of C−H bond activation reactions, oxidation addition to iron(0) and σ-bond metathesis with iron(II)−R (R = alkyl), have precedents in the literatures: for example, the reactions of [(PMe3)4Fe] with C−H bonds to produce iron(II) hydrides,17 the chelation-assisted methane elimination reactions of [(PMe3)4Fe(Me)2] with phosphines,18 and the C−H bond activation of arenes by [Cp*Fe(IPr2Me2′)] (IPr2Me2′ = cyclometalated 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene)19 and [Cp*Fe(CO)(NCMe)(Ph)].20 The second step, silylation of the cyclometalated iron complex, was performed by treatment of 1 with 1 equiv of the dihydrosilane H2SiPhMe or H2SiPh2 in THF (Scheme 2).

Figure 1. Molecular structure of 1 showing 30% probability ellipsoids. Key distances (Å) and angles (deg): Fe1−C1 1.926(2), Fe1−C2 2.066(2), C1−Fe1−C1A 177.3(1), C2−Fe1−C2A 148.4(1), C1−Fe1− C2 82.9(1).

distances in [trans-(IMes)2FeMe2] (1.959(5) and 2.125(5) Å),7b [((PrC)2Ph)2Fe]I2 (1.920(10) and 2.010(9) Å for the Fe− C(carbene) distances; (PrC)2Ph = o-phenylene-linked bis(NHC) ligand),10 and the six-coordinate macrocyclic tetracarbene iron(II) compounds [(NHC)4FeX2] (ca. 2.00 Å)11 but are much shorter than those of the reported tetrahedral iron(II)− NHC compounds.7,10,12 The absorption spectrum of 1 in THF has a broad d−d transition band around 1390 nm, close to that of 1420 nm observed for [trans-(IMes)2FeMe2]7b (Figure 2). Its 1H

Scheme 2. Preparation of [(CSi)Fe(IMes′)]

Both reactions can proceed at room temperature and afford the silylation products [(CSi)Fe(IMes′)] (2 and 3) in high yields, as indicated by NMR-scale reactions. However, the good solubility of these two compounds in the recrystallization solvents made the isolation yields much lower (30 and 37%).8 H2 has been detected as the byproduct by 1H NMR in NMR scale reactions. Complex 1 can react rapidly with phenylsilane to produce H2 and a diamagnetic mixture, and the attempts to isolate ironcontaining species were unsuccessful. In addition to the primary and secondary silanes, 1 was unreactive toward tertiary silanes, such as triphenylsilane, triethylsilane, and triethoxylsilane. The 1H NMR spectra of 2 and 3 feature 28 and 30 peaks in the ranges +83 to −59 and +56 to −63 ppm, respectively. The observed peak numbers suggest that the rotation of the mesityl groups is restricted in these silyl-functionalized NHC complexes when noting the presence of the diasterotopic protons on the CH2Si moieties and the absence of the CH2Fe signals. The solution magnetic moments of 2 and 3 measured in C6D6 are around 3.6 μB, again being slightly higher than that expected for the spin-only value of intermediate-spin iron(II) compounds.7b,12,14 The absorption spectra of 2 and 3 in THF feature two characteristic near-infrared absorptions at 1010, 1400 and 1020, 1370 nm, respectively (Figure 2). The resemblance of these near-infrared bands to those of 1 and [trans(IMes)2FeMe2] indicates that the iron centers in 2 and 3 should also have an intermediate spin state with a seesaw geometry, which has been confirmed by single-crystal X-ray diffraction studies. As shown in Figure 3, the newly formed CSi ligands in 2 and 3 bite to the metal centers to form boat-shaped seven-membered chelating rings. The further chelation of a IMes′ anion to the iron

Figure 2. Absorption spectra of 1−3 (in THF) and [trans(IMes)2FeMe2] (in benzene) in the near-infrared region measured at room temperature.

NMR spectrum in C6D6 features paramagnetically shifted resonances ranging from +7 to −30 ppm, and solution magnetic susceptibility measurements13 (in THF-d8 at 22 °C) revealed a magnetic moment of 3.7(1) μB, which is slightly larger than the spin-only value around 3.0 μB observed for intermediate-spin iron(II) compounds, e.g. [trans-(IMes)2FeMe2]7b and [trans(PEt2Ph)2FeAr2],14 probably due to second-order spin−orbit coupling. The conversion of [(IMes)2FeCl2] to 1 represents a rare example of iron-mediated C−H bond activation of NHC ligands, although great recent endeavors have been achieved in iron− NHC chemistry.15 The reduction-induced conversion, which 7269

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produce (IMes)Fe(IMes′-SiHR1R2) (D), which might then convert to (IMes)Fe(CSi)(H) (E) via oxidative addition of the Si−H bond to the iron(0) center.23 Finally, E could eliminate one molecule of H2 to generate the products [(CSi)Fe(IMes′)] (2 and 3) (pathway A in Scheme s2, Supporting Information). In addition to this, another route, starting from the reaction of 1 with the hydrosilanes to produce (IMes′)Fe(IMes′-SiHR1R2)(H) (F) that could subsequently undergo intramolecular reaction between the Si−H and Fe−H bonds to yield the products and H2 (pathway B in Scheme s2), could also be possible.20 With limited information in hand at this stage, we are unable to discriminate between the two pathways.24 Complexes 2 and 3 are rare examples of metal complexes featuring silyl donor functionalized NHC ligands5 and the first structurally characterized four-coordinate iron silyl complexes.25 To get further insight into their electronic structures, DFT calculations26 have been performed on 2 with a fixed geometry based on its crystal structure using the ORCA 2.8 program.8,27 Calculations on different spin states (S = 2, 1, and 0) indicate that the single-point energies of the quintet (S = 2) and singlet (S = 0) states are 37.3 and 30.0 kcal/mol higher than that of the triplet state (S = 1), suggesting that the triplet state is a reasonably stable state. The spin-density distribution plot of the triplet state (Figure s2, Supporting Information) clearly indicated that the two spins are located primarily on the iron center. Inspecting the molecular orbitals then revealed that the four frontier orbitals (MOs 209−206) are essentially metal-based, which points to a (dxy)2(dyz)2(dxz)1(dz2)1 electron configuration for the iron(II) center and that MOs 205 and 204 contain σ-type interactions between the metal center and the silyl and benzyl moieties (Figure s3). The Meyer bond orders of the Fe−Si, Fe− C(benzyl), and Fe−C(carbene) bonds are 0.77, 0.73, 0.85, and 0.88, respectively. Thus, the [CSi MePh ] − anion in this intermediate-spin iron(II) complex should be viewed as a bidentate σ-donating ligand. In conclusion, we have applied the sequential metal-mediated C−H bond activation and silylation synthetic protocol to an iron(II)−NHC complex [(IMes)2FeCl2], which has led to the successful preparation of the two silyl donor functionalized NHC−iron(II) complexes [(CSi)Fe(IMes′)]. Single-crystal Xray structure studies revealed unusual seesaw iron(II) centers in these [(CSi)Fe(IMes′)] complexes. Solution magnetic susceptibility measurements, bond distance data from X-ray diffraction studies, and absorption spectra point to an intermediate spin state for them. DFT calculations support this assignment and suggest that the silyl donor functionalized NHC ligand is essentially a σ-donating ligand in the iron complex. Reactivity studies on these cyclometalated NHC−iron(II) complexes are ongoing.

Figure 3. Molecular structures of 2 (top) and 3 (bottom) showing 30% probability ellipsoids. Key distances (Å) and angles (deg): for 2, Fe1− C1 1.943(2), Fe1−C2 1.968(2), Fe1−C3 2.082(2), Fe1−Si1 2.396(1), C1−Fe1−C2 173.9(1), C3−Fe1−Si1 154.7(1), C1−Fe1−Si1 91.0(1), C2−Fe1−C3 83.7(1); for 3:, Fe1−C1 1.938(3), Fe1−C2 1.940(2), Fe1−C3 2.093(2), Fe1−Si1 2.419(1), C1−Fe1−C2 176.2(1), C3− Fe1−Si1 157.5(1), C1−Fe1−Si1 94.6(1), C1−Fe1−C3 83.0(1).

center engenders seesaw-type FeSiC3 cores with C(carbene)− Fe−C(carbene) and Si−Fe−C(benzyl) angles of 173.9(1) and 154.7(1)° for 2 and 176.2(1) and 157.5(1)° for 3. The Fe−Si bond distances of 2 and 3 (2.396(1) and 2.419(1) Å) are at the long end of Fe−Si bonds observed in the reported complexes1c and are ca. 0.09 Å longer than the Co−Si bonds in [(CSi)Co(IMes′)].5 Similarly, all the Fe−C bonds in the iron compounds are slightly longer than their counterparts in the cobalt complexes (Table s2, Supporting Information). Interestingly, in spite of their long M−Si and M−C distances, the Si− Fe−C(benzyl) angles in 2 and 3 (154.7(1) and 157.5(1)°) are smaller than the Si−Co−C(benzyl) angles in the [(CSi)Co(IMes′)] complexes (160.6(1) and 161.7(1)°) (Table s2). A comparison of the Fe−C(benzyl) distances in 1−3 revealed that those of 2 and 3 (2.082(2) and 2.093(2) Å) are slightly longer than that in 1 (2.066(2) Å). The difference can be attributed to the strong trans effect of the silyl anions versus that of the benzyl anion.21 The formation of the silyl donor functionalized NHC− iron(II) complexes signifies the unique reactivity of the doubly cyclometalated compound 1 toward hydrosilanes in comparison with the reactions of Cp*Fe(IPr2Me2′) with silanes that afforded Cp*Fe(IPr2Me2)(SiR3) without C−Si bond formation.22 Noting this, we reason that the interactions of 1 with H2SiPhMe or H2SiPh2 might initially give (IMes)Fe(IMes′)(SiHR1R2) (C). A C(benzyl)−Si bond-forming reductive elimination from C could

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving X-ray crystallographic data for complexes 1−3, experimental procedures, characterization data, and computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*L.D.: tel, 86-21-54925460; fax, 86-21-54925460; e-mail, deng@ sioc.ac.cn. 7270

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Notes

(12) (a) Xiang, L.; Xiao, J.; Deng, L. Organometallics 2011, 30, 2018. (b) Danopoulos, A. A.; Braunstein, P.; Stylianides, N.; Wesolek, M. Organometallics 2011, 30, 6514. (c) Meyer, S.; Orben, C. M.; Demeshko, S.; Dechert, S.; Meyer, F. Organometallics 2011, 30, 6692. (d) Wang, X.; Mo, Z.; Xiao, J.; Deng, L. Inorg. Chem. 2013, 52, 59. (e) Danopoulos, A. A.; Monakhov, K. Y.; Braunstein, P. Chem. Eur. J. 2013, 19, 450. (f) Wu, J.; Dai, W.; Farnaby, J. H.; Hazari, N.; Le Roy, J. J.; Mereacre, V.; Murugesu, M.; Powell, A. K.; Takase, M. K. Dalton Trans. 2013, 42, 7404. (13) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Sur, S. K. J. Magn. Reson. 1989, 82, 169. (14) Hawrelak, E. J.; Bernskoetter, W. H.; Lobkovsky, E.; Yee, G. T.; Bill, E.; Chirik, P. J. Inorg. Chem. 2005, 44, 3103. (15) For examples, see: (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. (c) Day, B. M.; Pugh, T.; Hendriks, D.; Guerra, C. F.; Evans, D. J.; Bickelhaupt, M.; Layfield, R. A. J. Am. Chem. Soc. 2013, 135, 13338. (16) Vogel, C. S.; Heinemann, F. W.; Khusniyarov, M. M.; Meyer, K. Inorg. Chim. Acta 2010, 364, 226. (17) (a) Karsch, H. H.; Klein, H.-F.; Schmidbaur, H. Chem. Ber. 1977, 110, 2200. (b) Xu, G.; Sun, H.; Li, X. Organometallics 2009, 28, 6090. (c) Bhattacharya, P.; Krause, J. A.; Guan, H. Organometallics 2011, 30, 4720. (18) Beck, R.; Zheng, T.; Sun, H.; Li, X.; Flörke, U.; Klein, H.-F. J. Organomet. Chem. 2008, 693, 3471. (19) (a) Ohki, Y.; Hatanaka, T.; Tatsumi, K. J. Am. Chem. Soc. 2008, 130, 17174. (b) Hatanaka, T.; Ohki, Y.; Tatsumi, K. Chem. Asian J. 2010, 5, 1657. (20) Kalman, S. E.; Petit, A.; Gunnoe, T. B.; Ess, D. H.; Cundari, T. R.; Sabat, M. Organometallics 2013, 32, 1797. (21) (a) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973, 10, 335. (b) Coe, B. J.; Glenwright, S. J. Coord. Chem. Rev. 2000, 203, 5. (c) Zhu, J.; Lin, Z. Inorg. Chem. 2005, 44, 9384. (22) Hatanaka, T.; Ohki, Y.; Tatsumi, K. Eur. J. Inorg. Chem. 2013, 3966. (23) Oxidative addition reactions between iron(0) species and hydrosilanes have been well-documented in the literature. For related reviews, please see refs 1a and 1c. (24) Attempts to probe the mechanism with Ph2SiD2 revealed fast H− D exchange at the early stage of the interaction of 1 with Ph2SiD2 to produce Ph2SiH2 and partial deuterium incorporation into many of the methyl and methylene positions in the product. The low deuterium content at these positions renders little change of the relative integration of the 1H NMR peaks, but peak broadening can be seen. Due to the low deuterium content in the product, no apparent 2H NMR signal was observed. For the 1H NMR spectrum of the deuterium-incorporated product, please see the Supporting Information. (25) Tilley and coworkers reported the synthesis and characterization of three- and four-coordinate iron(II) complexes of the bulky silyl anion [Si(SiMe3)3]−. Among them, only the structure of a three-coordinate complex, [NEt4][FeCl(Si(SiMe3)3)2], has been established by XRD. See: Roddick, D. M.; Tilley, T. D.; Rheingold, A. L.; Geib, S. J. J. Am. Chem. Soc. 1987, 109, 945. (26) (a) Hohenberg, P.; Kohn, W. Phys. Rev. 1964, 136, B864. (b) Kohn, W.; Sham, L. J. Phys. Rev. 1965, 140, A1133. (27) Neese, F. ORCA: An Ab Initio, Density Functional and Semiempirical Program Package (v. 2.8); Universität Bonn, Bonn, Germany, 2011.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial supports from the National Basic Research Program of China (973 Program, No. 2011CB808705) and the National Natural Science Foundation of China (Nos. 20923005, 21121062, and 21222208).

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