Synthesis and Characterization of Novel Iron(II) Complexes with

Article pubs.acs.org/Organometallics

Synthesis and Characterization of Novel Iron(II) Complexes with Tetradentate Bis(N-heterocyclic carbene)−Bis(pyridine) (NCCN) Ligands Andreas Raba,† Mirza Cokoja,† Stefan Ewald,† Korbinian Riener,† Eberhardt Herdtweck,† Alexander Pöthig,† Wolfgang A. Herrmann,*,† and Fritz E. Kühn*,†,‡ †

Chair of Inorganic Chemistry and ‡Molecular Catalysis, Catalysis Research Center, Technische Universität München, Ernst-Otto-Fischer Straße 1, D-85747 Garching bei München, Germany S Supporting Information *

ABSTRACT: Three novel iron(II) complexes bearing tetradentate ligands of the type pyridine−bis(N-heterocyclic carbene)− pyridine (NCCN) have been synthesized. The compounds trans-diacetonitrile[bis(o-imidazol-2-ylidenepyridine)alkane]iron(II) hexafluorophosphate (alkane = methane (2a), ethane (2b)) and cis-diacetonitrile[1,3-bis(o-imidazol-2-ylidenepyridine)propane]iron(II) hexafluorophosphate (2c) have been characterized by single-crystal X-ray diffraction (XRD), nuclear magnetic resonance spectroscopy (NMR), and infrared spectroscopy (IR). Cyclic voltammetry (CV) measurements show reversible oxidation of Fe(II) to Fe(III). The rotational barrier of the bridge has been determined via variable-temperature NMR (VT-NMR) studies of 2b. In all complexes, the Fe centers coordinatein addition to the NCCN ligandstwo acetonitrile ligands in the solid state as well as in solution. The alkylene bridge connecting the two NHCs has an influence on the coordination mode of the NCCN ligands. Whereas the methylene- and ethylene-bridged NHC moieties lead to a nearly planar geometry of the NCCN ligand and two trans-positioned acetonitrile ligands, the propylene-bridged complex 2c exhibits a sawhorse-type coordination mode, with two cis-oriented acetonitrile ligands. The reactivity of the synthesized complexes toward substitution of the solvent ligands was investigated, showing that acetonitrile is readily substituted by benzonitrile. Upon addition of carbon monoxide, one acetonitrile ligand is replaced by CO to yield complexes 3a−c, as shown by NMR and IR spectroscopy, as well as by XRD in the case of compound 3c.

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INTRODUCTION Since the first report on the synthesis of N-heterocyclic carbenes (NHCs), this compound ranks among the most widely used ligands for transition-metal centers, as evidenced by more than 3500 scientific publications and patents up to now.1 NHCs usually exhibit strong bonds to metals and they are highly versatile ligands, whose steric and electronic properties are easy to modify. Thus, NHCs have become ubiquitous ligands at transition-metal centers, outcompeting phosphines in synthetic availability and functionalization, as well as catalysis.2 In comparison to the rich chemistry of NHC complexes of noble and coinage metals,3 iron complexes bearing NHCs are not widespread. Our research interest is the synthesis of Fe−NHC complexes exhibiting weakly bound ligands, such as donor solvents, or free coordination sites. There are only three reports on the synthesis of similar compounds (Figure 1), and their reactivities are, with the exception of compound A, still unknown. In 2004, © 2012 American Chemical Society

Danopoulos et al. reported a pincer-type NHC−pyridine− NHC ligand (CNC), which was successfully coordinated to iron.4 In 2008 Hahn et al. synthesized a N-functionalized NHC complex of iron with two acetonitrile molecules.5 In 2009, Chen et al. reported a square-planar 14-valence-electron (VE) Fe complex with a tetradentate NCCN ligand.6 In this work, we report the synthesis and characterization of three iron(II) NHC complexes, which are ligated by a tetradentate NCCN ligand with different alkylene bridges.

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RESULTS AND DISCUSSION Precursor Synthesis. The preparation of the NCCN ligands starts with the synthesis of the respective bisSpecial Issue: F. Gordon A. Stone Commemorative Issue Received: November 2, 2011 Published: March 6, 2012 2793

dx.doi.org/10.1021/om2010673 | Organometallics 2012, 31, 2793−2800

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Scheme 2. Synthesis of Complexes 2a−c

Figure 1. Di-, tri-, and tetradentate NHC-pincer complexes of Fe(II), synthesized by the groups of Danopoulos (A),4 Hahn (B),5 and Chen (C).6

(imidazolium) salts bearing pyridine wing tips and different alkylene bridges (Scheme 1). While the synthesis of imidazolium salt 1a with acceptable yields has been known since 2007,7 imidazolium salts 1b,c have been reported only recently, with relatively poor yields (38 and 15%, respectively).8 With pressure tube techniques, successfully utilized for this type of reaction before, we were able to increase the yields to above 70%. Interestingly, the ethylene-bridged imidazolium salt requires a longer reaction time for complete conversion. With a shorter reaction time, a monosubstituted byproduct was observed. Scheme 1. Synthesis of Imidazolium Salts 1a−c

Figure 2. ORTEP view of the cationic complex 2a showing vibrational ellipsoids at the 50% probability level. H atoms and PF6− are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1−C1 = 1.837(2), Fe1−N3 = 2.096(2), Fe1−N4 = 1.915(2); N3−Fe1−C1 = 79.45(7), N3−Fe1−N4 = 87.14(7), N3−Fe1−N3a = 115.30(6), N4− Fe1−N4a = 172.23(7), C1−Fe1−C1a = 85.85(9), N4−Fe1−C1 = 93.77(8). Symmetry code: (a) −x, y, 1/2 − z.

Synthesis and Characterization of Iron(II)−NCCN Complexes. The cationic Fe(II) complexes 2a−c were prepared by aminolysis of [Fe{N(SiMe3)2}2(THF)] with the imidazolium salts 1a−c in acetonitrile at −35 °C in very good yields (>90%; Scheme 2), as recently described by Danopoulos et al.9 The amide ligand of the educt acts as an internal base, deprotonating the imidazolium salt to generate in situ the free carbene, which coordinates to the Fe(II) center. The byproduct bis(trimethylsilyl)amine can be easily removed in vacuo. Complexes 2a,b are red solids, which dissolve well in acetonitrile and benzonitrile but not in apolar solvents. Whereas 2a is insoluble in dichloromethane, compounds 2b,c are slightly soluble. In contrast to 2a, complexes 2b,c are not stable to air and decompose after several hours. Elemental analyses are consistent with the calculated values expected for 2a, including one additional acetonitrile molecule. Compounds 2a−c are diamagnetic and are red in the solid state. X-ray Structures of Compounds 2a−c. Single crystals of complex 2a were obtained by cooling crystallization from a saturated acetonitrile solution. In the case of 2b,c, crystals were obtained by slow diffusion of diethyl ether into an acetonitrile solution. All complexes exhibit a distorted-octahedral geometry around the Fe center (Figures 2−4). Complexes 2a,b have

similar coordination properties: the NCCN ligand coordinates in a nearly square-planar fashion to the metal, exhibiting transsituated acetonitrile molecules. A structure similar to 2a (Figure 2) was already reported in 2009 (Figure 1, complex C).6 However, the authors reported that the complex is square planar and does not contain nitrile ligands, having only 14 VE, which was unknown at that point for Fe(II) complexes. Further, they stated that this compound was stable to air, which is certainly difficult to imagine for a coordinatively and electronically unsaturated 14-VE d6 Fe center. On a closer look of the structural features of complex C, it is quite striking that it exhibits nearly the same bond lengths and angles as the isostructural compound of Co, which was reported in the same paper by Chen et al. (see Table S9 in the Supporting Information). Given the distinctly different ionic radii of Fe(II) and Co(II), the structure of compound C has to be seriously questioned. Thus, a structural comparison of C with our compound 2a is not possible. Note, that despite our best efforts, we were unable to synthesize compound C according to the published route starting from Fe powder. Also, we 2794


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oxygen, hydrogen, and other reactive molecules, which would certainly react with an Fe(II) 14-VE complex. The Fe1−C1 bond lengths of 1.837(2) Å (2a) and 1.905(2) Å (2b) are elongated with higher alkylene bridges and are notably shorter than in A (1.944(5) Å)4 but are within the range of B (1.917(10) Å).5 The Fe1−C1 bond length of 2a (1.837(2) Å) is somewhat longer than in the analogous complex C (1.801(6) Å).6 The Fe1−N3 bond lengths of 2a (2.096(2) Å) and 2b (2.099(2) Å) are remarkably longer compared to those observed in A (1.911(4) Å)4 and B (1.956(8) Å).5 However, they are comparable to the Fepyridine bond length found in C (1.983(5) Å).6 The N3−Fe1− C1 bond angles are 79.45(7)° (2a) and 79.59(9)° (2b) and are virtually equal to those in A (79.7(2)°) but smaller compared to those in B (81.3(6)°) and C (82.2(2)°). In addition, the N4−Fe1−N4a bond angles of the trans-coordinated acetonitrile molecules are slightly bent (2a, 172.23(7)°; 2b, 173.93(9) °). Another interesting feature of complexes 2a,b is the torsion angle between the two NHC−pyridine planes spanned by N3− Fe1−C1 and N3a−Fe1−C1a (2a, 2.72°; 2b, 8.82°), which increases with the length of the alkylene bridge. In comparison to the analogous nickel complexes,8 the angle determined for complex 2a is in the same range. However, the angle of the ethylene-bridged complex 2b is smaller due to the preferred octahedral coordination of Fe(II) compared to Ni(II). As mentioned above, the steric encumbrance caused by the higher number of carbon atoms in the alkylene bridge induces a distortion of the planar coordination of the tetradentate NCCN ligand. This steric effect is maximized in this series at complex 2c, displaying a propylene bridge between the NHC ligands. The torsion angle between the two NHC−pyridine moieties becomes so large that a change of the ligand geometry is induced. The tetradentate NCCN ligand no longer coordinates in plane (Figure 4) but leads to a sawhorse-type coordination with two cis-oriented acetonitrile ligands. This is a very interesting fact in the light of the distinctly different reactivities of trans- vs cis-oriented free coordination sites in Fe(II) centers, as discussed by Que et al.10 As expected, two different lengths for the iron−carbene bond were found (Fe1−C1 = 1.897(2) Å, Fe1−C12 = 1.913(2) Å), as a consequence of the different trans influences of the pyridine and nitrile donors. Pyridine is a weaker σ-donor than acetonitrile (Fe1−N6 vs Fe1−N8 bond), and hence, the trans influence of pyridine leads to a shorter Fe−NHC bond. The bond lengths are similar to those in complexes 2a,b. Additionally, the two different iron− pyridine bond lengths are in the expected range (Fe1−N3 = 1.993(2) Å, Fe1−N6 = 2.037(2) Å). The bond angles of N3− Fe1−C1 (80.41(8)°) and of N6−Fe1−C12 (80.72(8)°) are also quite similar to those in 2a,b. One interesting feature of the crystal structure of 2c is the bending of one of the acetonitrile ligands (Fe1−N7−C20 = 166.9(2)°, Figure 4), which is probably caused by a steric interaction with the atoms of the bridge (Figure 5). Spectroscopic Characterization. Complexes 2a−c are diamagnetic and therefore suitable for NMR spectroscopic investigations. 1H NMR spectra of compounds 2a,b clearly show the absence of the C2 imidazolium proton resonance signals (at around 10 ppm), indicating the deprotonation: i.e., the coordination to the metal. Since compounds 2a,b both have C2 symmetry, one set of signals for the protons at the pyridine moiety of the carbenes, the imidazolylidene moiety, and the bridging alkylene unit can be found. The ligated acetonitrile molecules were not observed, due to fast exchange with

Figure 3. ORTEP view of the cationic complex 2b showing vibrational ellipsoids at the 50% probability level. H atoms and PF6− are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1−N3 = 2.099(2), Fe1−N4 = 1.927(2), Fe1−C1 = 1.905(2); N3−Fe1−C1 = 79.59(9), N3−Fe1−N4 = 85.64(8), N3−Fe1−N3a = 104.96(8), N4− Fe1−N4a = 173.93(9), C1−Fe1−C1a = 96.4(1), N4−Fe1−C1 = 95.1(1). Symmetry code: (a) 1 − x, y, 1/2 − z.

Figure 4. ORTEP view of the cationic complex 2c showing vibrational ellipsoids at the 50% probability level. H atoms and PF6− are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1−N3 = 1.993(2), Fe1−N6 = 2.037(2), Fe1−N7 = 1.952(2), Fe1−N8 = 1.969(2), Fe1−C1 = 1.897(2), Fe1−C12 = 1.913(2); N3−Fe1−C1 = 80.41(8), N6−Fe1−C12 = 80.72(8), N3−Fe1−N7 = 174.00(8), N3− Fe1−N8 = 85.04(8), N6−Fe1−N7 = 84.77(7), N6−Fe1−N8 = 93.33(7), N3−Fe1−N6 = 94.56(7), C1−Fe1−C12 = 93.41(9), N7− Fe1−C1 = 100.95(8), N7−Fe1−C12 = 85.45(8), N8−Fe1−C1 = 93.03(8), N8−Fe1−C12 = 172.22(7).

undertook intense attempts to recrystallize compound 2a and to remove the trans-coordinated nitrile ligands, without success, however, all tries only yielded 2a. This led us to the conclusion that an iron compound with the methylene-bridged NCCN ligand precursor 1a is most likely not stable as a 14-VE complex without solvent ligands. Since the complex reported by Chen et al. appears to a reliable structure analysis, we suggest that the true structure of this complex must be as displayed in Figure 2. We found that 2a is coordinated by two nitrile ligands, which exhibit a stable bond to the Fe center and cannot be removed in vacuo. This matches well with the poor reactivity of 2a toward 2795


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Figure 5. B3LYP-optimized ground-state structure of compound 2c (all hydrogen atoms except the interacting H atoms are omitted for clarity).

deuterated acetonitrile molecules, and an almost equimolar amount of free acetonitrile is found. An exchange experiment with the more strongly ligating benzonitrile gave a 1:5 ratio of the benzonitrile- to the acetonitrile-ligated complex (in excess of deuterated acetonitrile; see the Supporting Information). With regard to the 1H NMR resonance signals of 2a,b, one signal is shifted relatively downfield (2a, 9.56 ppm; 2b, 9.43 ppm), which can be assigned to the proton of the pyridine moiety in an ortho position. The imidazolylidene backbone signals (2a, 8.33/7.86 ppm; 2b, 8.22/7.54 ppm) appear as singlets due to the usually low coupling constant, which could not be resolved. The methylene bridge of complex 2a shows a clear singlet (6.99 ppm), whereas 2b displays a broadened virtual doublet (4.87 ppm), indicating two enantiomers, transformable into each other by rotation of the bridge within the NMR time scale (Figure 6). In a variable-temperature 1H NMR experiment, a clear singlet at higher temperatures (coalescence temperature Tc = 35 °C) and two doublets at lower temperatures (first temperature without line broadening T0 = −5 °C) are obtained with a coupling constant of 14.0 Hz, which is characteristic for a geminal coupling (Figure 6). On the basis of the obtained NMR data an enthalpic energy ΔH⧧ of 60.4 ± 3.1 kJ/mol was determined for the rotational barrier (determination of the rate of interconversion via line shape analysis (see the Supporting Information). The 13C{1H} signals of the carbene carbon atoms are at 216.2 ppm (2a) and 208.3 ppm (2b), which is expected for iron carbenes (210−190 ppm).4−6,11 The shift of 2a isto the best of our knowledgethe most downfield shifted Fe(II)−NHC carbon resonance reported to date. In contrast, the analogous complex C synthesized by Chen et al. shows the carbene signal at 159.1 ppm.6 All other 13C{1H} signals were observed and could be assigned by 2D-NMR experiments (HMQC, COSY; see the Supporting Information). The structures of compounds 2a,b are corroborated by ESI-MS and IR spectroscopy, where the CN stretching vibrations were observed at 2255.8 cm−1 (2a) and 2283.3 cm−1 (2b). The 1H NMR spectrum of compound 2c shows a complete set of signals for each position due to the broken symmetry. Interestingly, only one downfield-shifted ortho pyridine proton (at C19) is observed at 9.32 ppm. The second ortho pyridine proton (at C8; 6.97 ppm)in close proximity to the other

Figure 6. Variable-temperature 1H NMR spectra of compound 2b at temperatures from −20 to +70 °C. The rotation of the ethylene bridge (4.5−5.0 ppm) can be frozen at −5 °C.

pyridine ringis shifted relatively upfield (Δδ = −2.35 ppm). This can be explained by the magnetic anisotropy of the π system in the pyridine moiety caused by the ring current.12 Interestingly, the 1H NMR spectra also exhibits an unusual upfield shift of one of the bridge protons with a Δδ value of −1.53 ppm, in comparison to the second proton at the same carbon atom (C9). This shielding effect is again caused by magnetic anisotropy: i.e., the π bond of the acetonitrile ligand. For coordination compounds, there have been only a few investigations of this type of effect.13 Computational studies of 2c confirm the close proximity of one of the bridge protons at C9 to the π bond (N7−C20) of the acetonitrile ligand with a calculated distance of 2.532 Å (Figure 5). The two resonance signals of the carbene atoms of 2c in 13 C{1H} NMR spectroscopy were observed at 213.7 and 206.6 ppm, respectively, well within the expected range. Each carbon atom could be observed and clearly assigned except for one of the ipso carbon atoms of the pyridine moiety. An inverse-gated 13 C{1H} NMR experiment, which can be integrated, shows that both pyridine carbon atoms in ipso positions have identical chemical shifts at 156.0 ppm. Signals of ligated acetonitrile molecules of 2c were not observed via 1H NMR or via 13C NMR spectroscopy, probably due to fast exchange on the NMR time scale. The presence of coordinated solvent molecules was shown by the CN stretching vibrations in the IR spectrum at 2320.0 and 2286.7 cm−1. Cyclic Voltammetry Experiments. Electrochemical properties of complexes 2a−c were determined by CV in acetonitrile solution with 0.1 M nBu4NPF6 as the supporting 2796


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electrolyte from −0.2 V to +1.8 V vs Ag/AgCl and a scanning rate of 100 mV/s. All cyclic voltammograms show a totally reversible oneelectron redox process (FeII/FeIII) for at least 20 cycles (Figure 7). Therefore, all complexes should be suitable for one-

A single substitution of one acetonitrile ligand in 2a can be seen by comparing the signal of the rigid methylene bridge, which is a singlet in the starting material and two doublets in the product, showing a geminal coupling (2JHH = 12.7 Hz). According to the broken symmetry, a diastereotopic splitting of the signal is expected by single substitution. The ethylene bridge of 2b exhibits a different behavior: while the educt spectrum shows a broad virtual doublet, the product signal is a broad singlet, most probably due to the smaller steric hindrance caused by carbon monoxide in comparison to acetonitrile. To exclude a double CO substitution, a low-temperature NMR experiment was performed, giving an AA′BB′ coupling pattern of the ethylene bridge, which can only be caused by single CO substitution (see the Supporting Information). The 13C{1H} NMR spectra of 3a−c also show relative shifts of every carbon resonance signal in comparison to the educts. Dissolved carbon monoxide has a chemical shift of 185.2 ppm, while coordinated carbon monoxide is shifted relatively downfield (3a, 196.0 ppm; 3b, 211.6 ppm; 3c, 188.1 ppm). The two resonance signals of the ipso carbon atom of the pyridine moiety of compound 3c, having identical chemical shifts in the educt at 156.0 ppm, are now separated (154.3/154.0 ppm). IR spectroscopy shows absorption bands in the typical range for carbonyls as well as for nitriles for each compound (see the Experimental Section). Concerning the two possible substitution sites of complex 2c, complex 3c was crystallized by diffusion of diethyl ether into an acetonitrile solution. The obtained single-crystal structure of 3c shows the replacement of one acetonitrile by CO, trans to the N atom of the pyridine moiety (Figure 8). The preferred

Figure 7. Cyclic voltammograms of complexes 2a−c in 0.1 M n Bu4NPF6 acetonitrile solution under an argon atmosphere (scan rate 100 mV s−1; Pt electrodes).

electron-transfer (catalytic) reactions. The expected ratio of unity (ip,a/ip,c) is observed, and the ΔEp separation is in the range of 100−160 mV. Interestingly, the half-cell potential E1/2 increases with increased bridge lengths from 0.86 V (2a) to 0.94 V (2b) and 0.98 V (2c). Reactivity of Compounds 2a−c. NMR pressure tube experiments of complexes 2a−c and dihydrogen or propylene show no reaction. The addition of carbon monoxide (3.5 bar) in acetonitrile-d3 at room temperature led to the isolation of complexes 3a−c in quantitative yields (Scheme 3). The completion of the reaction was determined by NMR and varied between 4 and 16 h. Within this time, the color changed from red to orange. The 1H NMR signals are shifted relative to the educts 2a−c. Scheme 3. Carbonylation of Compounds 3a−c

Figure 8. ORTEP view of the cationic complex 3c showing vibrational ellipsoids at the 50% probability level. H atoms and PF6− are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1−N3 = 2.042(2), Fe1−N6 = 2.026(2), Fe1−N7 = 1.965(2), Fe1−C1 = 1.902(2), Fe1−C12 = 1.919(2), Fe1−C22 = 1.771(2); N3−Fe1−C1 = 79.87(9), N3−Fe1−C12 = 99.61(9), N3−Fe1−C22 = 172.45(9), N6−Fe1−C1 = 170.29(9), N6−Fe1−C12 = 80.71(9), N6−Fe1−C22 = 89.20(9), N7−Fe1−C1 = 93.16(9), N7−Fe1−C12 = 173.42(9), N7−Fe1−C22 = 88.83(10), N3−Fe1−N6 = 93.98(8), N3−Fe1−N7 = 84.14(9), N6−Fe1−N7 = 93.68(9), C1−Fe1−C12 = 92.8(10), C1− Fe1−C22 = 97.83(10), C12−Fe1−C22 = 87.66(11).

substitution site is also verified by the difference in the groundstate free energies with ΔG = 19.7 kJ/mol for both possible isomers (see the Supporting Information). The Fe1−C22 bond length (1.771(2) Å) is in the range of similar iron carbene CO complexes.14 Further, the CO ligand is slightly bent (N3−Fe1− C22 = 172.45(9)°) due to the steric pressure caused by the propylene bridge. No siginificant changes in the other metal− 2797


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Å, c = 13.4771(8) Å, α = 100.749(2)°, β = 105.640(2)°, γ = 118.072(1)°, V = 1611.20(13) Å3, Z = 2, λ(Mo Kα) = 0.710 73 Å, μ = 0.671 mm−1, ρcalcd = 1.648 g cm−3, T = 123(1) K, F(000) = 808, θmax = 25.35°, R1 = 0.0301 (5271 observed data), wR2 = 0.0801 (all 5789 data), GOF = 1.028, 482 parameters, Δρmax/min = 0.40/−0.24 e Å−3. 3c: yellow fragment, C22H21FeN7O·2F6P, Mr = 745.25, triclinic, space group P1̅ (No. 2), a = 11.5615(2) Å, b = 12.4910(3) Å, c = 13.2661(3) Å, α = 87.9783(8)°, β = 76.8642(8)°, γ = 63.4836(7)°, V = 1664.61(6) Å3, Z = 2, λ(Mo Kα) = 0.710 73 Å, μ = 0.644 mm−1, ρcalcd = 1.487 g cm−3, T = 123(1) K, F(000) = 748, θmax = 25.4°, R1 = 0.0362 (4829 observed data), wR2 = 0.0859 (all 6020 data), GOF = 1.042, 407 parameters, Δρmax/min = 0.89/−0.29 e Å−3. For more detailed information, see the Supporting Information. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication Nos. CCDC-851957 (2a), CCDC-851958 (2b), CCDC-851959 (2c), and CCDC-864195 (3c). Copies of the data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (+44)1223-336-033; e-mail, [email protected]. Computational Methods. All calculations used DFT methodology as implemented in Gaussian0317 using the density functional hybrid model B3LYP18 together with 6-31G** as the basis set.19 No symmetry or internal coordinate constraints were applied during optimizations. All reported structures were verified as being true minima by the absence of negative eigenvalues in the vibrational frequency analysis. Line shape analyses were performed with WINDNMR.20 General Procedure for Imidazolium Salts 1b,c. o-Imidazolylpyridine (1.00 g, 6.88 mmol) and α,ω-dibromoalkane (3.13 mmol) were dissolved in THF (p.a., not dried; 10 mL) in an ACE pressure tube in air. The sealed pressure tube was heated to 100 °C for 1 or 4 days. The slightly yellow precipitate that formed was filtered off and washed with ether and ethyl acetate to give an off-white powder, which was dried in vacuo. The solid was dissolved in water (20 mL), and an aqueous solution of ammonium hexafluorophosphate (1.28 g, 7.83 mmol, 20 mL) was added to the stirred solution to form a colorless precipitate, which was filtered off and washed with water (3 × 10 mL) and dried in vacuo. 1,2-Bis(o-imidazolylpyridine)ethane Hexafluorophosphate (1b). Reaction time: 4 days. Yield: 72%. The product can be recrystallized from methanol if necessary. 1H NMR (400 MHz, DMSO-d6, 300 K): δ 10.13 (s, 2H, NCHN), 8.66 (d, 3J = 4.8 Hz, 2H, Hpy), 8.57 (s, 2H, Him), 8.24 (dd, 2H, 3JHH = 8.1 Hz, 3JHH = 7.4 Hz, Hpy), 8.03 (d, 3JHH = 8.1 Hz, 2H, Hpy), 7.93 (s, 2H, Him), 7.67 (dd, 3JHH = 7.4 Hz, 3JHH = 4.8 Hz, 2H, Him), 4.90 (s, 4H, NCH2CH2N). 1,3-Bis(o-imidazolylpyridine)propane Hexafluorophosphate (1c). Reaction time: 1 day. Yield: 1.66 g, 85%. 1H NMR (400 MHz, DMSOd6, 300 K): δ 10.16 (s, 2H, NCHN), 8.65 (d, 3JHH = 4.8 Hz, 2H, Hpy), 8.59 (s, 2H, Him), 8.24 (dd, 2H, 3JHH = 8.1 Hz, 3JHH = 7.7 Hz, Hpy), 8.09 (s, 2H, Him), 8.05 (d, 3JHH = 8.1 Hz, 2H, Hpy), 7.66 (dd, 3JHH = 7.7 Hz, 3JHH = 4.8 Hz, 2H, Him), 4.42 (t, 3JHH = 6.7 Hz, 4H, CH2CHCH2), 2.62 (p, 3JHH = 6.7 Hz, 2H, CH2CH2CH2). trans-Diacetonitrile[bis(o-imidazol-2-ylidenepyridine)methane]iron(II) Hexafluorophosphate (2a). A suspension of bis(o-imidazolylpyridine)methane (1a; 215 mg, 362 μmol) in acetonitrile (10 mL) was slowly added to a suspension of [Fe{N(SiMe3)2}2(THF)] (162 mg, 362 μmol) in acetonitrile (5 mL) at −35 °C via a transfer cannula and gently warmed to room temperature. After stirring for 4 days the deep red solution was filtered via a filter cannula and the filtrate was evaporated to dryness. After the residue was redissolved in acetonitrile (5 mL), the solvent was again evaporated under vacuum to remove residual amine to leave an orange-red solid. The compound was recrystallized from acetonitrile at −25 °C. Single crystals were grown by cooling crystallization (60 °C to room temperature) from an oversaturated solution. Yield: 246 mg (93%). 1H NMR (400 MHz, CD3CN, 300 K): δ 9.56 (d, 3JHH = 4.8 Hz, 2H, Hpy), 8.36 (dd, 3JHH = 8.1 Hz, 3JHH = 6.5 Hz, 2H, Hpy), 8.33 (s, 2H, Him), 8.07 (d, 3JHH = 8.1 Hz, 2H, Hpy), 7.86 (s, 2H, Him), 7.76 (dd, 3JHH = 6.5 Hz, 3JHH = 4.8 Hz, 2H, Hpy), 6.99 (s, 2H, NCH2N).

ligand bond lengths of 3c in comparison to those in 2c were seen, except for the Fe1−N3 bond (2.042(2) Å), which is elongated more than 0.04 Å as a consequence of the better π back-donation of the CO ligand in comparison to acetonitrile. It is worth noting that the unusual anisotropic effect caused by the π system of the ligand on one of the bridge protons (at C9) can also be observed in the case of CO (Δδ = −1.61 ppm), as well as a second anisotropic effect on the proton at C8 (Δδ = −1.60 ppm; compare above).

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CONCLUSIONS The synthesis and characterization (XRD, NMR, IR) of three NCCN-ligated (NCCN = pyridine−NHC−NHC−pyridine) Fe(II) complexes 2a−c with different alkylene bridges are reported. The X-ray structures of 2a,b show a planar coordination of the NCCN ligand and two trans-coordinated acetonitrile ligands, while the propylene-bridged NCCN ligand in complex 2c shows an out-of-plane sawhorse coordination with cis coordination of two acetonitrile ligands. NMR experiments show an unusual anistotropic shielding of one of the protons of the bridge. The reactivity of the synthesized complexes 2a−c was investigated by the single exchange of one acetonitrile ligand with carbon monoxide. Further examinations of the reactivities of compounds 2a−c, particularly the oxidation of NCCN−Fe(II) to NCCN−Fe(IV)−oxo complexes, are currently being studied in our laboratories.

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EXPERIMENTAL SECTION

Materials. Unless otherwise noted, all manipulations were carried out under argon atmosphere using standard Schlenk and glovebox techniques. Solvents were obtained water and oxygen free by a MBraun MB SPS purification system. Acetonitrile-d3 was refluxed over phosphorus pentoxide and distilled prior to use. Imidazolylpyridine,15 bis(o-imidazolylpyridine)methane (1a),7 and [Fe{N(SiMe3)2}2(THF)] 16 were synthesized according to literature procedures. All reagents were purchased from commercial suppliers and used without further purification. Instruments. NMR spectra were recorded on a Bruker Avance DPX 400 (1H NMR, 400.13 MHz; 13C NMR, 100.53 MHz) and chemical shifts are reported relative to the residual signal of the deuterated solvent. NMR pressure tube reactions were carried out in a Wilmad NMR pressure tube. IR spectra were recorded on a Jasco FT/ IR-460 Plus using Nujol mulls between KBr plates. Elemental analyses (C/H/N) were obtained by the microanalytical laboratory at the Technische Universität München. ESI spectra were recorded on a Thermo Electron LCQ classic. Cyclic voltammetry measurements were performed on a GAMRY Reference 600 potentiostat, and eDAQ Electrochemical Reaction Vessels (3 mL) were used as electrochemical cells. Platinum electrodes were used as working/counter electrodes and Ag/AgCl (3.4 M in KCl) as reference electrode (all from eDAQ). Single-Crystal X-ray Structure Determinations. 2a: red fragment, C21H20FeN8·2F6P·2C2H3N, Mr = 812.35, monoclinic, space group C2/c (No. 15), a = 15.8935(6) Å, b = 15.9458(6) Å, c = 13.0754(5) Å, β = 102.558(2)°, V = 3234.5(2) Å3, Z = 4, λ(Mo Kα) = 0.710 73 Å, μ = 0.671 mm−1, ρcalcd = 1.668 g cm−3, T = 123(1) K, F(000) = 1640, θmax = 25.54°, R1 = 0.0343 (3034 observed data), wR2 = 0.0897 (all 3275 data), GOF = 1.054, 249 parameters, Δρmax/min = 0.75/−0.60 e Å−3. 2b: red-brown fragment, C22H22FeN8·2F6P·2C2H3N, Mr = 826.37, monoclinic, space group C2/c (No. 15), a = 15.7026(6) Å, b = 16.2632(6) Å, c = 13.9422(5) Å, β = 107.9400(17)°, V = 3387.4(2) Å3, Z = 4, λ(Mo Kα) = 0.710 73 Å, μ = 0.642 mm−1, ρcalcd = 1.620 g cm−3, T = 123(1) K, F(000) = 1672, θmax = 25.32°, R1 = 0.0380 (2663 observed data), wR2 = 0.0946 (all 3096 data), GOF = 1.079, 233 parameters, Δρmax/min = 1.10/−0.30 e Å−3. 2c: red fragment, C23H24FeN8·2F6P·C2H3N, Mr = 799.35, triclinic, space group P1̅ (No. 2), a = 12.2592(4) Å, b = 12.3475(4) 2798


Organometallics

Article

C{1H} NMR (100 MHz, CD3CN, 300 K): δ 216.2 (CFe), 155.3 (Cpy), 153.8 (Cpy), 142.2 (Cpy), 126.1 (Cim), 124.3 (Cpy), 120.3 (Cim), 113.1 (Cpy), 65.5 (CH2). IR (KBr/Nujol, cm−1): ν 2255.8 (CN). Anal. Calcd for C21H20N8P2F12Fe·CH3CN: C, 35.82; H, 3.01; N, 16.34. Found: C, 35.82; H, 3.13; N 16.28. ESI-MS ([M]+): m/z 377.1 [2a − 2MeCN + F−]+. trans-Diacetonitrile[1,2-bis(o-imidazol-2-ylidenepyridine)ethane]iron(II) Hexafluorophosphate (2b). A solution of 1,2bis(o-imidazolylpyridine)ethane (1b; 206 mg, 339 μmol) in acetonitrile (10 mL) was slowly added to a a suspension of [Fe{N(SiMe3)2}2(THF)] (152 mg, 339 μmol) in acetonitrile (5 mL) at −35 °C via a transfer cannula and gently warmed to room temperature. After it was stirred for 4 days, the deep red solution was filtered via a filter cannula and the filtrate was evaporated to dryness. After the residue was redissolved in acetonitrile (5 mL), the solvent was evaporated in vacuo to remove residual amine, yielding an orangered solid. The complex was purified by dissolving in dichloromethane/ acetonitrile (10/1), followed by filtration and evaporation of the filtrate under vacuum. Single crystals were grown by slow diffusion of Et2O into solution of 2b in acetonitrile. Yield: 232 mg (92%). 1H NMR (400 MHz, CD3CN, 300 K): δ 9.43 (d, 3JHH = 5.1 Hz, 2H, Hpy), 8.32 (dd, 3JHH = 8.2 Hz, 3JHH = 6.7 Hz, 2H, Hpy), 8.22 (s, 2H, Him), 8.02 (d, 3JHH = 8.2 Hz, 2H, Hpy), 7.71 (dd, 3JHH = 6.7 Hz, 3JHH = 5.1 Hz, 2H, Hpy), 7.54 (s, 2H, Him), 4.87 (br virt d, JHH = 36.9 Hz, 4H, NCH2CH2N). 13C{1H} NMR (100 MHz, CD3CN, 300 K): δ 208.3 (CFe), 156.1 (Cpy), 153.4 (Cpy), 142.1 (Cpy), 129.7 (Cim), 124.0 (Cpy), 119.0 (Cim), 112.8 (Cpy), 51.1 (CH2CH2). IR (KBr/Nujol, cm−1): ν 2283.3 (CN). Anal. Calcd for C22H22N8P2F12Fe: C, 35.50; H, 2.98; N, 15.06. Found: C, 35.69; H, 3.34; N 14.63. ESI-MS ([M]+): m/z 391.2 [2b − 2MeCN + F−]+. cis-Diacetonitrile[1,3-bis(o-imidazol-2-ylidenepyridine)propane]iron(II) Hexafluorophosphate (2c). The synthesis of 2c followed the procedure of 2b, starting with 1,3-bis-(o-imidazolylpyridine) propane 1c (175 mg, 280 μmol) and [Fe{N(SiMe3)2}2(THF)] (126 mg, 280 μmol), to give a red solid. Yield: 191 mg (90%). 1H NMR (400 MHz, CD3CN, 300 K): δ 9.32 (d, 3JHH = 5.2 Hz, 1H, Hpy1), 8.38 (dd, 3JHH = 8.3 Hz, 3JHH = 6.8 Hz, 1H, Hpy1), 8.22 (s, 1H, Him1), 8.18 (s, 1H, Him2), 8.09 (d, 3JHH = 8.3 Hz, 1H, Hpy1), 7.85 (dd, 3 JHH = 8.2 Hz, 3JHH = 6.8 Hz, 1H, Hpy2), 7.79 (dd, 3JHH = 6.8 Hz, 3JHH = 5.2 Hz, 1H, Hpy1), 7.68 (d, 3JHH = 8.2 Hz, 1H, Hpy2), 7.56 (s, 1H, Him2), 7.30 (s, 1H, Him1), 6.97 (d, 3JHH = 5.4 Hz, 1H, Hpy2), 6.87 (dd, 3 JHH = 6.8 Hz, 3JHH = 5.4 Hz, 1H, Hpy2), 4.57−4.38 (m, 2H, CH2CH2CHH), 4.02 (virt d, JHH = 15.4 Hz, 1H, CH2CH2CHH), 2.54−2.44 (m, 1H, CH 2 CH 2 CHH), 2.32−2.23 (m, 2H, CH2CH2CHH). 13C{1H} NMR (100 MHz, CD3CN, 300 K): δ 213.7 (Cim1Fe), 206.6 (Cim2Fe), 156.0 (2C, Cpy2/Cpy1; see Results and Discussion), 153.9 (Cpy1), 153.6 (Cpy2), 141.9 (Cpy1), 140.5 (Cpy2), 129.5 (Cim1), 126.8 (Cim2), 124.4 (Cpy1), 121.8 (Cpy2), 121.4 (Cim1), 119.3 (Cim2), 113.0(Cpy1), 112.4(Cpy2), 46.9 (CH2CH2CHH), 46.7 (CH2CH2CHH), 31.3 (CH2CH2CHH). IR (KBr/Nujol, cm−1): ν 2320.0 (CN), 2286.7 (CN). Anal. Calcd for C23H24N8P2F12Fe: C, 36.43; H, 3.19; N, 14.78. Found: C, 36.48; H, 3.30; N 15.03. ESI-MS ([M]+): m/z 405.2 [2c − 2MeCN + F−]+. Genreal Procedure for the Carbonylation of Complexes 2a− c. A NMR pressure tube was filled with a solution of complexes 2a−c (15 mg) in acetonitrile-d3 (0.7 mL) inside a glovebox. Subsequently, the NMR tube was degassed by several freeze−pump−thaw cycles before CO (3.5 bar) was added. After a reaction time of at least 4 h (depending on the complex), a complete conversion was observed. Acetonitrilecarbonyl[bis(o-imidazol-2-ylidenepyridine)methane]iron(II) Hexafluorophosphate (3a). Reaction time: 24 h. Yield: 100% (determined by NMR). 1H NMR (400 MHz, CD3CN, 300 K): δ 9.14 (d, 3JHH = 5.3 Hz, 2H, Hpy), 8.37 (dd, 3JHH = 8.3 Hz, 3JHH = 6.8 Hz, 2H, Hpy), 8.24 (s, 2H, Him), 8.04 (d, 3JHH = 8.3 Hz, 2H, Hpy), 7.82 (s, 2H, Him), 7.73 (dd, 3JHH = 6.8 Hz, 3JHH = 5.3 Hz, 2H, Hpy), 6.83 (d, 2 JHH = 12.7 Hz, 1H, CHH), (d, 2JHH = 12.7 Hz, 1H, CHH). 13C{1H} NMR (100 MHz, CD3CN, 300 K): δ 212.3 (CFe), 196.0 (CO), 154.2 (Cpy), 153.8 (Cpy), 143.3 (Cpy), 126.4 (Cim), 125.0 (Cpy), 120.8 (Cim), 114.3 (Cpy), 65.0 (CH2). IR (KBr/Nujol, cm−1): ν 2264.0 (CN), 2022.5 (CO). 13

Acetonitrilecarbonyl[1,2-bis(o-imidazol-2-ylidenepyridine)ethane]iron(II) Hexafluorophosphate (3b). Reaction time: 6 h. Yield: 100% (determined by NMR). 1H NMR (400 MHz, CD3CN, 300 K): δ 9.02 (d, 3JHH = 3.8 Hz, 2H, Hpy), 8.33 (dd, 3JHH = 8.2 Hz, 3JHH = 6.0 Hz, 2H, Hpy), 8.11 (s, 2H, Him), 7.97 (d, 3JHH = 8.2 Hz, 2H, Hpy), 7.69 (dd, 3JHH = 6.0 Hz, 3JHH = 3.8 Hz, 2H, Hpy), 7.52 (s, 2H, Him), 4.75 (br s, NCH2CH2N). 13C{1H} NMR (100 MHz, CD3CN, 300 K): δ 215.6 (CFe), 211.6 (CO; determined by labeled 13CO), 154.6 (Cpy), 152.8 (Cpy), 143.2 (CPy), 130.2 (Cim), 124.7 (Cpy), 119.2 (Cim), 114.0 (CPy), 50.8 (br s, CH2CH2). IR (KBr/Nujol, cm−1): ν 2260.7 (CN), 1988.3 (CO). Acetonitrilecarbonyl[1,3-bis(o-imidazol-2-ylidenepyridine)propane]iron(II) Hexafluorophosphate (3c). Reaction time: 4 h. Yield: 100% (determined by NMR). 1H NMR (400 MHz, CD3CN, 300 K): δ 8.76 (d, 3JHH = 5.4 Hz, 1H, Hpy1), 8.42 (dd, 3JHH = 8.4 Hz, 3 JHH = 6.8 Hz, 1H, Hpy1), 8.24 (d, 3JHH = 2.2 Hz, 1H, Him1), 8.17 (dd, 3 JHH = 8.2 Hz, 3JHH = 6.8 Hz, 1H, Hpy2), 8.04 (d, 3JHH = 8.4 Hz, 1H, Hpy1), 8.02 (d, 3JHH = 2.2 Hz, 1H, Him2), 7.95 (d, 3JHH = 8.2 Hz, 1H, Hpy2), 7.78 (dd, 3JHH = 6.8 Hz, 3JHH = 5.4 Hz, 1H, Hpy1), 7.53 (d, 3JHH = 2.2 Hz, 1H, Him2),7.25 (dd, 3JHH = 6.8 Hz, 3JHH = 5.4 Hz, 1H, Hpy2), 7.23 (d, 3JHH = 2.2 Hz, 1H, Him1), 7.16 (d, 3JHH = 6.8 Hz, 1H, Hpy2), 4.68−4.62 (m, 2H, CH 2 CH 2 CHH), 3.97−3.89 (m, 1H, CH2CH2CHH), 2.40−2.25 (m, 3H, CH2CH2CHH). 13C{1H} NMR (100 MHz, CD3CN, 300 K): δ 215.3 (Cim1Fe), 201.8 (Cim2Fe), 188.1 (CO), 154.3 (Cpy1), 154.0 (Cpy2), 152.7 (Cpy1), 150.5 (Cpy2), 143.6 (Cpy1), 143.0 (Cpy2), 130.3 (Cim1), 127.5 (Cim2), 125.5 (Cpy1), 123.9 (Cpy2), 122.0 (Cim1), 119.1 (Cim2), 114.2 (Cpy1), 114.0 (Cpy2), 48.0 (CH2CH2CHH), 46.8 (CH2CH2CHH), 30.8 (CH2CH2CHH). IR (KBr/Nujol, cm−1): ν 2359.5 (CN), 1997.9 (CO).

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ASSOCIATED CONTENT

S Supporting Information *

Text, tables, figures, and CIF files, line shape analysis, crystallographic data for 2a−c and 3c, comparison of calculated and measured bond lengths and angles, NMR and IR spectra, B3LYP-optimized ground-state structure of 3c, and the calculated energy difference of possible monosubstituted CO products of 2c. 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] (W.A.H.); fritz. [email protected] (F.E.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Osnat Younes-Metzler for valuable help with CV measurements. This work was financially supported by the EU Project Next-GTL. The Abu Dhabi Petroleum Institute and the TUM Graduate School are acknowledged for further financial support.

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REFERENCES

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Synthesis and Characterization of Novel Iron(II) Complexes with

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