Change the Redox Behavior of Iron(II) Complexes - ACS Publications

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NHC Versus Pyridine: How “Teeth” Change the Redox Behavior of Iron(II) Complexes Daniel T. Weiss,†,⊥ Markus R. Anneser,†,⊥ Stefan Haslinger,† Alexander Pöthig,‡ Mirza Cokoja,§ Jean-Marie Basset,∥ and Fritz E. Kühn*,† †

Chair of Inorganic Chemistry/Molecular Catalysis, Department of Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstraße 4, D-85747 Garching bei München, Germany ‡ Catalysis Research Center, Technische Universität München, Ernst-Otto-Fischer-Straße 1, D-85747 Garching bei München, Germany § Chair of Inorganic & Organometallic Chemistry, Department of Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstraße 4, D-85747 Garching bei München, Germany ∥ KAUST Catalysis Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia S Supporting Information *

ABSTRACT: A series of octahedral iron(II) complexes with tetradentate NHC/pyridine hybrid ligands containing up to three pyridyl units was designed to study the influence of NHC and pyridine donors on the electronic structure of the metal center. Structural analysis of the iron complexes by NMR spectroscopy and single-crystal X-ray diffraction reveals different coordination modes of the ligand depending on the linkage of the different donor moieties. The oxidation potentials of all complexes correlate linearly with the number of NHC moieties coordinated to iron, as shown by cyclic voltammetry. The influence, although minor, of structural properties on the oxidation potential and (in one case) the influence of the oxidation state of the coordination geometry of the hybrid ligand are also demonstrated.

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far: open-chain (D),29 macrocyclic (F),30−32 and mixed aryl- or heteroatom-bridged ligands.33−38 The coordination chemistry of the aforementioned acyclic type A di-NHC ligand derivatives has been well investigated. Complexes are known for group 10 metals,21,22,25,26,39−41 coinage metals,22,42 cobalt,41 ruthenium,43 and iron.41,44 Only one example of class B is known; it is a highly rigid ligand and does not permit mononuclear, chelated metal complexes, with only a silver complex being reported.23 The more flexible skeletal structure of type C offers chelating abilities,24,27,45 although the reported palladium complex is strongly distorted with N−Pd−N and N−Pd−C angles of 80° in comparison to C−Pd−C angles of 120°.24 Acyclic tetra-NHC ligands (D) are highly flexible, and complexes are known for coinage metals, group 10 metals, and iron.29,46 For macrocyclic ligands of type E and F a broad range of square-planar as well as linearly coordinated metal complexes is known.10,11,17,28,30−32,47−54 In this work, the synthesis of a series of open-chain, tetradentate NHC/pyridine hybrid ligands with different NHC/pyridyl ratios and their application in the coordination of iron(II) is presented. The compounds are analyzed by NMR

INTRODUCTION

N-heterocyclic carbene (NHC) ligands have become a multifunctional tool in organometallic chemistry and homogeneous catalysis, as steric and electronic properties can be easily influenced by modification of the substituents.1−6 During the past decade, iron NHC complexes have especially received increasing attention.7−9 Examples of remarkable reactivity were obtained on the basis of the use of chelating, polydentate NHC ligands,10−17 including oxidation catalysis with NHC/pyridine hybrid ligands.18,19 Recently, it could be shown that replacing acetonitrile ligands of the catalytically active iron(II) NCCN complex has a strong influence on the electronic structure of the metal center.20 However, the literature lacks comprehensive studies on the influence of NHC/pyridine hybrid ligands with different NHC/pyridyl ratios on the oxidation potential of the metal. Such manipulations would allow fine-tuning of the metal center without blocking labile binding sites. Several alkylene-bridged tetradentate NHC/pyridine hybrid ligands are known (Figure 1). Ligands of the general types A (NCCN) and B and C (both CNNC) as well as their arylated derivatives21−27 count for open-chain structures, while E fits into the class of macrocyclic ligands.28 Three types of alkylenebridged tetra-NHC ligands (CCCC) have been described so © XXXX American Chemical Society

Received: August 25, 2015

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DOI: 10.1021/acs.organomet.5b00732 Organometallics XXXX, XXX, XXX−XXX

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Scheme 2. Synthesis of Mono-NHC Ligand Precursors P1 and P2

ligand precursors P1 and P2, as 1 was not purified according to the literature but was deployed as a crude mixture with a product content of around 60%. Workup and anion exchange with ammonium hexafluorophosphate gave P1 and P2 as colorless solids. As mentioned before, a variety of di-NHC ligands of the NCCN type are known.21,25 Inversion of the general donor sequence leads to di-NHC ligands of the CNNC type. The first ligand of this type was reported by the group of Chen and was based on a naphthyridine unit.23 A slight flexibility was introduced by changing the fundamental unit to a bipyridine.24,27,45 A ligand precursor with two additional methylene bridges was synthesized in 2005 but was not applied as an NHC ligand in coordination chemistry.59 Despite the fact that numerous precursors of the CNNC type exist, no such iron NHC complexes have been reported. We modified the literature procedure for P5 in analogy to the syntheses of P1 and P2 (Scheme 3).

Figure 1. Different types of tetradentate NHC and NHC/pyridine hybrid ligands.

spectroscopy and single-crystal X-ray diffraction. Electrochemical investigations are used for the evaluation of the impact of the NHC/pyridyl ratio on the oxidation potentials of the iron(II) complexes in the context of the literature.32,44,46,51

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RESULTS AND DISCUSSION Synthesis of Ligand Precursors. Open-chain and tetradentate NHC/pyridine hybrid ligands have mostly been limited to symmetrically linked di-NHC ligands that are synthesized by either the reaction of two pyridyl-substituted imidazoles (Scheme 1i)21,25 or disubstitution of a centered dipyridine fragment with imidazole (Scheme 1ii).23,24 To investigate the influence of the NHC/pyridyl ratio in the respective hybrid ligand on the electronic structure of an iron(II) complex, a tetradentate and chelating mono-NHC ligand with three pyridine moieties was targeted. Synthetic paths toward a terminal imidazolium unit are challenging, as they either involve heterocoupling of a bipyridine and an imidazolyl-pyridine or the introduction of a terminal imidazolium moiety to a tripyridine. Neither approach has been reported so far. Thus, introduction of a centered imidazolium unit was chosen, as successfully implemented before.55−57 However, none of the ligands were able to form mononuclear κ4-coordinated complexes; in addition, no iron complexes have been reported. Therefore, modification of an approach described by Newkome et al.,58 who synthesized tetrapyridine ligands via bromomethylated bipyrdine 1, was executed (Scheme 2). The bromination of 6-methyl-2,2′bipyridine to yield 1 was performed in analogy to the literature procedure.58 Moderate yields of around 40% were obtained for

Scheme 3. Synthesis of Di-NHC Ligand Precursor P5

Bipyridine derivative 4 was synthesized and purified according to literature procedures.60 Reaction with excess methylimidazole in boiling THF led to precipitation of the diimidazolium salt from the reaction mixture. Anion exchange with ammonium hexafluorophosphate and precipitation from acetone yielded ligand precursor P5 as a colorless powder in 80% yield. The access to tetradentate, open-chain tri-NHC-pyridine ligands is again challenging. All attempts to target a CNCC ligand precursor with an internal pyridine moiety failed. At first glance, the synthesis of a ligand precursor with an external pyridine unit may seem even more complicated, as only one

Scheme 1. Retrosynthetic Access to Literature-Known NCCN (i) and CNNC (ii) Ligands

B


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Organometallics linearly linked triazolium fragment has been reported to date and modification of this precursor hardly appears possible.61 Nevertheless, a combination of two recently developed strategies to access nonsymmetrically substituted diimidazolium salts was found to be successful.29,62,63 Following the initial work of Nozaki et al.,62 who targeted asymmetrically substituted monoimidazolium salts, the group of Gardiner was able to access N,N′-asymmetrically substituted di-NHC ligands by isolating halo-methylated imidazolium salts.63 Instead of the arylimidazole precursors used in their work, we reacted 2-imidazolylpyridine with chloroiodomethane. In the next step, reaction with a recently reported poly-NHC building block29 allowed access to a nonsymmetric NCCC ligand precursor (Scheme 4). Scheme 4. Synthesis of Tricarbene Ligand Precursor P6

Figure 2. Precursors P3, P4, P7, and P8 known in the literature for tetradentate, open-chain poly-NHC ligands and P9 and P10 for tetradentate, macrocyclic poly-NHC ligands.

nating amide ligand. By modification of their approach and addition of 1 equiv of ammonium hexafluorophosphate working as proton and counterion sourceit was possible to isolate iron(II) complexes C1 and C2 (Scheme 5). Scheme 5. Synthesis of Iron Complexes C1 and C2

Halomethylation of imidazolylpyridine 2 using an excess of chloroiodomethane did not work out as well as was reported for other arylimidazole precursors in the literature.63 Even at a 50-fold excess of chloroiodomethanein comparison to an 8− 12-fold excess in the literature−distinct amounts of NCCN ligand precursor P3 were obtained, which could be separated from 5 by stepwise precipitation of the raw product. Alternatively, the crude product mixture of 5 was directly used for further reaction with the building block 6. The resulting raw product could be precipitated stepwise to separate P6 from unreacted 6 and impurities of P3. Further tetradentate NHC and NHC/pyridine hybrid ligand precursors investigated in this work are given in Figure 2 and can be accessed via literature procedures.21,22,28,29,32 Synthesis of Iron(II) Complexes. Synthetic access to iron NHC complexes can be achieved via a variety of different approaches.9 Recently, the use of imidazolium salts together with [Fe{N(SiMe3)2}2(THF)] as iron precursor with an internal base has especially attracted a significant amount of interest.17,32,44,51,64−67 Transmetalation starting from silver NHC complexes and iron halides has also been studied during the last few years.46,54,68 Both methods reach their limits when they are applied to mono-NHC ligand precursors P1 and P2. The silver complex of the similar NCMeCN ligand features three silver ions per two ligands.22 Thus, the stoichiometry for transmetalation with MIIX2 salts, which require two silver ions per molecule ligand, is not matched. On the other hand, the iron precursor [Fe{N(SiMe3)2}2(THF)] contains 2 equiv of internal base while only 1 equiv is needed. Danopoulos et al.64 showed that iron mono-NHC complexes accessed from [Fe{N(SiMe3)2}2(THF)] as a precursor contain one coordi-

The synthesis was performed in acetonitrile, and 1 h after the reaction was started, ammonium hexafluorophosphate was added as a suspension in acetonitrile. Complexes C1 and C2 have been carefully precipitated, washed with diethyl ether, and dried under high vacuum. 1H NMR spectroscopic investigation (see Figures S10 and S12 in the Supporting Information) in solution reveal all expected signals for the ligand and the presence of two weakly bound acetonitrile ligands, which are prone to ligand exchange with deuterated acetonitrile. An equatorial coordination of the ligand is revealed by sharp singlet signals obtained for the methylene bridges. The characteristic signals for carbene carbons in 13C NMR are found at 216.13 ppm for C1 and 196.70 ppm for C2 (see Figures S11 and S13 in the Supporting Information). Single crystals of complex C2 suitable for X-ray diffraction were obtained by diffusion of diethyl ether into an acetonitrile solution of the complex (Figure 3) and reveal an octahedral coordination of iron in C2. As shown by NMR spectroscopy in solution, in the solid state the ligand is coordinated in an equatorial geometry with two axial acetonitrile ligands. The Fe−C1 distance of 1.892(2) Å as well as Fe−N3 and Fe−N5 distances of 2.0231(17) and 2.0487(17) Å are in the range of those bonds in comparable iron complexes.44 Only the Fe−N4 distance of 1.9743(17) Å is somewhat shorter and closer to the bond lengths of Fe−NMeCN in the range 1.9358(17)−1.9372(17) Å. Interestingly, the cationic fragment of C2 is chiral, with both enantiomers being present in the crystal structure. This seems to be in contrast to the results of 1H NMR spectroscopy, showing singlet signals for C


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acetonitrile molecules are found by 1H NMR spectroscopy after drying supports this conclusion. The CNHC signals in 13C NMR spectroscopy are obtained at 196.14 and 189.94 ppm (see Figure S15 in the Supporting Information). The suggested κ3-C4 structure in solution is supported by Xray diffraction of single crystals obtained by slow diffusion of diethyl ether into an acetonitrile solution of C4 (Figure 4). The

Figure 3. ORTEP style representation of the dicationic fragment of C2 with ellipsoids shown at the 50% probability level. Hydrogen atoms, cocrystallized solvent molecules, and PF6− counterions are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1− C1 1.892(2), Fe1−N3 2.0231(17), Fe1−N4 1.9743(17), Fe1−N5 2.0487(17), Fe1−N6 1.9358(17), Fe1−N7 1.9372(17); C1−Fe1−N5 169.76(8), N4−Fe1−N3 176.73(7), N6−Fe1−N7 175.30(7).

Figure 4. ORTEP style representation of the dicationic fragment of κ3C4 with ellipsoids shown at the 50% probability level. Hydrogen atoms, cocrystallized solvent molecules, and PF6− counterions are omitted for clarity. The asymmetric unit contains an additional moiety. Selected bond lengths (Å) and angles (deg): Fe1−C1 1.883(4), Fe1− C11 1.953(4), Fe1−N3 2.049(3), Fe1−N7 1.991(3), Fe1−N8 1.928(3), Fe1−N9 1.937(3); C11−Fe1−N3 173.6(1), C1−Fe1−N7 176.5(2), N8−Fe1−N9 175.9(1).

both methylene bridges. However, facile interconversion of two enantiomers in solution is a possible explanation for these observations. While iron complexes are known for several NCCN ligands,44 the methylene-bridged NCCN ligand precursor P4 is an exception, offering a great deal of flexibility. By application of standard reaction conditions iron complex C4 was synthesized in 62% yield (Scheme 6).

ligand is coordinated as expected from the NMR experiments described above in a tridentate, meridional fashion. The three resulting open coordination sites are occupied by acetonitrile ligands. Iron−carbon distances of 1.884(4)−1.950(4) Å are in the usual range, as are the Fe−NPy bond length of 2.051(3) Å and the Fe−NMeCN distances of 1.938(3)−1.991(3) Å. The resulting C−Fe−N and N−Fe−N bond angles are in the range of 173−176° and indicate a nearly ideal octahedral coordination of the iron center. Again, chirality with two enantiomeric species is indicated by single-crystal XRD and on the basis of NMR spectroscopy rapid interconversion of these is likely to occur in solution. The reaction of CNNC ligand precursor P5 with 1 equiv of [Fe{N(SiMe3)2}2(THF)] led to the formation of monomeric metal complex C5 (Scheme 7). 1 H and 13C NMR spectroscopy in solution (see Figures S16 and S17 in the Supporting Information) indicates symmetry in the ligand. However, the 1H NMR signals of the methylene bridges show geminal coupling with a coupling constant of

Scheme 6. Synthesis of Iron Complex C4 Starting from P4

Interestingly, 1H NMR spectroscopic investigation in solution (see Figure S14 in the Supporting Information) revealed broad signals for all methylene bridges as well as sharp signals for backbone protons and complete asymmetry in the ligand. Neither square-planar coordination nor sawhorse-type coordination of the ligand can explain these findings. In the case of a conformational interconversion, as observed for iron tetra-NHC complex C7, only broad signals would be expected.46 These findings lead to the conclusion that in acetonitrile solution the ligand is only coordinated in a tridentate, meridional fashion with one pyridine moiety not attached to the iron (κ3-C4). However, this contrasts with the elemental analysis obtained after drying under high vacuum, which revealed the existence of only two acetonitrile ligands (κ4-C4). Apparently, drying under high vacuum removes one of the acetonitrile ligands. The fact that only two nondeuterated

Scheme 7. Synthesis of Iron Complex C5 Starting from Ligand Precursor P5

D


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Organometallics around 17 Hz, indicating magnetic inequality of the CH2 protons and pointing to an equatorial coordination of the ligand, which is fixed displaying a slight twist. In the 13C NMR spectrum of compound C5, the characteristic signal for the carbene carbon atoms is found at 191.88 ppm. X-ray diffraction performed with single crystals obtained from diffusion of diethyl ether into an acetonitrile solution of C5 confirms the results obtained by NMR spectroscopy (Figure 5). The octahedral structure of C5 is distorted with

Scheme 8. Synthesis of Iron Complex C6 from Ligand Precursor P6 and Excess [Fe{N(SiMe3)2}2(THF)]

the terminal CH3 group is strongly shifted upfield to 2.20 ppm, as observed for the similar sawhorse-type coordinated iron(II) tetra-NHC complex C8.46 Characteristic carbene carbon signals in 13C NMR spectra are detected at 218.30, 202.98, and 188.38 ppm (see Figure S19 in the Supporting Information). Single crystals for X-ray diffraction were obtained by slow diffusion of diethyl ether into an acetonitrile solution of C6 (Figure 6).

Figure 5. ORTEP style representation of the dicationic fragment of C5 with ellipsoids shown at the 50% probability level. Hydrogen atoms and PF6− counterions are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1−C1 1.937(2), Fe1−C17 1.946(2), Fe1−N3 2.0007(19), Fe1−N4 2.014(2), Fe1−N7 1.927(2), Fe1−N8 1.936(2); C1−Fe1−N4 164.55(9), C17−Fe1−N3 165.77(9), N7−Fe1−N8 178.48(8).

CNHC−Fe−NPy angles of around 165°. The ligand is indeed arranged in a twisted fashion best described as distortedequatorial geometry. The resulting free trans coordination sites are occupied by acetonitrile ligands. Iron−carbon distances are 1.937(2) and 1.946(2) Å, slightly higher than those in C2, while Fe−NPy and Fe−NMeCN bond lengths are in their usual ranges. Two enantiomeric complexes with different rotation directions of the spirally arranged ligand are present. NMR spectra indicate a frozen conformation or a very slow interconversion of the enantiomeric species on the basis of the observation of geminal coupling for the protons of the methylene bridges. The ligand precursor P6 features three imidazolium subunits and therefore exceeds the limits set by the stoichiometry of [Fe{N(SiMe3)2}2(THF)] as an iron source with 2 equiv of internal base. However, following the synthetic protocol for iron(II) complexes with cyclic tetra-NHC ligands,17,32 1.7 equiv of [Fe{N(SiMe3)2}2(THF)] was used to ensure complete deprotonation of ligand precursor P6 (Scheme 8). The resulting byproducts were separated by filtration over dried silica, and C6 was obtained as an orange solid in a yield of 90%. 1H NMR spectroscopy in solution (see Figure S18 in the Supporting Information) reveals a sawhorse-type coordination of the ligand within the octahedrally coordinated iron complex, as indicated by geminal couplings for the CH2 protons with coupling constants of 13 and 14 Hz. In addition, the signal for

Figure 6. ORTEP style representation of the dicationic fragment of C6 with ellipsoids shown at the 50% probability level. Hydrogen atoms and PF6− counterions are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1−C1 1.973(2), Fe1−C6 1.907(3), Fe1−C10 1.843(3), Fe1−N7 2.040(2), Fe1−N8 1.977(2), Fe1−N9 1.983(2); C1−Fe1−N8 168.53(10), C6−Fe1−N7 164.42(10), C10−Fe1−N9 167.77(10).

NMR spectroscopic findings in solution have been supported by X-ray diffraction. A sawhorse-type coordination of the ligand with two cis acetonitrile ligands is obtained. C−Fe−N angles are in the range of 164−169°, indicating a distorted-octahedral coordination of the iron center. Iron−carbon bond lengths vary between 1.843(3) and 1.973(2) Å. Again, the Fe−NPy distance is substantially higher (2.040(2) Å) than the Fe−NMeCN distances of 1.977(2) and 1.983(2) Å. As assumed for C5, two enantiomers of the cationic fragment of C6 are found, which do not interconvert on the NMR time scale. While the reaction of tri-NHC ligand precursor P6 with excess [Fe{N(SiMe3)2}2(THF)] yields iron complex C6, the reaction of tetra-NHC ligand precursor P7 with excess E


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transmetalation to FeBr2. However, even traces of moisture cause hydrolysis of C7 toward the tricationic fragment of κ3-C7. Iron(II) complexes C3, C7, and C8 known in the literature with open-chain, tetradentate NHC ligands that were applied in electrochemical tests are given in Figure 8 and were synthesized according to the reported procedures.44,46 It is worth noting that all investigated complexes C1−C10 are diamagnetic as a consequence of the low-spin state of iron(II). Electrochemical Investigations. In order to investigate the influence of the number of NHC groups on the electronic structure of the synthesized iron NHC compounds C1−C10, cyclic voltammetry (CV) measurements were carried out in acetonitrile solution with tetrabutylammonium hexafluorophosphate as the supporting electrolyte. All complexes show a quasireversible one-electron redox process, which is assigned to the Fe(II)/Fe(III) redox couple (Figure 9). The peak separation ΔE varies from 80 to 100 mV among the complexes, which is above the theoretical value (around 60 mV) for one-electron processes69 but is typical for related iron complexes.20,32,44,51 Recently it was shown that the half-cell potential of C3 can be strongly influenced (from 0.08 to 0.44 V) by axial ligand substitution with pyridines and phosphanes.20 The variation of the NHC/pyridyl ratio causes a decrease of the half-cell potential with an increasing number of NHC donors from 0.68 V for mono-NHC complex C1 to 0.42 V for di-NHC complex C3 and 0.25 V for tri-NHC complex C6 to 0.08 V for tetraNHC complex C7 (Table 1). The average decrease of potential per additional NHC donor is approximately 0.2 V, resulting in a linear correlation of the half-cell potentials and the number of NHC donors (Figure 10). For the iron(II) complexes bearing an equal number of NHC donors but different linkages, the half-cell potentials show small deviations of up to 0.1 V. On comparison of the NPy−Fe−NPy angle (Figure 11) of NCCN complex C3 (115.30°)44 and the analogous CNHC−Fe−CNHC angle of CNNC complex C5 (98.15°), it becomes clear that structural distortion has an influence on the oxidation potential (0.42 V vs 0.35 V). As for the di-NHC ligands, the mono-NHC complex with a more flexible ligand and therefore potentially less distortion of the octahedral coordination has a significantly lower oxidation potential (0.58 V for C2 vs 0.68 V for C1). Although we were unable to obtain crystals of C1 suitable for X-ray diffraction and therefore lack data to compare structures of C1 and C2, the distortion in C1 should be similar to that of C3, as both ligands feature a single methylene bridge. In addition to structural distortion of the complexes caused by steric reasons, trans-standing NHC donors increase the respective Fe(II/III) half-cell potential (C9 vs C3 and C10 vs C8; see Table 1 and Figure 12). As mentioned before, C7 is not entirely stable in the presence of protic reagents (e.g., secondary amines). Hence, during the synthesis of C7 from [Fe{N(SiMe3)2}2(THF)] a mixture of C7 and κ3-C7 is formed. Traces of water that are present in a solution of pure C7 have a similar influence and lead to protonation of C7 and the formation of the tricationic fragment of κ3-C7 (Scheme 10). Thus, the redox step that is observed at a half-cell potential of 0.48 V can be assigned to its monoprotonated form, the cationic fragment of tri-NHC-ligated complex κ3-C7 with three acetonitrile ligands coordinated to the iron center (Figure 13, top).

[Fe{N(SiMe3)2}2(THF)] was found to produce a mixture of C7 and tricationic complex κ3-C7 (Scheme 9). Scheme 9. Reaction of Ligand Precursor P7 with Excess [Fe{N(SiMe3)2}2(THF)]

The product mixture is nearly inseparable by precipitation and extraction. 1H NMR spectroscopic investigations in solution at room temperature (see Figure S20 in the Supporting Information) show broad signals for C7 which lay underneath the sharp signals of κ3-C7. Varying the excess of [Fe{N(SiMe3)2}2(THF)] also directly influences the product composition, but even with a 10-fold excess and stirring at 80 °C overnight, a considerable amount of κ3-C7 was formed. Single crystals of κ3-C7 were obtained by slow diffusion of diethyl ether into the product mixture obtained from the reaction of P7 with excess [Fe{N(SiMe3)2}2(THF)]. X-ray diffraction reveals a meridional coordination of the ligand and a protonated, noncoordinating imidazolium moiety (Figure 7).

Figure 7. ORTEP style representation of the tricationic fragment of κ3-C7 with ellipsoids shown at the 50% probability level. Hydrogen atoms, cocrystallized solvent molecules, and PF6− counterions are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1− C1 1.989(3), Fe1−C6 1.900(3), Fe1−C10 1.987(3), Fe1−N9 1.986(3), Fe1−N10 1.931(3), Fe1−N1 1.934(3); C1−Fe1−C10 171.54(13), C6−Fe1−N9 175.70(12), N10−Fe1−N11 175.11(11).

The complex is arranged in a nearly ideal octahedral geometry comparable to that of κ3-C4 and exhibits two similar enantiomeric species, whichaccording to NMRreadily interconvert in solution. Both ligands feature three methylene bridges, while the most favorable situation for both iron(II) complexes seems to be three-coordination by the tetradentate poly-NHC ligand. Iron−carbon distances of 1.900(3)− 1.989(3) Å are in the usual range for Fe−CNHC bonds, while Fe−NMeCN distances vary between 1.931(3) and 1.986(3) Å. To avoid a mixture of C7 and κ3-C7, we recently described a clean synthesis of C7 and C8 via the silver route and F


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Figure 8. Iron(II) complexes known in the literature with open-chain, tetradentate poly-NHC ligands (C3, C7, C8) and macrocyclic, tetradentate poly-NHC ligands (C9, C10).

Table 1. Dependence of Half-Cell Potentials for Complexes C1−C10 on the NHC/Pyridyl Ratio (One to Four NHC Donors) C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 a

ligand

E1/2 (V)

ΔE (mV)

NN∧CN NN∧C∧N NC∧CNa N∧C∧C∧N C∧NN∧C NC∧C∧C C∧C∧C∧C C∧C∧C∧C cNCNCc cCCCCd

0.68 0.58 0.42 0.36b 0.35 0.25 0.08 0.02 0.46 0.15

100 100 80 80 80 100 90 100 110 90

From ref 44. bFor the κ4 species. cFrom ref 51. dFrom ref 32.

Figure 9. Cyclic voltammograms of mono- and tricarbene complexes C1, C2, and C6 (top), dicarbene complexes C3, C4, and C5 (middle), and tetracarbene complexes C7, C8, and C10 (bottom). All potentials (Table 1) are given relative to the half-cell potential of the Fc/Fc+ redox couple.

At a scan rate of 100 mV/s, the cyclic voltammograms of C4 (Figure 13, bottom) show a single oxidation at 0.56 V and a single reduction at 0.31 V. The peak separation ΔE of 250 mV is too high to indicate a reversible redox step; thus, a conformational change of the C4 upon oxidation seems feasible. Since 1H NMR and elememtal analysis data suggest a fluctuating coordination for one of the pyridine donors (κ3C4 vs κ4-C4), it seems reasonable that the oxidation state of the iron center may also influence the coordination mode of the

Figure 10. Average E1/2 value plotted against the number of carbene donors. A linear relation for the compared complexes C1−C10 is shown. Error bars indicate standard deviations.

ligand. At higher scan rates of 400 and 1000 mV/s an additional reduction at 0.48 V and oxidation at 0.40 V are observed (Figure 13, bottom). As shown by single-crystal XRD, in acetonitrile solution the ligand of C4 shows a κ3 coordination, G


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Figure 11. Complexes with the same number of NHC donors but different degrees of distortion due to different linkages.

Figure 13. Cyclic voltammograms of complexes C7 (top) and C4 (bottom). All potentials are given relative to the half-cell potential of the Fc/Fc+ redox couple.

Scheme 11. Structural Transformations during Cyclic Voltammetry of C4 at a Low Scan Rate of 100 mV/s

high redox stability and show a good linear correlation of the half-cell potentials with the number of NHC donors (n = 1−4). In addition to the number of NHC donors as a major factor, only a minor influence on the redox potentials can be ascribed to their linkage and in consequence their structures (equatorial, sawhorse-type, or meridional coordination of the ligand). The dominant influence of the number of NHC moieties is further emphasized when taking a look at iron(II) complexes with ligands of the TPA (tris(2-pyridylmethyl)amine) class. Que et al.70 investigated the oxidation potentials of octahedrally coordinated, dicationic iron(II) complexes with tetradentate ligands containing three pyridine moieties. For the simplest of these complexes, a redox potential of 0.86 V was obtained, being approximately 0.2−0.3 V higher than the values for mono-NHC complexes C1 and C2 and fitting well in the linear correlation between the number of NHCs and the redox potentials (Figure 10). Interestingly, Que noticed that the introduction of a methyl group to the pyridyl units leads to formation of iron(II) high-spin complexes and an irreversible oxidation to iron(III) during CV at 0.94 V. As an effect of our findings, the half-cell potential of the presented Fe(II) NHC compounds can be fine-tuned from 0.68 to 0.02 V in step sizes of approximately 0.1 V (Table 1). Among these complexes, C4 is an interesting example of an

Figure 12. Complexes with the same number of NHC donors but different trans effects caused by different arrangements of the NHC donor moieties.

Scheme 10. Protonation of One NHC Moiety of C7 in the Presence of Traces of Water

resulting in a half-cell potential which is higher than expected for di-NHC complexes (E1/2 = 0.52 V). After oxidation of C4 to the respective Fe(III) derivative, however, a κ4 coordination is favored, most probably due to the increased Lewis acidity of Fe(III). The κ4-coordinated species (E1/2 = 0.36 V) is reduced in the potential range expected for a tetradentate di-NHC complex (Scheme 11). Overall, the CV experiments of the [Fe(NHC)n(Pyr)4−n(MeCN)2]2+ complexes prove the system’s H


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Organometallics Table 2. Crystallographic Data for Iron Complexes C2, κ3-C4, C5, C6, and κ3-C7 CCDC no. formula fw space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T (K) Dcalc (g cm−3) μ (mm−1) R1/wR2 (I > 2σ(I)) R1/wR2 (all data)

C2

κ3-C4

C5

C6

κ3-C7

1420288 C26H26F12FeN8P2 796.34 P1̅ (No. 2) 11.8824(2) 12.0225(2) 12.7153(2) 78.1070(10) 89.0410(10) 63.7640(10) 1588.62(5) 2 123 1.665 0.680 0.0317/0.0795 0.0374/0.0827

1420289 C58H74F24Fe2N18O2P4 1746.93 P21/n (No. 14) 21.9386(9) 11.1672(5) 30.9935(14) 90 98.106(2) 90 7517.3(6) 4 123 1.544 0.585 0.0596/0.1437 0.0900/0.1608

1420290 C24H26F12FeN8P2 772.32 C2/c (No. 15) 20.2864(3) 12.2796(2) 26.5473(5) 90 108.2130(10) 90 6281.86(19) 8 123 1.633 0.685 0.0377/0.0856 0.0509/0.0924

1420291 C21H23F12FeN9P2 747.27 P21/c (No. 14) 11.6146(2) 14.9966(3) 19.4741(4) 90 98.3740(10) 90 3355.83(11) 4 123 1.479 0.639 0.0408/0.0933 0.0552/0.0999

1420292 C21H23F12FeN9P2 1074.50 Pbca (No. 61) 10.9373(3) 16.5377(4) 48.5989(12) 90 90 90 8790.5(4) 8 123 1.624 0.571 0.0481/0.1103 0.0680/0.1192

C8,46 C9,51 and C1032 were synthesized according to previously reported procedures. Liquid NMR spectra were recorded on a Bruker Avance DPX 400 and Bruker DRX 400 spectrometers. Chemical shifts are given in parts per million (ppm), and the spectra were referenced by using the residual solvent shift as internal standard (acetonitrile-d3, 1 H δ 1.94, 13C δ 118.26; dimethyl sulfoxide-d6, 1H δ 2.50, 13C δ 39.52). MS-ESI analyses were performed on a Thermo Scientific LCQ/Fleet spectrometer by Thermo Fisher Scientific. Elemental analysis was obtained from the microanalytical laboratory of the Technische Universität München. Single-Crystal X-ray Diffraction. For crystallization, diethyl ether was slowly diffused into acetonitrile solutions of compounds C2, κ3C4, C5, C6, and κ3-C7, respectively. Data were collected on an X-ray single-crystal diffractometer equipped with a CCD detector (Bruker APEX II, κ-CCD), a rotating anode (Bruker AXS, FR591) with Mo Kα radiation (λ = 0.71073 Å), and a Montel optic (κ3-C4, κ3-C7) or a fine-focused sealed tube with Mo Kα radiation (λ = 0.71073 Å) and a graphite monochromator (C2, C5, C6) by using the APEX2 software package.74 The measurements were performed on a single crystal coated with perfluorinated ether. The crystal was fixed on the top of a glass fiber and transferred to the diffractometer. The crystal was frozen under a stream of cold nitrogen. A matrix scan was used to determine the initial lattice parameters. Reflections were merged and corrected for Lorenz and polarization effects, scan speed, and background using SAINT.75 Absorption corrections, including odd- and even-ordered spherical harmonics, were performed using SADABS.75 Space group assignments were based upon systematic absences, E statistics, and successful refinement of the structures. Structures were solved by direct methods with the aid of successive difference Fourier maps and were refined against all data using SHELXLE76 in conjunction with SHELXL-2014.77 Hydrogen atoms were assigned to ideal positions and refined using a riding model with an isotropic thermal parameter 1.2 times that of the attached carbon atom (1.5 times for methyl hydrogen atoms). If not mentioned otherwise, non-hydrogen atoms were refined with anisotropic displacement parameters. Full-matrix least-squares refinements were carried out by minimizing ∑w(Fo2 − Fc2)2 with the SHELXL-9778 weighting scheme. Neutral atom scattering factors for all atoms and anomalous dispersion corrections for the non-hydrogen atoms were taken from ref 79. Images of the crystal structures were generated by PLATON.80 Detailed information on crystallographic data for these complexes can be found in Table 2. Synthesis. Ligand Precursor P1. In air, in an ACE pressure tube 2.85 g of 6-bromomethyl-2,2′-dipyridine (1; 11.4 mmol, 1.00 equiv) and 1.66 g of 2-imidazolylpyridine (2; 11.4 mmol, 1.00 equiv) were dissolved in 25 mL of THF. The tube was closed tightly, heated to 85 °C with stirring for 16 h, and then cooled to room temperature. After

Fe(II) complex featuring a covalently connected labile pyridine donor, lost in the Fe(II) state but coordinated in the case of Fe(III).

■

CONCLUSION Four tetradentate NHC/pyridine hybrid ligands with different NHC/pyridyl ratios have been synthesized, and the corresponding iron(II) complexes have been prepared. The geometry around the iron is strongly dependent on the number and length of the alkylene linkers used to tether the NHC and pyridyl moieties. While mono-NHC complexes C1 and C2 and di-NHC complex C5 are coordinated equatorially by the tetradentate ligand, the tri-NHC complex C6 exhibits a sawhorse-type coordination of the ligand, resulting in a cis labile site. C4 in acetonitrile solution shows tridentate coordination of the di-NHC ligand, with one pyridine moiety not attached to the iron due to three methylene bridges within the ligand. A strong influence of the number of NHC donors on the electronic structure of the iron(II) metal is demonstrated. Cyclic voltammetry reveals a linear correlation between the half-cell potentials of the iron(II) complexes and the number of NHC donors being coordinated to the metal. Oxidation potentials of 0.68 and 0.58 V were found for monoNHC ligands, 0.35−0.42 V for complexes with di-NHC ligands, 0.25 V in the case of the tri-NHC ligand, and 0.02 or 0.08 V for open-chain tetra-NHC ligands. Complex C4 shows interesting behavior during cyclic voltammetry, as a conformational change from κ3 to κ4 coordination of the ligand is observed after oxidation to iron(III).

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EXPERIMENTAL SECTION

General Remarks. All chemicals were purchased from commercial suppliers and used without further purification. Anhydrous acetonitrile and diethyl ether were obtained from an MBraun solvent purification system, degassed by freeze−pump−thaw techniques, and stored over molecular sieves. Acetonitrile-d3 was refluxed over phosphorus pentoxide, distilled prior to use, and stored over molecular sieves. All syntheses were performed using standard Schlenk techniques, if not stated otherwise. 6-Bromomethyl-2,2′-dipyridine (1),71 2imidazolylpyridine (2),72 2-(1H-imidazol-1-ylmethyl)pyridine (3),21 3-((1H-imidazol-1-yl)methyl)-1-methyl-1H-imidazol-3-ium iodide (6),29 6,6′-bis(bromomethyl)-2,2′-bipyridine (4),60 ligand precursor P4,21 [Fe{N(SiMe3)2}2(THF)],73 and iron complexes C3,44 C7,46 I


Article

Organometallics

CH2), 387 (s, 6H, CH3). 13C{1H} NMR (100.62 MHz, 296.4 K, CD3CN): δ 156.11, 153.52, 139.67, 137.65, 124.55, 124.16, 124.03, 121.64, 54.51, 36.96. MS-ESI (m/z): [P5 − PF6]+ calcd, 491.15; found, 490.68, [P5 − 2 PF6]2+ calcd, 173.09; found, 173.02. Anal. Calcd: C, 37.75; H, 3.48; N, 13.21. Found: C, 37.89; H, 3.52; N, 12.85. Compound 5. In air, in a round-bottom flask 1.02 g of 2imidazolylpyridine (2; 7.09 mmol, 1.00 equiv) was dissolved in 50 g of chloroiodomethane (280 mmol, 40 equiv). The flask was closed tightly, heated to 85 °C with stirring for 16 h, and then cooled to room temperature. The solution was decanted and the precipitate extracted three times with acetonitrile. Precipitation of the combined solutions with diethyl ether and drying under high vacuum yielded the imidazolium salt 5 as a colorless raw product, which was used for further reactions. Purification could be achieved by stepwise precipitation to yield the imidazolium salt 5 as a colorless solid (683 mg, 30% yield). 1H NMR (400.13 MHz, 295.8 K, DMSO-d6): δ 10.36 (m, 1H, HNCHC), 8.72−8.58 (m, 2H), 8.30−8.15 (m, 2H, HNCHC), 8.10−8.00 (m, 1H), 7.73−7.63 (m, 1H), 6.30−6.15 (2x s, 2H, CH2). Ligand Precursor P6. In air, in an ACE pressure tube 1.20 g of 5 (3.73 mmol, 1.00 equiv) and 1.50 g of 3-((1H-imidazol-1-yl)methyl)1-methyl-1H-imidazol-3-ium iodide (6.02 mmol, 1.61 equiv) were dissolved in 25 mL of acetonitrile. The tube was closed tightly, heated to 95 °C with stirring for 16 h, and then cooled to room temperature. After the solution was discarded, the colorless precipitate was dissolved in methanol and precipitated stepwise by addition of diethyl ether. The first precipitate was collected, dried under high vacuum, dissolved in 10 mL of water, and slowly added to a vigorously stirred solution of ammonium hexafluorophosphate (1.67 g, 3.00 equiv) in 25 mL of water. The resulting colorless precipitate was carefully washed with water and dissolved in 10 mL of acetone. Precipitation with diethyl ether and drying under high vacuum yielded the imidazolium salt P6 as a colorless solid (650 mg, 23% yield). 1H NMR (400.13 MHz, 300.0 K, CD3CN): δ 9.64 (t, J = 1.6 Hz, 1H, HIm), 9.13 (t, J = 1.7 Hz, 1H, HIm), 8.75 (s, 1H, HIm), 8.63 (dt, J = 4.6, 1.5 Hz, 1H, HPy), 8.19 (t, J = 2.0 Hz, 1H, HIm), 8.15 (td, J = 7.9, 1.9 Hz, 1H, HPy), 7.82 (t, J = 2.0 Hz, 1H, HIm), 7.79 (d, J = 8.3 Hz, 1H, HPy), 7.77 (t, J = 2.0 Hz, 1H, HIm), 7.70 (t, J = 2.0 Hz, 1H, HIm), 7.64 (ddd, J = 7.5, 4.9, 0.9 Hz, 1H, HPy), 7.60 (t, J = 2.0 Hz, 1H, HIm), 7.46 (t, J = 1.9 Hz, 1H, HIm), 6.58 (s, 2H, CH2), 6.46 (s, 2H, CH2), 3.89 (s, 3H, CH3). 13C{1H} NMR (100.62 MHz, 300.0 K, CD3CN): δ 150.59, 146.78, 141.65, 139.30, 138.48, 136.55, 127.08, 125.96, 124.45, 124.26, 124.24, 123.15, 121.60, 115.39, 60.37, 59.89, 37.45. MS-ESI (m/z): [P6 − PF6]+ calcd, 612.11; found, 611.91, [P6 − 2 PF6]2+ calcd, 233.57; found, 233.31. Anal. Calcd: C, 26.96; H, 2.66; N, 12.95. Found: C, 26.60; H, 2.49; N, 12.60. Complex C1. A solution of 460 mg of P1 (871 μmol, 1.00 equiv) in 4 mL of acetonitrile was slowly added to a cooled suspension of [Fe{N(SiMe3)2}2(THF)] (410 mg, 914 μmol) in 4 mL of acetonitrile at −40 °C. The mixture was carefully warmed to room temperature and stirred for 30 min. A 149 mg portion of NH4PF6 (914 μmol, 1.05 equiv) in 2 mL of acetonitrile was added as a suspension, and the mixture was stirred overnight at room temperature. After addition of 5 mL of diethyl ether, the deep red mixture was filtered through a Whatman filter with exclusion of air. The black residue was discarded and the solution precipitated stepwise with vigorous stirring by addition of diethyl ether. The bright red solution was discarded, and the black precipitate was washed with diethyl ether and dried under vacuum. A 325 mg amount of C1 was obtained as a deep red solid (438 μmol, 50% yield). 1H NMR (400.13 MHz, 295.1 K, CD3CN): δ 10.34 (d, J = 5.4 Hz, 1H, HPy), 8.91 (d, J = 5.1 Hz, 1H, HPy), 8.73 (d, J = 8.1 Hz, 1H, HPy), 8.59−8.54 (m, 2H, HPy, HIm), 8.42 (d, J = 8.0 Hz, 1H, HPy), 8.27−8.17 (m, 2H, HPy), 8.09 (d, J = 2.3 Hz, 1H, HIm), 8.05 (t, J = 7.9 Hz, 1H, HPy), 8.02 (d, J = 8.5 Hz, 1H, HPy), 7.65 (d, J = 8.0 Hz, 1H, HPy), 7.48 (t, J = 6.8 Hz, 1H, HPy),6.31 (s, 2H, CH2), 1.96 (s, 6H, CH3CN). 13C{1H} NMR (100.62 MHz, 296.2 K, CD3CN): δ 216.13, 163.04, 161.83, 159.03, 156.41, 154.75, 154.63, 141.90, 140.51, 138.68, 128.26, 126.42, 126.19, 124.57, 122.81, 122.59, 120.24, 112.27, 53.96. Anal. Calcd: C, 37.27; H, 2.86; N, 13.23. Found: C, 36.94; H, 2.97; N, 12.90. Complex C2. A solution of 383 mg of P2 (809 μmol, 1.00 equiv) in 4 mL of acetonitrile was slowly added to a cooled suspension of

the solution was discarded, the oily residue was dissolved in 5 mL of methanol. Diethyl ether was added cautiously until the solution was colorless. The solution was decanted from the brown oil and precipitated by addition of diethyl ether to yield a colorless precipitate. The oily residue was redissolved in methanol and again carefully decolorized by addition of diethyl ether. The combined precipitates were dried under high vacuum, dissolved in 10 mL of water, and slowly added to a vigorously stirred solution of ammonium hexafluorophosphate (1.86 g, 1.00 equiv) in 25 mL of water. The resulting colorless precipitate was carefully washed with water and dissolved in 10 mL of acetone. Precipitation with diethyl ether and drying under high vacuum yielded the imidazolium salt P1 as a colorless solid (2.05 g, 39% yield). 1H NMR (400.13 MHz, 295.1 K, CD3CN): δ 9.56 (t, J = 1.7 Hz, 1H, HIm), 8.65 (dt, J = 4.8, 1.3 Hz, 1H, HPy), 8.61 (dd, J = 5.4, 1.1 Hz, 1H, HPy), 8.43 (d, J = 8.0 Hz, 1H, HPy), 8.29 (dt, J = 8.0, 1.1 Hz, 1H, HPy), 8.14−8.09 (m, 2H, HPy), 8.00 (t, J = 7.8 Hz, 1H, HPy), 7.84 (td, J = 7.8, 1.8 Hz, 1H, HPy), 7.79−7.75 (m, 2H, HIm), 7.59 (ddd, J = 7.6, 4.8, 0.8 Hz, 1H, HPy), 7.56 (d, J = 7.6 Hz, 1H, HPy), 7.39 (ddd, J = 7.6, 4.7, 1.2 Hz, 1H, HPy), 5.65 (s, 2H, CH2). 13C{1H} NMR (100.62 MHz, 295.5 K, CD3CN): δ 156.92, 155.82, 152.80, 150.45, 150.23, 147.41, 141.40, 139.65, 138.17, 135.83, 126.39, 125.38, 125.34, 123.80, 121.74, 121.60, 120.22, 115.09, 55.11.MS-ESI (m/z): [P1 − PF6]+ calcd, 314.14; found, 314.24. Anal. Calcd: C, 49.68; H, 3.51; N, 15.25. Found: C, 49.37; H, 3.69; N, 14.89. Ligand Precursor P2. In air, in an ACE pressure tube 1.07 g of 6bromomethyl-2,2′-dipyridine (1; 4.30 mmol, 1.00 equiv) and 684 mg of 2-(1H-imidazol-1-ylmethyl)pyridine (3; 4.30 mmol, 1.00 equiv) were dissolved in 25 mL of THF. The tube was closed tightly, heated to 85 °C with stirring for 16 h, and then cooled to room temperature. After the solution was discarded, the oily residue was dissolved in 5 mL of methanol. Diethyl ether was added cautiously until the solution was colorless. The solution was decanted from the brown oil and precipitated by addition of diethyl ether to yield a colorless precipitate. The oily residue was redissolved in methanol and again carefully decolorized by addition of diethyl ether. The combined precipitates were dried under high vacuum, dissolved in 10 mL of water, and slowly added to a vigorously stirred solution of ammonium hexafluorophosphate (701 mg, 1.00 equiv) in 25 mL of water. The resulting colorless precipitate was carefully washed with water and dissolved in 10 mL of acetone. Precipitation with diethyl ether and drying under high vacuum yielded the imidazolium salt P2 as a colorless solid (835 mg, 41% yield). 1H NMR (400.13 MHz, 295.1 K, CD3CN): δ 8.92 (t, J = 1.7 Hz, 1H, HIm), 8.68 (dt, J = 4.9, 2.1 Hz, 1H, HPy), 8.52 (dt, J = 4.9, 1.2 Hz, 1H, HPy), 8.44 (d, J = 7.9 Hz, 1H, HPy), 8.21 (dt, J = 8.0, 1.1 Hz, 1H, HPy), 8.00 (t, J = 7.8 Hz, 1H, HPy), 7.84 (tdd, J = 7.8, 4.6, 1.8 Hz, 1H, HPy), 7.59 (t, J = 1.8 Hz, 1H, HIm), 7.54 (t, J = 1.9 Hz, 1H, HIm), 7.51 (d, J = 7.6 Hz, 1H, HPy), 7.47 (t, J = 7.8 Hz, 1H, HIm), 7.43 (ddd, J = 7.6, 4.8, 1.2 Hz, 1H, HPy), 7.38 (ddd, J = 7.7, 4.9, 1.1 Hz, 1H, HPy), 5.57 (s, 2H, CH2), 5.51 (s, 2H, CH2). 13C{1H} NMR (100.62 MHz, 295.5 K, CD3CN): δ 156.91, 155.85, 153.80, 153.21, 150.84, 150.23, 139.56, 138.47, 138.12, 138.00, 125.28, 124.81, 124.20, 123.99, 123.71, 123.56, 121.62, 121.45, 54.82, 54.62. MS-ESI (m/z): [P2 − PF6]+ calcd, 328.16; found, 328.29. Anal. Calcd: C, 50.75; H, 3.83; N, 14.80. Found: C, 50.41; H, 3.83; N, 14.81. Ligand Precursor P5. In air, in an ACE pressure tube 125 mg of 6,6′-bis(bromomethyl)-2,2′-bipyridine (4; 365 μmol, 1.00 equiv) and 150 mg of 1-methylimidazole (1.83 mmol, 5.00 equiv) were dissolved in 25 mL of THF. The tube was closed tightly, heated to 85 °C with stirring for 16 h, and then cooled to room temperature. After the solution was discarded, the colorless precipitate was washed with diethyl ether, dried under high vacuum, dissolved in 5 mL of water, and slowly added to a vigorously stirred solution of ammonium hexafluorophosphate (105 mg, 2.00 equiv) in 10 mL of water. The resulting colorless precipitate was carefully washed with water and dissolved in 10 mL of acetone. Precipitation with diethyl ether and drying under high vacuum yielded the imidazolium salt P5 as a colorless solid (185 mg, 80% yield). 1H NMR (400.13 MHz, 295.9 K, CD3CN): δ 8.61 (t, J = 1.6 Hz, 2H, HIm), 8.26 (d, J = 7.5 Hz, 2H, HPy), 7.96 (t, J = 7.8 Hz, 2H, HPy), 7.52 (t, J = 1.9 Hz, 2H, HIm), 7.49 (d, J = 7.4 Hz, 2H, HPy), 7.38 (t, J = 1.8 Hz, 2H, HIm), 5.49 (s, 4H, J


Article

Organometallics

at −40 °C. The mixture was carefully warmed to room temperature and stirred overnight. After addition of 5 mL of diethyl ether, the resulting suspension was filtered over dried silica with exclusion of air. The orange filtrate was precipitated with vigorous stirring by addition of diethyl ether, and the precipitate was washed with diethyl ether and dried under vacuum. A 89 mg amount of C6 was obtained as an orange solid (119 μmol, 90% yield). 1H NMR (400.13 MHz, 294.9 K, CD3CN): δ 8.82 (d, J = 5.4 Hz, 1H, HPy), 8.22 (d, J = 2.2 Hz, 1H, HIm), 8.21−8.16 (m, 1H, HPy), 7.99 (d, J = 8.4 Hz, 1H, HPy), 7.60 (d, J = 2.3 Hz, 1H, HIm), 7.52 (ddd, J = 7.1, 5.6, 1.2 Hz, 1H, HPy), 7.44− 7.38 (m, 2H, HIm), 7.27 (d, J = 2.0 Hz, 1H, HIm), 6.59 (d, J = 2.0 Hz, 1H, HIm), 6.55 (d, J = 13.3 Hz, 1H, CH2), 6.39 (d, J = 12.2 Hz, 1H, CH2), 6.08 (d, J = 13.3 Hz, 1H, CH2), 6.02 (d, J = 12.2 Hz, 1H, CH2), 2.20 (s, 3H, CH3), 1.96 (s, 6H, CH3CN). 13C{1H} NMR (100.62 MHz, 294.7 K, CD3CN): δ 218.30, 202.98, 188.38, 155.60, 153.29, 140.54, 125.37, 125.31, 124.96, 123.73, 123.52, 123.20, 120.85, 112.89, 63.80, 63.09, 35.37. Anal. Calcd: C, 33.75; H, 3.10; N, 16.87. Found: C, 33.58; H, 3.10; N, 16.46.

[Fe{N(SiMe3)2}2(THF)] (399 mg, 890 μmol) in 4 mL of acetonitrile at −40 °C. The mixture was carefully warmed to room temperature and stirred for 30 min. A 158 mg portion of NH4PF6 (979 μmol, 1.20 equiv) in 2 mL of acetonitrile was added as a suspension, and the mixture was stirred overnight at room temperature. After addition of 5 mL of diethyl ether, the deep red mixture was filtered through a Whatman filter with exclusion of air. The black residue was discarded and the solution precipitated stepwise with vigorous stirring by addition of diethyl ether. The bright red solution was discarded, and the black precipitate was washed with diethyl ether and dried under vacuum. A 380 mg amount of C2 was obtained as a deep red solid (503 μmol, 62% yield). 1H NMR (400.13 MHz, 295.9 K, CD3CN): δ 9.63 (d, J = 5.6 Hz, 1H, HPy), 9.23 (br s, 1H, HPy), 8.66 (d, J = 8.1 Hz, 1H, HPy), 8.44 (d, J = 8.0 Hz, 1H, HPy), 8.38 (td, J = 7.4, 6.7 Hz, 1H, HPy), 8.08 (t, J = 7.9 Hz, 1H, HPy), 8.04−7.95 (m, 2H, HPy), 7.85 (d, J = 1.9 Hz, 1H, HIm), 7.80 (d, J = 1.9 Hz, 1H, HIm), 7.65 (t, J = 6.8 Hz, 1H, HPy), 7.56 (d, J = 6.6 Hz, 1H, HPy), 5.81 (s, 2H, CH2), 5.46 (s, 2H, CH2), 1.96 (s, 6H, CH3CN). 13C{1H} NMR (100.62 MHz, 296.1 K, CD3CN): δ 196.70, 162.87, 161.50, 161.30, 159.51, 159.21, 154.74, 139.93, 139.32, 139.19, 128.16, 126.38, 125.98, 125.41, 125.17, 125.07, 124.58, 123.05, 53.86, 53.18. Anal. Calcd: C, 38.17; H, 3.07; N, 12.98. Found: C, 37.92; H, 3.12; N, 12.74. Complex C4. A solution of 250 mg of P4 (402 μmol, 1.00 equiv) in 4 mL of acetonitrile was slowly added to a cooled suspension of [Fe{N(SiMe3)2}2(THF)] (216 mg, 482 μmol) in 4 mL of acetonitrile at −40 °C. The mixture was carefully warmed to room temperature and stirred overnight. After addition of 5 mL of diethyl ether, the orange-red mixture was filtered through a Whatman filter with exclusion of air. The black residue was discarded and the orange solution precipitated with vigorous stirring by addition of diethyl ether. The red precipitate was washed with diethyl ether and dried under vacuum. A 188 mg amount of κ4-C4 iswas obtained as an orange solid (248 μmol, 62% yield). 1H NMR (400.13 MHz, 296.8 K, CD3CN): δ 9.27 (d, J = 5.6 Hz, 1H, HPy), 8.59 (d, J = 5.0 Hz, 1H, HPy), 8.00 (td, J = 7.7, 1.6 Hz, 1H, HPy), 7.85 (td, J = 7.7, 1.8 Hz, 1H, HPy), 7.66 (d, J = 7.9 Hz, 1H, HPy), 7.62 (d, J = 2.0 Hz, 1H, HIm), 7.60 (d, J = 2.0 Hz, 2H, HIm), 7.59 (d, J = 2.0 Hz, 1H, HIm), 7.54 (ddd, J = 7.4, 5.7, 1.5 Hz, 1H, HPy), 7.36 (dd, J = 6.5, 4.8 Hz, 1H, HPy), 7.22 (d, J = 7.9 Hz, 1H, HPy), 7.17 (d, J = 2.1 Hz, 2H, HIm), 6.35−6.00 (br m, 2H, CH2), 5.76 (br s, 2H, CH2), 5.60−5.35 (br m, 2H, CH2), 1.96 (s, 6H, CH3CN). 13 C{1H} NMR (100.62 MHz, 297.4 K, CD3CN): δ 196.14, 189.94, 158.83, 157.94, 156.85, 150.45, 139.04, 138.10, 126.07, 125.60, 125.30, 124.93, 124.54, 123.75, 123.26, 122.56, 62.46, 55.18, 53.59. MS-ESI (m/z): [η4-C4 − 2 MeCN − 2 PF6 + CHO2]+ calcd, 431.09; found, 431.02, [η4-C4 − 2 MeCN − 2 PF6]2+ calcd, 193.05; found, 193.15. Anal. Calcd: C, 36.43; H, 3.19; N, 14.78. Found: C, 36.79; H, 3.31; N, 14.51. Complex C5. A solution of 100 mg of P5 (157 μmol, 1.00 equiv) in 2 mL of acetonitrile was slowly added to a cooled suspension of [Fe{N(SiMe3)2}2(THF)] (74.0 mg, 165 μmol) in 2 mL of acetonitrile at −40 °C. The mixture was carefully warmed to room temperature and stirred overnight. After addition of 3 mL of diethyl ether, the deep red mixture was filtered through a Whatman filter with exclusion of air. The black residue was discarded and the red solution precipitated with vigorous stirring by addition of diethyl ether. The dark purple precipitate was washed with diethyl ether and dried under vacuum. A 85 mg amount of C5 was obtained as a deep red solid (110 μmol, 67% yield). 1H NMR (400.13 MHz, 295.1 K, CD3CN): δ 8.56 (d, J = 8.0 Hz, 2H, HPy), 8.22 (t, J = 7.9 Hz, 2H, HPy), 7.83 (d, J = 7.7 Hz, 2H, HPy), 7.67 (d, J = 1.9 Hz, 1H, HIm), 7.40 (d, J = 2.0 Hz, 1H, HPy), 5.91 (d, J = 16.8 Hz, 2H, CH2), 5.47 (d, J = 16.8 Hz, 2H, CH2), 3.54 (s, 6H, CH3), 1.96 (s, 6H, CH3CN). 13C{1H} NMR (100.62 MHz, 295.3 K, CD3CN): δ 191.88, 161.01, 159.63, 139.23, 126.08, 125.93, 125.00, 123.03, 53.64, 36.78. MS-ESI (m/z): [C5 − 2 MeCN − 2 PF6 + CHO2]+ calcd, 445.11; found, 444.94, [C5 − 2 MeCN − 2 PF6]2+ calcd, 200.05; found, 200.17. Anal. Calcd: C, 37.33; H, 3.39; N, 14.51. Found: C, 36.93; H, 3.18; N, 14.28. Complex C6. A solution of 100 mg of P6 (132 μmol, 1.00 equiv) in 4 mL of acetonitrile was slowly added to a cooled suspension of [Fe{N(SiMe3)2}2(THF)] (101 mg, 224 μmol) in 4 mL of acetonitrile

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00732. These data can also be obtained free of charge from the Cambridge Crystallographic Data Centre (CCDC 1420288−1420292) via www.ccdc.cam.ac.uk/data_request/cif. NMR spectroscopic data of all compounds (PDF) X-ray crystallographic data of compounds C2, η3-C4, C5, C6, and C7a (CIF)

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AUTHOR INFORMATION

Corresponding Author

*F.E.K.: tel, +49 89 289 13096; e-mail, [email protected]. Author Contributions ⊥

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.T.W. and S.H. gratefully acknowledge support from the TUM Graduate School. REFERENCES

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