From Ylides to Doubly Yldiide-Bridged Iron(II) High Spin Dimers via Self-Protolysis

A synthetic strategy for the preparation of novel doubly yldiide bridged iron(II) high spin dimers ([(μ2-C)FeL]2, L = N(SiMe3)2, Mesityl) has been developed. This includes the synthesis of ylide-iron(II) monomers [(Ylide)FeL2] via adduct formation. Subsequent self-protolysis at elevated temperatures by in situ deprotonation of the ylide ligands results in a dimerization reaction forming the desired bridging μ2-C yldiide ligands in [(μ2-C)FeL]2. The comprehensive structural and electronic analysis of dimers [(μ2-C)FeL]2, including NMR, Mössbauer, and X-ray spectroscopy, as well as X-ray crystallography, SQUID, and DFT calculations, confirm their high-spin FeII configurations. Interestingly, the Fe2C2 cores display very acute Fe–C–Fe angles (averaged: 78.6(2)°) resulting in short Fe···Fe distances (averaged: 2.588(2) Å). A remarkably strong antiferromagnetic coupling between the Fe centers has been identified. Strongly polarized Fe–C bonds are observed where the negative charge is mostly centered at the μ2-C yldiide ligands.


■ INTRODUCTION
Nitrogenases are the only enzymes that are capable of reducing atmospheric N 2 to NH 3 . 1 This conversion occurs at a heterometallic Fe 7 MoS 9 C cluster, known as FeMoco, which contains a very unique interstitial μ 6 -carbide ligand (Chart 1A). 2 Biosynthetically, the carbide ligand originates from the methyl group of S-adenosylmethionine and is transferred to the iron sulfur cluster via a radical mechanism. 3 While the role of the carbide ligand for the catalytic activity of the FeMoco is unknown, it has been postulated to function as a stabilizing anchor for the cluster, 4 to allow for facile modulation of the belt iron interactions during substrate binding, 5 and even to form transient C−H bonds during catalysis. 6 However, a detailed understanding of the exact role of the carbide is in part limited by the absence of any synthetic models which support both a μ 6 -carbide ligand and locally high-spin ferrous (and ferric) iron atoms. 7 In fact, synthetic strategies that allow the introduction of C-based ligands bonded to more than one Fe atom (μ 2−6 -C-based ligands) are very rare, explaining the lack of suitable model systems. Known strategies are, for example, the conversion or substitution of CO ligands in polyironcarbonyl clusters into μ 3 -, μ 5 -, or μ 6 -C-ligands to yield, e.g., clusters of type B (Chart 1). 8 Unfortunately, these clusters contain strong-field CO ligands and are low-spin complexes, in contrast to the sulfide ligated high spin iron sites in FeMoco.
Carbido bridged, diamagnetic porphyrinato (C) and phthalocyanato Fe dimers have been synthesized using CI 4 as a μ 2 -C 4− ligand source, yielding high-valent Fe IV sites. 9 In addition, a dimeric but low spin [1,2,4-t Bu(C 5 H 2 )Fe II (μ 2 -CPh)] 2 complex containing μ 2 -C ligands was synthesized by alkyne insertion into an FeFe double bond. 10 In addition, a small number of low coordinated, high spin Fe dimers bridged by singly charged μ 2 -C − ligands were prepared by using (H 3 C) Li or ArylLi as [H 3 C] − and [Aryl] − sources functioning as two electron donor ligands. 11−13 Recently, Holland and co-workers have reported on the synthesis of the singly alkylidene bridged high-spin Fe II dimer of type D by using diazoalkanes as a μ 2 -C 2− ligand source, formally having been a four electron donor ligand. 14 In this contribution, we report on the development of a new synthetic strategy for generating μ 2 -C 2− yldiide ligands that support the coordination of high-spin iron, which enabled the preparation of novel doubly yldiide-bridged iron(II) high spin complexes. This has been accomplished by starting from novel low-coordinated Fe II −ylide complexes of type E containing both a strongly basic ligand (L) and a H-acidic ylide ligand facilitating self-protolysis and subsequent dimerization yielding dinuclear complexes of type F. The well-established chemistry of isolable ylides containing, e.g., phosphonio or imidazoliumyl substituents as electron withdrawing groups (EWG) allows precise substituent adjustment, 15 rendering these as potential precursors for μ 2 -C 2− ligands.

■ RESULTS AND DISCUSSION
The reaction of the phosphorus ylides 1a and 1b or the related N-heterocyclic olefin (NHO) 1c with [Fe(N(SiMe 3 ) 2 ) 2 ] results in the quantitative formation of the Fe II −ylide adducts 2a−2c, which are isolated in good yields (Scheme 1). The molecular structures for all three congeners were determined by X-ray crystallography and show a distorted trigonal planar coordination environment around the Fe atom ( Figure 1). The N−Fe−N angles decrease with higher steric demand of the ylide (1b > 1a > 1c, see Table 1). 16 Interestingly, the Fe−C bonds in all three Fe II −ylide complexes which are at 2.158(6) Å (2a), 2.176(2) Å (2b), and 2.159(3) Å (2c), respectively, are longer than the Fe−C bond distances in the related tricoordinated alkyl substituted [LFeR] complexes reported by the groups of Hessen and Holland (L = bulky β-diketiminate ligand, R = Me, i Pr, CH 2 t Bu, benzyl; Fe−C: 2.009(3)−2.048(3) Å) 17 or in the anionic congener [((Me 3 Si)CH 2 )Fe(N(SiMe 3 ) 2 ) 2 ]-[(THF) 3 Na(NHC Dipp )], which is at 2.065 Å. 18 This observation points to a weaker donor strength of the ylide ligands. In the case of the Ph 3 P substituted complexes 2a and 2b, this can be explained by the strong negative hyperconjugation of the lone pair of the ylidic C atoms into the σ*− P−C Ph bonds, as indicated by the shortened C−P bonds (∑ covalent_radii (C,P): 1.83 Å) of 1.751(6) Å and 1.736(1) Å in 2a and 2b, respectively. 19 In the NHO substituted complex 2c, the ylidic lone pair is strongly involved in forming the π−C− C2 bond at 1.417(5) Å, which is in between the expected values for a C−C single bond (1.54 Å) and a double (1.34 Å) bond. 20 This delocalization decreases the overall ylide donor strength in 2c. 15c The widened Fe−C−P angles of 133.4(5)°a nd 120.35 (7)°in 2a and 2b, respectively, can be explained by the steric repulsion of the Ph 3 P and the N(SiMe 3 ) 2 groups. The paramagnetic character of complexes 2a−2c is manifested in the broad and paramagnetically shifted resonances observed in their 1 H NMR spectra. No resonances could be found in the 31 P and 29 Si NMR spectra of 2a and 2b due to paramagnetism. The solution magnetic susceptibilities determined at room temperature by Evan's method 21 are calculated to be 4.9 μ B , 5.2 μ B , and 5.5 μ B for 2a−2c, respectively. These values are all close to the expected value of 4.9 μ B for high-spin Fe II complexes (S = 2). The 57 Fe Mossbauer spectra of 2a−2c recorded at 80 K display isomer shifts ranging from δ = 0.51 to 0.55 mm/s (see Supporting Information (SI), Figures  Complexes 2a−2c are extremely air and moisture sensitive and readily decompose in solvents such as CH 2 Cl 2 , CHCl 3 , and CH 3 CN, which is probably due to the high reactivity of the weak Fe−C bond in 2a−2c. This hypothesis is nicely corroborated by the formal insertion of benzonitrile into the Fe−C bond of 2a. Subsequent proton migration from the ylidic C atom to the nitrile N atom yields complex 3 (Scheme 2). Due to its high reactivity and thermal instability (decomposes at ambient temperature), 3 was not isolated in larger amounts. However, suitable single crystals for X-ray crystallography of 3 were obtained from a PhCN/n-pentane mixture at −35°C showing the E isomer with a distorted trigonal planar coordination environment around the Fe atom (angle sum around Fe: 359.8(2)°; Figure 2). Both, the shortened N1−C2 (1.328(2) Å), C2−C3 (1.398(2) Å), and C3−P4 (1.723(1) Å) bond lengths in 3 as well as the planarity of the N1−C2−C3−P4 unit (torsion angle 0.1(3)°) indicate a significant π-delocalization over this fragment. The presence of basic amido-and a H-acidic ylide ligands in 2a−2c should allow for an intramolecular deprotonation reaction generating an additional donor site at the ylide−C atom. Attempts to induce such a self-protolysis reaction by refluxing solutions of 2a−2c in toluene, THF, or C 6 H 5 F were unsuccessful and yielded deep brown reaction mixtures of unidentified products. However, melting solid 2a at 135°C results in condensation of a colorless liquid at the glass wall and the formation of a brown sticky residue. The colorless product is identified as HN-(SiMe 3 ) 2 (Scheme 3, I), while recrystallization of the brown residue from CH 2 Cl 2 results in the formation of analytically pure, orange-brown block-like crystals which are composed of the desired doubly yldiide-bridged Fe II dimer 4 (isolated yield: 27%). Following the same synthetic approach for 2b and 2c resulted in the formation of an unidentifiable reaction mixture.
However, a second yldiide-bridged iron dimer can be prepared by reacting a fluorobenzene solution of 1a with [Fe(Mes) 2 ] 2 13 (Mes = 2,4,6-trimethylphenyl) instead of [Fe(N(SiMe 3 ) 2 ) 2 ] as an iron precursor. In analogy to the basic N(SiMe 3 ) 2 ligands, the mesityl ligands in [Fe(Mes) 2 ] 2 act as proton acceptors inducing the self-protolysis reaction (Scheme 3, II). Upon the addition of two equivalents of 1a to a red solution of [Fe(Mes) 2 ] 2 , an immediate color change to yellow is observed. The 1 H NMR spectrum of the reaction mixture shows the presence of four broad resonances, presumably due to the formation of monomer 5 (Scheme 3, see also SI, Figure S7). These resonances decrease in intensity upon heating to 85°C accompanied by the appearance of eight new resonances with increasing intensity over time. Six of those rising resonances are assigned to the doubly yldiidebridged Fe II dimer 6, and the residual two resonances correspond to the couple product mesitylene. After 1 h,  quantitative conversion to 6 is observed, resulting in a deep brown reaction solution from which brown needle-shaped crystals are isolated after layering it with n-hexane (yield: 60%). Attempts to isolate analogous Fe 2 C 2 -dimers from reactions of 1b and 1c with with [Fe(Mes) 2 ] 2 were not successful. The molecular structures of 4 and 6 display centrosymmetric Fe dimers, containing a Fe 2 C 2 -diamond core ( Figure 3). A distorted trigonal planar bonding environment is observed for the iron atoms (angle sum around Fe: 360°, see Table 2), whereas the endocyclic C atoms render a distorted tetrahedral geometry. The Fe−C bonds at 2.050(5) Å and 2.046(4) Å in 4 and 2.037(2) Å and 2.039(2) Å in 6 are slightly shorter than those found in the precursors 2a and 2b. However, they are significantly longer than those observed in the alkylidene bridged Fe II dimer D reported by Holland et al. (Chart 1, Fe− C 1.973(6) Å, 1.956(6) Å) 14 and are of a comparable length to those observed in the nitrogenase FeMoco (averaged: 2.00 Å). 2a Remarkable is the very acute Fe−C−Fe angle in 4 (78.58(16)°) and 6 (78.64(6)°). For comparison, the related and literature known thiolato-and amido-bridged Fe II dimers 7 and 8 ( Figure 3) render slightly larger Fe−S−Fe (84. 34(2)°) 22 and Fe−N−Fe (79.4(1)°) 23 angles, respectively. The acute Fe−C−Fe angles in 4 and 6 result in an very short Fe···Fe distance of 2.5939(12) Å and 2.5826(5) Å, respectively, being far below the covalent radii of two Fe high spin atoms (∑ covalent_radii : 3.04 Å). 19a Larger Fe···Fe distances are observed in 7 (3.209(4) Å) and 8 (2.715(1) Å). The 1 H and 13 C{ 1 H} NMR spectra of 4 and 6 display broad and paramagnetically shifted resonances which are nevertheless unambiguously assignable (see SI).
Noticeably, the resonances of the ylidic C−H protons in 4 and 6 are strongly downfield shifted at δ = 131.58 and 144.31 ppm, respectively. No resonances can be found in the 31 P and 29 Si NMR (for 4) spectra due to paramagnetism. The zero field     electronic structures of the dimers, 6 was representatively analyzed by Fe K-edge X-ray absorption spectroscopy (XAS). The normalized XAS spectrum is displayed in Figure 5 and shows a pre-edge feature at 7112.4 eV originating from the dipole forbidden 1s to 3d transition typically observed for highspin Fe II centers. 24 The additional intense feature at 7116.2 eV can be assigned to the dipole allowed Fe 1s to 4p z transition, where the ligands around the Fe atoms lie in the xy plane. This transition is characteristic for the trigonal planar coordination environment of the Fe atoms. 25 To obtain further insight into the electronic nature of 4 and 6, broken-symmetry DFT calculations have been performed using the TPSSh functional. The geometry optimized structures of the full models accurately reproduce the molecular structures derived from X-ray crystallography with calculated Fe···Fe distances of 2.568 and 2.557 Å for 4 and 6, respectively (2.5939(12) Å and 2.5826(5) Å found by X-ray crystallography). Localized magnetic molecular orbitals (LMMO) 26 were chosen to investigate the antiferromagnetic coupling between both iron centers following the analysis concept of Ye et al. 27 The frontier orbitals with significant d character are displayed in Figure 6 for compound 4 (for 6, see SI, Figure S12). Both iron centers reveal a doubly occupied d z 2 orbital, whereas each d xz , d yz , d x 2 y 2 , and d xy orbital is singly occupied, consistent with a high spin Fe II assignment.
The LMMO (Figure 6) for the d yz and d xz pairs shows strong localized iron character (>95%) depending on their spin-state as a result of the antiferromagnetic coupling. In contrast to that, the high-valent d x 2 y 2 and d xy pairs behave rather covalently with strong interaction between the metals and carbide ligands, respectively. A similar trend of increasing covalency of the high-valent MOs was previously studied by Ye and co-workers for antiferromagnetic coupled mononucleoid transition metal complexes. 27,28 This indicates a mechanism for strong antiferromagnetic coupling in which the Fe···Fe interactions are formed by (a) direct σ-bond interaction of the d x 2 y 2 pair and (b) the Fe···C interactions provided over the yldiide ligands for both the d xy and the d x 2 y 2 pair. This magnetic exchange coupling results in a diamagnetic ground state (S 0 = 0), consistent with the SQUID measurements. The Yamaguchi spin-projection formula 29 was used to calculate antiferromagnetic Heisenberg coupling constants of J = −372 cm −1 and J = −429 cm −1 for 4 and 6, respectively, which qualitatively fits with the values observed for the SQUID measurements. We note that a predicted overestimation of the computed J values relative to experiment is quite common for single reference DFT methods. 30 The bonding interactions in 4 and 6 were analyzed by Pipek-Mezey localized orbitals displaying a strong Fe−C interaction (B Fe−C > 0.65), whereas the Fe−Fe interaction (B Fe−Fe < 0.18) is comparatively weak, additionally attested by the Mayer bond-order analysis (see SI, Table S2). 31 The endocyclic Fe−C bonds reveal "pear shaped" localized orbitals that indicate a strong polarization of these bonds where the binding pairs of electrons are mainly localized on the bridging C atoms (Figure 7). This is further reflected by the position of the charge centroid depicted as small red dots centered at the carbon atoms. The depicted localized orbitals are comprised of 19−26% Fe character and 81−74% C character.

■ CONCLUSION
The synthesis of the doubly yldiide bridged Fe II dimers 4 and 6 is presented facilitated by the self-protolysis reaction of low coordinated ylide-Fe II high-spin complexes 2a−2c. Dimers 4 and 6 represent the first examples of yldiide-supported Fe 2 C 2 -

Inorganic Chemistry
Article diamond cores. Very acute Fe−C−Fe angles are observed that result in short Fe···Fe distances. XAS confirms both the high spin Fe II configuration and its trigonal planar bonding environment. The Fe II centers are strongly antiferromagnetically coupled via the yldiide bridges to give a diamagnetic ground state as evidenced by SQUID magnetometry. Highly polarized Fe−C bonds are identified where the bonding pairs of electrons are mostly centered at the μ 2 -C ligands. The use of ylides as μ 2 -C ligand building blocks formed in a self-protolysis reaction is a very powerful synthetic strategy to access μ-C ligated multinuclear iron complexes. The introduction of additional S-based ligands into yldiide bridged Fe II dimers would yield very promising synthetic model systems for nitrogenase. The synthesis of such sulfur-ligated [Fe 2 C 2 ] clusters is an ongoing research subject in our group.

■ EXPERIMENTAL SECTION
General Remarks. All manipulations were performed in a Glovebox (MBrown Labmaster pro ECO) under an atmosphere of nitrogen or using Schlenk techniques under an atmosphere of purified argon. Dry, oxygen-free solvents (CH 2 Cl 2 , CH 3 CN, C 6 H 5 F (distilled from CaH 2 ); Et 2 O, toluene, THF (distilled from potassium/ benzophenone); and n-hexane and n-pentane (distilled from potassium)) were employed. Deuterated benzene (C 6 D 6 ) was purchased from Sigma-Aldrich and distilled from potassium/ benzophenone. Benzonitrile (PhCN) and hexamethyldisiloxane (Me 3 Si) 2 O were degassed and recondensed prior to use. Anhydrous deuterated THF (THF-d 8 ) and dichloromethane (CD 2 Cl 2 ) were purchased from Sigma-Aldrich. All distilled, recondensed, and deuterated solvents were stored over molecular sieves (4 Å). All glassware was oven-dried at 120°C prior to use. [Fe(N(SiMe 3 ) 2 ) 2 ], 32 [Fe(Mes) 2 ] 2 , 13 Ph 3 PCH 2 (1a), 33 Ph 3 PCH(SiMe 3 ) (1b), 33 and 1,3iso-propyl-2-methylenebenzimidazol (ImCH 2 , 1c) 34 29 Si (99.34 MHz)). All 13 C NMR spectra were exclusively recorded with composite pulse decoupling. If necessary, reported numbers assigning atoms in the 13 C spectra were indirectly deduced from the cross-peaks in 2D correlation experiments (HMBC, HSQC). Chemical shifts (δ) are reported in parts per million. Coupling constants (J) are reported in Hz. Melting points were recorded on an electrothermal melting point apparatus in sealed capillaries under a nitrogen atmosphere and are uncorrected. Infrared (IR) spectra were recorded at ambient temperature using a Thermo Scientific Nicolet iS 5 FT IR spectrometer equipped with a temperature stabilized NIR diode laser. An ATR unit (diamond) was used for recording IR spectra. The intensities are reported relative to the most intense peak and are given in parentheses using the following abbreviations: vw = very weak, w = weak, m = medium, s = strong, vs = very strong. Elemental analyses were performed on a Vario MICRO cube Elemental Analyzer from Elementar Analysatorsysteme GmbH in CHNS modus. UV/vis spectra were recorded with a Cary 8454 UV−vis Diode Array System of Agilent Technologies.
SQUID. Magnetic susceptibility data were measured from powder samples of solid material in the temperature range 2−270 K by using a SQUID magnetometer with a field of 1.0 T (MPMS-7, Quantum Design, calibrated with standard palladium reference sample, error < 2%). Sample holders of quartz with an O-ring sealing were used. The SQUID response curves (raw data) have been corrected for holder and solvent contributions by subtracting the corresponding response curves obtained from separate measurements without sample material. The experimental magnetization data obtained from independent simulation of the corrected SQUID response curves were corrected for underlying diamagnetism by use of tabulated Pascal's constants, 35 as well as for temperature-independent paramagnetism (TIP). Magnetic susceptibility data were analyzed and simulated using the julX software written by E. Bill (Max Planck Institute for Chemical Energy Conversion, Mulheim an der Ruhr).
X-ray Absorption Spectroscopy (XAS). Solid 6 was diluted in boron nitride, sealed in an aluminum spacer (1 mm layer thickness) between Kapton tape windows, stored in liquid nitrogen, and kept at this temperature until the measurement. XAS measurements were performed at the Stanford Synchrotron Radiation Lightsource facility (SSRL, beamline 9−3) operated at 3 GeV, with an electron beam current of ∼500 mA. A double-crystal monochromator (DCM) equipped with a Si(220) crystal pair was used to select the energy of the incoming X-rays with an energy resolution (ΔE/E) of ∼10 −4 . The monochromator was partially detuned (by ∼30% maximum incident flux) to reduce harmonics. A Rh-coated mirror with a cutoff of 9.5 keV was also used to reject higher harmonics. Samples were measured using a 1 × 4 mm 2 (vertical × horizontal) beam at the sample position. The samples were kept at approximately 15 K using a continuous-flow liquid helium cryostat (Oxford Instruments CF1208). The energy of the incident beam was calibrated by measuring the XAS spectrum of an iron foil and setting the first inflection point to 7111.2 eV. Transmission measurements were performed using an N2-filled ion gas chamber, and reference foil measurements were performed simultaneously. Prior to measurements, each sample was checked for signs of radiation damage by performing subsequent 2.5 min scans over the same sample spot. Background subtraction and normalization were performed using the Athena program of the Demeter package. 36 A second-order polynomial was subtracted from the pre-edge and postedge regions to account for the background.
Mossbauer Spectroscopy. Mossbauer spectra were recorded on a conventional spectrometer with an alternating constant acceleration of the γ-source. The minimum experimental line width was 0.24 mm/ s (full width at half-height). The sample temperature was maintained constant in an Oxford Instruments Variox cryostat. The 57 Co/Rh source (0.6 GBq) was kept at room temperature. Isomer shifts are quoted relative to iron metal at 300 K. Zero field spectra were measured at 80 K. For solid state measurements, the obtained solid was filled into a Delrin capsule in the glovebox. The capsules were frozen in liquid nitrogen directly after discharging from the glovebox. Handling and mounting of the samples was performed under liquid nitrogen. Data simulation was carried out using the program "mf.SL" written by E. Bill (Max Planck Institute for Chemical Energy Conversion, Mulheim an der Ruhr).
Theoretical Calculations. All DFT calculations were performed using the ORCA Quantum Chemistry Package version 4.1. 37 Single point calculations and full geometry optimizations (based on experimental X-ray structures obtained by X-ray diffraction) for 4 and 6 have been performed by unrestricted density functional theory (DFT). The TPSSh functional 38 in combination with the def2-TZVP basis set 39 and D3 dispersion correction 40 has been used. The RIJCOSX approximation was used to speed up all hybrid DFT calculations. 41 Broken-symmetry solutions (for both single-point calculations and geometry optimizations) were derived by first converging the ferromagnetic solution (M S = 9), then flipping the spins on one side of the iron atom, and finally reconverging to the  antiferromagnetic broken symmetry solution with M s = 0. The localized orbitals were calculated using the Pipek−Mezey scheme. 42 X-ray Crystal Structure Determination. Suitable single crystals of 2a−2c, 3, 4, and 6 were coated with perfluoropolyether (PFO-XR75), picked up with nylon loops, and immediately mounted in the nitrogen cold stream of the diffractometers at 100 K. A Bruker Nonius KappaCCD diffractometer with a Mo-target rotating-anode X-ray source and Incoatec Helios focusing multilayer optics and a Bruker Kappa Mach3 APEX-II diffractometer with a Bruker IμS and Incoatec Helios mirror was used (Mo Kα radiation; λ = 0.71073 Å). Final cell constants were obtained from least-squares fits of setting angles of several thousand strong reflections. Intensity data were corrected for absorption using intensities of redundant reflections using SADABS. 43 The structures were readily solved by direct and Patterson methods and subsequent difference Fourier techniques. The Bruker ShelXTL2 44 software package was used for solution, refinement, and artwork of the structures. All non-hydrogen atoms were anisotropically refined, and hydrogen atoms were placed at calculated positions and refined as riding atoms with isotropic displacement parameters. Crystal and data collection details are given in Tables S3 and S4 (see SI). The [((SiMe 3 ) 2 N) 2 FeC] unit of 2a was found to be severely disordered. A split atom model with restrained bond distances and displacement parameters using the EADP, SAME, and ISOR instructions of ShelXL was refined to account for the disorder (293 restraints; 0.52/0.48 occupation ratio). A SiMe 3 group in the structure of 2c was also disordered and treated similarly (31 restraints; 0.67/0.33 occupation ratio). Crystals of 6 contained a severely disordered fluorobenzene molecule, which was removed using Platon/SQUEEZE. 45 Preparation of [(Ph 3 PCH 2 )Fe(N(SiMe 3 ) 2 ) 2 ] (2a). A solution of [Fe(N(SiMe 3 ) 2 ) 2 ] (1.51 g, 4.00 mmol) in THF (4 mL) was added to a stirring solution of Ph 3 PCH 2 (1a, 1.11 g, 4.00 mmol) in THF (5 mL), and the resulting light green reaction mixture was stirred for 1 h at ambient temperature. All volatiles were removed in vacuo to afford a pale green solid. n-Pentane (5 mL) was added to give a pale green solution followed by the addition of (Me 3 Si) 2 O (4 mL) resulting in the formation of pale green crystalline material. The volume of the reaction mixture was reduced to 1/2 of its volume, and the supernatant was removed by decantation. The crystalline material was washed with (Me 3 Si) 2 O (3·0.5 mL) and dried in vacuo to give 2a as highly air sensitive, pale green crystals. 2a slowly decomposes at ambient temperatures by changing its color to brown and therefore needs to be stored at low temperatures (−35°C). Yield: 2. Preparation of [(Ph 3 PCH(SiMe 3 ))Fe(N(SiMe 3 ) 2 ) 2 ] (2b). Toluene (2 mL) was added to a mixture of [Fe(N(SiMe 3 ) 2 ) 2 ] (376.3 mg, 1.00 mmol) and Ph 3 PCH(SiMe 3 ) (1b, 348.5 mg, 1.00 mmol), and the resulting green-yellow solution was stirred for 1 h at ambient temperature. All volatiles were removed in vacuo to afford a pale green solid. n-Hexane (2 mL) was added to give a suspension, which was stored for 12 h at −35°C to give large amounts of crystalline, pale green solid. The supernatant was removed by decantation, the crystalline residue was washed with n-pentane (0.5 mL) and dried in vacuo to give 2b as analytically pure, highly air sensitive, pale green Preparation of [(Im)Fe(N(SiMe 3 ) 2 ) 2 ] (2c). 1,3-iso-Propyl-2methylene-benzimidazole (1c, 118.7 mg, 0.55 mmol) was added to a stirring solution of [Fe(N(SiMe 3 ) 2 ) 2 ] (206.0 mg, 0.55 mmol) in toluene (2 mL)/ and the resulting yellow solution was stirred for 10 min at ambient temperature. All volatiles were removed in vacuo to afford a colorless solid. n-Hexane (2 mL) was added to give an almost clear solution that was filtered. (Me 3 Si) 2 O (2 mL) was added to the filtrate, and the mixture was stored at −35°C for 7 days to give large amounts of crystalline material. The supernatant was removed by decantation and the residue was dried in vacuo to give 2c as highly air sensitive colorless needles. 2c slowly decomposes at ambient temperatures by changing its color to brown and therefore needs to be stored at low temperatures (−35°C). Preparation of [(μ-Ph 3 PCH)Fe(N(SiMe 3 ) 2 )] 2 (4). Method A: Solid 2a (210 mg, 0.32 mmol) was heated to 135°C for 12 h. The resulting brown melt containing crystalline material was stored for 24 h at −35°C. The residual orange-brown crystalline material was washed with CH 2 Cl 2 (1 mL) and dried in vacuo (yield: 29 mg, 18%). Method B: A solution of Ph 3 PCH 2 (1a, 110.6 mg, 0.40 mmol) in THF (1 mL) was added to a stirring solution of [Fe(N(SiMe 3 ) 2 ) 2 ] (74.0 mg, 0.2 mmol) in THF (1 mL). The reaction mixture was stirred for 10 min followed by removal of all volatiles in vacuo. The yellow, sticky residue was heated to 135°C for 24 h to give a brown residue. CH 2 Cl 2 (1 mL) was added, and the resulting suspension was stored for 1 week at −35°C. The brown suspension was filtered, and the residue was washed with CH 2 Cl 2 (4 × 0.2 mL) and dried in vacuo to give an air sensitive, orange-red powder (yield: 27 mg, 28%).