Flexibility of an Open Indenyl Ligand in Iron(II ... - ACS Publications

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Flexibility of an Open Indenyl Ligand in Iron(II) Complexes Andreas Glöckner,† Thomas Bannenberg,† Kerstin Ibrom,‡ Constantin G. Daniliuc,† Matthias Freytag,† Peter G. Jones,† Marc D. Walter,*,† and Matthias Tamm*,† †

Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106 Braunschweig, Germany ‡ NMR-Labor der Chemischen Institute, Technische Universität Braunschweig, Hagenring 30, 38106 Braunschweig, Germany S Supporting Information *

ABSTRACT: [FeI2(thf)2] was sequentially treated with Li(C5Me5) and the potassium salt of the phenylmethallyl (“open indenyl”) ligand oIndMe, leading to selective formation of the half-open ferrocene [(η5-C5Me5)Fe(η5-oIndMe)] (1). A variable-temperature NMR study revealed a barrier of ca. 12 kcal mol−1 for the rotation of the phenyl group. The apparent lability and susceptibility of the oIndMe ligand in 1 to undergo η5−η3 interconversion allowed the preparation of the complexes [(η5-C5Me5)Fe(η3-oIndMe)(L)] (2, L = CO; 3, L = IMe; 4, L = PMe3) by addition of carbon monoxide, 1,3,4,5tetramethylimidazolin-2-ylidene (IMe), and trimethylphosphine, respectively. The η3-bound phenylmethallyl ligand in these complexes initially adopts an anti orientation with regard to the relative positions of the phenyl and methyl substituents, followed by anti-to-syn isomerization. For L = CO, both isomers could be isolated, and the conversion of anti-2 into syn-2 was monitored by NMR spectroscopy at 35, 50, 65, and 80 °C, affording an enthalpy of activation of ΔH⧧ = 24(1) kcal mol−1 for this equilibrium reaction. On the basis of DFT calculations, a mechanism is proposed that proceeds via consecutive η3−η1−η3 interconversions and involves η3-benzyl intermediates. In contrast, rapid equilibration was observed for L = IMe and PMe3. Addition of 1,2-bis(dimethylphosphino)ethane (dmpe) to 1 gave [(η5-C5Me5)Fe(η1-oIndMe)(dmpe)] (5), containing the oIndMe ligand bound in an η1-allyl fashion. η5-to-η3 hapticity interconversion was also observed upon reaction of 1 with methyl iodide and CH2Cl2, which formed the Fe(III) complexes [(η5-C5Me5)Fe(η3-oIndMe)(X)] (6, X = I; 7, X = Cl); solid-state magnetic susceptibility measurements on 6 revealed an S = 1/2 ground state. The mixed indenyl−open indenyl complex [(η5-Ind″)Fe(η3oIndMe)(CO)] (9, Ind″ = 1,3-di(tert-butyl)indenyl) was isolated from the stepwise reaction of [FeI2(thf)2] with Na(Ind″), K(oIndMe), and CO. The molecular structures of 1, anti-2, syn-2, syn-3, 5, syn-6, syn-7, and syn-9 were established by single-crystal X-ray diffraction.

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shown that further enhancement by a factor of 106 can be achieved when the indenyl ligand is replaced by a hydronaphthalenyl ligand.5 Similarly, we have recently introduced the indenyl effect to the pentadienyl (“open Cp”) chemistry with the preparation and coordination of a phenylmethallyl ligand, oIndMe (Figure 1), which can be classified as an open indenyl ligand.6 Although

INTRODUCTION Following initial studies on CO insertion reactions in molybdenum complexes by Mawby,1 Basolo ascribed the enhanced ligand substitution rates observed in indenyl rhodium and manganese complexes, as compared to their cyclopentadienyl counterparts, to the “indenyl effect”.2 The indenyl ligand can relatively easily switch between a perturbed η5coordination mode and η3-allyl-type coordination, leading to a reduced electron count of two at the metal center and thus providing the possibility for an associative ligand substitution pathway. The origin of the indenyl effect is clearly a result of the rearomatization of the annulated benzene ring upon hapticity interconversion.3 However, recent theoretical studies also suggest that the higher stability of the (η5-Cp)−M bond versus the (η5-Ind)−M bond and the reversed order for the η3bound intermediates play a crucial role; thereby, the activation of an η5−η3 rearrangement requires less energy for indenyl complexes, resulting in higher reaction rates.4 Furthermore, substitution studies of [(η5-X)Mn(CO)3] complexes (X = cyclopentadienyl, indenyl, fluorenyl, hydronaphthalenyl) have © 2012 American Chemical Society

Figure 1. Indenyl and the open indenyl, oIndMe, ligand.

conventional pentadienyl ligands are already known to coordinate to transition metals in various bonding modes,7 the relatively weak C−X π-bond in heteropentadienyl systems usually results in even more facile η5−η3 haptotropic shifts.8 For Received: April 13, 2012 Published: June 6, 2012 4480

dx.doi.org/10.1021/om3003009 | Organometallics 2012, 31, 4480−4494

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instance, [(η5-C5Me5)Ru(η5-2,4-C6H9O)] (C6H9O = dimethyloxapentadienyl) reacts with PMe3 under reflux to yield [(η5C5Me5)Ru(η3-2,4-C6H9O)(PMe3)], whereas the corresponding 2,4-dimethylpentadienyl complex does not.9 However, as a result of the indenyl effect, the complex [(η5-C5Me5)Ru(η5oIndMe)] undergoes a hapticity shift significantly faster than its oxapentadienyl congener upon addition of a ligand L (L = CO, PMe3, CN-o-xylyl), resulting within minutes at room temperature in anti-[(η5-C5Me5)Ru(η3-oIndMe)(L)] complexes. For the PMe3 complex, slow isomerization to the syn isomer takes place, which suggests that η1-oIndMe intermediates are also accessible.6 As part of our continuing interest in metal− pentadienyl and metal−allyl chemistry,10 we wished to seek a deeper insight into the characteristic features of the open indenyl ligand, oIndMe, including detailed studies on the anti-tosyn isomerization and the isolation of an η1-complex. Therefore, we have extended our previous study to iron and present here the corresponding results.

halogen half-sandwich complexes are stable at room temperature and can be used for further transformations when bulky cyclopentadienyl ligands, such as tri(tert-butyl)-, tetra(isopropyl)-, or penta(isopropyl)cyclopentadienyl, are employed.19 Therefore, we attempted the synthesis of [(η5-C5Me5)Fe(η5oIndMe)] (1) by the stepwise addition of Li(C5Me5) and K(oIndMe) to [FeI2(thf)2] at −78 °C (Scheme 1). The Scheme 1. Synthesis of [(η5-C5Me5)Fe(η5-oIndMe)] (1) via an Iron Half-Sandwich Intermediate

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RESULTS AND DISCUSSION Preparation and Characterization of a Pentamethylcylopentadienyl Iron(II) Open Indenyl Complex. The complex [(η5-C5Me5)Ru(η5-oIndMe)] was readily obtained by salt metathesis of [(η5-C5Me5)RuCl]4 with phenylmethallyl potassium, K(oIndMe),6 which is based on a common procedure for the preparation of half-open ruthenocenes.11 In contrast, the synthesis of half-open ferrocenes, [(η5-Cp)Fe(η5Pdl)] (Cp = C5H5, C5Me5; Pdl = pentadienyl or substituted pentadienyl) appears to be more tedious and less general. One approach involves the equimolar and simultaneous reaction of Na(Cp) (Cp = C 5 H 5 , C 5 Me 5 ), K(Pdl) (Pdl = 2,4dimethylpentadienyl, 2,3,4-trimethylpentadienyl), and FeCl2 or FeBr2(dme) (dme = dimethoxyethane) at low temperatures, which results in an almost statistical mixture of [(η5-Cp)2Fe], [(η5-Cp)Fe(η5-Pdl)], and [Fe(η5-Pdl)2]; subsequent fractional crystallization allows the separation of the half-open ferrocene, albeit in low yields.7c,12 Furthermore, for Pdl = pentadienyl or hexadienyl, [(η5-C5H5)Fe(η5-Pdl)] can be obtained more selectively via an η3-intermediate by the successive photolysis of [(η5-C5H5)Fe(η1-Pdl)(CO)2], which first has to be prepared from [(η5-C5H5)Fe(CO)2]Na and either 1-chloro-2,4-pentadiene or 1-chloro-2,4-hexadiene.13 Since we were interested in isolating the iron analogue of [(η5-C5Me5)Ru(η5-oIndMe)], [(η5-C5Me5)Fe(η5-oIndMe)] (1), with pentamethylcyclopentadienyl as the coligand, the stable complexes [(η5-C5Me5)Fe(NCMe3)3][PF6] and [(η5-C5Me5)Fe(acac)(L)] (acac = acetylacetonate) appeared to be the most promising candidates for providing the (η5-C5Me5)Fe fragment.14 Both complexes are known to yield mixed ferrocenes upon reaction with cyclopentadienide salts,15 but require time-consuming multistep syntheses to avoid the formation of the thermodynamically highly stable ferrocene molecule.14 Alternatively, [(η5-C5Me5)FeCl(tmeda)] (tmeda = N,N,N′,N′-tetramethylethylenediamine) might be a suitable material,16 because it was used as an iron half-sandwich reagent in several examples.17 However, the stabilizing tmeda ligand might remain coordinated and thereby inhibit the formation of an η5-oIndMe complex. Nevertheless, other studies suggest that a nonstabilized version, [(η5-C5Me5)FeBr], is formed at low temperatures upon reaction of [FeBr2(dme)] with 1 equiv of Li(C5Me5) in THF, but that ligand scrambling takes place after quenching with Na(C5H5), leading to a mixture of decamethylferrocene, pentamethylferrocene, and ferrocene.18 Notably, such iron−

successive addition of reagents is a slight, but crucial, modification of the original procedure for the synthesis of half-open ferrocenes,7c because the 1H NMR spectrum of the crude product clearly shows the selective formation of 1; neither [(η5-C5Me5)2Fe)] nor [Fe(η5-oIndMe)2] can be observed (see the Supporting Information for the 1H NMR spectrum in C6D6). In agreement with earlier studies,18 this suggests the presence of [(η5-C5Me5)FeI] as a stable intermediate as long as the yellow solution is kept at −78 °C; the same color was observed when the structurally characterized red dimer [(η5-C5H2tBu3)Fe(μ-I)]2 was dissolved in THF.19d The use of [FeI2(thf)2] instead of [FeBr2(dme)] might actually lead to an enhanced kinetic stability of the [(η5C5Me5)FeI] intermediate because of the larger size of the halide, which then reacts with K(oIndMe) in a second salt metathesis step to yield the desired product. Usually, an oily product is obtained after extraction with pentane; purification can be achieved by crystallization from concentrated pentane solutions at −30 °C, affording 1 as a dark green solid in 44− 64% yield. Because of the extremely high solubility even in hydrocarbon solvents, reaction yields are probably considerably higher than isolated yields. Complex 1 is indefinitely stable under an inert atmosphere (N2), but appears to be extremely air-sensitive in solution. Sometimes, the precipitation of a colorless solid was observed in subsequent reactions with 1. We suspect that 1 may occasionally be contaminated with LiI if the extraction during workup is not performed carefully. Replacing Li(C5Me5) with Na(C5Me5) still yields 1, and the byproduct NaI can be removed more easily because of its lower solubility in comparison to LiI. However, this is at the expense of the purity; additional unidentified resonances at 1.75, 1.65, and 1.15 ppm (in C6D6) were always observed in the 1H NMR spectrum of 1 after crystallization. NMR spectroscopy suggests the presence of oIndMe in 1 as an η5-pentadienyl ligand, but there are striking differences relative to the ruthenium analogue, for which we did not observe dynamic behavior on the NMR time scale at ambient temperature.6 The 1H NMR spectrum of 1 in THF-d8 shows two individual sharp signals for the protons of the CH2 group, showing that the rotation about the C1−C2 axis is slow (see Scheme 1 for the numbering of 1). The two ortho protons of the phenyl ring (H5/H9), however, give rise to only one very 4481


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broad resonance at δ ≈ 4.49 ppm (width at half-height ca. 500 Hz at 400 MHz), suggesting moderately fast rotation of the phenyl ring about the C3−C4 bond. Upon lowering the temperature, the signal separated into two resonances, which, at 221 K, were sharp enough to allow the full assignment of all 1H and 13C NMR signals by standard 1D and 2D NMR procedures ( 1 H, 1 H-COSY, 1 H, 1 H-NOESY, 1 H, 13 C-HSQC, 1 H, 13 CHMBC). The other 1H and the 13C NMR chemical shifts at room temperature and at 221 K exhibit only minor temperature dependence; chemical shifts and assignments are given in Tables 1 and 2. At room temperature, the δ values of the

ppm) should entail a very broad signal at room temperature, which, for this reason, could not be detected at the predicted position of 99.3 ppm. The chemical shifts of C1−C4 and especially C5 (δ = 67.4) at 221 K indicate that the oIndMe ligand is bound to Fe in an η5-manner, which also explains the alternation of the size of the ortho spin−spin coupling constants between the phenyl ring protons. Interestingly, the HMBC spectrum shows a distinct cross peak between C5 and anti-H1, which are separated by five bonds. Although the exact 13C,1H spin−spin coupling constants cannot be extracted from a conventional magnitude HMBC spectrum where the cross peaks are obscured by the proton−proton couplings, the remarkable splitting of the cross peak in the indirect dimension of about 22 Hz suggests an additional coupling pathway between the nuclei, presumably via the Fe atom. Further low-temperature 1D and 2D 1H NMR measurements were carried out to study the dynamics of the phenyl ring rotation. Bandshape analysis of proton spectra measured at six temperatures evenly distributed over a temperature range of 197−256 K yielded free enthalpies of activation (ΔG⧧) between 11.7 and 12.1 kcal mol−1.20 These are, if at all, only very slightly temperature-dependent and to a lesser extent than the error of the analysis, which is estimated to be ±0.2 kcal mol−1. A similar barrier for phenyl rotation was found for the related benzyl complex [(η5-C5H5)Fe(η3-CH2Ph)(CO)] (ΔG⧧ = 11.8(4)−12.6(2) kcal mol−1),21 whereas a smaller value was determined for Li(oIndMe) (ΔG⧧ = 10.0(2) kcal mol−1).22 The large chemical shift difference between the two ortho protons of 1 (Δδ = 6.04 ppm) causes very severe signal broadening near the coalescence temperature. In combination with signal overlap, this does not permit a reliable line-shape analysis at temperatures higher than 256 K. Thus, rate constants for the chemical exchange of the two ortho and of the meta positions, respectively, were only obtained within a temperature range of about 60 K. The value for the entropy of activation (ΔS⧧) is close to zero, and therefore, ΔH⧧ ≈ ΔG⧧. This is consistent with an intramolecular process, but the error associated with ΔS⧧ is disproportionally high. Therefore, we prefer to refrain from giving an exact value. The molecular structure of 1 in the solid state was determined by X-ray diffraction analysis (Figure 2) and proved to be isostructural with the ruthenium analogue.6 It should be noted that the phenylmethallyl ligand has two enantiotopic

Table 1. 1H and 13C NMR Data of 1 at Room Temperature in THF-d8a position

δC

δH

HMBC

2.39, dd, Janti‑H1,syn‑H1 = 3.5, Jsyn‑H1,H3 ≥ 0.7 Hz, 1H, syn-H1; −0.78, d, 1H, anti-H1

5.66, 1.91, (−0.78)b,c (5.66), (2.39), 1.91, (−0.78) 5.66, 2.39, (1.91),c −0.78 (5.66),c 2.39, 1.91, −0.78 6.93, (5.66), (−0.78) n. o.d n. o.,d broad 13 C signal (6.83)c 1.39 1.39c

1

43.4, t

2

96.4, s

CH3

25.3, q

1.91, s, 3H

3

82.1, d

5.66, s, 1H

4

103.8, s

5/9 6/8

n. o.d 133.9, br d

≈ 4.49, v br s, Δν1/2 ≈ 500 Hz, 2H 6.93, m, 2H

7 C5Me5 C5Me5

121.1, d 80.1, s 10.0, q

6.83, m (“t”, splitting 7.3 Hz), 1H 1.39, s, 15H

Referenced to δH = 1.72 and δC = 25.3 ppm, respectively. bShifts in parentheses indicate weak crosspeaks. c1JCH correlation. dNot observed. a

Table 2. 1H and 13C NMR Data of 1 at 221 K in THF-d8a position

δC

δH 2.33, d, Janti‑H1,syn‑H1 ≥ 3.4 Hz, 1H, syn-H1; −0.91, d, 1H, anti-H1

1

43.5, t

2 CH3

96.3, s 25.3, q

1.95, s, 3H

3

82.4, d

5.64, s, 1H

4

103.2, s

5

67.4, d

1.46, d, JH5,H6 = 5.8 Hz, 1H

6 7 8 9 C5Me5 C5Me5

139.5, d 121.0, d 127.8, d 131.2, d 80.1, s 10.2, q

6.90, 6.83, 7.00, 7.50,

“t” (dd), JH6,H7 = 8.6 Hz, 1H “t” (dd), JH7,H8 = 5.7 Hz, 1H “t” (dd), JH8,H9 = 8.5 Hz, 1H d, 1H

1.37, s, 15H

HMBC 5.64, 1.95 (2.33),b 1.95 5.64, 2.33, (1.95),c −0.91 7.50, (5.64),c 2.33, 1.95, (−0.91) 7.00, 6.90, 5.64, (1.46) 7.50, 6.83, 5.64, (−0.91) 7.00 7.50, 1.46 (6.90) 6.83, 5.64, 1.46 1.37 1.37c

Referenced to δH = 1.72 and δC = 25.3 ppm, respectively. bShifts in parentheses indicate weak crosspeaks. c1JCH correlation. a

Figure 2. ORTEP diagram of 1 with thermal displacement parameters drawn at 50% probability. Selected bond lengths (Å) and angles (deg): Fe−C(C 5 Me 5 ) 2.058(2)−2.100(2), Fe−C1 2.086(2), Fe−C2 2.062(2), Fe−C3 2.055(2), Fe−C4 2.119(2), Fe−C5 2.254(2), C1− C2 1.426(3), C2−C3 1.417(3), C3−C4 1.430(3), C4−C5 1.436(3), C5−C6 1.427(3), C6−C7 1.362(3), C7−C8 1.425(3), C8−C9 1.346(3), C4−C9 1.444(3); C1−C2−C3 122.2(2), C2−C3−C4 127.0(2), C3−C4−C5 123.2(2).

chemically equivalent ortho (H5/H9) and meta (H6/H8) protons and also of the meta-carbon atoms (C6/C8) equal the average shifts of the corresponding positions at 221 K. As for the ortho protons H5 and H9, the large chemical shift difference between the ortho carbons C5 and C9 at 221 K (Δδ = 63.8 4482


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faces, and on coordination of the (η5-C5Me5)Fe fragment, a racemic mixture is formed. The oIndMe ligand exhibits the expected η5-coordination mode, clearly identifying 1 as a halfopen ferrocene with a typical sandwich structure. Both ligands are reasonably parallel, as shown by a small tilt angle of 6.4° between the least-squares planes of the metal-bound carbon atoms. The open indenyl ligand plane approaches the iron atom much closer than C5Me5 (1.486 vs 1.688 Å) in order to achieve similar Fe−C bond distances (Figure 2).7 In comparison to the only other structurally characterized halfopen ferrocene, [(η5-C5H5)Fe(η5-2,4-C7H11)] (C7H11 = dimethylpentadienyl),23,24 the opening of the acyclic ligand in 1 is wider, with a larger separation between the terminal carbon atoms C1 and C5 (2.861 vs 2.746 Å). The open nature of the oIndMe ligand may be the cause of its more distorted η5coordination mode compared to that of conventional group 8 indenyl complexes.3b,25 Some slippage of the Fe atom toward the C1−C3 allyl moiety is observed, with Fe−C bond distances ranging from 2.055(2) to 2.086(2) Å, while the values for C4 and C5 (2.119(2)/2.254(2) Å) are markedly longer. Nevertheless, significant bonding interaction of the metal with C4 and C5 is indicated by a noticeable bending of the phenyl group toward iron, as revealed by a fold angle between the C1− C3 and C4−C9 planes of 11.7°. Furthermore, the phenyl coordination is supported by a distinct long-short-long-shortlong pattern of the C−C bond lengths within the C5-C6-C7C8-C9-C4 moiety, associated with a perturbed electron delocalization within the six-membered ring. Overall, the structural features of 1 resemble those found for [(η5C5Me5)Ru(η5-oIndMe)], although the metal−carbon bond distances are naturally shorter in 1. η5-to-η3 Hapticity Interconversion by Ligand Addition. The reduction of the electron count of the metal by two upon η5−η3 hapticity interconversion, which then provides a vacant orbital for ligand coordination or substrate activation, is a known process for pentadienyl ligands.26 This proposal rather than an intimate associative mechanismis further substantiated by the fact that a rapid interconversion between the η5- and η3-coordination modes is already observed for 1 in solution (vide supra). One reason for the easier adoption of an η3-coordination mode of pentadienyl in comparison to cyclopentadienyl ligands is the smaller loss of resonance energy.7b,d However, recent studies suggest that the reactivity can further be enhanced by using heteropentadienyl ligands, which, according to photoelectron spectroscopy and theoretical studies,27 interact more weakly with late metals than do conventional pentadienyl ligands.8 Our studies on [(η5C5Me5)Ru(η5-oIndMe)] have shown that haptotropic shifts can be achieved even more readily for the open indenyl ligand; here, the rearomatization can be considered as the main driving force for facile η5−η3 interconversion.6 To extend our previous studies, potential reactions of the 18electron sandwich complex [(η5-C5Me5)Fe(η5-oIndMe)] (1) were explored utilizing various two-electron donor ligands (Scheme 2). Notably, a review article mentions the reaction of [(η5-C5H5)Fe(η5-C10H9)] (C10H9 = hydronaphthalenyl) with CO, resulting in [(η5-C5H5)Fe(η3-C10H9)(CO)], whereas the corresponding η5-hexadienyl complex appears to be inert.28 In our case, carbon monoxide was passed through a green pentane solution of 1 at room temperature, which gave an immediate color change to orange. Coordination of CO is clearly indicated by a resonance at 223.8 ppm in the 13C NMR spectrum. Additionally, the aromatic region in the 1H NMR spectrum

Scheme 2. Syntheses of 2−4 and Their Isomerizations to the syn Isomers

now integrates for five hydrogen atoms, which is in agreement with an uncoordinated phenyl group. Similar to the anti-η3oIndMe complexes of ruthenium,6 the downfield resonance for H3 at 4.10 ppm suggests that H3 adopts a syn orientation with respect to the methyl group. Consequently, the open indenyl ligand has undergone an η5−η3 hapticity interconversion resulting in anti-[(η5-C5Me5)Fe(η3-oIndMe)(CO)] (anti-2), which was also confirmed by X-ray diffraction analysis (Figure 3, Table 3). The phenyl group still resides in the anti position

Figure 3. ORTEP diagram of anti-2 with thermal displacement parameters drawn at 50% probability.

as in 1, albeit significantly tilted out of the allyl plane with an increase of the absolute C1−C2−C3−C4 torsion angle from 2.0° (synperiplanar) in 1 to 49.5° in anti-2. CO coordination to iron from the open edge of the allyl ligand corresponds to an exo conformation,29 which is usually preferred to the endo conformation in d6-[(η5-C5H5)M(η3-allyl)(L)] complexes (L = CO, PR3).30 The Fe−CO bond distance of 1.7464(14) Å compares well to the values reported for similar complexes, such as [(η5-C5H5)Fe(η3-hexadienyl)(CO)] (1.717(6) Å) and [(η 5 -C 5 H 5 )Fe{η 3 -C 3 H 4 (CO)OCH 3 }(CO)] (1.717(6) Å).13b,31 Photolysis of a pentane solution of anti-2 for 6 h resulted in partial CO loss and reformation of 1 by an η 3 −η 5 interconversion process. In contrast, thermal activation by refluxing anti-2 in hexane does not cause any CO loss, but nevertheless yields a new product, which can be identified as syn-[(η5-C5Me5)Fe(η3-oIndMe)(CO)] (syn-2) by NMR spectroscopy (see the Experimental Section for details).32 In particular, the syn orientation of the phenyl and methyl substiutents is evidenced by ortho-phenyl/CH3 and anti-H1/H3 cross peaks in the 1H−1H NOESY NMR spectrum and a 4483


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dramatic upfield shift from 4.10 to 1.83 ppm for H3, corresponding to its change to the anti position. X-ray diffraction studies reveal that the structural parameters of syn2 and anti-2 are very similar (Figure 4, Table 3). Apart from the

Cpplane and allylplane refer to the least-squares planes of the η5- and η3-ligands, respectively. bτ = angle beween the two ligand planes.

Figure 4. ORTEP diagram of syn-2 with thermal displacement parameters drawn at 50% probability.

altered position of the phenyl group, there are only two other major differences: (1) For syn-2, the C1−C2−C3−C4 torsion angle is almost antiperiplanar (176.9°), whereas the corresponding angle in anti-2 (49.5°) is far apart from synperiplanarity. (2) The allyl moiety adopts an endo conformation, which is accompanied by a significant deviation from a mutually parallel orientation of the π-ligands; the angle τ increases from 6.9° for anti-2 to 64.5° for syn-2 (Table 3). In the present case, DFT studies (vide infra) suggest that the exo conformation is destabilized by ΔΔG298 = 3.2 kcal mol−1 in comparison to the observed endo conformation, which contrasts with earlier studies.30 At room temperature, the anti−syn isomerization proceeds slowly and can conveniently be followed by NMR spectroscopy. Accordingly, sealed samples of anti-2 in C6D6 were heated to 35, 50, 65, and 80 °C, respectively, and the isomerization was monitored by 1H NMR spectroscopy (see the Supporting Information for details). The C5Me5 ligands in the two isomers have distinct chemical shifts; while the signal at 1.54 ppm for anti-2 progressively decreased, a new resonance at higher field (1.43 ppm), assigned to syn-2, increased with time. Eventually, no further change in the syn/anti ratio was observed, and we suspect that the system had reached its equilibrium at the chosen temperature. The presence of an equilibrium was also verified by the observation that pure syn-2, obtained by recrystallization, slowly reconverts to anti-2 at room temperature until the mixture is equilibrated (syn/anti = 93:7). For a first-order reaction proceeding to equilibrium, a plot of ln(Asyn‑2,∞ − Asyn‑2,t) against time gives a straight line.33 As expected for an intramolecular process, this was indeed found for the anti−syn isomerization of 2 at all four temperatures (Figure 5), and the corresponding rate constants and half-lives are listed in Table 4 and can be used to determine the activation parameters for the reaction. An Eyring plot (see the Supporting Information) affords an enthalpy of activation of ΔH⧧ = 24(1) kcal mol−1 and an entropy of activation of ΔS⧧ = −7(3) eu; the latter value probably results from a loss of some degree of freedom while the transition state is approached. Notably, the barrier for the isomerization of a planar chiral open ferrocene, [Fe(η5-2,3-C7H11)2], via the partial decoordi-

a

2.0596(15)−2.2413(15) 1.763 2.0967(16) 2.0984(15) 2.1321(16) 1.644 116.72(15) 179.0(2) 1.7226(17) 62.6 2.115(3)−2.164(3) 1.763 2.154(4) 2.061(4) 2.111(4) 1.750 114.2(3) 178.9(4) 2.3087(8) 63.0 Fe−C(C5Me5) Fe−Cpplanea Fe−C1 Fe−C2 Fe−C3 Fe−allylplanea C1−C2−C3 C1−C2−C3−C4 (abs. value) Fe−L τb

2.0795(13)−2.1401(13) 1.719 2.1028(13) 2.0408(13) 2.1021(13) 1.600 121.04(12) 49.5(2) 1.7464(14) 6.9

2.0941(19)−2.134(2) 1.728 2.076(2) 2.070(2) 2.114(2) 1.648 115.5(2) 176.9(2) 1.737(2) 64.5

2.0901(12)−2.1423(12) 1.731 2.0709(13) 2.0267(12) 2.1736(12) 1.722 114.93(11) 177.4(1) 1.9644(12) 17.8

2.116(3)−2.185(3) 1.772 2.102(3) 2.092(3) 2.132(3) 1.678 115.2(3) 176.3(3) 2.6001(5) 68.8

syn-9 syn-2 anti-2

Table 3. Selected Bond Lengths (Å) and Angles (deg) for the η3-oIndMe Complexes

syn-3

syn-6

syn-7

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Scheme 3. Proposed Mechanism for the anti−syn Isomerization

Figure 5. First-order kinetic plots for the conversion of anti-2 to syn-2 in C6D6 at four different temperatures.

Table 4. Kinetic Data for the anti−syn Isomerization T 35 50 65 80

°C °C °C °C

(308.15 (323.15 (338.15 (353.15

K) K) K) K)

k (10−6 s−1)

t1/2 (h)

anti/syn ratio

1.43(3) 6.96(8) 51.8(5) 227(1)

134.4 27.6 3.7 0.8

10:90 8:92 8:92 9:91

nation of the pentadienyl ligand is comparable (ΔH⧧ = 22 kcal mol−1).7d From a mechanistic point of view, experimental and theoretical studies on various metal−allyl complexes suggest that anti−syn isomerizations occur via an η1-intermediate, which connects the opposite isomers via a C−C bond rotation.34,35 However, starting from the kinetic product of the ligand addition to 1, anti-[(η5-C5Me5)Fe(η3-oIndMe)(L)] in the exo conformation (highlighted in gray in Scheme 3), two 16-electron η1-intermediates can be envisaged: The first, intermediate A1 (Scheme 3), can be expected to be thermodynamically favored and, therefore, more easily accessible than B1, because it is less sterically congested and additionally has the maximum conjugation in a styrene-type πsystem. However, the anti orientation of the methyl and the phenyl groups is preserved, and rotation around the C1−C2 bond, followed by η1−η3 interconversion, leads to A2, now adopting an endo conformation. If a concomitant rotation around the Fe−C1 bond takes place,30e,34a A3 is formed, which is indistinguishable from the original complex, although the two hydrogen atoms at C1 have formally exchanged their positions. In contrast, pathway B is productive, because rotation around the C2−C3 bond is possible and finally results in the syn isomer B2 or B3, depending on the thermodynamic preference for either the exo or the endo conformation. Unique for pathway B is the possibility of stabilizing the 16-electron η1-intermediate (or transition state) B1 as an 18-electron η 3 -benzyl intermediate B1′, which still allows for productive rotation around C2−C3. As an alternative to the η3−η1−η3 switch for the η3-allyl to η3-benzyl interconversion, a concerted mechanism proceeding directly to B1′ can also be envisaged. It is worth mentioning that stable iron η3-benzyl complexes have been isolated and their fluxional process studied.21,36 If an electron-deficient 16-electron intermediate (or transition state), such as A1 or B1, is involved, the choice of the ligand L should substantially affect the stability and, therefore, the rate of isomerization.35,37 Higher stability can be anticipated if strong σ-donor ligands are used, whereas good π-acceptors, such as CO, should lead to higher activation barriers and consequently

slower rates, while steric effects are also likely to play an important role. We then turned our attention to N-heterocyclic carbenes (NHCs), which are strong σ-donor ligands with a diminished π-acceptor capability (relative to carbon monoxide) associated with a high-lying p(π) orbital;38 in some cases, they have even been considered as pure donor ligands.39 The reaction of 1,3,4,5-tetramethylimidazolin-2-ylidene (:C[N(Me)C(Me)]2, IMe) with [(η 5 -C 5 Me5 )Fe(η 5-oIndMe )] (1) in pentane immediately gave a red solution, and crystallization was achieved upon storage at −30 °C. The molecular structure was determined by X-ray diffraction analysis and is shown in Figure 6. As in the reaction with CO, hapticity interconversion was induced by the coordination of the carbene to iron; the corresponding Fe−C bond distance is at 1.9644(12) Å, in agreement with [(η5-C5H5)FeI(CO)(IMes)] (1.980(5) Å, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene),40 but shorter than that in NHC adducts of [Fe{N(SiMe3)2}2].41 Surprisingly, the X-ray study shows that the anti−syn isomerization has already taken place at room temperature, resulting in syn-[(η5-C5Me5)Fe(η3-oIndMe)(IMe)] (syn-3). In contrast to syn-2, an exo conformation is adopted, most probably for steric reasons. NMR spectroscopy, however, did not indicate the presence of only one isomer in solution. Instead, both syn-3 and anti-3 are found in a ratio of ca. 68:32 as judged by the integration of the corresponding methyl groups; the resonances for the C5Me5 ligands are not sufficiently resolved for individual integration. Nevertheless, two-dimensional NMR spectroscopy helps to distinguish the resonances assigned to the major (syn4485


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P{1H} NMR spectrum, which exactly matches the reported shift for [(η5-C5Me5)Fe(η3-C3H5)(PMe3)],42 and couplings to the 31P nucleus in the 1H and 13C NMR spectra. Additionally, the 1H NMR spectrum shows three resonances in a 2:2:1 ratio in the aromatic region, confirming the presence of an uncoordinated phenyl group. Supported by 1H−1H NOESY NMR spectroscopy and also by the upfield shift for the H3 hydrogen atom to 0.79 ppm, the product of the reaction is identified as syn-[(η5-C5Me5)Fe(η3-oIndMe)(PMe3)] (syn-4).43 Clearly, the kinetic product, anti-4, quickly rearranges to the syn isomer at room temperature, and syn-4 is almost exclusively observed (Scheme 2).44 If the 31P NMR spectrum is directly recorded after mixing 1 and PMe3, an additional resonance at 25.1 ppm is found, which can be tentatively assigned to anti-4, because it disappears with time, while the resonance at 32.8 ppm increases. It should be noted that only slow conversion was observed at room temperature for the analogous ruthenium complex, and a rate constant of 6.57(2)·10−6 s−1 was deduced from a kinetic study at 50 °C.6 The higher rate observed for the Fe−PMe3 complex, and also for its CO analogue (vide infra), compared with that for the corresponding ruthenium complexes agrees with the general trend that first-row transition metals generate weaker metal−ligand bonds.45 Since η 1 -oInd Me intermediates are proposed in the mechanism of the anti-to-syn isomerization (Scheme 3), the isolation of such species was desirable. Therefore, [(η5C5Me5)Fe(η5-oIndMe)] (1) was treated with an excess (20 equiv) of PMe3 to enforce the formation of [(η5-C5Me5)Fe(η1oIndMe)(PMe3)2]. However, the formation of this complex could not be detected by NMR spectroscopy, and only the resonances associated with syn-4 were found. Bis(dimethylphosphino)ethane (dmpe) can be expected to be of similar size to two PMe3 ligands, but the chelate effect should facilitate the coordination of the second phosphorus atom as soon as the η3-oIndMe complex is formed. Indeed, the reaction of 1 with 1 equiv of dmpe instantaneously gave a brown solution, which eventually turned red (Scheme 5). Crystal-


3) or minor (anti-3) isomers. This is particularly useful for the allyl moiety, because exchange peaks in the 1H−1H NOESY NMR spectrum give further evidence for the proposed anti-tosyn isomerization mechanism (Scheme 3). Consistent with the relatively slow exchange rates at room temperature, no significant line broadening is observed in the standard 1H NMR spectrum. Exchange peaks for anti-H1/syn-H1 and antiH1′/syn-H1′ (Scheme 4, the apostrophe denotes the minor Scheme 4. Relevant 1H−1H NOESY Exchange Peaks for the Major, syn-3, and the Minor Isomer (anti-3)

Scheme 5. Synthesis of the η1-oIndMe Complex 5 isomer) support the unproductive pathway A (Scheme 3) that does not lead to the opposite isomer, but represents an (indistinguishable) exchange of both hydrogen atoms at C1. The most important exchange peak is anti-H3/syn-H3′; this exchange stems from the productive anti-to-syn, and vice versa, isomerization, which requires pathway B. Additionally, a second slow fluxional process can be identified: The methyl substituents on the nitrogen atoms of IMe give two separate resonances for each isomer in the 1H NMR spectrum, which are all interconnected by exchange peaks in the 1H−1H NOESY spectrum, consistent with hindered carbene rotation at room temperature. To underline further that the choice of the ligand L has an influence on the rate of isomerization, we have used trimethylphosphine (PMe3) as a third example. Reaction of 1 with 1 equiv of PMe3 resulted in an immediate color change from green to red-brown. After stirring for 30 min, the solvent was removed, and a brown oil was obtained, which resisted further purification and crystallization because of its extremely high solubility even in hexamethyldisiloxane. However, coordination of PMe3 is clearly evidenced by a downfield shift from −61.9 ppm for the free phosphine to 32.8 ppm in the

lization of the highly soluble product from pentane/ hexamethyldisiloxane afforded red crystals of [(η5-C5Me5)Fe(η1-oIndMe)(dmpe)] (5). In contrast to 1−4, the hydrogen atoms at C1 give rise to only one resonance at 1.22 ppm in the 1 H NMR spectrum, which suggests that they are bonded to an sp3-hybridized carbon atom; otherwise, a shift to lower field would be expected. Instead, a low-field resonance at 6.32 ppm can be assigned to H3. Furthermore, the corresponding resonances in the 13C NMR spectrum for C1 and C3 are found at 10.3 and 115.6 ppm, respectively, which indicates that the open indenyl ligand in 5 is bound to iron via C1 rather than C3. This is further substantiated by an X-ray diffraction study that establishes a Fe−C1 bond length of 2.160(2) Å and also an 4486


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Table 5. M06-L Energies and Enthalpies (in kcal mol−1) for 2, 3, and 4a

anti orientation of the phenyl and the methyl group across the C2−C3 double bond (Figure 7). The formation of 5 can be

formation of anti formation of syn

complex

L

ΔEel

ΔE298

ΔH298

ΔG298

(exo)b (exo) (exo) (exo)

CO IMe PMe3 CO CO

−33.3 −33.3 −13.0 −32.9 −35.6

−30.7 −30.7 −10.5 −30.0 −33.1

−31.2 −31.3 −11.1 −30.6 −33.7

−22.0 −17.5 1.9 −20.9 −24.1

IMe IMe PMe3 PMe3 L

−31.7 −28.8 −14.2 −7.5 ΔΔEel

−28.8 −26.5 −11.6 −4.9 ΔΔE298

−29.4 −27.1 −12.2 −5.5 ΔΔH298

−14.9 −14.7 1.0 7.8 ΔΔG298

−2.3 1.6 −1.3

−2.5 1.9 −1.1

−2.5 1.9 −1.1

−2.1 2.6 −0.9

2 3 4 2 2 3 3 4 4

anti-to-syn isomerizationc

(endo)b (exo)b (endo) (exo) (endo) complex 2 3 4

CO IMe PMe3

ΔEel: zero-point uncorrected energies. ΔE298: relative energies at 298 K. ΔH298: enthalpies at 298 K. ΔG298: Gibbs free energies at 298 K. b X-ray data used as start geometry. cCalculated for the most stable conformer. a

Figure 7. ORTEP diagram of one of the two independent molecules of 5 with thermal displacement parameters drawn at 50% probability. Selected bond lengths (Å) and angles (deg): Fe−C(C5Me5) 2.114(2)−2.135(2), Fe−C1 2.160(2), Fe−P1 2.1761(7), Fe−P2 2.1548(6), C1−C2 1.460(3), C2−C3 1.355(3), C3−C4 1.473(3); P1−Fe−C1 95.71(6), P1−Fe−P2 83.29(2), P2−Fe−C1 87.09(6). Values for the second molecule are similar (see the Supporting Information for details). A least-squares fit of both molecules gave an rms deviation of 0.16 Å.

for prolonged times. In line with the experimental results, the endo conformation is only more stable for syn-2, while the exo conformation is preferred for syn-3 as well as for syn-4. The ΔΔ values (Table 5) give information about the energy differences between the anti and syn isomers and, therefore, about the equilibrium constants. In 2 and 4, an equilibrium between anti and syn isomers is experimentally observed, favoring the anti isomer; this result is at least qualitatively consistent with the DFT calculations. However, in agreement with the experiment, 3 behaves differently, because the ΔΔG298 value is endergonic with +2.6 kcal mol−1. Despite the importance of anti−syn isomerizations in allyl chemistry,34 this fluxional behavior was addressed in only few computational studies involving group 10 transition metal−allyl complexes (M = Ni, Pd, Pt),34j,35 but similar investigations with iron or ruthenium remain, to the best of our knowledge, unprecedented. For this study, we have chosen the CO complex 2, [(η5-C5Me5)Fe(η3-oIndMe)(CO)], for which we have acquired the most detailed experimental and structural data (vide supra). As mentioned above (Scheme 3), the starting point of the isomerization is anti-2 in the exo conformation (Figure 8), which is the kinetic product from the reaction of 1 with CO. The productive pathway requires breaking the Fe− C1 rather than the Fe−C3 bond, leading to an η1-benzyl transition state (TS-1). Although we were not able to locate TS-1 on the potential energy surface, the identification of TS-3 (vide infra) suggests its existence.48 The second step represents the η1−η3 shift to the first intermediate (IM-1), an η3-benzyl complex with Fe−C bond lengths of 2.063, 2.130, and 2.303 Å for C3, C4, and C5, respectively; the C5−C4−C3−C2 torsion angle is 51.6° (Figure 9). IM-1 is equivalent to the intermediate B1′ proposed in Scheme 3 and is also in accord with the stability of iron−benzyl complexes.21,36 Rotation around the uncoordinated C2−C3 bond has a comparatively small energy barrier (ΔΔG298 = 6.1 kcal mol−1) with respect to IM-1 and changes the relative positioning of the methyl and phenyl groups from anti to syn. A potential energy surface scan (PES) connects IM-1 through the transition state TS-2 with the synη3-benzyl intermediate (IM-2), and their freely optimized structures are displayed in Figure 9. The final change from η3-

rationalized by pathway A (Scheme 3), which is unproductive with respect to the anti-to-syn isomerization, but results in the less sterically encumbered η1-oIndMe complex with η1coordination via C1. The coordination around iron is best described as a distorted three-legged piano-stool geometry with the chelating phosphine occupying two positions. The Fe−P bond distances, at 2.1761(7) and 2.1548(6) Å, resemble those found in other bis(phosphino) iron complexes, for example, 2.225(2) Å in [(η 3 -C 5 H 7 ) 2 Fe(depe)] (depe = bis(diethylphosphino)ethane) or 2.176(3) and 2.179(3) Å in [(η5-C5H4)SiMe2(η1-C5H4)Fe(dmpe)].46 DFT Studies. To shed further light on the formation of [(η5-C5Me5)Fe(η3-oIndMe)(L)] complexes (2: L = CO; 3: L = IMe; 4: L = PMe3) and their anti-to-syn isomerization, density functional theory (DFT) calculations were performed employing the M06-L functional.47 First, the molecular structures of 1, anti-2, syn-2, and syn-3 were optimized; the calculated structural parameters agree very well with the experimentally derived data (see the Supporting Information for details) and served as starting points for the calculation of the remaining anti and syn complexes (Table 5). Since, for syn-2, the endo conformer was unexpectedly observed (vide supra), the syn isomers were calculated in both the exo and the endo conformations. Table 5 lists the thermodynamic data for the adduct formation from 1 and the corresponding free ligand L. In all cases, breaking the comparatively labile Fe−C4 and Fe−C5 bonds in [(η5C5Me5)Fe(η5-oIndMe)] (1), followed by Fe−L bond formation, is exothermic, and a substantial contribution probably comes from the rearomatization of the phenyl group. The PMe3 complexes 4, however, were calculated to be significantly less stable, which agrees with the experimental observation that 4 loses PMe3 and reforms 1 when heated to 65 °C under vacuum 4487


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endo interconversions for d6 complexes also proceed via the η3−η1−η3 pathway and can, therefore, accompany the anti-tosyn isomerization, as observed for 3 and 4.30e,34a η5-to-η3 Hapticity Interconversion Induced by Oxidative Addition. For the ruthenium complex [(η5-C5Me5)Ru(η52,4-C6OH9)] (C6OH9 = dimethyloxapentadienyl), it was reported that the oxidative addition of SnCl4, I2, or CHCl3 results in Ru(IV) η3-oxapentadienyl complexes, whereas no reaction with CH3I was observed, even when a 100-fold excess was applied under reflux.9 Since our earlier studies demonstrated a greater reactivity for the open indenyl ligand,6 the reaction of [(η5-C5Me5)Fe(η5-oIndMe)] (1) with CH3I was investigated. At room temperature, no immediate reaction similar to that with PMe3 or CO was observed, but heating of 1 with 2 equiv of methyl iodide in hexane to 50 °C for 1 h yielded a dark red, paramagnetic solid after workup (Scheme 6). X-ray diffraction analysis revealed this compound to be syn-[(η5C5Me5)Fe(η3-oIndMe)(I)] (6), which was also confirmed by mass spectroscopy and elemental analysis (Figure 10). Figure 8. Reaction profile for the anti-to-syn isomerization of the CO complex 2; for exo,anti-2 and endo,syn-2, the calculated structures are very similar to those established by X-ray diffraction (Figures 3 and 4).

Scheme 6. Reaction of 1 with CH3I or CH2Cl2 Resulting in Complexes 6 and 7

Figure 9. Calculated structures for IM-1 and TS-2 (top row) and for IM-2 and TS-3 (bottom row).

benzyl to η3-allyl coordination is associated with two hapticity interconversions (η3−η1 and vice versa). Supported by intrinsic reaction coordinate (IRC) calculations, the singlet transition state (TS-3, equivalent to B1 in Scheme 3) with η1coordination shows that the iron atom stops at C3 and then moves on to the η3-coordination mode via C1−C3, affording syn-2 with an endo conformation. The energy difference between TS-3 and syn-2 amounts to ΔΔG = 34.2 kcal mol−1, and it can be anticipated that approximately the same activation energy is required for the η3−η1 hapticity interconversion on going from anti-2 to TS-1, which would then constitute the rate-determining step of the overall process. The involvement of η1-transition states nicely agrees with our observation that good σ-donor ligands, such as IMe or PMe3, support faster anti−syn isomerizations than π-acceptor ligands.35,37 Furthermore, our kinetic NMR study (vide infra) provides a smaller activation barrier (ΔG⧧298 = 26 kcal mol−1) than that derived computationally, probably resulting from stabilizing solvent effects, in particular, for η1-intermediates, which are not properly addressed by calculations under pseudo-gas-phase conditions. As a final comment, it should be noted that exo−


Similarly, dichloromethane does not behave as an inert solvent toward 1. A gradual color change from green to red occurs when 1 is dissolved in CH2Cl2, and the product was identified as syn-[(η5-C5Me5)Fe(η3-oIndMe)(Cl)] (7). Both 6 and 7 are rare examples of iron(III) allyl complexes with an electron count of 17 and formally stem from a one-electron oxidation of Fe(II). Similarly to the reaction of [(η5C5Me5)Fe(η3-C3H5)(C2H4)] and allyl chloride, which delivered [(η5-C5Me5)Fe(η3-C3H5)(Cl)] and hexadiene,49 the formation of 6 and 7 presumably also involves a radical process to give ethane and 1,2-dichloroethane as the byproducts. The molecular structure of 7 was established by X-ray diffraction analysis, and an ORTEP diagram is shown in Figure 11, while pertinent bonding parameters are listed in Table 3. 4488


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electron complex 6; the slightly higher value in comparison to the spin-only value (μeff ≈ 1.73 μB) can be ascribed to the contribution of some spin−orbit coupling. Competition Experiment: η5-to-η3 Hapticity Interconversion in an Indenyl Iron Open Indenyl Complex. Thus far, we have only studied the η5−η3 hapticity interconversion in [(η5-C5Me5)M(η5-oIndMe)] complexes (M = Fe, Ru). In these systems, the open indenyl ligand is much more likely to switch to an η3-coordination mode than the strongly bound pentamethylcyclopentadienyl ligand, although η3-Cp complexes are known, but rare.51 The question remains as to how the oIndMe ligand would behave in comparison to an indenyl ligand in an internal competition experiment. To investigate this, a complex of the type [(η5-Ind)M(η5-oIndMe)] is required. Consequently, the preparation of [(η5-C9H7)Fe(η5-oIndMe)] was attempted by the sequential addition of Na(C9H7) and K(oIndMe) to [FeI2(thf)2] at −78 °C, mimicking the procedure successfully applied for the synthesis of 1. Unfortunately, bis(indenyl)iron was the only isolated product in several attempts.25b We attribute this fact to the inherent instability of the half-sandwich intermediate [(η5-C9H7)FeI], and its direct dismutation to [(η5-C9H7)2Fe] and [FeI2(thf)2].52 However, iron half-sandwich complexes can be stabilized by bulky cyclopentadienyl ligands,19 and a similar concept should also be applicable to indenyl ligands. Recently, we have evaluated the size of differently substituted cyclopentadienyl and indenyl ligands using cone angle measurements on [(η7-C7H7)Zr(η5Cp)] and [(η7-C7H7)Zr(η5-Ind)] complexes.53 These studies have provided a valuable tool to assess the steric demand of these π-ligands and gave a cone angle of 102° for C9H7, whereas 122° was found for C5Me5. This implies that the unsubstituted indenyl ligand is significantly smaller than C5Me5, which is probably the reason for the instability of [(η5C9H7)FeI] versus [(η5-C5Me5)FeI] even at low temperatures. 1,3-Disubstitution of C9H7 with tert-butyl groups increases the cone angle to 131°, and the corresponding (1,3-di(tertbutyl)indenyl)sodium, Na(Ind″), can be obtained by an improved protocol from indene in three steps.53 Thus, the reaction of [FeI2(thf)2] with 1 equiv of Na(Ind″) at −78 °C afforded a red solution, which was subsequently reacted with 1 equiv of K(oIndMe). Because of the extremely high solubility of the product, we refrained from a full characterization and directly added carbon monoxide (Scheme 7). NMR spectroscopy indicates the formation of syn-[(η5-Ind″)Fe(η3oIndMe)(CO)] (9), since, for instance, only one resonance, at 7.30 ppm, for the ortho-hydrogen atoms of the phenyl ring attached to oIndMe is observed in the 1H NMR spectrum. Furthermore, the high-field shift of 0.22 ppm for H3 is typical for anti-H atoms and, consequently, leads to the assignment of a syn orientation for the phenyl group. As expected for a molecule with C1 symmetry, all positions of the Ind″ ligand are inequivalent, giving rise to seven peaks in the 1H NMR spectrum. Complex 9 is one of the very few metal complexes involving the 1,3-di(tert-butyl)indenyl ligand,53,54 and the first example for iron.55 The synthesis of 9 clearly demonstrates that stabilization of a half-sandwich intermediate can be achieved by taking advantage of sterically encumbered indenyl ligands, although low temperatures are required,56 since otherwise bis(indenyl)iron formation takes place.57 In addition, the more ready tendency of the open indenyl ligand, rather than an indenyl ligand, to undergo an η5−η3 rearrangement is illustrated. Similar to the arguments applied for comparatively facile hapticity interconversions of pentadienyl versus cyclo-


Besides [(η5-C5Me5)Fe(η3-C3H5)(X)] (X = Cl, CH3),49 6 and 7 represent, to the best of our knowledge, the only other structurally characterized Fe(III) allyl complexes to date. In both cases, isomerization has already taken place, because the phenyl group is found in the syn orientation with respect to the methyl group. In contrast to [(η5-C5Me5)Fe(η3-C3H5)(CH3)], the allyl ligand adopts an endo conformation, which results in large interplanar angles of τ = 68.8° and 63.0° for 6 and 7, respectively. For the former, the larger atomic radius of the iodide substituent causes a greater shift of the allyl ligand and is also reflected in the longer Fe−X bond distance (Fe−I = 2.6001(5) Å) in comparison to that in the chloride derivative (Fe−Cl = 2.3087(8) Å). The Fe−C bond lengths resemble those found in the Fe(II) complexes 2 and 3. Indeed, structures 7 and syn-2 are isotypic, whereby the chloro and carbonyl ligands play a similar structural role as acceptors of a “weak” hydrogen bond from C6−H6, with H6···Cl = 2.76 Å in 7 and H6···O = 2.49 Å in syn-2. Structure 6 is, however, not isotypic to 7 and syn-2. The iodo complex 6 was selected for solid-state magnetic susceptibility measurements using the “quartz tube technology”, which is particularly useful for highly air-sensitive compounds.50 Curie behavior is observed, because the mangnetic moment is reasonably temperature-independent throughout the measured range of 2−300 K, and a plot of χ−1 vs T gives a straight line (Figure 12). The magnetic moment of μeff ≈ 2.0 μB is consistent with one unpaired electron (S = 1/2) in the 17-

Figure 12. Plot of μeff and χ−1 vs T for [(η5-C5Me5)Fe(η3-oIndMe)(I)] (6). 4489


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Scheme 7. One-Pot Synthesis of syn-[(η5-Ind″)Fe(η3oIndMe)(CO)] (syn-9)

modes. Consequently, 1 can easily provide an empty coordination side, which was probed by the addition of CO, PMe3, and IMe to afford the cyclopentadienyl−allyl iron species 2−4. The resulting complexes undergo anti-to-syn isomerization with the rate depending on the σ-donor/πacceptor capability of the additional ligand. As expected, this process is slowest for the carbonyl complex 2, and a kinetic study afforded an enthalpy of activation of ΔH⧧ = 24(1) kcal mol−1. DFT calculations suggest that this isomerization proceeds via consecutive η3−η1−η3 interconversions and involves η3-benzyl intermediates. Stabilization of an η1-allyl complex could be achieved by addition of dmpe to 1. This flexibility shows that the open indenyl ligand is a pentadienyl ligand with enhanced reactivity and, therefore, nicely adds to the growing family of unconventional open cyclopentadienyl ligands.8,58 Future work aims at exploiting this concept for the preparation of labile transition-metal complexes for potential use in homogeneous catalysis.

■

EXPERIMENTAL PROCEDURES

All synthetic and spectroscopic manipulations were carried out under an atmosphere of purified nitrogen, either in a Schlenk apparatus or in a glovebox. Solvents were dried and deoxygenated either by distillation under a nitrogen atmosphere from sodium benzophenone ketyl (THF) or by an MBraun GmbH solvent purification system (all other solvents). NMR spectra were obtained on a Bruker Avance II 600, a Bruker DRX 400, a Bruker Avance III 400, or a Bruker Avance II 300 spectrometer at 600, 400, or 300 MHz (1H); 151, 101, or 75 MHz (13C); and 162 or 121 MHz (31P). Unless stated otherwise, the spectra were recorded at ambient temperature using C6D6 as solvent. Its residual solvent signal was used as a chemical shift reference (δH = 7.16) for the 1H spectra and the solvent signal (δC = 128.04 ppm) for the 13C spectra. 31P spectra were referenced to virtual external 85% phosphoric acid (δP = 0). The number of protons attached to each carbon was determined by 13C-DEPT135 experiments. If required, signal assignment was achieved by two-dimensional H,H-COSY, H,HNOESY, H,C-HSQC, and H,C-HMBC spectra. They were recorded using standard Bruker pulse programs; sweep widths, digital resolution, and pulse delays were optimized for the samples under investigation. Mixing times of 1000 and 2000 ms were used for the H,H-NOESY experiments. For the low-temperature measurements of 1, the temperature display of the spectrometer was calibrated against the standard methanol sample (4% methanol in methanol-d4).59 Bandshape analysis of the low-temperature 1H NMR spectra of 1 was performed with the DNMR module implemented in Bruker TopSpin2.1pl6.20 The quality of the iterative analysis is defined by the program parameter “maximum overlap”, for which values between 97.8 and 98.4% could be achieved. A Bruker Vertex 70 spectrometer was used for recording IR spectra. Elemental analyses were performed by combustion and gas chromatographical analysis with an Elementar varioMICRO instrument. [FeI2(thf)2],60 Li(C5Me5), Na(C5Me5),61 Na(Ind″),53 and K(oIndMe)6 were prepared according to the literature; Li(C5Me5) was synthesized from equimolar amounts of n-butyllithium and pentametylcyclopentadiene in pentane.62 All other reagents were obtained commercially and used as received. X-ray Diffraction Studies. Data were recorded at 100 K on Oxford Diffraction diffractometers using monochromated Mo Kα or mirror-focused Cu Kα radiation (Table 6). The structures were refined anisotropically using the SHELXL-97 program.63 Hydrogen atoms were either (i) located and refined isotropically (H1A, H1B, H3, and for 1 H5; in some cases, with constraints to C−H bond lengths); (ii) included as idealized methyl groups allowed to rotate, but not tip; or (iii) placed geometrically and allowed to ride on their attached carbon atoms. Compound 5 crystallized with two independent molecules per asymmetric unit. Theoretical Calculations. All computations were performed using the density functional method M06-L as implemented in the Gaussian

pentadienyl ligands,7b,d this behavior can be attributed to the smaller loss of resonance energy for oIndMe. The identity of syn-9 was unambiguously confirmed by X-ray diffraction analysis (Figure 13), which also shows the allyl

Figure 13. ORTEP diagram of syn-9 with thermal displacement parameters drawn at 50% probability; the hydrogen atoms of the tertbutyl groups have been omitted for clarity.

moiety in an endo conformation, similar to syn-2. Other bonding parameters differ only marginally from those of syn-2 (Table 3). The fold angle for the Ind″ ligand is, at 4.6°, less pronounced than that for oIndMe in 1, but the Fe−C14 and Fe−C15 bond distances are still slightly longer than the other three.

■

CONCLUSION This study provides further evidence for the ability of the phenylmethallyl ligand oIndMe to coordinate to transition metals as an η5-bound open indenyl ligand, as shown by the isolation of the half-open ferrocene [(η5-C5Me5)Fe(η5oIndMe)] (1). In contrast to the corresponding half-open ruthenocene,6 1 shows fluxional behavior already at room temperature on the NMR time scale, indicating that the oIndMe ligand can easily shuttle between the η5- and η3-coordination 4490


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Table 6. Crystallographic Data 1 empirical formula formula weight temperature (K) wavelength λ (Å) cryst syst space group a [Å] b [Å] c [Å] α [°] β [°] γ [°] volume [Å3] Z reflns collected independent reflns goodness of fit on F2 ρcalcd [g cm−3] μ [mm−1] R(Fo), [I > 2σ(I)] Rw (Fo2) Δρ [e Å−3]

anti-2

syn-2

syn-3

syn-6

5

syn-7

syn-9

C20H26Fe

C21H26OFe

C21H26OFe

C27H38N2Fe

C26H42P2Fe

C20H26IFe

C20H26ClFe

C28H34OFe

322.26 100(2) 1.54184

350.27 100(2) 1.54184

350.27 100(2) 1.54184

446.44 100(2) 0.71073

472.39 100(2) 0.71073

449.16 100(2) 1.54184

357.71 100(2) 0.71073

442.40 100(2) 1.54184

monoclinic P21/c 13.5328(8) 6.8730(4) 17.8642(12) 90 104.134(6) 90 1611.27(17) 4 22 699 3319 [Rint = 0.0567] 1.042

monoclinic P21/n 10.0070(2) 16.1642(2) 10.9946(2) 90 96.813(2) 90 1765.87(5) 4 28 820 3678 [Rint = 0.0349] 1.113


monoclinic P21/n 14.6168(2) 8.2424(2) 19.4457(4) 90 91.592(2) 90 2341.86(8) 4 92 882 5363 [Rint = 0.0344] 1.047

triclinic P1̅ 9.8454(2) 15.7886(4) 16.6900(4) 76.568(2) 85.622(2) 87.456(2) 2515.10(10) 4 87 159 11 098 [Rint = 0.0639] 1.026




1.328 7.402 0.0369

1.317 6.843 0.0254

1.294 6.720 0.0342

1.266 0.660 0.0261

1.248 0.737 0.0401

1.630 19.785 0.0363

1.324 0.984 0.0475

1.257 5.279 0.0321

0.0990 0.623/−0.314

0.0709 0.235/−0.404

0.0887 0.551/−0.223

0.0672 0.315/−0.273

0.0885 0.573/−0.255

0.0957 1.873/−0.946

0.1198 0.914/−0.423

0.0789 0.382/−0.329

09 program.47,64 For all main-group elements (C, H, N, O, and P), the all-electron triple-ζ basis set (6-311G**) was used,65 whereas for iron, a small-core relativistic ECP together with the corresponding double-ζ valence basis set was employed (Stuttgart RSC 1997 ECP).66 [(η5-C5Me5)Fe(η5-oIndMe)] (1). [FeI2(thf)2] (2.0 g, 4.407 mmol) was completely (!) dissolved in THF (60 mL); sometimes, ultrasound and heating were employed to accelerate the dissolution process. The brown solution was cooled to −78 °C, efficiently (!) stirred, and a pale yellow suspension of Li(C5Me5) (0.627 g, 4.407 mmol) in THF (30 mL) was slowly added with a syringe. The resulting yellow solution was stirred for 10 min at −78 °C, followed by the slow addition of K(oIndMe) (0.750 g, 4.407 mmol) in THF (15 mL), which resulted in a color change to brown. During the slow warm-up, the color turned to green, and the solvent was removed after 3.5 h. Pentane (∼100 mL) was added, and the mixture was cooled to −30 °C. While still cold, filtration through a pad of Celite gave a green solution, which was concentrated until saturation was achieved. The solution was transferred to the freezer (−30 °C), which resulted in the crystallization of large, round, green plates (0.794 g); further concentration of the mother liquor afforded a second crop. Total yield after drying: 0.914 g (64%). Single crystals were obtained from a concentrated pentane solution at −20 °C. 1 H NMR (400 MHz): δ = 6.94 (m, 2H, m-phenyl), 6.85 (m (“t”), splitting 7.2 Hz, 1H, p-phenyl), 5.60 (s, 1H, H3), ≈ 4.62 (v br s, ν1/2 ≈ 340 Hz, 2H, o-phenyl), 2.56 (d, Janti‑H1,syn‑H1 ≥ 3.4 Hz, 1H, syn-H1), 1.91 (s, 3H, CH3), 1.41 (s, 15H, C5Me5), −0.46 (d, 1H, anti-H1). 13C NMR (101 MHz): δ = 133.2 (br, m-phenyl) 120.7 (p-phenyl), 103.2 (C4), 95.9 (C2), 81.8 (C3), 79.7 (C5Me5), 43.4 (C1), 25.2 (2-CH3), 9.9 (C5Me5), the o-C atoms of the phenyl ring were not observed. 1H NMR, 13C NMR (400 and 101 MHz, respectively, THF-d8): see Tables 1 and 2 in the Results and Discussion section. Anal. Calcd for C20H26Fe: C, 74.54; H, 8.13. Found: C, 74.20; H, 8.20. anti-[(η5-C5Me5)Fe(η3-oIndMe)(CO)] (anti-2). Compound 1 (0.170 g, 0.528 mmol) was dissolved in pentane (10 mL). CO was then passed through the solution for half a minute, which resulted in a color change to orange. The solution was kept under CO for 20 min, the solvent was removed, and the resulting orange-red oil was

redissolved in a minimum amount of pentane. Filtration and crystallization at −30 °C afforded an orange-red solid (0.116 g, 63%). Single crystals were obtained from a concentrated pentane solution at −15 °C. 1 H NMR (400 MHz): δ = 7.09−7.03 (m, 2H, phenyl), 6.94−6.87 (m, 3H, phenyl), 4.10 (s, 1H, H3), 2.27 (m, 1H, syn-H1), 1.91 (s, 3H, CH3), 1.83 (d, 2JHH = 2.0 Hz, 1H, anti-H1), 1.54 (s, 15H, C5Me5). 13 C{1H} NMR (101 MHz): δ = 223.8 (CO), 149.5 (ipso-phenyl), 128.6 (phenyl), 125.1 (phenyl), 123.5 (phenyl), 89.9 (C5Me5), 87.3 (C2), 58.2 (C3), 39.3 (C1), 25.8 (CH3), 10.2 (C5Me5). Anal. Calcd for C21H26FeO: C, 72.01; H, 7.48. Found: C, 71.93; H, 7.41. IR (ATR): ν(CO/cm−1) = 1924. syn-[(η5-C5Me5)Fe(η3-oIndMe)(CO)] (syn-2). Compound 1 (0.123 g, 0.382 mmol) was dissolved in hexane (15 mL). CO was then passed through the solution for half a minute. The resulting orange solution was refluxed overnight, and the solvent was removed, which gave a slightly oily, orange solid. Recrystallization from a filtered pentane solution at −30 °C afforded orange-red crystals (0.076 g, 57%). 1 H NMR (400 MHz,): δ = 7.48 (m (“d”), splitting 7.7 Hz, 2H, ophenyl), 7.18−7.14 (m, 2H, m-phenyl), 7.06 (m (“t”), splitting 7.5 Hz, 1H, p-phenyl), 2.46 (s, 1H, syn-H1), 2.00 (s, 3H, CH3), 1.85 (s, 1H, H3), 1.43 (s, 15H, C5Me5), 0.40 (s, 1H, anti-H1). 13C{1H} NMR (101 MHz): δ = 220.1 (CO), 142.6 (ipso-phenyl), 130.8 (phenyl), 128.0 (phenyl), 125.5 (phenyl), 108.0 (C2), 91.9 (C5Me5), 65.6 (C3), 42.6 (C1), 21.4 (CH3), 10.0 (C5Me5). Anal. Calcd for C21H26FeO: C, 72.01; H, 7.48. Found: C, 72.29; H, 7.64. IR (ATR): ν(CO/cm−1) = 1876. [(η5-C5Me5)Fe(η3-oIndMe)(IMe)] (3). Tetramethylimidazolin-2-ylidene (IMe) (0.047 g, 0.372 mmol) was dissolved in pentane (10 mL), and 1 (0.120 g, 0.372 mmol) in pentane (6 mL) was added. The color changed immediately to red, and the solvent was removed after 30 min of stirring. The red, voluminous solid was redissolved in a minimum volume of pentane for crystallization at −30 °C, which gave dark red crystals overnight (0.101 g, 61%). Single crystals were grown from a concentrated pentane solution at −20 °C. 1 H NMR (400 MHz, resonances for the minor isomer are denoted with ′): δ = 7.48−7.42 (m, 2H, phenyl), 7.34−7.28 (m, 2H, phenyl), 4491


Organometallics

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6.78−6.71 (1H, m, phenyl′), 6.69−6.58 (2H, br m, phenyl′), 3.85 (s, 1H, H3′), 3.75 (s, 3H, N-CH3 NHC′), 3.67 (s, 3H, N-CH3 NHC), 3.67 (s, 3H, N-CH3 NHC), 3.06 (s, 3H, N-CH3 NHC′), 2.69 (s, 1H, syn-H1′), 2.58 (s, 3H, CH3), 2.43 (s, 3H, CH3′), 2.28 (s, 1H, syn-H1), 1.58 (s, C5Me5′), 1.57 (s, C5Me5), 0.42 (s, 1H, anti-H1′), −0.10 (s, 1H, H3), −0.65 (s, 1H, anti-H1), two resonances for the phenyl groups are hidden under the residual solvent peak, and the C-CH3 NHC groups overlap with the resonances for C5Me5. 13C{1H} NMR (101 MHz): δ = 204.8 (Fe-C), 202.4 (Fe-C′), 155.5 (ipso-phenyl′), 149.2 (ipsophenyl), 129.1 (phenyl), 128.0 (phenyl), 126.7 (phenyl′), 125.0 (Cq NHC), 124.9 (Cq NHC), 121.7 (phenyl), 119.9 (phenyl′), 88.7 (C2′), 86.3 (C2), 82.7 (C5Me5′), 82.2 (C5Me5), 52.1 (C3), 50.7 (C3′), 41.3 (C1′), 38.0 (C1), 36.7 (N-CH3 NHC), 36.4 (N-CH3 NHC), 36.2 (NCH3 NHC), 36.1 (N-CH3 NHC), 27.6 (CH3′), 20.7 (CH3), 11.0 (C5Me5′), 10.6 (C5Me5), 9.4 (C-CH3 NHC), 9.3 (C-CH3 NHC), 9.1 (C-CH3 NHC), one phenyl′ is probably hidden under the solvent, and the two Cq NHC′ and one C-CH3 were not observed. Anal. Calcd for C27H38FeN2: C, 72.64; H, 8.58; N, 6.27. Found: C, 72.61; H, 8.58; N, 6.23. syn-[(η5-C5Me5)Fe(η3-oIndMe)(PMe3)] (syn-4). Trimethylphosphine (PMe3) (0.025 g, 0.326 mmol) dissolved in pentane (4 mL) was added to a pentane solution (10 mL) of 1 (0.100 g, 0.310 mmol). The resulting red-brown solution was stirred for 30 min at room temperature, and the solvent was subsequently removed under vacuum. The red-brown oil was used without further purification for NMR studies. 1 H NMR (400 MHz): δ = 7.57 (m (“d”), splitting 7.7 Hz, 2H, ophenyl), 7.24 (m (“t”), 2H, m-phenyl), 7.10 (m (“t”), splitting 7.3 Hz, 1H, p-phenyl), 2.65 (s, 3H, CH3), 1.85 (s, 1H, syn-H1), 1.46 (s, 15H, C5Me5), 0.92 (d, 2JPH = 6.4 Hz, PMe3), 0.79 (d, 3JPH = 18.1 Hz, 1H, H3), −0.47 (d, 3JPH= 22.5 Hz, 1H, anti-H1). 13C{1H} NMR (101 MHz): δ = 148.3 (ipso-phenyl), 129.9 (phenyl), 127.8 (phenyl), 122.9 (phenyl), 83.5 (C5Me5), 83.1 (C2), 44.2 (d, 2JPC = 5.5 Hz, C3), 30.9 (d, 2JPC = 10.8 Hz, C1), 23.3 (CH3), 18.5 (d, 1JPC = 22.1 Hz, PMe3), 10.8 (C5Me5). 31P{1H} NMR (162 MHz): δ = 32.8. [(η5-C5Me5)Fe(η1-oIndMe)(dmpe)] (5). Complex 1 (0.120 g, 0.372 mmol) was dissolved in pentane (8 mL). The addition of dmpe (0.056 g, 0.372 mmol) in pentane (3 mL) initially resulted in a brown solution, which eventually turned to red within 2 h. After solvent removal, the remaining red oil was taken up in a mixture of pentane and hexamethyldisiloxane, filtered, and stored at −30 °C overnight, which resulted in the formation of red crystals (0.106 g, 60%). Single crystals for X-ray diffraction analysis were obtained from a pentane solution at −20 °C. 1 H NMR (400 MHz): δ = 7.38−7.33 (m, 2H, phenyl), 7.29−7.23 (m, 2H, phenyl), 7.05 (m (“t”,) splitting = 7.3 Hz, 1H, p-phenyl), 6.32 (s, 1H, H3), 2.01 (s, 3H, CH3), 1.61 (s, 15H, C5Me5), 1.53−1.47 (m, 2H, P-CH2), 1.27−1.24 (m, 6H, P-CH3), 1.23−1.20 (m, 2H, H1), 1.19−1.11 (m, 2H, P-CH2), 1.05−1.00 (m, 6H, P-CH3). 13C{1H} NMR (101 MHz): δ = 158.8 (C2 or ipso-phenyl), 141.9 (C2 or ipsophenyl), 128.5 (phenyl), 128.3 (phenyl), 123.8 (phenyl), 115.6 (C3), 84.3 (C5Me5), 28.9 (P-CH2), 22.5 (P-CH3), 22.3 (CH3), 14.8 (PCH3), 11.1 (C5Me5), 10.3 (C1). 31P{1H} NMR (162 MHz): δ = 76.0. Anal. Calcd for C26H42FeP2: C, 66.10; H, 8.96. Found: C, 65.83; H, 8.93. syn-[(η5-C5Me5)Fe(η3-oIndMe)(I)] (6). Complex 1 (0.145 g, 0.450 mmol) was dissolved in hexane (10 mL), followed by the addition of CH3I (0.900 mmol, 0.56 mL). The color changed to red within 20 min upon stirring at 50 °C. After a total time of 1 h, the red solution was filtered, and the solvent was concentrated to ∼3 mL, accompanied by the precipitation of the product. The supernatant was removed, and the dark red microcrystalline solid was dried under vacuum (0.142 g, 72%). The EI mass spectrum showed a molecular ion at m/z = 449 amu. The parent ion isotopic cluster was simulated: (calcd %, observed %) 451 (4, 4), 450 (26, 26), 449 (100, 100), 448 (2, 2), 447 (7, 7). Anal. Calcd for C20H26FeI: C, 53.48; H, 5.83. Found: C, 53.64; H, 5.88. syn-[(η5-C5Me5)Fe(η3-oIndMe)(Cl)] (7). Dichloromethane (8 mL) was added to 1 (0.120 g, 0.372 mmol). The green solution gradually changed to red within 50 min at room temperature. Filtration and

solvent removal gave a slightly oily solid, which was layered with pentane (5 mL) and stored at −30 °C overnight. The supernatant was removed, and the remaining dark red solid was dried (0.075 g, 56%). The EI mass spectrum showed a molecular ion at m/z = 357 amu. The parent ion isotopic cluster was simulated: (calcd %, observed %) 360 (10, 10), 359 (36, 36), 358 (35, 35), 357 (100, 100), 356 (2, 2), 355 (7, 7). Anal. Calcd for C20H26ClFe: C, 67.15; H, 7.33. Found: C, 67.36; H, 7.35. syn-[(η5-Ind″)Fe(η3-oIndMe)(CO)] (9). [FeI2(THF)2] (1.0 g, 2.203 mmol) was completely (!) dissolved in THF (30 mL), and Na(Ind″) (0.552 g, 2.203 mmol) in THF (10 mL) was slowly added with a syringe at −78 °C. The resulting red solution was stirred for 10 min at −78 °C, followed by the slow addition of K(oIndMe) (0.375 g, 2.203 mmol) in THF (10 mL). During the slow warm-up, the color changed from brown to red, and the solvent was removed under vacuum. Extraction with pentane, followed by filtration, gave a red solution. CO was then passed through the solution for 1 min, which was not accompanied by any obvious color change. After stirring for 15 min under a CO atmosphere, the solvent was removed. Crystallization at −30 °C from a pentane/hexamethyldisiloxane mixture yielded 0.442 mg (45%) of a red-brown solid. Single crystals were obtained by cooling a concentrated hexamethyldisiloxane solution to −30 °C for several days. 1 H NMR (600 MHz): δ = 7.39 (m, (“d”), splitting 7.9 Hz, 2H, ophenyl), 7.22 (m (“d”), splitting 8.9 Hz, 1H, indenyl), 7.14−7.06 (m, 4H, m- and p-phenyl + indenyl), 6.71 (m, 1H, indenyl), 6.63 (m, 1H, indenyl), 4.87 (s, 1H, indenyl), 3.33 (s, 1H, syn-H1), 1.91 (s, 3H, CH3), 1.44 (s, 9H, tert-butyl), 1.18 (s, 9H, tert-butyl), 0.22 (s, 1H, H3), 0.15 (s, 1H, anti-H1). 13C{1H} NMR (151 MHz): δ = 222.2 (CO), 143.5 (ipso-phenyl), 129.6 (phenyl), 128.3 (phenyl), 126.5 (CH indenyl), 125.8 (phenyl), 125.7 (CH indenyl), 125.1 (CH indenyl), 124.7 (CH indenyl), 108.6 (C2), 107.9 (Cq indenyl), 104.8 (Cq indenyl), 97.8 (Cq indenyl), 95.6 (Cq indenyl), 84.6 (CH indenyl), 71.0 (C3), 50.3 (C1), 33.2 (Cq tert-butyl), 33.0 (Cq tert-butyl), 32.2 (tert-butyl), 31.8 (tert-butyl), 20.0 (CH3). Anal. Calcd for C28H34FeO: C, 76.01; H, 7.75. Found: C, 75.14; H, 7.75. IR (Nujol): ν(CO/cm−1) = 1909.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic information files (CIF), NMR spectra of 1, details of the kinetic NMR study of 2, details of the conversion of syn-4, details of the DFT calculations and mol2-files of all calculated structures, and comparison of experimental and theoretical structural data. 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] (M.D.W.), [email protected] (M.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Richard D. Ernst (University of Utah) for helpful discussions and Richard A. Andersen and Greg Nocton (University of California, Berkeley) for SQUID measurements. This work was supported by the Fonds der Chemischen Industrie (AG) and the Deutsche Forschungsgemeinschaft (DFG) through the Emmy-Noether program (MDW, WA 2513/2-1). A.G. gratefully acknowledges the German Academic Exchange Service for a travel scholarship. 4492


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Organometallics

Article

(31) Cheng, M.-H.; Wu, Y.-J.; Wang, S.-L.; Liu, R.-S. J. Organomet. Chem. 1989, 373, 119. (32) A closer inspection of the NMR spectra reveals that anti-2 is also present (

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