Mono- and Dinuclear Neutral and Cationic Iron(II) Compounds

Article pubs.acs.org/Organometallics

Mono- and Dinuclear Neutral and Cationic Iron(II) Compounds Supported by an Amidinato-diolefin Ligand: Characterization and Catalytic Application Crispin Lichtenberg,*,† Mario Adelhardt,‡ Michael Wörle,† Torsten Büttner,† Karsten Meyer,‡ and Hansjörg Grützmacher*,† †

Department of Chemistry and Applied Biosciences, ETH Zürich, 8093 Zürich, Switzerland Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University Erlangen-Nuremberg, 91058 Erlangen, Germany

‡

S Supporting Information *

ABSTRACT: A new amidinato-diolefin ligand is presented along with its sodium salt as a starting point for the preparation of iron(II) compounds. Rational synthetic approaches to cationic and dinuclear iron species have been established and allowed for the first isolation of an amidinato stabilized cationic Fe species. The versatility of the potentially tetradentate ligand system is mirrored by structural authentification of three different binding modes. These include rare examples of Fe(II)−olefin interactions in neutral compounds. A set of iron compounds was shown to be active as precatalysts in the synthesis of poly silylethers by dehydrogenative alcoholysis of PhSiH3. The most active catalyst was generated from a dinuclear iron compound. Characterization techniques include NMR, IR, and Mössbauer spectroscopy, single crystal X-ray analysis, magnetic susceptibility measurements, and mass spectrometry.

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INTRODUCTION

We became interested in amidine ligands with olefin functionalities as additional donors. An olefin ligand in the coordination sphere of a metal center can serve as a σ-donor and as a π-acceptor. Bonding situations from purely sigma type (olefin) → M interactions all the way to metallacyclopropanes can be realized.9 These donor/acceptor properties depend on and respond to the nature and the electronic character of the organometallic fragment interacting with the olefin. Thus, the olefin can act as an “electron buffer” in the course of a stoichiometric or catalytic transformation. These characteristics make olefins attractive candidates for the application as steering ligands in catalysis, but they are underdeveloped compared to the plethora of nitrogen and phosphorus based ligand sets.10 Ligands bearing the olefinic trop (trop = 5H-dibenzo[a,d]cyclo-hepten-5-yl; cf., Scheme 1) substituent have been shown to stabilize metal centers with low oxidation states in unusual coordination geometries and bonding situations due to their electronic and steric properties.11,12 Herein, we report the synthesis of a new trop-based chelating amidine-diolefin ligand, mono- and dinuclear iron complexes thereof including rare examples of [Fe(II)(halide)]−(olefin) interactions, along with

Amidinates have emerged as versatile, tunable ligand platforms in the coordination sphere of iron centers.1 Early and also more recent works show that low-valent iron centers can be stabilized by amidinato ligands in mono- and dinuclear complexes.1c,d In the majority of iron amidinates, the metal center is found in the oxidation states of +II or +III.1−8 Variation of the substituents at the carbon and nitrogen atoms of the amidinate backbone can significantly alter the coordination chemistry of iron amidinates (e.g., square planar vs tetrahedral coordination geometry)2a as well as their reactivity (e.g., no reaction with CO vs adduct formation vs insertion).2c,d In most cases, the substituents were alkyl, aryl, or silyl moieties without additional functional groups. Notable exceptions are a bis(amidinato) ligand,4 a ferrocenyl,5 and an (ethylene)dimethylamino substituted amidine.6 In the latter case, the additional donor moiety allowed for the in situ generation and characterization in solution of the only literature-known discrete cationic iron amidinato species. Further reactivity studies on Fe(II) amidinates include transformations with Lewis bases,2c πacids,2b−d,4 and various oxidizing agents.7 Moreover, iron amidinates have been studied as volatile precursors for atomic layer deposition.3 Curiously, however, catalytic applications have not been reported to date. © XXXX American Chemical Society

Received: May 8, 2015

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DOI: 10.1021/acs.organomet.5b00395 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics catalytic applications in the dehydrogenative alcoholysis of PhSiH3.

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RESULTS AND DISCUSSION Synthesis and Deprotonation of Amidine-diolefin Ligand. Reaction of phenylamidine with two equivalents of tropCl in the presence of excess base allowed for the straightforward synthesis of the desired ligand N,N′-Bis-tropphenylamidine, H-trop2AM (1) (Scheme 1, top).

Scheme 1. Synthesis of the Amidin-diolefin Ligand Htrop2AM (1) and Sodium Derivative [Na(trop2AM)(THF)3] (2) Figure 1. Molecular structure of H-trop2AM (1). Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms except for H1 are omitted for clarity. Selected bond lengths [Å] and angles [deg]: C31−N1, 1.369(2); C31−N2, 1.284(2); C31−C32, 1.497(2); C4−C5, 1.343(3); C19−C20, 1.339(3); and N1−C31−N2, 119.66(14).

Deprotonation of H-trop2AM with [NaN(SiMe3)2] in THF afforded the sodium amidinate [Na(trop2AM)(THF)3] (2) in 93% yield. The THF ligands could not be removed in vacuo in the solid state but are labile in THF solution as shown by 1H NMR spectroscopy. At ambient temperature, the (trop2AM)− ligand gives rise to broad resonances in the 1H and 13C NMR spectra. This indicates interconversion of the three possible conformers (endo/endo, endo/exo, and exo/exo conformation of trop substituents) and rules out strong interactions between the sodium center and the olefinic part of the trop moieties. Compound 2 was analyzed by single crystal X-ray analysis and crystallized as a twin14 in the monoclinic space group P21/c with Z = 4 (Figure 2). The trop substituents do not interact with the sodium cation, and both adopt an exo conformation.

Compound 1 was isolated as a colorless crystalline material in 67% yield. In solution, 16 conformers of compound 1 could theoretically be formed (amine, syn-/antiperiplanar; imine, E/ Z; trop, endo/exo). 1H NMR spectroscopic analysis in CDCl3 at ambient temperature showed a broad set of resonances for the conformationally flexible NHtrop fragment and a sharper set of resonances for the conformationally more rigid trop-imine group. Lowering the temperature to −50 °C revealed the presence of three conformers in a ratio of 1.0:0.17:0.02, the major one being the conformer that is represented in Scheme 1 (for details, see Supporting Information). The molecular structure of compound 1 in the solid state was analyzed by single crystal X-ray diffraction. Compound 1 crystallizes in the monoclinic space group P21/n with Z = 4 (Figure 1). No short intermolecular hydrogen bonds are observed due to the steric bulk around the N−H and the CN group. Compound 1 shows one long amine (C31−N1, 1.369(2) Å) and one short imine type C−N bond (C31N2, 1.284(2) Å). It exibits an anti-conformation of the amine subsituents and an Econfiguration of the imine moiety, as commonly observed for dialkylamidines in the solid state.13 The amine- and iminebound trop substituents adopt an endo- and exo-conformation, respectively, i.e., the conformation of 1 in the solid state is identical to that of its major conformer in solution.

Figure 2. Molecular structure of [Na(trop2AM)(THF)3] (2). Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms, annelated benzo groups, and the part of a disordered THF ligand with minor occupancy are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Na1−N1, 2.371(3); Na1−N2, 2.407(3); Na1−O1, 2.387(3); Na1−O2, 2.341(3); Na1−O3A, 2.380(17); N1−C31, 1.326(4); N2−C31, 1.327(4); N1−Na1−N2, 57.08(10); N1−Na1−O1, 103.19(11); N1−Na1−O2, 156.77(13); N1−Na1−O3A, 98.1(7); N2−Na1−O1, 155.52(12); N2−Na1−O2, 104.94(12); N2−Na1−O3, 96.4(19); O1−Na1−O2, 89.31(11); O1− Na1−O3A, 97.6(6); and O2−Na1−O3A, 99.5(7). B


Article

Organometallics The sodium center is coordinated by three THF ligands and the chelating amidinate ligand. This results in a square pyramidal coordination geometry around Na1 (τ5 = 0.02)15 with a THF ligand in the apical position. The Na1−N1/2 bond lengths are similar (Δ = 0.03 Å), and the C31−N1/2 distances are identical within limits of error indicating delocalization of electron density in the amidinate functionality. Whereas a large number of lithium amidinates16 and a few of the corresponding potassium complexes have been investigated by single crystal Xray analysis,16b,17 structurally characterized sodium amidinates are less common.17a,18 The structure of compound 2 closely r e s em b l e s t h a t o f t h e f o r m a m i d i n a t e [ N a ( ( ( N (C6H3iPr2))2CH)(THF)3].17a It confirms the trend that among amidinate compounds the lithium species prefer CN = 4,19 sodium species prefer CN = 5,6, and potassium species prefer the formation of K−arene interactions.17a Synthesis and Structure of Fe Amidinato-diolefin Compounds. Reaction of 1 with dimesityl iron in toluene/ THF at room temperature led to the formation of the iron aryl amidinate [Fe(trop2AM)(Mes)(THF)] (3), which was isolated in 60% yield as a light brown powder (Scheme 2, top). Single

Figure 3. Molecular structure of [Fe(trop2AM)(C6H2Me3)(THF)]· THF (3·THF). Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms, annelated benzo groups, and one lattice bound THF molecule are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Fe1−N1, 2.0714(19); Fe1−N2, 2.1304(19); Fe1−O1, 2.1273(17); Fe1−C38, 2.061(2); N1−C31, 1.329(3); N2−C31, 1.328(3); C4−C5, 1.336(4); C19−C20, 1.334(4); N1−Fe1−N2, 63.45(7); N1−Fe1−O1, 95.82(7); N1− Fe1−C38, 131.66(9); N2−Fe1−O1, 96.55(7); N2−Fe1−C38, 119.99(9); and O1−Fe1−C38, 128.41(9).

Scheme 2. Synthesis of [Fe(trop2AM)(Mes)(THF)] (3) and the Reaction of 3 with Brønsted Acids [HNEt3][BPh4] or [H(OEt2)2][B(ArF)4]a

literature-known pyridine stabilized compound were unsuccessful due to ligand redistribution processes.2b This indicates a higher robustness of 3 toward ligand scrambling. Reactions of 3 with Brønsted acids were investigated aiming at the isolation of a discrete cationic species (Scheme 2, bottom). Protonolysis of 3 with [HNEt3][BPh4] in THF led to an off-white product in low yield, which was identified as cationic [Fe(trop2AM)(THF)3][BPh4] (4) by single crystal Xray analysis (monoclinic, P21/c, Z = 4; Figure 4).20 The iron

a

Mes = mesityl; Py = pyridine; ArF = (3,5-C6H3(CF3)2).

crystal X-ray analysis of 3 (monoclinic, P21/c, Z = 4) revealed close binding interactions of Fe1 with one THF, one mesityl, and the amidinato ligand, which shows a symmetrically chelating κ2N,N′ binding mode (Figure 3). The olefin units of the trop substituents are oriented toward the metal center. The Fe1−Colefin distances are rather large (3.33−3.43 Å) but below the sum of the van der Waals radii. The CC bonds are not elongated compared to those in the free ligand, which has been reported for purely σ-type M−olefin interactions.9c,d Thus, Fe1−Colefin bonding interactions are possible but weak if present. A distorted tetrahedral coordination geometry was assigned to the iron center. The bonding parameters and coordination geometry in 3 closely resemble those of the two literature-known Fe aryl/alkyl amidinates, [Fe(R)(amidinate)(L)] (R = mesityl, CH2SiMe3; L = NHC, Py).2b,g It is noteworthy that attempts to isolate the THF analogue of the

Figure 4. Cationic part of the molecular structure of [Fe(trop2AM)(THF)3][BPh4]·THF (4·THF). Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms and a lattice-bound THF molecule are omitted for clarity. C


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Organometallics center in 4 is five-coordinate and shows close direct interactions with three THF ligands and a chelating κ2N,N′-amidinato ligand. As in compound 3, the olefinic moieties of the trop substituents are oriented toward the metal center with possible weak Fe−Colefin interactions. Weak diffraction intensities at higher diffraction angles preclude a detailed discussion of bonding parameters of compound 4, but the connectivity is definite. The in situ generation and NMR spectroscopic characterization of a dinuclear, monocationic iron amidinato compound has been reported previously.6 Compound 4 is the first discrete mononuclear monocationic iron complex stabilized by an amidinato ligand platform demonstrating that such compounds are isolable species. Compound 4 is poorly soluble in common organic solvents with a polarity lower than that of THF and only moderately soluble in THF. Using more polar solvents such as pyridine led to ligand redistribution reactions, presumably similar to the Schlenk equilibrium in organometallic compounds of alkaline earth metals. [Fe(Py)6][BPh4]2 (5) was isolated as one of the products of this reaction (Scheme 2 and Supporting Information). Using a fluorinated counterion is a common strategy to improve the solubility of monocationic species. However, this also led to ligand redistribution in our system as indicated by the isolation of [Fe(THF)6][B(ArF) 4]2 (6) (Scheme 2 and Supporting Information). The large Fe−olefin distances in compounds 3 and 4 prompted us to investigate if closer Fe−olefin contacts could be realized by shifting the metal center from the central position in the binding pocket of the amidinato ligand (κ2N,N′-bonding mode) to one side of the binding pocket (κ2N,C-bonding mode). To achieve this, H-trop2AM (1) was reacted with equimolar amounts of [FeBr2(THF)2] to give [FeBr2(Htrop2AM)] (7) as an orange solid in quantitative yield (Scheme 3).

Figure 5. Molecular structure of [FeBr2(H-trop2AM)] (7). Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms (except for H2), annelated C6H4 groups, one chemically equivalent but crystallographically independent molecule of 7, and disordered parts with lower occupancy are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Fe1−N1, 2.047(4); Fe1−Br1, 2.3974(10); Fe1−Br2, 2.3765(12); Fe−olefin, 2.273(7);25 N1−C31, 1.312(6); N2−C31, 1.341(7); C4−C5, 1.387(8); C19−C20, 1.342(9); H2−Br1, 2.81;35 N1−Fe1−(C4−C5), 89.79(18); N1− Fe1−Br1, 113.57(13); N1−Fe1−Br2, 110.08(14); (C4−C5)−Fe1− Br1, 109.27(14); (C4−C5)−Fe1−Br2, 119.32(16); and Br1−Fe1− Br2, 112.90(5).

shorter than that in [FeCl2(κ-N-amidine)2] (2.08−2.11 Å)22,23 and only slightly longer than that in tetracoordinate, amidinate stabilized iron monobromides (2.02−2.03 Å).24 The Fe−olefin distance amounts to 2.273(7) Å,25 which is slightly larger than the Fe−alkinyl distances of 2.20−2.23 Å in the bimetallic “tweezer” compound [{Ti(C 5 H 4 SiMe 3 ) 2 (CCSiMe 3 ) 2 } {FeCl2}].26 The coordinating olefin bond in 7 is elongated by 0.04−0.05 Å compared to that in the free ligand. In the vast majority of literature-known iron olefin complexes, the metal center is low-valent (formal oxidation states of −2 to +1),12,27−31 or the organoiron fragment is cationic.32−34 Thus, M → (olefin) backbonding or an increased Lewis acidity of the organoiron fragment have been exploited as driving forces for the formation of Fe−olefin bonds. Compound 7 is the first example of an Fe(II) halide that forms an Fe−olefin bond with a purely organic olefin ligand. No Fe−olefin interactions were observed, when both nitrogen atoms of the (trop2AM)− ligand bind to an FeBr2 fragment, as shown by the synthesis and full characterization of [N(nBu) 4 ][FeBr2(trop2AM)] (8) (Scheme 3 and Supporting Information). Placing two metal centers in the binding pocket of ligand 1 would be another possibility to reduce Fe−olefin distances in compounds based on 1 (and thereby enforce Fe−olefin bond formation). The reaction of [Na(trop2AM)(THF)3] (2) with two equivalents of [FeBr2(THF)2] indeed gave dinuclear [Fe2Br3(trop2AM)] (9) in 75% yield as a deep red crystalline material (Scheme 4, top). Single crystal X-ray analysis of 9 revealed that two FeBr units bridged by a μ2-Br anion are situated in the binding pocket of the ligand, which shows a 1κ2N,C,2κ2N′,C′-bonding mode (monoclinic, C2/c, Z = 4; Figure 6). The compound exhibits crystallographic C2-symmetry (rotation around the C−Ph axis). The iron centers show strongly distorted tetrahedral coordination geometries (angles around Fe: 89°−125°) and are located on opposite sides of the NCN-amidinate plane (Fe1−N1− N1′−Fe1′, 73.16(7)°). The Fe1−N1 bond length (1.9914(16) Å) is by 0.11 Å shorter than the average Fe−N bond lengths in

Scheme 3. Synthesis of [FeBr2(H-trop2AM)] (7) and [N(nBu)4][FeBr2(trop2AM)] (8)

Indeed, single crystal X-ray analysis of 7 revealed a distorted tetrahedrally coordinated Fe center with the amidine ligand in a κ2N,C-bonding mode (monoclinic, P21/n, Z = 8; Figure 5).21 The ligand shows the bonding parameters of an amidine (C31−N2/1, 1.341(7) Å vs. 1.312(6) Å), but the difference in C−N bond lengths is less pronounced than that in the noncoordinate ligand 1. The imine-type nitrogen atom N1 binds to the metal center. The Fe1−N1 bond (2.047(4) Å) is D


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Organometallics

rare,18b,36 and to the best of our knowledge, 9 is the first paramagnetic compound of this type. When the synthesis of a mononuclear monohalide compound [FeBr(trop2AM)] was attempted by the reaction of 2 with one equivalent of [FeBr2(THF)2], ligand redistribution was observed instead to give [Fe2Br3(trop2AM)] (9) and [Fe(trop2AM)2] (10) (Scheme 4, middle). This is somewhat unexpected since monohalide compounds of type [FeBr(amidinate)] can readily be synthesized using amidinates without additional donor functionalities24 and hints at the special stability of the dinuclear iron complex 9 as a driving force for the observed ligand scrambling. Compound 10 could also be prepared in a rational approach in quantitative yield by the reaction of 2 with [FeBr2(THF)2] in a 2:1 stoichiometry (Scheme 4, bottom). Single crystal X-ray analysis revealed both amidinato ligands to be bound in a κ2N,N′-fashion (monoclinic, P21/c, Z = 4; Figure 7). One of the ligands shows a more

Scheme 4. Reaction of [Na(trop2AM)(THF)3] (2) with [FeBr2(THF)2] in Different Stoichiometries

Figure 7. Molecular structure of [Fe(trop2AM)2]·(CH2Cl2)3. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms, annelated C6H4 groups, and CH2Cl2 molecules in the lattice, which do not interact with the metal center, are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Fe1−N1, 2.030(2); Fe1− N2, 2.057(2); Fe1−N3, 1.996(2); Fe1−N4, 2.094(2); Fe1−(C41− C42), 3.246(3); C41−C42, 1.339(4); N1−C31, 1.338(3); N2−C31, 1.331(3); N3−C68, 1.337(3); N4−C68, 1.325(3); N1−Fe1−N2, 65.87(8); N1−Fe1−N3, 151.22(9); N1−Fe1−N4, 124.87(9); N1− Fe1−(C41−C42), 91.22(7); N2−Fe1−N3, 134.41(9); N2−Fe1−N4, 124.08(9); N2−Fe1−(C41−C42), 97.05(7); N3−Fe1−N4, 65.71(9); N3−Fe1−(C41−C42), 68.54(7); and N4−Fe1−(C41−C42), 132.49(7).

Figure 6. Molecular structure of [Fe2Br3(trop2AM)] (9). Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms and annelated C6H4 groups are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Fe1−N1, 1.9914(16); Fe1−Br1, 2.3681(4); Fe1−Br2, 2.5125(4); Fe1−(C4−C5), 2.223(2); C4−C5, 1.365(3); C16−N1, 1.327(2); Fe1−Br2−Fe1′, 76.339(16); Br1− Fe1−Br2, 107.410(13); Br1−Fe1−N1, 125.05(5); Br1−Fe1−(C4− C5), 111.65(6); Br2−Fe1−N1, 111.89(5); Br2−Fe1−(C4−C5), 109.89(6); N1−Fe1−(C4−C5), 89.33(7); and Fe1−N1−N1′−Fe1′, 73.16(7).

symmetrical bonding mode (ΔFe−N1/2, 0.03 Å vs ΔFe−N3/ 4, 0.10 Å). One of the trop substituents in the more asymmetrically bound ligand is oriented toward the metal center, which leads to strong distortion of the tetrahedral coordination geometry around Fe1. The angle between the two NCN planes of the ligands (86.7(2)°) is similar to that in less strained Fe-bis-amidinato compounds.2d,5a However, the ligands are tilted relative to each other due to the steric bulk of the trop substituent that is oriented toward Fe1 (C31−Fe1− N3, 164° vs C31−Fe1−N4, 127°). The Fe−C41/42 distances are large (3.26−3.37 Å) but below the sum of the van der Waals radii. The C41−C42 distance is not elongated compared to the free ligand 1. Thus, Fe−olefin bonding interactions are possible but weak if present and could not be confirmed by IR spectroscopy (vide infra). NMR and IR Spectroscopic Analysis. The NMR spectroscopic analysis of 1 and 2 has been discussed above. NMR spectra of the iron compounds 3, 4, and 7−10 show paramagnetically shifted resonances with maximum line widths (fwhm) ranging from 155 Hz (7) to 8000 Hz (8), i.e., integration of some signals is unreliable.37 Nevertheless, the

the mononuclear compounds [Fe(Mes)(trop2AM)(THF)] (3) and [N(nBu)4][FeBr2(trop2AM)] (8). The Fe−olefin distance of 2.223(3) Å is shorter than that in 7 (vide supra) and in the range of the Fe−alkinyl distances observed in [{Ti(C5H4SiMe3)2(CCSiMe3)2}{FeCl2}] (2.20−2.23 Å).26 The C4−C5 bond length of 1.365(3) Å is elongated by 0.024(3) Å compared to the free ligand 1 indicating significant Fe → olefin backbonding. As expected, the Fe−Brterminal bond length (2.3681(4) Å) is shorter than the Fe−Brbridging bond length (2.5125(4) Å). The Fe1−Br2−Fe1′ angle amounts to 76.339(16)°. Monoamidinate complexes in which two metal centers are connected by one bridging atom are extremely E


Article

Organometallics spectra are useful as NMR spectroscopic fingerprints of the compounds. Some more information can be deduced from the NMR spectra of [Fe(trop2AM)(THF)3][BPh4] (4). 1H NMR spectroscopy showed that the THF ligands are labile in THF-d8 solution. The resonances of the (BPh4)− counterion experience a slight paramagnetic shift indicating weak ion pair contacts,38 most likely in an association/dissociation equilibrium with the solvent separated ion pair as the dominating species. The 11B NMR spectrum of 4 displayed a sharp resonance at δ = −7.03 ppm. Solid state IR spectroscopic analysis of the amidine ligand 1 revealed a sharp band at 3454 cm−1 due to the NH stretch, which is in agreement with the absence of hydrogen bonds (vide supra; Table 1). Bands at 1619 and 1629 cm−1 were

Figure 8. Variable temperature SQUID magnetization data for [Fe2Br3(trop2AM)] (9) at 1 T; black squares represent experimental data, and the red line indicates simulation.

Table 1. Stretching Frequencies of NH, CN, and CC Functionalities Determined by Solid State IR Spectroscopy Cpd

ṽ(NH) (cm−1)

ṽ(CN) (cm−1)

ṽ(CC) (cm−1)

1 2 3 4 7 8 9 10

3453

1619

3282

1554

1629 1632 1622 1614 1629, 1604 1630 1601 1633

μB) and suggests the possibility of ferromagnetic interactions between the two iron centers.41 Upon lowering the temperature, μef f steadily increased to reach a value of 9.08 μB at 25 K confirming the presence of ferromagnetic interactions. This is in agreement with intramolecular electron exchange via the bridging bromido ligand in a superexchange mechanism (Fe1− Br2−Fe1′, 76°; cf. Figure 6), as ferromagnetic interactions are predicted for an M−X−M angle of 90°.42−44 Upon further lowering the temperature, the effective magnetic moment rapidly decreased to reach a value of 6.01 μB at 2 K, which was ascribed to intermolecular antiferromagnetic interactions and/ or zero field splitting. Simulation of the magnetization data revealed g = 2.078, J = 6.410 cm−1, D = 6.726 cm−1, and E/D = 0.299 (Figure 8, red line). Weak ferromagnetic coupling between two iron(III) centers bridged by two pseudohalide ligands, N3−, has been reported with J = 4.80 cm−1 and an Fe− N−Fe angle of 106.0(2)°.45 The solution effective magnetic moment of 9 at room temperature (μeff = 7.4(1) μB) is close to that in the solid state suggesting that its molecular structure is essentially maintained in solution. Zero field 57Fe Mössbauer spectroscopic analysis of 9 at 77 K revealed a quadrupole doublet (δ = 0.83(1) mm/s; ΔEQ = 1.81(1) mm/s) confirming the equivalence of the two iron centers and their high spin d6 electron configuration (Supporting Information).46 The significant decrease of the isomer shift compared to that of [FeBr2(H2O)4] (δ = 1.22(3) mm/s) or [FeBr2(terpyridine)] (δ = 1.10 mm/s)47 is ascribed to the higher degree of bond covalency in 9 and to d(Fe) → π*(olefin) back donation, which leads to a contraction of the iron centered s-orbitals. Catalyzed Dehydrogenative Alcoholysis of PhSiH3. The base metal catalyzed dehydrogenative alcoholysis of silanes has been discussed as a tool for introducing silyl protection groups in organic synthesis48 and as a means of on-demand-H2 generation.49 We became interested in this type of reaction for the synthesis of poly silylethers,12 an approach which was introduced by Kawakami et al. and has been only little exploited so far.50,51 The catalysts employed for this transformation were typically based on the precious metals Pd and Rh.52 We investigated the catalyzed dehydrogenative alcoholysis of phenylsilane with 1.5 equiv of 1,4-benzene-dimethanol in THF at room temperature in an open system (Table 2). Iron compounds 3, 4, 9, 10, and FeBr2 were tested as precatalysts (3 mol %, i.e., 1 mol % per Si−H bond) in the presence of [NaBHEt 3 ] (1−3 equiv with respect to precatalyst).53 The highest rate of reaction was achieved by dinuclear 9, which led to full conversion after 43 min (entry 1).

assigned to the imine and the tropolefin stretches, respectively.39 Compound 7, which also contains an amidine ligand shows the amine and the imine stretches at lower wave numbers due to coordination of the Lewis acidic Fe(II) center. The iron compounds which clearly show Fe−olefin interactions (7, 9) exhibit the expected decrease in the CC stretching frequencies by 25−28 cm−1. This decrease is small compared to that observed for complexes of electron poor olefins with low-valent Fe centers (e.g., Δ ≈ 150 cm−1 for [Fe(CO)4(alkene)]).40 In addition, compound 7 shows a CC band due to the noncoordinating trop substituent, the frequency of which is not altered compared to that in the free ligand 1. The compounds which do not show M−olefin contacts (2, 8) revealed CC stretching frequencies close to that of the free ligand 1. For compounds 3, 4, and 10, single crystal X-ray analysis suggested the possibility of weak Fe− olefin contacts (vide supra). The IR spectroscopic analysis does not support the presence of such bonding in 10 but lets us suggest weak Fe−olefin interactions in 3 and 4 (ṽ(CC) = 1622, 1614 cm−1). Magnetization Measurements and Mö ssbauer Spectroscopy. The mononuclear iron compounds 3, 4, 7, 8, and 10 showed effective magnetic moments in solution of μeff = 5.0(1)−5.3(1) μB at room temperature as determined by the Evans’ method (cf. Experimental Section). This indicates the expected high spin d6 electron configuration (S = 2) for all of these compounds. Because of its unusual binding mode (vide supra), the magnetic properties of dinuclear [Fe2Br3(trop2AM)] (9) were analyzed in more detail by SQUID magnetization experiments at 1 T in the temperature range of 2−300 K (Figure 8). At 300 K, an effective magnetic moment of μef f = 7.68 μB per molecule was observed. This is considerably larger than the spin only value expected for two magnetically isolated Fe2+ ions in a high spin d6 electron configuration (for g = 2: μeff = 6.93 F


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Organometallics

dehydrogenative coupling of PhSiH3 and 1,4-benzene-dimethanol (dae = N−CH2−CH2−N).12 Compared to 12, the use of precatalyst 9 results in a slower reaction (43 min vs 5 min until full conversion) but yields a product with a molecular weight that is more than two times higher (≥24 vs ≥9 repeating units).

Table 2. Dehydrogenative Alcoholysis of PhSiH3 with 1,4Benzene-dimethanol in the Presence of Catalytic Amounts of Fe Compounds (Open System)

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entry

precat.a

t [min]

conv. [%]

repeating unitsb

1 2 3 4 5

3 4 9 10 FeBr2

133 102 43 198 493

90 87 97 98 87

22 22 24 22 14

CONCLUSIONS A new amidinato-diolefin ligand [H-trop2AM] (1) and its sodium derivative [Na(trop2AM)(THF)3] (2) could be prepared. Both compounds are suitable starting materials for the preparation of a range of paramagnetic iron compounds. In rational synthetic approaches, one or two metal centers could be placed in the binding pocket of the ligand revealing versatile binding modes of this ligand platform (κ2N,N′; κ2N,C; 1κ2N,C,2κ2N′,C′). Iron−olefin interactions in these compounds are weak (if present at all), when both nitrogen atoms of the amidinato ligand bind to one metal center. A stronger interaction is observed in mononuclear [FeBr2(H-trop2AM)] (7) with an asymmetric binding mode or in dinuclear [Fe2Br3(trop2AM)] (9), which thus represent rare examples of Fe(II) halide olefin complexes. The iron centers of all compounds were found in a high spin d6 electron configuration with weak intramolecular ferromagnetic coupling in dinuclear 9. [Fe(trop2AM)(THF)3][BPh4] (4) represents the first isolated monocationic iron compound supported by an amidinate ligand, and ligand redistribution reactions analogous to the Schlenk equilibrium were identified as side reactions. Selected representatives of the new iron amidinato compounds were tested as precatalysts for the preparation of poly silylethers by dehydrogenative alcoholysis of PhSiH3 at room temperature in the presence of [NaBHEt3]. The amidinato-diolefin ligand proved suitable for stabilizing the catalytically active species (putatively Fe−H species) with dinuclear 9 leading to the fastest conversion. Future efforts will be directed toward the isolation of dinuclear iron hydride species supported by an amidinato-diolefin ligand.

a

3 mol % of Fe compound and 3 mol % (for compounds 3, 4, 10), 6 mol % (for FeBr2) or 9 mol % (for compound 9) of [NaBHEt3] were used with respect to PhSiH3. bMinimum number of repeating units in 11 (with respect to Si) as determined by mass spectrometry.

■

The rate of reaction hardly decreased upon lowering the catalyst loading to 1.5 mol %, but drops significantly with catalyst loadings of only 0.3 mol % (Supporting Information). For precatalyst 10, full conversion was detected after 198 min (entry 4). When using precatalysts 3, 4, or FeBr2, the hydrogen evolution stopped at 87−90% conversion (entries 2, 3, and 5). Compounds 3 and 4 were equally efficient at generating an active species and led to similar rates of reactions. FeBr2 as a precatalyst resulted in the slowest rate of reaction. Whereas the reaction of FeBr2 with [NaBHEt3] expectedly gave a dark precipitate (presumably Fe0),54 reactions with amidinato based compounds resulted in apparently THF soluble iron species55 suggesting the efficiency of the (trop2AM)− ligand to stabilize the Fe hydride species. The off-white polymeric materials obtained from these reactions were insoluble in common organic solvents such as THF, CHCl3, or DMF, once they had been isolated and dried in vacuo. IR spectroscopic analysis confirmed the absence of Si−H functionalities. Mass spectrometric analysis revealed similar values for the minimum number or repeating units (22−24) at similar conversions for entries 1−4. This suggests that the same number of active centers is present in the active species generated from mononuclear 3, 4, and 10 or from dinuclear 9. The Fe(I) compound [NaFe(trop2dae)(THF)3] (12) has very recently been reported as an efficient homogeneous precatalyst for the

EXPERIMENTAL SECTION

General Considerations. All air- and moisture-sensitive manipulations were carried out using standard vacuum line Schlenk techniques or in an MBraun inert atmosphere drybox containing an atmosphere of purified argon. THF, Et2O, n-hexane, CH2Cl2, and toluene were degassed and purified using an Innovative Technologies PureSolv system. C6D6, THF-d8, and MTBE were distilled before use from sodium benzophenone ketyl. CD2Cl2 and pyridine were distilled from CaH2. MeOH was dried by the addition of Na, distilled, and degassed prior to use. Phenylamidine, NEt3, anhydrous FeBr2, and a THF solution of NaBHEt3 were obtained from Sigma-Aldrich and used as received. PhSiH3 was obtained from Sigma-Aldrich and dried with molecular sieves (4 Å). [N(nBu)4]Br was obtained from SigmaAldrich and dried in vacuo prior to use. TropCl,56 [FeBr2(THF)2],57 and [Na(N(SiMe3)2)]58 were synthesized according to the literature. NMR spectra were recorded on Bruker instruments operating at 200, 250, 300, or 500 MHz with respect to 1H. 1H and 13C NMR chemical shifts are reported relative to SiMe4 using the residual 1H and 13C chemical shifts of the solvent as a secondary standard. 11B NMR chemical shifts are reported relative to BF3·OEt2. Infrared spectra were collected on a PerkinElmer-Spectrum 2000 FT-IR-Raman spectrometer. Mass spectrometric analyses were performed on a Bruker UltraFlex II instrument. Elemental analyses were performed at the Mikrolabor of ETH Zürich. Single crystals suitable for X-ray diffraction were coated with polyisobutylene oil in a glovebox, transferred to a nylon loop, and then transferred to the goniometer of an Oxford XCalibur, a Bruker APEX II, or a Bruker VENTURE diffractometer equipped with a molybdenum X-ray tube (λ = 0.71073 Å). The G


Article

Organometallics

[Na(trop2AM)(THF)3] (2). Solid [Na(N(SiMe3)2] (153 mg, 0.83 mmol) was added to a solution of H-trop2AM (417 mg, 0.83 mmol) in THF (4 mL) to give a yellow solution. All volatiles were removed from the reaction mixture under reduced pressure. The residue was washed with hexanes (2 × 2 mL) to give a pale yellow solid, which was dried in vacuo. Yield: 572 mg, 0.77 mmol, 93%. 1 H NMR (300 MHz, C6D6): δ = 1.28−1.34 (m, 12 H, β-THF), 3.35−3.41 (m, 12H, α-THF), 4.75 (br s, 1H, 5-Htrop1), 5.57 (br s, 1H, 5-Htrop2), 6.73−7.98 (m, 25H, HOlefin, HArom) ppm. 13C NMR (75 MHz, C6D6): δ = 25.64 (s, β-THF), 61.51 (s, α-THF), 67.83 (br s, 5Ctrop1), 70.01 (br s, 5-Ctrop2), 125.22 (br s, COlefin/CArom), 126.49 (br s, COlefin/CArom), 127.51 (br s, COlefin/CArom), 128.59 (br s, COlefin/CArom), 128.92 (br s, COlefin/CArom), 131.49 (br s, COlefin/CArom), 134.40(br s, COlefin/CArom), 140.78 (br s, COlefin/CArom), 146.55 (br s, COlefin/CArom), 176.07 (br s, NCPhN) ppm. Resonances due to quaternary carbon atoms of trop substitutents were not detected. mp = 129 °C. Anal. Calc. for C49H51N2NaO3 (738.94 g/mol): C, 79.65; H, 6.96; N, 3.79; found, C, 79.45, H, 6.93; N, 3.87. ATR IR: ṽ = 3052 (w), 3007 (w), 2958 (m), 2870 (m), 1632 (m), 1595 (m), 1465 (m), 1456 (s), 1435 (s), 1352 (m), 1306 (w), 1262 (w), 1192 (w), 1146 (w), 1110 (w), 1073 (w), 1049 (s), 1037 (s), 979 (w), 909 (m), 893 (m), 832 (w), 790 (s), 768 (s), 739 (s), 698 (s) cm−1. [Fe(mesityl)(trop2AM)(THF)] (3). A solution of H-trop2AM (1) (200 mg, 0.40 mmol) in toluene/THF (1:1; 2 mL) was added to a solution of [Fe(mesityl)2] (118 mg, 0.40 mmol) in toluene (1 mL). After 30 min, all volatiles were removed from the deep brown solution under reduced pressure. The residue was treated with hexanes (3 mL), which was subsequently removed under reduced pressure. The resulting light brown solid was washed with hexanes (3× 2 mL) and dried in vacuo. Yield: 179 mg, 0.24 mmol, 60%. Dark brown single crystals of [3·THF] were obtained by generating 3 in situ as described above but on a 0.20 mmol scale and layering a the deep brown solution of 3 in THF/toluene with hexanes after filtration. Yield: 34 mg, 42 μmol, 21%. Analytical data for 3: 1H NMR (200 MHz, C6D6): δ = −21.52 (br s), −5.54 (s), 2.16 (s), 2.67 (s), 4.35 (s), 6.74 (s), 14.47 (s), 18.53 (s), 20.93 (s), 28.93 (s), 36.31 (br s), 43.20 (br s), 121.22 (br s), 121.96 (s), 128.91 (br s) ppm. ATR IR: ṽ = 3057 (w), 3015 (w), 2911 (w), 1622 (w), 1593 (m), 1576 (w), 1442 (s), 1417 (s), 1328 (m), 1311 (m), 1259 (m), 1218 (w), 1157 (w), 1089 (w), 1026 (m), 944 (w), 920 (w), 887 (w), 845 (w), 827 (w), 797 (s), 781 (s), 763 (s), 736 (s), 700 (s) cm−1. Anal. Calc. for C50H46FeN2O (746.77 g/mol): C, 80.42; H, 6.21; N, 3.75; found, C, 80.22, H, 6.22; N, 3.76. μeff = 5.3(1) μB (Evans method, C6D6). Analytical data for [3·THF]: The 1H NMR spectrum of the singlecrystalline material was identical to that of 3 (except for extra THF signals). mp = 146 °C (decomp). Anal. Calc. for C54H54FeN2O2 (818.88 g/mol): C, 79.21; H, 6.65; N, 3.42; found, C, 79.03, H, 6.72; N, 3.31. [Fe(trop2AM)(THF)3]·(THF) (4·THF). A solution of H-trop2AM (1) (100 mg, 0.20 mmol) in THF (1.5 mL) was added to a solution of [Fe(mesityl)2] (59 mg, 0.20 mmol) in THF (1.5 mL). After 5 min, a solution of [HNEt3][BPh4] (79 mg, 0.18 mmol) in THF (2 mL) was slowly added. A colorless solid precipitated (presumably [Fe(THF)6][BPh4]2) and was filtered off after 30 min and dried in a stream of Argon. Yield: 88 mg, 0.078 mmol, 87%. The filtrate was layered with hexanes (6 mL). After 14 h, off-white crystals of (4·THF) had formed and were isolated by filtration and dried in a stream of Argon. Yield: 21 mg, 0.018 mmol, 10%. Analytical data for (4·THF): 1H NMR (250 MHz, THF-d8): δ = −31.15 (br s), −8.15 (br s), 1.72 (br s, β-THF), 3.58 (br s, α-THF), 5.60 (br s), 7.06 (br s, 4H, p-BPh4), 7.47 (br s, 8H, o/m-BPh4), 8.53 (br s, 8H, m/o-BPh4), 12.61 (br s), 16.14 (br s), 23.66 (br s), 36.06 (br s) ppm. 11B NMR (80 MHz, THF-d8): δ = −7.03 (s) ppm. ATR IR: ṽ = 3054 (w), 2982 (w), 2894 (w), 1614 (w), 1608 (w), 1598 (m), 1480 (m), 1428 (s), 1342 (m), 1308 (w), 1240 (m), 1177 (m), 1136 (w), 1093 (w), 1064 (m),1032 (m), 1010 (m), 945 (w), 918 (m), 844 (m), 809 (m), 782 (m), 763 (m), 749 (m), 731 (s), 703 (s) cm−1. mp = 114−116 °C (decomp.). Anal. Calc. for C77H79BFeN2O4 (1163.14 g/

structures were solved using direct methods (SHELXS) completed by Fourier synthesis and refined by full-matrix least-squares procedures. CCDC 1063643-1063653 contains the crystallographic information for compounds 1−10 and [8·(THF)1.5]. 57Fe Mössbauer spectra were recorded on a WissEl Mössbauer spectrometer (MRG-500) at 77 K in constant acceleration mode. 57Co/Rh was used as the radiation source. WinNormos for Igor Pro software has been used for the quantitative evaluation of the spectral parameters (least-squares fitting to Lorentzian peaks). The minimum experimental line widths were 0.20 mm/s. The temperature of the samples was controlled by an MBBC-HE0106 MÖ SSBAUER He/N2 cryostat within an accuracy of ±0.3 K. Isomer shifts were determined relative to α-iron at 298 K. Magnetism data of crystalline, finely powdered 9 (32.7 mg), contained within a polycarbonate gel capsule, were recorded with a Quantum Design MPMS−XL SQUID magnetometer. DC magnetization data were collected in the temperature range of 2−300 K with an applied DC field of 1 T. Values of the magnetic susceptibility were corrected for core diamagnetism of the sample estimated using tabulated Pascal’s constants.59 Fitting of the experimental data was performed using the program JulX, v1.4.1 by Eckhard Bill.60 [H-trop2AM] (1). This synthesis was performed under atmospheric conditions. Triethylamine (2.0 mL) and a suspension of tropCl (2.11 g, 9.31 mmol) in toluene (30 mL) were added to a suspension of phenylamidine (560 mg, 4.66 mmol) in toluene (20 mL). The reaction mixture was heated under reflux for 14 h, cooled to room temperature, and filtered. The residue was washed with toluene (3 × 30 mL). All volatiles were removed in vacuo from the combined organic fractions. The white residue was dissolved in CH2Cl2 (4 mL). The resulting solution layered with hexanes (60 mL) and stored at −20 °C overnight to give colorless crystalline 1, which was isolated by filtration and dried in vacuo. Yield: 1.56 g, 3.12 mmol, 67%. 1 H NMR (300 MHz, CDCl3): δ = 4.46 (s, 1H, 5-HtropNC), 5.04 (br s, 1 H, NH) 6.67 (s, 1H, 5-Htrop(NH)−C), 6.60−7.80 (m, 25H, HOlefin, HArom) ppm. 1H NMR (500 MHz, CDCl3, −50 °C): δ = 4.48 (s, 1H, 5-HtropNC), 5.07 (d, 1H, 3JHH = 7.6 Hz, NH), 6.70 (d, 2H, 3 JHH = 7.5 Hz, o-Ph), 6.84 (d, 1H, 3JHH = 7.6 Hz, 5-Htrop(NH)−C), 6.95 (s, 2H, 10,11-HtropNC), 7.10 (t, 2H, 3JHH = 7.5 Hz, m-Ph), 7.11 (s, 2H, 10,11-Htrop(NH)−C), 7.21 (t, 2H, 3JHH = 7.5 Hz, 2,8-HtropN C, overlapping with resonance for p-Ph), 7.21 (t, 1H, 3JHH = 7.5 Hz, pPh, overlapping with resonance for 2,8-HtropNC), 7.25 (d, 2H, 3JHH = 7.2 Hz, 1,9-HtropNC), 7.38 (t, 2H, 3JHH = 7.5 Hz, 3,7-HtropNC), 7.46 (t, 2H, 3JHH = 7.5 Hz, 2,8-Htrop(NH)−C), 7.53 (d, 2H, 3JHH = 7.4 Hz, 1,9-Htrop(NH)−C), 7.57 (t, 2H, 3JHH = 7.5 Hz, 3,7-Htrop(NH)− C), 7.66 (d, 2H, 3JHH = 7.8 Hz, 4,6-HtropNC), 8.05 (d, 2H, 3JHH = 7.6 Hz, 4,6-Htrop(NH)−C) ppm. Two more sets of signals corresponding to other conformers of 1 appear in the 1H NMR spectrum but could not clearly be assigned due to overlapping and weak intensities (intensities normalized to main conformer: 1.0:0.17:0.02). 13C NMR (126 MHz, CDCl3, −50 °C): δ = 60.75 (s, 5-Ctrop(NH)−C), 61.23 (s, 5-CtropNC), 124.66 (s, 4,6-CtropN C), 125.10 (s, 2,8-CtropNC), 126.96 (s, o-Ph), 127.04 (s, 1,9CtropNC), 127.14 (s, 2,8-Ctrop(NH)−C), 127.88 (s, m-Ph), 128.18 (s, 3,7-CtropNC), 128.73 (s, 3,7-Ctrop(NH)−C), 128.83 (s, p-Ph), 129.71 (s, 1,9-Ctrop(NH)−C), 130.39 (s, 4,6-Ctrop(NH)−C), 130.77 (s, 10,11-Ctrop(NH)−C), 130.98 (s, 10,11-CtropNC), 132.59 (s, 9a,11aCtropNC), 133.23 (s, 9a,11a-Ctrop(NH)−C), 134.18 (s, ipso-Ph), 138.82 (s, 4a,5a-Ctrop(NH)−C), 143.18 (s, 4a,5a-CtropNC), 155.64 (s, NCPhN) ppm. A second set of resonances corresponding to another conformer of 1 appears in the 13C NMR spectrum but could not clearly be assigned due to overlapping and weak intensities. mp = 219 °C. HR-ESI-MS: calc. for (C37H29N2)+ (M + H+), m/z = 501.2325; found, m/z = 501.2324 (27%). calc. for (C15H11)+ (trop+), m/z = 191.0855; found, m/z = 191.0860 (100%). Anal. Calc. for C37H28N2 (500.64 g/mol): C, 88.77; H, 5.64; N, 5.60; found, C, 88.44, H, 5.47; N, 5.24. ATR IR: ṽ = 3454 (w), 3057 (w), 3012 (m), 1629 (m), 1619 (s), 1594 (m), 1578 (w), 1493 (m), 1475 (s), 1456 (m), 1437 (m), 1330 (m), 1306 (w), 1284 (w), 1261 (w), 1194 (w), 1156 (w), 1117 (m), 1078 (w), 1040 (w), 971 (w), 919 (w), 899 (w), 829 (w), 799 (s), 784 (m), 766 (s), 737 (s), 728 (s), 697 (s) cm−1. H


Article

Organometallics mol): C, 79.51; H, 6.85; N, 2.41; found, C, 79.32, H, 6.98; N, 2.68. μeff = 5.1(1) μB (Evans method, THF-d8). Analytical data for colorless solid ([Fe(THF)6][BPh4]2): Anal. Calc. for C72H88B2FeO6 (1126.95 g/mol): C, 76.74; H, 7.87; N, 0.00; found, C, 76.54, H, 7.87; N, 0.29. mp = 111 °C (decomp.). [FeBr2(H-trop2AM)] (7). Toluene (4 mL) was added to a mixture of [FeBr2(THF)2] (359 mg, 1.0 mmol) and H-trop2AM (500 mg, 1.0 mmol) to give a suspension. After 16 h, all volatiles were removed from the reaction mixture under reduced pressure to give an orange powder which was dried in vacuo for 6 h. Yield: 730 mg, 1.0 mmol, quant. 1 H NMR (300 MHz, CD2Cl2): δ = −33.46 (br s), −12.14 (br s), −10.45 (br s), −7.09 (br s), −4.18 (br s), −3.06 (br s), −1.05 (br s), −0.84 (br s), 0.13 (br s), 1.29 (br s), 5.32 (br s), 6.08 (br s), 7.22 (br s), 7.49 (br s), 8.59 (br s), 8.94 (br s), 9.78 (br s), 11.21 (br s), 12.19 (br s), 13.60 (br s), 14.19 (br s), 16.06 (br s), 19.02 (br s), 21.65 (br s), 22.89 (br s), 23.79 (br s), 27.96 (br s) ppm. mp =215−220 °C (decomp.). μeff = 5.0(1) μB (Evans’ method). Anal. Calc. for C37H28Br2N2Fe (716.30 g/mol): C, 62.04; H, 3.94; N, 3.91; found, C, 61.74, H, 4.20; N, 3.79. ATR IR: ṽ = 3454 (w), 3282 (w), 3010 (w), 1629 (m), 1604 (m), 1581 (w), 1554 (s), 1495 (m), 1475 (m), 1456 (m), 1435 (m), 1413 (m), 1330 (m), 1317 (w), 1305 (w), 1237 (w), 1189 (w), 1155 (w), 1117 (w), 1080 (w), 1040 (w), 1026 (w), 992 (m), 950 (w), 895 (w), 875 (w), 827 (w), 798 (s), 783 (s), 765 (s), 746 (s), 738 (s), 726 (s), 701 (s) cm−1. [N(nBu)4][FeBr2(trop2AM)] (8). [N(nBu)4]Br (22 mg, 68 μmol) and [Na(THF)3(trop2AM)] (2) (69 μmol) were added subsequently to a solution of FeBr2 (15 mg, 70 μmol) in THF (4 mL). The reaction mixture was filtered after 2 h. The filtrate was layered with MTBE (6 mL). After 1 day, trace amounts of a colorless solid were filtered off and the filtrate stored at −30 °C for 1 d to give colorless plates, which were filtered off and dried in vacuo (36 mg). All volatiles were removed from the filtrate under reduced pressure to give a second crop of colorless solid, which was dried in vacuo (30 mg). Combined yield: 66 mg, 69 μmol, quantitative. 1 H NMR (300 MHz, CD2Cl2): δ = −66.41 (br s), −18.11 (br s), −8.97 (s), −5.64 (s), −0.87 (s), 0.70 (br s), 0.96 (br s), 2.43 (br s), 2.90 (br s), 4.26 (br s), 6.41 (s), 6.54 (s), 6.75 (s), 6.88 (s), 7.09 (s), 7.12 (s), 7.15 (s), 7.23 (s), 7.26 (s), 7.38 (s), 7.41 (s), 7.48 (s), 7.51 (s), 7.87 (s), 8.09 (br s), 9.72 (br s), 10.33 (br s), 12.57 (br s), 21.54 (br s), 24.92 (br s), 26.76 (br s), 36.17 (br s) ppm. mp >220 °C. Anal. Calc. for C53H63N3FeBr2: C, 66.47; H, 6.63; N, 4.39; found, C, 66.22, H, 6.69; N, 4.27. μeff = 5.0(1) μB (Evans’ method). ATR IR: ṽ = 3062 (w), 3021 (w), 2958 (m), 2872 (w), 1630 (w), 1595 (w), 1480 (m), 1456 (s), 1435 (m), 1377 (m), 1338 (w), 1310 (w), 1228 (w), 1190 (w), 1154 (w), 1115 (m), 1080 (w), 1039 (w), 1027 (w), 972 (w), 953 (w), 921 (w), 882 (w), 855 (w), 831 (w), 800 (s), 791 (m), 769 (s), 736 (s), 713 (m), 699 (s) cm−1. [Fe2Br3(trop2AM)] (9). [Na(THF)3(trop2AM)] (2) (200 mg, 0.271 mmol) was added to a suspension of [FeBr2(THF)2] (195 mg, 0.542 mmol) in Et2O (4 mL). After 1 h, all volatiles were removed from the reaction mixture under reduced pressure. CH2Cl2 (3 mL) was added to the yellow residue, and the suspension was filtered. The red filtrate was layered with hexanes (12 mL). Red single crystals of 9 formed within 16 h. Yield: 174 mg, 0.204 mmol, 75%. When the mother liquor is stored at −30 °C for 1 d, 11 mg of single crystalline [(Fe(trop2AM)2)·(CH2Cl2)3] is obtained (0.008 mmol, 6% based on [Na(THF)3(trop2AM)]). 1 H NMR (300 MHz, CD2Cl2): δ = −10.56 (br s), −6.36 (br s), −5.45 (br s), −3.10 (s), −1.66 (s), 1.26 (s), 2.98 (s), 4.38 (s), 6.61 (s), 7.44 (s), 8.92 (s), 9.74 (s), 11.16 (s), 12.14 (s), 13.57 (s), 15.98 (s), 18.98 (br s), 21.85 (br s), 22.89 (br s), 27.93 (br s) ppm. mp >220 °C. Anal. Calc. for C37H28N2Fe2Br3: C, 52.22; H, 3.20; N, 3.29; found, C, 52.12, H, 3.23; N, 3.42. μeff = 7.4(1) μB (Evans’ method). ATR IR: ṽ = 3067 (w), 3023 (w), 1601 (m), 1578 (w), 1560 (w), 1486 (w), 1449 (s), 1431 (s), 1409 (s), 1335 (m), 1315 (m), 1284 (w), 1271 (w), 1234 (w), 1224 (w), 1190 (m), 1162 (w), 1151 (w), 1111 (w), 1077 (m), 1037 (w), 1022 (m), 984 (w), 933 (w), 896 (m), 869 (m), 815 (s), 783 (m), 768 (s), 749 (m), 737 (s), 709 (s) cm−1.

[Fe(trop2AM)2] (10). THF (2 mL) was added to a mixture of [Na(THF)3(trop2AM)] (2) (62 mg, 84 μmol) and [FeBr2(THF)2] (15 mg, 42 μmol). After 5 min, all volatiles were removed from the resulting suspension under reduced pressure. The residue was extracted with toluene (2 × 1 mL). All volatiles were removed from the combined toluene phases under reduced pressure to give a pale orange solid which was dried in vacuo for 4 h. Yield: 44 mg, 42 μmol, quant. Single crystals of [Fe(trop2AM)2]·(CH2Cl2)3 were obtained by layering a CH2Cl2 solution of 10 with hexanes at ambient temperature. 1 H NMR (300 MHz, C6D6): δ = −40.18 (br s), −32.27 (br s), −22.80 (br s), −14.55 (br s), −5.37 (br s), 2.63 (br s), 3.60 (br s), 3.62 (br s), 3.64 (br s), 3.67 (br s), 4.85 (br s), 5.19 (br s), 5.95 (br s), 6.05 (br s), 6.50 (br s), 6.70 (br s), 6.75 (br s), 7.93 (br s), 9.08 (br s), 10.05 (br s), 10.58 (br s), 14.34 (br s), 18.57 (br s), 19.78 (br s), 22.79 (br s), 23.85 (br s), 26.44 (br s), 42.78 (br s) ppm. mp =200 °C (decomp.) [Samples of single crystalline [Fe(trop2AM)2]·(CH2Cl2)3 were used for melting point analyses. The crystals pale at 135 °C (presumably due to loss of lattice bound solvent molecules)]. μeff = 5.2(1) μB (Evans’ method). Anal. Calc. for C74H54N4Fe: C, 84.24; H, 5.16; N, 5.31; found, C, 84.02, H, 5.27; N, 5.21. ATR IR: ṽ = 3054 (w), 3016 (m), 1633 (m), 1596 (m), 1578 (w), 1494 (m), 1478 (m), 1421 (s), 1331 (m), 1260 (w), 1234 (w), 1156 (w), 1117 (w), 1087 (w), 1027 (m), 966 (w), 944 (w), 918 (m), 887 (m), 850 (w), 829 (w), 796 (m), 783 (m), 764 (s), 735 (s), 699 (s) cm−1. Dehydrogenative Alcoholysis of PhSiH3 (cf. Table 2). In a typical experiment, [NaBHEt3] (1 M solution in THF; for compounds 3, 4, and 10, 13 μL; for compound 9, 40 μL; for FeBr2, 28 μL) and PhSiH3 (45 mg, 0.42 mmol) were added consecutively to a solution of the iron compound (13 μmol), in THF (3 mL). For the catalytic experiments using 1.5 mol % of 9 with 4.5 mol % of [NaBHEt3] or 0.3 mol % of 9 with 0.9 mol % of [NaBHEt3], the appropriate amounts of 9 were added using stock solutions while keeping the total volume of the reaction mixture constant. The resulting dark brown solution (in the case of 3, 4, 9, and 10) or suspension of a dark solid in a colorless liquid phase (in case of FeBr2) was added to a solution of 1,4-benzenedimethanol (87 mg, 0.63 mmol) in THF (2 mL). The reaction vessel was open to an inverted buret, by which the volume change of the reaction could be determined. When no further expansion of the system was detected, the reaction mixture was poured into hexanes under atmospheric conditions. An off-white solid precipitated, was isolated by filtration, and dried in vacuo. Yield: precatalyst 3, 84 mg, 271 μmol, 65% (MALDI-TOF-MS (m/z) = 6864.2 [C396H378NaO68Si22]+); precatalyst 4, 98 mg, 317 μmol, 76% (MALDI-TOF-MS (m/z) = 6864.1 [C396H378NaO68Si22]+); precatalyst 9, 120 mg, 388 μmol, 93% (MALDI-TOF-MS (m/z) = 7483.3 [C432H412NaO74Si22]+); ATR IR, ṽ = 2918 (w), 2869 (w), 1618 (m), 1593 (w), 1515 (w), 1460 (w), 1429 (m), 1375 (m), 1262 (w), 1228 (w), 1052 (s), 1017 (s), 843 (s), 800 (s), 763 (s), 740 (s), 699 (s) cm−1; precatalyst 10, 118 mg, 381 μmol, 91% (MALDI-TOF-MS (m/z) = 6965.2 [C404H384NaO68Si22]+); precatalyst FeBr2, the polymeric material could not be isolated by filtration. Thus, all volatiles were removed in vacuo, and no meaningful isolated yield could be determined. The resulting solid was analyzed by mass spectrometry: (MALDI-TOF-MS (m/z) = 4426.4 [C254H247O46Si14]+).61

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ASSOCIATED CONTENT

S Supporting Information *

Conformational analysis of 1; molecular structures of 5, 6, and 8 in the solid state; Mössbauer spectrum of 9; and details for catalytic experiments. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00395.

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AUTHOR INFORMATION

Corresponding Authors

*(C. L.) E-mail: [email protected]. *(H. G.) E-mail: [email protected]. I


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Organometallics Notes

van Rijn, J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349− 1356. (16) For example: (a) Boeré, R. T.; Cole, M. L.; Junk, P. C. New J. Chem. 2005, 29, 128. (b) Loh, C.; Seupel, S.; Görls, H.; Krieck, S.; Westerhausen, M. Eur. J. Inorg. Chem. 2014, 1312−1321. (c) Villiers, C.; Thuéry, P.; Ephritikhine, M. Eur. J. Inorg. Chem. 2004, 4624−4632. (d) Hong, J.; Zhang, L.; Wang, K.; Chen, Z.; Wu, L.; Zhou, X. Organometallics 2013, 32, 7312−7322. (e) Cragg-Hine, I.; Davidson, M. G.; Mair, F. S.; Raithby, P. R.; Snaith, R. J. Chem. Soc., Dalton Trans. 1993, 2423−2424. (17) For example: (a) Cole, M. L.; Davies, A. J.; Jones, C.; Junk, P. C. J. Organomet. Chem. 2004, 689, 3093−3107. (b) Benndorf, P.; Preuß, C.; Roesky, P. W. J. Organomet. Chem. 2011, 696, 1150−1155. (18) (a) Stalke, D.; Wedler, M.; Edelmann, F. T. J. Organomet. Chem. 1992, 431, C1−C5. (b) Cole, M. L.; Junk, P. C.; Louis, L. M. J. Chem. Soc., Dalton Trans. 2002, 3906−3914. (19) For an exception with CN = 5, see ref 16e. (20) A colorless side product was isolated, which is very poorly soluble in THF and is suggested to be the product of a disproportionation reaction, [Fe(THF)6][BPh4]2 (see Experimental Section). (21) The asymmetric unit of 7 contains two chemically identical but crystallographically independent molecules of 7, which are associated by weak Br···HC interactions (e.g., Br2A···H64A, 2.57 Å). (22) Cotton, F. A.; Daniels, L. M.; Maloney, D. J.; Matonic, J. H.; Murillo, C. A. Polyhedron 1994, 13, 815−823. (23) Cotton, F. A.; Daniels, L. M.; Murillo, C. A. Inorg. Chim. Acta 1994, 224, 5−9. (24) (a) Fohlmeister, L.; Liu, S.; Schulten, C.; Moubaraki, B.; Stasch, A.; Cashion, J. D.; Murray, K. S.; Gagliardi, L.; Jones, C. Angew. Chem., Int. Ed. 2012, 51, 8294−8298. (b) Rose, R. P.; Jones, C.; Schulten, C.; Aldridge, S.; Stasch, A. Chem.Eur. J. 2008, 14, 8477−8480. (25) Average value for two chemically equivalent but crystallographically independent molecules in the unit cell (individual values: 2.238(5) Å and 2.307(7) Å). (26) Lang, H.; Herres, M.; Zsolnai, L.; Imhof, W. J. Organomet. Chem. 1991, 409, C7−C11. (27) Reihlen, H.; Gruhl, A.; Heßling, G. v.; Pfrengle, O. Liebigs Ann. Chem. 1930, 482, 161−182. (28) For example: (a) Jonas, K.; Klusmann, P.; Goddard, R. Z. Naturforsch. 1995, 50b, 394−404. (b) Grevels, F.-W.; Koerner, E.; Gustorf, v. Liebigs Ann. Chem. 1973, 1821−1838. (c) Koerner, E.; Gustorf, v.; Henry, M. C.; McAdoo, D. J. Liebigs Ann. Chem. 1967, 707, 190−202. (d) Brown, D. L. S.; Connor, J. A.; Leung, M. L.; PazAndrade, M. I.; Skinner, H. A. J. Organomet. Chem. 1976, 110, 79−89. (e) Angermund, H.; Grevels, F.-W.; Moser, R.; Benn, R.; Krüger, C.; Romão, M. J. Organometallics 1988, 7, 1994−2004. (f) Ros, J.; Viñas, J. M.; Mathieu, R.; Solans, X.; Font-Bardia, M. J. Organomet. Chem. 1986, 307, C7−C9. (g) Knölker, H.-J.; Baum, E.; Goesmann, H.; Klauss, R. Angew. Chem., Int. Ed. 1999, 38, 2064−2066. (29) Hill, D. H.; Parvez, M. A.; Sen, A. J. Am. Chem. Soc. 1994, 116, 2889−2901. (30) Fürstner, A.; Martin, R.; Krause, H.; Seidel, G.; Goddard, R.; Lehmann, C. W. J. Am. Chem. Soc. 2008, 130, 8773−8787. (31) Wesendrup, R.; Schalley, C. A.; Schröder, D.; Schwarz, H. Organometallics 1996, 15, 1435−1440. (32) Cutler, C.; Ehntholt, D.; Lennon, P.; Nicholas, K.; Marten, D. F.; Madhavarao, M.; Raghu, S.; Rosan, A.; Rosenblum, M. J. Am. Chem. Soc. 1975, 97, 3149−3157. (33) Schurig, V. Tetrahedron Lett. 1984, 25, 2739−2742. (34) (a) Agoston, G. E.; Cabal, M. P.; Turos, E. Tetrahedron Lett. 1991, 32, 3001−3004. (b) Jiang, S.; Turos, E. Tetrahedron Lett. 1991, 32, 4639−4642. (c) Chen, T.; Jiang, S.; Turos, E. Tehtrahedron Lett. 1994, 35, 8325−8328. (d) Clayton, H. S.; Moss, J. R.; Dry, M. E. J. Organomet. Chem. 2003, 688, 181−191. (35) This distance suggests intramolecular hydrogen bonding. The corresponding distances in other crystallographically inequivalent molecules of 7 in the unit cell range from 2.68 to 2.79 Å.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C. L. is grateful for a Feodor Lynen fellowship generously hosted by Prof. François Diederich.

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REFERENCES

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Mono- and Dinuclear Neutral and Cationic Iron(II) Compounds

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