Oxidation States, Stability and Reactivity of Organoferrate Com- plexes

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Oxidation States, Stability and Reactivity of Organoferrate Complexes Tobias Parchomyk, Serhiy Demeshko, Franc Meyer, and Konrad Koszinowski J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06001 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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Journal of the American Chemical Society

Oxidation States, Stability and Reactivity of Organoferrate Complexes Tobias Parchomyk†, Serhiy Demeshko‡, Franc Meyer‡, and Konrad Koszinowski*† †

Institut für Organische und Biomolekulare Chemie, Universität Göttingen, Tammannstraße 2, 37077 Göttingen, Germany ‡

Institut für Anorganische Chemie, Universität Göttingen, Tammannstraße 4, 37077 Göttingen, Germany

ABSTRACT: We have applied a combination of electrospray-ionization mass spectrometry, electrical conductivity measurements, and Mössbauer spectroscopy to identify and characterize the organoferrate species RnFem− formed upon the transmetallation of iron precursors (Fe(acac)3, FeCl3, FeCl2, Fe(OAc)2) with Grignard reagents RMgX (R = Me, Et, Bu, Hex, Oct, Dec, Me3SiCH2, Bn, Ph, Mes, 3,5-(CF3)2-C6H3; X = Cl, Br) in tetrahydrofuran. The observed organoferrates show a large variety in their aggregation (1 ≤ m ≤ 8) and oxidation states (I to IV), which are chiefly determined by the nature of their organyl groups R. In numerous cases, the addition of a bidentate amine or phosphine changes the distributions of organoferrates and affects their stability. Besides undergoing efficient intermolecular exchange processes, several of the probed organoferrates react with organyl (pseudo)halides R'X (R' = Et, iPr, Bu, Ph, p-Tol; X = Cl, Br, I, OTf) to afford heteroleptic complexes of the type R3FeR'−. Gas-phase fragmentation of most of these complexes results in reductive eliminations of the coupling products RR' (or, alternatively, of R2). This finding indicates that iron-catalyzed cross-coupling reactions may proceed via such heteroleptic organoferrates R3FeR'− as intermediates. Gas-phase fragmentation of other organoferrate complexes leads to β-hydrogen eliminations, the loss of arenes, and the expulsion of organyl radicals. The operation of both one- and two-electron processes is consistent with previous observations and contributes to the formidable complexity of organoiron chemistry.

1. Introduction Organoiron chemistry is characterized by a remarkable complexity.1 This complexity, together with the high reactivity and low stability of many organoiron species, has so far prevented a comprehensive understanding of their behavior in solution. At the same time, organoiron complexes attract a great deal of interest because of their role as intermediates in iron-catalyzed cross-coupling reactions.1,2 These reactions combine Grignard reagents RMgX and (pseudo)halides R'X for the expedient formation of new carbon-carbon bonds, eq. (1).3 In comparison to more conventional palladium or nickel catalysts, iron offers the advantages of low toxicity, wide abundance, and distinct reactivity patterns.4 However, with detailed insight into their mechanism lacking, iron-catalyzed cross-coupling reactions have not yet reached the stage, at which further progress could be achieved in a rational, knowledge-based manner. RMgX + R'X

R−R' + MgX2

(1)

To a large extent, our present insight into organoiron chemistry results from X-ray crystallography.5 This method has identified several organoiron complexes formed upon the transmetallation of simple iron precursors, such as FeCl2 or Fe(acac)3 (acac = acetylacetonato), with vari-

ous Grignard reagents or other organometallics. Interestingly, a significant number of these complexes are anionic (Chart 1). Thus, organoferrate anions are likely to play a key role in organoiron chemistry and iron-catalyzed cross-coupling. Despite the unmatched level of structural detail provided by X-ray crystallography, a limitation of this method lies in its confinement to the solid state and its inability to resolve dynamic features. Therefore, it is unclear whether the organoferrate complexes observed in the solid state remain intact in solution and which reactions they undergo. Here, we systematically examine the solution-phase behavior of organoferrates by a combination of electrospray-ionization (ESI) mass spectrometry, electrical conductivity measurements, and 57Fe Mössbauer spectroscopy. The first two of these methods selectively probe ionic species and, hence, are particularly well-suited for the analysis of organoferrate anions.6 We have previously demonstrated the success of this approach for selected examples of organoferrates7 as well as for other organometallic ate complexes.8 Mössbauer spectroscopy, in turn, has the advantage of probing the entire population of iron species, irrespective of their charge state. Using these methods, we identify the organoferrate species formed upon the transmetallation of various common iron precursors with Grignard reagents RMgX in tetrahydrofuran (THF), the solvent used most often in iron-catalyzed

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cross-coupling. We determine not only the effect of the iron precursor and the organyl group R on the constitution of the organoferrates, but also that of additives and ligands commonly applied in cross-coupling. Moreover, we investigate the dynamic behavior of the organoferrates in solution as well as their reactivity toward organyl (pseudo)halides as typical substrates in cross-coupling reactions. To obtain further insight into the stability of the sampled organoferrate complexes and elucidate their unimolecular reactivity, we also study the gas-phase fragmentation of the mass-selected ions.9 Chart 1. X-ray crystal structures of organoferrate complexes with (average) iron oxidation states in blue.5

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(1.0 M), (p-F-C6H4)MgCl (1.0 M)) or synthesized according to a literature procedure (MesMgBr (0.9 M), (ArF)MgBr (0.9 M, ArF = 3,5-(CF3)2-C6H3), (o-CF3-C6H4)MgBr (0.9 M), (p-CF3-C6H4)MgBr (0.5 M), (p-CN-C6H4)MgBr, HexMgCl (1.0 M), OctMgCl (1.0 M), DecMgCl (1.0 M), Me3SiCH2MgCl (0.8 M), BnMgCl (0.5 M)). 10 Exact concentrations were determined by iodometric titration.11 57FeCl2 was synthesized from 57Fe-enriched metal powder (95%, Isoflex) according to the literature.12 2.2 Sample Preparation Standard sample solutions were prepared by the addition of a Grignard reagent (4 equiv) to a solution of an iron precursor in THF at 273 K, stirring for 30 min, and dilution to 20 mM (ESI mass spectrometry) or 50 mM (electrical conductivity measurements). In experiments with additives (4 equiv), the latter were added before the Grignard reagent. For the formation of heteroleptic arylferrates, PhMgCl (2 equiv) and RMgX (2 equiv) were added to a Fe(acac)3/THF solution simultaneously. For probing intermolecular exchange reactions, organoferrate solutions were prepared separately before their combination. For investigating the reaction of organoferrates with organyl halides R'X, the Fe(acac)3/4 RMgX solutions were treated with R'X at 273 K. Mössbauer sample solutions were prepared from solutions of 57FeCl2 in THF (5 mM) in a glove-box at 298 K and directly transferred into the Mössbauer sample cell before freezing in liquid nitrogen (outside of the glove-box). 2.3 ESI Mass Spectrometry Sample solutions were injected into the ESI source of a HCT quadrupole-ion trap mass spectrometer (Bruker Daltonics) at a flow-rate of 8 µL min˗1 and transferred into the helium-filled ion trap under mild conditions.8a Mass spectra were recorded typically over an m/z range of 50-1200. In gas-phase fragmentation (collision-induced dissociation) experiments, ions of interest were massselected, subjected to excitation voltages of amplitudes Vexc (typically between 0 and 1.00 V) and allowed to collide with helium gas. Additional experiments were performed with a micrOTOF-Q II mass spectrometer (Bruker Daltonics) under comparable mild conditions. With an external calibration scheme (use of a mixture of CF3COOH and phosphazenes in H2O/MeCN), this instrument achieved average relative m/z errors of < 10 ppm for the organoferrate anions.

2. Experimental Section 2.1 Materials & General Methods In all cases, standard Schlenk techniques were applied. Tetrahydrofuran (THF) was dried over sodium/benzophenone and freshly distilled before use. TMEDA was dried first over calcium hydride, then over sodium, and finally distilled. Grignard reagents (all in THF) were used as purchased (MeMgCl (3.0 M), EtMgCl (2.0 M), BuMgCl (2.0 M), PhMgCl (2.0 M), PhMgBr (1.0 M), (p-OMe-C6H4)MgBr (1.0 M), o-TolMgBr (1.0 M), p-TolMgBr

2.4 Electrical Conductivity Measurements Electrical conductivity measurements were performed with a Seven Multi instrument (Mettler Toledo) with a stainless-steel electrode cell (κcell = 0.1 cm˗1) calibrated against an aqueous solution of KCl (0.1 M) at 298 K. Measurements were taken at 273 K to suppress decomposition and hydrolysis reactions. Iodometric titration of the Gri-


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gnard reagents immediately before the sample preparation found ≥ 90% of the expected activity.

2.5 Mössbauer Spectroscopy Mössbauer spectra were recorded with a 57Co source in a Rh matrix using an alternating constant acceleration Wissel Mössbauer spectrometer operated in the transmission mode and equipped with a Janis closed-cycle helium cryostat. Isomer shifts are given relative to iron metal at ambient temperature. Simulation of the experimental data was performed with the Mfit program using Lorentzian line doublets.13 2.6 Titration Experiments14

conductivities in all cases indicates significant quantities of free ions, in line with the high signal intensities of the ESI mass spectra. We also analyzed a frozen solution of 57 FeCl2/4 MeMgCl by Mössbauer spectroscopy (Figure 1, right). The obtained spectrum showed a very broad dou1 1 blet (δ = 0.29 mm s− , ΔEQ = 0.89 mm s− ), which closely − resembled that of Me12Fe8 reported by Neidig and coworkers.5h This agreement affords additional support − for the almost exclusive formation of Me12Fe8 under the present conditions and, thus, also confirms the validity of the ESI-mass spectrometric results. −

Table 1. Specific electrical conductivities κ / (µS cm 1) of methylferrates produced by the transmetallation of iron precursors (50 mM) with 4 equiv of MeMgCl in THF (T = 273 K). Sample solution in THF

HNO3 (conc., 5 mL) was used to oxidize all iron species to soluble iron(III). The sample solutions were diluted by water to overall volumina of 25 mL and made less acidic (pH ≈ 2-3) by the addition of p-chloroaniline. Chelatometric titrations were performed with aqueous solutions of ethylenediamine tetraacetate (0.1 M) and 5-sulfosalicylic acid (1 mM, 0.5 mL) as indicator (color change from violet to yellow) in triplicate.

3. Results 3.1 Effect of the Organyl Substituent R R = Methyl. As reported previously, the negative-ion mode ESI mass spectrum of a THF solution of Fe(acac)3 treated with 4 equiv of MeMgCl showed almost exclusive− ly Me12Fe8 with an average oxidation state of iron of 1.4 (Figure 1, left).7b Neidig and coworkers had previously demonstrated that this ion corresponds to an octanuclear cluster, whose iron centers adopt a heterocubane structure.5h Essentially identical negative-ion mode mass spectra were observed when FeCl3, FeCl2, or Fe(OAc)2 were transmetallated with MeMgCl (Figures S1-S3). This finding suggests that the nature of the iron precursor FeXn matters only very little for the resulting organoferrate species. The corresponding positive-ion mode mass spectra (Figures S4-S6) did not exhibit any iron-containing ions, but only magnesium complexes of the type MgmX2m−1(THF)n+ (m = 3,4; n = 1, 4-6). The same behavior was also found for the transmetallation with Grignard reagents other than MeMgCl (see below). The electrical conductivity of the sample solutions differed depending on the counter-ion X− of the iron precursor, but not on the oxidation state of the latter (Table 1). Presumably, the measured differences reflect different tendencies of the − MgmX2m−1(THF)n+ cations to interact with the Me12Fe8 anions. However, the observation of appreciable electrical

Fe(acac)3 + 4 MeMgCl

–1

κ / (µS cm ) 65 ± 17

FeCl3 + 4 MeMgCl

206 ± 17

FeCl2 + 4 MeMgCl

222 ± 30

Fe(OAc)2 + 4 MeMgCl

141 ± 46

R = Ethyl, Butyl, Hexyl, Octyl, and Decyl. Upon reaction of Fe(acac)3 with EtMgCl or BuMgCl, no organoferrate complexes could be detected by ESI mass spectrome− try. The reaction with HexMgCl produced Hex4Fe(III) , which quickly decomposed (Figure S7). Using OctMgCl and DecMgCl for the transmetallation, we found the − − analogous complexes Oct4Fe(III) and Dec4Fe(III) , respectively (Figures S8-S12). These species showed high, but gradually decreasing signal intensities. Besides − − Dec4Fe(III) , smaller amounts of Dec3Fe(II) were also present.15 R = (Trimethylsilyl)methyl. Upon the treatment of Fe(acac)3 with 4 equiv of Me3SiCH2MgCl and ESI-mass − spectrometric analysis, the complex (Me3SiCH2)4Fe(III) was observed as main species (Figure S13-S15). Additional ferrates of lower signal intensity included the mononucle− − ar ions (Me3SiCH2)3Fe(II) and (Me3SiCH2)3FeCl(III) as − well as the dinuclear complexes (Me3SiCH2)5Fe2Cl and − (Me3SiCH2)6Fe2Cl . The latter contained iron in average oxidation states of 2.5 and 3.0, respectively. The sample solution had an electrical conductivity significantly higher than those of the methylferrates, thus pointing to a higher concentration of free ions (Table S1). The Mössbauer spectrum of a frozen solution of 57FeCl2/4 Me3SiCH2MgCl was dominated by a single signature indicative of a highspin Fe(III) species (δ = 0.30 mm s−1, ∆EQ = 1.10 mm s−1; Figure S16). This finding is in accordance with − (Me3SiCH2)4Fe(III) being the most abundant ion detected by ESI mass spectrometry.



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Figure 1. Left: Negative-ion mode ESI mass spectrum of a solution of the products formed in the reaction of Fe(acac)3 (20 mM) − − with MeMgCl (4 equiv); a = Me13Fe8 , b = [Me12,Fe8,O2] . Right: Mössbauer spectrum of a frozen solution (T = 80 K) of the prod57 ucts formed in the reaction of FeCl2 (5 mM) with MeMgCl (4 equiv) in THF together with overall fit (black); components of the 1 1 1 1 fit: δ(blue) = 0.29 mm s− , ΔEQ(blue) = 0.89 mm s− , rel. int. = 97%; δ(red) = 0.82 mm s− , ΔEQ(red) = 2.94 mm s− , rel. int. = 3%.

R = Benzyl. Negative-ion mode ESI mass spectrometry of a solution of Fe(acac)3 treated with 4 equiv of BnMgCl − − afforded Bn3Fe(II) and Bn4Fe(III) as main species (Figure 2, left). These findings fully agree with previous investigations on this system, which also succeeded in the structural characterization of both complexes by X-ray crystallography.5f In addition, the dinuclear anions − − Bn5Fe2 and Bn6Fe2 with average oxidation states of 2.0 and 2.5, respectively, were observed.16 Again, the electrical conductivity of the sample solution exceeded that of the methylferrates, but was not as high as that of the (trimethylsilyl)methylferrates (Table S1). Similar to the ESI mass spectrometric results, the Mössbauer spectrum of a frozen solution of 57FeCl2/4 BnMgCl also pointed to the presence of two main species in the sample solution (δ = 1 1 0.27/0.26 mm s− , ΔEQ = 1.05/1.54 mm s− ; Figure 2, right), which could be assigned to low-spin Fe(II) and high-spin Fe(III) complexes. R = Phenyl. The transmetallation of Fe(acac)3 with − − PhMgCl furnishes a mixture of Ph3Fe(II) and Ph4Fe(III) , together with small amounts of the tetranuclear cluster Ph7Fe4− with iron in an average oxidation state of 1.5 (Figure S17).7a Replacing Fe(acac)3 for FeCl3 did not notably affect the formation of phenylferrate complexes detectable by ESI mass spectrometry (Figure S18). The use of FeCl2 or Fe(OAc)2 as iron precursor also resulted in the − − predominant formation of Ph4Fe(III) and Ph3Fe(II) , respectively (Figures S19 and S20), but in addition afforded the heterobimetallic complexes Ph6MgFe(III)− (in both − cases) and Ph5MgFe(III)Cl (only for FeCl2). This finding directly demonstrates the tendency of the organoferrate anions to interact with magnesium species. Again, the electrical conductivity of the sample solutions did not depend on the oxidation state of the iron precursor FeXn, but on the nature of its counter-ion X− (Table S2). Unlike the case of the methylferrates, the oxygen-containing

precursors now gave rise to higher conductivities than their chlorine-containing counterparts. The Mössbauer spectrum of a frozen solution of 57 FeCl2/4 PhMgCl displayed one broad feature indicative of at least one Fe(III) species (δ = 0.50 mm s−1, − ∆EQ = 1.06 mm s−1; Figure S21). Given that the Ph3Fe(II) 7a anion decomposes with a half-life of 50 ± 15 s, it is not expected to survive the preparation of the Mössbauer experiment, thus explaining why it was not observed. R = Mesityl. The transmetallation of Fe(acac)3 with 4 equiv of MesMgBr resulted in the formation of − Mes3Fe(II) (Figure S22 and S23). This ion showed a high ESI signal intensity, which did not decrease even after prolonged times (∆t ≥ 1 h). In addition, small amounts of − Mes2Fe(II)Br were also present. The high ESI signal intensity observed apparently reflects not only a high ESI − activity of Mes3Fe(II) , but also points to its abundance in solution, as the considerable electrical conductivity of the sample solution suggests (Table S1). Mesityliron compounds have been investigated quite − extensively,17 including the characterization of Mes3Fe(II) 5e,f by X-ray crystallography and Mössbauer spectroscopy. We also applied the latter technique to the analysis of a frozen solution of 57FeCl2/4 MesMgBr and did not find any − significant species besides Mes3Fe(II) (δ = 0.20 mm s−1, −1 ∆EQ = 1.45 mm s ; Figure S24). Thus, the Mössbauerspectroscopic experiment fully confirmed the ESI-mass spectrometric results. R = 3,5-Bis(trifluoromethyl)phenyl. Upon reaction of Fe(acac)3 with 4 equiv of ArFMgBr (ArF = 3,5-(CF3)2-C6H3) − and ESI-mass spectrometric analysis, (ArF)4Fe(III) was detected as base peak, together with the less abundant − − complexes (ArF)3Fe(II) and (ArF)2Fe(II)Br (Figures S25-


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Figure 2. Left: Negative-ion mode ESI mass spectrum of a solution of the products formed in the reaction of Fe(acac)3 (20 mM) − − with BnMgCl (4 equiv); a = [Bn,Fe,O2] , b = [Bn2,Fe,O2] . Right: Mössbauer spectrum of a frozen solution (T = 80 K) of the prod57 ucts formed in the reaction of FeCl2 (5 mM) with BnMgCl (4 equiv) in THF together with overall fit (black); components of the 1 1 1 1 fit: δ(blue) = 0.27 mm s− , ΔEQ(blue) = 1.05 mm s− , rel. int. = 44%; δ(red) = 0.26 mm s− , ΔEQ(red) = 1.54 mm s− , rel. int. = 43%; −1 −1 δ(green) = 1.04 mm s , ΔEQ(green) = 1.89 mm s , rel. int. = 13%.

S27). Of all organoferrate solutions probed, this one exhibited the highest electrical conductivity (Table S1). The Mössbauer spectra measured for frozen solutions of 57 FeCl2/4 ArFMgBr (T = 7 K, 80 K) pointed to the presence of two or more iron species (Figure S28 and S29). A more detailed analysis was prohibited by the complicating influence of slow paramagnetic relaxation. R = Phenyl and Aryl. We also treated Fe(acac)3 with 2 PhMgCl and 2 RMgBr (R = aryl) simultaneously to prepare and investigate mixed arylferrate complexes by ESI mass spectrometry (Figures S30-S39). These experiments provided a straightforward way to assess the influence of steric and electronic effects on the stability of the arylferrates. Starting with the electron-rich para-substituted aryl groups R = p-Me2N-C6H4 and p-MeO-C6H4, we detected, − − besides Ph4Fe(III) , the heteroleptic ferrates Ph2Fe(II)R , − PhFe(II)R2 (only in the case of R = p-MeO-C6H4), and − Ph3Fe(III)R (Figure S30 and S31), which rapidly declined in intensity. For R = p-Tol, the complete series − − PhnFe(II)R3−n (n = 0-3) and PhnFe(III)R4−n (n = 0-4) were detectable over a longer time period (Figure S32). When mixtures of PhMgCl and electron-poor parasubstituted aryl Grignard reagents were used for the transmetallation, the arylferrate(II) complexes largely disappeared. For R = p-F-C6H4, the mass spectrum − showed the full series of PhnFe(III)R4−n complexes (n = 0-4, Figure S33), whereas only ferrates containing mainly or exclusively the electron-poor aryl group R were detected for R = p-CF3-C6H4 and p-CN-C6H4 (Figures S34 and S35). In the case of R = p-CF3-C6H4, we also found − PhnFe(IV)R5−n complexes (n = 0 and 1), which represent rare examples of organoiron(IV) species.18

For comparison, we also studied the simultaneous transmetallation of Fe(acac)3 with PhMgCl and Grignard reagents bearing the ortho-substituted residues R = o-Tol, o-F3C-C6H4, and Mes (Figures S36-S39). The resulting ESI mass spectra mainly showed organoferrate(II) complexes − of the series PhnFe(II)R3−n . The predominance of these species over organoferrates(III) was particularly striking for R = o-Tol, as the latter prevailed upon the transmetallation with PhMgCl and p-Tol. Apparently, the substitution pattern of the aryl substituent has a major influence on the preferred oxidation state of the formed ferrates. In the case of R = Mes, we mainly observed complexes enriched in the Mes group. This enrichment persisted even when PhMgCl and MesMgBr were used in a 10:1 ratio (Figure S40).

3.2 Effect of Added TMEDA Methyl. The addition of TMEDA (N,N,N’,N’-tetramethylethylenediamine, 4 equiv) to a solution of Fe(acac)3/4 MeMgBr almost completely suppresses the formation of polynuclear organoferrates detectable by ESI – mass spectrometry (Figure S40).7b Although Me4Fe(III) and other mononuclear complexes persist in the presence of TMEDA, their low absolute signal intensity suggests that their concentration in the sample solution is rather low. While no TMEDA-containing anions are found in the negative-ion mode ESI mass spectra, TMEDA is incorporated in the magnesium cations detected by positive-ion mode ESI mass spectrometry (Figure S41). The same TMEDA-containing cations were also observed for the transmetallation reactions with all other Grignard reagents.



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In the presence of TMEDA, the Fe(acac)3/4 MeMgCl system showed a vanishingly small electrical conductivity of < 0.5 µS cm–1. This result provides even more direct evidence for the low concentration of free ions in the sample solution. Mössbauer spectroscopy also confirmed the strong effect of TMEDA. A frozen solution of this additive together with 57FeCl2 and MeMgCl did no longer show the − spectroscopic signature characteristic of the Me12Fe8 cluster, but displayed a sharp feature, which can be attributed to a single high-spin Fe(III) species (Figure 3). It seems unlikely that this species corresponds to – Me4Fe(III) , because Neidig and coworkers calculated an intermediate spin state for the latter.5g Similarly, the low – absolute ESI signal intensity of Me4Fe(III) and the drastic decrease of the electrical conductivity are at odds with the – presence of Me4Fe(III) as main species. Instead, the formation of a neutral methyliron complex appears more probable.

scopic appearance of the sample solution evolved in time. After the addition of MeMgBr, the yellow solution of FeCl2 and TMEDA darkened quickly and turned almost completely black after approx. 2 min (Figure 4). Upon removal of the solution, a solid black residue remained, which had not been noted in the experiments performed at smaller scale and which consequently had not been considered for the Mössbauer experiments. Titration showed that the precipitate contained 38 ± 4% of the employed iron while its fraction in solution was independently determined as 60 ± 7%. Analysis of the precipitate by Mössbauer spectroscopy found a sharp, but noncharacteristic doublet (Figure S42). Exposure of the precipitate to air (t = 30 min) resulted in a complete change of the spectrum and the appearance of a new feature indicative of a high-spin Fe(II) species formed by oxidation (Figure S43). This finding strongly suggests that the precipitate contained low-valent iron.

Figure 3: Mössbauer spectrum of a frozen solution 57 (T = 80 K) of the products formed in the reaction of FeCl2 (5 mM) with TMEDA (4 equiv) and MeMgBr (4 equiv) in THF together with overall fit (black); components of the fit: 1 1 δ(blue) = 0.53 mm s− , ΔEQ(blue) = 0.67 mm s− , rel. 1 1 int. = 98%; δ(red) = 0.50 mm s− , ΔEQ(red) = 2.22 mm s− , rel. int. = 2%.

Figure 4: (A) Yellow solution of FeCl2 and TMEDA in THF. The solution darkened immediately after addition of MeMgBr (B) and turned black after approx. 2 min (C). A black solid (D) remained after removal of the supernatant solution (E).

Unlike ESI mass spectrometry, Mössbauer spectroscopy detects the entire population of 57Fe centers present in the sample and, thus, is well suited to probe the average oxidation state of iron. The observed nearly quantitative formation of an Fe(III) species upon the reaction of the Fe(II) precursor with a Grignard reagent is quite remarkable. As the latter certainly cannot bring about an oxidation and as we have carefully excluded the intake of traces of atmospheric oxygen,19 the present finding as well as the observation of abundant Fe(III) species in most of the other experiments requires an alternative explanation. We therefore wondered whether we might have lost a fraction of the iron during sample preparation for the Mössbauer experiment. To address this problem, we repeated the transmetallation of FeCl2 in the presence of TMEDA at a larger scale and observed how the macro-

R = Ethyl, Butyl, Hexyl, Octyl, and Decyl. The presence of TMEDA significantly enhanced the stability of the alkylferrates formed upon transmetallation of Fe(acac)3 with RMgCl and, thus, permitted the ESI-mass spectrometric detection of several ferrate anions, which had not − been found without this additive. For R = Et, Et3Fe(II) − and Et4Fe(III) could be observed within the first 2 min after sample preparation (Figures S44). The analogous − − ions Bu3Fe(II) and Bu4Fe(III) , the latter constituting the base peak, were also detected for R = Bu (Figure S45). In − addition, the dinuclear complex Bu5Fe2(II) as well as the − low-valent complex Bu2Fe(I) could be identified. In the cases of R = Hex, Oct, and Dec, the addition of TMEDA did not afford any new species (Figures S46-S48). The Mössbauer spectrum recorded for a frozen solution of 57 FeCl2/DecMgCl pointed to the presence of a single highspin Fe(III) species (Figure S49). Again, this finding


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Journal of the American Chemical Society −

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agrees well with the prevalence of Dec4Fe(III) in the ESI mass spectrum. R = (Trimethylsilyl)methyl and Benzyl. Upon the addition of TMEDA, the transmetallation of Fe(acac)3 with Me3SiCH2MgCl or BnMgCl did no longer furnish any detectable dinuclear ferrate complexes (Figures S50 and S51). Otherwise, the obtained ESI mass spectra closely resembled those recorded in the absence of TMEDA and showed both organoferrate(II) and (III) complexes. For the case of the benzylferrates, the addition of TMEDA again strongly lowered the electrical conductivity (Table S3). R = Phenyl, Mesityl, and 3,5-Bis(trifluoromethyl)phenyl. The presence of TMEDA (4 equiv) stabilizes the − − mononuclear Ph3Fe(II) and Ph4Fe(III) complexes, but prevents the formation of higher aggregates (Figure S52).7a While the addition of TMEDA lowered the electrical conductivity of the phenylferrates only slightly (Table S3), it led to significant changes of the Mössbauer spectrum (Figure S53) measured for a frozen solution of 57 FeCl2/4 PhMgCl. First, we observed a marked sharpening of the feature at δ = 0.49 mm s−1, which we assigned to a high-spin Fe(III) species. Presumably, this sharpening resulted from altered magnetic properties of the sample induced by the coordination of TMEDA to the magnesium counter-ions. Such TMEDA-containing magnesium cations were directly observed by positive-ion mode ESI mass spectrometry (Figure S41). Second, the presence of TMEDA gave rise to an additional resonance signal, which apparently belonged to a low-spin Fe(II) complex. Like ESI mass spectrometry, Mössbauer spectroscopy, thus, showed TMEDA to stabilize iron(II) species although no evidence for a direct interaction between the two components was found. In contrast, TMEDA did not significantly affect the negative-ion mode ESI mass spectra of the organoferrates produced in the reactions of Fe(acac)3 with MesMgBr or (ArF)MgBr (ArF = 3,5-(CF3)2-C6H3, Figures S54 and S55).

sities. No iron-containing complexes were detected for solutions of FeCl2(dppbz)2 treated with RMgCl (R = Bu, Hex, Oct, Dec) (Figures S60-S63). When solutions of FeCl2(dppbz)2/RMgCl (R = Me3SiCH2 and BnMgCl) were probed, only the organoferrates already known from the reactions without dppbz were found (Figures S64 and S65). R = Aryl. The transmetallation of FeCl2(dppbz)2 with 4 equiv of PhMgCl gave rise to numerous low-valent phenylferrate complexes detectable by ESI mass spectrometry (Figure S66).7a The Mössbauer spectrum measured for a frozen solution of 57FeCl2/4 PhMgCl/2 dppbz also pointed to the formation of several different iron complexes (Fig− ure S67). While again almost exclusively Mes3Fe(II) was found upon the transmetallation of FeCl2(dppbz)2 with MesMgBr (Figure S68), the ESI-mass spectrometric analysis of the corresponding reaction of (ArF)MgBr (ArF = 3,5− (CF3)2-C6H3) detected (ArF)3Fe(II)(dppbz) in high signal intensity, together with the already known complexes − − (ArF)3Fe(II) and (ArF)4Fe(III) (Figure S69-S71).

3.4 Intermolecular Exchange Reactions To study the dynamic behavior of the organoferrates, we combined equimolar solutions of two different, preformed organoiron species and analyzed the resulting mixtures by ESI mass spectrometry (Figures 5 and S72S78). In all cases, we observed the formation of heteroleptic organoferrates. The exchange reactions occurred so fast (at a reaction temperature of 273 K) that we were not able to follow their kinetics.

3.3 Effect of Added dppbz R = Alkyl. ESI mass spectrometry did not find any ferrate anions formed upon the reaction of FeCl2(dppbz)2 (dppbz = 1,2-bis(diphenylphosphino)benzene) with 4 equiv of MeMgCl (Figure S56). Instead, the iron-containing cations Fe(I)(dppbz)2+ and MeFe(II)(dppbz)2+ were detected in low, but stable signal intensities (Figure S57). When FeCl2(dppbz)2 was transmetallated with EtMgCl, an ani− onic complex of the formula of [Et2Fe(dppbz)−2H] was found as base peak in the negative-ion mode ESI mass spectrum (Figure S58 and S59).20 Furthermore, the low− − valent organoferrates Fe(−I)(dppbz) , Fe(0)H(dppbz)2 , − and EtFe2(0)(dppbz) were observed in lower signal inten-

Figure 5: Negative-ion mode ESI mass spectrum of a solution of the products formed in the exchange reaction of Fe(acac)3 (10 mM)/TMEDA (4 equiv)/OctMgCl (4 equiv) with Fe(acac)3 (10 mM)/TMEDA (4 equiv)/DecMgCl (4 equiv) in − − THF; a = [Oct2,Fe,O2] , b = [Oct,Fe,Dec,O2] , − − c = [Fe,Dec2,O2] , d = FeDec3 .



Page 8 of 25

3.5 Reactivity of Organoferrates toward Organyl (Pseudo)Halides Methylferrates. For testing the reactivity of the system FeCl2/4 TMEDA/4 MeMgBr, we treated it with (p-bromobenzyl)triphenylphosphonium bromide. This charge-tagged substrate permits the straightforward detection of cross-coupling products by ESI mass spectrometry.7b,21 However, neither the supernatant solution separated from the rapidly forming precipitate (see above) nor a suspension of the latter in THF was found to be reactive (Figures S79 and S80). Butylferrates. When the phenyl halides PhX (X = Cl, Br, I; Figures S81-S83) and p-tolyl (pseudo)halides p-TolX (Figures S84-S87), respectively, were added to in-situ − formed butylferrates, the complexes Bu2nFen(II)X , n = 1 and 2, were observed in almost all cases (except for X = OTf). For the reactions with the aryl iodides, the ion − Bu2Fe(II)I even corresponded to the base peak. These − reactions also afforded Bu4Fe2H2I , which possibly result− ed from two consecutive β-H eliminations from a Bu6Fe2I precursor. For the reactions of the butylferrates with PhBr and p-TolOTf, we moreover observed the formation of − − Bu3Fe(III)Ph and Bu3Fe(III)(p-Tol) , respectively. Benzylferrates. The reaction of in situ prepared benzylferrates with 1 equiv of iPrCl and iPrBr, respectively, − afforded the heteroleptic Fe(III) complex Bn3FeiPr , which showed a low ESI signal intensity (Figures S88 and S89). The corresponding reaction with iPrI furnished − − Bn2Fe(II)I and BnFe(II)I2 (Figures S90-S92). Phenylferrates. We had previously shown that the addition of iPrCl to a solution of the in-situ formed phenylfer− rates affords the heteroleptic Fe(III) complex Ph3FeiPr and had also studied the evolution of this species in time.7a We now extended this reaction to a larger series of alkyl halides R'X (0.5 equiv; R' = Et, iPr, Bu; X = Cl, Br, I) − and detected the corresponding ferrates(III) Ph3FeR' in 22 all cases (Figures 6 and S93-S99). For several systems, − we also observed additional species of the type Ph4FeR' in low and rapidly declining signal intensities (Figures S93, S94, S97 and S99). To test whether these species − represented genuine Ph4Fe(IV)R' ferrates or, alternative− 6 ly, constituted Ph2Fe(II)(η -Ph2)R' complexes, we added 0.5 equiv of 4,4'-dimethylbiphenyl to the sample solutions (Figures S100 and S101). If the species in question contained a π-bound biphenyl moiety, the latter would supposedly undergo facile exchange reactions with 4,4'-dimethylbiphenyl. In no case was such an exchange observed. The negative outcome of these control experi− ments suggests that the Ph4FeR' species indeed correspond to genuine ferrate(IV) complexes. Further experiments using 1.0 and 10 equiv of EtBr gave rise to inorganic species only (Figures S102 and S103).

Figure 6: Negative-ion mode ESI mass spectrum of a solution of the products formed in the reaction of Fe(acac)3 i (20 mM) with PhMgCl (4 equiv) and PrCl (0.5 equiv) in THF.

Mössbauer spectroscopy of the products of the reaction between the phenylferrates and iPrCl found a predominant resonance signal identical to that of the Fe(III) species observed for 57FeCl2/PhMgCl in the absence of iPrCl (Figures S104 and S105). The lack of any clear evidence of heteroleptic Fe(III) complexes possibly reflects their lability. Our previous ESI-mass spectrometric experiments − had demonstrated that Ph3FeiPr decomposes at a time 7a scale of a few minutes. Next, we investigated the reactions of the in-situ formed phenylferrates with various aryl (pseudo)halides R'X (R'X = p-TolCl, p-TolBr, p-TolI, p-TolOTf, o-F3C-C6H4Br, and MesBr) by ESI mass spectrometry (Figures S106-S111). Except for the reaction with p-TolBr, we observed the − heteroleptic ferrates Ph3Fe(III)R' in all cases, although in − only rather low signal intensities. In addition, Ph4FeR' species were visible in two instances (Figures S108 and S110). Control experiments with added 4,4'dimethylbiphenyl did not show the occurrence of any exchange reactions and, thus, again indicated the presence of genuine Fe(IV) complexes (Figures S112 and S113).

3.6 Gas-Phase Fragmentation Reactions Radical Expulsion. To probe the unimolecular reactivity of organoiron species, we subjected all ions observed in significant intensity to gas-phase fragmentation experiments (Figures S112-S186). Among the different types of resulting reactions, the loss of organyl radicals R•, eq. (2), was particularly prominent for R = Me, Me3SiCH2, and Bn. Homoleptic organoferrates with these residues reacted in this way, with only little dependence on their oxidation or aggregation state.


Page 9 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

RmFex

Journal of the American Chemical Society −

→

Rm−1Fex

−

+ R•

(2)

Many of the heteroleptic organoferrates bearing these alkyl groups displayed a similar reactivity. For example, − the complexes Ph2FeR (R = Me, Bn) also lost R, but no Ph radicals (Figures S136 and S139). Indeed, the reluctance of organoferrates to release aryl radicals proved to be quite − general, an exception being the loss of ArF from (ArF)3Fe (Figure S147). The expulsion of organyl radicals was also observed for organoferrates containing longer alkyl chains, but strongly depended on the oxidation state of the iron center. − Thus, the homoleptic ferrate(III) complexes R4Fe (R = Bu, Hex, Oct, Dec) all underwent this reaction predominantly or even exclusively (Figures 7, left and S158, S161, S162), unlike their ferrate(II) counterparts (see below). The loss of Dec radicals moreover occurred for the het− − − eroleptic species PhFeDec3 MeFeDec3 and Oct4−nFeDecn (n = 1-3; Figures S154, S176 and S163-S165). Finally, the − ligated complexes EtFe(dppbz) and MeFe(dppbz)+ also released an Me or Et radical, respectively, upon fragmentation (Figures S115 and S151). Reductive Elimination. Reductive elimination was found to be the typical and essentially exclusive fragmentation of homo- and heteroleptic arylferrates(III) − R4−nFemR'n (R, R' = aryl). For complexes of the latter type, we have previously reported how electronic and steric effects determine the branching ratio between the formation of the homo- and the cross-coupling product, eq. (3a) and (3b), respectively.23 R4−nFemR'n

−

R2−nFemR'n

→

R3−nFemR'n−1

Phenyl-containing ferrate(IV) complexes likewise exhibit− ed non-uniform behavior. Ph4FeBu preferentially reacted via a reductive elimination and released Ph2 (Figure S184), − eq. (4), unlike Ph4Fe(p-Tol) (see below). Ph4FeBu

−

→

Ph2FeBu

−

+ Ph2

(4)

Elimination of RH. Mononuclear homoleptic arylfer− rate(II) complexes R3Fe (R = Ph, Mes, o-Tol) lost RH molecules upon fragmentation (Figures S134, S145 and S149), eq. (5). Obviously, these reactions involve a C−H activation step. R3Fe

−

→

[R2Fe−H]

−

+ RH

(5)

Analogous decomposition pathways were also observed − for the corresponding heteroleptic complexes Ph2FeMes , − − − PhFeMes2 , Mes2FeBr , and Ph2FeR (R = o-Tol, Figures S141-S143 and S146). Similarly, the Fe(IV) complexes − Ph4FeR' (R' = p-Tol, o-CF3-C6H4) eliminated benzene (Figure S185 and S186), eq. (6). −

Ph4FeR'

→

−

[Ph3FeR'−H]

+ PhH

(6)

−

−

→

−

probed R3Fe complexes (R = aryl) showed mainly or exclusively alternative fragmentation patterns, respectively − (see below). The homoleptic cluster anions Ph7Fe4 and − Ph8Fe5 both underwent reductive eliminations (Figures S116 and S117) as did their heteroleptic counterparts − Me12−nFe8Phn , 3 ≤ n ≤ 5.7b

+ R2 −

(3a)

+ R–R' (3b)

Heteroleptic ferrate(III) complexes bearing both aryl and alkyl groups also underwent reductive eliminations. Spe− cies of the type Ph3FeR' exclusively or predominantly afforded the cross-coupling product PhR' for R' = Me, Et, i Pr, Bu, and Dec (Figures S170-S172 and S174), whereas almost only the homo-coupling product Ph2 was formed − for R' = Bn (Figure S177). The related Ph2FeBn2 complex was more reluctant toward fragmentation and afforded the alternative homo-coupling product Bn2 (Figure S178). For the alkyl-rich complexes, additional fragmentation reactions occurred as well (see below). To a minor extent, − the alkylferrates R4Fe(III) (R = Bu, Hex, Oct, Dec) also underwent reductive eliminations.24 Arylferrates in oxidation states < III showed differing tendencies toward reductive elimination. The dinuclear − organoferrate(II) Ph5Fe2 reacted in this way (Figure S135), − whereas its mononuclear homologue Ph3Fe and all other

The loss of butane from Bu4Fe2H2I also corresponds to the elimination of an RH molecule (Figure S162). However, in this case the parent ion already bore a hydride moiety such that this reaction channel can be also considered a reductive elimination. −

β-H Elimination. The alkylferrate(II) complexes R3Fe (R = But, Hex, Oct, Dec) exclusively released the corresponding alkenes upon gas-phase fragmentation (Figures 7, right and S120, S126, S128), eq. (7). These reactions apparently involve β-H eliminations. R3Fe

−

→

−

R2FeH

+ (R–H)

(7)

Analogous behavior was also observed for the heteroleptic − − anions Ph2FeDec and PhFeDec2 (Figures S137 and S138) as well as for the low-valent ligated complexes EtFe2(dppbz)− (Figure S115). In contrast, none of the al− kylferrate(III) complexes R4Fe (R = But, Hex, Oct, Dec) reacted in this manner (see above).



Page 10 of 25

−

Figure 7: Left: Mass spectrum of mass-selected Dec4Fe (m/z 621) and its fragment ions produced upon collision-induced disso− ciation (Vexc = 0.60 V). Right: Mass spectrum of mass-selected Dec3Fe (m/z 479) and its fragment ions produced upon collisioninduced dissociation (Vexc = 0.70 V).

Loss of FeR2. Upon gas-phase fragmentation, the dinu− clear ferrates R5Fe2 (R = CH2SiMe3 and Bn) underwent deaggregation reactions and lost FeR2 units (Figures S129 and 133), eq. (8). Similarly, Me12Fe8− releases FeMe2.7b −

R5Fe2

→

R3Fe

−

+ R2Fe

(8)

Further Reactions. Additional decomposition reactions were observed for complexes containing the dppbz ligand. These complexes lost the intact ligand or fragments thereof (Figures S118 and S148). Furthermore, ferrates bearing ArF groups showed a high tendency to release CF2 units from the trifluoromethyl residue (Figures S144 and S150).

4 Discussion 4.1 Efficiency of Transmetallation In all cases, the treatment of iron precursors with a modest excess of a Grignard reagent resulted in complete transmetallation. This finding shows that transmetallation occurs essentially independent of the nature of the organyl substituent and the iron precursor. The formation of ate complexes moreover implies that the transmetallation does not halt at the stage of neutral Rn−1Fe species, but continues to transfer an additional organyl group to − the iron center. The detection of Ph6MgFe(III) and − Ph5MgFe(III)Cl indicates that the transmetallation can probably even lead to complexes with a dianionic − Ph5Fe(III)2 core. This strong tendency to add organyl groups and form anionic ate complexes reflects the high Lewis acidity of iron(III) centers.25 In contrast, magnesium readily affords cationic species. This distinction clear-

ly demonstrates the differing electropositivities of iron on the one hand and magnesium on the other. 4.2 Redox Behavior and Spin States of Organoiron Species The ligand-free organoferrates observed by ESI mass spectrometry (Table 2) exhibit a significant diversity in oxidation states ranging from Fe(I) to Fe(IV). While ESI mass spectrometry probes only the ions present in solution, Mössbauer spectroscopy detects the entire population of iron species. Both methods yielded largely consistent results in that they found the transmetallation reactions to afford mainly iron(II) and iron(III) species. Mössbauer spectroscopy has the additional advantage of providing information on the spin state of the iron centers. Thus, we obtained evidence of a low-spin character of the in-situ formed iron(II) species, whereas their iron(III) counterparts displayed high-spin states. The fact that abundant iron(III) species are produced from iron(II) precursors in the presence of a Grignard reagent, i.e., under potentially reducing conditions, warrants an explanation. Previous work has made similar observations and postulated the occurrence of redox disproportionations, which should concomitantly form lowvalent iron.5f,7a,26 However, only in very few cases could such low-valent species be observed. In the present study, we succeeded in the detection of the transient complex − Bu2Fe(I) , whose lability results from its extremely electron-rich character. Low-valent iron complexes can be stabilized by bidentate phosphine ligands, which lower their electron density by π back bonding and by coordinatively saturating the iron center. Our control experiments on the system FeCl2/MeMgBr/TMEDA moreover showed that significant amounts of low-valent iron can also be tied up in precipitates. Given the dark color of sample solutions of organoferrates, these precipitates are easily


Page 11 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Observed organoferrates formed upon the transmetallation of Fe(acac)3 with different Grignard reagents (4 equiv) and their oxidation states. Species R2Fe

−

1.0

–

c

e

a

Et

Bu

–

a

Hex

Oct

Dec

Bn

Me3SiCH2

Ph

Mes

ArF

+

d

–

–

–

–

–

–

–

–

b

1.4

++

–

–

–

–

–

–

–

–

–

–

1.5

–

–

–

–

–

–

–

–

+

–

–

−

1.5

+

–

–

–

–

–

–

–

–

–

–

2.0

+

+

+

+

+

+

+

+

+

++

++

R5Fe2

−

2.0

–

–

+

+

–

–

+

–

–

–

–

− R6Fe2

2.5

–

–

–

–

–

–

+

–

–

–

–

+

+

+

++

++

++

+

++

+

–

+

R7Fe4

R13Fe8 R3Fe

−

R4Fe d

R = Me

−

R12Fe8

a

−

Oxidation state

−

3.0 b

c

In the presence of TMEDA. ArF = 3,5-(CF3)2-(C6H3). Species not observed (signal intensity ≤ 2% relative to the base peak). e Species observed in low or medium signal intensity (2-50%). Species observed in high signal intensity (≥ 50%).

overlooked, particularly for small sample volumes. To determine the average oxidation state of the precipitated iron, we combine the information from the titration experiments (approx. 60:40 ratio between dissolved and precipitated iron) with the result of the Mössbauer measurement, which found almost exclusively iron(III) in the supernatant solution. Balancing the number of electrons requires the precipitated iron originating from the disproportionation of iron(II) to have an average oxidation state of 0.5. This low oxidation state is fully consistent with the ready oxidation of the precipitate upon exposure to air. Its lack of reactivity toward the charge-tagged phenyl bromide possibly results from the aggregation of individual iron centers during the precipitation process. The tendency of the organoferrates to undergo redox − disproportionation reactions and form R4Fe(III) species inversely correlates with the steric demand of the organyl group R. For sterically less demanding groups, such as R = − n-alkyl or para-substituted phenyl, R4Fe(III) complexes − predominated. In contrast, mainly or exclusively R3Fe(II) anions were found for ferrates bearing the sterically more demanding ortho-tolyl and mesityl group, respectively. This trend indicates that the observed redox disproportionation does not proceed via a long-range electron transfer, but instead requires the close proximity of the participating iron centers. Obviously, the iron centers cannot approach each other easily if they are bound to sterically demanding organyl groups. The detected organoferrate(IV) complexes presumably result from redox disproportionation reactions as well. Fürstner and coworkers had also suggested such an origin for the neutral tetraalkyliron species they had observed.18 In those cases, even sterically demanding adamantyl groups apparently participated in redox disproportionation reactions.

4.3 Aggregation and Association The steric demand of the organyl group R controls not − only the redox behavior of the organoferrates RnFe , but also their tendency to form higher aggregates: the smaller R, the larger the clusters formed. Furthermore, the present results show that the deaggregating effect of TMEDA is quite a general phenomenon. The absence of any TMEDA-containing organoferrate anions implies that this additive brings about its effect in an indirect fashion. Possibly, it could bind to neutral organoiron species. However, our Mössbauer experiments did not give any evidence for such an interaction, but instead suggested the coordination of TMEDA to the MgX+ counter-ions. Findings by Bedford and coworkers pointed into a similar direction.5f,27 Apparently, the incorporation of TMEDA in the counter-ions modulates the interaction of the latter with the mono- and polynuclear organoferrate species and, thus, affects their relative stability. Recent results of Neidig and coworkers indicate that N-methylpyrrolidone, another common additive in iron-catalyzed crosscoupling, shows quite a similar behavior.5j The ESI-mass spectrometric experiments find identical organoferrate anions produced from different iron precursors FeXn. However, the electrical conductivity as well as the Mössbauer measurements indicate that the precursor counter-ion X− influences the interaction between the ferrate anions and the magnesium cations. The fact that we observe hardly any magnesium incorporation in the ferrate anions suggests that the interaction between the two species is rather weak. Likewise, the known crystal structures of organoferrates only comprise solventseparated magnesium counter-ions (Chart 1). According to the HSAB concept, the hard MgX+ cations preferentially bind to hard Lewis bases, such as THF and Cl−, but not to the softer organoferrate anions. We have previously found a very similar situation for other cases of related magnesium organometallates.8d,28



4.4 Dynamic Behavior in Solution and Stability The present ESI mass-spectrometric measurements have identified several organoferrates known and structurally − − − characterized in the solid state (Me3Fe , Me4Fe , Me12Fe8 , − − − Bn3Fe , Bn4Fe , and Mes3Fe , Chart 1). This finding clearly shows that these complexes remain intact in THF solution. At the same time, they undergo efficient exchange processes, as the mixing experiments demonstrate. This dynamic behavior of the organoferrates is also in line with their high tendency toward redox disproportionation − (with the exception of Mes3Fe ). The formation of a precipitate observed for the system FeCl2/MeMgBr/TMEDA parallels the generation of iron nanoparticles, which Bedford and coworkers observed upon the in-situ reduction of FeCl3/polyethylene glycol.29 Both findings point to a limited stability of the initially formed organoiron species. In the absence of coordinating ligands, low-valent transition-metal complexes are indeed well-known to be rather unstable toward aggregation reactions, which eventually lead to the precipitated − free metal.30 Me12Fe8 and the other cluster ions detected can be considered early intermediates of these aggregation reactions. If no low-valent species can form, such as − in the case of Mes3Fe (see above), the stability of the organoiron species is much higher. Information on the nature of likely decomposition reactions is provided by the gas-phase fragmentations. These experiments identify the loss of alkyl radicals, reductive eliminations, and β-hydrogen eliminations as feasible decomposition pathways of the examined organoferrates. All of these processes are well known for transition-metal complexes in general31 and organoiron species in particular.2

Page 12 of 25

iron(II) complex to yield the product of the oxidative addition. Support for the stepwise nature of the oxidative addition is provided by the observation of halogencontaining iron(II) intermediates. However, these intermediates were detected not only in the reactions of alkyl halides, but also those of aryl halides. It appears less likely that these substrates could undergo homolytic bond cleavages because the resulting aryl radicals would be rather unstable. Therefore, the halogen-containing iron(II) intermediates do not necessarily originate from the direct transfer of halogen atoms, but possibly form via different pathways, such as a concerted oxidative addition followed by a transmetallation of the resulting halogen− containing species with R3Fe species. Scheme 1. Possible reaction pathways for the formation − of heteroleptic organoferrates R3FeR' .

Alternative mechanisms affording heteroleptic organoferrates(III) might involve two iron(II) centers instead of a single iron(I) species (Scheme 1, (b)). However, our present results do not give any evidence for such behavior.

4.6 One- versus Two-Electron Processes

4.5 Reactivity toward Organyl (Pseudo)Halides The formation of heteroleptic organoferrates(III) (and, in a few cases, of organoferrates(IV)) observed upon the reaction of the homoleptic ferrates with organyl (pseudo)halides is fully in line with previous results.7a Benzyland phenylferrates were found to react with alkyl halides whereas butyl and phenyl ferrates reacted with aryl (pseudo)halides. One conceivable pathway affording heteroleptic organoferrates(III) would involve an oxidative addition to organoiron(I) species followed by a transmetallation (Scheme 1, (a)). The detection of the − iron(I) complex Bu2Fe indicates that such a mechanism might indeed be possible. The putative oxidative addition could proceed in a concerted or stepwise fashion. In the stepwise pathway, the homolytic cleavage of the C−X bond would lead to the transfer of the halogen atom X onto the iron(I) species. The resulting organyl radical could then recombine with the halogen-containing

Organoiron chemistry is well-known to involve both oneand two-electron processes.1,2,3f,32 The stepwise oxidative addition of organyl halides mentioned before represents an example of the former type of reactions, whereas its concerted counterpart corresponds to the second type. As the discussion above shows, distinguishing between both mechanisms for solution-phase reactions can be quite difficult. In contrast, gas-phase reactions of mass-selected organoferrates lend themselves to a much more straightforward analysis. Radical expulsions, i.e., prototypical one-electron processes are observed upon the fragmentation of several alkylferrates. These reactions prevail for ferrates containing methyl, benzyl, and (trimethylsilyl)methyl groups. The two latter ones form relatively stable radicals and, thus, are well expected to react in this way. The loss of alkyl radicals is also prominent for − R4Fe(III) complexes, whereas the expulsion of aryl radicals scarcely occurs. Clearly, this deviating reactivity reflects the lower stability of aryl radicals.


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Instead of one-electron processes, arylferrates almost exclusively undergo two-electron reactions. For iron(III) complexes, reductive eliminations predominate. We have previously studied the behavior of a series of heteroleptic − tetraarylferrrates Ph3Fe(III)R for probing how this elementary step and, in particular, the competition between cross- and homo-coupling are influenced by electronic and steric effects.23 We found the reductive elimination to occur in such a way that the more electron-withdrawing aryl groups remain attached to the iron center. Thus, they help to counter-act the build-up of electron density at iron in the course of the reductive elimination. For the systems investigated in the present work, electronic effects apparently operate in a similar manner, as the ob− served elimination of PhR from the Ph3Fe(III)R complexes bearing simple alkyl groups R demonstrates (Ph being more electron-withdrawing than simple alkyl substituents). The strongly predominant loss of the Ph2 homo− coupling product from Ph3Fe(III)Bn is also in accordance with the explanation put forward because in this case, the benzyl group can better stabilize extra electron density than phenyl (judging from the gas-phase acidities of BnH and PhH, ΔHacid(BnH) = 1592 kJ mol−1 and ΔHacid(PhH) = 1669 kJ mol−1).33,34 Arylferrates in the +II oxidation state show a lower tendency toward reductive elimination. Indeed, it is quite typical that a decrease in the oxidation state of the transition metal renders reductive eliminations more difficult.35 − The loss of arenes RH from R3Fe(II) observed instead is not well known from solution-phase chemistry. −

The β-hydrogen eliminations predominating for R3Fe(II) complexes with alkyl chains also correspond to twoelectron processes. This reactivity stands in marked con− trast to the loss of radicals observed for their R4Fe(III) counterparts. Presumably, the deviating behavior of the latter results from their lack of a free coordination site, which is needed to accommodate the alkene released during the β-hydrogen elimination.

4.7 Implications for Cross-Coupling Chemistry The present findings have direct implications for our understanding of iron-catalyzed cross-coupling reactions. First, they demonstrate that the in-situ formed organoiron species and potential catalytic intermediates can adopt a wide range of oxidation states. For assessing the overall network of redox reactions relevant to iron catalysis, it is not sufficient to consider only the iron species in solution, but necessary to take into account the possibility of iron precipitates. In the present case, the precipitate of low-valent iron did not show any reactivity toward a typical cross-coupling substrate, unlike the polyethylene glycole-based nanoparticles investigated by Bedford and coworkers (see above).29

The relative prominence of heteroleptic organoferrates(III) observed upon the addition of organyl (pseudo)halides to solutions of the organoferrates formed by transmetallation suggests that the catalytic cycle of the cross-coupling reactions proceeds via these or closely related iron(III) species. The readiness of the arylferrates(III) to undergo reductive eliminations directly demonstrates their ability to afford C−C-coupled products. The branching between cross-coupling and homocoupling observed for the heteroleptic complexes shows the strong influence of electronic effects on the productforming step of the catalytic cycle (see above). The knowledge of this dependence promises to be quite useful for predicting the selectivity of cross-coupling reactions involving aromatic substrates. For the other elementary steps of iron-catalyzed crosscoupling reactions, not only two-electron, but also oneelectron processes must be considered. As the present results clearly show, the propensity of the organoiron species toward such reactions correlates with the stability of the corresponding organyl radicals. Obviously, the possible operation of both one- and two-electron processes in iron-catalyzed cross-coupling reactions greatly adds to the mechanistic complexity of these transformations.

5 Conclusion The transmetallation of iron precursors with an excess of a Grignard reagent proceeded efficiently and afforded organoferrate complexes in all cases investigated. The nature of the iron precursor was found to have only minor effects on the formed organoferrates whereas the organyl group strongly influenced the aggregation and oxidation state of the latter. Smaller organyl groups facilitated the aggregation of individual iron centers and, by permitting their close approach, apparently also led to redox disproportionation reactions. These disproportionation reactions gave rise to organoferrates in oxidation states ranging from I to IV. Both the aggregation and oxidation states were affected by the presence of TMEDA or dppbz, which are often used as additives or ligands in iron-catalyzed cross-coupling reactions. TMEDA prevented the formation of higher aggregates whereas dppbz stabilized organoferrates in lower oxidation states. For the system FeCl2/MeMgBr/TMEDA, we could moreover observe the formation of significant quantities of a lowvalent iron precipitate. Due to the dark color of solutions of organoiron species, such precipitates are rather difficult to detect, particularly for small sample volumes. Several of the organoferrates observed in solution were identical to complexes previously characterized in the solid state. This finding proves that these complexes remain stable in solution. Nevertheless, they undergo fast intermolecular exchange reactions. Upon treatment with organyl (pseudo)halides, several of the probed organofer-



rates showed the incorporation of the organyl substituent originating from these substrates. The far majority of heteroleptic organoferrate(III) complexes thus formed readily underwent reductive eliminations and released coupling products when subjected to fragmentation in the gas phase. Thus, they can be considered likely intermediates of iron-catalyzed cross-coupling reactions. The branching between the cross-coupling product and its homo-coupling counterpart was controlled by the electronic properties of the organyl groups and by the tendency of the organoferrates to avoid the accumulation of too much electron density at the iron centers during the reductive elimination. Further important gas-phase fragmentation reactions of the organoferrate complexes include β-hydrogen eliminations, the loss of arenes, and the expulsion of organyl radicals. The former two reactions, like the reductive eliminations, correspond to two-electron processes whereas the latter represents a one-electron process. This dichotomy highlights the diverse reactivity of organoiron species. The actual mode of reactivity observed for a given complex depended on its oxidation state and, in particular, on the propensity of the involved organyl groups to form radicals. The present findings significantly advance our fundamental understanding of organoiron chemistry. Furthermore, they afford insight into the mechanism of iron-catalyzed cross-coupling. This insight should help in the rational planning of these reactions and the systematic development and optimization of iron catalysts.

ASSOCIATED CONTENT Additional ESI mass spectra, electrical-conductivity data, Mössbauer spectra, overview of gas-phase fragmentation experiments, and fragmentation mass spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Acknowledgement We gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (KO 2875/10-1) and Universität Göttingen.

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Oxidation States, Stability and Reactivity of Organoferrate Com- plexes

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