Monofunctionalized Cobaltocenium Compounds by Dediazoniation

The peak potential of −2.295 V at v = 0.10 V/s is 170 mV negative of the E1/2 ..... After the mixture was stirred for 10 min at 0 °C, 6.3 mL of an aqu...
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Monofunctionalized Cobaltocenium Compounds by Dediazoniation Reactions of Cobaltoceniumdiazonium Bis(hexafluorophosphate) Stefan Vanicek,† Holger Kopacka,† Klaus Wurst,† Thomas Müller,‡ Christopher Hassenrück,§ Rainer F. Winter,§ and Benno Bildstein*,† †

Institute of General, Inorganic and Theoretical Chemistry, Center for Chemistry and Biomedicine, University of Innsbruck, Innrain 80-82, A-6020 Innsbruck, Austria ‡ Institute of Organic Chemistry, Center for Chemistry and Biomedicine, University of Innsbruck, Innrain 80-82, A-6020 Innsbruck, Austria § Department of Chemistry, University of Konstanz, Universitätsstrasse 10, D-78457 Konstanz, Germany S Supporting Information *

ABSTRACT: Monofunctionalized cobaltocenium salts are obtained for the first time from cobaltoceniumdiazonium bis(hexafluorophosphate) with various nucleophiles via Sandmeyer-type and related reactions. For successful conversions, reaction conditions are quite critical: either standard solution chemistry in nitromethane or solvent-free ball milling proved necessary, depending on the type of reactant. By this synthetic approach valuable synthons such as iodocobaltocenium and azidocobaltocenium salts are accessible that open up new vistas in cobaltocenium chemistry. Spectroscopic characterization by NMR, IR, HRMS, and single-crystal structure analysis as well as the results of electrochemical studies are reported. Derivatives with two reversible reductions show the expected relation of the half-wave potentials with the Hammett substituent parameter σp of the respective substituent with a slightly larger slope for the first reduction. The carboxylic acid (reductive deprotonation of the −COOH functionality), the iodo (protodehalogenation), and the azido derivatives undergo irreversible subsequent reactions after primary reduction.

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INTRODUCTION The pivotal role of ferrocene and its derivatives in organometallic chemistry is very well documented, with many uses and applications in various research areas such as ligand design in homogeneous catalysis, redox sensing by electrochemistry, bioorthogonal functionalization in medicinal chemistry, structural and electronic control in supramolecular chemistry and materials science, etc.1 Isoelectronic cobaltocenium salts have in many respects similarly advantageous properties such as air stability and a fully reversible redox couple in combination with opposite polarity due to the cationic charge. In contrast to ferrocenes, cobaltocenes may even display a second redox couple that is well-accessible even in “conventional” solvents such as THF and DME (DME = 1,2-dimethoxyethane). However, cobaltocenium chemistry is much less developed in comparison to ferrocene chemistry, mainly because the deactivating positive charge of the cobaltocenium moiety prevents convenient chemical functionalization by established methods in organic and/or ferrocene chemistry. Historically, some cobaltocenium carboxylic acid derivatives have been known since 1970 in the seminal work of Sheats and Rausch.2 Two general reviews on cobaltocenium chemistry are available: Sheats3 summarized most of the known cobaltocenium chemistry in a first review in 1979, and quite recently a second review by Tang4 focusing on macromolecular applications was published. © XXXX American Chemical Society

Herein we report the synthesis and characterization of the first (pseudo)halide and other cobaltocenium derivatives, including azidocobaltocenium, made by dediazoniation4 of cobaltoceniumdiazonium bis(hexafluorophosphate).2 These new cobaltocenium compounds are valuable synthons for further chemistry: e.g., carbon−carbon cross-coupling reactions, azide−alkyne cycloadditions (“click chemistry”), oxidative additions, metal−halogen exchange, etc. With this contribution we hope to foster the development of cobaltocenium chemistry in general and expect in particular future applications in the construction of neoteric cobaltocenium-containing materials.

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RESULTS AND DISCUSSION Starting Materials. In organic chemistry, diazonium salts are versatile substrates for various nucleophilic and/or free radical substitution reactions due to the preeminent leaving group ability of N2 under reducing conditions.5 Therefore, cobaltoceniumdiazonium salts (CcN22+(X−)2; Cc = (η5-C5H5)Co(η5-C5H4) = cobaltoceniumyl) are potential candidates for similar reactions (Scheme 1). Interestingly, although cobaltoceniumdiazonium bis(hexafluorophosphate), CcN22+(PF6−)2 (3), has been prepared by Sheats and Rausch as early as 1970,2 it has only recently been isolated and characterized by Received: April 25, 2016

A

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Organometallics Scheme 1. Synthetic Protocola

bonds, and (iii) a low pKB value of 15.6.2 After repeated attempts under a variety of conditions, finally suitable single crystals of aminocobaltocenium 2b were obtained containing hexafluorophosphate and tetraphenylborate counterions in a ratio of 2:1 (Figure 1). The fulvenoid iminium character of 2b

Figure 1. Molecular structure of 2b. The hexafluorophosphate counterion is omitted for clarity. Selected bond distances (Å): C(10)−N(1) = 1.340(3), Co(1)−centroid of unsubstituted Cp ring 1.633, Co(1)−centroid of substituted Cp ring 1.645, N(1)−centroid of phenyl A 2.326, N(1)−centroid of phenyl B 2.506.

a

Conditions: (i) SOCl2/NaN3/H2SO4; (ii) HPF6/NaNO2; (iii) NaN3/CH3NO2; (iv) KI/CH3NO2; (v) CuBr/KBr/ball milling; (vi) CuCl/KCl/ball milling.

single-crystal structure analysis by Geiger and co-workers in 2010.6 Chemical reactions of 3 are so far limited to in situ formation of an azo dye with phenol2 and to electrochemical grafting onto a glassy-carbon electrode.6 Clearly the chemical potential of 3 has not yet been addressed at allfor reasons to be discussed below. As a starting point, a chemoselective synthesis of cobaltocenium carboxylic acid hexafluorophosphate, CcCO2H+PF6− (1), is available from our recent work.7 In the present contribution, Curtius rearrangement to aminocobaltocenium hexafluorophosphate, CcNH2+PF6− (2), in analogy to published protocols2,6 was performed, affording 2a in 45% yield, a moderate improvement in comparison to the 37% reported by Sheats and Rausch.2 Further attempts to improve the yield by a modified Curtius reaction with diphenylphosphoryl azide8 proved in our hands to be without benefits. In addition to being the necessary precursor for the following chemistry in this work, aminocobaltocenium hexafluorophosphate (2a) is in general a valuable monofunctionalized cobaltocenium compound for further amine derivatives containing an electrochemically active cobaltocenium substituent. For example, amide/peptide derivatives in bioorthogonal chemistry or Schiff bases for ligand design in catalysis might be of interest in the future. Preliminary experiments showed that acylation of 2a is easily possible,9 but condensation reactions of 2a with aldehydes or ketones have met so far without success,9 even under forcing conditions. We note that attempted amine/ carbonyl condensation reactions with the more easily available 1,1′-diaminocobaltocenium hexafluorophosphate have also met with failure.10 Chemically, the insufficient nucleophilicity of 2a toward carbonyl functionalities can be explained by a fulvenoid iminium character of the amino group, evidenced by (i) the intense orange color of 2a in comparison to otherwise yellow to ocher monofunctionalized cobaltocenium salts (Figure S1 in the Supporting Information), (ii) C−N stretching vibrations of 1631 and 1533 cm−1 at significantly higher energies in comparison to other primary aryl amines with C−N single

is clearly evidenced by the short C−N bond length of 1.340(3) Å, even slightly shorter than in pentamethylated aminocobaltocenium hexafluorophosphate (C−N = 1.351(5) Å).11 The tetraphenylborate counterion in 2b seems to be nececessary for forming the single-crystal lattice, indicated by N−H···π-arene hydrogen bonding of the fulvenoid iminium group with two phenyl moieties of the counterion. In the next step, diazotization of 2a with nitrous acid, formed in situ from hexafluorophosphoric acid and sodium nitrite, gave cobaltoceniumdiazonium bis(hexafluorophosphate), CcN22+(PF6−)2 (3), in a satisfying yield of 95% as a very slightly air sensitive yellow solid. This is a major improvement in comparison to the published procedure using hydrochloric acid,5 where only 48% of 3 was obtained. Solid diazonium salts are in general potentially explosive; therefore, 3 should be handled with care. However, according to our experience and a report by Geiger,6 3 seems to be quite stable: melting under decomposition was observed at 118.4 °C, a shock test with a hammer showed no explosive behavior, and no indications of particular thermal instability was observed in this work. In marked contrast, the stability of 3 in solution is surprisingly low: as was first observed by Geiger,6 nitromethane is the most suitable solvent for 3. In all other common polar solvents, e.g. water, acetonitrile, acetone, etc., rapid decomposition to unidentified products is encountered, as indicated by a color change from yellow to red. One can only speculate about the reason for this peculiar behavior; common solvent parameters such as dipole moment and acidity cannot be solely responsible. Nevertheless, the anomalous incompatibility of 3 with common solvents is certainly the main cause for the previous lack of any solution-based chemistry. A single-crystal structure analysis of 3 has been reported by Geiger,6 showing a normal cobaltocenium moiety with a regular NN triple bond (NN = 1.094(3) Å) within the range of 1.09−1.11 Å usually observed in other aromatic diazonium salts.5b There are no indications of a fulvenoid structure; hence, the solid-state structure of 3 gives no evidence of an unusual B

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Organometallics

electrodes,16 but in an earlier review on organic solid-state reactions the following was explicitly stated: Caution! solid diazonium salts are heat and shock sensitive; do not ball-mill!17 While this warning is in principle justified, we never experienced any spontaneous decomposition of 3. In addition, we consider small-scale reactions of 3 in a ball mill as quite safe due to the closed metal apparatus in comparison to fragile glass vessels in standard solution chemistry. Ball milling Sandmeyer reactions of 3 with cuprous bromide or chloride were therefore investigated in this work. In general, ball milling is a very rapid synthesis methodology; product formation is usually complete within a few minutes. However, reaction control is an important issue: depending on the rotation speed, scale of the reaction, number of balls in relation to the amount of starting materials, and type of ball mill used, mechanical and/or thermal energy transfer varies, resulting in varied yields. Fortunately, in our case we can apply the high lability of cobaltoceniumdiazonium bis(hexafluorophosphate) (3) in most solvents (vide supra) as a convenient test to optimize reaction conditions: simply adding acetone after a ball-milling reaction to the solid product mixture indicates either complete consumption of 3 by yellow dissolved cobaltocenium derivatives or incomplete consumption of 3 by evolution of N2 with formation of a red acetone solution containing cobaltocenium radical decomposition products. By this method, optimized procedures for solid-state bromo-/chloro-dediazoniation reactions afforded bromo- and chlorocobaltocenium hexafluorophosphates 6 and 7 in excellent yields of 94%, together with 3% of unsubstituted cobaltocenium hexafluorophosphate due to hydro-dediazoniation of 3 (see the Experimental Section). Because no hydrogen-containing solvents are present in these solid-state reactions, the source of the hydrogen must be adventitious moisture or more likely residual water in 3. For most future synthetic applications the small amounts of unsubstituted, nonfunctionalized cobaltocenium hexafluorophosphate will not interfere. If desired or necessary, separation of 6 or 7 from the minor impurity CcH+PF6− can be achieved by crystallization from an acetonitrile/toluene solvent mixture. Naturally the scope of replacement of the diazonium group by other functional groups was of interest in this work, as briefly summarized in the following. Fluoro-Dediazoniation. The Balz−Schiemann reaction is the preferred method in organic chemistry to introduce fluorine into an aromatic ring.5 Usually this nucleophilic aromatic substitution is performed by thermal decomposition of solid arenediazonium tetrafluoroborates, but hexafluorophosphates are also suitable, giving in many cases better yields.18 The melting behavior of 3start of melting at 118 °C, decomposition under darkening at 141 °Cindicated that such a reaction might occur in this temperature range. However, preparative attempts under a variety of experimental conditions afforded no isolable fluorocobaltocenium hexafluorophosphate. Attempted fluoro-dediazoniation by Olah’s reagent, pyridinium poly(hydrogen fluoride),19 also proved unsuccessful. Hence, the series of halocobaltocenium salts CcX+PF6− (X = I, Br, Cl, F) cannot be completed at this point. Cyano-Dediazoniation and Thiocyanato-Dediazoniation. In copper(I)-catalyzed cyano- or thiocyanato-Sandmeyer reactions5 the pseudohalides CN− and SCN− are substrates. In our experience, no successful radical substitution reactions were observed with cobaltoceniumdiazonium bis(hexafluorophosphate) (3). We note that, in analogy to

electronic structure of the diazonium moiety that would explain the abnormal behavior of 3 in solution. Cobaltoceniumdiazonium bis(hexafluorophosphate) 3 was fully characterized, including 13C NMR data and melting behavior (vide supra) so far not available in the literature.6 Dediazoniation Reactions. In comparison to all other leaving groups in organic chemistry, dinitrogen is by far the best nucleofuge available. Therefore, arenediazonium salts have a rich substitution chemistry, allowing a range of useful functionalized compounds to be prepared by either nucleophilic or radical substitution reactions, often performed under Sandmeyer conditions.5 Water as the most common solvent for dediazoniation reactions is not applicable here, because 3 is unstable in water and reduces to unsubstituted cobaltocenium salts in addition to other nonidentified products. The identity of the actual reducing agent in this undesired hydrodediazoniation reaction remains unclear. Most likely, the incompatibility of 3 with water has hampered earlier applications of 3 in organometallic synthesis.2,3 Since 3 is most stable in nitromethane, all reactions in solution were performed in this medium (Scheme 1). Azido-Dediazoniation. Since 1893 it has been known that arenediazonium salts react with sodium azide with formation of azidoarenes.12 Mechanistically, this reaction involves nucleophilic attack of azide at diazonium with formation of labile pentazene and pentazole intermediates13 that release N2 with concomitant formation of azidoarenes. Reaction of 3 in nitromethane with sodium azide afforded azidocobaltocenium hexafluorophosphate (4) in an excellent yield of 86%. Iodo-Dediazoniation. One of the best preparations of aryl iodides is the reaction of isolated or in situ formed arenediazonium salts with at least 4 equiv of iodide ions.4,14 This is sometimes called the Sandmeyer reaction, although “true” Sandmeyer reactions are copper-assisted halo-dediazoniations (vide infra). Mechanistically, this reaction consists of a reduction of arenediazonium to aryl radicals with formation of dinitrogen and iodine radicals followed by radical coupling with formation of diiodine and aryl iodides.5 Reaction of 3 with potassium iodide in nitromethane afforded iodocobaltocenium salts 5a,b in excellent yields of 90%. Bromo-/Chloro-Dediazoniation (Sandmeyer Reactions). In organic chemistry, a good method of wide scope to synthesize aryl bromides or chlorides is the Sandmeyer reaction of arenediazonium salts with cuprous bromide or chloride, respectively.5 Mechanistically, Cu(I) serves as a catalytic reductant of arenediazonium, generating aryl radicals, N2, and cupric halides followed by abstraction of a halogen radical by the aryl radical with concomitant reduction of Cu(II) back to Cu(I).5 Reaction of 3 with cuprous bromide or chloride in nitromethane solution always afforded a mixture of halodediazoniation and hydro-dediazoniation products: with CuBr a mixture of CcBr+X− (6; X = Br, PF6, 75% yield by NMR) and unsubstituted cobaltocenium salts CcH+X− (X = Br, PF6, 25% yield by NMR) was obtained, whereas with CuCl only a 50/50 mixture of CcCl+X− (7) and CcH+X− was observed. At first sight, one could be more or less satisfied with these yields of ≥50%, but in practice it proved impossible to separate unsubstituted cobaltocenium salts from the desired halocobaltocenium products. Therefore, we strived for avoiding hydrodediazoniation altogether by a mechanochemical approach15 under solvent-free reaction conditions in a ball mill. There is only one report in the literature on ball milling of aryl diazonium salts for the functionalization of glassy-carbon C

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attempted correlation of δ(13C−X) versus the electronegativity of X and, where available, the functional group electronegativity25 of X, showed no simple linear trend. Single-crystal structure analyses are available for 4, 5a, and 6 (Figures 2−4), proving unambiguously their chemical identi-

standard organic chemistry, cyanocobaltocenium salts CcCN+PF6− may be synthesized most likely by the reaction sequence cobaltocenium carboxylic acid CcCO2H+, chloride CcCO2Cl+, amide CcCO2NH2+, and cyano CcCN+ with a dehydration of the primary amide by thionyl chloride in the final step. Nitro-Dediazoniation. The nitro-Sandmeyer reaction is in organic chemistry an optional method of making nitroarenes by copper(I)-catalyzed reaction of arenediazonium salts with sodium nitrite.5 In the analogous reaction of 3 with NaNO2 no CcNO2+PF6− was formed. However, this failure is tolerable, since nitrocobaltocenium hexafluorophosphate can be synthesized more conveniently by oxidation of aminocobaltocenium hexafluorophosphate (2) with hydrogen peroxide.2 Chlorosulfo-Dediazoniation. A Sandmeyer-type reaction of diazonium salts with sulfur dioxide in the presence of cupric chloride is known to afford sulfonyl chloride.5,20 Using 3 as the starting material, NMR and mass spetrometric analysis of the product mixture indicated partial formation of the desired CcSO2Cl+PF6−, but so far no reliable workup procedure affording the pure product could be developed. Arsono-Dediazoniation. In the Bart reaction5,21 arenediazonium salts are treated with sodium arsenite and copper sulfate to give arsonic acids. With 3 as substrate no cobaltocenium arsonic acid CcAsO3H2+PF6− or the potential zwitterionic cobaltoceniumarsonate CcAsO3H could be detected. Borono-Dediazoniation. Dediazoniation of arenediazonium salts with diboronic acid, (HO)2B−B(OH)2, is a new and elegant method to synthesize arylboronic acids,22 key synthons in modern carbon−carbon cross-coupling reactions. Given the great importance of such Suzuki reactions in modern organic synthesis, it was of high interest to apply this reaction to cobaltoceniumdiazonium bis(hexafluorophosphate) (3), possibly allowing access to the hitherto unknown cobaltocenium boronic acid CcB(OH)2+PF6− or its zwitterion CcBF3, formed by reaction of the intermediate CcB(OH)2+PF6− with KHF2. Interestingly, this reaction did indeed occur in part, resulting so far in inseparable product mixtures. Further work is needed to develop this reaction on a preparative scale, but this is beyond the scope of the present contribution. Physical, Spectroscopic, and Structural Properties. Compounds 4−8 are simple, monosubstituted cobaltocenium salts of yellow to ocher color, soluble in polar solvents such as water, dimethyl sulfoxide, acetonitrile, acetone, dichloromethane, etc. These organometallic salts have rather high melting/decomposition temperatures in the range of 184−242 °C, depending on their substituent. IR spectra show diagnostic stretching vibrations of the C−X dipoles (X = N3, I, Br, Cl) and strong νP−F absorptions of the hexafluorophosphate counterion (see the Experimental Section). 1H NMR spectra show the typical signal pattern of monosubstituted metallocenes (two pseudotriplets and one singlet), and due to the cationic charge 1 H and 13C chemical shift values are observed at lower field in comparison to those of their neutral ferrocenyl analogues (compare with the values given in the Experimental Section). Having now and from our earlier work7,23 a set of monosubstituted cobaltocenium compounds in hand (CcX+PF6−; X = H, CO2H, CO2−, CO2Cl, CCH, NH2, N2+, N3, I, Br, Cl), it was of interest to check if there is any linear correlation between the 13C chemical shift of the ipso carbon (C−X) and Hammett or related substituent parameters.24 However, no such correlation could be observed. Similarly,

Figure 2. Molecular structure of 4. Selected bond distances (Å): C(10)−N(1) = 1.403(2), N(1)−N(2) = 1.255(2), N(2)−N(3) = 1.123(2), Co(1)−centroid of unsubstituted Cp ring 1.644, Co(1)− centroid of substituted Cp ring 1.643.

Figure 3. Molecular structure of 5a. Selected bond distance (Å): C(6)−I(1) = 2.101(4), Co(1)−centroid of unsubstituted Cp ring 1.629, Co(1)−centroid of substituted Cp ring 1.634.

Figure 4. Molecular structure of 6. Selected bond distance (Å): C(10)−Br(1) = 1.872(3), Co(1)−centroid of unsubstituted Cp ring 1.634, Co(1)−centroid of substituted Cp ring 1.638.

ties. All three cobaltocenium salts show regular, undistorted metallocenium moieties with C−Co, C−C, and C−X bond distances in line with expectations. Electrochemistry. In ferrocene chemistry, numerous papers deal with the effects of ring substituents on their halfwave potentials,26 giving usually good correlations with Taft or Hammett σp or σm parameters.27 On the basis of such investigations, Lever and co-workers derived the ligand electrochemical parameter EL(L) for a broad series of substituted cyclopentadienide ligands.27 Similar studies on D

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Organometallics Table 1. Electrochemistry Data for the Investigated Cobaltocenium Salts CpCo(η5-C5H4R)+

8 7 6 5bc 1 9 2a 4d

R

σp

E1/2+/0 (V) (ΔEp (mV))

H CCH Cl Br I COOH COO− NH2 N3

0.00 0.23 0.23 0.23 0.18 0.45 0.00 −0.66 0.08

−1.330 (59) −1.175 (59) −1.170 (59) −1.169 (59) −1.185 (−) −1.175 (59)b −1.525 (60) −1.607 (73) −1.165b,e (−)

E1/20/− (V) (ΔEp (mV)) −2.480 −2.295 −2.300 −2.276 −2.155

(61) (−)b (59) (−)a,b (−)b−d

−2.62b −2.619 (87)

ΔE1/2 (mV) −1.150 −1.130 −1.107

−1.012

σp value taken from ref 24. bPeak potential of a chemically irreversible process at v = 0.1 V/s. cTentative assignment. dReduction is accompanied by formation of cobaltocene and of I−, as shown by the appearance of the corresponding redox waves. An additional reversible wave is observed at E1/2 = −1.957 V (v = 0.1 V/s). eAdditional reversible redox couples following primary reduction at E1/2 = −2.265 and −2.650 V. a

cobaltocenes are quite scarce but indicated that substituent effects on the first reduction potential of 1,1′-disubstituted cobaltocenium salts and the first oxidation potential of the corresponding ferrocene analogues are nearly identical.27,28 Some other more recent accounts on ring-subsituted cobaltocenium salts containing electrochemical data have been published,29 albeit without half-wave potential/substituent parameter correlations. The results of electrochemical investigations on the cobaltocenium ions of the present study and some of their precursors,7,23 including ethynylcobaltocenium,29d,f in THF/ NBu4PF6 (0.1 M) as the supporting electrolyte are collected in Table 1, while representative cyclic voltammograms are shown in Figures 5−7 or in the Supporting Information.

Figure 7. Cyclic voltammogram of the azidocobaltocenium ion (PF6− salt) in THF/NBu4PF6 (0.1 M) at room temperature; v = 0.1 V/s.

In good agreement with previous literature reports,29a,30 the parent cobaltocene is reduced in two widely spaced oneelectron waves at half-wave potentials of −1.320 and −2.475 V (Figure S31 in the Supporting Information). Due to the electron-withdrawing effect of the ethynyl substituent, reduction of the ethynylcobaltocenium ion occurs at a less negative potential and is observed as a chemically and electrochemically reversible process at E1/2 = −1.175 V, in good agreement with the value of −1.195 V in DMF/NBu4PF6 reported by Astruc and co-workers (note that −0.495 V has to be added to the reported value to account for the potential difference between the decamethylferrocene/-ferrocenium couple used in that report and the ferrocene/ferrocenium couple employed here).29f In contrast to the behavior in DMF, the second reduction proved to be chemically irreversible under our conditions. The peak potential of −2.295 V at v = 0.10 V/s is 170 mV negative of the E1/2 value in DMF (see Figure S32 in the Supporting Information). A sample of aminocobaltocenium salt 2a containing residual cobaltocenium formed in the Curtius rearrangement (vide infra and the Experimental Section) also showed two fully reversible one-electron reductions (Figure S33 in the Supporting Information) at −1.610 and −2.620 V, significantly negative of its parent, as expected on the basis of the strongly electrondonating capability of the amino substituent. In water as the solvent, the potential difference between the reduction of the cobaltocenium and aminocobaltocenium ions is reduced from −280 mV to only −150 mV.30a For the series of halogeno-substituted cobaltocenium salts, the stability of the reduction products decreases in the order Cl (7) > Br (6) > I (5a,b). 7 is reduced in two reversible or close

Figure 5. Cyclic voltammogram of the chlorocobaltocenium ion (PF6− salt) in THF/NBu4PF6 (0.1 M) at room temperature; v = 0.1 V/s.

Figure 6. Cyclic voltammogram of the iodocobaltocenium ion (PF6− salt) in THF/NBu4PF6 (0.1 M) at room temperature; v = 0.1 V/s. E

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product(s) of the electrochemically triggered degradation of 2a are therefore warranted. The electrochemical properties of cobaltocenium carboxylic acid in aqueous electrolytes has already been investigated a long time ago.30a Two waves with pH-dependent E1/2 values have been assigned as the reductions of the free acid and its dissociation product: i.e., the corresponding carboxylate. In dry THF, the free acid 1 is irreversibly reduced at a potential of −1.175 V (v = 0.1 V/s). This process is followed by a fully reversible couple at −1.525 V and a second, chemically irreversible process at −2.62 V (Figure S36 in the Supporting Information). Both of these waves match with the first and second reductions of the corresponding carboxylate (Figure S37 in the Supporting Information). Thus, primary reduction of the free acid follows the sequence Cc-COOH+ + e− → Cc+COO− + 1/2 H2. Such reduction-induced deprotonation reactions are rather frequent and have been widely applied to elucidating the pKa of inorganic and organic acids in nonaqueous environments.32 Within our limited data set the half-wave potentials of the first reduction show the expected linear relation to the simple Hammett parameter24 σp with a slope of [0.494(±0.023)]σp and with Lever’s electrochemical ligand parameter EL.27 For an even smaller set of only four derivatives, the analysis could be extended to the second reduction, providing a somewhat smaller slope of [0.362(±0.086)]σp (Figures S38 and S39 in the Supporting Information).

to reversible one-electron steps at half-wave potentials of −1.170 and −2.300 V (Figure 5). For 6, however, only the first reduction is reversible (Figure S34 in the Supporting Information) and occurs at a half-wave potential that is indistinguishable from that of 7 within the limits of the experimental error (±3 mV). The second reduction remains a chemically irreversible process, even at a sweep rate of 2 V/s. As expected under the action of an interfering chemical process, the corresponding reduction peak occurs at a less negative potential in comparison to that of the 7/7− couple.31 For iodocobaltocenium, even the first reduction constitutes an only partially reversible process, independent of whether the iodide or the hexafluorophosphate salt is employed. The halfwave potential of −1.185 V is very similar to those of 6 and 7. Four additional waves/peaks are observed on continuing the scan to more negative potentials. Two of these, waves B/B′ and E/E′ in Figure 6, were identified as being due to parent cobaltocene. Cleavage of the C−I bond and hydrogen abstraction from the solvent or from adventitious protic impurities (whose levels nevertheless do not interfere with the second reduction of cobaltocene itself) thus constitute a major degradation pathway of reduced 7. The finding that the reverse waves B′ and E′ are associated with peak currents higher than those for the forward waves B and E and that the peak currents for the couple E/E′ are much larger than those associated with the B/B′ couple (note that during the scan fresh iodocobaltocenium diffuses from the bulk solution to the electrode surface and is getting reduced at any potential past the A/A′ wave) provide further arguments for electrochemically initiated protodehalogenation. The iodide formed in that process can also be detected by virtue of the appearance of the I−/I2 oxidation peak at −0.250 V when, after reduction of 5b, the reverse sweep is extended to more positive potential (Figure S35 in the Supporting Information). As judged by the rather similar potential of peak D in comparison to that of the second reduction of 5 and 6 and the smaller peak current in comparison to peak A as well as by the increase of peak current D at the expense of those of peaks B and E at faster sweep rates (see Figure S35), we are drawn to assigning peak D as the further reduction of the reactive iodocobaltocene to its even less stable anion. The nature of the species giving rise to the reversible wave C/C′ at E1/2 = −1.957 V remains unclear at this point. Azidocobaltocenium displays a likewise complicated reductive behavior. Here, even the first reduction is a chemically fully irreversible process giving rise to peak A. It is followed by two consecutive reductions at much more negative potentials, both showing clear signs of chemical reversibility. The first of these waves comprising peaks B/B′ is a kinetically slow process, as is evident from a broadening of particularly the “reverse” peak B′. The half-wave potential of this wave is −2.265 V and is thus in a range expected for the reduction of a neutral cobaltocene. The same applies to the additional reduction wave, peaks C/C′, which constitutes a fully reversible couple at E1/2 of ca. −2.650 V (Figure 7). As both waves are associated with peak currents similar to that of the primary reduction peak A of the azidocobaltocenium ion, one is tempted to assign them to the stepwise reduction of a unitary product. Degradation of azidoferrocene likely involves extrusion of N2 and formation of the corresponding nitrene, whose follow-up chemistry may ultimately give rise to the new species causing waves B/B′ and C/C′. Further efforts to synthesize and to identify the

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CONCLUSION An improved synthesis of cobaltoceniumdiazonium bis(hexafluorophosphate) via a Curtius rearrangement of cobaltocenium carboxylic acid followed by diazoniation of aminocobaltocenium hexafluorophosphate has been developed. Interestingly and relevant for further chemistry, this organometallic dicationic diazonium salt is unstable in most common polar solvents, including water, but sufficiently stable in nitromethane. Dediazoniation reactions, either in nitromethane solution or by solvent-free ball milling, provided the first (pseudo)halogenated derivatives of cobaltocenium. Some of these new monofunctionalized cobaltocenium salts, e.g. iodoor azidocobaltocenium hexafluorophosphate, are expected to be of great value for future developments in cobaltocenium chemistry. Spectroscopic characterization by IR, HRMS, 1 H/13C NMR, and single-crystal structure analyses are reported. Electrochemistry showed that the first and second half-wave potentials (as far as they are accessible) show the expected dependence on the Hammett parameter σp or the electrochemical ligand parameter EL. On reduction, cobaltocenium carboxylic acid undergoes reductive deprotonation to the corresponding carboxylate. Iodo- and azidocobaltocene show potentially interesting subsequent chemistry.

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EXPERIMENTAL SECTION

General Procedures. Standard methods and procedures of organic/organometallic synthesis, spectroscopic characterization, electrochemistry, and single-crystal structure analysis were performed as described recently.33 Ball-milling reactions were performed in a Fritsch Pulverizette 7 ball mill. Chemicals were obtained commercially and used as received. Cobaltocenium carboxylic acid hexafluorophosphate (1)7 and reference compounds for electrochemistry, cobaltocenium hexafluorophosphate,6 ethynylcobaltocenium hexafluorophosphate (8),7 and cobaltocenium carboxylate (9),23 were synthesized as recently reported. F

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Organometallics Electrochemical Measurements. All electrochemical experiments were executed in a home-built cylindrical vacuumtight onecompartment cell in freshly distilled THF as the solvent under a protective blanket of purified argon gas. A spiral-shaped Pt wire and a Ag wire as the counter and pseudoreference electrodes were sealed into glass capillaries and fixed by Quickfit screws via standard joints. A platinum electrode was introduced as the working electrode through the top port via a Teflon screw cap with a suitable fitting. The working electrode was polished first with 1 μm and then with 0.25 μm diamond paste before measurements. The cell was attached to a conventional Schlenk line via a side arm equipped with a Teflon screw valve. NBu4PF6 (0.1 mM) was used as the supporting electrolyte. Electrochemical data were acquired with a computer-controlled BAS CV50 potentiostat. Referencing was done with addition of an appropriate amount of ferrocene (Cp2Fe, E1/2 = 0.000 V) as an internal standard to the analyte solution after all data of interest had been acquired. Representative sets of scans were repeated in the presence of the standard. Aminocobaltocenium Hexafluorophosphate (2). The following steps detail an improved synthesis on the basis of published protocols.2,6 Step 1: Cobaltocenium Carboxylic Acid Chloride Hexafluorophosphate/Chloride. A Schlenk flask equipped with a wash bottle filled with a 10 M sodium hydroxide solution was charged with 4.500 g of 1 (11.9 mmol, 1 equiv) and 68 mL of neat thionyl chloride (932,2 mmol, 78 equiv) under protection from air by an argon atmosphere. After the yellow reaction mixture was refluxed over 48 h, thionyl chloride was removed in vacuo, giving pure 2 in almost quantitative yield (note: mixture of anions). The lime yellow powdery product is air stable over hours. IR (ATR): 3126 (νC−H), 1764 (νC=O), 1737 (νC=O), 1418 (νC=C), 1238, 1049, 1012, 943, 809 (νP−F), 554 (νP−F), 503, 457 cm−1. IR data concur with literature values.2 Step 2: Cobaltocenium Acyl Azide Hexafluorophosphate/Azide. To the whole amount of dry cobaltocenium carboxylic acid chloride hexafluorophosphate/chloride in a 1 L Schlenk flask was added 500 mL of tert-butyl alcohol (99%) and NaN3 (7.737 g, 119.0 mmol, 10 equiv). After the mixture was stirred at 30 °C overnight, 200 mL of acetone was added for solubility enhancement, excessive NaN3 was filtered off, and the mixture was thoroughly washed three times with acetone. Solvent was removed on a rotary evaporator, and the yellow product was dried in vacuo, affording 4.512 g of cobaltocenium acyl azide hexafluorophosphate/azide (note: mixture of anions). 1H NMR (300 MHz, CD3CN): δ 5.80 (s, 5H, Cp), 5.84 (pseudo-t, 2H, J = 2.3 Hz, C3/C4 of substituted Cp), 6.11 (pseudo-t, 2H, J = 2.3 Hz, C2/C5 of substituted Cp) ppm. 13C NMR (75 MHz, CD3CN): δ 86.4 (C3/ C4 of substituted Cp), 87.7 (Cp), 88.5 (C2/C5 of substituted Cp), 90.8 (quat carbon of substituted Cp) ppm. IR (ATR): 3128 (νC−H), 2203, 2159 (νN3), 1687, 1462 (νC=C), 1419, 1400, 1374 (νN3− counterion), 1268 (νN3− counterion), 830 (νP−F), 555 (νP−F), 461 cm−1. These data concur with literature values.2 Caution! Although this acyl azide showed no explosive behavior and remained stable under the impact of a hammer, all acyl azides should be handled with care. Step 3: Aminocobaltocenium Hexafluorophosphate (2). A 250 mL round-bottom flask equipped with a reflux condenser was charged with the whole amount of cobaltocenium acyl azide hexafluorophosphate/azide (11.9 mmol, 1 equiv; referring to the starting amount of 1) and 60 mL of 96% sulfuric acid (1071.2 mmol, 90 equiv). The reaction mixture was stirred for 1 h at 100 °C followed by 1 h at 130 °C. After the acidic solution was cooled in an ice bath, careful neutralization with 214 mL of 10 M NaOH (2142.3 mmol, 170 equiv) gave a dark amber neutral solution which was transferred into a 2 L beaker. Addition of 360 mL of cold ethanol (4 °C) under vigorous mechanical stirring precipitated sodium sulfate, which was filtered off and thoroughly washed with 2 L of cold ethanol (−20 °C). The amber filtrate was concentrated on a rotary evaporator to a final volume of 10 mL, 1.9 mL of an aqueous NH4PF6 solution (2.328 g, 14.3 mmol, 1.2 equiv) was added, and the product precipitated as an amber powder. After drying in vacuo at room temperature and chromatography on a short neutral alumina column (eluent acetonitrile/diethyl ether 1/1 v/

v) the obtained orange oil was dried in vacuo, affording 1.957 g of powdery 2 with a purity of 95.1%, corresponding to 1.861 g of pure product (5.3 mmol, 44.8% yield; referring to 11.9 mmol of 1 as starting material). 2 is soluble in H2O, DMSO, acetonitrile, acetone, and dichloromethane. 1H NMR (300 MHz, CD3CN): δ 4.90 (s, 2H, NH2; signal of the NH2 group is only observable in the presence of water), 5.16 (pseudo-t, 2H, J = 2.1 Hz, C2/C5 of substituted Cp), 5.28 (pseudo-t, 2H, J = 2.1 Hz, C3/C4 of substituted Cp), 5.37 (s, 5H, Cp) ppm. 13C NMR (75 MHz, CD3CN): δ 66.5 (C2/C5 of substituted Cp), 79.0 (C3/C4 of substituted Cp), 84.8 (Cp), 104.2 (quat carbon of substituted Cp) ppm. MS (ESI pos): m/z 204.07 (M+ − PF6−). IR (ATR): 3501 (νNH2), 3401 (νNH2), 3124 (νC−H), 1631 (νNH2), 1533 (νNH2), 1415 (νCC), 807 (νP−F), 553 (νP−F), 449 cm−1. Mp: 243.7 °C dec. Single crystals of 2b were obtained by diffusion−crystallization from dichloroethane/diethyl ether. Spectral and crystallographic data are given in the Supporting Information. Cobaltoceniumdiazonium Bis(hexafluorophosphate) (3). A 50 mL round-bottom flask was placed in an ice bath and charged with 10 mL of an aqueous hexafluorophosphoric acid solution (60% w/w, 68.6 mmol, 36.0 equiv) and 0.700 g of 2 (1.9 mmol, 1 equiv; 95.1% purity of 2 was considered in the calculation). After the mixture was stirred for 10 min at 0 °C, 6.3 mL of an aqueous NaNO2 solution (0.197 g, 2.9 mmol, 1.5 equiv) was added, the reaction mixture was warmed to room temperature, and the obtained yellow precipitate was filtered off, thoroughly washed two times with ice water and with diethyl ether, and dried in vacuo, affording pure yellow powdery 3 in 95.2% yield (0.917 g, 1.8 mmol). 3 was stored under argon at 4 °C; it is slightly air/moisture sensitive in the solid state and highly labile in all solvents (color change from yellow to red) except for nitromethane and to a lesser extent dichloromethane or anhydrous methanol. Caution! Although 3 showed no explosive behavior and remained stable even under the impact of a hammer, all appropriate safety regulations should be obeyed. 1H NMR (300 MHz, CD3NO2): δ 6.49 (s, 5H, Cp), 6.50 (pseudo-t, 2H, J = 2.7 Hz, C3/C4 of substituted Cp), 7.18 (pseudo-t, 2H, J = 2.4 Hz, C2/C5 of substituted Cp) ppm. 13 C NMR (75 MHz, CD3NO2): δ 80.3 (quat carbon of substituted Cp), 90.9 (C3/C4 of substituted Cp), 91.4 (C3/C4 of substituted Cp), 92.6 (Cp) ppm. MS (ESI pos): only fragmentation signals were observed. IR (ATR): 3131 (νC−H), 2294 (νN+N), 1417 (νCC), 1206, 814 (νP−F), 555 (νP−F), 475, 443 cm−1. Mp: 118.4 °C dec. Spectra are given in the Supporting Information. Azidocobaltocenium Hexafluorophosphate (4). A 25 mL round-bottom flask was placed in an ice bath and charged with 10 mL of nitromethane (abs), 0.150 g of 3 (0.30 mmol, 1 equiv), and 0.058 g of NaN3 (0.89 mmol, 3 equiv). The yellow reaction mixture was stirred for 10 min at 0 °C and then warmed to room temperature overnight. The solvent was removed on a rotary evaporator, inorganic salts were separated by chromatography on a short neutral alumina column (eluent acetonitrile/diethyl ether 3/1 v/v), and drying in vacuo afforded 4 as a yellow powder in 86.3% yield (0.096 g, 0.26 mmol). 4 is soluble in H2O, DMSO, acetonitrile, acetone, dichloromethane, and THF. 1H NMR (300 MHz, CD3CN): δ 5.54 (pseudo-t, 2H, J = 2.3 Hz, C3/C4 of substituted Cp), 5.69 (pseudo-t, 2H, J = 2.1 Hz, C2/C5 of substituted Cp), 5.71 (s, 5H, Cp) ppm. 13C NMR (75 MHz, CD3CN): δ 75.1 (C3/C4 of substituted Cp), 82.0 (C2/C5 of substituted Cp), 85.9 (quat carbon of substituted Cp), 86.5 (Cp). MS (ESI pos): m/z 229.87 (M+ − PF6−). IR (ATR): 3133 (νC−H), 2128 (νN3), 1467 (νC=C), 1228 (νN3), 825 (νP−F), 556 (νP−F), 530, 462 cm−1. Single crystals of 4 were obtained by diffusion−crystallization from dichloromethane/diethyl ether at 4 °C. Mp: 241.9 °C dec. Anal. Found (calcd) for C10H9N3CoPF6: C, 33.07 (32.02); H, 2.95 (2.42); N, 11.25 (11.20). Caution! Even though 4 showed no explosive behavior and remained stable even under the impact of a hammer, all appropriate safety regulations should be maintained. Spectra and crystallographic details are given in the Supporting Information. Iodocobaltocenium Iodide (5a). A round-bottom flask was charged at 0 °C with 0.150 g of 3 (0.30 mmol, 1 equiv), 10 mL of dry nitromethane, and 0.197 g of KI (1.19 mmol, 4 equiv). The mixture was stirred for 10 min at 0 °C and then at room temperature G

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Organometallics overnight. Solvent was removed on a rotary evaporator, and the heterogeneous mixture was extracted with three 80 mL portions of dichloromethane. The brown mostly inorganic residue, still containing small amounts of the product, was dissolved in 20 mL of a saturated aqueous sodium thiosulfate solution accompanied by a color change to amber. This solution was combined with the organic layer from above and extracted once in a separating funnel to reduce excess iodine, indicated by a color change of the dichloromethane phase from brown to amber. The organic layer was separated, the aqueous solution was extracted with 30 mL of dichloromethane, the organic layers were combined, extracted once with 10 mL of ice water, and dried with anhydrous sodium sulfate, solvent was removed on a rotary evaporator, and the amber product was dried in vacuo, giving 5a in 89.4% yield, based on iodide (0.117 g, 0.26 mmol). 5a is soluble in H2O, DMSO, acetonitrile, acetone, dichloromethane, and THF. IR spectroscopy showed traces of PF6− due to the counterion of starting material 3. This impurity was removed by a short anion exchange column as described for 5b. 1H NMR (300 MHz, CD3CN): δ 5.65 (s, 5H, Cp), 5.66 (pseudo-t, 2H, J = 2.4 Hz, C2/C5 of substituted Cp), 5.97 (pseudo-t, 2H, J = 2.1 Hz, C3/C4 of substituted Cp) ppm. 13C NMR (75 MHz, CD3CN): δ 50.9 (quat carbon of substituted Cp), 86.6 (C2/C5 of substituted Cp), 88.1 (Cp), 92.0 (C3/C4 of substituted Cp) ppm. MS (ESI pos): m/z 315.06 (M+ − I−). IR (ATR): 3121 (νC−H), 1408 (νC=C), 1014 (νC−I), 830 (νP−F), 558 (νP−F), 494, 463 cm−1 (mixture of anions). Single crystals of 5a were obtained at room temperature from a 1/1 mixture of acetonitrile and toluene. Mp: 183.8 °C dec. Spectra and crystallographic details are given in the Supporting Information. Iodocobaltocenium Hexafluorophosphate (5b). 5a (0.070 g, 0.16 mmol) was dissolved in an Erlenmeyer flask in 5 mL of acetone, and the counterion was exchanged over a DOWEX Marathon A anion exchange column loaded with PF6−. After the column was washed with an additional 50 mL of solvent, the combined solutions were concentrated on a rotary evaporator, giving amber solid 5b in 94.7% yield (0.069, 0,15 mmol). IR (ATR): 3120 (νC−H), 2924, 1411 (νC=C), 1729, 1012 (νC−I), 809 (νP−F), 553 (νP−F), 456 cm−1. Mp: 174.5 °C dec. No satisfactory elemental analysis could be obtained. Bromocobaltocenium Hexafluorophosphate (6). A 0.010 g portion of 3 (0.02 mmol, 1 equiv), 0.006 g of copper(I) bromide (0.04 mmol, 2 equiv), and 0.005 g of potassium bromide (0.04 mmol, 2 equiv) were placed in a zirconium(IV) oxide vessel, 6 ZrO2 balls with a diameter of 0.3 cm were added, and the heterogeneous mixture was milled in a Fritsch Pulverizette 7 ball mill at 200 rpm for 10 min. The resulting brown powder was dissolved in acetone, most of the inorganic salts were filtered off, the solvent was removed on a rotary evaporator, and the product was purified on a neutral alumina column with acetonitrile/diethyl ether (3/1) as eluent. After drying in vacuo 6 was obtained as a dark yellow powder in 94% yield (0.008 g, 0.018 mmol), containing 3% unsubstituted cobaltocenium hexafluorophosphate. For most further synthetic applications, this impurity did not interfere. If necessary, the unsubstituted cobaltocenium byproduct can be removed by crystallization from a 1/1 mixture of acetonitrile and toluene. 6 is soluble in H2O, DMSO, acetonitrile, acetone, THF, and dichloromethane. 1H NMR (300 MHz, CD3CN): δ 5.62 (pseudo-t, 2H, J = 2.0 Hz, C3/C4 of substituted Cp), 5.69 (s, 5H, Cp), 5.96 (pseudo-t, 2H, J = 2.0 Hz, C2/C5 of substituted Cp) ppm. 13C NMR (75 MHz, CD3CN): δ 84.9 (C3/C4 of substituted Cp), 87.2 (C2/C5 of substituted Cp), 87.9 (Cp), 88.8 (quat carbon of substituted Cp) ppm. MS (ESI pos): m/z 267.07 (M+ − PF6−). IR (ATR): 3127 (νC−H), 1418 (νC=C), 1034 (νC−Br), 1012 (νC−Br), 821 (νP−F), 555 (νP−F), 460 cm−1. Single crystals of 6 were obtained from a mixture of acetonitrile and toluene (1/1 v/v). Mp: 193.5 °C dec. No satisfactory elemental analysis could be obtained. Spectra and crystallographic details are given in the Supporting Information. Chlorocobaltocenium Hexafluorophosphate (7). A zirconium(IV) oxide vessel was charged with 0.010 g of 3 (0.02 mmol, 1 equiv), 0.004 g of copper(I) chloride (0.04 mmol, 2 equiv), and 0.006 g of potassium chloride (0.08 mmol, 4 equiv). Six ZrO2 balls with a diameter of 0.3 cm were added, and the heterogeneous mixture was milled in a Fritsch Pulverizette 7 ball mill at 200 rpm for 10 min. The

resulting yellow powder was dissolved in acetone, most inorganic salts were filtered off, the solvent was removed on a rotary evaporator, and the product was purified on a neutral alumina column with acetonitrile/diethyl ether (3/1) as eluent. After drying in vacuo 7 was obtained as a golden yellow powder in 94% yield (0.007 g, 0.018 mmol) containing 3% unsubstituted cobaltocenium hexafluorophosphate. For most further synthetic applications, this impurity did not interfere. If necessary, the unsubstituted cobaltocenium byproduct can be removed by crystallization from methanol/toluene (1/1 v/v). 7 is soluble in H2O, DMSO, acetonitrile, acetone, THF, and dichloromethane. 1H NMR (300 MHz, CD3CN): δ 5.59 (pseudo-t, 2H, J = 2.1 Hz, C3/C4 of substituted Cp), 5.70 (s, 5H, Cp), 5.91 (pseudo-t, 2H, J = 2.1 Hz, C2/C5 of substituted Cp) ppm. 13C NMR (75 MHz, CD3CN): δ 83.8 (C3/C4 of substituted Cp), 84.6 (C2/C5 of substituted Cp), 87.7 (Cp), 106.0 (quat carbon of substituted Cp) ppm. MS (ESI pos): m/z 223.07 (M+ − PF6−). IR (ATR): 3126 (νC−H), 1418 (νC=C), 1031 (νC−Br), 1012 (νC−Br), 812 (νP−F), 554 (νP−F), 494, 459 cm−1. Mp: 235.1 °C dec. No satisfactory elemental analysis could be obtained. Spectra are given in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00329. Spectra (1H/13C NMR, IR, HRMS), cyclic voltammograms, and crystallographic data (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for B.B.: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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