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Feb 8, 2018 - compounds.4−7 This includes the development of protocols for the efficient synthesis ... as early as 1966.1 This compound was structur...
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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Perfluoroalkyl Cobaloximes: Preparation Using Hypervalent Iodine Reagents, Molecular Structures, Thermal and Photochemical Reactivity Phil Liebing,*,† Florian Oehler,‡ Mona Wagner,† Pascal F. Tripet,† and Antonio Togni† †

Swiss Federal Institute of Technology, ETH Zurich, Department of Chemistry and Applied Biosciences, Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland ‡ Martin-Luther-Universität Halle-Wittenberg, Institut für Chemie, Kurt-Mothes-Str. 2, 06120 Halle (Saale), Germany S Supporting Information *

ABSTRACT: Treatment of cobaloximes(II), [Co(Hdmg)2(L)2] (Hdmg = dimethylglyoximate, L = neutral ligand), with perfluoroalkyl iodane reagents leads to the formation of perfluoroalkyl cobaloximes(III), [CoRF(Hdmg)2(L)] (RF = CF3, C2F5, n-C3F7, CF2CF2Ph; L = Py, NH3, MeNH2, PhNH2, MeOH). The synthetic protocol can be significantly simplified to a one-pot procedure starting from cobalt(II) acetate− tetrahydrate. The products have been fully characterized by NMR, IR, and UV/vis spectroscopy as well as single-crystal Xray diffraction, and the thermal and photochemical reactivity has been studied. According to the Co−L distances in the crystal, the trans influence of the RF− ligands can be rated as C2F5− ≈ n-C3F7− < CF2CF2Ph− ≈ CF3− < CH3−. The thermal decomposition of the complexes is different from that of nonfluorinated analogues, probably including perfluoroalkylation of an Hdmg− ligand as the initial step. In the CF3 complexes, the Co−C bond is very resistant against photolysis, but the ligand L is readily exchanged by MeOH upon exposure to blue light. In the complexes with longer RF chains, the Co−C bond is more readily cleaved, and the product distribution depends strongly on the presence of O2. Thus, the alkane RFH is the main product under exclusion of O2, while a fluorinated methyl ester and HF are formed in a methanol solution exposed to air.

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INTRODUCTION Since their discovery in the 1960s,1 bis(diorganyldioximato) cobalt(III) complexes (“cobaloximes”; Chart 1) such as [CoR(Hdmg)2(L)] (Hdmg = dimethylglyoximate; R = e.g., CN, Alkyl; L = neutral ligand), which are structurally closely related to the corresponding cobalt(II) complexes [Co(Hdmg)2(L)2], have been in the focus of numerous research groups. The organometallic cobalt(III) complexes are not only useful models for cobalamines, biologically important compounds of the vitamin B12 family,2 but are also of interest as homogeneous catalysts for various radical reactions.3 An important feature of cobaloximes is the versatile reactivity of the Co−C bond. Thus, this bond is resistant against hydrolysis under physiological conditions, but is readily cleaved homolytically to afford alkyl radicals under mild conditions, e.g., by visible-light photolysis. The reactivity of the Co−C bond can be tuned widely by varying the axial ligands R and L, while the Co(Hdmg)2 scaffold is comparatively inert.3 Due to a continuously rising demand for selective late-stage perfluoroalkylation methods for organic molecules, there is a remarkable research interest in perfluorinated organometal compounds.4−7 This includes the development of protocols for the efficient synthesis of metal complexes with perfluoroalkyl © XXXX American Chemical Society

Chart 1. Molecular Structures of Cobaloximes(II) (a) and Alkyl Cobaloximes(III) (b) (L = Neutral Ligand, R = Alkyl)

ligands RF−, and corresponding extensive reactivity studies. [CoCF3(Hdmg)2(Py)], the first cobaloxime comprising a perfluoroalkyl ligand, was briefly mentioned in the literature as early as 1966.1 This compound was structurally characterized by Toscano et al. in 1989, revealing that the Co−C bond in this complex is remarkably shortened as compared to the CH3 analogue [CoCH3(Hdmg)2(Py)].8 The preparation and crystal structures of a few related complexes with RF = e.g., i-C3F79,10 and C2F4H,11 have been reported in the early 1990s, and the reactivity of the CF3 complexes has been investigated in detail. For instance, a weaker trans effect of the CF3− ligand as Received: December 14, 2017

A

DOI: 10.1021/acs.organomet.7b00892 Organometallics XXXX, XXX, XXX−XXX

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Organometallics compared to the CH3− ligand,12 and a facile cleavage of the C− F bonds by aqueous bases13 have been reported. Very recently, related cobalamines with RF chains up to n-C8F17 have been postulated as catalytically active species in the electrochemical perfluoroalkylation of arenes.14 Perfluoroalkyl cobaloximes have been predominantly prepared following the “classical” route,3 including the reaction of a nucleophilic CoI species with a perfluoroalkyl halide RFX (X = Br or I; Scheme 1a).8,9,11−13 Similar conditions have been used

CF2CF2X moieties (X = e.g., OAr, SAr, pyrazolyl) have been reported from our laboratory.21 All of these reagents have been successfully employed for the perfluoroalkylation of various C, O, N, and P nucleophiles, where reagents like 1a−e show different reactivities. On the one hand, coordination to a Lewisacidic metal or protonation enhances electrophilic reactivity as RF+ equivalents.18 On the other hand, perfluoroalkyl iodanes often show radical reactions, which are initiated by transition metal catalysis or by one-electron reduction of the reagent.18−20,22 Notwithstanding this possibility to vary the reactivity of the iodane reagents by metal catalysts, reactions with metal compounds themselves have been investigated to a much lesser extent thus far. The only documented example of transformations of metal complexes with perfluoroalkyl iodanes is the trifluoromethylation of a binuclear PdII complex employing reagent 1a or 1b. In this case, a two-electron oxidation of a Pd atom under formation of a PdIV-CF3 species takes place. However, mechanistically, this reaction goes beyond a simple CF3+ transfer, as it involves a binuclear PdIII intermediate.4,23,24 To gain a deeper insight into reactions of perfluoroalkyl iodane reagents such as 1a−e with different metal complexes, we have been looking for novel model reactions, where the reagent shows clean radical reactivity. A detailed knowledge about such reactions forms a valuable basis for the development of new metal-catalyzed perfluoroalkylations with iodane reagents. This knowledge includes similarities and differences between acid- and alcohol-type reagents, the behavior of different perfluoroalkyl groups, and the tuning of the reactivity by the ligands around the metal center. In the course of our research, we were particularly interested in complexes of cheap 3d metals with simple and readily accessible ligand systems, which are attractive for potential commercial applications. In two recent contributions, it has been shown that complexes of cobalt are suitable for radical perfluoroalkylations of arenes, based on photolytic homolysis of a Co−RF bond.14,25 In this work, we investigated the reactivity of the reagents toward cobaloximes(II), [Co(Hdmg)2(L)2], that can be expected to enable radical reactivity of the iodane reagents according to reaction (b) in Scheme 1. Complexes like [Co(Hdmg)2(H2O)2] (2) are very easily accessible from a CoII salt and dimethylglyoxime, and their redox potential is widely tunable by the axial ligands attached to Co.3,26 For instance, the pyridine complex [Co(Hdmg)2(Py)2] (3) is more readily oxidized than the dihydrate 2, while [Co(Hdmg)2(PPh3)2] is comparatively stable against oxidation.27,28 Herein, we report a unique model reaction for the one-electron oxidative perfluoroalkylation of a metal center. This study led us to a straightforward synthetic protocol for perfluoroalkyl cobaloximes(III) and the full characterization (1H, 13C, 19F, and

Scheme 1. Synthetic Routes to Perfluoroalkyl Cobaloximes(III) from Cobaloximes(II), Using a Nucleophilic Co(I) Intermediate (a) or an RF Radical Source Such as RFI (b)

for the derivatization of related cobalamines.15 However, this reaction bears various disadvantages, including the handling of a highly reactive and air-sensitive CoI intermediate, and harmful gases such as CF3Br or CF3I. The CoI compound is most frequently accessed by reduction of a CoII or CoIII precursor with NaBH4, which can undergo side reactions with the RF moiety to reveal defluorination products such as CHF2 complexes.8,16 [CoCF3(Hdmg)2(Py)] was also obtained directly by reaction of a CoII complex with CF3I,1 which acts formally as a source of CF3 radicals in this reaction (Scheme 1b). This synthetic strategy is also less attractive due to the use of light-sensitive, hazardous CF3I gas and the difficulty to separate the byproduct [CoI(Hdmg)2(Py)].3,16 Therefore, a facile, widely applicable synthetic protocol including a cheap and easy to handle perfluoroalkyl source is still lacking. Hypervalent iodine reagents for electrophilic perfluoroalkylation are readily accessible, shelf-stable crystalline solids, and their versatility has been widely demonstrated since the first report from our laboratory in 2006.17,18 The first developed and most popular members of this reagent class are 1-(trifluoromethyl)-1,2-benziodoxol-3(1H)-one (1a; “Togni’s reagent I” or “acid reagent”) and trifluoromethyl-1,3-dihydro-3,3-dimethyl1,2-benziodoxole (1b; “Togni’s reagent II” or “alcohol reagent”; cf. Scheme 2). During the past 5 years, the library of iodane reagents has continuously been growing and includes now compounds with perfluorinated groups other than CF3, e.g., the C2F5 analogue of the acid reagent (1c),19 as well as the corresponding n-C3F7 homologue (1d).20 In 2016, a large variety of acid and alcohol reagents with functionalized

Scheme 2. Reaction of Cobaloximes(II) with the Perfluoroalkyl Iodanes 1a−e

B

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lucky to find that similar reactions of 3 with acid reagents delivering RF groups larger than CF3 (1c−e; RF = C2F5, n-C3F7, CF 2 CF 2 Ph) lead to the corresponding perfluoroalkyl cobaloximes(III) (9, 12, 13) in significantly better CoIII-RF/ RF-H ratios than observed for RF = CF3. The reaction between the CoII precursor complex and the iodanes also depends strongly on the nature of ligand L. This is easily explained by the fact that the ligands around Co define the redox potential of the complex.27,28 Thus, the strongly reducing [Co(Hdmg)2(Py)2] (3) is readily oxidized by, e.g., 1a, while [Co(Hdmg)2(PPh3)2] is inert against oxidation and consequently CHF3 and CF3I are observed as the main reaction products (cf. Table S1 in the SI). Other suitable precursor complexes with L = NH3, MeNH2, and PhNH2, which reveal the corresponding CoIII-CF3 complexes 5−7 in a straightforward manner corresponding to Scheme 2, have been generated in situ from the dihydrate 2. Within the series of nitrogen ligands, the highest 19F NMR yields of the target CoIII-CF3 complexes are reached with Py and PhNH2, while the yields are somewhat lower in the case of NH3 and MeNH2 (cf. Table S1 in the SI). Consequently, the yield seems not to be determined exclusively by the redox potential of the CoII complex, as, e.g., the NH3 complex (E(CoII/CoIII) = −0.65 V vs SCE in H2O) is a stronger reducing agent than the Py complex (E(CoII/CoIII) = −0.22 V vs SCE in H2O).27 The reduction potential of reagent 1a has been reported to be −0.68 V (vs SCE) in MeCN solution.29 The reaction of the less reducing [Co(Hdmg)2(H2O)2] (2; E(CoII/ CoIII) = +0.31 V vs SCE in H2O28) with 1a proceeds to completion after a longer reaction time. However, the product turned out to be the MeOH complex [CoCF3(Hdmg)2(MeOH)] (8) instead of the expected H2O complex. This can possibly be attributed to expulsion of the comparatively weak H2O ligand in 2 by the solvent MeOH prior to reaction with 1a, together with a lower redox potential of the corresponding MeOH complex. For L = NH3 and PhNH2, the [Co(Hdmg)2(L)2](o-IC6H4COO) byproducts have been identified by single-crystal X-ray diffraction after isolation from the reaction mixture, while a corresponding complex salt was not isolable in the case of L = MeNH2 and H2O/MeOH. Preparation and Properties. The [CoRF(Hdmg)2(L)]type complexes with nitrogen ligands, L = Py, NH3, MeNH2, and PhNH2 (4−7 and 9−13), have been isolated in 31−67% yield from reactions of the appropriate acid reagent (1a, 1c−e) on a 3 mmol scale with a slight excess (2.05 equiv) of the corresponding CoII precursor in methanol (cf. Figures S1−S4 in the SI). The products can easily be separated from the specific ortho-iodobenzoate salt (14−16) or decomposition products thereof, by exploiting the different solubility in water, ethyl acetate, or acetone. Even though the potential iodide source CF3I was observed in the reaction mixtures in the case of reagent 1a (cf. Figure 1), the appropriate iodido complex [CoI(Hdmg)2(L)] was not detected in the isolated products. The MeOH complex 8 could be isolated in only 14% yield, which is due to the need for several purification steps (the hypothetical byproduct [Co(Hdmg)2(MeOH)2](o-IC6H4COO) is unstable; see below). It is worth mentioning that the perfluoroalkyl cobaloximes have been obtained in comparable yields from a one-pot reaction directly from cobalt(II) acetate−tetrahydrate, H2dmg, ligand L, and the appropriate iodane reagent, this representing a further

59

Co NMR spectroscopy, IR and UV/vis spectroscopy, singlecrystal X-ray structure analysis, as well as thermal and photochemical reactivity studies) of the literature-known [CoCF3(Hdmg)2(Py)] (4) and nine novel [CoRF(Hdmg)2(L)] complexes (5−13). The influence of both L and RF on the properties of the compounds is discussed.

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RESULTS AND DISCUSSION Reaction Screening. In an initial set of experiments, we explored the reactivity of the trifluoromethyl iodanes 1a and 1b toward [Co(Hdmg)2(Py)2] (3) under variation of solvent and stoichiometry of the starting materials. The product distribution can easily be estimated by 19F NMR spectroscopy (cf. Table S1 in the SI). The target complex [CoCF3(Hdmg)2(Py)2] (4) shows a signal at −31.8 ppm, which can simply be assigned as it is slightly broadened due to the quadrupolar moment of the 59Co nucleus. When equimolar amounts of 1a and 3 are used, a considerable amount of unreacted reagent 1a remains, but the latter is completely consumed when 2 equiv of 3 are employed (Figure 1a,b). The need for 2 mol of cobalt(II)

Figure 1. 19F NMR spectra of the reaction solution of [Co(Hdmg)2(Py)2] (3) after 15 h at r.t., using different iodane reagents and molar ratios of reagent/cobalt(II) complex (Co = target CoIII-CF3 complex 4, H = CHF3, I = CF3I).

complex per mol of iodane reagent arises from the formation of an ortho-iodobenzoate salt [Co(Hdmg)2(Py)2](o-IC6H4COO) together with the target CoIII-CF3 complex 4 (cf. right reaction in Scheme 2). The mentioned byproduct has been identified by single-crystal X-ray diffraction after crystallization from the reaction mixture. Fluoroform, CHF3, has been identified as the major side product, which appears in the 19F NMR spectrum as a sharp signal at −79.5 ppm in methanol. The formation of the latter depends strongly on the solvent, where methanol was found to be the best choice (best CoIII-CF3/CHF3 ratio). Another minor side product was identified as CF3I (δF = −10.7 ppm in methanol). Use of excess 3 in the attempt to suppress side reactions does not lead to a significant shift of the product spectrum in favor of the target complex 4 (cf. Table S1 in the SI). Replacing the acid reagent 1a by the corresponding alcohol reagent 1b leads to the Co-CF3 complex 4 as well, but the reaction does not proceed to completion even after a prolonged reaction time (Figure 1c). Moreover, the undefined byproducts are difficult to separate from the reaction mixture. Consequently, acid reagents like 1a are clearly preferable for preparative purposes. With this knowledge in hand, we were C

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Table 1. 19F and 59Co NMR Data of Compounds 4−13b

tremendous simplification of the synthetic procedure (Scheme 3). Scheme 3. One-Pot Synthesis of Compounds 4−13 from Cobalt(II) Acetate−Tetrahydrate

comp.

δ(19F)/ppm

|1JC,F|/Hz

δ(59Co)/ppm

w1/2(59Co)/kHz

4 5 6 7 8 9 10 11 12 13

−32.7 −35.1 −34.3 −31.3 −28.5 −99.0a −104.0a −103.2a −95.7a −87.7a

379 376 379 380 378 320a

3828 3856 3839 3856 4350 4040 4044 4034 4168 4146

3.42 3.89 3.42 3.89 6.83 5.03 3.70 3.13 5.79 6.74

314a 328a

α-CF2 signal. 4, 9, 12, and 13 measured in CDCl3, all other compounds in CD3OD. a

b

of the acidic OH moieties of the Hdmg− ligands are not observable due to fast H/D exchange with the solvent, while the NH moieties of NH3, MeNH2, and PhNH2 appear as broadened singlets in methanol-D4 solution (cf. Figures S5 and S8 in the SI). In the 13C NMR spectra, the signals of all CF2 and CF3 moieties are not visible due to quadrupolar coupling with the 59Co nucleus and additional coupling with the 19F nuclei (cf. Figures S6 and S9 in the SI). However, the 13C satellites in the 19F NMR spectra are usually clearly resolved (cf. Figures S7 and S10 in the SI). In the case of the CF3 complexes 4−8, the 19F NMR signals appear in a range between −28.5 and −35.1 ppm. The |1JC,F| values are in a narrow range of 376−380 Hz, which is large as compared to the values observed in other CF3 compounds (e.g., CHF3: |1JC,F| = 272 Hz; CF3Cl: |1JC,F| = 299 Hz; CF3I: |1JC,F| = 344 Hz).30 The 59Co NMR shifts of 4−7 are in a range of 3828−4350 ppm, which is significantly downfield-shifted as compared to related non-fluorinated cobaloximes like [CoR(Hdmg)2(Py)] (R = Me, Et, nPr, nBu; δCo = 3640−3680 ppm).31 The line widths of the signals are in the lower kilohertz range and, as expected, increase with increasing size of the RF group (e.g., within the series of the pyridine complexes 4, 9, 12, and 13; cf. Figure S11 in the SI). The signal of the MeOH complex 8 is unexpectedly broad at 6.83 kHz in CD3OD solution (cf. 4−7: w1/2 = 3.42−3.89 kHz). This may be due to a higher asymmetry of the electronic environment around the Co nucleus, or symmetry reduction by hydrogen bonding between coordinated and solvent methanol.32 Surprisingly, the 59Co NMR signals have not been found in the case of the very symmetric cationic complexes [Co(Hdmg)2(L)2]+ in 14−16, which may be due to symmetry reduction by contact ion pairing.33 Compounds 4−13 have been additionally characterized by IR and UV/vis spectroscopy. The IR spectra (cf. Figure S12 in the SI) display characteristic strong bands of the Co(Hdmg)2 scaffold around 1565 cm−1 (νCN), 1240 cm−1 (νN−O), and 510 cm−1 (νCo−N), which are very similar to those observed in related methyl cobaloximes [CoCH3(Hdmg)(L)].34 In the CF3 complexes 4−8, a strong band at ca. 1030 cm−1 can most likely be assigned to a νC−F vibration. In the C2F5 complexes 9−11, strong bands appear at approximately 1160, 1200, and 1300 cm−1 (each νC−F), and at 905 cm−1 (νC−C). Similar νC−F and νC−C bands have been reported for [CoCF3(CO)4]35 and [Co(C2F5)(CO)4],36 respectively. The UV/vis spectra of 4−13 are very similar, displaying a strong band at approximately 250 cm−1, which can be assigned to π transitions within the Hdmg− ligands (Figure 2).37 Further unassigned shoulders of medium intensity are present between 300 and 350 cm−1. In the visible

With the attempt to prepare authentic samples of the byproducts [Co(Hdmg)2(L)2](o-IC6H4COO), we treated a mixture of cobalt(II) ortho-iodobenzoate−dihydrate, H2dmg, and ligand L in ethanol with air (Scheme 4). The results were Scheme 4. Targeted Preparation of the ortho-Iodobenzoate Salts 14−16

surprisingly different depending on the employed ligand L, and the desired target products have only been obtained in the case of [Co(Hdmg) 2(NH 3) 2 ](o-IC 6H 4COO) (15) and [Co(Hdmg)2(PhNH2)2](o-IC6H4COO) (16). The reaction using pyridine as a ligand led, even with excess of the latter, to the ortho-iodobenzoic acid adduct [Co(Hdmg) 2(Py) 2 ][H(oIC6H4COO)2] (14·o-IC6H4COOH). In the case of methylamine, only (o-IC6H4COO)(MeNH3) and H2dmg could be isolated. When Co(o-IC6H4COO)2 and H2dmg were treated with air in the absence of any ligand L, ortho-iodobenzoic acid could be isolated as the only defined product from the reaction mixture. The perfluoroalkyl cobaloximes 4−13 are yellow to orange solids, which are stable against air and moisture. The solubility in methanol decreases with increasing size of the RF group, while the solubility in nonpolar solvents like chloroform and diethyl ether increases in the same direction (e.g., going from 4 to 9 to 12). Characteristic NMR parameters of 4−13 are summarized in Table 1. In all compounds, the 1H NMR signals D

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the Co−C bond in [Co(i-C4F7)(Hdmg)2(Py)] is even longer due to the steric bulk of the i-C3F7 residue (Co−C 208.4(4) pm).9 Inspection of the Co−N(Py) bond lengths in 4,8 9, 12, and 13 allows a comparison of the trans influence of the different RF− ligands. The Co−Py separation in 13 (203.9(2) pm) is virtually equal to that in 4 (204.3(3) pm),8 indicating a very similar trans influence of the CF3− and CF2CF2Ph− ligand. The trans influence of C2F5− and n-C3F7− seems to be weaker, as the corresponding Co−N(Py) bonds are slightly shorter in 9 (201.3(3) pm) and 12 (201.9(3) pm), respectively. Nonetheless, all the mentioned RF− ligands are considerably stronger trans ligands than, e.g., pyridine. Thus, the Co−N(Py) separations observed in the [Co(Hdmg)2(Py)2]+ complex cation in 14·o-IC6H4COOH are much shorter at 195.7(2) pm. On the other hand, all RF groups exert a weaker trans influence than alkyl ligands like CH3−. For instance, the Co− N(Py) bond length in [CoCH3(Hdmg)2(Py)] has been observed at 206.9(3) pm.39 This is also confirmed by the Co−O(MeOH) separation of 200.4(1) pm in 8, which is considerably shorter than in an earlier reported alkyl cobaloxime comprising a MeOH ligand (Co−O 208.19(5) pm).40 As shown by previous work, cobaloximes with the iC3F7 ligand display a larger steric contribution to the trans influence than corresponding i-C3H7 derivatives.10 The steric bulk of the i-C3F7 ligand leads to bending of the Co(Hdmg)2 scaffold toward the trans ligand, which can be quantified by the angle between the two dmg planes (≡ α), and by the displacement of the Co atom from the plane defined by the four dmg N atoms (≡ δ). Negative values of α and δ indicate a bending of the two Hdmg ligands toward the L ligand (e.g., [Co(i-C3F7)(Hdmg)2(Py)]: α = −10.3°, δ = −7.5 pm), while positive values mean bending away from the L ligand (e.g., [Co(i-C3H7)(Hdmg)2(Py)]: α = +4.0°, δ = +2.0 pm).10 For complexes 4−13, α values from −0.6(2)° to −5.3(4)° and δ values from −0.2(4) to −6.6(4) pm have been observed (Table 3). Consequently, a relatively small, but mostly significant, steric trans influence of the RF ligand can be inferred. The magnitude of this contribution is similar for the different RF groups. This can be expected since all of these groups comprise a CF2 moiety bonded to the Co atom. The C−F bonds in 4−8 cover a range of 131.4(8)−136.3(2) pm, which fits the reference value of a C−F single bond of 134 pm.41 However, the C−F bonds within the α-CF2 moiety in 9−13 are

Figure 2. UV/vis spectra of complexes 4−8 in methanol solution.

wavelength range of the spectrum, only a very weak absorption around 435 nm has been observed. This is almost identical to the visible-light absorption in related nonfluorinated complexes (e.g., maximum at 441 nm for [CoCH3(Hdmg)2(Py)] in methanol),37 where it has been assigned to a CT(Co→C) transition.37,38 The spectra of the pyridine complexes 9, 12, and 13 are virtually identical to that of 5, and therefore the absorbance properties are not significantly influenced by substitutions within the RF− ligand (cf. Figure S41 in the SI). Crystal Structures. All new perfluoroalkyl complexes 5−13 and the ortho-iodobenzoate salts 14−16 have been characterized by single-crystal X-ray diffraction. The crystal structure of 4 has already been determined by Toscano et al. in 1989.8 In the crystalline state, all cobaloximes 4−13 exist as monomeric molecules with an octahedral environment of the Co atom (Figure 3). Characteristic bond lengths are summarized in Table 2. In the CF3 complexes 4−8, the Co−C distance ranges from 193.0(2) pm in 8 to 195.3(4) pm 5. This finding meets the expectation that the Co−C bond is significantly influenced by the ligand L trans to CF3, and is particularly short in the case of weak trans ligand MeOH in 8. In 9−13 comprising longer RF chains, the Co−C bond is significantly longer at 197.2(4)− 200.0(2) pm. These values are comparable with that observed in [Co(C2F4H)(Hdmg)2(Py)] (Co−C 199.8(6) pm),11 while

Figure 3. Molecular structures of 5−13 in the crystal (H atoms attached to C atoms omitted for clarity). E

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Organometallics Table 2. Selected Interatomic Distances (pm) in 4−13

a

compound

Co−C

Co−L

C−F

48 5 6 7 8 9 10 11 12 13

194.9(4) 195.3(4) 194.4(2) 193.2(2) 193.0(2) 199.9(4) 197.2(4)−198.5(5) 197.7(2) 199.9(5) 200.0(2)

204.3(3) 201.4(3) 202.8(2) 209.0(2) 200.4(1) 201.3(3) 199.5(4)−200.6(3) 203.5(2) 201.9(3) 203.9(2)

131.4(8)−133.8(6) −a 134.7(2)−136.3(2) 135.5(3)−136.1(3) 134.3(2)−135.4(2) 138.1(5)b 136.2(5)−143.3(5)b 136.6(2), 138.6(2)b 137.1(5), 142.8(6)b 137.5(2), 138.2(2)b

Values not reliable due to CF3 disorder. bα-CF2 moiety.

Table 3. Selected Parameters of the Coordination Octahedron around the Co Atom in 4−13a

a

compound

C−Co−L/deg

α/deg

δ/pm

48 5 6 7 8 9 10 11 12 13

177.4(2) 179.7(1) 176.57(7) 179.0(1) 177.51(7) 175.1(1) 175.3(2)−176.1(2) 179.75(7) 174.1(2) 174.74(6)

−0.6(2) −2.3(1) −1.2(1) −2.8(1) −3.6(1) −3.7(1) −3.7(1) to −5.3(1) −3.2(1) −2.1(1) −2.3(1)

−0.2(4) −2.9(4) −1.4(2) −3.3(2) −3.8(2) −3.4(4) −4.1(5) to −6.6(1) −3.9(2) −2.7(4) −2.2(2)

Negative values of α and δ indicate bending of the Co(hdmg)2 scaffold towards L.10

Thermal Decomposition Studies. Compounds 4−13 have been investigated by thermogravimetric and differential thermo analysis (TG/DTA), and some volatile thermolysis products have been identified by mass spectrometry. Complexes 4−7 and 9−13 were found to decompose under an inert atmosphere of nitrogen at 231−256 °C (Table 4).

significantly longer at 136.2(5)−143.3(5) pm, and consequently, these C−F bonds can be expected to be more reactive. The observed values agree with those reported for other CoIII complexes with RF− ligands, e.g., [CoCp(X)(C2F5)(PR3)] (X = F, CF3; C−F(α) 136.7(2)−139.6(6) pm).42 While the crystal structures of the pyridine complexes 4,8 9, 12, and 13 do not feature any specific intermolecular interactions, the molecules are aggregated to supramolecular assemblies by hydrogen bonding in all other cases. For instance, in 7, dimers by N−H···O connections between MeNH2 N-H moieties and Hdmg O atoms were observed (Figure 4). Similar

Table 4. Thermogravimetric Analysis of Compounds 4−13a 4 5 6 7 8 9 10 11 12 13

Tdec/°C

Tend/°C

mass loss/%

calcd/%

250 256 251 245 170 237 246 246 236 249 231

619 631 653 671 207 638 651 648 660 622 678

61 59 59 58 9 68 64 63 56 71 66

60 53 55 61 9 (−MeOH) 64 (total) 64 59 60 67 68

a Total mass loss between Tdec and Tend given; calcd for release of RF, L, C4H6, and 2 NO.

Figure 4. Supramolecular dimer of [Co(CF3)(Hdmg)2(MeNH2)] (6) in the crystal formed by N−H···O hydrogen bonds, and further aggregation thereof through N−H···F interactions. N···O 289.2(2) pm, N···F 321.6(2) pm.

Therefore, the decomposition temperatures are similar as those reported for [CoCH3(Hdmg)2(Py)] (Tdec = 246−250 °C), and higher than for complexes with longer alkyl moieties, e.g., [Co(C2H5)(Hdmg)2(Py)] (Tdec = 196−208 °C).43 Just like for the non-fluorinated analogues, both axial ligands L and RF are released in this range, as indicated by two exothermic processes detected by DTA (cf. Figures S30−S39 in the SI). The RF ligand seems to be released at first, as it was detected by MS prior to the L ligand (e.g., CF3, m/z = 69 and Py, m/z = 79 for

dimers exist in 8 and 11 (cf. Figures S20 and S24 in the SI), while the NH3 complex 5 exhibits a polymeric chain structure (cf. Figure S14 in the SI). In cases where not all N-H donors are involved in N−H···O bonding (5, 6, and 11), the structures are further aggregated by N−H···F interactions. F

DOI: 10.1021/acs.organomet.7b00892 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

leads to increasing side reactions, including formation of free fluoride (Figure 5; cf. Figures S45−S48 in the SI).

4). While, in the case of the alkyl cobaloximes, alkanes are usually formed,1,43 the corresponding perfluoroalkanes have not been detected. We surmise that decomposition is initiated by transfer of the RF group to the Hdmg− ligand, followed by release of a perfluoroalkylated fragment of the latter. This hypothesis is supported by the finding that decomposition of the Hdmg ligands begins simultaneously with RF release, and is not completed until ca. 650 °C. In contrast, the Co(Hdmg)2 scaffold shows a sharp decomposition around 290 °C in the case of the alkyl complexes.43 For 4−13, butadiene (m/z = 54), MeCN (m/z = 41), and N2O (m/z = 44) were identified by MS among several decomposition products of the Hdmg− ligands. The overall mass loss between 230 and ca. 650 °C corresponds to liberation of L, RF, and fragments of the Co(Hdmg)2 scaffold with a molar mass of approximately 115 g/mol. Table 4 displays the calculated mass loss for the release of 1 equiv of butadiene and 2 equiv of NO, which agrees relatively well with the experimental values. However, the detection of MeCN indicates that there is more than one relevant decomposition pathway of the Co(Hdmg)2 core. Moreover, detection of N2O instead of NO indicates contribution of the N2 atmosphere to the decomposition processes. The release pathway of the RF group remains elusive, since only the RF group itself, but no perfluoroalkylated molecule, could be detected by mass spectrometry. The MeOH complex 8 behaves differently than the other compounds, showing clean MeOH release already at 170 °C. Further degradation was similar as for the other complexes. Photolysis Experiments. Not only the thermal decomposition, but also the photochemical reactivity of 4−13 is different from that of related non-fluorinated alkyl cobaloximes. The photolysis can easily be monitored by 19F NMR spectroscopy (cf. Figures S45−S51 in the SI), while UV/vis spectroscopy is less suited due to the similarity of the spectra before and after irradiation (cf. Figures S42−S44 in the SI). While alkyl cobaloximes tend to homolytic Co−C bond cleavage mediated by visible light,1,44,45 the trifluoromethyl analogues 4−8 show no significant decomposition upon exposure to blue light in the solid state and even in chloroform solution. The results are very different when irradiation is carried out in methanol solution. In the case of 4−7, a fast exchange of the nitrogen ligand to solvent methanol takes place, thus providing complex 8 in each case (Scheme 5a). The ligand exchange runs toward an equilibrium with 50−70% of 8 depending on the leaving ligand L, and prolonged irradiation

Figure 5. Degradation of complexes 4−7 upon irradiation (a), concentration of MeOH complex 8 vs irradiation time (b).

However, only very small amounts of CHF3 have been detected upon prolonged irradation, demonstrating a high resistance of the Co−C bond against photolysis. Under ambient conditions, the ligand exchange is slow, and only minor amounts of 8 (