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. 2022 Sep 28;61(46):18325–18334. doi: 10.1021/acs.inorgchem.2c02056

One Electron Changes Everything: Synthesis, Characterization, and Reactivity Studies of [Re(NCCH3)6]3+

Robin Bolliger 1, Olivier Blacque 1, Henrik Braband 1, Roger Alberto 1,*
PMCID: PMC9682483  PMID: 36169602

Abstract

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Oxidation of [Re(NCCH3)6]2+ with the thianthrene radical cation results in the formation of [Re(NCCH3)6]3+, one of the very rare cases of a fully solvated +3 complex. It was fully characterized by spectroscopy and X-ray structure analysis. In contrast to its reduced analogue, [Re(NCCH3)6]3+ exhibits a much faster CH3CN exchange. Hence, substitution reactions proceed at 20 °C within minutes. Its potential as a versatile precursor for ReIII chemistry was examined with a series of substitution reactions. The more lipophilic analogue [Re(NCPh)6]3+ was synthesized by nitrile exchange in benzonitrile (NCPh). The Re(II) analogue of [Re(NCPh)6]3+, [Re(NCPh)6]2+, forms by AgI-mediated oxidation of in situ formed [Re(η6-C6H6)(NCPh3)3]+ in NCPh. The same synthetic strategy is feasible for the synthesis of [Re(NCCH3)6]2+ as well. [Re(NCCH3)6]3+ reacts with 1,4,7-trithiacyclononane (C6H12S3) to yield sevenfold-coordinated [Re(κ3-C6H12S3)2(NCCH3)]3+. The reaction of [Re(NCCH3)6]3+ with 1 equiv of (NBu4)X produces the ReIII monohalide complexes [ReX(NCCH3)5]2+ (X = Cl, Br, I). Mixed ReIII dihalides (trans-[ReXY(NCCH3)4]+) were obtained by treating [ReX(NCCH3)5]2+ with a second equivalent of (NBu4)Y (if X = Cl, Y = Br, I; if X = Br, Y = I). Because of this fast CH3CN exchange, [Re(NCCH3)6]3+ is a very suitable precursor for new ReIII complexes.

Short abstract

Oxidation of [Re(NCCH3)6]2+ gives the ReIII analogue [Re(NCCH3)6]3+. This fully solvated ReIII complex is thoroughly characterized. Its reactivity is examined with other nitriles, halides, and thioethers. Electron self-exchange is in the microsecond range. In contrast to its ReII analogue, the acetonitrile exchange rate in [Re(NCCH3)6]3+ is amplified, and hence substitution reactions proceed at room temperature and within minutes. The title compound is thus a convenient precursor complex for ReIII chemistry.

Introduction

Fully solvated metal centers are useful precursors in coordination chemistry. The most prominent examples for fully solvated metal centers are the hexa-aquo complexes of the first row transition metals, for example, [Fe(OH2)6]2+ or [Cr(OH2)6]3+ and many others. The water exchange rates for these hexa-aquo complexes strongly depend on the electronic occupancy of the d orbitals.1 This is exemplified by the exchange rates of the FeII/FeIII hexa-aquo complexes. The exchange rate in the d6 high spin system [Fe(OH2)6]2+ is accelerated by a factor of approximately 104 in comparison to its d5 high spin analogue [Fe(OH2)6]3+.2,3

Hexa-aquo complexes are scarce for 4d and 5d transition-metal elements and have not yet been found for Re or Tc in particular. For these two elements there are examples for fully solvated complexes, though with CH3CN as the ligand, for example, [M2(NCCH3)10]4+ or [M(NCCH3)6]2+ (M = Re, 99Tc).4−7 Recently, we have examined the potential of [Re(NCCH3)6]2+ as a precursor for the rare oxidation state +2, but one of the major drawbacks of substitution reactions with [Re(NCCH3)6]2+ was its unexpected inertness.6 Thus, we have wondered if changing the electron configuration (e.g., by oxidation) would have a significant effect on substitution rates as found in, for example, FeII/FeIII hexa-aquo complexes.

Synthesis of a fully solvated, monomeric ReIII complex, for example, [Re(NCCH3)6]3+, should give a new precursor for reactions in the oxidation state +3. Fully solvated ReIII complexes do not exist so far. The oxidation state +3 is accessible by a limited number of starting materials such as mer-[ReCl3(PMe2Ph)3] or [ReCl3(PPh3)2(NCCH3)].8−10 A fully solvated ReIII precursor complex will broaden the scope of ReIII chemistry and pave the way for other ReIII complexes not accessible starting from one of the established precursors.

Here we present the synthesis, characterization, and reactivity of [Re(NCCH3)6]3+, synthesized in a two-step procedure from [ReH(η5-C6H7)(η6-C6H6)]+.

Results and Discussion

Dissolving [ReH(η5-C6H7)(η6-C6H6)]+ ([1]+) in CH3CN results in the formation of [Re(η6-C6H6)(NCCH3)3]+.11 We demonstrated that the ReII precursor complex [Re(NCCH3)6]2+ ([2a]2+) is accessible by AgI-mediated oxidation of [Re(η6-C10H8)2]+ in CH3CN. By DFT calculations, we have proposed that the actual oxidation step does not take place directly on [Re(η6-C10H8)2]+ but rather on a CH3CN adduct, for example, [Re(η6-C10H8)(NCCH3)3]+.6 By combining these two observations, we have developed a new procedure for [2a]2+ that circumvents the low-yield synthesis of [Re(η6-C10H8)2]+. Heating of [1](BF4) at 60 °C for 1 h in CH3CN results in the formation of [Re(η6-C6H6)(NCCH3)3]+ (identified by ESI-MS). Addition of AgBF4 to the reaction mixture forms [2a](BF4)2 in 92% yield. The same reaction pathway is also applicable to other nitriles, for example, benzonitrile (NCPh), giving [Re(NCPh)6](BF4)2 ([3a](BF4)2) in 64% yield (Scheme 1).

Scheme 1. Formation of Fully Solvated ReII Complexes [2a](BF4)2 and [3a](BF4)2 Starting from the ReIII-Hydride Complex [1](BF4) via AgI-Mediated Oxidation of the ReI Piano-Stool Complexes [Re(η6-C6H6)(NCX)3](BF4) (X = CH3 or Ph).

Scheme 1

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The paramagnetic 1H NMR for [3a](BF4)2 shows three signals (δ = 11.8, 3.4, and −0.4 ppm) in a 2:1:2 ratio (Figure S2, Supporting Information). The IR spectrum reveals an absorption that is attributed to the coordinated nitrile at 2222 cm–1, red-shifted in comparison to the corresponding absorption of [2a](BF4)2 (νCN = 2261 cm–1). [3a](BF4)2 is easier to reduce than its acetonitrile analogue (−0.56 V for [3a](BF4)2 and −1.00 V for [2a](PF6)2; in CH3CN (0.1 M NBu4(PF6)) vs Fc+/Fc), whereas oxidation becomes more challenging (+0.44 V for [3a](BF4)2 and +0.25 V for [2a](PF6)2, Figure 1).6 These electrochemical differences align with findings for other CH3CN/PhCN complexes.12,13 As expected, the PhCN ligands make [3a](BF4)2 much more lipophilic as compared to its acetonitrile analogue [2a]2+, increasing the solubility of [3a](BF4)2 in solvents such as CHCl3, CH2Cl2, or acetone. This better solubility might make [3a](BF4)2 a more promising candidate for a fully solvated ReII precursor complex than its acetonitrile analogue [2a](BF4)2.

Figure 1.

Figure 1

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Comparison of cyclic voltammetry data of [2a](BF4)2 (black) and [3a](BF4)2 (red) depicting their (a) reduction and (b) oxidation. Conditions: [2a](PF6)2 = 1 mM, [3a](BF4)2 = 0.8 mM; in CH3CN (0.1 M NBu4(PF6)); sweep rate = 0.1 V/s except for oxidation of [3a](BF4)2 (sweep rate = 0.3 V/s due to fast nitrile exchange; for more details, see Figure S12).

X-ray diffraction analysis of [3a](BF4)2 revealed a slightly distorted octahedral geometry with N–Re–N angles ranging from 86.58(7)° to 93.48(7)° (Figure 2). The Re–N bond lengths determined in [3a](BF4)2 (2.033(2)–2.0509(18) Å) are slightly shorter than those found in [2a](OTf)2 (2.041(4)–2.056(3) Å).6 Similar to those in [Ru(NCPh)6](BF4)2, the phenyl moieties of the PhCN ligands in the trans position with respect to each other are not in the same plane.14

Figure 2.

Figure 2

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Ellipsoid displacement representation of the cations in the structures of (a) [3a](BF4)2 and (b) [2b](BF4)3 (ellipsoids drawn at 50% probability). Hydrogen atoms and tetrafluoroborate anions omitted for clarity. Selected bond lengths [Å]: (a) Re(1)–N(1) 2.048(2), Re(1)–N(2) 2.0509(18), Re(1)–N(3) 2.0418(18), Re(1)–N(4) 2.033(2), Re(1)–N(5) 2.0410(18), Re(1)–N(6) 2.0449(18); (b) Re(1)–N(1) 2.049(2), Re(1)–N(2) 2.0506(15), Re(1)–N(3) 2.044(2), Re(1)–N(4) 2.0463(16).

Oxidation of [2a](BF4)2 with the thianthrene radical cation tetrafluoroborate (E1/2 = +0.86 V vs Fc+/Fc in CH3CN)15 in CH3CN results in the formation of [Re(NCCH3)6](BF4)3 ([2b](BF4)3) in 92% yield (Scheme 2). A similar synthetic strategy was also chosen for the formation of [Cr(NCCH3)6](BF4)3.16 The 1H NMR shows one narrow signal at δ = 77.3 ppm (Figure S1), low field shifted in comparison to its ReII analogue [2a]2+ (δ = 63.8 ppm). In contrast to [2a](BF4)2 (νCN = 2261 cm–1) the IR spectrum of [2b](BF4)3 depicts two absorptions attributable to the coordinated CH3CN (see Figure S15). The intense band at 2289 cm–1 is assigned to the C≡N stretching, whereas the less intense absorption at 2321 cm–1 belongs to the combination mode of the C–C stretching and the CH3 deformation. The band assignment was performed as in the case of uncoordinated CH3CN.17 Similar IR spectrum patterns are reported for [99Tc(NCCH3)6](BF4)2 as well as for various first-row transition-metal analogues.7,18 No fluoride abstraction from BF4– as found in [Cr(NCCH3)6](BF4)3 was observed for [2b](BF4)3 when dissolved in CH3CN over prolonged periods of time.19 [2b](BF4)3 is unstable under a normal atmosphere (O2 and moisture) and decomposes within minutes even as a solid, if not protected by, for example, an oil such as “Infineum V8512” used for X-ray diffraction analysis. An assessment of the identity of the dark purple decomposition product(s) was not achieved ([2b](BF4)3 is light brown). 1H NMR (measured in CH3CN and CD3CN) as well as the IR spectrum obtained after the exposure of [2b](BF4)3 to a normal atmosphere (O2 and moisture) can be found in the Figures S13, S14, and S16. Dissolution of [2b](BF4)3 in D2O under the exclusion of O2 results in two paramagnetic 1H NMR signals (δ = 64.0 and 50.9 ppm, Figure S17) and the formation of uncoordinated CH3CN (δ = 2.07 ppm). The first was identified as [2a]2+ because of its chemical shift as well as ESI-MS results. Potentially, the ReII analogue is formed by disproportionation of [2b]3+ paralleled by the formation of an NMR silent Re oxide such as ReO2. Characterization of the second species (δ = 50.9 ppm) was ambiguous as isolation attempts resulted in its decomposition. However, the signal must originate from coordinated CH3CN as no other 1H-containing species was present in the experiment. Hence, we hypothesize that this signal originates from a paramagnetic Renitrile-aquo/hydroxo/oxo complex.

Scheme 2. Formation of [2b](BF4)3 by the Oxidation of [2a](BF4)2 with the Thianthrene Radical Cation (C12H8S2)(BF4).

Scheme 2

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The exchange of coordinated CH3CN with PhCN under vacuum results in the formation of [3b](BF4)3. The addition of 1,4,7-trithiacyclononane (2 equiv) to [2b](BF4)3 results in the formation of [4](BF4)3·CH3CN.

Single-crystal X-ray diffraction analysis of [2b](BF4)3 (Figure 2) did not evidence any major differences compared to that of its reduced form [2a](OTf)2 despite the different oxidation states. The Re–N bond lengths range from 2.044(2) to 2.0506(15) Å (2.041(3)–2.059(3) Å for [2a](OTf)2) with N–Re–N bond angles from 88.26(6)° to 92.33(6)° (86.6(1)°–93.4(1)° for [2a](OTf)2).6 [2b](BF4)3 crystallized in the orthorhombic space group Pnma. Comparable complexes such as [Cr(NCCH3)6](BF4)3 and [Rh(NCCH3)6](BF4)3 crystallize in the same space group.16,20

Even though the structures of [2b]3+ and its reduced analogue [2a]2+ are very similar, their respective reactivities differ distinctly. This is evident in the CH3CN exchange rates of the two analogues. Although [2a]2+ has a very slow CH3CN exchange rate (4.13 × 10–7 ± 6.7 × 10–8 s–1, see Figure S18), recording a 1H NMR spectrum of [2b]3+ in CD3CN is not possible because the CH3CN/CD3CN exchange is already complete within the time frame required for sample and measurement preparation. According to the X-ray diffraction analysis, no significant Re–N bond length elongation/shortening is observed for [2b]3+. Hence, the strongly increased nitrile exchange rate does not originate from weakened Re–N bonds. Instead, we propose that the enhanced rate is due to a switch in exchange mechanisms. Although the nitrile exchange in the ReII complex [2a]2+ potentially adheres to a dissociative (or Id) mechanism, its oxidized analogue [2b]3+ exhibits an associative (or Ia) nitrile exchange. However, because of the very slow nitrile exchange in [2a]2+ and the very fast exchange in [2b]3+, verification of this hypothesis by NMR experiments was not yet possible.

When [2a]2+ and [2b]3+ are mixed in CD3CN, no 1H NMR signals are found for either complex. Instead, the only observed signal belongs to uncoordinated CH3CN with an intensity corresponding to a complete CH3CN/CD3CN exchange for both complexes. This is due to the fast CH3CN exchange in [2b]3+ combined with an equally rapid electron self-exchange between the two analogues. In CH3CN, they display broadened 1H NMR signals, originating from the electron self-exchange reaction. NMR line-broadening experiments have previously been used for determing electron self-exchange reactions between two paramagnetic species.21,22 After these procedures, an electron self-exchange rate, kex = 8.31 ± 0.60 × 105 M–1 s–1 is estimated (Figures S25–S27).

The fast CH3CN exchange in [2b]3+ can be used to introduce other nitrile ligands, for example, PhCN. Dissolving [2b](BF4)3 in a mixture of PhCN and CH3CN (1:1) under vacuum (CH3CN has a higher vapor pressure than PhCN) results in the formation of [Re(NCPh)6](BF4)3 ([3b](BF4)3) in 59% yield (Scheme 2). In comparison to its reduced form [3a]2+, the absorption corresponding to the nitrile stretching is blue-shifted (νCN = 2236 cm–1). Complex [3b](BF4)3 is soluble in PhCN but only marginally soluble in CH2Cl2. Other solvents cannot be used as [3b](BF4)3 is either insoluble or decomposes. The 1H NMR spectrum in CD2Cl2 shows two signals at δ = 12.11 and −0.88 ppm. A third 1H NMR signal is not observed (see Figure S3). In contrast to the [2a]2+/[2b]3+ couple, no NMR line broadening is observed for mixtures of [3a]2+/[3b]3+ in PhCN.

X-ray diffraction analysis reveals that [3b](BF4)3 has an almost ideal octahedral geometry, with N–Re–N angles ranging from 89.32(7)° to 90.68(7)° (Figure 3). The Re–N bond lengths are 2.0369(17)–2.0385(17) Å and are therefore similar to those in [3a](BF4)2. In the structure, the phenyl moieties of PhCN ligands in the trans position with respect to each other are now in the same plane. A similar orientation of the phenyl moieties was also found in [Cu(NCPh)6](SbCl6)2.23

Figure 3.

Figure 3

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Ellipsoid displacement representation of the cations in the structures of (a) [3b](BF4)3·PhCN and (b) [4](BF4)3 (ellipsoids drawn at 50% probability). Hydrogen atoms, tetrafluoroborate anions, and solvent molecules (one molecule of PhCN in (a)) are omitted for clarity. Selected bond lengths [Å] and angles [°] (for (b)): (a) Re(1)–N(1) 2.0385(17), Re(1)–N(2) 2.0374(16), Re(1)–N(3) 2.0369(17); (b) Re(1)–N(1) 2.097(13), Re(1)–S(1) 2.4072(13), Re(1)–S(2) 2.4501(14), Re(1)–S(3) 2.4670(14), Re(1)–S(4) 2.4489(14), Re(1)–S(5) 2.4685(13), Re(1)–S(6) 2.4039(13), N(1)–Re(1)–S(1) 7.12(15), N(1)–Re(1)–S(6) 89.11(15), N(1)–Re(1)–S(2) 69.28(14), N(1)–Re(1)–S(4) 69.56(14), S(2)–Re(1)–S(3) 78.24(5), S(3)–Re(1)–S(5) 65.55(5), S(4)–Re(1)–S(5) 78.27(5).

The reaction of [2b](BF4)3 with 2 equiv of 1,4,7-trithiacyclononane (C6H12S3) results in the immediate formation of the seven-coordinated and diamagnetic complex [Re(κ3-C6H12S3)2(NCCH3)](BF4)3·CH3CN ([4](BF4)3·CH3CN, presence of uncoordinated CH3CN (1 equiv) evidenced by 1H NMR; see Figure S4) in 90% yield. Performing the reaction with only 1 equiv gives a mixture of [4]3+ and the starting material [2b]3+, indicating that the coordination of the second trithiacyclononane to the ReIII core is a faster process than the initial coordination. [4](BF4)3 is unstable under a normal atmosphere (O2) and decomposes in water to [Re(κ3-C4H8S3)(κ3-C6H12S3)]+ (identified by 1H NMR comparison with reported data).24 The loss of an ethene group has been reported for [M(κ3-C4H8S3)2]2+ (M = Re, 99Tc), though only under reductive conditions.25

Complex [4]3+ has a distorted pentagonal bipyramidal geometry (Figure 3). The CH3CN ligand lies in the equatorial plane, whereas one sulfur atom of each trithiacyclononane ligands (S1 and S6) are in the axial positions. In the equatorial plane the angles diverge from the ideal 72° ranging from 65.55(5)°–78.27(5)°. For the axial positions the S–Re–S angle is 176.19(5)°. The Re–S bond lengths for the sulfur atoms in the axial positions are shorter (2.4039(1) and 2.4072(13) Å) than those for the ones in the equatorial plane (2.449(1)–2.468(1) Å). The Re–S bond lengths in [4](BF4)3 are longer than the ones reported for [Re(κ3-C6H12S3)2](PF6)2 (2.366(1)–2.375(2) Å).26

Generally, reactions with the ReIII precursor complex [2b]3+ proceed at room temperature and almost instantaneously. There are however some synthetic challenges related to [2b]3+; for example, with the addition of phosphines such as PPh3, complex product mixtures are observed by 1H NMR even with only 1 equiv of PPh3. This is different from similar reactions with the ReII precursor [2a]2+, for which well-defined products such as trans-[Re(PPh3)2(NCCH3)4]2+ are obtained after prolonged microwave heating.6 Potentially, the charge increase of the Re center in [2b]3+ leads to a stronger polarization of the nitrile bond of the coordinated CH3CN. This amplified polarization enables a nucleophilic attack of the phosphine to the triple bond instead of simple complexation to the ReIII core. Such additions were recently reported for the reactions of mer-[ReCl3(PMePh2)3] with CH3CN and aryl nitriles.27

The oxidative power of [2b]3+ (E1/2 = +0.25 V vs [Fe(η5-C5H5)2]+/[Fe(η5-C5H5)2]) influences substitution reactions as well. Treatment of [2b](BF4)3 with potential ligands such as lithium cyclopentadienyl, ethane-1,2-dithiol, or with the N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-yliden led to a reduction to the ReII complex [2a]2+ instead of coordination.

The CH3CN ligand is readily replaced by halides, yielding well-defined products. With treatment of [2b](BF4)3 with halides (Cl, Br, or I) in CH3CN at 20 °C (Scheme 3) with exactly 1 equiv of NBu4X, monohalides [ReX(NCCH3)5](BF4)2 ([5](BF4)2 for X = Cl; [6](BF4)2 for X = Br; [7](BF4)2 for X = I) are obtained. If more than 1 equiv of NBu4X is added, the previously reported trans-[ReX2(NCCH3)4]+ forms alongside the ReIII monohalides.6 Because of their very similar solubilities, separation of the two products is hardly possible, but separation of [2b]3+ and the ReIII monohalides is feasible because of the insolubility of [2b]3+ in CH2Cl2. Hence, the best results were obtained with substoichiometric amounts of NBu4X (e.g., 0.8 equiv). This strategy allowed for the synthesis of the bromide [6](BF4)2 and iodide complex [7](BF4)2. Performing the synthesis even at −10 °C with chloride, small amounts (approximately 10%, estimated by paramagnetic 1H NMR) of trans-[ReCl2(NCCH3)4]+ are formed besides [5](BF4)2 and leave behind some unsubstituted [2b]3+. This implies that the coordination of the second chloride to the ReIII core proceeds at least with a comparable rate to the initial coordination. In contrast to [5](BF4)2 and [6](BF4)2, a slow decomposition of the monoiodo complex [7](BF4)2 was observed even inside the glovebox, presumably induced by light irradiation.

Scheme 3. Formation of the ReIII Monohalides [5](BF4)2, [6](BF4)2, and [7](BF4)2 by the Addition of NBu4X (X = Cl, Br, I) to [2b](BF4)3 in CH3CN.

Scheme 3

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The 1H NMR spectra of all three monohalides consists of two signals in an approximate 1:4 ratio (δ = 101.0 and 67.9 ppm for [5]2+; δ = 88.1 and 64.2 ppm for [6]2+; δ = 71.5 and 59.2 ppm for [7]2+; Figures S6–S8). For all three complexes the more low field shifted signal belongs to the coordinated NCCH3 in the trans position to the halide. The NCCH3 ligands in the trans position exchange much faster than those in the cis position. In fact, with dissolution of the ReIII monohalides in CD3CN, the signal for the CH3CN in the trans position has almost disappeared by the time the first spectrum is recorded. Similar rate differences for CH3CN exchange in the cis position versus that in the trans position were also reported for [Re2(NCCH3)10]4+, [VO(NCCH3)5]2+, or [OsCl(NCCH3)5]+.28,29 For the latter, photolysis is required as no notable CH3CN exchange is observed in the ground state.30 The slower CH3CN exchange in the cis positions was followed by 1H NMR for all three ReIII monohalides. The CH3CN exchange rates for the chloride [5]2+ and iodide complex [7]2+ are almost identical (7.80 × 10–4 ± 2.2 × 10–5 s–1 for [5]2+ and 6.71 × 10–4 ± 7.5 × 10–6 s–1 for [7]2+), whereas the exchange on the bromide complex [6]2+ is approximately twice as fast (1.40 × 10–3 ± 1.3 × 10–5 s–1 (for more details see Figures S19–S21).

Further investigations of the CH3CN exchange in the chloride complex [5]2+ indicate an associative or interchange (Ia) nitrile exchange mechanism. Determination of the exchange rates obtained from variable temperature NMR experiments resulted in ΔS⧧ = −92.8 ± 1.6 J/mol·K after data analysis according to the Eyring equation (for more details see Figures S22–S24).31 If the nitrile exchange mechanism in the Re(III) monohalides is similar (or the same) to that in [2b]3+, this result supports our hypothesis that the rate exchange enhancement for [2b]3+ compared to that for its 17 electrons analogue [2a]2+ might originate from a switch in the exchange mechanism (associative vs dissociative).

As expected, the crystal structures of [5](BF4)2 and [6](BF4)2 show that the Re–Cl bond (2.3204(10) Å) is shorter than the Re–Br bond (2.4542(9) Å). The two crystal structures do not indicate why the CH3CN ligand in the trans position is substituted faster than those in the cis position (Figure 4). No elongation of the Re–N bond trans to the halide ligand was observed (for [5](BF4)2: trans, Re–N 2.057(4) Å; cis, Re–N 2.053(7)–2.061(8) Å; for [6](BF4)2: trans, Re–N 2.053(7) Å; cis, Re–N 2.043(7)–2.061(7) Å). Similar findings were made for the cation in the complex salt trans-[RuIICl(NCCH3)5][RuIIICl4(NCCH3)2].32 We hypothesized that the increased exchange rate of the CH3CN in the trans position does not originate from a structural effect but rather from a kinetic trans effect. If the exchange follows an associative (A) or interchange (Ia) mechanism, a sevenfold coordinated (short-lived) intermediate or transition state is expected. Based on the exchange rate differences, an attack of ligand L on an octahedron face adjacent to the NCCH3 in the trans position seems more plausible than an attack on an octahedron face adjacent to the halide. From this transition state or short-lived intermediate, dissociation of the CH3CN formerly in the trans position seems to be more favored. In Scheme 4 a potential associative substitution mechanism with a capped trigonal prismatic intermediate is depicted; however, other geometries like a capped octahedron are also plausible.

Figure 4.

Figure 4

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Ellipsoid displacement representation of the cations in the structures of (a) [5](BF4)2 and (b) [6](BF4)2 (ellipsoids drawn at 50% probability). Hydrogen atoms and tetrafluoroborate anions omitted for clarity. Selected bond lengths [Å]: (a) Re(1)–Cl(1) 2.3204(10), Re(1)–N(1) 2.061(8), Re(1)–N(2) 2.058(7), Re(1)–N(3) 2.057(8), Re(1)–N(4) 2.053(7), Re(1)–N(5) 2.057(4); (b) Re(1)–Br(1) 2.4542(10), Re(1)–N(1) 2.057(7), Re(1)–N(2) 2.043(7), Re(1)–N(3) 2.059(7), Re(1)–N(4) 2.061(7), Re(1)–N(5) 2.053(7).

Scheme 4. Depiction of a Potential Associative Substitution Mechanism with a Ligand L (e.g., L = CH3CN) on ReIII Monohalides.

Scheme 4

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The intermediate is depicted as a capped trigonal prism. The depiction of other potential geometries (e.g., capped octahedron) as well as charges were omitted for clarity.

The faster CH3CN exchange rate in the trans position also becomes evident when treating [5](BF4)2 with 1 equiv of NBu4X (X = Br, I) in CH3CN. Under these conditions, the mixed ReIII dihalides trans-[ReClX(NCCH3)4]+ ([8]+ for X = Br; [9]+ for X = I) form almost instantaneously (the first 1H NMR reaction control after 5 min already showed complete consumption of [5]2+). The mixed ReIII dihalides (as well as all reported homo ReIII dihalides) are oxygen- and water-stable and can be purified by preparative HPLC (aqueous gradient). After anion exchange with sodium tetraphenylborate (NaBPh4; the anion exchange is mainly done for crystallization purposes), [8](BPh4) and [9](BPh4) are obtained in 32% and 48% yield, respectively (Scheme 5).

Scheme 5. Formation of the Mixed ReIII Dihalides [8](BPh4), [9](BPh4), and [10](BPh4) by the Addition of NBu4X (X = Br, I) to [5](BF4)2 or NBu4I to [6](BF4)2 in CH3CN and Subsequent Anion Exchange with NaBPh4.

Scheme 5

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Purification by preparative HPLC is necessary because these transformations also form small amounts of the homo ReIII dihalides (trans-[ReCl2(NCCH3)4]+ and trans-[ReX2(NCCH3)4]+ (X = Br or I)). Obviously, the favored substitution of the trans-CH3CN is paralleled by substitution of the coordinated chloride itself, leading to the observed halide scrambling. This reaction pathway seems to be more dominant in the case of halide additions to the ReIII monobromide [6]2+. After treatment of [6](BF4)2 with NBu4I in CH3CN and after the same purification and anion exchange as described above, trans-[ReBrI(NCCH3)4](BPh4) ([10](BPh4)) was isolated in 19% yield (Scheme 5), alongside trans-[ReBr2(NCCH3)4]+ (7% yield) and trans-[ReI2(NCCH3)4]+ (15% yield).

The 1H NMR of the three mixed ReIII dihalides consists of a single signal (δ = 62.6 ppm for [8]+; δ = 59.1 ppm for [9]+; δ = 57.0 ppm for [10]+; Figures S9–S11). The chemical shifts for the mixed ReIII dihalides are essentially the mean value of the reported shifts of the corresponding homo ReIII dihalides (δ = 65.6 ppm for trans-[ReCl2(NCCH3)4]+, δ = 60.1 ppm for trans-[ReBr2(NCCH3)4]+, and δ = 54.7 ppm for trans-[ReI2(NCCH3)4]+).6 All three mixed ReIII dihalides ([8](BPh4), [9](BPh4), and [10](BPh4)) crystallized in the monoclinic space group C2/c (Figure 5). The same space group was also found for trans-[ReX2(NCCH3)4](BPh4) (X = Cl, I).6 The Re–N bond lengths are essentially the same for all three mixed ReIII dihalides (2.0554(14) and 2.0575(15) Å for [8]+, 2.0606(19) and 2.0629(19) Å for [9]+, and 2.064(3) and 2.067(3) Å for [10]+). In all three crystal structures the positions of the halogen atoms are disordered over two sites (cations and anions lie on twofold axes), preventing any discussion about the bond distances and angles involving the halogen atoms. Nonetheless, the determined Re halide bond lengths are listed in the caption of Figure 5.

Figure 5.

Figure 5

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Ellipsoid displacement representation of the cations in the structures of (a) [8](BPh4), (b) [9](BPh4), and (c) [10](BPh4) (ellipsoids drawn at 50% probability). Hydrogen atoms and tetraphenylborate anions omitted for clarity. Selected bond lengths [Å]: (a) Re(1)–Cl(1) 2.352(13), Re(1)–Br(1) 2.445(5), Re(1)–N(1) 2.0554(14), Re(1)–N(2) 2.0575(15); (b) Re(1)–Cl(1) 2.436(8), Re(1)–I(1) 2.591(2), Re(1)–N(1) 2.0629(19), Re(1)–N(2) 2.0606(19); (c) Re(1)–Br(1) 2.471(6), Re(1)–I(1) 2.660(4), Re(1)–N(1) 2.064(3), Re(1)–N(2) 2.067(3).

Conclusion

The complex [Re(NCCH3)6]3+ is one of the rare examples of a fully solvated +3 cation in the transition element series. It exchanges its solvent ligands very fast as compared to the ReII complex [Re(NCCH3)6]2+, illustrating that a single electron can change reactivities drastically. Potentially, a change in the nitrile exchange mechanism (associative vs dissociative) is responsible for the rate difference. This rapid exchange makes [Re(NCCH3)6]3+ a suitable starting material for subsequent ReIII chemistry by substitution reactions at room temperature. We have demonstrated this reactivity pattern by the preparation of ReIII complexes, which were synthetically inaccessible before. The title compound is thus a promising precursor for ReIII chemistry.

The synthetic strategy toward [Re(NCR)6]2+ starting from [ReH(η5-C6H7)(η6-C6H6)]+ allows access to further fully solvated ReII complexes such as [Re(NCPh)6]2+. Because of its lipophilicity, noncoordinating solvents such as dichloromethane are suitable for substitution reactions. This extends the options for exchange reactions, which were basically restricted to [Re(NCCH3)6]2+ in CH3CN only. Different solvents may give a different ligand exchange pattern because substitutions do not need to compete with the nitrile exchange. The herein presented precursor complexes will enable the synthesis of a plethora of new Re compounds in the low-valent oxidation states +2 and +3.

Experimental Section

General

All reactions were performed under an N2 atmosphere in a MBRAUN Labmaster DP glovebox or with use of standard Schlenk techniques. NMR sample preparations, crystallizations (unless otherwise stated), and KBr pellet preparation for IR spectroscopy were performed under an N2 atmosphere in a MBRAUN Labmaster DP glovebox. Reagent-grade chemicals from Sigma-Aldrich Chemie GmbH, Tokyo Chemical Industry, and Merck KGaA were used without further purification. NMR solvents (CD3CN, CD3NO2) were purchased from Cambridge Isotopes Laboratories, Inc. NMR solvents were dried by the addition of molecular sieves (4 Å, 20 wt %).331H NMR for compounds with fast nitrile exchange were measured in dry CH3CN. Dry diethyl ether (Et2O) and tetrahydrofuran (THF) were obtained from distillation over sodium. Dry toluene, dry CH3CN, dry pentane, and dry CH2Cl2 were obtained by chromatographical separation on a MB SPS system from MBRAUN. Freeze-drying was carried out with a Christ Alpha 2-4 LD plus lyophilizer. [1](BF4) and the thianthrene radical cation tetrafluoroborate were produced following literature procedures.11,34 [Re(η6-C6H6)(NCCH3)3](BF4) and [Re(η6-C6H6)(NCPh)3](BF4) were not isolated and were identified only by ESI-MS. 1H NMR spectra were recorded on a Bruker AV-400 400 MHz spectrometer. 13C NMR spectra were proton-decoupled, recorded, and measured on a Bruker DRX-500 500 MHz spectrometer. 1H and 13C chemical shifts are reported relative to residual protio solvent resonances.35 Special parameters for paramagnetic 1H NMR measurements: time of delay = 0.1 s; acquisition time = 0.1 s. FT-IR spectra were measured as KBr pellets (unless otherwise stated) on a PerkinElmer Spectrum Two spectrophotometer. High-resolution electrospray mass spectra (HR-ESI-MS) were recorded on a maXis QTOF-MS instrument (Bruker DaltonicsGmbH, Bremen, Germany). The samples were dissolved in CH3CN at a concentration of ca. 50 μg/mL and analyzed via continuous flow injection (2 μL/min). The mass spectrometer was operated in the positive (and negative) electrospray ionization mode at 4000 V (−4000 V) capillary voltage and −500 V (500 V) end plate offset, with a N2 nebulizer pressure of 0.8 bar and dry gas flow of 4 L min–1 at 180 °C. Mass spectra were acquired in the mass range from m/z 50 to m/z 2000 at 20 000 resolution (full width at half-maximum) and a 1.0 Hz rate. The mass analyzer was calibrated between m/z 118 and m/z 2721 using an Agilent ESI-L low-concentration tuning mix solution (Agilent, USA) at a resolution of 20 000 and a mass accuracy below 2 ppm. All the solvents used were purchased at the best LC-MS quality. Preparative HPLC on a Shimadzu HPLC system was equipped with a Dr. Maisch Reprosil C18 10 μm, 100 Å (250 × 40 mm) column. HPLC solvents were trifluoroacetic acid (0.1% in bidistilled water) (solvent A) and acetonitrile (solvent B). HPLC gradients were applied as follows: Gradient A: 0–5 min, 85% A, 15% B; 5–40 min, 85–75% A, 15–25% B; 40–50 min, 75–65% A, 25–35% B; 50–55 min, 65–0% A, 35–100% B. The flow rate was 40 mL min–1. Detection was performed at 260 nm. Gradient B: 0–5 min, 85% A, 15% B; 5–45 min, 85–65% A, 15–35% B; 45–50 min, 65–0% A, 35–100% B; 50–60 min, 0% A, 100% B. The flow rate was 40 mL min–1. Detection was performed at 260 nm. Elemental analysis was performed with a LECO Model 630-100-200 protein analyzer. No reliable values for the H content were obtained presumably because of the formation of HF. Elemental analysis samples were prepared inside a N2-filled glovebox. Electrochemical measurements were carried out with a standard three-electrode setup of a glassy carbon working electrode (i.d. = 3 mm), platinum auxiliary electrode, and Ag/AgCl reference electrode. Measurements were done in acetonitrile containing NBu4(PF6) (0.1 M) as a conducting electrolyte. The measurements were performed first by measuring the compound alone, and in a second step a small amount of [Fe(η5-C5H5)2] was added as an internal reference. The combined sample was then remeasured under the same condition. All potentials are given versus Fc+/Fc. The sweep rate was 0.1 V/s. Magnetic susceptibility measurements were performed either on a magnetic susceptibility balance from Sherwood Scientific (Gouys method) or by Evans method (for more information, see the Supporting Information). All crystal structures (except for [3a](BF4)2 and [4](BF4)3) were obtained by single-crystal X-ray diffraction analyses on a Rigaku OD Supernova-Atlas diffractometer equipped with an Oxford Instruments Cryojet XL cooler using a single-wavelength X-ray source from a microfocus sealed X-ray tube (Cu Kα radiation, λ = 1.54184 Å) or the molybdenum X-ray radiation (Mo Kα radiation, λ = 0.71073 Å) from a dual wavelength X-ray source. Single-crystal X-ray diffraction analyses of [3a](BF4)2 and [4](BF4)3 were performed on a Rigaku OD XtaLAB Synergy, Dualflex, Pilatus diffractometer equipped with an Oxford liquid-nitrogen Cryostream cooler using a single-wavelength X-ray source from a microfocus sealed X-ray tube (Cu Kα radiation, λ = 1.54184 Å). A pre-experiment, data collection, data reduction, and analytical absorption correction36 were performed with the program suite CrysAlisPro.37 With use of Olex2,38 the structure was solved with the SHELXT39 small-molecule structure solution program and refined with the SHELXL2018/3 program package40 by full matrix least-squares minimization on F2. The data collections and structure refinement parameters are summarized in Tables S1–S5. CCDC 2176129 for [2b](BF4)3, 2176130 for [3a](BF4)2, 2176125 for [3b](BF4)3, 2176128 for [4](BF4)3, 2176131 for [5](BF4)2, 2176127 for [6](BF4)22176132 for [8](BPh4), 2176124 for [9](BPh4), and 2176126 for [10](BPh4) contain the supplementary crystallographic data for these compounds and can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

[Re(NCCH3)6](BF4)2 ([2a](BF4)2)

Inside a glovebox in a 50 mL Schlenk flask, [1](BF4) (218.7 mg, 0.507 mmol) was dissolved in CH3CN (10 mL). The reddish-brown solution was heated outside the glovebox for 1 h at 60 °C, resulting in a color change to dark yellow. AgBF4 (99.2 mg, 0.510 mmol) suspended in CH3CN (1 mL) was added, and the brownish-yellow suspension was stirred at 20 °C for 12 h before it was heated to 60 °C for 2.5 h. Inside a glovebox, the formed elemental silver was removed by filtration, and the filtrate was concentrated in vacuo to approximately 1 mL. Et2O (10 mL) was added, and the formed precipitate was collected by filtration and washed with Et2O (2 × 3 mL) and THF (2 × 2 mL). Upon drying, [2a](BF4)2 (282.7 mg, 0.466 mmol) was obtained in 92% yield.

1H NMR (CD3CN): 63.8 ppm. IR (neat): 2999 (w), 2936 (w), 2261 (m), 1420 (w), 1282 (w), 1047 (s), 1011 (s), 945 (m) cm–1.

[Re(NCCH3)6](BF4)3 ([2b](BF4)3)

Inside a glovebox in a 25 mL Schlenk flask, [2a](BF4)2 (100.9 mg, 0.166 mmol) was dissolved in CH3CN (6 mL), giving a brown solution. Thianthrene radical cation tetrafluoroborate (55.2 mg, 0.182 mmol) was dissolved in CH3CN (2.5 mL), and the dark purple solution was added dropwise to the [2a](BF4)2 solution within 1 min. The purple solution was stirred at 20 °C for 25 min before the solvent was concentrated in vacuo to approximately 1 mL. CH2Cl2 (7 mL) was added, and the formed brown precipitate was separated from the purple solution by filtration and washed with CH2Cl2 (2 × 2 mL), giving [2b](BF4)3 (106.0 mg, 0.153 mmol) in 92% yield. Crystals suitable for X-ray diffraction analysis were obtained by vapor diffusion (CH3CN/CH2Cl2).

1H NMR (CH3CN): 77.3 ppm. IR (KBr): 2941 (m), 2321 (w), 2289 (m), 2250 (w), 2218 (w), 1626 (m), 1416 (m), 1083 (s), 1062 (s) cm–1. Magnetic susceptibility (Evans method, CH3CN, 25 °C): μeff = 2.45 μB (two unpaired electrons).

[Re(NCPh)6](BF4)2 ([3a](BF4)2

Inside a glovebox in a 25 mL Schlenk flask, [1](BF4) (50.0 mg, 0.12 mmol) was dissolved in PhCN (4 mL). Outside the glovebox, the greenish-yellow solution was heated to 70 °C for 1 h before the flask was again taken inside the glovebox. AgBF4 (26.0 mg, 0.13 mmol) suspended in PhCN (2 × 0.5 mL) and Et2O (2 × 0.5 mL) was added, immediately resulting in a brownish-yellow suspension. The suspension was stirred for 18 h before the produced elemental silver was removed by filtration through a glass filter frit. Pentane (15 mL) was added, and the resulting yellow suspension was filtered again. The filter cake was washed with Et2O (7 mL) and THF (3 × 3 mL). After extraction of the filter cake with acetone (8 × 2 mL) and removal of the solvent in vacuo, [3a](BF4)2 (72.8 mg, 74.4 μmol) was obtained in 64% yield as a yellow solid. Crystals suitable for X-ray diffraction analysis were obtained by vapor diffusion (PhCN/Et2O).

1H NMR (CD3CN): 11.85 (s, 12 H), 3.41 (s, 6 H), −0.41 (s, 12 H) ppm. IR (KBr): 3066 (w), 2922 (w), 2851 (w), 2222 (w), 1629 (w), 1447 (w), 1384 (w), 1182 (w), 1083 (s), 1055 (s), 757 (m), 684 (m), 562 (w), 510 (w) cm–1. HR-ESI-MS: C42H30N6Re2+ [3a]2+, calculated 402.60392, found 402.60399.

[Re(NCPh)6](BF4)3 ([3b](BF4)3

Inside a glovebox in a 25 mL Schlenk flask, [2b](BF4)3 (14.3 mg, 20.6 μmol) was dissolved in a mixture of CH3CN and PhCN (1:1, 2 mL), giving a brown solution. The solution was stirred at 20 °C for 35 min before it was concentrated in vacuo for 1 h. Pentane (6 mL) was added, resulting in a reddish-brown precipitate that was collected by filtration. The filter cake was washed with a CH2Cl2/pentane mixture (1:6, 2 × 7 mL) and CDCl3 (3 × 0.5 mL). Upon drying, the filter cake turned greenish yellow, giving [3b](BF4)3 (13.0 mg, 12.2 μmol) in 59% yield. Crystals suitable for X-ray diffraction analysis were obtained by evaporation of a PhCN solution of [3b](BF4)3in vacuo.

1H NMR (CD2Cl2): 12.11 (s, 12 H), −0.88 (s, 12 H) ppm. IR (KBr): 3068 (w), 2926 (w), 2236 (m), 1630 (w), 1593 (w), 1448 (w), 1384 (w), 1182 (w), 1084 (s), 1057 (s), 758 (m), 685 (m), 553 (w) cm–1. Magnetic susceptibility (Gouys method, neat, 23 °C): μeff = 2.73 μB (two unpaired electrons).

[Re(κ3-C6H12S3)2(NCCH3)](BF4)3·CH3CN ([4](BF4)3·CH3CN)

Inside a glovebox in a 25 mL Schlenk flask, [2b](BF4)3 (30.8 mg, 44.4 μmol) was dissolved in CH3CN (3 mL), giving a brown solution. 1,4,7-Trithiacyclononane (16.6 mg, 92.0 μmol) dissolved in CH3CN (2 mL) was added to the brown solution, turning the mixture light brown. After the solution was stirred at 20 °C for 25 min, it was concentrated in vacuo to approximately 0.5 mL. Et2O (6 mL) was added, and the resulting brown suspension was filtered. The filter cake was washed with a CH3CN/Et2O mixture (1:12, 6.5 mL), Et2O (2 × 3 mL), and CH2Cl2 (2 × 2 mL). After the cake was dried with a stream of N2, [4](BF4)3·CH3CN (35.5 mg, 39.9 μmol) was obtained as a light brown solid in 90% yield. Crystals suitable for X-ray diffraction analysis were obtained by vapor diffusion (CH3CN/Et2O).

1H NMR (CD3NO2): 3.56–3.47 (m, 24 H), 3.09 (s, 1 × CH3), 2.00 (1 × uncoordinated CH3CN) ppm. 13C NMR (CD3NO2): 147.2 (1 × NC), 43.8 (12 × CH2), 6.6 (1 × CH3) ppm. IR (KBr): 2966 (m), 2929 (m), 2880 (m), 2290 (w), 1629 (m), 1473 (w), 1386 (w), 1022 (s), 839 (s), 670 (w), 558 (m) cm–1. HR-ESI-MS: C10H20ReS6+ [4–NCCH3–C2H4]+, calculated 518.94413, found 518.94300.

[ReCl(NCCH3)5](BF4)2 ([5](BF4)2

Inside a glovebox in a 25 mL Schlenk flask, [2b](BF4)3 (49.0 mg, 70.7 μmol) was dissolved in CH3CN (3 mL), giving a brown solution. In a separate vial, (NBu4)Cl (14.6 mg, 52.5 μmol) was dissolved in CH3CN (6 mL). Both solutions were chilled in a freezer (−11 °C) for 10 min before the (NBu4)Cl solution was added dropwise to the Schlenk flask over a period of 3 min. After 45 min of stirring at 20 °C, the solvent was concentrated to approximately 0.5 mL in vacuo. The addition of Et2O (5 mL) resulted in the formation of a reddish-brown precipitate, which was collected by filtration through a glass filter frit. The solid was washed with THF (4 × 2 mL) and subsequently extracted with a CH3CN/CH2Cl2 mixture (1:5, 4 × 3 mL), giving a reddish-brown filtrate. Concentrating the filtrate in vacuo gave [5](BF4)2 (35.0 mg, 58.3 μmol) in 82% yield (based on [2b](BF4)3). Note: The amount of product was slightly larger than the quantity of employed (NBu4)Cl (yield-limiting reagent). This is likely due to weighing errors (balance inside the glovebox) or due to the presence of (lighter) chloride impurities in the used (NBu4)Cl. Elemental analysis evidences the purity of the product. Crystals suitable for X-ray diffraction analysis were obtained by vapor diffusion (CH3CN/Et2O).

1H NMR (CH3CN): 101.0 (s, 1 × trans-CH3), 67.9 (s, 4 × cis-CH3) ppm. IR (KBr): 2927 (m), 2853 (w), 2288 (m), 2250 (w), 2218 (w), 1633 (m), 1422 (w), 1263 (m), 1083 (s), 1062 (s), 576 (w) cm–1. Anal. Calcd for C10H15B2ClF8N5Re: C, 20.00; N, 11.66. Found: C, 20.12; N, 11.34.

[ReBr(NCCH3)5](BF4)2 ([6](BF4)2

Inside a glovebox in a 25 mL Schlenk flask, [2b](BF4)3 (63.6 mg, 91.8 μmol) was dissolved in CH3CN (6 mL), giving a dark brown solution. (NBu4)Br (23.8 mg, 73.8 μmol) was dissolved in CH3CN (3.5 mL) and added dropwise over a period of 5 min to the [2b](BF4)3 solution, resulting in a lighter brown color. After 2 h of stirring, the solvent was removed in vacuo, the resulting solid was suspended in CH3CN (0.2 mL), and Et2O (5 mL) was added. The brown suspension was filtered through a glass filter frit, and the dark brown solid was washed with THF (4 × 3 mL). Extraction of the solid with a CH3CN/CH2Cl2 mixture (1:5, 4 × 3 mL) resulted in a reddish solution. Evaporation of the solvent gave [6](BF4)2 (49.2 mg, 76.3 μmol, 83% (based on [2b](BF4)3)) as a brownish-red solid. Note: The amount of product is slightly larger than the quantity of employed (NBu4)Br (yield-limiting reagent). This is likely due to weighing errors (balance inside the glovebox) or due to the presence of (lighter) bromide impurities in the used (NBu4)Br. Elemental analysis evidences the purity of the product. Crystals suitable for X-ray diffraction analysis were obtained by vapor diffusion (CH3CN/Et2O).

1H NMR (CH3CN): 88.1 (s, 1 × trans-CH3), 64.2 (s, 4 × cis-CH3) ppm. IR (KBr): 2937 (m), 2342 (w), 2285 (m), 2250 (w), 2219 (w), 1627 (m), 1423 (m), 1371 (m), 1083 (s), 1062 (s), 620 (w) cm–1. Anal. Calcd for C10H15B2BrF8N5Re: C, 18.62; N, 10.86. Found: C, 18.76; N, 10.63.

[ReI(NCCH3)5](BF4)2 ([7](BF4)2

Inside a glovebox in a 25 mL Schlenk flask, [2b](BF4)3 (11.6 mg, 16.7 μmol) was dissolved in CH3CN (2.5 mL), giving a brown solution. (NBu4)I (4.9 mg, 13.3 μmol) was dissolved in CH3CN (1 mL), and the colorless solution was added dropwise to the solution of [2b](BF4)3 over the course of 2 min. The red solution was stirred for 3 h before the solvent was concentrated in vacuo to approximately 0.5 mL. Et2O (4 mL) was added, the resulting suspension was filtered, and the brown precipitate was washed with additional Et2O (1 mL) and THF (3 × 2 mL). The brown precipitate was extracted with CH2Cl2 (approximately 15 mL), and the evaporation of the solvent gave [7](BF4)2 (8.8 mg, 12.7 μmol) as a yellow-brown solid in 78% yield (based on [2b](BF4)3).

1H NMR (CH3CN): 71.5 (s, 1 × trans-CH3), 59.2 (s, 4 × cis-CH3) ppm. IR (KBr): 2936 (m), 2872 (m), 2345 (w), 2286 (w), 2250 (w), 2218 (w), 1633 (m), 1455 (w), 1083 (s), 1060 (s), 841 (w), 619 (w) cm–1. Anal. Calcd for C10H15B2BrF8N5Re: C, 17.36; N, 10.12. Found: C, 16.19; N, 9.02.

trans-[ReClBr(NCCH3)4](BPh4) ([8](BPh4))

Inside a glovebox in a 25 mL Schlenk flask, [5](BF4)2 (10.2 mg, 17.0 μmol) was dissolved in CH3CN (1.5 mL), giving a dark purple solution. (NBu4)Br (5.6 mg, 17.3 μmol) dissolved in CH3CN (1 mL) was added dropwise to the flask, resulting in a light purple solution. After 40 min of stirring, the solvent was removed in vacuo, and the crude product was purified by preparative chromatography (Gradient A). Fractions containing the same compound were combined, and the solvent was reduced by freeze-drying. Anion exchange was performed by dissolving the dried combined fractions in H2O (1 mL) and adding NaBPh4 (22.8 mg, 66.6 μmol) to give a yellow suspension. After filtration, washing with cold H2O (4 × 0.5 mL), and freeze-drying, [8](BPh4) (4.3 mg, 5.5 μmol) was obtained in 32% yield as a yellow solid. Crystals suitable for X-ray diffraction analysis were obtained by vapor diffusion (CH3CN/Et2O).

1H NMR (CD3CN): 62.6 (s, 4 × CH3CN), 7.37 (m, 8 arom. H), 7.05 (t, J = 7.4 Hz, 8 arom. H), 6.88 (t, J = 7.4 Hz, 4 arom. H) ppm. IR (neat): 3056 (m), 2999 (m), 2984 (m), 2912 (m), 2284 (w), 1580 (w), 1479 (s), 1428 (s), 1405 (w), 1366 (m), 1270 (m), 1183 (w), 1155 (m), 1067 (m), 1030 (s) cm–1. HR-ESI-MS: C8H12ClBrN4Re+ [8]+, calculated 464.94859, found 464.94723.

trans-[ReClI(NCCH3)4](BPh4) ([9](BPh4))

Inside a glovebox in a 25 mL Schlenk flask, [5](BF4)2 (10.6 mg, 17.7 μmol) was dissolved in CH3CN (1.5 mL), giving a dark purple solution. (NBu4)I (6.5 mg, 17.7 μmol) dissolved in CH3CN (1 mL) was added to the flask, resulting in a dark yellow solution. After stirring for 1.5 h, the solvent was removed in vacuo. The reaction mixture was suspended in CH3CN (0.3 mL), and Et2O (6 mL) was added, resulting in a brownish-red suspension. The suspension was filtered and washed with additional Et2O (2 mL) and THF (2 × 2 mL). The filter cake was extracted with a CH3CN/CH2Cl2 mixture (1:5, 5 × 3 mL), and the resulting dark yellow filtrate was concentrated in vacuo. Purification of the crude product was performed with preparative HPLC (Gradient B). Fractions containing the same compound were combined, and the solvent was reduced by freeze-drying. Anion exchange was performed by dissolving the dried combined fractions in H2O (1 mL) and adding NaBPh4 (19.6 mg, 57.3 μmol) to give an orange suspension. After filtration, washing with cold H2O (2 × 0.5 mL), and freeze-drying, [9](BPh4) (7.0 mg, 8.4 μmol) was obtained in 48% yield as an orange solid. Crystals suitable for X-ray diffraction analysis were obtained by vapor diffusion (CH3CN/Et2O).

1H NMR (CD3CN): 59.1 (s, 4 × CH3CN), 7.34 (m, 8 arom. H), 7.04 (t, J = 7.4 Hz, 8 arom. H), 6.88 (t, J = 7.4 Hz, 4 arom. H) ppm. IR (neat): 3052 (m), 2981 (m), 2909 (m), 2276 (w), 1580 (w), 1479 (s), 1428 (s), 1404 (m), 1363 (m), 1268 (w), 1181 (w), 1151 (m), 1067 (w), 1028 (s), 848 (m) cm–1. HR-ESI-MS: C8H12ClIN4Re+ [9]+, calculated 512.93472, found 512.93546.

trans-[ReBrI(NCCH3)4](BPh4) ([10](BPh4))

Inside a glovebox in a 25 mL Schlenk flask, [6](BF4)2 (7.2 mg, 11.2 μmol) was dissolved in CH3CN (1.5 mL). (NBu4)I (4.1 mg, 11.2 μmol) dissolved in CH3CN (1 mL) was added to the flask, resulting in a brownish-red solution. After stirring for 1.5 h, the solvent was removed in vacuo, and the reaction mixture was purified by preparative HPLC (Gradient B). Fractions containing the same compound were combined, and the solvent was reduced either by freeze-drying ([10]+) or by rotary evaporation (trans-[ReX2(NCCH3)4]+ (X = I, Br). Anion exchange reactions (for [10]+ and trans-[ReI2(NCCH3)4]+) were performed by dissolving the dried combined fractions in H2O (1 mL) and adding NaBPh4 (13 mg, 38 μmol) to give orange suspensions. After filtration and freeze-drying, [10](BPh4) (1.9 mg, 2.2 μmol) was obtained in 19% yield as an orange solid. Crystals suitable for X-ray diffraction analysis were obtained by vapor diffusion (CH3CN/Et2O). Other isolated products included trans-[ReI2(NCCH3)4](BPh4) (1.5 mg, 1.6 μmol, 15%) and trans-[ReBr2(NCCH3)4](TFA) (0.5 mg, 0.8 μmol, 7%).

1H NMR (CD3CN): 57.0 (s, 4 × CH3CN), 7.27 (m, 8 arom. H), 6.99 (t, J = 7.4 Hz, 8 arom. H), 6.84 (t, J = 7.4 Hz, 4 arom. H) ppm. IR (neat): 3020 (m), 2978 (m), 2909 (m), 2275 (w), 1940 (m), 1579 (s), 1479 (s), 1428 (s), 1404 (m), 1363 (m), 1268 (w), 1181 (m), 1151 (m), 1066 (w), 1024 (s), 975 (m), 947 (m), 848 (s) cm–1. HR-ESI-MS: C8H12BrIN4Re+ [10]+, calculated 556.88421, found 556.88418.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c02056.

  • NMR data of new compounds, electrochemistry data, description of kinetic experiments, and crystallographic details (PDF)

We thank the University of Zurich for financial support.

The authors declare no competing financial interest.

Supplementary Material

ic2c02056_si_001.pdf (1.1MB, pdf)

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