Article pubs.acs.org/Organometallics
Salts of the Dianions [Hg(12-X-closo-1-CB11H10)2]2− (X = I, CCH, CCFc, CCSiiPr3): Synthesis and Spectroscopic and Structural Characterization Alexander Himmelspach,† Jonas Warneke,‡ Marius Schaf̈ er,† Michael Hailmann,† and Maik Finze*,† †
Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany Institut für Angewandte und Physikalische Chemie, Universität Bremen, Leobener Straße, 28334 Bremen, Germany
‡
S Supporting Information *
ABSTRACT: Cesium or tetraethylammonium salts of the doubly negatively charged mercury(II) complexes [Hg(12-I-closo-1CB11H10)2]2− (1), [Hg(12-HCC-closo-1-CB11H10)2]2− (2), [Hg(12-FcCC-closo-1-CB11H10)2]2− (3), and [Hg(12-iPr3SiCCcloso-1-CB11H10)2]2− (4) were synthesized. The synthesis of the alkynyl-functionalized clusters was conducted via two different routes. The alkynyl moiety either was present before formation of the mercury(II) complex or was introduced via a Pd-catalyzed crosscoupling reaction using Cs21 as starting material. The compounds were characterized by multi-NMR and vibrational spectroscopy, mass spectrometry, and elemental analysis. Gas-phase reactions of the dianions 1, 2, and 3 were studied by collision-induced dissociation in (−)-ESI mass spectrometry experiments. Single crystals of Cs21· 2MeCN, Cs21·xMe2CO (x ≈ 2), [Et4N]22·yEt2CO (y ≈ 4), and [Et4N]23 were studied by X-ray diffraction. In the crystals of Cs21·xMe2CO (x ≈ 2) the dianions and half of the Cs+ cations form a stacked hexagonal structure with channels that contain the second half of the Cs+ cations and the solvent molecules. The formation of this supramolecular structure is rationalized by weak Hg···I and Cs···I interactions that are close to classical van der Waals interactions.
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CB11X11)2]2− (X = H, F, Cl, Br) and [PhHg(closo-1-CB11X11)]− (X = H, F, Cl, Br, I).26,27 In addition, monocarba-closododecaboranyl metal complexes with a metal−Ccluster σ-bond are intermediates in some functionalizations at the Ccluster vertex, e.g., in the copper-mediated Ccluster−C cross-coupling reaction.28 In this contribution we report first on doubly negatively charged di(carba-closo-dodecaboranyl) mercury(II) complexes with functional groups. The functional groups are alkynyl substituents that are bonded to the antipodal boron atoms of the carboranyl ligands. Two synthetic strategies toward these mercury(II) complexes were developed. (−)-ESI mass spectrometry provided a detailed insight into the gas-phase reactivity of these species. The crystal structures of the new anionic complexes [Hg(12-R-closo-1-CB11H10)2]2− (R = I (1), CCH (2), CCFc (3)) were studied by single-crystal X-ray diffraction. The structure of Cs21·xMe2CO (x ≈ 2) provided evidence on the potential of the new linear mercury(II) complexes to serve as building blocks for supramolecular chemistry. Furthermore, selected electrochemical properties of the di(ferrocenylalkynyl)-substituted anion 3 are discussed.
INTRODUCTION Metal complexes of closo-carboranyl ligands, in which the metal is exo-bonded to either a cluster carbon or boron atom,1 are currently of growing interest because of a number of different applications. For example, complexes of this type have been used as catalysts in organic reactions,2−4 as building blocks in supramolecular chemistry, and for the design of metal−organic frameworks (MOFs),5−10 or as reactive species for the preparation of novel, otherwise inaccessible carborane derivatives.11,12 Most of these complexes contain dicarba-closo-dodecaboranyl ligands based on the three isomeric clusters {closo-1,2C2B10}, {closo-1,7-C2B10}, and {closo-1,12-C2B10}, which have a metal−Ccluster σ-bond.1,9,10,13,14 A substantial number of mercury(II) complexes with these ligands have been described, and they have been applied, for example, in supramolecular chemistry9,10,15−19 and catalysis.20 In contrast, only very few examples of metal complexes that have a metal−Ccluster σ-bond with related monocarba-closo-dodecaboranyl ligands have been reported to date.21,22 The coinage metal complexes [ClCu(closo-1-CB11F11)]2− 23 and [Ag(closo-1-CB11I5Br6)2]3− 24 were the first examples. Only a few further complexes have been described, which are the gold(I) derivative [Ph3PAu(closo-1CB11H11)]− 25 and the mercury(II) complexes [Hg(closo-1© 2015 American Chemical Society
Received: October 1, 2014 Published: January 14, 2015 462
DOI: 10.1021/om5009956 Organometallics 2015, 34, 462−469
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Organometallics Scheme 1. Syntheses of Di(carba-closo-dodecaboranyl) Mercury(II) Complexes
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RESULTS AND DISCUSSION Synthetic Aspects. Two different synthetic approaches were developed for the preparation of salts of di(carba-closododecaboranyl) mercury(II) complexes with alkynyl substituents bonded to both antipodal boron atoms (routes A and B in Scheme 1). The key compound of the first approach is the iodinated derivative Cs2[Hg(12-I-closo-1-CB11H10)2] (Cs21). It was obtained from readily accessible Cs[12-I-closo-1-CB11H11]29,30 by deprotonation and subsequent reaction with mercury dichloride, similar to the synthesis of salts of the related dianions [Hg(closo-1-CB11X11)2]2− (X = H, F, Cl, Br).26,27 The conversion to 1 was always incomplete, and the starting anion [12-I-closo-1-CB11H11]− was present in the crude product mixtures (20−30 mol %). Because of the different solubility of the cesium salts of 1 and [12-I-closo-1-CB11H11]− in water, separation of the less soluble Cs21 was achieved and [12-I-closo1-CB11H11]− was recovered as the [Et4N]+ salt. Recrystallization of Cs21 afforded the salt in high purity. Microwave-assisted Kumada-type cross-coupling reactions yielded the corresponding alkynyl-substituted dianions as exemplified by the Pd-catalyzed syntheses of salts of [Hg(12HCC-closo-1-CB11H10)2]2− (2) and [Hg(12-FcCC-closo-1CB11H10)2]2− (3). The dianion [Hg(12-Me3SiCC-closo-1CB11H10)2]2−, which was formed by the cross-coupling reaction with Me3SiCCMgBr, was desilylated in the course of the basic aqueous workup to result in the diethynyl derivative 2.
Related microwave-assisted cross-coupling reactions to yield carba-closo-dodecaborate 31−33 and closo-dodecaborate anions31,34 with alkynyl groups have been reported earlier. Surprisingly, no Ccluster−Hg bond cleavage was observed during the cross-coupling reaction, which underlines the high chemical stability of this bond. However, attempted cross-coupling reactions using a phenyl instead of an alkynyl Grignard reagent lead to Ccluster−Hg bond cleavage, which in turn demonstrates the limits of stability. The alternative synthesis starts from the alkynyl-substituted clusters [12-R′3SiCC-closo-1-CB11H11]− (R′ = Et, iPr)34 that are deprotonated at the Ccluster vertex35 and reacted with HgCl2 to yield the HgII complexes [Hg(12-iPr3SiCC-closo-1CB11H10)2]2− (4) and [Hg(12-HCC-closo-1-CB11H10)2]2− (2). The latter anion 2 was formed from the triethylsilyl derivative during acidic aqueous workup. In contrast to [Hg(12-R′ 3SiCC-closo-1-CB11 H10 ) 2]2− (R = Me, Et), desilylation of [Hg(12-iPr3SiCC-closo-1-CB11H10)2]2− (4) was achieved neither under aqueous basic or acidic conditions nor with fluoride ions in ethanol at elevated temperatures. The reaction mixtures of the syntheses of 2 and 4 contained the anions [12-HCC-closo-1-CB11H11]− and [12-iPr3SiCCcloso-1-CB11H11]−, respectively. Similar to the preparation of Cs21, purification of 2 was achieved upon precipitation of the cesium salt from aqueous solution. In contrast, all attempts to purify Cs24 have failed so far, which is due to the low solubility of Cs[12-iPr3SiCC-closo-1-CB11H11] in water. Hence, 463
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Figure 1. Dianion 1 in crystals of Cs21·2MeCN and Cs21·xMe2CO (x ≈ 2) (displacement ellipsoids at the 30% probability level; left) and plot of the hexagonal packing of Cs21·xMe2CO (x ≈ 2) viewed along [001] (transparent Cs atoms correspond to disordered atoms with an occupancy of 50%; H atoms are omitted for clarity; right). Selected bond lengths [Å] and angles [deg] of Cs21·2MeCN: Hg−Ccluster 2.068(7), B−I 2.192(8), Ccluster− Hg−Ccluster 180; Cs21·xMe2CO (x ≈ 2): Hg−Ccluster 2.04(2) and 2.06(2), B−I 2.25(2) and 2.24(3), Ccluster−Hg−Ccluster 174.8(7).
space group was found to be P6̅2c with Z = 2. The two other space groups that have the same systematic absences P63mc and P63/mmc were ruled out. The material is composed of hexagonal layers orthogonal to [001]. The hexagonal layers are stacked along [001] with channels that are filled with solvent molecules and one-half of the cesium cations. These Cs+ cations, which are disordered over two positions, are located at the border of the channels in close proximity to the dianions. The solvent molecules are severely disordered. Therefore, their contribution to the structure factors was taken into account using the SQUEEZE routine as implemented in the Platon program.37,38 The diameter of the hexagonal channels is approximately 16 ± 2 Å. The total solvent-accessible void is 2199 Å3, as calculated by the Platon program, which corresponds to 34% of the unit cell volume. According to 1H and 13C{1H} NMR experiments performed on a large crystal of Cs21·xMe2CO (x ≈ 2) dissolved in CD3CN, the solvent molecules are acetone that was used as solvent for the crystallization. The number of acetone molecules per formula unit was assessed by comparison to the number of electrons calculated by the Platon program. The stability of the supramolecular structure of Cs21· xMe2CO (x ≈ 2) can be attributed to weak interactions between (i) the iodine atoms and the second half of the Cs+ cations, which are not disordered and are located on a 3-fold axis, and (ii) the iodine and mercury atoms (Figure S2 in the Supporting Information). The corresponding Cs···I distance of 3.890(14) Å is significantly shorter than the shortest Cs···I distance in Cs21·2MeCN. The Hg···I distance of 3.7494(15) Å is slightly shorter than the sum of the van der Waals radii (Hg: 1.70−2.00 Å;39 I: 2.06 Å in tert-alkyl groups40). However, this d(Hg···I) is longer than the Hg···I distances observed for related dicarba-closo-dodecaboranyl mercury(II) complexes
synthetic route A depicted in Scheme 1 provides an advantage in that a separation of the salt of the dianionic mercury complex from the respective salt of the carba-closo-dodecaborate monoanion is unnecessary. So, the synthesis of the di(ferrocenylalkynyl) derivative 3 was attempted from dianion 1 and not from the [12-FcCC-closo-1-CB11H11]− anion.31 Crystallographic Characterization of Cs21, [Et4N]22, and [Et4N]23. Cs2[Hg(12-I-closo-1-CB11H10)2] (Cs21) crystallizes as a solvate with two acetonitrile molecules per formula unit in the centrosymmetric orthorhombic space group Pnnm with Z = 2. The solvent molecule is located on a mirror plane. Two CH3CN molecules and two Cs+ cations form a diamondshaped planar four-membered Cs···N···Cs···N ring [d(Cs···N) = 327.0(5) pm, ∠(Cs···N···Cs) = 99.654(5)°, ∠(N···Cs···N) = 80.346(3)°; Figure S1 in the Supporting Information]. The same structural motif with weakly coordinating, bridging acetonitrile molecules and similar interatomic distances and angles was found in the crystal of Cs2[closo-B12I12]·2MeCN.36 The shortest Cs···I distance in Cs21·2MeCN of 418.51(9) pm is similar to d(Cs···I) in related cesium salts of iodinated boron clusters, e.g., Cs2[closo-B12I12]·2MeCN (402−428 pm)36 and Cs[1-H2N-12-I-closo-1-CB11H10] (389.8(2)−398.6(2) pm).32 The shortest d(Hg···I) of 571.10(10) pm is too long for any bonding interaction. The dianion 1 is located on a center of symmetry, which results in a linear arrangement with a Ccluster− Hg−Ccluster angle of 180° and a staggered conformation of the two carba-closo-dodecaboranyl ligands (Figure 1). Similarly, staggered conformations have been found for the related doubly negatively charged mercury(II) complexes [Hg(closo-1CB11X11)2]2− (X = H, Cl, Br) in their cesium salts.26 Crystallization of Cs21 from acetone by slow uptake of dichloromethane yielded hexagonal crystals that contained approximately two acetone molecules per formula unit. The 464
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Organometallics with intermolecular Hg···I interactions,9,10 e.g., in [{Hg(closo1,2-C2B10H8Et2)}4·(9,12-I2-closo-1,2-C2B10H10)2] (3.445(2)− 3.626(2) Å).41 The longer d(Hg···I) indicates a weaker Lewis acidity of the doubly negatively charged 1 compared to the neutral mercury complexes of the dicarba-closo-dodecaboranyl ligands, which are known to be strong electron-withdrawing substituents concerning bonds to the Ccluster atoms. Furthermore, this finding agrees well with our comparative study on the dianions [Hg(closo-1-CB11X11)2]2− (X = H, F), which revealed a pronounced Lewis acidic behavior for the polyfluorinated dianion only.26,27 Dianion 1 is located on a mirror plane. In contrast to Cs21· 2MeCN, the dianion is slightly bent [∠(Ccluster−Hg−Ccluster) = 174.8(7)°] and the conformation of the carba-closo-dodecaboranyl ligands is almost eclipsed (Figure 1). The inner-cluster atomic distances of the clusters in Cs21·2MeCN and Cs21· xMe2CO (x ≈ 2) are very similar, and d(B−I) and d(Hg− Ccluster) are close as well (Figure 1). Furthermore, d(B−I) is similar to values observed for related [1-R-12-I-closo-1CB11H10]− anions (R = H (2.200(2) Å),42 NH2 (2.190(12) Å)32) and the d(Hg−Ccluster) in [Hg(closo-1-CB11H11)2]2− (2.085(4) Å).26 The tetraethylammonium salt of [Hg(12-HCC-closo-1CB11H10)2]2− (2; Figure 2) crystallizes as a solvate with approximately four diethyl ether molecules in the orthorhombic space group Pbcm with Z = 4. Since it was not possible to locate the atom positions of the Et2O molecules, their contribution to the structure factors was included using the SQUEEZE routine of the Platon program.37,38 The solvent molecules were confirmed to be Et2O by 1H NMR spectroscopy, and their number was estimated on the basis of the number of electrons in the voids calculated by the Platon program. [Et4N]2[Hg(12-FcCC-closo-1-CB11H10)2]2− ([Et4N]23) crystallizes in the orthorhombic space group Pnma with Z = 4 (Figure 2). The tetraethylammonium cations and a major fraction of the dianionic mercury(II) complex are disordered. One of the ferrocenylalkynyl groups is not disordered. The lengths of the dianions [Hg(12-HCC-closo-1CB11H10)2]2− (2) and [Hg(12-FcCC-closo-1-CB11H10)2]2− (3) are approximately 18 and 25 Å, respectively. Bond parameters of the alkynyl-functionalized complexes 2 and 3 are similar, and they are close to interatomic distances and angles that have been reported for (i) analogous carba-closododecaborate anions [12-FcCC-closo-1-CB11H11]− (R = H,43 Fc31) and (ii) the related mercury(II) complex [Hg(closo-1CB11H11)2]2−.26 Electrochemical Properties of [Hg(12-FcCC-closo-1CB11H10)2]2− (3). The dianion 3 undergoes a reversible oxidation at E1/2(CH3CN) = +0.04 V (Figure 3). There is no evidence for any electronic coupling between the ferrocenyl moieties through the di(carba-closo-dodecaboranyl) mercury(II) spacer since only one oxidation event is observed by cyclic as well as differential pulse voltammetry. The redox potential of the redox couple 3/32+ is the same as the one found for [1-R12-FcCC-closo-1-CB 11 H 10 ] − /1-R-12-FcCC-closo-1CB11H10 (R = H, H2N).31 Mass Spectrometry. The doubly negatively charged mercury(II) complexes [Hg(12-I-closo-1-CB11H10)2]2− (1), [Hg(12-HCC-closo-1-CB11H10)2]2− (2), and [Hg(12-FcC C-closo-1-CB11H10)2]2− (3) were studied in the gas phase by collision-induced dissociation (CID) in (−)-electrospray-iontrap [(−)-ESI-IT] mass spectrometry experiments.
Figure 2. Dianions 2 and 3 in crystals of [Et4N]22·yEt2O (y ≈ 4) and [Et4N]23 (displacement ellipsoids at the 30% probability level; the partial disorder of dianion 3 is omitted for clarity). Selected bond lengths [Å] and angles [deg] of [Et4N]22·yEt2O (y ≈ 4): Hg−Ccluster 2.064(7) and 2.057(7), B−CC 1.591(12) and 1.557(12), CC 1.200(14) and 1.165(14), Ccluster−Hg−Ccluster 179.5(3), B−CC 177.3(9) and 177.5(9); [Et4N]23 (only values of the ordered part are listed where applicable): Hg−Ccluster 2.086(11) and 2.100(11), B−CC 1.553(14), CC 1.181(12), CC−C 1.450(12), Fe−CpcentroidCC 1.6351(12), Fe−Cpcentroid 1.6410(12), Ccluster−Hg−Ccluster 176.5(4), B−CC 178.1(9), CC−C 177.3(9).
Figure 3. (a) Cyclic and (b) differential pulse voltammogram of [Et4N]2[Hg(12-FcCC-closo-1-CB11H10)2] ([Et4N]23).
Figure 4a shows the mass spectrum after CID of the dianion 1 with the signal of 1 at approximately m/z = 369. After loss of an iodide ion, one of the carba-closo-dodecaborate units possesses a free boron vertex (m/z ≈ 609) that readily adds 465
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Figure 4. (−)-ESI-IT mass spectra showing (a) products after CID of [Hg(12-I-closo-1-CB11H10)2]2− and (b) isolation and selective fragmentation of ions with m/z = 609 from the isotopic pattern of [Hg(12-I-closo-1-CB11H10)(closo-1-CB11H10)]− (green arrows: reactions observed without applying excitation energy; red arrows: reactions initiated by CID).
water from the residual gas in the ion trap (m/z ≈ 627). In general, iodinated boron clusters easily lose either iodine radicals or iodide ions.26,44 All ions with m/z = 609 were isolated from the isotopic mixture of [Hg(12-I-closo-1CB11H10)(closo-1-CB11H10)]− and subsequently activated by CID (Figure 4b). The masses of the detected product ions can be explained only by losses of HgH and HgH2. The loss of such a neutral fragment leads to a coupling of the carborate units, as evident from the set of peaks at around m/z = 405. [Hg(12-FcCC-closo-1-CB11H10)2]2− (3) loses an electron under “soft” CID conditions to result in the monoanion [Hg(12-FcCC-closo-1-CB11H10)2]− (Figure S4 in the Supporting Information). Probably, the oxidation takes place at one of the iron centers of the ferrocenylalkynyl moieties. Since the intensities of the peaks that correspond to [Hg(12-FcCCcloso-1-CB11H10)2]− were low, a detailed study of its gas-phase reactions was impossible. However, isolation and excitation of a small m/z range followed by CID resulted in a product signal with a broad isotopic pattern and the same mass difference as the one found for the fragmentation of [Hg(12-I-closo-1CB11H10)(closo-1-CB11H10)]− in Figure 3b (see Figure S4b in the Supporting Information). This indicates that an analogous coupling reaction of two carboranyl units under loss of HgH or HgH2 had occurred. For the di(ethynyl)-substituted dianion 2 a different behavior was observed from that for 1 (Figure S3 in the Supporting Information). The excitation of 2 in the gas phase leads to homolytic cleavage of the Hg−Ccluster bonds, as evident from the peaks that most likely correspond to [12-HCC-closo-1CB11H10]−. An analogous behavior was reported for the related complex [Hg(closo-1-CB11H11)2]2−.26 No coupling of the ions to yield the dimer [(12-HCC-closo-1-CB11H10)2]2− was observed. Presumably, this is due to the Coulomb repulsion between the two negatively charged species. Hence, formation of a monoanionic species by CID as found for 1 and 3 via
elimination of an anion or oxidation, respectively, enables the coupling of both carboranyl fragments after elimination of a mercury fragment.
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SUMMARY First di(carba-closo-dodecaboranyl) mercury(II) complexes with functional groups are described. Two different synthetic approaches have been developed, one of which includes a microwave-assisted Kumada-type cross-coupling reaction starting from the diiodinated complex [Hg(12-I-closo-1CB11H10)2]2− (1) to yield the dialkynyl-substituted derivatives [Hg(12-HCC-closo-1-CB11H10)2]2− (2) and [Hg(12-FcC C-closo-1-CB11H10)2]2− (3). The synthesis of 2 and 3 using 1 as starting material is indicative of a high chemical stability of the Hg−Ccluster bond, and it shows the possibility to use these complexes for further chemistry. The gas-phase studies reveal that elimination of a Hg fragment can proceed under formation of a bond between two {closo-1-CB11} units if one of the cages has lost its negative charge either by oxidation of the ferrocenyl moiety or by loss of an iodide ion. Especially, the diethynyl-substituted dianion [Hg(12-HC C-closo-1-CB11H10)2]2− (2) is a promising building block for coordination and supramolecular chemistry because of the versatile functional groups at the antipodal boron atoms. However, even the diiodinated derivative 1 can serve as a building block in supramolecular chemistry, as evident from the hexagonal structure of Cs21·xMe2CO (x ≈ 2), which has channels filled with the solvent molecules and half of the cesium cations.
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EXPERIMENTAL SECTION
General Methods. 1H, 11B, 13C, 29Si, and 199Hg NMR spectra were recorded at 25 °C in (CD3)2CO or CD3CN on a Bruker Avance 500 NMR, a Bruker Avance III 400 NMR, a Bruker Avance 600, or a 466
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Organometallics Bruker Avance III HD 300 NMR spectrometer. The NMR signals were referenced against TMS (1H and 13C), BF3·OEt2 in CDCl3 with Ξ(11B) = 32.083 974 MHz, TMS in CDCl3 with Ξ(29Si) = 19.867 187 MHz, and HgMe2 with Ξ(199Hg) = 17.910 822 MHz as external standards. 1H and 13C chemical shifts were calibrated against the residual solvent signal and the solvent signal, respectively (δ(1H): (CD 3 )(CD 2 H)CO 2.05 ppm, CD 2 HCN 1.94 ppm; δ( 13 C): (CD3)2CO 206.26 and 29.84 ppm, CD3CN 118.26 and 1.32 ppm).45 The assignment of the 11B and 1H NMR signals is aided by 11 1 B{ H}−1H{11B} 2D,46,47 11B{1H}−11B{1H} COSY,48,49 and 1H{11Bselective} experiments. 1H−13C HMBC and HSQC studies were performed to support the interpretation of the 13C NMR spectroscopic data. nJ(13C,1H) coupling constants derived from 1H−13C HMBC are listed with the 13C{1H} NMR spectroscopic data. IR spectra were recorded at room temperature with a Bruker Alpha spectrometer with an apodized resolution of 2 cm−1 in the attenuated total reflection (ATR) mode in the region of 4000−500 cm−1. Raman spectra were recorded at room temperature on a Bruker IFS-120 spectrometer with an apodized resolution of 2 cm−1 using the 1064 nm excitation line of a Nd/YAG laser on crystalline samples contained in melting point capillaries in the region of 3500−100 cm−1. UV/vis spectra of solutions of [Et4N]23 in CH2Cl2 were recorded on a UV-1650PC spectrometer (Shimadzu) in the range of 200−1000 nm. Elemental analyses (C, H, N) were performed with a Euro EA3000 instrument (HEKA-Tech, Germany). Chemicals. All standard chemicals were obtained from commercial sources. Tetrahydrofuran was distilled from K/Na alloy under an argon atmosphere and stored in a flask equipped with a valve with a PTFE stem (Young, London) under an argon atmosphere. A solution of Me3SiCCMgBr in THF (0.75 mol L−1) was prepared from Me3SiCCH (Apollo Scientific) by the reaction with EtMgBr (1 mol L−1 in THF), and it was kept in a round-bottom flask with a valve with a PTFE stem (Young, London) at 4 °C. Ethynylferrocene was synthesized according to a literature procedure.50−52 Cs[12-I-closo-1CB11H11]29,30 and Cs[12-R3SiCC-closo-1-CB11H11] (R = Et, iPr),33 were prepared as described elsewhere. Single-Crystal X-ray Diffraction. Colorless crystals of Cs21· 2MeCN suitable for X-ray diffraction were grown from acetonitrile by slow evaporation of the solvent. Colorless crystals of Cs21·xMe2CO (x ≈ 2) and [Et4N]22·yEt2O (y ≈ 4) were obtained from solutions of acetone by slow uptake of dichloromethane and diethyl ether, respectively. Slow uptake of pentane into a solution of [Et4N]23 in acetone resulted in orange crystals. A crystal of Cs21·xMe2CO (x ≈ 2) was studied with a Stoe IPDS I diffractometer using Mo Kα radiation (λ = 0.710 73 Å). Crystals of Cs21·2MeCN, [Et4N]22·yEt2O (y ≈ 4), and [Et4N]23 were investigated with CCD diffractometers using Mo Kα radiation as well (Oxford Xcalibur equipped with an EOS detector for Cs21·2MeCN and [Et4N]22·yEt2O (y ≈ 4), Bruker Apex I for [Et4N]3). All structures were solved by direct methods,53,54 and refinement is based on full-matrix least-squares calculations on F2.54,55 All non-hydrogen atoms were refined anisotropically. Most of the positions of the hydrogen atoms in the crystal structures were located via ΔF syntheses with the exception of the crystal structure of [Et4N]23. All hydrogen atoms were refined using idealized bond lengths as well as angles. The acetonitrile molecule in Cs21·2MeCN is located on a mirror plane, resulting in a disorder of the hydrogen atoms of the methyl group over two positions. The crystals of Cs21·xMe2CO (x ≈ 2) and [Et4N]22·yEt2O (y ≈ 4) contained large voids (2199 and 2271 Å3 per unit cell) filled with disordered acetone and diethyl ether molecules, respectively. 1H and 13 C{1H} NMR spectra of the crystals in CD3CN proved the identity of the solvent molecules. Their contributions to the structure factors were secured by back-Fourier transformation using the SQUEEZE routine of the program PLATON, resulting in 405 and 702 e− per unit cell for Cs21·xMe2CO and [Et4N]22·yEt2O, respectively.37,38,56 The number of acetone and Et2O molecules was estimated as 2 (Cs21· xMe2CO) and 4 ([Et4N]23·yEt2O) by comparison of the electron numbers of the molecules to the aforementioned numbers of electrons per unit cell.
In the crystal of [Et4N]23 both crystallographically independent cations are disordered over two positions. Dianion 3 is also disordered over two positions, with one of the ferrocenylalkynyl groups being the only exception. Because of the disorder, some distance restraints and some similarity restraints for Uij had to be applied to achieve a stable refinement. Calculations were carried out using either the ShelXle graphical interface57 or the WinGX program package.58 Molecular structure diagrams were drawn with the program Diamond 3.2i.59 Experimental details, crystal data, and CCDC numbers are collected in Table S1 in the Supporting Information. Supplementary crystallographic data for this publication are deposited in the Supporting Information or can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Mass Spectrometry. ESI-MS measurements were performed on a Bruker Esquire-LC ion trap mass spectrometer (Bruker Daltonik, Bremen, Germany). Samples were dissolved in acetonitrile (LCMS grade; VWR, Darmstadt, Germany) at concentrations of approximately 10−5−10−6 mol L−1 and injected into the mass spectrometer via a syringe pump at a flow rate of 3 μL min−1. Spectra were recorded in the negative ion mode for 3 to 5 min and averaged. Collision-induced dissociation was induced in an ion trap. Ions with a defined m/z value are isolated and specifically activated by collisions with the background gas (mainly He, and some N2 and H2O) in the ion trap. The collision is induced by applying an adjustable ac voltage to the ion trap end-caps. The resulting mass spectrum shows only the reaction products of the isolated and activated ions. Cyclic Voltammetry. Cyclic voltammetry experiments were performed using a Gamry Instruments Reference 600 potentiostat. A standard three-electrode cell configuration was employed using a platinum disk working electrode, a platinum wire counter electrode, and a silver wire, separated by a Vycor tip, serving as the reference electrode. Formal redox potentials are referenced to the ferrocene/ ferrocenium redox couple by using [Cp*2Fe] (E1/2 = −0.505 V, Cp* = η-C5Me5) as an internal standard.60 Tetra-n-butylammonium hexafluorophosphate ([nBu4N][PF6]) was employed as the supporting electrolyte. Compensation for resistive losses (iR drop) was employed for all measurements. Synthesis of Cs2[Hg(12-I-closo-1-CB11H10)2] (Cs21). A glass finger equipped with a valve with a PTFE stem (Young, London), fitted with a PTFE-coated magnetic stirring bar, was charged with Cs[12-I-closo-1-CB11H11] (300 mg, 0.75 mmol) and THF (20 mL). After addition of a solution of nBuLi in hexane (0.50 mL, 1.6 mol L−1, 0.79 mmol) at 0 °C the reaction mixture was warmed to room temperature and stirred for 1 h. A solution of HgCl2 (102 mg, 0.38 mmol) in THF (15 mL) was added, and the resulting mixture was stirred for a further 2 h. The reaction mixture was poured into water (50 mL), and the THF was removed using a rotary evaporator. An aqueous solution of CsCl (3.0 g, 17.8 mmol, 10 mL) was added slowly, resulting in the immediate formation of a white precipitate. Solid Cs21 was filtered off and dried in a vacuum. Addition of a solution of [Et4N] Br (1.0 g, 4.8 mmol) in water (10 mL) resulted in the formation [Et4N][12-I-closo-1-CB11H11] (no yield determined for this byproduct). The crude Cs21, which contained a small amount of Cs[12-I-closo-1-CB11H11] (
