Synthesis, Spectroscopy, and Redox Studies of Ferrocene

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Synthesis, Spectroscopy, and Redox Studies of FerroceneFunctionalized Coinage Metal Alkyne Complexes Tim P. Seifert,† Johannes Klein,‡ Michael T. Gamer,† Nicolai D. Knöfel,† Thomas J. Feuerstein,† Biprajit Sarkar,‡ and Peter W. Roesky*,† †

Institute of Inorganic Chemistry, Karlsruhe Institute of Technology, Engesserstraße 15, D-76131 Karlsruhe, Germany Institut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin, Fabeckstraße 34−36, D-14195 Berlin, Germany

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S Supporting Information *

ABSTRACT: Ethynylferrocene (FcCCH) was utilized as a redoxactive metalloligand for the synthesis of polynuclear coinage metal complexes. The reaction of [FcCCLi] with tri-tert-butylphosphine metal chlorides [tBu3P-MCl] (M = Au, Ag, Cu) yielded different heteronuclear ferrocene-funtionalized alkyne complexes featuring metallophilic interactions. Furthermore, the redox properties of the ferrocenyl-functionalized tetracopper complex were investigated by cyclic voltammetry and UV−vis−near-IR spectroelectrochemistry. They indicate the compounds’ redox-rich nature and a weak electronic coupling between the redox-active ferrocenyl units over a large distance.

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INTRODUCTION The chemistry of gold is one of the most rapidly evolving fields in chemistry mainly because of its wide range of application potential in different areas such as biosciences, photophysics, or catalysis.1−4 The application of single-crystal X-ray diffraction analyses in the last decades revealed unexpectedly short Au−Au distances in multinuclear gold(I) compounds and thus triggered fundamental research on gold(I) chemistry.5−8 These “aurophilic interactions” are just one class of “metallophilic interactions”, which are also known for other closed-shell metal ions such as thallium(I) and mercury(II) and the other monovalent d10 coinage metals silver(I) (“argentophilic interactions”) and copper(I) (“cuprophilic interactions”).9 Yet, the number of gold compounds containing metallophilic interactions is significantly higher compared to its lighter homologues. An interaction between metal centers is considered to be metallophilic if the M−M distance is shorter than the sum of their corresponding van der Waals radii (2.80 Å for Cu−Cu, 3.44 Å for Ag−Ag, and 3.32 Å for Au−Au).10,11 In addition, metallophilic interactions are usually classified as ligand-supported, semisupported, or unsupported interactions. Supporting ligands are mostly organic compounds exhibiting two or more donor sites or, as shown in this work, derivatives of acetylene.1 Because of their ability to coordinate to metal ions via both σ- and π-bonding modes, acetylenes allow a close proximity of the metal centers; thus, the formation of strong metal−metal interactions is promoted.12,13 In coinage metal compounds, metallophilicity is regarded as an important criterion for photoluminescence, molecular recognition, optical switches, and catalysis.14−17 Furthermore, metal alkyne complexes are believed to be key intermediates in metalcatalyzed transformations of alkynes.18−20 However, to date, © XXXX American Chemical Society

comparably few redox studies dealing with coinage metal complexes have been carried out.18,21−28 Investigation of the redox properties of copper complexes, in particular, is beneficial for the development and understanding of copperpromoted cross-couplings (e.g., Chan−Lam coupling).29,30 Therefore, an in-depth understanding of the cooperative interaction between metals, including the rationalization of structures and varying bonding modes, is essential. Ferrocene is the epitome of an organometallic moiety that displays a wellbehaved, reversible, one-electron redox process. Additionally, the incorporation of ferrocene into molecular structures can render them with additional stability but at the same time open up other reactive sites. For these reasons, ferrocenyl substituents are often incorporated into molecular structures to influence their redox properties and also sometimes to investigate through-space electronic interactions between ferrocenyl units that are spatially separated from each other.31−35 Herein, we report the synthesis of coinage metal alkyne complexes exhibiting strong metallophilic interactions and the formation of different structural motifs. Analysis includes single-crystal X-ray diffraction and electrochemical and spectroelectrochemical properties of ethynylferrocene-based di- and tetranuclear coinage-metal complexes. The aim here is to generate comparative data on the reactivity, metallophilic interactions, spectroscopic and electrochemical properties, and long-range electronic coupling through studies on these systems. Received: October 10, 2018

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DOI: 10.1021/acs.inorgchem.8b02873 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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cell under an inert atmosphere. Anhydrous and degassed THF, which was freshly distilled from sodium/benzophenone and purged with argon prior to use, with 0.1 M [NBu4][PF6] or 0.1 M [NBu4][BArF4] as the electrolyte, was used as the solvent. To avoid decomposition of [NBu4][BArF4], the UV light was cut off by placing glass slides between the sample and light source. Synthesis. General Procedure for the Synthesis of [MCl(PtBu3)] (M = Au, Ag, Cu). tBu3P (1.00 mmol) and MX (1.10 mmol for AgCl and CuCl; 1.00 mmol for [AuCl(tht)]) were suspended in CH2Cl2 (10 mL) and stirred for 18 h at room temperature. The mixture was filtered with a cannula and evaporated to dryness. The resulting colorless solids were used without further purification. The characterization data are consistent with the literature values.42,43 Synthesis of [Au(CCFc)(PtBu3)] (1). Ethynylferrocene (100 mg, 0.48 mmol, 1.00 equiv) was dissolved in THF (10 mL), and nBuLi (0.20 mL, 2.5 M in n-hexane, 0.48 mmol, 1.00 equiv) was added dropwise at room temperature. The mixture was stirred for 30 min and added to solid [AuCl(PtBu3)] (207 mg, 0.48 mmol, 1.00 equiv). After stirring overnight, the solvent was evaporated under reduced pressure. CH2Cl2 (10 mL) was added, and the mixture was filtered with a cannula. The solvent was removed under reduced pressure, and the crude product was redissolved in THF (10 mL). Slow evaporation of the solution yielded 220 mg (75%) of 1 as dark-red crystals. 1 H NMR (300 MHz, CDCl3, 298 K): δ 4.40−4.45 (m, 2H, Cp− H), 4.20 (s, 5H, Cp−H), 4.05−4.10 (m, 2H, Cp−H), 1.51 (d, 3JHP = 13.2 Hz, 27H, CH3). 13C{1H} NMR (75 MHz, CDCl3, 298 K): δ 71.8 (Cp), 69.9 (Cp), 67.5 (Cp), 39.0 (d, 1JCP = 17.6 Hz, C(CH3)3), 32.4 (d, 2JCP = 4.5 Hz, CH3). Resonances of the quarternary carbon atoms could not be detected. 31P{1H} NMR (121 MHz, CDCl3, 298 K): δ 91.6 (s). IR (ATR) (cm−1): ν̃ 3092 (vw), 3002 (w), 2953 (w), 1547 (w), 1481 (s), 1445 (w), 1393 (vs), 1369 (s), 1228 (w), 1171 (s), 1105 (s), 1019 (w), 997 (s), 918 (vs), 818 (vs), 669 (w), 583 (w), 523 (vs), 493 (w), 483 (vs), 446 (w). Raman (cm−1): ν̃ 3101 (w), 2909 (w), 2154 (vw), 2124 (s), 2105 (vs), 1451 (s), 1374 (w), 1231 (s), 1107 (s), 1059 (w), 923 (w), 811 (w), 669 (w), 639 (w), 586 (w), 442 (w), 382 (w), 315 (w). ESI-MS. Calcd ([M]+): m/z 608.1570. Found: m/z 608.1802. Elem anal. Calcd for C24H36AuFeP: C, 47.39; H, 5.97. Found: C, 45.85; H, 6.16. Synthesis of [{Au(PtBu3)}2(μ-η1-CCFc)][SbF6] (2). To [AuCl(PtBu3)] (98.6 mg, 0.23 mmol, 1.00 equiv) and AgSbF6 (77.9 mg, 0.23 mmol, 1.00 equiv) was added CH2Cl2 (5 mL), and the resulting solution was stirred for 5 min. The solution was directly filtered with a cannula into a solution of 1 (138 mg, 0.23 mmol, 1.00 equiv) in benzene (20 mL) and stirred overnight, and then the mixture was filtered with a cannula again. Slow evaporation of the solution gave 140 mg (49%) of 2 as bright-red crystals. Because of insufficient solubility, no NMR data are displayed. IR(ATR) (cm−1): ν̃ 2998 (w), 2953 (w), 2922 (vw), 2005 (w), 1481 (vw), 1471 (s), 1394 (s), 1371 (s), 1260 (w), 1170 (s), 1106 (w), 1022 (s), 913 (w), 807 (s), 730 (s), 694 (w), 653 (vs), 607 (w), 581 (w), 521 (s), 478 (vs). Raman (cm−1): ν̃ 2911 (w), 2007 (vs), 1472 (w), 1432 (w), 1374 (w), 1241 (w), 1172 (vw), 1107 (w), 992 (w), 808 (w), 668 (w), 644 (w), 627 (w), 608 (vw), 585 (w), 388 (vw), 309 (w). ESI-MS (positive). Calcd ([M]+): m/z 1007.309. Found: 1007.309. Elem anal. Calcd for C42H69Au2F6FeP2Sb: C, 38.17; H, 5.26. Found: C, 38.78; H, 5.42. Synthesis of [{Ag(PtBu3)}2(η2-FcCCAgCCFc)2] (3). Ethynylferrocene (140 mg, 0.67 mmol, 1.00 equiv) was dissolved in THF (10 mL), and nBuLi (0.30 mL, 2.5 M in n-hexane, 0.67 mmol, 1.00 equiv) was added dropwise at room temperature. The mixture was stirred for 30 min and added to solid [AgCl(PtBu3)] (230 mg, 0.67 mmol, 1.00 equiv). After stirring overnight, the solvent was evaporated under reduced pressure. CH2Cl2 (10 mL) was added, and the mixture was cannula-filtered. The solvent was removed under reduced pressure, and the residue was redissolved in THF. Slow evaporation of the solution gave 51 mg (18%) of 3 as bright-red crystals. Because of decomposition in solution, no NMR data are available. IR(ATR) (cm−1): ν̃ 2865 (w), 2037 (w), 1614 (w), 1481 (w), 1469 (vw), 1409 (w), 1391 (w), 1368 (w), 1246 (vw), 1226 (vw), 1202 (vw), 1173 (w), 1105 (s), 1022 (w), 999 (s), 914 (w), 820 (vw), 808

EXPERIMENTAL SECTION

General Procedures. All manipulations were performed under rigorous exclusion of moisture and oxygen in dried Schlenk-type glassware or in an argon-filled MBraun glovebox. Furthermore, for the most part, the compounds were handled with the exclusion of light by wrapping the compound-containing flasks in aluminum foil. Prior to use, CH2Cl2 was distilled under nitrogen from CaH2. Hydrocarbon solvents [tetrahydrofuran (THF) and benzene] were distilled from potassium/benzophenone prior to use. Deuterated solvents were obtained from Carl Roth GmbH (99.5 atom % D). NMR spectra were recorded on a BrukerAvance II 300 MHz or Avance 400 MHz spectrometer. 1H and 13C{1H} chemical shifts were referenced to the residual 1H and 13C resonances of the deuterated solvents and are reported relative to tetramethylsilane, and 31P{1H} resonances were referenced to external 85% phosphoric acid. IR spectra were obtained on a Bruker Tensor 37 spectrometer. Raman spectra were recorded on a Bruker MultiRam spectrometer. Electrospray ionization mass spectrometry (ESI-MS) spectra were obtained using a Fourier transform ion-cyclotron-resonance IonSpec Ultima mass spectrometer equipped with a 7 T magnet (Cryomagnetics) or on a LTQ Orbitrap XL Q Exactive mass spectrometer (Thermo Fisher Scientific, San Jose, CA) equipped with an HESI II probe. The instrument was calibrated in the range m/z 74−1822 using premixed calibration solutions (Thermo Scientific). A constant spray voltage of 4.6 kV, a dimensionless sheath gas of 8, and a dimensionless auxiliary gas flow rate of 2 were applied. The capillary temperature and S-lens radiofrequency level were set to 320 °C and 62.0, respectively. Elemental analyses were carried out with an Elementar MICRO cube instrument. [AuCl(tht)]36 and ethynylferrocene37 were prepared according to literature procedures. PtBu3, CuCl, AgCl, nBuLi, and AgSbF6 were purchased from commercial suppliers and used as received. X-ray Crystallographic Studies. Suitable crystals were covered in mineral oil (Aldrich) and mounted onto a glass fiber. The crystals were transferred directly into the cold stream of a STOE IPDS 2 or StadiVari diffractometer. All structures were solved by using the programs Olex238 and SHELXS/T.39,40 The remaining non-hydrogen atoms were located from successive difference Fourier map calculations. The refinements were carried out by using full-matrix least-squares techniques on F2 by using the program SHELXL.39,40 The hydrogen-atom contributions of all compounds were calculated but not refined. In each case, the locations of the largest peaks in the final difference Fourier map calculations, as well as the magnitudes of the residual electron densities, were of no chemical significance. Data collection parameters are given in Table S1. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre, and the relevant codes are CCDC1872432−1872435. Electrochemistry. Cyclic voltammograms (CVs) were recorded with a PAR VersaStat 4 potentiostat (Ametek) by working in THF, which was freshly distilled from sodium/benzophenone and purged with argon prior to use, with 0.1 M [NBu4][PF6] (dried, >99.0%, electrochemical grade, Fluka) or 0.1 M [NBu4][B{3,5-(CF3)2C6H3}4] (further referred to as [NBu4][BArF4]) as the electrolyte. [NBu4][BArF4] was prepared by a reported procedure, followed by cation exchange.41 The concentrations of the complexes were about 1 × 10−4 M. A three-electrode setup was used with a glassy carbon working electrode, a coiled platinum wire counter electrode, and a coiled silver wire pseudoreference electrode. The ferrocene/ferrocenium couple was used as the internal reference. UV−Vis Spectroscopy and Spectroelectrochemistry. UV−vis spectra were recorded with an Avantes spectrometer consisting of a light source (AvaLight-DH-S-Bal), a UV−vis detector (AvaSpecULS2048), and a near-IR (NIR) detector (AvaSpec-NIR256-TEC). Spectroelectrochemical measurements were carried out in an optically transparent thin-layer electrochemical cell (CaF2 windows) with a gold-mesh working electrode, a platinum-mesh counter electrode, and a silver-foil pseudoreference electrode. The cell is airtight and has a path length of 0.02 cm, and the solution of samples was filled in the B


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Inorganic Chemistry (vs), 744 (w), 645 (vw), 616 (vw), 600 (w), 527 (s), 499 (w), 481 (vs), 444 (w). Raman (cm−1): ν̃ 3102 (w), 2075 (vs), 1444 (s), 1372 (w), 1227 (s), 1105 (s), 808 (w), 653 (w), 623 (vw), 307 (w). ESIMS (positive). Calcd ([Ag(PtBu3)2]+): m/z 511.2752. Found: m/z 511.2620. Calcd ([M − Fc-CC-Ag-CC-Fc]+): m/z 1143.096. Found: m/z 1143.187. ESI-MS (negative). Calcd ([Fc-CC-Ag-CC-Fc]): m/z 524.9158. Found: m/z 524.9290. Elem anal. Calcd for C72H90Ag4Fe4P2: C, 51.71; H, 5.42. Found: C, 51.85; H, 5.52. Synthesis of [{Cu(PtBu3)}2(η2-FcCCCuCCFc)2] (4). Ethynylferrocene (90.0 mg, 0.43 mmol, 1.00 equiv) was dissolved in THF (10 mL), and nBuLi (0.17 mL, 2.5 M in n-hexane, 0.43 mmol, 1.00 equiv) was added dropwise at room temperature. The mixture was stirred for 30 min and poured onto solid [CuCl(PtBu3)] (129 mg, 0.43 mmol, 1.00 equiv). After stirring overnight, the solvent was evaporated under reduced pressure. CH2Cl2 (10 mL) was added, and the mixture was filtered with a cannula. Slow evaporation of the solution gave 127 mg (91%) of 4 as bright-red crystals. 1 H NMR (300 MHz, THF-d8, 298 K): δ 4.63−4.00 (m, 36H, Cp− H), 1.53 (m, 54H, CH3). 31P{1H} NMR (121 MHz, THF-d8, 298 K): δ 55.4 (bs). IR(ATR) (cm−1): ν̃ 2865 (s), 2349 (w), 2053 (w), 1658 (vw), 1631 (w), 1552 (vw), 1481 (w), 1469 (w), 1441 (w), 1410 (vw), 1390 (s), 1226 (vw), 1170 (s), 1104 (vs), 1036 (w), 1021 (w), 1001 (s), 918 (s), 818 (vw), 807 (vs), 662 (w), 599 (w), 572 (w), 529 (s), 481 (vs), 465 (w), 443 (s), 415 (w). Raman (cm−1): ν̃ 2057 (vs), 1443 (s), 1373 (w), 1228 (w), 1106 (w), 658 (vw), 311 (w). ESI-MS (positive). Calcd ([Cu(PtBu3)2]+): m/z 467.2997. Found: m/ z 467.2989. Calcd ([M − Fc-CC-Cu-CC-Fc]+): m/z 1011.1696. Found: m/z 1011.1685. ESI-MS (negative). Calcd ([Fc-CC-Cu-CCFc]−): m/z 480.9403. Found: m/z 480.9407. Elem anal. Calcd for C72H90Cu4Fe4P2: C, 57.84; H, 6.07. Found: C, 57.62; H, 5.87.

Figure 1. Molecular structure of 1 in the solid state. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: C1−C2 1.205(10), Au−P 2.306(2), Au−C1 2.006(7); C1−Au−P 177.5(2), C2−C1−Au1 178.4(6).

cation, such as [(L)Au]+, which forms an ion pair with a weakly coordinating anion. Thus, the ionic species [AuPtBu3][SbF6] (generated in situ from [AuCl(PtBu3)] and AgSbF6 in CH2Cl2) was added to a benzene solution of 1, resulting in a color change from orange to red. Filtration and crystallization by slow evaporation of the solvent resulted in bright-red crystals in 49% yield, which were identified as 2 (Scheme 2). Complex 2 crystallizes in the triclinic space group P1̅ with one molecule of benzene in the asymmetric unit. As shown in the solid-state structure (Figure 2), the two gold atoms are geminally bonded to the terminal carbon atom of the CC triple bond with an Au1−C1−Au2 angle of 96.0(6)°. Realizing the α,α-diaurated arrangement instead of the “classical” σ- and π-coordination mode can be explained by the high steric demand of the two phosphines coordinated at the gold atoms and the additional stabilization by aurophilic interactions, as indicated by the short Au1−Au2 distance [3.0691(6) Å].7 It is worth noting that the distances between the terminal carbon atom C1 and the two gold atoms [Au1−C1 2.072(13) Å; Au2−C1 2.057(12) Å], as well as the C1−C2 distance [1.206(2) Å], are not significantly influenced compared to monoaurated compound 1. The characteristic C1−C2 stretching frequency is observed at ν̃CC = 2005 cm−1 in the IR spectrum of 2. This is at lower wavenumbers compared to only π-bonded gold alkynes, 18 revealing a significant weakening of the CC triple bond. This three-center, twoelectron bonding system is of particular interest because such complexes can be employed as active catalysts in homogeneous catalysis and represent a silver-free pathway for alkyne transformations, as recent studies show.45−48 Because of the ionic character of complex 2, the solubility in benzene is limited, and thus a complete analysis in solution by NMR spectroscopic methods is impeded. In more polar solvents such as THF, CHCl3, CH2Cl2, and acetonitrile, rapid decomposition of 2 and formation of [Au(PtBu3)2][SbF6] were observed, presumably because of a solvent dependent equilibrium between two molecules of 2 and [Au(η2alkyne)2][SbF6] + [Au(PtBu3)2][SbF6], which is also known for other gold alkyne π complexes.12 The exact mass of the cation could, however, be validated by high-resolution ESI-MS. Furthermore, the composition of 2 is confirmed by elemental analysis. The sensitivity of bulky and electron-rich phosphinegold(I) π adducts in solution seems to be an intrinsic problem, as other studies show.49 Additionally, no solution data of the corresponding silver and copper complexes are found in the literature. We therefore assume that this trend is valid for all coinage metals (see the discussion below). Because the α,α-diaurated complex 2 could be readily prepared, we extended our investigations on the lighter

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RESULTS AND DISCUSSION With the aim of synthesizing coinage metal complexes with a redox-active site for subsequent electrochemical studies, ethynylferrocene was chosen as a metalloligand. The ligand was obtained by the reaction of acetylferrocene with DMF/ POCl3 and NaOAc, following a two-step literature-known procedure.37 To obtain a monoaurated acetylide, ethynylferrocene was treated with nBuLi, leading in situ to [FcCCLi] in a first step. Subsequently, by the addition of [AuCl(PtBu3)], 1 was isolated via salt metathesis as an orange, air-stable solid in 75% yield (Scheme 1). Compound 1 served as a reactant for further reactions and as a reference system. Scheme 1. Synthesis of the Gold(I) Acetylide 1

Single crystals of 1 suitable for X-ray analysis were obtained by the slow evaporation of a THF solution. Complex 1 crystallizes in the monoclinic space group P21/c with one molecule in the asymmetric unit. The molecular structure of 1 in the solid state (Figure 1) shows the expected nearly linear arrangement of the gold acetylide moiety [C1−Au−P angle of 177.5(2)°] with a C1− C2 distance of 1.205(10) Å, which is slightly longer than that in ethynylferrocene (1.170 Å, averaged over three values).44 Because the gold atom in 1 is exclusively σ-bonded to the acetylide function, the CC moiety is still available for an additional π coordination. The additional coordination of a gold ion can be realized by conversion with a “naked” metal C


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Inorganic Chemistry Scheme 2. Synthesis of the Diaurated, Cationic Complex 2

2.356(4) Å; Ag2′−C13 2.374(4) Å]. Furthermore, argentophilic interactions between the silver atoms are present [Ag1− Ag2 2.8698(9) Å; Ag1−Ag2′ 3.1441(10) Å; Figure 3].

Figure 2. Molecular structure of 2 in the solid state. Hydrogen atoms and solvent molecules (C6H6) are not displayed for clarity. Selected bond lengths [Å] and angles [deg]: Au1−Au2 3.0691(6), Au1−P1 2.290(3), Au2−P2 2.297(3), Au1−C1 2.072(13), Au2−C1 2.057(12), C1−C2 1.206(2); Au1−C1−Au2 96.0(6), Au1−C1−C2 127.4(9), Au2−C1−C2 136.6(10).

Figure 3. Molecular structure of 3 in the solid state. Hydrogen atoms and solvent molecules (THF) are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ag1−Ag2 2.8698(9), Ag1−Ag2′ 3.1441(10), Ag1−C1 2.066(4), Ag1−C13 2.058(4), Ag2−P1 2.4395(11), Ag2−C1 2.356(4), Ag2′−C13 2.374(4), C1−C2 1.224(5), C13−C14 1.206(5); Ag1−Ag2−Ag1′ 78.62(2), Ag2− Ag1−Ag2′ 101.38(2), Ag1−C1−Ag2 80.63(12), Ag1−C13−Ag2′ 90.09(14).

silver(I) and copper(I) homologues. Using the same synthetic strategy as that for 1, conversion of lithiated ethynylferrocene with [AgCl(PtBu3)] surprisingly did not result in the expected σ-bonded silver acetylide. Instead, the tetrasilver complex 3 was isolated in a 18% yield (Scheme 3). Single crystals suitable for structural analysis were obtained by slow evaporation of a THF solution of 3. Compound 3 crystallizes in the triclinic space group P1̅ with one molecule of 3 and two molecules of THF in the unit cell. The solid-state structure reveals two bis(ferrocenylacetylide)silver moieties, capped by two η2-coordinated {Ag(PtBu3)}+ units [Ag2−C1

The centrosymmetric planar arrangement of the four silver atoms, stabilized by coordinating CC triple bonds, is an already known structural motif for similar tetranuclear complexes with varying coinage metals.50−52 The CC bond lengths for the alkyne units C1−C2 and C13−C14 [C1−C2 1.224(5) Å; C14−C13 1.206(5) Å] are in the same range compared to those of compound 2. The increased CC bond stretching frequency (ν̃CC = 2038 cm−1) detected in the IR spectrum of 3, however, indicates a stronger bond. Unfortunately, even after several attempts, full characterization in solution was not possible because of rapid decomposition upon solvent exposure. Crystalline 3 immediately transforms into an insoluble orange powder by the addition of THF-d8, CDCl3, CD2Cl2, or C6D6. 31P{1H} NMR investigations of the supernatant reveals several species with chemical shifts of δ 123.4, 64.6, 62.2, 47.1, 44.2, and 42.8, as well as a doublet signal at δ = 81.2 (J = 543 Hz), which, most likely, corresponds to [Ag(PtBu3)2]+ (Figure S10).53 In the ESI-MS spectrum, the fragments [Ag(PtBu3)2]+ and [Ag(CCFc)2]− were identified; however, the actual composition of 3 in solution remains unclear. Finally, the reactivity of the analogous copper complex [CuCl(PtBu3)] toward lithiated ethynylferrocene was investigated. When the same reaction conditions as those for the

Scheme 3. Synthesis of the Heteronuclear Tetrasilver Complex 3

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Inorganic Chemistry Scheme 4. Synthesis of the Heteronuclear Tetracopper Complex 4

significant decomposition in solution, is suitable for further electrochemical characterization.

synthesis of 3 were applied, the isostructural tetranuclear copper complex 4 was isolated as single crystals in 91% yield by slow evaporation of a CH2Cl2 solution (Scheme 4 and Figure 4).

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ELECTROCHEMICAL PROPERTIES The presence of one or more redox-active ferrocenyl units in all of the synthesized complexes 1−4 prompted us to investigate their electrochemical and spectroelectrochemical properties. The ferrocenyl-containing precursors, which do not contain additional metal centers, were initially investigated electrochemically as benchmark compounds for the more complex multinuclear systems. Ethynylferrocene, which is the precursor for all of the complexes investigated, displays a reversible one-electron oxidation wave at 0.21 V in THF/0.1 M [NBu4 ][BArF4 ] versus FcH/FcH+ (Figure S28; all subsequent potentials given below are also referenced against the FcH/FcH+ couple). This redox step can be safely assigned to oxidation of the ferrocenyl unit in ethynylferrocene. As the next benchmark, we investigated the deprotonated and lithiated form of ethynylferrocene, [FcCCLi], the anion of which is the real ligand in all of the coinage metal complexes reported here. As expected, this species is highly reactive, and recording/calibrating its CV turned out to be a challenging task. We eventually managed to record a reasonable CV of this species (Figure S29). As with ethynylferrocene, [FcCCLi] also displays a reversible one-electron oxidation wave, which we attribute to oxidation of the ferrocenyl unit. Referencing of this redox couple was not possible because this highly reactive compound displayed reactions with all external standards that we tested. As the last benchmark, we investigated the mononuclear gold(I) complex 1. Similar to the previous two cases discussed above, 1 displays a reversible one-electron oxidation at 0.11 V (Figure 6), which can be assigned to oxidation of the ferrocenyl moiety. The highly reactive nature of complexes 2 and 3 in solution precluded their investigation by electrochemical methods. Having set the electrochemical benchmarks with the simpler building blocks, the heteronuclear tetracopper complex 4 was investigated. The initial cyclic voltammetric measurements showed that the separation between the various redox waves was not optimal when using [NBu4][PF6] as the supporting electrolyte. Thus, all subsequent measurements on 4 were carried out with the electrolyte [NBu4][BArF4], which contains a weakly coordinating anion.55 Better separations were obtained when using the weakly coordinating anion, a fact that points toward the through-space interaction between the redox centers. The heteronuclear tetracopper complex 4 displays reasonably well-separated oxidation steps in THF/0.1 M [NBu4][BArF4] (Figure 7). Differential pulse voltammetric experiments on 4 provide indications that the waves observed at 0.05, 0.25, and 0.46 V correspond to one-, two-, and oneelectron oxidation waves, respectively. The separation between the first and second waves is 200 mV, and that between the second and third waves is 210 mV. Because the redox stability

Figure 4. Molecular structure in the solid state of the neutral tetracopper complex 4. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Cu1−Cu2 2.5487(11), Cu1−Cu2′ 2.5944(12), Cu1−C1 1.903(7), Cu1−C13 1.897(8), Cu2−C1 2.068(6), Cu2−P1 2.241(2), Cu2′−C13 2.090(7), C1−C2 1.209(10), C13−C14 1.201(10); Cu1−Cu2−Cu1′ 75.79(4), Cu2− Cu1−Cu2′ 104.21(4), Cu1−C1−Cu2 79.7(3), Cu1−C13−Cu2′ 81.0(3).

Complex 4 crystallizes in the monoclinic space group P21/c with two halves of a molecule in the asymmetric unit. Similar to compound 3, the copper analogue features a centrosymmetric planar arrangement of the metal atoms with short Cu− Cu distances, indicating cuprophilic interactions [Cu2−Cu1 2.5487(11) Å; Cu2′−Cu1 2.5944(12) Å]. The CC bond lengths of the acetylide units [C1−C2 1.209(10) Å; C14−C13 1.201(10) Å] are in the range of that of unsubstituted acetylene 54 but are elongated compared to that of ethynylferrocene (1.170 Å). The weakening of the triple bonds compared to ethynylferrocene is also confirmed by IR spectroscopy (ν̃ = 2053 cm−1). The poor solubility of 4 impedes complete characterization by NMR spectroscopy. The 1 H NMR spectrum of 4 is not well-resolved; however, the characteristic resonances for the ferrocenyl and phosphine moieties can be detected. Like complex 3, the crystalline tetracopper compound 4 decomposes when exposed to solvent but on a significantly longer time scale. Thus, decomposition kinetics of 4 were monitored by periodic 31P{1H} NMR measurements THF-d8 (Figures 5 and S11). The decomposition of complex 4 begins after about 2 h, when a significant change in the NMR spectrum can be detected and an additional broader resonance at δ 59.9 was seen (Figure S11). This most likely corresponds to [Cu(PtBu3)2]+.42 This 2 h time frame, in which 4 does not show E


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Figure 5. Stacked 31P{1H} NMR spectra of 4 in THF-d8 at selected times from 0h15m to 12h15m.

Figure 6. CV of 1 in CH2Cl2 at 100 mV s−1 with 0.1 M [NBu4][PF6].

Figure 8. Changes in the vis−NIR absorption of 4 in the course of the first oxidation measured in THF.

electron oxidation to 4+, the original band at 480 nm shifts to lower energy to about 500 nm and gains in intensity. No detectable absorption is seen for the native and one-electronoxidized forms of 4 beyond 700 nm. Surprisingly, the second oxidation step, which was observed as a two-electron step in the CV/differential pulse voltammogram (DPV; Figure 7), could be resolved into two separate (one-electron) steps with the help of the UV−vis−NIR spectroelectrochemical data. This result was obtained by separating the entire bunch of spectra recorded during the two-electron oxidation step based on isosbestic points. Additionally, the absorptions between 315 and 355 nm were first seen to decrease during the two-electron oxidation but again seen to increase in the “second half” of this step (Figure 9, inset). UV−vis−NIR spectroelectrochemistry has already been successfully used in the past to separate a seemingly two-electron-transfer step, as observed by cyclic voltammetry, into two one-electron-transfer steps.56−58 Thus, upon evaluation of these spectra, the spectroscopic signatures can be assigned to a doubly oxidized species 42+ as well as to a triply oxidized species 43+. For 42+, a band is observed at 510 nm, which is reminiscent of a similar band observed for both 4 and 4+. Additionally, for 42+, a further band is observed at 760

Figure 7. CV (black) and DPV (red) of 4 in THF at 100 mV s−1 with 0.1 M [NBu4][BArF4].

of 4 in solution was found to be reasonable (∼ 2 h), we ventured into performing UV−vis−NIR spectroelectrochemical measurements on this complex. The idea behind these measurements was to generate spectroscopic signatures for the various redox states of 4. Additionally, the spectroscopic data, together with the electrochemical benchmarking of the building blocks reported above, should help us in deciphering the loci of the various oxidation steps. The starting complex 4 displays a strong absorption band at around 280 nm and a comparatively rather weak band at 480 nm in THF/0.1 M [NBu4][BArF4] (Figure 8). Upon oneF


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coupling over a distance of at least 6 Å (see below).60−63 The poor solubility of the complex precluded determination of the bands’ exact extinction coefficients. The spectroscopic data thus seem to indicate that the separation of the redox waves is not only due to charge-based Coulomb interactions. Additionally, electronic coupling between the ferrocenyl units contributes to the separation as well. Whereas 4+ is also expected to be a mixed-valent species, we were not able to spectroscopically detect any IVCT bands for that form. At this point, it is not totally clear as to why this is the case. However, considering the very weak intensity of the band at 760 nm and the broad absorption between 1000 and 1800 nm for both 42+ and 43+, it is conceivable that these bands for 4+ lie below the detection limit of the spectrometer. While the CV of 4 is not electrochemically reversible, upon turning the potential back to the starting potential during spectroelectrochemcial measurements the starting spectrum was regenerated almost quantitatively. The apparent irreversible nature of the CV is likely associated with structural reorganization at the copper(I) centers that would definitely react to a change in the electron density in their vicinity (the ferrocenyl units). Taking the above results into consideration, the following simplified rectangular redox scheme can be proposed to explain the electrochemical and spectroelectrochemical properties of complex 4 (Figure 11).

Figure 9. Changes in the vis−NIR absorption of 4 in the course of the second (top; first part of the two-electron oxidation) and third (bottom; second part of the two-electron oxidation) oxidation measured in THF. For the combined total changes, see Figure S30.

nm and a broad and rather weak absorption is seen between 1000 and 1800 nm (Figure 9). Upon further oxidation to 43+, both the band at 760 nm and the broad NIR absorption between 1000 and 1800 nm increase in intensity (Figures 9 and S30). On the final oxidation to 44+, the band at 760 nm is seen to further increase in intensity, whereas the broad absorption band between 1000 and 1800 nm disappears (Figure 10). Figure 11. Rectangular scheme to explain the redox properties of complex 4 (see the text for an explanation). The Fe1 center is arbitrarily taken as the starting point of the discussion. See Scheme 4 for a description of the molecular structure in the crystal of 4. The distances are taken only as tentative benchmarks to explain the experimental observations because the dynamics in solution will likely alter these distances to a certain extent.

One-electron oxidation at Fe1 is followed by a second oxidation at Fe2, which is around 6 Å apart from and hence very weakly coupled to Fe1. The third oxidation is assigned to the oxidation of Fe2′, which is more than 11 Å separated from Fe1 (the second and third oxidations fall together in the CV and were separated via spectroelectrochemistry). Finally, the fourth oxidation takes place at Fe1′, generating the fully oxidized complex 44+. Thus, a weak electronic coupling in these systems is observed over a distance between 6 and 11 Å. Additionally, we have shown that spectroelectrochemistry can be used to resolve a two-electron step (see the CV) into two one-electron steps (see the UV−vis−NIR spectroelectrochemistry). It should be mentioned that similar redox schemes were also proposed in the past for cyclic multiferrocenyl-containing compounds.64 The electrochemical response of those systems is similar to what we have observed for complex 4.

Figure 10. Changes in the vis−NIR absorption of 4 in the course of the fourth oxidation measured in THF (last one-electron oxidation in CV).

On the basis of the electrochemical benchmarking of the building blocks and the spectroscopic data presented above, we assign the oxidation steps to the stepwise oxidations of the four ferrocenyl units in 4. The band at 760 nm observed for 42+, 43+, and 44+, which progressively increases in intensity with each unit of oxidation, is assigned to an absorption associated with the oxidized ferrocenium unit.33,59 Remarkably, the broad and weak absorption bands between 1000 and 1800 nm appear only for the mixed-valent forms 42+ and 43+, whereas these absorption bands are absent for the homovalent forms 4 and 44+. Thus, we assign the broad and weak absorption bands to intervalence charge-transfer (IVCT) bands for a weakly coupled class II species that displays long-range electronic G


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(2) Schmidbaur, H.; Schier, A. Aurophilic interactions as a subject of current research: an up-date. Chem. Soc. Rev. 2012, 41 (1), 370−412. (3) Hermann, H. L.; Boche, G.; Schwerdtfeger, P. Metallophilic Interactions in Closed-Shell Copper(I) CompoundsA Theoretical Study. Chem. - Eur. J. 2001, 7 (24), 5333−5342. (4) Jerabek, P.; von der Esch, B.; Schmidbaur, H.; Schwerdtfeger, P. Influence of Relativistic Effects on Bonding Modes in M(II) Dinuclear Complexes (M = Au, Ag, and Cu). Inorg. Chem. 2017, 56 (23), 14624−14631. (5) Das, A.; Dash, C.; Yousufuddin, M.; Celik, M. A.; Frenking, G.; Dias, H. V. R. Isolable Tris(alkyne) and Bis(alkyne) Complexes of Gold(I). Angew. Chem., Int. Ed. 2012, 51 (16), 3940−3943. (6) De La Rica, R.; Stevens, M. M. Plasmonic ELISA for the ultrasensitive detection of disease biomarkers with the naked eye. Nat. Nanotechnol. 2012, 7 (12), 821−4. (7) Hooper, T. N.; Green, M.; Russell, C. A. Cationic Au(i) alkyne complexes: synthesis, structure and reactivity. Chem. Commun. 2010, 46 (13), 2313−2315. (8) Jahnke, A. C.; Pröpper, K.; Bronner, C.; Teichgräber, J.; Dechert, S.; John, M.; Wenger, O. S.; Meyer, F. A New Dimension in Cyclic Coinage Metal Pyrazolates: Decoration with a Second Ring of Coinage Metals Supported by Inter-ring Metallophilic Interactions. J. Am. Chem. Soc. 2012, 134 (6), 2938−2941. (9) Walker, S. B.; Lewis, J. A. Reactive Silver Inks for Patterning High-Conductivity Features at Mild Temperatures. J. Am. Chem. Soc. 2012, 134 (3), 1419−1421. (10) Barillo, D. J.; Marx, D. E. Silver in medicine: A brief history BC 335 to present. Burns 2014, 40, S3−S8. (11) Che, C.-M.; Lai, S.-W. Structural and spectroscopic evidence for weak metal−metal interactions and metal−substrate exciplex formations in d10 metal complexes. Coord. Chem. Rev. 2005, 249 (13), 1296−1309. (12) Ciano, L.; Fey, N.; Halliday, C. J. V.; Lynam, J. M.; Milner, L. M.; Mistry, N.; Pridmore, N. E.; Townsend, N. S.; Whitwood, A. C. Dispersion, solvent and metal effects in the binding of gold cations to alkynyl ligands: implications for Au(I) catalysis. Chem. Commun. 2015, 51 (47), 9702−9705. (13) Koshevoy, I. O.; Karttunen, A. J.; Kritchenkou, I. S.; Krupenya, D. V.; Selivanov, S. I.; Melnikov, A. S.; Tunik, S. P.; Haukka, M.; Pakkanen, T. A. Sky-Blue Luminescent AuI−AgI Alkynyl-Phosphine Clusters. Inorg. Chem. 2013, 52 (7), 3663−3673. (14) Carlos Lima, J.; Rodriguez, L. Applications of gold(I) alkynyl systems: a growing field to explore. Chem. Soc. Rev. 2011, 40 (11), 5442−5456. (15) Yam, V. W.-W. Luminescent coinage metal clusters of acetylides and chalcogenides. J. Photochem. Photobiol., A 1997, 106 (1), 75−84. (16) Buschbeck, R.; Low, P. J.; Lang, H. Homoleptic transition metal acetylides. Coord. Chem. Rev. 2011, 255 (1), 241−272. (17) Chen, Z.-N.; Zhao, N.; Fan, Y.; Ni, J. Luminescent groups 10 and 11 heteropolynuclear complexes based on thiolate or alkynyl ligands. Coord. Chem. Rev. 2009, 253 (1), 1−20. (18) Shapiro, N. D.; Toste, F. D. Synthesis and structural characterization of isolable phosphine coinage metal π-complexes. Proc. Natl. Acad. Sci. U. S. A. 2008, 105 (8), 2779−2782. (19) Hashmi, A. S. K.; Hutchings, G. J. Gold Catalysis. Angew. Chem., Int. Ed. 2006, 45 (47), 7896−7936. (20) Worrell, B. T.; Malik, J. A.; Fokin, V. V. Direct Evidence of a Dinuclear Copper Intermediate in Cu(I)-Catalyzed Azide-Alkyne Cycloadditions. Science 2013, 340 (6131), 457−460. (21) Das, A.; Dash, C.; Celik, M. A.; Yousufuddin, M.; Frenking, G.; Dias, H. V. R. Tris(alkyne) and Bis(alkyne) Complexes of Coinage Metals: Synthesis and Characterization of (cyclooctyne)3M+ (M = Cu, Ag) and (cyclooctyne)2Au+ and Coinage Metal (M = Cu, Ag, Au) Family Group Trends. Organometallics 2013, 32 (11), 3135−3144. (22) Kovács, A.; Frenking, G. Bonding Interactions of a Molecular Pair of Tweezers with Transition Metals. Theoretical Study of Bis(ç2alkyne) Complexes of Copper(I), Silver(I), and Gold(I)1. Organometallics 1999, 18 (5), 887−894.

CONCLUSIONS Ethynylferrocene was utilized to synthesize various coinage metal complexes. In contrast to the gold acetylide compound 1, conversion of the respective silver and copper salts yielded tetrasilver and tetracopper ferrocenyl complexes 3 and 4. For compounds 2−4, instability in solution was observed, which is attributed to the use of the bulky and electron-rich PtBu3 ligand. Ferrocenyl-based oxidation steps were involved for all of the compounds that were electrochemically investigated. For the heteronuclear tetracopper complex 4, a total of four oxidations were observed. Whereas cyclic voltammetric measurements showed that the second and third oxidation steps fall together, we were able to separate those two steps by UV−vis−NIR spectroelectrochemical measurements, thus showing the power of that method for investigating complex redox processes. Additionally, the spectroelectrochemical measurements showed that weak electronic coupling is observed between the remote ferrocenyl units in the heteronuclear tetracopper complex, leading to the generation of weakly coupled class II species. These results thus show that the combination of copper(I) and ethynyl bridges is ideally suited for generating long-range electronic coupling between remote redox-active units.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02873. Additional crystallographic data, NMR, IR, Raman, and MS spectra, and electrochemical data (PDF) Accession Codes

CCDC 1872432−1872435 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +49-721-608-46117. ORCID

Biprajit Sarkar: 0000-0003-4887-7277 Peter W. Roesky: 0000-0002-0915-3893 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the DFG-funded transregional collaborative research center SFB/TRR 88 “Cooperative Effects in Homo and Heterometallic Complexes (3MET)” project C3 is gratefully acknowledged.

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REFERENCES

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Synthesis, Spectroscopy, and Redox Studies of Ferrocene

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