Syntheses, Structures, and Antimicrobial Activities of Gold(I)– and

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

Syntheses, Structures, and Antimicrobial Activities of Gold(I)− and Copper(I)−N‑Heterocyclic Carbene (NHC) Complexes Derived from Basket-Shaped Dinuclear Ag(I)−NHC Complex Kenji Nomiya,* Soichiro Morozumi, Yuki Yanagawa, Misa Hasegawa, Kaori Kurose, Kenshiro Taguchi, Ryosuke Sakamoto, Kohei Mihara, and Noriko Chikaraishi Kasuga Department of Chemistry, Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa 259-1293, Japan Downloaded via DURHAM UNIV on August 31, 2018 at 16:11:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Novel dinuclear gold(I)- and copper(I)-N-heterocyclic carbene (NHC, L-1a) complexes [M2(L-1a)](PF6)2 (M = AuI (Au-1·PF6) and M = CuI (Cu-1·PF6)) were synthesized by transmetalation of the dinuclear silver(I)−NHC complex [Ag2(L-1a)](PF6)2 (Ag-1·PF6) with [AuCl(Me2S)] or CuI in over 70% yield. These NHC complexes were characterized by CHN elemental analysis, Fourier transform infrared spectroscopy, thermogravimetry/differential thermal analysis, and solution (1H and 13C) NMR spectroscopy. X-ray crystallography revealed that Au-1· PF6 and Cu-1·PF6 are dinuclear molecules consisting of two linear intramolecular C−M−C bonds (M = AuI and CuI), one M···M interaction, and two metal atoms arranged in a T-shaped geometry; their molecular structures are very similar to the basket-shaped cage structure of the parent complex Ag-1·PF6. Because of the smaller ionic radius of copper(I), Cu-1· PF6 has the smallest upper space of the basket among the three complexes. Au-1·PF6 and Cu-1·PF6 were very stable in the solid state and in solution. They did not undergo nucleophilic reaction with thiols 2-mercaptoethanol and 2-benzimidazolethiol (Hbmt) at least for several hours, whereas Ag-1·PF6 did react, forming precipitates of silver(I) thiolate and the free ligand L-1a· PF6. The minimum inhibitory concentration values toward a panel of bacteria, yeasts, and molds were examined in a watersuspension system and a solution system containing dimethyl sulfoxide. The antimicrobial spectra of the three compounds were metal-dependent, and Au-1·PF6 showed the greatest activity toward Gram-positive bacteria.

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INTRODUCTION The chemistry of metal complexes with N-heterocyclic carbene (NHC) ligands, which contain a carbene carbon incorporated in a nitrogen-containing 5-membered heterocycle, has been extensively studied because many of these complexes are effective catalysts of various chemical reactions.1,2 The complexes with NHC ligands are often air- and photostable due to their strong σ-donating and weak π-accepting characters, and their properties have often been compared with those of phosphane ligands.3 Among them, the complexes with group 11 elements, namely the coinage metal−NHC complexes, have aroused intense interest as potential antimicrobial and antitumor metallodrugs4−6 as well as catalysts.7,8 For example, silver(I)−NHC complexes not only have potential medicinal applications9,10 but also are useful as carbene transfer agents. NHC complexes of many other transition metals, including gold, copper, palladium, nickel, and ruthenium, have been similarly prepared.11 We previously studied silver(I) complexes with amino acids as model compounds of silver(I)−peptide and silver(I)− protein complexes.12 We suggested that ligand exchangeability plays a key role in the broad spectrum of antimicrobial activities of Werner-type silver(I) complexes based on the © XXXX American Chemical Society

relationship between molecular structure and antimicrobial activities. Recently, we also became interested in organometallic silver(I) complexes and found that dinuclear NHC silver(I) complexes showed antimicrobial activities toward a panel of selected bacteria (Escherichia coli, Bacillus subtilis, Staphylococcus aureus, and Pseudomonas aeruginosa), yeasts (Candida albicans and Saccharomyces cerevisiae), and molds (Aspergillus brasiliensis (niger) and Penicillium citrinum).13 In particular, Ag-1·PF6, which forms a basket-like shaped dinuclear complex (Scheme 1), exhibited a broad spectrum of activity toward the test panel. Au(I)−NHC complexes have been extensively developed as catalysts of various organic reactions7 and also as potential chemotherapeutic agents.14 On the other hand, Cu(I)−NHC also has various catalytic activities.8 Cu(I)−NHC complexes were reported to exhibit cytotoxicity higher than that of clinically used cisplatin toward various cell lines.15 Like other Ag(I)−NHC complexes,11 dinuclear Ag(I)− NHC complex Ag-1·PF613 is also expected to work as a metaltransfer agent, affording metallodrug candidates of other group Received: January 2, 2018

A

DOI: 10.1021/acs.inorgchem.8b00011 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

procedure used for L-1·PF613 but with NaBF4 and KTf2N instead of KPF6 (see Properties and Figure S1 in Supporting Information). Broad signals of the precursors of L-1·X in the 1 H NMR spectra indicate that interconversion of several conformers takes place on the NMR time scale.16 As shown in the upper line of Scheme 2, the dinuclear silver(I)−NHC complexes (Ag-1·X; X = BF4 and Tf2N) were isolated from mixtures of the corresponding salt of L-1·X and Ag2O, followed by vapor diffusion. The synthetic procedure was almost the same as that used for Ag-1·PF6. The 1H NMR signals of Ag-1·X (Figure S2) were complex and sharp compared to those of L-1·X and also indicated that the conformations of the three Ag-1·X complexes were similar in solution. Metal-transfer reaction of CuI with Ag-1·PF6 was performed in acetone at ambient temperature under an inert atmosphere as shown in Scheme 2. AgI precipitated immediately as a yellow solid, indicating that metal exchange occurred readily. The Ag-1·PF6 generated in situ from L-1·PF6 and Ag2O also reacted with CuI to give the same complex with similar yield, though it was contaminated. Cu-1·PF6 is highly soluble in acetone and DMSO, soluble in CH3CN, and sparingly soluble in MeOH but insoluble in water, CHCl3, EtOAc, and ether. Vapor diffusion in the CH3CN/ether system gave single crystals suitable for X-ray diffraction measurements. The single-crystal X-ray analysis showed that Cu-1·PF6 is a dinuclear complex, as illustrated in Figure 1, and has a similar basket-shaped structure to that of Ag-1·PF6. Au-1·PF6 was also prepared by metal-transfer reaction of [AuCl(SMe2)] with Ag-1·PF6 in acetonitrile. This complex was also obtained from the reaction of in situ-generated Ag-1· PF6 and [AuCl(SMe2)], but minor contaminants were generated in the in situ system. Au-1·PF6 is highly soluble in acetone, CH3CN, and DMSO but insoluble in water, MeOH,

Scheme 1. Schematic Drawing of Ag-1·PF6

11-containing complexes, i.e. Au(I)−NHC and Cu(I)−NHC complexes, as in the case for other NHC complexes. Herein, we report the preparation of light-stable complexes containing group 11 metals, Au-1·PF6 and Cu-1·PF6, by metal-transfer reaction of Ag-1·PF6 with counteranions BF4 and (CF3SO2)2N (Tf2N)). These Au(I) and Cu(I) complexes and their NHC precursors with several counterions were characterized by CHN elemental analysis, Fourier transform infrared spectroscopy (FTIR), thermogravimetry/differential thermal analysis (TG/DTA), solution (1H and 13C) NMR spectroscopy, and Xray crystallography. Their antimicrobial activities in a watersuspension system and in homogeneous DMSO solution were evaluated by minimum inhibitory concentration (MIC: μg mL−1) assay and compared.

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RESULTS AND DISCUSSION The BF4 and Tf2N salts of the ligand, L-1·X (X = BF4 and (CF3SO2)2N (Tf2N))) containing both major product L-1a·X and minor product L-1b·X, were synthesized by the same Scheme 2. Synthetic Scheme of M-1·Xa

a

M = CuI, AgI, and AuI, X = PF6, BF4, and (CF3SO2)2N (Tf2N). B

DOI: 10.1021/acs.inorgchem.8b00011 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Local structure of Cu-1·PF6 with 50% probability thermal ellipsoids. Hydrogen atoms, two anions outside the Cu-1 cavity, and solvent molecules are omitted for clarity.

Figure 2. Local structure of Au-1·PF6 with 50% probability thermal ellipsoids. One PF6 anion is located inside the cavity of the basket structure. Hydrogen atoms, the other anion, and solvent molecules are omitted for clarity.

NHC complexes with BF4− and Tf2N− anions (Au-1·BF4 and Au-1·Tf2N) were also obtained by the trans-metalation of Ag1·BF4 and Ag-1·Tf2N, respectively. Among these NHC complexes, X-ray analysis was successful for BF4 salts Ag-1·

CHCl3, EtOAc, and ether. Vapor diffusion in the CH3CN/ ether system gave single crystals suitable for X-ray measurement. The molecular structure of the dinuclear complex Au-1· PF6 (Figure 2) was very similar to that of Ag-1·PF6. Gold(I)− C

DOI: 10.1021/acs.inorgchem.8b00011 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. Local structure of Ag-1·BF4 with 50% probability thermal ellipsoids. One anion is located inside the cavity of the basket structure. Hydrogen atoms and the other anion are omitted for clarity.

Figure 4. Local structure of Au-1·BF4 with 50% probability thermal ellipsoids. One BF4 anion is located inside the cavity of the basket structure. Hydrogen atoms and the other anion are omitted for clarity.

ions based on the observation that the metal−metal separation was less than twice the van der Waals radii (the values for Cu, Ag, and Au are 2.80, 3.44, and 3.32 Å, respectively17). Overall, the molecular structures of the four complexes looked like a basket with two 1,2-phenylenebis(methylene) “handles”, as shown in Figures 1−4. The atoms in the complexes were numbered as shown in Scheme 3. One anion was located in the space between the two handles in Ag-1·BF4, Au-1·PF6, and

BF4 and Au-1·BF4, whose molecular structures are shown in Figures 3 and 4, respectively. The crystal data and structure refinement of the complexes are summarized in Table 1. X-ray analysis of the four compounds revealed that (1) all of these NHC complexes were dinuclear, (2) each metal atom was coordinated by two carbenes in a linear arrangement, (3) metal−π interaction was seen for each metal atom, and (4) metal−metal interaction was observed in the dinuclear metal D

DOI: 10.1021/acs.inorgchem.8b00011 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the phenylene rings A and B and those of A and C as well as the distances of C4−C24 and C14−C34 in Table 2 indicated that the aromatic planes of B and C for Cu-1·PF6 are closer to each other than those of the silver(I)− and gold(I)−NHC complexes. Therefore, the space between the handles might be too small to accommodate PF6−. Accordingly, four PF6− were observed around the dinuclear copper cation, [Cu2(L-1a)]2+. The Cu···Cu distance of Cu-1·PF6 is longer than that of other similar Cu−NHC complexes.18,19 In contrast, Au···Au distance of Au-1·PF6 (3.0727(11) Å) is shorter than that of previously reported noncaged Au−NHC dimers.20 Although single crystals of Ag-1·Tf2N and Au-1·Tf2N suitable for X-ray analysis could not be grown, the 1H NMR overlay spectra in DMSO-d6 resembled those of the corresponding PF6 and BF4 salts, indicating that the complexes of Tf2N salts may also form similar caged structures, as illustrated in Figures S2 and S3. NHCs are stronger σ-donors and weaker π-acceptors than phosphane ligands, as suggested by density functional study.21 To investigate the M−C bond properties of the obtained dinuclear complexes, aliphatic and aromatic thiol compounds, which contain a mercapto group having a soft sulfur atom, were added to a solution of the dinuclear complexes, as summarized in Scheme 4. When 2-mercaptoethanol or 2benzimidazolethiol (Hbmt) was added to a solution of Ag-1· PF6, broad 1H NMR signals of the released ligand L-1a·PF6 were observed, and insoluble Ag−thiolate complex was precipitated within a few hours (Figure S5). However, no reaction was observed with Cu-1·PF6 or Au-1·PF6. From the mixed solution of Ag-1·PF6 and Hbmt followed by vapordiffusion crystal method, single crystals of the silver complex were obtained. It was determined to be the hexanuclear silver(I)−thiolate complex Ag6(bmt)6·4DMSO (Figure 5), which was a polymorph of the reported structure with different solvates (THF).22,23 Formation of the imidazolium salt Ag6(bmt)6 means that Ag-1·PF6 has reactivity for action as base to deprotonate thiols.24,25 Nucleophilic substitution of dinuclear Ag-1·PF6 by thiol, as well as transmetalation, is consistent with the behavior of previously reported mononuclear Ag−NHC complexes. These results are consistent with the idea that the strength of the metal−NHC bonds is in the order Ag < (Cu, Au), as suggested by density functional theory (DFT) calculation.26 To compare the antimicrobial activities of the group 11 metal−NHC complexes (Cu-1·PF6, Au-1·PF6, Ag-1·BF4, Au1·BF4, Ag-1·Tf2N, and Au-1·Tf2N) and the ligand precursors (L-1·X X = PF6, BF4, and Tf2N), the minimum inhibitory concentrations (MIC: μg mL−1) toward the selected bacteria, yeasts, and molds were evaluated in a water-suspension system and a DMSO solution system.27,28 The ligand precursors showed modest antibacterial and antiyeast activities (Table 3). All of the dinuclear complexes showed activity, though the antimicrobial activities were metal-dependent; the broadest spectrum was observed for Ag-1·PF6, which showed activities toward bacteria and yeasts, while Cu-1·PF6 was active only toward bacteria. Au-1·PF6 was more active than Ag-1·PF6 toward the Gram-positive bacteria (B. subtilis and S. aureus), whereas that of Cu-1·PF6 showed only modest activities. Next, we examined the effect of the anion on the antimicrobial activities using salts of Ag-1 and Au-1. The MIC values of the complexes with BF4 and Tf2N counteranions indicated that the effect of the anion was greater for Au1 than for Ag-1. Santoni and coworkers reported that gold(I) and silver(I) NHC complexes inhibited TrxR, a selenoenzyme,

Table 1. Summary of Crystal Data and Structure Refinement Parameters for Cu-1·PF6, Au-1·PF6, Ag-1·BF4, and Au-1·BF4a

empirical formula formula weight crystal system space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Dcalcd/g·cm−3 Z μ mm−1 T/K total unique (I > 2σ(I)) Rint R1 (I > 2σ(I)) wR2 (I > 2σ(I)) GOF

empirical formula formula weight crystal system space group a/Å b/Å c/Å α/° γ/° γ/° V/Å3 Dcalcd/g·cm−3 Z μ mm−1 T/K total unique (I > 2σ(I)) Rint R1 (I > 2σ(I)) wR2 (I > 2σ(I)) GOF

Cu-1·PF6

Au-1·PF6

{[Cu2(L-1a)](PF6)2}2· 3CH3CN C82H77Cu4F24N19P4

[Au2(L-1a)](PF6)2· 3CH3CN C44H43Au2F12N11P2

2162.71 triclinic P1̅ (no. 2) 11.7967(2) 11.8710(2) 15.5198(3) 100.807(2) 94.010(2) 93.446(2) 2123.64(7) 1.69 1 1.175 150 no. of reflections 34 546 9726 no. of observations 8021 0.0401 0.0430 0.1093 1.022 Ag-1·BF4 [Ag2(L-1a)](BF4)2 C38H34Ag2B2F8N8 992.09 triclinic P1̅ (no. 2) 11.887(5) 13.266(5) 14.015(6) 108.401(6) 114.782(6) 98.2530(7) 1803.7(13) 1.84 2 1.171 120 no. of reflections 29 218 8236 no. of observations 6431 0.0488 0.0297 0.0753 0.831

1409.76 triclinic P1̅ (no. 2) 12.365(3) 14.003(5) 15.410(4) 93.933(15) 104.32(2) 114.474(12) 2308.2(12) 2.03 2 6.515 120 10 465 10 465 8872 0.0453 0.0343 0.0809 1.070 Au-1·BF4 [Au2(L-1a)](BF4)2 C38H34Au2B2F8N8 1170.28 triclinic P1̅ (no. 2) 11.9363(3) 13.0193(3) 14.0685(2) 108.573(2) 114.759(2) 97.870(2) 1786.70(7) 2.18 2 8.288 120 29 920 9734 8639 0.0616 0.0341 0.0872 1.039

a R1 = ∑{|Fo| − |Fc|}/∑|Fo|, wR2 = [∑ω(|Fo| − |Fc|)2/∑ωFo2]1/2, GOF = [∑ω(|Fo| − |Fc|)2/(m − n)]1/2. m, no. of reflections; n, no. of parameters.

Au-1·BF4. One solvent molecule of Cu-1·PF6 was highly disordered. The dihedral angles between the mean planes of E

DOI: 10.1021/acs.inorgchem.8b00011 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 3. Numbering of Coinage Metal Complexes L-1·X

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at nanomolar concentration.29 Au-1·PF6 and Cu-1·PF6 appear to inhibit the growth of microorganisms via through-space interaction with biomolecules such as enzymes because of their strong C−M−C bonds, as suggested by the thiol addition experiments described above. In the case of Ag-1·PF6, both through-space and ligand-exchange interactions with nucleophilic biomolecules could be possible. The MIC values of Ag1·PF6 in a DMSO solution were similar to those in the watersuspension system, indicating that the dinuclear complex remains a suitable molecular structure in solution for interaction with target biomolecules.

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EXPERIMENTAL SECTION

Materials. The following materials were used as received: Ag2O, CuI, imidazole, potassium hexafluorophosphate, potassium tetrafluoroborate, sodium hydroxide, Celite (no. 503), CD3CN (99.9 D atom %), dimethyl sulfoxide (DMSO), EtOH, Et2O, CH2Cl2, MeOH, CH3CN, THF, pentane, toluene, and acetone (Wako); α,α′-dibromoo-xylene, tetrakis(bromomethyl)benzene, and 2-benzimidazolethiol (Hbmt) (Aldrich); chloro(dimethylsulfide)gold(I), 2-mercaptoethanol (TCI), potassium bis(trifluoromethanesulfonyl)imide, and CDCl3-d6 (99.8 D atom %) (Kanto Kagaku); and DMSO-d6 (99.9 D atom %, Isotec). Instrumentation/Analytical Procedures. CHN elemental analyses were performed using a PerkinElmer PE2400 series II CHNS/O analyzer. TG and DTA were performed under air with a temperature ramp of 4 °C min−1 using a Rigaku Thermo Plus 2 TG 8120 instrument between 30 and 500 °C. Infrared spectra were recorded on a JASCO FT-IR 4100 spectrometer in KBr disks at room temperature. 1H and 13C{1H} NMR spectra in solution were recorded at ambient temperature on a JEOL EX-400 NMR or a JEOL ECP500 NMR spectrometer. 1H and 13C{1H} NMR spectra of the complexes were measured in a DMSO-d6 solution with reference to internal TMS. X-ray Crystallography. Crystallization of four kinds of M−NHC complexes (Cu-1·PF6, Au-1·PF6, Ag-1·BF4, and Au-1·BF4) was carried out by vapor diffusion of an internal aqueous solution of the metal complex with an external solvent (diethyl ether). Waterinsoluble colorless crystals of M−NHC complexes were obtained, and single crystals of the metal complexes were mounted on a loop and used for measurements of cell constants and for the collection of intensity data on a Rigaku VariMax with Saturn CCD diffractometer. The structures were solved by direct methods followed by difference Fourier calculations; they were refined by a full-matrix least-squares method on F2 using the ShelXle program package.30 All nonhydrogen atoms were refined anisotropically. The hydrogen atoms were placed geometrically or identified on a difference Fourier map and were treated using a riding model. The crystal data and structure refinement of the complexes are summarized in Table 1. The details of the crystal data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication (Cu-1· PF6 (CCDC no. 1590621), Au-1 PF6 (CCDC no. 1560141), Ag-1· BF4 (CCDC no. 1560142), Au-1·BF4 (CCDC no. 1560140), and Ag6(bmt)6·4DMSO (CCDC no. 1560134)). Reaction with Thiol Compounds. To investigate the M−C bond properties of the dinulcear complexes with thiols, 10 μmol of each coinage complex was mixed with 0.6 mL CD3CN or acetone-d6 in a 5 mm NMR tube. To the resulting clear solution was added 20 μL of 2 M CD3CN solution of 2-mercaptoethanol, and the NMR spectra were measured. In the case of reaction using 2benzimidazolethiol (Hbmt), 100 μL of 0.2 M acetone-d6 solution of Hbmt was added to each coinage complex in 0.6 mL of acetone-d6 solution. Although a number of silver−thiolate complexes were hard to crystallize, we obtained single crystals of Ag6(bmt)6·4DMSO (16.3% isolated yield) by vapor-diffusion method and determined the crystal structure.

CONCLUSIONS

Novel dinuclear metal complexes of the group 11 elements with NHC ligand, [M2(L-1a)](PF6)2 (M = Au and Cu), were synthesized by metal exchange reactions of Ag-1·PF6 with [AuCl(Me2S)] and CuI in yields of 75% (0.485 g scale) and 73% (0.131 g scale), respectively. Au-1·PF6 and Cu-1·PF6 were unequivocally characterized by CHN elemental analysis, FTIR, TG/DTA, X-ray crystallography, and solution (1H and 13 C) NMR spectroscopy. X-ray crystallography revealed that Au-1·PF6 and Cu-1·PF6 contain two intramolecular C−M−C bonds, one M···M interaction, and two M(I) atoms arranged in a T-shaped geometry, resembling the basket-shaped cage structure of the parent complex Ag-1·PF6. Such isostructural complexes of group 11 elements are rarely seen with other ligands. Ligand exchange experiments by adding 2-mercaptoethanol and 2-benzimidazolethiol indicated that the Ag-1·X complex has different properties from the other two M-1·X complexes. The Au-1·PF6 and Cu-1·PF6 complexes were both very stable in the solid state and in solution. Antimicrobial activities toward selected bacteria, yeasts, and molds evaluated in terms of MIC in a water-suspension system and a DMSO solution system indicated that all three dinuclear complexes showed activity, although the antimicrobial spectra were metaldependent: Ag-1·PF6 showed the widest activity spectrum; Au-1·PF6 was active against bacteria and yeasts, while Cu-1· PF6 was active only against bacteria. In particular, the activities of Au-1·PF6 against Gram-positive bacteria were superior to those of Ag-1·PF6, while those of Cu-1·PF6 were only modest. The counteranion had a greater effect on the antimicrobial activities of Au-1 over Ag-1. The stability of the M−C bonds in the M−NHC complexes suggests that they act on biomolecules such as amino acids and peptides via throughspace interactions rather than through-bond interactions, and they may denature enzymes or form pits in the cell walls of target microorganisms. F

DOI: 10.1021/acs.inorgchem.8b00011 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 2. Selected Distances (Å), Angles (deg), and Torsion Angles (deg) of Crystals of Cu-1·PF6, Ag-1·PF6, Au-1·PF6, Ag-1· BF4, and Au-1·BF4 Cu-1·PF6

Ag-1·PF613

Au-1·PF6

{[Cu2(L-1a)](PF6)2}2·3CH3CN Cu1···Cu2 2.8502(5) Cu1−C1 1.914(2) Cu1−C21 1.910(3) Cu2−C11 1.909(3) Cu2−C31 1.914(3) Cu1···C10 2.611(3) Cu2···C20 2.622(3) C4 C24 4.308(4) C14 C34 4.333(5) C21−Cu1−C1 171.66(11) C21−Cu1−Cu2 88.37(8) C11−Cu2−C31 171.29(11) C11−Cu2−Cu1 97.76(8) N1−C4−C5 115.2(3) N3−C14−C15 113.9(3) N5−C24−C25 113.0(2) N7−C34−C35 113.1(2) Cu1−C1−N1−C4 7.9(5) Cu2−C11−N3−C14 −2.4(5) Cu1−C21−N5−C24 −4.5(4) Cu2−C31−N7−C34 4.0(5) A and B 54.7 (1) A and C 48.2 (1) Ag-1·BF4

[Ag2(L-1a)](PF6)2·2CH3CN Ag1···Ag2 3.0460(4) Ag1−C1 2.109(3) Ag1−C21 2.102(3) Ag2−C11 2.096(3) Ag2−C31 2.095(3) Ag1···C10 2.797(3) Ag2···C20 2.766(3) C4 C24 5.080(5) C14 C34 4.998(4) C21−Ag1−C1 176.69(12) C21−Ag1−Ag2 88.87(8) C11−Ag2−C31 177.22(11) C11−Ag2−Ag1 89.10(8) N1−C4−C5 114.6(3) N3−C14−C15 112.9(2) N5−C24−C25 112.5(3) N7−C34−C35 112.5(2) Ag1−C1−N1−C4 4.9(4) Ag2−C11−N3−C14 −5.3(4) Ag1−C21−N5−C24 −7.1(4) Ag2−C31−N7−C34 5.1(4) A and B 49.1 (1) A and C 42.4 (1)

[Au2(L-1a)](PF6)2·3CH3CN Au1···Au2 3.0727(11) Au1−C1 2.039(4) Au1−C21 2.039(4) Au2−C11 2.032(4) Au2−C31 2.033(4) Au1···C10 2.810(4) Au2···C20 2.873(4) C4 C24 4.932(6) C14 C34 4.875(7) C21−Au1−C1 175.61(15) C21−Au1−Au2 94.33(11) C11−Au2−C31 177.81(15) C11−Au2−Au1 92.85(11) N1−C4−C5 111.5(3) N3−C14−C15 112.9(3) N5−C24−C25 111.1(3) N7−C34−C35 111.5(3) Au1−C1−N1−C4 −5.4(6) Au2−C11−N3−C14 5.8(6) Au1−C21−N5−C24 10.3(6) Au2−C31−N7−C34 −4.1(6) A and B 40.2 (1) A and C 39.2 (2) Au-1·BF4

[Ag2(L-1a)](BF4)2 Ag1···Ag2 Ag1−C1 Ag1−C21 Ag2−C11 Ag2−C31 Ag1···C10 Ag2···C20 C4 C24 C14 C34 C21−Ag1−C1 C21−Ag1−Ag2 C11−Ag2−C31 C11−Ag2−Ag1 N1−C4−C5 N3−C14−C15 N5−C24−C25 N7−C34−C35 Ag1−C1−N1−C4 Ag2−C11−N3−C14 Ag1−C21−N5−C24 Ag2−C31−N7−C34 A and B A and C

[Au2(L-1a)](BF4)2 Au1···Au2 Au1−C1 Au1−C21 Au2−C11 Au2−C31 Au1···C10 Au2···C20 C4 C24 C14 C34 C21−Au1−C1 C21−Au1−Au2 C11−Au2−C31 C11−Au2−Au1 N1−C4−C5 N3−C14−C15 N5−C24−C25 N7−C34−C35 Au1−C1−N1−C4 Au2−C11−N3−C14 Au1−C21−N5−C24 Au2−C31−N7−C34 A and B A and C

3.0890(15) 2.110(3) 2.098(2) 2.100(3) 2.101(2) 2.803(3) 2.770(3) 5.020(5) 5.138(5) 177.56(9) 87.78(6) 175.64(9) 89.73(6) 113.29(19) 112.84(19) 112.2(2) 111.66(19) 5.4(3) −4.7(3) −3.3(3) 5.9(3) 45.6 (1) 40.5 (1)

Antimicrobial Activity. The antimicrobial activities were estimated in terms of the minimum inhibitory concentration (MIC: μg mL−1), as described elsewhere.27,28,31 Bacteria were inoculated into 5 mL of a liquid medium (Soybean Casein Digest (SCD)) and cultured for 24 h at 35 °C. Yeasts were inoculated into 5 mL of a liquid medium (Glucose Peptone (GP)) and cultured for 48 h at 30 °C. The cultures were adjusted to a concentration of 106−107 mL−1 by dilution-culture method and used for inoculation in the MIC test. Molds were cultured on agar slants (Potato Dextrose (PD) agar medium), for 1 week at 27 °C and then gently washed with saline

3.1137(2) 2.035(4) 2.028(4) 2.020(4) 2.020(4) 2.828(4) 2.838(4) 4.935(4) 4.722(4) 176.66(16) 88.11(12) 178.23(16) 92.35(11) 111.3(3) 111.5(4) 112.1(3) 112.3(3) 6.8(6) −1.9(6) −5.1(6) 5.4(6) 41.0 (1) 45.8 (1)

containing 0.05% Tween 80. The spore suspension thus obtained was adjusted to a concentration of 106 mL−1 and used for inoculation in the MIC test. The test materials were dissolved in DMSO (Ag-1·PF6) or suspended (Au-1·X, Ag-1·X, and Cu-1·X) in water, and solutions were then diluted with SCD medium for bacteria or GP medium for yeast and molds to give concentrations of 1000 to 2 μg mL−1. To these solutions (1 mL) were added 0.1 mL of microorganism suspension prepared above. Bacteria were cultured for 24 h at 35 °C, yeasts for 48 h at 30 °C, and molds for 1 week at 25 °C, and then the growth of the microorganisms was observed. If no growth was G

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Inorganic Chemistry Scheme 4. Summary of Thiol Addition Experiment of M-1·PF6a

a

M = CuI, AgI, and AuI.

Figure 5. Local structure of the hexanuclear silver(I) complex, Ag6(bmt)6·4DMSO, with 2-benzimidazolethiol (Hbmt) obtained from the mixed solution with 50% probability thermal ellipsoids. Symmetry operation i = 2−x, −y, 2−z. Hydrogen atoms and the other anion are omitted for clarity. observed visually in the medium containing the lowest concentration of test materials, the MIC was taken to be this concentration. Synthesis of BF4 and Tf2N Salts of the Precursors of NHC Ligand L-1·X. The precursors of ligand L-1·X containing L-1a·X (major component) and L-1b·X (minor component) were synthesized according to the procedure reported for L-1·PF613 using NaBF4 or KTf2N instead of KPF6. Preparation of L-1·PF6 and properties of precursors H4L-1·(BF4)4 and H4L-1·(Tf2N)4 are described in Supporting Information. Synthesis of BF4 and Tf2N Salts of the Dinuclear Silver(I) Complexes, Ag-1·BF4 and Ag-1·Tf2N. [Ag2(L-1a)](BF4)2 (Ag-1· BF4) and [Ag2(L-1a)](Tf2N)2 (Ag-1·Tf2N) were synthesized according to the procedure reported for Ag-1·PF613 using L-1·BF4 or L-1·Tf2N instead of L-1·PF6. Syntheses of Ag-1·X are described in Supporting Information. Synthesis of the Dinuclear Gold(I) and Copper(I) NHC Complexes by trans-Metalation of the Corresponding Silver(I) Complexes. Synthesis of [Cu2(L-1a)](PF6)2 (Cu-1·PF6). To a solution of Ag-1·PF6 or [Ag2(L-1a)2] (PF6)2 (70 mg, 63 μmol) dissolved in 5 mL of acetone was added CuI (35 mg, 184 μmol)

under an N2 atmosphere. The yellow suspension was stirred for 1 h at room temperature and then was passed through Celite (Wako No. 503) and through a membrane filter (JV 0.1 μm). The colorless clear filtrate was evaporated to ca. 4 mL in a rotary evaporator at ca. 30 °C, which was poured into 50 mL of Et2O. The resultant pale-yellow powder was collected on a membrane filter (JV 0.1 μm), washed with Et2O (20 mL × 2), and dried thoroughly by suction. Yield 52 mg (81%). The compound was crystallized by vapor diffusion of an internal solution of the powder (52 mg, 51 μmol) in 5 mL of acetonitrile, with Et2O as an external solvent. After 1 day, pale-yellow clear block crystals were collected on a membrane filter (JV 0.1 μm), washed with Et2O (20 mL × 2), and dried in vacuo for 2 h. Yield: (40 mg, 78%). The crystals obtained were soluble in acetone, acetonitrile, and DMSO and slightly soluble in MeOH, but insoluble in water, EtOH, CH2Cl2, CHCl3, Et2O, and hexane. They were light-stable for more than one month in the solid-state. Anal. Calcd for C38H34N8F12P2Cu2 or [Cu2(L-1a)](PF6)2: C, 44.76; H, 3.36; N, 10.99. Found: C, 44.60; H, 3.21; N, 11.06%. TG/DTA data: a weight loss of 0.68% was H

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Table 3. Minimum Inhibitory Concentration (MIC; μg mL−1) Values of Coinage(I) Complexes and Free Ligands Towards a Panel of Selected Microorganismsb Ag-1·PF613

Cu-1·PF6 E. coli B. subtilis S. aureus P. aeruginosa C. albicans S. cerevisiae A. brasiliensis (niger) P. citrinum E. coli B. subtilis S. aureus P. aeruginosa C. albicans S. cerevisiae A. brasiliensis (niger) P. citrinum

(ATCC8739) (ATCC6633) (ATCC6538) (ATCC9027) (ATCC9763) (ATCC10231) (ATCC16404) (NBRC6352) (ATCC8739) (ATCC6633) (ATCC6538) (ATCC9027) (ATCC9763) (ATCC10231) (ATCC16404) (NBRC6352)

>1000 250 62.5 >1000 >1000 >1000 >1000 >1000 Ag-1·BF4

15.7 31.3 31.3 62.5 62.5 31.3 62.5 125

Ag-1·PF6a

Au-1·BF4

L-1·BF4

31.3 31.3 31.3 31.3 7.9 15.7 31.3 31.3 Ag-1·Tf2N

250 7.9 4.0 >1000 500 125 62.5 >1000

>1000 62.5 300 1000 62.6 62.6 >1000 >1000

62.6 125 125 250 62.5 31.3 250 250

62.5 31.3 15.7 31.3 7.9 4.0 62.5 >1000

Au-1·PF6

L-1·PF613

125 1000 >1000 Au-1·Tf2N

62.5 125 1000 >1000 125 31.3 >1000 >1000 L-1·Tf2N

>1000 15.7 7.9 >1000 >1000 >1000 >1000 >1000

250 125 >1000 >1000 250 125 >1000 >1000

a

The MIC tests of the above complexes were conducted under DMSO solution conditions. bThe MIC tests of the above complexes were conducted under suspension conditions. observed at below 163 °C. Decomposition began at around 274 °C with exothermic peaks at 312, 350, and 432 °C. Prominent IR bands in the 1800−400 cm−1 region (KBr disk): 1617 vw, 1568 w, 1480 w, 1466 w, 1429 w, 1402 m, 1378 w, 1340 vw, 1299 w, 1261 w, 1236 w, 1222 w, 1201 w, 1169 m, 1117 w, 1054 vw, 1041vw, 1013 vw, 957 vw, 841 vs, 795 w, 777 m, 741 s, 729 m, 702 w, 671 w, 651 vw, 627 vw, 614 vw, 608vw, 558 s, 506 vw, 411 vw cm−1. 1H NMR (DMSO-d6, 21.7 °C): δH 5.15 (CH2, d, 4H, J = 13.5 Hz), 5.28 (CH2, d, 4H, J = 13.5 Hz), 5.72 (CH2, d, 2H, J = 13.5 Hz), 5.80 (CH2, d, 4H, J = 13.5 Hz), 6.30 (CH, s, 4H), 7.07 (CH, s, 4H), 7.58 (CH, m, 4H), 7.73 (CH, m, 4H), 8.24 (CH, s, 2H). 13C NMR (DMSO-d6, 21.7 °C): δC 50.40 (CH2), 54.15 (CH2), 119.28 (CH), 122.76 (CH), 129.56 (CH), 133.27 (CH), 135.48 (C), 136.47 (C), 138.63 (CH), 179.04 (C). Synthesis of [Au2(L-1a)](PF6)2 (Au-1·PF6). To a solution of Ag-1· PF6 or [Ag2(L-1a)2] (PF6)2 (109 mg, 98 μmol) in ca. 10 mL of CH3CN was added [(CH3)2SAuCl] (60 mg, 204 μmol). The paleviolet suspension was stirred overnight at ca. 60 °C and then filtered through Celite (Wako No. 503) and a membrane filter (JV 0.1 μm). The filtrate was poured into 100 mL of Et2O, and the resultant paleviolet powder was collected on a membrane filter (JV 0.1 μm), washed with Et2O (20 mL × 2), and dried thoroughly by suction. Yield 98 mg (77%). The compound was crystallized by vapor diffusion of an internal solution of the powder (88 mg, 68 μmol) in 10 mL of CH3CN with Et2O as an external solvent. After 4 days, colorless clear block crystals were collected on a membrane filter (JV 0.1 μm), washed with Et2O (20 mL × 2), and dried in vacuo for 2 h. Yield: (67 mg, 76%). The crystals obtained were soluble in acetone, acetonitrile and DMSO, but insoluble in water, MeOH, EtOH, CH2Cl2, CHCl3, Et2O, and hexane. They were light-stable for more than one year in the solid-state. Anal. Calcd for C38H34N8F12P2Au2 or [Au2(L-1)](PF6)2: C, 35.47; H, 2.66; N, 8.71. Found: C, 35.36; H, 2.18; N, 8.63%. TG/DTA data: a weight loss of 0.64% was observed at below 164 °C. Decomposition began at around 258 °C with am exothermic peak at 290 °C. Prominent IR bands in the 1800−400 cm−1 region (KBr disk): 1636 vw, 1566 vw, 1476 w, 1454 w, 1439 vw, 1409 w, 1385 vw, 1308 vw, 1243 vw, 1223 vw, 1210 vw, 1191 vw, 1170 w, 1121 vw, 1053 vw, 1021 vw, 958 vw, 924 w, 844 vs, 822 s, 795 w, 779 m, 742 m, 727 m, 706 vw, 682 w, 632 vw, 611 vw, 552 s, 500 vw cm−1. 1H NMR (DMSO-d6, 21.9 °C): δH 5.09 (CH2, d, 4H, J = 14.0 Hz), 5.38 (CH2, d, 4H, J = 14.0 Hz), 5.92 (CH2, d, 4H, J = 14.0 Hz), 6.03 (CH2, d, 4H, J = 14.0 Hz), 6.30 (CH, s, 4H), 7.17 (CH, s, 4H), 7.61 (CH, m, 4H), 7.75 (CH, m, 4H), 8.14

(CH, s, 2H). 13C NMR (DMSO-d6, 21.8 °C): δC 51.28 (CH2), 54.24 (CH2), 119.90 (CH), 124.45 (CH), 129.84 (CH), 133.60 (CH), 134.95 (C), 135.89 (C), 143.29 (CH), 183.65 (C). Synthesis of [Au2(L-1a)](BF4)2 (Au-1·BF4). The complex was obtained through the reaction of Ag-1·BF4 with (CH3)2SAuCl. Yield (0.219 g, 94%). Anal. Calcd for C38H39N8F8B2O2.5Au2 or [Au2(L-1a)](BF4)2 2.5H2O: C, 37.56; H, 3.23; N, 9.22. Found: C, 37.56; H, 2.91; N, 9.15%. TG/DTA data: a weight loss of 3.71% was observed at below 204 °C. Calcd for 2.5H2O 3.71%. Decomposition began at around 205 °C with exothermic peaks at 359, 409, and 501 °C. Prominent IR bands in the 1800−400 cm−1 region (KBr disk): 1635 vw, 1559 m, 1474 w, 1422 w, 1355 w, 1290 w, 1226 m, 1153 vs, 1059 vs, 1020 vw, 853 w, 797 m, 659 vw, 623 vw, 522 m, 500 vw, 409 vw cm−1. 1H NMR (DMSO-d6, 21.4 °C): δH 5.09 (CH2, d, 4H, J = 14.0 Hz), 5.38 (CH2, d, 4H, J = 14.0 Hz), 5.92 (CH2, d, 4H, J = 14.0 Hz), 6.03 (CH2, d, 4H, J = 14.0 Hz), 6.30 (CH, s, 4H), 7.17 (CH, s, 4H), 7.61 (CH, m, 4H), 7.75 (CH, m, 4H), 8.14 (CH, s, 2H). 13C NMR (DMSO-d6, 22.6 °C): δC 51.28 (CH2), 54.24 (CH2), 119.91 (CH), 124.45 (CH), 129.86 (CH), 133.61 (CH), 134.96 (C), 135.90 (C), 143.31 (CH), 183.66 (C). Synthesis of [Au2(L-1a)](Tf2N)2 (Au-1·Tf2N). The complex was obtained through the reaction of Ag-1·Tf2N with (CH3)2SAuCl. Yield (0.131 g, 85%). Anal. Calcd for C42H34N10F12O8S4Au2 or [Au2(L1)](Tf2N)2: C, 32.40; H, 2.20; N, 9.00. Found: C, 32.78; H, 2.22; N, 9.05%. TG/DTA data: a weight loss of 0.05% was observed at below 200 °C. Prominent IR bands in the 1800−400 cm−1 region (KBr disk): 1570 w, 1502 vw, 1474 w, 1438 w, 1411 m, 1384 w, 1351 vs, 1225 s, 1179 vs, 1130 vs, 1055 s, 1018 vw, 918 vw, 829 vw, 815 vw, 788 w, 738 s, 704 w, 681 vw, 648 w, 594 m, 571 m, 510 m, 418 vw, 406 vw cm−1. 1H NMR (DMSO-d6, 21.3 °C): δH 5.09 (CH2, d, 4H, J = 14.0 Hz), 5.35 (CH2, d, 4H, J = 14.0 Hz), 5.92 (CH2, d, 4H, J = 14.0 Hz), 6.03 (CH2, d, 4H, J = 14.0 Hz), 6.30 (CH, s, 4H), 7.17 (CH, s, 4H), 7.61 (CH, m, 4H), 7.75 (CH, m, 4H), 8.14 (CH, s, 2H). 13 C NMR (DMSO-d6, 21.9 °C): δC 51.30 (CH2), 54.27 (CH2), 119.40 (CF3, J = 1281 Hz), 119.92 (CH), 124.46 (CH), 129.86 (CH), 133.62 (CH), 134.96 (C), 135.91 (C), 143.42 (CH), 183.68 (C). I

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(4) Lin, J. C.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A. B.; Hwang, W. S.; Lin, I. J. B. Coinage metal-N-hetetorocyclic carbene complexes. Chem. Rev. 2009, 109, 3561−3598. (5) Oehninger, L.; Rubbiani, R.; Ott, I. N-Heterocyclic carbene metal complexes in medicinal chemistry. Dalton Trans. 2013, 42, 3269−3284. (6) Cisnetti, F.; Gautier, A. Metal/N-Heterocyclic carbene complexes: Opportunities for the development of anticancer metallodrugs. Angew. Chem., Int. Ed. 2013, 52, 11976−11978. (7) Nolan, S. P. Development and catalytic uses of N-heterocyclic carbene gold complexes. Acc. Chem. Res. 2011, 44, 91−100. (8) Lazreg, F.; Nahra, F.; Cazin, C. S. J. Copper−NHC complexes in catalysis. Coord. Chem. Rev. 2015, 293−294, 48−79. (9) Budagumpi, S.; Haque, R. A.; Endud, S.; Rehman, G. U.; Salman, A. W. Biologically relevant silver(I)−N-heterocyclic carbene complexes: Synthesis, structure, intramolecular interactions, and applications. Eur. J. Inorg. Chem. 2013, 2013, 4367−4388. (10) Medici, S.; Peana, M.; Nurchi, V. M.; Lachowicz, J. I.; Crisponi, G.; Zoroddu, M. A. Noble metals in medicine: Latest advances. Coord. Chem. Rev. 2015, 284, 329−350. (11) Andrew, R. E.; Storey, C. M.; Chaplin, A. B. Well-defined coinage metal transfer agents for the synthesis of NHC-based nickel, rhodium and palladium macrocycles. Dalton Trans. 2016, 45, 8937− 8944. (12) Nomiya, K.; Kasuga, N. C.; Takayama, A. Synthesis, structure and antimicrobial activities of polymeric and nonpolymeric silver and other metal complexes. From synthesis to applications. RSC Polymer Chemistry Series 10, Polymeric Materials with Antimicrobial Activity; Muñoz-Bonilla, A., Cerrada, M. L., Fernandez-Gárcia, M., Eds.; RSC Publishing: Cambridge, UK, 2014; pp 153−207. (13) Sakamoto, R.; Morozumi, S.; Yanagawa, Y.; Toyama, M.; Takayama, A.; Kasuga, N. C.; Nomiya, K. Synthesis, characterization, and structure−activity relationship of the antimicrobial activities of dinuclear N-heterocyclic carbene (NHC)-silver(I) complexes. J. Inorg. Biochem. 2016, 163, 110−117. (14) Glišić, B. D.; Djuran, M. I. Gold complexes as antimicrobial activities in relation to the oxidation state of the gold ion and the ligand structure. Dalton Trans. 2014, 43, 5950−5969. Bertrand, B.; Casini, A. A golden future in medicinal inorganic chemistry: the promise of anticancer gold organometallic compounds. Dalton Trans 2014, 43, 4209−4219. (15) Teyssot, M.-L.; Jarrousse, A.-S.; Manin, M.; Chevry, A.; Roche, S.; Norre, F.; Beaudoin, C.; Morel, L.; Boyer, D.; Mahiou, R.; Gautier, A. Metal−NHC complexes: a survey of anti-cancer properties. Dalton Trans 2009, 6894−6902. (16) Baker, V. M.; Brown, D. H.; Haque, R. A.; Skelton, B. W.; White, A. H. Dinuclear N-heterocyclic carbene complexes of silver(I), derived from imidazolium-linked cyclophanes. Dalton Trans 2004, 3756−3764. (17) Bondi, A. (1964). Van der Waals volumes and radii. J. Phys. Chem. 1964, 68, 441−451. (18) Gierz, V.; Seyboldt, A.; Maichle-Mössmer, C.; Törnroos, K. W.; Speidel, M. T.; Speiser, B.; Eichele, K.; Kunz, D. Dinuclear coinagemetal complexes of bis(NHC) ligands: Structural features and dynamic behavior of a Cu-Cu complex. Organometallics 2012, 31, 7893−7901. (19) Liu, B.; Chen, C.; Zhang, Y.; Liu, X.; Chen, W. Dinuclear copper(I) complexes of phenanthrolinyl-functionalized NHC ligands. Organometallics 2013, 32, 5451−5460. (20) Wedlock, L. E.; Barnard, P.; Filipovska, A.; Skelton, B. W.; Berners-Price, S. J.; Baker, M. V. Dinuclear Au(I) N-heterocyclic carbene complexes derived from unsymmetrical azolium cyclophane salts: potential probes for live cell imaging applications. Dalton Trans. 2016, 45, 12221−12236. (21) Lee, M.-T.; Hu, C.-H. Density functional study of Nheterocyclic and diamino carbene complexes: Comparison with phosphines. Organometallics 2004, 23, 976−983. (22) Yue, C.; Yan, C.; Feng, R.; Wu, M.; Chen, L.; Jiang, F.; Hong, M. A polynuclear d10-d10 metal complex with unusual near-infrared

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00011. Synthesis of NHC precursors with BF4 and (CF3SO2)2N (Tf2N) anion, H4L-1·(BF4)4 and H4L-1·(Tf2N)4, and H4L-1·(PF6)413; synthesis of Ag-1·X (X = PF6,13 BF4, and Tf2N); overlay of 1H NMR spectra of H4L-1·X4 (X = PF6,13 BF4, and Tf2N) in DMSO-d6 (Figure S1); overlay of 1H NMR spectra of Ag-1·X4 (X = PF6,13 BF4, and Tf2N) DMSO-d6 (Figure S2); overlay of 1H NMR spectra of Au-1·X4 (X = PF6,13 BF4, and Tf2N) DMSOd6 (Figure S3); 1H NMR spectral change of M-1·PF6 (M = CuI, AgI, and AuI) after addition of 4 equiv of 2mercaptoethanol in CD3CN and H4L-1·PF6 (Figure S4); 1H and 13C NMR spectra of H4L-1·X4 with PF6, BF4, and Tf2N anion in DMSO-d6 (Figures S6−S10); 1 H and 13C NMR spectra of Ag-1·X; X = PF6, BF4 and Tf2N in DMSO-d6 (Figures S11−S16), 1H and 13C NMR spectra of Au-1·X; X = PF6, BF4 and Tf2N in DMSO-d6 (Figures S17−S22); 1H and 13C NMR spectra of Cu-1·PF6 (Figures S23 and S24); crystal data of H4L1a·PF6 (Table S1), and Ag6(bmt)6·4DMSO isolated after thiol addition (Table S2) (PDF) Accession Codes

CCDC 1560134, 1560140−1560142, and 1590621 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, 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

*Phone: 81-463-59-4111; Fax: 81-463-58-9684; E-mail: [email protected]. ORCID

Kenji Nomiya: 0000-0003-0225-877X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge a grant from the Strategic Research Base Development Program for Private Universities of the Ministry of Education, Culture, Sports, Science and Technology of Japan, and also a grant from the Research Institute for Integrated Science, Kanagawa University (RIIS201604). We thank Hiroshi Yokoyama of Kanagawa University for NMR measurements. We also thank Dr. Tadashi Matsumoto of Rigaku Corporation for helping single-crystal X-ray analysis.

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K

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