NHC Ligated Group 11 Metal-Arylthiolates Containing an Azide

Aug 16, 2018 - ... reaction with bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN−OH) and ... the new azide-modified arylthiol 1-HSCH2-2,5-Me2-4-N3CH2-C6H2,...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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NHC Ligated Group 11 Metal-Arylthiolates Containing an Azide Functionality Amenable to “Click” Reaction Chemistry Vaishnavi Somasundaram,† Praveen N. Gunawardene,† Alexander M. Polgar,† Mark S. Workentin,†,‡,* and John F. Corrigan†,‡,* †

Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7 Canada Centre for Advanced Materials and Biomaterials Research, The University of Western Ontario, London, Ontario N6A 3K7 Canada

‡

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/21/18. For personal use only.

S Supporting Information *

ABSTRACT: The reaction of N-heterocyclic carbene (NHC) Group 11 metal complexes, [(NHC)M−X] (X = chloride, acetate), with the new azide-modified arylthiol 1-HSCH2-2,5Me2-4-N3CH2-C6H2, 1 (for M = Au; X = Cl), or 1-Me3SiSCH2-2,5-Me2-4-N3CH2-C6H2, 2 (for M = Cu, X = Cl; M = Ag, X = OAc), affords the “clickable” NHC-metal thiolates [(iPr2bimy)Au-(1-SCH2-2,5-Me2-4-N3CH2-C6H2)], 5; [(IPr)Au-(1-SCH2-2,5-Me2-4-N3CH2C6H2)], 6; [(IPr)Ag-(1-SCH2-2,5-Me2-4-N3CH2-C6H2)], 7; and [(IPr)Cu-(1-SCH2-2,5Me2-4-N3CH2-C6H2)], 8 (iPr2-bimy = 1,3-di-isopropylbenzimidazol-2-ylidene, IPr = 1,3bis(2,6-di-iso-propylphenyl)imidazol-2-ylidene). Single-crystal X-ray analysis of all metal complexes show that they are two-coordinate, nearly linear, with a terminally bonded thiolate ligand possessing an accessible azide (−N3) moiety. The strain-promoted alkyne−azide cycloaddition (SPAAC) reaction of complex 6 with bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN−OH) and dibenzocyclooctyne-amine (DBCO−NH2) illustrated the reactivity of the azide moiety toward strain-promoted cycloaddition. The rate of the SPAAC reaction between complex 6 and BCN−OH was determined via 1H NMR spectroscopy under second order conditions, and was compared to that of BCN−OH with PhCH2N3.

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INTRODUCTION The coordination flexibility of arylthiolate ligands, that is their ability to bridge two or more metal centers, results in Group 11 metal−SR complexes of Cu(I) and Ag(I) typically adopting structures with varying nuclearity, size, and shape.1 This propensity for cluster formation can be circumvented if the size of the substituent on the chalcogen atom is sufficiently large to promote terminal coordination of −SR or occupation of available coordination sites around the metal are completed by additional ligands.2 Recently, it has been shown that Nheterocyclic carbenes (NHCs) can serve as effective ancillary ligands for stabilizing either monometallic, linear Group 11 complexes [(NHC)M−SR] with sufficiently large NHCs;3 cyclic structures (clusters) are isolated when the size of the NHC is reduced sufficiently.3d,4 For the heavier metal gold− thiolates, the preferred linear coordination of Au(I) results in monometallic complexes, independent of the size of the NHC ligands used.5 Thiolate ligands that are incorporated onto metal complexes can be designed with a functional handle for potential “click” or bio-orthogonal reactions.6 The azide moiety is very useful in this regard as it undergoes chemoselective and rapid reactivity as a 1,3 dipole toward cyclooctyne dipolarophiles in the strainpromoted alkyne−azide cycloaddition (SPAAC) reaction. The robustness of the azido handle and the fact that there is virtually no cross-reactivity with other functional groups in biological systems gives it great potential to be exploited in labeling studies.7 The modification of a thiolate, through the © XXXX American Chemical Society

introduction of an azide moiety would render the corresponding metal−thiolate coordination complex amenable to the SPAAC reaction.8 Herein we describe the preparation and characterization of the new azide functionalized thiol 1HSCH2-2,5-Me2-4-N3CH2-C6H2, 1, together with its silylated analogue 1-Me3SiSCH2-2,5-Me2-4-N3CH2-C6H2, 2. These were prepared from 1,4-(BrCH2)2-2,5-Me2-C6H2 via the intermediates 1-BrCH2-2,5-Me2-4-AcSCH2-C6H2, 3, and 1AcSCH2-2,5-Me2-4-N3CH2-C6H2, 4. The reaction chemistry of 1 and 2 was developed with NHC ligated Group 11 metals (Cu, Ag, and Au) to yield the metal−thiolates [(NHC)M-(1CH2S-2,5-Me2-4-N3CH2-C6H2)]. The complexes [(iPr2-bimy)Au-(1-SCH2-2,5-Me2-4-N3CH2-C6H2)], 5 (iPr2-bimy = 1,3diisopropylbenzimidazolin-2-ylidene), and [(IPr)M-(1-SCH22,5-Me2-4-N3CH2-C6H2)] (IPr = 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene; M = Au 6, Ag 7, Cu 8) were characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The SPAAC reactivity of complex 6 was demonstrated through reaction with two electronically distinct strained-alkyne moieties: the aliphatic bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN−OH) and the benzoannulated dibenzocyclooctyne-amine (DBCO− NH2). Successful cycloaddition of the two different strainedalkynes to complex 6 was confirmed by NMR spectroscopy and mass spectrometry. The rate of reaction between complex Received: June 24, 2018

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DOI: 10.1021/acs.inorgchem.8b01750 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry 6 and BCN−OH was determined through a 1H NMR kinetic study under second-order conditions, and was compared to that between a model azide, PhCH2N3, and BCN−OH, with both reactions having similar reaction kinetics.

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1-BrCH2-2,5-Me2-4-AcSCH2-C6H2 (3).

EXPERIMENTAL SECTION To a solution of 1,4-(CH2Br)2-2,5-Me2-C6H2 (14.8 g, 0.051 mol) in 150 mL of CH3CN was added KSAc (2.30 g, 0.020 mol). After the mixture was stirred for 24 h at reflux, the CH3CN was removed by rotary evaporation, and the mixture was redissolved in 80 mL of CH2Cl2. The salt byproduct was removed by gravity filtration. Crude 3 was purified by column chromatography using 1:3 CH2Cl2:hexanes as the eluent and obtained as a yellow oil in 30% yield (5.2 g). 1H NMR (400 MHz, CDCl3): δ 7.10 (s, 2H, Ar−H), 4.47 (s, 2H, CH2Br), 4.09 (s, 2H, CH2S(CO)CH3), 2.35 (s, 6H, Ar−CH3), 2.28 (s, 3H, CH2S(CO)CH3). 13C{1H} NMR (150 MHz, CDCl3): δ 195.1, 135.2, 134.3, 132.5, 132.2, 101.4, 51.3, 32.2, 31.3, 30.5, 18.9, 18.3. HRMS (ESI): Calcd [M] 287.0027, found 287.0031. 1-AcSCH2-2,5-Me2-4-N3CH2-C6H2 (4).

General Synthetic Techniques and Starting Materials. Standard Schlenk techniques for all reactions were used under a dry nitrogen atmosphere unless specified otherwise. Nonchlorinated solvents (tetrahydrofuran (THF), toluene, pentane, hexane, heptane, and diethyl ether) were dried by passage through packed columns of activated alumina using an MBraun MBSP Series solvent purification system. Chlorinated solvents (CHCl3, and CH2Cl2) were distilled and dried over P2O5. Trimethylsilyl chloride (TMSCl) was distilled and dried over P2O5. Triethylamine was purchased from Aldrich and dried over CaH2. Chloroform-d and dichloromethane-d2 were purchased from Aldrich and distilled over P2O5. Dibenzocyclooctyne-amine was purchased from Aldrich and used as received. [(iPr2-bimy)AuCl],9 [(IPr)AuCl],10 [(IPr)AgOAc],11 [(IPr)CuCl],12 and 1,4-(BrCH2)22,5-Me2-C6H213 were prepared according to literature procedures. A literature procedure was also followed for the preparation of bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN−OH);14 this yields both the exo and endo isomers. Only the exo isomer was used here in order to simplify NMR analyses. 1H and 13C{1H} NMR spectra were recorded on Varian Mercury 400 MHz, and Inova 400 MHz spectrometers and the chemical shifts (δ) were internally referenced by the (residual) solvent signals relative to tetramethylsilane (1H and 13 C). Infrared spectra were collected using a PerkinElmer Spectrum Two ATR-FTIR spectrometer. Mass spectra and exact mass determinations were performed on a Bruker micrOTOF II instrument or Finningan MAT 8400. UV−vis absorption spectra were acquired with a Varian Cary 300 Bio Spectrophotometer (solutions; 10−5 M; 1.00 cm matched cells). For single crystal X-ray analysis, the crystals were mounted on a Mitegen polyimide micromount with a small amount of Paratone N Oil. All X-ray measurements were performed on a Bruker Kappa Axis Apex2 diffractometer at a temperature of 110 K. The data collection strategies were a number of ω and φ scans. The frame integration was performed using SAINT.15 The resulting raw data were scaled and absorption corrected using a multiscan averaging of symmetry equivalent data using SADABS.16 The structure was solved by using a dual space methodology using the SHELXT program.17 All nonhydrogen atoms were obtained from the initial solution. The hydrogen atoms were introduced at idealized positions and were allowed to ride on the parent atom. The calculated structure factors included corrections for anomalous dispersion from the usual tabulation. The structure was refined using the SHELXL-2014 program from the SHELXTL suite of crystallographic software.18 The −CH2N3 moiety in 5 displayed a 2 site disorder that was satisfactorily modeled with 0.72:0.28 occupancy. Disordered atoms C21−N3−N4−N5/C21A−N3A−N4A−N5A were refined as rigid groups with common restraints (DFIX). CCDC files 1548144− 1548147 contain the supplementary crystallographic data for this Article. Elemental analysis was performed by Laboratoire d’Analyze É lémentaire de l’Université de Montréal, Montréal, Canada. All computations were carried out in Gaussian 09, revision B.01.19 All calculations were performed at the B3LYP/Def2SVP level of theory.20 Kohn−Sham orbitals were plotted from Gaussian.cub files using Visualization for Electronic and Structural Analysis,21 version 3.3.2. Time-dependent density functional theory (TD-DFT) calculations were performed in Gaussian 09 using the adiabatic approximation on structurally determined geometry of 5 and 6 for singlet excitations.

Caution: Sodium azide may react with lead and copper plumbing to form highly explosive metal azides! To a solution of 3 (5.20 g, 0.018 mol) in 50 mL of CH3CN was added NaN3 (4.59 g, 0.070 mol). The reaction flask was purged with Ar and heated to reflux overnight. CH3CN was removed on a rotary evaporator and redissolved in 10 mL of CH2Cl2. Salt byproducts were removed by extraction with distilled H2O to yield 4 as a yellow oil in 83% yield (6.0 g). 1H NMR (400 MHz, CDCl3): δ 7.14 (s, 1H, Ar− H), 7.05 (s, 1H, Ar−H), 4.29 (s, 2H, CH2N3), 4.11 (s, 2H, CH2SC(O)CH3), 2.36 (s, 3H, CH2S(CO)CH3), 2.30 (s, 6H, Ar− CH3). 13C{1H} NMR (150 MHz, CDCl3): δ 195.3, 135.5, 134.6, 134.4, 132.8, 132.3, 101.5, 52.8, 31.3, 30.5, 19.9, 18.5. HRMS (ESI): Calcd [M] 250.0936, found 250.0935. 1-HSCH2-2,5-Me2-4-N3CH2-C6H2 (1).

To a solution of 4 (2.00 g, 8 mmol) in 20 mL of ethanol was added 8 mL of 1 M NaOH (8 mmol) in ethanol. The reaction was left stirring for 15 min under an Ar atmosphere after which 8 mL of 2 M HCl (16 mmol) was added; the reaction was left to stir for an additional 15 min. The cloudy mixture was extracted into 5 mL of CH2Cl2, and the resultant yellow solution was washed with 3 × 10 mL of distilled H2O. The organic solvent was removed under vacuum, and the resultant yellow solid was purified by column chromatography using 1:1 CH2Cl2 hexanes as the eluent. The thiol 1 was obtained as a white solid in 82% yield (1.60 g). 1H NMR (400 MHz, CDCl3): δ 7.10 (s, 1H, Ar−H), 7.07 (s, 1H, Ar−H), 4.30 (s, 2H, CH2N3), 3.71 (d, 2H, CH2SH, 3JHH = 8 Hz), 2.38 (s, 3H, Ar−H), 2.32 (s, 3H, Ar−H), 1.67 (t, 1H, SH, 3JHH = 8 Hz). 13C{1H} NMR (150 MHz, CDCl3): δ 139.5, 134.8, 133.6, 132.5, 131.8, 131.1, 52.8, 26.6, 18.6, 18.5. IR (cm−1): 2161, 2550. HRMS (ESI): Calcd [M] 208.0664, found 208.0662. λmax 285 (ε = 11 600 M−1 cm−1), 290 nm (ε = 11 400 M−1 cm−1). B

DOI: 10.1021/acs.inorgchem.8b01750 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry 1-Me3SiSCH2-2,5-Me2-4-N3CH2-C6H2 (2).

vacuo and layered with 8 mL of pentane to obtain as colorless crystals in 77% yield (0.097 g). 1H NMR (400 MHz, CDCl3): δ 7.52 (t, 2H, 3 JHH = 8 Hz, (iPr)2Ar−H), 7.29 (d, 4H, 3JHH = 8 Hz, (iPr)2Ar−H), 7.16 (s, 2H, HC = CH), 6.93 (s, 1H, Ar−H), 6.84 (s, 1H, Ar−H), 4.20 (s, 2H, CH2N3), 3.52 (s, 2H, CH2SAu), 2.61 (sept, 4H, 3JHH = 7 Hz, (CH3)2CH), 2.21 (s, 3H, Ar−CH3), 2.07 (s, 3H, Ar−CH3), 1.35 (d, 12H, 3JHH = 8 Hz, (CH3)2CH), 1.23 (d, 12H, 3JHH = 8 Hz, (CH3)2CH). 13C{1H} NMR (150 MHz, CDCl3): δ 187.5, 145.9, 144.0, 134.5, 133.8, 133.3, 131.2, 130.6, 130.3, 129.2, 128.4, 124.2, 122.9, 53.2, 34.3, 28.9, 28.6, 27.7, 24.6, 24.2, 22.5, 18.8, 18.6, 14.2. mp: 180−184 °C. Anal. Calcd for C37H48AuN5S: N, 8.84; C, 56.13; H, 6.11; S, 4.04. Found: N, 8.13; C, 55.70; H, 6.42; S, 3.58. UV−vis (CH2Cl2): λmax = 286 (ε = 23 000 M−1 cm−1), 291 (22 400 M−1 cm−1), 300 nm (20 400 M−1 cm−1). [(IPr)Ag-(1-SCH2-2,5-Me2-4-N3CH2-C6H2)] (7).

To a solution of 1 (0.150 g, 0.720 mmol) in 8 mL of toluene was added trimethylsilyl chloride (0.3 mL, 2.18 mmol), followed by triethylamine (0.1 mL, 0.740 mmol). The reaction was stirred for 18 h after which the mixture was filtered through a plug of glass wool. The solvent and excess trimethylsilyl chloride were removed under vacuum to yield 2 as a colorless oil in 92% yield (0.190 g). 1H NMR (CDCl3, 400 MHz): δ 7.12 (s, 1H, Ar−H), 7.04 (s, 1H, Ar− H), 4.29 (s, 2H, CH2N3), 3.67 (s, 2H, CH2Si(CH3)3), 2.38 (s, 3H, Ar−CH3), 2.31 (s, 3H, Ar−CH3), 0.34 (s, 9H, CH2Si(CH3)3). 13 C{1H} NMR (150 MHz, CDCl3): δ 138.8, 134.4, 134.2, 132.2, 131.8, 131.7, 52.9, 28.3, 18.7, 18.5, 1.2. IR: 2162 cm−1. HRMS (ESI): Calcd [M] 280.4822, found 280.4862. [(iPr2-bimy)Au-(1-SCH2-2,5-Me2-4-N3CH2-C6H2)] (5).

To a 6 mL solution of [(IPr)AgOAc] (0.10 g, 0.1 mmol) in THF was added the trimethylsilylsulfide reagent 2 (0.061 g, 0.22 mmol) in 6 mL of THF. The reaction was stirred for 5 h at room temperature after which the solvent was removed under vacuum. The white solid was redissolved in 4 mL of toluene and layered with 10 mL of pentane to yield colorless crystals of 7 in 68% yield (0.048 g). 1H NMR (400 MHz, CDCl3): δ 7.51 (t, 2H, 3JHH = 8 Hz, (iPr)2Ar−H), 7.30 (d, 4H, 3 JHH = 8 Hz, (iPr)2Ar−H), 7.20 (s, 2H, HCCH), 6.98 (s, 1H, Ar− H), 6.82 (s, 1H, Ar−H), 4.20 (s, 2H, CH2N3), 3.41 (s, 2H, CH2SAg), 2.58 (sept, 4H, 3JHH = 8 Hz, (CH3)2CH), 2.20 (s, 3H, Ar−CH3), 2.09 (s, 3H, Ar−CH3), 1.30 (d, 12H, 3JHH = 8 Hz, (CH3)2CH), 1.23 (d, 12H, 3JHH = 8 Hz, (CH3)2CH). 13C{1H} NMR (150 MHz, CDCl3): δ 145.8, 145.3, 138.0, 134.9, 131.6, 131.26, 131.0, 130.7, 129.2, 128.4, 125.4, 124.3, 53.2, 28.9, 28.7, 27.7, 24.9, 24.7, 24.1, 21.6, 18.7. mp: 174−179 °C. Anal. Calcd for C37H48AgN5S.C7H8: N, 9.17; C, 66.48; H, 7.10; S, 4.03. Found: N, 9.17; C, 66.00; H, 7.10; S, 3.53. UV−vis (CH2Cl2): λmax = 286 (ε = 23 600 M−1 cm−1), 290 (22 900 M−1 cm−1), 300 nm (20 700 M−1 cm−1). [(IPr)Cu-(1-SCH2-2,5-Me2-4-N3CH2-C6H2)] (8).

To [(iPr2-bimy)AuCl] (0.10 g, 0.16 mmol) in 10 mL of THF was added a solution of the azide terminated thiol, 1 (0.040 g, 0.19 mmol) in 5 mL of THF. To the solution was added NEt3 (40 μL, 0.32 mmol) dropwise. The reaction was stirred at room temperature over 18 h. The solvent was evaporated under vacuum and redissolved in 8 mL of toluene. The white solid was filtered off by pipetting solution through glass wool and the supernatant was concentrated to ∼2 mL in vacuo and layered with 8 mL of pentane to obtain as colorless needles of 5 in 74% yield (0.072 g). 1H NMR (400 MHz, CDCl3): δ 7.53 (m, 2H, Arcarbene−H), 7.26 (m, 2H, Arcarbene−H), 7.19 (s, 1H, Ar−H), 6.91 (s, 1H, Ar−H), 5.30 (m, 2H, (CH3)2CH), 4.19 (s, 2H, CH2N3), 4.02 (s, 2H, CH2SAu), 2.43 (s, 3H, Ar−CH3), 2.21 (s, 3H, Ar−CH3), 1.59 (d, 12H, 3JHH = 8 Hz, (CH3)2CH). 13C{1H} NMR (150 MHz, CDCl3): δ 190.3, 145.2, 134.2, 133.3, 132.7, 131.8, 131.5, 130.8, 123.7, 113.16, 53.8, 53.6, 53.2, 29.5, 21.8, 19.4, 18.7. mp: 142−146 °C. Anal. Calcd for C23H30AuN5S: N, 11.56; C, 45.63; H, 4.99; S, 5.28. Found: N, 11.27; C, 45.78; H, 5.00; S, 4.80. UV−vis (CH2Cl2): λmax = 284 (ε = 21 100 M−1 cm−1), 292 (20 700 M−1 cm−1), 318 nm (17 500 M−1 cm−1). [(IPr)Au-(1-SCH2-2,5-Me2-4-N3CH2-C6H2)] (6).

To a 6 mL solution of [(IPr)CuCl] (0.13 g, 0.25 mmol) in THF was added 2 (0.085 g, 0.30 mmol) in 6 mL of THF. The reaction was stirred for 5 h at room temperature after which the solvent was removed under vacuum. The yellow-white solid was redissolved in 4 mL of toluene and layered with 10 mL of pentane to obtain colorless crystals of 8 in 65% yield (0.10 g). 1H NMR (400 MHz, CDCl3): δ 7.50 (t, 2H, 3JHH = 8 Hz, (iPr)2Ar−H), 7.31 (d, 4H, 3JHH = 8 Hz, (iPr)2Ar−H), 7.14 (s, 2H, HC = CH), 6.89 (s, 1H, Ar−H), 6.82 (s, 1H, Ar−H), 4.19 (s, 2H, CH2N3), 3.19 (s, 2H, CH2SCu), 2.62 (sept, 4H, 3JHH = 8 Hz, (CH3)2CH), 2.20 (s, 3H, Ar−CH3), 2.08 (s, 3H, Ar−CH3), 1.33 (d, 12H, 3JHH = 8 Hz, (CH3)2CH), 1.25 (d, 12H, 3JHH = 8 Hz, (CH3)2CH). 13C{1H} NMR (150 MHz, CDCl3): δ 179.2, 145.9, 145.1, 134.8, 133.7, 131.2, 131.1, 130.6, 129.2, 128.4, 125.44, 124.3, 122.9, 53.2, 29.3, 29.1, 28.9, 24.6, 24.2, 22.5, 18.8, 18.6, 14.2. mp: 174−178 °C. Anal. Calcd for C37H48CuN5S: N, 10.64; C, 67.49; H, 7.35; S, 4.87. Found: N, 10.62; C, 67.01; H, 7.49; S, 5.34. UV−vis (CH2Cl2): λmax = 285 (ε = 22 200 M−1 cm−1), 289 (21 500 M−1 cm−1), 302 nm (18 000 M−1 cm−1).

Method 1a. To a 10 mL suspension of NaH (0.0050 g, 0.21 mmol) in THF was added 0.075 g of 1 (40 mg, 0.19 mmol) in 5 mL of THF. The reaction was left to stir over 5 h. A 5 mL solution of [(IPr)AuCl] (0.12 g, 0.19 mmol) in THF was added to the reaction mixture and stirred for 18 h at room temperature. The THF solvent was evaporated under vacuum, and 10 mL of toluene was added. The white solid (NaCl) was filtered off by pipetting solution through glass wool and the supernatant was concentrated to ∼2 mL in vacuo and layered with 8 mL of pentane to obtain 6 as colorless crystals in 72% yield (0.10 g). Method 1b. To [(IPr)AuCl] (0.10 g, 0.16 mmol) in 10 mL of THF was added a solution of 1 (0.040 g, 0.19 mmol) in 5 mL of THF. To the solution was added NEt3 (42 μL, 0.32 mmol) dropwise. The reaction was stirred at room temperature over 18 h. The solvent was evaporated under vacuum and redissolved in 8 mL of toluene. The white solid (Et3NHCl) was filtered off by pipetting solution through glass wool and the supernatant was concentrated to ∼2 mL in C

DOI: 10.1021/acs.inorgchem.8b01750 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1

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Cycloadducts of 6 with BCNexo−OH (9).

RESULTS AND DISCUSSION The preparation of the mixed thiol/azide ligand 1 utilized the dibromide 1,4-(BrCH2)2-2,5-Me2-C6H2, as the template onto which stepwise substitution reactions were completed. A protected thiol was first introduced by heating 1,4-(BrCH2)22,5-Me2-C6H2 with potassium thioacetate (KSAc) over 24 h in CH3CN to yield 3 (Scheme 1). To maximize the yield of 3, the ratios of the two reagents, the solvent system, temperature, and reaction times were optimized. In all experiments, the disubstituted thioacetate together with unreacted 1,4(BrCH2)2-2,5-Me2-C6H2 were found to be present, in addition to 3. Adjusting the ratio of 1,4-(BrCH2)2-2,5-Me2-C6H2:KSAc from 2:1 to 1.4:1 resulted in the highest yields of 3. Yields were highest (30%) with this ratio for reactions completed in CH3CN at reflux for 24 h. After purification by flash column chromatography, 3 was isolated as a white solid. When 3 was treated with NaN3 in CH3CN, the azide functionality was introduced to yield 4 in high yields (Scheme 1). Purification was achieved through a simple extraction to give 4 as an air-stable, yellow oil. The acetate group of compound 4 can be subsequently hydrolyzed by the addition of NaOH to yield anionic 1-−SCH2-2,5-Me2-4-N3CH2-C6H2 (not isolated), which can itself be protonated to generate the thiol 1 as an air sensitive, white solid. The −SH functionality is easily converted to −SSiMe3 via reaction with Me3SiCl/NEt3 and the elimination of [Et3NH]Cl to yield 1-Me3SiSCH2-2,5Me2-4-N3CH2-C6H2 2. The synthesis of 1−4 was followed by 1H and 13C{1H} NMR spectroscopy. As shown in Figure S1, the aromatic proton NMR signals were all observed within a narrow range as singlets at ∼7 ppm, the methyl groups yielded signals ∼2.2− 2.4 ppm, whereas the resonances assigned to methylene protons vary in accordance with the nature of their substituents, observed between δ = 3.5 and 4.5 ppm. Substitution of one bromide in 1,4-(BrCH2)2-2,5-Me2-C6H2 with −SAc to yield asymmetric 3 was confirmed by the presence of two methylene resonances. The shielding effect of the thioacetate shifted the AcSCH2−methylene signal slightly upfield versus −CH2Br. Upon substitution of the second bromide with the azide group in 4, both the inequivalent aromatic and methyl proton signals were resolved for each. After hydrolysis of the thioacetate moiety, the resonance corresponding to proton of the thiol functionality in 1 appears as a triplet at 1.67 ppm. Generation of the thiol 1 from 4 also leads to a shift in the signal of the adjacent methylene protons, upfield from 4.11 to 3.71 ppm. Finally, the reaction of 1 with Me3SiCl/NEt3 to give 2 yields a singlet at 0.34 ppm for −SSiMe3 while the chemical shifts of the methylene groups do

To a 5 mL solution of 6 (0.064 g, 0.080 mmol) in CH2Cl2 was added BCN−OH (0.016 g, 0.11 mmol) and left to stir at room temperature for 12 h. The solvent was removed under vacuum. The white solid was washed with 5 × 5 mL of Et2O and redissolved in 3 mL of THF and a few drops of heptane to obtain the white solid in 76% yield (0.057 g). 1H NMR (400 MHz, CD2Cl2): δ 7.54 (t, 2H, 3JHH = 8 Hz, (iPr)2Ar−H), 7.34 (d, 4H, 3JHH = 8 Hz, (iPr)2Ar−H), 7.23 (s, 2H, HCCH), 6.84 (s, 1H, Ar−H), 6.19 (s, 1H, Ar−H), 5.31 (s, 2H, CH2N3), 3.43 (s, 2H, CH2SAu), 3.43 (m, 4H, 3JHH = 7 Hz), 3.03 (m, 1H), 2.87 (m, 1H), 2.68 (m, 1H), 2.61 (sept, 4H, 3JHH = 7 Hz, (CH3)2CH), 2.50 (m, 1H), 2.38 (1H, m), 2.21 (s, 3H, Ar−CH3), 1.93 (s, 3H, Ar−CH3), 1.80 (br s, 1H), 1.34 (d, 12H, 3JHH = 8 Hz, (CH3)2CH), 1.24 (d, 12H, 3JHH = 8 Hz, (CH3)2CH), 1.17 (t, 3H, 3 JHH = 7 Hz), 0.77 (m, 1H), 0.65 (m, 1H). 13C{1H} NMR (150 MHz, CD2Cl2): δ 187.4, 146.2, 145.3, 134.7, 134.0, 133.9, 132.6, 131.2, 131.0, 130.8, 128.3, 124.5, 123.4, 66.53, 66.04, 49.89, 29.14, 28.55, 28.18, 27.60, 26.63, 26.08, 24.53, 24.09, 23.24, 22.44, 22.15, 18.83, 18.77, 15.48. mp 93−97 °C. HRMS (ESI): Calcd [M] 1035.5123, found 1035.5124. Cycloadducts of 6 with DBCO−Amine (10).

To a 0.3 mL solution of 6 (0.028 g, 0.036 mmol) in CD2Cl2 was added DBCO−amine (0.010 g, 0.036 mmol) in 0.3 mL of CD2Cl2. The reaction was left for 2 h, and consumption of both starting materials was monitored by 1H NMR spectroscopy. The solvent was removed under reduced pressure, and the resulting crude solid was washed with several portions of cold Et2O to give 10 as a white solid in 89% yield (0.027g). 1H NMR (400 MHz, CD2Cl2): See Figure S6. 13 C{1H} NMR (150 MHz, CD2Cl2): 188.0, 172.3, 145.8, 143.8, 143.6, 140.3, 135.8, 134.3, 133.9, 133.6, 132.4, 131.4, 131.0, 130.7, 130.4, 130.2, 130.0, 129.9, 129.6, 129.3, 128.9, 128.1, 127.1, 127.0, 125.2, 124.1, 122.0, 52.8, 51.3, 50.4, 37.7, 29.6, 28.8, 28.2, 28.1, 24.1, 23.7, 21.1, 18.4, 18.2. HRMS (ESI): Calcd [M + 1] 1068.4637, found 1068.4649. D

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ppm) compared to the signal at 3.71 in 1. As depicted in Figure S2, the integrated 1H NMR spectra for 6−8 exhibit resonances consistent with the monomeric, 1:1 (NHC:SAr) structure of each of the coinage-metal complexes. The resonances from the methylene alpha to the azide are unaffected by coordination to any of the metals. However, there is again a notable change to those for M−SCH2−, which appears at 3.52 (Au, 6), 3.41 (Ag, 7), and 3.19 (Cu, 8) ppm. The position of these methylene protons in the 1H NMR spectrum is thus indicative of the coinage metal to which the thiolate is bonded. The resonances for the CH3−Ar and the H−Ar protons closest to the thiolate functionality in 6 (2.07 and 6.84 ppm), 7 (2.09 and 6.82 ppm), and 8 (2.08 and 6.82 ppm, respectively) also shifted upfield from 1 (2.38 and 7.07 ppm) upon coordination to the metal centers. There were also small shifts observed for CH3−Ar and the H−Ar resonances closest to the azide moiety in 6−8 versus those for the free thiol (see Figure S2). Crystals of 5−8 were obtained by layering toluene solutions with pentane. Compound 5 crystallizes in the monoclinic space group P21/c with a Z = 4 (Figure 1). Isostructural 6-8 crystallize in the triclinic space group P1̅ with Z = 2 (Figures 2 and S3−S4). The three latter samples are in fact isomorphous. Selected bond angles and lengths are listed in Table 1; values are consistent with those reported for NHC terminated complexes of Cu, Ag, and Au.3−5 The geometry around the metal atom was found to be nearly linear for all complexes, with the bond angles for S−M−C deviating greatest away from linearity from Au to Ag to Cu. The metal to carbene (M1−C1) bond distances were found to be 2.0093(14) (6) and 2.0168(16) Å (5) for Au and 2.0815(9) Å for Ag (7). Expectedly, they were observed to be markedly shorter in the case of Cu (8) at 1.8957(9) Å. The metal to sulfur (M1−S1) bond distances were 2.3243(3) Å for 7, 2.2792(6) Å and

not change. The incorporation of the azide functionality in 1 was also evident using infrared spectroscopy, with a strong, narrow band observed at 2161 cm−1 characteristic of the azide asymmetric stretch (virtually unchanged in 2).22 A broad signal at 2550 cm−1 was also observed in 1 due to the presence of the −SH.23 The synthesis of the NHC−gold−thiolate complexes [(iPr2bimy)Au-(1-SCH2-2,5-Me2-4-N3CH2-C6H2)], 5, and [(IPr)Au-(1-SCH2-2,5-Me2-4-N3CH2-C6H2)], 6, proceeded by reaction of 1, with either [(iPr2-bimy)AuCl] (to give 5) or [(IPr)AuCl] (to give 6) in the presence of Et3N (Scheme 2). The thiolates 5 and 6 can also be formed by first deprotonating the thiol with sodium hydride and subsequent reaction with the gold salts although we find the former method is more convenient. The complexes were characterized by NMR spectroscopy and X-ray crystallography, the latter confirming the monomeric nature of the complexes and the near linear coordination about Au(I) (vide infra). The Me3SiS- containing reagent 2 was found to be the most suitable for the preparation of the Ag(I) and Cu(I) complexes as deprotonating thiol 1 and reaction with [(IPr)AgX] or [(IPr)CuX] led to elimination of the NHC ligands from these complexes and the formation of insoluble precipitates. It has been demonstrated that the Cu/Ag−OAc bond can be cleaved by trimethylsilylchalcogen reagents to form complexes of the type [(IPr)AgER] (E = S, Se) and AcOSiMe3.3c,d,g,h The complexes [(IPr)Ag-(1-SCH2-2,5-Me2-4-N3CH2-C6H2)], 7, and [(IPr)Cu-(1-SCH2-2,5-Me2-4-N3CH2-C6H2)] 8, were readily purified by crystallization from mixtures of toluene:pentane in comparable yields (68% and 65% yields, respectively) The 1H NMR spectrum of 5 had a trend similar to the other NHC-metal−thiolate complexes in terms of the significant shift of the −CH2−S−M resonance (which appeared at 4.02 E

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Figure 1. Molecular structure of 5 in the crystal. Thermal ellipsoids for non-hydrogen atoms are drawn at 50% probability. Au, blue; S, yellow; N, green; C, gray; H, white.

Figure 3. Absorption spectra of 1 and 5−8 in CH2Cl2 solution.

These bands are very similar in terms of shape and position to the NHC−metal precursors and thiol 1. The additional band at 305 nm in the IPr complexes 6−8 and at 320 nm in the iPr2bimy complex 5 were tentatively assigned as arising from charge transfer transition (ArS → NHC), similar what was reported for [IPr-Au-NRR′].25 DFT calculations (@ B3LYP/Def2SVP)20 indicate that for both complexes, the HOMO is localized largely on SAr and the main contributions to the LUMO come from the π system of the respective NHC ligand (Figure 4 and Table S1). The IPr complex 6 has a fairly delocalized LUMO but with little contribution from sulfur to this electronic state. The TD-DFT results for the iPr2-bimy 5 complex indicate reasonably strong, lowest energy excitation at 414 nm, which is entirely HOMO− LUMO in nature (the SAr → NHC charge transfer). The lowest energy excitation in 6 is blue-shifted (consistent with experimental data) relative to 5 with mixed SAr → NHC and SAr π → π* character. Copper-catalyzed alkyne−azide cycloaddition (CuAAC) at the periphery of NHC ligands bound to metal ions has been demonstrated using alkynyl appended ligands on a NHC−Au− Cl complex with PhCH2N3.26 Gold-azides L−Au−N3 (L = 2 electron ligand) have also been prepared and used for cycloaddition reactions with terminal alkynes and metalalkynyl complexes.27 Veige and co-workers illustrated that L = NHC suppresses the cyclization reactions with metal-

Figure 2. Molecular structure of 8 in the crystal. Thermal ellipsoids are drawn at 50% probability; H atoms are omitted. Cu, blue; S, yellow; N, green; C, gray.

2.2901(5) Å for 6 and 5, respectively, and (2.1371(3) Å) in 8. The S−M−C bond angle was greatest in 5 (178.23(4)°) with the smaller NHC and longer C−M and M−S. Conversely, the larger IPr complexes displayed significant deviation from linear coordination about the metal, with C−M−S angles varying from 174.88(4)° in 6 to 173.00(3)° in 8. In all four complexes the intermolecular bond distances were greater than the sum of the van der Waals radii for the metals with no metallophilic interaction present.24 Figure 3 shows the electronic absorption spectra of complexes 5−8, together with that of thiol 1 in CH2Cl2 solution. The optical solution state spectra exhibit an onset at approximately 310 nm and two maxima at 285 and 290 nm.

Table 1. Selected Bond Angles (deg) and Bond Lengths (Å) for Complexes 5−8 angles S−M1−C S1−C28−C29 M1−S1−C28 C32−C35−N3 N1−C1−M1 N2−C1−M1

C1−M1 M1−S1

5

6

7

8

178.23(4) 112.03(11) 101.20(6) 111.74(19) 126.57(11) 126.43(11)

174.88(4) 115.48(11) 106.97(5) 111.90(15) 124.86(10) 130.45(10) lengths

174.46(3) 115.32(8) 106.15(4) 112.08(10) 124.94(7) 130.89(8)

173.00(3) 115.51(7) 107.94(4) 111.95(9) 124.98(7) 131.16(7)

5

6

7

8

2.0168(16) 2.2901(5)

2.0093(14) 2.2792(6)

2.0815(9) 2.3243(3)

1.8957(9) 2.1371(3)

F

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seen in the characteristic downfield shift in the 1H NMR signal of the methylene protons alpha to the azide moiety from 4.20 to 5.31 ppm after formation of the new triazole ring in cycloadduct 9.28 This is accompanied by a concomitant shift of the methylene resonance alpha to the hydroxyl group from 3.55 ppm to between 2.0 and 2.5 ppm as BCN−OH is consumed. Expectedly, the 1H NMR signal of −CH2S− showed little change on going from 6 to 9. It should be noted that while we used the exo-diastereomer of BCN−OH to simplify the NMR analysis, the same reaction proceeds with the endo-diastereomer. To further demonstrate the scope of SPAAC reactivity, 6 was also reacted with the benzoannulated, kinetically more reactive DBCO-NH2 in CD2Cl2 (2 h), resulting in the formation of cycloadduct 10 (again present as two regioisomers) which was also purified using trituration with ether. As illustrated in Figure S6, the characteristic downfield shift of the 1H NMR resonance of the methylene alpha to −N3 was again observed (4.20−5.31 ppm), indicating successful SPAAC reaction. The formation of the cycloadduct 10 was also confirmed by mass spectrometry. As illustrated in the space-filling model of 6 in Figure S7, the azide functionality appears as though it may be sterically restricted from reaction with BCN−OH and so the SPAAC reactivity of 6 was compared to that of the smaller organic azide PhCH2N3. However, the regression analysis of the 1H NMR kinetic analysis of both 6 and benzyl azide with BCN− OH (Figure S8) yields a second order rate constant between 6 and BCN−OH of 0.031 M−1 s−1, which is very similar to that determined for the smaller, yet electronically similar, PhCH2N3 (0.035 M−1 s−1) under identical reaction conditions.

Figure 4. Representation of the frontier orbitals for complexes 6 (left) and 5 (right). The HOMO is localized largely on the thiolate ligands and the main contributions to the LUMO come from the π* system of the respective NHC ligand.

alkynyls because of the lack of ligand dissociation required for alkynyl activation.27a With 5−8, the azide moiety on the coordinated thiolate also offers capability for alkyne−azide cycloaddition reactions. Here, we explored copper-free strainpromoted cycloaddition between complex 6 and two different strained-alkyne moieties to illustrate the general reactivity of our azide-modified metal-arylthiolates (Scheme 3). First, 6 was reacted with the aliphatic, kinetically less reactive BCN−OH in CD2Cl2 for 12 h, after which residual starting material was removed by trituration in ether to give cycloadduct, 9 (present as two regioisomers). The successful formation of 9 was confirmed by mass spectrometry. As illustrated in Figure S5, the expected changes to the 1H NMR signals of the newly formed cyclooctene ring in 9 in the 0.5−3.0 ppm region were observed,28 which is a key indication of a cycloaddition reaction between the azide-terminated complex and BCN− OH. Additional confirmation of the SPAAC reaction can be

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CONCLUSION A set of substitution reactions on a dibrominated template produced a “clickable” thiol that can be incorporated onto coinage metals as a thiolate ligand. Because of the relative inertness of the azide moiety, it was unaltered during the preparation of metal complexes, providing a facile approach to

Scheme 3

G

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Henkel, G. Transition-Metal Thiolates: From Molecular Fragments of Sulfidic Solids to Models for Active Centers in Biomolecules. Angew. Chem., Int. Ed. Engl. 1991, 30, 769−788. (2) Rungthanaphatsophon, P.; Barnes, C. L.; Walensky, J. R. Copper(I) Clusters with Bulky Dithiocarboxylate, Thiolate, and Selenolate Ligands. Dalton Trans. 2016, 45, 14265−14276. (3) (a) Delp, S. A.; Munro-Leighton, C.; Goj, L. A.; Ramirez, M. A.; Gunnoe, T. B.; Petersen, J. L.; Boyle, P. D. Addition of S-H Bonds across Electron-Deficient Olefins Catalyzed by Well-Defined Copper(I) Thiolate Complexes. Inorg. Chem. 2007, 46, 2365−2367. (b) Wu, J.; Gu, Y.; Leng, X.; Shen, Q. Copper-Promoted Sandmeyer Difluoromethylthiolation of Aryl and Heteroaryl Diazonium Salts. Angew. Chem., Int. Ed. 2015, 54, 7648−7652. (c) Partyka, D. V.; Deligonul, N. Phosphine- and Carbene-Ligated Silver Acetate: EasilyAccessed Synthons for Reactions with Silylated Nucleophiles. Inorg. Chem. 2009, 48, 9463−9475. (d) Azizpoor Fard, M.; Levchenko, T. I.; Cadogan, C.; Humenny, W. J.; Corrigan, J. F. Stable -ESiMe3 Complexes of Cu(I) and Ag(I) (E = S, Se) with NHCs: Synthons in Ternary Nanocluster Assembly. Chem. - Eur. J. 2016, 22, 4543−4550. (e) Ferrara, S. J.; Wang, B.; Haas, E.; Wright LeBlanc, K.; Mague, J. T.; Donahue, J. P. Synthesis and Structures of [LCu(I)(SSiiPr3)] (L = Triphos, Carbene) and Related Compounds. Inorg. Chem. 2016, 55, 9173−9177. (f) Zhai, J.; Filatov, A. S.; Hillhouse, G. L.; Hopkins, M. D. Synthesis, Structure, and Reactions of a Copper-Sulfido Cluster Comprised of the Parent Cu2S Unit: {(NHC)Cu}2(μ-S). Chem. Sci. 2016, 7, 589−595. (g) Zhai, J.; Hopkins, M. D.; Hillhouse, G. L. Synthesis and Structure of a Cu(I)3S Cluster Unsupported by Other Bridging Ligands. Organometallics 2015, 34, 4637−4640. (h) Fard, M. A.; Weigend, F.; Corrigan, J. F. Simple but Effective: Thermally Stable Cu-ESiMe3 via NHC Ligation. Chem. Commun. 2015, 51, 8361− 8364. (4) (a) Humenny, W. J.; Mitzinger, S.; Khadka, C. B.; Najafabadi, B. K.; Vieira, I.; Corrigan, J. F. NHC Stabilized Copper- and SilverPhenylchalcogenolate Clusters. Dalton Trans. 2012, 41, 4413−4422. (b) Khalili Najafabadi, B.; Corrigan, J. F. N-Heterocyclic Carbenes as Effective Ligands for the Preparation of Stabilized Copper- and Silvert-Butylthiolate Clusters. Dalton Trans. 2014, 43, 2104−2111. (5) See, for example: Baker, M. V.; Barnard, P. J.; Berners-Price, S. J.; Brayshaw, S. K.; Hickey, J. L.; Skelton, B. W.; White, A. H. Synthesis and Structural Characterization of Linear Au(I) N-Heterocyclic Carbene Complexes: New Analogues of the Au(I) Phosphine Drug Auranofin. J. Organomet. Chem. 2005, 690, 5625−5635. (6) Gobbo, P.; Mossman, Z.; Nazemi, A.; Niaux, A.; Biesinger, M. C.; Gillies, E. R.; Workentin, M. S. Versatile Strained Alkyne Modified Water-Soluble AuNPs for Interfacial Strain Promoted Azide-Alkyne Cycloaddition (I-SPAAC). J. Mater. Chem. B 2014, 2, 1764−1769. (7) (a) McKay, C. S.; Finn, M. G. Click Chemistry in Complex Mixtures: Bioorthogonal Bioconjugation. Chem. Biol. 2014, 21, 1075− 1101. (b) Sletten, E. M.; Bertozzi, C. R. Bioorthogonal Chemistry: Fishing for Selectivity in a Sea of Functionality. Angew. Chem., Int. Ed. 2009, 48, 6974−6998. (8) (a) Agard, N. J.; Prescher, J. A.; Bertozzi, C. R. A StrainPromoted [3 + 2] Azide−Alkyne Cycloaddition for Covalent Modification of Biomolecules in Living Systems. J. Am. Chem. Soc. 2004, 126, 15046−15047. (b) Dommerholt, J.; Rutjes, F. P. J. T.; van Delft, F. L. Strain-Promoted 1,3-Dipolar Cycloaddition of Cycloalkynes and Organic Azides. Top. Curr. Chem. 2016, 374, 16. (c) Patterson, D. M.; Nazarova, L. A.; Prescher, J. A. Finding the Right (Bioorthogonal) Chemistry. ACS Chem. Biol. 2014, 9, 592−605. (9) Jothibasu, R.; Huynh, H. V.; Koh, L. L. Au(I) and Au(III) complexes of a Sterically Bulky Benzimidazole-derived N-Heterocyclic Carbene. J. Organomet. Chem. 2008, 693, 374−380. (10) Visbal, R.; Laguna, A.; Gimeno, M. C. Simple and Efficient Synthesis of [MCl(NHC)] (M = Au, Ag) Complexes. Chem. Commun. 2013, 49, 5642−5644. (11) Wong, V. H.; Vummaleti, S. V.; Cavallo, L.; White, A. J.; Nolan, S. P.; Hii, K. K. Synthesis, Structure and Catalytic Activity of NHCAg(I) Carboxylate Complexes. Chem. - Eur. J. 2016, 22, 13320− 13327.

generating such functionalized ligands. Four mononuclear, linear coinage-metal complexes were prepared containing Cu(I), Ag(I), and Au(I) centers with ancillary NHC ligands. X-ray crystallography confirmed the terminal coordination of the azide-terminated thiolate to the NHC-metal centers. The reactivity of the −CH2N3 moiety toward cycloaddition with strained alkynes was demonstrated with one of the complexes. The second order rate constant for the formation of the cycloadduct formed via reaction with BCN−OH was determined to be similar to that of the model azide PhCH2N3.

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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.8b01750. 1 H NMR spectra of 1−4 and 6−10, molecular structures of 6 and 7 in the crystal, space filling model of 6 in the crystal, TDDFT (B3LYP/Def2SVP) excitation energies and oscillator strengths for the lowest symmetry-allowed singlet excitations of 5 and 6, and regression analysis of estimated second order rate kinetics from 1H NMR spectra between BCN−OH and 6 and benzyl azide (PDF) Accession Codes

CCDC 1548144−1548147 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 Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Mark S. Workentin: 0000-0001-8517-6483 John F. Corrigan: 0000-0003-2530-5890 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Natural Sciences and Engineering Research Council Canada (NSERC) for supporting this research. NSERC, the Government of Ontario, The University of Western Ontario and the Canada Foundation for Innovation are each thanked for equipment funding. A.M.P. thanks the NSERC for a postgraduate scholarship. The authors thank Dr. Paul D. Boyle (Western) for assistance with the refinement of the structure of 5.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01750 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01750 Inorg. Chem. XXXX, XXX, XXX−XXX