Six-Membered Janus-type Ditopic N-Heterocyclic Carbene Coinage

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Six-Membered Janus-type Ditopic N‑Heterocyclic Carbene Coinage Metal Complexes Zejun Hu,#,† Xufeng Ma,#,† Jiwei Wang,† Han Wang,† Xiaoyan Han,‡ Min Shi,†,§ and Jun Zhang*,† †

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Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China University of Science and Technology, 130 Mei Long Road, Shanghai 200237, China ‡ Testing and Analysis Center, Soochow University, Suzhou 215123, China § State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, China S Supporting Information *

ABSTRACT: Homo (Au2)- and heterodinuclear coinage metal complexes (AuAg) ligated by a six-membered Janustype ditopic N-heterocyclic carbene (NHC) have been prepared by deprotonation of aNHC (abnormal NHC) gold complex followed by complexation. The two ditopic NHC coinage metal complexes were structurally characterized by single crystal X-ray diffraction. The carbene character for the C2 carbon and C5 carbon of the ditopic NHC ligand in 3 and 4 was confirmed by the NMR and structural data.

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INTRODUCTION

Over the past decade, N-heterocyclic carbene (NHC) has emerged as one of the most important ligands in organometallic complexes, mainly due to its structural diversity, wide variety of coordination modes, and its capability to form stable complexes with a large number of metals.1 Recently, there has been an increased interest in exploration of unusual coordination geometries of NHC ligands. Dicarbenes incorporating two carbene centers in the same heterocycle ring has caused considerable interest, as they create tunable molecular scaffolds that can bridge transition metals.2 Bertrand and co-workers first reported a Janus-type diNHC ligand 1,2,4-trimethyltriazolidine-3,5-diylidene (ditz) as a building block for organometallic polymer synthesis (A, Figure 1).3 Subsequently, the homo- and heterodimetallic complexes based on the ditz ligand have also been successfully employed by the group of Peris in a range of challenging catalytic tandem reactions.4 Robinson et al. reported a Janus-type ditopic carbene bearing both normal (C2) and abnormal carbene (C4) centers in the same imidazole ring (B, Figure 1), bridging two lithium metals.5 To date, the ditopic NHCs of type B have been welldeveloped to synthesize a wide range of bimetallic complexes containing two main-group elements6 or one main-group element and one transition metal7 and those featuring two transition metal centers.8 A heterobimetallic Pd−Ru complex containing an NHDC ligand of type B has also been employed as efficient catalyst in tandem reactions.9 Recently, we have developed a new class of six-membered NHC ligands with a pyrimidinone core, and their potential application in coordination chemistry has been studied.10 We also reported the direct synthesis of gold complex D ligated © XXXX American Chemical Society

Figure 1. Known complexes ligated by five-membered ditopic NHCs A and B and the targeted six-membered ditopic NHC C, and related aNHC gold complex D.

by the six-membered abnormal N-heterocyclic carbine (aNHC) with a pyrimidinone core through the goldmediated cyclization of N-propiolic formamidines (D, Figure 1).10a We envisioned that the acidic nature of the C2−H proton in D could allow access to the Janus-type ditopic NHCs of type C. Gratifyingly, deprotonation of an aNHC complex of type D allowed us to synthesize for the first time homo- and heterodinuclear coinage metal complexes C ligated by a six-membered Janus-type ditopic NHC. Received: February 25, 2019

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DOI: 10.1021/acs.organomet.9b00124 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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RESULTS AND DISCUSSION We first synthesized aNHC gold complex 2, an analogue of D, by gold-promoted cyclization of N-propiolic formamidine 1, according to a synthetic method10a for complex D (Scheme 1). The 1H NMR spectrum of complex 2 shows a Scheme 1. Synthesis of Ditopic NHC Au2 Complex 3 from Complex 2

Figure 2. Molecular structure of 3 with 40% probability. H atoms in aryl rings have been omitted for clarity. Selected bond distances (Å): Au1−C2 2.074(7), Au2−C5 2.051(8), O1−C6 1.202(9), N3− C2 1.336(10), N3−C4 1.401(9), N1−C2 1.355(10), N1−C6 1.426(10), C5−C6 1.465(11), C4−C5 1.328(10). Selected bond angles [deg]: C(2)−Au(1)−P(1) 178.5(2); C(5)−Au(2)−P(2) 177.1(2); C(2)−N(3)−C(4) 121.2(6); C(2)−N(1)−C(6) 122.9(6); N(3)−C(2)−Au(1) 121.6(5); N(3)−C(2)−N(1) 119.1(6); N(1)−C(2)−Au(1) 119.3(5); N(1)−C(6)−C(5) 114.9(6); O(1)−C(6)−N(1) 118.1(7); O(1)−C(6)−C(5) 127.0(7); C(6)−C(5)−Au(2) 117.0(5); C(4)−C(5)−Au(2) 123.7(6); C(4)−C(5)−C(6) 119.0(7); C(5)−C(4)−N(3) 122.0(7).

characteristic signal at 8.29 ppm for the azolium C−H proton. The upfield shift of the formamidine (C2) proton from 10.10 ppm in the corresponding 6-membered formamidinium salt10c to 8.29 ppm in 2 was observed, indicative of the loss of cationic character. Similar to its methyl analogue, an abnormal carbene (aNHC) complex,10a the resonance for the remote carbon C5 complex 2 appears at 154.0 ppm, more than 20 ppm downfield relative to that for the corresponding 6-membered formamidinium salt. Deprotonation of 2 followed by metalation provided Janustype ditopic NHC complex 3 (Au2) in 53% yield. Our synthetic methodology was also used for the synthesis of ditopic NHC-based heterodinuclear coinage complex, giving the Au/Ag complex 4 (AuAg) in 66% yield (Scheme 2). Both Scheme 2. Synthesis of Ditopic NHC Au/Ag Complex 4 from Complex 2

3 and 4 are air stable and could be purified by column chromatography. The identity of 3 and 4 is ascertained through the lack of the azolium C−H proton resonance in their 1H NMR spectra. The new Janus-type ditopic NHC complexes 3 and 4 were characterized by X-ray diffraction (Figure 2 and Figure 3). In 3, the Au−C bond length at the normal C2 position (2.074(7) Å) is typical of NHC-Au complexes11 and slightly longer than that at the remote C5 position (2.051(8) Å) (Figure 4). The Au−C5 bond (2.063(3) Å) in 4 is similar to that in 3, and the Ag−C bond distance (2.087(3) Å) in 4 is in the typical range for NHC-Ag complexes.1h Different from the 5-membered ditopic NHCs of type A and type B, both 6membered 3 and 4 consist two linearly diametrically opposed

Figure 3. Molecular structure of heterometallic 4 with 40% probability. H atoms in aryl rings have been omitted for clarity. Selected bond distances (Å): Au1−C5 2.063(3), Ag1−C2 2.087(3), O1−C6 1.218(4), N3−C2 1.342(4), N3−C4 1.421(4), N1−C2 1.360(4), N1−C6 1.429(4), C5−C6 1.426(5), C4−C5 1.355(5). Selected bond angles [deg]: C(5)−Au(1)−P(1) 173.12(10); C(2)− Ag(1)−Cl(1) 177.04(11); C(2)−N(3)−C(4) 123.3(3); C(2)− N(1)−C(6) 125.4(3); C(6)−C(5)−Au(1) 121.0(3); C(4)−C(5)− Au(1) 119.8(3); C(4)−C(5)−C(6) 119.2(3); N(3)−C(2)−Ag(1) 123.7(2); N(3)−C(2)−N(1) 115.7(3); N(1)−C(2)−Ag(1) 120.6(2); O(1)−C(6)−N(1) 117.4(3); O(1)−C(6)−C(5) 126.7(4); C(5)−C(6)−N(1) 115.8(3); C(5)−C(4)−N(3) 120.5(3). B


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MHz spectrometer. Chemical shifts are reported in ppm from tetramethylsilane with the solvent resonance as the internal standard (1H NMR CDCl3: 7.26 ppm; 13C NMR CDCl3: 77.0 ppm). Mass spectra were recorded on the HP-5989 instrument by EI/ESI methods. X-ray diffraction analyses were performed by using a Bruker APEX-II CCD X-ray diffractometer. Compound 1 was prepared according to the literature method.10c Synthesis of Gold Complex 2. A mixture of 1 (100 mg, 0.20 mmol) and AuCl·SMe2 (60 mg, 0.23 mmol) was stirred in the DCE (2.5 mL) at 80 °C for 20 min. All volatiles were removed under vacuum, the resultant solid was dissolved in DCM, and filtered through a pad of Celite, and the rude product was washed twice with diethyl ether to afford pure 2 as a yellow solid (136 mg, 94%). 1 H NMR (400 MHz, CDCl3) δ = 8.29 (s, 1H, NC(H)N), 7.52 (t, J = 7.8 Hz, 1H, HAr), 7.41 (d, J = 8.0 Hz, 2H, HAr), 7.36 (t, J = 7.8 Hz, 1H, HAr), 7.33 (d, J = 7.8 Hz, 2H, HAr), 7.25−7.21 (m, 1H, HAr), 7.20−7.15 (m, 2H, HAr), 7.13 (d, J = 8.0 Hz, 2H, HAr), 2.90−2.81 (m, 2H, CH(CH3)2), 2.72−2.62 (m, 2H, CH(CH3)2), 1.26 (d, J = 6.8 Hz, 12H, CH3), 1.23 (d, J = 6.8 Hz, 6H, CH3), 1.14 (d, J = 6.8 Hz, 6H, CH3); 13C{1H} NMR (100 MHz, CDCl3) δ = 160.14 (s, CO), 154.00 (s, CC(Au)C), 149.38 (s, C−N), 149.01 (s, C−N), 145.05 (s, N−CAr), 144.33 (s, N-CAr), 134.95 (s, CAr), 133.99 (s, CAr), 132.08 (s, CAr), 131.57 (s, CAr), 131.05 (s, CAr), 129.61 (s, CAr), 129.55 (s, CAr), 127.55 (s, CAr), 124.98 (s, CAr), 124.79 (s, CAr), 29.62 (s, CH(CH3)2), 29.16 (s, CH(CH3)2), 26.30 (s, CH3), 24.92 (s, CH3), 24.22 (s, CH3), 23.62 (s, CH3), 22.26 (s, CH3). Anal. Calcd for C34H40AuClN2O(0.75 CH2Cl2): C, 52.91; H, 5.30; N, 3.55; found: C, 53.24; H, 5.48; N, 3.23; HRMS (MALDI) m/z [M − Cl]+ calcd. for C34H40AuN2O+: 689.2806; found: 689.2831. Synthesis of Digold Complex 3. In a nitrogen-filled roundbottom flask, a solution of 2 (200 mg, 0.28 mmol) in THF (5 mL) was cooled to −78 °C, and KHMDS (1 M in THF, 308 μL, 1.1 equiv) was added dropwise. The yellow solution was stirred for 30 min, AuCl·SMe2 (82 mg, 0.28 mmol) and triphenylphosphine (73 mg, 0.28 mmol) were added. After the mixture was stirred for 1 h at −78 °C, the cooling bath was removed, and the solution was warmed to room temperature. After 3 h at room temperature, all volatiles were evaporated in vacuo to afford the crude product. The latter was dissolved in DCE (6 mL), AgOTf (PPh3) (154 mg, 0.28 mmol) was added, and the mixture was stirred at 25 °C for 2 h. All volatiles were removed under vacuum, the resulting solid was dissolved in DCM, and the solution was filtered through a pad of Celite and then purified by column chromatography using silica gel (v/v, DCM/EtOH = 20:1) to afford pure 3 as a yellow solid (231 mg, 53%). 1H NMR (400 MHz, CDCl3) δ = 7.57−7.43 (m, 8H, HAr), 7.43−7.32 (m, 15H, HAr), 7.32−7.27 (m, 5H, HAr), 7.25− 7.18 (m, 3H, HAr), 7.18−7.09 (m, 4H, HAr), 6.80 (dd, J = 13.2, 8.0 Hz, 6H, HAr), 2.95−2.87 (m, 2H, CH(CH3)2), 2.87−2.77 (m, 2H, CH(CH3)2), 1.27 (d, J = 6.4 Hz, 6H, CH3), 1.18 (dd, J = 13.2, 6.8 Hz, 12H, CH3), 1.11 (d, J = 6.8 Hz, 6H, CH3); 13C{1H} NMR (100 MHz, CDCl3) δ = 206.78 (d, 2JC−P = 117.2 Hz, NC(Au) N), 163.17 (s, CO), 158.70 (s, Ph−C−N), 157.50 (d, 2JC−P = 109.8 Hz, CC(Au)C), 145.55 (s, N-CAr), 145.15 (s, N-CAr), 138.23 (s, CAr), 136.50 (s, CAr), 134.03 (d, 2JC−P = 13.6 Hz, PPh3), 133.51 (d, 2JC−P = 13.6 Hz, PPh3), 132.29 (s, CAr), 131.49 (s, CAr), 131.08 (s, CAr), 130.26 (s, CAr), 129.85 (s, CAr), 129.27 (d, 2JC−P = 11.6 Hz, PPh3), 129.05 (d, 2JC−P = 11.2 Hz, PPh3), 127.49 (s, CAr), 127.43 (s, CAr), 126.58 (s, CAr), 124.97 (s, CAr), 124.50 (s, CAr), 28.95 (s, CH(CH3)2), 28.89 (s, CH(CH3)2), 26.42 (s, CH3), 24.76 (s, CH3), 23.87 (s, CH3), 23.03 (s, CH3); 31P NMR (162 MHz, CDCl 3 ) δ = 41.10, 37.16. Anal. Calcd for C71H69Au2F3N2O4P2S: C, 54.69; H, 4.46; N, 1.80; found: C, 54.40; H, 4.56; N, 1.70; HRMS (MALDI) m/z [M − OTf]+ calcd. for C70H69Au2N2OP2+: 1409.4216; found: 1409.4242. Synthesis of Heterodinuclear Gold-Sliver Complex 4. In a nitrogen-filled round-bottom flask, a solution of 2 (200 mg, 0.28 mmol) in THF (5 mL) was cooled to −78 °C, and KHMDS (1 M in THF, 308 μL, 1.1 equiv) was added dropwise. The yellow solution was stirred for 30 min, and AgOTf(PPh3) (145 mg, 0.28

Figure 4. Comparative metrical data in 3 and 4.

carbenes. So far, most of Janus-type bis-NHC ligands are those containing two NHC moieties linked by a rigid πconjugated system,12 such as benzobis(imidazolylidene). The intermetallic distances in the two complexes are 6.920 (Au− Au for 3) and 6.994 Å (Au−Ag for 4), respectively. Using NMR data, we could evaluate the degree of carbene character of the donor atoms in the new ditopic NHC ligand in 3 and 4. The 13C signal for the carbon C2 in 3 shows a doublet at 206.8 ppm with 2JP−C = 117.2 Hz in the usual range for normal NHC metal complexes.1h The resonance for the remote carbon C5 appears at 157.5 ppm with 2JP−C = 109.8 Hz, more than 49 ppm upfield relative to the C2 resonance. These data support the normal NHC character for the C2 carbon of the ditopic NHC ligand in 3 (see below resonance forms I−III in Figure 5). A similar trend in

Figure 5. Various mesomeric structures for the ditopic NHC complexes 3 and 4.

chemical shifts is also observed when the 13C signal of the carbon C2 of 4 is compared with that for the C5 carbon. Thus, in complex 4, the 13C signal for the carbon C2 shows at 209.1 ppm with 1JAg−C = 232.5 Hz, and that for the remote C5 position at 155.5 ppm with 2JP−C = 110.3 Hz. NMR and structural data indicate that I is the major contributing resonance structure in the ditopic NHC complexes (Figure 5).

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CONCLUSIONS We have prepared homo (Au2)- and heterodinuclear coinage metal complexes (AuAg) ligated by the first six-membered ditopic NHC by deprotonation of aNHC gold complex and structurally characterized by single-crystal X-ray diffraction. The carbene character of the novel ditopic NHC was confirmed by the NMR and structural data. Future work will be aimed at synthesizing other dinuclear transition-metal complexes supported by the ditopic NHC and investigating the applications of the novel ditopic NHC ligands in transition-metal catalyzed reaction.

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EXPERIMENTAL SECTION

General Methods. Unless otherwise stated, all reactions and manipulations were performed using standard Schlenk techniques. All solvents were purified by distillation using standard methods. Commercially available reagents were used without further purification. NMR spectra were recorded by using a Bruker 400 C


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Organometallics mmol) was added. After the mixture was stirred for 1 h at −78 °C, the cooling bath was removed, and the solution was warmed to room temperature. After 3 h at room temperature, all volatiles were evaporated in vacuo, and the residue was purified by column chromatography using silica gel (v/v, PE/EtOAc = 5:1) to afford 4 as a yellow solid (202 mg, 66%). 1H NMR (400 MHz, CDCl3) δ = 7.49−7.41 (m, 5H, HAr), 7.41−7.33 (m, 7H, HAr), 7.33−7.26 (m, 9H, HAr), 7.16 (t, J = 7.4 Hz, 1H, HAr), 7.11−7.03 (m, 4H, HAr), 2.96−2.85 (m, 4H, CH(CH3)2), 1.34 (dd, J = 10.2, 6.8 Hz, 12H, CH3), 1.27 (d, J = 6.8 Hz, 6H, CH3), 1.17 (d, J = 6.8 Hz, 6H, CH3); 13C{1H} NMR (100 MHz, CDCl3) δ = 209.14 (d, 1JC−Ag = 232.5 Hz, NC(Ag)N), 163.73 (d, 3JC−Ag = 5.7 Hz, CO), 158.37 (dd, 3JC−P = 8.1 Hz, 3JC−Ag = 2.4 Hz, Ph−C−N), 155.50 (d, 2 JC−P = 110.3 Hz, CC(Au)C), 145.99 (s, CAr), 144.50 (s, NCAr), 139.97 (s, CAr), 138.46 (s, CAr), 137.81 (s, CAr), 134.08 (d, 2 JC−P = 13.9 Hz, PPh3), 131.26 (s, CAr), 130.42 (s, CAr), 130.19 (s, CAr), 129.89 (s, CAr), 129.85 (s, CAr), 129.66 (s, CAr), 128.94 (d, 2 JC−P = 11.2 Hz, PPh3), 128.46 (s, CAr), 127.10 (s, CAr), 124.65 (s, CAr), 124.04 (s, CAr), 28.71 (s, CH(CH3)2), 26.57 (s, CH3), 24.74 (s, CH3), 24.20 (s, CH3), 22.95 (s, CH3); 31P NMR (162 MHz, CDCl3) δ = 41.45. Anal. Calcd For C52H54AgAuClN2OP (CH2Cl2): C, 53.98; H, 4.79; N, 2.38; found: C, 54.14; H, 4.87; N, 2.35; HRMS (MALDI) m/z [M − Cl]+ calcd. for C52H54AgAuN2OP+: 1057.2690; found: 1057.2676.

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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.organomet.9b00124. Full experimental details, including NMR spectra for new compounds, and X-ray crystal structure data (PDF) Accession Codes

CCDC 1514557−1514558 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]. ORCID

Min Shi: 0000-0003-0016-5211 Jun Zhang: 0000-0002-4558-4391 Author Contributions #

ZH and XM contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (21171056 and 21671066) is greatly acknowledged.

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REFERENCES

(1) (a) Arduengo, A. J. Looking for Stable Carbenes: The Difficulty in Starting Anew. Acc. Chem. Res. 1999, 32, 913−921. (b) Garrison, J. C.; Youngs, W. J. Ag(I) N-Heterocyclic Carbene Complexes: Synthesis, Structure, and Application. Chem. Rev. 2005, D


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Organometallics Tritopic Carbanionic N-Heterocyclic Dicarbene and Its Homo- and Heterometallic Coinage Metal Complexes. Chem. Commun. 2018, 54, 5736−5739. (11) (a) De Frémont, P.; Scott, N. M.; Stevens, E. D.; Nolan, S. P. Synthesis and Structural Characterization of N-Heterocyclic Carbene Gold(I) Complexes. Organometallics 2005, 24, 2411−2418. (b) Ai, P.; Mauro, M.; De Cola, L.; Danopoulos, A. A.; Braunstein, P. A Bis(Diphosphinyl N-Heterocyclic Carbene) Gold Complex: A Synthon for Luminescent Rigid AuAg2 Arrays and Au5 and Cu6 Double Arrays. Angew. Chem., Int. Ed. 2016, 55, 3338−3341. (12) (a) Boydston, A. J.; Williams, K. A.; Bielawski, C. W. A Modular Approach to Main-Chain Organometallic Polymers. J. Am. Chem. Soc. 2005, 127, 12496−12497. (b) Khramov, D. M.; Boydston, A. J.; Bielawski, C. W. Synthesis and Study of Janus Bis(carbene)s and Their Transition-Metal Complexes. Angew. Chem., Int. Ed. 2006, 45, 6186−6189. (c) Tennyson, A. G.; Ono, R. J.; Hudnall, T. W.; Khramov, D. M.; Er, J. A. V.; Kamplain, j. w.; Lynch, V. M.; Sessler, J. L.; Bielawski, C. W. Quinobis(imidazolylidene): Synthesis and Study of an Electron-Configurable Bis(N-Heterocyclic Carbene) and Its Bimetallic Complexes. Chem. Eur. J. 2010, 16, 304−315. (d) Tennyson, A. G.; Rosen, E. L.; Collins, M. S.; Lynch, V. M.; Bielawski, C. W. Bimetallic NHeterocyclic Carbene-Iridium Complexes: Investigating Metal-Metal and Metal-Ligand Communication via Electrochemistry and Phosphorescence Spectroscopy. Inorg. Chem. 2009, 48, 6924−6933.

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Six-Membered Janus-type Ditopic N-Heterocyclic Carbene Coinage

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