A N-Heterocyclic Carbene-Stabilized Coinage Metal-Chalcogenide

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A N‑Heterocyclic Carbene-Stabilized Coinage Metal-Chalcogenide Framework with Tunable Optical Properties Alexander M. Polgar,† Florian Weigend,‡,§ Angel Zhang,† Martin J. Stillman,† and John F. Corrigan*,†,⊥ †

Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada Institut für Nanotechnologie, Karlsruher Institut für Technologie, Hermann-von Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany § Institut für Physikalische Chemie, Karlsruher Institut für Technologie, Kaiserstr. 12, 76131 Karlsruhe, Germany ⊥ Centre for Advanced Materials and Biomaterials Research, The University of Western Ontario, London, Ontario N6A 3K7, Canada ‡

S Supporting Information *

phosphines.8 This is in stark contrast to other areas of gold chemistry, where N-heterocyclic carbene (NHC) ligands rival phosphines as the ligand of choice for applications in catalysis,9 medicine,10 supramolecular assembly,11 surface chemistry,12 and recently nanocluster chemistry.13 NHC−Au(I) complexes demonstrate fascinating and tunable optical effects such as mechano-, iono-, and vapochromism.14 Furthermore, reports have emerged recently where NHCs stabilize chalcogenide clusters of other d-block metals.15 Herein, we promote a fusion of the fields of luminescent gold(I) chalcogenide clusters and NHC−Au(I) complexes, by using gold(I) trimethylsilylchalcogenolate reagents [(IPr)AuESiMe3] (E = S (1a), Se (1b), Te (1c); IPr = 1,3-bis(2,6diisopropylphenyl)imidazole-2-ylidene) as precursors to homoand heterometallic clusters with NHC ancillary ligands. Treatment of these synthons with [LM(OAc)] (L = Ph3P, (ArO)3P; M = Ag, Au) led to the formation of the isostructural octanuclear clusters [Au4Ag4(μ3-E)4(IPr)4] (E = S (2a), Se (2b)) and [Au8(μ3-E)4(IPr)4] (E = S (3a), Se (3b), Te (3c)). The enhanced σ-donating ability of NHCs compared to phosphines conferred exceptional stability to the silylchalcogenolate precursors and resulting cluster framework. This allowed for extensive tuning of the metal/chalcogen ion composition, with a marked shift in the luminescence upon controlled variation of E and M. The present work demonstrates that NHCs have a special role in the continued development of gold(I) chalcogenide chemistry and its application toward new light emitting metal complexes. The NHC-stabilized gold trimethylsilylchalcogenolates 1a−c were prepared by salt metathesis of [(IPr)AuCl] with Li[ESiMe3] in tetrahydrofuran (THF; Scheme 1). Repeated crystallization attempts produced fragile and rapidly desolvating crystals unsuitable for diffraction analysis. The molecular structures were thus confirmed by 1D and 2D multinuclear NMR spectroscopy and elemental analysis (Supporting Information). Given the propensity for gold(I) to adopt linear, terminally coordinated species, we propose mononuclear complexes with gold(I) ligated to one IPr ligand and one E(SiMe3). Compared to previous metal trimethylsilylchalcogenolate reagents,15b,c,16 complexes 1a−c exhibit excep-

ABSTRACT: A new class of coinage-metal chalcogenide compounds [Au4M4(μ3-E)4(IPr)4] (M = Ag, Au; E = S, Se, Te) has been synthesized from the combination of Nheterocyclic carbene-ligated gold(I) trimethylsilylchalcogenolates [(IPr)AuESiMe3] and ligand-supported metal acetates. Phosphorescence is observed from these clusters in glassy 2-methyltetrahydrofuran and in the solid state at 77 K, with emission energies that depend on the selection of metal/chalcogen ion composition. The ability to tune the emission is attributed to electronic transitions of mixed ligand-to-metal-metal-charge-transfer (IPr → AuM2) and interligand (IPr → E) phosphorescence character, as revealed by time-dependent density functional theory computations.N-heterocyclic carbenes (NHCs) have been applied as ancillary ligands in the synthesis of luminescent gold(I) chalcogenide clusters and this approach allows for unprecedented selectivity over the metal and chalcogen ions present within a stable octanuclear framework.

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ver the past 50 years, complexes of gold in the +1 oxidation state have been studied for their rich photophysical properties.1 Early crystallographic studies of linear Au(I) complexes revealed intermolecular aggregation of Au+ centers into supramolecular chains and sheets through what is now understood as a dispersive d10−d10 attraction strengthened by relativistic effects, which has been termed “aurophilicity”.2 It was soon recognized that this interaction is intimately connected to photoluminescence in Au(I) complexes.3 It was also shown that aurophilicity could exist intramolecularly in the clustering of Au+ centers about main group donor atoms.4 Of these, gold(I) chalcogenide clusters have demonstrated particular promise for their stability, synthetic accessibility, and intense photoluminescence.5 When combined with phosphine ancillary ligands, gold(I) chalcogenides form clusters typically based on [(μ3-E)Au3]+ units aggregated through aurophilic interactions.6 By varying the steric and electronic parameters of the stabilizing phosphine, clusters of varying size and geometry have been prepared.7 Curiously, there have been very few reports on gold(I) chalcogenide clusters with ancillary ligands other than © 2017 American Chemical Society

Received: August 23, 2017 Published: September 27, 2017 14045

DOI: 10.1021/jacs.7b09025 J. Am. Chem. Soc. 2017, 139, 14045−14048

Communication

Journal of the American Chemical Society Scheme 1. Synthesis of 1a−ca

and residing about a crystallographic 4̅ center (Figures 1 and S4). The molecules consist of a central {M4E4} cycle (M = Ag

a

See the Supporting Information for detailed experimental conditions. THF = tetrahydrofuran. Dipp = 2,6-diisopropylphenyl.

tional stability in solution, as shown by monitoring in CDCl3 at room temperature with 1H NMR spectroscopy (Supporting Information). Both 1a and 1b show negligible degradation after 2 weeks in solution. 1c is, expectedly, less stable but decomposes only after several days to produce Te(SiMe3)2 and insoluble polynuclear gold-tellurides. Stoichiometric reactions of 1a with a suspension of AgOAc at −78 °C resulted in instant darkening of the reaction solutions, and ultimately production of intractable mixtures. The reaction was subsequently performed via ligation of AgOAc with a tertiary phosphine to retard uncontrolled cluster growth. The weak σ donor PPh3 was chosen so that the phosphine would not be present in the final product bound to Ag and may be removed by recrystallization. Thus, reaction of 1a and 1b with [(Ph 3 P)AgOAc] yielded the ternary Au-E-Ag clusters [Au4Ag4(μ3-S)4(IPr)4] (2a) and [Au4Ag4(μ3-Se)4(IPr)4] (2b), respectively, as the only crystalline materials (Scheme 2). In a

Figure 1. Molecular structures of [Au4Ag4S4(IPr)4] 2a (left) and [Au8Te4(IPr)4] (3c) (right) in the crystal (50% thermal ellipsoids for core atoms; all protons have been omitted for clarity). Molecules of 2a reside about a crystallographic 4̅ center.

(2a,b), Au (3a,b)) with the four “M” atoms close to planarity (torsion angles between 0.63° and 1.47°), and the chalcogen atoms lying alternately above and below this plane. Each chalcogen atom is ligated to two M and one Au. In 2a and 2b, these arrangements result in unprecedented [Ag2Au(μ3-S)]+ and [Ag2Au(μ3-Se)]+ tetrahedra as structural units, respectively. Aurophilic and/or metallophilic interactions are present in all clusters,17 with the interaction becoming weaker (as judged by longer metal−metal contacts) when the size of the bridging chalcogen increases. Compound 3c crystallizes as a THF solvate in the orthorhombic space group P212121 and its geometry is distorted compared to the lighter congeners of this series, owing to both the longer Au−Te bonds and aurophilic contacts present (Figure 1). Selected structural parameters are presented in Table S3. In contrast to many coinage metal-chalcogenide clusters stabilized by phosphine ligands,18 crystals of NHC-stabilized clusters 2a,b and 3a−c readily redissolve in solution without evidence of ligand dissociation, allowing for detailed solutionstate spectroscopic characterization. Multinuclear NMR spectra have been recorded for all clusters and are consistent with the molecules remaining intact in solution (Supporting Information). The luminescence of clusters 2a,b and 3a,b in glassy 2methyl tetrahydrofuran (2-Me-THF) and the solid state spectra for 2a,b and 3c have been measured at 77 K (Figure 2). The spectroscopic results are summarized in Table S5. Except for 3c, all clusters exhibit a single excitation band with λexmax between 310 and 327 nm, increasing in wavelength in the order 2a < 2b < 3a < 3b. The same ordering applies to the lowenergy λemmax of the emission spectra in glassy 2-Me-THF. This gives rise to emission colors ranging from yellow-green (2a) to red (3b). Each of the clusters exhibited large Stokes’ shifts (>14 000 cm−1) and microsecond lifetimes of ca. 10 to 40 μs, indicating a triplet parentage for the emissions. Bathochromic shifting of the emission maxima are observed in 2a, 2b, 3b, and 3c (Figures 2 and S11) from glassy 2-Me-THF to the solid state and is attributed to strengthened Au···Ag/Au···Au interactions in the absence of solvent.19 The cluster 2b displays dual luminescence in glassy 2-Me-THF and as a powder with an additional weak band at ∼500 nm; such features have been observed in complementary gold(I)−chalcogenide systems and were attributed to metal-perturbed ligand-centered phosphorescence.7e The relative intensity of this band decreases markedly at temperatures above 77 K (Figure S6). The

Scheme 2. Synthesis of 2a,b and 3a−ca

a

See the Supporting Information for detailed experimental conditions.

similar reaction of 1a with [(Ph3P)AuOAc], the crystallization of the octanuclear gold(I)-sulfido cluster [Au8(μ3-S)4(IPr)4] (3a) was observed. However, a major byproduct was [(IPr)Au(PPh3)](OAc) (ca. 33%), resulting in low yields of the cluster (Figures S1 and S2). A weaker Lewis base (Ar*O)3P (Ar* = 2,4-tBu(C6H3)) was employed instead to promote P− Au bond scission. This modified procedure selectively yielded [Au8(μ3-S)4(IPr)4] and [Au8(μ3-Se)4(IPr)4] (3b). Surprisingly, reaction of 1c with [(Ph3P)AuOAc] allowed for the clean isolation of [Au8(μ3-Te)4(IPr)4] (3c) without observation of any cationic gold(I) species. The binary gold(I) chalcogenide clusters 3a−c represent a rare series for which all the heavier chalcogen congeners have been structurally characterized, another being [{(Ph3P)Au}3(μ3-E)]+.7l,n,o The benefits of using metal trimethylsilylchalcogenolate reagents 1a−c are apparent in the retention of reaction stoichiometry in the resultant cluster, and the ease of purification by simply layering the reaction solutions with hydrocarbons. Compounds 2a, 2b, 3a, and 3b are isostructural and isomorphous, crystallizing in the tetragonal space group I 4̅ 14046

DOI: 10.1021/jacs.7b09025 J. Am. Chem. Soc. 2017, 139, 14045−14048

Communication

Journal of the American Chemical Society

Figure 3. Difference density plot for the singlet excitations of 2a using the method of ref 25. Red regions indicate a surplus of electron density in the ground electronic state and blue indicates a surplus in the excited singlet state. Figure 2. (Top) Photoluminescence excitation and emission spectra of compounds 2a (red), 2b (blue), 3a (green), and 3b (purple) in glassy 2-Me-THF at 77 K. Samples were excited at 315 nm (2a and 2b) and 320 nm (3a and 3b). Photoluminescence excitation spectra were recorded by monitoring emissions at 600 nm (2a and 2b) and 650 nm (3a and 3b). (Bottom) Solid state photoluminescence excitation and emission spectra of compounds 2a (red), 2b (blue), and 3c (dark red) at 77 K. Samples were excited at 300 nm. Photoluminescence excitation spectra were recorded by monitoring emissions at 650 nm (2a) and 700 nm (2b and 3c).

gold(I) chalcogenide clusters. NHCs are found to be valuable for stabilizing a particular metal cluster framework, allowing the photophysical properties to be fine-tuned by selective doping of metal ions and chalcogen centers. Furthermore, the nature of the emission is found to influenced by the electron accepting ability of the NHC ligands, so further tuning of the optical properties through ancillary ligand modifications should be possible.

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ASSOCIATED CONTENT

S Supporting Information *

emission maxima of 3c in the solid state occur at low energies, which are similar to those reported for the octanuclear gold(I) tellurolates [Au 8(μ-TePh)8(PR 3)4] and [Au 8(μ-TeTol) 8(PR3)4].20 For cluster 2a, the character of the excitations causing the excitation band starting at a wavelength of ca. 350 nm was determined via time-dependent density functional theory (TDDFT). Calculations were carried out with TURBOMOLE21 at the B3LYP/def2-SV(P) level.22,23 These transitions dominantly involve the highest 8 occupied and the lowest 16 unoccupied orbitals. The latter are part of the π-system of the NHCs, with the former mainly showing contributions of the 3p orbitals of sulfur (55%), the 5s/4d orbitals of silver (29%), and the 6s/5d orbitals of the gold centers (13%), as determined from Mulliken population analysis.24 The character of this transition is also evident from the difference density (Figure 3) between the excited states and the ground state.25 The electronic excitations in 2a may thus be formulated as metal−metal to ligand charge transfer (1MMLCT; AuAg2 → IPr) with an interligand (1IL; S → IPr) admixture. Based on the photophysical data, it is expected that the reverse process (3LMMCT; IPr → AuAg2 and 3IL; IPr → S) is responsible for emission from the first excited triplet state. As more electron density is injected into the {M4E4} core (via replacement of S with Se or Ag with Au), a higher-lying S0 state should induce a bathochromic shift of the electron transition energy, as is indeed observed in the ordering of λem max. A similar involvement of the ancillary ligands in the triplet excited state has been documented recently in copper(I) chalcogenide clusters.26 In summary, NHC ligands provide a remarkable increase in thermal stability to [LAuESiMe3] reagents, while these complexes retain their high cluster-forming reactivity, readily yielding highly crystalline, solution processable, and functional

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b09025. Experimental details, NMR and combustion analysis of the reported complexes, stability studies of 1a−c in solution, crystal structure data for 2a,b and 3a−c and electronic spectroscopy data (PDF) Crystallographic data for 2a,b and 3a−c (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Florian Weigend: 0000-0001-5060-1689 John F. Corrigan: 0000-0003-2530-5890 Notes

The authors declare no competing financial interest. CCDC 1526645−1526646 for 2a and 2b, respectively, and CCDC 1559057−1559059 for 3a−c, respectively, contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Center.

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ACKNOWLEDGMENTS Dedicated with best wishes to Prof. Dr. Dieter Fenske on the occasion of his 75th birthday. We are grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canada for supporting this research in the form of Discovery and Research Tools and Instruments to J.F.C. and M.J.S. A.M.P. thanks the NSERC for scholarship support and the Chemistry Department at the University of Western Ontario for an ASPIRE travel grant. A.Z. acknowledges scholarship support from a Queen Elizabeth II Graduate Scholarship. The 14047

DOI: 10.1021/jacs.7b09025 J. Am. Chem. Soc. 2017, 139, 14045−14048

Communication

Journal of the American Chemical Society

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authors thank Dr. Sergei Lebedkin and Prof. Dr. Manfred M. Kappes at the Karlsruher Institut für Technologie for access to equipment and instruction in low temperature luminescence spectroscopy on solid samples. Dr. Paul Boyle (Western) is thanked for his assistance with the analysis of X-ray structural data.

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DOI: 10.1021/jacs.7b09025 J. Am. Chem. Soc. 2017, 139, 14045−14048