and Bidentate NHC Ligands - ACS Publications - American Chemical

Article Cite This: Inorg. Chem. 2017, 56, 14771−14787

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Gold(I) Alkynyls Supported by Mono- and Bidentate NHC Ligands: Luminescence and Isolation of Unprecedented Ionic Complexes Alexander A. Penney, Galina L. Starova, Elena V. Grachova, Vladimir V. Sizov, Mikhail A. Kinzhalov, and Sergey P. Tunik* Saint Petersburg State University, Institute of Chemistry, Universitetsky pr. 26, Saint Petersburg 198504, Russian Federation

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S Supporting Information *

ABSTRACT: Reactions of NHC·HX (NHC = 1-benzyl-3methylbenzimidazol-2-ylidene, X = Br−, PF6−) and (AuC CR)n (R = Ph, C3H6OH) in the presence of Cs2CO3 initially afford compounds of the general formula [(NHC)2Au]2[(RC2)2Au]X, which can be isolated by crystallization. With increased reaction time, only the expected mononuclear complexes of the type [NHCAuCCR] are produced. The crystal structure of [(NHC)2Au]2[(PhC2)2Au]PF6 reveals an unprecedented triple-decker array upheld by a remarkably short (2.9375(7) Å) unsupported Au···Au···Au contact. The mononuclear complex [NHCAuCCPh] was found to crystallize as three distinct polymorphs and a pseudopolymorph, which depending on the intermolecular Au···Au distances emit blue, green, or yellow light. Two synthetic approaches were employed for the preparation of a series of dinuclear NHC-ligated Au(I) alkynyl complexes of the general formula [NHC-(CH2)n-NHC(AuCCR)2], where NHC = Nbenzylbenzimidazol-2-ylidene, R = Ph, C3H6OH, C6H10OH, and n = 1−3. In solution, the complexes with aliphatic substituents on the alkynyl fragment are nonemissive, whereas their phenyl-bearing congeners demonstrate characteristic metal-perturbed 3 [IL(CCPh)] emission. In the solid state, a clear correlation between intermolecular aurophilic interactions and luminescence was established, including their role in the luminescent thermochromism of the phenylalkynyl complexes. The relationship between the Au···Au distance and emission energy was found to be inverse: i.e., the shorter the aurophilic contact, the higher the emission energy. We tentatively attribute this behavior to a smaller extent of excited-state distortion for a structure with a shorter Au···Au separation.

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INTRODUCTION Aurophilicity, once thought of as a curious feature of the structural chemistry of gold(I), has emerged as a versatile tool for the design of diverse functional systems, many of which present tangible opportunities for practical application.1,2 Relatively weak metallophilic interactions often produce interesting phenomena, such as solid-state vapochromic,3−11 temperature-dependent,12−14 and mechanochromic15−19 luminescent responses. Importantly, reports have appeared on the utilization of “off−on” Au···Au interactions for solution-state sensing applications.20−26 Polymorphism, a feature not uncommon for gold(I) compounds, is likewise often related to variations in aurophilic attractions.27−29 However, there are examples of polymorphic and/or stimuli-responsive d10 metal complexes, in which changes in emission are not associated with the modulation of metallophilic contacts.30−35 The construction of elaborate supramolecular architectures enabled by auro- and/or metallophilic interactions is an active area of investigation, not least due to the desirable photophysical characteristics often found for such systems.36,37 In this respect, both homo- and heteroleptic alkynyl complexes of gold(I) are at the forefront of research efforts, as these species © 2017 American Chemical Society

are readily accessible, are often highly emissive, and may themselves serve as building blocks for the preparation of assemblies of yet higher complexity.38−45 A variety of coordination modes available to acetylide ligands has proven to be a fruitful feature that greatly enhances structural diversity of the complexes containing these coordinating functions.46−48 Metal-perturbed alkynyl-localized triplet excited states operative in gold(I) alkynyl complexes lead to long-lived phosphorescence accompanied by large Stokes shifts, making these compounds suitable for time-gated bioimaging applications.49−55 Enduring interest in ligand-supported gold(I) alkynyl complexes and the paucity of reports dealing with dinuclear Au(I) alkynyls with bridging NHC auxiliaries56 encouraged us to study this class of compounds. Intrigued by the role aurophilicity plays in the structural arrangement and luminescence of these complexes, conformationally flexible bidentate NHC ligands were used. In order to examine the effects intramolecular aurophilic interactions may have on the Received: June 13, 2017 Published: November 27, 2017 14771

DOI: 10.1021/acs.inorgchem.7b01508 Inorg. Chem. 2017, 56, 14771−14787

Article

Inorganic Chemistry Scheme 1. Synthesis of Dinuclear Complexes 1A−C, 2A−C, and 3A−C

the general formula [diNHC(AuX)2] (X = Cl, Br) in effectively quantitative yield (Scheme 1, method I). Unfortunately, the extremely low solubility of these species precluded their complete characterization, although, in the case of the propylene-bridged compound it was possible to record a 1H NMR spectrum (Figure S1 in the Supporting Information), which indicated sufficient purity of the material obtained and confirmed its chemical identity. We note that the formulation of these species as a mixture of chlorido and bromido derivatives is based on ESI+ MS measurements, which consistently revealed signals originating from both compounds (Figures S2 and S3 in the Supporting Information). While our newly developed protocol appears to be amenable to scale-up, we observed that these [diNHC(AuX)2] complexes tend to be rather unstable and decompose upon prolonged storage; therefore, it was deemed prudent to make small batches that would be immediately consumed for the synthesis of the corresponding heteroleptic complexes 1A−C, 2A−C, and 3A−C. The discovery of the unique reactivity of [NHCAu(OH)] species, pioneered by the group of Nolan, paved the way for a straightforward route to a multitude of functionalized Au(I) derivatives,61,62 the requisite condition being that the pKa of the acidic substrate be lower than that of the gold(I)-NHC hydroxide itself.63 [Au(OH)(IPr)] (IPr = N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) was found to have a pKa value of 30.3 in DMSO,63 and since the pKa of the terminal proton of most monosubstituted acetylenes is below this value, access to mixed-ligand Au(I)-NHC alkynyl species is granted via a simple acid−base reaction.64 A number of synthetic procedures have recently appeared, which apparently leverage this reactivity but at the same time forego the necessity to isolate the [NHCAu(OH)] complex itself. Thus, mononuclear Au(I) acetylides supported by NHC ligands have been prepared starting from more readily available [NHCAuCl] complexes and terminal acetylenes with an alkali hydroxide as a base.65−69 The drawbacks of these protocols are

photophysics of these complexes, reference mononuclear species were investigated. For all the compounds under study, an attempt was made to establish a correlation between structural organization and photophysical properties in the solid state.

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RESULTS AND DISCUSSION Synthesis and Characterization. The investigation started with the preparation of dinuclear complexes 1A−C, 2A−C, and 3A−C (Scheme 1), to which end a number of different synthetic approaches were examined. Transmetalation between the respective silver(I) NHC species, prepared by the classic Ag2O protocol, and a Au(I) acetylide is the first immediately obvious possibility.56 In our hands, however, reactions between the corresponding [diNHC(AgBr)2] complexes and 2 equiv of (AuCCR)n in dichloromethane consistently afforded material contaminated with some unidentified soluble silver-containing impurities, and in all cases the yields were far from optimal. This observation prompted us to search for an alternative silver-free synthetic procedure. A few novel methods of preparing mononuclear complexes of the type [NHCAuX] (X = Cl, Br, I) that bypass the silver NHC precursor have recently appeared in the literature.57−60 Of these, particularly interesting are the direct reactions of imidazolium salts with the commonly used [Au(THT)Cl] (THT = tetrahydrothiophene) or [Au(Me2S)Cl] gold sources in the presence of a base. The protocol reported by Nolan and co-workers,58 which prescribes the use of K2CO3 and employs acetone as the solvent, fails for our dibenzimidazolium salts, turning nearly all of the starting [Au(THT)Cl] into colloidal gold. If the reaction is carried out in dichloromethane with K2CO3 as a base, as proposed by Gimeno and co-workers,59 conversion is impractically slow. We found that the treatment of [Au(THT)Cl] (2 equiv) with diNHC·2HBr (1 equiv) in the presence of NaOAc·3H2O (4 equiv) in methanol at room temperature affords complexes of 14772


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Inorganic Chemistry Scheme 2. Synthesis of Mononuclear Complexes 4A−4C, 5, 6Br, and 6PF6

the need for reflux conditions65 and in some cases somewhat involved workup and purification procedures: e.g., column chromatography. As our attempts to isolate [diNHC(AuOH)2] complexes from the reaction mixtures of the respective [diNHC(AuX)2] compounds and alkali hydroxides were unsuccessful, and also in light of possible complications arising from the need to purify the mixed-ligand Au(I)-NHC alkynyl complexes obtained in this way, we abandoned this strategy and turned our attention to a simpler alternative. A milder approach would involve a reaction of [diNHC(AuX)2], a terminal acetylene, and a weaker base. Preliminary screening revealed that K2CO3 is suitable for this purpose and performs better than NaOAc or Et3N, and methanol was found to be the solvent of choice. While this reaction is unclear mechanistically, nevertheless, it appears to be quite robust, affording complexes 1A−C, 2A−C, and 3A−C in good yield at room temperature with short reaction times (Scheme 1, method I). A simple recrystallization furnishes analytically pure compounds. We note that this methodology is reminiscent of that originally proposed by Lin and co-workers.70 With this two-stage pathway to compounds 1A−C, 2A−C, and 3A−C established, we became intrigued by the possibility of obtaining these complexes by a one-step procedure. We have developed a hitherto unreported method comprising a direct reaction between benzimidazolium salts and polymeric Au(I) acetylides (Scheme 1, method II). This transformation proceeds readily in dichloromethane at room temperature with Cs2CO3 as a base, and the yields of the target compounds are generally good. In the case where K2CO3 is used in place of Cs2CO3, conversion is considerably slower. Mononuclear complexes 4A−4C were readily obtained by method II developed for the dinuclear compounds (Scheme 2). Upon filtration and crystallization of reaction mixtures that were stirred for ca. 1 h, the ionic species [(NHC)2Au]2[(RC2)2Au]X (R = Ph, X = PF6−, complex 7; R = C3H6OH, X = Br−, complex 8) were obtained (Scheme 3). Prolonged stirring (ca. 5 h) afforded only the expected mononuclear complexes of the type [NHCAuCCR]. The presence of PF6− or Br− anions in the structures of complexes 7 and 8 seems puzzling at first. However, as there are two [(NHC)2Au]+ cations per [(RC2)2Au]− anion, an additional anion is required to ensure electroneutrality. A plausible way to explain the inclusion of PF6− or Br− counteranions into the structures of complexes 7 and 8 is to propose that partial decomposition of [(RC2)2Au]− anions results in the uptake of

Scheme 3. Proposed Reactions That Lead to the Formation of Complexes 4A,C, 7, and 8 Starting from NHC·HX, (AuCCR)n, and Cs2CO3 in CH2Cl2

external PF6− or Br− (Scheme 3, reaction pathway II). Upon dissolution of complexes 7 and 8, [(NHC)2Au]+ and [(RC2)2Au]− react with each other to produce 2 equiv of [NHCAuCCR], and as the number of [(NHC)2Au]+ cations is twice that of [(RC2)2Au]− anions, 1 equiv of [(NHC)2Au]X is left. This transformation was conveniently monitored for complex 7 by 1H NMR (Figure S4 in the Supporting Information). Further crystallization of the mother liquor, from which crystals of complex 7 were obtained, afforded crystals of both [NHCAuCCPh] (4A) and [(NHC)2Au]PF6 (6PF6), the identities of which were established by XRD analyses. The fact that at long reaction times (5 h) the formation of [(NHC)2Au]X was not observed pointed to the possible reversibility of the CsHCO3-mediated decomposition of [(RC2)2Au]− (Scheme 3, reaction pathway III). Indeed, it was found that the reaction of [(NHC)2Au]X with (AuC CPh)n and HCCPh in the presence of Cs2CO3 produces [NHCAuCCPh], which unambiguously demonstrates the reversible nature of the transformation of [(RC2)2Au]− into [AuCCR] and HCCR. Mononuclear complexes 5, 6Br, and 6PF6 were prepared by reacting the corresponding NHC ligand precursors with [Au(THT)Cl] (1 or 0.5 equiv, respectively) in dichloromethane with Cs2CO3 as a base (Scheme 2). The NMR spectra of the compounds under study demonstrate sets of signals whose number, multiplicity, and relative intensity are consistent with structures depicted in 14773


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Figure 1. Thermal ellipsoid plot (30%) of two symmetry-equivalent molecules in the solid-state structure of 3A. Hydrogen atoms and CH2Cl2 molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Au2−Au2′ 3.3491(4), Au1−C1 2.008(6), Au1−C15 1.988(6), Au2− C26 2.020(5), Au2−C40 2.005(5); C1−Au1−C15 178.7(3), C26−Au2−C40 176.4(2).

Figure 2. Thermal ellipsoid plot (30%) of the asymmetric unit of 4Ablue. Selected bond lengths (Å) and angles (deg): Au1−C1 2.018(5), Au1−C16 1.996(5), Au1A−C1A 2.012(5), Au1A−C16A 1.991(5); C1−Au1−C16 178.5(2), C1A−Au1A−C16A 178.8(2).

exist as isolated ions. The positive-mode ESI mass spectra of both compounds feature the same monocationic signal corresponding to the composition of the [(NHC)2Au]+ moiety (Figures S34 and S35 in the Supporting Information). The negative-mode mass spectrum of 7 displays two monoanions with the m/z values and isotopic patterns of PF6− and [(PhC C)2Au]−, whereas that of 8 demonstrates a signal corresponding to the [(HOC3H6CC)2Au]− anion (Figures S36 and S37 in the Supporting Information). Single-Crystal XRD Studies. Of all the dinuclear alkynyl complexes under study, only 3A gave crystals suitable for XRD analysis. In contrast, preparative recrystallizations of the mononuclear complexes, including homoleptic complexes 6Br and 6PF6 and acetylides 4A,C, readily afforded X-ray-quality crystals. Crystallographic data are summarized in Table S2 in the Supporting Information, and molecular structures and selected structural parameters are given in Figures 1−7. The asymmetric unit of complex 3A contains the whole molecule in a nonsymmetric conformation along with a cocrystallized CH2Cl2 molecule (Figure 1). The values of

Schemes 1 and 2 (Figures S5−S17 in the Supporting Information). The ESI+ mass spectra of the alkynyl complexes (Figures S21−S32 in the Supporting Information) display signals characteristic of the respective molecular ions of [M + Na]+ and/or [M + K]+ composition with m/z values and isotopic patterns perfectly matching those calculated. The 1H NMR spectra of complexes 7 and 8 reveal a group of resonances corresponding to the [(NHC)2Au]+ fragment, identical with those found for complexes 6Br and 6PF6 (Figure S33 in the Supporting Information). For 7, two groups of aromatic second-order multiplets are present, which are assigned to the [(PhCC)2Au]− anion. In the case of 8, an aliphatic singlet ascribed to the methyl groups of the [(HOC3H6CC)2Au]− anion is observed, along with a resonance of the hydroxyl proton. For both complexes, relative integral intensities of the signals support the proposed stoichiometry. Unchanged chemical shifts and appearance of the resonances of the [(NHC)2Au]+ cations in the spectra of 7 and 8 in comparison to those of 6Br and 6PF6 indicate that in solution these trinuclear complexes dissociate completely and 14774


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Inorganic Chemistry Au−Ccarbene and Au−Calkynyl bond distances are in line with those reported previously for structurally analogous compounds.56,65 An intermolecular aurophilic contact of 3.3491(4) Å is present along with π-stacking (the distance between the planes is 3.724 Å, and the dihedral angle is 1.56°), which links the two neighboring molecules in a head-to-tail mode. The two C carbene −Au−C alkynyl angles are almost linear, and the coordination geometry of the Au atom engaged in the aurophilic interaction deviates from linearity to a greater extent than that of the noninteracting metal center, for which the distance to its symmetry-equivalent counterpart is 3.8338(4) Å. Three polymorphs and a pseudopolymorph of mononuclear complex 4A were isolated, aided in large part by their readily distinguishable luminescence. Colorless crystals of all four forms were grown under seemingly identical conditions by slow room-temperature evaporation of CH2Cl2/heptane solutions of 4A. The blue-glowing polymorph, denoted 4Ablue, crystallizes as plates in the monoclinic P2/c space group with two molecules in the asymmetric unit, and no aurophilic or π-stacking interactions are found in the crystal structure (Figure 2). The two independent molecules demonstrate slightly differing Au− Ccarbene and Au−Calkynyl bond lengths and almost linear Ccarbene−Au−Calkynyl angles. Block-shaped crystals of the two green-emissive forms, 4Agreen and 4Agreen DCM, appear identical in crystal habit and luminescence. The chief differences are in the space group, the presence of 0.38 CH2Cl2 molecule per formula unit in 4Agreen DCM, and the angle at which the two independent molecules in the asymmetric unit are disposed relative to each other (Figures 3 and 4). The Au···Au separations found in the dimeric units of both forms are very similar, and no π-stacking interactions are observed. The yellow-emitting polymorph, 4Ayellow, crystallizes as blocks or sometimes needles in the triclinic P1̅ space group

Figure 4. Thermal ellipsoid plot (30%) of the asymmetric unit of 4Agreen DCM. Hydrogen atoms and the CH2Cl2 molecule are omitted for clarity. Selected bond lengths (Å) and angles (deg): Au1−Au1A 3.4190(3), Au1−C1 2.015(8), Au1−C16 2.001(9), Au1A−C1A 2.015(7), Au1A−C16A 1.997(8); C1−Au1−C16 173.0(3), C1A− Au1A−C16A 175.3(3).

with a single molecule in the asymmetric unit (Figure 5). The Au(I) centers of two adjacent symmetry-equivalent molecules engage in a long aurophilic contact of 3.6164(3) Å. Additionally, each molecule is π-stacked to its symmetry-equivalent counterpart, and the two neighboring NHC ligands are mutually disposed in a perfectly parallel head-to-tail arrangement with the distance between the planes being 3.350 Å. The dihedral angle between the NHC plane and the plane of the phenylacetylide both coordinated to the same gold(I) center is almost perpendicular (89°), in contrast to the situation observed for 3A and 4Ablue, in which those planes are close to being parallel. The solid-state structure of complex 4C reveals no intermolecular interactions, and the bond lengths and angles fall in the expected ranges65 (Figure 6). Crystallization of complex 7 is complicated by its irreversible transformation into 4A in solution (vide supra). Repeated attempts (see the Experimental Section) afforded poor-quality crystals, and the situation is further exacerbated by a severe disorder of the phenyl rings. Nevertheless, the obtained diffraction data proved to be sufficient to unambiguously determine all structural parameters. Complex 7 crystallizes in the triclinic P1̅ space group with half the cation and one PF6− anion in the asymmetric unit (Figure 7). In the trinuclear cation, each peripheral Au(I) center is coordinated by two NHC ligands, and the Ccarbene−Au−Ccarbene angle appreciably departs from linearity at 172.5(8)°. The central Au atom is adjoined by two phenylacetylides and interacts strongly with the two adjacent NHC-bound gold(I) centers (the Au···Au separation is 2.9375(7) Å), leading to an effectively square planar coordination geometry around the central metal atom and a T-shaped motif at the peripheral ones. The + − + arrangement of the charged fragments within the trinuclear cation likely contributes to the shortness of the Au···Au···Au contact. Intermolecular π stacking between the NHC ligands of adjacent trinuclear cations (interplane distance is 3.306 Å) forms an infinite one-dimensional chain of interpenetrated cations propagating along the crystallographic a axis (Figure S38 in the Supporting Information).

Figure 3. Thermal ellipsoid plot (30%) of the asymmetric unit of 4Agreen. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Au1−Au1A 3.3603(4), Au1−C1 2.005(4), Au1−C16 1.992(5), Au1A−C1A 2.021(5), Au1A−C16A 1.988(5); C1−Au1−C16 174.6(2), C1A−Au1A−C16A 174.6(2). 14775


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Figure 5. Thermal ellipsoid plot (30%) of four symmetry-equivalent molecules in the solid-state structure of 4Ayellow. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Au1−Au1′ 3.6164(3), Au1−C1 2.027(5), Au1−C16 1.998(6); C1−Au1−C16 174.0(2).

Figure 7. Molecular view of complex 7. Disorder is omitted for clarity. Selected bond lengths (Å) and angles (deg): Au1−Au2 2.9375(7), Au1−C1 2.04(2), Au1−C16 2.00(2), Au2−C31 2.08(2); C1−Au1− C16 172.5(8), C31−Au2−C31 180.0, C31−Au2−Au1 90.2(7).

(Figure S42 in the Supporting Information). Unlike the mononuclear and ethylene-bridged dinuclear compounds, the methylene- and especially propylene-linked dinuclear complexes demonstrate weak low-energy shoulders, which are tentatively attributed to the presence of an intramolecular aurophilic interaction. Such absorption bands are found for systems with intramolecular aurophilic bonding71 and may be assigned to either an MMLCT transition72 or a [dσ*pσ] Au− Au-centered state.73,74 In solution, mononuclear complexes 5, 6Br, and 6PF6 as well as alkylalkynyl complexes 1B,C−4B,C exhibit no appreciable luminescence, which indicates that intraligand NHC-localized excited states deactivate via nonradiative pathways. In dichloromethane solution, complexes 1A−4A present vibronically structured emission bands (Figure 8), which, in accordance with the literature, are ascribed to a Au-perturbed 3[IL(C CPh)] state.56,75−77 The observed 1100 and 2000 cm−1 vibronic spacings are attributed to the ground-state phenyl

Figure 6. Thermal ellipsoid plot (30%) of complex 4C. Selected bond lengths (Å) and angles (deg): Au1−C1 1.990(9), Au1−C16 2.034(8); C1−Au1−C16 177.9(3).

Photophysical Studies. In solution, both mono- and bidentate NHC ligand precursors absorb in the UV region with λmax at ca. 270 nm, and the absorption bands feature vibronic progressions of ca. 1000 cm−1 (Figure S39 in the Supporting Information). The UV/vis spectra of mononuclear complexes 5, 6Br, and 6PF6 reveal red-shifted absorption maxima at ca. 290 nm, and in the case of 5, a pronounced vibronic structure with a ca. 1100 cm−1 spacing is observed (Figure S40 in the Supporting Information). The UV/vis spectra of alkylalkynyl complexes 1B,C−4B,C are almost identical in appearance with that of 5 (Figure S41 in the Supporting Information). Phenylalkynyl complexes 1A−4A absorb with λmax at ca. 290 nm, and no clearly resolved vibronic structure is present 14776


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temperature emission spectrum of 4Ablue appears very similar to that found in solution and features a well-resolved vibronic structure (Figure 10). Vibronic spacings of 1100, 1600, and

Figure 8. Emission (solid line) and excitation (dashed line) spectra of phenylalkynyl complexes 1A−4A in CH2Cl2 solution at room temperature. λexc is 290 nm.

ring deformation and CC stretching frequencies, respectively.76 The emission spectrum of 3A features a prominent low-energy component at ca. 500 nm, possibly pointing to a greater contribution of an excited state, in which an intramolecular Au−Au contact is involved. In the solid state, mononuclear alkylalkynyl complexes 4B,C are nonemissive, whereas their dinuclear congeners 1B,C− 3B,C are luminescent (Figure 9). Unfortunately, in the absence of XRD data for the dinuclear alkylacetylides, no correlation between the solid-state structural organization and emission could be made. Of the four identified polymorphic modifications of complex 4A (vide supra), only the blue-emissive form could be isolated as a pure bulk phase, as evidenced by the comparison of its PXRD pattern to that calculated from single-crystal data (Figure S43 in the Supporting Information). The room-

Figure 10. Room-temperature solid-state emission (solid line) and excitation (dashed line) spectra of the polymorphs of mononuclear phenylalkynyl complex 4A. λexc is 350 nm. The low-intensity shoulder at ca. 570 nm in the emission spectrum of 4Agreen corresponds to a small admixture of crystals of 4Ayellow.

2100 cm−1 are identified, which correspond to the ground-state phenyl ring deformation, symmetric phenyl ring stretching, and triple-bond stretching frequencies, respectively.76 The observed emission is thus assigned to a gold-perturbed 3[π → π*](C CPh) state, the triplet character of which is evidenced by the 3.9 μs excited-state lifetime. The spectrum designated 4Agreen (Figure 10) was recorded for a crystalline sample containing small amounts of 4A green DCM and 4A yellow; the phase composition is known because numerous crystals from this crystallization vial were examined by single-crystal XRD. UV light aided inspection of the crystals under a microscope showed that the emissions of crystals of 4Agreen and 4Agreen DCM are indistinguishable by the human eye. Unfortunately, the amount of material in this batch was insufficient for a powder XRD investigation. Figure S44 in the Supporting Information presents the emission spectrum of a different batch, which was analyzed by PXRD. The comparison of the measured powder pattern to those calculated from single-crystal structures of the polymorphs revealed that this sample is best formulated as a mixture of 4Agreen DCM and 4Ayellow (Figure S45 in the Supporting Information). This emission spectrum was deconvoluted using two Gaussian curves to establish the true positions of the emission maxima of 4Agreen DCM and 4Ayellow (Figure 11 and Table 1). Unlike 4Ablue, the emission bands of the green- and yellowemissive forms are structureless, which points to a different nature of the electronic transitions involved: e.g., the excited state may be localized on the Au···Au contact and not on the phenylalkynyl moiety. Of note is the observation that, in the dimeric units of 4Agreen and 4Agreen DCM , the Au···Au separations are relatively short (3.3603(4) and 3.4190(3) Å, respectively), yet dimers of 4Ayellow exhibit a considerably longer Au···Au contact of 3.6164(3) Å. Such an apparently

Figure 9. Room-temperature solid-state emission (solid line) and excitation (dashed line) spectra of dinuclear alkylalkynyl complexes 1B,C−3B,C. λexc is 350 nm. 14777


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“pure” heavy-atom-facilitated 3[IL(CCPh)] transition. The same logic applies to the luminescence of the polymorphs of complex 4A: without intermolecular aurophilicity, high-energy intraligand phosphorescence of the phenylalkynyl fragment is observed, whereas in the presence of an aurophilic contact the emission is red-shifted, probably due to the direct participation of a Au−Au-centered state. In this conjecture we disagree with Lima,72 who concluded that the electronic nature and energy of intraligand arylalkynyl phosphorescence are not affected by an aurophilic interaction, and only the emission intensity increases on decreasing the Au···Au distance. All of the phenylalkynyl complexes, including the three aurophilic forms of complex 4A, exhibit intriguing solid-state luminescent thermochromism (Figure 12). At 77 K, their phosphorescence becomes remarkably different from that observed at room temperature and resembles that found in solution. In the case of 1A, the vibronic structure characteristic of the CCPh fragment becomes exceptionally well resolved. A qualitative description of this unprecedented temperature dependence is that at low temperature the efficiency of intraligand phenylalkynyl-localized phosphorescence increases, whereas the influence of Au···Au interactions is diminished. In order to establish whether the observed effect is related to some kind of structural change, we investigated a crystal of 4Agreen by single-crystal XRD at 260 K and found no significant differences between the obtained structure and that for which the diffraction data were collected at 100 K (Figure S48 in the Supporting Information). Furthermore, the cell constants of 4Agreen DCM at 260 K were practically identical with those found at 100 K, and the PXRD pattern of 3A recorded at room temperature is in good agreement with that predicted from the 100 K solid-state structure. Given that on varying the temperature the structures remain unchanged, it is unlikely that the electronic nature of the ground state should be different. The structured emission bands at 77 K suggest that the HOMO of the ground state resides on the CCPh fragment. A preliminary explanation might be that, depending on temperature, different excited states become populated one localized on the phenylalkynyl moiety (an IL state) and the other on the Au···Au contact (an LMMCT state). A simple assumption that the cooling-induced suppression of molecular vibrations may be responsible for the blue shift in emission is inconsistent with the fact that the observed Stokes shifts are independent of temperature. In the solid state, trinuclear complexes 7 and 8 are intensely emissive (Figure 13). Their blue phosphorescence is attributed to the presence of a strong Au···Au···Au contact. We note that, in the absence of solid-state structural data for 8, we assume that its structure is effectively the same as that of 7. A 10 nm red shift of the emission maximum of the phenylalkynyl complex relative to that of its aliphatic congener may be indicative of the involvement of a state in which the phenylacetylide ligands participate. However, this must not necessarily be so, as the difference in emission energy may be alternatively explained by variation in the intermetallic distances. Relatively small Stokes shifts (2500 and 3300 cm−1 for 7 and 8, respectively) suggest minor distortion in the triplet emissive state. Surprisingly, a plot of the room-temperature solid-state emission energy versus the Au···Au distance for the complexes in which aurophilic interactions are found exposes an inverse linear relationship (Figure 14), similar to that reported by Elder for a series of bis(thiocyanato)aurate(I) salts.78 Likewise, the

Figure 11. Fit of two Gaussian curves to the emission spectrum of material in batch 2, formulated as a mixture of 4Agreen DCM and 4Ayellow.

anomalous dependence of emission energy on the Au···Au distance, i.e., the longer the distance, the lower the emission energy, was previously observed in a series of bis(thiocyanato)aurate(I) salts with various cations78,79 as well as in a number of neutral isocyanide complexes of the type [(p-tosyl)CH2N CAuX] (X = Cl, Br, I).80,81 For the latter series of compounds, this behavior was rationalized by considering the difference in the extent of excited-state distortion in dimeric versus extended-chain exciplexes.80,81 The present result underlines that the structural pattern of infinite aurophilic chains is not a prerequisite for the manifestation of the “inverse” structure− luminescence correlation, as isolated dimeric units are sufficient for this phenomenon to occur. Earlier, two polymorphs of an isocyanide Au(I) complex, [PhNCAuAr], were reported.82 In this case, however, the luminescence follows the conventional pattern: the aurophilic contact in the dimeric unit of the blue-emissive form is longer at 3.504(1) Å, while the Au···Au separation in the yellowemitting polymorph shortens to 3.2955(6) Å. These observations demonstrate that the factors that control the relationship between the Au···Au distance and emission energy in the solid state are not yet fully understood. Whereas the room-temperature solid-state luminescence of complex 3A is rather conventional, the emission profiles of 1A and 2A appear more complicated: the spectrum of 1A presents two discernible shoulders and is remarkably broad, and in the case of 2A, a somewhat concealed vibronic structure in the high-energy part is observed (Figure 12). We suspect that such behavior of these two complexes may result from the occurrence of several crystalline phases in the solid-state samples. The phase purity of 3A, however, is attested by a good agreement between its PXRD pattern and that predicted from single-crystal data (Figure S46 in the Supporting Information). Alternatively, it is possible that, in the crystals of 1A and 2A, there exist crystallographically independent molecular units with different types of intermolecular association, leading to multiple emissive states. Unfortunately, in the absence of crystal structures, neither of these hypotheses can be substantiated. The emission at ca. 520 nm may be attributed to a state in which Au···Au contacts are involved, whereas the high-energy band of 2A and the high-energy shoulder of 1A arise from a 14778


272, 279 291

293

292

292

283, 281, 284, 283, 283, 281, 283, 283, 281, 291 291

NHC·HBr 1A

2A

3A

4A

1B 2B 3B 4B 1C 2C 3C 4C 5 6Br 6PF6 7 8

14779

423, 444, 487sh 422, 446, 487sh 424, 449, 484sh 421, 446, 482sh 461sh,

465sh,

462sh,

462sh,

solution

467 456

149 (40%), 41 (60%) 235

10 33

18 7 3

287 529 348

468 465 478

3 (4Ablue)

5

6 5 7

349 (86%), 1500 (14%)

522 (room temp); 436, 462sh, 478 (77 K)

0.5

0.1

quantum yield (%)

420, 440, 448, 460, 483sh (4Ablue, room temp); 524 (4Agreen, room temp); 527 (4Agreen DCM, room temp); 433, 471 3908 (4Ablue); 271 (36%), 1991 (4Agreen DCM, 77 K); 573 (4Ayellow, room temp) (64%) (4Agreen) 468 300 465 523 469 500

267 (82%), 1647 (18%)

198 (86%), 1471 (14%)

excited state lifetime (ns)

433, 456sh, 471sh, 529 (room temp); 438, 466sh, 477sh (77 K)

454sh, 510, 630sh (room temp); 425, 447, 455, 467, 490sh, 504sh, 517sh (77 K)

solid state

emission λmax (nm)

a λexc for solution-state emission spectra is 290 nm. λexc for solid-state emission spectra is 350 nm. Solution-state UV/vis and emission spectra were recorded in dichloromethane. Solid-state emission spectra and excited-state lifetimes were measured for powder samples. Excited-state lifetimes were recorded at room temperature by monitoring emission decay at the respective emission maxima. Solid-state quantum yields were measured in KPF6 pellets at room temperature.

292 290 292 292 292 290 292 292 289

absorption λmax (nm)

compound

Table 1. Photophysical Data for the Investigated Compoundsa

Inorganic Chemistry Article


Article

Inorganic Chemistry

Stokes shifts trend with the intermetallic distances, and the shorter the Au···Au contact, the smaller the Stokes shift (Figure 14). We surmise that the observed correlation is associated with the extent of distortion the triplet excited state undergoes relative to the ground-state geometry. Assuming that the Au− Au bond formed in the triplet excimer is of similar length for all the structures, it must be that, for a complex in which the ground-state Au···Au separation is shorter, the distance the two Au atoms need to move toward each other to form the bond would be reduced. A lesser amount of the initially absorbed excitation energy would be expended on this motion, which in turn should lead to lower energy losses and subsequently higher emission energy. The results of Elder for a series of [Au(SCN)2]− salts78 demonstrate that Au···Au-linked isolated dimers (Bu4N+ salt), trimers (Me4N+ salt), and infinite aurophilic chains (K+, Rb+, Cs+ salts) all correlate Au···Au distances and emission energies similarly, and one approximating equation is suitable for these three structural patterns. On the basis of this finding, Elder argues that a single gold−gold pair must serve as the emissive source. In our case, the situation appears to be identical, as the trinuclear species 7 correlates the emission energy and Au···Au distance in the same way as for the complexes that crystallize as dimers. Regarding the long ground-state Au···Au distance in 4Ayellow, Balch found that the isonitrile complex [(CyNC)AuI], which does not exhibit significant ground-state aurophilic bonding (the shortest Au··· Au distance is 3.7182(11) Å), displays orange luminescence (λmax is 625 nm) with a large Stokes shift (ca. 21000 cm−1), in line with an exciplex emission.83 Balch further argues that short ground-state Au···Au contacts are not required for the observation of exciplex emissions. It appears reasonable that, in the Au−Au-bonded triplet excimer, the intermetallic distance can shorten considerably relative to that in the ground state, as has been observed computationally for the [Au(SCN)2]− salts.79 Computational Studies. DFT geometry optimizations and TDDFT calculations were performed for mononuclear complexes 4A−C and 5. TDDFT calculations resulted in absorption wavelengths between 250 and 278 nm, which are overestimated by 15−30 nm, in comparison to the experimental data. For complex 5, absorption originates from a combination of interligand Br → NHC charge transfer with LMCT and NHC intraligand excitations. Upon substitution of Br by a CCR ligand, the nature of absorption does not

Figure 12. Solid-state emission spectra of phenylalkynyl complexes 1A−4A at room temperature (λexc is 350 nm) and 77 K (λexc is 310 nm).

Figure 13. Room-temperature solid-state emission (solid line) and excitation (dashed line) spectra of complexes 7 and 8. λexc is 350 nm.

Figure 14. (a) Dependence of the room-temperature solid-state emission energy on the Au···Au distance and (b) dependence of the Stokes shift (calculated as the difference between the respective emission and excitation maxima at room temperature in the solid state) on the Au···Au distance for complexes 3A, 4Agreen, 4Agreen DCM, 4Ayellow, and 7. 14780


Article

Inorganic Chemistry

experimentally measured emission wavelength in solution. In the lowest triplet state of 4A, the excitation is localized on the CCPh fragment, which is not present in the other mononuclear species. Relocation of the triplet excitation to the NHC fragment in 5 and to the AuCCR moiety in alkylalkynyl complexes 4B,C results in substantially larger energy gaps between the lowest triplet and the ground state. Therefore, it is not surprising that 4A is the only mononuclear complex capable of displaying luminescence in solution. DFT calculations for dinuclear complexes 1A−3A, 1B−3B, and 1C−3C show that these molecules may exist either in “closed” conformations with an approximately parallel arrangement of the AuCCR fragments or in “open” conformations, in which these two fragments are located far from each other (Figure S51 in the Supporting Information). The “open” conformations are more stable than the “closed” ones; for example, for 1A the energy of the “open” conformation is 6.9 kJ/mol lower than that for the “closed” conformation, for 2A 6.6 kJ/mol, and for 3C 15.4 kJ/mol. The relatively small energy difference between the “open” and “closed” structures suggests that the latter may be present in solution in noticeable quantities (about 6%, as estimated from a Boltzmann

change significantly, as the mixed LLCT + LMCT + IL character is preserved for 4A−C (Figure 15). The IL(NHC)

Figure 15. Highest occupied and lowest unoccupied molecular orbitals for complex 4A.

contribution may be responsible for the vibronic structure observed in the experimental absorption spectra of complex 5 and those bearing alkyl substituents at the triple bond. DFT calculations for the lowest triplet states indicate that the energy gaps between these states and the ground states of the respective complexes are very sensitive to the nature of the ligand: 348 nm for 5, 414 nm for 4A, and 311 and 312 nm for 4B,C. Thus, the phenylalkynyl complex shows the lowest emission energy, which is in good agreement with the

Figure 16. Molecular orbitals involved in singlet excitations for complexes 3A (a) and 3C (b). 14781


Inorganic Chemistry

Article

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SUMMARY We investigated a series of neutral mono- and dinuclear NHCsupported Au(I) alkynyl complexes of the general formulas [NHCAuCCR] and [NHC-(CH2)n-NHC(AuCCR)2], where R = Ph, C3H6OH, C6H10OH and n = 1−3. The development of a facile one-step procedure for the preparation of these species led to the isolation of unprecedented ionic complexes of the type [(NHC)2Au]2[(RC2)2Au]X (R = Ph, C3H6OH and X = PF6−, Br−), which are stable in the solid state and constitute a new class of highly emissive materials. In solution, mono- and dinuclear phenylalkynyl complexes 1A−4A are emissive, whereas their alkylalkynyl congeners 1B,C−4B,C are not. The luminescence of the phenylacetylides is assigned to a metal-perturbed 3[IL(CCPh)] state. All of the dinuclear complexes are emissive in the solid state, and the emission of the phenylalkynyls is considerably red shifted relative to that of their aliphatic counterparts. Three polymorphs and a pseudopolymorph of mononuclear phenylalkynyl complex 4A were isolated, which, depending on the presence and strength of aurophilic contacts, luminesce blue, green, or yellow light. The polymorph devoid of aurophilic interactions demonstrates blue emission, which is unambiguously ascribed to the CCPh-localized phosphorescence. The two forms with similar Au···Au separations of ca. 3.4 Å emit green light, but the polymorph with a longer Au···Au distance of 3.6164(3) Å is yellow-emissive. The counterintuitive structure−luminescence relationship is tentatively explained by the different extents of distortion the Au−Au-bonded triplet excited state undergoes. The observed trend holds for dinuclear complex 3A and trinuclear species 7 as well. Interesting luminescent thermochromism was found for all of the phenylalkynyl complexes investigated, with the exception of the nonaurophilic polymorph of complex 4A. When the solid samples of these complexes are cooled to 77 K, the emission bands undergo a considerable blue shift and become vibronically resolved. Importantly, no structural changes take place. We attribute this behavior to an increase in the efficiency of the CCPh-centered phosphorescence, induced by a decrease in the influence Au···Au contacts exert on the emissive transitions. No definitive explanation can be given thus far.

distribution for 1A and 2A at room temperature). For the propylene-bridged dinuclear complexes, several “open” structures could be found, since the rotation around C−C bonds in the relatively long spacer can yield a significant number of possible structures. For some of these conformers the energies were higher than that for the lowest-energy structure by only 5 kJ/mol or even less. Vibrational analysis reveals the absence of imaginary frequencies, which indicates that these structures correspond to local minima on the potential energy surface. For the lowest triplet excited states, the situation is largely similar: the “closed” structures are reproducibly less stable than their “open” counterparts. It is worth mentioning, however, that the “closed” triplets tend to display aurophilic contacts, which cannot emerge in the “open” structures. Ground-state frontier orbitals of the optimized structures of complexes 1A−3A are given in Figures S52−S54 in the Supporting Information. UV/vis spectra of the dinuclear complexes obtained from TDDFT calculations show good agreement with the experimental data (Table S1 in the Supporting Information). The nature of the lowest-energy transitions can be described as a combination of LLCT (RC C → NHC) and LMCT with admixtures of intraligand charge redistribution (Figure 16). Luminescent properties of the dinuclear complexes were investigated by DFT optimizations of their lowest triplet states. The wavelengths corresponding to the calculated energy difference between the lowest triplet excited state and the singlet ground state are given in Table S1 in the Supporting Information. The nature of the triplet state for phenylacetylides 1A−3A is drastically different from that of the alkylalkynyl dinuclear complexes, since the spin density in the former compounds is localized on the PhCC fragment. While this result is essentially similar to that obtained for mononuclear complex 4A, it should be noted that, in the dinuclear species, the excitation involves only a half of the complex rather than both of its parts simultaneously (Figure S55 in the Supporting Information). For 1B−3B and 1C−3C, the excitation is typically localized on one of the AuCCR fragments (predominantly on the CC bond). The energy gap between the lowest triplet state and the ground state for the dinunclear alkylalkynyl complexes is significantly higher than that for the phenylacetylides and is, typically, between 330 and 340 nm, in comparison to 413−415 nm for 1A−3A. This difference might be largely responsible for the observed emission of 1A−3A in solution and the lack of luminescence for the other dinuclear species. Since the photophysical properties in the solid state could not be investigated in the framework of the same computational approach, single-point DFT calculations were carried out for the dimeric units of 4Agreen and 4Ayellow and the trinuclear cationic part of complex 7. Figures S56−S58 in the Supporting Information present the ground-state frontier orbitals obtained for these structures. The electronic structures of these compounds are thus largely similar, as indicated by the nature of the orbitals which are involved in lowest-energy transitions. The highest occupied orbitals are predominantly localized on gold atoms and phenylalkynyl fragments, while the lowest virtual orbitals are localized on the NHC fragment. Despite the obvious differences in the nature and composition of the complexes, the similarity of electronic structure enables a meaningful comparison of their photophysical properties and justifies the inclusion of trinuclear species 7 in the relationship depicted in Figure 14.

■

EXPERIMENTAL SECTION

General Notes. Chloro(tetrahydrothiophene)gold(I),84 gold(I) phenylacetylide,39 (3-hydroxy-3-methylbut-1-yn-1-yl)gold(I),39 ((1hydroxycyclohexyl)ethynyl)gold(I),39 and bromide85 and hexafluorophosphate86 salts of 1-benzyl-3-methylbenzimidazolium (NHC·HBr and NHC·HPF6) were prepared according to literature procedures. The preparation of bidentate NHC ligand precursors was reported previously.15 Other reagents were obtained commercially and used as received. Solvents were obtained from commercial suppliers and distilled over suitable drying agents. NMR spectra were recorded on Bruker Avance 400 and 500 MHz instruments. Chemical shifts are reported in parts per million (ppm) and referenced to tetramethylsilane (0 ppm) using residual proton solvent peaks as internal standards for 1H NMR experiments or the characteristic resonances of the solvent nuclei for 13C NMR experiments. High-resolution ESI mass spectra were measured on a Bruker Daltonik MaXis ESI-QTOF instrument. Elemental analyses were performed using a Euro EA3028HT elemental analyzer. UV/vis absorption spectra were recorded with a Shimadzu UV-1800 spectrophotometer; excitation and emission spectra, excited state lifetimes, and emission quantum yields were measured on a HORIBA Scientific FluoroLog-3 spectrofluorometer. X-ray powder diffraction experiments were carried out using a Rigaku MiniFlex II instrument with a Cu Kα1 X-ray source. Density functional theory (DFT) calculations were carried out for complexes 1A−4A, 14782


Article

Inorganic Chemistry

OHC6H10OH), 1.70−1.63 (m, 4H, CH2C6H10OH), 1.57−1.48 (m, 8H, CH2C6H10OH), 1.44−1.36 (m, 6H, CH2C6H10OH), 1.21−1.11 (m, 2H, CH2C6H10OH). HR-ESI+ MS (m/z): 1091.2894 [M + Na]+, 2013.5298 [2 M − HOC6H10CC]+, 2159.6088 [2 M + Na]+; calcd for [C45H46N4O2Au2Na]+, 1091.2850. NHC-(CH2)2-NHC(AuCCC6H10OH)2 (2B). Yield: method I, 32 mg (60%); method II, 35 mg (64%). Anal. Calcd for C46H48N4O2Au2· CH2Cl2: C, 48.34; H, 4.32; N, 4.80. Found: C, 47.62; H, 4.19; N, 4.54. 1 H NMR (400 MHz, DMSO-d6): δ (ppm) 7.48 (d, 3JH,H = 8 Hz, 2H, 4 CHbenzimidazole), 7.41 (d, 3JH,H = 8 Hz, 2H, 7CHbenzimidazole), 7.34−7.19 (m, 14H, CHAr), 5.67 (s, 4H, CH2benzyl), 5.08 (s, 4H, CH2−CH2), 4.67 (s, 2H, OHC6H10OH), 1.74−1.68 (m, 4H, CH2C6H10OH), 1.64−1.50 (m, 8H, CH2C6H10OH), 1.46−1.38 (m, 6H, CH2C6H10OH), 1.24−1.12 (m, 2H, CH2C6H10OH). HR-ESI+ MS (m/z): 1105.2982 [M + Na]+, 1121.2724 [M + K]+, 1721.4931 [2 M − HOC6H10CC]+, 2187.6182 [2 M + Na]+, 2203.5926 [2 M + K]+; calcd for [C46H48N4O2Au2Na]+, 1105.3006. NHC-(CH2)3-NHC(AuCCC6H10OH)2 (3B). Yield: method I, 43 mg (79%); method II, 44 mg (80%). Anal. Calcd for C47H50N4O2Au2: C, 51.47; H, 4.59; N, 5.11. Found: C, 51.02; H, 4.01; N, 4.75. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.83 (d, 3JH,H = 8 Hz, 2H, 4 CHbenzimidazole), 7.58 (d, 3JH,H = 8 Hz, 2H, 7CHbenzimidazole), 7.46−7.27 (m, 14H, CHAr), 5.87 (s, 4H, CH2benzyl), 4.80 (s, 2H, OHC6H10OH), 4.60 (t, 3JH,H = 7 Hz, 4H, CH2-CH2-CH2), 2.71 (quint, 3JH,H = 7 Hz, 2H, CH2-CH2-CH2),1.74−1.68 (m, 4H, CH2C6H10OH), 1.64−1.50 (m, 8H, CH2C6H10OH), 1.46−1.38 (m, 6H, CH2C6H10OH), 1.24−1.12 (m, 2H, CH2C6H10OH). 13C{1H} NMR (100 MHz, DMSO-d6): δ (ppm) 194.27, 136.03, 133.11, 132.80, 128.74, 127.90, 127.02, 124.33, 124.24, 120.21, 112.44, 112.22, 66.77, 51.62, 44.92, 40.77, 28.20, 25.25, 22.87. The signal of one quaternary carbon from the CC moiety was not observed. HR-ESI+ MS (m/z): 973.2457 [M − HOC6H10CC]+, 1119.3163 [M + Na]+, 2069.5766 [2 M − HOC6H10CC]+, 2216.6490 [2 M + Na]+; calcd for [C47H50N4O2Au2Na]+, 1119.3163. NHC-CH2-NHC(AuCCC3H6OH)2 (1C). Yield: method I, 32 mg (65%); method II, 33 mg (68%). Anal. Calcd for C39H38N4O2Au2: C, 47.38; H, 3.87; N, 5.67. Found: C, 47.14; H, 3.72; N, 5.71. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.90 (d, 3JH,H = 8 Hz, 2H, 4 CHbenzimidazole), 7.68 (d, 3JH,H = 8 Hz, 2H, 7CHbenzimidazole), 7.45−7.28 (m, 16H, CHAr, N-CH2-N), 5.86 (s, 4H, CH2benzyl), 4.78 (s, 2H, OHC3H6OH), 1.33 (s, 12H, CH3C3H6OH). HR-ESI+ MS (m/z): 1011.2244 [M + Na]+, 1999.4693 [2 M + Na]+; calcd for [C39H38N4O2Au2Na]+, 1011.2224. NHC-(CH2)2-NHC(AuCCC3H6OH)2 (2C). Yield: method I, 26 mg (52%); method II, 30 mg (59%). Anal. Calcd for C40H40N4O2Au2: C, 47.91; H, 4.02; N, 5.59. Found: C, 47.23; H, 4.00; N, 5.44. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.49 (d, 3JH,H = 8 Hz, 2H, 4 CHbenzimidazole), 7.37 (d, 3JH,H = 8 Hz, 2H, 7CHbenzimidazole), 7.33−7.19 (m, 14H, CHAr), 5.67 (s, 4H, CH2benzyl), 5.07 (s, 4H, CH2-CH2), 4.76 (s, 2H, OHC3H6OH), 1.36 (s, 12H, CH3C3H6OH). HR-ESI+ MS (m/z): 919.2088 [M − HOC3H6CC]+, 1025.2476 [M + Na]+; calcd for [C40H40N4O2Au2Na]+, 1025.2380. NHC-(CH2)3-NHC(AuCCC3H6OH)2 (3C). Yield: method I, 41 mg (81%); method II, 38 mg (75%). Anal. Calcd for C41H42N4O2Au2: C, 48.43; H, 4.16; N, 5.51. Found: C, 48.09; H, 4.25; N, 5.76. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.84 (d, 3JH,H = 8 Hz, 2H, 4 CHbenzimidazole), 7.62 (d, 3JH,H = 8 Hz, 2H, 7CHbenzimidazole), 7.47−7.27 (m, 14H, CHAr), 5.87 (s, 4H, CH2benzyl), 4.88 (s, 2H, OHC3H6OH), 4.58 (t, 3JH,H = 6 Hz, 4H, CH2-CH2-CH2), 2.71 (quint, 3JH,H = 6 Hz, 2H, CH2-CH2-CH2), 1.36 (s, 12H, CH3C3H6OH). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) 194.72, 134.94, 133.48, 133.37, 129.07, 128.48, 127.42, 124.90, 124.78, 117.49, 112.69, 111.47, 111.01, 65.81, 53.01, 45.62, 32.94, 30.00. HR-ESI+ MS (m/z): 1039.2578 [M + Na]+; calcd for [C41H42N4O2AuNa]+, 1039.2537. Synthesis of Mononuclear Complexes [NHCAuCCR] (4A− C). NHC·HBr (40 mg, 0.13 mmol) or NHC·HPF6 (50 mg, 0.13 mmol), the respective gold(I) acetylide (0.13 mmol), and Cs2CO3 (300 mg, 0.92 mmol) were stirred in CH2Cl2 at room temperature for 5 h. The mixture was filtered through Celite, and the filtrate was diluted with heptane and left in an open vessel for 2−3 days to

1B−4B, 1C−4C, and 5 using the CAM-B3LYP long-range-corrected hybrid functional.87 Gold atoms were described by the Stuttgart− Dresden basis set with effective core potential;88 for all other atoms the 6-31G* basis set was used. Geometries for singlet ground states and lowest triplet excited states were fully optimized by DFT and verified by vibrational analysis. UV/vis absorption spectra of the complexes were obtained for optimized structures by time-dependent DFT calculations (TDDFT) with the same combination of functional and basis set. All quantum chemical calculations were performed using the Gaussian 09 package.89 Synthesis of Dinuclear Complexes [NHC-(CH2)n-NHC(AuC CR) 2 ] (1A−C, 2A−C, and 3A−C). Method I. Chloro(tetrahydrothiophene)gold(I) (0.10 mmol), the corresponding bidentate NHC ligand precursor (0.05 mmol), and NaOAc·3H2O (0.20 mmol) were stirred in methanol at room temperature for 2 h; the precipitate that formed was collected by filtration and washed with small portions of methanol. The residue was suspended in methanol, and the respective acetylene (0.30 mmol) and K2CO3 (0.30 mmol) were added. The mixture was stirred at room temperature for 1 h, and the newly formed precipitate was collected by filtration, washed with methanol and then with acetone, redissolved in CH2Cl2, filtered, and evaporated to dryness. Recrystallization by diffusion of pentane into the CH2Cl2 solutions of the respective complexes with subsequent drying under vacuum afforded the title compounds as white powders. Method II. Gold(I) acetylide (0.10 mmol), the respective bidentate NHC ligand precursor (0.05 mmol) and Cs2CO3 (0.5 mmol) were stirred in CH2Cl2 at room temperature for 5 h. The suspension was filtered through a pad of Celite, the solvent was removed under reduced pressure, and the residue was washed with acetone. Diffusion of pentane into the CH2Cl2 solutions of the respective complexes, followed by filtration and vacuum drying, afforded the title compounds as white powders. NHC-CH2-NHC(AuCCPh)2 (1A). Yield: method I, 33 mg (64%); method II, 37 mg (73%). Anal. Calcd for C45H34N4Au2: C, 52.75; H, 3.34; N, 5.47. Found: C, 52.25; H, 3.42; N, 5.11. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.90 (d, 3JH,H = 8 Hz, 2H, 4CHbenzimidazole), 7.73 (d, 3JH,H = 8 Hz, 2H, 7CHbenzimidazole), 7.53−7.16 (m, 26H, CHAr, N− CH2-N), 5.89 (s, 4H, CH2benzyl). HR-ESI+ MS (m/z): 923.1718 [M − PhCC]+, 1047.2007 [M + Na]+, 1063.1746 [M + K]+; calcd for [C45H34N4Au2Na]+, 1047.2007. NHC-(CH2)2-NHC(AuCCPh)2 (2A). Yield: method I, 36 mg (71%); method II, 45 mg (88%). Anal. Calcd for C46H36N4Au2: C, 53.19; H, 3.49; N, 5.39. Found: C, 52.85; H, 3.73; N, 5.01. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.54 (d, 3JH,H = 8 Hz, 2H, 4CHbenzimidazole), 7.47 (d, 3JH,H = 8 Hz, 2H, 7CHbenzimidazole), 7.36−7.16 (m, 24H, CHAr), 5.66 (s, 4H, CH2benzyl), 5.16 (s, 4H, CH2-CH2). 13C{1H} NMR (125 MHz, CDCl3): δ (ppm) 194.65, 134.61, 133.78, 133.05, 132.46, 129.16, 128.65, 128.17, 128.11, 127.62, 126.69, 125.62, 125.18, 124.92, 112.31, 111.28, 105.87, 53.02, 47.72. HR-ESI+ MS (m/z): 937.1880 [M − PhCC]+, 1061.2172 [M + Na]+, 2099.4680 [2 M + Na]+; calcd for [C46H36N4Au2Na]+, 1061.2163. NHC-(CH2)3-NHC(AuCCPh)2 (3A). Yield: method I, 38 mg (74%); method II, 43 mg (85%). Anal. Calcd for C47H38N4Au2: C, 53.62; H, 3.64; N, 5.32. Found: C, 53.41; H, 3.48; N, 5.42. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.88 (d, 3JH,H = 8 Hz, 2H, 4CHbenzimidazole), 7.62 (d, 3JH,H = 8 Hz, 2H, 7CHbenzimidazole), 7.47−7.19 (m, 24H, CHAr), 5.82 (s, 4H, CH2benzyl), 4.69 (t, 3JH,H = 7 Hz, 4H, CH2-CH2-CH2), 2.72 (quint, 3JH,H = 7 Hz, 2H, CH2-CH2-CH2). 13C{1H} NMR (125 MHz, CDCl3): δ (ppm) 194.65, 135.02, 133.60, 133.40, 132.41, 129.04, 128.43, 128.12, 128.11, 127.52, 126.58, 125.81, 124.91, 124.75, 112.74, 111.52, 105.82, 53.09, 45.63, 29.99. HR-ESI+ MS (m/z): 951.2064 [M − PhCC]+, 1075.2284 [M + Na]+; calcd for [C47H38N4Au2Na]+, 1075.2320. X-ray-quality single crystals of complex 3A were grown by slow evaporation of its CH2Cl2/MeOH solution at room temperature. NHC-CH2-NHC(AuCCC6H10OH)2 (1B). Yield: method I, 34 mg (63%); method II, 38 mg (71%). Anal. Calcd for C45H46N4O2Au2· 2CH2Cl2: C, 45.57; H, 4.07; N, 4.52. Found: C, 45.09; H, 3.74; N, 4.55. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.90 (d, 3JH,H = 8 Hz, 2H, 4CHbenzimidazole), 7.67 (d, 3JH,H = 8 Hz, 2H, 7CHbenzimidazole), 7.45− 7.28 (m, 16H, CHAr, N-CH2-N), 5.86 (s, 4H, CH2benzyl), 4.70 (s, 2H, 14783


Article

Inorganic Chemistry

diethyl ether vapors were allowed to diffuse into the filtrate. In ca. 12 h intensely blue-glowing yellow crystals formed, the best of which were investigated by X-ray diffraction. All of the crystals tried were found to have identical cell parameters. The remaining crystals were collected by filtration, washed with diethyl ether, and dried. Further crystallization of the mother liquor afforded crystals of both [NHCAuCCPh] (4A) and [(NHC)2Au]PF6 (6PF6), as confirmed by XRD analyses. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.86 (dd, 3 JH,H = 7 Hz, 4JH,H = 1 Hz, 4H, 4CHbenzimidazole), 7.83 (dd, 3JH,H = 7 Hz, 4 JH,H = 1 Hz, 4H, 7CHbenzimidazole), 7.54 (ddd, 3JH5,H4 = 3JH5,H6 = 7 Hz, 4 JH,H = 1 Hz, 4H, 5CHbenzimidazole), 7.50 (ddd, 3JH6,H5 = 3JH6,H7 = 7 Hz, 4 JH,H = 1 Hz, 4H, 6CHbenzimidazole), 7.41−7.38 (m, 8H, o-CHbenzyl), 7.33−7.28 (m, 12H, m,p-CHbenzyl), 7.22−7.17 (m, 8H, o,m-CHCCPh), 7.14−7.09 (m, 2H, p-CHCCPh) 5.84 (s, 8H, CH2benzyl), 4.14 (s, 12H, CH3). HR-ESI+ MS (m/z): 641.1994 [(NHC)2Au]+; calcd for [C30H28N4Au]+, 641.1974. HR-ESI− MS (m/z): 144.9628 [PF6]−, 399.0383 [(PhCC)2Au]−; calcd for [PF6]−, 144.9647; calcd for [C16H10Au]−, 399.0454. Isolation of [(NHC)2Au]2[(HOC3H6C2)2Au]Br (8). NHC·HBr (20 mg, 0.065 mmol), (3-hydroxy-3-methylbut-1-yn-1-yl)gold(I) (18 mg, 0.065 mmol), and Cs2CO3 (100 mg, 0.31 mmol) were stirred in CH2Cl2 at room temperature for 1 h. The mixture was filtered through Celite, and diethyl ether vapors were allowed to diffuse into the filtrate. In ca. 12 h intensely blue-glowing yellow crystals (not suitable for XRD) formed, which were collected by filtration, washed with diethyl ether, and dried. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.86 (dd, 3 JH,H = 7 Hz, 4JH,H = 1 Hz, 4H, 4CHbenzimidazole), 7.83 (dd, 3JH,H = 7 Hz, 4 JH,H = 1 Hz, 4H, 7CHbenzimidazole), 7.54 (ddd, 3JH5,H4 = 3JH5,H6 = 7 Hz, 4 JH,H = 1 Hz, 4H, 5CHbenzimidazole), 7.50 (ddd, 3JH6,H5 = 3JH6,H7 = 7 Hz, 4 JH,H = 1 Hz, 4H, 6CHbenzimidazole), 7.41−7.38 (m, 8H, o-CHbenzyl), 7.33−7.28 (m, 12H, m,p-CHbenzyl), 5.84 (s, 8H, CH2benzyl), 4.57 (s, 2H, OHC3H6OH), 4.15 (s, 12H, CH3), 1.24 (s, 12H, CH3C3H6OH). HR-ESI+ MS (m/z): 641.1999 [(NHC)2Au]+; calcd for [C30H28N4Au]+, 641.1974. HR-ESI− MS (m/z): 363.0681 [(HOC3H6CC)2Au]−; calcd for [C10H14O2Au]−, 363.0665.

produce colorless crystals, which were collected by filtration, washed with diethyl ether, and dried. NHCAuCCPh (4A). Yield: 51 mg (75%). Anal. Calcd for C23H19N2Au·0.5CH2Cl2: C, 50.15; H, 3.58; N, 4.98. Found: 49.40; H, 3.58; N, 5.01. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.78 (d, 3 JH,H = 8 Hz, 1H, 4CHbenzimidazole), 7.68 (d, 3JH,H = 8 Hz, 1H, 7 CHbenzimidazole), 7.48−7.17 (m, 12H, CHAr), 5.78 (s, 2H, CH2benzyl), 4.08 (s, 3H, CH3). HR-ESI+ MS (m/z): 543.1105 [M + Na]+, 939.2031 [2 M − PhCC]+, 1063.2306 [2 M + Na]+; calcd for [C23H19N2AuNa]+, 543.1106. NHCAuCCC6H10OH (4B). Yield: 48 mg (68%). Anal. Calcd for C23H25N2OAu: C, 50.93; H, 4.65; N, 5.16. Found: C, 50.30; H, 4.45; N, 4.48. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.76 (d, 3JH,H = 8 Hz, 1H, 4CHbenzimidazole), 7.64 (d, 3JH,H = 8 Hz, 1H, 7CHbenzimidazole), 7.47−7.28 (m, 7H, CHAr), 5.74 (s, 2H, CH2benzyl), 4.70 (s, 1H, OHC6H10OH), 4.05 (s, 3H, CH3), 1.68−1.62 (m, 2H, CH2C6H10OH), 1.55−1.49 (m, 4H, CH2C6H10OH), 1.44−1.36 (m, 3H, CH2C6H10OH), 1.22−1.12 (m, 1H, CH2C6H10OH). HR-ESI+ MS (m/z): 565.1536 [M + Na]+, 961.2484 [2 M − HOC6H10CC]+, 1107.3203 [2 M + Na]+; calcd for [C23H25N2OAuNa]+, 565.1530. NHCAuCCC3H6OH (4C). Yield: 50 mg (77%). Anal. Calcd for C20H21N2OAu: C, 47.82; H, 4.21; N, 5.58. Found: C, 48.01; H, 4.17; N, 5.36. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.75 (d, 3JH,H = 8 Hz, 1H, 4CHbenzimidazole), 7.64 (d, 3JH,H = 8 Hz, 1H, 7CHbenzimidazole), 7.47−7.28 (m, 7H, CHAr), 5.73 (s, 2H, CH2benzyl), 4.77 (s, 1H, OHC3H6OH), 4.04 (s, 3H, CH3), 1.32 (s, 6H, CH3C3H6OH). HR-ESI+ MS (m/z): 525.1214 [M + Na]+, 1027.2525 [2 M + Na]+; calcd for [C20H21N2OAuNa]+, 525.1212. Synthesis of [NHCAuBr] (5). NHC·HBr (50 mg, 0.165 mmol), chloro(tetrahydrothiophene)gold(I) (53 mg, 0.165 mmol), and Cs2CO3 (300 mg, 0.92 mmol) were stirred in CH2Cl2 at room temperature for 5 h. The mixture was filtered through Celite, and the filtrate was diluted with heptane and left in an open vessel for 2−3 days to produce colorless crystals, which were collected by filtration, washed with diethyl ether, and dried. Yield: 51 mg (62%). 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.80 (d, 3JH,H = 8 Hz, 1H, 4 CHbenzimidazole), 7.72 (d, 3JH,H = 8 Hz, 1H, 7CHbenzimidazole), 7.51−7.41 (m, 4H, 5,6CHbenzimidazole, o-CHbenzyl), 7.38−7.28 (m, 3H, m,p-CHbenzyl), 5.75 (s, 2H, CH2benzyl), 4.06 (s, 3H, CH3). HR-ESI+ MS (m/z): 917.0842 [2 M − Br]+; calcd for [C30H28N4BrAu2]+, 917.0828. Synthesis of [(NHC)2Au]X (X = Br−, PF6−; 6Br and 6PF6). NHC· HBr (40 mg, 0.13 mmol) or NHC·HPF6 (50 mg, 0.13 mmol), chloro(tetrahydrothiophene)gold(I) (21 mg, 0.065 mmol), and Cs2CO3 (300 mg, 0.92 mmol) were stirred in CH2Cl2 at room temperature for 12 h. The mixture was filtered through Celite, and the filtrate was diluted with heptane and left in an open vessel for 2−3 days to produce colorless crystals, which were collected by filtration, washed with diethyl ether, and dried. [(NHC)2Au]Br (6Br). Yield: 69 mg (73%). 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.87 (dd, 3JH,H = 7 Hz, 4JH,H = 1 Hz, 1H, 4 CHbenzimidazole), 7.84 (dd, 3JH,H = 7 Hz, 4JH,H = 1 Hz, 1H, 7 CHbenzimidazole), 7.54 (ddd, 3JH5,H4 = 3JH5,H6 = 7 Hz, 4JH,H = 1 Hz, 1H, 5CHbenzimidazole), 7.50 (ddd, 3JH6,H5 = 3JH6,H7 = 7 Hz, 4JH,H = 1 Hz, 1H, 6CHbenzimidazole), 7.41−7.38 (m, 2H, o-CHbenzyl), 7.33−7.28 (m, 3H, m,p-CHbenzyl), 5.84 (s, 2H, CH2benzyl), 4.15 (s, 3H, CH3). HR-ESI+ MS (m/z): 641.1974 [M]+; calcd for [C30H28N4Au]+, 641.1974. [(NHC)2Au]PF6 (6PF6). Yield: 81 mg (76%). 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.86 (dd, 3JH,H = 7 Hz, 4JH,H = 1 Hz, 1H, 4 CHbenzimidazole), 7.83 (dd, 3JH,H = 7 Hz, 4JH,H = 1 Hz, 1H, 7 CHbenzimidazole), 7.54 (ddd, 3JH5,H4 = 3JH5,H6 = 7 Hz, 4JH,H = 1 Hz, 1H, 5CHbenzimidazole), 7.50 (ddd, 3JH6,H5 = 3JH6,H7 = 7 Hz, 4JH,H = 1 Hz, 1H, 6CHbenzimidazole), 7.41−7.38 (m, 2H, o-CHbenzyl), 7.33−7.28 (m, 3H, m,p-CHbenzyl), 5.83 (s, 2H, CH2benzyl), 4.14 (s, 3H, CH3). HR-ESI+ MS (m/z): 641.1994 [M]+; calcd for [C30H28N4Au]+, 641.1974. HRESI− MS (m/z): 144.9647 [PF6]−; calcd for [PF6]−, 144.9647. Isolation of [(NHC)2Au]2[(PhC2)2Au]PF6 (7). NHC·HPF6 (20 mg, 0.05 mmol), gold(I) phenylacetylide (16 mg, 0.05 mmol), and Cs2CO3 (100 mg, 0.31 mmol) were stirred in CH2Cl2 at room temperature for 1 h. The mixture was filtered through Celite, and

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01508. Spectroscopic data (NMR, HR-ESI MS, UV/vis, emission and excitation spectra), computational results, PXRD patterns, and crystallographic information (PDF) Accession Codes

CCDC 1504380−1504382, 1504386−1504389, 1528375, 1528680, and 1528736 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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Corresponding Author

*S.P.T.: e-mail, [email protected]; fax, +7 (812) 3241258. ORCID

Alexander A. Penney: 0000-0003-3932-1816 Mikhail A. Kinzhalov: 0000-0001-5055-1212 Sergey P. Tunik: 0000-0002-9431-0944 Notes

The authors declare no competing financial interest. 14784


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Inorganic Chemistry

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ACKNOWLEDGMENTS The authors appreciate financial support from the grant program of the President of the Russian Federation (grant MK-7425.2016.3) and St. Petersburg State University (grant 12.40.1679.2016). NMR, photophysical, analytical, crystallographic, and computational investigations were performed using the following core facilities of St. Petersburg State University Research Park: Centre for Magnetic Resonance, Centre for Optical and Laser Materials Research, Centre for Chemical Analysis and Materials Research, X-ray Diffraction Centre, and Computing Centre, respectively.

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