Aurophilicity in Action: Fine-Tuning the Gold(I)âGold(I) Distance in the

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Aurophilicity in Action: Fine-Tuning the Gold(I)−Gold(I) Distance in the Excited State To Modulate the Emission in a Series of Dinuclear Homoleptic Gold(I)−NHC Complexes Alexander A. Penney,† Vladimir V. Sizov,† Elena V. Grachova,† Dmitry V. Krupenya,† Vladislav V. Gurzhiy,‡ Galina L. Starova,† and Sergey P. Tunik*,† †

Saint Petersburg State University, Institute of Chemistry, Universitetsky pr. 26, Saint Petersburg 198504, Russian Federation Saint Petersburg State University, Institute of Earth Sciences, University emb. 7/9, Saint Petersburg 199034, Russian Federation

‡

S Supporting Information *

ABSTRACT: The solution-state emission profiles of a series of dinuclear Au(I) complexes 4−6 of the general formula Au2(NHC-(CH2)n-NHC)2Br2, where NHC = N-benzylbenzimidazol-2-ylidene and n = 1−3, were found to be markedly different from each other and dependent on the presence of excess bromide. The addition of excess bromide to the solutions of 4 and 6 leads to red shifts of ca. 60 nm, and in the case of 5, which is nonemissive when neat, green luminescence emerges. A detailed computational study undertaken to rationalize the observed behavior revealed the determining role aurophilicity plays in the photophysics of these compounds, and the formation of exciplexes between the complex cations and solvent molecules or counterions was demonstrated to significantly decrease the Au−Au distance in the triplet excited state. A direct dependence of the emission wavelength on the strength of the intracationic aurophilic contact allows for a controlled manipulation of the emission energy by varying the linker length of a diNHC ligand and by judicial choice of counterions or solvent. Such unique stimuli-responsive solution-state behavior is of interest to prospective applications in medical diagnostics, bioimaging, and sensing. In the solid, the investigated complexes are intensely phosphorescent and, notably, 5 and 6 exhibit reversible luminescent mechanochromism arising from amorphization accompanied by the loss of co-crystallized methanol molecules. The mechano-responsive properties are also likely to be related to changes in bromide coordination and the ensuing alterations of intramolecular aurophilic interactions. Somewhat surprisingly, the photophysics of NHC ligand precursors 2 and 3 is related to the formation of groundstate associates with bromide counterions through hydrogen bonding, whereas 1 does not appear to bind its counterions.

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INTRODUCTION Aurophilicity continues to attract considerable theoretical and experimental interest largely due to the diverse photophysical effects it often produces.1 Phosphine and N-heterocyclic carbene (NHC) gold(I) complexes are known to be intensely luminescent, and their emission profiles may be profoundly influenced by the presence of intra- or intermolecular aurophilic interactions.2 The high emission efficiency of these species paves the way for applications in such important research areas as bioimaging3,4 and OLED devices.5 Strong spin−orbit coupling greatly facilitated by the heavy atom effect places gold(I) compounds among those in a unique class of brightly phosphorescent emitters.6 More recently, reversible stimuli-responsive luminescence switching arising from the modulation of aurophilic interactions, both in solution and in the solid state, instigated an increase in efforts to further the understanding of these phenomena that are potentially capable of revolutionizing sensing devices and medical diagnostics.7−9 Being weak © 2016 American Chemical Society

dispersion forces, aurophilic interactions are well-suited to form the basic machinery through which relevant analytical signals can be transduced with the added benefit of their relatively simple optical detection. Numerous reports have highlighted the possibility for ligand-supported Au(I) systems to be employed as environmental sensors,8−10 ion probes,11,12 molecular switches,13 etc. A particularly elegant example from the recent literature is that of a vapochromic heterobimetallic system described by Catalano et al., in which coordination of solvent molecules to coordinatively unsaturated Cu(I) centers induces modulation of metallophilic interactions with concomitant changes in emission.14 Upon exposure to solvent vapors, some phosphine-ligated and isocyanide Au(I) complexes have been documented to demonstrate remarkable alterations in emission without the uptake of solvent molecules.15,16 Koshevoy et al. reported a polymorphic Received: November 23, 2015 Published: February 16, 2016 4720

DOI: 10.1021/acs.inorgchem.5b02722 Inorg. Chem. 2016, 55, 4720−4732

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Inorganic Chemistry

conditions, leading to singly charged species formulated as [M2+ − CH2Ph+]+ (Figures S6−S8). The solid-state structure of 2 was determined by singlecrystal X-ray diffraction (Figure 1 and Table S1). The bromide

Au(I)−Cu(I) alkynyl cluster that undergoes significant changes in emission as a result of the action of vapors of different solvents.17 A series of tetranuclear Au(I)−Cu(I) clusters was found to transform from the amorphous state into crystalline solid phases upon exposure to vapors of some polar solvents, leading to a yellow to blue shift of the emission color.18 Often, the underlying basis for the observed stimuli-responsive behavior is related either to the modulation of auro- or metallophilic interactions or variations in counterion/solvent coordination.19 However, the influence counterions exert on the electronics of d10 complexes and exciplex formation tends to be overlooked despite ample evidence that such possibilities may be paramount to the photophysics of coinage metal compounds.20−24 Rarely encountered and somewhat elusive, luminescent mechanochromism also presents promising practical applications.25 In this regard, gold(I) compounds have similarly proven to be at the vanguard of research efforts driven by deep curiosity to unravel the basic mechanisms responsible for the observed grinding-induced changes in emission.26,27 In this article, we report on the synthesis, characterization, and photophysical properties of a series of dinuclear bis(diNHC) Au(I) complexes based on the bridging bidentate benzimidazol-2-ylidene scaffold. Related publications5,28−34 prompted us to systematically investigate the effect of ligand linker length and counterions on the photophysics of dinuclear bis(dicarbene) Au(I)−NHC complexes. Being aware of polymorphism frequently encountered in gold(I) chemistry,35 flexible ethylene and propylene spacers capable of adopting different conformations were employed to examine the possibility of distinct structural arrangements associated with changes in intramolecular organization.

Figure 1. Thermal ellipsoid (50%) plot of 2. Selected bond distances (Å) and angles (deg): C1−N1 1.232, C1−N2 1.327, C1−H1 0.930, H1−Br1 2.750, H3−Br1 2.780, N1−C1−N2 110.60.

counterion is hydrogen-bonded to the hydroxyl hydrogen of the methanol molecule and also demonstrates short contacts to H1 and H3. All intramolecular bond lengths and angles are typical for benzimidazolium salts reported in the literature,36,37 and no intermolecular π-stacking interactions are observed. Synthesis and Characterization of Au(I)−NHC Complexes. Homoleptic complexes 4−6 were prepared by a modified procedure originally reported by Baker et al.38 (Scheme 2). Equimolar amounts of chloro(tetrahydrothiophene)gold(I) and the corresponding NHC ligand precursor with a small excess of NaOAc·3H2O as a base were heated at 90 °C in degassed DMF for 1 h under an atmosphere of Ar. The resulting mixture was precipitated with diethyl ether, washed with water to remove inorganic salts, and dried. The material obtained contains the target complex with scrambled counterions (i.e., a mixture of bromide and chloride), as evidenced by the presence of two monocationic signals of the [M2+ + Br−]+ and [M2+ + Cl−]+ compositions in the ESI+ mass spectra. Refluxing the mixture with excess KBr in methanol for 5 min followed by the addition of water, filtration, and recrystallization affords complexes 4−6 as analytically pure bromide complexes, as confirmed by mass spectrometry. Anion metathesis between compounds 4−6 and KPF6 in methanol with the subsequent addition of water precipitates complexes 7−9 in quantitative yield (Scheme 2). The presence of PF6− counterions is confirmed by 31P NMR spectroscopy along with mass spectra featuring the [M2+ + PF6−]+ ion pair, and the absence of left-over bromide is ensured by the lack of the [M2+ + Br−]+ signal. Single-Crystal X-ray Crystallography of Gold(I)−NHC Complexes. The solid-state structures of 4−6 and 9 were determined by single-crystal XRD studies. Crystallographic data are summarized in Table S1, and molecular structures and selected structural parameters are given in Figures 2−6. Upon gas-phase diffusion of diethyl ether into a methanol/ DMSO solution, complex 4 crystallizes as a disolvate containing one molecule of DMSO and one molecule of methanol (Figure 2), neither of which interact with the cation. However, DMSO hydrogens reveal short contacts to one bromide counterion, whereas the hydroxyl group hydrogen of the methanol molecule is hydrogen-bonded to the oxygen atom of DMSO. C2 effective symmetry identified in solution (vide infra) is lost in the solid state, where the cation of 4 is devoid of any

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RESULTS AND DISCUSSION Synthesis and Characterization of NHC Ligand Precursors. NHC ligand precursors 1−3 were synthesized by the reaction of N-benzylbenzimidazole with corresponding α,ω-dibromoalkanes (Scheme 1). Compound 1 was obtained Scheme 1. Synthesis of NHC Ligand Precursors 1−3

by refluxing N-benzylbenzimidazole in neat dibromomethane, whereas refluxing excess N-benzylbenzimidazole with either 1,2-dibromoethane or 1,3-dibromopropane in acetonitrile afforded 2 and 3, respectively. The 1H NMR spectra of NHC ligand precursors 1−3 (Figure S1) demonstrate a characteristic downfield signal at ca. 10 ppm corresponding to the H2 proton of the benzimidazolium moiety. The aromatic region of the spectra presents a number of well-resolved multiplets, which were assigned on the basis of 2D 1H−1H COSY experiments (Figures S3−S5). The ESI+ mass spectra of 1−3 contain signals corresponding to doubly charged cations [M]2+, along with those of monocations [M2+ − H+]+ and ion pairs of the [M2+ + Br−]+ composition. Some fragmentation is observed under the ESI+ ionization 4721


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Inorganic Chemistry Scheme 2. Synthesis of Homoleptic Complexes 4−9

aurophilic contacts or π-stacking interactions are observed in the crystal cell. A single crystal of 5 was grown by cooling its methanolic solution (Figure 3). The four Au−Ccarbene distances range from

Figure 2. Thermal ellipsoid (30%) plot of 4. Selected bond distances (Å) and angles (deg): Au1−C1 2.006, Au1−C45 2.029, Au2−C16 2.023, Au2−C30 2.035, Au1−Au2 3.396, Au1−Br1 3.499, Au2−Br2 3.412, C1−Au1−C45 174.97, C16−Au2−C30 173.49.

Figure 3. Thermal ellipsoid (30%) plot of 5. Selected bond distances (Å) and angles (deg): Au1−C1 2.006, Au1−C47 1.999, Au2−C17 2.029, Au2−C31 2.033, Au1−Au2 3.362, Au1−Br1 3.586, Au2−Br2 3.545, C1−Au1−C47 178.17, C17−Au2−C31 178.03.

symmetry. Nevertheless, the observed Au−Ccarbene distances and Ccarbene−Au−Ccarbene angles are in good agreement with those reported in literature for other dinuclear benzimidazol-2ylidene-based bis(carbene) Au(I) complexes.28 Due to a moderate aurophilic interaction (Au1−Au2 distance of 3.396 Å), the two gold atoms bow toward each other to distort the linearity of the Ccarbene−Au−Ccarbene axes. The inequivalence of hydrogen atoms on each of the methylene spacers between the two donor functions of the dicarbenes manifests itself in the presence of endo and exo hydrogens. The gold(I) centers are approached by bromide counterions, each of which demonstrates a short contact to an adjacent hydrogen residing on a benzyl methylene. Additionally, one bromide forms a short contact with the endo hydrogen of one methylene linker. Although the Au−Br distances of ca. 3.5 Å are too long to be considered bona fide coordinated, we argue that it is the abundance of hydrogen atoms in the vicinity of metal centers that prevents closer approach of anions. No intermolecular

1.999 to 2.033 Å with almost linear Ccarbene−Au−Ccarbene angles. The interatomic distance of 3.362 Å between the two gold centers is indicative of the presence of aurophilic attraction. One bromide approaches the gold atom and demonstrates contacts to one of the methylene hydrogens of a benzyl group and to a hydrogen on one ethylene spacer. Likewise, the other bromide adjoins the second metal center and interacts with a hydrogen atom on a benzyl methylene and a hydrogen on the other ethylene linker. In the solid state, 6 was found to exist in two polymorphic modifications. Both polymorphs were crystallized in a similar manner by cooling methanolic solutions of 6. One form, denoted 6-syn, crystallizes in the monoclinic C2/c space group with half of the cation and one bromide counterion in the 4722


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Inorganic Chemistry asymmetric unit (Figure 4). The bromide is disordered over two close positions with equal occupancies and is uncoordi-

Figure 5. Thermal ellipsoid (30%) plot of 6-anti. Disorder was omitted for clarity. Selected bond distances (Å) and angles (deg): Au1−C1 2.020, Au1−C49 2.025, Au2−C18 2.026, Au2−C32 2.036, Au2−Br2 3.916, C1−Au1−C49 179.40, C18−Au2−C32 178.06.

indicates the absence of aurophilic interaction. No intermolecular aurophilic contacts or π-stacking interactions are found. A single crystal of 9 was grown by cooling its acetonic solution. 9 crystallizes in the monoclinic C2/c space group with half of the cation and one PF6− counterion in the asymmetric unit and with some disordered acetone molecules present (Figure 6). The structural motif of the complex is essentially

Figure 4. Thermal ellipsoid (30%) plot of 6-syn. Disorder was omitted for clarity. Selected bond distances (Å) and angles (deg): Au1−C1 2.023, Au1−C18 2.009, Au1A−Au1B 3.132, C1−Au1−C18 178.30.

nated at 6.387 Å away from the nearest metal center. One benzyl group is disordered as well, and some residual density was treated as disordered methanol molecules. The distance between the two gold(I) atoms is 3.132 Å, indicative of a relatively strong intramolecular aurophilic interaction. Flexible propylene spacers between the two NHC donor functions allow for an almost parallel orientation of the two benzimidazole planes (dihedral angle 10.56°), with the distance between them being 3.572 Å, which points to the presence of an intramolecular π-stacking interaction. Two propylene spacers are both in the syn conformation, which leads to the closed or folded geometry of the cation. The bromide interacts with two hydrogen atoms residing on different benzyl methylenes and additionally demonstrates short contacts with two hydrogens in the ortho positions of two benzyl groups. Another short contact is also present between the bromide and a hydrogen atom of one benzimidazole ring. The other polymorph, 6-anti, crystallizes in the monoclinic P21/c space group with the whole molecule in the asymmetric unit with one phenyl ring disordered (Figure 5). C2 putative symmetry observed in the solid state for 6-syn is absent, and both propylene linkers adopt an anti conformation, leading to the nonsymmetric geometry of the cation. One bromide counterion appears to approach the gold atom (Br2−Au2 3.916 Å), with the distance being well over the sum of the van der Waals radii of the two atoms. Br2 also exhibits short contacts to a hydrogen atom of an adjacent benzyl methylene, an aromatic hydrogen from another benzyl substituent, and also to a hydrogen atom residing on the propylene spacer. The other bromide is removed yet farther from the metal center and interacts with a hydrogen atom of the propylene bridge and with hydrogen atoms residing on two benzimidazole moieties. The long distance between the two gold atoms (4.308 Å)

Figure 6. Thermal ellipsoid (30%) plot of 9. Disorder was omitted for clarity. Selected bond distances (Å) and angles (deg): Au1−C1 2.019, Au1−C18 2.023, Au1A−Au1B 3.189, C1−Au1−C18 178.02.

identical to that revealed by 6-syn, including relatively strong intramolecular aurophilic interaction (Au−Au distance of 3.189 Å) and the syn conformation of both propylene spacers that leads to a folded C2 symmetric geometry of the cation. Intramolecular π-stacking between the two benzimidazole fragments of the dicarbene ligand is present, with the distance between the two planes being 3.600 Å. One PF6− fluorine atom forms short contacts with two hydrogens residing on two 4723


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Figure 7. 1H NMR spectra (CD3OD, rt) of complexes 7, 4, and 4 with the addition of excess TMAB. Note that for 7 the signal from the endo protons is overlapped with the signals of the H5 and H6 protons of the benzimidazole fragment.

different benzyl methylenes, whereas another fluorine atom interacts with a hydrogen on the benzimidazole fragment. Tubaro et al.28 reported the solid-state structure of a related propylene-bridged Au(I)−NHC complex with methyl (instead of benzyl) wingtip substituents at the nitrogen atoms of the benzimidazol-2-ylidene fragment. This compound was found to adopt a planar or open geometry without any intra- or intermolecular aurophilic contacts. It would seem that smaller wingtip methyl groups create less steric hindrance for the dication to exist in a folded conformation, but, in this case, an intermolecular π-stacking interaction possibly facilitated by the small size of the methyl group is the determining factor that leads to such structural arrangement. In the same communication,28 a similar complex was described but with a nonsymmetrical imidazole/benzimidazole-based NHC ligand, which revealed a folded conformation with a short Au−Au distance (3.095 Å) and intramolecular π-stacking in the absence of any intermolecular interactions. A propylene-bridged imidazole-based dinuclear bis(carbene) Au(I)−NHC complex bearing PF6− counterions was also reported.5 In this case, the solid-state structure exhibited folded geometry of the cation with an intramolecular aurophlic interaction (Au−Au distance 3.272 Å) along with both intra- and intermolecular π-stacking interactions. Solution Structure of Gold(I)−NHC Complexes. The ESI+ mass spectra of 4−9 (Figures S9−S14) reveal doubly charged species with m/z characteristic of the respective dications. Depending on the instrument operating conditions, singly charged particles corresponding to the [M2+ + Br−]+ and [M2+ + PF6−]+ stoichiometry are detected as well. In all cases, m/z values and isotopic patterns are in perfect agreement with those calculated. 1H and 13C NMR spectra of complexes 4−9 (Figures S15−S22) demonstrate sets of signals, whose number, multiplicity, and relative intensity are consistent with symmetrical structures depicted in Scheme 2. The 1H NMR signals of congeners 4 and 7 (CD3OD, rt) (Figure 7) are sharp except for the resonance of the H4 protons

of the benzimidazole moiety and the signals of the methylene spacer between the NHC functions. For 4, the latter appear as two well-separated doublets (2JH−H = 14.5 Hz) at 7.97 and 7.27 ppm. The assignment of endo and exo resonances is based on the assumption that the exo protons are shielded by the ring currents of two adjacent benzimidazole backbones, whose proximity to the exo hydrogens is evidenced by the solid-state structure of 4 (vide supra). The presence of two unequal methylene protons indicates that at ambient temperature the molecule is effectively rigid on the NMR time scale and exists in a folded conformation with C2 putative symmetry, and the broadened shape of these resonances points to the presence of a slow, yet tangible process of interconversion between the two equivalent forms of the cation. Similar dynamic behavior was found in a structurally analogous methylene-bridged dinuclear Au(I)−NHC complex with the aid of variable-temperature 1H NMR spectroscopy, which revealed the coalescence of two doublets from methylene protons into a singlet in the hightemperature limiting spectrum.38 Noteworthy is the fact that, in the 1H NMR spectrum of 4 (CD3OD, rt), the endo protons from the methylene linker undergo a significant downfield shift as compared to those of 7 (Figure 7); this observation might indicate that in solution, as in the solid state, bromide counterions interact with the gold centers. Solution-state coordination of Br− to a Au(I) center in a dinuclear NHC complex has been investigated previously with the aid of EXAFS spectroscopic measurements and luminescence spectroscopy.39 However, this does not exclude an alternative explanation, which would implicate hydrogenbond formation between the bromide and the two endo hydrogens. It is reasonable to suggest that should such hydrogen bonding be present, the bromide would approach both of the gold atoms sufficiently closely so that some sort of interaction between the anion and the metal centers would be produced. A schematic representation of the possible structure of a hydrogen-bonded complex between the endo protons of 4 and Br− is given in Figure 8. The further downfield shift of the 4724


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the approximate solid-state position of bromides appears to be retained as the same hydrogen atoms are affected. Photophysical Studies. Benzimidazolium salts 1−3 absorb in the UV region with λmax at 270 nm, and the absorption bands demonstrate a pronounced vibronic fine structure (vibronic spacings of ca. 1000 cm−1) (Figure S25). These bands are assigned to π−π* transitions inside the benzimidazole moiety. Compared to the UV/vis spectra of NHC ligand precursors 1− 3, the absorption maxima of complexes 4−9 are slightly redshifted and the vibronic structure is absent (Figure S26). These bands may be similarly ascribed to metal-perturbed ligandcentered π−π* transitions. The change of PF6− counterions for Br− as well as the addition of excess tetramethylammonium bromide (TMAB) does not affect the UV/vis spectra of 4−9 (Figures S26 and S27). NHC ligand precursors 1−3 are fluorescent in solution (Figure S28), and, notably, 1 luminesces in the higher energy region of the spectrum, whereas the spectra of 2 and 3 are almost identical. The major band in the emission spectrum of 1 features a maximum at 294 nm and a shoulder at 302 nm, with the distance between the two components being ca. 1000 cm−1, which is identical to the vibronic progressions found in the UV/vis spectra of all three NHC ligand precursors. Additionally, the spectrum of 1 presents a low-intensity tail extending into the region where 2 and 3 emit (λmax ca. 370 nm). Such a striking discrepancy in the emission profiles of closely related congeners may be rationalized by taking into account the role of bromide counterions. Hydrogen bonding between imidazolium cations and various anions is well-documented and is often accompanied by dramatic changes in emission.41−46 The addition of excess TMAB to the methanolic solutions of 1−3 leads to some alterations in the UV/vis spectrum of 1, whereas the spectra of 2 and 3 remain unaffected (Figure S30). On the other hand, the removal of bromide counterions achieved by the addition of excess AgOTf to the methanolic solutions of 1− 3 leaves the UV/vis spectrum of 1 unchanged, whereas those of 2 and 3 undergo marked changes: the bands become redshifted and the vibronic structure is preserved (Figure S30). This observation points out that, in neat solution, the ground state of 1 does not bind bromides (or binds them in such a way that the HOMO−LUMO orbitals are not appreciably affected), whereas in the case of 2 and 3, bromides do interact with cations. Upon the addition of excess AgOTf, the emission spectrum of 1 loses the low-energy tail and retains the major high-energy component, whereas those of 2 and 3 acquire intensive high-energy bands with λmax at 303 and 297 nm, respectively (Figure S31). The above demonstrates that bromide counterions are involved in the lower energy emission of 1−3. This hypothesis is fully corroborated by computational investigations (vide infra). The investigated complexes are luminescent in solution (Figure 9) with the notable exceptions of 5 and 8, which are nonemissive. As is the case with the UV/vis spectra, emission spectra are also unaffected by the change of counterions from Br− to PF6−. However, the addition of excess TMAB causes marked changes in the emission spectra of the complexes (Figure S32), i.e., in the case of complexes 4 or 7 and 6 or 9, the emission band becomes red-shifted, and 5 and 8, which are nonemissive in neat solutions, become luminous. We note that the addition of excess KPF6 does not affect the emission spectra of the complexes. The addition of TMAB leads to a substantial red shift of the emission band of 7 (Figure 10). Overall, going from a neat

Figure 8. Rendition of possible hydrogen bonding between the endo hydrogens of 4 and a bromide ion.

endo protons (from 7.97 to 8.12 ppm) upon the addition of excess tetramethylammonium bromide (TMAB) to the CD3OD solution of 4 (Figure 7) indicates the existence of a fast exchange equilibrium between the bromide-bound and uncoordinated forms of the complex. In contrast to 4 and 7, the 1H NMR signals of 5 and 8 (CD3OD, rt) are sharp, including those of the ethylene linker (Figure S23), which points to the faster dynamics of the bridge flip in comparison to the analogous process in 4 and 7. The change of PF6− counterions for Br−, going from 8 to 5, causes a perceptible downfield shift of the signals corresponding to the benzyl methylene and ethylene spacer protons, which might indicate that some sort of interaction between the gold centers and bromides takes place whereby the aforementioned hydrogen atoms become proximate to the anion. While the 1H NMR spectrum of 5 demonstrates a set of wellresolved multiplets, the aromatic region of 6 exhibits poorly resolved second-order multiplets (Figure S24). Interestingly, phenyl ring protons in 9 give rise to what essentially is a singlet, and on change of the counterions from PF6− to Br−, going from 9 to 6, the phenyl signal assumes the appearance of a doublet. The addition of excess TMAB leads to a further complication of the phenyl pattern with the segregation of ortho protons. The presence of excess bromide also leads to a downfield shift of benzyl methylene protons as well as those from the terminal methylenes of the propylene bridge. The 13C{1H} NMR spectra of complexes 4−6 (CD3OD, rt) (Figure S16) feature a characteristic downfield signal from the C2 carbon at ca. 190 ppm, which is in good agreement with the literature data for other benzimidazol-2-ylidene-based homoleptic Au(I)−NHC complexes.40 Signals from aromatic carbons lie in the range from 140 to 110 ppm, whereas benzylic methylenes and CH2 groups from the alkyl spacers resonate at ca. 50 and 60−30 ppm, respectively. Thus, the NMR data obtained for the investigated complexes indicate that the stoichiometry and the general structural patterns found in the solid state remain unchanged in solution, although, on the NMR time scale, unlike in the solid, the conformations of the cations are highly symmetrical. Additionally, all of the complexes demonstrate an affinity for bromide ions with which they take part in an association equilibrium, which manifests in a perceptible downfield shift of the signals corresponding to the protons in the vicinity of gold centers. We note that the short contacts between bromide counterions and hydrogen atoms of the ligand backbones revealed by singlecrystal XRD for complexes 4−6 are generally in line with the changes induced by the presence of bromide observed in the NMR spectra, i.e., upon solution-state bromide coordination, 4725


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(Figure S34). The addition of TMAB causes the band at 479 nm to disappear with a concomitant emergence of a new lowerenergy component at 542 nm (Figure S34). In solution, the emission profiles of complexes 4−9 demonstrate remarkable changes upon the addition of TMAB. Although it would appear logical to conclude that, on the basis of the identical nature of the UV/vis spectra of neat solutions of the complexes and those to which excess TMAB is added, exciplex formation is the underlying cause of the observed phenomena, 1H NMR experiments (vide supra) point to ground-state bromide coordination. We tentatively suggest that the coordination of bromides to the gold(I) centers does not alter the chromophore, i.e., the orbitals involved in the absorption of UV excitation, but the resulting triplet excited state binds the adjacent anions more strongly and that ultimately it is this exciplex that gives off emission. In neat solutions without excess bromide, it is probable that emission actually occurs from the exciplexes formed between the excited state of the cation and solvent molecules. Such a hypothesis has been proposed by Che et al. to explain the visible emission of a series of dinuclear bis(diphosphine) Au(I) complexes.47,48 In solution, the 3[dσ*pσ] excited state of [Au2(dcpm)2]2+ (dcpm = bis(dicyclohexylphosphino)methane) was found to bind solvent molecules and various counterions including halides to produce phosphorescence with λmax ranging from 490 to 530 nm.47 Complexes 4−9 are emissive in the solid state, and changing the counterion has a pronounced effect on the photophysical characteristics of the corresponding solid-state samples (Table 1 and Figure S36). Thus, upon substitution of Br− for PF6−, going from complexes 5 and 6 to 8 and 9, respectively, the emission wavelength undergoes a substantial red shift, and in the case of 4 and 7, in which λmax remains unchanged, the emission quantum yield is considerably lowered (from 46 to 19%). It is noteworthy that the emission intensity decreases for all complexes as Br− is replaced with PF6−. Such differences in photophysical properties may indicate that the nature of counterions plays a major role in the excited state of the complexes. In accordance with Che et al.,47 we assume that, as in solution, in the solid counterion binding by the excited state of the cations is also responsible for the observed emission. Notably, complexes 5 and 6 exhibit reversible luminescent mechanochromism (Figure 11). Grinding quickly changes the emission color from blue to green, and the action could be reversed by the addition of a drop of methanol or ethanol or by

Figure 9. Emission spectra of complexes 4, 6, 7, and 9 in methanol. Excitation wavelength was 290 nm.

Figure 10. Emission spectra of 7 in methanol as TMAB is added. Excitation wavelength was 290 nm.

solution of 7 to a solution containing ca. 200 equiv of TMAB, a red shift of 62 nm is observed. A neat methanolic solution of 5 is nonemissive; however, upon the addition of TMAB, luminescence at 525 nm emerges (Figure S33). The emission spectrum of a neat methanolic solution of 6 features an intense band at 479 nm and a high-energy component at 412 nm Table 1. Photophysical Data for Compounds 1−9a

emission λmax (nm) compound 1 2 3 4 5 6 7 8 9

absorbance λmax (nm) (extinction (L·mol−1·cm−1)) 270 270 270 285 289 290 285 289 280

(14 100) (14 400) (13 500) (45 700) (47 900) (48 700) (35 900) (37 500) (38 200)

solution 294, 302 sh 369 363 476 412, 479 476 412, 479

solid state

excited-state lifetime (ns)

quantum yield (%)

463 463 440 463 496 472

17 (61%), 285 (39%) 116 (29%), 322 (71%) 57 (67%), 223 (33%) 173 (30%), 830 (70%) 86 (39%), 743 (61%) 68 (38%), 436 (62%)

46 15 26 19 8 3

a

Solid-state emission spectra and excited-state lifetimes were measured from unground crystalline powders. Solid-state quantum yields were measured in KBr or KPF6 pellets (depending on the counterion). Excitation wavelength for solid-state emission spectra was 370 nm. Excitation wavelength for solution-state emission spectra was 290 nm. Solution-state UV−vis and emission spectra were recorded in methanol. 4726


Article

Inorganic Chemistry

the detailed mechanism of the mechanochromic process is unclear, but it is possible that, once bromides become unencumbered with methanol molecules (to which they may be hydrogen-bonded) upon the grinding-induced removal of the latter, the propensity of counterions to form stronger adducts with the metal centers is increased and hence the emission is red-shifted. Interestingly, while the XRD pattern of 5 is fully interpretable in terms of the calculated one, the pattern of 6 is markedly less similar to the ones predicted from single-crystal data. In addition, 6 appears to be significantly more prone to amorphization than 5, possibly due to the ease with which it desolvates. The general features of the powder pattern of 6 do not allow for a straightforward assignment of which polymorph comprises the bulk powder, and it is probable that the bulk powder contains both 6-syn and 6-anti. In the solid state, the emission energy of complexes 4−6 appears to trend with the strength of aurophilic interactions within the cations, i.e., Au−Au distances revealed by singlecrystal X-ray diffraction studies. As a stronger aurophilic contact would be expected to lead to a lower emission energy, it is indeed logical that the emission maxima of complexes 4 and 5 are the same (λmax is 463 nm; Au−Au distances are 3.396 and 3.362 Å for 4 and 5, respectively), whereas that of 6 is considerably blue-shifted (λmax is 440 nm; Au−Au distance for 6-anti is 4.308 Å, assuming the bulk of the sample contains 6anti as the prevailing polymorph). Closer gold−gold separations are also typically expected to increase emission intensity; however, as 4 has the highest quantum yield and 5 has the lowest among the three complexes, in this case it must be that other factors influence luminescence efficiency to a greater extent. Similarly, emission quantum yields do not correlate with Au−Br distances found in the solid state. Notably, there might be some correlation between Au−Br separations and the emission wavelengths, as complex 4 features Au−Br contacts of 3.499 and 3.412 Å, 5 has slightly longer distances of 3.545 and 3.586 Å, and in 6-anti, only one bromide approaches the gold center and is located the farthest at 3.916 Å. It would appear reasonable to propose that should exciplex formation between counterion and the excited state of the cation be operative structures with shorter Au−Br contacts would be predisposed to form stronger adducts and hence emit in the lower energy spectral region. Although, following this logic, it is unclear why complexes 8 and 9 bearing weakly coordinating PF 6 − counterions produce emission at substantially lower energies compared to that of their Br− counterparts. A possible explanation is that, similarly to the ground samples of 5 and 6, 8 and 9 are partially amorphous, although this hypothesis could not be verified experimentally as the addition of methanol to powder samples of 8 and 9 does not lead to changes in emission in the same way as it does not cause any differences in the XRD powder patterns. The fact that complexes 4 and 7 emit at the same wavelength is also difficult to rationalize so far. Baron et al. reported a related propylene-bridged dinuclear Au(I)−NHC complex bearing PF6− counterions that features a nearly unit quantum yield.5 However, because in that study the quantum yield was measured in KBr pellets, in our opinion counterion exchange might have taken place and the actual emissive compound had bromide counterions instead. As we have demonstrated, the change of PF6− for Br− may have a dramatic effect not only on the emission intensity but also on the emission wavelength.

Figure 11. Photographic images of samples of 5 and 6 taken under 365 nm UV light before and after grinding and following the exposure of the ground samples to methanol vapors.

brief exposure of the ground powder to the vapors of these solvents, whereas other common solvents do not produce such an effect. Repeated grinding of the vapor-exposed blue-glowing powder leads to the change of the emission color to green, and blue emission can be restored upon exposure to methanol or ethanol vapors. The transformation cycle may be repeated at least 10 times without the loss of reversibility. As evidenced by Figure 12, grinding crystalline samples of complexes 5 and 6 leads to a red shift of 39 and 73 nm,

Figure 12. Emission spectra of unground samples of 5 and 6, ground samples of 5 and 6, and ground samples of 5 and 6 exposed to the vapors of methanol. Excitation wavelength was 370 nm.

respectively, whereas methanol vapors return the emission to its original state. In order to establish whether the changes in emission are associated with the loss of methanol molecules, we recorded 1H NMR spectra (DMSO-d6) of samples of 5 and 6 before and after grinding and found that grinding indeed removes residual solvent. X-ray powder diffraction experiments revealed that in complexes 5 and 6 grinding accompanied by the loss of co-crystallized methanol molecules results in partial amorphization, whereas the addition of methanol reconstitutes the original crystal structure (Figures S39 and S40). However, 4727


Article

Inorganic Chemistry Computational Studies. DFT and TDDFT calculations were carried out to obtain optimized structures and electronic spectra for NHC ligand precursors 1−3. According to computational results, the structure of 2 in solution resembles that in the solid state, as evidenced by X-ray data. The calculated spectra show the existence of a group of low-lying triplet excited states between 310 and 360 nm, followed by another group of singlets and triplets with wavelengths below 285 nm. While these spectra can assign the high-energy luminescence band observed for 1 to the emission from the lowest singlet state (284 nm), the nature of the low-energy emission of 2 and 3 cannot be readily explained by phosphorescence from low-lying triplets. However, the emission profiles of 2 and 3 in the presence of excess AgOTf can be attributed to fluorescence from the lowest singlet states. In order to account for the effects of counterions on photophysical properties, DFT geometry optimizations and TDDFT calculations were performed for ion associates of 1−3 with two Br− anions. The counterions display close contacts (2.3−2.6 Å) to carbenoid hydrogen atoms and longer-range contacts (ca. 3.1 Å) with aromatic hydrogens and CH2 groups. The formation of relatively short hydrogen bonds upon the addition of bromide ions leads to a pronounced decrease of the energies of the lowest singlet states (Table 2), which provides

fact may provide an explanation for the nonemissive nature of 5 in neat solution. As suggested by NMR measurements, a more realistic representation of complexes in solution may be drawn by extending the computational model from isolated complexes to their associates with counterions or solvent molecules. DFT calculations indicate that in singlet ion associates of 4 and 5 the Br− anions are located at a distance of 3.50 Å from gold atoms and 2.7−2.9 Å from hydrogen atoms of CH2 groups, which is in good agreement with X-ray data. For 6, the Au−Br distances are significantly longer (4.00 Å); no contacts with CH2 groups are observed, although contacts with aromatic hydrogens can be found. As compared to X-ray data, the optimized ion associate structure of 6 bears semblance both to 6-syn (the structural arrangement/conformation of the complex) and 6anti (the position of anions and Au−Br distances). In triplet exciplexes, the Au−Br distances decrease to 3.05−3.08 Å. For all three complexes, at least 80% of the total spin density is localized at the central fragment of the exciplex, which comprises gold atoms, bromide anions, and four Ccarbene atoms. In solvent associates of complexes 4−6, the two methanol molecules occupy approximately the same positions as Br− anions with typical Au−O(MeOH) distances of 3.5 Å in singlet states and 3.6 Å in triplets. Structural relaxation for exciplexes upon de-excitation is a more complicated process than that for the excited states of free complexes. Since the positions of solvent molecules or counterions associated with the complex in the excited state may differ significantly from those in the ground state, the relaxation of an exciplex is likely to involve translational motion with a typical travel distance of a few angstroms. Furthermore, the lack of strong bonding between the complex and associated solvent molecules or counterions may lead to a complete breakup of the associate before the ground state is reached. The abundance of possible terminal states for exciplex decay caused by the migration of loosely bound ions or solvent molecules would result in a relatively wide emission band, such as that observed for the complexes under investigation. A straightforward attempt to evaluate the emission energy for exciplexes as the energy difference ET→GS between optimized triplet and ground-state singlet states would probably result in a significant overestimation. A lower estimate of the emission energy can be obtained as the energy difference ET→S between the optimized lower triplet state and a singlet state with identical molecular geometry. In reality, the probable emission energy is likely to lie in between these two estimates. The emission wavelengths corresponding to the calculated values of ET→GS and ET→S for complexes 4−6 (7−9) are presented in Table 3. The difference in ET→GS and ET→S values between MeOH exciplexes and Br− exciplexes is at least 100 nm, which is in good agreement with the pronounced red shift of emission

Table 2. Wavelengths (nm) Corresponding to the Lowest Singlet Excited States of 1−3 Obtained from TDDFT Calculations NHC ligand precursor

free ligand

ligand + 2 Br−

1 2 3

284 272 267

349 330 335

sufficient evidence for the assignment of the low-energy emission of 2 and 3 to fluorescence from the lowest excited singlet. For 1, the lowest excited singlet state at 349 nm appears to be responsible for the low-energy, low-intensity emission tail observed in experimental measurements. In agreement with NMR data, DFT geometry optimizations for complexes 4−6 yield relatively symmetrical structures. The general layout of the core part of the complex and most of the calculated distances and angles are in line with X-ray data, although the Au−Au distances for 5 and 6 appear to be noticeably longer. As shown by TDDFT calculations for complexes 4−6, the lowest singlet excited states can be found at relatively high energies (298 nm for 4, 288 nm for 5, and 307 nm for 6), whereas the low-energy part of the electronic spectrum is dominated by a large group of triplet states. The wavelengths of the lowest triplets are close to 350 nm for all three complexes, which makes them the likely source of luminescence observed in experimental studies. Geometry optimization carried out for the triplet states of 4−6 resulted in structures that were generally similar to the ground state but differed significantly in the arrangement of the metal core. For complexes 4 and 6, the Au−Au distances decreased from 3.4 to 2.8 Å, suggesting the emergence of a strong aurophilic interaction in the triplet state. Approximately 70% of the total spin density is localized on two gold atoms and four Ccarbene atoms, providing further evidence of the key contribution of the metal core to the excitation. However, DFT calculations for complex 5 found no appreciable aurophilicity in the lowest triplet or in the ground state. This

Table 3. Emission Wavelengths (nm) for Exciplexes with Solvent or Counterions Obtained from DFT Calculations exciplex with 2 MeOH molecules

4728

exciplex with 2 Br− ions

complex

ET→GS

ET→S

ET→GS

ET→S

4 5 6

342 334 364

380 407 397

424 405 457

551 501 577


Article

Inorganic Chemistry

luminous. A summary of the data on computed Au−Au distances is given in Table 4.

wavelength accompanying the addition of excess bromide to complexes 4−6 (Figure S32). The ET→S values for exciplexes with bromide anions are quite close to the emission maxima for complexes 4−6 with excess bromide, whereas the ET→S values calculated for MeOH exciplexes appear to be somewhat lower than the experimentally measured emission maxima of complexes 4−6 or 7−9. A deeper insight into the photophysical behavior of the investigated complexes can be obtained from the analysis of the distances between gold atoms in ion and solvent associates of complexes 4−6 (Table 4). As was already mentioned,

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CONCLUSIONS A series of dinuclear homoleptic Au(I)−NHC complexes based on the benzimidazol-2-ylidene scaffold with alkyl spacers of varying length was prepared and characterized by conventional methods. Solid-state structures of complexes 4, 5, 6, and 9 were determined by single-crystal XRD studies. In the solid state, 6 was found to exist in two polymorphic modifications, which is attributed to the flexible propylene linker between the two donor functions of the diNHC ligand. Bromide-binding behavior of the complexes was qualitatively probed in solution by means of 1H NMR and luminescence spectroscopy. On the basis of literature data47 and our theoretical investigations, we propose that emission in solution originates from the binding of solvent molecules or bromide counterions by the triplet excited state of the cations. For the first time, a detailed DFT study established a clear link between bromide coordination to the gold(I) centers and the strength of the intracationic aurophilic interaction in the triplet excited state. Additionally, computational studies revealed the formation of exciplexes between the complex cations and methanol molecules, and the process was found to affect the emission energy through the same mechanism of modulating the excited state Au(I)−Au(I) distances. The direct dependence of the emission wavelength on the Au(I)−Au(I) distance in the excited state paves the way for fine-tuning the emission energy simply by varying the linker length of a diNHC ligand and employing a suitable solvent or anion. Complexes 4−9 were found to be emissive in the solid state, and, notably, 5 and 6 demonstrate reversible luminescent mechanochromism arising from amorphization accompanied by the loss of co-crystallized methanol molecules. Exposure of the ground powders of 5 and 6 to methanol vapors restores the original crystal structure, and, accordingly, the emission characteristic of unground crystals returns. Interestingly, in the solid state, there also may be a link between bromide coordination and the degree of aurophilic interaction in the excited state, which in turn affects the emission wavelength. Bromide binding was implicated in the photophysics of NHC ligand precursors 2 and 3 but not 1. Hydrogen bonding between acidic carbenoid hydrogens and bromides was found

Table 4. Distances between Gold Atoms (Å) in Optimized Structures of Complexes 4−6 and Their Ion or Solvent Associates Obtained from DFT Calculations complex

free complex

4 5 6

3.45 3.82 3.41

4 5 6

2.86 3.74 2.83

complex + 2 MeOH Singlets 3.44 3.84 3.33 Triplets 2.87 3.58 2.83

complex + 2 Br− 3.55 3.96 3.31 2.98 3.03 2.96

complexes 5 and 8 are nonemissive in solution, whereas in the presence of excess TMAB, these complexes demonstrate emission similar to that of 4 and 6 with a small blue shift (525 nm for 5 vs 542 nm for 4 and 6). As shown above, DFT calculations for complexes 4−6 point out the absence of aurophilic interaction in the lowest triplet state of 5, which explains the lack of emission. However, the emission of 5 in the presence of excess TMAB can be explained only on the basis of calculations for exciplexes with bromide anions. While in the MeOH exciplex the absence of aurophilic interaction is reproduced (Au−Au distance is 3.58 Å), in the Br− exciplex the distance between the two gold atoms decreases to 3.03 Å, suggesting the emergence of an aurophilic interaction (Figure 13). Thus, the formation of an exciplex with bromides in the presence of excess TMAB switches on the aurophilic interaction in the triplet state of 5, which, accordingly, becomes

Figure 13. HOMO (A) and LUMO (B) orbitals of the ground state of 5 with two coordinated Br− ions and select frontier orbitals of the triplet excited state of the Br− exciplex of 5: (C) HOMO − 1 (MO 287β), (D) HOMO (MO 288β), (E) LUMO (MO 289α), and (F) LUMO + 1 (MO 290α). 4729


Article

Inorganic Chemistry 3

JH,H = 7.7 Hz, 2H, 5CHbenzimidazole), 7.50−7.46 (m, 4H, o-CHbenzyl), 7.42 (t, 3JH,H = 7.5 Hz, 2H, 6CHbenzimidazole), 7.41−7.36 (m, 6H, m,pCHbenzyl), 5.80 (s, 4H, CH2-Ph), 5.26 (s, 4H, CH2-CH2). 13C{1H} NMR (100 MHz, DMSO-d6): δ (ppm) 144.32, 133.58, 131.17, 130.57, 128.93, 128.74, 128.42, 126.82, 126.69, 113.96, 113.05, 50.06, 45.86. ESI+ MS: (m/z) 222.12 (C30H28N42+), 353.18 (C23H21N4+), 443.23 (C30H27N4+), 523.15 (C30H28N4Br+). 1,3-Bis(1-benzyl-1H-benzimidazol-1-ium-3-yl)propane Dibromide (3). To a suspension of N-benzylbenzimidazole (4.08 g, 20 mmol) in acetonitrile (30 mL) was added 1,3-dibromopropane (1.31 g, 7 mmol), and the mixture was refluxed for 48 h. The precipitate formed was filtered, washed with a small amount of acetonitrile, and dried in air. Recrystallization from boiling methanol with subsequent vacuum drying afforded the title compound as a white powder. Yield: 2.71 g (68%). Anal. Calcd for C31H30Br2N4: C, 60.21; H, 4.89; N, 9.06. Found: C, 60.41; H, 4.84; N, 9.09. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 10.28 (s, 2H, 2CHbenzimidazole), 8.23 (d, 3JH,H = 6.8 Hz, 2H, 4CHbenzimidazole), 7.99 (d, 3JH,H = 6.8 Hz, 2H, 7CHbenzimidazole), 7.71− 7.63 (m, 4H, 5,6CHbenzimidazole), 7.58 (d, 3JH,H = 8.2 Hz, 4H, oCHbenzyl), 7.43−7.35 (m, 6H, m,p-CHbenzyl), 5.85 (s, 4H, CH2-Ph), 4.81 (t, 3JH,H = 7.2 Hz, 4H, CH2-CH2-CH2), 2.71 (quint, 3JH,H = 7.2 Hz, 2H, CH2-CH2-CH2). 13C{1H} NMR (100 MHz, DMSO-d6): δ (ppm) 142.61, 133.93, 131.32, 130.84, 128.90, 128.66, 128.34, 126.71, 126.62, 113.97, 113.90, 49.90, 44.13, 28.07. ESI+ MS: (m/z) 229.12 (C31H30N42+), 367.19 (C24H23N4+), 457.24 (C31H29N4+), 537.16 (C31H30N4Br+). Synthesis of Gold(I)−NHC Complexes with Br− Counterions (4−6). A Schlenk flask was charged with chloro(tetrahydrothiophene)gold(I) (96 mg, 0.30 mmol), the corresponding NHC ligand precursor (0.30 mmol), NaOAc·3H2O (88 mg, 0.65 mmol), and DMF (10 mL). The mixture was degassed with three freeze−pump−thaw cycles, the vessel was backfilled with argon, and the reaction mixture was heated at 90 °C for 1 h. The resulting solution was added dropwise into diethyl ether (30 mL), and the precipitate that formed was filtered, washed with water (2 × 5 mL), methanol/diethyl ether 1:4 (2 × 5 mL), and diethyl ether (2 × 5 mL), and dried under vacuum. The residue was taken up in methanol, and solid KBr (360 mg, 3 mmol) was added. The mixture was refluxed for 5 min, and water was introduced to produce a precipitate that was filtered, washed with water (2 × 5 mL), methanol/diethyl ether 1:4 (2 × 5 mL), and diethyl ether (2 × 5 mL), and dried under vacuum. Recrystallization by gasphase diffusion of diethyl ether into the methanolic solution of the residue, followed by filtration and vacuum drying, afforded the title compounds as white crystalline powders. Au2(NHC−CH2−NHC)2Br2 (4). Yield: 180 mg (85%). Combustion analysis was performed on vacuum-dried ground X-ray quality crystals, which contained co-crystallized DMSO. Anal. Calcd for C58H48Au2Br2N8·(CH3)2SO: C, 48.40; H, 3.66; N, 7.53. Found: C, 47.78; H, 3.50; N, 7.55. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 8.23 (d, 2JH,H = 14.5 Hz, 2H, Hendo N-CH2-N), 8.10 (d, 3JH,H = 7.2 Hz, 4H, 4CHbenzimidazole), 7.86 (d, 3JH,H = 8.1 Hz, 4H, 7CHbenzimidazole), 7.62 (t, 3JH,H = 7.7 Hz, 4H, 5CHbenzimidazole), 7.55 (t, 3JH,H = 7.7 Hz, 4H, 6CHbenzimidazole), 7.50 (d, 2JH,H = 14.5 Hz, 2H, Hexo N-CH2-N), 7.31 (d, 3 JH,H = 6.7 Hz, 8H, o-CHbenzyl), 7.25−7.17 (m, 12H, m,p-CHbenzyl), 5.77−5.69 (m, 8H, CH2-Ph). 13C{1H} NMR (100 MHz, MeOD-d4): δ (ppm) 192.75, 136.57, 134.86, 134.34, 130.04, 129.49, 128.19, 127.21, 127.09, 114.37, 113.05, 60.37, 53.72. ESI+ MS: (m/z) 625.17 (C58H48N8Au22+). Au2(NHC−CH2−CH2−NHC)2Br2 (5). Yield: 160 mg (74%). Anal. Calcd for C60H52Au2Br2N8: C, 50.09; H, 3.64; 7.79. Found: C, 50.28; H, 3.80; N, 7.76. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.68 (d, 3 JH,H = 8.1 Hz, 4H, 4CHbenzimidazole), 7.62 (d, 3JH,H = 8.2 Hz, 4H, 7CHbenzimidazole), 7.40 (t, 3JH,H = 7.8 Hz, 4H, 6CHbenzimidazole), 7.32 (t, 3 JH,H = 7.7 Hz, 4H, 5CHbenzimidazole), 7.17 (t, 3JH,H = 7.4 Hz, 4H, pCHbenzyl), 7.07 (t, 3JH,H = 7.6 Hz, 8H, m-CHbenzyl), 6.79 (d, 3JH,H = 7.5 Hz, 8H, o-CHbenzyl), 5.53 (s, 8H, CH2-Ph), 5.48 (s, 8H, CH2-CH2). 13 C{1H} NMR (100 MHz, MeOD-d4): δ (ppm) 191.96, 136.35, 134.70, 134.42, 130.05, 129.26, 127.26, 126.65, 126.51, 113.83, 112.79, 53.07, 48.21. ESI+ MS: (m/z) 639.18 (C60H52N8Au22+), 1357.28 (C60H52N8Au2Br+).

to lower the emission energy, whereas the removal of bromides leads to high-energy fluorescence.

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EXPERIMENTAL SECTION

General Notes. Benzimidazole,49 N-benzylbenzimidazole,50 and chloro(tetrahydrothiophene)gold(I)51 were prepared according to literature procedures. Other reagents were obtained commercially and used as received. Solvents were obtained from commercial suppliers and distilled over suitable drying agents immediately prior to use. Solution 1D and 2D NMR spectra were recorded on a Bruker Avance 400 instrument. Chemical shifts are reported in parts per million (ppm) and referenced to TMS (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. An 85% aqueous solution of H3PO4 was used as an external standard for 31P NMR spectra. Mass spectra were measured on a Bruker Daltonik MaXis ESI-QTOF instrument operating in positive mode. Combustion analyses were carried out using a Euro EA3028-HT 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. An integrating sphere was used to measure the solid-state emission quantum yields of the KBr or KPF6 pellets containing the compounds studied. X-ray powder diffraction experiments were carried out using a Rigaku MiniFlex II instrument with a Cu Kα1 X-ray source (1.540562 Å). Density functional theory (DFT) calculations were carried out using the B3LYP hybrid functional,52−54 Stuttgart/Dresden effective core potential (MWB60), and a basis set for Au atoms55 and the 6-31G* basis set for all other atoms. Solvent effects were taken into account via the polarizable continuum model (PCM) with methanol as solvent. Full geometry optimizations were carried out for the lowest singlet and triplet states of all ligand precursors, complexes, and their associates with two solvent molecules or bromide anions. Open-shell systems were studied using spin-unrestricted DFT formulation (UDFT). Electronic spectra were obtained from timedependent density functional theory (TDDFT) calculations. All calculations were performed using the Gaussian 09 package.56 Synthesis of NHC Ligand Precursors. 1,1-Bis(1-benzyl-1Hbenzimidazol-1-ium-3-yl)methane Dibromide (1). N-Benzylbenzimidazole (3.00 g, 14 mmol) was refluxed in dibromomethane (20 mL) for 24 h. Excess dibromomethane was removed under reduced pressure; the resulting sticky brown residue was triturated with acetonitrile to yield a white solid, which was filtered, washed with acetonitrile, and dried in air. Recrystallization from boiling water followed by vacuum drying at elevated temperature afforded the title compound as a white solid. Yield: 1.65 g (40%). Anal. Calcd for C29H26Br2N4: C, 59.00; H, 4.44; N, 9.49. Found: C, 58.98; H, 4.55; N, 9.32. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 10.75 (s, 2H, 2CHbenzimidazole), 8.49 (d, 3JH,H = 8.2 Hz, 2H, 4CHbenzimidazole), 8.05 (d, 3 JH,H = 8.2 Hz, 2H, 7CHbenzimidazole), 7.78 (t, 3JH,H = 7.6 Hz, 2H, 5CHbenzimidazole), 7.71 (t, 3JH,H = 7.7 Hz, 2H, 6CHbenzimidazole), 7.61 (d, 3 JH,H = 6.6 Hz, 4H, o-CHbenzyl), 7.56 (s, 2H, N−CH2-N), 7.45−7.37 (m, 6H, m,p-CHbenzyl), 5.91 (s, 4H, CH2-Ph). 13C{1H} NMR (100 MHz, DMSO-d6): δ (ppm) 144.17, 133.46, 130.76, 130.70, 128.93, 128.85, 128.51, 127.50, 127.20, 114.32, 113.96, 55.40, 50.27. ESI+ MS: (m/z) 215.11 (C 29 H 26 N 4 2+ ), 339.16 (C 22 H 19 N 4 + ), 429.21 (C29H25N4+), 509.13 (C29H26N4Br+). 1,2-Bis(1-benzyl-1H-benzimidazol-1-ium-3-yl)ethane Dibromide (2). To a suspension of N-benzylbenzimidazole (3.44 g, 17 mmol) in acetonitrile (25 mL) was added 1,2-dibromoethane (1.16 g, 6 mmol), and the mixture was refluxed for 72 h. The precipitate formed was filtered, washed with a small amount of acetonitrile, and dried in air. Recrystallization from boiling methanol with subsequent vacuum drying afforded the title compound as a white powder. Yield: 1.87 g (50%). Anal. Calcd for C30H28Br2N4·H2O: C, 57.89; H, 4.86; N, 9.00. Found: C, 57.57; H, 4.64; N, 9.03. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 10.24 (s, 2H, 2CHbenzimidazole), 7.92 (d, 3JH,H = 8.4 Hz, 2H, 4CHbenzimidazole), 7.87 (d, 3JH,H = 8.4 Hz, 2H, 7CHbenzimidazole), 7.57 (t, 4730


Inorganic Chemistry

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Au2(NHC−CH2−CH2−CH2−NHC)2Br2 (6). Yield: 170 mg (76%). Anal. Calcd for C62H56Au2Br2N8·CH3OH: C, 50.48; H, 4.03; N, 7.48. Found: C, 50.72; H, 3.96; N 7.55. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.92−7.90 (m, 4H, 4CHbenzimidazole), 7.73−7.70 (m, 4H, 7CHbenzimidazole), 7.37−7.34 (m, 8H, 5,6CHbenzimidazole), 7.24 (d, 3JH,H = 7.3 Hz, 8H, o-CHbenzyl), 7.15−7.09 (m, 12H, m,p-CHbenzyl), 5.78 (s, 8H, CH2-Ph), 4.99 (t, 3JH,H = 5.9 Hz, 8H, CH2-CH2-CH2), 2.87 (quint, 3JH,H = 5.9 Hz, 4H, CH2-CH2-CH2). 13C{1H} NMR (100 MHz, MeOD-d4): δ (ppm) 191.44, 136.35, 134.36, 134.15, 130.03, 129.46, 127.95, 126.39, 126.33, 113.62, 113.32, 52.96, 49.50, 28.56. ESI+ MS: (m/z) 653.21 (C62H56N8Au22+), 1385.32 (C62H56N8Au2Br+). Synthesis of Gold(I)−NHC Complexes with PF6− Counterions (7−9). To a methanolic solution of the corresponding homoleptic gold(I) complex with Br− counterions (0.05 mmol) was added a filtered solution of KPF6 (0.50 mmol) in methanol. To the solution water was introduced, and the resulting precipitate was collected by filtration, washed with water (2 × 5 mL), methanol/diethyl ether 1:4 (2 × 5 mL), and diethyl ether (2 × 5 mL), and dried under vacuum, affording the title compounds as white powders in quantitative yield. Vapor diffusion of diethyl ether into the acetonic solutions of 7−9 produced small crystals, which were filtered and dried under vacuum. Au2(NHC−CH2−NHC)2(PF6)2 (7). Anal. Calcd for C58H48Au2F12N8P2·(CH3)2CO·2H2O: C, 44.81; H, 3.58; N, 6.85. Found: C, 44.64; H, 3.54; N, 6.88. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 8.07 (d, 3JH,H = 7.5 Hz, 4H, 4CHbenzimidazole), 7.90 (d, 3JH,H = 8.1 Hz, 4H, 7CHbenzimidazole), 7.66 (d, 2JH,H = 14.5 Hz, 2H, Hendo N-CH2N), 7.65 (t, 3JH,H = 7.6 Hz, 4H, 5CHbenzimidazole), 7.58 (t, 3JH,H = 7.7 Hz, 4H, 6CHbenzimidazole), 7.49 (d, 2JH,H = 14.5 Hz, 2H, Hexo N-CH2-N), 7.28 (d, 3JH,H = 7.2 Hz, 8H, o-CHbenzyl), 7.26−7.18 (m, 12H, m,pCHbenzyl), 5.78−5.69 (m, 8H, CH2-Ph). 31P{1H} NMR (121 MHz, DMSO-d6): δ (ppm) −144.18 (sept, 1JP,F = 711 Hz, PF6−). ESI+ MS: (m/z) 625.17 (C58H48N8Au22+). Au 2 (NHC−CH 2 −CH 2 −NHC) 2 (PF 6 ) 2 (8). Anal. Calcd for C60H52Au2F12N8P2·(CH3)2CO·H2O: C, 46.00; H, 3.68; N, 6.81. Found: C, 46.31; H, 3.71; N, 7.04. 1H NMR (400 MHz, DMSOd6): δ (ppm) 7.68 (d, 3JH,H = 7.9 Hz, 4H, 4CHbenzimidazole), 7.62 (d, 3 JH,H = 8.2 Hz, 4H, 7CHbenzimidazole), 7.41 (t, 3JH,H = 7.7 Hz, 4H, 6CHbenzimidazole), 7.33 (t, 3JH,H = 7.7 Hz, 4H, 5CHbenzimidazole), 7.18 (t, 3 JH,H = 7.4 Hz, 4H, p-CHbenzyl), 7.08 (t, 3JH,H = 7.5 Hz, 8H, mCHbenzyl), 6.79 (d, 3JH,H = 7.5 Hz, 8H, o-CHbenzyl), 5.53 (s, 8H, CH2Ph), 5.47 (s, 8H, CH2-CH2). 31P{1H} NMR (121 MHz, DMSO-d6): δ (ppm) −144.17 (sept, 1JP,F = 711 Hz, PF6−). ESI+ MS: (m/z) 639.18 (C60H52N8Au22+), 1423.33 (C60H52N8Au2PF6+). Au2(NHC−CH2−CH2−CH2−NHC)2(PF6)2 (9). Anal. Calcd for C62H56Au2F12N8P2·(CH3)2CO·H2O: C, 46.66; H, 3.86; N, 6.70. Found: C, 46.31; H, 3.98; N, 6.86. 1H NMR (400 MHz, DMSOd6): δ (ppm) 7.84−7.82 (m, 4H, 4CHbenzimidazole), 7.65−7.63 (m, 4H, 7CHbenzimidazole), 7.36−7.34 (m, 8H, 5,6CHbenzimidazole), 7.22 (d, 3JH,H = 7.6 Hz, 8H, o-CHbenzyl), 7.16−7.09 (m, 12H, m,p-CHbenzyl), 5.75 (s, 8H, CH2-Ph), 4.98 (t, 3JH,H = 6.1 Hz, 8H, CH2-CH2−CH2), 2.90 (quint, 3JH,H = 6.1 Hz, 4H, CH2-CH2-CH2). 31P{1H} NMR (121 MHz, DMSO-d6): δ (ppm) −144.17 (sept, 1JP,F = 711 Hz, PF6−). ESI+ MS: (m/z) 653.20 (C62H56N8Au22+), 1451.36 (C62H56N8Au2PF6+).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Fax: +7 (812) 3241258. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate financial support from the St. Petersburg State University (research grant 0.37.169.2014) and the Russian Foundation for Basic Research (grants 13-0312411 and 13-04-40342). NMR, photophysical, analytical, and crystallographic measurements were performed using the following core facilities at St. Petersburg State University Research Park: Centre for Magnetic Resonance, Centre for Optical and Laser Materials Research, Centre for Chemical Analysis and Materials Research, and X-ray Diffraction Centre, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02722. Spectroscopic data (NMR, ESI+ MS, UV/vis, and emission and excitation spectra), PXRD patterns, and crystallographic information (PDF) Crystallographic data for 2, 4, 5, 6-anti, 6-syn, and 9 (CIF) Optimized structures of 1−6 in different states (ZIP) 4731


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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on February 16, 2016, with a minor text error on page twelve, subheading eight of the document. The corrected version was reposted on February 19, 2016.

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