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Very Short Metallophilic Interactions Induced by Three-Center−TwoElectron Perhalophenyl Ligands in Phosphorescent Au−Cu Complexes José M. López-de-Luzuriaga,* Miguel Monge, M. Elena Olmos, David Pascual, ́ and Marıá Rodrıguez-Castillo Departamento de Quı ́mica, Universidad de La Rioja, Grupo de Sı ́ntesis Quı ́mica de La Rioja, UA-CSIC, Complejo Cientı ́fico-Tecnoloǵ ico, 26004 Logroño, Spain S Supporting Information *

ABSTRACT: The synthesis, structural characterization, and study of the photoluminescent properties of the complexes [Au3Cu(C6F5)4(NCCHCHPh)3]n (1) and [Au2Cu(C6Cl5)3(NCCHCHPh)2] (2) have been carried out. The crystal structures of complexes 1 and 2 display a common [Au2Cu(C6X5)2(μ-C6X5)(NCCHCHPh)2] fragment built up through a trinuclear Au2Cu unit with a perhalophenyl group acting as a threecenter−two-electron bridging ligand. In the case of 1 an additional [Au(C6F5)(NCCHCHPh)] unit is also present in the asymmetric unit. Both complexes are brightly luminescent in the solid state at room temperature and at 77 K with lifetimes in the nanosecond range for 1 and in the microsecond range for 2. The presence of very short metallophilic interactions seems to be responsible for the low-energy luminescence observed. DFT and time-dependent DFT calculations agree with the experimental results and support the idea of that the origin of the emissions arises from metal-based orbitals and arrive at ligand-based orbitals as charge transfer transitions.

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INTRODUCTION Metallophilic Au(I)···M closed-shell interactions (M = Ag(I), Cu(I), Tl(I), Hg(II), Bi(III), etc.) have been a subject of intense research in the last few years.1−5 The interest in this type of complexes covers several areas, including supramolecular structural analysis, theoretical calculations on the nature of the metallophilic interactions,6 photoluminescent properties associated to the metallophilicity,7 and even potential applications as VOC sensors.8,3a An important part of our interest has been focused on metallophilic Au(I)···Cu(I) interactions.2a−d Although both copper and gold belong to the coinage metal group, the closedshell interactions between both ions in their +1 oxidation state is much less represented than the corresponding aurophilic Au(I)···Au(I) or the Au(I)···Ag(I) interactions. Apart from this fundamental interest, the development of new Au−M compounds bearing Cu(I) ions opens new fields of research related to the intrinsic properties of this metal.2e−j For example, the high affinity of Cu(I) for N-donor ligands makes this metal very attractive for biological studies, such as its use as luminescent probes in the interaction with DNA, among others.9 Also Cu(I) is being used for the development of emitting materials for MOLEDs (molecular light-emitting devices)10 or for catalytic applications.11 In this regard, the presence of gold(I) moieties promoting metallophilic Au(I)···Cu(I) interactions and luminescent properties of complexes would give rise to new and interesting photophysical properties and applications. © 2012 American Chemical Society

Another recurrent feature is that metallophilic interactions usually coexist with other types of secondary weak interactions in the same molecule. Thus, for example, we have described complexes bearing Au(I)···M interactions concomitant with hydrogen bonding,12 π stacking,13 Cipso···M interactions,14 and X···M interactions15 (X = halogen atom, M = closed-shell metal center). In all cases both interactions are complementary and responsible for the structural arrangements found in the solid state. Therefore, a delicate tuning of the luminescent emission energies can be established upon slight structural modifications. In this context, a poorly represented weak bonding trend in gold chemistry is the use of aromatic C-donor ligands for promoting M−C−M (M = closed-shell metal center) threecenter−two-electron (3c2e) deficient bonds.16 This type of supporting interaction promotes the formation of strong and very short metallophilic interactions that may lead to a metalbased HOMO−LUMO gap decrease. The combination of metallophilic Au(I)···M interactions and M−C−M threecenter−two-electron bonds could become an interesting tool for the synthesis of low-energy emissive compounds, since very short metal−metal interactions would give rise to red-shifted emissions. In a recent report we analyzed the influence of different nitrile ligands on the structural arrangement and the photoluminescent properties of Au−Cu complexes of the type Received: March 15, 2012 Published: April 26, 2012 3720

dx.doi.org/10.1021/om300204x | Organometallics 2012, 31, 3720−3729

Organometallics

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Scheme 1. Synthesis of the Complexes [Au3Cu(C6F5)4(NCCHCHPh)3]n (1) and [Au2Cu(C6Cl5)3(NCCHCHPh)2] (2)

[AuCu(C6F5)2(NCR)2]x (R = CH3, x = 1; R = Ph, x = 2; R = CHCHPh, x = 1).2c These complexes display dinuclear or tetranuclear arrangements built up through unsupported metallophilic interactions between the basic aurate fragment [Au(C6F5)2]− and the acidic Cu(I) center that completes its coordination sphere with nitrile ligands. The luminescent properties were only observed for the tetranuclear complex with benzonitrile ligands that shows a Cu−Au−Au−Cu arrangement with a blue luminescence arising from the aurophilic interactions. The addition of different amounts of nitrile ligands with respect to the metal centers would lead to different reaction mechanisms and new structural dispositions, in which the presence of possible 3c2e bonding would play a key role. Herein we report the synthesis of new gold−copper heterometallic complexes of the type [Au3Cu(C6F5)4(N CCHCHPh)3]n (1) and [Au2Cu(C6Cl5)3(NCCH CHPh)2] (2), bearing cinnamonitrile ligands added in a 1:1 molar ratio with respect to copper chloride (see Scheme 1). This type of reaction leads to a molecular rearrangement and collateral formation of [Cu(C6F5)] in different amounts which decreases the 2:1 ratio of C6F5 ligands with respect to Au(I), promoting the formation of 3c2e bonding. We have analyzed the influence of the perhalophenyl ligand (C6F5 or C6Cl5) on the structural arrangement obtained in the solid state and on the photoluminescent properties. DFT and TDDFT calculations have been used for the interpretation of the experimental results.

Figure 1. Crystal structure of 1·0.5(toluene). Hydrogen atoms have been omitted for clarity.

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RESULTS AND DISCUSSION Synthesis and Structural Characterization. The complexes [Au3Cu(C6F5)4(NCCHCHPh)3]n (1) and [Au2Cu(C6Cl5)3(NCCHCHPh)2] (2) were prepared by the reaction of the corresponding heterometallic gold−silver precursor [AuAg(C6X5)2(NCMe)]n (X = F (1), Cl (2)) with CuCl in acetonitrile. After 2 h of reaction the evaporation of the donor solvent acetonitrile and the addition of toluene (20 mL) and 1 equiv of cinnamonitrile ligand per copper(I) center led to the formation of complexes 1 and 2 by loss of 2/3 equiv of [Cu(C6F5)] and 1 equiv of [Cu(C6Cl5)], respectively. The 1H NMR spectra of complexes 1 and 2 display the signals corresponding to the cinnamonitrile ligands (see Experimental Section). The observed chemical shifts for the gold−copper cinnamonitrile species are similar to those of the free cinnamonitrile ligand, perhaps due to dissociation processes in solution. Complex 1 displays a 19F NMR spectrum similar to that of the precursor complex NBu4[Au(C6F5)2], showing signals corresponding to the C6F5 groups bonded to Au(I) in the [Au(C6F5)2]− units at −114.9 (Fo), −161.7 (Fp),

Figure 2. Crystal structure of 2. Hydrogen atoms have been omitted for clarity.

and −162.9 ppm (Fm), suggesting the rupture of the Au···Cu metallophilic interaction in solution. The IR spectra of 1 in Nujol mulls show absorptions arising from [Au(C6F5)2]− groups at 1496, 962, and 784 cm−1. Complex 2 shows the corresponding absorptions at 835 and 614 cm−1, in accord with the presence of [Au(C6Cl5)2]− units. In both complexes, the absorptions due to the cinnamotrile ligands coordinated to the Cu(I) centers in the solid state can be assigned. These bands appear at 2254 (1) and 2248 cm−1 (2) and are assigned to the 3721


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Figure 3. One-dimensional structure of 1·0.5(toluene). Hydrogen atoms have been omitted for clarity.

Table 1. Spectroscopic and Photophysical Properties of Complexes 1 and 2 complex 1

2

medium (T (K)) acetonitrile (298) solid (298) solid (77) butyronitrile (77) acetonitrile (298) solid (298) solid (77) butyronitrile (77)

λabs (nm) (ε (M−1 cm−1))

λem (λexc) (nm)/τ

215 (13.1 × 104), 237 (5.6 × 104), 264 (7.0 × 104) 559(403, 497)/τ1 = 330 ns, τ2 = 59 ns 600 (390, 485) 450 (315−330) 217 (18.6 × 104), 231 (10.6 × 104), 269 (7.9 × 104) 635 (372, 463, 557)/1.62 μs 652 (376, 425, 472) 462 (313)

Figure 4. Excitation and emission spectra for complexes 1 (top) and 2 (bottom) in the solid state at room temperature.

ν(CN) stretching vibration. These absorptions appear at energies significantly different from the absorption of the uncoordinated cinnamonitrile ligand (2218 cm−1). The crystal structures of 1·0.5(toluene) and 2 were determined by X-ray diffraction methods from single crystals obtained by slow diffusion of n-hexane in a saturated solution of the complex in toluene. Both crystal structures display a common [Au2Cu(C6X5)2(μ-C6X5)(NCCHCHPh)2] fragment (see Figures 1 and 2), although in the case of 1·0.5(toluene) an additional [Au(C6F5)(NCCHCHPh)] unit is also present in the asymmetric unit (Figure 1). Two unsupported Au(I)···Cu(I) interactions and one metallophilic contact supported by the C6X5 bridging ligand are observed within the trinuclear units. In the case of 1·0.5(toluene) the Au···Cu interactions, at 2.7177(8) and 2.7077(8) Å, are more symmetric than in 2, which shows Au−Cu distances of 2.7184(13) and 2.6841(13) Å. The latter is similar to that found in the complex [AuCu(C6F5)2(NCCHCHPh)2]2c (2.6727(4) Å) and all are shorter than in the acetonitrile derivatives [AuCu(C6F5)2(NCCH3)2]2c (2.9335(11) Å) and [Cu{Au(C6F5)2}(NCMe)(μ2-C4H4N2)]n2a (2.8216(6) Å) and are also shorter than the Au−Cu distances observed in [{AuCu-(μ-Spy)(μPPh2py)}2](PF6)217 (3.108(8) Å) and NBu4[Au3Cu2(C CC6H4Me-p)6]18 (2.743(1)−2.980(1) Å), where the Au···Cu contact is supported by the presence of a bridging ligand. However, there are other Au/Cu complexes with stronger

Au···Cu interactions, such as [AuCu(C6F5)2(NCPh)2]22c (2.6163(12) and 2.6092(12) Å), [Au 2 Cu 2 (C 6 F 5 ) 4 (N CCH3)2]n2b (2.5741(6) and 2.5876(5) Å) (where one of the pentafluorophenyl ligands of the adjacent [Au(C6F5)2]− units

Figure 5. Absorption UV−vis spectra of complexes 1 (black) and 2 (red). 3722


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Figure 6. Theoretical model systems [Au3Cu(C6F5)4(NCCHCHPh)3] (1a) (left) and [Au2Cu(C6Cl5)3(NCCHCHPh)2] (2a) (right).

difference between both structures. It is worth mentioning that this fragment does not exist as a pure complex and also that the coordination of a nitrile ligand to gold is rather uncommon.22−26 In addition, this unit connects two Au2Cu fragments through unsupported Au···Au contacts, resulting in the formation of a one-dimensional polymer that runs parallel to the crystallographic y axis (Figure 3), while the crystals of complex 2 contain discrete molecules. As expected, these aurophilic interactions are weaker than those found within the trinuclear Au2Cu unit, and thus, the Au−Au distances are considerably longer (2.9494(3) and 2.9553(3) Å). Finally, the Cu−N distances of 1.891(5) Å in 1·0.5(toluene) and 1.879(9) and 1.882(9) Å in 2 are shorter than in {[Au(C 6 F 5 ) 2 ][Cu(NCCHCHPh)(μ 2 -C 4 H 4 N 2 )]} 2 d (2.000(4) Å) and in {[Au(C 6 Cl 5 ) 2 ][Cu(NCPh)(μ 2 C4H4N2)]}2d (1.969(5) Å) and are of the same order as in the complexes [AuCu(C6F5)2(NCCHCHPh)2] (1.866(3) and 1.871(3) Å) 2 c and [AuCu(C 6 F 5 ) 2 (NCPh) 2 ] 2 (1.869(8)−1.877(8) Å).2c Photophysical Properties. Complexes 1 and 2 show interesting photophysical properties, which can be related to their structures (Table 1). Both of them are luminescent in the solid state at room temperature when they are irradiated with UV−vis radiation. Complex 1 displays an orange emission centered at 559 nm when it is excited in the 375−530 nm range, and complex 2 shows a red-shifted emission at 635 nm with a maximum of excitation at 557 nm. As expected, both emissions shift to red when the temperature is decreased at 77 K; thus, complex 1 shifts to 600 nm and complex 2 to 652 nm. The larger shift for complex 1 in comparison to that for complex 2 is in accordance with a greater rigidity in the structure of the latter,2c,3b,26,27 possibly associated with the short metal−metal distances observed in the crystal structure (see above), preventing the shortening of the atom distances when the sample is cooled (Figure 4). The lifetime determined for complex 1 by the single photon counting technique in the solid state at room temperature displays two components within the nanosecond time scale (330 and 59 ns). These values, together with the short Stokes shift (ca. 2200 cm−1) between the maxima of excitation and emission peaks, suggest a fluorescent emission. On the other hand, complex 2 displays a monoexponential decay in the microsecond range (1.62 μs) but a similar Stokes shift;

Table 2. Population Analysis for the Model System [Au3Cu(C6F5)4(NCCHCHPh)3] (1a): Contribution from Each Part of the Molecule to the Occupied Orbitals amount (%)a 307 306 305 304 303

HOMO HOMO-1 HOMO-2 HOMO-3 HOMO-4

Auext

Au2

Cu

Rbridge

Rext

Rterm

L

24.5 0.8 0.9 17.3 0.3

34.4 0.8 21.2 1.9 5.9

7.2 0.1 4 1.6 47.8

5.5 0.7 14.6 0 1.2

9.8 94.8 2.9 74.9 0.7

13.5 2.7 55.5 5.5 33.2

5.1 0 0 2.8 9.6

a

The percent contribution of each part of the molecule to the occupied orbitals refers to Auext (gold center from the isolated [Au(C6F5)(NCCHCHPh)] unit), Au2 (gold centers from the Au2Cu metal core), Cu (copper center), Rbridge (C6F5 bridging ligand), Rext (C6F5 ligand from the isolated [Au(C6F5)(NCCHCHPh)] unit), Rterm (C6F5 terminal ligands bonded to the gold centers from the Au2Cu core), and L (cinammonitrile ligand).

acts as a bridge between Au and Cu), and some Au/Cu clusters19,20 (2.589 and 2.584 Å). It is worth noting that these complexes display among the shortest Au−Au distances found in the literature for aurophilic contacts, with values of 2.7325(3) Å in 1·0.5(toluene) and 2.7186(5) Å in 2 for the Au−Au distance within the Au2Cu unit. These distances are slightly larger than that observed in the organometallic complex [Au(Mes)]516b (Mes = 2,4,6-CH3− C6H2) of 2.697(1) Å but shorter than that observed in the complex [Au2μ-(C6F3H2)(PPh3)2]ClO416e of 2.759(1) Å. This situation is favored by the presence of a C6X5 group acting as bridging ligand between both gold atoms, giving rise to a threecenter−two-electron bond. This type of bond was previously observed for Au(I)−Ag(I) and Au(I)−Cu(I),21 but, as far as we know, this is the first time that this kind of bond with pentachlorophenyl or pentafluorophenyl ligands has been observed between two gold atoms. It is also in accordance with the longer Au−C distances observed between the gold centers and the carbon atom of the bridge (2.183(6) and 2.166(6) Å in 1·0.5(toluene) and 2.162(9) and 2.174(9) Å in 2) in comparison to those corresponding to the terminal aryl groups (2.048(6) and 2.037(6) Å in 1·0.5(toluene0 and 2.044(10) and 2.036(9) Å in 2). As noted above, in the crystal structure of 1·0.5(toluene) an additional [Au(C6F5)(NCCHCHPh)] unit is also present in the asymmetric unit (Figure 1), which represents the main 3723


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Figure 7. Frontier molecular orbitals and HOMO-3 to LUMO+2 for model system 1a.

loss of the metal−metal interactions in solution, which are responsible for the emissive behavior in the solid state. In glass media at 77 K the results depend on the solvent used. Complexes dissolved in an EtOH/MeOH/CH2Cl2 mixture (8/2/1) and cooled at 77 K do not show any emission, probably due to the breaking of the complexes and coordination of the alcohol molecules in the coordination sphere of copper, preventing the interaction with gold. Interestingly, if the measurement is carried out in butyronitrile at 77 K, both glasses are luminescent, showing blue-shifted emissions at 462 (1) and 450 nm (2), by excitation at 315 nm. This result is interpreted in terms of the formation of species of the type [{Au(C6X5)2}{Cu(butyronitrile)2}], in which the gold and copper centers keep the interaction at 77 K, since the species [{Au(C6X5)2}{Cu(CH3CN)2}]2c show similar emissions in butyronitrile at 77 K (X = F, 468 nm; X = Cl, 445 nm), probably by substitution of both acetonitrile ligands for butyronitrile ones. On the other hand, the behavior at room temperature is different. In fact, according to the UV−vis absorption measurements and the NMR data, it seems that both complexes dissociate in solution. Thus, the 1H NMR spectra show signals due to the cinnamonitrile ligands at a chemical shift similar to that for the free nitrile and the 19F NMR spectrum of 1 displays the signals of the anionic [Au(C6F5)2]− unit similar to those in the precursor complex (see the Experimental Section). In the

Table 3. Population Analysis for the Model System [Au2Cu(C6Cl5)3(NCCHCHPh)2] (2a): Contribution from Each Part of the Molecule to the Occupied Orbitals amount (%)a 283 282 281 280 279 275

HOMO HOMO-1 HOMO-2 HOMO-3 HOMO-4 HOMO-8

Au2

Cu

Rbridge

Rterm

L

23.2 51.4 10.3 12.1 7.1 2.3

1.5 12.7 19.2 20.3 10.3 37.2

29.5 0.7 0.3 0.7 0.2 0.1

44.9 33.2 65.9 62.0 79.7 0.3

0 1.2 3.1 3.4 1.6 56.9

a

The percent contribution of each part of the molecule to the occupied orbitals refers to Au2 (gold centers from the Au2Cu metal core), Cu (copper center), Rbridge (C6F5 bridging ligand), Rterm (C6F5 terminal ligands bonded to the gold centers from the Au2Cu core), and L (cinammonitrile ligand).

therefore, taking into account these comments, a conclusive assignment of the nature of the emission cannot be done. Very interestingly, in spite of the strength of the metal−metal interactions in both complexes, which is favored by the threecenter−two-electron perhalophenyl bridges, these compounds do not display a luminescent emission in solution (THF, dichloromethane, acetone, etc.). This trend is interpreted as the 3724


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Figure 8. Frontier molecular orbitals and HOMO-2 to LUMO+2 for model system 2a.

of both compounds, 1a and 2a, have been built up from the Xray diffraction results (Figure 6). In the case of the polymeric complex [Au 3Cu(C 6F 5 ) 4(NCCHCHPh) 3]n (1), the model 1a represents the asymmetric unit, where the metallophilic interactions Au···Au and Au···Cu, and also the different coordination environments, are represented. In the model system 2a, the asymmetric unit of the complex and [Au2Cu(C6Cl5)3(NCCHCHPh)2] (2) are fully represented. In the first step we have analyzed the electronic structures of the models [Au3Cu(C6F5)4(NCCHCHPh)3] (1a) and [Au2Cu(C6Cl5)3(NCCHCHPh)2] (2a) obtained through single-point DFT calculations at the B3LYP level of theory. With this analysis we can check the contribution of each part of the molecule to the frontier orbitals. In the case of model 1a the occupied orbitals from HOMO (307a) to HOMO-4 (303a) have been analyzed through a population analysis for each model, while the shape of the virtual orbitals is given by visual analysis (see Table 2 and Figure 7). The contribution from each part of the molecule to the occupied MO's shows that the HOMO is mostly located at the Au(I) centers (ndz2σ* character) with some contribution from the perhalophenyl rings and from the Cu(I) center, the HOMO-1 is mostly located at one of the C6F5 rings bonded to Au(I), HOMO-2 and HOMO-3 are mostly placed at the C6F5 ligands with some contribution from gold, and HOMO-4 is mainly located at the Cu(I) center with a contribution from the perhalophenyl ligands. On the other hand, the lowest unoccupied orbitals (LUMO to LUMO+2) for model 1a are all located at π* orbitals of the cinammonitirile ligands. Similarly, the highest occupied MO’s for model system 2a, HOMO (283a) to HOMO-4 (279a), are found at the Au(I) centers and the C6F5 rings (HOMO and HOMO-1) or at the C6F5 ligands with some contribution from the metal centers (HOMO-2 to HOMO-4) (see Table 3 and Figure 8). The lowest occupied empty orbitals LUMO and LUMO+1 found

case of the UV−vis absorption spectra in degassed acetonitrile (see Figure 5), three bands are observed for each complex, 215 nm (ε = 13.1 × 104 M−1 cm−1), 237 nm (ε = 5.6 × 104 M−1 cm−1), and 264 nm (ε = 7.0 × 104 M−1 cm−1) (1) and 217 nm (ε = 18.6 × 104 M−1 cm−1), 231 nm (ε = 10.6 × 104 M−1 cm−1), and 269 nm (ε = 7.9 × 104 M−1 cm−1) (2), which appear in the absorption spectra of the starting material of each compound: NBu 4 [Au(C 6 X 5 ) 2 ] (X = F (1), Cl (2)). Consequently, these absorptions could be associated with π → π* or Au → π* electronic transitions in the aryl groups (C6X5), as has been previously described in the literature.1a,2a,3d However, the band edge in each compound is red-shifted if it is compared to those in the precursor gold complexes. This effect can be assigned to bands due to free cinnamonitrile, which has an absorption band centered at 272 nm with a band edge at 303 nm, similar to those found in 1 and 2.2c Consequently, the lowest energy band in the absorption spectra for both compounds can be assigned to a mixture of electronic transitions that involves the (C6X5)2Au groups and/or the cinnamonitrile ligand. Taking into account the previous comments, we can propose that the emissive behavior in both complexes arises from transitions from the electron-rich (d10) metal centers, which are favored by the anionic aryl groups bonded to them, and go to π* orbitals located in the cinnamonitrile ligands or on the perhalophenyl groups. This proposal is consistent with previous work carried out in our laboratory on [{Au(C6X5)2}{Cu(nitrile)2}]2c systems in which the emissive character is strongly dependent on the metal−metal interactions and the nitrile bonded to the copper atoms. In addition, theoretical calculations agree well with this proposal (see below). Theoretical Studies. Theoretical DFT and TDDFT calculations have been used as a tool for studying the origin of the luminescent properties of complexes 1 and 2 (see Computational Details). For these calculations, model systems 3725


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Table 4. TD-DFT First 20 Singlet Excitation Calculations for the Model [Au3Cu(C6F5)4(NCCHCHPh)3] (1a) excitation

λcalcd (nm)

oscil strength (s)a

1

472

0.0388

2

463

0.0283

3 4

426 399

0.0024 0.0124

5

395

0.0473

6

391

0.0025

7

388

0.0025

8

384

0.0029

9

375

0.0681

10

372

0.0082

11

368

0.0123

12

366

0.0298

13 14

365.6 364.5

0.0067 0.0097

15

362

0.0025

16

360

0.0026

17 18

355 350.4

0.0059 0.0150

19

350.2

0.0148

20

349.7

0.0012

Table 5. TD-DFT First Triplet and First 20 Singlet Excitation Calculations for the Model [Au2Cu(C6Cl5)3(N CCHCHPh)2] (2a)

contribnsb 307a → 309a (52), 307a → 308a (47) 307a → 308a (52); 307a → 309a (47) 307a → 310a (99) 305a → 308a (64); 303a → 308a (19) 303a → 308a (67); 305a → 308a (22) 306a → 309a (56); 306a → 308a (31) 305a → 309a (67); 306a → 308a (25) 306a → 309a (41); 306a → 308a (30) 304a → 308a (50); 304a → 309a (41) 304a → 309a (44); 302a → 308a (25) 306a → 310a (61); 303a → 310a (19) 306a → 310a (20); 303a → 310a (19) 303a→309a (65); 301a→308a (13) 303a → 310a (39); 306a → 310a (14) 305a → 310a (82); 303a → 310a (11) 301a → 308a (47); 302a → 308a (41) 302a → 309a (96) 300a → 308a (65); 307a → 311a (21) 307a → 311a (68); 300a → 308a (22) 304a → 310a (81)

excitation

λcalcd (nm)

1 triplet

469.9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

420.7 417.8 403.2 401.2 399.0 386.4 380.4 379.1 368.3 364.3 361.8 352.7 348.1 334.9 332.4 330.5 329.3 322.0 321.6 320.5

oscil strength (s)a

0.0157 0.0006 0.027 0.0050 0.0090 0.0016 0.0011 0.0005 0.0080 0.0017 0.0007 0.0008 0.0004 0.0009 0.1020 0.0122 0.0043 0.4626 0.042 0.082

contribnsb 275a → 284a (23); 282a → 284a (9) 282a → 284a (75); 283a → 284a (73); 282a → 285a (75); 281a → 284a (41); 283a → 285a (75); 281a → 285a (41); 280a → 284a (49); 279a → 284a (76); 278a → 284a (82) 281a → 285a (51); 279a → 285a (67); 278a → 285a (96) 277a → 284a (97) 277a → 285a (88) 274a → 284a (42); 276a → 284a (68) 282a → 286a (94) 275a → 284a (55); 283a → 287a (51); 274a → 285a (50);

275a → 285a (23); 283a 282a 283a 280a 282a 280a 281a 280a

→ → → → → → → →

284a (23) 284a (24) 285a (20) 284a (34) 285a (20) 285a (36) 284a(48) 284a (11)

280a → 285a (30) 280a → 285a (21)

275a → 284a (26)

274a → 284a (28) 282a → 287a (25) 276a → 285a (24)

a

Oscillator strength shows the mixed representation of both velocity and length representations. bValue is |coeff|2 × 100.

from C6F5 orbital HOMO-1 to the ligand-based orbital LUMO +2. Finally, high-energy excitations occur between the goldbased orbital HOMO and ligand-based orbitals LUMO+1 and LUMO+3. All these data suggest that the theoretical excitations responsible for the luminescent behavior for complex 1 could arise from mixed metal to ligand charge transfer 1(MLCT) and ligand metal to ligand charge transfer 1(LMLCT) transitions. Although the repetition unit of complex 1 and the structure of complex 2 are not very different, the photophysical behaviors differ in the emission lifetimes. Complex 1 shows an emission with a lifetime in the nanosecond range, while complex 2 displays an emission in the microsecond range. Taking this into account, we have evaluated both the lowest triplet excitation and the first few singlet electronic excitations for model system 2a (Table 5). The first triplet transition arises, as the main contribution, from an orbital located at the cinnamonitrile ligands (HOMO-8) and, with a less important contribution, from the metal centers (HOMO-1). In all contributions to the lowest triplet excitation the electronic density arrives at orbitals (LUMO, LUMO+1) centered at the cinnamonitrile ligands. This result seems to indicate that the electronic transition responsible for the phosphorescent emission observed for complex 2 could have its origin in a mixed metal to ligand charge transfer 3(MLCT) and an intraligand 3(IL) transition. In addition, the analysis of the first singlet electronic transitions also points to a similar contribution being the most intense theoretical transitions, these arising from metal pentachlorophenyl moieties (HOMO to HOMO-4) and arriving at cinammonitrile-based orbitals LUMO and LUMO+1. Comparison between the experimental and theoretically predicted excitation spectra for both complexes is given in Figure 9. As can be observed, the theoretical excitation energies

a

Oscillator strength shows the mixed representation of both velocity and length representations. bValue is |coeff|2 × 100.

for model 2a are also located at the cinammonitrile ligands, while LUMO+2 is mostly located at the bridging C6Cl5 ligand with some contribution from the gold centers. Therefore, the electronic structures found for both model systems suggest that the most favored electronic transitions could be charge-transfer transitions from the electron-rich metals and/or the pentafluorophenyl ligands to π* orbitals of the cinammonitrile or pentafluorophenyl ligands, as has been described in the experimental photophysical results. In order to confirm the origin of the electronic transitions responsible for the luminescent emissions for complexes 1 and 2, the first few spin-allowed singlet excitations have been computed at the TDDFT level of theory for model 1a. Also, the first triplet excitation and the first few spin-allowed singlet excitations has been estimated for model 2a, in view of its longer lifetime in the microsecond range. The analysis of the most intense theoretical excitations can be classified into different groups (see Table 4). Thus, the lowest energy theoretical excitations at 472 and 463 nm arise from the gold-based orbital HOMO and arrive at LUMO and LUMO+1 placed at the nitrile ligand. Excitations at 399, 395, and 375 nm arise from Au−C6F5 (HOMO-2 and HOMO-3) and Cu(C6F5) moieties (HOMO-4) and arrive at the LUMO (nitrile). Excitations at 368 and 366 nm suggest a transition 3726


Organometallics

Article

Table 6. Data Collection and Structure Refinement Details for Complexes 1·0.5(toluene) and 2 1·0.5(toluene) C51H21Au3CuF20N3·0.5 C6H5CH3 cryst habit orange plate cryst size/mm 0.35 × 0.15 × 0.07 cryst syst monoclinic space group C2/c a/Å 28.9642(2) b/Å 14.5041(1) c/Å 26.9041(3) β/deg 110.843(1) U/Å3 10562.83(16) Z 8 Dc/g cm−3 2.209 Mr 1756.21 F(000) 6552 T/°C −100 2θmax/deg 56 μ(Mo Kα)/mm−1 8.817 no. of rflns measd 83 636 no. of unique rflns 12 515 Rint 0.0881 Ra (I > 2σ(I)) 0.0387 Rwb(F2, all rflns) 0.0898 no. of params 729 no. of restraints 208 Sc 1.020 max Δρ/e Å−3 2.163 chem formula

Figure 9. Comparison between the theoretical first few singlet excitations (red bars) and the experimental excitation profiles in solid state at room temperature (black lines) for complexes 1 (top) and 2 (bottom). The asterisk corresponds to the lowest singlet−triplet excitation for complex 2 (the oscillator strength is not taken into account).

orange cube 0.16 × 0.15 × 0.15 monoclinic P21/c 12.5473(3) 17.8938(9) 21.8989(9) 119.914(3) 4261.7(3) 4 2.2841 1463.72 2744 −173 55 8.336 30 527 9689 0.0596 0.0568 0.1430 505 154 1.034 3.494

R(F)= ∑||Fo| − |Fc||/∑|Fo|. bRw(F2) = [∑{w(Fo2 − Fc2)2}/ ∑{w(Fo2)2}]0.5; w−1 = σ2(Fo2) + (aP)2 + bP, where P = [Fo2 + 2Fc2]/3 and a and b are constants adjusted by the program. cS = [∑{w(Fo2 − Fc2)2}/(n − p)]0.5, where n is the number of data and p the number of parameters. a

NMR spectra were recorded on a Bruker ARX 300 instrument in CD3CN. Chemical shifts are quoted relative to SiMe4 (1H external) and CFCl3 (19F, external). Absorption spectra in solution were registered on a Hewlett-Packard 8453 diode array UV−visible spectrophotometer. Excitation and emission spectra as well as lifetime measurements were recorded with a Jobin-Yvon Horiba Fluorolog 322 Tau-3 spectrofluorimeter. The lifetime measurements in the microsecond range were performed by operating in the phosphorimeter mode (with an F1-1029 lifetime emission PMT assembly, using a 450 W Xe lamp).The lifetime measurements in the nanosecond range were performed by operating in the phase-modulation mode. The phase shift and modulation were recorded over the frequency range 0.2−50 MHz. The lifetime data were fitted using the Jobin-Yvon software package and the Origin 6.1 program. Preparation of [Au3Cu(C6F5)4(NCCHCHPh)3]n (1). To an acetonitrile solution (20 mL) of [AuAg(C6F5)2(NCMe)]n (95 mg, 0.112 mmol) was added CuCl (11 mg, 0.112 mmol), and a white precipitate was observed (AgCl). The mixture was stirred for 2 h, and the solid was eliminated by filtration. The acetonitrile was evaporated to dryness, and an orange residue was obtained. This solid was dissolved in toluene (20 mL), and then cinnamonitrile (14.3 μL, 0.112 mmol) was added, leading to a light yellow solution. After 30 min of stirring the solvent was evaporated to dryness, giving complex 1 as an orange solid. Yield: 58%. Anal. Calcd for 1 (C51H21Au3CuF20N3·0.5C6H5CH3): C, 37.27; H, 1.43; N, 2.39. Found: C, 37.42; H, 1.39; N, 2.20. 1H NMR (298 K, CD3CN): 7.31 (m, 5H, C6H5), 7.22 (m, 1H, PhCHC), 5.86 ppm (m, 1H, C CHCN). MS (ES-): m/z 530.7 [Au(C6F5)2]−, 1124.7 [Au2Cu(C6F5)4]−. FT-IR (Nujol mull): ν 2254 cm−1 (CN), ν 1496, 962, 784 cm−1 (Au(C6F5)2).

are slightly overestimated for both cases, although the profiles are fairly similar and are divided into two main energy zones for both experimental and theoretical spectra. In conclusion, the presence of bridging perhalophenyl ligands between Au(I) centers permits the synthesis of complexes with constrained Au2Cu trinuclear cores displaying very short intermetallic distances. This structural trend permits the design of organometallic complexes with electronic structures suitable for possible low-energy charge transfer transitions between highly destabilized orbitals located at the electron-rich parts of the molecules (Au(I), Cu(I), and perhalophenyl ligands) and the π* orbitals of nitrile ligands, leading to a new class of lowenergy emissive compounds.

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2 C36H14Au2Cl15CuN2

EXPERIMENTAL SECTION

General Considerations. The compound [{AuAg(C6F5)2(N CMe)}2]n2b was synthesized according to a published procedure. The complex [AuAg(C6Cl5)2(NCMe)]n was prepared similarly to the analogous one with pentafluorophenyl ligands by changing the aurate precursor. Solvents (spectroscopic grade) used in the spectroscopic studies were degassed prior to use. Instrumentation. Infrared spectra were recorded in the 4000−200 cm−1 range on a Perkin-Elmer FT-IR Spectrum 1000 spectrophotometer, using Nujol mulls between polyethylene sheets. C, H, N analyses were carried out with a C.E. Instrument EA-1110 CHNS-O microanalyzer. Mass spectra were recorded on a HP-5989B APIElectrospray mass spectrometer with 59987A interface. 1H and 19F 3727


Organometallics

Article

Table 7. Selected Bond Lengths (Å) and Angles (deg) for Complex 1·0.5(toluene)a Au(1)−Cu Au(2)−Cu Au(1)−Au(2) Au(1)−Au(3) Au(2)−Au(3)#2 Au(1)−C(1) Au(1)−C(11) C(1)−Au(1)−C(11) Cu−Au(1)−Au(2) Cu−Au(1)−Au(3) C(21)−Au(2)−C(11) Cu−Au(2)−Au(1) a

2.7177(8) 2.7077(8) 2.7325(3) 2.9494(3) 2.9553(3) 2.048(6) 2.183(6) 174.9(2) 59.58(2) 160.26(2) 176.2(2) 59.94(2)

Au(2)−C(21) Au(2)−C(11) Au(3)−C(31) Au(3)−N(1) Cu−N(2) Cu−N(3) Au(1)−Au(3)−Au(2)#1 C(31)−Au(3)−N(1) N(2)−Cu−N(3) Au(2)−Cu−Au(1)

2.037(6) 2.166(6) 1.997(6) 2.035(6) 1.891(5) 1.891(5) 166.25(1) 178.9(3) 138.9(2) 60.49(2)

Symmetry transformations used to generate equivalent atoms: (#10 −x + 1/2, y + 1/2, −z + 1/2; (#2) −x + 1/2, y − 1/2, −z + 1/2. distances, angles and dihedral angles kept frozen, single-point DFT calculations were performed on the models. In the single-point ground-state calculations and the subsequent calculations of the electronic excitation spectra, the default Beck−Perdew (BP) functional29 as implemented in TURBOMOLE30 was used. The excitation energies were obtained at the density functional level by using the time-dependent perturbation theory approach (TD-DFT),31−35 which is a DFT generalization of the Hartree−Fock linear response (HF-LR) or random-phase approximation (RPA) method.36 In all calculations, the Karlsruhe split-valence quality basis sets37 augmented with polarization functions were used (SVP).38 The Stuttgart effective core potentials in TURBOMOLE were used for Au and Cu.39 Calculations were performed without any assumption of symmetry for 1a and 2a.

Table 8. Selected Bond Lengths (Å) and Angles (deg) for Complex 2 Au(1)−C(1) 2.044(10) Au(1)−C(11) 2.162(9) Au(1)−Cu 2.6841(13) Au(1)−Au(2) 2.7186(5) Au(2)−C(21) 2.036(9) C(1)−Au(1)−C(11) 171.8(3) Cu−Au(1)−Au(2) 60.42(3) C(21)−Au(2)−C(11) 164.0(3)

Au(2)−C(11) Au(2)−Cu Cu−N(1) Cu−N(2) Cu−Au(2)−Au(1) N(1)−Cu−N(2) Au(1)−Cu−Au(2)

2.174(9) 2.7184(13) 1.879(9) 1.882(9) 59.16(3) 146.5(4) 60.42(3)

Preparation of [Au2Cu(C6Cl5)3(NCCHCHPh)2] (2). To an acetonitrile solution (20 mL) of [AuAg(C6Cl5)2(NCMe)]n (95 mg, 0.112 mmol) was added CuCl (11 mg, 0.112 mmol), and a white precipitate was immediately formed (AgCl). The mixture was stirred for 2 h, and the solid was eliminated by filtration. Acetonitrile was evaporated to dryness, and a white residue was obtained. This solid was dissolved in toluene (20 mL), giving rise to a white suspension. Addition of cinnamonitrile (14.3 μL, 0.112 mmol) led to a light yellow solution that was stirred for 2 h. Then the solvent was evaporated to ca. 5 mL, and the addition of n-hexane gave rise to complex 2 as a reddish solid. Yield: 60%. Anal. Calcd for 2 (C36H14Au2CuCl15N2): C, 29.54; H, 0.96; N, 1.91. Found: C, 29.70; H, 1.02; N, 2.01. 1H NMR (298 K, CD3CN): 7.35 (m, 5H, C6H5), 7.26 (m, 1H, PhCHC), 5.91 ppm (m, 1H, CCHCN). MS (ES-): m/z 694.61 [Au(C6Cl5)2]−. FT-IR (Nujol mull): ν 2248 cm−1 (CN), ν 834, 614 cm−1 (Au(C6Cl5)2). Crystallography. Single crystals of 1·0.5(toluene) and 2 were mounted in mineral oil on a glass fiber and transferred to the cold stream of a Nonius Kappa CCD diffractometer equipped with an Oxford Instruments low-temperature attachment. Data were collected by monochromated Mo Kα radiation (λ = 0.710 73 Å) with scan types ω and ϕ and semiempirical absorption correction (based on multiple scans). The structures were solved by direct methods and refined on F2 using SHELXL-97.28 All non-hydrogen atoms were anisotropically refined, and hydrogen atoms were included using a riding model. Further details regarding the data collection and refinement methods are given in Table 6. Selected bond lengths and angles are given in Tables 7 and 8, and the crystal structures of 1·0.5(toluene) and 2 are shown in Figures 1−3. Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC-870660 and CCDC-870661. Copies of the data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (0.44) 1223-336-033; e-mail, [email protected]). Computational Details. The molecular structures used in the theoretical studies on [Au3Cu(C6F5)4(NCCHCHPh)3] (1a) and [Au2Cu(C6Cl5)3(NCCHCHPh)2] (2a) were taken from the Xray diffraction data for [Au3Cu(C6F5)4(NCCHCHPh)3]n (1) and [Au2Cu(C6Cl5)3(NCCHCH-Ph)2] (2), respectively. Witgh all

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data in CIF format for 1 and 2 (CCDC870660 and CCDC-870661). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The DGI (MEC)/FEDER (CTQ2010-20500-C02-02) project is acknowledged for financial support. M.R.-C. and D.P. thank the CAR for a grant.

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REFERENCES

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