Tailor-Made Luminescent Polymers through Unusual Metallophilic

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Tailor-Made Luminescent Polymers through Unusual Metallophilic Interaction Arrays Au···Au···Ag···Ag María Gil-Moles,† M. Concepción Gimeno,*,‡ José M. López-de-Luzuriaga,*,† Miguel Monge,† M. Elena Olmos,† and David Pascual† †

Departamento de Química, Universidad de La Rioja, Centro de Investigación de Síntesis Química (CISQ), Complejo Científico Tecnológico, 26004 Logroño, Spain ‡ Departamento de Química Inorgánica, Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), CSIC-Universidad de Zaragoza, E-50009 Zaragoza, Spain S Supporting Information *

ABSTRACT: A novel and efficient strategy for the synthesis of luminescent polymers bearing metallophilic interactions with unprecedented charge sequences has been designed. For this end suitable basic gold units such as [AuR2]−, bearing perhalophenyl derivatives, and dinuclear acid silver terpyridine species, [Ag2(terpy)2](CF3SO3)2, have been chosen. Their combination originates the polymeric derivatives [{AuR2}2Ag2(terpy)2]n (R = C6F5, C6Cl2F3) or [{Au(C6Cl5)2}Ag(terpy)]n. The change of the perhalophenyl group in the gold complex modulates the strength in the metallophilic contacts and, consequently, the polymer arrays and luminescent properties. The X-ray diffraction studies of these derivatives revealed that there are polymers with unusual + + − − + + − − charge sequences for the R = C6F5 and C6Cl2F3 species, whereas the more classical + − + − disposition was found for the bulkiest C6Cl5 derivative. Their luminescent properties also vary depending on the formation of these polymer arrays, and time-dependent density functional theory calculations were performed to determine the origin of the luminescence.

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INTRODUCTION Metallophilic interaction is a specific term coined to describe the interaction that appears between metals with closed-shell configurations at distances shorter than the sum of their van der Waals radii.1 This concept is an extension of the primary term aurophilicity, which describes the interactions that usually appeared between gold(I) atoms.2 This effect, in principle, generated a great controversy in the scientific community, since two gold(I) atoms in formally +1 oxidation states would normally repel each other, taking into account the classical Coulomb rules. Additionally, if we refer only to their electronic configurations (d10), the interaction between the gold centers should arise from weak van der Waals forces. Instead, far from repulsions between similar charges or weak attractive forces between closed-shell configurations, the interactions were theoretically calculated to be similar in energy to the standard hydrogen bonding,3 much stronger than expected taking into account the previous considerations. A second point to be considered is the formal charge of the whole complex. For instance, anionic gold(I) complexes can react with metallic salts in which the cationic metallic ion is accompanied by a noncoordinating anion, in what can be defined as acid−base neutralization reactions.4−7 In this case, the reactions lead to complexes in which, parallel to the electrostatic interaction between anionic and cationic counterparts, and reinforcing this, the metallic centers often keep a © XXXX American Chemical Society

metallophilic interaction between similar or different metal centers. On other occasions, similar results can be obtained in reactions between metallic precursors when the exchange of cations and anions gives rise to the formation of insoluble salts in the reaction medium, which can be named transmetalation reactions.8,9 Thus, by means of these kinds of reactions, it is possible to synthesize complexes displaying metallophilic interactions between centers with closed-shell configurations (d10−d10 or d10−s2) as, for instance, Au(I) and Ag(I),4 Tl(I),5 Hg(II),6 Bi(III),7 Cu(I)8 or Pb(II),9 in which the usual disposition is an alternate positive−negative ordering of complex counterions in different molecular dispositions as discrete molecules, onedimensional polymers, or two- or three-dimensional networks. In these arrangements, in addition to the metallophilic interactions, the highest contribution to the stabilization comes from the electrostatic interaction between the positive−negative sequences of complexes. This has been calculated in selected examples, and it is responsible for ca. 80− 85% of the total interaction energy.10 Therefore, any other ordering resulting in a positive−positive or negative−negative interaction between ionic complexes would in principle result in a more repulsive force than the attraction that arises from the Received: May 26, 2017

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DOI: 10.1021/acs.inorgchem.7b01342 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

structures with a great variety of metals, showing a high thermal and oxidative stability. One of these metals is silver, which forms complexes with a different structure depending on the counterion used in the reaction.15 Consequently, dinuclear silver complexes with bridging terpyridine ligands could react with the adequate anionic gold complexes to produce the acid− base neutralization reaction, thus leading to the synthesis of polynuclear complexes with an unusual charge disposition as shown in Figure 1. At this point it is also worth mentioning that the choice of the anionic complexesnormally bis(perhalophenyl)auratesis also important since both the control of their basicity and the steric hindrance of the perhalophenyl groups can be important factors affecting the metallophilic arrangements.

dispersive interactions and relativistic effects between heavy metallic centers with closed-shell configurations. This is a particularly interesting point that constitutes a step forward in the study of the metallophilic interactions and their quantification in different environments. Up to now, we had previously reported several heterometallic structural arrangements disobeying Coulomb’s rules, such as M+−Au−−Au−−M+ (M = Cu,11 Ag,12 or Tl13) or the interaction between anionic [AuR 2 ] − ···[Ag 4 X 5 ] − (R = C6F3Cl2; X = CF3CO2) fragments (Figure 1).14 Nevertheless, in all cases, the observed patterns relied on nondeliberated strategies.

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RESULTS AND DISCUSSION Synthesis and Characterization. In order to build luminescent materials with a preorganized metallophilic array, basic gold(I) species of the type [AuR2]− (R = C6F5, C6F3Cl2, and C6Cl5) together with acid dinuclear silver(I) complexes containing the N-donor tridentate ligand terpy have been chosen. The reaction of the dinuclear Ag(I) complex [Ag2(terpy)2](CF3SO3)2 (1) with the corresponding (NBu4)[AuR2] (R = C6F5, C6F3Cl2, and C6Cl5) precursors in the appropriate molar ratio in tetrahydrofuran (THF) led to the synthesis of three new heteronuclear gold−silver metallopolymers: [{Au(C 6 F 5 ) 2 } 2 Ag 2 (terpy) 2 ] n (2), [{Au(C6F3Cl2)2}2Ag2(terpy)2]n (3), and [{Au(C6Cl5)2}Ag(terpy)]n (4) (see Scheme 1). Complex 1 is a white solid, soluble in acetone, partially soluble in THF, dichloromethane or acetonitrile, and insoluble in n-hexane or diethyl ether. Its 1H NMR spectrum was recorded in d8-THF. The signals attributed to the presence of the terpyridine ligand bonded to the Ag(I) centers appear shifted with respect to those of the free ligand, confirming the coordination of the N-donor ligand to the metal centers. We have evaluated the possibility of the existence of complex 1 as a mononuclear species in solution. For this, we have carried out a conductivity study in solution for this complex at different concentrations, applying the Onsager law. The results (see Supporting Information) indicate that the formulation of

Figure 1. Design of luminescent polymers with unusual charge sequences.

A purposeful strategy to achieve a different metallic order that leads to a different charge sequence from the more usual positive−negative (see Figure 1) could be the use of polydentate neutral ligands bearing donor centers in close proximity. These can bind metal atoms of similar positive charge leading to polycharged complexes displaying metallophilic interactions. Further reaction with anionic complexes in the adequate molar ratio will produce charge neutralization, and heterometallic complexes could be obtained, in which the metallic order puts together similar formal charges. A suitable polydentate ligand to develop the last strategy is 2,2′:6′,2″-terpyridine (terpy), which is one of the most extensively used ligands for the synthesis of supramolecular Scheme 1. Synthesis of Polynuclear Complexes 2−4

B


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Inorganic Chemistry

Information). The molecular structure of 1 is depicted in Figure 2.

complex 1 is in accordance with a 1:2 electrolyte, which is in agreement with the existence of only the dinuclear species in solution. The three heterometallic complexes 2−4 are white solids, but their physical properties and characteristics are fairly different. Complexes 2 and 3 are partially soluble in THF, dichloromethane or acetonitrile, and insoluble in n-hexane or diethyl ether, while compound 4 is nearly insoluble in all solvents tested. Their 1H NMR spectra were recorded in d8-THF (2, 3) or in d6-DMSO (4). The coordination between both acid−base fragments seems to be evident in the 1H NMR spectra of 2 and 3 because the resonances due to the terpy ligand are different from those for complex 1 and for the free ligand. In contrast, the resonances of protons in compound 4 have also changed related to 1 but are more similar to those of free terpy ligand. This fact suggests that the coordination of terpy to the silver centers is different in complex 4 in comparison with compounds 2 and 3 (see Experimental Section). The analytical and spectroscopic data for complexes 1−4 are in accordance with the proposed stoichiometries. In the IR spectrum of complex 1 the bands corresponding to ν(CN) 1592 cm−1, ν(CF3SO3−) 1277 cm−1, and ν(O−SO2CF3) 1159, 1140 cm−1 were observed. The corresponding ν(CN) absorptions arising from the terpy ligands appear at 1628 cm−1 (2) and 1589 cm−1 (4). In the case of complex 3 we cannot unequivocally assign this band due to the overlapping with that due to the ν(Au−(C6F3Cl2)) absorption at 1588 cm−1. The presence of the perhalophenyl groups bonded to gold(I) in complexes 2−4 is evident from their IR spectra, which show, among others, absorptions due to the C6F5, C6F3Cl2, or C6Cl5 ligands at 1502, 954, and 781 cm−1 (2), 1588, 1562, 1087, and 771 cm−1 (3), and 1319, 1084, and 755 cm−1 (4). The conductivity measurements of solutions of complexes 2 and 3 in acetonitrile are in accordance with an ionic formulation of the complexes, since they behave as 1:1 electrolytes, which suggests dissociation in solution into [AuR2]− (R = C6F5, C6F3Cl2, and C6Cl5) anions and [Ag(terpy)]+ cations. The low solubility of complex 4 precluded the measurement of its molar conductivity in solution. In the mass spectra MALDI-TOF(−) of the new products 2−4 the base peak corresponding to the aurate(I) anion [AuR2]− (R = C6F5, C6F3Cl2, and C6Cl5) is observed at m/z = 530.826, 596.634, and 694.508, respectively. Their MALDI-TOF(+) mass spectra display a base peak corresponding to the fragment [Ag(terpy)]+ at ca. m/z = 339.9 (2−4), confirming the presence of the silver derivative. In all of them, the experimental isotopic distributions are in agreement with the calculated ones. In addition, in their mass spectra ESI(−), the experiment of exact mass for complex 1 shows the peak at m/z = 148.9521 corresponding to the CF3SO3− anion, while for complexes 2−4 the base peak corresponds to the aurate(I) anion [AuR2]− (R = C6F5, C6F3Cl2 or C6Cl5) at m/z = 530.9506, 596.8301, or 694.6486, respectively. In the ESI(+), the experiment of exact mass for complex 1 shows the peaks corresponding to [Ag(terpy)]+ at m/z = 340.000 and [Ag(terpy)2]+ at m/z = 573.0929, respectively. For complexes 2 and 3, the base peak assigned to the fragment [{AuR2}Ag2(terpy)2]+ (R = C6F5, C6F3Cl2) at m/z = 1210.9508 or 1274.8336 was observed, respectively. Complex 4 presents a peak at m/z = 573.0964 associated with the [Ag(terpy)2]+ fragment. Crystal Structures. The crystal structures of complexes 1− 4 have been established by X-ray diffraction (see Supporting

Figure 2. Molecular structure of complex 1. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ag(1)−N(3) 2.223(2), Ag(1)−N(1) 2.270(2), Ag(1)−N(2) 2.4510(18), Ag(1)−Ag(1)#1 2.9412(4), N(1)−C(1) 1.345(3), N(1)−C(5) 1.349(3), N(2)−C(6) 1.341(3), N(2)−C(10) 1.342(3), N(3)−C(15) 1.347(3), N(3)−C(11) 1.357(3); N(3)−Ag(1)−N(1)147.76(7), N(3)−Ag(1)−N(2) 132.09(7), N(1)−Ag(1)−N(2) 71.29(7); #1 −x + 2, y, −z + 1/2.

The complex is a dimer in which the silver atoms are bridged by the tridentate terpy ligands, each silver center is bound to four nitrogen atoms with dissimilar distances, the shortest ones are those of the outside nitrogen atoms, 2.223(2) and 2.270(2) Å, and there are two long ones corresponding to the central nitrogen atom, 2.4510(18) Å, and a very weak bond of 2.861 Å to the other central nitrogen. Additionally, the silver centers make weak bonds to the oxygen of the triflate anion, 2.625 Å. Consequently, the silver atoms have a very irregular molecular geometry that can be considered as tetrahedral or trigonal bipyrimidal. There is a close Ag···Ag bonding interaction of 2.9412(4) Å. The structure of complex 2 is shown in Figure 3. There are two independent molecules in the asymmetric unit, each one

Figure 3. Molecular structure of one of the independent molecules in complex 2. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Au(1)−C(31) 2.038(6), Au(1)−C(37) 2.049(6), Au(1)−Ag(1) 2.8979(5), Au(1)−Au(2) 3.2477(4), Au(2)− C(43) 2.041(7), Au(2)−C(49) 2.040(7), Au(2)−Ag(2)#1 2.7765(6), Ag(1)−N(6) 2.265(5), Ag(1)−N(3) 2.285(5), Ag(1)−N(2) 2.482(5), Ag(1)−N(5) 2.634(5), Ag(1)−Ag(2) 2.9598(7), Ag(2)− N(1) 2.254(6), Ag(2)−N(4) 2.259(5); #1 −x + 1, y + 1/2, −z + 3/2. C


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Inorganic Chemistry composed of a dinuclear silver species joined to a [Au(C6F5)2]− unit through metallophilic Ag···Au interactions, which is further bonded to another [Au(C6F5)2]− unit though aurophilic interactions, thus forming a Ag···Ag···Au···Au moiety. The Ag−N bond lengths in the dinuclear silver unit have changed slightly with the coordination of the gold fragments. Again, the shortest ones correspond to the outside nitrogen atoms, and there are different situations with the central nitrogen atoms; in one of the terpy ligands they are weak and similar, Ag(1)−N(5) 2.634(5) and Ag(2)−N(5) 2.651 Å, whereas the other terpy makes dissimilar bonds of Ag(1)−N(2) 2.482(5) and the very weak contact of Ag(2)−N(2) of 2.933 Å. The molecules are arranged in two linear polymers bearing these unusual metallophilic interactions (Figure 4). The

(Figure 5). Such a small change has the result that, although the whole structure has a similar pattern, the building blocks for the formation of the chain polymers are different. In this case each silver center in the dinuclear fragment is bonded to a gold moiety, with a short Ag···Au distance of 2.9456(4) and 2.9762(4) Å, respectively. These tetranuclear fragments are further bonded to others through weak aurophilic interactions of ca. 4 Å. Consequently, a similar pattern + + − − + + − − is achieved with differences in the metallophilic interactions. The Ag(1)−Ag(2) bond distance of 3.0349(6) Å is longer than in the starting material and complex 2. The complete substitution of the fluorine by chlorine atoms in the perhalophenyl ligands gives a totally different result: the steric hindrance of the chlorine atoms prevents the formation of polymers with metallophilic interactions preserving the dinuclear silver units. In this case complex 4 forms a polymer chain through the more classical Au···Ag···Au···Ag interactions, alternating a bis(pentachlorophenyl)gold moiety and a mononuclear silver−terpyridine unit (Figure 6). The silver center is bonded symmetrically to the three nitrogen atoms of the terpy ligand, with distances Ag(1)−N(1) 2.393(9), Ag(1)−N(3) 2.402(9), or Ag(1)−N(2) 2.404(9) Å. The Ag···Au bonding interactions are the shortest in the three complexes, Ag(1)−Au(1)#1 2.8970(10) and Ag(1)−Au(1) 2.9070(10) Å. Photophysical Properties. The heterometallic complexes 2−4 display a rich optical behavior in the solid state, in which the intermetallic interactions seem to play an important role. The apparent loss of these interactions in coordinating solvents, as acetonitrile or DMSO, which is in agreement with the NMR data and conductivity measurements, leads to the disappearance of the emissions. In contrast, the bimetallic silver precursor 1 does not show luminescence either in the solid state or in solution, even at low temperature, which is likely to suggest that the intramolecular silver(I)−silver(I) interactions are not responsible for the luminescent behavior found in the heterometallic derivatives. The absorptions of these complexes in acetonitrile solutions are dominated by ligand transitions. Thus, the terpy ligand

Figure 4. One of the chain polymers formed by complex 2 through metallophilic interactions.

formation of this complex produces a slight lengthening of the Ag···Ag bonding interaction, 2.9598(7), very short Ag··Au contacts of 2.8979(5) and 2.7765(6) Å, and an aurophilic interaction of 3.2477(4) Å. This leads to a + + − − + + − − arrangement of charges, opposite to Coulomb’s rules, which has been observed for the first time. The structure of complex 3, in which two of the fluorine atoms in the gold fragment have been substituted by the bulkiest chlorine ones, has been confirmed by X-ray diffraction

Figure 5. Molecular structure of complex 3 and formation of the polymer chain using the tetranuclear array. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Au(1)−C(37) 2.058(5), Au(1)−C(31) 2.069(5), Au(1)−Ag(1) 2.9456(4), Au(2)−C(49) 2.051(5), Au(2)−C(43) 2.051(6), Au(2)−Ag(2) 2.9762(4), Ag(1)−N(6) 2.222(4), Ag(1)−N(1) 2.236(4), Ag(1)−N(2) 2.604(4), Ag(1)−Ag(2) 3.0349(6), Ag(2)−N(3) 2.248(4), Ag(2)−N(4) 2.261(4), Ag(2)−N(5) 2.561(4); C(37)−Au(1)−C(31) 177.39(19), C(31)−Au(1)−Ag(1) 112.02(14), C(49)−Au(2)−C(43) 176.7(2), C(49)−Au(2)−Ag(2) 102.73(14), C(43)−Au(2)−Ag(2) 78.93(14), N(6)−Ag(1)−N(1) 169.01(14), N(6)− Ag(1)−N(2) 108.96(14), N(1)−Ag(1)−N(2) 70.49(14), N(3)−Ag(2)−N(4) 171.06(14), N(3)−Ag(2)−N(5) 112.97(14), N(4)−Ag(2)−N(5) 70.10(14). D


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the metallic complexes extending to more than 400 nm that are likely to be due to transitions involving the metal centers (see Supporting Information). In contrast, the luminescence studies show that the emissive properties of these complexes are greatly influenced by the metallic ordering, and it is important to discuss how the presence of Au···Au interactions affects the emission energies if it is compared with complexes where the aurophilicity phenomenon is absent. Thus, in the case of the heterometallic complexes, only two of the three compounds present emissive properties in the solid state at room temperature and at 77 K (Figure 8), and none of them show luminescence in solution,

Figure 6. Molecular structure of complex 4. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ag(1)−N(1) 2.393(9), Ag(1)−N(3) 2.402(9), Ag(1)−N(2) 2.404(9), Ag(1)−Au(1)#1 2.8970(10), Au(1)−Ag(1)#2 2.8971(10), Ag(1)−Au(1) 2.9070(10), Au(1)−C(7) 2.072(11), Au(1)−C(1) 2.083(11); #1 −x + 1/2, y − 1/2, −z + 1/2, #2 −x + 1/2, y + 1/2, −z + 1/2.

displays absorptions at 233 (ε = 2.81 × 104), 250 (ε = 2.19 × 104), 277 (ε = 2.63 × 104), 300 (ε = 1.81 × 104), and 315 nm (ε = 1.00 × 104 M−1·cm−1), which are likely to arise from π → π* and n → π* transitions. Identical bands appear in the spectrum of the silver complex [Ag2(terpy)2](CF3SO3)2 (1) as well as in the heterometallic derivatives [{Au(C6F5)2}2Ag2(terpy)2]n (2), [{Au(C6F3Cl2)2}2Ag2(terpy)2]n (3), and [{Au(C6Cl5)2}Ag(terpy)]n (4), with an additional intense band at 220 nm in the latter, which is assigned to a π → π* transition located in the C6Cl5 rings.4b,c Therefore, the absorption maxima obtained for the heterometallic complexes 2−4 is more intense in the ca. 250 nm region due to the intense absorption of the perhalogenated part of the molecules that does not occur for complex 1 (see Figure 7). Their absorption spectra in the solid state are featureless with intense absorptions around 300 nm and higher energy bands, which are assigned to the π → π* and n → π* transitions in the perhalophenyl or in the terpyridine ligands, similarly to those appearing in the solution experiments, and low energy tails for

Figure 8. Luminescence in the solid state at room temperature and at 77 K for complex 2.

which suggests that the loss of the intermetallic interactions in solution (see in the Experimental Section, the molar conductivity values typical of 1:1 electrolytes) leads to the loss of the emissive properties, as we have repeatedly observed in previous examples.16 Thus, for instance, while complexes 2 and 3, at room temperature, show emissions at 556 (quantum yield of 0.047) and 545 nm (quantum yield of 0.041) by excitation at 370 nm, respectively, and at 77 K both complexes shift the emission bands at lower energy, 565 nm (2) and 570 nm (3) (Figures 8 and 9), complex 4 does not show any emission in the solid state. The more pronounced emission red-

Figure 7. Absorption spectra in acetonitrile solutions of free terpy and complexes 1−4 at concentrations 6.7 × 10−5 M (terpy); 5.0 × 10−5 M (1); 2.5 × 10−5 M (2); 1.3 × 10−5 M (3); 1.0 × 10−6 M (4).

Figure 9. Luminescence in the solid state at room temperature and at 77 K for complex 3. E


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Inorganic Chemistry shift observed for complex 3 if compared to that of complex 2 could be due to a larger intermetallic Au−Au distance in the former that allows a higher degree of contraction, leading to a higher structural distortion when the temperature is lowered. These results are likely to be related to the presence of gold− gold interactions in the formers, interactions that do not appear in the latter, which shows an ordering of alternate gold and silver centers in the solid state. Thus, the energies of both emissions agree well with this assignation, since the lowest energy emission appears in the complex displaying the shortest metal−metal interactions (d(Au−Au) = 3.248 (2) vs 4.047 Å (3); d(Ag−Ag) = 2.961 (2) vs 3.035 Å (3)). Significantly, complex [Ag2(terpy)2]2+ (1) does not show any emission in the solid state, suggesting that the emissions found in both heterometallic complexes have their origins in the gold centers. Regarding this assignation, although a distance of 4.047 Å between the gold centers must be considered a very weak interaction because it is longer than the sum of their van der Waals radii, it is worth noting that usually the emissions having their origins in two gold centers at a short distance occur from orbitals of antibonding character and arrive to an orbital of bonding character with contributions of both gold atoms producing an appreciable shortening of the metal−metal distance.8b,12 In this regard, our DFT results (see DFT and TD-DFT Calculations) show that the character of the highest occupied orbital HOMO is located at the internal gold centers showing the above-mentioned antibonding character, even at such a long distance, in the same way that the interacting Au− Au centers behave. It is also worth mentioning that the different Au−Au distances found experimentally for complexes 2 and 3 and the lack of aurophilic interactions for complex 4 would be related to the different basicity of the bis(perhalophenyl)aurate moieties. Thus, when C6F5 ligands are used, the less donor ability of this aurate anion would favor an additional stabilization of the Au(I) centers through aurophilic interactions. This tendency is less pronounced when 2,3-C6F3Cl2 ligands are used, leading to a larger Au−Au distance, and, finally, when C6Cl5 ligands are employed, the higher basicity of the corresponding aurate units, among other factors, would prevent the formation of aurophilic interactions. This tendency in the basicity would also be responsible for the different emission energies found for 2 and 3 and for the lack of emissive properties in the case of complex 4. Finally, the lifetimes determined for both complexes at room temperature fall within the range of microseconds (0.85 μs (2) and 0.79 μs (3)). This fact together with the large Stokes shifts (9000 cm−1 (2) and 8500 cm−1 (3)) and the presence of heavy metal centers that promote strong spin−orbit couplings suggests phosphorescent processes. DFT and TD-DFT Calculations. To explain the origin of the emissive properties observed for complexes 2 and 3 we have carried out DFT and TD-DFT calculations on model systems representing the solid state structures for these complexes. The studied model systems used for the calculations were built up from the X-ray diffraction structures of the complexes, and they represent all types of metallophilic interactions characterized experimentally, i.e., Ag···Ag, Au··· Ag, and Au···Au interactions for both complexes. Thus, octanuclear model systems [{Au(C6F5)2}2Ag2(terpy)2]2 (2a) and [{Au(C6F3Cl2)2}2Ag2(terpy)2]2 (3a) had to be used, which prevented the full optimization of such large theoretical models, using the experimentally obtained structural parameters as a valid approach (Figure 10).

Figure 10. Theoretical model systems 2a and 3a.

The study of the most important frontier molecular orbitals (MOs) through their population analysis was carried out in order to check the contribution of each atom or group of atoms to the frontier MOs. Tables 1 and 2 and Figures 11 and 12 summarize the obtained results. Table 1. Population Analysis for Model System 2aa LUMO+2 LUMO+1 LUMO HOMO HOMO−1 HOMO−2

Auext

C6F5ext

Auint

C6F5int

Ag

terpy

0 0 0 1 51 0

0 0 0 0 12 0

0 0 0 59 2 11

0 0 0 21 1 86

1 2 2 10 19 2

98 97 97 9 15 1

a

Contribution from each part of the molecule to the frontier orbitals (%).

Table 2. Population Analysis for Model System 3aa LUMO+2 LUMO+1 LUMO HOMO HOMO−1 HOMO−2

Auext

C6F3Cl2ext

Auint

C6F3Cl2int

Ag

terpy

0 0 0 1 0 36

0 0 0 0 0 31

0 1 0 48 27 6

0 1 0 31 73 7

2 3 2 13 0 11

97 96 97 7 0 9

a

Contribution from each part of the molecule to the frontier orbitals (%).

Figure 11. Frontier molecular orbitals for model system 2a.

Thus, the population analysis of the highest occupied MOs for model 2a shows that the HOMO is mostly located (59%) at the Au(I) centers of the internal aurate units (Auint) with a secondary contribution from the internal C6F5− ligands of 21% and from the [Ag2(terpy)2]+ units (19%). HOMO−1 is mainly located (51%) at one of the Au(I) centers of the external aurate units (Auext) with some contribution from the external C6F5− ligands (12%) and a secondary contribution from the [Ag2(terpy)2]+ moiety of 34%. HOMO−2 is mainly placed almost exclusively at the internal aurate units with Auint and F


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have its origin in a metal (aurate unit) to ligand (terpy) charge transfer 3(MLCT) transition. In the case of the S0 → T1 excitation for model 2a, the computed energy value appears red-shifted with respect to that obtained for model 3a. This result is in agreement with the shortest gold−gold distance found for complex 2, leading to a destabilization of the antibonding HOMO orbital between the gold interacting centers. Then, in order to reproduce the whole absorption spectra for complexes 2 and 3 we also computed the first few singlet−singlet excitations. The results and the corresponding comparison with the experimental data are depicted in Figure 13 and summarized in Table 3 and Tables S1 and S2. As can be Figure 12. Frontier molecular orbitals for model system 3a.

C6F5−int contributions of 11% and 86%, respectively. In the case of the lowest empty MOs LUMO, LUMO+1, and LUMO+2, an almost exclusive contribution from the terpyridine ligands is computed. In the case of model 3a, the results found in the population analysis of the highest occupied MOs show a similar distribution to that of model 2a, with the main difference of an exchange of character between HOMO−1 and HOMO−2. Thus, HOMO is again mainly located at Auint (48%) and C6F3Cl2−int ligands (31%), being 79% of the MO located at the internal aurate units, with a minor contribution from the [Ag2(terpy)2]+ units (20%). In this case, HOMO−1 is only located at the internal aurate units (Auint 27% and C6F3Cl2−int 73%); meanwhile HOMO−2 is mainly placed at the external aurate units (Auext 36% and C6F3Cl2−ext 31%) with minor contributions from the internal aurate units (13%) and the [Ag2(terpy)2]+ units (20%). Again, LUMO, LUMO+1, and LUMO+2 are located at the terpyridine ligands. If we analyze the obtained results, we can anticipate that the lowest electronic transitions will mainly arise from the [AuR2]− (R = C6F5, C6F3Cl2) units, arriving to terpyridine-based MOs, leading to charge transfer type electronic transitions. In order to confirm the origin of the electronic transitions responsible for the luminescent emissions for complexes 2 and 3 we performed an analysis of the energy and oscillator strength of the most intense vertical singlet−singlet electronic excitations computed using the TDDFT approach, and we compared them with the experimental UV−vis solid-state spectra of the complexes. Moreover, since the emissive behavior of these complexes could arise from phosphorescent processes in view of their lifetimes, in the microsecond range, we also computed the lowest singlet−triplet excitation at the TD-DFT level (Table 3 and Tables S1 and S2).

Figure 13. Experimental UV−vis spectra in the solid state (black profile) for complexes 2 (top) and 3 (bottom); TD-DFT first singlet− singlet (black bars) excitation calculations for model systems 2a (top) and 3a (bottom); experimental excitation spectra in the solid state (red profile) for complexes 2 (top) and 3 (bottom); TD-DFT lowest singlet−triplet (red bar) excitation for model systems 2a (top) and 3a (bottom). The red bars only represent the energy of the lowest singlet−triplet transitions since the oscillator strength cannot be estimated.

Table 3. TD-DFT First Triplet. Excitation Calculations for the Models 2a and 3a exc

λcal (nm)

contributions %

1 triplet (2a) 1 triplet (3a)

404 360

H → L + 1 (56) H → L + 1 (41)

If we analyze first the lowest energy excitations, the first triplet transition arises, as the main contribution for both model systems 2a and 3a, from HOMO to LUMO+1. If we take into account the above-mentioned population analysis for these orbitals in both model systems, these computed results seem to indicate that the electronic transition responsible for the phosphorescent emission observed for complexes 2 and 3 could

observed, the lowest singlet−triplet excitation computed for both model systems matches the lower energy absorption tails of the solid UV−vis spectra (black profiles) and also matches the experimental excitation spectrum (red profiles). In addition, the computed singlet−singlet excitations can be classified in two groups at lower and higher energy, respectively, which also match the experimental profile obtained for the solid state G


Article

Inorganic Chemistry

nanoLED controller and DAS6 software. The nanoLED employed for lifetime measurements was one of 370 nm. The lifetime data were fitted with the Jobin-Yvon software package. Quantum yields were measured using a F-3018 Integrating Sphere mounted on a Fluorolog 3-11 Tau-3 spectrofluorimeter. Multinuclear regression analyses were carried out using the Origin 8.0 program. Synthesis of [Ag2(terpy)2](CF3SO3)2 (1). To a diethyl ether solution of AgOSO2CF3 (0.3854 g, 1.5 mmol) was added terpy (0.3499 g, 1.5 mmol). A white solid was formed immediately and was filtered after 1 h of stirring. Yield: 94%. 1H NMR (300 MHz, d8-THF, ppm): δ 8.65 (m, 2H, H1), 8.57 (m, 2H, H4), 8.49 (d, 2H,H5, 3JH5−H6 = 7.86), 8.03 (t, 1H, H6, 3JH6−H5 = 7.80), 7.90 (ddd, 2H, H3, 3JH3−H2 ∼ 3 JH3−H4 = 7.71, 4JH3−H1 = 1.79), 7.37 (ddd, 2H, H2, 3JH2−H3 = 7.42, 3 JH2−H1 = 4.71, 4JH2−H4 = 1.16). MS (ESI+) m/z: calculated for [Ag(terpy)]+ = 339.9998, found 340.000; calculated for [Ag(terpy)2]+ = 573.0951, found 573.0929. MS (ESI−) m/z: calculated for [CF3SO3]− = 148.9526, found 148.9521. Analytical data (%): C32H22Ag2F6N6O6S2 (843.90) requires 39.20, C; 2.26, H; 8.57, N. Found: 39.58, C; 2.47, H; 8.73, N. FTIR: ν(CN) 1592 cm−1, ν(ring mode vibrations) 1449 cm−1; ν(CF3SO3−) 1277 cm−1, ν(O−SO2CF3) 1159, 1140 cm−1. ΛM = 292 Ω−1·cm2·mol−1. Synthesis of [{Au(C6F5)2}2Ag2(terpy)2]n (2). To a suspension of [Ag2(terpy)2](CF3SO3)2 (0.1000g, 0.1 mmol) in tetrahydrofuran was added (NBu4)[Au(C6F5)2] (0.1547g, 0.2 mmol). Immediately a small amount of a white solid was formed. After 2 h of stirring the reaction mixture was concentrated under vacuum and [{Au(C6F5)2}2Ag2(terpy)2]n was precipitated with n-hexane as a white solid. Yield: 69%. 1H NMR (300 MHz, d8-THF, ppm): δ 8.28 (d, 2H, H1, 3JH1−H2 = 4.32), 8.20 (m, 3H, H4 + H6), 8.12 (m, 2H, H5), 7.96 (ddd, 2H, H3, 3JH3−H2 ∼ 3JH3−H4 = 7.60, 4JH3−H1 = 1.68), 7.39 (ddd, 2H, H2, 3JH2−H3 = 7.5, 3JH2−H1 = 5, 4JH2−H4 = 1.1). 19F NMR (282 MHz, d8-tetrahydrofuran, ppm): δ −88.44 (m, 4F, Fo), −119.66 (m, 2F, Fp). MS (ESI+) m/z: calculated for [{Au(C6F5)2}Ag2(terpy)2]+ = 1210.9511, found 1210.9508. MS (ESI−) m/z: calculated for [Au(C6F5)2]− = 530.9500, found 530.9506. Analytical data (%): C54H22Ag2Au2F20N6 (1210.95) requires 37.18, C; 1.27, H; 4.82, N. Found: 36.75, C; 1.48, H; 4.41, N. MALDI-TOF(+) m/z: [Ag(terpy)]+ = 339.835. MALDI-TOF(−) m/z: [Au(C6F5)2]− = 530.826. FTIR: ν(Au−(C6F5)) at 1502, 954, 781 cm−1; ν(ring mode vibrations) 1452 cm−1; ν(CN) 1628 cm−1. ΛM = 159 Ω−1·cm2·mol−1. Synthesis of [{Au(C6F3Cl2)2}2Ag2(terpy)2]n (3). To a suspension of [Ag2(terpy)2](CF3SO3)2 (0.1000 g, 0.1 mmol) in tetrahydrofuran was added (NBu4)[Au(C6F3Cl2)2] (0.1678 g, 0.2 mmol). Immediately a small amount of a white solid was formed. After 2 h of stirring the reaction mixture was concentrated under vacuum and [{Au(C6F3Cl2)2}2Ag2(terpy)2]n was precipitated with n-hexane as a white solid. Yield: 61%. 1H NMR (300 MHz, d8-tetrahydrofuran, ppm): δ 8.37 (d, 2H, H1, 3JH1−H2 = 4.43), 8.19 (m, 3H, H4 + H6), 8.17 (m, 2H, H5), 7.96 (ddd, 2H, H3, 3JH3−H2 ∼ 3JH3−H4 = 7.68, 4JH3−H1 = 1.72), 7.41 (ddd, 2H, H2, 3JH2−H3 = 7.54, 3JH2−H1 = 4.97, 4JH2−H4 = 1.03). 19F NMR (282 MHz, d8-tetrahydrofuran, ppm): δ −88.44 (m, 4F, Fo), −119.66 (m, 2F, Fp). MS (ESI+) m/z: calculated for [{Au(C6F3Cl2)2}Ag2(terpy)2]+ = 1274.8326, found 1274.8336. MS (ESI−) m/z: calculated for [Au(C6F3Cl2)2]− = 596.8289, found 596.8301. MALDITOF(+) m/z: [Ag(terpyridine)]+ 339.924. MALDI-TOF(−) m/z: [Au(C6F3Cl2)2]− 596.634. Analytical data (%): C54H22Ag2Au2Cl8F12N6 (1274.83) requires 34.57, C; 1.18, H; 4.48, N. Found: 34.12, C; 1.15, H; 3.95, N. FTIR: ν(Au−(C6F3Cl2)) at 1588,1562, 1087, 771 cm−1; ν(ring mode vibrations) 1432 cm−1. ΛM = 154 Ω−1·cm2·mol−1. Synthesis of [{Au(C6Cl5)2}Ag(terpy)]n (4). To a suspension of [Ag2(terpy)2](CF3SO3)2 (0.1000g, 0.1 mmol) in tetrahydrofuran was added (NBu4)[Au(C6Cl5)2] (0.1876g, 0.2 mmol). Immediately a small amount of a white solid was formed. After 2 h of stirring the reaction mixture was concentrated under vacuum and [{Au(C6Cl5)2}Ag(terpy)]n was precipitated with n-hexane as a white solid. Yield: 83%. 1H NMR (300 MHz, d6-DMSO, ppm): δ 8.73 (m, 2H, H1), 8.58 (d, 2H,H5, 3JH5−H6 = 7.93), 8.50 (d, 2H, H4, 3JH4−H3 = 7.40), 8.22 (t, 1H, H6, 3JH6−H5 = 7.93), 8.09 (m, 2H,H3), 7.60 (m, 2H, H3). MS (ESI +) m/z: calculated for [Ag(terpy)2]+ = 573.0951, found 573.0964. MS (ESI−) m/z: calculated for [Au(C6Cl5)2]− = 694.6486, found

absorption spectra for complexes 2 and 3 (see Figure 13 and Tables S1 and S2). Regarding the character of the singlet−singlet excitations, a general tendency is observed for the most intense computed excitations, which corresponds to electronic transitions from aurate-based orbitals to terpyridine based orbitals, suggesting that charge-transfer transitions may explain the most important contributions to the absorption spectra for complexes 2 and 3, although some minor contributions from π → π* transitions in the terpy or perhalophenyl ligands cannot be excluded.

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CONCLUSIONS A novel and efficient strategy for the synthesis of luminescent polymers bearing metallophilic interactions with unprecedented charge sequences has been designed. For this end suitable, basic gold units such as [AuR2]−, bearing perhalophenyl derivatives, and dinuclear acid silver terpyridine compounds, [Ag2(terpy)2](CF3SO3)2, have been chosen. Their combination originates the polymeric derivatives [{AuR2}2Ag2(terpy)2]n (R = C6F5, C6Cl2F3) or [{Au(C6Cl5)2}Ag(terpy)]n. The change of the perhalophenyl group in the gold complex modulates the strength in the metallophilic contacts and, consequently, the polymer arrays and luminescent properties. The X-ray diffraction analysis of these derivatives revealed that there are polymers with unusual + + − − + + − − charge sequences for the R = C6F5, C6Cl2F3 species, whereas the more classical + − + − disposition was found for the bulkiest C 6Cl5. The luminescent properties also vary depending on the formation of these polymer arrays, and time-dependent density functional theory calculations were performed to determine the origin of the luminescence. In conclusion, this unprecedented heterometallic structural arrangement has allowed us to design new organometallic complexes with electronic structures suitable for possible charge transfer transitions between orbitals located at the electron-rich parts of the molecules (Au(I) and perhalophenyl ligands) and the π* orbitals of terpyridine ligands, leading to a new class of emissive compounds.

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EXPERIMENTAL SECTION

General. 2,2′:6′2″-Terpyridine and AgOSO2CF3 were purchased from Alfa Aesar and used as received. NBu4[AuR2] (R = C6F5, 3,5C6F3Cl3, and C6Cl5) were prepared according to the literature.14,17 Instrumentation. Fourier transform infrared (FT-IR) spectra were recorded in the 4000−450 cm−1 range on a PerkinElmer μ-ATR Spectrum II. C, H, and N analyses were carried out with a PerkinElmer 240C microanalyzer. Molar conductivities were measured in ca. 5 × 10−4 M acetonitrile solutions with a Jenway 4510 conductimeter. Mass spectra were recorded on a HP-5989B API electrospray mass spectrometer with 59987A interface; exact mass experiments were carried out in the same instrument as ESI mass experiments, and a Bruker Microflex matrix-assisted laser desorption ionization time-offlight mass spectrometer (MALDI-TOF MS) using 11-dicyano-4-tertbutylphenyl-3-methylbutadiene (DCTB) as matrix. 1H and 19F NMR spectra were recorded with a Bruker Avance 400 or ARX 300 spectrometer in d8-tetrahydrofuran. Chemical shifts are quoted relative to SiMe4 (1H, external) and CFCl3 (19F, external). Absorption spectra in solution were recorded on a Hewlett-Packard 8453 diode array UV−vis spectrophotometer. Diffuse reflectance UV−vis spectra of pressed powder samples diluted with KBr were recorded on a Shimadzu UV-3600 spectrophotometer with a Harrick Praying Mantis accessory and recalculated following the Kubelka−Munk function. Excitation and emission spectra in the solid state were recorded with a Jobin-Yvon Horiba Fluorolog 3-22 Tau-3 spectrofluorimeter. Lifetime measurements were recorded with a Datastation HUB-B with a H


Inorganic Chemistry

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694.6486. MALDI-TOF(+) m/z: [Ag(terpy)]+ = 339.909. MALDITOF(−) m/z: [Au(C6Cl5)2]− = 694.508. Analytical data (%): C27H11AgAuCl10N3 (1036.75) requires 31.28, C; 1.07, H; 4.05, N. Found: 30.89, C; 0.98, H; 3.65, N. FTIR: ν(Au−(C6Cl5)) at 1319, 1084, 755 cm−1; ν(ring mode vibrations) 1442 cm−1; ν(C = N) 1589 cm−1. Crystallography. Crystals suitable for X-ray diffraction studies were obtained layering saturated THF (1) or dichloromethane (2−4) solutions with diethyl ether at room temperature. The crystal was mounted in inert oil on a glass fiber and transferred to the cold gas stream of a Nonius Kappa CCD diffractometer equipped with an Oxford Instruments low-temperature attachment. Data were collected using monochromated Mo Kα radiation (λ = 0.71073 Å). Scan type: ω and ϕ. Absorption correction: semiempirical (based on multiple scans). The structure was solved by direct methods and refined on F2 using the program SHELXL-97.18 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included using a riding model. Computational Details. All calculations were carried out using the Gaussian 09 package.19 DFT and TD-DFT calculations were carried out using the M06-2X20 hybrid functional. The following basis set combinations were employed for the metal Au and Ag: the 19-VE pseudopotentials from Stuttgart21 and the corresponding basis sets augmented with two f polarization functions.22 The heteroatoms were treated by SVP23 basis sets. All the calculations were performed on model systems for complexes built up from their corresponding X-ray structures. Overlap populations between molecular fragments were calculated using the GaussSum 2.2.5 program.24

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REFERENCES

(1) (a) Pyykkö, P. Strong Closed-Shell Interactions in Inorganic Chemistry. Chem. Rev. 1997, 97, 597−636. (b) Gold Chemistry, Applications and Future Directions in the Life Sciences; Mohr, F., Ed.; Wiley-VCH: Weinheim, 2009. (c) Modern Supramolecular Gold Chemistry; Laguna, A., Ed.; Wiley-VCH: Weinheim, 2009. (d) Sculfort, S.; Braunstein, P. Intramolecular d10−d10 interactions in heterometallic clusters of the transition metals. Chem. Soc. Rev. 2011, 40, 2741−2760. (2) (a) Schmidbaur, H.; Schier, A. Aurophilic interactions as a subject of current research: an up-date. Chem. Soc. Rev. 2012, 41, 370−412 and references therein. (b) Schmidbaur, H.; Schier, A. A briefing on aurophilicity. Chem. Soc. Rev. 2008, 37, 1931−1951. (c) Schmidbaur, H.; Graf, W.; Müller, G. Weak Intramolecular Bonding Relationships: The Conformation-Determining Attractive Interaction between Gold(I) Centers. Angew. Chem., Int. Ed. Engl. 1988, 27, 417−419. (3) Pyykkö, P.; Zhao, Y.-F. Ab initio Calculations on the (ClAuPH3)2 Dimer with Relativistic Pseudopotential: Is the “Aurophilic Attraction” a Correlation Effect? Angew. Chem., Int. Ed. Engl. 1991, 30, 604−605. (4) (a) Fernández, E. J.; Gimeno, M. C.; Laguna, A.; López-deLuzuriaga, J. M.; Monge, M.; Pyykkö, P.; Sundholm, D. Luminescent Characterization of Solution Oligomerization Process Mediated GoldGold Interactions. DFT Calculations on [Au2Ag2R4L2]n Moieties. J. Am. Chem. Soc. 2000, 122, 7287−7293. (b) Fernández, E. J.; López-deLuzuriaga, J. M.; Monge, M.; Olmos, M. E.; Puelles, R. C.; Laguna, A.; Mohamed, A.; Fackler, J. P., Jr. Vapochromic Behavior of {Ag2(Et2O)2[Au(C6F5)2]2}n with Volatile Organic Compounds. Inorg. Chem. 2008, 47, 8069−8076. (c) Fernández, E. J.; Laguna, A.; López-de-Luzuriaga, J. M.; Olmos, M. E.; Puelles, R. C. Vapochromism in Complexes of Stoichiometry [Au2Ag2R4L2]n. Z. Naturforsch. 2009, 64b, 1500−1512. (d) Laguna, A.; Lasanta, T.; López-de-Luzuriaga, J. M.; Olmos, M. E.; Monge, M.; Naumov, P. Combining Aurophilic Interactions and Halogen Bonding To Control the Luminescence from Bimetallic Gold−Silver Clusters. J. Am. Chem. Soc. 2010, 132, 456−457. (e) Lasanta, T.; Olmos, M. E.; Laguna, A.; López-deLuzuriaga, J. M.; Naumov, P. Making the Golden Connection: Reversible Mechanochemical and Vapochemical Switching of Luminescence from Bimetallic Gold−Silver Clusters Associated through Aurophilic Interactions. J. Am. Chem. Soc. 2011, 133, 16358−16361. (5) See for example: (a) Fernández, E. J.; López-de-Luzuriaga, J. M.; Monge, M.; Olmos, M. E.; Pérez, J.; Laguna, A. [Au2Tl2(C6Cl5)4]· (CH3)2CO: A Luminescent Loosely Bound Butterfly Cluster with a Tl(I)−Tl(I) Interaction. J. Am. Chem. Soc. 2002, 124, 5942−5943. (b) Fernández, E. J.; Laguna, A.; López-de-Luzuriaga, J. M.; Monge, M.; Montiel, M.; Olmos, M. E. Photophysical Studies and ExcitedState Structure of a Blue Phosphorescent Gold−Thallium Complex. Inorg. Chem. 2007, 46, 2953−2955. (c) Fernández, E. J.; Laguna, A.; Lasanta, T.; López-de-Luzuriaga, J. M.; Montiel, M.; Olmos, M. E. 1,2Dibromo- and 1,2-Diiodotetrafluorobenzene as Precursors of Anionic Homo- and Heterometallic Gold Complexes. Organometallics 2008, 27, 2971−2979. (d) Blake, A. J.; Donamaria, R.; Fernández, E. J.; Lasanta, T.; Lippolis, V.; López-de-Luzuriaga, J. M.; Manso, E.; Monge, M.; Olmos, M. E. Heterometallic gold(I)−thallium(I) compounds with crown thioethers. Dalton Trans. 2013, 42, 11559− 11570. (6) See for example: (a) López-de-Luzuriaga, J. M.; Monge, M.; Olmos, M. E.; Pascual, D.; Lasanta, T. Amalgamating at the molecular level. A study of the strong closed-shell Au(I)···Hg(II) interaction. Chem. Commun. 2011, 47, 6795−6797. (b) Lasanta, T.; López-deLuzuriaga, J. M.; Monge, M.; Olmos, M. E.; Pascual, D. Experimental and Theoretical Evidence of the Existence of Gold(I)···Mercury(II) Interactions in Solution through Fluorescence-Quenching Measurements. Chem. - Eur. J. 2013, 19, 4754−4766. (7) Fernández, E. J.; Laguna, A.; López-de-Luzuriaga, J. M.; Monge, M.; Nema, M.; Olmos, M. E.; Pérez, J.; Silvestru, C. Experimental and theoretical evidence of the first Au(I)···Bi(III) interaction. C. Chem. Commun. 2007, 571−573. (8) See for example: (a) Fernández, E. J.; Laguna, A.; López-deLuzuriaga, J. M.; Monge, M.; Montiel, M.; Olmos, M. E. Unsupported

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01342. Graph of conductivity in solution for 1 vs C1/2, details of data collection and structure refinement, TD-DFT singlet−singlet excitation calculations, and UV−vis solid-state spectra (PDF) Accession Codes

CCDC 1551915−1551918 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

M. Concepción Gimeno: 0000-0003-0553-0695 José M. López-de-Luzuriaga: 0000-0001-5767-8734 Miguel Monge: 0000-0002-9672-8279 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the D.G.I. MINECO/ FEDER (Project Numbers CTQ2016-75816-C2-1-P and CTQ2016-75816-C2-2-P (AEI/FEDER, UE)) and DGA-FSE (E77) for financial support. M.G.-M. also acknowledges MINECO for a FPI grant. We gratefully thank CESGA for computer support. I


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