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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Cooperative Au(I)···Au(I) Interactions and Hydrogen Bonding as Origin of a Luminescent Adeninate Hydrogel Formed by Ultrathin Molecular Nanowires Daniel Blasco, José M. López-de-Luzuriaga,* Miguel Monge, M. Elena Olmos, David Pascual, and María Rodríguez-Castillo Departamento de Química, Universidad de La Rioja, Centro de Investigación en Síntesis Química (CISQ), Complejo Científico Tecnológico, 26004 Logroño, Spain S Supporting Information *

ABSTRACT: Two water-soluble [Au(9N-adeninate)(PR3)] complexes (PR3 = PMe3 (1); PTA (3)) were synthesized by the coordination of the respective cationic [Au(PR3)]+ fragment to the 9N position of the adeninate anion. Both complexes crystallize as dimers by aurophilic contacts of 3.2081(6) Å in 1 and 3.0942(7) and 3.0969(7) Å in 3, but different packings are observed due to the crystallizing solvent choice and the nature of the ancillary phosphine ligand. At this regard, different supramolecular behavior is observed in water, ranges from the formation of ultrathin nanowires of 5.3 ± 1.9 nm of diameter and up to 1.5 μm in length and leads to a blue-luminescent hydrogel for 1, to the single-crystallization of 3. Parallel computational studies carried out show that aurophilicity and N−H···N or O−H···N hydrogen bonding are comparable in strength, suggesting a competition between all types of weak forces in the final observed macroscopic properties.

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H-bonding, with the extra possible formation of AuI···AuI bonds.9 The supramolecular gels formed by the self-assembly of low molecular weight gelator molecules (LMWGs) are arising as new and promising materials, since they are able to show smart and tunable response to stimuli of different nature.10−12 Among all well-known types of LMWGs, nucleobase-containing molecules are a very interesting class of building blocks for gelation, since nucleobases are naturally optimized for interacting with other molecules mainly through strong Hbonds (from 20 to 40 kJ mol−1) but also by π−π stacking interactions.13 Despite this type of molecules being expected to act as excellent hydrogelators, only guanosine-based compounds are the typical ones, and most nucleobase-containing gelators consist of derivatized nucleosides or nucleotides with long alkyl chains or other hydrophobic moieties. In this regard, very few reports have dealt with the use of nucleobase adenine as a LMWG.14,15 Thus, the first example described in the

INTRODUCTION The self-assembly of coordination or organometallic compounds through weak interactions such as π−π stacking, hydrophobic interactions, or hydrogen bonds gives rise to new types of multifunctional supramolecular materials of controlled structures and compositions.1 One of the metals that allows a better control of the molecular aggregation leading to homo- or heteropolymetallic species is gold. The large relativistic effects displayed by this metal in its +1 oxidation state is the driving force that permits the molecular oligomerization through the so-called aurophilic interactions, sometimes even in solution.2−4 In addition, this surprising behavior is not only circumscribed to AuI but is a general feature to all closed-shell late transition metal cations (such as AgI, CuI, HgII, PbII, etc.) and their combinations, although the stability provided by metallophilicity is maximum between gold(I) atoms.5−8 The strength of aurophilicity has been repeatedly estimated and compared with that of strong H-bonds.2−4 In fact, [AuL]+ units (where L is a neutral, two-electron donor ligand) are isolobal with H+ itself and should then be reasonably good candidates to give rise to supramolecular structures similar to those obtained by © XXXX American Chemical Society

Received: December 15, 2017

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DOI: 10.1021/acs.inorgchem.7b03131 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis of Complexes 1−3

approaches (complex 1 and 3) are carried out in organic solvents due to the insolubility of the chloro-gold(I) precursors in water. However, when both complexes are isolated, they now display solubility in water but behave in different ways. Thus, surprisingly, complex 1 forms a hydrogel, whereas complex 3 forms single crystals (see below). Regarding the analytical and spectroscopic data, the most interesting aspects are that the IR spectra of 1 and 3 reveal a broad band between 3249 and 2898 cm−1 (for 1) or between 3320 and 2930 cm−1 (for 3) due to 6 C-amino N−H stretching, the presence of which has been previously suggested as indicative that the corresponding [Au(PR3 )]+ moiety does not coordinate through that position.20 Moreover, the disappearance of the 9N−H stretching band at 2791−2690 cm−1 in both cases respect from free adenine suggests that gold(I) coordinates to that atom as in similar reported complexes.20−22 Adeninate proton resonances are observed in CDCl3 for 1 at 8.32 (2CH), 7.75 (8CH), and 5.33 (NH2) ppm and for 3 at 8.29 (2CH), 7.69 (8CH), and 5.37 (NH2) ppm, accompanied by those of the respective phosphine moiety at 1.70 (P(CH3)3) for 1 and the methylene groups of PTA at 4.56 (PCH2N) and 4.37 (NCH2N) ppm for 3. A sharp singlet in their respective 31 1 P{ H} NMR spectra at −11.47 ppm in 1 and −59.66 ppm in 3 shows that only one species is present in each sample. Finally, the ESI-MS spectrum of 1 displays the molecular peak [Au(adeninate)(PMe3)] + H+ (408.1 Da, 51%), whereas that of 3 is dominated by the [Au2(adeninate)(PTA)2]+ peak (842 Da, 100%). However, the molecular peak [Au(adeninate)(PTA)] + H+ (489 Da, 46%) and that of [Au(PTA)2]+ (511 Da, 6%) are also present what undoubtedly confirms the formation of the desired complex. X-ray Crystal Structure Determination. Suitable single crystals for the elucidation of the structure of 1 by X-ray diffraction were obtained by slow diffusion of n-hexane into a saturated dichloromethane solution of 1 at room temperature. Experimental details and selected bond lengths and angles are collected in Tables S1−S3. The complex crystallizes in the orthorhombic P212121 space group with eight formula units per unit cell. Thus, the asymmetric unit of 1 is integrated by two independent linear molecules formed by the coordination of the gold(I) atom to the P atom of trimethylphosphine and to the 9N atom of the adeninate ligand (see Figure 1). The Au−N bond lengths of 2.048(10) and 2.043(10) Å compare well with those observed in related (adeninate)gold(I) complexes prepared with different phosphines ([Au(9N-adeninate)(PEt3)],21 Au−N: 2.057(5); Au−P: 2.238(2) Å; [Au(9Nadeninate)(PPh3)],22 Au−N: 2.038(4); Au−P: 2.240(1) Å; [Au2(9N-adeninate)2(dppp)],20 Au−N: 2.040(4), 2.036(6); Au−P: 2.2347(11), 2.2361(10) Å). Also, the Au−P bond lengths (2.233(3) and 2.231(3) Å) are identical to those observed in the complexes cited above. The AuI···AuI distance of 3.2081(6) Å introduces aurophilicity as a new structural motif not found in the aforementioned complexes, with bulkier

literature displays the formation of adenine supramolecular hydrogels in the presence of 1,3,5-benzenetricarboxylic acid, with the hydrogen bonding between these two molecules being responsible for the hydrogel formation.14 Also, a recent report by Liang and co-workers shows that the adenine-containing nucleotide adenosine monophosphate (AMP) coordinated to Zn2+ forms an interesting type of supramolecular hydrogels displaying self-healing and encapsulating properties.15 In addition, metallogelators, which are coordination polymers or discrete metal complexes with gelating abilities, only represent a small portion of the whole of LMWGs.16 In this sense, only a few examples of metallogelators formed through aurophilic interactions have been reported.16,17 In these examples, the analysis of the metallogel morphology displays micro- or nanostructures, but as far as we are aware, all of them are above the ultrathin nanosize in their diameter and are not self-organized. Taking all the above comments into account, we wondered whether the modification of the nucleobase adenine through the coordination of small hydrosoluble [Au(PR3)]+ moieties (PR3 = PMe3; 1,3,5-triaza-7-phosphaadamantane, PTA) would favor the formation of hydrogels or other supramolecular structures through the cooperative self-assembly of the (adeninate)gold(I) complexes through both H-bonds and aurophilic interactions, which are similar in strength but different in nature. PTA phosphine and its derivatives are widely employed in coordination and organometallic chemistry to increase the solubility of metallic compounds in water, thus favoring further applications;18 and it is also a highly versatile ligand since it can be chemoselectively alkyl- or arylated at different positions.19 Therefore, herein we show for the first time that the cooperative and sequential assembly of (adeninate)gold(I) units through aurophilic and H-bonding interactions constitute an original approach for the supramolecular building-up in water solution into hydrogelating self-assembled ultrathin molecular nanowires or single crystals, depending on the nature of the phosphine ligand of simple amphiphilic systems [Au(9N-adeninate)(PR3)].

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RESULTS AND DISCUSSION Synthesis and Characterization. Complex [Au(9Nadeninate)(PMe3)] (1) was synthesized by a modified procedure from that previously reported.20 In addition, the title complex was in origin insufficiently characterized, and in this paper, we fully address the synthesis of the pure compound and its structural characterization. [AuCl(PTA)] reacts with a slight excess of Tl(acac) in dichloromethane, forming [Au(acac)(PTA)] (2), with which an equimolecular reaction with adenine in absolute ethanol affords [Au(9N-adeninate)(PTA)] (3) in an excellent yield (see Scheme 1). This two-step synthetic procedure can also be applied for an analogous synthesis of complex 1, but with a lower yield. Both synthetic B


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Inorganic Chemistry

described for [AuLX] (X = anionic ligand) complexes [Au(C 6 F 5 )(PTA)] and [Au(C 6 F 5 )(DAPTA)]·0.5H 2 O (DAPTA = diacetyl PTA).24a Single crystals of 3·3H2O were obtained by cooling a supersaturated hot aqueous solution of 3 to room temperature (see Tables S1, S4, and S5 for further experimental details and bond lengths and angles) The complex crystallizes in the triclinic P1̅ space group with four independent molecules of 3 and 12 molecules of water (half of them disordered over two different positions) in the asymmetric unit. As in the case of 1, [Au(9N-adeninate)(PTA)] crystallizes as dimers through aurophilic interactions (Figure 3) with the P−Au−N units

Figure 1. X-ray structure of complex 1 with the labeling scheme for the atom positions. Selected bond lengths and angles (Å, °): Au(1)− Au(2) 3.2081(6), Au(1)−N(1) 2.048(10), Au(1)−P(1) 2.233(3), Au(2)−N(6) 2.043(10), Au(2)−P(2) 2.231(3), N(1)−Au(1)−P(1) 175.3(3), N(6)−Au(2)−P(2) 173.4(3).

phosphines. Both molecules show the usual linear environment of gold(I) centers but slightly distorted (as demonstrated by the deviation of the P−Au−N angles from 180°) in the direction of approximating the gold(I) centers, as a possible consequence of aurophilicity. The structural arrangement of complex 1 in its crystalline packing consists of the stacking of parallel folded sheets at distances ranging from 8.58 to 9.11 Å, formed by the combination of the previously described aurophilic contacts, which are nearly parallel to the crystallographic a axis, with N− H···N hydrogen bonds mediated by adenine (see Figure 2). Figure 3. X-ray structure of 3·3H2O with the labeling scheme for the atom positions. Water molecules have been omitted for clarity. Selected bond lengths and angles (Å, °): Au(1)−Au(2) 3.0942(7), Au(3)−Au(4) 3.0969(7), Au(1)−P(1) 2.224(3), Au(2)−P(2) 2.225(3), Au(3)−P(3) 2.227(3), Au(4)−P(4) 2.225(3), Au(1)− N(13) 2.050(9), Au(2)−N(18) 2.049(10), Au(3)−N(23) 2.049(10), Au(4)−N(28) 2.055(10), N(13)−Au(1)−P(1) 173.5(3), N(18)− Au(2)−P(2) 176.6(3), N(23)−Au(3)−P(3) 177.6(3), N(28)− Au(4)−P(4) 172.8(3).

not so close to an orthogonal disposition (P1−Au1−Au2−N18 66.2(3), P3−Au3−Au4−N28 67.1(3)°). The AuI···AuI distances of 3.0942(7) and 3.0969(7) Å fit well with that observed in [Au(3,5-Ph2pz)(PTA)]24b (AuI···AuI: 3.0962(6) Å), but they are considerably shorter than in 1 in spite of the presence of the bulkier phosphine PTA. The Au−P bond lengths, between 2.224(3) and 2.227(3) Å, are similar to those observed in other [Au(N-donor ligand)(PTA)] reported complexes ([Au(PTA)(saccharinate)],24c Au−P: 2.215(3) Å; [Au(pbi)(PTA)],24d Au−P: 2.205(4) Å; [Au(3,5-Ph 2 pz)(PTA)], 24b Au−P: 2.227(2), 2.213(2) Å). The Au−N bond distances (2.049(10)−2.055(10) Å) are identical to those found in 1 and compare well with those described for the related (adeninate)phosphinegold(I) derivatives containing PEt3 (2.057(5) Å),21 PPh3 (2.038(4) Å),22 or dppp (2.040(4) and 2.036(6) Å)20 as neutral ligand. As shown in Figure 4, these dimers are associated in couples through N−H···N hydrogen bonds (H···N 2.16 Å, N···N 3.017(15) Å, N−H···N 178.5°; H···N 2.29 Å, N···N 3.087(15) Å, N−H···N 154.6°). Moreover, these aggregates are further connected through additional hydrogen bonds, in which both

Figure 2. 2D arrangement of 1 through aurophilic contacts and hydrogen bonding. Color code: C, gray; H, light gray; Au, yellow; N, blue; P, orange.

Since adenine possesses two faces of donor and acceptor atoms, a highly rigid and ordered ribbon-like pattern is formed.13a Moreover, the existence of aurophilic interactions in the structure, possibly favored by the small cone angle of 118° of PMe3,23 permits the expansion of the structure in two directions, what gives rise to the folded sheet motif (see Figure S13). The distance between consecutive Au(I)···Au(I) dimers in the fold line of 4.91 Å is long enough to ensure the nonexistence of extended aurophilic interactions between them. A similar folded sheet-like packing has been very recently C


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Inorganic Chemistry

Figure 4. Arrangement of 3 into couples of dimers through aurophilic contacts and N−H···N hydrogen bonds. Color code: C, gray; H, light gray; Au, yellow; N, blue; P, orange.

Scheme 2. Theoretical Model Systems 1a, 1b, and 3a

Figure 5. Interaction energy (BSSE considered with counterpoise correction) of models 1a and 1b calculated at different (a) AuI···AuI, (b) N−H···N distances, at RHF (black) and MP2 (blue) levels of theory.

with a similar strength for both types of interactions and with the possibility of a cooperative assembly leading to the supramolecular arrangements found in the solid state for complexes 1 and 3. In view of the different structural arrangements found for complexes 1 and 3 and, specially, the different tendency displayed by both complexes toward gelation (see below), we also studied the possible influence of the H-bonds formed between H2O molecules and N atoms of the PTA ligand in complex 3 (see Scheme 2 and Figure S18). These additional interactions, among many other H-bonds with H2O molecules and complex 3, seem to be responsible for the very complex 3D arrangement found for this compound preventing it to adopt an analogous structure to that of 1. Indeed, as it is shown in Table 1, the presence of H-bonds between H2O molecules and the N

the molecules of 3 and the crystallization water are involved, giving rise, unexpectedly in this case, to a three-dimensional network (see Figure S14). Computational Studies. A computational analysis of the aurophilic and H-bonding interactions responsible for the supramolecular arrangements found in 1 and, partially, in 3 has been carried out at RHF and MP2 levels of theory on two simplified dinuclear [Au(9N-adeninate)(PH3)]2 model systems, representing the aurophilic interaction (model 1a) and the N− H···N H-bonding interactions between adeninate units (model 1b) (see Scheme 2 and Figures 5 and S15−S17). The results show that each unsupported AuI···AuI interaction accounts for an additional stabilization of ca. −31.3 kJ·mol−1, whereas each N−H···N interaction leads to an extra stabilization of ca. −26 kJ·mol−1. As we have commented above, these results agree D


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Inorganic Chemistry Table 1. Interaction Energies and Equilibrium Distances for Models 1a, 1b, and 3a at MP2 Level of Theory

a

model

Re(AuI···AuI) (Å)

1a 1b 3a

3.39

Re(N−H···N) (Å)

V(Re) (kJ mol−1)

Re(O−H···N) (Å)

2.14 3.09b

O−H···5N O−H···PTAN O−H···3N

1.73 1.79 1.78

−31.35 −52.35a −38.80 −22.92 −46.94

Values for two hydrogen bonds. boptimized at DFT−D3 level.

Figure 6. Extinction coefficient (ε) and maximum wavelength values of 1 (left) and 3 (right) superimposed with adenine, in water.

Figure 7. Normalized excitation (dashed line) and emission (solid line) spectra of complexes 1 (left) and 3 (right) at RT (black) and 77 K (blue) in solid state.

Luminescent Properties in Water Solution. In view of the important stabilization provided by the aurophilic and the H-bonding interactions, the complexity of the 3D network in the crystalline form of complex 1, and the existence of aurophilicity in complex 3 despite being surrounded by water molecules, we envisaged the possibility of the existence of these interactions in solution. In addition, taking advantage of the amphiphilic character of both complexes, we decided to study the interactions in water as solvent. Thus, to gain insight into the self-assembly in water solution we carried out a combined study of the luminescence in water and 1H pulsed-gradient spin−echo (PGSE) NMR experiments for both complexes in D2O as solvent, at different concentrations. These experiments allow the study of the possible aggregation in solution through aurophilic and/or H-bonding interactions.25 Figure 8 depicts a summary of the information obtained in the luminescence studies for 1 (see Figure S19 for complete information). First, we have observed concentration dependence of both the excitation and emission spectra in water, which have been measured in the 0.25−15.0 mM range. In this range, we observe a red-shift of the excitation (293−325 nm) and emission (415−440 nm) profiles between 0.25 and 5.0 mM

atoms of the PTA ligand provides an additional stabilization of ca. −23 kJ·mol−1, which is comparable with the O−H···N bonds found between adeninate ligands and slightly lower than the one obtained through aurophilic interactions. Optical Properties. Absorption spectra of free adenine and complexes 1 and 3 were recorded in water at concentration 5 × 10−5 M, and are depicted in Figure 6. The absorption spectra of 1 and 3 display very similar profiles to that of free adenine, suggesting that only ligand-centered transitions are responsible for the UV absorption. In this sense, Tiekink et al. also showed that the UV−vis spectrum of [Au(9N-adeninate)(PEt3)] was essentially the sum of the absorption of the adenine and phosphinegold moieties, while the latter does not severely perturb the electronic structure of the π-delocalized system of adenine.21 Referring to the emissive properties in the solid state, complex 1 displays blue luminescence with emissions at 441 nm (exc. 338 nm) at RT and at 432 nm (exc. 311 nm) at 77 K, while complex 3 is only luminescent at 77 K, with a sharp emission at 420 nm with a shoulder at 490 nm (exc. 356 nm). The lifetime of the emission at RT of 1 is 10.3 ns, what suggests a fluorescent process from a singlet excited state (Figure 7). E


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Inorganic Chemistry

Pulsed-Gradient Spin−Echo Experiments. Pulsed-gradient spin−echo NMR allows the calculation of the diffusion coefficient (Dt), and it has shown to be a powerful tool to determine molecular sizes of organometallic complexes in solution.25 Figure 9 displays the summary of the PGSE results of 1.

Figure 8. Concentration-dependent excitation spectra for complex [Au(9N-adeninate)(PMe3)] (1) in water (top). Evolution of the excitation (black) and emission (red) wavelength for 1 at different concentrations, in the 0.25−15.0 mM range (bottom). The red area (0.25−5.0 mM) represents the dimerization of 1 through aurophilic interactions; the blue area represents that dimers of 1 are kept in solution at higher concentrations (5.0−15 mM).

concentrations, whereas from 5.0 to 15.0 mM both excitation (325−335 nm) and emission (440−435 nm) profiles remain almost constant. Considering previously reported results2c and the obtained data we propose that this red-shift in a low concentration range would arise from the formation of oligomers in solution through aurophilic interactions. Indeed, DFT and TD-DFT calculations support this assumption. Thus, we have computed both the electronic structure and the absorption spectra of model systems of different nuclearities of complex 1 in water (PCM-model). Among [Au(9N-adeninate)(PMe3)]n models with n = 1−3, the dimeric model (n = 2, model 1d) provides a very good computed simulation of the behavior found experimentally, with a predicted excitation maximum at 330 nm, in good agreement with the experimental results in the 5.0−15.0 mM range. Also, the molecular orbitals involved in the HOMO−LUMO singlet−singlet transition for the dimeric model system agree with a metal-perturbed intraligand transition 1(IL) that takes place when the aurophilic interaction is held (see the Supporting Information). Therefore, we propose that at low concentrations dimeric [Au(9Nadeninate)(PMe3)]2 units are formed through aurophilic interactions. Importantly, at concentrations higher than 5.0 mM the luminescence has a similar value and, therefore, we propose a similar origin, what agrees with the fact that dimeric units remain at higher concentrations (see Figure 4 and the Supporting Information). When this experiment is replicated with complex 3, no luminescence response is recorded even at concentrations that conducted to precipitation, and the solutions turned cloudy when aged.

Figure 9. PGSE-1H NMR spectra for complex [Au(9N-adeninate)(PMe3)] (1) in water at 25 and 100 mM concentrations (top). Graphical representation (bottom) of the decrease of Dt with increasing concentration in the 5.0−100 mM range (black) and estimation of the number of molecules at each concentration using the rH and VH calculated data (red).

In this case, the dependence of Dt with respect to the concentration provides interesting information. As it can be observed, a clear decrease in Dt is obtained upon increasing its concentration in water in the 5.0−100.0 mM range, what confirms the aggregation of [Au(9N-adeninate)(PMe3)] units in water. Also, using the Stokes−Einstein equation we can evaluate the molecular size of the aggregates through their hydrodynamic radii (rH). Indeed, the estimated hydrodynamic volume of the aggregates (VH) can be compared with the molecular volume of the complexes in the solid state from the X-ray diffraction data (VX‑ray), which is obtained dividing the volume of the crystallographic cell by the number of molecules of each compound that are present. The results are depicted in Figure 9 (bottom) showing that upon increasing the concentration in the 5.0−100 mM range the size of the molecular aggregates increase from approximately 2−11 molecules. If we focus on the experimental data of the 5.0 mM sample, then we obtained a Dt of 4.92 × 10−10 ms−1, which corresponds, applying the Stokes−Einstein equation, to a rH of 5.26 Å and a VH of 609.5 Å3. If we compare the obtained value of VH with the one corresponding to VX‑ray of 315.64 Å3, then describing a F


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Inorganic Chemistry

Figure 10. (A) Image of the hydrogel under ambient light (left) and UV light of 365 nm (right). (B) Emission spectra (excitation at 365 nm) of the hydrogel of 1 at different temperatures. (C) Emission spectra (excitation at 336 nm) of a film of the hydrogel of 1 exposed to HCl vapors. The emission spectrum was recorded in 2 min intervals approximately. The inset in (C) shows the effect of HCl vapors on the texture of the gel, forming a white solid. (D) Recovery of the luminescent emission of the hydrogel of 1 after exposition to NH3 vapors. Note that the high volatility of ammonia precluded the obtaining of intermediate data. The inset in (D) shows the final aspect of the former acidified hydrogel after being exposed to NH3 vapors showing the regeneration of the hydrogel texture.

[Au(9N-adeninate)(PMe3)] molecule in the unit cell, we obtain a VH/VX‑ray ratio of 1.93 that approximately corresponds to the existence of dimeric [Au(9N-adeninate)(PMe3)]2 units at 5.0 mM concentration, as we described in the concentration dependence of the luminescence in solution. Since the luminescent emission energy remains constant at concentrations higher than 5.0 mM and the PGSE measurements display a clear decrease of Dt up to 100.0 mM concentration related to an aggregation process, we propose that this dimeric [Au(9N-adeninate)(PMe3)]2 units polymerize in water solution through N−H···N H-bonds in a similar way to what it is observed in the solid state structure, giving rise to supramolecular entities from the association of dimers. Therefore, a sequential self-assembly of 1 starting from aurophilic interactions at low concentrations followed by H-bonding interactions at higher concentrations is responsible for the supramolecular arrangement found in water. At higher concentrations, the 1H signals appear broadened, probably due to an initial stage of hydrogelation, what precluded the estimation of Dt at concentrations close to the gelation of the complex but confirm the possibility of aggregation at lower concentrations. Similar conclusions can be extracted from the analysis of the PGSE results of 3 (see Figure S26 and Table S8), although its lower solubility in D2O respect to 1 prevents the obtaining of data above 25.0 mM. In this case, an increasing from approximately 1 to 3 molecules is observed, showing a great concordance with the possible formation of dimers, and no bigger aggregates, by aurophilicity, as observed in the crystalline

structure obtained in water. Therefore, we propose that a correlation between the crystalline structure and the oligomerization in water can be established, supported by these two examples. Hydrogelation Properties. Unexpectedly, at unusually large concentrations (ca. 10% wt of 1 in water, 250 mM) complex [Au(9N-adeninate)(PMe3)] behaves as a metallohydrogelator, leading to a transparent and slightly yellow stable hydrogel, which displays blue luminescence at similar energies to that of its crystalline form (Figure 10a). Since gels are known to be responsive to microenvironmental changes as its structure relies in weak and thus reversible interactions, we have decided to study the effect of temperature and pH variations in the luminescence of the hydrogel. With respect to the effect of temperature, no shifting of the excitation or emission maxima is observed upon temperature increasing, but a continuous decrease of its emission intensity occurs as a natural result of thermal deactivation of the excited states (Figure 10b). The commonly observed gel-to-sol thermoreversible transition is not present in this material, even at temperatures that lead to the evaporation of the solvent. In contrast, the response to pH variations of the material was evaluated by recording its luminescent emission in two different approaches. The first experiment consisted in the titration of a sample of the hydrogel with small aliquots of diluted 1 M HCl and NaOH solutions (see the Supporting Information for further experimental details). In summary, a pH-dependent reversible protonation−deprotonation sequence in the adenine moiety is observed, what leads to the subsequent formation of G


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Inorganic Chemistry Scheme 3. Proposed Reversible pH Dependence of the Hydrogel of 1

Figure 11. Cryo-STEM (A) and (B) images, TEM (C) image and proposed morphology of the UNWs of [Au(9N-adeninate)(PMe3)] (1) proposed morphology of the UNWs metallohydrogel based on its X-ray crystal structure (D) and (E).

[AuCl(PMe3)] (as unequivocally confirmed by 1H and 31P{1H} NMR spectroscopy, see the Supporting Information) and the loss of luminescence in acidic conditions. Surprisingly, the addition of aqueous NaOH to the mixture led to the recovery of both luminescence and gel texture since as we propose the introduction of a base regenerates the adeninate anion, which can react with [AuCl(PMe3)] in the gel matrix. These results are summarized in Scheme 3. In view of the results of this first experiment, we decided to study the effect of HCl and NH3 vapors in the luminescent response of the material, speculating with the possibility of the hydrogel to act as an acid−base vapors sensor. Therefore, we developed an experimental setup analogous to that devised by Che for the proton sensing of a solid PtII complex,26 consisting in our case in a drop-casted hydrogel sample disposed in a quartz plate subjected to the acidic or basic atmosphere generated inside a sealed quartz cell. The results are summarized in Figures 10c,d. As expected, HCl vapors react with [Au(9N-adeninate)(PMe3)], reverting it to free adenine and nonluminescent [AuCl(PMe3)] in accordance with the first experiment, as demonstrated by the luminescence quenching. Again, NH3 vapors, as aqueous NaOH before, can switch on the luminescence of the hydrogel by regenerating adeninate anion, the reaction of which with [AuCl(PMe3)] reaffords complex 1. This acid-to-basic cycle was also essayed qualitatively exposing a hydrogel drop to concentrated HCl

and NH3 vapors, and we could repeat it up to 10 times without apparent gel degradation. Also, upon lyophilization of the hydrogel a stable xerogel was formed. The analysis of the xerogel through TEM showed the formation of agglomerated fibrous-like nanostructures, what prompted us to analyze in detail this type of molecular nanostructure. In this regard, the use of the cryo-STEM technique provides very interesting and definitive results for a deep analysis of the morphology of the self-assembled hydrogel. Figure 11 depicts the cryo-STEM results of diluted 1:4 (hydrogel/water) hydrogel form of 1. To our delight, the molecular complex [Au(9N-adeninate)(PMe3)] self-assembles into one-dimensional ultrathin nanowires (UNWs) of 5.3 ± 1.9 nm of diameter and up to 1.5 μm length. In addition, the very good control over the aspect ratio of the UNWs gives rise to its self-assembly into parallel arrays. The intercalation of water between the UNWs would be responsible for the stabilization of the hydrogel leading to an unprecedented organization from the molecular to the mesoscopic level. Figure 11 shows cryo-STEM images, a TEM image, and a representation of the nanowires composition taking into account diameters and X-ray diffraction parameters. The mean diameter of ca. 5 nm fits well with four [Au(9Nadeninate)(PMe3)]2 dimeric units displaying the aurophilic interactions perpendicular to the nanowire growing axis and the H


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Inorganic Chemistry

previous study by some of us, in which Au UNPs where synthesized by a solvent free thermolysis of a pentafluorophenylgold(I) carbene complex.29 The results show that Au UNPs of ca. 1.4 ± 0.7 nm (Figure 13c,d) can be obtained deposited at the hydrogel bundles following the anisotropic pattern imposed by the molecular UNWs. In this sense, this material has proven its usefulness in the synthesis of self-assembled gold nanomaterials, what opens a very promising area of research in the use of soft metal-based materials as precursors for the synthesis of ordered gold-based nanostructures.

hydrogen bonding interactions between adeninate units parallel to it. In order to gain insight into the molecular nature of the [Au(9N-adeninate)(PMe3)] UNWs, we used X-ray photoelectron spectroscopy (XPS) to probe the oxidation state of gold and the composition of the hydrogel. A summary of the results is depicted in Figure 12 and in the Supporting Information.

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CONCLUSIONS We have demonstrated in this paper that the number, strength, and nature of the weak intermolecular and molecule−solvent interactions play a critical role in the character of supramolecular gold(I) materials built up from discrete molecules. In this sense, we have reported a new bottom-up approach for the building up of a gold(I)-based luminescent hydrogel from a low molecular weight gelator molecule (LMWG) such as the simple [Au(9N-adeninate)(PMe3)] complex. This soft material consists of unprecedented ordered arrays of ultrathin molecular nanowires (UNWs). The bottom-up strategy has consisted of the delicate control of weak intermolecular interactions such as aurophilic and hydrogen bonding. The sequential formation of molecular dimers through aurophilic interactions first and further assembly of these dimers through hydrogen bonding has allowed the formation of the anisotropic UNWs. The packing of these UNWs into bundles gives rise to the formation of a very stable blue emissive hydrogel, which gives a reversible visual response to the presence of acid or bases both in liquid media or in the gas phase. Similarly, the hydrogel can act as a molecular template and metal precursor for the synthesis of ultrasmall Au NPs, deposited at the hydrogel bundles following its anisotropic pattern. In contrast, the presence of a hydrophilic phosphine such as PTA in complex 3, in which N atoms are present, may preclude the formation of anisotropic assemblies as in the case of complex 1 leading instead to complex 3D crystalline networks. Future ligand variations in the complexes will provide the possibility of forming hydrogels controlling the size, shape and composition of these type of nanostructures and therefore the tuning of their photophysical properties and their response to different types of stimuli. Their use as precursors for the synthesis of Au-based nanomaterials is also envisaged.

Figure 12. XPS Au 4f core level spectra of a lyophilized sample of [Au(9N-adeninate)(PMe3)] UNWs (1).

The experimentally obtained spectrum displays some interesting features. First, the analysis of the high-resolution scan of the Au 4f peak reveals the presence of a main doublet signal Au 4f7/2 at 85.2 eV corresponding the +1 oxidation state for gold. A progressive increase of a minor signal at 83.5 eV is attributed to the 4f7/2 peak assigned to Au(0), which arises from the X-ray induced reduction of the main Au(I) species. In addition, the P 2p spectrum shows two P 2p3/2 peaks at 132.5 and 131.6 eV that can be assigned to PMe3 ligands.27 Also, the N 1s core level displays two peaks that would correspond to nitrogen atoms in the adeninate ligands with more than one bonding configuration, i.e., the amine and imino nitrogen groups.28 Finally, the C 1s core level peak consists of two components at 284.9 and 286.2 eV, which could be assigned to the different carbon atoms in the adeninate and PMe3 ligands. Synthesis of Au Ultrasmall Nanoparticles (Au UNPs). During the cryo-STEM experiments of the hydrogel of [Au(9Nadeninate)(PMe3)] (1), we also tested the stability of the molecular UNWs. Thus, long-time exposures to the electron scanning probe leads to the formation of Au UNPs of ca. 1.6 ± 0.9 nm (Figures 13a,b). We could observe that the UNWs acted as a size and directionality template, since the formed Au UNPs follow the pattern imposed by the UNWs, what can be seen in Figure 13a. That suggested to us the possible viability of this material to act as both molecular precursor and template for synthetic approaches leading to self-organized Au UNPs following the anisotropic shape of the molecular UNWs precursor. Therefore, we carried out a test by the direct thermolysis of the hydrogel of 1 at 150 °C for 35 min, following a similar strategy of a

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EXPERIMENTAL SECTION

General Procedures. Na(adeninate),30 [AuCl(PMe3)],31 [AuCl(PTA)],32 and Tl(acac)33 were prepared according to previously reported methods. Adenine (6-aminopurine, Sigma-Aldrich) was used as received. Distilled water was saturated with Ar for 10 min prior to photophysical measurements in solution. Physical Measurements. 1H (δ (SiMe4) = 0.0 ppm) and 31P{1H} (δ (85% H3PO4) = 0.0 ppm) NMR spectra were recorded on a Bruker ARX 300 spectrometer. UATR-IR spectra were recorded in the range 4000−400 cm−1 on a PerkinElmer Two spectrophotometer equipped with a diamond crystal-UATR accessory. ESI-MS spectra were obtained with a Bruker MicroTOF-Q spectrometer with ESI ionization source. CHNS elemental analysis were carried out with a PerkinElmer 240C microanalyser. UV−vis absorption measurements were registered with a Hewlett-Packard 8453 Diode Array spectrophotometer in quartz cells (optical path = 1 cm). Steady-state solid, solution, and gel luminescence measurements were carried out on a Jobin-Yvon Horiba Fluorolog 3−22 Tau3 spectrofluorimeter. Single-photon counting I


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Inorganic Chemistry

Figure 13. Cryo-STEM (A) and TEM (B) images of Au UNPs formed after long-time exposure of [Au(9N-adeninate)(PMe3)] UNWs (1) to the electron scanning probe. TEM images (C and D) of Au UNPs formed after direct thermolysis of the [Au(9N-adeninate)(PMe3)] UNWs (1). spectra were obtained at room temperature. The binding energy (BE) scale was internally referenced to the C 1s peak (BE for C−C = 284.9 eV). Crystallography. Suitable single crystals were mounted in inert oil on a glass fiber and transferred to the cold nitrogen stream of a Nonius Kappa CCD area-detector diffractometer equipped with an Oxford Instruments low-temperature controller system (Mo Kα = 0.71073 Å, graphite monochromator). Data was collected in ω and φ scan mode. Absorption correction was based on multiple scans: the program Scalepack was applied.35 The structure was solved by using direct methods36 and refined on F20 with SHELXL.37 All non-hydrogen atoms were treated anisotropically, and all hydrogen atoms were included as riding bodies. Computational Details. Interaction energy calculations were performed with the Gaussian 09 package program.38 Model system [Au(9N-adeninate)(PH3)] was built up from the X-ray structure of [Au(9N-adeninate)(PMe3)] by substituting PMe3 with PH3 to keep computational cost feasible and initially optimized at the DFT level of theory with the PBE1PBE functional.39 Model system [Au(9Nadeninate)(PH3)]2 (1a) describing aurophilicity was prepared from two separate fragments, disposing them perfectly orthogonal (P−Au··· Au−P dihedral angle of 90.314°) and in a similar fashion to that found in the X-ray aurophilic dimers. The interaction energy at the RHF and MP2 levels of theory was obtained according to eq 1 at different values of distance R, defined as the distance between Au(I)···Au(I) atoms. A counterpoise (cp) correction for the basis set superposition error (BSSE)40 on ΔE was thereby performed.

lifetime measurements were recorded with a Datastation HUB-B using a 320 nm nanoLED (0.8−1.4 ns pulse length) and the DAS6 software. 1 H PGSE-NMR measurements were carried out with the doublestimulated echo-pulsed sequence (Double STE)34 on a Bruker AVANCE 400 equipped with a BBI H-BB Z-GRD probe at 298 K without spinning at different concentrations in D2O. Microscopy. Samples for transmission electron microscopy (TEM) were directly drop-casted from a diluted hydrogel sample (2−3 drops) to carbon-coated Cu grids. TEM images were obtained using a JEOL JEM 2100. For a direct observation of liquid samples in its original state, specimens were vitrified in liquid ethane and analyzed in a STEM microscope at low temperature. The vitrification method is a very fast sample cooling that prevents the formation of crystalline ice. Moreover, the thin layer of amorphous ice formed during the vitrification process protects the material from electron beam damage. The vitrification process was performed in an FEI Vitrobot: A 3 μL drop of an aqueous suspension of the material was placed on a TEM quantifoil carbon grid, the excess of water was blotted away at the Vitrobot with filter paper, and finally the grid was freeze-plunged in liquid ethane. Samples were then transferred under liquid nitrogen atmosphere to a Gatan TEM cryo-holder equipped with a liquid nitrogen reservoir. That way samples were handled and observed at T = 100 K. STEM images were obtained in a Tecnai F30 (FEI) operated at 300 kV, coupled with a high-angle annular dark field (HAADF) detector. X-ray Photoelectron Spectroscopy (XPS). XPS experiments were performed in a Kratos AXIS Supra spectrometer, using a monochromatized Al Kα source (1486.6 eV) operating at 12 kV and 10 mA. Wide scans were acquired at analyzer pass energy of 160 eV, whereas high-resolution narrow scans were performed at constant pass energy of 20 eV and steps of 0.1 eV. The photoelectrons were detected at a takeoff angle of Φ = 0° with respect to the surface normal. Basal pressure in the analysis chamber was less than 5 × 10−9 Torr. The

(AB) ΔE = EAB − EA(AB) − E B(AB) = V (R )

(1)

Calculation points were fitted using the four-parameter (A, B, C, n) eq 2, which had been previously used to derive the Herschbach− Laurie relation.41 J


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Inorganic Chemistry ΔE = V (R ) = A exp(− BR ) − CR−n

concentrated in vacuo. Addition of 20 mL of n-hexane gives rise to the precipitation of [Au(9N-adeninate)(PTA)] (0.0908 g, 0.19 mmol) as a white solid. Yield: 85%. 1H NMR (300 MHz, CDCl3): 8.29 (1H, s, 2 CH), 7.69 (1H, s, 8CH), 5.37 (2H, s, NH2), 4.56 (6H, m, PCH2N), 4.37 (6H, s, NCH2N). 31P{1H} NMR (121 MHz, CDCl3): −60.02 (s, PTA). UATR-IR (cm−1): 3320 (NH2), 3130 (NH2), 2936 (NH2), 578 (PC). ESI-MS(+) m/z: 489.1 ([Au(9N-adeninate)(PTA)] + H+, calcd: 488.1), 511.1 ([Au(PTA)2]+, calcd: 511.2), 842.1 ([Au2(9Nadeninate)(PTA)2]+, calcd: 842.1). Anal. Calcd for C11H16AuN8P· 4H2O: C, 23.58; H 4.32; N, 20.00. Found: C, 23.77; H, 4.25; N, 19.96.

(2)

An analogous study was carried out on a second model (1b) fully optimized at DFT/PBE1PBE level, where [Au(9N-adeninate)(PH3)]2 fragments interacted by N−H···N hydrogen bonding, with distance R being that between two ghost atoms disposed in the parallel line to the hydrogen bonds that dissects the minimum distance between them. Model 3a was built up from the X-ray structure of 3·3H2O keeping in the dimer structure the three most strongly bounded water molecules and optimizing its structure completely at the DFT/ PBE1PBE level of theory with the third dispersion correction by Grimme (DFT-D3).42 Single-point cp corrections were carried out to estimate the interaction energy of each water molecule with the rest of the model. The following basis set combinations were employed: For gold, the quasi-relativistical (QR) 19-valence electrons (VE) pseudopotential (PP) from Andrae43 and the corresponding basis sets augmented with two f polarization functions were used.44 Carbon, nitrogen, and phosphorus were treated by Stuttgart pseudopotentials,45 including only the VE for each atom. For these atoms, the double-ζ basis set were used,46 augmented by d-type polarization functions.47 For hydrogen, a double-ζ plus a p-type polarization function was used.48 Optimization and TD-DFT calculations on oligomerization models 1c−1e were performed with TURBOMOLE v.6.4,49 with the PBE functional38 and the third dispersion correction by Grimme (DFTD3).42 The following basis set combinations were employed: for gold, the def2-TZVP50,51 with its correspondent effective core potential, whereas for carbon, nitrogen, and phosphorus, SVP basis sets52 were used. Excitation energies were obtained at the DFT level of theory through time-dependent RPA approximation,53−56 and implicit treatment of solvent (water, ε = 78.3553) by COSMO continuous solvation model.57 Synthesis and Characterization. Synthesis of [Au( 9 Nadeninate)(PMe3)] (1). Solid Na(adeninate) (0.1629 g, 1.04 mmol) is added to a solution of [AuCl(PMe3)] (0.3184 g, 1.03 mmol) in 30 mL of tetrahydrofuran. The mixture is heated to reflux for 2.5 h, and then the solvent is removed in vacuo. The obtained residue is treated with 30 mL of absolute ethanol for 0.5 h and then filtered over Celite to remove unreacted [AuCl(PMe3)] and formed NaCl. The transparent filtrate is reduced in vacuo, and the addition of 20 mL of n-hexane leads to the precipitation of [Au(9N-adeninate)(PMe3)] (0.3707 g, 0.91 mmol) as a white solid. Yield: 88%. An alternative synthetic route of 1 by the equimolecular reaction of [Au(acac)(PMe3)] (prepared in an analogous manner as complex 2) with adenine in absolute ethanol was also tested, but lower yields were achieved. 1H NMR (300 MHz, CDCl3): 8.32 (1H, s, 2CH), 7.75 (1H, s, 8CH), 5.33 (2H, s, NH2), 1.72−1.68 (9H, d, 2JPH = 12 Hz, P(CH3)3). 31P{1H} NMR (121 MHz, CDCl3): −11.47 (s, P(CH3)3). UATR-IR (cm−1): 3249 (NH2), 3076 (NH2), 2898(NH2), 964 (PC). ESI-MS(+) m/z: 408.1 ([M + H]+, calcd: 408.1). Anal. Calcd for C8H13AuN5P: C, 23.60; H, 3.22; N, 17.20. Found: C, 23.14; H, 3.03; N, 17.52. Synthesis of [Au(acac)(PTA)] (2). A suspension of [AuCl(PTA)] (0.2523 g, 0.65 mmol) in 10 mL of dichloromethane is prepared, and then a slight excess of Tl(acac) (0.2372 g, 0.78 mmol) and 10 mL of dichloromethane are added. The mixture is then let to react for 6 h protected for direct light. The TlCl formed is filtered over Celite, and the obtained yellow filtrate is concentrated in vacuo. Addition of 20 mL of n-hexane gives rise to the precipitation of [Au(acac)(PTA)] (0.2629 g, 0.58 mmol) as a yellowish solid. Yield: 90%. 1H NMR (300 MHz, d6-acetone): 4.58 (6H, AB q, PCH2N), 4.41 (6H, s, NCH2N), 4.10 (1H, s, CH). 31P{1H} NMR (121 MHz, d6-acetone): −47.18 (s, PTA). UATR-IR (cm−1): 1638 (CO). ESI-MS(+) m/z: 157.0 ([PTA]+, calcd: 157.1), 174.1 ([PTAO] + H+, calcd: 174.1). Calcd for C11H19AuN3O2P·0.5 CH2Cl2: C, 27.86; H, 4.07; N, 8.48. Found: C, 27.43; H, 3.91; N, 8.72. Synthesis of [Au(9N-adeninate)(PTA)] (3). Solid adenine (0.0297 g, 0.22 mmol) is added to a solution of [Au(acac)(PTA)] (0.1004 g, 0.22 mmol) in 30 mL of absolute ethanol, and the mixture is let to react for 6 h. The suspension is filtered over Celite, and the uncolored filtrate is

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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.7b03131. IR spectra, NMR results, ESI-MS spectra, structure data, models for 1 and 3, excitation and emission spectra, TDDFT data, HOMO/LUMO orbitals, plots of 1H ln(I/I0) (2CH signal) vs arbitrary units, PSGE-NMR experimental results, effect of pH on luminescence, emission spectra, diameter histograms, XPS data, xyz data (PDF) Accession Codes

CCDC 1563745 and 1586887 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

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 We gratefully acknowledge the D.G.I. MINECO/FEDER (project number CTQ2016-75816-C2-2-P (AEI/FEDER, UE)) for financial support. D. Blasco also acknowledges MECD for a FPU grant. We thank the Laboratorio de Microscopias Avanzadas, Instituto de Nanociencia de Aragón (LMA-INA) and the SERMET-Universidad de Cantabria for microscopy facilities. The computational work was carried out in the Beronia cluster (Universidad de La Rioja), which is supported by FEDER-MINECO grant number UNLR-094E2C-225.

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REFERENCES

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