A new family of clusters containing a silver-centered tetracapped [Ag@Ag₄(μ₃-P)₄] tetrahedron, inscribed within a N₁₂ icosahedron

Abstract

An unprecedented silver-centered P-tetracapped [Ag@Ag₄(μ₃-P)₄] tetrahedron inscribed within a N₁₂ icosahedral cage has been discovered in the novel family of luminescent clusters. The latter are easily self-assembled by reacting Ag^I salts with tris(2-pyridyl)phosphine (Py₃P).

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Currently, silver(I) complexes and clusters attract considerable attention because of their intriguing structural, photophysical, catalytic and biological properties.¹ One of the most powerful approaches to the synthesis of well-defined silver(I) clusters is self-assembly² from Ag⁺ ions and diverse ligands such as N-heterocycles,³ carboxylates,⁴ sulfides and selenides,⁵ 1,1-dithiolates,⁶ and alkynyl anions.⁷ Among a plethora of ligands that are used for this purpose, phosphines containing additional donor sites are of special interest because they allow assembling a number of unique architectures.⁸ For instance, based on diphenyl-2-pyridylphosphine (dppy), a series of highly luminescent heterometallic Ag/Au clusters have been synthesized, e.g. [Au₃(E)Ag(dppy)₃]²⁺,⁹ [(CCN)₂Au₈Ag₄(dppy)₈(MeCN)₂]⁶⁺,¹⁰ and [RCOCAu₄Ag₄(dppy)₄]⁵⁺.¹¹ Recently,¹² using bis(2-pyridyl)phenylphosphine, the original 1D polymer [Ag₂(PPhPy₂)₂⋯Cl⁻⋯Ag₂(PPhPy₂)₂]_n has been designed. Also, it is pertinent to note that silver(I) complexes with pyridylphosphines are prospective antitumour agents.¹³

Herein, we report on a new family of luminescent Ag(I) clusters with an unprecedented core structure, which can be viewed as a silver-centered phosphorus-tetracapped tetrahedron [Ag@Ag₄(μ₃-P)₄], inscribed within a N₁₂ icosahedral cage. These clusters have been readily self-assembled by reacting silver(I) salts with tris(2-pyridyl)phosphine under mild conditions. The photophysical and electrochemical properties as well as the electronic structure of the novel complexes have also been investigated.

We have discovered that tris(2-pyridyl)phosphine easily reacts with AgOTf in a 4 : 5 molar ratio (room temperature, CH₂Cl₂/MeCN, 1 h) to afford the unprecedented cluster [Ag@Ag₄(Py₃P)₄(OTf)₄](OTf) (1) in 84% yield (the synthetic details are in the ESI†). It is noteworthy that the variation of the reactant molar ratio (e.g. Ag/L = 1 : 1, 2 : 1, 1 : 2 or 2 : 3) does not lead to the formation of other clusters apart from 1. X-Ray diffraction (XRD) analysis reveals that this compound crystallizes from acetone solution as the 1·4Me₂CO solvate. The [Ag@Ag₄(Py₃P)₄(OTf)₄]⁺ cation of the latter (Fig. 1a) contains a structurally remarkable [Ag@Ag₄(μ₃-P)₄] skeleton which is best described as a C₃-symmerical Ag-centered Ag₄ tetrahedron (τ₄ = 0.954 (ref. 14)) whose each triangular face is capped by a phosphorus atom (μ₃-P) of the Py₃P ligands (Fig. 1b). Overall, the [Ag@Ag₄(μ₃-P)₄] core is inscribed within a N₁₂ icosahedron, constituted by the twelve pyridine nitrogen atoms of four Py₃P ligands. The central Ag atom has a slightly distorted P₄ tetrahedral environment (τ₄ = 0.984 (ref. 14)), constituted by the four P atoms of the Py₃P ligands [Ag–P, 2.5766(16)–2.6095(15) Å]. Each vertex Ag atom shows three short bonds with the N atoms of the three neighbouring Py₃P ligands [Ag–N, 2.272(5)–2.306(5) Å] and a one longer bond with a triflate O atom, thus adopting a [3 + 1] trigonal pyramidal geometry. The three Ag–O bond lengths are similar (ca. 2.620 Å), which suggests a weak association of the triflate anions with the Ag3, Ag4 and Ag5 atoms, whereas the Ag1–O3 distance [2.7400(53) Å] is clearly non-bonded. The Ag_central–Ag_vertex distances are quite short ranging from 3.0917(7) to 3.1638(6) Å {cf. with twice the van der Waals radius of Ag, 3.4 Å}. Meanwhile, the Ag–Ag distances along the edges of the Ag₄ tetrahedron are too long [5.0031(7)–5.2553(7) Å] for any argentophilic interactions. Thus, each Py₃P ligand binds the central metal (via the P atom) and the three adjacent Ag_vertex ions (via pyridine N donors), exhibiting a P,N,N′,N′′-coordination pattern.

Fig. 1 (a) The structures of the [Ag@Ag₄(Py₃P)₄(OTf)₄]⁺ core of 1·4Me₂CO (the pyridine H atoms are omitted for clarity). (b) Ag-centered P-tetracapped tetrahedron, [Ag@Ag₄(μ₃-P)₄], inscribed in a N₁₂ icosahedral cage. Selected bond lengths [Å]: Ag(1)–Ag(2) 3.1622(6), Ag(2)–Ag(4) 3.0917(7), Ag(2)–Ag(5) 3.1313(6), Ag(2)–Ag(3) 3.1638(6), Ag(1)–O(3) 2.7400(53), Ag(3)–O(9) 2.6229(53), Ag(4)–O(6) 2.6675(47), Ag(5)–O(1) 2.5711(56), Ag–P (av.) 2.593, Ag–N (av.) 2.288.

The [Ag@Ag₄(Py₃P)₄] core of 1 is chiral due to a propeller-like arrangement of the pyridine cycles around four vertex silver atoms. Interestingly enough, exclusively ΔΔΔΔ- and ΛΛΛΛ-helical isomers are found in the packing of 1 (Fig. S1†), i.e. the Ag_vertex atoms in each stereoisomer have an identical configuration.

Encouraged by the first result, we next have examined whether the disclosed approach could be used for the assembly of similar aggregates from other silver(I) salts. The experiments have shown that [Ag(MeCN)₄]BF₄ and [Ag(MeCN)₄]PF₆ interact with Py₃P in the same way to afford structurally related clusters, [Ag₄(Ag)(Py₃P)₄](BF₄)₅ (2) and [Ag₄(Ag)(Py₃P)₄](PF₆)₅ (3), in 99 and 86% yield, respectively.† The data of elemental analysis, ¹H NMR and FT-IR spectroscopy of samples 2 and 3 confirm the above-mentioned formulations. The diffusion of hexane into a solution of 2 in an acetone/MeCN mixture results in X-ray quality crystals, the asymmetric unit of which consists of two cations, [Ag@Ag₄(Py₃P)₄(Me₂CO)(MeCN)₃]⁵⁺ (Fig. 2a) and suchlike [Ag@Ag₄(Py₃P)₄(Me₂CO)(MeCN)₂]⁵⁺ (Fig. S2†) along with ten BF₄⁻ anions. Similarly, the equal number of ΔΔΔΔ- and ΛΛΛΛ-enantiomers for both the cations is presented in the centrosymmetric crystals of 2. On the whole, the [Ag@Ag₄(Py₃P)₄] skeleton 2 is almost superimposable with that of 1 (see the overlay of these fragments in Fig. S4†). One of the Ag_vertex atoms of 2 is additionally ligated by O from acetone, whereas the remaining Ag_vertex atoms are weakly bonded with MeCN molecules (Fig. 2a and S2†). At that, two basal atoms, Ag8 and Ag9, have quite similar Ag–N distances (2.542 and 2.550 Å), whereas the corresponding bond for Ag6 is somewhat longer: 2.681 Å.

Fig. 2 The structures of the [Ag@Ag₄(Py₃P)₄(Me₂CO)(MeCN)₃]⁵⁺ (a) and [Ag@Ag₄(Py₃P)₄(MeCN)]⁵⁺ (b) cations in 2·Me₂CO·2.5MeCN and 3·MeCN, correspondingly (the pyridine H atoms are omitted for clarity).

The crystals of [Ag@Ag₄(Py₃P)₄(MeCN)](PF₆)₅ (3·MeCN) have been obtained by diffusion of diethyl ether into an MeCN solution of 3. XRD analysis revealed (Fig. 2b) that the [Ag@Ag₄(Py₃P)₄] unit in [Ag@Ag₄(Py₃P)₄(MeCN)]⁵⁺ is nearly the same as that in 1 (Fig. S5†). As in the case in 2, the PF₆⁻ anions of 3 remain non-coordinated. Therefore, the three basal Ag atoms are saturated by pyridyl N donors in a trigonal planar manner, while the fourth Ag centre (Ag3) is additionally ligated by a MeCN solvent molecule (Ag–N 2.497 Å).

Significantly, using two different Ag(I) salts in the reaction with Py₃P allows [Ag@Ag₄(Py₃P)₄] clusters bearing two types of counterions to be synthesized. So, we have found that [Ag(MeCN)₄]PF₆ and four equivalents of [Ag(MeCN)₄]BF₄ may be easily self-assembled into the pentanuclear cluster [Ag₄(Ag)(Py₃P)₄](PF₆)(BF₄)₄ (4) in 85% yield by the addition of four equivalents of Py₃P.† From the MeCN/Et₂O system, cluster 4 crystallizes as solvate 4·6MeCN containing [Ag@Ag₄(Py₃P)₄(MeCN)₄]⁵⁺ cations (Fig. 3), the core of which is comparable to that of 1–3 (Fig. S6†).

Fig. 3 The structures of the [Ag@Ag₄(Py₃P)₄(MeCN)₄]⁵⁺ cation in 4·6MeCN, correspondingly (the pyridine H atoms are omitted for clarity).

Our attempts to obtain a similar polynuclear cluster using AgCl in reaction with Py₃P in a 5 : 4 molar ratio lead to the isolation of the dinuclear complex [Ag(Cl)(μ-Py₃P)]₂ (5) in quantitative yield (Scheme 1, the synthetic details, and structural and luminescence properties are given in the ESI†). According to the XRD study, two Ag atoms in 5 are bridged by two Py₃P ligands in a head-to-tail fashion so that each metal coordinates two N atoms and one P atom form two adjacent ligands (Scheme 1 and Fig. S3†). Note that complexes of a similar structure, but with the PhPy₂P ligand, were previously described.^12,15 The Ag–Ag distances in both independent molecules of 5 [3.2615(8) and 3.2701(9) Å] imply argentophilic interactions. In the solid state, this cluster exhibits blue-green emission (λ_max = 500 nm, Fig. S30†) with the modest PLQY (8% at 298 K).†

Scheme 1 Synthesis of dinuclear cluster 5.

The pentanuclear clusters obtained are air- and light-stable powders and show good solubility in acetonitrile. All these clusters exhibit a high stability in solutions and can be recrystallized from them. The ESI-mass spectra of the clusters exhibit a number of low intensive peaks assigned to polynuclear Ag-containing species, thus indicative of a fragmentation of the [Ag₄(Ag)(Py₃P)₄]⁵⁺ cation under ESI-conditions.

The NMR data reveal that the cluster cations of 1–4 remain intact in the solution state. So, in the ³¹P{¹H} NMR spectrum of 1 dissolved in CD₃CN at 298 K, only one signal is visible at ca. 14.6 ppm with a large doublet splitting due to the ³¹P–^109/107Ag coupling. The magnitude of this coupling (238 Hz for ³¹P–¹⁰⁹Ag and 207 Hz for ³¹P–¹⁰⁷Ag) clearly suggests that all four P atoms are bonded to the Ag atom.¹⁶ Moreover, couplings with other Ag atoms of the complex are resolved as well (see. Fig. 4, top, for the spectra and assignment of the couplings; we have tentatively assigned all 6–7 Hz splittings to the ²J(P–Ag–Ag) couplings, assuming that they are similar in value and that the ³J(P–C–N–Ag) coupling is too small to be resolved). The high resolution constant time ¹H,¹⁵N-HMBC spectrum, shown in Fig. 4, bottom, further reveals the ¹J(¹⁵N,^109/107Ag) and ²J(¹⁵N,^109/107Ag) couplings consistent with the structure of the complex, as well as the ²J(³¹P,¹⁵N) coupling of ca. 56 Hz. Note that within the spectral line width the differences in couplings to ¹⁰⁹Ag and ¹⁰⁷Ag are not always resolved. The complete set of ¹H, ³¹P{¹H}, ¹⁰⁹Ag and lower resolution ¹H,¹⁵N-HMBC NMR spectra for complexes 1, 2, 3 and 4 dissolved in CD₃CN is given in the ESI (Fig. S7–S22†). The ¹⁰⁹Ag NMR signals of 1–4 do not show clear Ag–P splitting patterns. However, these signals have similar line shapes with a pedestal (see Fig. S9, S13, S17 and S21†) indicating the presence of some kind of splitting. At the given signal-to-noise ratio (up to 4 days of acquisition), it is hard to speculate about the number of signals and the types of spin systems. In turn, the signal of the central silver atom is not observed in the ¹⁰⁹Ag NMR spectra because it should be four times less intensive and additionally split into complicated multiplets due to the one-bond couplings with phosphorous and silver nuclei.

Fig. 4 ³¹P{¹H} (top) and constant time ¹H,¹⁵N-HMBC (bottom) NMR spectra of 1 dissolved in CD₃CN at 300 K.

The UV-Vis spectra of clusters 1–4 are characterized by a strong band at 260 nm and a weak shoulder expanding from 275 to 340 nm (Fig. S29†). The high-energy band could be assigned to intraligand transitions because the Py₃P itself displays the same absorption. The weak band in the region 275–340 nm is tentatively assigned to metal-to-ligand charge transfer transitions that is in accordance with DFT calculations (vide infra).

The luminescence properties of clusters 1–4 have been preliminarily investigated in the solid state. All these exhibit a greenish-blue emission at room temperature with the quantum yields up to 24% (Table 1). The excitation and emission spectra of 1–4 are depicted in Fig. 5. Clusters 4, 3, 1 and 2 show a broad emission with the maxima at 470, 480, 490 and 510 nm, respectively, i.e. the distinct red-shift of the emission maxima is observed in this range, while the free Py₃P displays a broad emission with λ_max at about 420 nm. Taking into account the literature data,^3c the origin of luminescence of 1–4 can be ascribed to metal-to-ligand charge transfer states, perturbed by the presence of Ag⋯Ag interactions.

Fig. 5 Normalized excitation and emission spectra of 1–4 in powder states at 300 K.

Table 1 Photophysical properties of clusters 1–4 at 300 K

	λ ^exmax (nm)	λ ^emmax (nm)	ϕ PL (%)
1	350	490	14
2	360	510	24
3	350	480	18
4	350	470	2

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The redox properties of clusters 1–4 have been investigated by cyclic voltammetry (CV). All compounds demonstrate similar redox behavior in the negative range (up to −2 V) and reveal irreversible reduction processes which can be assigned to the Ag^I/Ag⁰ couple. The cathodic potential values (Table S3†) strongly depend on the number of scans and dramatically change on going from the first scan to the second one. In particular, the CV of 1 shows two cathodic peaks at −1.2 and −1.5 V (vs. Ag/AgCl) at the first scan which are further transformed to the broad wave centered at −0.77 V at the second scan (Fig. 6). This behavior can be explained by absorption of the reduction products on the surface of the working electrode.

Fig. 6 CV of 1 in CH₃CN between 0 ↔ −2 V at 0.1 V s⁻¹ scan rate (dashed line is the first scan, solid line is the second scan).

Conclusions

In summary, we have presented the novel family of luminescent clusters containing a remarkable Ag-centered P-tetracapped tetrahedral [Ag@Ag₄(μ₃-P)₄] unit inscribed within a N₁₂ icosahedron. Taking into account the intriguing structure of these clusters and their facile and almost quantitative self-assembly from available reagents, one expects that the reported finding would stimulate intense research interest in this area. Further investigation of the silver clusters is in progress.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial support from the Russian Foundation for Basic Research (Grant 15-03-05591). We also are grateful to Multi-Access Chemical Service Center SB RAS for XRD analysis. Supported by Russian Ministry of Science and Education under 5-100 Excellence Programme.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, characterization data, NMR and FT-IR spectra, photophysical and electrochemical data, computation details. CCDC 1472281, 1472282, 1532341 and 1538903. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt02597a

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