Silver Alkynyl-Phosphine Clusters: An Electronic Effect of the Alkynes

Article pubs.acs.org/Organometallics

Silver Alkynyl-Phosphine Clusters: An Electronic Effect of the Alkynes Defines Structural Diversity Yi-Ting Chen,§ Ilya S. Krytchankou,‡ Antti J. Karttunen,*,∥ Elena V. Grachova,‡ Sergey P. Tunik,‡ Pi-Tai Chou,*,§ and Igor O. Koshevoy*,⊥ ‡

St.-Petersburg State University, 7/9 Universitetskaya nab., 199034, St. Petersburg, Russia National Taiwan University, Department of Chemistry, Taipei TW 106, Taiwan ∥ Aalto University, Department of Chemistry, FI-00076 Aalto, Finland ⊥ University of Eastern Finland, Department of Chemistry, Joensuu 80101, Finland §

S Supporting Information *

ABSTRACT: The face-capping triphosphine, 1,1,1-tris(diphenylphosphino)methane (tppm), together with bridging alkynyl ligands and the counterions, facilitates the formation of a family of silver complexes, which adopt cluster frameworks of variable nuclearity. The hexanuclear compounds [Ag6(C2C6H4-4-X)3(tppm)2(An−)3] (X = H (1), CF3 (2), OMe (3), An− = CF3SO3−; X = OMe (4), An− = CF3COO−) are produced for the electron-accepting to moderately electrondonating alkynes and the appropriate stoichiometry of the reagents. 1 and 3 undergo an expansion of the metal core when treated with 1 equiv of Ag+ to give the species [Ag7(C2C6H4-4-X)3(tppm)2(CF3SO3)3](CF3SO3) (X = H (5), OMe (6)). The electron-donating substituent (X = NMe2) particularly favors this Ag7 arrangement (7) that undergoes geometry changes upon alkynylation, resulting in the capped prismatic cluster [Ag7(C2C6H4-4-NMe2)4(tppm)2(CF3SO3)](CF3SO3)2 (8). Alternatively, for the aliphatic tBu-alkyne, only the octanuclear complex [Ag8(C2But)4{(PPh2)3CH}2(CF3SO3)2](CF3SO3)2 (9) is observed. The structures of 1−4 and 6−9 were determined by X-ray diffraction analysis. In solution, all the studied compounds were found to be stereochemically nonrigid that prevented their investigation in the fluid medium. In the solid state, clusters 2, 3, 5−8 exhibit room temperature luminescence of triplet origin (maximum Φem = 27%, λem = 485−725 nm). The observed emission is assigned mainly to [d(Ag) → π*(alkyne)] electronic transitions on the basis of TD-DFT computational analysis.

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INTRODUCTION The closed-shell interactions between d10 metal ions are a unique type of chemical bonding that has been extensively studied both experimentally and theoretically, particularly for gold(I)-containing compounds.1 An impressive development of organometallic chemistry of coinage metals produced numerous polynuclear aggregates that show fascinating geometries due to the formation of metallophilicity-assembled frameworks enclosed in suitable ligand environments.2 The existence of possibly attractive interactions between silver(I) ions, later termed argentophilicity, has been demonstrated in the early 50s of the last century,3 but was considered insignificant until the early 1990s when cluster chemistry of silver was considerably stimulated by the rapid growth of the related class of gold(I) multimetallic complexes.1d Since then, a large number of Ag(I) systems employing metal−metal interaction has been prepared.1f,4 Comprehensive analysis of these objects provided a convincing amount of structural, spectroscopic, and theoretical data to prove energetic favorability of the Ag−Ag bonding that effectively operates both in the solid state and in solution.1a,5 © XXXX American Chemical Society

An inherent trait of Ag−Ag interactions is their poorly expressed directionality that, together with a tendency of silver(I) ions to adopt a coordination number four, often leads to the aggregation of several metal centers in a rather convoluted and, therefore, hardly predictable fashion. Moreover, clustering of Ag atoms is notably enhanced by the bridging groups capable of κ1-coordination (thiolates/selenates, alkynyls, cyanides, chalcogenides, halides)2b,e,6 that can serve as an additional instrument to direct the assembly processes. Thus, the appropriate choice of bridging and ancillary ligands allows for the successful synthesis of various polymeric materials and discrete molecules of different dimensionality, arrangement, and nuclearity. A recent review by Schmidbaur gives a detailed account of polynuclear silver(I) associates, for the design of which different combinations of the ligands have been employed.1f Received: November 18, 2016

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DOI: 10.1021/acs.organomet.6b00866 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Synthesis of Complexes 1−9 (Acetone, 298 K, N2 Atmosphere)

−CCR ligands are reported to be emissive. The representative examples contain the heterodonors like phosphines, imines, or caboxylates,15a,16 possibly incorporated into the alkyne backbone,8d,e,17 while the luminescence of homoleptic Ag alkynyls remains an extremely rare phenomenon.6a,10 During the past years, we have been studying families of homo- and heterometallic coinage metal clusters, stabilized by a number of multidentate phosphine ligands. Among others, the series of tetranuclear Au−Cu and hexanuclear Cu complexes, which possess a triphosphine tris(diphenylphosphino)methane (tppm), were found to exhibit attractive stimuli-responsive (vapochromic) luminescence.18 Additionally, this tppm ligand was employed by Chen and coauthors in the synthesis of related Ag−Cu luminophores [Ag6Cu(CCR)3(PPh2O2)3(tppm)2]+ as dopants for highly efficient OLEDs.19 As an extension of this chemistry, herein, we present the preparation of novel hexa-, hepta-, and octanuclear homometallic silver(I) alkynyl complexes supported by the tppm triphosphine, their structural characterization and photophysical investigations together with computational analysis of the electronic structures.

The alkynyl anions [R−(CC)n]n− are a popular set of ligands in a view of supramolecular construction involving silver(I) due to the following main reasons: (a) high affinity of the RCC− moiety to Ag(I) ions and versatility of its coordination modes to stabilize the cluster frameworks; (b) rich opportunities for stereochemical modulation of the R group that makes possible varying the ligand denticity (i.e., the number of terminal CC fragments), electronic properties, bulkiness, and introducing the secondary binding functions. Several research laboratories, namely, those of Mak and Zhang, have been largely focused on the structural chemistry utilizing mono- and dialkynyl ligands of homo-4,7 and heterodentate8 nature for the preparation of silver aggregates, in which RC C− units are generally surrounded by 3−5 Ag cations. The concept was further extended to tri-(1,2,3-tris(prop-2-ynyloxy)benzene),9 star-like tris(4-ethynylphenyl)amine,10 and higher alkynes (tetraethynylethene, penta-/hexaethynylbenzene)11 to generate sophisticated nanoclusters and 3D organometallic networks. Eminently interesting silver alkynyl compounds have been prepared via an anion-templated approach2g pioneered by Mingos, who reported the assembly of rhombohedral cage complex [Ag14(CCBut)12Cl]+ encapsulating a chloride anion.12 Fruitful recent advances of this methodology can be illustrated by high-nuclearity molecular clusters [Agn(C CR)m](n−m)+ (n ranges from 17 to 70) and coordination polymers, which accommodate simple (CO32−, NO3−, CrO42−, SO42−)13 and polyoxometalates anions (e.g., Mo6O228−, V10O286−, PW9O349−).14 In contrast to gold homo- and heteroleptic alkynyl complexes, which often show intriguing photoluminescence properties,15 only few homometallic silver congeners bearing

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RESULTS AND DISCUSSION

Synthesis. The reaction of the tppm ligand with silver(I) triflate and phenylacetylene in a 2:6:3 molar ratio in the presence of trimethylamine under a nitrogen atmosphere produces a pale yellow solution, from which a nearly colorless microcrystalline solid can be precipitated. Recrystallization gives the hexanuclear cluster [Ag6(C2Ph)3(tppm)2(CF3SO3)3] (1) in good yield (Scheme 1). Similarly, complexes [Ag6(C2C6H4-4-X)3(tppm)2(CF3SO3)3] (X = CF3 (2), OMe (3)) were B


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Figure 1. Molecular views of clusters 1, 2, and 4. One of two independent molecules found in the unit cell of 4 is shown. Symmetry transformations to generate equivalent atoms in 2: (′) 1 − x + y, 1 − x, z; (″) 1 − y, x − y, z.

evidently points to an important effect of the electronic properties of the −C2R ligands on the preferential topology of the silver cluster core. Structural Characterization. The complexes 1−4 and 6− 9 have been characterized crystallographically. The structures of hexanuclear species 1−4 are given in Figures 1 and S1, and the selected interatomic distances are listed in Tables S2−S5. The triflate clusters 1−3 adopt twisted trigonal prismatic Ag6 cores where two tripod tppm ligands cap the bases of the prism. The observed coordination mode of the triphosphine was described earlier for some homo-18b,20 and heterometallic coinage metal compounds.19,21 The twisting angle, estimated as an average of torsion angles (e.g., Ag(1)−Ag(2)−Ag(5)−Ag(4), Ag(2)− Ag(3)−Ag(6)−Ag(5), Ag(3)−Ag(1)−Ag(4)−Ag(6) in 1), is 27° in 1 and 31° in both 2 and 3. The metal frameworks display clearly visible argentophilic contacts supported by the capping phosphines along with bridging bonding of alkynyl and, to a less extent, CF3SO3− ligands. The intermetallic distances within the Ag6 motifs for the congener complexes 1−3 lie in a range from 2.9344 to 3.2287 Å, which are quite normal for Ag(I)− Ag(I) interactions.1f Three alkynyl groups in 1−3 occupy generally unsymmetrical μ3-positions on the side faces of the cluster cores with one C−Ag bond being systematically longer than two others (2.337−2.465 Å vs 2.116−2.332 Å); the values are in line with the corresponding parameters reported for other silver(I) alkynyl species.16a,c,22 The triflate anions semi-bridge the Ag− Ag edges to further enhance the connectivity of Ag3(tppm) units, though the O−Ag separations exceeding 2.6−2.7 Å are expected to have only a weakly bonding character.23 Complex 1 was crystallized as an acetonitrile solvate with the NCMe molecule attached to a silver ion, resulting in substitution of one CF3SO3− anion from the coordination sphere of the Ag6 core (Figure 1). However, the acetonitrile ligand is only weakly bound to the metal ion and can be easily removed from the crystalline sample by vacuum drying as confirmed by elemental analysis data. Overall, variation of the substituent X of the alkynyl ligand C2C6H4-4-X (X = H, CF3, OMe) does not result in appreciable systematic alteration of the structural parameters. A trifluoroacetate compound 4 (X = OMe), prepared for the sake of comparison, has essentially the same structural arrangement as its triflate analogue 3. The prismatic metal

prepared as colorless crystalline materials following this simple protocol. Using the alkyne with X = OMe and Ag trifluoroacetate in place of the triflate allowed for the isolation of an akin compound [Ag6(C2C6H4OMe)3(tppm)2(CF3CO2)3] (4). This cluster was synthesized more efficiently from the (AgC2C6H4-4-OMe)n/tppm/AgCF3SO3 3:2:3 mixture containing homoleptic silver alkynyl polymer that simplified separation of 4 from the side products (see the Experimental Section). Interestingly, 1 and 3 undergo nearly quantitative expansion of the metal core when treated with 1 equiv of AgCF3SO3 to afford yellow-greenish heptanuclear complexes [Ag7(C2C6H4-4X)3(tppm)2(CF3SO3)3](CF3SO3) (X = H (5), OMe (6)). Alternatively, 5 and 6 are formed in a single step if the appropriate stoichiometry (AgC2C6H4-4-X)n/tppm/Ag triflate 3:2:4 is obeyed (see the Experimental Section). However, no Ag7 congener was observed in the case of electron-accepting alkyne (X = CF3) even in the presence of an excess of the silver salt. On the other hand, for the electron-donating ligand −C2C6H4-NMe2, no Ag6 cluster was identified under the conditions of the synthesis of 1−3, i.e., when the ratio of the phosphine/ Ag+ was 1:3 or somewhat less. Instead, only a deep red complex [Ag7(C2C6H4-4-NMe2)3(tppm)2(CF3SO3)3](CF3SO3) (7) was obtained irrespectively of the amount of the metal salt loaded into the reaction mixture. However, the amount of the NMe2-functionalized alkyne appeared to have a crucial effect on the geometry of the cluster framework. Thus, an addition of 1 equiv of the alkyne to 7 or carrying out the reaction in tppm/AgCF3SO3/acetylene 2:7:4 ratio produces a yellow-orange solution, from which a bright yellow cluster [Ag7(C2C6H4-4-NMe2)4(tppm)2(CF3SO3)](CF3SO3)2 (8) was separated. In order to extend the list of the alkynyl ligands and further probe their effect on the assembly of the clusters under study, we investigated the reaction of (AgC2But)n polymer with tppm and AgCF3SO3 in a 3:2:3 ratio. The only product crystallized from the reaction mixture was the octanuclear complex [Ag8(C2But)4{(PPh2)3CH}2(CF3SO3)2](CF3SO3)2 (9). The attempts to influence the arrangement of the metal centers for the −C2But ligand via changing the proportion of the starting regents were unsuccessful, leading to the formation of 9 only. It is noteworthy that the geometries of 8 and 9 were not reproduced for other alkynes utilized in the current work. This observation C


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Figure 2. Molecular views of clusters 7, 8, and 9. Noncoordinated counterions are omitted for clarity. Symmetry transformations to generate equivalent atoms in 7: (′) 1 − x + y, 1 − x, z; (″) 1 − y, x − y, z. In 9: (′) 2 − x, 1 − y, 1 − z.

single-crystal X-ray diffraction analysis. Nevertheless, the collected data allowed for the determination of the space group (P63/m), which belongs to the hexagonal crystal system. Together with Z = 2, it requires the presence of at least 3-fold rotation axis in molecular symmetry that is completely compatible with the idealized structural type of complexes 6 and 7. The unit cell parameters for 5 (a = b = 18.3 Å, c = 20.7 Å; α = β = 90°, γ = 120°, V = 5992 Å3) are rather similar to those of cluster 2 (P3̅, a = b = 18.4199(10) Å, c = 21.9233(11) Å, α = β = 90°, γ = 120°, V = 6441.9(8) Å3, Z = 2), indicating a comparable molecular volume of these complexes, which is not contradictory with the proposed arrangement of 5. Despite a very poor quality of the diffraction data, the general structural motif of the cluster core in 5 was estimated (Figure S3), albeit no structural details could be evaluated. The results of microanalysis (see the Experimental Section) also support the suggested composition of 5. Altogether, it is reasonable to conclude that 5 possesses the same structural motif as 6 and 7. As mentioned above, only compound 7 from the isostructural series 5−7 is capable of undergoing a metal core transformation upon further alkynylation to give cluster 8 of the same nuclearity but of unprecedented geometry (Figure 2). Seven silver(I) ions in 8 form a nearly ideal trigonal prism, with an anionic [Ag(C2C6H4NMe2)2]− unit capping one rectangular face. The alkynyl ligands of this fragment are predominantly σbound to the capping Ag(7) atom and show the C−Ag distances of 2.082(6) and 2.078(6) Å, which are the shortest ones among the compounds studied in this work (Table S8). In addition, these −C2C6H4NMe2 groups also participate in binding to adjacent silver ions with the C−Ag contacts of 2.366−2.494 Å, thus forming nonsymmetric μ3-bridges over the corresponding triangular Ag3 faces. Two other alkynyl ligands occupy μ4-bridging positions over the remaining rectangular faces of the prism, while the only coordinated CF3SO3− anion spans the sterically unhindered Ag−Ag edge opposite to the capping moiety (O−Ag bond lengths are 2.472(3) and 2.494(4) Å). The metal−metal contacts in 8 range from 2.897 to 3.062 Å that is in line with the values found for 5−7. The cocrystallized acetone molecule, coordinated to the Ag(7) center, can be easily removed from the solid sample under a reduced pressure, which is confirmed by the analytical data (see the Experimental Section).

core shows a somewhat smaller degree of twisting than found for 1−3; the Ag···Ag contacts in 4 (av. 2.97 Å) are on average only slightly shorter than those in 3 (av. 3.04 Å). The alkynyl groups in 4 demonstrate a tendency to be coordinated both in μ3- and μ4-modes presumably due to a smaller distortion of the Ag6 framework. In contrast to triflates, which have one Ag−O distance significantly elongated with respect to another in 1−3, CF3COO− ligands are bound to the Ag6 edges in a more symmetrical fashion with the O−Ag separations spanning the range from 2.408 to 2.589 Å (Table S5), which reflects a stronger binding ability of trifluoroacetate anions to silver(I).24 It is worth noting that the related [Cu6(C2R)4(tppm)2]2+ species demonstrate quite different distorted octahedral cluster geometry, alternatively seen as a result of fusing two tetrahedra via a common edge.18b The heptanuclear complexes 6 and 7 (Figures 2 and S2) possess the metal cores, which are derived from the trigonal prismatic one by inserting an additional Ag+ ion in between the trigonal faces capped with triphosphines. As a result, the Ag7 framework can be considered as a combination of two distorted tetrahedra sharing one vertex. The congener heterometallic complexes [Ag 6 Cu(CCR) 3 (PPh 2 O 2 ) 3 (tppm)2]+ were recently reported to possess a resembling topology, in which the copper ion is incorporated into the Ag6 prism.19 The central silver atom bears σ-bonded alkynyl groups to form a nearly planar trigonal dianionic moiety [Ag(C2C6H44-X)3]2− that holds together the entire cluster by means of μ3bridging −C2C6H4-X ligands and a network of argentophilic interactions. The corresponding Ag−C bond distances in 6 and 7 (2.177−2.280 Å; see Tables S6, S7) are comparable with the values determined for 1−4 species discussed above. The intermetallic distances between the central and peripheral silver atoms (av. 2.87 and 2.83 Å for 6 and 7, respectively) are visibly shorter than the Ag···Ag separations within the Ag3(tppm) fragments (av. 3.55 Å (6) and 3.46 Å (7)); the latter values exceed the sum of two van der Waals radii for silver (3.44 Å). A slightly looser cluster arrangement in 6 in regard to 7 might arise due to coordination of methanol solvent molecules in 6, which leads to predominantly a monodentate binding of the CF3SO3− anions (Figure S2) and, therefore, a less efficient bridging support of the Ag7 framework. Unfortunately, the crystals of the Ag7 complex 5 (X = H) appeared to be extremely susceptible toward the loss of crystallization solvent and, therefore, were poorly suitable for D


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Organometallics Interestingly, changing the aromatic to aliphatic alkyne was also accompanied by the substantial modification of the preferential cluster type (Figure 2). The dication 9, which contains tBu-alkynyl ligands, displays an octanuclear metal framework formed by edge-to-edge interaction of two butterfly Ag4 cores, with two (Ag(3)−Ag(4′) and Ag(3′)−Ag(4)) rather short argentophilic contacts (3.0096(3) Å, Table S9) and two μ3-alkynyl bridges. The rest of the alkynyls and two triphosphines are placed over the trigonal faces of the butterfly Ag4 motifs, which are additionally stabilized by the bridging triflates. The attempts to investigate the solution behavior of all title clusters 1−9 were unsuccessful due to their instability in solution with respect to possible dissociation and related dynamic equilibria. The ESI-MS did not allow detecting the molecular ions. The 1H and 31P NMR spectroscopy also provided unsatisfactory results, revealing complicated mixtures of several species for each cluster, which could not be reliably identified even at low temperature. Moreover, leaving the solutions of these complexes in air with no light protection results in fast appearance of a blue-greenish color, indicative of possible decomposition and formation of the nanoparticles. Nevertheless, all the complexes can be reproducibly recrystallized with minimum losses. This proves an effectiveness of selfassembly processes, leading to the crystallographically determined cluster cores. Photophysical Properties. Because the title clusters do not retain their structures in solution and show substantial photodegradation, the analysis of the photoemissive properties was carried out for the crystalline samples only. In contrast to the fluid medium, the solid materials were stable under the conditions of the photophysical measurements and could be handled at room temperature for prolonged periods of time. Complexes 1, 4, and 9 are not luminescent neither under ambient conditions nor at 77 K. The absence of detectable photoemission could be tentatively assigned to the efficient nonradiative relaxation pathways of the excited state, which might result from a number of lattice effects. In particular, in cluster 1, one of the triflate anions is clearly not coordinated to the metal core, in contrast to the emissive isostructural species 2 and 3. In the case of complex 4, a different origin of the counterion (trifluoroacetate) probably introduces a detrimental influence and increases the nonradiative decay rate. The last, but not the least, cluster 9 adopts a more open and, therefore, less rigid metal framework that facilitates vibrational relaxation, along with rather different (aliphatic) substituents at alkynyl ligands, which are sterically and electronically distinct from the aromatic alkynyls present in the rest of the complexes studied. The relevant photophysical data for the rest of the compounds are given in Table 1; the solid-state spectra are presented in Figures 3 and 4. Clusters 2, 3, 5−8 display weak to moderate emission intensity at room temperature in the solid state. The determined quantum yields range from 0.34% to 26.7% (8 exhibits too low intensity to determine Φem), while the radiative lifetimes of several microseconds and longer evidence in favor of a triplet origin of luminescence. Excitation spectra of compounds 2, 3, 5, 6 display one major band each in a low energy region from 372 to 440 nm, while, NMe2-functionalized 7 and 8 demonstrate the dominating broad band maxima at 576 and 508 nm, respectively, being considerably red-shifted compared to other species. In general, the excitation bands reveal a clear effect of the alkynyl substituentsan increase of

Table 1. Solid-State Photophysical Properties of Clusters 2, 3, 5−8 298 K 2 3 (N2) 3 (air) 5 5c 6 7 8

77 K

λexc/nm

λem/nma

τobs/μs

Φem/%

372 410 410 405 405 440 576 508

485 500 500 515 454 510 725 570

9.51 9.24 5.25 3.90 2.95 2.78 0.058

0.34 7.7 4.3 26.7 24.5 17.6 2.8

b

b

kr/s−1

λem/nm

× × × × × × ×

490

3.58 8.29 8.20 6.85 8.31 6.33 4.76

102 103 103 104 104 104 106

496 490 500 725 b

The excitation wavelength is 400 nm for 2, 3, 5, ̧6; 600 nm for 7; 406 nm (from a GaN diode laser) for 8. bCould not be reliably determined due to very low intensity of the emission. cTreated with acetone vapors. Emission quantum yield was measured under aerated condition except for compound 3 in N2. a

their electron-donating ability results in a noticeable decrease of excitation energy for the same structural motif. This influence of the −C2C6H4X ligands allows for a tentative assignment of the lower-lying absorptions mainly to MMLCT [d(Ag) → π*(alkyne)] transitions probably mixed with a small contribution of MMLCT processes involving the triphosphine moieties; see the computational analysis below. The hexanuclear clusters 2 and 3 show quite low luminescence efficiency; the emission energy of 2 bearing alkynes with CF3 substituents is expectedly higher than that of OMe-containing complex 3 due to a larger HOMO−LUMO gap for the electron-withdrawing ligand environment. The emission profile for 2 is poorly structured with vibronic progression of ca. 2100 cm−1, indicating a significant participation of the alkynyl triple bond in the electronic transitions. It has to be taken into account that the congener species 1 and 4 are virtually nonemissive, manifesting a detrimental impact of the given prismatic Ag6 geometry on the photoluminescence performance of this class of compounds. Noteworthy, among 1−4, the crystalline sample of complex 3 shows significant O2-dependent emission properties. The emission quantum yield, Φem, of this material is measured to be 7.7% under an inert N2 atmosphere, whereas it is quenched by 44% (Φem = 4.3%) on air. Similarly, the emission lifetime of 3, which is measured to be 9.24 μs under nitrogen, is visibly reduced to 5.78 μs in the presence of oxygen, showing a potential for the application of this kind of solid materials for O2 sensing. Complexes 5 and 6, which possess an extended Ag7 core, display considerably brighter emission than 2 and 3. An improved quantum efficiency might be attributed to a more rigid metal framework in 5 and 6 that decreases the number of radiationless deactivation pathways. No less important could be the rigidity of the ligand sphere and more pronounced πcoordination of the CC triple bonds to the silver ions that provides a better overlap of the ligands and metal orbitals. Because the alkynyl π* orbitals have a large contribution to the excited states (computational support, vide inf ra), the effective metal−ligand interaction facilitates spin−orbit coupling, consequently S1 → T1 intersystem crossing and emissive T1 → S0 transition that affords enhanced luminescence. Evidence for enhanced T 1 → S 0 transition is given by the phosphorescence radiative lifetime for 5 and 6, which is significantly shorter than that of 2 and 3 (see Table 1). The E


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Organometallics

Figure 3. Normalized solid-state excitation (left) and emission (right) spectra of 2, 3, 5−8 at room temperature.

to deep blue (λem = 454 nm, Figure 4) with essentially the same Φem. The observed alteration is reversible, and thorough vacuum drying restores the original lower energy band. The absence of satisfactory crystallographic data for 5, however, prevents unambiguous explanation of luminescence vapochromism. This sort of stimuli-responsive behavior has been described for a variety of organometallics compounds.26 In particular, for coinage metal alkynyl clusters, a vapor-induced variation of emission energy could be assigned to a number of reasons, namely, to the changes of intermolecular packing18a,19,27 and modulation of intramolecular metal−metal interactions18b,28 with or without solvent binding to the metals. The TD-DFT optimization of acetone-solvated cluster 5 both in the ground S0 and in the excited T1 states did not reveal an increase in emission energy. Therefore, in the absence of structural and theoretical evidence on the cluster ligation by the VOC molecules, the experimental finding presumably could be related to the modulation of solid-state packing and intermolecular interactions. Finally, the reorganization of the heptanuclear cluster framework of 7 leading to compound 8 has a dramatic influence on the photoemission and decreases the quantum yield along with hypsochromic shift of the emission band. The computational approach elaborated in the next section indicates that the contribution from d(Ag) to the lowest energy excited state in complex 8 is relatively small. This may lead to smaller spin−orbit coupling and hence low emission intensity. Though pending clear fundamental explanation, this observation highlights a crucial role of the metal core geometry on the physical properties of polynuclear aggregates. Computational Results. We also investigated the photophysical properties of the Ag(I) complexes 1−8 with quantum chemical methods. We first optimized the geometries of the studied complexes using the DFT-PBE0 method and studied the excited states by means of time-dependent DFT calculations (TDDFT-PBE0; see the Experimental Section for full computational details). The optimized geometries of the complexes 1−8 are in line with the available X-ray structures (the coordinates of the optimized structures are included as Supporting Information). The wavelengths predicted for the S0 → S1 and T1 → S0 electronic transitions of all studied complexes are listed in Table 2, and the corresponding electron density difference plots are shown in Figure 5 for representative complexes 3 and 6 (X = OMe). The electron density difference plots of other complexes are illustrated in the Supporting Information

Figure 4. Solid-state emission spectra of solvent-free and solvated forms of 5 at room temperature (inset shows appearance of 5 under UV excitation, λexc = 365 nm).

expediency of the cluster geometry of 5 and 6 for the design of luminophores is also supported by impressive values of quantum yields (up to 78%) reported for the heterometallic Ag6Cu complexes.19 The emission energies of 5 and 6 are slightly changed upon alteration of the −C2C6H4X alkynyl substituents X from H to OMe (515 and 510 nm, respectively). Complex 7 (X = NMe2) is isostructural to 5 and 6. However, the emission band of 7 is substantially red-shifted (λmax ∼ 725 nm) that reflects destabilization of the HOMO level and narrowing of the HOMO−LUMO gap due to the presence of electron-rich NMe2 functions. The emission with such a small energy gap may be quenched via a common mechanism dubbed as the “energy gap law”.25 This law specifies that, in the absence of a zero-order surface crossing, the deactivation pathway between the lowest-lying singlet (S1) or triplet (T1) states and the ground state (S0) can be facilitated by coupling between the zero vibration level of the S1 (or T1) state and higher vibration levels of the S0 state. This nonradiative process is greatly enhanced if the emission gap becomes smaller, particularly in the near-infrared region, rationalizing the much lower emission quantum yield of complex 7 (Φem = 4.3%) than that of 5 (24.5%) and 6 (17.6%). Interestingly, cluster 5 exhibits a distinct luminescence response when treated with certain volatile organics. While solvent-free crystalline sample 5 exhibits greenish emission (λem = 515 nm), exposure to acetone or methanol switches its color F


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Organometallics

from the triphosphine ligands, especially for the S0 → S1 excitation (for the relaxed T1 geometry and the T1 → S0 emission, the triphosphine ligand no longer plays a role). For the heptanuclear complexes 5−7, the triphosphine is also negligibly involved into the S0 → S1 excitation and T1 → S0 emission, which are almost mirror images of each other. The central Ag atom of the heptanuclear core has the largest contribution of all metal atoms, and the interplay between d(Ag) and π*(alkyne) orbitals is stronger than in the cases of 1−4. These clear electronic differences between S0 → S1 transitions of the hexa- and heptanuclear compounds along with alteration of the degree of metallophilic bonding are reflected by the bathochromic shift of λ (S0 → S1) absorption wavelengths. The calculated trend is completely in line with a distinct color change from white to yellow-green, observed for 5 and 6 with respect to their Ag6 counterparts 1 and 3.

Table 2. Computational Photophysical Results for the Complexes 1−8 in the Gas Phase (TDDFT-PBE0) λ (S0 → S1) (nm) 1 2 3 4 5 6 7 8 a

λ (T1 → S0) (nm)

theor.

exp.a

theor.

exp.a

353 355 369 364 393 401 450 448

n/a 372 410 n/a 405 440 576 508

497 511 496 502 524 537 592 608

n/a 485 500 n/a 515 510 725 570

Excitation and emission wavelengths from the solid state.

(Figures S5, S6). The predicted S0 → S1 excitation energies are slightly underestimated, but the relative trends are reproduced rather well. The T1 → S0 emission wavelengths listed in Table 2 are in the vicinity of the experimentally observed values, except for complex 7, where the predicted emission wavelength is clearly underestimated. For all complexes except 8, the lowest energy triplet state is clearly dominated by the d(Ag) and π*(alkyne) orbitals. In the case of 8, the contribution from d(Ag) to the lowest energy excited state is relatively small in comparison to the alkyne orbitals. The smaller metal−ligand interaction could result in smaller spin−orbit coupling, reduced ISC, and the experimentally observed low emission intensity. The hexanuclear clusters 1−4 show some contributions also

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CONCLUSIONS A series of novel polynuclear silver alkynyl-phosphine complexes were obtained using a simple protocol, which is based on the assembly of a metal core, stabilized by the bridging nature of the ligand environment, which involves the alkynyl and triphosphine moieties along with coordinating anions (CF3SO3−, CF3COO−). The nuclearity and the geometry of the cluster frameworks revealed their unusual dependence on the electronic properties of the constituting alkynyl groups. For the range of aromatic alkynes −C2C6H4-4X with the substituents varied from electron-accepting to

Figure 5. Electron density difference plots for the lowest energy singlet excitation (S0 → S1) and the lowest energy triplet emission (T1 → S0) of the complexes 3 and 6 (isovalue 0.002 a.u.). During the electronic transition, the electron density increases in the blue areas and decreases in the red areas. Hydrogen atoms omitted for clarity. G


Article

Organometallics

diethyl ether (4 cm3). The opaque mixture was filtered through cotton wool to remove some oily yellow admixture. The crude colorless product was precipitated by adding an excess of diethyl ether. Recrystallization by gas-phase diffusion of diethyl ether into an acetone solution of 2 gave colorless crystalline material (74 mg, 76%). IR (KBr, νCC, cm−1): 1964 (w), 1902 (w). Anal. Calcd for C104H74Ag6F18O9P6S3 (%): C, 45.61; H, 2.72; S, 3.51. Found: C, 45.67; H, 2.85; S, 3.53. [Ag6(C2C6H4-4-OMe)3{(PPh2)3CH}2(CF3SO3)3] (3). Prepared similarly to 1 from silver triflate (50 mg, 0.195 mmol), 1,1,1tris(diphenylphosphino)methane (37 mg, 0.065 mmol), and HC2− C6H4-4-OMe (13 mg, 0.098 mmol). Recrystallization by gas-phase diffusion of diethyl ether into an acetone solution of 3 gave colorless crystalline material (81 mg, 95%). IR (KBr, νCC, cm−1): 1999 (s), 1895 (w). Anal. Calcd for C104H83Ag6F9O12P6S3 (%): C, 47.59; H, 3.19; S, 3.67. Found: C, 47.53; H, 3.28; S, 3.63. [Ag6(C2C6H4-4-OMe)3{(PPh2)3CH}2(CF3CO2)3] (4). (AgC2C6H4-4OMe)n (32 mg, 0.134 mmol) was suspended in acetone (8 cm3) under a nitrogen atmosphere, followed by the addition of 1,1,1-tris(diphenylphosphino)methane (50 mg, 0.088 mmol) and silver(I) trifluoroacetate (30 mg, 0.136 mmol). The reaction mixture was stirred overnight to give a colorless suspension. It was diluted with hexane (12 cm3); the white precipitate was collected and washed with diethyl ether. Recrystallization by gas-phase diffusion of pentane into an acetone solution of 4 gave colorless crystalline material (91 mg, 81%). IR (KBr, νCC, cm−1): 2000 (s), 1956 (sh), 1891 (m). Anal. Calcd for C107H83Ag6F9O9P6 (%): C, 51.06; H, 3.32. Found: C, 50.95; H, 3.26. [Ag7(C2Ph)3{(PPh2)3CH}2(CF3SO3)3](CF3SO3) (5). (AgC2Ph)n (28 mg, 0.134 mmol) was suspended in acetonitrile (15 cm3) under a nitrogen atmosphere, followed by the addition of 1,1,1-tris(diphenylphosphino)methane (50 mg, 0.088 mmol) and silver(I) triflate (46 mg, 0.179 mmol). The reaction mixture was stirred for 2 h, then briefly heated to give a nearly clear pale solution. The solvent was evaporated; the solid residue was washed with diethyl ether and recrystallized by gas-phase diffusion of diethyl ether into an acetone/methanol solution of 5 at +5 °C to give colorless crystalline material (97 mg, 78%). IR (KBr, νCC, cm−1): 2016 (s), 1963 (w), 1894 (m). Anal. Calcd for C102H77Ag7F12O12P6S4 (%): C, 43.88; H, 2.78; S, 4.59. Found: C, 43.88; H, 2.82; S, 4.49. [Ag 7 (C 2 C 6 H 4 -4-OMe) 3 {(PPh 2 ) 3CH} 2 (CF 3 SO 3 ) 3 ](CF 3 SO 3 ) (6). (AgC2C6H4-4-OMe)n (24 mg, 0.100 mmol) was suspended in acetone (6 cm3) under a nitrogen atmosphere, followed by the addition of 1,1,1-tris(diphenylphosphino)methane (37 mg, 0.065 mmol) and silver(I) triflate (34 mg, 0.132 mmol). The reaction mixture was stirred overnight at room temperature. The resulting yellow-greenish solution was concentrated to the volume of ca. 4 cm3; the crude product was precipitated with diethyl ether (8 cm3) and sequentially washed with acetone/diethyl ether (1:2 v/v mixture, 2 × 6 cm3) and diethyl ether (2 × 5 cm3). Recrystallization by gas-phase diffusion of diethyl ether into an acetone/methanol solution of 6 at +5 °C gave yellow crystalline material (72 mg, 75%). IR (KBr, νCC, cm−1): 1997 (s), 1895 (vw). Anal. Calcd for C105H83Ag7F12O15P6S4 (%): C, 43.76; H, 2.90; S, 4.45. Found: C, 43.73; H, 2.98; S, 4.51. [Ag7(C2C6H4-4-NMe2)3{(PPh2)3CH}2(CF3SO3)3](CF3SO3) (7). Silver triflate (59 mg, 0.230 mmol) and 1,1,1-tris(diphenylphosphino)methane (37 mg, 0.065 mmol) were dissolved in acetone (4 cm3) under a nitrogen atmosphere. A colorless reaction mixture was treated dropwise with a solution of HC2C6H4-4-NMe2 (14.5 mg, 0.100 mmol) and NEt3 (3 drops, ca. 0.25 mmol) in acetone (2 cm3). The resulting deep red suspension was stirred for 1 h and diluted with diethyl ether (6 cm3). The precipitate was collected by centrifugation, washed with acetone/diethyl ether (1:2 v/v mixture, 2 × 6 cm3), diethyl ether (2 × 5 cm3) to give pure 7 (84 mg, 88%). Single crystals suitable for an Xray diffraction analysis were obtained by gas-phase diffusion of diethyl ether into an acetone/methanol solution of 7 at room temperature. IR (KBr, νCC, cm−1): 1968 (s). Anal. Calcd for C108H92Ag7F12N3O12P6S4 (%): C, 44.41; H, 3.18; N 1.44; S, 4.39. Found: C, 44.32; H, 3.26; N, 1.40; S, 4.12. [Ag7(C2C6H4-4-NMe2)4{(PPh2)3CH}2(CF3SO3)](CF3SO3)2 (8). Prepared similarly to 7 from silver triflate (59 mg, 0.230 mmol), 1,1,1-

moderately electron-donating (X = CF3, H, OMe), a twisted trigonal prismatic Ag6 (1−4) motif was identified. For X = H (5) and OMe (6), a cluster-to-cluster transformation was observed upon treating these species with silver ions to give a heptanuclear core, which is formed by incorporating an additional Ag+ center into the parent Ag6 prism. Such an Ag7 topology was found to be preferential for the electron-rich alkynyl (X = NMe2, 7), for which no Ag6 congener could be identified. On the contrary, for the particular case of NMe2functionalized alkyne, the Ag7 core can be transformed upon further alkynylation to give a capped prismatic arrangement (8) retaining the cluster nuclearity. In turn, changing the aromatic groups of the alkynes to aliphatic ones (tBu) results in the formation of an octanuclear metal core via edge fusing two butterfly Ag4 units (9). The obtained aggregates were found to be structurally nonrigid in the fluid medium due to the dynamic behavior associated with possible dissociation and stereochemical nonrigidity. In solid, the title clusters (except 1, 4, and 9) are phosphorescent at ambient conditions with maximum quantum efficiency reaching 27% and emission energies covering the range 485−725 nm. The heptanuclear cluster 5 (X = H, OMe) exhibits a distinct response to the vapors of acetone or methanol, which cause a remarkable hypsochromic shift of emission of nearly 160 nm with respect to the solvent-free sample, tentatively assigned to the rearrangement of the solidstate packing and thus modulation of the intermolecular interactions. The photophysical behavior of the photoemissive compounds was rationalized by means of TD-DFT computational studies, which point to a dominating contribution of the d(Ag) and π*(alkyne) orbitals into the emissive lowest energy triplet state for the prismatic compounds 2, 3, 5−7. The metal−alkynyl interactions, theoretically determined to be the strongest for the heptanuclear species 5−7, presumably account for the efficient spin−orbit coupling and pertinent emission of this structural pattern. On the other hand, a substantially smaller involvement of the d(Ag) orbitals to the T1 state for the capped prismatic core (8) could result in the observed dramatic drop of luminescence intensity.

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EXPERIMENTAL SECTION

General Comments. All reagents and solvents were used as received. The polymeric complexes (AgC2R)n (R = Ph, −C6H4-4OMe, −C6H4-4-NMe2, But) were prepared according to published procedures.29 Microanalyses were carried out at the analytical laboratory of the University of Eastern Finland. [Ag6(C2Ph)3{(PPh2)3CH}2(CF3SO3)3] (1). Silver triflate (50 mg, 0.195 mmol), 1,1,1-tris(diphenylphosphino)methane (37 mg, 0.065 mmol), and phenylacetylene (10 mg, 0.098 mmol) were dissolved in acetone (6 cm3) under a nitrogen atmosphere. A nearly colorless reaction mixture was treated with NEt3 (3 drops, ca. 0.25 mmol) to give a pale yellow solution, which was stirred for 1 h and then diluted with diethyl ether (10 cm3). The pale precipitate was collected by centrifugation, washed with acetone/diethyl ether (1:2 v/v mixture, 2 × 6 cm3), diethyl ether (2 × 5 cm3), and recrystallized by gas-phase diffusion of diethyl ether into an acetonitrile solution of 1 to give colorless crystalline material (76 mg, 91%). IR (KBr, νCC, cm−1): 2026 (m), 2005 (s), 1967 (m), 1900 (m). Anal. Calcd for C101H77Ag6F9O9P6S3·2NCMe (%): C, 48.19; H, 3.20; N 1.07; S, 3.68. Found: C, 48.38; H, 3.26; N, 1.20; S, 3.63. [Ag6(C2C6H4-4-CF3)3{(PPh2)3CH}2(CF3SO3)3] (2). Prepared similarly to 1 from silver triflate (55 mg, 0.214 mmol), 1,1,1tris(diphenylphosphino)methane (41 mg, 0.072 mmol), and HC2− C6H4-4-CF3 (19 mg, 0.112 mmol). The resulting pale yellow acetone solution was concentrated to the volume of ca. 2 cm3 and diluted with H


Article

Organometallics tris(diphenylphosphino)methane (37 mg, 0.065 mmol), and HC2− C6H4-4-NMe2 (19 mg, 0.131 mmol). Bright yellow microcrystalline material (91 mg, 95%). Single crystals suitable for an X-ray diffraction analysis were obtained by gas-phase diffusion of diethyl ether into an acetone/methanol solution of 8 at 5 °C. IR (KBr, νCC, cm−1): 1978 (s, br). Anal. Calcd for C117H102Ag7F9N4O9P6S3 (%): C, 48.19; H, 3.53; N 1.92; S, 3.30. Found: C, 48.03; H, 3.58; N, 1.86; S, 3.08. [Ag8(C2But)4{(PPh2)3CH}2(CF3SO3)2](CF3SO3)2 (9). (AgC2But)n (25 mg, 0.132 mmol) was suspended in acetone (6 cm3) under a nitrogen atmosphere, followed by the addition of 1,1,1-tris(diphenylphosphino)methane (37 mg, 0.065 mmol) and silver(I) triflate (34 mg, 0.132 mmol). The reaction mixture was stirred overnight at room temperature. The resulting pale suspension was diluted with diethyl ether (8 cm3); the colorless precipitate was collected and sequentially washed with acetone/diethyl ether (1:2 v/v mixture, 2 × 6 cm3) and diethyl ether (2 × 5 cm3). Recrystallization by gas-phase diffusion of diethyl ether into an acetone/methanol solution of 9 at +5 °C gave nearly colorless crystalline material (84 mg, 87%). IR (KBr, νCC, cm−1): 2032 (w), 1896 (vw). Anal. Calcd for C102H98Ag8F12O12P6S4 (%): C, 41.94; H, 3.38; S, 4.39. Found: C, 41.70; H, 3.36; S, 4.36. X-ray Structure Determinations. The crystals of 1−4 and 6−9 were immersed in cryo-oil, mounted in a Nylon loop, and measured at a temperature of 120 K (1 − at 150 K). The X-ray diffraction data were collected on a Bruker Kappa Apex II, Bruker SMART APEX II, or Bruker Kappa Apex II Duo diffractometers using Mo Kα radiation (λ = 0.71073 Å). The APEX230 program package was used for cell refinements and data reductions. The structures were solved by direct methods using the SHELXS-201431 programs with the WinGX32 graphical user interface. A semiempirical absorption correction (SADABS)33 was applied to all data. Structural refinements were carried out using SHELXL-2014.31 The counterions (in 1, 4, 8, and 9), the phenyl rings (in 1−3, 9), a CF3 group (in 2), and an alkynyl ligand (in 4) were modeled to be disordered over two sites each. In 3, two CF3SO3− fragments were placed in three positions with occupancies 0.57/0.54/0.89. In 7, the CF3SO3− anion was found near a special position and was refined with 0.333 occupancy. Some of the acetonitrile (in 1), acetone (in 4, 7, 8), and diethyl ether (in 8) crystallization solvent molecules were partially lost and, therefore, were refined with 0.5 occupancy. The geometry and displacement constraints and restraints were applied to these moieties, as well as to one alkynyl ligand in 3. Due to a significant disorder, the silver atoms Ag(10) in 4 and Ag(2) in 9 were found to populate two sites each with occupancies 0.89/0.11 and 0.65/0.35, respectively. The unspecified solvent molecules in the crystals of 2, 3, and 9 could not be resolved unambiguously and were omitted. The missing solvent was taken into account by using a SQUEEZE routine of PLATON.34 The contribution of the solvent to the cell content was not taken into account. All hydrogen atoms in 1−4, 6−9 were positioned geometrically and constrained to ride on their parent atoms, with C−H = 0.95−1.00 Å, and Uiso = 1.2−1.5Ueq (parent atom). The crystallographic data are given in Table S1 (Supporting Information). Photophysical Measurements. Steady-state emission measurements were recorded on an Edinburgh (FS920) fluorimeter. The conventional Xe lamp coupled with the fluorimeter was used as an excitation source for 2, 3, 5, ̧6, and 7, while the luminescence of 8 was excited with a diode laser. Both the wavelength-dependent excitation and emission responses of the fluorimeter were calibrated. The photoluminescence quantum yields in the solid state were determined on a calibrated integrating sphere with a luminescence spectrometer (HORIBA FluoroMax-4P). The uncertainty of the quantum yield measurement was in the range of ±5% (an average of three replications, which correspond to different orientations of the sample). Lifetime studies were performed with an Edinburgh FL 900 photon counting system using a hydrogen-filled lamp as the excitation source. The emission decays were fitted by the sum of the exponential functions with a temporal resolution of 300 ps by deconvolution of the instrument response function. Computational Details. The Ag(I) complexes 1−8 were studied using the hybrid PBE0 density functional method (DFT-PBE0).35 Ag

atoms were described by a triple-ζ-valence quality basis set with polarization functions (def2-TZVP). A split-valence basis set with polarization functions on non-hydrogen atoms was used for the other atoms (def2-SV(P)).36 A multipole-accelerated resolution-of-theidentity technique was used to speed up the calculations.37 To facilitate comparisons with the experiments, point group symmetry was applied as follows: 1−7: C3; 8: Cs. The geometries of all complexes were fully optimized. The excited states were investigated using the Time-Dependent DFT formalism.38 The singlet excitations were determined at the optimized ground state S0 geometries, while the lowest energy triplet emissions were determined at the optimized T1 geometry. All electronic structure calculations were carried out with the TURBOMOLE program package (version 7.0).39

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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.organomet.6b00866. Molecular views of selected complexes, excitation and emission spectra (PDF) X-ray crystallographic data for 1−4 and 6−9 (CIF) Optimized Cartesian coordinates of the studied systems (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: antti.j.karttunen@aalto.fi (A.J.K.). *E-mail: [email protected] (P.-T.C.). *E-mail: igor.koshevoy@uef.fi (I.O.K.). ORCID

Pi-Tai Chou: 0000-0002-8925-7747 Igor O. Koshevoy: 0000-0003-4380-1302 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Academy of Finland (grant 268993, I.O.K.) and the Ministry of Science and Technology in Taiwan are gratefully acknowledged. Computational resources were provided by CSC, the Finnish IT Center for Science (A.J.K.).

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REFERENCES

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