Structural Characterization and Luminescence Properties of a

Article pubs.acs.org/Organometallics

Structural Characterization and Luminescence Properties of a Triphosphine-Stabilized Ag16Cu9 Heterometallic Alkynyl Cluster Zhong-Hui Chen,† Li-Yi Zhang,† and Zhong-Ning Chen*,†,‡ †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People's Republic of China ‡ Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, People's Republic of China S Supporting Information *

ABSTRACT: A triphosphine-stabilized Ag16Cu9 heterometallic alkynyl cluster complex was prepared by the reaction of polymeric silver(I) (4-tert-butylphenyl)acetylide with a copper(I) dpepp (dpepp = bis(2-(diphenylphosphino)ethyl)phenylphosphine) complex and characterized by X-ray crystallography. This Ag16Cu9 heterometallic alkynyl complex exhibits an unprecedented structural topology stabilized by three auxiliary triphosphine ligands. The (4-tert-butylphenyl)acetylide exhibits five types of asymmetric bonding modes, including μ-η1, μ-η1(σ):η2(π), μ3-η1, μ3-η1(σ):η1(σ):η2(π), and η1-μ4. This Ag16Cu9 complex exhibits visible to near-infrared luminescence in both fluid CH2Cl2 solution and the solid state.

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INTRODUCTION The chemistry of metal alkynyl complexes continues to be an actively investigated topic which has significantly progressed over the past decade.1 In particular, owing to ubiquitous metallophilic interactions and σ/π bonding of the alkynyl ligands, coinage-metal alkynyl complexes exhibit rich structural diversity and remarkable photophysical and photochemical properties.2−7 On the one hand, numerous homonuclear coinage-metal cluster species have been successfully prepared through various synthetic procedures.1−5 On the other hand, heteronuclear coinage-metal alkynyl complexes have been much less explored because of the difficulty in controlling the heterometallic arrays and the tendency to form homonuclear arrays.6−22 To the best of our knowledge, the number of AgI− CuI heteronuclear alkynyl arrays23−28 is particularly limited, although a number of silver(I) or copper(I) acetylide homopolynuclear complexes with various nuclearities and topologies have been described. In our previous studies, a series of diphosphine-participating AuI−AgI and AgI−CuI heteronuclear alkynyl cluster complexes were obtained by the reaction of metal diphosphine (Ph2PCH2PPh2 or Ph2PNHPPh2) components with polymeric metal acetylides.8,9,23,24 It appears that the participation of diphosphine is favorable for stabilizing heteropolynuclear coinage metal alkynyl cluster structures. With different polyphosphines as auxiliary ligands, it is anticipated that heterometallic alkynyl cluster structures with various nuclearities and topologies are attainable.6−22 With this in mind, the flexible triphosphine dpepp (bis(2-(diphenylphosphino)ethyl)phenylphosphine) was utilized to prepare AgI−CuI hetero© 2011 American Chemical Society

nuclear alkynyl complexes. We describe herein the synthesis, structural characterization, and luminescence properties of the unprecedented Ag I −Cu I cluster complex [Ag 16 Cu 9 (μdpepp)3(CCC6H4But-4)20]5+ (Scheme 1) with (4-tertbutylphenyl)acetylide, stabilized by dpepp.

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RESULTS AND DISCUSSION The Ag16Cu9 complex (Scheme 1) was prepared by the reaction of 2 equiv of dpepp and 3 equiv of [Cu(CH3CN)4](ClO4) with ca. 8 equiv of polymeric silver acetylide (AgC CC6H4But-4)n in CH2Cl2. When silver acetylide was gradually dissolved in CH2Cl2 with stirring, the colorless suspension became red. Diffusion of diethyl ether into the dichloromethane solution afforded the Ag16Cu9 complex as deep red crystals in a few days, which were suitable for single-crystal Xray diffraction. ICP analysis of this heterometallic complex indicated that the molar ratio of Ag to Cu is 1.81, coinciding well with the calculated value (1.78). The Ag16Cu9 complex is soluble in dichloromethane, chloroform, and 1,2-dichloroethane. The ESI-MS exhibits a molecular ion peak at [M − (ClO4)5]5+ (m/z 1409) together with [M − (ClO4)4]4+ (m/z 1786) and [M − (ClO4)2]2+ (m/z 3672.5). The 1H NMR spectrum (Figure S1, Supporting Information) of the Ag16Cu9 cluster complex in CD2Cl2 at ambient temperature indicates that there exist 155 protons of C6H5 and C6H4, 24 protons of −CH2CH2−, and 180 protons of C4H9. The 31P NMR spectrum (Figure S2, Supporting Information) in CD2Cl2 Received: September 6, 2011 Published: December 7, 2011 256

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Scheme 1

Figure 1. Perspective view of the [Ag 16Cu 9(μ-dpepp) 3 (C CC6H4But-4)20]5+ cluster complex.

shows the presence three P signals at 6.3, 3.7, and 0.34 ppm with identical intensities for the triphosphine dpepp, implying that the Ag16Cu9 cluster structure is sufficiently stable in solution. Selected atomic distances and bond angles are presented in Table 1. Perspective views are shown in Figures 1 and 2. As Table 1. Selected Atomic Distances (Å) and Bond Angles (deg) of the Ag16Cu9 Complex Ag1−Cu1 Ag6−Cu3 Ag6−Cu1 Ag1−Ag4 Ag2−Ag4 Ag2−Ag3 Ag3−Ag5 Cu2−Cu3 Ag1−C35 Ag2−C35 Ag2−C108 Ag3−C71 Ag4−C59 Ag4−C35 Ag5−C118 Ag6−C96 Cu1−P1 Cu2−P2 Cu2−Cu3 Cu3−P3 C47−Ag1−C35 C35−Ag1−C71 C35−Ag2−C108 C47−Ag3−C71 C71−Ag3−C118 C59−Ag4−C71 C96−Cu1−P1 C83−Cu2−P2 C96−Cu3−P3 P3−Cu3−C59

2.725(2) 2.777(2) 3.055(2) 2.764(2) 2.7921(18) 3.2598(16) 2.9655(16) 3.0277(17) 1.896(18) 2.201(15) 2.610(4) 2.287(14) 1.831(16) 2.478(15) 1.94(2) 1.98(2) 2.403(4) 2.396(4) 3.0277(17) 2.423(4) 145.9(6) 102.4(6) 101.9(4) 147.9(5) 102.7(3) 160.2(6) 153.0(4) 163.4(4) 128.9(4) 131.3(4)

Ag4−Cu2 Ag6−Cu2 Ag1−Ag2 Ag1−Ag3 Ag2−Ag5 Ag3−Ag4 Ag3−Ag6 Ag1−C47 Ag1−C71 Ag2−C59 Ag3−C47 Ag3−C118 Ag4−C71 Ag5−C108 Ag6−C83 Cu1− C96 Cu2−C83 Cu2−C84 Cu3−C96 Cu3−C59 C47−Ag1−C71 C35−Ag2−C59 C59−Ag2−C108 C47−Ag3−C118 C59−Ag4−C35 C71−Ag4−C35 C96−Cu1−Ag1 P2−Cu2−C84 C96−Cu3−C59

3.0771(19) 3.1094(19) 2.8141(19) 2.8143(17) 3.0354(17) 2.8435(17) 3.223(2) 1.830(14) 2.243(15) 2.241(14) 2.255(13) 2.534(4) 1.931(17) 1.90(3) 1.915(18) 2.284(14) 2.229(14) 2.535(17) 2.380(15) 2.473(14) 110.0(6) 149.5(5) 106.7(4) 108.5(3) 106.1(6) 93.4(6) 101.9(4) 138.9(4) 99.1(5)

Figure 2. Views of (a, top) the Ag16Cu9 cluster complex with the phenyl groups omitted and (b, bottom) the triphosphine-stabilized Ag16Cu9 cluster core.

which are encircled by 20 (4-tert-butylphenyl)acetylide and 3 dpepp groups. It is obvious that the Ag16Cu9 cluster structure is remarkably stabilized by the three auxiliary triphosphine

depicted in Figure 1, the Ag16Cu9 heterometallic cluster complex consists of 16 silver(I) and 9 copper(I) atoms, 257

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ligands. The Ag5 atom (Figure 2) is situated at the center of the Ag16Cu9 cluster with a C3 axis through this atom. The dpepp adopts a bridging mode with three P donors bound to three copper(I) centers, forming six-membered and eight-membered coordination rings. The acetylides exhibit five types of asymmetric bonding modes, including μ-η1, μ-η1(σ):η2(π), μ3η1, μ3-η1(σ):η1(σ):η2(π), and η1-μ4. The AgI−CuI centers are linked by the acetylides in μ(Ag,Cu)-η 1 (σ):η 2 (π), μ3(Ag,Ag,Cu)-η1(σ), and μ3(Ag,Cu,Cu)-η1(σ):η1(σ):η2(π) modes, in which the Cu−C lengths (2.22−2.54 Å) are always much longer than those of Ag−C (1.83−2.61 Å), implying that silver(I)−acetylide bonds are more stable than copper(I)− acetylide bonds. The copper(I) centers are surrounded by one C and one P donor to afford a curved linear geometry for the Cu1 and Cu2 centers and by two C and one P donor to give a distortedtrigonal-planar environment for the Cu3 center (Figure 2a). The C−Cu−P angle is 153.0(4)° for Cu1 and 163.4(4)° for the Cu2 center. The bond angles around the Cu3 center are 131.3(4), 128.9(4), and 99.1(5)° for C59−Cu3−P3, C96− Cu3−P3, and C59−Cu3−C96, respectively. The silver(I) centers are coordinated by three acetylide C donors to form a trigonal-planar geometry for the Ag1−Ag4 centers, whereas the Ag5 and Ag6 centers are located in a quasi-linear environment built by two acetylide C donors. Figure 2b depicts an unprecedented fanlike Ag16Cu9 cluster structure formed through Ag−Ag and Ag−Cu linkages, which is stabilized by the flexible triphosphine Ph2CH2CH2PPhCH2CH2PPh2. This is significantly different from those of diphosphine PPh2NHPPh2- or PPh2PCH2PPh2participating Ag4Cu2 or Ag6Cu2 alkynyl cluster complexes23 or Ag8Cu2 cluster complexes stabilized by the rigid diphosphine pPh2PC6H4PPh2.25 The Ag16Cu9 cluster is C3 symmetric through the Ag5 center. The Ag···Cu distances are in the range 2.72− 3.11 Å, and the Cu···Cu distance is 3.0277(17) Å. The Ag···Ag distances are in the range 2.76−3.26 Å. The considerably short Ag···Cu, Cu···Cu, and Ag···Ag distances imply the presence of significant metallophilic interactions in the Ag16Cu9 cluster core.23−28 The UV−vis spectrum of the Ag16Cu9 complex (Figure 3) in CH2Cl2 solution exhibits an intense absorption with a

Table 2. Luminescence Data for the Ag16Cu9 Complex λem/nm (τem/μs) medium

298 K

77 K

CH2Cl2 crystalline ground

650 (0.36) 675 (0.43), 812 (0.40) 693 (0.99)

687 (76.7) 678 (67.1), 860 (23.6) 720 (69.2)

state. With reference to heterometallic CuI−AgI alkynyl complexes described in the literature,23,25 the emission of the Ag16Cu9 complex is likely derived from a Ag16Cu9 clustercentered triplet excited state modified by metal−metal interactions, in view of the short Cu−Cu, Cu−Ag, and Ag− Ag contacts, mixed probably with a 3[CCC6H4Bu-4→ Ag16Cu9] 3LMCT transition. In comparison with Ag4Cu2, Ag6Cu2, and Ag8Cu2 alkynyl cluster complexes that usually emit at 450−600 nm in solution,23,25 the emission of the highnuclearity Ag16Cu9 cluster (650−860 nm) is largely red-shifted to the infrared region due likely to much more significant metallophilic interactions. The relatively weak emission with a maximum at 650 nm (Figure 3) and a lifetime of 0.36 μs in fluid CH2Cl2 solution at 298 K arises likely from vibrational and rotational nonradiative relaxation of the peripheral methylene in dpepp. Another possible factor lowering the emissive efficiency is the unsaturated coordination of copper(I) (two-coordination for Cu1 and Cu2) and silver(I) (two-coordination for Ag5 and Ag6) atoms which are susceptible to the solvents, thus quenching significantly the solution luminescence. In frozen CH2Cl2 glass at 77 K, however, the Ag16Cu9 complex is highly emissive with a maximum at 687 nm and a lifetime of 76.7 μs, since the vibration or wobbling from the peripheral methylene in dpepp would be significantly constrained in a rigid medium such as a frozen glassy state. The Ag 16 Cu 9 complex exhibits intriguing solid-state luminescence. Upon irradiation in the solid state with excitation light λex >300 nm at ambient temperature, bright red luminescence is observed with an emission maximum at 675 nm (lifetime 0.43 μs) together with a lower energy emission shoulder at 812 nm (lifetime 0.40 μs) extending to 1300 nm (Figure 4). At 77 K, the solid-state emission occurs at 678 and 860 nm and the corresponding lifetimes are prolonged to 67.1 and 23.6 μs, respectively. When the crystals of Ag16Cu9 complex were mechanically ground at ambient temperature, the near-infrared emission at 812 nm disappeared, whereas that at 675 nm was red-shifted to 693 nm, as depicted in Figure 4. Similarly, the near-infrared emission of the crystal species at 860 nm is unobserved in the ground sample at low temperature (77 K), whereas the emission at 678 nm is significantly red-shifted to 720 nm in response to mechanical grinding. Interestingly, the XRD (X-ray diffraction) pattern in the crystalline species disappeared entirely when it was thoroughly ground. This suggests that the regular crystalline packing in the crystal lattices was totally destroyed by mechanical grinding, so that the ground sample was in a metastable amorphous phase.29−33 Nevertheless, the crystalline arrangement could be perfectly reverted when the ground powder sample was crystallized in dichloromethane solution. The significant emission spectral changes, including the disappearance of a near-infrared emission shoulder together with a distinct red shift of the emission band with a maximum from 675 nm to 693 nm (298 K) or 678 nm to 720 nm (77 K),

Figure 3. UV−vis absorption (blue) and emission (red) spectra of the Ag16Cu9 complex in fluid CH2Cl2 solution at 298 K.

maximum at 270 and a shoulder at 310 nm, together with a low-energy shoulder at ca. 370 nm tailing to 550 nm. Upon excitation at λex >300 nm, the Ag16Cu9 complex is luminescent in both dichloromethane solution and the solid state at 298 and 77 K (Table 2). Considerably large Stokes shifts and a microsecond range of luminescent lifetimes indicate that the emission is phosphorescent in character with a triplet excited 258

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Table 3. Crystallographic Data of the Ag16Cu9 Complex empirical formula formula wt cryst syst space group a, Å b, Å c, Å γ, deg V, Å3 Z ρcalcd, g/cm−3 μ,mm−1 radiation (λ, Å) temp, K R1(Fo)a wR2(Fo2)b GOF a

R1 = ∑|Fo − Fc|/∑Fo. bwR2 = ∑[w(Fo2 − Fc2)2]/∑[w(Fo2)]1/2.

(ClO4)5]5+, 1786 [M − (ClO4)4]4+, 3672.5 [M − (ClO4)2]2+. 1H NMR (400 MHz, CD2Cl2, ppm): 6.08−7.48 (m, 155H, C6H5 and C6H4), 2.28−3.43 (m, 24H, CH2), 0.98−1.39 (m, 180H, CH3). 31P NMR (202.3 MHz, CD2Cl2, ppm): δ 6.3, 3.7, 0.34. IR (KBr, cm−1): 2007 m (CC), 1093s (ClO4−). Physical Measurements. The 1H and 31P NMR spectra were measured on a Bruker Avance III (400 MHz) spectrometer. The UV− vis absorption spectra were measured on a Perkin-Elmer Lambda 25 UV−vis spectrophotometer. The infrared spectra (IR) were recorded on a Magna 750 FT-IR spectrophotometer with KBr pellets. The elemental analyses (C, H, N) were carried out on a Perkin-Elmer Model 240 C elemental analyzer. The Ag and Cu content analyses were performed on an Ultima2 Inductively Coupled Plasma (ICP) OES spectrometer. The electrospray ion mass spectra (ESI-MS) were recorded on a Finnigan DECAX-30000 LCQ mass spectrometer using dichloromethane−methanol as the mobile phase. The emission and excitation spectra in the UV−vis region were recorded on a PerkinElmer LS 55 luminescence spectrometer with a red-sensitive photomultiplier, type R928. The near-infrared (near-IR) emission spectra were measured on an Edinburgh Analytical Instrument FLS920 fluorescence spectrometer. The emission lifetimes in the solid state and degassed solutions were determined on a FLS920 fluorescence spectrometer using an LED laser at 397 nm excitation, and the resulting emission was detected by a thermoelectrically cooled Hamamatsu R3809 photomultiplier tube. The instrument response function at the excitation wavelength was deconvolved from the luminescence decay. Crystal Structure Determination. Single crystals sealed in capillaries with mother liquors were measured on a Mar CCD 165 nm diffractometer by ω-scan techniques at 293 K at the Beijing Synchrotron Radiation Facility with the 3W1A beam (λ = 0.800 00 Å). The structures were solved by direct methods, and the heavy atoms were located from an E map. The remaining non-hydrogen atoms were determined from the successive difference Fourier syntheses. The nonhydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were generated geometrically with isotropic thermal parameters. The structures were refined on F2 by full-matrix least-squares methods using the SHELXTL-97 program package.35 The unit cell contains 60 perchlorate anions which have been treated as a diffuse contribution to the overall scattering without specific atom positions by SQUEEZE/PLATON. The crystallographic data are summarized in Table 3.

Figure 4. Emission spectra of crystalline Ag16Cu9 complex (blue) and mechanically ground sample (red) at (a) 298 K and (b) 77 K.

are probably relevant to molecular twisting, unordered packing, or lattice energy loss induced by mechanical stimuli.29−33

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CONCLUSIONS A triphosphine-stabilized Ag16Cu9 alkynyl cluster complex with an unprecedented structural topology was successfully obtained and characterized by X-ray crystallography. This Ag16Cu9 cluster complex is emissive in the visible to near-infrared region with distinctly mechanical grinding responsive emission switching. The isolation of the Ag16Cu9 cluster affords a new approach to accessing heterometallic alkynyl clusters using polyphosphine ligands to stabilize aggregating structures with higher nuclearity.

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C342H359Ag16Cl5Cu9O20P9 7543.05 rhombohedral R3c 33.935(5) 112.33(2) 112.33(2) 120.00 112031(32) 12 1.342 1.442 0.800 00 293(2) 0.1041 0.2423 1.193

EXPERIMENTAL SECTION

General Procedures and Materials. All operations were carried out under a dry argon atmosphere by using Schlenk techniques at ambient temperature and a vacuum-line system unless specified. The solvents were dried, distilled, and degassed prior to use, except those for spectroscopic measurements were of spectroscopic grade. [Cu(MeCN)4](ClO4) and (AgCCC6H4But-4)n were prepared by the literature procedures. 3 4 (HCCC 6 H 4 Bu t -4) and bis(2(diphenylphosphino)ethyl)phenylphosphine (dpepp) were commercially available. Caution! Silver acetylide and copper(I) perchlorate are potentially explosive and should be handled with care and in small amounts. [Ag16Cu9(μ-dpepp)3(CCC6H4But-4)20](ClO4)5. In a Schlenk flask containing 30 mL of dichloromethane was added dpepp (106 mg, 0.2 mmol) and [Cu(MeCN)4](ClO4) (98 mg, 0.3 mmol) with stirring at ambient temperature for 3 h. To the solution was added (AgCCC6H4But-4)n (212 mg, 0.8 mmol), with the color changing gradually to deep red. The suspended solution was then stirred at room temperature for 12 h to afford a clear red solution. Layering diethyl ether onto the concentrated dichloromethane solution gave the product as red crystals. Yield: 43%. Anal. Calcd for C342H359Ag16Cl5Cu9O20P9: C, 54.45; H: 4.80. Found: C, 54.53; H, 5.13. ICP analysis: Ag, 21.53 (calcd 22.88); Cu 6.98 (calcd 7.58) with Ag/Cu content 1.81 (calcd 1.78). ESI-MS (m/z): 1409 [M − 259

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving NMR spectra, additional crystal structure views, and excitation and emission spectra and a CIF file giving X-ray crystallographic data for the structure determination. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

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ACKNOWLEDGMENTS

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REFERENCES

Corresponding Author *E-mail: [email protected].

We acknowledge financial support from the NSFC (Nos. 20931006, U0934003 and 91122006), the 973 project (No. 2007CB815304) from the MSTC, and the NSF of Fujian Province (No. 2011J01065).

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