Cyclometalated Platinum(II) Cyanometallates: Luminescent Blocks for

Article pubs.acs.org/IC

Cyclometalated Platinum(II) Cyanometallates: Luminescent Blocks for Coordination Self-Assembly Leon Schneider,†,‡,¶ Vasily Sivchik,‡,¶ Kun-you Chung,≠ Yi-Ting Chen,≠ Antti J. Karttunen,*,§ Pi-Tai Chou,*,≠ and Igor O. Koshevoy*,‡ †

Institut für Anorganische Chemie, Julius-Maximilians-Universität, Würzburg, Germany Department of Chemistry, University of Eastern Finland, 80101 Joensuu, Finland ≠ Department of Chemistry, National Taiwan University, Taipei 106, Taiwan § Department of Chemistry, Aalto University, FI-00076 Aalto, Finland ‡

S Supporting Information *

ABSTRACT: A family of cyanide-bridged heterometallic aggregates has been constructed of the chromophoric cycloplatinated metalloligands and coordinatively unsaturated d10 fragments {M(PPh3)n}. The tetranuclear complexes of general composition [Pt(C^N)(CN)2M(PPh3)2]2 [C^N = ppy, M = Cu (1), Ag (2); C^N = tolpy (Htolpy = 2-(4-tolyl)pyridine), M = Cu (4), Ag (5); C^N = F2ppy (HF2ppy = 2-(4, 6-difluorophenyl)-pyridine), M = Cu (7), Ag (8)] demonstrate a squarelike arrangement of the molecular frameworks, which is achieved due to favorable coordination geometries of the bridging ligands and the metal ions. Variation of the amount of the ancillary phosphine (for M = Ag) afforded compounds [Pt(C^N)(CN)2Ag(PPh3)]2 (C^N = ppy, 3; C^N = tolpy, 6); for the latter one an alternative cluster topology, stabilized by the Pt−Ag metallophilic and η1-Cipso(C^N)−Ag bonding, was observed. The solid-state structures of all of the title species 1−8 were determined crystallographically. The complexes exhibit moderately strong room-temperature phosphorescence as crystalline powders (Φem = 16−34%, λem = 470−511 nm). The luminescence studies and time-dependent density functional theory computational analysis indicate that the photophysical behavior is dominated by the 3π−π* electronic transitions localized on the cyclometalated fragment and mixed with MPtLCT contribution, while the d10-phosphine motifs have a negligible contribution into the frontier orbitals and therefore show a little influence on the emission performance of the described compounds.

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the area of spin-crossover and ferromagnetic materials.6,9−15 From the photophysical viewpoint, cyano ligands extend the conjugation of the molecular system that promotes electronic communication and intramolecular charge/energy transfer between the connected components, opening a pathway to metal-based materials with attractive optical characteristics.16−18 For instance, the d10 cyanometallates have been involved in the fabrication of birefringent crystals,19 luminescent compounds,20−24 and stimuli-responsive materials, which exhibit a detectable alteration of light-emissive properties upon interaction with a variety of analytes.25−27 Furthermore, cyanide-containing d8 complexes, particularly those of PtII, were used as nodes for the construction of homoand heteronuclear polymetallic species, which adopt versatile structural arrangements and demonstrate tunable photophysical behavior.28 The notable examples include solid-state sensitization of lanthanide emission by tetracyanoplatinate [Pt(CN)4]2− units29,30 and mixed [Au2Pt2(CN)10]4−/[Au2Pt4(CN)20]10−

INTRODUCTION The cyanide anion as a compact and robust bridging ligand has been extensively used in coordination chemistry to afford numerous inorganic and organometallic supramolecular aggregates. Its pronounced heterodentate nature offers facile opportunities to combine electronically different metal centers and therefore to design functional materials with intriguing physical and chemical behavior.1−5 Homoleptic cyano complexes (cyanometallates) [M(CN)x]y− have proven to be exceptionally efficient building blocks for the preparation of a wide range of ordered assemblies (polymeric chains, layers, and three-dimensional frameworks) due to a high directionality and rigidity of bonding with a variety of cations of main group, transition, and rare-earth elements.3 Similarly, heteroleptic [LzM(CN)x]y− motifs have been successfully employed to produce an impressive diversity of infinite arrays of lower dimensionality along with discrete molecular architectures, which comprise clusters, cages, and polygons.1,6−8 In addition to the synthetic benefits, the short cyanide spacers are capable of providing efficient exchange coupling between the magnetic units that have substantially stimulated © 2017 American Chemical Society

Received: January 3, 2017 Published: March 30, 2017 4459

DOI: 10.1021/acs.inorgchem.7b00006 Inorg. Chem. 2017, 56, 4459−4467

Article

Inorganic Chemistry cyano clusters,31 metallophilicity-driven assembly of luminescent coordination polymers incorporating main-group metal ions (Tl, Pb) and [Pt(R)2(CN)2]2− linkers (R = C6F5, C2Tol),32,33 formation of CN-bridged macrocyclic supramolecular systems [Pt(L)2(CN)]n (L = dicarbene, diamine).5,34 Noteworthy, the PtII compounds bearing π-conjugated cyclometalated ligands (e.g., of C^N, N^N^C, and N^C^N types) represent an appealing class of luminophores with exceptional optical properties,28,35 mainly determined by the 3 ππ* intraligand and 3metal-to-ligand charge transfer states. Despite some cyclometalated complexes [Pt(C^N) (CN)2]−,36−38 [Pt(C^N)(CN)(CNR)] (HC^N = phenylpyridine, benzoquinoline),39 [Pt(N^C^N)(CN)] (N^CĤ N = 1,3-di(2-pyridyl)benzene), 40 and [Pt(C^N^C)(CN)] (HC^N^CH = 1,6-diphenylpyridine)41 functionalized with cyanide ligands have been reported and showed an important role of CN− groups in modulating and enhancing photoemission performance, there has been virtually no subsequent development with respect to supramolecular construction using these sort of chromophore blocks. Currently, the only related report concerns [PtTl(C^N)(CN)2] polymeric species, which show thermochromic luminescence.42 Herein, we present a facile utilization of the photofunctional cis-[Pt(C^N)(CN)2]− anionic metalloligands for the construction of heterometallic tetranuclear Pt2M2 (M = CuI, AgI) aggregates. The resulting complexes, which contribute to a virtually unexplored family of PtII−CN−MI bridged compounds,43 were obtained via cyanide-driven coordination selfassembly. Their crystallographic characterization reveals a subtle dependence of the structural motif on the electronic properties of metalated C^N moiety, which also determines the photoluminescent behavior as shown by the computational studies.

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8.92 (br s, 2H), 8.07 (d, JHH 4.3 Hz, 4H), 7.73 (d, JHH 7.7 Hz, 4H), 7.45−7.30 (m, 60H), 7.17 (br s, 2H), 7.08 (dd, JHH ca. 7.5 Hz, 2H), 6.96 (br, 2H). IR (KBr, ν(CN), cm−1): 2145, 2131. Anal. Calcd for C98H76Cu2N6P4Pt2·CH2Cl2: C, 57.62; H, 3.81; N, 4.07. Found: C, 57.43; H, 3.87; N, 4.06%. [Pt(ppy) (CN)2Ag(PPh3)2]2 (2). Prepared analogously to 1 using AgNO3 (24 mg, 0.135 mmol) instead of [Cu(NCMe)4]PF6. Light green solid, 102 mg, 74%. Single crystals suitable for XRD analysis were obtained by a slow evaporation of a dilute dichloromethane/ acetonitrile solution of 2 at room temperature. 1H NMR (DMSO-d6, 298 K, δ): 9.15 (dd JHPt ca. 30 Hz, JHH 5.5 Hz, 2H, 6-H py), 8.09−8.03 (m, 4H, 3,4-H py), 7.86 (ddd, JHPt ca. 46 Hz, JHH 7.1 and 1.7 Hz, 2H, 3-H Ph), 7.71 (dd, JHH 7.5, 1.5 Hz, 2H, 6-H Ph), 7.50 (t, JHH 7.2 Hz, 12H, para-H PPh3), 7.44−7.34 (m, 48H, ortho + meta-H PPh3), 7.30 (ddd, JHH 5.8, 5.5, and 2.7 Hz, 2H, 5-H py), 7.06 (ddd, JHH 7.3, 7.1, and 1.5 Hz, 2H, 4-H Ph), 7.02 (ddd, JHH 7.5, 7.3, and 1.7 Hz, 2H, 5-H Ph). IR (KBr, ν(CN), cm−1): 2141, 2126. Anal. Calcd for C98H76Ag2N6P4Pt2·CH2Cl2: C, 55.24; H, 3.65; N, 3.90. Found: C, 55.43; H, 3.76; N, 3.95%. [Pt(ppy) (CN)2Ag(PPh3)]2 (3). [Pt(ppy) (DMSO)Cl] (60 mg, 0.130 mmol) was suspended in acetonitrile (5 cm3), and a solution of AgPF6 (33 mg, 0.130 mmol) in acetonitrile (5 cm3) was added. The resulting suspension was stirred for 30 min in the absence of light; then, the precipitate of AgCl was filtered off, and the pale yellow transparent reaction mixture was treated with a solution of K[Ag(CN)2] (26.0 mg, 0.131 mmol) and PPh3 (34 mg, 0.130 mmol) in methanol/ dichloromethane (10 cm3, 4:1 v/v mixture). The stirring was continued for 1 h. Then the volatiles were evaporated; solid residue was washed with methanol (2 × 5 cm3) and acetone (2 × 5 cm3) and recrystallized by a slow evaporation of a dichloromethane/acetonitrile solution of 3 at room temperature to give light yellow-green crystalline material (51 mg, 51%). 1H NMR (DMSO-d6, 298 K, δ): 9.18 (dd, JHPt ca. 30 Hz, JHH 4.8 Hz, 2H, 6-H py), 8.11−8.06 (m, 4H, 3,4-H py), 7.90 (dm, JHPt ca. 44.0 Hz, 2H, 3-H Ph), 7.73 (m, 2H, 6-H Ph), 7.56−7.44 (m, 30H, PPh3), 7.37 (ddd, JHH 5.7, 4.8, and 2.8 Hz, 2H, 5-H py), 7.07 (m, 4H, 4,5-H Ph). IR (KBr, ν(CN), cm−1): 2141, 2124. Anal. Calcd for C62H46Ag2N6P2Pt2: C, 48.26; H, 3.01; N, 5.45. Found: C, 48.28; H, 3.07; N, 5.44%. [Pt(tolpy) (CN)2Cu(PPh3)2]2 (4). Prepared similarly to 1 from [Pt(tolpy) (DMSO)Cl] (48 mg, 0.101 mmol). Light green solid, 88 mg, 87%. Single crystals suitable for XRD analysis were obtained by a gas-phase diffusion of methanol into a dilute solution of 4 in DMSO at room temperature. 1H NMR (DMSO-d6, 298 K, δ): 9.00 (br, 2H), 8.08−8.01 (m, 4H), 7.63 (m, 4H), 7.49−7.35 (m, 60H), 7.30 (br, 2H), 6.91 (d, JHH 8.0 Hz, 2H), 2.19 (s, 6H). IR (KBr, ν(CN), cm−1): 2151, 2131. Anal. Calcd for C100H80Cu2N6P4Pt2: C, 59.85; H, 4.02; N, 4.19. Found: C, 59.49; H, 4.05; N, 4.20%. [Pt(tolpy) (CN)2Ag(PPh3)2]2 (5). Prepared similarly to 1 from [Pt(tolpy) (DMSO)Cl] (60 mg, 0.126 mmol), AgNO3 (21.5 mg, 0.126 mmol) instead of [Cu(NCMe)4]PF6, and using 3 equiv of PPh3 (99 mg, 0.378 mmol). The resulting reaction mixture was filtered to remove small amount of a yellow precipitate and left to evaporate slowly at room temperature. The greenish crystals of 5 were collected and were additionally recrystallized by a slow evaporation of a dichloromethane/methanol/acetonitrile solution of 5 at room temperature (52 mg, 39%). 1H NMR (DMSO-d6, 298 K, δ): 9.12 (dd, JHPt ca. 30 Hz, JHH 5.6 Hz, 2H, 6-H py), 8.02 (ddd, JHH 8.3, 7.2, and 1.6 Hz, 2H, 4-H py), 7.98 (d, JHH 8.3 Hz, 2H, 3-H py), 7.70 (dd, JHPt ca. 43, JHH 1.2 Hz, 2H, 3-H MePh), 7.59 (d, JHH 7.9 Hz, 2H, 6-H MePh), 7.50 (t, JHH 7.4 Hz, 12H, para-H PPh3), 7.43−7.34 (m, 48H, ortho + metaH PPh3), 7.25 (ddd, JHH 7.2, 5.7, and 1.6 Hz, 2H, 5-H py), 6.87 (dd, JHH 7.9, 1.2 Hz, 2H, 5-H MePh), 2.16 (s, 6H, CH3). IR (KBr, ν(CN), cm−1): 2141, 2124. Anal. Calcd for C100H80Ag2N6P4Pt2: C, 57.32; H, 3.85; N, 4.01. Found: C, 57.67; H, 3.91; N, 3.82%. [Pt(tolpy)(t-CN)(μ-CN)Ag(PPh3)]2 (6). Prepared similarly to 5 using 1 equiv of PPh3 (33 mg, 0.126 mmol). After all the components were mixed, the reaction mixture was stirred overnight at room temperature resulting in a yellow suspension. The precipitate was collected by centrifugation, washed with methanol (2 × 5 cm3) and dichloromethane (2 × 5 cm3), and dried to give yellow crystalline material (72

EXPERIMENTAL SECTION

General Comments. The cyclometalated complex [Pt(ppy) (DMSO)Cl] (DMSO = dimethyl sulfoxide, Hppy = 2-phenylpyridine) was prepared according to the reported procedures.44 The congener compounds [Pt(tolpy) (DMSO)Cl] and [Pt(F2ppy) (DMSO)Cl] (Htolpy = 2-(4-tolyl)-pyridine, HF2ppy = 2-(4, 6-difluorophenyl)pyridine) were obtained similarly from the previously synthesized [Pt(tolpy)Cl]2 45 and [Pt(F2ppy)(H−F2ppy)Cl]46 species by dissolving in a minimum volume of DMSO and precipitating with water. Other reagents were used as received. The solution 1H and 31P{1H} NMR and 1H−1H COSY spectra were recorded on Bruker 400 MHz Avance and AMX 400 spectrometers. The infrared spectra were measured on Shimadzu FTIR-8400S. Microanalyses were performed at the analytical laboratory of the University of Eastern Finland. The syntheses described below were performed under aerobic conditions at room temperature. Caution! Metal cyanides are very toxic and dangerous for the environment. [Pt(ppy) (CN)2Cu(PPh3)2]2 (1). [Pt(ppy) (DMSO)Cl] (62 mg, 0.130 mmol) was suspended in dichloromethane (4 cm3), and a solution of AgNO3 (23 mg, 0.135 mmol) in acetonitrile/methanol (6 cm3, 2:1 v/v mixture) was added. The resulting suspension was stirred for 30 min in the absence of light; then, the precipitate of AgCl was filtered off, and pale yellow transparent reaction mixture was treated with a solution of NaCN (13.3 mg, 0.271 mmol) in methanol (4 cm3), followed by a solution of [Cu(NCMe)4]PF6 (50 mg, 0.134 mmol) and PPh3 (71 mg, 0.271 mmol) in dichloromethane (4 cm3). Stirring was continued for 3 h; the resulting light yellow-greenish precipitate was collected by centrifugation, washed with methanol (2 × 5 cm3) and dichloromethane (2 × 5 cm3), and vacuum-dried to give pure 1 (98 mg, 75%). Single crystals suitable for X-ray diffraction (XRD) analysis were obtained by a gas-phase diffusion of methanol into a dilute solution of 1 in DMSO at room temperature. 1H NMR (DMSO-d6, 298 K, δ): 4460


Article

Inorganic Chemistry Scheme 1. Synthesis of Complexes 1−8

respectively; the geometry and displacement constraints and restraints were applied to these moieties. The dichloromethane solvent molecule in 3a was as well-disordered between two sites and was refined with occupancies of 0.56/0.44; both components were geometrically restrained. The crystallization solvent in the crystals of 1, 2, 7, and 8 was heavily disordered and could not be resolved unambiguously. The contribution of the missing solvent to the calculated structure factors was taken into account by using a SQUEEZE routine of PLATON.52 The missing solvent was not taken into account in the unit cell content. All H atoms in 1−8 were positioned geometrically and constrained to ride on their parent atoms, with C−H = 0.95−0.99 Å and Uiso = 1.2−1.5Ueq (parent atom). The crystallographic details are summarized in Table S1, Supporting Information. Photophysical Measurements. Steady-state absorption and emission measurements were recorded on a Hitachi UV−vis spectro-photometer (U-2900) and Edinburgh (FS920) fluorimeter, respectively. Both the wavelength-dependent excitation and emission responses of the fluorimeter were calibrated. The photoluminescence quantum yields in the solid state were measured on a calibrated integrating sphere with 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. Upon deconvolution of the instrument response function the temporal resolution of the system is ∼300 ps. Computational Details. Complexes 1−8 were studied using the hybrid PBE0 density functional theory method (DFT-PBE0).53,54 Pt, Cu, and Ag atoms were described by a triple-ζ-valence quality basis set with polarization functions (def2-TZVP).55 Scalar relativistic effects were taken into account by employing 28- and 60-electron relativistic effective core potentials for Ag and Pt, respectively.56 A split-valence basis set with polarization functions on non-hydrogen atoms was used for the other atoms (def2-SV(P)).55 Multipole-accelerated resolutionof-the-identity technique was used to speed the calculations.57−59 To facilitate comparisons with the experiments Ci point group symmetry was applied for all complexes. The geometries of all complexes were first fully optimized using the DFT-PBE0 method. 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 excited states were investigated using the time-dependent (TD) DFT formalism.60,61 The singlet excitations were determined at the optimized ground-state S0 geometries, while the lowest-energy triplet emissions were determined at the optimized

mg, 73%). Single crystals suitable for XRD analysis were obtained by a gas-phase diffusion of methanol into a solution of 6 in DMSO at room temperature. 1H NMR (DMSO-d6, 298 K, δ): 9.17 (dd, JHPt ca. 30 Hz, JHH 5.7 Hz, 2H, 6-H py), 8.03 (ddd, JHH 8.0, 7.2, and 1.2 Hz, 2H, 4-H py), 7.99 (d, JHH 8.0 Hz, 2H, 3-H py), 7.71 (d, JHPt ca. 44 Hz, 2H, 3-H MePh), 7.60 (d, JHH 8.0 Hz, 2H, 6-H MePh), 7.57−7.41 (m, 30 H, PPh3), 7.32 (dd, JHH 7.2 and 5.7 Hz, 2H, 5-H py), 6.88 (d, JHH 8.0 Hz, 2H, 5-H MePh), 2.23 (s, 6H, CH3). IR (KBr, ν(CN), cm−1): 2149, 2118. Anal. Calcd for C64H50Ag2N6P2Pt2: C, 48.93; H, 3.21; N, 5.35. Found: C, 48.77; H, 3.23; N, 5.30%. [Pt(F2ppy) (CN)2Cu(PPh3)2]2 (7). Prepared similarly to 1 from [Pt(F2ppy) (DMSO)Cl] (60 mg, 0.120 mmol). Pale green-bluish solid, 110 mg, 89%. Single crystals suitable for XRD analysis were obtained by a gas-phase diffusion of water into a solution of 7 in DMSO at room temperature. 1H NMR (DMSO-d6, 298 K, δ): 9.20 (br, 2H), 8.16 (dd, JHH 8.2 and 7.5 Hz, 2H), 8.09 (d, JHH 7.5 Hz, 2H), 7.62 (m, 2H), 7.56 (m, 2H), 7.51−7.32 (m, 60 H), 6.90 (ddd, JHF 12.3 and 9.2 Hz, JHH 2.6 Hz, 2H). IR (KBr, ν(CN), cm−1): 2147, 2137. Anal. Calcd for C98H72Cu2F4N6P4Pt2·CH2Cl2: C, 55.67; H, 3.49; N, 3.93. Found: C, 55.59; H, 3.58; N, 3.92%. [Pt(F2ppy) (CN)2Ag(PPh3)2]2 (8). Prepared similarly to 7 using AgNO3 (20.5 mg, 0.120 mmol) instead of [Cu(NCMe)4]PF6. Recrystallization by a slow evaporation of a dichloromethane/ acetonitrile/methanol solution of 8 at room temperature gave pale green-blue crystalline material (108 mg, 84%). 1H NMR (DMSO-d6, 298 K, δ): 9.24 (dd, JHPt ca. 30 Hz, JHH 5.7 Hz, 2H, 6-H py), 8.15 (dd, JHH 8.5 and 7.2 Hz, 2H, 4-H py), 8.08 (d, JHH 8.5 Hz, 2H, 3-H py), 7.64−7.53 (m, 2H, F2C6H4), 7.52−7.47 (m, 2H, 5-H py), 7.51 (t, JHH 7.0 Hz, 12H, para-H PPh3), 7.44−7.33 (m, 48H, ortho + meta-H PPh3), 6.89 (ddd, JHF 12.5 and 9.4 Hz, JHH 2.5 Hz, 2H, F2C6H4). IR (KBr, ν(CN), cm−1): 2145, 2133. Anal. Calcd for C98H72Ag2F4N6P4Pt2·CH2Cl2: C, 53.46; H, 3.35; N, 3.78. Found: C, 53.68; H, 3.48; N, 3.67%. X-ray Structure Determinations. The crystals of 1−8 were immersed in cryo-oil, mounted in a Nylon loop, and measured at a temperature of 150 K. The X-ray diffraction data were collected with Bruker Kappa Apex II, Bruker SMART APEX II, or Bruker Kappa Apex II Duo diffractometers using Mo Kα radiation (λ = 0.710 73 Å). The APEX247 program package was used for cell refinements and data reductions. The structures were solved by direct methods using the SHELXS-201448 program with the WinGX49 graphical user interface. A semiempirical absorption correction (SADABS)50 was applied to all data. Structural refinements were performed using SHELXL-201448 and OLEX2.51 Some of the PPh3 phenyl rings were disordered in 1, 2, 7, and 8. Additionally, the F2ppy ligands in 7 and 8 were found to have two orientations each with occupancies of 0.68/0.32 and 0.81/0.19, 4461


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Inorganic Chemistry

Figure 1. Molecular views of complexes 1, 3, and 6. Thermal ellipsoids are shown at the 50% probability level. H atoms are omitted for clarity. ́ in 1, −x, 1 − y, −z; in 3, −x, 1 − y, 1 − z; in 6, 1 − x, 1 − y, 1 − z. Symmetry transformations used to generate equivalent atoms (): T1 geometry. All electronic structure calculations were performed with the TURBOMOLE program package (version 7.1).62

etry, which comprises nearly square metallacycles formed by platinum and d10 metal units as nodes. The vertices are connected by the CN− ligands to afford Pt−CN−M edges, which is in line with high affinity of platinum(II) ion to carbanionic donor functions. However, the crystallographic data do not allow for an unequivocal assignment of CN orientation because of a comparable electron density around C and N atoms. Nevertheless, the refinement parameters (R1, wR2) for Pt(−CN)2 models for 1−8 are systematically better than those for CN−Pt−CN and Pt(−NC)2 alternatives, suggesting that the cyanide bridges are positioned as shown in Figure 1 and Figure S1. In this respect it is worth pointing to a visible deviation from the linearity of the Pt−C N−M fragments due to a noticeable bending of CN−Cu/Ag angles, which are found in the ranges of 160−164° and 148− 156° for copper (1, 4, 7) and silver (2, 5, 8) species, respectively. Similar angular distortions have been observed earlier for a number of cyanide polynuclear complexes, including the CuI/AgI phosphine−cyanide arrays.20,21,23 On the contrary, the NC−Pt angles for 1−8 lie in between 174 and 179° that are typical for Pt−CN species29,39,42,63,64 and also support the suggested binding mode of CN ligands. This η2-bidentate coordination of cyanide groups provides the intramolecular Pt···M separations of 5.02−5.11 Å (M = Cu; 1, 4, 7) and 5.27−5.35 Å (M = Ag; 2, 5, 8), which are comparable to the characteristics of the related cyanide-built squares.5,65−68 The d10 ions in [Pt(C^N) (CN)2M(PPh3)2]2 complexes adopt typical tetrahedral coordination environment, which is furnished by two N-bound cyanides and two triphenylphosphine ligands. The structural parameters for the copper and silver fragments are close to those found for the coordination polymers containing [(CN)2M(PPh3)2] (M = CuI, AgI) motifs.21,69 The geometry of the platinum centers is also not exceptional and is very much alike to the arrangement of mononuclear [Pt(C^N) (CN)2]− 36−38 and heterometallic [PtTl(C^N) (CN)2]42 analogues. Complexes 1, 2, 4, and 5 form a ladderlike packing via intermolecular π-stacking interactions between the planar systems of the metalated C^N ligands (see an example in Figure S2, Supporting Information). The aggregate 3 crystallizes as a mixture of polymorphs (P21/c and P1̅ space groups), having practically identical

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RESULTS AND DISCUSSION Synthesis and Characterization. The conventionally prepared cycloplatinated precursors [Pt(C^N) (DMSO)Cl] (DMSO = DMSO, HC^N = 2-phenylpyridine Hppy, 2-(4tolyl)-pyridine Htolpy, 2-(4,6-difluorophenyl)-pyridine HF2ppy) were converted into the labile cationic derivatives [Pt(C^N) (NCMe)2]+ upon treatment with Ag+ salt (AgNO3 or AgPF6) in the presence of acetonitrile (Scheme 1). Subsequent addition of stoichiometric amounts of sodium cyanide, [Cu(NCMe)4]PF6, or AgNO3 along with 2 equiv of PPh3 results in precipitation of poorly soluble tetrametallic squarelike complexes [Pt(C^N) (CN)2M(PPh3)2]2 [C^N = ppy, M = Cu (1), Ag (2); C^N = tolpy, M = Cu (4), Ag (5); C^N = F2ppy, M = Cu (7), Ag (8)]. The cyclic compounds were isolated as pale green to yellow solids in good yields (74− 89%) except 5 (39%). For the latter complex, an excess of the phosphine ligand was used to achieve the formation of the target square molecule due to a competing reaction, which leads to the assembly of a yellow cluster [Pt(tolpy)(t-CN)(μCN)Ag(PPh3)]2 (6) of a different structural arrangement. In an attempt to generate a similar motif with ppy metalated ligand, we followed the molar ratio [Pt(ppy)]+/Ag+/PPh3 of 1:1:1. However, the complex [Pt(ppy) (CN)2Ag(PPh3)]2 (3), obtained as a main product, was found to adopt a squarelike geometry as 2 but having one phosphine per silver ion. The efficiency of the reaction was improved by using K[Ag(CN)2] as a source of precoordinated cyanide ligands (see the Experimental Section). Along with 3 a yellow solid was observed as a side product, which presumably belongs to the structural type of cluster 6. However, very poor solubility in common organic solvents prevented its characterization. In the case of [Pt(F2ppy)]+ synthon only the isostructural molecular squares 7 and 8 were identified irrespectively of the amount of the phosphine ligand loaded in the reaction mixtures. The structures of the complexes 1−8 in the solid state were determined by single-crystal XRD analysis (Figure 1 and Figure S1 in the Supporting Information; selected bond lengths and angles are given in Table S2). The congener compounds [Pt(C^N)(CN)2M(PPh3)2]2 show essentially the same geom4462


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Inorganic Chemistry Table 1. Photophysical Properties of 1−8 at 298 K in CH2Cl2 and in Solida solution 1 2 3 4 5 6 7 8 a

solid

λem, nm

Φem, %

τobs, ns

kr (1 × 104), s−1

knr (1 × 106), s−1

λem, nm

Φem, %

τobs, μs

480 480 479 487 486

1.01 0.23 0.44 1.89 0.47

190 59 69 435 187

5.29 3.82 6.38 4.35 2.52

5.20 17.0 14.4 2.26 5.31

464 464

0.57 0.26

102 58

5.57 4.51

9.74 17.2

492 511 497 479 478 497 470 470

31 27 24 28 27 16 34 31

13.12 12.01 14.12 15.84 11.95 3.28 9.61 7.64

λexc = 355 nm; the uncertainty of the quantum yield measurement was in the range of ±5% (an average of three replicas).

PPh3 ligands (see the Experimental Section). Low concentrations and presumably stereochemical nonrigidity of these complexes at room temperature did not allow collecting interpretable 31P NMR spectra, which display only featureless broad signals. However, the mass spectra, measured in atmospheric pressure photoionization mode, showed only fragmentation signals. Therefore, the available spectroscopic data do not allow making clear conclusions concerning the structure of heterocycles in solution, and possible dissociation of 1−8 in coordinating solvent cannot be excluded. Photophysical Properties and Computational Studies. In solution, the normalized absorption and emission spectra of complexes 1−5, 7, and 8 are shown in Figure S5; the relevant emission data are listed in Table 1. Note that the precise absorption molar extinction coefficient (ε) for these compounds could not be obtained due to their extremely sparse solubility. We also investigated the photophysical properties of the complexes 1−8 with quantum chemical methods (DFTPBE0 and TDDFT-PBE0; see Experimental for full computational details). In general, the high-energy absorption bands at less than 350 nm can be reasonably assigned to metal-perturbed π−π* ligand-centered (1LC) transitions within the aromatic systems of both metalated C^N36,37 and PPh3 ligands,20 which is in accordance with previous reports on the related PtII and d10 coinage metal phosphine complexes. On the basis of DFT calculations, the lower-energy band of more than ∼350 nm is assigned to the spin-allowed metal-to-ligand charge transfer (1MLCT) mixed with the ligand 1ππ* transition. This assignment is consistent with that of the mononuclear [Pt(C^N) (CN)2]− congeners.36,37 Ag atoms contribute very little to the lower-lying electronic transition, while in the case of Cu complexes 1, 4, and 7 the contribution from Cu is slightly larger. The predicted S0→S1 excitation wavelengths are around 360−380 nm (Table 2), which is in line with the experimental spectra shown in Figure S5. The Ag species have slightly higher excitation energy in comparison to the analogous Cu species. All of the studied compounds exhibit weak luminescence in the fluid medium under the ambient conditions (Figure S5). The dominant nonradiative deactivation may be rationalized by two possible quenching mechanisms. First, the observed emission properties resemble those of the monuclear [Pt(C^N)(CN)2](NBu4) complexes (C^N = ppy, F2ppy),37,38,73 which also display very low quantum efficiencies in solution but intense emission in the solid state. This behavior was assigned earlier to the thermal population of a higher-lying metalcentered dd* state37 leading to geometry distortions and Pt− C/Pt−N bond elongations that is also reasonable to expect from 1−8, particularly if they undergo extensive dissociation.

molecular structures. In both forms, the metal ions are found at the corners of the distorted squares, analogously to the tetraphosphine compound 2. The lack of PPh3 ligands in 3 leads to a trigonal coordination geometry of AgI ions and substantial shortening of P−Ag bond lengths (2.3712 and 2.3698 Å) with respect to 2 (2.4735 and 2.4366 Å). The major difference between the polymorphs can be seen in the packing and intermolecular interactions. The monoclinic form does not show appreciable π-stacking, and the molecules constitute infinite chains via some π-ppy−π-CN attraction (Figure S3). On the contrary, the triclinic modification of 3 demonstrates rather extensive π−π bonding, which involves phenyl−pyridine fragments and the phosphine phenyl rings of the adjacent molecules (Figure S3). Cluster 6 with tolylpyridine platinated moiety contains one phosphine ligand per silver ion as in 3. However, 6 has a different topology facilitated by three-center Pt···Ag···C(3) bonding. The complex shows a dimeric structure (Figure 1, Figure S4 shows its crystal packing) consisting of two [Pt(tolpy)(t-CN)(μ,η2-CN)Ag(PPh3)] motifs, which are held together by short Pt−Ag contacts (2.9507(2) Å) additionally stabilized by Ag−Cipso(tolyl) interactions (Ag(1)−C(3) distance is 2.482(2) Å). This sort of η1-C−M bonds between coordinatively unsaturated d10 ions and metalated carbanion of the aromatic system has been previously recognized to play an important role in construction of heterometallic Pt−M compounds.70,71 The akin Pt−Au dinuclear complex has been recently analyzed in detail by the DFT studies,72 which revealed an ionic character of this interaction. The preferential formation of the cluster molecule 6 versus square assembly 3 can be ascribed to a higher electron-donating ability of the tolpy system in comparison to the ppy and particularly F2ppy relatives. This conclusion is supported by the DFT calculations, which show that the partial charge of the Cipso atom increases from −0.25 (3) to −0.35 (6, partial charges from natural population analysis). The higher basicity of anionic Cipso of tolpy ligand therefore favors η1-C−Ag interaction leading to the cluster framework. The IR spectra of the title compounds show two strong ν(CN) stretching vibrations between 2149 and 2118 cm−1, in agreement with the presence of two types of CN groups in these molecules (i.e., each CN has a different trans-partner). The largest Δν difference corresponds to complex 6, containing both terminal and bridging cyanide ligands. All the complexes 1−8 are only poorly soluble in common organic solvents that prevented their complete characterization in fluid medium. The 1 H NMR spectra in DMSO-d6 are not contradictory to the solid-state structures and display the set of well-separated resonances of metalated motifs along with grouped signals of 4463


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Inorganic Chemistry Table 2. Computational Photophysical Results for the Complexes 1−8 (TDDFT-PBE0) λaba S0→S1 (nm) 1 2 3 4 5 6 7 8 a

384 376 371 382 376 371 379 360

(0.03) (0.05) (0.06) (0.03) (0.06) (0.08) (0.02) (0.03)

λem T1→S0 (nm) 525 524 524 532 530 536 500 500

Wavelengths in nanometers, oscillator strengths given in parentheses.

Second, these PtII-containing complexes possess d8 electronic configuration with the nonbonding dz2 orbital being perpendicular to the square-planar framework. As a result, this orbital is subject to collisional interaction with solvent molecules that are capable of inducing radiationless deactivation and therefore causing a dramatic decrease of emission intensity.74 In the solid state, both dd*- and solvent collision-induced quenching are drastically reduced, as evidenced by a substantial increase of emission intensity acquired for the powders of the studied complexes. Therefore, our investigation on the photophysical behavior of compounds 1−8 is focused on their solid-state characteristics. The relevant data are listed in Table 1. The emission spectra of the crystalline samples are shown in Figure 2, where the Pt−Cu and Pt−Ag complexes are presented separately for the sake of clarity. The isostructural square tetraphosphine complexes [Pt(C^N)(CN)2M(PPh3)2]2 demonstrate moderately strong emission with quantum yields of 27−34%. The spectral profiles for these species are structured and feature vibronic progressions of ca. 1350−1500 cm−1, which indicates a dominating contribution of aromatic chromophores into the emissive states and are often observed for platinum cyclometalated compounds.36,37,75 The TDDFT-PBE0 calculated T1→S0 emission wavelengths listed in Table 2 are slightly overestimated in comparison to the experiment. The electron density difference plots are shown in Figure 3 for representative complexes 2, 6, and 8, while the corresponding illustrations of other complexes are given in Figure S6. The trend of the gas-phase emission wavelengths is in a very good agreement with the emission wavelengths measured in solution (for complex 6, the emission wavelength in solution is not available), the deviations from the solid-state data are mainly attributed to the packing effects and

Figure 3. Electron density difference plots for the lowest-energy singlet excitation (S0→S1) and the lowest-energy triplet emission (T1→S0) of the complexes 2, 6, and 8 (isovalue 0.002 au). During the electronic transition, the electron density increases in the blue areas and decreases in the red areas. Hydrogen atoms are omitted for clarity.

intermolecular π-stacking interactions, which were not taken into account in modeling approach. For the complexes 7 and 8, the TDDFT-PBE0 calculations reproduce the experimentally observed slight blue shift in comparison to complexes 1−6. The long excited-state lifetimes for all the title complexes (τobs span from 3.28 to 15.84 μs) point to a triplet parentage of emission (i.e., phosphorescence). According to the DFT calculations (Figure 3), the emission originates from C^N intraligand 3ππ* transitions mixed with metal(Pt)-to-ligand charge transfer (MLCT) character. This assignment is

Figure 2. Normalized solid-state emission spectra of Pt−Cu (1, 4, 7, left) and Pt−Ag (2, 3, 5, 6, 8, right) complexes at 298 K. 4464


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dramatically enhanced reaching the value of Φem = 34% at room temperature, accompanied by up to 200-fold increase of excited-state lifetimes. Analysis of the spectroscopic characteristics, supported by the computational TDDFT-PBE0 studies of the electronic structures, indicates that intense triplet emission T1→S0 originates mainly from the 3IL transitions localized on the cyclometalated {Pt(C^N)} chromophores with small to negligible contribution of the {M(PPh3)n} moieties into the frontier orbitals of the investigated species.

supported by a visible dependence of emission maximum on the electronic properties of the metalated ligands. The F2ppycontaning compounds 7 and 8 exhibit the highest energy luminescence among 1−8 that complies with a stabilizing effect of electron-accepting fluoro-substituents. Both Ag and particularly Cu atoms have virtually negligible contribution to the lowest-energy emission of the studied complexes (Figures 3 and S6). Also, the emission patterns of 1 (M = Cu)/2 (M = Ag) and 7 (M = Cu)/8 (M = Ag) are nearly identical to those of monometallic platinum [Pt(ppy) (CN)2]− and [Pt(F2ppy) (CN)2]− constituents,36,37 pointing to a minor impact of [(CN)2M(PPh3)2] blocks on the frontier molecular orbitals involved in the electronic transitions. Comparison of the photophysical behavior of 2 and 3 having different number of phosphine ligands reveals only slight deviations of the measured parameters, which is in line with proposed hypothesis. However, cluster 6 with metallophilic interactions shows a detectable bathochromic shift of the emission band with regard to square 5. However, computational analysis of the electronic structure of 6 as well points to a very minor role of Ag ions in the S0→S1 and T1→S0 transitions (Figures 3 and S7). Therefore, the observed increase of λem for 6 might be assigned to the combination effect of (a) solid-state packing, in which the extensive CHPh−πC^N interactions can be seen (Figure S4) and (b) disturbance of the C^N chromophore π-system, induced by the Ag−Cipso bonding. The latter is somewhat reflected by the predicted minor decrease of T1→S0 emission energy for 6 versus 5 (Table 2). This hypothesis correlates with the data for other Pt−Ag aggregates, for which small shift of emission can be observed in comparison to monometallic Pt precursors,70,71,76,77 pointing to a subtle but important influence of the lattice on the physical behavior of cluster 6. Simultaneously, 6 clearly displays a visible decrease of the quantum efficiency and the lifetime, indicative of the appearance of a competing radiationless pathway induced by the intermolecular interactions.

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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.7b00006. Molecular views of selected complexes, absorption and emission spectra in solution, electron density difference plots, and frontier MOs of 6 (PDF) Optimized Cartesian coordinates of the studied systems (XYZ) X-ray crystallographic data for 1−8 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: antti.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 Author Contributions ¶

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Russian Science Foundation (Grant No. 16-13-10064, synthesis and structural characterization), the Ministry of Science and Technology in Taiwan, and ERASMUS + programme (L.S.) are gratefully acknowledged. Computational resources were provided by CSC, the Finnish IT Center for Science (A.J.K.).

CONCLUSIONS In this work, we have demonstrated a facile strategy to utilize the cycloplatinated [Pt(C^N) (CN)2]− building units for the construction of the photofunctional heterometallic assemblies. The coordination-driven formation of the molecular squares as a prevailing structural motif has been realized due to the bridging bonding of the cyanide ligands. The bifunctional nature of CN linkers and a pronounced affinity of the platinum and coinage metal ions to carbon and nitrogen donors, respectively, allowed for efficient coupling the anionic cyanometallate chromophores with cationic phosphine-stabilized d10 ions to afford a family of neutral [Pt(C^N)(CN)2M(PPh3)n]2 (M = CuI, AgI; n = 1, 2) aggregates. In the case of silver(I) compounds, an alternative arrangement of the complex framework has been observed, which comprises a cluster core evidently supported by the metal−metal interactions in addition to the bridging coordination of the constituting cyanide and C^N ligands. The metallophilic and η1-C−Ag contacts, promoting a more compact topology, are presumably facilitated by the tendency of silver ions to attain a tetracoordinate geometry at a lack of ancillary phosphine ligands, as well as by the increase of electron-donating ability of the C^N metalated fragment. The title complexes display only faint photoluminescence in solution, while in the solid state the emission intensity is

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Cyclometalated Platinum(II) Cyanometallates: Luminescent Blocks for

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