Article pubs.acs.org/IC
1D Polymeric Platinum Cyanoximate: A Strategy toward Luminescence in the Near-Infrared Region beyond 1000 nm Danielle R. Klaus,† Matthew Keene,† Svitlana Silchenko,‡ Mikhail Berezin,*,§ and Nikolay Gerasimchuk*,† †
Department of Chemistry, Missouri State University (MSU), Temple Hall 456, Springfield, Missouri 56897, United States Absorption Systems, Inc.; 440 Creamery Way, S. 300, Exton, Pennsylvania 19341, United States § Department of Radiology, Washington University of St. Louis Medical School, St. Louis, Missouri 63110, United States ‡
S Supporting Information *
ABSTRACT: We report the synthesis and properties of the first representative of a new class of PtL2 complexes with ambidentate mixed-donor cyanoxime ligands [L = 2-cyano-2oximino-N,N′-diethylaminoacetamide, DECO (1)]. Three differently colored polymorphs of “Pt(DECO)2” (3−5) were isolated, with the first two being crystallographically characterized. The dark-green complex [Pt(DECO)2]n (5) spontaneously forms in aqueous solution via aggregation of yellow monomeric complex 3 into the red dimer [Pt(DECO)2]2 (4), followed by further oligomerization into coordination polymer 5. A spectroscopic and light-scattering study revealed a “poker-chips”-type 1D polymeric structure of 5 in which units are held by noncovalent metallophilic interactions, forming a Pt---Pt wire. The polymer 5 shows a broad absorption at 400−900 nm and emission at unusually long wavelengths in the range of 1000−1100 nm in the solid state. The near-infrared (NIR) emission of polymer 5 is due to the formation of a small amount of nonstoichiometric mixed-valence PtII/PtIV species during synthesis. A featureless electron paramagnetic resonance spectrum of solid sample 5 recorded at +23 and −193 °C evidences the absence of PtIII states, and the compound represents a “solid solution” containing mixed-valence PtII/PtIV centers. Exposure of KBr pellets with 5% 5 to Br2 vapors leads to an immediate ∼30% increase in the intensity of photoluminescence at 1024 nm, which confirms the role and importance of mixed-valence species for the NIR emission. Thus, the emission is further enhanced upon additional oxidation of PtII centers, which improves delocalization of electrons along the Pt---Pt vector. Other polymorph of the “Pt(DECO)2“ complexmonomerdid not demonstrate luminescent properties in solutions and the solid state. An excitation scan of 5 embedded in KBr tablets revealed an emission only weakly dependent on the wavelength of excitation. The NIR emission of quasi-1D complex 5 was studied in the range of −193 to +67 °C. Data showed a blue shift of λmax and a simultaneous increase in the emission line intensity with a temperature rise, which is explained by analogy with similar behavior of known quasi-1D K2[Pt(CN)4]-based solids, quantum dots, and quantum wells with delocalized carriers. The presented finding opens a route to a new class of platinum cyanoxime based NIR emissive complexes that could be used in the design of novel NIR emitters and imaging agents.
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particles,14 single-wall carbon nanotubes,15,16 and, recently, supramolecular assemblies with noncovalent metal−metal (metallophilic) interactions have become an attractive way to synthesize NIR luminescent materials that feature palladium nanowires17 or platinum self-assembled aggregates.18 In these structures, a significant metal−metal interaction extended over multiple metal centers leads to lowering of the gap between the ground and excited states, resulting in longer wavelengths of emission. Such materials are especially promising because of their tunable optical properties, which can be adjusted by the type of ligand and controlled synthesis regulating the number of assembled complexes (polymorphs). Recently, some platinum complexes with their known propensity to metal− metal interactions and polymorphism have attracted attention because of their emissive properties up to 800 nm.19−22
INTRODUCTION In the past decade, near-infrared (NIR) technology opened up a variety of novel directions in industrial, scientific, military, and medical applications as new optical devices,1−3 sensors,4,5 contrast agents,6,7 and imaging techniques.8,9 As a result of the development of new technology, a diverse set of photoluminescent (PL) compounds emitting in NIR from small molecules to nanoparticles and aggregates have emerged. However, the great majority of these materials emits up to 800−850 nm, while the number of NIR emitters with emission above 1000 nm is still limited. Fundamental constraints, such as the energy gap law, stating that radiationless transitions at longer wavelengths increase because of vibrational overlaps between the ground and excited states, causing a decrease in the luminescence efficiency,10 significantly restrict the number of luminescent materials beyond 1000 nm. Reported compounds including some lanthanide complexes,11 cyanine dyes,12 certain types of quantum dots,13 small gold nano© XXXX American Chemical Society
Received: November 22, 2014
A
DOI: 10.1021/ic502805h Inorg. Chem. XXXX, XXX, XXX−XXX
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Spectroscopic Studies. NMR Spectroscopy. The synthesized organic compound 1 and its substituted acetonitrile precursor were characterized by 1H and 13C NMR spectroscopy [solutions in DMSOd6; tetramethylsilane (TMS) was an internal standard; Varian INova 400 MHz spectrometer]. Variable-temperature experiments for 1, 3, and 6 were conducted in the 20−95 °C range. IR Spectroscopy. IR spectra were recorded in KBr pellets for compounds 1 and 3−6 in the range of 400−4000 cm−1, using the Bruker Vertex S70 Fourier transform infrared (FTIR) spectrophotometer with 64 repetitions at 23 °C and 4 cm−1 resolution. Absorption Spectroscopy. The UV−visible spectra of the cyanoxime 1 and its anion 2 (as NHEt3+ salt) in solutions were recorded at room temperature (293 K) using an HP 8354 spectrophotometer in the range of 200−1100 nm, in 1 and 10 mm quartz cuvettes (Starna, Inc., Atascadero, CA). The solid-state UV− visible spectra of compounds of interest [Magnus Green Salt (MGS), K2[Pt(CN)4] (KCP), and mixed valence partially oxidized with bromine K2[Pt(CN)4]·0.3Br·H2O (later POCP), as well as synthesized complexes 3−5, and Pd(DECO)2 complex 6; Scheme 1] were
The key factor in the development of supramolecular emitters with the desired optical properties is to control the aggregation process. Herein, we developed a new class of 1D platinum-based luminescent complexes with strong emission beyond 1000 nm that challenges the energy gap law. The choice of a suitable ligand system is critical for the successful preparation of such 1D complexes. Among ambidentate ligands, oximes used in this work take a special place because of their strong affinity for nickel triad metals that favor square-planar geometry.23−26 On the basis of our previous studies,27−30 we selected cyanoximes28,31−33 as ligands for binding PtII centers. Cyanoximes are particularly flexible mixed-donor ligands that allow modular design and are the platform of choice for developing metal complexes with controlled self-assembly in solutions and the solid state.34−36 In the design of long-wavelength PtII emitters, we rationalized that the environment facilitates metallophilic interactions, leading to a red shift of the emission21 and fulfilling the necessary geometric requirements for the formation of mixed-valence species. These interactions are kinetically controlled and lead to 1D aggregation along the M--M vector, resulting in a red-to-NIR luminescent 1D polymer. Recently, palladium-based mixed-valence 1D “metallic wire” complexes have shown a very intense NIR emission.17 We also rationalized that the anion’s bulkiness from substituents would control the distance between the metal centers. Therefore, among known cyanoxime ligands,36 those with amide groups provide a greater variety of electronic and steric properties and solubility.37−39 Thus, novel bivalent palladium and platinum complexes were synthesized using a cyanoxime ligand well soluble in organic solvents, 2-cyano-2-oximino-N,N′-diethylaminoacetamide [HDECO, 1]. As we demonstrate below, the platinum complex self-assembles in solutions into a luminescent 1D polymeric “poker-chips” structure. In this work, we report the synthesis and emission properties of a new PL platinum member of the cyanoxime family. We also report the synthesis and characterization of related bivalent palladium and platinum monomeric complexes of M(DECO)2 composition to understand the unique luminescence of the new material.
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Scheme 1. Synthetic Route to Metallocyanoximates and Their Properties
EXPERIMENTAL SECTION
General Considerations. All of the necessary chemicals, such as cyanoacetic acid ester, K2CO3, K2[PdCl4], K2[PtCl4], and organic solvents, were obtained from commercial sources and used without further purification. The elemental composition of the starting cyanoxime and its palladium and platinum complexes on the C, H, and N content was determined using a combustion method at Atlantic Microlab (Norcross, GA). Melting points were measured on a Digimelt apparatus without correction. Solution Studies. pKa of the New Cyanoxime and Conductivity Measurements. The ionization constants for the new cyanoxime 1 were determined using a Sirius Analytical Instruments automated titration station (Sussex, U.K.) equipped with a temperature-controlled bath with details provided in the Supporting Information (SI), p S3. The aqueous pKa value was calculated with a Yasuda−Shedlovsky extrapolation in RefinementPro software. The pKa values for 1 and two other closely related amidocyanoximes are summarized in the SI, pp S4 and S5. Electrical conductivity measurements were carried out at 22 °C in dimethyl sulfoxide (DMSO) solutions at 1 mM concentrations of synthesized palladium and platinum complexes, using the Vernier LabQuest digital conductivity meter. An electrode was calibrated with 1 mM DMSO solutions of N(C4H9)4+Br−, P(C6H5)4+Br−, and K2PtCl4. The values of the conductivities are listed in Table S6 in the SI.
recorded as absorbance spectra from their fine suspensions in mineral oil using the above diode-array spectrophotometer. Absorption spectra of the tablets were recorded using a custom setup based on an integrating sphere fiber-optically connected to the silicon-based diodearray CCD camera Synapse (Horiba). Halogen light (HL-2000, Ocean Optics) was used as a light source. An integration time of 0.05 s and a slit of 2 nm were used. All spectra were collected 10 times and averaged. The spectra were collected for light intensity. Absorption spectra were referenced to the spectrum of a pure, neat KBr tablet. Emission Spectroscopy. The PL of solid metal complexes was investigated using the following experimental setup based on a Horiba B
DOI: 10.1021/ic502805h Inorg. Chem. XXXX, XXX, XXX−XXX
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Electron Microscopy. The scanning electron microscopy (SEM) images from a fine dark-green powder of polymeric complex 5 were obtained using four randomly selected areas at 200, 100, 50, 20, 10, and 5 μm resolution. Synthesis. Ligands. The cyanoxime ligand 1 (HDECO) was obtained in high yield according to known procedures from the respective diethylamide, as shown in the SI, p S32. The pure compound represents clear, ice-looking crystals soluble in most organic solvents except hydrocarbons. Yield: 68%. Mp: 87−90 °C. Rf = 0.32 in a 1:1 EtOAC/hexane mixture. Anal. Calcd (found) for C7H11N3O2: C, 49.70 (49.53); H, 6.55 (6.61); N, 24.84 (24.77). UV− visible [λnm, nm (ε, M−1·cm−1): CH3OH, 226 (10331; π → π*); deprotonated DECO− (as NHEt3+ salt), 394 (73; n → π*). 1H NMR (DMSO-d6): δ 1.16 (3H, t, methyl group), 1.12 (3H, t, methyl group), 3.40 (2H, quartet, methylene), 3.47 (2H, quart, methylene), 14.21 (1H, s, oxime); the value of a 3J(H−H) coupling constant in the ethyl group is 7.1 Hz. 13C NMR: two methyl groups at δ 12.74 (CH3), 14.63 (CH3), two ethyl groups at δ 40.95 (CH2), 43.99 (CH2), 127.83 (C N−OH), 109.77 (CN), 158.82 (amide). IR (KBr pellet, cm−1): 3205 [ν(OH)], 2992 [νas(CH)], 2830 [νs(CH)]; 2250 [ν(CN)], 1630 [ν(CO, amide I)], 1478 [ν(CN, oxime)], 1468 [ν(CO, amide II)], 1034 [ν(N−O)]. Metal Complexes. The preparation of MGS,40,41 KCP,42 and POCP43 (Krogman’s salt) was carried out according to published procedures. Their microscopic photographs (at 40×) are shown in the SI, p S25. All of these complexes were used as standards of known 1D solids with well-defined Pt---Pt distances and acted as model compounds to compare with the newly obtained complexes 3−6. For the synthesis of 6, 0.330 g (1.95 mM) of HDECO were placed in a 100 mL Erlenmeyer flask and 5 mL of distilled water was added to the flask. Pure ligand 1 is not well soluble in water, but after being deprotonated, it becomes well soluble and reacts with transition-metal salts (Scheme 1). Therefore, the required stoichiometric amount of a 1.01 M KOH solution (1.93 mL) was added to the sludge of 1 in water. The cyanoxime dissolved, and the reaction mixture immediately changed to bright yellow. A solution of 0.318 g (0.98 mM) of K2[PdCl4] in 5 mL of water was added dropwise to a yellow solution of 2 under intense stirring. A thick yellow precipitate formed instantaneously, and after ∼5 min of stirring, it was filtered, washed three times with water, and then dried in a desiccator charged with concentrated H2SO4 (see the SI, p S32). Elem anal. Found (calcd) for PdC14H22N6O4: N, 18.24 (18.24); C, 37.05 (37.05); H, 4.50 (4.50). IR (KBr pellet, cm−1): 2983 [νas(CH)], 2940 [νs(CH)], 2211 [ν(C N)], 1631 [ν(CO, amide I)], 1440 [ν(CNO), oxime)], 1579 [ν(CO, amide II)], 1200 [ν(CNO)]. Complex 6 is soluble in CH3CN, well soluble in DMF, DMSO, Py, and its homologues, but not well soluble in acetone and alcohols. For the synthesis of Pt(DECO)2, 0.217 g (1.28 mM) of 1 in 8 mL of water was treated with 1.28 mL of a 1.01 M KOH solution, forming a yellow solution of deprotonated cyanoxime 2, to which 0.268 g (0.65 mM) of K2PtCl4 in 5 mL was added. When all components were mixed, the formation of the Pt(DECO)2 complex involving several stages was observed after an extended period of time. After ∼24 h, a very fine dark-green precipitate was filtered (see the SI, p S32), washed three times with water, and then dried in a vacuum desiccator under concentrated H2SO4. Elem anal. Found (calcd) for green-form PtC14H22N6O4 (5): N, 15.75 (15.78); C, 31.52 (31.46); H, 4.16 (4.03). IR (KBr pellet, cm−1): 2985 [νas(CH)], 2943 [νs(CH)], 2213 [ν(CN)], 1709 [ν(CO, carbonyl)], 1622; 1442 [ν(CNO), oxime)], 1585 [ν(CO, amide II)], 1227 [ν(CNO)]. The dark-green form 5 is not soluble in water but has some solubility in alcohols, slowly dissolves in DMSO (with the help of an ultrasound bath), CH2Cl2, and CHCl3, forming at first a dark-red solution of dimer 4. Also, complex 5 is well soluble in pyridine and its homologues with the formation of yellow solutions. The yellow form 3 of the same complex can be obtained after dissolution of the green form in CH3CN or DMF, which slowly changes color to red and yellow-orange. Crystals of 3 were grown from an overlap with a heptane/acetonitrile solution or by the vapor-diffusion method when ether slowly dilutes the solution. In both cases, nicely shaped yellow prism-type crystals can be isolated in moderate yield. Elem anal. Found (calcd) for the yellow
spectrofluorimeter using the CCD liquid-nitrogen-cooled InGaAs diode-array camera Symphony (Horiba) sensitive in the 600−1600 nm range and a 3 s integration time with 20 times signal accumulation in summation mode. In both types of experiments, emission and excitation slits were kept constant at a width of 10 nm. Excitation was conducted with a xenon lamp using a double-grating (2 × 1200 g/ mm at 500 mm) monochromator. A long-band-pass 830 nm Schott RG 830 filter was placed in front of the iHR-820 spectrograph with a grating of 100 g/mm at 800 mm. The system was calibrated with the laser (neodymium-doped) glass before every set of measurements. A custom-built anodized aluminum vacuum-pumped cryostat filled with liquid nitrogen/oil was used for variable-temperature experiments in the −195 to +70 °C range, where a J-Kem Scientific digital thermometer with a T-type thermocouple was used to monitor the temperature. 3D excitation−emission scans of the tablets (KBr matrix with 5% of metal complexes) were recorded with the simultaneous measurement of the light intensity (R) and correction of the emission by light intensity. The absolute fluorescence quantum yield of the tablet was measured using a large 150 mm (6 in.) integrating sphere with a fiber-optic bundle with high transmission in the NIR. The tablet was placed at the bottom-loading circular shallow drawer in the sphere. All tablets were pressed as 13 mm disks from a thoroughly homogenized mixture of the IR-spectroscopic-grade KBr and studied complexes 3−6 using a Carver hydraulic press at 23 °C and a pressure of 9 torr (∼6000 psi). All glassware cuvettes and hardware parts that were in contact with either KBr pellets or powders of studied metal complexes 3−6 were washed using warm to ∼65 °C N,N-dimethylformamide (DMF) followed by deionized water to avoid contamination and exclude anything related to misinterpretation of the results of PL measurements. All synthesized complexes are well soluble in DMF. Electron Paramagnetic Resonance (EPR) Spectroscopy. Spectra were recorded on Bruker EMXplus X-band EPR spectrometer with dual-mode cavity and an Oxford cryostat system at +20 and −193 °C using a field sweep from 200 to 4000 G. The field was calibrated using DFPG, while the sensitivity of instrument was checked using a solid standard containing 1% of Cr3+ in Al2(SO4)3. Spectra were recorded as a sum of five repetitions with the time constant for each set at 160 ms. X-ray Crystallography. Crystal structures were determined for the cyanoxime 1 and compounds 3, 4, and 6, suitable single crystals of which were grown by an ether vapor diffusion method in CH3CN solutions. All attempts to grow suitable crystals of 5 were unsuccessful (see the SI, p S24). Crystals of all studied compounds were mounted on a thin glass fiber or plastic MiTeGen holders attached to the copper pin positioned on the goniometer head of the Bruker APEX 2 diffractometer, equipped with a SMART CCD area detector. The intensity data were collected at 120 K in ω-scan mode using a molybdenum tube (Kα radiation; λ = 0.71073 Å) with a highly oriented graphite monochromator. Intensities were integrated from four series of 364 exposures, each covering 0.5° in ω at 20 s of acquisition time, with the total data set being a sphere. The space group determination was done with the aid of XPREP software. The absorption correction was performed by a crystals face-indexing procedure with a videomicroscope (see the SI, pp S8, S11, and S15) followed by a numerical input into the SADABS program that was included in the Bruker AXS software package. All structures were solved by direct methods and refined by least squares on weighted F2 values for all reflections using the SHELXTL program. In the structures of the free ligand HDECO, and in its Pd(DECO)2 complex 6, all hydrogen atoms were placed in calculated positions in accordance with the hybridization state of a hosting carbon atom and refined isotropically. However, in the structure of yellow Pt(DECO)2 (3), all hydrogen atoms were found in the electron difference map and refined anisotropically. No apparent problems or complications were encountered during structure solution and refinement. The crystal data for 1, 3, 4, and 6 are summarized in the SI, p S19, while selected bond lengths and valence angles are presented in the SI, p S21. Representative drawings of the crystal structures and packing diagrams were done using the ORTEP and Mercury software packages. C
DOI: 10.1021/ic502805h Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry form 3 (for 5): N, 15.75 (15.89); C, 31.52 (31.42); H, 4.16 (4.27). The data of UV−visible spectra for all synthesized compounds are summarized in the SI, p S43. Safety Note! Complexes of bivalent palladium and platinum are known to be cytotoxic agents. Gloves at all times are required during work with these compounds.
The dimension of aggregates formed is in the nanoscale, as is evident from their light scattering (Tindal effect) and particle size growth (see the SI, p S39). The linear trend in the increase of its size suggests the Oswald ripening mechanism of particle growth. Structures and NMR Spectra of the New Cyanoxime and Its Pd(DECO) and Pt(DECO) Complexes. The crystal structure of the cyanoxime 1 (see the SI, p S7) reveals an adoption of trans−anti oxime geometry in the solid state. The molecule’s core is not planar, with a significant dihedral angle of 53.06° between planes of the cyanoxime and carbonyl fragments due to the steric strain induced by two ethyl groups on the amide fragment. The ligand becomes significantly more planar when coordinated to the platinum metal as described below but adopts cis−anti geometry to adjust its chelate binding in the complex. Variable-temperature 1H NMR spectra of the free ligand 1 and monomeric complexes 3 (and its palladium counterpart 6) in DMSO-d6 revealed a significant increase of ∼6 kJ/mol in the rotation energy barrier of the N,Ndiethylamino fragment around the C−N amide bond in the complex as opposed to the ligand. Such a barrier indicates an increased rigidity of the anion in the platinun complex (see the SI, p S4) and explains the overall propensity of the flattened complex to dimerization and further polymerization. Crystal structures revealed that different colors of complexes in the Pt(DECO) system relate to distinct polymorphs (see the SI, p S27). The crystal and refinement data are presented in the SI, pp S19 and S20. The yellow complex was found to correspond to monomeric 3 and adopts a shallow bowl-shapes structure (Figure 2). The planar coordination [PtN2O2] environment of metal consists of two chelate ligands in a cis arrangement (Figure 2). Cyanoxime monoanions 2 (DECO−) in all metal complexes presented here (3, 4, and 6) are in the nitroso form, as is evident from the rather short N−O distance of ∼1.24 Å compared to the much longer ∼1.36 Å C−N bonds in the CNO fragment (Table S23 in the SI). The DECO− anion 2 in the monomeric complex 3 is considerably more planar than the structure of 1, with the values of the dihedral angles between the cyanoxime and amide fragments at ∼19−22° (see the SI, p S10). The structure represents “slipped stacks” of the cisPt(DECO)2 units, where the shortest intermetallic distance is 7.204 Å, suggesting minimum interaction between the platinum centers (see the SI, p S9). Yellow monomeric platinum complex 3 and palladium complex 6 are isostructural (see the SI, p S18). The red complex was found to be the dimeric form of 3: [Pt(DECO)2]2 (4). The crystal and molecular structure of 4 is depicted in Figure 2, and the complex’s actual appearance as clear red prisms can be seen in the SI, pp S11 and S27. This centrosymmetric dimer is formed by two bowl-shaped monomeric units of cis-Pt(DECO)2 that form an elegant, but shallow, double-bowl convex structure (Figure 2). The DECO− anion in the structure of 4 is even more planar than that in 3, with the values of the dihedral angles between the two cyanoxime and amide fragments decreasing to 4.4 and 14.7° (see the SI, p S12). Such increased planarity is apparently due to the demand of the metallophiliic Pt---Pt interactions to form the dimer. Two chelate rings in the cis-Pt(DECO)2 units form the dihedral angle of 23.1°, which is responsible for the adoption of the bowl shape by the individual units, but being combined in the dimer takes the convex structure (Figure 2). The Pt---Pt intermetallic distance of the dimer was found to be
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RESULTS AND DISCUSSION Intertransformation of Complexes in the Pt(DECO) System. One of the most remarkable observations during the Pt(DECO)2 synthesis (Scheme 1) is the color change. The ligand HDECO itself is colorless but turns into the brightyellow anion DECO− (2) in the presence of a base. Shortly after the addition of the platinum salt, the color of the solution changes to red, then slowly the reaction mixture becomes turbid, and a very fine dark-green precipitate starts to form in the still red solution. The green precipitate makes up the bulk of the pure platinum complex 5 without any admixture of the red or yellow complexes 4 and 3, which are polymorphs. The spectra of the products in solutions are shown in Figure 1. The
Figure 1. UV−visible absorption spectra of Pt(DECO) complexes in solution: polymer 5 in n-PrOH, dimer 4 in CHCl3, and monomer 3 in diethylacetamide; T = +20 °C. An arrow shows the “blue band” in the NIR region.
green form of 5 can be completely converted into the yellow monomeric form 3 by a simple dissolution of the complex in donor solvents such as pyridine and DMSO. Finally, all three forms of Pt(DECO)2 can be isolated (see the SI, pp S26 and S27). The UV−visible spectra of the monomer 3 were similar to those for the anionic ligand 2 in the same solvent (see the SI, Table S43). The red and yellow forms can be obtained from the green one using trituration of the solids with solvents such as CH3CN, CHCl3, and CH2Cl2, followed by either controlled crystallization or mechanical separation. The process of disaggregation appears to be reversible, showing spontaneous formation of the dark-green 5 from the red 4 or yellow complex 3 upon solvent evaporation (see the SI, p S26). The platinum complexes overall remain stable after dissolution: electrical conductivity in DMSO (as the donor solvent that disrupts metallophilic contacts and then dissolves all of the studied compounds) showed nonconductive solutions, which implies that the complexes keep their platinum ligand integrity (see the SI, p S6). For comparison, the bright-yellow 6 complex instantly forms upon mixing stoichiometric amounts of 2 and K2PdCl4 in water (see the SI, p S32). Compound 6 was prepared as a structurally similar, but nonaggregating, model complex used as a control (see the SI, pp S16−S18). D
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interconversion has not been carried out. It is important to mention that other platinum metals (rhodium, for example) in their complexes also exhibit yellow-to-red color changes when the intermetallic distance becomes shorter in the 1D chain.57 That peculiar color change upon the formation of aggregated structures with relatively short metallophilic contacts seems to have a rather general trend independent of the metal center. The color changes to a dark green (such as in MGS)41 or an intense blue like in platinum, rhodium, and iridium when a further decrease in the intermetallic separation occurs.58,59 Our efforts to grow crystals of the “green” complex 5 suitable for X-ray study were not successful. We were able to obtain very thin dichroic needles (green in reflected light and brownyellow in passing through light; see the SI, p S28), but these were insufficient to determine the structure. This is, perhaps, due to significant problems of alignment and packing in the crystal dimeric units of cis-Pt(DECO)2 into polymer with the formation of a long-ordered lattice capable of diffracting X-rays, as suggested in the SI, p S24. According to SEM images, a very fine dark-green powder of the coordination polymer 5 represents fibrous microcrystalline materials without visible 5 μm resolution impurities or foreign phase (see the SI, p S29). Electronic Spectra of Platinum Cyanoximates. The yellow monomeric complex 3 shows typical π → π* transitions in UV−visible spectra around 250−400 nm (Figure 1; see the SI, p S43). The red dimeric complex 4 has three pronounced bands in the UV−visible spectrum at 258 nm (ε = 13670 M−1 cm−1), 374 nm (ε = 18500 M−1 cm−1), and 542 nm (ε = 4400 M−1 cm−1) with a weak shoulder at ∼420 nm (Figure 1). The uncommon dark-green color of the polymeric complex 5 deserves some careful consideration because the origin of such an intense transition is not obvious. The color of this complex is comprised of a “blue” band at ∼800 nm and a “yellow” band of tailing from the UV region of spectra of π → π* transitions at ∼380 nm and a weak n → π* transition at ∼420 nm, which is typical of all deprotonated cyanoximes.28,39,60 The presence of a broad band centered at ∼800 nm in the absorption spectra of 5 in solutions (Figure 1) and in the solid state (see the SI, pp S34, S44, and S45) immediately suggested the formation of a 1D coordination polymer observed in some other platinum complexes such as a well-known MGS, [Pt(NH3)4][PtCl4] with a Pt---Pt chain, and Millon’s salt [Cu(NH3)4][PtCl4] with alternating Cu---Pt chains.61 The color of both green compounds is attributed to the charge-transfer (CT) band in “poker-chip stacks” formed by alternating donor [PtCl4]2− and acceptor [Pt(NH3)4]2+ or [Cu(NH3)4]2+ units.41 These exhibit metallophilic interactions and are separated by 3.23 Å in MGS, or 3.22 Å in Millon’s salt compared to a typical covalent single Pt−Pt bond (2.6−2.8 Å in length). Similarly, in the absence of direct structural data, we propose the formation of 1D stacks for the “green” Pt(DECO)2 complex 5 in which short (∼3.1 A) metallophilic Pt---Pt contacts are present. We found that the green color of complex 5 quickly disappears upon its dissolution in donor solvents such as DMSO, DMF, CH3CN, pyridine, and 2-picoline, which evidenced a loss of the 1D polymeric structure in these solvents. Yellow solutions formed have UV−visible spectra identical with those for monomeric complex 3. However, the DMF and CH3CN solutions upon drying regenerate green solid 5 (often contaminated with red polymorph 4; see the SI, p S26), which suggests the transformation shown in Scheme 1 and Figure 3. Thus, the red dimer 4 is an intermediate product of such aggregation/ disaggregation processes and acts as a building block with the
Figure 2. (A) Top and side views and the numbering scheme in the structure of yellow monomeric 3. (B) Red dimeric 4, which adopts a “head-to-tail” arrangement.
3.1208 Å, which is shorter than the sum of the ionic and van der Waals platinum radii and significantly shorter than that for other published platinum dimers.44−47 The overall structure is best described as a column that consists of “slipped dimers” (see the SI, p S13), the geometrical details of which are shown in the SI, p S14. The formation of red and yellow polymorphs of PtLX2 [L = dipy, phen, bis(isoquinoline); X = Cl, Br, CN] has been well documented in the past48−55 and lately for cyclometalated platinum isonitriles.56 However, a study of their E
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solid-state solution of the famous MGS upon partial oxidation of PtII and formed mixed-valence species, dramatically improving its conductivity.68 As mentioned above, dark-green polymeric Pt(PiPCO)2 and Pt(MCO)2 complexes being excited at 770 nm also emit in the NIR region at 1020 and 1038 nm, respectively (see the SI, p S53). These new cyanoxime ligands also represent N-acetamides formed with piperidine and morpholine, respectively.27,29,30,43 Details of syntheses, structures, and spectroscopic properties of these new polymeric platinum cyanoximates will be published elsewhere. The formation of PtIII species as impurities in the dark-green complex 5 is excluded based on the EPR silence of this coordination polymer at 293 and 80 K (see the SI, p S46). The PdIII centers have readily observable low-spin d7 configurations in a square-planar environment.69−73 The formation of variable-length aggregates in 5, which were held together by metallophillic Pt---Pt interactions, can provide an explanation for some range of λmax of the “blue band” at ∼720−820 nm in visible/NIR spectra attributed to “pokerchip” stacks of variable size. Thus, a difference between the values of maxima in spectra of 5 in the solid state and different solvents, including micelles, can be as big as ∼100 nm. However, donor solvents such as DMSO and Py facilitate a fast and monotonic decrease of the “blue band” intensity and its eventual disappearance (see the SI, p S34), with the remaining spectrum identical with that of yellow complex 3. It is important to note the similar behavior of mixed-valence tetrameric platinum blue complex [Pt4(NH3)8(DMGI)4]· (NO3)5·2H2O (DMGI = 3,3-dimethylglutaimidate), where an intense band at ∼750 nm decreases upon complex disaggregation and reduction to bivalent species.74 The red dimer 4 has a pronounced band at 542 nm (Figure 1). The latter may be attributed to the metal-to-ligand CT band, as was observed for several red PtII dimers of [PtLX2] (L = dipy, phen; X = Cl, Br, CN) composition in the past.44 The phenomenon of a size-dependent change of λmax in the PL of quantum dots, wells, and nanoparticles is well established75 and also associated with extensive charge-carrier delocalization. It is likely that both cases outlined above take place in our particular example of polymeric 1D platinum cyanoximates, where mixed-valence species with IVCT have the dominant effect. In that sense, the presence of small quantities of PtIV centers in complex 5 is similar to a doping ef fect well-known and widely used for semiconductors. In our case, however, it is manifested by a significant red shift of the emission into the extended NIR region beyond 1000 nm. Additional and specifically designed studies of complexes 4 and 5 and other similar platinum cyanoximates are necessary for obtaining a more definite answer regarding the origin of the observed transition. These studies were out of the scope of the current investigation but will be carried out for other recently discovered 1D platinum cyanoximates and reported elsewhere. PL of Solid Complexes in the NIR Region. Crystal data for complexes 3, 4, and 6 revealed that palladium and platinum cyanoximates are typical Werner-type complexes of a nickel triad. Many complexes of this family are known to form dimers45−47 or extended 1D columnar structures that exhibit strong anisotropy of their physical and optical properties.3,42,76 For instance, they possess electrical conductivity and demonstrate unusual recently detected photoemission in the red part of the visible spectrum.77−80 The presence of an NIR absorption band associated with metallophilic interactions in the UV−visible spectra of 5 prompted us to investigate the PL
Figure 3. Proposed structure of a dark-green 1D polymeric Pt(DECO)2 complex 5 and its transformations in solutions.
observed short Pt---Pt interactions necessary for the formation of the green 1D polymer 5. It should be noted that the formation of dark-green, or blue-green, aggregates of other platinum-based alkynyl and terpyridyl62−65 complexes in solutions has been previously observed and correlated with the leading role of metallophilic interactions. There is a plausible explanation for the origin of such an intense broad band in the NIR region of spectra for the polymeric complex 5 both in the solid state and in solutions in which the compound is stable: formation of a mixed-valence coordination polymer that contains a small amount of nonstoichiometric PtIV species. Thus, we suggest the formation of a “solid solution” of 5 containing other PtII metal centers. In this case, a small amount of higher-oxidation-state platinum would provide conditions for the appearance of a low-energy intravalence CT (IVCT) band in the PtII/PtIV or PtII/PtIII centers. This scenario was well documented during extensive investigations of “platinum blues” and related compounds in the past. Air oxygen or chemical oxidizers such as H2O2, FeIII,47 or AgI 46 cations were successfully employed in the past for the generation of mixed-valence complexes that had intense colors and also demonstrate anisotropic properties in the solid state, with electrical conductivity being one of them. In our case, we tried to address the issue of the partial air oxidation of the initial Pt(DECO)2 complex by carrying out its preparation at strictly anaerobic conditions using a specially designed 1 cm quartz cuvette that allows deaeration of starting solutions via repeated freeze/thaw cycles, accompanied by a system flash with argon (see the SI, p S33). The presence of air oxygen does not seem to be the sole source of the PtII oxidation because the “blue band” started to grow after ∼10 min, followed anaerobic mixing of the components. There is, however, a possibility for the chemical oxidation of PtII by the cyanoxime, which, perhaps, cannot be excluded from consideration (see the SI, p S58). Thus, a partial reduction of the ligand in an aqueous environment to hydroxylamine with the formation of a small amount of PtIV may lead to the formation of a mixed-valence complex, which self-aggregates using Pt---Pt interactions into 1D polymer 5. The latter shows intense bands in the visible region and emits in the NIR region of the spectrum (Figures 1 and 5). Interestingly enough, similar to complex 5 presented here, other solid dark-green Pt(PiPCO)2 and Pt(MCO)2 complexes66,67 conduct electricity in the upper range for semiconductors showing values of the solid-state conductivity at room temperature as 22 and 33 S/cm, respectively (see the SI, pp S30 and S31). As observed for the Pt(PiPCO)2 and Pt(MCO)2 complexes, the electrical conductivity at the high end of the semiconductors indicates the presence of delocalized carriers at room temperature, which is typical of mixed-valence compounds. Such carriers, for example, were introduced in the F
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which leads to a lowering of the measured quantum yield especially with the absolute method.81 The platinum and palladium monomeric complexes 3 and 6 were not emissive in the NIR region in accordance with the absence of the absorption band at the wavelength of excitation (770 nm). Similarly, known 1D platinum-coordination polymers such as MGS, POCP, and KCP (Figures 4 and 5), which were used as control compounds, did not show any measurable PL in the NIR region of the spectrum being excited in the 350−700 nm range (see the SI, pp S54 and S55). This range is important for potential biological applications of the NIR emitters. The absence of PL of the above control compounds and the presence of PL in the emissive complex 5 demonstrate the critical importance of the cyanoxime DECO− ligand 2 in the complex. This is perhaps due to its ability to alter the electronic structure of polymer 5 in the manner of dramatic lowering of the energy of key molecular orbitals corresponding to transitions. Also, the electron-withdrawing character of the cyanoxime can make the formation of mixedvalence species easier and thus facilitate electron “hopping” between two different oxidation states. We suggest that the observed PL of 5 originated from lowenergy transitions in an extended mixed-valence complex with metallophilic interactions between the centers. In order to prove this, a small drop of bromine was added to the chamber with the KBr pellet containing 5. The intensity of the emission in the first 1 min of such a treatment increased by ∼30%, with the pellet changing from dark green to copper-red, followed by a slow decrease in the NIR emission and the pellet turning yellow (see the SI, p S56). Thus, bromine partially oxidized PtII to PtIV and facilitated a significant increase in the degree of formation of the mixed-valence complex. The decrease in the intensity is associated with the complex decomposition in the presence of halogen (see the SI, p S57). In the context of the role of an oxidizer and experimental conditions during the syntheses of quasi-1D platinum complexes, it is appropriate to mention inconsistencies of both emission intensities and its energies referred to in the past as a “sample history”.82 No attempts to resolve and study this phenomenon were made. In our view, this is a clear manifestation of the formation or disappearance of mixedvalence species in that particular system. Under the same synthetic conditions as those for Pt(DECO) complexes, Pd(DECO)2 forms exclusively a yellow monomer 6 (see the SI, pp S16 and S21), which has no NIR emission. Importantly, there are some significant differences in time between the platinum and palladium complexes’ formation. For instance, the Pd(DECO)2 complex 6 formed within several minutes, as opposed to the much slower reaction of the formation of the green Pt(DECO)2 complex 5. This is in line with the much greater kinetic lability of bivalent palladium as opposed to platinum.83 Hence, kinetic lability explains why the PdII complexes did not form the extended 1D polymers necessary for emission in the NIR region. The emission−excitation scan for the KBr pellet of 5 is shown in Figure 6A,B and evidenced that the position and intensity of the emission signal remain around 1000 nm and are weakly dependent on the excitation wavelength. This is typical of systems with delocalized charge carriers such as mixed-valence species, quantum dots, and wells. The emission spectrum of 5 was registered at the range of 825−1400 nm using a step of 5 nm and an integration time of 0.5 s with the help of a diode-array CCD detector. Moreover, the emission scan does not resemble
of this compound in the NIR range. To avoid the observed interconversion of the species 3 ↔ 5 in solution, we investigated the emission in the solid state. The solids were embedded in the KBr matrix at 5% by weight concentration to minimize the uncertainties associated with the powder density and particle size and enhance the reproducibility of the results. Such a technique is commonly used in FTIR measurements but was originally also proposed for absorption spectra.22 Control compounds, such as model 1D platinum salts with known different Pt---Pt distances (Figure 4), as well as Pd(DECO)2
Figure 4. KBr pellets containing studied metallocyanoximates and model complexes.
complex 6 (see the SI, p S16), were also investigated. The pellets shown in Figure 4 had a long shelf-life stability (more than 1 year) if stored in a desiccator at room temperature. Details of their preparation and experimental setup can be found in the SI, pp S35 and S37. All pellets were excited at 770 nm with emission measured in the range of 800−1600 nm. Among the studied complexes, only the dark-green platinum complex 5 was found to emit in the NIR range with λmax ∼ 1060 nm (Figure 5). The effect of
Figure 5. Emission of Pt(DECO) and Pd(DECO) complexes embedded in KBr pellets (5 wt %). λex = 770 nm; the detector was a liquid-nitrogen-cooled CCD camera.
the KBr matrix was marginal; the powder also exhibited a similar NIR emission although at a slightly shorter emission maximum (∼1047 nm; see the SI, p S47). Also, it was found that in a vacuum the intensity of the NIR emission is much higher, which implies the role of air oxygen in the triplet state as the phosphorescence quencher (see the SI, p S48). The quantum yield of the emission from a solid pellet of 5 in KBr was measured absolutely and was found to be 0.51% (see the SI, p S49). Although this quantum yield appears to be low, it might be significantly higher in the tablets with a lower concentration of the active component. The graphs shown in the SI, p S45, demonstrate significant absorption from the tablet, thus pointing to potential reabsorption by the sample, G
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7 indicate a pronounced bathochromic shift of λmax of the emission band and a significant intensity decrease upon
Figure 7. Temperature dependence of the emission of green platinum complex 5 embedded in KBr pellets (5 wt %). λex = 770 nm; monitored at 1061 nm. The inset shows a large linear response in the emission intensity change with the temperature.
lowering of the temperature from +50 to −170 °C. The observed signal intensity change for complex 5 is opposite to the common trend.78 However, a very similar red shift of the emission energy was found earlier for other quasi-1D solids such as numerous KCP-type compounds84 and columnar red polymorphs of PtLX2 (L = dipy, phen; X = Cl, Br, CN),49,52,55 or nonaggregated [Pt(ter)Y] (ter = terpyrinine; Y = Cl, solvent molecule).85 Nevertheless, in the latter series of complexes, the opposite to that observed for the complex 5 trend of a dramatic increase in the emission intensity with a temperature decrease was documented. As we stated above, the yellow monomeric complex 3 is not emissive; only complex 5, which is assembled in a 1D polymer, shows pronounced luminescence beyond 1000 nm. We explain the simultaneous increase in the emission intensity with the temperature by analogy with the well-documented behavior of quantum dots and quantum wells with delocalized carriers. Thus, multicore and multishell CdSe/CdS/ZnS/ZnSe quantum dots, as well as single quantum wells GaAsSb/AlGaAs, demonstrate very similar temperature-induced changes in the emission energies and intensity profiles. Consequently, excitons with a temperature increase gain sufficient thermal energy to overcome small potential barriers in the local potential minimum, become mobile, and transfer to higher energy states of the band until the edge of the conduction band is reached.86,87 Hence, the emission energy undergoes a blue shift with a temperature increase. We suggest that at low temperatures excitons mostly localize in a mixed-valence aggregate, which is small in size, or even in the mixed-valence dimer. Elevation of the temperature creates delocalized excitons that can now reach higher energy states and, because of electron hopping, spread on a larger distance, increasing the probability of emission from a larger number of mixed-valence centers along the Pt---Pt wire. In general, increasing with temperature, electronic hopping interactions between sites lead to a greater degree of delocalization in both the ground and excited states.88 The energy that corresponds to an observed shift of 54 nm is 472 cm −1 (58 meV, or 5.65 kJ/mol) for complex 5 (Figure 7) and is consistent with delocalization of
Figure 6. (A) 3D excitation−emission scan of green 1D polymeric [Pt(DECO)2]n in KBr tablets, (B) its 2D projection showing the position of the emission at 900−1100 nm practically independent of the excitation wavelength (the intensity was corrected for the light intensity), and (C) the excitation spectrum recorded in the range 350−810 nm.
the absorption spectra of the green [Pt(DECO)2]n complex 5 in KBr pellets (see the SI, p S45) because of severe distortion of the former due to strong chromophore coupling in the solid state. Dependence of the Emission Profile from Temperature. Transition Energy and Line Intensity. Investigation of the temperature-dependent PL of the dark-green complex 5 in the KBr pellet shed some light on the mechanism of the emission. The experiments were carried out using a custombuilt liquid-filled cryostat (see the SI, p S36) with the sample under vacuum at ∼6 × 10−2 torr to prevent moisture condensation on the quartz windows. Data displayed in Figure H
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ligand affects the emissive properties of the complexes. This knowledge will expand the family of emitters to a variety of materials with different wavelengths of emission. Equally important is to demonstrate the utility of the complexes in real-life applications. Instead of embedding the complexes in KBr tablets, which are excellent models for the basic research, more realistic matrix materials should be explored.
electrons along the 1D chain in compounds with metallophillic interactions.3,89 Ultimately, these results can be explained in terms of an increase of the distance traveled by electrons along the 1D Pt--Pt chain (e-delocalization) with rising temperature, which requires higher energy. Lower temperatures tend to localize this motion to a smaller distance. Similar blue-shift behavior was reported for a series of cadmiun halcogenide90,91 and In−As quantum dots92 and wells86 where even values of the PL temperature shifts are in the same range of 30−60 meV.93 The intermetallic Pt---Pt distances in KCP, MGS, and POCP were previously reported as 3.5, 3.25, and 2.89 Å, respectively. The crystal structure of the dimer 4 showed the Pt---Pt distance as 3.1208 Å, and apparently the emitting 1D polymeric complex 5 has the same or a similar intermetallic distance. Our data indicate rather clearly that the length of the metal−metal bond alone cannot explain the observed emission. Rather, the combination of several factors is important. The unique role of the cyanoxime ligand in the emission is suggested to be critical to (1) the formation of an appropriate low-energy electronic state including metal orbitals due to the asymmetric cis-PtN2O2 environment with significant covalent character in Pt−N bonding, (2) the electron-withdrawing character of the ligand, which allows depletion of the electron density from the metal centers and helps in the formation of mixed-valence centers due to an intramolecular redox process, and (3) providing a favorable geometrical configuration, leading to alignment of 1D polymeric “poker-chip stacks” of certain length and necessary for the NIR emission. Other ligands with different substituents are currently being investigated to evaluate the role of the ligand in the assembly. Line Width (Full Width at Half-Maximum). The almost symmetrical signal of the emission from polymeric complex 5 can best be described as two Gaussian-type lines (see the SI, pp S51 and S52). The minor slightly higher energy component contributes from 2.2 to 5.4% intensity to the overall fit at +50 and −165 °C. During variable-temperature studies, we also observed line narrowing upon a temperature decrease (see the SI, p S50), which is in agreement with the literature data for all the above-mentioned quasi-1D systems. Practicality of Studied Compounds and Future Plans. Similar to other platinum complexes such as cisplatin and oxaliplatin, platinum cyanoximates also demonstrate cytotoxicity27,28,30 and, in combination with their emissive properties, can be potentially utilized as theranostic agents. For these biomedical imaging and theranostics applications, the complexes have to be formulated for better bioavailability. Therefore, encapsulation of these NIR-emissive complexes in micelles, or water-soluble polymeric shells, is considered for future studies. Our preliminary data indicate just facile formation of monodispersed micelles based on sodium salts of long-chain carboxylic acids (see the SI, pp S40 and S41). This finding opens a new area of research for application of this and similar polymeric platinum cyanoximates. Future work also will be focused on a better understanding of the nature of the emission and tuning of the structures toward brighter complexes with a diverse range of emission. Such work will include, for example, characterization and measurement of the PL lifetime to characterize the type of emission. Given that the 1D aggregate might have a relatively large length distribution, it is also important to identify the relationship between the brightness of the emission and the size of the “poker-chip” stack. It is also critical to understand how the nature of the
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CONCLUSIONS In summary, we developed a unique supramolecular assembly based on the PtII complex of PtL2 composition (L = new cyanoxime ligand of the N-acetamides family, namely, 2oximino-2-cyano-N,N′-diethylacetamide, HDECO). The complex has three polymorphic forms: monomeric yellow Pt(DECO)2, red dimeric [Pt(DECO)2]2, and dark-green [Pt(DECO)2]n. The latter complex represents a quasi-1D coordination polymer that strongly absorbs in the 400−900 nm range and luminesces at 1000−1200 nm. This compound also appears to be a solid solution containing nonstoichiometric amounts of PtIV centers. Thus, aggregation of monomeric Pt(DECO)2 units into a stacked polymer in aqueous solution is driven by metallophilic interactions and accompanied by the partial oxidation of PtII to PtIV. Contrary to other previously reported unstable mixed-valence palladium-17 or platinumbased72,74 “metal wires”, presented in this work dark-green polymeric platinum cyanoximate is stable in the solid state at room temperature for many months. This and similar NIR emissive platinum cyanoximate complexes can be used in lightemitting devices and optical sensors to cover wavelengths that are currently unavailable. In addition, because of the transparency of the biological tissue at 1060−1150 nm (second optical window)8,9,94 and established cytotoxicity of platinum cyanoximates,27,30,95 the emissive platinum complexes can be utilized as theranostic agents for anticancer treatment and diagnostics.
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ASSOCIATED CONTENT
S Supporting Information *
Thermodynamic properties of new compounds (section 1), crystal structures (section 2), microscopy images of crystals of studied complexes (section 3), synthesis/additional information (section 4), hardware for spectroscopy measurements (section 5), optical and spectroscopic properties of the complexes (section 6), and CIF files for all structures presented herein [CCDC 983924 (for 1), 983927 (for 3), 983928 (for 4), and 983926 (for 6)]. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS N.G. is very grateful to Prof. V. Yam (University of Hong Kong) for the earlier recording of the emission spectra of other platinum(II) cyanoximates in solutions, to M. Hilton for his careful and diligent work on the KBr pellet preparations, to B. Grindstaff for custom manufacturing specialty cuvettes and other hardware necessary for photophysical measurements, to I
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H. Zhang for her help with the photoemission measurements, and to A. Corbett for technical help during manuscript preparation. We also acknowledge MSU Graduate College for partial financial support of this work throughout several years and financial support from the NCI/NIH (Grant R21CA149814) and NSF (Grant 1355466). We also thank the Washington University Optical Spectroscopy Core facility (Grant NIH 1S10RR031621-01) for spectroscopy measurements. We are grateful to Dr. Justin Douglas from the University of Kansas for recording the EPR spectra of solid samples of 5 (support for the EPR instrumentation was provided by NSF Chemical Instrumentation Grant 0946883). This material is also based upon work supported by the NSF under Award Number IIA-1355406. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF.
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DOI: 10.1021/ic502805h Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/ic502805h Inorg. Chem. XXXX, XXX, XXX−XXX
