Influences of Molecular Structure on Supramolecular Selection during

Nov 19, 2014 - School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland 4001, Australia. Cry...
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Influences of Molecular Structure on Supramolecular Selection during Cocrystallization of Polypyridyl Metal Complexes Published as part of the Crystal Growth & Design virtual special issue IYCr 2014 - Celebrating the International Year of Crystallography Jocelyne Bouzaid, Madeleine Schultz, Zane Lao, Thor Bostrom, and John McMurtrie* School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland 4001, Australia ABSTRACT: Heteroleptic complexes of the type [RuL2L′](PF6)2 (L, L′ = combinations of 1,10-phenanthroline (phen) and 2,2′-bipyridine (bipy)) were found to cocrystallize with [Ni(phen)3](PF6)2 to produce cocrystals of [Ni(phen)3]x[RuL2L′]1−x(PF6)2. In this report we show that the ability of the complexes to cocrystallize is influenced by the number of common ligands between complexes in solution. Supramolecular selection is a phenomenon caused by molecular recognition through which cocrystals can grow from the same solution but contain different ratios of the molecular components. It was found that systems where L = phen displayed less supramolecular selection than systems where L = bipy. With increasing supramolecular selection, the composition of cocrystals was found to vary significantly from the initial relative concentration in the cocrystallizing solution, and therefore it was increasingly difficult to control the final composition of the resultant cocrystals. Consequently, modulation of concentration-dependent properties such as phase was also found to be less predictable with increasing supramolecular selection. Notwithstanding the complication afforded by the presence of supramolecular selection, our results reaffirm the robustness of the [M(phen)3](PF6)2 structure because it was maintained even when ca. 90% of the complexes in the cocrystals were [Ru(phen)(bipy)2](PF6)2, which in its pure form is not isomorphous with [M(phen)3](PF6)2. Experiments between complexes without common ligands, i.e., [Ru(bipy)3](PF6)2 cocrystallized with [Ni(phen)3](PF6)2, were found to approach the limit to which molecular recognition processes can be confused into cocrystallizing different molecules to form single cocrystals. For these systems the result was the formation of block-shaped crystals skewered by a needle-shaped crystals.

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INTRODUCTION An unsolved challenge for supramolecular chemists is to reliably predict the crystal structures and tune the properties of functional materials.1 Cocrystallization has been used to effectively dilute metal complexes that have common ligands; i.e., the only difference between complexes is the metal center such that [MA(L)n]z is cocrystallized with [MB(L)n]z to form systems with the general formula such as [MAxMB1−x(L)n]z (MA, MB = different metals; L = ligand; x = metal complex fraction; n = number of ligands; z = overall charge). These types of systems are typically solid solutions of molecules and hence are also referred to as molecular alloys. Because of the similarity between the molecular components, they usually form with predictable crystal structures and with properties intermediate between those of the pure complexes.2−9 We have been investigating the cocrystallization of metal complexes, taking advantage of well-known supramolecular motifs displayed by polypyridyl complexes (Figure 1), with the goal of manipulating concentration-dependent physical properties. Pairs of [M(phen)3]2+ complexes interact through one offset face-to-face (OFF) and two edge-to-face (EF) © 2014 American Chemical Society

interactions (Figure 1a) which propagate to give the phen motif. Pairs of [M(bipy)3]2+ interact through six EF interactions (Figure 1b) and propagate to give the bipy motif. Dance et al. define the phen embrace as a parallel-4-fold-aryl-embrace (P4AE)10,11 and the bipy motif as a 6-fold-aryl-embrace (6AE).12−14 In 2012, we reported the synthesis and single crystal characterization of molecular alloys of [MAxMB1−x(phen)3](PF6)2·0.5H2O and [MAxMB1−x(bipy)3](PF6)2 (MA and MB = combinations of Fe(II), Ni(II), and Ru(II); phen = 1,10-phenanthroline; bipy = 2,2′-bipyridine) where the interactions between complexes are built upon πinteractions.15 In that report, molecular alloys were grown from evaporating solutions of [MA(L)3](PF6)2 and [MB(L)3](PF6)2 (L = phen or bipy) in acetone and water with relative molar ratios of the two metal complexes in solution ranging from 0.9:0.1 to 0.1:0.9 in 0.1 increments. Characterization using Xray diffraction techniques revealed that the cocrystallized Received: June 16, 2014 Revised: October 21, 2014 Published: November 19, 2014 62

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Figure 1. Motifs observed in [M(phen)3]2+ structures differ from those observed in [M(bipy)3]2+ systems as shown by the comparison of (a) the phen motif comprising one offset face-to-face and two edge-to-face aryl−aryl interactions between two [M(phen)3]n+ complexes with (b) the bipy motif which comprises six edge-to-face interactions (three in front and three obscured) between two [M(bipy)3]n+ complexes.

transition metal, are labile and undergo ligand exchange in solution. Therefore, the experiments described above could not be conducted by mixing solutions of two first row transition metal complexes with different ligands. For example, if [Ni(phen)3]2+ is mixed with [Fe(bipy)3]2+, exchange of the phenanthroline and bipyridine components would rapidly result in formation of a large number of mixed ligand complexes (including [Ni(phen) 2 (bipy)] 2+ , [Ni(phen)(bipy)2]2+, etc.). Ruthenium(II) complexes with polypyridyl ligands are inert under the experimental conditions employed for cocrystallization and therefore are not expected to exhibit ligand exchange.16,17 In this report, we interpret the influence of variations of molecular structure on supramolecular selection and hence on the composition of cocrystals comprised of mixed ligand ruthenium polypyridyl complexes and [Ni(phen)3](PF6)2.

samples were isomorphous with the pure compounds and that they were comprised of only one phase. Crystals of [MAxMB1−x(phen)3](PF6)2 contained the same ratio of metals as originally mixed in the crystallizing solution. However, and surprisingly, data for [MAxMB1−x(bipy)3](PF6)2 cocrystals indicated that different cocrystals harvested from the same solution contained ratios of metals that were sometimes significantly different to the ratio present in solution and also different from each other. To explain these unprecedented results, we introduced the phenomenon of supramolecular selection, a process that is driven by molecular recognition whereby identical molecules crystallize slightly more favorably with each other than with structurally similar molecules. So, for a mixture containing two similar molecules, say A and B, cocrystallization proceeds with A crystallizing more favorably (but not exclusively) with A and B crystallizing more favorably (though not exclusively) with B. The result is that some of the cocrystals formed have a higher concentration of A than B and vice versa despite all crystals being grown at the same time and from the same solution. This phenomenon was observed in the cocrystallization of [M A x M B 1−x (bipy) 3 ](PF 6 ) 2 but not in the analogous [MAxMB1−x(phen)3](PF6)2·0.5H2O systems. For these homoleptic complexes, differences between the geometric parameters such as bite and twist angles for the different metal complexes are almost negligible. It is therefore very interesting that the disparity between the metal centers in simple binary [MAxMB1−x(bipy)3](PF6)2 systems is recognized beyond the point of difference because intermolecular interactions occur between the ligands of neighboring coordinating spheres and not the metals. Given the lack of apparent supramolecular selection in [MAxMB1−x(phen)3](PF6)2·0.5H2O systems, it was of interest to cocrystallize [M(phen)3]2+ complexes with heteroleptic complexes of the type [M(phen)2(bipy)]2+ and [M(phen)(bipy)2]2+, to see whether supramolecular selection is observed, and if so, to what degree. It was also of interest to attempt the cocrystallization of [MII(bipy)3]2+ cations with [MII(phen)3]2+ cations. These experiments were designed to introduce varying degrees of structural difference between complexes by decreasing the number of common ligands in the system, i.e., stepwise substitution of 1,10-phenanthroline by 2,2′-bipyridine. Metal complexes with polypyridyl ligands such as [MII(phen)3]2+ and [MII(bipy)3]2+, where MII is a first row

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EXPERIMENTAL SECTION

The metal complexes [Ni(phen)3](PF6)2·0.5H2O,18 cis-[Ru(L)2Cl2]· 2H2O (L = phen, bipy), [Ru(phen)(bipy)2](PF6)2, [Ru(phen)2(bipy)](PF6)2, [Ru(bipy)3](PF6)2 (phen = 1,10-phenanthroline and bipy = 2,2′-bipyridine)19 were synthesized using established literature methods. All solvents were of reagent grade quality and used as supplied by the manufacturers. [Ni(phen)3](PF6)2·0.5H2O Pink Prisms. Yield 0.76 g (93%). Mass spectrum: (HR-ESI) MeCN [ML 3 ]2+ m/z 299.0715 [100%] (calculated for [C36H24N6Ni1]2+, 299.0708), [ML3 + X]+ m/z 743.1063 [35%] (calculated for [C36H24N6Ni1 + PF6]+, 743.1063). Found: C, 47.68; H, 2.60; N, 9.11. Calculated for C36H24N6NiP2F12· (H2O)0.5: C, 48.14; H, 2.81; N, 9.36.15 cis-[Ru(phen)2Cl2]·2H2O Yield 2.79 g (67%). This complex was used as a synthetic precursor without further purification. 1H NMR (400 MHz; DMSO-d6) 10.32 (d, 3JHH = 4.7 Hz, 2H), 8.69 (d, 3JHH = 8.2 Hz, 2H), 8.40 (d, 3JHH = 8.2 Hz, 2H), 8.33 (d, 3JHH = 8.2 Hz, 2H), 8.28 (d, 3JHH = 8.2 Hz, 2H), 8.21 (m, 2H), 7.77 (dd, 3JHH = 4.7 Hz, 2H), 7.32 (ddd, 3JHH = 8.2 Hz, 2H). This is a match to the spectrum reported in the literature.20 cis-[Ru(bipy)2Cl2]·2H2O Yield 2.58 g (69%). This complex was used as a synthetic precursor without further purification. 1H NMR (400 MHz; DMSO-d6) 9.95 (unresolved dd, 3JHH = 4.8 Hz, 2H), 8.63 (d, 3JHH = 8.2 Hz, 2H), 8.47 (d, 3JHH = 8.2 Hz, 2H), 8.06 (dt, 3JHH = 7.6 Hz, 4JHH = 1.2 Hz, 2H), 7.77 (unresolved dt, 3JHH = 7.6 Hz, 2H), 7.68 (unresolved dt, 3JHH = 8.2 Hz, 2H), 7.51 (d, 3JHH = 5.3 Hz, 2H), 7.10 (unresolved td, 3JHH = 6.8 Hz, 2H). This is a match to spectra reported in the literature.20,21 63

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[Ru(phen)2(bipy)](PF6)2 Red Prisms. Yield 0.62 g (69%) based on cis-[Ru(phen)2Cl2]·2H2O. Found: C, 44.91; H, 2.62; N, 9.40. Calculated for C34H24N6RuP2F12: C, 44.99; H, 2.67; N, 9.26. [Ru(phen)(bipy)2](PF6)2·EtOH Red Prisms. Yield 0.63 g (68%) based on cis-[Ru(bipy)2Cl2]·2H2O. Found: C, 43.89; H, 3.29; N, 9.87. Calculated for C32H24N6RuP2F12·C2H5O: C, 43.93; H, 3.25; N, 9.04. [Ru(bipy)3](PF6)2 Red Prisms. Yield 1.16 g (70%). HR-ESI-MS [ML3]2+ m/z 285.0548 [100%] (calculated for [C30H24N6Ru1]2+, 285.0553), [ML3 + X]+ m/z 715.0739 [35%] (calculated for [C30H24N6Ru1 + PF6]+, 715.0753). 1H NMR (400 MHz; DMSOd6): δH 8.84 (d, 3JHH = 8.2 Hz, 6H), 8.17 (unresolved td, 3JHH = 7.6 Hz, 6H), 7.73 (d, 3JHH = 5.3 Hz, 6H), 7.53 (unresolved td, 3JHH = 6.7 Hz, 6H). Found: C, 41.66; H, 2.59; N, 9.71. Calculated for C30H24N6RuP2F12: C, 41.92; H, 2.81; N, 9.78.15 Cocrystallization Experiments. Binary metal complex cocrystals were prepared as previously reported15 by cocrystallization from evaporating solutions of the complexes [Ni(phen)3](PF6)2 with [Ru(phen)2(bipy)](PF6)2, [Ru(phen)(bipy)2](PF6)2 or [Ru(bipy)3](PF6)2 in acetone and water. Cocrystallization experiments were performed with relative molar ratios of the two complexes in solution ranging from 90:10 to 10:90 in 20% increments. All experiments were performed in duplicate, and in each case, crystallization was allowed to continue until no appreciable color remained in the mother liquor. Characterization of Metal Complex Cocrystals. Powder X-ray diffraction data of both the pure and the cocrystallized samples were collected to examine phase purity. Powdered samples were prepared as thin layers adhered to Vaseline on silicon 511 wafers, creating a thin film of fine powder. Crystallized samples were visually assessed under an optical microscope equipped with a polarizer for evidence of cocrystallization. Crystals deemed by visual inspection to be single and representative of the entire batch (as indicated predominantly by size and morphology) were subjected to elemental analysis using energy dispersive X-ray spectroscopy (SEM-EDX). Crystals were mounted on aluminum stubs using double-sided carbon tape and then coated with a thin layer of evaporated carbon to enhance surface conductivity and reduce charging. At least three crystals were harvested from the same crystallizing solution (batch), and each were analyzed in three different areas. The ratio of the atomic fractions of the metals was calculated from the energy dispersive X-ray spectra, thus providing the ratio of the two metals and by extension the metal complexes in each crystal. The instrument was calibrated using a set of standard blocks mounted with natural minerals containing elements found in the samples analyzed. It should be noted that SEM-EDX is a surface technique that analyzes a crystal to a depth of approximately 6 μm and does not provide direct information about the composition at greater depths. The ratios of the metal complexes found in each binary cocrystal were plotted as a molar fraction of [Ni(phen)3](PF6)2 in crystallizing solution versus mole fraction of [Ni(phen)3](PF6)2 determined by SEM-EDX in the resulting cocrystals. Least squares regression analysis was applied to the plots to provide semiquantitative comparisons between the cocrystallization behavior of the metal complex mixtures. Instrument Details. Energy-dispersive X-ray (EDX) analysis and scanning electron microscope (SEM) imaging were conducted on a JEOL 840A analytical SEM set at 20 kV accelerating voltage coupled with a JEOL 2300 microanalyzer. The electron beam current was set at 1.2 nA, which was measured with a Faraday cup, while the acquisition live time was programmed for 120 s with an average of 3000 counts per second. The molar fraction of the metal complexes in the solid state (χMsolid) were calculated by dividing the observed atomic percentages (obtained from a standardized quantitative method using phi-rho-z X-ray depth distribution function) for MA by the sum of the values obtained for the two metals MA + MB. For example, a single cocrystal grown from a solution containing [Ni(phen)3]2+ and [Ru(phen)2(bipy)]2+ with a molar ratio of 90:10, respectively, which analyzed to give EDX atomic percentage readings of 1.47 and 0.19 for Ni and Ru, respectively, was calculated to have a solid state composition of [Ni(phen)3]0.89[Ru(phen)2(bipy)]0.11(PF6)2. X-ray diffraction (XRD) patterns were collected using a PANalytical X’Pert PRO MPD X-ray diffractometer (radius of 240 mm) in Bragg−

Brentano geometry. The incident X-ray radiation was sourced from a line-focused PW3373/10 copper (Cu) X-ray tube at 40 kV and 40 mA, generating a Kα wavelength of 1.54 Å. The incident beam was conditioned by being passed through a 0.04 rad Soller incident slit, a variable divergence slit set to 20 mm irradiated length, a 15 mm fixed mask, and a 1° fixed antiscatter slit. The diffracted beam had a 5° antiscatter slit, a 0.04 rad Soller slit, and an X’Celerator RTMS detector set to 2°. Patterns were acquired over a range of 3.5−40° 2θ with a nominal step size of 0.0167° 2θ at 95 s per step. Calculations and analysis of diffraction patterns were made using the X’pert Highscore (V3, PANanalytical) software package. High resolution electrospray ionization mass spectrometry (ESI-MS) was conducted on an Agilent 6520 Accurate-Mass Q-TOF LC/MS. Samples were dissolved in HPLC-grade MeCN and diluted to concentrations of 10 μg·cm−3. Ultrapure nitrogen gas was used as both the drying and nebulizing gas, and the source temperature was set to 350 °C. The fragmentor and skimmer voltages were set at 100 and 20 V respectively. Mass spectra were collected from m/z 100 to 1000 Da. 1 H NMR spectra were recorded on a Bruker Avance 400 MHz at room temperature. Only the pure compounds where characterized using this method. Samples were prepared in DMSO-d6. Chemical shifts are reported in ppm and referenced against the solvent peak. Elemental microanalysis (C, H, N) was performed by the University of Queensland Microanalytical Service. Difference calculations were employed to determine the solvate species.

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RESULTS AND DISCUSSION Cocrystallization of mixtures of pink [Ni(phen)3](PF6)2 and red [Ru(phen)2(bipy)](PF6)2 leads to red crystals that contain the metals in similar proportions to the original solution. Figure 2 illustrates the results of SEM-EDX analyses for the cocrystallization experiments, in which five of the six ligands are phenanthroline.

Figure 2. SEM-EDX results for cocrystals grown from solutions containing [Ni(phen)3](PF6)2 and [Ru(phen)2(bipy)](PF6)2 in relative molar ratios from 0.10 to 0.90 in 0.20 increments.

Consider the data points at 0.1 on the x axis. These arise from the experimental determination of the relative concentration of [Ni(phen)3]2+ complexes in cocrystals from a batch prepared by crystallization of 0.1:0.9 ratio of [Ni(phen)3]2+ to [Ru(phen)2(bipy)]2+. The measurements at 0.30 on the x axis are from crystals obtained from a solution containing a ratio of 64

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0.30:0.70 of the same complexes, respectively, and so on. Within each batch, the data for each crystal are colored differently. For example, in the 0.10:0.90 cocrystallization batch, there are clusters of points colored in red, green, and blue. The red points represent measurements taken from the same crystal. The blue points from another crystal and the green from a third crystal. The colors are a convenient way to identify data measured from individual crystals. There is no relationship between colors of the data points in different batches. It can be seen from Figure 2 that measurements taken from each cocrystal (indicated by same color markers within a specific metal complex ratio) displayed similar Ni:Ru ratios (within 2%) but differed slightly from measurements obtained from other single crystals harvested from the same solution. The equation for the least-squares regression line together with the errors in the slope and the intercept for this Ni:Ru mixed-ligand system were found to be y = (0.98x ± 0.01) − (0.01 ± 0.01). Although some variation within each metal ratio batch was observed, the gradient and y intercept are still close to one and zero, respectively. This indicates that the relationship between the [Ni(phen)3](PF6)2 concentration in the solid was largely dependent on the proportion of the metal complexes in solution. Note that cocrystals formed with solid-state concentrations both above and below that of the initial relative molar ratios in solution. As observed for the [NixRu1−x(bipy)3](PF6)2 systems,15 the largest variation, albeit only by a slight margin, is observed for the 0.50:0.50 sample (±0.10). Compared to the cocrystals of [NixRu1−x(bipy)3](PF6)2,15 these crystals showed much less variation in metal complex concentration, even though the former involve only a single ligand type. The pairwise arrangement of [Ni(phen)3]2+ cations in the phen motif involves only two of the three ligands engaging in aryl−aryl interactions (see Figure 1). The motif extends in two dimensions such that all three ligands are involved in interactions of the same type with adjacent complexes. It appears that since the phenanthroline ligand is larger than bipyridine the crystal packing can easily withstand the replacement of a single phenanthroline ligand by bipyridine. Notwithstanding this, the results presented here suggest the presence of a small degree of supramolecular selection as indicated by the variation in relative concentrations of the metal complexes in crystals harvested from the same batch. Powder X-ray diffraction patterns for pure [Ni(phen)3](PF6)2 and [Ru(phen)2(bipy)](PF6)2 as well as those obtained from the cocrystals are shown in Figure 3 and are graphed so that the position °2θ (shown on the x axis) is plotted against the relative intensity “counts” (shown on the y axis). Miller indices have been assigned to the main peaks in Figure 3 based on the indexing obtained from the [Ni(phen)3](PF6)2 pattern simulated from SCXRD data (deposited with the Cambridge Structural Database (CCDC reference number 869923) and can be obtained free of charge via www.ccdc.cam.ac.uk/conts/ retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or [email protected]). The labeled dotted lines emphasize the positions of the prominent peaks. The diffraction patterns of pure [Ni(phen)3](PF6)2 and [Ru(phen)2(bipy)](PF6)2 and those of the cocrystallized samples are very similar to each other, showing that the crystals are isomorphous and all crystallize in the monoclinic space group Cc. The similarity of the experimental patterns to the simulated patterns indicate that the cocrystal samples are phase pure.

Figure 3. Powder XRD of pure [Ni(phen)3](PF6)2 (blue), [Ru(phen)2(bipy)](PF6)2 (red) interleaved with the patterns of cocrystals resulting from the cocrystallization of these complexes in relative molar ratios of 0.10:0.90, 0.30:0.70, 0.50:0.50, 0.70:0.30, and 0.90:0.10.

The vertical dotted lines shown in Figure 3 highlight the slight differences between the peak positions of the reflections obtained from these samples. Generally, the equivalent peaks in the [Ni(phen)3](PF6)2 pattern occur at lower angles than the corresponding peaks for [Ru(phen)2(bipy)](PF6)2 peaks. This indicates that the unit cell of the former is slightly larger than that of the latter, as might be expected. Close inspection of the patterns obtained from the cocrystallized samples suggests a gradual change in the positions of these reflections and hence in the unit cell parameters. These peaks are shown to generally shift leftward (toward lower angles) as the concentration of [Ni(phen)3](PF6)2 in solution increases, indicating an increase in the size of the unit cell in the resultant cocrystals. Therefore, it appears that for these systems, the unit cell parameters are able to be modulated in a systematic and predictable manner by controlling the initial relative concentration of the complexes in the crystallizing solution. It was of interest to observe if the substitution of a further 1,10-phenanthroline ligand in the ruthenium complex with 2,2′bipyridine leads to greater supramolecular selection. Cocrystallization experiments between [Ni(phen)3](PF6)2 and [Ru(phen)(bipy)2](PF6)2 were carried out, and Figure 4 shows the SEM-EDX results for a representative set of experiments. It can be seen from Figure 4 that these single crystals gave results that are reminiscent of those obtained for [MAxMB1−x(bipy)3](PF6)2;15 that is, individual cocrystals vary within any given batch and can differ greatly in their composition from the initial relative ratio of metal complexes 65

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Figure 4. SEM-EDX results for cocrystals grown from solutions containing [Ni(phen)3](PF6)2 and [Ru(phen)(bipy)2](PF6)2 in relative molar ratios from 0.10 to 0.90 in 0.20 increments.

in solution. The results of this system display even larger variations of concentrations of metal complexes between cocrystals harvested from the same solution than that of [M AxM B1−x(bipy)3](PF 6)2 systems as evidenced by the correlation coefficient (R2 = 0.92) and the errors in the slope and intercept (y = (1.02x ± 0.04) − (0.03 ± 0.02)). Again, cocrystals formed with solid-state concentrations both above and below that of the initial relative molar ratios in solution. As with the previously investigated systems that displayed supramolecular selection, the largest variation is observed for the 0.50:0.50 sample at ca. 30% (that is, relative concentrations of metal complexes in these cocrystals can vary by up to 15% from the relative concentrations of the complexes in the crystallizing solutions), which is in fact the largest variation that we have so far observed for these types of systems. This indicates that supramolecular selection is playing a significant role in determining the metal complex ratios in the single crystals of [Ni(phen)3]x[Ru(phen)(bipy)2]1−x(PF6)2. In light of the SEM-EDX data, it was especially important to investigate the crystal phase of cocrystals resulting from these systems. Powder X-ray diffraction patterns for pure [Ni(phen)3](PF6)2 and [Ru(phen)(bipy)2](PF6)2 as well as those obtained from cocrystals grown from solutions containing both complexes are shown in Figure 5. To highlight similarities (and differences) between patterns, Miller indices have been assigned to the main peaks based on the indexing obtained from the [Ni(phen)3](PF6)2 pattern simulated from SCXRD data. The labeled dotted lines emphasize the positions of the prominent peaks in like patterns. The diffraction pattern of pure [Ni(phen)3](PF6)2 (blue) is very different from the [Ru(phen)2(bipy)](PF6)2 pattern (red). The patterns obtained from the cocrystallized samples are very similar to each other and to the experimentally obtained [Ni(phen)3](PF6)2 pattern indicating that they are isomorphous and crystallize in the monoclinic Cc space group. None of the cocrystal patterns have additional peaks that can be attributed to the [Ru(phen)(bipy)2](PF6)2 phase. Furthermore,

Figure 5. Powder XRD of pure [Ni(phen)3](PF6)2 (blue), [Ru(phen)(bipy)2](PF6)2 (red) interleaved with the patterns of cocrystals resulting from the cocrystallization of these complexes in relative molar ratios of 0.10:0.90, 0.30:0.70, 0.50:0.50, 0.70:0.30, and 0.90:0.10.

the similarity of these patterns to that of the simulated [Ni(phen)3](PF6)2 pattern indicate that they are phase pure. The vertical dotted lines shown in Figure 5 draw attention to the gradual leftward shift (toward lower angles) of the prominent peaks with increasing initial solution concentrations of [Ni(phen)3](PF6)2. Again, this is an indication that an increase of [Ni(phen)3]2+ complexes that are incorporated into the crystals causes an increase of the unit cell parameters. It is interesting that cocrystals with a relative solid state [Ni(phen)3](PF6)2 concentration as low as 6% that were formed from solutions containing an initial [Ni(phen)3](PF6)2 molar ratio of 10% (as evidence from SEM-EDX data shown in Figure 3) solely adopt the [Ni(phen)3](PF6)2 phase. Thus, the [Ni(phen)3](PF6)2 structure can accommodate [Ru(phen)(bipy)2]2+ ions in place of [Ni(phen)3]2+ ions, while the [Ru(phen)(bipy)2](PF6)2 phase cannot accommodate [Ni(phen)3]2+ ions in place of [Ru(phen)(bipy)2]2+. This is perhaps not surprising given the relative size difference between the complexes which means that the smaller [Ru(phen)(bipy)2]2+ can fit in a space normally occupied by [Ni(phen)3]2+, while the reverse is not possible. It appears that for this system, the unit cell parameters are also able to be systematically modulated by controlling the initial relative concentration of the complexes in the crystallizing solution, albeit in a less predictable manner due to the significant degree of supramolecular selection. These results suggest that the [M(phen)3]2+ motif can withstand significant changes to the molecular components, both the ligands and the metals, but that supramolecular 66

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Table 1. Summary of SEM-EDX Results for Mixed-Ligand Cocrystallization Experiments Compared with Homoleptic Experiments (in Bold) entry

complex A

complex B

1 2 3 4

[Ni(phen)3](PF6)2 [Ni(phen)3](PF6)2 [Ni(phen)3](PF6)2 [Ni(bipy)3](PF6)2

[Ru(phen)2(bipy)](PF6)2 [Ru(phen)(bipy)2](PF6)2 [Ru(phen)3](PF6)2 [Ru(bipy)3](PF6)2

regression line equation y y y y

= = = =

0.98x + 0.01 1.02x − 0.03 0.994x − 0.001 1.02x + 0.03

error in slope (±)

error in intercept (±)

R2

0.01 0.04 0.003 0.03

0.01 0.02 0.002 0.01

0.99 0.92 0.999 0.93

Figure 6. (a) From left to right, single crystal of pure [Ru(bipy)3](PF6)2 (red needle) [Ni(phen)3](PF6)2 (pink prism) and a cocrystal formed by the crystallization of a 1:1 ratio of [Ru(bipy)3](PF6)2 and [Ni(phen)3](PF6)2. (b) A batch of skewered cocrystals formed by the cocrystallization of a 1:1 mixture of [Ru(bipy)3](PF6)2 and [Ni(phen)3](PF6)2.

Figure 7. Secondary electron image (a) and backscattered image (b) of a well-defined block shaped crystal skewered by a needle grown from a solution containing an equimolar mixture of [Ni(phen)3](PF6)2 and [Ru(bipy)3](PF6)2.

(PF6)2 was cocrystallized with [Ru(phen)(bipy)2](PF6)2 (entry 2). Although [Ni(bipy)3]2+ and [Ru(bipy)3]2+ have similar geometric parameters, varying only in the metal center and by the slightest margins, previous cocrystallization experiments have demonstrated that this variation between complexes is being recognized, although the metal center is completely shrouded by ligands, giving rise to a system that displays supramolecular selection. This is noteworthy because unlike the homoleptic [NixRu1−x(bipy)3](PF6)2 system, molecular recognition processes between [Ni(phen)3]2+ and either [Ru(phen)2(bipy)]2+ or [Ru(phen)(bipy)2]2+ are influenced by differences at the metal center and ligand, the latter difference being a variation that is exposed to other complexes during cocrystallization. This suggests that the degree of supramolecular selection that is observed it is not only influenced by the differences introduced between the complexes being cocrystallized but also by the type of intermolecular interactions involved in propagating the structure (bipy vs phen motif). Given the increasing supramolecular selection observed as the number of common ligands reduced, we continued by investigating the cocrystallization of [Ni(phen)3]2+ with [Ru(bipy)3]2+. Crystals of pure [Ni(phen)3](PF6)2·0.5H2O

selection begins to play an important role in determining the concentration of each metal complex in the resulting crystals. Our previous work has shown that the [M(bipy)3]2+ motif is substantially less robust than that of [M(phen)3]2+ with respect to substitution of different metals within cocrystals.15 Table 1 summarizes the regression lines, respective errors, and correlation coefficients for the mixed-ligand cocrystallization experiments (entries 1−2) and compares these with our published results for homoleptic systems (entries 3−4). Errors in the gradient and intercept and correlation coefficients for SEM-EDX measurements of single cocrystals are interpreted to indicate the degree of supramolecular selection displayed for a given system. For cocrystallized systems which involved [Ni(phen)3](PF6)2, a trend from almost perfect statistical cocrystallization with [Ru(phen)3](PF6)2 (entry 3) through to greater supramolecular selection was observed as the number of phenanthroline ligands in the second complex was reduced (entries 1 and 2). It is interesting that the degree of variation that was observed for [NixRu1−x(bipy)3](PF6)2 cocrystals (entry 4) was greater than when [Ni(phen)3](PF6)2 was cocrystallized with [Ru(phen)2(bipy)](PF6)2 (entry 1) and comparable to when [Ni(phen)3]67

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Figure 8. Secondary electron image (a) and backscattered image (b) of a more complicated morphology skewered crystal grown from a solution containing an equimolar mixture of [Ni(phen)3](PF6)2 and [Ru(bipy)3](PF6)2.

These results suggest that cocrystallization of [Ni(phen)3](PF6)2 with [Ru(bipy)3](PF6)2 occurs at the very limit at which structural differences prohibit cocrystallization. The skewered cocrystals only form from acetone and water solvent mixtures. Crystallization from other solvent mixtures such as acetonitrile and water only produced single crystals of pure metal complexes. Regardless of the structural differences between each of the compounds involved, the molecular structures of complexes are theoretically capable of engaging in some combination of intermolecular interactions to yield cocrystals containing both complexes. The SEM-EDX data confirms that this is the case but that the needles predominantly contain [Ru(bipy)3]2+ and the prisms predominantly contain [Ni(phen)3]2+. The results therefore suggest that the [Ni(phen)3]2+ complexes have a high selectivity for other [Ni(phen)3]2+ complexes in solution over [Ru(bipy)3]2+ complexes and vice versa. This leads to formation of two different single crystal phases, each containing a majority of one metal complex and displaying the morphology usually exhibited by that complex, connected by regions in which there are significant concentrations of both metal complexes. It is important to note that exclusive supramolecular selection is the usual result of recrystallization and is why recrystallization is considered such a good technique for purification. Ultimately, it seems that under these conditions, both the phen and bipy supramolecular motifs were not robust enough to allow the formation of a single phase of cocrystals.

are pale pink block-like prisms, while crystals of pure [Ru(bipy)3](PF6)2 are intense red colored needles. The 0.50:0.50 ([Ni(phen)3]2+:[Ru(bipy)3]2+) ratio crystallization batch displayed a new and surprising form of cocrystal morphology in which reddish-orange block-shaped prisms were, in every case, skewered by needles of the same color, as shown in Figure 6. To our knowledge, this complicated crystal morphology has only been reported by Dechambenoit et al. in what they call “necklace crystals”.22 Both the block and needle components of the crystals shown in Figure 6 are red and so presumably both contain some [Ru(bipy)3]2+ complex. It is not clear by visual inspection alone whether the crystals also contain [Ni(phen)3)]2+ and if they do, to what extent. In order to probe this, SEM-EDX was performed on different areas of the crystals. Figures 7 and 8 are secondary and backscattered SEM images of skewered crystals. Figure 7 shows a single well-defined block-shaped crystal penetrated by a needle, while Figure 8 shows a more complicated conglomerate. Backscattered electrons can be used to detect contrast between areas with different chemical compositions, especially when the average atomic number of the elements of interest differ substantially. In a backscattered image, uniform shading indicates compositional homogeneity. The backscattered images of the crystals in Figures 7b and 8b clearly show the difference in composition between the prisms and the needles that pass through them (as indicated by the contrasting shades of gray). The darker shade of gray displayed by the prisms indicates that they are rich in nickel, while the lighter shade of the needles indicates they are richer in ruthenium. Similar metal complex ratios were found for different sections of the skewered cocrystal pictured in Figure 8. SEM-EDX analysis of the area on the prism outlined in red in Figure 7a indicated a [Ni(phen)3](PF6)2 concentration of ca. 90% (i.e., 10% [Ru(bipy)3](PF6)2). The portion near the interface of the needle and prism (outlined in blue) was found to contain a nickel fraction of about 30−40%, while the area on the needle further away from the interface boxed in yellow was almost exclusively ruthenium complex (ca. 90%). That is, the skewered crystals display morphologies of both the pure complexes, and both parts contain both metals although in different proportions. Although the mechanism of the crystal growth which produced these crystals is unknown, it is possible that the crystal nucleated with a metal complex ratio close to what was in solution (ca. 0.50:0.50), but at some stage in the growth process the crystallization interface began to select one complex over the other, selecting for the Ni(II) complex in one dimension and the Ru(II) complex in the other.

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CONCLUSIONS

We have demonstrated that varying the number of common ligands between complexes being cocrystallized had a marked impact on supramolecular selection during cocrystallization. We have also shown that it is possible to modulate the properties of single cocrystals, such as phase and unit cell parameters, by incorporating various amounts of [Ni(phen)3](PF6)2 in the cocrystallizing solution. However, modulation of these properties was found to be less predictable with increasing supramolecular selection. Experiments between [Ni(phen)3](PF6)2 and [Ru(bipy)3](PF6)2 were found to approach the limit to which molecular recognition processes can be confused into cocrystallizing different molecules to form single cocrystals. These results reaffirm the robustness of the phen motif compared to the bipy motif because the [M(phen)3](PF6)2 structure was maintained even when ca. 90% of the complexes in the cocrystals contained only a single 1,10-phenanthroline ligand. 68

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the QUT Institute for Future Environments and the Central Analytical Research Facility for access to equipment and instrumentation for sample analysis. The authors wish to thank Mr. Tony Raftery for his help and assistance with powder XRD. J.B. acknowledges an Australian Postgraduate Award and the office of the Deputy Vice Chancellor of QUT for financial support.

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REFERENCES

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