Supramolecular Selection in Molecular Alloys - Crystal Growth & Design

Article pubs.acs.org/crystal

Supramolecular Selection in Molecular Alloys Jocelyne Bouzaid, Madeleine Schultz, Zane Lao, John Bartley, Thor Bostrom, and John McMurtrie* School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia S Supporting Information *

ABSTRACT: Complexes of the type [M(phen)3](PF6)2 (M = Ni(II), Fe(II), Ru(II) and phen = 1,10-phenanthroline) were found to co-crystallize to form molecular alloys (solid solutions of molecules) with general formula [MAxMB1−x(phen)3](PF6)2·0.5H2O in which the relative concentrations of the metal complexes in the crystals closely match those in the crystallizing solution. Consequently, the composition of the co-crystals can be accurately predicted and controlled by modulating the relative concentrations of the metal complexes in the crystallizing solution. Although they are chemically and structurally similar, complexes of the type [M(bipy)3](PF6)2 (M = Ni(II), Fe(II), Ru(II) and bipy = 2,2′bipyridine) display markedly different behavior upon co-crystallization. In this case, the resulting co-crystals of general formula [MAxMB1−x(bipy)3](PF6)2 have relative concentrations of the constituent complexes that are markedly different from the relative concentrations of the complexes initially present in the crystallizing solution. For example, when the nickel and iron complexes are co-crystallized from a solution containing a 50:50 ratio of each, the result is the formation of some crystals with a higher proportion of iron and others with a higher proportion of nickel. The relative concentrations of the metal complexes in the crystals can vary from those in the crystallizing solutions by as much as 15%. This result was observed for a range of combinations of metal complexes (Ni/Fe, Ni/Ru, and Fe/Ru) and a range of starting concentrations in the crystallizing solutions (90:10 through to 10:90 in 10% increments). To explain this remarkable result, we introduce the concept of “supramolecular selection”, which is a process driven by molecular recognition that leads to the partially selective aggregation of like molecules during crystallization.

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INTRODUCTION One of the continuing challenges in supramolecular chemistry and crystal engineering is the development of reliable methods for tuning the physical properties of target materials. The physical and chemical properties of the molecular components in crystalline materials are modulated by their spatial distribution and geometric arrangement. It follows that if the physical properties of crystalline materials are to be efficiently exploited, then structural predictability is an imperative component of their synthesis. However, such predictability is difficult to achieve. One approach that is gaining momentum is through the synthesis of molecular alloys (i.e., solid solutions of molecules). This has been achieved by co-crystallization of species that have similar molecular and crystal structures but different physical properties. The resulting crystals form with predictable structures, and their physical properties may be tuned by varying the relative concentrations of the components in the crystallizing solution. Examples of this approach were reported by Gütlich more than 30 years ago,1,2 although the term molecular alloy was not used. In the 1982 report, the effects of metal dilution on the spin-crossover properties of [Fe(phen)2(NCS)2] were examined for metal complex alloys of the form [FexM1−x(phen)2NCS)2], where M = Mn(II), Co(II), Ni(II), and Zn(II). Gütlich and others have thus demonstrated the potential in this approach for the modulation of physical © XXXX American Chemical Society

properties of crystalline solids, and the area has been reviewed.3−10 An important research milestone was published in 2000 by MacDonald et al.11 In their report, isomorphous salts of the form (imidazolium)2[M(dipic)2]·2H2O (dipic = pyridine-2,6dicarboxylate) metal complexes were co-crystallized in varying ratios. Analysis of single crystals, which had the form (imidizolium)2[MAxMB1−x(dipic)2]·2H2O (MA and MB = combinations of Mn(II), Co(II), Ni(II), Cu(II), and Zn(II)), by atomic absorption spectroscopy (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), and neutron diffraction, indicated that the concentration of the different metal complexes (associated with MA and MB) in the crystallizing solution directly determined the concentration of the metal complexes in the resulting co-crystals. For example, co-crystals obtained from a solution containing equimolar mixtures of (imidazolium)2[MA(dipic)2] and (imidazolium)2[MB(dipic)2] were found to have a composition of [MA0.50MB0.50(dipic)2]·2H2O while co-crystals from a solution with an 80:20 molar ratio of the same two complexes had the composition [MA0.80MB0.20(dipic)2]·2H2O. The physical properties of the crystals containing two different metals, including Received: March 7, 2012 Revised: June 18, 2012

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dx.doi.org/10.1021/cg300320r | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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color, metal−ligand distances, and effective magnetic moment, were demonstrated to lie between the properties of the corresponding homometallic systems and were proportional to their relative solid state concentrations. MacDonald used the term “mixed crystals” to describe these materials. This term has since been used extensively in the literature to describe a variety of types of materials, from simple salts,12−16 through doped polyoxometallates,17−19 to crystals containing similar discrete molecules or ions in similar ratios that co-crystallize in a single lattice.1,20−25 To avoid confusion, we prefer to use the terms “co-crystal” to describe crystals with multiple molecular components and “molecular alloy” to refer specifically to the case where the molecular components are interchangeable.26−30 The approach, described above, for forming molecular alloys through molecular substitution is appealing due to the predictability of the resulting structure. In almost all reports on molecular alloys in the literature, physical measurements were made on bulk crystalline samples, with the assumption being that the co-crystals were homogeneous and the relative concentrations of the molecules in the co-crystals were the same as those in the solutions from which they were crystallized. The report published by MacDonald is the only example in the literature in which single co-crystals of synthetic molecular alloys have been specifically analyzed to directly determine the relative concentrations of the different molecular components.11 In these co-crystals, the complexes are connected via charge assisted hydrogen bonds between the carboxylic acid groups of the ligands and the imidazolium cations. The assumptions about uniform crystal composition made previously by others were supported by the results of MacDonald, and we were inspired to see if the same was true for some other common metal complexes also known to form isostructural arrays in crystals. Metal complexes of the type [M(phen)3]X2, where X is a small monoanionic counterion, are known to crystallize with a recurring supramolecular motif comprising one offset face-toface π-interaction and two edge-to-face π-interactions between each pair of cations. This motif is described as a parallel 4-fold aryl embrace (P4AE), and the result is 1D chain-like connectivity between cations along the P4AE axis,31,32 as illustrated in Figure 1. Importantly, this motif is observed in a range of different crystal symmetries, (C2/c, P21/c, P1̅),33−41 which indicates that it is an energetically favorable arrangement for cations of this type. In contrast, cations of the form [M(bipy)3]2+ typically crystallize with concerted sets of six edge-to-face interactions per pair of complexes in a motif described as a 6-fold aryl embrace (6AE),42 as shown in Figure 2. As with the phen motif, the bipy motif is almost universal for [M(bipy)3]2+ and occurs in a range of crystal systems and symmetries (P3c1, C2/c, P1̅).43−53 Thus, our initial objective in this work was to assess the potential for formation of metal complex alloys where the primary structural motifs are based upon π-interactions between ligands of the metal complexes. We report here the preparation and analysis of two series of molecular alloys: [M A xM B 1−x (phen)3 ](PF 6 ) 2 and [M A x M B 1−x(bipy) 3 ](PF 6 ) 2 (where MA, MB = Fe(II), Ni(II), Ru(II)). The results include the discovery and investigation of an intriguing phenomenon for which we introduce the term “supramolecular selection”.

Figure 1. (a) The “phen motif” is a P4AE embrace characterized by one offset face-to-face π···π interaction (red arrow) and two edge-toface π interactions (orange arrows) between each pair of M(phen)3 complexes. (b) The motif typically propagates to create chains (black arrows) which can (as in this structure) be linked by (usually less welldeveloped) P4AE interactions (white arrows) to create 2D sheets of complexes.

dissolved in HPLC grade acetonitrile and diluted to concentrations of 10 μg·cm−3. Ultrapure nitrogen gas was used as both the drying and nebulizing gas while the source temperature was set to 350 °C. The fragmentor and skimmer voltages were set at 120 and 20 V, respectively. Mass spectra were collected in positive ion mode from m/z 100 to 1000 Da. 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) was calculated by dividing the observed atomic percentages (obtained from a standardized quantitative method using the 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 co-crystal grown from a solution containing [Ni(phen)3](PF6)2 and [Ru(phen)3](PF6)2 with a molar ratio of 90:10, respectively, which analyzed to give EDX atomic percentage readings of 1.20 and 0.15 for Ni and Ru respectively, was calculated to have a solid state composition of [Ni0.89Ru0.11(phen)3](PF6)2. X-ray diffraction (XRD) patterns were collected using a PANalytical X’Pert PRO X-ray diffractometer (radius of 240 mm). The incident X-ray radiation was sourced from a linefocused PW3373/00 copper (Cu) X-ray tube at 40 kV and 40 mA, generating a Kα wavelength of 1.54 Å. The incident beam is conditioned with a W/Si parabolic X-ray mirror that produces a parallel beam, which is passed through a 0.04 rad Soller incident slit, a 0.5° divergence slit, a 20.0 mm fixed mask,

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INSTRUMENT DETAILS Electrospray ionization mass spectrometry was conducted on an Aligent 6520 Accurate-Mass Q-TOF LC/MS. Samples were B



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(3). Similarly, data were collected for [Ni(bipy)3](PF6)2 (4), [Ru(bipy)3](PF6)2 (5), and a crystal produced from a solution containing a 50:50 mixture of [Ni(bipy)3](PF6)2 and [Ru(bipy)3](PF6)2 (6). Data were collected at 173(2) K for the phenanthroline complexes and at 225(2) K for the bipyridine complexes under the software control of CrysAlis CCD61 on an Oxford Diffraction Gemini Ultra diffractometer using Mo Kα radiation generated from a sealed tube. Data reduction was performed using CrysAlis RED. 61 Multiscan empirical absorption corrections were applied using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm, within CrysAlis RED,61 and subsequent computations were carried out using the WinGX-3262 graphical user interface. The structures of [Ni(phen)3](PF6)2 and [Ni(bipy)3](PF6)2 were solved by direct methods using SIR97.63 The preliminary refinement models for [Ru(phen)3](PF6)2 and the 50:50 Ni/ Ru phenanthroline co-crystal were prepared using the solution for [Ni(phen)3](PF6)2, and the preliminary refinement models for [Ru(bipy)3](PF6)2 and the 50:50 Ni/Ru bipyridine cocrystal were adapted from the solution for [Ni(bipy)3](PF6)2. The structures were refined with SHELXL-97.64 Full occupancy non-hydrogen atoms were refined with anisotropic thermal parameters. C−H hydrogen atoms were included in idealized positions, and a riding model was used for their refinement. Rotationally disordered PF6− anions were modeled in two orientations. Occupancies of each orientation were estimated by placing the same free variable constraint on all affected atoms while refining their occupancies. Once determined, the occupancies were fixed for the final refinement cycles. The P−F bond lengths in the disordered anions were restrained to 1.57(1) Å, and F···F distances were restrained to 2.23(2) Å. The Ni and Ru atoms in the molecular alloys shared the same sites. Their coordinates and anisotropic thermal parameters were fixed to be equivalent in each case, and occupancies were refined to determine an estimate of the relative proportion of each metal in the crystals. The CIF files have been deposited with the Cambridge Structural Database (CCDC reference numbers 869923−869928) 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, U.K.; fax (+44) 1223-336-033; or [email protected]). Crystal and refinement data are reported in Table 3.

Figure 2. The “bipy motif”: (a) The pairwise interactions comprise six edge-to-face π interactions (6AE). Four such interactions are indicated by the orange arrows, while the other two are obscured from this viewpoint. (b) The motif typically propagates in heterochiral chains (alternate complexes colored green and gray for clarity). (c) Common space groups are trigonal, and in these cases the chains run parallel to the c axis.

and a 1.4 mm fixed antiscatter slit. The diffracted beam was detected with a 009° parallel plate collimator used with a 0.02 rad solar slit and proportional detector. Patterns were acquired using a symmetric parallel beam geometry over a range of 3.5− 40° 2θ with a step size of 0.02° 2θ at 2.0 s per step. Calculations and analysis of diffraction patterns were made using the X’pert Highscore (PANanalytical) software package.

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EXPERIMENTAL SECTION

General Procedure for the Synthesis of [ML3](PF6)2 (M = Fe(II), Ni(II), Co(II), Cu(II); L = phen, bipy). These compounds have been known and used for over 50 years.65−70 However, a full synthetic procedure and characterization of all members of the series has not been published in an accessible form, to our knowledge. We include the full procedure and data here because of the importance, in our work, of being absolutely confident of the analytical purity of the complexes used. In each case, solutions containing 3 mol equiv of the ligand dissolved in ethanol (15 mL) were heated to ca. 60 °C and added dropwise with stirring to solutions of the metal chlorides in water (10 mL) also at 60 °C. Addition of saturated aqueous KPF6 resulted in immediate precipitation of the complexes as strongly colored hexafluorophosphate salts. The complexes were collected by vacuum filtration and washed with water (2 × 5 mL), at which time a sample of the filtrate remained clear upon addition of 2 drops of an aqueous solution of AgNO3 (ca. 1 M). The solid was then washed with ethanol (2 × 5 mL). Recrystallization from solutions of the complexes in evaporating mixtures of acetone and water produced large prismatic

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SINGLE CRYSTAL X-RAY CRYSTALLOGRAPHY Although the single crystal X-ray structures for most of the metal complexes used in this work have been reported,33,43,44,46,48,55−60 we repeated some of them using our instrument/conditions to minimize the effect of errors on structural parameters. This allowed a very close comparison of structural data such as unit cell dimensions and bond distances between the pure crystals and the co-crystals. Single crystal X-ray structures were determined for [Ni(phen)3](PF6)2·0.5H2O (1), for [Ru(phen)3](PF6)2·0.5H2O (2), and for a crystal obtained from a solution containing a 50:50 mixture of [Ni(phen)3](PF6)2 and [Ru(phen)3](PF6)2 C



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crystals with general formula [M(phen)3](PF6)2·0.5H2O for L = phen, and [M(bipy)3](PF6)2 for L = bipy. [Fe(phen)3](PF6)2·0.5H2O. Red prisms. Yield 0.77 g (94%). HR-ESIMS [ML3]2+ m/z 298.0714 [100%] (calculated for [C36H24N6Fe1]2+, 298.0706), [ML3 + X]+ m/z 741.1055 [32%] (calculated for [C36H24N6Fe1 + PF6]+, 741.1059). Found: C, 47.81; H, 2.69; N, 9.20. Calculated for C36H24N6FeP2F12·(H2O)0.5: C, 48.29; H, 2.81; N, 9.39. [Ni(phen)3](PF6)2·0.5H2O. Pink prisms. Yield 0.76 g (93%). HR-ESIMS [ML3]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. [Co(phen)3](PF6)2·0.5H2O. Yellow/brown prisms. Yield 0.74 g (90%). HR-ESI-MS [ML3]2+ m/z 299.5700 [100%] (calculated for [C36H24N6Co1]2+, 299.5697), [ML3 + X]+ m/z 744.1048 [27%] (calculated for [C36H24N6Co1 + PF6]+, 744.1042). Found: C, 47.89; H, 2.67; N, 9.17. Calculated for C36H24N6CoP2F12·(H2O)0.5: C, 48.12; H, 2.80; N, 9.35. [Cu(phen)3](PF6)2·0.5H2O. Blue/green prisms. Yield 0.81 g (92%). HR-ESI-MS [ML 3 ] 2+ m/z 299.5700 [100%] (calculated for [C36H24N6Co1]2+, 299.5697), [ML3 + X]+ m/z 744.1048 [27%] (calculated for [C36H24N6Co1 + PF6]+, 744.1042). [Fe(bipy)3](PF6)2. Red needles. Yield 0.82 g (95%). HR-ESI-MS [ML3]2+ m/z 262.0702 [100%] (calculated for [C30H24N6Fe1]2+, 262.0706), [ML3 + X]+ m/z 669.1054 [38%] (calculated for [C30H24N6Fe1 + PF6]+, 669.1059). Found: C, 43.99; H, 2.77; N, 10.10. Calculated for C30H24N6FeP2F12: C, 44.25; H, 2.97; N, 10.32. [Ni(bipy)3](PF6)2. Pink needles. Yield 0.79 g (91%). HR-ESI-MS [ML3]2+ m/z 263.0705 [100%] (calculated for [C30H24N6Ni1]2+, 263.0703). Found: C, 43.77; H, 3.03; N, 9.98. Calculated for C30H24N6NiP2F12: C, 44.09; H, 2.96; N, 10.28. General Procedure for the Synthesis of the Ruthenium Complexes [Ru(phen)3](PF6)2 and [Ru(bipy)3](PF6)2. Again, while these compounds have been long known and used,54 detailed synthetic procedures with full characterization and spectral data are difficult to find, so the details are included here. In each case, equimolar quantities of cis-[Ru(L)2Cl2] and the ligand were heated at reflux for 1 h in methanol (scale ca. 0.3 g of ligand per ca. 20 mL of methanol). The resulting deep red solution was cooled to room temperature and filtered through a pad of Celite. Addition of saturated methanolic NH4PF6 to the filtrate resulted in precipitation of an intensely colored red powder, which was collected by vacuum filtration, washed with water (2 × 5 mL), methanol (2 × 5 mL), and diethyl ether (2 × 5 mL), and air-dried. Recrystallization from an evaporating mixture of water and acetone gave large red prisms. cis-[Ru(phen)2Cl2]·2H2O. Yield 2.77 g (67%). 1H NMR (400 MHz; d6-DMSO; 300 K): δH 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 = 8.4 Hz, 2H), 8.28 (d, 3JHH = 8.4 Hz, 2H), 8.21 (m, 2H), 7.77 (dd, 3JHH = 4.7 Hz, 2H), 7.32 (dd, 3JHH = 8.2 Hz, 2H). This matches the spectrum reported in the literature.71 cis-[Ru(bipy)2Cl2]·2H2O. Yield 2.59 g (69%). 1H NMR (400 MHz; d6-DMSO; 300 K): δH 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 (td, 3JHH = 8.4 Hz, 4JHH = 1.6 Hz, 2H), 7.77 (unresolved td, 3JHH = 6.0 Hz, 2H), 7.68 (td, 3JHH = 7.8 Hz, 4JHH = 1.2 Hz, 2H), 7.51 (d, 3JHH = 5.2 Hz, 2H), 7.10 (td, 3JHH = 6.8 Hz, 4JHH = 1.2 Hz, 2H). This matches the spectra reported in the literature.71,72 [Ru(phen)3](PF6)2·0.5H2O. Yield 1.14 g (73%). HR-ESI-MS [ML3]2+ m/z 321.0558 [98%] (calculated for [C36H24N6Ru1]2+, 321.0553), [ML3 + X]+ m/z 787.0749 [33%] (calculated for [C36H24N6Ru1 + PF6]+, 787.0753). 1H NMR (400 MHz; d6-DMSO; 300 K): δH 8.77 (d, 3JHH = 8.2 Hz, 6H), 8.39 (s, 6H), 8.08 (d, 3JHH = 5.3 Hz, 6H), 7.77 (dd, 3JHH = 8.2, 5.3 Hz, 6H). Found: C, 46.28; H, 2.48; N, 8.97. Calculated for C36H24N6RuP2F12·(H2O)0.50: C, 45.97; H, 2.68; N, 8.93. [Ru(bipy)3](PF6)2. 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; d6-DMSO; 300 K): 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. Co-crystallization Experiments. Binary metal complex alloys of the form [MAxMB1−x(phen)3](PF6)2 and [MAxMB1−x(bipy)3](PF6)2 were prepared by co-crystallization from evaporating solutions of the complexes [MA(L)3](PF6)2 with [MB(L)3](PF6)2 in acetone and water. The co-crystallization experiments were carried out using a systematic range of molar ratios of the two complexes from 90:10 to 10:90 in 10% increments. The MA/MB combinations thus prepared were Fe/Ni, Fe/Ru, and Ni/Ru. All experiments were performed in duplicate, and in each case, crystallization was allowed to continue until no appreciable color remained in the mother liquor, indicating that all but a negligible amount of complex remained in the solution phase (less than 1% by mass). Characterization of Metal Complex Alloys. Powder X-ray diffraction data of both the single metal and the co-crystallized samples were collected to investigate phase purity. Powdered samples were prepared as thin layers adhered to Vaseline on silicon 511 wafers, creating a thin film of fine powder. Single co-crystals were screened by electrospray mass spectrometry to (qualitatively) confirm cocrystallization of the metal complexes. 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. 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). At least three crystals from every batch were analyzed in this manner. Measurements were processed against a set of standard blocks mounted with natural minerals containing elements found in the samples analyzed. It is important to note that with the exception of the ratio of the two metals in the binary alloys, the elemental compositions of the crystals are well-known. The ratio of the atomic fractions of the metals (which is the only variable between the crystals) was calculated from the energy dispersive X-ray spectra, thus providing the ratio of the two metal complexes in each crystal. The individual crystals were examined for, and found to maintain, compositional homogeneity by comparison of measurements from at least three different regions on the same crystal. It should be noted that this technique probes the surface of the crystal to a depth of approximately 6 μm and does not provide direct information about the composition of the interior of the crystal. The ratios of the metal complex components for each binary alloy were plotted as percent [MA(L)3](PF6)2 in crystallizing solution versus percent [MA(L)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 co-crystallization behavior of the metal complex mixtures. Additionally, [Ru(bipy)3](PF6)2 and [Ni(bipy)3](PF6)2 were co-crystallized from 50:50 solution mixtures in four separate batches. For these batches, every crystal was subjected to EDX analysis to determine the range of different concentration ratios for the complexes in the crystals.

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RESULTS AND DISCUSSION The synthetic procedures for the single crystals detailed in the Experimental Section lead to large quantities of pure, crystalline materials. It is important to note that all of the phenanthroline complexes crystallize with half a molecule of water, while the bipyridine complexes do not contain any water of crystallization when crystallized as described. Mixtures of two pure single metal phenanthroline complexes co-crystallize to make single co-crystals of the form [MAxMB1−x(phen)3](PF6)2·0.5H2O (MA/MB combinations Fe/Ni, Fe/Ru, and Ni/Ru). The co-crystallization experiments were performed with the ratios of metal complexes MA/MB in D



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the crystallizing solution at 90:10, 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, 20:80, 10:90. In cases where the crystals of the pure forms differ significantly in color, visual inspection can be a useful tool for (qualitatively) confirming co-crystallization. For example, cocrystals of two complexes, one blue and one yellow, would be expected to be green. An absence of either yellow or blue crystals also provides evidence that co-crystallization is ubiquitous. In the systems used here, the complexes are not sufficiently different in color to make visual inspection a reliable tool for confirming co-crystallization. The complexes of [Ni(phen)3]2+ (and [Ni(bipy)3]2+) are pale pink while those of [Fe(phen)3]2+ (and [Fe(bipy)3]2+) are dark brown and [Ru(phen)3]2+ (and [Ru(bipy)3]2+) are dark red. When the pale pink nickel complexes were co-crystallized with either ruthenium or iron complexes, the resulting crystals are all very similar in color to the more intensely absorbing iron or ruthenium complexes, as shown in Figure 3 (see Figure S1 of

Figure 4. Mass spectrum obtained for a single crystal produced from a solution containing a 50:50 mixture of [Ni(phen)3](PF6)2 and [Ru(phen)3](PF6)2. The spectrum has peaks indicative of the presence of ions associated with each metal complex, thus confirming that both complexes were present in the single crystal.

PF6]+). ESI-MS was used to screen at least three crystals from every batch of the co-crystallization experiments, and in every case, the crystals were found to contain both metal complexes, confirming co-crystallization to form metal complex alloys of the mixed [M(phen)3](PF6)2. The X-ray diffraction patterns of powdered crystals of pure [Fe(phen)3](PF6)2·0.5H2O, [Ni(phen)3](PF6)2·0.5H2O, and [Ru(phen)3](PF6)2·0.5H2O as well as for crystals produced by co-crystallization of these complexes in 20:80, 50:50, and 80:20 ratios, respectively, are presented in Figure 5. The patterns closely match patterns simulated from the single

Figure 3. Crystals of [Ni(phen)3](PF6)2·0.5H2O, [Ru(phen)3](PF6)2·0.5H2O, and co-crystals of the form [NixRu1−x(phen)3](PF6)2·0.5H2O obtained by co-crystallization of a 50:50 mixture of the Ni(II) and Ru(II) complexes. While the crystals of the nickel complex are significantly lighter and different in color from those of the ruthenium complex, the crystals obtained after co-crystallization are visually indistinguishable from the pure ruthenium complexes.

the Supporting Information for additional images). Notably though, in none of the co-crystallization experiments involving nickel were pale pink crystals observed to form, which is indicative that the nickel complexes co-crystallized. The complexes of Fe and Ru were of similar color and intensity, and as such, visual inspection proved useless as a qualitative tool for screening co-crystals. We therefore looked for an alternative method for screening the batches of co-crystals to determine if co-crystallization was ubiquitous. Analysis of single crystals by ESI-MS proved very effective. The presence of peaks corresponding to ions specific for each metal complex in the mass spectra provided rapid and conclusive confirmation that both metal complex species were present in individual crystals. For example, Figure 4 shows the electrospray mass spectrum obtained from a single crystal produced by co-crystallization of a 50:50 mixture of [Ni(phen)3](PF6)2 and [Ru(phen)3](PF6)2. The trace clearly shows the presence of peaks for (m/z) 299.0704 ([Ni(phen)3]2+) and 743.1057 ([Ni(phen)3 + PF6]+) and (m/z) 321.0556 ([Ru(phen)3]2+) and 787.0748 ([Ru(phen)3 +

Figure 5. Powder XRD patterns for pure [Fe(phen)3](PF6)2·0.5H2O (red), [Ru(phen) 3 ](PF 6 ) 2 ·0.5H 2 O (blue), and [Ni(phen) 3 ](PF6)2·0.5H2O (green) interspersed with the powder XRD patterns for crystals obtained from solutions containing 80:20, 50:50, and 20:80 molar ratios of the metal complexes. E



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crystal X-ray structures of [Ni(phen)3](PF6)2·0.5H2O (1) and [Ru(phen)3](PF6)2·0.5H2O (2) (Figure S2 of the Supporting Information contains simulated powder XRD patterns). The similarity of the pure samples with each other and with their simulated patterns indicates that they share the same phase and that the individual samples comprise only one pure phase. The co-crystals also display the same powder patterns and thus are phase pure and isomorphous with the pure samples. There are some subtle differences in the peak positions for reflections from the pure crystals due to slight variations in unit cell parameters. Interestingly, close examination of the powder patterns for the co-crystals shows the gradual change in these peak positions (and therefore unit cell parameters) as a function of the proportion of the two metal complexes that were present in the crystallizing solutions. Once it was established that co-crystallization was definitely taking place, elemental analysis of single co-crystals was performed using SEM-EDX spectroscopy to quantify the ratio of the two different metals in the co-crystals. At least three crystals from each batch were analyzed to determine the extent of variation of concentration ratios between crystals from each batch. In addition, each crystal was measured at three separate locations to determine the extent (if any) of intracrystal concentration variation. The results for each series of cocrystallization experiments were analyzed graphically in a 2D scatter plot, showing the fraction of MA complex in the solid state (χMAsolid) versus the fraction of MA in the crystallizing solution (χMAsolution), and the results were extended by linear regression. (The atomic percentages of both metals were measured using EDX, and their ratios were used to determine the relative molar fractions (χ) in the solid. Consequently, the relationship between χMBsolid and χMBsolution is redundant, since χMA + χMB = 1, and therefore, only one of these need be considered for each set of metal complex combinations.) Figure 6 shows the scatter plot of χNisolid versus χNisolution for the co-crystallization of [Ni(phen)3](PF6)2 with [Fe(phen)3](PF6)2. The scatter plots for all combinations are provided in

Figure S3 of the Supporting Information. The EDX measurements from different regions of the same crystals showed almost identical ratios of metals. It is also immediately apparent that the ratio of metal complexes in the co-crystals is the same as the ratio in the crystallizing solutions. For example, three crystals from the batch of co-crystals obtained by preparing a 10:90 crystallizing mixture of [Ni(phen)3](PF6)2 and [Fe(phen)3](PF6)2 (i.e., χNisolution = 0.10) were analyzed and found to contain 10% [Ni(phen)3]2+ (and 90% [Fe(phen)3]2+). Similarly, the analysis of crystals from other co-crystallization batches confirms a direct relationship between the ratio of the metal complexes in the solids and their respective ratios in the solutions from which they crystallized across the entire range of concentration ratios examined. This is apparent from the scatter plot because the slope of the line of regression is one and the intercept is zero. The errors in the slope and intercept are very small, and they are summarized in Table 1 for each metal complex combination (Ni/Fe, Fe/Ru, and Ni/Ru). These results confirm that the distribution of the metal complexes in the crystals from each batch are uniform. Table 1. Summary of SEM-EDX Analyses and Linear Regression Parameters for Co-crystallization Experiments To Produce Crystals of the Form [MAxMB1−x(phen)3](PF6)2 and [MAxMB1−x(bipy)3](PF6)2 complex A

complex B

regression line equation

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

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

y = 0.994x − 0.001 y = 0.992x − 0.010 y = 0.994x + 0.008 y = 0.98x + 0.03 y = 1.01x − 0.02 y = 1.02x + 0.03

error in slope (±)

error in intercept (±)

R2

0.003

0.002

0.999

0.003

0.002

0.999

0.003

0.002

0.998

0.02

0.01

0.96

0.03

0.01

0.94

0.03

0.01

0.93

The crystals of the pure [M(phen)3](PF6)2 (M = Fe, Ni, Ru) complexes are isomorphous and, as explained in the introduction, exhibit the parallel 4-fold aryl embrace (P4AE) between cations. The results described above suggest that this motif is flexible enough to allow co-crystallization of [M(phen)3](PF6)2 complexes with a range of different metals (including transition metals from both the first row and the second row), with no apparent inhibition or selectivity. Thus, the resulting crystals have relative concentrations of metals determined solely by their relative concentrations in the crystallizing solutions. To provide final confirmation of this result, samples of [Co(phen)3](PF6)2 and [Cu(phen)3](PF6)2 were also prepared and co-crystallized simultaneously with [Ni(phen)3](PF6)2, [Fe(phen)3](PF6)2, and [Ru(phen)3](PF6)2. The ratio of the five complexes in the crystallizing solution was 1:1:1:1:1. Ten crystals were then analyzed using SEM-EDX (3 measurements per crystal). An example of an EDX spectrum showing peaks associated with each of the five metal complexes can be seen in Figure S4 of the Supporting Information. The crystals were found to comprise all five complexes in almost exactly the same ratios as in the crystallizing solution (Fe 20(2)%, Co 22(1)%, Ni 20(1)%, Cu 20(1)%, and Ru 18(1)%).

Figure 6. Scatter plot and regression line for SEM-EDX analyses of cocrystals of the form [NixFe1−x(phen)3](PF6)2. There is a very strong correlation (R2 = 0.999) between the ratio of each metal complex in the crystallizing solution and the ratio of those metal complexes in the co-crystals. F



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It was of interest then to investigate whether bipyridyl complexes, which are known to crystallize with a different structural motif (6AE) compared with phenanthroline complexes (P4AE), are also flexible enough to co-crystallize with mixtures of different metals. The co-crystallization experiments to produce co-crystals of the form [MAxMB1−x(bipy)3](PF6)2 were carried out using the same protocol as described above for the [MAxMB1−x(phen)3](PF6)2 systems. Qualitative screening of crystals from all batches by ESI-MS as described above confirmed co-crystallization in every case. Powder XRD patterns for the pure [M(bipy)3](PF6)2 crystals and their cocrystals all show the same phase as each other and agree with patterns simulated from the structures of [Ni(bipy)3](PF6)2 (4) and [Ru(bipy)3] (5) (P3̅c1), as was observed for the [M(phen)3(PF6)2 analogues. Figure 7 shows the powder

Figure 8. Scatter plots and regression lines for SEM-EDX analyses of co-crystals of the form (a) [NixFe1−x(bipy)3](PF6)2 and (b) [NixRu1−x(bipy)3](PF6)2. Metal ratios in the co-crystals differ markedly from those in their respective crystallizing solutions.

To understand this data, it is important to note that each crystal was analyzed at three separate locations on its surface. These measurements were very similar to each other, generally within 2%. In Figure 8, the color coding is used to identify the three measurements from the same crystals at each concentration ratio. For example, it can be seen in Figure 8a that the measurements (enclosed in the black rectangle) for three co-crystals analyzed from the batch of crystals produced from the 30:70 Ni/Fe ratio in crystallizing solution contain very different ratios of the two metals. The color coding highlights measurements from the same crystal. There is no relationship between the color codes used for crystals from different crystallization batches. The individual crystals appear to be relatively homogeneous (i.e., the measurements from the same crystals show little variation). However, what is surprising is that the ratio of the metal complexes in the crystals significantly differs from those in the crystallizing solution. Specifically, one of the crystals in the 30:70 Ni/Fe batch (blue points) is 40% Ni complex (and therefore 60% Fe complex), another (red points) is 31% Ni complex (69% Fe complex), and the third crystal

Figure 7. Powder XRD patterns for pure [Fe(bipy)3](PF6)2 (red), [Ru(bipy)3](PF6)2 (blue), and [Ni(bipy)3](PF6)2 (green) interspersed with the powder XRD patterns for crystals obtained from solutions containing 80:20, 50:50, and 20:80 molar ratios of the metal complexes.

XRD patterns for the pure [Fe(bipy)3](PF6)2, [Ni(bipy)3](PF6)2, and [Ru(bipy)3](PF6)2 crystals as well as the patterns for the co-crystals obtained from solutions with 20:80, 50:50, and 80:20 molar ratios of these complexes. Analysis of single crystals by SEM-EDX for batches of crystals comprising binary combinations of [M(bipy)3](PF6)2 complexes gave results in stark contrast to those obtained for the analogous phenanthroline complexes. The results from the co-crystallization of [Ni(bipy)3](PF6)2 with [Fe(bipy)3](PF6)2 and of [Ni(bipy)3](PF6)2 with [Ru(bipy)3](PF6)2 are illustrated in parts a and b, respectively, of Figure 8 and are summarized in Table 1. G



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To explain this, we introduce the term supramolecular selection, which applies to co-crystallization of two molecules A and B, that are chemically and structurally very similar. When A and B are crystallizing in the same vessel, like molecules organize through supramolecular interaction with slightly more favorable arrangements of intermolecular interactions. That is, intermolecular interactions of the type A···A and B···B are slightly more favorable than A···B. As a consequence, some crystal nuclei are expected to contain a higher proportion of (though not exclusively) A, while other crystal nuclei originate with a higher proportion of B (again not exclusively so). During crystallization, the crystals with a higher proportion of A are partially selective (through molecular recognition) for molecules of A while the reverse is true for B. The crystallization process, which is being driven (in large part) by the formation of intermolecular interactions, is thus influenced by the partial preferences for molecules of A to crystallize with other molecules of A and the same for molecules of B. Thus, it is possible by this mechanism to obtain crystals from a 50:50 mixture of A and B, where some have a higher proportion of A and others have a higher proportion of B and where both grew simultaneously from one solution with a fixed starting ratio of the two molecules. It is interesting that the [M(phen)3](PF6) complexes display no supramolecular selection during co-crystallization while the [M(bipy)3](PF6)2 complexes do. Outwardly, tris-phen and trisbipy complexes are similar. They have similar interaction surfaces (aromatic ligands) and the same molecular symmetry (D3). To make some close structural comparisons between the complexes co-crystallized, single crystal X-ray structures were determined for crystals of pure [Ni(phen)3](PF6)2·0.5H2O (1), [Ru(phen) 3 ](PF 6 ) 2 ·0.5H 2 O (2), and a crystal of [NixRu1−x(phen)3](PF6)2·0.5H2O (3) obtained from a 50:50 mixture of the nickel and ruthenium complexes. Crystal and refinement data are provided in Table 3. The structures (1−3) are isomorphous, as expected given the similarity between their powder patterns, as discussed earlier. There are two complete molecules in the asymmetric unit and one water molecule. In the structure of (3), the positions of the metal centers were modeled as a disordered mixture of nickel and ruthenium and their relative occupancies were refined (with fixed and equivalent atomic displacement parameters) to be 0.48 Ru and 0.52 Ni. This agrees reasonably well with the SEM-EDX results, which showed that co-crystals in this batch were all very close to 50:50. When the relative occupancies were fixed at 60:40 or 40:60 prior to refinement, there were only very slight changes in the refinement residuals, and thus, despite the difference in electron density of Ni and Ru, it is clear that simple refinement of positional occupancy is not a very accurate indicator of their relative concentrations in crystals. As a result, final refinement cycles were carried out with the relative occupancies at 50:50, in accord with the results obtained through SEM-EDX analysis. A comparison of some of the structural parameters provided some interesting insights. The average M−N bond length for the Ni(II) complex (1) was 2.093(10) Å, which was (perhaps counterintuitively) longer than that of the Ru(II) complex at 2.065(8) Å. Predictably, the average M−N bond length in [Ni0.50Ru0.50(phen)3](PF6)2·0.5H2O (3) was between these values at 2.073(8). What is particularly interesting though is that the unit cell volume for the co-crystal (6795.4(4) Å3) is slightly smaller than the volumes for either of the pure compounds (7037.1(5) Å3 for the Ni complex (1) and

from that same batch (green points) is 23% Ni complex (77% Fe complex). The SEM-EDX results for all batches of the Ni/Fe co-crystals show the same deviation in solid state ratios from those in the crystallizing solutions. Similar results were obtained for the other metal combinations (Fe/Ru and Ni/Ru) (see Table 1 and Supporting Information Figure S5). That is, cocrystallization of [M(bipy)3](PF6)2 complexes resulted in the formation of crystalline molecular alloys in which the solid state concentrations of metal complexes in individual crystals deviate markedly from those in the crystallizing solution from which they formed. Furthermore, each batch contains some cocrystals with a higher concentration of each metal complex than was present in the crystallizing solution as well as some crystals with a lower concentration. Visual inspection of the scatter plots indicates that the extent of deviation of the metal concentrations in the co-crystals from their solution concentrations is largest for the ratios close to 50:50 ratio mixture and progressively smaller for the concentration extremes 90:10 and 10:90. This makes sense given that the closer the starting ratio is to 50:50, the greater the possible deviation in either direction. Four separate batches of co-crystals of the form [NixRu1−x(bipy)3](PF6)2 were prepared from solutions containing 1:1 ratios of the complexes. Every crystal in these four batches was subjected to EDX analysis to examine the range of variation of metal complex concentrations. The results of these experiments are presented in Table 2. The average ratio of Table 2. Concentration Variation in Four Batches of [NixRu1−x(bipy)3](PF6)2 batch 1 2 3 4

x̅ 0.52 0.52 0.50 0.51

σ

range

n

0.08 0.10 0.07 0.08

0.37−0.66 0.38−0.70 0.36−0.67 0.37−0.65

28 55 40 36

nickel to ruthenium complex in each batch of crystals was very close to 0.5. All batches showed a wide range of ratios from around 35% Ni complex (i.e., x = 0.36) through 70% Ni complex (i.e., x = 0.70). This set of results indicates that the maximum possible variation of solid state concentrations from that in the crystallizing solution is quite large at around 20%. Before we propose a mechanism for these remarkable and unprecedented results, consider that crystallization is arguably the most common and experimentally simple procedure used for purification. The reason that crystallization works so well as a purification technique is that in most cases molecules display completely selective molecular recognition for their own kind during crystallization, leading to production of pure crystals. Conversely, in the co-crystallization experiments involving [M(phen)3](PF6)2 complexes, the resulting crystals were found to have concentrations of components exactly the same as in the crystallizing solutions. This means that, for the purposes of crystallization, the complexes are completely interchangeable. Any subtle difference in molecular structure apparently makes no difference to the viability and quality of intermolecular interactions that drive the crystal packing process. The question then arises as to what happens when molecules are similar enough to co-crystallize but not so similar that they are completely interchangeable in the crystal packing motifs. This is the exact scenario that we propose is evident in the cocrystallization of the [M(bipy)3](PF6)2 complexes. H



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Table 3. Crystal and Refinement Data for Structures 1, 2, 3, 4, 5, and 6

formula M crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Dc/(g cm−3) Z color habit dimensions μ(Mo Kα)/ mm−1 Tmin,max Nind (Rint) Nobs (I > 2σ(I)) Nvar R1a wR2a A, B Flack parameter GoF Δρmin,max/(e− Å−3)

[Ni(phen)3] (PF6)2·0.5H2O (1)

[Ru(phen)3] (PF6)2·0.5H2O (2)

[Ni0.5Ru0.5(phen)3] (PF6)2·0.5H2O (3)

[Ni(bipy)3](PF6)2 (4)

[Ru(bipy)3](PF6)2 (5)

[Ni0.65Ru0.35(bipy)3](PF6)2 (6)

C72H50F24N12OP4Ni2 898.27 monoclinic Cc 37.3364(14) 15.7791(8) 12.1979(5)

C72H50F24N12OP4Ru2 940.63 monoclinic Cc 37.149(2) 15.8742(10) 12.1522(9)

C36H25F12N6O0.5P2Ni0.5Ru0.5 919.45 monoclinic Cc 37.1678(13) 15.7813(5) 12.1409(3)

C30H24F12N6NiP2 817.20 trigonal P3̅c1 10.6804(3)

C30H24F12N6P2Ru 859.56 trigonal P3̅c1 10.7046(2)

C30H24F12N6Ni0.65P2Ru0.35 832.03 trigonal P3̅c1 10.6849(2)

16.5274(5)

16.3796(4)

16.4633(3)

101.693(4)

101.574(6)

101.619(3)

7037.1(5) 1.696 8 pink prism 0.26 × 0.20 × 0.16 0.747

7020.6(8) 1.780 8 red prism 0.20 × 0.15 × 0.12 0.643

6975.4(4) 1.751 8 red prism 0.20 × 0.15 × 0.14 0.700

1632.73(9) 1.662 2 pink needle 0.50 × 0.13 × 0.11 0.794

1625.4(2) 1.756 2 red needle (cut) 0.35 × 0.22 × 0.20 0.684

1627.8(2) 1.698 2 red needles 0.38 × 0.21 × 0.20 0.757

0.9498, 1 14328 (0.018) 12920

0.9799, 1 14783 (0.0235) 12918

0.8837, 1 19201 (0.0431) 15444

0.9617, 1 1247 (0.0235) 879

0.9917, 1 1253 (0.0146) 1012

0.9725, 1 1149 (0.0157) 996

1030 0.0361 0.0899 0.05, 4 0.316(8)

1030 0.0388 0.0834 0.040, 2 0.300(18)

1030 0.0477 0.100 0.043, 2.5 0.377(12)

78 0.0362 0.0845 0.030, 1

78 0.0268 0.0703 0.035, 1

80 0.0313 0.771 0.03, 1

1.073 −0.866, 0.948

1.039 −0.65, 1.15

1.094 −0.64, 1.12

1.011 −0.338, 0.436

1.024 −0.472, 0.459

1.029 −0.463, 0.378

Reflections with [I > 2σ(I)] considered observed. R1 = ∑||Fo| − |Fc||/∑|Fo| for Fo > 2σ(Fo) and wR2(all) = {∑[w(Fo2 − Fc2)2]/∑[w(Fc2)2]}1/2, where w = 1/[σ2(Fo2) + (AP)2 + BP], P = (Fo2 + 2Fc2)/3.

a

7020.6(8) Å3 for the Ru complex (2)). The unit cell dimensions a and b for the co-crystal are between those of a and b for the pure crystals while the unit cell dimension, c, is shorter in the co-crystal than in either of the pure complexes. The P4AE motif between metal complexes propagates along the c axis, and this result indicates that there is more efficient packing along the axis of motif propagation in the co-crystal than in either of the pure crystals. This is a very subtle indicator that co-crystallization of these two complexes leads to slightly more efficient crystal packing. Structure overlay of the pure nickel complex cation with the pure ruthenium complex cation in Mercury provided a root-mean-square difference between equivalent atoms of only 0.044, indicating that there is very little difference in molecular structure of the complexes, with the most obvious (though still small) difference being the extent of the twist of the 3-fold propeller arrangement of the ligands with respect to the (pseudo) 3-fold axis of the complex, which is 1° greater for the ruthenium complex (51.6°) compared to the nickel complex (50.6°). It is important to note, however, that propagation of the P4AE motif in the crystals does not occur along the molecular 3-fold axis, and as such, it is not particularly surprising that this difference does not lead to selectivity during crystallization. A similar comparison was made between the crystal structures of [Ni(bipy)3](PF6)2 (4), [Ru(bipy)3](PF6)2 (5), and a crystal of [NixRu1−x(bipy)3](PF6)2 (6) obtained from a solution containing a 50:50 mixture of the two complexes.

Crystal and refinement data are included in Table 3. All three structures are isomorphous in the trigonal space group P3̅c1. The metal complex resides on a 3-fold special position. The relative occupancies for the nickel and ruthenium atoms in [NixRu1−x(bipy)3](PF6)2 were refined with fixed thermal parameters giving values close to 0.65:0.35 Ni/Ru. As discussed earlier, the SEM-EDX measurements for these crystals showed a considerable variation in the ratio of metals from the ratio of the complexes in the crystallizing solution, and so it was not surprising to find that the occupancies of the metals confirm this. That said, and as mentioned earlier, refinement of the occupancies of the two metals is not greatly perturbed by adjusting the occupancies, and so we estimate this ratio to be accurate to about ±10%. The occupancies of the metals were fixed at 0.65 and 0.35 (for Ni and Ru, respectively) for the final refinement cycles. As with the tris-phen analogues, the average M−N bond lengths for the Ni(II) complex (2.073(2) Å) are slightly longer than those in the Ru(II) complex (2.056(2) Å) while those in the [Ni0.65Ru0.35(bipy)3](PF6)2 crystal were between these values (2.066(2) Å). Unlike the tris-phen analogues, however, the unit cell parameters a, b, and c and, therefore, unit cell volume for the co-crystalline molecular alloy were between those of the pure crystals. The structure overlay of the pure complexes again shows only very minor differences (rms difference 0.0448), and again, the most notable distinction is the twist angle of the ligands from the 3-fold axis of the I



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complexes. The twist angles for the tris-bipy complexes are identical to those in the analogous tris-phen complexes (i.e., 50.6° for the Ni complex and 51.6° for the ruthenium complex). While the difference is again only 1° (as it was for the tris-phen analogues), it is significant that in these crystals the molecules interact via the 6PE motif, which propagates coincident with the 3-fold axes of the complexes (and the unit cell). We postulate that this is the main reason why the tris-bipy complexes display supramolecular selection while the tris-phen complexes do not. The reasoning is that the slight difference in the torsional arrangement of the ligands in the complexes has greater influence over the molecular recognition process during crystallization when the complexes are aggregating along their 3-fold axes. As a result, the complexes become somewhat selective for molecules with exactly the same twist angle over those with a slightly different degree of twist. While differences of only 1° may seem small, it is conceivable that greater differences could readily prohibit co-crystallization from occurring at all.

AUTHOR INFORMATION

Corresponding Author

*Phone: +61 7 3138 1220. Fax: +61 7 3138 1804. E-mail: j. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Mr. Tony Raftery for his help and assistance with powder XRD.

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REFERENCES

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CONCLUSIONS We have shown that co-crystallization of similar molecules does not, as previously believed, necessarily result in the formation of crystals with the same homogeneous distribution of components. This has significant implications, since any measurements of physical properties on bulk crystalline samples would show the average results for the range of crystals and their compositional distributions rather than specific attributes resulting only from the presence of multiple species in fixed ratios and their interaction with each other. We strongly recommend future research that involves the production of molecular alloys to incorporate some experiments to determine the relative homogeneity of single crystals obtained prior to making conclusions about the physical properties of bulk crystalline samples. In the example reported herein, tris-bipy metal complexes were found to crystallize with ratios in single crystals that vary significantly from those in the crystallizing solution. We believe this hitherto undocumented phenomenon to be the result of subtle molecular recognition processes that lead to what we have called “supramolecular selectivity” during crystallization. While supramolecular selection introduces complications when attempting to produce crystals with uniform molecular distribution, it is possible that supramolecular selectivity can also lead to crystals that contain solid state concentration gradients, since the selectivity is expected to increase as crystallization progresses, and it is to this possibility that we now turn our attention.

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ASSOCIATED CONTENT

S Supporting Information *

Crystals of [Ni(bipy)3](PF6)2, [Ru(bipy)3](PF6)2, and cocrystals [NixRu1−x(bipy)3](PF6)2 obtained from a 50:50 mixture; powder XRD patterns simulated from the single crystal X-ray structures of [Ni(phen)3](PF6)2·0.5H2O (1) and [Ru(phen)3](PF6)2·0.5H2O (2); scatter plot and regression line for SEM-EDX analyses of [NixRu1−x(phen)3](PF6)2 and [FexRu1−x(phen)3](PF6)2 co-crystals; an EDX spectrum of a single co-crystal grown from a solution containing equimolar amounts of [Fe(phen)3](PF6)2, [Co(phen)3](PF6)2, [Ni(phen)3](PF6)2, [Cu(phen)3](PF6)2, and [Ru(phen)3](PF6)2; and scatter plot and regression lines for SEM-EDX analyses of co-crystals of the form [FexRu1−x(bipy)3](PF6)2. This material is available free of charge via the Internet at http://pubs.acs.org. J



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