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Electronic Effects on Narcissistic Self-Sorting in Multicomponent SelfAssembly of Fe-Iminopyridine meso-Helicates Calvin A. Wiley,⊥ Lauren R. Holloway,⊥ Tabitha F. Miller, Yana Lyon, Ryan. R. Julian, and Richard J. Hooley* Department of Chemistry, University of CaliforniaRiverside, Riverside, California 92521 United States S Supporting Information *

ABSTRACT: Small changes in the electron donating ability of coordinating groups have substantial effects on the multicomponent self-assembly of Fe (II)-iminopyridine-based meso-helicate complexes. Both the nature of the internal diamine core and the terminal formylpyridine reactants control the rate of the assembly process, the thermodynamic favorability of the meso-helicate products, and the selective incorporation of different aldehyde termini into the assembly. Steric congestion at the coordinating ligands can prevent assembly altogether, and favorable incorporation of electron-rich aldehyde termini is observed, even though the rate of reaction is accelerated by the use of electron-poor aldehyde reactants. NMR and electrospray ionization mass spectrometry analyses were employed to determine the synergistic nature of narcissistic self-sorting in this system, which depends on both the rigidity of the central core and the electronic donor ability of the aldehyde terminus. These experiments illustrate that significant control of selfsorting and self-assembly is possible upon extremely small variations in ligand structure, rigidity, and donor ability.

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narcissistic30,31) in the literature36−43 involve relatively large changes in ligand structure. Recently, we investigated the possibility of controlled selfsorting between constituent ligands displaying extremely small differences, where the contributions to the selectivity can be difficult to tease out. We have previously shown that controlled self-sorting is possible in Fe-iminopyridine-based assemblies involving fluorenone and dibenzosuberone-derived ligand scaffolds.44−46 Small changes in geometry, and the presence of complementary hydrogen-bonding groups, showed exquisite control of the self-assembly properties. However, only modifications to the ligand backbone have been studied as yet, and the effects of small changes in the coordinators themselves are unknown. We asked the question: can small electronic changes have any meaningful effect on self-assembly? We had previously attempted similar variations on selective assembly in Pd-pyridyl M2L4 paddlewheel assemblies and found that electronic variation of the pyridyl coordinators was ineffective in controlling assembly.47 Fe-iminopyridine-based assemblies are far more sensitive to structural changes, however, and so here we investigate the effect of coordinator variation on the self-assembly properties of Fe2L3 meso-helicates.

INTRODUCTION Multicomponent self-assembly is an essential strategy for the formation of complex metal−ligand coordination cages.1 These systems can range from macrocycles and polyhedra to coordination polymers, catenanes, and molecular switches.2−8 Introducing function to these self-assembled complexes is an enticing goal, and one that can introduce a far greater range of applications, but there are a number of challenges to introducing functional groups in self-assembled systems. A number of these challenges have been recently addressed, such as endohedral orientation of functional groups,9−16 useful exofunctionalization motifs,17,18 incompatibility of reactive functions with coordinating groups,19,20 and controlled postsynthetic modification of fragile, reversible self-assemblies.21−23 A major obstacle in the introduction of function into the ligand scaffolds used in cage formation is that controlled multicomponent self-assembly is far simpler if the components are all identical, symmetrical, and display as few degrees of freedom as possible.24 In addition, selective incorporation of single functional groups or multiple different functional groups into a cagelike assembly requires self-sorting of the individual components. This is well-precedented for Hbonded systems25 and can be easily introduced by significant differences in the denticity26,27 or the geometry28,29 of the coordinating motif, but it is far more challenging when differentiating between similar, if not identical, components. Slight changes in ligand backbone30,31 can have drastic effects on the outcomes of self-assembly, leading to highly controlled narcissistic self-sorting between similar individual components and even control of reactivity by the self-assembly process.32 Many examples of self-sorting behavior (either social33−35 or © XXXX American Chemical Society

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RESULTS AND DISCUSSION The constituent pieces of the assemblies are shown in Figure 1. Three different cores were employed: 2,7-diaminosuberone A, 2,7-diaminosuberol B, and 2,7-diaminosuberenone C. Each of these cores is capable of forming the corresponding Fe2L3 mesoReceived: July 9, 2016

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DOI: 10.1021/acs.inorgchem.6b01644 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the various formylpyridines 4−7 to form suberone mesohelicates 1·Br6 and 1·Me6, suberol meso-helicates 2·Br6 and 2· Me6, and suberenones 3·Br6 and 3·Me6. Specifically, diamine (A−C, 39 mM), 2-formylpyridine, and Fe(ClO4)2 were mixed in a 1:2:0.66 ratio in CH3CN and heated overnight at 55 °C to allow full equilibration. Precipitation from the solution with Et2O gave clean samples of the various meso-helicates, and the structures were characterized by NMR and electrospray ionization mass spectrometry (ESI-MS). Each new mesohelicate complex was highly similar in behavior to the known meso-helicates 1−3·H6, which had previously been characterized via X-ray diffraction analysis.21 While the 5-substituted formylpyridines 4−6 were all capable of forming assemblies with the diamine ligands A−C, 2-formylquinoline 7 was not. When suberone A was heated with 2-formylquinoline 7 and Fe(ClO4)2, the expected purple solution (a clear marker of the formation of Fe-iminopyridine-based assemblies) was not observed. After 12 h of reaction time, the solution had attained an orange/brown color and formed a brown solid upon precipitation. Dissolution in CD3CN yielded a red solution: no starting material peaks were present, but no discrete mesohelicate was observed. Broad peaks at high chemical shift (up to 50 ppm) were observed, hinting at a paramagnetic system (see Supporting Information for spectra). Increased reaction time (24 h) or temperature (80 °C) had no effect. Molecular modeling analysis of the putative 1·Qn6 meso-helicate suggested that substitution at the 2-position of the pyridine ring caused steric clashes between the three ligands around the Fe center. The rigid, constrained suberone meso-helicate assembly has little flexibility, preventing structural reorganization at the metal center. As such, the assembly process does not tolerate 2substitution on the pyridyl group. While the variation in termini had no obvious effect on the stoichiometry of assembly (all systems formed meso-M2L3 complexes with full diastereocontrol at the metal centers), the relative stability of the systems, and most notably the effect on self-sorting, was unknown. Previous work with ligands A, B, and C and 2-formylpyridine showed distinct variations in favorability between the central, core ligands. The general stability trend in the formation of the final, fully assembled products was meso-helicate 1 > meso-helicate 3 > meso-helicate 2.46 The relative rate of formation of the different mesohelicates had not been analyzed, however. There are some exquisite examples of mechanistic and kinetics analysis of supramolecular assemblies in the literature for Pt/Pd-pyridyl polygons49,50 and polyhedra.51 The challenge in determining rates of reaction in this system is that the assembly reaction is a highly complex multicomponent kinetic process with multiple different ligand-based reaction steps that occur before (or in tandem with) the metal−ligand assembly. As such, accurate determination of individual rate constants is extremely challenging in this system. However, sharp, easily definable peaks for fully assembled product 1·H6 as well as unreacted formylpyridine can be observed in the 1H NMR spectrum throughout the reaction process. We therefore monitored the rate of the self-assembly process by NMR. The formation of all nine meso-helicates was performed in NMR tubes in CD3CN using 1,3,5-trimethoxybenzene as an internal standard. The concentrations of all components were constant, and the reaction progress after 2.0 h at 55 °C was determined by analysis of the resulting NMR spectra. All reactions were performed in triplicate, and the errors were determined from the variations in each run. As there are many possible “dead-

Figure 1. (a) Synthesis of the mixed meso-helicate assemblies from diamine cores and formylpyridine derivatives; (b) X-ray crystal structures (1·H6, 2·H6) and DFT-minimized structures (3·H6, 1· Me6, 1·H6, 1·Br6) of representative meso-helicate assemblies.

helicate complexes 1−3 upon multicomponent self-assembly with 2-formylpyridine (PyCHO) and Fe(ClO4)2 upon mild heating in CH3CN. Cores A−C can be synthesized in 2−3 steps from dibenzosuberone, and the synthesis and characterization of the standard PyCHO-based meso-helicates 1·H6, 2· H6, and 3·H6 is described in our previous publications.21,46 To determine the effects of electronic and steric substitution at the coordinating termini on the assembly process, we employed four different coordinating formylpyridines to the system: three highly similar formylpyridines 4−6 (see Figure 1), which display only small electronic differences, and 2formylquinoline 7, which provides increased steric repulsion around the metal centers in the assembly. Aldehydes 4−6 should display only minor steric variations: substitution is varied at the 5-position, remote from the coordinating iminopyridine unit. Space-filling representations of the minimized structures of 1·H6, 1·Me6, and 1·Br6 are shown in Figure 1, which indicate no steric clashes between the substituents on the pyridyl groups upon self-assembly. 5Bromo-2-formylpyridine (BrPyCHO 5) and 5-methyl-2-formylpyridine (MePyCHO 6) provide small (and opposite) variations in electron donating capacity of the pyridyl coordinator. The 5-substituents (Br, Me) have Hammett parameters of similar magnitude yet opposite sign: σpara (CH3) = +0.23, σpara (Br) = −0.17.48 The various assemblies were formed via the standard multicomponent assembly process. Dianiline precursors suberone (A), suberol (B), and suberenone (C) were mixed with B


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Inorganic Chemistry

example, after 2.0 h at 55 °C, meso-helicate 1·H6 is formed in 65% yield, whereas 1·Br6 is formed in 92% yield, and mesohelicate 1·Me6 is formed in only 43% yield. Under these incomplete reaction conditions, peaks for the relevant aldehyde (4−6) are clearly observable (see Supporting Information and Figure 2 for spectra), but the sum of aldehyde and product does

end” pathways and unproductive mechanistic routes (not to mention the possibility of different mechanisms for the formation of different meso-helicates), we did not focus on determination of reaction rates, rather the conversion observed after 2.0 h of reaction time (at constant concentration and reaction temperature), which can be used as a simple measure of the average reaction rate, as shown in Table 1. The same Table 1. Effect of Varying Aldehyde Terminus on Rate of Conversion

helicate product

helicate yield at 2.0 h, %

unreacted PyCHO, % (4/5/6)

intermediates, %

time to full completion (approx, h)

1·H6 2·H6 3·H6 1·Br6 2·Br6 3·Br6 1·Me6 2·Me6 3·Me6

65 41 62 92 67 89 43 22 37

29 40 32.5 3.5 15.5 6.5 15.5 27.5 20

6 19 5.5 4.5 17.5 4.5 41 50.5 43.5

3 5 3 2.5 3 2.5 5 9 5.5

Figure 2. Downfield regions of the 1H NMR spectra of the selective formation of meso-helicate 1·Br6 (blue dots) in the presence of equimolar amounts of ligands B (red dots) and A (green dots) at varying times (CD3CN, 400 MHz, spectra taken at 298 K).

not equal 100% (with respect to the standard). Table 1 shows the discrepancy as the percentage of “intermediates” present after 2.0 h of reaction. There are generally few easily assignable peaks in the NMR spectra for these intermediates, merely broad mounds. Presumably a large range of different intermediate structures is present in small amounts, under equilibration conditions. Metastable intermediates of this type have been previously shown to display broad, undefined 1H NMR spectra.52 The slower reactions (involving MePyCHO 6) contain a greater proportion of intermediates, as might be expected: initial imine formation and consumption of the aldehyde is relatively rapid, and full equilibration to mesohelicate is slower. We repeated the constant time experiments, but lowered the reaction time to 1.0 h. The shorter reaction time reduced the yield of formed helicate, as would be expected (see Supporting Information for data). The observed yields of meso-helicates were (within error) half that observed at 2.0 h of reaction. A much larger proportion of intermediates was observed after 1.0 h of reaction than at 2.0 h, which agrees with the observation that initial imine formation is rapid, followed by a slower equilibration to final meso-helicate product. The ligand core self-sorting experiments were then performed under controlled conditions to determine the relative rates of the competitive assembly processes. Here, two core ligands (A, B, or C, 39 mM each), PyCHO (4, 5, or 6), and Fe(ClO4)2 were mixed in a 1:1:2:0.66 ratio in CD3CN and heated at 55 °C in the presence of 1,3,5-trimethoxybenzene as internal standard. The concentrations of all components were constant, and the reaction progress was measured after 2.0 h at 55 °C. Again, all reactions were performed in triplicate, and the results were shown in Table 2 and Figures 2 and 3. Two major observations were immediately apparent: in the presence

process was repeated for the other successful PyCHO derivatives 5 and 6, to determine whether the electron withdrawing or donating nature of the methyl or bromo substituents would affect the rate and outcome of these sorting experiments. As can be seen in Table 1, the relative rate of formation is far more dependent on the nature of terminus PyCHO than on the core diamine ligand. While the reactivity of the core diamine ligands follows the order of final product stability, there are only small differences in rate between ligands A, B, and C (e.g., after 2.0 h at 55 °C, meso-helicate 1·H6 is formed in 65% yield, whereas 3·H6 is formed in 62% yield), no matter the aldehyde terminus used. The high degree of self-sorting seen between competing backbone ligands (e.g., when A and C are combined, 100% selectivity for 1·H 6 is observed after complete equilibration) cannot be due to these small rate differences, rather the relative thermodynamic stability of the final products. Varying the nature of the terminal aldehyde has a far greater effect on assembly rate than varying the internal diamine. Use of the more electron-poor (and thus more electrophilic) BrPyCHO 5 increased the rate of meso-helicate formation, whereas use of the more electron-rich (and less electrophilic) MePyCHO derivative 6 slowed the rate for formation. For C


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Inorganic Chemistry

Table 2. Effect of Varying the Aldehyde Terminus on the Rate of Conversion in Competitive Self-Sorting Experiments

reactants A A A A A A

+ + + + + +

B+4 C+4 B+5 C+5 B+6 C+6

major product

major product yield, %

PyCHO (4−6, %)

minor product yield, %

intermediates, %

1·H6 1·H6 1·Br6 1·Br6 1·Me6 1·Me6

32 28 65 38 17 25

22 5 12 10 18 14

5 6 0 4 7 10

41 61 23 48 58 51

1·H/Me/Br6) was significantly slower than when the reaction was performed with ligand A alone. For example, after 2.0 h at 55 °C, 65% yield of 1·H6 was formed when only A is present, but when both A and B are used, 32% yield of 1·H6 is formed under the same conditions. The relative trends in reactivity upon using the other aldehydes 5 and 6 are retained, but the overall reaction is slower in the presence of competing diamine in each case. In addition to this, peaks in the 1H NMR spectrum corresponding to disfavored homocomplex (i.e., either suberol or suberenone meso-helicates 2/3·H/Me/Br6) are present (e.g., green dots in Figure 3) in all cases except that of the reaction of ligands A and B with BrPyCHO 5. Even though no disfavored homocomplex is formed after full equilibration, the complex multicomponent self-assembly process appears to involve all possible species in the reaction. The disfavored meso-helicates are formed as part of the equilibrium mixture, but only transiently, and they are not present after complete reaction. It is not clear from the NMR spectra whether any heterocomplexes are also formed, as the remainder of the peaks in the spectra are broad and difficult to assign. No evidence of heterocomplex formation was observed in the ESI-MS spectra.46 In each case, a significant proportion of “intermediates” are present (20−60%, Table 2), and it is highly likely that these intermediates consist, at least in some part, of fully assembled heterocomplexes as well as oligomers, coordination polymers, and incompletely assembled iminopyridine-Fe species. This is not necessarily surprising, as the presence of transient intermediates and “dead end” pathways in this type of complex multicomponent equilibrium assembly process has been observed in other systems containing oligo-bipyridyl or iminopyridine units,53−61 but it is interesting to observe them here, especially considering the high degree of narcissistic selfsorting seen in the assembly process after full equilibration. These observations lead to the question of whether the termini could be discriminated in the self-assembly process and what effect the various internal ligand backbones had on this process. To investigate the propensity for terminus-based selectivity and explain the unusual behavior of MePyCHO 6, meso-helicate assembly was performed with two dif ferent PyCHO derivatives 4−6 in the reaction mixture. As the NMR spectra of suberone ligand 1 were the cleanest and simplest to analyze, the initial experiments focused on that backbone. Two experiments were performed (Figure 4): 1 equiv of diamine ligand A (40 mM) and 0.66 equiv of Fe(ClO4)2 were dissolved in CH3CN in in the presence of

Figure 3. (a) Illustration of the possible pathways to final favored product 1·X6 after full equilibration. (b) Sorting of the meso-helicate assembly 1·H6 from diamine cores A and B. Downfield regions of the 1 H NMR spectra of the selective formation of meso-helicate 1·Me6 (blue dots) in the presence of equimolar amounts of ligands A and C (c) after full equilibration (13 h) and (d) 2.0 h. Downfield regions of the 1H NMR spectra of freshly prepared cages (e) 1·Me6 and (f) 3· Me6. (CD3CN, 400 MHz, spectra taken at 298 K). Red = unreacted MePyCHO 6, purple = unreacted ligand C, green = 3·Me6, blue = 1· Me6. Note: 3·Me6 is present only transiently at 2.0 h and not observed after full equilibration.

of a second competitive ligand, the rate of formation of the preferred product species (in each case, suberone meso-helicate D


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Inorganic Chemistry

Figure 4. Product distribution upon incorporation of different PyCHO groups to the self-assembly process. Downfield regions of the 1H NMR spectra of (a) [1·H6]; (b) [1·Br6]; products formed from reaction of 0.66 equiv of (FeClO4)2 and 1.0 equiv of A with (c) 1.0 equiv of 4 + 1.0 equiv of 5; (d) 2.0 equiv of 4 + 2.0 equiv of 5 (500 MHz, CD3CN, 298 K). Note: “positional isomers” refers to the different possible isomers of 1·H4Br2, 1·H3Br3, and 1·H2Br4 not shown in the cartoon: these isomers contribute to the broad peaks in spectra (c) and (d).

Figure 5. ESI-MS of the products formed from reaction of 0.66 equiv of (FeClO4)2 and 1.0 equiv of A with (a) 1.0 equiv of 4 + 1.0 equiv of 5; (b) 2.0 equiv of 4 + 2.0 equiv of 5 (in CH3CN). Assemblies 1· H4Br2, 1·H3Br3, and 1·H2Br4 have different isomeric possibilities that are all of the same mass and cannot be discriminated by MS.

either two total equivalents PyCHO 4 and Br-PyCHO 5 (1:1, 40 mM each, Figure 4c) or four total equivalents of PyCHO 4 and Br-PyCHO 5 (1:1, 80 mM each, Figure 4d). These two concentrations of PyCHO correspond to (1) a stoichiometric reaction where all available PyCHO is incorporated in the reaction (to determine relative favorability of hetero- vs homocomplexes), and (2) a super-stoichiometric reaction whereby the system can select which terminus is more favorable. Unsurprisingly, the initial NMR tests of combining two similar terminus aldehydes gave spectra that were somewhat broad and challenging to analyze. While 1H NMR spectra of the individual meso-helicates 1·H6 and 1·Br6 are symmetrical and sharp, the product NMR spectra are significantly broader. There are numerous different product possibilities from the multicomponent self-assembly of two aldehydes and one ligand (in an M2L3 stoichiometry), shown in cartoon form in Figure 4. Evidently the magnetic anisotropy from the aromatic rings causes small changes in the chemical shifts of the protons on aromatic rings in close proximity, but those shifts are not sufficient for obvious discrimination and analysis. Clearly, however, both aldehydes are incorporated into the assembly in the stoichiometric reaction (Figure 4c). It is also clear that, in the super-stoichiometric reaction (Figure 4d), one aldehyde is predominantly incorporated: the peaks corresponding to 1·H6 (labeled with red dots) are significantly enhanced as compared to the stoichiometric experiment. Quantitative discrimination between the heterocomplexes is challenging via 1H NMR, but the use of Br-PyCHO 5 allows simple analysis by ESI-MS. Figure 5 shows the M4+ region of the ESI-MS (see Supporting Information for full spectra) gained from ionization of a sample of 1·HxBry in CH3CN. Two samples were analyzed as before, formed from addition of either 2 or 4 equiv of a 1:1 PyCHO mixture. As can be seen in

Figure 5a, a mixture of all possible isomers was formed in the stoichiometric reaction, and the spectrum shows seven distinct peaks for each possible combination of the two different termini (4 and 5). The two PyCHO termini are sufficiently similar to allow self-assembly into the different mixed products, and in this case, all aldehydes were consumed to form selfassemblies. The ESI data show very little selectivity between the two aldehydes 4 and 5: although the product is not a completely statistical mixture, no appreciable preference is observed. However, as can be seen from the superstoichiometric reaction outcome (Figure 5b), the self-assembly process is able to discriminate between the two termini if given the opportunity. While some Br-PyCHO 5 is incorporated in the assembly mixture, a significant excess of more electron-rich terminus 4 is incorporated. The most intense (and therefore populous) peak is that of 1·H6. Some 1·H5Br and 1·H4Br2 is formed (along with a miniscule amount of 1·H3Br3), but no assemblies are present that incorporate an excess of terminus 5. The comparison of the two ESI-MS spectra in Figure 5 is illustrative, as it shows that the ionization potential of the different assemblies is highly similar, all things being equal, and that the peak intensity is a somewhat quantitative method of investigating the favorability of incorporating different termini in the assembly. Unfortunately, there is no obvious way to discriminate between isomers of the different assemblies (i.e., the different positional variants in 1·H3Br3, for example), but the nonselective nature of the stoichiometric reaction suggests that there is no bias to one isomer over the other. It is also interesting to note that, while the rate of formation of the selfE


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Inorganic Chemistry assembly is increased upon the use of electron-poor aldehydes, the thermodynamic stability of the more electron-rich product is greater. While discrimination between the electron-poor Br-PyCHO 5 and PyCHO 4 was pronounced, the discrimination between PyCHO 4 and the electron-rich Me-PyCHO 6 was much less. Figure 6 shows the ESI-MS spectra for the incorporation of 4

Figure 7. ESI-MS of the products formed from reaction of 0.66 equiv of (FeClO4)2 and 1.0 equiv of C with (a) 1.0 equiv of 5 + 1.0 equiv of 6; (b) 2.0 equiv of 5 + 2.0 equiv of 6 (in CH3CN).

the electron-rich terminus into the assembly. The ratio of 6:5 incorporation is similar to that of 4:5. Even though there is a wider difference in donating/withdrawing nature between BrPyCHO 5 and Me-PyCHO 6 than there is between Br-PyCHO 5 and PyCHO 4, the system does not discriminate better between the methyl and unsubstituted substituents (as seen in Figure 6). All possible combinations of the termini and the ligand cores were tested. The specific examples shown in the text were chosen for their cleanliness and ease of viewing, hence the variety in cores in Figures 5−7. The ESI-MS spectra of all other combinations of cores A−C and termini 4−6 can be found in the Supporting Information, and there are some interesting variations, although the differences are relatively minor. The favorability of incorporating the different termini is somewhat dependent on the ligand core. Mixing experiments performed with suberol ligand B showed heightened favorability toward the electron-rich Me-PyCHO 6, more so than the suberone ligand A. Likewise, incorporation of electron-poor BrPyCHO 5 is disfavored more by the suberol ligand than the suberone ligand. The differences are small, so concrete explanations are unclear, but this may be due to the overall stability differences in the meso-helicate assemblies. The suberol meso-helicate 2·H6 is less stable than the suberone meso-helicate 1·H6 and can benefit more from the extra stability of the electron-donating methyl group in 2·Me6. Likewise, the destabilizing effect of the Br-PyCHO 5 makes the bromosuberol meso-helicate 2·Br6 more highly disfavored. The equilibrium nature of the process, especially with the similar termini 4 and 6 where Me-PyCHO-based intermediates are observed during the reaction, led us to investigate whether the aldehydes could be selectively displaced af ter assembly. The

Figure 6. ESI-MS of the products formed from reaction of 0.66 equiv of (FeClO4)2 and 1.0 equiv of B with (a) 1.0 equiv of 4 + 1.0 equiv of 6; (b) 2.0 equiv of 4 + 2.0 equiv of 6 (in CH3CN).

and 6 in either stoichiometric (Figure 6a) or superstoichiometric (Figure 6b) amounts into the self-assembly of suberol backbone B with Fe(ClO4)2. The stoichiometric reaction gave an approximately statistical mixture of peaks, as expected, but when 2 equiv each of 4 and 6 were used, only a slight favorability for incorporating the electron-rich terminus was observed. While no peaks for 2·H6 are seen, that is the only missing assembly, and all other combinations are present in almost statistical amounts. Interestingly, while the Hammett parameters of Me and Br are (almost) equal and opposite, the relative effect of the donor/acceptor abilities of the substituents on the assembly process does not linearly track with the Hammett parameter. In this case, the outcomes can be qualitatively explained, but the effect is not due solely to buildup of charge at the pyridine nitrogen, as might be expected for a complex assembly system such as this. The final variation in termini is between the electron-rich and electron-poor termini 5 and 6. The ESI-MS spectra for the incorporation of 5 and 6 in either stoichiometric or superstoichiometric amounts into the self-assembly of suberenone backbone C with Fe(ClO4)2 are shown in Figure 7 and follow the same trends as before. The stoichiometric process gives an approximately statistical mixture of products, and the superstoichiometric reaction shows the preferential incorporation of F


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Inorganic Chemistry

4 and 6, the results followed the expected pattern, within experimental error. Treating 1·Me6 with PyCHO at 80 °C resulted in the displacement of 40% of the initial Me-PyCHO aldehydes, resulting in the expected mixture of heterocomplexes 1·HxMey. Full equilibration was observed after 5 h, and no additional change was observed after a further 20 h of heating. Interestingly, the favorability of the displacement process was dependent on the central core, as well as the aldehyde itself: displacement of Br-PyCHO from suberol-based meso-helicate 2·Br6 was far more facile. After addition of 6 equiv of 4 to 2·Br6 and heating at 80 °C for 15 min, no remaining meso-helicate containing Br-PyCHO was observed by 1H NMR analysis. The more effective equilibration process is most likely due to the lower energetic favorability of meso-helicates formed from suberol B. Computational analysis using DFT minimizations showed that there was a significant increase in strain conferred on the backbone of 2·H6 upon self-assembly, as compared to suberone meso-helicate 1·H6.21 The less favorable suberol-based meso-helicate 2·H6 is evidently “spring-loaded”, and when ligand A is added, the system takes advantage of the strain release gained upon disassembly (relative to that gained from disassembly of suberone meso-helicate 1·H6) to undergo the equilibration reaction more easily. This observation introduces the question of how the displacement process occurs. The mechanism of aldehyde displacement most likely occurs through initial, reversible cleavage of the imine bonds by trace water in the CD3CN solvent. This equilibrium will lie to the left, but once some fragmentation has occurred, the presence of uncomplexed diamine allows equilibration to the thermodynamic product over time. To corroborate this theory, the displacement experiment between 1·Br6 and 4 was performed in the presence of 20 μL of H2O. In this case, the equilibration occurred significantly faster. Aldehyde 4 was added to a CD3CN solution of 1·Br6 in CD3CN/D2O (25:1), stirred for 30 s at 25 °C, then heated to 80 °C. After 5 min, significant displacement had occurred, and the reaction proceeded to completion after only 15 min, giving 85% displacement of the disfavored aldehyde 5, similar to that seen in the anhydrous case. Evidently some water is required to “grease the wheels” and get the displacement equilibration started. Interestingly, however, the addition of water has no effect on the equilibration of uphill reactions. If bromoformylpyridine 5 is added to either 1·H6 or 2·H6 in CD3CN/H2O (25:1), no incorporation of the disfavored aldehyde is seen, even after 3 d of heating at 80 °C (see Supporting Information for spectra). While the addition of water accelerates favorable equilibration, it cannot aid in the equilibration of disfavored processes. As the energy differences between meso-helicates containing termini 4 or 5 are small (e.g., see Figure 5) and the mixed meso-helicates are easily formed from the combination of diamine and aldehyde (as opposed to aldehyde displacement), 1·H6 and 2·H6 are evidently “kinetically trapped” upon formation, and harsher conditions are required to effect equilibration. Unfortunately, decomposition occurs in other solvents and at higher temperatures, especially in the presence of water, and this limits further investigation of this phenomenon.

mechanism of exchange between different diamines is obvious, as the terminal amines can easily attack the imine groups in 1· H6 (for example), allowing simple equilibration.46 Other Feiminopyridine assemblies are susceptible to added amine, and structural switching between different amine termini62 or cores12 is commonplace. Whether it is possible to exchange the aldehyde termini after equilibrium is reached is less obvious, however. As we have a clear delineation between favored and disfavored PyCHO termini, we attempted to perform displacement experiments on preformed, isolated meso-helicates with different aldehydes. Preformed meso-helicates 1·Br6, 1·Me6, and 2·Br6 were combined with aldehydes 4−6 in CD3CN and heated, and 1H NMR analysis was used to determine the displacement. When the meso-helicate formed from A and electron-poor Br-PyCHO 5 (i.e., 1·Br6) was heated at 80 °C with 6 equiv of PyCHO 4, incorporation of the more electronrich aldehyde occurred relatively quickly (see Figure 8). The

Figure 8. Aldehyde displacement experiment. Downfield regions of the 1H NMR spectra of the displacement of Br-PyCHO 5 from mesohelicate 1·Br6 (blue dots) by PyCHO 4 over time at 80 °C (CD3CN, 400 MHz, spectra taken at 298 K).

displacement was nearly complete after 5 h, and after 8 h no further change was observed, after which 88% of Br-PyCHO termini had been displaced by PyCHO 4. When analyzed by ESI-MS (see Supporting Information), the proportion of products was similar to that observed for the superstoichiometric assembly experiment shown in Figure 5b. As might be expected, “uphill”, energetically disfavored displacement was not possible: when meso-helicate 1·H6 was treated with Br-PyCHO 5 in the same manner, no displacement was observed, even after extensive heating. When the same experiment was performed between the quite similar termini

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CONCLUSIONS In conclusion, we have shown that small changes in the electron donating ability of coordinating groups can have substantial effects on the multicomponent self-assembly of Fe G


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Inorganic Chemistry

mg, 0.056 mmol) were combined in anhydrous CH3CN (2 mL) in an 8 mL vial. The solution was then heated at 55 °C for 12 h with stirring. The purple solution was diluted with Et2O (3 mL), placed in a freezer at 0 °C for 3 h, followed by filtration of the resulting precipitate. After it was dried, the product was isolated as a purple solid (51.3 mg, 87%). 1 H NMR (500 MHz; CD3CN) δ 8.67 (s, 2H), 8.36 (d, J = 7.0 Hz, 2H), 8.21 (d, J = 6.9 Hz, 2H), 7.19 (d, J = 7.5 Hz, 4H), 6.3 (s, 2H), 5.56 (s, 2H), 3.33 (m, 2H), 3.20 (m, 2H), 2.43 (s, 6H). 13C NMR (150 MHz, CD3CN): δ 189.7, 174.8, 174.2, 155.6, 148.5, 144.2, 141.5, 139.8, 138.2, 130.7, 130.2, 125.1, 123.3, 34.7, 18.6. HRMS (ESI) m/z calcd. for C87H72Fe2N12O3 ([1·Me6]4+) (361.0688), found 361.1037. Bromopyridine-Suberol meso-Helicate (2·Br6). 3,7-Diaminodibenzosuberol (diamine B; 20 mg, 0.083 mmol), 5-bromo-2-formylpyridine 5 (29 mg, 0.16 mmol), and Fe(ClO4)2·3H2O (17 mg, 0.055 mmol) were combined in anhydrous CH3CN (2 mL) in an 8 mL vial. The solution was then heated at 55 °C for 12 h with stirring. The purple solution was diluted with Et2O (3 mL), placed in a freezer at 0 °C for 3 h, followed by filtration of the resulting precipitate. After it was dried, the product was isolated as a purple solid (60.7 mg, 92%). 1H NMR (500 MHz; CD3CN) δ 8.78 (s, 2H), 8.25 (d, J = 8.2 Hz, 2H), 7.86 (m, 4H), 7.76 (s, 2H), 6.90 (d, J = 7.9 Hz, 2H), 6.19 (d, J = 7.6 Hz, 2H), 6.10 (d, J = 4.7 1H), 3.36 (m, 2H), 2.88 (m, 2H), 2.52 (d, J = 4.7, 1H). 13 C NMR (150 MHz, CD3CN): δ 170.4, 157.6, 156.7, 144.8, 143.5, 141.7, 137.7, 130.6, 129.8, 125.1, 120.7, 120.2, 66.8, 30.4. HRMS (ESI) m/z calcd. for C81H60Fe2N12O3Br6·Cl2O8 ([2·Br6]2+) (1017.2901), found 1017.8162. Methylpyridine-Suberol meso-Helicate (2·Me6). 3,7-Diaminodibenzosuberol (diamine B; 20 mg, 0.083 mmol), 5-methyl-2formylpyridine 6 (19 mg, 0.16 mmol), and Fe(ClO4)2·3H2O (17 mg, 0.055 mmol) were combined in anhydrous CH3CN (2 mL) in an 8 mL vial. The solution was then heated at 55 °C for 12 h with stirring. The purple solution was diluted with Et2O (3 mL), placed in a freezer at 0 °C for 3 h, followed by filtration of the resulting precipitate. After it was dried, the product was isolated as a purple solid (49.8 mg, 89%). 1 H NMR (500 MHz; CD3CN) δ 8.7 (s, 2H), 7.98 (s, 2H), 7.83 (m, 4H), 7.35 (s, 2H), 6.83 (d, J = 5.9 Hz, 2H), 6.13 (d, J = 6.9 Hz, 2H), 6.09 (s, 1H), 3.34 (m, 2H), 2.85 (m, 2H), 2.43 (s, 1H), 2.33 (s, 6H). 13 C NMR (150 MHz, CD3CN): δ 189.2, 174.7, 156.3, 155.7, 148.5, 144.3, 141.5, 139.9, 138.2, 130.6, 130.0, 125.0, 123.3, 34.6, 18.7. HRMS (ESI) m/z calcd. for C87H78Fe2N12O3 ([2·Me6]4+) (362.5906), found 362.6257. Bromopyridine-Suberenone meso-Helicate (3·Br6). 3,7-Diaminodibenzosuberenone (diamine C; 20 mg, 0.085 mmol), 5-bromo-2formylpyridine 5 (31 mg, 0.17 mmol), and Fe(ClO4)2·3H2O (18 mg, 0.056 mmol) were combined in anhydrous CH3CN (2 mL) in an 8 mL vial. The solution was then heated at 55 °C for 12 h with stirring. The purple solution was diluted with Et2O (3 mL), placed in a freezer at 0 °C for 3 h, followed by filtration of the resulting precipitate. After it was dried, the product was isolated as a purple solid (64.1 mg, 93%). 1 H NMR (500 MHz; CD3CN) δ 8.74 (s, 2H), 8.68 (d, J = 7.6 Hz, 2H), 8.43 (d, J = 7.9 Hz, 2H), 7.61 (d, J = 8.1 Hz, 2H), 7.55 (s, 2H), 7.24 (s, 2H), 6.56 (s, 2H), 5.77 (d, J = 7.5 Hz, 2H). 13C NMR (150 MHz, CD3CN): δ 185.9, 176.1, 157.7, 156.7, 149.7, 143.5, 138.0, 135.5, 134.5, 132.4, 131.6, 126.5, 125.3, 122.6. HRMS (ESI) m/z calcd. for C 81 H48 Fe 2N 12 O3Br 6 ([3·Br6 ] 4+ ) (456.6315), found 456.7847. Methylpyridine-Suberenone meso-Helicate (3·Me6). 3,7-Diaminodibenzosuberenone (diamine C; 20 mg, 0.085 mmol), 5-methyl-2formylpyridine 6 (21 mg, 0.17 mmol), and Fe(ClO4)2·3H2O (18 mg, 0.056 mmol) were combined in anhydrous CH3CN (2 mL) in an 8 mL vial. The solution was then heated at 55 °C for 12 h with stirring. The purple solution was diluted with Et2O (3 mL), placed in a freezer at 0 °C for 3 h, followed by filtration of the resulting precipitate. After it was dried, the product was isolated as a purple solid (53.1 mg, 91%). 1 H NMR (500 MHz; CD3CN) δ 8.77 (s, 2H), 8.53 (s, 2H), 8.47 (s, 2H), 7.59 (d, J = 7.8 Hz, 2H), 7.47 (d, J = 4.3 Hz, 2H), 7.23 (s, 2H), 6.58 (s, 2H), 5.65 (d, J = 7.2 Hz, 2H), 2.16 (s, 6H). 13C NMR (150 MHz, CD3CN): δ 186.2, 175.5, 156.7, 156.4, 155.7, 150.2, 141.8,

(II)-iminopyridine meso-helicate complexes. While the addition of steric congestion at the coordinating ligands prevents assembly, subtle electronic changes in aldehyde termini can be discriminated by the assembly process. Ligand rigidity affects this selectivity, indicating a synergistic effect between the electron donating ability of the coordinators and the deformation of the ligand backbone upon self-assembly. Selective, favorable incorporation of electron-rich aldehyde termini is observed, even though the rate of assembly is accelerated by the use of electron-poor aldehyde reactants. The selective displacement of aldehyde termini is possible in the presence of small amounts of water in the solution. The water acts as a poor nucleophile, but is sufficient to start the equilibrium process, allowing shuttling from less-favored complexes to more-favored products. Aldehyde termini as similar as 2-formyl-pyridine and 5-bromo-2-formylpyridine can be discriminated by the assembly process, as shown by 1H NMR and ESI-MS analysis. These experiments illustrate that significant control of self-sorting and self-assembly is possible upon extremely small variations in ligand structure, rigidity, and donor ability: these effects can conceivably be exploited in the selective formation of large, more complex systems with defined functions. Further study on the formation of functionalized cage complexes is underway in our laboratory.

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EXPERIMENTAL SECTION

General Information. 1H and 13C NMR spectra were recorded on a Varian Inova 500 MHz NMR spectrometer. Proton (1H) chemical shifts are reported in parts per million (δ) with respect to tetramethylsilane (TMS; δ = 0), and referenced internally with respect to the protio solvent impurity. Deuterated NMR solvents were obtained from Cambridge Isotope Laboratories, Inc., Andover, MA, and used without further purification. Mass spectra were recorded on an Agilent 6210 LC TOF mass spectrometer using ESI with fragmentation voltage set at 115 V and processed with an Agilent MassHunter Operating System, or on a Thermo LTQ linear ion trap mass spectrometer (Thermo Fisher Scientific, San Jose, CA) using ESI and processed with Thermos Xcalibur 2.0 software. Predicted isotope patterns were prepared using ChemCalc.63 All other materials were obtained from Aldrich Chemical Co, St. Louis, MO, or TCI, Tokyo, Japan, and were used as received. Solvents were dried through a commercial solvent purification system (Pure Process Technologies, Inc). The synthesis and characterization of ligands A−C and unsubstituted formylpyridine meso-helicates 1·H6, 2·H6, and 3·H6 can be found in our previous reports.21,46 The minimized structures of the other meso-helicates were obtained via density functional calculations and were optimized using the dispersion-corrected B97-D density functional64,65 in the 6-31G(d) basis set.66 Synthesis of New Compounds. Bromopyridine-Suberone meso-Helicate (1·Br6). 3,7-Diaminodibenzosuberone (dianiline A; 20 mg, 0.084 mmol), 5-bromo-2-formylpyridine 5 (31 mg, 0.17 mmol), and Fe(ClO4)2·3H2O (18 mg, 0.056 mmol) were combined in anhydrous CH3CN (2 mL) in an 8 mL vial. The solution was then heated at 55 °C for 12 h with stirring. The purple solution was diluted with Et2O (3 mL), placed in a freezer at 0 °C for 3 h, followed by filtration of the resulting precipitate. After it was dried, the product was isolated as a purple solid (61.4 mg, 89%). 1H NMR (500 MHz; CD3CN) δ 8.69 (s, 2H), 8.60 (d, J = 8.2 Hz, 2H), 8.36 (t, J = 8.2 Hz, 2H), 7.49 (s, 2H), 7.19 (d, J = 8.0 Hz, 2H), 6.25 (s, 2H), 5.61 (d, J = 6.8 Hz, 2H), 3.29 (m, 2H), 3.19 (m, 2H). 13C NMR (150 MHz, CD3CN): δ 189.0, 175.3, 157.5, 156.8, 148.1, 144.8, 143.2, 138.3, 131.6, 130.8, 126.20, 125.0, 123.4, 34.7. High-resolution mass spectrometry (HRMS) (ESI) m/z calcd. for C81H48Fe2N12O3Br6 ([1·Br6]4+) (457.9810), found 458.0136. Methylpyridine-Suberone meso-Helicate (1·Me6). 3,7-Diaminodibenzosuberone (diamine A; 20 mg, 0.084 mmol), 5-methyl-2formylpyridine 6 (21 mg, 0.17 mmol), and Fe(ClO4)2·3H2O (18 H


Article

Inorganic Chemistry Notes

140.0, 138.1, 135.3, 134.2, 130.5, 125.3, 122.8, 18.6. HRMS (ESI) m/z calcd. for C87H66Fe2N12O3 ([3·Me6]4+) (359.8059), found 359.3785. General Procedure for Formation Kinetics Experiments. Suberone ligand A (1 equiv, 3.7 mg, 0.016 mmol, initial concentration = 39 mM) was placed in an NMR tube with 400 μL of CD3CN. 5Bromo-2-formylpyridine 5 (2 equiv, 5.8 mg, 0.031 mmol), 0.66 equiv of Fe(ClO4)2·xH2O (3.4 mg, 0.010 mmol), and 1 equiv of 1,3,5trimethoxybenzene standard (2.6 mg, 0.016 mmol, concentration = 39 mM) were added to the tube, and the sample was shaken to facilitate dissolution. The sample was heated at 55 °C for 2.0 h. After it was heated, the tube was rapidly chilled in a 0 °C ice bath to halt the reaction progress, and a 1H NMR spectrum of the mixture was obtained. Initial kinetic rates were determined from the integration of cage peaks versus the internal standard. Each experiment was performed in triplicate, and the product values were averaged to determine experimental error. General Procedure for Ligand Self-Sorting Experiments. All general sorting experiments were performed in an NMR tube. Suberone Ligand A (1 equiv, 3.72 mg, 0.0156 mmol) and 1 equiv of suberol Ligand B (3.75 mg, 0.0156 mmol) were placed in an NMR tube with 400 μL of CD3CN. 5-Bromo-2-formylpyridine 5 (2 equiv, 5.81 mg, 0.0312 mmol) and 0.66 equiv of Fe(ClO4)2·xH2O (3.37 mg, 0.0103 mmol) were added to the tube, and the sample was shaken to facilitate dissolution. A 1H NMR spectrum of the mixture was taken prior to heating. The sample was then heated at 80 °C, and 1H NMR spectra were taken at regular intervals to determine the sorting end point. Semiquantitative rate data was determined by integrating representative peaks in the 1H NMR spectrum during the reaction and the known concentrations of reactants. Sorting experiments were repeated with 1H NMR spectrum obtained after heating at 55 °C for 2.0 h. These experiments were performed in triplicate, and the percentages of homocomplexes, unreacted formylpyridine, and intermediates were recorded. General Procedure for Aldehyde Terminus Mixing Experiments. All mixing experiments were performed in glass 8 mL vials. Diamine A (20 mg, 0.084 mmol), 1 mol equiv of 5-bromo-2formylpyridine 5 (15.5 mg, 0.084 mmol), 1 mol equiv of 2formylpyridine 4 (10.1 μL, 0.084 mmol), and 0.66 mol equiv of Fe(ClO4)2·3H2O (18 mg, 0.056 mmol) were placed in a vial with a stir bar. CH3CN (2 mL) was added to the vial, and the mixture was heated at 55 °C for 12 h. Diethyl Et2O (2 mL) was added to the vial, the mixture was placed in a freezer at 0 °C for 3 h, and the resulting purple solid was filtered and dried. Super-stoichiometric experiments were also performed using 2 equiv (0.168 mmol) of each formylpyridine. General Procedure for Aldehyde Displacement Experiments. One equivalent of preformed meso-helicate (e.g., 5 mg 1·Br6 (0.0022 mmol)) was dissolved in 600 μL of CD3CN and 6 equiv of aldehyde solution (e.g., 13 μL of 1.05 M PyCHO 4 in CH3CN (0.0135 mmol)) were added via micro syringe. The solution was heated to 80 °C, and 1H NMR spectra were taken at regular intervals to determine the reaction end point. Proportional displacement data were determined by integrating representative peaks in the 1H NMR spectrum during the reaction.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the National Science Foundation (CHE-1151773 to R.J.H., CHE-1401737 to R.R.J.), the UCR Center for Catalysis, and the UCR Office for Research and Economic Development for support.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01644.

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REFERENCES

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