Stepwise Construction of Heterobimetallic Cages by an Extended

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Stepwise Construction of Heterobimetallic Cages by an Extended Molecular Library Approach Matthias Hardy,† Niklas Struch,† Filip Topić,‡ Gregor Schnakenburg,§ Kari Rissanen,‡ and Arne Lützen*,†,# †

Kekulé-Institut für Organische Chemie und Biochemie and §Institut für Anorganische Chemie, Rheinische Friedrich-Wilhelms-Universität Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany ‡ Nanoscience Center, Department of Chemistry, University of Jyväskylä, P.O. Box 34, 40014 Jyväskylä, Finland S Supporting Information *

ABSTRACT: Two novel heterobimetallic complexes, a trigonal-bipyramidal and a cubic one, have been synthesized and characterized using the same C3-symmetric metalloligand, prepared by a simple subcomponent self-assembly strategy. Adopting the molecular library approach, we chose a mononuclear, preorganized iron(II) complex as the metalloligand capable of self-assembly into a trigonal-bipyramidal or a cubic aggregate upon coordination to cis-protected C2symmetric palladium(II) or unprotected tetravalent palladium(II) ions, respectively. The trigonal-bipyramidal complex was characterized by NMR and UV−vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and single-crystal Xray diffraction. The cubic structure was characterized by NMR and UV−vis spectroscopy and ESI-MS.

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geometry and number) for two different metal cations;15 (b) the use of a heterotopic ligand that bears hard and soft donor motifs and two metal cations that differ significantly in their charge densities;16 (c) the use of an inert or highly preorganized complex with free coordination sites in the periphery that acts as a metalloligand to coordinate a second metal cation. The latter complex-as-a-ligand strategy was already proven useful in the construction of several heterometallic 2D and 3D architectures.17 Herein, we report the design and synthesis of a heterobimetallic trigonal bipyramid and a heterobimetallic cube, using a new C3-symmetric metalloligand (Scheme 1). Iron(II) and palladium(II) cations can be distinguished effectively because of their different preferred coordination spheres. We chose the heterotopic ligand building block 1 and commercially available tris(2-aminoethyl)amine (2; TREN) to generate a covalently bridged octahedral tris(pyridylimine) binding site to coordinate iron(II) cations. TREN was chosen as the chelating backbone because of the many examples of its use in combination with 2-carbaldehyde-substituted heteroarenes to create an octahedral coordination sphere. 18 Furthermore, the resulting formally heptadentate metalloligand ML is preorganized for a fac coordination. The suppressed mer

INTRODUCTION The selective self-assembly of metal fragments and organic ligands to coordination cages is a widely studied field in supramolecular chemistry, and many beautiful examples of sophisticated metallosupramolecular complexes have been reported.1 In this context, the molecular library approach2 is a useful tool to design aggregates with defined structures3 and functionality.4 Despite the undisputed beauty and functionality of these assemblies, the ligands to build these often require a complicated and demanding synthesis.5−9 The subcomponent self-assembly approach simplifies the ligand synthesis in metallosupramolecular chemistry by using two ligand subunits that form the desired ligand in situ via the reversible formation of covalent bonds, e.g., imine bonds.10 Here, aldehyde and amine components form the ligand, which further binds suitable metal cations, yielding discrete complexes11 from three different components. In addition to the many organic ligands with different donor motifs that have been investigated, there is growing interest in heterobimetallic structures because the combination of two types of metal cations in one discrete aggregate can expand its electrochemical,12 photophysical,13 and magnetic properties.14 However, the synthesis of heterobimetallic structures is a challenging task. The metal cations need to be distinguished effectively in order to achieve the formation of discrete supramolecular structures. There are mainly three strategies to achieve this: (a) the use of a heterotopic ligand that generates two different coordination spheres (coordination © XXXX American Chemical Society

Special Issue: Self-Assembled Cages and Macrocycles Received: September 29, 2017

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DOI: 10.1021/acs.inorgchem.7b02516 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

assembly reactions of mononuclear iron(II) complexes.22 The reaction of 3 equiv of the aldehyde building block 1 with 1 equiv of 2 and iron(II) tetrafluoroborate hexahydrate yielded the deep-violet metalloligand [ML](BF4)2 in 86% yield (Scheme 3).

Scheme 1. Design of Heterobimetallic Trigonal-Bipyramidal (HM1) and Cubic (HM2) Complexes Based on a C3Symmetric Iron(II) Complex (ML)

Scheme 3. Synthesis of the Metalloligand [ML](BF4)2

configuration improves the predictability by excluding numerous possible side products,19 and the preorganization may speed up self-assembly. Furthermore, the high connectivity stabilizes the aggregate, with regard to both thermodynamic and stereochemical views.20 Being inherently chiral, metalloligands like ML can also lead to chiral aggregates and exhibit self-sorting behavior.21 While the ligand 1 also possesses a 4pyridyl binding site capable of also binding iron(II) ions, the formally heptadentate ligand is strongly favored. Hence, the remaining pyridyl moiety is capable of binding palladium(II) cations in the second step. The easy availability of TREN and easy three-step synthesis of 1 are also great advantages of this approach.

The heterobimetallic complexes were obtained by mixing the metalloligand [ML](BF4)2 with the regarding palladium(II) species in acetonitrile under an argon atmosphere. A total of 2 equiv of [ML](BF4)2 and 3 equiv of cis-protected [Pd(dppp)(OTf)2] yielded the trigonal bipyramid [HM1](BF4)4(OTf)6 as a blue solid in 83% isolated yield (see Scheme 4). On the other hand, the mixture of 4 equiv of [ML](BF4)2 and 3 equiv of tetravalent [Pd(MeCN)4](BF4)2 resulted in formation of the cubic complex [HM2](BF4)28 as a blue solid in 91% isolated yield (Scheme 4). Both complexes were isolated by the diffusion of diethyl ether vapor into an acetonitrile solution of the complex to yield deep-blue microcrystalline solids. Interestingly, a one-pot reaction using all nonpreorganized subcomponents to form the trigonal bipyramid was not successful, elucidating the crucial preorganization of ML (see the Supporting Information, SI). Structural Characterization. The metalloligand [ML](BF4)2 was obtained as a deep-violet solid by precipitation from an acetonitrile solution of the complex upon the addition of diethyl ether. Its identity and purity were confirmed by 1H, 13C, and DOSY NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS; see the SI), and UV−vis spectroscopy (vide infra). The slow diffusion of diethyl ether vapor into an acetonitrile solution yielded violet X-ray-quality single crystals (Figure 1). The metalloligand [ML](BF4)2 crystallizes in the monoclinic space group P21/c with four cationic units per unit cell. Six nitrogen atoms of the chelating tris(pyridylimine) binding site form an octahedral coordination sphere around the iron(II) cation, with the average Fe−N bond lengths of 1.97 Å being consistent with a diamagnetic iron(II) complex (see Table 1). Covalently bridged and preorganized binding sites dictate the fac coordination of the cation. The crystals occur in racemic form because both Λ- and Δ-configured complexes are present in the unit cell, as was already observed with analogous TRENbased mononuclear iron(II) complexes.22 The trigonal-bipyramidal complex [HM1](BF4)4(OTf)6 was obtained as a blue solid, whose identity and purity were proved by 1H, 13C, and DOSY NMR spectroscopy, ESI-MS (see the SI), and UV−vis spectroscopy (vide infra). All signals referring to [HM1]10+ were already observed in the 1H NMR spectrum 30 min after the addition of [(dppp)Pd(OTf)2] to a solution of the metalloligand [ML](BF4)2 in acetonitrile, revealing the fast formation of [HM1]10+ at room temperature. Heating the solution to 50 °C for 3 days

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RESULTS AND DISCUSSION Synthesis. The heterotopic ligand building block 1 can be easily obtained in a straightforward three-step synthesis (Scheme 2) starting from readily available 5-bromo-2formylpyridine (3). Scheme 2. Three-Step Synthesis of the Ligand Building Block 1

3 and ethylene glycol gave the acetal 4, which could be readily coupled with 4-pyridineboronic acid in a Suzuki− Miyaura cross-coupling reaction. Finally, the 3,4′-bipyridine 5 could be deprotected using hydrochloric acid to give the ligand building block 1 with an overall yield of 86% over three steps. The synthesis of ML turned out to be quite simple, which could already be shown in analogous subcomponent selfB


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Inorganic Chemistry Scheme 4. Synthesis of [HM1]10+ and [HM2]28+, Starting with [ML]2+

acidic palladium cations. The UV−vis spectrum of the heterobimetallic complex [HM1]10+ had to be measured with a comparatively high concentration above its critical selfassembly concentration (see the SI). This underlines the thermodynamic lability of this complex, whereas the fast formation of [HM1]10+, proven by 1H NMR spectroscopy, shows the kinetic lability of the bipyramid. From the 1H NMR spectrum, it can be deduced that the aggregate possesses both C3 symmetry along the iron(II)− iron(II) axis and C2 symmetry orthogonal to this axis because only one set of signals was detected (Figure 2). This indicates a narcissistic self-sorting of the two enantiomers of [ML]2+, yielding a racemic mixture of D3d-symmetric Δ,Δ- and Λ,Λconfigured complexes. Figure 1. Structure of [ML](BF4)2 as determined by XRD analysis shown with 50% probability ellipsoids. Color code: red, iron; blue, nitrogen; gray, carbon. Anions, hydrogen atoms, and solvate molecules are omitted for clarity.

Table 1. Comparison of Selected Bond Lengths (Å) and Angles (deg) in [ML](BF4)2 and [HM1](BF4)4(OTf)6 [ML](BF4)2

[HM1](BF4)4(OTf)6

Fe1−N1 Fe1−N3 Fe1−N5 Fe1−N7 Fe1−N8 Fe1−N10

1.987 1.967 1.972 1.954 1.976 1.948

Fe1−N1 Fe1−N3 Fe1−N5 Fe1−N7 Fe1−N8 Fe1−N10

1.958 1.955 1.983 1.955 1.971 1.950

N6e−Fe−N3e N6e−Fe−N6f N3e−Fe−N3d

81.42 94.94 95.25

N1−Fe−N3 N1−Fe−N5 N3−Fe−N10

81.47 94.07 97.75

Figure 2. Top: 1H NMR spectrum of [HM1](BF4)4(OTf)6 (700 MHz, acetonitrile-d3, 297 K). Middle: 1H NMR spectrum of [ML](BF4)2 (500 MHz, acetonitrile-d3, 297 K). Bottom: 1H NMR spectrum of [HM2](BF4)28 (700 MHz, acetonitrile-d3, 297 K).

did not cause any changes of the shifts or the integrals (see the SI). The high preorganization of the metalloligand, which already dictates the necessary symmetry to form the trigonal bipyramid, and the kinetically labile Pd−N bonds should be responsible for the rapid formation of this heterobimetallic complex. This is further corroborated by the fact that the formation of [HM1]10+ did not occur when all subcomponents were mixed in a single step. While the metalloligand [ML](BF4)2 has a deep-violet color, the heterobimetallic aggregate is dark blue, both in the solid state and in solution. The UV−vis spectra show a shift of the 1 A1 → 1T1 transition of iron(II) cations23 from 580 nm in [ML]2+ to 595 nm in [HM1]10+, which is responsible for the color change. This shift could be the result of the reduced electron density due to the additional coordination of Lewis

The slow diffusion of diethyl ether vapor into an acetonitrile solution of [HM1](BF4)4(OTf)6 yielded blue, good-quality single crystals. The crystal structure of the cationic heterobimetallic complex [HM1]10+ is shown in Figure 3. [HM1](BF4)4(OTf)6 crystallizes in the triclinic space group P1.̅ The structure is best described as a trigonal bipyramid, where the two iron(II) cations occupy the axial corners on the top and at the bottom and three palladium(II) cations occupy the corners in the equatorial plane of the aggregate. The unit cell contains two homochiral enantiomers, (Δ,Δ)-[HM1]10+and (Λ,Λ)[HM1]10+; hence, [HM1](BF4)4(OTf)6 might as well be described as a helicate, furthermore corroborating the selfsorting observed in solution.24 Iron(II) cations are coordinated C


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The progress of the self-assembly of this system was followed using 1H NMR spectroscopy by heating the complex solution and recording the corresponding NMR spectra at different points in time (Figure 4). Unlike with other cubic complex-

Figure 4. 1H NMR spectra of a mixture of [ML](BF4)2 and [Pd(CH3CN)4](BF4)2. Top: 30 min after mixing at room temperature (300 MHz, acetonitrile-d3, 297 K). Middle: after 16 h at 50 °C (400 MHz, acetonitrile-d3, 297 K). Bottom: after 5 days at 65 °C (700 MHz, acetonitrile-d3, 297 K). Figure 3. Structure of [HM1](X)10 (X− = OTf−, or BF4−) as determined by XRD analysis: (top) view along the palladium plane; (bottom) view along the Fe−Fe axis. Color code: red, iron; green-blue, palladium; blue, nitrogen; orange, phosphorus; gray, carbon. Anions, solvent molecules, and hydrogen atoms are omitted for clarity.

es,15b,25 a relatively long time is needed for self-assembly. The very broad signals in the NMR spectrum shown in Figure 4 indicate that, at first, only nondiscrete oligomers or a mixture of different stereoisomers are formed. Heating the solution overnight at 50 °C gave rise to first discrete signals but with the background signals caused by undefined aggregates still significant. Finally, after the complex solution was heated for 5 days at 65 °C, the solution reached an equilibrium state, with one prominent well-defined set of signals for the thermodynamically most stable aggregate in the 1H NMR spectrum indicating an O-symmetric aggregate and only trace signals for minor undefined species. The identity of the cubic complex [HM2](BF4)28 could also be shown by ESI-MS (Figure 5), DOSY NMR spectroscopy (Figure 6), and UV−vis spectroscopy (Figure 7). The diffusion constant was determined to be D = 5.11 × 10−10 m2 s−1. The hydrodynamic radius of the cubic aggregate was calculated using the Stokes−Einstein equation for spherical particles (see

by six nitrogen atoms that come from the tris(pyridylimine) binding site that are part of the octahedral coordination sphere. The average Fe−N bond length of 1.96 Å is again consistent with a diamagnetic iron(II) complex (Table 1). The palladium(II) cations exhibit square-planar coordination, coordinated by two 4-pyridyl moieties of [ML]2+ and one bidentate dppp ligand. While the cavity of the highly charged complex would be big enough to enclose a tetrafluoroborate anion, it was found to be filled with solvent molecules instead. Moreover, the C3 symmetry observed in solution on the NMR time scale is broken in the solid state, with one of the dppp ligands facing in the opposite direction relative to the other two. Nevertheless, the core of the aggregate, i.e., the two metalloligands and the palladium cations, is found to be in good agreement with the D3d symmetry (see Figure 3). A comparison of the X-ray crystal structures of the metalloligand [ML](BF4) and [HM1](BF4)4(OTf)6 shows that the metalloligand is highly preorganized to form the heterobimetallic trigonal bipyramid because the binding angles between iron(II) and coordinated nitrogen atoms do not change significantly between the two (see Table 1). This high preorganization of the building block could be one of the reasons for the very fast assembly of [HM1](BF4)4(OTf)6 as observed by NMR because the conformation of the metalloligand does not have to change significantly upon complexation with palladium. The heterobimetallic cubic complex [HM2](BF4)28 was also obtained as a dark-blue solid. The formation of this aggregate requires the self-assembly of six palladium cations and eight [ML]2+ metalloligands, yielding a very highly charged cationic complex with 14 metal centers.

Figure 5. ESI(+)-MS spectrum of the cubic complex [HM2](BF4)28 from an acetonitrile solution. The counteranion is tetrafluoroborate in every marked signal. D


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A comparison of the UV−vis spectra of the metalloligand [ML]2+, bipyramidal complex [HM1]10+, and cubic complex [HM2]28+ nicely shows the effect of the additional coordination of palladium(II) cations (see Figure 7). The significant maximum shift from 580 nm in [ML]2+ to 595 or 594 nm, respectively, in the heterobimetallic species [HM1]10+ and [HM2]28+ proves that the palladium−pyridyl bond is indeed formed in both cases. Unfortunately, despite our great efforts, we were not able to grow X-ray-quality single crystals of [HM2](BF4)28. Therefore, this complex could not be characterized by X-ray diffraction (XRD) experiments. Figure 8 shows an MMFF-minimized structure26 of [HM2]28+ instead, which is in accordance with the analytical data that we obtained by NMR and UV−vis spectroscopy and ESI-MS.

Figure 6. DOSY NMR spectrum of [HM2](BF4)28 (700 MHz, acetonitrile-d3, 297 K, τ = 175 ms).

Figure 8. MMFF-minimized structure of the cationic cubic aggregate (all-Δ)-[HM2]28+. Color code: red, iron; green-blue, palladium; blue, nitrogen; gray, carbon. Hydrogens are omitted for clarity.

The 1H NMR spectrum of the complex [HM2](BF4)28 indicates an O-symmetric aggregate in solution. In analogy to the crystal structure of [HM1]10+ and several analogous cubes,14d,15c,17c we computed the MMFF-minimized structure of [HM2]28+ as a homochiral aggregate, in which all iron centers have the same configuration. However, there should be two enantiomers: all eight iron centers with the Δ configuration and all eight iron centers with the Λ configuration. The crystal structure of [HM1]10+ showed that the metalloligand [ML]2+ is perfectly preorganized to form the bipyramidal aggregate. This suggests that the angles in [ML]2+ have to adjust to form the cubic complex because one would expect that the angles required to build up the cube are larger than those found in the bipyramidal complex. This could be the reason responsible for the obviously kinetically less favorable formation of [HM2]28+ and the significant amounts of oligomeric species found at first in NMR studies.

Figure 7. Top: Comparison of the UV−vis spectra of [ML]2+ and [HM1]10+ in acetonitrile. Bottom: Comparison of the UV−vis spectra of [ML]2+ and [HM2]28+ in acetonitrile.

the SI) because the rotating complex should nearly describe a sphere and resulted in a calculated diameter of d = 23.7 Å. This diameter is in very good agreement with the diagonal distance of the MMFF-minimized model of [HM2]28+, which was estimated to be 24.3 Å. As was already observed with the pyramidal complex [HM1]10+, the cubic aggregate [HM2]28+ is blue in solution as well as in the solid state, whereas the metalloligand [ML]2+ has a dark-violet color. Again, the 1A1 → 1T1 transition shifts from 580 nm in [ML]2+ to 594 nm in [HM2]28+. The UV−vis spectrum had to be measured with a relatively high concentration again because of the high critical self-assembly concentration of the complex.

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CONCLUSIONS In conclusion, we were able to synthesize two heterobimetallic complexes that differ greatly in their shape and size, using the same C3-symmetric mononuclear iron(II) metalloligand and varying palladium(II) moieties. We showed that the molecular library approach can be applied to heterobimetallic aggregates in a very effective way that allows the design and assembly of well-defined heterobimetallic metallosupramolecular complexes. In addition E


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Inorganic Chemistry to other established methods,17 we were able to introduce 4pyridyl as a typical binding motif from the classic monometallic molecular library approach.2,3 Hence, the structures of the resulting heterometallic cages are highly predictable because all design principles can easily be adopted. We described and characterized a small, yet highly charged trigonal-bipyramidal complex that contains two iron(II) and three palladium(II) centers. We also presented a heterobimetallic cube consisting of 14 metal cations with a 28-fold positive charge. Although we were not able to grow single crystals of this complex, all other analytical results clearly indicate its successful formation. The employed ligand system enables modifications of both the ligand system and metal centers, resulting in a broad structural diversity that paves the avenue for new discrete assemblies with interesting magnetic properties and host−guest chemistry.

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quantitative yield (121.56 mg, 0.66 mmol). 1H NMR (500 MHz, acetonitrile-d3, 297 K): δ 10.07 (d, 4J1,3 = 5J1,4 = 0.8 Hz, 1 H, H-1), 9.12 (dd, 4J4,6 = 2.2 Hz, 5J3,6 = 0.8 Hz, 1 H, H-6), 8.73 (dd, 3J8.9 = 4.5 Hz, J = 1.7 Hz, 2 H, H-9), 8.27 (ddd, 3J3,4 = 8.1 Hz, 4J4,6 = 2.2 Hz, 5J1,4 = 0.8 Hz, 1 H, H-4), 8.05 (dd, 3J3,4 = 8.1 Hz, 4J1,3 = 5J3,6 = 0.8 Hz, 1 H, H-3), 7.71 (dd, 3J8,9 = 4.5 Hz, J = 1.7 Hz, 2 H, H-8). 13C NMR (126 MHz, acetonitrile-d3, 297 K): δ 193.1 (C-1), 152.8 (C-2), 150.6 (C-9), 148.8 (C-6), 143.9 (C-7), 137.5 (C-5), 137.9 (C-4), 131.9 (C-8), 121.6 (C-3). Metalloligand [ML](BF4)2. A solution of (3,4′-bipyridine)-6carboxaldehyde (100.00 mg, 0.54 mmol, 3.00 equiv) and tris(2aminoethyl)amine (2, TREN; 27.09 μL, 0.18 mmol, 1.00 equiv) in 15 mL of acetonitrile was degassed by applying a vacuum and flushing with argon three times and stirred for 30 min at 65 °C under an argon atmosphere. After the reaction mixture was cooled to room temperature, iron(II) tetrafluoroborate hexahydrate (61.08 mg, 0.18 mmol, 1.00 equiv) was added. The deep-purple solution was degassed again and stirred at 65 °C for 20 h under an argon atmosphere. After cooling to room temperature, the solution was poured into 100 mL of diethyl ether. The violet solid was collected and carefully washed with diethyl ether several times. After drying in air, the product was obtained as a deep-violet solid in 86% yield (136.51 mg, 0.16 mmol). 1 H NMR (500 MHz, acetonitrile-d3, 297 K): δ 9.44 (s, 1 H, H-3), 8.60 (dd, 3J10,11 = 4.5 Hz, J = 1.7 Hz, 2 H, H-11), 8.50 (dd, 3J5,6 = 8.2 Hz, 4 J6,8 = 1.7 Hz, 1 H, H-6), 8.45 (d, 3J5,6 = 8.2 Hz, 1 H, H-5), 7.43 (m, 1 H, H-8), 7.33 (dd, 3J10,11 = 4.5 Hz, J = 1.7 Hz, 2 H, H-10), 3.87 (s, 1 H, H-2), 3.72 (d, 2J1,1′ = 12.5 Hz, 1 H, H-1), 3.26 (td, 2J2,2′ = 11.8 Hz, 3 J1,2= 3.7 Hz, 1 H, H-2, H-2), 3.16 (td, 2J1,1′ = 12.5 Hz, 3J3.7 = 3.7 Hz, 1 H, H-1, H-1). 13C NMR (128 MHz, acetonitrile-d3, 297 K): δ 172.3 (C-4), 157.7 (C-3), 154.2 (C-8), 151.7 (C-11), 143.4 (C-9), 139.6 (C7), 138.2 (C-6), 129.8 (C-5), 122.4 (C-10), 60.3 (C-2), 54.6 (C-1). ESI(+)-MS: m/z 787.3 ({[M] + BF4}+), 762.3 ({[M] + NO3}+), 745.3 ({[M] + HCO3}+), 719.3 ({[M] + F}+), 699.3 ({[M] − H}+), 350.1 ({[M]}2+). UV−vis [CH3CN, 100 μM; λ (nm)]: 294, 385, 538, 580. Heterobimetallic Trigonal Bipyramid [HM1](BF4)4(OTf)6. A solution of [ML](BF4)2 (5.00 mg, 5.72 μmol, 2.00 equiv) and [1,3bis(diphenylphosphino)propanyl]palladium(II) triflate ([Pd(dppp)(OTf)2]; 7.01 mg, 8.58 μmol, 3.00 equiv) in 0.7 mL of acetonitrile was degassed by applying a vacuum and flushing with argon three times and heated under an argon atmosphere at 65 °C for 65 h. The resulting solution was filtered, and the product was precipitated by the diffusion of diethyl ether vapor into an acetonitrile solution of the complex. The blue solid was filtered off and carefully washed with diethyl ether several times. After the solid was dried in a stream of air, the product was obtained as a blue solid in 83% yield (9.95 mg, 2.37 μmol). 1H NMR (700 MHz, acetonitrile-d3, 297 K): δ 9.33 (bs, 6 H, H-3), 8.66 (s, 12 H, H-11), 8.46 (d, 3J5,6 = 8.3 Hz, 6 H, H-5), 8.23 (d, 3 J5,6 = 8.3 Hz, 6 H, H-6), 7.60−7.44 (m, 36 H, Phdppp), 7.34 (m, 12 H, H-10), 7.33−7.26 (m, 24 H, Phdppp), 6.97 (bs, 6 H, H-8), 3.68 (bs, 6 H, H-2), 3.52 (m, 6 H, H-1), 3.13−3.05 (m, 30 H, H-1, H-1′, H-2, H2′, H-12, H-13). 13C NMR (176 MHz, acetonitrile-d3, 297 K): δ 173.0 (C-3), 158.6 (C-4), 153.2 (C-8), 151.6 (C-11), 145.7 (C-9), 139.3 (C6), 136.5 (C-7), 133.9 (Phdppp), 133.4 (C-10), 130.5 (Phdppp), 129.8 (C-5), 60.8 (C-2), 54.8 (C-1), 22.3 (propyldppp), 18.2 (propyldppp). ESI(+)-MS: m/z 2074.7 ({[M] + 8OTf}2+ + {[M]2 + 16OTf}4+), 2043.7 ({[M] + 7OTf + BF4}2+ + {[M]2 + 14OTf + 2BF4}4+), 2028.2 ({[M]2 + 13OTf + 3BF4}4+), 2013.2 ({[M] + 6OTf + 2BF4}2+ + {[M]2 + 12OTf + 4BF4}4+), 1997.2 ({[M]2 + 11OTf + 5BF4}4+), 1981.7 ({[M] + 5OTf + 3BF4}2+ + {[M]2 + 10OTf + 6BF4}4+), 1966.2 ({[M]2 + 9OTf + 7BF4}4+), 1950.3 ({[M] + 4OTf + 4BF4}2+ + {[M]2 + 8OTf + 8BF4}4+), 1936.3 ({[M]2 + 7OTf + 9BF4}4+), 1919.3 ({[M] + 3OTf + 5BF4}2+ + {[M]2 + 6OTf + 10BF4}4+), 1666.2 ){[M] − Pd(dppp) + 6OTf}2+), 1635.2 ({[M] − Pd(dppp) + 5OTf + BF4}2+), 1604.2 ({[M] − Pd(dppp) + 4OTf + 2BF4}2+), 1573.2 ({[M] − Pd(dppp) + 3OTf + 3BF4}2+), 1542.3 ({[M] − Pd(dppp) + 2OTf + 4BF4}2+), 1511.3 ({[M] − Pd(dppp) + OTf + 5BF4}2+), 1333.8 ({[M] + 7OTf}3+), 1313.1 ({[M] + 6OTf + BF4}3+), 1292.2 ({[M] + 5OTf + 2BF4}3+), 1271.5 ({[M] + 4OTf + 3BF4}3+), 1250.5 ({[M] + 3OTf + 4BF4}3+), 1061.1 ({[M] − Pd(dppp) + 5OTf}3+), 1040.5 ({[M] −

EXPERIMENTAL SECTION

General Procedures. All reagents and solvents were purchased from commercial sources and used as received without any further purification. NMR spectra were recorded on a Bruker Avance I 400, a Bruker Avance III 500, a Bruker Avance I 500, or a Bruker Avance III 700 spectrometer. Chemical shift values are reported relative to the residual solvent peak. 1H NMR data are reported as follows: chemical shift (δ) in ppm, multiplicity (s = singlet, bs = broad singlet, d = doublet, dd = doublet of doublets, ddd = doublet of doublet of doublets, td = triplet of doublets, and m = multiplet), coupling constant (J) in hertz (Hz), integral, correlation of the proton. Lowand high-resolution electrospray ionization mass spectrometry (ESIMS) spectra were recorded on a Bruker Daltonic LTQ Orbitrap XL or a Bruker Daltonic micro-TOF-Q. UV−vis spectra were recorded on a Specord 200 spectrometer (Analytik Jena AG). 5-Bromo-2-(1,3-dioxolan-2-yl)pyridine (4) was prepared by a slightly modified procedure reported in the literature (see the SI).27 4-(1,3-Dioxolan-2-yl)-3,4′-bipyridine (5). A suspension of 4 (50.00 mg, 0.22 mmol, 1.00 equiv), 4-pyridineboronic acid (32.50 mg, 0.24 mmol, 1.20 equiv), tetrakis(triphenylphosphine)palladium(0) (20.49 mg, 16.00 μmol, 0.08 equiv), and potassium carbonate (37.17 mg, 0.24 mmol, 1.20 equiv) in a solvent mixture of 5 mL of tetrahydrofuran and 0.5 mL of water was degassed three times and stirred under an argon atmosphere for 48 h at 75 °C. After the reaction mixture was cooled to room temperature, a 1:1 mixture of a saturated sodium chloride solution and a disodium ethylenediaminetetraacetic acid solution was added. The aqueous layer was extracted with ethyl acetate. The combined organic layers were dried with magnesium sulfate (MgSO4), and the solvent was removed under reduced pressure. Purification by column chromatography on silica using a cyclohexane/ethyl acetate/ triethylamine gradient (1:1:0.05 → 1:5:0.05, v/v) as the eluent yielded the desired compound as an off-white solid in 97% yield (47.95 mg, 0.21 mmol). 1H NMR (500 MHz, dichloromethane-d2, 297 K): δ 8.86 (d, 3J5,7 = 2.2 Hz, 1 H, H-7), 8.68 (d, 3J9,10 = 6.2 Hz, 2 H, H-10), 8.00 (dd, 3J4,5 = 8.2 Hz, 4J5,7 = 2.2 Hz, 1 H, H-5), 7.65 (d, 3J4,5 = 8.2 Hz, 1 H, H-4), 7.53 (d, 3J9,10 = 6.2 Hz, 2 H, H-9), 5.85 (s, 1 H, H-2), 4.13 (m, 4 H, H-1). 13C NMR (126 MHz, dichloromethane-d2, 297 K): δ 158.5 (C-3), 151.1 (C-10), 148.1 (C-7), 145.4 (C-8), 135.7 (C-5), 134.7 (C-6), 122.1 (C-9), 121.3 (C-4), 104.2 (C-2), 66.2 (C-1). ESI(+)-MS: m/z 229.1 ([M] + H+), 251.1 ([M] + Na+). (3,4′-Bipyridine)-4-carboxaldehyde (1).28 A solution of 5 (150.00 mg, 0.66 mmol, 1.00 equiv) in 20 mL of 4 N hydrochloric acid was stirred for 30 h at 70 °C. After the reaction mixture was cooled to room temperature, the pH value was adjusted to 9 by the addition of a saturated sodium carbonate solution. The reaction mixture was extracted with ethyl acetate, and the combined organic layers were dried over MgSO4. The solvent was removed under reduced pressure. The product was purified by column chromatography on silica using cyclohexane/ethyl acetate/triethylamine (1:2:0.05, v/v) as an eluent and dried in vacuo. The product was obtained as a white solid in F


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Å3, Z = 4, ρ = 1.517 g cm−3, μ = 3.900 mm−1, F(000) = 1792, 53904 reflections (2θmax = 135.486°) measured (6928 unique, Rint = 0.1146, completeness = 99.9%), final R indices [I > 2σ(I)] of R1= 0.0653 and wR2 = 0.1607, R indices (all data) of R1= 0.0850 and wR2 = 0.1738, GOF = 1.014 for 560 parameters and 20 restraints, and largest difference peak/hole 2.12/−0.60 e Å−3. [HM1](BF4)4(OTf)6: crystal dimensions 0.457 × 0.335 × 0.078 mm, [C159H150Fe2N20P6Pd3]2+·5.1(CF3SO3)−·4.9(BF4)−·8.75C2H3N, M = 4502.55, T = 123.0(1) K, triclinic, space group P1̅, a = 19.0094(3) Å, b = 19.2043(4) Å, c = 35.1158(6) Å, α = 74.7559(16)°, β = 87.3719(14)°, γ = 69.7033(16)°, V = 11585.8(4) Å3, Z = 2, ρ = 1.291 g cm−3, μ = 4.383 mm−1, F(000) = 4579, 131362 reflections (2θmax = 148.974°) measured (46024 unique, Rint = 0.0324, completeness = 99.8%), final R indices [I > 2σ(I)] of R1 = 0.0750 and wR2 = 0.2077, R indices (all data) of R1 = 0.0827 and wR2 = 0.2161, GOF = 1.031 for 3424 parameters and 13552 restraints, and largest difference peak/hole 2.154/−1.569 e Å−3.

Pd(dppp) + 4OTf + BF4}3+), 1019.8 ({[M] − Pd(dppp) + 3OTf + 2BF4}3+), 999.2 ({[M] − Pd(dppp) + 2OTf + 3BF4}3+), 978.2 ({[M] − Pd(dppp) + OTf + 4BF4}3+), 957.2 ({[M] − Pd(dppp) + 5BF4}3+). HRMS. Calcd for {M + 4OTf + 3BF4}3+: m/z 1271.1608. Found: m/z 1271.1657. UV−vis [CH3CN, 1425 μM; λ (nm)]: 298, 390, 546, 596. Heterobimetallic Cube [HM2](BF4)28. A solution of [ML](BF4)2 (5.00 mg, 5.72 μmol, 8.00 equiv) and [tetrakis(acetonitrile)]palladium(II) tetrafluoroborate ([Pd(CH3CN)4](BF4)2; 1.91 mg, 4.29 μmol, 6.00 equiv) in 0.7 mL of acetonitrile was degassed by applying a vacuum and flushing with argon three times and heated under an argon atmosphere at 65 °C for 5 days. The resulting solution was filtered, and the product was precipitated by the diffusion of diethyl ether vapor into an acetonitrile solution of the complex. The blue solid was filtered off and carefully washed with diethyl ether several times. After the solid was dried in a stream of air, the product was obtained as a blue solid in 91% yield (5.67 mg, 0.65 μmol). 1H NMR (700 MHz, acetonitrile-d3, 297 K): δ 9.11 (bs, 24 H, H-3), 8.41 (bs, 72 H, H-11, H-5), 8.23 (bs, 24 H, H-6), 7.73 (bs, 48 H, H-10), 7.46 (bs, 24 H, H-8), 3.49 (bs, 24 H, H-2), 3.36 (bs, 24 H, H-1), 3.02 (bs, 48 H, H-1, H-2). 13C NMR (176 MHz, acetonitrile-d3, 297 K): δ 173.5 (C-3), 159.0 (C-4), 153.6 (C-8), 152.9 (C-11), 148.4 (C-9), 138.0 (C-6), 130.2 (C-7), 127.3 (C-10), 114.7 (C-5), 59.9 (C-2), 55.3 (C-1). ESI(+)-MS: m/z 1647.9 ({[M] + 23BF4}5+), 1358.8 ({[M] + 22BF4}6+), 1152.2 ({[M] + 21BF4}7+), 997.6 ({[M] + 20BF4}8+), 877.1 ({[M] + 19BF4}9+), 780.6 ({[M] + 18BF4}10+), 701.7 ({[M] + 17BF4}11+), 636.1 ({[M] + 16BF4}12+), 580.4 ({[M] + 15BF4}13+). HRMS. Calcd for {[M] + 20BF4}8+: m/z 997.4443. Found: m/z 997.4356. UV−vis [CH3CN, 1152 μM; λ (nm)]: 295, 388, 594. Crystal Structure Determination. X-ray crystallographic analysis of [ML](BF4)2: Data were collected on a Bruker D8-Venture diffractometer, equipped with a low-temperature device (Kryoflex I, Bruker AXS GmbH, Karlsruhe, Germany) using graphite-monochromated Cu Kα radiation (λ = 1.54178 Å). A semiempirical (mulabs) absorption correction was carried out on the data set. The structure was solved by a dual-space method (SHELXT-2014) and refined by full-matrix least squares on F2 (SHELXL-2014).29 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms at carbon were placed in calculated positions and refined isotropically using a riding model. X-ray crystallographic analysis of [HM1](BF4)4(OTf)6: Data were collected on an Agilent SuperNova Dual diffractometer, equipped with a low-temperature device (Cryostream 700, Oxford Cryosystems, Oxford, U.K.) using mirror-monochromated Cu Kα radiation (λ = 1.54184 Å). CrysAlisPro30 software was used for data collection and reduction as well as the application of numerical absorption correction based on Gaussian integration. The structure was solved by a dualspace algorithm using SHELXT-2014/529 and refined by full-matrix least squares on F2 using SHELXL-2017/129 within the Olex231 and WinGX32 environments. All non-hydrogen atoms were refined anisotropically. All carbon-bound hydrogen atoms were calculated to their optimal positions and treated as riding atoms using isotropic displacement parameters 1.2 (or 1.5 in the case of methyl groups) times larger than the respective parent atoms. The appropriate geometry restraints were applied to the ligands, anions, and solvent molecules, making the 1,2 and 1,3 distances equal as well as ensuring the planarity of the aromatic rings. Rigid bond and similarity restraints were applied to the atomic displacement parameters of the dppp ligands, anions, and solvents. Anions (triflate and tetrafluoroborate) were found to exhibit both substitutional and positional disorder and were modeled accordingly, with their relative occupancies freely refined. Some of the solvent (acetonitrile) molecules were also found to be disordered and were modeled as such. However, not all of the solvent could be explicitly modeled, and the SQUEEZE routine33 of PLATON34 was used to calculate the contribution of electron density in these regions to the structure factors in the form of FAB files, which were used in subsequent refinement cycles. [ML](BF4)2: crystal dimensions 0.11 × 0.08 × 0.04 mm, C39H36FeN102+·2(BF4)−, M = 874.25, T = 100.0 K, monoclinic, space group P21/c, a = 13.9363(4) Å, b = 19.4995(6) Å, c = 14.1077(4) Å, α = 90°, β = 93.1426(17)°, γ = 90°, V = 3828.01(19)

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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.7b02516. Experimental details concerning 4, 1H and 13C NMR spectra of all organic compounds, 1H, 13C, and 2D NMR, ESI-MS, and UV−vis spectra of ML, HM1, and HM2 (if not already shown in this manuscript), and 1H NMR spectra of the one-pot reaction concerning HM1(PDF) Accession Codes

CCDC 1550913 and 1575238 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kari Rissanen: 0000-0002-7282-8419 Arne Lützen: 0000-0003-4429-0823 Present Address #

Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montréal, Québec H3A 0B8, Canada. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Deutsche Forschungsgemeinschaft (Grant SFB813; Chemistry at Spin Centers) and the Academy of Finland (Projects 263256, 298817, and 265328 to K.R.) is gratefully acknowledged. M.H. thanks the Manchot Foundation for a doctoral scholarship. N.S. thanks the Evonik Foundation for a doctoral scholarship. We thank Prof. Dr. Stefan Grimme and M.Sc. Fabian Bohle for trying some high-level computational structural optimizations of the heterobimetallic cubic aggregate.

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REFERENCES

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Stepwise Construction of Heterobimetallic Cages by an Extended

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