Design, Synthesis, and Structural Characterization of a New Class of

Dec 28, 2012 - The new ditopic organoiron ligand, [3,5-bis(1-ferrocenyl-prop-3-enol-1-one)(pyridine)] (H2L3,5), has been prepared and the reactions of...
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Design, Synthesis, and Structural Characterization of a New Class of Ferrocene-Containing Heterometallic Triple-Stranded Helicates Muthukrishna Raja,*,† Ratnasabapathy G. Iyer,† Chengeto Gwengo,† Daniel L. Reger,‡ Perry J. Pellechia,‡ Mark D. Smith,‡ and Andrea E. Pascui‡ †

Department of Chemistry, Claflin University, Orangeburg, South Carolina 29115, United States Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208, United States

‡

S Supporting Information *

ABSTRACT: The new ditopic organoiron ligand, [3,5-bis(1-ferrocenyl-prop-3-enol-1-one)(pyridine)] (H2L3,5), has been prepared and the reactions of its dianion (Na2L3,5) with M3+ ions (M = Ga or In) yield a new class of “3d-np block” heterometallic triple-stranded helicates, M2(L3,5)3, by the self-assembly process. The X-ray structural analysis of the new ligand shows that it is in the enolic form with each enolic carbon bonded to the pyridine ring and each carbonyl carbon connected to a ferrocene moiety; overall, the nonferrocenyl part of the molecule is nearly planar. The M2(L3,5)3 (M = Ga or In) complexes are helicates with three ligand strands, each of which is twisted into an S-shape, coordinating to two metal ions, each of which is in a distorted octahedral geometry. The new helicates are observed as a racemic mixture in the solid state by single-crystal X-ray analysis, and in solution by NMR, with both the left-handed Λ,Λ- and the right-handed Δ,Δ-isomers present. Variabletemperature 1H NMR study of the Ga2(L3,5)3 helicate indicates that the right-handed Δ,Δ-isomer and left-handed Λ,Λ-isomer equilibrate through a heterochiral Λ,Δ-intermediate by a concerted twist motion of one-half of the dinuclear complex through a trigonal prismatic transition state, according to the Bailar twist mechanism. Electrochemical properties of the ligand (H2L3,5) and the M2(L3,5)3 helicates were investigated through cyclic voltammetry, and the results indicate the lack of communication between the ferrocene units, because the separation between any two ferrocene units is greater than the 5−6 Å range in both the free ligand and the helicates.

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INTRODUCTION Elegant structural motifs ubiquitous in nature from the microscopic to the macroscopic level, man-made art, and architectures continue to inspire chemists to design and build similar synthetic molecules from smaller building blocks by the self-assembly processes.1 Over the past two decades, numerous multitopic ligands have been developed and their coordination chemistry with metal ions has been used to synthesize aesthetically attractive and functionally useful metal-containing polyhedra of various shapes ranging from simple grids to complex catenates and knots.2 The helical architecture has, in particular, fascinated chemists ever since the discovery of DNA’s double helix structure. Triple-stranded metallohelicates are considered simple models for more complex biopolymers, such as DNA and viruses.3 Building triple-stranded helicates with one type of metal center has led to a large number of reports in the literature.4 © 2012 American Chemical Society

Currently, no simple synthetic methodology is available for the synthesis of triple-stranded helicates with mixed valence metals and/or different metal centers. Challenges associated with the synthesis of heterometallic triple-stranded helicates have been met either by assembling them in a multistep process5 or by using segmented ligands with a bidentate site coordinated to the pseudo-octahedral d-block cation and a tridentate site coordinated to the pseudotricapped trigonal prismatic lanthanide.6 One possible solution to the challenge of synthesizing heterometallic triple-stranded helicates is to use metal-containing ligands as building blocks. Multidentate ligands built from organometallic moieties will be useful not only because they influence the shapes of the helicates but also because of their properties. Use of both metalloligands and Received: August 30, 2012 Published: December 28, 2012 95

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under a nitrogen atmosphere, and the reaction of 1.5 equiv of the resulting dianion with M(CF3SO3)3 yields the polynuclear heterometallic complexes M2(L3,5)3 (M = Ga, 1, 3 or In, 2, 4) in good yield (Scheme 2). The electrospray mass spectra show the expected isotopic pattern for the [M2(L3,5)3 + H]+ peaks. Elemental analyses and NMR spectra also match this formulation. Solid-State Structures. Crystals of H2L3,5 were formed by layering hexane over a cold CH2Cl2 solution. Figure 1a shows the two independent molecules in the unit cell. Even though the β-diketonate fragment is known to be in equilibrium between keto and enol forms, all analytical data clearly demonstrate that it is in one of the enolic forms. Each enolic [C(OH)] carbon in the ligand is bonded to the pyridine ring, and each carbonyl (CO) carbon is connected to a ferrocene moiety. The observed bond length for the C−O single bond of each enolic moiety is 1.31 Å, and the observed bond length for each CO double bond of the carbonyl moiety is 1.27 Å. These values are in agreement with reported values for similar bonds.10 Additional support for the enolic structure is offered by both 1H NMR and 13C NMR. The enolic hydrogen atoms resonate at 16.53 ppm as a broad peak, and the ene carbon atoms (−CH) are observed at 94.7 ppm. The central, nonferrocenyl part of the molecule is nearly planar. The planar acetylacetone (acac) portions are twisted only slightly away from the pyridine ring plane; measured torsion angles are 3.5, −6.2, 2.8 and 5.2°, respectively (Table 1). Given this rigid geometry and the short link between the two acac groups, the deprotonated ligand is designed to bind two separate metal centers, the arrangement needed to produce M2L3 helicates, and given the charge, should form neutral molecules with metals in the +3 oxidation state. Single crystals of M2(L3,5)3 suitable for X-ray diffraction analysis were grown by layering Et2O over a CHCl3 solution. Xray diffraction analysis of the gallium(III) and indium(III) complexes prepared from the NaH preparations reveal that they are nearly isostructural solids (different solvent of crystallization) in the rhombohedral space group R3̅ and composed of neutral M2(L3,5)3 molecules. Two different views of the structure of In2(L3,5)3 are shown in Figure 1. The molecule is a helicate composed of three ligand strands coordinating to two metal ions, each of which is in a distorted octahedral geometry. This geometry at the metal centers is confirmed by the large twist angles, the angles between the two

organometallic ligands in the design and synthesis of helicates is still in its preliminary stage.7 Recently, we have communicated the synthesis and structural characterization of a new organometallic ligand, 1-ferrocenyl-3(5-bromopyridyl)-prop-3-enol-1-one (H2L), and a new bimetallic 2-D coordination polymer, [Cu(L)2]n, derived from it.8 Results from this study, and the reports detailing the efficacy of polydiketonates9 in crystal engineering, prompted us to explore the syntheses and reactions of a new bimetallic ligand, in which two ferrocene moieties are connected to a pyridine ring through two-ketoenol linkers. Herein, we describe the synthesis of a new ditopic organometallic ligand, 3,5-bis(1-ferrocenylprop-3-enol-1-one)(pyridine) (H2L3,5), and its reactions with gallium(III) and indium(III) to form new polynuclear/ heterobimetallic triple-stranded helicates.

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RESULTS AND DISCUSSION Synthesis. The protonated form of a new ditopic organoiron ligand, [3,5-bis(1-ferrocenyl-prop-3-enol-1-one)(pyridine)] (H2L3,5), was synthesized as shown in Scheme 1 Scheme 1. Synthesis of Ligand H2L3,5

from diethyl-3,5-pyridinedicarboxylate and acetylferrocene in good yield (82%). The composition and structure of H2L3,5 was fully characterized by NMR, MS, elemental analysis, and singlecrystal X-ray diffraction. Both enolic protons in H2L3,5 are easily deprotonated in THF using NaH or DBU (see the Supporting Information) Scheme 2. Synthesis of M2(L3,5)3 [M = Ga or In]

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Figure 1. (a) Structure of two independent moleucles of H2L3,5. (b) Side view of In2(L3,5)3. (c) View of In2(L3,5)3 down the In···In axis. (d) S-shape of the ligand upon coordination to In. Hydrogen atoms are removed for clarity of the figures.

Table 1. Important Metrics for the Ligand and Metal Complexes twist angle (deg)

a

a

H2L3,5

Ga2(L3,5)3

In2(L3,5)3

Ga2(L3,5)3·[DBU-H][CF3SO3]

In2(L3,5)3·[DBU-H][CF3SO3]

NA

45.32 44.33 39.51 38.58 40.26 36.21 7.53 16.96

45.71 42.30 35.81 40.81 39.49 30.68 7.67 21.89

52.43 52.43 35.05 42.18 42.18 26.50 7.42 NA

53.46 53.46 32.89 42.84 42.84 22.99 7.49 NA

average torsion angle (deg)b

4.8

intramolecular M···M distance (Å) cavity diameter (Å)

NA NA

Twist angle is defined as the angle between the Cmethine···M···M planes. bTorsion angle is defined as the O−C−Cpy−Cpy angle.

Δ,Δ configurations or a diastereomeric meso form with a Δ,Λ configuration. Both complexes crystallize as racemates, with both the left-handed Λ,Λ- and the right-handed Δ,Δ-isomers present in the unit cell. Figure 2 shows the two forms. Despite the short connection between the two acac portions of the ligands and the near planarity of the protonated ligand, the helicity of the ligand occurs by ligand distortion upon coordination. Upon deprotonation and coordination, the ligands distort into an S-shape (Figure 1d). In this configuration, the small torsion angle that the acac groups make with the pyridyl ring in the protonated ligand increases dramatically (Table 1) in the metal complexes. In all cases, one

Cmethine···M···M planes (Table 1),11 angles emphasized by the view down the M···M axis (Figure 1c). This arrangement results in a small central intramolecular void with a maximum diameter, as measured by the M···M distance, of approximately 7.5 Å (for Ga) and 7.6 Å (for In). These values are similar to those reported in the literature for neutral M2L3 types of helicates with analogous organic ligands.12 The structures of the M2(L3,5)3 molecules from the DBU preparations, which cocrystallize with one [DBU-H][CF3SO3] per helicate, are also similar (Table 1). Three forms of these molecules containing two trischelate metal centers can exist, two enantiomeric forms with Λ,Λ or 97

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currently exploring the plausible use of these cavities in “host− guest” chemistry. Variable-Temperature NMR. Compared to the free ligand, both 1H NMR and 13C NMR spectra show slight shifts in proton and carbon resonances in the Ga2(L3,5)3 and In2(L3,5)3 complexes with the largest shifts noted for the 4pyridyl carbon and hydrogen atoms and the enolic carbon. However, there were notable differences in the 1H NMR and 13 C NMR spectra of the resonances of the substituted cyclopentadienyl ring of the metal complexes. In the case of Ga2(L3,5)3 at 20 °C in deuterated tetrachloroethane (TCE-d2) solution, one of the two proton singlet resonances of the cyclopentadienyl ring linked to the acac moiety in the free ligand splits into two distinct resonances. This spliting (5.03 and 4.97 ppm) is a result of two nonequivalent resonances corresponding to diastereotopic hydrogens of the substituted cyclopentadienyl ring in the ligand, likely in the 2,2′-position, thus confirming the chirality of the molecule in solution. In toluene, these resonances are split by a larger amount (0.42 ppm) and the 3,3′-position resonance is also slightly split (0.04 ppm). The observation of diastereotopic hydrogen atoms has been reported previously in somewhat similar gallium helicates by Raymond and co-workers. They also explained the observation as evidence for the existence of two enantiomeric isomers and showed by variable-temperature NMR the interconversion of two enantiomeric isomers.13 Variable-temperature 1H NMR studies in TCE-d2 and toluene were carried out to investigate the possible interconversion of the two enantiomeric forms of the dinuclear gallium(III) and indium(III) metallohelicates reported here. Figure 4 shows the results for compound 1 in TCE-d2. Although most of the resonances show the normal small changes in chemical shifts with temperature, the 2,2′diastereotopic ferrocene resonances broaden above 40 °C, pass through a coalescence point around 65 °C, and finally form a relatively sharp singlet at 5.04 ppm at higher temperatures. Similar results were obtained in toluene in this temperature range. Simulation of the exchanging resonances using DNMR program as implemented in SpinWorks 3.1.8.214 resulted in the determination of the corresponding rate constants. The temperature dependence of the chemical shifts was estimated from the data collected between −20 and 40 °C, and a correction for this change was introduced into the simulation of the spectra used in the variable-temperature analysis. The

Figure 2. Λ,Λ- (left, red) and Δ,Δ (right, blue)-isomers looking down the In···In axes. The hydrogen atoms are removed for clarity of the figure, and the ligand ferrocene molecules are represented by the large balls.

of the three ligands is less S-shaped, presumably to accommodate the bulky ferrocene side arms. Interestingly, it is this S-shaped distortion that allows the octahedral coordination at the metals; if these torsion angles remained as small as that in H2L3,5, the metals would be in a nearly trigonal prismatic arrangement. In the overall packing of the dinuclear helicates from the NaH preparations, large “holes” form filled with solvent (Figure 3a). Six molecules with alternating chirality form a circular arrangement with a center hole approximately 20 Å across. These rings are organized in stacked layers with the resulting “columns” close packing with six adjacent columns. It is interesting to compare this arrangement with that of (M2(L3,5)3·[DBU-H][CF3SO3]) that was prepared using the organic base DBU to deprotonate the ligand. From this preparation, the compound crystallizes in a different space group, although the helicate molecules are present in essentially the same racemic configuration. In this form of the molecule, the dinuclear molecules close pack (Figure 3b) with the one [DBU-H][CF3SO3] present per dinuclear molecule, demonstrating the substantial impact of having the [DBU-H][CF3SO3] present during the crystallization procedure. One significant difference between the new ferrocenecontaining M2L3 helicates reported here and similar neutral M2L3 helicates with pure organic ligands is the overall packing of the dinuclear helicates in a unit cell. Unlike their organic analogues, the ferrocene-containing helicates pack with large cavities (diameter ∼ 20 Å) filled with solvent molecules. We are

Figure 3. Overall packing of two forms of In2(L3,5)3: (a) In2(L3,5)3 and (b) In2(L3,5)3·[DBU-H][CF3SO3]. Right-handed (Δ,Δ)-enantiomers are colored shades of blue, left-handed (Λ,Λ) are colored shades of red, DBU-H+ is colored green, and CF3SO3− is colored yellow. Interstitial solvent and the hydrogen atoms are removed for clarity of the figure. 98

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cene (Fc*) internal standard and each of the ligand or helicates dissolved in freshly distilled CH2Cl2 using 0.1 M Bu4NPF6 as the supporting electrolyte. A summary of the electrochemical data is presented in Table 2. The ligand and helicates exhibit a Table 2. Electrochemical Data for the Ligand and Helicates compound

E1/2 (V)

ΔEp (V)

ipa/ipc

n

ferrocene (Fc) decamethylferrocene (Fc*) H2L3,5 Ga2(L3,5)3 In2(L3,5)3 Ga2(L3,5)3·[DBU-H][CF3SO3] In2(L3,5)3 ·[DBU-H][CF3SO3]

0.374 −0.173 0.623 0.553 0.538 0.554 0.547

0.073 0.076 0.118 0.246 0.272 0.156 0.170

0.94 0.95 0.57 0.33 0.35 0.25 0.29

2.30 5.96 5.89 5.94 6.12

single quasi-reversible oxidation wave, corresponding to the Fc/ Fc+ redox couple, as shown in Figure 5 for H2L3,5, and

Figure 4. Variable-temperature 1H NMR (400 MHz) spectra of 1 in TCE-d2.

activation parameters derived from the Eyring plot at 25 °C are ΔG⧧ = 16.5(9) kcal mol−1, ΔH⧧ = 12(3) kcal mol−1, and ΔS⧧ = −13(9) cal mol−1 K−1. Previously, it was shown that similar dinuclear homohelical gallium(III) trischelates of the form K6Ga2L3, where L is a biscatecholamide or bisterephthalamide derivative, isomerize by a concerted twist motion along the 3-fold axis of one-half of the dinuclear complex through a trigonal prismatic transition state according to the Bailar twist mechanism.13 The inversion of the Λ,Λ- and Δ,Δ-helicates involves the heterochiral Λ,Δintermediate, which is produced by a single twist event at one metal. The energy of activation for the inversion in this system was reported as ranging from ΔG⧧ = 17.4−19.1 kcal mol−1 at 25 °C. The activation parameters for 1 are similar to the ones measured for K6Ga2L3, indicating that the system reported here also undergoes a similar intramolecular inversion, where the Λ,Λ- and Δ,Δ-homochiral helicates interconvert through a heterochiral Λ,Δ-intermediate. The negative entropy value and the fact that these parameters are not solventdependent support this intramolecular inversion mechanism. Spectra recorded below 20 °C show line broadening that we attribute to increases in the viscosity of the solvent; the impact of solvent viscosity is larger than one normally observes, presumably because of the unusually large size of the molecule. The effect was particularly large in toluene, resulting in broadening of the resonances that are clearly unrelated to the Δ,Δ−Λ,Λ inversion. In toluene at −75 °C, the lowest temperature measured, the data indicate a possible second dynamic process, where one of the broad resonances starts to split in two, but further analyses of these resonances was not possible as toluene freezes at −95 °C. The indium compound 2 undergoes a similar inversion in toluene. The 2,2′-ferrocene hydrogen atoms are at the fast exchange limit at room temperature and broaden as the temperature is lowered. The limiting low-temperature spectrum would be below the freezing point of toluene, but from the observation of a coalescence point at about −45 °C, the ΔG⧧ can be estimated to be about 10 kcal/mol at this temperature. This value is clearly much smaller than that observed with the gallium complex 1. This difference is explained by the increase in the size of the metal ion, which facilitates the Bailar twist mechanism. Electrochemistry. Cyclic voltammograms were collected at a scan rate of 50 mV/s for a 1:1 mixture of decamethylferro-

Figure 5. Cyclic voltammograms of 1 mM solutions (Fc/Fc+) of H2L3,5 (red), Ga2(L3,5)3·[DBU-H][CF3SO3] (pink), and Ga2(L3,5)3 (purple) in CH2Cl2 using 0.1 M Bu4NPF6 as supporting electrolyte and 1 mM decamethylferrocene (Fc*/Fc*+) as the internal reference. Scan rate: 50 mV/s.

Ga2(L3,5)3 helicates with or without DBU.15 A comparison of redox peak current of the internal standard Fc* and H2L3,5 or helicates gives a reasonable estimate for the number of electrons (n) being transferred during the redox process, as shown in Table 2. The oxidation of ferrocene groups occurs at more positive potentials compared to ferrocene due to the negative inductive effects of the acetylacetonate moiety. In comparison with H2L3,5, the half-wave potentials of helicates are shifted to less positive potentials by at least 70 mV. The presence of the neutral [DBU-H][CF3SO3] salt in 3 and 4 does not significantly influence oxidation potentials of the helicates, a similar feature noted with NMR, mass spectrometry, and elemental analysis data. The single-wave redox signatures observed in these multiferrocene systems are consistent with the absence of electronic interaction between ferrocene units, a phenomenon commonly dictated by a separation distance between ferrocene units, which should ideally be in the 5−6 Å range.16 Single-crystal Xray diffraction results indicate the Fe···Fe distance to be greater than this range in the free ligand (14.48 Å), the coordinated ligand (13.50 Å for 1 and 13.89 Å for 2), and the Fe atoms on the same side of the helicate (8.12 Å for 1 and 8.58 Å for 2). Because of the large intramolecular Fe···Fe distances, ferrocene units in both the ligand and the helicates undergo concurrent 99

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redox change and the Fc/Fc+ couple appears at the same potential. The lack of electronic communication has been reported for similar meta-substituted arene derivatives functionalized with ferrocene on the periphery17 and other multiferrocene compounds.18 The quasi-reversibility in these systems is usually associated with large cathodic−anodic peak separation (ΔEp) values and is common in systems where conformational reorganization is required to minimize electrostatic repulsions during redox processes.19,16a As expected in our multiferrocene systems, the exceptionally large peak separation (ΔEp ≥ 118 mV) is characteristic of the amount of intramolecular reorientation required for di- or hexaferrocenium cations upon oxidation.

mV/s. CH2Cl2 was freshly distilled prior to use. All measurements were conducted in a Faraday cage connected to a GAMRY Framework station. Syntheses. 3,5-Diethyl-pyridinedicarboxylate. This compound was prepared as previously reported by heating 3,5-pyridinedicarboxylic acid (5.00 g, 29.9 mmol), and H2SO4 (1.5 mL) in ethanol (70 mL) at reflux for 12 h. Yield: 4.79 g, 72%. mp 47−48 °C. 1H NMR (CDCl3): δH 9.35 (d, J = 2 Hz, 2H), 8.85 (t, J = 2 Hz, 1H), 4.44 (q, J = 7.2 Hz, 4H), 1.42 (t, J = 7.2 Hz, 6H). 13C NMR (CDCl3): δC 164.6, 154.2, 138.0, 126.3, 61.9, 14.4. 3,3′-(3,5-Pyridyl)bis(1-ferrocenyl-prop-3-enol-1-one) (H2L3,5). A solution of acetylferrocene (3.07 g, 13.4 mmol) dissolved in THF (20 mL) was added dropwise to a suspension of KH (0.898 g, 22.4 mmol) in THF (20 mL). After stirring at room temperature for 1 h, a solution of the 3,5-diethyl-pyridinedicarboxylate (1.00 g, 4.48 mmol) in THF (20 mL) was added, and the mixture was stirred at room temperature for 16 h. Excess KH was neutralized by adding 0.1 M HCl (50 mL), and the product was extracted with three 50 mL portions of ethyl acetate, washed with brine solution and water, dried over MgSO4, and concentrated. The crude product was washed three times with boiling hexane to remove unreacted acetylferrocene. The final purple powder obtained was redissolved in a minimum amount of CH2Cl2 and crystallized as purple needles by layering hexane. Yield: 2.15 g, 82%. mp 184−185 °C. 1H NMR (CDCl3): δH 16.53 (s, 2H), 9.23 (s, 4H), 8.67 (s, 2H), 6.44 (s, 2H), 4.93 (d, J = 2 Hz, 4H), 4.63 (d, J = 2 Hz, 4H), 4.24 (s, 10H). 13C NMR (CDCl3): δC 195.0, 176.5, 150.3, 132.2, 131.2, 94.7, 78.4, 73.0, 70.6, 69.1. Anal. Calcd (Found) for {[C31H25O4NFe2]4·CH2Cl2}, 61.69 (62.31); H, 4.22 (4.10); N, 2.30 (2.28). EI(+)-MS Calcd (Found) m/z: 587.0483 (587.0494). X-ray quality crystals were grown by layering hexane over a cold CH2Cl2 solution of H2L3,5. The two phases were allowed to slowly diffuse at 4 °C over 4 days. General Procedure for the Synthesis of M2(L3,5)3 Complexes. A solution of H2L3,5 (3 equiv) dissolved in THF (20 mL) was added to a suspension of NaH (7 equiv) in THF (10 mL), and the mixture was stirred at room temperature for 30 min. A solution of M(CF3SO3)3 (2 equiv) dissolved in THF (20 mL) was added, and the mixture was stirred at room temperature for 16 h. The reaction mixture was filtered over a pad of Celite, concentrated, washed with three 50 mL portions of ether, and dried under vacuum to afford a deep red powder. X-ray quality crystals were grown by layering Et2O over a saturated CHCl3 solution of each M2(L3,5)3 complex to yield Ga2(L3,5)3·(CHCl3)1.44(C4H10O)3.29, and In2(L3,5)3·(CHCl3)1.44(C4H10O)3.29. Ga2(L3,5)3, 1. Ga2(L3,5)3 was synthesized according to the general procedure from H2L3,5 (0.400 g, 0.681 mmol), NaH (0.064 g, 1.59 mmol), and Ga(CF3SO3)3 (0.235 g, 0.454 mmol). Yield: 0.328 g, 76%. mp > 260 °C. 1H NMR (CDCl3): δH 8.85 (br, 3H), 8.80 (br, 6H), 6.17 (s, 6H), 5.10 (br, 6H), 4.90 (br, 6H), 4.54 (br, 12H), 4.21 (s, 30H), 3.47 (q, 4H, −CH2 of ether), 1.20 (t, 6H, CH3 of ether). 13C NMR (CDCl3): δC 193.3, 181.9, 147.9, 137.6, 136.5, 96.5, 79.8, 72.6, 70.7, 70.1, 69.4, 66.0 (−CH2 of ether), 15.4 (CH3 of ether). Anal. Calcd (Found) for C97H79O13N3Fe6Ga2: C, 59.16 (58.96); H, 4.04 (3.46); N, 2.13 (2.21). ES(+)-TOF MS Calcd for [Ga2(L3,5)3 + H]+ (Found) m/z: 1896 (1896). In2(L3,5)3, 2. In2(L3,5)3 was synthesized according to the general procedure from H2L3,5 (0.200 g, 0.341 mmol), NaH (0.027 g, 1.14 mmol), and In(CF3SO3)3 (0.128 g, 0.227 mmol). Yield: 0.216 g, 95%. mp > 260 °C. 1H NMR (CDCl3): δH 8.94 (br, 3H), 8.85 (br, 6H), 6.22 (s, 6H), 5.00 (br, 12H), 4.54 (br, 12H), 4.21 (s, 30H). 13C NMR (CDCl3): δC 195.9, 183.3, 148.2, 137.7, 136.8, 96.9, 80.7, 72.5, 70.8, 69.7. Anal. Calcd (Found) for C94H70O12N3Cl3Fe6In2: C, 53.64 (54.20); H, 3.35 (3.21); N, 2.00 (2.05). ES(+)-TOF MS Calcd for [In2(L3,5)3] + H]+. (Found) m/z: 1986 (1986). General Procedure for the Synthesis of [M2(L3,5)3]·[DBUH][CF3SO3] Helicates. DBU (6 equiv) was added to a degassed solution of H2L3,5 (3 equiv) dissolved in THF (20 mL), and the resultant mixture was stirred at room temperature for 30 min. A solution of M(CF3SO3)3 (2 equiv) dissolved in THF (20 mL) was added, and the mixture was stirred at room temperature for 16 h. The

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CONCLUSION We have synthesized a new organometallic, ditopic ligand designed for the formation of triple-stranded helicates and report here the successful preparation of the first polynuclear heterometallic triple-stranded helicates that contain both d- and p-block metals, using a ferrocene-based ketoenolate dianion as the basic ligand building block and gallium(III) and indium(III) as the p-block metal. The new helicates form a racemic mixture in both the solid state and solution of the left-handed Λ,Λ- and the right-handed Δ,Δ-isomers. Variable-temperature NMR methods have established that the two enantiomers equilibrate in solution through a series of single metal Bailar twist rearrangements. Electrochemical data are consistent with the absence of electronic interaction between the ferrocene units in the ligand and both helicates. We note that the methods reported here can be generally applied to synthesize polynuclear heterometallic helicates using a variety of organometallic ligands as building blocks. We are actively exploring the reactions of our new ligands with other transition, lanthanide, and main-group metal ions. The electrochemistry of helicates with other electroactive metal ions is being investigated.

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EXPERIMENTAL SECTION

General Considerations. All preparations were carried out under a nitrogen atmosphere using standard Schlenk techniques. Acetylferrocene, ferrocene, decamethylferrocene, tetra-n-butylammonium hexafluorophosphate (Bu4NPF6), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), In(CF3SO3)3, KH, NaH, Ga(CF3SO3)3, and 3,5-pyridinedicarboxylic acid were purchased from Alfa Aesar and were used without further purification. Solvents were dried using conventional methods and were freshly distilled prior to use. 1H and 13C NMR spectra were recorded on a JEOL 300 MHz spectrometer, and chemical shifts were referenced to solvent resonances at δH 7.25, δC 77.12 for CDCl3. Variable-temperature 1H spectra were recorded on a Varian Mercury/ VX 400 spectrometer. All chemical shifts are in parts per million and were referenced to residual undeuterated solvent signals (1H). Robertson Microlit Laboratories performed all elemental analyses. Melting points were measured on an MSRS MPA160 DigiMelt apparatus and are uncorrected. The high-resolution mass spectrum of the ligand was measured using a VG70SQ mass spectrometer in the EI mode and mass spectra of the complexes on a MicroMass QTOF spectrometer, at the University of South Carolina Mass Spectrometry Center. Electrochemical activity studies were performed using a GAMRY PCI 600 potentiostat. Cyclic voltammograms were recorded for CH2Cl2 solutions containing 1.0 mM analyte, 1.0 mM decamethyferrocene (Fc*) internal standard, and 0.1 M Bu4NPF6 supporting electrolyte using a three-electrode cell consisting of a platinum working electrode, a glassy carbon counter electrode, and a Ag/AgCl reference electrode. Half-wave potentials (E1/2) were calculated from the average of the cathodic and anodic potentials at a scan rate of 50 100

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C114H104Cl3F6 Fe6Ga2N7O18S2 2619.05 monoclinic C2/c 100(2) 19.332(7) 17.846(7) 30.979(11) 90 94.366(11) 90 10656(7) 4 0.0581 0.1194 C107.61H103.36Cl4.33 Fe6In2N3O15.29 2401.67 trigonal R3̅ 100(2) 67.440(2) 67.440(2) 12.8588(9) 90 90 120 50648(4) 18 0.0701 0.1704 C101.36H87.69Cl6.39 Fe6Ga2N3O13.56 2265.76 trigonal R3̅ 100(2) 68.226(5) 68.226(5) 12.7521(18) 90 90 120 51406(9) 18 0.0627 0.1578 C31.5H26Cl Fe2NO4 629.69 tetragonal P42 100(2) 29.741(2) 29.741(2) 5.8917(5) 90 90 90 5211.4(7) 8 0.0562 0.1312 formula fw, g·mol−1 cryst syst space group T, K a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z R1 (I >2σ(I)) wR2 (I >2σ(I))

1·(CHCl3)2.13(C4H10O)1.56 H2L3,5·(CH2Cl2)0.5

Table 3. Selected Crystal Data and Structure Refinement

2·(CHCl3)1.44(C4H10O)3.29

3·CHCl3

4·CHCl3

reaction mixture was filtered over a pad of Celite, and the filtrate was concentrated to afford a sticky oily residue. The residue was washed with three 50 mL portions of ether, and dried under vacuum to afford a deep red sticky solid. The sticky solid was recrystallized over 2 days in the freezer by layering Et2O over a saturated CH2Cl2 solution. X-ray quality crystals were grown by layering Et2O over a saturated CHCl3 solution of each [M2(L3,5)3]·[DBU-H][CF3SO3] helicate to yield [Ga2(L3,5)3]·[DBU-H][CF3SO3]·CHCl3 and [In2(L3,5)3]·[DBU-H][CF3SO3]·CHCl3. [Ga2(L3,5)3]·[DBU-H][CF3SO3], 3. [Ga2(L3,5)3]·[DBU-H][CF3SO3] was synthesized according to the general procedure described above from H2L3,5 (0.200 g, 0.341 mmol), DBU (0.1 mL, 0.682 mmol), and Ga(CF3SO3)3 (0.118 g, 0.227 mmol). Yield: 0.172 g, 68%. mp > 260 °C. 1H NMR (CDCl3): δH 9.76 (br, 1H), 8.84 (br, 3H), 8.80 (br, 6H), 6.20 (s, 6H), 5.10 (br, 6H), 4.90 (br, 6H), 4.53 (br, 12H), 4.21 (s, 30H), 3.48 (m, 4H), 2.80 (br, 2H), 2.07 (br, 2H), 1.79−1.64 (m, 10H), 1.20 (t, 3H). 13C NMR (CDCl3): δC 193.3, 181.9, 166.6, 147.9, 137.6, 136.5, 96.5, 79.9, 72.5, 70.7, 70.1, 69.3, 66.0 (−CH2 of ether), 54.8, 48.9, 38.4,, 33.0, 31.1, 29.1, 26.8, 23.9, 19.5, 15.4 (CH3 of ether). Anal. Calcd (Found) for C104H87O15N5SCl3F3Fe6Ga2: C, 53.91 (52.38); H, 3.78 (4.05); N, 3.02 (3.76). ES(+)-TOF MS Calcd for [Ga2(L3,5)3] + H]+ (Found) m/z: 1896 (1896). [In2(L3,5)3]·[DBU-H][CF3SO3], 4. [In2(L3,5)3]·[DBU-H][CF3SO3] was synthesized according to the above general procedure from H2L3,5 (0.200 g, 0.341 mmol), DBU (0.1 mL, 0.682 mmol), and In(CF3SO3)3 (0.128 g, 0.227 mmol). Yield: 0.205 g, 82%. mp > 260 °C. 1H NMR (CDCl3): δH 9.48 (br, 1H), 8.94 (br, 3H), 8.85 (br, 6H), 6.22 (s, 6H), 5.00 (br, 12H), 4.54 (br, 12H), 4.21 (s, 30H), 3.48 (m, 4H), 2.78 (br, 2H), 2.06 (br, 2H), 1.78−1.72 (m, 10H). 13C NMR (CDCl3): δC 195.8, 183.3, 166.7, 148.2, 137.7, 136.7, 96.8, 80.7, 72.5, 70.8, 69.7, 61.4, 54.8, 48.9, 38.4, 33.0, 29.1, 26.8, 23.9, 19.5. Anal. Calcd (Found) for C104H87O15N5SCl3F3Fe6In2: C, 51.90 (51.41); H, 3.64 (3.66); N, 2.91 (3.50). ES(+)-TOF MS Calcd for [In2(L3,5)3] + H]+. (Found) m/z: 1986 (1986). Crystallographic Studies. X-ray diffraction intensity data for the ligand, H2L3,5·(CH2Cl2)0.5, was measured on a Bruker Apex II DUO diffractometer using Cu Kα radiation (λ = 1.54 Å) at 100 K on a red crystal with approximate dimensions of 0.030 mm × 0.041 mm × 0.191 mm. Details of the data collection are given in Table 3. A total of 1985 frames were collected. The frames were integrated with the Bruker SAINT+ software package using a narrow-frame algorithm. Because of the thin needle morphology and weak diffracting power of the available crystals, no reflections were observed above a 2θmax value of ca. 44° (Cu radiation). Data were truncated at this value. Data were corrected for absorption effects using the multiscan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.747. The data were integrated using a tetragonal unit cell. The structure was solved and refined using the Bruker SHELXTL20 Software Package, using the space group P42, with Z = 8. PLATON indicated that the space group was correct with no additional symmetry. The asymmetric unit consists of two crystallographically independent, but chemically similar, molecules of H2L3,5, and a region of disordered dichloromethane. The dichloromethane species are disordered around a 2-fold axis of rotation and were refined with two independent positions with a total site occupancy of 0.5. Disordered atoms were refined with a common isotropic displacement parameter, with the aid of C−Cl and Cl−Cl distance restraints. All atoms of the H2L3,5 molecules were refined anisotropically. Some anisotropic displacement parameters were restrained to approximate a spherical electron density distribution using SHELX ISOR instructions. This was necessary to prevent unacceptably oblate and prolate displacement parameters and is a direct result of the weak diffraction data. Reasonable positions for all four enolic protons were located in difference maps. These were refined freely, resulting in normal atomic positions and parameters. The C−O bond distances are intermediate between enolic/carbonyl forms and are consistent with similar bonds. All other hydrogen atoms were placed in geometrically idealized positions and refined as riding atoms. At convergence, the absolute structure (Flack) parameter was 0.038(15). The largest peak in the

C114H104Cl3F6 Fe6In2N7O18S2 2709.25 monoclinic C2/c 100(2) 19.2023(11) 17.9718(9) 31.0005(15) 90 94.224(4) 90 10669.2(10) 4 0.0664 0.1492

Article

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final difference electron density synthesis was 0.45 e−/Å3, located near Cl(3) of the disordered dichloromethane molecules. The Ga and In complexes were measured on a Bruker SMART APEX CCD-based diffractometer (Mo Kα radiation, λ = 0.71073 Å). Raw area detector data frame processing was performed with the SAINT+ and SADABS programs. Final unit cell parameters were determined by least-squares refinement of large sets of strong reflections taken from each data set. Direct methods structure solution, difference Fourier calculations, and full-matrix least-squares refinement against F 2 were performed with SHELXS/L as implemented in OLEX2. Non-hydrogen atoms were refined with anisotropic displacement parameters, the exception being disordered species. The hydrogen atoms were placed in geometrically idealized positions and included as riding atoms. Details of the data collection are given in Table 3. The compounds Ga2(L3,5)3·(CHCl3)2.13(1)(C4H10O)1.56(1) and In2(L3,5)3·(CHCl3)1.44(1)(C4H10O)3.29(1) crystallize in the trigonal system. The pattern of systematic absences in the intensity data indicated an R-centered lattice, but were inconsistent with a c-glide symmetry element, leaving the space groups R3, R3,̅ R32, R3m, and R3̅m. The correct space group was eventually determined to be the centrosymmetric group R3̅ (No. 148). The two compounds are essentially isostructural, but with a different disordered solvent content and distribution. The asymmetric unit consists of one complete M2(C31H23NO4Fe2)3 (M = Ga, In) complex and several independent regions of disordered solvent species. For the Ga compound, three solvent volumes near the gallium complex could be satisfactorily modeled; another region was too heavily disordered for identification. The identifiable disorder areas were modeled as chloroform molecules, diethylether molecules, or a mixture of both on the same site. In general, the disordered atoms in a particular volume were refined with a common isotropic displacement parameter, and with distance restraints to maintain chemically reasonable geometries. In total, 67 restraints were used to assist in modeling the disorder. There is a fully ordered chloroform molecule (Cl1−Cl3) occupying the cavity between pyridyl rings with nitrogen atoms N2 and N3. The cavity between pyridyl ring nitrogen atoms N1 and N2 was modeled as containing two partially occupied ether molecules (O102 and O103, populations 67.6% and 32.4%, respectively) along with two partially occupied chloroform molecules (Cl7−Cl9 and Cl10−Cl12, populations 32.4% and 36.3%, respectively). Between pyridyl rings N1 and N3, the disorder was modeled as a mixture of chloroform (Cl4−Cl6) and ether (O101) with site occupancies of 0.444(4)/0.556(4), respectively, which were constrained to sum to unity. Another region of non-negligible electron density occupies channels along the c axis at (0,0,z) and symmetryequivalent positions. No good disorder model could be found for the atoms in this region. They were accounted for using the Squeeze technique in PLATON. Squeeze calculated a solvent-accessible void volume of 9122.1 Å3 (17.7% of the total unit cell volume), amounting to 4345 electrons per unit cell. The contribution of the atoms in this region was removed from the structure factor calculations. The reported F.W., d(calc), and F(000) were calculated from refined species only. The largest residual electron density peak of 0.99 e−/Å3 in the final difference map is located in the modeled disordered region, 1.02 Å from C114. For the In compound, five solvent volumes near the indium complex could be satisfactorily modeled; another region was too heavily disordered for identification. The known disorder areas were modeled as chloroform molecules, diethylether molecules, or a mixture of both on the same site. In general, the disordered atoms in a particular volume were refined with a common isotropic displacement parameter, and with distance restraints to maintain chemically reasonable geometries. In total, 62 restraints were used to assist in modeling the disorder. Near pyridyl nitrogen atom N2, a chloroform molecule was modeled as occupying three distinct orientations with a total site occupancy summing to 0.79(1). The cavity between pyridyl nitrogen atoms N1 and N3 is filled by a fully occupied ether molecule (O101). Another fully occupied ether (O102) is located between pyridyl nitrogen N2 and ferrocene group Fe4; this ether displayed

large displacement parameters, indicating positional uncertainty and was refined without hydrogen atoms. Between pyridyl nitrogen atoms N2 and N3, the species were modeled as a mixture of chloroform and ether with site occupancies of 0.783(4)/0.217(4), respectively, which were constrained to sum to unity. The cavity between N1 and N2 was also modeled as a chloroform/ether mixture with respective occupancies of 0.438(6)/0.51(1); no hydrogen atoms were included for these atoms. Another region of non-negligible electron density occupies channels along the c axis at (0,0,z) and symmetry-equivalent positions. No good disorder model could be found for the atoms in this region. They were accounted for using the Squeeze technique in PLATON. Squeeze calculated a solvent-accessible void volume of 5883.2 Å3 (11.6% of the total unit cell volume), amounting to 2403 electrons per unit cell. The contribution of the atoms in this region was removed from the structure factor calculations. The reported F.W., d(calc), and F(000) were calculated from refined species only. The largest residual electron density peak of 1.20 e−/Å3 in the final difference map is located 0.98 Å from In2. The compound [Ga2(L3,5)3]·[DBU-H][CF3SO3]·CHCl3 crystallizes in the monoclinic system. The space group C2/c was confirmed by the pattern of systematic absences in the intensity data and by the successful solution and refinement of the structure. The asymmetric unit consists of half of one Ga2(L3,5)3 complex, which resides on a crystallographic 2-fold axis of rotation oriented perpendicular to the Ga−Ga vector, one DBUH cation, one triflate anion, and a disordered chloroform molecule. The ligand of the gallium complex that contains N2 is bisected by the C2 axis, and therefore, only half of this ligand is present per asymmetric unit. Atom N2 and C17 are located on the 2fold. The C2 axis generates one “N2” ligand per complete complex. Atoms of the second independent ligand (containing N1) are on general positions, generating two ligands per symmetry-expanded gallium complex. The chloroform occupies two symmetry-independent sites within the asymmetric unit. Both sites are further disordered about a C2 axis of rotation, and therefore, only half of one CHCl3 is present per asymmetric unit. The independent sites have occupancy values of C61/C62 = 0.313(4)/0.187(4), which were constrained to sum to 0.5. The C2 axis generates four CHCl3 sites per gallium complex. Twelve C−Cl and Cl−Cl distance restraints were used to model the chloroform disorder, and atoms of the two independent sites were assigned common isotropic displacement parameters. The compound [In2(L3,5)3]·[DBU-H][CF3SO3]·CHCl3 crystallizes in the monoclinic system. The space group C2/c was confirmed by the pattern of systematic absences in the intensity data and by the successful solution and refinement of the structure. The asymmetric unit consists of half of one In2(L3,5)3 complex, which resides on a crystallographic 2-fold axis of rotation oriented perpendicular to the In−In vector, one DBUH and triflate cation−anion pair, and a disordered chloroform molecule. The ligand of the gallium complex that contains N2 is bisected by the C2 axis, and therefore, only half of this ligand is present per asymmetric unit. The C2 axis generates one “N2” ligand per complete complex. The other ligand (containing N1) is on a general position, generating two ligands per symmetryexpanded gallium complex. The chloroform occupies two symmetryindependent sites in the asymmetric unit. Both sites are further disordered about a C2 axis of rotation, and therefore, only half of one CHCl3 is present per asymmetric unit. The independent sites have occupancy values of C61/C62 = 0.248(8)/0.252(8), which were constrained to sum to 0.5. The C2 axis of rotation generates four CHCl3 sites per gallium complex. Twelve C−Cl and Cl−Cl distance restraints were used to model the chloroform disorder, and the two independent sites were each assigned a common isotropic displacement parameter.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format for the structural determinations and mass spectrometry data, NMR data, and cyclic voltammograms for the In2(L3,5)3 helicates. This material is available free of charge via the Internet at http://pubs.acs.org. 102

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(c) Clegg, J. K.; Jolliffe, K. A.; Lindoy, L. F.; Meehan, G. V. Pol. J. Chem. 2008, 82, 1131. (10) (a) Dudek, M.; Clegg, J. K.; Glasson, C. R. K.; Kelly, N.; Gloe, K.; Gloe, K.; Kelling, A.; Buschmann, H.-J.; Jolliffe, K. A.; Lindoy, L. F.; Meehan, G. V. Cryst. Growth Des. 2011, 11, 1697. (b) Burrows, A. D.; Mahon, M. F.; Renouf, C. L.; Richardson, C.; Warren, A. J.; Warren, J. E. Dalton Trans. 2012, 41, 4153. (11) Grillo, V. A.; Seddon, E. J.; Grant, C. M.; Aromi, G.; Bollinger, J. C.; Folting, K.; Christou, G. Chem. Commun. 1997, 1561. (12) (a) Saalfrank, R. W.; Seitz, V.; Heinemann, F. W.; Gobel, C.; Herbst-Irmer, R. J. Chem. Soc., Dalton Trans. 2001, 599. (b) Saalfrank, R. W.; Seitz, V.; Caulder, D. L.; Raymond, K. N.; Teichert, M.; Stalke, D. E. J. Inorg. Chem. 1998, 1313. (c) Saalfrank, R. W.; Dresel, A.; seitz, V.; Trummer, S.; Hampel, F.; Teichert, M.; Stalke, D.; Stadler, C.; Daub, J.; Schunemann, V.; Trautwein, A. X. Chem.Eur. J. 1997, 3, 2058. (13) (a) Meyer, M.; Kersting, B.; Powers, R. E.; Raymond, K. N. Inorg. Chem. 1997, 36, 5179. (b) Kersting, B.; Meyer, M.; Powers, R. E.; Raymond, K. N. J. Am. Chem. Soc. 1996, 118, 7221. (c) Kersting, B.; Telford, J. R.; Meyer, M.; Raymond, K. N. J. Am. Chem. Soc. 1996, 118, 5712. (14) Marat, K. SPINWORKS 3, v.3.1.8.2, University of Manitoba, 2011. This freeware can be downloaded from ftp://davinci.chem. umanitoba.ca/pub/marat/SpinWorks/. (15) Cyclic voltammograms for the In2(L3,5)3 helicates with or without DBU is provided in the Supporting Information. (16) Solntsev, P. V.; Dudkin, S. V.; Sabin, J. R.; Nemykin, K. N. Organometallics 2011, 30, 3037. (17) (a) Diallo, A. K.; Absalon, C.; Ruiz, J.; Astruc, D. J. Am. Chem. Soc. 2011, 133, 629. (b) Funk, H.; Long, N. J.; Martin, A. J.; Opromolla, G.; White, A. J. P.; Williams, D. J.; Zanello, P. Organometallics 1997, 16, 2646. (18) (a) Dulatas, L. T.; Brown, S. N.; Ojomo, E.; Noll, B. C.; Cavo, M. J.; Holt, P. B.; Wopper, M. M. Inorg. Chem. 2009, 48, 10789. (b) Chen, L.-J.; Li, Q.-J.; He, J.; Tan, H.; Abliz, Z.; Yang, H.-B. J. Org. Chem. 2012, 17, 1148. (19) Auger, A.; Swarts, J. C. Organometallics 2007, 26, 102. (20) SHELXTL; Bruker Analytical X-ray Systems: Madison, WI, 2003.

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge with thanks the financial support of the Army Research Office Grant number 59402-59042-CHREP.

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