Reversible Formation of a Planar Chiral Ferrocenylboroxine and Its

Article pubs.acs.org/Organometallics

Reversible Formation of a Planar Chiral Ferrocenylboroxine and Its Supramolecular Structure Pakkirisamy Thilagar,†,‡ Jiawei Chen,† Roger A. Lalancette,† and Frieder Jak̈ le*,† †

Department of Chemistry, Rutgers University-Newark, 73 Warren Street, Newark, New Jersey 07102, United States Inorganic and Physical Chemistry Department, Indian Institute of Science, Bangalore, 560 012, India

‡

S Supporting Information *

ABSTRACT: The boronic acid (pS)-1,2-NpFcB(OH)2 (1) was obtained by treatment of the lithiated species (pS)-1,2NpFcLi with B(OiPr)3, followed by acidic workup; subsequent dehydration gave the enantiomerically pure boroxine [(pS)1,2-NpFcBO]3 (2) in 49% isolated yield. Multinuclear and 2D NMR spectroscopies, single-crystal X-ray diffraction, and elemental analysis served to confirm the structure of 2. In the solid-state structure, all three of the naphthyl groups point in one direction and all of the ferrocenyl moieties are placed on the opposite face of the boroxine ring, which is also the preferred conformation in solution according to a 1H, 1H-NOESY experiment. Cyclic voltammetry revealed three separate reversible oxidation events, which suggests significant communication between the ferrocenyl moieties. These redox processes experience a cathodic shift upon addition of 4-dimethylaminopyridine (DMAP) as a Lewis base. The six-membered ring is opened upon treatment with hot CHCl 3/MeOH to form the methoxy species (pS)-1,2-NpFcB(OH)(OMe) (3), which can be converted back to the cycle 2 by dissolution in wet CHCl3, followed by column chromatography on silica gel.

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the electron-withdrawing boryl group. Upon oxidation, the interaction between the (formerly electron-rich) Fe and the electron-deficient B, which is characteristic of this class of molecules,6 is interrupted, and hence the Lewis acidity of the boron center is expected to increase.7 Boronic acids, on the other hand, have been widely used as reagents in organic transformations, such as Suzuki−Miyaura coupling reactions, as catalysts, for example, for amide and ester bond formation, 8 as well as in the recognition and immobilization of saccharides and other polyols.9 They are also known to establish a reversible equilibrium with their respective anhydrous trimersthe boroxines.10−12 The latter are of interest in small molecule recognition13,14 and have been used as flame retardants15 and as additives16 in lithium-ion batteries. The tripodal architecture of boroxines is also advantageous in the reversible assembly of conjugated materials with interesting nonlinear optical and emissive properties and of 3D networks.11,17,18 For instance, in 2005, the Yaghi group reported that crystalline covalent organic framework materials can be prepared based on simple arylboroxine building blocks.19 Ferrocenyl boroxine has first been reported by Kotz20 in 1970 and later crystallographically characterized by Wagner 21,22 (B, Chart 1). In the solid state, all three ferrocene moieties are positioned on one side of the boroxine ring,23,24 suggesting the possibility of strong electronic coupling between the metal centers.25 However, the redox chemistry of B has not been

INTRODUCTION While planar chiral metallocenes play important roles in asymmetric synthesis, most of the compounds investigated thus far incorporate Lewis-basic phosphine and amine ligands. 1 They have been extensively studied as ligands to transitionmetal complexes. Chiral systems based on metallocene Lewis acids are still limited to relatively few examples.2,3 Prior efforts in our group have focused on developing planar chiral Lewis acids based on the ferrocenylborane framework for asymmetric synthesis, keeping in mind that the Lewis acidity of the boron center could be tuned by taking advantage of the reversible redox behavior of the ferrocene unit.3−5 Most recently, we reported the synthesis of the highly Lewis acidic, chiral ferrocenylborane derivatives (pS)-1,2-NpFcBCl(C 6F5) and (pS)-1,2-NpFcB(C6F5)2 (A, Chart 1; Np = naphthyl). 5 Chart 1

According to cyclic voltammetry studies, the triarylborane derivative undergoes reversible oxidation at a significantly more

Received: October 8, 2011 Published: November 30, 2011

positive potential than ferrocene, as a result of attachment of © 2011 American Chemical Society

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dx.doi.org/10.1021/om200947v | Organometallics 2011, 30, 6734−6741

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Scheme 1. Synthesis of Compounds 2 and 3

explored. Moreover, the planar chirality of substituted ferrocenylboroxine derivatives has not been utilized extensively.26 In this context, we report here the reversible formation of a chiral boroxine derived from the enantiomerically pure boronic acid precursor (pS)-1,2-NpFcB(OH)2. Single-crystal Xray diffraction shows that the resulting boroxine forms a supramolecular helix with large chiral cavities that are occupied by solvent molecules. Voltammetric experiments were also carried out to examine the redox behavior of the ferrocene moieties in the absence and presence of an external Lewis base.

with two oxygen-containing substituents attached to a tricoordinate boron. The cyclic species 2 is easily distinguished from the respective boronic acid (1) based on the absence of an OH signal and the unusual upfield shift of one of the Cp protons in 2 to 2.77 ppm. The trimeric structure of 2 was further confirmed by high-resolution MALDI mass spectrometry, which showed the molecular ion peak as the major signal, and single-crystal X-ray diffraction (vide infra). In wet CDCl3, the boroxine 2 readily converts back to the boronic acid 1, as is evident from the appearance of a second set of signals at 4.70, 4.64, and 4.60 ppm for the substituted and at 4.33 ppm for the free Cp ring and a singlet at 4.05 ppm corresponding to the hydroxyl protons. In an alternative workup procedure, the crude product after column chromatography was taken up in hot CHCl3/MeOH (3:1) and the resulting solution was allowed to slowly evaporate, leaving behind an orange crystalline solid. To our surprise, a different product formed, which was identified by multinuclear NMR spectroscopy, single-crystal X-ray diffraction, and elemental analysis to be (pS)-1,2-NpFcB(OH)(OMe) (3). Alternatively, compound 3 is also accessible by heating the cycle 2 in a mixture of CHCl3 and MeOH.30 In contrast to the 1 H NMR pattern of 2, all resonances for the substituted Cp ring of 3 fall into a narrow range from 4.54 to 4.67 ppm and two additional singlets are observed at 3.84 and 3.61 ppm for the hydroxy and methoxy group, respectively. In the MALDI-MS, only a small signal was detected for compound 3. The most intense signals can be assigned to the formation, under the high-vacuum conditions used for MS analysis, of boroxine 2 and three different types of dimeric species (NpFcB(OMe)-O(OMe)BNpFc, NpFcB(OH)-O-(OMe)BNpFc, and NpFcB(OH)-O-(OH)BNpFc), which correspond to condensation with formal loss of H2O, MeOH, and MeOMe (see the Supporting Information). Compound 2 crystallizes together with two chloroform solvent molecules in the chiral space group P32 with a highly symmetric hexagonal crystal lattice (see Table 1). The boroxine molecules adopt a double propeller-like structure by placing the three ferrocenyl and the three naphthyl substituents on opposite sides of the central boroxine ring (B3O3), which forms a nearly perfectly planar hexagon (Figure 1). The largest deviations from an ideal plane are observed for B3 and O1 with

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RESULTS AND DISCUSSION In earlier work, we demonstrated that Kagan’s ferrocenyl p-tolyl sulfinate27 serves as a powerful precursor to ferrocene-based planar chiral and/or bidentate Lewis acids. Two complementary methods allow for stereoselective functionalization of ferrocene with main group Lewis acids: (1) lithiation of ferrocenyl p-tolyl sulfinate with LDA, followed by treatment with R3SnCl (R = Me, n-Bu), leads to stereoselective formation of (pR)-1-stannyl-2-sulfinylferrocenes; (2) the sulfinyl group itself can be replaced by treatment with tert-butyl lithium at −78 °C and subsequent quenching with organometallic halides, such as R3SnCl.28 In both cases, the stannyl groups can then be further elaborated via highly selective transmetalation reactions to yield ferrocene-based organomercury, organocopper, or organoborane Lewis acids. To incorporate a boronic acid group into the planar chiral 1,2-disubstituted ferrocene framework, we chose a more expeditious approach in which we reacted (pR,SS)-2-(1-naphthyl)-1-(p-tolylsulfinyl)ferrocene27 with tertbutyl lithium, followed by direct quenching with the boric ester B(OiPr)3 (Scheme 1). Hydrolysis under acidic conditions led to the boronic acid (pS)-1,2-NpFcB(OH)2 (1), which formsas typically observed for boronic acidsan equilibrium with the boroxine species 2 in organic solvents.29 The crude product was subjected to chromatography on silica gel using a mixture of CHCl3/MeOH as the eluent. The resulting solution was concentrated and then layered with THF to give 2 in the form of orange crystals. The 1H NMR spectrum in dry CDCl3 shows the typical pattern of a 1,2-disubstituted ferrocene derivative with signals at 4.45, 4.31, and 2.77 ppm for the substituted Cp ring and a singlet at 3.98 ppm for the free Cp ring. A single resonance at 31.3 ppm in the 11B NMR spectrum is consistent 6735


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Table 1. Crystal Data and Structure Refinement Details for (pS)-1,2-[NpFcBO]3 (2) and (pS)-1,2-NpFcB(OH)(OMe) (3) 2 empirical formula MW T, K wavelength, Å cryst syst, space group a, Å b, Å c, Å a, deg β, deg γ, deg V, Å3 Z, ρcalc, g cm−3 μ, mm−1 (Cu Kα) F(000) cryst size, mm θ range, deg limiting indices

reflns collected/unique completeness absorption correction max. and min transmission refinement method data/restraints/ parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] a R indices (all data) a absolute structure parameter peak/hole (eÅ−3)

3

C62H47B3Cl6Fe3O3 1252.68 100(2) 1.54178 hexagonal, P32 17.3350(2) 17.3350(2) 15.6486(3) 90 90 120 4072.43(14) 3, 1.532 9.435 1914 0.28 × 0.19 × 0.13 2.94−67.68 −20 ≤ h ≤ 20 −20 ≤ k ≤ 20 −18 ≤ l ≤ 17 25609/8988 [R(int) = 0.067] 97.8% to θ = 67.08° numerical 0.374 and 0.178

C21H19BFeO2 370.02 100(2) 1.54178 monoclinic, P21 7.9679(1) 9.8347(2) 10.6624(2) 90 90.858(1) 90 835.43(3) 2, 1.471 7.314 384 0.24 × 0.10 × 0.08 4.15−67.37 −9 ≤ h ≤ 9 −11 ≤ k ≤ 9 −12 ≤ l ≤ 12 6931/2253 [R(int) = 0.045] 95.6% to θ = 67.37° numerical 0.603 and 0.273

full-matrix least-squares on F2 8988/1/695

full-matrix least-squares on F2 2253/1/229

1.00 R1 = 0.053, wR2 = 0.118

1.08 R1 = 0.021, wR2 = 0.058

R1 = 0.075, wR2 = 0.128 0.000(5)

R1 = 0.022, wR2 = 0.057 0.006(3)

0.94/−0.60

0.27/−0.19

Figure 1. (a) Molecular structure of 2. Selected bond lengths [Å] and angles [°]: B1−O2 1.386(7), B1−O3 1.382(7), B2−O1 1.389(6), B2−O3 1.385(7), B3−O1 1.384(6), B3−O2 1.383(6), B1−C1 1.528(7), B2−C21 1.524(7), B3−C41 1.532(7), Cp(Fe1)//B 3O3 11.7, Cp(Fe2)//B3O3 14.4, Cp(Fe3)//B3O3 6.3, Cp//Cp(Fe1) 2.4, Cp//Cp(Fe2) 2.6, Cp//Cp(Fe3) 2.0, B2−O1−B3 121.0(4), B1− O2−B3 120.8(4), B1−O3−B2 121.7(4), O1−B3−O2 119.1(5), O1− B2−O3 118.3(4), O2−B1−O3 118.7(5), O2−B1−C1 121.8(5), O3− B1−C1 119.5(5), O1−B2−C21 120.1(5), O3−B2−C21 121.7(5), O1−B3−C41 120.6(5), O2−B3−C41 120.2(5). (b) Molecular structure of 3. Selected bond lengths [Å] and angles [°]: B1−C1 1.562(3), B1−O1 1.354(3), B1−O2 1.371(3), C21−O1 1.436(3), Cp//Np 16.7, Cp//Cp 2.7, C1−B1−O1 117.7(2), C1−B1−O2 123.3(2), O1−B1−O2 119.0(2).

a

R1 = ∑∥F o | − |F c ∥/∑|F o |; wR2 = (∑[w(F o 2 − F c 2 ) 2 ]/ ∑[w(Fo2)2])1/2.

angle for the boroxine 2 (6.1°), but still significantly smaller than for strongly Lewis acidic ferrocenylboranes, such as (pS)1,2-NpFcB(C6F5)2 (β = 15.5°).5 The boronic acid unit is positioned almost coplanar to the attached Cp ring (Cp// B1O1O2 = 9.4°). Interestingly, H-bonding between OH groups of neighboring molecules is not observed, which stands in sharp contrast to the observation of multiple H-bonding motifs that result in a polymer of dimers in FcB(OH)2.32 Instead, we observe an intramolecular O−H···π interaction between the OH group and the naphthyl unit (O−H···centroid of substituted benzene ring, 3.041 Å). The OH and OMe moieties also show weak C−H···O contacts to neighboring molecules (OCH3···OH 2.659 Å; NpH···OCH3 2.690, 2.792 Å). The extended structure of boroxine 2 in the solid state is displayed in Figure 2. An interesting supramolecular structure results from several cumulative intermolecular interactions between the aromatic rings of the naphthyl substituents and the cyclopentadienyl rings of the ferrocenes. Most notably, extended chainlike structures are formed as a result of C−

0.036 and 0.035 Å, respectively. The B−O bond distances lie in the range from 1.382(6) to 1.389(6) Å, which is comparable to those of previously reported boroxines.21,23,24,31 The substituted Cp rings adopt a conformation in which they are almost coplanar with the B3O3 ring (Cp//B3O3 11.7° (Fe1), 14.4° (Fe2), 6.3° (Fe3)). For each of the individual ferrocene moieties, the Cp rings are essentially coplanar with Cp//Cp angles ranging from 2.0 to 2.6°. The naphthyl groups are oriented nearly orthogonal to the substituted cyclopentadienyl rings of the ferrocenes (Np//Cp 87.0, 85.4, 80.4°) and the central boroxine ring (Np//B3O3 81.4, 85.5, 82.0°). Importantly, short intramolecular C−H···π contacts are evident between the Cp protons next to the boron substituents and the substituted phenyl rings of the naphthyl moieties (2.78, 2.87, 2.91 Å). In compound 3, the bond lengths B1−O1 = 1.354(3) Å and B1−O2 = 1.371(3) Å are significantly shorter than the B−O distances within the boroxine ring. The tilt angle of boron toward the iron center defined as β = 180° − angle(CpCENT− Ci−B) is about 7.8°, which is slightly larger than the average 6736


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Figure 2. Different views of the supramolecular structure of 2. Left: view along the crystallographic c axis (CHCl3 molecules are omitted for clarity). Right: illustration of the pocket containing two CHCl3 molecules.

H···π interactions between one of the C−H protons of each naphthalene moieties and the unsubstituted Cp ring of one of the ferrocenes in adjacent molecules, as illustrated in Figure 2 (Fc1: 2.782 Å; Fc2: 2.723 Å; Fc3: 2.882 Å). The naphthalene moiety acts as a hydrogen-bond donor and the ferrocenyl Cp ring acts as a hydrogen-bond acceptor. The propagation of these interactions along the crystallographic c axis generates an infinite triple-helical structure with chiral pockets. The planar aromatic rings of the naphthalene and cyclopentadienyl rings of ferrocene render these chiral pockets hydrophobic, thus allowing two chloroform molecules to be trapped in each pocket. The guest molecules are noninnocent and further augment the supramolecular interactions. Thus, one of the chloroform molecules shows short C−Cl···C contacts to two naphthyl moieties (3.294, 3.335 Å) and to two boron centers (3.381, 3.526 Å), while the other chloroform molecule shows two C−Cl···H contacts (2.936, 2.949 Å) to the substituted Cp rings of two ferrocene moieties. In addition, there is a short C− Cl···H contact of 2.599 Å between the two chloroform molecules themselves. As discussed above, in the solid state, all of the naphthalene moieties of 2 point into the same direction and the Cp-H5 protons (ortho to boron) are situated adjacent to the neighboring Np units. In solution, on the other hand, the unusual upfield shift for the Cp-H5 protons to 2.77 ppm (cf. 4.1 ppm for ferrocene) indicates a strong shielding effect as a result of the ring current of the Np units. This implies that the preferred conformation of 2 in solution might be similar to that in the solid state derived from X-ray crystallography. To gain further insight, we carried out an 1H, 1H-NOESY experiment (Figure 3). The protons Cp-H5 show moderately strong NOE peaks to the naphthyl protons Np-H3, Np-H4, and Np-H5, whereas the protons Cp-H3 display very strong crosspeaks with Np-H2 and Np-H8. Moreover, the protons on the free Cp rings show only crosspeaks to Np-H2. These observations taken together with the absence of crosspeaks of the free Cp ring protons with any other naphthalene protons suggest that, in solution, the preferred isomer is also the one with all three ferrocene moieties positioned on the same side of the boroxine ring and the naphthyl groups pointing away from the Fe centers of the ferrocenes. A conformation with one of the naphthylferrocene moieties on the opposite side of the boroxine ring is highly unfavorable due to severe steric interactions between the naphthyl substituents, which would point toward each other.

Figure 3. (a) 1H, 1H-NOESY NMR spectrum for boroxine 2 (Np/Fc region; mixing time = 500 ms). (b) Illustration of short H···H contacts in 2 (only two of the Fc groups are shown for clarity). Interatomic distances from X-ray structure (Å): Np5···Cp-H5: 3.804, 3.854, 4.213; Np4···Cp-H5: 3.225, 3.272, 3.399; Np8···Cp-H3: 3.72, 3.583, 3.452; Np2···Cp-H3: 3.67, 3.601. 3.498; Np2···C5H5: 2.511, 2.409, 2.498.

Kua and Iovine demonstrated that Lewis bases facilitate the formation of boroxines by binding preferentially to one of the boron centers.10,14,18 Lewis base interaction with multiple borane centers in the boroxine ring is rare and only observed when supported by chelate effects.33 On the other hand, an interesting aspect of ferrocenylboranes is that the Lewis acidity of the borane moieties can be altered through redox-switching of the ferrocene groups.7 It seemed conceivable that such an increase in the Lewis acidity of the boron centers might favor binding to two or even three boron centers. We, therefore, decided to further investigate the redox chemistry of 2 in the absence and in the presence of varying amounts of 4dimethylaminopyridine (DMAP). 6737


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We acquired both cyclic and square-wave (see the Supporting Information) voltammetry data for 2 in CH2Cl2 with Bu4N[B(C6H3(CF3)2)4] as the supporting electrolyte. The cyclic voltammetry data show three well-separated redox waves for the three Fc groups (Figure 4, Table 2). Oxidation of the

rich and thus more easily oxidized. The effect on the subsequent oxidation waves is less pronounced, presumably because the other two boron centers remain tricoordinate. Upon addition of 3 equiv of base, the first oxidation wave is not significantly shifted, whereas the second and third redox processes shift slightly and appear closer together. We attribute this observation to the effect of changes in the DMAP concentration on the comproportionation equilibria for partially oxidized and partially complexed species. Clearly, a further cathodic shift as expected for the binding of DMAP to multiple boron centers is not observed.

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CONCLUSION In conclusion, the synthesis of a chiral redox-active boroxine (2) was accomplished and its structure, reactivity, and redox behavior were studied in depth. The boroxine 2 adopts an interesting C3-symmetic arrangement, in which all three ferrocenyl moieties are positioned on one side of the boroxine ring system, while the naphthyl substituents point into the other direction. Supramolecular interactions in the solid state result in a highly unusual triple-helical structure and the formation of large cavities that are occupied by solvent molecules. The presence of three redox-active ferrocenyl moieties that are in close vicinity leads to three well-separated waves in the cyclic voltammogram with significant redoxsplitting. These are shifted to lower potentials upon addition of DMAP as a Lewis base. However, despite the expected strong Lewis acidity increase of the boron centers upon oxidation, only one of them appears to be coordinated even in the presence of excess DMAP. The results described further indicate that chiral boroxines, such as 2, and especially their oxidized counterparts might serve as powerful hosts for chiral anions. The strong binding of Lewis basic substrates to boron also indicates potential utility in Lewis acid catalyzed processes.

Figure 4. Cyclic voltammetry data of 2 in the absence and in the presence of varying amounts of DMAP; 0.13 M Bu 4 N[B(C6H3(CF3)2)4] in CH2Cl2 as electrolyte, reported vs Fc/Fc+, which is taken as +610 mV versus Cp*2Fe/Cp*2Fe+ used as an internal reference (indicated with an asterisk).

Table 2. Redox Potentials Derived from Cyclic Voltammetry Data of 2a

2 2 + DMAP ΔE = E(2) − E(2·DMAP) 2 + 3DMAP

E1/2 (1) (mV)

E1/2 (2) (mV)

E1/2 (3) (mV)

+98 −269 +367 −257

+284 +67 +217 +139

+504 +325 +179 +283

a

Conditions: CH2Cl2, 0.13 M Bu4N[B(C6H3(CF3)2)4] as electrolyte, reported vs Fc/Fc+.

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EXPERIMENTAL SECTION

Reagents and General Methods. B(OiPr)3 and tert-butyl lithium (1.7 M in hexanes) were purchased from Aldrich and used without further purification. (pR,SS)-2-(1-naphthyl)-1-(p-tolylsulfinyl) ferrocene27,35 was prepared according to literature procedures. All reactions and manipulations were carried out under an atmosphere of prepurified nitrogen using either Schlenk techniques or an inertatmosphere glovebox (MBraun). The 499.9 MHz 1H NMR, 125.7 MHz 13C NMR, and 160.4 MHz 11B NMR spectra were recorded on a Varian INOVA NMR spectrometer (Varian Inc., Palo Alto, CA) equipped with a boron-free 5 mm dual broad-band gradient probe (Nalorac, Varian Inc., Martinez, CA). Solution 1H and 13C NMR spectra were referenced internally to solvent signals (for CDCl 3: 1H NMR, 7.27 ppm; 13C NMR; 77.0 ppm). 11B NMR spectra were acquired with boron-free quartz NMR tubes and referenced externally to BF3·Et2O (δ = 0). MALDI-MS data were obtained in positive mode on an Apex Ultra 7.0 Hybrid FTMS (Bruker Daltonics) using benzo[α]pyrene as the matrix. UV/vis absorption data were acquired on a Varian Cary 500 UV/vis/NIR spectrophotometer. Cyclic voltammetry measurements were carried out on a BAS CV-50W analyzer. The three-electrode system consisted of a Au disk as the working electrode, a Pt wire as the secondary electrode, and a Ag wire as the pseudoreference electrode. The scans were referenced after the addition of a small amount of decamethylferrocene as the internal standard. The potentials are reported relative to the ferrocene/ ferrocenium couple (−610 mV for Cp*2Fe/Cp*2Fe+ in CH2Cl2/ Bu4N[B(C6H3(CF3)2)4]). Elemental analyses were performed by Quantitative Technologies Inc., Whitehouse, NJ. X-ray diffraction intensities of 2 and 3 were collected on a Bruker SMART APEX CCD diffractometer using Cu Kα (1.54178 Å)

first Fc moiety takes place at +98 mV, whereas the second and third oxidation processes occur at significantly higher potentials of +284 and +504 mV. This is indicative of significant Coulombic interactions between the oxidized Fe centers and in contrast to studies by Elschenbroich on a related trovacenylboroxine species,25 for which redox splittings could not be resolved (this could be due to differences in the electrolyte; [Bu4N]ClO4 in DMF was used by Elschenbroich). Noteworthy is that the currents for the second and especially the third redox processes are significantly lower than those for the first oxidation, which we attribute to the existence of diffusioncontrolled comproportionation equilibria for the mixed-valent species 2+ and 22+. Similar phenomena have been observed by Compton et al. when they investigated redox systems with multiple well-separated processes in the absence of excess supporting electrolyte.34 In our case, even in the presence of a relatively large amount of Bu4N[B(C6H3(CF3)2)4] in CH2Cl2 (0.13 M; [2] = 1 mM), an unequal current intensity was clearly observed, which might be due to inefficient screening of charges in the presence of the arylborate electrolyte. Upon addition of 1 molar equiv of DMAP, all three oxidation events are shifted to more negative potentials, suggesting that binding of the Lewis base leads to more facile oxidation of the Fc units. The effect is strongest for the first redox wave, consistent with binding of DMAP to one of the boron centers, which renders the attached ferrocenyl moiety more electron6738


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pyrene): m/z 370.0859 ([M]+, 3%, Calcd for 12C211H1911B116O256Fe1: 370.0827), 694.1289 ([NpFcB(OH)-O-(OH)BNpFc] +, 65%, Calcd for 12C401H3211B216O356Fe2: 694.1236), 708.1473 ([NpFcB(OH)-O(OMe)BNpFc]+, 45%, Calcd for 12C411H3411B216O356Fe2: 708.1393), 722.1599 ([NpFcB(OMe)-O-(OMe)BNpFc] +, 55%, Calcd for 12 C421H3611B216O356Fe2: 722.1549), 1014.145 (2+, 100%, Calcd for 12 C 60 1 H 45 11 B 3 16 O 3 56 Fe 3 : 1014.1714). Elemental analysis for C21H19BO2Fe Calcd: C, 68.16; H, 5.18. Found: C, 68.05; H, 5.00. Conversion of 3 into Boroxine 2. Compound 3 (7.0 mg, 19 μmol) was dissolved in a small amount of CHCl3 and passed through a short silica gel column with THF/CHCl3 (1:1). The resulting solution was dried over Na2SO4 and concentrated to yield an orange solid, which was washed with a minimum amount of THF and dried under high vacuum. Yield: 5.0 mg (78%). Ring-Opening of 2 with Formation of 3. A solution of boroxine 2 (1.00 g, 0.80 mmol) in 20 mL of CHCl3/MeOH (3:1) was heated to reflux for 2 h. The solvent was removed and the resulting residue taken up in MeOH. Orange crystals of 3 were obtained upon slow evaporation of the solvent. Yield: 0.72 g (81%).

radiation at 100 K. The structures were refined by full-matrix leastsquares based on F2 with all reflections (Sheldrick, G. SHELXTL V5.10; Siemens XRD: Madison, WI). Non-hydrogen atoms were refined with anisotropic displacement coefficients, and hydrogen atoms were treated as idealized contribution. SADABS (Sheldrick, G. 12 G.M. SADABS (2.01): Bruker/Siemens Area Detector Absorption Correction Program; Bruker AXS: Madison, WI, 1998) absorption correction was applied. Crystallographic data for the structures of 2 and 3 have been deposited with the Cambridge Crystallographic Data Center as supplementary publications CCDC-852845 and CCDC852846, respectively. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (Fax: (+44) 1223-336-033. E-mail: [email protected]). Synthesis of [(pS)-1,2-NpFcBO]3 (2). To a precooled (−78 °C) solution of (pR,SS)-2-(1-naphthyl)-1-(p-tolylsulfinyl) ferrocene (1.00 g, 2.22 mmol) in THF (60 mL) was added tert-butyl lithium (1.44 mL, 2.44 mmol, 1.10 equiv) dropwise under stirring. The reaction mixture was kept stirring for 10 min at the same temperature, and B(O iPr)3 (0.67 mL, 2.89 mmol, 1.30 equiv) was added via a syringe. The resulting solution was stirred for 1 h, and the temperature was slowly raised to 25 °C. After hydrolysis with dilute HCl, the mixture was extracted with dichloromethane (3 × 25 mL). The combined organic layers were washed with brine solution, followed by water, dried over sodium sulfate, and concentrated. The residue was chromatographed on silica gel with CHCl3/MeOH (3:1) as the eluent. The resulting solution was concentrated and layered with THF to obtain the product as orange crystals. Yield: 485 mg (49%). 1H NMR (499.9 MHz, CDCl3, 25 °C): δ 8.05 (d, J = 7.1 Hz, 3H, Np), 7.93 (2 × d, J = 8.5 Hz, 6H, Np), 7.71 (br, 3H, Np), 7.58 (pst, J = 7.5 Hz, 3H, Np), 7.46 (pst, J = 8.1 Hz, 3H, Np), 7.32 (d, J = 8.1 Hz, 3H, Np), 4.45 (br, 3H, Cp), 4.31 (pst, J = 2.5 Hz, 3H, Cp), 3.98 (s, 15H, free Cp), 2.77 (br, 3H, Cp). 13C NMR (125.69 MHz, CDCl3, 25 °C): δ 131.5, 128.5, 127.9, 123.5, 122.4, 121.7, 121.6, 120.3, 120.1, 119.7 (Np), 89.5 (ipso-Cp), 72.8 (Cp), 70.6 (Cp), 66.3 (Cp), 64.8 (free Cp). 11B NMR (160.4 MHz, CDCl3, 25 °C): δ 31.3 (w1/2 = 1300 Hz). UV−vis (CHCl3): λmax = 446 nm (ε = 970 M−1 cm−1). High-resolution MALDI-MS (+ mode, benzo[α]pyrene): m/z 1014.1758 ([M] + , 100%, Calcd for 12 C 60 1 H 45 11 B 3 16 O 3 56 Fe 3 : 1014.1714). Elemental analysis for C60H45B3O3Fe3 (2CHCl3) Calcd: C, 59.44; H, 3.78. Found: C, 60.51; H, 3.72. For (pS)-1,2-NpFcB(OH)2. 1H NMR (499.9 MHz, CDCl3, 25 °C): δ 8.12 (br, 1H, Np), 7.86 (d, J = 6.5 Hz, 2H, Np), 7.78 (br, 1H, Np), 7.55 (pst, J = 6.5 Hz, 1H, Np), 7.46 (pst, J = 6.5 Hz, 1H, Np), 7.37 (pst, J = 5.0 Hz, 1H, Np), 4.70 (br, 1H, Cp), 4.64 (br, 1H, Cp), 4.60 (br, 1H, Cp), 4.34 (s, 5H, free Cp), 4.05 (s, 2H, OH). Synthesis of (pS)-1,2-NpFcB(OH)(OMe) (3). A solution of (pR,SS)-2-(1-naphthyl)-1-(p-tolylsulfinyl) ferrocene (0.30 g, 0.66 mmol) in THF (20 mL) was kept at −78 °C, and tert-butyl lithium (0.46 mL, 0.72 mmol, 1.10 equiv) was added dropwise under stirring. The reaction mixture was kept stirring for 10 min at the same temperature, and B(OiPr)3 (0.18 mL, 0.86 mmol, 1.30 equiv) was added via a syringe. The resulting solution was stirred for 1 h, and the temperature was slowly raised to 25 °C. After hydrolysis with dilute HCl, the mixture was extracted with dichloromethane (3 × 10 mL). The combined organic layers were washed with brine solution, followed by water, dried over sodium sulfate, and concentrated. The residue was subjected to silica gel column chromatography with CHCl3/MeOH (3:1) as the eluent, concentrated, and then taken up in hot CHCl3/MeOH (3:1). Slow evaporation of the solvent gave orange crystals suitable for X-ray diffraction analysis. Yield: 130 mg (53%). 1H NMR (499.9 MHz, CDCl3, 25 °C): δ 8.13 (d, J = 6.5 Hz, 1H, Np), 7.85 (2 × d, J = 8.5 Hz, 2H, Np), 7.75 (br, 1H, Np), 7.55 (pst, J = 7.5 Hz, 1H, Np), 7.46 (pst, J = 7.5 Hz, 1H, Np), 7.37 (d, J = 7.5 Hz, 1H, Np), 4.67 (br, 1H, Cp), 4.60 (br, 1H, Cp), 4.54 (br, 1H, Cp), 4.32 (s, 5H, free Cp), 3.84 (s, 1H, OH), 3.61 (s, 3H, OMe). 13C NMR (125.69 MHz, CDCl3, 25 °C): δ 135.5, 133.7, 133.6, 129.3, 128.5, 128.4, 126.4, 126.3, 126.1, 125.4 (Np), 94.4 (ipso-Cp), 76.4 (Cp), 73.8 (Cp), 71.24 (Cp), 69.9 (free Cp), 50.6 (OMe). 11B NMR (160.4 MHz, CDCl3, 25 °C): δ = 30.5 (w1/2 = 330 Hz). UV−vis (CHCl3): λmax = 443 nm (ε = 300 M−1 cm−1). High-resolution MALDI-MS (+ mode, benzo[a]-

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ASSOCIATED CONTENT S Supporting Information * One- and two-dimensional 1H NMR spectra and UV−vis data of 2 and 3, MALDI-MS spectrum of 3, and square-wave voltammetry data for 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

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ACKNOWLEDGMENTS

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REFERENCES

We thank the Petroleum Research Fund administered by the American Chemical Society for financial support and the National Science Foundation for providing funds for acquisition of the X-ray diffractometer (NSF-CRIF 0116066). F.J. thanks the Alfred P. Sloan Foundation for a research fellowship. We are grateful to Dr. Lazaros Kakalis for help with the acquisition of 2D NMR spectra.

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Reversible Formation of a Planar Chiral Ferrocenylboroxine and Its

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