Reactivity and Electronic Properties of a Ferrocene Molecule Bearing

Article pubs.acs.org/Organometallics

Reactivity and Electronic Properties of a Ferrocene Molecule Bearing an N,C-Chelated BMes2 Unit Ying-Li Rao,† Tetsuro Kusamoto,‡ Ryota Sakamoto,‡ Hiroshi Nishihara,‡ and Suning Wang*,† †

Department of Chemistry, Queen’s University, Kingston, Ontario K7M 3N6, Canada Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

‡

S Supporting Information *

ABSTRACT: A dimesitylboron-functionalized benzimidazolylferrocene, B(2-ferrocenyl-N-Me-benzimidazolyl)Mes2 (1), has been synthesized and fully characterized. The B−N bond in 1 was found to undergo a dynamic dissociation/association process in solution, leading to a dynamic exchange of the two mesityls bound to the boron atom and slow hydrolysis of 1 under ambient conditions. The hydrolyzed product 2-BMes(OH)-1-(N-methylbenzimidazol-2-yl)ferrocene (2) was isolated and characterized, in which the boron center has a trigonal-planar geometry with one of the mesityls being replaced by an OH− group. The photoisomerization process of the boron unit in 1 was fully inhibited by the low lying d−d/MLCT states of the ferrocene unit. Compound 1 can be oxidized readily by I2, forming the ferrocenium species [1+]I3‑ (3). NMR and EPR data for 3 indicated a notable spin delocalization through space from the Fe(III) center to a flanking mesityl group.

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INTRODUCTION Organoboron compounds are an emerging class of photochromic materials.1 In particular, N,C-chelated dimesitylboranes exhibit tunable colors and rich photochemical and chemical reactivity.1c−g The charge transfer transition from the mesityl to the chelated backbone is a key driving force for the photoisomerization of N,C-chelated BMes2 compounds. To further tune the photochromic properties of N,C-chelated BMes2 compounds, we incorporated transition-metal ions such as Au(I), Pt(II), and Re(I) to the chelated backbone, because this strategy was used successfully in tuning other photochromic systems such as dithienylethenes (DTE).2 However, we observed that, in most instances, the introduction of a metal ion unit greatly inhibited the photoreactivity of the boron unit because of the low-lying metal-to-ligand charge transfer (MLCT) state.3 On the other hand, redox-active transition metal ions have been extensively explored as a control to modulate photochromic systems by influencing the electronic structure of the photoswitch through the oxidation states of the metal ion.4 One of the most commonly used metal units for regulating photoswitching systems is ferrocene.5 In addition to influencing the properties of photochromic systems, the ferrocene unit was also incorporated into a variety of maingroup systems to create new materials that possess interesting electronic, chemical, and photophysical properties and applications.6,7 In particular, three-coordinate boryl units were found to be very effective in promoting electronic communication with or between the ferrocene units,7 leading to important applications such as electrochemical sensing of anions.7f−h © 2014 American Chemical Society

To examine the mutual influence of a ferrocene unit and a N,C-chelated BMes2 unit on the electronic, chemical, and photochemical properties, we designed and synthesized the new molecule B(2-ferrocenyl-N-Me-benzimidazolyl)Mes2 (1), in which a four-coordinate boryl moiety is connected to the ferrocene. Compound 1 was found to display an unusual dynamic exchange and ring-opening hydrolysis. Furthermore, 1 can be oxidized readily by I2, generating the new ferrocenium species [1+]. Details of our investigation on the reactivity and the electronic properties of 1 and [1+] are presented herein.

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RESULTS AND DISCUSSION Synthesis and Structure of 1. Compound 1 was synthesized according to Scheme 1. The key starting material, (N-methylbenzimidazol-2-yl)ferrocene (1a), was obtained by a procedure modified from that used for a related compound reported previously.8 The benzimidazolyl-directed ortho lithiation of 1a, followed by addition of 1 equiv of Scheme 1. Synthesis of Compound 1

Received: February 7, 2014 Published: March 21, 2014 1787

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dimesitylboron fluoride, led to the formation of compound 1, which was isolated in ∼60% yield as a reddish crystalline solid. The 11B NMR spectrum of 1 has a peak at 0.60 ppm that is characteristic of a four-coordinate boron. The four aryl protons and the six methyls on the two mesityl groups display wellresolved and distinct peaks in the 1H NMR spectrum of 1, in agreement with the asymmetric nature of the molecule and the highly congested environment around the boron center. However, variable-temperature 1H NMR and 2D NOESY NMR spectra indicated the existence of a dynamic exchange between the two mesityls and between the two aryl protons on the same mesityl ring. From NOESY/EXSY experiments (see the Supporting Information), the Gibbs free energy for the exchange between two mesityl groups was determined to be 67.0 kJ/mol at 273 K, while that for the exchange between two aryl protons of the same mesityl is 70.0 kJ/mol. The proton chemical shift exchange within the same mesityl ring can be explained by the rotation around the B−C bond.9a The dynamic exchange between the two mesityl groups is most likely caused by the dissociation of the B−N bond, leading to the formation of the intermediate A, followed by the rotation of B−CCp bond and the re-formation of the B−N bond, as shown in Scheme 2. The activation energy for the B−N bond

Figure 1. Crystal structure of one of the independent molecules in the asymmetric unit of 1 with 50% thermal ellipsoids. H atoms are omitted for clarity.

Table 1. Selected Bond Lengths (Å) and Angles (deg) compd

Lengths 2.024(2) 2.027(2) 2.033(2) 2.054(2) 2.125(2) 2.029(2) 2.035(3) 2.035(3) 2.039(3) 2.047(3) 1.665(3) 1.646(3) 1.659(3) 1.658(3)

Fe−C (Cp-B)

Scheme 2. Proposed Mechanism for the Interconversion of the Two Mesityl Groups in 1

Fe−C (Cp)

B−CCp B−CMes B−N B−O

dissociation in 1 is similar to that reported previously for a class of o-(aminomethyl)arylboronate compounds9b which undergo a reversible B−N dative bond dissociation/association process similar to that observed for 1. The B−N dissociation in 1 is quite unusual, since the same phenomenon was not observed in the previously reported N,C-chelated BMes2 compounds involving the phenyl−benzimidazolyl and the related phenyl− azolyl chelate ligands.1f The greater steric congestion imposed by the ferrocene unit in 1, in comparison to the nonferrocenecontaining N,C-chelated BMes2 compounds, may be responsible for the unusual B−N bond dissociation/reassociation process of 1. The crystal structure of 1 was examined by a single-crystal Xray analysis, which confirmed the congested nature of 1, as shown in Figure 1. There are two independent molecules in the asymmetric unit of 1. Selected bond lengths and angles for one of them are shown in Table 1. The B−N bond length (1.658(3), 1.644(3) Å) in 1 is indeed much longer than those observed previously in the phenyl−azolyl chelated BMes2 compounds (1.620−1.635 Å),1f while the B−CCp and B−CMes bonds are longer (by ∼0.01 Å) than the typical B−C bond lengths observed in previously reported N,C-chelated BMes2 compounds.1c−f The Fe−C bonds on the Cp ring that is bound to the boron atom vary significantly in bond lengths, as shown in Table 1. The Fe−C(2) bond (2.125(2) Å) is ∼0.1 Å longer than other Fe−C bonds, which can be attributed to the binding of C(2) to the bulky BMes2 unit. The two Cp rings in 1 are nearly parallel to each other with tilt angles of 8.7 and 4.8°,

1

2

3

2.030(3) 2.035(3) 2.035(3) 2.035(3) 2.036(3) 2.027(3) 2.044(3) 2.044(3) 2.046(2) 2.052(3) 1.566(4) 1.590(4)

2.033(8) 2.067(8) 2.080(8) 2.148(7) 2.200(7) 2.052(8) 2.059(8) 2.109(8) 2.116(8) 2.129(9) 1.635(11) 1.637(11) 1.653(11) 1.647(10)

1.351(4) Angles 94.14(17) 113.36(19) 117.22(18) 105.82(18) 106.33(18) 119.00(19)

CCp−B−N CMes−B−CMes CMes−B−N CMes−B−CCp CCp−B−O CMes−B−O tilt angle between the two Cp rings

8.7/4.8

118.2(3) 123.4(3) 118.4(3) 2.9

93.7(5) 115.8(6) 114.1(6) 105.5(6) 118.1(6) 107.7(6)

13.7

respectively, for the two independent molecules in the asymmetric unit. Chemical and Photochemical Reactivity of 1. Under ambient conditions, we observed that the solution color of 1 slowly changed from red to yellow-orange. NMR spectra indicated that 1 was converted to the new species 2 quantitatively over a period of a few weeks if the solvent used was not dried rigorously. NMR and single-crystal X-ray diffraction analyses established that 2 is a ring-opened, hydrolyzed product of 1 (Scheme 3). As shown in Figure 2, the boron atom in 2 is no longer bound to the benzimidazolyl nitrogen atom and one of the mesityls on the boron is replaced by a hydroxyl group. The geometry of the boron atom is a trigonal plane. The B−O bond length (1.351(4) Å) is characteristic of a BO bond due to the formation of a π bond between the O and the B atoms.10 The 1788

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would lead to the product 2. The fact that the addition of excess H2O to the solution of 1 did not accelerate the hydrolysis process supports that the formation of the intermediate A is likely the rate-determining step. All attempts to trap the intermediate A by adding other Lewis donors such as NBu4F and NBu4Cl to a rigorously dried solution of 1 led to the formation of 2, in which the OH− group came from the H2O molecules associated with the NBu4F and NBu4Cl compounds. This could be attributed to the high stability of compound 2. The facile hydrolysis of 1 is in sharp contrast with the previously reported non-ferrocene-containing N,C-chelated BMes2 compounds, which did not exhibit the same behavior at all. In addition, similar hydrolysis was not observed for a related Fe(Cp)[(Cp-3,5-Me2py)BMes2] compound in which, instead of a benzimidazolyl ring, a pyridyl ring is bound to the BMes2 unit.11 Since N-methylbenzimidazole has a donor strength which is comparable to that of 3,5-Me2pyridine and the steric environment around the boron atom in 1 is similar to that in Fe(Cp)[(Cp-3,5-Me2py)BMes2], the greater hydrolysis tendency by 1 is likely caused by the more strained chelation to the boron atom by the two five-membered rings in 1, in comparison to that by a five-membered and a six-membered ring in Fe(Cp)[(Cp-3,5-Me2py)BMes2], which may also be a key factor in the dynamic exchange of the two mesityl groups in 1. Compounds 1 and 2 have distinct electronic properties. As shown in Figure 3, the characteristic ferrocene low-energy

Scheme 3. Proposed Mechanism for the Formation of 2

Figure 2. Crystal structure of 2 with 50% thermal ellipsoids. H atoms are omitted for clarity.

B−CCp and B−CMes bonds are much shorter than those in 1, due to the much reduced congestion. The Fe−C bond lengths in 2 show much less variation in comparison those in 1. The Fe−C(6) bond length (2.052(3) Å) in 2 is also much shorter than the corresponding length (Fe−C(2)) in 1, further supporting that steric congestion is the main cause for the long Fe−C and B−N bonds in 1. As a consequence of the reduced steric congestion, the tilt angle between the two Cp rings in 2 is smaller (2.9°). The H atom of the OH group in 2 was located in the difference Fourier map and refined successfully. It forms a fairly strong intramolecular hydrogen bond with the benzimidazolyl N atom, as indicated by the O(1)···N(1) distance (2.686(7) Å). The chemical shift of this H atom appears at ∼14 ppm in the 1H NMR spectrum of 2, which is in agreement with the H bond formation. The boron atom in 2 displays a broad peak at 48.6 ppm in the 11B NMR spectrum, consistent with a three-coordinated boron center. The formation of the hydrolyzed product 2 is consistent with the proposed B−N bond dissociation process of 1 shown in Scheme 2. It is conceivable that, after the formation of the intermediate A, external donor molecules such as H2O can compete with the internal nitrogen donor atom for binding to the boron center, forming the intermediate B shown in Scheme 3. An intramolecular H bond between the H2O and the N atom would stabilize the intermediate B, making the hydrolysis a highly competitive pathway, in comparison to the re-formation of the B−N bond. A similar internal H bond facilitated hydrolysis phenomenon involving a BMes2 group was observed previously.10b Elimination of a mesitylene molecule from B

Figure 3. (left) UV−vis absorption spectra of 1 and 2 in CH2Cl2. (right) CV diagrams of 1 and 2 showing the oxidation wave (vs Fc/ Fc+) at a 75 mV/s scan rate with [Bu4N][PF6] (0.1 M) as the electrolyte in CH3CN.

absorption band appears at ∼490 nm for 1 and at ∼450 nm for 2 with ε ≈ 1200 M−1 cm−1, which accounts for the distinct colors of these two compounds. The 450 nm absorption band of 2, in fact, is similar to that of ferrocene. Both compounds also display the characteristic reversible 1e oxidation peaks at E1/2 = −67 and 205 mV, respectively (relative to Fc/Fc+), in the CV diagram shown in Figure 3. The 270 mV increase of the oxidation potential from 1 to 2 indicates a considerable decrease of the electron density on the Fe(II) center, which may be attributed to the BMes(OH) unit, since the same trend was observed in the previously reported ferrocene compounds functionalized by an electron-deficient three-coordinate boron unit.7a The loss of chelation between the Cp and the benzimidazolyl ring is believed to be another key factor in the higher oxidation potential of 2, because the oxidation potential of the nonborylated precursor compound 1a was found to be more positive than that of ferrocene (E1/2 = 128 mV, relative to Fc/Fc+), albeit to a lesser degree, in comparison 1789

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to that of 2. The attachment of the relatively electron negative benzimidazolyl ring to the Cp ring would certainly decrease the π electron density on the Cp ring, which in turn would reduce the electron density on the Fe(II) center, leading to the more positive oxidation potential of 1a, relative to ferrocene. Adding the BMes2 chelating unit enhances the π conjugation of the Cp and the benzimidazolyl ring. However, because the mesityls are electron-donating groups, the net consequence of the BMes2 chelation is to put more electron density on the Fe(II) center; thus, the oxidation potential of 1 is more negative, relative to 1a. TD-DFT computational studies in fact confirmed that one of the mesityls has a large contribution to the HOMO of 1 (see the Supporting Information). TD-DFT data further established that the weak absorption band of 1 at ∼490 nm is mainly from d → π*Cp‑benzimidazolyl and d → π*Cp transitions (MLCT) with a small contribution from a Mes→ π*Cp transition (see the Supporting Information). The BMes2 unit in 1 lowers the π* energy by enhancing the π conjugation of the Cp− benizimidazolyl unit via chelation and raises the filled d orbital energy via σ donation (as evidenced by the oxidation potential of 1), which leads to the relatively low energy MLCT band and the distinct red color of 1. Well-defined MLCT transitions in borylated ferrocenes have been observed previously.12 For 2, the key contributions to the absorption band at ∼450 nm were found to involve mainly d → π*Cp‑B and d → π*benzimidazolyl‑B transitions (MLCT). The boron center in 2 has a significant contribution to the π* level, which should lower the π* energy. This is, however, offset by the loss of the chelation between the Cp and the benzimidazolyl ring in 2. Thus, the higher MLCT energy of 2 relative to that of 1 is mainly caused by the decreased electron density (or the stabilization of the filled d orbitals) on the Fe(II) center induced by the unsaturated boron unit. Despite the high degree of steric congestion, compound 1 is very stable toward irradiation by light. No change was observed at all when the dry C6D6 solution of 1 was irradiated under nitrogen at 365 nm or using white light for several hours. Although the HOMO level of 1 has appreciable contributions from a mesityl and the LUMO has a large contribution from the π* orbital of the benzimidazolyl−Cp chelate, the HOMO → LUMO transition contributes very little to the first five excited states, which are dominated by d−d and MLCT transitions (see the Supporting Information). Thus, the inactivity of the boron unit in 1 toward photoisomerization can be explained by the presence of the low-lying electronic states involving the ferrocene unit that effectively quench the photoisomerization pathway by trapping the excitons. Compound 1 can be oxidized readily by a mild oxidant such as I 2 , generating the dark brown species [1 + ]I 3 (3) quantitatively (Scheme 4), which was fully characterized by NMR, elemental, and single-crystal X-ray diffraction analysis. The crystal structure of 3 is shown in Figure 4.

Figure 4. Crystal structure of 3 with 50% thermal ellipsoids. The H atoms are omitted for clarity.

In comparison to those of 1, the Fe−C bonds in 3 are much longer, which is in agreement with the trend reported in the literature for ferrocenium cations and can be attributed to the decreased π back-bonding between the Fe(III) ion and the Cp rings.13 As observed in the structure of 1, one of the mesityls (the C(28) ring) is on the same side as the Fe(III) center, thus being very close to the Fe atom (Fe(1)···C(28) = 3.52(1) Å). Significantly, the Fe−C bonds that are close to the C(28) mesityl ring are all longer than other Fe−C bonds (Fe(1)− C(1) 2.200(7) Å, Fe(1)−C(5) 2.148(7) Å, Fe(1)−C(6) 2.129(9) Å, Fe(1)−C(7) 2.116(8) Å). Furthermore, the two Cp rings in 3 have a much greater tilt angle (13.7°) and are more open toward the C(28) mesityl ring in comparison to those in 1. Electronic and Chemical Properties of 3. To examine the electronic structures of 3, we recorded the UV−vis−near-IR absorption spectra of [1 +] generated in situ using a stoichiometric amount of tris(4-bromophenyl)ammoniumyl hexachloroantimonate and iodine, respectively. Upon the addition of the oxidant, a broad band covering the range 600−1300 nm appeared with λmax ∼950 nm (ε = 400 M−1 cm−1), which matches well with that observed for the isolated pure compound 3 (Figure 5). This low-energy band may be assigned to a ligand-to-metal CT transition, according to the well-documented work for ferrocenium ions in the literature (∼617 nm, ε = 390 M−1 cm−1 for the nonsubstituted ferrocenium at 300 K).14 This band is known for its sensitivity to substituent groups on the Cp ring.14b

Scheme 4. Formation of 3 via the Oxidation of 1 by I2

Figure 5. UV−vis−near-IR absorption spectra change of 1 upon the addition of an oxidant: [N(4-Br-C6H4)3][SbCl6] (left) and I2 (right) in CH2Cl2. 1790

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The EPR spectra of 3 was recorded at 21 and 9 K, respectively. At both temperatures, a highly anisotropic EPR signal with an axial character was observed at g∥ = 3.11 and g⊥ = 1.94 (Figure 6), which is characteristic of ferrocenium cations.15

Figure 7. 1H NMR spectra of 3 in CD3CN at 298 K; (bottom) full spectrum (the asterisks indicate all the methyl groups); (top) enlarged region from +13.0 to −1.0 ppm showing the assigned peaks (color coded). Figure 6. EPR spectrum of 3 recorded at 21 K. Inset: spin density distribution diagram of [1+] obtained from DFT data (plotted with an isospin value of 0.004).

delocalization/electronic interaction between the Fe(III) center and the flanking C(28) mesityl ring. 2D NOESY NMR data indicated that there is a dynamic exchange between the two mesityl groups in 3 similar to that observed in compound 1, which is likely through the same B− N bond dissociation process proposed for 1. Compound 3 is stable in the solid state under ambient conditions for months. However, it can be reduced back to compound 1 in solution by weak donor solvent molecules such as DMSO. A similar phenomenon was observed for some of the previously reported ferrocenium compounds.16,17 The presence of the low-lying LMCT/d−d states in 3 could retard the photoisomerization of the boron unit in 3. In fact, 3 was found to be sensitive toward light in solution and rapidly decomposes to unidentified species upon exposure to light, and no evidence of photoisomerization of the boron unit was observed.

Nonetheless, the much smaller g∥ value in comparison to that (∼3.8) of a nonsubstituted ferrocenium indicates that the spin density extends partially to the organic substituent.5a,15 The spin density distribution obtained from the DFT calculation data of [1+] corroborated the EPR results. Single-point calculations on the radical cation [1+] without the counterion were performed at the theory level of the BP86 function, using the basis set of def2SVP for Fe and 6-31G(d) for other atoms. The tilt angle of the two Cp rings from the optimized geometry of [1+] is 13°, similar to that observed in the crystal structure. As shown in Figure 6, the calculated spin density in [1+] is mainly localized on the ferrocenium center with a notable delocalization to the flanking mesityl group (the C(28) ring). The NMR spectrum of 3 in CD3CN was recorded to further examine the influence of the paramagnetic Fe(III) center on the surrounding organic ligands.16 The 11B NMR signal of 3 appears at −30 ppm, which is shifted ∼30 ppm upfield relative to that of 1. As shown in Figure 7, the 1H chemical shifts of the aromatic protons on the benzimidazole moiety in 3 are similar to those in 1. The chemical shifts of all seven methyls are distinct and are spread over the range of +26 to −8 ppm. Assignments for some of the chemical shifts of 3 were made possible with the aid of 2D COSY and NOESY data. The chemical shifts for two of the methyls and the two aryl H atoms on the C(19) mesityl ring that is far away from the Fe(III) center were assigned without ambiguity. Because the difference in the chemical shifts of these protons in comparison to those in 1 is relatively small (