Influence of Substituent Type and Position on the Adsorption

Article pubs.acs.org/JPCA

Influence of Substituent Type and Position on the Adsorption Mechanism of Phenylboronic Acids: Infrared, Raman, and SurfaceEnhanced Raman Spectroscopy Studies Natalia Piergies,† Edyta Proniewicz,*,† Yukihiro Ozaki,‡ Younkyoo Kim,§ and Leonard M. Proniewicz†,‡,⊥ †

Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krakow, Poland Department of Chemistry, School of Science and Technology, Kwansei-Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan § Department of Chemistry, Hankuk University of Foreign Studies, Yongin, Kyunggi-Do, 449-791, Korea ⊥ The State Higher Vocational School, ul. Mickiewicza 8, 33-100 Tarnów, Poland ‡

S Supporting Information *

ABSTRACT: This paper shows systematic spectroscopic studies using Fourier-transform infrared absorption (FT-IR), Fourier-transform Raman (FT-Raman), and surface-enhanced Raman (SERS) in an aqueous silver sol of fluoro and formyl analogues of phenylboronic acids: 2-fluorophenylboronic acid (2-FPhB(OH)2), 3-fluorophenylboronic acid (3-F-PhB(OH)2), 4-fluorophenylboronic acid (4-F-PhB(OH)2), 2formylphenylboronic acid (2-CHO-PhB(OH)2), 3-formylphenylboronic acid (3-CHO-PhB(OH)2), and 4formylphenylboronic acid (4-CHO-PhB(OH)2). To produce an extensive table of vibrational spectra, density functional theory (DFT) calculations with the B3LYP method at the 6-311++G(d,p) level of theory were performed for the ground state geometry of the most stable species, dimers in cis−trans conformation. On the basis of the SERS spectral profile, the adsorption modes of the phenylboronic acid isomers were proposed. The type of substituent and its position in the phenyl ring have a strong influence on the geometry of isomers on the silver nanoparticle’s surface. This effect was especially evident in the case of 4-CH-PhB(OH)2, for which dearomatization of the phenyl ring took place upon adsorption.

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INTRODUCTION

The addition of various substituents onto the phenyl rings of boronic acids allows the pKa to be tuned so that boronic acidcontaining compounds can be employed at a physiologically relevant pH range23 and broadens the compounds’ biological applications. For example, phenylboronic acid derivatives with a fluorine atom substitution are known as a strong antagonist of bacterial quorum sensing.2,24,25 In addition, the introduction of a fluorine atom in the 2-position (orthoisomer) of phenylboronic acid increases the Lewis acidity of this isomer and thus leads to its increased base affinity.26,27 In contrast, formylphenylboronic acids are inhibitors of γ-glutamyltranspeptidase28 and are important intermediates in the synthesis of active compounds in the agrochemical and pharmaceutical industries. The amination−reduction reaction of o-formylphenylboronic acid would appear to be the least expensive and most straightforward preparative strategy for the synthesis of secondary aromatic amine derivatives, which are important because of their possible fluorescence and resulting analytical utility in sensing.29 Formylphenylboronic acids are also of especially high efficiency and importance as enzyme stabilizers, inhibitors, and bactericides.30 o-Substituted formylboronic acids show a tendency to form boron heterocycles bearing a resemblance to naturally occurring purines.31 The fluorine or formyl group substituted at the 2-position in the phenylboronic

In recent years, scientific attention has been focused on phenylboronic acids because of their versatile reactivity and stability.1,2 These properties are associated with the classification of boronic acid as a strong Lewis acid.1 Boron’s open shell allows the easy conversion of the boron atom from trigonal (sp2 hybridization) to tetrahedral (sp3) form in the presence of Lewis bases, which allows new applications of boronic acid derivatives in many areas.1,3 Numerous structural similarities between the carbon and boron atoms and low toxicity contribute to the boron atom’s extremely useful nature in organic chemistry (i.e., in C−C bond formation, acid catalysis, asymmetric synthesis, carbohydrate analysis, or molecular sensing)4−11 and in medicine. For example, phenylboronic acids are used as inhibitors of serine protease and kinase enzymes that, when overexpressed, increase growth, progression, and metastasis of tumor cells.11−17 One of these boronic-acid-based proteasome inhibitors, bortezomib, has been approved as a drug for the treatment of multiple myeloma.18 These compounds are also used as agents in boron neutron capture therapy (BNCT)19 because of the 10B isotope’s ability to emit α-particles over a short-range in tissue as a result of neutron radiation.11,20 In addition, compounds containing the boronic acid group have a special role in glucose monitoring systems (glucose chemosensors) due to the strong covalent interaction between this group and the diols that exist in sugars.11,21,22 © 2013 American Chemical Society

Received: April 28, 2013 Revised: June 7, 2013 Published: June 12, 2013 5693

dx.doi.org/10.1021/jp404184x | J. Phys. Chem. A 2013, 117, 5693−5705

The Journal of Physical Chemistry A

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Figure 1. Molecular structures of 2-F-PhB(OH)2, 3-F-PhB(OH)2, 4-F-PhB(OH)2, 2-CHO-PhB(OH)2, 3-CHO-PhB(OH)2, and 4-CHO-PhB(OH)2 dimers.

acid ring also contributes to the formation of intramolecular hydrogen bonds with a boronic acid moiety.31−35 Because of the significance of the fluorine and formylsubstituted phenylboronic acid, we undertake Fourier-transform Raman spectroscopy (FT-Raman), Fourier-transform absorption infrared spectroscopy (FT-IR), and surfaceenhanced Raman spectroscopy (SERS) studies, using an aqueous silver sol, for a group of three isomers (2- (orthoisomer), 3- (meta isomer, and 4- (para isomer)) of fluoro- and formyl-substituted phenylboronic acids: 2-fluorophenylboronic acid (2-F-PhB(OH)2), 3-fluorophenylboronic acid (3-F-PhB(OH)2), 4-fluorophenylboronic acid (4-F-PhB(OH)2), 2formylphenylboronic acid (2-CHO-PhB(OH)2), 3-formylphenylboronic acid (3-CHO-PhB(OH)2), and 4-formylphenylboronic acid (4-CHO-PhB(OH)2) (see Figure 1 for molecular structures). For these compounds, we also performed vibrational analysis using DFT (density functional theory) calculations with the B3LYP method at the 6-311++G(d,p) level of theory. This procedure is commonly employed in calculations for similar compounds and yields reliable results.36 Our aim was to produce an extensive look-up table of infrared and Raman spectra that can make structural determination of the investigated molecules a fast and accurate process. The result of this work is an enhanced understanding of the adsorption modes of these molecules at a solid/liquid interface. The liquid/solid interface refers to a water/metal interface. The interaction between the molecule and the specially prepared metal surface controls the stability and architecture of the adsorbed species and identifies groups of atoms that are close or directly involved in the bonding at the surface. Thus, the composition of the molecule is a factor that determines the mode of molecule adsorption onto a given metal surface. This

interaction is believed to be of great significance in the context of nanobiomedicine, protein screening, and other therapeutic applications and, hence, should be fully understood. It is clear that, with a sufficiently sensitive and selective SERS method, the molecular behavior at the solid/liquid interface and changes in this behavior due to specific modifications (electro-donor substituents at the 2-, 3-, and 4-positions) can be detected in response to these specific regions near or on a metal surface. In addition, the use of SERS techniques assures the enhancement of the scattered signal37 but also opens new possibilities for the Raman effect, such as the investigation of molecules at a very low concentrations (down to single molecule detection).38,39 This technique is an increasingly powerful spectroscopic tool notably used in the fields of biomedicine and pharmacology, especially in the detection of DNA, 40 detection and identification of neurotransmitters,41−46 substrate-receptor modeling,47,48 detection of microorganisms,49 and monitoring of intracellular drug distribution.50 Because the SERS spectrum strongly depends on the adsorption mode of the investigated molecule, it provides information about the functional groups of the molecule that are involved in an interaction with the metal substrate, the strength of the interaction, and the molecular fragment’s orientation. The prediction of the orientation of aromatic molecules adsorbed onto SERS-active surfaces is governed by surface selection rules that state that vibrations with large tensor components oriented along the perpendicular axis to the metal surface will be most enhanced.51−54 However, it was shown that, for highly symmetric molecules, a better approach to predicting the correct SERS intensities is the use of the Herzberg−Teller selection rules.55 5694



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EXPERIMENTAL AND THEORETICAL METHODS Phenylboronic Acid Analogues Synthesis. The 2-, 3-, and 4-fluorophenylboronic acids were purchased from CombiBlocks, Inc. in the USA. The 2-, 3-, and 4-formylphenylboronic acids were synthesized according to the standard method56 shown in Scheme 1.

AgNO3 and NaBH4 were purchased from Wako (Japan) and used without further purification. Three batches of Ag sols, as mentioned above, were prepared by the simple borohydride reduction of silver nitrate.57 Briefly, 42.5 mg of AgNO3 dissolved in 50 mL of deionized water (18 MΩ cm) at 4 °C was added dropwise to 150 mL of 2 mM NaBH4 immersed in an ice bath while the mixture was stirred vigorously. After the silver nitrate was added in its entirety, the final dark-yellow colloidal silver solution was stirred continuously at 4 °C for approximately 1 h. Aqueous solutions of the investigated compounds were prepared by dissolution of each sample in deionized water. Before mixing with the silver colloid, the concentration of the prepared solutions was 10−4 M. Ten microliters of sample was mixed with 20 μL of the silver nanoparticle solution. At this concentration, we expect only partial coverage of a surface (less than a monolayer). To prove this hypothesis, we measured the SERS spectra for species adsorbed from a solution with a concentration 10 times more dilute and from a solution with a concentration 10 times more concentrated (appearance of mono/bilayer). The SERS measurements for the molecules adsorbed onto the colloidal silver surface were performed using a HoloSpec f/ 1.8i spectrograph (Kaiser Optical Systems) equipped with a liquid-nitrogen-cooled CCD detector (Princeton Instruments). The 785.0 nm line of a NIR diode laser (Invictus) was used as an excitation source, with an output laser power of 15 mW. The typical exposure time for each SERS measurement in this study was 20 s with eight accumulations (a series of 8 spectra lasting 20 s = 160 s). Typically, the SERS spectra were collected 1 h after mixing the sample with the silver colloid from three spots on the surfaces of the colloidal silver nanoparticles. The eight spectra from the series possess high repeatability, except for small differences (up to 5%) in some band intensities. There were no spectral changes associated with sample decomposition or desorption processes during these measurements. Transmission Electron Microscopy. A transmission electron microscope (TEM) from Hitachi High-Technologies, model H-7650, at an accelerating voltage of 100 kV was employed to collect TEM images of synthesized Ag nanoparticles and sample/Ag nanoparticles. Theoretical Analysis. To optimize the ground-state geometry of the investigated phenylboronic acids and to calculate their FT-Raman and FT-IR spectra, the Gaussian 03 suite58 was used. Our earlier analysis of various types of Nbenzylamino(boronphenyl)methylphosphonic acids36 and a literature study for similar compounds59,60 noted that the most stable structure of the boronic acid derivatives is a cyclic dimer formed by a pair of intermolecular hydrogen bonds between the boron hydroxyl groups of two monomers.36 On the basis of previous experience, in this paper, we present the theoretical calculations for the most stable structure of the substituted phenylboronic acid dimers (Figure 1). We also performed calculations for the monomers (not shown) and compare their energy with the energy of the corresponding dimers. The calculated stabilization of the energy (calculations are based on the method proposed by Dr. Paweł Dziekoński in his Ph.D. thesis at the Technical University of Wroclaw, 2003) indicates that dimers are more stable than monomers (see Table 1). A DFT method with the B3LYP level of theory was employed to optimize the molecular structure of the

Scheme 1. Synthesis of Formylphenylboronic Acids

The purity of the formyl derivatives and their chemical structures were confirmed by use of elemental analysis and 1H and 11B NMR spectroscopy. FT-Raman Measurements. The FT-Raman measurements were performed on a Nicolet spectrometer (model NXR 9650) equipped with a liquid-nitrogen-cooled germanium detector and a continuous-wave Nd3+:YAG laser as an excitation source (1064 nm line). The laser power at the output was set at 500 mW. During each measurement, 1000 scans were collected with a resolution of 4 cm−1. FT-IR Measurements. FT- IR spectra were collected for thin pellets that contain 200 mg KBr and about 1 mg of each phenylboronic acids. Each of the investigated compound’s samples was dispersed in KBr at room temperature. The Bruker spectrometer (EQUINOX 55) equipped with a DT-GS detector and Nernst rod as a excitation source was used to perform the FT-IR measurements. SERS Measurements. SERS spectra were recorded in silver colloidal dispersions. Silver colloidal dispersions prepared by simple borohydride reduction of silver nitrate were used as substrates. Figure 2 provides an overview of UV−vis spectra and the morphology of the substrate used in this work.

Figure 2. Excitation spectra (UV−vis) (A) and transmission electron microscopy (TEM) (B) images of a clear silver colloid used in this work (a) and sample/Ag nanoparticles system (b). 5695



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give information about the geometry of these molecules and also answer the question of how the geometry of the molecule changes when the H atom in the 2-, 3-, or 4-position of the phenylboronic ring is substituted with a fluorine atom or formyl group. The phenyl rings (Ph and Ph′) and the O8−B7−O10 and O′8−B′7−O′10 moieties in all of the investigated dimers form a plane. The hydrogen atoms of both boronic groups in the dimers (H9, H10, H′9, and H′10) lie in the O−B−O plane (Figure 1). This could be explained on the basis of the oxygen lone-electron pairs having a resonance interaction with the empty p orbital of the boron atom, forcing the hydrogen atoms to be in the O−B−O plane.66 Thus, in the lowest-energy conformation, all of the investigated dimer molecules are planar. The calculated B−O and C−B bond lengths (Table 2) in fluorophenylboronic and formylphenylboronic acids are in good agreement with those found in the X-ray structure of phenylboronic (B−O: 1.362 Å− cis H, 1.378 Å− trans H; C−B: 1.568 Å)3 and pentafluorophenylboronic (B−O: 1.362 Å− cis H, 1.355 Å− trans H; C−B: 1.579 Å) acids.66 The calculated B7−O8 distance (∼1.350 Å) for all of the investigated compounds is slightly shorter than the typical B−O (∼1.360 Å)3 and B7−O10 (Table 2) bonds, suggesting a relatively stronger π interaction for the former bond. The calculated C− H bond lengths are approximately equal (Table 2) and match those obtained from theoretical calculations for the 2fluorophenylboronic acid monomer very well;67 however, these calculated lengths are greater (1.082−1.088 Å) than those obtained from the X-ray study for the 2-and 3fluorophenylboronic acid monomer (0.930 Å).67,68 In addition, the calculated ring C−C bond lengths and angles for the investigated dimers are very similar to the results from X-ray investigations on 2-fluorophenylboronic acid. These values (for the dimer) better reproduce the bond lengths and angles than those calculated for 2-fluorophenylboronic acid as a monomer species. Bond angles at the B and C atoms correspond to sp2 hybridization; however, these angles show some deviations from the expected 120° angle (Table 2). For fluoro- and formylphenylboronic acids, the value of the C1−B7−O10 angle is slightly higher than those of the C1−B7−O8, C−C−C, and C−C−H angles. The presence of the substituents in the 2position, acting as proton acceptor/donor capable of forming a weak intramolecular H-bond with respect to the boronic acid group creates steric hindrance for the intramolecular Hbonding. Thus, in 2-F-PhB(OH)2 and 2-CHO-PhB(OH)2, this bond angle in the boronic group can change, and the entire group may rotate about the C−B bond. As shown in Table 2, the C−F bond length for the 2- isomer of fluorophenylboronic acid (1.377 Å) is slightly longer than that for the 3- (1.357 Å) and 4- (1.354 Å) isomers, implying that the fluorine atom of the 2- isomer is involved in hydrogen bonding with H11 (forming a six-segmented ring). The F···H11 distance is 1.982 Å, which is close to the typical length of a hydrogen bond in water (1.970 Å). According to Jeffrey, who categorized H bonds with varying donor−acceptor distances, this value corresponds to a “strong, mostly covalent” H-bond.69 In the case of the 2- isomer of formylboronic acid, the C−C* (C* is the carbon atom in the formyl group) bond length (1.474 Å) is slightly shorter than that for the other two isomers, whereas the C*O* (1.221 Å) and O10−H11 (0.984 Å) distances are slightly longer than that for the 3- and 4- isomers (Table 2), causing a decrease in the length of the O*···H11 hydrogen bond (1.653 Å, seven-segmented ring).

Table 1. Calculated Stabilization of the Energy for Monomers and Dimers of the Investigated Phenylboronic Acids energy (hartree)

energy (kcal/mol)

compd

monomer

dimer

monomer

dimer

2-F-PhB(OH)2 3-F-PhB(OH)2 4-F-PhB(OH)2 2-CHO-PhB(OH)2 3-CHO-PhB(OH)2 4-CHO-PhB(OH)2

−507.670 −507.664 −507.665 −521.758 −521.753 −521.753

−1015.36 −1015.34 −1015.35 −1043.53 −1043.52 −1043.52

−318 568 −318 564 −318 565 −327 408 −327 405 −327 405

−637 146 −637 138 −637 139 −654 828 −654 819 −654 819

investigated phenylboronic acids. The triple-split valence basis with diffuse s- and p-functions on the heavy atoms and a diffuse s-function on the hydrogens with a polarization function on both heavy atoms and hydrogens (6-311++G(d,p)) was used as the basis set.61 This type of basis was applied for the calculations of similar phenylboronic acid derivatives and provides reliable results.59,62 No imaginary wavenumbers were observed during optimization, which demonstrates that the calculated structures correspond to energy minima on the potential energy surface for nuclear motion. Theoretical Raman intensities were calculated by the Raint program, which uses the following relationship:63 IiR = 10−12(ν0 − νi)4 νi−1Si

in which Ii is given in arbitrary units, ν0 is the laser excitation wavenumber (cm−1) (9398.5 cm−1 for a Nd:YAG laser), νi is the frequency of the normal mode obtained from the DFT calculation, and Si is the Raman scattering activity of the normal mode. The theoretical vibrational spectra were generated by the free GaussSum 0.8 software package.64 To better match the theoretical and experimental spectra, the calculated wavenumbers were scaled by a scaling factor of 0.980, and the theoretical spectra were plotted by setting the fwhm (full width at half-maximum) at 10 cm−1 (the average value of a typical fwhm for these compounds in the condensed phase with a 50%/50% Gaussian/Lorentzian band shape). To obtain normal mode assignments for the calculated vibrational bands, the PED (potential energy distribution) for the optimized structures were determined with the freeware Gar2ped program65 in conjunction with a visualization script.

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RESULTS AND DISCUSSION Geometric Structures. The most stable structure of the 2-, 3-, and 4-fluorophenylboronic and 2-, 3-, and 4-formylphenylboronic acids is a cyclic dimer formed by a pair of intermolecular hydrogen bonds between the boron hydroxyl groups of two monomers. Thus, all theoretical spectra presented are calculated for dimers. For these dimer species, there are two possible conformers (cis−trans and trans−cis), depending on the positions of the hydrogen atoms bonded to the oxygen atom of the boronic group, whether they are directed away from (trans) or toward (cis) the phenylboronic ring. Our calculations show that the cis−trans conformation for all of the investigated compounds has the lowest energy; thus, it is the most stable. The molecular structures and numbering scheme of the atoms of the investigated compounds are given in Figure 1, and Table 2 lists some geometric parameters for these molecules. The theoretical bond lengths and bond angles 5696



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Table 2. Select Calculated Bond Lengths and Angles of the Investigated Phenylboronic Acids bond

2-F-PhB(OH)2

3-F-PhB(OH)2

4-F-PhB(OH)2

bond

2-CHO-PhB(OH)2

Bond Length (Å) C1−C2 C2−C3 C3−C4 C4−C5 C5−C6 C6−C1 C1−B7 B7−O8 O8−H9 H9−O′10 B7−O10 O10−H11 O10−H′9 C2−F C2−H12 C3−F C3−H12 C3−H13 C4−F C4−H13 C5−H14 C6−H15 F ···· H11

1.392 1.383 1.393 1.395 1.391 1.407 1.573 1.350 0.976 1.838 1.385 0.964 1.838 1.377

C3−C2−F C4−C3−F C5−C4−F F−C2−C1 F−C3−C2 F−C4−C3 C1−C2−H C2−C3−H C3−C4−H C4−C5−H14 C5−C6−H15 C1−B7−O8 B7−O8−H9 O8−B7−O10 C1−B7−O10 B7−O10−H11 B7−O10−H′9 B′7−O′10−H9

116.7

1.403 1.384 1.385 1.394 1.392 1.404 1.569 1.351 0.974 1.859 1.391 0.961 1.852

1.403 1.392 1.386 1.386 1.391 1.405 1.565 1.352 0.974 1.855 1.393 0.960 1.856

1.085 1.357

1.086

1.082 1.354 1.083 1.084 1.083

1.083 1.083

C1−C2 C2−C3 C3−C4 C4−C5 C5−C6 C6−C1 C1−B7 B7−O8 O8−H9 H9−O′10 B7−O10 O10−H11 O10−H′9 C2−C* C2−H12 C3−C* C3−H12 C3−H13 C4−C* C4−H13 C5−H14 C6−H15 C*O* C*−H* O*···H11

1.425 1.406 1.388 1.391 1.395 1.402 1.598 1.355 0.978 1.810 1.368 0.984 1.810 1.474

C3−C2−C* C4−C3−C* C5−C4−C* C*−C2−C1 C*−C3−C2 C*−C4−C3 C1−C2−H C2−C3−H C3−C4−H C4−C5−H14 C5−C6−H15 C2−C*−H* C2−C*−O* C3−C*−H* C3−C*−O* C4−C*−H* C4−C*−O* C1−B7−O8 B7−O8−H9 O8−B7−O10 C1−B7−O10 B7−O10−H11 B7−O10−H′9 B′7−O′10−H9

113.2

118.8 118.5 118.7

119.8 119.4 119.9 119.4 115.1 118.0 122.4 114.9 127.4 127.6

1.407 1.387 1.400 1.398 1.390 1.404 1.571 1.350 0.975 1.855 1.390 0.961 1.854

1.088 1.480

1.087

1.083 1.482 1.083 1.084 1.082 1.221 1.105 1.653

1.083 1.084 1.083 1.210 1.111

1.085 1.083 1.210 1.110

Angle (°)

118.8

119.9 119.5 120.0 120.1 118.4 114.9 119.1 122.4 113.1 127.8 127.7

1.401 1.397 1.401 1.388 1.395 1.405 1.568 1.350 0.975 1.856 1.391 0.961 1.856

1.085

Angle (°)

122.5

4-CHO-PhB(OH)2

Bond Length (Å)

1.082

1.083 1.083 1.083 1.982

3-CHO-PhB(OH)2

118.7 120.7 122.0 119.7 119.4 119.5 115.0 117.8 122.5 114.7 127.9 127.9

FT-Raman and DFT Studies. Fluorophenylboronic acid (2-F-PhB(OH)2, 3-F-PhB(OH)2, and 4-F-PhB(OH)2) dimers consist of 32 atoms (16 atoms per molecule), whereas formylphenylboronic acid (2-CHO-PhB(OH)2, 3-CHO-PhB(OH)2, and 4-CHO-PhB(OH)2) dimers contain 36 atoms (18 atoms per molecule). Therefore, 90 and 102 normal vibrational modes are expected, respectively. Because these dimers have a symmetric center, half of the numbers of fundamental

120.7 119.6 126.3 119.5 120.8 118.9 120.3 120.1 119.3 113.4 128.5

118.7 120.0 119.7

120.7 120.4 121.4 119.5 120.1

114.5 125.0

115.8 114.4 119.4 124.7 112.8 127.6 127.6

119.3 115.1 118.1 122.5 115.0 127.6 127.6

114.5 124.8 119.3 115.0 118.1 122.4 114.9 127.3 127.3

vibrations are active in the FT-Raman and FT-IR spectra presented in Figures 3 and S1 (Supporting Information), respectively. In addition, Tables S1 and S2 (Supporting Information) give a detailed overview of the PEDs (in percent) of these IR or Raman active fundamental modes based solely on the B3LYP/6-311++G(d,p) calculations. However, the complete vibrational analysis of the observed modes is also based on our earlier investigations of N-benzylamino5697



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Figure 3. The experimental (black solid line) and theoretical (red dashed line) FT-Raman spectra.

in wavenumber for the ν8a and ν8b modes of 2-CHOPhB(OH)2 indicates the donor behavior of the formyl group in the 2-position of the phenylboronic ring. It is worth mentioning that, for 2-F-PhB(OH)2 and 3-F-PhB(OH)2, only one of the ν8 modes (ν8a at 1616 cm−1) is clearly enhanced, whereas for 4-F-PhB(OH)2, the ν8b mode (1590 cm−1) also occurs as a lower-wavenumber shoulder on ν8a. The opposite situation appears for the formylphenylboronic acid isomers; thus, for the 2- and 3- isomers, the ν8a and ν8b modes (at 1593 and 1564 cm−1 and at 1609 and 1579 cm−1, respectively) are observed and only the ν8a mode (at 1611 cm−1) for the 4isomer. The relative intensities of the ν18a and ν12 modes are also affected by the position (2-, 3-, or 4-) of the substituent. For the 2- isomer, ν18a (at 1034 cm−1 for 2-F− and at 1033 cm−1 for 2-CHO−) is strong, whereas ν12 is unnoticed. For the 3- isomer, the ν12 mode (at 1001 cm−1 for 3-F− and at 998 cm−1 for 3-CHO−) is intense, and ν18a is absent. In the case of 4-F-PhB(OH)2 and 4-CHO-PhB(OH)2, both bands (at ∼1012 and 1020 cm−1) are medium-weak and negligibly scattered, respectively. Boronic Acid Group Vibrations. One of the strongest bands in the FT-IR spectra (Supporting Information Figure S1) of the investigated phenylboronic acid analogues occurs at ∼1362− 1343 cm−1 and is due to the asymmetric B−O stretching vibration [νas(BO)] (Supporting Informationv Tables S1 and S2). It has been suggested that the high relative intensity of νas(BO) could indicate some double bond character for the B− O bond in phenylboronic acid (1.58 in the BO33‑ ion).78 On the other hand, the 838−825 cm−1 absorption is assigned to the symmetric B−O stretching vibration [νs(BO)]. Our calculations suggest that the ν(BO) mode is also manifested by the rather strong bands observed in the range of 1457−1370 cm−1. Santucci et al.79 and Faniran and Shurvell78 have assigned the band in the spectral range between 1103 and 1082 cm−1 to the

(boronphenyl)methylphosphonic acid analogues,36 fluorobenzene,70,71 fluorophenylboronic acids,66,67,72 benzaldehyde derivatives,73 and aromatic formyl compounds.74−76 Below, we briefly discuss only the main spectral features in the FT-Raman and FT-IR spectra presented. This discussion provides basic information about the molecular structure of the investigated compounds that is also useful in the analysis of the SERS spectra that are shown later in this paper. Only unambiguous assignment of the SERS bands allows us to suggest the possible mode of interaction between the investigated phenylboronic acid derivatives and the silver nanoparticle’s surface and changes in this mode upon substitution in the 2-, 3-, or 4-position by an electronwithdrawing substituent of the investigated phenylboronic acid analogues in the spectral range of 3650−300 cm−1. Phenyl Ring Vibrations. The assignment of the well-resolved and rather narrow 3075−3061 (ν2, according to Wilson’s nomenclature77), 1616−1593 (ν8a), 1579 −1564 (ν8b), 1217− 1195 (ν7a), 1172−1153 (ν9a), 1059−1027 (ν18a), 1012−998 (ν12), and ∼517 (ν6a) (see Supporting Information Tables S1, S2, and 3 for detailed band wavenumbers, PEDs, and full width at half-maximum) Raman bands (Figure 3) of the investigated phenylboronic acid analogues is straightforward. These bands are represented by the corresponding medium-weak absorptions at 3094−3053, 1617−1594, 1580−1562, 1219−1191, 1154−1149, 1045−1028, 1015−1009, and ∼526 cm−1 that overlap with intense bands due to the polar fragment oscillations of the investigated molecules. In the FT-Raman spectra (Figure 3), the noticeable spectral shift to higher wavenumbers for the ν8a mode of 2-F-PhB(OH)2 and 3-FPhB(OH)2 compared with that of phenyl reflects some redistribution of the π-electrons caused by the electron acceptor character of the fluorine atom being some distance from the boronic acid group (acceptor), whereas the downshift 5698



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Figure 4. The SERS spectra of the investigated phenylboronic acid analogues adsorbed onto colloidal silver nanoparticles surface in the spectral range of 1750−300 cm−1.

the fluorine and formyl substituents, especially at the 2position, which shift the wavenumber of these modes to a higher value. However, it has been proven that the introduction of the fluorine atom into the phenyl ring should not appreciably affect the strength of the intramolecular H bond.80 On the basis of DFT calculations, we propose that hydrogen bonds influence the 3375−3341 cm−1 band. There are no other ν(OH) absorption or Raman bands in the spectra in Supporting Information Figures S1 and 2. Several bands associated with the ρr(BO(H)H)bridge modes of the intermolecular hydrogen-bonds between the two monomers of the investigated phenylboronic acid derivatives (Figure 1) were calculated to be in the spectral range of 1117−1081 cm−1. In addition, the deformation and torsion vibrations of the HOBO···H fragment are noticed. The coupled δp(bridge) and γas/s(bridge) modes appear at 1304−1280, 1172−1170, 859−840, 812−762, 747−725, 699−622, 591−534, 506,−494, and 427− 416 cm −1 , whereas the symmetric and antisymmetric HOBO···H bridge deformation vibrations [δas/s(bridge)] are calculated to be at 1137−1126 and 1070−1067 cm−1. The δp(bridge) + γas/s(bridge) and the δas(bridge) vibrations also influence the bands at 1368−1361 cm−1 and 718 and 497− 471 cm−1, respectively. Substituent Vibrations. The presence of the formyl group in the formylphenylboronic acids is manifested by the relatively weak vibrational bands in the spectral range of 2920−2810 cm−1 [ν(CH)] (Figures 3 and Supporting Information) by the medium scattering at 1460−1430 cm−1 due to the C−H deformations [δ(CH)] (Figure 3) and by characteristic bands

ν(BC) mode in arylboronic acids and natural and deuterated diphenyl phenylboronate. Our calculations agree with this assignment. In addition, our calculations show that ν(CB) also contributes to the 718−694 cm−1 spectral features. The bending vibrations involving the B−C fragment oscillations [ρr(OB(CL)O) + ρr(CCL(B)C)ϕ] are manifested by the medium intensity bands at 463−428 cm−1, whereas the outof-plane deformations of the −OB(C)O− fragment [δoop(OB(CL)O)] influence the bands in the range of 634−622 cm−1 (Supporting Information Tables S1 and S2). The medium-weak absorptions at approximately 1195 and 1015−1009 cm−1 could be identified as the B−O−H deformations [δ(BOH)] (Supporting Information Figure S1). The −BO2 out-of-plane and in-plane deformations give rise to the absorptions at approximately 631 and 590 cm−1. The former band shows a medium relative intensity in the FT-IR spectra of 2-F-PhB(OH)2 and 2-CHO-PhB(OH)2, whereas it is absent for the remaining isomers. The FT-IR spectra (Supporting Information Figure S1) of all investigated phenylboronic acid analogues show a very broad O−H stretching vibration band centered in the spectral range of 3449−3371 cm−1, at a much higher wavenumber range than that for phenylboronic acid (3350 and 3279 cm−1).78 This complex spectral pattern is associated with the formation of intra− or intermolecular hydrogen-bonds among the −BOH, −CHO, and −F molecular fragments. In addition, for 2-FPhB(OH)2 and 2-CHO-PhB(OH)2, the ν(OH) spectral feature clearly exhibits additional maxima at 3341 and 3375 cm−1, respectively. This means that the ν(OH) modes are sensitive to 5699



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Table 3. Wavenumbers and Full Width at Half Maximum of Selected Bands in the FT-Raman and SERS Spectra 2-F-PhB(OH)2 Raman assignment ν 8a ν8b ν(CF) ν(BO) ν7a ν9a ν18a ν12 ν17b δoop(CH)ϕ δoop(CH)ϕ ν6b ν6a

ν8a ν8b ν(CF) ν(BO) ν18a ν12 ν17b δoop(CH)ϕ

4-F-PhB(OH)2

Raman

SERS

Raman

SERS

ν (cm−1)

fwhm (cm−1)

ν (cm−1)

fwhm (cm−1)

ν (cm−1)

fwhm (cm−1)

ν (cm−1)

fwhm (cm−1)

ν (cm−1)

fwhm (cm−1)

ν (cm−1)

fwhm (cm−1)

1616 1569

6 12

1602

18

1616

6

1602

20

1600

14

1600

20

1438 1398

30 51

1434

4

1442 1396

40 31 1217

6

1439 1390 1215

31 33 17

1197 1034 1001 933 808 785

20 15 10 38 12 47

1020 1012

13 8

1029 1001 934

12 10 9

809

6

629

4

640

12

1195 1034

10 9

521

1001

7 2-CHO-PhB(OH)2 Raman

assignment

3-F-PhB(OH)2 SERS

522

SERS

ν (cm−1)

fwhm (cm−1)

1593 1564 1429

6 6 6

1033

9

ν (cm−1)

5

1445

18

1032 1002

18 10

783

30

22 8 38

784

28

6 3-CHO-PhB(OH)2

Raman

fwhm (cm−1)

1034 1000 938

4-CHO-PhB(OH)2

SERS

Raman

SERS

ν (cm−1)

fwhm (cm−1)

ν (cm−1)

fwhm (cm−1)

ν (cm−1)

fwhm (cm−1)

1609 1579

6 9

1599

15

1399 1033 998 934

1611 1567 1407 1172

5 5 11 7

11 11 31

998

5

at approximately 1703 (weak) and 1670 cm−1 (strong) of the CO absorption moieties [ν(CO)] (Supporting Information Figure S1). The wavenumber of the 1670 cm−1 band is significantly lower in comparison with the standard wavenumber of ν(CO) for benzaldehyde.73 This reduction can be explained by the intra- and intermolecular H-bonding interactions that decrease the CO bond order.81 The intense absorption at ∼1364 cm−1 can also be assigned to the C−H bending oscillations of the formyl group [ρr(CCHOC(O)H)]. In addition, the ν(CCHO) and ρr(CCCHO(C)C)ϕ modes contribute to the bands at 1195−1193, 1187−1185, and 480− 466 cm−1. The bands originating from the C−F bond stretching vibrations [ν(CF)] are observed at 834−811 cm−1. The C−F out-of-plane bending mode [δoop(CLCF(F)C)ϕ] is identified as the band at ∼526 cm−1. This mode also influences the spectral features at 497−480 cm−1. SERS Studies. The addition of sample (10 μL of 10−4 M) to silver colloid (20 μL) changed the color of the colloid from yellow to red. This color change resulted from aggregation of the colloid particles, as evidenced by the excitation spectra (UV−vis) (the surface plasmon band of these nanoparticles at 397 nm)82 and the transmission electron microscopy (TEM) images at different magnifications (scale bars at 50 and 100 nm) of the clear silver colloid and sample/Ag nanoparticles system shown in Figure 2A and B, respectively. The unaggregated colloid primarily contained silver spheres with diameters of ∼20−25 nm (Figure 2B(a)). Aggregation did not produce fusion into larger particles, but rather, produces assemblies of apparently randomly adhering spheres, each of which with dimensions approximately equal to the original dimensions

ν(CC) ν(CC) ν(BO) ρip(CH) ν(C−C)

ν (cm−1)

fwhm (cm−1)

1595 1555 1399 1174 902

16 15 27 15 38

(Figure 2B(b)). These large assemblies are responsible for the color change (the metal particles become electronically coupled) that results from the broadness and red-shift to 565 nm of the plasmon resonance (Figure 2A). As mentioned earlier, at this concentration of sample, investigated molecules form a monolayer, at most; thus, this thin layer will result in a weak charge-transfer transition, which would be most likely overwhelmed by the tail of the UV−vis coagulation peak. That is why it is not clearly observed in this spectrum. Figure 4 presents the SERS spectra of the investigated phenylboronic acid analogues adsorbed onto the colloidal silver surface, whereas Table 3 summarizes the wavenumbers and fwhm’s of the modes enhanced in these spectra. These values are compared with those in the respective FT-Raman spectra. This is necessary to predict the phenyl ring orientation with respect to the silver colloidal substrate, according to so-called surface selection rules. These rules postulate that, in the case of the flat orientation of an aromatic molecule adsorbed onto a metal surface, its out-of-plane vibrational modes will be more enhanced when compared with its in-plane vibrational modes, and vice versa when it is adsorbed perpendicular to the surface.53,83,84 It is further seen that vibrations involving atoms that are close to the silver surface will be enhanced. The changes in the bandwidth and position of the ν12 mode were also used to determine the orientation of the aromatic ring with respect to the metal substrate.53,54 For example, the intense ν12 SERS signal is considered a sign of vertical ring orientation,55 whereas the significant downshift in wavenumber (by 10−30 cm−1) of ν12 (comparison between FT-Raman and SERS spectra), together with this band-broadening, correspond to the flat adsorption mode of the aromatic ring.54 5700



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Figure 5. Suggested adsorption geometry of the investigated molecules onto colloidal silver surface.

these δoop(CH)ϕ vibrations are enhanced in the 2-F-PhB(OH)2 and 3-F-PhB(OH)2 SERS spectra. For the former isomer, the 808 cm−1 band is the strongest in the spectrum, whereas the 785 cm−1 SERS signal is of medium relative intensity. In the case of the latter isomer, both of the δoop(CH)ϕ bands most likely overlap to produce one broad spectral feature centered at 784 cm−1 (fwhm = 47 cm−1). These observations suggest a rather tilted orientation of the phenyl ring with respect to the silver substrate surface for the 2- and 3- isomers. This statement could be supported by the relatively small changes in the spectral position and width of the ν12 SERS band (Figure 4) for 2-F-PhB(OH)2 and 3-F-PhB(OH)2. Similar to the 2-FPhB(OH)2 Raman spectrum (Figure 3), in the SERS spectrum (Figure 4) of this acid, the ν12 mode is slightly enhanced, whereas the SERS relative intensity of ν12 for 3-F-PhB(OH)2 decreases in comparison to its Raman intensity. These observations indicate that the phenyl ring for the 2-FPhB(OH)2 and 3-F-PhB(OH)2 isomers are in close proximity to the silver nanoparticle’s surface; however, that of 2-FPhB(OH)2 is located closer to the surface than that of 3-FPhB(OH)2. In addition, the weak Raman band at ∼1397 (ν(B−O)) cm−1 in the SERS spectra of all of the fluorophenylboronic acids investigated is broadened (Table 3) and strengthened (I2− > I3− > I4−). Other bands influenced by δip(BO2) (∼575 cm−1) and ν(B−O) + ν(C−B) (∼785 cm−1) are also enhanced in the 2-FPhB(OH)2 and 3-F-PhB(OH)2 SERS spectra, implying that the

The SERS spectra of the fluorophenyl boronic acid analogues (Figure 4) show some important differences if compared with Figure 3. For example, the X-sensitive phenyl ring 1616 cm−1 band broadens, decreases in relative intensity, and red-shifts to 1602 cm−1 (Table 3) for 2-F-PhB(OH)2 and 3-F-PhB(OH)2, whereas the equivalent band for 4-F-PhB(OH)2 does not move (1600 cm−1). However, the 1663 cm−1 SERS signal due to the ν(CC) vibrations also appears for the latter isomer. This implies that the delocalized π-electron system of the 4-FPhB(OH)2 is perturbed. This effect could be due to the combination of the two electron-withdrawing groups, −F and −B(OH)2, located symmetrically on opposite sites of the phenyl ring at the 1- and 4- positions and the direct interaction of the positively charged silver nanoparticles, which enhances the withdrawing abilities of F, along with the vertically oriented fluorophenyl boronic acid that is directed to the surface through the F atom. This assumed orientation of the phenyl ring of 4-F-PhB(OH)2 with respect the silver surface could be confirmed by the 1001 cm−1 SERS spectral feature, which shifts in wavenumber by −11 cm−1 and practically does not change in relative intensity and fwhm (Table 2), in comparison to the Raman spectrum (Figure 3). Additional arguments for this proposed orientation of the 4fluorophenyl ring are the appearance of the prominent 1439 cm−1 band, which involves the ν(C−F) vibrations, and the lack of out-of-plane C−H ring deformation modes [δoop(CH)ϕ] (approximately 808 and 785 cm−1). Unlike for 4-F-PhB(OH)2, 5701



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boronic acid moiety for 4-F-PhB(OH)2 is moved away from the silver surface, which is in agreement with the proposed interaction via the F atom. On the other hand, the −B(OH)2 group of 2-F-PhB(OH)2 and 3-F-PhB(OH)2 is in contact with the silver surface because of the tilted orientation of the ring. It is noteworthy that benzaldehyde in the silver sol does not produce a SERS spectrum. The lack of benzaldehyde SERS bands has been explained by the absence of a surface plasmon resonance band from the mixture of benzaldehyde and the colloidal silver solution.85 In the case of the benzaldehyde substituted by a boronic acid group in the 2-, 3-, and 4positions that we investigated, SERS spectra were obtained (Figure 4). Interestingly, the 4-CHO-PhB(OH)2 SERS pattern differs markedly from that of 2-CHO-PhB(OH)2 and 3-CHOPhB(OH)2. According to the time-dependent imine formation experiment,86 the very weak 1681 cm−1 (ν(CO)), weak 1637 cm−1 (ν(CC)), and strong 1595 cm−1 (ν(CC)) SERS signals observed in the 4-CHO-PhB(OH)2 spectrum are signs of double bond formation between the phenylboronic acid and a formyl group (CC(H)−OH). Thus, dearomatization of the phenyl ring occurs upon adsorption onto the colloidal silver surface. The lack of the characteristic ring breathing mode (ν12) and the appearance of the ν(CC) (1555 cm−1), in-plane C− H bending [ρip(CH)] (1174 cm−1), and ν(C−C) (902 cm−1) modes confirm the above statement. In addition, the medium enhancement of the 1399 and 500 cm−1 [δ(OBO) + δ(CBO)] SERS signals suggest that the boronic acid group participates in the adsorption process of 4CHO-PhB(OH)2 onto the silver nanoparticle substrate. The former band decreases in relative intensity in going from 4CHO-PhB(OH)2 to 3-CHO-PhB(OH)2 and then to 2-CHOPhB(OH)2, whereas the latter band is not enhanced for the 2and 3- isomers. Instead, the 576 cm−1 spectral feature appears for 2-CHO-PhB(OH)2. Therefore, it could be stated that the −B(OH)2 moiety, being in some proximity to the silver surface, assists in the interaction with this surface. However, on the basis of the characteristic phenyl ring profile, it seems that the phenyl ring of 2-CHO-PhB(OH)2 and 3-CHO-PhB(OH)2 adsorbs on this substrate, albeit in a different manner. The strong 998 cm−1 and medium-weak 1599 and 1033 cm−1 spectral features in the 3-CHO-PhB(OH)2 SERS spectrum indicate, rather, the perpendicular orientation of the phenyl ring. The negligible enhancement of the ν8a and ν12 modes and the medium and strong relative intensities of the ν18a and δoop(CH)ϕ modes, respectively, imply a more-or-less horizontal arrangement of the phenyl ring of 2-CHO-PhB(OH)2. The suggested adsorption geometry of the investigated molecules onto the colloidal silver surface is shown in Figure 5.

(ii) The cis−trans conformation for the dimer of all of the investigated compounds has the lowest energy and, thus, is the most stable. (iii) The phenyl ring of 4-F-PhB(OH)2 adopts a vertical orientation with respect to the colloidal silver surface, whereas the phenyl ring of the 2- and 3- isomers, being in close proximity to the silver surface, accepts a rather tilted orientation to this surface. However, it seems that the distance between the ring and surface is longer for the 3- isomer compared with the 2- isomer. (iv) Both the 2- and 3-CHO-PhB(OH)2 isomers adsorb onto the colloidal silver surface through the phenyl ring. The aromatic ring of the 2- isomer is lying on the silver nanoparticle’s surface, whereas the arrangement for the 3- isomer is perpendicular with regard to this substrate. Surprisingly, the phenyl ring of 4-CHO-PhB(OH)2 undergoes dearomatization during the adsorption process. (v) The −B(OH)2 group of the 2- and 3-F-PhB(OH)2 isomers is in contact with the silver nanoparticles and away from this surface for 4-F-PhB(OH)2. (vi) The −B(OH)2 group of all of the fluorophenyl acid analogues interacts with the silver nanoparticles, whereas that of 2-CHO-PhB(OH)2 and 3-CHO-PhB(OH)2 is slightly removed from the substrate surface. (vii) The direct interaction of 4-F-PhB(OH)2 with the silver nanoparticles, via the fluorine atom, is proposed.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental and theoretical FT-IR spectra of the investigated phenylboronic acid analogues in the spectral range of 3650− 400 cm−1. Calculated wavenumbers and potential energy distribution for the FT-Raman and FT-IR spectra of the investigated fluorophenylboronic acid analogues and formylphenylboronic acid analogues. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +48-12-663-2077. Fax: +48-12-634-0515. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Center of the Ministry of Science and Higher Education (Grant No. N N204 544339 to E.P. and No. 2011/03/N/ST4/00777 to N.P. and E.P.). The authors thanks Professor Andrzej Sporzyński from the Warsaw University of Technology for the donation of samples of phenylboronic acid analogues investigated in this work. The authors are grateful to the Academic Computer Center “Cyfronet” in Krakow for the opportunity to perform calculations and to Dr. M. Andrzejak (Jagiellonian University) for access to the visualization script. Y. Kim gratefully acknowledges HUFS for financial support.

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CONCLUSIONS The molecular structure and adsorption mode onto the silver colloidal surface of the two groups of three isomers (2-, 3-, and 4-) of fluoro- and formyl-substituted phenylboronic acids were investigated by experimental (FT-Raman, FT-IR, and SERS) and theoretical (DFT, B3LYP/6-311++G(d,p)) methods. These investigations imply the following: (i) The most stable structure of the 2-, 3-, and 4-fluoro- and formylphenylboronic acids is a cyclic dimer formed by a pair of intermolecular hydrogen bonds between the boron hydroxyl groups of two monomers. Thus, all presented theoretical spectra are calculated for dimers.

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