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Experimental and Computational Investigation of Salophen−Zn Gas Phase Complexes with Cations: A Source of Possible Interference in Anionic Recognition Alessandra Ciavardini,†,# Simonetta Fornarini,† Antonella Dalla Cort,‡ Susanna Piccirillo,§ Debora Scuderi,∥ and Enrico Bodo*,⊥ †

Dipartimento di Chimica e Tecnologie del Farmaco, Università di Roma “La Sapienza”, Rome, Italy Dipartimento di Chimica and IMC−CNR Sezione Meccanismi di Reazione, Università La Sapienza, 00185 Roma, Italy § Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma “Tor Vergata”, Rome, Italy ∥ Laboratoire de Chimie Physique, UMR 8000, Université Paris Sud, 91405 Orsay Cedex, France ⊥ Dipartimento di Chimica, Università La Sapienza, 00185 Roma, Italy ‡

S Supporting Information *

ABSTRACT: We explore the possibility that protonated molecular ions might be an unexpected source of interference in the recognition process of anions and neutral species by Zn−salophen receptors. Zn−salophen complexes are known to bind anions and neutral molecules in solution. We present here evidence (from computational work and IRMPD spectroscopy) that these complexes can also be the binding site for protonated pyridine or quinuclidine. The resulting binding pattern does not involve the Zn ion, but one of the oxygen atoms directly attached to it. The resulting complex therefore turns out to have a positive charge adjacent to the Zn−salophen binding site. This finding seems to point to the existence of an interfering factor in the quantification of the experimental data about the association constant.

1. INTRODUCTION The low toxicity and biocompatibility of Zn2+ ions with respect to other transition metals have made its complexes with organic ligands extremely useful in various areas of medicine.1 In addition, Zn is the second (after iron) abundant transition metal in living organisms, where it is crucial to stabilize the structure (and the function) of biomolecules including several enzymes.1,2 From a molecular point of view, the determination of the chemical and physical properties of Zn complexes aims at the description of the bonding patterns whose understanding, in turn, might help to improve the procedures for the detection, extraction, and transport of biologically relevant molecules.3,4 The Zn complexes with Schiff base ligands such as N,N′bis(salicylidene)1,2-phenylenediamine), known as salophen, have shown an interesting affinity toward different biological targets, anions and neutral molecules.5−8 These ligands are synthesized9 by condensation of o-phenylendiamine with 2 equiv of salicylaldehyde and form stable complexes with many transition and main group metals. The advantage of these ligands relies on their chemical versatility. Starting materials, i.e., phenylenediamine and salicylaldehyde, are easily functionalized, providing access to a large variety of structures with subtle variations in their steric and electronic properties10−17 © XXXX American Chemical Society

that can be tuned in order to obtain tailor-made ligands for the recognition of specific task molecules. While the typical coordination number of Zn2+ in solution is four, when coordinated by a salophen unit, it turns out to form an additional bond with a fifth donor atom such as, for instance, an oxygen atom of phosphate anions or other electron-rich atoms such as halides. Recently, we have engaged ourselves in the study of biological phosphate ion recognition and halide anions18,19 by metal−salophen complexes. It is clear from these works, that the successful design of receptors based on Zn−salophen complexes depends upon the understanding of the specific metal−ligand interactions. In order to obtain insights into the bonding nature of the complexes we have adopted an approach that is based on the investigation of tailor-made complexes in the gas phase, avoiding in this way the interference deriving from the environment. In our approach, mass-selected spectroscopic techniques and quantum chemical modeling are used to provide an accurate picture of the forces at play in the binding processes that, in turn, should be able to characterize Received: June 14, 2017 Revised: August 29, 2017 Published: August 30, 2017 A

DOI: 10.1021/acs.jpca.7b05825 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

CH2Cl2 solutions of 1 or 2 were added to methanol solutions of pyridine or quinuclidine in order to obtain a final concentration of 10−4 mol/L of each component. The solutions were acidified with a few drops of formic acid and introduced into the electrospray ionization (ESI) source by means of direct infusion with a syringe pump at a flow rate of 200 μL/h. The ESI conditions used were as follows: a spray voltage equal to −4500 V and capillary temperature at 423 K. The cationic complexes delivered in the gas phase by ESI were trapped in a Paul ion trap and probed by IRMPD spectroscopy in the 950−1800 cm−1 spectral range. All IRMPD experiments have been performed using a modified quadrupole Paul ion trap (Esquire 3000+, Bruker) coupled to the tunable infrared radiation produced by the free electron laser (FEL) at the Centre Laser Infrarouge d’Orsay (CLIO, Orsay, France).30,31 This facility is based on a 10−44 MeV electron accelerator and provides radiation over the infrared wavelength range of 3−120 μm with a relative bandwidth of 0.5% (full-width at half-maximum). The coupling of the Paul ion trap and FEL has been described in detail previously.32 The FEL wavelength was scanned by 5 cm−1 steps. The laser power was around 1.2 W, about constant in the 950−1800 cm−1 range. Ions were irradiated for typically 500 ms. Spectra were acquired also by reducing the laser intensity with DB3. Considering the complex multiphoton character of the IRMPD process, no fluence correction has been applied to the measured data. The IRMPD efficiency on a particular mode is affected by several factors: the amount of energy deposited by the laser pulse, the rate of intramolecular vibrational relaxation, the dissociation rates into the various channels and the presence of nearby bands.33,34 The reported spectra are obtained by monitoring the abundance of parent and fragment ions versus the IR frequency. If F is the sum of the abundances of the fragment ions produced by IRMPD and P the one of the parent ion, our IRMPD spectra correspond to the plot of −ln(P/(F + P)) as a function of the IR radiation wavenumber. 2.2. Computational Section. For each of the two salophen structures in Scheme 1, we have calculated various possible geometries of the resulting complex with H+, pyridine−H+ (pyrH+), and quinuclidine−H+ (quinH+). Geometric optimization has been carried out using B3LYP with a 6311+G(d,p) basis. The final minima were fully characterized by

the dynamics of conceivable bonding patterns. In particular, the infrared multiple photon dissociation (IRMPD) spectroscopy has been successfully employed to characterize the structure of gaseous metal complexes20−28 including salophens.18,19 Most of the recent literature focuses on the role of Zn− salophen complexes as potential receptors for anionic and neutral molecules, while very little is known on the role played by Zn−salophens as binding agents for cations such as molecules containing protonated aza groups. This is an important issue since the possibility for cations to interact with the ligand framework where highly electronegative groups are present, e.g., O and N, might result in a variation of the complex affinity toward anionic and neutral targets causing an unexpected and poorly quantifiable interference effect. As we shall see in the following, the binding motif to the Zn− salophen complexes that we have found involves the salophen oxygen atom that is adjacent to the Zn site. This bonding pattern has the evident effect of altering the affinity of the receptors toward the target due to the acquisition of an additional positive charge. To shed light on these points, we investigated the interactions eventually present between zinc-salophen receptors 1 and 2, Scheme 1, and protonated pyridine (pyrH+) and quinuclidine (quinH+) bases using IRMPD spectroscopy of trapped ions and quantum calculations. Scheme 1

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Experimental Section. Zn−salophen complexes 1 and 2 were available from a previous investigation.29 Concentrated

Figure 1. Protonated structures of 1 and 2. The additional proton is shown in purple color. B


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complete assignment of the bands will be provided for the spectra coming from the adducts with the protonated bases in the following sections. 3.2. The [Pyr]H+ Complex with Salophen−Zn. The geometry of the complex with protonated pyridine follows from the structural evidence of the previous section. The protonated pyridine does not occupy the apical position of the Zn atom that is a typical geometry of neutral molecular complexes. Instead, the calculated structures show the pyridine moiety directly attached to one of the oxygen atoms of the salophen unit via the proton that acts as a bridge between the pyridine nitrogen and the salophen basic oxygen atoms. The two structures 1-pyrH+ and 2-pyrH+ are reported in Figure 3. They are characterized by a hydrogen bonding interaction linking the proton on the pyridine aza group to one of the two oxygen atoms with H−O distances of 1.60 and 1.59 Å respectively (1.56 and 1.55 Å when including empirical corrections for the dispersion interactions). The pyridine plane is approximately perpendicular with respect to the plane defined by zinc and the two oxygen atoms with an angle that, in both compounds is about 100 degrees. The adiabatic interaction energies (corrected for the zero point energies) between the Zn−salophen and the protonated base are 25.2 kcal/mol for compound 1 and 25.3 kcal/mol for compound 2 (35.2 and 35.6 kcal/mol respectively when dispersion energies are taken onto account). Despite the wellknown preference for metal-salophen complexes to bind anions, the cationic interaction energies that we have found are large enough to allow the binding of the cations to the salophen units and are comparable in magnitude to the typical binding energies of anions such as phosphate18 (about 30−40 kcal/mol). This result may therefore lead to the consideration that in solution the protonated base can easily attach to the salophen receptor and induce a positively charged environment near its binding site that might (unpredictably) alter the Lewis acidic character of zinc and consequently the resulting association constant with anions and neutral targets. In addition, by looking at the structures in Figure 1, one clearly sees that, once the protonated pyridine attaches itself to the salophen, the typical apical binding site of Zn for anionic or neutral molecules is sterically hindered. Therefore, the protonated base, once attached, not only might alter the Zn affinity for specific ligands but also could act as a competitor for the binding site since the simultaneous presence of the protonated base and of the ligand is sterically unfavorable. The computed and measured vibrational spectra for 1-pyrH+ and 2-pyrH+ are reported in Figure 4. As we see, the agreement between the calculated absorption and the IRMPD measurements is very good over the reported frequency region. As in the previous case of protonated salophen−Zn, most of the absorption lines in the fingerprint region are due to complex collective motions of the salophen unit. We have provided a detailed assignment of the bands in the Supporting Information, and here, we limit ourselves to few general comments. The vibrational normal modes are quite delocalized over the aromatic structure of the salophen unit. The presence of the protonated pyridine, however, breaks the symmetry of the salophen−Zn complex and some of the vibrational modes localize on either the side of the salophen moiety near the protonation site or on the opposite one. In order to allow for a simple interpretation of the resulting vibrational spectra, we have divided them into seven groups of absorption bands. The

a vibrational analysis and a set of harmonic vibrational spectra has been obtained. As usual, to account for anharmonicity effects, the B3LYP predicted spectra were scaled by 0.98.35 Although in the presence of ions, dispersion energy generally has a minor impact upon the final structures, we have repeated the optimization computations using the B3LYP functional empirically corrected using Grimme’s D3 method.36 The final geometries turned out to be very similar (with RMSD deviations between 0.3 and 0.6 Å, see the Supporting Information, Table S5). The absorption spectra calculated with and without dispersion corrections are substantially superimposable (see the Supporting Information, Figure S1) except for the band relative to the N−H+ stretching of the protonated bases which is red-shifted for less than 1% in the D3-B3LYP spectra. This band falls around 2600 cm−1 that is well outside the range of our experimental data. Natural bond orbital (NBO) analysis37 for the 1-quinH+ and 1-pyriH+ complexes has been performed at the RHF/6-31G** level where we have chosen a compact basis to help the interpretation of the results and the HF scheme to perform the second order perturbative analysis of the delocalization energy. The calculations were performed using Gaussian09.38

3. RESULTS AND DISCUSSION 3.1. Protonation Site. As a first step in our study we have interpreted the spectra of the bare salophen−Zn protonated molecules. We have explored three different protonation geometries for both compounds: protonation at the oxygen and nitrogen atoms of the salophen system and at the apical position of the Zn ion. We have found three stable minima, but only the oxygen atom resulted a viable protonation site since the other positions led to species that were more than 30−60 kcal/mol higher in energy. We report in Figure 1 the six protonated structures along with their relative energy. The protonation at the nitrogen atom did produce a protonated salophen specie, but the final structure showed that the proton binds to the adjacent carbon. The gas-phase proton affinity of the three possible protonation sites for each compound is reported in Table 1 where the high proton affinity of the salophen oxygen atom is indicative of a site whose basicity is comparable to that of a nitrogen base such as pyridine. Table 1. Gas-Phase Proton Affinity (kcal/mol) of the Three Different Protonation Sites As Identified by ab Initio Computations for the Six Structures Reported in Figure 1 protonation site

1

2

O C Zn

229.2 194.2 168.6

230.0 192.3 167.8

As further evidence that the protonation takes place on the oxygen atom of the salophen units, we show in Figure 2 the calculated and measured IR spectra for the two lowest energy structures of Figure 1. The spectra match reasonably well with each other. The calculated spectra for the other structures significantly disagree from the experimental pattern (they are reported in Figure S2 and S3 in the Supporting Information). The few strong absorption bands are due to complicated motions of the salophen aromatic system. Those at high frequencies are mainly composed by C−C and C−N stretching coupled with C−H in-plane bending. The absorptions at lower frequencies are mainly due to the in-plane C−H motions. A C


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Figure 2. Calculated (B3LYP/6-311+G(d,p)) and measured vibrational spectra for the most stable protonated structure of 1 and 2 compounds.

data, the spectra of 1-pyrH+ and of 2-pyrH+ are very similar, the only differences being due to the more complicated vibrational patterns induced by the additional back ring. The differences are, in fact, concentrated in bands C where the C−C stretching motions of the back rings appear and in band G where collective motions of the aromatic skeleton are dominant. In order to provide evidence pointing to the geometry found through computations we have reported in Figure 5 (only for 1-pyrH+) the calculated spectra of the complex, of the isolated protonated pyridine and of the isolated salophen−Zn. As we can see the IR spectrum of protonated pyridine has a very small intensity and turns out to be irrelevant for the discussion since the oscillator strength of its normal modes remains low also in the complex. The spectra of the bare salophen−Zn and of the 1-pyrH+ complexes are very similar in the absorption patterns (blue and red lines). We spot however several differences that are characteristic of the 1-pyrH+ complex and that, in our opinion, validate the proposed structure shown above. The first evidence is a red-shift of the band at 1420 cm−1 which turns out in agreement with the experimental pattern where the latter has coalesced with the other band at 1390 cm−1 (group D) that, in turn, is blue-shifted in the protonated complex with respect to the bare Zn−salophen. A second, more evident difference is the appearance of three bands in the region between 1270 and 1330 cm−1 (group E, indicated by a red arrow in Figure 5) that are due to collective motions of the aromatic system that involve a certain degree of CO stretching motions. These motions, in the bare salophen−Zn, have a lower oscillator strength and fall at 1340 cm−1 (blue arrow). In addition, while in the bare salophen−Zn these absorptions fall almost at the same frequency due to the symmetry of the molecule, they turn out to be largely split in the 1-pyrH+ complex because one of the oxygen atoms is now involved in the H-bond with pyridine. The interaction with pyridine, in addition, alters significantly the dipole moment of the system leading to an enhanced absorption along these modes with respect to the bare salophen−Zn. As we can see in Figure 5, the experimental spectrum has a strong multiplet feature around 1320 cm−1 (black arrow, group E) that is a clear indicator of the formation

Figure 3. Protonated pyridine complexes with Zn−salophen. 1-pyrH+ on the left and 2-pyrH+ on the right. For clarity, we have reported the complexes from two different points of view (upper and lower panels).

first two, labeled A and B, are mainly composed by stretching motions of the C−C bonds on the two side rings coupled with C−N stretching motions in the salophen unit. The less intense group C contains localized motions of the i-Pr groups (mainly CH3 scissoring) and C−C stretching motions of the back aromatic ring (the one opposite to the Zn ion). In group D, we find the N−C−H bending motions of the salophen unit. Group E is particularly important (see below) and represents a complex group of absorption bands that include to various extent the CO stretching motions of the salophen unit. Within group E, the lower energy vibrations are coupled with the stretching of the CO bond which is protonated while the ones at higher energy are coupled with the nonprotonated C O bond, a situation which is compatible with a weakening of the CO bond upon oxygen protonation. Various collective motions compose the F and G group and only few assignments have been attempted. The assignment of all the intense absorption bands is reported in the Supporting Information in section S1 and Tables S1 and S2. Judging from the theoretical D


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Figure 4. Vibrational spectra of the 1 and 2 protonated pyridine complex. The experimental data are reported in arbitrary units (ion yield) and have been vertically shifted and scaled for visualization purposes. The theoretical data are at the B3LYP/6-311+G(d,p) level.

Figure 5. Comparison of various theoretical spectra along with the measured spectra on complex 1-pyrH+. Green line: isolated protonated pyridine. Blue line: neutral salophen−Zn. Red line: complex 1-pyrH+.

of the salophen oxygen atoms. The proton-oxygen distance is 1.68 Å for both compounds (1.61 and 1.60 Å when dispersion correction are included), slightly larger than in the pyridine complexes. The vertical axis of quinuclidine in both compounds is substantially perpendicular to the plane identified by the Zn and the oxygen atoms. The adiabatic interaction energies of the complex 1-quinH+ and 2-quinH+ are 21.1 and 21.5 kcal/mol respectively (34.9 and 35.7 kcal/mol when including dispersion energies). Again, in these complexes the binding energy is high enough to compete with the typical binding energies of anions and neutral species to the Zn ion. In the quinuclidine case, the steric hindrance is more pronounced with respect to the pyridine case above, given the larger size of the protonated base. As we see in Figure 6, the steric dimension of the

of the H-bridged complex and validates the proposed structure of Figure 3. The same discussion can be done for compound 2-pyrH+ where we have found the same behavior albeit it turned out much less evident because the bands between 1200 and 1400 cm−1 are much less intense (see detailed assignment in the Supporting Information). We are, however, even in this case very confident of the assignment of the structure because of the very good agreement of all major bands as shown in Figure 4. 3.3. The Quinuclidine−H+ Complex with Salophen− Zn. The structures of the 1-quinH+ and 2-quinH+ that we have found through computations have similar features to the pyridine complexes and are shown in Figure 6. Also in this case, a proton-mediated bridge links the basic nitrogen atom to one E


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strongly coupled with C−C stretching on the side rings. The agreement between these experimental bands and the theoretical absorption lines (see Table S3 in the Supporting Information), can be considered a clear mark of the formation of the complex reported in Figure 6. A second difference with respect to the pyridine case, is that the C−O stretching motions absorptions appear between band E and F as a double structure in the theoretical calculations, but these vibrations do not seem to provide fragmentation channels efficient enough to contribute significantly to the experimental ion yield, although two very weak signals can be traced in that region (see Tables S3 and S4 in the Supporting Information). This leaves little space for the validation of the proposed structures of Figure 6. It is possible, however, as we did for 1-pyrH+, to trace a few bands that arise in the protonated complex spectra that are not present in the bare salophen−Zn system. In particular the E groups of signals slightly above 1300 cm−1, that are due to complex patterns of motions (C−C stretching and C−H in plane bending), in the isolated salophen−Zn, are blue-shifted and are much less intense. We believe (as for pyridine in the previous section) that these bands, and their agreement both in position and intensity with the measured spectra, can be considered a clear signature of the structures that we have identified using the ab initio methods. 3.4. Electronic Structure of the Bound Complexes and Their Bonding Features. In order to qualitatively characterize the intramolecular features in the structures, we have performed a natural bond orbital (NBO) analysis for the 1quinH+ and 1-pyriH+ complexes. The localization of the molecular orbitals has two consequences: (i) the shape of the resulting orbitals is simpler to interpret because of the maximization of the directional overlap of the monoelectronic functions along the chemical bonds and (ii) the Fock operator representation in the basis of the localized orbitals is nondiagonal, so that the off-diagonal elements of the operator can be used to trace the “delocalization energy” between the two orbitals involved which can be interpreted as a measure of

Figure 6. Optimized geometries of 1-quinH+ (left) and 2-quinH+ (right). Each compound is shown from two different points of view (upper and lower panels).

protonated base is such that it substantially occupies the binding site of the Zn. We expect that, besides the effects due to charge and polarization, the presence of quinuclidine or another bulky protonated species could reduce substantially the bonding ability of Zn−salophen toward the target species. The recorded and calculated vibrational spectra are reported in Figure 7. The agreement is very good although most of the absorption bands, as in the case of pyridine, have to be assigned to complex, collective motions. As in the previous case, we have provided the assignment of the major bands in the spectra in the Supporting Information. As shown in Figure 7, also in the case of quinuclidine, we can divide the spectra into seven groups of bands. The interpretation of the spectra is similar to what we have seen for pyridine albeit few differences can be traced: the band B can be assigned to bending motions of the N(quin)−H+ bond although these motions turn out to be

Figure 7. Vibrational spectra of the 1 and 2 protonated quinuclidine complex vibrational spectra. The experimental data are reported in arbitrary units (ion yield) and have been vertically shifted and scaled for visualization purposes. The theoretical data are at the B3LYP/6-311+G(d,p) level. F


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Table 2. Natural Charge of the Atoms Involved in the N−H−O Hydrogen Bond and Natural Composition of the Orbitals Involved and “Delocalization Energy” N

H

O

natural charge 1-pyrH+ 2-pyrH+ 1-quinH+ 2-quinH+

−0.57 −0.57 −0.54 −0.54

1-pyrH+

0.53 0.53 0.53 0.53 natural orbital (type, occupation, % character) LP, 1.60, s (18%) and p (81%) LP*, 0.45, s (100%)

2-pyrH+

LP, 1.60, s (19%) and p (81%)

LP*, 0.46, s (100%)

1-quinH+

LP, 1.60, s (13%) and p (87%)

LP*, 0.46, s (100%)

2-quinH+

LP, 1.60, s (13%) and p (87%)

LP*, 0.46, s (100%)

1-pyrH+ 2-pyrH+ 1-quinH+ 2-quinH+

74.0 75.4 54.8 55.3

−0.97 −0.97 −0.97 −0.97 LP, 1.94, s (12%) and p(88%) LP, 1.88, s (49%) and p (50%) LP, 1.88, s (1%) and p (99%) LP, 1.94, s (12%) and p (88%) LP, 1.88, s (50%) and p (50%) LP, 1.81, s (1%) and p (99%) LP, 1.93, s (11%) and p (89%) LP, 1.88, s (51%) and p (49%) LP, 1.81, p (100%) LP, 1.93, s (11%) and p (89%) LP, 1.89, s (51%) and p (49%) LP, 1.81, p (100%)

O(LP) → H(s) second order perturbation energy kcal kcal kcal kcal

in blue/red color code while the empty orbital on H is in green/orange color. The bonding between the protonated base and the salophen is substantially mediated by the interaction between the three oxygen lone pairs and the partially empty s orbital of the proton. The latter is a very distorted orbital (1s + 2s) that has a nodal surface in between the N−H bond and acts as an N−H antibonding orbital. The proton carries half a positive charge, which turns out to be the same in all compounds. The oxygen atom, on the other hand remains strongly negatively charged as expected since it forms an ionic bond with the Zn2+ cation. Summing up the three delocalization energies coming from the O(LP) → H(s) coupling, we obtain a measurement of the bonding energy between the protonated base and the salophen unit that is reported at the bottom of Table 2. The delocalization energy turns out to be higher for the pyrH+ compounds where, for steric reasons, the distance between the N and O atoms is smaller (2.66 Å for 1-pyrH+ vs 2.72 Å for 1quinH+).

the additional interaction energy with respect to the Lewis structure. We have found that the interaction of the salophen complex with the protonated bases can be rationalized in terms of a recurring bonding scheme that involves an almost filled lone pair on the protonated nitrogen atom of the base (which is roughly 80% p in character), an half empty s orbital on the proton and the three filled lone pairs of the bound oxygen in the salophen unit (which are two sp hybrids and one pure p orbital). The detailed outcomes of the NBO analysis are reported in Table 2, where the natural charges pertaining to the three involved atoms and their relevant natural orbitals composition are listed. Although the NBO analysis does not identify a covalent bond between the base’s nitrogen and the proton, the second order perturbative energy for the N(2p) → H(1s) delocalization is extremely large therefore pointing to a strong ionic bond between the two atoms. The bonding pattern is very similar along the series of our compounds and we report in Figure 8 only the 1-pyrH+ and 1-quinH+ cases. The filled orbitals (the N(2p) and one of the 2p lone pairs of oxygen) are

4. CONCLUSIONS The binding of protonated pyridine or quinuclidine to two prototypical Zn−salophen complexes has been investigated using IRMPD spectroscopy and ab initio calculations. We have found that the minimum energy structures, as confirmed by comparison between experimental and theoretical vibrational spectra, have the proton attached to one of the salophen oxygen atoms acting as a bridge between the base and the organic structure. We have been able to identify clear spectral features in the protonated base−Zn−salophen complex, which do not appear in the spectra of the separated species. The appearance of these theoretically predicted features in the experimental spectra, together with an overall good agreement between the experimental and theoretical spectra provides a strong evidence for the validation of the geometries that we

Figure 8. Shape of the natural orbitals centered on the oxygen and the H involved in the H-bond between the protonated base and the salophen unit. Compound 1-pyrH+ on the left; compound 1-quinH+ on the right. G



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have attributed to the complexes. In these complexes, the protonated base binds to the salophen moiety through an Hbond bridge. The interaction energy (adiabatically calculated) is high enough to consider the possibility that these kinds of cations, with an affinity for the oxygen atoms, might substantially interfere with anionic/neutral targets recognition. The presence of this cationic ligand near the Zn atom of the complex may alter significantly (though not necessarily lower, but rather possibly increase) its ability to coordinate anions or neutral molecules. The concluding remark that we draw from the work presented above is that the Zn−salophen, through the ligand unit, can directly bind protic cations with a high affinity for oxygen atoms. The binding pattern is characterized by a noncovalent interaction between the salophen oxygen and the protonated nitrogen atom that can be rationalized in terms of a donation of electrons from the oxygen lone pairs to the half empty s orbital on the proton. With respect to the nonbonded Lewis structure the delocalization energy is very high around 75 and 55 kcal/mol for pyridine and quinuclidine, respectively.

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REFERENCES

(1) Vahrenkamp, H. Why does nature use zinc − a personal view. Dalton Trans. 2007, 4751−4759. (2) Klug, A. The discovery of zinc fingers and their development for practical applications in gene regulation and genome manipulation. Q. Rev. Biophys. 2010, 43, 1−21. (3) Terenzi, A.; Lauria, A.; Almerico, A. M.; Barone, G. Zinc complexes as fluorescent chemosensors for nucleic acids: new perspectives for a “boring” Element. Dalton Trans. 2015, 44, 3527− 3535. (4) Pace, N. J.; Weerapana, E. Zinc-binding cysteines: diverse functions and structural motifs. Biomolecules 2014, 4, 419−434. (5) Terenzi, A.; Bonsignore, R.; Spinello, A.; Gentile, C.; Martorana, A.; Ducani, C.; Högberg, B.; Almerico, A. M.; Lauria, A.; Barone, G. Selective G-quadruplex stabilizers: Schiff-base metal complexes with anticancer activity. RSC Adv. 2014, 4, 33245−33256. (6) Jing, J.; Cai, Y.-B.; Tang, J.; Chen, J.-J.; Zhang, J.-L.; Xie, D. Construction of an orthogonal ZnSalen/Salophen library as a colour palette for one- and two-photon live cell imaging. Chem. Sci. 2014, 5, 2318−2327. (7) Gasbarri, C.; Angelini, G.; Fontana, A.; De Maria, P.; Siani, G.; Giannicchi, I.; Dalla Cort, A. Kinetics of demetallation of a zinc− salophen complex into liposomes. Biochim. Biophys. Acta, Biomembr. 2012, 1818, 747−752. (8) Cano, M.; Rodriguez, L.; Lima, J. C.; Pina, F.; Dalla Cort, A.; Pasquini, C.; Schiaffino, L. Specific Supramolecular Interactions between Zn2+-Salophen complexes and biologically relevant anions. Inorg. Chem. 2009, 48, 6229−6235. (9) Vigato, P. A.; Tamburini, S. The challenge of cyclic and acyclic schiff bases and related derivatives. Coord. Chem. Rev. 2004, 248, 1717. (10) Ma, C. T. L.; MacLachlan, M. J. Supramolecular assembly and coordination-assisted deaggregation of multimetallic macrocycles. Angew. Chem., Int. Ed. 2005, 44, 4178. (11) Shen, Y. M.; Duan, W. L.; Shi, M. Chemical fixation of carbon dioxide catalyzed by binaphthyldiamino Zn, Cu, and Co salen-type complexes. J. Org. Chem. 2003, 68, 1559−1562. (12) Zeglis, B. M.; Pierre, V. C.; Barton, J. K. Metallointercalators and Metalloinsertors. Chem. Commun. 2007, 4565−4579. (13) Campbell, N. H.; Karim, N. H. A.; Parkinson, G. N.; Gunaratnam, M.; Petrucci, V.; Todd, A. K.; Vilar, R.; Neidle, S. Molecular basis of structure-activity relationships between salphen metal complexes and human telomeric DNA quadruplexes. J. Med. Chem. 2012, 55, 209−222. (14) Barone, G.; Terenzi, A.; Lauria, A.; Almerico, A. M.; Leal, J. M.; Busto, N.; García, B. Coord. DNA-binding of nickel(II), copper(II) and zinc(II) complexes: structure-affinity relationships. Coord. Chem. Rev. 2013, 257, 2848−2862. (15) Kleij, A. W.; Kuil, M.; Tooke, D. M.; Lutz, M.; Spek, A. L.; Reek, J. N. H. Zn(II)−salophen complexes as versatile building blocks for the construction of supramolecular box assemblies. Chem. - Eur. J. 2005, 11, 4743−4750. (16) Wezenberg, S. J.; Kleij, A. W. Material applications for salen frameworks. Angew. Chem., Int. Ed. 2008, 47, 2354−2364. (17) Salassa, G.; Coenen, M. J. J.; Wezenberg, S. J.; Hendriksen, B. L. M.; Speller, S.; Elemans, J. A. A. W.; Kleij, A. W. Extremely strong selfassembly of a bimetallic salen complex visualized at the singlemolecule level. J. Am. Chem. Soc. 2012, 134, 7186−7192. (18) Ciavardini, A.; Dalla Cort, A.; Fornarini, S.; Scuderi, D.; Giardini, A.; Forte, G.; Bodo, E.; Piccirillo, S. Adenosine monophosphate recognition by zinc-salophen complexes: IRMPD spectroscopy and quantum modeling study. J. Mol. Spectrosc. 2017, 335, 108− 116. (19) Bodo, E.; Ciavardini, A.; Dalla Cort, A.; Giannicchi, I.; Yafteh Mihan, F.; Fornarini, S.; Vasile, S.; Scuderi, D.; Piccirillo, S. Anion recognition by uranyl-salophen derivatives as probed by IRMPD spectroscopy and ab-initio modeling. Chem. - Eur. J. 2014, 20, 11783− 11792. (20) Moore, D. T.; Oomens, J.; Eyler, J. R.; von Helden, G.; Meijer, G.; Dunbar, R. C. Infrared spectroscopy of gas-phase Cr+ coordination

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b05825. Jmol files for B3LYP and D3-B3LYP structures (ZIP) Tables S1−S4, assignment of experimental and theoretical frequencies for the four compounds; Table S5, RMSD between the structures computed with and without dispersion corrections; Table S6, proton− oxygen distances; Figure S1, example of an IR spectra computed with and without dispersion corrections; Figures S2 and S3, spectra of protonated salophen−Zn complexes (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*(E.B.) E-mail: [email protected]. ORCID

Simonetta Fornarini: 0000-0002-6312-5738 Debora Scuderi: 0000-0003-3931-8481 Enrico Bodo: 0000-0001-8449-4711 Present Address #

Synchrotron SOLEIL Saint-Aubin 91192, France

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are very grateful for the valuable assistance of the CLIO team. Philippe Maitre, Vincent Steinmetz and the Mass Spectrometry platform SMAS and Jean-Michel Ortega and the CLIO staff of the Laboratory of Physical Chemistry are gratefully acknowledged for their assistance. E.B. acknowledges the computational support of CINECA through Grants IsC44_AAOX and IsC48_PTILS. A.D.C. acknowledges the financial support of Università La Sapienza, Project “Ricerca scientifica di Ateneo 2015” (C26A15437J). D.S. acknowledges the support by a public grant from the “Laboratoire d’Excelence Physics atomic mater (LabEx PALM) CHIRAUX UV-IR No. 2013-0562T. S.P. acknowledges financial support from Italian MIUR (PRIN 2010ERFKXL_006). H


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The Journal of Physical Chemistry A complexes: determination of binding sites and electronic states. J. Am. Chem. Soc. 2005, 127, 7243−7254. (21) Fridgen, T. D. Infrared consequence spectroscopy of gaseous protonated and metal ion cationized complexes. Mass Spectrom. Rev. 2009, 28, 586−607. (22) MacAleese, L.; Maitre, P. Infrared spectroscopy of organometallic ions in the gas phase: from model to real world complexes. Mass Spectrom. Rev. 2007, 26, 583−605. (23) Chiavarino, B.; Crestoni, M. E.; Fornarini, S.; Taioli, S.; Mancini, I.; Tosi, P. Infrared spectroscopy of copper-resveratrol complexes: A joint experimental and theoretical study. J. Chem. Phys. 2012, 137, 024307. (24) Piccirillo, S.; Ciavardini, A.; Bodo, E.; Rondino, F.; Scuderi, D.; Steinmetz, V.; Paladini, A. Probing the competition among different coordination motifs in metal-ciprofloxacin complexes through IRMPD spectroscopy and DFT calculations. Inorg. Chem. 2013, 52, 103−112. (25) De Petris, A.; Ciavardini, A.; Coletti, C.; Re, N.; Chiavarino, B.; Crestoni, M. E.; Fornarini, S. Vibrational signatures of the naked aqua complexes from platinum(II) anticancer drugs. J. Phys. Chem. Lett. 2013, 4, 3631−3635. (26) Lanucara, F.; Scuderi, D.; Chiavarino, B.; Fornarini, S.; Maitre, P.; Crestoni, M. E. IR signature of NO binding to a ferrous heme center. J. Phys. Chem. Lett. 2013, 4, 2414−2417. (27) Lanucara, F.; Chiavarino, B.; Crestoni, M. E.; Scuderi, D.; Sinha, R. K.; Maitre, P.e; Fornarini, S. Naked five-coordinate FeIII(NO) porphyrin complexes: vibrational and reactivity features. Inorg. Chem. 2011, 50, 4445−4452. (28) Chiavarino, B.; Crestoni, M. E.; Fornarini, S.; Lanucara, F.; Lemaire, J.; Maitre, P.; Scuderi, D. Direct probe of no vibration in the naked ferric heme nitrosyl complex. ChemPhysChem 2008, 9, 826− 828. (29) Dalla Cort, A.; Mandolini, L.; Pasquini, C.; Rissanen, K.; Russo, L.; Schiaffino, L. Zinc−salophen complexes as selective receptors for tertiary amines. New J. Chem. 2007, 31, 1633−1638. (30) Prazeres, R.; Glotin, F.; Insa, C.; Jaroszynski, D. A.; Ortega, J. M. Two-colour operation of a free-electron laser and applications in the mid-infrared. Eur. Phys. J. D 1998, 3, 87−93. (31) Ortega, J. M.; Glotin, F.; Prazeres, R. Extension in Far-Infrared of the Clio free-electron laser. Infrared Phys. Technol. 2006, 49, 133− 138. (32) Aleese, L. M.; Simon, A.; McMahon, T. B.; Ortega, J. M.; Scuderi, D.; Lemaire, J.; Maitre, P. Mid-IR spectroscopy of protonated leucine methyl ester performed with an FTICR or a Paul type ion-trap. Int. J. Mass Spectrom. 2006, 249-250, 14−20. (33) Oomens, J.; Sartakov, B. G.; Meijer, G.; von Helden, G. Gasphase infrared multiple photon dissociation spectroscopy of massselected molecular ions. Int. J. Mass Spectrom. 2006, 254, 1−19. (34) Parneix, P.; Basire, M.; Calvo, F. Accurate modeling of infrared multiple photon dissociation spectra: The dynamical role of anharmonicities. J. Phys. Chem. A 2013, 117, 3954−3959. (35) Alecu, I. M.; Zheng, J.; Zhao, Y.; Truhlar, D. G. Computational thermochemistry: scale factor databases and scale factors for vibrational frequencies obtained from electronic model chemistries. J. Chem. Theory Comput. 2010, 6, 2872−2887. (36) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parameterization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (37) Foster, J. P.; Weinhold, F. Natural hybrid orbitals. J. Am. Chem. Soc. 1980, 102, 7211−7218. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009.

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