DOI: 10.1002/chem.201402788

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& Gas-Phase Reactions

Anion Recognition by Uranyl–Salophen Derivatives as Probed by Infrared Multiple Photon Dissociation Spectroscopy and Ab Initio Modeling Enrico Bodo,[a] Alessandra Ciavardini,[b, c] Antonella Dalla Cort,[a] Ilaria Giannicchi,[a] Francesco Yafteh Mihan,[a] Simonetta Fornarini,[b] Silvana Vasile,[a] Debora Scuderi,[d] and Susanna Piccirillo*[c]

Abstract: The vibrational features and molecular structures of complexes formed by a series of uranyl–salophen receptors with simple anions, such as Cl , H , and HCOO , have been investigated in the gas phase. Spectra of the anionic complexes were studied in the n˜ = 800–1800 cm 1 range by mass-selective infrared multiple photon dissociation (IRMPD) spectroscopy with a continuously tunable free-electron laser. The gas-phase decarboxylation of the formate adducts pro-

Introduction The use of hexavalent uranyl dication (UO22 + ) coordinated with different ligands, in either reactions or separation processes,[1] as well as in supramolecular assemblies,[2] presents challenging issues regarding the nature of uranyl–ligand coordination and bonding properties.[3–5] In the past few years, the number of artificial receptors able to host UO22 + has increased remarkably and unsuspected structural and reactivity features have been revealed.[6] Several ligands can be coordinated to the linear UO22 + dication in a mono- or multidentate fashion. Among the vast array of organic ligands, tetradentate salophen ligands (1) have proven very robust and have shown peculiar complexing prop-

[a] Prof. E. Bodo, Prof. A. Dalla Cort, Dr. I. Giannicchi, Dr. F. Yafteh Mihan, Dr. S. Vasile Dipartimento di Chimica, Universit di Roma “La Sapienza” P.le A. Moro 5, 00185 Rome (Italy) [b] Dr. A. Ciavardini, Prof. S. Fornarini Dipartimento di Chimica e Tecnologie del Farmaco Universit di Roma “La Sapienza” P.le A. Moro 5, 00185 Rome (Italy) [c] Dr. A. Ciavardini, Prof. S. Piccirillo Dipartimento di Scienze e Tecnologie Chimiche Universit di Roma “Tor Vergata” Via della Ricerca Scientifica, 00133 Rome (Italy) E-mail: [email protected] [d] Dr. D. Scuderi Laboratoire de Chimie Physique, Universit Paris Sud UMR8000 CNRS, Bt.350, 91405 Orsay cedex (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402788. Chem. Eur. J. 2014, 20, 11783 – 11792

duces uranyl–salophen monohydride anions, which have been characterized for the first time and reveal a strong U H bond, the nature of which has been elucidated theoretically. The spectra are in excellent agreement with the results obtained from high-quality ab initio calculations, which provided the structure and binding features of the anion–receptor complexes.

erties.[7] These ligands are diimino tetradentate Schiff bases with two oxygen and two nitrogen donor atoms in their dianionic form. Coordination around uranium is pentagonal bipyramidal with the four donor atoms of the salophen ligand occupying the equatorial plane and the uranyl oxygen atoms in the axial positions. The fifth equatorial site of the metal remains available for coordination by an additional group, either an anion or a neutral molecule. The presence of the Lewis acidic metal center makes these complexes particularly efficient in the molecular recognition of several substrates, namely, inorganic and organic salts,[8] neutral molecules,[9] and anions.[10] Thus, uranyl–salophen complexes have found several applications as receptors,[11] catalysts,[12] carriers,[13] and sensors,[14] and their structural characterization has been extensively performed in the solid state and in solution.[8] Although it can be argued that the properties of isolated complexes cannot be directly extended to the condensed phase, spectroscopic studies of these complexes in the gas phase can help to elucidate their structure at the molecular level, in the absence of solvent effects and in direct connection with high-level computational modeling. They might also serve as benchmarks to develop, validate, and refine the theoretical models and approximations employed to characterize such complexes. This approach has been successfully exploited to characterize the structure of organometallic gaseous complexes.[15] Indeed, no reasonable assessment of the effect of solvent and/or other environmental factors can be obtained without proper evaluation of the gas-phase interactions.

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Full Paper Experimental Section IRMPD vibrational spectroscopy All IRMPD experiments were performed by using a modified quadrupole Paul ion trap (Esquire 3000 + , Bruker) coupled to the tunable IR radiation produced by the free-electron laser (FEL) at the Centre Laser Infrarouge CLIO (Orsay, France).[18] This facility is based on a 10–44 MeV electron accelerator and provides radiation over the IR wavelength range of 3–120 mm 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.[19] The FEL wavelength was scanned by 5 cm 1 steps. Ions were irradiated for typically 250 ms. The laser power was around 1200 mW, about constant in the n˜ = 1000–1800 cm 1 range, and fell off to about 1000 mW at n˜ = 850 cm 1. Unless otherwise specified, spectra were acquired by reducing the laser intensity with a DB3 filter. Considering the complex multiphoton character of the IRMPD process, no fluence correction was applied to the measured data.

Scheme 1. Structures of uranyl–salophen receptors A–D discussed herein.

Herein, the gas-phase structure of the complexes formed by the uranyl–salophen receptors, A–D (Scheme 1), with chloride and formate anions is examined. The binding ability of these derivatives towards such anions in solution was already known.[9f] The complexes were delivered in the gas phase by ESI, trapped in a Paul ion trap, and probed by infrared multiple photon dissociation (IRMPD) spectroscopy in the n˜ = 800– 1800 cm 1 spectral range. We discovered that both IR photodissociation and the collision-induced dissociation (CID) of the formate adducts produce, through decarboxylation, the monohydride anion of the uranyl–salophen complexes, which, as far as we know, has not been reported previously. A thorough literature survey provided only H2UO2[16] and uranyl borohydride[17] complexes as known hydrides of the uranyl dication. We found that the monohydride anions of the uranyl–salophen complexes were stable, could be observed in long ion trap dwell times, and their IR spectra could be recorded. The IRMPD spectra have been analyzed and interpreted in the light of DFT quantum chemical calculations. There is an excellent agreement between measured spectra and those calculated. Typical U Cl and U OCOH stretching frequencies fall in the n˜ = 200–300 cm 1 range, whereas the U H stretching frequency appears at n˜  1300 cm 1. An extensive theoretical investigation of the metal–ligand bonding features shows how, compared with chloride and formate complexes, the hydride anion seems to form a more strongly bound complex with uranyl, as characterized by a strong (mostly ionic) metal–ligand bond. In addition, the U H bond is also characterized by a strong anharmonicity due to the peculiar nature of the chemical bonding involved. The reaction leading to the uranyl–salophen hydrides from the uranyl–salophen formates has also been investigated theoretically.

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Mass spectra were recorded after 10 accumulations, in the mass range 300–1000, and this sequence was repeated twice at each photon energy. Unless noted otherwise, the spectra were reported as a function of the IRMPD fragmentation efficiency, R = log(Iparent/ (Iparent+SIfragment), in which Iparent and Ifragment were the parent and fragment ion intensities, respectively. The mass spectrum was subject to a low-mass cutoff fixed in all cases to 25 % of the ion mass isolated in the trap. Uranyl–salophen complexes A–D were synthesized and characterized according to standard procedures already reported.[5a] The 1:1 solutions (3  10 5 m) of A, B, C, or D with tetrabutylammonium chloride (TBACl) or tetrabutylammonium formate (TBAHCO2) in CH3CN were prepared and introduced into the ESI source by means of direct infusion with a syringe pump at a flow rate of 150 mL h 1. The ESI conditions used were as follows: spray voltage = 4500 V, capillary temperature = 423 K.

Computational details A total of 12 structures were investigated by using DFT methods: for each of the 4 possible salophen structures in Scheme 1, we calculated the most stable geometry of the resulting complex with Cl , H , and HCO2 . An initial geometric optimization was carried out at the B3LYP level with a 6-31 + G(d) basis set on first-row atoms and a CRENBL effective core potential (ECP) basis set on U.[20] The final minima were fully characterized by vibrational analysis and a first set of harmonic vibrational spectra were obtained by using this procedure. For the hydride anions, a second set of optimizations were performed by using the 6-31G + + G(d,p) basis set to explore the influence of a richer basis set on the hydrogen atoms. In the latter case, a control calculation was also performed, but only on compound A, by using ECP78MWB on uranium.[21] To assess the role of weak dispersion forces, an additional set of geometry optimizations and frequency calculations was performed (only for the compounds with H and Cl ) by using a dispersioncorrected version of B3LYP employing the DTF-D3 approach by Grimme et al.[22] The final set of optimization and frequency calculations was accomplished for adducts with Cl and H by using the M06-2X functional with a triple-zeta quality basis (6-311 + G(d)) coupled with a small-core ECP for U (ECP60MWB).[23] As usual, to account for anharmonicity effects, the B3LYP-predicted spectra were scaled by 0.97 and those computed by M06-2X by 0.944.[24] The calculations were performed by using both Gaussian 09 and NWChem suites of programs.[25, 26]

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Full Paper Results and Discussion Uranyl–salophen receptor complexes It is helpful to begin our discussion by reporting the calculated spectra of the isolated uranyl–salophen compounds to elucidate some of their prominent spectroscopic features. Because these species are neutral molecular complexes (salophen dianion + uranyl dication), their spectra cannot be observed with the technique employed herein. The salophen unit is an aromatic structure, which is highly distorted from planarity when coordinated to the bulky UO2 + 2 molecular ion. As a consequence, asymmetrically substituted salophens, such as compound C, are chiral, as already reported.[27] The spectra of the four uranyl–salophen receptors, predicted at the B3LYP/6-31G(d) level of theory, are shown in Figure 1. We report only the region that concerns our experimental investigations, that is, the region that goes from n˜ = 700 to 1800 cm 1. Very few prominent features can be easily attributed to a specific fragment motion because the normal modes of the salophen system are distributed among many atoms. A specific “diagnostic” spectral feature is the absorption due to the asymmetric stretching of uranyl indicated by nAUO2 and located at n˜  900 cm 1. Another prominent feature is represented by the strong absorptions at n˜ = 1600 cm 1, which are due to various complicated collective oscillations with unidentifiable contributions attributed to C=N stretching motions.

correspond to complexation of the uranyl–salophen compound with chloride. Conversely, a solution of compound A with TBACl does not show the signal corresponding to the chloride complex. By CID of bare B Cl , C Cl and D Cl ions, no significant fragments were observed in the mass spectra because CID produces almost solely the corresponding neutral uranyl–salophen compounds B, C, and D and Cl as the ionic product, which is not detectable due to the cutoff of the ion trap. Only in the case of the C Cl complex is a negative uranyl–salophen C ion (m/z 660), of very low intensity, observed in the CID mass spectrum. Attempts to produce and trap a sufficient amount of the C ion to measure an IRMPD spectrum were unsuccessful. No fragment ions were observed when the uranyl–salophen chloride species stored in the ion trap were irradiated with FEL light, and the IRMPD spectra of B Cl , C Cl , and D Cl were acquired by recording the depletion spectra of the parent ions. Figure 2 shows the inverse of the

Figure 2. Computed (dashed lines) and experimental (solid line) IR spectra of the complexes of compounds A–D with chloride. For the ACl complex (upper-left panel), for which no experimental data are available, we show the results of different sets of computational models. For compounds BCl , CCl , and DCl , for clarity, we report only the scaled M06-2X theoretical results.

Figure 1. Calculated IR spectra for the neutral, bare salophen–uranyl compounds A–D.

In the case of compound D, we can also easily identify the C OMe stretching frequencies by virtue of the strong absorption at n˜  1240 cm 1. The other absorption lines are due to complicated and mixed motions of the molecular conjugated system and of the aromatic C H bonds. Absorptions involving the collective, relative motion of UO2 towards the salophen ligand fall at n˜  600 cm 1, which is just below the experimental window.

Uranyl–salophen chlorides The ESI mass spectra of the uranyl–salophen and TBACl solutions recorded in negative mode contain pronounced signals at m/z 771/773 for B, m/z 695/697 for C, and m/z 679/681 for D; these Chem. Eur. J. 2014, 20, 11783 – 11792

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IRMPD depletion spectra of the chloride complexes together with their corresponding calculated spectra. Figure 2A, which refers to compound A Cl (no experimental spectra), shows part of the calculations carried out to test the quality of the computations. In particular, we show the spectra calculated with B3LYP and D-B3LYP with a double-zeta quality basis set and those for which we used the M06-2X functional and a triple-zeta quality basis. The differences due to the inclusion of dispersion corrections (comparing B3LYP and D-B3LYP sets of data) are very limited, especially in the range of frequencies shown. Moving to the M06-2X functional with a larger basis allows a shift of the UO2 asymmetric stretching to be seen because of the different number of core electrons included in the ECPs, and consequently, to the different description of the U= O bonding feature. Herein, we limit ourselves to the presentation of the M06-2X results for B Cl , C Cl , and D Cl (Figure 2B–D). The diagnostic peak of the asymmetric uranyl stretch falls at a frequency of n˜ = 910, 914 and 926 cm 1 for B Cl , C Cl , and D Cl , respectively. All IRMPD spectra show common absorption bands in the n˜ = 1180–1190 and 1550–1600 cm 1 regions, which are characteristic of aromatic C C and C=N stretching modes. A feature in

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Full Paper the range n˜ = 1420–1430 cm 1 can be attributed to the N=C H bending mode. For complexes of D with chloride, the strong absorption band at n˜ = 1250 cm 1 is attributable to the C OMe stretching motions. In general, very good agreement between the IRMPD and calculated absorption spectra is found for the chlorinated complexes. A major deviation is observed for the asymmetric uranyl stretch due to a combination of the approximations involved in the use of both a pseudopotential and an empirical uniform scaling factor that might be unable to account for the necessarily different degrees of anharmonicity involved in the different bonds. Interestingly, asymmetric O=U=O stretching is observed in the n˜ = 988– 1019 cm 1 range in the IRMPD spectra of doubly charged [UO2(L)n]2 + complexes (L = acetone or acetonitrile; n = 2–5).[28] Moreover, an increasing number of ligands (in either the acetone or acetonitrile series of complexes) caused a redshift in the frequency of this mode, which was ascribed to increasing donation of electron density to the uranyl center. In this view, it is not surprising that, in the anionic adducts of uranyl–salophen complexes, in which the uranyl unit is pentacoordinated and includes three negatively charged ligands, the observed shift, relative to [UO2(L)n]2 + complexes, is of the order of n˜ = 80–100 cm 1. The optimized geometries of the uranyl–salophen chlorides are shown in Figure 3. The Cl anion is bound in the same plane as the four donor atoms of the salophen ligand, occupying the fifth position of the equatorial plane of the pentagonal bipyramid. The

distance between the U atom and the Cl anion is independent of the computational method and is around 2.7  (see Table 1). This distance is typical of a Lewis acid/base interaction in which dispersion forces play a limited role given that the geometry does not change significantly upon switching to the dispersion-corrected version of B3LYP. A more marked difference can be found instead when moving to the large basis set employed with the M06-2X functional. The changes are not restricted to only the U Cl distance, but there is a certain shortening of the N U distances that change from 2.74 to 2.68  and of the O U ones, which shorten from 2.33 to 2.32 .

Nature of the U Cl bond in the uranyl–salophen chloride complexes The uranyl–salophen unit behaves as an electron-pair acceptor in the Lewis acid–base complex with the chloride anion. To gain further insight into the nature of the interaction between the two partners, natural bond orbital (NBO) analysis[29] was performed for all chloride complexes at the M06-2X/6-311 + G(d) and M06-2X/631G(d) levels. The two sets of NBOs do not differ much in qualitative terms and we refer herein only to the second method because of the smaller number of orbitals involved, which makes interpretation much easier. Evaluation of the Fock matrix elements between the NBOs allows the quantification of the so-called “delocalization contribution” to the energy and to the wave function. This contribution is related to the energy spent to promote an electron pair of an occupied NBO to an empty one. In the complex of uranyl–salophen with chloride, the largest contribution to the delocalization energy comes from coupling of the lone-pair orbitals of the chloride anion (the Lewis base) with the empty, nonbonding orbitals localized on the U atom (the Lewis acid). As an example, in Figure 4a we show one of the filled lone-pair orbitals of the chloride anion and three selected uranium nonbonding orbitals (Figure 4b–d) chosen among those with the largest delocalization energies. The composition of these orbitals is evident from the symmetry of the nodal planes. The orbital on Cl is one of the two sp hybrids (32 % s + 67 % p) and is directed along the U Cl bond direction. Uranium orbitals are more complex in nature: those in Figure 4b and c are a combination of d and f orbitals; the one in Figure 4d comes from a combination of p and f atomic orbitals. For the other three compounds, the shape of the orbitals involved in U Cl bonding is very similar and we do not show them herein.

Uranyl–salophen formates Figure 3. M06-2X-optimized structures of uranyl–salophen complexes A, B, C, and D with chloride. The ligand–metal bonds have been added to the representation for clarity and to highlight the bis-pyramidal geometry of the uranyl center.

The negative ESI mass spectra of solutions containing the uranyl– salophen compounds B, C, and D with formate in CH3CN show a distinct signal of the pertinent complexes with formate. Upon irradiation with the FEL, several fragments are observed. The most

Table 1. Optimized U Cl distances [in ], U Cl stretching frequencies [unscaled in cm 1], and adiabatic interaction energies [in kcal mol 1, including zeropoint energy (ZPE) harmonic corrections] for uranyl–salophen complexes A–D with chloride.

U···Cl distance

U···Cl frequency U=O asymmetric stretching frequency interaction energy

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Method

A

B

C

D

B3LYP/6-31G(d)/CRENBL D-B3LYP/6-31G(d)/CRENBL M06-2X/6-311 + G(d)/ECP60MWB M06-2X/6-311 + G(d)/ECP60MWB M06-2X/6-311 + G(d)/ECP60MWB M06-2X/6-311 + G(d)/ECP60MWB

2.72 2.71 2.68 240.7 997.4 47.1

2.71 2.70 2.69 239.8 996.7 48.9

2.71 2.71 2.69 243.0 991.2 48.0

2.74 2.73 2.70 232.8 1002.0 55.8

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Figure 6. B3LYP-optimized structures of the four compounds with the formate anion. The ligand–metal bonds have been added to the representation for clarity and to highlight the pyramidal geometry of the uranium center.

Figure 4. NBOs for the U Cl bond. The surface has been drawn at the 0.04 isovalue.

intense peaks in all mass spectra correspond to the loss of a CH radical (13 u) and to the loss of CO2 (44 u) from the complex. These results are consistent with CID experiments. The loss of the intact anionic ligand from bare uranyl–salophen compounds B, C, and D, if it occurs, cannot be detected due to the cutoff of the ion trap. The loss of CO2 from formate complexes generates the anions BH , CH , and DH . Figure 5B–D shows the experimental spectra of the anionic complexes of B, C, and D with formate in the n˜ = 700–1800 cm 1 range compared with the pertinent calculated spectra. The asymmetric uranyl stretching frequency is observed at n˜ = 926, 919, and 932 cm 1 for [B···OCOH] ,[C···OCOH] , and [D···OCOH] , respectively. All IRMPD spectra present intense absorptions between n˜ = 1670 and 1690 and 1260–1270 cm 1. These bands can be assigned to carboxylate UOC=O and U OCO stretching motions and are typical for a monocoordinated carboxylate. In the experimentally obtained spectrum of [C···OCOH] , the U OCO stretching band is split, which suggests the presence of a second conformer of the

formate with a different orientation of the carboxylate with respect to the phenyl ring. The DFT geometries of the resulting complexes with formate are reported in Figure 6. The formate anion coordinates one oxygen atom to the fifth equatorial position of uranyl in the plane orthogonal to the OUO direction. The coordinated oxygen atom is located at approximately 2.3  from the U atom (Table 2). This distance is significantly shorter than that found in chlorides.

Table 2. B3LYP/6-31G(d)/CRENBL-optimized U···O(formate) distances [in ], the U···O(formate) stretching frequencies, C=O carbonyl stretching and O=U=O asymmetric stretching frequencies [unscaled in cm 1], and adiabatic interaction energies [in kcal mol 1, with ZPE harmonic corrections] for complexes A–D with formate.

U···O distance U···OCO frequency UOC=O frequency U=O asymmetric stretching frequency interaction energy

A

B

C

D

2.28 243.5 1768.4 931.5 53.2

2.29 324 1741.7 935.5 54.7

2.29 294.5 1751.3 933.2 54.4

2.30 298.8 1760.1 931.5 58.90

Generation of the hydride complex by CID of the uranyl–salophen complex with formate

Figure 5. B3LYP/6-31G(d)/CRENBL computed (dashed line) and experimental (solid line) IR spectra of complexes A, B, C, and D with the formate anion. Chem. Eur. J. 2014, 20, 11783 – 11792

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As previously described, the monohydride anions of the uranyl parent compounds B, C, or D are produced by CID of the uranyl– salophen complexes with formate. As far as we know, such hydrides have never been observed or characterized before. They are probably formed by a reaction that involves hydride transfer from formate to the uranium atom with the formation of CO2. Decarboxylation processes are not uncommon in gas-phase ion chemistry. In a recent example, twofold loss of CO2 from alkali metal and silver oxalates has been discovered and exploited to generate bare metal anions,[30, 31] whereas in another very interesting application

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Full Paper a decarboxylation reaction promoted by CID resulted in the formation of novel organometallic ions.[32, 33] To explain the experimental results, we characterized the reactive path leading to the hydride adduct by ab initio calculations. Initially we chose a model system composed of a formate anion and a bare uranyl cation. We started by placing the two partners at a distance of 4  and optimized the resulting complex by using a M06-2X/cc-pvdz level. The result of such minimization is that H is transferred to the uranyl cation with the formation of a CO2 molecule. We found that the gas-phase decarboxylation reaction between uranyl and formate was exothermic by about 300 kcal mol 1 and barrierless. This is not surprising because the two reactive partners have opposite charges and the reaction proceeds through electrostatic attraction. To verify the possibility of having the same reaction with neutral uranyl complexes, we carried out a series of stationary-point calculations (M06-2X/cc-pvdz) by using the simplified model system reported in Figure 7.

Figure 8. Energetic profile for the decarboxylation reaction of the formate anion complex.

drides depend on the fluence regime of the FEL. As an example, the mass spectra of the hydride anion CH (m/z 661) are reported together with the fragmentation spectra obtained by CID and by irradiation at different fluence regimes (Figure 9). By irradiation of

Figure 7. Simplified model system for simulating the uranyl–salophen unit.

The model is an unsaturated tetradentate organic ligand that has been devised to resemble the presence of the salophen moiety and its coordination effect on the uranyl cation. The stationary points of the decarboxylation reactions have been characterized in the gas phase: the starting point is represented by the infinitely separated reactants. As they approach, they create a prereaction complex that sees the formate attached to the uranyl through the oxygen atom. This prereaction complex stabilizes the separate reactants by about 57.7 kcal mol 1. The geometric rearrangement that this complex must undergo to get to the transition state involves rotation of the carboxylate anion and requires 38.8 kcal mol 1 (Figure 8). This transition state is characterized by a geometry very similar to that of the product pair, namely, the hydride complex and a CO2 molecule. The reaction profile, energies, and geometries are reported in Figure 8. The decarboxylation reaction in the model is exothermic by 14.9 kcal mol 1.

Uranyl–salophen hydrides The IRMPD spectra of the hydride complexes BH , CH , and DH were acquired by isolation of the fragment anions obtained by CID in the ion trap in a MS3 experiment. Upon irradiation with the FEL, the hydride complexes photodissociate through the loss of several radicals from the uranyl–salophen unit. An interesting aspect of the photofragmentation processes is that the mass spectral patterns obtained from the irradiated hyChem. Eur. J. 2014, 20, 11783 – 11792

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Figure 9. Mass spectra of the CH hydride acquired under different experimental conditions: a) m/z = 661 isolation + IRMPD, 700 mW at 2.9 mm; b) m/z = 661 isolation + IRMPD, 1210 mW at 2.9 mm; c) m/z = 661 isolation + CID; d) m/z = 661 isolation.

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Full Paper isolated CH at a power of 700 mW (Figure 9 a), fragment ions at m/z 573 and 497 are generated, and these are the same fragments as those produced by MS3 CID of the isolated hydride anion (Figure 9 c). These fragments correspond to the loss of neutral C6H3N and C13H7, respectively. At higher laser fluence (laser power 1210 mW; Figure 9 b), different species are formed: m/z 586 (loss of C6H3), 554 (loss of C6N2H7), 510 (loss of C6H7), 478 (loss of C12N2H11), m/z 270 (corresponding to UO2 ). Thus, the low fluence regime results are consistent with CID experiments, whereas in the high fluence regime different ionic products are formed. Such behavior is not uncommon in IRMPD experiments[34] and can be explained by competition between dissociation and further photon absorption. In the low-fluence regime, the highest rate is for the reactions with lower activation energy, whereas in the higher laser fluence regime the rate of further photon absorption increases and eventually leads the molecule to an average internal energy, which exceeds that of higher energy dissociation channels. In this situation, the rate of reactions with a higher dissociation threshold may predominate over the rate of lower energy exit channels. However, it cannot be excluded that some fragments are produced in a sequential absorption–dissociation process, that is, additional photon absorption and dissociation from the product ions of the primary photofragmentation event.[35] Figure 10 reports the IRMPD spectra of CH hydride (m/z 661) taken in two different fluence regimes; the intensity of different specific fragments formed in each fluence regime was monitored. The relative band intensities in the channels of the various fragments show remarkable differences, depending on both the fluence regime and the specifically observed fragment ion. Indeed, 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.[36, 37] It can also be noted from Figure 10 that the band at n˜  823 cm 1, which corresponds to the symmetric U=O stretching mixed with U H stretching, is observable only in the IRMPD spectra acquired in the higher energy regime. Hence, the increase in the laser fluence can have the effect of boosting the rate of photon absorption at a specific spectral frequency, heating the ion up to the dissociation threshold. It is also evident from Figure 10 that various bands acquired in the high-fluence regime (Figure 10 a), which require higher internal energy for fragmentation to occur, are generally more redshifted than those present in the spectra obtained at low energy (Figure 10 b). The specific position of some IRMPD bands is particularly sensitive to the monitored fragmentation channels in the different fluence conditions. For example, the bands located at n˜  1600 cm 1, which correspond to aromatic C C and C=N stretching, are shifted by about 25 cm 1 when observing different fragmentation channels. The band located at n˜  1300 cm 1, and those at n˜  900 cm 1, which we attribute to U H and U=O stretching motions, respectively (see below), are also slightly redshifted in the high-fluence regime. The redshifts with respect to the frequency position of the bands calculated for harmonic linear absorption spectra originate from intrinsic anharmonicities and anharmonic couplings. These are a consequence of heating in the multiple photon absorption process. Indeed, they depend on the dynamic response of the system to the laser pulse.[37] Chem. Eur. J. 2014, 20, 11783 – 11792

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Figure 10. IRMPD spectra of CH hydride acquired in different fluence regimes and recorded in different mass channels: Top) m/z 586, 554, 510, 478, 270; laser power 1200 mW; bottom) m/z 573 and 497; laser power 700 mW.

Comparisons with calculated frequencies and the nature of the U H bond in the hydride The fragmentation efficiency (low-energy regime) and theoretical spectra are reported in Figure 11; the calculated relevant frequencies and distances are given in Table 3. The diagnostic asymmetric uranyl stretch falls at n˜ = 901, 904, and 908 cm 1 for B H , C H , and D H , respectively. All IRMPD spectra of the BH , CH , and DH hydrides present a new strong feature located at n˜  1300 cm 1 that can be assigned to U H stretching frequency. This is in line with the fact that several matrix-assisted compounds containing U H bonds show strong U H stretching absorptions in this frequency region.[38] In Figure 11A, we report the spectra calculated for compound AH with different methods. The result from the B3LYP functional coupled with a 6-31 + + G(d,p) basis set is shown by the dotted line in Figure 11A. B3LYP predicts a frequency for the U H stretching motion that is too low, located at n˜ = 1200 cm 1, whereas the measured band is at n˜  1300 cm 1. A functional rich in HF exchange, such as M06-2X, yields a significantly higher U H stretching frequency, at a value of n˜  1350 cm 1. The frequency values resulting from the M06-2X functional for the three compounds are generally

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Full Paper NBO analysis was repeated for the hydride compounds to further clarify the nature of the U H bonding feature. The presence of a strong chemical bond (very different from the interaction found for chlorine) is signaled by the fact that the hydride anion cannot be separated from the uranyl unit during NBO analysis. Therefore, the bond between the U and H atoms cannot be analyzed by using the second-order approach used for chloride, but can be investigated by looking at the molecular orbital mainly responsible for the bond. Such an orbital is reported in Figure 12. The NBO is

Figure 11. Computed (dashed lines) and experimental (solid lines) IRMPD spectra of the AH , BH , CH , and DH hydrides. Figure 12. NBO responsible for the U H bond. The surface has been drawn at the 0.04 isovalue.

Table 3. Calculated optimized U···H(hydride) distances [in ], U hydride stretching frequencies [unscaled in cm 1], O=U=O uranyl frequencies [asymmetric stretching, unscaled in cm 1], and adiabatic interaction energies [in kcal mol 1, with ZPE harmonic corrections] in complexes of A–D with hydride at the M06-2X/6-311 + G(d) level.

U···H distance U···H frequency U=O frequency interaction energy

A

B

C

D

2.05 1428.0 984.6 80.9

2.04 1430.7 974.3 81.9

2.04 1430.3 978.7 81.3

2.06 1394.7[a] 985.4 89.4

composed of an s orbital on H and a hybrid orbital on U, which is mostly composed of s (23.07 %), p (15.55 %), and d (38.74 %) atomic orbitals. Analysis of the electronic density (not shown herein) reveals that the Laplacian of the density has one of its nodal surfaces between the U and H atoms; thus pointing to a bond with an ionic character more than a covalent one. The Laplacian shows a density concentration on H and depletion on the uranyl (the Laplacian of the electronic density is reported in the Supporting Information).

[a] This frequency is mixed with other molecular motions.

higher (  50 cm 1) than the experimental values; a discrepancy that can be attributed to anharmonicity and to the character of the multiple photon dissociation process. From data collected in Table 3, we also see that the U H bond is characterized by a dissociation energy that is remarkably higher than the binding energies found for the Cl and formate complexes. At the same time, the U H distance turns out to be around 2.0 . Difficulties in correctly locating the stretching frequency of U H prompted us to carry out an extensive series of test computations to further characterize the bond. Most of these tests are reported in the Supporting Information. In particular, we computed the properties of the U H bond in a bare UO2H + cation and on the already presented model system shown in Figure 7, in which the UO2H + cation is complexed by the simplified tetradentate ligand. In both cases, we found that the calculated characteristic stretching frequency of the U H bond was strongly dependent on the particular method/basis used. By using MP2 as a reference, we saw that M06-2X and MP2 essentially provided the same results. A further series of tests, by using the second-order anharmonic analysis of the vibrational frequencies, allowed us to conclude that the U H bond was characterized by very strong anharmonicity. In the isolated UO2H + cation, the ratio of the anharmonic to harmonic frequency of U H is as low as 0.84, whereas it is 0.96 for the complexed uranyl cation in the model system; a value that is still remarkably low because the nearby frequencies have ratios around 0.99–0.98. Chem. Eur. J. 2014, 20, 11783 – 11792

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Conclusion Negatively charged adducts of simple anions with uranyl–salophen complexes were formed and characterized as isolated species in the gas phase. The chloride and formate adducts were delivered in the gas phase by ESI of solutions of the anion dissolved in acetonitrile as the tetrabutylammonium salt and the neutral uranyl–salophen complex. The hydride adduct was obtained from a gas-phase reaction, namely, CID of the formate complex. The process involved hydride transfer to form a U H bond with concomitant loss of CO2. Calculations on a simplified model revealed details of the reaction that led to the formation of the hydride uranyl–salophen complex. The chloro-, formato-, and hydridouranyl–salophen complexes were thoroughly characterized by DFT-based methods, by using a functional rich in HF exchange, a relatively large basis set, and by taking care to account for dispersion corrections. Experimentally, the gaseous ions were sampled by IRMPD spectroscopy, which provided information about vibrational modes in the fingerprint region in the n˜ = 800–1800 cm 1 wavenumber range. The match between calculated IR spectra for the optimized structures and experimental IRMPD spectra provided support for the computational results and allowed assignment to the observed bands. A distinct feature in the IR spectra of the sampled chloro-, formato-, and hydridouranyl–salophen complexes was the one associated with the asymmetric stretching vibration of the O=U=O unit, which appeared at n˜ = 900–932 cm 1. The frequency of this mode was also at n˜  900 cm 1 in the calculated IR spectra of neutral salophen complexes A–D. The novel hydridouranyl–salophen complex was

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Full Paper also characterized by a strong feature at n˜  1300 cm 1 for the three BH , CH , and DH sampled species; this was assigned to the U H stretching vibration. Calculations showed that the U H bond was strong and ionic in character and cleavage of hydride ion was much more demanding in energy than the corresponding anion ligand dissociation processes from the chloride and formate complexes. As a final comment, quantum chemical calculations and IRMPD spectroscopy have proven their well-documented, notable potential in characterizing the structure of gaseous metal complexes.

Acknowledgements The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP7/2007–2013) under grant agreement no. 226716. S.P., A.D.C., I.G., and F.Y.M. acknowledge Italian MIUR(PRIN 2010CX2TLM and PRIN 2010ERFKXL_006) and COST Action 1006 “EUFEN: European F-Element Network”. Thanks are due to Philippe Matre for helpful discussions. Vincent Steinmetz and the Mass Spectrometry Platform SMAS, Jean-Michel Ortega and the CLIO staff of the Laboratory of Physical Chemistry are gratefully acknowledged for their assistance. E.B. acknowledges the CINECA calculation support through the ESP-IL grant. Keywords: ab initio calculations · anions reactions · uranium · vibrational spectroscopy

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gas-phase

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Received: March 26, 2014 Revised: June 2, 2014 Published online on August 5, 2014

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