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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Aqueous Solvation of SmI3: A Born−Oppenheimer Molecular Dynamics Density Functional Theory Cluster Approach Alejandro Ramirez-Solis,*,† Jorge Iván Amaro-Estrada,‡ Jorge Hernández-Cobos,‡ and Laurent Maron§ †
Depto. de Física, Centro de Investigación en Ciencias-IICBA, Universidad Autónoma del Estado de Morelos, Cuernavaca, Morelos 62209, México ‡ Instituto de Ciencias Físicas, UNAM, Cuernavaca, Morelos 62210, México § Laboratoire de Physique et Chimie de Nano-objets, Université de Toulouse INSA-CNRS-UPS, 135, Avenue de Rangueil, 31077 Toulouse, France ABSTRACT: We report the results of Born−Oppenheimer molecular dynamics (BOMD) simulations on the aqueous solvation of the SmI3 molecule and of the bare Sm3+ cation at room temperature using the cluster microsolvation approach including 37 and 29 water molecules, respectively. The electronic structure calculations were done using the M062X hybrid exchange-correlation functional in conjunction with the 6-31G** basis sets for oxygen and hydrogen. For the iodine and samarium atoms, the Stuttgart−Köln relativistic effective-core potentials were utilized with their associated valence basis sets. When SmI3 is embedded in the microsolvation environment, we find that substitution of the iodine ions by water molecules around Sm(III) cannot be achieved due to an insufficient number of explicit water molecules to fully solvate the four separate metal and halogen ions. Therefore, we studied the solvation dynamics of the bare Sm3+ cation with a 29-water molecule model cluster. Through the Sm−O radial distribution function and the evolution of the Sm−O distances, the present study yields a very tightly bound first rigid Sm(III) solvation shell from 2.3 to 2.9 Å whose integration leads to a coordination number of 9 water molecules and a second softer solvation sphere from 3.9 to 5 Å with 12 water molecules. No water exchange processes were found. The theoretical EXAFS spectrum is in excellent agreement with the experimental spectrum for Sm(III) in liquid water. The strong differences between the solvation patterns of Sm(III) vs Sm(II) are discussed in detail.
I. INTRODUCTION Reductive electron transfer (ET) to organic substrates is a very important tool to obtain chemoselective reactions. This is often achieved by generating either radicals, radical anions, anions, or dianions.1 Samarium(II) diiodide (SmI2) introduced by Kagan2 in 1977 is one of the most promising reductive ET reagents available.3,4 Kagan2b first noted that H2O addition was important to control the reactivity of SmI2, and this was further confirmed by the work carried out by Curran5 in 1993. However, only until very recently, the coordination mode of SmI2 in water was not well defined experimentally, and numerous proposals have emerged in the literature on the effect of water. Among others, Flowers showed a high affinity of water to samarium but also, quite unexpectedly, showed that water was not saturating the samarium coordination sphere.6,7 Hoz and coworkers suggest that water was acting as a proton donor,8 and Procter suggested that water was crucial for these reactions to take place by stabilizing the metal-bound radical anions.9 However, recently, an important breakthrough in the knowledge of the SmI2 reactivity was provided by the theoretical study of Zhao and coworkers.10 Indeed, in this work, the authors used an incremental model to build the water coordination sphere around the SmI2 moiety. They demonstrated that the © XXXX American Chemical Society
water molecule was a far better ligand than iodide, which appears to be displaced to the second coordination sphere of the metal. Moreover, these authors proposed that in the course of the reduction of ketone iodide, ligands could be trapped by the trivalent samarium metal yielding a formal SmI3 molecule. Therefore, in the same study, Zhao and coworkers studied the solvation of SmI3 and compared it with SmI2. It was again demonstrated that water was displacing the iodide ligands but also that Sm(III) required more water molecules than Sm(II) to saturate its first coordination sphere. The results of the work on SmI2 in water by Zhao and coworkers were further confirmed by an ab initio molecular dynamics microsolvation study by our group.11 The latter study clearly demonstrated the very swift displacement of both iodide ligands by water at room temperature. Therefore, in the same spirit as the Born−Oppenheimer molecular dynamics study of SmI2 in water, we present here the first attempt at theoretically describing the solvation features of SmI3 in an aqueous environment using ab initio molecular dynamics to take into account the finite temperature and Received: December 26, 2017
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DOI: 10.1021/acs.inorgchem.7b03220 Inorg. Chem. XXXX, XXX, XXX−XXX
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of several molecules.13−15 However, when a molecular process occurs exclusively on the ground adiabatic potential surface, the relevant Schrödinger equation provides solutions leading to accurate nuclear dynamics, which is the main reason for the great success that the Born−Oppenheimer molecular dynamics approach has seen in the past decade. As in our previous study of the SmI2 case,11 the microsolvation problem we deal here with can accurately be described using the nuclear dynamics on the adibatic singlet electronic ground state PES. In order to address the aqueous solvation of SmI3 through the microsolvation scheme, we perform Born−Oppenheimer molecular dynamics density functional theory (BOMD-DFT) simulations of the SmI3−(H2O)37 model system. The BOMDDFT molecular dynamics simulations were carried out with the Geraldyn2.1 code,16 which has been coupled to the electronic structure modules of Gaussian09.17 The BOMD algorithm in Geraldyn uses the velocity-Verlet integration scheme.18 The simulations were done with a time step of 0.5 fs. A chain of four Nosé−Hoover thermostats19,20 was used to control the temperature at 300 K. Electronic structure and energy gradient calculations were performed at the DFT level through the hybrid M06-2X exchange-correlation functional with the 6-311G(d,p) basis sets for oxygen and hydrogen, since these yield a good compromise between accuracy and computational efficiency. The iodine21 and samarium atoms22 were treated with the 7 and 12 active valence electrons Stuttgart−Köln relativistic effective-core potentials (RECP) respectively, in combination with their adapted valence basis sets. The simulations started from a random distribution of 37 water molecules placed around the equilibrium structure of SmI3 (Sm−I, Re = 2.93 Å) and 29 water molecules placed around the bare Sm3+ cation without any preferred velocity vectors other than the thermal energy using a Boltzmann distribution at 300 K. 25 000 and 40 000 steps were performed for a total of 12.5 and 20 ps simulations for microsolvated SmI3 and Sm(III), respectively. The BOMD simulations on the potential surface of the lowest singlet electronic states required 78 and 61 CPU days on 128 processors for SmI3 and Sm(III), respectively. The EXAFS production run was started following a thermalization period of 5 ps, and reliable data were extracted to obtain the Sm−O
entropic effects at room temperature. The main aim of this investigation is to study the displacement of the iodide ligands by water but also to determine the number of water molecules in the first coordination sphere of Sm(III). This will be compared with the previous results found for microsolvated Sm(II). Crucially, the accuracy of this refined theoretical description can be validated by comparison with experiments, as EXAFS data are available for Sm(III) in aqueous solution. This investigation is the cornerstone of further study/understanding of the effects of water additives to SmI2 reactivity.
II. COMPUTATIONAL DETAILS A. Electronic Structure Calculations and Born− Oppenheimer Molecular Dynamics. The Born−Oppenheimer (BO) approximation12 separates the treatment of the fast electrons and the slow moving nuclei leading to two important concepts, viz., the adiabatic potential energy surfaces (PES) and the nonadiabatic coupling terms (NACT). Recent research has focused on the construction of diabatic PES to deal with NACT between the ground and the low-lying excited states
Figure 1. Initial microsolvation pattern for the SmI3−(H2O)37 system at 300 K. Samarium (yellow), oxygen (red), and iodine (green) atoms.
Figure 2. Evolution of the Sm−I distances at 300 K for the SmI3−(H2O)37 model. B
DOI: 10.1021/acs.inorgchem.7b03220 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry radial distribution function from the last 15 ps of the Sm(III)− (H2O)29 simulation. B. Theoretical EXAFS Spectrum. To produce the extended X-ray absorption fine structure (EXAFS) spectrum from the molecular dynamics simulation, we followed same the procedure as the one we used previously to address the aqueous solvation of As(OH)3,23 HgCl2,24 and SmI211 which, in turn, is based on the one originally presented by Merkling et al.25 The EXAFS spectrum (L3 edge) for the bare Sm3+ cation solvated with 29 water molecules was calculated as an average of the spectra produced by a number of system decorrelated configurations obtained during the simulation, thus incorporating the thermal disorder effects (the Debye−Waller factor) naturally occurring in the experiment. After thermalization was achieved, 500 snapshots each separated by 300 configurations were used to obtain the theoretical EXAFS spectrum. A cutoff centered around the Sm atom was applied to each structure in order to include water molecules whose oxygen atoms lie at distances up
to 5.0 Å, and paths with lengths up to this value were included considering multiple scattering. The EXAFS calculations were performed using the FEFF program (version 9.03)26 with an amplitude reduction factor S02 = 1.
III. RESULTS AND DISCUSSION The first BOMD simulation started from the initial SmI3− (H2O)29 microsolvated structure shown in Figure 1. The analysis of the Sm−I distances shows that, although with rather large Sm−I bond oscillations (0.8 Å, see Figure 2), the three iodine atoms remain coordinated to the Sm3+ cation all along the simulation. Figure 3 shows a typical structure of the microsolvation pattern around Sm after thermalization has been achieved. We stress that for the aqueous solvation of SmI2, we recently found a rather rapid dissociation of the Sm−I bonds and the ensuing substitution of both iodine ions by four solvating water molecules in the first 1.5 ps of the simulation.11 Here the iodide anions are more tightly held by the triply charged Sm cation and not even the first Sm−I dissociation was observed during the 10 ps simulation. Clearly, the molecular model used is limited and this phenomenon requires a significantly larger number of explicit water molecules so that there are enough of them to ensure that each dissociating iodide anion can be separately solvated besides those needed to build the first (and perhaps the second) solvation shells of the bare Sm(III) cation. A reasonable estimation yields that no less than 60 water molecules are required to achieve full solvation of the four separate ions. However, even a short BOMD simulation including 60 or more explicit water molecules is, in practice, unfeasible; here, using 37 water molecules, each simulation step (energy, gradient, and velocity-Verlet) with 115 atoms and ca. 1200 basis functions already takes 326 CPU min, or 5 real minutes with the present computational resources. Figure 4 shows the temporal evolution of the six shortest Sm−O distances, where it can be seen that after thermalization was achieved around 3 ps, five water molecules remain tightly bound inside a 2.7 Å sphere around Sm(III), and a sixth molecule oscillates slightly further away, leading to an overall Sm coordination number CN = 8.5 at R = 3.3 Å. Figure 5 shows the Sm−O radial distribution function (RDF) obtained for this simulation. The results led us to
Figure 3. Typical microsolvation pattern for the SmI3−(H2O)37 system at 300 K. Samarium (yellow), iodine (green), and oxygen (red) atoms.
Figure 4. Evolution of the six shortest Sm−Ow distances for the SmI3−(H2O)37 model. Note the intermittent Sm coordination of the sixth solvating water molecule after thermalization has been achieved, leading to an average water coordination number of 5.5 below R(Sm−O) = 2.9 Å. C
DOI: 10.1021/acs.inorgchem.7b03220 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 5. Sm−O (black) and Sm−H (red) radial distribution functions for the SmI3−(H2O)37 model and water coordination number (CN in green) as a function of distance from the Sm(III) center at 300 K.
artificially dissociate the three iodine ions from the SmI3 solute in order to analyze the solvation dynamics of the bare Sm(III) cation, for which experimental information is available.24 In this case, 29 explicit water molecules were considered to build the first, second, and third solvation shells for this goal. The same simulation conditions were applied, and 40 000 simulation steps for the Sm(III)−(H2O)29 model were done for a total of 20 ps. Figures 6 and 7 show the initial and typical microsolvation
Figure 7. Typical microsolvation pattern for the Sm(III)−(H2O)29 system at 300 K. Samarium (yellow) and oxygen (red) atoms.
less than 1.8 ps, a ninth molecule was inserted into the first solvation shell of Sm(III) (see Figure 8). We emphasize that, after thermalization was achieved, all nine water molecules directly interacting with the metal cation remain coordinated to Sm3+ and that no water exchange events were observed during the 20 ps simulation. This suggests a very tightly bound first solvation shell around Sm(III), which is significantly different from that found for the Sm(II). For Sm(III), the first nine water molecules remain bound within a 2.8 Å sphere, while in the Sm(II) case, the Sm−O distance for the ninth water molecule oscillates up to 3.7 Å, revealing that this molecule is much less bound to the doubly charged metallic center (see Figure 4 in ref 11). After a 5 ps thermalization period, the Sm−O radial distribution function (RDF), obtained from the last 30 000 configurations, is shown in Figure 9. Note that the first solvation shell extends from 2.3 to 2.9. Å, and as mentioned above, its integration leads to a CN of 9 water molecules around the
Figure 6. Initial microsolvation pattern for the Sm(III)−(H2O)29 system at 300 K. Samarium (yellow) and oxygen (red) atoms.
pattern around the Sm(III) cation. Figure 8 shows the evolution of the nine shortest Sm−O distances. While eight water molecules were initially placed around Sm(III) (a random typical structure extracted from the trajectory for Sm(II) from ref 11) it was found that very swiftly, in D
DOI: 10.1021/acs.inorgchem.7b03220 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 8. Evolution of the nine shortest Sm−Ow distances for the Sm(III)−(H2O)29 model at 300 K.
Figure 9. Sm−O (arbitrary units in black) and Sm−H (red) radial distribution functions and water coordination number (CN in green) as a function of distance from the Sm(III) center at 300 K.
for Sm(III) in liquid water, although we emphasize that our spectrum has been obtained with a finite microsolvation environment around Sm(III). This good agreement can be explained by the fact that the triply charged Sm3+ cation strongly interacts with the immediate aqueous solvation environment, leading to a rather rigid solvation sphere containing nine water molecules, which are the ones responsible for most of the EXAFS signal around Sm(III). At this point, a comparison with the previously obtained EXAFS spectrum for Sm(II) is in order.11 Note that, considering the smaller intensity of the signal, the Sm(II) spectrum shows a slower k-decay and a higher frequency with respect to the present theoretical Sm(III) spectrum, which is in much better agreement with that obtained in liquid water.27 These changes are consistent with what can be expected, because as shown by the comparison of the RDFs, the triply charged Sm(III) cation binds water molecules much more closely than the Sm(II) center. The Sm(II) spectrum reveals more multicenter dispersion contributions as compared with the Sm(III) spectrum, where a single contribution (i.e., the Sm−O paths of the significantly more rigid first solvation shell) largely dominates the EXAFS signal; this is evidenced by the
Sm(III) cation. The second solvation sphere is much broader, extending from 3.9 to around 5 Å and integrating at least 12 water molecules; the remaining 8 water molecules belong to the third solvation shell and are mostly asymmetrically located in structures as that shown in Figure 7. Again, the RDF for Sm(III) is significantly different from that found for the Sm(II) case;11 in the latter case, the first solvation shell is much broader, extending up to 3.5 Å, and the RDF does not vanish between the first and second solvation shell (see Figure 5 in ref 11). In the present case, two clearly distinct solvation shells are revealed by the coordination number n(r). Note the slight shoulder in the RDF around 5.2 Å, which could be indicative of the superposition and water exchange process between the second and third solvation shells, a feature that is completely absent in the Sm(II) case. The theoretical EXAFS spectrum using a set of 500 decorrelated configurations after thermalization was achieved is shown in Figure 10 along with the experimental one reported for Sm(III) in water from ref 24. An excellent agreement can be seen between our theoretically derived EXAFS spectrum with the experimental data E
DOI: 10.1021/acs.inorgchem.7b03220 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 10. Theoretical EXAFS spectra for the Sm(II) and Sm(III) cations with the microsolvation models at 300 K (red and blue curves, respectively). The black dotted line corresponds to the Sm(III) spectrum in liquid water.27 The Sm(II) spectrum is taken from ref 11
rather sharp decay of the intensity of the oscillation between 7 and 8.5 Å−1 in the Sm(II) case.
(c) The Sm(III)−O radial distribution function reveals a well-defined and rather rigid first solvation shell from 2.5 to 2.9 Å and a more diffuse second sphere located between 3.9 and 5 Å, which integrates at least 12 water molecules. (d) The water coordination number extracted from the integration of the radial distribution function for Sm(III) up to R(Sm−O)=2.9 Å is 9, while that obtained for Sm(II) is 8.4.11 (e) A theoretical EXAFS spectrum was obtained from 500 decorrelated cluster−microsolvation configurations extracted from the BOMD simulations for the Sm(III)− (H2O)29 system. Clearly distinct simulated spectra were obtained for Sm(II) and Sm(III) under very similar microsolvation conditions, both at 300 K. Although we use a finite cluster to model the aqueous solvation around the Sm3+ cation, the theoretically derived spectrum is in excellent agreement with the experimental EXAFS spectrum for Sm(III) in liquid water,27 presumably due to the rather rigid first solvation shell which largely dominates the EXAFS signal in this case. This comparison allowed us to conclude that the present results for Sm(III) using the microsolvation cluster model are consistent with what is known for Sm(III) in the liquid aqueous phase. Finally, as a future perspective, we believe that when a large enough microsolvation SmI3−(H2O)n model can be studied through ab initio Born−Oppenheimer molecular dynamics, the triple iodide dissociation process might actually be revealed in computationally available simulation times, leading to the four fully microsolvated separate ions (Sm3+ + 3 I−)−(H2O)n, as has been recently revealed for the SmI 2 case.11
IV. CONCLUSIONS AND PERSPECTIVES The question of the stability of SmI3 in aqueous media and the substitution of water molecules as ligands to the metallic center instead of the iodide anions is still a matter of debate. However, to date no theoretical data are available to describe at the molecular level the solvation pattern around the Sm(III) ion in aqueous environments. For this reason, we report here the results of ab initio Born−Oppenheimer molecular dynamics through the microsolvation scheme using a cluster model including 37 water molecules around the SmI3 neutral solute at room temperature. The electronic structure and molecular dynamics approach used here has been successfully applied before to address the aqueous solvation of As(OH)3,23 Hg(OH)2,24 and, very recently, for SmI2.11 Starting form the equilibrium configuration of isolated SmI3 and 37 randomly located water molecules around the solute, the BOMD simulation at 300 K shows that not even the first Sm−I dissociation can be achieved using this limited molecular model. These results reveal two important facts: the much stronger attractive interaction of the iodide ions with the central metallic Sm(III) cation as compared with that present for the SmI2 reagent, where two Sm−I dissociations were readily found to occur in the first 2 ps; second, that a significantly larger number of water molecules (>60) are needed to allow for a minimal solvation around the four separate ions resulting from the dissociation of the three iodide anions from the Sm(III) cation. Clearly, a BOMD simulation including such a large number of solvating water molecules is out of reach with the present computational resources. Therefore, in order to study the solvation dynamics of the bare Sm(III) cation, we performed another BOMD simulation utilizing the Sm(III)−(H2O)29 cluster model. The main results of this simulation can be summarized as follows (a) The system is stable showing a rather strong Sm(III)− water interaction; no water evaporation was observed in the 20 ps of the simulation. (b) A total of 9 water molecules remain tightly coordinated to Sm(III) and no first-to-second solvation shell water exchange events were observed.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Alejandro Ramirez-Solis: 0000-0002-8428-5714 Laurent Maron: 0000-0003-2653-8557 F
DOI: 10.1021/acs.inorgchem.7b03220 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.R.S. thanks support from CONACYT Basic Science project number 253679. A.E.J.I. thanks a DGAPA-UNAM postdoctoral fellowship. J.H.C. thanks support from DGAPA-UNAM grant No. IG100416.
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