On the Aqueous Solvation of SmI2: A Born-Oppenheimer

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On the Aqueous Solvation of SmI: A Born-Oppenheimer Molecular Dynamics Density Functional Theory Cluster Approach Alejandro Ramirez-Solis, Jorge Ivan Amaro-Estrada, Jorge Hernandez-Cobos, and Laurent Maron J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b00910 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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The Journal of Physical Chemistry

On the Aqueous Solvation of SmI2: A Born-Oppenheimer Molecular Dynamics Density Functional Theory Cluster Approach a* b b c A. Ramirez-Solis , J. I. Amaro-Estrada , J. Hernández-Cobos and L. Maron

a

Depto. de Física, Centro de Investigación en Ciencias-IICBA Universidad Autónoma del Estado de Morelos, Cuernavaca, Morelos. 62209, México. bInstituto de Ciencias Físicas, UNAM. Cuernavaca, Morelos. 62210, México.

c Laboratoire de Physique et Chimie de Nano-objets, Université de Toulouse, INSACNRS-UPS, 135,Avenue de Rangueil, 31077 Toulouse, France.

We report the results of Born-Oppenheimer molecular dynamics (BOMD) simulations on the aqueous solvation of the SmI2 molecule at room temperature using the cluster microsolvation approach including 32 water molecules. 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. Starting from the optimized geometry of SmI2 embeded in the microsolvation environment, we find a swift substitution of the iodine ions by eight tightly bound water molecules around Sm(II). Through the Sm-O radial distribution function and the evolution of the Sm-O distances, the present study predicts a first rigid Sm(II) solvation shell from 2.6 to 3.4 Å whose integration leads to a coordination number of 8.4 water molecules, and a second softer solvation sphere from 3.5 to ca. 6 Å.

The Sm(II)-O radial distribution function is in excellent agreement with that reported for Sr2+ from EXAFS studies, a fact that can be explained since Sr2+ and Sm2+ have almost identical ionic radii (ca. 1.26 Å) and coordination numbers, 8 for Sr2+ and 8.4 for Sm2+. The theoretical EXAFS spectrum was obtained from the BOMD trajectory and is discussed in the light of the experimental spectra for Sm(III). Once microsolvation is achieved, no water exchange events were found to occur around Sm2+, in agreement

with the experimental data for Eu2+ (which has a nearly identical charge-to-ionic radius relation as Sm2+), where the mean residence time of a water molecule in [Eu(H2O)8]2+ is known to be ca. 230 ps.

*e-mail:[email protected]

ACS Paragon Plus Environment


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I. Introduction. Reductive electron transfer (ET) to organic synthesis is one of the most versatile methods for highly chemoselective reductions and reductive coupling reactions via the formation of radicals, radical anions, anions and dianions.1 In this regard, samarium(II) diiodide (SmI2) was introduced to the synthetic organic community by Kagan2,3 in 1977, and since then it has emerged as one of the most promising reductive ET reagents available4-12. In his seminal paper3, Kagan first noted that H2O additive is instrumental in SmI2-mediated ET reactions, which was then further studied by Curran et al. in 199313. But only recently, H2O has been found to open up new chemical reactions that were previously thought to be impossible promoted by SmI214. For instance, although SmI2 is widely used in the reduction of aldehyde and ketones to radical anions, the carboxylic acid derivatives have for a long time been thought to lie outside the reducing range of the SmI2 reagent. Procter’s work on H2O activation of the reagent makes a significant breakthrough to shatter this dogma15. To understand the role of H2O in promoting reductions mediated by SmI2, several groups have made great contributions. First, the pioneering works by Flowers showed that water has a high affinity to samarium and doesn’t locally saturate the coordination sphere of the substrate, thus increasing the rate of substrate reduction substantially.16-18 The reduction potential of SmI2-H2O is further accelerated when amine is mixed with water.19-21 Secondly, in 2009 Hoz and co-workers reported a detailed study of the reduction of α-cyanostilbenes, suggesting that H2O molecules coordinated to samarium act as proton donors.22 Thirdly, Procter reported a key finding whereby H2O enables the productive ET from Sm(II) by stabilizing the metal-bound radical anions.23 In spite of the above seminal reports,16-23 the coordination of H2O to SmI2, the ligand displacement and the role of the additive in Sm(II)-mediated reductions remains poorly understood at the molecular level, a fact which is presently limiting full exploitation of the reagent system in organic chemistry. We present here the first attempt at theoretically describing the solvation features of this important molecule in an aqueous environment using ab Initio Born-Oppenheimer molecular dynamics within the explicit microsolvation scheme through cluster models. This study is the cornerstone of further studies aimed at understanding the effect of water additives to SmI2 reactivity.


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II. Computational details A. Electronic structure calculations and Born-Oppenheimer molecular dynamics. In order to address the aqueous solvation of SmI2 through the microsolvation scheme, we perform Born-Oppenheimer molecular dynamics (BOMD) Density Functional theory simulations of the SmI2-(H2O)32 model system. The BOMD-DFT molecular dynamics simulations were carried out with the Geraldyn2.1 code,24 which has been coupled to the electronic structure modules of GAUSSIAN09.25 The BOMD algorithm in Geraldyn uses the velocity-Verlet integration scheme.26 The simulations were done with a time step of 0.5 fs. A chain of four Nosé–Hoover thermostats27,28 was used to control the temperature at 300 K. Electronic structure and energy gradient calculations were performed at the DFT level through the hybrid M062X exchange-correlation functional with the 6-31G(d,p) basis sets for oxygen and hydrogen, since these yield a good compromise between accuracy and computational efficiency. The hybrid M062X functional29 was chosen since, although not explicitly designed to precisely deal with dispersion effects, it has proven to yield results in good agreement with experimental data for systems where such effects are important. The iodine30 and samarium31 atoms 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 32 water molecules placed around the equilibrium linear structure of SmI2 (Sm-I Re=3.08 Å) without any preferred velocity vectors other than the thermal energy using a Boltzmann distribution at 300K. The MD simulations on the potential surface of the lowest singlet electronic state took 36 CPU days on 64 processors running the Linux versions of Geraldyn2.1-G09. The production run was started following an initial



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thermalization period of 5 ps, so that reliable data were extracted to obtain the Sm-O radial distribution function from the last 10 ps of the simulation.

B. Theoretical EXAFS spectrum. To produce the 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)332 and HgCl233 which, in turn, is based on the one originally presented by Merkling et al..34 The EXAFS spectrum was calculated as an average of the spectra produced by a number of system configurations obtained during the simulation, thus incorporating the disorder factor (the Debye-Waller factor) naturally occurring in the experiment. After thermalization was achieved, 500 snapshots each separated by 200 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)35 with an amplitude reduction factor S02 =1.

III. Results and discussion The BOMD simulation starting from the initial microsolvated structure (see Figure 1) shows that both iodine atoms readily dissociate as I- anions and that the samarium center becomes a +2 cation, quickly solvated by ca. 8 water molecules. It is interesting to note that as the iodine ions leave the vicinity of Sm, very quickly (within the first ps) four water molecules fill in the space left by the anions, approaching the Sm center and they remain coordinated (along with other four molecules) to the metal ion throughout the simulation.


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Figure 2 shows a typical structure of the microsolvation pattern around Sm after thermalization has been achieved.

Figure 1. Initial structure of the SmI2-(H2O)32 cluster model used for aqueous microsolvation. Sm (yellow), oxygen (red), iodine atoms (green).

Figure 2. Typical microsolvation pattern for the SmI2-(H2O)32 system at 300K after thermalization has been achieved. Sm (yellow), oxygen (red), iodine atoms (green).



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While Figure 2 shows a representative configuration of the system at 300K after thermalization has been achieved, Figure 3 shows the evolution of the Sm-I distances starting from the equilibrium geometry of the solute.

Figure 3. Evolution of the Sm-I distances at 300K.

Note the rather rapid dissociation of the Sm-I bonds and the ensuing substitution of both iodine ions by four solvating water molecules in the first 2 ps of the simulation. Figure 4 shows the temporal evolution of the shortest Sm-O distances, where it can be seen that 8 water molecules remain tightly bound inside a 3.2 Å sphere around Sm(II), and that an ninth molecule (orange curve) is intermittently coordinated, oscillating in and out of this region every 0.7 ps, on average.


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Figure 4. Evolution of the shortest Sm-O distances. Note the intermittent Sm coordination of the ninth solvating water molecule after thermalization has been achieved (5ps), leading to an average water coordination number of 8.4 below R(Sm-O)=3.5 Å.

After a 5 ps thermalization period, the Sm-O radial distribution function (RDF) obtained from the last 20000 configurations is shown in figure 5. Note that the first solvation shell extends from 2.4 to ca. 3.5 Å and its integration leads to a coordination number CN of 8.4 water molecules around the Sm dication. The second solvation sphere is much broader extending from 3.5 to around 6 Å although, with only 32 water molecules in the present model, it is difficult to distinguish the limit between the second and the third solvation shells around Sm(II). Note the slight shoulder around 5.2 Å which could be indicative of the superposition of the second and third solvation shells in this case.



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Figure 5. The Sm-O radial distribution function (arbitrary units in black) and water coordination number (CN in green) as a function of distance from the Sm(II) center at 300K.

Figure 6 presents the evolution of the I-Sm-I angle; note that as water molecules replace both iodine ions, the initial 180 degrees evolve also because water molecules surround both iodine anions through a dynamical network of hydrogen-halogen bridges and hydrogen bonds. This evolution is to be taken with care since the iodine ions cannot be fully solvated given the small number of water molecules in the present model.


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Figure 6. Evolution of the I-Sm-I angle in the SmI2-(H2O)32 cluster The present simulated EXAFS spectrum is shown in figure 7. Unfortunately, to the best of our knowledge, no experimental EXAFS data have been reported for Sm(II) in water, so that we can only qualitatively compare our theoretical prediction with the EXAFS spectrum obtained for Sm(III) in liquid water.36



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Figure 7. EXAFS spectrum of the SmI2-(H2O)32 cluster at 300K. The dotted line corresponds to the Sm(III) spectrum in liquid water from ref.36

We emphasize that our spectrum has been obtained with a microsolvation environment around Sm(II). We can point out that the Sm(II) spectrum presents two main differences, it shows a slower k-decay and with a higher frequency with respect to the Sm(III) spectrum in liquid water. These changes are consistent with can be expected since Sm(III) binds water molecules more closely than the Sm(II) cation. Clearly the present Sm(II) spectrum reveals more multi-center dispersion contributions as compared with the Sm(III) spectrum where there is, basically, a single contribution (i.e. the Sm-O paths of the first solvation shell); this is evinced by the rather sharp decay of the intensity of the oscillation between 7 and 8.5Å-1. Finally, the shape of the signal between 11 and 14 Å-1


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could be indicative of either a lack of disorder in the simulated spectrum or that the solvation structure around Sm(II) is significantly rigid at room temperature. A final comparison can be made with the structural data obtained from the present

BOMD simulations with EXAFS data reported for Sr2+.37 Actually, the radial distribution function obtained from BOMD simulations (Fig. 5) is in excellent agreement with that reported for Sr2+ from EXAFS studies. This is not surprising considering the almost identical ionic radii of Sr2+ (IR=1.26 Å and a coordination number of 8) while that of Sm2+ is IR=1.27 Å.38 Here we note that another metal ion with virtually identical charge-to-radius ratio is Eu2+, for which the water exchange reaction has been investigated by using

17

O NMR.39 The mean residence time of a water molecule in

[Eu(H2O)8]2+ was reported to be ca. 230 ps. In the present study, where the simulation time was 15ps, no water exchange events were found to occur around Sm2+, in agreement with the experimental NMR data for Eu2+. IV. Conclusions and perspectives SmI2 in aqueous medium is widely used in the reduction of aldehyde and ketones to radical anions and, more recently, even for the reduction of carboxylic acid derivatives. It has been found that water plays a key role in promoting reductions mediated by SmI2, since water has a high affinity to samarium and doesn’t locally saturate the coordination sphere of the substrate, thus increasing the rate of substrate reduction substantially.8 Other detailed studies of the reduction of α-cyanostilbenes suggest that H2O molecules coordinated to samarium act as proton donors22 and a recent a key finding revealed that H2O enables the productive electron transfer from Sm(II) by stabilizing the metal-bound radical anions.23



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However, to date no experimental or theoretical data are available to describe at the molecular level the solvation pattern around the Sm(II) ion in aqueous environments. For this reason we report here the first theoretical results from ab initio Born-Oppenheimer molecular dynamics (BOMD) through the microsolvation scheme using a cluster model including 32 water molecules at room temperature. The electronic structure (here using the hybrid M062X exchange-correlation functional) and molecular dynamics approach here presented has been successfully applied before to address the aqueous solvation of As(OH)332 and Hg(OH)2 .33 Starting form the equilibrium configuration of isolated SmI2 surrounded by 32 randomly located water molecules, the BOMD simulation yields results which can be summarized as follows: a) The system is stable showing a rather strong Sm-water interaction and no water evaporation was observed in the 15 ps of the simulation at 300K. b) The iodine anions are readily substituted by ca. 8 water molecules tightly coordinated to Sm(II). A ninth molecule oscillates in and out interacting with the Sm(II)-(H2O)8 coordination structure every 0.7 ps on average. c) The Sm-O radial distribution function reveals a well defined first solvation shell from 2.5 to 3.4 Å and a more diffuse second sphere located between 3.5 and 6 Å. d) The water coordination number extracted from the integration of the radial distribution function up to r(Sm-O)=3.5 Å is 8.4. e) The small number of water molecules in the present microsolvation model prevents complete solvation of the isolated iodine anions and a significantly larger number of water molecules would be necessary to achieve this. f) We have also obtained a theoretical EXAFS spectrum from 500 decorrelated clustermicrosolvation configurations extracted from the BOMD simulations. However, to the best


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of our knowledge, no experimental EXAFS spectrum for Sm(II) in water is available yet; therefore, we have presented a qualitative comparison with the experimental EXAFS spectrum for Sm(III) in liquid water.36 This comparison allowed us to conclude that the present results for Sm(II) using the microsolvation cluster model are consistent with what is known for Sm(III) in the liquid phase. g) The Sm(II)-O radial distribution function obtained from our BOMD simulations is in

excellent agreement with that reported for Sr2+ from EXAFS studies. This fact can be explained since Sr2+ and Sm2+ have almost identical ionic radii (ca. 1.26 Å) and coordination numbers, 8 for Sr2+ and 8.4 for Sm2+. h) Here the simulation time was 15ps and no water exchange events were found to

occur around Sm2+, in agreement with the experimental data for Eu2+ (which has a nearly identical charge-to-ionic radius relation as Sm2+), where the mean residence time of a water molecule in [Eu(H2O)8]2+ is known to be ca. 230 ps.

Finally, we mention that work is under way to obtain the equivalent cluster model microsolvation environment for the Sm(III) case. This will allow a balanced comparison of the spectra for the Sm(II) and Sm(III) ions in microsolvated aqueous environments to obtain more refined molecular pictures of the Sm(II,III)-water interactions.

Acknowledgments We thank E. Sánchez-Marcos and the reviewers for their valuable comments. ARS thanks support from CONACYT Basic Science project number 253679. AEJI thanks a DGAPA-UNAM postdoctoral fellowship.

References [1] Chatgilialoglu, C.; Studer, A. Encyclopedia of Radicals in Chemistry, Biology and Materials; Wiley-Blackwell: Chichester, U.K., 2012.



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On the Aqueous Solvation of SmI2: A Born-Oppenheimer

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