actinide ions substitution

Accepted Manuscript Title: Computational Study of Organo-Cesium Complexes and the Possibility of Lanthanide/Actinide ions Substitution Author: Walter A. Rabanal-Le´on Guillermo Martinez-Ariza Sue A. Roberts Christopher Hulme Ramiro Arratia-P´erez PII: DOI: Reference:

S0009-2614(15)00804-0 http://dx.doi.org/doi:10.1016/j.cplett.2015.10.048 CPLETT 33373

To appear in: Received date: Revised date: Accepted date:

5-8-2015 17-10-2015 20-10-2015

Please cite this article as: Walter A. Rabanal-Le´on, Guillermo Martinez-Ariza, Sue A. Roberts, Christopher Hulme, Ramiro Arratia-P´erez, Computational Study of OrganoCesium Complexes and the Possibility of Lanthanide/Actinide ions Substitution, Chemical Physics Letters (2015), http://dx.doi.org/10.1016/j.cplett.2015.10.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Highlights (for review)

Highlights •

A relativistic two-component theoretical of organo-cesium (lanthanide and actinide) is presented. Energy decomposition analysis evidences an ionic character on the ligand-metal

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•

interaction in organo-cesium compounds. •

The substitution of Cs+ ions by the isoelectronic La3+ and Th4+ produce an increment of

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organo-lanthanide/actinide by Ugi-type of synthesis.

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The increment of the covalency previously reported suggests the viability of obtain

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the covalency on the studied compound.

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*Graphical Abstract (pictogram) (for review)

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La3+ Cs++

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Cs++ Th4+

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Computational Study of Organo-Cesium Complexes and the Possibility of Lanthanide/Actinide ions Substitution

b

Universidad Andr´es Bello, Facultad de Ciencias Exactas, Ph.D. Program in Molecular Physical ´ Chemistry, ReMoPhys group, Av. Repblica 275, Santiago 8370146, Chile.

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a

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Walter A. Rabanal-Le´ona , Guillermo Martinez-Arizab , Sue A. Robertsc , Christopher Hulmeb,c,∗, Ramiro Arratia-P´ereza,∗

Department of Pharmacology and Toxicology, The University of Arizona. Biological Sciences West Room # 351, 1041 E. Lowell St., Tucson, AZ 85721, USA. Department of Chemistry and Biochemistry, 1041 E. Lowell St., The University of Arizona, Tucson, AZ 85721, USA.

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Abstract

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Relativistic DFT calculations suggest that two organo-Cesium complexes studied herein afford large HOMO-LUMO gaps of around 2.4 eV with the PBE xc-functional, which accounts for their stability. Energy decomposition studies suggest these two complexes are largely ionic with about 20 % covalency. However, when the Cs+ ions are substituted by the isoelectronic La3+ and Th4+ , their predicted ionicity decreases significantly. The significant increase in covalence indicates that employing Ugi reaction cascades that afford tetramic acid-based organo-Cesium complexes may be extended to La3+ and Th4+ organometallics. Keywords: Relativistic-DFT, Energy Decomposition Analysis, TD-DFT

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1. Introduction

The s-Block elements are one of the largest and most reactive metal families in the periodic table and as such, limited research has been performed in the area due to inherent synthetic challenges.[1] In particular, the synthesis of Cesium organometallics is a difficult task because it often involves the use of Cesium metal that is highly sensitive to air and moisture and their preparations are carried out at low temperatures with vacuum air-free or ∗

Correspding Author(s): • R. Arratia-P´erez E-mail: [email protected] Phone: +56-02-2661-8232 • C. Hulme E-mail: [email protected] Phone: +1-520-626-5322 Preprint submitted to Elsevier

October 17, 2015

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glove/box inert atmosphere protocols.[2] Moreover, the bonding in organometallic compounds of the heavier s-block elements is difficult to describe because they do not follow the criteria of conventional coordination chemistry since large cations interact with large anions in which charge is delocalized and there are strong cation-ligand interactions.[3]

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In spite of the experimental difficulties, it has been recently reported that the synthesis of stable, complex organo-Cesium tetramic acids is feasible via a sequential Ugi reaction[4] and Cesium carbonate promoted three-step one-pot cascade.[5] The Ugi reaction was carried out with p-methoxyaniline, ethyl glyoxalate, 2,6-dimethylphenylisonitrile and cyanoacetic acid, and the crude adduct was subsequently treated with Cesium carbonate in DMF yielding unprecedented organo-Cesium complexes.[5] In particular, although several organo-Cesium complexes were reported in this article, we will focus our attention on organo-Cesium species with unique coordination spheres that involve binding phenyl rings in two different modes, and simultaneously picking up interactions with carbonyl and nitrile groups. These two complexes, 1a: 3-cyano-5-(2-((2,6-dimethylphenyl) amino)-2-oxoethyl)-1-(4-methoxyphenyl)-5-methyl-2,4- dioxopyrrolidin-3-ide Cesium and 1b: 1-(4-bromophenyl)-3-cyano-5-(2-((2,6-dimethylphenyl)amino)-2-oxoethyl)-5-methyl2,4-dioxopyrrolidin-3-ide Cesium, as well as the metal susbstitution are depicted in Figure 1.[5]

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Herein, we report studies of the molecular and electronic structure and optical properties of complexes 1a and 1b. Furthermore, the possibility of replacing the Cs+ ions by the isoelectronic La3+ and Th4+ ions by performing relativistic DFT and TD-DFT calculations is evaluated.

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M

M

M

M

M = Cs+ , La3+ , Th4+ 1a: 3-cyano-5-(2-((2,6-dimethylphenyl)amino)-2oxoethyl)-1-(4-methoxyphenyl)-5-methyl-2,4dioxopyrrolidin-3-ide

1b: 1-(4-bromophenyl)-3-cyano-5-(2-((2,6dimethylphenyl)amino)-2-oxoethyl)-5methyl-2,4-dioxopyrrolidin-3-ide

Figure 1: Molecular structure for the [Cs-1a] and [Cs-1b] complexes and their analogous with La3+ and Th4+ ions.

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2. Computational Details

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The computational study was carried out using the molecular structure obtained from Xray crystallographic analyses of the organo-Cesium complexes with the 1a and 1b ligands, reported by Hulme et al (see Figure 1).[5] All this work was developed on the framework of the relativistic density functional theory (R-DFT) by using the Amsterdam Density Functional (ADF 2012.01) code,[6] where the scalar (SR) and spin-orbit (SO) relativistic effects were incorporated by means of a two-component Hamiltonian with the zerothorder regular approximation (ZORA).[7] All the molecular structures presented here were studied via the generalized gradient approximation (GGA) employing the non-local correction proposed by Perdew, Burke and Erzenhof (PBE).[8] Furthermore, uncontracted triple-ζ quality Slater-type orbitals (STO) basis set, augmented with two sets of polarization functions (TZ2P) were used for all atoms.[9]

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To study the bonding nature of these systems, we looked for a methodology to quantify the interactions between the organic ligand and the metallic centre. Therefore, an analysis of bonding energetics were performed by combining a fragment approach to the molecular structure of a chemical system with the decomposition of the total bonding energy (EBE ), according to Morokumas energy partitioning scheme,[10, 11, 12] as: EBE = ∆EPauli + ∆VElestat + ∆EOrb . Here, ∆EPauli , ∆VElestat and ∆EOrb are the Pauli’s repulsion, electrostatic interaction, and orbital-mixing terms, respectively. In this way, the electrostatic component is calculated from the superposition of the unperturbed fragment densities at the molecular geometry and corresponds to the classical electrostatic effects associated with Coulombic attraction and repulsion. The electrostatic contribution is most commonly dominated by the nucleus- electron attraction and therefore has a stabilizing influence. The Pauli term is obtained by requiring that the electronic antisymmetry conditions be satisfied and has a destabilizing character, whereas the orbital-mixing component represents a stabilizing factor originating from the relaxation of the molecular system due to the mixing of occupied and unoccupied orbitals and can involve electron pair bonding, charge-transfer or donor-acceptor interactions, and polarization.[13]

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As a perspective, and in order to estimate the possibility of substitution of the Cs+ ions with isoelectronic La3+ or Th4+ ions, we calculated the energy decomposition by Morokuma-Ziegler scheme, substituting the Cs+ ions by La3+ and Th4+ ions but keeping the same structure as the organo-cesium complexes. Additionally, we calculated the excitation energies and absorption spectra using the time-dependent density functional theory (TD-DFT)[14] at scalar relativistic level for both systems. The excitation energies were calculated for all the studied complexes using the statistical average of orbital exchange-correlation model potential (SAOP),[15] which was designed for this response property calculations. The calculated absorption spectra were plotted by using Lorentzian weighted functions by the respective oscillator strengths with a peak width of 20 nm.

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3. Results and Discussion 3.1. Electronic Structure

−1.50

−1.75

−1.75

−2.00

−2.00

−2.25

−2.25

−2.50

−2.50

−3.25

−2.75 −3.00

−3.50

−3.75

−3.75

−4.00

−4.00

−4.25

−4.25 −4.50

141ag

−4.75

LUMS

2.40 eV

HOMS

564a1/2 (99% 141ag + 1% 140ag)

141au

563a1/2 (99% 141ag + 1% 140ag) 562a1/2 (99% 141au + 1% 140au)

140ag

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−4.75

2.40 eV

−3.25

−3.50

−4.50

565a1/2 (100% 142au)

142au

ed

−3.00

566a1/2 (100% 142ag)

565a1/2 (100% 142ag) 566a1/2 (100% 142au)

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−2.75

142ag

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−1.50

M

−1.25

Energy (eV)

−1.25

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The electronic structures of the complexes [Cs-1a] and [Cs-1b] are quite similar. For example, their frontier molecular orbitals in both cases are centred on the ligand orbitals with a predominantly π-character with zero contribution from the cesium atomic orbitals, which are actually strongly destabilized in the region of unoccupied states. The HOMOLUMO energy gaps at the scalar and spin-orbit relativistic level of calculation have similar values, being 2.40 eV and 2.41 eV for [Cs-1a] and [Cs-1b] complexes, respectively. It is clear to see from Figure 2, that the molecular spinors (representations obtained considering the spin-orbit coupling) are formed by exactly the same contribution that the molecular orbitals on the scalar relativistic level. We obtained a similar energy level diagram and electronic structure for the [Cs-1b] complex.

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

561a1/2 (99% 141au + 1% 140au)

140au

Spin-Orbit (SO)

Scalar Relativistic (SR)

Figure 2: Molecular orbital (SR) and molecular spinor (SO) energy diagram for the [Cs-1a] complex.

3.2. Bonding Nature and Energetics In order to understand the nature of the interaction between the organic ligand and the cesium ions, we used the energy-partitioning scheme (EDA) proposed by MorokumaZiegler. We choose the fragment approach to evaluate this interaction considering the unitary cell composed by twice the coordination complex formed by the interaction between the organic ligand and the cesium ion. The fragmentation implemented here adopted the two organic ligands and the two-cesium ions like different fragments, as indicated in Figure 3 below and the results are shown in Table 1. 4

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Fragment A (Lig. - 1a) (Lig. - 1b)

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R = -OCH3 R = -Br

Fragment B

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M = Cs+, La3+, Th4+

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Figure 3: Fragmentation scheme for both the organo-Cesium compounds, with the possibility of Cs+ substitution by La3+ and Th4+ ions. Contour density isovalue at 0.03 a.u.

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Table 1: Energy decomposition analysis (EDA) for the [Cs-1a] and [Cs-1b] complexes at scalar relativistic level. All energies are in kcal/mol.

[Cs-1a]

[Cs-1b]

∆EPauli

53.73

58.11

∆VElestat

-261.51

-263.81

∆EOrb

-69.64

-80.94

EBE

-277.65

-286.87

%Ionicity

78.9

76.5

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∆EPauli : Pauli’s energy term. ∆VElestat : Electrostatic interaction energy. ∆EOrb : Orbital interaction energy. EBE : Interaction bonding energy.

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From Table 1 we observe that the interaction between the Fragment A (the two-organic ligands) and the Fragment B (the two-cesium ions) is mainly ionic with more than 75 % ionicity in both complexes, which is in agreement with the electronic structure described above, because there are no bonding molecular orbitals with contributions from both ligand and cesium ion orbitals. These results also favour the presence of an electric dipole among the carbanion present on the organic ligand and the positive charge of the Cs+ ion. It is also observed from Table 1, that due to the substitution of the aromatic ring by Br on the 1b ligand, the ionic character slightly decreased. If we now consider the electrondonor character of the -OCH3 (1a ligand) and the electron-withdrawing character of the -Br (1b ligand), it is easy to understand that the ionic behaviour of the interaction could be slightly modified by this substitution, where electron-donor -OCH3 substituent favour the dipole formation (charge separation) and therefore the ionic interaction.

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In Addition, the energetics of projected lanthanide/actinide substitution in the complexes with 1a and 1b organic ligands is shown in Table 2 and 3. From these tables we observed that the La3+ ion (with both 1a and 1b organic ligand) still has an ionic interaction with the organic ligand; which is in agreement with the traditional concepts of coordination chemistry of the organo-lanthanide complexes. In addition to the ionic behaviour of the lanthanum ion, there is also an incremental increase in the orbital energy, which means that orbital overlapping is more favored than in the case of the cesium ion.

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We can also deduce from Table 2 and 3 that the Th4+ ion completely changed the bonding nature. In this case the interaction is more covalent (53.7 and 60.4 % for the [Th-1a]6+ and [Th-1b]6+ complexes, respectively), which is also in agreement with traditional concepts of actinide coordination chemistry. In these two cases, we speculate that the interaction of the thorium ion with the phenyl acetamide group of the organic ligand is favored because of the overlap among the f -orbitals and π-orbitals, which is covalent and stronger than the interaction with the fragment containing the carbanion. Moreover, EDA calculations were also computed for the relaxed (optimized) structures with the ligand 1a. The results obtained from this analysis showed that the orbital interaction have a sligthly increase with respect to the electrostatic interaction term, due to the minimization of the steric interaction during the geometry optimization. In the case of the Lanthanum complex the system is still ionic but it presents lower ionic character than the non-optimized system; furthermore, the Thorium complex have almost the same covalent behaviour in the optimized and the non-optimized systems (See Supplementary Material Table S1).

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The analysis mentioned above suggests that it is possible to substitute the Cs+ ion by La3+ or Th4+ ions (taking into account their ionic radii that follows RCs+ > RLa3+ > RTh4+ )[16] and the new complexes obtained from the same Ugi-cascade pathway would be equal or more stable than the original organo-cesium complex. Nevertheless, from the Morokuma-Ziegler energy decomposition it is also observed that this substitution is accompanied by an increment of electrostatic energies due to the higher nuclear charge of the actinide ion, which could produce a structural distortion involving the organic ligand and the actinide ion. Thus, we can predict lanthanide/actinide organometallic complexes with a greater structural diversity. 6

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[La-1a]4+

[Th-1a]6+

∆EPauli

53.73

129.83

48.89

∆VElestat

-261.51

-306.71

-1000.60

∆EOrb

-69.64

-157.04

-1237.43

EBE

-277.65

-333.92

-2189.14

%Ionicity

78.9

66.1

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∆EPauli : Pauli’s energy term. ∆VElestat : Electrostatic interaction energy. ∆EOrb : Orbital interaction energy. EBE : Interaction bonding energy.

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[Cs-1a]

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Table 2: Energy decomposition analysis (EDA) for the [Cs-1a], [La-1a]4+ , and [Th-1a]6+ complexes at scalar relativistic level. All energies are in kcal/mol.

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Table 3: Energy decomposition analysis (EDA) for the [Cs-1b], [La-1b]4+ , and [Th-1b]6+ complexes at scalar relativistic level. All energies are in kcal/mol.

[La-1b]4+

[Th-1b]6+

∆EPauli

58.11

47.97

56.96

∆VElestat

-263.81

-759.38

-1005.21

∆EOrb

-80.94

-712.34

-1391.70

EBE

-286.87

-1423.53

-2339.95

%Ionicity

76.5

51.6

41.9

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[Cs-1b]

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∆EPauli : Pauli’s energy term. ∆VElestat : Electrostatic interaction energy. ∆EOrb : Orbital interaction energy. EBE : Interaction bonding energy.

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3.3. Excitation Energies and Absorption Spectra

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The results of the calculated excitation energies are presented in Figures 4a, 4b and Table 4. It is observed, from the analysis of the active molecular orbitals composition, that the electronic transitions can be characterized as intra ligand charge transfer (ICLT) bands. For the [Cs-1a] complex, the electronic transitions arise from orbitals located on the 1(4-methoxyphenyl) pyrrolidine-2,5-dione carbanion group fragment toward the orbitals localized on the phenyl acetamide group of the organic ligand. In the case of the [Cs-1b] complex, the transitions arise from the phenyl acetamide group and go toward the orbitals extended to encompass all the organic ligand. The [Cs-1a] complex spans the ultraviolet region and the [Cs-1b] complex spans the UV-visible region of the electromagnetic spectra. It is important to mention that the Cs-orbitals do not contribute to any of the electronic transitions assigned here.

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Table 4: Excitation energies (eV), wavelengths (nm), oscillator strengths (f ), active molecular orbitals and electronic transition assingment for the complexes Cs-1a and Cs-1b at scalar relativistic level.

%

Assignment

139au → 147ag 140au → 150ag 139ag → 147au 140au → 149ag

23 17 13 10

ILCT ILCT ILCT ILCT

141ag → 145au 141au → 145ag 141ag → 146au

34 32 22

ILCT ILCT ILCT

Active MOs

%

Assignment

[Cs-1a]

4.749

b

3.521

261

352

pt

a

band

Active MOs

M

Energy (eV) λ (nm) f (x10) 0.837

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band

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

0.775

[Cs-1b]

Energy (eV) λ (nm) f (x10)

a

5.148

241

1.124

145au → 155ag 145ag → 153au 145ag → 152au 145au → 153ag

18 13 11 11

ILCT ILCT ILCT ILCT

b

4.388

283

1.245

147au → 152ag 148au → 153ag

30 21

ILCT ILCT

c

3.124

397

0.993

150au → 153ag 150ag → 153au

62 18

ILCT ILCT

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0.5

0.3

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Oscillator Strength (f)

0.4

0.2

0.0 150

200

250

300

350

λ (nm)

0.5

450

M

0.4

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0.3

0.2

pt

Oscillator Strength (f)

400

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(a)

0.1

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0.1

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

0.0

150

200

250

300

350

400

450

500

λ (nm)

(b)

Figure 4: Calculated absorption spectra for both complexes at scalar relativistic level using the SAOP xcpotential. a) [Cs-1a], and b) [Cs-1b] systems.

Finally, the excitation energies were also calculated for the [La-1a]4+ , [Th-1a]6+ , [La1b] , and [Th-1b]6+ systems, under the same considerations as in the case of the Cesium complexes showed previously. In these spectra we can observed a red-shift of the bands with respect to the Cesium complexes. In general all the absorption spectra with the 1a ligand are similiar in shape, but the highest absorptive bands exhibit a shift to the red of 230 and 330 nm for the Lanthanum and Thorium complexes with respect to the Cesium analogous, these can be observed in the Figure 5. Furthermore, the Lanthanum and Thorium complexes with the 1b ligand exhibit a similar trend as the mentioned before with respect to Cesium complex (See Supplementary Material - Figure S1). 9 4+

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1.2

[Cs-1a] [La-1a]4+ [Th-1a]6+

1

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0.8

cr

0.6

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0.4

0.2

300

400

M

200

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Oscillator Strength / f

500

600

700

λ / nm

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Figure 5: Comparison of the normalized absorption spectra for the [Cs-1a], [La-1a]4+ , and [Th-1a]6+ .

4. Conclusions

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A detailed description of the excitation energies for the complexes mentioned on this paragraph could be found on the Tables S2 and S3 in the supplementary material.

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Relativistic DFT calculations revealed that organo-cesium 1a and 1b complexes present large HOMO-LUMO gaps of around 2.40 eV accounting for their stability. The energy decomposition results give evidence that these two complexes are largely ionic with about 20 % covalency. However, when the Cs ions are substituted by La3+ and Th4+ their ionicity decreases significantly, i.e. for complex 1a which is about 76 % ionic decreasing to about 66 % when Cs+ is substituted by La3+ and to about 45 % when substituted by Th4+ . The complex 1b which is about 79 % ionic decreases to about 52 % when Cs+ is substituted by La3+ and to about 42 % when substituted by Th4+ . The significant increase in covalence indicates that the synthetic protocol, namely sequential Ugi reaction/cascades to afford tetramic acid organo-Cesium species may be extended to La3+ and Th4+ organometallics. Finally, the excitation energies computed to the these extended systems put on evidence the localization of this excitations on the near-infrared region of the spectra which could be used for the development of optical technologies. 10

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Acknowledgements

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G.M.A. is grateful for the financial support from CONACyT/UA doctoral scholarship 215981 (CVU: 464318) and Professors Denis Lichtenberger (UA), and Zhiping Zheng (UA) are acknowledged for useful discussions. R.A.P. is grateful for the financial support from Millennium Nucleus 120001 and Fondecyt 1150629. W.A.R.L. acknowledges CONICYT/ PCHA / Doctorado Nacional / 2013 N◦ 63130118 for his Ph.D. fellowship.

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