Coordination Structure and Fragmentation Chemistry of the Tripositive

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Coordination Structure and Fragmentation Chemistry of the Tripositive Lanthanide-Thio-Diglycolamide Complexes Xiuting Chen, Qingnuan Li, and Yu Gong J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08094 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 24, 2017

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The Journal of Physical Chemistry

Coordination Structure and Fragmentation Chemistry of the Tripositive Lanthanide-Thio-Diglycolamide Complexes Xiuting Chen1,2, Qingnuan Li1, and Yu Gong1*

1 Department of Radiochemistry, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai

201800, China

2 University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: Tripositive Ln(TMTDA)33+ complexes (Ln = La-Lu except Pm, TMTDA= tetramethyl 3-thio-diglycolamide) were observed in the gas phase by electrospray ionization (ESI) of LnCl3 and TMTDA mixtures. Collision induced dissociation (CID) was employed to investigate their fragmentation chemistry, which revealed the influence of metal center as well as ligand on the ligated complexes. Ln(TMTDA)2(TMTDA-45)3+ resulting from Ccarbonyl-N bond cleavage of TMTDA and hydrogen transfer was the major CID product for all Ln(TMTDA)33+ except Eu(TMTDA)33+

which

predominantly

formed

charge

reducing

product

EuII(TMTDA)22+ via electron transfer from TMTDA to Eu3+. Density functional theory calculations on the structure of La(TMTDA)33+ and Lu(TMTDA)33+ revealed that Ln3+ was coordinated by six Ocarbonyl atoms from three neutral TMTDA ligands, and both complexes possessed C3h symmetry. The Sether atom deviating from the ligand plane was not coordinated to the metal center. Based on the CID results of Ln(TMTDA)33+, Ln(TMGA)33+ and Ln(TMOGA)33+, the fragmentation chemistry associated with the ligand depends on the coordination mode while the redox 1

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chemistry of these tripositive ions is related to the nature of both metal centers and diamide ligands.

Email: [email protected] 2


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Introduction Over

the

past

three

decades,

diamide

ligands

especially

tetraalkyl-3-oxa-diglycolamide have gained much attention in the field of actinide partitioning.1,2 In addition to the studies on the extraction performances of these ligands, the coordination structures of the corresponding lanthanide-3-6 and actinide-diglycolamide complexes5-9 were also well characterized, which provide further insights into their behaviors during the extraction process. In contrast to the rich studies regarding the diglycolamide complexes, the diamide ligands bearing sulfur atom between two amide groups (thio-diglycolamide, Figure 1) have been of limited

interest.1,10-14

It

was

N,N’-dimethyl-N,N’-dihexyl-3-thio-diglycolamide

reported

that and

N,N’-dihexyl-3-thio-diglycolamide showed no extraction ability to Eu3+, Th4+, UO22+, NpO2+ and Am3+ but can extract Th4+ synergistically when combined with thenoyltrifluoroacetone (HTTA).1,10,11 For ligands where the substituent groups (R in Figure 1) are iso-propyl, n-butyl or iso-butyl, they showed a slight but perceptible extraction for UO22+ from the nitric acid medium but very weak complexing ability for both lanthanides and americium.12,13 All these research results indicate that the extraction capacities of thio-diglycolamide ligands are not as good as those of diglycolamides. Structural studies on the coordination complexes involving thio-diglycolamide ligand are limited to those of U(VI) and La(III), where thio-diglycolamide acted as a bidentate ligand with Sether uncoordinated.13,14 No

3



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systematic study has been performed on the lanthanide-thio-diglycolamide complexes as well as the effect of the metal center on the chemistry of these complexes. Our recent investigations have demonstrated that the coordination structure and chemistry of ligated multiply charged diglycolamide complexes can be investigated in the gas phase.15-17 For trivalent lanthanides and actinides, the fragmentation chemistry of

nine

coordinate

Ln(TMOGA)33+

and

An(TMOGA)33+

(TMOGA

=

tetramethyl-3-oxa-diglycolamide) complexes parallels the redox chemistry of the corresponding trivalent metal ions in condensed phase.18 The very recent investigation on the lanthanide-TMGA (TMGA = tetramethyl glutaramide) complexes revealed that tripositive Ln(TMGA)33+ complex was stable in the gas phase although the lanthanide center was coordinated by only six Ocarbonyl, and the fragmentation patterns of the Ln(TMGA)33+ complex strongly depended on the reduction potentials of the Ln3+/Ln2+ couples.19 To get further insight into the effect of lanthanide metal cation as well as diamide ligand on the coordination mode and fragmentation chemistry of the tripositive lanthanide diamide complexes, we report the formation and gas phase chemistry

of

the

Ln(TMTDA)33+

complex

(TMTDA

=

tetramethyl-3-thio-diglycolamide, R = CH3 in Figure 1), and the coordination structures of selected complexes were investigated by density functional calculations. Comparisons on the behaviors of Ln(TMTDA)33+ with previously characterized Ln(TMGA)33+ and Ln(TMOGA)33+ were made as well.18,19

4


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Figure 1. Structure of thio-diglycolamide (TDA).

Experimental and Theoretical Methods All the experiments on the gas phase chemistry of Ln(TMTDA)33+ were performed on a ThermoScientific (San Jose, CA) LTQ-XL LIT mass spectrometer equipped with an Heated Ion Max ESI (electrospray ionization) source. LnCl3 (200 µM) and TMTDA mixtures (2:1 to 1:10) in methanol were prepared for ESI experiments. LnCl3 and TMTDA stock solutions were prepared by dissolving LnCl3 (Sigma-Aldrich) and TMTDA in Milli-Q water. TMTDA was synthesized according to the literature.20 ESI mass spectra were acquired in the positive polarity mode and the detailed instrumental parameters are described in Supporting Information. Cations of interest were mass-selected and subjected to collision induced dissociation (CID). High purity nitrogen (99.999%) gas was used for nebulization and drying in the ESI source and helium (99.999%) was used as the collision gas. The pressure inside the ion trap is about 7.5 × 10-6 Torr during all the experiments. Density functional theory (DFT) calculations on the La(TMTDA)33+ and Lu(TMTDA)33+ complexes were performed with the Gaussian 09 package using the hybrid B3LYP density functional.21-23 The 28 and 60 electron core pseudopotential basis sets were used for La and Lu, and the 6-31G(d) basis set was used for all the 5



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remaining atoms (C, H, O, N, S).24-27 All of the geometrical parameters were optimized and the harmonic vibrational frequencies were obtained analytically at the optimized structures. Zero-point energy (ZPE) corrections were included in the calculations of relative energies. Results and Discussion ESI of LnCl3 and TMTDA The ESI mass spectra of 1:3 LnCl3 (200 µM) and TMTDA mixtures reveal the formation of a series of lanthanide dependent species such as Ln(TMTDA)33+, Ln(TMTDA)(TMTDA-H)2+ and Ln(TMTDA)2Cl2+ (Figure S1, Supporting Information), and the mass spectra for selected lanthanides are shown in Figure 2. Identifications of the charge of the complexes for La, Ce, Pr, Tb, Ho, Tm, and Lu are based on isotopic peaks arising from 12C/13C isotopes, and the assignment of the corresponding Nd, Sm, Gd, Dy, Er, Yb complexes can be made based on the several naturally occurring stable isotopes with comparable abundances. As shown in Figure S2 (Supporting Information), the calculated mass distributions of selected Ln(TMTDA)33+ are in good agreement with the experimental observations. Two stable isotopes (151Eu and 153Eu) of europium with similar abundance help identify the europium complexes. Note that Ln(TMTDA)(TMTDA-H)2+ is prevalent in the ESI mass spectra of LnCl3-TMTDA mixtures but not in those of LnCl3-TMGA mixtures.19 Besides, both EuIII(TMTDA)(TMTDA-H)2+ and EuII(TMTDA)22+ were observed in the spectrum of EuCl3-TMTDA mixture (Figure S3, Supporting Information) while EuIII(TMGA)(TMGA-H)2+ was not detected in the experiment of EuCl3 and TMGA.19 6


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The tripositive Ln(TMTDA)33+ complex is of particular interest since it is the binary tripositive complex with least TMTDA ligands. In addition to the evidence from isotopic peaks, assignment of such tripositive ions can be also confirmed by the results from CID during which di- and monocations are formed. Weak peaks due to Ln(TMTDA)43+ were also observed in the ESI mass spectra for all the lanthanides, and its abundance remained about the same when the LnCl3/TMTDA ratio varied from 2:1 to 1:4, in contrast to the increase in abundance of the Ln(TMTDA)33+. Formation of Ln(TMTDA)33+ was favored over Ln(TMTDA)43+ when the capillary temperature was increased from 20 to 300︒C and the latter almost disappeared when the sweep gas was turned on, indicating that the fourth TMTDA ligand was probably weakly bound in the second coordination sphere as observed in the TMGA case.19 Neither Ln(TMTDA)23+ nor Ln(TMTDA)53+ was detected in the experiments even though the LnCl3/TMTDA ratio was increased to 1:10. In addition, lanthanide independent species such as Na(TMTDA)+ and Fe(TMTDA)Cl+ were always present in the mass spectra.

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Figure 2. Selected ESI mass spectra of 1:3 LnCl3 and TMTDA mixtures in methanol. A: Ln(TMTDA)2Cl2+; B: Fe(TMTDA)Cl+.

Coordination Structure of the Ln(TMTDA)33+ Complex To understand the coordination structure of the Ln(TMTDA)33+ complex, DFT calculations were carried out at the B3LYP level of theory. La(TMTDA)33+ and Lu(TMTDA)33+ with the largest and smallest Ln3+ ionic radii were selected as representatives whose optimized geometries were obtained. As shown in Figures 3 and S4 (supporting information), five isomers were obtained for both La(TMTDA)33+ and Lu(TMTDA)33+. The most 8


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stable structure of the La(TMTDA)33+ complex possesses a C3h symmetry with La3+ coordinated by six Ocarbonyl atoms. Two other isomers with much longer La-S distances were predicted to be 14.2 (C3) and 9.1 (D3) kcal/mol higher in energy. In addition, computations also found two isomers which contain two cis sulfur atoms, and they were computed to be slightly less stable than the C3h isomer by 2.7 and 12.6 kcal/mol respectively. For the most stable C3h isomer, the calculated La-Ocarbonyl distance is 2.477 Å, close to the values reported for other lanthanide containing systems including lathanide-diglycolamide complexes.2,4,5,13 However, it is apparent that the coordination mode of Sether atom is completely different from Oether atom in TMOGA. Although the TMTDA ligand itself was computed to have a planar geometry, the CH2-S-CH2 moiety deviates from the molecular plane upon coordination with La3+. Such geometry results in a rather long La-Sether distance of 3.321 Å compared with those around 2.9 ~ 3.0 Å in the compounds where sulfur atoms are bonded to La3+.28,29 The computed La-S distance of La(TMTDA)33+ is about 0.5 Å longer than the sum of single-bond radii of La and S (2.83 Å) while that of La and O (2.43 Å) is very close to the computed La-Ocarbonyl distance (2.477 Å),30 suggesting the bonding interaction is negligible between La and Sether in comparison to the La-Ocarbonyl bond. This is also consistent with the case of the praseodymium dibenzothiophene complexes where the sulfur atoms are nonbonded to the metal center owing to the long Pr-S distances around 3.35 Å.31 It should be noted that much longer La-Sether distances (5.05-5.07 Å) were reported in the crystal structures of [La(NO3)3(iPr2NCOCH2SCH2CONiPr2)2(H2O)] 9


and


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[La(NO3)3(iBu2NCOCH2SCH2CONiBu2)].13 A similar structure of La(TMTDA)33+ with D3 symmetry was obtained which possesses a rather long La-Sether distance of 5.06 Å (Figure S4, supporting information), identical with the values in those crystal structures. Although this isomer is 9.1kcal/mol less stable in the gas phase, it appears that such structure is preferred in the solid state, which is probably due to steric effects arising from the three nitrate ligands. Geometry optimization on the La(TMTDA)33+ complex with the lanthanum center coordinated by both Ocarbonyl and Sether atoms ended up with a structure with three imaginary frequencies according to which the Sether atoms tend to deviate from the ligand plane. The optimization results suggest the Sether atoms prefer not to bond the lanthanum center and the nine coordinate structures analogous to Ln(TMOGA)33+ and its homologues do not exist.2,4,5 A similar geometry with C3h symmetry is most stable for the Lu(TMTDA)33+ complex as well (Figure 3), and all the other isomers are higher in energy with the energy difference ranging from 1.8-7.2 kcal/mol (Figure S4, supporting information). The Lu-Ocarbonyl and Lu-Sether distances were computed to be 2.284 and 3.150 Å, both of which are ~0.2 Å shorter than those of lanthanum due to lanthanide contraction. The sum of single-bond radii of Lu and O (2.25 Å) is almost the same as the Lu-Ocarbonyl distance while that of Lu and S (2.65 Å) is 0.5 Å shorter,30 suggesting negligible bonding interactions between Lu and Sether. Compared with the nine coordinate Ln3+ center in Ln(TMOGA)33+ and its homologues, both La(TMTDA)33+ and Lu(TMTDA)33+ are six-coordinate complexes while the three Sether atoms are not

10


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coordinated to the metal center, which reflects the softer nature of sulfur than oxygen.32 The observation of Ln(TMTDA)33+ in the mass spectra indicates these tripositive complexes are stable against charge reduction in the gas phase. Since the Sether atoms are not coordinated to the metal center, the structure of Ln(TMTDA)33+ should be very similar to that of Ln(TMGA)33+ where the Ln3+ center is coordinated by six Ocarbonyl atoms.19 Although tripositive Ln(TMOGA)33+ complex is also a stable gaseous species,18 it is obvious that the presence of Oether atom in TMOGA is not crucial for the stabilization of tripositive lanthanide complex in the gas phase.

Figure 3. Optimized geometries of La(TMTDA)33+ and Lu(TMTDA)33+ with C3h symmetry at the B3LYP level of theory. (O: red, N: blue, C: gray, S: yellow, Ln light blue; hydrogen atoms are omitted for clarity.)

CID of Ln(TMTDA)33+ The observation of tripositive Ln(TMTDA)33+ complex in the gas phase allows the investigation of the fragmentation behaviors of Ln(TMTDA)33+ by CID, which makes it possible to probe the influence of Ln3+ cation 11



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as well as diamide ligand on the chemistry of these triply charged species. Figure 4 shows the CID spectra of selected Ln(TMTDA)33+ complexes under the same normalized collision energy (NCE). Increasing the NCE only affected the yield of the fragments while their relative abundance remained the same. Two different fragmentation channels were observed for all the Ln(TMTDA)33+ complexes except Eu(TMTDA)33+. The major channel is formation of Ln(TMTDA)2(TMTDA-45)3+ and neutral dimethylamine (45 Da) via the cleavage of Ccarbonyl-N bond and hydrogen transfer (Reaction 1). Since (TMTDA-45) is a non-radical neutral ligand, the III oxidation state of Ln is retained in the Ln(TMTDA)2(TMTD-45)3+ complex. Loss of protonated TMTDA (HTMTDA+) to form LnIII(TMTDA)(TMTDA-H)2+ is the minor fragmentation channel (Reaction 2) upon CID of Ln(TMTDA)33+, and the yield of LnIII(TMTDA)(TMTDA-H)2+ is about 1/10 of Ln(TMTDA)2(TMTDA-45)3+. Different from other Ln(TMTDA)33+ complexes, Eu(TMTDA)2(TMTD-45)3+ was not observed

when

Eu(TMTDA)33+

was

subjected

to

CID,

and

EuIII(TMTDA)(TMTDA-H)2+ as well as EuII(TMTDA)22+ are the only CID products with an approximate ratio of 1:3 (Reaction 3). Such difference in the CID behavior between Eu(TMTDA)33+ and other Ln(TMTDA)33+ complexes can be rationalized in terms of the difference in Ln3+/Ln2+ reduction potentials. Europium possesses the highest reduction potential (-0.36 V) across the lanthanide series,33 suggesting divalent europium should be most easily accessible, in line with our experimental observations. It is possible that all the other Ln(TMTDA)33+ complexes fragmented to

12


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give LnIII(TMTDA)(TMTDA-H)2+ via concerted mechanism before a stable LnII(TMTDA)22+ complex could be formed. The presence of stable II oxidation state for Eu should be responsible for the absence of Eu(TMTDA)2(TMTDA-45)3+ which would undergo charge reduction via the loss of (TMTDA-45)+. Similar reduction processes for ligated Eu3+ complexes have been observed in other systems.18,19,34,35 LnIII(TMTDA)33+ → LnIII(TMTDA)2(TMTDA-45)3+ + NH(CH3)2 (45 Da)

(1)

LnIII(TMTDA)33+ → LnIII(TMTDA)(TMTDA-H)2+ + HTMTDA+

(2)

EuIII(TMTDA)33+ → EuII(TMTDA)22+ + TMTDA+

(3)

In addition to the properties of Ln3+, the fragmentation chemistry of tripositive lanthanide-diamide complexes was also affected by the diamide ligands especially the central moiety of diamides. With the exception of Eu, the major fragmentation patterns of Ln(TMTDA)33+ are very similar to those of Ln(TMGA)33+ which preferred the formation of Ln(TMGA)2(TMGA-45)3+ via the loss of dimethyl amine,19 but different from those of Ln(TMOGA)33+ in which case Ln(TMOGA)2(TMOGA-86)3+ was the major fragmentation product.18 Such difference in the fragmentation patterns for these ligated lanthanide complexes indicates that C-N bond cleavage is more important for the complexes with TMGA and TMTDA ligands while the lanthanide-TMOGA complex preferred C-Oether bond cleavage. Note that the Oether atom of TMOGA is coordinated to Ln3+ in the Ln(TMOGA)33+ complex while the Sether and CH2 moieties in TMTDA and TMGA are not directly bonded to the lanthanide center. It is obvious that the major fragmentation patterns of 13



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lanthanide-diamide complexes are affected by the coordination modes especially the central moiety of diamide ligands, and it is possible to distinguish the difference on the coordination structure of these ligated complexes by comparing their fragmentation behaviors in the gas phase. For EuIII(TMGA)33+, EuIII(TMOGA)33+ and EuIII(TMTDA)33+, formation of divalent EuII(TMGA)22+, EuII(TMOGA)22+ and EuII(TMTDA)22+ via charge reduction is predominant in all cases,18,19 which is consistent with the highest reduction potential of Eu3+/Eu2+ couple (-0.36 V).33 For Yb and Sm which have the second and third highest reduction potentials (Yb3+/Yb2+: -1.05 V, Sm3+/Sm2+: -1.55 V) across the lanthanide series,33 their fragmentation behaviors are strongly affected by the ligands. Our

previous

investigations

revealed

that

CID

of

YbIII(TMGA)33+

and

SmIII(TMGA)33+ led to the formation of YbII(TMGA)22+ and SmII(TMGA)22+ as well as YbIII(TMGA)(TMGA-H)2+ and SmIII(TMGA)(TMGA-H)2+, and it depended on the nature of metal center whether formation of charge reducing or conserving products was preferred.19 In our current experiments, divalent product LnII(TMTDA)22+ was observed only for Eu(TMTDA)33+, and the fragmentation patterns of Yb(TMTDA)33+ and Sm(TMTDA)33+ were similar to those of other Ln(TMTDA)33+ complexes (Figure

S5,

Supporting

Information).

The

appearance

of

EuIII(TMTDA)(TMTDA-H)2+ upon CID of Eu(TMTDA)33+ demonstrated that charge reduction is in competition with charge conserving process even for europium, suggesting it is more favorable for the TMTDA ligand to lose proton compared with

14


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TMGA ligand due to the presence of Sether atom in TMTDA. As a result, it should be much more difficult to form YbII(TMTDA)22+ and SmII(TMTDA)22+ as revealed by the CID results in Figure 4, and the gas phase behaviors of all the Ln(TMTDA)33+ complexes are similar except Eu(TMTDA)33+. As far as the redox chemistry is concerned, the Ln(TMOGA)33+ complex behaved more like Ln(TMGA)33+ but the TMOGA ligand tended to be deprotonated more easily, which resulted in the formation of more LnIII(TMOGA)(TMOGA-H)2+ than LnII(TMOGA)22+ for Yb and Sm. The fact that TMTDA is more prone to be deprotonated than TMOGA and TMGA was also demonstrated by the CID spectra of Ln(TMTDA)43+ (Figure S6, Supporting Information) where loss of neutral and protonated TMTDA ligands to form Ln(TMTDA)33+ and Ln(TMTDA)2(TMTDA-H)2+ were observed for all the lanthanides. This contrasts the fragmentation behaviors of Ln(TMGA)43+ which only formed

Ln(TMGA)33+

via

neutral

TMGA

loss.19

The

formation

of

Ln(TMTDA)2(TMTDA-H)2+ during CID of Ln(TMTDA)43+ is consistent with the appearance of Ln(TMTDA)(TMTDA-H)2+ in the ESI mass spectra of LnCl3 and TMTDA mixtures. Unlike

the

Ln(TMOGA)23+

and

Ln(TMGA)23+

complexes,18,19

no

Ln(TMTDA)23+ analog was detected in the CID spectra of Ln(TMTDA)33+, suggesting at least three neutral TMTDA ligands are necessary to stabilize Ln3+ in the gas phase as a tripositive complex while two TMTDA ligands are not sufficient. 15



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Figure 4. CID mass spectra of selected Ln(TMTDA)33+. B1: LnIII(TMTDA)(TMTDA-H)2+, B2: LnII(TMTDA)22+. The asterisk denotes Eu(TMTDA)2(OH)2+. For Sm, Gd, and Yb, a single isotopomer of Ln(TMTDA)33+ was mass selected for CID.

Conclusion In summary, ESI of LnCl3 and TMTDA mixtures in methanol resulted in the formation of Ln(TMTDA)33+ in the gas phase. Theoretical calculations predicted that both La(TMTDA)33+ and Lu(TMTDA)33+ complexes possess C3h symmetry with the Ln3+ center coordinated by six Ocarbonyl atoms from three TMTDA molecules. The Sether atoms are not bonded to Ln3+, which is different from the roles of Oether atoms in 16


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TMOGA.

The

major

CID

product

of

most

Ln(TMTDA)33+

is

Ln(TMTDA)2(TMTDA-45)3+ formed via Ccarbonyl-N bond cleavage and hydrogen transfer, and Ln(TMTDA)(TMTDA-H)2+ is the minor product. For Eu(TMTDA)33+, charge

reducing

and

conserving

products

EuII(TMTDA)22+

and

EuIII(TMTDA)(TMTDA-H)2+ are in competition with the former being more favorable due to the highest Eu3+/Eu2+ reduction potential across the lanthanide series. Therefore, it is reasonable that no other LnII(TMTDA)22+ complex including YbII(TMTDA)22+ and SmII(TMTDA)22+ was observed upon CID of Ln(TMTDA)33+. The observation of Ln(TMGA)33+, Ln(TMOGA)33+ and Ln(TMTDA)33+ indicates the central moiety of the diamide ligand is not crucial to the stabilization of Ln3+ as a tripositive cation in gas phase. The fragmentation chemistry associated with the ligand for these three complexes depends on its coordination mode while their redox chemistry is influenced by the nature of metal center as well as ligand. Supporting Information: Complete citations for ref 21, detailed instrumental parameters in ESI experiments, ESI mass spectra of all LnCl3 and TMTDA mixtures in methanol, experimental and calculated mass spectra of La(TMTDA)33+, Sm(TMTDA)33+, Eu(TMTDA)33+, Eu(TMTDA)(TMTDA-H)2+ and Eu(TMTDA)22+, CID mass spectra of all Ln(TMTDA)33+ and Ln(TMTDA)43+, optimized geometries of La(TMTDA)33+ and Lu(TMTDA)33+. This material is available free of charge via the internet at http://pubs.acs.org. Acknowledgements This work was supported by the “Strategic Priority Research 17



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Coordination Structure and Fragmentation Chemistry of the Tripositive

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