Volume 27, Issue 3 p. 1103-1112
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Open Access

Gas-Phase Structures of Potassium Tetrakis(hexafluoro- acetylacetonato) Lanthanide(III) Complexes [KLn(C5HF6O2)4] (Ln=La, Gd, Lu)

Prof. Dr. Georgiy V. Girichev

Corresponding Author

Prof. Dr. Georgiy V. Girichev

Ivanovo State University of Chemistry and Technology, Sheremetevsky avenue 7, Ivanovo, 153000 Russia

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Prof. Dr. Nina I. Giricheva

Prof. Dr. Nina I. Giricheva

Ivanovo State University, Ermaka Street 39, Ivanovo, 153025 Russia

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Alexey E. Khochenkov

Alexey E. Khochenkov

Ivanovo State University of Chemistry and Technology, Sheremetevsky avenue 7, Ivanovo, 153000 Russia

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Dr. Valery V. Sliznev

Dr. Valery V. Sliznev

Ivanovo State University of Chemistry and Technology, Sheremetevsky avenue 7, Ivanovo, 153000 Russia

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Prof. Dr. Natalya V. Belova

Prof. Dr. Natalya V. Belova

Ivanovo State University of Chemistry and Technology, Sheremetevsky avenue 7, Ivanovo, 153000 Russia

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Prof. Dr. Norbert W. Mitzel

Corresponding Author

Prof. Dr. Norbert W. Mitzel

Bielefeld University, Universitätsstrasse 25, 33615 Bielefeld, Germany

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First published: 22 October 2020
Citations: 3

Graphical Abstract

Gas phase and solid state of potassium tetrakis(hexafluoroacetylacetonato) lanthanid complexes [KLn(hfa)4] (Ln: lanthanide) are very different. The structures of the of salt-like [KLn(hfa)4] (Ln=La, Gd, Lu) have now been determined by gas electron diffraction, and the nature of the bonding by quantum-chemical methods. The gas phase of such compounds is seldom studied, but highly relevant to applications in gas-phase metal transport or deposition techniques.

Abstract

The molecular structures of potassium tetrakis(hexafluoroacetylacetonato)lanthanide(III) complexes [KLn(hfa)4] (Ln=La, Gd, Lu; hfa=C5HF6O2,) were studied by synchronous gas-phase electron diffraction/mass spectrometry (GED/MS) supported by quantum-chemical (DFT/PBE0) calculations. The compounds sublime congruently and the vapors contain a single molecular species: the heterobinuclear complex [KLn(hfa)4]. All molecules are of C1 symmetry with the lanthanide atom in the center of an LnO8 coordination polyhedron, while the potassium atom is coordinated by three ligands with formation of three K−O and three K−F bonds. One hfa ligand is not bonded to the potassium atom. Topological analysis of the electron-density distributions confirmed the existence of ionic-type K−O and K−F bonding. The structures of the free [KLn(hfa)4] molecules are compared with those of the related compounds [KDy(hfa)4] and [KEr(hfa)4] in their crystalline state. The complex nature of the chemical bonding is discussed on the basis of electron-density topology analyses.

Introduction

Metal complexes with β-diketonate ligands have attracted considerable interest due to their stabilities under normal conditions and their exceptionally high volatilities. These properties allow their use for low-temperature gas-phase metal transport (metal–organic chemical vapor deposition: MOCVD, plasmaenhanced CVD: PECVD) in the thin-film, semiconductor, and superconductor industries.1-4

Polyfluorinated complexes, in particular those containing the hexafluoroacetylacetonato ligand [M(hfa)n] (hfa=1,1,1,5,5,5-hexafluoropentane-2,4-dionate) have considerably higher volatilities than the nonfluorinated β-diketonate counterparts.5 However, a drawback of using the hexafluoroacetylacetonato complexes of the lanthanides for gas-phase technologies is the frequently observed complicated composition of their saturated vapors. According to mass spectrometry studies,6-8 the vapors generally consist of monomeric, dimeric, and trimeric forms of [Ln(hfa)3]. The reason for oligomerization is the tendency of the central lanthanide atom to form a coordinatively saturated coordination polyhedron with eight or nine ligating oxygen atoms.9

In the monomeric [Ln(hfa)3] complexes, the lanthanide atom has a coordination number of six. Saturation of the LnIII coordination sphere can be used to prevent polymerization in the gas and solid phases as well as during hydrolysis processes. Making use of binuclear alkali metal–lanthanate complexes as volatile lanthanide-containing species is one of the strategies to solve this problem.7 Saturation of the coordination sphere of the central Ln3+ ion can in this case be achieved by addition of a fourth hexafluoroacetylacetonate chelating ligand, which results in complexes of the composition [MLn(hfa)4]. Such heterobinuclear β-diketonates remain poorly studied up to now, despite their high potential for creating new materials with specified properties.10, 11

Detailed information not only on the volatility of such precursors, but also on the types and molecular structures of species present in the gas phase, is necessary for rational design, prediction, and control of CVD processes as well as for other practical perspectives of this class of compounds. However, up to now there is no information available in the literature on the molecular structures of [MLn(hfa)4] compounds in the gas phase, although this is the relevant phase for gas-phase applications.

By contrast, the crystal structures of two [KLn(hfa)4] (Ln=Dy, Er) complexes have been studied.10 The structures of these compounds are very similar and depend only slightly on the type of lanthanide atom involved. According to ref. 10, both complexes form chain structures in their crystals in which the neighboring [Ln(hfa)4] ions are linked by alkali metal ions (Figure S1 of the Supporting Information ). The LnO8 coordination polyhedra in these complexes adopt distorted square-antiprismatic structures of pseudo-D4 symmetry. Two [Ln(hfa)4] units coordinate the potassium ions through six oxygen and six fluorine atoms. Consequently, the coordination numbers of the Ln atoms are eight and that of the potassium atom is 12.10 Interestingly, the potassium ions are located at the center of an approximate hexagon of fluorine atoms formed by two [Ln(hfa)4] units (Figure S1 of the Supporting Information). This is due to similar bonding of three ligands bridging the Ln and K atoms, leading to an approximate threefold symmetry and bringing the plane defined by three F atoms of one [Ln(hfa)4] unit to a position approximately containing the K atom; matching two of these units about one K atom results in this approximately hexagonal-planar surrounding of the K atom by six F atoms.

Recently, the mass spectra of the potassium tetrakis(hexafluoroacetylacetonato) lanthanide(III) complexes [KLn(hfa)4] with Ln=La, Gd, and Lu have been investigated.12 Their vaporization was studied over temperature ranges of 414–471 K for [KLa(hfa)4], 401–453 K for [KGd(hfa)4], and 399–442 K for [KLu(hfa)4]. The sublimation enthalpies ΔsubH°(T) were determined by the Knudsen effusion method in the framework of the second law of thermodynamics. They are 170.6±2.1, 166.4±3.6, and 161.8±2.3 kJ mol−1 for the La, Gd, and Lu complexes, respectively, and correlate with the lanthanide contraction.12 This study reports the congruent sublimation of these complexes, and this behavior should allow their structures to be studied in the gas phase.

Consequently, we aim here at determining the geometrical and electronic structure and the nature of chemical bonding of three complexes [KLn(hfa)4] (Ln=La, Gd, and Lu), applying synchronous gas-phase electron diffraction/mass spectrometry (GED/MS) augmented by data obtained from DFT calculations.

Results and Discussion

Sound knowledge of the volatilities of precursors in MOCVD technologies, the types of molecular species present in the gas phase, and their structures is necessary to predict and control CVD processes. For [KLn(hfa)4] (Ln=La, Gd, Lu), we recorded mass spectra simultaneously with electron-diffraction patterns; pure mass spectra have been reported earlier;12 they show high intensities of K+ and [KLn(hfa)3]+ ions generated by electron-impact elimination of one ligand from the parent species. The loss of one ligand by dissociative ionization is also a typical event for other types of metal β-diketonates (e.g., see ref. 13). For the complexes in this study, the formation of the [Ln(hfa)(hfa−CF2)]+ ion was found to be characteristic; in this process a difluorocarbene (CF2) fragment is eliminated from a CF3 group.

The observed mass spectra indicate that the saturated vapors of all three complexes consist of the parent [KLn(hfa)4] species. The good agreement between the experimental scattering intensities sM(s) and radial distribution curves f(r) with their theoretical analogues for all three complexes under study (Figure 1) supports the earlier proposed congruent sublimation.12 The experimental data agree well with C1 symmetry of the molecular complexes.

Details are in the caption following the image

Left: experimental (dots) and theoretical (solid) molecular intensity curves sM(s) and the difference (experimental−theoretical) at two nozzle-to-plate distances, L1=598 and L2=338 mm. Right: radial distribution curves f(r) and the difference (experimental−theoretical) for [KLa(hfa)4], [KGd(hfa)4], and [KLu(hfa)4].

Table 1 summarizes the structural parameters of [KLn(hfa)4] complexes determined by analysis of the gas electron-diffraction (GED) patterns obtained at sample temperatures of 425(7) K for [KLa(hfa)4], 418(9) K for [KGd(hfa)4], and 459(5) K for [KLu(hfa)4]. All three molecules have triply oxygen-coordinated potassium ions (C1(t) configuration in Figure 2 a). The good agreement between calculated and experimental molecular structure parameters is noteworthy, and this fact confirms both the correctness of the performed structural analyses and calculations. The most pronounced difference between GED and optimized PBE0 structures was found for the Ln⋅⋅⋅K internuclear distances in the case of La and Lu complexes as well as for (C′β−C)B, and K−O′’B bond lengths in [KLa(hfa)4].

Table 1. Optimized (DFT/PBE0) and experimental geometric parameters of [KLn(hfa)4][a] [Å, °].

[KLa(hfa)4]

[KGd(hfa)4]

[KLu(hfa)4]

Parameter

GED

QC

GED

QC

GED

QC

(Ln−O′)B p1

2.572(31)[b]

2.599

2.467(12)

2.471

2.368(12)

2.387

(Ln−O)B (p1)[c]

2.455(31)

2.482

2.369(12)

2.373

2.275(12)

2.295

(O′−C′β)B p2

1.254(8)

1.255

1.260(12)

1.253

1.250(17)

1.250

(O−Cβ)B (p2)[c]

1.237(8)

1.238

1.244(12)

1.237

1.232(17)

1.232

(C′β−Cr)B p3

1.387(13)

1.384

1.379(15)

1.382

1.381(7)

1.394

(Cβ−Cr)B (p3)[b]

1.406(13)

1.404

1.401(15)

1.404

1.400(7)

1.413

(C′β−C)B p4

1.573(6)

1.539

1.539(9)

1.538

1.548(4)

1.537

(C−F1)B p5

1.331(3)

1.322

1.323(3)

1.322

1.316(3)

1.322

(Cr−H)B p6[d]

1.077

1.077

1.077

1.077

1.077

1.077

K−O′B p7

2.59(26)

2.728

2.71(15)

2.721

2.749(17)

2.712

Ln⋅⋅⋅K[e]

3.867(78)

3.993

3.917(40)

3.916

3.927(18)

3.763

K⋅⋅⋅Fav[f]

2.91(24)

2.896

2.96(27)

2.867

2.89(6)

2.862

O’A-Ln-O’B p8

68.6(6)

69.8

72.5(6)

71.3

70.8(8)

72.0

(Ln-O′-C′β)A p9

133.2(15)

134.4

131.9(13)

133.6

135.7(12)

133.0

(C-C′β-Cr)B p10

114.0(10)

117.8

115.0(18)

118.1

114.1(12)

118.3

(F1-C-C′β)B p11

115.2(3)

114.5

113.9(15)

114.4

115.7(13)

114.4

(F3-C-F1)B p12

108.2(3)

107.5

108.4(15)

108.8

111.3(5)

107.6

(F3-C-C′β)B p13

110.4(3)

109.7

108.4(15)

108.8

106.2(4)

108.8

(O-Ln-O′-C′)A p14

25.7(50)

15.8

14.4(58)

16.3

13.8(36)

17.7

(C-C′β-Cr- Cβ)B p15

−178(8)

−178.0

177(6)

179.4

−160.1(13)

179.7

(F1-C-C′β-Cr)B p16

−12.7(74)

−14.3

−13.5(82)

−14.3

−29.6(26)

−14.2

K-O′B-O′C-Ln p17

134(6)

132.5

132(8)

132.6

132.4(16)

132.4

Rf, %

5.62

4.86

4.77

  • [a] For atom notation, see Figures 2 and 3. A, B, C, D: notation for different chelate rings. [b] Uncertainties given in parentheses were estimated as σ=[urn:x-wiley:09476539:media:chem202004010:chem202004010-math-0001 +(2.5σLS)2]1/2 for distances, where the scale error σscale=0.002 r, and σLS is the least-squares deviation; 3 σLS for valence and 2 σLS for torsion angles. [c] pi: refinable independent parameter; (pi): parameter refined in the i-th group. [d] Fixed value from (DFT/PBE0) calculations. [e] Dependent parameter. [f] Average value of internuclear distances.
Details are in the caption following the image

Molecular structures of free [KLn(hfa)4] and atom notation. a) C1(t) structure (A, B, C, D: notation of chelate rings) and b) C4(q) structure. The calculated structural parameters are given in Tables S1 and S2 of the Supporting Information.

Structural peculiarities

Both calculations and GED analyses resulted in the C1(t) configuration for all three [KLn(hfa)4] complexes under investigation (Figure 2 a). The structural peculiarities are very similar for these complexes. Thus, each of the three chelate rings (A, B, C) in [KLn(hfa)4] bears one three-coordinate oxygen atom O′, bonded to the potassium ion. This leads to longer Ln−O′ and O′−C′β bonds compared to Ln−O and O−Cβ. The average differences r(Ln−O′)−r(Ln−O) for rings A, B, and C decrease along the series from 0.088 Å for [KLa(hfa)4] to 0.068 Å for [KLu(hfa)4]. The fourth chelate ring, D, is not bonded to potassium, and the dissimilarity of the bond lengths in this ring is insignificant, that is, the differences between the distances r(Ln−O) in ring D do not exceed 0.004 Å.

For each [KLn(hfa)4] complex (Ln=La, Gd, Lu), the structural parameters of the chelate rings are largely independent on the nature of the lanthanide atom, and differences in the same type of bond lengths and bond angles between the three complexes are 0.001–0.003 Å and 0.1–2.0°, respectively.

All four chelate rings in the free [KLn(hfa)4] complexes are folded about their O⋅⋅⋅O lines. Although the folding angles do not exceed 14° in any case, this distortion of the chelate rings distinguishes heteronuclear β-diketonates from the homoleptic tris-complexes, [Ln(thd)3] and [Ln(hfa)3], which have D3 symmetry with planar chelate rings.14-16 The distortions of the [KLn(hfa)4] chelate rings appear to be caused by binding of the potassium ion to oxygen atoms of three of the four chelate ligands, leaving one of these not involved in this binding of potassium.

An essential feature of the gas-phase complexes is that the CF3 groups of the hfa ligands occupy positions in which the C−F bonds are actually eclipsed to the C−Cr bonds, that is, the torsion angles τ(F-C-Cβ-Cr) are close to 0°, except for the angles for the three CF3 groups surrounding the K+ ion. These three groups are rotated by 10–15° about the C′β−C axes (see Figure 3, Tables 1). Three F3 atoms of these groups form an almost equilateral F3 triangle (Figure 3), and the C−F3 bonds are significantly longer (1.350–1.359 Å) than the other C−F bonds (Table 1). The potassium ion is located slightly below the center of this F3 triangle by 0.437, 0.213, and 0.144 Å for the complexes of La, Gd, and Lu, respectively (Figure 3). Apparently, these rotations of the CF3 groups and the longer C−F3 bonds are caused by attractive K⋅⋅⋅F interactions. More evidence of this K⋅⋅⋅F bonding was established by analyses of the electron-density topologies (vide infra).

Details are in the caption following the image

Location of the potassium atom relative to the F3 triangle made up by three CF3 groups in free [KLn(hfa)4] molecules. The hfa ligand not bonded to K is omitted for clarity.

The internuclear distances r(Ln⋅⋅⋅K) and the distances r(K⋅⋅⋅F) in the three complexes studied in this work are rather similar. The average K−O′ bond lengths decrease slightly along the lanthanide series from 2.705 for [KLa(hfa)4] to 2.693 Å for [KLu(hfa)4].

From crystal to gas

The crystal structures of [KLa(hfa)4], [KGd(hfa)4], and [KLu(hfa)4] have not yet been determined. However, the similarity of the free-molecule structures of these three complexes allows us to assume such similarity also for potential crystal structures as well as to compare them with the reported crystal structures of [KDy(hfa)4] and [KEr(hfa)4].10

The central LnO8 coordination geometry of the molecules in both the crystal and in the free molecules in the gas phase is close to that of a square antiprism. Interestingly, the distortion from an ideal square-antiprismatic structure is much more significant for the gaseous complexes than for the crystalline complexes. In the crystal of [KEr(hfa)4] all O-Er-O bond angles in the ligands are 73(0.5)°, and the O-Er-O angles between the ligands are about 73–78° (Figure 4 b).

Details are in the caption following the image

Comparison of molecular structures of [KLn(hfa)4]. a) Structure of the lutetium complex in a gas phase, b) fragment of the chain in the crystal of Er(hfa)4 (only six CF3 groups coordinating the K atom are shown), c) frame of the chain in the crystal of [Er(hfa)4] (only coordination bonds are shown).

In the gaseous complexes, the potassium cations pull together three ligands, due to the Coulomb interactions with the negatively charged oxygen atoms; this decreases the O′-Ln-O′ angles between the ligands to about 70° and increases the O-Ln-O angles to approximately 85° (Figure 2 a). As a result, in comparison with the crystal data, the K−O bond lengths and Ln⋅⋅⋅K distances become shorter.

The main dissimilarity of the crystal and gas-phase structures is the different mutual coordination of the potassium ion and the [Ln(hfa)4] moiety (Figures 4 a–c). The crystals contain chains of alternating K+ and [Ln(hfa)4] ions, approximately equally coordinated to both sides. Such coordination leads to a relatively symmetric distortion of the LnO8 coordination polyhedron. The K+ ions in the crystals are bound to two [Ln(hfa)4] anions and occupy the centers of hexagons of six fluorine atoms (Figure 4 b), whereas in the gas-phase complexes they are only coordinated to a triangle of three fluorine atoms (Figure 4 a). The coordination number of potassium atom is thus six in the free molecules, whereas in crystals the K+ ions settled between two [Ln(hfa)4] anions adopt coordination numbers of twelve.

The dipole moments of the studied neutral gas-phase complexes [KLn(hfa)4] are large (≈10 D), and doubtless this affects the processes of embedding, packing, and chain formation of molecules in the crystals.

Lanthanide contraction

The trend of the Ln−O distances in the [KLn(hfa)4] complexes studied in this work reveals the effect of the lanthanide contraction. In the fragments K-O′-Ln (Figure 2 a), the difference between the metal–oxygen bond lengths Δr(Ln−O′)=r(La−O′)−r(Lu−O′) is 0.204(33) Å and 0.212 Å according to the experimental and theoretical data, respectively. For the Ln−O bond lengths, the difference Δr(Ln−O)=r(La−O)−r(Lu−O) derived from PBE0 is 0.187 Å and is close to the experimental value (Table 1). The last value is in a good agreement with those of Δr(Ln−O)=0.187(6) Å for β-diketonato complexes in the tris-dipivaloylmethanides [Ln(thd)3] and Δr(Ln−O)=0.190(9) Å in the hexafluoroacetylacetonates [Ln(hfa)3],15, 16 as well as Δr(Ln−Cl)=0.186(8) Å, Δr(Ln−Br)=0.186(9) Å, and Δr(Ln−Cl)=0.190(9) Å for lanthanide trihalides.17

Thus, the magnitude of the lanthanide contraction weakly depends on the coordination number of the lanthanide atom and appears to be rather similar for three-, six-, and eight-coordinate Ln atoms in the trichlorides, the tris(β-diketonates), and the binuclear complexes [KLn(hfa)4], respectively.

We also note that the lanthanide contraction for Ln3+ ions is Δr(Ln3+)=r(La3+)−r(Lu3+)=0.187 Å.18 The good correlation of Δr(Ln−O) with the trend of the Ln3+ ionic radii provides evidence for a significantly ionic nature of the chemical Ln−O bonds in the potassium tetrakis(hexafluoroacetylacetonato) lanthanate(III) complexes [KLn(hfa)4] and other β-diketonato compounds.

The O’-Ln-O bond angles increase along the series Ln–Lu from 67 to 72°, reflecting the lanthanide contraction and rigidity of the chelate fragments at the same time.

Chemical bonding

The quantum theory of atoms in molecules (QTAIM)19 and the natural bond orbital (NBO)20 analysis are the most widely used approaches for analyzing the electron-density distribution in molecules. The results of applying these approaches to the complexes [KLn(hfa)4] are summarized in Tables 2 and 3 and Tables S5–S7 of the Supporting Information.

Table 2. Averaged net atomic charges q [e] according to QTAIM and NPA calculations for [KLn(hfa)4] (Ln=La, Lu) complexes (C1(t) structures).

QTAIM

NPA

La

Lu

La

Lu

q(K)

0.92

0.93

0.96

0.97

q(Ln)

2.44

2.35

2.50

1.98

q(O)

−1.22

−1.21

−0.69

−0.62

q(O′)

−1.24

−1.23

−0.75

−0.69

q(Cβ)

0.96

0.96

0.44

0.44

q(C′β)

0.91

0.90

0.40

0.40

q(Cr)

−0.04

−0.04

−0.50

−0.49

q(H)

0.06

0.06

0.27

0.27

q(chel)[a]

−0.59

−0.57

−0.82

−0.69

q(C)[b]

1.87

1.88

0.89

0.89

1.84

1.84

0.89

0.89

q(F1)[b]

−0.67

−0.66

−0.31

−0.31

−0.66

−0.66

−0.29

−0.30

q(F2)[b]

−0.66

−0.66

−0.30

−0.30

−0.66

−0.66

−0.31

−0.31

q(F3)[b]

−0.65

−0.65

−0.30

−0.29

−0.66

−0.66

−0.32

−0.32

q(CF3)[b]

−0.10

−0.10

−0.01

−0.01

−0.16

−0.15

−0.03

−0.03

  • [a] q(chel): total charge of the chelate fragment (-O-Cβ-Cr(H)-Cβ-O-). [b] Upper and lower values assigned to the same parameter correspond to CF3 groups linked to Cβ and C′β atoms, respectively.
Table 3. Selected averaged electron-density topological parameters at BCPs and Wiberg bond indexes Q for [KLn(hfa)4] (Ln=La, Lu) complexes (C1(t) structures).

Interaction

QTAIM

NBO

ρb[a]

2ρb[b]

ϵ[c]

ntopo[d]

δ[e]

Q

Ln

La

Lu

La

Lu

La

Lu

La

Lu

La

Lu

La

Lu

K−F

0.062

0.058

1.257

1.189

0.103

0.123

0.05

0.04

0.00

0.00

K−O′

0.118

0.121

1.993

2.0470

0.028

0.031

0.09

0.09

0.01

0.01

Chelate ring

Ln−O

0.341

0.384

4.211

5.653

0.015

0.013

0.26

0.23

0.12

0.22

Ln−O′

0.298

0.341

3.674

4.992

0.030

0.020

0.22

0.21

0.10

0.20

O−Cβ

2.655

2.660

−8.349

−7.846

0.043

0.047

1.15

1.15

1.19

1.19

1.52

1.51

O′−C′β

2.576

2.568

−10.807

−10.646

0.046

0.048

1.15

1.15

1.14

1.14

1.44

1.43

Cβ−Cr

2.097

2.097

−20.913

−20.941

0.206

0.203

1.21

1.21

1.25

1.24

1.31

1.30

C′β-Cr

2.155

2.165

−21.788

−21.971

0.241

0.244

1.27

1.28

1.32

1.33

1.40

1.40

Cr−H

1.980

1.980

−28.321

−28.340

0.030

0.030

0.86

0.85

0.93

0.93

0.90

0.90

CF3 group

C−Cβ[e]

1.758

1.761

−16.649

−16.726

0.036

0.036

0.76

0.76

0.79

0.79

0.89

0.89

1.763

1.770

−16.724

−16.858

0.041

0.042

0.78

0.78

0.80

0.80

0.90

0.90

C−F1[e]

1.951

1.950

−10.551

−10.579

0.127

0.127

-

0.65

0.65

0.92

0.92

1.997

1.999

−10.021

−10.025

0.130

0.129

0.66

0.66

0.94

0.94

C−F2[e]

1.976

1.969

−10.869

−10.926

0.130

0.130

0.66

0.65

0.93

0.92

1.933

1.940

−10.881

−10.895

0.140

0.138

0.65

0.65

0.92

0.92

C−F3[e]

1.983

1.992

−10.816

−10.757

0.129

0.127

0.66

0.66

0.93

0.93

1.906

1.900

−10.855

−10.905

0.142

0.142

0.64

0.64

0.90

0.90

F3(A)-F2(D)

0.002

0.004

0.030

0.058

0.690

0.175

0.0016

0.0037

F2(A)−F3(D)C′β

0.002

0.003

0.026

0.049

1.383

0.278

0.0013

0.0030

  • [a] ρb [e Å−3]: electronic density. [b] ∇2ρb [e Å−5]: the Laplacian. [c] ϵ: bond ellipticity: ϵ=λ1/λ2−1, where λ1, λ2 are eigenvalues of the electronic-density Hessian matrix. [d] ntopo: topological bond-order index; δ(e): the electron delocalization index (average number of electrons delocalized between the pair of atoms. [e] Values in the upper and lower rows correspond to CF3 groups linked to atoms Cβ and C′β, respectively.

Table 2 lists net atomic charges calculated within the QTAIM and NBO (NPA) schemes. Some electron-density topology parameters (electron density ρb, Laplacian ∇2ρb, bond ellipticity ϵ, topological bond-order indices ntopo and δ) at the bond critical points (QTAIM) and Wiberg bond indices Q (NBO) are listed in Table 3.

The increasing Wiberg bond indices Q for the Ln−O and Ln−O′ bonds from [KLa(hfa)4] to [KLu(hfa)4] indicate a decrease of the covalent contribution to the Ln−O bond (Table 3). The energy of the donor–acceptor interaction between the lone pair at oxygen LP(O), which does not interact with the potassium atom, and vacant orbitals at the Ln atoms is about 45 and 54 kcal mol−1 for the La−O and Lu−O bonds, respectively, and these values are typical for the other β-diketonates. This energy for even longer La−O′ and Lu−O′ bonds is lower (ca. 32 and 37 kcal mol−1, respectively). In contrast, the QTAIM analysis revealed only small changes in charge at the Ln atoms (Table 2). It classifies the Ln−O bonds as ionic on the basis of low electron densities ρb and small positive values of the Laplacians ∇2ρb at the bond critical points (BCPs) (Table 3). The molecular graph established by QTAIM analysis (Figure 5) found three K−O and three K⋅⋅⋅F atomic interaction lines and located corresponding BCPs. Furthermore, the densities ρb, Laplacians ∇2ρb, and topological bond-order indices δ at these all BCPs prove the classification of the K−O and K−F bonds as ionic.19 The NBO analysis confirmed this conclusion, since the energy of the donor–acceptor interaction between the lone pairs at oxygen LP(O) or at fluorine LP(F) and vacant orbitals at the potassium atom is very small (1.5 and 2.2 kcal mol−1 for K−O′ and K⋅⋅⋅F3, respectively).

Details are in the caption following the image

The molecular graph for the [KLa(hfa)4] molecule. The BCPs are dark blue, the ring critical points are red, and the cage critical point is blue. The intramolecular K⋅⋅⋅F and F⋅⋅⋅F bond paths are represented by dashed lines.

In addition to K−O′ bonding, the potassium ions are involved in ionic K−F interactions, and thus the coordination number of the potassium ion is six. The existence of ionic coordination bonds in the complexes studied herein leads to cyclic structures in the molecular graph (Figure 5). Along with the ring critical points, one cage critical point is located inside the polyhedron that is formed by Ln−O′ and K−O′ atomic interaction lines.

Within the ligands, QTAIM analysis discovered atomic interaction lines and corresponding BCPs for the O−Cβ, Cβ−Cr, Cr−H, Cβ−C, and C−F interactions. The topological parameters for these bonds (Table 3) clearly indicate covalent nature for all of them.

According to the model of Howard and Lamarche21 the topological bond-order indices ntopo can be calculated by using the equation ntopo=a0+a1(λ1+λ2)+a2λ3+a3ρb, which connects the covalent bond order with the electron density and the electron-density Hessian matrix eigenvalues λ1, λ2, and λ3 at the BCPs. The coefficients a0, a1, a2, and a3 for C−C, C−O, and C−H bonds were taken from ref. 22, which contains an improved and expanded coefficient set. The ntopo values are rather close to the δ values, which are calculated directly from the electron-density distribution between a pair of atoms.

The topological bond-order indices ntopo and δ as well as the Wiberg bond indices Q (Table 3) do not correspond to exact single or double bonds for O−Cβ and Cβ−Cr in the chelate rings. This circumstance confirms the presence of π conjugation in the -O-Cβ-Cr(H)-Cβ-O- fragment. As additional criterion for π-bonding contributions that is used in QTAIM is the bond ellipticity ϵ. The bond ellipticity determines the ratio between negative curvatures λ1 and λ2 at the BCPs and can be computed by using the equation ϵ=λ1/λ2−1. For ideal single and triple bonds (λ1=λ2) the ϵ value is zero, and bonds have a cylindrically symmetrical electron-density distribution. Figure 6 shows the electron-density Laplacian zero isosurface. One can note noticeable bond ellipticity (see also Table 3) for O−Cβ and Cβ−Cr bonds.

Details are in the caption following the image

Zero-value surface of the electron-density Laplacian in [KLa(hfa)4].

According to the NPA analysis, the total charges of the CF3 groups are close to zero (−0.01 e and −0.03 e). At the same time, QTAIM provides charges q(CF3) between −0.10 and −0.16 e indicating the pronounced electron-withdrawing properties of trifluoromethyl groups (Table 2).

The QTAIM charges on the fluorine atoms are about −0.66 e, whereas on the carbon atoms of the CF3 groups they lie between 1.82 and 1.87 e. The topological features of the C−F bonds are accompanied by significant electron-density redistributions in the CF3 groups. The electron-density Laplacian zero isosurface around the fluorine atoms looks like a sphere, and the BCP is moved towards the carbon atom (Figure 6). Thus, the C−F bond must be described as a strongly polar bond. The pronounced negative charges on the fluorine atoms indicate that the electron shell of fluorine atoms in the CF3 group is diffuse. Clearly, this diffuse shell on fluorine atoms is the reason for existence of BCPs on two F⋅⋅⋅F lines. The low electron density and the positive Laplacian for these critical points indicate that a van der Waals interaction occurs between the fluorine atoms of A and D chelate rings.

Conclusion

The structure of the binuclear complexes [KLn(hfa)4] (Ln=La, Gd, Lu) in the gas phase was determined by synchronous MS and ED experiments augmented by quantum-chemical calculations. While earlier studies on crystalline [KLn(hfa)4] compounds have established structures of endless chains consisting of alternating units of [Ln(hfa)4] anions and K+ cations, our new MS and GED data show that, on sublimation, the continuous chains [⋅⋅⋅K+⋅⋅⋅[Ln(hfa)4]⋅⋅⋅K+⋅⋅⋅] are cleaved into isolated molecular species [KLn(hfa)4]. In these molecules the potassium cations find fewer oxygen and fluorine atoms to bond to, but still enough contacts to exist as molecular entities in the gas phase. Thus, in gaseous binuclear complexes [KLn(hfa)4], the coordination number of the K+ ion is six, whereas in the crystal the K+ cation is situated in the middle between two [Ln(hfa)4] anions and has a coordination number of twelve. It is probably the possibility of enveloping the potassium ion in the ligand shell of the anion that allows molecular units to pass from the solid to the gaseous state without decomposition.

Quantum-chemical calculations (PBE0) predict the presence of two isomers C1(t) and C4(q) with three and four K−O bonds. These isomers are close in relative energy, but rather different in their relative Gibbs free energies at the temperature of the GED/MS experiment. However, the experimental ED data clearly correspond solely to the structure C1(t), in which the ion K+ is located between three of the four ligands. In these species the LnO8 coordination polyhedron adopts the structure of a distorted square antiprism. All four chelate rings in a free [KLn(hfa)4] complex are folded about O⋅⋅⋅O line, whereas the homoleptic tris-ligand complexes [Ln(thd)3] and [Ln(hfa)3] contain planar chelate rings and have overall D3 symmetry.

The potassium ion is located slightly below the center of an F3 triangle made up by fluorine atoms of the three CF3 groups and is mainly bonded to three oxygen atoms of the LnO8 coordination polyhedron. The asymmetry caused by binding to three of the four hfa ligands leads to a noticeable nonequivalence of the C−O and C−Cr bonds. The three potassium-bonded ligands are attracted so much by the potassium ion that narrowing of the O-Ln-O angles between pairs of ligands results.

The nature of bonding between [Ln(hfa)4] anions and K+ cations, analyzed by the QTAIM concept, is characterized by three K⋅⋅⋅F and three K−O atomic interaction lines. The topological characteristics for K⋅⋅⋅F and K−O BCPs reveal the predominantly ionic nature of these interactions. The trend of Ln−O distances along the series [KLa(hfa)4] to [KLu(hfa)4] reveals the effect of the lanthanide contraction. The value of the lanthanide contraction Δr(Ln−O) is in a good agreement with that for the β-diketonato complexes [Ln(hfa)3] and [Ln(thd)3] as well as for the lanthanide trihalides LnX3 with X=Cl, Br, I. The lanthanide contraction was shown to depend only weakly on the coordination number of the lanthanide atoms, that is, it is very similar for three-, six-, and eight-coordinate Ln atoms. A clear correlation of Δr(Ln−O) with the Ln3+ ionic radii provides further proof for the predominantly ionic nature of Ln−O chemical bond.

Experimental Section

Quantum-chemical calculations

Quantum chemical studies on the structures and calculations of vibrational frequencies of the [KLn(hfa)4] complexes were carried out with Gaussian 03.23 DFT methods with hybrid PBE0 functional24 were employed, because an earlier theoretical study on the related complexes of lanthanides with dipivaloylmethane, [Ln(thd)3], showed that the results of PBE0 calculations are in better agreement with gas-phase experimental values than those obtained with the B3LYP functional.14

The inner electronic shells of Ln (1s22s22p63s23p63d104s24p64d104fk, k=ZLn−57, where ZLn is the atomic number of Ln) were described by the relativistic effective core potentials (ECPnMWB, n=ZLn−11).25, 26 For the outer shells of Ln (5s25p65d16s2) the basis sets were taken from refs. 25, 27, 28 (8s7p6d3f2g/6s5p5d3f2g). The segmented polarization consistent valence-triple-zeta basis sets pcs-229 were taken for K (28s18p6d1f/ 7s5p4d1f), all F, O, and C (11s6p2d1f/4s3p2d1f), and H (5s2p1d/3s2p1d) atoms. According to ref. 29, the pcs-n basis sets were optimized for DFT calculations and provide higher accuracy for calculations of the molecular system properties in comparison with the often-used 6-31G*, 6-311G*, and cc-pVXZ basis sets.

Preliminary calculations of possible molecular configurations of [KLn(hfa)4] were performed at the Hartree–Fock (HF) level of theory. Different positions of the potassium ion with di-, tri-, and tetracoordination by the oxygen atoms in the anionic Ln(hfa)4 ligands were probed (Figure 7). The energetically most preferable structure of [KLn(hfa)4] is C1(t) with three-coordinate potassium atom. In configurations C1′(t) and C2(q) the potassium ions are coordinated by three and four oxygen atoms, respectively, and two of these oxygen atoms belong to the same hfa ligand. Both structures correspond to minima on the potential energy surface (PES), but have rather high relative energies. A structure with a two-coordinate potassium ion, C2v(b), corresponds to a second-order saddle point on the PES.

Details are in the caption following the image

Configurations of molecular [KLn(hfa)4] species corresponding to different points on the PES (nSP, where n is the order of saddle point, min: minimum) and the relative energy ΔE [kJ mol−1] by HF calculations.

The DFT/PBE0 approximation was used for further geometry optimization of structures C1(t) and C2v(b) and subsequent force-field and vibrational-frequency calculations. The PBE0 calculations also result in second-order saddle points on the PES for structure C2v(b). The relative energy of these are 19.4, 20.1, and 22.5 kJ mol−1 for [KLa(hfa)4], [KGd(hfa)4], and [KLu(hfa)4], respectively. Analysis of the vibrational modes corresponding to imaginary frequencies shows that the displacement of the potassium atom for one of the modes corresponds to a rearrangement to the three-coordinate configuration C1(t), while a second mode can be attributed to the rotation of the chelate rings, followed by lowering of the symmetry from C2v to C2.

Subsequent optimization of the structure with C2 symmetry led to a further configuration C4(q) (Figure 2 b) with four-coordinate potassium ion. This configuration corresponds to a minimum on the PES like C1(t), but the relative energy for C4(q) structure is higher than that for C1(t) and can be considered to be an isomerization energy. This isomerization energy decreases from 5.1 kJ mol−1 for [KLa(hfa)4] to 2.7 kJ mol−1 for [KLu(hfa)4]. The calculated geometric parameters for structures C1(t) and C4(q) of all of the three complexes studied here are listed in Tables S1 and S2 of the Supporting Information. The atom notation for structure C1(t) is shown in Figures 2 a and 3.

Natural bond orbitals analyses for structures C1(t) and C4(q) of the [KLn(hfa)4] complexes were performed with the NBO 3.1 program,20 incorporated in the G03 program suite, and were used to obtain net atomic charges and Wiberg bond indexes. Topological analyses of the electron-density distribution functions ρ(r) were carried out with the AIMAll Professional software.30

Synthesis

The complexes were synthesized in accordance with known protocols.31, 32 The sources and purity of the materials used in the experiments have been described earlier in detail12 and are provided in Table S3 of the Supporting Information.

Gas electron diffraction

Electron-diffraction patterns and mass spectra of [KLn(hfa)4] complexes were recorded simultaneously by using the techniques described elsewhere33, 34 at two nozzle-to-film distances: L1=598 and L2=338 mm. The samples were evaporated at 425(7) K for [KLa(hfa)4], 418(9) K for [KGd(hfa)4], and 459(5) K for [KLu(hfa)4] from an X18H10T stainless steel effusion cell. The effusion channel had a diameter of 0.5 mm and a length of 1.6 mm. Six ED patterns of each substance and two ED patterns of the polycrystalline ZnO standard were recorded for each nozzle-to-plate distance. The electron wavelengths were determined from the ZnO diffraction pattern. The conditions of the GED/MS experiment are listed in Table S4 of the Supporting Information. The microphotometric measurements of optical densities of exposed films were carried out by means of automatic techniques35 with a step of 0.1 mm along the diagonal of the plate. A 10×130 mm region was scanned in 33 equidistant scans. The molecular intensities and radial distribution curves are shown in Figure 1.

The mass-spectral monitoring of the vapor over the solid samples at the temperature of the experiment proved the presence of molecular species with a composition [KLn(hfa)4]. The relative abundances of the characteristic ions in the mass spectra of [KLa(hfa)4], [KGd(hfa)4], and [KLu(hfa)4] are listed in Table 4. Volatile impurities were not detected under the conditions of the GED experiment, except for the case of the lutetium complex, for which two ions were observed at approximately 1200 and 1600 Da. However, the intensities of these ions were at the experimental noise level, that is, the relative abundance of both ions was less than 0.5 %. Therefore, species that were the parent molecules associated with an appearance of these ions were ignored in the refinement of the GED data.

Table 4. The relative abundance of ions in mass-spectra of [KLn(hfa)4] recorded simultaneously with the diffraction pattern registration (Uionization=50 V).

Ion[a]

m/z

Irel. [%]

La

Gd

Lu

nozzle-to-plate distance [mm]

338

598

338

598

338

598

[K]+

39

100

100

100

100

100

100

[CF3]+

69

0.5

1.0

1.0

0.5

[Ln(hfa)−F]+

Ln+188

0.5

0.5

0.5

0.5

[Ln(hfa)+F]+

Ln+226

0.5

0.5

0.5

0.5

[Ln(hfa)2−2 CF2]+

Ln+314

0.5

0.5

[Ln(hfa)2−CF2]+

Ln+364

1.0

1.0

1.0

1.0

3.0

3.5

[Ln(hfa)2]+

Ln+414

1.0

1.0

1.0

1.0

1.5

1.5

[Ln(hfa)3−2 CF2]+

Ln+521

1.0

1.0

[Ln(hfa)3−CF3]+

Ln+552

2.5

2.5

[KLn(hfa)3−2 CF2]+

Ln+560

1.0

1.0

[Ln(hfa)3]+

Ln+621

0.5

0.5

0.5

0.5

1.0

1.0

[KLn(hfa)3]+

Ln+660

9.0

8.0

8.0

7.0

7.0

6.5

[KLn(hfa)4]+

Ln+867

1.0

GED data analysis

The mass spectra recorded during the GED/MS experiments for all three complexes reproduce the peculiarities noted earlier.12 A theoretical study on the structure of [KLn(hfa)4] resulted in two configurations, C1(t) and C4(q), that correspond to minima on the PES (Figure 2). Although the relative energies of these structures are rather close, the relative Gibbs free energy of configuration C4(q) is considerably higher, notably 21.8, 19.5, and 19.0 kJ mol−1 for complexes of La, Gd, and Lu, respectively. As a result, the theoretical mole fractions of structures C1(t) are 99.8, 99.6, and 99.3 % for complexes of La, Gd, and Lu, respectively. Consequently, only the C1(t) configurations were examined in the GED data analyses.

The structures of the complexes were refined by least-squares (LS) analysis of the experimental sM(s) functions. Structural starting parameters were taken from the PBE0 calculations. The following independent parameters were used for the constructions of structural models (Figure 2 a): bond lengths rh1: (Ln−O′)B, (O′-−C′β)B, (C′β−Cr)B, (C′β−C)B, (C−F1)B, (Cr−H)B, (K−O′)B; bond angles ∡: (O′A-Ln-O′B), (Ln-O′-C′β)A, (C-C′β-Cr)B, (F1-C-C′β)B, (F3-C-F1)B, (F3-C-C′β)B; dihedral angles τ: (O-Ln-O′-C′)A, (C-C′β-Cr-Cβ)B, (F1-C-C′β-Cr)B, and (K-O′B-O′C-Ln). The differences between each of the rh1(Ln−O), rh1(O−C), rh1(K-−O), rh1(C−F), and rh1(C−C) values of the same type, as well as between rh1(C−H), were constrained to calculated values (PBE0). Vibrational amplitudes and corrections, Δr=rh1ra, were derived by using the theoretical force fields (PBE0) by taking into account the nonlinear kinematic effects at the level of first-order perturbation theory for the transformation of Cartesian coordinates into internal coordinates by the VibModule program.36 Vibrational amplitudes were refined in groups corresponding to the different peaks on the radial distribution curve with fixed differences. Thus, seven bond lengths, six bond angles, and four dihedral angles were refined simultaneously and independently with eleven groups of vibrational amplitudes. The LS analyses of the molecular intensity functions sM(s) was performed by the modified KCED-35 program with an algorithm similar to that described earlier.37 Results of the LS analysis are listed in Table 1 along with the theoretical (PBE0) values.

Acknowledgements

This work was supported by RSF (grant 20-13-00359). Preliminary laborious quantum chemical calculations were supported by RMSE (grant FZZW-2020-0007). The authors thank Prof. N. P. Kuzmina for providing samples. Open access funding enabled and organized by Projekt DEAL.

    Conflict of interest

    The authors declare no conflict of interest.