New complexes of lanthanide chlorides.

www.elsevier.nl/locate/poly Polyhedron 19 (2000) 199–204

New complexes of lanthanide chlorides. Reversible isomerization in octahedral [LaCl3(HMPA)3] and the crystal structure of fac-[SmCl3(HMPA)3] ˇ *, A. Demsar, ˇ L. Golic, ˇ J. Kosmrlj ˇ S. Petricek ˇ ˇ University of Ljubljana, Faculty of Chemistry and Chemical Technology, Askerceva 5, 1001 Ljubljana, Slovenia Received 9 August 1999; accepted 10 November 1999

Abstract Complexes of [LnCl3L] (LnsLa, Nd, Sm, Eu, and Lsdiethylene glycol dimethyl ether, diglyme; LnsLa, Pr, and Lsdimethoxyethane, DME) were prepared from the lanthanide oxides and hydrogen chloride, which was formed in situ from chlorotrimethylsilane and water. When hexamethylphosphoramide (HMPA) was added to the suspension of the prepared DME and diglyme complexes in toluene, fac[LnCl3(HMPA)3] (LnsLa, Pr, Nd, Sm, Eu, Gd) complexes, isomorphous according to the unit cell determination, were formed. The crystal structure of [SmCl3(HMPA)3] was determined. Each samarium atom is octahedrally coordinated by three chlorine atoms and three HMPA molecules, which are distributed in a facial mode. The exchange of coordinated and free HMPA, as well as the reversible fac–mer isomerization of [LaCl3(HMPA)3], was observed by variable temperature 1H NMR spectroscopy. The associative mechanism of the exchange and isomerization was proposed. q2000 Elsevier Science Ltd All rights reserved. Keywords: Crystal structures; Fac isomers; Isomerization; Lanthanide chloride complexes; Octahedral complexes

1. Introduction An important source of information regarding the reactivity of coordination and organometallic compounds are studies of fluxionality, isomerization [1] and ligand exchange [2]. Transition metal six-coordinate complexes are rather rigid [3]. On the other hand, the small crystal-field splitting and larger ionic radii of six-coordinate lanthanide complexes promise enhanced fluxionality and facile coordination sphere extension [4]. The isolation of solid state six-, seven- and eight-coordinate complex ions [Ln(NCS)n](ny3)y (ns6, 7, 8) [5], six-coordinate [LnCl3(HMPA)3] (HMPAs hexamethylphosphoramide) [6] and seven-coordinate [LaBr3(HMPA)4] [7] suggests that little energy is required for transition among six-, seven- and eight-coordinate species. The facile dissociation and binding of ligands are important for the catalytic activity of coordination and organometallic compounds. The growing interest in the reactivity of lanthanide compounds is stimulated by their use as versatile reagents and catalysts [8–12]. The solution dynamics of eight- and higher * Corresponding author. Tel.: q386-61-176 0556; fax: q386-61-125 8220; e-mail: [email protected]

coordinate solvated lanthanide ions [4] and the lanthanide magnetic resonance imaging agents [13] have attracted much interest, but six-coordinate lanthanide compounds have been studied less intensively. We report on the synthesis of new [LnCl3(DME)] (LnsLa, Pr; DMEsdimethoxyethane) and [LnCl3(diglyme)] (LnsLa, Nd, Sm, Eu; diglymesdiethylene glycol dimethyl ether) complexes directly from the corresponding lanthanide oxides and chlorotrimethylsilane. Two similar synthetic routes for the preparation of lanthanide chloride complexes have been reported, the first starting from lanthanide metals and chlorotrimethylsilane [14] and the second from lanthanide oxides using tionyl chloride [15]. The [LnCl3(DME)] (LnsLa, Pr) and [LnCl3(diglyme)] (LnsLa, Nd, Sm, Eu) complexes were used in the preparation of fac-[LnCl3(HMPA)3] (LnsLa, Pr, Nd, Sm, Eu, Gd) isomers. The solution dynamics of [LaCl3(HMPA)3] are also described. 2. Experimental 2.1. Materials All manipulations were carried out under an atmosphere of purified argon using Schlenk tubes or in a dry box. Lan-

0277-5387/00/$ - see front matter q2000 Elsevier Science Ltd All rights reserved. PII S 0 2 7 7 - 5 3 8 7 ( 9 9 ) 0 0 3 4 5 - 9

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thanide oxides (Aldrich), diglyme (Fluka), DME (Fluka), chlorotrimethylsilane (Aldrich) and HMPA (Aldrich) were used as received. Tetrahydrofuran (THF) and toluene were dried with Na–K alloy and distilled before use. 2.2. Physical measurements The samples were ground in a Nujol mull and IR spectra were recorded on a Perkin-Elmer 1720X instrument between 400 and 4000 cmy1. The chlorine content was determined by a potentiometric titration of chlorine ions with silver nitrate. The lanthanide content was determined by gravimetric analysis. Elemental analyses were obtained using a Perkin-Elmer 2400 CHN analyser at the University of Ljubljana (Department of Organic Chemistry). The 1H, 31P and 139La NMR spectra were recorded on a Bruker DPX 300 spectrometer. Variable temperature spectra were recorded using the variable temperature controller of the spectrometer. Deuterated toluene was dried with potassium and distilled under reduced pressure.

MHz, [D8]toluene): d 2.50, J(PH) 9Hz. MHz, [D8]toluene): d 26.1.

31

P NMR (121

2.3.3.2. Method B. HMPA (0.700 g, 3.91 mmol) was added to a suspension of [LaCl3(diglyme)] (0.338 g, 0.891 mmol) in toluene (20 ml). A clear solution resulted immediately. Bright, colourless crystals of the complex [LaCl3(HMPA)3] grew out of the solution during slow evaporation of the solvent. Found: La, 17.4; C, 27.41; H, 7.00; N, 15.86. Calc. for [LaCl3(HMPA)3]: La, 17.7; C, 27.62; H, 6.95; N, 16.10%. IR (Nujol) (cmy1): 1303 s, 1194 m, 1163 m, 1130 vs, 1067 w, 988 vs, 753 s, 482 m. 2.3.4. [PrCl3(DME)] To 0.495 g (0.48 mmol) of Pr6O11 were added 13.6 g (151 mmol) of DME, 13.6 g (125 mmol) of (CH3)3SiCl and 0.10 g (5.55 mmol) of water. The suspension was stirred for 4 days and after drying in vacuo 0.950 g (96.8% yield) of the dark brown complex [PrCl3(DME)] was gained. Found: Cl, 31.0. Calc. for [PrCl3(DME)]: Cl, 31.5%. IR (Nujol) (cmy1): 1082 s, 1039 vs, 1019 s, 1001 s, 859 s.

2.3. Synthesis 2.3.1. [LaCl3(DME)] To 0.440 g (1.35 mmol) of La2O3 were added 12.91 g (143 mmol) of DME, 13.09 g (121 mmol) of (CH3)3SiCl and 0.10 g (5.55 mmol) of water. The suspension was stirred for 4 days and after drying in vacuo 0.904 g (99.8% yield) of the complex [LaCl3(DME)] was gained. Found: La, 42.0; Cl, 31.5. Calc. for [LaCl3(DME)]: La, 41.4; Cl, 31.7%. IR (Nujol) (cmy1): 1082 s, 1041 vs, 1019 s, 1001 s and 858 s. 2.3.2. [LaCl3(diglyme)] To 0.330 g (1.01 mmol) of La2O3 were added 35 ml of THF, 13.71 g (126 mmol) of (CH3)3SiCl, 0.850 g (6.3 mmol) of diglyme and 0.120 g (6.7 mmol) of water. The suspension was stirred for 4 days and after drying in vacuo 0.769 g (100% yield) of the complex [LaCl3(diglyme)] was gained. Found: La, 36.5; C, 18.96; H, 3.55. Calc. for [LaCl3(diglyme)]: La, 36.6; C, 18.99; H, 3.72%. IR (Nujol) (cmy1): 1289 w, 1261 m, 1213 vw, 1090 vs, 1064 m, 1048 vs, 1005 m, 951 m, 873 s, 832 m, 801 m. 2.3.3. [LaCl3(HMPA)3] 2.3.3.1. Method A. HMPA (0.850 g, 4.74 mmol) was added to a suspension of [LaCl3(DME)] (0.238 g, 0.710 mmol) in toluene (25 ml). A clear solution resulted immediately. Bright, colourless crystals of the complex [LaCl3(HMPA)3] grew out of the solution during slow evaporation of the solvent. The crystals were filtered off and washed with pentane three times. Found: La, 17.4; C, 27.58; H, 7.00; N, 15.83. Calc. for [LaCl3(HMPA)3]: La, 17.7; C, 27.62; H, 6.95; N, 16.10%. IR (Nujol) (cmy1): 1302 s, 1193 m, 1163 m, 1130 vs, 1067 w, 986 vs, 802 w, 752 s, 482 m. 1H NMR (300

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2.3.5. [PrCl3(HMPA)3] HMPA (1.06 g, 5.92 mmol) was added to a suspension of [PrCl3(DME)] (0.289 g, 0.86 mmol) in toluene (25 ml). A clear solution resulted immediately. Bright, colourless crystals of the complex [PrCl3(HMPA)3] grew out of the solution during slow evaporation of the solvent. Found: C, 27.51; H, 6.89; N, 15.68. Calc. for [PrCl3(HMPA)3]: C, 27.55; H, 6.93; N, 16.06%. IR (Nujol) (cmy1): 1302 s, 1261 w, 1195 m, 1165 m, 1131 vs, 1068 w, 986 vs, 802 w, 759 s, 481 m. 2.3.6. [NdCl3(diglyme)] To 0.340 g (1.01 mmol) of Nd2O3 were added 20 ml of THF, 12.86 g (118 mmol) of (CH3)3SiCl, 0.890 g (6.6 mmol) of diglyme and 0.120 g (6.7 mmol) of water. The suspension was stirred for 4 days and after drying in vacuo 0.777 g (100% yield) of the pale violet complex [NdCl3(diglyme)] was gained. Found: Nd, 37.6; C, 18.90; H, 3.75. Calc. for [NdCl3(diglyme)]: Nd, 37.5; C, 18.73; H, 3.67%. IR (Nujol) (cmy1): 1290 w, 1260 s, 1213 w, 1091vs, 1048 vs, 1006 s, 953 m, 875 s, 834 m, 803 m. 2.3.7. [NdCl3(HMPA)3] HMPA (0.640 g, 3.75 mmol) was added to a suspension of [NdCl3(diglyme)] (0.338 g, 0.878 mmol) in toluene (15 ml). A clear solution resulted immediately and the colour changed to light blue. Bright, pale blue crystals of the complex [NdCl3(HMPA)3] grew out of the solution during slow evaporation of the solvent. Found: Nd, 18.7; C, 27.12; N, 15.43; H, 6.81. Calc. for [NdCl3(HMPA)3]: Nd, 18.3; C, 27.43; N, 15.99; H, 6.91%. IR (Nujol) (cmy1): 1302 s, 1193 m, 1163 m, 1133 vs, 1064 w, 986 vs, 759 s, 481 m.

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2.3.8. [SmCl3(diglyme)] To 0.490 g (1.41 mmol) of Sm2O3 were added 20 ml of THF, 12.96 g (119 mmol) of (CH3)3SiCl, 0.930 g (6.93 mmol) of diglyme and 0.080 g (4.44 mmol) of water. The suspension was stirred for 4 days and after drying in vacuo 1.099 g (100% yield) of the complex [SmCl3(diglyme)] was gained. Found: Sm, 38.0; C, 18.47; H, 3.55. Calc. for [SmCl3(diglyme)]: Sm, 38.4; C, 18.44; H, 3.61%. IR (Nujol) (cmy1): 1290 w, 1261 w, 1245 w, 1092 vs, 1064 s, 1048 vs, 1006 m, 954 m, 876 vs, 834 m, 824 m, 803 w.

2.3.11. [EuCl3(HMPA)3] HMPA (1.10 g, 8.20 mmol) was added to a suspension of [EuCl3(diglyme)] (0.77 g, 1.96 mmol) in toluene (40 ml). A clear solution resulted immediately. Bright, colourless crystals of the complex [EuCl3(HMPA)3] grew out of the solution during slow evaporation of the solvent. Found: Eu, 19.30; C, 27.23; N, 15.38; H, 6.86. Calc. for [EuCl3(HMPA)3]: Eu, 19.09; C, 27.16; N, 15.84; H, 6.84%. IR (Nujol) (cmy1): 1301 m, 1195 m, 1170 m, 1137 vs, 1068 w, 986 vs, 759 s, 482 m.

2.3.9. [SmCl3(HMPA)3]

2.3.12. [GdCl3(HMPA)3] To 0.410 g (1.13 mmol) of Gd2O3 were added 20 ml of THF, 13.18 g (121 mmol) of (CH3)3SiCl, 0.850 g (6.33 mmol) of diglyme and 0.130 g (7.2 mmol) of water. The suspension was stirred for 5 days and dried in vacuo. The dried residual was suspended in toluene (25 ml) and 1.60 g (10.9 mmol) HMPA was added. The white powder partly dissolved. The insoluble substance was filtered off. Bright, colourless crystals of the complex [GdCl3(HMPA)3] grew out of the solution during slow evaporation of the solvent. Found: Gd, 19.9; C, 26.90; N, 15.56; H, 6.93. Calc. for [GdCl3(HMPA)3]: Gd, 19.6; C, 26.98; N, 15.73; H, 6.79%. IR (Nujol) (cmy1): 1303 s, 1193 m, 1170 m, 1138 vs, 1067 w, 985 vs, 798 vw, 753 m, 481 m.

2.3.9.1. Method A. HMPA (0.710 g, 3.96 mmol) was added to a suspension of [SmCl3(DME)2] (0.296 g, 0.677 mmol) [16] in toluene (28 ml). A clear solution resulted immediately. Bright, colourless crystals of the complex [SmCl3(HMPA)3] grew out of the solution during slow evaporation of the solvent. Found: Sm, 19.3; C, 27.39; H, 6.82; N, 15.42. Calc. for [SmCl3(HMPA)3]: Sm, 18.9; C, 27.22; H, 6.85; N, 15.87%. IR (Nujol) (cmy1): 1303 s, 1196 m, 1169 m, 1135 vs, 1067 w, 986 vs, 759 s, 482 m. 2.3.9.2. Method B. HMPA (0.580 g, 3.24 mmol) was added to a suspension of [SmCl3(diglyme)] (0.265 g, 0.678 mmol) in toluene (25 ml). A clear solution resulted immediately. Bright, colourless crystals of the complex [SmCl3(HMPA)3] grew out of the solution during slow evaporation of the solvent. Found: Sm, 19.4; C, 27.05; N, 15.64; H, 6.97. Calc. for [SmCl3(HMPA)3]: Sm, 18.93; C, 27.22; N, 15.87; H, 6.85%. IR (Nujol) (cmy1): 1302 s, 1193 m, 1167 m, 1134 vs, 1068 m, 985 vs, 798 w, 752 s, 481 m. 2.3.10. [EuCl3(diglyme)] To 0.340 g (0.97 mmol) of Eu2O3 were added 20 ml of THF, 10.54 g (97 mmol) of (CH3)3SiCl, 0.790 g (5.89 mmol) of diglyme and 0.130 g (7.2 mmol) of water. The suspension was stirred for 4 days and after drying in vacuo 0.760 g (99.8% yield) of the white complex [EuCl3(diglyme)] was gained. Found: Eu, 39.0; C, 18.2; H, 3.52. Calc. for [EuCl3(diglyme)]: Eu, 38.72; C, 18.36; H, 3.60%. IR (Nujol) (cmy1): 1290 w, 1261 m, 1245 w, 1213 vw, 1092 vs, 1049 vs, 1006 m, 955 m, 877 s, 835 m, 825 w, 803 vw.

2.4. Crystal structure determination Hygroscopic crystals of [LnCl3(HMPA)3] (LnsLa, Pr, Nd, Sm, Eu, Gd) were fixed and covered by nail polish and used for cell determination on an Enraf-Nonius CAD-4 diffractometer with graphite monochromatized Mo Ka radiation, ls0.7107. Accurate unit cell parameters of [LnCl3(HMPA)3] (LnsLa, Pr, Nd, Sm, Eu, Gd) were obtained from a least-squares refinement of the angular settings of 75 reflections 8-u-158. Crystals of [LnCl3(HMPA)3] (LnsLa, Pr, Nd, Sm, Eu, Gd) are monoclinic, with space group P21/c, no. 14, C18H54N9Cl3O3P3Ln, Zs4; unit cell parameters are presented in Table 1. A colourless block-shaped crystal of [SmCl3(HMPA)3], with dimensions 0.4=0.5=0.6 mm, sealed in a capillary purged with argon was used for data collection. Intensities were collected at 293(2) K in the v scan mode to 2umaxs288 in the range h 0 to 20, k 0 to 26, l y31 to 31; 19 139 total data were measured; equivalent reflections merged into a set

Table 1 Unit cell parameters of [LnCl3(HMPA)3] (LnsLa, Pr, Nd, Sm, Eu, Gd) Ln

˚ a (A)

˚ b (A)

˚ c (A)

b (8)

˚ 3) V (A

La Pr Nd Sm Eu Gd

15.885(1) 15.826(4) 15.794(9) 15.783(1) 15.772(3) 15.774(2)

20.326(2) 20.285(5) 20.215(7) 20.179(2) 20.152(3) 20.147(3)

23.820(2) 23.679(6) 23.686(7) 23.606(2) 23.571(3) 23.557(3)

90.04(1) 90.00(2) 90.14(8) 90.18(1) 90.01(1) 90.03(1)

7705(2) 7602(6) 7562(6) 7518(2) 7492(3) 7486(3)

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of 11 007 independent reflections, Rints0.029, 6961 with I)2.5s(I) were observed. Correction was made for analytical absorption. The structure was solved by direct methods, with full-matrix least-squares refinement; H atoms at calculated positions with isotropic temperature factors were included only in structure factor calculations. In the final least-squares cycle there were 6961 reflections and 667 variables. The final R factors were Rs0.086 and Rws0.091, and goodness-of-fit was 1.103. All calculations were performed with the Xtal3.4 system of crystallographic programs [17].

3. Results and discussion 3.1. Synthesis and characterization Chlorotrimethylsilane reacts with water, generating hydrogen chloride in situ. The generated hydrogen chloride reacts with lanthanide oxides in dimethoxyethane giving the complexes [LnCl3(DME)] (LnsLa, Pr), as well as with diglyme in tetrahydrofuran resulting in [LnCl3(diglyme)] (LnsLa, Nd, Sm, Eu). The method is very simple since the syntheses are carried out at room temperature and the side products, HCl and (CH3)3SiOSi(CH3)3, are volatile and easily removed. All prepared products are anhydrous based on the absence of OH stretching bands in the IR spectra. [LaCl3(DME)] and [PrCl3(DME)] were prepared for the first time in the same way as previously described [SmCl3(DME)2] [16]. The very low solubilities of [LnCl3(DME)] in common solvents hamper the preparation of single crystals and also suggest the polymeric structure of the compounds, in contrast to the monomeric [LnCl3(DME)2] [18–20]. This indicates a possible structural diversity in [LnCl3(DME)n] complexes as previously reported for [LnCl3(THF)n] [20–28]. The stoichiometry of the prepared diglyme complexes has been found in the octahedral [ScCl3(diglyme)] [29]. We tried to prepare more soluble [LnCl3L] (Lsdiglyme, DME) derivatives by introducing ligand HMPA. When HMPA was added to a suspension of the prepared [LnCl3L] (LsDME, diglyme) in toluene, the complexes [LnCl3(HMPA)3] were formed in high yield in all cases. The syntheses of [MCl3(HMPA)3] (Msall rare earth, Sc) have been already reported [6]. The X-ray structure determination of the previously prepared LnCl3(HMPA)3 (LnsPr, Dy, Yb) revealed the meridional distribution of the ligands in the octahedron [30–32]. The 1 H NMR spectrum of [LaCl3(HMPA)3] from our synthesis suggested a facial isomer. The X-ray structure analysis of the prepared [SmCl3(HMPA)3] confirmed the proposed facial distribution of the ligands. The prepared [LnCl3(HMPA)3] (LnsLa, Pr, Nd, Eu, Gd) complexes are isomorphous to the fac-[SmCl3(HMPA)3] isomer according to the unit cell determination (Table 1). Decreasing volumes of the unit cells from lanthanum to gadolinium complexes are expected, due to lanthanide contraction.

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3.2. Crystal structure of [SmCl3(HMPA)3] The molecular structure of [SmCl3(HMPA)3] is shown in Fig. 1 and selected bond lengths and angles are listed in Table 2. An asymmetric unit consists of two slightly different molecules. This is in contrast to the mer isomers with only one molecule in the asymmetric unit [30–32], resulting in the doubling of the unit cell volume in the fac compounds. The coordination polyhedron around samarium in fac[SmCl3(HMPA)3] is a slightly distorted octahedron with angles Cl–Sm–O from 168.0(4) to 173.1(4)8. The angles between chlorine atoms are wider, Cl–Sm–Cl, from 93.2(2) to 98.0(3)8 and sharper between oxygen atoms, O–Sm–O, from 81.4(5) to 87.6(5)8. The bond lengths are from ˚ for Sm–Cl and from 2.290(13) to 2.657(8) to 2.680(7) A ˚ for Sm–O. The values for bond lengths are 2.388(18) A between those listed for mer-[PrCl3(HMPA)3] [30] and mer-[DyCl3(HMPA)3] [31], as expected. Sm–Cl bond lengths are also similar to those in the seven-coordinate com-

Fig. 1. The molecular structure of [SmCl3(HMPA)3]. Hydrogen atoms are omitted for clarity. Table 2 ˚ and bond angles (8) in [SmCl3(HMPA)3] Selected bond lengths (A) Sm1–Cl1 Sm1–Cl2 Sm1–Cl3 Sm1–O1 Sm1–O2 Sm1–O3

2.680(7) 2.668(7) 2.657(8) 2.388(18) 2.338(14) 2.290(13)

Sm2–Cl4 Sm2–Cl5 Sm2–Cl6 Sm2–O4 Sm2–O5 Sm2–O6

2.660(6) 2.665(7) 2.662(8) 2.349(15) 2.305(13) 2.331(17)

Cl1–Sm1–Cl2 Cl1–Sm1–Cl3 Cl1–Sm1–O1 Cl1–Sm1–O2 Cl1–Sm1–O3 Cl2–Sm1–Cl3 Cl2–Sm1–O1 Cl2–Sm1–O2 Cl2–Sm1–O3 Cl3–Sm1–O1 Cl3–Sm1–O2 Cl3–Sm1–O3 O1–Sm1–O2 O1–Sm1–O3 O2–Sm1–O3

94.0(2) 93.2(2) 172.9(5) 91.7(4) 89.1(4) 98.0(3) 87.0(5) 172.4(4) 93.6(4) 93.7(5) 86.8(4) 168.0(4) 86.8(6) 83.8(6) 81.4(5)

Cl4–Sm2–Cl5 Cl4–Sm2–Cl6 Cl4–Sm2–O4 Cl4–Sm2–O5 Cl4–Sm2–O6 Cl5–Sm2–Cl6 Cl5–Sm2–O4 Cl5–Sm2–O5 Cl5–Sm2–O6 Cl6–Sm2–O4 Cl6–Sm2–O5 Cl6–Sm2–O6 O4–Sm2–O5 O4–Sm2–O6 O5–Sm2–O6

94.4(2) 93.4(2) 173.1(4) 91.3(4) 88.7(4) 96.4(3) 86.2(4) 172.7(4) 93.7(5) 93.3(4) 87.9(4) 169.5(5) 87.6(5) 84.4(5) 81.8(6)

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Scheme 1. The proposed mechanism of fac–mer isomerization and HMPA exchange in [LaCl3(HMPA)3]. The central seven-coordinate species is either an interchange transition state, an intermediate or a stable molecule.

plexes [SmCl3(THF)4] (2.635–2.683) [26] and [SmCl3(DME)(THF)2] (2.646–2.668) [33]. The six-coordinate complexes of lanthanide chlorides include mer-[LnCl3(HMPA)3] (LnsPr, Dy, Yb) [30–32] and mer-[LnCl3(THF)3] (LnsYb, Lu) [28,34], as well as complexes with bulky ligands in a facial conformation: [CeCl3(OAsPh3)3] [35], [GdCl3(C7H8O2)3] [36], [YbCl3(C6H10O2)(THF)2] [27] and [PrCl3(CCl3C(O)N(H)P(O)(NEt2)2)3] [37]. 3.3. The NMR solution dynamics study of [LaCl3(HMPA)3] The existence of fac and mer isomers of [PrCl3(HMPA)3] prompted us to study fac–mer isomerization. The 1H NMR spectrum of [LaCl3(HMPA)3] shows a doublet consistent with three equivalent HMPA ligands in the fac isomer (Fig. 2a). The doublet in the 1H NMR spectrum of the sample containing only traces of free HMPA1 is converted (at 362 K in 2.5 h) to two doublets in the approximate ratio of intensities of 2:1, consistent with the formation of the mer isomer (Fig. 2b, c). The change is reversible, the slow reappearance of the fac isomer being observed at room temperature (Fig. 2d, e). Splitting of the doublet at 362 K was not detected, either with the sample containing pure [LaCl3(HMPA)3] or in the solution containing [LaCl3(HMPA)3] and HMPA in the molar ratio 1:0.5. The addition of free HMPA to the [D8]toluene solution of [LaCl3(HMPA)3] causes a shift of the HMPA resonance in the 1H NMR spectra to the lower frequency, suggesting the fast exchange of free and coordinated HMPA. The shifts of 0.043 and 0.055 ppm were observed after addition of 3 and 9 moles of HMPA to 1 mole of complex, respectively. The separated resonances of free HMPA at lower frequency and of coordinated HMPA at higher frequency were observed at 182 K in a 0.01 M toluene solution containing equimolar amounts of [LaCl3(HMPA)3] and free HMPA (Fig. 3). The resonances coalesced at 196 K. We propose a reversible fac–mer isomerization of [LaCl3(HMPA)3] by an associative mechanism (Scheme 1). However, we are not able to distinguish between the associative interchange mechanism and seven-coordinate intermediate. An equilibrium of [LaCl3(HMPA)3] and [LaCl3(HMPA)4] also can not be excluded, since [LaBr3(HMPA)4] was isolated in the solid state [7]. The 1 Crystals of [LaCl3(HMPA)3] were taken in a dry box from the mother liquor containing free HMPA, gently dried with paper and dissolved in deuterated toluene.

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Fig. 2. The 1H NMR spectra of [LaCl3(HMPA)3] with traces of free HMPA during thermal treatment ([D8]toluene solution). (a) Starting spectrum at 303 K. The sample was then heated to 362 K, and after 0.5 and 2.5 h at this temperature spectra (b) and (c) were recorded. The sample was cooled down to 303 K and after 1 h the spectrum (d) was recorded. The sample was then left at 298 K for 36 h, and the spectrum (e) resulted.

Fig. 3. The variable temperature 1H NMR spectra of [LaCl3(HMPA)3] with added free HMPA (molar ratio 1:1, [D8]toluene solution).

fac isomer is dominant at room temperature and the mer isomer at 362 K. According to the proposed mechanism, the exchange and isomerization are second-order reactions, depending on the concentrations of the complex and free HMPA. The exchange of two different environments of HMPA ligand in mer[LaCl3(HMPA)3] is slow on the 1H NMR time scale only in the presence of traces of free HMPA (Fig. 2c, d). At a 1:0.5 molar ratio of complex to free HMPA, only one HMPA resonance was observed at 362 K, indicating the fast exchange of all the coordinated HMPA molecules in mer[LaCl3(HMPA)3] and free HMPA. The coalescence of resonances of free and coordinated HMPA at 196 K gave a second-order rate constant k2(196

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K) of 1.0(2)=104 My1 sy1 and DG/(196 K)s32"1 kJ/ mol. Only a broad 31P NMR singlet was observed for fac and mer isomers of [LaCl3(HMPA)3]. We were not able to detect a 139La NMR resonance.

4. Conclusions The fac-[LnCl3(HMPA)3] (LnsLa, Pr, Nd, Sm, Eu, Gd) complexes were prepared by the reaction of [LnCl3L] (Lsdiglyme and DME) with excess HMPA at room temperature in toluene solution. The previously reported mer[PrCl3(HMPA)3] was obtained by the reaction of PrCl3 in hot ethanol, crystallized at 08C and then recrystallized from benzene [6,30]. The 1H NMR study of [LaCl3(HMPA)3] shows exchange of coordinated and free HMPA. In addition, fac-[LaCl3(HMPA)3] observed in toluene solution at room temperature is reversibly converted to the mer isomer at 362 K in the presence of traces of free HMPA. The conversion was not observed without free HMPA. On this basis, the mechanism was proposed for reversible fac–mer isomerization in the presence of free HMPA. However, the polar solvent ethanol, used in the synthesis of mer-[PrCl3(HMPA)3] [6,30], could alter the relative stabilities of fac and mer isomers, making the mer isomer more stable at 08C. mer[PrCl3(HMPA)3] could not be converted to the fac isomer during recrystallization from non-polar benzene since no free HMPA was present.

Supplementary data Crystallographic data concerning the crystal structure determination of [SmCl3(HMPA)3] are available from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK, on request, quoting deposition number CCDC 132917.

Acknowledgements This work was supported by the Ministry of Science and Technology, Republic of Slovenia.

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