Linear, Zigzag, and Helical Cerium(III) Coordination Polymers

Article pubs.acs.org/crystal

Linear, Zigzag, and Helical Cerium(III) Coordination Polymers Iurie L. Malaestean,† Meliha Kutluca-Alıcı,† Arkady Ellern,‡ Jan van Leusen,† Helmut Schilder,† Manfred Speldrich,† Svetlana G. Baca,† and Paul Kögerler*,† †

Institute of Inorganic Chemistry, RWTH Aachen University, D-52074 Aachen, Germany Ames Laboratory, Iowa State University, Ames, Iowa 50011, United States

‡

S Supporting Information *

ABSTRACT: Five novel one-dimensional cerium(III) carboxylate coordination polymers, [Ce(O2CCH2CHMe2)3(EtOH)2]n (1), {[Ce(O2CCH2Me)3(H2O)]· 0.5(4,4′-bpy)}n (2; 4,4′-bpy = 4,4′-bipyridine), {[Ce2(O2CCHMe2)6(H2O)3]}n (3), {[Ce3(O2CCHMe2)9(nPrOH)4]}n (4), and {[Ce3(O2CCHMe2)9(HO2CCHMe2)2(H2O)2]·2Me2CHCO2H}n (5), showcase the surprisingly consistent tendency of Ce(III) coordination network structures to adopt one-dimensional connection modes. The type of carboxylate as well as the reaction solvents determines the exact bridging versus end-on coordination modes for the carboxylates and, in turn, discriminate between linear, zigzag, and helical arrangements. Detailed magnetochemical analyses reveal pronounced single-ion effects and the expected weak antiferromagnetic coupling.

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INTRODUCTION Cerium coordination compounds display significant flexibility in both coordination numbers and geometries of the Ce(III) and Ce(IV) coordination environments. Thus, unlike classical 3d transition metal coordination networks in which such geometric constraints are operative, allowing for their use as synthons, for example, in the construction of 3D metal−organic frameworks, Ce coordination networks should in principle be subject to a wide range of secondary structure-directing effects and accommodate a wide range network topologies. At the same time, from a materials properties point of view, polynuclear cerium structures are motivated by their use in, for example, electroluminescence1 or corrosion inhibition.2 However, despite the seemingly flexible coordination chemistry, most of the known polynuclear cerium coordination compounds are heterometallic coordination clusters containing CeIV/MnIII/IV3−7 (including a single-molecule magnet8), as well as trinuclear CeIII/FeIII, CeIII/Zn, CeIII/Co,9 or tetranuclear MnIII/CeIII and CeIII/CoII cores.10 Only a few homonuclear hexanuclear {CeIV6O8} clusters are known so far,11 and only very recently we reported two giant Ce(III/IV) clusters comprising {Ce10} and {Ce22} cores.12 Some mononuclear Ce(III) carboxylate complexes are known for bulky ligands (octanoate to octadecanoate),13 binuclear clusters were also reported (e.g., acetate, pivalate),14 and a few Ce(III) polymers based on monocarboxylic acids were reported so far (e.g., acetate, trifluoroacetate, trichloroacetate).15 Herein, we exemplify how a recently developed strategy toward polynuclear Mn(II/III) coordination cluster-based 1D chains utilizing competing ligands (which are eventually not integrated into the product)16 can be extended to generate cerium(III) carboxylate-based chains, and we report five novel Ce(III) coordination polymers based on isovaleric, isobutyric, and propionic acid. For the first © 2012 American Chemical Society

time, a magnetochemical analysis for Ce(III) coordination polymers is presented.

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EXPERIMENTAL SECTION

Materials and General Methods. All reagents were purchased from commercial sources and used without further purification. The zirconium oxocluster [Zr6O4(OH)4(O2CCH2Me)12]2·6MeCH2CO2H was synthesized according to the previously reported procedure.17 The precursor [Ce(O2CCH2CHMe2)3]·Me2CHCH2CO2H was synthesized by heating Ce(MeCO2)3·H2O (2.0 g, 6.3 mmol) in 10 mL (90.5 mmol) of isovaleric acid for 4 h. The thus-obtained microcrystalline product was filtered off, washed with pentane, and dried in air (yield: 1.78 g, 50% based on Ce). Anal. Calcd for C20H39CeO9: C, 42.62; H, 6.97. Found: C, 43.20; H, 6.76. IR data (KBr pellet, v/cm−1): 3396 br/m, 2957 s, 2871 w, 1689 w, 1543 vs, 1416 s, 1384 w, 1325 w, 1261 w, 1216 w, 1168 w, 1123 w, 1103 w, 978 w, 922 w, 893 w, 839 w, 729 w, 642 w. All IR spectra were recorded with a Perkin-Elmer Spectrum One spectrometer using KBr pellets in the region 4000−400 cm−1. TGA/DTA measurements were carried out with a Mettler Toledo TGA/SDTA851 instrument under dry N2 (60 mL min−1) at a heating rate of 10 K min−1. Syntheses. [Ce(O2CCH2CHMe2)3(EtOH)2]n (1). 0.08 g (0.14 mmol) of [Ce(O2CCH2CHMe2)3]·Me2CHCH2CO2H and N-butyldiethanolamine (0.085 mL, 0.53 mmol) were refluxed for 1 h in 5 mL of a 1:1 ethanol/THF mixture. The resulting solution was filtered and left in a closed vial. Pale-yellow crystals of 1 were obtained in several days, washed with ethanol, and dried in air (yield: 0.025 g, 33% based on Ce). Anal. Calcd for C38H78Ce2O16: C, 42.6; H, 7.34. Found: C, 41.8; H, 6.92. IR (KBr pellet, v/cm−1): 3422 br/m, 2957 s, 2871 m, 1541 vs, 1417 s, 1383 w, 1367 w, 1340 w, 1325 m, 1261 w, 1218 w, 1170 w, 1125 w, 1107 w, 979 w, 921 w, 892 w, 839 w, 728 m, 644 w, 499 w. Received: December 9, 2011 Revised: January 1, 2012 Published: January 31, 2012 1593

dx.doi.org/10.1021/cg2016337 | Cryst. Growth Des. 2012, 12, 1593−1602

Crystal Growth & Design

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Table 1. Summary of Crystallographic Details, Data Collection, and Refinement Details for 1−5 empirical formula molecular weight/g mol−1 T/K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z ρ/g cm−3 μ/mm−1 crystal size/mm3 index ranges

reflns collected independent reflns completeness to θ data/restraints/parameters GOF on F2 final R1, wR2 R indices (all data) largest peak and hole, e Å−3

1

2

3

4

5

C38H78Ce2O16 1071.24 173(2) triclinic P1̅ 9.3022(7) 11.2733(8) 23.9813(17) 100.730(1) 93.913(1) 91.412(1) 2463.3(3) 2 1.442 1.884 0.20 × 0.18 × 0.11 −11 ≤ h ≤ 11 −13 ≤ k ≤ 13 −28 ≤ l ≤ 28 26 003 8586 [R(int) = 0.1431] 98.9% 8586/17/540 1.050 R1 = 0.0529, wR2 = 0.1391 R1 = 0.0590, wR2 = 0.1425 2.790 and −1.697

C42H63Ce3N3O21 1366.31 173(2) monoclinic P2(1)/n 20.704(5) 7.933(2) 32.877(8) 90 106.526(3) 90 5177(2) 4 1.753 2.671 0.30 × 0.12 × 0.11 −24 ≤ h ≤ 24 −9 ≤ k ≤ 9 −39 ≤ l ≤ 39 35 635 9114 [R(int) = 0.0347] 99.9% 9114/0/631 1.068 R1 = 0.0258, wR2 = 0.0530 R1 = 0.0340, wR2 = 0.0584 1.101 and −0.872

C26H54Ce2O16 902.93 173(2) monoclinic P2(1)/n 14.6915(11) 18.5748(14) 15.3352(12) 90 113.766(1) 90 3830.0(5) 4 1.566 2.408 0.32 × 0.12 × 0.08 −17 ≤ h ≤ 17 −19 ≤ k ≤ 22 −18 ≤ l ≤ 18 23 450 6742 [R(int) = 0.1351] 99.9% 6742/0/382 0.938 R1 = 0.0562, wR2 = 0.1410 R1 = 0.0688, wR2 = 0.1499 1.810 and −2.018

C48H95Ce3O22 1444.60 153(2) orthorhombic Pna2(1) 24.6004(13) 12.4601(7) 21.0665(12) 90 90 90 6457.4(6) 4 1.486 2.145 0.42 × 0.26 × 0.19 −35 ≤ h ≤ 34 −17 ≤ k ≤ 17 −30 ≤ l ≤ 30 75 470 19 770 [R(int) = 0.0285] 99.6% 19 770/33/718 1.031 R1 = 0.0244, wR2 = 0.0570 R1 = 0.0304, wR2 = 0.0604 0.986 and −0.938

C52H99Ce3O28 1592.67 173(2) monoclinic C2/c 27.212(2) 13.2769(11) 22.3004(18) 90 120.300(1) 90 6956.4(10) 4 1.521 2.005 0.24 × 0.19 × 0.18 −32 ≤ h ≤ 32 −15 ≤ k ≤ 15 −26 ≤ l ≤ 26 36 239 6134 [R(int) = 0.0822] 100.0% 6134/48/378 1.145 R1 = 0.0308, wR2 = 0.0796 R1 = 0.0387, wR2 = 0.0848 1.285 and −0.947

{[Ce(O2CCH2Me)3(H2O)]·0.5(4,4′-bpy)}n (2). Ce(NO3)3·6H2O (0.17 g, 0.4 mmol), [Zr6O4(OH)4(O2CCH2Me)12]2·6MeCH2CO2H (0.36 g, 0.1 mmol), N-methyldiethanolamine (0.088 g, 0.7 mmol), and 4,4′bipyridine (0.08 g, 0.5 mmol) were refluxed in 5 mL of MeCN for 1 h, and the resulting solution was filtered and stored in a closed flask. Pale-yellow crystals of 2 were obtained after 2 weeks (yield: 0.13 g, 77%). Anal. Calcd for C42H63Ce3N3O21: C, 36.92; H, 4.64; N, 3.07. Found: C, 36.21; H, 4.61; N, 3.13. IR (KBr pellet, v/cm−1): 3383 br/m, 2974 m, 2939 m, 1599 m, 1552 s, 1465 s, 1417 s, 1375 w, 1325 w, 1284 s, 1241 w, 1222 w, 1079 s, 1041 w, 999 w, 820 m, 893 s, 738 w, 663 w, 620 s, 495 m. [Ce2(O2CCHMe2)6(H2O)3]n (3). A warm solution of isobutyric acid (1.26 mL, 13.8 mmol) and 0.54 g (13.5 mmol) of NaOH in 20 mL of EtOH and 5 mL of H2O was added to a warm solution of 2.0 g (4.6 mmol) of Ce(NO3)3·6H2O in 20 mL of H2O. The resulting solution was left in a covered flask overnight. The formed colorless crystals were filtered off, washed with water and ethanol, and dried in air (yield: 1.25 g, 63% based on Ce). Anal. Calcd for C24H48Ce2O15: C, 33.64; H, 6.02. Found: C, 34.18; H, 5.29. IR (KBr pellet, v/cm−1): 3423 br/m, 2967 m, 2926 sh, 2871 w, 1592 vs, 1472 m, 1419 s, 1372 w, 1285 m, 1225 w, 1095 m, 927 m, 840 m, 802 w, 607 m. {[Ce3(O2CCHMe2)9(nPrOH)4]}n (4). A solution of 0.1 g (0.06 mmol) of 5 in 5 mL (67 mmol) of n-propanol was combined with 0.085 mL (0.51 mmol) of N-butyldiethanolamine. The resulting solution was refluxed for 1 h, filtered off, and left in a covered vial. Yellow crystals were isolated after 3 days, washed with propanol, and dried in air (yield: 0.03 g, 36%). Anal. Calcd for C48H95Ce3O22: C, 39.90; H, 6.62. Found: C, 39.56; H, 6.85. IR (KBr pellet, v/cm−1): 3424 m/br, 2971 m, 2933 sh, 2873 w, 1537 vs, 1477 m, 1422 s, 1375 w, 1361 w, 1284 m, 1169 w, 1096 m, 928 m, 850 w, 659 w, 530 w. {[Ce3(O2CCHMe2)9(HO2CCHMe2)2(H2O)2]·Me2CHCO2H}n (5). 2.0 g (6.3 mmol) of Ce(MeCO2)3·H2O was refluxed in 10 mL (110 mmol) of isobutyric acid for 2 h. Yellow single crystals formed after a few days, were filtered off, washed with hexane, and dried in air (yield: 2.6 g, 75%).

Anal. Calcd for C52H99Ce3O28: C, 39.21; H, 6.26. Found: C, 39.63; H, 6.34. IR (KBr pellet, v/cm−1): 3422 m/br, 2971 m, 2933 sh, 2873 w, 1685 w, 1537 vs, 1477 m, 1427 s, 1376 w, 1362 w, 1286 m, 1168w, 1096 m, 927 m, 852 m, 782 w, 663 w, 531 w. Physical Measurements. Magnetic susceptibility data were recorded for three compounds (1, 2, and 5) representing the three different polymer types (linear, zigzag, and helical, respectively). Measurements used a Quantum Design MPMS-5XL SQUID magnetometer and were performed for T = 2.0−290 K at 0.1 T. Experimental data were corrected for the sample holder (PTFE tubes), and diamagnetic contributions were calculated from tabulated values (χdia(1) = −0.52 × 10−3 cm3 mol−1; χdia(2) = −0.68 × 10−3 cm3 mol−1; χdia(5) = −0.80 × 10−3 cm3 mol−1). X-ray Crystallography. Single-crystal X-ray diffraction experiments (Table 1) were carried out on a Bruker diffractometer (APEX CCD detector) using graphite-monochromated Mo Kα radiation with a detector distance of 50.09 mm. Full-sphere data collection was performed with ω scans in the range 0−180° at φ = 0°, 120°, and 240°. A semiempirical absorption correction was based on a fit of a spherical harmonic function to the empirical transmission surface as sampled by multiple equivalent measurements18a using SADABS software.18b All structures were solved using direct methods resulted in the model with almost all expected cluster core atoms present (Figures S1−S11, Tables S1−S5). The remaining non-hydrogen atoms were located on difference Fourier maps in an alternating series of leastsquares cycles. All non-hydrogen atoms were refined in full-matrix anisotropic approximation. Oxo-carboxylate ligands are highly disordered; therefore, various restrains were applied to obtain reasonable geometrical parameters and thermal displacement coefficients for all structures except 2. The structure of 3 was refined to R1 = 0.0730; however, the existing electron density in voids pointed to possible existence of diffused solvent. Water and ethanol were two candidates based on the synthesis route. The SQUEEZE routine18c was applied to estimate electron density in voids, resulting in about 25 uncounted e− per asymmetric unit, which is very 1594



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close to one EtOH molecule per void (as reflected by the empirical formula for 3 in Table 1). All hydrogen atoms for all structures were placed in the structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coefficients. All expected hydrogen atoms were placed on calculated positions and were refined in isotropic approximation using riding model. The Uiso(H) values have been set at 1.2−1.5 times the Ueq value of the carrier atom. All calculations were performed using the Bruker APEX II software suite.18d

Scheme 1. Reaction Pathways Resulting in Compounds 1−5a

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RESULT AND DISCUSSION Synthesis and Preliminary Characterization. The reaction of Ce(NO3)3·6H2O or Ce(MeCO2)3·H2O with corresponding carboxylic acids gave rise to five one-dimensional coordination polymers. Compound 1 was synthesized from the precursor [Ce(O2CCH2CHMe2)3]·Me2CHCH2CO2H, which was subsequently recrystallized from THF/EtOH to yield [Ce(O2CCH2CHMe2)3(EtOH)2]n (1). Synthesis of {[Ce(O2CCH2Me)3(H2O)]·0.5(4,4′-bpy)}n (2) employed the zirconium(IV) complex [Zr 6 O 4 (OH) 4 (O 2 CCH 2 Me) 12 ] 2· 6MeCH2CO2H as a source of propionate ligands; note that the direct reaction of simple Ce(III) salts with propionic acid or the use of cerium(III) propionate did not result in the formation of single crystals. Compounds 3, {[Ce2(O2CCHMe2)3(H2O)3]}n, and 5, {[Ce3(O2CCHMe2)9(HO2CCHMe2)2(H2O)2]·2Me2CHCO2H}n, were obtained by the reaction of Ce(III) salts with isobutyric acid in H2O/EtOH or pure acid solutions. The recrystallization of 5 from propanol resulted in {[Ce3(O2CCHMe2)9(PrOH)4]}n (4). For compounds 1, 2, and 4, N-alkyldiethanolamines were used in the reactions; the amine did not coordinate or was otherwise integrated in the isolated products but was essential to the formation of crystalline product (no precipitation was observed in the absence of the amines). Reaction conditions are illustrated in Scheme 1. The IR spectra of complexes 1−5 show strong and broad bands in the 1599−1537 and 1465−1417 cm−1 regions, which are assigned to the asymmetric and symmetric vibrations of the coordinated carboxylic groups. A medium intensity peak in 5 at 1696 cm−1 is consistent with the presence of protonated solvate carboxylic groups.19 Medium-intensity bands in all complexes in the regions 2974−2871 cm−1 and bands at 1477−1325 cm−1 are assigned to characteristic v(C−H) stretching and bending modes. TGA/DTA (10 mL N2/min, 25−600 °C) graphs display all-endothermic decomposition steps (Figures S12−S16). The relatively low temperatures at which ligands are separated are potentially related to the comparably weak coordinative bonds typical for Ce(III) complexes. For 1, two mass loss steps are observed, at 40−110 °C (loss of two EtOH; calcd 8.6%, found 9.8%) and at 255−600 °C (53.1%). For 2, two sequences of overlapping steps are found, with a first mass loss between 40 and 230 °C (loss of one coordinated H2O and 0.5 4,4′-bpy solvate; calcd 17.1%, found 21.1%) and at 235−600 °C (41%). For 3, mass loss steps are observed at 35−110 °C (loss of three H2O and one isobutyrate; calcd 16.5%, found 15.6%) and at 110−600 °C (four overlapping steps) corresponding to the loss or decomposition of the remaining ligands. For 4, after a first mass loss at 30−95 °C (one PrOH; calcd 4.2%, found 5.9%), a second mass loss is observed at 255−600 °C (54%); for 5, mass loss steps are observed at 35−105 °C (two H2O; calcd 2.3%, found 2.6%), at 110−135 °C (4.9%), at 140−200 °C (9.8%), and at 365−600 °C (49%). Structure Description. Single-crystal X-ray diffraction analyses of compounds 1−5 reveal that all compounds

a

Blue bonds indicate terminal ligands in end-on coordination modes (H2O, EtOH, PrOH, isobutyric acid). Red arches represent chelating carboxylate groups. mda, N-methyldiethanolamine; bda, N-butyldiethanolamine.

exclusively contain one-dimensional chain structures of carboxylate-bridged Ce(III) centers (Scheme 1). In the crystal lattices, the polymer strands are all aligned in uniform directions along the a (1, 4), b (2), or c (5) axis, or parallel to the [1,0,1] (3) direction. Crystallographic data and structure refinement parameters for 1−5 are summarized in Table 1, and selected bond distances for 1−5 in Table 2. The coordination polyhedra around the Ce(III) centers are exclusively defined by O donor sites, with an O9 coordination environment for 1 and 2, O9 and O10 coordination environments for 3 and 5, and O8 and O10 coordination environments for 4; Ce−O bond lengths range from 2.401(4) to 2.760(3) Å. The average Ce−O bond distances and bond angles are consistent with those observed in other Ce carboxylates.15 On the basis of bond valence sum calculations (BVS) for Ce centers,20 a uniform valence state of +3 is assigned to all Ce positions (Table S6, Supporting Information). Structure of [Ce(O2CCH2CHMe2)3(EtOH)2]n (1). Compound 1 comprises a linear chain structure consisting of [Ce(O2CCH2CHMe2)(EtOH)2] entities that are interlinked by bridging carboxylate groups (Figure S1). The [Ce(O2CCH2CHMe2)(EtOH)2] building units are interlinked by two isovaleric ligands in different coordination modes: two carboxylate groups in standard μ2-η1:η1-bridging modes connect two CeIII atoms with a Ce···Ce separation of 4.977(6) Å, whereas two 1595



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Table 2. Selected Ce−O Bond Distances (Å) in Compounds 1−5 1 Ce1−O6 Ce1−O5 Ce1−O11 Ce1−O8 Ce1−O7 Ce1−O1 Ce1−O2 Ce1−O3 Ce1−O4

2.409(4) 2.498(4) 2.547(4) 2.551(4) 2.562(4) 2.592(4) 2.601(4) 2.617(4) 2.650(4)

3 Ce2−O13 Ce2−O14 Ce2−O4 Ce2−O15 Ce2−O12 Ce2−O16 Ce2−O9 Ce2−O10 Ce2−O11

2.401(4) 2.468(4) 2.477(4) 2.532(4) 2.534(4) 2.547(4) 2.593(4) 2.607(4) 2.688(4)

Ce1−O4 Ce1−O5 Ce1−O2 symmetry codes: #1, z − 1/2

4 Ce1−O9 Ce1−O11 Ce1−O6 Ce1−O7 Ce1−O3 Ce1−O2 Ce1−O1 Ce1−O5 Ce1−O4 Ce1−O8 Ce2−O16 Ce2−O8 Ce2−O14 Ce2−O10 symmetry codes:

2 Ce1−O5 2.416(2) Ce2−O8 2.560(2) Ce1−O2#1 2.456(2) Ce2−O10 2.601(3) Ce1−O3#1 2.497(2) Ce2−O11 2.604(2) Ce1−O6#2 2.520(2) Ce2−O9 2.642(2) Ce1−O3 2.560(2) Ce3−O19 2.439(2) Ce1−O1 2.566(2) Ce3−O8#4 2.480(2) Ce1−O4 2.585(2) Ce3−O20 2.493(2) Ce1−O7 2.591(2) Ce3−O11 2.497(2) Ce1−O2 2.624(2) Ce3−O15 2.545(2) Ce2−O12 2.444(2) Ce3−O16 2.560(2) Ce2−O14 2.479(2) Ce3O13 2.570(2) Ce2−O18#3 2.501(2) Ce3−O17 2.590(2) Ce2−O21 2.515(2) Ce3−O14 2.603(2) Ce2−O17#3 2.518(2) symmetry codes: #1, −x + 1/2, y − 1/2, −z + 1/2; #2, −x + 1/2, y + 1/2, −z + 1/2; #3, x, y − 1, z; #4, x, y + 1, z 3 Ce1−O11#1 Ce1−O7 Ce1−O6 Ce1−O1 Ce1−O10 Ce1−O9 Ce1−O3

2.502(5) 2.525(5) 2.529(5) 2.540(5) 2.566(5) 2.576(6) 2.592(5)

Ce2−O5 Ce2−O2#2 Ce2−O15 Ce2−O14 Ce2−O12 Ce2−O8 Ce2−O13

2.604(5) Ce2−O11 2.626(4) 2.679(4) Ce2−O7 2.647(4) 2.691(4) x + 1/2, −y + 1/2, z + 1/2; #2, x − 1/2, −y + 1/2,

Ce1−O9#1 Ce1−O5 Ce1−O12 Ce1−O7 Ce1−O2 Ce1−O1#2 Ce1−O3 Ce1−O4 Ce1−O1

2.435(2) 2.500(2) 2.523(2) 2.535(2) 2.579(2) 2.591(2) 2.616(2) 2.6187(19) 2.6360(19) 2.732(2) 2.4135(18) 2.416(2) 2.421(2) 2.424(2) #1, x + 1/2, −y + 3/2, 5

Ce2−O6 Ce2−O6#1 Ce2−O4#1 Ce2−O4 Ce2−O10 Ce2−O10#1 Ce2−O11 Ce2−O11#1 Ce2−O9#1 Ce2−O9 symmetry codes: #1, −x, y, −z + 1/2; #2, −x, −y + 1, −z

2.435(5) 2.455(5) 2.488(6) 2.512(6) 2.535(6) 2.542(6) 2.565(6)

other carboxylates bridge two CeIII atoms in a μ2-η1:η2 coordination mode resulting in {Ce(μ2-O)2Ce} dimers with a shorter Ce···Ce distance of 4.437(6) Å. This results in a linear Ce1−Ce2−Ce1# sequence with alternating intrachain Ce···Ce separations (Figure 1). Stacking of neighboring chains primarily optimizes van der Waals interactions (Figure S2), with a shortest interchain Ce···Ce distance of 12.449(3) Å. Each Ce position additionally coordinates to two ethanol ligands and one chelating terminal isovalerate ligand. The coordinated ethanol molecules form additional intrachain hydrogen bonds (O···O = 2.643(6)− 2.749(6) Å) with the O atoms of the isovalerate ligands (Table 3). The two crystallographically independent Ce centers within each chain (Ce1 and Ce2) are nonacoordinated with an O9 atom set from two O atoms of two ethanol molecules, two O atoms of the terminal carboxylate ligand, and five oxygens from the four bridging carboxylate ligands: two O from the two μ2-η1:η1-bridging groups, three O from the μ2-η1:η2-bridging groups. The Ce1/2−Ocarb distances range from 2.409(4) to 2.650(4) Å and 2.401(4) to 2.688(4) Å, respectively, comparable to those reported for other cerium(III) carboxylates.15 For the coordinated ethanol ligands, the Ce1−O and Ce2−O distances are equal to 2.551(4)−2.562(4) and 2.532(4)−2.547(4) Å, respectively (Table 2). Structure of {[Ce(O2CCH2Me)3(H2O)]·0.5(4,4′-bpy)}n (2). The solid-state structure of 2 consists of neutral zigzag cerium carboxylate chains and 4,4′-bipyridine (4,4′-bpy) solvate

2.413(3) 2.4839(12) 2.491(3) 2.5179(11) 2.5209(11) 2.5228(14) 2.5863(11) 2.6035(10) 2.6382(11)

Ce2−O18 2.4550(19) Ce2−O2 2.5175(19) Ce2−O12 2.571(2) Ce2−O13 2.597(2) Ce3−O15 2.456(2) Ce3−O5#1 2.500(2) Ce3−O17 2.552(2) Ce3−O21 2.571(2) Ce3−O22 2.586(2) Ce3−O4#1 2.5873(19) Ce3−O19 2.591(2) Ce3−O20 2.656(2) Ce3−O16 2.694(2) Ce3−O18 2.7029(18) z; #2, x − 1/2, −y + 3/2, z 2.4468(11) 2.4467(11) 2.5334(11) 2.5334(10) 2.563(3) 2.563(3) 2.595(3) 2.595(3) 2.760(3) 2.760(3)

molecules. The lattice comprises two different chain types built up from [Ce(O2CCH2Me)3(H2O)] or [Ce2(O2CCH2Me)6(H2O)2] asymmetric units and 1.5 4,4′-bpy solvent molecules (Figures S3 and S4). Selected bond distances for 2 are given in Table 2. Each of the independent units forms an independent zigzag 1D chain oriented along the c axis (Figure 2). Whereas the first chain type consists of the Ce1 sites, the second chain type contains two independent Ce atoms (Ce2 and Ce3). The Ce1···Ce1# separation (4.168(1) Å) is slightly shorter than Ce2···Ce3 (4.176(1) Å) and Ce3···Ce2# (4.185(1) Å) separations (Figure S3). All CeIII atoms (C1, Ce2, and Ce3) are nonacoordinated with an O9 donor set (eight carboxylate oxygen atoms: four oxygens from two chelating propionates, two oxygen atoms from two bridging propionates, two oxygen atoms from two carboxylates chelated to the neighboring Ce atoms, and one water ligand). The Ce−Ocarb distances are in the range of 2.416(2)−2.642(2) Å; the Ce−Owater distance equals 2.591(2), 2.515(2), and 2.493(2) Å for Ce1, Ce2, and Ce3, respectively (Table 2). The main difference between the chains is that two carboxylates are chelated to each Ce1 metal ions in trans positions, whereas they have a cis orientation about Ce2 and Ce3 sites as shown in Figure S5 with respect to the coordinated water molecule. Crystal solvent 4,4′-bpy molecules connect the 1D chains into supramolecular 2D sheets: one 4,4′-bpy molecule bridges the Ce1-containing chain via N···O hydrogen bonds with the water molecule O7 1596



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Figure 1. Side (a) and top (b) views of a section of the 1D chain in 1. Hydrogen atoms omitted for clarity. Color scheme: Ce, blue; O, red; C(isovalerate), light gray; C(EtOH), dark gray.

Structures of {[Ce 2 (O 2 CCHMe 2 ) 3 (H 2 O) 3 ]} n (3), {[Ce3(O2CCHMe2)9(nPrOH)4]}n (4), and {[Ce3(O2CCHMe2)9(HO2CCHMe2)2(H2O)2]·2Me2CHCO2H}n (5). Structural analysis of compounds 3−5 revealed the formation of single-stranded helical chains in all solid-state structures (Figure 3; see Figures S7−S9 for atomic labeling schemes). Each helical twist consists of five (3) or seven (4 and 5) cerium(III) ions. The pitch of the helix is 16.4 Å for 3, 24.6 Å for 4, and 22.3 Å for 5. In the coordination polymer 3, the [Ce(O2CCHMe2)3] and [Ce(O2CCHMe2)(H2O)3] building units are connected into a helical chain by two isobutyrate ligands in a μ2-η1:η2 mode with the separations between CeIII atoms equal to 4.330(5) Å [Ce1···Ce2] and 4.308(5) Å [Ce2···Ce1#]. In the coordination polymer 4, the adjacent [Ce(O2CCHMe2)(PrOH)] and [Ce(PrOH)] units are linked by three or two bridging carboxylates. The separations between Ce···Ce atoms bridged by three carboxylates (two isobutyrate ligands in a μ2-η1:η2 coordination mode and one carboxylate in a standard μ2-bridging fashion) equal 4.172(3) Å [Ce1···Ce2] and 4.232 Å [Ce2···Ce3], whereas the distance between Ce···Ce centers linked by only two carboxylates (in μ2-η1:η2 coordination mode) is slightly longer with 4.335(3) Å [Ce3···Ce1#]. Similar to 4, in 5 the adjacent [Ce(HO2CCHMe2)(H2O)] and [Ce(O2CCHMe2)] units are also bridged by three and two carboxylate ligands with Ce···Ce distances of 4.279(4) and 4.347(4) Å, respectively. These carboxylates are di- or tridentate ligands in μ2-bridging and μ2-η1:η2 chelating bridging coordination modes. There are two crystallographically independent CeIII centers in the crystal structure of 3 and 5, and three crystallographically independent CeIII ions in 4. For 3, Ce1 is decacoordinated with an O10 donor set from the carboxylate oxygen atoms of six isobutyrate ligands: four of them are chelating and two remaining are bridging ones, whereas Ce2 center is nonacoordinated by four carboxylate oxygen atoms from two chelating isobutyrates, two carboxylate oxygen atoms from two bridging carboxylates, and three water molecules. Similar to 3, the Ce(III) ions in 5 are in nonacoordinated (Ce1) and decacoordinated (Ce2) environment with O9 and O10 atom set, respectively. Ce1 is coordinated by four chelating carboxylate oxygen atoms from two isobutyrates, three bridging oxygen atoms from three isobutyrates, one monodentate carboxylate oxygen atom, and a water molecule, while Ce2 is ligated by 10 carboxylate oxygen atoms. In 4, Ce1 and Ce3 atoms are decacoordinated by the carboxylate oxygen atoms from five isobutyrate ligands: four of them are chelating and one remaining is bridging one, and an

Table 3. Hydrogen Bonds in 1−5 (Å, deg) D−H···A

d(D−H)

1 O7−H7···O10#1 0.91 O8−H9···O9 0.74 O15−H15···O2 0.88 O16−H16···O1#2 0.80 symmetry codes: #1, x − 1, y, z; #2,

d(H···A)

d(D···A)

∠(DHA)

1.86 2.07 1.78 1.92 x + 1, y, z

2.749(6) 2.721(6) 2.643(6) 2.710(6)

165.6 147.2 165.3 171.1

2 O7−H7D···O6#4 0.83 2.18 3.003(4) 170.4 O7−H7E···N3#6 0.92 1.94 2.862(4) 175.5 O20−H20D···N1 0.84 1.95 2.795(4) 176.1 O20−H20E···O18#3 0.93 1.86 2.778(4) 169.5 O21−H21D···O13#3 0.89 1.95 2.848(4) 177.2 O21−H21E···N2#7 0.83 1.98 2.785(4) 164.6 symmetry codes: #3, x, y − 1, z; #4, x, y + 1, z; #6, −x + 1, −y + 1, −z + 1; #7, x + 1/2, −y + 3/2, z + 1/2 3 O13−H13A···O4#2 0.89 1.80 2.671(7) 165.6 O14−H14B···O9 0.98 1.78 2.702(7) 157.1 O15−H15D···O3 0.84 1.86 2.692(7) 176.9 O15−H15E···O10#2 0.94 1.97 2.617(7) 124.2 symmetry codes: #2, x − 1/2, −y + 1/2, z − 1/2 4 O11−H11···O21#2 0.80 1.85 2.631(3) 165.6 O12−H12···O20 0.80 1.95 2.718(3) 162.1 O13−H13···O3 0.80 1.91 2.714(3) 176.3 O22−H22···O1#1 0.80 1.94 2.740(3) 173.6 symmetry codes: #1, x + 1/2, −y + 3/2, z; #2, x − 1/2, −y + 3/2, z 5 O8−H8···O11 0.84 1.77 2.591(3) 166.0 O12−H12B···O5#1 0.87 1.84 2.698(3) 171.7 O12−H12A···O14 0.89 1.90 2.785(5) 174.5 O13−H13···O3 0.84 1.85 2.648(4) 159.3 symmetry codes: #1, −x, −y + 1, −z

(O7···N3[−x + 1, −y + 1, −z + 1]: 2.862(4) Å) (Figure S6), whereas the other 4,4′-bpy molecule connects the Ce2/Ce3based chains (N···O: 2.795(4) and 2.785(4) Å). The coordinated water molecules (O7, O20, and O21) also form intrachain O···O hydrogen bonds with the bridging carboxylate oxygen atoms (Table 3). The coordination modes of carboxylate groups for 2 are similar to 1, with one carboxylic group adopting the μ2-η1:η1-bridging mode between two Ce sites, whereas two other carboxylates bridge to neighboring Ce sites in a μ2-η1:η2 coordination mode. 1597



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Figure 2. The two distinct parallel polymer chains in 2. Hydrogen atoms and 4,4′-bpy groups omitted for clarity. Color code: Ce, blue; O, red; O(H2O), yellow; C(μ2-η1:η1 propionate), light gray; C(μ2-η1:η2 propionate), dark gray.

Figure 3. Side (left) and top (right) views of the polymer chains in 3, 4, and 5. Hydrogen atoms and solvent molecules are omitted for clarity. Color scheme: Ce, blue; O, red; O(H2O), yellow. 3: C(terminal isobutyrate), dark gray; C(μ2-isobutyrate), light gray. 4: C(nPrOH), dark gray; C(μ2isobutyrate), light gray; C(terminal isobutyrate), green. 5: C(μ2-isobutyrate), light gray; C(terminal chelating isobutyrate), dark gray; C(terminal isobutyric acid), green. 1598



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oxygen atom of propanole, whereas Ce2 is octacoordinated by six carboxylate oxygen atoms from six isobutyrates and two oxygen atoms from two propanole molecules. The Ce−Ocarb distances are in the range of 2.435(5)−2.691(4) Å (3), 2.416(2)−2.732(2) Å (4), and 2.413(3)−2.760(3) Å (5). The Ce−OPrOH distances range from 2.500(2) to 2.597(2) Å in 4. For coordinated water ligands, Ce−Owater ranges from 2.488(6) to 2.565(5) Å in 3, and equals 2.491(3) Å in 5 (Table 2). Coordinated H2O molecules (O13, O14, and O15) in 3 and coordinated PrOH molecules (O11, O12, O13, and O22) in 4 form intramolecular O···O hydrogen bonds with the bridging carboxylate oxygen sites. In coordination polymer 5, the solvate isobutyric acid and the coordinated water molecule form intrachain hydrogen O···O interactions (Table 3). The nearest interchain Ce···Ce separation was found to be 8.268(9) Å for 3, 10.682(6) Å for 4, and 10.473(9) for 5.

Figure 4. Comparison of the CeOn environments in 1 and 5.

within this coordination polyhedron is D3h, reducing the number of independent ligand field parameters. The corresponding Hlf operator for the angular part of the wave function then reads:22 N

N

N

i=1

i=1

hex Ĥ lf = B02 ∑ C02(i) + B04 ∑ C04(i) + B06 ∑ C06(i)

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MAGNETIC PROPERTIES The discussion of the magnetism of the Ce(III) compounds 1−5 requires the consideration of single-ion effects, in particular the role of spin−orbit (so) coupling and ligand field (lf) effects. Generally, we determine the magnetic properties of fN lanthanide ions by first considering magnetically isolated metal ions surrounded by ligands imposing a distinct ligand field point symmetry upon the magnetic center. In a static magnetic field B, the Hamiltonian of the metal ion is then represented by

+

i=1 N B66 (C −6 6(i) i=1

∑

+ C66(i))

The computational simulations are based on the full basis set of 14 microstates. As discussed above, the polymer strands in 1 display alternating Ce···Ce distances and bridging modes between neighboring Ce sites, with two μ-O centers or two μ-carboxylate groups, respectively, mediating nearest-neighbor exchange coupling. Given the neglible coupling via extended exchange pathways between lanthanide ions, exchange coupling is effectively limited to the Ce(μ2-O)2Ce pairs and modeled using a standard Heisenberg-type exchange Hamiltonian (Hex = −2JexS1̂ ·Ŝ2). Figure 5 displays the variation χmT versus T (inset: χm−1 vs T) for 1. The solid lines display the result of the fitting procedures, utilizing our computational framework CONDON 2.0,23 which took into account the aforementioned single-ion effects in addition to the exchange interactions between the Ce3+ atoms (Hex) and the Zeeman effect of an applied magnetic field (Hmag). Compound 1, with the CeO9 environments approximated as tricapped trigonal prismatic, approaches a roomtemperature χmT value of 0.698 cm3 K mol−1, within the range of 0.55−0.78 cm3 K mol−1 typical for mononuclear Ce3+− oxygen donor complexes. The observed increase of χmT with increasing temperature is due to the thermal population of higher multiplet states and thus of single-ion origin. Note that for the determination of the corresponding interdependent ligand field parameters Bkq, the assumption of the idealized D3h symmetry of the CeO9 environment was sufficient to yield an excellent fit (Figure 5; goodness of fit: 0.6%), yielding B20 = 47 cm−1, B40 = −725 cm−1, B60 = −2100 cm−1, B66 = 1350 cm−1, and Jex = −0.06 cm−1. The ligand field parameters are in good agreement with other Ce3+ compounds. Jex corresponds to a Weiss temperature θ = −0.03 K for an adaptation of a Curie− Weiss expression to the low-temperature (T = 2−10 K) susceptibility. We note that interdimer magnetic exchange, mediated via carboxylate groups, cannot be ruled out, but does not become significant in this model description. For the individual Ce(III) centers, the D3h-symmetric ligand field splits the 2F5/2 ground multiplet into the three doublets |±mJ⟩ (where mJ = 1/2, 3/2, 5/2; see Table 4). The doublet |±1/2⟩ represents the ground state, followed by |±5/2⟩ at 32 cm−1 and |±3/2⟩ at 260 cm−1. This splitting of the doublets can also be correlated with the different Ce−O distances in the CeO9 coordination polyhedron. The |±3/2⟩ state of Ce3+ results as the highest state because of the maximum of corresponding 4f electron density directed toward the six O donors forming the

While H(0) represents the energy in the central field approximation, Hee and HSO account for interelectronic repulsion and spin−orbit coupling (modified by the orbital reduction factor κ), respectively. The former is effectively parametrized by Slater− Condon parameters (F2, F4, F6), the latter by the one-electron spin−orbit coupling parameter ζ. These interelectronic repulsion parameters, as well as ζ and κ, are set as constants for the fitting procedure. Hlf models the electrostatic effect of the ligands in the framework of ligand field theory, based on the global parameters Bkq (in Wyborne notation21,22). The spherical tensors Ckq in Hlf are directly related to the spherical harmonics, Ckq = (4π/(2k + 1))1/2Ykq, and the ligand field parameters Bkq are given by Akq⟨rk⟩, where Akq is a numerical constant describing the charge distribution in the environment of the metal ion and ⟨rk⟩ specifies the expectation value of the radial part of the wave function. For a Ce(III) site, HSO splits the 14-fold degenerate 4f1 ground state into the ground multiplet 2F5/2 and the excited multiplet 2F7/2 (ΔEso ≈ 2200 cm−1). With respect to the ligand field effect (Hlf), a model for compound 1 that reflects the individual lf parameters of the structurally different Ce3+ ions in the space group P1̅ (the donor environments of Ce1 and Ce2 are Cs-symmetric with 13 lf parameters) is impracticable, as this would result in too many independent lf parameters. On the basis of simulations using a point charge model, the coordination polyhedra of both sites (Ce1 and Ce2) can be approximated by a uniform tricapped trigonal prism (Figure 4). This polyhedron is relatively common for 9-fold coordinated lanthanide ions, and the site symmetry of the magnetic center 1599



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Figure 5. Temperature dependence of χmT and χm−1 (inset) for 1 at 0.1 T. ○, experimental data; −, least-squares fit to the isolated-dimer model described in the text.

Table 4. Magnetochemical Parameters and Ligand Field Splitting Energies of the 2F5/2 Ground State

prism, while corresponding 4f lobes for |±5/2⟩ are in the plane perpendicular to the main axis, defined by the three capping O sites, and for |±1/2⟩ the 4f lobes are oriented between both O donor sets.24 While compound 1 can be effectively treated as a dimer due to the large discrepancy between intradimer and interdimer linking modes (and also Ce−Ce distances), compounds 2−5 do not exhibit such pronouncedly alternating motifs within their polymer strands. Therefore, 2 and 5 were analyzed as extended spin systems. For compound 2, the three crystallographically distinct Ce ions are coordinated by 9 O atoms, and the ligand environments can be represented as tricapped trigonal prisms similar to 1. In 5, Ce1 is coordinated by 9 O sites that form a tricapped trigonal prism, whereas Ce2 is coordinated by 10 O sites, the geometry of which is represented by an O5 pentagon around Ce2, an O3 triangle coplanar to the pentagon, and an O2 dumbell on the opposite face of the pentagon (Figure 4). To avoid overparametrization, the susceptibility data for 2 and 5 were analyzed assuming a uniform Ce

environment, that is, a trigonal tricapped coordination coordination polyhedron. Furthermore, all coupling interactions between cerium ions were taken into account by a phenomenological mean field model. As shown in Figure 6, χmT approaches a value equal to 0.69 cm3 K mol−1 (2) and 0.71 cm3 K mol−1 (5) at room temperature. The corresponding ligand field parameters Bkq are determined assuming local D3h symmetry of the ligand fields. Note that the goodness-of-fit parameters SQ of 0.8% (2) and 0.9% (5) are slightly higher than that for 1, as the model assumption of a uniform ligand field appears to be more adequate for 1 than for 2 and 5. The energetic distance of the ground state 2F5/2 and the first excited state 2F7/2 is ΔE = 2221 (2) and 2241 cm−1 (5). The ligand fields in 2 and 5 split the Ce(III) 2F5/2 ground multiplet into three |±mJ⟩ doublets in the same sequence as in 1. The small, negative mean field parameters λmf in both cases confirm the expected very weak antiferromagnetic coupling between the Ce(III) spin centers. 1600



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ACKNOWLEDGMENTS We thank Yutian Wang for collecting crystallographic data sets for compounds 1, 3, and 5. Work at the Ames Laboratory was supported by the Department of Energy-Basic Energy Sciences under Contract No. DE-AC02-07CH11358.

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Figure 6. Temperature dependence of χmT and χm−1 (inset) for 2 (red) and 5 (blue) at 0.1 T. ○, experimental data; −, least-squares fit to the model described in text.

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CONCLUSIONS The presented five cerium(III) carboxylate coordination polymers based on smaller, yet slightly flexible isovalerate (1), propionate (2), and isobutyrate (3, 4, and 5) ligands exclusively form one-dimensional chains with linear (1), zigzag (2), and helical (3, 4, 5) geometries. This prevalence of 1D chains is in stark contrast to the flexibility of Ce(III) coordination environments that accommodate a wide range of coordination numbers and geometries, and in turn should in principle also allow for the formation of 2D or 3D coordination networks. The five examples demonstrate the structurecontrolling effects of the carboxylate groups that bind in bridging and end-on modes and discriminate between linear, zigzag, and helical shapes. As exemplarily shown by the three isobutyrate-based compounds 3−5, secondary structure-directing effects (e.g., induced by solvents or competing ligands) can drastically affect the Ce coordination environments; yet the overall helical structure of the resulting coordination polymer is retained. The magnetism of the presented compounds highlights the importance of single-ion effects for the 4f1based title compounds, where in particular the ligand fieldinduced splitting of the mJ states of the 2F5/2 ground multiplet determines the temperature-dependent magnetic susceptibility.

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ASSOCIATED CONTENT

S Supporting Information *

ORTEP-style plots with atom labeling schemes and crystal packing diagrams (Figures S1−S11), TGA and DTA curves (Figures S12−S16), selected bond distances and angles (Tables S1−S5), BVS calculation results (Table S6), and complete crystallographic details (CIF files). This material is available free of charge via the Internet at http://pubs.acs.org. CCDC 809323−809327 contain the supplementary crystallographic data for 1−5. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc. cam.ac.uk/data_request/cif.

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Corresponding Author

*Tel.: +49-241-80-92642. Fax: +49-241-80-92642. E-mail: paul. [email protected]. 1601



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Linear, Zigzag, and Helical Cerium(III) Coordination Polymers

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