Structural Evolution in the RE(OAc) 3 · 2AcOH Structure Type. A Non-Linear, One-Dimensional Coordination Polymer with Unequal Interatomic Rare Earth Distances

Coordination Abstract: The existing range of the centrosymmetric, triclinic RE(OAc) 3 · 2AcOH structure type has been extended for RE = Eu and Gd while the structure data of the Nd- and Sm-compounds have been revised and corrected, respectively, using low temperature (T = 100 K), well resolved (2 θ max = 56 ◦ ), highly redundant SCXRD data in order to evaluate the structural evolution within this class of acetic acid solvates by statistical methods. Within the nine-fold mono-capped square-antiprismatic coordination spheres of the RE 3+ ions, RE-O distances decrease as a result of lanthanide contraction; some with different rates depending on the coordination modes (2.11/2.21) of the acetate ions. The experimental data show that the internal structural parameters of the acetate ions also correlate with their coordination modes. Both acetic acid molecules act as hydrogen bond donors but only one as monodentate ligand. The geometries of the hydrogen bonds reveal that they are strongly inﬂuenced by the size of the rare earth atom. The non-linear, one-dimensional coordination polymer propagates with unequal RE ··· RE distances along the a-axis. Rods of the coordination polymer are arranged in layers congruently stacked above each other with the hydrogen bonded acetic acid molecules as ﬁller in between. In most cases, data ﬁtting is best described in terms of a quadratic rather than a linear regression analysis. the desired anhydrous triacetates —solvates with acetic acid and/or acetic anhydride are formed during the reaction. The formation of such compounds does not seem implausible as several acetic acid solvates , RE(OAc) 3 · m AcOH and acetic acid solvates hydrates , RE(OAc) 3 · m AcOH · n H 2 O are already known in addition to various hydrates , RE(OAc) 3 · n H 2 O acetic elongated along the a-axis, the propagation direction of the coordination polymer (see below). Besides the great inﬂuence of the reaction and crystallization conditions on the compounds formed, isolation from the mother liquor also turned out to be problematic as in mother liquor most of the compounds are very sensitive towards moisture whereas they are stable at ambient conditions over periods of hours and days when free of remaining solvents. In most cases we could achieve this challenge when taking the crystals out of the mother liquor by use of a spoon spatula and pouring on a ﬁlter paper.


Introduction
Rare earth triacetates are widely used as precursors in the synthesis of NaREF 4 and LiYF 4 core and core/shell nanocrystals. Usually, rare earth triacetate hydrates, RE(OAc) 3 nH 2 O, are employed since they are commercially available or can easily be prepared by dissolving the appropriate rare earth oxide in acetic acid. When nanocrystals of this kind are doped with lanthanide, ions emitting in the near infrared, anhydrous reaction conditions are advantageous since the luminescence is quenched by hydroxyl groups in the crystal lattice [1][2][3]. An example is NaYF 4 :Yb,Er/NaYF 4 core/shell nanocrystals showing upconversion emission with high quantum yield when anhydrous rare earth triacetates are used in the synthesis of the core particle and the shell [4].
In principle, drying of RE(OAc) 3 nH 2 O can be achieved by removal of their water content by extensive water exhausting in a vacuum at higher temperatures or by use of the reaction of the hydrated molecules with acetic acid anhydride, Ac 2 O, in glacial acetic acid, AcOH. With regard to a common, reproducible, easily upscaled protocol for the preparation of a large number of various anhydrous rare earth triacetates, RE(OAc) 3 , we started our attempts with the second method.
We became rapidly aware, however, that extensive drying under vacuum is required after synthesis to obtain a product of constant mass. This indicated that-in addition to the desired anhydrous triacetates-solvates with acetic acid and/or acetic anhydri formed during the reaction. The formation of such compounds does not seem impl as several acetic acid solvates, RE(OAc)3 · mAcOH and acetic acid solvates hydrates, RE · mAcOH · nH2O are already known in addition to various hydrates, RE(OAc)3 · nH2 ure 1). All attempts, however, to identify our products by PXRD failed.
We therefore decided to study the reaction products by single crystal X-ray diffraction (SCXRD) in order to identify the different phases formed as a result of the applied preparation and crystallization conditions. Last but not least we hoped to find unknown phases and compounds in the three phase system shown in Figure 1.
While exploring the reaction products with SCXRD we obtained the structural data for many new compounds, some for new composition opening new structure types, some extending or fulfilling the existence range of well-known structure types and others improving or correcting former results. Here we present our results on the crystal structure determinations of compounds belonging to the class of rare earth triacetates acetic acid solvates with composition RE(OAc) 3 · 2AcOH belonging to structure type 6, a non-linear one-dimensional coordination polymer with unequal interatomic RE···RE distances, observed in the case of the earlier lanthanides RE = Nd, Sm, Eu and Gd. Preparation and solid state structures of two compounds (RE = Nd, Sm) of this structure type have been formerly described in a doctoral thesis [5] but have never been published. In the Cambridge Structural Database [33] their structural data are deposited under the CSD-numbers 653,311 for RE = Nd (data base identifier: JIPLOK) and 653,320 for RE = Sm (data base identifier: JIPNIG). Although it was measured at the same temperature, the deposited data set of the Sm compound exhibited a significantly larger unit cell volume (793.01 Å 3 ) than the Nd compound (779.15 Å 3 ) and in sharp contrast to the so called lanthanide contrac- tion [34], the decrease of the RE 3+ ion radius with increase of the atomic number of the rare earth element.

Materials and Methods
Single crystals of the compounds described here have been obtained alongside crystals of other compounds by dissolving the corresponding rare earth oxides in acetic acid and using acetic anhydride to remove all water. In the cases of Eu and Gd, 15 mmol of RE 2 O 3 was dissolved in 100 mL of aqueous acetic acid (50% acetic acid by volume) by heating the suspension to reflux temperature. The solvent of the resulting clear solution was then removed with a rotavap avoiding prolonged drying of the resulting solid. Subsequently, 50 to 60 mL of glacial acetic acid were added and the solid dissolved by heating at 90 • C. After cooling the clear solution to 40 • C, 30 mL of acetic anhydride was added and the flask tightly closed. In the case of Nd and Sm, similar solutions could be prepared in a simpler way by directly dissolving the rare earth oxide in a mixture of glacial acetic acid and acetic anhydride. In this case, 10 mmol of RE 2 O 3 was refluxed in 35 mL of acetic acid and 15 mL of acetic anhydride under nitrogen atmosphere until a clear solution was obtained. After cooling to 40 • C, 30 mL of acetic anhydride was added and the flask tightly closed.
Parts of these solutions were mixed with additional acetic anhydride in a ratio, by volume, of 1:1, 1:2 or 1:3. All solutions were left for crystallization at room temperature. In cases where no crystals were detected after one week, the solution was concentrated on a rotavap by a factor of two. No attempts were made to further optimize the reaction conditions to exclusively obtain the substances described here. In the case of Gd, for instance, crystals of Gd(OAc)3 · 4AcOH also form, in addition to the title compound.
Morphology of the crystals was characterized by a prismatic habit ( Figure 2) most often resulting from the combination of the pinacoids {100}, {010} and {001}. Crystals are elongated along the a-axis, the propagation direction of the coordination polymer (see below). Besides the great influence of the reaction and crystallization conditions on the compounds formed, isolation from the mother liquor also turned out to be problematic as in mother liquor most of the compounds are very sensitive towards moisture whereas they are stable at ambient conditions over periods of hours and days when free of remaining solvents. In most cases we could achieve this challenge when taking the crystals out of the mother liquor by use of a spoon spatula and pouring on a filter paper.

Materials and Methods
Single crystals of the compounds described here have been obtained alongside cry tals of other compounds by dissolving the corresponding rare earth oxides in acetic ac and using acetic anhydride to remove all water. In the cases of Eu and Gd, 15 mmol RE2O3 was dissolved in 100 mL of aqueous acetic acid (50% acetic acid by volume) heating the suspension to reflux temperature. The solvent of the resulting clear soluti was then removed with a rotavap avoiding prolonged drying of the resulting solid. Su sequently, 50 to 60 mL of glacial acetic acid were added and the solid dissolved by heati at 90 °C. After cooling the clear solution to 40 °C, 30 mL of acetic anhydride was add and the flask tightly closed. In the case of Nd and Sm, similar solutions could be prepar in a simpler way by directly dissolving the rare earth oxide in a mixture of glacial ace acid and acetic anhydride. In this case, 10 mmol of RE2O3 was refluxed in 35 mL of ace acid and 15 mL of acetic anhydride under nitrogen atmosphere until a clear solution w obtained. After cooling to 40 °C, 30 mL of acetic anhydride was added and the flask tigh closed.
Parts of these solutions were mixed with additional acetic anhydride in a ratio, volume, of 1:1, 1:2 or 1:3. All solutions were left for crystallization at room temperatu In cases where no crystals were detected after one week, the solution was concentrated a rotavap by a factor of two. No attempts were made to further optimize the reaction co ditions to exclusively obtain the substances described here. In the case of Gd, for instan crystals of Gd(OAc)3 · 4AcOH also form, in addition to the title compound.
Morphology of the crystals was characterized by a prismatic habit ( Figure 2) mo often resulting from the combination of the pinacoids {100}, {010} and {001}. Crystals a elongated along the a-axis, the propagation direction of the coordination polymer (s below). Besides the great influence of the reaction and crystallization conditions on t compounds formed, isolation from the mother liquor also turned out to be problematic in mother liquor most of the compounds are very sensitive towards moisture where they are stable at ambient conditions over periods of hours and days when free of rema ing solvents. In most cases we could achieve this challenge when taking the crystals o of the mother liquor by use of a spoon spatula and pouring on a filter paper. Single crystals suitable for X-ray measurements were selected under a microsco and mounted on a 50 μm MicroMesh MiTeGen Micromount TM using FROMBLIN Y p fluoropolyether (LVAC 16/6, Aldrich) before they were centered on a Bruker Kappa AP II CCD-based 4-circle X-ray diffractometer using graphite monochromated Mo Kα rad tion (λ = 0.71073 Å) of a fine focus molybdenum-target X-ray tube operating at 50 kV a 30 mA. The crystal-to-detector distance was 40 mm and exposure time was 5 s per fram for all samples with a scan width of 0.5°. Samples were cooled down to 100 K by use o Kryoflex low temperature device. Single crystals suitable for X-ray measurements were selected under a microscope and mounted on a 50 µm MicroMesh MiTeGen Micromount TM using FROMBLIN Y perfluoropolyether (LVAC 16/6, Aldrich) before they were centered on a Bruker Kappa APEX II CCD-based 4-circle X-ray diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) of a fine focus molybdenum-target X-ray tube operating at 50 kV and 30 mA. The crystal-to-detector distance was 40 mm and exposure time was 5 s per frame for all samples with a scan width of 0.5 • . Samples were cooled down to 100 K by use of a Kryoflex low temperature device.
Data were integrated and scaled using the programs SAINT and SADABS [35] within the APEX2 software package of Bruker [36]. Special care was taken regarding the data collection strategy in order to reduce absorption effects. A maximum reduction of absorption effects and remaining electron density was achieved by a high data redundancy and numerical absorption corrections.
Structures were solved by direct methods of SHELXS ( [37]) and completed by difference Fourier synthesis with SHELXL [38]. Structure refinements were carried out on F 2 using full-matrix least-squares procedures, applying anisotropic thermal displacement parameters for all non-hydrogen atoms.
All H atoms including those of the acetic acid molecules were clearly identified in difference-Fourier syntheses. Hydrogen atoms of the methyl groups were refined with idealized positions and allowed to ride on their parent carbon atoms with d(C-H) = 0.98 Å and common isotropic temperature factors for all hydrogen atoms of each methyl group. Hydrogen atoms of the carboxyl groups were refined with a common O-H distance of 0.96 Å before they were fixed and allowed to ride on the corresponding oxygen atom with one common isotropic temperature factor.
Details on the crystallographic data, data collection parameters and structure refinement results are summarized in Table 1 where n is the number of reflections and p is the total number of parameters refined.
The listed CCDC numbers (Table 1) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures (accessed on 28 June 2021). Molecular graphics were prepared using DIAMOND [39], Mercury [40] and POV-Ray [41], respectively.
In order to compare bond lengths and angles in a simple manner, atoms of all four compounds were labeled according to the common numbering scheme depicted in Figure 3. The listed CCDC numbers (Table 1) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures (June, 2021). Molecular graphics were prepared using DIAMOND [39], Mercury [40] and POV-Ray [41], respectively.
In order to compare bond lengths and angles in a simple manner, atoms of all four compounds were labeled according to the common numbering scheme depicted in Figure  3. . Unit cell and labelling scheme of the atoms in the asymmetric unit of the RE(OAc)3 · 2AcOH structure type visualized for RE = Nd. All atoms are drawn as thermal displacement ellipsoids of the 40% level. Hydrogen bonds are indicated as broken sticks in red, additional RE-O-bonds to RE atoms outside the asymmetric unit as shortened sticks.

Results and Discussion
Our measurements of the RE(OAc)3 · 2AcOH structure type confirmed the previous results of a triclinic unit cell with two formula units therein, but in contrast to the depos- Figure 3. Unit cell and labelling scheme of the atoms in the asymmetric unit of the RE(OAc) 3 · 2AcOH structure type visualized for RE = Nd. All atoms are drawn as thermal displacement ellipsoids of the 40% level. Hydrogen bonds are indicated as broken sticks in red, additional RE-O-bonds to RE atoms outside the asymmetric unit as shortened sticks.

Results and Discussion
Our measurements of the RE(OAc) 3 · 2AcOH structure type confirmed the previous results of a triclinic unit cell with two formula units therein, but in contrast to the deposited data (see above) the unit cell volume of the Gd compound was-in accordance with the lanthanide contraction-smaller than the unit cell volume of the Nd compound.

Unit Cell
As expected, the unit cell volumes decreased continuously from Nd to Gd (Table 1, Figure 4). The mathematical correlation of unit cell volume with the atomic number of the rare earth element can be calculated by a linear regression analysis (y = ax + b) with a goodness of fit factor R 2 of 0.9931. Data, however, are better fitted by use of a quadratic expression (y = ux 2 + vx + w) with R 2 = 1.000.
With four data points the reliability of these calculations is limited but the trend is in accordance with the observations of Greis and Petzel [42], whose data on the unit cell volumes of isostructural REF 3 -compounds (orthorhombic YF 3 -structure type, RE = Sm − Lu, 10 data points; hexagonal LaF 3 -structure type, RE = Ce − (Pm) − Eu, 5 data points) support With four data points the reliability of these calculations is limited but the trend is in accordance with the observations of Greis and Petzel [42], whose data on the unit cell volumes of isostructural REF3-compounds (orthorhombic YF3-structure type, RE = Sm − Lu, 10 data points; hexagonal LaF3-structure type, RE = Ce − (Pm) − Eu, 5 data points) support the fit of the unit cell volume against the atomic number of the rare earth element by use of a quadratic equation instead of a linear one.

RE Coordination
Rare earth atoms of the RE(OAc)3 · 2AcOH structure type are ninefold, mono-capped square-antiprismatic coordinated ( Figure 5) with a narrow RE-O bond length distribution (Table 2), with the mean value decreasing from RE = Nd to Gd. Statistically, a linear regression analysis of the nine different RE-O distances results in R 2 -values in the range 0.9996 to 0.9838 but-as in the case of unit cell shrinkage-decrease of bond lengths is best described in all cases by a quadratic regression analysis with R 2 values in the range of 1.000 to 0.9984. The coefficients a, b (linear regression) and u, v, w (quadratic regression) of the different approaches are summarized in Table S1.

RE Coordination
Rare earth atoms of the RE(OAc) 3 · 2AcOH structure type are ninefold, mono-capped square-antiprismatic coordinated ( Figure 5) with a narrow RE-O bond length distribution (Table 2), with the mean value decreasing from RE = Nd to Gd. Statistically, a linear regression analysis of the nine different RE-O distances results in R 2 -values in the range 0.9996 to 0.9838 but-as in the case of unit cell shrinkage-decrease of bond lengths is best described in all cases by a quadratic regression analysis with R 2 values in the range of 1.000 to 0.9984. The coefficients a, b (linear regression) and u, v, w (quadratic regression) of the different approaches are summarized in Table S1. With four data points the reliability of these calculations is limited but the trend is in accordance with the observations of Greis and Petzel [42], whose data on the unit cell volumes of isostructural REF3-compounds (orthorhombic YF3-structure type, RE = Sm − Lu, 10 data points; hexagonal LaF3-structure type, RE = Ce − (Pm) − Eu, 5 data points) support the fit of the unit cell volume against the atomic number of the rare earth element by use of a quadratic equation instead of a linear one.

RE Coordination
Rare earth atoms of the RE(OAc)3 · 2AcOH structure type are ninefold, mono-capped square-antiprismatic coordinated ( Figure 5) with a narrow RE-O bond length distribution (Table 2), with the mean value decreasing from RE = Nd to Gd. Statistically, a linear regression analysis of the nine different RE-O distances results in R 2 -values in the range 0.9996 to 0.9838 but-as in the case of unit cell shrinkage-decrease of bond lengths is best described in all cases by a quadratic regression analysis with R 2 values in the range of 1.000 to 0.9984. The coefficients a, b (linear regression) and u, v, w (quadratic regression) of the different approaches are summarized in Table S1. Figure 5. (a) Distorted mono-capped square antiprismatic coordination polyhedron of the rare earth atoms in the RE(OAc)3 · 2AcOH structure type; example RE = Nd. All atoms are drawn as thermal displacement ellipsoids of the 40% level. Bonds from oxygen to carbon are drawn in two colors, bonds from oxygen to neighboring rare earth atoms as short sticks. (b) Figure 5. (a) Distorted mono-capped square antiprismatic coordination polyhedron of the rare earth atoms in the RE(OAc) 3 · 2AcOH structure type; example RE = Nd. All atoms are drawn as thermal displacement ellipsoids of the 40% level. Bonds from oxygen to carbon are drawn in two colors, bonds from oxygen to neighboring rare earth atoms as short sticks. (b) Evaluation of RE-O distances as function of the atomic number of the RE element with trend lines; empty raw of RE = Pm included in order to get an adequate representation; Symmetry codes to generate equivalent atoms: 1 1 − x, 1 − y, 1 − z; 2 −x, 1 − y, 1 − z.
There are, however, some remarkable exceptions from parallelism in bond length shrinkage: (i) the RE-O(11) distance decreases slower than the RE-O (12)  Besides this global view on the RE-O bond lengths, their detailed inspection in terms of the acetate coordination modes will give a deeper insight into the structural evolution of this structure type as a consequence of lanthanide contraction. In the following we will use the Harris symbol as described by Coxall et al. [43] for the different acetate coordination modes to distinguish the three different crystallographic acetate groups shown in Figure 6. The first one (atoms labelled 1n) exhibits a bridging 2.11 (1κO; 2κO') coordination mode. In addition, this acetate ligand acts as a hydrogen acceptor in a hydrogen bridge bond.
Both the second and third acetate group (atoms labeled 2n and 3n, respectively) belong to the three-dentate, bridging-chelating coordination mode 2.21 (1:2κ2O; 1κO'), but only the third one behaves as a hydrogen acceptor too.

2.21-Coordination Mode
The influence of lanthanide contraction on the structural parameters associated with this kind of coordination mode differs for both 2.21-coordinating acetate groups. While all RE-O distances decrease in both cases when the atomic number of the rare earth element increases (see above), the corresponding bonding angles seem to be unaffected for the third acetate group but vary in the case of the second one, indicating a change of their orientation in relation to the coordinated rare earth atoms. Thus the C-O···RE angle of the bridge decreases from 148.

2.21-Coordination Mode
The influence of lanthanide contraction on the structural parameters associated with this kind of coordination mode differs for both 2.21-coordinating acetate groups. While all RE-O distances decrease in both cases when the atomic number of the rare earth element increases (see above), the corresponding bonding angles seem to be unaffected for the third acetate group but vary in the case of the second one, indicating a change of their orientation in relation to the coordinated rare earth atoms. Thus the C-O···RE angle of the bridge decreases from 148.7(1) • (RE = Nd) to 148.0(1) • (RE = Gd) equally as the chelating C-O ···RE angle decreases on the opposite side from 96.0(1) • (RE = Nd) to 95.5(1) • (RE = Gd).
In contrast to the foregoing coordination mode, Janiki et al. [44] report on the subset of nine-fold coordinated rare earth atoms and 2.21-coordination mode mean RE-O distances which are to some extent greater than in the present study (Janiki et al. [44]

Internal Structural Parameters and Hydrogen Bonding of the Acetate Groups
The present SCXRD data not only give us a deeper insight into acetate coordination modes and the structural evolution of the rare earth coordination sphere as a result of lanthanide contraction but also allow us a detailed look at the internal structural parameters of the acetate groups (Table 3). For C-C bonds of type C(sp 2 )-C(sp 3 ) RCOO-, Allen et al. [45] reported an overall value of 1.520 (11) Å. This value is somewhat longer than the C-C bonds in the present study [1.498(1)-1.506(1) Å], which may be ascribed to the fact that our compounds exclusively have acetate groups. There is an indication that the C-C bond is longer for the bridging 2.11-coordination mode [1.506(1) Å] compared with the corresponding bond length in the 2.21-coordination mode [1.498(1) Å], but the data situation is too poor for a clear statement.

Coordination Modes, Internal Structural Parameters and Hydrogen Bonding of the Acetic Acid Molecules
While both crystallographical different acetic acid molecules (Figure 7) of the RE(OAc) 3 · 2AcOH structure type act as hydrogen donors in hydrogen bonds, only the first one (atoms labeled 4n) also acts as an electron donor towards RE in a monodentate fashion (1.10 coordination mode). In the latter case, the RE-O distances [RE-O(41), 2.497(1)-2.445(1) Å; Table 2, Figure 5] are of medium strength.    In comparison with the uncoordinated acetic acid molecule (atom labeled 5n), the C=O bond of the coordinated one is significantly (+0.036 Å) longer, while-on the other hand-the C-OH bond is shorter (−0.009 Å) (Table 4). In their review on bond lengths in organic compounds, Allen et al. [45] recorded a value of 1.308 (19)   Donor acceptor distances (Table 5)   Although the oxygen atoms of the OH-groups are not involved in rare earth coordination, the structural parameters of both hydrogen bonds show a strong correlation with the size of the rare earth atom. As we could localize the hydrogen atoms of the OH-groups from difference Fourier synthesis (see above), the analysis of the structural evolution of the hydrogen bonds as a function of the size of the rare earth element reveals some remarkable features (Table 5, Figure S1

Packing
The {REO 9 } building units are connected with each other to a non-linear, onedimensional coordination polymer along the a-axis with unequal interatomic RE···RE distances (Table 6, Figure 8). Within the series of the investigated compounds, both distances decrease (Table 4) continuously with increasing atomic number as the interatomic RE···RE···RE bond angle decreases from 138. 15(1) • (RE = Nd) to 137.80(1) • (RE = Gd).
nation, the structural parameters of both hydrogen bonds show a strong correlati the size of the rare earth atom. As we could localize the hydrogen atoms of the OHfrom difference Fourier synthesis (see above), the analysis of the structural evolu the hydrogen bonds as a function of the size of the rare earth element reveals so markable features (

Packing
The {REO9} building units are connected with each other to a non-linear, onesional coordination polymer along the a-axis with unequal interatomic RE···RE di (Table 6, Figure 8). Within the series of the investigated compounds, both distan crease (Table 4)   In spite of the zig-zag-arrangement of the rare earth atoms, the overall onesional coordination polymers exhibit-neglecting the uncoordinated acetic acid cules-a remarkable circular, rod-like shape with a diameter of about 1.22 nm (Fi In this context, the uncoordinated, only hydrogen bonded acetic acid molecules be knobs that fill the space in the square primitive arrangements of the rods (Figure zipper-like way. In spite of the zig-zag-arrangement of the rare earth atoms, the overall one-dimensional coordination polymers exhibit-neglecting the uncoordinated acetic acid moleculesa remarkable circular, rod-like shape with a diameter of about 1.22 nm (Figure 9). In this context, the uncoordinated, only hydrogen bonded acetic acid molecules behave as knobs that fill the space in the square primitive arrangements of the rods ( Figure 10) in a zipper-like way.  . Arrangement of the one-dimensional coordination polymers (simplified visualized as rods) in the crystal structure of the RE(OAc)3 · 2AcOH structure type, looking down the a-axis; example RE = Nd. Uncoordinated, but hydrogen bonded acetic acid molecules filling the space between the rods in a zipper-like manner are shown as ball-and-stick model.

Conclusions
Our low temperature, well resolved single-crystal X-ray data of the rare earth triacetates acetic acids solvates belonging to the RE(OAc)3 · 2AcOH structure type allow a more detailed insight not only into the coordination behavior of the rare earth element but also into the influence of their coordination on the internal structural parameter of the acetate ligands and acetic acid molecules and vice versa.  . Arrangement of the one-dimensional coordination polymers (simplified visualized as rods) in the crystal structure of the RE(OAc)3 · 2AcOH structure type, looking down the a-axis; example RE = Nd. Uncoordinated, but hydrogen bonded acetic acid molecules filling the space between the rods in a zipper-like manner are shown as ball-and-stick model.

Conclusions
Our low temperature, well resolved single-crystal X-ray data of the rare earth triacetates acetic acids solvates belonging to the RE(OAc)3 · 2AcOH structure type allow a more detailed insight not only into the coordination behavior of the rare earth element but also into the influence of their coordination on the internal structural parameter of the acetate ligands and acetic acid molecules and vice versa. Figure 10. Arrangement of the one-dimensional coordination polymers (simplified visualized as rods) in the crystal structure of the RE(OAc) 3 · 2AcOH structure type, looking down the a-axis; example RE = Nd. Uncoordinated, but hydrogen bonded acetic acid molecules filling the space between the rods in a zipper-like manner are shown as ball-and-stick model.

Conclusions
Our low temperature, well resolved single-crystal X-ray data of the rare earth triacetates acetic acids solvates belonging to the RE(OAc) 3 · 2AcOH structure type allow a more detailed insight not only into the coordination behavior of the rare earth element but also into the influence of their coordination on the internal structural parameter of the acetate ligands and acetic acid molecules and vice versa.
For the nine-fold mono-capped square-antiprismatic coordination of the rare earth atoms, lanthanide contraction represents the most prominent factor for RE-O bond lengths and RE-O-C bond angles in the different coordination modes of the acetate groups as the specific values most often decrease in accordance with the size of the rare earth atom. The corresponding relationships can be fitted by use of a linear regression analysis but are more often better described by a quadratic equation, an observation that was formerly already observed in the case of some isostructural rare earth trifluorides, REF 3 [42], trisethylsulfate nonahydrates, RE(C 2 H 5 SO 4 ) 3 · 9H 2 O [47], and tris-trifluoromethanesulfonate nonahydrates, RE(CF 3 SO 3 ) 3 · 9H 2 O [48,49]. Based on these data, the parabolic decay of structural parameters associated with lanthanide contraction has been revisited [50] and theoretically reinforced [51]. Our data also show that not all RE-O bond lengths decrease uniformly. The exceptions show that the corresponding acetate groups occupy a somewhat different orientation in space in order to optimize their interactions with the rare earth atoms.
For the acetate ligands our data indicate that the internal structural parameters strongly depend on their coordination modes (2.11, 2.21) and the hydrogen bonds they are involved in, and in particular the strongly different bond angles between the oxygen atoms of the different acetate groups constitute important indicators for this assumption. Unexpected results came from the hydrogen bridging bonds of the acetic acid molecules as their structural parameters strongly correlated with the size of the lanthanide atom, an observation that has to be confirmed by further experiments. With respect to the extrapolation of these data it seems possible that the hydrogen bridging bonds confine the existence range of the RE(OAc) 3 · 2AcOH structure type of the compounds described herein.