Stability of the Compounds with the Octahedral [M(κ-L)2] Coordination Building Blocks of Late 3d Elements via Solvate Water Supramolecular Arrangement

Five new coordination compounds with the common formulae [M(bdmpza)2] · xH2O (bdmpza = bis(3,5-dimethylpyrazol-1-yl)acetate, C11H15N4COO ; x = 2 for Co (1), Ni (2) and Zn (3), x = 3 for Mn (4) and Fe (5)), were synthesized from the reaction mixtures containing appropriate metal salt, bis(3,5-dimethylpyrazol-1-yl)acetic acid (Hbdmpza), methanol and water. The X-ray single crystal diffraction analysis reveals that all five compounds (1–5) contain mononuclear homoleptic [M(bdmpza)2] coordination molecules, in which the central metal ions are surrounded with two chelating bdmpza ligands. Each ligand is bound tridentately facially via κ-N,N,O sites and centrosymmetric coordination chromophore is thus obtained. In the dihydrates [M(L)2] · 2H2O, 1–3, every coordination molecule is linked by two water molecules, i.e. two pairs of O–HO hydrogen bonds, on each of the two sides, thus forming infinite chain. In case of the trihydrates [M(L)2] · 3H2O, 4–5, with an assistance of the additional water molecule, such chains are further connected into extended supramolecular sheets. A stability of both similar hydrate types is confirmed by powder XRD data and IR analysis. The ionic metal radius and the coordination M–O, M–N bonds were related to the void dimension among the coordination molecules of the title [M(bdmpza)2] · xH2O.


Introduction
Although the coordination chemistry of the first row transition metals with bis(3,5-dimethylpyrazol-1-yl)acetate anions (bdmpza) and related ligands has progressed in recent years due to the extensive search for enzyme-mimicking compounds, 1-5 a detailed investigation of CCDC has shown that only a few hydrates with mononuclear coordination cores have been structurally characterized till now. 6 These are hemihydrate of manganese(I), [Mn(bdmpza)(CO) 3 ] · 0.5H 2 O, 7 dihydrate of copper(II), [Cu(bdmpza) 2 ] · 2H 2 O, 8 and trihydrate of zinc, [Zn(bdmpza) 2 ] · 3H 2 O. 9 Outside the family of first row transition metal compounds, hemihydrate of Re(I), [Re(bdmpza) (CO) 3 ] · 0.5H 2 O, 7 and trihydrate of magnesium, [Mg (bdmpza) 2 ] · 3H 2 O, 10 were reported as well. The small number of such described compounds is in accordance with the fully com-pleted octahedral coordination sphere of the [M(κ 3 -L) 2 ] type with two strongly bound tridentate bdmpza, thus disabling any potential controlled replacement at one or two coordination sites. 2 Despite of no obvious catalytic potential of such compounds, the aqueous medium of isolation and a prospect of possible catalytic activation 11 may suggest increased interest toward this family of compounds. Further investigation into this direction would thus be motivated. Nevertheless, the number of fully characterized [M(bdmpza) 2 ] · xH 2 O compounds is not large enough to draw any unambiguous suggestion about the influences of water molecules, co-crystallized and/or trapped among the coordination moieties, or the specific metal role in [M II (κ 3 -L) 2 ] coordination sphere regarding the coordination species potential activation. This may well be related with a stability of such hydrogen bonding supramolecular networks, especially outside the mother liquid.
Herein we represent five new hydrates, having [M II (κ 3 -L) 2 ] coordination building blocks of the late 3d elements.

1. Materials and Synthesis
All commercial starting compounds and solvents were of analytical grade quality and used as received. Hbdmpza (C 12

Physical Measurements
CHN elemental analyses were performed by a PERKIN ELMER Elemental Analyzer Series ll CHNS/O.
The infrared spectra of the solid samples were measured in a region between 4000-600 cm -1 using a Perkin-Elmer Spectrum 100 series FT-IR spectrometer equipped with Specac Golden Gate ATR as a sample support.
The electronic spectra were measured between 950-200 nm using Perkin-Elmer Lambda 750, UV-Vis-NIR Spectrometer. Each finely ground solid sample was homogenized with nujol oil and the resulting suspension was smeared on a filter paper band. The band was transferred to quartz cuvette (1 cm). Filter paper with nujol oil only was used as a reference.
X-ray powder diffraction data were collected on Panalytical X'Pert PRO MPD powder diffractometer equipped with Ge(111) Johannson type monochromator, in reflection mode using CuKa1 radiation and the full range of the 128 channel linear RTMS detector.

1. Coordination Molecules
The determined crystal structures of all five title compounds consist of homoleptic [M II (bdmpza) 2 ] coordination molecules (Fig. 1) and water molecules residing among them. Each bdmpza ligand is bound tridentately to the central metal ion, via two pyrazole nitrogen atoms and one deprotonated oxygen atom from the carboxylic group in facial manner. Such coordination via N,O,N sites giving [M II (κ 3 -L) 2 ] is often observed for bdmpza ligand. 1,3 The central metal ion within each structure occupies a special crystallographic position, i.e. an inversion centre for 1-3, while a twofold axis for 4 and 5, respectively. Accordingly, the asymmetric unit comprises one half of the coordination molecule, while two tridentate bdmpza ligands enable a formation of an octahedral environment around the central metal ion. Within the M II N 4 O 2 chromophore in 1-5, the coordination bond distances do differ to some extent ( Table 2, Fig. 2), though less distinguished than in analogous copper compound [Cu(L) 2 ] · 2H 2 O, 8 where the basal plane of the coordination octahedron comprises of four nitrogen atoms, while the two oxygen atoms occupy the remaining axial positions, showing lon-ger bond distances due to the Jahn-Teller distortion. Interestingly, the water free copper analogue [Cu(L) 2 ] 8 shows the long N-M-N axis, thus having M II N 2 O 2 N 2 chromophore. Contrary to the hydrated copper compound, the M-O distances in the other hydrates 1-5 and related zinc compound [Zn(L) 2 ] · 3H 2 O 9 are shorter than their respective M-N distances. The M-N distances in 4-5 and [Zn(L) 2 ] · 3H 2 O are almost of the same length, while in 1-3 even these bond distances differ, giving them slight compressed or rhombic distorted octahedral character for both groups of compounds, respectively. Figure 2 shows the differences of the unit cell volume and coordination bonds, respectively, depending on the type of the central metal ion and its ionic radius. The unit cell volume of 4, 5 and [Zn(L) 2 ] · 3H 2 O, which cry- Table 2. Bonding distances within the first coordination sphere of central metal ions in 1-5 and their hydrated copper and zinc analogues. 8 Figure 2, revealing copper compound especially differing at the coordination bond lengths. Its ground state is typical with d(x 2 -y 2 ) 1 and d(z 2 ) 2 , while more spherical influence of the ligands towards d orbital occupancy, for the other metal species investigated herein, is noticed. Such characteristic copper(II) phenomenon may be related with clear ease of copper(II) species isolation, perhaps being the principal activation driving force.

2. Dihydrates [ [M(L)2] ] · 2H 2 O; 1, 2, 3
In the crystal structure of the dihydrates 1, 2 and 3, besides one half of the coordination molecule, one symmetry independent molecule of water is present. Thus, between the two adjacent coordination molecules, two net water molecules are positioned, being linked with four hydrogen bonds O-H ... O of moderate strength. As being so on each of the two sides, infinite chains are therefore formed (Fig. 3a). Details on hydrogen bond geometry are collected in Table 3.
The packing of the infinite chains being further inter-connected via weak intermolecular C-H ... C van der Waals interactions is presented in Fig. 3b.

Trihydrates [ [M(L)2] ] · 3H 2 O; 4, 5
The first and probably the most obvious difference between di-and trihydrates from the crystallographic point of view is the change of unit cell from triclinic P-1   with V ∼ 700 Å 3 to the monoclinic C2/c with V ∼ 2800 Å 3 . From the first sight onwards, it must be related with an inclusion of an additional water molecule into the crystal lattice. In 4 and 5, therefore, two symmetry independent water molecules, namely O3 and O4, appear (in dihydrates, only one with occupancy 1, i.e. O3, is present per one half of [M(L) 2 ]). The first one in the trihydrates (occupancy 1) is disordered over two positions (O3a and O3b) with relative occupancies 0.799(4):0.201(4) in 4 and 0.758(4):0.242(4) in 5, respectively, while the O4 is symmetry disordered, being positioned at a close vicinity of the twofold axis (occupancy 0.5).
The role of O3a in 4 and 5 is almost the same as the role of water O3 in the dihydrates. Namely, within these ∼80% of the unit cells, where water O3a is present and O3b is absent, the same building motif, i.e. infinite chains of the coordination molecules bridged by two water molecules as in the dihydrates 1-3, appear (see Fig. 3). Additionally to the dihydrates feature, these chains are differently further inter-connected via symmetry disordered water molecules O4 into supramolecular layered structure (Fig. 4). The details on hydrogen bonding in trihydrates 4 and 5 are in Table 4.
Considering the ∼20% of the unit cells in 4 and 5, where only O3b is present, a hydrogen bond to the carboxylate O2 atom from the adjacent coordination molecule is formed (Fig. 5). Two O3b water molecules per coordination molecule are attached this way. Another water molecule (i.e. the symmetry disordered O4) is hydrogen bonded to this structural segment. Sections consisting of a   coordination molecule and two pairs of water molecules (O4, occupancy 0.5) are thus obtained in ∼20% of the unit cells (Fig. 5). This analogy of both stable hydrate structural types, differing in water H-bonding networks, is further confirmed by the XRD powder analysis. It shows almost identical features within each group, though clear difference among all (see Supplementary Information). The measured zinc powder XRD reveals a presence of both, i.e. di-and trihydrate, showing both species may be obtained at similar conditions, possibly in relation with a very stable zinc(II) oxidation state. The network water molecules thus generally enable stable different types of packing. However, it seems the network water molecules do not activate additionally the catalytic potential of the coordination species by their impact towards the first coordination sphere, since this sphere remains closely sealed octahedral ML 6  Interestingly, in the structures of Mg and Zn trihydrate analogues, 9,10 the O4 water was symmetrically di-sordered, while O3 was not disordered at all. It is important to stress once again that the hydrogen atoms attached to O4 in Zn trihydrate compound 9 were not observed in the difference Fourier map, while those on O3 were, refined one by one with the appropriate bond length restraints and fixed at the final refinement cycles due to high shifts/unstable refinement. Figure 6 shows the infrared spectra of the compounds 1-5 and copper dihydrate analogue. At first glance, the vibrational spectra of all six compounds are very similar. However, a detailed view shows almost identical feature for 4 and 5 on one hand, as well as the spectra of 1-3 and related copper species on the other hand, respectively. Most obvious-differences among both groups of compounds are seen in the shapes of O-H stretching absorptions around ∼3300 cm -1 and asymmetric stretching absorptions of the carboxylate group at 1650 cm -1 . In case of the dihydrates 1-3, there are two ν(O-H) bands that are well-resolved. For the trihydrates 4-5, in which hydrogen bonding is more diverse, the aforementioned bands get broader and are not resolved to the distinguished lines anymore. Similar phenomenon can be observed at approximately twice smaller energy as due to the asymmetric carboxylate stretching absorptions, where an additional band can be observed for the title dihydrates and one broad band for the respective trihydrates.

Spectroscopy
The electronic spectra are in accordance with the metal oxidation states and ligands attached. 18 As such, they are not significantly related with the water arrangement outside the coordination sphere.

Conclusions
Five new coordination compounds with the common formulae [M II (bdmpza) 2 ]·xH 2 O (bdmpza = bis(3,5dimethylpyrazol-1-yl)acetate, Co (1), Ni (2), Zn (3), Mn (4) and Fe (5) were isolated and compared with the other two known species in this system, namely [Cu II (bdmpza) 2 ] · 2H 2 O 8 and [Zn II (bdmpza) 2 ] · 3H 2 O. 9 The typical octahedral coordination sphere is found in all cases showing only small differences among each other. Interestingly, the expectedly weaker stabile Mn II (4) and Fe II (5) species 19,20 were isolated as trihydrates, Co (1), Ni (2) and Cu 8 as dihydrates, while Zn as di-(3) and trihydrate. 9 The two stable hydrate groups are distinguished with obvious resemblance (even isostructurality) also at the powder XRD and IR analysis. All trihydrates clearly show longer coordination bonds, along with the expected larger unit cell volume (more net water molecules). Ionic radius thus seems to be important, though solely shell does not favour any kind of the hydrate, as both types were isolated with zinc. Also the reaction conditions may not be the decisive reason for the diferenciation, due to many alternative procedure tryouts, but only limited number of the final products for each late 3d metal. It seems more probable that alternative hydrate types are plausible in situ as the partial motif of the dihydrates is found also within one of the trihydrate structural patterns. Both, namely the dihydrate and trihydrate type, or even more alternative pathways (e.g. a and b trihydrate) are thus simultaneously competing by packing the same available structural building blocks. More disordered option, namely trihydrate type, seems to enable additional stabilization to the oxygen sensitive Mn II and Fe II species, while firmly stable Zn II can adopt either of the two forms.