2D Supramolecular Structure for a Chiral Heterotrinuclear Zn II2 Ho III Complex through Varied H-Bonds Connecting Solvates and Counterions

: We report the crystal structure of [Zn II 2 Ho III (L)(ald)(HO)(H 2 O) 3 (MeCN)](NO 3 ) 2 ·EtOH [H 3 L = 2-(5-bromo-2-hydroxy-3-methoxyphenyl)-1,3-bis[4-(5-bromo-2-hydroxy-3-methoxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine) and Hald = 5-bromo-2-hydroxy-3-methoxy-benzaldehyde]. Despite the presence of two bulky multidentate ligands, as well as several monodentate ligands surrounding the nonacoordinate holmium cation, and the two pseudooctahedral zinc ions, the intricate H-bonded system formed by this chiral heterotrinuclear complex is only expanded in a 2D supramolecular structure. The interactions involve the nitrate counterions and the solvated ethanol, in such way that each complex unit is connected to an identical enantiomer, and to two units of inverted chirality through H bonds.


Introduction
Coordination chemistry of lanthanoids has experimented a substantial development in past years. The interest in this area is closely related to that in single-molecule magnets (SMMs) and singleion magnets (SIMs), and their potential applications [1]. Concurrently, a particular interest has been devoted to heteronuclear {s-/3d-4f}-coordination complexes, as their combination can afford different properties, and hence, new polyfunctional molecules could arise [2][3][4][5]. For instance, the combination of zinc ions, and changes in the ancillary ligands could influence not only the anisotropy barrier [6], but it can additionally afford luminescent properties [6][7][8][9][10]. In this sense, we are particularly interested in obtaining lightluminescent molecular magnets. Thus, we have been recently involved in a research programme aiming to prepare, characterise and study not only discrete pure lanthanoid (Dy, Er, Tb) complexes, but also hybrid Zn-Ln polynuclear complexes containing one of the two polytopic ligands shown in Scheme 1 [11,12].
As an extension of this work, in this occasion we have tried to combine zinc(II) and holmium(III) ions with H3L (Scheme1), with the aim of getting further insight into the features of new Zn-Ho systems, as they could provide interesting results at a magnetic level [1,5,13,14]. Scheme 1. Ligands used in our work. In this particular case we have employed H3L.

Materials and Methods
All chemical reagents were purchased from commercial sources and used as received without further purification.
The structure was solved by standard direct methods, employing SHELXT [16], and then it was refined by full-matrix leastsquares techniques on F 2 , using SHELXL [17]. All non-hydrogen atoms, including counterions and solvates, were anisotropically refined. Hydrogen atoms were included in the structure factor calculations in geometrically idealised positions in most of cases. Hydrogen atoms potentially involved in classic H bonds were located in Fourier maps, and then they were refined with thermal factors depending on the parent atoms.
Crystal data and experimental parameters relevant to the structure determinations are listed in Table 1. Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre (CCDC-1843238) and can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

Synthetic Method
Keeping in mind the magnetic results previously obtained with H3L and H3L', our goal was obtaining heterotrinuclear holmium(III) and zinc(II) complexes with H3L. The synthetic method was designed on the basis of our previous experience with H3L [11], and H3L' [12] (Scheme 1). A template method using appropriate molar ratios of the corresponding aldehyde, tetraamine and metal salts had resulted as being useful to obtain mononuclear and even heteronuclear complexes with H3L' (Scheme 1) [12], but this method was not suitable for H3L, as the previous isolation of the tricondensed ligand was necessary to prepare its complexes.
Unfortunately, and despite multiple attempts made with different methods, no pure samples of di-or hetetrironuclear analogous with holmium(III) ions could be isolated. In fact, the only pure and crystalline compound that has been accurately identified was a by-product identified as [Zn II 2Ho III (L 3 )(ald)(HO)(H2O)(MeCN)](NO3)2·EtOH by its crystal structure (Figure 1). It is clear that both the deprotonated aldehyde ligand (ald − ), and the hydroxide ion appear resulting from some hydrolysis that occurred during either the reaction, or the recrystallisation processes.
Despite the high stability shown by this type of three-armed ligands when we had worked with them ( [11,12] and own references therein), the reason for these fruitless results could be related to reaction and/or crystallisation periods longer than usual, with a partial decomposition of H3L. This fact had been previously detected for another related ligand [18]. Unfortunately, the lack of a sufficient amount of sample, with a guaranteed purity, prevented us from performing further studies of this complex.

Spatial Arrangement of [Zn II 2Ho III (L 3 )(ald)(HO)(H2O)(MeCN)] 2+
Despite the symmetry of the three-armed H3L ligand (Scheme 1), and even of both Zn···Ho distances, [Zn II 2Ho III (L)(ald)(HO)(H2O)(MeCN)] 2+ is chiral. In Figure 1, we can see the enantiomer where the chiral C12, N2 and N3 atoms are displaying S configurations. The main geometric parameters are listed in Table 2. Chirality is related not only to the irregular coordination environment of the holmium atom, but also to the two different solvent molecules (MeCN and water) coordinated to each internal zinc atom. These zinc ions show distorted octahedral environments, which are sharing one of their faces with the pseudo-polyhedron around Ho1 (Figure 2  With regard to the holmium(III) ion, it is surrounded by nine O atoms that belong to the phenoxy and methoxy groups attached to the two terminal arms of L 3− (O1, O2, O5, O6) and to the deprotonated aldehyde residue (O7, O8), as well as to the hydroxide ion (O1h), and to two water molecules (O1w, O3w). The degree of distortion of this HoO9 coordination sphere with respect to an ideal nine-vertex polyhedron was calculated with the SHAPE software [19], and it indicates that nearly corresponds to a "muffin-like" geometry ( Figure 2).

Packing Scheme for [Zn2Ho III (L)(ald)(HO)(H2O)(MeCN)](NO3)2·EtOH
With the presence of so many O and N atoms in the ligands, three coordinated water molecules, an ethanol solvate, and two nitrate counterions is not surprising that this complex can give rise to an intricate H bonding scheme. However, this packing scheme presents some interesting features at a supramolecular level. To simplify its study, only classic O-H···O bonds are listed in Table 2, but several bifurcations, and many C-H···A interactions (A = O or Br) have been also detected. The basic O-H···O network is shown in Figure 3.
It is evident that both the enveloping three-armed ligand and the ancillary aldehyde ligand can exert a considerable steric hindrance for the intermolecular propagation of classic H bonds. Nevertheless, this does not avoid the multidirectional expansion of multiple O-H···O bonds from the three water molecules and the hydroxide anion to form an extended supramolecular structure. Curiously, this growth exclusively occurs in a perfect 2D arrangement, parallel to the b axis of the unit cell (Figures 3 and 4).   To form these supramolecular layers, each heterotrinuclear cation is simultaneously acting as Hdonor, through the μ3-HO group and a water molecule coordinated to the holmium atom, connected to two O acceptors belonging to one of the nitrate counterions, while the water molecule is also linked to a solvated ethanol molecule. This substructure is mutually connected to an inverted one, as the third O atom of each nitrate is acting as an H-acceptor for the ethanol solvate of the other substructure. Additionally, a second water molecule coordinated to the holmium atom displays an intramolecular H-bond to the methoxy group of the central arm of L 3− , and an intermolecular one involving a second nitrate counterion.
Finally, the water molecule coordinated to one of the zinc atoms participates in an intramolecular bifurcated H-bond with the deprotonated aldehyde ligand, while it is additionally connected to an O atom of a third nitrate anion, which also participate in the second type of interactions described above. Thus, each one of the enantiomers of the complex is H-bonded to three nitrate ions, and it is also connected to an identical enantiomer, and to two units of inverted chirality, but all of them extended on infinite 2D sheets. These interactions are illustrated by Figures 3 and 4.
The surface of each layer thusly formed presents a hydrophobic character, as the predominant bonds are mostly C-H, corresponding to imidazolidine rings, ethylene chains and aromatic rings. Likewise, C-Br bonds and some nitrate counterions that are scarcely accessible are forming part of this surface (Figure 3). Consequently, these layers are not connected via classic bonds, but through C-H···A bonds (A = O or Br). This behaviour has been previously observed for other related ligands [24].

Conclusions
The heterotrinuclear complex [Zn2Ho III (L)(ald)(HO)(H2O)(MeCN)](NO3)2·EtOH is chiral due to the asymmetry of the different coordination environments. Despite the presence of multiple and varied potential donors and acceptors for H bonding, the packing scheme is basically bidimensional, and mostly based on classic O-H···O bonds. The surface of the layers formed is rather hydrophobic, and the interaction between layers mostly depends on C-H···Br and C-H···O interactions.
Supplementary Materials: Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre (CCDC-1843238) and can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

Author Contributions:
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.