Syntheses and crystal structures of the one-dimensional coordination polymers formed by [Ni(cyclam)]2+ cations and 1,3-bis(3-carboxypropyl)tetramethyldisiloxane anions in different degrees of deprotonation

The title coordination polymers show different degrees of deprotonation of the disiloxane-dicarboxylate bridging ligands: both contain tetragonally distorted trans-NiN4O2 octahedra.

The asymmetric units of the title compounds, namely, catena-poly [[(1,4,8,11tetraazacyclotetradecane-4 N 1 ,N 4 ,N 8 ,N 11 )nickel(II)]--1,3-bis(3-carboxylatopropyl)tetramethyldisiloxane-2 O:O 0 ], [Ni(C 10 H 24 O 5 Si 2 )(C 12 H 24 N 4 )] n (I), and catena-poly [[[(1,4,8,11-tetraazacyclotetradecane-4 N 1 ,N 4 ,N 8 (C 10 H 25 O 5 Si 2 )(C 12 H 24 N 4 )]ClO 4 } n (II), consist of one (in I) or two crystallographically non-equivalent (in II) centrosymmetric macrocyclic cations and one centrosymmetric dianion (in I) or two centrosymmetric monoanions (in II). In each compound, the metal ion is coordinated by the four secondary N atoms of the macrocyclic ligand, which adopts the most energetically stable trans-III conformation, and the mutually trans O atoms of the carboxylate in a slightly tetragonally distorted trans-NiN 4 O 2 octahedral coordination geometry. The crystals of both types of compounds are composed of parallel polymeric chains of the macrocyclic cations linked by the anions of the acid running along the [101] and [110] directions in I and II, respectively. In I, each polymeric chain is linked to four neighbouring ones by hydrogen bonding between the NH groups of the macrocycle and the carboxylate O atoms, thus forming a threedimensional supramolecular network. In II, each polymeric chain contacts with only two neighbours, forming hydrogen bonds between the partially protonated carboxylic groups of the bridging ligand. As a result, a lamellar structure is formed with the layers oriented parallel to the (111) plane.
In contrast to the widespread rigid aromatic carboxylates, flexible spacers incorporating polymethylene chains have rarely been used for the design of MOFs, although this could potentially lead to frameworks possessing unusual properties, the most intriguing of which is a 'breathing' phenomenon (Elsaidi et al., 2018;Lee et al., 2019). A representative example of such a highly flexible ligand is 1,3-bis(3-carboxypropyl)tetramethyldisiloxane -a member of a rather restricted family of silicon-containing carboxylic acids. However, no attempt has been made so far to combine this ligand with macrocyclic complexes in MOF synthesis.

Structural commentary
The molecular structures of the title compounds are shown in Figs. 1 and 2. Both complexes are one-dimensional coordina-
tion polymers consisting of centrosymmetric macrocyclic [Ni(L)] 2+ cations coordinated by the oxygen atoms of the carboxylic groups of the centrosymmetric acid, completely deprotonated (in I) and monoprotonated (in II), in the axial positions. In the latter case, there are two crystallographically independent cations and anions and the H2C and H5C acidic H atoms are distributed over two carboxylic groups with site occupancies of 50%. The macrocyclic ligands in the complex cations adopt the most energetically favourable trans-III (R,R,S,S) conformation (Bosnich et al., 1965) with five-membered chelate rings in gauche and six-membered chelate rings in chair conformations. As a result of the presence of the inversion centres, all Ni(N 4 ) fragments are strictly planar. The equatorial Ni-N bond lengths and bite angles fall in a range typical of high-spin 3d 8 nickel(II) complexes with 14-membered tetraamine ligands (Table 1). The axial Ni-O bond lengths are slightly longer than the Ni-N ones, and the geometry of the nickel(II) polyhedra can be described as tetragonally distorted trans-N 4 O 2 octahedra.
In two cases (Ni1 in I and Ni2 in II), a monodentate coordination of the carboxylate to the complex cation is complemented by strong hydrogen bonding between the noncoordinated O atom of the carboxylic group and the NH group of the macrocycle, which is often observed in complexes of cyclam-like ligands. For the [Ni1(L)] 2+ cation in II, the noncoordinated O2 atom is almost equidistant from the N1 and N2 centres [3.225 (5) and 3.143 (4) Å , respectively], so that two weak hydrogen bonds are formed in this case (Figs. 1 and 2, Tables 2 and 3).

Table 3
Hydrogen-bond geometry (Å , ) for II.  5) and are characterized by a slightly larger (14.684 Å ) intra-chain separation between the Ni II ions.
In the crystals, the interactions between the polymeric chains in I and II are characterized by markedly different features. In the first case, each chain is linked to four neighbouring ones as a result of hydrogen bonding between the N2-H2 groups of the macrocycles and carboxylate O2 atoms (Table 2), resulting in a three-dimensional supramolecular network. On the other hand, in II each polymeric chain contacts with only two neighbours via paired O2-H2CÁ Á ÁO5/ O2Á Á ÁH5C-O5 hydrogen bonds. The bonding is reinforced by the perchlorate anions bridging macrocyclic units: N1-H1Á Á ÁO8-Cl1-O7Á Á ÁH4-N4 (plus an additional very weak O2-H2CÁ Á ÁO8 contact) ( Table 3). As a result, a lamellar structure is formed with the layers lying parallel to the (111) plane (Fig. 6).

Database survey
A search of the Cambridge Structural Database (CSD, version 5.40, last update February 2019; Groom et al., 2016) indicated that seven compounds formed by 1,3-bis(3-carboxypropyl)tetramethyldisiloxane itself or its anions have been characterized structurally. Two of them are co-crystals of the acid with organic bases derived from pyridine [refcodes NERTOV (Vlad et al., 2013a) and VIPZUR (Racles et al., 2013)]. Other complexes represent one-or two-dimensional coordination polymers formed by Cu II (YIGXOD; Vlad et al., 2013b), Co II (NERTIP; Vlad et al., 2013a), Zn II [NERTUB (Vlad et al., 2013a), GIWSAI (Vlad et al., 2014) and GAPKOA (Zaltariov et al., 2016)]. Except for the last complex, in which the secondary building unit is a hexametal oxocluster bridged by salicylaldoxime ligands, all of the other compounds contain additional heterocyclic co-ligands. No attempt was made to combine this carboxylic acid with macrocyclic cations in MOF synthesis, and thus the title compounds I and II are the first examples of such compounds described so far.

Synthesis and crystallization
All chemicals and solvents used in this work were purchased from Sigma-Aldrich and were used without further purification. The macrocyclic nickel(II) complex Ni(L)(ClO 4 ) 2 (Barefield et al., 1976) and 1,3-bis(3-carboxypropyl)tetramethyldisiloxane (H 2 Cx) (Mulvaney & Marvel, 1961) were prepared by the reported methods. The packing in I viewed down the [101] direction with polymeric chains cross-linked by N-HÁ Á ÁO hydrogen bonds (dotted lines) to form a threedimensional supramolecular network. C-bound H atoms are omitted for clarity.

Figure 5
The packing in II viewed down the [110] direction with polymeric chains cross-linked by hydrogen bonds (dotted lines).

Figure 6
The hydrogen-bonded sheet in II parallel to the (111) plane. C-bound H atoms are omitted for clarity.
{Ni(L)(Cx)} n , (I). To a solution of 48 mg (0.24 mmol) of the ligand L in 4 ml of water, 30 mg of nickel(II) hydroxide (0.32 mmol) were added and the suspension stirred for 4 d at room temperature to give a yellow-coloured solution. The excess of Ni(OH) 2 was filtered off and the filtrate was treated with the solution of 75 mg (0.24 mmol) of H 2 Cx in 2 ml of MeOH. This solution was rotary evaporated to give an oily material. The residue was dissolved in 2 ml of MeOH, and the product precipitated with acetonitrile. It was recrystallized in a similar fashion from a MeOH/MeCN (1:15 v/v) solvent mixture. Yield 54 mg (40%). Analysis calculated for C 22 H 48 N 4 NiO 5 Si 2 : C,46.89;H,8.59;N,9.94%. Found: C,46.76;H,8.64;N,9.85%. Single crystals of I suitable for X-ray diffraction analysis were obtained analogously, except that precipitation was carried out using a diffusion regime (a methanolic solution of complex was layered with MeCN).
{[Ni(L)(HCx)]ClO 4 } n (II). A solution of 100 mg (0.26 mmol) of K 2 Cx in 1 ml of water was added to a solution of 130 mg (0.28 mmol) of [Ni(L)](ClO 4 ) 2 in 3 ml of water and the mixture was left at room temperature. Potassium perchlorate crystals, which formed after ca two weeks, were removed by filtration and the filtrate was allowed to evaporate slowly at room temperature. The crystals of the product formed after about one month. Yield 59 mg (34% Single crystals of II suitable for X-ray diffraction analysis were selected from the sample resulting from the synthesis.
Safety note: Perchlorate salts of metal complexes are potentially explosive and should be handled with care.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4 (Macrae et al., 2020); software used to prepare material for publication: publCIF (Westrip, 2010). [[(1,4,8,11-tetraazacyclotetradecane-κ 4 N 1 ,N 4 ,N 8 ,N 11 ) where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.56 e Å −3 Δρ min = −0.61 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refined as a 2-component twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.51 e Å −3 Δρ min = −0.44 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refined as a 2-component twin.