Self-assembly of zinc(II) tetraaza macrocyclic complex and 1,3,5-cyclohexanetricarboxylic acid

Introduction

Self-assembly of the transition metal(II) macrocyclic complexes and aromatic polycarboxylate ligands has been proved to be good building blocks for the construction of covalent coordination polymer and multidimensional networks (Choi and Suh, 1998; Choi et al., 1999, 2001, 2003; Eddaoudi et al., 2001; Cao et al., 2002; Choi, 2002; Kim et al., 2002; Park et al., 2007; Choi and Kim, 2008; Kwag et al., 2010; Sen et al., 2013). As previously reported, aromatic and aliphatic polycarboxylate ligands have long been used for the construction of covalent coordination polymers. For example, the self-assembly of Zn(L)(NO₃)₂ (Park et al., 2007) with oxalate or fumarate leads to 1D carboxylate-bridged nickel(II) complexes {[Zn(L)(oxalate)·3.5H₂O}_n (Park et al., 2007) and {[Zn(L)(fumarate)·2H₂O}_n (Kwag et al., 2010), in which the zinc(II) ions have a distorted octahedral geometry. In comparison, the reaction of [Cu(L)]Cl₂·2H₂O (Choi et al., 1996) and 2,3-pyrazinedicarboxylic acid (H₂pdc) gives rise to a 3D copper(II) complex {[Cu₂(L)(pdc)₂(H₂O)₂]·12H₂O}_n (Choi, 2002), in which each copper(II) atom reveals an axially elongated octahedral geometry. Generally, self-assembly involves the spontaneous aggregation of molecules into stable, noncovalent joined ensembles displaying multidimensional networks (Tecila et al., 1990). Hydrogen bonding is one of the key interactions generating molecular aggregation and creates novel molecular assemblies (Lehn et al., 1992). For example, the reaction of [M(L)]Cl₂·2H₂O (M=Ni²⁺ and Cu²⁺) (Choi et al., 1996) with 1,1-cyclobutanedicarboxylic acid (H₂cbdc) generates 1D and 2D hydrogen-bonded polymers [Ni(L)(Hcbdc^-)₂] and [Cu(L)(Hcbdc^-)₂]Cl₂·2H₂O (Choi and Kim, 2008), which show a distorted octahedral environment. The different molecular topologies in these complexes may be due to the different coordination modes of the ligands.

To investigate further the effect of polycarboxylate ligands on self-assembly, we synthesized and characterized the 3D hydrogen-bonded polymer [Zn(L)(H₂chtc^-)]H₂chtc^-·H₂O (1) (Scheme 1), which was assembled by [Zn(L)(H₂O)₂]·Cl₂ (L=3,14-dimethyl-2,6,13,17-tetraazatricyclo[14,4,0^1.18,0^7.12]docosane) and 1,3,5-cyclohexanetricarboxylic acid (H₃chtc).

Scheme 1:

Self-assembly of [Zn(L)(H₂chtc^-)]H₂chtc^-·H₂O (1).

Results and discussion

An Oak Ridge Thermal Ellipsoid Plot (ORTEP) (Farrugia, 1997) of [Zn(L)(H₂chtc^-)]H₂chtc^-·H₂O (1) with the atomic numbering scheme is shown in Figure 1. Selected bond lengths and angles are listed in Table 1. The macrocyclic ligand skeleton of the present compound adopts the trans-III(R,R,S,S) conformation with two chair six-membered and two gauche five-membered chelate rings. The complex consists of a [Zn(L)(H₂chtc^-)]⁺ cation and H₂chtc^- anion. The zinc atom is five-coordinate, distorted square pyramidal geometry with bonds to four nitrogen atoms of macrocycle and to the oxygen atom of H₂chtc^- ligand. The basal plane is slightly distorted [deviation N(1) -0.0235(19), N(2) 0.0225(18), N(3) -0.0228(18), N(4) 0.0238(19) Å from the least-squares plane through these basal donor atoms] with zinc atom displaced 0.4602(19) Å toward the axial oxygen atom of H₂cbtc^- ligand. The average Zn-N distance of 2.117(4) Å lies within previously reported values in related systems [Zn(L)(NCS)]NCS (2.124(10) Å) (Choi and Suh, 1997) and [Zn(L1)Cl][ClO₄] (L1=5,12-dimethyl-1,4-8,11-tetraazacyclotetradecane, 2.121(9) Å) (Choi, 1997), which have a distorted square pyramidal geometry. Zn-O(1) bond distance of 2.026(3) Å is ca. 0.1 Å shorter than equatorial Zn-N (secondary amines). The axial Zn-O(1) linkage is not perpendicular to the ZnN₄ plane with the four N-Zn-O(1) angles ranging from 99.82(14) to 105.80(13)°. The N-Zn-N angles of the six-membered chelate ring are larger than those of the five-membered chelate ring. The Zn-O(1)-C(21) angle and C(21)-O(1) distance relative to the H₂chtc^- ligand are 125.0(3)° and 1.257(5) Å, respectively. The carboxylate oxygen atom O(2) in the H₂chtc^- ligand forms the intramolecular hydrogen bond to an adjacent secondary amine N(3) of the macrocycle [N(3)-H(35)···O(2); 2.864(5) Å, 161(5)°]. Interestingly, the two secondary amines N(1) and N(4) form intermolecular hydrogen bonds with the carboxylate oxygen atoms of H₂chtc^- ligand [N(1)-H(33)···O(9)#i 2.930(5) Å, 175(4)2°; N(4)-H(36)···O(10)#i 3.005(5) Å, 174(5)°; symmetry code (#i): x+1, -y+3/2, z+1/2]. Furthermore, the protonated oxygen atoms O(3), O(5), O(7), and O(11) in the H₂chtc^- ligand form the intermolecular hydrogen bonds to an adjacent deprotonated oxygen atoms O(2), O(4), O(9), and O(10) of the H₂chtc^- ligand [O(3)-H(37)···O(10)#iii; 2.509(4) Å, 177.0(2)°; O(5)-H(38)···O(2)#iv; 2.622(5) Å, 175(6)°; O(7)-H(39)···O(9)#v; 2.556(4) Å, 157(5)°; O(11)-H(40)···O(4)#i; 2.735(5) Å, 166(5)°; symmetry codes (#iii) x, -y+3/2, z-1/2; (#iv) –x+1, -y+2, -z+1; (#v) x, -y+3/2, z+1/2]. The water molecules also form the intermolecular hydrogen bonds to secondary amine N(2) and the deprotonated oxygen atoms O(6) and O(12) of the H₂chtc^- ligand [N(2)-H(34)···Ow#ii; 2.998(5) Å, 171(5)°; Ow-HOw2···O(6)#vi; 2.793(5) Å, 153.1(3)°; Ow-HOw1···O(12)#vii; 2.907(5) Å, 172.9(3)°; symmetry codes (#vi): x+1, y, z; (#vii) –x+2, y+1/2, -z+3/2]. This interaction gives rise to a 3D hydrogen-bonded polymer (Figure 2 and Table 2).

Figure 1:

An ORTEP drawing (30% probability ellipsoids) of [Zn(L)( H₂chtc^-)]H₂chtc^-·H₂O (1) with the atomic numbering scheme.

The hydrogen bonds are shown as dashed lines. Symmetry codes: (#viii) x+2, y, z+1; (#ix) x+1, y, z+1.

Figure 2:

Packing diagram of [Zn(L)( H₂chtc^-)]H₂chtc^-·H₂O (1).

The hydrogen bonds are shown as dashed lines.

The IR spectrum of 1 shows a band at 3156 cm^-1 corresponding to the ν(NH) of the coordinated secondary amines of the macrocycle. Two strong bands exhibit ν_as(COO) stretching frequency at 1544 cm^-1 and ν_sym(COO) at 1301 cm^-1, respectively. The value of Δν (243 cm^-1) indicates that the carboxylate groups coordinated to the zinc(II) ion only as a monodentate ligand (Bakalbassis et al., 1991; Cao et al., 2002). The TGA diagram of 1 further supports the structure determined by the X-ray diffraction method (Figure 3). The compound was heated in the temperature range 25–1000°C in nitrogen gas. The first weight loss is observed from 25 to 125°C, which is due to the loss of two water molecules (observed, 2.0%; calculated, 2.1%). A second weight loss corresponding to two H₂chtc^- ligands (observed, 48.3%; calculated, 48.7%) is found in the temperature range 125–380°C (266–342°C). The third weight loss is observed from 380 to 581°C, which is due to the macrocycle (observed, 39.8%; calculated, 39.6%). Further weight loss is observed in the temperature range 581–1000°C corresponding to the ZnO residue (observed 9.9%, calculated 9.6%). The formation of ZnO accompanies the decomposition of the macrocycle ligand in the zinc(II) complex (Dong et al., 1999).

Figure 3:

Thermogravimetric curve of [Zn(L)( H₂chtc^-)]H₂chtc^-·H₂O (1).

Conclusions

The crystal structure of complex 1 shows a distorted square-pyramidal geometry around the zinc(II) atom, with the four nitrogen atoms of the macrocycle occupying the basal sites and one carboxylate oxygen atom of the H₂chtc^- ligand at the axial position. Interestingly, the intramolecular and intermolecular hydrogen bonds in the complex yield a 3D hydrogen-bonded polymer. The IR spectrum and the TGA behavior of the complex are significantly affected by the H₂chtc^- ligand.

Experimental section

X-ray crystallography

X-ray data were collected on a Bruker APEX II CCD diffractometer using graphite-monochromated Mo-Kα radiation (λ=0.71073 Å). Intensity data were measured at 100(2) K by the ω-2θ technique. Accurate cell parameters and an orientation matrix were determined by the least-squares fit of 25 reflections. The intensity data were corrected for Lorentz and polarization effects. An empirical absorption correction was applied with the multi-scan (Sheldrick, 1996). The structure was solved by direct methods (Sheldrick, 2008a), and the least-squares refinement of the structure was performed by the SHELXL-97 program (Sheldrick, 2008b). All atoms except all hydrogen atoms were refined anisotropically. The hydrogen atoms were placed in calculated positions allowing them to ride on their parent C atoms with U_iso(H)=1.2U_eq(C). The N1-H33, N3-N35 and N4-H36 were refined freely. The H37 of O3 was fixed. The N2-H34 and O5-H38 were used a DFIX command. The crystal parameters and details of the data collection and refinement are summarized in Table 3. Crystallographic data for the structural analysis have been deposited to the Cambridge Crystallographic Data Center, CCDC No. 1036819 for 1. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2,1EZ, UK (fax: +44-1223-336033; e-mail: deposit@ccdc.cam.uk or http://www.ccdc.cam.ac.uk).